The Intrinsic Kinetic Study Strategy For Gas-li--Quid Reactions And Its Application Into Oxida--Tive Reactions

The liquid-phase air oxidation is a heterogeneous reaction occurred in the liquid phase with air or oxygen using as the oxidant agent.Thus the mass transfer should be considered to explore the oxidation process,kinetics,mechanistic investigation,and catalys design,which was ingnored by many researchers.The intrinsic kinetics is not affected by mass transfer,which is the intrinsic information for heterogeneous reactions.The strategy to obtain the intrinsic kinetics can be described as the description below.The gas-liquid reaction can be seemed as homogeneous reaction when the intrinsic reaction is slow reaction.Considering the intermidate fast reaction or fast reaction,the intrinsic kinetics can be obtained by the decoupling the apparent kinetics due to the difficulty to exclude the mass-transfer resistances thoroughly.The intrinsic kinetics cannot be obtained in case the instaneous reactions,thus we should adjust the reaction scheme by changing the reaction condtions.Then the intrinsic kinetics can be obtained.Based on the conclusion of the strategy to obtain the intrinsic kinetics of gas-liquid phase oxidation,both the intrinsic kinetic study on the liquid-phase oxidation of 4-meththyl guaiacol to vanillin and the oxidation of(3S,7R,7aR)-6-benz--yl-7-(1-hydroxy-2-oxocyclohexyl)-3-phenyltetrahydro-5H-imidazo[1,5-c][1,3]thiazol-5-one to corresponding 6-oxohexanoic acid involved in the synthesis of vanillin and biotin were explored.The conclusions of the intrinsic kinetic were also applied into the investigation of mechanism.The consecutive oxidation of 4-methyl guaiacol via the ether intermediate towards vanillin was demonstrated by logical experiment,column analysis,and HPLC-MS.Pseudo-first-order fast reactions were observed for both steps of the consecutive reactions,thus the oxidation occurred mainly in the liquid film.The intrinsic kinetic experiments were carried out based on the results of carbon balance experiment.The kinetic results showed that the activation energy for the two steps of the consecutive reaction was 31.95 kJ/mol,and 29.77 kJ/mol respectively.The 1.0 rate dependence on oxygen and catalyst for the consecutive reaction were derived by exploring the influence of oxygen pressure,and catalyst concentration.The concentration of ethylene glycol had no influence on the oxidation rate.The side-product analysis indicated that the oligomers formed by the radical polymerization were the main side product.The mechanistic investigation based on the intrinsic kinetic investigation was conducted.The mechanism showed that the oxidation rate of 4-methyl guaiacol to vanillin was mainly controlled by the oxidation of phenolate anion to afford phenoxyl radical.The highly reactive p-benzoquinone methide was produced due to the catalytic oxidation of phenoxyl radical,which could react with alcohol solvent to form aromatic ether by 1,6-addition reaction.The selectivity of vanillin was mainly decided by the oxidative reaction of phenolate anion to phenoxyl radical.Vanillyl acid was formed due to the slow Cannizzaro reaction.Alkali played an important role in the catalytic oxidative of p-cresols.The alkali can activate 4-methyl guaiacol by acid-base reaction to form phenolate anion.The oxidative rate of phenolate anion to phenoxyl radical could be also enhanced by alkali or the combination of alkali and catalyst,thus the coupling reaction between phenoxyl radicals was decreased leading to the improved selectivity of vanillin.Alkali cannot inhibit the further oxidation of vanillin to vanillyl acid verified by control experiments.The usage amount of alkali was affected by different kinds of solvent and water due to the low pKa values was more favorable for the oxidation than high values.The experiments under cobalt-free conditions was also deigned to prove the above conclusions.Additionally,a new protocol to synthesis of p-hydroxyl benzaldehydes was also proposed by base-catalyzed oxidation.The oxidation of(3S,7R,7aR)-6-benzyl-7-(1-hydroxy-2-oxocyclohexyl)-3-ph--enyltetrahydro-5H-imidazo[1,5-c][1,3]thiazol-5-one to corresponding 6-oxohexanoic acid could be described by single reaction due to no intermediated was found during the reaction process.The mass-transfer experiments under different stirring rate and oxygen flow rate demonstrated a slow reaction occurred in the main body of the liquid phase.The oxidation of(3S,7R,7aR)-6-benzyl-7-(1-hydroxy-2-oxocyclohexyl)-3-p--henyltetrahydro-5H-imidazo[1,5-c][1,3]thiazol-5-one can be described by three parallel reactions consisted by the main reaction to form 6-oxohexanoic acid,and the side reactions due to the halogenation reaction and the C-C bond cleavage from another direction.The halogenation reaction showed zero reaction order with respect to oxygen while the C-C bond cleavage reaction exhibit first order.A higher concentration of catalyst was favorable to all the three parallel reactions due to the first order dependence on catalyst concentration.The aviation energy for the main reaction,the halogenation reaction,and the C-C bond cleavage from another direction was 54.67 kJ/mol,105.44 kJ/mol,and 88.67 kJ/mol respectively.The high values of the activation energy and the low values of hatta number demonstrated the low reaction regime.The structure of catalyst was proved to be[Fe(DMSO)4Cl2]Cl by IR and UV-vis.The DMSO solvent served as two functions:the solvent and the ligand.A higher efficient yield of 6-oxohexanoic acid for the oxo-ligand was obtained when compared with other ligands.Our case was quite different from the methyl-substituted cyclohexanone due to the steric hindrance.The explanations for why sufficient amount of catalyst was necessary to overcome the steric effect were provided informed by the experimental verifications and side product analysis.Thus a strategy to trigger different reaction pathways by steric hindrance was presented.Finally,the deeper exploration of the mechanism was shown with control experiments carried out and important intermediates captured.The mechanism disclosed that the oxidation proceeded via the combination of ionic and free radical route with the ratio of 52:48,which was also accordance with the conclusions from intrinsic kinetics.