Hydroperoxide Formation and Thermal Stability of Ethyl t

Hydroperoxide Formation and Thermal Stability of Ethyl t...
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Hydroperoxide Formation and Thermal Stability of Ethyl t‑Butyl Ether Oxidation Qiang Zhang, Yan-Fei Zheng, Xiong-Min Liu,* Bo Wang, Li Ma, Fang Lai, and Xiao-Di Zhou College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China ABSTRACT: A customer-designed mini closed pressure vessel test (MCPVT) consisting of a pressure sensor and a temperature sensor connected to recorder was applied to evaluate the isothermal stability along with the formation of hydroperoxide in the ethyl t-butyl ether (ETBE) oxidation process at low temperatures. A new type of hydroperoxide, named 1-tert-butoxy-ethyl hydroperoxide (TBEHP), was separated from ETBE oxidation products via column chromatography, which was further characterized by mass spectrometry (MS), 1H and 13C nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR). The thermal characteristics of TBEHP were assessed via differential scanning calorimetry (DSC). Results showed that the exothermic onset temperature (T0) and thermal decomposition heat (QDSC) of TBEHP were 99.12 °C and 1523.89 J·g−1, respectively. Moreover, a jet-stirred reactor (vessel volume: 500 mL) was used to evaluate the explosive risk of ETBE oxidation. The corresponding result indicated that detonation would arise in conditions of reaching system temperature of 140.0 °C, sample mass of 5.0 g, and oxygen pressure of 1.0 MPa, respectively. Finally, it was confirmed that ETBE thermal oxidation was a three-step exothermic reaction including the formation of hydroperoxide by absorbing oxygen, followed by the thermal decomposition of hydroperoxide, and subsequently deep oxidation reactions or detonation caused by reactive free radicals.

1. INTRODUCTION Gasoline depletion and greenhouse gas emission control, which are believed to potentially aggravate global warming, put forward new demands for sustainable energy.1 The aim of developing a new generation of renewable energy sources and higher security of energy has been issued by the European Union (EU).2 Ethyl tbutyl ether (ETBE), an oxygenate additive,3 has attracted considerable attention due to its antiknock ability in engine performance, referring to a higher boiling point (70 °C at 0.1 MPa), lower flash point, lower blending Reid vapor pressure, lower volatility, water insolubility, and reasonably high oxygen content.4 Currently, ETBE-blend gasoline is widely used in developed countries with an annually increasing share in the energy market,5 including the United States, Germany, France, Spain, Italy, and Japan;6 for instance, it accounts for 7% of total energy consumption in Japan.7 However, concerns about the potential security of ETBE do exist as a result of the dual effect of the oxygen atom in ETBE. The oxygen atom in ETBE plays two roles. On one hand, it is essential in reduction of carbon monoxide (CO), nitrogen oxides (NO, NO2), particular matter (PM), and complete combustion.8 On the other hand, oxygen in air destabilizes ETBE upon contact, which may cause potential safety issues during its production, storage, transportation, and usage. Accordingly, studies on the low temperature oxidation of ETBE can also provide a better understanding of the combustion and oxidation mechanism.9 Therefore, this indicates the significance to the study on thermal stability of ETBE and its reactivity with oxygen, as well as the thermal stability of the intermediate (mainly hydroperoxide) formed during the oxidation reaction. The autoignition characteristic of ETBE-blend fuels in a gasoline engine was studied by Kaminaga et al.10 ETBE acted as an ignition accelerator for all intake temperatures. Ogura et al. calculated the rate coefficients of H atom abstraction from ETBE © XXXX American Chemical Society

based on transition state theory (TST) and quantum chemical calculations.11 In their studies, ether-peroxy radicals (etherQOO·) were assumed as intermediates for the calculations and the threshold energies (E0) for the H atom abstraction reactions from ETBE by an H atom, and OH radicals were also obtained. The first direct detection of hydroxyl hydroperoxide (HHP) formation from the reaction of H2O2 with glyoxal (GLY) and methylglyoxal (MG) was reported by Zhao et al.12 The combustion and ignition of ETBE were studied using a shock tube and spherical bomb from 1280 to 1750 K.13 Liu et al. studied the oxidation characteristic and oxidation kinetics of ETBE using accelerating rate calorimetry (ARC) from 30 to 250 °C under adiabatic conditions.14 An exothermic peak due to the decomposition of hydroperoxides and deep oxidation process was detected by monitoring the pressure and temperature changes. Most of the oxidation products were determined by gas chromatrography mass spectrometry (GC-MS), but a few of them remain unidentified. Winfough et al. investigated the oxidation of ETBE initiated by Cl atoms using synchrotron photoionization radiation at selected temperatures.15 BattinLeclerc achieved significant success on the experimental confirmation of peroxides at low temperature oxidation scheme of alkanes mainly present in petrol and diesel fuel.16 ETBE is frequently present accompanied by alkanes, and it is more ready to form hydroperoxides when performed in engine combustion due to the enhanced reactivity by conjugates of oxygen atom in ether. It has been widely accepted that autoignition of engine performance, explosive reactions, and complex oxidation processes can be well explicated by free radical reaction Received: March 1, 2017 Revised: June 15, 2017 Published: July 3, 2017 A

