Elucidation of the reaction mechanism helps in selecting more efficient catalysts, optimizing the reaction intermediates, selecting reaction conditions, and designing of a commercial reactor. This process also generates ideas for further research and development. The oxidation of hydrocarbons is usually explained in terms of the classical free-radical chain mechanism involving the initiation, propagation, and termination steps.31
The rate of RPhCH2• generation is much faster with Br• than either Co(III) or Mn(III). It means in the oxidation process, the rates of reaction (4) is much faster than that of reactions (2) and (3), and the radical reactions (2) and (3) can be neglected. Partenheimer has emphasized that these two metals greatly enhance the selectivity of the oxidation i.e, reduce by-product formation.13, 14, 31 The addition of cobalt by itself greatly reduces the rate of generation of the unselective carbon dioxide, carbon monoxide, and cresols as well as decarboxylation products such as toluene. The addition of Br to Co, further increase the selectivity of the reaction (as well as the activity) by reducing the steady-state concentration of Co(III) through the following reaction13, 14, 38:
The effect of Mn addition to Co/Br increases the activity and selectivity to the catalyst through the following reactions13, 14, 38:
We can conclude that the initiation mechanism is predominantly hydrogen abstraction from methyl groups by Br• (see Eq. 4). The Mn and Co ions oxidize the bromine ions formed to bromine radical thus ensuring the availability of bromine radical for initiation (see Eqs. 5–7).31 Br• radical is the reaction initiator, and the radical chain propagation and termination reactions from MX to MPA can be illustrated by the following reactions. To simplify the treatment, similar to that for the oxidation of para-xylene to terephthalic acid,29 some assumptions are necessary: (a) there is no inclusion of RCH2O• and •OH radicals in the propagation and termination steps; (b) chain termination takes places by reactions of Br• radical and other active radicals, and other radicals are not allowed to undergo termination reactions.
Oxidation of MX to m-TALD
The major chain termination reactions are that between two radicals.6, 11 Because Br• radical acts the most important role in chain initiation and propagation, here only the following reactions between Br• radical and other radicals are considered as the chain termination reactions
Oxidation of m-TALD to m-TA
The reducibility of aldehyde group is strong and the following self-catalyzed Baeyer-Villiger reaction between aldehyde group and aromatic peracid may occur to generate aromatic acid24, 27
Besides the way to generate the aromatic acid via the Baeyer-Villiger reaction (see reaction Eq. 24), the peroxyacid reacts with Co(II) or Mn(II) and also gives the aromatic acid by the following reactions7, 16, 36
Oxidation of m-TA to m-CBA
Oxidation of m-CBA to MPA
There must be many other chain reactions that are not considered in the above mechanism, but the radical chain reactions (2)–(43) might be expected to approximately represent the reaction mechanism for the oxidation of MX to MPA.
The oxidation of MX to MPA is a typical consecutive reaction, and coupling effects exist among reactants, radicals and catalyst. Rounded consideration of these interactions is necessary for the mechanistic kinetic model establishment. In this work, by accounting for the most important intermediates and final products of the process as shown in Figure 1, the reaction rates of MX, m-TALD, m-TA, m-CBA, and MPA are considered. According to the reaction scheme (Figure 1) and mechanism reaction (8), the consumption rate for MX may be determined as
According to the reaction scheme (Figure 1) and the mechanism reactions (20) and (24), the generating rate for m-TALD may be determined as
According to the reaction scheme (Figure 1) and the mechanism reactions (29), the generating rate for m-TA may be determined as
According to the reaction scheme (Figure 1) and the mechanism reactions (35) and (39), the generating rate for m-CBA may be determined as
The major radicals produced in the radical chain reactions (2)–(43) are Br•, mCH3PhCH2•, mCH3Ph CH2O2•, mCH3PhCO•, mCH3PhCO3•, mHOOCPhCH2•, mHOOCPhCH2O2•, m HOOCPhCO•, and mHOOCPhCO3•, and the aromatic peroxyacid produced in reactions (2)–(36) are mCH3PhCOOOH and mHOOCPhCOOOH. From reactions (5), (7), (8), (18)–(20), (27)–(29), (33)–(35), and (42)–(43), the generating rate of Br• radical may be determined as
represents the linear assembled total concentration of all the reactive radicals that are produced in the oxidation process.
