Zeolitic imidazolate framework

In 2006, Omar M. Yaghi and his collaborators published a series of Zeolitic imidazolate frameworks (ZIFs), including the iconic ZIF-8, building on his pioneering work in metal-organic frameworks (MOFs).{{Cite journal |last1=Park |first1=Kyo Sung |last2=Ni |first2=Zheng |last3=Côté |first3=Adrien P. |last4=Choi |first4=Jae Yong |last5=Huang |first5=Rudan |last6=Uribe-Romo |first6=Fernando J. |last7=Chae |first7=Hee K. |last8=O’Keeffe |first8=Michael |last9=Yaghi |first9=Omar M. |date=2006-07-05 |title=Exceptional chemical and thermal stability of zeolitic imidazolate frameworks |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=27 |pages=10186–10191 |doi=10.1073/pnas.0602439103 |doi-access=free |pmid=16798880 |issn=0027-8424|pmc=1502432 |bibcode=2006PNAS..10310186P }} Yaghi introduced ZIFs as a novel class of materials that combined the structural characteristics of zeolites — such as their tetrahedral coordination and robust chemical stability— with the tunability and porosity of MOFs. By linking metal ions (typically zinc or cobalt) with imidazolate linkers, ZIFs achieved the zeolite-like topology while maintaining the modularity and versatility of MOF chemistry. This innovation significantly broadened the potential applications of porous materials in areas such as gas storage, separation, and catalysis.{{Cite journal |last1=Kouser |first1=Salma |last2=Hezam |first2=Abdo |last3=Khadri |first3=M. J. Nagesh |last4=Khanum |first4=Shaukath Ara |date=2022-06-01 |title=A review on zeolite imidazole frameworks: synthesis, properties, and applications |url=https://link.springer.com/article/10.1007/s10934-021-01184-z |journal=Journal of Porous Materials |language=en |volume=29 |issue=3 |pages=663–681 |doi=10.1007/s10934-021-01184-z |issn=1573-4854}}

The breakthrough material ZIF-8, composed of zinc ions and 2-methylimidazolate linkers, exemplifies the unique properties of ZIFs. ZIF-8 demonstrated exceptional chemical and thermal stability, coupled with a highly selective pore system, making it suitable for demanding applications like carbon dioxide capture and hydrocarbon separation.{{Cite journal |last1=Zheng |first1=Zhiling |last2=Rong |first2=Zichao |last3=Nguyen |first3=Ha L. |last4=Yaghi |first4=Omar M. |date=2023-12-25 |title=Structural Chemistry of Zeolitic Imidazolate Frameworks |url=https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c02322 |journal=Inorganic Chemistry |volume=62 |issue=51 |pages=20861–20873 |doi=10.1021/acs.inorgchem.3c02322 |pmid=38063312 |issn=0020-1669|url-access=subscription }} The introduction of ZIFs not only expanded the capabilities of MOFs but also bridged the gap between traditional zeolites and modern framework materials, solidifying Yaghi’s reputation in the development of advanced porous materials.{{Cite journal |last1=Chen |first1=Binling |last2=Yang |first2=Zhuxian |last3=Zhu |first3=Yanqiu |last4=Xia |first4=Yongde |date=2014-09-23 |title=Zeolitic imidazolate framework materials: recent progress in synthesis and applications |url=https://pubs.rsc.org/en/content/articlelanding/2014/ta/c4ta02984d |journal=Journal of Materials Chemistry A |language=en |volume=2 |issue=40 |pages=16811–16831 |doi=10.1039/C4TA02984D |issn=2050-7496|url-access=subscription }}

The structure of melt-quenched ZIF glasses maintains a certain amount of short-range order, although the chemical configuration and coordination environments, after melting, lose long-range order completely. From a microscopic view, the linkages between metal nodes and organic ligands (e.g., Zn-N linkages) partially break at high temperature and the resulting undercoordinated metal ions have the potential to link with other neighboring organic ligands for exchange.

