Environmental Engineering Reference
In-Depth Information
has been the establishment of reliable and systematic methods of studying critical
phenomena by simulation, although typically the Monte Carlo method is more
useful for this type of study.
1.3.2.7 PREDICTING GAS DIFFUSIVITY USING MOLECULAR
DYNAMICS SIMULATIONS IN MOFS
1.3.2.7.1 EQUILIBRIUM MOLECULAR DYNAMICS SIMULATIONS IN
THIS CASE STUDY
Over approximately the last decade, metal organic framework (MOF) materials
have attracted a great deal of attention as a new addition to the classes of nano-
porous materials. MOFs, also known as porous coordination polymers (PCPs) or
porous coordination networks (PCNs), are hybrid materials composed of single
metal ions or polynuclear metal clusters linked by organic ligands through strong
coordination bonds. Due to these strong coordination bonds, MOFs are crystal-
lographically well-defined structures that can kept their permanent porosity and
crystal structure after the removal of the guest species used during synthesis. In
the literature, MD simulations have been used to predict three different types of
gas diffusivities in MOFs. These are transport diffusivity, corrected diffusivity and
self-diffusivity. MOFs have become attractive alternatives to traditional nanopo-
rous materials specifically in gas storage and gas separation since their synthesis
can be readily adapted to control pore connectivity, structure and dimension by
varying the linkers, ligands and metals in the material. The enormous number of
different possible MOFs indicates that purely experimental means for designing
optimal MOFs for targeted applications is inefficient at best. Efforts to predict the
performance of MOFs using molecular modeling play an important role in select-
ing materials for specific applications. In many applications that are envisioned
for MOFs, diffusion behavior of gases is of paramount importance. Applications
such as catalysis, membranes and sensors cannot be evaluated for MOFs without
information on gas diffusion rates. Most of the information on gas diffusion in
MOFs has been provided by molecular dynamics (MD) studies. The objective of
this chapter is to review the recent advances in MD simulations of gas diffusion in
MOFs. In Sectio ns 1.3.2.7.2 and 1.3.2.7.3 the MD models used for gas molecules
and MOFs will be introduced. Studies which computed single component and
mixture gas diffusivities in MOFs will be reviewed in Section 1.3. The discussion
of comparing results of MD simulations with the experimental measurements and
with the predictions of theoretical correlations will be given in Section 1.2.2.11,
respectively. Finally, opportunities and challenges in using MD simulations for
examining gas diffusion in MOFs will be summarized in Section Gas diffusion is
an observable consequence of the motion of atoms and molecules as a response to
external force such as temperature, pressure or concentration change. Molecular
dynamics (MD) is a natural method to simulate the motion and dynamics of atoms
and molecules. The main concept in an MD simulation is to generate successive
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