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels mechanism.17 Le Tan investigated the quantification of HO2 and other products of dimethyl ether (DME) oxidation (H2O2, H2O, and CH2O) in a jet-stirred reactor at elevated temperatures.18 In addition, the formation of hydroperoxide and thermal decomposition are abiding by free-radical reaction mechanism. To the best of our knowledge, hydroperoxides possess the properties of strong oxidizability, explosibility, and thermal instability due to the fragility of the O−O bond. The O−O bond, with dissociation energy of about 43 kcal·mol−1,19 can easily break at increasing temperatures, which makes hydroperoxides promising candidates for the deterioration of branched-chain reactions, autoignition, and the occurrence of cool flames in engine combustion.19 Following the thermal decomposition of hydroperoxide, detonation reaction, runaway chemical reactions, and sophisticated oxidation reactions arise. Incidents caused by hydroperoxide had been reported frequently.20 Therefore, it is necessary to study the thermal stability of ETBE and its reactivity with oxygen, as well as the thermal stability of intermediates (mainly hydroperoxide) formed in the oxidation reaction. Therefore, the purpose of this study is to investigate the hydroperoxides formed in the ETBE oxidation process and provide a solid support to the molecular structure, calculation assumptions, and reaction mechanism. Furthermore, the production process, storage, transportation, and consumptions of ETBE rely on good understanding of its resistibility to air or oxygen. Distinguishing the species of hydroperoxides formed in the ETBE oxidation process by thin liquid chromatography (TLC) analysis is reasonable and feasible considering the fact that hydroperoxides are thermally unstable to lights, impact, acid−base, and heat.21 In addition, the thermal safety parameters including exothermic onset temperature (T0), peak temperature (Tp), heat of thermal decomposition (QDSC), maximum self-heat rate [(dT/dt)max], and maximum self-heat rate temperature (T(dT/dt)max), which are critical to chemicals, are determined by differential scanning calorimetry (DSC) measurement.22 According to mini closed pressure vessel test, the hazard of ETBE (sample mass: 5.0 g) oxidation reaction was investigated using a jet-stirred reactor (vessel: 500 mL) at different temperatures.23

Figure 1. Custom-designed MCPVT (1−stainless steel vessel, 2−glass casing, 3−temperature sensor, 4−pressure sensor, 5−pressure transducer, 6−recorder and signal conditioner, 7−intake valve, 8−safety valve, 9−heating furnace) instrument for evaluation of the isothermal stability of ETBE oxidation.

titration of sodium hyposulfite solution [eq 2]. Results were quantified as milligrams per kilogram (ppm) of hydroperoxide:25

2KI + ROOH + H 2O = I 2 + 2KOH + ROH

(1)

I 2 + 2Na 2S2O3 = Na 2S4 O6 + 2NaI

(2)

The oxidation products were analyzed by TLC analysis, in which starch potassium iodide solution was used as color developing reagent to determine the species of hydroperoxides formed in ETBE oxidation process as the hydroperoxides were unknown compounds.26 A silica gel precoated TLC plate with measuring area of 3 cm × 5 cm (Sinowax, Shanghai, China) was applied to analysis. Samples collected from ETBE oxidation process on different temperatures were dipped on the silica gel plate with a space interval of 1 cm between spots. The spots were then air-dried for 2 min before development. The plate was developed in 10 mL of cyclohexane−ethyl acetate (7.5:2.5, v/v) for approximately 5 min and sprayed with starch potassium iodide solution. Areas that turned blue suggested species of hydroperoxide. 2.4. Separation of Hydroperoxide. The separation of hydroperoxide was implemented by column chromatography. A 15 mm i.d. glass column, 250 mm long, was filled with 200−300 mesh silica gel and equilibrated with 500 mL of cyclohexane. The collected ETBE oxidation samples at 85 °C were submitted and remained in contact with the silica for 20 min. Later, samples were eluted using a mixed agent (cyclohexane−ethyl acetate, 90:10, v/v) at a flow of 0.3 mL/min. The presence of hydroperoxide was monitored via chromogenic method. The eluate was then collected upon detection of hydroperoxide. The collected eluotropic product was distilled using a rotary evaporator at room temperature with vacuum (0.08 MPa) in order to further purify hydroperoxide. 2.5. Molecular Structure of Hydroperoxide. The mass spectra of hydroperoxide was determined by a Thermo LCQ Fleet (Thermo Fisher Scientific Inc., USA) with an electro-spray ionization (ESI) running in positive-ionization mode. The typical ion source parameters were as follows: spray voltage, 3500 V; sheath gas pressure (N2), 5 units; ion trans tube temperature, 300 °C; collision gas (Ar), 1.5 mTorr; mass spectra range, 15 to 200 m/z. The scan dwell time was set to 50 ms for each ion pair. 1-tert-Butoxy-ethyl hydroperoxide (TBEHP) dissolved in methanol was directly introduced into the MS machine for measurement. The molecular structure of hydroperoxide was confirmed by 1H and 13 C nuclear magnetic resonance (NMR), MS, and Fourier transform infrared spectroscopy (FTIR). For the 1H and 13C NMR spectra study of hydroperoxide and the reactive site of ETBE, spectra were recorded on an AVANCE III HD 600 spectrometer (Bruker, Switizerland) with a 5 mm liquid conventional probe operating at 600 MHz (1H) and 150 MHz (13C) at 25 °C.27 For sample preparation, 10 mg hydroperoxide separated from ETBE oxidation products was dissolved in CDCl3 for the NMR measurements.

