Cobalt Doped Mesoporous SBA-15 Immobilized NDHPI As A Composite Catalyst For Selective Oxidation Of Toluene

Benzaldehyde and benzyl alcohol are important organic intermediates with high added value,which are widely used in dyes,pharmaceuticals,food stuff and other industries.Benzaldehyde is mainly produced from toluene chlorination followed by hydrolysis in the domestic industry.However,the products produced from the above process inevitably contain traces of chlorine,which are prohibited to be used in the pharmaceuticals and food stuff industries,leading to large market demand gap.Compared with the traditional preparation method,the direct oxidization of toluene to benzaldehyde and benzyl alcohol with molecular oxygen is an environmental-friendly synthesis route.However,the majority of the oxidation process proceeds under harsh conditions such as high temperature,high pressure.Benzoic acid is the main product and only a small amount of benzaldehyde and benzyl alcohol are obtained,therefore how to realize the selective oxidation of toluene to benzaldehyde and benzyl alcohol under mild conditions is a hotspot issue.Based on the previous studies,we constructed two kinds of catalytic system,one is immobilizing active site NDHPI on SBA-15 molecular sieve combined with Cosalen complex as co-catalyst,another is immobilizing active site NDHPI on Co-doped mesoporous sieve SBA-15 as composite catalyst.Then the structure of obtained catalysts was characterized by various methods and the catalytic performance of toluene oxidation was evaluated.In addition,the factors affecting the stability of the catalyst were investigated.All the work was aimed to provide technical accumulation for the development of new system of toluene catalytic oxidation.Firstly,the active component NDHPI was immobilized on SBA-15 molecular sieve by chemical grafting method,then the structure of catalysts was characterized by FT-IR,XRD,SEM,N2 physical adsorption,TEM,TG,29Si(CP-MAS).The results showed that the immobilized structure was realized through the reaction between two methoxy groups of one GPTMS molecule with the surface silanol groups of SBA-15,and another methoxy group was unchanged.The obtained catalyst kept highly ordered 2-D hexagonally mesostructure.The loading amount of the active components was above 0.14g/g carrier.The catalytic performance of the NDHPI-epoxy/SBA-15 was evaluated in the toluene aerobic oxidation moderately at 90 oC with acetonitrile as the solvent for 7 h,toluene conversion rate reached6.9%,the total selectivity of benzaldehyde and benzyl alcohol was up to 78%.After three times recycling,little change in activity was occurred,but the active component loss problem was still existed.Secondly,active site NDHPI was immobilized on the co-catalyst Co-doped mesoporous sieve SBA-15,the structure of the composite catalyst was characterized by FT-IR,XRD,TGand so on.The catalytic performance was evaluated in toluene oxidation reaction and the effects of the reaction conditions on the catalytic performance were also investigated.The results showed that the content of Cobalt in immobilized catalyst was 1.53% and the loading amount of the active components was 0.1g/g carrier.Under the optimized reaction conditions,namely the mole ratio of catalyst active component NDHPI to substrate was 2mol%,the temperature was 90 oC,the pressure was 1.60 MPa,reaction time was 7 h,the conversion of toluene was up to 38.5%,the selectivity of benzaldehyde,benzene alcohol and benzoic acid were 13.3%,3.4% and 83.3%,respectively.After recycling,the activity of the catalyst decreased apparently,which was due to the loss of active component.Finally,the factors that may influence the stability of the catalyst were discussed.During the preparation process of the catalyst,toluene is more suitable as the solvent than ethanol,the kinds of the silane reagents and order of the catalyst synthetic route had no effect on the stability of the catalyst;The stability of N-O-C bond condensation between NDHPI and silanization reagent and Si-O-Si bond from siloxane groups and Si-OH groups of molecular sieve carrier were investigated independently.There was no bond broken under the reaction conditions;The reaction product benzoic acid had a certain toxic effect on the catalyst,but this influence had no apparent effect on the stability of the catalyst under the reaction condition;acetonitrile as the solvent can slow the rate of the loss of active components,and was more favorable to improve the selectivity of benzaldehyde and benzyl alcohol.