Modification Of Graphitic Carbon Nitride Materials And Their Catalytic Application For Benzene Hydroxylation And Base Catalysis

As a new nitrogen material,graphitic carbon nitride（g-C₃N₄）has been extensively applied in multiple research fields including fuel cell,photocatalysis,thermocatalysis,gas storage,etc,owing to its combination of various unique physicochemical properties.In viewpoints of chemical composition and structure,g-C₃N₄ is constituted of highly conjugated heptazine tectonic units,which can activate benzene molecules.On the other hand,the nitrogen atoms bridging tectonic units and amino groups located at the edges of graphitic sheets of g-C₃N₄ are typical basic sites,which therefore can promote Knoevenagel condensation,transesterification and some base-mediated reactions.However,the common g-C₃N₄ materials prepared through ordinary thermal condensation have very lower surface areas(<10 m² g^-1)and no rich porous structures.As a result,the catalytic activity of the common g-C₃N₄materials in benzene-involving and base-mediated reactions is very low.In this thesis,g-C₃N₄ is applied as a research platform of the catalytic material.According to the demands of catalytically active sites of hydroxylation of benzene,transesterification,and cycloaddition of CO₂,a series of g-C₃N₄-based supported catalysts were designed and prepared,and then evaluated in the abovementioned catalytic reactions.The thesis included the following three sections.In the first part,exfoliated g-C₃N₄ material（eg-C₃N₄）was initially prepared by second calcination of g-C₃N₄,and then Fe Cl₃/eg-C₃N₄materials were fabricated by a wet impregnation method.The physicochemical properties of the Fe Cl₃/eg-C₃N₄ materials were analyzed by X-Ray Diffraction（XRD）,N₂ adsorption–desorption,and X-Ray Photoelectron Spectroscopy（XPS）techniques.The characterization results indicated that thermal exfoliation could significantly enhance the surface areas as well as pore volume of g-C₃N₄.Fe Cl₃ could be firmly supported on the surface of g-C₃N₄ via the coordination between Fe Cl₃and nitrogen in heptazine of g-C₃N₄.In the hydroxylation of benzene using H₂O₂ as an oxidant,the Fe Cl₃/eg-C₃N₄ catalysts demonstrated high catalytic activities.The maximum of phenol yield was up to 22%under60°C of reaction temperature and 3 h of reaction time.Combining the characterization results and catalytic performances,a possible catalytic mechanism was proposed.Next,mesostructured g-C₃N₄（mpg-C₃N₄）materials were synthesized using cyanamide as a precursor by a silica-templating approach.The research work indicated that Na OH solution could not only eliminate the silica template,but also improve the in-plane condensation of graphitic sheets of g-C₃N₄ and thus increase the percentage of bridging nitrogen species.The catalytic performances of mpg-C₃N₄ materials were examined in the transesterification of ethylene carbonate（EC）and methanol（CH₃OH）.The maximal conversion of EC and selectivity of the target molecule（dimethyl carbonate,DMC）were up to 70%,and 81%,respectively,under 100°C and 3 h of reaction time.The catalysts were able to be recycled and reused for five times.Combining the characterization results and catalytic performances,a possible catalytic mechanism was proposed.In the last section,ZrO₂/mpg-C₃N₄ catalysts were prepared using mpg-C₃N₄ as a catalyst support via a wet impregnation method.The physicochemical properties of the ZrO₂/mpg-C₃N₄ materials were analyzed by XRD,N₂ adsorption–desorption,Fourier Transform Infrared Spectroscopy（FT-IR）,and X-Ray Photoelectron Spectroscopy（XPS）techniques.The catalytic performances of ZrO₂/mpg-C₃N₄ materials were evaluated in the cycloaddition of CO₂.The catalytic results revealed that ZrO₂/mpg-C₃N₄ could catalyze the cycloaddition reaction of CO₂.As the CO₂ pressure was 2.0 MPa,reaction time was 6 h,and reaction temperature was 140°C.The maximal cvonversion of propylene oxide（PO）and selectivity of propylene carbonate were 54%,and 99%,respectively.