Tuning aldol condensation product selectivity by controlling diffusion and reaction rates in zeolites

Abstract

This study presents a method for quantifying mass-transfer limitations in microporous acid zeolites that combines a titration technique and a reaction–diffusion model. The method involves partially exchanging H+ sites with Na+ sites to adjust the site density and observing the resulting reaction rates for the aldol condensation of acetone and cyclopentanone. When the intrinsic activity per site is uniform throughout the zeolite and in the absence of mass-transport limitations, a linear decrease in the reaction rate is expected as the density of sites is systematically lowered by gradual titration. Conversely, if mass-transport limitations are present, the initial decrease in rate with increasing number of titrated sites is much lower than that observed when a large fraction of the active sites have been titrated. This non-linear variation of observed rate as a function of the density of titrated sites indicates the presence of mass-transport limitations in the system. Near the limit where the density of remaining active sites is low enough to avoid generating a concentration gradient inside the zeolite crystallite, the variation in rate becomes linear. The titration results can be coupled with reaction–diffusion modeling to fit and calculate the Thiele modulus, which provides quantitative information on the extent of mass-transport limitations in the studied porous acidic domains. In the case of acetone-cyclopentanone cross-condensation reaction, the selectivity towards the different products was found to be influenced by the degree of mass transfer limitations. That is, kinetically driven systems favor the cyclopentanone self-condensation product while diffusion-limited environments favor products of the reactant with faster diffusion rates. Accordingly, the manipulation of site density was found to be useful in tuning product selectivity towards specific desired products, which demonstrates the potential of this method for developing and optimizing microporous catalysts with controllable acid site density and provide valuable insight into the relationship between mass-transfer limitations and product selectivity.