Structural properties and chemical bonds in double metal cyanide catalysts

Abstract

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 ZnCl 2 and potassium hexacyanocobaltate(III) solution, in presence of the organic ligand ( t BuOH), 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 Wojdeł et al. Moreover, the generally accepted formula Zn 3 [Co(CN) 6 ] 2 • xZnCl 2 • ytBuOH • zH 2 O 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. Results and discussion X-ray diffractionTrizinc bishexacyanocobaltate(III), used as a reference material, has been reported to be dimorphic, cubic (Fm-3 m) and rhombohedral Catalytic centre in double metal cyanide catalysts

Transparente Filme aus CO2‐basierten polyungesättigten Polyethercarbonaten: eine neue Synthesestrategie und schnelle Vernetzung

“…Die Verwendung von DMC‐Katalysatoren für die Homopolymerisation von Epoxiden wie auch für die Copolymerisation von CO 2 mit Epoxiden und anderen Comonomeren, wie cyclischen Anhydriden und Lactonen, ist bekannt . Die katalytisch aktiven Metallzentren sind tetraedrisch koordinierte Zn‐Atome mit benachbarten Cl‐Atomen, wie durch Röntgenfeinstrukturanalyse gezeigt wurde . Die Möglichkeit, unterschiedliche Substrate mit DMC‐Katalysatoren zu polymerisieren, veranlasste uns, funktionelle Comonomere (MA und AGE) in einer Mehrkomponentenpolymerisation einzusetzen.…”

DMC-Mediated Copolymerization of CO2 and PO—Mechanistic Aspects Derived from Feed and Polymer Composition

“…It is found that the induction time is not influenced by the presence of TEA or DDA. Propoxylation reactions mediated by DMC catalysts [71][72][73] generally feature an induction period as substrates from synthesis bind to the catalyst surface, which prevent PO coordination and therefore catalytic activity [93,94]. The catalyst activation is commonly carried out by addition of a small amount of epoxide to a DMC catalyst suspended in a starter (mainly diols as CTA) of low molecular weight [95][96][97][98], such as here in form of Lupranol1200.…”

“…Proposed reaction routes in double metal cyanide (DMC)-mediated propoxylation of alcohols, external attack of CTA ROH and proton transfer (a) and "insertion" of PO in a zinc alkoxide bond after (reversible) RO − dissociation (b). Formulated in the essence of single zinc site catalysis for simplicity, more interactions will be present; structures would be derived from the 001/100 plane of DMC crystals [69][70][71].…”

Photophysical Investigation of Silver/Gold Dicyanometallates and Tetramethylammonium Networks: An Experimental and Theoretical Investigation

“…To accurately model the luminescence behavior of K 3 10 ]. This anionic model was selected based on its ground-state structural agreement with the crystal structure, as shown in Figure 5 and Table S2.…”