One notable discovery regarding the structure of ZIF glass was made by Rasmus et al. Before this research was published, the short-range structural order at the scale of the cation-ligand units remained unknown given the limitations of the analytical techniques available. The short-range structural disorder of the tetrahedral ligand environment around metal nodes in the ZIF glass was detected for the first time by performing zinc-67 nuclear magnetic resonance. This finding clearly showed that ZIF glasses are structurally very different from the other known glass types, overturning the traditional view that a glass structure has short-range order and long-range disorder, providing a broader view of what qualifies as a glass.

ZIFs are mainly 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.{{Cite journal|last1=Park|first1=Kyo Sung|last2=Ni|first2=Zheng|last3=Côté|first3=Adrien P.|last4=Choi|first4=Jae Yong|last5=Huang|first5=Rudan|last6=Uribe-Romo|first6=Fernando J.|last7=Chae|first7=Hee K.|last8=O’Keeffe|first8=Michael|last9=Yaghi|first9=Omar M.|display-authors=3|date=2006-07-05|title=Exceptional chemical and thermal stability of zeolitic imidazolate frameworks|journal=Proceedings of the National Academy of Sciences|language=en|volume=103|issue=27|pages=10186–10191|doi=10.1073/pnas.0602439103|issn=0027-8424|pmc=1502432|pmid=16798880|bibcode=2006PNAS..10310186P|doi-access=free}} Functionalized ImH linkers allow for control of ZIF structure.{{Cite journal|last1=Hayashi|first1=Hideki|last2=Côté|first2=Adrien P.|last3=Furukawa|first3=Hiroyasu|last4=O’Keeffe|first4=Michael|last5=Yaghi|first5=Omar M.|display-authors=3|date=2007-07-01|title=Zeolite A imidazolate frameworks|journal=Nature Materials|language=en|volume=6|issue=7|pages=501–506|doi=10.1038/nmat1927|pmid=17529969|issn=1476-1122|bibcode=2007NatMa...6..501H}} This process is ideal for generating monocrystalline materials for single-crystal X-ray diffraction.{{Cite journal|last1=Banerjee|first1=Rahul|last2=Phan|first2=Anh|last3=Wang|first3=Bo|last4=Knobler|first4=Carolyn|last5=Furukawa|first5=Hiroyasu|last6=O'Keeffe|first6=Michael|last7=Yaghi|first7=Omar M.|display-authors=3|date=2008-02-15|title=High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture|journal=Science|language=en|volume=319|issue=5865|pages=939–943|doi=10.1126/science.1152516|issn=0036-8075|pmid=18276887|bibcode=2008Sci...319..939B|s2cid=22210227}}{{Cite journal|last1=Wang|first1=Bo|last2=Côté|first2=Adrien P.|last3=Furukawa|first3=Hiroyasu|last4=O’Keeffe|first4=Michael|last5=Yaghi|first5=Omar M.|display-authors=3|date=2008-05-08|title=Colossal cages in zeolitic imidazolate frameworks as selective carbon dioxide reservoirs|journal=Nature|language=en|volume=453|issue=7192|pages=207–211|doi=10.1038/nature06900|pmid=18464739|issn=0028-0836|bibcode=2008Natur.453..207W|doi-access=free}} A wide range of solvents, bases, and conditions have been explored, with an eye towards improving crystal functionality, morphology, and dispersity.{{Cite journal|last1=Madhav|first1=Dharmjeet|last2=Malankowska|first2=Magdalena|last3=Coronas|first3=Joaquin|date=2020-11-06|title=Synthesis of nanoparticles of zeolitic imidazolate framework ZIF-94 using inorganic deprotonators|journal=New Journal of Chemistry|language=en|volume=44|issue=46|pages=20449–20457|doi=10.1039/D0NJ04402D|s2cid=229232268|issn=1144-0546|url=https://lirias.kuleuven.be/handle/123456789/664581}} 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. 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journal|last1=Pan|first1=Yichang|last2=Liu|first2=Yunyang|last3=Zeng|first3=Gaofeng|last4=Zhao|first4=Lan|last5=Lai|first5=Zhiping|display-authors=3|date=2011-02-01|title=Rapid synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals in an aqueous system|journal=Chemical Communications|language=en|volume=47|issue=7|pages=2071–3|doi=10.1039/C0CC05002D|pmid=21206942|issn=1364-548X}}{{Cite journal|last1=Tanaka|first1=Shunsuke|last2=Kida|first2=Koji|last3=Okita|first3=Muneyuki|last4=Ito|first4=Yosuke|last5=Miyake|first5=Yoshikazu|display-authors=3|date=2012-10-05|title=Size-controlled Synthesis of Zeolitic Imidazolate Framework-8 (ZIF-8) Crystals in an Aqueous System at Room Temperature|journal=Chemistry Letters|volume=41|issue=10|pages=1337–1339|doi=10.1246/cl.2012.1337|issn=0366-7022|doi-access=free}}{{Cite 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controllable size and morphology|website=ResearchGate|language=en|access-date=2017-05-01}} sodium formate,{{Cite journal|last1=Cravillon|first1=Janosch|last2=Schröder|first2=Christian A.