Superstructure of a Metal–Organic Framework Derived from Microdroplet Flow Reaction: An Intermediate State of Crystallization by Particle Attachment

Understanding the crystallization pathway is of fundamental importance in controlling structures and functionalities for metal-organic frameworks (MOFs), but only few studies have been reported on the mechanism of crystallization for MOFs to date. Here, by using a microdroplet flow (MF) reaction technique, we successfully revealed the different status of HKUST-1 during its crystal growth process. The morphologies and structures of crystals at different stages were recorded and characterized by scanning electron microscopy, transmission electron microscopy, and small-angle X-ray diffraction. Experimental observations clearly demonstrate a process of crystallization by particle attachment (CPA) for crystal growth of HKUST-1 under MF conditions. The superstructure of HKUST-1, which is assembled from oriented attachment of nanosized particles of HKUST-1, is observed at early stage of crystal growth. This type of superstructure gradually transforms to true single crystals through a ripening effect upon increasing residence time, accompanied by increase in dimensions of crystals. Thus, the superstructure is the intermediate state during crystallization and acts as the bridge between disordered reactants and highly ordered single crystals. Based on these findings, the crystal growth of HKUST-1 in MF reaction can be elucidated as a process involving three steps: the generation of nanosized primary particles, the following assembly of the primary particles into a superstructure, and the ripening of superstructure into a crystal. Furthermore, the superstructure of HKUST-1 shows superior performance for CO₂ and CH₄ adsorptions. The CPA mechanism in the crystallization of HKUST-1 demonstrated in this work is in clear contrast to the monomer-by-monomer addition mechanism in classic models of crystal growth. This mechanism could have important reference meaning for understanding the crystal growth mechanism of other type of MOFs or other special morphologies.