# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 2 Synthesis of Hydrophobic Catalysts
### Polymeric Catalyst  
Polymers are widely employed in the field of catalysis. Cross-linked polymers are used as supports for different kind of active phases thanks to their insolubility. In fact, the recycle of most catalyst is not always possible as in the case of organometallic catalysts. The use of a cross-linked polymer as the support allows separation and reuse of the catalyst [35]. The active phase is adsorbed or incorporated directly into the polymeric chain as functional groups like in the case of ion-exchange resins. Acidic and basic ion-exchange resin catalysts promote a large variety of organic reactions. The most widespread are styrene-based sulfonic such as Amberlyst(r) (acid) and Dowex(r) (acids and basics) or perfluorinated sulfonic acids such as Nafion(r) in which the active species is sulfonic acid. These are able to substitute mineral acid in condensation, alkylation and isomerization reaction [36; 37]. Like the other catalysts cited in this review, the presence of water is detrimental for the active sites of polymeric catalyst. In the case of sulfonic resins for example, the -SO\({}_{3}\)H functional group can be deactivated due to water adsorption. Moreover, hydrophobic or weakly polar organic compounds have poor affinity with the hydrophilic active sites, so it is crucial to increase the surface hydrophobicity also in the case of resin catalysts, preventing deactivation of active sites and improving the activity towards organic substrates. Styrene-based resins have an intrinsic hydrophobicity because of their hydrocarbon-based structure. However, acidic or basic functional groups incorporated in the polymeric structure impart a certain hydrophilicity to the surface. To properly allocate the number of functional groups in the resin network is a crucial aspect in order to tune the surface hydrophobicity. It was demonstrated that a novel synthetized sulfonic acid porous resin (PS) and a fluoride sulfonic acid resin (PCS) have a better hydrophobicity and affinity towards organic molecules compared to the commercial analogues Amberlyst-15 and Nafion-212 as confirmed by measuring the contact angle of water [38]. This is related to a lower presence of sulfonic and perfluorosulfonic groups respectively with respect to the commercial resins. The enhanced hydrophobicity not only improves the catalytic activity of the materials but also increases the selectivity to the desired product (FCF) by inhibiting the secondary reaction that involves water in the hydroxylalkylation /alkylation of 2-methylfuran (2-MF) with ketones. Thus, conversion of 2-MF goes from 45% to 61% and selectivity to FCF from 72% to 84% moving from Amberlyst-15 (contact angle = 65\({}^{\circ}\)) to the 4.28% PS resin (contact angle = 158\({}^{\circ}\)) (Figure 4). A similar effect on selectivity was obtained in the same reaction in the presence of phenylsulfonated biochar. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Solid Materials** & **Hydrophobic Functionality** & **Reference** \\ \hline Zeolites & octadecyltrichlorosilane & [21] \\  & -CH\({}_{3}\), -Ph, -CF\({}_{3}\) & [24] \\ \hline Silicas & dichloromethylsilane & [32] \\  & diphenyldichlorosilane & [28] \\  & Tetra ethoxy silane combined with methyltriethoxysilane & [33] \\  & trimethoxymethylsilane and & [34] \\  & 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Hydrophobic functionalizations for zeolites and silicas materials. Thus, the hydrophobic biochar modified with 6.63% PhSO\({}_{3}\)H acid (contact angle = 114\({}^{\circ}\)) allowed to obtain 70.5% of FCF owing to a 84.4% selectivity whereas Amberlyst-15 gave only a 46.5% yield [39]. [PERSON] et al. proposed a method in which a sulfonic acid styrene-based resin was synthetized under solvothermal conditions and copolymerization of divinylbenzene with sodium \(p\)-styrene sulfonate followed by ion-exchange of a H\({}_{2}\)SO\({}_{4}\).