Low Temperature‐High Selectivity Process over Supported Pd Nanoparticles in Partial Oxidation of Methanol
Low Temperature‐High Selectivity Process over Supported Pd Nanoparticles in Partial Oxidation of Methanol
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DOI：
10.1002/cctc.201100215
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发表时间：
2012-01
期刊：
影响因子：
4.5
通讯作者：
R. Wojcieszak;E. Gaigneaux;P. Ruiz
中科院分区：
文献类型：
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作者：
R. Wojcieszak;E. Gaigneaux;P. Ruiz
气相多相催化中最重要的挑战是在所需过程中达到高选择性。目前，人们做了许多努力来提高高温反应（>350 ℃）的选择性;然而，在高温下，似乎很难达到很高（约100%）的选择性。其主要原因可能是难以（并且可能是不可能）分离催化剂的选择性和非选择性活性位点的反应性。通常，两个位点形成活性相的相同结构的一部分，并且它们位于非常接近的位置。然后，对反应条件下的动力学现象（氧化还原速率、酸碱性质、活性中心和相的重构、新相的形成、化学物种的迁移和反应性、中间体的形成等）进行了讨论。选择性取决于什么，同时涉及并涉及选择性和非选择性位点。由于这些原因，可以说，在气相多相催化中获得高选择性的唯一途径可能是在“低温条件”下工作。众所周知，催化剂颗粒的纳米尺寸可以在新的催化过程的设计中发挥重要作用。尺寸的减小增加了催化活性。小的尺寸可以提供大量的表面原子，这导致每单位量的金属的高催化活性。因为表面原子倾向于配位不饱和，所以存在与该表面相关的大能量。纳米颗粒越小，表面能对系统总能量的贡献就越大。另一方面，当粒子太小时，量子效应胜过经典尺寸效应，赋予粒子新的和意想不到的性质。因此，使用能够生产具有定制尺寸分布的催化活性相的纳米颗粒的合成技术是绝对必要的。相比之下，甲醇的部分氧化（POMeOH）通常在相对高的温度（高于250 ° C）下进行，但是反应的选择性非常低。可以得到几种反应产物，例如甲醛和二甲氧基甲烷（在氧化还原位点上）以及二甲醚和碳氧化物（在碱性和酸性位点上）。我们报告，甲酸甲酯（MF）可以直接从甲醇的氧化反应条件下，具有非常高的选择性，在一步反应中进行的催化剂上形成的Pd纳米粒子负载在TiO 2在低温和常压下。Pd/TiO 2催化剂通过使用油包水微乳液法（方案1）制备。这个方法很有趣-
The most important challenge in gas phase heterogeneous catalysis is to reach high selectivity in a desired process. Many efforts are made at present to improve selectivity in high temperature reactions (>350 8C); however, under high temperature, it seems very difficult to reach a very high (about 100 %) selectivity. The main reason for this is probably the difficulty (and maybe the impossibility) to separate the reactivity of the selective and non-selective active sites of the catalyst. In general, both sites form a part of the same structure of the active phase and they are located in a near proximity. Then, under the reaction conditions, the dynamic phenomena (oxidation and reduction rates, acid and base properties, restructuration of active sites and phases, formation of new phases, migration and reactivity of chemical species, formation of intermediates etc.) on which selectivity depends, are simultaneously involved and concern selective and non-selective sites. For these reasons, it could be stated that probably the only way to get high selectivity in gas phase heterogeneous catalysis is to work under “low temperature conditions”. It is well known that the nanosize of the catalytic particles could play an important role in the design of new catalytic processes. The decrease in size increases the catalytic activity. The small size can provide a large amount of surface atoms, which results in the high catalytic activity per unit amount of metal. Because surface atoms tend to be coordinatively unsaturated, there is a large energy associated with this surface. The smaller the nanoparticles, the larger the contribution made by the surface energy to the overall energy of the system will be. On the other hand, when the particles are too small, quantum effects prevail over the classical size effect giving the particles new and unexpected properties. Therefore, it is absolutely necessary to use synthesis techniques capable of producing nanoparticles of the catalytic active phases with a tailored-size distribution. In contrast, the partial oxidation of methanol (POMeOH) is usually performed at a relatively high temperature (above 250 8C), but the selectivity of the reaction is very low. Several reaction products, such as formaldehyde and dimethoxymethane (on the redox sites) and dimethyl ether and carbon oxides (on the basic and acidic sites), can be obtained. We report that methyl formate (MF) could be produced directly from methanol under the oxidation reaction conditions, with a very high selectivity, in a one step reaction performed on catalysts formed by Pd nanoparticles supported on TiO2 at low temperature and under atmospheric pressure. The Pd/TiO2 catalysts were prepared by using the water-inoil microemulsion method (Scheme 1). This method is interest-