Hydrothermal catalytic processing of saturated and unsaturated fatty acids to hydrocarbons with glycerol for in situ hydrogen production

Abstract

Lipids are a promising feedstock to produce renewable hydrocarbon fuels and H₂via catalytic hydrothermal processing. Upon exposure to hydrothermal media (e.g., 300 °C, 8–11 MPa), lipids rapidly hydrolyze to produce saturated and unsaturated free fatty acids in varying ratios, depending on the feedstock, as well as glycerol. This report demonstrates the potential of Pt–Re/C for the hydrothermal conversion of saturated and unsaturated fatty acids to hydrocarbons, using glycerol reforming for in situ H₂ production to meet process demands. Experiments showed that deoxygenation of stearic acid, a model saturated fatty acid, was significantly enhanced with Pt–Re/C under a reducing atmosphere compared to Pt/C. The coupled hydrogenation and deoxygenation (HYD–DOX) of oleic aid, a model unsaturated fatty acid, was also moderately enhanced under an inert atmosphere using glycerol for in situ H₂ production, with DOX as the rate-limiting step. Characterization of Pt–Re/C showed that Re had a significant effect on CO : H uptake ratio (2.2) compared to commercial Pt/C (1.3), with the metals dispersed as small crystallites (∼3–4 nm) throughout carbon support. Experiments revealed that the initial system H₂ headspace loading <3.45 MPa greatly enhances fatty acid DOX kinetics via decarboxylation/decarbonylation without net H₂ consumption. At higher initial H₂ loadings (≥3.45 MPa), fatty acid reduction was also observed as a minor DOX pathway. Experiments also showed that oleic acid HYD–DOX and glycerol reforming are affected by initial glycerol concentration and catalyst loading. Under optimized process conditions, complete HYD–DOX of oleic acid to heptadecane was achieved within 2 h with a net-zero H₂ consumption using a 1 : 3 glycerol-to-fatty acid ratio (i.e., the native ratio in triacylglycerides). X-ray photoelectron spectroscopy showed that H₂ in the reactor headspace results in lower oxidation states of Pt and Re, suggesting a possible mechanism for enhanced DOX kinetics. This approach holds promise for overcoming the high external H₂ demands of conventional lipid hydrotreatment processes.