Catalytic Consequences of Isolated and Paired Acid Sites in CHA Zeolites on Monomolecular Propane Cracking

Catalytic Consequences of Isolated and Paired Acid Sites in CHA Zeolites on Monomolecular Propane Cracking

Philip M. Kester, John R. Di Iorio, and Rajamani Gounder*

Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907-2100 (USA)

*rgounder@purdue.edu

Turnover rates for monomolecular cracking and dehydrogenation of alkanes in zeolites are sensitive to local void environments [1] and to the local density of Brønsted acid sites as inferred from changes in bulk Si/Al ratio [2]. Here, we utilize single tetrahedral-site chabazite (CHA) zeolites of fixed bulk composition (Si/Al = 15) with controlled arrangements of framework Al atoms (Al-O(-Si-O)_x-Al) in paired (x = 1, 2) and isolated (x ≥ 3) configurations [3] to decouple the effects of void environment and active site proximity. Turnover rates of monomolecular propane cracking (per H⁺, 748 K) increase systematically in CHA zeolites as the fraction of Al pairs increases from 0 to 44%. Apparent rate constants (per H⁺) on CHA with all isolated Al are constant, suggesting that all isolated Brønsted acid sites are equivalently reactive. Apparent rate constants (per H⁺) are an order of magnitude larger for paired Al sites (4.0 × 10^-3 mol (mol H⁺)^-1 s^-1 bar^-1) than for isolated Al sites (3.3 × 10^-4 mol (mol H⁺)^-1 s^-1 bar^-1). Furthermore, rate constants decrease upon partial Na⁺ poisoning of H⁺ sites in CHA zeolites containing paired Al sites, consistent with the different reactivity of isolated and paired Brønsted acid sites. Infrared spectra are used to detect and quantify paired proton sites in CHA zeolites using molar extinction coefficients estimated from a suite of model samples of varying paired acid site density, allowing for the quantification of the distribution of residual protons in partially ion-exchanged CHA zeolites. Apparent activation energies are independent of acid site proximity, while apparent activation entropies are less negative at paired acid sites, suggesting that transition states are later and more loosely bound at such sites. This work illustrates that the catalytic behavior of zeolites for alkane activation can be tuned by synthetically manipulating the local arrangement framework aluminum atoms and, in turn, of active Brønsted acid sites.

[1] R. Gounder, E. Iglesia. J. Am. Chem. Soc. 131 (2009) 1958-1971.

[2] C. Song, et al. J. Catal. 349 (2017) 163-174.

[3] J. R. Di Iorio, R. Gounder. Chem. Mater. 28 (2016) 2236-2247.