Challenging Synthesis of High-Selectivity MOF-5 Crystals for Separation of CO2/N2 and CO2/CH4

MOF-5 is a classic metal-organic framework (MOF) crystal. However, synthesis of high-quality crystals usually requires an expensive N,N′-diethylformamide (DEF) solvent, and a high molar ratio of a raw material (Zn2+ : BDC2– = 3‒5 : 1). More critically, its extreme instability against water vapor has caused researchers to do not use it. In this study, we achieved some important breakthroughs by using inexpensive N,N′-dimethylformamide (DMF) solvent and meticulously optimizing synthesis parameters combined with a specialized micro-open solvothermal method. With a reduced molar ratio of a raw material (1.47 : 1), we successfully synthesized crystals exhibiting excellent phase structure and morphology, high surface area, outstanding water vapor stability, and strong CO2 adsorption capacity. The results show that the DEF replacement with DMF and reduction of the raw material molar ratio from 3 : 1 to 1.47 : 1 provides an increase of the BET surface area from ~935 to 2388 m2/g, and an improvement of the CO2 adsorption capacity from ~0.8 to 2.5 mmol/g (298 K, 100 kPa). Breakthrough curves reveal that in simulated separation experiments for the flue gas and natural gas containing low CO2 concentrations (10‒20%), the dynamic separation selectivity for CO2/N2 reaches 22, while that for CO2/CH4 remains stable at 4. Notably, the selectivity remained nearly unaffected by the pressure variation; this phenomenon is promising for enhancing the capture and purification of a low-concentration CO2 from the flue gas and natural gas. It is important that the improved MOF-5 crystals prepared under optimal synthetic conditions maintained the stable phase structure and morphology even after 14-day exposure to humid air (relative humidity >70%), successfully overcoming the challenge posed by a water vapor.