5 years ago

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

Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity
Rajamani Gounder, Claire T. Nimlos, John R. Di Iorio
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 Brønsted acid-catalyzed reaction. Specifically, synthetic routes are reported that result in systematic changes in the arrangement of aluminum atoms (Al–O(−Si-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 Brønsted acid sites, respectively, and normalize rates of methanol dehydration to dimethyl 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 ∼10× higher at paired protons. Such behavior reflects faster dissociative (surface methoxy-mediated) 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.

Publisher URL: http://dx.doi.org/10.1021/acscatal.7b01273

DOI: 10.1021/acscatal.7b01273

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