Regulation Of Performance And Preparation Strategy Of Heterogenerous Photocatalysts For Aromatic Alcohols Selective Oxidation

The carbonyl compounds（aldehydes and ketones）obtained by the conversion of functional groups of alcohol hydroxyl groups are widely used in chemical production,but the traditional preparation process may cause side reactions and low selectivity due to high temperature,high pressure and strong oxidants.With the development of science and technology,people are facing increasingly serious problems of environmental pollution and energy crisis.An important research direction of chemical industry development in the future is to develop a synthesis process of green,environmental protection,non-toxic and energy saving.Photocatalytic organic conversion is a new green catalytic method in organic synthesis reactions due to its mild reaction conditions.The photo-induced selective oxidation of alcohol with oxygen as oxidant is in line with the requirements of environmental protection policy and green chemistry development,but the low catalytic activity and slow reaction rate of photocatalyst are the scientific problems to be solved in photocatalysis.Therefore,it is of great significance to explore environmentally friendly,stable and efficient photocatalysts for photocatalytic conversion.Graphite phase carbon nitride polymer（g-C₃N₄）has become a hot spot in photocatalysis research because of its non-toxic,cheap,simple synthesis process,suitable band gap and stable physical and chemical properties.The main purpose of this thesis is to solve the problems（such as low selectivity and low conversion rate）in the process of photocatalytic aromatic alcohols selective oxidation.The modification of photocatalytic materials can regulate photo-absorption efficiency and photogenerated charge recombination efficiency to improve the catalytic activity.Firstly,an inorganic metal photocatalyst bismuth tungstate（Bi₂WO₆）modified by oxygen vacancy was designed to improve the photocatalytic efficiency of aromatic alcohols.However,Bi₂WO₆ has some disadvantages such as layered perovskite structure,high raw material cost,low conduction band position and limited band gap regulation,which limtis the development of its diversity.Therefore,cheap,non-toxic,simple synthesis process,adjustable band gap and good stability of organic semiconductor g-C₃N₄ was selected as photocatalytic material,and a series of g-C₃N₄ photocatalysts with porous structure were synthesized though structure design,element doping and other strategies.Under the irradiation of simulated sunlight,the photocatalytic activity and product selectivity of photocatalytic aromatic alcohols selective oxidation were systematically studied.The main contents and results of this thesis are listed as follows:（1）The flower-like bismuth tungstate（Bi₂WO₆-C/S-x）with oxygen vacancy was synthesized by the strategy of negative and cationic double surfactants.The growth of Bi₂WO₆was controlled by micelles formed by anionic and cationic surfactants in aqueous solution,so as to regulate the morphology and surface defects of Bi₂WO₆.The results show that the presence of oxygen vacancy expands the photoresponse range,and can capture electrons to promote photoelectron-hole separation,and finally realize the improvement of its photocatalytic performance.In the experiment of photocatalytic benzyl alcohol selective oxidation,the photocatalytic efficiency of benzyl alcohol is highest when the molar ratio of cationic surfactant to anionic surfactant is 2（Bi₂WO₆-C/S-2）.And the conversion of starting material benzyl alcohol reaches 91.5%after 12 hours of simulated solar radiation,selectivity of the product benzaldehyde is over 99.0%.Additionally,Bi₂WO₆-C/S-2 showed good photocatalytic stability during five cycles of reactions.After the fifth cycle reaction,the conversion of benzyl alcohol still reaches 86.0%,and the seleselectivity of benzaldehyde is more than 99.0%.