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Functionalized metal-organic frameworks as selective metal adsorbents

Jeroen De Decker UGent (2017)
abstract
A sustainable future for our world, with the help of renewable energy and green technologies... For a long time, this sounded like an idealism, a concept at best. But now, a little over 250 years since the first industrial revolution, it has become clear that all those decades of mindless resource consumption have left their mark on our planet. Sustainability is no longer a prestigious school project, but a dire necessity, at least if we want to keep our species alive for a few more centuries. However, as the Greek tragedian Euripides once said: “Nothing has more strength than dire necessity”, and thankfully the world is gradually shifting its views. We have developed and optimized technologies to help us achieve such a sustainable future. Electric cars, wind energy, solar and hydropower… The world is ready for its gradual shift towards clean energy, leaving fossil fuels behind. Sadly, the problems do no end there. While our intentions are noble this time, we have come to the realization that, in order to keep using these clean technologies, a sufficient supply of several important resources is required. Resources such as metals and other elements that are indispensable in the design and operation of ‘clean’ devices and machinery. As it turns out, some of these elements have become increasingly scarce over the past decades, up to a point where complete exhaustion is expected if we do not act in time. In this context, a group of metals, called the rare earths elements (REEs), play a central role. They are amongst the most critical elements in several sustainable technologies, whilst also being critical with respect to their (future) availability. It is shown throughout this work that it is difficult to mine them in a cost-effective way, and that a lot of geopolitics come into play to acquire them. Moreover, their recycling rate is disappointingly low, mainly because they are present in small quantities in their respective devices, and the current recycling technologies, in most cases, do not allow a cost-effective recovery. It is nonetheless of extreme importance that a steady REE supply is ensured, because in most cases these elements have no worthy replacements. In the domain of recycling, more and more interest is shown in the concept of urban mining, where the technosphere (i.e., electronic waste) serves as an actual mine, full of valuable resources. This ‘e-waste’ has the potential to supply a significant amount of rare earths to help satisfy an increasing demand. On the other hand, next to urban mining, improved (sustainable) primary mining should also be able to increase our REE supply. It is in this context that the core of this work resides. Both in primary REE mining and recycling processes, aqueous streams are often generated, containing varying fractions of these metals. It is shown that in the treatment of these solutions, a lot of improvement can be made in terms of applied technologies. While industrial techniques like solvent extraction will remain an obvious choice for highly concentrated solution processing (e.g., g/L scale), they prove to be cost-ineffective for more dilute streams (e.g., mg/L scale). In addition, when dealing with REE recycling or the processing of secondary deposits (e.g., resources with a typically lower REE content than primary ores), these techniques would face the similar issues. Therefore, adsorption technology is proposed in this work, as a worthy alternative. The advantages of this technique are discussed, and it is shown that adsorbents are ideal materials for low concentration recovery or removal or certain species of interest. With respect to REE mining and recovery, different domains are proposed where selective adsorption could earn its place as the prevailing technology, and the requirements for such adsorbents are discussed. Whether it is to recover rare earths from process or waste streams, or to purify REE-containing solutions from toxic elements (uranium, thorium…), adsorbents can be tailored readily towards a certain application. Further on, an introduction is given into the world of metal-organic frameworks (MOFs). The concept of using these versatile coordination polymers in aqueous environment as adsorbents is explored and documented with existing literature. The stability of MOFs will play a major role in their possible application as aqueous adsorbents. It was therefore investigated which structural parameters have a significant influence on this stability. In short words; What makes a MOF suitable for water-based applications? In addition, a study was performed on the long-term stability of several popular MOFs, regarding their behavior in different aqueous conditions, i.e., acidic, alkaline, oxidative... With this information a rational choice can be made in the selection of suitable MOFs for aqueous adsorption applications. This work then describes the application of a specific cage-type MOF, namely MIL-101(Cr), as a potential ideal support for aqueous adsorption. It was chosen based on its incredible stability, high porosity, and ready functionalizability. The MIL-101(Cr) was functionalized via two different routes with a specific ligand and its affinity for certain critical metals (REEs, uranium…) was investigated. In one approach, a carbamoylmethylphosphine oxide-type ligand (CMPO) was covalently anchored onto the surface of MIL-101 via a three-step method. Another approach encapsulates another CPMO-type ligand in-situ in the cages of MIL-101, where it is confined and cannot leach out. Both materials have been subjected to a comprehensive adsorption study, in which their performance in terms of selectivity, stability, uptake, kinetics, recyclability… is investigated. In conclusion, it is shown that several MOFs do possess the required characteristics to serve as aqueous-phase adsorbents. They can be tailored towards a specific application, via different functionalization methods. This was proven for the ultra-stable MIL-101(Cr), which could be functionalized with selective ligands for critical metals recovery. As there are several stable MOFs to explore, and various strategies to tailor them, it is safe to say that MOF-based adsorption opens up exciting avenues in the field of aqueous-phase metal recovery.
Please use this url to cite or link to this publication:
author
promoter
UGent, UGent and UGent
organization
year
type
dissertation
publication status
published
subject
pages
XXXIV, 223 pages
publisher
Ghent University. Faculty of Sciences
place of publication
Ghent, Belgium
defense location
Gent : Aula universiteit
defense date
2017-06-12 15:00
language
English
UGent publication?
yes
classification
D1
copyright statement
I have transferred the copyright for this publication to the publisher
id
8523501
handle
http://hdl.handle.net/1854/LU-8523501
date created
2017-06-13 09:55:31
date last changed
2017-06-14 06:58:10
@phdthesis{8523501,
  abstract     = {A sustainable future for our world, with the help of renewable energy and green technologies... For a long time, this sounded like an idealism, a concept at best. But now, a little over 250 years since the first industrial revolution, it has become clear that all those decades of mindless resource consumption have left their mark on our planet. Sustainability is no longer a prestigious school project, but a dire necessity, at least if we want to keep our species alive for a few more centuries. However, as the Greek tragedian Euripides once said: {\textquotedblleft}Nothing has more strength than dire necessity{\textquotedblright}, and thankfully the world is gradually shifting its views. We have developed and optimized technologies to help us achieve such a sustainable future. Electric cars, wind energy, solar and hydropower{\textellipsis} The world is ready for its gradual shift towards clean energy, leaving fossil fuels behind.
Sadly, the problems do no end there. While our intentions are noble this time, we have come to the realization that, in order to keep using these clean technologies, a sufficient supply of several important resources is required. Resources such as metals and other elements that are indispensable in the design and operation of {\textquoteleft}clean{\textquoteright} devices and machinery. As it turns out, some of these elements have become increasingly scarce over the past decades, up to a point where complete exhaustion is expected if we do not act in time.
In this context, a group of metals, called the rare earths elements (REEs), play a central role. They are amongst the most critical elements in several sustainable technologies, whilst also being critical with respect to their (future) availability. It is shown throughout this work that it is difficult to mine them in a cost-effective way, and that a lot of geopolitics come into play to acquire them. Moreover, their recycling rate is disappointingly low, mainly because they are present in small quantities in their respective devices, and the current recycling technologies, in most cases, do not allow a cost-effective recovery. It is nonetheless of extreme importance that a steady REE supply is ensured, because in most cases these elements have no worthy replacements.
In the domain of recycling, more and more interest is shown in the concept of urban mining, where the technosphere (i.e., electronic waste) serves as an actual mine, full of valuable resources. This {\textquoteleft}e-waste{\textquoteright} has the potential to supply a significant amount of rare earths to help satisfy an increasing demand. On the other hand, next to urban mining, improved (sustainable) primary mining should also be able to increase our REE supply.
It is in this context that the core of this work resides. Both in primary REE mining and recycling processes, aqueous streams are often generated, containing varying fractions of these metals. It is shown that in the treatment of these solutions, a lot of improvement can be made in terms of applied technologies. While industrial techniques like solvent extraction will remain an obvious choice for highly concentrated solution processing (e.g., g/L scale), they prove to be cost-ineffective for more dilute streams (e.g., mg/L scale). In addition, when dealing with REE recycling or the processing of secondary deposits (e.g., resources with a typically lower REE content than primary ores), these techniques would face the similar issues.
Therefore, adsorption technology is proposed in this work, as a worthy alternative. The advantages of this technique are discussed, and it is shown that adsorbents are ideal materials for low concentration recovery or removal or certain species of interest. With respect to REE mining and recovery, different domains are proposed where selective adsorption could earn its place as the prevailing technology, and the requirements for such adsorbents are discussed. Whether it is to recover rare earths from process or waste streams, or to purify REE-containing solutions from toxic elements (uranium, thorium{\textellipsis}), adsorbents can be tailored readily towards a certain application. Further on, an introduction is given into the world of metal-organic frameworks (MOFs). The concept of using these versatile coordination polymers in aqueous environment as adsorbents is explored and documented with existing literature. The stability of MOFs will play a major role in their possible application as aqueous adsorbents. It was therefore investigated which structural parameters have a significant influence on this stability. In short words; What makes a MOF suitable for water-based applications? In addition, a study was performed on the long-term stability of several popular MOFs, regarding their behavior in different aqueous conditions, i.e., acidic, alkaline, oxidative... With this information a rational choice can be made in the selection of suitable MOFs for aqueous adsorption applications.
This work then describes the application of a specific cage-type MOF, namely MIL-101(Cr), as a potential ideal support for aqueous adsorption. It was chosen based on its incredible stability, high porosity, and ready functionalizability. The MIL-101(Cr) was functionalized via two different routes with a specific ligand and its affinity for certain critical metals (REEs, uranium{\textellipsis}) was investigated. In one approach, a carbamoylmethylphosphine oxide-type ligand (CMPO) was covalently anchored onto the surface of MIL-101 via a three-step method. Another approach encapsulates another CPMO-type ligand in-situ in the cages of MIL-101, where it is confined and cannot leach out. Both materials have been subjected to a comprehensive adsorption study, in which their performance in terms of selectivity, stability, uptake, kinetics, recyclability{\textellipsis} is investigated.
In conclusion, it is shown that several MOFs do possess the required characteristics to serve as aqueous-phase adsorbents. They can be tailored towards a specific application, via different functionalization methods. This was proven for the ultra-stable MIL-101(Cr), which could be functionalized with selective ligands for critical metals recovery. As there are several stable MOFs to explore, and various strategies to tailor them, it is safe to say that MOF-based adsorption opens up exciting avenues in the field of aqueous-phase metal recovery.},
  author       = {De Decker, Jeroen},
  language     = {eng},
  pages        = {XXXIV, 223},
  publisher    = {Ghent University. Faculty of Sciences},
  school       = {Ghent University},
  title        = {Functionalized metal-organic frameworks as selective metal adsorbents},
  year         = {2017},
}

Chicago
De Decker, Jeroen. 2017. “Functionalized Metal-organic Frameworks as Selective Metal Adsorbents”. Ghent, Belgium: Ghent University. Faculty of Sciences.
APA
De Decker, J. (2017). Functionalized metal-organic frameworks as selective metal adsorbents. Ghent University. Faculty of Sciences, Ghent, Belgium.
Vancouver
1.
De Decker J. Functionalized metal-organic frameworks as selective metal adsorbents. [Ghent, Belgium]: Ghent University. Faculty of Sciences; 2017.
MLA
De Decker, Jeroen. “Functionalized Metal-organic Frameworks as Selective Metal Adsorbents.” 2017 : n. pag. Print.