Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid‐State NMR Spectroscopy

Lang, Swen; Benz, Michael; Obenaus, Utz; Himmelmann, Robin; Hunger, Michael

doi:10.1002/cctc.201600372

Cited by 47 publications

(63 citation statements)

References 31 publications

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“…More recently, Lang et al performed a systematic study of alumina and metal-exchanged zeolites using various probe molecules, such as 2-13 C-acetone, TMPO, and ammonia. [32] By comparison with the results of other probe molecules, these authors suggested that adsorption of ammonia on LAS causes 1 H MAS NMR signals in the chemical shift range of δ 1H = À 0.5 to 3.0 ppm, which agrees well with the current observation of ammonia adsorbed on LAS in ASAs.…”

Section: Resultssupporting

confidence: 81%

“…This signal is accompanied by strong and broad spinning sidebands, which was not observed with SiOH groups or BAS. The strong and broad sidebands indicate that ammonia molecules are strongly adsorbed on these surface sites, i. e. on surface LAS via electron pairs …”

Section: Resultsmentioning

confidence: 99%

“…The strong and broad sidebands indicate that ammonia molecules are strongly adsorbed on these surface sites, i. e. on surface LAS via electron pairs. [32] The signal at δ 1H = 3.0 ppm was further investigated using ASAs with different Al content. As shown in Figure 4, the signal of ammonium ions at δ 1H = 6.7 ppm became more prominent with increasing Al content up to 70 %.…”

Section: Resultsmentioning

confidence: 99%

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NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

et al. 2019

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Amorphous silica‐aluminas (ASAs), possessing both Brønsted acid sites (BAS) and Lewis acid sites (LAS), are important bifunctional catalysts in various industrial applications. Solid‐state NMR spectroscopy has been widely used for characterizing the local structure and the surface sites of ASAs with probe molecules. In this work, four‐, five‐ and six‐coordinated Al species have been observed on the flame‐made ASAs by 27Al MQ MAS NMR experiment. 1H/27Al TRAPDOR MAS NMR experiments confirmed that surface Al species contribute to the formation of BAS and protonate ammonia probe molecules. The adsorption of ammonia on Lewis acidic Al sites (δ1H=3.0 ppm) was evidenced by various 1H MAS NMR experiments on samples dehydrated at different temperatures, allowing the distinction from ammonium ions (δ1H=6.7 ppm) formed at BAS. The signal of ammonia adsorbed on LAS increased with increasing Al content in the ASAs. These properties together with the absence of pore diffusional constraints render the flame‐made ASAs excellent catalysts for cyclohexanol dehydration and the conversion of glyceraldehyde in ethanol to ethyl lactate, outperforming the performance of other ASAs or zeolites.

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Section: Resultssupporting

confidence: 81%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

et al. 2019

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“…The significant increase of the broad autocorrelation signal at (2.6, 5.2) ppm with increasing Al/Si ratio from 1/9 to 5/5 (compare Fig. 1d and Supplementary Note 2) allows assigning it to ammonia adsorbed on LAS [51][52][53][54] . Such assignment is supported by previously reported 2D 27 Al-1 H through-space correlations of SA/10 and SA/50 loaded with ammonia 16 .…”

Section: Articlementioning

confidence: 99%

Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks

Wang

Jiang

et al. 2020

Nat Commun

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Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C-H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis.

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“…Both signals are attributed to the high content of Al cations, particularly, three‐coordinate Al 3+ species as revealed in the above 27 Al MAS NMR study. Notably, the assignment of the signal at δ 31P =37 ppm was considered as TMPO at LAS by Rakiewicz et al., however, was not observed with TMPO‐loaded γ‐Al 2 O 3 . According to Zheng et al .…”

Section: Resultsmentioning

confidence: 99%

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

et al. 2019

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Commercial bioethanol can be readily converted into ethylene by ad ehydration process using solid acids,s uch as Brønsted acidic H-ZSM-5 zeolites,a nd thus,i ti sa ni deal candidate to replace petroleum and coal for the sustainable production of ethylene.Now,strong Lewis acidic extra-framework three-coordinate Al 3+ species were introduced into H-ZSM-5 zeolites to improve their catalytic activity.Remarkably, Al 3+ species working with Brønsted acid sites can accelerate ethanol dehydration at amuchlower reaction temperature and shorten the unsteady-state period within 1-2 h, compared to > 9h for those without Al 3+ species,w hich can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid-state NMR, shows that strong Lewis acidic EFAl-Al 3+ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics-based cycle to produce ethylene.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

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Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid‐State NMR Spectroscopy

Cited by 47 publications

References 31 publications

NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

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