Deactivation and regeneration studies of activated carbon during continuous decarboxylation of oleic acid in subcritical water

Decarboxylation of fatty acids using sub or supercritical water is a renewable pathway for producing fuel range hydrocarbons with high selectivity without an added external source of H₂. Activated carbon (AC) has been shown as an effective catalyst for decarboxylation of fatty acids to fuels but it has limited life and an unknown surface chemistry for this catalysis. This study investigates the stability (time on stream) and surface chemistry of AC for the decarboxylation of oleic acid in a bench scale continuous reactor system at 400 °C, 2 h of space time and water to oleic acid ratio of 4:1. AC was found to be effective until 30 h time on stream (out of 45 h), providing 91% degree of decarboxylation constantly. The degree of decarboxylation declined afterwards, dropping from 91 to 50% after 45 h time on stream due to deactivation. The thermal regeneration of deactivated AC was examined using potassium hydroxide (KOH) treatment at 750 °C which helped to regain the physical and catalytic properties of AC. The degree of decarboxylation of oleic acid using fresh and regenerated AC were 91% and 87%, respectively. On the other hand, the selectivity of heptadecane obtained using fresh and regenerated activated carbon was 89.3% and 81.2%, respectively. Decarboxylated liquid products using fresh and regenerated activated carbon gave a similar density and HHV's as commercial fuels, showing the utility of this approach.