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45141-B5
Studying the Behavior of Carbon Dioxide Within Zeolites: Atomistic Simulations

Daniela Kohen, Carleton College

Summary of funded research:

During this grant period several research projects have continued to be developed with the aim of understanding and characterizing at the molecular level how small gas molecules interact with pure CO₂ in pores of molecular sieves and how this interaction changes in the presence of other gases present in our atmosphere. These studies will help to improve our basic understanding of the use of molecular sieves as filters to remove CO₂ from the atmosphere. Following initial work on the adsorption behavior of carbon dioxide and nitrogen mixtures within silicon only zeolites, the projects have focused on the properties of these gases within related zeolites. Two projects have focused on the properties of zeolites containing only silicon and oxygen: the search for preferential sites for adsorption and the examination of diffusion of CO₂ and other gases within these zeolites. A third related project is a systematic study of the effect of substituting silicon atoms for aluminum atoms within zeolites on the adsorption and diffusion behavior of gas mixtures. Significant progress has been made in all these projects. We have been able to demonstrate that within some siliceous zeolites that are highly selective for carbon dioxide, diffusion is fast enough to enable the use of these materials in practical separations. We have also been able to understand the diffusion and adsorption behavior of CO₂/N₂ mixtures in terms of preferential adsorption sites and have established that the role of entropy is minor compared to that of competing enthalpic factors. Our study of substituted zeolites is in its preliminary phase, but for at least the one zeolite studied the inclusion of Al atoms in the material framework induces a promising increase of CO₂ adsorption. Finally, a new project has stemmed from our desire to develop better gas-zeolite interaction potentials. In collaboration with Mark Gordon (Iowa State University) we are starting to explore the use of the Effective Fragment Potential to model the interaction of zeolites and CO₂. This last project is its infancy, but we are quite excited about the prospect of obtaining a better description of the interactions.

Two students have worked on these projects during the grant period, and our work has resulted in students' talks at the MU3C (Midwest Undergraduate Computational Conference - a small regional meeting), talks to the members of the Carleton chemistry department doing research over the summer, and a poster to be presented at the AICHE later this fall.

Research activities:

The nature of our investigations (atomistic simulations) allows us to gain macroscopic as well as microscopic information about the adsorption and diffusion processes; this information can provide guidance in the design of an adsorbent selective to CO₂. Some of the funding from this grant has been used in projects inspired by this notion of a guided search. One such a project involves simulating the adsorption behavior and diffusion of CO₂, N₂ and CO₂/N₂ mixtures onto zeolites that have been altered by modifying the Al/Si ratio in the rigid zeolite framework. During this last year we have adapted our Grand Canonical Monte Carlo code to handle these materials, and we have also obtained preliminary results for adsorption of CO₂ onto Al containing silicalite. These results are quite promising as the presence of Al induces in increase in adsorption. The next phase of this project includes calculations in other zeolites but also modifying our Molecular Dynamics code so we can look at diffusion in these materials.

A second project, identifying the most likely adsorption sites within the zeolites, was also inspired by our ability to gain macroscopic as well as microscopic insight about the adsorption and diffusion processes. In the last year we have learned that N₂ and CO₂ have the same preferred sites, but that CO₂ adsorbs much more strongly to them – which explains the strong selectivity of these materials. Also, we learned that in all materials the preferred sites are close to the walls, which strongly suggests that an ideal zeolite is one with intermediate pore sizes. Some of these results will to be included in a manuscript that is already in preparation.

A third project, studying diffusion of CO₂, N₂ and CO₂/N₂ mixtures within silica only zeolites, stems from our desire to establish if diffusion is fast enough to enable the use of these materials in practical separations. All our results show that diffusion is not too slow, and that the details depend on the geometry of the materials. Interestingly, we have also shown that the presence of CO₂ affects the diffusion of N₂ but that the reverse is not true. We are currently looking at the causes of this phenomenon. I am currently in the process of writing these results up, and expect I will have submitted the manuscript by the end the 2007 calendar year.

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