Deoxygenation of Non-Edible Vegetable Oil to Produce Hydrocarbons Over Mg-Al Mixed Oxides

Abstract

The transition from the use of fossil fuels to alternative and innovative fuels is a complex process involving two main points: the research of a new generation of raw materials and the development of the related technology. Regarding to raw materials, vegetable oils have the capacity to storage large a+mounts of energy, a capacity directly related to their chemical structure, which have similar carbon chains to the common fuels such as gasoil or diesel. However, vegetable oils, which are mostly constituted by triglycerides, cannot be easily used as such. Additionally, they contain significant amounts of oxygen atoms incorporated in the form of carboxyl groups. Consequently, they need to be treated through a deoxygenation process to be converted into useful fuels. Selective deoxygenation can be obtained by promoting the reactions of decarboxylation and decarbonylation in limited presence of hydrogen or even in absence of this gas. Selective decarboxylation of fatty acids results in the elimination of carboxylic groups producing CO2 and a paraffin hydrocarbon (n-alkane with one less carbon atom than the starting fatty acid) while selective decarbonylation leads to the formation of CO, H2O and an olefinic hydrocarbon (the corresponding alkene). When realized over triglycerides these reactions imply also the cracking of the glycerol moiety producing light hydrocarbons. Additionally, ketonization occurs producing high molecular weight ketones that can later be cracked to lighter methyl-ketones. Such pyrolytic processes may bring different benefits, such as the production of high-cetane-number linear hydrocarbons within the diesel range coming from biological sources, which are fully compatible with existing engines and infrastructure. Also, by-products as CO2 could be recovered and used in an integrated biorefinery. Previously, we studied the deoxygenation of oil a non-edible oil (Jatropha curcas oil) and waste cooking vegetable oil in a hydrogen free atmosphere using ?-Al2O3, CaO and Mg-Al mixed oxides (obtained from treated hydrotalcite) as catalysts. Mg-Al mixed oxides (calcined hydrothalcite) revealed interesting catalytic activity. This communication reports on the analysis of the chemical composition of the products obtained during the catalytic pyrolysis of Jatropha curcas oil, at different temperatures and reaction times, using the same catalyst. In the experiments, it was evidenced that the liquid biofuel recovered at 400 °C was composed mainly by hydrocarbons (66 mol % including n-paraffins and aliphatics), this amount increased for the product recovered at 425 °C (77 mol % including n-paraffins and aliphatics). Those compositions are not very different from that of commercial Diesel B7 (91 mol % of hydrocarbons including n-paraffins and aliphatics). It is suggested that Jatropha curcas oil deoxygenation proceeds mainly through the reactions of decarboxylation and ketonization (also involving a decarboxylation). While, ketonization reaction was favored at the lowest temperature tested, it was evident that decarboxylation reaction was predominantly by increasing the temperature.