Modifying OMS-2 Manganese Dioxide Molecular Sieve And Its Catalytic Benzyl Oxide Oxidation Performance

Volatile organic compounds(VOCs)as one of the main atmospheric pollutions have caused many environmental pollutions such as photochemical smog,PM_2.5 and other familiar problems due to the rapid development of industry and economy in recent years.Catalytic oxidation technology is deemed to one of the most effective methods to eliminate low concentration VOCs since its lower cost and higher efficiency,and the development of high-efficiency catalyst is the core of this technology.Mn-based oxides have variable oxidation state and strong oxidation ability due to its special d electron structure,which can be applied in many oxidation reactions of VOCs.Manganese oxides with different crystal phases and structures show different catalytic properties.The cryptomelane-type octahedral molecular sieve(OMS-2)has unique pore structure which attracts more and more attentions in the worldwide.However,its small surface area leads to the less surface active sites and poor mass transfer,affecting its catalytic performance.In this dissertation,OMS-2 with the high activity,great stability and water resistance was obtained by adjusting its structure and surface properties though various treatment methods.To tune its structure and surface properties,a series of characterizations were used to analyze the influence of modification on the physical and chemical properties of OMS-2.The catalytic oxidation of toluene or benzene was used as a model reaction for purpose of exploring the promoting effect of the modification on the catalytic activity of catalyst,and further discussing the relationship between the activity and structure of the catalyst.OMS-2 with hierarchical pore structure(OMS-2-OA)was prepared by etching process with the moderate redox reaction between oxalic acid and OMS-2.A series of characterizations such as XRD,SEM,TEM,H₂-TPR etc.was performed on OMS-2and OMS-2-OA.Moreover,OMS-2 and OMS-2-OA were applied in catalytic toluene oxidation process.The results suggest the formation of hierarchical pore structure in OMS-2-OA can effectively enhance the mass transfer and the utilization of active sites.The surface properties of the OMS-2-OA catalyst are changed due to the slow redox reaction process between low concentration oxalic acid and OMS-2 catalyst.The increase of the surface Mn⁴⁺and lattice oxygen content on OMS-2-OA effectively improves its catalytic activity.The OMS-2-OA catalyst which is used after different reaction conditions was analyzed by XRD,H₂-TPR and XPS to explore the reasons for the different activity stability for OMS-2-OA catalyst with or without H₂O in the reaction feed.A series of OMS-2-x A catalysts was obtained by modifying K via using one-pot hydrothermal method.Many sorts of characterizations such as XRD,XRF,N₂absorption-desorption and XPS were carried out to explore the effect of K addition on the structure,composition,surface properties,oxidation ability and other physical and chemical properties of OMS-2 and OMS-2-x A.In addition,these catalysts were applied in catalytic toluene and benzene oxidation process in order to explore the effect of the K introduction on their catalytic activity and stability.Among all synthesized catalysts,OMS-2-2.5A shows the largest surface area and abundant mesopore which can enhance the utilization of active sites.Moreover,in situ DRIFTS tests confirm the K modification can improve the activity and mobility of the oxygen species.Thus,OMS-2-2.5A displays the best catalytic performance.The analysis of the used OMS-2-2.5A catalyst after long-term stability test suggests the K modification can solve the deactivation problem of the traditional OMS-2 caused by the lack of surface lattice oxygen species.A series of Cex-OMS-2 catalysts was prepared by hydrothermal method via using Ce₂(CO₃)₃ as Ce source.The structure,morphology,surface properties and oxidation capacity of the OMS-2 and Cex-OMS-2 catalysts were analyzed for exploring the effect of the Ce introduction in Cex-OMS-2.The strong interaction between Ce and Mn increases the amount of surface lattice oxygen and Mn⁴⁺on the Cex-OMS-2catalysts.Ce3-OMS-2 shows the best catalytic toluene and benzene activity,stability even in presence of H₂O.The results of in situ DRIFTS and MS confirm the addition of Ce not only leads to the fast oxidation process of toluene on the surface of Ce3-OMS-2 but also results in the fast oxygen cycle speed.Moreover,the characterizations of the used Ce3-OMS-2 suggest that the Ce introduction stabilizes the surface properties of Ce3-OMS-2,which can effectively solve the poor water resistance of the traditional OMS-2.