:arrow: [color=red][size=18]Green chemistry[/size][/color]
In most countries the general public associate the word 'green' with 'all things good for the environment'. The 'green' word is also representative of a certain political philosophy in some countries. So what is green chemistry?
Green chemistry is essentially chemistry for the environment; it is most definitely apolitical. In many ways green chemistry is a philosophy and a way of thinking. It is not a new branch of chemistry but is a pulling together of tools, techniques and technologies that can help chemists in research and production to develop more eco-friendly and efficient products and processes. The term, first defined by the US Environmental Protection Agency1 as 'the utilisation of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products', is now widely accepted.
The EPA issued 12 principles of green chemistry (see Box 1), which go someway in explaining what the definition means in practice. The principles cover such concepts as:
the design of processes to maximise the amount of raw material that ends up in the product;
the use of safe, environmentally-benign solvents where possible;
the design of energy efficient processes;
the best form of waste disposal - aiming not to create it in the first place.
Box 1. The 12 principles of green chemistry
Prevention - it is better to prevent waste than to treat or clean up waste after it has been created.
Atom economy - synthetic methods should be designed to maximise the incorporation of all materials used in the process into the final product.
Less hazardous chemical synthesis - wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to people or the environment.
Designing safer chemicals - chemical products should be designed to affect their desired function while minimising their toxicity.
Safer solvents and auxiliaries - the use of auxiliary substances (egg solvents or separation agents) should be made unnecessary whenever possible and innocuous when used.
Design for energy efficiency - energy requirements of chemical processes should be recognised for their environmental and economic impacts and should be minimised. If possible, synthetic methods should be conducted at ambient temperature and pressure.
Use of renewable feedstock's - a raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
Reduce derivatives - unnecessary dramatisation (use of blocking groups, protection/de-protection, and temporary modification of physical/chemical processes) should be minimised or avoided if possible because such steps require additional reagents and can generate waste.
Catalysis - catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
Design for degradation - chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
Real-time analysis for pollution prevention - analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
Inherently safer chemistry for accident prevention - substances and the form of a substance used in a chemical process should be chosen to minimise the potential for chemical accidents, including releases, explosions, and fires.
Although much of this may appear to be common sense, it is surprising how many chemists in research and process development do not think in these terms. Given a free choice, how many researchers in academia would choose a chlorinated or noxious solvent for a reaction? Of 29 papers published in a recent issue of Perkin Transactions 1, a highly acclaimed international journal of organic and bio-organic chemistry, for example, 31 per cent cited chlorinated solvents, 35 per cent dipolar aprotic solvents, such as DMF, and 24 per cent cited other noxious solvents such as benzene and pyridine for carrying out reactions. Only one paper mentioned water as the solvent. So it seems, there is much to be done in educating chemists to think in terms of green chemistry principles.
Some progress, however, is being made through social and legislative pressure, and industry is moving towards clean technology. Chemists are becoming better educated in the finer points of economics and the financial workings of industry; they are increasingly using this knowledge to justify R&D into cleaner and greener processes. In academia, initiatives such as EPSRC's Clean Technology Programme and the Government's Sustainable Technology Initiative have encouraged researchers to enter this area. The Royal Society of Chemistry has recently launched a Green Chemistry Network and the associated Green Chemistry Journal.
In this article I will focus on examples of green chemistry from both industry and academic research to illustrate some of the principles.
The maleic anhydride story
There are many ways to define the efficiency of a chemical reaction. While yield and selectivity (ie the yield of product divided by amount of substrate converted) are commonly used, these are not especially useful for measuring the amount of waste generated in a process. From an environmental, and increasingly economic, point of view it is more important to know how many atoms of the starting material end up in useful products and how many in waste.2,3 In an attempt to measure this Barry Trost at Stanford University used an atom economy or atom utilisation approach, looking at, for example, the percentage of reactant carbon atoms that end up in the desired products. One of the benefits of this approach is that it can be done theoretically; before any practical work is done chemists can evaluate alternative routes. While yield and selectivity will also be important, a route that gives only a maximum of 30 per cent atom utilisation with a quantitative yield will still produce a lot of waste.
Consider, for example, the production of maleic anhydride (MA). Maleic anhydride is widely used in the manufacture of polyester resins and paint, and is a valuable intermediate in the manufacture of 1,4-butanediol (used in polyurethane manufacture/solvents) and butyrolactone, (a solvent, paint remover and fine chemical intermediate). Total world production of MA is ca 900,000 tonnes per year via three technically similar routes, see Scheme 1. All three routes essentially involve passing the feedstock - benzene, butene or butane - with air over a vanadium pentoxide catalyst at 3-5 bar pressure and 350-450?C.
The history of maleic anhydride production makes interesting reading. Pre-1960 MA was a high value product with limited markets and little manufacturing competition. As the polyester and paint uses grew so more producers came on stream. The Denka plant in the US was built to produce MA from butene, not for environmental reasons but because it was less expensive than benzene. When butene prices rose the Denka plant was converted back for use with the old benzene technology. Everything changed with the 1970s oil crisis when Monsanto brought a new plant on stream, which used butane as a feedstock. To remain competitive Denka quickly followed by adopting the butane process. By the early 1980s all plants in the US using benzene had been either shut down or converted for use with aliphatic feedstocks. The advent of the 1990s saw environmental concerns growing and much more emphasis being put into waste minimisation. Two companies, UCB Chemicals and BASF, started to sell MA that had been produced as a byproduct during the oxidation of naphthalene to phthalic acid and anhydride.
