| Benzaldehyde is the second largest perfume in the world,and widely used in food,medicine,cosmetics and other fields.With more attention about quality from consumers,natural benzaldehyde is more and more popular and represents a strong market advantage.As a result,the synthesis of benzaldehyde is drawing much attention.Based on the rich natural cinnamal oil,of which cinnamaldehyde is the main composition,we have developed a clean and efficient process to produce benzaldehyde from cinnamaldehyde under much milder conditions,using ozone as oxidant.Moreover,the mechanism has been investigated by experiments,various characteristic methods,kinetics methods and in-situ characterization.Firstly,the transition metal oxide is used to selectively ozonation cinnamaldehyde to produce benzaldehyde.CaO shows the highest catalytic activity and selectivity in the studied metal oxides.This is because CaO has a high basic strength,which can make the reaction more conducive to the formation of benzaldehyde.The effects of catalyst calcination temperature,calcination time and reaction temperature,reaction time,catalyst amount and oxygen flow rate on the selective oxidation of cinnamaldehyde to benzaldehyde by CaO catalyzed were investigated.The catalysts were characterized by XRD,FT-IR,N2 adsorption-desorption and TG-DTG.Under the optimum conditions,the cinnamaldehyde conversion and benzaldehyde selectivity were 97.97%and 60.87%,respectively.The effects of different catalyst reusability methods on the catalytic performance were investigated.The results showed that the deactivation of the catalyst was mainly due to the adsorption and blocking of the organic matter on the catalyst surface and the hydration and carbonation of the catalyst.The experimental results show that the ozonation process does not follow the mechanism of hydroxyl radicals,and follows the Criegee ozonation mechanism.First,the primary ozonide is formed,and then it is converted into a stable secondary ozonide to produce benzaldehyde.Furthermore,the addition of CaO catalyst can reduce the activation energy of ozonation from 19.17 kJ·mol-1 to 11.96 kJ·mol-1.Secondly,the kinetic study of the formation of benzaldehyde by CaO as calcined at 900℃ was studied.It was found that there was no external diffusion effect when the oxygen flow rate was more than 750 mL·min-1,and the influence of internal diffusion was excluded according to Mears criterion.In the experimental range,the power function results show that the reaction is 0.57 for cinnamaldehyde,1.36 for ozone and 0.19 for CaO catalyst.The instrinsic kinetic analysis shows that the Eley-Rideal model of only cinnamaldehyde adsorption can be in line with the intrinsic dynamics test results.Finally,the adsorption of CO2 and O3 on the surface of CaO,ZnO,Al2O3,CuO and Fe2O3 were investigated by in situ diffuse reflectance infrared spectroscopy.The effects of the surface properties of the catalyst on the adsorption/dissociation of O3 and the experimental mechanism of the ozonation of cinnamaldehyde and the excessive oxidation of benzaldehyde were discussed.The results show that the adsorption of ozone molecules on the surface of metal oxides is accompanied by the formation of ozonates,atomic oxygen and superoxide,and the strong acid/base sites will accelerate the decomposition of ozone molecules.Cinnamaldehyde can not only be oxidized to cinnamic acid under the action of ozone,but also oxidized to benzaldehyde,benzoic acid and maleic anhydride,and finally mined into COx at room temperature. |
