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MCM-41 Supported Bis[(perfluoroalkyl)sulfonyl]imides Based Catalysts And Their Applications

Posted on:2006-08-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y B YuanFull Text:PDF
GTID:1101360182969924Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Bis[(perfluoroalkyl)sulfonyl]imide is a kind of novel "super acid"rising from the middle of 1980s. The bis[(perfluoroalkyl)sulfonyl]imides anion exhibit a "weakly coordinating"characteristic since negative charge of nitrogen atom can be highly delocalized onto the whole S-O-N framework due to strongly electron-drawing and steric effects from the perfluorinated alkyl chains. Consequently, their complexes can exhibit strong Lewis acidity as well. As a kind of highly effective catalysts, bis[(perfluoroalkyl)sulfonyl]imides and their complexes were used for such organic synthetic reactions as Diels-Alder, Friedel-Crafts acylation, esterification, Baeyer-Villiger oxidation, and so on. With the awareness by public and the compulsive implement by legislation for environmental reason, chemists focused their interesting on highly effective, environmentally friendly and recyclable "green"clean catalysts recently. Amongst which, immobilizing homogeneous catalysts onto inorganic materials is an important approach. In this work, at first we synthesized series of bis[(perfluoroalkyl)sulfonyl]imides and their salts as well as MCM-41 silica by using different materials and methods, and then immobilized bis[(perfluoroalkyl)sulfonyl]imides and their salts onto MCM-41 to form a kind of solid acid catalysts of organic-inorganic hybrid. The supported catalysts were subsequently characterized by FTIR, pyridine-FTIR, XRD, SEM, TG-DTA and N2-adsorption techniques. Finally the MCM–41-supported bis[(perfluoroalkyl)sulfonyl] imides and their salts were employed for tert-butylation of hydroquinone and aromatic nitration, respectively. The loaded amounts of supported catalysts prepared by different methods are different: the loading of MCM-41 supported bis[(perfluoroalkyl)sulfonyl]imides or their salts by impregnation is usually above 1.0 mmol/g, while the loading of MCM-41 supported ytterbium bis[(trifluoromethyl)sulfonyl]imide by surface modification of support using functional reagents is 0.25 mmol/g or so. The results of characterization revealed that starting materials and preparation methods have little influences on structure and texture of MCM-41, but the yield is only 30% when sodium silicate (NaSiO3) was used as the source of silicon while the yield is nearly 100% when tetraethoxyorthosilicate (TEOS) was used as the source of silicon. Supported active component can lead to damages to crystallinity of MCM-41 or cause decreases in specific surface, pore volume and pore size to different extent. Thermal analysis showed that the decomposing temperature of active component is lower than its melting point. MCM-41 supported metal bis[(perfluoroalkyl)sulfonyl]imides exhibited much better catalytic performances than other Lewis acid catalysts when employed for aromatic nitration with 1.0 equiv. 65 wt% nitric acid. A conversion of more than 95% could be obtained when 1 mol% solid acid catalysts were used for activated aromatics, but too small catalytic amount (no more than 10 mol%) is generally not satisfying when substrates are strongly deactivated aromatic compounds (δp+>+0.4). Furthermore, MCM-41 supported solid acid catalysts showed no regio-selectivity at all while catalyzing some aromatics nitration such as toluene. Solvents with high εor low DN value such as 1,2-dichloroehnane or nitromethane can promote catalytic effects of supported metal bis[(perfluoroalkyl)sulfonyl]imides. The supported catalysts by impregnation could be recycled at least 5 times without substantial loss of activity, whereas MCM-41 supported ytterbium bis[(trifluoromethyl)sulfonyl]imide by surface modification of support using functional reagents could be recycled only 3 times. The mechanism of aromatic nitration in the presence of MCM-41 supported metal bis[(trifluoromethyl)sulfonyl]imides is a mode of forming nitronium ion: hydrated cations existing on support's surface can bind nitric acid to its center and then release an active proton which will continue to combine with another nitric acid molecule to produce nitronium ion. MCM-41 supported bis[(perfluoroalkyl)sulfonyl]imides showed better catalytic performances than conventional protic acid catalysts as well when used for tert-butylation of hydroquinone. The total yield of TBHQ could be as high as up to 52.0% under the optimized reaction conditions, and the catalytic performances can not be influenced substantially even if the supported catalyst was recycled 6 times. Although MCM-41 supported copper salts can promote the conversion of hydroquinone to some extent, the selectivity of TBHQ reduced because of increase of oxidated products due to copper (Ⅱ)cation. MCM-41 supported bis[(perfluoroalkyl)sulfonyl]imides and their salts can be disposed of in the air conveniently, implying that the hygroscopic property of which was improved obviously. In summary, MCM-41 supported bis[(perfluoroalkyl)sulfonyl]imides and their salts are highly effective and environmentally friendly, and they can be recycled when employed for aromatic nitration and tert-butylation of hydroquinone. Their further potential application in organic synthesis as a kind of novel solid acid catalysts can be optimistically expected.
Keywords/Search Tags:Bis[(perfluoroalkyl)sulfonyl]imides, MCM-41, Supported solid acid catalysts, Aromatics, Hydroquinone, Nitration, Tert-butylation
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