Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 115, Issue 47, Pages 23354-23362Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp203813f
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Funding
- U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences
- DOE's Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory (PNNL)
- DOE by Battelle Memorial Institute [DE-AC06-76RLO 1830]
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The acidic sites in dispersed tungsten oxide supported on SBA-15 mesoporous silica were investigated using a combination of pyridine titration, both fast- and slow-MAS N-15 NMR, static H-2 NMR, and quantum chemistry calculations. It is found that the bridging acidic -OH groups in surface-adsorbed tungsten dimers or multimers (i.e., W-OH-W) are the Bronsted acid sites. The unusually strong acidity of these Bronsted acid sites is confirmed by quantum chemistry calculations. In contrast, terminal W-OH sites are very stable and only weakly acidic as are terminal Si-OH sites. Furthermore, molecular interactions between pyridine molecules and the Bronsted and terminal W-OH sites for dispersed tungsten oxide species are strong. This results in restricted molecular motion for the interacting pyridine molecules even at room temperature, that is, a reorientation mainly about the molecular C-2 symmetry axis. The restricted reorientation results in efficient H-1-N-15 cross-polarization, making it possible to estimate the relative ratio of the Bronsted to the weakly acidic terminal W-OH sites in the catalyst using the slow-MAS H-1-N-15 CP PASS method.
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