Kinetics and Mechanism of Catalytic Oxidation of 2,6

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Kinetics, Catalysis, and Reaction Engineering

Kinetics and Mechanism of Catalytic Oxidation of 2,6Dimethylnaphthalene to 2,6-Naphthalenedicarboxylic Acid Heng Ban, Shengdong Yang, Youwei Cheng, Lijun Wang, and Xi Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05630 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Industrial & Engineering Chemistry Research

Kinetics and Mechanism of Catalytic Oxidation of 2,6-Dimethylnaphthalene to 2,6Naphthalenedicarboxylic Acid Heng Ban†, Shengdong Yang‡, Youwei Cheng∗†, Lijun Wang† and Xi Li† †

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture

Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡

Yisheng Dahua Petrochemical Co., Ltd. Dalian, 116100, P. R. China

ABSTRACT: The liquid-phase catalytic oxidation of 2,6-dimethylnaphthalene (2,6-DMN) to 2,6naphthalenedicarboxylic acid (2,6-NDA) were carried out over Co/Mn/Br catalysts in acetic acid solvent. Considering the complex free-radical oxidation mechanism of 2,6-DMN, detailed reaction pathways for the oxidation of 2,6-DMN to the main 2,6-NDA product and byproducts were elucidated. Then, a detailed fractional kinetic model was proposed to describe the liquid-phase oxidation of 2,6-DMN to 2,6-NDA with side reactions involved. Furthermore, the kinetic model parameters were determined by a nonlinear optimization method, minimizing the difference between the experimental data and calculated values. Several key factors such as substrate concentration, reaction temperature and catalyst concentration, influencing the reaction rate were investigated systematically. Finally, semi-continuous experiments were carried out at different ∗ Corresponding author. E-mail: [email protected]. ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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reactant feed rates and temperatures to evaluate the reliability of the derived fractional kinetic model. The determined fractional kinetic model can provide fundamental data for the industrial process optimization and scale-up of 2,6-DMN oxidation to 2,6-NDA.

1. INTRODUCTION 2,6-Naphthalenedicarboxylic acid (2,6-NDA) is often used as a significant monomer for preparing various advanced polyesters and polyurethane materials, especially polyethylene 2,6naphthalate (PEN).1-6 Since 2,6-NDA has a larger conjugated naphthalene nucleus than the benzene ring of terephthalic acid (TA), PEN exhibits more superior structural stiffness and planarity than PET synthesized by TA and ethylene glycol (EG). Consequently, compared with PET, PEN is characterized by more outstanding mechanical strength, heat resistance and gas impermeability and can be widely used as radial tires, electrical insulation applications and bottle containers.7-15 Hence, the preparation of 2,6-NDA has drawn considerable attention from many major corporations, like BP Amoco, Teijin, Sun Oil, and Eastman Chemical, with a bright application prospect and encouraging market value.1, 16 Many methods for preparing 2,6-NDA have been reported, such as Henckel method,16-19 the oxidation of 2-acyl-6-methylnaphthalene,20-21 and 2,6-dialkylnaphthalenes.22-23 Because Henckel method is applied at the temperature range from 350 to 500 ℃ under high pressure with the highly toxic cadmium salt as a catalyst, it has been eliminated in industry.16-19 2-Acyl-6methylnaphthalene is mostly derived from the expensive 2-methylnaphthalene with amounts of pollutants as byproducts, so it is not cheaply available in large quantities.20-21 2,6Dialkylnaphthalenes, like 2,6-dimethylnaphthalene (2,6-DMN),24-26 2,6-diethylnaphthalene (2,6DEN), and 2,6-diisopropylnaphthalene (2,6-DIPN) are often used as raw materials for preparing 2,6-NDA with the comparatively higher yield and purity and less pollution. It was proved by Kamiya et al.22-2322-23 that the yield of 2,6-NDA from 2,6-DMN oxidation is the highest (up to 88 [键入文字]

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mol %) under the same reaction conditions, compared with 2,6-DEN and 2,6-DIPN oxidaiton.27 Additionally, 2,6-DMN can be catalytically synthesized using o-xylene and butadiene as raw materials or directly separated from cheap light cycle oil or coal tar distillates.16 Therefore, the preparation of 2,6-NDA from 2,6-DMN has been recognized as the most technically feasible and economically beneficial process with high atom economy and 2,6-NDA yield. Besides, BP Amoco has established a commercial plant of 60 000 t/a 2,6-NDA using 2,6-DMN as the feedstock at Decatur, Alabama site using Mid-Century (MC) process since 1995.1 The oxidation of 2,6-DMN to 2,6-NDA by molecular oxygen in the presence of cobalt and manganese salts and bromine with acetic acid as the solvent is a liquid-phase catalytic oxidation process. As proposed by Partenheimer et al.,28 the classical free-radical reaction mechanism is applicable to the liquid-phase catalytic oxidation of methyl groups of aromatic hydrocarbons including 2,6-DMN and p-xylene (PX) with Co/Mn/Br catalysts involving three steps, namely initiation, propagation and termination. It has also been testified by many researcher that the liquid-phase catalytic oxidation of 2,6-DMN to 2,6-NDA and PX to TA proceeds with two methyl groups oxidized into carboxyl groups in sequence.22-26 In the MC process, over 99% of PX is oxidized and gives a greater than 95 mol % yield of TA. While the conversion of 2,6-DMN is more than 99% for 2,6-DMN oxidation, the 2,6-NDA yield is approximately 81-88 mol %, much less than the TA yield from PX oxidation in the same process.27-28 Although the C-H bond energy of the methyl group on naphthalene nucleus is 86 kcal/mol, less than that of the corresponding PX (88-89 kcal/mol), yet the activity of the methyl of 2,6-DMN is significantly less than expected for the inhibitive effect of peroxy radical attacking the nucleus.28 Additionally, Kamiya et al.27 also carried out the experiment of 2,6-DMN oxidation using Co/Mn/Br catalysts in acetic acid at 150 ℃ under oxygen partial pressure of 10 bar and found that the self-inhibiting effect of the naphthalene nucleus leads to a low 2,6-NDA yield. It was also suggested by Kamiya et al. that the substrate concentration has to be controlled within a



