Mechano-synthesis Of Double Metal Cyanide Catalysts For The Coupling Reaction Of Carbon Dioxide And Propylene Oxide

The cyclic carbonates and aliphatic polycarbonates can be obtained through thecopolymerization of CO2and epoxides with the presence of hetero-/homogeneous catalysts.To date, many systems have been reported and demonstrated effective utilization of CO2.One of the successful examples is double metal cyanide (DMC) catalyst, which so far hasbeen effective not only for ring-opening polymerization of epoxides but also forcopolymerization with CO2(Scheme1). Thus, a green, strikingly efficient and practicallyapplicable technique for DMC synthesis under mechanochemical conditions was reportedhere, which revealed a clear shortcut in terms of free or limited solvent employed. As hasbeen widely regarded as a clean, efficient and environmentally friendly synthetic method, themechanical grinding (ball milling, also known as mechanochemistry) is of great interest ininorganic and organic synthesis, cocrystalsand even successful industrialization inpharmaceutical aspects.Scheme1.Reaction of CO2and Propylene OxideFirstly, two new kinds of active DMCs (Zn-Ni DMC and Co-Ni DMC) based on ZnNi(CN)4and CoNi (CN)4were successfully synthesized by ball milling at50Hz for10min.Zn-Ni DMC was obtained by K2Ni (CN)4and ZnCl2; Co-Ni DMC was obtained by K2Ni (CN)4and CoCl2.Their structures were characterized by FTIR spectrometer, X-ray diffractometer,SEM images and elemental analysis. The results showed that Zn-Ni DMC and Co-Ni DMCwere amorphous and lower crystallinity. The alternating copolymerization of CO2andpropylene oxide with the two catalysts obtained anticipated poly (propylene carbonate)(PPC)with very high catalytic activity. The PPCs produced by optimized Zn-Ni DMC and Co-NiDMC had molecular weight (Mn) up to10,344g/mol and8,478g/mol, polydispersity of1.45and1.44with83.5%and73.7%carbonate linkages respectively. The temperatures ofmaximum rate of degradation (Tmax) of PPC catalyzed by Zn-Ni DMC and Co-Ni DMC wereseparately292.57°C and274.91°C.As a continuing study, different co-complexing agents (co-CAs) were introduced toDMC catalysts under the protocol of ball milling. Zn-Fe DMCs were successfully obtained byemploying ball milling. The co-complexing agents (co-CAs) were incorporated into thestructure of DMC simultaneously during the grinding process. The FT-IR spectra and elemental analysis indicated the incorporation of various co-CAs within DMCs. It was foundthat the resulting DMC catalysts were amorphous with a monoclinic phase and had moderatecatalytic activity. The DMC catalysts were proven to be effective for both cycloaddition andcopolymerization between CO2and propylene oxide under controlled reaction conditions. TheDMCs exhibited high conversion and yield from93.6to97.3%with high selectivity for thecyclic carbonate. For copolymerization, the DMCs showed relatively modest activity butyielded poly (propylene carbonate) with high molecular weight (Mn61K) and narrowpolydispersity index (PDI1.46).Base on these, cetyltrimethylammonium bromide (CTAB) was introduced. Zn-Fe DMCswith co-complexing agents (co-CAs) and CTAB were successfully obtained via ball millingfor the solvent-free cycloaddition of CO2and propylene oxide (PO).The effect of CTAB onDMCs with different co-CAs using grinding method for the cycloaddition of CO2and PO wasalso reported. From the SEM image, it was clear that all the catalysts were nanoscale.Elemental analysis, TGA and FT-IR confirmed CTAB was incorporated into the structure ofDMC simultaneously during the grinding process. The DMC complex was proven to be aneffective catalyst for the cycloaddition of CO2and propylene oxide (PO)(140°C,3MPa,6hours). The DMCs with CTAB exhibited high conversion from88.9to98.3%with highselectivity for the cyclic carbonate. During the whole process, CTAB incorporated into thecatalyst matrix made them more active during ball milling for DMCs, and for cycloaddition, itplayed an important role of nucleophile to promote the reaction speed and gain high yields ofcyclic carbonates.