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Metal Organic Frameworks (MOF) are one of the most exciting classes of chemical structures to be discovered in the past decade. They’ve been used to great effect in an incredible variety of applications: water splitting, gas separation, water filtration, therapeutics delivery, conversion of CO2 to economically valuable products, photovoltaics, Lithium ion electrodes, and high capacity storage of H2, methane, and CO2. Even more surprising than their broad applicability is the simple chemical concepts that underlie MOF synthesis and utilization. In this post, I’ll give a high-level overview of their synthesis, properties, and applications.
MOF’s are composed of metal atoms (nodes) and organic ligands (edges), which form a highly porous, interconnected, pseudo-crystalline structure. But to synthesize such a complicated and ordered structure, all we need to do is toss a metal salt and an organic linker into a simple organic solvent, and wait! After separating via centrifugation or filtration, we end up with our MOF crystals.
As an example, we could use Zinc Nitrate and 2-methylimidazole(Hmim) as our metal salt and organic linker respectively, Dimethylformamide(DMF) as our organic solvent, and apply 140 degrees Celsius heating for 24 hours. This particular synthesis procedure would lead to the formation of Zeolitic Imidazolate Framework(ZIF) 8, an archetypal MOF. This procedure was actually used in the original paper(which has been cited over 2000 times !) that first characterized ZIF’s.
As time went on, this already simple synthesis procedure was improved upon so that now MOF’s can now be synthesized solvent-free, at low temperatures and shorter time periods, in aqueous solutions, and in continuous processes. Their simple synthesis procedure and broad application have led to several startup ventures already, such as NuMat Technologies and Mosaic Materials.
The facile synthesis of MOF’s also means they are easily integrated into other advanced nanoscale materials. For instance, nanoparticles can be easily encapsulated by MOF crystals simply by adding nanoparticles to a solution of metal nodes and linkers. This encapsulation strategy can also be used to synthesize core-shell nanostructures, where the MOF forms a shell around a core nanoparticle. This often serves to prevent agglomeration of the otherwise naked nanoparticles or to improve adsorption of reagents on active sites by taking advantage of MOF’s high surface area.
Another natural morphology we might desire is thin films/membranes. Such a morphology is particularly prominent in separation and filtration applications. A general strategy for synthesizing porous thin films is to use a porous substrate, typically Alumina (Al2O3), and add ligands and metal salts to the solution. Current challenges include crack-free membranes, controlling uniform growth, and film adhesion to the substrate, although substantial progress has been made in understanding how to counteract such problems.
In the below paper published in Scientific Reports, Hmim-coated ZnO nanoparticles were electrodeposited on an etched pencil bar. A Zinc salt and Hmim were added to a methanol solution and the substrate was immersed in the solution for a short duration of time. The resulting ZIF-8 membrane was then used to adsorb and monitor drug levels in urine.
There are two main serial themes to MOF synthesis. The first is that they are easily synthesizable using an assortment of common reagents e.g. transition metals and common organic molecules, and simple, non-toxic, wet chemistry(although a vast array of other methods to synthesize MOF’s are also under active investigation). The second is that their versatile yet simple synthesis procedure allows for facile incorporation of MOF’s in complex nanomorphologies. Encapsulation and embedding are central design themes, as transition metal based nanostructures can be used as sacrificial templates to grow MOF’s.
Given our knowledge of the general structure of MOF’s, we can now describe some of their properties. Perhaps their most notable property is their incredibly high surface area, which can range from 1500–7000 m²/g. In contrast, graphene has a max theoretical surface area of 2360 m²/g. This incredibly high surface area results from the ordered distribution of nano-scale pore cavities with the majority of pore diameters less than 1nm. At this scale, we can count the number of atoms that would fit inside these pores!
Another hallmark of MOF’s is their high degree of tunability. This tunability of properties(pore size and distribution, cavity size, adsorption selectivity) has allowed their simple synthesis template has been applied to a wide variety of metals, such as Zirconium, Aluminum, Zinc, Copper, and even multi-metal based MOF’s. Indeed, this is often how new MOF’s are discovered e.g. ZIF-8 and ZIF-67 which are Copper and Cobalt based respectively but share a common organic linker. Ligand modification or exchange can also be utilized to alter pore distributions, cavity sizes, mechanical properties, etc.
