Green Chemistry

Green Chemistry: Ensuring A Sustainable Future whilst Protecting the Environment Mike Lancaster RSC Green Chemistry Network, Dept of Chemistry, University of York, Heslington, York Y01 5DD
Introduction
In general the public usually holds chemists and the chemical industry in fairly low esteem. There are a number of reasons for this, not least of which is that the chemical industry is seen as a major source of pollution and major disasters such a Bhopal and Flixborough linger in peoples minds. Nevertheless chemists have been involved with many of the great leaps forward society has seen in the last 100 years and has been a great source of wealth creation for most Western societies. Without modern chemistry things we take for granted such as health care, transport and clothing would be unrecognisable by today's standards. Recent advances in science and technology now enable the chemist to understand better the effect their work has on the environment, both in the short and longer term. We can no longer make excuses for developing new products and processes that adversely effect our environment. Green Chemistry is all about developing tools to enable new products and processes to be environmentally benign. With this in mind the Royal Society of Chemistry has recently launched the Green Chemistry Network to promote the awareness of Green Chemistry in schools, universities and industry¹.
Green Chemistry Concepts
Green Chemistry is not a new branch of chemistry, it is more of a thought process, using the existing tools of chemistry in a manner which will continue to provide the societal and economic wealth we have come to expect whilst protecting the environment. The following are three of the more important concepts that hopefully chemists of the future will think of daily.
Sustainable development. With oil production due to decline within the next 30 years, production methods to get chemicals from renewable resources such as crops will need to be developed. Atom Efficiency. It is an obvious statement but every atom that goes into a chemical reaction must come out somewhere. Be developing synthetic methods that maximise the number of atoms going into the reaction that end up in the product waste will be minimised. Solvent selection. When preparing an organic chemical most chemists will automatically carry out the reaction in an organic solvent such as toluene or dichloromethane. These solvents are often difficult to recover and purify and often form the major part of the chemical industry's waste. In fact many reactions can be carried out without solvent, in water or in environmentally friendly supercritical fluids such as carbon dioxide. It is sound economic and environmental practice not to use solvents unless absolutely essential.
The remainder of this article will illustrate how these concepts are being put in practice.
Sustainable Development
In terms of providing a mix of chemical raw materials plants can be viewed as non-toxic, biodegradable and CO2 neutral alternatives to oil. One common chemical derived from plants, which is non-toxic and much under used, as a chemical feedstock is glucose. A good example where this could be used is in the manufacture of adipic acid, which is used in the manufacture of nylon 6.6². The conventional route for making adipic acid is shown in Figure 1. Benzene, a carcinogenic chemical coming from the refinery process, is catalytically hydrogenated under high pressure to cyclohexane, which in turn is oxidised to give adipic acid.
Figure 1 Conventional route to adipic acid
An alternative Route developed by John Ford at Michigan State University (figure 2) starts from glucose and uses bacteria to convert it to muconic acid that is easily reduced to the product. Advantages of this route include: Sustainable feedstock Avoids use of carcinogenic benzene Avoids requirement for costly high pressure equipment Uses water as solvent Avoids production of waste N₂O, from use of nitric acid, which contributes to global warming
Figure 2 Alternative route to adipic acid from glucose
Vanillin which is widely used in flavouring and fragrances as well as a raw material for pharmaceuticals is currently made by two routes, one from biomass the other from oil. The biomass route involves a series of extraction and distillation steps from waste produced during pulp manufacture. The non-sustainable route involves a series of fairly complex steps staring from 2-methoxyphenol (Figure 3) but is used because of the currently cheaper process economics.
Figure 3 Vanillin production
Furfural is an excellent example of production of a major chemical from biomass. The Quaker Chemical Company have been producing furfural from pentosans (5 carbon sugars for many years through a series of acidic dehydration and rearrangement steps (Figure 4). This is by far the major process for furfural production, which is used as a solvent in petroleum refining and as a pharmaceutical intermediate.
Figure 4 Production of furfural from pentose
Atom Efficiency³
Too often when chemical process have been developed in the past chemists have not considered by-product generation or the fate of so called 'catalysts' which have be used in near stoichiometric amounts. With increasing costs of waste disposal and tighter control of effluent industry is becoming increasingly concerned that what goes into a reactor should come out as a useable product. A classic example of this is the industrial manufacture of phenol. For many years phenol was made by sulphonation of benzene with oleum to give benzene sulphonic acid (Figure 5). This was reacted with sodium hydroxide, first to give the sodium salt and then at high temperature to eliminate sodium sulphite, which was, disposed of a s a waste product.
Figure 5 Old phenol manufacturing process
The overall equation for the manufacture of phenol is then:

