# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 2 Synthesis of Hydrophobic Catalysts
### Silicas  
Silicas are materials widely employed in heterogeneous catalysis mainly as support for metals or metal compounds due to their high surface area and to their inert nature. Silicas are materials made of interlinked tetrahedral SiO\({}_{4}\). The structure pattern can end either with a siloxane group (Si-O-Si) or with a silanol group (Si-OH). The latter is defined as isolated if no adjacent silanols are present, vicinal if two silanol groups are present or different silicon atoms are close enough to make a hydrogen bond and, finally, geminal if the two silanols are attached to the same silicon [26]. Due to their polar nature, the quantity of silanols group influences the silica surface hydrophilicity and, therefore, the affinity with water. However, a high population of silanol groups on the silica surface could lead to a poor moist tolerance and hydrothermal stability of the material. In fact, the presence of water adsorbed on the -OH of the silanol groups can cause the hydrolysis of the siloxane bonds and, as a consequence, a structure collapse [27]. After these considerations, the improvement of the stability of a silica material is crucial to achieve a more stable and long-life catalyst, especially if the catalyst has to be used in a moist or watery reaction environment. Thus, in order to increase the structural stability and moist resistance of a silica, the population of surface silanols can be diminished through functionalization, mainly with organic compound, to arise the hydrophobicity of the surface. Similarly, for the case of zeolites, diminishing the number of hydrophilic silanols makes silica surface more accessible for organic molecules. The hydrophobization of silica materials could be achieved without compromising the activity of a catalyst but also increasing its stability. As reported, a method to obtain hydrophobic phenyl sulfonic acid functionalized mesoporous SBA-15 silica (SBA-15-Ph-SO\({}_{3}\)H) consists in the silanization of activated mesoporous SBA-15 with dichlorodiphenylsilane followed by silylation and sulfonation [28]. This procedure allows to obtain a Bronsted acidic catalyst with a boosted stability towards water and leaching compared to the silica sulfuric acid and the simply sulfonated SBA-15. Analogically, in the work of [PERSON] et al., propylsulfonic functionalized benzenesilicas were prepared, and acidic sites content was monitored with \({}^{1}\)H nuclear magnetic resonance (NMR). 2D proton spin-exchange NMR explained also that the proximity of the hydrophobic phenyl ring to the acidic sites could protect them against water deactivation, explaining the fact that water has no negative effect on catalytic activity concerning low proton-loaded catalysts [29]. The introduction of metal nanoparticles on the surface of a hydrophobic silica is not so trivial. A way to produce hydrophobic supported metal catalyst is to hydrophobize the surface after the anchoring of the metal. Ru/SiO\({}_{2}\) can be synthetized with an impregnation method and subsequently functionalized with trimethylchlorosilane by using a post grafting method [30]. Another strategy to obtain a metal supported hydrophobic catalyst consists in preparing phenyl-modified amine-bridged silica with a water-in-oil reverse microemulsion. In fact, the phenyl groups create the hydrophobic layer on the silica surface, and the amine groups in the framework allow to anchor platinum nanoparticles that act as active sites for the oxidation of aliphatic alcohol in acids [31]. [PERSON] et al. reported a method to functionalize hydrophilic silica with dichlorodomethyl silane at various silane/silica ratios to ensure different degrees of hydrophobicity. Using this strategy, a hydrophobic silica supported Pd catalyst was achieved by impregnation of the metal after silanization. Hydrophobic Pd/SiO\({}_{2}\) catalyst was also prepared impregnating the silica and subsequently functionalizing it with silane.