Aerosol Synthesis of Metal-Organic Frameworks Under Low Pressures

JIANPING CHEN, Zan Zhu, Da-Ren Chen, Wei-Ning Wang, Virginia Commonwealth University

     Abstract Number: 5
     Working Group: Nanoparticles and Materials Synthesis

Abstract
The development of synthesis methods of metal-organic frameworks (MOFs) creates many new opportunities in the utilization of porous materials. Recently, a microdroplet spray process for MOF synthesis has been developed. This aerosol process offers a way for engineering nanoscale MOFs with controlled structures and properties for applications such as gas adsorption. In this aerosol process, atomized microdroplets of a MOF precursor solution undergo well-controlled physiochemical transformations, with each microdroplet serving as a microreactor. Reaction parameters, such as temperature, pressure, can have a strong influence on solvent evaporation, and eventually, affect the rate of nucleation and growth of MOFs. In particular, the synthesis pressure is of importance since it can determine the flow regime of aerosol droplets (i.e., continuum or free molecular). The evaporative cooling effect occurs in this free molecular regime (i.e., Knudsen number Kn ≥ 1), which complicates the heat and mass transfer accounting for the MOF formation process. Nevertheless, this problem remains complex both from a computational and experimental standpoint. This work studies the rapid formation of copper-based MOFs under changing flow regimes. 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 from continuum to free-molecular, suggest that these variations are partially related to different microdroplet evaporation rates. In addition, we found that the dissociation of solvent molecules from MOFs under sub-ambient pressure generates open metal sites which result in high binding affinity for CO₂ molecules. Our study 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.