# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 4 Application of Hydrophobic Catalysts  
The chance to tune the hydrophobic/hydrophilic nature of the catalyst surface has a significant impact on synthetic strategies. Thus, when facing with processes involving reagents and products markedly different in the polar character, their contrasting adsorption/desorption behavior can rule the reaction as far as both activity and selectivity are concerned. Figure 12 shows the main fields of action related with the design of hydrophobic materials for catalytic purposes. Figure 12: Main effects on catalysis obtained by designing proper hydrophobic materials. Great part of the work on this topic relies on the relationship between hydrophilicity/hydrophobicity and catalyst stability in terms of both acidity and hydrothermal properties. Acidic materials have seen a rapid and increased interest in their application due to their use in biomass transformation. In this respect, a proper design and modulation of the hydrophobicity of the surface allows one to have at hand an acidic catalyst with enhanced water tolerance, thus granting minor deactivation during reaction. It has been reported that hydrophobic porous void space in Sn-Beta-F and Ti-Beta-F zeolites are able to catalyze glucose isomerization to fructose in aqueous medium showing also high stability while hydrophilic Ti-Beta-OH and TiO\({}_{2}\)-SiO\({}_{2}\) exhibit no catalytic power in this case. In fact, Sn and Ti Lewis acidic centers are active when they are situated in hydrophobic channels of Beta zeolite but not when located within hydrophilic void spaces or at hydrophilic surfaces of TiO\({}_{2}\)-SiO\({}_{2}\) and Ti-Beta-OH. The hydrophobicity of the internal environment shields water penetration preventing deactivation of the Lewis Ti and Sn centers during glucose isomerization [74]. Similarly, hydrophobic functionalization of HY zeolites enhances its catalytic activity, stability and reusability in hot aqueous media like in the case of refining of biomass pyrolysis oil. Hydrophobization of the zeolite surface prevents the contact with water; hence, the disruption of the crystalline structure leaving acidic active sites unaltered [22]. [PERSON] et al. reported that ethane-containing PMO-based propysulfonic acid catalysts were more stable than non-organic functionalized analogue catalysts and commercial pSO\({}_{3}\)H-SC in the aqueous fructose dehydration to give 5-hydroxymethyl furfural. The main deactivation cause was found to be the hydrolytic cleavage of the acidic sites. Again, hydrophobic microenvironment decreases local water concentration and, thus, acid sites hydrolysis [75]. Likewise, hydrophobicity has a strong influence on zeolites hydrothermal stability. Thus, hydrophilic silanol groups formed during zeolite synthesis are known to be the main active sites responsible for the hydrolysis of the matrix framework occurring when working under hot liquid water conditions, namely >150 \({}^{\circ}\)C, that are typically employed in many biomass transformations. In this respect, the hydrophobization of zeolites reveals to be an interesting strategy alternative to the un-green one based on to use charge-balancing anions such as F\({}^{-}\) in order to prevent -SiOH sites formation. The hydrophobization process hinders water molecules to approach to zeolites surfaces, thus preserving the framework against hydrothermal conditions. Several hydrophobization approaches have been used for this purpose and are very well described in a review by [PERSON] et al. [11]. Another important aspect relates to the influence of hydrophobicity on adsorption and interface phenomena. Once again, the need to design efficient catalysts for biomass conversion has stimulated a growing interest in the understanding of solid-liquid interactions, a field less explored with respect to the one of gas-phase reactions. Very interesting insights into these phenomena have been reported by [PERSON] et al. in a perspective paper [4].