Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment,Journal of the American Chemical Society

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Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2018-09-30 , DOI: 10.1021/jacs.8b08630
Fan Zhang , Kai C. Szeto ₁ , Mostafa Taoufik ₁ , Laurent Delevoye ₂ , Régis M. Gauvin ₂ , Susannah L. Scott

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Inorganic oxides play a crucial role in the activation of atomically dispersed metal oxides for catalytic olefin transformations, but the inefficient activation processes remain poorly understood. Activation of methyltrioxorhenium (MTO) for propene metathesis via its deposition on the surface of γ-Al2O3 typically results in <5% active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina ( a-Al2O3) results in ca. 4× more activity and at least 10× more productivity. On both types of alumina, metathesis is initiated only at specific sites, whose availability limits the catalytic activity. While the two aluminas have similar total numbers of Lewis acid sites, the less crystalline support activates twice as many grafted MTO sites. Interestingly, a-Al2O3 has nearly double the number of strong Lewis acid sites. However, the number of active sites is ca. 10× lower than the total number of strong Lewis acid sites, and metathesis proceeds even when most are occupied by pyridine. DQSQ and D-HMQC 1H and 27Al solid-state NMR reveal that many Lewis acid sites are co-located with surface hydroxyl groups, which prevent activation and/or cause rapid deactivation. Undercoordinated Al sites on dominant (110) facets, which retain hydroxyl groups under catalyst preparation conditions, are therefore unlikely to lead to stable active sites. In contrast, the minor (100) facets of γ-Al2O3, which are completely dehydroxylated, contain strongly Lewis-acidic five-coordinate Al sites that are necessarily remote from surface hydroxyl groups. Such sites, which are relatively more abundant on less well-crystallized aluminas, are inferred to be responsible for generating stable metathesis sites.

中文翻译：

甲基三氧铼对大部分无定形氧化铝的复分解活性和稳定性的增强：局部嫁接环境的作用

无机氧化物在原子分散的金属氧化物催化烯烃转化的活化中起着至关重要的作用，但对低效的活化过程仍然知之甚少。用于丙烯复分解的甲基三氧铼 (MTO) 通过其沉积在 γ-Al2O3 表面上的活化通常会导致 <5% 的活性位点，并且这些位点会迅速失活。用较少结晶（主要是无定形）的氧化铝 (a-Al2O3) 简单替代载体导致约。活动增加 4 倍，生产力至少增加 10 倍。在这两种类型的氧化铝上，复分解仅在特定位点开始，其可用性限制了催化活性。虽然两种氧化铝具有相似的路易斯酸位点总数，但较少结晶的载体激活的接枝 MTO 位点数量是其两倍。有趣的是，a-Al2O3 的强路易斯酸位点数量几乎翻了一番。然而，活性位点的数量是大约。比强路易斯酸位点总数低 10 倍，即使大部分被吡啶占据，复分解也会进行。DQSQ 和 D-HMQC 1H 和 27Al 固态 NMR 显示许多路易斯酸位点与表面羟基位于同一位置，这会阻止活化和/或导致快速失活。因此，在催化剂制备条件下保留羟基的主要 (110) 面上的未配位 Al 位点不太可能导致稳定的活性位点。相比之下，γ-Al2O3 的次要 (100) 晶面完全脱羟基，包含强路易斯酸性五配位 Al 位点，这些位点必然远离表面羟基。这样的网站，

更新日期：2018-09-30

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