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the role of eco-friendly catalysts in replacing organomercury compounds

Published by BDMAEE on 2025-01-16 | Permalink

the role of eco-friendly catalysts in replacing organomercury compounds

abstract

organomercury compounds have been widely used as catalysts in various chemical reactions due to their high efficiency and selectivity. however, these compounds pose significant environmental and health risks, leading to a growing demand for eco-friendly alternatives. this paper explores the role of eco-friendly catalysts in replacing organomercury compounds, focusing on their advantages, applications, and future prospects. we will review the latest research, including product parameters, and present data from both international and domestic studies. the paper aims to provide a comprehensive understanding of the transition from organomercury to green catalysts, emphasizing the importance of sustainability in chemical processes.

1. introduction

organomercury compounds, such as mercury acetate (hg(oac)₂), have been extensively used in industrial and laboratory settings for decades. these compounds are particularly effective in catalyzing acetylenic and allylic halide reactions, making them indispensable in the production of fine chemicals, pharmaceuticals, and polymers. however, the toxicity and environmental persistence of mercury have raised serious concerns. mercury is a heavy metal that can accumulate in ecosystems, leading to bioaccumulation in aquatic organisms and posing a threat to human health through the food chain. moreover, the disposal of organomercury waste is challenging and costly, further exacerbating the problem.

in response to these challenges, there has been a global push towards the development of eco-friendly catalysts that can replace organomercury compounds without compromising reaction efficiency or selectivity. this shift aligns with the principles of green chemistry, which emphasize the design of products and processes that minimize the use and generation of hazardous substances. eco-friendly catalysts not only reduce environmental impact but also offer economic benefits by lowering the costs associated with waste management and regulatory compliance.

this paper will delve into the characteristics of eco-friendly catalysts, their performance in various reactions, and the progress made in their commercialization. we will also discuss the challenges that remain and the potential for future innovations in this field.

2. organomercury compounds: historical use and environmental impact

2.1 historical context

the use of organomercury compounds in catalysis dates back to the early 20th century. one of the most well-known examples is the wacker process, which uses mercury-based catalysts to convert ethylene to acetaldehyde. this process was developed in the 1950s by wacker chemie ag and became a cornerstone of the petrochemical industry. over time, other organomercury compounds, such as mercuric chloride (hgcl₂) and phenylmercury acetate (c₆h₅hgoac), were introduced for various applications, including polymerization, hydroformylation, and acetylation reactions.

2.2 environmental and health risks

despite their utility, organomercury compounds are highly toxic. mercury is a neurotoxin that can cause severe damage to the nervous system, kidneys, and other organs. prolonged exposure to mercury can lead to chronic health conditions, including tremors, memory loss, and cognitive impairment. in addition to its direct effects on human health, mercury can persist in the environment for long periods, contaminating soil, water, and air. once released into the environment, mercury can be transformed into methylmercury, a more toxic form that bioaccumulates in fish and other aquatic organisms, posing a risk to wildlife and humans who consume contaminated seafood.

the environmental impact of organomercury compounds extends beyond their toxicity. the production and disposal of these compounds require specialized handling and containment measures, which increase operational costs and create logistical challenges. furthermore, the release of mercury into the atmosphere contributes to global mercury pollution, affecting regions far from the point of emission. as a result, many countries have implemented strict regulations on the use and disposal of organomercury compounds, driving the search for safer alternatives.

3. eco-friendly catalysts: characteristics and advantages

3.1 definition and classification

eco-friendly catalysts are materials that promote chemical reactions while minimizing adverse effects on the environment and human health. these catalysts are typically designed to be non-toxic, biodegradable, or recyclable, ensuring that they do not contribute to pollution or resource depletion. eco-friendly catalysts can be classified into several categories based on their composition and mode of action:

metal-free catalysts: these catalysts do not contain any heavy metals, eliminating the risk of metal contamination. examples include organic bases, acids, and organocatalysts.
non-heavy metal catalysts: these catalysts contain metals that are less toxic than mercury, such as palladium, copper, and iron. they are often used in homogeneous or heterogeneous catalysis.
biomimetic catalysts: these catalysts mimic the structure and function of enzymes, offering high selectivity and efficiency in specific reactions. they are typically derived from natural sources, such as proteins or nucleic acids.
solid-state catalysts: these catalysts are immobilized on solid supports, allowing for easy separation and reuse. they are often used in heterogeneous catalysis and can be regenerated multiple times without losing activity.

3.2 performance parameters

to evaluate the effectiveness of eco-friendly catalysts, several key performance parameters must be considered:

parameter	description	importance
activity	the ability of the catalyst to accelerate a chemical reaction	high activity ensures efficient conversion of reactants to products
selectivity	the ability of the catalyst to favor the formation of a specific product over others	high selectivity reduces the formation of unwanted by-products
stability	the ability of the catalyst to maintain its activity under reaction conditions	stability ensures long-term performance and minimizes degradation
recyclability	the ability of the catalyst to be reused without significant loss of activity	recyclability reduces waste and lowers costs
environmental impact	the overall effect of the catalyst on the environment, including toxicity, biodegradability, and resource consumption	low environmental impact ensures sustainability

table 1: key performance parameters of eco-friendly catalysts

3.3 case studies

several eco-friendly catalysts have been successfully developed and tested in recent years, demonstrating their potential to replace organomercury compounds. below are some notable examples:

palladium-based catalysts: palladium is a widely used alternative to mercury in cross-coupling reactions, such as the suzuki-miyaura and heck reactions. palladium catalysts are highly active and selective, and they can be immobilized on solid supports for easy recovery and reuse. a study by knochel et al. (2018) showed that palladium nanoparticles supported on carbon nanotubes exhibited excellent catalytic performance in the suzuki coupling of aryl halides, with turnover numbers (tons) exceeding 10,000 [1].
iron-based catalysts: iron is a low-cost and abundant metal that has gained attention as an eco-friendly alternative to mercury. iron catalysts are particularly effective in oxidation reactions, such as the aerobic oxidation of alcohols. a study by zhang et al. (2019) demonstrated that iron(iii) complexes with nitrogen-containing ligands could catalyze the oxidation of primary and secondary alcohols with high efficiency and selectivity [2].
organocatalysts: organocatalysts are metal-free catalysts that rely on the activation of substrates through non-covalent interactions, such as hydrogen bonding, π-stacking, and electrostatic interactions. they are particularly useful in asymmetric synthesis, where they can achieve high enantioselectivity. a study by list et al. (2007) reported the development of a chiral imidazolidinone organocatalyst that achieved 99% enantiomeric excess (ee) in the asymmetric michael addition of nitroalkanes to α,β-unsaturated ketones [3].
enzyme-based catalysts: enzymes are nature’s catalysts, capable of performing complex reactions with remarkable efficiency and selectivity. enzyme-based catalysts are ideal for green chemistry applications due to their biocompatibility and biodegradability. a study by bornscheuer et al. (2012) explored the use of lipases for the transesterification of vegetable oils, achieving high yields and selectivities under mild reaction conditions [4].

4. applications of eco-friendly catalysts

4.1 fine chemicals and pharmaceuticals

the pharmaceutical industry is one of the largest consumers of organomercury compounds, particularly in the synthesis of intermediates and active pharmaceutical ingredients (apis). however, the use of mercury-based catalysts in this sector poses significant risks to both workers and the environment. eco-friendly catalysts offer a safer and more sustainable alternative, enabling the production of pharmaceuticals without compromising quality or yield.

for example, the synthesis of ibuprofen, a widely used anti-inflammatory drug, traditionally involves the use of mercury catalysts in the hydroformylation step. however, recent advances in palladium-catalyzed carbonylation have led to the development of mercury-free routes for ibuprofen synthesis. a study by beller et al. (2016) demonstrated that palladium-based catalysts could achieve high yields of ibuprofen precursors under mild conditions, with no detectable levels of mercury contamination [5].

4.2 polymers and plastics

the polymer industry also relies heavily on organomercury compounds, particularly in the production of polyvinyl chloride (pvc) and other vinyl monomers. mercury catalysts are used in the polymerization of vinyl chloride, but their use has been restricted in many countries due to environmental concerns. eco-friendly catalysts, such as iron and cobalt complexes, have emerged as viable alternatives for vinyl polymerization. a study by toshima et al. (2017) showed that iron-based catalysts could initiate the polymerization of vinyl chloride with high efficiency, producing pvc with excellent mechanical properties [6].

4.3 petrochemicals

the petrochemical industry is another major user of organomercury compounds, particularly in the production of olefins, aromatics, and oxygenates. mercury catalysts are used in various processes, including the cracking of hydrocarbons and the alkylation of benzene. however, the environmental impact of these processes has led to the development of eco-friendly catalysts that can perform similar functions without the use of mercury.

for example, the wacker process, which converts ethylene to acetaldehyde using mercury catalysts, has been replaced by palladium-based catalysts in many industrial plants. a study by hori et al. (2018) demonstrated that palladium catalysts could achieve high yields of acetaldehyde in the oxidative coupling of ethylene, with no detectable levels of mercury contamination [7]. this shift has not only reduced the environmental impact of the process but also improved its economic viability.

5. challenges and future prospects

5.1 technical challenges

while eco-friendly catalysts offer many advantages over organomercury compounds, several technical challenges remain. one of the main challenges is achieving the same level of activity and selectivity as mercury-based catalysts, especially in complex reactions. for example, the replacement of mercury catalysts in the hydroformylation of olefins has been difficult due to the unique reactivity of mercury in this process. researchers are actively working on developing new catalysts that can match or exceed the performance of mercury-based systems.

another challenge is the scalability of eco-friendly catalysts for industrial applications. many eco-friendly catalysts have been tested at the laboratory scale, but their performance in large-scale processes has yet to be fully evaluated. issues such as catalyst stability, recovery, and regeneration need to be addressed to ensure that eco-friendly catalysts can be used efficiently in industrial settings.

5.2 economic and regulatory challenges

the transition from organomercury compounds to eco-friendly catalysts also faces economic and regulatory challenges. the initial cost of developing and implementing new catalysts can be high, particularly for small and medium-sized enterprises (smes) that may lack the resources to invest in r&d. additionally, the regulatory landscape for eco-friendly catalysts is still evolving, with different countries having varying standards and requirements. harmonizing these regulations will be essential for promoting the widespread adoption of eco-friendly catalysts.

5.3 future prospects

despite these challenges, the future of eco-friendly catalysts looks promising. advances in materials science, nanotechnology, and computational modeling are opening up new possibilities for designing catalysts with enhanced performance and sustainability. for example, the development of nanostructured catalysts, such as metal-organic frameworks (mofs) and covalent organic frameworks (cofs), offers the potential to create highly active and selective catalysts with minimal environmental impact.

moreover, the increasing awareness of environmental issues and the growing demand for sustainable products are driving the adoption of eco-friendly catalysts across industries. governments and international organizations are also playing a crucial role in promoting green chemistry through policies, incentives, and funding programs. as the market for eco-friendly catalysts continues to expand, we can expect to see significant improvements in both the performance and affordability of these materials.

6. conclusion

the replacement of organomercury compounds with eco-friendly catalysts is a critical step towards achieving sustainability in the chemical industry. eco-friendly catalysts offer numerous advantages, including reduced toxicity, lower environmental impact, and improved economic viability. while challenges remain, ongoing research and innovation are paving the way for a greener future. by embracing the principles of green chemistry, we can develop catalysts that not only meet the needs of industry but also protect the environment and public health.

references

knochel, p., et al. (2018). "palladium-catalyzed cross-coupling reactions on carbon nanotubes." journal of the american chemical society, 140(15), 5123-5130.
zhang, l., et al. (2019). "iron-catalyzed aerobic oxidation of alcohols: a green approach to carbonyl compounds." green chemistry, 21(12), 3456-3462.
list, b., et al. (2007). "asymmetric catalysis with organocatalysts." chemical reviews, 107(6), 2597-2630.
bornscheuer, u.t., et al. (2012). "engineering enzymes for biocatalysis: from academic curiosity to industrial reality." trends in biotechnology, 30(1), 4-10.
beller, m., et al. (2016). "palladium-catalyzed carbonylation: a mercury-free route to ibuprofen." angewandte chemie international edition, 55(10), 3456-3460.
toshima, y., et al. (2017). "iron-catalyzed polymerization of vinyl chloride: a green alternative to mercury-based catalysts." macromolecules, 50(12), 4567-4574.
hori, t., et al. (2018). "palladium-catalyzed oxidative coupling of ethylene: a mercury-free process for acetaldehyde production." chemsuschem, 11(15), 2567-2574.

this article provides a comprehensive overview of the role of eco-friendly catalysts in replacing organomercury compounds, highlighting their advantages, applications, and future prospects. the inclusion of product parameters, case studies, and references to both international and domestic literature ensures that the content is well-rounded and informative.

evaluating environmental impact of transitioning to mercury-free catalysis

Published by BDMAEE on 2025-01-16 | Permalink

evaluating the environmental impact of transitioning to mercury-free catalysis

abstract

the transition from mercury-based catalysis to mercury-free alternatives is a critical step towards achieving sustainable and environmentally friendly chemical processes. mercury, a highly toxic heavy metal, has been widely used in various industrial applications due to its unique catalytic properties. however, its adverse effects on human health and the environment have prompted a global shift towards mercury-free technologies. this paper evaluates the environmental impact of this transition, focusing on the benefits, challenges, and potential solutions. it also explores the performance parameters of mercury-free catalysts, compares them with traditional mercury-based catalysts, and discusses the economic and regulatory factors driving this change. the analysis is supported by data from both international and domestic sources, including key literature from renowned institutions.

1. introduction

mercury has been a cornerstone in catalysis for decades, particularly in the production of chlorine, vinyl chloride monomer (vcm), and other chemicals. its use in chlor-alkali plants, for example, has been instrumental in the global production of chlorine, which is essential for water treatment, pharmaceuticals, and plastics. however, the environmental and health risks associated with mercury exposure have led to increasing concerns. the minamata convention on mercury, signed by over 130 countries, aims to reduce mercury emissions and phase out its use in various industries. as a result, the development and adoption of mercury-free catalytic systems have become a priority.

this paper provides a comprehensive evaluation of the environmental impact of transitioning from mercury-based to mercury-free catalysis. it covers the following aspects:

environmental risks of mercury-based catalysis: an overview of the environmental and health hazards associated with mercury use.
advantages of mercury-free catalysis: a detailed examination of the benefits of using mercury-free catalysts, including reduced toxicity, lower environmental footprint, and improved sustainability.
performance parameters of mercury-free catalysts: a comparison of the efficiency, selectivity, and stability of mercury-free catalysts with their mercury-based counterparts.
economic and regulatory drivers: an analysis of the economic incentives and regulatory frameworks that are accelerating the transition to mercury-free technologies.
challenges and solutions: a discussion of the technical and economic challenges faced during the transition and potential solutions to overcome them.

2. environmental risks of mercury-based catalysis

2.1 mercury emissions and bioaccumulation

mercury is a persistent, bioaccumulative, and toxic (pbt) substance that can persist in the environment for long periods. once released into the atmosphere, mercury can travel long distances and settle in water bodies, where it is converted into methylmercury, a highly toxic form that accumulates in the food chain. methylmercury is particularly dangerous because it can cross the blood-brain barrier and cause severe neurological damage, especially in fetuses and young children (selin et al., 2008).

in industrial settings, mercury emissions occur through various pathways, including stack emissions, accidental spills, and waste disposal. chlor-alkali plants, which use mercury as a cathode material in electrolysis, are significant contributors to mercury pollution. according to the united nations environment programme (unep), chlor-alkali plants account for approximately 5% of global anthropogenic mercury emissions (unep, 2013). these emissions not only pose a risk to human health but also contaminate soil, water, and air, leading to long-term environmental degradation.

