Selective Synthesis of para-Xylene in the Alkylation of Toluene via Regulation of MgO Location in Co-modification Over HZSM-5.

The alkylation of toluene catalyzed by modified HZSM-5 was an economical and efficient way to produce para-xylene (PX) due to the full utilization of toluene and fewer by-products. It has attracted significant attention in optimizing the modification method for both high activity and PX selectivity. In this study, we employed SiO₂ and MgO to co-modify HZSM-5 for the alkylation of toluene with dimethyl carbonate. The catalysts in which SiO₂ was deposited first with trimethoxysilane (TMOS) as the precursor, showed significantly higher activity and PX selectivity than that of depositing MgO first. When the content of SiO₂ and MgO was both 5%, the conversion of toluene was 12%, with the selectivity of PX up to 85% (400 °C) for the 5Si-5Mg/ZSM-5 catalyst (depositing SiO₂ first). However, the conversion and selectivity were only 5% and 56% for the 5Mg-5Si/ZSM-5 catalyst (depositing MgO first). The analysis of structure and acid properties indicated that the pre-deposition of SiO₂ can regulate the location of MgO, leading to the suppression of the external acid sites and the inhibition of the pore blockage by MgO. Owing to the pore size of ZSM-5 being slightly smaller than the molecular size of TMOS, TMOS molecules would be embedded in the pore entrance, making SiO₂ tend to prevent MgO from entering the micropore of ZSM-5. [ABSTRACT FROM AUTHOR]