# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 3 Characterization of Hydrophobic Catalyst
### Infrared Spectroscopy  
Infrared spectroscopy (IR) is one of the most commonly used characterization techniques in catalysis. IR can be used as a direct way to investigate the surface composition of a solid catalyst or as an indirect way, by studying the interaction between the surface and an adsorbing species and the way in which they are chemisorbed. Concerning surface hydrophobicity, IR can be used as a direct method to quantify the polar or non-polar groups present on a solid surface, for example the hydroxyl groups [64], thus having an insight on the surface polarity. In the work of [PERSON] et al., Beta-zeolites with different Si/Al ratio were synthetized in fluoride media in order to achieve a smaller amount of silanol groups on the surface. These samples were compared to the Beta-zeolites synthetized in OH\({}^{-}\) media by recording their IR spectra. It is noticeable that the silanol band at 3745 cm\({}^{-1}\) is less pronounced for the fluoride media synthetized zeolites respect to the one synthetized in OH\({}^{-}\) media (Figure 8). Moreover, the Beta-zeolites with higher Si/Al ratio (25) present a smaller peak compared with Beta-zeolites with lower Si/Al ratio synthetized in the same medium, and they exhibit the highest activity in the acetalization of HMF with n-octanol [65]. A more recent example shows the disappearance in Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectra of the free surface silanol band after silane functionalization of MCM-41 both in the case of high temperature grafting and low temperature grafting. In fact, silane binds to surface -OH groups forming Si-O bonds. Moreover, C-H vibration band appears after organosilane functionalization. Thus, the decrease of band intensity can be used as an indicator of functionalization success together with increase of hydrophobicity (Figure 9) [66]. If IR is used as an indirect method, it allows to investigate the interaction between the surface of the catalyst and a polar or non-polar probe molecule. 1-Hexene was used for this scope because, in principle, a hydrophobic zeolite should have a higher affinity towards an organic compound. In Figure 10, the IR 1-hexene adsorption spectra on two type of zeolites, the more hydrophobic Ti-MFI-MW and the more hydrophilic Ti-MFI-CH, are shown. The 1-hexene adsorbed on Ti-MFI gave a series of adsorption bands at 3100-2780 cm\({}^{-1}\). It is easy to observe how the intensity of the peak is much higher for the more hydrophobic zeolite indicating that it has a higher capacity of adsorption of organic molecules [15]. Figure 7: Weight-loss profile of chlorosilane modified ZSM-5 adapted from Reference [62] (O = octyltrichlorosilane, D = decyltrichlorosilane, DD = dodecyltrichlorosilane, HD = hexadecylthridroxosilane). The major water loss is experimented by the bare UZSM-5. As showed, the decomposition of chlorosilane happens in the range of 500 \({}^{\circ}\)C, a higher temperature than the silane boiling point (from 233 \({}^{\circ}\)C to 269 \({}^{\circ}\)C). Infrared spectroscopy is also helpful to investigate how the hydrophobicity influences the affinity with substrates and products in a catalytic reaction. [PERSON] et al. adopted IR to understand the role of wettability of silicalite-1 zeolite supported palladium catalyst in the selective hydrogenation of furfural to give furane. In this case, a more hydrophilic catalyst was more effective in this type of reaction. To justify this behavior, they studied the adsorption of furfural and furan on the bare Pd@S-1  
Figure 8: IR spectra of Beta-zeolites synthetized in fluoride (F) and OH\({}^{-}\) media (B). The silanol band at 3745 cm\({}^{-1}\) is less pronounced for Beta-zeolites prepared in a fluoride medium and for Beta-zeolite with higher Si/Al ratio. Adapted from Reference [65].