Balance between Catalyst Acidity and Hydrophilicity in Biofuel Production from Fatty Acid Esterification over Al-SBA-15

A collection of Al-SBA-15 mesoporous catalysts (Si/Al = 13-174) was investigated to overcome typical accessibility constraints of microporous solids. Al-27 MAS NMR confirmed the existence of tetrahedrally coordinated Al in the catalyst's framework, and the fraction of such species increased as the Si/Al ratio decreased. Bronsted acidity followed the same pattern found using n-propylamine thermodecomposition. Mesoporous catalysts with lower Si/Al ratios exhibited a higher affinity to water as quantitatively determined using water adsorption experiments. Those surface properties were correlated to the catalytic performance on oleic acid esterification. The introduction of Al into the SBA-15 framework significantly improved esterification activity, leading to conversions ranging from 70 to 93%. This was explained by the acidity engendered upon Si substitution by Al. However, a turning point from which activity started dropping was registered and it was proposed to be associated with catalyst hydrophilicity. The balance between acidity and hydrophilicity was unveiled to be of paramount importance to accomplish the best catalytic efficiency and uppermost biofuel yield. Catalyst activity was also assessed for the esterification of stearic and linoleic acids. Higher conversion rates were accomplished with unsaturated fatty acids (oleic acid > linoleic acid > stearic acid) due to the higher reactivity of the carboxylic acid moieties as a consequence of the polarity of the double bonds in the carbon chain.