On the Meaning and Origins of Lignin Recalcitrance: A Critical Analysis of the Catalytic Upgrading of Lignins Obtained from Mechanocatalytic Biorefining and Organosolv Pulping

In the broad context of catalysis for lignin valorization, the term “recalcitrance” is often used to describe the resistance of lignin to undergo chemical transformations (generally, reductive processes) rendering small molecules soluble in the reaction medium. Unfortunately, the current usage of the term “recalcitrance” often remains vague in meaning, hindering the search for better catalysts for lignin valorization. In the quest to address the research question—What is lignin recalcitrance?—we present our search for the factors responsible for the resistance of lignin to reductive catalytic processes, from various perspectives. In this study, lignins isolated as a precipitate obtained from the saccharification of water-soluble lignocelluloses (produced by solvent-free mechanocatalytic depolymerization of beechwood, pinewood, or sugarcane bagasse) and their counterparts isolated by solvent extraction (organosolv pulping) are investigated. The critical analysis of structure and bonding, in addition to the in-depth understanding of results from the catalytic upgrading of lignin streams, in the presence of Raney Ni and H2 pressure under mild and extreme conditions, reveals that the simple evaluation of the total yield of liquid products provides no quantitative measure of the lignin recalcitrance. Our results shed light on the real meaning, origins and implications of “lignin recalcitrance” for catalysis research. The results demonstrate that lignin recalcitrance is associated not only with its intrinsic properties (i.e. molecular weight, the occurrence of native linkages, and their bond dissociation enthalpies) but also with its extrinsic properties (e.g. residual polysaccharides and solubility). Overall, this study presents a detailed evaluation of recalcitrance of lignin through the critical analysis of the product mixture properties (e.g. H/C and O/C ratios, molecular weight distribution, yield of key individual products, and several others).