Characterization of the adsorbents

The adsorbents were characterized using the adsorption of N₂ at 77 K. The results obtained were reported in previous studies (^{Hernández-Huesca et al. 1999}).

Adsorption kinetics measurement

Adsorption kinetic uptake was measured using a conventional high vacuum volumetric system constructed entirely of Pyrex glass and equipped with grease-free valves. Vacuum was created by a mechanical pump E2M1.5 (Edwards) reaching residual pressures of 10^-3 torr. Pressure was registered by a digital display pressure transducer TPR 017 (Balzers) for pressures in the interval of 10^-4 to 1 torr, as well as by a pressure transducer CDG Gauge (Varian) Multi Gage digitally displayed for pressures between 1 and 1000 torr. Before measurement, about 500 mg of each sample (fraction 0.38-0.54 mm) was dehydrated in situ at 300 °C in an oven under constant evacuation, down to a residual pressure of 10^-3 torr, and these conditions were maintained for 3 h. Simultaneously, the determination of the weight loss of the adsorbents was evaluated by heating samples up to 300 °C at atmospheric pressure in a conventional oven. After dehydration, the temperature of the samples was reduced until it reached the desired measuring temperature and it was then maintained constant for at least 1 h before starting any measurement. The 0 °C temperature was created by a bath of ice water. The 20 and 60 °C temperatures were controlled by a Haake L. ultra thermostat with a precision of ± 0.15 °C, while the 100 °C temperature was controlled by an oven (Lindberg) with a precision of ± 0.2 °C.

Gas adsorption uptake as a function of time was obtained as the difference between the initial amount of gas introduced into the cell and the amount of gas remaining in the dead space of the cell at any given time between 0 and t_eq (equilibrium time). The initial pressure was 400 torr in all experiments.

During the kinetic measurements, the pressure decrease in the system was measured manually and this allowed us to write down time and pressure data. The procedure used to perform the kinetic measurements was described in detail in previous papers (^{Aguilar-Armenta et al. 2001}, ^{Aguilar-Armenta et al. 2006}). After evacuation of the adsorbent in situ at 300 °C, a kinetic adsorption curve was measured at a given temperature. Subsequently, the sample was evacuated at 300 °C, as in the previous experiment, until a residual pressure of < 10^-3 torr was reached for another study.

Adsorption kinetics

Adsorbate uptake curves for N₂O were measured from t = 0 up until equilibrium was reached (t → ∞, α = constant).

a) ZAPS.Figure 1 shows the adsorption kinetic curves for N₂O on ZAPS at 0, 20, 60, and 100 °C. It can be seen that the adsorbed amount and the adsorption rate diminish with increasing temperature. This behavior allows us to deduce that the adsorption process of N₂O is not of the activated type and that it is probably not influenced by kinetic effects, possibly because the kinetic diameter of the N₂O molecule is 3.3 Å (^{Breck 1974}) and ZAPS adsorbs molecules with kinetic diameters that do not exceed 4.3 Å.

Fig. 1 Uptake rates of N₂O on erionite at different temperatures 

To study more carefully the effect of temperature on the adsorption rate of gas, the fractional pore filling αlα_∞, where α and α_∞ are the amounts adsorbed at a given time t and at equilibrium, respectively, was analyzed as a function of time. The adsorption rate of the fractional uptake of N₂O on ZAPS is shown in Figure 2. The results show that for times as short as 100 seconds, a 90 % of the total capacity of N₂O adsorption is observed. In addition, the adsorption rate of N₂O does not depend on temperature in the range of 0-100 °C (Fig. 2), confirming that this process is not an activated diffusion.

