Brønsted acid sites based on penta-coordinated aluminum species

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.


Supplementary Note 1
The 1 H MAS NMR spectra of dehydrated SA/10 and SA/50 (Supplementary Fig. 1a and b) are dominated by a strong signal of SiOH groups (δ1H = 1.9 ppm), which masks that of BAS. After loading with ammonia, BAS protonates adsorbed ammonia to ammonium ions, leading to a strong signal at δ1H = 6.7 ppm (Supplementary Fig. 1c and d). It evidences the presence of BAS in ASA materials under study. Moreover, the densities of BAS in SA/10 and SA/50 (9.7 and 13.1 mmol.g -1 ) have been determined from the integrated intensity of ammonium signals.

Supplementary Note 2
We estimated the quadrupolar parameters of Al IV and Al V nuclei in ASA loaded with ammonia from the 27 Al MQMAS NMR 2D spectra ( Supplementary Fig. 2) and the direct excitation 27 Al 1D spectrum acquired under quantitative conditions. 3,5 Based on previous work, 6  where I = 5/2 is the spin quantum number of 27 Al and v0 its Larmor frequency. Then, the quadrupolar coupling constant can be estimated from the SOQE value since: (1) where, the range of asymmetry parameters is limited to 0 ≤ ηQ ≤ 1, and thus the range of the quadrupolar coupling constant is restricted to SOQE ≤ CQ ≤ 1.15.SOQE. As a result, the SOQE parameter directly determined from the MQMAS 2D spectra can be used as an initial input CQ value for the fitting and deconvolution of the 27 Al 1D MAS spectra acquired under quantitative conditions (see Supplementary Fig. 3

), whereas Supplementary Equation 2
provides an initial input value for the isotropic chemical shift value. The fits of the 1D NMR spectra were carried out using DMfit software 7 using as input parameters those determined from the analysis of the corresponding 2D MQMAS spectrum. 8 Supplementary Table 2 However, Al IV and Al V sites have similar CQ values (see Supplementary Table 2) and similar number of protons in their vicinity (see Fig. 4). Under those conditions, the relative amount of BAS-Al IV and BAS-Al V can be estimated from the relative integrated intensities of the deconvoluted signals of those sites in 27 Al-{ 1 H} D-HMQC experiment (Fig. 3). The fits shown in Supplementary Fig. 4 used the same parameters determined previously by fitting the 1D MAS ( Supplementary Fig. 3), and only the fractions of the aluminum species were allowed to vary. Combined with the densities of BAS (0.097 and 0.131 mmol.g -1 for SA/10 and SA/50) determined from Supplementary Fig. 1, the population of BAS-Al IV and BAS-Al V are listed in Supplementary Table 2. Clearly, when increasing Al content from 10 to 50 %, (i) the density of BAS-Al V increases from 0.041 to 0.071 mmol.g -1 , but (ii) that of BAS-Al IV also increases from 0.057 to 0.063 mmol.g -1 . Obviously, on ASA samples the formation of BAS-Al V does not replace that of BAS-Al IV , and both BAS-Al IV and BAS-Al V co-exist on the surface.

Supplementary Note 4
The indicates that Al coordination is unchanged before and after ammonia loading on samples.
Therefore, Si-OH⋅⋅⋅Al V and Si-OH⋅⋅⋅Al IV coordinations remained the same after the protonation of ammonia. No Si-OH⋅⋅⋅Al IV has been transferred to Si-OH-Al V permanently after the adsorption of ammonia. The Al V -NH4 + cross-peak (30, 6.7) ppm in Fig. 3 was directly from the protonated ammonia on Si-OH⋅⋅⋅Al V BAS (Si-O -(NH4) + ⋅⋅⋅Al V ) rather than bridging Si-O -(NH4) + -Al V (transferred from Si-OH⋅⋅⋅Al IV BAS). These NMR results indicate that no permanent bridge between Al site and silanol is formed after the deprotonation of BAS via the adsorption of ammonia.