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- Main focus article: https://en.wikipedia.org/wiki/Zeolitic_imidazolate_framework - Main focus = Comparing ZIFs to other structures.

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


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