Complete Oxidation Of O-xylene Over OMS-2 Catalysts

Benzene hydrocarbons is one of the most common types of VOCs?Volatile Organic Compounds?,which can cause serious harm to human and animal health.Catalytic oxidation technology has many advantages,it has been paid much attention by many researchers in the treatment of benzene pollutants.In this paper,o-xylene is used as the target pollutant to prepare manganese oxide octahedral molecularsieve?OMS-2?catalyst with good catalytic performance.Specific contents are as follows:?1?OMS-2 catalysts were synthesized via KMn O4/benzyl alcohol and KMn O4/Mn?NO3?2?the products were denoted as B-OMS-2 and N-OMS-2,respectively?.The effects of reducing agent on the structure and catalytic oxidation of o-xylene were investigated.Scanning Electron Microscope?SEM?analysis showed that the catalyst B-OMS-2 exhibited a loose cubic structure.X-ray Photoelectron Spectroscopy?XPS?indicated that B-OMS-2 contained almost 100% Mn4+.Thermogravimetric Analysis?TGA?confirmed that B-OMS-2 contained some bulk oxygen vacancies.Fourier Transform Infrared Spectoscopy?FT-IR?denoted that benzyl alcohol might be adsorbed on the surface of Mn O2 nuclei acting as a ligand or structure-directing agent,and the desorption of the organic compounds led to the formation of bulk oxygen vacancies in B-OMS-2.While XPS showed that Mn4+ and Mn³⁺ co-existed in N-OMS-2,it could be deduced that there was Mn³⁺-oxygen vacancy associates on its surface.The activity test results revealed that,B-OMS-2 could convert 100% o-xylene into at CO2 at 190 ?at a space velocity of 8000h-1,50 ?lower than N-OMS-2.The results of H2-TPR and O2-TPO showed that the catalyst B-OMS-2 had good low-temperature reducibility and high oxygen exchange ability with gas phase oxygen,this may be the reason for its better catalytic performance.In the steam resistance ability tests,two kinds of catalysts of o-xylene conversion remained at 80% in the absence of water vapor.Introduction of 5% water vapor resulted that the conversion of B-OMS-2 decreased by 39%,while N-OMS-2 decreased by 27%.When water vapor was removed,the conversion of o-xylene recovered to its initial level.In situ diffuse reflectance infrared Fourier transform spectroscopy?in suit DRIFTS?revealed that the catalytic oxidation mechanism of the two catalysts was in accordance with MVK?Mars-van Krevelen?mechanism.In the presence of lattice oxygen,the catalyst B-OMS-2 could oxidize the o-xylene to phthalyl alcohol at 120?,which was 30?lower than that of N-OMS-2.After introduction of molecular oxygen,B-OMS-2 could oxidize o-xylene side chain at 50?,and produced low molecular weight carboxylates and carbonate at 120?,and could completely decompose carboxylates at 300?.While the intermediate product had not been broken down at 300? in the N-OMS-2.The above results showed that B-OMS-2 has better catalytic performance than N-OMS-2.?2?The doped Ag or Cu can improve the catalytic performance of OMS-2.The results of catalatic activity showed that Ag-OMS-2 could completely oxidize o-xylene at 190?,10?lower than OMS-2 and Cu-OMS-2,respectively.While T20?the temperature at which the yield of CO2 was 20%?of Cu-OMS-2 was 147?,which was 36? lower than OMS-2 and 15? lower than Ag-OMS-2.The characterization of XPS before and after the catalytic oxidation reaction confirmed that Ag played a role in the active center of Ag-OMS-2,accelerated the circulation of reactive oxygen species and the process of catalytic oxidation.While Mn O2 in Cu-OMS-2 was the active center,the incorporation of copper increased the adsorption capacity of molecular oxygen on Cu2 O,promoted the exchange between Mn O2/Mn2O3,which was benifited for catalytic oxidation of o-xylene.In the steam resistance ability tests,three kinds of catalysts of o-xylene conversion remained at 80% in the absence of water vapor.Introduction of 5% water vapor resulted that the conversion of Ag-OMS-2 and OMS-2 decreased by 25%,while Cu-OMS-2 decreased by 6%.When water vapor was removed,the conversion of o-xylene recovered to its initial level.In situ DRIFTS revealed that the catalytic oxidation mechanism of the three catalysts was in accordance with MVK mechanism.In the presence of lattice oxygen,could oxidize the side chain of o-xylene to phthalyl alcohol at 100?,20? lower than that of OMS-2,but Ag-OMS-2 could generate maleate and carbonate at 150?.After introduction of molecular oxygen,Cu-OMS-2 could oxidize o-xylene side chain at 80?,and produce low molecular weight carboxylates and carbonate at 120?,which was 20-30? lower than that of Ag-OMS-2.While Ag-OMS-2 could completely decompose the intermediates of o-xylene at 300?.The doping of silver species was beneficial to the decomposition of carboxylate and carbonate,while copper doping could accelerated the oxidation of o-xylene.The above results showed that the incorporation of silver species and copper species can improve the catalytic activity of OMS-2.