In Situ NMR, Ex Situ XRD and SEM Study of the Hydrothermal Crystallization of Nanoporous Aluminum Trimesates MIL-96, MIL-100, and MIL-110

In an attempt to understand the relationship between the chemistry occurring in solution and the topology of final crystalline structures, this paper investigates through ex situ and in situ Al-27 and H-1 NMR methods, the formation mechanisms of three porous aluminum trimesates MIL-96, MIL-100, and MIL-110, the hydrothermal synthesis conditions of which are very similar, despite distinct crystallographic structures. The nature of the starting organic reactant-trimesic acid (H(3)btc) or trimethyltrimesate ester (Me(3)btc)-is of paramount importance for the targeted product. Both the solution and solid part of the reactive mixture were studied along the synthesis process. In situ speciation in solution of both inorganic and organic parts during hydrothermal crystallization were monitored using Al-27 and H-1 NMR spectroscopy. The nature and morphology of the various intermediate solid phases, at different reaction times, were monitored by means of ex situ powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), in relation with the solution speciation. This multimodal approach allowed assignment of the Al-27 NMR signals. Besides the dominant signal of the aluminum monomers (0-1 ppm), two additional small resonances at 4 and 7 ppm were assigned to two different types of Al dimers-a corner-shared mu(2)-O and an edge-shared mu(2)-O-stabilized by two and one trimesate ligands, respectively. These units can be found in MIL-96 and MIL-100 structure for the first type of dimer and in MIL-110 structures for the second type of dimer. Depending on the nature of the solid product, the structure of the soluble species is proposed, and a scheme of the reaction pathways occurring in solution is proposed. The final structure is strongly dependent on the nature of the species formed in solution.