# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 4 Application of Hydrophobic Catalysts
### Sorbitoldehyd  
To scavenge water formed during a condensation reaction is definitely the main application of hydrophobization as functionalization strategy not only to preserve the acidic properties of the catalyst ([PERSON] et al., 2017) but also to push the reaction to higher conversions by leveraging the preferential adsorption/desorption of reactants and products. Several cases are reported in this respect when facing with condensation or dehydration reactions, particularly with biomass derived materials. One interesting example is the dehydration of sorbitol into isosorbide. This reaction is deeply studied due to the interest in isosorbide as high added value intermediate for the preparation of polymer, functional material, solvent of cosmetics, pharmaceutical molecules, surfactants, plasticizers, food additives and even as a fuel or fuel additive ([PERSON] et al., 2017). Its preparation starting from sorbitol, in turn obtained by reduction of glucose, is hampered by a double-step dehydration as shown in Scheme 1. The acidic properties and density of the catalyst surely play a major role in the efficiency of this process, but a proper design of the catalyst surface in terms of hydrophilicity/hydrophobicity appears to be another important point to be looked at. In particular, the right balance of acidic sites and hydrophobicity has to be carefully tuned in order to reach the best trade-off for high activity. Some studies based on the use of zeolites have been reported. [PERSON] et al. reported that the use of high-silica large pore zeolites such as Mordenite (Si/Al = 110), beta(Si/Al = 75) and beta(Si/Al = 150) exhibited a high catalytic performance, and especially beta(75) showed selectivities to sorbitan and isosorbide of 44 and 33%, respectively, at sorbitol conversion of 87% [85]. The comparison between different Si/Al zeolites shows that that catalytic activity based on the catalyst is determined by a balance between the number of acid sites, mainly relying on the Al content, and the degree of hydrophobicity, increasing with the increase in Si content. The gradual decrease in water and methanol adsorption with the increase in Si/Al had been already described as an important feature in these kind of materials [12]. The importance of finding the best trade-off between acid sites density and hydrophobicity in this reaction has been clearly evidenced in a paper by [PERSON], showing a volcano-shaped curved obtained for of sorbitol conversion and isosorbide yield vs Si/Al ratio, with a maximum corresponding to Si/Al = 75 of H\(\beta\) zeolite, able to gain 76% yield (Figure 15) [86]. Therefore, both the acidic density and the hydrophobicity have to be properly balanced in order to exploit the catalytic activity and the capacity to remove water from the surface. In this respect, the very different interaction substrate/catalyst taking place for the two steps involved in isosorbide formation as depicted in Scheme 1 can play a major role as deeply discussed by [PERSON] et al. in the use of propyl-sulfonic functionalized SBA-15 [87]. The study demonstrates that the two steps are strongly and differently influenced by both the acidic surface density and the hydrophobicity. In particular, a higher -SO\({}_{3}\)H density favors the second dehydration step sorbitan \(\rightarrow\) isosorbide, whilst TOFs values for the first step sorbitol \(\rightarrow\) sorbitan decrease by increasing the acid sites density. The main reason for this observation has to be ascribed to the interaction of the substrates with the acidic sites. The extended molecular structure of sorbitol allows its interaction with several catalytic sites at the same time. The multiple terminal protonation of -OH groups of sorbitol when the acidic sites are denser disfavors the cyclization process occurring via an intramolecular nucleophilic attack. The opposite effect is reasonably observed when sorbitan is the substrate and therefore in the second step.