Location of CO₂during its uptake by the flexible porous metal–organic framework MIL-53(Fe): a high resolution powder X-ray diffraction study

Abstract

The interaction of CO 2 with the porous metal-organic framework material MIL-53(Fe), Fe III (OH) 0.8 F 0.2 [O 2 CC 6 H 4-CO 2 ] 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 CO 2 adsorption occurs in three steps, with firstly the formation of an "Intermediate" (INT) form [S. G. P1; V = 916.80(6) Å 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) Å 3 ] at 10 bar. The "Large Pore" (LP) form [S. G. Imcm; V = 1563.10(4) Å 3 ] is obtained also at 10 bar but by decreasing the temperature to 220 K. Crystal structures of the three CO 2 materials MIL-53(Fe)[nCO 2 ], with n = 0.22, 0.63 and 2.72, have been solved and refined, which has allowed precise localisation of guest CO 2 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 CO 2 molecules. In the (NP) and (LP) forms, all tunnels become equivalent and are occupied by CO 2. The huge unit cell volume increase of the (LP) form leads to drastic increase in the amount of CO 2 adsorbed. In the three forms, CO 2 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 CO 2 are observed.

Effect of the structural constituents of metal organic frameworks on carbon dioxide capture

“…As pressure is increased, CO 2 fills up channels and positions itself in the middle of the pores. As pressure increased even further, the framework of MIL-53 is fully expanded and the CO 2 adsorption process is predominantly governed by the guest-guest molecules interactions [121].…”

MIL‐53 and its Isoreticular Analogues: a Review of the Chemistry and Structure of a Prototypical Flexible Metal‐Organic Framework

“…Selective binding of gases is important for practical applications, and for MIL‐53(Cr) an interesting observation was made on the influence of hydration: the selectivity of CO 2 over CH 4 was dramatically improved if water was also present, unlike in zeolite adsorbents where water often blocks the adsorption sites for carbon dioxide . In the case of MIL‐53(Fe) analysis of high‐resolution powder X‐ray diffraction patterns allowed accurate location of CO 2 guests at different loadings: this showed that intermediate structures can be isolated with not every pore of the structure filled by guest molecules, Figure . Furthermore a fully‐open form of MIL‐53(Fe) could be induced with high CO 2 loadings, not possible with water or by just heating, as noted above.…”

“… MIL‐53(Fe) with different adsorbed CO 2 loadings with crystal structures refined during increasing gas pressure from ambient to 10 bar and on cooling to 220 K …”

State‐of‐the‐Art Advances and Challenges of Iron‐Based Metal Organic Frameworks from Attractive Features, Synthesis to Multifunctional Applications

“…b) Two views of the structure of MIL‐53(Fe) in the ab plane showing stacked CO 2 molecules at 2 bar and 10 bar. Reproduced with permission . Copyright 2015, Royal Society of Chemistry.…”

“…Pore size of MOFs is correlated with the adsorption affinity for CO 2 . For example, Guillou et al reported that MIL‐53(Fe) under higher pressure (10 bar) presented larger pore structure, leading to drastic increase of adsorbed CO 2 molecules inside the framework (Figure b). On the one hand, the catenation or interpenetration of several frameworks are regarded as obstacles to prepare porous MOFs due to the reduction of pore volume .…”