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Kwak, Ja Hun
Molecular Catalysis lab
Research Interests
  • Heterogeneous catalysis, molecular catalysis, ASlumima, zeolites

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Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported Tungsten Oxide Catalysts Using Solid-State NMR and Quantum Chemistry Calculations

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Title
Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported Tungsten Oxide Catalysts Using Solid-State NMR and Quantum Chemistry Calculations
Other Titles
Characterizing Surface Acidic Sites in Mesoporous-Silica-Supported Tungsten Oxide Catalysts Using Solid-State NMR and Quantum Chemistry Calculations
Author
Hu, Jian ZhiKwak, JahunWang, YongHu, Mary Y.Turcu, Romulus V.Peden, Charles H. F.
Keywords
CHEMICAL-SHIFT-ANISOTROPY; TENSOR PRINCIPAL VALUES; ANGLE TURNING EXPERIMENT; MAGIC-ANGLE; N-15 NMR; SKELETAL ISOMERIZATION; SPINNING NMR; ACTIVATED CARBON; POWDER PATTERNS; GAMMA-ALUMINA
Issue Date
2011-12
Publisher
AMER CHEMICAL SOC
Citation
JOURNAL OF PHYSICAL CHEMISTRY C, v.115, no.47, pp.23354 - 23362
Abstract
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
URI
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DOI
10.1021/jp203813f
ISSN
1932-7447
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ECHE_Journal Papers
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