Monitoring the Activation Process of the Giant Pore MIL-100(Al) by Solid State NMR

The structure of mesoporous MIL-100 consists of Al-3 aluminum octahedra trinuclear units sharing a common oxo vertex/mu(3)-O, interconnected through 1,3,5-benzenetricaboxylate (btc) ligands. Each Al is bonded to four framework carboxylate functions, and the corresponding octahedra exhibit therefore a terminal position occupied either by a water molecule or by a hydroxide group. The ability of a reversible removal of coordinated water upon dehydration/rehydration process has been investigated using solid state NMR techniques. Double resonance techniques such as H-1{Al-27} TRAPDOR (transfer population in double resonance) and Al-27-{H-1} HETCOR (heteronucleus correlation) were used to probe the proximity between species containing protons and the inorganic framework. On the other hand, H-1-H-1 correlation experiments using the DQ-BABA and RFDR sequences were employed to investigate the interaction between the different kinds of protonic species, including those of the organic framework. The compound shows a remarkable thermal stability up to 370 degrees C, with small structural alterations leading to a lowering of its crystallographic symmetry. Only one water molecule per Al-3 trimer was found to leave the trimer, producing only one coordinatively unsaturated site (cus) at 350 degrees C. Compared to the case for chromium isotype MIL-100 (Cr), the difference of generated number of cus site per trimer explains at the same time the different origin of the stability of the aluminum trimer versus chromium, and the different chemistry that it will induce between both MIL-100(Al) and MIL-100(Cr). The as-synthesized compound contains an important amount of extra-framework (EF) trimesic acid encapsulated into the large pores, and most of it could be removed upon DMF/water activation. However, 0.3-0.5 molecules of (H(3)btc)(EF) per Al-3 trimer were found strongly interacting with the framework and their complete removal was difficult to achieve.