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Kinetics, Catalysis, and Reaction Engineering

Kinetics and Mechanism of Catalytic Oxidation of 5-Methylfurfural to 2,5-Furandicarboxylic Acid with Co/Mn/Br Catalyst Heng Ban, Shuaibo Chen, Youdi Zhang, Youwei Cheng, Lijun Wang, and Xi Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03573 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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Industrial & Engineering Chemistry Research

Kinetics and Mechanism of Catalytic Oxidation of 5Methylfurfural to 2,5-Furandicarboxylic Acid with Co/Mn/Br Catalyst Heng Bana, Shuaibo Chena, Youdi Zhanga, Youwei Chenga,b*, Lijun Wanga and Xi Lia aZhejiang

Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology,

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

of Zhejiang University-Quzhou, 78 Jiuhua Boulevard North, Quzhou, China 324000

ABSTRACT: 2,5-Furandicarboxylic acid (FDCA), a versatile platform chemical, can be widely used for synthesizing various polymers and has the potential to replace terephthalic acid in green polymer industry. To replace part of relatively expensive and chemically unstable 5hydroxymethylfurfural (HMF), 5-methylfurfural (MF) was used as the starting material for preparing FDCA. The aerobic oxidation of MF to FDCA over the Co/Mn/Br catalyst system was investigated in both batch and semicontinuous reactors with acetic acid as solvent. On the basis of free-radical chain reaction mechanism, a fractional kinetic model was developed to describe the oxidation process of MF to FDCA and the relevant model parameters were determined by data fitting using the least squares method. Under the same reaction conditions, the FDCA yield of 73.5% and purity of 99.5% were achieved, lower than that of HMF oxidation to FDCA. The influence of stirring rates

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1was

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evaluated to eliminate the liquid-gas transfer resistance. Furthermore, the effect of substrate

concentration, reaction temperature and catalyst concentration on the reaction rate was investigated systematically. Finally, the developed fractional kinetic model was proved to be reliable for predicting the product distribution satisfactorily by the semicontinuous experiments. The kinetic data and reactor model play an essential role in the optimization for industrially producing FDCA.

KEYWORDS: 5-Methylfurfural, 2,5-Furandicarboxlylic acid, Co/Mn/Br catalyst, Catalytic oxidation.

1. Introduction The depleting fossil fuels and the concern about their environmental impact are driving people to pay increasing attention to chemical production from renewable biomass.1-4 2,5-Furandicarboxylic acid (FDCA), a new chemical building block derived from biomass, has been recognized as one of the top 12 bio-based compounds by U.S. Department of Energy due to its multi-functionality with a cyclic structure and two carboxyl groups.5 For instance, the application opportunities of FDCA are multiple, such as polyesters, polyurethanes, polyamides and biochemicals.6-8 One of the most popular applications is its copolymerization with ethylene glycol (EG) for producing green polymer polyethylene furanoate (PEF), which is superior to the counterpart polyethylene terephthalene (PET) synthesized from petroleum-based terephthalic acid (TA) and EG in terms of gas barrier property, mechanical strength and thermal stability. As a consequence, PEF can also be extensively used for engineering plastics, packaging, coatings, and fibers by replacing PET.7, 9, 10 Additionally, PEF with excellent biodegradability is more appealing than PET for environmental protection. A FDCA pilot plant with an annual capacity of 500 tons has been established by Avantium since 2011 in the Netherlands and the first commercial FDCA plant with yearly capacity 50 000 tons is under

 Corresponding author. E-mail: [email protected].


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construction in Antwerp, Belgium by Avantium.11, 12 Thus, PEF is regarded as a promising platform chemical with a bright market prospect and increasing demand.6, 10, 13 Currently, 5-hydroxymethylfurfural (HMF) is usually used as the starting material for producing FDCA via catalytic oxidation.14 Many efforts have been made to produce HMF by the dehydration of monose, like glucose and fructose.15 Although high yield of HMF can be achieved in high-boiling solvents including dimethylsulfoxide (DMSO) and dimethylformamide (DMF),16 yet the separation and purification of HMF is an energy-intensive process, which makes the HMF production in limited volume with relatively high operation cost.14 biomass HO

H 3C

(OH)3

(OH)3 carbohydrates

(OH)3 OH

OH

OH

xylose

6-deoxy carbohydrates H 90

dehydration

2,

HI

,R

℃

,0

.5-

O

uC

1.0

l3

dehydration or

h

O

Pd

/C

2-furaldehyde O

O

OH

O

O

CH3

HCOOH, PVP-assisted Pd catalyst MF

HMF catalyst

catalyst

O

O O

OH

HO

FDCA HMF founctional group chemical stability solubility boiling point separation method separation

MF

hydroxymethyl and aldehyde unstable

methyl and aldehyde stable

soluble in both water and hydrocarbon

soluble in hydrocarbon and insoluble in water 51 ℃ (1 torr),188 ℃ (760 torr)

110 ℃ (0.02 torr) vacuum distillation and extraction difficult with low yield and much energy

atmospheric distillation and extraction easy with high yield



Page 4 of 35

Scheme 1. Comparison between HMF and MF.17-21 As shown in Scheme 1, although HMF and 5-methylfurfural (MF) have different functional groups, namely hydroxymethyl group and methyl group, respectively, both hydroxymethyl group and methyl group can be catalytically oxidized into carboxyl group. With the same furan ring and similar functional groups, both HMF and MF can be catalytically oxidized into FDCA with two functional groups converted into carboxyl groups.22 Compared with HMF, MF is an industrially on-going product from biomass and often used as a useful intermediate for the synthesis of pharmaceuticals,23 chemicals for agriculture,24 perfumes,24 fuels,17,

