铂/氧化铝/氧化硅介孔材料的限域与载体效应及其羰基化合物液相加氢反应性能的研究

Pt catalysts are active for liquid-phase hydrogenation of carbonyl compounds, especially for asymmetric hydrogenation of alfa-functionalized ketones after chirally modified with cinchona alkaloids. Higher than 90% ee value can be achieved for the asymmetric hydrogenation of pyruvate esters on chirally modified Pt/alumina catalyst in acetic acid. For the enantioselective hydrogenation of ethyl 2-oxo-4-phenylbutyrate (EOPB), of which the hydrogenated product is the intermediate for synthesis of ACE inhibitors, about 80% ee value could be obtained with chirally modified Pt/alumina catalyst under optimal conditions such as in acetic acid. However, traditional Pt/alumina catalysts will be easily peptized under the acidic conditions, resulting in poor reusablity of Pt/alumina catalyst for the asymmetric hydrogentation of alfa-functionalized carbonyl compounds in acetic acid..In addition, enormous attention has been paid to confinement effect of mesopores on catalysis. The activity or selectivity may be changed via the confinement effect. In this project we will introduce highly-dispersed amorphous alumina into confined mesopores of ordered mesoporous silicas such as SBA-15, KIT-6 and SBA-16, which have relatively high hydrothermal ability and are quite stable under acidic conditions, to prepare mesoporous silica-alumina composites. The stability of alumina composites will be increased by modulating preparation parameters. Furthermore, the mesopore confinement effect might affect the catalytic performance. .Pt nanoparticles will be supported on the mesoporous silica-alumina composites to prepare Pt/alumina/mesoporous silica catalysts for the chiral hydrogenation of ethyl pyruvate and EOPB after chirally modified with cinchona alkaloids. Then to broaden the substrate scope, selective hydrogenation of a typical alfa,beta-unsaturated aldehyde, such as cinnamaldehyde, will be carried out. The effect of coordination state of Al species (four-coordinated, five coordinated or six-coordinated), the host-guest interaction, the electronic state of Pt nanoparticles and the mesopore size (4.6-30 nm of SBA-15) or symmetry (P6mm symmetry of SBA-15, Ia3d symmetry of KIT-6 or Im3m symmetry of SBA-16) on the target reactions will be investigated in detail. Moreover, based on the catalytic performance and the characterization results including 27Al NMR, IR spectra of pyridine and CO adsorption, XPS, EXAFS etc., the host-guest interaction and its effect on the electronic state of Pt nanoparticles will be studied as well. In combination with the IR results of reactant adsorption, the TPD results of intermeidate or product desorption and the relevant TPSR information about the reaction intermediates, the mechamisms about adsorption, activation and hydrogenation of carbonyl compounds on the Pt nanoparticles confined inside the mesopores will be proposed.

本项目拟在水热稳定性和耐酸性都相对较好的氧化硅介孔材料孔道限域空间中涂覆氧化铝层，制备介孔氧化硅-氧化铝复合材料，并负载铂纳米粒子，制备铂/氧化铝/氧化硅介孔材料催化剂。这样不仅可以保留氧化铝负载铂催化剂的优良性能，还可以引入氧化硅介孔材料的孔道限域效应，同时也可以通过制备方法的调变提高氧化铝负载铂催化剂的耐酸性。铂/氧化铝/氧化硅介孔材料催化剂经过手性修饰后用于丙酮酸乙酯和2-氧-4-苯基丁酸乙酯的不对称氢化反应或直接用于肉桂醛选择加氢反应。将详细考察限域空间中铝配位状态对铂纳米粒子电子状态的影响、探明主客体氧化物相互作用及其对铂纳米粒子电子状态的影响机制、研究氧化硅介孔材料孔道尺寸及孔道对称性对负载铂纳米粒子上醛酮化合物加氢反应性能的影响，提出限域空间中铂纳米粒子表面醛酮化合物吸附、活化和加氢反应机理。申请者近年来开展了相关前期工作，研究积累为项目顺利完成打下了扎实的基础。

传统氧化铝负载铂催化剂在酸性条件下易胶溶，导致性能优异的Pt/Al2O3催化剂在酸性条件下对醛酮化合物液相加氢反应重复使用效果较差。针对以上问题，本项目在水热稳定性和耐酸性都较好的氧化硅介孔材料（如SBA-15、MCM-41、SBA-16和KIT-6等）孔道中，通过固相研磨、超声浸渍等方法在介孔氧化硅孔道中涂覆无定形氧化铝层制备介孔氧化硅-氧化铝复合材料。介孔氧化硅-氧化铝复合材料不仅保留了介孔氧化硅的结构对称性，还引入了氧化铝的优良性能。除了氧化铝原来的六配位铝物种外，还产生了四配位的铝物种，表明部分铝原子进入介孔氧化硅的孔壁，通过和氧化硅表面的硅羟基作用形成了Si-O-Al-O-Si四配位铝物种，因此介孔复合材料不仅保留了缘自氧化铝的大量L酸中心，还在氧化硅-氧化铝界面处产生了B酸中心。以介孔氧化硅-氧化铝复合材料为载体负载铂纳米粒子后，经过手性分子辛可尼丁修饰后用于2-氧-4-苯基丁酸乙酯的多相不对称氢化或不经手性修饰用于不饱和醛酮化合物（如苯甲醛和肉桂醛）的液相选择加氢反应。考察了介孔氧化硅不同结构对称性和不同孔径对铂/氧化铝/介孔氧化硅上不饱和醛酮化合物选择加氢性能的影响，发现具有直型孔道的SBA-15由于具有较大的介孔孔径，氧化铝极易在其介孔孔道中分散，且由于载体具有较多的Ｌ酸中心和少量Ｂ酸中心，负载的铂纳米粒子表面呈现部分缺电子状态，经过辛可尼丁修饰后用于2-氧-4-苯基丁酸乙酯的不对称氢化反应时性能最佳，TOF可以达到11000 h-1以上，光学选择性高于87%ee。而介孔氧化硅-氧化铝复合材料负载的单金属铂催化剂上肉桂醛选择加氢性能都较差，添加少量Fe后可以进一步调变负载铂纳米粒子表面的电子性质，有助于对不饱和醛酮化合物中羰基官能团的选择吸附，提高羰基官能团加氢的选择性；并与铂/氧化钛/介孔氧化硅以及铂/纳米碳化硅-碳复合材料催化体系进行了对比，发现载体和第二金属组元都会影响铂粒子表面的电子状态，铂纳米粒子表面的部分缺电子状态有利于肉桂醛中羰基官能团的选择加氢。