Benzene Methylation by the Coupled Protolytic Cracking Reaction of C 5+ Alkanes

Producing high-value-added toluene and xylene via benzene methylation technology is a promising route to valorize C5+ nonaromatic (C5+NA), optimize the structure of aromatic products and reduce benzene content. In this study, the methylation process of benzene with C5+NA into toluene and xylene was carried out using SSZ-13, ZSM-5, USY, and Beta zeolite-based catalysts, respectively, in a fixed-bed flow reactor under high pressure (3.0 MPa) in a hydrogen atmosphere. Among the various zeolite-based catalysts tested, medium-pore ZSM-5 shows the best catalytic activity due to its appropriate pore structure and acidity. Furthermore, ZSM-5 zeolite-based catalysts with varying SiO2/Al2O3 ratios (denoted as HZ-X, where X represents the SiO2/Al2O3 ratios of the ZSM-5 zeolite, with values of 24, 35, 93, and 239) were well-characterized by NH3-TPD and Py-IR. HZ-24 displays high conversion, selectivity, and stability in the methylation of benzene with C5+NA. The lower SiO2/Al2O3 ratios of ZSM-5 zeolite-based catalysts give higher acidity and superior catalytic activity, where HZ-24 is 2.18 times larger in the conversion to benzene than HZ-239 and shows the highest yield to toluene and xylene. Similarly, the reaction temperature, weight hourly space velocity (WHSV) and reaction pressure have a significant effect on the benzene methylation process. Moreover, the alkylation of benzene with C5+NA obeys the carbenium ion mechanism, following the protolytic cracking, the electrophilic substitution, and the deprotonation processes. More importantly, HZ-24 exhibits a long lifetime and good stability, which allows it to be used for at least 200 h without a significant loss of conversion and selectivity at 500 °C and 10 h–1. Our findings pave the way for large-scale industrial applications in enhancing the value-added chemical products.