Biomass is an important feedstock for the renewable production of fuels, chemicals, and energy. As of 2005, over 3% of the total energy consumption in the United States was supplied by biomass, and it recently surpassed hydroelectric energy as the largest domestic source of renewable energy. Similarly, the European Union received 66.1% of its renewable energy from biomass, which thus surpassed the total combined contribution from hydropower, wind power, geothermal energy, and solar power. In addition to energy, the production of chemicals from biomass is also essential; indeed, the only renewable source of liquid transportation fuels is currently obtained from biomass.

The Catalytic Valorization of Lignin for the Production of Renewable Chemicals

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

/pdf/chemicals-from-lignin-an-interplay-of-lignocellulose-1th1dk0l26.pdf

Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Our severe dependence on fossil resources for the production of fuels and chemicals is responsible for two major global challenges: declining the fuel supply and increasing the anthropogenic greenhouse gas emissions. Conversion of biomass to fuels and chemicals can be a part of the low-carbon solution to both issues. Among various biomass species, inedible biomass such as lignocellulosics is the preferred choice for such applications due to their minimal impact on the food security. While technologies for the conversion of carbohydrates to value-added materials such as pulp, sugar monomers, and ethanol are well-established, lignin upgrading and valorization processes are significantly less-developed, and technical lignins are almost entirely burnt to generate heat and steam. The economic viability of biorefineries – which will receive significant amounts of lignin in future – can potentially improve significantly when advanced technologies are available that aid the conversion of lignin to value-added compounds. In this paper we assess how thermochemical processes can be used to isolate lignin from the lignocellulosic biomass, and subsequently convert it to liquid fuels, hydrogen, and aromatic monomers. To this end, different depolymerization, gasification and upgrading technologies for lignin conversion will be considered. Finally, the foreseeable applications of lignin-based products, the future directions for development, and the potential supportive interventions from policy makers are critically assessed.

D Meier

Papers

Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin)