In recent years, metal-organic frameworks (MOFs) have been identified as promising
adsorbents in a number of industrially relevant, yet challenging, separations,
including the removal of propane from propane/propylene mixtures and the
separation of mixtures of xylene isomers. The highly tuneable nature of MOFs -
wherein structures may be constructed from a variety of diverse building blocks β
has resulted in the publication of a staggering number of frameworks incorporating a
wide range of network topologies, pore shapes and pore diameters. As a result, there
are a huge number of candidate adsorbents to consider for a given separation.
Molecular simulation techniques allow the identification of those structural features
and characteristics of a MOF which exert the greatest influence on the adsorption and
separation of the compounds of interest, providing insights which can both guide the
selection and accelerate the development of adsorbents for a specific application.
The separation of propane/propylene mixtures via adsorption has typically focused
on selective adsorption of the olefin, propylene, via specific olefin-adsorbent
interactions. These propylene-selective MOFs result in processes which selectively
remove the most abundant species in the process stream and are typically
characterised by high heats of adsorption, resulting in large adsorption units and
adsorbents which are difficult to regenerate. In this work, the capability of MOFs to
selectively adsorb propane over propylene is explored, potentially allowing for the
design of smaller and more energy-efficient adsorption units. By studying a range of
different MOFs as well as carbon-based model pores, it was found that the low-pressure
selectivity of the structure is determined by the strength of the electrostatic
interaction between propylene and the framework, while the adsorptive preference at
industrially-relevant pressures is dominated by the enhanced packing efficiency of
propylene over propane. The confinement of C3 molecules, however, may be
employed to negate this entropic advantage and guide the development of materials
which selectively adsorb propane over propylene.
It has recently been reported that the adsorptive preference of a MOF for one xylene
isomer over another may be predicted based solely on the pore size distribution of
the structure. In this work, the impact of pore size on selectivity was studied
systematically in both one-dimensional model pore systems of varying geometries
and analogous published MOF structures. The ability of the framework to
discriminate between xylene molecules in these systems was found to be determined
primarily by the different packing arrangements available to the different isomers β
while small pores were found to favour the slimmest of the isomers, larger pores
were found to favour the more compact ortho- isomer.
Finally, the adsorption and diffusion of xylene isomers in a more complex MOF,
UiO-66(Zr), was studied in depth. Simulations were able to correctly predict the
previously-reported preference of the MOF for ortho-xylene (oX). The smaller
volume of the oX molecule compared to the other isomers was found to be
responsible both for an enhanced entropic contribution and higher guest-host
interaction energies. The importance of framework flexibility in the diffusion of
xylene isomers in UiO-66(Zr) was also explored, with distortion of the structure in
response to interaction with adsorbed molecules found to be essential in allowing
xylenes to diffuse through the pore space
