Hydrogen Safety (Green Chemistry and Chemical Engineering)

Green Chemistry and Chemical Engineering

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Hydrogen Science and Engineering: Added to Your Shopping Cart. Description Authored by 50 top academic, government and industry researchers, this handbook explores mature, evolving technologies for a clean, economically viable alternative to non-renewable energy. In so doing, it also discusses such broader topics as the environmental impact, education, safety and regulatory developments. Stolten received his doctorate from the University of Technology at Clausthal, Germany. Stolten's research focuses on fuel cells, implementing results from research in innovative products, procedures and processes in collaboration with industry, contributing towards bridging the gap between science and technology.

Biocatalysis has become a standard synthetic technique across a wide range of the chemicals and pharmaceutical industries [ , ]. While enzyme catalysis in non-aqueous solvents has been known for a long time [ ], water is the solvent of choice for biocatalytic processes. Hence, the use of enzyme-catalysed reactions is often accompanied by a replacement of non-aqueous solvents with water and so is included here.

The use of enzymes in water has also enabled improvements in other environmental impacts of many processes [ — ]. Pregabalin, S aminomethyl methylhexanoic acid, is a treatment for central nervous system disorders. This demonstrates how the avoidance of hazardous solvents can reduce the cost of chemicals production. The original commercial synthesis of pregabalin [ ]. Reports can be found in the literature from both Pfizer [ ] and Dowpharma [ ] of the development of asymmetric hydrogenation-based routes to avoid this problem. However, Pfizer's eventual solution was an enzyme-catalysed process scheme 4 [ ].

The unreacted diester is then recycled and racemized in toluene to be reused, while the carboxylic acid is thermally decarboxylated in the aqueous solution.

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Hydrogenation in aqueous i -PrOH completes the synthesis. Perhaps some concern remains at the use of toluene in the racemization process, but the environmental performance of the synthesis has been significantly improved. The enzymatic synthesis of pregabalin [ ]. Acrylamide is a commodity chemical used as the monomer for the polymer polyacrylamide. It is prepared by the hydration of acrylonitrile. The traditional synthesis used copper catalysts and exhibited problems such as incomplete reaction of the acrylonitrile, requiring its recovery from the product mixture, and the formation of by-products, such as acrylic acid nitrylotrispropionamide, ethylene cyanohydrin and polymers of both the starting material and product [ ].

This was the first commercial example of an enzyme-catalysed reaction being used to produce a commodity chemical. Biocatalysis can also be performed using whole microorganisms. The biocatalytic methods use less energy, reduce waste and use renewable resources, such as sugar or plant oil, as the starting materials and produce the riboflavin at approximately half the cost of the synthetic chemistry route. When discussed in the context of the environment, solvents are usually seen as a problem to be overcome.

However, it is possible for the selection of an appropriate solvent to provide a sustainable solution to a process problem. In the following sections, I attempt to show examples of how solvents have been used to deliver sustainable chemicals processes.

1. Introduction

These have been grouped by the advantage that the particular solvent provides. The polymer most often referred to simply as polycarbonate PC is an aromatic carbonate polymer based on the monomer bisphenol-A Bis-A. It has increased in use and importance with the spread of modern electronic devices. Asahi Kasei introduced a new process for the production of PC scheme 5 that is acclaimed for replacing phosgene COCl 2 as the source of the carbonate link in the polymer with CO 2 [ — ]. However, this process also led to the removal of dichloromethane DCM as a solvent.

While one might not choose one of the components to be the solvent for the others, this is undoubtedly a solution process. The reaction occurs at the interface of these two immiscible solutions. The DCM is a solvent for the PC product, thus maintaining a homogeneous solution throughout the process. DCM also contaminated the product, leading to the release of this toxic solvent to the environment and a lower quality product.

Also, although forming two layers, DCM has some solubility in water and water has some solubility in DCM, leading to energy-intensive and expensive separations. The new process is conceptually simple. However, a number of intermediates are used to achieve this, which are either consumed or recycled in the process. First, the ethylene oxide and CO 2 are reacted to give ethylene carbonate, which is then reacted with methanol to produce ethylene glycol co-product and dimethyl carbonate. The dimethyl carbonate is reacted with phenol to yield diphenyl carbonate and regenerate the methanol. A final transesterification reaction between diphenyl carbonate and Bis-A yields the PC and regenerates phenol [ ].

