(377g) Computational Modelling of CO2 and CH4 Diffusion in Bulk ZIF-8

font-family:" times new roman>Computational Modelling of CO₂
and CH₄ Diffusion in Bulk ZIF-8

Hari P.
Paudel, Wei Shi, David Hopkinson, Yuhua Duan

justify;line-height:normal;text-autospace:none"> font-family:" times new roman>Zeolite imidazolate frameworks (ZIFs)
have selective adsorption properties and are useful materials for gas
separation. As compared to traditional chemical sorbents (mostly amines), ZIFs
have (1) high CO₂ loading capacity, (2) lower-energy consumption,
(3) high thermal and chemical stability, (4) fewer mechanical requirements and
lower corrosion rates (Banerjee et al, Science 2008, 319, 939; Park et
al, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186, Nordin, et al RSC Adv., 2014,4,
52530-52541). There have been several studies, both theoretical and
experimental, that reveal that ZIF-8 material is highly responsive to changes
of the external physical and chemical environments due to its hybrid
metal/linker type structure. Interaction of sorbent materials with gas
molecules plays a significant role in identifying materials’ nature of
selective adsorption and diffusion for some gas molecules over others.
Therefore, understanding of ZIF-8 interaction with different gas molecules
plays crucial role for further experimental and theoretical studies while
considering the bulk, surface and interface composite model designs. With the
understanding of gas adsorption and diffusion in bulk, surface and at interface
of ZIF-8 helps in screening sorbent materials of similar or with no significant
difference in their properties.

justify;line-height:normal;text-autospace:none"> font-family:" times new roman>Several experimental and theoretical
studies have been performed to understand the gas adsorption and diffusions in
ZIF-8.  While those reported theoretical studies are helpful for assessing the
computational methodology and atomistic models at large, the basic theoretical
understanding of adsorption processes and gas diffusion selectivity using
quantum mechanical calculations is still in question. The preferential
adsorption of gas molecule of one type in a gas mixture by sorbent material can
hinder the diffusion of other type of gas molecule. On the other hand, due to a
variation in size of gas molecules, diffusion process is affected for different
type of gas molecules with different measure as they diffuse through the pores.
Overall effect on transport and selectivity demands a deep understanding of
interaction picture of framework with gas molecules.

justify;line-height:normal;text-autospace:none"> font-family:" times new roman>Computational studies based on the
density functional theory (DFT) can capture the fundamental properties of
interactions and gas diffusion mechanisms that can be directly correlated with
the experimental observations. We present our studies based on DFT calculations
on CO₂ and CH₄ gas adsorption and diffusion in the bulk
of ZIF-8. We evaluate the structural and electronic properties of bulk ZIF-8,
determine the strong adsorption sites for gas molecules, and calculate the
diffusion barriers for CO₂ and CH₄. Our calculations
include long-range dispersion interactions. We analyze adsorption properties by
fixing as well as relaxing both the framework atoms and gas molecules, examine
their effects on gas diffusion properties, and compare with the experimental
results where available. We find that the CO₂ and CH₄
adsorption energies at the strongest adsorption sites are 5.01 and 4.65
kcal/mol, respectively.  The diffusion energy barriers for CO₂ and
CH₄ are attractive, and are calculated to be -3.65 and -4.01
kcal/mol, respectively. With the above value of diffusion barrier, the
diffusion coefficient for CO₂ is calculated to be 1×10^-10
m²/s.

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