# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 2 Synthesis of Hydrophobic Catalysts
### Zeolites  
Zeolites are one of the most widely employed class of acidic catalysts used in industrial chemistry, especially in oil refinery and production of chemical commodities [17]. They are very versatile due to shape selectivity and ability to be subjected to ion-exchange. Zeolites are basically crystalline aluminosilicates possessing a different Si/Al ratio that influences the physical and chemical properties of the solid. Zeolites typically present both Bronsted and Lewis acidic sites that can, however, experiment a severe deactivation mechanism in the presence of aqueous environment. Moreover, zeolites under hydrothermal conditions suffer also of dealumination drawback with subsequently structural instability [12; 13]. Enhancing the hydrophobic surface properties of a zeolite weakens the binding of water to the active sites and prevents dealumination, thus increasing the catalyst life. Furthermore, a more hydrophobic surface can enhance the affinity with organic substrates. There are diverse methodologies to increase the hydrophobicity of a zeolite. It has been reported that wettability of zeolites can be tuned by modulating the Si/Al ratio: in fact, hydrophobicity linearly increases with the silica content [18; 19]. High-silica content zeolites can be obtained subjecting a zeolitic material to an HNO\({}_{3}\) treatment in order to dealuminate the structure [20]. Another reported methodology to increase zeolite hydrophobicity is to coat the surface with a non-polar silane compound such as octadecyltrichlorosilane. This hydrophobic functionalization improves hydrocarbon contaminant adsorption in water [21]. Moreover, it also makes zeolites able to stabilize water/oil emulsion and to catalyze reactions in this environment, such as alcohol dehydration [22] and alkylation, with an enhanced hydrothermal stability [23].  
[PERSON] et al. described a method to obtain hydrophobic Ti-incorporated Y-zeolites, for formaldehyde removal, anchoring organic functionalities, i.e., -CH\({}_{3}\), -Ph and -CF\({}_{3}\), on the framework of faujasite during the crystallization process and successively fixing TiO\({}_{2}\) within the crystals [24].  
With respect to the total synthesis method, [PERSON] et al., instead, experimented that a Ti-incorporated MFI zeolite crystallized in a microwave oven was more hydrophobic if compared to the one prepared with the traditional hydrothermal method. The IR spectra analysis shows the presence of a lower amount of hydroxyl group on the surface imparting to the catalyst the capacity to adsorb an higher amount of 1-hexene [15].  
In the case of zeolite supported metal catalysts, a recent work shows the synthesis of AuPd alloy hydrophobic zeolite catalyst allowing to enhance the methanol productivity in methane oxidation. AuPd alloy nanoparticles can be fixed within aluminosilicate zeolite crystals, and subsequently, the external surface is functionalized with organosilanes able to increase hydrophobicity. The silanes make the diffusion of hydrogen, oxygen and methane to the catalyst active sites easier, while blocking the peroxide to increase its reaction probability [25].  
Figure 3: Example of trichlorosilane grafting procedure on surface hydroxyl groups.