# A focus on the observation of bridging SiOHA1 Brønsted acid sites in amorphous silica-alumina

Olivier Lafon

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Olivier Lafon. A focus on the observation of bridging SiOHA1 Brønsted acid sites in amorphous silica-alumina. Chemical Synthesis, 2025, 5 (1), pp.23. 10.20517/cs.2024.141. hal-05504701

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# A focus on the observation of bridging SiOHAl Brønsted acid sites in amorphous silica-alumina

Olivier Lafon*

Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, Lille 59000, France.

*Correspondence to: Prof. Olivier Lafon, Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, 42 rue Paul Duez, Lille 59000, France. E-mail: olivier.lafon@univ-lille.fr

**How to cite this article:** Lafon, O. A focus on the observation of bridging SiOHAl Brønsted acid sites in amorphous silica-alumina.
*Chem. Synth. **2025**, *5*, 23. https://dx.doi.org/10.20517/cs.2024.141

**Received:** 30 Sep 2024 **First Decision:** 5 Nov 2024 **Revised:** 3 Dec 2024 **Accepted:** 4 Jan 2025 **Published:** 8 Feb 2025

**Academic Editor:** Teng Ben **Copy Editor:** Pei-Yun Wang **Production Editor:** Pei-Yun Wang

**Keywords:** Amorphous silica-alumina, Brønsted acid sites, solid-state NMR

Aluminosilicate catalysts, including zeolites and amorphous silica-alumina (ASA), are popular solid-acid catalysts, which are widely employed in petroleum refining, biomass conversion and organic synthesis[1,2]. In particular, ASA is advantageous for the conversion of large molecules, notably those derived from biomass, which are often complex and viscous, and hence, diffuse slowly into the micropores of zeolites. ASA can be prepared by different approaches, including sol-gel synthesis[3], coprecipitation[4], flame-spray pyrolysis[5,6] and atomic-layer deposition[7] but they exhibit milder Brønsted acidity than zeolites.

Despite extensive studies, the structure of Brønsted acid sites (BAS) in ASA is still highly debated. It has been proposed, notably on the basis of molecular modeling studies, that ASA contains BAS, called pseudo-bridging silanols (PBS), consisting of a silanol in close proximity to an aluminum atom, which stabilizes the silanolate anion and, hence, increases the Brønsted acidity of the PBS[8-10]. In contrast with bridging Si-O(H)-Al BAS found in zeolites, there is a lack of covalent bonds between O and Al atoms in PBS. Solid-state nuclear magnetic resonance (NMR) experiments, which allowed to probe proximities between ¹H and ²⁷Al nuclei and to measure ¹H-¹⁷O distances in ASA, provided experimental evidence for the existence of PBS in these catalysts and their ability to protonate ammonia[11-13].

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© The Author(s) 2025. **Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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<figure><img src="image_5.png" /><figcaption>**Figure 1.** (A) ¹H NMR spectra under magic-angle spinning of (i) thermally activated ZSM-5, (ii) protonated and (iii) deuterated ASA: as-prepared catalyst (red line) and after H/D exchange with benzene (dashed blue line). The shaded spectra correspond to the NMR spectra of protons close to Al atoms, i.e., BAS. It was obtained as the difference between ¹H NMR spectra recorded with and without the recoupling of ¹H-²⁷Al dipolar interactions; (B) Solid-state NMR measurement of ¹H-²⁷Al distances for (i and ii) BAS and (iii and iv) other silanol groups in (i and iii) thermally activated ZSM-5 and (ii and iv) partially deuterated ASA. Reprinted with permission from Ref. [17]. Copyright 2023, Royal Chemical Society. NMR: Nuclear magnetic resonance; ASA: amorphous silica-alumina; BAS: Brønsted acid sites.</figcaption></figure>

Nevertheless, the existence of BAS in ASA was suggested by their catalytic activity for the propane cracking and H/D exchange and the presence of sites with distinct acidities, as detected by infrared spectroscopy using probe molecules[14,15]. Recently, solid-state NMR experiments were conducted at an ultra-high magnetic field (35.2 T), i.e., a ¹H Larmor frequency of 1.5 GHz, to observe the proximity between ¹H and ²⁷Al nuclei and provided the first direct spectral evidence for the existence of BAS in ASA[16]. However, even at this ultra-high magnetic field, the ¹H NMR spectra of ASA exhibited limited resolution owing to the amorphous nature of these catalysts and the distribution of proton local environments.

