Through catalytic hydrogen transfer reactions, a new biorefining method results in the isolation of depolymerized lignin--a non-pyrolytic lignin bio-oil--in addition to pulps that are amenable to enzymatic hydrolysis. Compared with organosolv lignin, the lignin bio-oil is highly susceptible to further hydrodeoxygenation under low-severity conditions and therefore establishes a unique platform for lignin valorization by heterogeneous catalysis. Overall, the potential of a catalytic biorefining method designed from the perspective of lignin utilization is reported.

Catalytic upstream biorefining (CUB) encompasses processes for plant biomass deconstruction through the early-stage conversion of lignin by the action of a hydrogenation catalyst. CUB processes produce lignin as an extensively depolymerised product (i.e., a viscous lignin oil) and render highly delignified pulps. In this report, we examine CUB from the pulp perspective. Notably, Raney Ni plays an indirect role in the processes that occur within the lignocellulose matrix. As there are negligible points of contact between the poplar wood chips and Raney Ni, the catalyst action is limited to the species leached from the matrix into the liquor. Nevertheless, the substantial changes in the liquor composition (through the decomposition of carboxylic acids and H-transfer reductive processes on the lignin fragments) have significant implications for the pulp composition, degree of polymerisation and morphology. Compared with organosolv pulps, CUB pulps show higher xylan retention, higher delignification, and higher polymerisation degree. Moreover, the correlation between these properties and the performance of the enzymatic hydrolyses of CUB and organosolv pulps reveals that the high susceptibility of CUB pulps is mostly caused by their lower residual lignin contents.

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We recently introduced catalytic upstream biorefining, a fractionation process performed on whole lignocellulosic materials, based on the early‐stage conversion of lignin by hydrogen‐transfer reactions (using Raney Ni as the catalyst and 2‐PrOH as a hydrogen‐donor). The process fractionates lignocellulose, isolating lignin as an extensively depolymerised oil, opening up new avenues in the catalytic upgrading of bio‐derived phenolic streams to chemicals and fuels. In addition, highly delignified holocellulose pulps are obtained, holding potential as a feedstock for the production of paper, chemicals, and biofuels. Herein, we report our first results on the chemistry underlying this process under nearly neutral to slightly alkaline conditions achieved by the addition of inorganic bases. This report sheds light on the influence of hemicellulose sugars on the product distribution obtained from the early‐stage catalytic conversion of lignin oligomers released from lignocellulose. The increase in the pH value of the medium suppressed the hydrolysis of xylans. As a result, a dramatic increase in the xylans retention from 10 % (at pH 4.5) up to 60 % (at pH>7.5) was achieved. Interestingly, the pH value of the liquor did not affect the delignification extent of lignocellulose or the absolute content of glucans retained in the holocellulose. By enhancing xylans retention, we provide evidence that hemicellulose sugars decrease the activity of Raney Ni towards full hydrogenation of the aromatic species composing the lignin stream. In fact, the yield of selected cyclohexanols increases from 0.8 % (no added bases) to 4.4 % (added NaOH), whereas the yield of selected phenols decreases from 12.9 % (no added bases) to 7.2 % (added NaOH).

Through catalytic hydrogen transfer reactions, a new biorefining method results in the isolation of depolymerized lignin-a non-pyrolytic lignin bio-oil-in addition to pulps that are amenable to enzymatic hydrolysis. Compared with organosolv lignin, the lignin bio-oil is highly susceptible to further hydrodeoxygenation under low-severity conditions and therefore establishes a unique platform for lignin valorization by heterogeneous catalysis. Overall, the potential of a catalytic biorefining method designed from the perspective of lignin utilization is reported.