A new publication, entitled ‘Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small-Pore Zeolites’ has recently been accepted as a hot paper in the journal Angewandte Chemie and has been promoted on the journal cover. Our study presents new insight into the influence of the acid site distribution of zeolite catalysts on the diffusion rate of small alkenes and alkanes in a confined porous environment. The cover shows that the interaction of ethene with Brønsted acid sites (yellow dots) facilitates the diffusion through the catalyst, leading to a shorter diffusion path for ethene (blue) compared to the diffusion of ethane (orange) which follows a random trajectory. Our findings highlight that the acid distribution in the zeolite catalyst is an important design parameter for diffusion limitations which may open up interesting perspectives for catalysis or separation purposes.
Image: Ella Maru Studio
Designing new zeolite catalysts with a higher selectivity and lifetime remains one of the biggest challenges for the chemical industry. The selectivity for catalytic processes in acid zeolites is not only determined by the chemical reactivity but also by the transport phenomena of the reactants and products, which may become hindered in the confined zeolite environment. The diffusion and residence time of small (un)saturated hydrocarbons through the nanopores of the zeolite is of fundamental importance for many catalytic conversions, resulting in a significant impact on the ultimate product selectivity and separation. In this context, strategies to enhance the diffusion of desired products such as light olefins while limiting the diffusion of undesired products such as paraffins look very promising when tuning the catalyst design. By means of a synergistic theoretical and experimental approach, we showed that the presence of Brønsted acid sites on the catalyst promotes the diffusion of ethene and propene through the pores of H-SAPO-34, whereas the diffusion of ethane and propane is insensitive for the acid site density. The enhanced diffusivity of unsaturated hydrocarbons can be ascribed to the formation of favorable alkene π-H interactions with the acid sites, as confirmed by IR spectroscopy measurements. Our results demonstrate that dedicated acid site distributions in the confined framework may significantly affect the product selectivity, thus leading the way for future developments in the field of zeolite catalysis.
This publication is the result of a joint international collaboration, led by the Center for Molecular Modeling (CMM) at Ghent University, together with the Center for Material Science and Nanotechnology (SMN) at the University of Oslo, the National Institute for Advanced Materials at Nankai University, the Institute of Chemical Technology at the University of Stuttgart and the Department of Chemistry, NIS centre of excellence, at the University of Turin. Read the full article here.