Immobilization Of NHPI/Co(?) Catalysts And Their Catalytic Performance In Liquid-phase Selective Oxidation Of Benzylic C-H Bonds Using Molecular Oxygen

Direct selective oxidation of benzylic C-H bonds is one of the most effective ways to prepare aromatic aldehydes,ketones,acids,alcohols and peroxides.Benzaldehyde and acetophenone are important chemical intermediate raw materials.However,at present,there are few industrial production methods to prepare aromatic aldehydes or ketones by this process,which need further development and application.Commercial benzaldehyde is mainly produced by chlorination of toluene followed by saponification.The large amount of waste acid produced by this process will seriously corrode the equipment and increase the cost of sewage treatment.At the same time,the benzaldehyde produced by the chlorination/hydrolysis of toluene method contains chlorine,which prevents it from being used in medicine and food industries.Industrially,acetophenone is mainly synthesized by Friedel-Crafts acylation reaction of benzene.However,the process has the disadvantages of harsh reaction conditions,large amount of catalyst and high cost of sewage treatment.the Direct selective oxidation of toluene and ethylbenzene to produce benzaldehyde and acetophenone using molecular oxygen as oxidant has the advantages of high atomic economy and process environmental protection.Under mild reaction conditions,it has become an urgent task for the current research to develop a high-efficiency catalytic oxidation system of toluene and ethylbenzene benzylic C-H bonds and realize the recycling of the catalyst.In this paper,firstly,the effect of different solvents on the selective oxidation of toluene and ethylbenzene in liquid phase was studied based on the homogeneous NHPI/Co catalysts,and the mechanism of the oxidation of toluene and ethylbenzene catalyzed synergistically by NHPI/Co was explored.Then the cobalt oxide catalysts were prepared by co-precipitation,and the immobilized NHPI catalysts were prepared by covalent bond grafting.The composition,structure,morphology and chemical state of the heterogeneous catalysts were characterized,and the catalysts were used in the liquid-phase selective oxidation of toluene and ethylbenzene with molecular oxygen.Under reaction conditions,the catalytic activity of the catalysts and its stability were tested.The structure-activity relationship of the catalysts was studied.The mechanism of selective oxidation of toluene and ethylbenzene under heterogeneous catalytic reaction conditions was discussed.1.Effect of solvents on the oxidation of benzylic C-H bonds to aromatic aldehydes(ketones)At room temperature,the effects of different solvents on the selective oxidation of toluene to benzaldehyde were compared.Compared with dimethyl malonate and acetic acid,fluorinated alcohol was used as solvent can significantly improve the selectivity to the product benzaldehyde.This may be due to the formation of an H-bond adduct between fluorinated alcohol and benzaldehyde,which seriously increased the bond energy of the aldehydic C-H bonds and thereby inhibited its breakage and subsequent further oxidation.At the same time,fluorinated alcohol also promoted the conversion of toluene.Under the optimal reaction conditions,the conversion of toluene reached 98.0%,and the selectivity to benzaldehyde was 56.0%.In order to further study the effect of fluorinated alcohol on NHPI/Co catalytic oxidation of benzylic C-H bonds,fluorinated alcohol solvent was applied to the liquid phase oxidation reaction of ethylbenzene in this thesis.The solvent HFIP was found to significantly enhance the oxidation efficiency in the transformation from ethylbenzene to acetophenone in comparison with acetic acid and ethanol,and the conversion of ethylbenzene and the selectivity to acetophenone was high up to 87.9%and 61.2%,respectively,at room temperature.A higher concentration of Phthalomide-N-oxyl(PINO)radicals observed by an in situ electron paramagnetic resonance(EPR)in fluorinated alcohol with respect to other solvents in the initial stage of the reaction suggested that fluorinated alcohol may facilitate the generation of the nitroxyl radical and thus promote the selective oxidation of ethylbenzene to acetophenone.Furthermore,phenylethl radical(PhCH-CH3)was captured by tetramethylpiperidine nitroxyl radical(TEMPO)and also observed by high resolution mass spectrometry.The findings of both PINO and PhCH-CH3 under reaction conditions indicated that the selective oxidation of ethylbenzene to acetophenone catalyzed by NHPI/Co(II)might proceed according to the picture proposed by Ishii.2.Immobilization of cobalt(?)and its catalytic performance for selective oxidation of toluene to benzaldehydeCoOx/SiO2 catalyst was prepared by co-precipitation using cobalt nitrate as precursor and sodium