## 3 Results and Discussion  
### Synthesis of Silicalite-1 Zeolites  
The zeolite crystallization from the synthesis gels containing tetrapropylammonium (TPA), tetrapropylphosphonium (TPP), or mixtures thereof as OSDAs lead to the formation of pure silica MFI as the unique crystalline phase without any amorphous material, as confirmed by the XRD patterns shown in Figure 2. The X-ray diffractograms show slight differences in the relative intensities of the peaks at low angles, which can be attributed to the use of different OSDAs, and the peaks between 22\({}^{\circ}\) and 26\({}^{\circ}\) (see magnification, inset in Figure 2) are broader for the sample synthesized with TPP than that with TPA. These differences suggest that smaller crystals are formed when the TPP cation is used than in the presence of TPA. The chemical composition of the silicalite-1 samples is displayed in Table 2. The P/(P + N) molar ratio contained into the zeolites synthesized using mixtures of TPP and TPA cations is close to that employed in the synthesis gel. This result indicates that, contrary to the observations for the silicalite-1 samples synthesized using mixtures of TEP/TEA, there is no preferential incorporation of phosphonium over ammonium cations in the zeolite [64]. Independently from the organic cation used in the synthesis, all silicalite-1 samples contain about four molecules of OSDA per unit cell. All silicalite-1 samples contain a F/OSDA molar ratio below 1, indicating that they must contain SiO\({}^{-}\) defects to completely balance the positive charge introduced by the organic molecules. The SiO\({}^{-}\) defect concentration in the silicalite-1 increases as the occluded TPP inside of zeolite sample does. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **P/(P + N) 1 Gel** & **P/(P + N) 1 Solid** &
\begin{tabular}{c} **OSDA/** \\ **u.c. 2** \\ \end{tabular} & **F/u.c. 3** & **F/OSDA** \\ \hline TPA-MFI & 0.00 & 0.00 & 4.5 & 3.3 & 0.74 \\ TPX-MFI\({}_{25}\) & 0.25 & 0.26 & 4.1 & 2.6 & 0.64 \\ TPX-MFI\({}_{45}\) & 0.50 & 0.45 & 4.1 & 2.5 & 0.60 \\ TPP-MFI & 1.00 & 1.00 & 4.3 & 2.4 & 0.56 \\ \hline \hline \end{tabular}  
* \({}^{\dagger}\) Molar ratios. 2 96 Si atoms per unit cell (Sig\({}_{0}\)O\({}_{192}\)). 3 Quantified by 19F MAS-NMR.  
\end{table} 
Table 2: Chemical composition of the silicalite-1 samples prepared using TPA, TPP, and mixtures of them as OSDAs. Figure 2: XRD patterns of silicalite-1 samples synthesized using TPA, TPP, and mixtures of them as OSDAs. The inset corresponds to the magnification of the range between 22–26\({}^{\circ}\). Figure 3 shows images of the silicalite-1 samples obtained by FESEM, which shows a homogeneous distribution of crystal sizes which are notably smaller for the TPP-MFI (1 \(\upmu\)m \(\times\) 0.5 \(\upmu\)m \(\times\) 0.2 \(\upmu\)m) than for the TPA-MFI sample (10 \(\upmu\)m \(\times\) 5 \(\upmu\)m \(\times\) 1 \(\upmu\)m), in agreement with the differences observed in the XRD patterns (see inset in Figure 2). Crystals of intermediate sizes are observed for the TPX-MFI\({}_{25}\) and TPX-MFI\({}_{45}\) zeolites synthesized with mixtures of OSDAs. It is worth noting that the morphology of the crystals is different from those synthesized with TEP/TEA, which are more elongated and narrower, presenting the typical coffin-shape [64]. Additionally, the analysis of the P content in the TPX-MFI\({}_{25}\) and TPX-MFI\({}_{45}\) samples by FESEM-EDX was verified in several crystals to be 0.6 \(\pm\) 0.1 wt.% P and 1.1 \(\pm\) 0.2 wt.% P, respectively. These values are fully consistent with those obtained by ICP analysis, indicating that the two types of OSDA molecules are well distributed along the MFI crystals supporting their co-templating effect during the zeolite crystallization.