25

and other applications.26-28 The commercial

production of MF is involved with the conversion of platform chemical furfural derived from carbohydrates containing raw materials like corncobs, oat, wheat bran and sawdust, etc, which does not compete with human food supply.20 Additionally, MF can also be synthesized via the catalytic dehydration of renewable 6-deoxy hexoses or fructose with high selectivity, as shown in Scheme 1.17, 20, 21

Additionally, HMF (boiling point: 110 °C, 0.02 torr) is sensitive to light and prone to be

converted into byproducts at high temperature with acid via nucleophilic addition of its reactive hydroxyl group and aldehyde group.29 On the contrary, MF (boiling point: 51 °C, 1 torr) exhibits better chemical stability than HMF with methyl group replacing hydroxymethyl group and can be directly separated by atmospheric distillation.30 In addition, HMF with a hydrophilic hydroxyl has good solubility in both water and organic phase, so the HMF yield from extraction separation is usually far from satisfactory for its poor partition coefficient of HMF.31 Compared with HMF, MF with a hydrophobic methyl group can hardly mix with water and be easily separated via extraction operation with high yield.17 Therefore, chemically stable MF with wide sources can also serve as the feedstock for industrially preparing FDCA and replace part of HMF, increasing the diversity of raw materials. Noble metal catalysts, such as Pt,32,

33

Pd,34,

35

Au,36-38 and zeolite,39 have been found to be

effective for the oxidation of HMF to FDCA. Although the oxidation experiments could be


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conducted at mild reaction temperature (65-130 ℃), yet FDCA tends to precipitate as the solid phase and blocks the active sites, resulting in the deactivation of the catalysts.34-37 To improve the yield, base solution is usually used as the reaction medium for HMF conversion into FDCA’s salt, which has to be further acidified into FDCA by extra adding strong acids. But the consumption of large amounts of base and acid for the production of FDCA exerts an adverse impact on environment protection.37 Furthermore, the high price of noble metal catalysts also remains an obstacle for the industrial application of above methods.32-37 To avoid using large amounts of acid and base and highly expensive noble catalysts, many researchers have conducted investigations on the efficient heterogeneous catalytic systems based on non-noble metal catalysts without the need for base additives or noble metal catalysts.40-45 Although high FDCA yield (85-99%) could be obtained via HMF oxidation over non-noble catalysts, yet most of the oxidation processes took a long period (528 h).40-45 The majority of the production of various aromatic dicarboxylic acids, like TA, iso-phthalic acid and 2,6-naphthalenedicarboxylic acid (2,6-NDA) is via the aerobic catalytic oxidation of dimethylaromatics with air as the oxidant in acetic acid medium, catalyzed by cobalt, manganese, and bromide compounds, in a process commonly known as the AMOCO Mid-Century (MC) process.46-48 Industrial production of TA is achieved through the liquid-phase oxidation of p-xylene (PX) with TA yield more than 95 mol %.49 By extending the MC process to FDCA production, Partenheimer has proved that HMF was successfully oxidized into FDCA by oxygen over the Co/Mn/Br catalysts in acetic acid at 125 ℃ and the FDCA yield was about 60%.50 Zuo et al. also applied the traditional MC process to the oxidation of HMF to FDCA and enhanced the FDCA yield to 90% by optimizing operating conditions, such as catalyst composition, temperature, water content in the solvent, and pressure.51 Zuo et al. also further investigated the reaction mechanism and kinetics of the liquid-phase oxidation of HMF to FDCA in a semi-batch oxidation reactor.52



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To the best of our knowledge, there is little investigation on MF oxidation to FDCA in the MC process, not to mention the relevant kinetics. For the rational design and the scale-up of reactors, it is essential to gain some insight into the cascade reaction kinetics of MF oxidation to FDCA. In this work, the chemical stability of HMF and MF was compared in acetic acid solution and both HMF and MF oxidation to FDCA were also carried out by the AMOCO MC method. On the basis of free-radical chain reaction mechanism, the detailed pathways and a fractional kinetic model for MF oxidation to FDCA were developed for describing the product distribution over time. Some operating variables including substrate concentration, reaction temperature, and catalyst concentration, influencing the oxidation process were investigated to maximize FDCA yield. Finally, the semicontinuous experiments of MF oxidation to FDCA were carried out to estimate the reliability of the developed kinetic model. 2. Experimental Section 2.1. Materials. 5-Hydroxymethylfurfural (HMF) with a purity of 98 % used in all experiments was purchased from Hefei Leaf Biological Technology Co., Ltd. 5-Methylfurfural (MF) and 5methylfuran-2-carboxylic acid (MCA) used in all experiments were purchased from Ailan (Shanghai) Chemical Technology Co., Ltd. with 98–99% purity. 5-Formyl-2-furancarboxylic acid (FFCA) was purchased from Shanghai Macklin Biochemical Co., Ltd. Air (oxidant) and nitrogen were bought from Hangzhou Jingong Tezhong Gas Company. Other reagents containing p-xylene (PX), terephthalic acid (TA), 2,5-furandicarboxylic acid (FDCA), Co(CH3COO)2·4H2O(catalyst), Mn(CH3COO)2·4H2O (catalyst), HBr (40% in water, promoter) and acetic acid (HAc) as the solvent were all chemically pure reagents from Sinopharm Chemical Reagent Co., Ltd. 2.2. Apparatus and Procedure. The experimental apparatus used in this work was described detailedly in our previous works.48,

