ZSM-5@Mesoporous Silica核壳复合结构分子筛的制备及其甲苯甲醇烷基化择形催化性能的研究

doi:10.6023/A12110980

摘要/Abstract

采用阳离子表面活性剂模板法在ZSM-5晶体颗粒表面外延生长介孔氧化硅壳层来调变其外表面酸性, 制备了具有高择形催化性能的介孔氧化硅包裹ZSM-5分子筛的微孔-介孔核壳结构复合材料ZSM-5@mesosilica. 扫描电镜和高分辨透射电镜表征显示, 具有无序孔道结构的介孔壳层均匀包覆于ZSM-5晶粒的外表面, 而且壳层的厚度在一定范围内可调变; 另外, 壳层介孔的孔道走向垂直于分子筛核, N₂吸附曲线表明复合材料的微孔和介孔具有互通性. 吡啶吸附和氨吸脱附实验结果证明这些分子可以自由扩散进入分子筛的微孔道, 并且介孔壳层包覆以后ZSM-5分子筛内的酸性位和强度基本没有变化. 用固定床评价了该复合分子筛对甲苯甲醇烷基化反应的催化性能, 结果表明, 与常规ZSM-5相比, ZSM-5@mesosilica核壳材料表现出了较高的对位选择性. 核壳材料独特的择形催化性能归因于介孔氧化硅壳层将ZSM-5非择形性的外表面酸性位部分覆盖, 从而抑制了对二甲苯在外表面的二级异构化反应.

关键词: 核壳结构分子筛, ZSM-5, 介孔氧化硅, 对二甲苯, 择形烷基化

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.

Key words: core/shell-structured zeolite, ZSM-5, mesoporous silica, para-xylene, shape-selective alkylation

引用此文

纪永军, 张斌, 张坤, 徐乐, 彭洪根, 吴鹏. ZSM-5@Mesoporous Silica核壳复合结构分子筛的制备及其甲苯甲醇烷基化择形催化性能的研究[J]. 化学学报, 2013, 71(03): 371-380.

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.

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