## 1 Introduction
As mentioned above, tetraalkylphosphonium cations can also be used as OSDAs in the synthesis of zeolites, and, in these cases, phosphorous remains inside the zeolite pores after the calcination process required for burning-off the organic material and plays the stabilizing role of Al in the framework [55; 56; 57; 27; 35]. Interestingly, the use of P-containing OSDAs favors a homogeneous distribution of P across the zeolite crystals and allows the control of the amount of P incorporated by means of the _dual-template_ methodology for the synthesis of zeolites [55; 58; 59]. This approach consists of using two OSDAs of different nature, in this case, one containing phosphorous and the other nitrogen, thus modulating the P/Al ratio of the zeolite and therefore its acidity. Figure 1 shows the location of the tetrapropylammonium cations in the MFI structure which are placed in the intersection of the two different channels that the structure has [60]. Figure 1: MFI structure containing the TPA molecules at the intersections of the channels. ZSM-5 is a medium pore size zeolite which has been assigned the three-letter code MFI by the International Zeolite Association (IZA), and its structure contains a sinusoidal channel along the _a_-axis that intersects with straight channels running along \(b\) with pore apertures of 5.1 \(\times\) 5.5 A and 5.3 \(\times\) 5.6 A, respectively [61]. The structure can be built by the linkage of four different Composite Building Units (CBUs) and contains a small cage, known as _mel_, which usually hosts the fluorine atoms incorporated when the synthesis is done in fluoride medium [62; 63]. This zeolite can be synthesized in a wide range of chemical compositions and in the presence of a great variety of molecules as OSDAs or even in the absence of any organic, being the first case of an inorganic structure directing effect. Recently, we have studied the synthesis of the pure silica MFI zeolite, known as silicalite-1, by applying the _dual-template_ method using tetraethylammonium (TEA) and tetraethylphosphonium (TEP) cations in which both molecules operated as co-templates and the characteristics of the zeolites were modified by changing the TEA/TEP ratio inside of their channels [64]. In this work, we extend the synthesis of silicalite-1 using tetrapropylammonium (TPA) and tetrapropylphosphonium (TPP) cations and mixtures thereof as OSDAs, since TPA is the most used organic molecule for the synthesis of MFI materials, and it is worth studying their preparation in the presence of TPA/TPP. Thus, the main objective of this work is the characterization of pure silica MFI zeolite (silicalite-1) prepared by the _dual-template_ route with TPA, TPP, and mixtures of both cations in different ratios aiming to understand how the nature of the organic influences the physico-chemical properties of the zeolite. This study has been carried out by exhaustive characterization of the zeolites prepared with special emphasis on the use of solid state nuclear magnetic resonance techniques which are very powerful for this purpose. In particular, a multinuclear magic angle spinning-nuclear magnetic resonance (MAS NMR) analysis has allowed for investigating the framework structure, the status of the guest species located within the void volume of the zeolites, as well as the interaction between them. This MAS NMR study includes the resonance spectra of \({}^{13}\)C, \({}^{1}\)H, \({}^{31}\)P, \({}^{29}\)Si, and \({}^{19}\)F nuclei for the characterization of the guest species and the host zeolite, and bidimensional (2D) heteronuclear (HETCOR) and homonuclear (DQ-SQ) NMR experiments for shedding light on the interactions of the OSDAs with the zeolite. The results allow us to conclude that the _dual-template_ route with TPA/TPP for the synthesis of MFI is achievable, both organics are hosted inside the pores forming a homogeneous mixture, and the concentration of structural defects (i.e., silanol groups) depends on the amount of P incorporated in the zeolite.