固相合成法制备甲苯甲醇烷基化的ZSM-5@silicalite-1核壳分子筛催化剂

Preparation of ZSM-5@silicalite-1 core-shell zeolite catalysts by solid-phase synthesis method for alkylation of toluene with methanol

以ZSM-5晶体为核,将二氧化硅(SiO_(2))、四丙基溴化铵(TPABr)和氟化铵(NH4F)固体研磨成粉作为原料,采用固相合成法制备ZSM-5@silicalite-1核壳分子筛催化剂;并借助X射线衍射仪(XRD)、扫描电镜(SEM)、X射线能谱仪(EDS)、透射电子显微镜(TEM)、X射线光电子能谱仪(XPS)和2,6-二叔丁基吡啶吸附红外光谱(DTBPy-IR)等手段,对核壳分子筛催化剂的结构进行表征,考察其对甲苯甲醇烷基化反应的催化性能。结果表明,通过固相合成法,ZSM-5核晶体外表面外延生长出了全硅MFI型分子筛(silicalite-1)壳层,且该核壳催化剂表现出较高的对二甲苯(PX)选择性。还考察了核晶体尺寸对核壳结构催化剂的影响。研究发现,随着核晶体尺寸的增大,核壳分子筛的甲苯转化率下降、PX选择性上升。作为对比,采用传统水热合成法(液相合成法)制备了核壳分子筛,考察了核晶体尺寸的影响。研究发现,随核晶体尺寸增大,水热合成法制备的核壳分子筛的甲苯转化率呈现先减后增的趋势,PX选择性则相反。由此得出,固相合成法对于较大核晶体的壳层生长具有更好的适用性,表现出广阔的应用前景。

The ZSM-5@silicalite-1 core-shell zeolite catalyst was prepared by solid-phase synthesis method with ZSM-5 crystal as the core,and powders of silica(SiO_(2)),tetrapropylammonium bromide(TPABr)and ammonium fluoride(NH4F)as raw materials.The structure of core-shell zeolite catalyst was characterized by means of X-ray energy spectrum analysis(EDS),transmission electron microscope(TEM),X-ray photoelectron spectroscopy analysis(XPS)and adsorption infrared spectrum of 2,6-di-tert-butylpyridine(DTBPy-IR),and its catalytic performance on alkylation of toluene with methanol was investigated.The results showed that an all-silicon MFI zeolite(silicalite-1)shell formed on the outer surface of ZSM-5 core crystal by solid-phase synthesis method and that the core-shell catalyst showed high selectivity for p-xylene(PX).The effect of core size on the core-shell catalyst was also studied.It was found that the conversion of toluene decreased and PX selectivity increased as the core size increased.As a contrast,the core-shell zeolite catalyst was synthesized with traditional hydrothermal synthesis(liquid-phase synthesis method),and the effect of core crystal size was investigated.With the increase of core size,the toluene conversion over the core-shell zeolites synthesized by hydrothermal synthesis decreased first and then increased,while the PX selectivity was on the contrary.This study shows that the solid-phase synthesis method has better applicability for the shell coating of larger core crystals.