Design of highly stable metal/ZSM-5 catalysts for the shape-selective alkylation of toluene with methanol to para-xylene

This work aims to achieve high para-selectivity while maintaining catalytic stability for the alkylation of toluene with methanol to para-xylene over ZSM-5 catalysts under demanding reaction conditions. A high para-selectivity of greater than 97% is achieved by applying consecutive SiO₂-P₂O₅-MgO (SPM) and SiO₂ surface coatings on the surface of pristine ZSM-5. This surface modification passivates the external acid sites and at the same time narrows the pore openings of the ZSM-5 particles, which are both beneficial for the high para-selectivity. The catalytic stability of the surface modified catalysts is achieved by loading with metals including Pt, Ni and (Pt + Ni). This study shows that catalysts co-loaded with (Pt + Ni) exhibit superior catalytic stability, not only in pure H₂ but also in a carrier gas containing a very low H₂ fraction of 5% H₂ and 95% Ar. This is because the co-existence of Pt and Ni effectively inhibits the formation of low-carbon olefins (the coke precursors) via the dehydrogenation of methanol to carbon oxides and the hydrogenation of olefins to paraffins. By studying the different locations of metal loading on the catalysts, this work also confirms that the most effective location of metal loading is on the external surface of the modified catalysts.