Investigating the Coke Formation Mechanism of H-ZSM-5 during Methanol Dehydration Using Operando UV–Raman Spectroscopy

Methanol dehydration on solid acid catalysts is a fundamental step in many industrial chemical processes, such as methanol to dimethyl ether (MTD) and methanol to olefin (MTO). The performance of catalysts often encounters the detrimental effect of coke deposition. However, the heterogeneous distribution of feedstock and product in a fixed-bed reactor usually brings in difficulties in the study of the coking mechanism. In this work, the coking progress of H-ZSM-5 in a fixed-bed reactor under MTD conditions is investigated using operando UV-Raman spectroscopy. Methylbenzenium carbenium ions (MB+), a key precursor for coke formation, was identified by UV resonance Raman spectroscopy and isotope exchange experiments. At higher temperature (473 K), MB+ rapidly transforms into "hard coke" at the beginning of the catalyst bed. The relative intensity of the 1605 cm–1 peak can serve as an indicator for the catalyst deactivation. Moreover, water formed during MTD can suppress the transformation of MB+ into "hard coke" at the later parts of the bed. These results provide important information for the key steps and intermediates about coke formation on solid acid catalysts during methanol conversion, and the findings will contribute to improved catalytic performance in the related catalytic reaction.