Small-angle X-ray scattering documents the growth of metal-organic frameworks

Abstract

We present a combined in situ small- and wide-angle scattering (SAXS/WAXS) study on the crystallization of two topical metal-organic frameworks synthesized from similar metal and organic precursors: NH2-MIL-53(Al) and NH2-MIL-101(Al). A thorough analysis of SAXS data reveals the most important phenomena occurring during crystallization and unravels the effect of the solvent. NH2-MIL-53(Al) growth follows two routes: (i) through direct hydrolysis of AlCl3·6H2O in water, and (ii) via the intermediate NH2-MOF-235(Al), which forms in pure DMF or DMF/H2O mixtures. In the case of pure H2O as solvent, formation of NH2-MIL-53(Al) crystals proceeds through steady growth in all three dimensions. The addition of DMF to the synthesis mixture results in amorphous scattering entities forming very rapidly and subsequently arranging into the intermediate phase, NH2-MOF-235(Al). In DMF/H2O mixtures, amorphous precursors develop in rapid fashion with fractal character dominating, followed by densification, crystallization of NH2-MOF-235(Al) and slow transformation into NH2-MIL-53(Al). Formation of NH2-MIL-101(Al) only occurs when pure DMF is used as solvent, and it always proceeds through the formation of the intermediate NH2-MOF-235(Al). In this case a smooth scatterer surface is observed, with morphology and size constant in time. --------------------------------------------------------------------------------