The Effect Of K Content Tuning On The Structure And Performance Of OMS-2 Catalysts

Benzene,Toluene and Xylene?BTX?are the main representatives of volatile organic compounds?VOCs?in the atmosphere,which pose a fatal threat to human health and the environment.Therefore,the rapid and efficient treatment of BTX pollutants has become one of the important hot spots in environmental catalysis.In recent years,considerable attention has been paid to the catalytic oxidation of BTX over type II manganese oxide octahedral molecular sieves?OMS-2?,but most of them are carried out at high temperatures.Therefore,the preparation of catalysts for complete catalytic oxidation BTX at low temperature??150??has become an important goal of environmental governance.In this paper,o-xylene was used as the goal pollutan.KOH and HNO₃ were used to dope and displace K in OMS-2 catalysts.The effects on the structure and performance of the catalysts were studied,and the catalysts with low temperature and high activity were prepared.The details are as follows:1.The effect of KOH reflux on t he structure and performance of OMS-2catalysts:?1?The effect of KOH reflux on t he structure and performance of OMS-2_I catalyst:OMS-2_I precursor was synthesized via KMnO4/benzyl alcohol.The precursor was refluxed in KOH solution then the catalyst K-OMS-2_Ipre was obtained.The effects of K doping methods?impregnation and reflux?,reflux substrates?OMS-2_I precursor and OMS-2_I?,reflux solvents?KOH,K₂CO₃,NaOH and H₂O?on t he structure and properties of the catalyst were investigated.X-ray diffraction?XRD?,Brunauer-Emmett-Teller?BET?,Fourier transform infrared spectoscopy?FTIR?results showed that K-OMS-2_Ipre was microcrystalline OMS-2.its specific surface area increased from 44.93 m²/g to 120.77 m²/g.And the Mn-O bond energy could be weakened.Scanning electron microscope?SEM?result showed that the catalytic changed from short rod to small granular after KOH reflux.Energy dispersive X-ray spectroscopy?EDX?result showed that of OMS-2_I was 0.20,while K/Mn ratio increased to 0.28 of K-OMS-2_Ipre,indicating that K was doped into the catalyst.X-ray photoelectron pectroscopy?XPS?result showed that the average oxidation state of K-OMS-2_Ipre was lower than OMS-2_I,and the K was mainly existed in the form of K⁺.Thermo gravimetric analysis?TGA?experiment result showed K-OMS-2_Ipre was significant weight loss at 200-400?,indicating that K-OMS-2_Iprepre had more surface adsorbed water or surface oxygen.The results of various temperature programmed adsorption-desorption experiments showed that K-OMS-2_Ipre had good low temperature reducing and reoxidation ability.The catalyst surface oxygen content increased,and the lattice oxygen was easily released.The results of activity test showed that K-OMS-2_Ipre catalytic oxidized o-xylene,the temperature of carbon dioxide formation ratio 100%(T₁₀₀)was 170?,and the temperature of carbon dioxide formation ratio 20%(T₂₀)was 130?.Compared with T₁₀₀ and T₂₀ of OMS-2_I,the temperature was reduced 10?and 23?,respectively.It can be seen that the catalytic oxidation o-xylene of K-OMS-2_Ipre ability,especially at low temperature,was better than OMS-2_I.In the water vapor resistance test,K-OMS-2_Ipre was injected with 5%water vapor.The carbon dioxide formation ratio from 80%to 64%,but when water vapor was removed from the reaction system,the carbon dioxide formation ratio returned to 80%,indicated that the effect of water vapor on the activity of K-OMS-2_Ipre was reversible.The stability test showed that the activity remained unchanged for 60 hours under T₁₀₀of K-OMS-2_Ipre catalyst.In situ diffuse reflectance infrared Fourier transform spectroscopy?In situ DRIFTS?showed that K-OMS-2_Ipre had strong lattice oxygen and molecular oxygen oxidation ability.K-OMS-2_Ipre can activated benzene ring of o-xylene at 80?under the lattice oxygen.The mainly intermediate product was carboxylate species,and it can be completely oxidized to carbon dioxide and water at 350?.K-OMS-2_Ipre can convert part of o-xylene into maleate at 80?under the molecular oxygen,and completely oxidized to carbon dioxide and water at 250?.?2?The effect of KOH reflux on t he structure and performance of OMS-2_III catalyst:In order to proved the universality of KOH reflux method for optimizing the performance of OMS-2 catalysts,we synthesized the precursor of OMS-2_II via KMnO4/Mn?Ac?