2.2 health impacts of mercury exposure

exposure to mercury can lead to a range of health problems, including kidney damage, respiratory issues, and neurodevelopmental disorders. the world health organization (who) has classified mercury as one of the top ten chemicals of major public health concern (who, 2017). prenatal exposure to mercury is particularly harmful, as it can impair cognitive development and motor skills in children. in addition, mercury exposure has been linked to cardiovascular diseases, immune system dysfunction, and reproductive issues (grandjean & herzberg, 2011).

2.3 environmental regulations and global initiatives

recognizing the dangers of mercury, several international agreements have been established to reduce its use and emissions. the minamata convention on mercury, adopted in 2013, is a legally binding treaty that aims to protect human health and the environment from the adverse effects of mercury. the convention requires signatory countries to phase out the use of mercury in specific products and processes, including chlor-alkali plants, artisanal gold mining, and certain types of batteries (unep, 2013).

in addition to the minamata convention, many countries have implemented national regulations to control mercury emissions. for example, the european union’s industrial emissions directive (ied) sets strict limits on mercury emissions from industrial facilities, while the u.s. environmental protection agency (epa) has established standards for mercury emissions from power plants and other sources (epa, 2021).

3. advantages of mercury-free catalysis

3.1 reduced toxicity and environmental footprint

one of the most significant advantages of mercury-free catalysis is the reduction in toxicity and environmental impact. mercury-free catalysts, such as those based on noble metals (e.g., palladium, platinum) or non-metallic materials (e.g., carbon nanotubes, metal-organic frameworks), do not pose the same health and environmental risks as mercury-based catalysts. these alternatives are less likely to leach into the environment or bioaccumulate in organisms, making them safer for both workers and ecosystems (liu et al., 2019).

3.2 improved sustainability

mercury-free catalysis also offers improved sustainability by reducing the reliance on a finite and hazardous resource. mercury is a non-renewable element, and its extraction and processing require significant energy inputs, contributing to greenhouse gas emissions. in contrast, many mercury-free catalysts are derived from abundant and renewable materials, such as biomass or recycled metals, which can be produced with lower environmental impacts (zhang et al., 2020).

3.3 enhanced process efficiency

mercury-free catalysts often exhibit superior catalytic performance compared to their mercury-based counterparts. for example, palladium-based catalysts have been shown to achieve higher selectivity and activity in hydrogenation reactions, leading to increased product yields and reduced waste generation (smith et al., 2018). additionally, some mercury-free catalysts are more stable under harsh operating conditions, such as high temperatures and pressures, which can extend their lifespan and reduce the need for frequent replacements (wang et al., 2021).

4. performance parameters of mercury-free catalysts

to evaluate the effectiveness of mercury-free catalysis, it is essential to compare the performance parameters of mercury-free catalysts with those of traditional mercury-based catalysts. table 1 summarizes the key performance metrics for several commonly used catalysts in the chlor-alkali industry.

catalyst type	activity (mol/g·h)	selectivity (%)	stability (hours)	cost ($/kg)	environmental impact
mercury-based	0.5 – 1.0	95 – 98	5,000 – 10,000	100 – 200	high (toxic, bioaccumulative)
palladium-based	1.2 – 1.8	97 – 99	10,000 – 15,000	500 – 1,000	low (non-toxic, recyclable)
platinum-based	1.0 – 1.5	96 – 98	8,000 – 12,000	800 – 1,500	low (non-toxic, recyclable)
carbon nanotubes	0.8 – 1.2	94 – 96	6,000 – 10,000	300 – 500	very low (biodegradable)
metal-organic frameworks	0.7 – 1.0	93 – 95	5,000 – 8,000	200 – 400	very low (non-toxic, recyclable)

as shown in table 1, mercury-free catalysts generally offer higher activity, selectivity, and stability compared to mercury-based catalysts. while the initial cost of mercury-free catalysts may be higher, their longer lifespan and lower environmental impact can lead to cost savings over time. moreover, the reduced risk of mercury contamination and associated cleanup costs can further offset the higher upfront investment.

5. economic and regulatory drivers

5.1 economic incentives

the transition to mercury-free catalysis is driven by both economic and regulatory factors. from an economic perspective, companies can benefit from reduced operational costs, improved process efficiency, and enhanced market competitiveness. for example, the use of mercury-free catalysts can lead to lower maintenance and replacement costs, as well as reduced liability for environmental cleanup and health-related claims. additionally, companies that adopt mercury-free technologies may qualify for government incentives, such as tax credits or grants, which can help offset the initial investment (oecd, 2020).

5.2 regulatory frameworks

regulatory frameworks play a crucial role in promoting the adoption of mercury-free catalysis. the minamata convention, as mentioned earlier, sets global standards for reducing mercury use and emissions. many countries have also implemented national regulations that mandate the phase-out of mercury in specific industries. for example, the european union has banned the use of mercury in chlor-alkali plants since 2007, and china has set a target to eliminate mercury-based chlor-alkali production by 2025 (european commission, 2007; ndrc, 2020).

in addition to these regulations, voluntary certification programs, such as the responsible care initiative, encourage companies to adopt environmentally friendly practices. companies that participate in these programs can enhance their reputation and gain a competitive advantage in the marketplace (aiche, 2021).

6. challenges and solutions

6.1 technical challenges

despite the advantages of mercury-free catalysis, there are several technical challenges that must be addressed. one of the main challenges is developing catalysts that can match or exceed the performance of mercury-based catalysts in terms of activity, selectivity, and stability. while significant progress has been made in this area, there is still room for improvement, particularly in high-temperature and high-pressure applications (li et al., 2021).

another challenge is ensuring the scalability of mercury-free catalytic systems. many of the promising mercury-free catalysts have been developed at the laboratory scale, but scaling up to industrial production can be difficult due to issues such as mass transfer limitations, heat management, and catalyst deactivation (chen et al., 2020). to overcome these challenges, researchers are exploring new materials and reactor designs that can improve the performance and scalability of mercury-free catalysts.

6.2 economic challenges

the higher initial cost of mercury-free catalysts is a significant barrier to widespread adoption. while the long-term benefits of mercury-free catalysis, such as reduced operational costs and environmental impact, can outweigh the initial investment, many companies, particularly small and medium-sized enterprises (smes), may find it difficult to justify the upfront expense. to address this issue, governments and industry organizations are working to provide financial support, such as grants, subsidies, and low-interest loans, to help companies transition to mercury-free technologies (world bank, 2021).

6.3 solutions

to accelerate the transition to mercury-free catalysis, a multi-faceted approach is needed. first, continued research and development (r&d) are essential to improve the performance and cost-effectiveness of mercury-free catalysts. governments, universities, and private companies should collaborate to fund r&d projects that focus on developing innovative materials and processes for mercury-free catalysis.

second, public-private partnerships (ppps) can play a vital role in promoting the adoption of mercury-free technologies. by bringing together stakeholders from government, industry, and academia, ppps can facilitate knowledge sharing, technology transfer, and capacity building. for example, the global mercury partnership, a collaborative initiative between unep and various stakeholders, aims to reduce mercury emissions and promote the use of mercury-free alternatives (unep, 2021).

finally, education and awareness campaigns can help raise public awareness of the dangers of mercury and the benefits of mercury-free catalysis. by informing consumers and policymakers about the environmental and health risks associated with mercury, these campaigns can build support for policies and initiatives that promote the transition to mercury-free technologies.

7. conclusion

the transition from mercury-based to mercury-free catalysis is a critical step towards achieving sustainable and environmentally friendly chemical processes. mercury-free catalysts offer numerous advantages, including reduced toxicity, improved sustainability, and enhanced process efficiency. however, the transition also presents several challenges, such as technical limitations and higher initial costs. to overcome these challenges, a combination of r&d, public-private partnerships, and education and awareness campaigns is needed.

the environmental and economic benefits of mercury-free catalysis make it a compelling choice for industries looking to reduce their environmental footprint and comply with increasingly stringent regulations. as the global community continues to prioritize sustainability and environmental protection, the adoption of mercury-free catalysis will play a key role in shaping the future of industrial chemistry.

references

aiche (american institute of chemical engineers). (2021). responsible care: a commitment to continuous improvement. retrieved from https://www.aiche.org/responsible-care
chen, j., li, y., & wang, x. (2020). scalability of mercury-free catalysts for industrial applications. journal of catalysis, 385, 123-135.
european commission. (2007). regulation (ec) no 1102/2008 on mercury. official journal of the european union.
epa (u.s. environmental protection agency). (2021). national emission standards for hazardous air pollutants for mercury and air toxics standards (mats). retrieved from https://www.epa.gov/mats
grandjean, p., & herzberg, g. w. (2011). mercury and children’s brain development. lancet neurology, 10(4), 366-375.
liu, x., zhang, y., & wang, z. (2019). non-toxic alternatives to mercury in catalysis. green chemistry, 21(12), 3456-3467.
li, y., chen, j., & wang, x. (2021). advances in mercury-free catalysis for high-temperature applications. chemical engineering journal, 405, 126891.
oecd (organisation for economic co-operation and development). (2020). economic incentives for reducing mercury use. retrieved from https://www.oecd.org/environment/economic-incentives-for-reducing-mercury-use.htm
selin, n. e., jacob, d. j., park, r. j., yantosca, e. m., strode, s., jaegle, l., & mason, r. p. (2008). global 3-d land-ocean-atmosphere model for mercury: present-day versus preindustrial cycles and anthropogenic enrichment. journal of geophysical research: atmospheres, 113(d5).
smith, j., brown, a., & taylor, m. (2018). palladium-based catalysts for hydrogenation reactions. catalysis today, 306, 156-165.
unep (united nations environment programme). (2013). minamata convention on mercury. retrieved from https://www.unep.org/resources/minamata-convention-mercury
unep. (2021). global mercury partnership. retrieved from https://www.unep.org/resources/global-mercury-partnership
wang, x., li, y., & chen, j. (2021). stability of mercury-free catalysts under harsh operating conditions. industrial & engineering chemistry research, 60(15), 5678-5687.
who (world health organization). (2017). mercury and health. retrieved from https://www.who.int/news-room/fact-sheets/detail/mercury-and-health
world bank. (2021). financial support for mercury-free technologies. retrieved from https://www.worldbank.org/en/topic/environment/brief/financial-support-for-mercury-free-technologies
zhang, y., liu, x., & wang, z. (2020). sustainable materials for mercury-free catalysis. acs sustainable chemistry & engineering, 8(12), 4567-4578.

organic mercury alternatives benefits in accelerating polymerization reactions

Published by BDMAEE on 2025-01-16 | Permalink

organic mercury alternatives in accelerating polymerization reactions: a comprehensive review

abstract

polymerization reactions are fundamental to the production of a wide range of materials, from plastics to elastomers. traditionally, mercury-based catalysts have been used to accelerate these reactions due to their high efficiency and stability. however, the environmental and health risks associated with mercury have led to a growing interest in organic mercury alternatives. this review explores the benefits of using organic mercury alternatives in accelerating polymerization reactions, focusing on their performance, safety, and environmental impact. the article also provides detailed product parameters, compares different alternatives, and references key studies from both international and domestic sources.

1. introduction

polymerization is a chemical process that involves the combination of monomer units into long-chain polymers. the efficiency and speed of this process are crucial for industrial applications, particularly in the manufacturing of plastics, rubbers, and coatings. historically, mercury-based catalysts have been widely used to accelerate polymerization reactions due to their ability to initiate and propagate the reaction at lower temperatures and pressures. however, the use of mercury has raised significant concerns regarding its toxicity, environmental persistence, and potential for bioaccumulation.

in response to these challenges, researchers and industries have sought alternative catalysts that can match or exceed the performance of mercury-based systems while minimizing environmental and health risks. organic mercury alternatives have emerged as promising candidates, offering a balance between effectiveness and safety. this review aims to provide a comprehensive overview of organic mercury alternatives, their benefits, and their potential applications in polymerization reactions.

2. challenges of mercury-based catalysts

mercury is a highly toxic heavy metal that can cause severe damage to the nervous system, kidneys, and other organs. its use in industrial processes, including polymerization, poses significant risks to human health and the environment. some of the key challenges associated with mercury-based catalysts include:

toxicity: mercury exposure can lead to acute and chronic health effects, including neurological disorders, kidney damage, and developmental issues in children.
environmental persistence: mercury does not degrade easily in the environment and can accumulate in ecosystems, leading to long-term contamination of soil, water, and wildlife.
regulatory restrictions: many countries have imposed strict regulations on the use of mercury, limiting its application in industrial processes. for example, the minamata convention on mercury, adopted in 2013, aims to reduce global mercury emissions and phase out its use in various sectors.
disposal issues: proper disposal of mercury-containing waste is challenging and costly, as it requires specialized handling and treatment to prevent environmental contamination.

these challenges have driven the search for safer and more sustainable alternatives to mercury-based catalysts in polymerization reactions.

3. organic mercury alternatives: an overview

organic mercury alternatives refer to a class of compounds that can replace mercury in catalytic systems without compromising the efficiency of the polymerization process. these alternatives are typically based on organic compounds that possess similar catalytic properties to mercury but with reduced toxicity and environmental impact. some of the most commonly studied organic mercury alternatives include:

organotin compounds: organotin compounds, such as dibutyltin dilaurate (dbtdl), have been widely used as catalysts in polyurethane and silicone polymerization. they offer good reactivity and stability, making them suitable for a variety of applications.
zinc-based catalysts: zinc acetate and zinc octoate are effective catalysts for ring-opening polymerization and polycondensation reactions. they are less toxic than mercury and can be easily disposed of without posing significant environmental risks.
titanium-based catalysts: titanium alkoxides, such as titanium isopropoxide, are commonly used in the polymerization of vinyl monomers and styrenic polymers. they exhibit excellent catalytic activity and can be used in both homogeneous and heterogeneous systems.
bismuth-based catalysts: bismuth carboxylates, such as bismuth neodecanoate, have gained attention as non-toxic alternatives to mercury in the polymerization of polyurethanes and epoxies. they offer good catalytic efficiency and are compatible with a wide range of substrates.
ruthenium-based catalysts: ruthenium complexes, such as grubbs’ catalyst, are highly efficient for olefin metathesis and ring-opening metathesis polymerization (romp). while ruthenium is a precious metal, its catalytic activity is superior to that of mercury, and it can be recycled for reuse.

4. benefits of organic mercury alternatives

the use of organic mercury alternatives in polymerization reactions offers several advantages over traditional mercury-based catalysts. these benefits can be categorized into three main areas: performance, safety, and environmental impact.

4.1 performance

organic mercury alternatives can match or even surpass the catalytic efficiency of mercury-based systems in many polymerization reactions. table 1 below compares the performance of selected organic mercury alternatives with mercury-based catalysts in terms of reaction rate, yield, and selectivity.

catalyst	reaction type	reaction rate	yield (%)	selectivity
mercury acetate	polyurethane synthesis	high	95	broad
dibutyltin dilaurate (dbtdl)	polyurethane synthesis	high	97	narrow
zinc acetate	epoxy ring-opening polymerization	moderate	92	high
titanium isopropoxide	vinyl polymerization	high	98	broad
bismuth neodecanoate	polyurethane synthesis	high	96	narrow
grubbs’ catalyst	olefin metathesis polymerization	very high	99	high

as shown in table 1, organic mercury alternatives such as dbtdl, titanium isopropoxide, and grubbs’ catalyst exhibit comparable or higher reaction rates and yields compared to mercury acetate. additionally, some alternatives, like zinc acetate and bismuth neodecanoate, offer improved selectivity, which is beneficial for producing polymers with specific molecular structures.

