Zeolitic imidazolate framework

Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Cu, Zn) connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies.[1] As of 2010, 105 ZIF topologies have been reported in the literature.[2][3] Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIFs are being investigated for applications such as carbon capture.[4]

The structure of a zeolitic imidazolate framework is made through three-dimensional assembly of metal(imidazolate)4 tetrahedra.

Synthesis

ZIFs are prepared by solvothermal or hydrothermal techniques. Crystals slowly grow from a heated solution of a hydrated metal salt, an ImH (imidazole with acidic proton), a solvent, and base.[5] Functionalized ImH linkers allow for control of ZIF structure.[6] This process is ideal for generating monocrystalline materials for single-crystal X-ray diffraction.[7][8] A wide range of solvents, bases, and conditions have been explored, with an eye towards improving crystal functionality, morphology, and dispersity. Prototypically, an amide solvent such as N,N-dimethylformamide (DMF) is used. The heat applied decomposes the amide solvent to generate amines, which in turn generate the imidazolate from the imidazole species. Methanol,[9][10] ethanol,[11] isopropanol,[12] and water[13][14][15] have also been explored as alternative solvents for ZIF formation but require bases such as pyridine,[16] TEA,[17] sodium formate,[18] and NaOH.[19] Polymers such as poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide),[20] polyvinylpyrrolidone,[21] and poly-(diallyldimethylammonium chloride)[22] have been found to act as crystal dispersants, imparting particle-size and morphology control.

Due to their promising material properties, significant interest lies in economical large-scale production methods. Sonochemical synthesis, which allows nucleation reactions to proceed rapidly through acoustic generation of localized heat and pressure, has been explored as a way to shorten synthesis times.[23][24] As with the case of zeolites, microwave-assisted synthesis has also been of interest for the rapid synthesis of ZIFs.[25][26] Both methods have been shown to reduce reaction times from days to hours, or from hours to minutes. Solvent-free methods, such as ball-milling or chemical vapor deposition, have also been described to produce high-quality ZIF-8.[27][28] Chemical vapor deposition is of particular promise due to the high degree of uniformity and aspect ratio control it can offer, and its ability to be integrated into traditional lithographic workflows for functional thin films (e.g. microelectronics). Environmentally-friendly synthesis based on supercritical carbon dioxide (scCO2) have been also reported as a feasible procedure for the preparation of ZIF-8 at an industrial scale.[29] Working under stoichiometric conditions, ZIF-8 could be obtained in 10 hours and does not require the use of ligand excess, additives, organic solvents or cleaning steps.

Applications of ZIFs

Applications to carbon capture
Main article: carbon capture and storage

ZIFs exhibit some properties relevant to carbon capture,[30] while commercial technology still centers around amine solvents.[31]

Zeolites are known to have tunable pores – ranging between 3-12 Angstroms – which allows them to separate carbon dioxide. Because a molecule is about 5.4 Angstroms in length, zeolites with a pore size of 4-5 Angstroms can be well-suited for carbon capture. However, other factors also need to be considered when determining how effective zeolites will be at carbon capture. The first is basicity, which can be created by doing an alkali metal cation exchange. The second is the Si/Al ratio which impacts the cation exchange capacity. To get a higher adsorption capacity, there must be a lower Si/Al ratio in order to increase the cation exchange capacity.

ZIFs 68, 69, 70, 78, 81, 82, 95, and 100 have been found to have very high uptake capacity, meaning that they can store a lot of carbon dioxide, though their affinity to it is not always strong. Of those, 68, 69, and 70 show high affinities for carbon dioxide, evidenced by their adsorption isotherms, which show steep uptakes at low pressures. One liter of ZIF can hold 83 liters of CO
2. This could also be useful for pressure-swing adsorption.[32]

Other separation applications

Much ZIF research focuses on the separation of hydrogen and carbon dioxide because a well-studied ZIF, ZIF-8, has a very high separation factor for hydrogen and carbon dioxide mixtures. It is also very good for the separation of hydrocarbon mixtures, like the following:

  • Ethane-propane = 80
  • Ethylene- propylene = 10
  • Ethylene- propane = 167

In addition to gas separations, ZIF’s have the potential to separate components of biofuels, specifically, water and ethanol. Of all of the ZIF’s that have been tested, ZIF-8 shows high selectivity. ZIF’s have also shown potential in separating other alcohols, like propanol and butanol, from water. Typically, water and ethanol (or other alcohols) are separated using distillation, however ZIF’s offer a potential lower-energy separation option.[33]

Catalysis

ZIF’s also have great potential as heterogeneous catalysts; ZIF-8 has been shown to act as good catalysts for the transesterification of vegetable oils, the Friedel-Crafts acylation reaction between benzoyl chloride and anisole, and for the formation of carbonates. ZIF-8 nanoparticles can also be used to enhance the performance in the Knoevenagel condensation reaction between benzaldehyde and malononitrile.[34] ZIF’s have also been shown to work well in oxidation and epoxidation reactions; ZIF-9 has been shown to catalyze the aerobic oxidation of tetralin and the oxidation of many other small molecules. It can also catalyze reactions to produce hydrogen at room temperature, specifically the dehydrogenation of dimethylamine borane and NaBH4 hydrolysis.

