Oxygen-modified Defective Carbon-based Catalysts For Baeyer-Villiger Oxidation Of Cyclic-ketones

Polymer materials have become an indispensable part in our modern live because of unique advantages.However,it takes hundreds of years for traditional plastics to be completely degraded in their natural state,which will seriously jeopardize the global water ecosystem.Therefore,open up degradable materials into people’s field of vision.Polyε-caprolactone（PCL）is becoming more and more popular since it has good degradability and biocompatibility.The key to the production of PCL lies in the synthesis of the monomerε-caprolactone.At present,the industry mainly preparesε-caprolactone through the oxidation of cyclohexanone via Baeyer-Villiger（B-V）reaction process.In recent years,people have developed a variety of processes for cyclohexanone B-V oxidation,such as peroxyacid method,direct/indirect H₂O₂ method,biological enzymatic method and molecular oxygen/aldehyde system method.The peroxyacid method has the disadvantages of high safety hazards and a lot of waste acid.The direct oxidation ability of H₂O₂ is weak,forming an explosive ketone dimer.The indirect method is prone to product hydrolysis,poor H₂O₂ stability,and complicated technical routes,which limits its large-scale production.Biological enzyme oxidation method is expensive and easy to inactivate.Molecular oxygen is cheap and easy to obtain,safe and green,and has high atomic economy,which is very attractive for industrial applications.Using the environmentally friendly molecular oxygen/aldehyde oxidation system,high-efficiency activation of oxygen is the core issue.Therefore,it is very important to develop a cheap and easy-to-obtain catalyst with high catalytic activity and good stability.This dissertation mainly studies the preparation ofε-caprolactone by B-V oxidation of cyclohexanone under the molecular oxygen/benzaldehyde system.A simple pyrolysis method is used to successfully construct a new type of metal-modified nitrogen-doped carbon material catalyst.The structure-activity correlation and reaction mechanism was proposed and verified after combining the catalytic performance and catalyst characterization results.The main results are divided into the following parts.1.Construct a model Ni-Zn（2:1）/O-NC catalyst through pyrolysis of the melamine-nickel/zinc metal salt mixture.The effect of the types of metal salt precursors and the metal doping with different nickel/zinc molar ratios on the catalyst was explored.The changes in the catalyst before and after the H₂O₂ treatment were compared through XRD,Raman,TG,XPS,BET characterizations.The multiple effects of the H₂O₂ treatment were summarized,（a）Eliminate amorphous carbon and increase the degree of graphitization;（b）Etch carbon materials to increase and optimize catalyst pore structure;（c）Introduce（-C=O）,（-OH）,（-COOH）and other oxygen-containing groups on the surface of the catalyst to enhance the hydrophilicity of the catalyst.2.Carry out catalytic activity evaluation and reaction mechanism study.The reaction conditions,such as the dosage of catalyst,reaction temperature and time,solvent type,and mole ratio of aldehyde/ketone are optimized.The reaction was optimized with the following conditions,the dosage of catalyst is 50 mg,the reaction temperature is 45 ℃,using1,2-dichloroethane as solvent,and 2 equivalents of benzaldehyde as co-oxidant.The conversion rate of cyclohexanone is 99.5%after reaction for 12 h,and theε-hexane lactone selectivity is greater than 99.0%.On this basis,through oxygen source exploration experiment,free radical control experiment,EPR test and product qualitative analysis,a reasonable reaction mechanism is given.Benzaldehyde self-oxidative dehydrogenation generates benzoyl radicals（C₆H₅CO·）,activated oxygen molecules are inserted into C₆H₅CO·to generate benzoyl peroxy radicals（C₆H₅COOO·）,and C₆H₅COOO·snap the hydrogen of benzaldehyde molecules to form peroxybenzoic acid,peroxybenzoic acid attacks cyclohexanone to undergo a nucleophilic addition reaction to form a Criegee adduct,the adduct decomposes to produceε-caprolactone and benzoic acid.We believe that the first step of the auto-oxidation of aldehydes is the key and the decisive step of the reaction.Based on the reaction mechanism,we proposed the multi-functional synergistic catalytic effect of the catalyst,（a）Nickel-based sites combined with molecular oxygen activates oxygen to peroxygen,（b）Carbon material buffers and stabilizes free radicals in favor of the benzoyl radical chain growth reaction,（c）Zinc sites plays the role of Lewis acid,electrophilic activation of cyclohexanone carbonyl facilitates nucleophile（peroxy acid）to attack the carbonyl carbon,（d）Modified oxygen-containing group plays the role of Br（?）nsted acid,nucleophilic activation of Criegee addition.It promotes its decomposition to form lactone products.3.Systematic optimization of catalyst preparation process.Using Ni-Zn（2:1）/O-NC as a model catalyst and cyclohexanone B-V oxidation as a model reaction,we have explored the preparation process of the catalyst,such as pyrolysis temperature,transition metal loading,different carbon-based supports and hydrochloric acid activation.The results show that the pyrolysis temperature is 750 ℃,the metal loading is 30.0 wt.%,and the H₂O₂ oxidation modification is the optimal catalyst preparation process.In conclusion,this thesis provides a simple and inexpensive synthesis strategy for the preparation of metal-modified defective carbon material catalysts,proposes a multi-functional/multi-active sites synergistic catalytic effect,and advances the design concept of a multi-functional catalyst for cyclohexanone B-V oxidation.