Studies On The Synthesis And Catalytic Application Of Vanadium-based Metal Oxides In The Selective Oxidation Of Methane And Propane

Studies on the synthesis and catalytic application of vanadium-based metal oxides in the selective oxidation of methane and propaneBecause of the worsening global energy crisis and limited natural oil resource, more and more attention has been paid on other alternative energy sources.One of the promising ways is the effective utilization of natural gas,and as a result, investigations focused on this area are carried on by many groups worldwide.The natural gas consists of light alkanes,such as methane and propane,so one of the most popular ways in utilization of natural gas is to directly produce organic products through catalytic conversion of light alkanes.There are direct and indirect ways of transforming methane to other products. The indirect process has been commercialized.For example,methane is firstly converted to synthesis gas and then to liquid fuel and organic chemicals.Indirect process suffers some drawbacks,such as the request of heavy investment,high temperature and pressure,etc.So,much effort has been paid in the direct transforming of methane.Among this research,methane selective oxidation(MPO)to formaldehyde is one of the attractive reactions.The active species in the catalysts are mainly molybdenum and vanadium oxide as well as other transition metal oxides. Various supports are used,such as silica,Al₂O₃ and TiO₂.However,catalytic performance of MPO over these catalysts is still under expectation.Propene is one of the most important chemicals,and a variety of derivatives can be produced from propene,such as polypropene,acrylonitrile,acrolein,acrylic acid, etc.Propene is mainly produced from oil-cracking,and as a result of the increasingly shortage of oil resource,the production of propene in this way cannot meet the growing demand.As there is a considerable amount of propane in natural gas, producing of propene from catalytic oxidative dehydrogenation of propane(ODP)is now very desirable.It is reported that catalysts containing vanadium oxide have remarkable activity in ODP and the supports may be SiO₂,Al₂O₃,TiO₂,ZrO₂ and MgO,etc.The results show that catalytic performance in ODP can be affected by a series of aspects,such as the surface acidity,the redox state of vanadium species, surface area and porous structure of the catalyst.It is found that better catalytic activity can obtained in silica with large pore diameter and pore volume.As the discovery and application of MCM-41 type mesoporous silica,many supported metal oxide catalysts are prepared using mesoporous silica as supports.Most of these catalysts are obtained through a simple two-step process:support is prepared first and then active components are loaded on its inner surface by impregnation.However,the mesopores may be easily blocked and the active species may sinter at high temperature.These problems can be solved if the active components are embedded in the wall of mesopores in a highly dispersive state.The main purpose of this dissertation is,therefore,to develop some methods for preparing mesoporous silica based metal oxides catalysts for the selective oxidation of methane and propane,in which the dispersion of the active species has been improved and the interaction between the active species and the supports has been strengthened. All the catalysts were characterized using various characterization techniques and the relationship between the intrinsic state of metal oxide species and their catalytic activity were studied.1.Synthesis of mesoporous materials containing PMoV mixed oxides by impregnation and co-synthesis methods using hyteropolyacid as precursor and their application in the selective oxidation of methane to formaldehydeRecently,some investigations indicated that MoO₃ or V₂O₅ supported on ordered mesoporous silicas showed better catalytic activities than that supported on commercial amorphous silica.It is also showed that the supported phosphorus-assisted vanadium or molybdenum oxide exhibited better activity due to the formation of active dispersed oxide species or molybdophosphoric acid-like species on the support.We prepared PMoV mixed oxide supported on SBA-15 with different loadings using wet impregnation method and hyteropolyacid H₅PMo₁₀V₂O₄₀as the precursor and studied its catalytic activity in the MPO reaction.In order to understand their catalytic performance in MPO reaction,we studied the influences of various factors to the conversion and selectivity,such as loading,temperature and space velocity.It showed that the higher loading and temperature,the higher conversion of methane, while the selectivity of formaldehyde reached a highest value at certain loading.When increasing space velocity,the conversion of methane increased while the selectivity of formaldehyde decreased.The best reaction condition is as follows:T=640℃,GHSV =48320 L/(Kg·h).The maximum space yield of HCHO reached 295 g·Kg_cat^-1h^-1at 2.89%loading.Characterization result of XRD,as well as N₂ adsorption,NH₃-TPD, shows that when the loading of oxides is below 2.89%,P-Mo-V oxides exist in a highly dispersed state in the mesopores of the support,and the acidity,redox property of catalyst are related with the loading.All these aspects and property can affect the catalytic activity.It is known that during