期刊
ACS CATALYSIS
卷 6, 期 5, 页码 3061-3071出版社
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.6b00389
关键词
catalyst; supported; tungsten oxide; silica; metathesis; propylene; spectroscopy; in situ; operando; Raman; UV-vis; XAS; TPSR
资金
- NSF [CHE 1301262]
- U.S. DOE [DE-FG02-03ER15476]
- U.S. Department of Energy, Office of Basic Energy Sciences [DE-SC0012704]
- Synchrotron Catalysis Consortium (U.S. Department of Energy, Office of Basic Energy Sciences) [DE-SC0012335]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1301262] Funding Source: National Science Foundation
Supported WOx/SiO2 catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by. incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman spectroscopy under dehydrated conditions revealed that below 8% WOx/SiO2, only surface WOx sites are present on the silica support: dioxo (O=)(2)WO2 and mono-oxo O=WO4. The in situ XANES analysis showed that dioxo surface WO4 sites were the dominant surface WOx sites on SiO2 (>90%). The isolated nature of the surface WOx sites was confirmed with in situ UV-vis spectroscopy. The surface WOx sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WOx site that can perform olefin metathesis at similar to 150-250 degrees C. For 8% WOx/SiO2 and higher tungsten oxide loading, crystalline WO3 nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO3 NPs, however, are not active for propylene metathesis. The acid character of the surface WOx sites (Lewis) and WO3 NPs (Bronsted) is responsible for formation of undesirable reaction products (C-4-C-6 alkanes and dimerization of C-2(=) to C-4(=)). This study represents the first time that molecular level structure activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WOx/SiO2 catalysts.
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