TY - JOUR
T1 - Spectroscopic and computational study of cr oxide structures and their anchoring sites on ZSM-5 zeolites
AU - Gao, Jie
AU - Zheng, Yiteng
AU - Tang, Yadan
AU - Jehng, Jih Mirn
AU - Grybos, Robert
AU - Handzlik, Jaroslaw
AU - Wachs, Israel E.
AU - Podkolzin, Simon G.
N1 - Publisher Copyright:
© 2015 American Chemical Society.
PY - 2015/5/1
Y1 - 2015/5/1
N2 - Cr/ZSM-5 catalysts with 0.5-2.6 wt % Cr loadings and zeolites with 15-140 Si/Al ratios were characterized with in situ UV-vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773-1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol3, Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si-O-)2Cr(=O)2 and mono-oxo (Si-O-)4Cr(=O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.
AB - Cr/ZSM-5 catalysts with 0.5-2.6 wt % Cr loadings and zeolites with 15-140 Si/Al ratios were characterized with in situ UV-vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773-1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol3, Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si-O-)2Cr(=O)2 and mono-oxo (Si-O-)4Cr(=O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.
KW - Cr/ZSM-5
KW - DFT calculations
KW - Raman spectroscopy
KW - methane dehydroaromatization
KW - zeolite
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U2 - 10.1021/acscatal.5b00333
DO - 10.1021/acscatal.5b00333
M3 - Article
AN - SCOPUS:84928911501
SN - 2155-5435
VL - 5
SP - 3078
EP - 3092
JO - ACS Catalysis
JF - ACS Catalysis
IS - 5
ER -