制备条件对钒硅中孔分子筛的催化氧化性能的影响

Effect of Preparation Method on the Catalytic Activity of Vanadium-substituted Silicate Mesoporous Molecular Sieves

通过溶胶-凝胶的方法在室温下制备出结晶度较好的钒硅中孔分子筛V-MCM-41,并以苯乙烯的液固相催化氧化反应作为指针反应, 研究了制备条件影响V-MCM-41催化氧化性能的本质。在催化剂制备的前处理阶段,降低Si/V比直至Si/V=50(mol比),并将合成凝胶的pH值控制在11.5左右时能获得较高的催化活性,同时也不会降低催化剂的循环使用寿命。另外,碱源用氢氧化钠比用四甲基氢氧化胺更能提高催化剂活性。在催化剂制备的后处理阶段, 晶化时间, 晶化温度和焙烧温度对于提高催化剂的活性都有一个最佳值,其最佳值分别为4d, 100℃和650℃。研究结果表明,V在分子筛骨架的搀杂量以及分子筛的结晶度是制备条件影响钒硅中孔分子筛催化氧化性能的重要决定因素。

The well-crystallized vanadium-substituted silicate mesoporous molecular sieves (V-MCM-41) were prepared at the room temperature by the method of sol-gel. The catalytic activity on the oxidation reaction of styrene and its relationship with preparation method were studied. The preparation process can be divided into two steps. At the first step, increasing the vanadium content up to V/Si (mol)=0.02 and controlling the pH value precisely at 11.5, a high catalytic activity could be gotten, and the long cycle life could be kept. It's found that different types of vanadium-silicate, e.g. V-MCM-41, V-MCM-48 and amorphous vanadium-silicate, can be formed under different pH values. The catalytic activities of those materials decrease by the order of V-MCM-41>V-MCM-48>vanadium-silicate. The type of alkali used for preparing the catalyst was also found to be an important factor that influence the catalytic activity. The factors that influence the catalytic activity at the second step of the preparation include the crystallization time, crystallization temperature, and calcined temperature. It is indicated that the vanadium content and crystallinity of V-MCM-4, which can be controlled by the preparation method, account for its catalytic activity.