## 1 Introduction  
Zeolites are crystalline microporous materials typically in aluminosilicate composition whose structures are formed by silicon and aluminum tetrahedra linked by corner-sharing oxygen atoms. These tridimensional structures generate channels and cavities of molecular dimensions leading to materials largely utilized in industrial applications, such as catalysis, adsorption, and ion exchange, among others [1, 2, 3]. Their well-defined porosity, thermal stability, and the possibility of modifying the chemical composition and properties account for the success of these materials [4, 5, 6, 7, 8]. Zeolites exist in nature as low Si/Al ratio materials, and can be synthesized in a wide range of chemical compositions from Si/Al ratios of 1 up to purely siliceous, and also incorporating other elements, thus generating an extensive variety of materials with diverse properties [9]. Purely siliceous zeolites are neutral materials, but the presence of Al in the framework generates a negative charge that must be compensated by cations that can be inorganic, organic, or protons. In the latter case, acidic sites are generated, and this is one of the main properties of zeolites that allows their application as catalysts in acid catalyzed processes [10, 11, 12, 13, 14]. Synthesis of zeolites is usually done in hydrothermal conditions starting from alkaline mixtures that contain the aluminosilicate precursors in which the hydroxyls mobilize the species in solution to build the final zeolite structure [15; 16; 17; 18]. However, it is also possible to carry out the synthesis from fluoride containing synthesis mixtures wherein the fluoride anions play the mobilizing role [19; 20; 21]. Zeolites synthesized in fluoride medium generally contain a lower concentration of structural connectivity defects than their hydroxide counterparts, and fluorine tends to occupy the smallest cage present in the structure and compensates the positive charge of the organic cations [22; 23]. High Si/Al ratio zeolites are commonly synthesized using tetraalkylammonium cations as organic structure directing agents (OSDAs) that participate during the crystallization process and stabilize the zeolite structure by filling the pores and cavities [24; 25; 26; 27; 28]. These ammonium cations frequently undergo Hoffmann degradation in the highly alkaline medium that exists during the synthesis at high temperature and prolonged time, the use of tetraalkylphosphonium cations being an alternative route that has been successfully used in the last several years leading to the discovery of new zeolite structures [29; 30; 31; 32; 33; 34; 35]. ZSM-5 is one of the most widely applied zeolites as an acid catalyst for industrial processes in many reactions, such as fluid catalytic cracking, hydrocarbon isomerization and alkylation, methanol to olefins, among many others [36; 37; 38; 39; 40]. The distribution and location of Al in different crystallographic positions of the zeolite framework is highly relevant as it can affect the final catalytic properties and, specifically, ZSM-5 is one of the most studied zeolites in this regard [41; 42; 43; 44; 45; 46]. In addition, the control of the acidity is of paramount importance for maximizing the activity of the catalyst that is frequently submitted to very high temperatures and leads to partial dealumination processes in which aluminum atoms migrate to extraframework positions with the subsequent decrease in the acidity. In relation to this, the stabilization of Al in framework sites by post-synthesis modification of the zeolite with phosphorous containing reactants has been reported [38; 47; 48; 49; 50; 51; 52]. These P-species interact with Al atoms and stabilize them in the structure when they are submitted to severe thermal or hydrothermal treatments [53; 54].