Metal-Organic Frameworks are an exciting class of materials with unparalleled properties that can be customised to suit a variety of applications. The ease with which these properties can be tuned, make them a versatile material in gas separation and storage, energy storage, catalysis, drug delivery and sensing among others.
First synthesised in 1995 by Prof Omar Yaghi, Metal-Organic Frameworks (MOF) have become one of the most widely investigated materials of the 21st Century. MOFs are hybrid inorganic-organic crystalline porous materials with a positively charged metal ion surrounded by organic ligands commonly called “linkers”. Because of their cage-like hollow structure, they have an extraordinarily large internal surface area; MOFs with a whopping surface area of 7800 square meters per gram have been synthesised. A plethora of applications in diverse fields have been developed to exploit the unique structural properties of MOFs.
Unique Properties
With metal centres and organic linkers MOFs form infinite crystalline networks. The crystalline structure is characterised by very high surface to volume ratio and extraordinary flexibility in pore morphology and chemical functionality. Their unique structure endows on MOFs properties that are not available to other conventional porous materials, for example zeolites. The geometry and connectivity of the linker molecules influence the structure of the MOF. The size, shape and properties of the MOF can be tuned to suit a targeted application by modulating the linker geometry and functional groups. The ease with which these tuning can be done make MOFs a very versatile material for diverse applications.Among the numerous applications to which MOFs have been put are gas separation, gas storage, catalysis, drug delivery and sensing.
Gas Separation
Gas separation is one application of MOFs that has attracted considerable interest. These include carbon dioxide separation, oxygen purification, separation of light hydrocarbons and separation of noble gases. A particularly exciting application is CO2 capture. It is possible to tailor the functionality of MOFs to obtain high adsorption selectivity and capacity for CO2 capture. Various types of MOF-based membranes have been developed and studied for CO2 capture. The early MOF-based membrane configurations like thin film and hollow fibre suffered from stability issues. New hybrid configurations developed recently combine high separation performance with robust mechanical strength. MOF Technologies, a spin-off from Queen’s University, Belfast is a case in point. The company claims to have developed a carbon capture system using MOF-based membrane filters at 80% less energy compared to conventional technologies.
Gas Storage
The exceptionally high porosity of MOFs has captured the imagination of scientists and engineers to consider them as low-cost media for high-density storage of gases. Hundreds of MOF materials have been investigated as potential candidates for storing hydrogen. Compared with traditional porous materials like zeolites and activated carbons, the performance of MOFs has been vastly superior with record uptakes of gases like hydrogen and methane. However, they are still short of the ambitious targets set by the US Department of Energy.
Catalysis
MOFs having many open metal sites have been used as highly active catalysts. Open metal sites for catalyst applications are generated by replacing some of the original organic linkers. While the open metal sites in the framework act as active sites for catalytic reactions, the well-developed porous network in MOFs provide an excellent platform for mass transfer. Ziegler-Natta polymerisation, Diels-Alder reaction, and photochemical reactions are some excellent examples where MOF-based catalysts have been successfully used. Molybdenum-based MOFs have been used as catalysts for biodiesel production from oleic acid and palmitic acid via esterification reaction with methanol. Zinc-based MOFs have been developed as catalysts to synthesise high-purity precursors of pharmaceutical and bio-pharmaceutical compounds. MOFs and MOF-derived materials are emerging as excellent catalysts for water splitting both by electrochemical and photoelectrochemical routes, with a number of early studies yielding extremely promising results.
Drug Delivery
The highly ordered structure of MOFs together with the ultra-high surface area and large pore volume make it possible for them to adsorb functional molecules on their external surface as well as trap them inside the framework. MOFs can be functionalised with therapeutic agents for biological applications. Anti-cancer drugs have been incorporated into MOFs for intracellular delivery. MOF nanocarriers have been studied for delivering biomolecules like proteins, nucleic acids, lipids and carbohydrates. MOFs are now regarded as one of the best candidates for drug delivery.
Sensing
The most recent application of MOFs is in sensing. Precise chemical modifications can infuse MOFs with specific functions that make it possible to develop a new generation of sensors. MOF-based sensors have been used to detect solvents, pesticides, explosives and biological markers. MOF-based sensors have been extensively deployed for food safety control, where they have been able to detect pesticide residues, mycotoxins, pathogens, heavy metals, illegal additives and other contaminants. Hydration and dehydration of MOFs can significantly alter their 3D structure and this property has been tapped for detecting different levels of humidity in gaseous environments. MOF sensors have tailor made pockets to absorb a particular molecular species. Occlusion of different molecules or ions into the pores of MOFs lead to changes in their optical and electrical properties which can be exploited for sensitive detection of these species. Considerable amount of work is currently being done to study the optical and electrical response of MOFs to different species and understand the underlying mechanisms.
Energy Storage
MOFs have the potential to meet the needs of the next-generation energy storage technologies. They are already being used in electrodes of batteries and supercapacitors. A unique feature of MOFs is the ability to tweak select functionalities for improving electrochemical properties and thus lead to better performance in energy storage devices.
Synthesis
The size and architecture of MOF, and hence its properties, depend on how it is synthesised. Rather, MOFs are “designed” for a specific application by selecting the appropriate metal centre and the organic linkers. The chemical properties are further modified by chemical functionalisation of the linkers. Typically, the metal precursors and the organic linkers are dissolved in a suitable solvent and placed in a reaction vessel for the formation and self-assembly of the MOF crystals. The crystallisation time can vary from several hours to several days. The synthesis and crystallisation conditions play a very important role in determining the crystal morphology and size. Introduction of additives during crystallisation is an important strategy to control the morphology and size of MOF crystals. Many novel methods like electrochemical, sonochemical, mechanochemical, microwave-assisted, microfluidics etc. have been used in recent years for MOF synthesis. Over 90,000 MOFs have been synthesised so far. At least 500,000 have been predicted. The classification and characterisation of MOFs continues to be a work in progress.
Epilogue
MOFs are an intriguing class of materials with properties that sets them apart from other porous substances. The inherently unprecedented structural and compositional diversity of MOFs make them extremely exciting for a variety of applications. MOF research is still a nascent area and many exciting materials are yet to move from laboratory to industry. But it is a matter of time, before they make their presence felt in many cutting edge technologies
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