|last3=Bux|first3=Helge|last4=Rothkirch|first4=André|last5=Caro|first5=Jürgen|last6=Wiebcke|first6=Michael|display-authors=3|date=2011-12-12|title=Formate modulated solvothermal synthesis of ZIF-8 investigated using time-resolved in situ X-ray diffraction and scanning electron microscopy|journal=CrystEngComm|language=en|volume=14|issue=2|pages=492–498|doi=10.1039/C1CE06002C|issn=1466-8033|url=http://www.repo.uni-hannover.de/handle/123456789/2191}} and NaOH.{{Cite journal|last1=Peralta|first1=David|last2=Chaplais|first2=Gérald|last3=Simon-Masseron|first3=Angélique|last4=Barthelet|first4=Karin|last5=Pirngruber|first5=Gerhard D.|date=2012-05-01|title=Synthesis and adsorption properties of ZIF-76 isomorphs|journal=Microporous and Mesoporous 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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.{{Cite journal|last1=Seoane|first1=Beatriz|last2=Zamaro|first2=Juan M.|last3=Tellez|first3=Carlos|last4=Coronas|first4=Joaquin|date=2012-04-02|title=Sonocrystallization of zeolitic imidazolate frameworks (ZIF-7, ZIF-8, ZIF-11 and ZIF-20)|journal=CrystEngComm|language=en|volume=14|issue=9|pages=3103|doi=10.1039/C2CE06382D|bibcode=2012CEG....14.3103S |issn=1466-8033|hdl=11336/53044|hdl-access=free}}{{Cite journal|last1=Cho|first1=Hye-Young|last2=Kim|first2=Jun|last3=Kim|first3=Se-Na|last4=Ahn|first4=Wha-Seung|date=2013-03-15|title=High yield 1-L scale synthesis of ZIF-8 via a sonochemical route|journal=Microporous and Mesoporous Materials|volume=169|pages=180–184|doi=10.1016/j.micromeso.2012.11.012|bibcode=2013MicMM.169..180C }} As with the case of zeolites, microwave-assisted synthesis has also been of interest for the rapid synthesis of ZIFs.{{Cite journal|last1=Bux|first1=Helge|last2=Liang|first2=Fangyi|last3=Li|first3=Yanshuo|last4=Cravillon|first4=Janosch|last5=Wiebcke|first5=Michael|last6=Caro|first6=Jürgen|display-authors=3|title=Zeolitic Imidazolate Framework Membrane with Molecular Sieving Properties by Microwave-Assisted Solvothermal Synthesis|journal=Journal of the American Chemical Society|language=en|volume=131|issue=44|pages=16000–16001|doi=10.1021/ja907359t|pmid=19842668|year=2009|bibcode=2009JAChS.13116000B }}{{Cite journal|last1=Hillman|first1=Febrian|last2=Zimmerman|first2=John M.|last3=Paek|first3=Seung-Min|last4=Hamid|first4=Mohamad R. A.|last5=Lim|first5=Woo Taik|last6=Jeong|first6=Hae-Kwon|display-authors=3|date=2017-03-28|title=Rapid microwave-assisted synthesis of hybrid zeolitic–imidazolate frameworks with mixed metals and mixed linkers|journal=Journal of Materials Chemistry A|language=en|volume=5|issue=13|pages=6090–6099|doi=10.1039/C6TA11170J|issn=2050-7496}} 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.{{Cite journal|last1=Bennett|first1=Thomas D.|last2=Cao|first2=Shuai|last3=Tan|first3=Jin Chong|last4=Keen|first4=David A.|last5=Bithell|first5=Erica G.|last6=Beldon|first6=Patrick J.|last7=Friscic|first7=Tomislav|last8=Cheetham|first8=Anthony K.|display-authors=3|title=Facile Mechanosynthesis of Amorphous Zeolitic Imidazolate Frameworks|journal=Journal of the American Chemical Society|language=en|volume=133|issue=37|pages=14546–14549|doi=10.1021/ja206082s|pmid=21848328|year=2011|bibcode=2011JAChS.13314546B }}{{Cite journal|last1=Stassen|first1=Ivo|last2=Styles|first2=Mark|last3=Grenci|first3=Gianluca|last4=Gorp|first4=Hans Van|last5=Vanderlinden|first5=Willem|last6=Feyter|first6=Steven De|last7=Falcaro|first7=Paolo|last8=Vos|first8=Dirk De|last9=Vereecken|first9=Philippe|display-authors=3|date=2016-03-01|title=Chemical vapour deposition of zeolitic imidazolate framework thin films|journal=Nature Materials|language=en|volume=15|issue=3|pages=304–310|doi=10.1038/nmat4509|pmid=26657328|issn=1476-1122|bibcode=2016NatMa..15..304S|url=https://lirias.kuleuven.be/handle/123456789/551545|url-access=subscription}} 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.{{Cite journal|last1=López-Domínguez|first1=Pedro|last2=López-Periago|first2=Ana M.|last3=Fernández-Porras|first3=Francisco J.|last4=Fraile|first4=Julio|last5=Tobias|first5=Gerard|last6=Domingo|first6=Concepción|display-authors=3|date=2017-03-01|title=Supercritical CO2 for the synthesis of nanometric ZIF-8 and loading with hyperbranched aminopolymers. Applications in CO2 capture|journal=Journal of CO2 Utilization|volume=18|pages=147–155|doi=10.1016/j.jcou.2017.01.019}} 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.