Selling the ethylene glycol co-product of this reaction provides much of both the environmental and economic benefits of this process. The new process saves energy and the capital cost of the plant for this process is less than half that of similar scale plants that use phosgene [ , ]. However, ethylene oxide is a hazardous material and phenol is environmentally harmful and the environmental impact of this synthesis is reliant upon containment of these.

An example of the use of the reactivity of a solvent to enable a process is the use of an ionic liquid to remove mercury from natural gas [ , ]. Fossil fuel production and use is a major source of environmental Hg pollution [ ]. Mercury's corrosive nature can also lead to disastrous production plant failures [ ]. The Hg is present in tiny concentrations in the gas stream, but the enormous volume of natural gas production leads to large absolute amounts of Hg passing into the production plant. Consequently, a Hg removal process that can operate at these low concentrations is required. This was solved by developing a chlorocuprate II ionic liquid system that was capable of absorbing the mercury and combining this with the supported ionic liquid phase SILP technology [ , , ].

SILP technology was originally developed to enable catalysts dissolved in ionic liquids to be contacted with gaseous reactants [ ]. The same ability enabled this Hg removal system to be brought to the full production plant scale. Full elucidation of the chemistry involved has proved difficult, but the inventors have deduced that scheme 6 is the most likely. Some might question whether the production of natural gas can ever be thought of as sustainable. However, given that the use of natural gas is unlikely to significantly decline in the near future, it is vital that its production is conducted as sustainably as possible.

The introduction of this technology has led i to a reduction in the pollution generated and ii to savings in the costs of the production of natural gas. This identifies it as a likely sustainable process only time will tell. This is because the water produced as a by-product of the reaction forces the equilibrium back towards the starting alcohol. When the starting material is a diol, a number of possible partially chlorinated and ether by-products are formed.

However, these are toxic, difficult to handle and environmentally damaging. BASF has recently commercialized an ionic liquid process for nucleophilic substitutions for the conversion of alcohols to halogenoalkanes that allows HCl to be used as the chlorinating agent [ ]. When used in the chlorination of 1,4-butanediol this yields the dichloride without the formation of by-products scheme 7.

This salt is the chlorinating agent. However, this does not explain why the water produced in the reaction no longer causes a problem. These interactions lead to ionic liquids being able to stabilize water-sensitive solutes [ ] or prevent water from reacting with a solute [ ]. The introduction of this process has led to the elimination of the highly toxic gas COCl 2 , with the attendant savings that derive from not needing to put in place the necessary engineering controls to handle it safely. In the latter half of the twentieth century, health concerns over the effects of caffeine led to increased demand for decaffeinated coffee.

Early forms of decaffeinated coffee were produced by caffeine extraction with dichloromethane [ ]. The direct decaffeination of green coffee beans occurs before their roasting, which removed the DCM from the beans to levels of a few ppm. It was not the environmental concern that led to the replacement of this process. Alongside caffeine the DCM also removed important flavour components of the coffee, giving a poor quality product. This led to a number of other less environmentally concerning solvents being used for coffee decaffeination, but with the commercial driver being the search for a better product.

Ethyl acetate is an environmentally preferred solvent [ 80 ] used for coffee bean decaffeination [ ]. First, the unroasted green beans are wetted with steam to increase their water content and to release the caffeine. Then the EtOAc is added to separate the caffeine from the moistened beans, from which residual EtOAc is removed by further steam treatment [ ]. EtOAc is also used to decaffeinate tea [ ]. The green beans are treated with hot water, which not only removes the caffeine, but also several other flavour chemicals.

The caffeine is then extracted from the water with an activated charcoal filter. The water, still bearing many of the flavour chemicals, is reused for subsequent extractions of fresh beans. As this process is repeated, the water solution becomes saturated in the flavour compounds, so caffeine is extracted from the fresh beans, but the flavour compounds are not [ ], giving a high-quality product.

