Brønsted acid sites based on penta-coordinated aluminum species

Wang, Zichun; Jiang, Yijiao; Lafon, Olivier; Trébosc, Julien; Kim, Kyungduk; Stampfl, Catherine; Baiker, Alfons; Amoureux, Jean‐Paul; Huang, Jun

doi:10.1038/ncomms13820

Cited by 122 publications

(161 citation statements)

References 55 publications

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“…As shown in Figure , three signals at 56.7, 27.7, and 3.6 ppm could be identified, which were assigned to Al IV , Al V , and Al VI species, respectively. It has been widely accepted that BAS are formed by SiOH groups having Al IV /Al V species in the vicinity . The presence of Al V and Al VI species is often related to the formation of Al domains in the ASAs, which may lead to the formation of surface LAS …”

Section: Resultsmentioning

confidence: 99%

“…Recently, ASAs before and after ammonia loading has been characterized by 27 Al‐{ 1 H} dipolar‐mediated heteronuclear multiple quantum correlation (D‐HMQC) two‐dimensional (2D) NMR experiments . Upon loading with ammonia, the correlation between the occurrence of Al IV /Al V species and NH 4 + ions revealed the formation of BAS based on the interaction of Al IV /Al V with neighboring SiOH groups, which has been evidenced by various DNP‐enhanced homonuclear and heteronuclear correlation NMR experiments .…”

Section: Introductionmentioning

confidence: 99%

“…Pure silica possesses no or only very weak acidity. Incorporation of Al atoms into the silica network/framework promotes the formation of Brønsted acid sites (BAS) . BAS can protonate hydrocarbons, forming carbocation intermediates, which promote important reactions, such as cracking, isomerization, alkylation, and aromatization .…”

Section: Introductionmentioning

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: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

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 formation of BAS in silica-aluminas is based on aluminum centers distributed in the silica framework or network 6,[16][17][18][19][20] , as (i) a tetra-coordinated aluminum species (Al IV ), replacing a Si 4+ atom in the zeolite framework, can introduce a negatively charged framework oxygen to be balanced by a proton, resulting in bridging OH groups 6,17,18 , and (ii) the interaction between Al IV atoms and neighboring silanols in the silica network can provide acidic OH groups, acting as BAS in ASA 5,[21][22][23][24] . In crystalline zeolites, it is well accepted that increasing the Si/Al ratio can enhance the BAS strength by increasing the overall electronegativity 25 .…”

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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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“…The 27 Al MAS NMR spectra of Pd/SA is similar to that reported for amorphous silica-alumina supports in literature. [33,36,37] Since the metal-support interaction induced by acidic support (e. g. alumina and silica-alumina) can strongly influence the catalytic performance of Pd catalysts, [3,9,19] the surface acidity of supported Pd catalysts are characterized by the temperature programmed desorption of ammonia (NH 3 -TPD) experiments. Often, Al VI species in alumina and silica-alumina are associated to the formation of surface Lewis acid sites (LAS).…”

Section: Resultsmentioning

confidence: 99%

The Comparative Effect of Particle Size and Support Acidity on Hydrogenation of Aromatic Ketones

et al. 2019

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A comparative study was reported for both the effects of shape-confined cubic Pd particle size (8, 13, and 21 nm) and surface property of most commonly used supports (SiO 2 , Al 2 O 3 , and silica-alumina) on catalytic performance in the chemoselective hydrogenation of three model bio-oil chemicals (benzaldehyde, acetophenone, and butyrophenone). The results showed that the size of Pd particles could be more associated with the hydrogenation reaction than acidities of the supports. Smaller size of Pd particles, regardless of the type of the support, provided the higher catalytic performance. XPS data showed that the electronic properties of Pd particles were similar, therefore, the possible reasons were the higher fraction of Pd atoms on corner in smaller particles, the lower accessibility of hydrogen atom to reactant on bigger particles, and the more low-coordinated sites in the small-size particles due to the short-range ordering. In addition, Pd/SA catalysts (Brønsted acid sites on the support) showed the highest conversion and TOF compared to Pd/Al 2 O 3 and Pd/SiO 2 catalysts. This might be due to the enhanced the diffusion rates of the chemicals on the surface of the catalysts although they could not induce the ionic effect from the metal surface. Pd/ SiO 2 catalysts performed better than Pd/Al 2 O 3 catalysts (Lewis acid sites on the support). The flexible SiOH groups on surface made the easy interaction with the metal particles and promote the reaction.[a] Dr.

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Brønsted acid sites based on penta-coordinated aluminum species

Cited by 122 publications

References 55 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

The Comparative Effect of Particle Size and Support Acidity on Hydrogenation of Aromatic Ketones

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