Magic MOFs

Since the first ‘MOF’ structure was made over 20 years ago, researchers have created more than 20,000 different versions using a variety of metals and organic ligands, Maria Burke reports

Metal-organic frameworks (MOFs) are versatile structures made up of metal ions connected to organic ligands, forming one-, two-, or three-dimensional compounds. Their crystal structure hosts nano-sized pores and they have the largest internal surface area of any known class of material. Their ability to selectively adsorb ions and gases makes them useful for a range of applications, such as carbon capture and removing heavy metals from water. They can be modified to mimic the functions of DNA, one of which includes hydrogen bonding interactions between nucleobases.

One potential application, for example, is in water purification, where traditional polymeric membranes for separating ions from water often have limited selectivity. Nanoporous membranes with uniform nanopores that act like a sieve may overcome this limitation. But fabricating ultrathin – less than 100nm – MOFs membranes for water-related processing is challenging because of their 3D crystal constitution, plus most reported MOFs membranes are insufficiently stable in water.

In June 2020, however, an international research team claimed ‘a world first’ when it reported an ultrathin MOFs membrane that could completely separate potentially harmful ions, such as lead and mercury, from water. The membrane performed steadily for more than 750 hours using limited energy.

The team, led by Monash University and Australia’s Nuclear Science and Technology Organisation, developed 2D nanosheets from water-stable aluminium tetra-(4-carboxyphenyl) porphyrin frameworks.¹ These Al-MOFs nanosheets were then exfoliated to have a lateral size between 200nm and 2μm in thickness. The exfoliated nanosheets were used as building blocks to create ultrathin water-stable membranes.

‘In this world-first study, we were able to use these ultrathin Al-MOFs to create a membrane that is permeable to water while achieving maximum porosity with nearly 100% rejection of [inorganic] ions,’ says Xiwang Zhang of Monash University. The team reports that most water molecules pass through vertically aligned channels formed by pores in the nanosheets, but ions, such as metal and chloride ions, are blocked. Zhang says Al-MOFs nanosheets could be used to enhance desalination processing and clean up contaminated water, as well as in the separation of gases and solvents.

Researchers at the Ecole Polytechnique Fédérale de Lausanne in Switzerland are focusing on MOFs to remove toxic metals from water. In May 2020, a team led by Wendy Queen reported on a novel technique to capture hexavalent chromium (Cr(VI)) from real-world water samples and reduce it to a less mobile and more benign species. The team believes this is the first demonstration of MOF-based adsorption–photoreduction of Cr(VI) in a single process.

The team used a known Zr-MOF (UiO-66) and functionalised it with double amino groups, which allows the new material to act as both adsorbent and photocatalyst. Next, they made their material into beads using polyethersulfone (PES) chemically modified with carboxylic acid groups to improve hydrophilicity. This modification enhances the Cr(VI) extraction rate of the beads by a factor of about three.

Queen reports that the sponge-like material offers one of the highest Cr(VI) uptake capacities reported to date, rapid extraction rates, high selectivity for Cr(VI) in real-world water samples, and full recyclability.² The team demonstrated that the Cr(VI) concentration could be brought below drinkable levels and Cr(VI) could be photoreduced to Cr(III) species using light during adsorbent regeneration.

‘The great thing about our sponges is they are relatively easy and cheap to make,’ explains Queen. ‘The next step is to test our sponges at larger scales.’ She acknowledges further developments are needed to implement the technology for decontaminating water outside the lab.

Carbon capture

Currently, power plants strip CO₂ from flue emissions by bubbling flue gases through organic amines in water, which bind to and extract the CO₂. The liquid is then heated to release the CO₂ gas, after which the liquids are reused. However, the process consumes about 30% of the power generated. Researchers in the US at the University of California, Berkeley, Lawrence Berkeley National Laboratory and ExxonMobil have been working on using MOFs as an alternative.

