Characterization and Catalytic Performance of Co-MCM-41 for Styrene Epoxidation

分别采用模板剂离子交换法(TIE法)、水热合成法(DHT法)及浸渍法合成了Co-MCM-41,此催化剂能够在无共还原剂存在条件下催化以氧气为氧化剂的烯烃环氧化反应.另外考察了催化剂的结构特别是钴的存在状态与催化性能之间的关系,以期能够弄清催化环氧化反应中催化剂活性位的本质.X射线衍射及N2吸附-脱附等结果证实,当导入分子筛中的钴的量在2.2%以下时,所有的Co-MCM-41均具有规整排列的六方孔道结构.漫反射光谱和H2程序升温还原等结果表明,通过TIE法合成的样品中钴主要以孤立态钴离子(single-site Co(Ⅱ))形式存在,而以DHT法合成的样品中钴主要以微晶态Co2SiO4形式存在,浸渍法合成的样品中则存在大量颗粒状氧化钴.催化反应结果表明,当钴含量均接近1.0%时,采用TIE法合成的样品的苯乙烯转化率为45%,而用DHT法和浸渍法合成的样品的苯乙烯转化率均低于30%.考虑到氧化钴和Co2SiO4的低活性以及DHT法和浸渍法合成样品中可能存在一小部分孤立态钴离子,我们推测MCM-41中的孤立态钴离子是催化活化分子氧发生环氧化反应的高效活性位.The cobalt-containing MCM-41 can be used as the catalyst for the liquid-phase epoxidation of styrene with molecular oxygen.The correlations between its structures,especially the states of Co in it,and its catalytic performance were studied.It may be helpful to make clear the nature of active sites for the epoxidation.(Co-MCM-41) samples synthesized by a template-ion exchange(TIE) method, a direct hydrothermal(DHT) method and a conventional impregnation method have been characterized in detail.The X-ray diffraction and(N_2-adsorption) results indicate that the hexagonal array of mesopores in these samples is sustained as the cobalt contents are lower than 2.2%.The results of diffuse reflectance UV-Vis spectroscopy and H_2 temperature-programmed-reduction suggest that the cobalt introduced into Co-MCM-41 by the TIE method exists mainly as single-site(Co(Ⅱ)) ions,whereas the cobalt in the sample synthesized by the DHT method exists mainly as Co_2SiO_4 in a microcrystalline form and the samples prepared by the impregnation method contain a large proportion of cobalt oxide.With a similar cobalt content(ca 1.0%),the sample prepared by the TIE method gave a conversion of 45% in the epoxidation of styrene with molecular oxygen,however,the sample prepared by the DHT method or impregnation method gave a conversion lower than 30%.Since cobalt oxide and Co_2SiO_4 show a lower activity for the epoxidation reaction and there may also exist a small part of single-site Co(Ⅱ) ions in the samples prepared by the DHT method or impregnation method,we thus suggest that the single-site Co(Ⅱ) ions in MCM-41 are the real active sites for the epoxidation with molecular oxygen.国家自然科学基金资助项目(20273054,20373055