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), Fe^III(OH)_0.8F_0.2[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. P; V = 916.80(6) ų] at room temperature and 2 bar, followed by the transition to a “Narrow Pore” (NP) form [S. G. C2/c; V = 1083.01(2) ų] at 10 bar. The “Large Pore” (LP) form [S. G. Imcm; V = 1563.10(4) ų] 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.