Study on the Influence of MnO2 Crystal Phases on Low-Temperature Microwave Catalytic Mineralization of Toluene

Developing low-temperature catalytic mineralization technology has become an important research direction for the treatment of volatile organic pollutants (VOCs). Using toluene as the model pollutant, we combine MnO2, which has both catalytic oxidation activity and microwave absorption ability, with microwave irradiation to mineralize toluene into CO2 and H2O at low temperatures ranging from 100 ℃ to 200 ℃. Four different crystalline phases of MnO2 catalysts (α-MnO2, β-MnO2, δ-MnO2, and γ-MnO2) were successfully synthesized by the hydrothermal method. The catalytic oxidation activity and microwave utilization potential of the different catalysts were evaluated quantitatively, considering both physicochemical and microwave properties. XRD results confirm the successful synthesis of the four distinct crystalline phases. SEM and BET results show that δ-MnO2 has a higher specific surface area (115.3 m2/g) and larger pore volume (0.458 cm3/g) due to its porous structure. Through the heating experiment, it was found that δ-MnO2 shows better microwave conversion ability. When the microwave output power was 400 W, 600 s was required for δ-MnO2 to rise from room temperature to 300 ℃, which was lower than that of α-MnO2, β-MnO2, and γ-MnO2. Combined with the vector network test results, we found that δ-MnO2 exhibits the strongest reflection loss, impedance matching, and maximum attenuation constant, indicating better microwave absorption and utilization ability. By comparing the mineralization performance under microwave irradiation, we conclude that the crystalline structure significantly affects the catalytic activity of MnO2. δ-MnO2 exhibits a superior low-temperature mineralization performance, achieving complete mineralization temperature at 195 ℃ with a gas hourly space velocity (GHSV) of 18 000 h−1. The order of toluene mineralization activity is determined to be: δ-MnO2 > α-MnO2 > γ-MnO2 > β-MnO2. Moreover, δ-MnO2 shows an outstanding stability, whose toluene mineralization efficiency remains stable with increasing reaction time. Moreover, we used GC-MS to analyze the degradation products of toluene at different catalytic temperatures. GC-MS results reveal that the main by-products of toluene degradation are esters, ketones, and other organic compounds. The type of toluene degradation by-products decreases as the reaction temperature increases. At a temperature of 200 ℃, toluene is completely oxidized to CO2 and H2O without the generation of organic products. Through comprehensive characterization, analysis of electromagnetic properties, and experimental results, we found that the excellent low-temperature oxidation characteristics of δ-MnO2 are related to its unique microstructure, including crystallinity, specific surface area, pore volume, and pore size. The rich void structure of δ-MnO2 enhances the absorption and attenuation of microwaves, exhibiting optimal microwave absorption and utilization properties.