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 (Zn 2+ : BDC 2– = 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, out standing water vapor stability, and strong CO 2 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 m 2 /g, and an improvement of the CO 2 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 CO 2 concentrations (10‒20%), the dynamic separation selectivity for CO 2 /N 2 reaches 22, while that for CO 2 /CH 4 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 CO 2 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.