In Situ Flow MAS NMR Spectroscopy and Synchrotron PDF Analyses of the Local Response of the Brønsted Acidic Site in SAPO‐34 during Hydration at Elevated Temperatures

Abstract

In situ flow magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and synchrotron-based pair distribution function (PDF) analyses were applied to study water's interactions with the Brønsted acidic site and the surrounding framework in the SAPO-34 catalyst at temperatures up to 300 °C for NMR spectroscopy and 700 °C for PDF. Si enrichment of the sample enabled detailed NMR spectroscopy investigations of the T-atom generating the Brønsted site. By NMR spectroscopy, we observed dehydration above 100 °C and a coalescence of Si peaks due to local framework adjustments. Towards 300 °C, the NMR spectroscopy data indicated highly mobile acidic protons. In situ total X-ray scattering measurements analyzed by PDF showed clear changes in the Al local environment in the 250-300 °C region, as the Al-O bond lengths showed a sudden change. This fell within the same temperature range as the increased Brønsted proton mobility. We suggest that the active site in this catalyst under industrial conditions comprises not only the Brønsted proton but also SiO . To the best of our knowledge, this is the first work proposing a structural model of a SAPO catalyst by atomic PDF analysis. The combination of synchrotron PDF analysis with in situ NMR spectroscopy is promising in revealing the dynamic features of a working catalyst.

Water‐Induced Structural Dynamic Process in Molecular Sieves under Mild Hydrothermal Conditions: Ship‐in‐a‐Bottle Strategy for Acidity Identification and Catalyst Modification

“…In very recent study,K alantzopoulos et al found that the framework of molecular sieves become labile with subtle changes of Ta toms when interacting with water at 250-300 8 8C. [44] In the present work, aw ater-induced dynamic and reversible process of breaking and forming of T-O-T bonds is directly and definitely confirmed by using H 2 17 Of or hydrothermal treatment. Moreover,asevidenced in Figure 1a,the breaking and forming of T-O-T bonds becomes more easily to be realized in the H 2 17 Oa tmosphere at enhanced temperature,asisindicated that more 17 Oatoms from H 2 17 Ocan be incorporated into the molecular-sieve framework.…”

Supramolecular Organization in Confined Nanospaces

“…Modern crystallographic techniques can integrate single crystal, powder, or neutron diffraction data with PDF analyses ,,. Such advances have fostered structural studies on disordered phases, (such as the recently reported liquid MOFs), dye‐zeolite materials, or catalysts under industrial conditions, unveiling, for example, the interaction of water with SAPO‐34 acidic sites and their environment at high temperature (700 °C) …”

Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials

“…Another application is the desorption of surface-bound methanol from zeolites in the presence of water, for example after methane oxidation to methanol (Dyballa et al 2019a, b;Pappas et al 2017;Ravi et al 2017). The presence of bulk species and the interaction sites of water and methanol are also of interest for improving the stability of zeolites and related materials of different pore size, as the interaction sites like silanols can react with water at higher temperatures and damage the framework (Kalantzopoulos et al 2018(Kalantzopoulos et al , 2020Prodinger et al 2016).…”