Synthesis Of C₃N₄-based Composites And Their Catalytic Application In The Cycloaddition Reaction Of CO₂ And Knoevenagel Condensation

Owing to its abundant nitrogen-containing functional groups,chemical stability,and several unique physicochemical properties,graphitic carbon nitride（g-C₃N₄）can catalyze multiple reactions including CO₂ conversion,Knoevenagel condensation,etc.Thermal polymerization is the most used approach to prepare g-C₃N₄.However,the specific surface area and pore volume of g-C₃N₄ obtained by direct thermal polymerization are very low.The major functionalities on the surface of g-C₃N₄ are basic nitrogen-containing groups.In the case of those catalytic reactions dependent on acidic sites,the low specific surface area and single basic species of g-C₃N₄ inevitably restrict its performance in catalytic reactions.Given the problems of g-C₃N₄ in catalyst preparation along with the catalytic application,in this dissertation,g-C₃N₄ material was used as catalyst support and active component in cycloaddition reaction of CO₂ with epoxide,and Knoevenagel condensation reaction,respectively.According to the catalytic mechanism characteristics of different thermal catalytic reactions,functionalized g-C₃N₄ materials were designed and synthesized in order to improve the catalytic activity and expand to wide thermal catalytic reactions.Moreover,the relation between physiochemical properties and catalytic activity had been correlated.The research work contains four sections as follows.（1）g-C₃N₄ was synthesized using dicyandiamide as a precursor,and then a series of Zr O₂/g-C₃N₄ materials were prepared by the impregnation method.XRD,N₂ adsorption–desorption,SEM,FT-IR,UV–vis,and XPS were employed to analyze the physicochemical properties of g-C₃N₄ and Zr O₂/g-C₃N₄ materials.The interaction between Zr and N in Zr O₂/g-C₃N₄ materials was particularly studied.The results showed that the crystal structure and main chemical functional groups of g-C₃N₄ had almost remained after the loading of Zr O₂.Zr might coordinate with N of g-C₃N₄.The catalytic performance of Zr O₂/g-C₃N₄ catalyst in the reaction between CO₂ and propylene oxide（PO）was investigated.Under optimized reaction conditions,the conversion of PO and selectivity to PC were 69%and 98%,respectively.Zr O₂ was the key component to activate PO molecules,whereas g-C₃N₄ not only served as a solid base to adsorb and activate CO₂ but also anchored Zr O₂ species.（2）A series of Zr_xAl_yO/g-C₃N₄ materials were prepared by impregnation method adopting g-C₃N₄ material as catalytic support.The high-resolution XPS spectra of Zr,Al,and N indicated coordination between metal ions and nitrogen of g-C₃N₄.The loading of Zr_xAl_yO brought acidic sites to g-C₃N₄,enabling the catalysts to possess acid-base bifunctional properties.As the reaction time was 6 h,the CO₂ pressure was 2.0 MPa,and the reaction temperature was 140°C,the conversion of PO and selectivity to propylene carbonate（PC）obtained in Zr₂Al₃O/g-C₃N₄catalyst were 92%,and 97%,respectively.The activity of Zr₂Al₃O/g-C₃N₄ catalyst exhibited no significant decreases after six consecutive experiments.The activation energies of Zr_xAl_yO/g-C₃N₄ and Zr Al O₂/g-C₃N₄ in this reaction were compared.Compared with 5Zr O₂/g-C₃N₄,the introduction of Al enhanced the acidic and basic strengths and thus showed superior catalytic activity.（3）Mesoporous g-C₃N₄（mpg-C₃N₄）material was prepared by nanocasting method using guanidine hydrochloride as a precursor and colloidal silica nanoparticles as hard templates.Afterward,mpg-C₃N₄ was subjected to thermal treatment by HCl（a.q.）,generating hydroxyl functionalized mpg-C₃N₄-OH.As a metal-free heterogeneous catalyst,mpg-C₃N₄-OH material showed high catalytic activity in the cycloaddition of CO₂ with PO.It was found that the concentration of hydrochloric acid and heat treatment temperature had obvious effects on the catalytic activity of mpg-C₃N₄-OH.Under optimized reaction conditions,the conversion of PO and selectivity to PC were 78%and 97%,respectively.In comparison with the pristine mpg-C₃N₄,the introduced hydroxyl groups onto mpg-C₃N₄ were able to activate PO molecules and exhibited higher catalytic activity.（4）A series of CN materials loaded on SBA-15（CN/SBA15）were prepared by thermal polymerization adopting various calcination temperatures.The physicochemical characterization results verified that the loading of CN had not altered the original ordered mesoporous structure of SBA-15 support.The distribution of nitrogen-containing functional groups of CN could be adjusted by changing the calcination temperature.CN/SBA15-400owned superior total nitrogen content as well as a higher percentage of bridging nitrogen species.As a solid base,CN/SBA15-400 showed high catalytic activity in the Knoevenagel condensation reaction.Under optimized reaction conditions,the conversion of benzaldehyde and selectivity to benzyl malononitrile were 88.6%and 95.1%,respectively.No obvious catalytic deactivation was observed after several catalytic uses.The catalyst also showed good catalytic activity for various aldehydes of Knoevenagel condensation.In this dissertation,g-C₃N₄ material,as a catalyst platform,is used as catalyst carrier and active component in cycloaddition reaction of CO₂ and Knoevenagel condensation reaction,respectively.In view of the requirements of the above two target reactions for catalytic active components,g-C₃N₄ supported with metal oxides,hydroxyl functionalized g-C₃N_4,and a series of CN materials loaded on SBA-15 were prepared,respectively.The crystal structure,specific surface and pore structure,chemical functional groups,surface chemical bonding,micromorphology,solid acid-base properties and other physical and chemical properties of the catalyst were analyzed in detail based on a variety of catalyst characterization techniques.The interaction between metal ions and g-C₃N₄ and the effect of preparation conditions on the interaction were analyzed and a synergic catalytic mechanism of cycloaddition of CO₂ towards cyclic carbonates was proposed.Also,the influence of preparation conditions on the distribution of nitrogen-containing functional groups in g-C₃N₄ was learned.The research work provides a reference for the design and development of efficient conversion of CO₂ and new metal-free solid base catalysts.