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

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

John R. Di Iorio, Claire T. Nimlos, Rajamani Gounder*

Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

Distinct Brønsted acid site (H⁺) ensembles within a given zeolite arise from differences in the arrangement (Al−O(−Si−O)_x−Al) of framework Al atoms between isolated (x ≥ 3) or paired configurations (x = 1, 2), the latter defined functionally by their ability to exchange extraframework divalent cations. The effects of proton proximity on hydrocarbon catalysis have been previously studied in MFI zeolites,^1-2 but the concomitant changes in Al distribution among different crystallographically distinct tetrahedral sites (T-sites) and void environments (i.e., straight and sinusoidal channels, and their intersections) have precluded unambiguous kinetic assessments of proton proximity in MFI. Here, we use chabazite (CHA) zeolites, a high-symmetry framework with only 1 unique lattice T-site, to develop a predictive relationship that describes how synthetic methods can be developed to prepare zeolites, of fixed topology and elemental composition, with systematically different framework Al arrangements. Such arrangements, in turn, generate different proton active site ensembles that markedly influence turnover rates for the Brønsted acid-catalyzed dehydration of methanol to dimethyl ether, a known probe reaction of solid acids.³ Specifically, we extend previously reported synthetic procedures⁴ to crystallize CHA zeolites at similar composition (Si/Al = 15), but with fractions of paired Al sites that systematically varied from the limit of site isolation (0% paired Al) to nearly half of the sites (44% paired Al) in paired arrangements. This series of CHA zeolites is used to demonstrate that first and zero-order rate constants (415 K, per total H⁺) of methanol dehydration are one order of magnitude larger on paired than on isolated protons. In situ IR spectra measured during steady-state dehydration catalysis are used to observe the formation of surface methoxy species at paired Al sites in CHA, which are not observed on isolated protons in CHA or MFI, providing evidence that alternate dehydration pathways are accessible on paired protons in CHA zeolites. These results represent a step toward the synthetic control of active site atomic arrangement and establish a predictive synthesis-structure-function relation for Brønsted acidic CHA zeolites, demonstrating that catalytic diversity can arise solely from differences in the atomic arrangement of active sites, even among single T-site zeolites of fixed composition.

References:

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2.      Bernauer, M.; Tabor, E.; Pashkova, V.; Kaucký, D.; Sobalík, Z.; Wichterlová, B.; Dedecek, J. J. Catal. 2016, 344, 157-172.

3.      Carr, R. T.; Neurock, M.; Iglesia, E. J. Catal. 2011, 278, 78-93.

4.      Di Iorio, J. R.; Gounder, R. Chem. Mater. 2016, 28, 2236-2247