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reasonable range for inhibiting side reactions.22 But he did not further investigate the side reaction mechanism and kinetics. Under such harsh conditions (185-200 ℃, 2.5 MPa and acetic acid as solvent), some reactive reactant and intermediates tend to get lost by side reactions of cleavage of the naphthalene nucleus, decarboxylation and decarbonylation, generating a significant quantity of trimellitic acid (TMA) with small amounts of TA, o-phthalic acid (OA), naphthalene (NP) and 2naphthoic acid (NA), which were experimentally confirmed by high performance liquid chromatography (HPLC) in our work. These various byproducts generated in side reactions not only have a great influence on the oxidation process, but also result in the loss of 5-20 mol % 2,6-NDA yield, which can hardly be overlooked, especially for the industrial production of 2,6-NDA. For instance, TMA that serves as an effective chelating agent can precipitate and deactivate the catalyst metals, reducing the oxidation rate.29 TA with an extremely low solubility in acetic acid is likely to precipitate as a solid and mix with the desired product 2,6-NDA, deteriorating the quality of product. Degradation of 2,6-DMN to aromatic byproducts, H2O and carbon oxides also contributes to a great economic yield decrease. Thus, the kinetic data of side reactions are especially essential and necessary for optimizing reaction conditions and recycling mother liquor in a commercial process. To the best of our knowledge, there are few studies on the kinetics for 2,6-DMN oxidation to 2,6-NDA, not to mention the side reactions. Although Xu et al. developed the 2,6-DMN fractional kinetics at the temperature range from 185 to 200 ℃, yet she overlooked the investigation into the effect and kinetics of side reactions.30 In this work, for the optimization of operation parameters and reliable scale-up of reactors, a comprehensive kinetic model for the oxidation of 2,6-DMN to 2,6-NDA including side reactions was developed and some key factors influencing the reaction rate (initial 2,6-DMN concentration, reaction temperature and catalyst concentration) were investigated systematically. Finally, semi[键入文字]


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continuous experiments with different feeding rates and temperatures were conducted for estimating the reliability of the developed fractional kinetic model. 2. EXPERIMENTAL SECTION 2.1. Materials. 2,6-DMN prepared in the lab was purified by crystallization and its purity (＞ 99.5%) was determined by gas chromatography (GC). Standard air used as the oxidant and nitrogen were purchased from Hangzhou Jingong Tezhong Gas Company. Other reagents containing cobalt (ΙΙ) acetate tetrahydrate (catalyst), manganese (ΙΙ) acetate tetrahydrate (catalyst), hydrogen bromide (40% in water) (promoter) and acetic acid as the solvent were all chemically pure reagents from Sinopharm Chemical Reagent Co., Ltd. 2.2. Batchwise Operation. The description of experimental apparatus used in this work was quite detailed in our previous works.31-40 Briefly, the oxidation experiment was carried out in a 0.5 L titanium reactor as shown in Figure 1. The system was equipped with a sampler, a mechanically driven agitator, cooling coil, three condensers, a thermocouple and et al. The gas mass flow meter was in charge of the measurement and adjustment of air flow rate. The pressure in the reactor was monitored by a pressure probe and controlled by the back pressure valve with an uncertainty of ± 0.1 MPa. The temperature of the reactor was well controlled by both the wall electric heating and the circulation of diathermic oil. With the advanced PID system and data acquisition system combined, the fluctuation of temperature was maintained within ± 1 ℃. In a typical experimental run, the reactor was first charged with 280 mL acetic acid and the designed 2,6-DMN and Co/Mn/Br catalysts. Then the reactor was purged with nitrogen and pressurized to 1.6 MPa. When the reactor temperature reached the desired value, the gas (standard air) was introduced for replacing nitrogen through the liquid continuously. By adopting different agitation rates, the experimental results confirmed that the reaction rates were free of gas-liquid transfer limitation with the agitation rate above 750 rpm. In this work, an agitation rate of 1000 rpm was adopted for ensuring the gas and liquid well mixed. Furthermore, an air flow rate of 16 L/min and an O2 partial



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pressure of 0.5 MPa inside the reactor were also employed for guaranteeing that the reaction rate was controlled by kinetics. All the reaction conditions have been summarized in Table S1. In each experimental run, samples were taken through a three-way valve at the interval from 1.0 to 1.5 min. In order to avoid residual solid-liquid mixtures in the sampling tube, high-pressure nitrogen was used to sweep the sampling tube after each sampling. To verify the reproducibility of the experimental results, each experiment was repeated at least twice.

