Continuous Synthesis Of Metal-Organic Frameworks Via Microdroplet Flow Reaction And Structural Modification

Metal-organic frameworks?MOFs?,an emerging class of multifunctional porous materials,have attracted extensive attentions due to their unprecedented surface areas,variable structures and tunable functionalities.Thanks to these characteristics,they have found plenty of applications including CH₄ storage,CO₂ adsorption and separation of gas mixture.However,preparation of MOFs by conventional solvothermal/hydrothermal method always require several hours or days,the scale-up meet its limitation due to the size of the vessel and the conventional methods often provides an risk for the inflexibility controlled crystallization process of MOFs.When considering the formation of MOFs,the mechanism of crystallization process,the controllable synthesis and the large scale production become increasingly important.The microdroplets flow reaction hold the great advantages in material synthesis as a result of its distinguishing features,such as,precise reaction control,high heat and mass transfer,high surfaceto-volume ratio and continuous scale-up production.Therefore,the development of microdroplets flow reaction for studying the formation mechanism of MOFs crystal growth,and then synthesizing MOFs with controllable become very promising.In this thesis,a series of MOFs with superstructure,hierarchically porous structures and ultrathin 2D nanosheets were designed and synthesized.The structures of these MOFs were characterized and the gas adsorption behaviors on these MOFs were investigated.Firstly,we successfully revealed the different status of HKUST-1 during its crystal growth process by using a microdroplet flow?MF?reaction technique.The morphologies and structures of crystals at different stages clearly demonstrate a process of crystallization by particle attachment?CPA?for crystal growth of HKUST-1 under MF conditions.The superstructure of HKUST-1,assembled from oriented attachment of nano-sized particles of HKUST-1,is observed at the early stage of crystal growth.This type of superstructure gradually transforms to true single-crystals through a Ostwald ripening effect upon increasing residence time.Thus,the superstructure is the intermediate state during crystallization and acts as the bridge between disordered reactants and highly ordered single-crystals.Furthermore,the superstructure of HKUST-1 shows superior performance for CO₂ and CH₄ adsorptions.Secondly,a hierarchical porous zirconium metal-organic framework?UiO-66?was prepared continuously through a microdroplet flow reaction strategy for the first time.The existence of metal-node defects arising from incomplete coordination in UiO-66 was found to be the main reason for the formation of mesopores.The dimensions of mesopores could be facilely tuned by adjusting the residence time,and the portion of mesopores was linearly correlated with the missing-nodes.The surface acidity was enhanced due to a large amount of pendant coordinated-free carboxylate groups in pores.Therefore,the existence of hierarchical porous structure and the enhanced surface acidity provided the superior storage capacities for CO₂ and CH₄ on MOFsThirdly,we present the first facile,continuous bottom-up strategy for preparing ultrathin?³ nm?2D MOF nanosheets with high crystallinity comprising of assemblies of a few layers.Unlike conventional solvothermal synthetic methods for 2D MOFs,the weak interlayer interaction in the vertical direction of the 2D materials is restricted under the microdroplet flow reaction conditions.2D MOF nanosheets with a large lateral area and a few layers thick were directly synthesized by suppressing the lamellar stacking of the nanosheets under the dynamic growth conditions.The ultrathin 2D MOF nanosheets have greater external surface area,resulting in a far higher gas adsorption?³ times for CO₂ and 2.6 times for CH₄?.Furthermore,the continuous microdroplet flow reaction technique can be extended easily to realize large-scale production for materials.Therefore,this method represents a breakthrough for the synthesis of ultrathin 2D MOF nanosheets.Finally,we report an efficient method of synthesizing a series of Zr-MOFs through a microdroplet flow reaction,which is more accommodate the requirements of industrial production.Four types of Zr-based MOFs with different ligands and topologies were produced as a pure phase of high quality crystalline with uniform morphologies.Furthermore,this series of Zr-MOFs were obtained in a continuous way and at the conversion rates is high up to 85.7%and the space-time yield?STY?highly up to 367.2 kg·m^-3·d^-1.The continuous synthesis,good product quality and high productivity of microdroplet flow reaction are expected to realize large-scale production for Zr-MOFs in practical applications...