Deciphering Faujasite Zeolite Dealumination at the Atomic Scale

Zeolites are widely used as solid acid catalysts from the laboratory to the industrial scale. Their thermal stability, mesoporous volume, and catalytic properties can be significantly enhanced by framework dealumination, resulting in the formation of framework defects and extra-framework aluminum (EFAL) species, altering surface acidity and microporous volume. Understanding dealumination of as-synthesized zeolite crystals is critical to tuning their catalytic activities. However, the atomic-scale structural evolution of the zeolite surface in the course of dealumination treatments remains elusive. Here, we examined a series of four faujasite zeolite samples, ranging from Y to USY zeolites, collected at key steps of the manufacturing process. High-resolution quantitative 1H NMR spectra were obtained at high field and fast magic angle spinning frequency and interpreted with the help of 1H multiple-quantum (up to four-quanta) and 1H–27Al dipolar-based correlation experiments. An extensive array of surface species was resolved, identified, and monitored during dealumination with a high level of structural detail. This was achieved through a joint interpretation of the NMR data acquired across the entire sample set and DFT calculations spanning an exceptionally broad range of environments. Key insight is provided into the environment of various hydroxyl groups as well as the atomic-scale structure of mononuclear EFAL species. While alumina-like domains are not observed, the presence of multinuclear EFAL species is evidenced. The structures of several defect silanol sites were also characterized. The quantitative evolution of these various surface sites during dealumination is discussed based on the peak intensity changes in the 1H spectra. Ultimately, in situ IR spectra were obtained. Clear correlations were observed between the IR bands and 1H peaks, offering valuable perspectives to refine the interpretation of both IR and NMR spectra.