| Benzaldehyde was the second flavour in the world after vanillin in quantity,and a valuable precursor or intermediate in the chemical industry.It was widely used in food,medicine,cosmetics,and other fields.The generalized natural flavor was gradually concerned and loved by consumers due to the dual influence of the change of consumers'consumption habits and the stricter laws and regulations.At present,CaO was commonly used as a catalyst in the catalysis of cinnamon oil?the main component was cinnamaldehyde?to benzaldehyde.However,it was found that the trace water generated in the reaction might affect the reaction.Based on the fact that CaO could producein the presence of water,the selective oxidation of cinnamaldehyde to benzaldehyde by Ca?OH?2 catalyst was studied.Firstly,Ca?OH?2 catalyst without carbonate was prepared by adding Millipore water under the protection of anhydrous ethanol with CaO as a precursor,and its microstructure and characteristics were characterized by XRD,TEM,N2 adsorption-desorption,XPS and in situ DRIFTS,and it was applied to the cinnamaldehyde ozonation to benzaldehyde.Under the optimum reaction conditions(the reaction time was 210 min,the reaction temperature was 0?,the catalyst dosage was 0.2 g,and the oxygen flow rate was 400 m L·min-1),the cinnamaldehyde conversion,benzaldehyde selectivity,and yield were 78.27%,68.60%,and 53.59%,respectively,which were 2.27%,10.93%,and 13.23%higher than those of CaO catalyst under the same conditions.The specific surface area,average pore size,and pore diameter of Ca?OH?2 were 1.74,1.44,and 3.06 times of CaO,respectively.Meanwhile,in the Ca?OH?2 catalytic reaction,the ozone utilization efficiency was 67.27%,which was 2.28%higher than that of CaO.The activation energy was 6.900 k J·mol-1 and was 35.45%lower than that of CaO.Secondly,the adsorption-desorption process of O3 on Ca?OH?2surface was investigated by in situ DRIFTS,and the adsorption and ozonation of cinnamaldehyde and benzaldehyde on Ca?OH?2 surface were studied.The results showed that the O3 adsorption on the Ca?OH?2 surface was chemical adsorption except for a small amount of physical adsorption.Ozone was mainly adsorbed on the weak Lewis acid site,i.e.,it formed alkali metal odorous oxide with Caion.With the increase of temperature,the adsorbed ozone would decompose into atomic oxygen,O2-,and other oxygen species.Under the condition of 0? and anhydrous ethanol,the ozonide which formed on the Ca?OH?2 surface by ozone adsorption preferentially oxidized the carbon-carbon double bond group?>C=C<?of cinnamaldehyde,rather than the aldehyde group?-CHO?.At the same time,in the process of the benzaldehyde ozonation to benzoic acid,the activation energy of the reaction increased by 0.82 times with the presence of Ca?OH?2 compared with that without catalyst,which indicated that the presence of Ca?OH?2 could also inhibit the benzaldehyde ozonation to benzoic acid,which had a protective effect on the main product.Finally,the density functional theory?DFT?was used to simulate the O3adsorption on the Ca?OH?2 dominant?101?surface.The results showed that the terminal oxygen atom of O3 was easily adsorbed on the Caion,and a few of them were dissociated into atomic oxygen and molecular oxygen.By adding t-butyl alcohol?TBA?,a hydroxyl radical inhibitor,it was proved that this process was not a hydroxyl radical mechanism.Combined with the experimental results,the process could be described by the Criegee mechanism.In the reaction,Ca?OH?2 played a dual role catalyst at the same time:to promote the alkene ozonation,to promote the zwitterion peroxides rearrangement to produce nonoxidative acids,to promote the formation of acetals.At the same time,Ca?OH?2 could inhibit the excessive oxidation of benzaldehyde to benzoic acid,the formation of benzoic acid by primary ozonates,to improve the benzaldehyde selectivity. |
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