Pressure-regulated synthesis of Cu(TPA)·(DMF) in microdroplets for selective CO₂ adsorption

The synthesis of metal-organic frameworks (MOFs) by using traditional wet-chemistry methods generally requires very long durations and still suffers from non-uniform heat and mass transfer within the bulk precursor solutions. Towards addressing these issues, a microdroplet-based spray method has been developed. In a typical spray process, an MOF's precursor solution is first atomized into microdroplets. These droplets serve as microreactors to ensure homogeneous mixing, fast evaporation, and rapid nucleation and crystal growth to form MOF particles. However, the fundamental MOF formation mechanisms by using this strategy have not been fully understood. In this work, the role of the operating pressure in the synthesis of a representative MOF (i.e., Cu(TPA)·(DMF); TPA: terephthalic acid, DMF: dimethylformamide) was systematically investigated. Detailed characterization showed that the pressure variations significantly affected both the morphologies and crystalline structures of Cu(TPA)·(DMF). Numerical simulations revealed that the morphology changes are mainly attributed to the variations in supersaturation ratios, which are caused by different microdroplet evaporation rates due to the regulation of operating pressure, while the crystalline structure variations are closely related to the dissociation of DMF molecules at lower operating pressures. Besides, the dissociation of DMF molecules decreased the surface area of the MOF crystals, but gave rise to massive coordinatively unsaturated metal sites, which greatly enhanced the interaction of CO2 with the MOF crystal and thus led to improved CO2 adsorption capacity and selectivity. The outcome of this work would contribute to the fundamental understanding of MOF synthesis using the microdroplet-based spray method.