49, 53-61

The kinetic experiments were carried out in a 0.5 L

titanium reactor as shown in Figure 1. The air flow rate was measured by the gas mass flow meter. With the advanced PID system and data acquisition system combined, the temperature was well


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controlled with the temperature fluctuation of ± 1.0 ℃ . In a typical batch run, the reactor was preloaded with a certain amount of substrate, acetic acid (HAc) and the Co/Mn/Br catalysts. Then the reactor was purged with nitrogen and pressurized to a certain pressure. When the temperature of the solution reached the desired value, gas (air) was introduced into the liquid phase to replace nitrogen continuously. Air was continuously fed through the liquid until the O2 concentration of tail gas didn’t change. In a typical semicontinuous run, the reactor was firstly charged with a certain amount of HAc and the Co/Mn/Br catalysts. When the reactor temperature reached the set value, gas was fed into the liquid phase to replace nitrogen continuously and the feedstock containing HAc and substrate was added into the reactor by a metering pump continuously. When all feedstock was fed into the reactor, the metering pump was switched off and air was continuously introduced through liquid phase until the O2 concentration of tail gas remained constant. In each experiment, a three-way valve was used for taking 8-10 samples out at the interval from 0.5 to 3 min and the volume of every sample was from 1.0 to 2.0 mL. After each sampling, highpressure nitrogen was used to sweep the sampling tube to avoid residual solid-liquid mixtures in the sampling tube. Samples withdrawn from the reactors that were a slurry at the reaction temperature were separated into the liquid phase and the solid phase by disposable filter needles. Then the liquid phase was diluted with methanol and the solid phase was also dissolved with methanol. Both the diluted liquid phase and the dissolved solid phase were analyzed by high-performance liquid-phase chromatography (HPLC). When the concentration of oxygen in tail gas remained constant, the oxidation reaction was thought to stop.



Page 8 of 35

Gas Mass Flow Meter

CO Analyzer

3-Way Valve TC P

Back Pressure Regulator

Needle Valve Thermocouple

CO2 Analyzer

Pressure Gauge Stirring Motor Rotameter

O2 Analyzer

Condenser Gas Dryer Agitator Gas-liquid Separator TC

O2 Reservoir

Data Acquisition and Control

Computer

P

Pump

HPLC Analysis

TC

Feedstock Reservoir Flow Rate Controlling

O2

N2

Pump

Diathermic Oil Reservoir

Figure 1. Experimental apparatus for kinetic investigation. 2.3. Analytical Method. On the basis of the external standard method, the reactant 5methylfurfural (MF), 5-methylfuran-2-carboxylic acid (MCA), 5-formyl-2-furancarboxylic acid (FFCA) and desired 2,5-furandicarboxylic acid (FDCA) were quantified by high-performance liquidphase chromatography (HPLC) equipped with Agilent ZORBAX Eclipse XDB-C18 column. 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 composed of three eluents: water (0.2 wt % H3PO4), methanol and acetonitrile. The detailed gradient elution program for separating different components is presented as Table S1 in Supporting Information (SI). The products were quantified by a UV detector at 278 nm. The concentration of O2 in the tail gas was monitored by a magnetic oxygen analyzer (Shanghai Encel Instrument Co., Ltd. EN 560). The concentrations of CO and CO2 in the tail gas were


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monitored by the infrared gas analyzer (Shanghai Encel Instrument Co., Ltd. EN 308). The purity of oxidation product (FDCA and TA) was determined by HPLC using an external method. 2.4. Yield Calculation. In this work, the yield of FDCA and by-products were defined as The oxidation product FDCA yield, % = (mol of FDCA detected in liquid phase + mol of FDCA detected in solid phase)/(mol of initial HMF) × 100 % The oxidation byproducts yield, % = (mol of CO and CO2 detected in gas phase + mol of byproducts detected in liquid phase)/(mol of initial HMF) × 100 % 3. Lumped Reaction Scheme and Kinetic Model Investigation into the reaction mechanism is essential for optimizing reaction conditions and scaleup of reactors in industry. The liquid-phase catalytic oxidation of aromatic hydrocarbons including pxylene (PX), 2,6-dimethylnaphthalene (2,6-DMN), 5-hydroxymethylfurfural (HMF) and 5methylfurfural (MF) to aromatic dicarboxylic acids is widely recognized to belong to the free-radical chain reaction mechanism, in which the initiation, propagation and termination steps play an important role.47, 50 As shown in reactions 1 to 3, although Co3+ and Mn3+ are effective oxidants, they are not strong enough to directly oxidize MF into 5-methylfuran-2-carboxylic acid (MCA). However, the hydrogen abstraction reaction is initiated by bromine free radicals generated from the electron transfer between Co and Mn ions. Additionally, Co and Mn ions can also make hydroperoxides decomposed, generating free radicals and facilitating the rapid generation of free radicals, which is called the synergic effect between Co and Mn. Since aldehyde group is comparatively more reactive than methyl group, the aldehyde group of MF is firstly catalytically oxidized into carboxyl group followed the conversion of methyl group into carboxyl group via aldehyde group, as shown in Scheme 2.



