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).

During saccharification of water-soluble wood, a dual 2-MeTHF/water solvent system enables extraction of low molecular weight lignin fragments.

There are severalestablished approaches for the reductive fractionation of lignocellulose (e.g.," catalytic upstream biorefining" and "lignin-first"approaches) that lead to alignin oil product that is composed primarily of dihydro-p-monolignols [e.g., 4-(3-hydroxypropyl)-2-methoxyphenol and 4-(3-hydroxypropyl)-2,6-dimethoxyphenol].A lthough effective catalytic methods have been developed to perform reductive or deoxygenative processeso nt he lignin oil, the influence of the 3-hydroxypropyls ubstituent on catalysta ctivity hasp reviously been overlooked. Herein, to better understand the reactivity of the depolymerized lignin oil obtained from catalytic upstream biorefiningp rocesses, dihydro-p-coumaryl alcohol was selected as am odel compound. Hydrogenation of this speciesi nt he presence of Raney Ni with molecular hydrogen led to ring saturation (100 %s electivity) in the absence of hydrodeoxygenation, whereas under hydrogen-transfer conditions with 2-propanol, hydrogenation occurred ( % 55 %s electivity) simultaneously with hydrodeoxygenation ( % 40 %s electivity). In ab roader context,t his study sheds light not only on the reactivity of dihydro-p-monolignols but also on the intricacies of the catalytic upstream biorefiningr eaction network in which these species are revealed to be key intermediates in the formation of lessfunctionalized p-alkylphenols.Undoubtedly,l ignin constitutest he primary renewable source of bulk and specialty aromatic chemicals.[1] Recently,s everal catalytic routes have been developed to convert lignocellulosic biomass into am ixture of compounds that show potential to replacep etroleum-basedr aw materials.[2] In this context,t he emerging field of "catalytic upstream biorefining" (CUB, sometimes referred to as "reductive fractionation" or the "ligninfirst" approach) has attracted increasinga ttention.[1a, 3] CUB encompasses processes for plant biomass deconstruction through the early-stage catalytic conversion of lignin (ECCL) by the action of ah ydrogenation catalyst.[1a] In our CUB process, on the basis of the ECCL by hydrogen-transfer( H-transfer) reactions catalyzed by Raney Ni, the lignin fractioni si solated as av iscous oil.[4] This lignin stream is rich in monophenolic species (< 65 %). Notably,t he type of functionalg roups on the propyls ide chain of the monophenolic compounds depends on both the selected catalysta nd the hydrogen source employed in the process. [4a, 5] Furthermore, the lignin oil also contains dimers and low-molecular-weight lignin oligomers (M w < 1000 Da) as minor products (> 35 %). Amid the monophenolic products obtained by H-transfer CUB in the presence of Raney Ni, two major products,d ihydro-p-sinapyl alcohol (1, Figure 1) and dihydro-p-coniferyl alcohol (2,F igure 1), are obtained from the biorefining of hardwoods. Understandably,t he CUB of (G-lignin-enriched) softwoods generates 2 as the major product.Products 1 and 2 incorporate three classes of oxygenated functional group, namely,the phenolic OH substituent, the methoxy group(s) ortho to the ph...