Using the traditional melt-quench of metals or sintering of ceramics would cause the collapse of MOF structure as its thermal decomposing temperature is lower than its melting temperature. Moreover, the amorphous form of MOF can be achieved through pressurization or heating, but its network feature would be significantly broken during the amorphization process. Bennett et al found certain members from MOF family (ZIF-4, etc.) can be made into a glassy state. Those carefully selected ZIF crystals are able to form a glassy solid after heating and cooling in an argon atmosphere. Moreover, the melting range can be tuned by their network topologies.

The crystal form of ZIF, or MOF in general, is known for its porosity, but is difficult to mass-produce and incorporate in actual applications due to unavoidable intercrystalline defects.{{Cite journal|last=Pera-Titus|first=Marc|date=2014-01-22|title=Porous Inorganic Membranes for CO2 Capture: Present and Prospects|url=https://doi.org/10.1021/cr400237k|journal=Chemical Reviews|volume=114|issue=2|pages=1413–1492|doi=10.1021/cr400237k|pmid=24299113|issn=0009-2665|url-access=subscription}} There are several interesting characters about ZIF glasses addressing those challenges to potentially realize promised applications achievable. The first intriguing one is that ZIF glass maintains the porous structure as its crystalline form after melt-quench process, which means it can be applied for applications such as gas separation and storage. The glassy form would also offer unique opportunities for easy processability and mass production. Last but not least, besides pure ZIF glass, composites based on it by tuning the composition and structure has the distinct advantage of a broad design space.