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Supercritical CO 2 sc-CO 2 decaffeination is also often described as solvent-free [ , , ]. The green coffee beans are wetted and then the sc-CO 2 is used to extract the caffeine. The sc-CO 2 process is much more selective for the removal of caffeine than any of the other processes, leading to a high-quality product without the need for the additional steps to isolate it that are required for other methods. Sc-CO 2 processing has become a widely used method in the food industry, such as in the decaffeination of tea [ ], the removal of fat to produce low-fat varieties, the removal of alcohol to produce low-alcohol beers and wines and the removal of pesticides from rice and the extraction of flavours and fragrance compounds [ , ].

The number of intermediate product isolations in a multi-step chemical synthesis can greatly negatively affect the environmental impact of a process. This usually occurs because individual steps are independently optimized and then connected in a chain of reactions to yield the final product. Thus, one step can be followed by another with the solvent for the first being unsuitable for the second. However, it may be possible to select a solvent so that it is capable of supporting several consecutive reactions and lead to a significant reduction in the waste generated by the overall process.

The use of ethanol, together with adjustment of the synthetic route, allowed the final three steps of Pfizer's sertraline synthesis scheme 8 to be conducted without intermediate product isolation [ , ]. The first commercial route used Most of these were used in the purifications of the isolated intermediates. The synthesis of sertraline [ ]. The new synthesis changed the reactions to affect each transformation rather than change the intermediates in the process. It avoided the use of TiCl 4 and eliminated TiO 2 waste, removing the need for a costly and wasteful filtration.

Ethanol was not the optimum solvent choice for this step if considered in isolation, showing the importance of considering the overall process, not just the individual parts. It is not possible from the available information [ , ] to compare the details of the performances achieved in these processes in order to calculate their green metrics, but it is possible to estimate the low end of the likely range of values.

These calculations assume that there is no solvent recovery in either process. The first-generation synthesis of sitagliptin, a treatment for type 2 diabetes, was conducted in multiple steps [ ]. First 3-trifluoromethyl-[1,2,4]triazolo[4,3-a]piperazine was prepared, so that it could be reacted with the hydrolysed form of the lactam N -benzyloxy-4 R -[1-methyl- 2,4,5-trifluorophenyl ]oxoazetidine. It is the formation of this lactam intermediate and its subsequent reaction with the triazole that was redesigned for the second-generation synthesis.

The original synthesis required three isolations including the product , two aqueous—organic liquid separations and two solvent switches. This synthesis was replaced with multi-step one-pot synthesis in high concentration in acetonitrile scheme 9 [ ]. This process led to a reduction in the E-factor from to 50 for the overall synthesis, including a complete elimination of organic-contaminated aqueous wastes. One-pot synthesis of sitagliptin [ ].

Despite this process being a considerable improvement over its predecessor, the late-stage hydrogenation was only moderately stereoselective and required high-pressure conditions [ ]. The final version of the sitagliptin synthesis avoided this hydrogenation by using a transaminase enzyme to directly aminate the prositagliptin diketone precursor with iso -propylamine scheme 10 [ ], giving a highly enantiopure product.

In addition to these environmental advantages, the biocatalytic process eliminated the need for specialized high-pressure equipment, leading to reductions in both capital and running costs.

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Enzymatic synthesis of sitagliptin [ ]. It uses readily available reactive starting materials in reliable reactions to give high yields. The cycloaddition reactions of azides have been particularly of interest. However, these reactions are not always straightforward. One such case is the formation of 4-cyano-1,2,3-triazoles from organic azides and 2-chloroacrylonitrile scheme 11 [ 28 ]. The initial 1,3-dipolar cycloaddition yields a triazoline which eliminates HCl to form the triazole product.

If conducted in a single homogeneous solution, yields are disappointing, because the by-product HCl reacts with 2-chloroacrylonitrile to initiate its polymerization. Controls such as conducting the reaction at low concentration of starting materials and with excess 2-chloroacrylonitirle are not effective and lead to polymeric waste.

Hydrogen Science and Engineering: Materials, Processes, Systems, and Technology, 2 Volume Set

This is because the starting materials and product are not soluble in water, but HCl is. As the HCl is generated it is rapidly dissolved in the water, removing it from the reaction solution so that it cannot initiate the 2-chloroacrylonitrile polymerization. The synthesis of tadalafil, a treatment for erectile dysfunction, begins with a Pictet—Spengler reaction of tryptophan methyl ester.

