Metal-Organic Framework Films and Their Potential Applications in Environmental Pollution Control

Metal-organic frameworks (MOFs), an emerging class of porous hybrid inorganic-organic crystals, exhibit very important application prospects in gas storage and separation, heterogeneous catalysis, sensing, drug release, environmental decontamination, etc., due to their competitive advantages over other traditional porous materials (e.g., activated carbon and zeolite), including high surface areas, adjustable pore size, uniformly distributed metal centers, and tunable functionalities. However, MOF particles are usually difficult to be processed into application-specific devices because of their brittleness, insolubility, difficulty in molding, and low compatibility with other materials. It is an urgent need to shape MOF nanocrystals into various useful configurations by developing effective fabrication methods. Specifically, versatile functional MOF films with robustness and operation flexibility are highly desired. Although an increasing number of MOF films and their diverse applications have been demonstrated, this field is still at an emerging stage with challenging issues. In this Account, we describe our recent research progress on controllable synthesis of MOF films, highlighting postsynthetic polymerization, in situ interweaving, and solvent-free hot-pressing methods. Basically, two main synthesis concepts are involved, including incorporation of the performed MOF particles into polymer matrix and in situ growth of MOF coatings on surface. In MOF/polymer hybrid films, MOF nanocrystals were covalently linked by flexible polymer chains via graft copolymerization, interconnected by functional polymer chains via in situ polymerization, or adhered to polymer matrix via specific interactions at interface, consequently leading to a molecular-level homogeneous membrane or functional coating layer or foam. In these examples, the existence of polymer endows MOF films with favorable features of processability and flexibility, along with new functions. Moreover, we developed an in situ solvent-free hot-pressing method as a general approach for efficient fabrication of MOF coatings on various commercial substrates (e.g., cloth and metal foils), where metal ions or ligands were chemically bonded to the surface functional groups or metal sites at the early stage of nucleation and subsequently assembled into continuous, uniform, and stable MOF layers under confined conditions. We further extended it to a scalable manufacturing method, roll-to-roll production. MOF films severing as filters (MOFilters) have significant applications in air and water purification. They show high and stable performance in PM capture along with a low pressure drop, holding promise of application in both industrial and residential environments. Moreover, MOFilters can remove SO₂ and O₃ from air by adsorption and catalytic decomposition, respectively. Given the functional diversity of MOFs, mixed pollutants in solution could also be efficiently trapped by multifunctional MOF hollow tubes. We believe this Account will offer new insights for design and preparation of functional MOF films and coatings and accelerate the practical applications of MOFs.