Unraveling the role of operating pressure in the rapid formation of Cu-BDC MOF via a microdroplet approach

Abstract

A microdroplet-based spray process has been recently developed for the synthesis of metal–organic frameworks (MOFs) which features a short processing time and scalable manner. In this process, atomized microdroplets of the MOFs precursor solution undergo well-controlled physiochemical transformations with each droplet serving as a microreactor. Droplet transformation is closely related to the flow patterns of the spray system. A conventional spray system is operated at atmospheric pressure (under the continuum flow regime). By lowering the operating pressure, the spray system is transitioned to a free-molecular flow regime, where the transport phenomena of the microdroplet become distinct due to the discrete surrounding gas. It is hypothesized that the MOFs formed under this condition could have unique morphological and structural features. Nevertheless, this topic has rarely been investigated. This work studies the formation of the copper 1,4-benzenedicarboxylate (Cu-BDC) MOF under changing flow regimes by modulating the operating pressure. Specifically, changes in crystal size, morphology, and orientation, as well as surface oxidation state, are observed with decreasing operating pressure. Heat and mass transfer calculations, accounting for the transition of flow regimes, suggest that these variations are related to different evaporation rates of the microdroplets. Our study also suggests that the dissociation of solvent molecules from Cu-BDC under sub-ambient pressure increases open metal sites which lead to a strong CO2 binding affinity. This work illustrates the synthesis of MOFs under substantially low operating pressures and offers many new opportunities in the controlled synthesis of MOFs for selective gas adsorption.