Study On V-MCM-41 Mesoporous Molecular Sieves For Selective Oxidation Of Toluene To Benzaldehyde

Benzaldehyde is important intermediates in the organic synthesis industry. Selective oxidation of toluene to benzaldehyde is one of the typical reactions in the selective oxidation of aromatic hydrocarbon. At present, alkyl chlorination of toluene in the industrial production, which prevents an admriable product-benzaldehyde from further usage in the line of pharmaceutics and flavor, due to some chlorides included among products. Oxidation of toluene by air is an envrionment benign technology for synthesizing benzaldehyde.MCM-41 mesoporous molecular sieve has attracted much attention as catalyst support since it possesses well-ordered channels with controllable uniform pore size of 2～10nm as well as large surface area. Even though it is an attractive candidate, pure siliceous MCM-41 shows limited applications in organic transformations because neutral frame-structure of this material is lack of lacuna, acid sites and acidity. These lacunas, acid sites and acidities could give rise to the higher cation-exchange capacities and reactivity. Modification of this material by the introduction of various active metal incorporated into MCM-41 seemed to be a more interesting task for applications, owing to more acidic centers arising from the incorporated metals in the mesoporous materials.This study on direct partial oxidation of toluene to benzaledhyde was presented, taking Cerium,Lanthanum-incorporated Ce(La)/V-MCM-41 mesoporous molecular sieves as reaction catalysts, and air as an oxidative reagent.(1) A high vanadium contest of V-MCM-41 molecular sieve has been prepared by the atrane method. Gas phase partial oxidation of toluene to benzaldehyde on the V-MCM-41 molecular sieves catalysts was studied. Effects of mole ratio of silica to vanadium, reaction temperature, space velocity and mole ratio of air to toluene on V-MCM-41 catalyst performances were investigated. The structure and character of catalyst were characterized by means of XRD, UV-vis and so on. The experimental results show that the V contents in the framework relates to mole ratio of silica to vanadium. The V contents in the framework is the highest, when the mole ratio of Si to V is 30. The V in the framework may be active centers of catalytic oxidation of toluene to benzaldehyde. Under normal pressure, the best reaction condition of gas phase partial oxidation of toluene to benzaldehyde on V-MCM-41 molecular sieve has been obtained: mole ratio of Si to V 30, space velocity 800 h^-1, n(air): n(toluene)5 and reaction temperature 450℃. Under these conditions, the conversion of toluene, selectivity to benzaldehyde and its yield are 20.8%, 41.8% and 8.7%, respectively.(2) Ce/V-MCM-41 mesoporous molecular sieves were synthesized, which were used as catalyst in selective oxidation of toluene to benzaldehyde in gas phase under the same process conditions, yield to benzaldehyde increased by a factor. At the same time, a plausible mechanism of gas phase oxidation of toluene was preliminary study. It showed that electron transfer between Ce⁴⁺ and V⁵⁺ to maintain an effective catalyst surface oxidation state. It enhance the selectivity of benzaldehyde by electronic cycle of Mars-Van Krevelen mechanism.(3) Liquid phase partial oxidation of toluene to benzaldehyde on the La/V-MCM-41 molecular sieves catalysts was studied. Effects of La loading and various process conditions were investigated. The structure and character of catalyst were characterized by means of XRD, IR, TPR and so on. Under optimal condition: toluene 0.1mol,acetic acid 10ml,1% benzaldehyde,O₂ flow rate 60ml/min,6La/V-MCM-41catalyst dosage 0.1g,reaction temperature 80℃,reaction time 1.5h, conversion of toluene and selectivity to benzaldehyde are 42.5% and 75.7%, respectively. According to the analysis results of the oxidation products, a plausible mechanism was proposed.