Low-silica faujasite (FAU) zeolites (with Si/Al ratio of ca. 1.2-1.8) sustain framework integrity and porosity upon moderate ion exchange (0.01 M NH4NO3solution for 1 h at ambient temperature), which introduces two kinds of protons, distinctive in reactivity and coordination to the zeolite framework, within supercages (HSUP). Moderate ion exchange limited within supercages transpires while maintaining full occupancy of Na+cations within associated sodalite cages; this in turn helps stabilize the framework of low-silica H-FAU zeolites. Protons located on site II (H3630) and site III (H3650) within supercages on low-silica FAU zeolites can be classified and enumerated by virtue of infrared spectroscopy, providing an opportunity to compare reactivities of these distinct protons for monomolecular protolytic reactions of propane. Protons on site II exhibit prominently higher reactivity for monomolecular propane dehydrogenation and cracking than protons on site III. Low-silica proton-form FAU zeolites (zeolite X) upon moderate ion exchange possess protons on site III that are unavailable on high-silica FAU zeolites (zeolite Y) and limit ion exchange within supercages, providing unprecedented high-temperature structural and chemical stability (>773 K) and enabling their application as solid-acid catalysts.
Bibliographical noteFunding Information:
We acknowledge partial support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award No. DE-SC0001004. Partial support was also provided by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences (Award DE-FG02-12ER16362), and by the U.S. Department of Energy, Office of Basic Energy Science, Catalysis Science Program (Award DE-SC00019028). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial NSF support through the MRSEC and NNIN programs (DMR-1420013). Solid-state MAS NMR measurements were provided by the NMR facility at Caltech. The synchrotron XRD data were collected through the mail-in program at Beamline 17-BM of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors also thank the reviewers for their helpful technical suggestions during the revision of this manuscript.
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