The Study On Photocatalytic Hydrogenation Of Unsaturated C=C And C=O Bond And Swirl-Flow Effect

The selective catalytic hydrogenation of carbonyl compounds containing unsaturated C=C bond and C=O bond is an important step during the synthesis of fine chemicals,which is widely used in industrial processes such as pharmaceuticals,perfumes and fragrances.The noble metal,as the catalysts,was used in the catalytic hydrogenation reactions,however,the harsh reaction conditions,high production cost,the difficult post-processing,and serious environmental pollution could not meet the requirements of sustainable development.Photocatalytic hydrogenation reaction possesses mild reaction conditions,green regeneration and easy product separation.In this work,ultraviolet light source and TiO₂ were used as power sources and catalysts for hydrogenation of α,β-unsaturated aldehydes which is the typical structure of carbonyl compounds containing unsaturated C=C bond and C=O bond.The effects of metal loading,preparation temperature and crystal plane structure of the photocatalyst on the conversion and selectivity were studied.The swirl-flow effect of photocatalytic hydrogenation of cinnamaldehyde was discussed in detail based on the experiment and simulation.Then,the interfacial exchange mechanism of photocatalytic hydrogenation of α,β-unsaturated aldehyde under rotating flow field was revealed.The main results and conclusions are as follows:The effects of different calcination temperature,calcination time,metal loading and content of TiO₂ on the photocatalytic hydrogenation of α,β-unsaturated aldehydes were studied.The results showed that the TiO₂ photocatalyst calcined at 500 °C for 2h has a high conversion of α,β-unsaturated aldehyde.The conversion of cinnamaldehyde,citral and leaf aldehyde can reach to 88.7%,99% and 97.4%,respectively.At the same time,the unsaturated C=O bond hydrogenation shows a high selectivity.The selectivity of cinnamyl alcohol,unsaturated alcohol（geraniol and nerol）and trans-2-hexenol can reaches to 91.7%,100% and 95.7%,respectively.The effects of different crystal planes of TiO₂ on the photocatalytic hydrogenation of α,β-unsaturated aldehydes was investigated.The anatase（101）facets has high conversion for cinnamaldehyde,citral and leaf aldehyde,and shows a high selectivity for unsaturated C=O bond.Although the conversion of α,β-unsaturated aldehydes is not high under the action of the anatase（001）facets photocatalysts,the anatase（001）facets with higher surface energy could simultaneously reduce the unsaturated C=C bond and C=O bond and has a certain selectivity for the corresponding saturated alcohols.The regularity of photocatalytic hydrogenation of α,β-unsaturated aldehydes with time and the reuse of photocatalysts were studied.Among them,the photocatalytic hydrogenation of cinnamaldehyde and citral was completed in 8 hours and 17 hours,respectively,which shortened the photocatalytic hydrogenation time,reduced energy consumption.The recycling results of photocatalyst showed that the TiO₂ photocatalyst has a good function of recycling,and the number of recycling can reach more than 3times.The reasonable dosage of hydrogen producing agent triethanolamine was 5 m L.The simulation results of the swirl-flow environment of photocatalytic hydrogenation of α,β-unsaturated aldehydes showed that the swirl-flow has the mixing effect of reaction solution.Meanwhile,the change of swirl-flow shape would increase the effective area of light irradiation at the interface of photocatalytic hydrogenation.A low speed turning point and a high speed turning point were formed during the process of speed increase.When the speed is greater than the low speed turning point,the cinnamaldehyde conversion rate increases significantly with the increasing of the speed,while when the speed is greater than the high speed turning point,the cinnamaldehyde conversion rate slows down with the increase of speed.The comprehensive statistical table of influencing factors of fluid dynamics was obtained based on the simulation of the different physical quantities at different rotational speeds,such as the circumferential average rate,the rate of secondary flow,the overall average rate,the average rate in z direction and the area of gas-liquid interface.Among them,the material exchange between the inner layer and the interface are formed in reaction solution under the action of the secondary flow.The change of the gas-liquid interfacial area would obviously effect the area receiving light of the reaction solution.The definition of the interfacial flux of illumination is the area of interface carried away by the secondary flow per unit time,which is the product of the rate of secondary flow and the area of gas-liquid interface.The correlation coefficient between the interfacial flux of illumination and the conversion of cinnamaldehyde is 0.974 according to the statistical calculation.The interfacial flux of illumination allows for the effective combination of light source and the swirl-flow,indicating the reason for the conversion of photocatalytic hydrogenation of cinnamaldehyde.The interfacial exchange mechanism of photocatalytic hydrogenation of α,β-unsaturated aldehydes under rotating flow field was proposed.A fully mixed between the reaction solution and catalyst TiO₂ under the action of rotating flow field.The secondary flow drives the reaction solution fluid up and down and loads the photocatalyst TiO₂ adsorbing the reactants to the upper gas-liquid interface of the solution.Then,the photocatalytic hydrogenation reduction reaction be carried out by fully absorbing the irradiation from axial ultraviolet light source.The photoelectronhole pairs were generated in TiO₂ and diffused to the surface of the photocatalyst.The photoelectrons in the conduction band had strong reducability,reducing the cinnamaldehyde adsorbed on the interface of the photocatalyst to cinnamyl alcohol.The photogenerated holes have strong oxidation ability and oxidize the sacrificial agent in the reaction solution.Meanwhile,the reaction products and photocatalyst TiO₂ were unloaded to the inner layer of the solution by the secondary flow.This cycle is repeated until the photocatalytic hydrogenation is completed.In this paper,the photocatalytic hydrogenation of unsaturated C=C bond and C=O bond was studied.The influence of photocatalysts on α,β-unsaturated aldehydes photocatalytic hydrogenation reaction were revealed as well as the interfacial exchange mechanism of photocatalytic hydrogenation reaction under rotating flow field,which would provide reference for the industrial application of photocatalytic hydrogenation.