Catalytic activities and structures of silicalite-1/H-ZSM-5 zeolite composites

Polycrystalline silicalite layers were formed on H-ZSM-5 with different crystal sizes of 5–30 μm. The silicalite-1/H-ZSM-5 composite with a crystal size of 5 μm showed high catalytic activity and excellent of para-xylene selectivity. But, para-selectivity slightly decreased with increasing the crystal size, indicating that the surface of the large H-ZSM-5 crystals was not fully covered by the silicalite-1 layer. The toluene conversion over the catalyst with a small crystal size (5 μm) was very stable with reaction time, while the catalysts with large crystal sizes were rapidly deactivated. The crystalline structure of the interface between silicalite-1 and H-ZSM-5 crystals was studied by FE-SEM and TEM observations. The silicalite-1 crystals were epitaxially grown on the surface of the H-ZSM-5 crystals and their pores must be directly connected to the pores of H-ZSM-5.