H2/CO2 separation is the key point in the acquisition of green power and CO2 capture. A fast and precise separation with the application of selective membrane materials has been a long-cherished dream for the industrial world. Conventional membrane materials are subject to a “trade-off” phenomenon between permeance and selectivity. Hence it is a big challenge to improve the two of selective membranes simultaneously.
Recently, the group of membranes led by Prof. Weishen Yang and Prof. Yanshuo Li from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has successfully fabricated molecular sieving membranes comprised of 1-nanometer-thick nanosheets for the first time. The membrane thickness is only 1/1000 of that of cicada’s wings, even ten thousand times thinner than that of conventional molecular sieving membranes. These ultrathin nanosheets possess highly regular “screen mesh” like pores, which can precisely sieve hydrogen and carbon dioxide molecules with difference in size of only 0.04 nm, thereby intercept the bigger CO2 molecules effectively. Moreover, the selectivity and permeance of the molecular sieving nanosheets membranes have far exceeded those of all the membranes for H2/CO2 separation reported to date, and are the only ones that meet the requirements of pre-combustion CO2-capture application as demonstrated in literature. This research finding is published in the journal Science (10.1126/science.1254227) on December 12th.
Y. Lin, as the famous American scientist of inorganic membranes and also the associate director of the Journal of Membrane Science, gave a evaluation that this work is “a major advance in membrane science”. Besides, the famous German scientist of zeolite membranes and membrane catalysis, J. Caro, assessed this research finding to be “a ground-breaking work” in the molecular sieve membrane field. T. Tsuru, as the well-known scientist of microporous membranes and the chairman of Japanese membrane association, evaluated that this work has developed “a new generation” molecular sieve membranes.
The key issue of increasing the selective membranes permeance is to decrease the membrane thickness substantially; on the other hand, constructing pores in molecule-size scale can lead to the improvement of membrane selectivity. According to the two issues, molecular sieving nanosheets could be the most appropriate building unit for high-performance separation membranes. How to achieve large-area nanosheets with high crystallinity and how to control the assembly morphology of nanosheets in the membranes have significant influence on molecular sieving nanosheets membranes fabrication. The research group from Dalian Institute of Chemical Physics, Chinese academy of sciences, obtained an hydrothermal stable two-dimensional material, denoted Zn2(bim)4, via hydrothermal transformation of ZIF-7 nanoparticles which have been studied extensively as a typical zeolitic imidazolate framework. Zn2(bim)4 as precursor, methanol and n-propanol as dispersing agents, The group successfully achieved single-layer MOF nanosheets(~ 1-nm-thick) for the first time in the world with the aid of low-power wet-ball-milling and ultrasonic technology, and further fabricated ultrathin molecular sieving membranes via hot drop-coating method. For 50: 50 H2/CO2 feedstock molar ratio, the membranes exhibited H2/CO2 selectivity greater than 200 with H2 permeance for more than 2000 GPUs (Gas Permeation units, 1GPU=1 × 10-6 cm3/cm2·sec·cmHg，STP), which far surpassed the separation performance of organic and inorganic membranes reported to date. Moreover, a nanosheets membrane was tested in different heating and cooling cycles (room temperature to 200 °C) and a hydrothermal condition (150 °C, 4 mol % steam) for cal. 400 h, it showed good stability.
In recent years, two dimensional porous materials have become a research focus in the fields of low dimensional materials and nanoporous materials. Layered MOFs materials with abundant porous structure and functional surface groups could serve as a diverse source of MOFs molecular sieving nanosheets membranes. This work exhibited the significant application of layered MOF material in the ultrathin molecular sieving membranes field. The as-synthesized membranes are expected to play a role in the integrated gasification combined cycle (IGCC) system to achieve pre-combustion CO2 capture.
This work was supported by the National Natural Science Foundation of China and the Strategic Priority Research Program of the Chinese Academy of Sciences.(Photo/ Text LI Yanshuo)