Preparation Of Highly Active Manganese-based Bimetallic Oxides For Low-temperature Catalytic Oxidation Of Aromatic VOCs

Volatile organic compounds?VOCs?as global pollutants has caused many environmental pollution.China's Ecology and Environment Ministry has recently issued the"Key Points for National Air Pollution Prevention and Control in 2019"to accelerate the control of VOCs in key industries.Catalytic oxidation technology is considered to be the one of the most promising treatment methods for oxidizing toxic and harmful VOCs to CO₂,H₂O or other substances at low temperature.Manganese-based oxides,as inexpensive and environmentally friendly transition metal oxides,have the efficient low-temperature catalytic oxidation activity for aromatic VOCs?benzene,toluene,ethylbenzene,o-xylene,chlorobenzene,etc.?.However,the current catalytic system still has problems such as low catalytic activity at low temperature,complicated preparation process,idealized reaction atmosphere,short evaluation time for the stability,and insufficient research on catalytic reaction mechanism,limiting the further application of this technology.In this paper,a simplified and scalable preparation process of manganese-based catalysts for catalytic oxidation of aromatic VOCs at low temperature was developed;the effect of complex reaction conditions on the catalytic activity of manganese-based catalysts was investigated;by means of various characterization,the relationship between the microstructure of manganese-based catalysts and the activity of catalysts was investigated,and the mechanism between manganese-based catalysts and probe pollutants was also revealed.The main results of this paper are as follows:?1?A facile and effective molten-polymerization strategy via utilizing the molten metal salts from heated solid metal salt sources as the solvents to dissolve citric acid which acted as a mingling agent had been successfully applied to rapidly synthesize the highly active perovskite-type SmMnO₃oxides.Compared with the traditional preparation methods of SmMnO₃ such as co-precipitation,sol-gel and impregnation methods,the SmMnO₃ catalyst prepared by this method has the higher Mn⁴⁺/Mn³⁺and O_latt/O_ads ratios,better low-temperature reduction and the most catalytic activity for toluene oxidation.The T_50%and T_90%values of toluene oxidation over SmMnO₃prepared by self-molten-polymerization reached 250°C and 278°C at 48000ml?g^-1?h^-1 space velocity.The activity evaluation of catalytic oxidation of toluene without shutdown showed that the catalyst had high catalytic stability.?2?A morphology-controlled molten-polymerization route was developed to synthesize SmMnO₃?SMO?perovskites with porous netlike?SMO-N?,granular-like?SMO-G?and bulk?SMO-B?for toluene oxidation.The results showed that the morphology had a significant effect on the removal of toluene:the catalytic activity of SMO-N with porous network was significantly higher than that of SMO-G,while SMO-B showed the lowest activity of toluene oxidation.The mineralization of different kinds of VOCs over SmMnO₃ was also different,and toluene was the most easily mineralized?240oC?,while benzene?270oC?and o-xylene?300oC?were relatively difficult to be mineralized.The mineralization efficiency of toluene over SmMnO₃ was affected by water vapor,and increasing humidity can lead to a significant decrease in CO₂ yield,but when the humidity restored to the original value,the mineralization efficiency of toluene can also be restored.By the mechanism study,it was found that the lattice oxygen in SmMnO₃participated in the catalytic oxidation of VOCs.The high lattice oxygen content and Mn⁴⁺/Mn³⁺ratios were beneficial to the improvement of catalytic activity.?3?In order to further enhance the activity of the catalyst prepared by the molten-polymerization method,the?-MnO₂/SmMnO₃ catalyst was prepared by in-situ etching and applied to the catalytic oxidation of toluene,benzene,ethylbenzene and o-xylene.The synergistic mechanism between the composites was explored.The reduction ability of the modified catalyst was strengthened and the molar ratios of Mn⁴⁺/Mn³⁺and O_latt/O_ads increased from1.02 to 1.28 and 2.17 to 2.68,respectively.Compared with those of SmMnO₃and?-MnO₂,?-MnO₂/SmMnO₃ possessed the better total oxidation and mineralization of toluene.The apparent activation energy for toluene conversion and mineralization can be as low as 55.0 and 81.9 kJ?mol^-1,respectively.?-MnO₂/SmMnO₃ had different catalytic oxidation ability for different kinds of VOCs:the higher conversion and mineralization of toluene were achieved by?-MnO₂/SmMnO₃,followed by the oxidation of ethylbenzene,and the mineralization of benzene was better than that of o-xylene.The in situ DRIFTS confirmed that the intermediate products of the four VOCs contained carboxyl functional groups,and they follows a successive oxidation route at low-temperature reaction,resulting in a series of products:the acetic acid,benzaldehyde,2-?5H?