# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 3 Characterization of Hydrophobic Catalyst
### Thermogravimetric Analysis  
Thermogravimetric analysis (TGA) is an experimental technique based on measuring the mass variation of a sample using a microbalance as a function of the temperature. The experiments are carried out at constant heating rate or at constant temperature in inert or oxidizing atmosphere setting the desired temperature range. The output is a graph in which the mass percentage is plotted against temperature. The mass of a sample changes whenever it loses material due to the temperature changes [59]. TGA is useful to quantify the carbon, hydrogen or oxygen content on a solid surface for example. It is a direct technique to investigate the nature of the solid because it avoids the measurement of some kind of interaction between the catalyst and a compound. The advantages are the ease of setup and the possible automatization. On the other hand, it is a destructive technique and materials cannot be recovered after the analysis. In the case of hydrophobicity studies, TGA is mainly used for investigating the water affinity of a solid catalyst or for the quantification of the -OH groups present on the solid surface that however represent an indirect water affinity quantification as seen in the previous section. Setting an appropriate method, it is possible to assess and quantify the water loss of a solid together with the dehydroxylation. In the case of silica and titania, [PERSON] et al. set up a method to determine physisorbed water and silanol groups into two steps: The solid is in nitrogen from 25 \({}^{\circ}\)C to T\({}_{1}\) = 120 \({}^{\circ}\)C (average value comparing the literature) at 10 \({}^{\circ}\)C/min, holding this temperature for 10 min and then heated at 20 \({}^{\circ}\)C/min to T\({}_{2}\) = 800 \({}^{\circ}\)C in the case of silica or 500 \({}^{\circ}\)C in the case of titania and held at this temperature for 10 min [60]. The weight loss experimented in the first step is related to the physisorbed water; however, it strongly depends on the measure environment humidity. The second represents the weight loss derived from silanol groups removal from the surface, and the quantification for unit of area can be obtained with the following formula:  
\[\text{OH/nm}^{2}=\alpha\left\{\left(\text{OH/nm}^{2}\right)_{\text{T}_{2}} \times\text{SSA}\times\text{wt}_{\text{T}_{2}}+\left[\left(\text{wt}_{\text {T}_{1}}-\text{wt}_{\text{T}_{2}}\right)/\text{MW}_{\text{H}_{2}\text{O}}/ \text{N}_{\text{A}}\times 2\right]\right\}/\text{SSA}\times\text{wt}_{\text{T}_{1}} \tag{3}\]  
where \(\text{wt}_{\text{T}_{1}}\) is the sample weight at the corresponding temperature T\({}_{1}\), \(\text{MW}_{\text{H}_{2}\text{O}}\) is the molecular weight of water, N\({}_{\text{A}}\) is Avogadro's number, and \(\alpha\) is a calibration factor. In the case of silica, 1 OH/nm\({}^{2}\) remains on the surface at T\({}_{2}\) while in the case of titania the surface is free from hydroxyl groups at T\({}_{2}\)[60]. This method could be used not only for silica and titania but also for fumed silica, barium glass and zirconia [61]. A more straightforward way consists in measuring the loss of physisorbed water in the range of 30\({}^{\circ}\) and 900 \({}^{\circ}\)C and normalizing for the final mass registered. The comparison between non-modified titania-silica and a methyl modified titania-silica showed that the free material has a higher quantity of physisorbed water if compared to the modified ones. The methylated catalyst has a lower water affinity and demonstrated also a higher catalytic activity in the epoxidation of cyclootene [33]. Similarly, a simpler method to measure the surface silanol density consists in always measure the water loss of Ti-SiO\({}_{2}\) and Ti-MCM-41 but in the range between 150 \({}^{\circ}\)C and 900 \({}^{\circ}\)C instead [58]. [PERSON] et al. also indicated that the bare ZSM-5 zeolite exhibited a sharper mass decrease in the temperature range around 100 \({}^{\circ}\)C respect to silane modified ZSM-5 (Figure 7).