Solvent-free oxidation of benzyl alcohol using Au-Pd catalysts prepared by sol immobilisation
Solvent-free oxidation of benzyl alcohol using Au-Pd catalysts prepared by sol immobilisation
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DOI：
10.1039/b900151b
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发表时间：
2009-01-01
影响因子：
3.3
通讯作者：
Hutchings, Graham J.
中科院分区：
文献类型：
--
作者：
Dimitratos, Nikolaos;Lopez-Sanchez, Jose Antonio;Hutchings, Graham J.
我们报道了用三种不同的制备策略，通过溶胶固定化技术制备了负载在TiO2和碳上的Au-Pd纳米晶催化剂；即，同时形成两种金属的溶胶或初始形成其中一种金属的种子溶胶，然后在其中添加第二种金属的涂层溶胶的单独步骤。利用透射电子显微镜和x射线光电子能谱对催化剂进行了结构表征。对催化剂在无溶剂条件下氧化苯甲醇进行了评价。与浸渍法制备的催化剂相比，用溶胶固定化技术制备的催化剂表现出更高的活性，特别是当金属浓度较低时。Au-Pd催化剂的活性均高于相应的单金属负载Au或Pd催化剂。对于1 wt%的Au-Pd/TiO2，在制备中金属添加的顺序对选择性或活性没有显著影响。然而，在等转化过程中，1 wt% Au-Pd/碳催化剂比tio2负载的催化剂更有活性，但对苯甲醛的选择性更低。此外，对于碳负载型催化剂，金属加成的顺序对活性的影响非常显著。碳负载型催化剂受热处理的影响也更明显，例如，在400℃下煅烧导致活性下降一个数量级，而tio2负载型催化剂的活性下降50%。甲苯被观察到是反应的副产物，并且已经确定了使其形成最小化的条件。甲苯和苯甲醛是由最初的苯基中间体通过吸附的可氢化或氧化的苄基物质相互竞争的平行反应形成的。因此，使催化剂表面氧气可用性最大化的条件有利于苯甲醛的合成。
We report the preparation of Au-Pd nanocrystalline catalysts supported on TiO2 and carbon prepared via a sol-immobilisation technique using three different preparation strategies; namely, simultaneous formation of the sols for both metals or initial formation of a seed sol of one of the metals followed by a separate step in which a coating sol of the second metal is added. The catalysts have been structurally characterised using a combination of transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts have been evaluated for the oxidation of benzyl alcohol under solvent-free conditions. The catalysts prepared using the sol immobilisation technique show higher activity when compared with catalysts prepared by impregnation, particularly as lower metal concentrations can be used. The Au-Pd catalysts were all more active than the corresponding monometallic supported Au or Pd catalysts. For 1 wt% Au-Pd/TiO2 the order of metal addition in the preparation was not observed to be significant with respect to selectivity or activity. However, the 1 wt% Au-Pd/carbon catalysts are more active but less selective to benzaldehyde than the TiO2-supported catalysts when compared at iso-conversion. Furthermore, for the carbon-supported catalyst the order of metal addition has a very marked affect on activity. The carbon-supported catalysts are also more significantly affected by heat treatment, e. g. calcination at 400 degrees C leads to the activity being decreased by an order of magnitude, whereas the TiO2-supported catalysts show a 50% decrease in activity. Toluene is observed as a by-product of the reaction and conditions have been identified that minimise its formation. It is proposed that toluene and benzaldehyde are formed by competing parallel reactions of the initial benzyl intermediate via an adsorbed benzylidene species that can either be hydrogenated or oxidised. Hence, conditions that maximise the availability of oxygen on the catalyst surface favour the synthesis of benzaldehyde.