Figure 1. Experimental apparatus for kinetic investigations. 2.3. Semi-continuous Operation. Similar to the batchwise operation apparatus, the apparatus in semi-continuous operation consisted of a 2000 mL titanium reactor. In a typical semi-continuous experiment, the reactor was first charged with half of 1200 mL acetic acid dissolving all Co/Mn/Br catalysts. The agitation rate was set at 1000 rpm for ensuring full mixing of the gas and liquid phase. When the temperature inside the reactor remained stable, O2 was immediately introduced through the liquid phase to initiate the oxidation reaction. After the air flow rate remained constant, the other 600 mL acetic acid solution dissolving a certain amount of 2,6-DMN was fed into the reactor by a metering pump at a flow rate of 60 mL/min. When all feedstock was fed into the reactor, the metering pump was turned off and air was continuously fed through liquid for another [键入文字]


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15 minutes to ensure the complete oxidation of remaining 2,6-DMN and intermediates. All the relevant reaction conditions of semi-continuous experiments are shown in Table S1. 2.4. Analytical Method. The reactant 2,6-DMN, intermediates 6-methyl-2-naphthaldehyde (2,6MND), 6-methyl-2-naphthalenecarboxylic acid (2,6-MNA), 6-formyl-2-naphthoic acid (2,6-FNA) and 2,6-NDA, byproducts trimellitic acid (TMA), terephthalic acid (TA), o-phthalic acid (OA), 2naphthoic acid (NA), and the desired 2,6-NDA were analyzed by high-performance liquid-phase chromatography (HPLC) equipped with an Agilent ZORBAX Eclipse XDB-C18 column. The external standard method was used for quantifying the concentrations of 2,6-DMN and 2,6-NDA and various byproducts including TMA, TA, OA and NA. 2-Formyl-6-naphthoic acid (2,6-FNA) and 6-methyl-2-naphthalenecarboxylic acid (2,6-MNA) were determined by liquid chromatography-mass spectrometry (LC-MS). The column temperature was set at 30 ℃ and the flow rate of the mobile phase was 1 mL/min. The mobile phase of HPLC was comprised of three eluents: water (0.1 wt % phosphoric acid), methanol and acetonitrile. The products were quantified by a UV detector at 270 nm and the corresponding gradient elution program for separating different components. Every sample was analyzed at least twice until the relative error was within ± 1%. 3. LUMPED REACTION SCHEME AND KINETIC MODEL Many researchers22 demonstrated that the oxidation of the methyl substituents of 2,6-DMN to carboxyl groups is mainly accomplished via aldehyde groups and partially formyl groups. Additionally, the concentration of the intermediate 6-methyl-2-hydroxy methyl naphthalene is so low that it is not included into the kinetic scheme of 2,6-DMN oxidation to 2,6-NDA for simplification. In Figure 2a, the concentration of intermediates 6-methyl-2-naphthaldehyde (2,6MND), 6-methyl-2-naphthalenecarboxylic acid (2,6-MNA), 6-formyl-2-naphthoic acid (2,6-FNA) and final product 2,6-NDA during the reaction process reached a maximum value successively at



different time, which is consistent with the characteristics of intermediates and final product

Concentration (mol/kg)

concentration variation for typical cascade reactions. 0.6

2,6-DMN 2,6-MND 2,6-MNA 2,6-FNA 2,6-NDA

a

0.4 0.2 0.0 0

2

4

6

8

10

8

10

Time (min) Concentration (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.03

b

0.02

TMA NA OA+TA

0.01 0.00 0

2

4

6

Time (min)

Figure 2. Concentration profiles of reactant, main intermediates and all products as a function of time. (a) the reactant 2,6-DMN, intermediates 2,6-MND, 2,6-MNA and 2,6-FNA, and desired product 2,6-NDA; (b) main byproducts TMA, OA+TA and NA. [Reaction conditions: T = 190 °C, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. It has been proved by Cheng et al.39 that a small part of reactive RCO• and RCOO• generated during the liquid-phase catalytic oxidation of aromatics undergo decarboxylation and decarbonylation and produce CO, CO2 and mono-decarboxylated product. After one methyl of 2,6DMN is oxidized to carboxyl, the naphthalene nucleus is significantly deactivated by the electronwithdrawing effect of carboxyl. As suggested by Partenheimer et al.,28 any reagents with initially low electron density are particularly prone to decarboxylation and decarbonylation. Hence, compared with 2,6-MND, 2,6-FNA with a COOH group is regarded to be more vulnerable to decarboxylation and decarbonylation, generating undesirable byproducts, like naphthalene (NP) [键入文字]


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and 2-naphthoic acid (NA). Considering the trace amounts of NP, NP can be neglected in the oxidation network for simplification. The mechanism is given by CO·

- CO

HO

- CO 2

O HO COO ·

- CO 2

(1) O

NA

NP

HO O

From Figure 2b, the concentration profiles of terephthalic acid (TA), o-phthalic acid (OA) and trimellitic acid (TMA) show a similar increasing trend to NA from 0 to 4 min, which indicates that they might be converted from some common intermediates. It can also be noted from Figure 2a and 2b that the generation rates of NA, OA+TA and TMA were almost in proportion to the concentration of 2,6-FNA from 0 to 4 min, which testifies that 2,6-FNA might be over-oxidized into these byproducts. Besides, Kamiya et al.22 proposed that the oxidation of 2,6-FNA to 2,6NDA is the most difficult step, because 2,6-FNA is likely to precipitate in the oxidation process due to its extremely low solubility in acetic acid. Partenheimer et al.28 proposed that one of condensed rings tends to be attacked by peroxy radical and is ultimately destroyed, yielding TMA as byproduct. It was also reported by Kamiya et al.22 also that TMA is converted from 2,6-FNA with the scission of the naphthalene nucleus. In Figure 2a and 2b, when the oxidation of 2,6-FNA was nearly complete at 8.5 min, OA+TA concentration tended to level off and TMA concentration began to increase quite slowly, which suggests that OA+TA and TMA are possibly converted from 2,6-FNA. As shown in Figure 2b, NA concentration descended slightly from 5 to 10 min due to the instability of NA, whereas TMA concentration continued to rise, which indicates that chemically unstable NA might be further oxidized into relatively stable TMA, causing TMA concentration to increase. By combining the concentration evolution of all components with time and the radical chain reaction mechanism, a simplified scheme for the liquid-phase catalytic oxidation of 2,6-DNM to 2,6-NDA including side reactions was proposed accordingly, as shown in Figure 3. ACS Paragon Plus Environment