Industry got there in the end; maleic anhydride manufacture is now as 'green' as current technology permits but the industrial drivers until recently were totally financial. The history of MA production may have been different if green chemistry ideas and, more importantly, the threat of carbon taxes had been around 40 years ago. Consider now the atom efficiency of the three processes (see Table 1).
Table 1. Atom efficiencies for maleic anhydride manufacture
From benzene From butene From butane
Carbon per cent 67 100 100
Hydrogen per cent 33 25 20
Oxygen 33 50 43
Apart from the toxicity of benzene, it is clear from Table 1 that in terms of carbon atom efficiency this is the worst route, with 33 per cent of the starting material forming carbon dioxide, which contributes to global warming. Similarly, oxygen efficiencies for the benzene route are poor. Hydrogen efficiencies from the aliphatic feedstock are lower than from benzene but at least the butane route only produces water as a byproduct. As with most oxidation processes the selectivity is not quantitative, but the latest processes from benzene and butane operate at ca 75 per cent selectivity while that for butene is a little lower at 65 per cent. Overall, the aliphatic feedstock - butane - provides the most 'green' route.
Catalysis is at the core of green chemistry thinking. Not only can catalysts replace stoichiometric reagents but they can also lead to significant selectivity improvements.
Friedel-Crafts reactions, 'catalysed' by aluminium chloride, are fondly remembered for the gelatinous precipitates and copious amounts of HCl produced on work-up by generations of undergraduate chemists. For many such reactions, particularly acylations, greater than stoichiometric amounts of AlCl3 are required and produce large quantities of acidic aluminous waste. However, over the past 10 years or so, clean technology, through use of zeolites, clays and supported catalysts, has been increasingly employed by industry for such reactions. A typical example is the alkylation of benzene with octene (Scheme 2).4 By supporting the aluminium chloride on silica, significant increases in yield are obtained, but more importantly the catalyst can be filtered off and reused, thus providing significant environmental benefits in terms of waste reduction.
The nitration of aromatics is a common reaction in the fine chemicals industry, producing intermediates for dyes, plastics and pharmaceuticals. Typical industrial processes involve using a large excess of fuming nitric acid with sulphuric acid, leading to large amounts of acidic waste. Recent research, however, using lanthanide triflate catalysts eg Yb(CF3SO3)3, has opened up the possibility for industry to use molar equivalents of nitric acid, thus avoiding the use of sulphuric acid.5 These so-called rare earth elements are not that rare or expensive and offer real commercial possibilities; their value lies in the fact that they behave as Lewis acids but are water tolerant and can be readily recovered and recycled.
One of the great 'chemical' tragedies of the 20th century involved the use of thalidomide, in which one optically active isomer had beneficial therapeutic effects while the other resulted in birth defects. Today chemists understand much more about the effects of the different optical isomers and there is an increasing requirement for industry to produce optically pure pharmaceuticals. However, making these compounds by conventional methods, followed by purification procedures, such as preferential crystallisation, leads to large amounts of waste because often 50 per cent of unwanted isomer is produced.
In recent years much research has gone into catalytic asymmetric synthesis with the aim of producing only the desired isomer.6 To minimise waste the best processes introduce the asymmetric centre as early as possible in the synthesis. Homogeneous metal catalysts containing chiral ligands have proved extremely beneficial in producing optically pure compounds and are at the core of many elegant syntheses. One of the most general reactions is the asymmetric hydrogenation of alkenic compounds. Chemists at Monsanto, for example, discovered that L-dopa derivatives, which are used in the treatment of Parkinson's disease, could be produced in high optical purity by using a rhodium hydrogenation catalyst containing a chiral phosphine ligand (1).7 Another elegant example of asymmetric hydrogenation employs a chiral ferrocene catalyst (2) in the key step of the synthesis of the herbicide (S)-metolachlor.8 Another related reaction of industrial significance, involving addition to a double bond, is the production of Naproxen, an important anti-inflammatory drug, by Dupont.9 The key step in this process involves asymmetric hydrocyanation of an alkene using a nickel catalyst containing chiral carbohydrate ligands.
The replacement of toxic and noxious solvents by more environmentally-benign ones is an important area for the 'green' chemist. Over recent years the use of harmful solvents, such as benzene, carbon tetrachloride and chloroform, have been phased out by industry and have gradually become less commonly used in academic research. Many solvents, which can be harmful to workers if not adequately protected and to the environment if releases occur, are still used however. With the exception of solvents such as supercritical fluids and ionic liquids, solvent replacement is not seen as a 'sexy' subject by academics, so most of the research in this area is done by industrial process development chemists. And because of increasingly strict legislation on the release of volatile organic compounds and on trace solvents in water, industry is focusing on the use of water as a solvent. Recently, for example, Hickson & Welch in Castleford replaced acetone with water in its optical brightners plant.10 The payback time for the R&D involved in establishing a water-based process was less than one year, proof that green chemistry does pay.
Water is not normally a good solvent for organic molecules. At high temperatures, however, the ionic product of water increases dramatically, being 1000 times higher at 240?C than at room temperature, making it a stronger acid and base. At the same time, the polarity is significantly reduced as the temperature increases. These properties suggest that water maybe a useful solvent for some high temperature organic reactions, including condensations, isomerisations, decarboxylations and hydration of alkenes.11
High-temperature water may be of use in the flavour and fragrance industry where the expert 'nose' can detect even the faintest trace of an unwanted solvent. Isomerisation of geraniol (Scheme 3) to the useful fragrance materials linalol and