Six years ago, Jeffrey Long’s group at UC Berkeley reported that a chemically modified MOF could capture CO₂ from concentrated power plant flue emissions, potentially reducing the cost by half. They added diamine molecules to a magnesium-based MOF to catalyse the formation of polymer chains of CO₂. A major advantage is that the amines can be tweaked to capture CO₂ at different concentrations, ranging from 12-15% – typical of coal plant emissions – to 4% as in natural gas plants, or even the much lower concentrations in air. But the hot stream of water and CO₂ needed to flush out the captured CO₂ eventually drives off the diamine molecules, shortening the life of the material.

Now the team has reported a new version, which uses a tetraamine that is much more stable at high temperatures and in the presence of steam.³ In experiments, it showed a six times greater capacity for removing CO₂ from flue gas than current amine-based technology, and was highly selective, capturing more than 90% of the CO₂ emitted. The materials could then be regenerated with low-grade steam for repeated use, providing a pathway for a viable solution for carbon capture at scale.

‘The tetraamines are so strongly bound within the MOF that we can use a very concentrated stream of water vapour with no CO₂, and if you tried that with the previous adsorbents, the steam would start destroying the material,’ Long says. Studies of the structure of the modified MOF using Berkeley Lab’s Advanced Light Source revealed that the CO₂ polymers lining the pores of the MOF are actually linked by the tetraamines, like a ladder with tetraamines as the rungs.

Meanwhile, researchers at Monash University in Australia are focusing on carbon capture from the air using MOFs. A major barrier for existing commercial carbon capture technologies is how much energy they need, particularly the energy required to regenerate the capture media.

While MOFs have high adsorption capacities, significant energy and time is needed to regenerate them because of their thermally insulating nature and the relatively strong adsorption interactions with guest molecules, explains Matthew Hill. Now, Hill and his colleague Muhammad Munir Sadiq report a MOF nanocomposite that can be regenerated at high speed and low energy cost while also removing CO₂ efficiently.

The team says energy costs are 45% below commercially deployed materials. ‘Our research shows the lowest reported regeneration energy calculated for any solid porous adsorbent, including monoethanolamine, piperazine and other amines,’ says Hill. ‘This makes it a cheap method that can be paired with renewable solar energy to capture excess CO₂ from the atmosphere. Essentially, we can capture CO₂ from anywhere. Our current focus is for capture directly from the air.’

The material is a combination of a MOF (Mg-MOF-74) with high adsorption capacity, a magnetic nanoparticle (MgFe2O4), and a porous hydrophobic polymer, which acts as a matrix to make the material more stable and slow down moisture uptake. The team use the magnetic induction swing adsorption (MISA) process to generate heat for regenerating the MOFs.⁴

Magnetic induction heating delivers heat remotely to the magnetic particles dispersed in the composite. Heat produced this way is localised and rapid compared with the slow and gradual heat transfer in conventional heating methods.

Overall, our study highlights the utility of biologically derived MOFs as nanoreactors for capturing biological molecules through specific interactions, and for transforming them into other molecules.
Kyriakos Stylianou Ecole Polytechnique Fédérale de Lausanne

Bio MOFs

Researchers have started to investigate MOFs made of biological building blocks, such as amino acids or nucleic acids. These biologically derived MOFs (bio-MOFs) can be used in catalysis and as models for complex biomolecules that are difficult to isolate and study. Researchers can gain insights into how biomolecules respond when they contain guest molecules and how guest molecules behave within confined spaces. This could ultimately lead to novel bio-MOFs with ‘tunable’ pores, allowing researchers to target specific chemical reactions and isolate desired products. Recently, researchers have exploited the functional pore surface in MOFs in applications such as carbohydrate separation and labelling, and photochemical transformations.