{{Main|carbon capture and storage}}

ZIFs exhibit some properties relevant to carbon dioxide capture,{{Cite journal|last1=Venna|first1=Surendar R.|last2=Carreon|first2=Moises A.|date=2010-01-13|title=Highly Permeable Zeolite Imidazolate Framework-8 Membranes for CO2/CH4 Separation|journal=Journal of the American Chemical Society|volume=132|issue=1|pages=76–78|doi=10.1021/ja909263x|pmid=20014839|bibcode=2010JAChS.132...76V |issn=0002-7863}} while commercial technology still centers around amine solvents.{{cite book|last1=Smit|first1=Bernard|last2=Reimer|first2=Jeffrey A.|last3=Oldenburg|first3=Curtis M.|last4=Bourg|first4=Ian C.|title=Introduction to Carbon Capture and Sequestration|date=2014|publisher=Imperial College Press|location=Hackensack, NJ|isbn=978-1-78326-328-8|edition=1}}

Zeolites are known to have tunable pores – ranging between 3-12 Angstroms – which allows them to separate carbon dioxide. Because a CO2 molecule is about 5.4 Angstroms in length, zeolites with a pore size of 4-5 Angstroms can be well-suited for carbon dioxide capture. However, other factors also need to be considered when determining how effective zeolites will be at carbon dioxide 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 {{co2}}. This could also be useful for pressure-swing adsorption.{{Cite journal|last1=Phan|first1=Anh|last2=Doonan|first2=Christian J.|last3=Uribe-Romo|first3=Fernando J.|last4=Knobler|first4=Carolyn B.|last5=O'Keeffe|first5=Michael|last6=Yaghi|first6=Omar M.|display-authors=3|date=2010-01-19|title=Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks|journal=Accounts of Chemical Research|volume=43|issue=1|pages=58–67|doi=10.1021/ar900116g|issn=1520-4898|pmid=19877580}}

ZIF-62 was made into a glassy membrane on the nanoporous alumina support for gas separation for the first time by Yuhan et al in 2020.{{Cite journal|last1=Wang|first1=Yuhan|last2=Jin|first2=Hua|last3=Ma|first3=Qiang|last4=Mo|first4=Kai|last5=Mao|first5=Haizhuo|last6=Feldhoff|first6=Armin|last7=Cao|first7=Xingzhong|last8=Li|first8=Yanshuo|last9=Pan|first9=Fusheng|last10=Jiang|first10=Zhongyi|date=2020-03-09|title=A MOF Glass Membrane for Gas Separation|url=https://onlinelibrary.wiley.com/doi/10.1002/ange.201915807|journal=Angewandte Chemie|language=en|volume=132|issue=11|pages=4395–4399|doi=10.1002/ange.201915807|bibcode=2020AngCh.132.4395W|s2cid=226676528|issn=0044-8249|url-access=subscription}} The vitrification process effectively eliminates grain boundaries formation within the glass, and the molecular sieving ability of such membrane is significantly improved. The value of the ideal selectivities of several gas pairs, e.g. CO₂/N₂, are much higher than Knudsen selectivities, and the excellent performance of the ZIF-62 glass membrane not only far exceeds the Robeson upper bound, but also exceeds most of other pure polycrystalline MOF materials reported so far.

