# Catalytic Evaluation of Hafnium Modified SiO\({}_{2}\) for the Dehydration of Alcohols
## 3 Results and Discussion
### Catalytic Activity  
Ethanol was used as a model molecule to determine the acid-base nature of SiO\({}_{2}\), 3.6 Hf/SiO\({}_{2}\), and 12 SHf/SiO\({}_{2}\). The dehydration reaction of ethanol was carried out in an inert atmosphere. Dehydration can be performed through two possible pathways: (1) intra-molecular dehydration to produce ethylene, (2) intermolecular dehydration to generate diethyl ether, and and others like dehydrogenation of the alcohol to produce acetaldehyde [31; 32], and as a result, a reaction selectivity parameter depending on the relative number of active site types in the catalyst is obtained. Table 3 summarizes the results of the ethanol dehydration reaction with selectivity to ethylene and diethyl ether at different temperatures. SiO\({}_{2}\) did not show catalytic activity as it lacks acid sites, as demonstrated in the potentiometric titration test with n-butylamine (Figure 8). When hafnium is present in the silicate alcohol decomposition is favored, however, at low temperatures ethanol conversion is lower than 50% with a tendency to form diethyl ether, conversely, when the reaction temperature is increased, selectivity of alcohol conversion to ethylene is increased as reported in literature [31; 33]. According to the results obtained by titration with n-beta and pyridine adsorption, 12 SHf/SiO\({}_{2}\) contains very strong acid sites. Therefore, they are responsible for ethanol dehydration at low temperatures. At 300 \({}^{\circ}\)C ethanol conversion was 100% with selectivity to ethylene. It should be noted that, in alcohol dehydration processes, the reaction temperature is a key factor to consider since a slight change in this operating variable can significantly affect the selectivity of the alcohol dehydration reaction. In the present study, no products generated by ethanol dehydrogenation were identified, confirming that hafnium-modified silicates either lack basic sites or are too weak to carry out a dehydrogenation reaction. On the other hand, the ethanol dehydrogenation reactions were carried out for 3 h, the catalysts showed good thermal stability in the course of the reaction, and no deactivation by organic carbon deposition was observed.  
Figure 9: FT-IR spectroscopy with adsorption-desorption of pyridine (**a**) Hf/SiO\({}_{2}\); (**b**) 12 SHf/SiO\({}_{2}\). L: [PERSON]’s acid sites (*), B: Brönsted acid sites (+), B + L: Lewis-Brönsted-type acid sites (\({}^{\circ}\)).  
Additionally, the catalytic behavior of methanol dehydration was studied, which can decompose intramolecularly or intermolecularly depending on the reaction conditions and the type of catalyst [34]. Methanol dehydration reaction requires catalysts with strong acid sites, and as observed in the pyridine adsorption-desorption tests (Figure 9) and ethanol dehydration tests (Table 3), 3.6 Hf/SiO\({}_{2}\) and 12 SHf/SiO\({}_{2}\) have very strong acid sites suitable for catalyzing decomposition reactions of said molecule. Figure 10a shows ethanol conversion over catalyst 3.6 Hf/SiO\({}_{2}\) at 340 \({}^{\circ}\)C, with 30% ethanol conversion and dimethyl ether as the main product. The reaction was carried out for 240 min without deactivation of the material. Catalyst 12 SHf/SiO\({}_{2}\) exhibited higher methanol conversion, up to 70%, but selectivity to dimethyl ether was lower compared to 3.6 Hf/SiO\({}_{2}\). This is attributed to a decrease in the number of very strong acid sites in 12 SHf/SiO\({}_{2}\) over the course of the reaction, resulting in lower selectivity to the formation of hydrocarbon compounds (Figure 10b) [35]. This reduction in acid sites is attributed to coke deposition on the catalyst surface, however conversion reaction and selectivity to dimethyl ether remained stable for 200 min.