Gentle Exfoliation Yields Mof Membranes

Thin sheets of metal-organic framework (MOF) compounds could be useful as gas-separating membranes. But researchers have had limited success in preparing these crystalline solids in two-dimensional form. Yanshuo Li and Weishen Yang of China’s Dalian Institute of Chemical Physics and coworkers have now demonstrated that MOFs with a layered structure can be exfoliated in a way that exposes 1-nm-thick sheets with large lateral dimensions and a high degree of crystallinity (Science 2014, DOI: 10.1126/science.1254227). MOFs are porous crystalline materials composed of metal ions or metal clusters bridged by organic linking groups. Some MOFs exhibit exceptional surface areas and gas storage and separation properties because of their elaborate system of pores and internal channels. For those applications, MOFs are typically used as powders. Easily processed membranes would be more convenient, but common methods for preparing membranes often damage the MOF structure, which is key to gas permeation. So Li, Yang, and coworkers devised a gentle exfoliation method that includes low-speed wet ball milling and ultrasonicating in a methanol-propanol mixture. In tests with hydrogen-CO₂ mixtures, the team showed that a membrane formed from exfoliated sheets of a Zn-benzimidazole MOF efficiently separates the gases, allowing hydrogen to permeate easily and selectively.