The Effects of Acid and Hydrogen Peroxide Stabilizer on the Thermal Hazard of Adipic Acid Green Synthesis

Abstract

The synthesis of adipic acid, which is formed by the reaction of cyclohexene oxidized by hydrogen peroxide (H₂O₂), is hazardous because of the highly exothermic nature of this reaction and H₂O₂ decomposition. The objective of this comprehensive study was to investigate and illustrate the effects of sulfuric acid (H₂SO₄) and H₂O₂ stabilizer (EDTA) on the thermal hazard of H₂O₂ decomposition and the green synthesis of adipic acid, which also provided a reference to reduce the risk of the reactions. Various calorimetry techniques were carried out to characterize the exothermic behavior of the reactions. An HPLC device was used to characterize the yield of adipic acid and the conversion rate of the raw materials, cyclohexene and H₂O₂. Meanwhile, density functional theory calculations were performed to understand the reaction mechanism and the associated energies of H₂O₂ decomposition catalyzed by sodium tungstate dihydrate (Na₂WO₄·2H₂O). Finally, combined with the calorimetry results, the risk of the adipic acid synthesis reaction was assessed using the intrinsic control index method (ITHI). The results show that the addition of H₂SO₄ and EDTA can reduce the exothermic heat of the H₂O₂ decomposition reaction and the green synthesis reaction of adipic acid. The yield of adipic acid was also increased. The hazard level of stage A was IV, and to remove more reaction heat, it was recommended to enhance the reflux cooling of stage A. The hazard level of stage B was I, which was very low and no further measures could be taken.

1. Introduction

As the most practical dicarboxylic acid, adipic acid has important application values in the organic synthesis of plastics, synthetic resins, food, and other fields. Adipic acid is mainly used as an intermediate in the manufacture of nylon 66, polyurethane, plasticizers, and other materials [1].

At present, the two main adipic acid synthesis processes are the cyclohexane [2] and the cyclohexene methods [3], both of which have low atomic utilization and the serious pollution problem of “the three wastes”. The oxidant used in these two methods is nitric acid, which generates a large amount of NO_x exhaust and acidic wastewater [2]. The catalyst is cuprum and vanadium, which generates waste residue. Therefore, the research and development of new technologies for the green synthesis of adipic acid has received extensive attention from experts and scholars at home and abroad. Among these new technologies, the hydrogen peroxide (H₂O₂) oxidation method could align with the requirements of green chemistry, with its mild conditions and simple operations, and which can ensure a high conversion rate, high yield, and high selectivity while maintaining clean production [4,5,6]. Jiang et al. [7] investigated the thermal hazards of adipic acid synthesis achieved by the oxidation of cyclohexene with H₂O_2, combined with reaction calorimetry and mechanism analysis, and found that the process could be divided into two reaction stages, both of which were assessed as a class 5 risk according to the Stoessel criticality diagram method, indicating that the process had a high risk of explosion.

Typically, H₂O₂ is unstable with highly exothermic hazards. The initial decomposition temperature of H₂O₂ ranges from 47 to 81 °C, and the decomposition heat is about 192–1079 J/g [8,9]. Wu et al. [10] suggested that the sulfuric acid (H₂SO₄) at a low concentration could slow down the decomposition rate of H₂O_2, and the rate could be increased with an increase in H₂SO₄ concentration. Jung et al. [11] found that the addition of C₈H₆O₄ as a stabilizer to a Fenton reagent or Fenton-like reagent could extend the lifetime of H₂O₂ and reduce the rate of H₂O₂ autolysis. Wu et al. [12] studied the effects of inorganic salts and organic acids on the thermal runaway behavior of hydrogen peroxide, and found that the order of catalytic effect from large to small was Fe²⁺ > Fe³⁺ > Cu²⁺, and the order of risk of thermal runaway from large to small was H₂O₂/HCOOH > H₂O₂ > H₂O₂/CH₃COOH. Zhang et al. [13] investigated the effects of free manganese ions and manganese on H₂O₂ decomposition in the presence of precipitated lignin under typical pulp bleaching conditions, and found that adding only Ethylene Diamine Tetraacetic Acid (EDTA) was the most effective method for reducing the decomposition of H₂O₂.

