Preparation of crown ether complexing highly active double metal cyanide catalysts and copolymerization of CO2 and propylene oxide

Double metal cyanide (DMC) catalysts are generally prepared by coprecipitation of potassium hexacyanocobaltate(III) with zinc chloride followed by complexation with tert-butanol, and these materials have been used for several decades in the copolymerization of CO2 and epoxides. However, the catalytic efficiency of DMC catalysts can be adversely affected by the presence of excess K+, and the preparation of these catalysts can therefore become complicated and time-consuming because of the multiple washing and centrifugation stages required for the removal of the excess K+. In this study, 18-crown-6 ether was used as an effective co-complexing agent for the removal of K+. A series of DMCs containing 18-crown-6 were prepared with different quantities of the crown ether and different washing times. The resulting crown ether-complexing catalysts (CDMCs) and DMC catalysts without crown ether were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis-IR and X-ray diffraction. These characterization results showed that the inclusion of 18-crown-6 allowed for the formation of uniform and highly dispersed CDMC catalysts. In contrast, the DMC catalysts prepared in the absence of 18-crown-6 became uneven and delaminated during the purification by centrifugation, with high- and low-density portions of the material forming on the bottom and top of the catalyst cakes, respectively. The inclusion of 18-crown-6 not only trapped K+ but also participated in the complexion process. The complexion of tert-BuOH and 18-crown-6 led to a less crystalline form of the CDMC catalyst. Elemental analysis revealed that CDMC1 contained 1.2% K+. The copolymer was obtained by the copolymerization of CO2 with propylene oxide using CDMC3 catalyst, which was superior to the copolymer prepared using DMC1. CDMC3 was as active as DMC2 prepared without the crown ether but with seven washing steps. A hypothetical two-stage catalytic mechanism was proposed.

Graphical AbstractA new series of Zn–Co double metal cyanide catalysts with K+ fixed by 18-crown-6 showed catalytic activity as high as 5122 g/g for the copolymerization of CO2 with propylene.Figure optionsDownload as PowerPoint slide