Study on Photo-Excitation Process of Mo Species Anchored on Mesoporous Support

本研究以介孔二氧化硅(Mo/MCM-41)为载体，制备了高分散Mo催化剂，并对其进行了表征。以1-丁烯为反应物考察了催化剂的光复分解活性。光催化的活性中心是金属卡宾物种，因此选择性很难控制。研究了光激发过程中的结构变化、钼氧化物的失活过程以及钼原子分布对光复分解活性的影响。我已经制备了一种可以在辐照和反应气体通道下使用的原位X射线吸收精细结构(XAFS)池。用原位XAFS技术观察了1-丁烯光解反应中Mo原子周围结构的变化：(1)通过改变烷基铵(C10-C16)的碳数来控制MCM-41的中孔直径。以MoCl5为前驱体制备了Mo/MCM-41催化剂。在这些催化剂中，Mo在MCM-41上高度分散。XAFS分析表明，在MCM-41上形成的Mo团簇由少量原子组成。此外，MCM-41中孔直径越大，Mo团簇尺寸越小。2)在紫外光照射下，Mo/MCM-41上发生了1-丁烯光复分解反应。介孔直径越小，交叉歧化反应活性越高。在PF处的XAFS原位测量表明，MCM-41的中孔直径影响了Mo=O位的消耗速率。也就是说，光复分解活性取决于活性中心在孔中的分布。(3)部分预还原态Mo/MCM-41催化剂的光复分解活性和交叉复分解活性都得到了提高。研究结果表明，Mo(4+)、Mo(5+)等还原价态物种能够激活Mo-O-Mo桥位的烯烃光复分解反应。

In this research, highly dispersed Mo catalysts supported on mesoporous silica (Mo/MCM-41) were prepared and characterized. The photo-metathesis activity of the catalyst was examined by using 1-butene as reactant. The active center of the photo-catalysis is the metal-carbene species, and hence, the control of the selectivity is difficult. The structural change in photo-excitation, the deactivation process of Mo-oxide and the effect of Mo atom distribution on the photo-metathesis activity was also investigated. I have already prepared an in-situ X-ray absorption fine structure (XAFS) cell that could be used under irradiation and reaction gas passage. The structural change around Mo atoms under the 1-butene photo-metathesis reaction was observed by using the in-situ XAFS technique.(1)Mesopore-diameter of MCM-41 was controlled by changing the carbon number of alkyl ammonium (C10-C16). Mo/MCM-41 catalysts were prepared by using MoCl_5 as Mo precursor. Mo was highly dispersed on MCM-41 in these catalysts. XAFS revealed that Mo cluster formed on MCM-41 consists a few atoms. Moreover, the larger the diameter of MCM-41 mesopore became, the smaller the Mo cluster size became.2)1-Butene photo-metathesis reaction proceeded on Mo/MCM-41 under UV irradiation. The activity of cross-metathesis reaction was promoted as the mesopore-diameter became shorter. In-situ XAFS measurement at PF revealed that the mesopore-diameter of MCM-41 affected the consumption rate of Mo=O site. In other words, photo-metathesis activity depended on the distribution of active site in the pore.(3)Photo-metathesis activity as well as cross-metathesis activity was promoted on partially pre-reduced Mo/MCM-41 catalysts. It was suggested that reduced-valence species, such as Mo(4+) and Mo(5+), could activate the Mo-O-Mo bridged site toward olefin photo-metathesis reaction.