Study of Porous Adsorbents for Carbon Capture via Molecular Simulation

Increasing atmospheric CO2 levels are of concern due to potential links/nto negative environmental impacts such as increasing global temperatures and/nocean acidification. Strategies for reducing CO2 emissions involve both/nreducing the use of fossil fuels a primary energy sources and employing/nprocesses to capture CO2 from gas streams incidental to energy/ngeneration and store in deep geologic features. The one of the main targets/nfor CO2 separation are post-combustion gas streams at electricity/ngenerating plants, which represent a large fraction of the CO2 emitted./nWhile current process technology (amine scrubbing) could be scaled to the/naccomplish the task, it is a relatively inefficient process and would/nsubstantially reduce the efficiency of electricity generation./nAdsorption-based processes have the potential to reduce the parasitic load on/ngenerating plants by reducing the amount and quality of heat diverted from/ngenerating cycle./nAdsorption processes involve the use of solid porous materials with large/ninternal surface areas to separate components of a gas mixture. One or more/ncomponents will preferentially adsorb and be enriched in the adsorbed mixture,/nwhich can then be desorbed in a separate part of the process. While a variety/nof adsorbents have wide application in industry, there is a need to identify/nthe most efficient materials for this process to ensure its economic/nviability. Some materials of interest are zeolites and metal-organic/nframeworks, and recent experimental and theoretical work have identified/nthousands of possible new materials. To evaluate this large range of/nmaterials, molecular simulation techniques useful for quickly generating/nthermodynamic data and understanding the molecular-level mechanisms/nresponsible for selectivity./nThis work details efforts addressing several aspects using Monte Carlo/nsimulations to evaluate different materials for CO2 separations. One/naspect is the proper description of how mixtures adsorb in materials with/nheterogeneous surfaces where components can competitively adsorb at spatially/ndistinct sites. By applying ideal adsorbed solution theory (IAST) to separate/nLangmuir sites, it is possible to improve predictions of mixture adsorption/nisotherms compared to applying IAST to the whole isotherm. One critical/nelement of applying IAST accurately is ensuring the saturation loadings of/ndifferent components in a mixture are estimated as accurately as possible./nAnother part addresses how to apply simulation techniques to millions of/nrelated structures simultaneously and evaluate them with a simple model of/ngenerating plant performance. Using GPU-accelerated Monte Carlo simulations, a/ndatabase containing thousands of hypothetical zeolite structures was screened/nfor CO2/N2 separations, and many structures were identified that/npotentially would have a lower energy penalty to operate than the standard/namine scrubbing process./nNext, a method for fitting parameters for a classical force field from ab/ninitio calculations was developed and used to predict the adsorption of/nCO2 in Mg-MOF-74, a promising MOF material for CO2 separations./nUsing a modified Buckingham potential with an additional r-5 attractive/nterm was able to describe the enhanced interaction between CO2 molecules/nand the coordinative-unsaturated Mg atoms. Finally, the adsorption of water in/nzeolite 13X was studied, showing the strong effect it has on the co-adsorption/nof CO2. Rearrangement of sodium cations in the zeolite pores was/nimportant for predicting the correct isotherms, and at the highest water/nloadings, some sodium cations are removed from the pore walls become/ncoordinated by water closer to the center of the pore. This rearrangement/nmay explain the the steep elbow of the isotherm.

Advisor: Smit, Berend