Experimental evidence for three rate-controlling regions of the non-oxidative methane dehydroaromatization over Mo/HZSM-5 catalyst at 1073 K

Abstract

Mo/HZSM-5 catalyst offers high selectivity for the non-oxidative dehydroaromatization of methane to benzene. This strongly suggests the deep involvement of the zeolite channels in controlling of the overall reaction rate. Therefore, two 5 wt% Mo/HZSM-5 catalysts based on two zeolite samples of different average crystal sizes were tested for the methane dehydroaromatization reaction over a wide range of space velocities (from 3500 to 60 000 mL g⁻¹ h⁻¹) at 1073 K to examine the effect of superficial velocity on the benzene formation rates. Additionally, a recently developed on-line sampling and off-line analysis approach was employed to follow the maximum outlet benzene concentrations reached over very short time frames. Multiplying the obtained maximum outlet benzene concentrations by the corresponding inlet gas flow rates that were corrected for a temperature factor allowed approximate estimation of the maximum benzene formation rates. Consequently, two rate–space velocity curves were obtained to reveal that three rate-controlling regions exist for the title reaction: <20 000, between 20 000 and 40 000 and >40 000 mL g⁻¹ h⁻¹. Moreover, some specifically designed tests and detailed data analysis were performed to further reveal that these three rate-controlling regions correspond, respectively, to the external mass transfer, intracrystalline diffusion and kinetic desorption controlling steps of the reaction.