a while back rhodium posted regarding our dependancy on finite petroleum supplies, i found this extract from the Rsc journal of green chemistry. The use of glucose and pentose as feedstock is quite interesting.
(edit: just realised this is the 30000th post in chem discourse- surely symbolic or something?)
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
