| Polytetrahydrofuran (PTHF) is one of the most useful intermediates for producing thermoplastic elastomers, such as polyesters and polyurethanes (Spandex) with good hydrolytic stability, gas permeability and wear resistance, and rubber with excellent endurance and oil resistance, which are widely used in the textile, chemical and war industries. Generally, PTHF is obtained by the catalytic polymerization of tetrahydrofuran (THF) in the presence of various acid catalysts. The traditional mineral acids (FSO3H, HClO4, H2SO4, etc.) suffer from a number of drawbacks such as inadequate safety, corrosion activity and the need to separate and dispose waste products. Accordingly, these acids are being increasingly replaced by solid acid catalysts, which are safer and environmentally friendly.In this thesis, SiO2-Al2O3 composite oxides with mild acidity and tunable properties were employed as the catalysts for the polymerization of THF to PTHF. The effects of the preparation conditions on the texture, structure and surface acidity of SiO2-Al2O3 composite oxides were systematically studied, and the influences of the texture, structure and surface acidity of SiO2-Al2O3 composite oxides on the performance of THF polymerization were deeply investigated based on the results of catalyst characterization and catalytic evaluation. The catalytic performance of THF polymerization was well developted by the preparation of SiO2-Al2O3 composite oxides with suitable acidity and porosity. The main contributions are summarized as follows:1. The SiO2-Al2OC3 composite oxides with different chemical compositions were prepared and the effects of the Al/Si ratio on the texture, structure, surface acidity and THF polymerization performance of SiO2-Al2O3 composite oxides were studied. The results showed the acid amount increased with the increase of Al/Si ratio due to the increase of Si4+ substituted by Al3+. As the Al/Si ratio increased from 0.025 to 0.065, the products yield of THF polymerization increased from 11% to 54%. As the Al/Si ratio continued to increase, the acidity of SiO2-Al2O3 composite oxides kept increasing, but the activity of THF polymerization dropped obviously. As Al/Si ratio increased to 0.90, the product yield was only 15%. This was not consistent with the reported results that the activity of THF polymerization increased with the increase of the acidity. Based on the characterization result of the catalyst structure and texture, it was found that the specific surface area, the pore volume and the pore diameter decreased as the As the Al/Si ratio increased from 0.065 to 0.90. This was consistent with the trend of the decrease of catalytic activity of THF polymerization. It may be due to the different process of diffusion and mass transfer and different utilization rate of acid sites. Thus, pore structure was one of the important factors to affect the catalytic performance of THF polymerization of SiO2-AlO3 composite oxides.2. Based on the above study, SiO2-Al2O3 composite oxides with different acid and textural properties were prepared by changing the calcination temperature. The structure-activity relationship of SiO2-Al2O3 composite oxides for the THF polymerization was investigated. The results showed that the acidity of the SiO2-Al2O3 composite oxides decreased as the calcination temperature increased due to the break of Si-O-Al bonds and the deprivation of the hydroxy, while the pore channel collapsed and the pore size increased as the calcination temperature increased. Although the acid amount of SiO2-Al2O3 composite oxide calcined at 900℃ was only one third than that of SiO2-Al2O3 composite oxide calcined at 700℃, the catalytic activity of both samples was similar. This was because that the THF polymerization was a chain growth process and the large pore size of SiO2-Al2O3 composite oxide calcined at 900℃ enhanced the process of diffusion and mass transfer, accelerated the reaction rate and increased the product yield. This further confirmed that besides the acidity, the pore structure was one of the important factors to affect the catalytic performance of THF polymerization of SiO2-Al2O3 composite oxides.3. The tuning of pore structure of SiO2-Al2O3 composite oxides was achieved by changing the pH of the sol of precursors. The SiO2-Al2O3 composite oxides with similar acidity and different pore structures were successfully prepared and the catalytic performance of THF polymerization of these SiO2-Al2O3 composite oxides was investigated. The results exhibited that due to the different hydrolysis-condensation mechanisms of the Si, Al precursors under different pH, the gel types were different, so that the textural properties of SiO2-Al2O3 composite oxides were different. At pH 2 and 5, the most probable pore sizes of SiO2-Al2O3 composite oxides were 3.2 nm and 3.3 nm, respectively; at pH 11 and 13, the most probable pore sizes of SiO2-Al2O3 composite oxides were 7.1 nm and 6.7nm, respectively. All these samples showed uniform pore distribution. While at pH 7 and 9, the hierarchically porous SiO2-Al2O3 composite oxides composed of mesopores (7 nm) and macropores (54 nm) were obtained. In the THF polymerization, the SiO2-Al2O3 composite oxides with hierarchical pores exhibited more excellent catalytic performance than SiO2-Al2O3 composite oxides with single pore structure.4. The comparison of catalytic performances of SiO2-Al2O3 composite oxides with different porosity (one with practical macropores, another with the coexistence of mesopores and macropores), Al-MCM-41 and Al-SBA-15 with the typical ordered mesopores was further carried out. The effect of the hierarchical pore system composed of mesopores and macropores on the catalytic performance of SiO2-Al2O3 composite oxides in the THF polymerization was investigated. The results showed the mesopores in the catalyst not only act as the main reaction places, their pore sizes were also decisive to the value of Mn of the polymerization products. The existence of macropores enhanced the accessibility of acid sites and diffusion of reactants and products, which led to high utilization of active sites and long catalytic life.5. The SiO2-Al2O3 composite oxide with excellent catalytic performance was selected to investigate the influence of catalyst concentration, reaction temperature, reaction time on reaction performances of THF polymerization. The optimum rection condition was the catalyst concentration 5 wt%, the reaction temperature 40℃ and the reaction time 6 h.Moreover, the obtained SiO2-Al2O3 composite oxides with appropriate acidity were employed as the supports of Ni based catalysts for the selective hydrogenation of maleic anhydride. The influence of the supports properties on the hydrogenation performance of Ni based catalysts was investigated. The results showed that not only the dispersion degree of Ni but also the adsorption and activation capability for C=O were influenced by using supports with different surface properties. Subsequently, the selectivity for maleic anhydride hydrogenation was significantly affected. The main product over Ni/SiO2 catalyst was succinic anhydride (SA), while Ni/Sio2-Al2O3 catalysts with abundant L acid sits enhanced the adsorption and activation of C=O bonds. The synergistic effect of L acid sites and Ni active sites highly improved the selectivity of y-butyrolactone. |
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