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DRAFT OF INFORMATION FOR ZIF PAGE

Introduction of ZIFs

ZIFs are a subclass of MOF’s that contain 3-dimensional structures made from tetrahedral metal ions and bridged by Imidazolate. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIF’s are being looked into for applications such as carbon capture and storage. ZIFs are made by joining four coordinated transition metals with imidazolate according to existing tetrahedral topologies.

http://yaghi.berkeley.edu/pdfPublications/09zeolite.pdf

Synthesis of Powder-Based ZIFs

Solvothermal Synthesis:

• Pioneered by Yaghi et. al using organic solvents (e.g. DMF, DEF, NMP)
• Bases such as TEA, pyridine, and NaOH have been used to facilitate  material formation (Yaghi papers)
• MeOH, ethanol, iPa is also used as solvent, in combination with ligands such as sodium formate to control crystal size and yield
• Organic solvents are expensive, flammable, not green
• ZIF-8 can be synthesized

Hydrothermal Synthesis:

• Done with water
• Interest is in achieving reaction with stoichiometric molar ratio of zinc and imidazolates
• Bases have been used (TEA, NH4OH, NH3) to do this
• Triblock copolymers also allowed stoichiometric ratios
• ZIF-8 can be synthesized

Ionothermal Synthesis:

• Ionic liquids can act as solvents + templates, which avoids competing interactions between solvent-framework and template-framework in hydrothermal prep
• Additional benefits include negligible vapor pressure (can be used in open system), non-flammability, and recyclability.
• ZIF-8 can be synthesized

Sonochemical Synthesis:

• Uses sonication to generate acoustic cavitation conditions.  Improved crystal size and PS distribution at short duration and lower temperature.  ZIF-8 was synthesized among others.  

Mechanochemical Synthesis:

• Ball and mill grinding with little to no solvent.  Pretty cool.  

Synthesis of ZIF-based films and membranes

ZIF-based films and membranes are important for gas separation and chemical sensors

Secondary growth crystallization:  Depositing crystal seeds onto a support, or by pre-treating a support with reactive seeding or organic functional groups

Applications

Applications to carbon capture

Main article carbon capture and storage [link to ccs page]

ZIF’s have great potential to be used in capturing carbon from flue gases of many different processes because of their stability and pore size tunability. [3] Currently, most carbon capture methods use amine based solvents, which are [insert negative traits of amines]; ZIF’s have the potential to be used in pre-combustion capture, post-combustion capture [link to wiki page], and oxyfuel combustion [link to wiki page]. [4] This is because the ideal selectivity of pure ZIF membranes can be highly variable and dependant on the gases involved; it can be affected by the target gas, the type of ZIF, and the operating conditions under which the process is being run (i.e. temperature and pressure). [1] Additionally, they are more chemically stable than other MOFS [link to wiki page] and have better maximum adsorbances at lower pressures.

Zif’s 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 even if they don’t have extremely high affinities. Of those, 68, 69, and 70 show high affinities for carbon dioxide, evidenced by their adsorption isotherms, which show steep uptakes at low pressures. This could also be useful for pressure-swing adsorption [link to wiki page]. [2]

Other separation applications

Much ZIF research focuses on the separation of hydrogen and carbon dioxide separation 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.

[insert chart maybe] [1]

Hydrocarbon mixture vs. separation factor:

• 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 [5].

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’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 [table number x] gives a more comprehensive list of ZIF’s that can act as catalysts for different organic reactions. [1]

I’ll make a table of this that we can then put into the wiki page (i believe)

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. [1] 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 Cu2+, and Cd2+ ions as well as acetone. ZIF nanoparticles can also sense fluorescently tagged single stranded pieces of DNA. [1]

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. [1]

1. ZIFs compared to other structures

ZIFs compared with 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 which they were created in.[6] However, ZIFs have been considerably limited until recently due to their limitation to zeolite-like topology when the crystallize with imidazolate frames.[6]

[8] Most ZIF alterations up to this point have involved changing the linkers - bridging O2- anions and imizazolate-based ligands [7] - or combining two types of linkers to change bond angles or pore size due to limitations in synthesizing methods and production.[8] 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.[9] Compare this to MOFs, which have a much larger degree of variety in the types of their building units.[8] Recently, the high through-put methods previously used in drug activity research has been applied to MOF and ZIF synthesis, allowing consistent ZIF structure yields.[3] Despite these advancements, both MOFs and ZIFs require highly intensive methods for synthesis, making them unfavorable in comparison to current commercial products that are less intensive.[9]

Despite these similarities with MOFs, ZIFs have significant properties that distinguish these structures as uniquely to be applied 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.[6] A recent study compared ZIF and MOF abilities to catalyze a carbon dioxide photoreduction reaction, and it was found that when MOFs were used, the product yield declined in comparison to that of ZIFs that underwent the same experiment.[9] Repeated with other MOFs - none of which were able to fully catalyze the reaction - determined that the main component of ZIFs ability to catalyze the photosplitting reaction is connected to the framework of the ZIF catalyst.[9]

Perhaps the most important difference is the ZIFs hydrophobic properties and water stability. A man issue with zeolites and MOFs, to a certain extent, was their adsorption of water along with CO2.[7] Water vapor is often found in carbon-rich exhaust gases, and MOFs would absorb the water, lowering the amount of CO2 required to reach saturation.[6] MOFs are also less stable in moist and oxygen rich environments due to metal-oxygen bonds performing hydrolysis.[6] ZIFs, however, have nearly identical performances in dry and humid conditions, showing much higher CO2 selectivity over water, allowing the adsorbent to store more carbon before saturation is reached.[8]

ZIFs compared with commerically available materials

SImilar to with MOFs, the ZIFs most attractive quality is its hydrophobic properties. When compared to ZIFs in dry conditions, activated carbon was nearly identical with its uptake capacity.[8] However, once the conditions were changed to wet, the activated carbon’s uptake was halved.[8] 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 resusability.[8]

However, ZIFs tend to be expensive to synthesize. MOFs require “solvothermal methods and long reaction periods,” which aren’t method easy to scale-up.[6]  ZIFs do tend to be more affordable than commercially available non-ZIF MOFs.[6]

When combined with polymer-sorbent materials, research determined that hybrid polymer-ZIF sorbent membranes no longer following the upper bound of the Robeson plot (wiki link).[7] In 2010, a hybrid ZIF membrane with a-Al2O3 metal framework for support was tested by the Yaghi group and found to have high gas permeance and selectivity.[9]

References:

1. Zeolitic imidazolate framework materials: recent progress in synthesis and applications
2. Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks By Phan et al
3. Highly Permeable Zeolite Imidazolate Framework-8 Membranes for CO2/CH4 Separation
4. = Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks =
5. Biofuel purification in zeolitic imidazolate frameworks: the significant role of functional groups
6. Carbonate-Based Zeolitic Imidazoloate Frame for Highly Selective CO2 Capture
7. CCS Textbook
8. Mixed-Metal ZIFs and their Selective Capture of Wet Carbon Dioxide over Methane
9. IIL, IHC, and ZIFs for CO2 Capture and Photochemical Reduction