Preparation Of Supported Silver-based Catalyst And Study On Their Catalytic Oxidation Performance

In recent years,with the rapid development of polyester industry,ethylene glycol shows a high demand in the field of chemical industry.The epoxidation of ethylene to the ethylene oxide has become a very important carbon dioxide chemical process route,which promotes much research interest in new catalysts design as well as structure-property correlation.At the same time,green synthesis of organic aldehydes,especially for octanal,which serve as important intermediates for drugs and spices,has attracted considerable attention.However,several problems remain unsolved in the study of supported Ag-based catalysts:1)How to effectively control the active stucture of silver during the preparation of catalysts so as to improve the reaction performance;2)How to deeply understand the regulation of active site structure of supported Ag-based catalysts and corresponding reaction mechanism.In this dissertation,the Ag-based catalysts were prepared by A negative pressure complex impregnation method in the epoxidation of ethylene.In a typical procedure,the ratio of ethylenediamine to silver species,activation temperature and time were optimized.The amount and mode of additives Re and Cs were adjusted.The Re3|Ag/Al2O3 sample exhi-bits the optimal performance performance toward ethylene epoxidation reaction,with a EO selectivity of 97%.In the reaction of dehydrogenation of n-octanol to dehyde,the Ag-based catalysts were prepared by using organic complexation impregnation method were prepared by precisely controlling the activation temperature and atmosphere.The optimum selectivity of hydrogen-activated Ag/Mg2Al1MMO catalyst was 96%in the reaction of dehydroaldehyde of n-octanol Dehydrogenation to Aldehyde.In addition,by using a variouty of experimental methods,the effects of promoters and activation atmosphere were studied towards the state of active sites of catalysts and their structure-activity relationship with reaction performance.Each effects of promoters and activation atmosphere on the active sites of metals was revealed respectively.The main research contents are as follows:1.Preparation of Supported Ag-based Catalysts and Their Catalytic Performance toward Ethylene Epoxidation to Ethylene oxideIn this thesis,A negative pressure complex impregnation method was used to prepare Ag-based catalyst with controllable composition and enhanced performance,by accurately controlling the material ratio of ethylenediamine with silver species as well as the calcination conditions(temperature and time).Subsequently,by tuning the amount and manner of additive Re,four kinds of Ag-based catalysts supported on Al2O3 were prepared,and the Re3|Ag/Al2O3 sample exhibits the optimal performance performance toward ethylene epoxidation reaction,with a EO selectivity of 87%.SEM and XRD results show that the introduction of Re additive impose no obvious influence on structure and morphology of Ag.H2-TPR,UV-vis,XPS and Raman spectra demonstrate that the addition of Re promotes the formation of positive Ags+species on the surface,which results in electrophilic oxygen species and enhanced selectivity toward ethylene epoxidation reaction.Moreover,a positive correlation between reaction selectivity and Ag?+ species(or electrophilic oxygen species)is obtained,indicating that Ag?+ species serves as the active site.The introduction of Re(300 ppm)facilitates the formation of Ag?+ species,and therefore improves the selectivity of catalytic.2.Preparation of Supported Ag-based Catalysts and Their Catalytic Performance toward n-octanol Dehydrogenation to AldehydeBy virtue of the topological transformation behavior of hydrotalcites,Mg2Al1 mixed metal oxide was prepared via high temperature roasting treatment.Subsequently,three kinds of Ag/Mg2Al1MMO catalysts with uniform dispersion were prepared by using organic complexation impregnation method.XRD and TEM characterizations show that the Ag particles were well dispersed in different activation atmospheres,and their particle sizes were about 5 nm.The optimum selectivity of hydrogen-activated catalyst was 96%at 150? and 4 h by systematically adjusting the activation atmosphere.UV-vis and H2-TPR analysis show that the catalyst in oxygen atmosphere has relatively high Ag?+species,while the catalyst in hydrogen activation induces relatively stronger interaction between Ag and carrier.Further XPS analysis shows that the Ag and A1 species undergo electron transfer in different atmosphere.Electron transfer occurs from A1 to Ag in hydrogen atmosphere,accompanied with the formation of Ag?-species.In addition,the Ag/Mg2Al1MMO(H)catalyst provides the strongest electron transfer from A1 to Ag,which is favorable for the enhanced selectively of aldehyde.