Both high temperatures and the catalysts used can accelerate the ineffective decomposition of H₂O₂ during the oxidation of cyclohexene by H₂O₂ for the preparation of adipic acid, resulting in the concentration of H₂O₂ decreasing, and the heat generated by the H₂O₂ decomposition reaction increasing. Consequently, the thermal hazard of the process is greatly increased. Therefore, it is necessary to select an effective H₂O₂ stabilizer to inhibit the decomposition reactions of excessive H₂O₂, so that the H₂O₂ can fully and effectively participate in the adipic acid synthesis.

In this work, various calorimetric and analytical techniques were carried out to investigate the effects of H₂SO₄ concentration and the addition of EDTA on the thermal hazard of the adipic acid synthesis reaction: (1) Differential scanning calorimetry (DSC) was implemented to obtain the thermal decomposition parameters of H₂O₂ under different catalytic conditions. (2) An automatic reaction calorimeter (EasyMax^TM102) was employed to measure the enthalpy of the H₂O₂ decomposition reaction and the adipic acid synthesis reaction. Meanwhile, high-performance liquid chromatography (HPLC) was used to assess the conversion rate of the substances used in the reaction and the yield of adipic acid. (3) An adiabatic calorimeter (Phi-TEC II) was implemented to simulate the adiabatic decomposition behavior and parameters of H₂O₂ and the reaction products obtained by calorimetric experiments. Finally, the reaction risks of the target reactions were assessed using the ITHI method [14].

2. Materials and Methods

2.1. Materials

Hydrogen peroxide (30 wt.%) and H₂SO₄(98 wt.%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The cyclohexene and sodium tungstate dehydrate (Na₂WO₄·2H₂O) used in this work were of analytical grade and made by Shanghai Macklin Biochemical (Shanghai, China). Adipic acid (HPLC, Shanghai Aladdin Bio-Chem Technology, Shanghai, China), methanol, (HPLC, Shanghai Aladdin Bio-Chem Technology, Shanghai, China), acetonitrile (HPLC, Shanghai Aladdin Bio-Chem Technology, Shanghai, China), phosphoric acid (AR, Shanghai Macklin Biochemical, Shanghai, China), potassium dihydrogen phosphate (AR, Shanghai Macklin Biochemical, Shanghai, China), sodium dodecyl sulfate (SDS) (AR, Sinopharm Chemical Reagent, Shanghai, China), Ethylene Diamine Tetraacetic Acid (EDTA) (AR, tcichemicals) were used as supplied. All reagents were used without further purification.

2.2. Methods

The reaction equation for the catalytic oxidation of cyclohexene by H₂O₂ to synthesize adipic acid is shown in Scheme 1.

During the synthesis of adipic acid, a total of four oxidation and two hydrolysis reactions occur. At 73 °C for two hours, cyclohexene is oxidized and hydrolyzed to produce 1,2-cyclohexanediol. The 1,2-cyclohexanediol continues to be oxidized to 2-hydroxycyclohexanone, which is recorded as stage A. At 90 °C for five hours, 2-hydroxycyclohexanone undergoes two steps of oxidation and hydrolysis to produce the final product, adipic acid, and the first three hours of this period are recorded as stage B [15].

2.2.1. HPLC Experiment

An HPLC device (Agilent-1260, Agilent Technologies Co., Ltd., Santa Clara, CA, USA) was used to characterize the yield of adipic acid and the conversion rate of the raw materials, cyclohexene and H₂O₂ [16]. The HPLC conditions for the adipic acid testing were as follows: The injection volume was 20 μL. The mobile stage was phosphoric acid-potassium dihydrogen phosphate buffer (50 mmol/L) and methanol. The elution rate was 0.3 mL/min. The column temperature was 30 °C. The chromatographic conditions for the cyclohexene and H₂O₂ tests were as follows: The injection volume was 5 μL. The mobile stage was acetonitrile and ultrapure water. The elution rate was 0.8 L/min. The column temperature was 30 °C and the UV detection wavelength was 209 nm [17].

2.2.2. Calorimetric Experiments

An EasyMax^TM102 calorimeter (Mettler Toledo Ltd., Greifensee, Switzerland) was used to determine the exothermic characteristics of adipic acid synthesis and the decomposition of H₂O₂ using a 100 mL atmospheric pressure glass reactor. The reactor temperature (T_r), jacket temperature (T_j), and heat release rates (q_r) were all tracked automatically to analyze the reaction enthalpy, adiabatic temperature rises ($∆T_{\mathrm{ad}}$), and other thermodynamic parameters.

