Development of accurate computational methods for simulations of adsorption and diffusion in zeolites

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Awati, Rohan Vivek
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The overall objective of this thesis has been to develop accurate computational methods for the diffusion and adsorption of small gases in zeolites. Firstly, the effect of the zeolite framework flexiblity on the single component and binary diffusion of various gases were discussed. Results indicate that for tight fitting molecules the rigid framework approximation can produce order(s) of magnitude difference in diffusivities as compared to the simulations performed with a fully flexible framework. We proposed two simple methods in which the flexible structure of a zeolite is approximated as a set of discrete rigid snapshots. Both methods are orders of magnitude more efficient than the simulations with the fully flexible structure. Secondly, we use a combined classical and quantum chemistry based approach to systematically develop the force fields based on DFT calculations for interactions of simple molecules like CH4, N2, linear alkanes, and linear alkenes in zeolites. We used a higher level of theory known as the DFT/CC method to correct DFT energies that were used in the periodic DFT calculations to develop force fields. Our results show that DFT-derived force fields give good predictions of macroscopic properties like adsorption isotherms in zeolites. The force fields are transferrable across zeolites and hence can be further used to screen materials for different storage and separation applications.
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