Construction Of Metal/Carbon-based Composites For Heterogeneous Catalysis

Catalysis plays an important role in chemical industry,and more than 90%of the production of chemicals is related to this process.Traditional transition metals and their compounds are widely used in many catalytic reactions due to their unique properties.However,in practical process,metal particles always suffer from some problems(such as agglomeration,immigration,and even leaching deactivation),making the catalytic performance decrease sharply.Therefore,it is highly essential to immobilize the metal nanoparticles,improve their recycling ability and leaching resistance capability.More recently,graphene,graphitic carbon nitride(g-C3N4),porous N-doped carbon have attracted increasing attention in catalysis due to the advantages,such as unique structure,large specific surface area,modifiable surface,and good stability.It has been reported that they can serve as excellent catalyst supports or cocatalysts.Inspired by the above considerations,this dissertation focuses on the three carbon-based materials,and a series of metal/carbon-based composite materials were then prepared via carbon supported or encapsulated,as well as micro-nano material structural design strategies.Moreover,the catalytic performance of the prepared composites for heterogeneous catalytic reactions and the intrinsic correlations between catalytic activity and catalyst property were also investigated.The present work could be concluded as follows:First,a series of graphene supported Sn-W bimetallic oxide catalysts were prepared via a conventional hydrothermal method,which exhibited excellent catalytic activity for the selective oxidation of benzyl alcohol to benzaldehyde(Chapter 2).In the reaction system,water was used as solvent,H2O2 was used as oxidant,and no base additives were used,which met the requirements of green chemistry.It was found that 94.0%conversion of benzyl alcohol and 94.3%selectivity of benzaldehyde could be achieved within 3 h over 0.3Sn-W/RGO catalyst,and graphene served as an excellent catalyst support for the metal nanoparticles and greatly promoted their dispersion.Owing to the introduction of Sn,WO3 nanocrystal was transferred into hexagonal phase with dominant exposed(001)and(200)planes,and the interaction between graphene and WO3 was also enhanced.Such results led to the improved stability of the prepared catalysts,and no obvious loss of catalytic activity after five cycles was observed.In Chapter 3,a series of leaf-like WO3 nanoflake decorated on graphene-like g-C3N4 composite catalysts were prepared for the selective oxidation of various alcohols to corresponding carbonyl compounds.The effects of solvent,reaction temperature,reaction time,catalyst amount and oxidant dosage on catalytic activity were also investigated.It was found that the WO3/g-C3N4 composites catalysts displayed much better catalytic performance compared to these of the bare WO3 and bare g-C3N4,which resulted from the synergistic catalysis effect and strong interaction between WO3 and g-C3N4.XPS indicated the existence of electron transfer between g-C3N4 and WO3,and HRTEM suggested that specific interface was formed between them.On the other hand,the prepared catalysts were not only highly selective for the selective oxidation of benzyl alcohol,but also applicable to the selective oxidation of various aryl and aliphatic alcohols.Besides,the catalysts could be recovered from reaction system via centrifugation,making it a promising candidate in the future industrial applications.To gain more insights into the interaction between metal and g-C3N4,in Chapter 4,graphene-like g-C3N4 supported Cu-Cr bimetallic oxide catalysts were prepared via an in-situ heating treatment method,and the catalytic activity of prepared catalysts for methyl sp3 C-H activation of toluene to benzaldehyde was studied.The highest yield of benzaldehyde was obtained as 42.9%within 5 h over CuCr2/g-C3N4 under mild reaction conditions.It was found that some metal nanoparticles were encapsulated by g-C3N4 to form unique core/shell-like nanostructure,impeding the metal leaching during liquid-solid catalytic reaction process.Additionally,electron transfer between Cu and Cr species occurred,resulting in the existence of a certain amount of low-valence Cu+ among the bimetallic catalysts,which was beneficial to the improvement of catalytic activity.Moreover,the reaction kinetics of methyl sp3 C-H activation of toluene was studied and the apparent activation energy was determined in the presence of the prepared catalysts.In addition,the reusability and leaching resistance capacity of the prepared g-C3N4 supported catalyst was also investigated.Finally,a possible mechanism for this reaction was also preliminarily proposed.To further reveal the interaction between metal and N-doped carbon material,in Chapter 5,a honeycomb-like porous Ce-Cr oxide/N-doped carbon nanostructure was successfully prepared via an in-situ pyrolysis method by using glucose and melamine as the carbon and nitrogen precursors,respectively,and combined with metal precursors.The prepared catalysts were used for the selective oxidation of cyclohexane to cyclohexanone/cyclohexanol(KA oil),and the effects of reaction parameters and calcination temperature on catalytic performance were also investigated.The optimized reaction conditions were as follows:acetonitrile as solvent,H2O2:cyclohexane molar ration(2:1),temperature(60?)and reaction time(5 h).Over CeCr/NC-500 catalyst,the cyclohexane conversion and KA oil selectivity could be obtained as 56.5%and 95.5%,respectively.It was found that the porous nanostructure contributed to the increase of specific surface area and pore volume,and the pyridinic N and pyrrolic N species of N-doped carbon provided main active sites for the metal immobilization.In addition,the reaction mechanism showed that,with the activation of the prepared catalyst,hydroxyl radical(·OH)generated by homolysis of H2O2 molecules played an important role during the present cyclohexane oxidation system and which was the main active oxygen species.Moreover,the prepared catalysts also showed excellent stability and leaching resistance capability during the catalytic reactions,and negligible loss in catalytic activity was observed after seven runs.In conclusion,a series of metal/carbon-based composite catalysts were constructed via carbon supported or encapsulated metal nanoparticles,and micro-nano material structural design strategies,which enhanced their activities and reusability in heterogeneous catalytic reactions and also provided some new insights into the design of high-performance heterogeneous carbon-based supported catalysts.