Metal Organic Frameworks: The highly porous and versatile future material

 

Metal Organic Frameworks

Metal organic framework (MOFs) are a class of porous crystalline materials composed of metal ions or metal-containing clusters coordinated to organic ligands. The large internal surface area of a MOF lends it well to storage of gases such as hydrogen, methane, or carbon dioxide. In addition, MOFs offer tunable pores, large surface areas, and excellent chemical and thermal stability. These promising properties have spurred intense research on MOFs for gas storage and separation, catalysis, drug delivery, sensing, and more.

Porous Structure

The high porosity or internal surface area of MOFs is one of their most advantageous properties. Metal Organic Framework crystals feature metal ion centers connected by organic linkers into two- or three-dimensional framework structures with nanometer-sized channels and cavities. Careful selection of the metal ions and organic ligands allows for systematic tuning of the pore size, functionality, and geometry. Measured Brunauer–Emmett–Teller (BET) surface areas of MOFs are routinely above 1500 m2/g with some reported to have surface areas exceeding 5000 m2/g. For comparison, typical surface areas of other porous solids like zeolites and activated carbon are 500-1500 m2/g and 1000 m2/g respectively. The extremely large internal void spaces of MOFs afford massive storage capacities and high adsorption selectivities for gases.

Tunability

 

The modular nature of MOF synthesis, involving the self-assembly of metal centers and organic struts, affords an exceptional degree of tunability. Slight modifications to the metal ions or organic ligands can significantly alter the framework topology, pore size, and functionality. Common strategies for tunability include: varying the metal ion radius, changing the length/geometry of the organic linker, introducing functional groups to the linkers, and post-synthetic modifications. Researchers have synthesized over 70,000 distinct MOF structures to date by altering the metal and organic components. This tunability at the molecular level grants unprecedented control over material properties for targeting specific applications.

Gas Storage

The ability to tune pore size and chemical functionality makes MOFs promising candidates for improved gas storage. MOFs have achieved some of the highest reported gravimetric and volumetric adsorption capacities for hydrogen, the cleanest burning fossil fuel. However, further improvements are still needed to meet DOE targets for usable H2 storage in vehicles. MOFs also display outstanding uptake capacities and selectivities for other industrially and environmentally important gases like natural gas, carbon dioxide, and methane. Beyond pure gas storage, the design of MOFs optimized for mixed-gas systems offers possibilities for improved separation. Overall, the enormous internal surface areas, systematic tunability, and tailored functionalities of MOFs position them as leading contenders for next-generation gas applications.

Catalysis

 

Metal sites within MOFs can act as Lewis-acid catalysts while organic linkers offer possibilities for base-catalyzed transformations or shape selectivity. By placing reactive metal centers in precise arrangements surrounded by organic pores and functional groups, MOFs present a new design paradigm for solid catalysts. MOF catalysts have shown promising activities and reusability for varied reactions including hydrolysis, carbon-carbon coupling, hydrogenation, epoxidation, and more. Furthermore, MOFs allow incorporation of multiple catalytic species for cooperative or tandem catalysis not accessible with typical solid frameworks. Areas of current research include designing MOF structures optimized for reaction selectivity, activity, and stability under process conditions. MOF catalysts offer exciting prospects for improved performance and resource efficiency.

Drug Delivery

MOFs properties are well suited for biomedical applications like drug delivery and imaging. Their high surface areas, tunable pore sizes, and biocompatible constituents permit controlled encapsulation and release of pharmaceutical compounds. Drugs can be loaded into the porous interior or coordinated to open metal sites of MOFs for sustained or triggered action. Further, surface functionalization allows targeted delivery. Research aims to optimize MOF carriers for improved drug loading, minimal burst release, protection of actives, biodegradability, and in vivo circulation. Some studies demonstrate MOFs effectively enhancing drug solubility while maintaining therapeutic efficacy in animal models. Other work applies MOFs for bioimaging, with near-infrared responsive MOF nanoparticles demonstrating in vivo tumor tracking. Continued advances in biocompatibility and load/release performance position MOFs at the forefront of developing efficient drug and therapeutic delivery nanomaterials.

Sensing

MOFs have tremendous potential as sensing platforms because their physical and chemical properties are readily modulated. Tunable porosity enables selective detection of analytes via size exclusion effects. Photoactive MOFs fluoresce in response to chemical stimuli, giving colorimetric or luminescent signals. Redox-active MOFs change conductivity upon analyte binding. Surface-immobilized recognition sites provide molecular selectivity. Researchers are developing MOF-based sensors for important targets including explosives, toxic gases, pharmaceutical residues, and biological species. Recent examples include fluorescent MOFs for imaging cancer biomarkers or quantifying neurotransmitters, electronic skin sensors utilizing conductive MOFs, and paper-based detection kits leveraging patterned MOF arrays. The early-stage field of MOF sensing holds great promise for low-cost, rapid, portable, and reusable detection technologies impacting security, healthcare, and more.

In metal organic framework represent a highly versatile new class of porous crystalline materials with enormous potential spanning gas storage, catalysis, drug delivery, sensing, and beyond. Their systematic tunability at the molecular level allows tailoring properties like pore size, functionality, and crystal structure. This tunability, along with the immense porous surface areas facilitated by metal-organic coordination, underpins MOFs ability to outperform traditional porous solids. Significant advances have already been made utilizing MOF properties, but continued research promises even greater impact in the development of next-generation sustainable technologies and biomedical solutions. With rapid expansion of applications and design possibilities, metal-organic frameworks are emerging as undoubtedly important materials driving innovation for the future.

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