2.2.3. DSC Experiment

DSC is among the most used techniques for thermal analysis [18]. In this experiment, DSC 250 (TA Instruments, Eden Prairie, MN, USA) was carried out to investigate the exothermic behavior of H₂O₂, the intermediates, and the products in stages A and B under Na₂WO₄·2H₂O, H₂SO_4, and EDTA conditions. The scanning temperature range was 40~180 °C and 40~300 °C. The ramp-up rate was 5 °C/min. Nitrogen was used for protection and the flow rate was 50 mL/min.

2.2.4. Adiabatic Tests

An adiabatic calorimeter (Phi-TEC II, HEL Limited, Hertford, UK) was used to identify the adiabatic decomposition characteristics of the reactants. Samples (~1 g) of the stage A product, the stage B product, and the raw H₂O₂ mixtures from the Calorimetric experiments were loaded into a 10 mL Hastelloy test cell. The temperature ranged from 30 to 300 °C with a step-upward trend of 5 °C. The temperature and pressure data from the adiabatic experiments were tracked to record any exotherms. The detection sensitivity was 0.02 °C/min [19].

3. Results and Discussion

3.1. The Working Curves Obtained from the HPLC Experiments

Three sets of adipic acid, cyclohexene, and H₂O₂ solutions with concentrations ranging from 1 mg/g to 50 mg/g were prepared and used to obtain the working curves using HPLC (mg/g is the ratio of the mass of the substance to the mass of the prepared standard solution). The experimental results are shown in Figure 1, Figure 2 and Figure 3.

3.2. The Decomposition Mechanism of H₂O₂ Catalyzed by Na₂WO₄·2H₂O

Typically, in addition to participating in the catalytic oxidation of cyclohexene to adipic acid, H₂O₂ can also be auto catalyzed for decomposition. This leads to the reduced utilization of H₂O₂ during the whole process, and the heat released from the decomposition increase the thermal hazard of the reaction. Hence, it is necessary to identify the decomposition mechanism of H₂O₂ to reduce the thermal hazard of the process and to improve the utilization of H₂O₂.

Nardello V et al. [20] investigated the disproportionation reaction of H₂O₂ catalyzed by Na₂WO₄·2H₂O to form singlet oxygen (¹O₂). The mono-, di-, and tetraperoxotungstate intermediates ${W{\left(O_2\right)}_n{O}_{4-n}}^{2-}$ (n = 1, 2, 4) have been characterized using UV and 183W NMR spectroscopies. The diperoxo species was proposed as the precursor of ¹O₂. Yoshiro O et al. [21] carried out the kinetics for the oxidation of dimethyl sulfoxide (DMSO) by H₂O₂ using iodometry. Meanwhile, the major protons were formed at a solution pH of 3.5–5.0 acidity (H₂WO₅ and H₂WO₈) with using Na₂WO₄·2H₂O as the catalyst. The effect of acidity on the rate of oxidation and the rate determining mechanism of peroxotungstic acid were also discussed.

According to Nardello V et al. and Yoshiro O et al., the decomposition of H₂O₂ catalyzed by sodium tungstate was studied because of the presence of H₂O₂, Na₂WO₄·2H₂O, and H₂SO₄ in the oxidative synthesis of adipic acid. In this work, as shown in Figure 4, a possible decomposition mechanism of H₂O₂ catalyzed by Na₂WO₄·2H₂O was proposed on the basis of existing research. First, $W{O}_4^{2-}$ is hydrolyzed to produce $HW{O}_4^{-}$, after which $HW{O}_4^{-}$ is gradually oxidized by H₂O₂ to form $HW{O}_3{(O_2)}^{-}$ and $HW{O}_2{\left(O_2\right)}_2^{-}$. $O_2$ is generated through $HW{O}_2{\left(O_2\right)}_2^{-}$, and then $HW{O}_4^{-}$ is produced, leading a catalytic decomposition cycle. As the pH of the solution decreases, $HW{O}_2{\left(O_2\right)}_2^{-}$ and $W{(O_2)}_4^{2-}$ are in dynamic equilibrium in the presence of 2 mol of H₂O₂.