the process of synthesizing mesoporous materials,HCl and HNO₃ are usually applied as acidity regulator.Since hyteropolyacid could be dissolved in water or alcohol and release hydrogen-ion,we used hyteropolyacid as the acidity regulator and mixed oxide precursor to synthesis mixed oxide-containing mesoporous materials.Another part of work was mainly focused on the synthesis of PMoV-silica mesoporous composite and its catalytic application.H₅PMo₁₀V₂O₄₀was used as the acid regulator and inorganic precursor and ethanol as solvent.We obtained PMoV-mesoSiO₂ catalytic materials with orderly mesopores.The amount of added H₅PMo₁₀V₂O₄₀can affect the morphology and structure of the product.XRD and TEM characterization results showed these mesoporous materials are MCM-48-type structure.Oxides composite is highly dispersed in mesoporous silica when its content is not high than 6.68 wt%.Under certain content of oxide composites,the products exhibit good catalytic activity in MPO to formaldehyde.Compared the catalysts prepared with co-synthesis and wet impregnation methods,the former have better catalytic activity at nearly the same content of the active component.The maximum space yield of HCHO 330 g·Kg_cat^-1h^-1over the catalyst prepared with co-synthesis method with a PMoVO loading of 3.40%.2.Synthesis of V-containing mesoporous materials and their application in the selective oxidation of propaneMesoporous SBA-15 has been widely applied in catalytic research for its high surface area and pore diameter.The silica supported vanadium oxides are good catalysts for the propane selective oxidation based on the previous researches.This part of work mostly consist of the synthesis of V-containging mesoporous materials with VOSO₄ as a precursor and to apply in the selective oxidation of propane.In Chapter 5,we developed pH-adjusting method to synthesize V-containing mesoporous silica using VOSO₄ and TEOS as the V and Si sources,P123 as template,and ammonia solution to adjust the pH of the mixture.Characterizations,such as TG-DTA, XRD,XRF,N₂ adsorption,TEM,Raman spectroscopy,NH₃-TPD and H₂-TPR are carried out.It showed that the material has a BET specific surface area of approximately 350 m²/g,average pore diameter of 10 nm and a SBA-15 type 2D hexagonal arranged pore structure.V species are highly dispersed in a form of V-O tetrahedron in mesopores.Catalytic performance in the selective oxidation of propane reaction under optimized conditions are yield of propene reached 13.3%at T=600 and GHSV=15000 L·Kg^-1·h^-1over the catalyst of 0.98V-SBA-15.Mesoporous HMS is a material of high surface area,thermostability and hydrothermostability.We obtained V-HMS using VOSO₄ as the precursor.Its BET surface area is approximately 930 m²/g and pore volume is 0.8 cm³/g,and vanadium is embedded in silica matrix.The sample show better catalytic performance in the selective oxidation of propane reaction than the V-containg mesoporous SBA-15 with yield of HCHO reached 16.6%over the catalyst of 6.21V-HMS.And the sample also has better catalytic performance than the Vanadium oxide impregnated with HMS reported on the before literature.Triblock copolymer is widely used as soft template in synthesis of mesoporous materials.However,the cost of this template is relatively high.We employ a cheaper organic acid-glutaric acid as template,through a sol-gel process,to synthesize V-containing silica catalyst.Vanadium oxide embedded into the silica matrix and highly dispersed and the catalyst has high surface area.approximately 600 m²/g, average pore diameter of 5 nm.Characterizations denote that the vanadium has little loss and a form of V-O tetrahedron in mesopores.The yield of propene reached 13.3% at T=600 and GHSV=15000 L·Kg^-1·g^-1over the catalyst of 0.98V-SBA-15.3.Double solvent system used in preparing of effective catalysts with large pore diameter and its application in the selective oxidation of propaneThe most widely used mesoporous materials in catalytic field are MCM-41 and SBA-15 which are both 2D hexagonal,P6mm group.However,their pore sizes are limited to hundred nanometers.It was reported that larger pore size benefits the molecular diffusion and transportation even in the reactions involving small molecules,such as selective oxidation of propane.Therefore,people seek to find materials with macropore and 3D continuous porous structure.Recently a noval foam material MCF is synthesized,which possesses ultra large pore diameter and 3D continuous pores,and shows thermal stability.This kind of material has a narrow distribution of pore diameter and'cells' are connected through'window',which greatly favor diffusion and transportation.We synthesized vanadium oxide loaded MCF by impregnation with the double solvent method using n-hexane as organic phase and VOSO₄ aqueous solution as water phase.Catalytic performance in the selective oxidation of propane reaction under optimized conditions are yield of propene reached 17.3%over the catalyst of 7.03V/MCF The catalytic activity of VOx/MCF in the selective oxidation of propane reaction to propene reaction,along with the catalyst with the same V loading by commom impregnation were studied.The results showed that catalyst prepared by double solvent impregnation has a better activity which can be explained by vanadium oxides dispersed better on the catalyst prepared by double solvent impregnation.