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It had been verified that the liquid-phase catalytic oxidation of alkyl-aromatics was zero-order with respect to oxygen with sufficiently high partial pressure of oxygen (above 50-100 Torr).41-44 Consequently, the reaction rate of each step was supposed to be zero-order for oxygen by maintaining oxygen concentration in stoichiometric excess (an air flow rate of 16 L/min). Some researchers41-45 adopted the pseudo-first-order kinetic model for the liquid-phase oxidation of PX to TA catalyzed by Co/Mn/Br catalysts, where all the reactions were zeroth order with respect to oxygen and first order with respect to reactants oxidized. The experimental results from our work group have proved that the concentration of liquid reactants had little influence on the reaction rate and the apparent reaction order was far less than 1.0, which has been testified by both batchwise and semi-continuous operation.31, 35-36, 39 Considering the catalyst cycle and interaction of radicals, a fractional kinetic model for the liquid-phase oxidation of PX to TA was proposed by our workgroup on the basis of a complicated radical chain reaction mechanism.32-33 Additionally, our workgroup compared the first-order and nth-order kinetic model for PX oxidation with the fractional kinetic model and found that the fractional kinetic model could well describe the distribution of PX oxidation products, especially under industrial conditions.34, 37-38 With similar reaction pathways following the classical radical reaction mechanism, the fractional kinetic model for PX oxidation to TA can also be applied to 2,6-DMN oxidation to 2,6NDA. Although there are a great number of radical and molecular species generated during the reaction process, only part molecular species can be measured instead of hardly detected reactive radicals. For avoiding over-fitting, only important reactants, intermediates and products, i.e., 2,6DMN, 2,6-MND, 2,6-MNA, 2,6-FNA and 2,6-NDA are involved in the simplified fractional kinetic model for the main reaction of 2,6-DMN oxidation. Since side reactions can not only exert a great impact on the main reaction rate, but also generate a certain amount of byproducts, reducing the yield and purity of the desirable product 2,6-NDA. For better reducing byproducts and predicting the product distribution, a detailed fractional kinetic [键入文字]


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model for the oxidation of 2,6-DMN to 2,6-NDA involving side reactions was established with systematic experiments and the regression of model parameters in this work. To gain better insight into the kinetics of 2,6-DMN oxidation to 2,6-NDA, some key factors (initial 2,6-DMN concentration, catalyst concentration and reaction temperature) influencing the reaction rate were investigated systematically in this work.

Figure 3. Scheme of catalytic oxidation of 2,6-DMN to 2,6-NDA. k1, k2, k3, and k4 are the main reaction constant of each step respectively, ks1, ks2, ks3, ks4, and ks5 are the side reaction constant of each

step

respectively;

2,6-DMN:

2,6-dimethylnaphthalene,

2,6-MND:

6-methyl-2-

naphthaldehyde, 2,6-MNA: 6-methyl-2-naphthalenecarboxylic acid, 2,6-FNA: 6-formyl-2naphthoic acid, 2,6-NDA: 2,6-naphthalenedicarboxylic acid, TA: terephthalic acid, OA: o-phthalic acid, NA: 2-naphthoic acid, TMA: trimellitic acid.

rj =

k jC j 4

(∑ di Ci +ε )

βj

j = 1− 4

(2)

i =1

rs1 = k s1C4

(3)

rs 2 = k s 2C4

(4)



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rs 3 = ks 3C7

(5)

rs 4 = k s 2C4

(6)

rs 5 = k s 4C2

(7)

dC1 = −r1 dt

(8)

dC2 = r1 − r2 − rs 5 dt

(9)

dC3 = r2 − r3 dt

(10)

dC4 = r3 − r4 − rs1 − rs 2 − rs 4 dt

(11)

dC5 = r4 dt

(12)

dC6 = rs1 + rs 3 dt

(13)

dC7 = rs 2 + rs 5 − rs 3 dt

(14)

dC8 = rs 4 dt

(15)

with the following initial conditions: C1 = C10, C2 = C3 = C4 = C5 = C6 = C7 = C8 = 0

(16)

where the subscript 1-7 represent 2,6-DMN, 2,6-MND, 2,6-MNA, 2,6-FNA, 2,6-NDA, TMA and NA, respectively, and 8 represents OA+TA; rj and rsj（j = 1-4）are the main and side reaction rate of the step j, repectively, mol/(kg•s); kj and ksj (j = 1-4) are the main and side reaction rate constant of the step j affected by temperature, catalyst concentration, etc., repectively, s-1; Cj (j = 1-8) is the concentration of the component j, mol/kg; di, ε and βj (i = 1-4, j = 1-4) are the modified parameters given the catalyst cycle and radical interaction. Eight experimental runs corresponding to the conditions listed in Table S1 were carried out to regress the parameters of the fractional kinetic model for 2,6-DMN oxidation. The nonlinear least[键入文字]


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squares method was used to optimize the parameters of the kinetic model, minimizing the difference between the calculated values and experimental results as follows:

= min E

∑∑ ( C m

n

=i 1 =j 1

cal ij

− Cijexp )

2

(17)

exp are the calculated and experimental concentrations of the component j in the where Ccal ij and Cij

ith experimental run, respectively. m and n denote the number of all batch experiments and all involved components, respectively. The fourth-order Runge-Kutta method installed in the Matlab software was also adopted to solve the batch reactor model describing the reaction process, and the calculated concentration distribution curves of each component were compared with experimental values. 4. RESULTS AND DISCUSSION 4.1. Kinetic Parameter Evaluation. The kinetic experiments with three values of the initial concentrations of 2,6-DMN (runs 1-3 of Table S1) were performed to evaluate the kinetic parameters. For making comparison and determining the reaction order, all component concentrations were divided by the initial 2,6-DMN concentration. The normalized concentration profiles of reactant, important intermediates and products with different 2,6-DMN initial concentrations are shown in Figures 4 and 5.