In 2019, a team of chemical engineers at Ecole Polytechnique Fédérale de Lausanne synthesised a new biologically derived MOF called SION-19, based on a zinc framework. Led by Kyriakos Stylianou, the researchers constructed the new MOF with adenine molecules – one of the nucleobases present in both DNA and RNA. Adenine interacts through hydrogen bonds with another nucleobase, thymine, in the formation of the DNA double helix, and contributes to the folding of DNA and RNA inside the cell.

Using rational design, the team made the MOF with unobstructed faces of adenine pointing towards the MOF cavities. They showed, through experimental and computational approaches, that thymine molecules diffuse through the pores of the MOF and become base-paired with adenine molecules on the MOFs’ cavities through hydrogen bonding, successfully mimicking what happens in DNA.

‘The adenine molecules act as structure-directing agents and “lock” thymine molecules in specific positions within the cavities of our MOF,’ Stylianou explains.

The thymine molecules are packed within the channels and dimerise when the MOFs are irradiated with UV light. The team was then able isolate and study di-thymine, which is linked to skin cancer.⁵

‘Overall, our study highlights the utility of biologically derived MOFs as nanoreactors for capturing biological molecules through specific interactions, and for transforming them into other molecules,’ says Stylianou. It also extends the use of MOFs as nanoreactors for the synthesis of molecules that are otherwise challenging to isolate.

The team believes the ability to photo-induce the dimerisation of solid-state thymine within the constrained pores of SION-19 will ultimately provide new fundamental insights on the governing mechanism of this process. They say their approach could be applied to a variety of MOFs to ‘lock’ molecules in specific positions, opening up new avenues for the synthesis of organic molecules that may otherwise be difficult to obtain through traditional routes.

Motorised MOFs

Meanwhile, chemists at the University of Groningen in the Netherlands have turned to MOFs to get the most out of molecular motors. To work effectively, molecular motors need to operate in unison. However, creating an ordered array of billions of motors in a 3D solid-state material has proved challenging.

In March 2019, the Groningen team reported that it had housed numerous unidirectional light-driven rotary motors in a MOF. This means it is now possible to create a ‘motorised MOF’, in which large numbers of molecular motors are packed densely together to create macroscopic crystals.

The team, involving Ben Feringa, Sander Wezenberg, and Wesley Browne, produced a working system containing 3x1020 rotary motors/cm3, which all run in unison. First, they made crystalline MOFs, 3D stacks of molecular cages made from metals with interconnecting struts of organic molecules. The team replaced these vertical pillars with motors using a process known as solvent-assisted linker exchange. The stationary components of the molecular motors function as the pillars of the cages, while the rotor components remain free inside the cages. The cages were designed to be large enough to allow the motors to run freely. The motors themselves were powered by illuminating the crystal with UV light.

Tests showed that the motors were predominantly oriented in the same direction and that their rotational speed was similar to the speeds achieved in liquids.⁶ The team was ‘delighted’, as previous attempts by other groups to incorporate another type of molecular machine into MOFs showed the motors were not able to run freely.

In theory, crystals like this could be used to control the diffusion of gases, or they could function as light-powered pumps in microfluidic systems. Another potential application would be to feed the motorised MOF with materials that would then react inside the cages before being pumped out again. However, much more research is needed. One issue, for example, is that materials passing through the cages could clog up the motors.

Meanwhile, the remarkable properties of MOFs make them increasingly attractive for a range of applications. Researchers continue to find new uses while working to make them more efficient.

References
1 X Zhang et al, Science Advances 2020, 6, 23, eaay3998
2 Journal of Materials Chemistry A, doi: 10.1039/d0ta01046d
3 Science, doi: 10.1126/science.abb3976
4 MR Hill et al; Cell Reports Physical Science 2020, 1, 6, 100070
5 Nature Communications, doi: 10.1038/s41467-019-09486-2
6 Nature Nanotechnology, doi: 10.1038/s41565-019-0401-6

Image credit: RAMON ANDRADE 3DCIENCIA/SCIENCE PHOTO LIBRARY

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