Immobilization Of Co-Schiff Base Catalysts And Their Performance Of Oxidation Of Toluene Solvent-free Using Molecular Oxygen

The side chain oxidation products of toluene,benzaldehyde,benzyl alcohol and benzoic acid,are widely used as spices,pharmaceuticals,pesticides and other chemical intermediates,so the study of this reaction has an important industrial application prospect.Traditional hydrocarbon oxidation often involves the use of organic solvents and strong oxidizing agents such as potassium permanganate and nitric acid,resulting in waste water and equipment corrosion that increases the cost of the enterprise.Molecular oxygen is cheap and readily available,and its oxidation byproduct is usually water,which does not pollute the environment.Therefore,molecular oxygen is considered a green oxidizing agent.However,there is a spin restriction between the oxygen atoms in the ground state and the sp3 hybridized C-H in toluene at room temperature,which makes it difficult for molecular oxygen to react with toluene.In order to solve the problem of molecular oxygen activation,we often need to add a catalyst in the reaction system to activate the inert ground state dioxygen.Homogeneous catalysts have low efficiency in industrial application because of their high cost and cumbersome separation and recovery process.Compared with homogeneous catalysts,heterogeneous catalysts can be separated from the reaction solution through simple operation such as centrifugation or filtration,which decreases the difficulty of catalyst recovery and has a wider prospect of industrial application.In this disseration,the effects of different initiators and phase transfer catalysts on the catalytic oxidation of toluene molecular oxygen with NHPI were firstly explored in a solvent-free homogeneous system.Then,the initiator cobalt ion was immobilized by grafting method to prepare the immobilized cobalt Schiff base catalysts.On this basis,the substitution group on the Schiff base ligand was modulated to study the effect of substitution group on the catalytic activity of toluene.The catalytic activity and stability of the supported cobalt Schiff base catalyst were tested by a heterogeneous system,and the structure-activity relation was discussed in combination with the reaction mechanism of the homogeneous system.1.Effect of phase transfer catalysts and initiators on the solvent-free oxidation of toluene in NHPIThe effects of different phase transfer catalysts and initiators on the catalytic oxidation of toluene with NHPI were compared in the solvent-free condition.The results show that the catalytic activity of NHPI can be improved by adding a proper amount of phase transfer catalysts into the oxidation reaction system of toluene,and the main product is benzoic acid.Among them,the performance of the phase transfer catalyst BS-12 is the best,and the conversion of toluene and the selectivity to benzoic acid is 29.9%and 86.4%,repectively.The role of phase transfer catalysts in toluene molecular oxygen catalytic oxidation reaction of NHPI was also discussed.Through experimental data and GC-MS analysis results,the reason why phase transfer catalysts can promote toluene oxidation catalyzed by NHPI was speculated.The phase transfer catalysts can improve the solubility of NHPI in toluene under the solvent-free conditions,accelerate the generation of PINO free radicals,and thus improve the catalytic activity.Effects of the initiators Co(acac)2,benzaldehyde m-chloroperoxybenzoic acid and nitric acid on toluene molecular oxygen catalytic oxidation were compared in presence of NHPI and the phase transfer catalyst BS-12 under solvation-free conditions.It was found that when Co(acac)2 was used as initiator,the toluene oxidation performance of NHPI was the best.In this chapter,the promotional mechanism of the four initiators in toluene solvent-free molecular oxygen catalytic oxidation reaction was further discussed.2.Covalent anchoring Co-Schiff base catalysts and their performance in solvent-free oxidation of toluene3-Aminopropyltriethoxysilane(APTES)was used as a linking molecule to functionalize the commerical SiO2,and the resulting surface ammoniated SiO2 carrier(SiO2-APTES)was obtained.The selectd ligand o-Hydroxyacetophenone was dehydrated and condensed with primary amine on SiO2-APTES to synthesize supported Schiff base catalyst with a imine grafting bond.Finally,Co2+ions were coordinated to form the immobilized cobalt Schiff base catalyst.The prepared catalyst was used for toluene solvent-free molecular oxygen oxidation catalyzed by NHPI.Under the experimental conditions,the conversion of toluene was 18.5%,the selectivities to benzaldehyde,benzyl alcohol and benzoic acid were 13.6%,13.1%and 69.3%,respectively.After being recycled for 5 times,the catalytic activity of the prepared supported cobalt Schiff base remained