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

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].

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)].

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.

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].