高活性甲醇氧化羰基化CuY催化剂的结构及催化活性中心

Structure and Catalytic Active Center of High Catalytic Activity CuY Catalysts in Oxidative Carbonylation of Methanol

采用硝酸铜溶液和NaY分子筛溶液离子交换制备了CuY催化剂,通过加入氨水提高交换溶液的pH值以及高温焙烧活化,显著提高了该催化剂对甲醇氧化羰基化合成碳酸二甲酯的催化活性,与固相离子交换、沉积和浸渍法制备的催化剂相比,虽然铜的负载量较低,但催化活性较高.通过元素分析、XRD、H2-TPR、XPS和Auger电子能谱(Auger electron spectroscopy,AES)等手段对CuY催化剂微观结构的表征结果表明,在Cu(NO3)2离子交换溶液中加入氨水,可促进Cu2+离子交换的进行,提高CuY催化剂的Cu交换量,并且交换的Cu2+主要位于分子筛的超笼中.在惰性气氛中焙烧活化CuY催化剂,Cu2+自还原为Cu+,氨促进了自还原过程的进行,显著提高了催化剂的活性.焙烧活化温度越高,越有利于超笼中Cu2+→Cu+的自还原过程,使超笼中Cu+的含量增加,CuY催化活性增加.进一步的研究结果表明,Y分子筛超笼中的Cu+是主要的催化活性中心.

In this work,ammonia was added to the ion-exchange solution to adjust the pH in the preparation of ion-exchanged CuY by ion-exchange of NaY zeolite with aqueous solution of copper nitrate,and then the ion-exchanged CuY was activated by high temperature calcination.The prepared CuY catalysts exhibited high catalytic activity in the oxidative carbonylation of methanol to dimethyl carbonate.Compared with Cu catalysts prepared by solid-state ion-exchange,precipitation and impregnation,CuY catalysts loaded less Cu and showed good catalytic performance.Based on the characterization of the microstructure of CuY catalysts by XRD,XPS,AES,H2-TPR and elementary analysis,it is shown that by adding ammonia into the ion-exchange solution,the ion-exchanged reaction is promoted,resulting in the increase of the ion-exchanged Cu^2＋ and the most of Cu^2＋ in the supercage of CuY catalyst.In the process of calcinating activation of CuY catalysts in inert atmosphere,Cu^2＋ was auto-reduced to Cu＋ and ammonia promoted the auto-reduction,which was conducive to the improvement of catalytic activity.The high activating temperature facilitate the auto-reduction progress of Cu^2＋ to Cu＋ located in the supercage of zeolite,Cu＋ content in the supercage of zeolite increase more quickly and the catalytic activity of CuY catalysts increase in the meantime.It is concluded that Cu＋ in the supercage of Y zeolite is the main catalytic active center.