Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small‐Pore Zeolites. (12th March 2021) - Cnudde, Pieter
Abstract: The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined zeolite pores can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments, and pulse‐response temporal analysis of products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π–H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations. Abstract : The influence of the acid site density on the diffusivities of alkanes and alkenes through the 8‐ring windows of SAPO‐34 was probed using a complementary set of ab initio MD simulations and experimental techniques (PFG‐NMR and TAP pulse‐response measurements). The presence of Brønsted acid sites was found to have a clear promotional effect on alkene diffusion but it does not influence alkane diffusion.