0.45

a

b 2,6-DMN:HAc=1:7 2,6-DMN:HAc=1:10 2,6-DMN:HAc=1:14 —— model fitting

0.75

C2,6-MND/C2,6-DMN,0

C2,6-DMN/C2,6-DMN,0

1.00

0.50

2,6-DMN:HAc=1:7 2,6-DMN:HAc=1:10 2,6-DMN:HAc=1:14 —— model fitting

0.30

0.15

0.25

0.00 0.00 0

5

10

15

0

5

Time (min)

10

15

Time (min)

0.8 0.15

c


C2,6-FNA/C2,6-DMN,0

0.6

C2,6-MNA/C2,6-DMN,0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

d


0.10

0.05

0.2

0.00 0.0 0

5

10

15

0

Time (min)

5

10

15

Time (min)

Figure 4. Concentration profiles of reactant and intermediates over time with different initial 2,6DMN concentrations [reaction conditions: T = 190 °C, P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm].

[键入文字]


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1.00

a

0.15

b 2,6-DMN:HAc=1:7 2,6-DMN:HAc=1:10 2,6-DMN:HAc=1:14 —— model fitting

CTMA/C2,6-DMN,0

C2,6-NDA/C2,6-DMN,0

0.75

0.50

0.10

0.05

0.25


0.00 0

5

10

0.00

15

0

5

Time (min)

10

15

Time (min)

c

0.08


COA+TA/C2,6-DMN,0

0.024

CNA/C2,6-DMN,0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.016

0.008

d 2,6-DMN:HAc=1:7 2,6-DMN:HAc=1:10 2,6-DMN:HAc=1:14 —— model fitting

0.06

0.04

0.02

0.00

0.000 0

5

10

15

0

Time (min)

5

10

15

Time (min)

Figure 5. Concentration profiles of desirable product and byproducts over time with different initial 2,6-DMN concentrations [reaction conditions: T = 190 °C, P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. From Figure 4, the half-life for reactant 2,6-DMN with different initial 2,6-DMN concentrations is not identical, which is contrary to the universally recognized characteristics of the first-order reaction that the half-life is independent of the initial concentration of reactant, indicating that the oxidation reaction rate is not the first order with respect to reactant. When a fractional kinetic model was adopted to fit the concentrations of important components as a function of time (Figures 4 and 5), the obtained agreement is generally satisfactory, and the obtained kinetic model parameters are listed in Table 1. The rate constant k2 is one order of magnitude greater than the rate constant k1, which indicates that the oxidation of the methyl to aldehyde (2,6-DMN→2,6-MND) is remarkably harder than the aldehyde to carboxyl (2,6-MND→2,6-MNA). This also confirms that aldehyde group is



Page 16 of 41

more reactive than methyl group and prone to be further oxidized to carboxyl group. Additionally, similar phenomenon has been observed in the oxidation of PX to TA. From Table 1, the rate constant k2 related to the step 2,6-MND→2,6-MNA is relatively greater than the rate constant k4 corresponding to the step 2,6-FNA→2,6-NDA. Because the COOH group is an electronwithdrawing group, contrary to the electron-donating methyl group and tends to attract electrons of the ring to move towards itself, making 2-carboxyl-6-naphthylformyl radical hardly stabilize through resonance structures.46 Then the further oxidation of 2,6-FNA is more difficult owing to the strengthened methyl group C-H bond of 2,6-FNA. As shown in Figure 5, the yield of 2,6-NDA dropped with the increasing 2,6-DMN initial concentration, while the yield of TMA, NA and OA+TA increased. As reported by Kamiya,22 part of 2,6-FNA might precipitate as a solid and even be trapped in the solid product during the oxidation course due to its low solubility in acetic acid. Thus, solid 2,6-FNA was oxidized to 2,6-NDA at a slow rate due to the restricted O2 diffusion into the solid phase, which substantially impeded the main reaction. Then more 2,6-FNA tended to be converted into byproducts, like TMA, OA+TA and NA, resulting in the decreasing 2,6-NDA yield. Therefore, 2,6-DMN concentration has to be controlled in a reasonable range for obtaining a high yield of 2,6-NDA. Table 1. Estimated Kinetic Model Parameters with Different Initial 2,6-DMN Concentrationsa main reaction i

ki, s-1

di

βi

1

6.32×10-3

1.05

1.85

2

1.47×10-2

0.859

0.767

3

3.14×10-3

0.381

4

8.50×10-3

7.36

side reaction ε

i

ki, s-1

s1

1.69×10-3

s2

8.07×10-4

0.558

s3

3.50×10-3

1.082

s4

1.30×10-3

s5

1.12×10-3

4.17×10

-3

Reaction conditions: T = 190 ℃, P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. a

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4.2. Effect of Temperature. The effect of temperature on reaction rate was investigated systematically by performing experiments (runs 3-6 of Table S1) at 185 ℃, 190 ℃, 195 ℃ and 200 ℃ in sequence.