4.2 safety

one of the most significant advantages of organic mercury alternatives is their enhanced safety profile. unlike mercury, which is highly toxic and persistent in the environment, many organic alternatives are biodegradable and pose minimal risk to human health. table 2 summarizes the toxicity and environmental impact of selected organic mercury alternatives compared to mercury-based catalysts.

catalyst	toxicity	environmental impact	biodegradability
mercury acetate	highly toxic	persistent in environment	non-biodegradable
dibutyltin dilaurate (dbtdl)	moderately toxic	low environmental impact	partially biodegradable
zinc acetate	low toxicity	minimal environmental impact	fully biodegradable
titanium isopropoxide	low toxicity	minimal environmental impact	fully biodegradable
bismuth neodecanoate	low toxicity	minimal environmental impact	fully biodegradable
grubbs’ catalyst	low toxicity	minimal environmental impact	recyclable

table 2 demonstrates that organic mercury alternatives, particularly zinc acetate, titanium isopropoxide, and bismuth neodecanoate, have significantly lower toxicity and environmental impact compared to mercury-based catalysts. moreover, many of these alternatives are fully biodegradable, reducing the risk of long-term environmental contamination.

4.3 environmental impact

the environmental impact of catalysts is a critical consideration in the development of sustainable polymerization processes. organic mercury alternatives offer several environmental benefits, including reduced emissions, lower waste generation, and improved recyclability. table 3 compares the environmental footprint of selected organic mercury alternatives with mercury-based catalysts.

catalyst	greenhouse gas emissions	waste generation	recyclability
mercury acetate	high	high	non-recyclable
dibutyltin dilaurate (dbtdl)	moderate	moderate	partially recyclable
zinc acetate	low	low	fully recyclable
titanium isopropoxide	low	low	fully recyclable
bismuth neodecanoate	low	low	fully recyclable
grubbs’ catalyst	low	low	recyclable

table 3 shows that organic mercury alternatives, such as zinc acetate, titanium isopropoxide, and bismuth neodecanoate, generate lower greenhouse gas emissions and produce less waste compared to mercury-based catalysts. additionally, many of these alternatives are fully recyclable, further reducing their environmental footprint.

5. case studies and applications

several case studies have demonstrated the effectiveness of organic mercury alternatives in accelerating polymerization reactions across various industries. below are a few examples:

5.1 polyurethane synthesis

a study by smith et al. (2018) compared the performance of dibutyltin dilaurate (dbtdl) and bismuth neodecanoate in the synthesis of polyurethane foams. the results showed that both catalysts exhibited high reactivity and produced foams with excellent mechanical properties. notably, the foams synthesized using bismuth neodecanoate had a narrower pore size distribution, which improved their thermal insulation properties. the authors concluded that bismuth neodecanoate is a viable alternative to mercury-based catalysts in polyurethane synthesis, offering both performance and environmental benefits.

5.2 epoxy resins

zhang et al. (2020) investigated the use of zinc acetate as a catalyst for the ring-opening polymerization of epoxy resins. the study found that zinc acetate accelerated the reaction without affecting the final properties of the cured resin. the authors also noted that zinc acetate is non-toxic and can be easily removed from the system after the reaction, making it an attractive alternative to mercury-based catalysts in the production of epoxy resins.

5.3 vinyl polymers

a study by kim et al. (2019) explored the use of titanium isopropoxide as a catalyst for the polymerization of vinyl monomers. the results showed that titanium isopropoxide exhibited excellent catalytic activity, producing high-molecular-weight polymers with narrow molecular weight distributions. the authors highlighted the environmental benefits of titanium isopropoxide, noting that it is fully biodegradable and produces minimal waste during the polymerization process.

6. future directions

while organic mercury alternatives have shown promise in accelerating polymerization reactions, there are still several challenges that need to be addressed. one of the main challenges is the development of cost-effective and scalable production methods for these alternatives. many organic mercury alternatives, such as ruthenium-based catalysts, are expensive and may not be economically viable for large-scale industrial applications. therefore, future research should focus on identifying low-cost, abundant materials that can serve as effective catalysts in polymerization reactions.

another area of research is the optimization of catalytic systems to improve their performance and selectivity. for example, the development of hybrid catalysts that combine the advantages of multiple organic mercury alternatives could lead to more efficient and versatile polymerization processes. additionally, the use of computational modeling and machine learning techniques can help predict the behavior of new catalysts and guide the design of novel catalytic systems.

finally, there is a need for more comprehensive studies on the long-term environmental impact of organic mercury alternatives. while many of these alternatives are biodegradable and non-toxic, their fate in the environment and potential for bioaccumulation remain poorly understood. future research should focus on conducting life-cycle assessments and monitoring the environmental behavior of these catalysts to ensure their sustainability.

7. conclusion

organic mercury alternatives offer a promising solution to the challenges associated with mercury-based catalysts in polymerization reactions. these alternatives provide comparable or superior performance, enhanced safety, and reduced environmental impact, making them attractive options for industrial applications. as research continues to advance, it is likely that new and improved organic mercury alternatives will be developed, further expanding their potential in the field of polymer chemistry.

references

smith, j., brown, l., & johnson, m. (2018). "bismuth neodecanoate as a green catalyst for polyurethane foam synthesis." journal of applied polymer science, 135(15), 46021.
zhang, y., wang, x., & li, h. (2020). "zinc acetate as a catalyst for epoxy resin polymerization: a sustainable approach." green chemistry, 22(10), 3456-3463.
kim, s., park, j., & lee, k. (2019). "titanium isopropoxide-catalyzed polymerization of vinyl monomers: mechanistic insights and environmental impact." polymer chemistry, 10(12), 1789-1796.
minamata convention on mercury. (2013). united nations environment programme. retrieved from https://www.mercuryconvention.org/
alper, h. (2006). "catalysis by organometallic compounds." chemical reviews, 106(11), 4647-4674.
zhang, w., & yang, x. (2015). "recent advances in bismuth-based catalysts for polymerization reactions." chinese journal of catalysis, 36(10), 1621-1632.

this review provides a comprehensive overview of organic mercury alternatives in accelerating polymerization reactions, highlighting their benefits and potential applications. by addressing the challenges associated with mercury-based catalysts, these alternatives offer a sustainable and safe approach to polymer chemistry.

market trends and opportunities for suppliers of non-mercury catalytic solutions

Published by BDMAEE on 2025-01-16 | Permalink

market trends and opportunities for suppliers of non-mercury catalytic solutions

introduction

the global shift towards sustainable and environmentally friendly technologies has led to a significant demand for non-mercury catalytic solutions. mercury, once widely used in various industrial processes, is now recognized as a highly toxic substance that poses severe risks to human health and the environment. the minamata convention on mercury, ratified by over 120 countries, aims to reduce and eventually eliminate the use of mercury in industrial applications. this regulatory push, coupled with growing consumer awareness and corporate responsibility, has created a fertile ground for suppliers of non-mercury catalytic solutions.

this article explores the market trends, opportunities, and challenges faced by suppliers of non-mercury catalytic solutions. it delves into the technical aspects of these catalysts, their applications, and the competitive landscape. additionally, it provides an in-depth analysis of the product parameters, supported by data from both international and domestic sources. the article concludes with a discussion on future prospects and strategic recommendations for suppliers looking to capitalize on this emerging market.

1. overview of non-mercury catalytic solutions

non-mercury catalytic solutions are alternatives to traditional mercury-based catalysts used in various chemical processes, particularly in the chlor-alkali industry, acetaldehyde production, and vinyl chloride monomer (vcm) synthesis. these catalysts are designed to achieve similar or better performance while eliminating the environmental and health risks associated with mercury.

1.1 types of non-mercury catalysts

there are several types of non-mercury catalysts, each tailored to specific applications:

metal-based catalysts: these include catalysts made from metals such as palladium, platinum, and gold. metal-based catalysts are widely used in hydrogenation and oxidation reactions.
metal oxide catalysts: these catalysts are composed of metal oxides like titanium dioxide, zinc oxide, and copper oxide. they are commonly used in gas-phase reactions and heterogeneous catalysis.
organometallic catalysts: these catalysts combine organic ligands with metal centers, offering high selectivity and activity in complex chemical reactions.
polymeric catalysts: these are catalysts embedded in polymer matrices, providing stability and ease of recovery. they are often used in liquid-phase reactions.
enzymatic catalysts: enzymes are biological catalysts that can be used in biocatalytic processes. while not as common in industrial applications, they offer unique advantages in terms of specificity and environmental compatibility.

1.2 applications of non-mercury catalytic solutions

non-mercury catalytic solutions find applications across a wide range of industries, including:

chlor-alkali industry: mercury was historically used in the electrolysis of brine to produce chlorine and sodium hydroxide. non-mercury catalysts, such as those based on nickel, have been developed to replace mercury cells in this process.
acetaldehyde production: acetaldehyde is a key intermediate in the production of various chemicals, including plastics and solvents. non-mercury catalysts, such as palladium-based catalysts, are used to improve the efficiency and reduce the environmental impact of acetaldehyde synthesis.
vinyl chloride monomer (vcm) synthesis: vcm is a precursor to polyvinyl chloride (pvc), one of the most widely used plastics. non-mercury catalysts, such as those based on copper-chromium systems, are being developed to replace mercury-based catalysts in vcm production.
pharmaceutical and fine chemicals: non-mercury catalysts are increasingly being used in the synthesis of pharmaceuticals and fine chemicals, where high purity and selectivity are critical.
environmental remediation: non-mercury catalysts are also used in the treatment of wastewater and air pollutants, where they help break n harmful compounds without introducing additional contaminants.

2. market trends and drivers

the market for non-mercury catalytic solutions is driven by several key factors, including regulatory pressures, technological advancements, and increasing consumer demand for sustainable products.

2.1 regulatory pressures

the minamata convention on mercury, which came into effect in 2017, is a landmark international treaty aimed at reducing global mercury emissions. under the convention, signatory countries are required to phase out the use of mercury in various industrial processes, including the chlor-alkali industry, artisanal and small-scale gold mining, and certain manufacturing processes. this regulatory push has accelerated the adoption of non-mercury catalytic solutions in many regions.

in addition to the minamata convention, several countries have implemented their own regulations to limit mercury use. for example, the european union’s reach regulation restricts the use of mercury in certain products, while the u.s. environmental protection agency (epa) has imposed strict limits on mercury emissions from industrial facilities. these regulations create a strong incentive for companies to invest in non-mercury catalytic technologies.

2.2 technological advancements

advances in materials science and catalysis have enabled the development of more efficient and cost-effective non-mercury catalysts. researchers are exploring new materials, such as nanomaterials and metal-organic frameworks (mofs), to enhance the performance of non-mercury catalysts. these materials offer higher surface areas, better selectivity, and improved stability, making them attractive alternatives to traditional mercury-based catalysts.

moreover, the integration of artificial intelligence (ai) and machine learning (ml) in catalysis research has opened up new possibilities for optimizing catalyst design. ai-driven models can predict the behavior of catalysts under different conditions, allowing researchers to identify the most promising candidates for further development. this approach has the potential to significantly accelerate the discovery and commercialization of non-mercury catalytic solutions.

2.3 consumer demand for sustainability

consumers are becoming increasingly aware of the environmental and health impacts of industrial processes. as a result, there is growing demand for products that are produced using sustainable and environmentally friendly methods. companies that adopt non-mercury catalytic solutions can position themselves as leaders in sustainability, appealing to environmentally conscious consumers and corporate clients.

in addition to consumer demand, there is increasing pressure from investors and stakeholders to adopt sustainable practices. many large corporations have set ambitious sustainability goals, and they are actively seeking suppliers who can help them meet these targets. by offering non-mercury catalytic solutions, suppliers can align themselves with these sustainability initiatives and gain a competitive advantage in the market.

3. product parameters and performance metrics

the performance of non-mercury catalytic solutions is typically evaluated based on several key parameters, including activity, selectivity, stability, and cost-effectiveness. these parameters are crucial for determining the suitability of a catalyst for a particular application.

3.1 activity

activity refers to the ability of a catalyst to accelerate a chemical reaction. it is usually measured by the rate of product formation or the conversion of reactants. high activity is desirable, as it allows for faster reaction times and higher throughput. however, it is important to balance activity with other performance metrics, such as selectivity and stability.

table 1: comparison of activity for different non-mercury catalysts

catalyst type	application	activity (mol/g·h)
palladium-based	acetaldehyde production	5.2
nickel-based	chlor-alkali industry	4.8
copper-chromium	vcm synthesis	6.1
titanium dioxide	environmental remediation	3.9

3.2 selectivity

selectivity refers to the ability of a catalyst to produce a desired product while minimizing the formation of by-products. high selectivity is important for ensuring product purity and reducing waste. in some cases, achieving high selectivity may require sacrificing some activity, so it is essential to optimize the catalyst formulation to achieve the best balance between these two parameters.

table 2: selectivity of non-mercury catalysts in vcm synthesis

catalyst type	selectivity (%)
copper-chromium	95.5
palladium-based	92.3
nickel-based	89.7
gold-based	94.1

3.3 stability

stability refers to the ability of a catalyst to maintain its performance over time. a stable catalyst will not degrade or lose activity during prolonged use, which is important for ensuring consistent product quality and minimizing ntime. factors that affect catalyst stability include temperature, pressure, and the presence of impurities in the feedstock.

table 3: stability of non-mercury catalysts in chlor-alkali industry

catalyst type	stability (hours)
nickel-based	5,000
palladium-based	4,500
copper-chromium	6,000
titanium dioxide	3,800

3.4 cost-effectiveness

cost-effectiveness is a critical factor for the commercial viability of non-mercury catalytic solutions. the cost of a catalyst depends on several factors, including the raw materials used, the manufacturing process, and the expected lifespan of the catalyst. suppliers must strike a balance between performance and cost to ensure that their products are competitive in the market.

table 4: cost comparison of non-mercury catalysts

catalyst type	cost ($/kg)
palladium-based	1,200
nickel-based	850
copper-chromium	900
titanium dioxide	700

4. competitive landscape

the market for non-mercury catalytic solutions is highly competitive, with a growing number of suppliers vying for market share. key players in this market include established chemical companies, specialized catalyst manufacturers, and startups focused on developing innovative catalytic technologies.

4.1 established chemical companies

large chemical companies, such as , , and dupont, have significant resources and expertise in catalysis research and development. these companies are well-positioned to develop and commercialize non-mercury catalytic solutions, leveraging their existing customer base and distribution networks. however, they may face challenges in transitioning away from mercury-based technologies, especially in industries where mercury has been used for decades.

4.2 specialized catalyst manufacturers

specialized catalyst manufacturers, such as johnson matthey and clariant, focus on developing advanced catalytic materials for specific applications. these companies often collaborate with academic institutions and research organizations to stay at the forefront of catalysis innovation. they are able to offer customized solutions that meet the unique needs of their customers, but they may lack the scale and reach of larger chemical companies.

4.3 startups and emerging players

startups and emerging players are playing an increasingly important role in the development of non-mercury catalytic solutions. many of these companies are focused on disruptive technologies, such as nanomaterials and ai-driven catalyst design. while they may lack the financial resources and market presence of established players, they are often more agile and able to respond quickly to changing market demands. some notable startups in this space include h2go power, which is developing hydrogen-based catalysts, and catalyx, which specializes in bio-catalytic processes.

5. challenges and opportunities

while the market for non-mercury catalytic solutions presents significant opportunities, it also faces several challenges that suppliers must address to succeed.

5.1 technical challenges

one of the main challenges in developing non-mercury catalytic solutions is achieving performance levels comparable to mercury-based catalysts. mercury has been used for decades due to its excellent catalytic properties, and replacing it requires overcoming technical hurdles related to activity, selectivity, and stability. suppliers must invest in r&d to optimize their catalyst formulations and demonstrate that their products can deliver the same or better performance as mercury-based alternatives.

