Synthesis Of Nanostructured Manganese Oxides And Their Catalytic Activity In Esterification

Nanostructured manganese oxides are of considerable importance in technological applications including ion-exchange, molecular adsorption, catalysis, as well as electrochemical and magnetic uses, due to their outstanding structural flexibility combined with novel chemical and physical properties. However, few studies have been conducted on the catalysis of manganese oxides for esterification. In this paper, we introduce the catalytic synthesis of isoamyl acetate with manganese oxides as catalyst, to our knowledge, which has not been reported yet. Nanostructured manganese oxides are proved to have high catalytic efficiency and good selectivity for esterification.This paper focuses on synthesis of nanostructured manganese oxides and their catalytic activity in esterification.β-MnO2 nanorod catalyst was prepared by calciningγ-MnOOH nanorod precursor. The sample was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Brunauer– Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distribution. TEM image shows thatβ-MnO2 sample is composed of nanorods with diameters of 50~200 nm and lengths of tens of micrometers. Its surface area is 52.297 m2/g, total pore volume reaches 0.32395 cm3/g and pore sizes mostly distribute between 5~60 nm. Compared with other manganese oxides,β-MnO2 nanorod catalyst achieves a 92.76% yield of isoamyl acetate under optimum condition: molar ratio of acetic acid to isoamyl alcohol is 1.8:1, catalyst dosage is 0.2% of total reagents weight, reaction temperature is 126℃and reaction time is 3.5 h. The mechanism and kinetics of the catalytic reaction are studied as well. Kinetic study indicates that this catalytic esterification reaction coupled with distillation and adsorption is a second-order reaction. The corresponding rate constant is 1.199×10-2 L·mol-1·min-1.To overcome the shortcomings of the manganese oxides with low surface area and non-uniform distribution, we loaded manganese oxides onto porous alumina. The obtained catalyst has a high surface area of 349 m2/g, total pore volume of 0.9755 cm3/g and pore size distribution between 0.17~30 nm. Energy dispersive X-Ray fluoresence spectrometer (EDX) test indicates that Mn element distributes evenly on the alumina support. The effects of different synthesis route to porous alumina, the amount of loaded Mn and the reaction parameters on the yield are investigated. The catalyst with 20% Mn(AC)2 onto Al2O3 (neutral route) has the highest activity. When catalyst dosage is 0.37% of total reagents'weight, reaction temperature is 126℃and reaction time is 3.5 h, the yield of isoamyl acetate reaches 97.28%. The kinetic study shows that this catalytic reaction is also a second-order reaction. The corresponding rate constant is 2.025×10-2 L·mol-1·min-1...