O O

O

O

[O]

O OH

k1 MCA

MF

O

[O] OH

k2

k3

OH

HO

FFCA

ks2

O

O

O O

[O]

Page 10 of 35

FDCA

ks3

ks1

Byproducts

Scheme 2. Scheme of catalytic oxidation of MF to FDCA. ki and ksi (i = 1-3) are the reaction rate constant of each step respectively. MF: 5-methylfurfural, MCA: 5-methylfuran-2-carboxylic acid, FFCA: 5-formyl-2-furancarboxylic acid, FDCA: 2,5-furandicarboxylic acid. Co 3  Br    Co 2  Br g

（1）

Co 3  Mn 2   Co 2  Mn 3

（2）

Mn 3  Br    Mn 2  Br g

（3）

For the liquid-phase catalytic oxidation of alkyl-aromatics, the reaction rate of each step was supposed to be zero-order for oxygen when the oxygen was in stoichiometric excess and its partial pressure was above 100 Torr.62,

63

The pseudo-first-order kinetic model was widely used for the

liquid-phase oxidation of PX to TA catalyzed by Co/Mn/Br catalysts, where all the reactions were first order with respect to all reactants involved.62, 63 However, the first order kinetic model was actually empirical without considering the relevant reaction mechanism. Taking the catalyst cycle and interaction of radicals into account, the detailed reaction mechanism and a fractional kinetic model for MF oxidation to FDCA derived from the freeradical chain reaction mechanism were deduced step-by-step and illustrated as Sections Reaction Mechanism of MF oxidation to FDCA and Kinetic Model in SI, as shown in equations 4-9. Similar fractional kinetic models for the oxidation of PX to TA and 2,6-DMN to 2,6-naphthalendicarboxylic acid have also been proposed by our workgroup,48, 49, 53-61 and proved to describe the distribution of oxidation products satisfactorily, especially under industrial conditions.48, 49, 53-61


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rj 

rsj 

k jC j     d iCi    i 1  3

j

k sjC j     d iCi    i 1  3

j

j  1 3

(4)

j  1 3

(5)

dC1   r1  rs1 dt

(6)

dC2  r1  r2  rs 2 dt

(7)

dC3  r2  r3  rs 3 dt

(8)

dC4  r3 dt

(9)

where the subscripts 1-4 represent MF, MCA, FFCA, and FDCA, respectively; rj and rsj (j = 1-3) is the reaction rate of the step j, mol/(kg•min); kj and ksj (j = 1-3) is the reaction rate constant of the step j affected by temperature, catalyst concentration, etc. , min-1; Cj (j = 1-4) is the concentration of the component j, mol/kg; di, βi and ε (i = 1-3) are the modified parameters given the catalyst cycle and radical interaction; t is the reaction time, min; with the following initial conditions: C1= C10, C2 = C3 = C4 = 0

(10)

The kinetic model parameters were regressed by the nonlear least-squares method to minimize the difference between the calculated values and experimental results as follows: m

n

min E    Cijcal  Cijexp 

2

(11)

i 1 j 1

where Ccal ij and Cexp ij represent the calculated and experimental concentrations of the component j in the ith experimental run, respectively. m and n denote the number of the kinetic experiments and key components, respectively. 4. Results and Discussion



0.5

MF concentration (mol/kg)

a

MF concentration

0.4

0.3

0.2

0.1

0.0 0

30

60

90

120

Time (min) 0.4

HMF concentration AMF concentration HMF+AMF concentration

b 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

Page 12 of 35

0.3

0.2

0.1

0.0 0

30

60

90

120

Time (min)

Figure 2. Comparison of MF and HMF chemical stability [reaction condition: T = 140 ℃; PN2 = 2.0 MPa; 210 g HAc solution with Co/Mn/Br catalysts added into reactor in advance; 40 g HAc solution containing HMF (10 g) or MF (10 g) added at 10 g solution/min via a metering pump; [Mn] = 4 mmol/kg, Mn/Co/Br (mol/mol/mol)= 1/5/5; stirring rate = 1000 rpm]. O

O

O

O

+ HAc

HO

AMF

HMF

+ H2O

O H3 C

Condensation products (Polymers,humins)

+ H2O

AcO

OH O Levulinic acid

+ HCOOH Formic acid


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Scheme 3. Esterification, condensation and hydrolysis process of HMF. 4.1. Comparison between HMF and MF chemical stability. HMF with an active hydroxyl group can not only be oxidized by oxygen, but also condense into brown polymers and humins, especially with Bronsted acid or Lewis acid catalysts, as shown in Scheme 3. So the separation and storage of HMF had better be performed without acid. Additionally, the hydrolysis of HMF can also occur with acids as the catalyst including acetic acid, leading to the generation of levulinic acid (LA) and formic acid, as shown in Scheme 3. As shown in Figure 2 and Scheme 3, after the mixture of HMF and acetic acid was fed into acetic acid solution containing Co/Mn/Br catalysts, 5-acetoxymethylfurfural (AMF) was formed via the esterification of reactive HMF and acetic acid in the presence of the Co/Mn/Br catalyst system and its concentration increased fast from 0 to 90 min. As shown in Figure 2, the concentration of MF remained constant, while the total concentration of HMF and AMF decreased continuously from 0 to 120 min, due to HMF decomposition into LA and oligomerization, which further confirms that MF is significantly more chemically stable than HMF under acidic conditions. Finally, after HMF dissolving in acetic acid was fed into the acetic acid solution containing the Co/Mn/Br catalysts, the concentration of AMF also increased slowly after 90 min. So both AMF and MF are more chemically stable than HMF under acetic conditions in the presence of Co/Mn/Br catalyst system. 4.2. Comparison of PX, HMF and MF Oxidation to Aromatic Diacid. In the MC process, the liquid-phase catalytic oxidation of PX plays an important role in the industrial production of TA. Additionally, the oxidation of HMF to FDCA has also been proved to be successful with the FDCA yield of 90% in the MC process.51,

