Silicalite-1 membrane on millimeter-sized HZSM-5 zeolite extrudates: Controllable synthesis and catalytic behavior in toluene disproportionation

Silicalite-1 membrane was synthesized onto the millimeter-sized HZSM-5 zeolite extrudates by a hydrothermal method. The key step of the synthesis is to pretreat the rough external surface of extrudates with the cationic poly-flocculent poly-dimethyldiallylammonium chloride followed with the adsorption of silicalite-1 seeds. The seed size, and the concentrations of the seed and PAC were adjusted to get the suitable synthesis conditions. The resulting HZSM-5/silicalite-1 composite catalysts were characterized by SEM, XRD and N2 adsorption. Catalytic performances were measured in the disproportionation of toluene with a pressured fixed-bed reactor. HZSM-5/silicalite-1 core/shell composites are proved to be successfully and controllably synthesized, giving much higher selectivity for para-xylene than the thermodynamic equilibrium value. The selectivity for para-xylene is enhanced by passivating acid sites of the external surface of HZSM-5 by the non-acidic silicalite-1 membrane, as well as the shape-selective diffusion of para-xylene inside silicalite-1 pores, and the conversion of toluene is decreased due to the additional diffusion resistance from the silicalite-1 membrane. The extent of these changes depends on the density and thickness of the membrane.