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From waste biomass to valuable products

In 2019, most people can agree that continuing to exploit fossil fuels to generate energy and fuels is a problem. Not only because of the damaging effect on our climate, but also owing to the finite nature of fossil fuels: most forecasts predict that they will run out within our lifetimes, possibly by 2060.

Renewable energy sources, like solar and wind energy, are becoming key players in their field; recently, Britain spent two weeks without using coal to generate electricity (the first time since 1882). However, we must also consider that the vast majority of fuels, chemicals and materials which we use in our daily lives also originate from fossil fuels. Some of these are obvious, like the petrol we use in our cars and the plastic water bottles we drink from. But how about when we spray a perfume, wash our hair with shampoo or swallow a medicine? Many consumer products such as these contain chemical ingredients which are also derived from petroleum.

Although it’s important to think about recycling or reducing our consumption, ultimately we need to find new, sustainable ways of making the products we are used to. Most scientists consider biomass as the best alternative to fossil fuels for this purpose. Biomass is any matter coming from living things, such as trees, plants and crops. It is both renewable – and therefore sustainable – and hugely abundant. Some people are concerned that growing special crops to produce biofuels will cause deforestation or compete with food crops. While this is an issue, there are many sources of waste biomass – like wood chippings, sawdust and even animal droppings – which could also be utilised, if effective processes can be developed. This is because all sources of biomass have the same key chemical components,
in varying amounts.

All lignocellulosic biomass – making up our trees, plants and crops – has three main chemical components: cellulose, hemi-cellulose and lignin. Cellulose and hemi-cellulose are both carbohydrate polymers made up of sugar units, but they have very different properties from each other. They are tightly bound to lignin, a complex polymer of aromatic molecules. Of these components, cellulose has the most well-ordered structure. Therefore, it is the easiest to use as a feedstock to produce fuels and chemicals. However, it can be non-trivial to separate and isolate cellulose from the other biomass components.

Forecasts predict that fossil fuels will run out within our lifetimes, possibly by 2060.

Once cellulose has been isolated from biomass, it can be converted into a number of chemical feedstocks. Chemically, there is a large difference between molecules derived from fossil fuels and those derived from biomass. We can think of crude oil as a mixture of blank carbon chains (alkanes and alkenes). To transform crude oil into useful products, we need to add functional groups to modify the properties of the chains. However, most biomass molecules have many different functional groups already. We must perform chemical reactions to alter and combine the biomass molecules into products which have the properties we desire.

The increased complexity of feedstocks derived from biomass (relative to those from fossil fuels) means that they are more complicated to work with, but there is also a large benefit. It is possible to access a wider range of useful products from a single chemical feedstock, by performing a range of different chemical transformations. However, most of these reactions require a catalyst to actually work. Without a catalyst, the reaction might be prohibitively slow or not proceed at all.

Britain recently spent two weeks without using coal to generate electricity. Instead using renewable energy sources like solar and wind energy.

The development of catalysts that can convert biomass feedstocks into high-value fuels and chemicals is a very active area of research. There are many considerations to take into account. The catalyst must be able to obtain high yields of product, in high purity, under suitably mild conditions. The catalyst should be inexpensive, and not so difficult to prepare that it can’t be applied industrially. Different processes typically require different catalysts, but they should be as versatile and durable as possible. Most catalysts use a precious metal, either by itself or combined with a chemical framework allowing it to interact with the reactant molecules. Some processes use enzymes (biological molecules) as catalysts.

So, what potential is there for biomass to replace fossil fuels completely? Whilst biofuels are becoming more commonplace, there remains some controversy over their use. The public are becoming more conscious of the need to recycle and reduce their consumption of particular products. However, most people don’t realise that almost every product we use or consume can be traced back to fossil fuels in some way. Using biomass is the most likely solution, although difficulties remain. It can be difficult to obtain the individual components of biomass, and it is complicated to turn these into the desired products selectively. However, these processes are becoming more and more effective over time. We could imagine a machine with wood chippings going in and a series of refined liquids coming out at the other end, into our perfume, shampoo and medicine bottles. And we’ll be able to continue doing so indefinitely, thanks to biomass.

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Samuel Page
PhD student, Imperial College London

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