Theoretically from Eqs. 48–59, the evolution relations of concentrations for radicals and peroxyacids coupled with time can be obtained. If substituting these relations into Eqs. 44–47, the detailed mechanistic kinetic models can be obtained. However, the models obtained in this manner are too complex and difficult to be used practically. Some simplifications are necessary and the following assumptions are made:
a. The quasi-steady-state approximation is used for these radicals and aromatic peracids. This means the differential items in the left side of Eqs. 48–59 can be assumed to be zero, and the total radical concentrations in Eq. 49 can be approximately a constant, i.e.
From Eqs. 48 and 60, by using the quasi-steady-state approximation, we can get
Inserting Eq. 61 into Eqs. 44 and 46, we can get the mechanism kinetic model for the oxidation of methyl to aldehyde group in the oxidation of MX to MPA as
b. Because oxidation operates at high oxygen pressure, the concentration of oxygen in liquid may be much higher than that for Br• radical, i.e. ;
c. The products of chain termination reactions (18)–(19), (27)–(28), (33)–(34), and (42)–(43) are side or by-products, which are in low concentrations compared with the main reaction products such as m-TALD or m-PT. The concentrations of these by-products are assumed negligible. By this assumption, the items represent termination reactions in Eqs. 48–59 can be neglected.
By using these three assumptions, the concentrations of peroxyacids produced in the oxidation process can be obtained from Eqs. 48–59, and the obtained expressions are shown in the following equation.
Inserting Eqs. 61 and 63 into Eqs. 45 and 47, we can get the mechanism kinetic model for the oxidation of aldehyde to carboxylic group in the oxidation of MX to MPA as
Equations 62 and 64 are the kinetic models for the reaction steps shown in Figure 1. Comparing Eq. 64 with Eq. 62, we can find the oxidation rate of aldehyde group is not only affected by the concentration of Br• radical, but also affected by concentrations of peroxyacids. The oxidation aromatic aldehyde to aromatic acid has two sideways, one is the self-catalyzed Baeyer-Villiger reaction, [Reactions (24) and (39)], the other is the reduction of peroxyacid by Co(II) or Mn(II) [Reactions (25)–(26) and (40)–(41)]. In the recent works of Wang, the similar equation as Eq. 64 was also obtained for the oxidation of para-xylene to terephthalic acid. In that work, Eq. 64 was empirically expressed by the following equation39, 40
By combining the results from semi-continuous and batch experiments, the optimal α and β for the oxidation of para-xylene to terephthalic acid parameters are all approximately to be 1.39, 40 It indicated that although two oxidation sideways might occur to produce carboxylic acid in the oxidation of aromatic hydrocarbon to aromatic acid, only one sideway reaction might be dominated in specific operation conditions. The oxidations of MX to MPA and the oxidation of para-xylene to terephthalic acid both belongs to the aromatic hydrocarbon oxidation to aromatic acid, and for the generating of carboxylic group from aldehyde group in the oxidation of MX to MPA, we could also expect that only one sideway reaction might be dominated in specific operation oxidation conditions. That is to say, we can expect the following kinetic model for the oxidation of aldehyde group to carboxylic group
From Eqs. 62 and 66, the kinetic model for reactions in the MX liquid-phase catalytic oxidation to MPA can be obtained in the following general expression
where ε and λi are model parameters, and are independent of catalyst concentrations. ki is the reaction rate constant, and factors affecting ki are temperature, solvent composition, catalyst concentration, and catalyst ratios Co/Mn and Br/(Co+Mn). Comparing model Eq. 67 with the empirical model Eq. 1, we find the number of model parameters needed to be regressed are greatly decreased from 28 to 9 at a certain operation condition, and the nine model parameters can easily be uniquely determined by data fitting.
Mechanistic model Eq. 67 is a radical competition model and reveals the following the special competition mechanism. Radical Br• is a key factor to initiate the oxidation process, and its concentration determines the reaction rate directly. When the concentration of reactants increases, from Eq. 61 we can see that the concentration of radical Br• decreases, which restricts the acceleration of the reaction rate. When the concentration of reactants decreases, from Eq. 61 we can see that the concentration of radical Br• increases, which restricts the deceleration of the reaction rate. Because of the restriction effect of radical Br•, the change of apparent reaction rate is not sensitive with the change of reactants concentration. This unique characteristic is consistent with the large quantity of experimental data in our group for aromatic hydrocarbon oxidation to aromatic acid.25, 26, 29, 30, 39, 40