摘要
用XRD、电导率测定和TEM/SAD法研究了12-钨磷酸(PTA)在γ-Al_2O_3上的分散性,测得PTA在γ-Al_2O_3上的最大分散量(阈值)为2.88×10^(-6)mol PTA/m^2,表明PTA可在γ-Al_2O_3上形成多层铺展,满4层后才能生成PTA晶相。
A series of γ-Al2O3-supported phospho-tungstic acid (PTA) catalysts prepared by impregnating γ-Al2O3 with PTA solution and drying at 423K for 4h (and then preserved in a desiccator without drier) have been studied by X-ray diffraction, conductivity measurement and transmission electron microscopy/selected area electron diffraction (TEM/SAD). The maximum dispersion capacity (the dispersion threshold value) of PTA on γ-Al2O3 with specific surface area 243m2/g is found to be 2.16g PTA/g γ-Al2O3, equivalent to 2.88×10-6mol PTA/m2 γ-Al2O3 surface. As considering a close-packed monolayer model of PTA with a-cross-sectional area of 2.44nm2 per PTA molecule, the monolayer dispersion value should be 6.80×10-7 mol PTA/m2 γ-Al2O3 surface, therefore, the experimental dispersion threshold value of PTA on γ-Al2O3 surface corresponds to a value of 4 layers. The TEM/SAD results provide full support to the existance of multilayer dispersivity of PTA on γ-Al2O3. Relationship between the conductivity of the water solution, with a definite amount of PTA contained in PTA/γ-Al2O3 and PTA loading reveals that the solubility of PTA/γ-Al2O3 is small and increases slightly with the increase of PTA loading while the PTA loading is less than the threshold value, however, the solubility increases rapidly with increasing the PTA loading while the PTA loading exceeds the threshold value, and finally, it reaches a steady value as if all the PTA in the PTA/γ-Al2O3 dissolves into the water. This fact implies that the dissolution of the crystalline PTA may accelerate the dissolution of the multilayer PTA on γ-Al2O3.
基金
国家自然科学基金
