The Preparation Of α-,β-MnO₂ Nanowires And Their Catalytic For Deep Catalytic Oxidation Of O-xylene

Volatile organic compounds (VOCs) which were widely applicated in the petrochemical, paint, coating, printing, pesticides, adhesives, pharmaceuticals and other industries were the most important pollutants in the air. Benzene pollutants are one of the VOCs. They caused serious environmental problems and hurt human health. Therefore, the removement of benzene pollutants has become an important research in recent environmental treatments. Deep catalytic oxidation that can directly oxidize benzene pollutants into CO₂ and H₂O is one of the most effective and promising research methods. In this paper a series ofα-MnO₂ andβ-MnO₂ manganese oxides nanowires were prepared using the hydrothermal method. The as-preparedα-MnO₂ andβ-MnO₂ were used for o-xylene deep catalytic oxidation. The catalytic activities were evalued in term of o-xylene conversion and the yileld of CO₂. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller(BET) were used to characterize the catalysts. The details contents were as follows:(1)α-MnO₂ nanowires were successfully synthesized by a simple hydrothermal method based on the redox reactions of KMnO₄ and CO(NH₂)₂. The influence of the reaction temperature, reaction time, reactant molar ratio and concentration on the the product phase and morphology of obtainedα-MnO₂ nanowires were discused. Expermental results indicate thatα-MnO₂ nanowires can be prepared under the conditions: molar ratio of n (KMnO₄): n (CO(NH₂)₂) = 1:3, at 120oC for12h. The possible formation mechanism ofα-MnO₂ nanowires were also proposed. The CO₂ and NH₃ gas which produced by the excess of urea hydrolysis play an important role in the formation ofα-MnO₂ nanowires.(2) Nanowire-like MnOOH andβ-MnO₂ nanowires were synthedized by a indirect hydrothermal method using potassium permanganate (KMnO₄) and oxalic acid (H₂C₂O₄·2H₂O) as raw materials. The reaction conditions are as follows: n (KMnO₄): n(H₂C₂O₄·2H₂O) = 1:1.8. The reactants were in 60oC water bath for 30min, then reacted in a stainless-steel autoclave at 150oC for 12h. The intermediate MnO(OH) nanowires were received firstly, and then they were alcined 5h at 400oC to obtained the final product ofβ-MnO₂ nanowires. The influence of the reaction temperature, reaction time and the reactant molar ratio to the product phase and morphology of obtained MnO(OH) nanowires were discussed. The CO₂ gas which produced by the excess of oxalic acid hydrolysis play an important role in the formation ofβ-MnO₂ nanowires.(3) The as-preparedα-MnO₂ andβ-MnO₂ were used for the deep catalytic oxidation of the o-xylene. Then, the better catalytic were selected add the alkali metal K to improve the activity of these catalysts. The result showed that the dopingα-MnO₂ catalyst could converse o-xylene into carbon dioxide and water at 230oC, which is 10oC lower than that ofα-MnO₂ itself.β-MnO₂ catalyst could converse o-xylene into carbon dioxide and water at 200oC, which is 20oC lower than that of itself.