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Microwave Chemistry Helps Recycle Plastics
John Newell
Former Science Editor, BBC World Service

ngineers normally think in terms of engineering structures. But the chemists and engineers who work in one UK research centre make... holes.

These engineered holes are the pores and cavities in zeolites - crystalline aluminium silicates - that are widely used as catalysts in industry. By engineering zeolites, it is making it possible to use them as catalysts in the distillation of fuel from wastes including mixed plastics waste.

It is now becoming possible to manufacture zeolites more quickly and cheaply using microwaves and this is expanding the range of clean technology.

Dr Vladimir Zholobenko is shown here with the centre's Fourier transform IR spectrometer that is fitted with a microscope attachment for recording the spectra of small single crystals.


Conserving Resources
In the near future, cars may be running on a fuel that will solve two environmental problems simultaneously. The fuel will be distilled from dirty, mixed plastic waste - the hardest of all waste products to recycle and one which does not break down naturally in the environment . By helping to replace petrol distilled from crude oil, it will help to conserve natural resources.

The Centre for Microporous Materials in Manchester, northwest England , has been established by a consortium of leading UK-based companies including British Oxygen, British Nuclear Fuels and Imperial Chemical Industries . The centre, headed by Dr John Dwyer, already has major projects running on the development of zeolites for use in the distillation of plastics.

One project involves the recycling of mixed plastic waste. The target of centre operations manager, Dr Arthur Garforth, is finding ways to distil this into a gasoline-type fuel. This would solve two environmental problems simultaneously: helping to dispose of a non-biodegradable waste, which is highly visible and on the increase, and helping to conserve natural resources by replacing some fuel distilled from crude oil.

Distilling Success
Dr Garforth's team has already succeeded in distilling plastic waste composed of a single type of plastic to produce a mixture of oils suitable for use as fuel . The distillation process is just as that used to distil fuel from crude oil; the plastic is heated up with no oxygen present so it cannot burn and the hydrocarbons in it are distilled off as vapours, cooled and collected.

But a big problem in extending this to mixed plastic waste. The form in which it is usually found is PVC (polyvinyl chloride plastic) that forms hydrochloric acid when it is heated.

The Manchester team has shown that the acid produced in this way can be broken down to harml es s compounds by adding an appropriate zeolite catalyst during distillation. More research will be needed to perfect the process but fuel from plastic waste is now clearly on the horizon.

Green Fuels
Zeolites have many other uses including the manuracture or more efficient washing powders and the production of hydrocarbon fuels that contain no lead but have high octane ratings - the so-called Green fuels.

But their use in such roles has been limited by their high cost, which is largely due to the time needed to produce the crystalline materials in exactly the right forms. This takes about 12 hours using conventional chemistry.

Now, instead, researchers at the Manchester Centre for Microporous Materials have shown that zeolite crystals can be made by putting them in a giant microwave oven and microwaving them for about 15 minutes. This will cut the cost of making zeolites substantially and so should also cut the cost of lead-free petrol and some detergents.

Another relatively new focus for environmentaiiy friendly research in UMIST - the University of Manchester Institute of Science and Technology - is the Satake Centre for Grain Process Engineering, established in 1994 in collaboration with the Satake Corporation of Japan. Much research there focuses on developing the use of grain to produce industrial chemicals. Starch is already widely extracted from grain, mainly from corn, traditionally, but wheat is now becoming an increasingly attractive alternative.

Starch is widely used in the making of pharmaceuticals, paints, plastics and agrochemicals. But the most exciting prospects for the future lie in the use of starch as a carbohydrate energy source in fermentation processes in biotechnology.

"It is possible," says Professor Colin Webb, Director of the Satake Centre, "to foresee a future in which some industrial products will be directly extracted from grain (sucn as oils, starches) while others are produced by organisms fed by the fermentation of grain flour."

Clear Advantages
Professor Webb and his colleagues envisage the emergence of a complete processing concept in which wheat, for example, is milled primarily to produce flour, but with lower-value chemical streams being tapped off for processing to non-food products, rather than being blended into higher-value streams.

The concept of grain processing as a process analogous to petroleum processing is being developed at the Satake Centre in a similar way to the development of plastic waste distillation as a process parallel to the distillation of crude oil in the Centre for Microporous Materials. There are clear environmental advantages in replacing non-renewable resources with renewable grains as feedstocks for chemical processes.

Central to the non-food products would be a fermentation plant. The key to this would be the development of a fermentation medium from which a wide range of fermentation products could be produced by biotechnology.

A main research activity at the Satake Centre is the production of such a medium, based on whole wheat flour. "Results have been extremely encouraging," says Professor Webb. And a process has been developed that produces separate giucose-rich and nitrogen-rich streams . Improved fuels and biodegradable plastics are among the products that are likely to emerge from this continuing research.

Paper From Straw
Another research project that aims to conserve resources this time again by replacing a limited natural resource with a waste product as raw material - is work in UMIST's Paper Sciences Department, which aims to replace trees with straw as the raw material for paper.

Straw, the stalks left behind after grain harvesting, is increasingly hard to dispose of now that burning it is widely forbidden . At present, straw is little used in paper-making because the stalks contain silica in the form of sand-like granules that damage paper-making machinery and cause pollution. UMIST researchers have discovered that the silica in straw is not widely dispersed but is concentrated in some cell walls, held in place by specific chemical bonding to carbohydrates in the cell walls.

The paper sciences team is now identifying enzymes that specifically break the bonds holding silica granules in cell walls and developing these for addition to paper-making processes. This will help to make the manufacture of paper from straw a practicable proposition.

For more information contact:
Centre of Excellence for Research on Microporous Materials
Department of Chemistry, UMIST, Manchester, United Kingdom, M60 1QD
Tel: +44 161 200 4512 | Fax: +44 161 236 7677 | E-mail: Colin.Cundy@umist.ac.uk.



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