3.3. Reaction Calorimetry Results and Analysis

3.3.1. The Exothermic Results and Analysis of the Catalytic Decomposition Reaction of H₂O₂

In reaction condition 1, the molar ratio of H₂O₂ to Na₂WO₄·2H₂O was 4.4:0.06. In reaction condition 2 the molar ratio of H₂O₂ to Na₂WO₄·2H₂O was 4.4:0.06 and the H₂SO₄ concentration was 0.22 mol/L. In reaction condition 3, the molar ratio of H₂O₂ to Na₂WO₄·2H₂O was 4.4:0.06, the H₂SO₄ molar concentration was 0.22 mol/L, and the EDTA mass concentration was 18.3 g/L.

The experimental tests were all carried out at room temperature with stirring at 300 rpm, then T_r was heated to 73 °C and held for 120 min. Subsequently, the reactions were maintained for 240 min at 90 °C. Samples were taken at every 60 min intervals and analyzed the conversion of H₂O₂ by HPLC.

As depicted in Figure 5, when T_j was raised to about 60 °C, T_r increased rapidly, indicating that the reaction was violently exothermic, and a large number of bubbles were also generated during the process. After temperature stabilization, the decomposition of H₂O₂ was basically complete. The maximum temperature difference between T_r and T_j during the process was about 59 °C. Thus, if a cooling failure occurred, the reaction temperature could rise sharply, resulting in runaway accidents.

The amount of H₂SO₄ was increased under reaction conditions 2 to reduce the potential of hydrogen (pH) of the solution. Hence, the process of catalytic decomposition could be inhibited under the dynamic equilibrium of $HW{O}_2{\left(O_2\right)}_2^{-}$ and $W{(O_2)}_4^{2-}$ in the presence of 2 mol of H₂O₂. The decomposition rate of H₂O₂ slowed down as is shown in Figure 6. The decomposition rate of H₂O₂ at the isothermal stage of 90 °C is significantly higher than at the isothermal stage of 73 °C. This indicates that the addition of acid effectively reduced the catalytic decomposition of H₂O₂, while the high temperature still led the H₂O₂ to decompose completely; the conversion rate reached 100% after 240 min of reaction time at the 90 °C isothermal stage. Therefore, it is essential to add H₂O₂ stabilizers into H₂O₂ solutions to inhibit the ineffective decomposition of H₂O₂ during the oxidation reaction.

EDTA, which is commonly used as a stabilizer of H₂O₂ in the field of paper bleaching, is rarely used in the chemical industry. In test 3, EDTA was added and the other conditions were kept consistent with those of test 2. As depicted in Figure 7, after 240 min of reaction time at the 90 °C isothermal stage, only about 41.24% of the H₂O₂ was converted, the results indicate that the addition of EDTA can improve the thermal stability of H₂O₂ Therefore, the application of EDTA in the process of catalytic oxidation of cyclohexene to adipic acid with H₂O₂ was relatively feasible.

3.3.2. Calorimetry and Theoretical Calculation of Catalytic Decomposition of H₂O₂

Gaussian is a professional calculation software in the field of quantum chemistry, which can be used to calculate thermodynamic parameters such as enthalpy change ΔH and free energy change ΔG of chemical reactions, and the calculated values are very close to the experimental values, thus accurately predicting the thermodynamic properties of reactions. Based on the mechanism of the catalytic decomposition of hydrogen peroxide, density functional theory (DFT) calculations of the enthalpy of reaction for the catalytic decomposition of H₂O₂ solution were performed. The standard molar enthalpies of production for each step of the reaction are shown in

Table 1. The standard enthalpy of decomposition of H₂O₂ catalyzed by sodium tungstate.

Reaction Steps	Standard Molar Enthalpy of Production (kJ/mol)
$2H_2{O}_2\to 2H_2\mathrm{O}+O_2\uparrow$	61.98
$W{O}_4^{2-}+H_2\mathrm{O}\to HW{O}_4^{-}+O{H}^{-}$	168.18
$HW{O}_4^{-}+H_2{O}_2\to HW{O}_3{(O_2)}^{-}+H_2\mathrm{O}$	68.55
$HW{O}_3{(O_2)}^{-}+H_2{O}_2\to HW{O}_2{\left(O_2\right)}_2^{-}+H_2\mathrm{O}$	59.48
$HW{O}_2{\left(O_2\right)}_2^{-}\to HW{O}_4^{-}+O_2\uparrow$	−66.05
$HW{O}_2{\left(O_2\right)}_2^{-}+2H_2{O}_2\to W{(O_2)}_4^{2-}+H_3{O}^{+}+H_2\mathrm{O}$	−1025.97

. $HW{O}_2{\left(O_2\right)}_2^{-}$ gives $HW{O}_4^{-}$ after releasing $O_2$ and $HW{O}_2{\left(O_2\right)}_2^{-}$ is oxidized to $W{(O_2)}_4^{2-}$ by 2 mol of hydrogen peroxide as a heat-absorbing reaction. The standard molar enthalpy of production for the entire hydrogen peroxide decomposition reaction is 61.98 kJ/mol.