stable.According to the elemental analysis,there was no appreciable loss of cobalt content in the retrieved catalyst.Three kinds of nitrogens with different chemical states in the catalyst were calculated by XPS and compared,which was basically consistent with the N contents in different structures calculated by element analyses of C,H and N elements.Conclusively,the covalently anchored cobalt Schiff base catalyst synthesized here exhibilted good catalytic activity and stability under the applied reaction conditions,which indicates that the preparation method of the supported cobalt Schiff base catalyst is reliable,and the prepared catalyst has a promising application prospect.3.Effect of substituents on the performance of anchoring Co-Schiff base catalysts in solvent-free oxidation of tolueneIn this chapter,the effect of substituents on the molecular oxygen oxidation of toluene over grafted cobalt Schiff base catalyst was studied under the solvent free conditions.Four supported cobalt Schiff base catalysts with different substituents were prepared by using salicylaldehyde,o-hydroxyacetophenone,4-(diethylamino)salicylaldehyde and 5-nitro salicylaldehyde as ligands.In order to avoid the influence of adsorbed cobalt species on the surface of the prepared catalyst,the remaining cobalt content of the catalyst after the first reaction was detected by elemental analysis.According to the content of cobalt left after the first reaction,another toluene oxidation reaction was carried out without the addition of fresh catalyst.The results of the second catalytic reaction showed that the cobalt Schiff base catalyst prepared with 4-(diethylamino)salicylaldehyde as the complex possessed the best oxidation activity of toluene.The cobalt Schiff base catalysts with o-hydroxyacetophenone,salicylaldehyde as ligand follow.The catalysts with electron-withdrawing substituent nitro groups as the ligand showed the lowest toluene oxidation performance.This indicates that the cobalt Schiff base with electron-donating groups on the ligand is more favorable than that with electron-withdrawing groups for the solvent-free oxidation of toluene.The reason may be that the electron-donating group accelerates the activation rate of molecular oxygen by increasing the density of electron cloud around the cobalt ion,and promotes the generation of PINO free radical,thus improving the activity of the catalysts.The cobalt Schiff base catalyst with 4-(diethylamino)salicylaldehyde as a ligand was characterized by XPS,which indicated that the chemical state of cobalt ions did not change significantly before and after the reaction.The catalyst was evaluated for 5 times under the optimized reaction conditions,and the toluene conversion and the selectivities to the main products remain relatively stable except that the toluene conversion rate of the first time decreases from 31.3%to 26.3%.A cobalt Schiff base catalyst prepared with 4-(diethylamino)salicylaldehyde was mechanically mixed with the immobilized catalyst NHPI,and its catalytic performance in molecular oxygen oxidation of toluene was testet.When acetonitrile was used as a solvent,the toluene conversion was up to 11.2%and the predominating product was benzoic acid.In summary,the addition of phase transfer catalysts can promote the dissolution of NHPI in toluene in the solventt-free molecular oxygen catalytic oxidation reaction of toluene with NHPI in homogeneous conditions,thus improve the toluene oxidation activity of NHPI.In addition to the common transition metal cobalt as initiator,the performance of benzaldehyde,nitric acid and m-chloroperoxybenzoic acid as initiators in the catalytic oxidation of toluene with NHPI was investigated.The cobalt Schiff base catalyst were prepared by coordinating the initiator cobalt into a supported Schiff base complex.The catalyst showed good catalytic activity and stability in toluene solvent-free molecular oxygen catalytic oxidation synergestically with NHPI.The catalytic activity is closely related to the electron cloud density around the cobalt ions.In this study,the electron cloud density around cobalt ions was modulated by changing the substituents on the ligand.The toluene oxidation activity of the catalysts were consistent with the electron donating ability of the ligand under the solvent-free toluene aerobic oxidation reaction conditions.The order of electron donating ability was 4-(diethylamino)salicylic aldehyde>o-hydroxyacetophenone>salicylic aldehyde>5-nitrosalicylic aldehyde.This suggests that the greater the electron-donating capacity,the denser the electron cloud surrounding the cobalt,and thus the better its ability to activate the molecular oxygen.Therefore,the cobalt Schiff base catalyst prepared with 4-(diethylamino)salicylaldehyde as the coordination system has the best catalytic oxidation performance for toluene.