(691a) Effects of Acid Site Proximity in CHA Zeolites on Monomolecular Propane and n-Butane Activation Kinetics

Brønsted acid (H⁺) sites in zeolites catalyze monomolecular alkane cracking and dehydrogenation via protolytic pathways involving carbocationic transition states. Propane dehydrogenation also occurs via H-transfer routes on product-derived carbonaceous deposits. These deposits form in H-deficient regions of catalyst beds, with their H-abstraction capacity depending on their H-saturation. Co-feeding excess H₂ suppresses the number and reactivity of these carbonaceous residues, enabling the measurement of protolytic dehydrogenation rates. In CHA framework, first-order propane cracking rate constants (per H⁺) increase with increasing six-membered ring (6-MR) H⁺ site pairing. Previous proposals suggest that this rate increase is due to the entropic stabilization of carbocationic transition states at 6-MR paired H⁺ sites. Herein, we explore the catalytic consequences of H⁺ site proximity on monomolecular propane and n-butane cracking and dehydrogenation.

CHA zeolites were synthesized with varying 6-MR paired H⁺ sites by varying the compositions of structure directing agents (SDAs). First-order apparent rate constants (per H⁺) for propane and n-butane cracking and dehydrogenation increased linearly with 6-MR paired H⁺ sites content. Assuming measured rates reflect contributions from independent paired and isolated site ensembles, apparent propane and n-butane activation rate constants (748 K, per H⁺) were estimated to be ~6–10× higher at paired sites. Estimated apparent cracking and dehydrogenation activation enthalpies were similar for both site ensembles, while activation entropies were systematically less negative at 6-MR paired H⁺ sites (> 15 J mol^-1 K^-1). This increase in activation entropy appears to reflect loosely bound carbocationic transition states at 6-MR paired H⁺ sites resulting from attenuated ion-pair interaction between the carbocationic transition state and the anionic framework charge, likely via stabilization of the anionic lattice charge by the spectating H⁺ site. Our findings further the understanding of how H⁺ site proximity and multi-ion-pair interactions influence the chemical properties and catalytic reactivity of acid zeolites.