Aerobic Liquid Phase Oxidation Of Xylene Catalyzed By Co-O/SiO₂

The selective catalytic oxidation of xylene and the application of Cobalt oxide and its correlative species as oxidation catalysts are reviewed in this dissertation. Based on the judgement that the research of aerobic liquid phase oxidation of xylene catalyzed by Cobalt oxide are not developed heretoforely, we attempted to oxidize xylene using Co-O/SiO₂ as catalyst and molecular oxygen as oxidation reagent in the condition of liquid Phase and atmosphere pressure. The results indicated that Co-O/SiO₂ was provided with better catalytic quality and stability. The main aspects and discussion are presented as follows: 1.Preparation of catalysts: Using CoCl₂·6H₂O as precursor of active component and dilute NaOH solution as precipitating reagent, active component is supported on the different supports in proportion and calcined for 3 hours at certain temperature. 2.Performance of the catalysts are measured by oxidation reaction of p-xylene, The experiment results showed that the best catalyst is Co-O/SiO₂, which contains 0.12mmol/g cobalt and was calcined at 350℃(marked as 0.12 Co-O/SiO₂-350, the following markers are similar). Such catalyst is prepared easily , has better catalytic activity and,especially, relatively good stability. Even after the catalyst was reused for 4 times, catalytic activity dropped at low content. 3.Effects of reaction conditions(such as time,temperature,catalyst dosage, agitating rate, etc.) on catalytic performance of Co-O/SiO₂ are based on high conversion of xylene and high selectivity of toluic acid. Under appropriate conditions and without any additive ,the conversion of p-,m-,o-xylene are 47.4%,27.5%,40.2% respectively,and selectivity of p-,m-,o-toluic acid are 84.0%,72.3%,55.2% respectively. When appropriate content of sodium bromide was added, catalytic activity increased at some content4.By comparison,the conclusion can be draw that catalytic oxidation activity of xylene drop in order: p-xyleneï¹¥o-xyleneï¹¥m-xylene. The difference of the activities can be explained from stereo-electronic effect. 5.A series of Co-O/SiO2 catalyst samples were characterized by XRD and FT-IR techniques. The results indicated that the active components of the catalysts are cobalt oxide (mainly Co3O4) .The characteristic peaks of Co3O4 and SiO2 can be only observed in XRD pattern. The XRD patterns further indicated that Cobalt oxide disperses greater on 0.12Co-O/SiO2-350 than on 0.36Co-O/SiO2-350 and on 0.60Co-O/SiO2-350,no agglomeration is observed on the 0.12Co-O/SiO2-350. The intensities of the characteristic peaks of Co3O4 tend to increase at higher temperature, which indicates the increasing of crystallinity. As a result, the stability of catalyst was promoted. 6.Possible reaction mechanism for the oxidation of xylene is proposed ,the role of Co(â…¢) on the support is to initiate the chain reaction of xylene.