以Ca-M-Al(M=Mg,La,Ce,Y)层状双氢氧化物为前驱体的固体碱用于r酯交换合成碳酸二甲酯

摘要

Composite solid base catalysts derived from Ca-M-Al (M = Mg, La, Ce, Y) layered double hydroxides (LDH) were synthesized, characterized and applied to the transesterification of methanol with pro-pylene carbonate. X-ray diffraction analyses of the catalysts show that all of the catalysts were in the form of composite oxides. Compared with the Ca-Al LDH catalyst, the specific surface areas and pore volumes of the catalysts were increased with the introduction of Mg, La or Ce. The catalytic perfor-mance of these catalysts increases in the order of Ca-Y-Al < Ca-Al < Ca-Ce-Al < Ca-La-Al < Ca-Mg-Al, which is consistent with the total surface basic amounts of these materials and the formation of especially strong basic sites following modification with Mg and La. The Ca-Mg-Al catalyst shows the highest (Ca+Mg):Al atomic ratio, indicating that it likely contains more unsaturated O2- ions, providing it with the highest concentration of very strong basic sites. The recyclability of these cat-alysts is improved following the addition of Mg, La, Ce or Y, with the Ca-Mg-Al maintaining a high level of activity after ten recycling trials. X-ray diffraction analyses of fresh and used Ca-Mg-Al demonstrate that this catalyst is exceptionally stable, which could be of value in practical applica-tions related to heterogeneous catalysis.%碳酸二甲酯(DMC)是一种环境友好型绿色化学品,可作为甲基化和羰基化试剂用于取代传统剧毒的硫酸二甲酯和光气.另外,DMC具有良好的溶解性能,可用于高级溶剂;DMC分子中具有高的氧含量,可用作汽油添加剂来提高汽油的辛烷值;DMC还可用作聚碳酸酯的原料.随着人们环保意识的不断增强,DMC的生产和应用呈现出巨大的吸引力和市场潜力.DMC合成方法主要有光气法、甲醇氧化羰化法、尿素醇解法及酯交换法等.酯交换法具有反应条件温和、产率高等优点,是目前工业制备DMC的主要方法.研究发现,相对于酸性催化剂,碱性催化剂更有利于酯交换法合成DMC.金属氧化物催化剂具有活性高、热稳定性高及可连续重复回收利用等优点,因而引起了广泛关注.CaO对于酯交换合成DMC反应具有良好的催化活性,但其稳定性差.因此,通常采用复合金属氧化物来促进CaO的分散,并增加金属间相互作用以防止CaO流失.研究发现,经煅烧后的Mg-Al,Ca-Al和Ca-Mg-Al催化剂对于酯交换反应具有高的活性和稳定性.此外,通过碱性稀土金属(La,Ce和Y)的引入可以修饰催化剂上的碱性位点,从而调变催化剂的碱性.本文合成了一系列以Ca-M-Al(M=Mg,La,Ce,Y)层状双氢氧化物为前驱体的固体碱催化剂,将其用于甲醇与碳酸丙烯酯酯交换合成DMC.通过X射线衍射、热重分析、红外光谱、X射线光电子能谱、电感耦合等离子体、CO2程序升温脱附和Hammett指示剂对催化剂进行了表征.研究发现,各催化剂的活性高低依次为:Ca-Y-Al