52

Little research has been conducted for synthesizing FDCA

using MF as the starting material. So, to begin, MF was catalytically oxidized by air with acetic acid as the solvent in the presence of Co/Mn/Br catalysts to evaluate the feasibility of MF oxidation to FDCA. To gain insight into the difference of HMF and MF oxidation to FDCA, the oxidation



Page 14 of 35

experiments were carried out under the same conditions. HMF (solid at room temperature) dissolved in acetic acid and MF (liquid at room temperature) mixed with acetic acid were continuously fed into the reactor containing catalyst solution through a pump separately. HO

O

O O

[O]

O

O O

[O]

O

DFF

O

[O] OH

HMF

O

O

O

FFCA

OH

HO

FDCA

Scheme 4. Pathway for HMF oxidation to FDCA. DFF: 2,5-diformylfuran. As shown in Figure 3a, although reactant (HMF, MF and PX) conversion and oxidation product (FDCA and TA) purity are almost 100% (>= 99.5%), the FDCA yield from MF oxidation and HMF oxidation are 73.5 mol % and 86.1 mol %, respectively, which are much lower than TA yield (97.9 mol %) from PX oxidation. This could be attributed to the fact that the furan ring is not as chemically stable as benzene ring. For instance, the electron cloud distribution of furan is not as even as benzene ring, and the resonance energy of furan is 17 kcal/mol, much lower than that of benzene (36 kcal/mol).50 Additionally, the oxygen of the furan ring has an electron donating effect, so the electrophilic substitution reaction activity of furan is stronger than that of benzene. It is proved by the resonance theory that the electron cloud density on the furan ring is relatively larger than that of benzene ring, supporting the theory of electrophilic reactivity. So the furan ring bromination and the furan ring attack by radicals are exacerbated and the furan ring tends to undergo its cleavage, generating more byproducts, such as COx (CO and CO2), as shown in Figure 3b. The FDCA yield from MF oxidation is only 73.5 mol %, quite lower than that of HMF oxidation due to more overoxidation of substrate to byproducts (Figure 3). As shown in Schemes 2 and 4, the HMF conversion to FFCA via 2,5-diformylfuran (DFF) and MF conversion to FFCA via MCA over a Co/Mn/Br catalyst system follow the classical free-radical chain reaction mechanism in the MC process. In the HMF oxidation to FDCA process, the hydroxymethyl group rather than the aldehyde group is firstly converted into a carboxyl group, which suggests that the hydroxymethyl group is


Page 15 of 35

more reactive than the aldehyde group in the MC process. While the aldehyde group rather than the methyl group is firstly converted into a carboxyl group in the MF oxidation to FDCA process, which suggests that the aldehyde group is more reactive than the methyl group in the MC process. So the reactivity of hydroxymethyl group of HMF is higher than the methyl group of MF and the conversion of MCA to FFCA is more difficult than the conversion of DFF to FFCA. As a result, undesirable side reactions are exacerbated in the MF oxidation process, generating more byproducts, such as COx and reducing FDCA yield. 120

HMF oxidation MF oxidation PX oxidation

a

(mol %)

90

60

30

0 Conversion

0.8

0.6

(mol/mol)

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


Diacid purity

Diacid yield

b HMF oxidation MF oxidation PX oxidation

0.4

0.2

0.0 CO/substrate

CO2/substrate

Figure 3. Comparison of PX oxidation, MF oxidation, and HMF oxidation [reaction conditions: Pair = 2.0 MPa; [Mn] = 4 mmol/kg; Co/Mn/Br (molar ratio) = 1/5/5; stirring rate = 1000 rpm; 260 g HAc solution containing catalyst added into reactor at first; 40 g solution containing 0.1 mol substrate fed into the reactor at 5 (g solution)/min; Fair (air flow rate) = 5 L/min; PX oxidation: T = 180 ℃; MF and HMF oxidation: T = 150 °C].



4.3. Influence of Stirring Rate. The oxidation of MF to FDCA under the MC process conditions with fast reaction rates might be restricted by gas-liquid mass transfer, especially with inadequate mixing of gas and liquid. Four experiments with different stirring rates (stirring rate: 500 rpm, 750 rpm, 1000 rpm and 1250 rpm) were carried out in the batch reactor to evaluate the influence of the stirring rate on the reaction rate. Since the oxidation of MCA to FFCA is the slowest step among the sequential oxidation steps, the gas-liquid mass transfer resistance exerts a greater impact on the oxidation step of MCA→FFCA. For instance, the measurement uncertainty associated with the rate constant of the slowest step MCA→FFCA (k2) is much smaller than those of other steps. The temporal product distributions of the intermediate MCA and desired product FDCA at varying stirring rates are shown in Figure 4. 0.4

a

n = 500 rpm n = 750 rpm n = 1000 rpm n = 1250 rpm —— calculated values

CMCA (mol/kg)

0.3

0.2

0.1

0.0 0

5

10

15

Time (min)

