材料期刊网

首次在共沉淀过程中添加18-冠-6醚络合生成的钾离子得到了均一的高活性冠醚络合的锌-钴双金属催化剂，并用红外光谱(FTIR)、扫描电镜(SEM)、热重红外(TGA-IR)和X射线衍射(XRD)进行了表征.元素分析发现K含量为1.2%. FTIR表明未加冠醚络合的双金属催化剂离心后上下部分呈现不同的络合状态，而冠醚络合的双金属催化剂仍保持均一. SEM表明冠醚络合的双金属催化剂为均一松散的结构.由于生成的钾离子被冠醚络合，不影响聚合反应效果. TGA-IR表明冠醚不仅络合K离子，还参与对金属活性中心的络合. XRD表明此催化剂具有低的结晶度.所制冠醚络合的锌-钴双金属催化剂能成功催化CO2与环氧丙烷共聚，其中CDMC3催化得到的共聚物碳酸酯含量为47.8%，副产物环状碳酸酯为1.5%，催化效率高达5122 g/g催化剂(32600 g/g Zn)，明显优于不添加冠醚以同样工艺制备的DMC1(共聚物碳酸酯含量29.2%，副产物环状碳酸酯3.3%，催化效率4100 g/g催化剂(16300 g/g Zn).与不添加冠醚8次洗涤离心得到的DMC2相当(共聚物碳酸酯含量48.3%，副产物环状碳酸酯含量2.4%，催化效率5073 g/g催化剂(16400 g/g Zn)).基于此结果提出了两步的反应机理假设.

Double metal cyanide (DMC) catalysts are generally prepared by coprecipitation of potassium hex-acyanocobaltate(III) with zinc chloride followed by complexation with tert-butanol, and these ma-terials 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 cata-lysts 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, respec-tively. 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.