Structural effects of WO 3 incorporation on USY zeolite and application to free fatty acids esterification

Abstract

Zeolites have occupied a distinguished position due to their unique properties as solid acids and catalytic results achieved in several industrial reactions. This work studied the influence of supported WO 3 on USY zeolite structure, acidity and activity towards an esterification reaction. High dispersion of WO 3 species on USY was achieved, but at higher loading (⩾11.4%), microcrystalites of WO 3 were detected below the theoretical monolayer coverage (∼32%). Tungsten species were deposited preferentially inside the zeolite structure and interacted with the Brønsted sites of USY as well as on silanol surface groups with the formation of small aggregates. In addition, dealumination took place, especially in the samples with high WO 3 loading. USY had the most and the strongest acidic sites (Brønsted type), but the incorporation of WO 3 decreased the amount and the strength of the new sites. However, all WO 3/USY catalysts were more active than USY in the esterification of oleic acid with ethanol (conversion above 74%, 2 h at 200 °C). The calculation of the TOF for a 1 h reaction demonstrated that 11.4% WO 3/USY was the most active catalyst. Furthermore, it had the lowest rate of deactivation of acid sites after the reaction (∼13% after four cycles). The better performance of the 11.4% WO 3/USY sample was also attributed to a better distribution of strength of the acidic sites and a more hydrophobic character of the synthesized material.