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Ab Initio Carbon Capture in Open-Site Metal-Organic Frameworks

Allison Dzubak, a graduate student in Professor Laura Gagliardi's group at the University of Minnesota, together with Li-Chang Lin, a graduate student in Professor Berend Smit's group at the University of California, Berkeley, have developed a computational method to predict adsorption properties of gases in porous materials.    

The article describing this work has been published in Nature Chemistry, and was available as an Advance Online Publication on Sunday, August 19. To read, visit Nature Chemistry's website.

Most energy scenarios predict a significant increase in the role of renewable energy sources and an even higher increase in energy needs. As a consequence, while the relative consumption of fossil fuels will be decreasing, in absolute terms, we will continue to burn more coal. In such a scenario carbon capture and sequestration will be one of the only viable technologies to mitigate carbon dioxide emissions.

At present, the cost associated with the capture of carbon dioxide from flue gas is one of the bottlenecks in the large-scale deployment of this technology. The search for novel materials that can make this process more efficient is thus of particular importance.

A promising class of materials is metal organic frameworks (MOFs). MOFs are crystalline materials consisting of metal centers connected by organic linkers. These materials have an extremely large internal surface area and, compared to other common adsorbents, promise very specific customization of their chemistry. By changing the metal and the linker one can generate many millions of possible materials. In practice one can only synthesize a very small fraction of these materials, and it is important to develop a theoretical method that supports experimental efforts to identify an ideal MOF for carbon capture. A key aspect is the ability to predict the properties of a MOF before the material is synthesized. This type of information can be obtained from molecular simulation using classical force fields, following the method developed by Dzubak and Lin. By fitting force fields against high-level quantum chemical calculations, they performed classical simulations to obtain single and mixed component adsorption data for CO2 and N2 in MOFs.  

The approach is based on a methodology that decomposes the total electronic interaction energy obtained from quantum chemical calculations into the various contributions (electrostatic, repulsive, dispersion, etc.).

In summary, a novel methodology that yields accurate force fields for CO2 and N2 in an open site MOF from high-level quantum chemical calculations has been developed. These force fields take into account the subtle changes in the chemical environment induced by the presence of open-metal sites in metal organic frameworks. This new method allowed researchers to reproduce the experimental adsorption isotherms for both CO2 and N2 in Mg-MOF-74 and to predict the mixture isotherms at flue gas conditions. Researchers have also shown that their methodology is transferable to systems containing different metals, linkers, and different topologies. The same approach will be used to predict properties of open-site MOFs that have not yet been synthesized.