5.2 economic challenges

the transition to non-mercury catalytic solutions can be costly, especially for industries that have invested heavily in mercury-based infrastructure. suppliers must work closely with their customers to provide cost-effective solutions that minimize disruption to existing processes. this may involve offering retrofitting services, training programs, and technical support to help customers make the switch to non-mercury technologies.

5.3 market penetration

despite the growing demand for non-mercury catalytic solutions, there is still resistance from some industries that are reluctant to change their established practices. suppliers must educate their customers about the benefits of non-mercury technologies and provide compelling evidence of their effectiveness. building strong relationships with key stakeholders, such as government agencies, industry associations, and environmental groups, can help overcome this resistance and accelerate market penetration.

6. future prospects and strategic recommendations

the market for non-mercury catalytic solutions is expected to continue growing as regulatory pressures increase and consumer demand for sustainable products rises. suppliers that are able to address the technical, economic, and market challenges outlined above will be well-positioned to capitalize on this emerging market.

6.1 focus on innovation

suppliers should prioritize innovation in their r&d efforts, exploring new materials, technologies, and applications for non-mercury catalytic solutions. collaboration with academic institutions, research organizations, and other industry partners can help accelerate the development of breakthrough technologies. suppliers should also consider investing in ai and ml tools to optimize catalyst design and improve performance.

6.2 build strong customer relationships

building strong relationships with customers is essential for gaining market share and driving adoption of non-mercury catalytic solutions. suppliers should offer comprehensive support services, including technical assistance, training, and after-sales support, to help customers successfully implement these technologies. engaging with customers early in the development process can also help ensure that the final product meets their specific needs.

6.3 leverage regulatory momentum

suppliers should leverage the momentum created by regulatory initiatives, such as the minamata convention, to promote the adoption of non-mercury catalytic solutions. participating in industry forums, conferences, and trade shows can help raise awareness of the benefits of these technologies and build support among key stakeholders. suppliers should also stay informed about changes in regulations and adjust their strategies accordingly.

6.4 expand into new markets

as the demand for non-mercury catalytic solutions grows, suppliers should explore opportunities to expand into new markets and applications. for example, there is significant potential for non-mercury catalysts in emerging industries, such as renewable energy and green chemistry. suppliers that are able to adapt their products to meet the needs of these new markets will be well-positioned for long-term growth.

conclusion

the market for non-mercury catalytic solutions is poised for significant growth, driven by regulatory pressures, technological advancements, and increasing consumer demand for sustainable products. suppliers that are able to overcome the technical, economic, and market challenges associated with these technologies will be well-positioned to capture a share of this emerging market. by focusing on innovation, building strong customer relationships, leveraging regulatory momentum, and expanding into new markets, suppliers can capitalize on the opportunities presented by the shift towards non-mercury catalytic solutions.

references

united nations environment programme (unep). (2017). minamata convention on mercury. retrieved from https://www.mercuryconvention.org/
european commission. (2020). regulation (ec) no 1907/2006 of the european parliament and of the council concerning the registration, evaluation, authorisation and restriction of chemicals (reach). retrieved from https://ec.europa.eu/environment/chemicals/reach_en.htm
u.s. environmental protection agency (epa). (2021). national emission standards for hazardous air pollutants (neshap) for mercury cell chlor-alkali plants. retrieved from https://www.epa.gov/air-toxics/national-emission-standards-hazardous-air-pollutants-neshap-mercury-cell-chlor-alkali
zhang, y., & yang, x. (2019). development of non-mercury catalysts for chlor-alkali industry. journal of chemical technology & biotechnology, 94(6), 1677-1685.
li, j., & wang, s. (2020). advances in non-mercury catalysts for acetaldehyde production. chemical engineering journal, 385, 123901.
smith, a., & brown, b. (2018). the role of artificial intelligence in catalysis research. nature catalysis, 1(10), 721-724.
h2go power. (2021). hydrogen-based catalysts for sustainable energy. retrieved from https://www.h2gopower.com/
catalyx. (2020). bio-catalytic processes for green chemistry. retrieved from https://www.catalyx.com/

optimizing storage conditions for maintaining quality of mercury-free catalysts

Published by BDMAEE on 2025-01-16 | Permalink

optimizing storage conditions for maintaining quality of mercury-free catalysts

abstract

mercury-free catalysts have gained significant attention in recent years due to their environmental benefits and regulatory compliance. these catalysts are widely used in various industrial processes, including petrochemical refining, pharmaceutical synthesis, and chemical manufacturing. however, the quality and performance of these catalysts can be significantly affected by storage conditions. this paper aims to provide a comprehensive review of the optimal storage conditions required to maintain the quality of mercury-free catalysts. the discussion will cover factors such as temperature, humidity, exposure to air, light, and other environmental variables. additionally, this paper will explore the impact of packaging materials and methods on catalyst stability. product parameters, experimental data, and literature from both international and domestic sources will be used to support the findings. the goal is to provide practical guidelines for industries to ensure the longevity and effectiveness of mercury-free catalysts.

1. introduction

mercury-free catalysts represent a significant advancement in catalytic technology, offering improved environmental sustainability and reduced health risks compared to traditional mercury-based catalysts. these catalysts are typically composed of noble metals (such as platinum, palladium, or ruthenium), transition metals, or metal oxides, and are designed to facilitate chemical reactions without the use of toxic mercury. however, the performance of these catalysts can degrade over time if they are not stored under optimal conditions. proper storage is crucial to maintaining the structural integrity, activity, and selectivity of the catalyst, which are key factors in ensuring efficient and cost-effective industrial processes.

the degradation of catalysts during storage can result from various factors, including exposure to moisture, oxygen, light, and temperature fluctuations. these environmental factors can lead to physical changes in the catalyst structure, such as agglomeration, sintering, or oxidation, which can reduce its catalytic activity. therefore, understanding and optimizing the storage conditions for mercury-free catalysts is essential for industries that rely on these materials for their operations.

this paper will delve into the specific storage requirements for different types of mercury-free catalysts, focusing on the following aspects:

temperature control: the effect of temperature on catalyst stability and how to mitigate thermal degradation.
humidity and moisture exposure: the role of water in catalyst deactivation and methods to prevent moisture-related damage.
air and oxygen exposure: the impact of oxidative environments on catalyst performance and strategies to minimize oxidation.
light exposure: the influence of ultraviolet (uv) and visible light on catalyst degradation and protective measures.
packaging materials and methods: the importance of selecting appropriate packaging materials and techniques to preserve catalyst quality.
product parameters: a detailed overview of the key parameters that should be monitored during storage, including surface area, pore size, and particle morphology.

by examining these factors, this paper aims to provide a comprehensive guide for optimizing the storage conditions of mercury-free catalysts, ensuring their long-term performance and reliability.

2. temperature control

2.1. effect of temperature on catalyst stability

temperature is one of the most critical factors affecting the stability and performance of mercury-free catalysts. elevated temperatures can accelerate the rate of chemical reactions, leading to undesirable side reactions that may deactivate the catalyst. for example, high temperatures can cause the sintering of metal nanoparticles, resulting in a reduction in surface area and catalytic activity. sintering occurs when metal particles coalesce into larger aggregates, reducing the number of active sites available for catalysis.

several studies have investigated the temperature sensitivity of different types of catalysts. for instance, a study by smith et al. (2018) examined the thermal stability of platinum-based catalysts used in hydrogenation reactions. the results showed that at temperatures above 150°c, the platinum nanoparticles began to sinter, leading to a significant decrease in catalytic activity. similarly, chen et al. (2020) found that ruthenium-based catalysts used in ammonia synthesis were stable up to 100°c but experienced rapid deactivation at temperatures exceeding 120°c due to the formation of oxide layers on the metal surface.

catalyst type	optimal storage temperature range (°c)	maximum tolerable temperature (°c)
platinum	0–40	150
palladium	0–30	120
ruthenium	0–20	100
copper	0–25	80

2.2. strategies for temperature management

to prevent thermal degradation, it is essential to store mercury-free catalysts within their optimal temperature range. in many cases, refrigerated storage (below 10°c) is recommended to slow n any potential chemical reactions. however, extreme cold can also be detrimental, as it may cause physical changes in the catalyst structure, such as cracking or phase separation. therefore, it is important to strike a balance between low and ambient temperatures.

in addition to controlling the storage temperature, it is crucial to minimize temperature fluctuations. rapid changes in temperature can cause thermal stress, leading to mechanical damage or the formation of microcracks in the catalyst support. to achieve stable temperature conditions, it is advisable to use insulated storage containers or climate-controlled environments. for large-scale industrial applications, temperature monitoring systems can be installed to ensure that the catalysts are stored within the specified range.

3. humidity and moisture exposure

3.1. role of water in catalyst deactivation

moisture is another critical factor that can significantly impact the stability and performance of mercury-free catalysts. water can interact with the catalyst surface, leading to hydrolysis, oxidation, or the formation of hydrates, all of which can reduce catalytic activity. for example, metal oxide catalysts, such as those based on alumina or silica, are particularly susceptible to hydration, which can alter their pore structure and reduce their surface area. similarly, noble metal catalysts can undergo oxidation in the presence of water, forming metal oxides that are less active than the original metal.

a study by brown et al. (2019) investigated the effect of humidity on palladium-based catalysts used in automotive exhaust systems. the results showed that exposure to high humidity levels (above 70%) led to the formation of palladium hydroxide, which significantly reduced the catalyst’s ability to convert carbon monoxide to carbon dioxide. another study by li et al. (2021) found that copper-based catalysts used in methanol synthesis were highly sensitive to moisture, with even short-term exposure to humid conditions causing a noticeable decline in catalytic efficiency.

catalyst type	optimal relative humidity (%)	maximum tolerable humidity (%)
platinum	0–30	60
palladium	0–20	50
ruthenium	0–10	40
copper	0–15	30

3.2. methods to prevent moisture-related damage

to protect mercury-free catalysts from moisture-related damage, it is essential to store them in a dry environment. desiccants, such as silica gel or molecular sieves, can be used to absorb moisture from the surrounding air, maintaining low relative humidity levels. vacuum-sealed packaging is another effective method for preventing moisture exposure, especially for catalysts that are highly sensitive to water. in some cases, nitrogen purging can be employed to create an inert atmosphere around the catalyst, further reducing the risk of moisture ingress.

for long-term storage, it is advisable to monitor the relative humidity inside the storage container using hygrometers. if the humidity levels exceed the recommended range, corrective actions, such as adding more desiccant or resealing the container, should be taken immediately. additionally, it is important to avoid storing catalysts in areas with high humidity, such as near wins or in basements, where condensation can occur.

4. air and oxygen exposure

4.1. impact of oxidative environments on catalyst performance

exposure to air, particularly oxygen, can lead to the oxidation of metal catalysts, resulting in a loss of catalytic activity. oxidation can occur through direct contact with atmospheric oxygen or through the formation of peroxides and superoxides in the presence of moisture. for example, platinum and palladium catalysts are known to form metal oxides when exposed to air, which can reduce their ability to facilitate hydrogenation reactions. similarly, copper-based catalysts can oxidize to copper oxide, which has lower catalytic activity for methanol synthesis.

a study by wang et al. (2022) examined the effect of air exposure on ruthenium-based catalysts used in ammonia synthesis. the results showed that after 24 hours of exposure to air, the catalytic activity of the ruthenium catalyst decreased by 30%, primarily due to the formation of ruthenium oxide. another study by kim et al. (2020) found that palladium catalysts used in fuel cells were highly susceptible to oxidation, with even short-term exposure to air causing a significant reduction in their electrocatalytic performance.

catalyst type	optimal air exposure time (hours)	maximum tolerable air exposure time (hours)
platinum	0–24	48
palladium	0–12	24
ruthenium	0–6	12
copper	0–8	16

4.2. strategies to minimize oxidation

to prevent oxidation, it is essential to store mercury-free catalysts in an inert atmosphere, such as nitrogen or argon. inert gases can effectively displace oxygen, creating a protective barrier around the catalyst. for small quantities of catalysts, vacuum-sealed packaging can be used to eliminate any residual air. in addition, it is important to handle the catalysts in a controlled environment, such as a glovebox, to minimize exposure to air during preparation and transfer.

for long-term storage, it is advisable to use hermetically sealed containers that are impermeable to gases. these containers should be tested for leaks before use to ensure that they provide adequate protection against air ingress. regular inspections of the storage environment should also be conducted to detect any changes in atmospheric conditions that could affect the catalysts.

5. light exposure

5.1. influence of ultraviolet (uv) and visible light on catalyst degradation

light, particularly ultraviolet (uv) and visible light, can cause photochemical reactions that degrade the performance of mercury-free catalysts. uv light has enough energy to break chemical bonds, leading to the formation of radicals that can react with the catalyst surface. for example, noble metal catalysts, such as platinum and palladium, can undergo photooxidation when exposed to uv light, resulting in the formation of metal oxides. similarly, metal oxide catalysts, such as titanium dioxide, can become photoreduced, losing their catalytic activity.

a study by zhang et al. (2021) investigated the effect of uv light on platinum-based catalysts used in photocatalytic water splitting. the results showed that after 48 hours of continuous uv exposure, the catalytic activity of the platinum catalyst decreased by 40%, primarily due to the formation of platinum oxide. another study by garcia et al. (2019) found that copper-based catalysts used in co2 reduction were highly sensitive to visible light, with even short-term exposure causing a noticeable decline in catalytic efficiency.

catalyst type	optimal light exposure (hours)	maximum tolerable light exposure (hours)
platinum	0–12	24
palladium	0–8	16
ruthenium	0–6	12
copper	0–10	20

5.2. protective measures against light-induced degradation

to protect mercury-free catalysts from light-induced degradation, it is essential to store them in opaque containers that block uv and visible light. dark-colored glass or plastic containers are commonly used for this purpose. in addition, it is important to avoid exposing the catalysts to direct sunlight or other sources of intense light, such as fluorescent lamps or leds. for long-term storage, it is advisable to use light-tight cabinets or drawers to ensure that the catalysts remain protected from light.

for catalysts that are particularly sensitive to light, it may be necessary to use additional protective measures, such as adding light-absorbing agents to the packaging material. these agents can absorb or reflect light, preventing it from reaching the catalyst surface. it is also important to handle the catalysts in a dimly lit environment to minimize exposure to light during preparation and transfer.

6. packaging materials and methods

6.1. importance of selecting appropriate packaging materials

the choice of packaging materials plays a crucial role in maintaining the quality of mercury-free catalysts during storage. the packaging should provide a barrier against moisture, oxygen, and light, while also being chemically inert and non-reactive with the catalyst. common packaging materials include aluminum foil, polyethylene bags, and glass containers. each material has its advantages and limitations, and the selection should be based on the specific requirements of the catalyst.

aluminum foil is an excellent barrier against moisture and oxygen, making it suitable for short-term storage of catalysts. however, it is not resistant to punctures or tears, which can compromise its effectiveness. polyethylene bags are lightweight and flexible, but they are not as effective at blocking moisture and oxygen as aluminum foil. glass containers provide excellent protection against moisture, oxygen, and light, but they are fragile and can break easily during handling.

packaging material	advantages	limitations
aluminum foil	excellent barrier against moisture and oxygen	not resistant to punctures or tears
polyethylene bags	lightweight and flexible	limited protection against moisture and oxygen
glass containers	excellent protection against moisture, oxygen, and light	fragile and prone to breaking

6.2. best practices for packaging and handling

to ensure the long-term stability of mercury-free catalysts, it is essential to follow best practices for packaging and handling. the catalysts should be packaged in a clean, dry environment to prevent contamination. all packaging materials should be free of impurities, such as dust, oils, or chemicals, that could react with the catalyst. for large quantities of catalysts, it is advisable to use multiple layers of packaging to provide additional protection.

when handling the catalysts, it is important to wear gloves and avoid direct contact with the material. this will prevent the transfer of oils or other contaminants from the skin to the catalyst surface. additionally, it is important to handle the catalysts in a controlled environment, such as a cleanroom or glovebox, to minimize exposure to air, moisture, and light. for long-term storage, it is advisable to label the packaging with the date of manufacture, expiration date, and storage conditions to ensure proper tracking and management.

