介孔分子筛中过渡金属氧化物的可控制备及催化性能研究

【作者】 朱卡克；

【导师】 贺鹤勇；

【作者基本信息】 复旦大学 ， 物理化学， 2003， 博士

【摘要】 大比表面的固体多孔材料可用作吸附剂、催化剂和新型功能材料，其中90年代发展起来的介孔材料尤其引人注目。介孔分子筛具有规则的孔道结构、纳米范围内可调变的孔径（2-50 nm）以及大的比表面。较大的介孔孔径有利于催化剂的负载、反应物和产物的扩散，大的比表面可实现催化活性组份的高分散度。介孔氧化硅材料表面存在大量的可供修饰的硅羟基，可用于表面的功能化。另外，介孔孔径属于纳米尺度范围，为纳米材料的限制性生长提供了可能。所以，自从介孔材料报道以来，对它的表面修饰、功能化及其应用研究引起了广泛的重视。本论文利用介孔材料的大比表面和丰富的硅羟基，制备了具有孤立态结构的钒催化剂，研究了含不同钒物种催化剂的丙烷氧化脱氢活性；利用有机物官能团的嫁接将多酸通过库仑作用固定在介孔分子筛孔道中，以此为前驱体首次制备出具有三维纳米多孔结构的氧化钨和氧化铬晶体以及氧化钒和氧化钨纳米线，研究了氧化钒和氧化铬在乙苯氧化脱氢中的催化活性；利用嫁接的有机胺的配位作用在介孔分子筛孔道中固定Fe3+和Cu2+，并在一定条件下制备了相关的金属氧化物纳米线，以苯酚羟基化制苯二酚为探针反应，探索了均相催化反应的多相化。具体结论如下：1． 以茂金属为前驱体，采用嫁接法制备了载体分别为介孔材料MCM-41和SBA-15及普通SiO2的一系列只含有孤立态钒的催化剂，而利用浸渍法得到的催化剂中含有多种钒物种。通过比较二者在丙烷氧化脱氢反应中的催化活性，发现在大多数情况下，嫁接样品的丙烷转化率，丙烯产率要略高于普通浸渍样品，钒的含量对催化活性起着重要作用。NH3-TPD、UV-DRS及活性测试证实，当钒以高分散度物种存在时，表面酸性弱，丙烯产率高；而当钒物种为聚集态时，具有中强酸性，丙烯的产率下降；强酸性位的存在会导致积碳而使催化剂失活。2． 通过嫁接的3-氨基丙硅酸三乙酯对介孔分子筛SBA-15的表面进行修饰，在其表面形成大量碱性的氨基。对表面氨基质子化，利用库仑作用把多酸离子固定在介孔内。焙烧去除有机物和分解多酸后，可在孔道内限制性生长纳米前过渡金属氧化物。我们分别以(NH4)6V10O28, K2Cr2O7和H3PW12O40为原料，在SBA-15分子筛孔道内得到了V2O5和WO3纳米线及三维Cr2O3和WO3纳米材料。除去介孔分子筛模板后，首次获得了Cr2O3和WO3的三维纳米多孔晶体。Cr2O3和WO3三维纳米多孔晶体中纳米线排列有序，通<WP=6>过线之间的“桥”相互连接为三维网格结构，具有与SBA-15互补的纳米孔道结构。另外，多酸可固定在氨基修饰的介孔分子筛表面并保持其原有结构。通过质子化氨基与多酸离子的静电作用，带高负电荷的多酸离子可被优先固定在介孔分子筛表面。多酸和正硅酸乙酯在有机表面活性剂的作用下可共沉淀，焙烧后生成金属氧化物和氧化硅的固体混合物，该混合物在通过HF选择性溶解氧化硅后，能获得金属氧化物的纳米颗粒，通过这种方法制备了纳米WO3颗粒。利用以上方法获得的V2O5，Cr2O3纳米材料对CO2乙苯脱氢具有很高的活性。3． 在嫁接3-（2-氨乙基）氨丙基三甲氧基硅烷修饰介孔分子筛SBA-15表面后，可以使Fe3+、Cu2+等金属阳离子与胺基配位，固定在介孔分子筛孔道内，实现金属中心在介孔分子筛表面上的高度分散。经焙烧嫁接有金属离子的SBA-15，利用介孔内限制性生长，可获得相应的金属氧化物纳米线。含金属阳离子配合物或金属氧化物纳米线的SBA-15均可以作为苯酚羟基化反应的催化剂，使均相催化反应多相化，提高产物的选择性。更多还原

