文献类型：期刊文献

英文题名：Spectroscopic and structural characterization of Cr(II)/SiO₂ active site precursors in model Phillips polymerization catalysts

作者：Zhong, Lei[1,2];Lee, Ming-Yung[1];Liu, Zhen[2];Wanglee, Ying-Jen[3];Liu, Boping[2];Scott, Susannah L.[1,3]

机构：[1]Univ Calif Santa Barbara, Dept Chem Engn, Santa Barbara, CA 93106 USA;[2]E China Univ Sci & Technol, State Key Lab Chem Engn, Shanghai 200237, Peoples R China;[3]Univ Calif Santa Barbara, Dept Chem & Biochem, Santa Barbara, CA 93106 USA

年份：2012

卷号：293

起止页码：1

外文期刊名：JOURNAL OF CATALYSIS

收录：;WOS:【SCI-EXPANDED(收录号:WOS:000308836800001)】；

基金：This work was supported by the National Science Foundation under Grant No. CBET08-54425. L.Z. thanks the China Scholarship Council (CSC) for a fellowship. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

语种：英文

外文关键词：Polymerization; Phillips catalyst; Chromyl chloride; Silica; X-ray absorption spectroscopy; ONIOM

摘要：Compositionally and structurally uniform chromate sites grafted onto silica via the ambient temperature reaction of CrO2Cl2 with silica pretreated at either 500 or 800 degrees C, followed by mild heating at 300 degrees C, were reduced by CO to the corresponding Cr(II) sites. The resulting structures were investigated by IR spectroscopy of adsorbed CO as well as X-ray absorption spectroscopy and compared with model structures predicted by ONIOM calculations (B3PW91*/BS1: B3PW91*/STO-3G). The IR spectra suggest that there are two distinct and non-interconverting Cr(11) sites whose relative amounts differ on the two types of pretreated silica. The calculated upsilon(CO) frequencies for two model structures, ( SiO)(2)Cr(CO)(2) and ( SiO)(2)( Si2O)Cr(CO), agree well with experimental data, although these structures do not necessarily reflect the coordination of Cr(II) in the absence of CO. The X-ray absorption spectra reveal that the CO-free Cr(II) sites are coordinated by additional siloxane ligands, which may influence their ability to initiate ethylene polymerization. (C) 2012 Elsevier Inc. All rights reserved.