Generation and characterization of catalytically active sites of heteropolyacids on zeolite Y for liquid-phase esterification

Zeolites and polyoxometalates are catalysts used in important environmental processes and their combination produces a new solid tailoring the advantages of both catalysts. In this work, supported H3PW12O40 or H4SiW12O40 on zeolite Y was prepared by incipient and aqueous impregnation, which maintained the Keggin structure. The dispersion of H4SiW12O40 or H4SiW12O40 on zeolite Y surface lead to nanocrystals of 5 nm and 23 nm, respectively. Although the crystallinity decreased with HPA loading, the SEM images showed that the original morphology of zeolite Y was conserved and a strong interaction between the solids could be inferred from the MAS NMR spectra. Enhanced formation of secondary mesopores on the zeolite Y structure was related to the partial dealumination process that took place during the impregnation. Heteropolyacid impregnation on zeolite Y resulted on the disappearance of the Lewis sites, because of the interaction with the Keggin anion, thus causing the solid to have predominantly Bronsted type sites. The new sites kept the strengths higher than 100 kJ mol(-1) with a generally increased number because the individual solids had about the same strength of sites (-147 kJ mol(-1)). Higher conversion (77%) for esterification reaction was observed for the samples prepared by incipient impregnation. (C) 2016 Elsevier B.V. All rights reserved.