Ruddy Daniel A, Tilley T Don
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Am Chem Soc. 2008 Aug 20;130(33):11088-96. doi: 10.1021/ja8027313. Epub 2008 Jul 29.
The reaction kinetics of cyclohexene epoxidation using aqueous H2O2 oxidant and the highly selective epoxidation catalyst Bu(cap)TaSBA15 were studied. The reaction was determined to be first-order in Ta(V) surface coverage. The reaction rate exhibited saturation with respect to increasing concentrations of cyclohexene and H2O2. An Eley-Rideal mechanism and rate equation may be used to describe the epoxidation kinetics, which are similar to those for Ti(IV)SiO2-catalyzed epoxidations. The observed kinetics may also be modeled by a double-displacement mechanism typically associated with saturation enzyme catalysts. In addition, (1)H NMR spectroscopy was employed to investigate H2O2 decomposition by Bu(cap)TaSBA15 and the unmodified TaSBA15 catalysts. Little decomposition occurred over the surface-modified material, but the unmodified material catalyzed a 30% conversion of H2O2 after 6 h. UV-visible absorbance and diffuse reflectance UV-visible (DRUV-vis) spectroscopy were used to investigate the structure of the Ta centers on the TaSBA15 catalysts. DRUV-vis spectroscopy was also used to identify a Ta(V)-based epoxidation intermediate, proposed to be a Ta(V)(eta(2)-O2) species, which forms upon reaction of the TaSBA15 and Bu(cap)TaSBA15 materials with H2O2.
研究了使用过氧化氢水溶液氧化剂和高选择性环氧化催化剂Bu(cap)TaSBA15进行环己烯环氧化的反应动力学。该反应被确定为对Ta(V)表面覆盖率呈一级反应。反应速率随着环己烯和过氧化氢浓度的增加而呈现饱和。可以使用Eley-Rideal机理和速率方程来描述环氧化动力学，这与Ti(IV)SiO2催化的环氧化反应类似。观察到的动力学也可以用通常与饱和酶催化剂相关的双取代机理来建模。此外，采用(1)H NMR光谱研究了Bu(cap)TaSBA15和未改性的TaSBA15催化剂对过氧化氢的分解情况。在表面改性材料上几乎没有发生分解，但未改性材料在6小时后催化了30%的过氧化氢转化。紫外可见吸收光谱和漫反射紫外可见(DRUV-vis)光谱用于研究TaSBA15催化剂上Ta中心的结构。DRUV-vis光谱还用于鉴定一种基于Ta(V)的环氧化中间体，推测为Ta(V)(η(2)-O2)物种，它在TaSBA15和Bu(cap)TaSBA15材料与过氧化氢反应时形成。
