Chapter 5: Early-stage Conversion of Lignin over Hydrogenation Catalysts

The deconstruction of lignocellulosic materials producing high-quality pulps together with lignin streams of low molecular weight (M_w) with controlled molecular diversity constitutes the overarching objective of catalytic upstream biorefining (CUB) based on the early-stage catalytic conversion of lignin (ECCL). The CUB process based on ECCL or, simply, a “lignin first” approach builds on the lignin extraction with organic solvents in the presence of a hydrogenation catalyst under hydrogen pressure or employing an H-donor solvent (e.g. 2-propanol). Subjected to a hydrogenation catalyst, the lignin fragments undergo reductive processes, which decrease their reactivity and propensity to recondensation, directly upon their release from the lignocellulosic matrix. As a result, the isolated lignin fraction is a mixture of compounds of M_w 100–600 Da, with a high content of individual components. Herein, key concepts required for the rational deconstruction of lignocellulose based on ECCL are addressed. Notably, catalysis is now applied to the “pulping process” itself, opening up new horizons for the realistic valorization of lignin and full utilization of lignocellulose via downstream catalytic processing of better-defined lignin streams.