Synthesis Of Nitrogen-doped Manganese Oxide And Its Catalytic Oxidation Of Hydroxyl

Many variable valence states of transition metals play a vital role in heterogeneous catalytic reactions.Among them,manganese oxide（MnO_x）,as the most typical transition metal oxide catalyst,exhibits good catalytic activity in many catalytic oxidation reactions.At the same time,constructing new catalytic active sites by doping heteroatoms to form a synergistic relationship with metal atoms to improve the catalytic performance of metal oxides has become a new research hotspot.Since nitrogen（N）atoms have similar electronic structure and atomic radius to oxygen（O）atoms,the nitrogen-metal（N-M）bond is highly stable.Therefore,it has potential to regulate the catalytic performance of metal oxides by nitrogen doping.This thesis mainly studies the N-doped MnO_x catalyst（N-MnO_x）,using XRD,SEM,TEM,EDS-mapping,BET,XPS,H₂-TPR,XANES to study the detailed characterization of the catalyst structure,and obtain the N sites in N-MnO_x And the regulation mechanism of oxygen vacancy sites;taking the oxidation reaction of 5-hydroxymethylfurfural（5-HMF）and p-methylbenzyl alcohol as a model,combined with in-situ infrared characterization methods,the correlation between the reaction sites and catalytic activity of the catalyst was studied.The main research results obtained are listed in the following.（1）Synthesis of nitrogen-doped manganese dioxide（N-MnO₂）catalyst by hydrothermal method using Urea as N sources.Under the action of surfactants,high-temperature crystallization forms a spherical flower-like catalyst N_x-MnO₂（X is the amount of urea doped with different contents）.Through XRD and BET characterization,it is determined that the catalyst is beta-type MnO₂,and its specific surface area is 103.4-118.5 cm²/g,and the mesoporous pore volume is 0.25-0.26 m³/g,which is relative to commercial MnO₂ without doped N（specific surface area is 1.9 cm²/g）,the specific surface area of MnO₂ is increased 54-62 times after doping with N atoms.（2）Studies on the oxidation of 5-HMF to 2,5-diformylfuran（DFF）show that the selectivity and yield of DFF over N-MnO₂ at room temperature are as high as 99.9%.The as-synthesized N-MnO₂ also exhibit excellent recyclability.According to XPS characterization,it is found that oxygen vacancies are greatly increased after MnO₂ was doped with heteroatom N.XANES data shows that nitrogen doping makes the Mn-O bond slightly elongated and the Mn-O coordination number decreases,which leads to the generation of unsaturated manganese sites in the crystal,and sequent increases the catalyst surface defects and improves the catalytic performance.The catalytic performance of N-MnO₂ is much higher than that of MnO₂ due to its abundant oxygen vacancies.（3）Using oxidation of p-methylbenzyl alcohol as model reaction,it is found that the catalytic performance of N-MnO₂was correlated with its site of N dopants.Studies on different calcining temperature show that,interstitial N dopants are formed in N-MnO₂ at the calcining temperature of 300-400℃,while both interstitial N and substitutional N dopants are formed when the calcining temperature is greater than 600℃.Compared with the substitutional ones,N-MnO₂ catalysts with interstitial N doping show better catalytic performance for alcohol oxidation.And the alcohols can fully convert to the corresponding aldehydes or ketones over interstitial N doped N-MnO₂ catalysts at room temperature.The better catalytic performance of interstitial N doped N-MnO₂ catalysts than those of substitutional N doped ones could be ascribed to their more abundant oxygen vacancies.Basis of the above analysis,it can be concluded that the catalytic performance of N doped MnO_x catalysts is greatly improved in the oxidation reaction after heteroatoms N doping.Considered of no decay at catalytic activity after many cycles,it is suggested that the N-MnO_x are potential catalysts for the oxidation of hydroxyl groups from biomass and aromatic alcohols.