Mechanochemical-synthesis And Property Of Double Metal Cyanide Catalysts For Carbon Dioxide Polymerization

Owing to increasing awareness of energy,environment and sustainable development,fixation and utilization of carbon dioxide?CO₂?has increasingly becoming a worldwide topic.Various polycarbonates can be synthesized by the sequential copolymerization of CO₂ and strained heterocyclic molecules with the presence of catalysts.Among the developed catalyst systems,double metal cyanide?DMC?complexes are well-known heterogeneous catalysts for the effective utilization of CO₂.However,tedious procedures of DMCs preparation in conventional solvent-based methods are time-,energy-and materials-consuming.With an objective of simplifying the preparation and improving the selectivity,the work employed mechanochemistry,which is regard as bearing the concept "green chemistry",to synthesize DMC catalysts.The work greatly simplified the preparation processes,and presented a novel method for DMC syntheses.The first class Fe-Zn DMC was obtained by solvent-free grinding ZnCl₂ and K₃Fe?CN?₆ at 50 Hz for 14min.The copolymerization of propylene oxide?PO?and CO₂ demonstrated that the Fe-Zn DMC obtained at this condition tended to be completely amorphous and showed highest catalytic activity with TOF?Turnover frequency?19.34 h-1.Moreover,the produced poly?propylene carbonate??PPC?had relatively favorable properties,with Mn 98000,PDI 1.93 and 40%mass fraction of CO₂.Prolonging of grinding time and shortening of grinding strength will result in increase of crystallinity and decrease of catalytic activity of the Fe-Zn DMC.The second class Fe-Zn DMC was obtained by solvent-free grinding Zn?CH₃COO?₂ and K₃Fe?CN?₆ for 7min.Tuning of grinding time and strength had obscure impact on the crystallinity of this Fe-Zn DMC.By mutual comparison between the first and second Fe-Zn DMCs with regard to IR spectrum,XRD patterns,catalytic activity and properties of the product PPCs,it has been indirectly proved that the negative ions of zinc salts would participate in the structure of DMC during the formation,and thereby had impacts on catalytic efficiency and selectivity in the copolymerization.By employing liquid-assisted grinding?LAG?,the work obtained the first class Fe-Zn DMC and Co-Zn DMC.Two pathways,'BuOH-after and 'BuOH-with,were attempted to introduce the tert-butyl alcohol?'BuOH?that was a necessity to improvement of catalytic activity under mechanochemical conditions.Under the same mechanochemical conditions,it was found that the DMCs obtained by the way'BuOH-with had much lower crystallinity but higher efficiency.Furthermore,the syntheses of Fe-Zn and Co-Zn DMC could be accomplished within 12min.The adoption of 'BuOH-with pathway,overall,demonstrated that a "one-step" way was found to synthesize active DMC.It has been shown from microstructure that the DMC catalyst obtained under grinding conditions displayed smaller particle sizes and some crystal defects,which would be advantageous to the ring-opening reaction of PO.Meanwhile,it was found that the crystallinity decreased,while catalytic activity and selectivity increased with increasing the polarity of the used liquids?with an exemption of H₂O and 1,1,1,2,3,4,4,5,5,5-decafluoropentane as assisted liquid?.Overall,as a green,convenient and rapid method,the mechanochemistry has shown interesting potential in chemical syntheses.The DMC catalysts in this work were successfully synthesized by using such a mechanochemical method,which has promising guidance to extend the application fields of both mechanochemistry and DMC catalysts.