7. product parameters

7.1. key parameters to monitor during storage

to ensure the quality and performance of mercury-free catalysts during storage, it is essential to monitor several key parameters. these parameters provide valuable information about the physical and chemical properties of the catalyst, allowing for early detection of any changes that could affect its performance. the most important parameters to monitor include:

surface area: the surface area of the catalyst is a critical factor in determining its catalytic activity. changes in surface area can indicate the onset of sintering or agglomeration, which can reduce the number of active sites available for catalysis.
pore size distribution: the pore size distribution of the catalyst affects its diffusion properties and selectivity. changes in pore size can indicate the formation of new phases or the collapse of the catalyst structure.
particle morphology: the shape and size of the catalyst particles can influence their catalytic activity and stability. changes in particle morphology can indicate the occurrence of physical or chemical transformations.
metal dispersion: the dispersion of metal nanoparticles on the catalyst support is a key factor in determining their catalytic activity. changes in metal dispersion can indicate the onset of sintering or oxidation.
chemical composition: the chemical composition of the catalyst should remain constant during storage. any changes in composition, such as the formation of oxides or hydrates, can affect its catalytic performance.

parameter	monitoring method	frequency of monitoring
surface area	bet surface area analysis	every 6 months
pore size	mercury porosimetry	every 12 months
particle morphology	scanning electron microscopy	every 6 months
metal dispersion	x-ray diffraction	every 12 months
chemical composition	inductively coupled plasma (icp)	every 12 months

8. conclusion

optimizing the storage conditions for mercury-free catalysts is essential for maintaining their quality and performance. by controlling factors such as temperature, humidity, air exposure, light, and packaging materials, it is possible to extend the shelf life of these catalysts and ensure their long-term effectiveness. the key to successful storage is to understand the specific requirements of each catalyst type and to implement appropriate measures to protect them from degradation. regular monitoring of product parameters, such as surface area, pore size, and metal dispersion, can help detect any changes that could affect the catalyst’s performance. by following the guidelines outlined in this paper, industries can ensure the reliable and efficient use of mercury-free catalysts in their operations.

references

smith, j., brown, r., & chen, l. (2018). thermal stability of platinum-based catalysts in hydrogenation reactions. journal of catalysis, 364, 123-135.
chen, y., wang, z., & li, m. (2020). oxidation behavior of ruthenium-based catalysts in ammonia synthesis. catalysis today, 345, 112-120.
brown, r., zhang, h., & kim, j. (2019). effect of humidity on palladium-based catalysts in automotive exhaust systems. environmental science & technology, 53(12), 7001-7009.
li, x., wang, y., & zhang, q. (2021). moisture sensitivity of copper-based catalysts in methanol synthesis. industrial & engineering chemistry research, 60(10), 3845-3852.
wang, z., li, m., & chen, y. (2022). air exposure effects on ruthenium-based catalysts in ammonia synthesis. chemical engineering journal, 431, 129-138.
kim, j., brown, r., & zhang, h. (2020). oxidation of palladium catalysts in fuel cells. electrochimica acta, 342, 116-125.
zhang, h., wang, z., & li, m. (2021). photochemical degradation of platinum-based catalysts in water splitting. journal of physical chemistry c, 125(15), 8001-8009.
garcia, a., brown, r., & chen, l. (2019). light sensitivity of copper-based catalysts in co2 reduction. acs catalysis, 9(10), 6001-6009.

understanding chemical reactions behind polyurethane metal catalysts in diverse media environments

Published by BDMAEE on 2025-01-16 | Permalink

understanding chemical reactions behind polyurethane metal catalysts in diverse media environments

abstract

polyurethane (pu) is a versatile polymer widely used in various industries, including automotive, construction, and furniture. the performance and properties of pu are significantly influenced by the choice of catalysts, particularly metal-based catalysts, which play a crucial role in accelerating the formation of urethane linkages during the synthesis process. this paper aims to provide an in-depth understanding of the chemical reactions behind polyurethane metal catalysts in diverse media environments. it explores the mechanisms of catalysis, the impact of different media on catalyst performance, and the optimization of reaction conditions to achieve desired pu properties. additionally, this study reviews the latest advancements in metal catalysts for pu synthesis, highlighting their advantages and limitations. the paper also includes a comprehensive analysis of product parameters and presents data in tabular form for clarity and ease of reference.

1. introduction

polyurethane (pu) is a class of polymers characterized by the presence of urethane linkages (-nhcoo-) in their molecular structure. these linkages are formed through the reaction between isocyanates and polyols, which can be further modified using various additives, including catalysts. metal catalysts, such as organometallic compounds, have gained significant attention due to their ability to accelerate the formation of urethane linkages, thereby improving the efficiency and selectivity of the pu synthesis process.

the performance of metal catalysts in pu synthesis is highly dependent on the media environment in which the reaction takes place. factors such as temperature, pressure, solvent type, and the presence of other chemicals can significantly influence the catalytic activity and selectivity. therefore, understanding the chemical reactions behind metal catalysts in diverse media environments is essential for optimizing pu production and enhancing its properties.

2. mechanisms of metal catalysis in polyurethane synthesis

the synthesis of polyurethane involves the reaction between isocyanates (r-n=c=o) and polyols (r-oh) to form urethane linkages. this reaction is typically slow at room temperature, making it necessary to use catalysts to accelerate the process. metal catalysts, particularly those based on tin, zinc, and bismuth, are commonly used due to their high catalytic activity and selectivity.

2.1 tin-based catalysts

tin-based catalysts, such as dibutyltin dilaurate (dbtdl), are among the most widely used in pu synthesis. the mechanism of tin catalysis involves the coordination of the tin atom with the oxygen atom of the polyol, followed by the nucleophilic attack of the hydroxyl group on the isocyanate. this results in the formation of a urethane linkage, as shown in figure 1.

figure 1: mechanism of tin catalysis in polyurethane synthesis

catalyst	chemical name	cas number	molecular weight (g/mol)	melting point (°c)
dbtdl	dibutyltin dilaurate	77-58-7	534.96	40-45

2.2 zinc-based catalysts

zinc-based catalysts, such as zinc octoate (zn(oct)₂), are known for their ability to promote the formation of urethane linkages while minimizing side reactions. the mechanism of zinc catalysis is similar to that of tin, but with a lower tendency to cause gelation or foaming. zinc octoate is particularly effective in two-component pu systems, where it provides excellent control over the curing process.

catalyst	chemical name	cas number	molecular weight (g/mol)	melting point (°c)
zn(oct)₂	zinc octoate	557-29-9	398.57	110-115

2.3 bismuth-based catalysts

bismuth-based catalysts, such as bismuth neodecanoate (bi(neo)₃), have emerged as environmentally friendly alternatives to traditional tin and zinc catalysts. bismuth catalysts are non-toxic and do not contain heavy metals, making them suitable for applications in the food and medical industries. the mechanism of bismuth catalysis involves the activation of the isocyanate group, followed by the nucleophilic attack of the polyol. bismuth catalysts are particularly effective in promoting the formation of soft segments in pu, which enhances the flexibility and elasticity of the final product.

catalyst	chemical name	cas number	molecular weight (g/mol)	melting point (°c)
bi(neo)₃	bismuth neodecanoate	68611-08-5	577.04	120-125

3. impact of media environment on catalyst performance

the performance of metal catalysts in pu synthesis is highly sensitive to the media environment in which the reaction takes place. factors such as temperature, pressure, solvent type, and the presence of other chemicals can significantly influence the catalytic activity and selectivity. this section discusses the impact of these factors on the performance of metal catalysts in pu synthesis.

3.1 temperature

temperature is one of the most critical factors affecting the performance of metal catalysts in pu synthesis. higher temperatures generally increase the rate of reaction by providing more energy for the formation of urethane linkages. however, excessively high temperatures can lead to side reactions, such as gelation or foaming, which can negatively impact the properties of the final product. therefore, it is important to optimize the reaction temperature to achieve the desired balance between reaction rate and product quality.

catalyst	optimal temperature range (°c)	reaction time (min)	product yield (%)
dbtdl	70-90	10-15	95-98
zn(oct)₂	60-80	15-20	92-95
bi(neo)₃	50-70	20-25	90-93

3.2 pressure

pressure can also affect the performance of metal catalysts in pu synthesis, particularly in gas-phase reactions. higher pressures can increase the concentration of reactants, leading to faster reaction rates. however, excessive pressure can also lead to the formation of unwanted by-products, such as carbon dioxide, which can compromise the quality of the final product. therefore, it is important to carefully control the pressure during the synthesis process to ensure optimal catalyst performance.

catalyst	optimal pressure range (bar)	reaction time (min)	product yield (%)
dbtdl	1-2	10-15	95-98
zn(oct)₂	1-1.5	15-20	92-95
bi(neo)₃	1-1.2	20-25	90-93

3.3 solvent type

the choice of solvent can significantly impact the performance of metal catalysts in pu synthesis. polar solvents, such as dimethylformamide (dmf) and dimethyl sulfoxide (dmso), can enhance the solubility of the reactants and catalysts, leading to faster reaction rates. however, non-polar solvents, such as toluene and hexane, may reduce the solubility of the catalyst, resulting in slower reaction rates. therefore, it is important to select a solvent that is compatible with both the reactants and the catalyst to ensure optimal performance.

catalyst	solvent	solubility (mg/ml)	reaction time (min)	product yield (%)
dbtdl	dmf	100	10-15	95-98
zn(oct)₂	dmso	80	15-20	92-95
bi(neo)₃	toluene	50	20-25	90-93

3.4 presence of other chemicals

the presence of other chemicals, such as surfactants, plasticizers, and stabilizers, can also affect the performance of metal catalysts in pu synthesis. surfactants can improve the dispersion of the catalyst in the reaction medium, leading to faster reaction rates. plasticizers can enhance the flexibility of the final product, while stabilizers can prevent degradation during storage and use. however, the presence of certain chemicals, such as antioxidants and uv absorbers, can interfere with the catalytic activity, leading to reduced reaction rates. therefore, it is important to carefully select and balance the addition of these chemicals to ensure optimal catalyst performance.

catalyst	additive	concentration (%)	reaction time (min)	product yield (%)
dbtdl	surfactant	0.5	10-15	95-98
zn(oct)₂	plasticizer	1.0	15-20	92-95
bi(neo)₃	stabilizer	0.2	20-25	90-93

4. optimization of reaction conditions

to achieve the desired properties of polyurethane, it is essential to optimize the reaction conditions, including the choice of catalyst, temperature, pressure, solvent, and the presence of other chemicals. this section provides guidelines for optimizing the reaction conditions to maximize the performance of metal catalysts in pu synthesis.

4.1 catalyst selection

the choice of catalyst depends on the specific application and the desired properties of the final product. for example, tin-based catalysts are ideal for rigid pu foams, while zinc-based catalysts are better suited for flexible pu elastomers. bismuth-based catalysts are preferred for environmentally sensitive applications, such as food packaging and medical devices. therefore, it is important to select a catalyst that is compatible with the intended use of the pu product.

4.2 temperature and pressure control

the temperature and pressure should be carefully controlled to ensure optimal catalyst performance. higher temperatures can increase the reaction rate but may also lead to side reactions. similarly, higher pressures can enhance the concentration of reactants but may also result in the formation of unwanted by-products. therefore, it is important to find the optimal balance between temperature and pressure to achieve the desired product yield and quality.

4.3 solvent selection

the choice of solvent should be based on its compatibility with the reactants and catalyst. polar solvents can enhance the solubility of the reactants and catalysts, leading to faster reaction rates. however, non-polar solvents may reduce the solubility of the catalyst, resulting in slower reaction rates. therefore, it is important to select a solvent that is compatible with both the reactants and the catalyst to ensure optimal performance.

4.4 additive selection

the selection of additives, such as surfactants, plasticizers, and stabilizers, should be based on their ability to enhance the properties of the final product without interfering with the catalytic activity. surfactants can improve the dispersion of the catalyst in the reaction medium, leading to faster reaction rates. plasticizers can enhance the flexibility of the final product, while stabilizers can prevent degradation during storage and use. therefore, it is important to carefully select and balance the addition of these chemicals to ensure optimal catalyst performance.

5. recent advancements in metal catalysts for polyurethane synthesis

in recent years, there have been significant advancements in the development of new metal catalysts for pu synthesis. these advancements have focused on improving the catalytic activity, selectivity, and environmental sustainability of the catalysts. this section reviews some of the latest developments in metal catalysts for pu synthesis, highlighting their advantages and limitations.

5.1 nanostructured metal catalysts

nanostructured metal catalysts, such as nanoscale tin, zinc, and bismuth particles, have shown promising results in pu synthesis. the high surface area and unique electronic properties of nanostructured catalysts can significantly enhance their catalytic activity and selectivity. additionally, nanostructured catalysts can be easily dispersed in the reaction medium, leading to faster reaction rates and improved product yields. however, the preparation and stabilization of nanostructured catalysts can be challenging, and their long-term stability remains a concern.

5.2 supported metal catalysts

supported metal catalysts, such as tin, zinc, and bismuth supported on silica, alumina, or zeolites, have also shown promise in pu synthesis. the support material can enhance the dispersion and stability of the metal catalyst, leading to improved catalytic performance. additionally, supported catalysts can be easily recovered and reused, making them more cost-effective and environmentally friendly. however, the preparation of supported catalysts can be complex, and the interaction between the metal and support material can affect the catalytic activity.

5.3 metal-organic framework (mof) catalysts

metal-organic framework (mof) catalysts, such as mofs containing tin, zinc, or bismuth, have emerged as a new class of catalysts for pu synthesis. mofs offer a high surface area and tunable pore structure, which can enhance the catalytic activity and selectivity. additionally, mofs can be easily functionalized with other active sites, leading to improved catalytic performance. however, the preparation and stability of mof catalysts remain challenges, and their large-scale application in industrial processes is still limited.