The heat of decomposition and $∆T_{ad}$ of the H₂O₂ decomposition reaction were obtained using test 1. The whole reaction heat was 19.04 kJ and $∆T_{ad}$ was 170.77 °C. As shown in Figure 8, the trend of q_r curve showed two severe peaks of exothermic heat. The maximum heat release rate was about 55.72 W. This indicates that the decomposition rate of H₂O₂ catalyzed by Na₂SO₄·2H₂O was great and acute. The large amount of gas produced could easily trigger an accidental explosion. The mass of the 30 wt.% H₂O₂ used in the experiment was 36.434 g, so the enthalpy of reaction was 59.5 kJ/mol. The theoretical calculation gives a standard molar enthalpy of production of 61.98 kJ/mol. The theoretical value is close to the calculated value.

3.3.3. Adipic Acid Green Synthesis Reaction Results and Analysis

Sodium dodecyl sulfate (SDS) is an anionic surfactant with good emulsification. In the present work, to investigate the effect of H₂SO₄ concentration and the addition of EDTA on the reaction process of adipic acid synthesis, nine experiments were conducted on the synthesis of adipic acid using calorimetric experiments, and the yield of adipic acid was analyzed using HPLC. The molar ratio of the H₂O₂, cyclohexene, and Na₂WO₄·2H₂O used in the tests was all 4.4:1:0.06. Different concentrations of H₂SO₄ (C_H2SO4), SDS(ω_SDS), and EDTA(ω_EDTA) were used in the different tests, as shown in

Table 2. Thermal chemical parameters under different reaction conditions for adipic acid synthesis reaction.

Run	CH2SO4 (mol/L)	ωSDS (g·L−1)	ωEDTA (g·L−1)	$-{∆}{\mathit{H}}_{\mathit{A}}$ (kJ)	$-{∆}{\mathit{H}}_{\mathit{B}}$ (kJ)	$-{∆}{\mathit{H}}_{\mathit{r}}$ (kJ)	ΔTA (°C)	ΔTB (°C)	YAA (%)
1	0.22	0	0	-	-	-	-	-	8.67
2	0.22	18.3	0	22.635	13.134	35.77	10.42	3.17	72.31
3	0.22	18.3	9.15	24.41	3.185	27.595	14.25	−0.35	75.21
4	0.22	9.15	18.3	21.274	3.406	24.68	11.96	−0.57	78.25
5	0.22	9.15	0	22.415	7.307	29.722	7.87	2.23	73.62
6	0.12	9.15	9.15	22.457	5.234	27.69	7.12	6.53	81.25
7	0.12	9.15	18.3	13.229	7.809	21.038	5.348	6.067	82.78
8	0.12	9.15	0	15.567	5.899	21.466	5.45	6.54	76.65
9	0.12	9.15	9.15	21.231	4.7003	25.931	6.62	4.95	81.83

.

In experiment 1, the incomplete yield of the entire synthesis reaction, without the addition of SDS, was only 8.67%. Since the process is a liquid–liquid non-homogeneous reaction, both the lack of organic solvents and the ultimate stirring speed could lead to the poor mass transfer of the reaction. The mass transfer could be enhanced by SDS emulsification, and the yields of adipic acid were significantly increased, as depicted in experiments 2 and 3. The comparison of experiments 2, 3, 4, and 5 indicated that the addition of EDTA could reduce the total heat release while increasing the yield of adipic acid. By comparing experiments 4 and 7, it was found that the decrease of H₂SO₄ concentration also reduced the total heat release and increased the yield of adpic acid. Experiment 7, which had the lowest total heat release and temperature difference, the H₂SO₄ concentration of 0.12 mol/L, the SDS concentration of 9.15 g/L, and the EDTA concentration of 18.3 g/L, was determined to be the optimum process for adipic acid synthesis after a comprehensive comparison of experiments 1–9.

