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Epoxidation of styrene on Si/Ti/SiO2 catalysts prepared by chemical grafting

机译:Epoxidation of styrene on si/Ti/siO2 catalysts prepared by chemical grafting

摘要

Titania-silica (Ti/SiO2) and silica-titania-silica (Si/Ti/SiO2) catalysts were:prepared by chemical grafting using TiCl4 and tetraethyl orthosilicate (TEOS) as precursors and SiO2 as support. The prepared catalysts were characterized by UV Raman and visible Raman spectroscopies, XRD and the epoxidation of styrene; Ti/SiO2: catalyst grafted with only titanium species is not very active for epoxidation using H2O2 (30%), but is active and-selective when one uses tert-butyl hydroperoxide (TBHP). The catalyst grafted at high temperatures shows better epoxide selectivity. Si/Ti/SiO2 catalyst, the titanium-silica grafted further with TEOS, is active and selective for the epoxidation of styrene using either dilute H2O2 or TBHP, possibly due to the fact that the grafting of Ti/SiO2 with TEOS modifies the coordination structure of titanium and makes the titanium sites of Si-O-Ti-O-Si species less hydrophilic. A characteristic band at 1085cm(-1) due to Ti-O-Si species is detected for the grafted catalysts by UV resonance Raman spectroscopy. Reaction between TiCl4 and SiO2 at high temperatures favors the formation of Ti-O-Si species. Better activity and selectivity to epoxide,is found for the catalysts with more Ti-O-Si species. It is assumed that the active sites are the highly isolated Ti-O-Si species. For Si/Ti/SiO2 catalyst, the gas phase O-2 can participate in the catalytic oxidation of styrene when H2O2 is present ana:ii causes the formation of benzaldehyde. (C) 2000 Elsevier Science B.V. All rights reserved.
机译:以TiCl4和原硅酸四乙酯(TEOS)为前驱体,以SiO2为载体,通过化学接枝制备了二氧化钛(Ti / SiO2)和二氧化钛(Si / Ti / SiO2)催化剂。制备的催化剂通过紫外拉曼光谱和可见拉曼光谱,XRD和苯乙烯的环氧化反应进行表征。 Ti / SiO2:仅接枝钛物种的催化剂对于使用H2O2(30%)进行环氧化的活性不是很高,但是当使用叔丁基氢过氧化物(TBHP)时则具有活性和选择性。在高温下接枝的催化剂表现出更好的环氧选择性。 Si / Ti / SiO2催化剂,即用TEOS进一步接枝的钛-二氧化硅,对使用稀H2O2或TBHP的苯乙烯环氧化具有活性和选择性,这可能是由于Ti / SiO2与TEOS的接枝会改变配位结构从而使Si-O-Ti-O-Si物种的钛位点亲水性降低。通过紫外共振拉曼光谱检测到的接枝催化剂在1085cm(-1)处由于Ti-O-Si物种的特征谱带。 TiCl4和SiO2在高温下反应有助于形成Ti-O-Si物质。对于具有更多Ti-O-Si物种的催化剂,发现了更好的活性和对环氧化物的选择性。假定活性部位是高度隔离的Ti-O-Si物种。对于Si / Ti / SiO2催化剂,当存在H2O2时,气相O-2可以参与苯乙烯的催化氧化,ii:导致苯甲醛的形成。 (C)2000 Elsevier Science B.V.保留所有权利。

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