Core/shell-Structured ZSM-5@Mesoporous Silica Composites for Shape-Selective Alkylation of Toluene with Methanol
Received date: 2012-11-29
Online published: 2013-02-01
Supported by
Project supported by the the National Natural Science Foundation of China (Nos. 20925310, 21003050, U1162102), the Science and Technology Commission (No. 2012BAE05B02), and the Shanghai Leading Academic Discipline Project (No. B409).
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 Br?nsted 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.
Ji Yongjun , Zhang Bin , Zhang Kun , Xu Le , Peng Honggeng , Wu Peng . Core/shell-Structured ZSM-5@Mesoporous Silica Composites for Shape-Selective Alkylation of Toluene with Methanol[J]. Acta Chimica Sinica, 2013 , 71(03) : 371 -380 . DOI: 10.6023/A12110980
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