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From the desorption curves displayed in Figure 5, it is seen, obviously, that, under the same experimental conditions, the time spent for desorption to a certain c/c0 reduce with the increase of mesoporosity in Beta zeolites, reflecting the enhancement of the speed of desorption by introduction of mesopores inside zeolites. For three different xylene isomers adsorbates, the desorption rates are in the order of p-xylene > m-xylene > o-xylene. A summary of the effective diffusion time constants (Deff/R2) with parameters L and β extracted from the ZLC model fittings corresponding to Figure 5 are listed in Table 5. It is seen that the fitted L values, which were used as a criterion for the ZLC model to guarantee that the desorption process is controlled by the kinetics [34,35], are all greater than 5. The effective diffusivity values (Deff/R2) of all sorbates are of the same magnitude around 10−4·s−1, which is in reasonably good agreement with the reported results [36,37]. Figure 6 reveals the variation of the effective diffusion time constants for p-xylene, m-xylene and o-xylene with the mesopore volume at 333 K. It can be seen that there is a similar trend for all adsorbates that the values of Deff/R2 approximate linearly increase with mesopore volumes of the Beta zeolites. The values of effective diffusion time constants Deff/R2 for three xylene isomers increase with the mesoporosities of the Beta zeolites at the measured temperature. This confirms that the molecular transport in mesoporous Beta zeolites is obviously enhanced. In addition, the values of Deff/R2 for the three xylene isomers in the four Beta zeolite samples at 373 K is in the following order: p-xylene > m-xylene > o-xylene, which shows the same trend as expected from their molecular kinetic diameter and Henry’s constants. The favorable molecular configuration of adsorbate and weak interaction of adsorbate-adsorbent will be beneficial on the diffusion rate of adsorbates. Moreover, the relation of corresponding activation energy values calculated according the relationship between the temperature and diffusivity to mesopore volume are displayed in Figure 6. It is seen for the opposite trend with diffusivity, i.e., the Ea values decrease with the increase of the mesopores in four samples, which is similar to the previous reported results [8]. In addition, a significant downtrend can be seen for all adsorbates from Beta-0 to Beta-3 (e.g., from 24.7 kJ/moL to 15.6 kJ/moL for p-xylene, from 30.3 kJ/moL to 20.9 kJ/moL for m-xylene, from 31.1 kJ/moL to 21.9 kJ/moL for o-xylene, respectively). Such an observation reveals a general pattern that the activation energy involved in diffusion in a mesoporous zeolite would be relatively lower than that in a microporous zeolite. 249PDF Image | Zeolite Catalysis
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