Spectroscopic and computational study of cr oxide structures and their anchoring sites on ZSM-5 zeolites

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.