Given that such Metal Organic Frameworks form through covalent and dipole interactions between their metal nodes and organic linkers, we can also demonstrate that such structures are quite chemically stable. Indeed, MOF’s have been shown to have high thermal and chemical stability (although some modifications are required for stability under moisture). This stability is also tied into its generally high resistivity and large bandgap(>5eV), although discovering conductive MOF’s is an ongoing field of research, particularly of interest for photovoltaics and batteries.
In the next several sections, we’ll use examine several papers to see how these properties of MOF’s are being applied to real world problems.
Applications — Gas storage(CO2,H2)
The high porosity and tunability of MOF’s makes them a natural choice for investigating gas storage, both for Carbon Capture and Storage(CCS) and hydrogen energy storage. In “Fluorous Metal-Organic Frameworks with Enhanced Stability and High H2/ CO2 Storage Capacities” published in Scientific Reports 2013, the authors describe a novel set of MOF’s, which they denote as TKL-101 to 107. Composed of nickel metal nodes and a 1:1 molar ratio of pthalic acid and 2,4,6- tri(4-pyridinyl)-1,3,5-triazine (TPT) ligands, they can be easily prepared by adding Nickel nitrate, pthalic acid, and TPT to a solution of DMF heated at 100 deg C for 2 days. Note how even very recent and novel discoveries of new MOF’s still use the same, simple, wet-chemistry based procedures!
The authors synthesized TKL-104 to 107 by adding varying concentrations of fluorine to opthalic acid. Simply by modifying one of the ligands with a dopant, we obtain excellent results for gas capture. Note that MOF’s also tend to demonstrate excellent selectivity — in this paper, the ratio of CO2 uptake to CH4 uptake is almost 5:1. These MOF’s also demonstrate excellent hydrogen storage potential for fuel cell applications. For context, the Dept. of Energy set 4.5wt% H2 storage as a specific system goal for portable fuel cells, which this paper far surpasses, albeit at very low temperatures and high pressure.
Applications — Electrodes
In addition to storing gases, the high surface area of MOF’s also makes them of interest for battery and supercapacitor electrodes, which store electrical charge. The main challenge for integrating MOF’s into electrodes is their high resistivity.
A paper recently published in Nature Communication demonstrates how to overcome these challenges and presents state-of-the-art results for Lithium Sulfur batteries. Currently, the market is dominated by cobalt-based lithium batteries, which is much more expensive and geopolitically contentious compared to the abundant sulfur. This paper utilizes a method called “solid precursor assisted-confinement conversion process” to integrate various MOF’s with Carbon Nanotubes(CNT). CNT’s are a highly conductive material commonly used in electrode research.
Specifically, metal hydroxide nanostrands are prepared and mixed with CNT’s. Because the nanostrands are positively charged, the CNT’s are well-integrated within these nanostrands. A porous anodic alumina oxide substrate is immersed in this solution and mixed with a solution containing the organic ligands for each respective MOF. By varying ligand concentration, temperature, and immersion time, the weight ratio of MOF to CNT can be easily controlled. By using an porous anodic alumina oxide substrate, these complex nanomorphologies can be synthesized binder-free. Sulfur is then added drop-wise afterwards.
This nanomorphology results in a dense interweaving of highly conductive CNT’s and polysulfides within a porous structure, allowing for fast ion transport and higher sulfur loading. MOF’s also endow the structure with mechanical robustness, allowing it to withstand volumetric changes that have plagued Li-ion batteries. Finally, the porosity of this nanostructure also allows for good electrolyte contact and wettability, allowing for easy Li penetration.
In the above figure, the authors demonstrate in parts a,b the mechanical durability of the cell by bending it at varying angles. In e), we can see a clear performance comparison between this reference and other MOF’s in Li-Sulfur systems. For comparison, the maximum theoretical volumetric capacity of cobalt-based electrodes for Li ion batteries is 1360 Ah/L.
Hopefully this article helped you gain some insight into how simple chemical processes can create wonderful assortments of different compounds and nanomorphologies. Simply by tossing some common reagents into a solution, we can synthesize nanoscale structures with incredible properties! Characterized by their extremely high surface area and easy tunability of properties, Metal Organic Frameworks are set to radically improve the materials we use in our world.
If you want to see some of the work I’ve done on MOF’s, check out this paper I helped co-author that was on the back cover feature of Crystal Engineering Communications or this report that I wrote on ZIF-8 formation mechanisms.