（2）Due to the problems of high coat,limited band gap adjustment and fixed lamellar structure of inorganic metal Bi₂WO₆,cheap,adjustable band structure,simple synthesis process and non-toxic organic semiconductor g-C₃N₄ was selected as the photocatalytic material.In this chapter,the synthesis strategy of hydrogen bond self-asembly was adopted to obtain rhe coral-like porous g-C₃N₄ photocatalyst with nitrogen vacancy（CN coral-like-x,x is the calcination temperature）through the formation of hydrogen bond between melamine and cyanic acid for intermolecular self-assembly.The results showed that CN coral-like T550 had porous structure,high specific surface area(123.7 m²·g^-1)and nitrogen vacancy.The porous structure can inhibit the recombination of photogenerated charge,the high specific surface area can increase more active aites and adsorb more reaction substrates in the photocatalytic reaction process,and the nitrogen vacancy can act as electron capture center to accelerate the separation and migration of photogenerated charge.These characteristics can effectively improve the photocatalytic performance of g-C₃N₄.In the photocatalytic selective oxidation experiment of benzyl alcohol,CN coral-like-x shows better catalytic efficiency than conventional g-C₃N₄,especially CN coral-like-T550.The conversion rate of benzyl alcohol reaches 91.6%after 12 h of simulated sunlight irradiation,which is 3.2-fold that of bulk g-C₃N₄（29.1%）and 2.6-fold that of lamellar g-C₃N₄（35.4%）.In addition,CN coral-like T550 has better photocatalytic stability,the conversion of benzyl alcohol is still more than 90.0%after 5 cycles,benzaldehyde selectivity>96.0%.（3）Ammonium molybdate,uracil and melamine were chelated by hydrogen bond to form Mo（VI）as the center of the chelate precursor by a simple hydrothermal method.The transition metal Mo doped layered porous g-C₃N₄ phtocatalyst（CN-Mo-x）was designed.The vacant orbital of Mo（VI）is used as electron acceptor to capture photogenerated electrons.The results show that the doping of Mo expands the absorption range of light,narrows band gap,and accelerates the separation and migration of photogenerated charge.In the photocatalytic selective oxidation of benzyl alcohol,CN-Mo-2 showed better catalytic activity for benzyl alcohol.The conversion of benzyl alcohol reaches 58.9%after 12 h of simulated sunlight irradiation,which is 2.1 times that of bulk g-C₃N₄（28.7%）,and is 1.7 times that of undoped Mo lamellar g-C₃N₄（35.4%）,and the selectivity of benzaldehyde reaches 99.0%.（4）In order to improve the photocatalytic performance of porous g-C₃N₄ for aromatic alcohols,the cerium dioxide nanoparticles（Ce O₂-NPs）doped porous g-C₃N₄ photocatalysts（Ce-PCN-x）was designed by means of the interconversion between the mixed valence states of Ce³⁺/Ce⁴⁺in rare earth metal cerium.XPS analysis shows that the valence of Ce in Ce O₂-NPs is Ce³⁺and Ce⁴⁺.In the photocatalytic reaction process,Ce³⁺/Ce⁴⁺transform each other by capturing electrons,promoting the separation and transfer of photogenerated charges.In addition,the high specific aurface area(145.7 m²·g^-1)of the photocatalyst Ce-PCN-30 provides more active sites for the reaction and promotes the adsorption of substrates.Taking selective oxidation of benzyl alcohol as probe reaction,the conversion of benzyl alcohol catalyzed by Ce-PCN-30 reaches 74.9%after 8 h of simulated sunlight irradition,which is 1.3 times that of porous g-C₃N₄.Moreover,the conversion of benzyl alcohol reaches 93.2%after 12 h of simulated sunlight irradiation,and the selectivity of benzaldehyde exceeds 98.0%.（5）The phenyl was successfully doped into basic structural unit heptaazine ring of g-C₃N₄by means of copolymerization,and then the porous g-C₃N₄ was modified from the chemical composition,so as to prepare the phenyl-doped porous g-C₃N₄ photocatalyst（CN-P_x）.the results show the doping of phenyl improves the absorption efficiency of visible light and promotes the separation and transfer of photogenerated charge.The photocatalyst CN-P_0.3shows excellent photocatalytic performance for the benzyl alcohol selective oxidation.After 8h of simulated sunlight irradiation,the conversion of benzyl alcohol reaches 94.5%,which is2.1-fold that of single porous g-C₃N₄（44.3%）,and the selectivity of benzaldehyde overs 99.0%.Additionally,CN-P_0.3 has high catalytic stability,and the photocatalytic efficiency hardly changes after 5 cycles.