To confirm the presence of BAS in ASA, Salvia *et al.* have compared the one-dimensional (1D) ¹H NMR spectra of thermally activated ZSM-5 and ASA [Figure 1A][17]. As seen in Figure 1A(i), the ¹H NMR spectrum of ZSM-5 exhibits two resolved resonances at 4.2 and 2.0 ppm, assigned to BAS and non-acidic silanol groups distant from aluminum atoms[18]. Conversely, the ¹H NMR spectrum of ASA [Figure 1A(ii)] is dominated by a narrow peak of silanol groups, including PBS but also exhibits a broad signal extending from 2 to 10 ppm. This broad signal must stem from hydroxyl groups with a distribution of local environment, which is consistent with the amorphous nature of ASA. Furthermore, these hydroxyl groups are close to ²⁷Al nuclei. The intensity of this broad signal is also reduced when ASA is exposed to deuterated benzene, suggesting that these protons are exchanged with the deuterons of C₆D₆. To facilitate the observation of these highly acidic sites in ASA, the authors partially deuterated this catalyst with deuterated water and then exposed it to isotopically unmodified benzene. This strategy allowed them to label selectively with protons the Brønsted sites, which are active towards H/D exchange with benzene [Figure 1A(iii)].
Then, they compared the dephasing of ¹H NMR signals under ¹H-²⁷Al dipolar interactions in ZSM-5 and ASA [Figure 1B]. Using this approach, they measured H-Al distance of 2.5 ± 0.3 Å for the BAS protons in ZSM-5, which resonates at 4.2 ppm [Figure 1B(i)]. A similar distance of 2.4 ± 0.2 Å was obtained for protons resonating at the same isotropic chemical shift in deuterated ASA exposed to benzene [Figure 1B(ii)]. These NMR observations confirm the existence of BAS in ASA. Interestingly, for the silanol groups, negligible dephasing was observed for ZSM-5, whereas distances ranging from 4.2 to 5.7 Å were found for ASA [Figure 1B(iii) and (iv)]. These distances are consistent with those predicted using molecular modeling for PBS in ASA.

In conclusion, Salvia *et al*. convincingly demonstrate, using solid-state NMR and selective isotope labeling, that besides PBS, ASA contains BAS similar to those found in zeolites, which can activate C-H bonds[17]. The employed NMR experiments are expected to be useful in improving the acidity of ASA in a rational way. In particular, it will be beneficial to apply them to understand the structure of BAS in ASA with higher aluminum content, which exhibits higher catalytic activity toward H/D exchange[19].

## DECLARATIONS

### Authors' contributions

The author contributed solely to this manuscript.

### Availability of data and materials

Not applicable.

### Financial support and sponsorship

The Chevreul Institute is gratefully acknowledged for its support of CPER projects funded by the "Ministère de l'Enseignement Supérieur et de la Recherche", the region "Hauts-de-France", the ERDF program of the European Union and the "Métropole Européenne de Lille". Olivier Lafon also acknowledges financial support from contract ANR-22-CE29-0007-01 (IVAN) and CNRS international research project (IRP) CYAN.

### Conflicts of interest

The author declared that there are no conflicts of interest.

### Ethical approval and consent to participate

Not applicable.

### Consent for publication

Not applicable.

### Copyright

©The Author(s) 2025.

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<figure><img src="image_6.png" /><figcaption>Olivier Lafon</figcaption></figure>

Professor Olivier Lafon earned his BSc in 2002 from Ecole Normale Supérieure de Lyon and his PhD in 2006 at the University of Paris-Saclay. Following his doctoral studies, he worked as a postdoctoral fellow at CEA Saclay. From 2007 to 2011, he served as an assistant professor at Centrale Lille. In 2010, he obtained his habilitation in 2010 at the University of Lille. Since 2011, he has been a professor at Lille, where he also leads the “NMR and inorganic materials” team in the UCCS research institute.

Professor Lafon received the Magnetic Resonance in Chemistry Award for Young Scientists from the Euromar scientific committee in 2013. In 2016, he was named a fellow of the Institut Universitaire de France (IUF). He currently directs the Lille high-field NMR facility, part of the INFRANALYTICS infrastructure, and coordinated the installation of a 1.2 GHz NMR spectrometer in Lille. Additionally, he leads the French network of research infrastructures in physical sciences and engineering and serves on the editorial board of the journal *Solid-State Nuclear Magnetic Resonance*.

His research focuses on advancing solid-state NMR spectroscopy to uncover unique insights into the structure-property relationships of materials, with applications in energy, bio-economy and health.