Research Development Fellowship. Towards biorefineries based on wastes: efficient enzymatic lignin degradation

点击翻译按钮获取中文摘要

Dwindling oil reserves and climate change represent major challenges for society over the next 50-100 years. A number of solutions are being developed, one of which is the replacement of fossil fuels with liquid biofuels. Biofuels are produced by fermenting plant-derived sugars to produce ethanol and butanol. The easiest way to produce biofuels is to ferment food crops, such as sugar cane, cereals, rice or maize, but the increase in industrial biofuel production from starch-containing food crops is already leading to increasing food prices. This raises the spectre of widespread starvation if the developed world continues to meet the need for liquid transportation fuels at the expense of food supplies for developing countries. One solution might be to increase the area of land cultivated to produce starch-containing crops. However, this would reduce biodiversity, with the inevitable environmental consequences. Furthermore, increasing crop cultivation would increase water use, at a time when there is already a serious problem with providing enough fresh water for the human population to drink. Therefore, the solution is obvious: instead of using starch as the primary feedstock for biofuel production, we must use waste materials that are left over after harvesting food crops and after processing food for human consumption. We must also learn to make use of materials that we currently send to landfill as rubbish. The big problem is that agricultural and food wastes are composed primarily of lignocellulose, and municipal wastes are also mixed with plastics from packaging. Whilst cellulose can already be used as a feedstock for biofuel production, lignin and waste plastics are much more difficult to break down into useful chemicals except under harsh reaction conditions and at high temperatures. Therefore, the aim of this project is to develop new enzymatic methods to degrade lignin and plastics which will work at low temperatures with environmentally benign reagents. We shall use unusual enzymes called ligninases, which are able to produce reactive reaction products, called free radicals. Free radicals are chemical terrorists - they literally blow apart any molecule that they come into contact with. Ligninases are produced by wood rotting fungi, and their natural function is to break down lignin in wood. However, ligninases are also exploited in industry to smash up all sorts of molecules, including dyes, chemical intermediates and plastics. The enzymes do not work very well under natural conditions because lignin is not soluble in water and the enzymes cannot get close enough to smash the lignin efficiently. Therefore, we plan to use ionic liquids instead of water to bring the enzyme and the lignin into contact. Ionic liquids are salts, but unlike the familiar crystalline salts (e.g. sodium choride), they are unable to form proper crystals except at sub-zero temperatures. At room temperature, they are liquids, and they have very unusual properties, including the ability to dissolve lignin and support enzymatic activity. Therefore, the ionic liquids will help to dissolve the lignin and the enzyme will break it down to form useful products. This will make it possible to degrade lignin much more quickly than in water. In this project, we shall develop new processes to break down lignin and other waste polymers for use as feedstocks in biorefineries to produce useful chemicals and fuels.