A Premilary Study On Catalytic Oxidation Of Dimethyl Ether

The present work made a primary study on the catalytic oxidation of dimethyl ether (DME),two parts included,namely selective oxidation of DME to formaldehyde (HCHO)and dimethoxyethane(DMET)and total oxidation of DME.The former study was focused on the synthesis of down-stream products from DME,and the latter on the application of DME as a new and environmental energy.The standard enthalpy of formation and thermal capacity of DMET as a result of DME oxidation were calculated by Benson group contribution.The molar enthalpy changes,the Gibbs free energies and the equilibrium constants of the selective oxidation and total oxidation of DME were calculated as a function of temperature.The effect of the pressure and the mole ratio of the source gas on the DME equilibrium conversion were calculated as well.This provides a thermodynamic basis for the synthesis of high added-value product such as DMM and DMET from dimethyl ether and for its total oxidation namely catalytic combustion of DME.Two series of VOx/Al₂O₃ catalysts were prepared by the modified-impregnation method,one used pseudo-boehmite and its calcination product Al₂O₃ as carrier,the other used the precursor of Al₂O₃ and Al₂O₃ its self synthesized by coprecipitation method.Selective oxidation of DME to formaldehyde was carded on such catalysts. The results show that VOx species were highly distributed on the Al₂O₃ carder and no independent V₂O₅ exits.The modified-impregnation can lead to a well distribution of VOx species on the carrier than that of the conventional way,thus result in a better DME conversion and HCHO selectivity.wo groups of VOx-SnO₂/MgO catalysts were made by hydrothermal assisted by Super Critical Liquid technology and the modified-impregnation means,and the catalysts were used for the selective oxidation of DME to DMET.The VOx-SnO₂/MgO catalysts by hydrothermal have proper redox activity and acidity,thus show a better DMET selectivity and well stability.However,the VOx-SnO₂/MgO by impregnation has strong redox activity and catalyzes DME mainly to CH₃OH and HCHO.Different variable valence doped manganese oxide octahedral molecular sieve (OMS-2)was prepared and applied to the DME catalytic combustion reaction.The doped-metals change the fine structure of the OMS-2,enhance the surface defects, reduce the strength of Mn-O bond,increase the flow of lattice oxygen,and thus improve the DME catalytic combustion activity.Over all the M-OMS-2 catalysts,the T₉₀are all lower than 200℃with only CO₂ and H₂O as the products.There are about three types of oxygen desorption peaks on the M-OMS-2 catalysts, namely the physisorbed oxygen at low temperature,the chemisorped oxygen and surface lattice oxygen at middle temperature and the bulk lattice oxygen at high temperature.The abundance and facility of oxygen species contributed to the DME catalytic combustion activity.The doped-metal improve the redox between the Mn⁴⁺and Mn³⁺.The physisorbed and chemisorped oxygen have a relationship with the T₁₀ temperature,and the surface and bulk lattice have a relationship with the T₉₀ temperature.DME catalytic combustion on the OMS-2 follows a mechanism proposed by Mars-van Krevelen.DME was absorbed and activated on the active sites of the M-OMS-2 catalysts,its C-O bond was broken to form CH₃O^* and CH₃^* species.At the same time,the Mn⁴⁺was reduced to Mn³⁺,and then the molecular oxygen absorbed on the Mn³⁺catalyzed it to Mn⁴⁺again.Or the doped-metal transfer electron through M^m++Mn³⁺→M^(m-1)++Mn⁴⁺and reduce the Mn⁴⁺was reduced to Mn³⁺,and the M^(m-1)+was catalyzed by oxygen to M^m+again.The formed CH₃O^* and CH₃^* species were catalyzed to CO₂ and H₂O at last.The OMS-2 was also synthesized by the sol-gel method with KMnO4 and maleic as raw materials with different surfactant such as SDS,CTAB,P123 and MPEG100 as structure-directing agent.The nonionic surfactant and the mixed cationic/anionic surfactants can modulate the morphology of the OMS-2,and lead to the formation of nano-rods or nano-needles from nano-particles.The modification of the DME catalytic combustion activity was achieved by the modification of the OMS-2 structure.The OMS-2 catalysts were also prepared by super sonic technique and applied to the DME combustion.The influence of super sonic time,power and frequency on the structure and activity of the OMS-2 catalyst was evaluated.The super sonic technique can shorten the reaction time to form OMS-2 crystal to as low as 2h.And such synthesized OMS-2 catalysts have large surface area with 2 to 3 times of that prepared by conventional ways.With large surface area and more surface defects,the as-prepared OMS-2 catalysts show excellent activity on the DME combustion with a T₁₀at 149℃ and T₉₀at 155℃.