Study On Catalysts Effective For Deoxygenating Vegetable Oils Into Diesel-like Hydrocarbons And Corresponding Reaction Mechanisms

Energy is an essential material basis for human survival and development, In recent years, diminishing fossil fuel reserves, rising international oil prices and growing concerns about global warming make the utilization of biomass-based energy resources a hot domain all over the world. Vegetable oils and animal fats existing widely in nature are mainly composed of free fatty acids (FFA) and their corresponding triglycerides. Usually, the carbon atom number of the alkyl chains of FFA is in the range from8to22. Although triglyceride-based biomass possess high energy density and abundant sources, they are not suitable for direct combustion in modern diesel engines due to high oxygen content, high viscosity, high cloud point and low stability. Hence natural oils and fats must be upgraded with diverse approaches. Fossil diesel fuels are hydrocarbon mixtures of alkanes, alkenes and cyclanes containing10-22carbon atoms. Based on similarity in structure between vegetable oils and diesel fuels, i.e. the long alkyl chain, diesel-like hydrocarbons could be produced from vegetable oils and animal fats by decarboxylation, decarbonylation or hydrodeoxygenation.In the present dissertation, we investigated the highly selective deoxygenation of vegetable oils over a series of palladium-based and molybdenum carbide-based catalysts including their preparations, characterizations, performance tests and corresponding reaction mechanisms. Some conclusions drawn from the work are provided as follows:(1) An effective and highly selective decarbonylation approach to convert higher aliphatic esters into diesel-like hydrocarbons was developed using Pd/BaSO4as the catalyst and methyl stearate as model compounds. During the reaction course, Pd nanoparticles would migrate together and grow larger, which resulted in the deactivation of Pd/BaSO4catalyst.(2) Colloidal Pd nanoparticles were prepared by reducing the precursor Pd2+ions with ethanol in micelles of hydrophilic block copolymers Poly (N-vinyl-2-pyrrolidone)(PVP). The as-synthesized Pd nanoparticles were then supported onto BaSO4, and the catalyst obtained was denoted as PdNP/BaSO4. The synthetic method reported here is a significant improvement to the traditional preparation approach of the Rosenmund catalyst. Due to the protection of PVP to Pd nanoparticles, there was no deactivation detected after six consecutive tests. In addition, PdNP/BaSO4showed high activity and selectivity for the decarbonylation reaction of food-grade and industrial-grade vegetable oils.(3) Based on comprehensive analysis to gas and liquid products, the deoxygenation mechanism of fatty acid esters was proposed, which included decarboxylation, decarbonylation and hydrodeoxygenation and decarbonylation was the main reaction route. By using the proposed mechanism the gas and liquid products could be reasonably explained. Preliminary kinetics investigation indicated that decarbonylation of fatty acid esters on palladium in a hydrogen-rich atmosphere showed a zero-order rate. We speculated that hydrogen species on the catalyst surface might participate in the rate-determining reaction.(4) We reported for the first time that conversion of fatty acids, fatty acid esters and vegetable oils into diesel-like hydrocarbon mixtures could be realized on the activated charcoal-supported molybdenum carbide catalyst (Mo2C/AC) with high activity and selectivity. The molybdenum carbide-based catalyst exhibited much better resistance to leaching than palladium-based catalyst and could be reused consecutively for sixteen times without deactivation.(5) Ordered mesoporous carbon (OMC) supported molybdenum carbide catalysts [MoxC/OMC (x=1or2)] were successfully prepared in one-pot and prepared catalysts were characterized by N2-sorption, XRD, TEM and XPS. By changing the additive amount of Mo precursor from less than2%to more than5%, molybdenum carbide structures could be easily regulated from Mo2C to MoC. Compared with Mo2C, MoC exhibited high product selectivity and excellent resistance to leaching in converting vegetable oils to diesel-like hydrocarbons.(6) Nanostructured molybdenum carbides supported on multi-walled carbon nanotubes (Mo2C/CNTs) with different loadings were prepared by carbothermal hydrogen reduction method and characterized with SEM, Raman, HAADF-STEM and XRD. Compared with amorphous activated charcoal, the specific G-band structure of carbon nanotubes promoted the formation of molybdenum carbide at lower temperatures. The Mo2C/CNTS catalyst also showed high activity and selectivity for one-step conversion of vegetable oils into branched diesel-like hydrocarbons, which provided a promising approach to prepare high-grade diesel fuels from renewable resources.(7) Mechanism investigation indicated that deoxygenation reaction of fatty acid esters on molybdenum carbide-based and palladium-based catalysts showed different reaction selectivity:hydrodeoxygenation was the main reaction route on molybdenum carbide-based catalysts, while decarbonylation was the main reaction route on palladium-based catalysts. Based on the experimental results of H2-TPD and probe molecules, we speculated that the level of difficulty in acyl-to-alkyl rearrangement of surface acyl intermediates on molybdenum carbide and palladium resulted in the different product selectivity.