Ketonization of Mixed Carboxylic Acid-Rich Feedstock from Kraft Black Liquor-Derived Hydroxy Acids

The ketonization of carboxylic acids offers a promising pathway toward the production of valuable intermediate platform chemicals. Building on previous work involving the dehydroxylation of black liquor (BL)-derived hydroxy acids to generate a carboxylic acid-rich stream, this study explores the ketonization of (1) a model mixture comprising linear C2–C9 and branched C4–C6 acids and (2) a realistic carboxylic acid-rich mixture obtained from kraft BL, both on metal oxide catalysts. Initial screening of equimolar mixed-metal oxides ZrMOy (M = Mg, Nb, Ce, Mn) reveals ZrMnxOy as the most active and selective catalyst under varied reaction conditions. Subsequent optimization of ZrMnxOy catalysts shows that ZrMnxOy (Mn = 90 mol %) as well as Zr-free Mn3O4 delivered the highest ketonization activity, achieving up to a 10-fold activity increase compared to t-ZrO2. Mechanistic studies of C2–C9 acids show ketonization rates decline from C2–C6, then deviate nonmonotonically for C7–C9 due to steric and adsorption effects. Catalyst characterization demonstrates that increasing Mn content enhances the formation of moderate Mn2+–O2– acid–base and defect-associated lattice oxygens, which correlate with catalytic activity, and account for the strong performance of the Mn-rich catalysts. Both 90 mol % Mn and Zr-free Mn3O4 catalysts show stable ketonization performance over extended time-on-stream with minimal Mn leaching and without observable structural degradation. However, the Zr-incorporated catalyst exhibited significantly greater leaching resistance in liquid phase tests, underscoring its enhanced robustness. Using the optimized ZrMnxOy catalyst (90 mol % Mn) for the ketonization of a real BL-derived feedstock, more than 93% selectivity to mixed C3–C17 linear and branched ketones is achieved at complete acids conversion. These ketones meet the criteria for downstream condensation and hydrodeoxygenation, suggesting the viability of hydroxy acids derived from kraft BL as a feedstock for synthesis of biolubricants.