In order to accurately assess the thermal hazard of the adipic acid synthesis process, the entire reaction process was divided into two stages, A and B, as shown in Figure 9.

The severity of the reaction runaway is usually determined using $∆T_{ad}$ due to the exothermic of the target reaction. Since the yield of adipic acid was 82.78%, the accurate adiabatic temperature rise $∆T_{ad,r}$ should be corrected by the yield $Y$ [22]. The equations for calculating $∆T_{ad}$ and $∆T_{ad,r}$ are listed in Equations (1) and (2). The calorimetric experimental data and results are shown in

Table 3. Thermal parameters of oxidation reaction in EasyMax^TM102 experiment.

Stage	$-{∆}{\mathit{H}}_{\mathit{r}}$ (kJ)	${\mathit{C}}_{\mathit{p}}$ (J/(K·g))	${∆}{\mathit{T}}_{\mathit{a}\mathit{d},\mathit{r}}$ (°C)	$\mathit{M}\mathit{T}\mathit{S}{\mathit{R}}_{,\mathit{r}}$ (°C)
A	13.229	2.88	122.82	192.33
B	7.809	2.88	72.5	161.95

, where $∆H$ is the heat of the oxidation reaction, kJ; $m_r$ is the total mass of the oxidation system, g; and $C_p$ is the specific heat capacity of the system after the oxidation reaction J/(K·g).

$$∆T_{ad}=\frac{∆H_r}{C_p{m}_r}$$

(1)

$$∆T_{ad,r}=\frac{∆H_r}{C_p{m}_{r}Y}$$

(2)

$$T_{cf}=T_p+\frac{∆H_r-{{\displaystyle \int }}_0^t{q}_r\left(t\right)dt}{C_p{M}_t}$$

(3)

$$T_{cf,r}=T_p+\frac{\frac{∆H_r}{Y}-{{\displaystyle \int }}_0^t{q}_r\left(t\right)dt}{C_p{M}_t}$$

(4)

In Equations (3) and (4), $T_p$ is the process operating temperature, which depends on the process conditions, where $T_p$ is the temperature in the kettle during the synthesis process, °C. $M_t$ is the total mass of the oxidized reaction substance. $T_{cf}$ is the temperature that can be reached by the oxidation reaction after cooling failure, and the maximum value of $T_{cf}$ is MTSR [23].$T_{cf}$ should also be corrected by the yield $Y$, which is $T_{cf,r}$, and $q_r$ is the heat release rate of the reaction, W. The changes in $T_{cf,r}$ and $T_{cf,r}$ are shown in Figure 10 and Figure 11. The stage A MTSR_r was 192.33 °C and the stage B MTSR_r was 161.95 °C.

3.4. DSC Experimental Results and Analysis

DSC was used to investigate the effect of H₂SO₄ and EDTA conditions on the catalytic decomposition characteristics of 30 wt.% H₂O₂ and the thermal stability of the products from stages A and B. Sample 1 was a mixture of 30 wt.% H₂O₂ and tungsten Na₂WO₄·2H₂O. Sample 2 was a mixture of 30 wt.% H₂O₂, Na₂WO₄·2H₂O, and H₂SO₄. Sample 3 was a mixture of 30 wt.% H₂O₂, Na₂WO₄·2H₂O, H₂SO4, and EDTA. The proportion of each substance in the mixture followed the process parameters of experiment 7. Samples 4 and 5 were the products from stages A and B. The experimental results are shown in Figure 12 and Figure 13.

As indicated in

Table 4. Thermal decomposition parameters for different products.

Sample Sequence	${\mathit{T}}_{\mathit{o}\mathit{n}\mathit{s}\mathit{e}\mathit{t}}$ (°C)	${\mathit{T}}_{\mathit{peak}}$ (°C)	$-{∆}{\mathit{H}}_{\mathit{d}}$ (J/g)
1	83.61	89.71	825.72
2	72.20	93.84	508.80
3	106.08	115.6	455.92
4	83.54	123.3	587.3
5	82.2, 135.7	81.1, 114.4	−1143.24

, the mixed system became more acidic with the addition of H₂SO₄, and slowed the decomposition rate of H₂O₂. The rate of heat generation and the heat of liberation could be decreased, although the initial decomposition temperature ($T_{onset}$) was advanced. The autolytic reaction of H₂O₂ was effectively inhibited by the addition of the H₂O₂ stabilizer, EDTA. $T_{onset}$ was delayed and the heat released from decomposition was decreased. The product of stage A was exothermic due to the presence of unfinished H₂O₂ and intermediate products. However the product of stage B, was mostly adipic acid, and consequently there was no exothermic heat during the test.

