## 3 Results and Discussion
### Structural Characterization of Silicalite-1 Zeolites by MAS NMR Spectroscopy
#### 3.2.2 Characterization of the Interactions OSDA-Zeolite by Bidimensional NMR  
The interactions of the zeolite framework with the organic molecules can be studied by using heteronuclear or homonuclear bidimensional NMR experiments. Figure 8 shows the \({}^{1}\)H-\({}^{31}\)P 2D HETCOR MAS NMR experiment of the TPP-MFI sample containing two main correlation signals at \(\delta^{31}\)P-\({}^{1}\)H \(\approx\) (29.9, 1.4) and \(\delta^{31}\)P-\({}^{1}\)H \(\approx\) (29.9, 10.4) ppm, corresponding to the interactions of the \({}^{31}\)P atom with the \({}^{1}\)H of the organic molecule and with the connectivity defects, respectively. The last correlation suggests that the zeolitic defects should be located close to the positively charged organic molecule.  
To get a better knowledge of the location of the zeolitic defects in the MFI channels, the \({}^{1}\)H 2D DQ-SQ MAS NMR spectra were measured for the zeolites containing just one type of organic cation. The DQ-SQ experiments are based on homonuclear dipolar interactions and require at least two \({}^{1}\)H atoms in a very close proximity to observe the signal; the  
Figure 8: \({}^{1}\)H-\({}^{31}\)P 2D HETCOR NMR experiment of the silicalite-1 zeolite synthesized with TPP.  
Figure 7: \({}^{19}\)F and \({}^{29}\)Si MAS NMR spectra of silicalite-1 samples synthesized using different OSDAs: (**a**) \({}^{19}\)F MAS NMR of the zeolites prepared with TPA, TPP (including the deconvolution of the spectra) and mixtures of them; (**b**) \({}^{29}\)Si MAS NMR of the same zeolites.  
chemical shift in the second dimension is equal to the sum of the chemical shifts of the signals observed in the first dimension [66; 72; 73].  
The \({}^{1}\)H 2D DQ-SQ NMR spectra of TPP-MFI and TPA-MFI samples are shown in Figure 9a,b which are very similar. The spectra show three correlation signals: (1) a very intense and wide one at \(\delta^{1}\)H-\({}^{1}\)H \(\approx\) (1.2, 2.4) ppm that includes the auto-correlation signals of protons of the same group and of different groups of the alkyl chains; (2) the signal at \(\delta^{1}\)H-\({}^{1}\)H \(\approx\) (10.4, 20.8) ppm is the auto-correlation of the connectivity defects in the zeolite, indicating that the SiO- group is close to, at least, two SiOH groups; and (3) a weak correlation signal is seen at \(\delta^{1}\)H-\({}^{1}\)H \(\approx\) (1.2, 11.6) ppm, which is attributed to the interaction of SiOH zeolitic defects with -CH\({}_{3}\) groups exclusively. This result indicates that the zeolitic defects are much closer to the methyl groups, unlike what was observed through the NOESY or DQ-SQ NMR experiments for the zeolites synthesized with TEA [64; 66], where the correlation with all the \({}^{1}\)H of the organic cation was seen.  
The nature of the framework connectivity defects formed in zeolites with F/OSDA ratios below 1 is a question that is under discussion, since the formation, location, and number of SiOH groups that are located close to the SiO- species remain unclear. Currently, two models are accepted, the first one proposes two SiOH groups around two SiO- groups in nanocrystalline silicalite-1, and the second model assumes the presence of three SiOH surrounding the SiO- group in zeolites conventionally synthesized without F [67; 72; 73; 74]. The results obtained here do not allow us to undoubtedly confirm whether these samples correspond to one model or the other.  
The bidimensional NMR study has shown that structural defects are placed close to the OSDA as shown by \({}^{1}\)H-\({}^{31}\)P 2D HETCOR. Moreover, much better understanding is gained by \({}^{1}\)H 2D DQ-SQ NMR results, which indicate that the zeolitic defects are located near the methyl groups of the OSDA, as previously reported for the silicalite-1 zeolite synthesized by the OH- route, and also in the close vicinity of at least two silanol groups.