(30a) Hydrothermal Hydrogenation of Crotonic Acid over Activated Carbon and Nickel Catalysts

Hydrogenation of lipid biomass into fuel-like hydrocarbons is an attractive method in response to the increasing global energy demand. Current research focuses on developing methods to promote the reaction by employing H₂-donor solvents. However, solvent consumption makes the process costly and environmentally harmful. In this study, we explored the capability of sub- and supercritical water as a medium for hydrogenation process. Hydrogenation of crotonic acid in sub- and supercritical water was investigated (T=300–400 °C, P=25 MPa, RT=30–120s) using a continuous flow process. Temperature dependence results of non-catalytic reactions showed propene as the main gas product. This indicates hydrogenation via decarbonylation route was favored. The propene yield (Y=78–100%) was found to decrease with the increase in temperature and residence time. On the contrary, the yield of decomposition products such as ethene and ethane increased. At high temperature (T≥350 °C), ethane formation as a consequence of hydrogenation of a portion of the originally formed ethene was observed. This suggests that sufficient H₂ was available which aids in facilitating the reaction. Catalytic experiments using activated carbon and nickel were conducted to compare with the results of non-catalyzed reactions. Gas yields increased by 2–5 times during catalytic reactions. Both catalysts promoted the formation of propene through decarbonylation route; however, activated carbon also supported the consequent hydrogenation of propene to form propane. Meanwhile, nickel catalyst promoted the water-gas shift reaction as H₂ and CO₂ were more evident in the product gases. Depending on catalyst used, the decomposition pathway of crotonic acid differed. Nevertheless, activated carbon and nickel showed catalytic activity for the hydrogenation of crotonic acid. Product distributions were analyzed, and a reaction scheme for non-catalyzed reaction was deduced (Fig. 1).