3.5. Adiabatic Tests Results and Analysis

In this work, the adiabatic tests of the stage A and B products and sample 3 were executed using Phi-TEC II, as depicted in Figure 14. No exothermic heat was detected in the stage B products. The characteristic parameters of the adiabatic decomposition reaction of the stage A products and sample 3 are shown in

Table 5. Thermal characteristic parameters of the first decomposition reaction of stage A products and hydrogen peroxide in the Phi-TEC II experiment.

Thermal Characteristic Parameters	Stage A Products	Sample 3
$T_{0,d,1}$ (°C)	50.03	41.27
$T_{max,1}$ (°C)	93.1	90.44
$∆T_{ad,d,l}$ (°C)	43.07	49.17
${\left(\frac{dT}{dt}\right)}_{max,1}$ (°C/min)	0.18	2.97
$\phi$	3.41	3.28
${(∆T_{ad,d,1})}_{\mathit{correct}}$ (°C)	146.87	161.28
${({\left(\frac{dT}{dt}\right)}_{max,1})}_{\mathit{correct}}$ (°C)	0.62	9.74

.

In order to assess the thermal hazard of the adipic acid green synthesis process, $TM{R}_{ad}$ and $T_{D24}$ were calculated for the stage A products to access the possibility and reaction risk of the target process. $TM{R}_{ad}$ is the time required to reach the maximum rate under adiabatic conditions [24], calculated using Equation (5), and $T_{D24}$ is the temperature at which $TM{R}_{ad}$ equals 24 h [25].

$$TM{R}_{ad}=\frac{R{T}^2}{A{\left(\frac{T_f-T}{∆T}\right)}^n∆Texp\left(-\frac{E}{RT}\right)E}$$

(5)

$$\frac{d_T}{d_t}=A{\left(\frac{T_f-T}{∆T}\right)}^n∆Texp\left(-\frac{E}{RT}\right)$$

(6)

Here, T_f is the maximum temperature obtained by self-heating under adiabatic conditions, K; R is the gas constant, 8.314 J/(mol·K); E is the apparent activation energy, kJ/mol; A is pre-exponential, s⁻¹; and n is the number of reaction stages.

According to Equation (6), the thermodynamic parameters were obtained by non-linear fitting. The correlation coefficient (R²) was 0.954 and the fit was essentially the same as the experimental data. E was 98.215 kJ/mol, A was 7.688 × 10¹² s⁻¹, and n was 1.238, as depicted in Figure 15. As indicated in Figure 16, the curve of $TM{R}_{ad}$ vs. Temperature was fitted from the adiabatic kinetic data, and $TM{R}_{ad}$ was 34.8 min and $T_{D24}$ was 27.95 °C.

3.6. ITHI Method for Assessing the Thermal Hazards of Adipic Acid Synthesis Reactions

The ITHI method, an intrinsic control index method that considers the combination of substance hazard and reaction heat hazard, was chosen to provide a comprehensive and accurate assessment of the hazards of adipic acid synthesis [12]. The two stages, A and B, of the adipic acid synthesis process were assessed, respectively.

As depicted in

Table 6. The MF of the adipic acid synthesis process in stages A and B.

Working Conditions	${\mathit{T}}_{\mathit{onset}}$ (°C)	${\mathit{I}}_{\mathit{Tonset}}$	$-{∆}{\mathit{H}}_{\mathit{d}}$ (J/g)	${\left({\mathit{m}}_{\mathit{T}}\right)}_{\mathit{m}\mathit{a}\mathit{x}}$ (°C/min)	${∆}{\mathit{T}}_{\mathit{a}\mathit{d}}$ (°C)	$\mathit{M}\mathit{P}\mathit{D}$ (W/mL)	${\mathit{I}}_{\mathit{M}\mathit{P}\mathit{D}}$	MF
Stage A	106.8	2	451.35	9.74	161.20	31.47	2	1.25
Stage B	83.54	3	422.98	0.62	146.87	1.90	1	1.188

,

Table 7. The probability of runaway adipic acid synthesis process in stages A and B.

