Top Chemical Opportunities from Carbohydrate Biomass: A Chemist's View of the Biorefinery

Cheap fossil oil resources are becoming depleted and crude oil prices are rising. In this context, alternatives to fossil fuel-derived carbon are examined in an effort to improve the security of carbon resources through the development of novel technologies for the production of chemicals, fuels, and materials from renewable feedstocks such as biomass. The general concept unifying the conversion processes for raw biomass is that of the biorefinery, which integrates biofuels with a selection of pivot points towards value-added chemical end products via so-called "platform chemicals". While the concept of biorefining is not new, now more than ever there is the motivation to investigate its true potential for the production of carbon-based products. A variety of renewable chemicals have been proposed by many research groups, many of them being categorized as drop-ins, while others are novel chemicals with the potential to displace petrochemicals across several markets. To be competitive with petrochemicals, carbohydrate-derived products should have advantageous chemical properties that can be profitably exploited, and/or their production should offer cost-effective benefits. The production of drop-ins will likely proceed in short term since the markets are familiar, while the commercial introduction of novel chemicals takes longer and demands more technological and marketing effort. Rather than describing elaborate catalytic routes and giving exhaustive lists of reactions, a large part of this review is devoted to creating a guideline for the selection of the most promising (platform) chemicals derived via chemical-catalytic reaction routes from lignocellulosic biomass. The major rationale behind our recommendations is a maximum conservation of functionality, alongside a high atom economy. Nature provides us with complex molecules like cellulose and hemicellulose, and it should be possible to transform them into chemical products while maintaining aspects of their original structure, rather than taking them completely apart only to put them back together again in a different order, or turning them into metabolites and CO2. Thus, rather than merely pursuing energy content as in the case of biofuels, the chemist sees atom efficiency, functional versatility, and reactivity as the key criteria for the successful valorization of biomass into chemicals. To guide the choice of renewable chemicals and their production, this review adopts the original van Krevelen plots and develops alternative diagrams by introducing a functionality parameter F and a functionality index F:C (rather than O:C). This index is more powerful than the O index to describe the importance of functional groups. Such plots are ideal to assess the effect of several reaction types on the overall functionality in biomass conversion. The atom economy is an additional arbitrator in the evaluation of the reaction types. The assessment is illustrated in detail for the case of carbohydrate resources, and about 25 chemicals, including drop-ins as well as novel chemicals, are selected. Most of these chemicals would be difficult to synthesize from petrochemicals feeds, and this highlights the unique potential of carbohydrates as feedstocks, but, importantly, the products should have a strong applied dimension in existing or rising markets. Ultimately, the production scales of those markets must be harmonized to the biomass availability and its collection and storage logistics.