| Selective oxidation of toluene to benzaldehyde is one of the typical reactions inthe selective oxidation of aromatic hydrocarbon. At present, alkyl chlorination oftoluene and dehydrogenations is mostly one of main routes of obtaining thebenzaldehyde in the industrial production, which prevent an admirableproduct-benzaldehyde from further usage in the line of pharmaceutics, flavor andfragrance, due to some chlorides included among products.According to relevant literatures, micro-porous manganese oxide octahedralmolecular sieve (OMS) has been one of new catalytic materials that possessed poredimensions close to molecular size like zeolite, wherein (2×2) matrix of edge-sharedMnO6 octahedral double chains which were corner connected to form onedimensional channels of about 0.46 nm×0.46 nm., K-OMS-2 abridged. It wasphysically composed of their porous structure, mild surface acidity-basicity andperfect ion-exchange ability, and chemically made of mixed valence state manganese(Mn2+, Mn3+, and Mn4+), which has been widely used as one of effective catalystmaterials for the selective oxidation of benzyl alcohol to benzaldehyde, wet oxidationof phenol compounds, formaldehyde oxidation at low temperature.This study on direct partial oxidation of toluene to benzaldehyde was presented,taking Lanthanum, Cerium-incorporated OMS-2 (La-OMS-2, Ce-OMS-2 abridged) asreaction catalysts, and air as an oxidative reagent. Some concrete details as following:At first, manganese oxide octahedral molecular sieve (OMS-2) was synthesizedthrough refluxing method; and Lanthanum incorporated OMS-2 catalysts wereprepared by means of ion exchange and impregnation methods (La-OMS-2-A,La-OMS-2-B abridged respectively), which were used as catalyst in selectiveoxidation of toluene to benzaldehyde in liquid phase. Effects of preparation methods,namely catalyst dosage, reaction temperature, oxygen flow rate, reaction time,addition of benzoic acid on the conversion of toluene and selectivity of benzaldehydewere investigated. Catalysts were characterized by means of XRD, FTIR and BETmethod. Catalytic performances of La-OMS-2-A catalyst prepared by ion exchangewere better than these of La-OMS-2-B catalyst prepared by impregnation. Underoptimal condition: toluene 0.1mol, O2 flow rate 60 ml/min, La-OMS-2-A catalystdosage 0.10g, reaction temperature 353K, reaction time 1.0h and addition of benzoicacid 0.4％(wt％), conversion of toluene and yield of benzaldehyde could reach 61％and 56％, respectively. Possible reaction principle of toluene oxidation to benzaldehyde was presented through some relevant literatures and conclusions drawnduring this research: liquid phase oxidation of toluene to benzaldehyde matched withfree radical linear reaction mechanism: peroxy-benzoic acid was prepared whenbenzoic acid as an activator took action with oxygen, and then decomposed into freeradical C6H5C-O-0·that continued with toluene to produce free radical C6H5CH2·,admired product benzaldehyde was brought about through the transfer of line afterfree radical C6H5CH2·reacted with oxygen.Secondly, according to relevant literatures, Cerium-incorporated OMS-2catalysts were synthesized by means of ion exchange method and impregnation(Ce-OMS-2-A, Ce-OMS-2-B abridged respectively), which were tested as catalysts inthe selective vapor phase oxidation of toluene to benzaldehyde. Effects of preparationmethods, Ce3+/OMS-2(wt％), reaction temperature, space velocity and molar ratio ofair to toluene on catalyst performances were investigated, and some properties ofefficient catalyst have been characterized using XRD, FTIR, TG, O2-TPD.The resultsshowed that under appropriate conditions, Ce3+/OMS-2 (wt％)46.0％by impregnation,reaction temperature 723 K, space velocity 2 400 h-1, n(air):n (toluene) 4.2,conversion of toluene, selectivity of benzaldehyde could be obtained 14.0％and85.0％, respectively. Synthesized Cerium-incorporated OMS-2 catalysts byimpregnation were matched with the well-crystallized octahedral molecular sieve(OMS-2) crystal structure by means of XRD, FTIR,TG,O2-TPD, and some phases ofMn3O4, CeO2, CexMn1-xO2-y were existed in the Ce-OMS-2-4, and altemate redox ofcerium ions Ce3+/Ce4+ provided active species Mn with lattice oxygen, which favoredthe electrical transfer of active species, and enhanced the conversion of toluene andselectivity of benzaldehyde. Possible reaction principle of vapor phase oxidation oftoluene to benzaldehyde was presented through some relevant literatures andconclusions drawn during this research: when toluene decomposed into carboncation C6H5(?)H2 by depriving ofα-H while it adsorbed around Mn4+, an admired productbenzaldehyde was attained when carbon cation C6H5(?)H2 entered the octahedral tunnel, and tookaction into with free lattic oxugen O2- that diffused through the tunnel.
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