Catalytic property of hetero-elements isolated in crystalline mesoporous transition metal oxides

本研究旨在评价晶化介孔过渡金属氧化物中杂原子的特征。首先，作为参考，考察了未掺杂的纯介孔过渡金属氧化物对过氧化氢氧化烯烃的催化性能。研究发现，介孔氧化钽是环己烯在十六腈溶剂中氧化反应的有效催化剂，其催化活性高于非多孔材料的比表面积。钛掺杂介孔Ta氧化物也进行了尝试，但由于另一种材料本身的高活性，没有观察到任何改善。2007年，我们对介孔Nb氧化物进行了详细的研究，在无定形介孔Nb氧化物上，环己烯与过氧化氢的氧化反应对环己二醇的选择性超过80%，而在晶态介孔Nb氧化物上对环氧环己烷的选择性也达到了同样的高水平。这些结果表明，通过改变介孔过渡金属氧化物的晶体结构可以控制催化反应的选择性。通过优化反应条件，环己烯与过氧化氢在晶态介孔Nb氧化物上氧化反应，在40℃下反应2 h，环己烯转化率为12%，环氧环己烷选择性为97%。目前工业上常用的含钛沸石用于直链烯烃的环氧化反应，但与以环己烷为代表的沸石孔相比，它们并不能有效地催化大分子的环氧化反应。从这个角度来看，介孔过渡金属氧化物可以催化大分子烯烃的环氧化反应，特别是晶态介孔Nb氧化物，这一发现除了具有学术意义外，还将有助于催化体系的实际发展。

The evaluation of characteristic features of hetero-atoms embedded in crystallized mesoporous transition metal oxides are aimed in the present research. At the beginning as references, the catalytic performance of non-doped pure mesoporous transition metal oxides was evaluated by oxidation of olefins with hydrogen peroxide. Mesoporous tantalum oxide was found to be an efficient catalyst for oxidation of cyclohexene in cetonitrile solvent, whose activity exceeded the one expected from the surface area in comparison with non-porous materials. Ti doping to the mesoporous Ta oxide was also attempted, but no improvement was observed due to the high activity of the other material itself. In the term 2007, mesoporous Nb oxide was studied in detail.The oxidation of cyclohexene with hydrogen peroxide resulted in over 80 % selectivity for cyclohexanediol over amorphous mesoporous Nb oxide, while a similarly high selectivity was observed for epoxycyclohexane over crystalline mesporous Nb oxide. These results indicate that the selectivity of catalytic reactions can be controlled by changing the crystal structure of mesoporous transition metal oxides. By optimizing the reaction conditions, 12 % of conversion of the reactant ant 97 % of selectivity for epoxycyclohexane were obtained by the oxidation of cyclohexene with hydrogen peroxide over crystalline mesoporous Nb oxide even at 40 ℃ for 2 h.While Ti-containing zeolites are commercially utilized for the epoxidation of linear olefins, they are not effectively catalyze larger-sized molecules than zeolitic pores represented by cyclohexe. In this point of view, the present finding that mesoporous transition metal oxides can function the epoxidation of larger olefins, especially the crystalline mesoporous Nb oxide, would contribute to the practical development of catalytic systems in addition to the academic importance.