Assuming that a 100% yield is obtained i.e. all the benzene is converted to phenol then the atom efficiencies are:
ATOM Carbon Hydrogen Oxygen Sodium Sulphur			% EFFICIENCY 100 50 20 0 0
What is even worse if we look at the relative molecular masses, is that for every tonne of phenol produced we produce over 1.5 tonnes of waste! We can compare this with the modern process for production of phenol shown in Figure 6. This involves reaction of benzene with propene with a reusable heterogeneous catalyst to form isopropyl benzene. This is reacted with oxygen to give a hydroperoxide, which decomposes to phenol and acetone. Unlike sodium sulphite the acetone is a valuable chemical produced in high purity. The overall equation for this reaction is:

Again if we assume 100% yield, then the atom efficiencies are 100% in each case.
Figure 6 Modern phenol manufacturing process
Another example of where the chemical industry has developed a more atom efficient process in recent years is in the production of maleic anhydride (used in the manufacture of polyester resins). Historically the process involved oxidation of benzene to maleic anhydride using a vanadium pentoxide catalyst doped with molybdenum or tungsten as shown in Figure 7. Although this process worked well it is readily seen that only four of the six carbon atoms in benzene are required to produced the product, the other two carbons form end up as carbon dioxide. Not only is this process bad from the point of view of atom efficiency, it produces significant amounts of CO₂ which lead to global warming.
Figure 7 Maleic anhydride production from benzene
The current process, which has dominated maleic anhydride production for the last 15 years, uses either butene or butane as feedstock (Figure 8). The benefit here is that all the carbon atoms of these C4 feedstocks end up in the product.
Figure 8 Maleic anhydride from butene or butane
ATOM	From Benzene % Eff	From Butene % Eff	From Butane % Eff
Carbon	67	100	100
Hydrogen	33	25	20
Oxygen	27	50	43
Solvent Selection
The choice of solvent in a reaction or process should be given as much thought as the choice of reagent. Solvents are often used in large quantities and either end up as waste or need purification before recycling which is both inefficient in terms of energy and time. Solvents are, however, sometimes essential and frequently careful choice will have beneficial effects both in terms reaction rate and selectivity. The question do I need a solvent? should always be asked even if the starting material is a solid. A simple illustration of a process where a solvent is not required is in the manufacture of phenol sulphonic acid, used extensively as a plating electrolyte in tin can manufacture. The process involves reaction of phenol (a solid) with concentrated sulphuric acid (Figure 9).
Figure 9 Manufacture of phenol sulphuric acid
Phenol is preheated to a temperature above its melting point (42^oC) and charged to a reactor. Sulphuric acid is added and the reaction temperature increased, as this happens the phenol and acid become miscible without the need for solvent and reaction takes place. Water is removed by distillation to drive the reaction forward. When the reaction is complete the product is cooled to about 50^oC before water is added and the final product - a 60% solution of phenol sulphonic acid in water - discharged from the reactor. Supercritical fluids are gasses compressed until they become nearly as dense as liquids. Under these conditions gases such as carbon dioxide exhibit some exciting solvent effects. They are extremely 'green' solvents since by reducing the pressure they are returned to their former gaseous state and can be readily separated from the product and recycled. A striking example of the use of supercritical carbon dioxide is in the production of decaffeinated coffee (Figure 10). The process is carried out at 160 - 200 atmospheres pressure by mixing wet coffee beans with supercritical CO₂, which is an extremely good solvent for caffeine but does not extract any other components from the coffee. After extraction the caffeine solution is mixed with water and the C0₂ recycled. Water is removed by distillation to give pure caffeine which is used as a stimulant. The coffee beans are removed from the reactor and treated as other coffee beans for those who prefer 'unstimulating' coffee. Before the advent of this process chlorinated solvents were used to extract the caffeine and extreme care was needed to ensure that the beans were completely free of solvent before they could be used.
Figure 10 Caffeine extraction using supercritical carbon dioxide
Summary
The Green Chemistry Network aims to promote the concepts of green, environmentally friendly chemistry in schools, universities and industry. If chemists of the future are to be seen as protectors not polluters of the environment then we all need to think carefully of the long-term consequences of the products and processes we develop. The tools and techniques required to make chemistry environmentally benign are being developed at a rapid pace, as you progress in your chosen area of chemistry please take care to ensure that you have an overall positive effect on the environment.
References
1. J H Clark, Chemistry in Britain, 1998, 34(10), 43 2. K M Draths & J W Frost, J. Am. Chem. Soc., 1991, 113, 9361 3. B M Troste, Science, 1991, 254, 1471

Further Resources
Journal of Green Chemistry published bimonthly by The Royal Society Of Chemistry P T Anastas & J Warner, Green Chemistry Theory and Practice; Oxford University Press, New York 1998. www.epa.gov/greenchemistry www.lanl.gov/Internal/projects/green www.rsc.org/journals