12-Tungstophosphoric Acid Modified-Zirconia Aerogel As A Solid Acid For THF Polymerization

Polytetrahydrofuran (PTHF), which is usually obtained by the polymerization of tetrahydrofuran (THF) catalyzed by strong acids, is a versatile intermediate for fibers (Spandex, etc.) and thermoplastic elastomers. In the case of homogeneous mineral acid systems, the catalysts such as perchloric acid, fluorosulphonic acid and oleum are highly efficient. But they corrupt equipment, generate a substantial amount of waste and is not easy to recycle. Therefore, the eco-friendly solid acid catalysts gradually replace them.Heteropoly acid (HPA) has proved to a potential catalysts because of its strong proton acidity, the capability of controlling configuration and performance and low causticity. But the shortcomings of heteropoly acid, such as low surface area and pore diameter, have restricted their further application. As a key approach to enhance surface area and improve the catalytic properties of HPA, the supported form can keep the configuration and acidity furthest and apply in the case of heterogeneous catalytic system. However, leaching of HPA from the catalysts is serious for the strong polarity reaction system. Thus selecting a suitable support to realize the strong interaction for HPA is the solution of this problem. ZrO2 have unique surface hydroxyl group, which can act strongly with HPA. So the solution of active compent can be resolved in a certain extent.The present work is devoted to the synthesis of tungstophosphoric acid (TPA) modified-zirconia aerogel solid acids and examining their performances for use as catalysts in the production of PTHF. A detailed investigation on the TPA loading and heat-treating temperature effects on the structure of the catalysts, acidity and catalytic performance of polymerization of THF has been made. The main contributions are summarized as follows:1. By dispersing TPA on zirconium oxyhydroxide and calcinating at 120℃, a series of solid acid materials, zirconia-supported TPA, can be synthesized. At the moderate TPA loading (mass fraction of 25～45%), TPA on zirconia mainly exist as highly dispersed, somewhat distorted but intact Keggin species. Both Bronsted and Lewis acid sites appeared on the surface of catalysts. The structure of intact Keggin species and the Bronsted acid sites originated from them is indispensable to THF polymerization. Along with the increase of Keggin-anion density and the weaken of destruction, the catalyst performances will greatly enhance. In the scope of experimental research, the catalyst with 35% TPA loading and calcined at 120℃showed the highest acidity and catalytic activity. Under the reaction conditions of 40℃for 5 h, the polymer yield is 60.9%, and the number average molecular weight is 3113.2. Polyoxotungstate-modified zirconia aerogel calcinated at 750℃as another kind of solid acid materials have favorable catalytic activity and recycle. Although the TPA Keggin units have decomposed, the decompositions are not all oxides due to the strong interaction between TPA and zirconia. At the moderate TPA loading, TPA on zirconia mainly exist as zirconia-anchored heteropoly tungstates and/or Zr-containing pseudo-heteropolyanion, WO3 nanoparticles and bulk-like WO3 crystals. Both Bronsted and Lewis acid sites are found on the surface of catalysts. Along with the increase of TPA loading, the catalytic activity and acid density have the same trend. In the scope of experimental research, the catalyst with 20% TPA loading and calcined at 750℃showed the highest acidity and due to the monolayer coverage of the active tungstate species. Under the reaction conditions of 40℃for 20 h, the most active catalyst, 20TPZ-750, showed high polymer yield of 30.9% and number average molecular weight of 2698.3. To TPA-modified zirconia aerogel solid catalysts with 20% TPA loading, the vast majority of tungsten species, in catalysts treated at low-temperatures, exists as highly dispersed, somewhat distorted but intact Keggin species, and the catalysts show both Bronsted and Lewis acidity. When the treating temperature increases to 400-500℃, TPA lose all acid proton and the Keggin units decompose. As a result, the Br(?)nsted acid sites of the catalysts then disappears. A further increase in the heat-treated temperature will bring about a large number of W-O-Zr bonds due to the reaction between zirconia and the surface tungsten species. With the emergence of zirconium-anchored polymeric tungstate species, new Bronsted acid sites develop. The treating temperature have distinct effects on the catalytic performance for THF polymerization. The results showed that, along with the increase of treating temperature, the polymer yield diminishes and then increases. It may relate with the existence condition on the surface of zirconia and density of tungsten species.