A new paper by Behrens and co-workers in CrystEngComm details the modulated syntheses of a Zinc-fumarate MOF in both water and DMF. The authors performed kinetic studies for each synthesis, showing that, contrary to what was expected, the modulator increased the rate of formation of the MOF in DMF.
Modulating agents (usually monocarboxylic acids) are added to a MOF reaction mixture to increase the reproducibility and crystallinity of the final product. In coordination modulation, the modulating agent competes with the organic linkers in binding to the metal centre, preventing the formation of impurities but slowing down the reaction. In this current work, the authors chose formic acid as their modulator and performed in situ energy dispersive x-ray diffraction, which allowed for quantitative kinetic data to be produced.
When the authors carried out the synthesis in water, the modulating agent behaved as expected, decreasing the nucleation and growth rates as the formic acid concentration increased. However, when formic acid was added to the DMF-containing reaction mixture, the rate of growth increased. The authors theorise this occurs due to trace water in their commercial formic acid which they investigated by keeping the formic acid concentration constant but increasing water content. This showed remarkable results, increasing the rate constant by 2 orders of magnitude.
By observing that both the presence of a modulator and the water concentration have a large effect on the crystal formation, the authors added to the body of evidence that successful MOF syntheses are highly dependent on subtle changes in reagents and conditions.
Read the full article to find out more
Insight into the mechanism of modulated syntheses: in situ synchrotron diffraction studies on the formation of Zr-fumarate MOF
Gesa Zahn, Philip Zerner, Jann Lippke, Fabian L. Kempf, Sebastian Lilienthal, Christian A. Schröder, Andreas M. Schneidera and Peter Behrens
CrystEngComm, 2014, 16, 9198-9207
|Josh Campbell is a PhD student, currently at the University of Southampton, UK studying crystal structure prediction of organic semiconductors. He received his BSc from the University of Bradford.|