0.20

b

a 185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.30

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.15

C2,6-MND (mol/kg)

C2,6-DMN (mol/kg)

0.45

0.15

0.00

0.10

0.05

0.00

0

2

4

6

8

10

0

2

Time (min)

4

6

8

10

Time (min)

0.3

0.06

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.2

0.1

0.0

d

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.04

C2,6-FNA (mol/kg)

c

C2,6-MNA (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02

0.00

0

2

4

6

8

10

0

2

Time (min)

4

6

8

10

Time (min)

Figure 6. Concentration profiles of reactant and intermediates versus time at different reaction temperatures [reaction conditions: P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm].



b

a

0.4

0.03

CTMA (mol/kg)

C2,6-NDA (mol/kg)

0.3

0.2

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.1

0.0

0

2

4

6

8

0.02

0.01

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.00

10

0

2

Time (min)

0.010

6

0.015

0.004

0.010

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

0.005

0.002

0.000

0.000

2

4

10

d

0.006

0

8

0.020

COA+TA (mol/kg)

0.008

4

Time (min)

185 ℃ 190 ℃ 195 ℃ 200 ℃ —— model fitting

c

CNA (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

6

8

10

0

2

Time (min)

4

6

8

10

Time (min)

Figure 7. Concentration profiles of desirable product and byproducts versus time at different reaction temperatures [reaction conditions: P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. As can be seen from Figures 6 and 7, both main and side reaction rates were significantly accelerated by increasing temperature. The kinetic parameters of the fractional kinetic model at each temperature were estimated by fitting the experimental results. The relationship between rate constants ki and temperature T can be generally described by Arrhenius equation, as shown in Table 2. Table 2. Pre-exponential Factor and Activation Energya i

Eai, kJ/mol ki,0, s-1

1

62.9

7.75×104

variance (R2) i

Eai , kJ/mol ki,0, s-1

0.995

125.0

s1

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2.09×1011

variance (R2) 0.986

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2

66.7

4.82×105

0.991

s2

105.3

6.09×108

0.986

3

66.5

9.66×104

0.994

s3

93.5

1.19×108

0.989

4

79.4

7.53×106

0.993

s4

109.9

3.35×109

0.984

s5

114.0

8.20×108

0.992

a

Reaction conditions: P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm,

Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm. It can be seen from Table 2 that the activation energy of the 2,6-MNA to 2,6-FNA step and 2,6FNA to 2,6-NDA step are 66.5 kJ/mol and 79.4 kJ/mol, larger than that of the 2,6-DMN to 2,6-MND step and 2,6-MND to 2,6-MNA step respectively, confirming that the oxidation of the second methyl group of 2,6-DMN is more sensitive to temperature variation than the first one. In addition, the side reaction steps show higher activation energy (93.5-125.0 kJ/mol) than the main reaction steps (62.979.4 kJ/mol), which suggests that temperature variation exerts a greater impact on side reactions than on main reactions and high temperature is conductive to yielding more byproducts, like TMA and OA+TA, thus reducing 2,6-NDA yield (Figures 6-7). Additionally, high temperature also speeds up solvent burning, increasing the operational cost. Hence, relatively low temperature is favorable for 2,6-DMN oxidation to 2,6-NDA with less byproducts and higher yield. 4.3. Effect of Catalyst Concentration. Different total catalyst concentrations with Co/Mn/Br molar ratio (2/4/3) kept constant in the kinetic experiments for 2,6-DMN oxidation to 2,6-NDA were employed to examine the effect of catalyst concentration on the reaction rate.



0.24

a

b [Co]=150 ppm [Co]=200 ppm [Co]=250 ppm —— model fitting

C2,6-DMN (mol/kg)

0.36

[Co]=150 ppm [Co]=200 ppm [Co]=250 ppm —— model fitting

0.18

C2,6-MND (mol/kg)

0.48

0.24

0.12

0.12

0.06

0.00

0.00

0

3

6

9

12

0

3

Time (min)

6

9

12

Time (min)

0.32

c

d [Co]=150 ppm [Co]=200 ppm [Co]=250 ppm —— model fitting


0.06

C2,6-FNA (mol/kg)

0.24

C2,6-MNA (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

0.16

0.04

0.02

0.08

0.00

0.00 0

3

6

9

12

0

3

6

9

12

Time (min)

Time (min)

Figure 8. Concentration profiles of reactant and intermediates over time with different catalyst concentrations [reaction conditions: T =190 ℃, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm].

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Page 21 of 41

0.4

b

a 0.03

CTMA (mol/kg)

C2,6-NDA (mol/kg)

0.3

0.2

0.1


0.0

0

3

6

9

0.02

0.01


0.00

12

0

3

Time (min)

0.009

c

6

9

12

Time (min)


0.020

d

COA+TA (mol/kg)

0.015

CNA (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.006

0.003

0.010

0.005


0.000

0.000

0

3

6

Time (min)

9

12

0

3

6

9

12

Time (min)

Figure 9. Concentration profiles of desirable product and byproducts over time with different catalyst concentrations [reaction conditions: T =190 ℃, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. In Figure 8, the concentration curves of each component advances as the concentration of Co catalyst increases, indicating that all reaction steps are sped up by increasing total catalyst concentration. It can be seen from Table 3 that the main reaction rate constant k3 is always smaller than the other rate constants k1, k2 and k4 at the Co concentration range from 150 to 250 ppm, which suggests that the step of 2,6-MNA to 2,6-FNA is the rate-limiting step. Besides, when Co concentration increased from 150 to 250 ppm, the main reaction rates were increased by 1.7-2.1 times, while the side reaction rates were accelerated by 2.0-2.5 times, which demonstrates that the total catalyst concentration exerts a greater effect on the side reaction rather than the main reaction. Hence, given the side reactions and solvent burning, catalyst concentration has to be controlled



Page 22 of 41

within a reasonable range for suppressing the side reactions. There is satisfactory agreement between the simulated values and experimental data, as shown in Figures 8 and 9. Table 3. Relationship between ki and the Total Catalyst Concentrationa [Co], mg/kg

k1, s-1

k2, s-1

k3, s-1

k4, s-1

ks1, s-1

ks2, s-1

ks3, s-1

ks4, s-1

ks5, s-1

150

4.52×10-3 8.74×10-3 2.40×10-3 5.50×10-3 1.09×10-3 5.07×10-4 2.40×10-3 9.85×10-4 7.91×10-5

