Location of CO2 during its uptake by the flexible porous metal-organic framework MIL-53(Fe): A high resolution powder X-ray diffraction study

Abstract

The interaction of CO2 with the porous metal-organic framework material MIL-53(Fe), FeIII(OH)0.8F0.2[O2C-C6H4-CO2] has been studied by complementary gas adsorption and high resolution powder X-ray diffraction as a function of gas pressure. It has been shown that CO2 adsorption occurs in three steps, with firstly the formation of an "Intermediate" (INT) form [S. G. P1; V = 916.80(6) A˚3] at room temperature and 2 bar, followed by the transition to a "Narrow Pore" (NP) form [S. G. C2/c; V = 1083.01(2) A˚3] at 10 bar. The "Large Pore" (LP) form [S. G. Imcm; V = 1563.10(4) A˚3] is obtained also at 10 bar but by decreasing the temperature to 220 K. Crystal structures of the three CO2 materials MIL-53(Fe)[nCO2], with n = 0.22, 0.63 and 2.72, have been solved and refined, which has allowed precise localisation of guest CO2 molecules, not previously determined. This shows that the (INT) form presents two types of tunnels of different sizes, with only the largest ones are occupied by the CO2 molecules. In the (NP) and (LP) forms, all tunnels become equivalent and are occupied by CO2. The huge unit cell volume increase of the (LP) form leads to drastic increase in the amount of CO2 adsorbed. In the three forms, CO2 molecules are located in order to favour interactions between their oxygen atoms and the OH/F groups of the framework and there is no evidence for guest-guest interactions until the highest loading where short contacts of a similar distance found in solid CO2 are observed.