Investigation of the stability of Zn-based HZSM-5 catalysts for methane dehydroaromatization

Non-oxidative methane conversion into aromatic compounds was studied over Zn/HZSM-5 catalysts at 700 degrees C, 3000 scc/g(cat)/h and atmospheric pressure. In addition to reaction studies, the stability of Zn at different loadings (1, 2, 3, and 8 wt%) was investigated by XRD, ICP-OES, EDS, TGA, BET, and NH3-TPD characterization techniques. The results suggest the presence of two Zn species during reaction: (1) loosely bound and easily reduced ZnO particles; (2) anchored and thermally stable [Zn(OH)](+). At low loading (1 and 2 wt%) anchored Zn is the dominant, thermally stable specie on the catalyst surfaces showing the most retained Zn after the reaction. At high loading (3 and 8 wt%) most of the Zn is in the form of ZnO particles susceptible to reduction to Zn metal, which slowly vaporized under reaction conditions. The catalyst with 3 wt%Zn produced the highest benzene yield; however, it decreased rapidly, due to coke formation, compared to the 1 wt%, which showed more yield stability. Small amounts of CO2 (0.5-2%) were added to the reaction stream to help stabilize ZnO and reduce coke formation during the reaction over 3 wt% Zn/HZSM-5. Results showed that the addition of CO2 resulted in retaining more Zn on the spent catalyst and improved the catalytic performance stability, but it significantly decreased the aromatic yield, indicating that the ZnO particles are not the active Zn species. Instead, the reactive specie was concluded to be the anchored [Zn(OH)](+) acting as a strong Lewis acid. (C) 2015 Elsevier B.V. All rights reserved.