Investigating the Relative
Influences of Molecular Dimensions and Binding Energies on Diffusivities
of Guest Species Inside Nanoporous Crystalline Materials
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Abstract
The primary objective of this article is to investigate
the relative influences of molecular dimensions and adsorption binding
energies on unary diffusivities of guest species inside nanoporous
crystalline materials such as zeolites and metal–organic frameworks
(MOFs). The investigations are based on molecular dynamics (MD) simulations
of unary diffusivities, along with configurational-bias Monte Carlo
(CBMC) simulations of the isosteric heats of adsorption (−<i>Q</i><sub>st</sub>) of a wide variety of guest molecules (CO<sub>2</sub>, H<sub>2</sub>, N<sub>2</sub>, He, Ne, Ar, Kr, CH<sub>4</sub>, C<sub>2</sub>H<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, C<sub>3</sub>H<sub>6</sub>, C<sub>3</sub>H<sub>8</sub>, and <i>n</i>C<sub>4</sub>H<sub>10</sub>) in 24 different host materials spanning
a wide range of pore sizes, topologies, and connectivities. For cage-type
materials with narrow windows, in the 3.2–4.2 Å size range,
separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8), the diffusivities
are primarily dictated by the molecular dimensions, bond lengths,
and bond angles. However, for channel structures (e.g., AFI, MFI,
MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and “open”
frameworks with large windows separating adjacent cavities (NaY, NaX,
CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest
species in any given host material are strongly dependent on the binding
energies of the guest species that can be quantified by −<i>Q</i><sub>st</sub>. The stronger the binding energy, the higher
the “sticking tendency”, and the lower the corresponding
diffusivity. The insights gained from our study are used to rationalize
published experimental data on diffusivities and trans-membrane permeances.
The results of our study will be valuable in choosing the right material
with the desired diffusion characteristics for a given separation
application