By replacing the DCM with iso -propylalcohol and starting with d -tryptophan methyl ester the cis -isomer could be obtained in high yield. Both isomers are formed during the reaction, but the cis -isomer is poorly soluble in the i -PrOH and spontaneously precipitates, leaving the trans -isomer in solution. However, the two isomers are in equilibrium in solution, so heating the solution over time generates more of the cis -isomer, which precipitates further and so on until the reaction is complete.

This elimination of by-product formation led to a dramatic reduction in the waste formed and eliminated the need for flash chromatography, hence greatly reducing solvent use. The alkoxyphenylphosphanes are liquid and the [Et 3 NH]Cl solid, resulting in a thick slurry that required separation using filter presses that regularly blocked. This eliminated the costly and unreliable filtration step. The by-product [HC 1 im]Cl is deprotonated to recycle the 1-methylimidazole, again reducing costs. A recent ecoefficiency analysis has shown that the BASIL technology is far more environmentally sustainable than the process using tertiary amines http: Homogeneous catalysis is inherently more efficient all metal centres are involved in catalysis, flexibility of ligand design to optimize catalyst performance, etc.

Despite this, solid catalysts are usually preferred. This is because it can be very difficult and costly to separate a homogeneous catalyst from the reaction products. One approach to solving this problem is biphasic catalysis [ — ]. Thus, the separation of the catalyst from the reaction products is achieved.

The reactants are contacted with the catalyst by rapid stirring to give a useful rate, with the reaction occurring at the liquid—liquid interface, not by transfer into one or the other bulk phases [ ]. This process replaced a previous industrial process, which used a cobalt catalyst at high pressure, giving several advantages including: As well as giving an improved commercial performance this reduced the environmental impact with the biphasic process having an estimated E-factor of 0.

Aqueous biphasic conditions can also be used with heterogeneous catalysts. Asahi Kasei has commercialized a process for the hydrogenation of benzene to cyclohexene [ ]. The hydrogenation takes place in an aqueous phase that is in contact with a solid ruthenium catalyst. While benzene forms a separate phase from the water, it is sufficiently soluble in water to be contacted with the catalyst. The less soluble cyclohexene product transfers to the benzene phase before it can react further, preventing the formation of cyclohexane.

The process can be tuned to produce any preferred product distribution. SHOP replaced earlier thermal cracking of petroleum-derived wax. Having a low environmental impact is necessary for a product or process to be sustainable, but it is not on its own sufficient for it to be so; it must also be a commercial success. There are a number of examples of technically excellent processes that have been introduced, only later to be withdrawn due to commercial pressures.

Solvents and sustainable chemistry

One such example is Thomas Swan Ltd's hydrogenation of isophorone in sc-CO 2 over a supported palladium catalyst [ ]. The sc-CO 2 system gave selective hydrogenation of isophorone to 3,3,5-trimethylcyclohexanone, with no 3,3,5-trimethylcyclohexanol or 3,3,5-trimethylcyclohexane by-products [ , ].

This eliminated an expensive and energy-intensive separation of these from the product. A similar fate befell the Eastman Chemical Company process for the isomerization of 3,4-epoxybutene to 2,5-dihydrofuran in a phosphonium iodide ionic liquid [ 44 , 47 ].

The cost of the implementation of a new technology can also prevent a technically excellent process from being adopted because of commercial pressures. This is a biphasic process for the dimerization of olefins, in which a nickel catalyst is dissolved in an ionic liquid phase with the ionic liquid acting as both solvent and co-catalyst. The product is separated as a liquid layer that forms above the ionic liquid.

The Difasol process can either be used as an addition to the previous homogeneous Dimersol process or as a replacement for it. Despite the fact that the Difasol process offers more efficient catalyst use, higher yield, better dimer selectivity, enhanced reactor space time yield and energy savings over its predecessor, it appears that the cost of capital equipment has prevented it from yet being put into commercial application. The environmental concerns that surround the use of solvents for chemicals processing will ensure that this remains an active area for research for some time to come. The examples that I have shown above demonstrate that it is possible to make considerable advances in the reduction of the amounts of solvents used in chemicals processing.

They also go beyond this to demonstrate the potential of appropriate solvent selection to improve other areas of a process's performance and hence its overall sustainability. These examples also demonstrate that the implementation of the concept of sustainability in the production and use of chemicals and chemical products requires that chemicals processing must be both environmentally and commercially sustainable.

