Sub-second HKUST-1 Synthesis in Continuous Flow Supercritical CO2 Reactor

Abstract

The efficient, reliable, and environmentally friendly synthesis of Metal-Organic Frameworks (MOFs) can trigger their wide adoption in many practical applications. Addressing this challenge, this study focuses on optimizing the continuous flow synthesis of Copper Benzene-1,3,5-Tricarboxylate (HKUST-1) in a supercritical carbon dioxide (scCO2) environment. The effects of the synthesis parameters on the physiochemical characteristics of MOF were investigated over a wide range of CO2 injection temperature Tinj = 50 300 C while maintaining sub-second reactor residence time ~ 650 ms. The X-ray diffraction analysis (XRD) verified a well-defined MOF crystal structure. Analysis of the MOFs' surface area and pore sizes indicated that the optimal properties were obtained at a CO2 injection temperature of 150 C corresponding to an average reactor temperature of ~80 C with the maximum BET-specific surface area ~ 1,550 m2/g and pore size ~ 10.55 . At lower synthesis temperatures, the pore sizes and BET-specific surface area are lower due to insufficient activation, while at temperatures Tinj >250C, BET surface area decreases significantly due to the degradation of organic precursors. The increase in the synthesis temperature results in a fast MOF synthesis and activation due to a single-phase supercritical environment; however, high CO2 injection temperatures may create a high-temperature gradient leading to rapid degradation of organic precursors. This work demonstrates the feasibility of sub-second synthesis of high-quality MOFs and highlights the need to optimize the continuous flow process for HKUST-1 and other MOFs.