Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity

We report a synthesis structure function relation describing how different routes to crystallize single tetrahedral-site (T-site) zeolites of fixed composition lead to different arrangements of framework Al atoms and, in turn, of extraframework proton active site ensembles that markedly influence turnover rates of a Bronsted acid-catalyzed reaction. Specifically, synthetic routes are reported that result in systematic changes in the arrangement of aluminum atoms (Al-O(SiO) O)(x)-Al) in isolated (x > 2) and paired (x = 1, 2) configurations within chabazite (CHA) zeolite frameworks of effectively fixed composition (Si/Al = 14-17). Precursor solutions containing different structure-directing agents and aluminum sources crystallize CHA zeolites with one organic N,N,N-trimethyl-1-adamantylammonium cation occluded per CHA cage, and with amounts of occluded Na+ cations that increase linearly with paired framework Al content (0-44%). Ammonia and divalent cobalt ion titrations are used to quantify total and paired Bronsted acid sites, respectively, and normalize rates of methanol dehydration to dirnethyl ether. First order and zero-order methanol dehydration rate constants (per H+, 415 K) systematically increase with the fraction of paired protons in CHA zeolites and are similar to 10x higher at paired protons. Such behavior reflects faster dissociative (surface methoxymediated) pathways that prevail at paired protons over slower associative (methanol dimer-mediated) pathways at isolated protons, consistent with in situ infrared spectra. These findings demonstrate that zeolites of fixed elemental composition, even when crystalline frameworks contain one unique T-site, can exhibit catalytic diversity when prepared via different synthetic routes that influence their atomic arrangements.