Study On Sb/LDHs Catalyst And Memory Effect Of Layered Double Hydroxides

Layered double hydroxides (LDHs) or hydrotalcite-like compounds are a class oflayered materials consisting of positively charged brucite-like layers and the chargecompensating interlayer exchangeable anions. When calcined at moderate temperatures,LDHs form mixed oxides (LDO), which are able to recover the layered structure afterimmersed in water or other solutions containing anions. This important feature is called'memory effect' of layered double hydroxides. LDHs and LDO can be used as solid baseand redox catalysts, which have recently gained much attention owing to their potentialindustrial applications. The Baeyer-Villiger (BV) oxidation of carbonyl compounds tolactones or esters is a significant process in synthetic organic chemistry. For example, theBV oxidation of cyclohexanone to ε-caprolactone is an attractive process becausepoly(ε-caprolactone) is one of the most commonly used biodegradable polymers.Traditional BV oxidizing agents are peroxycarboxylic acids, which industrial use arecostly and require significant safety considerations. Moreover, they yield thecorresponding carboxylic acids, raising separation and recycling problems. With theincrease in environmental concern, much more researches are focused on developingefficient catalytic method using green and cheaper oxidant like hydrogen peroxide insteadof peroxycarboxylic acids.In this thesis work, the catalytic behaviors and 'memory effect' of Mg-Al layereddouble hydroxides were systematically studied.A series of Sb-impregnated LDHs were prepared by wetness impregnation of LDOwith a solution of SbCl3, then dried and calcined at 450 C for 5 h. The samples were ocharacterized by XRD, TG-DTG,BET,FT-IR and used to catalyze BV oxidation ofcyclohexanone. The result of XRD suggests the Sb/LDO recovered the original layered i南京工业大学硕士学位论文structure during the reaction process. Baeyer-Villiger oxidation of cyclohexanone toε-caprolactone on Sb/LDO catalysts was carried out in a three-necked glass flask,equipped with a reflux condenser and a magnetic stirrer. The results indicate that Sb/LDOshow much a higher activity in BV oxidation of cyclohexanone in acetonitrile medium.The conversion of cyclohexanone over Sb(1.5)/LDO(3)-450 is about 42 % and theselectivity to the ε-caprolactone is over 94 %. In a set of reuse experiments, there wasalmost no reduction of the catalytic activity. The selectivity to the ε-caprolactone ofregenerated Sb(1.5)/LDO(3)-450 is almost the same as the fresh catalyst.In addition, the 'memory effect' of Mg-Al layered double hydroxides was studiedand used to dehydrate hydrous ethanol. The results indicate that calcination of the layereddouble hydroxides in excess of 400 C yields a mixed oxide (LDO) structure. These omixed oxides derived from the LDHs can restore the lamellar structure in water when thecalcination temperature ranges from 400 oC to 700 oC. When the calcination temperaturewas raised up to ca. 800 C, the reconstruction ability of LDO disappears. After two ocycles of calcination and rehydration, the LDO exhibits steady hydration ability. Theresults of dehydration of ethanol/water mixtures suggest LDO can recover the layeredstructure in the ethanol solution, which ethanol concentration is ca. 88 wt %. After treatedwith LDO, the ethanol concentration can be raised 4.8 %, which is higher than the resultsof hydrous ethanol treated with 5A molecular sieve. However, in the ethanol/watermixtures of ca. 93 % ethanol, LDHs are difficult to recover and the LDO has almost thesame dehydration ability as 5A molecular sieve.