200

6.32×10-3 1.47×10-2 3.14×10-3 8.50×10-3 1.69×10-3 8.07×10-4 3.50×10-3 1.30×10-3 1.12×10-4

250

7.61×10-3 1.67×10-2 4.28×10-3 1.15×10-2 2.19×10-3 1.21×10-3 4.80×10-3 1.99×10-3 1.96×10-3

Reaction conditions: T = 190 ℃, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm. a

4.4. Verification by Semi-continuous Operation. The liquid-phase catalytic oxidation of 2,6DMN to 2,6-NDA in industry is usually a continuous commercial process. To further evaluate the reliability of the proposed fractional kinetic model for 2,6-DMN oxidation to 2,6-NDA, semicontinuous experiments with varying temperatures and feeding rates were carried out. For the semicontinuous oxidation process, the mass balance equation describing the concentration of liquid phase components for the feeding period can be expressed as follow:

(V0 + t × F ) ×

dCi + F × Ci = F × Ci ,0 + (V0 + t × F ) × Ri dt

(20)

where V0 is the initial volume of the liquid phase and t is the feeding time. F represents the volume flow rate and Ci,0 and Ci stand for the component i concentration of the inlet and liquid phase in the reactor, respectively. Ri is the net formation rate of the component i. Combined with the fractional kinetic model obtained previously, the concentration distribution of reactant, intermediates and products was calculated using software COMSOL Multiphysics. Table 4. Model Predictions against Experimental Results run

temp, ℃

yield of TMA, %

yield of OA+TA, %

yield of 2,6-NDA, %

exptl

exptl

exptl

calcd

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calcd

calcd

Page 23 of 41

9a

190

6.7

6.3

2.7

3.3

88.4

90.5

10b

182

5.9

5.4

3.2

2.9

89.1

91.7

Reaction conditions: P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm; a2,6-DMN/HAc (mass ratio) = 1/5, T = 190 ℃; b2,6-DMN/HAc (mass ratio) = 1/6, T = 182 ℃.

C2,6-NDA (mol/kg)

0.6

(a) Experimental data Calculated values

0.4

0.2

0.0 0

5

10

15

20

25

20

25

Time (min) 0.6

(b)

C2,6-NDA (mol/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental data Calculated values

0.4

0.2

0.0 0

5

10

15

Time (min)

Figure 10. 2,6-NDA concentration evolution over time by experiments and calculation [reaction conditions: P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm. (a) 2,6-DMN/ HAc (mass ratio) = 1/5, T = 190 ℃; (b) 2,6-DMN/HAc (mass ratio) = 1/6, T = 182 ℃]. It can be seen from Figure 10 and Table 4 that the model predictions agree with the experimental results satisfactorily. The yields of TMA from experiments were higher than the model predictions and the 2,6-NDA yield from experiments was consequently lower than expected. Because in a typical semi-continuous experiment, longer residence time and complete dissolution of 2,6-FNA with low reactant concentration by continuous feeding could not only exacerbate decarboxylation and decarbonylation of the reactant and intermediates, but also lead to more cleavage of the naphthalene nucleus, reducing the yield of 2,6-NDA. There is no variation in 2,6-NDA concentration



Page 24 of 41

after 15 min and 17.5 min in experimental runs 9 and 10 respectively, which indicates that the complete oxidation of the reactant and intermediates and also demonstrates the chemical stability of 2,6-NDA in such severe conditions. It is worth pointing out that in the experimental run 10 the calculated values could even predict the concentration evolution of the desired product 2,6-NDA with the temperature out of the investigated range for the derived kinetic model. Hence, the reliability of the fractional kinetic model is further confirmed by the semi-continuous experimental results. 5. CONCLUSIONS Based on the complicated radical chain reaction mechanism and the analysis of the oxidation process, detailed pathways for the oxidation of 2,6-DMN to the main 2,6-NDA product and various byproducts were elucidated. In order to evaluate the influence of experimental parameters on the oxidation process, a comprehensively fractional kinetic model for the oxidation of 2,6-DMN to 2,6NDA involving side reactions was established accordingly. Key factors including substrate concentration, temperature and catalyst concentration, influencing product distribution were systematically investigated. The kinetic model parameters were determined in a nonlinear optimization, and the calculated values agreed well with the experimental data. Additionally, it is testified by experiments that the 2,6-MNA to 2,6-FNA step is the rate-limiting step for the cascade reactions of 2,6-DMN oxidation to 2,6-NDA, which could be attributed to the inhibiting effect of the electron-withdrawing carboxyl group of 2,6-MNA. Finally, the proposed fractional kinetic model has been proven to be reliable by satisfactory agreement between the results of semi-continuous experiments and kinetic predictions. The determined fractional kinetic model can provide fundamental data and valuable insight, which could be used in the reactor scale-up and the process optimization for the oxidation of 2,6-DMN to 2,6-NDA.

ASSOCIATED CONTENT [键入文字]


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Supporting Information The Supporting Information is available free of charge on the ACS Publications. Table S1 showing operation conditions used in kinetic studies; Table S2 presenting solubility data for 2,6naphthalenedicarboxylic acid (2,6-NDA) in water and acetic acid; Table S3 presenting the response factor of reactant 2,6-dimethylnaphthalene (2,6-DMN), intermediates and the final product 2,6-NDA; Figure S1 showing experimental and simulated product distributions at different stirring speeds; Figure S2-S5 showing typical HPLC chromatogram of product samples; Figure S6-S7 showing liquid chromatography-mass spectrometry results of intermediate 6-methyl-2-naphthalenecarboxylic acid (2,6-MNA) and 6-formyl-2-naphthoic acid (2,6-FNA).

