Selective formation of para-xylene over H-ZSM-5 coated with polycrystalline silicalite crystals

H-ZSM-5 crystals with various Si-to-Al ratios were coated with polycrystalline silicalite-1 layers by a repeated hydrothermal synthesis. The thus-formed silicalite-1 layers were affected by the morphology of the substrate H-ZSM-5 crystals. Applied to the alkylation of toluene with methanol, the silicalite coating significantly enhanced para-selectivity up to 99.9% under all reaction conditions. The enhanced para-selectivity may originate from diffusion resistance through the inactive silicalite layer on the H-ZSM-5, resulting in increased diffusional length. The silicalite coating on the H-ZSM-5 catalysts not only improved para-selectivity, but also prevented catalyst deactivation.