Ce,La和Cs离子对CuY催化甲醇氧化羰基化活性中心的影响

Effects of Cerium,Lanthanum and Cesium Cations on Catalytic Active Center of CuY Catalyst in Oxidative Carbonylation of Methanol

将NaY分子筛在Ce,La和Cs的硝酸溶液中进行离子交换制得CeY,LaY和CsY分子筛,然后在pH=9.5的硝酸铜氨溶液中进行二次离子交换,并在惰性气氛下于600℃高温活化得到CuCeY,CuLaY和CuCsY催化剂.与CuY催化剂相比,加入Ce,La和Cs催化剂的甲醇氧化羰基化合成碳酸二甲酯的催化活性均显著提高.采用元素分析,XRD,H2-TPR,CO-TPD,CH3OH-TPD,XPS等对催化剂微观结构进行了表征分析.结果表明,阳离子Ce和La促进了Cu2+在分子筛超笼内的落位,在高温活化中有利于转化为更多的活性物质Cu+,从而提高了催化剂的活性;阳离子Cs阻碍了Cu2+在分子筛超笼内的落位,不利于催化活性的提高.但阳离子的引入,减弱了催化剂对CO的吸附,加强了对CH3OH的吸附(CuCeY除外),而CuCsY催化剂对CO的吸附最弱,对CH3OH的吸附最强,使其催化活性最佳.另外,阳离子的引入加大了催化剂Cu2+的自还原难度,但相比CuCeY,CuLaY催化剂,CuCsY催化剂中Cu2+自还原难度最小,高温活化中更容易转化为活性物质Cu+,利于其活性的提高.

CeY,LaY and CsY zeolites were firstly prepared by ion-exchange of NaY zeolite with aqueous solution of cerous nitrate,lanthanum nitrate and cesium nitrate.Then CuCeY,CuLaY and CuCsY catalysts were prepared by secondary ion-exchange of CeY,LaY and CsY zeolites with ammonia solution of copper nitrate（pH=9.5）,and activated by calcinating at the temperature of 600 ℃.Compared with CuY catalyst,all of the prepared CuCeY,CuLaY and CuCsY catalysts exhibited higher catalytic activity in the oxidative carbonylation of methanol to dimethyl carbonate.Based on the characterization of the microstructure of catalysts by XRD,H2-TPR,CO-TPD,CH3OH-TPD,XPS and elementary analysis,it was shown that cerium and lanthanum cations promoted Cu2＋ to locate in the supercage of zeolite,and more Cu2＋ auto-reduced to Cu＋ in the process of calcinating activation in inert atmosphere,which was conducive to the improvement of catalytic activity.Cesium cation blocked Cu2＋ locating in the supercage of zeolite,which was not likely to the improvement of catalytic activity.However,the introduction of the Ce,La or Cs cations resulted in CO weakening adsorption and CH3OH intensifying adsorption other than CuCeY.CuCsY catalyst exhibited highest catalytic activity for it adsorped CO the weakest and adsorped CH3OH the strongest.Besides,the introduction of the cations were all not conducive to auto-reduction progress of Cu2＋ to Cu＋,but compared with CuCeY and CuLaY catalysts,CuCsY had a easier auto-reduction progress of Cu2＋ to Cu＋,which was likely to the improvement of catalytic activity.