【Abstract】 Porous materials are frequently used as catalyst, catalyst support, and adsorbent as well, for their high surface areas or high active phase dispersions as well as fast mass transfer of the reactants and products to and from the catalytic sites, which are required from any active catalyst. Among them the mesoporous materials discovered by Mobil Company in 1992 received great attention for their very high surface area, monodispersed pore diameters in the range 2-50 nm, and a stereoregular arrangement of these channels which mimics the liquid crystals formed by these surfactants used in their preparation. The large amounts of Si-OH inside the mesopores also facilitate the modification of the surface with metals and organic groups to form heterogenerous catalyst. The nano-scale mesopores of these materials make it possible to grow nano-scale materials within them, thus developed the "ship in bottle" synthesis of nano-scale materials.The research of this work developed the preparation of isolated vanadium oxides by grafting method and formation of nano-scale transition metal oxide within the mesopores of mesoporous materials. The activity of isolated vanadium oxide upon modified mesoporous materials were tested, and compared with conventional impregnated catalysts in the catalytic oxidative dehydrogenation of propane. Through grafting of 3-aminopropyltrimethoxysilane upon the surface Si-OH, amino- groups can be immobilized on the surface of mesoporous materials, subsequent interaction of polyacid anions with amino- groups can fix these metal centers within the mesoporous channels, hydrolysis of polyacid within these channels can produce nano-scale transition metal oxides. The grafted amino- groups can also immobilize ions through the formation of coordination bond between, which provide a route for the heterogenization of homogenerous catalysis process.1. A route for the preparation of isolate vanadium species upon mesoporous materials is developed, Bis-(cycloPentayl) vanadium was used as precursor was dissolved in chloroform in anhydrous glove box, after diffusing for 2 hr, Et3N was added to activate the surface silanols, calcinations after the grafting process formed isolated vanadium species with C3V symmetry, UV-Raman and 51V MAS NMR unambiguously proved that only isolated species exist in grafted samples, however, impregnated samples consist of mixed<WP=8>vanadium species. In the catalytic oxidative dehydrogenation of propane, the conversion of propane and yield of propene of samples prepared by two different methods are similar, the surface acidity of these samples influence selectivity to propene dramatically, weak acidity are beneficial for propene yield, intermediate acidity decrease the selectivity to propene, and strong acidity will form coal and poison the catalytic activity. Thus, we conclude that it is acidity that effect the selectivity to propene.2. Transition metal oxides are widely used as functional materials and catalysts in many areas, nano-scale metal oxides exhibit extraordinary properties due to quantum size effect, thus draw great attention in recently years. The conventional nano- materials are nano- particles, which grow along one dimension will produce nano- wires, or along two dimension to form nano- thin films, three dimensional nano- material will be difficulty to form because growing along three dimension fully will form bulk maters. Since many of these transition metals can form poly acids, which is easily attached to protoned amino- groups upon modified mesoporous silica by static electric interaction, by this way metal centers can be immobilized within mesopores. Hydrolysis of this material by heating in air can produce nano- scale transition metal oxide within channels of mesoporous silica, after the removal of the silica wall, pure nano- scale materials of wires and 3-dimensional nanowire arrays can produce. We successfully synthesized WO3, Cr2O3, V2O5 nano- materials. Coprecipitation of silica and H3PW12O40 can produce nano- particles of WO3. Cr2O3, V2O5 nano- mat更多还原