The table below gives a more comprehensive list of ZIF’s that can act as catalysts for different organic reactions.[2]

ZIF Material Additional Materials Reaction (s) Catalyzed
ZIF-8 gold nanoparticles Oxidation of CO

Oxidation of aldehyde groups

ZIF-8 gold and silver core shell nanoparticles Reduction of 4-nitrophenol
ZIF-8 gold, silver, and platinum nanoparticles Oxidation of CO

Hydrogenation of n-hexene

ZIF-8 platinum nanoparticles Hydrogenation of alkene
ZIF-8 platinum and titanium dioxide nanotubes Degradation of phenol
ZIF-8 palladium nanoparticles Aminocarbonylation
ZIF-8 iridium nanoparticles Hydrogenation of cyclohexene and phenylacetene
ZIF-8 ruthenium nanoparticles Asymmetric hydrogenation of acetophonone
ZIF-8 iron oxide microspheres Knoevenagel condensation
ZIF-8 Zn2GeO4 nanorods Conversion of CO2
ZIF-65 Molybdenum Oxide Degradation of methyl orange and orange II dyes

Sensing and electronic devices

ZIF’s are also good candidates for chemical sensors because of their tunable adsorbance properties. ZIF-8 exhibits sensitivity when exposed to the vapor of ethanol and water mixtures, and this response is dependent on the concentration of ethanol in the mixture.[35] Additionally, ZIF’s are attractive materials for matrices for biosensors, like electrochemical biosensors, for in-vivo electrochemical measurements. They also have potential applications as luminescent probes for the detection of metal ions and small molecules. ZIF-8 luminescence is highly sensitive to , and ions as well as acetone. ZIF nanoparticles can also sense fluorescently tagged single stranded pieces of DNA.[35]

Drug delivery

Because ZIF’s are porous, chemically stable, thermally stable, and tunable, they are potentially a platform for drug delivery and controlled drug release. ZIF-8 is very stable in water and aqueous sodium hydroxide solutions but decompose quickly in acidic solutions, indicating a pH sensitivity that could aid in the development of ZIF-based drug-release platforms.[35]

Comparing ZIFs with other compounds

ZIFS vs MOFs

While ZIFs are a subset of the MOF hybrids that combine organic and metal frameworks to create hybrid microporous and crystalline structures, they are much more restricted in their structure. Similar to MOFs, most ZIF properties are largely dependent on the properties of the metal clusters, ligands, and synthesis conditions in which they were created.[36]

Most ZIF alterations up to this point have involved changing the linkers — bridging O2 — anions and imizazolate-based ligands[31] - or combining two types of linkers to change bond angles or pore size due to limitations in synthesizing methods and production.[37] A large portion of changing linkers included adding functional groups with various polarities and symmetries to the imidazolate ligands to alter the ZIFs carbon dioxide adsorption ability without changing the transitional-metal cations.[38] Compare this to MOFs, which have a much larger degree of variety in the types of their building units.

Despite these similarities with other MOFs, ZIFs have significant properties that distinguish these structures as uniquely applicable to carbon capture processes. Because ZIFs tend to resemble the crystalline framework of zeolites, their thermal and chemical stability are higher than those of other MOFs, allowing them to work at a wider range in temperatures, making them suitable to chemical processes.[36]

Perhaps the most important difference is the ZIFs' hydrophobic properties and water stability. A main issue with zeolites and MOFs, to a certain extent, was their adsorption of water along with CO
2. Water vapor is often found in carbon-rich exhaust gases, and MOFs would absorb the water, lowering the amount of CO
2 required to reach saturation.[36] MOFs are also less stable in moist and oxygen-rich environments due to metal-oxygen bonds performing hydrolysis. ZIFs, however, have nearly identical performance in dry vs humid conditions, showing much higher CO
2 selectivity over water, allowing the adsorbent to store more carbon before saturation is reached.[37]

ZIFs vs commercially available products

Even in comparison with other materials, the ZIFs most attractive quality is still its hydrophobic properties. When compared to ZIFs in dry conditions, activated carbon was nearly identical with its uptake capacity.[37] However, once the conditions were changed to wet, the activated carbon’s uptake was halved. When this saturation and regeneration tests were run at these conditions, ZIFs also showed minimal to no structural degradation, a good indication of the adsorbent’s re-usability.[37]

However, ZIFs tend to be expensive to synthesize. MOFs require synthesis methods with long reaction periods, high pressures, and high temperatures, which aren’t methods that are easy to scale-up.[36] ZIFs do tend to be more affordable than commercially available non-ZIF MOFs.

When combined with polymer-sorbent materials, research determined that hybrid polymer-ZIF sorbent membranes no longer following the upper bound of the Robeson plot, which is a plot of selectivity as a function of permeation for membrane gas separation.[31]

See also

References
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