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Double metal cyanide (DMC) catalysts are commonly applied at industrial ring-opening polymerization of the epoxides, being the initial stage of the polyurethanes manufacturing route. This group of catalysts is frequently used in industry, but the knowledge on the molecular nature of their high activity and selectivity is limited to some phenomenological hypotheses based on overall chemical premises. To shine some light on the relation between structural and chemical properties of DMC catalysts and their activity, the complementary X-ray powder diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy and X-ray absorption spectroscopy studies were performed. The comprehensive characterization of DMC catalyst synthesized from ZnCl2 and potassium hexacyanocobaltate(III) solution, in presence of the organic ligand (tBuOH), confirmed that a significant part of the catalyst material reveals the features of the non-crystalline structure. Extended X-ray absorption fine structure (EXAFS) analysis established that only Zn atoms are the active metallic centers in the DMC catalyst. The coordination around Zn was changed from octahedral in reference material to tetrahedral in catalysts, and Cl atoms were detected near some of the Zn atoms, but no significant amount of oxygen atoms was detected, which agrees with XPS chemical analysis. The performed experimental studies give direct experimental evidence for the model deduced by Zhang et al., and for the calculation performed by Wojdel et al. Moreover, the generally accepted formula Zn3[Co(CN)6]2•xZnCl2•ytBuOH•zH2O describing this catalyst is not valid, because none of the compounds were detected by XRD or EXAFS techniques. Regarding the relation between structural and chemical activity of catalyst, we conclude that for the chemical activity of DMC catalyst not only is the content of Cl very important, but also the fraction of non-crystalline phase, the formation of which is stimulated by ligand. The non-crystalline fraction of catalyst may offer the easier access to the active center with Cl. Copyright © 2015 John Wiley & Sons, Ltd. |
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