Fig. 2 N₂O fractional uptakes on erionite at different temperatures 

The values of the diffusion coefficients (D, cm²/s) were found by fitting the experimental data by the Barrer equation R = α_t - α₀/ α_∞ - α₀ = 2S_BET/V(1 + K/K) (Dt/π)^1/2 (^{Barrer 1971}). Where K =[(α ₀)g - ( α _∞ - α ₀]/ α _∞ is the ratio of the adsorbate in the gas phase to that in the adsorbed phase at equilibrium; α_t, α₀ and α _∞ are the amounts of gas adsorbed at times t, t = 0, and t = ∞ (equilibrium), respectively (in our experiments α₀ = 0); 2_BET and V are the specific external surface area of the particles (cm²/g) and the volume of the crystals (cm³/g), respectively; (α ₀ )g is the amount of gas initially available for adsorption; D is the diffusion coefficient (cm²/s). Table I summarizes the diffusion time constants of N₂O on ZAPS at different temperatures. These results show that the diffusion of N₂O rises faster with increases of temperature between 20 and 100 °C.

Table I Diffusion coefficients (D) and diffusion activation energies (Eα) of N₂O on the studied zeolites: erionite (ZAPS), mordenite (ZNT), and clinoptilolite 

The activation energy can be calculated by the equation Eα = [RT₁T₂/T₁]1n[τ ₂(_0.5)/ τ_2(0.5)] (^{Timofeev et al 1961}) Calculations revealed that the energy of activation in ZAPS is 1.8 kJ/mol (Table I).

b) ZSL. The adsorption kinetic behavior at 0, 20, 60, and 100 °C on the ZSL sample for N₂O is presented in Figure 3. These results show that the adsorbed amount and the adsorption rate diminish with increasing temperature, as in the case of the ZAPS sample. It can be seen that the adsorbed amounts of N₂O on the ZSL sample is lower than that on the ZAPS sample (Fig. 1). The latter could be due to the decrease in micropore accessibility (3.5 Å). The fractional uptakes of N₂O on ZSL at different temperatures reveal a great difference in the adsorption kinetic behavior on the ZAPS and ZSL samples (Fig. 4). The adsorbed amounts of N₂O on ZAPS, in the range from t = 0 to t = t_eq (equilibrium) at different temperatures, were greater than that on ZSL. The activation energy on ZSL was 4.6 kJ/mol. These results show that the diffusion of N₂O rises faster on ZSL than that on ZAPS as the temperature increases (Table I). Taking into account that the kinetic diameter of N₂O is about 3.3 Å, while the ZSL diameter is around 3.5 Å, we believe that these results could be due to the lower micropore participation in the N₂O adsorption process.

Fig. 3 Uptake rates of N₂O on clinoptilolite at different temperatures 

Fig. 4 N₂O fractional uptakes on clinoptilolite at different temperatures 

c) ZNT. The results shown in Figures 5 and ⁶ reveal a decrease in adsorption uptake when the temperature goes up from 0 to 100 °C in ZNT. These results show that the adsorption process of N₂O is not of the activated type and that it is probably not influenced by kinetic effects. Unlike for ZAPS and ZSL, the activation energy was 4.3 kJ/mol. This could be due to the reduction of pore volume that greatly influences the adsorption of the N₂O molecule (^{Hernández-Huesca et al. 1999}). The diffusion constants of N₂O are of a higher order as temperature increases, these values are also higher to one another (Table I).

Fig. 5 Uptake rates of N₂O on mordenite at different temperatures 

Fig. 6 N₂O fractional uptakes on mordenite at different temperatures 

Influence of the temperature in the adsorption of N₂O

The N₂O adsorption capacities decrease in the order ZAPS > ZSL > ZNT at 20 °C (Figs. 1, ³ and ⁵). Therefore, this adsorption behavior is mainly due to the limiting volume of the micropores of these zeolites and to the quantity of cations available per unit mass of the dehydrated zeolites (cationic density). The cationic density depends on the Si/ Al ratio, which follows precisely the order ZSL ≈ ZNT > ZAPS (^{Hernández-Huesca et al. 2009}). In other words, the adsorption capacity of ZAPS is the greatest because there is a larger amount of Al in this zeolite and, therefore, its cationic density is greater than that of the other two zeolites.

There is a difference between the adsorbed amounts of N₂O at 20 °C than at 100 °C. The results show that the adsorption capacities decrease in the order ZSL > ZAPS > ZNT at 100 °C. This behavior suggests that the adsorption of N₂O is favored at low temperatures on ZAPS, close to room temperature, whereas the adsorption of N₂O is recommended at 100 °C on ZSL.