0.4

CFDCA (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 16 of 35

0.3 0.2

b n = 500 rpm n = 750 rpm n = 1000 rpm n = 1250 rpm —— calculated values

0.1 0.0 0

5

10

15

Time (min)


Page 17 of 35 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


Figure 4. Experimental and simulated product distributions at different stirring rates [reaction conditions: MF = 20 g; MF/HAc (mass ratio) = 1/16; T = 140 °C; P = 2.0 MPa; [Mn] = 4 mmol/kg; Co/Mn/Br (molar ratio) = 1/5/5; Fair (air flow rate) = 16 L/min; calculated values from the proposed fractional kinetic model shown in Table 1]. When stirring rates of the agitator are in the range from 750 to 1250 rpm, the stirring rates have little influence on the reaction rate, even the slowest oxidation step MCA → FFCA. So when the stirring rates are above 750 rpm, the gas-liquid mass transfer limitation could be eliminated. In the oxidation experiment, the stirring rate of 1000 rpm and excessive air (air flow rate: 16 L/min) were adopted for eliminating the gas-liquid mass transfer limitation. 4.4. Influence of Substrate concentration. The oxidation experiments with varying concentrations of substrate MF (MF:HAc (mass ratio) =1:13, 1:16 and 1:21) were carried out to determine the kinetic parameters.



a

b

1.00

MF:HAc = 1:21 MF:HAc = 1:16 MF:HAc = 1:13 —— model fitting

0.75


0.6

CMCA/CMF,0

CMF/CMF,0

0.4

0.50

0.2 0.25

0.0

0.00

0

5

10

15

0

5

Time (min)

10

15

Time (min)

0.8

c

d


0.3

0.6

0.2

CFDCA/CMF,0

CFFCA/CMF,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

Page 18 of 35

0.4

0.1 0.2

0.0


0.0

0

5

10

Time (min)

15

0

5

10

15

Time (min)

Figure 5. Concentration profiles of reactant, intermediates and product over time with different MF concentrations [reaction conditions: HAc = 320 g; T = 140 °C; Pair = 2.0 MPa; [Mn] = 4 mmol/kg; Co/Mn/Br (molar ratio) = 1/5/5; stirring rate = 1000 rpm; Fair = 16 L/min]. For comparison, the concentrations of all involved components were divided by the initial concentration of MF under the corresponding conditions. As shown in Figure 5, the lower the initial MF concentration is, the shorter the reaction time is. The half-life for reactant MF depends on the substrate concentration, which is obviously inconsistent with the first-order reaction law. On the basis of the free-radical chain reaction mechanism, the fractional kinetic model was developed to describe the oxidation process, and the kinetic model parameters were regressed by minimizing the difference of calculated values and experimental results. It is shown in Table 1 that the reaction rate constant k2 is relatively smaller than the rate constants k1 and k3, which suggests that the oxidation step of MCA to FFCA is the rate-limiting step. This also confirms that the aldehyde group is more


Page 19 of 35 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


reactive than a methyl group and prone to be further oxidized into a carboxyl group. Additionally, similar phenomenon has been observed in the oxidation of p-toluic acid to 4-carboxybenzaldehyde.49, 54, 58, 59

As shown in Table 1, the reaction rate constant k3 of FFCA → FDCA is comparably smaller

than the reaction rate constant k1 of MF → MCA, due to the replacing of –CH3 group by -COOH group. For instance, the -COOH group as a strong electron-withdrawing group tends to attract electron of the furan ring and makes 2-carboxyl-5-furanmethyl radical stabilize more difficultly as the resonance structure. After the aldehyde group of MF is oxidized into a carboxyl group, the furan nucleus is significantly deactivated by the electron-withdrawing effect of the carboxyl group. Similar phenomenon has also been observed in the oxidation of PX to TA.56-58 Table 1. Parameters for the Fractional Kinetic Model with 95% confidence levela main reactions

side reactions

i

ki, min -1

di

βi

1

3.07×10-1

0.709

0.741

2

8.38×10-2

0.827

0.944

3

1.22×10-1

0.752

0.776

aReaction

conditions: HAc = 320 g; T = 140 °C; Pair = 2.0 MPa; [Mn] = 4 mmol/kg; Co/Mn/Br

ε

3.49×10-3

(molar ratio) = 1/5/5; stirring rate = 1000 rpm; Fair = 16 L/min. 4.5. Effect of Temperature.


i

ki, min -1

s1

1.38×10-2

s2

1.59×10-2

s3

1.61×10-2


0.4 0.60

a

0.30

135 ℃ 140 ℃ 145 ℃ 150 ℃ —— model fitting

0.3

CMCA (mol/kg)

CMF (mol/kg)

b

135 ℃ 140 ℃ 145 ℃ 150 ℃ —— model fitting

0.45

0.2

0.1

0.15

0.0

0.00

0

5

10

15

0

5

Time (min)

10

15

Time (min)

0.20

0.5 135 ℃ 140 ℃ 145 ℃ 150 ℃ —— model fitting

c

d 0.4

CFDCA (mol/kg)

0.15

CFFCA (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 35

0.10

0.3

0.2

0.05

135 ℃ 140 ℃ 145 ℃ 150 ℃ —— model fitting

0.1

0.00

0.0

0

5

10

15

0

5

Time (min)

10

15

Time (min)

