# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 4 Application of Hydrophobic Catalysts
### Fatty Acid Esterification  
Reactions involving fatty acids represent a class of processes very well-fitting with the design of a properly modified surface in order to favor the preferential adsorption of a substrate and in turn to rule on the activity or selectivity of a reaction. In particular, the preparation of ethers and esters of fatty acids or alcohols with polyols such as glycerol has been the matter of study in this respect. Monoglycerides (MG) are a class of compounds highly requested at industrial level due to their application as emulsifier in the food industry, as surfactants in detergency, as well as antibacterial agents in animal husbandry and aquaculture [77]. Two main strategies can be used for their preparation, namely, transesterification of triglycerides with alcohols in the presence of a basic catalyst or direct esterification of glycerol with a fatty acid in the presence of an acidic system. The latter route is one of the most pursued in the case of solid catalysts, but its success strongly depends on the capacity of the protocol to optimize the yield in mono- and diglycerides while reducing as much as possible the formation of triglycerides. Thus, their presence reduces the emulsifier properties due to the lack of hydrophilic functionality. As an example of a proper final product composition, the food additive commercially labelled as E471 and derived from C16 and C18 fatty acids is a mixture typically in the range monoglycerides:diglycerides of 40:40, whereas for more profitable applications such as cosmetic ones, higher concentrations of monoglycerides are required. To obtain high selectivity values is not trivial, and several strategies have been exploited such as the selective precipitation of monoglycerides, the protection of 1,2-hydroxyl groups of glycerol via ketal formation, as well as the adsorption/desorption of hydrophobic/hydrophilic molecules. A significant increase in selectivity, for example, has been reported to be obtained through the hydrophobization of mesoporous silica-supported sulfonic acid, due to the better diffusion of the fatty acids inside the porous framework of the catalyst (Figure 14) [78]. As already reported from the groups of [PERSON] [79], the difficult diffusion of fatty molecules inside the pores hinders the positive effect of the shape selectivity. Therefore, a strategy aimed to improve the contact of fatty reactants with the framework surface could be a good point to look at. Figure 14: Schematic representation of the shape-selectivity effect in monoglyceride formation assisted by the surface hydrophobization. Figure 13: Preferential adsorption of fatty acids and preferential desorption of water on hydrophobic catalysts. The same effect has been claimed by [PERSON] et al. (2018) in the use of a modified silica zirconia catalyst (Me&Et-PhSO\({}_{3}\)H-SiO\({}_{2}\)-ZrO\({}_{2}\)) in the esterification of glycerol with oleic acid for the preparation of the corresponding monoglycerides. Under the optimized conditions, the obtain a selectivity of 84.5% in MG at 39% of conversion. The paper relies on the fine design of a modified ZrO\({}_{2}\)-SiO\({}_{2}\) catalyst able to maintain interesting acidic properties in spite of the surface modification with trimethoxymethylsilane (TMMS) and 2-(4-chlorosuofonylphenyl)ethyltrimethoxysilane, while also favoring the preferential framework diffusion of oleic acid. The significant effect of hydrophobization when having in hand fatty substrates and their competition with water for the catalytic site has been also exploited in the preparation of catalysts for biodiesel production. In this case, the water produced during esterification unavoidably contributes to the reverse reaction of hydrolysis, thus reducing the yield in the desired esters. A zirconium-containing periodic mesoporous organosilicas (Zr-PMOs) has been proposed for this purpose, revealing significant differences if compared with the analogous Zr-SBA-15 ([PERSON] et al., 2017).