Solid-State NMR Characterization of the Structure and Catalytic Reaction Mechanism of Solid Acid Catalysts

Li, Shenhui; Li, Jing; Zheng, Anmin; Deng, Feng

doi:10.3866/pku.whxb201611022

Cited by 3 publications

(2 citation statements)

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“…Solid-state NMR spectra were recorded for both aluminas after modification by adsorbed pyridine and its desorption at 350 °C, in order to observe the adducts formed between pyridine and the strong Lewis acid sites. While IR monitoring of pyridine adsorption is commonly used for exploring acidity in inorganic supports, corresponding studies using 1 H and 27 Al NMR have attracted much less attention . Instead, NMR studies have focused primarily on the 13 C-, 15 N-, and 2 H-nuclei of the adsorbed pyridine itself. − Recent technological and methodological advances have provided the resolution enhancements to allow us to probe details of the alumina itself at the atomic level …”

Section: Results and Discussionmentioning

confidence: 99%

“…While IR monitoring of pyridine adsorption is commonly used for exploring acidity in inorganic supports, 45 corresponding studies using 1 H and 27 Al NMR have attracted much less attention. 46 Instead, NMR studies have focused primarily on the 13 C-, 15 N-, and 2 H-nuclei of the adsorbed pyridine itself. 47−50 Recent technological and methodological advances have provided the resolution enhancements to allow us to probe details of the alumina itself at the atomic level.…”

Section: Journal Of the American Chemical Societymentioning

confidence: 99%

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Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment

Zhang¹,

Szeto

Taoufik

et al. 2018

J. Am. Chem. Soc.

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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-Al 2 O 3 typically results in <5% active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina (a-Al 2 O 3 ) 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-Al 2 O 3 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 1 H and 27 Al 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-Al 2 O 3 , 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.

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