₂.The product was also refluxed in KOH solution,then the catalyst K-OMS-2_IIpre was obtained.XRD,BET and FTIR results showed that the K-OMS-2_IIpre was microcrystalline OMS-2,and its specific surface area increased from 41.32m²/g to 115.51m²/g.The Mn-O bond energy could be weakend.SEM result showed that the catalytic changed from short rod to granular after KOH reflux.EDX result showed that K/Mn ratio of OMS-2_II was 0.21,and the K/Mn ratio of K-OMS-2_IIpre increased to 0.28,indicating that K was doped into the catalyst.TGA experiment result showed K-OMS-2_IIpre had a significant weight loss between 200-400?,indicating that K-OMS-2Ipre had more surface adsorbed water or surface oxygen.The results of various temperature programmed adsorption-desorption experiments showed that K-OMS-2_IIpre has good low temperature reducibility and redox ability,the surface oxygen species on the catalyst increased,and the lattice oxygen also easily released.The T₁₀₀ and T₂₀ of K-OMS-2_IIpre were 200?and 116?,respectively.Compared with OMS-2_II,The T₁₀₀ and T₂₀ of K-OMS-2_IIpre were reduced 10?and45?,respectively.The K-OMS-2_IIpre catalytic oxidation ability of o-xylene was also optimized.The results showed that the KOH reflux method to optimize the performance of OMS-2 catalyst was universal.2.The influence of dilute HNO₃ reflux on t he structure and performance of OMS-2_I catalyst:The OMS-2_I precursor was refluxed in dilute HNO₃ solution,the product was denoted 0.02H-OMS-2_Ipre.XRD and BET result showed that the 0.02H-OMS-2_Ipre was microcrystalline OMS-2.Its specific surface area increased from 44.93m²/g to 113.59m²/g.However,SEM result showed that the morphology changed from short rod to long rod.Combined with the activity,it was speculated that the growth towards long rod would made the activity reduced.EDX result showed that the K/Mn ratio of0.02H-OMS-2_Ipre was 0.17,which was 0.03 lower than OMS-2_I,indicating that part of K was replaced.Chemical titration result showed that the average oxidation state of manganese in catalyst 0.02H-OMS-2_Ipre increased to 4.0.The results of temperature programmed adsorption-desorption experiments showed that the low-temperature reducibility of 0.02H-OMS-2_Ipre was reduced,the content and the removal ability of the lattice oxygen were reduced.The T₁₀₀ and T₂₀ of 0.02H-OMS-2_Ipre were 200?and 164?,respectively.Compared with the T₁₀₀ and T₂₀ of OMS-2_I,the temperature of catalytic oxidation o-xylene was increased by 20?and 11?,which indicated that the catalytic oxidation o-xylene ability was reduced.In summary,the precursor of OMS-2_I was refluxed in KOH solution can incorporated K⁺into OMS-2,which reduced the average oxidation state of manganese.Moreover,the surface oxygen was increased and lattice oxygen was easily removed.As the oxygen species of oxidation o-xylene,surface oxygen and lattice oxygen reduced the energy for activated o-xylene and improved the catalytic oxidation ability.Inaddtion,the increased specific surface area of K-OMS-2_Ipre may also supply more active sites and improved the catalytic oxidation ability.When OMS-2_I precursor was refluxed in dilute HNO₃ solution,part of K could be displaced,and the average oxidation state of manganese increased.With the decreased content and the bad removal ability of lattice oxygen,the catalytic oxidation ability was deteriorated.This dissertation was supported by Hebei Natural Science Foundation?No.B2018205099?.