Supported MoO_x/Al₂O₃ catalysts were synthesized by incipient-wetness impregnation of aqueous ammonium heptamolybdate, dried at room temperature and 110 °C, and finally calcined at 500 °C in air. The catalysts were spectroscopically characterized with in situ Raman, UV–vis, DRIFTS, and TPSR, both after calcination and during propylene metathesis reaction conditions. Three distinct MoO_x species on the Al₂O₃ support were identified: isolated surface dioxo (O═)₂MoO₂, anchored to the basic HO-μ₁-Al_IV sites (<1 Mo atom/nm²), oligomeric surface mono-oxo O═MoO_4/5 anchored to more acidic HO-μ₁-Al_V/VI sites (1–4.6 Mo atoms/nm²), and crystalline MoO₃ nanoparticles also present above monolayer coverage (>4.6 Mo atoms/nm²). During propylene metathesis, activation proceeds by removal of oxo Mo═O bonds and insertion of ═CH₂ and ═CHCH₃ alkyls, which maintain the surface MoO_x species in the Mo⁶⁺ oxidation state. The surface oligomeric mono-oxo O═MoO_4/5 species easily activate at mild temperatures of 25–200 °C, while the isolated surface dioxo (O═)₂MoO₂ species require very high temperatures for activation (>400 °C). The crystalline MoO₃ nanoparticles decrease the number of accessible activated surface MoO_x sites by their physical blocking. This study establishes the structure–reactivity relationship for olefin metathesis by supported MoO_x/Al₂O₃ catalysts and demonstrates the significant role that the anchoring surface hydroxyl sites on alumina have on the reactivity of surface MoO_x species.

中文翻译：

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Al ₂ O ₃支撑的表面MoO _x位点进行烯烃复分解的分子结构-反应性关系

通过初湿浸渍七钼酸铵水溶液合成负载型MoO _x / Al ₂ O ₃催化剂，在室温和110°C下干燥，最后在500°C下在空气中煅烧。在煅烧后和丙烯复分解反应条件下，用原位拉曼光谱，紫外可见光谱，DRIFTS和TPSR对催化剂进行光谱表征。三个不同的MoO _X在Al物种₂ ö ₃支持被确定：孤立表面二氧代（O =）₂的MoO ₂，锚定到基本HO-μ _1个-Al _IV位点（<1个原子Mo /纳米²），低聚表面单氧O═MoO_4/5锚定到多个酸性HO-μ ₁ -Al _{V / VI}位点（1-4.6沫个/ nm ²），和结晶的MoO ₃纳米颗粒还存在上述单层覆盖（> 4.6沫个/ nm ²）。在丙烯的复分解，通过除去氧代Mo═O键和= CH的插入激活前进₂和= CHCH ₃烷基，其中维持表面的MoO _X在Mo物种⁶⁺氧化态。表面低聚一元氧O═MoO4 _/5物种在25-200°C的温和温度下很容易活化，而孤立的表面二氧杂（O═）₂ MoO ₂物种需要极高的温度才能激活（> 400°C）。结晶的MoO ₃纳米颗粒通过物理阻塞减少了可访问的活化表面MoO _x位置的数量。这项研究建立了负载型MoO _x / Al ₂ O ₃催化剂对烯烃复分解的结构-反应性关系，并证明了氧化铝上的锚固表面羟基对表面MoO _x物种的反应性具有重要作用。