Furthermore, reducing the cost of chemicals production and hence the price of chemicals is vital for the application of chemistry and chemical products to enable sustainable development. The successful introduction of a truly sustainable chemicals industry is one of the great challenges that we face today. There are relatively few examples described in the open literature of the introduction of processes based upon sustainable solvent use, particularly when compared with the thousands of commercial chemical processes that exist.

This does not necessarily mean that so few have been implemented; it is likely that some companies have chosen to hide these behind a wall of commercial confidentiality. It would, however, be helpful to see more of these described so that they can act as an inspiration to others trying to achieve this important aim for us all. Energy and the Environment at Imperial College London for their helpful discussions. National Center for Biotechnology Information , U.

Proc Math Phys Eng Sci. Author information Article notes Copyright and License information Disclaimer. Received Jul 22; Accepted Oct This article has been cited by other articles in PMC. Abstract Solvents are widely recognized to be of great environmental concern.

Introduction In , the United Nations defined sustainable development as development that enabled the current generation to meet its own needs, without compromising the ability of future generations to meet their needs [ 1 ]. Green metrics The sustainability of a chemical product or process is necessarily the result of a complex interaction of environmental, technological and economic factors and is difficult to predict.

Open in a separate window. The value of sales per category for both Europe and the US are broadly similar, as shown in Table 1. An example is methanol, commonly produced from oil and natural gas in the US and Europe but from coal in China. Another is ethene, derived from oil and gas in the US and Europe but increasingly from biomass in Brazil. Other examples are described in the units on this web site.

Basic chemicals, produced in large quantities, are mainly sold within the chemical industry and to other industries before becoming products for the general consumer. For example, ethanoic acid is sold on to make esters, much of which in turn is sold to make paints and at that point sold to the consumer. Huge quantities of ethene are transported as a gas by pipeline around Europe and sold to companies making poly ethene and other polymers.

These are then sold on to manufacturers of plastic components before being bought by the actual consumer. Figure 3 shows a plant producing chemicals which it then immediately uses to manufacture other chemicals. Figure 3 Many companies use some of their chemical products as intermediates in their own manufacturing processes. There are often clusters of processes which use the output of one as the input to another. This site, at Billingham in the north-east of England, is a good example of such an integrated chemical plant.

All the plants are also interconnected by steam pipes to make the most efficient use of energy released during manufacturing processes. Ammonia is made from natural gas which is imported by pipeline from the North Sea. Some ammonia is used to make nitric acid. Ammonia and nitric acid are used to make the fertilizer, ammonium nitrate. Ammonia is also converted into hydrogen cyanide. Hydrogen cyanide is used in the process to make methyl 2-methylpropenoate , a key monomer for the manufacture of various acrylic polymers.

The tank farm stores imported reactants and products prior to export. The production of chemicals from petroleum and increasingly from coal and biomass has seen many technological changes and the development of very large production sites throughout the world. The hydrocarbons in crude oil and gas, which are mainly straight chain alkanes, are first separated using their differences in boiling point, as is described in the unit Distillation.

They are then converted to hydrocarbons that are more useful to the chemical industry, such as branched chain alkanes, alkenes and aromatic hydrocarbons. These processes are described in the unit, Cracking and related refinery processes. In turn, these hydrocarbons are converted into a very wide range of basic chemicals which are immediately useful petrol, ethanol, ethane-1,2-diol or are subjected to further reactions to produce a useful end product for example, phenol to make resins and ammonia to make fertilizers.

Many examples are found in the group of units on this web site devoted to Basic chemicals. The main use for petrochemicals is in the manufacture of a wide range of polymers. Due to their importance of these they are given their own section of units, Polymers.

These are relatively low cost chemicals used throughout manufacturing and agriculture. They are produced in very large amounts, some in millions of tonnes a year, and include chlorine, sodium hydroxide, sulfuric and nitric acids and chemicals for fertilizers. As with petrochemicals, many emerging countries are now able to produce them more cheaply than companies based in the US and Europe.

This has led to tough competition and producers of these chemicals worldwide work continuously to reduce costs while meeting ever more stringent environmental and safety standards. The units on basic inorganics can be found within the Basic chemicals section of the site. This category covers a wide variety of chemicals for crop protection , paints and inks, colorants dyes and pigments. It also includes chemicals used by industries as diverse as textiles, paper and engineering.

