Core/shell-structured ZSM-5@Mesoporous silica composites for shape-selective alkylation of toluene with methanol

A shape-selective core/shell-structured ZSM-5@mesosilica composite catalyst has been synthesized by a facile surfactant-directed sol-gel coating strategy in basic medium using cetyltrimethylammonium bromide as a template. Typically, ZSM-5 with Si/Al ratio of 80 was hydrothermally synthesized using sodium aluminate, tetraethylorthosilicate, and tetrapropylammonium hydroxide (25% aqueous solution). Then it was functionalized with positively charged polymer of polydiallyldimethylammonium chloride (PDDA). Finally, the PDDA-modified ZSM-5 powder was dispersed into a gel of mesoporous silica, and reacted at 303 K for 6 h, thus the ZSM-5@mesosilica composite catalysts were obtained. The characterizations with scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) evidence that mesoporous silica with disordered pores can be evenly coated on the external-surface of ZSM-5 crystals with defined shell thickness. N₂ adsorption isotherms implies the connectivity between the nanopores of mesoporous silica and subnano pores of ZSM-5 zeolite. Additionally, the adsorption-desorption measurement using pyridine or ammonia as probing molecules further demonstrates that, the acid sites within ZSM-5 nanochannels are still accessible and the numbers and intensity of which are nearly unvaried. In the probe reaction of the alkylation of toluene and methanol on a fixed-bed reactor, the ZSM-5@mesosilica composite exhibits higher para-xylene selectivity in comparison to pristine ZSM-5. The unique catalytic behaviors of ZSM-5@mesosilica are ascribed to an effective removal of non-shape-selective acid sites located on the external surface of ZSM-5 by coating nonacidic mesoporous silica shell. In addition, the relationship between acidity and catalytic performance is discussed in detail, particularly in correlation to the enhancement of the para-selectivity by covering the external Brønsted acid sites with the generated layer of mesoporous silica. The present synthetic strategy was also applicable to preparation of other materials having micro-mesoporous core-shell composite with different topology and catalytic application.