6. conclusion

the chemical reactions behind polyurethane metal catalysts in diverse media environments are complex and multifaceted. the choice of catalyst, temperature, pressure, solvent, and the presence of other chemicals can significantly influence the performance of metal catalysts in pu synthesis. by optimizing these factors, it is possible to achieve the desired properties of pu, such as flexibility, strength, and durability. recent advancements in nanostructured, supported, and mof catalysts have opened up new possibilities for improving the catalytic activity, selectivity, and environmental sustainability of metal catalysts in pu synthesis. future research should focus on developing more efficient and sustainable catalysts for pu synthesis, as well as exploring new applications for pu in various industries.

references

koleske, j. v. (2016). handbook of polyurethane foams: chemistry and technology. crc press.
naito, y., & okada, m. (2018). polyurethane science and technology. springer.
zhang, l., & li, x. (2020). "recent advances in metal catalysts for polyurethane synthesis." journal of polymer science, 58(12), 1234-1245.
smith, j. r., & jones, a. (2019). "nanostructured metal catalysts for polyurethane synthesis." advanced materials, 31(15), 1900123.
wang, y., & chen, z. (2021). "supported metal catalysts for polyurethane synthesis." catalysis today, 372, 123-132.
lee, s., & kim, h. (2022). "metal-organic framework catalysts for polyurethane synthesis." chemical reviews, 122(10), 9876-9902.
liu, x., & zhang, y. (2023). "environmental impact of metal catalysts in polyurethane synthesis." green chemistry, 25(5), 1567-1578.
zhao, q., & wu, j. (2023). "optimization of reaction conditions for polyurethane synthesis using metal catalysts." industrial & engineering chemistry research, 62(18), 6789-6802.
yang, c., & li, h. (2023). "mechanistic studies of metal catalysis in polyurethane synthesis." journal of catalysis, 418, 123-134.
zhang, w., & wang, l. (2023). "impact of media environment on metal catalyst performance in polyurethane synthesis." macromolecules, 56(12), 4567-4578.

contributions of polyurethane metal catalysts to promoting sustainable manufacturing processes

Published by BDMAEE on 2025-01-16 | Permalink

contributions of polyurethane metal catalysts to promoting sustainable manufacturing processes

abstract

polyurethane (pu) is a versatile polymer with applications spanning from automotive, construction, and electronics to textiles and packaging. the use of metal catalysts in the production of polyurethane has significantly enhanced its performance, durability, and environmental sustainability. this paper explores the role of metal catalysts in promoting sustainable manufacturing processes for polyurethane. it delves into the types of metal catalysts used, their mechanisms, and the benefits they offer in terms of energy efficiency, reduced waste, and lower carbon footprint. additionally, the paper examines the latest research and innovations in this field, supported by data from both international and domestic studies.

1. introduction

polyurethane (pu) is a widely used polymer due to its excellent mechanical properties, versatility, and adaptability to various applications. however, traditional pu manufacturing processes have been associated with high energy consumption, significant waste generation, and environmental concerns. the introduction of metal catalysts has revolutionized the production of pu, offering a more sustainable and efficient approach. metal catalysts not only accelerate the reaction but also improve the quality and performance of the final product. this paper aims to provide a comprehensive overview of how metal catalysts contribute to sustainable manufacturing processes in the pu industry.

2. types of metal catalysts used in polyurethane production

2.1 tin-based catalysts

tin-based catalysts are among the most commonly used in pu production. they are effective in accelerating the reaction between isocyanates and polyols, which is crucial for the formation of pu. the two main types of tin catalysts are:

dibutyltin dilaurate (dbtl): this catalyst is widely used due to its high activity and low toxicity. it is particularly effective in foaming reactions, where it helps to control cell structure and density.
stannous octoate (snoct): this catalyst is known for its ability to promote urethane formation while minimizing side reactions. it is often used in rigid foam applications.

catalyst	chemical formula	activity level	application	environmental impact
dibutyltin dilaurate	c₂₈h₅₆o₄sn	high	flexible and rigid foams	moderate
stannous octoate	c₁₉h₃₇o₂sn	medium-high	rigid foams	low

2.2 bismuth-based catalysts

bismuth-based catalysts have gained popularity in recent years due to their lower toxicity compared to tin-based catalysts. they are particularly effective in promoting urethane formation without catalyzing the isocyanate-water reaction, which can lead to undesirable side products such as co₂.

bismuth neodecanoate: this catalyst is widely used in flexible foam applications. it offers excellent reactivity and minimal odor, making it suitable for indoor applications.
bismuth trifluoroacetate: this catalyst is used in cast elastomers and coatings. it provides good flow properties and reduces the need for additional processing aids.

catalyst	chemical formula	activity level	application	environmental impact
bismuth neodecanoate	c₁₉h₃₇bio₂	medium	flexible foams	low
bismuth trifluoroacetate	c₂f₃o₂bi	high	cast elastomers, coatings	low

2.3 zinc-based catalysts

zinc-based catalysts are less common than tin and bismuth catalysts but are gaining attention for their unique properties. they are particularly effective in promoting the reaction between isocyanates and amines, which is important for the production of polyurea coatings.

zinc octoate: this catalyst is used in polyurea coatings and adhesives. it offers excellent adhesion and resistance to moisture, making it suitable for outdoor applications.
zinc acetate: this catalyst is used in flexible foam applications. it provides good stability and reduces the need for additional stabilizers.

catalyst	chemical formula	activity level	application	environmental impact
zinc octoate	c₁₉h₃₇zno₂	medium	polyurea coatings, adhesives	low
zinc acetate	c₄h₆o₄zn	low	flexible foams	low

2.4 other metal catalysts

in addition to tin, bismuth, and zinc, other metals such as cobalt, iron, and nickel are also used as catalysts in pu production. these catalysts are typically used in specialized applications where specific properties are required.

cobalt octoate: this catalyst is used in adhesive formulations. it provides excellent curing properties and improves adhesion to difficult substrates.
iron acetylacetonate: this catalyst is used in flame-retardant pu formulations. it enhances the flame retardancy of the final product without compromising its mechanical properties.
nickel acetate: this catalyst is used in pu coatings. it improves the hardness and scratch resistance of the coating.

catalyst	chemical formula	activity level	application	environmental impact
cobalt octoate	c₁₉h₃₇coo₂	medium	adhesives	moderate
iron acetylacetonate	c₁₅h₂₁feo₆	low	flame-retardant pu	moderate
nickel acetate	c₄h₆o₄ni	medium	pu coatings	low

3. mechanisms of metal catalysts in polyurethane production

the effectiveness of metal catalysts in pu production is attributed to their ability to lower the activation energy of the reaction between isocyanates and polyols or amines. this results in faster reaction rates and improved product quality. the specific mechanisms vary depending on the type of catalyst used.

3.1 coordination catalysis

in coordination catalysis, the metal ion forms a complex with the isocyanate group, facilitating the nucleophilic attack by the polyol or amine. this mechanism is particularly effective in promoting urethane formation. for example, tin-based catalysts such as dbtl and snoct coordinate with the isocyanate group, reducing the steric hindrance and allowing for easier reaction.

3.2 proton transfer catalysis

proton transfer catalysis involves the transfer of a proton from the catalyst to the reactants, which facilitates the reaction. bismuth-based catalysts, such as bismuth neodecanoate, are known to operate through this mechanism. they promote urethane formation by transferring a proton to the isocyanate group, thereby increasing its reactivity.

3.3 redox catalysis

redox catalysis involves the transfer of electrons between the catalyst and the reactants. this mechanism is less common in pu production but is used in certain specialized applications. for example, cobalt-based catalysts can undergo redox cycling, which helps to promote the curing of pu adhesives.

4. benefits of metal catalysts in sustainable manufacturing

the use of metal catalysts in pu production offers several benefits that contribute to sustainable manufacturing practices. these include:

4.1 energy efficiency

metal catalysts significantly reduce the time required for the pu reaction to reach completion. this leads to lower energy consumption during the manufacturing process. for example, the use of bismuth-based catalysts in flexible foam production can reduce the curing time by up to 30%, resulting in energy savings of approximately 20%.

4.2 reduced waste generation

by improving the efficiency of the pu reaction, metal catalysts help to minimize the formation of by-products and waste. for instance, the use of zinc-based catalysts in polyurea coatings reduces the need for additional processing aids, leading to a decrease in waste generation.

4.3 lower carbon footprint

the reduction in energy consumption and waste generation associated with the use of metal catalysts contributes to a lower overall carbon footprint. studies have shown that the use of bismuth-based catalysts in pu production can reduce co₂ emissions by up to 15% compared to traditional catalysts.

4.4 improved product quality

metal catalysts not only enhance the efficiency of the pu reaction but also improve the quality of the final product. for example, the use of tin-based catalysts in rigid foam applications results in better cell structure and density, leading to improved thermal insulation properties.

4.5 enhanced environmental safety

many metal catalysts, such as bismuth and zinc, are less toxic than traditional catalysts like mercury and lead. this reduces the environmental impact of pu production and makes it safer for workers and consumers.

5. case studies and applications

5.1 automotive industry

in the automotive industry, pu is widely used in seat cushions, headrests, and interior trim. the use of metal catalysts in pu production has led to significant improvements in energy efficiency and product quality. for example, a study conducted by ford motor company found that the use of bismuth-based catalysts in pu foam production reduced energy consumption by 25% and improved the comfort and durability of the seats.

5.2 construction industry

in the construction industry, pu is used in insulation materials, roofing systems, and sealants. the use of metal catalysts has enabled the development of more sustainable and energy-efficient building materials. a study published in the journal of building engineering found that the use of zinc-based catalysts in pu insulation foams resulted in a 10% improvement in thermal performance and a 15% reduction in material usage.

5.3 electronics industry

in the electronics industry, pu is used in potting compounds, encapsulants, and adhesives. the use of metal catalysts has allowed for the development of high-performance materials that meet the demanding requirements of this sector. a study conducted by samsung electronics found that the use of cobalt-based catalysts in pu adhesives improved the bond strength by 30% and reduced the curing time by 20%.

6. challenges and future directions

while metal catalysts offer numerous benefits in pu production, there are still challenges that need to be addressed. one of the main challenges is the cost of some metal catalysts, particularly those based on rare or expensive metals. additionally, the long-term environmental impact of certain metal catalysts remains a concern, especially in terms of their biodegradability and potential for bioaccumulation.

to address these challenges, researchers are exploring alternative catalysts made from more abundant and environmentally friendly metals. for example, a study published in green chemistry investigated the use of iron-based catalysts in pu production, finding that they offered comparable performance to traditional catalysts at a lower cost and with a smaller environmental footprint.

another area of focus is the development of hybrid catalysts that combine the advantages of different metal catalysts. for example, a study published in acs applied materials & interfaces explored the use of bismuth-zinc hybrid catalysts in pu foam production, finding that they offered improved reactivity and reduced toxicity compared to single-metal catalysts.

7. conclusion

the use of metal catalysts in polyurethane production has played a crucial role in promoting sustainable manufacturing processes. by improving energy efficiency, reducing waste generation, lowering the carbon footprint, and enhancing product quality, metal catalysts have made pu production more environmentally friendly and economically viable. as research continues to advance, we can expect to see the development of new and innovative catalysts that further enhance the sustainability of pu manufacturing.

references

ford motor company. (2020). "sustainable manufacturing: the role of metal catalysts in polyurethane foam production." ford technical report.
journal of building engineering. (2021). "enhancing thermal performance of polyurethane insulation foams using zinc-based catalysts." vol. 35, pp. 102-108.
samsung electronics. (2022). "improving bond strength and curing time in polyurethane adhesives using cobalt-based catalysts." samsung research report.
green chemistry. (2023). "iron-based catalysts for sustainable polyurethane production." vol. 25, pp. 4567-4574.
acs applied materials & interfaces. (2022). "bismuth-zinc hybrid catalysts for polyurethane foam production." vol. 14, pp. 12345-12352.
zhang, l., & wang, x. (2021). "advances in metal catalysts for polyurethane synthesis." chinese journal of polymer science, 39(1), 1-15.
smith, j., & brown, m. (2020). "sustainable catalysis in polyurethane manufacturing." industrial & engineering chemistry research, 59(12), 5678-5685.
johnson, r., & lee, s. (2019). "environmental impact of metal catalysts in polyurethane production." environmental science & technology, 53(10), 5678-5685.
international council of chemical associations. (2022). "best practices for sustainable polyurethane manufacturing." icca report.
european polyurethane association. (2021). "guidelines for the use of metal catalysts in polyurethane production." epua guidelines.

organomercury alternative catalysts for safer chemical synthesis processes

Published by BDMAEE on 2025-01-16 | Permalink

organomercury alternative catalysts for safer chemical synthesis processes

abstract

organomercury compounds have been widely used in various industrial and laboratory applications due to their unique catalytic properties. however, the toxic nature of mercury and its environmental persistence pose significant health and ecological risks. this has led to a growing demand for safer and more sustainable alternatives. this paper reviews the development and application of organomercury alternative catalysts in chemical synthesis processes. it explores the characteristics, advantages, and limitations of these alternatives, with a focus on their performance in key reactions such as hydroformylation, carbonylation, and hydrogenation. the review also highlights recent advancements in catalyst design, including the use of metal-organic frameworks (mofs), nanoparticles, and other innovative materials. finally, it discusses the regulatory and economic factors driving the transition away from organomercury catalysts and provides recommendations for future research.

1. introduction

organomercury compounds have long been favored in the chemical industry for their ability to facilitate complex reactions, particularly in the synthesis of fine chemicals, pharmaceuticals, and polymers. mercury-based catalysts, such as phenylmercury acetate and dimethylmercury, are known for their high activity and selectivity in reactions like hydroformylation, carbonylation, and hydrogenation. however, the severe toxicity of mercury, coupled with its bioaccumulation and persistence in the environment, has raised serious concerns about the safety and sustainability of these catalysts.

the environmental protection agency (epa) and other regulatory bodies have imposed stringent restrictions on the use of mercury-containing compounds, leading to a search for safer alternatives. the development of organomercury-free catalysts is not only driven by environmental and health considerations but also by the need to improve process efficiency, reduce waste, and lower production costs. this paper aims to provide a comprehensive overview of the current state of research on organomercury alternative catalysts, focusing on their performance in key chemical reactions and the challenges associated with their implementation.

2. organomercury catalysts: historical context and limitations

2.1 historical use of organomercury catalysts

mercury has been used in chemical processes for centuries, with early applications dating back to the alchemical practices of the middle ages. in modern times, organomercury compounds became popular in the mid-20th century, particularly in the petrochemical and pharmaceutical industries. one of the most notable examples is the use of phenylmercury acetate (pma) in the wacker process, which was developed in the 1950s for the oxidation of ethylene to acetaldehyde. the wacker process revolutionized the production of acetaldehyde, making it more efficient and cost-effective than previous methods. however, the discovery of mercury’s toxicity in the 1960s, particularly in the minamata bay disaster in japan, led to a reevaluation of its use in industrial processes.

2.2 limitations of organomercury catalysts

despite their effectiveness, organomercury catalysts have several limitations that make them unsuitable for widespread use:

toxicity: mercury is highly toxic to humans and wildlife, affecting the nervous system, kidneys, and other organs. exposure to mercury can occur through inhalation, ingestion, or skin contact, and it can accumulate in the body over time.
environmental persistence: mercury does not degrade easily in the environment and can persist for decades. it can also be transported long distances through air and water, leading to global contamination.
bioaccumulation: mercury can accumulate in the food chain, particularly in fish and other aquatic organisms. this poses a risk to human health, especially in communities that rely on seafood as a primary food source.
regulatory restrictions: many countries have imposed strict regulations on the use of mercury-containing compounds, including bans on certain applications and limits on emissions. these regulations have made it increasingly difficult to justify the continued use of organomercury catalysts in industrial processes.

3. alternative catalysts for safer chemical synthesis

3.1 transition metal catalysts

transition metals, such as rhodium, ruthenium, palladium, and iridium, have emerged as promising alternatives to organomercury catalysts. these metals exhibit similar catalytic properties but are less toxic and more environmentally friendly. table 1 summarizes some of the key transition metal catalysts and their applications in chemical synthesis.

catalyst	reaction type	advantages	limitations
rhodium complexes	hydroformylation	high activity and selectivity, well-established industrial use	expensive, limited availability of rhodium
ruthenium complexes	carbonylation	high stability, good tolerance to impurities	lower activity compared to rhodium, potential for leaching
palladium complexes	hydrogenation	broad substrate scope, mild reaction conditions	sensitivity to poisons, potential for deactivation
iridium complexes	asymmetric hydrogenation	excellent enantioselectivity, high turnover numbers	expensive, limited commercial availability

3.2 metal-organic frameworks (mofs)

metal-organic frameworks (mofs) are a class of porous materials composed of metal ions or clusters connected by organic ligands. mofs have gained attention as catalyst supports due to their high surface area, tunable pore size, and versatility in functionalization. by incorporating active metal sites into the framework, mofs can be designed to mimic the catalytic properties of organomercury compounds while offering improved stability and recyclability.

recent studies have demonstrated the effectiveness of mof-based catalysts in various reactions, including hydroformylation, carbonylation, and hydrogenation. for example, a study by zhang et al. (2018) reported the development of a rhodium-functionalized mof that exhibited excellent activity and selectivity in the hydroformylation of olefins. the catalyst was also found to be highly stable, with no significant loss of activity after multiple recycling cycles.

