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

Daniela Kohen, Carleton College

PRF #45141 – B5                                     2008 Report
Summary of funded research:
We continue to make progress in 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.  Our atomistic simulations provide basic scientific insights that complement experiments and facilitate the development of new materials that can effectively (and cheaply) remove atmospheric CO₂.  Following initial work simulating the adsorption behavior of carbon dioxide and nitrogen mixtures within silicon only zeolites, the projects funded by this grant have focused on the properties of these gases within related zeolites.  Two of these research 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 with aluminum atoms within zeolites on the adsorption and diffusion behavior of gas mixtures.
The bulk of the work my group has done this past year, however, deals with augmenting our understanding of the behavior of CO₂ and N₂ within silica-only zeolites.  Materials that are useful in separations allow gas species to move throughout them at reasonable speed.  We have been able to demonstrate conclusively that this is the case in the systems that we study.  In addition we have been able to understand the dissimilarity in the diffusion behavior of CO₂ and N₂ within the different zeolites in terms of thermodynamic properties such as free energy, enthalpy and entropy.  This understanding emerged from our search for preferential carbon dioxide adsorption sites within the materials that we had identified as promising CO₂ “removers”.  Expressing the siting problem in term of potential energies surfaces lead to a complete picture of a very interesting situation as gases behaves very unusually within one of the materials, ITQ-3.  In this manner, two projects that had somehow separate origins blended into one and resulted in journal article that is currently in press in the Journal of Physical Chemistry C.
Furthermore, this past summer, we addressed questions that stemmed from the findings described in this paper as we continue to deepen our understanding of a problem that proved richer than we had anticipated.  We are now investigating the behavior of a fourth all-silica zeolite with geometry similar to ITQ-3 with the goal of learning how general ITQ-3’s unusual behavior is among zeolites with cages and narrow pores connecting them.  We are also investigating the importance of coulombic interactiosn and of rotational barriers among these types of systems.  
Next summer we plan to continue working on these projects but also to make significant progress in 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 (the third project described earlier)
Two students were supported last year by this grant. Besides the publication mentioned earlier our work has resulted in students’ posters at a MERCURY conference and at a national ACS meeting, students’ talks to the members of the Carleton chemistry department doing research over the summer and two posters to be presented at an all-science poster session at Carleton College. It has also resulted in a poster presented at a Zeolite’s Gordon Conference and another at an ACTC Conference.

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