Preparation Of Cerium Manganese Composite Oxides And Its Catalytic Performance For Liquid Phase Selective Oxidation Of Toluene

The partial oxidation of toluene is an important method for the preparation of benzaldehyde（BAL）,benzyl alcohol（BOL）and benzoic acid（BAC）,which are widely used as raw materials in organic chemistry.BAL and BOL are important chemical intermediates with high added value,which are used in the fields of food,fragrance and other industries.Meanwhile,BAC is one of the important raw materials for the preparation of sodium benzoate preservatives,and the additives for drugs,dyes.Industrially,BAL is generally obtained by toluene chlorination hydrolysis,but the product obtained by this method contains chlorine,and high environmental pollution,which limited its extensive application.At present,the researches on toluene oxidation mainly focuses on the photo/electrochemical,vapor and liquid oxidation.However,photo/electrochemical catalytic oxidation and gas phase oxidation processes had problems,such as poor catalyst stability,low activity,harsh reaction conditions,and high production costs.Due to advantages of environmental friendliness,high atom economy and mild reaction conditions,liquid-phase selective oxidation of toluene is an important method for the preparation of oxygen-containing compounds such as aromatic alcohols,aromatic aldehydes and aromatic acid.Therefore,a solvent-and additive-free green catalytic system is constructed for the selective oxidation of toluene to BAL,BOL and BAC employing molecular oxygen as oxidant and cerium-manganese composite oxide as catalyst.The research conclusions are as follows:First,the sheet cerium-manganese composite oxides catalyst（Ce MnO_x）was prepared by ultrasonic-assisted coprecipitation method with cerium nitrate and manganese nitrate,and the reaction conditions were optimized.It showed that the catalyst（Ce:Mn=2:1,mol:mol）had preferable catalytic activity,and the manganese oxides（MnO_x）dominated by high valence were active species.7.0%conversion of toluene with 51.1%selectivity to BAL and BOL,and 95.6%total selectivity to three products when the amount of toluene was 21.0g（the subsequent amount of toluene is the same as this）,the temperature was 180°C,4.0 h reaction time,0.6 MPa oxygen pressure and 0.1 g catalyst.The catalytic performance of the catalyst remained basically unchanged after five recycled times.In addition,the relationship between the structure and performance of the catalyst was revealed by N₂adsorption-desorption,XRD,XPS,H₂-TPR,TEM,SEM-EDS-Mapping and FT-IR,and the active species of manganese oxide were highly dispersed in CeO₂.The possible mechanism of catalytic oxidation of toluene was speculated.In order to further improve the catalytic performance of cerium-manganese composite oxide catalyst,cerium and manganese active species were dispersed on the carrier,and CeO₂-MnO₂/NC catalyst was prepared by citric acid complexation method.The catalytic performance of different metal composite oxide catalysts was investigated,and the results showed that CeO₂-MnO₂/NC exhibited good catalytic activity.As the temperature was 180°C,0.1 g catalyst,0.9 MPa oxygen pressure and1.5 h reaction time,the conversion of toluene was 7.1%,the selectivity to BOL and BAL was 59.5%,and the total selectivity to three products was 95.4%.The stability of CeO₂-MnO₂/NC was tested,and the results showed that the catalyst was good reusability.Various characterization methods,such as XRD,N₂ adsorption-desorption,FT-IR,UV-Vis DRS,H₂-TPR,TEM and SEM-EDS-Mapping were used to analyze the microstructure and physicochemical properties of the catalyst,and MnO₂ were active species in the catalyst,the structure-activity relationship of the catalyst was revealed.In addition,a spongy CeO₂-MnO_x catalyst was prepared by improved sol-gel method.It showed that CeO₂-MnO_x（Ce:Mn=1:1,mol:mol）had good catalytic performance,and the toluene conversion was 6.9%,the selectivity to BAL and BOL was 64.4%,and the total selectivity to BAL,BOL and BAC was 96.8%as the temperature was 180°C,0.1 g catalyst,1.0 MPa oxygen pressure,2.0 h reaction time,the catalyst showed good stability.Series of characterization（N₂adsorption-desorption,ICP,XRD,H₂-TPR,FT-IR,XPS,UV Raman,SEM and TEM-Mapping）were used to investigate the structure of the catalysts.Combined with the experimental results,it showed that the active species of MnO_x（mainly manganese trivalent）were highly dispersed in CeO₂,and the redox pair formed by trivalent and divalent Mn in the catalyst had the synergistic catalysis effect.Meanwhile,the existence of trivalent Mn could also inhibit the further oxidation of BAL and BOL,and the possible catalytic reaction mechanism was also speculated.Finally,spindle shaped MnO_x/Ce（OH）CO₃ composite oxides catalyst was prepared by water thermal-redox method.It showed that the temperature was 180°C,0.1 g catalyst,0.9 MPa oxygen pressure,2.0 h time,the conversion of toluene was6.7%,the selectivity to BOL and BAL was 69.7%,and the total selectivity to three products was 94.9%.The characterization techniques（N₂ adsorption-desorption,ICP,XRD,XPS,H₂-TPR,UV Raman,SEM-EDS-Mapping,TEM,FT-IR,UV-Vis DRS,TG-DTG）were used to study the microstructure of the catalyst,the results showed that MnO_x species were uniformly dispersed in basic cerium carbonate（Ce（OH）CO₃）,and the redox pairs formed between the multiple valence（Mn（+4）,Mn（+3）and Mn（+2））of Mn had the synergistic catalysis effects,which further improved the catalytic performance of the catalyst.Density functional theory（DFT）was used to calculate the adsorption energy of toluene on the active species of the catalyst,it showed that toluene preferentially adsorbed on Mn active species,and the possible reaction mechanism was speculated.In summary,a novel cerium manganese composite oxide catalyst with low-cost,high catalytic activity,good heat resistance and stability was constructed,and used for the selective partial oxidation of toluene to BOL,BAL and BOL.This process has the advantages of simple route,high selectivity of target products,green and cheap catalyst with stable performance,which has potential in the industrial application.