Effects of Brønsted acid site proximity in chabazite zeolites on OH infrared spectra and protolytic propane cracking kinetics

Abstract

The arrangement of framework Al heteroatoms in zeolite lattices influences the energetics of proton siting and thus the distribution of protons among crystallographically-distinct oxygen atoms and void environments, as shown here for H-CHA zeolites of fixed bulk Al content (Si/Al = 15) but varying amounts of proximal Al sites (Al-O-(Si-O)x-Al, x = 1,2) in a six-membered ring (6-MR). Stretching vibrations of Brønsted acidic OH groups give rise to composite infrared (IR) features whose shapes and integrated intensities are invariant with temperature (448–748 K) on H-CHA zeolites containing predominantly 6-MR isolated Al sites. In sharp contrast, IR peak shapes change and integrated OH areas decrease reversibly with increasing temperature on H-CHA zeolites containing 6-MR paired Al sites (quantified by Co2+ titration). Periodic density functional theory (DFT) calculations show that the four distinct OH configurations at an isolated Al site exhibit distinct vibrational frequencies and molar absorptivities, but are isoenergetic (within 10 kJ mol−1) rendering equilibrium proton populations and OH IR spectra insensitive to temperature. In contrast, paired proton configurations differ in energy by up to ~40 kJ mol−1, with higher energy configurations having lower oscillator strengths; as a result, increasing temperatures shift proton populations toward higher energy configurations that result in a decrease in composite OH IR peak areas. First-order rate constants of protolytic propane cracking (748 K, per H+) are ~12× higher on paired than isolated protons, despite similar apparent activation energies, reflecting apparent activation entropies that are less negative at paired proton sites. Propane cracking rates (748 K, per g) decrease linearly with Na+ content upon partial titration of H+ sites in CHA zeolites containing 6-MR isolated sites, but decrease more strongly at low Na+ contents on CHA zeolites containing 6-MR paired sites, consistent with experimental OH IR spectra and DFT calculations evincing the preferential exchange of Na+ at 6-MR paired Al sites. These conclusions about proton proximity, founded on the interrogation of well-defined and characterized CHA zeolite samples by experiment and theory, provide new insights regarding its consequences for the temperature sensitivity of OH IR spectra and protolytic alkane cracking rates in Brønsted acidic zeolites. The finding that proximal proton sites accelerate high-temperature alkane activation turnovers via non-polar transition states represents a mechanistically distinct extension of our prior reports, wherein proximal protons accelerate low-temperature alkanol dehydration turnovers via polar transition states stabilized by hydrogen bonding interactions, heralding the catalytic diversity among zeolites containing different framework Al arrangements.