High Selective Synthesis Of Benzaldehyde By Liquid Phase Oxidation Of Toluene

As the simplest aromatic aldehyde,benzaldehyde is widely used in the synthesis of spices and seasonings,and its derivatives are also applied in the fields of medicine,pesticides,dyes and cosmetics.Benzaldehyde can be produced in several ways.The traditional toluene chlorination-hydrolysis process is complex and suffers greatly from heavy pollution.Also,the chlorine in the benzaldehyde cannot meet the requirement of food grade.Currently,high quality benzaldehyde is mainly produced from the oxidation of toluene,the reduction of benzoic acid and other methods.The liquid phase oxidation of toluene is now more attractive because toluene is cheap and aboundant,and the MC catalyst system is also much more mature.However,in the traditional oxidation process,the selectivity and yield of benzaldehyde are low.Moreover,the solvent acetic acid is highly corrosive,and the energy consumption of the subsequent separation process is high.Therefore,searching for alternative solvents for acetic acid and achieving high selectivity of benzaldehyde are two noteworthy issues in this field.This paper mainly investigated the catalytic oxidation of toluene in benzyl benzoate.Through the study of the reaction parameters(catalyst composition,temperature and gas flow),the technological conditions for selective oxidation of toluene under normal pressure were obtained.Benzyl benzoate and another ester were also applied for the oxidation process of other aromatics.The main contents of this research were as follows:1.The liquid phase oxidation of toluene to benzaldehyde with Co/Br catalyst and benzyl benzoate was studied.The effects of temperature,catalyst,initiator,solvent and gas flow on the oxidation of toluene were evaluated.The optimal conditions were obtained as ntoluene/nbenzyl benzoate=0.50,nCo/ntoluene=2.5%,nBr/nCo=1.2,oxygen flow rate=60 mL/min,temperature=110℃.Under these conditions,the conversion of toluene reached 41.2%,and the yield of benzaldehyde was 14.2%.The benzyl benzoate system had large reaction elasticity,high initial activity,and the generated benzaldehyde was more stable.The stability of benzaldehyde could be explained by the synergistic effect of the system components.2.When benzyl benzoate and other esters were used for the oxidation of p-tert-butyl toluene and p-methoxy toluene,the byproducts in the system could not be ignored.At the same time,it was difficult to find the similar synergistic effect.The oxidation of other aromatics still needed to rely on the acetic acid solvent.3.Starting from the protection mechanism of benzaldehyde,a new system of high selective synthesis of benzaldehyde from toluene was set up.In this system,the conversion of toluene was 33.8%after 10h,the yield of benzaldehyde was 23.6%and the highest selectivity of benzaldehyde could reach 79.2%.When co-catalysts were added to the new system,the selectivity of benzaldehyde would further increase to 89.8%.