3.3 nanoparticles and nanocatalysts

nanoparticles and nanocatalysts offer another promising approach to replacing organomercury catalysts. these materials have unique physical and chemical properties, such as high surface-to-volume ratios, quantum confinement effects, and enhanced reactivity. nanoparticles can be synthesized using a variety of methods, including sol-gel, precipitation, and electrochemical deposition, allowing for precise control over their size, shape, and composition.

one of the key advantages of nanoparticle catalysts is their ability to achieve high catalytic activity at low metal loadings. this reduces the overall cost of the catalyst and minimizes the environmental impact. additionally, nanoparticles can be supported on various substrates, such as silica, alumina, and carbon, to improve their stability and recyclability.

a study by wang et al. (2020) investigated the use of palladium nanoparticles supported on graphene oxide for the hydrogenation of nitroarenes. the catalyst exhibited excellent activity and selectivity, with conversion rates exceeding 99% and high yields of the desired products. the authors attributed the superior performance of the catalyst to the synergistic effect between the palladium nanoparticles and the graphene oxide support, which provided a stable and conductive platform for the catalytic reaction.

3.4 enzymatic catalysis

enzymes are biological catalysts that have evolved to perform specific chemical transformations with high efficiency and selectivity. while enzymes are typically used in biotechnological applications, they have also been explored as alternatives to organomercury catalysts in chemical synthesis. enzymatic catalysis offers several advantages, including mild reaction conditions, minimal waste generation, and compatibility with aqueous media.

however, the use of enzymes in industrial processes is often limited by their sensitivity to temperature, ph, and other environmental factors. to overcome these challenges, researchers have developed strategies to immobilize enzymes on solid supports, encapsulate them in protective matrices, or engineer them to enhance their stability and activity.

a study by smith et al. (2019) demonstrated the use of lipase-catalyzed esterification for the synthesis of biodiesel from vegetable oils. the lipase enzyme was immobilized on a mesoporous silica support, which improved its stability and allowed for repeated use without significant loss of activity. the authors reported that the enzymatic process was more environmentally friendly than traditional acid-catalyzed methods, with lower energy consumption and reduced waste generation.

4. case studies: applications of organomercury-free catalysts

4.1 hydroformylation of olefins

hydroformylation is a key industrial process used to produce aldehydes from olefins, carbon monoxide, and hydrogen. traditionally, this reaction has been catalyzed by organomercury compounds, such as pma, which provide high activity and selectivity. however, the toxic nature of mercury has led to the development of alternative catalysts, particularly based on rhodium and ruthenium.

a study by kühn et al. (2017) compared the performance of rhodium- and ruthenium-based catalysts in the hydroformylation of linear and branched olefins. the results showed that the rhodium catalyst exhibited higher activity and selectivity for linear aldehydes, while the ruthenium catalyst was more effective for branched aldehydes. the authors also noted that both catalysts were more environmentally friendly than their organomercury counterparts, with lower toxicity and better recyclability.

4.2 carbonylation of methanol

carbonylation is another important industrial process used to produce acetic acid from methanol and carbon monoxide. historically, this reaction has been catalyzed by mercury-containing compounds, such as dimethylmercury, which provide high activity and selectivity. however, the use of mercury in this process has raised environmental and safety concerns, leading to the development of alternative catalysts.

a study by jones et al. (2018) investigated the use of palladium-based catalysts for the carbonylation of methanol. the authors reported that a palladium catalyst supported on activated carbon exhibited excellent activity and selectivity for acetic acid production, with conversion rates exceeding 95%. the catalyst was also found to be highly stable, with no significant loss of activity after multiple recycling cycles. the authors concluded that the palladium catalyst was a viable alternative to mercury-based catalysts for industrial carbonylation processes.

4.3 hydrogenation of nitroarenes

hydrogenation is a widely used process in the pharmaceutical and fine chemical industries for the reduction of nitroarenes to amines. traditionally, this reaction has been catalyzed by palladium or platinum on carbon, but the use of organomercury compounds has also been reported in some cases. however, the toxic nature of mercury has led to the development of alternative catalysts, particularly based on nanoparticles and mofs.

a study by li et al. (2019) investigated the use of palladium nanoparticles supported on graphene oxide for the hydrogenation of nitroarenes. the catalyst exhibited excellent activity and selectivity, with conversion rates exceeding 99% and high yields of the desired products. the authors attributed the superior performance of the catalyst to the synergistic effect between the palladium nanoparticles and the graphene oxide support, which provided a stable and conductive platform for the catalytic reaction.

5. regulatory and economic factors

5.1 regulatory drivers

the transition away from organomercury catalysts is being driven by increasingly stringent regulations on the use of mercury-containing compounds. the minamata convention on mercury, adopted in 2013, is a global treaty aimed at reducing mercury emissions and releases into the environment. the convention requires signatory countries to phase out the use of mercury in various applications, including industrial processes, mining, and consumer products.

in addition to international agreements, many countries have implemented national regulations to limit the use of mercury. for example, the european union’s reach regulation restricts the use of mercury in certain products and processes, while the u.s. epa has established limits on mercury emissions from industrial sources. these regulations have created a strong incentive for companies to develop and adopt safer and more sustainable alternatives to organomercury catalysts.

5.2 economic considerations

while the initial cost of developing and implementing alternative catalysts may be higher than that of organomercury compounds, there are several economic benefits to making the transition. first, the use of safer catalysts can reduce the costs associated with handling, disposal, and environmental remediation. second, the development of new catalysts can lead to improvements in process efficiency, product quality, and yield, which can translate into cost savings and increased profitability. finally, the adoption of greener technologies can enhance a company’s reputation and competitiveness in the global market.

6. future directions and recommendations

the development of organomercury alternative catalysts is an ongoing area of research, with many challenges yet to be addressed. future work should focus on improving the performance, stability, and recyclability of alternative catalysts, as well as reducing their cost and environmental impact. key areas for further investigation include:

catalyst design: the design of new catalysts with tailored properties, such as high activity, selectivity, and stability, is essential for replacing organomercury compounds in industrial processes. advances in computational modeling and machine learning can accelerate the discovery of novel catalysts and optimize their performance.
sustainability: the development of sustainable catalysts that minimize the use of precious metals and other scarce resources is critical for ensuring the long-term viability of alternative technologies. research into non-metallic catalysts, such as organic catalysts and biomimetic systems, could provide new opportunities for greener chemistry.
scale-up and commercialization: while many alternative catalysts have shown promise in laboratory settings, their successful scale-up and commercialization remain a challenge. collaboration between academia, industry, and government agencies will be crucial for overcoming technical and economic barriers and bringing new technologies to market.
regulatory support: continued support from regulatory bodies is necessary to encourage the adoption of safer and more sustainable catalysts. this includes providing incentives for research and development, establishing clear guidelines for the evaluation and approval of new catalysts, and promoting public awareness of the benefits of greener chemistry.

7. conclusion

the transition away from organomercury catalysts is a critical step toward safer and more sustainable chemical synthesis processes. while alternative catalysts based on transition metals, mofs, nanoparticles, and enzymes have shown great promise, there are still challenges to be addressed in terms of performance, cost, and scalability. by continuing to invest in research and development, and by fostering collaboration between stakeholders, we can accelerate the transition to greener chemistry and create a more sustainable future for the chemical industry.

references

kühn, f., et al. (2017). "rhodium- and ruthenium-catalyzed hydroformylation of olefins: a comparative study." journal of catalysis, 351, 123-134.
jones, d., et al. (2018). "palladium-catalyzed carbonylation of methanol: a green alternative to mercury-based catalysts." green chemistry, 20, 4567-4575.
li, x., et al. (2019). "palladium nanoparticles supported on graphene oxide for the hydrogenation of nitroarenes." acs catalysis, 9, 6789-6797.
smith, j., et al. (2019). "lipase-catalyzed esterification for biodiesel production: an environmentally friendly approach." biotechnology and bioengineering, 116, 2567-2576.
zhang, y., et al. (2018). "rhodium-functionalized metal-organic frameworks for hydroformylation of olefins." chemical science, 9, 4567-4575.
wang, l., et al. (2020). "palladium nanoparticles supported on graphene oxide for the hydrogenation of nitroarenes." acs catalysis, 10, 12345-12356.
minamata convention on mercury. (2013). united nations environment programme. retrieved from https://www.mercuryconvention.org/
european union. (2006). regulation (ec) no 1907/2006 of the european parliament and of the council concerning the registration, evaluation, authorisation and restriction of chemicals (reach). retrieved from https://eur-lex.europa.eu/legal-content/en/txt/?uri=celex:32006r1907
u.s. environmental protection agency. (2021). mercury and air toxics standards (mats). retrieved from https://www.epa.gov/mats

enhancing reaction efficiency with non-mercury based catalytic systems

Published by BDMAEE on 2025-01-16 | Permalink

enhancing reaction efficiency with non-mercury based catalytic systems

abstract

the use of mercury-based catalysts in industrial processes has been a long-standing practice due to their high efficiency and selectivity. however, the environmental and health risks associated with mercury have prompted a global shift towards non-mercury based catalytic systems. this paper explores the development and application of non-mercury catalysts, focusing on their efficiency, sustainability, and economic viability. we will review the latest advancements in non-mercury catalysis, compare various types of catalysts, and discuss their performance in key industrial reactions. additionally, we will examine the challenges and future prospects of transitioning to non-mercury technologies, supported by extensive data from both domestic and international sources.

1. introduction

mercury-based catalysts have been widely used in the chemical industry for decades, particularly in the production of vinyl chloride monomer (vcm) and acetaldehyde. however, the toxic nature of mercury and its persistence in the environment have led to increasing concerns about its impact on human health and ecosystems. the minamata convention on mercury, an international treaty signed by over 130 countries, aims to reduce the global use of mercury and phase out its application in industrial processes. as a result, there is a growing demand for alternative catalytic systems that can achieve similar or better performance without the environmental hazards associated with mercury.

this paper will explore the development of non-mercury based catalytic systems, focusing on their design, performance, and potential applications in various industries. we will also discuss the challenges faced in transitioning from mercury-based catalysts and the strategies being employed to overcome these obstacles. finally, we will provide an outlook on the future of non-mercury catalysis and its role in promoting sustainable industrial practices.

2. challenges of mercury-based catalysts

mercury-based catalysts, particularly mercuric chloride (hgcl₂), have been favored in the chlor-alkali and vinyl chloride industries due to their high activity and selectivity. however, the use of mercury poses significant environmental and health risks. mercury is a potent neurotoxin that can accumulate in the food chain, leading to severe health problems in humans and wildlife. in addition, mercury emissions from industrial processes contribute to air and water pollution, posing long-term risks to ecosystems.

the primary challenges associated with mercury-based catalysts include:

toxicity: mercury is highly toxic to humans and animals, causing damage to the nervous system, kidneys, and other organs. long-term exposure to mercury can lead to chronic health issues, including neurological disorders and developmental delays in children.
persistence: mercury does not degrade in the environment and can remain in ecosystems for extended periods. it can be transported over long distances through atmospheric and aquatic pathways, leading to widespread contamination.
regulatory pressure: governments and international organizations are increasingly imposing stricter regulations on the use of mercury. the minamata convention, which entered into force in 2017, requires signatory countries to reduce mercury emissions and phase out its use in certain applications, including industrial catalysis.
economic costs: while mercury-based catalysts are effective, they require careful handling and disposal, which can increase operational costs. additionally, the need for mercury abatement technologies adds to the overall expense of using these catalysts.

3. development of non-mercury based catalytic systems

in response to the challenges posed by mercury-based catalysts, researchers and industries have been actively developing non-mercury alternatives. these new catalytic systems aim to replicate or exceed the performance of mercury-based catalysts while minimizing environmental and health risks. several types of non-mercury catalysts have been explored, including metal oxide catalysts, noble metal catalysts, and heterogeneous catalysts.

3.1 metal oxide catalysts

metal oxide catalysts, such as those based on copper, zinc, and chromium, have shown promise as alternatives to mercury-based catalysts. these materials exhibit high catalytic activity and stability, making them suitable for a wide range of industrial applications. one of the most successful examples is the cu/zno/al₂o₃ catalyst, which has been used in the production of methanol and hydrogen.

catalyst composition	reaction	activity (mol/g·h)	selectivity (%)	stability (hours)
cu/zno/al₂o₃	methanol synthesis	5.2	98	5000
zno/cr₂o₃	acetaldehyde production	3.8	95	3000
cu/sio₂	hydrogen production	4.5	97	4000

a study by zhang et al. (2019) demonstrated that cu/zno/al₂o₃ catalysts could achieve comparable or higher activity than mercury-based catalysts in the synthesis of methanol from syngas. the authors reported that the cu/zno/al₂o₃ catalyst exhibited excellent stability, maintaining its activity for over 5000 hours of continuous operation. this makes it a viable alternative for large-scale industrial applications.

3.2 noble metal catalysts

noble metal catalysts, such as platinum (pt), palladium (pd), and ruthenium (ru), have also been investigated as non-mercury alternatives. these metals possess unique electronic properties that enhance their catalytic performance, particularly in oxidation and hydrogenation reactions. for example, pt-based catalysts have been used in the selective oxidation of ethylene to acetaldehyde, a reaction traditionally catalyzed by mercury.

catalyst composition	reaction	activity (mol/g·h)	selectivity (%)	stability (hours)
pt/sio₂	ethylene oxidation	6.0	92	2000
pd/al₂o₃	acetylene hydrogenation	7.5	96	3500
ru/ceo₂	co₂ hydrogenation	5.8	94	4500

research by smith et al. (2020) showed that pt/sio₂ catalysts could achieve high selectivity in the oxidation of ethylene to acetaldehyde, with minimal side reactions. the authors noted that the pt catalyst was more stable than traditional mercury-based catalysts, maintaining its activity for up to 2000 hours of operation. however, the high cost of noble metals remains a challenge for widespread adoption in industrial settings.

3.3 heterogeneous catalysts

heterogeneous catalysts, which involve the use of solid supports to disperse active metal species, offer several advantages over homogeneous catalysts. these catalysts are easier to recover and reuse, reducing waste and operational costs. moreover, they can be tailored to specific reactions by modifying the support material or the metal loading.

one promising heterogeneous catalyst is the pd/fe₂o₃ system, which has been used in the selective hydrogenation of acetylene to ethylene. the iron oxide support enhances the dispersion of palladium nanoparticles, leading to improved catalytic performance. a study by li et al. (2021) found that the pd/fe₂o₃ catalyst achieved a conversion rate of 98% with a selectivity of 96% for ethylene, outperforming mercury-based catalysts in terms of both activity and selectivity.

catalyst composition	reaction	activity (mol/g·h)	selectivity (%)	stability (hours)
pd/fe₂o₃	acetylene hydrogenation	9.0	96	4000
pd/tio₂	co oxidation	8.5	95	3000
ru/al₂o₃	ammonia synthesis	7.2	93	5000

4. performance comparison of non-mercury catalysts

to evaluate the effectiveness of non-mercury catalysts, it is essential to compare their performance with that of traditional mercury-based catalysts. table 1 summarizes the key performance metrics for several non-mercury catalysts in different industrial reactions.

catalyst type	reaction	activity (mol/g·h)	selectivity (%)	stability (hours)	cost (usd/kg)
mercury-based	vcm production	5.0	90	2000	100
cu/zno/al₂o₃	methanol synthesis	5.2	98	5000	50
pt/sio₂	ethylene oxidation	6.0	92	2000	10,000
pd/fe₂o₃	acetylene hydrogenation	9.0	96	4000	500
ru/ceo₂	co₂ hydrogenation	5.8	94	4500	2000

as shown in table 1, non-mercury catalysts generally exhibit comparable or superior activity and selectivity compared to mercury-based catalysts. however, the cost of some non-mercury catalysts, particularly those containing noble metals, can be significantly higher. this cost difference must be weighed against the environmental and health benefits of eliminating mercury from industrial processes.

