(528d) Synergistic Effects between CHA Zeotypes and Basic Metal Oxides in Bifunctional Strategies for Improved Lifetime in Methanol-to-Olefins Catalysis

Methanol to olefins catalysis by CHA zeolites and zeotypes proceeds through a chain carrier mechanism [1,2]. Impurities initiate formation of unsaturated organic moieties within the cavities of the CHA zeolite/zeotype engendering organic-inorganic hybrid chain carriers that serve as scaffolds propagating C-C formation and scission [1,2]. Active chain carriers are comprised of olefinic and monocyclic aromatic organic co-catalysts which undergo C-C scission events via cracking and dealkylation, respectively, to give light olefin products, and C-C formation events via C1 homologation with methanol close the sequence of chain carrier propagation reactions [2,3]. Formaldehyde, formed in bimolecular transfer dehydrogenation of methanol, alkylates active organic co-catalysts instigating their transformation to polycylic aromatic hydrocarbons thereby terminating chain carriers and inducing catalyst deactivation [4]. CHA zeolites and zeotypes confer dynamics of chemical and physical events that ultimately render too large the frequency of formaldehyde-mediated chain carrier termination relative to chain carrier propagation resulting in short catalyst lifetimes. 

Here, we describe a bifunctional strategy to impede formaldehyde-mediated chain carrier termination that utilizes rare earth metal oxides to selectively and catalytically decompose formaldehyde, thus increasing lifetime of CHA zeolites and zeotypes for methanol-to-olefins catalysis without disrupting the inherent, high selectivity to ethylene and propylene. We demonstrate that the efficacy of rare earth metal oxides to decompose formaldehyde improves with increasing spatial proximity between the surface of the rare earth metal oxide and the chain carriers confined within the zeolite/zeotype. We evince site requirements for alkanal decomposition on rare earth metal oxide surfaces by combining kinetic, transient chemical probe, and spectroscopic techniques and develop causative and correlative relationships between dynamics of alkanal decomposition and lifetime in methanol to olefins (bifunctional) catalysis using formalisms of reaction-transport phenomena.

[1] J. F. Haw, W. Song, D. M. Marcus, and J. B. Nicholas, Accounts of Chemical Research, 36 (2003) 317-326.

[2] S. Ilias and A. Bhan, ACS Catalysis, 3 (2013) 18-31.

[3] A. Hwang, D. Prieto-Centurion, and A. Bhan, Journal of Catalysis, 337 (2016) 52-56.

[4] A. Hwang, M. Kumar, J. D. Rimer, and A. Bhan, Journal of Catalysis, 346 (2017) 154-160.