| Producing valuable phenolic chemicals with selective hydrogenation is an important way to utilize lignin and its derivatives.The key is the development of efficient catalysts to selectively reduce part of the oxygen-containing functional groups without saturating C=C in the benzene ring(hydrodeoxygenation),by which phenolic compounds can be obtained with high yield under low hydrogen consumption conditions.In this study,the selective hydrogenation performances of a series of Au catalysts were investigated with guaiacol as a model compound of lignin.It was found that anatase-type TiO2(TiO2-A)supported Au nanoparticle(Au/TiO2-A)can hydrodeoxygenate guaiacol with high selectivity into phenolic compounds.Characterizations,such as TEM,XRD and XPS,and batch reactor reactions were used to study the effects of catalyst composition and structure on the catalytic performance in detail.The guaiacol reaction conditions were optimized,and the guaiacol reaction pathways over the Au/TiO2-A catalyst were deduced.Furthermore,the catalyst functions and synergistic effect were deeply analyzed combined with the probe reaction of hydroquinone hydrogenation.The main results of the study are summarized as follows:Au catalysts were prepared with anatase-type TiO2(TiO2-A),rutile-type TiO2(TiO2-R),γ-Al2O3 and activated carbon(AC)as the supports,and applied to the guaiacol hydrodeoxygenation.The results of XPS,TEM and XRD characterization are as follows:the Au species on the support surfaces exists in the form of metal nanoparticles.There are no significant change in the support structure after Au loading.The supports have important effects on the catalytic performances of Au catalysts in the guaiacol hydrodeoxygenation reaction.Among the four supports,only the Au/TiO2-A,prepared with TiO2-A,obtains the improvement in the capability of hydrogenation,while the Au catalysts prepared with the other three supports do not show any improving hydrogenation performance.The conversion rate of guaiacol over Au/TiO2-A is 67%under the reaction conditions of 3.0 MPa H2 initial pressure,300℃ and 3 h,and the selectivity of partially hydrodeoxygenated phenolic compounds such as phenol and cresol is more than 96%.At the same time,no benzene ring saturated product,such as cyclohexane,was detected.The Au catalysts prepared with TiO2-R and γ-Al2O3 generate similar products as each pure supports,with catechol and methylcatechol as the main products.The activated carbon(catalytical inert support)shows no hydrodeoxygenation performance after supporting Au particles,indicating that Au nanoparticles alone cannot catalyze the guaiacol hydrodeoxygenation reaction.The Au particle size in Au/TiO2-A catalyst gradually increases with the increase of the calcination temperature(350℃～650℃)from 2.7 nm(calcined under 350℃)to 7.9 nm(calcined under 650℃).The TiO2-A support maintains the anatase structure and contains oxygen vacancy.The increase in the Au particle size reduces the catalytic activity of Au/TiO2-A and affects the guaiacol reaction pathway.Increasing the hydrogen pressure(1.0 MPa～5.0 MPa)and the reaction temperature(250℃～330℃)can improve the guaiacol conversion,but it has little effect on the selectivity of phenolic products;water can inhibit the guaiacol hydrodeoxygenation reaction over Au/TiO2-A;the catalytic performance of Au/TiO2-A catalyst remains the same as that of the fresh one after three successive runs.There are three possible reaction pathways for guaiacol hydrodeoxygenation over the Au/TiO2-A catalyst:(1)bimolecular methyl transfer between guaiacol and phenolic compounds to form catechol and cresol compounds,followed by catechol hydrodeoxygenation into phenol;(2)direct demethoxylation of guaiacol into phenol and methanol;(3)when the Au particle size of the Au/TiO2-A catalyst is less than 6 nm,in addition to the above mentioned reaction pathways,catechol and methane can be formed from guaiacol hydrogenation.(1)should be the main reaction pathway for guaiacol hydrodeoxygenation on Au/TiO2-A.Hydroquinone hydrogenation was used as a probe reaction to extensively study the catalytic functions of Au/TiO2-A and synergistic effect.The position of hydroxy groups in benzenediol molecule has an important influence on the catalytic performance of Au/TiO2-A,in which catechol can be hydrodeoxygenated to phenol(the conversion of catechol is 24%),hydroquinone can be hydrogenated to p-benzoquinone,cyclohexanol,cyclohexane,etc.(the conversion of hydroquinone is 57%),and resorcinol does not react at all.Under the same reaction conditions(3.0 MPa initial H2 pressure,280℃ and 3 h),the Au catalysts prepared by the γ-Al2O3,SiO2 and TiO2-R supports cannot catalyze the reaction of hydroquinone hydrogenation.It is comfirmed that p-benzoquinone is a key intermediate in the reaction of hydroquinone hydrogenation over Au/TiO2-A through the probe reaction result of p-benzoquinone hydrogenation.The possible hydrogenation route of hydroquinone over the Au/TiO2-A catalyst are proposed as follows:hydroquinone is first dehydrogenated to form p-benzoquinone.Then the C=C and C=O bonds in p-benzoquinone are hydrogenated to obtain 1,4-cyclohexanediol.Cyclohexane is finally formed through a series of reactions,such as alcohol dehydration and olefin hydrogenation.Combining the results of experiments and characterizations,four catalytic functions of the Au/TiO2-A catalyst are proposed:(1)hydrodeoxygenation function on the oxygen vacancy of the TiO2-A surface,such as hydrodeoxygenation of catechol into phenol;(2)dehydrogenation function on the TiO2-A surface and near the interface between the Au particles and the support,such as dehydrogenation of hydroquinone into p-benzoquinone;(3)hydrogenation function of Au particles,such as hydrogenation of C=C and C=O double bonds;(4)dehydration functionon the TiO2-A surface and the interface between the Au particles and the support,such as dehydration of cyclohexanol into cyclohexene.The catalytic performance of Au/TiO2-A in the hydroquinone hydrogenation was the result of the synergy of the above mentioned functions(2),(3)and(4). |
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