| Imines, also named Shiff Base, are crucial electrophilic intermediates to synthesis fine chemicals, active biological nitrogen; pesticide and pharmaceuticals. As significant industrial synthetic building blocks, imines can derive alternative synthesis routes for sustainable chemistry with less downstream processing. Generally, the aldol condensation was uesd to synthesis the imines, in addition, the dehydrogenation of secondary amineprepartion can also obtain the imines. To avert the problems caused by harmful reagents and excessive emission of pollutants, a great surge interest in "green" photocatalytic processes to oxidize amines efficiently under mild conditions, which obtain high efficiency and pure products.Several photocatalytic systems have been designed for oxidation of amines to imines. However the polyoxometallate (POM) photocatalytic systems have not been reported. POMs have been known to be promosing catalysts for photocatalytic organic synthesis and photodegradation because of the strong oxidizing, especially the unique coordination stabilizing the intermediates. To improve the drawbacks such as low surface area(<5m2g-1) causing low efficiency and high solubility leading recycling difficulty, we take the following two steps:(a) change the cation of 12-phosphotungstic acid with potassium ion. (b) immobilize on mesoporous carrier SBA-15 possessing large specific surface area. Then we got the new composite photocatalysts exhibiting high activity and good recyclability.Herein, we report the new efficient photocatalysts, which are synthesized by unsaturated POM coordinated rare earth, can function on selective photooxidation of amine into corresponding imine with atmosphere. The properties of photology, photochemistry and thermodynamics, including structures and surfaces of our photocatalysts were studied by several charactertions. The main works were showed as the following two stages:First stage:A straightforward, environment friendly procedure is designed to synthesis the target catalyst using cerous nitrate and 12-phosphotungstic acid of different mole ratios in potassium acetate buffer solution through degradation method and double decomposition method. The phosphotungstate containing rare earth showed a higher activity than phosphotungstic acid and phosphotungstate without rare earth for the oxidation of n-butylamine under the conditions:a flow of atmospheric oxygen(1atm,2.5mL/min) at 10～15℃ using methylbenzene as solvent. Moreover, The phosphotungstate containing rare earth with a molar ratio of n (Ce):n (P)=1:2 gave the imine as a main product in high selectivity(>98%). Based on the foregoing preparation route, we synthesis six kinds of complexes catalysts using the 12-phosphotungstic acid, H4SiW12O40 with cerous nitrate, nitric acid lanthanum and nitric acid yttrium through cross reaction method. Besides, we also devote to research the optimal reaction conditions, such that the molar ration of rare earth and heteropolyacid, different oxidants, several different polar solvents, various concentrations of substrate etc. Solvent has much effect on the yield, at the identical condition, we get the highest yield when the methylbenzene was used as solvent.Second stage:Aminopropyl or acetylacetonate was successfully functionalized on SB A-15 by one pot, respectively. Polyoxometalate containing rare earth catalyst was immobilized on the organic-modified mesoporous carrier using impregnation method. FTIR and DRS results indicated that polyoxometalate containing rare earth catalyst was homodispersed on the mesoporous carrier. The composite catalysts K-2.0CeP/SBA-NH2 and K-2.0CeP/SBA-acac still kept the initial pore structure of SBA-15, and the later showed a better photocatalytic activity and better stability of recycling. Different kinds of amines, for example, benzylamine, aniline, diphenylamine and triethylamine, were also investigated to evaluate the catalytic activities and the scope of application of these composite catalysts, based on these, the possible mechanism was deduced. |
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