O-xylene Oxidation Process And Mechanism Over Oms-2 Catalysts:Investigation Through In-situ DRIFTS

Benzene,toluene and xylene?BTX?are the main volatile organic compounds?Volatile Organic Compounds,VOCs?,which are released in various industrial and commercial processes and cause serious harm to human health and the environment.Catalytic oxidation is considered to be the most effective and economical way to convert pollutants into harmless products?CO₂ and H₂O?.In recent years,researchers have developed many catalysts for the catalytic oxidation of aromatic VOCs,such as Manganese oxide octahedral molecular sieve?OMS-2?.OMS-2 has the characteristics of mixed valence states(Mn²⁺,Mn³⁺,and Mn⁴⁺),hydrophobicity,porous structure,lattice oxygen easy release and acid sites.Our previous studies have known that doping transition metal Ni,Ag and alkali metal K⁺can improve the catalytic performance of OMS-2 at low temperature.The prepared Ni and Ag co-doped and K-doped catalysts had showed high activity at low temperature in the catalytic oxidation of o-xylene.However,the differences in the reaction paths of high performance catalysts have not been studied too much.In this paper,the catalytic oxidation path of o-xylene over high active catalyst was studied byIn situ DRIFTS.The details are as follows:1?The study of o-xylene catalytic combustion over Ag/NiO_x-MnO₂ catalyst by In situ DRIFTS?1?NiO_x-MnO₂ and Ag/NiO_x-MnO₂ catalysts were prepared with potassium permanganate,manganese acetate,nickel acetate and silver nitrate as raw materials,all of which showed high activity at low temperature.Two kinds of reaction pathways in N₂ and O₂atmosphere were proposed through in situ DRIFTS.The results showed that the adsorption capacity of NiO_x-MnO₂ and Ag/NiO_x-MnO₂ catalysts for o-xylene was stronger than that of OMS-2 catalysts,and the reaction path was also changed.?2?The reaction path of o-xylene catalytic combustion over OMS-2 catalyst was o-xylene?benzyl alcohol?aromatic carbonyl compound,carboxylate and CO₂ in N₂atmosphere.The reaction path of o-xylene catalytic combustion over OMS-2 catalyst was o-xylene?benzyl alkoxide?aliphatic carboxylate and benzoate?maleate,aliphatic carboxylate,anhydride,carbonate and CO₂ in O₂ atmosphere.?3?The reaction path of o-xylene catalytic combustion over NiO_x-MnO₂ catalyst was o-xylene?activated benzyl alcohol?activated benzoate?maleateandaliphatic carboxylate?polymaleate,maleate,carbon acid salt and aliphatic carboxylic acid salt in N₂ atmosphere.The reaction path of o-xylene catalytic combustion over NiO_x-MnO₂ catalyst was o-xylene?activated benzoate?maleateandcarbonate?formate?CO₂ and H₂O in O₂ atmosphere.After the transition metal Ni was doped into OMS-2 catalyst,a large number of hydroxyl groups characteristic peaks appeared in NiO_x-MnO₂ catalyst,and the mobility ability of lattice oxygen was enhanced.Due to the participation of hydroxyl groups and active oxygen in the reaction,the catalyst could break benzene ring into maleate in N₂ atmosphere.After the introduction of O₂,the intermediate species were decomposed into CO₂ and H₂O.The addition of Ni could improve the desorption ability of the catalyst to carbon dioxide.?4?The reaction path of o-xylene catalytic combustion over Ag/NiO_x-MnO₂ catalyst was o-xylene?activated benzoate?maleate?polymaleate,maleate,acetateand carbonate in N₂atmosphere.The reaction path of o-xylene catalytic combustion over Ag/NiO_x-MnO₂catalyst was o-xylene?activated benzoate?maleateand carbonate?formate?CO₂ and H₂O in O₂ atmosphere.It was found that the introduction of Ag increased the amount of electrophilic oxygen and the ability to activate molecular oxygen,and produced a large number of hydroxyl groups.The reaction was accelerated by the interaction of hydroxyl groups and active oxygen.The Ag/NiO_x-MnO₂ catalyst could produce active oxygen species and participate in the process of catalytic oxidation of o-xylene.So that the intermediate species could be oxidized and decomposed more quickly.Under the atmosphere of N₂ and O₂,the reaction of OMS-2,NiO_x-MnO₂ and Ag/NiO_x-MnO₂ catalysts to catalytic combustion of o-xylene conforms to the Mars-van Krevelen?MvK?mechanism.2?The study of o-xylene catalytic combustion over KOH-OMS-2 catalyst by In situ DRIFTS?1?KOH-OMS-2 was obtained by refluxing of OMS-2 precursor in 6 mol/L KOH solution.In order to explore the reasons for the improved catalytic oxidation of o-xylene by KOH-OMS-2,the related catalyst was investigated byIn situ DRIFTS.In-situ DRIFTS results showed that the ability of OMS-2 catalyst modified by KOH to adsorb o-xylene became stronger,and the adsorption mode and reaction path changes.?2?The reaction path of o-xylene catalytic combustion over OMS-2 catalyst was o-xylene?benzyl alcohol?aromatic carbonyl compound and CO₂?aromatic carboxylate,aromatic carbonyl compound and CO₂ in N₂ atmosphere.The reaction path of o-xylene catalytic combustion over OMS-2 catalyst was o-xylene?benzyl alkoxide?aliphatic carboxylate?aliphatic carboxylateand acid anhydride?acid anhydride in O₂atmosphere.?3?In situ DRIFTS results showed that The reaction path of o-xylene catalytic combustion over KOH-OMS-2 catalyst was o-xylene?activated benzoic acid?maleateand carbonate?polymaleate in N₂ atmosphere.The reaction path of o-xylene catalytic combustion over NiO_x-MnO₂ catalyst was o-xylene?maleateand a small amount of carbonate?formate?CO₂ and H₂O in O₂ atmosphere.Compared with OMS-2 catalyst,in N₂ atmosphere o-xylene benzene ring could be cracked to maleate by KOH-OMS-2 catalyst.There were a lot of hydroxyl groups and electrophilic oxygen species in OMS-2 catalyst modified by refluxing in KOH solution.Hydroxyl groups and active oxygen species worked together to break the benzene ring,which was the key step of catalytic oxidation of o-xylene.The addition of K⁺made the KOH-OMS-2catalyst generate oxygen vacancy,which could be transformed into active oxygen species quickly after the introduction of O₂.Reactive oxygen species could to participate the reaction,further accelerating the reaction.The addition of K⁺and-OH also improved the desorption of carbon dioxide.Under the atmosphere of N₂ and O₂,the reaction of OMS-2 and KOH-OMS-2catalysts to catalytic combustion of o-xylene conforms to the Mars-van Krevelen?MvK?mechanism.