The Influence of Hemicellulose Sugars on Product Distribution of Early-Stage Conversion of Lignin Oligomers Catalysed by Raney Nickel

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 pH4.5) 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).