(109d) Catalytic Upgrading of Kraft Black Liquor-Derived Hydroxy Acids

Lignocellulosic feedstocks are widely studied for sustainable liquid fuels and chemical production. The pulp and paper industry generates large amounts of kraft black liquor (BL) from which a high volume of hydroxy acids (HAs) can be separated for further catalytic processing. While the separation of HAs from BL has been recently demonstrated, little attention has been paid to their conversion. This study explores the stepwise catalytic upgrading of (1) a model HA, gluconic acid; (2) a model mixture of HAs, and (3) a realistic mixture of kraft BL-derived hydroxy acids. Specifically, the removal of the -OH functional group of hydroxy acids via C-OH bond scission reactions on bifunctional catalytic materials (M/Nb₂O₅, M = Pd, Pt, Rh, and Ru) is targeted. The hydrodeoxygenation of model gluconic acid on supported metal catalyst candidates reveals C₂ and C₃ carboxylic acids, levulinic acid, and cyclic esters as the main products, with Pd/Nb₂O₅ showing the best activity and selectivity to valuable intermediate products. The model mixture of HAs showed a wide range of reactivity to temperature, with the product selectivity strongly correlating to reaction temperatures. Finally, the 0.25%Pd/Nb₂O₅ catalyst is tested using the real BL-derived HAs feed, revealing stable product selectivity for 15 h time on stream, without notable deactivation. The catalyst successfully removed the -OH functional group of most components in the HA mixture to selectively produce a C₃ – C₈ -rich carboxylic acids mixture that may be amenable to further catalytic upgrading via ketonization (a pathway to increase carbon chain lengths). Hence, we propose enriched kraft BL as a suitable sustainable platform for producing mixed carboxylic acids from highly oxygenated by-product feed.