Study On Vanadium Based Catalyst For One Step Synthesis Of Methylal From Catalytic Oxidation Of Methanol

Methylal?dimethoxymethane,DMM?is a eco-friendly chemical,which can be used as the gasoline additive,organic solvent and intermediate.A direct one-step synthesis of DMM from methanol oxidation using vanadium based catalyst was studied in this paper.The results of thermodynamic calculation showed that all the six reactions in the DMM synthesis system from methanol oxidation were independent and exothermic,the Gibbs free energy of these reactions were negative,which implied that the reactions might occur spontaneously.Except for the reaction of methanol dehydration to dimethyl ether,which depended on the thermodynamics,the equilibrium constant of other reactions were large and these irreversible reactions could be influenced by dynamics.The V₂O₅/SiO₂-Al₂O₃ catalysts were prepared with silicon aluminum composite oxide as the carrier.And the effects of carrier structure on the catalyst surface acidity,the presence form of vanadium species,redox property and catalytic activity for methanol oxidation reaction were investigated.The results showed that the carrier structure had significant effect on the presence form of vanadium species and the property of redox.The carrier calcinated at 800?,which formed Al₆Si₂O₁₃ crystal was beneficial to the formation of isolated VO₄ active species.It could improve the oxidation activity of the catalyst.The weak acid center was effective active center for the condensation of methanol and formaldehyde to DMM.The generation of excessive strong acid centers would obviously promote dehydration of methanol to generate the by-product dimethyl ether.The reaction temperature,reaction pressure,feed fraction and space velocity had significant effects on the activity of the V₂O₅/SiO₂-Al₂O₃ catalyst and the distribution of product.Under the optimized reaction conditions,the yield of DMM was 45.3%.The V₂O₅/AC catalyst was prepared using high specific surface area activated carbon as the carrier.The oxidation activity of V₂O₅/AC catalyst was obviously improved compared to the V₂O₅/SiO₂-Al₂O₃ catalyst.In addition to DMM,a large amount of intermediate product formaldehyde was formed using the V₂O₅/AC catalyst.The reaction activity was improved and more formaldehyde was formed with increasing the loading of V₂O₅.The result of characterization indicated that the vanadium which was highly dispersed on the surface of activated carbon had higher oxidization.The vanadium was existed in V⁵⁺andV⁴⁺valence state on the catalyst.With increasing the loading of vanadium oxide,both the ratio of V⁴⁺/V⁵⁺and the amount of V⁴⁺valence state were increased.There was a positive correlation between the activity of the catalyst and the ratio of V⁴⁺/V⁵⁺.The reaction temperature had important effect on the selectivity of DMM.It was more favorable to the aldol condensation reaction under the low reaction temperature.At the reaction temperature of 140?,the conversion of methanol was 35.8%and the yield of DMM reached 24.6%.