| Benzaldehyde and its derivatives were widely used in food,medicine,cosmetics,dyes,and other fields.Moreover,natural benzaldehyde was in short supply due to the mild green chemical reaction requirements and consumer demand.The preparation of higher value-added natural benzaldehyde from cinnamon oil or natural cinnamaldehyde,a characteristic product of Guangxi,could make full use of rich cinnamon resources and be fully used to of great significance to promote the local economic and social development of Guangxi.The heterogeneous catalytic ozonation of cinnamaldehyde to natural benzaldehyde with Ca（OH）2 as the catalyst had high benzaldehyde selectivity.However,the trace amount of water produced in the reaction process would affect the ozonation.The effect of water on cinnamaldehyde ozonation to prepare benzaldehyde under Ca（OH）2 catalysis was studied by in situ diffuse reflectance infrared Fourier transform spectra（in situ DRIFTS）,Density Function Theory（DFT）,and ozonation experiments.Firstly,the effect of water on the preparation of benzaldehyde by ozonation of cinnamaldehyde under Ca（OH）2 catalysis was investigated.Thermogravimetric analysis and X-ray diffraction（XRD）data showed that it would exist in Ca（OH）2with the form of adsorbed and interlayer water after the water,generated by ozonation,reacted with Ca O to form Ca（OH）2.Pretreated at 300°C for 30 min to remove most of adsorbed and interlayer water in Ca（OH）2 to obtain pre-processed Ca（OH）2.The catalysts were characterized by N2 adsorption-desorption.Although the specific surface areas of both lab-made and pre-processed Ca（OH）2were more extensive than three times that of Ca O,the specific surface area,average pore diameter,and pore volume of lab-made Ca（OH）2 were reduced by about 19.6%,24.8%,and 38.9%respectively compared with those of pre-processed Ca（OH）2.Therefore,benzaldehyde selectivity under the catalysis of lab-made Ca（OH）2 was reduced by 8.1%compared with the catalysis of pre-processed Ca（OH）2.The ozonation experimental data showed that:（1）In the initial stage of the catalytic ozonation,when Ca O transitioned Ca（OH）2,the presence of water improved the selectivity of benzaldehyde in the ozonation.（2）However,the continuously generated water would inhibit the ozonation reaction and reduce the benzaldehyde selectivity.Secondly,the effect of water on the adsorption of ozone on Ca（OH）2 surface was studied by DFT and in situ DRIFTS.The DFT study showed that when Ca（OH）2 adsorbed with ozone or water,Ca atoms first formed spd hybrid orbitals with the 4s,3p,and 3d orbitals and then interacted with the 2p orbital of O atoms in ozone or water molecules.However,the water did not affect the cinnamaldehyde ozonation through competitive adsorption with ozone in the presence of water and ozone on the surface of Ca（OH）2,due to a 4.5-fold gap in the adsorption energy when water and ozone alone were adsorbed on the Ca（OH）2surface.According to DFT and in situ DRIFTS data,water affected the cinnamaldehyde ozonation by decomposing the ozonide formed at the ozone adsorption on the Ca（OH）2 surface to yield hydroxyl radical and changing the Criegee ozonation mechanism to the hydroxyl radical mechanism.Finally,a series of continuous cinnamaldehyde ozonation reactions were studied by in situ DRIFTS.The results showed that the presence of water could quickly decompose the physisorption ozone molecules and alkaline-earth metal ozonides generated by chemical adsorption to yield oxygen atom and superoxide ions O2–.These two decomposition products entered the reaction system to generate hydroxyl radicals and changed the mechanism of cinnamaldehyde ozonation.The free-radical trap experiment data further confirmed that after water completely transformed Ca O into Ca（OH）2,its inhibitory effect became more and more evident with the increase of water content in the system.Stability and deactivation studies indicated that the catalytic activity of Ca（OH）2 decreased significantly after four cycles.IR data showed that the organic compounds in the reaction system were adsorbed on the Ca（OH）2 surface,resulting in the coverage of some active sites on the catalyst surface,the blocking of pores,and the shift of acidity and alkalinity on the catalyst surface,which caused the deactivation of the catalyst and the reduction of the benzaldehyde selectivity. |
