制备条件对MoO_3/γ-Al_2O_3结构的影响及ZnO改性作用的LRS研究

LRS Studies on Effect of Preparation Variables on Structure of MoO_3 /γ-Al_2O_3 and Promotion of ZnO

当MoO_3含量低于单层分散容量时,MoO_3/γ-Al_2O_3喇曼谱图上两个最突出的峰是～950cm^(-1)的宽峰和876cm^(-1)的较尖锐峰。前者代表“二维聚铝酸”(其中的铝主要是八面体配位的),是加氢脱硫(HDS)等活性中心的前身;后者代表孤立的、四面体配位的钿物种,在加氢脱硫反应中没有活性。用定性和定量的方法系统研究了浸渍液的pH值、焙烧温度、时间、MoO_3总含量等因素对MoO_3/γ-Al_2O_3中两种钼物种相对含量的影响。探讨了在制备过程中减少这种四面体配位钼物种相对含量的方法。指出添加ZnO,使γ-Al_2O_3变成缺锌铝酸锌可以在一定程度上抑制此种非活性钼物种的生成,从而达到提高催化剂活性的目的。

When MoO_3 loading is below the monolayer capacity, there are two intense peaks in the Raman spectrum of MoO_3/γ-Al_2O_3: a broad band at 950cm^(-1) and a sharp peak at 876cm^(-1). The first is assigned to'two-dimensional polymolybdate'(the majority of Mo in it is octahedrally coordinated), which is the precursor of HDS active phase. The second is associated with tetrahedrally coordinated monomeric molybdenum species which are inactive in HDS reaction. The effect of pH value of impregnation solution, calcination time, calcination temperature and the total content of MoO_3 etc. on the relative content of two kinds of Mo has been studied systematically with both qualitative and quantitative methods. The ways to decrease the tetrahedral coordinated-Mo species in the process of preparation have been explored. It has been pointed out that adding ZnO to change γ-Al_2O_3 into Zn-deficiency zinc aluminate can restrict the formation of this inactive Mo-phase to some extent, and as a result, the activity of the catalyst can be improved.