-furanone,citric acid and benzoic acid for toluene oxidation;the acetic acid,benzaldehyde,citric acid and benzoic acid for ethylbenzene oxidation;the 5-hexen-1-ol,ethanedioicacid,acetic acid,n-butyric acid,phenol,and malic acid for benzene oxidation;the acetic acid,citric acid,and 1,3-isobenzofurandione for o-xylene oxidation.under a high weight hourly space velocity of 120000 ml?g^-1?hr^-1,?-MnO₂/SmMnO₃ can completely catalyze the mineralization of toluene at 260°C.Moreover,the change in humidity did not affect the stability of the catalyst,but the higher humidity was not conducive to mineralization of toluene.?4?In order to improve the concentration of adsorbed oxygen on the surface of manganese-based catalysts,the Sm-Mn bimetallic oxides with high defects were synthesized via molten-polymerization route and applied to the catalytic oxidation of toluene and chlorobenzene.The amorphous Mn_ySm_2-yO₃ catalyst was formed by the combination of Sm and Mn,and excessive Mn was converted to MnO_x which was dispersed on the surface of Mn_ySm_2-yO₃.Compared with that of Cu,the introduction of Sm could make high-surface defects and more vacancies in manganese-based oxides,enhancing the adsorbed oxygen content and further strengthened the catalytic ability of manganese-based catalysts.The Sm content had a great influence on the microstructure of the catalyst.An increase in Sm content was beneficial to promote the amount of Mn⁴⁺on the catalysts surface,and a small amount addition of Sm could lead to an appropriate amount of structural defects,which was beneficial to the improvement of the reducibility of the catalysts.However,excessive introduction of Sm could aggrandize the amount of SmO_x oxide films on the catalyst surface,reducing the reducibility of the catalysts.When the molar ratio of Mn to Sm was 4,the catalyst has the superior low-temperature reducibility,higher surface Mn dispersion and better catalytic oxidation activity.Besides,the main products of chlorobenzene oxidation over the most active catalyst were Cl₂,HCl,CO,CO₂ and H₂O.At higher catalytic reaction temperature,the yields of CO and HCl were reduced and the production of Cl₂ was increased.The phenol,hydroquinone,and oxalic acid were co-existing in the exhaust gas after chlorobenzene oxidation at low temperature.According to the characterization and performance of catalysts,it is inferred that the surface Cl^-can be removed by the following two paths:One was that the lattice oxygen from the bulk diffused to the catalysts surface,and pushed the Cl atoms in oxygen vacancies to the outer surface and transformed into Cl₂,and the other was that the surface hydrogenated lattice oxygen migrated toward oxygen vacancies and squeezed the Cl atoms in oxygen vacancies onto the outer surface of the catalyst to form HCl.The study on the stability of catalyst showed that water vapor could promote the oxidation of chlorobenzene,and the adsorption of chloride ions on the surface of catalyst was the main reason for the deactivation of catalyst and the catalytic activity of the catalyst for chlorobenzene tended to be stable after high temperature reaction at 350°C.?5?In order to study the effect of metal doping with low ionic radius on the structure and catalytic activity of manganese-based catalyst,the Mn_3-xFe_xO₄ spinel,as a catalyst for the catalytic oxidation of VOCs,was synthesized by molten-polymerization route wherein the Fe was selected as a dopant.The results showed that Mn_3-xFe_xO₄ spinel has higher activity of toluene oxidation compared to that of Mn₃O₄ spinel,while?-Fe₂O₃ spinel did not show obvious toluene oxidation;the insertion of Fe in the lattice of Mn₃O₄ caused the lattice structure distortion,which created more oxygen defects on the catalyst surface,increased the oxygen vacancy and active oxygen concentration.The change in the insertion amount of iron had a significant effect on the structure and catalytic ability of the catalyst.According to the analysis results,Mn_2.6Fe_0.4O₄ had the higher amount of surface adsorbed oxygen(O_ads/O_latt=0.89),better low-temperature reducibility and higher surface Mn dispersion(D_Mn=56%),and the most superior efficiency of toluene oxidation.The activity of Mn_2.4Fe_0.6O₄ was still not attenuated after recycle and the crystal structure after 120 hours of reaction did not change,further suggesting that Mn_2.4Fe_0.6O₄ hold a high stability.In addition,Sm-containing catalysts are more susceptible to inactivation by SO₂poisoning than pure manganese oxides,but the Fe insertion could effectively inhibit the deactivation of manganese-based oxides.