Selective Oxidation Of Toluene To Benzaldehyde Under Mild Conditions

Benzaldehyde, as an important synthetic flavoring agent approved by the U.S. Food and Drug Administration for food and perfume additives, has also been widely used in the fields of medicine, pesticides and dyes manufacture, etc. In China, chlorine-free benzaldehyde is in short supply and its marketing demand grows at an average rate of 7% annually, approaching approximately 30 kt in 2015. Refined from petroleum or separated from tar, toluene is cheap, easily available and economically potential. Therefore, conversion of toluene to value-added benzaldehyde and its derivatives has attracted global interest.Nowadays, benzaldehyde is mainly produced by the hydrolysis of benzal chloride in China. However, considerable use of Cl2 results in complicated process, environmental problems and failure to be applied as food additives due to chlorine residual. In recent years, benzaldehyde has mainly been prepared by direct liquid oxidation of toluene with air, excluding chlorine from the products. However, benzoic acid is the main product and benzaldehyde is just a by-product with low yield at high pressure and high temperature. On one hand, liquid oxidation of toluene is a radical chain reaction, and radicals are prone to binding rapidly at high temperature, which produce copious byproducts such as benzene, biphenyl, methyl biphenyl, dimethyl biphenyl, phthalate, benzyl acetate and benzyl benzoate. On the other hand, benzaldehyde, which is more oxidative than toluene is, is mostly oxidized to benzoic acid, so the reaction is of low selectivity. Under normal conditions, the selectivity of benzaldehyde decreases with rising toluene conversion, on which high temperature exerts adverse effects. Generally, liquid phase oxidation of toluene is a radical chain reaction affected by many factors. Allowing high benzaldehyde selectivity at high toluene conversion seems very difficult with common methods. Notably, jet reactor can accelerate the reaction by greatly enhancing the mass transfer process, so it is possible to augment the conversion and selectivity simultaneously for toluene oxidation under mild conditions in a suitable reaction system.In this thesis, mild conditions for toluene oxidation in a jet reactor were found. Pure oxygen at normal pressure was used as the oxidant, thus the reaction could be carried out at below 100℃. Other reaction conditions (including catalyst formulations, gas flow rate and solvents, etc.) were optimized to fit the reactor design. The results provide evidence and guidance for toluene oxidation in the jet reactor. This thesis mainly focused on the following aspects:(1) Toluene was selectively oxidized to benzaldehyde over Co/Mn/Br in acetic acid solvent at 1 atm oxygen. The effects of catalyst type, amount of Co, mole ratio of Co/Mn and amount of Br on toluene oxidation were studied to determine the optimal ratio of Co/Mn/Br. Thereafter, the influences of temperature, the amount of acetic acid, gas flow rate and water content on the selectivity and yield of benzaldehyde were discussed in detail. The optimum reaction conditions were as follows:catalyst, Co(OAC)2/MnSO4-Mn(OAC)2/C2H2Br4; reaction temperature, 98℃; oxygen flow rate,60mL/min; nCo/ntoluene=3%; nBr/nco=0.24; nco/nMn=3; n(MnSO4):n(Mn(OAc)2)=1:1. Under the conditions,48.69% of toluene was converted and the yield of benzaldehyde reached up to 21.02%.(2) In order to reduce the corrosion of the solvent acetic acid and to increase the yield of benzaldehyde, acetic anhydride, ethyl acetate and butyl acetate were added into the original system respectively, of which acetic anhydride functioned as the best co-solvent. The role of acetic anhydride in the reaction was investigated and the reaction conditions were optimized. Under these conditions, the selectivity and yield of benzaldehyde reached as high as 68.69% and 26.12%, respectively.(3) A simple and effective method for catalyst recovery was also developed, which had the advantages of short recycling process, low catalyst loss and no byproducts. Moreover, after addition of bromide, the recycled catalyst still managed to catalyze the toluene oxidation well.