CP/MAS 29Si NMR

Fig. 12.12 29Si CP-MAS NMR spectra of star gels obtained from Star A and Star B precursors. Trifunctional silicon centers were named with the conventional T notation, where T refers to (SiO) R Si(OR)3 units and n to the number of bridging oxygen atoms surrounding the central silicon atom. In both BSC materials, the T2, T1 and T° relative contribution was...

Chemical shifts in ppm spectra recorded at room temperature. b Isotropic chemical shifts obtained by 29Si CP/MAS NMR experiments. c Experiment failed. d No experiment performed. 

Sepiolite clay (<100 mesh) was heated in air at 120°C in order to remove the zeolitic and surface bound water molecules. The partially dehydrated clay mineral was subsequently exposed to acetone vapor at room temperature for a period of four days. H and 29Si CP MAS-NMR experiments revealed that the acetone molecules penetrated into the microporous channels of the sepiolite structure. Broad line 2H NMR studies using acetone-d6 revealed that, in addition to fast methyl group rotations, the guest acetone-d6 molecules were also undergoing 2-fold re-orientations about the carbonyl bond. The presence of acetone-d6 molecules adsorbed on the exterior surfaces of the sepiolite crystals was also detected at room temperature. 

The 29Si CP/MAS-NMR spectrum of untreated sepiolite is presented in Fig. 2a. Three well-resolved resonances of approximately equal intensity occur at -92.7, -94.3 and -98.2 ppm, as well as a significantly less intense resonance (<5%) around -85 ppm, in excellent agreement with previous reports [6,23-24], The resonance at -92.7 ppm is assigned to type 2 (near edge) silicon nuclei, as denoted in Fig. la. Furthermore, the resonances at -94.3 and -98.2 ppm are assigned to type 1 (edge) and type 3 (center) silicon nuclei, respectively [25],... 

Fig. 2 29Si CP/MAS-NMR spectra of sepiolite a) unheated b) heated to 120°C in air for 20 h and c) sample b after exposure to acetone vapor for 4 days.

Finally, the 29Si CP/MAS-NMR spectrum of a partially dehydrated sepiolite that was subsequently exposed to acetone vapor is presented in Fig. 2c, and is strikingly similar to the spectrum of the original, untreated sepiolite (Fig. 2a). Since zeolitic water molecules are not present in this sample, and in light of the discussion of the partially dehydrated sepiolite sample, it appears that the acetone molecules have penetrated inside the microporous channels and reversed the structural changes that were caused by partial dehydration. Thus Fig. 2c confirms that acetone molecules enter the microporous channels of sepiolite, and are not simply adsorbed on the crystallite exterior surfaces. 

Figure 3.4 29Si CP MAS NMR spectra ofLSA silica gel before (a) and after (b and b ) H/D exchange in liquid D20, (b) plotted on the same intensity scale as (a), (b ) at 26 times this intensity scale. Taken from ref. (51) with permission.

Figure 5.10 shows the structural resolution obtainable in a 29Si CP MAS NMR experiment in the most favourable cases. The low intensity peak at -89 ppm is assigned to geminal silanol sites, the peak at -100 ppm to single silanol sites and the peak at -109 ppm to siloxane bridges. 

Surface silanol types with their 29Si CP MAS NMR and FTIR peak position and names... 

It is the merit of Maciel and Sindorf21,22 to have studied these effects in detail, and to have provided methods and algorithms to perform quantitative measurements with 29Si CP MAS NMR. The final outcome of this study is shown in figure 5.11. 

Figure 9.6 Hydrated silica, modified in toluene solvent, air cured, (a) 29Si CP MAS NMR spectrum (b) 13C CP MAS NMR spectrum.

After characterization of the physisorption, we wish to get information on the ultimate chemical structure of the coating. Spectroscopic analysis of the modified substrate is used to complement the indirect analytical data, reported above. Further elucidation of the chemical bonding of the silanes to the surface is obtained from 29Si CP MAS NMR. With this technique, the formation of siloxane bonds with the surface can be modelled. 

Figure 9.19 29Si CP MAS NMR spectra of modified silica (a) APTS modified, vacuum cured (b) APTS modified air cured (c) APDMS modified, air cured.

Si CP MAS NMR peak positions, with relative contributions of bonding form, for APTS and APDMS modified silica gel... 

Figure 9.20 29Si CP MAS NMR spectra ofmesoporous silica gel, pretreated at 473 K and modified with APTS in toluene solvent, after curing under vacuum at indicated temperature taken from ref. (22) with permission.

In figure 9.33 the 29Si CP MAS NMR spectra of the APTS modified silica samples with variable pretreatment temperature are displayed. The spectrum contains two broad bands, which may be decomposed into three or four peaks. The surface Si atoms are found in the (-85, -125 ppm) region, while the (-45, -80 ppm) signals are due to silane Si atoms. Band assignments of the silane Si region have been made previously as indicated in table 9.8. 

Figure 9.33 29Si CP MAS NMR spectra of APTS modified silica gel with variable silica pretreatment temperature (a) 473 K, (b) 673 K, (c) 973 K.

Deuterated silicas were modified with APTS. 29Si CP MAS NMR spectra of the deuterated modified substrates are analogous to the spectra of the non-predeuterated... 

Figure 9.35 29Si CP MAS NMR spectra of silica gel after pretreatment at 973 K, deuteration and modification with APTS, measured with variable contact time, (a) 8 ms, (b) 5 ms, (c) 2.5 ms, (d) same sample before APTS modification at same scale,with 5 ms contact time.

This perturbation is used as a probe to estimate the concentration of the N-containing surface species, using Partial Least Squares regression (PLS) and to characterize these different species by infrared curve fitting, combined with 29Si CP MAS NMR. 

A distinction between primary and secondary species can be made using 29Si CP MAS NMR. Primary species cause a chemical shift at -36 ppm (reference TMS), whereas secondary species cause a shift at -60 ppm. Following the procedures of Maciel and Sindorf,64,65 which take into account the dependence of the CP (Cross - Polarisation) line intensities with the CP contact time, the percentage of primary and secondary species is calculated. 

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