Selective Alkylation of Toluene with Diethyl Carbonate over MgO- and Pd-Modified ZSM-5 Catalysts

The alkylation of toluene with diethyl carbonate (DEC) was studied over ZSM-5 (MFI) zeolite catalysts modified with magnesium oxide and palladium. MgO modification effectively suppressed strong Brønsted acid sites, leading to enhanced para-selectivity by inhibiting disproportionation, isomerization, and deep alkylation reactions. The introduction of palladium provided hydrogenation–dehydrogenation functionality, markedly improving catalyst stability by suppressing olefin oligomerization and coke formation. As a result, high selectivity toward p-ethyltoluene was achieved at moderate toluene conversion under optimized conditions. The influence of reaction temperature, space velocity, and toluene/DEC molar ratio on catalytic performance was systematically investigated. Toluene conversion exhibited non-monotonic behavior with respect to these parameters, reflecting a balance between DEC activation, reactant contact time, and catalyst deactivation. Optimal operating conditions provided high para-selectivity while limiting secondary isomerization and polyalkylation reactions. Fresh and spent catalysts were characterized by XRD, N2 physisorption, NH3-TPD, Py-IR, TEM, TGA, and DSC. These analyses revealed that MgO modification alone leads to the formation of condensed, pore-blocking coke, whereas palladium alters the nature of carbonaceous deposits toward less condensed species, thereby enhancing catalyst stability. Compared with literature systems based on alcohols and other carbonates, the developed catalysts operate at lower alkylating-agent excess and higher space velocities while maintaining competitive conversion and para-selectivity.