| Propylene oxide(PO)is an important chemical feedstock.Compared with the traditional chlorohydrin and co-oxidation processes,and the emerging hydrogen peroxide(H2O2)direct oxidation process,gas-phase epoxidation of propylene is regarded as the most promising process for PO production,owing to the environmental friendliness,atomic economy and simple process.However,the nano-gold catalysts used in direct propylene epoxidation are reported to show low activity and poor stability.In this dissertation,the catalytic performance of nano-gold and nano-gold/palladium respectively supported on titanium silicalite-1(denoted as Au/TS-1 and Au-Pd/TS-1)have been investigated.Au/TS-1 was prepared by the deposition precipitation method and the following three aspects of our work were performed.First,based on the previous work,a kind of core-shell catalyst with small core and thin shell,denoted as Au/S-1/TS-1(0.26),was designed to confine Au clusters inside the nanoporous channels to obtain more smaller Au clusters(<1.0 nm),as well as decrease the internal mass transfer limitation.This core-shell catalyst exhibited a PO formation rate of 410 g PO h-1 kg-1TS-1 at 200°C under atmospheric pressure,which is much greater than 300 g PO h-1 kg-1TS-1 of Au/TS-1(0.26).Obviously,the catalytic activity of Au/S-1/TS-1(0.26)was improved significantly.Second,the main deactivation reason for nano-gold catalysts was discussed by comparing the catalytic stabilities of Au/S-1/TS-1 and Au/TS-1.The activity of Au/TS-1decreased by 35%at 200°C for 50 h under atmospheric pressure,while that of Au/S-1/TS-1 only decreased by 25%.In addition,the activity of Au/S-1/TS-1 decreased by35%after being tested at 200°C for 100 h.Results showed that the migration as well as agglomeration of Au clusters was the primary reason for catalyst deactivation,followed by the effect of carbonaceous deposits.Au agglomeration resulted in losing the proper size for propylene epoxidation and then decreasing the catalytic activity.Furthermore,higher reaction temperature and longer reaction time were responsible for the gradual deactivation of nano-gold catalysts.Third,the effect of TS-1 crystal planes on catalytic activity of Au/TS-1 was explored.Results showed that the relative intensities of crystal planes of(101)and(501)(denoted as I(101)/I(501))increased gradually with the removal of organic template in the support.Correspondingly,the activity of nano-gold catalyst increased at 200°C under atmospheric pressure as the template was removed.To exclude the effect of exposed active sites with the removal of template,two catalysts with similar Ti contents and Au loadings were prepared.It was found that the catalyst with greater I(101)/I(501)ratio exhibited the higher activity for propylene epoxidation.This was because that the greater I(101)/I(501)ratio contributed to the adsorption of propylene,which was of benefit to enhance the activity.Therefore,the activity of nano-gold catalysts could be improved by optimizing the support.To further improve the avtivity of direct propylene epoxidation,a second metal Pd was introduced into Au/TS-1 to form Au-Pd/TS-1 bimetallic catalyst via the alcohol reduction-mediated supported method.The synergistic enhancement over Au-Pd/TS-1 for gas-phase epoxidation of propylene was revealed by comparing the activities of bimetallic and monometallic catalysts at 200°C under atmospheric pressure.As the Pd/Au molar ratio increased,H-spillover process became more pronounced on Au-Pd/TS-1,and the calculated apparent activation energy of Au-Pd/TS-1 decreased.The origin of synergy between Au and Pd could be elaborated by H-spillover process.The active hydrogen(H2)on Pd could spill over to Au sites and react with the adsorbed oxygen(O2),then the formed H2O2 transferred to the adjacent Ti sites for epoxidizing propylene.In-situ DRIFT spectra confirmed that the synergy significantly promoted the formation of H2O2 from H2 and O2,thereby improving the activity of Au-Pd/TS-1.The higher PO formation rate per gram of Au for Au-Pd/TS-1 was elaborated by combining experimental characterization with density functional theory(DFT)calculations.There existed a volcano-shaped relationship between Pd/Au molar ratio and catalytic activity.Au0.68Pd0.32/TS-1 exhibited the highest PO formation rate of 1000 g PO h-1 g Au-1 at200°C under atmospheric pressure.It was found that charge transfer from Pd to Au atoms occured on cuboctahedron-structured Au-Pd alloy nanoparticles,then the charge was transferred from the formed negatively charged Au atoms to the adsorbed O2.This process had a positive effect on the generation of H2O2,thus improving the PO formation rate per gram of Au for Au-Pd/TS-1 alloy catalysts.A four-stage model was proposed according to the performance of Au-Pd/TS-1 at100°C under atmospheric pressure.In the first stage,propylene hydrogenation dominanted and the propylene epoxidation dominanted in the second stage.Afterwards,these two reactions achieved a balance in the third stage and the activity decreased in the fourth stage.The four-stage model reveals the competition between propylene hydrogenation and epoxidation reactions,which may help us understand the reaction mechanism over Au-Pd/TS-1 and further optimize the supported Au-based bimetallic catalysts. |
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