Chabazite (CHA) zeolites were synthesized with mixtures of structure directing agents (TMAda+ and Na+) to alter the proximity of Al atoms within the framework. Co2+ uptakes, measured by UV-visible spectroscopy, were proportional to the Na+:TMAda+ ratio. These Co2+ species selectively titrate ‘paired’ Al sites separated by 1–2 Si atoms in 6-member-ring (6-MR) CHA structures as determined by exchange energies calculated by density functional theory (DFT) across all paired Al arrangements. These synthesis protocols form CHA samples with constant Si:Al ratios (15) and varying amounts of paired Al in 6-MR structures. Methanol dehydration to dimethyl ether (415 K, per proton) was used here as a well-studied probe of acid strength and confinement effects in zeolites. Kinetic turnover rate measurements and in situ spectroscopic studies of this reaction in small-pore 8-MR zeolites (e.g., CHA and AEI) and in medium-pore 10-MR MFI zeolites indicate that paired acid sites in 8-MR zeolites dehydrate methanol with larger turnover rates (in first- and zero-order regimes), lead to the formation of surface methoxy species, and lead to rate inhibition at higher methanol pressures. These consequences of site pairing are observed in CHA and AEI frameworks but not in MFI frameworks. Co2+ titrants selectively bind to paired sites in the 6-MRs of CHA. Other proximal arrangements of Al pairs (e.g., in 8-MRs) or of 3 or more Al atoms may also be formed during synthesis with Na+, although titrations of these 8-MR multi-site configurations have not yet been reported. DFT, however, can directly probe differences in acid strength and reactivity between isolated and proximal Al in a variety of multi-site configurations. DFT calculations demonstrate that proximal acid sites can directly influence one another through H-bonding directly (at close distances) and indirectly (via CH3OH*) to alter acid strength as calculated by deprotonation energies. Furthermore, DFT data indicate that paired protons dehydrate methanol with lower barriers and provide insights into the formation of surface methoxy and high-pressure inhibition observed on such sites. These findings highlight the catalytic diversity of paired and isolated Brønsted acid site ensembles for methanol conversion catalysis, even for single T-site frameworks (e.g., CHA), caused by differences in the stabilities of reactive intermediates and transition states, which can alter the prevailing reaction mechanism.

Effects of acid site proximity on acid strength and the rates and mechanism of methanol dehydration in zeolites

2019 Spring National Meeting

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