Selective hydrocarboxylation of fatty acids into long-chain hydrocarbons catalyzed by Pd/Al-SBA-15

Abstract

Pd/Al-SBA-15 materials, with different Si/Al atomic ratios, have been investigated as catalysts in the hydrodecarboxylation (HDC) of fatty acids for the production of long-chain paraffinic hydrocarbons. The catalytic tests were performed in a stirred batch reactor under H2 atmosphere using stearic, oleic and palmitic acids as substrates. Characterization of the catalysts indicates that the Pd nanoparticles are well distributed on the support, being partially located within their ordered mesopores. Increasing the Al-SBA-15 acidity enhances the stearic acid conversion but provokes a reduction in the selectivity towards hydrodecarboxylation versus hydrodeoxygenation (HDO). Variation of the reaction temperature within 225 - 300 ºC enhanced the catalytic activity, while the HDC selectivity remained over 90%, being just reduced when working at 300 ºC due to the occurrence of cracking reactions. The H2 pressure also increased the stearic acid conversion, in particular when passing from 3 to 6 bar, confirming the positive role of the hydrogen atmosphere in the decarboxylation process. However, when operating at 25 bar of H2 pressure, the HDC selectivity decreases, as the HDO pathway is favored. Comparing the results obtained with stearic, oleic and palmitic acids shows that the Pd/Al-SBA-15 system is an excellent catalyst for this reaction, providing in all cases a significant catalytic activity and a very high HDC selectivity. These findings can be related to the features of the Al-SBA-15 support (high surface area, uniform mesopores and mild Lewis acidity), which provides a good Pd dispersion, a high substrate accessibility and limit the extension of cracking reactions.