The process of purifying hydrogen gas using pressure swing adsorption columns
heavily relies on highly efficient adsorbents. Such materials must be able to
selectively adsorb a large amount of impurities, and must also be regenerated with
ease. The work presented in this thesis focuses on a novel class of porous solids,
metal-organic frameworks (MOFs), and their potential for use as adsorbents in
hydrogen purification processes. MOFs are tuneable structures, a property that can
be exploited in order to achieve the desired characteristics that are beneficial for a
specific application. The design or selection of MOFs for any separation process
however, relies on a thorough understanding of the relationship between a
framework’s characteristics and its adsorption and selective properties.
In order to identify favourable MOF characteristics for the separation of hydrogen
from typical impurities a systematic molecular simulation study is performed on a
large group of MOFs. Features such as the presence of short linkers, amine groups
and additional aromatic rings, and a high density of linker groups are found to
increase the adsorbate - framework interaction strength, and reduce the free volume
available inside the pores. Both of these effects are shown to enhance MOF
selectivity for impurities. Two promising materials, exhibiting desirable features, Mn
MIL-53 and MIL-47, are studied further through a variety of approaches.
A combination of experimental work and molecular simulations are employed in
order to assess the level of flexibility in Mn MIL-53 on uptake of CO2 and CH4. An
investigation of the experimental and simulation adsorption and characterization data
indicates that the framework undergoes structural changes, in order to accommodate
CO2 molecules, but not CH4. The form of the framework during CO2 uptake is also
shown to be strongly influenced by temperature.
In the case of MIL-47, adsorption isotherms simulated for a wide range of gases
overpredict experimental adsorption data, leading to an in-depth investigation of
non-porous effects, force field suitability, and framework rigidity. Ab initio
molecular dynamics studies of MIL-47 indicate that the benzene dicarboxylate
linkers rotate about their symmetry axis to reach more energetically favourable
configurations, an effect responsible for the discrepancies between simulated and
experimental isotherms.
The effect of MOF flexibility on adsorption is further highlighted in a study of
Sc2BDC3, a material able to undergo structural changes in order to accommodate a
variety of adsorbates. Molecular simulations show that structural changes in the
framework are responsible for the creation of additional CO2 adsorption sites as
pressure is increased, whereas methanol adsorption sites occupied at extreme
pressure are stabilized by the formation of hydrogen bonds.
Finally, the exceptionally robust UiO-66(Zr) and UiO-67(Zr) families of MOFs are
analysed using a multi-scale simulation study combining molecular level and
process-scale computational work, seeking to compare the materials to commercial
adsorbents, and assess whether they are suitable for H2 purification through pressure
swing adsorption (PSA). Of the four MOFs studied, UiO-66(Zr)-Br is the most
promising, as it significantly outperforms commercial zeolites and activated carbons
in H2 purification from steam methane reformer offgas