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(32) Wang, Q. B.; Cheng, Y. W.; Wang, L. J.; Li, X. Semicontinuous studies on the reaction mechanism and kinetics for the liquid-phase oxidation of p-xylene to terephthalic acid†. Ind. Eng. Chem. Res. 2007, 46, 8980-8992. (33) Wang, L.; Li, X.; Xie, G.; Cheng, Y.; Sima, J. Kinetics of p-xylene liquid phase catalytic oxidation. (I) Reaction mechanism and kinetic model. J. Chem. Ind. Eng. 2003, 54, 946-952. (34) Xie, G.; Sima, J.; Wang, Q. B.; Li, X. Effect of oxygen volume fraction on p-xylene liquid phase catalytic oxidation. J. Zhejiang Univ. 2004, 38, 1024-1028. (35) Cheng, Y. W.; Li, X.; Niu, J. F. Kinetics of p-xylene liquid phase catalytic oxidation (Ⅲ) Catalyst composition and concentration. J. Chem. Ind. Eng. 2004, 55, 580-585. (36) Cheng, Y. W.; Li, X.; Sima, J. Kinetics of p-xylene liquid phase catalyic oxidation. (IV). Kinetics for PX and solvent buring. J. Chem. Ind. Eng. 2004, 55, 1894-1899. (37) Wang, Q. B.; Li, X.; Wang, L. J.; Cheng, Y. W.; Xie, G. Effect of water content on the kinetics of p-xylene liquid-phase catalytic oxidation to terephthalic acid. Ind. Eng. Chem. Res. 2005, 44, 4518-4522. (38) Wang, Q. B.; Li, X.; Wang, L. J.; Cheng, Y. W.; Xie, G. Kinetics of p-xylene liquid-phase catalytic oxidation to terephthalic acid. Ind. Eng. Chem. Res. 2005, 44, 261-266. (39) Cheng, Y. W.; Peng, G.; Wang, L. J.; Li, X. Kinetics of burning side reaction in the liquidphase oxidation of p-xylene. Chin. J. Chem. Eng. 2009, 17, 181-188. (40) Wang, Q. B.; Zhang, Y. Z.; Cheng, Y. W.; Li, X. Reaction mechanism and kinetics for the liquid-phase catalytic oxidation of meta-xylene to meta-phthalic acid. AIChE J. 2010, 54, 2674-2688.



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Only used as the Table of the Contents. Scheme of catalytic oxidation of 2,6-DMN to 2,6-NDA and byproducts, and concentration profiles of reactant, main intermediates and all products as a function of time.

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Figure 1. Experimental apparatus for kinetic investigations. 127x76mm (600 x 600 DPI)


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Figure 2. Concentration profiles of reactant, main intermediates and all products as a function of time. (a) the reactant 2,6-DMN, intermediates 2,6-MND, 2,6-MNA and 2,6-FNA, and desired product 2,6-NDA; (b) main byproducts TMA, OA+TA and NA. [Reaction conditions: T = 190 °C, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 161x169mm (300 x 300 DPI)



Figure 3. Scheme of catalytic oxidation of 2,6-DMN to 2,6-NDA. k1, k2, k3, and k4 are the main reaction constant of each step respectively, ks1, ks2, ks3, ks4, and ks5 are the side reaction constant of each step respectively; 2,6-DMN: 2,6-dimethylnaphthalene, 2,6-MND: 6-methyl-2-naphthaldehyde, 2,6-MNA: 6methyl-2-naphthalenecarboxylic acid, 2,6-FNA: 6-formyl-2-naphthoic acid, 2,6-NDA: 2,6naphthalenedicarboxylic acid, TA: terephthalic acid, OA: o-phthalic acid, NA: 2-naphthoic acid, TMA: trimellitic acid. 139x76mm (600 x 600 DPI)


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Figure 4. Concentration profiles of reactant and intermediates over time with different initial 2,6-DMN concentrations [reaction conditions: T = 190 °C, P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 224x197mm (300 x 300 DPI)



Figure 5. Concentration profiles of desirable product and byproducts over time with different initial 2,6-DMN concentrations [reaction conditions: T = 190 °C, P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 225x197mm (300 x 300 DPI)


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Figure 6. Concentration profiles of reactant and intermediates versus time at different reaction temperatures [reaction conditions: P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 224x197mm (300 x 300 DPI)



Figure 7. Concentration profiles of desirable product and byproducts versus time at different reaction temperatures [reaction conditions: P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 225x195mm (300 x 300 DPI)


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Figure 8. Concentration profiles of reactant and intermediates over time with different catalyst concentrations [reaction conditions: T =190 ℃, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 224x197mm (300 x 300 DPI)



Figure 9. Concentration profiles of desirable product and byproducts over time with different catalyst concentrations [reaction conditions: T =190 ℃, P = 2.5 MPa, 2,6-DMN/HAc (mass ratio) = 1/14, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm]. 225x195mm (300 x 300 DPI)


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Figure 10. 2,6-NDA concentration evolution over time by experiments and calculation [reaction conditions: P = 2.5 MPa, [Co] = 200 ppm, Co/Mn/Br (molar ratio) = 2/4/3, stirrer speed = 1000 rpm. (a) 2,6-DMN/ HAc (mass ratio) = 1/5, T = 190 ℃; (b) 2,6-DMN/HAc (mass ratio) = 1/6, T = 182 ℃]. 228x268mm (300 x 300 DPI)


Kinetics and Mechanism of Catalytic Oxidation of 2,6

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