(429b) C2-C4 Alkane/Alkene Diffusion and C2-C4 Alkene Methylation Reactions in Zeolites and Zeotypes Studied in TAP Experiments

Coupled diffusion and acid-catalyzed reactions in zeolites control a multitude of industrially-important chemistries. The catalytic performance of different materials and/or different operating conditions is typically compared based on steady-state turnover frequencies reflecting the global rates of multistep reaction sequences. However, more specific information about the intrinsic kinetics of individual reaction steps is needed in order to harvest the full potential of computational catalysis and advanced materials synthesis for rational catalyst design and optimization. Such data are highly desirable for single-parameter variation studies of structure-performance relationships and for benchmarking ab initio calculations of reaction rates. Kinetic characterization of individual reaction steps is particularly challenging for zeolite-mediated hydrocarbon conversions, whereby several multistep reaction pathways usually co-exist and influence each other in a highly convoluted network. To make matters even more intriguing, these complex reaction networks are strongly coupled to diffusion phenomena within the spatially non-uniform nanoporous media of a zeolite crystal.

Recently, we used Temporal Analysis of Products (TAP) experiments to study transport phenomena of low weight guest molecules in ALPO-5 (AFI), SAPO-34 (CHA), ZSM-22 (TON), ZSM-5 (MFI), and self-pillared pentasils (MFI) materials. Properly selected reaction conditions and pulse design enabled us to subsequently study the reactions between C₂-C₄ alkenes and a well-defined population of methylation-competent surface species consistent with Surface Methoxy Species (SMS) to precisely measure the intrinsic kinetic parameters of isolated alkene methylation steps in ZSM-22. The resulting parameters were compared to theoretical (DFT) estimates. These results contribute to our fundamental understanding of and the practical ability to control shape-selective chemistry occurring in zeolites and zeotypes under confinement.