Wang Feng, Ueda Wataru
CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan.
Chemistry. 2009;15(3):742-53. doi: 10.1002/chem.200801153.
The synthesis and characterization of nanostructured MoO(3) with a thickness of about 30 nm and a width of about 450 nm are reported. The composition formula of the MP (precipitation method) precursor was estimated to be (NH(4))(2)O x MoO(3) x (H(2)O)(0.239). The calcination of the precursor in air afforded nanostructured pellets of the alpha-MoO(3) phase. The nanostructured MoO(3) catalyst exhibited high efficiency in catalyzing the benzylation of various arenes with substituted benzyl alcohols, which were strikingly different to common bulk MoO(3). Most reactions offered >99% conversion and >99% selectivity to monoalkylated compounds. MoO(3) is a typical acid catalyst. However, the benzylation reaction over nanostructured MoO(3) does not belong to the acid-catalyzed type or defect site-catalyzed type, since the catalyst has no acidity and defect site on surface. Characterization with thermal, spectroscopic, and electronic techniques reveal that the catalyst contains fully oxygen-coordinated MoO(6) octahedrons on the surface but partially reduced species (Mo(5+)) within the bulk phase. The terminal oxygen atoms of Mo=O bonds on the (010) basal plane resemble oxygen anion radicals and act as active sites for the adsorption and activation of benzyl alcohols by electrophilic attack. Such sites are indispensable for catalytic reactions since the blocking of these sites by electron acceptors, such as tetracyanoethylene (TCNE), can greatly decrease catalytic activity. This work represents a successful example of combining a heterogeneous catalysis study with nanomaterial synthesis.
报道了厚度约为30 nm、宽度约为450 nm的纳米结构MoO₃的合成与表征。MP（沉淀法）前驱体的组成式估计为[(NH₄)₂O]₀.₁₆₉·MoO₃·(H₂O)₀.₂₃₉。前驱体在空气中煅烧得到α-MoO₃相的纳米结构颗粒。该纳米结构MoO₃催化剂在催化各种芳烃与取代苄醇的苄基化反应中表现出高效率，这与普通块状MoO₃有显著不同。大多数反应对单烷基化化合物的转化率>99%，选择性>99%。MoO₃是一种典型的酸催化剂。然而，纳米结构MoO₃上的苄基化反应不属于酸催化类型或缺陷位催化类型，因为该催化剂表面没有酸性和缺陷位。通过热学、光谱学和电子技术表征表明，催化剂表面含有完全氧配位的MoO₆八面体，但体相内含有部分还原物种（Mo⁵⁺）。(010)基面Mo=O键的端基氧原子类似于氧阴离子自由基，通过亲电攻击作为苄醇吸附和活化的活性位点。这些位点对于催化反应是必不可少的，因为电子受体如四氰基乙烯（TCNE）对这些位点的阻断会大大降低催化活性。这项工作代表了将多相催化研究与纳米材料合成相结合的一个成功例子。