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.{{Cite journal|last1=Zhang|first1=Kang|last2=Nalaparaju|first2=Anjaiah|last3=Chen|first3=Yifei|last4=Jiang|first4=Jianwen|date=2014-04-23|title=Biofuel purification in zeolitic imidazolate frameworks: the significant role of functional groups|journal=Physical Chemistry Chemical Physics|language=en|volume=16|issue=20|pages=9643–55|doi=10.1039/C4CP00739E|pmid=24727907|issn=1463-9084|bibcode=2014PCCP...16.9643Z}}

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.{{Cite journal|last1=Guan|first1=Yebin|last2=Shi|first2=Juanjuan|last3=Xia|first3=Ming|last4=Zhanga|first4=Jun|last5=Pang|first5=Zhenfeng|last6=Marchetti|first6=Alessandro|last7=Wang|first7=Xiaohong|last8=Cai|first8=Jingsong|last9=Kong|first9=Xueqian|display-authors=3|date=2017-11-30|title= Monodispersed ZIF-8 particles with enhanced performance for CO2 adsorption and heterogeneous catalysis |journal=Applied Surface Science|language=en|volume=423|pages= 349–353 |doi=10.1016/j.apsusc.2017.06.183|bibcode=2017ApSS..423..349G}} 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.

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.{{Cite journal|last1=Chen|first1=Binling|last2=Yang|first2=Zhuxian|last3=Zhu|first3=Yanqiu|last4=Xia|first4=Yongde|date=2014-09-23|title=Zeolitic imidazolate framework materials: recent progress in synthesis and applications|journal=Journal of Materials Chemistry A|language=en|volume=2|issue=40|pages=16811–16831|doi=10.1039/C4TA02984D|issn=2050-7496}} 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.

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.

{{Original research section|date=May 2017}}

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.{{Cite journal|last1=Basnayake|first1=Sajani A.|last2=Su|first2=Jie|last3=Zou|first3=Xiadong|last4=Balkus|first4=Kenneth J.|date=2015-02-04|title=Carbonate-Based Zeolitic Imidazolate Frame for Highly Selective CO2 Capture|journal=Inorganic Chemistry|language=en|volume=54|issue=4|pages=1816–1821|doi=10.1021/ic5027174|pmid=25650775}}

Most ZIF alterations up to this point have involved changing the linkers {{--}} bridging {{O2}} {{--}} anions and imidazolate-based ligands - or combining two types of linkers to change bond angles or pore size due to limitations in synthesizing methods and production.{{Cite journal|last1=Nguyen|first1=Nhung T. T.|last2=Lo|first2=Tien N. H.|last3=Kim|first3=Jaheon|date=2016-04-04|title=Mixed-Metal Zeolitic Imidazolate Frameworks and their Selective Capture of Wet Carbon Dioxide over Methane|url=http://globalscience.berkeley.edu/sites/default/files/mm-zif-2016.pdf|journal=Inorganic Chemistry|language=en|volume=55|issue=12|pages=6201–6207|doi=10.1021/acs.inorgchem.6b00814|pmid=27248714}} 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.{{cite journal|last1=Wang|first1=Sibo|last2=Wang|first2=Xinchen|title=Imidazolium Ionic Liquids, Imidazolylidene Heterocyclic Carbenes, and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction|journal=Angewandte Chemie|date=2015-12-08|volume=55|issue=7|pages=2308–2320|doi=10.1002/anie.201507145|pmid=26683833}} 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.

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 {{CO2}}. 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. 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 {{CO2}} selectivity over water, allowing the adsorbent to store more carbon before saturation is reached.

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. 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.

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. 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.

{{Portal|Energy}}

• Metal-organic framework
• Covalent organic framework
• Omar M. Yaghi
• Zeolite
• Hydrogen-bonded organic framework

{{Reflist|40em}}

• [http://helios.princeton.edu/mofomics/cgi-bin/list_structures.pl?src=zif&page=1 Pore characterizations of ZIFs]

{{DEFAULTSORT:Zeolitic Imidazolate Framework}}

Category:Carbon capture and storage

Category:Sustainable technologies

Category:Crystal engineering

Category:Metal-organic frameworks