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

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 BrOnsted acidic site and the surrounding framework in the SAPO-34 catalyst at temperatures up to 300 degrees C for NMR spectroscopy and 700 degrees C for PDF. Si-29 enrichment of the sample enabled detailed NMR spectroscopy investigations of the T-atom generating the BrOnsted site. By NMR spectroscopy, we observed dehydration above 100 degrees C and a coalescence of Si peaks due to local framework adjustments. Towards 300 degrees C, the NMR spectroscopy data indicated highly mobile acidic protons. Insitu total X-ray scattering measurements analyzed by PDF showed clear changes in the Al local environment in the 250-300 degrees C region, as the Al-O bond lengths showed a sudden change. This fell within the same temperature range as the increased BrOnsted proton mobility. We suggest that the active site in this catalyst under industrial conditions comprises not only the BrOnsted proton but also SiO4. 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 insitu NMR spectroscopy is promising in revealing the dynamic features of a working catalyst.