Carbon Nitride-based Photocatalysts:Design,Synthesis And Applications For C-H Bond Activation

The controllable oxidation of alkanes to more valuable organic chemicals is a challenge in academic research and industrial production.Although a number of traditional methods had been developed for the activation of alkanes,they require severe reaction conditions,and suffer from over-oxidation,low selectivity and low controllability of the reaction process,which ultimately lead to low yields,high cost,and difficulties in separation.The photocatalytic approach,with its attractive evironmental sustainability and ease of operation,is considered a promising alternative for this process.Photocatalysts are the vital component of the catalytic process,and can be catagorized into homogeneous and heterogeneous catalysts.The former is higher in activity,but difficult to separate from the products.The latter is cheaper and readily separatable for reuse,which is preferred in real applications.Among the numerous existing photocatalysts,carbon nitride?g-C₃N₄?as a metal-free semiconductor,has drawn much attention owing to its desirable visible-light response chemical stability,and low cost with earth-abundant carbon and nitrogen elements.However,its photocatalytic efficiency is still limited due to the moderate bandgap as well as fast recombination of its photoinduced charge carriers.Therefore,the development of economic,green and efficient C₃N₄-based photocatalysts that can transform inert alkanes is significant and highly desirable.In this thesis,the methods of doping non-metal atoms,controlling morphology,constructing heterojunctions and loading cocatalysts on the surface were adopted respectively to prepare heterogeneous photocatalysts for photocatalytic selective oxidation of alkanes to ketones,aldehydes and phenol,and have achieved satisfactory catalytic activities.Moreover,the physical and chemical properties of the as-prepared catalysts were studied through a variety of characterization techniques such as XRD,XPS,SEM,TEM,FT-IR el at.The relationships between strucutures and catalytic activities were also investigated.The main efforts and innovative results are listed as follows:1)Using the phosphorous acid-assisted hydrothermal method,partial hydrolysis of melamine into cyanuric acid led to the formation of melamine-cyanuric acid supramolecular precursor.By controlling the H₃PO₄ amount and thermal treatment temperature,the phosphorus-doped C₃N₄ with nitrogen vacancy?P-V_N-C₃N₄?were obtained from supramolecular precursor.It was found that the optimized condition for getting the P-V_N-C₃N₄ composite is:550°C with 9.8 g of H₃PO₄.Excellent benzaldehyde production rate of 958.8?mol g^-1 h^-1 at a selectivity of up to 94%in the selective aerobic oxidation of toluene was achieved at room temperature.It was demonstrated that the incorporated P and N in g-C₃N₄ could significantly enhance the triplet-exciton yield and the oxidizability of the valence band.2)The precursors of unstable helical AgNO₃/melamine belts could be twisted in a certain direction by controlling the pH values and the generation of BiVO₄ under hydrothermal treatment,and eventually led to the formation of Ag/C₃N₄/BiVO₄composite with different nanoarchitecture.In weak acidic or strong alkaline hydrothermal treatment,the Ag/C₃N₄/BiVO₄ composite was a hexagonal structure.The helical Ag/C₃N₄/BiVO₄ column could be obtained under weak alkaline conditions.We evaluated helical Ag/C₃N₄/BiVO₄ for the selective aerobic oxidation of C–H bonds,and achieved nearly 100%selectivity to target product.Benefited from its unique structural features?BiVO₄ spiraled on top of the helical nanostructure of sliver and carbon nitride?,the helical Ag/C₃N₄/BiVO₄ exhibited outstanding ability for the separation of photogenerated carriers as well as light absorption,enabling enhanced alkane conversion.In addition,the helical Ag/C₃N₄ provided suitable sites for the photocatalytic oxidation of alcohols to aldehydes and ketones,endowing BiVO₄/Ag/C₃N₄ with excellent selectivity in alkanes oxidation3)The release of CdS,cyanuric acid?CA?,and sulfur by partial decomposition of octahedral Cd₃?C₃N₃S₃?₂,and resulted in the generation of melamine-cyanuric acid supramolecular that wrapped Cd₃?C₃N₃S₃?₂ through hydrogen bonding.By varying the hydrothermal temperature,time and NaOH amount,the hollow,porous and double-shell octahedron of CdS@C₃N₄ composition?2-CSCN?was successful fabricated by several crystallization and calcination processes.Using 2-CSCN as the photocatalyst and O₂ as the oxidant,the satisfactory catalytic activity(benzaldehyde production rate of 475?mol g^-1 h^-1 at a selectivity of up to 95%.)in the selective aerobic oxidation of toluene was achieved at room temperature.Owing to the porous and hollow double shell structure,the light absorption and the separation of photoinduced charge carriers was significantly enhanced.Moreover,the heterostructures of CdS with C₃N₄ and core-shell structure prevented the self-photooxidation of CdS and thus resulted in high photocatalyst stability.4)The traditional generation of benzophenone methods are often costly and environmentally-consuming.A novel and efficient route for the preparation of aryl ketones by photocatalysis at mild conditions has been developed.In general,the melamine molecules are bonded to the Ag atom through the ring nitrogen,forming asymmetric N–Ag–N chains.Then,the added cyanuric acid reacts with melamine to form a supramolecular structure that wrapped silver nitrate.With the addition of Co?NO₃?₃ and subsequent calcination,porous C₃N₄ nanorods decorated with spatially separated Ag and Co₃O₄ nanoparticles on the interior and exterior surface of C₃N₄hollow sphere?ACNC?was obtained.Using this porous material as photocatalyst and O₂ as oxidant,aryl ketones could efficiently generate?with a yield up to 75.7%and selectivity to 97%?via toluene and halobenzene under visible-light irradiation at room temperature.5)The partial decomposition of spindle Zn₃?C₃N₃S₃?₂ precursor lead to the formation of zinc sulfide,cyanuric acid?CA?,and sulfur under strong alkali condition.By selecting the hydrothermal temperature,time and NaOH amount,the added melamine will rapid promote the polymerization of melamine between cyanuric acid and melamine by intermolecular hydrogen bonding,and finally leading to the formation of a flower-like protective layer on the Zn₃?C₃N₃S₃?₂.After several crystallization and calcination processes,the flower-like and double-shell ZnS@C₃N₄composition?2-ZSCN?were successful fabricated.2-ZSCN exhibited enhanced photocatalytic activity and stability in both the selective aerobic oxidation of amines to imines and benzene to phenol under visible light irradiation.The enhanced efficiency could be attributed to the efficiently facilitated charge carriers by the formation of the novel flower-like and double-shell structure.6)Using the structural characteristics of Bi₂WO₆ parallel growth along the?001?plane,the hierarchical Bi₂WO₆/CdWO₄ composite?BCW?were prepared by adjusting the concentration of Bi³⁺,which influenced the growth of Bi₂WO₆ on the surface of CdWO₄.When the molar ratio of Bi₂WO₆ to CdWO₄ is 4:10,CdWO₄ microrods were decorated with Bi₂WO₆ nanoplates that were uniform in size and shape.Using BCW-4as the photocatalyst and O₂ as the oxidant,a satisfactory catalytic activity?benzene conversion at 7.3%and selectivity of phenol>99%?in the hydroxylation of benzene to phenol was achieved at room temperature.It was found that the unique hierarchical heterostructure is beneficial for the absorption of visible light and the separation of photoinduced charge carriers.