Abstract
Reversible structural transition between the Large (LP) and Narrow Pore (NP) forms (breathing phenomena) of the MIL-53(X, X = Al, Cr, Fe, Ga) Metal Organic Framework (MOF) is probably one of the most amazing physical properties of this class of soft-porous materials. Whereas great attention has been paid to the elucidation of the physical mechanism ruling this reversible transition, the effect of the functionalization on the flexibility has been less explored. Among functionalized MIL-53(Al) materials, the case of NH-MIL-53(Al) is undoubtedly a very intriguing structural transition rarely observed, and the steadier phase corresponds to the narrow pore form. In this work, the flexibility of the NH-MIL-53(Al) metal organic framework was investigated by means of molecular dynamics simulations. Guest (methanol) and thermal breathing of the NH-MIL-53(Al) was thus explored. We show that it is possible to trigger a reversible transition between NP and LP forms upon adsorption, and we highlight the existence of stable intermediate forms and a very large pore phase. Furthermore, the NP form is found thermodynamically stable from 240 to 400 K, which is the result of strong intramolecular hydrogen bonds.
Highlights
The flexibility of soft-porous Metal-Organic Frameworks (MOFs) is probably one of the most fascinating structural features in the field of nanoporous materials in comparison with the most common materials such as active carbons and zeolites [1,2,3,4,5,6]
For several MOF materials, this flexibility induces structural transitions triggered by several external stimuli such as light [7,8,9], electrical fields [10,11], mechanical pressure [12,13,14] and adsorption of gases [2,3,4,5,6,15,16,17,18,19,20,21,22,23]
Among functionalized aluminum hydroxo terephthalates (Al(OH)(BDC-X)) (with X -H, -CH3, -Cl, -Br, -NH2, -NO2, -(OH)2, -CO2H) crystallizing in the MIL-53-type structure, it was recently exhibited that NH2-MIL-53(Al) did not present structural transition, which explains the little attention given to its flexibility [27,28], Ahmfelt and coworkers have found similar breathing behavior to that unfunctionalized MIL-53 [29]
Summary
The flexibility of soft-porous Metal-Organic Frameworks (MOFs) is probably one of the most fascinating structural features in the field of nanoporous materials in comparison with the most common materials such as active carbons and zeolites [1,2,3,4,5,6]. For several MOF materials, this flexibility induces structural transitions triggered by several external stimuli such as light [7,8,9], electrical fields [10,11], mechanical pressure [12,13,14] and adsorption of gases [2,3,4,5,6,15,16,17,18,19,20,21,22,23]. Let us mention that, the comparison of the effect of the adsorption of polar and no polar molecules on the flexibility of NH2-MIL-53(Al) can be very interesting, it is not the scope in this work, and that is under investigation
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