(374e) Saturation Loadings on Zeolites Vary Significantly with Reduced Temperature in the Range 0.60 to 3.5

Starting in 2009, we have worked to evaluate and model saturation loadings on three zeolites: 4A, 5A, and 13X from data posted in the literature over the past 64 years. Isotherms posted in the literature for 49 species were taken from tabulated data or by digitizing the relevant figures. Saturation loadings determined from log-log plots for data exhibiting a slope of approaching zero only.

Experimental Design/Method

Our observations indicate that T_CAR, the reduced temperature at which adsorbed species transition to supercritical behavior, is often lower than T_R=1, the critical reduced temperature. The saturation loadings for subcritical temperatures can be theoretically modeled for microporous zeolites from first principles for zeolite crystals assuming 100% accessibility for the adsorbate and the Rackett model for the sorbate density as a function of ε_Z (the zeolite void fraction), ρ_Z (the zeolite crystallographic density), and Z_RA (a particular constant for the modified Rackett equation). Values of Z_RA are given in the paper by (Spencer & Danner, 1972). The saturation loadings, q_max, are then normalized by the theoretical saturation loading at the critical temperature, q_max,c.

Since supercritical data appear relatively linear, a simple linear model for the supercritical range can be derived, and the intersection of this line with the subT_CAR model provides T_CAR.

Major Findings/Results

The major findings are summarized in the Figures 1 to 4. The subT_CAR data for n-alkanes (C₁-C₈) on zeolites 5A and 13X is plotted in Figure 1 along with the model curves for saturation loading at subT_CAR temperatures for the largest and smallest Z_RA for these alkanes. The data fits the model well; some of the low points below the C₁ model may be attributed to the difficulty in measuring q_max in glass apparata such that saturation was not completely attained. A similar plot for 4A, 5A and 13X zeolites is illustrated for all the SubT_CAR data with similar observations to those in Figure 1.

The superT_CAR data for diatomic species on zeolites 4A, 5A and 13X is plotted in Figure 3. For reference, the subT_CAR model curves are also included for the largest and smallest Z_RA for these species. The data for individual species have very high linear regression coefficients (often > 0.95). The slopes of the regressed lines are remarkably similar across many species and over the three zeolites, although the intercepts vary. The spread observed is due to the many species and zeolites portrayed. The intersection of the line with the subT_CAR model provides T_CAR and is in the range of 0.6 to 1 mainly. A further interesting point is that the ratio q_max/q_max,c ranges from 2.75 to 0.5 indicating a significant change of density or molecular volume with reduced temperature. A plot of all available superT_CAR plot for 4A, 5A and 13X zeolites is in Figure 4; the observations are similar in all respects to Figure 3.

Conclusions

The data indicate that saturation loadings for all sorbates studied on zeolites is not constant but varies significantly over a large T_R range reducing by more than a factor of 2. This implies that that the sorbate density decreases with T_R and the mean molecular volume increases by the same factor. And so, the area occupied by a molecule of any species may also change with temperature. For example, nitrogen occupies an area of 16.2 Ų at 77K (T_R of 0.61), but its occupied area may be significantly different at a temperature of 700K (T_R of 5.56), which is a typical temperature during catalytic operations. We will continue to study saturation loadings as well as the Henry constant area to portray this data (see reference list).

References

Abouelnasr, D. M., & Loughlin, K. F. (2020). Saturation Loadings on 5A Zeolite above and below the Critical Conditions: Diatomic Species Data Evaluation and Modeling. Adsorption, in preparation.

Abouelnasr, D.M., & Loughlin, K.F. (2020). Saturation Loadings on 5A Zeolite above and below the Critical Conditions: Monatomic Species Data Evaluation and Modeling. Submitted to Adsorption.

Abouelnasr, D., Loughlin K., & Al Mousa, A. (2017). Saturation loadings on 13X (faujasite) zeolite above and below the critical conditions. Part III: Inorganic monatomic and diatomic species data evaluation and modeling. Adsorption, 23, 945-961.

Al Mousa, A., Abouelnasr, D., & Loughlin, K. F. (2015). Saturation Loadings on 13X (Faujasite) above and below the critical conditions. Part II: Unsaturated and Cyclic Hydrocarbons, Data Evaluation and Modelling. Adsorption, 21(4), 321-332.

Al Mousa, A., Abouelnasr, D., & Loughlin, K. (2015). Saturation Loadings on 13X zeolite. Part I: Alkane Hydrocarbons Data Evaluation and Modelling. Adsorption, 21(4), 307-320.

Dirar, Q. H., & Loughlin, K. F. (2013). Intrinsic adsorption properties of CO2 on 5A and 13X zeolite. Adsorption, 19(6), 1149-1163.