Study On Catalysts And Catalytic Activities In Liquid-Phase Oxidation Of Toluene

Toluene is the simplest substituted aromatic compound and its oxidation reaction is a typical example of the aromatic hydrocarbon oxidation. The toluene oxidation products, such as benzyl alcohol, benzaldehyde and benzoic acid, are important intermidiates in organic synthesis industry and widely used in pesticides, perfumes, pharmaceuticals, dyes industry and so on. At present, the liquid-phase oxidation of toluene by air is the commonly used process, which is environmentally friendly for its mild reaction conditions and good security. In this paper, the work is to investigate the catalytic activities in the liquid-phase oxidation of pure toluene without halogen and solvent by using Co(acac)2, Co/Mn,[ace-min]Co, CoSBA-15as catalysts separately and benzoic acid as initiator in a stirred tank reactor.Study on aerobic liquid-phase oxidation of toluene with Co(acac)2was investigated and the effects of operation parameters, such as reaction time, reaction temperature, Co concentration, benzoic acid addition were studied. A phenomenon of ’catalyst-inhibitor’transition was found for Co(acac)2catalyst like other Co salt catalyst. The appropriate range of Co(acac)2dosage is<0.011w%. Optimal operation conditions are reaction temperature165℃and Co concentration0.0044w%. At this situation, the life of catalyst can reach6h. There is an obvious induction period when the reaction system was absence of catalyst and benzoic acid. Co(acac)2catalyst has less catalyst dosage and longer lifetime than other Co salt catalysts like Co(OAc)2, etc., which could slow down the cobalt oxalate scaling during reaction. Therefore, Co(acac)2catalyst has a good application prospect as catalyst in liquid-phase oxidation of toluene.The catalytic activities of Co/Mn binary and Co/Mn/Zr ternary catalytic system in toluene liquid-phase oxidation were studied. The results show that the appropriate ratio of Co/Mn is1:1. In this situation, reaction selectivity increase50%compared with only cobalt catalyst. But the Co/Mn/Zr ternary catalyst system does not show the catalytic activities.On the basis of existed kinetic models, we took into account the toluene oxidation products, such as benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate, also considered the reverse reaction of benzyl benzoate and promotion of benzoic acid. A new reaction network was proposed, on which a kinetic model was established. The fitting results show that this kinetic model can agree well with the experimental data. Cobalt acetylcaetonate catalyst could reduce activation energy of the reaction of toluene oxidation and make more conducive to the reaction compared to cobalt naphthenate catalyst.[Ace-min]Co catalyst was synthesized successfully, characterized by1H-NMR and used as catalyst for toluene liquid-phase oxidation. The catalytic activity of [ace-min]Co catalyst used for the first time was the same as the Co(acac)2catalyst, but greatly reduced when the [ace-min]Co catalyst was used again. The deactivation of cobalt catalysts is due to the generation of COC2O4·2H2O. And the oxalate ions come from the catalyst and excessive oxidation of hydrocarbon. The precipitation of cobalt catalyst in toluene oxidation is inevitable.Mesoporous CoSBA-15molecular sieves were directly synthesized and characterized using XRD, N2adsorption. CoSBA-15materials were used as catalyst for toluene liquid-phase selective oxidation to benzyl alcohol and benzaldehyde using dioxygen as the oxidant. At160℃, the sum selectivity of benzaldehyde and benzyl alcohol is higher than86%.