silicate as silicon source,and their composition,structure,morphology and surface chemical states were systematically characterized.The selective oxidation of toluene to benzaldehyde was catalyzes by the prepared CoOx/SiO2 catalysts together with N-hydroxyphthalimide(NHPI)using molecular oxygen in the presence of fluorinated alcohol as solvent.The CoOx/SiO2 catalysts exhibited excellent textural properties and a highly dispersion of cobaltous ions even at a Co:Si ratio up to 0.44.It was concluded that the predominant cobalt species could be cobalt silicate(CoSiO3).The prepared catalysts highly efficiently catalyzed the selective oxidation of toluene to benzaldehyde at room temperature.Under optimized experimental conditions,the conversion of toluene and the selectivity to benzaldehyde were high-up to 98.5%and 62.5%,respectively.It was confirmed that the reaction proceeded by a free radical mode under the experimental conditions,and the solvent fluorinated alcohol significantly inhibited the further oxidation of benzaldehyde to benzoic acid.There was no obvious decrease observed in the catalytic performance of the recycled catalyst,indicating an excellent reusable capability of the prepared catalysts.Excellent toluene selective oxidation activity and stable recyclability indicated that the prepared CoOx/SiO2 catalysts possess a promising industrial application.3.Immobilization of NHPI and its catalytic performance for selective oxidation of ethylbenzene to acetophenoneThe surface aminated SiO2 carrier by ?-Aminopropyl triethoxysilane was imidized with pyromellitic dianhydride,3,3',4,4'-biphenyltetracarboxylic dianhydrid and 1,4,5,8-naphthalenetetracarboxylic dianhydride,respectively,and different dianhydride precursors were grafted onto the carrier surface.Then,the acid anhydrides in the precursors grafted on the surface of the carrier were imidized again by hydroxylamine hydrochloride to generate the active functional group N-OH.SiO2-APTES-PDMA-NOH,SiO2-APTES-BDP-NOH and SiO2-APTES-NDTA-NOH catalysts were prepared respectively.The prepared catalysts were used for the liquid-phase selective oxidation of ethylbenzene to acetophenone,and the activity and stability of the catalyst under the reaction conditions were explored.The catalysts were characterized by XPS,TG,FT-IR and elemental analysis,and it was found that the N element binding energy of the catalysts during the preparation process changed significantly,and the prepared catalysts had a higher the decomposition temperature of organic matter.The formation of O=C-N-C=O imide graft bonds were observed in the Fourier transform infrared spectrum of the catalysts.Elemental analysis results showed that the catalysts have highly density of grafting sites and active functional groups,and in the process of use,no obvious loss of organic matter in the catalysts was found after reaction.Under the reaction conditions of molecular oxygen as oxidant and acetic acid as solvent,all the immobilized NHPI catalysts prepared by different precursors exhibited catalytic activity oxidation of ethylbenzene in the presence of Co(OAc)2.Among them,SiO2-APTES-PDMA-NOH was the best catalyst for the liquid-phase selective oxidation of ethylbenzene.The highest conversion of ethylbenzene was 63.8%,and the selectivity to acetophenone was 79.0%.The catalyst was repeatedly used 4 times under the reaction conditions,the conversion of ethylbenzene and the selectivity to acetophenone remained stable,and the catalyst showed good stability.This shows that the preparation of immobilized NHPI catalysts by imide bond grafting is a reliable method and shows good application prospects.In summary,in the NHPI/Co catalyzed liquid-phase selective oxidation of toluene and ethylbenzene,fluorinated alcohols can promote the formation of PINO radicals and thus increase the oxidation rate of toluene and ethylbenzene.The formation of an H-bond adduct between fluorinated alcohol and benzaldehyde,which seriously increased the bond energy of the aldehydic C-H bonds,and thereby inhibited its breakage and subsequent further oxidation,increased the selectivity to benzaldehyde.The heterophasic cobalt catalysts with high specific surface area and high cobalt loading were prepared by co-precipitation.The cobalt catalysts exhibited excellent catalytic performance and good stability in toluene liquid-phase selective oxidation reaction.In addition,this paper developed a method for preparing immobilized NHPI catalysts by imide bond grafting.All the immobilized NHPI catalysts prepared by different precursors exhibited catalytic activity oxidation of ethylbenzene in the presence of Co(OAc)2.Among them,SiO2-APTES-PDMA-NOH was the best catalyst for the liquid-phase selective oxidation of ethylbenzene.In addition,the active components had not been lost in the catalysts during multiple uses.