Figure 6. Concentration profiles of reactant, intermediates and product versus time at different reaction temperatures [reaction conditions: MF = 20 g; MF/HAc (mass ratio) = 1/16; Pair = 2.0 MPa; [Mn] = 4 mmol/kg; Co/Mn/Br (molar ratio) = 1/5/5; stirrer speed = 1000 rpm; Fair = 16 L/min]. As can be seen from Figure 6, the reaction rates were greatly increased and the reaction time for reaching the maximum yield of FDCA was significantly shortened with increasing temperature. The relevant kinetic parameters for the fractional kinetic model were estimated by data fitting. Additionally, the effect of temperature on the reaction rate of every step can be well described by Arrhenius equation (equation 12), as shown in Table 2.

ki  ki 0 exp( 

Eai ) RT

Table 2. Pre-exponential Factor and Activation Energya


(12)

Page 21 of 35 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


i

Eai, kJ/mol

ki,0, min-1

variance (R2)

i

Eai, kJ/mol

ki,0, min-1

variance (R2)

1

46.7

2.39×105

0.992

s1

33.1

2.09×102

0.988

2

56.0

9.55×105

0.980

s2

32.1

1.81×102

0.992

3

39.6

1.27×104

0.996

s3

27.9

5.39×101

0.996

aReaction

conditions: MF = 20 g; MF/HAc (mass ratio) = 1/16; Pair = 2.0 MPa; [Mn] = 4 mmol/kg;

Co/Mn/Br (molar ratio) = 1/5/5; stirring rate = 1000 rpm; Fair = 16 L/min. It can be seen from Table 2 that the activation energy of MCA oxidation to FFCA step is 56.0 kJ/mol, larger than other steps, indicating that increasing temperature exerts greater impacts on the oxidation of the methyl group than the adehyde group. Most importantly, as shown in Figure 6, when the reaction temperature increased from 135 to 150 ℃, FDCA yield was improved from 68.8% to 73.5%, suggesting that increasing reaction temperature is favorable for enhancing the FDCA selectivity. 4.6. Effect of Catalyst Concentration.



0.4 0.60

a

b

[Co+Mn] = 18 mmol/kg [Co+Mn] = 24 mmol/kg [Co+Mn] = 30 mmol/kg —— model fitting


0.3

CMCA (mol/kg)

CMF (mol/kg)

0.45

0.30

0.2

0.1

0.15

0.0

0.00

0

4

8

12

16

0

4

Time (min)

8

12

16

Time (min)

0.20


c

d 0.45

CFDCA (mol/kg)

0.15

CFFCA (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 22 of 35

0.10

0.30

0.15

0.05


0.00

0.00

0

4

8

12

16

Time (min)

0

4

8

12

16

Time (min)

Figure 7. Concentration of reactant, intermediates and product over time with varying catalyst concentrations [reaction conditions: MF = 20 g; MF/HAc (mass ratio) = 1/16; Pair = 2.0 MPa; Co/Mn/Br (molar ratio) = 1/5/5; stirring rate = 1000 rpm; Fair = 16 L/min]. Catalyst concentration has a great influence on all reaction steps. As shown in Figures 7 and 8, the main reaction rate of MCA to FFCA was accelerated by 1.45 times, when the total concentration of (Co+Mn) increased from 18 to 30 mmol/kg. So increasing catalyst concentration is beneficial for enhancing reaction rates. But high concentration of bromine in acetic acid solution is corrosive for the equipment, especially at high temperature. So suitable catalyst concentration should be determined for avoiding severe corrosion.


Page 23 of 35

0.4

a

k1 k2 k3

ki (min-1)

0.3

0.2

0.1

0.0 16

20

24

28

32

28

32

[Co+Mn] (mmol/kg) 0.020

b

ki (min-1)

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


ks1 ks2 ks3

0.015

0.010 16

20

24

[Co+Mn] (mmol/kg)

Figure 8. Relationship between ki and the total catalyst concentration [reaction conditions: MF = 20 g; MF/HAc (mass ratio) = 1/16; Pair = 2.0 MPa; Co/Mn/Br (molar ratio) = 1/5/5; stirring rate = 1000 rpm; Fair = 16 L/min]. 4.7. Verification by Semicontinuous Experiments. In industry, the liquid-phase catalytic oxidation of MF to FDCA is usually carried out via a continuous process. For evaluating the reliability of the proposed fractional kinetic model for MF oxidation to FDCA, some experiments with different MF concentrations and temperatures were performed. In the semicontinuous



Page 24 of 35

experiment, the concentration variation of components in the reaction media can be described as Table 3: Table 3. Concentration Variation of Components in Semicontinuous Process by Model Prediction

during substrate feed

after substrate feed

dC1 F  C1Qm   r1   rs1 dt mt

(13)

dC1   r1  rs1 dt

(18)

dC2 CQ  r1  r2  rs 2  2 m dt mt

(14)

dC2  r1  r2  rs 2 dt

(19)

dC3 CQ  r2  r3  rs 3  3 m dt mt

(15)

dC3  r2  r3  rs 3 dt

(20)

dC4 CQ  r3  4 m dt mt

(16)

dC4  r3 dt

(21)

mt  m0  Qmt

(17)

mt  mT

(22)

where the subscript 1-4 represent MF, MCA, FFCA, and FDCA, respectively, rj and rsj (j = 1-3) is the reaction rate of the step j, mol/(kg•min); t is the reaction time, min; Qm is the mass flow rate of HAc solvent, kg HAc/min; mt is the mass of solvent at t, kg; F is the molar feed rate of the substrate, mol/min; m0 is the initial mass of solvent, m0 = 0.3 kg; mT is the mass of solvent after the substrate feed, kg. By adopting the fractional kinetic model obtained previously, the concentration variation of all molecular components was calculated with COMSOL software.


