Probing Acid Sites in MOR Zeolite Using Low-Temperature 13C Solid-State NMR Spectroscopy of Adsorbed Carbon Monoxide

Abstract

Understanding the structure of Lewis and Brønsted acid sites in zeolites is a key step toward unraveling mechanisms of catalytic reactions, establishing molecular-level structure–activity relationships and thereby rational improvement of their performance. Acid sites are typically studied indirectly, using probe molecules, whose spectroscopic signatures are linked to the nature of specific sites. In this context, carbon monoxide and pyridine (Py) are the most frequently used probe molecules due to their sensitivity toward the nature and strength of surface acid sites. Although infrared and solid-state NMR spectroscopies of adsorbed Py are now widely used, the utilization of carbon monoxide to study surface sites has been mainly limited to infrared spectroscopy. Considering that NMR spectroscopy parameters, such as isotropic chemical shift (δiso) and chemical shift anisotropy (CSA), provide detailed structural information, we have explored the capabilities of low-temperature carbon-13 NMR spectroscopy of adsorbed 13CO on dehydrated mordenite. One-dimensional (1D) and two-dimensional (2D) 13C low-temperature magic-angle spinning NMR spectra show that carbon monoxide interacting with Lewis (LAS) and Brønsted (BAS) acid sites displays distinct 13C NMR signatures, in line with density functional theory (DFT) calculations. Furthermore, {1H}13C HETCOR NMR allows the observation of surface OH groups interacting with CO and points to the spatial proximity (within 1 nm) of BAS to both Si-OH and Al-OH groups, while LAS predominantly comprises the Al-OH group. These observations suggest the formation of defective aluminum sites with one and/or two OH groups bound to aluminum, upon calcination of a NH4-MOR with Si/Al = 6 at high temperature (580 °C). The NMR protocol discussed is perfectly suited to explore the surface sites of other solid materials.