New highly selective catalytic process for conversion of vegetable oils to diesel-range alkanes under mild conditions
A team led by researchers from the University of Oxford has developed a simple but highly selective catalytic process for the direct hydrodeoxygenation of vegetable oils (triglycerides) into diesel-range alkanes under mild conditions over a Pd/NbOPO4 catalyst. As reported in their paper in the RSC journal Chemical Communications The mass yields of diesel-range alkanes from palm oil and soybean oil can approach to quantitative values.
A number of approaches are being developed and commercialized to convert vegetable oils into diesel fuels. The current primary commercial pathway is the production of first-generation biodiesel—the transesterification of triglycerides with methanol to form fatty acid methyl ester (FAME), with glycerol as the by-product.
FAME has several drawbacks, including high viscosity and poor calorific value, while the by-product glycerol has low commercial demand and is creating environmental problems.
Another alternative is the catalytic cracking of triglycerides over zeolites such as HZSM-5 to produce gasoline- and diesel-range fuel. This approach has low selectivity, however.
The hydrotreating of triglycerides at high temperature over supported metal sulfide catalysts such as NiMo and CoMo sulfide is selective and existing infrastructure and application experience from the petroleum industry can be used. However, the leaching of sulfur from the sulfide catalysts is an issue at high temperature—;leading inevitably to product contamination and catalyst deactivation.
An alternative approach—decarboxylation and/or decarbonylation of carboxylic acids at high temperature over non-sulfide catalysts—shows low activities towards triglycerides conversion.
Hydrodeoxygenation of triglycerides over metal supported on acidic zeolites (H-beta, H-ZSM-5) as bi-functional catalysts produces diesel-range alkanes as high-grade transportation fuels. However, rapid carbon deposition in the zeolitic cavity leads to catalyst deactivation. A Nb2O5-modified Pd/SiO2 has been reported to be active for the total hydrodeoxygenation of fatty acid and triglycerides, but the detailed mechanism remains to be studied.
Herein, we report that a simple methodology for the direct hydrodeoxygenation of vegetable oils over Pd/NbOPO4 catalyst under mild conditions (180 ˚C, 30 bar). Total conversions with near quantitative yields for alkane formation from triglycerides can be obtained over this efficient catalyst. The low reaction temperature preserves C-C bond cleavage, giving >96% corresponding n-alkane selectivity from fatty acid and propane from glycerol moieties.
Catalyst characterization suggests that the glassy nature of NbOPO4 phase can loss its lattice oxygen in H2 readily at mild temperature to expose strong Lewis acid Nb5+ sites hence generating oxygen deficient phase with a high propensity for the adsorption of [O] from triglyceride. In combination with active hydrogen spilled from Pd surface, a highly efficient cooperative catalysis for direct hydrodeoxygenation reaction of triglyceride can be achieved.—Tsang et al.
E. Tsang, Q. Xia, X. Zhuang, M. M. Li, Y. Peng, G. Liu, T. Wu, Y. L. Soo, X. Gong and Y. Wang (2016) “Cooperative Catalysis for Direct Hydrodeoxygenation of Vegetable Oils into Diesel-range Alkanes over Pd/NbOPO4” Chem. Commun. doi: 10.1039/C5CC10419J