多钨酸盐修饰ZrO2气凝胶催化四氢呋喃开环聚合

采用过量浸渍结合溶剂蒸发将磷钨杂多酸(TPA)分散于ZrO₂气凝胶表面(TPA的质量分数为5%-45%), 再经750 °C空气气氛焙烧得到多钨酸盐修饰ZrO₂固体酸催化剂. 借助N₂吸附、X射线衍射(XRD)、傅里叶变换红外(FTIR)光谱、氨程序升温脱附(NH₃-TPD)和吡啶吸附红外(Py-IR)光谱对催化剂的结构及酸性质进行表征, 针对四氢呋喃(THF)开环聚合反应考察其酸催化性能. 研究发现, TPA与ZrO₂之间较强的相互作用抑制了ZrO₂的晶化, 同时也在一定程度上稳定了TPA的Keggin(凯金)-阴离子结构. 高温焙烧的催化剂中, 活性组分以ZrO₂锚定的表面相(包括含有畸变或缺陷型Keggin单元的杂多酸盐及以Zr为杂原子的类杂多酸物种等)和TPA完全分解形成的氧化物体相存在, 各物种的相对量取决于TPA的负载量. 催化剂表面同时具有中等强度的布朗斯特德(Brönsted)酸与路易斯(Lewis)酸中心, 且初始TPA负载量为20%的催化剂实现了活性组分在载体表面的单层覆盖, 因而显示最大的总酸量, 对THF聚合反应也表现出最高的催化活性. 在反应温度为40 °C、时间为20 h条件下, 聚合物收率达30.9%±2%, 数均相对分子质量为2698±100; 在催化剂重复使用6次过程中, 活性未见明显降低.

Polyoxotungstate-Modified Zirconia Aerogel as a Solid Catalyst for Tetrahydrofuran Polymerization

Catalysts with different loadings (mass fractions from 5% to 45%) of tungstophosphoric acid (TPA) on zirconia were prepared by suspending zirconia aerogel in an ethanol solution of TPA, removing the solvent via evaporation, drying and then calcining the powder at 750 °C. These catalysts were characterized by N₂ adsorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, FTIR spectroscopy of adsorbed Pyridine (Py-IR), and temperature programmed desorption of NH₃ (NH₃-TPD). The catalytic performances of these materials were examined for the polymerization of tetrahydrofuran (THF). The experimental results indicated that the strong interaction between TPA and zirconia retarded both the crystallization of zirconia and the destruction of the Keggin-unit of TPA. In these materials, the major tungsten species were found to be zirconia-anchored heteropolytungstates (with distorted intact and/or partially fragmented Keggin-units) and Zr-containing pseudo-heteropolyanions produced by the chemical bonding of Zr⁴⁺ with the WO_x fragments from TPA decomposition as well as some amount of WO₃. The catalysts showed both Brönsted and Lewis acidity, and the catalyst with 20% TPA loading had the highest total acidity and catalytic activity because of the monolayer coverage of the active species. Under the reaction conditions of 40 °C for 20 h, the most active catalyst, 20TPZ-750, gave a high polymer yield of 30.9%±2%. During recycling for 6 times, no obvious activity loss was observed.