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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 some zeolite’s silicon atoms with aluminum atoms on the adsorption and diffusion behavior of gas mixtures. This past year my group has made significant progress in all these projects.

Materials that are useful in separations allow gas species to move throughout them at reasonable speed.  Previously, we have been able to demonstrate conclusively that this is the case in the systems that we study.  However, we discovered that the diffusion behavior of CO₂ and N₂ within the different zeolites was very different.  In particular, when looking at diffusion rates and siting we noticed that in the one material with cages and narrow channels we studied (ITQ-3) gases behaved quite unusually.  During this last year we characterized the behavior of a fourth all-silica zeolite with geometry similar to ITQ-3 (zeolite LTA), and we discovered that ITQ-3’s behavior is not unusual among zeolites with cages and narrow pores connecting them.  

Doing simulations with and without coulombic interactions we demonstrated that CO₂’s quadrupole strong attraction to the partial charges within the zeolites is responsible for its unusual behavior.  This interaction is so strong that the narrow channels are not the location of the diffusion barrier –and furthermore became the preferred site – while when coulombic interactions are neglected CO₂’s behavior becomes the most usual one where the barrier to diffusion coincides with the “bottle necks” – the narrow channels connecting the cages.  The behavior of N₂ in these materials is also quite unexpected: when coulombic interactions are neglected diffusion rates decreases, contradicting the usual assumption that weaker interactions should result in faster diffusion. The solution to this puzzle lies in the diffusion free energy profiles. These profiles show that when coulombic forces are present the interaction of nitrogen with the walls in the narrow pores is so attractive that the free energy barrier to diffusion, located at the “bottle neck”, decreases with the consequent increase in rates.

To further understand the nature of the unusual barrier to diffusion we have uncover in zeolites with narrow pores connecting larger cages we also looked at the role of orientations.  These studies allowed us to understand the unusual location of the free energy barrier for CO₂ in these materials.  Free energy and probability profiles show that, in these systems, most molecules have a very defined orientation with respect to the walls of the zeolite so molecules re-orient themselves quite dramatically as they diffuse throughout the zeolite.  Potential energy profiles and free energy profiles show that this molecular reorientation has a large entropy cost that results in the large free energy barriers we observe.

These results underscore the power of coupling macroscopic behavior (diffusion rate constants) with molecular understanding (free energy, potential energy and probability profiles) when trying to understand complex behavior.

This past year we also made significant progress in studying the behavior of CO₂, N₂ and CO₂/N₂ mixtures within zeolites that have been altered by modifying the Al/Si ratio in the rigid zeolite framework (the third project described earlier).  In particular the code has been developed so we are now able to study both diffusion and adsorption in these systems.  We are now in a position that would allow my students to start running simulations, so I expect to have preliminary results soon.

Two students were supported last year by this grant. Their work has resulted in students’ presentations at MU3C conference, 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 contributed to a poster presented at FOMMS conference.  A manuscript, currently in preparation, will also describe this work.