(80f) Conversion of Vegetable and Algae Oils to Hydrocarbons Over Supported Metal Catalysts
As an alternative to hydrotreating, we have investigated metal-catalyzed deoxygenation for the production of hydrocarbon fuels from triglycerides. In this approach, deoxygenation of the triglyceride is accomplished ? in the absence of hydrogen ? via removal of the carboxyl group in the fatty acid structure as CO2 and/or CO, thereby producing linear hydrocarbons. Upgrading of tristearin, containing saturated C18 fatty acid chains, over 20 wt% Ni/C at 350°C was found to result in near quantitative conversion of the triglyceride. Alkanes, corresponding to mainly n-heptadecane and n-pentadecane, constituted ca. 60% of the liquid product (by weight), the remainder consisting of fatty acids and unidentified heavy residues. Analysis of the gaseous products confirmed that oxygen was rejected as CO and CO2, while small amounts of light alkanes (C1?C4) and H2 were also formed. Similar results were obtained with triolein (a triglyceride containing only monounsaturated C18 chains); however, compared to tristearin, the selectivity to light hydrocarbons was slightly increased, reflecting the greater reactivity of the unsaturated fatty acid chains towards cracking. This trend was maintained in the deoxygenation of soybean oil; the greater unsaturation of the triglycerides present resulted in increased yields of C2-C10 hydrocarbons relative to triolein. 13C NMR spectroscopic analysis of the liquid product from soybean oil deoxygenation at intermediate reaction times suggested that one pathway for triglyceride deoxygenation involves liberation of fatty acids via C-O bond scission and concomitant H transfer, followed by elimination of CO2 from the acids in a later step. Compared to Ni, catalysts containing Pd or Pt supported on activated carbon showed lower activity for both triglyceride decarboxylation and for cracking of the fatty acid chains, although significant yields of linear alkanes and alkenes were obtained when soybean and algae oils were treated with these catalysts at 350 °C.