Working Conditions	${\mathit{T}}_{\mathit{p}}$ (°C)	$\mathit{M}\mathit{T}\mathit{T}$ (°C)	$\mathit{M}\mathit{T}\mathit{S}\mathit{R}$ (°C)	${\mathit{T}}_{\mathit{D}24}$ (°C)	${\mathit{I}}_{\mathit{C}\mathit{C}}$ (°C)	$\mathit{T}\mathit{M}{\mathit{R}}_{\mathit{a}\mathit{d}}$ (h)	${\mathit{I}}_{\mathit{T}\mathit{M}\mathit{R}}$	P
Stage A	40–78	73–75	192.33	27.95	5	0.58	5	10
Stage B	90–96	100	161.95	300	3	>24	1	4

and

Table 8. The severity of runaway adipic acid synthesis process in stages A and B.

Working Conditions	$-{∆}{\mathit{H}}_{\mathit{r}\mathit{x}}$ (kJ/kg)	${\mathit{I}}_{\mathit{H},\mathit{r}\mathit{x}}$	${∆}{\mathit{T}}_{\mathit{a}\mathit{d},\mathit{r}\mathit{x}}$ (°C)	${\mathit{I}}_{{∆}\mathit{T}\mathit{a}\mathit{d},\mathit{r}\mathit{x}}$	$-{∆}{\mathit{H}}_{\mathit{r},\mathit{d}\mathit{e}\mathit{c}}$ (kJ/kg)	${\mathit{I}}_{\mathit{H},\mathit{d}\mathit{e}\mathit{c}}$	${∆}{\mathit{T}}_{\mathit{a}\mathit{d},\mathit{d}\mathit{e}\mathit{c}}$ (°C)	${\mathit{I}}_{{∆}\mathit{T}\mathit{a}\mathit{d},\mathit{d}\mathit{e}\mathit{c}}$
Stage A	292.83	2	122.82	2	422.98	3	161.20	2
Stage B	172.84	2	72.5	2	0	1	0	1

, the material coefficients (MF) of stages A and B were 1.25 and 1.188.

The probability of runaway (P) in stages A and B were 10 and 4, respectively, and the severity of runaway (S) is the sum of (I_H,rx,I_ΔTad,rx)_max and (I_H,dec,I_ΔTad,dec)_max. The accident consequence severity S in stage A was 5, and the post-accident severity S in stage B was 3.

The runaway consequence severity (RI) is the product of S and P. The ITHI value is the product of MF and RI. Thus, the ITHI of stages A and B were 62.5 and 13.42, respectively. The risk level of stage A was IV, since stage A was carried out under the reflux period. Consequently, the cooling of reflux should be increased to remove more of the heat generated from stage A and to more steadily control the temperature. The risk level of Stage B was I, which indicated the risk was low and no action should be taken.

4. Conclusions

We used various calorimetric equipment and material analysis instruments to conduct a thermal hazard analysis of the catalytic decomposition reaction of hydrogen peroxide and the green synthesis reaction of adipic acid. The conclusions are as follows:

(1)

The ineffective decomposition of hydrogen peroxide during the green synthesis reaction of adipic acid not only wastes raw materials, but also greatly increases the thermal risk of the reaction. It was found that the addition of sulfuric acid and the hydrogen peroxide stabilizer EDTA improved the thermal stability of hydrogen peroxide, and reduced the ineffective decomposition of hydrogen peroxide.

(2)

The addition of sulfuric acid and EDTA to the DSC experiment delayed the onset of decomposition temperature (T_onset) and reduced the heat separation.

(3)

In the green synthesis process of adipic acid, by varying the concentration of sulfuric acid, SDS, and EDTA, the process conditions were optimized to 0.12 mol/L sulfuric acid, 9.15 g/L SDS, and 18.3 g/L EDTA, resulting in the lowest overall heat release and temperature difference, and an adipic acid yield of 82.78%.

(4)

A thermal hazard assessment of the optimized adipic acid green synthesis reaction was carried out using the ITHI method for stages A and B. The hazard level of stage A is Ⅳ, and improvement of the heat transfer capacity of the reflux unit is recommended. The hazard level of stage B is I, and no additional measures are needed due to the low hazard level.