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In 1983 Chuck Hussey wrote a review, Room Temperature Molten Salt Systems,1  that reported everything that was known at that time about ionic liquids by referencing only 82 articles. A relatively small band of devotees continued to make steady progress for the next 15 years until, at the turn of this century, interest in ionic liquids began to explode. By the end of 2014, Web of Science™ listed 58771 articles with the phrase ‘ionic liquid’ (or ‘ionic liquids’) in the title. If one wanted to produce a text that covered all that is known about ionic liquids today, it would undoubtedly be a multivolume collection and, like the never-ending task of painting the Forth Bridge, no sooner would it be finished than it would be time to start again. Consequently, in recent years we have been provided with a number of monographs and collections that tell us about one aspect or another of ionic liquids and their applications. Ionic Liquids in the Biorefinery Concept complements some other excellent texts, notably: Electrodeposition in Ionic Liquids, edited by Frank Endres, Andy Abbott and Doug MacFarlane; Electrochemical Aspects of Ionic Liquids, edited by Hiro Ohno; Ionic Liquids in Synthesis, edited by Peter Wasserscheid and myself; Handbook of Green Chemistry, Vol. 6: Ionic Liquids, edited by Peter Wasserscheid and Annegret Stark, and (recently) Topics in Current Chemistry: Ionic Liquids, edited by Barbara Kirchner.

Over a very similar timeframe the concept of the biorefinery has also been being actively developed to a very similar scale, with 74091 articles with ‘biorefinery’ or ‘biofuels’ recorded as topics in Web of Science™ by the end of 2014. Ionic Liquids in the Biorefinery Concept complements some other excellent texts, notably: Biorefineries – Industrial Processes and Products, edited by Birgit Kamm, Patrick R. Gruber and Michael Kamm, Biorefinery Co-Products, edited by Chantal Bergeron, Danielle J. Carrier and Shri Ramaswamy, Integrated Forest Biorefineries: Challenges and Opportunities, edited by Lew Christopher, and Renewable Resources for Biorefineries, edited by Carol Lin and Rafael Luque.

It was perhaps inevitable that these two fields of study would have been brought together at some point; the first papers bringing the two together appeared in 2001. However, that they should generate so much interest so quickly could not have been so easily predicted. It is this activity that is reviewed so effectively here.

Of course, it is first necessary to understand what is meant by the term ‘biorefinery’ and how it fits into the history of the utilization of biomass, which is by no means a new activity (Chapter 1). We discover that these older technologies were heavily polluting and far from any modern definition of Green Chemistry, in spite of their use of a bio-renewable starting material. We also discover that there is more than one type of biorefinery – many versions are beginning to spring up around the world, tuned to match a local biomass source. We must not forget the importance of applying rigorous Green analysis to any proposed processes (Chapter 6).

The very first step in the use of lignocellulose biomass is to break it down into its component parts, predominantly the polymers cellulose, hemicellulose and lignin. This is also the most difficult step in the economic exploitation of biomass. Ionic liquid pre-treatments have shown much promise in this area (Chapter 2). Ionic liquids with imidazolium cations and basic (mostly acetate) anions have received the greatest attention, with the intention being the dissolving and reprecipitation of cellulose. A whole variety of process variables have been investigated – biomass type, loading and particle size, experimental conditions, and ionic liquids’ physical properties.

Technoeconomic analysis based on the results of such studies have emphasized the importance of ionic liquid recycling in achieving an economically viable ionic liquid-based biomass pre-treatment process (Chapter 3).

To obtain fermentable sugars, it is necessary to depolymerize the carbohydrate polymers by hydrolysis (Chapter 4). Acid-catalysed hydrolysis of either whole biomass (without prior pre-treatment) or cellulose itself has been shown to be a viable process option, either by adding acid to an ionic liquid or using an ionic liquid that has an acidic functionality. The use of extremophile-derived enzymes to catalyse the hydrolysis has also been shown to be possible.

When using plant biomass as a source of chemicals and fuels, it is vital that its hugely variable nature is understood. Not only does the composition of biomass depend upon its type (hardwood, softwood, grass, different species, …), but variables such as point of harvest in the growing season have the potential to lead to the need for different processing conditions (Chapter 5). One should also not forget that different crops might be selected to produce different potential products.

Although ethanol is often thought of as the primary output of the biorefinery, many other potential platform chemicals have been identified. 5-Hydroxymethylfurfural (HMF) production has received particular attention in ionic liquids (Chapter 7).

A number of ionic liquid-based approaches have also been suggested for the extraction of high-value products from biomass (Chapter 8).

In spite of all of these advances and promising results, no ionic liquid based biorefining process has yet been commercialised. It is, of course, very early days for these approaches. It is necessary to think about what barriers exist to the implementation of ionic liquids in this area (Chapter 10) and to work on technological approaches that can overcome these.

I have no doubt that research into the applications of ionic liquids in the utilization of biomass has a very exciting future.

Tom Welton

Professor of Sustainable Chemistry

Imperial College London

C. L.
Adv. Molten Salt Chem.
, vol. 
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