Preparation And Catalytic Performance Of ZrO₂-SiO₂ Mixed Oxide For THF Polymerization

ZrO2-SiO2 is one of the binary mixed oxides with many excellent properties. It has high BET surface area of SiO2, particular surface properties of ZrO2 and many superior physicochemical properties which are not owned by ZrO2 or SiO2 such as improved mechanical strength, high stability of ZrO2 crystalline structure and strong surface acidity. For these reasons, ZrO2-SiO2 mixed oxide had been applied widely as catalyst or catalyst carrier. So the work of investigating the preparation of ZrO2-SiO2 mixed oxide with excellent performance has important theoretical and practical significance.Recently, the preparation methods of ZrO2-SiO2 mixed oxide include co-precipitation, deposition-precipitation and sol-gel. Sol-gel processes are widely used in the preparation of atomic mixing ZrO2-SiO2. However, the materials, zirconium alkoxides, which are commonly used in sol-gel method, are expensive and toxic. Further more, the preparation procedure are complex. So, in this report, using zirconyl nitrate as precursors, the effects of preparation parameters on the mixing extent were studied in an alcohol-aqueous system. The mechanism for Zr-O-Si hetero-linkages formation of the mixed oxide was also discussed in detail. Then series of homogeneous ZrO2-SiO2 mixed oxides were obtained, and their structure and surface properties were characterized. Catalytic performances of these materials were examined for the polymerization of tetrahydrofuran (THF). The relation among the structure, surface properties and catalytic performances were investigated in depth. The main results and conclusions are as follows:1. The alcohol-aqueous heating method was workable in preparing highly homogeneous ZrO2-SiO2 mixed oxides. Highlighting, this process was achieved without using any acid or base catalyst, chelating agent. Among all the preparation conditions, the volume ratio of alcohol to water and the prehydrolysis of TEOS have important effects on the homogeneity of ZrO2-SiO2 mixed oxide. In the alcohol-aqueous system, the hydrolysis/ condensation rate of zirconyl nitrate was accelerated when the alcohol-aqueous volume ratio in the reaction system was increased. The increase of alcohol/water volume ratio will on the contrary slow the hydrolysis rate of TEOS down. So the mixing homogeneity of 50ZS decreased as the alcohol-water volume ratio increasing. Although the prehydrolysis of TEOS make the hydrolysis/condensation rates of the two precursors matched more correctly, the Si-OH produced during the prehydrolysis of TEOS may began to condense if the time of prehydrolysis of TEOS is too long. The optimized preparation parameters are taht the alcohol-water volume ratio is 4 and the time of prehydrolysis of TEOS is 12 h.2. The structure and surface property of the mixed oxides were studied combined with a variety of characterizations. The results show that when ZrO2 content in mixed oxide was smaller than 30 mol%, all the Zr-O linkages were involved in Zr-O-Si hetero-linkages, and the average pore size and the amount of total surface acidity of mixed oxides increased as ZrO2 content increased, moreover, the mole ratio of Brφnsted acidity and Lewis acidity was almost 1:1. When ZrO2 content in mixed oxide was 50 mol%, portion of the Zr-O linkages were not involved in Zr-O-Si hetero-linkages and deposited in the pore, so the average pore size decreased apparently and the total surface acidity was almost equal to that of mixed oxide with ZrO2 content of 30 mol%, and the amount of Lewis acidity was evidently larger than Bronsted acidity.3. The polymerization of THF indicated that all of the mixed oxide showed good catalytic performance. With the congtent of ZrO2 increasing, the yield and the average molecular weight of the polymer showed increased firstly and then decreased. When the content of ZrO2 was 30mol%, the yield and the average molecular weight of the polymer were the highest. Combining with the characterization, it was considered that both B and L acid sites of the mixed oxide catalysed THF ring-opening polymerization. With the calcination temperature increasing, the average pore size of the mixed oxide increased absolutely, and catalytic activity also increased obviously.These results indicated that the pore size had important effect on the catalytic activity. Mass transfer was considerd to be the speed-control step.