Insights into effects of zeolite framework topology on the cross aldol reaction of benzaldehyde with 3-Pentanone

Abstract

Aldol reactions are important in chemical syntheses and have high potential for use in the conversion of biomass to fuels and chemicals. Zeolites of various framework topologies (MFI, MWW, MOR, BEA, FAU) were investigated for their ability to direct aldol chemistry towards the dehydration and formation of the classical condensation product, or towards fission to an olefin and a carboxylic acid. The performance of these solid acids was benchmarked against homogeneous catalysis by benzenesulfonic acid. At reaction temperatures between 80 °C and 180 °C, autogenous pressure in toluene as the solvent, the zeolite-catalyzed cross-aldol reaction between benzaldehyde and 3-pentanone gave the fission products β-methyl-styrene and propionic acid with selectivity of up to 99 %, whereas catalysis by benzenesulfonic acid resulted in the dehydration product 2-methyl-1-phenyl-1-penten-3-one with a selectivity of 100 %. Selectivity-conversion analysis showed fission and dehydration to be parallel reactions of the intermediate ketol 1-hydroxy-2-methyl-1-phenyl-3-pentanone. Poisoning experiments with pyridine, 2,6-lutidine, triphenylphosphine and 2,4,6 tri-tert-butylpyridine demonstrate that fission is catalyzed by Brønsted acid sites and occurs in the zeolite interior of FAU and BEA, in the external cups of MWW, and to some extent in the pore mouths of all topologies. Arrhenius analysis revealed an activation energy of 107 ± 2 kJ/mol for fission in FAU. Fission selectivity in zeolites is found to be enhanced by the inability of the dehydration product to leave the pores. In the reaction-controlled regime, as determined by Weisz-Prater analysis and inspection of the Arrhenius plot, the fission rate is independent of the site density over a wide range of compositions (Si/Al from 6 to 40 for FAU). This result is interpreted as a limitation through molecular crowding in the pores, which prevents full participation of all sites in a cage. The high yields achievable to either product at maximum conversion – up to 85 % fission or 90 % dehydration – simply by choice of catalyst make acid catalysis an attractive choice in aldol chemistry.