John Di Iorio

2018 Faculty Lectureship Award Winner

Davidson School of Chemical Engineering, Purdue University

 

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

 

Abstract

Single-site heterogeneous catalysts are characterized by turnover rates, normalized by the total number of active sites that are independent of the spatial density or proximity of active sites. We explore this concept by studying chabazite (CHA) zeolites, which are crystalline silica frameworks comprised of a single, symmetry-equivalent lattice tetrahedral site (T-site), that contain a fraction of their framework silicon atoms substituted with aluminum. This isomorphous substitution generates an anionic charge on a framework oxygen that compensates an extra-framework proton or metal cation, which act as catalytic active sites. Crystallization of CHA zeolites using mixtures of organic and inorganic structure-directing agents are shown to influence the proximity of framework Al atoms at a fixed bulk composition, which, in turn, generates different proton active site ensembles. The consequences of Brønsted acid site arrangement are probed using methanol conversion catalysis, which is practiced industrially to produce chemical precursors (alkenes, aromatics) and fuels (gasoline). Turnover rates of methanol dehydration to dimethyl ether (per proton, 415 K) on CHA zeolites systematically increase with the fraction of protons located in close proximity, which stabilize methoxy intermediates that mediate lower free energy methanol dehydration pathways than hydrogen-bonded methanol intermediates do at isolated protons. These results represent a significant step toward predictive synthetic control of active site arrangement in zeolites and can benefit a broad range of chemistries practiced in the petrochemical industry for hydrocarbon and oxygenate conversion to fuels and chemicals, and in the automotive industry for NOx pollution abatement.