Page 25 of 35

100

a

b calculated CMF calculated CMCA calculated CFFCA

0.2

experimental CMF experimental CMCA experimental CFFCA

CFDCA (mol/kg)

Concentration×103 (mol/kg)

75

0.1

50

25

Experimental data Calculated values 0.0

0 0

5

10

15

0

5

Time (min)

10

15

Time (min) 120

Calculated CMF Calculated CMCA Calculated CFFCA

d

c

Concentration×103 (mol/kg)

0.3

CFDCA (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


0.2

0.1

Experimental CMF Experimental CMCA Experimental CFFCA

80

40

Experimental data Calculated values

0.0

0

0

5

10

15

0

Time (min)

5

10

15

Time (min)

Figure 9. Concentration evolution over time by semicontinuous operation and model predication [semicontinuous experiments: Pair = 2.0 MPa; stirring rate = 1000 rpm; [Mn] = 4 mmol/kg; Co/Mn/Br (molar ratio) = 1/5/5; 300 g acetic acid containing Co/Mn/Br catalysts added into the reactor in advance; 60 g acetic acid mixed with MF fed into the reactor by a pump at 10 g/min; Fair = 6 L/min; (a) and (b): T = 133 ℃; MF feed = 12 g; (c) and (d): T = 142 ℃; MF feed = 15 g]. It can be seen from Figure 9 that there is good agreement between model predication and experimental results. The concentration of FDCA remained constant after 12 min in Figures 9a and 8c, suggesting that the oxidation process of reactant and intermediates were completely oxidized and FDCA is chemically stable even under such severe conditions. As shown in Figures 9a and 9b, calculated values of the kinetic model could well predict all component concentration variation at the



Page 26 of 35

temperature out of the investigated range. Hence, the fractional kinetic model is further proved to be reliable by the semicontinuous experimental results. 5. CONCLUSIONS MF with excellent chemical stability, the similar structure and wide resources derived from biomass is proved to be a promising alternative of bio-based HMF for producing the platform chemical FDCA. Under the same oxidation condition, 73.5% FDCA yield was obtained from MF oxidation, which is lower than that (86.1%) from HMF oxidation due to more difficulty of methyl group oxidation to carboxyl group than hydroxymethyl group oxidation. On the basis of the freeradical chain reaction mechanism, a comprehensively fractional kinetic model for the oxidation of MF to FDCA was established accordingly. Key factors including substrate concentration, temperature and catalyst concentration, influencing product distribution were systematically investigated. The kinetic model parameters were regressed by data fitting with MATLAB software. Additionally, it was confirmed by experiment results that MCA oxidation to FFCA step is the rate determining step for the cascade oxidation reactions of MF to FDCA, due to more difficulty of methyl group oxidation than aldehyde group oxidation. Since the electron-withdrawing carboxyl group inhibits the further oxidation of methyl group to carboxyl group via aldehyde group, FFCA conversion to FDCA is significantly slower than MF conversion to MCA. Finally, it was validated by the results of semicontinuous experiments that the proposed fractional kinetic model is effective for predicting product distributions. The determined fractional kinetic model can provide fundamental data and method, which could be used in the reactor scale-up and the process optimization for the oxidation of MF to FDCA.

ASSOCIATED CONTENT Supporting Information


Page 27 of 35 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


The Supporting Information is available free of charge on the ACS Publications. Figure S1. Pathways of MF oxidation to FDCA. MF: 5-methylfurfural, MCA: 5-methylfuran-2-carboxylic acid, FFCA: 5-formyl-2-furancarboxylic acid, FDCA: 2,5-furandicarboxylic acid; Table S1. Gradient Elution of High-Performance Liquid-Phase Chromatograph for MF Oxidation to FDCA; Figure S2. High-performance liquid-phase chromatography of sample from the liquid oxidation of MF to FDCA; Figure S3. High-performance liquid-phase chromatography of a sample containing intermediate FFCA and final product FDCA; Figure S4. High-performance liquid-phase chromatography of final product

FDCA

sample;

Figure

S5.

High

performance

liquid

chromatography

of

5-

hydroxymethylfurfural (HMF) and its ester 5-acetoxymethylfurfural (AMF); Figure S6. Pathways of MF oxidation to FDCA and byproducts; Table S2. Mobile Phase of High-Performance Liquid-Phase Chromatograph for HMF and AMF; Table S3 Influence of Reaction Temperature on FDCA Yield; Section 1: Mechanism of MF oxidation to FDCA. Section 2: Kinetic Model REFERENCES (1) Corma, A.; Iborra, S.; Velty, A., Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 2007, 107, 2411-2502. (2) Climent, M. J.; Corma, A.; Iborra, S., Converting carbohydrates to bulk chemicals and fine chemicals over heterogeneous catalysts. Green Chem. 2011, 13, 520-540. (3) Gallezot, P., Conversion of biomass to selected chemical products. Chem. Soc. Rev. 2012, 41, 1538-1558. (4) Zhang, Z.; Huber, G. W., Catalytic oxidation of carbohydrates into organic acids and furan chemicals. Chem. Soc. Rev. 2018, 47, 1351-1390.



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Only used as the Table of the Contents. Scheme of catalytic oxidation of MF to FDCA


Kinetics and Mechanism of Catalytic Oxidation of 5 ... - ACS Publications

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