The rapid accumulation of carbon dioxide, a greenhouse gas, is becoming a major environmental issue worldwide. [1] The rise in sea level, increase in average temperature and ocean acidity, abnormal climate change, and other unpredictable natural occurrences are all signs of global warming as a direct consequence of such accumulation. Since fossil fuels will remain a major energy source for some time, the world can expect continued industrial emissions of CO 2 that will only make the situation more severe. It is, therefore, imperative to develop effective ways to selectively capture and sequester CO 2 to reduce its green house effect in the atmosphere.[2] Current methods to capture CO 2 from flue gases include adsorption, membrane separation, and chemical absorption. [3][4][5][6][7][8][9][10] Among them, adsorption is considered a very promising technology for commercial and industrial applications because of its low energy, low cost, and wide applicability over a range of temperatures and pressures. [4,11] However, the success of this approach is strongly dependent on the development of suitable adsorbents with very high selectivity and adsorption capacity of CO 2 .Microporous metal organic frameworks (MMOFs) are a new type of adsorbent materials and promising candidates for selective CO 2 capture and separation. [12][13][14] Recent studies have demonstrated their high CO 2 uptake at low temperature or high pressure. [15][16][17][18][19][20][21][22][23][24][25][26][27][28] However, their CO 2 selectivity at relative low pressure and high temperature conditions (e.g., 0.1-0.2 atm, 55-65 8C), which reflect those in flue gas mixtures, remains low and unsuitable for industrial processes. Herein, we present a flexible MMOF material that exhibits remarkably high adsorption selectivity of CO 2 over other small gases under such conditions, with a separation ratio of 294, 190, 257, and 441 (v/v) for CO 2 /N 2 , CO 2 /H 2 , CO 2 /CH 4 and CO 2 /CO, respectively, at 0.16 atm and 25 8C and of 768 for CO 2 /O 2 (v/v) at 0.2 atm and 25 8C, calculated based on single-component gas adsorption data using the method by Choi and Suh.[15] With a gas mixture of 20 % CO 2 and 80 % air, the CO 2 /N 2 separation ratio reaches a value of 84 at 50 8C desorption temperature, demonstrating an outstanding selectivity and good separation capability of this material towards CO 2 under conditions that mimic industrial flue gas compositions. To the best of our knowledge, these values represent the highest separation selectivity among all MOFs reported to date.