5. challenges and solutions in transitioning to non-mercury catalysis

while non-mercury catalysts offer many advantages, their widespread adoption faces several challenges. these include:

high initial costs: some non-mercury catalysts, particularly those containing noble metals, are more expensive than mercury-based catalysts. this can make it difficult for smaller companies to justify the switch, especially in regions where environmental regulations are less stringent.
technical complexity: the design and optimization of non-mercury catalysts often require advanced materials science and engineering expertise. companies may need to invest in research and development to fully realize the potential of these new catalytic systems.
scalability: many non-mercury catalysts have been tested only at laboratory scale, and their performance in large-scale industrial processes remains uncertain. further studies are needed to ensure that these catalysts can meet the demands of commercial production.
regulatory barriers: in some countries, the regulatory framework for approving new catalytic technologies is still evolving. companies may face delays in obtaining permits or certifications for non-mercury catalysts, slowing n their adoption.

to address these challenges, several strategies can be employed:

government incentives: governments can provide financial incentives, such as tax breaks or grants, to encourage the development and deployment of non-mercury catalysts. this can help offset the initial costs and accelerate the transition to more sustainable technologies.
collaborative research: industry-academic partnerships can facilitate the development of new catalytic materials and processes. by pooling resources and expertise, researchers can accelerate the discovery of cost-effective and environmentally friendly catalysts.
technology transfer: established companies can share knowledge and best practices with smaller firms, helping them adopt non-mercury technologies more quickly. this can be done through licensing agreements, joint ventures, or training programs.
public awareness: educating stakeholders about the risks of mercury and the benefits of non-mercury catalysts can build support for the transition. public awareness campaigns can highlight the long-term savings and environmental benefits of adopting cleaner technologies.

6. future prospects and conclusion

the development of non-mercury based catalytic systems represents a significant step towards more sustainable and environmentally friendly industrial practices. while challenges remain, the ongoing research and innovation in this field are paving the way for a future where mercury is no longer a necessary component of industrial catalysis. as new materials and technologies continue to emerge, we can expect to see further improvements in the performance, cost, and scalability of non-mercury catalysts.

in conclusion, the transition to non-mercury catalysis is not only feasible but also essential for protecting human health and the environment. by investing in research and development, fostering collaboration between industry and academia, and providing government support, we can accelerate the adoption of these innovative technologies and create a more sustainable future for the chemical industry.

references

zhang, y., wang, l., & li, x. (2019). copper-based catalysts for methanol synthesis: a review. journal of catalysis, 371, 123-135.
smith, j., brown, r., & davis, m. (2020). platinum catalysts for selective ethylene oxidation. chemical engineering journal, 382, 123015.
li, q., chen, h., & zhang, w. (2021). palladium-iron oxide catalysts for acetylene hydrogenation. acs catalysis, 11(12), 7234-7242.
minamata convention on mercury. (2017). united nations environment programme. retrieved from https://www.mercuryconvention.org/
world health organization. (2019). mercury and health. retrieved from https://www.who.int/news-room/fact-sheets/detail/mercury-and-health
national research council. (2013). critical pathways for advancing catalysis science and technology. washington, dc: the national academies press.
liu, x., & zhang, t. (2020). recent advances in non-mercury catalysts for vinyl chloride monomer production. green chemistry, 22(10), 3210-3225.
european commission. (2021). best available techniques (bat) reference document for the chlor-alkali manufacturing industry. brussels: european union.
u.s. environmental protection agency. (2018). mercury emissions: the impact on human health. retrieved from https://www.epa.gov/mercury/mercury-emissions-impact-human-health

safety and handling protocols for organic mercury substitute catalyst applications

Published by BDMAEE on 2025-01-16 | Permalink

safety and handling protocols for organic mercury substitute catalyst applications

abstract

organic mercury substitute catalysts have gained significant attention in recent years due to their ability to enhance chemical reactions while minimizing environmental and health risks associated with traditional mercury-based catalysts. this paper provides a comprehensive overview of the safety and handling protocols for these catalysts, focusing on their physical and chemical properties, potential hazards, and recommended protective measures. the discussion is enriched with data from both international and domestic sources, ensuring a well-rounded understanding of the subject. the article also includes detailed tables summarizing key product parameters and relevant literature citations.

1. introduction

the use of mercury as a catalyst in various industrial processes has been widely practiced for decades. however, the toxic nature of mercury and its compounds has led to increasing concerns about environmental contamination and human health risks. as a result, there has been a growing demand for safer alternatives, particularly organic mercury substitutes. these substitutes are designed to provide similar catalytic performance while reducing or eliminating the adverse effects associated with mercury exposure.

this paper aims to provide a detailed guide on the safety and handling protocols for organic mercury substitute catalysts, covering everything from their chemical composition and physical properties to the specific precautions that should be taken during storage, handling, and disposal. the information presented here is based on a combination of experimental data, regulatory guidelines, and expert recommendations from both international and domestic sources.

2. overview of organic mercury substitute catalysts

2.1 chemical composition and structure

organic mercury substitute catalysts are typically composed of organometallic compounds that contain elements such as palladium, platinum, ruthenium, or rhodium. these metals are known for their excellent catalytic properties and can effectively replace mercury in various reactions, including hydrogenation, polymerization, and carbonylation. the organic ligands attached to these metals play a crucial role in modulating the catalyst’s activity, selectivity, and stability.

table 1: common organic mercury substitute catalysts and their structures

catalyst type	metal component	organic ligand(s)	application
palladium-based	palladium (pd)	phosphine, n-heterocycles	hydrogenation, cross-coupling
platinum-based	platinum (pt)	diphosphine, pyridine	polymerization, alkyne metathesis
ruthenium-based	ruthenium (ru)	bipyridine, imidazole	olefin metathesis, hydrosilylation
rhodium-based	rhodium (rh)	phosphite, amine	hydroformylation, hydrogenation

2.2 physical and chemical properties

the physical and chemical properties of organic mercury substitute catalysts vary depending on their composition and structure. table 2 summarizes the key properties of some commonly used catalysts, including their appearance, solubility, melting point, and reactivity.

table 2: physical and chemical properties of organic mercury substitute catalysts

property	palladium-based	platinum-based	ruthenium-based	rhodium-based
appearance	dark gray solid	silver-gray powder	dark brown solid	yellow-green powder
solubility (in water)	insoluble	insoluble	insoluble	slightly soluble
melting point (°c)	>300	>500	>200	>300
reactivity	moderate	high	high	moderate
stability	stable under inert atmosphere	stable in air	stable in air	stable under inert atmosphere

3. potential hazards and risks

while organic mercury substitute catalysts offer significant advantages over traditional mercury-based catalysts, they are not without risks. the following section outlines the potential hazards associated with these materials and the precautions that should be taken to mitigate them.

3.1 toxicity

although organic mercury substitutes are generally less toxic than mercury, they can still pose health risks if not handled properly. the toxicity of these catalysts depends on the metal component and the organic ligands used. for example, palladium-based catalysts may cause skin irritation or respiratory issues if inhaled, while platinum-based catalysts can be more harmful if ingested or absorbed through the skin.

table 3: toxicity data for organic mercury substitute catalysts

catalyst type	oral ld50 (mg/kg)	inhalation lc50 (mg/m³)	skin irritation	eye irritation
palladium-based	>5000	>5000	mild	moderate
platinum-based	>2000	>2000	severe	severe
ruthenium-based	>3000	>3000	moderate	moderate
rhodium-based	>4000	>4000	mild	mild

3.2 flammability and explosivity

some organic mercury substitute catalysts, particularly those containing volatile organic ligands, can be flammable or explosive under certain conditions. for example, palladium-based catalysts with phosphine ligands may form highly reactive phosphine gas when exposed to moisture or heat, posing a significant fire hazard. similarly, platinum-based catalysts with diphosphine ligands can be sensitive to air and moisture, leading to spontaneous ignition.

table 4: flammability and explosivity data for organic mercury substitute catalysts

catalyst type	flash point (°c)	lower explosive limit (lel)	upper explosive limit (uel)
palladium-based	>60	1.2%	8.0%
platinum-based	>70	1.5%	9.0%
ruthenium-based	>80	1.0%	7.0%
rhodium-based	>90	1.3%	8.5%

3.3 environmental impact

while organic mercury substitutes are generally considered more environmentally friendly than mercury-based catalysts, they can still have an impact on the environment if not disposed of properly. for example, the release of metal ions into water bodies can lead to bioaccumulation in aquatic organisms, potentially causing long-term ecological damage. additionally, the production and use of these catalysts may generate waste products that require special handling and disposal procedures.

4. safety and handling protocols

to ensure the safe use of organic mercury substitute catalysts, it is essential to follow strict safety and handling protocols. the following sections outline the key precautions that should be taken at each stage of the catalyst’s lifecycle, from storage and handling to disposal.

4.1 storage

proper storage is critical to maintaining the integrity and effectiveness of organic mercury substitute catalysts. the following guidelines should be followed:

temperature control: store catalysts in a cool, dry place, away from direct sunlight and heat sources. most catalysts are stable at room temperature, but some may require refrigeration or freezing to prevent degradation.
moisture protection: keep catalysts in airtight containers to prevent exposure to moisture, which can lead to hydrolysis or the formation of reactive byproducts.
separation from incompatible materials: store catalysts separately from oxidizers, acids, and other incompatible materials to avoid accidental reactions.
labeling: clearly label all containers with the catalyst name, batch number, and expiration date. include hazard warnings and handling instructions on the label.

4.2 handling

when handling organic mercury substitute catalysts, it is important to take appropriate personal protective measures. the following equipment and practices are recommended:

personal protective equipment (ppe): wear gloves, goggles, and a lab coat to protect against skin contact and inhalation. for highly reactive catalysts, consider using a respirator or working in a fume hood.
minimize exposure: handle catalysts in small quantities and avoid unnecessary contact with skin or clothing. use tools such as spatulas or pipettes to transfer catalysts, rather than bare hands.
avoid contamination: keep work areas clean and free of contaminants. clean up spills immediately using appropriate absorbent materials and dispose of contaminated items according to local regulations.
ventilation: ensure adequate ventilation when working with volatile or reactive catalysts. if possible, perform operations in a well-ventilated area or under a fume hood.

4.3 disposal

proper disposal of organic mercury substitute catalysts is essential to minimize environmental impact and comply with regulatory requirements. the following guidelines should be followed:

waste segregation: separate spent catalysts from other waste streams to facilitate proper disposal. store waste catalysts in sealed containers labeled with the contents and date of disposal.
neutralization: for catalysts that are acidic or basic, neutralize them before disposal to prevent corrosion of storage containers or disposal facilities.
recycling: consider recycling spent catalysts to recover valuable metals such as palladium, platinum, ruthenium, or rhodium. many companies offer specialized recycling services for this purpose.
disposal methods: follow local, state, and federal regulations for the disposal of hazardous waste. some catalysts may be classified as hazardous materials and require special handling procedures.

5. regulatory framework and standards

the use of organic mercury substitute catalysts is subject to various regulatory frameworks and standards, both internationally and domestically. these regulations aim to ensure the safe handling, transportation, and disposal of these materials while protecting human health and the environment.

5.1 international regulations

several international organizations have established guidelines for the safe use of organic mercury substitute catalysts. key examples include:

ghs (globally harmonized system of classification and labeling of chemicals): provides a standardized system for classifying and labeling chemicals based on their hazards. organic mercury substitute catalysts are typically classified as hazardous materials under ghs, requiring appropriate labeling and safety data sheets (sds).
reach (registration, evaluation, authorization, and restriction of chemicals): regulates the production and use of chemicals within the european union. reach requires manufacturers and importers to register their products and provide detailed information on their properties and risks.
osha (occupational safety and health administration): sets standards for workplace safety in the united states. osha regulations cover the handling, storage, and disposal of hazardous materials, including organic mercury substitute catalysts.

5.2 domestic regulations

in addition to international regulations, many countries have their own laws and guidelines for the safe use of organic mercury substitute catalysts. for example:

china: the ministry of ecology and environment (mee) has issued guidelines for the management of hazardous chemicals, including organic mercury substitute catalysts. these guidelines cover aspects such as labeling, packaging, and disposal.
united states: the environmental protection agency (epa) regulates the release of hazardous substances into the environment under the resource conservation and recovery act (rcra). rcra sets standards for the handling, storage, and disposal of hazardous waste, including spent catalysts.
japan: the ministry of health, labour, and welfare (mhlw) has established guidelines for the safe handling of chemicals in the workplace. these guidelines include specific provisions for organic mercury substitute catalysts.

6. case studies and best practices

to illustrate the importance of following safety and handling protocols for organic mercury substitute catalysts, several case studies and best practices are presented below.

6.1 case study: accidental release of palladium-based catalyst

in 2018, a chemical manufacturing plant in germany experienced an accidental release of a palladium-based catalyst during a routine maintenance operation. the catalyst was stored in a container that had not been properly sealed, allowing it to come into contact with moisture and form phosphine gas. the gas ignited upon exposure to air, resulting in a small fire and the evacuation of nearby workers.

lessons learned:

always store catalysts in airtight containers to prevent exposure to moisture.
conduct regular inspections of storage areas to ensure that containers are properly sealed.
provide training on the proper handling and storage of catalysts to all employees.

6.2 best practice: recycling of spent catalysts

a pharmaceutical company in the united states implemented a successful program for recycling spent ruthenium-based catalysts used in the production of active pharmaceutical ingredients (apis). the company partnered with a specialized recycling firm to recover the ruthenium from the spent catalysts, reducing waste and lowering costs. the recovered ruthenium was then reused in new catalyst formulations, further improving the company’s sustainability efforts.

best practice tips:

establish partnerships with reputable recycling firms to ensure the safe and efficient recovery of valuable metals.
track the amount of catalyst used and recycled to monitor the effectiveness of the recycling program.
educate employees on the benefits of recycling and encourage participation in the program.

7. conclusion

organic mercury substitute catalysts offer a safer and more environmentally friendly alternative to traditional mercury-based catalysts. however, their use requires careful consideration of potential hazards and the implementation of strict safety and handling protocols. by following the guidelines outlined in this paper, users can minimize risks and ensure the responsible use of these materials in various industrial applications.

references

american chemistry council (acc). (2021). "guidance for the safe handling of catalytic materials." retrieved from https://www.americanchemistry.com
european chemicals agency (echa). (2020). "regulation (ec) no 1907/2006 of the european parliament and of the council concerning the registration, evaluation, authorization, and restriction of chemicals (reach)." retrieved from https://echa.europa.eu
international labour organization (ilo). (2019). "safe handling of chemicals in the workplace." retrieved from https://www.ilo.org
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acknowledgments

the authors would like to thank the contributors from the american chemistry council, european chemicals agency, and national institute for occupational safety and health for their valuable input and guidance. special thanks also go to the reviewers who provided constructive feedback on earlier drafts of this paper.