A method for controlled growth of large MIL-88A crystals†

The most challenging aspect of the structural elucidation of novel metal–organic frameworks (MOFs) is presumably the small crystal size of the obtained compounds, since nanocrystallites cannot be analysed by X-ray single crystal diffraction without special equipment. In this paper, we present a new method based on the example of the well-known MIL-88A with which the size of the MOF crystals can be significantly increased by varying the reactants. It was shown that by replacing the originally used fumaric acid with fumaric acid esters, the fumarate ions are successively added to the reaction mixture because of the required ester hydrolysis, which results in a controlled growth of large MIL-88A crystals. In addition to the previously existing Rietveld refinements, it was thereby possible to obtain the first single crystal structure analysis of MIL-88A. This new method for growing large MOF crystals may thus provide access to the structural elucidation of further MOFs and offers a field of applications, such as the investigation of chemical reactions on defined MOF crystal surfaces.