Application Of Composite Metal Oxides In Toluene Oxidation Reaction

The toluene oxidation reaction has always been one of the most classic reactions among the selective oxidation of C-H.Its oxidation products have broad application in all walks of life.The current toluene oxidation process is mainly carried out by homogeneous catalysis.However,there are significant defects in homogeneous catalysis,such as excessive amounts,too high reaction temperatures and complicated separation processes.Therefore,exploring the environment-friendly catalyst and reusable heterogeneous catalytic toluene oxidation reaction system has become a long-term goal pursued by many researchers.In this paper,co-precipitation,in-situ precipitation and other methods were used to prepare composite metal oxides such as manganese-cerium,cobalt-chromium,copper-chrome and chromium oxide-supported cobalt hydroxide,which were then used to catalyze the oxidation of toluene.The optimal experimental results were obtained by optimizing the preparation and reaction conditions of the catalyst,and a series of characterizations of the catalyst were analyzed.The main research contents are as follows:（1）Manganese-cerium composite oxides with different molar ratios were preparedby co-precipitation method and used in toluene oxidation reaction.The experimental results show the best effect is achieved when the molar ratio of manganese to cerium is 1:1.The toluene conversion rate reached 23.4%;the main product was benzoic acid with a selectivity of 80.5%.The catalyst was characterized by various characterization techniques.The analysis shows the active site plays the main catalytic role in the reaction is Mn³⁺,it was found that the release of lattice oxygen in cerium can be accelerated and the flow of active oxygen on the catalyst surface can be facilitated after doping manganese into cerium.（2）Supported catalysts with different cobalt hydroxide loadings were prepared byin situ precipitation method and calcined at different temperatures to obtain cobalt-chromium composite metal oxides for toluene oxidation reaction.The experimental results show the uncalcined catalyst can obtain toluene conversion of37.8%and the selectivity of main product benzoic acid is 94.4%.The optimal data for the calcined catalyst is:toluene conversion of 10.7%,and the main product is benzaldehyde,selectivity of 61.9%.In order to explain this phenomenon,we used a variety of techniques to perform a more comprehensive characterizations of the two catalysts.The results show the high content of Co²⁺in the catalyst is the catalytic active center of the reaction,the acid-base and oxygen species concentrations on the catalyst surface can be changed by calcination,and the distribution of oxidation products can be adjusted accordingly.（3）Copper-chromium composite metal oxides with different molar ratios wereprepared by co-precipitation method and used in toluene oxidation reaction.According to a series of process conditions optimization,it can be seen that the molar ratio of copper to chromium is 1:1 shows the best experimental results,the conversion rate of toluene is as high as 49.1%;the selectivity of benzaldehyde is 50.7%.The catalyst was characterized in many aspects by a series of characterization techniques.The analysis shows the two phases of Cu Cr₂O₄and Cu O cooperative catalysis strongly promotes the reaction,and cause more lattice defects in the catalyst to lead an increase in the content of active oxygen,while the rich Cu²⁺increases the amount of acid on the surface of the catalyst,then achieve the purpose of improving catalytic activity.Acetonitrile,a solvent added in the reaction,can also promote the transfer of active oxygen decomposed by hydrogen peroxide and facilitate the reaction.