## 3 Results and Discussion
### N\({}_{2}\) adsorption-Desorption Isotherms: Porosity and Surface Area  
The graphs obtained from the N\({}_{2}\) adsorption-desorption isotherms test are shown in Figure 2. In the case of the Dt adsorption curve (Figure 2a), a slight increase in the amount of N\({}_{2}\) adsorbed is observed when the relative pressure is practically zero, and then the adsorption increases at a low rate until approximately P/Po = 0.7. Above this relative pressure, the adsorption rate increases up to about P/Po = 0.95, above which the slope of the curve is again somewhat reduced. In the case of the Dt-Z adsorption curve (Figure 2b), the pathway is similar to that of the Dt sample, with the largest differences observed when P/Po > 0.7, with a less pronounced rise in the rate of adsorbed amount and no final deceleration if compared with Dt. Despite this, during desorption a hysteresis loop is observed for both Dt and Dt-Z, decoupling from the adsorption curve from relative pressures of approximately 0.7 to 0.95 for Dt, and 0.5 to 0.95 for Dt-Z. Figure 2: N\({}_{2}\) adsorption–desorption isotherms of: (**a**) initial diatomic, Dt; (**b**) the material obtained after the zeolitization, Dt-Z. The International Union of Pure and Applied Chemistry (IUPAC) defines six possible types of adsorption isotherms, from which the type of the dominant porosity (if any) in the sample can be deduced [19; 20; 21; 22]. The Dt and Dt-Z isotherms would be classified within type IV, typical of mesoporous materials, i.e., in theory, they would present a predominance of pores with sizes between 2 and 50 nm according to the IUPAC [22; 23]. In the case of Dt, this type of isotherm (Figure 2a) differs from others for diatomics presented in the literature [8], which show type I isotherms, typical of non-porous or macroporous materials. Probably, the pretreatment at 550 \({}^{\circ}\)C may have helped to unclog the micro- and mesopores in Dt, hence their curves show a different trajectory than expected. From the four types of hysteresis loops classified by IUPAC [19; 20; 22], Dt presents the H1 type, which is typical of materials with well-defined cylindrical-type porous channels or agglomerates of more or less uniform spheres. For its part, Dt-Z exhibits an H3-type hysteresis loop, which is usually linked to slit-shaped pores and non-rigid aggregates of plate-like particles [22]. The main characteristics deduced from the N\({}_{2}\) isotherms test are summarized in Table 3. As can be seen, the alkaline hydrothermal treatment has produced a substantial increase in the specific surface area according to the BET method. \(S_{\text{BET}}\) has increased from 39.4 m\({}^{2}\)/g for Dt to 62.6 m\({}^{2}\)/g for Dt-Z, which in percentage terms is almost a 60% increase in specific surface area. This specific surface area value is slightly higher than the maximum \(S_{\text{BET}}\) obtained by [PERSON] et al. [9] in zeolites P also produced from diatomics (59.5 m\({}^{2}\)/g) by using a more laborious method, which also included the addition of an aluminum source. Returning to the results of the present investigation, although the microporosity (represented in this case as the accumulated volume of pores smaller than 2 nm) is not too high in either sample, an increase is again observed, from 0.002 cm\({}^{3}\)/g in Dt to more than double in Dt-Z (0.005 cm\({}^{3}\)/g). However, the data estimated from the BJH method indicate that the total pore volume has hardly varied between both samples, even decreasing slightly (\(-4.9\%\)) this property in the Dt-Z sample with respect to the untreated diatomic. The fact that the volume of accumulated porosity has hardly changed when applying the hydrothermal treatment, while the specific surface area has increased is an indicator that there has been a change in the pore size, so that in Dt-Z the pores would present a smaller size, being also more numerous than in Dt as a result of the zeolitization process (Figure 3).