Silicalite-1 Shell Synthesized onto Cylinder-Shaped ZSM-5 Extrudate for Disproportionation of Toluene into Para-Xylene

The cylinder-shaped ZSM-5 extrudate with a diameter 2 mm and a length 4-5 mm was coated with a silicalite-1 shell at its external surface by a hydrothermal synthesis with TEOS ( tetraethylorthosilicate) as a silica source and TPAOH ( tetrapropylammonium hydroxide) as a structure-directing agent. Catalytic performance of the obtained core/shell zeolite composite was measured in disproportionation of toluene using a pressured fixed-bed reactor. Ratios of H2O/SiO2 and TPAOH/SiO2, crystallization time and temperature, and synthesis cycles were adjusted to get optimal synthesis conditions, at which the selectivity to para-xylene is enhanced from the thermodynamic equilibrium value 24% to nearly 80%. SEM and XRD results further confirm the formation of the ZSM-5/silicalite-1 core/shell structure, which is responsible for the evident shape-selective production of para-xylene.