- Metal Organic Framework (MOF) materials, consisting of metal ions with organic linkers, have a functional cavity structure which can be utilized in applications such as catalyst, micro sensing, and gas absorption. Due to MOF materials' selective gas adsorption property, interest in MOF materials has intensified in the last few years, particularly for CO, CO₂, N₂, CH₄, and H₂. MOF materials are typically synthesized by reaction under hydrothermal conditions which yields a highly crystalline product. However, reaction under solvothermal condition typically requires long reaction times - from 8 hours up to several days depending upon the particular MOF material and the reaction conditions, such as solvent, temperature, and concentration. Other synthesis methods that have been developed to address these issues include microwave synthesis, sonochemical synthesis, and mechanochemical synthesis. Reaction time can be reduced to minutes under the high energy conditions of a microwave synthesis method. A solvent free synthesis can be achieved using the mechanochemical synthesis. The sonochemical synthesis method provides an environmentally friendly process.
However, all of these synthesis methods above are batch processes and meet several difficulties in scalability and controllability. Herein, we introduce a new synthesis method for MOF materials which utilizes a continuous flow reactor process. To reduce the reaction time and solvent usage, and to maintain a high degree of the crystallinity are the goals of this study. Cu-BTC (BTC = Benzene, -1,3,5-Tricarboxylate ) or HKUST-1 Metal Organic Framework material was chosen to demonstrate the continuous flow reactor process since it has a simple MOF structure, consisting of Cu⁺² ions and BTC linkers, and has been widely studied for catalyst applications. The continuous flow synthesis method shows successful results of reduced residence time as low as 5 minutes, high crystal quality obtained, size control, and high yield with recycle solvent cooperation. The particle size control of MOF material has been shown crucial contributions in absorption application and is accomplished by adjusting the system temperature, flow rate, and solvent composition ratio. A water/ethanol mixture as the solvent in Cu-BTC synthesis reaction is environmentally friendly and easy to separate from the MOF product. In addition, the composition of water in solvent is the most influential factor to the crystal growth rate specifically in crystallization rate and nucleation rate. BTC is used in excess to achieve a production yield of about 97% based on Cu ion consumption. Since the Cu-BTC particles have a low solubility in the ethanol/water solution, they can be obtained easily using a dispersion/sonication method. The BTC rich supernatant can be recycled for use in the feed stream to maintain a high production rate, which can be beneficial for quick economic production in laboratory, as well as, commercial scale applications.