Enhanced ketonic decarboxylation of fatty acids using vanadia-modified nickel on zirconia catalysts

Abstract

Ketonic decarboxylation of carboxylic acids over ZrO₂-modified catalysts has proven highly effective in yielding ketones and bio-hydrocarbons – key intermediates in biofuel and chemical manufacturing. However, most studies have focused on carboxylic acids with C₁–C₄ chains. This study explores the effect of modifying ZrO₂ catalysts with cost-effective transition metals to enhance the cross-ketonization of triglyceride-derived C₁₈ fatty acids with acetic acid to the C₁₉ fatty ketone, nonadecanone. Further deoxygenation of nonadecanone – with ∼18–23% carbon retention (C₁₂–C₁₈) – signifying preservation of the green diesel energy density was achieved. We introduce an in situ hydrogen generation strategy via formic acid decomposition, which preferentially hydrogenates tall oil fatty acid (TOFA) into stearic acid, significantly improving cross-ketonization. This approach afforded up to 93% conversion, with a catalyst turnover frequency (TOF) of 69 h⁻¹, yielding 64% nonadecanone and ∼20% green diesel-range bio-hydrocarbons (C₁₂–C₁₈) in a stirred-batch reactor (SBR) system using 10 wt% Ni/ZrO₂ at 350 °C for 5 h. The inherently low bio-hydrocarbon selectivity from unsaturated TOFA feedstock was improved by applying 10 bar of hydrogen pressure, coupled with hydrogen from formic acid decomposition, leading to a 1.5-fold increase in bio-hydrocarbon yield – confirming a saturated fatty acid-favoured cross-ketonization pathway. Furthermore, vanadia (V₂O₅) modification of the Ni/ZrO₂ catalyst enhanced bio-hydrocarbon selectivity (∼45%) by facilitating nonadecanone deoxygenation. These findings highlight the role of acid–base tuning in Ni/ZrO₂ catalysts, demonstrating that vanadia doping effectively promotes ketonization and deoxygenation of fatty acids, advancing sustainable green diesel and biochemical (nonadecanone) production.