Heterogeneous Liquid-Phase Oxidation Of Toluene To Benzaldehyde

In this study,liquid-phase aerobic oxidation of toluene catalyzed by Mn-Mo oxide was conducted in a 1.0 L batch reactor.The macroscopic kinetics of toluene consumption and benzaldehyde generation at 140¹70?were obtained from a combination of experimental observation and hypothetical models.The results showed that both the oxidation rate of toluene and generation rate of the aromatic product were proportional to the concentration of the substrate,the partial pressure of oxygen and the surface area of the catalyst.The energy barrier of toluene oxidation to benzyl alcohol was the highest(?81 kJ·mol^-1),while that of benzyl alcohol oxidation to benzaldehyde was the lowest(?57 kJ·mol^-1).Moreover,the activation energy of further oxidation of benzaldehyde in an acetic acid solvent was only slightly lower(?1.9 kJ·mol^-1)than that of toluene oxidation.Significantly,the transformation of benzyl alcohol indeed contributed to the generation of benzaldehyde and this step conformed to a first-order parallel-consecutive model.Increased reaction temperature and residence time favored the transformation of benzyl alcohol to benzaldehyde.In addition,doping with molybdenum at Mn/Mo=3/1 enhanced the catalytic performance of the heterogeneous catalyst and was attributed to the presence of a synergetic effect between the different metal cations.Regarding the microscopic kinetics,the LH-OS-ND mechanism?Langmuir-Hinshelwood adsorption of reagents on the same type of active sites and non-dissociative adsorption of oxygen?was verified as responsible for the heterogeneous oxidation of toluene.Oxygen and benzaldehyde were weakly adsorbed(?H_ads,Oxy=-15 kJ·mol^-1,?H_ads,Bald=-30 kJ·mol^-1)but showed strong mobility(?S_ads,Oxy=-22 J·mol^-1·K^-1,?S_ads,Bald=-39 J·mol^-1·K^-1).The fundamental intrinsic rates were deduced based on the LH-OS-ND mechanism and showed great consistency with the macroscopic results.In addition,a systematic study on the synthesis of the Cu_xMn_3.66-xMo₃O₁₂?CMMO?using citrate sol-gel technique is presented in this work.The influence of the copper substitution,citric acid concentration and pH of the precursor solution on the exclusive formation of Cu-Mn-Mo polymetallic oxide are studied.The physicochemical properties of as-prepared oxides are characterized by XRD,ICP-MS,SEM,TEM,XPS,FT-IR,BET,and H₂-TPR,and their dependence on the conditions of synthesis is explored.It is found that these three synthesis factors remarkably alter the crystal phases and surface structures.The catalytic behaviors of synthesized catalysts for liquid-phase toluene oxidation are evaluated with toluene-catalyst mixture under uniformly distributed conditions.The evaluation reveals that the ratio of Mn³⁺/Mn²⁺as well as O_ads/O_lat on catalysts play a crucial role in the activity and selectivity.Factors effecting reaction parameters,such as reaction temperature,residence time,catalyst weight,promoter weight,airflow rate and the volume ratio of substrate to solvent,are investigated in detail.The reusability of the catalyst is examined through conducting repeat runs with the same catalyst,exhibiting no significant decline in its activity even after 8 runs.A toluene conversion of 60%with 57.4%selectivity toward benzaldehyde is observed after 4hours over the high-crystalline CMMO nanoparticles.