Hydrothermal post-synthesis of HZSM-5 zeolite to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization reaction: Reconstruction of pore structure and modification of acidity

Hydrothermal post-synthesis of a commercial HZSM-5 zeolite in Al(NO3)3 aqueous solution resulted in the creation of a uniform microporous network and the formation of Brönsted acid sites with suitable acidic strengths through the dynamic extraction and incorporation of aluminum species between the solution and the framework. NMR and N2 adsorption–desorption isotherms measurements revealed that a dynamic extraction and incorporation of aluminum from the framework occurred during the post-synthesis treatment. XRD, XP spectra and TPR measurements of the resulting Mo/HZSM-5 catalyst indicated that the dispersion of Mo species was greatly promoted over the post-synthesized HZSM-5 zeolite, significantly facilitating the migration of Mo species into the channels. As a result, the Mo/HZSM-5 catalyst showed rather high methane conversion and selectivity of aromatics by effectively inhibiting the formation of coke during the methane dehydroaromatization reaction. Comparatively, the Mo/HZSM-5 catalyst, where the HZSM-5 zeolite was obtained by conventional hydrothermal post-synthesis, exhibited lower methane conversion and heavy coke deposits, apparently due to the substantial loss of the Brönsted acid sites and the creation of secondary pores.