Brønsted acid sites (H+) in zeolites catalyze monomolecular alkane cracking and dehydrogenation through protolytic mechanisms involving carbocationic transition states. In parallel with the protolytic dehydrogenation pathway, propane dehydrogenation is catalyzed via H-transfer routes with product-derived carbonaceous deposits, which form in H2-deficient regions of catalyst beds during reaction. The H-abstraction capacity of these extrinsic carbonaceous residues depends on their H2 deficiency; therefore, co-feeding excess H2 suppresses their number and reactivity, allowing for the sole measurement of protolytic dehydrogenation rates. In the chabazite (CHA) zeolite framework, first-order cracking rate constants (per H+) have been reported to increase with the fraction of six-membered ring (6-MR) paired H+ sites. Previous proposals suggest that propane cracking rates increase because carbocationic transition states are entropically stabilized at 6-MR paired H+ sites due to electrostatic interactions from spectating H+ sites. Herein, we investigate the effect of H+ proximity on monomolecular propane dehydrogenation rates. CHA zeolites with varying 6-MR paired H+ sites were synthesized using previously reported methods, where the compositions of structure directing agents (SDAs) was varied. First-order apparent rate constants (per H+) for propane cracking and dehydrogenation linearly increased with six-membered ring (6-MR) paired H+ site content. Assuming measured rates reflect the weighted-average contributions from independent paired and isolated site ensembles, propane cracking and dehydrogenation rate constants (748 K) were estimated to be ~7–10× higher at paired sites. Rate measurements at different temperatures (728–778 K) allowed estimating apparent cracking and dehydrogenation activation enthalpies that were the same for both site ensembles (ΔH‡app,crack. ~180 kJ mol-1; ΔH‡app,dehyd. ~ 205 kJ mol-1), while activation entropies were systematically less negative at 6-MR paired H+ sites (~15-20 J mol-1 K-1 higher for paired sites). The increase in activation entropy appears to reflect an 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. These insights further our understanding of the consequences of H+ site proximity and multi-ion-pair interaction in acid catalysis.

Effects of acid site proximity in CHA zeolites on protolytic propane cracking and dehydrogenation kinetics

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