(632b) Spectroscopic and Kinetic Assessment of Brønsted Acid Site Proximity in Chabazite Zeolites

The siting of framework Al heteroatoms in zeolites at crystallographically-unique tetrahedral sites (T-sites) introduces diversity in the location of charge-compensating Brønsted acid protons within different void environments, and in the relative proximity of these proton sites. Differences in monomolecular alkane activation rates have independently been attributed to the location¹ or proximity² of protons in MFI zeolites, highlighting the convolution of these material properties in current analysis of kinetic data. Chabazite (CHA) zeolites of similar bulk composition (Si/Al ~ 15) but synthesized to purposefully contain different populations of paired Al sites (0 – 44% Al in pairs, from Co²⁺ titration)³ provide a suite of model catalysts to deconvolute the effects of acid site location from proximity, because CHA contains only one unique T-site. Here, we use infrared (IR) spectroscopy to directly characterize the structure and number of isolated and proximal Brønsted acidic OH groups, and provide evidence that the integrated molar absorbance of asymmetric OH stretching vibrations (~3600 cm^-1) in H-CHA zeolites (Si/Al ~ 15) increases systematically with the fraction of Al in paired configurations. OH IR absorbances decrease with increasing temperature (448 – 748 K) on H-CHA zeolites that contain paired Al sites, while those containing predominantly isolated Al are insensitive to temperature. Density functional theory (DFT) calculations suggest that protons associated with each of the four lattice O atoms in isolated AlO_4/2^- tetrahedra are nearly isoenergetic and have similar dipole moment first derivatives. In contrast, the sixteen proton configurations at paired Al configurations differ by ~0.5 eV in energy, and lower-energy configurations in paired proton ensembles have greater dipole moment first derivatives, on average, resulting in larger molar extinction coefficients at lower temperatures for H-CHA zeolites that contain paired Al sites. IR spectra of Co-titrated H-CHA zeolites are similar to those for H-CHA zeolites containing mostly isolated Al atoms, consistent with the titration of paired protons by Co²⁺. These observations appear to clarify previous reports of widely ranging molar extinction coefficients (2-20 cmμmol^-1) measured under different conditions on acidic zeolites.⁴ Monomolecular propane cracking rates in CHA zeolites (748 K, per H⁺) are ~10× higher at protons associated with paired Al sites, because of apparent activation entropies that are ~20 J mol^-1 K^-1 less negative than at isolated sites. These observations are consistent with prior reports that attributed entropic gains to polarization of carbonium ion-like transition states by an adjacent proton.² Propane cracking rates (per H⁺) decrease upon partial removal of H⁺ for Na⁺ in CHA zeolites containing paired Al, reflecting thermodynamic preferences for Na⁺ to titrate paired proton sites and consistent with DFT results. These results highlight how differences in framework Al and charge-compensating proton arrangement, as a result of the synthetic methods used to prepare CHA zeolites, influences their catalytic properties for high temperature hydrocarbon catalysis.

References:

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