ZSM-5分子筛的粒径可控合成及其在甲醇转化中的催化作用

Size-controllable synthesis of ZSM-5 molecular sieves and their catalytic performance in the conversion of methanol to hydrocarbons

以四丙基氢氧化胺为模板剂、聚乙二醇为添加剂,水热合成了粒径在0.1～14μm且分布均一的ZSM-5分子筛,并采用XRD、SEM、BET、Py-IR、NH3-TPD和ICP等技术对其进行了表征,考察了不同粒径分子筛在甲醇转化制碳氢化合物反应中的催化作用。结果表明,通过调变PEG添加量、硅源种类和水含量以及控制陈化和晶化条件,可以在较大范围内调变分子筛晶粒尺寸。随着粒径的减小,ZSM-5晶粒的聚集程度、比表面积和Si/Al比提高,而结晶度、Brnsted酸浓度和总酸量有所下降。在甲醇催化转化制碳氢化合物的反应中,粒径为0.1μm的分子筛催化稳定性最好;随着粒径的增大,其稳定性逐渐下降,粒径为14μm的分子筛催化剂寿命明显降低。同时,粒径对甲醇转化产物分布也有较大影响;小粒径分子筛有利于生成轻质烃类(C1～C4),而大粒径分子筛对C5以上烷烃和芳烃的选择性较高。

ZSM-5 molecular sieves with a uniform particle size from 0.1 to 14μm were hydrothermally synthesized by using tetrapropylammonium hydroxide（TPAOH） as the template and polyethylene glycol（PEG） as a co-template.The ZSM-5 samples were characterized by XRD,SEM,BET,Py-IR,NH3-TPD and ICP and their catalytic performance in the conversion of methanol to hydrocarbons were investigated.The results indicated that the crystal size of ZSM-5 molecular sieves can be regulated by adjusting the quantity of PEG,silica source and water content in the precursor gel as well as controlling the aging and crystallizing conditions.The ZSM-5 molecular sieves with smaller crystal sizes exhibit higher conglomeration degree,Si/Al ratio and surface area than those with large crystal sizes,but lower crystallinity,Brnsted acid density and total acid quantity.When used as the catalysts in methanol conversion to hydrocarbons,the crystal size of ZSM-5 molecular sieves has a significant effect on their catalytic stability and the products distribution.The catalyst with smaller crystal size shows much higher stability and selectivity to the light alkanes of C1~C4;an increase in the crystal size can enhance the formation of C5＋ and aromatic products but lead to a significant decline in its stability.