Structural Transitions and Flexibility during Dehydration-Rehydration Process in the MOF-type Aluminum Pyromellitate Al2(OH)2[C10O8H2] (MIL-118)

A new metal-organic framework (MOF)-type aluminum pyromellitate (MIL-118) has been hydrothermally synthesized in water at 210 degrees C for 24 h. The crystal structure of the as-synthesized phase (Al-2(OH)(2)(H2O)(2)[C10O8H2] or MIL-118A) has been solved from single-crystal analysis using synchrotron radiation with a specific microdiffraction setup at ESRF ID13 beamline. It consists of infinite chains of trans-connected aluminum-centered octahedra linked to each other through the pyromellitate ligand. The organic linker is not fully connected, and one carboxyl oxygen of two carboxylate functions is hydrogen bonded to the terminal water molecule attached to aluminum cations. Upon heating of the sample, the structure analysis (from powder X-ray diffraction data) showed that the terminal water is removed and the previously nonbonded carboxylate groups are now linked to aluminum reflecting a solid-state transition with the formation of Al-OCO bonding (Al-2(OH)(2)[C10O8H2] or MIL-118B). This dried phase is able to reversibly adsorb and desorb ambient water. This hydration process induces a second phase transition with the encapsulation of water molecules within the channels delimited by the infinite chains and the aromatic ligands. In the third hydrated phase (Al-2(OH)(2)[C10O8H2]center dot 2.75H(2)O or MIL-118C), whose structure was solved from powder X-ray diffraction data collected at ESRF ID31), the inorganic chains are shifted to each other with a approximate to b/6 translation along the b axis, corresponding to a tilt angle of 90 degrees between the benzene rings and the chains, instead of 61 degrees in the dried phase MIL-118B. A 36.5 degrees rotation of the benzene rings is also observed along the c axis. The occurrence of such a flexible network (6% variation for cell volume) was fully characterized by X-ray thermodiffraction, thermogravimetric analysis, and solid-state NMR (Al-27, H-1).