Modification of ZSM-5 with B, P, Mg, and Ca for Selective Methylation of Toluene to p -Xylene

Selective p -xylene formation in toluene methylation over ZSM-5 is limited by excessive Brønsted acidity and rapid isomerization. Here, we report a synergistic triple-modification strategy (B-P-Mg/Ca) to independently tune acidity and pore architecture. B-P modification eliminates medium-strength acid sites and suppresses strong Brønsted acidity, while Mg induces controlled pore-mouth narrowing that enhances diffusion constraints without disrupting microporosity. The optimized 5%P-6%B-3%Mg/H-ZSM-5 catalyst achieves ∼90% p -xylene selectivity at 14–16% toluene conversion across a wide WHSV range, outperforming parent H-ZSM-5 (∼85% selectivity at ∼20% conversion). In contrast, Ca modification increases external surface area and pore size, reducing shape selectivity (∼70–75%). Kinetic analysis reveals an apparent activation energy of ∼67.7 kJ mol –1, consistent with a mixed kinetic–diffusion regime. Modifier sequence strongly influences performance, underscoring the importance of synthesis pathways. These results establish a direct link between nanoscale acid site passivation, pore confinement, and para-selectivity.