Theoretical and experimental study of adsorption of fluids in zeolites: Molecular simulation and calorimetry

Jude Anthony Dunne, University of Pennsylvania

Abstract

Factors such as adsorbate size, shape and polarity coupled with adsorbent pore structure, pore size distribution and energetic heterogeneity play a fundamental role in determining adsorption equilibrium. In this thesis, the effect of these factors on both single and multicomponent adsorption equilibrium has been studied through molecular simulation and calorimetry. Partial exclusion of adsorbate molecules was studied by grand canonical Monte Carlo molecular simulation of a Lennard-Jones fluid mixture adsorbed in zeolite 13X. The molecules forming the binary mixture are non-polar and have a 2.4 ratio of saturation capacities. In the molecular simulation, adsorption occurs on the energetically-homogeneous surface of a spherical shell. Binary adsorption is centered on separate shells with the larger molecule in the inner shell. At high coverage, the simulations yield moderate negative deviations from ideal mixing similar to experimental data. Both molecules can enter the cavity, but adsorbed-phase nonidealities are caused by the exclusion of the larger molecule from part of the micropore space accessible to the smaller molecule. The partial exclusion effect is calculated from the cavity radius and molecular diameters of the adsorbate molecules. The effect of differences in polarity of adsorbate molecules was also investigated through GCMC simulations of adsorption of N$\sb2$ and O$\sb2$ and their mixtures in a model zeolitic cavity. Micropore condensation is observed for oxygen but not for nitrogen. The condensation of oxygen is similar to a first-order phase transition but because of the small number of molecules that can fit into a micropore, co-existence of two phases is replaced by oscillations between gas- and liquid-like densities. To study the effect of adsorbate size and polarity on the energetics of adsorption in zeolites a Tian-Calvet type calorimeter was developed to determine adsorption isotherms and heats of adsorption. The adsorbate gases used in this thesis are quadrupolar (N$\sb2$, CO$\sb2$) and nonpolar (Ar, O$\sb2$, CH$\sb4$, $\rm C\sb2H\sb6$, and SF$\sb6$). Adsorbents include silicalite, NaX, Na-ZSM-5 and H-ZSM-5 and allow for a study of the impact of ion concentration and type as well as pore size on heats of absorption. In silicalite, the heats of adsorption of both polar and nonpolar gases are either constant or increase with coverage and may be classified as a relatively homogeneous adsorbent compared to X or Y type zeolites. Both the heats of adsorption and the loading are reversible and have been reproduced within 2% with different samples. Heats of adsorption of non-polar molecules (CH$\sb4$, $\rm C\sb2H\sb6$, SF$\sb6$) increase in the order NaX, silicalite, H-ZSM-5, Na-ZSM-5 at low coverage. Heats of adsorption of non-polar molecules are almost identical on silicalite, H-ZSM-5, and Na-ZSM-5 at high coverage. Heats of adsorption of the quadrupolar molecule CO$\sb2$ increase in the order silicalite, H-ZSM-5, Na-ZSM-5, NaX with ion type and concentration having a very strong impact on Co$\sb2$ loadings and heats of adsorption. Equilibrium loadings have been measured for many multicomponent systems but heats of adsorption are rarely determined. A second calorimeter was constructed to determine multicomponent equilibrium loadings and heats of adsorption simultaneously. (Abstract shortened by UMI.)

Subject Area

Chemical engineering|Materials science

Recommended Citation

Dunne, Jude Anthony, "Theoretical and experimental study of adsorption of fluids in zeolites: Molecular simulation and calorimetry" (1996). Dissertations available from ProQuest. AAI9636147.
https://repository.upenn.edu/dissertations/AAI9636147

Share

COinS