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

Article abstract of DOI:10.1016/j.cattod.2012.08.044

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.