Faujasite (FAU) zeolites (with Si/Al ratio of ca. 1.7) undergo mild dealumination at moderate ion exchange conditions (0.01 to 0.6 M of NH4NO3 solutions) resulting in protons circumscribed by sodalite cages becoming accessible for reaction without conspicuous changes to bulk crystallinity. The ratio of protons in sodalite cages (HSOD) to supercages (HSUP) can be systematically manipulated from 0 to ca. 1 by adjusting ammonium concentrations used in ion exchange. The fraction of accessible protons in the sodalite cages is assessed by virtue of infrared spectra for H-D exchange of deuterated propane based on the band area ratio of OD2620/OD2680 (ODSOD/ODSUP). Protons in sodalite cages (HSOD) show higher rate constants of propane dehydrogenation (kD) and cracking (kC) than protons in supercages (HSUP) plausibly due to confinement effects being more prominent in smaller voids. Rate constants of dehydrogenation and cracking including kD/kC ratios are also augmented as the fraction of accessible protons in the sodalite cages is enhanced. These effects of accessibility and reactivity of protons in sodalite cages hitherto inconspicuous are revealed herein via methods that systematically increase accessibility of cations located in sodalite cages.
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). Partial support was also provided by the US 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 support from the NSF 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.
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