New products are being created to meet both customer needs and new environmental regulations. An everyday example is household paints which have evolved from being organic solvent-based to being water-based. Another is the latest ink developed for ink-jet printers. Units on selected speciality chemicals can be found within the Materials and Applications section of this site. Consumer chemicals are sold directly to the public.

They include, for example, detergents, soaps and other toiletries. The search for more effective and environmentally safe detergents has increased over the last 20 years, particularly in finding surfactants that are capable of cleaning anything from sensitive skin to large industrial plants. Parallel to this, much work has been done in producing a wider range of synthetic chemicals for toiletries, cosmetics and fragrances.

Units on selected consumer chemicals can be found within the Materials and Applications section. The chemical industry is a very important contributor to the wealth of a country. Generally personnel in the industry are among the most well rewarded of all manufacturing industries because the industry has the largest proportion of highly qualified people and generally it is the most productive. Production in China and other Asian economies is rising rapidly Table 2. China itself in the space of just 10 years has increased its percentage share from 8.

In contrast the proportion has shrunk in Europe from Overall they are taking smaller slices of a much larger cake, but the mass of the slice is still growing. Nevertheless, the manufacturing core of the industry is now decisively in Asia. Table 3 shows the sales of the countries which have large sales. It can be seen Table 5 that the head offices are spread around the world and reflect not only the high growth of chemical markets in the Middle East and in Asia but also the desire of oil producers to participate in making chemicals.

Table 5 Chemical companies: Sales in and the location of their head office. The chemical industry is highly multi-national. This company exemplifies the changes wrought in the chemical industry. In fact there are very good reasons for the choice of sites, reasons which also reflect the industrial and consumer landscape of the day. At first sight it seems strange that what are currently the fourth and seventh largest chemical companies in the world, Dow and DuPont , are situated in two small US cities, Midland, Michigan and Wilmington, Delaware.

However, the reason that Henry Dow founded his company at Midland in was because the salt deposits in the area contain particularly high concentrations of bromide ions, and Dow had patented two methods for obtaining elemental bromine from these deposits. The falling water drove the machinery of the mill and the willow trees on the riverbanks were turned into charcoal, one of the three ingredients of gunpowder. The site was far enough away from Wilmington in case of explosion but near enough to wharves on the river to ship out the powder.

A perfect and entirely logical location. There Friedrich Bayer and Johann Friedrich Weskott, one a salesman and the other a master dyer, set up a factory to manufacture synthetic dyestuffs from coal-tar for the textile industry. BASF, like Bayer, was founded to make dyes but its location was influenced by civic utilities and an early instance of industrial recycling. In Friedrich Engelhorn built a gasworks in Mannheim and installed the street lighting for the town council.

At the same time he seized the opportunity to use the by-product, coal-tar, to make dyes. The company also began to make the other chemicals necessary for dye production, notably alkalis and acids. With environmental foresight, the city fathers of Mannheim did not want any pollution of their city and so the plant was actually built across the Rhine at Ludwigshafen. For example, the concentration of the chemical industry in the Northeast of England was influenced by the location of coal mines, the availability of iron ore for the steel industry and the closeness to ports.

Similarly, the strong chlor-alkali industry chlorine, sodium hydroxide, sodium carbonate in the Northwest of England developed because of local coal and salt mines and the proximity of a major canal leading to a main port of England. The great cotton mills in Lancashire gave the obvious location for the dyestuff industry around Manchester, the largest city in Lancashire.

All of the sites mentioned above are flourishing today, although the companies expanded during the subsequent years to make many other chemicals ranging from plastics to pharmaceuticals. They have also added many new plants all over the world to be near their customers. Nevertheless, exactly the same range of factors that influenced locations in the nineteenth century are active today, for example:. This explains why some installations are sited adjacent to oil fields.

For example, there is a cluster of companies adjacent to the oil fields in Texas, and the discoveries and development of gas shale still a controversial process in many countries in places like Texas, Colorado and Pennsylvania are leading to new investment in chemical plants nearby. Shale gas is extracted by a process called 'fracking' which is still a controversial processes in many countres. Fracking is discussed in detail in the unit Extracting crude oil and natural gas.

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