# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 5 Conclusions and Perspectives  
In the last few years, the hydrophilic/hydrophobic features of solid catalysts have attracted unprecedented attention particularly due to increasing relevance of renewables as raw materials. On one hand, reactions of lipidic starting materials such as triglycerides to give biofuels, biolubricants or surfactants require hydrophobic surfaces to adsorb on in an effective way; on the other one, when polar molecules are the products, their desorption improves over such catalytic surfaces. Analogous considerations apply when water is a reactant or a product. In the first case, a hydrophilic catalyst would be preferred, whereas in the second, higher hydrophobicity of the system would be advantageous. The preparation and characterization of hydrophobic and superhydrophobic solids to be used as catalysts or catalyst supports has reached a maturity level. This review deals only with zeolites, mesoporous silica and sulfonic resins as solid acids. In general, zeolites and mesoporous silica can be modified post synthesis. The hydrophobicity of zeolites can be tuned through dealumination or \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & **Conditions** & \begin{tabular}{c} **Sorbitol** \\ **Conversion** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Sorbitan** \\ **Yield (\%)** \\ \end{tabular} & \begin{tabular}{c} **Isosorbide** \\ **Yield (\%)** \\ \end{tabular} & **Reference** \\ \hline \begin{tabular}{c} Zeolite Beta \\ (Si/Al = 13) \\ \end{tabular} & 200 \({}^{\circ}\)C, 2 h & 23 & 13.8 & 1.5 & [85] \\ \begin{tabular}{c} Zeolite Beta \\ (Si/Al = 75) \\ \end{tabular} & 200 \({}^{\circ}\)C, 2 h & 87 & 38.3 & 28.7 & \\ \begin{tabular}{c} Zeolite H-Beta \\ (Si/Al = 15) \\ \end{tabular} & 100 & 9.4 & 80 & \\ \begin{tabular}{c} Zeolite H-Beta \\ (Si/Al = 15) \\ \end{tabular} & 127 \({}^{\circ}\)C, 1 h & 44 & & 11 & [86] \\ \begin{tabular}{c} Zeolite H-Beta \\ (Si/Al = 75) \\ \end{tabular} & 127 \({}^{\circ}\)C, 1 h & 96 & & 53 & \\ \begin{tabular}{c} SBA15-Pr-SO\({}_{3}\)H \\ + Si(CH\({}_{3}\))\({}_{3}\) \\ \end{tabular} & 150 \({}^{\circ}\)C, 24 h & 94 & 51 & 44 & [87] \\ \begin{tabular}{c} SBA15-Pr-SO\({}_{3}\)H \\ + Si(CH\({}_{3}\))\({}_{3}\) \\ \end{tabular} & 150 \({}^{\circ}\)C, 24 h & 90 & 20 & 70 & \\ \begin{tabular}{c} Amberlyts-15 \\ \end{tabular} & 140 \({}^{\circ}\)C, 10 h, 0.1 atm & 94 & 2.4 & 71.8 & [40] \\ \
\begin{tabular}{c} P-SO\({}_{3}\)H \\ PDS (0.025) \\ \end{tabular} & 150 \({}^{\circ}\)C, 12 h, 0.3 atm & 79.1 & 66.9 & 11.4 & [89] \\
\begin{tabular}{c} PDS (0.3) \\ \end{tabular} & 150 \({}^{\circ}\)C, 12 h, 0.3 atm & 94.1 & 10.4 & 81.7 & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Selected examples of sorbitol dehydration with hydrophobized material: direct comparison with the parent (or less hydrophobic) catalyst. coating with non-polar silanes, while less hydrophilic silicas can be obtained through functionalization with organic molecules and particularly through silanization. In few cases, sol-gel processes can give superhydrophobic silica aerogels. On the contrary, the surface hydrophilic/hydrophobic balance of sulfonic resins can be adjusted through the arrangement of sulfonic groups, that is, the sulfur content of the material. However, these features should be very finely tuned in order to keep the acidic properties of the catalytic material. Several methods of characterization involving both the surface composition and the measure of surface interactions with water or other solvents are available. Moreover, X-ray reflectivity studies, Sum frequency generation spectroscopy and atomic force microscopy are useful tools to characterize solid-liquid interfaces. Finally, some selected examples are reported to show how the hydrophobic-hydrophilic properties of a catalytic material or of a catalyst support may play a significant role in addressing not only activity but also selectivity, particularly in reactions involving water formation.