Stability and Adsorption Properties of Metal-Organic Frameworks for Acid Gases
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Carter, Eli A.
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Abstract
Research into the chemical stability properties of metal-organic frameworks undergoing
acid gas adsorption was conducted.
Chapter 1 conducts a general overview of research progress made to date in areas relevant
to applying MOFs as solid adsorbents in acid gas adsorption. An overview of porous solid
adsorbents is given to situate MOFs in this broader context, including advantages and
disadvantages with respect to other common solid adsorbents. Factors which have been found to
impact the general chemical stability of MOFs are also discussed. Also surveyed are the general
extent and limits thus far of research into the adsorption of SO2 and NOx by MOFs and the
performance of amine-tethered MOFs with respect to adsorption and stability.
Chapter 2 surveys the metal-organic framework materials that were used in this work,
including structural illustrations and brief surveys of applications toward which these materials
have been proposed. General comments on the characterization techniques deployed in this work
are also provided to familiarize the reader with the kinds of data collected from them and an
introductory knowledge of how the techniques obtain these data.
Chapter 3 presents the adsorption and stability performance of MIL-101(Cr) for SO2 and
NO2 in both dry and humid conditions. Both gases are demonstrated to diminish the adsorption
performance of MIL-101(Cr) under dry conditions by successive breakthrough experiments with
regeneration conducted in situ. The first-pass stability parameters (surface area calculated from
nitrogen physisorption at 77K using the BET method and impact on crystallinity observed in
PXRD scans) are assessed and it is concluded that impact of SO2 and NO2 increases as the bulk phase concentrations of these gases are increased and when adsorbed in humid streams. XPS scans
of MIL-101(Cr) samples after SO2 and NO2 adsorption and regeneration conditions show new
sulfur and nitrogen species retained in the case of both gases, further demonstrating the adsorption
of each gas in MIL-101(Cr) to be irreversible. The nature of the adsorption is investigated in situ
using DRIFTS spectroscopy where chemical changes caused by NO2 are observed. It is proposed
that some metal-ligand bond cleavage occurs in MIL-101(Cr) during NO2 adsorption in a pathway
similar to that previously proposed by other researchers for NO2 adsorption in HKUST-1, UiO-67,
and the stable MOF UiO-66. Finally, CO2 adsorption at 196 K is conducted on post-SO2 and post NO2 adsorption MIL-101(Cr) samples, where it is observed that CO2 adsorption in MIL-101(Cr)
is not affected to the same extent as N2 adsorption in MIL-101(Cr); from this it concluded that the
adsorption of each acid gas results in only limited structural degradation, if any.
Chapter 4 extends the study of NO2 adsorption to other MOFs which, like MIL-101(Cr),
feature coordinatively unsaturated sites (CUS): MIL-100(Fe), MIL-100(Fe)-HT, MOF-74(Co),
MOF-74(Ni), and HKUST-1. Although studies involving NO2 adsorption have been conducted in
literature for both MOF-74 structures and HKUST-1, to the author’s knowledge chapter 4 presents
the first complete (i.e. proceeding to saturation) breakthrough adsorption curves of NO2 for all
MOFs in the chapter. The reactive nature of the NO2 adsorption is demonstrated from observation
of nitric oxide in the breakthrough effluent of all MOFs during NO2 adsorption and the presence
of nitrate groups detected by XPS in all MOFs after NO2 adsorption. The impact of NO2 adsorption
on the stability of all MOFs is assessed by N2 physisorption and PXRD, and inertness of the metal
centers is proposed as an explanation for the trend in stability of the MOFs to dry NO2 adsorption.
It is also proposed that the presence of more inert metals may exert a stabilizing influence on less
inert metals when they are incorporated in the same structure.
Chapter 5 expands the work conducted in chapters 3 and 4 in studying the adsorption of
NO2 in CUS MOFs by tethering of triethylamine (TEA) to five of the MOFs with CUS studied
across chapters 3 and 4: MIL-101(Cr), MIL-100(Fe)-HT, MOF-74(Co), MOF-74(Ni), and
HKUST-1. Reduction of BET surface area after immersion of samples in TEA is proposed as
indicative of successful amine tethering to the five structures. Complete breakthrough profiles of
dry NO2 adsorption are collected for all five amine-MOF sorbents, which to the author’s
knowledge constitutes the first recorded NO2 adsorption data in amine-tethered MOFs, and the
impact of amine tethering on NO2 adsorption performance is discussed for all five MOFs against
the background of the adsorption performance of the “neat” structures observed in chapters 3 and
4. Stability metrics are also assessed for the five amine-tethered MOFs and it is demonstrated that
amine tethering may stabilize otherwise vulnerable MOF structures. As in chapters 3 and 4 the
observation of nitric oxide in the effluent and nitrate groups retained in the amine-tethered MOFs
signal that the adsorption of NO2 is reactive, with the SBU-ligand binding sites as the most likely
sites of reaction. The proposed explanation for this persistence in reactive NO2 adsorption at these
sites is lack of saturation in amine tethering.
The main conclusions of this work are summarized in chapter 6. This dissertation advances
research into metal-organic frameworks in the following topics: (1) Demonstrating the impact of
SO2 and NO2 adsorption in the stability of MIL-101(Cr), (2) advancing the understanding of
performance and stability under NO2 adsorption of MOFs featuring coordinatively unsaturated
sites, and (3) applying amine tethering in MOFs to NO2 adsorption with initial assessment of this technique's impact on MOF stability and performance under these conditions.
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Date
2023-04-30
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Dissertation