Efficient upgrading of palmitic acid in supercritical conditions: Effect of metal promoters and the impact of solvents as a hydrogen source

With their unique physicochemical properties and high solubility, supercritical alcohols represent effective media in the thermochemical conversion of liquid fuels and value-added chemicals from a variety of biomass-derived feedstocks. Short-chain alcohols (C₁–C₃) can enable hydrogenolysis of various feedstocks through in situ hydrogen supply while mitigating char formation by stabilizing reactive intermediates. However, their use is limited by excessive decomposition and high solvent consumption, issues that have been minimally explored. This study seeks to explain the role of supercritical alcohols as hydrogen donors regarding their self-reactivity, interaction with feedstocks, and conversion under catalytic and non-catalytic conditions, including pathways to by-product formation. Herein, we seek to optimize solvothermal upgrading of palmitic acid by maximizing hydrocarbon yield while conserving solvent. Optimizing the choice of solvent and addition of promoter metals can minimize consumption of solvent while increasing conversion by providing additional active hydrogen. At 325 °C, a brief 90-min reaction achieved near-complete decomposition of palmitic acid, yielding high hydrocarbon selectivity while minimizing both solvent consumption and solvent-derived by-products. Under optimum conditions, the liquid product exhibited a low O/C of 0.03, a yield of 67.53 %, and a high heating value of 45.78 MJ kg⁻¹. The hydrodeoxygenation rate of esters to hydrocarbons increased significantly with reaction time. NiCu/AC demonstrated promising catalytic activity in esterification, hydrogenation, hydrodeoxygenation, cracking, and alkylation. Based on identified products, simplified reaction mechanisms were proposed for the supercritical upgrading of palmitic acid.