2. MATERIAL AND METHODS 2.1. Materials. ETBE (99.50%, Nanjing HaBo Medical Technology Co., LTD, China), O2 (99.99%, Nanning Yu Da Tian Ke Tradec., LTD, China), CDCl3, KI (99.50%, Aladdin Industrial Corporation, China), Na2S2O3 (99.95%, Aladdin Industrial Corporation, China) were used for the testing. 2.2. Isothermal Thermal Stability of ETBE Oxidation by MCPVT. A custom-designed mini closed pressure vessel test (MCPVT, vessel volume: 10 mL) equipped with pressure sensor and thermocouple connected to a recorder was used for evaluating the thermal stability of ETBE in an oxygen atmosphere, as shown in Figure 1. The thermal stability of ETBE oxidation reaction was evaluated by monitoring the T and P behaviors of the container. The amount of substance was calculated with the ideal gas equation to monitor the decomposition reaction. A glass lining (diameter 0.8 cm) containing an ETBE sample (about 1.5 g) was loaded into the container.24 Isothermal experiments were conducted at 70, 85, and 110 °C for 8 h in an oxygen atmosphere. The initial pressure of the container was set to 1.00 MPa. When the reaction was finished, ice water was utilized to cool the container to ambient temperature. 2.3. Hydroperoxide Analysis by Iodimetry and TLC. Hydroperoxide concentration was determined via iodimetry.25 For this test, a quantity of sample obtained from ETBE oxidation reaction was added to aqueous potassium iodide, which reduced the hydroperoxides present [eq 1]. An equivalent amount of iodine was liberated, followed by the B

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Plots of T, P, and n versus oxidation time (t) for ETBE oxidation at 70, 85, and 110 °C. The functional groups of hydroperoxide were analyzed with FTIR (FTIR-8400S, Shimadzu, Japan). The FTIR resolution and number of scans were set as 4 cm−1 and 40, respectively. The samples were ground with KBr power and pressed into pellets. The FTIR spectra were recorded in the range of 4000−500 cm−1. 2.6. Determination of Thermal Stability of Hydroperoxide by DSC Analysis. DSC is an effectively thermal analytical device to evaluate the danger of energetic materials.28 Dynamic temperatureprogrammed screening experiments were conducted on a Mettler Toledo professional DSC-1 coupled with a stainless steel crucible. Crucibles sealed with a lid (maximum withstood pressure: 15.0 MPa) served as containers for samples during thermal analytical measurements. The build-in STARe software was used to acquire thermal curves.29 DSC was first stabilized for 30.0 min, then the experiments were started with a specific scanning grate (3 °C·min−1) for better thermal equilibrium.30 The detection sensitivity is 0.04 μW. Measuring temperature ranged from 30 to 250 °C, and the flow of dynamic N2 was 30 mL·min−1. The sample mass was 1.00 mg. After the DSC test, the crucible was weighed again to confirm that there was no leakage during the test. 2.7. Safety Test of the ETBE Oxidation Process. A high-pressure reactor (vessel volume: 500 mL) equipped with a piezometer was used to investigate the potential explosive property of the ETBE oxidation reaction at higher temperatures (100−200 °C).31 In the test, 5 g ETBE sample was introduced into a round-bottom flask (vessel volume: 100 mL), and the flask was loaded into the reactor.32 The degree of crushing of the round flask was an index of the brisance in the ETBE oxidation process. In the next test, the reactor was sealed and filled with pure oxygen (1.00 MPa). Isothermal experiments were conducted at selected temperatures for 8 h to detect the detonation reaction.

Figure 3. TLC analysis of hydroperoxides formed at different temperatures.

3. RESULTS AND DISCUSSION 3.1. Thermal Stability of ETBE Oxidation Process. To study the thermal stability of ETBE oxidation reaction,

Figure 4. Hydroperoxide concentration versus oxidation time (t) at different oxidation temperatures.

Table 1. Oxygen Consumption of ETBE Oxidation at Different Temperatures reaction temperature/deg C oxygen consumption/10−2 mol

70 0.015

85 0.023

110 0.356

Figure 5. Assumed molecular structure of unknown hydroperoxide.

isothermal experiments were conducted using a customdesigned MCPVT device.30,31 According to the US Recommendations on the Transport of Dangerous Goods and using MCPVT, the thermal stability of ETBE oxidation has been well studied.32 The MCPVT was a closed type experiment when the oxidation reaction was one of the following: R1OR 2 + O2 → reaction products

[R1 = C2H5, R 2 = C(CH3)3 ]

Reaction 3 is a complex oxidation process. Does the oxidation occur? It is possible using the gas molar number or pressure change. The amount of gas substance was calculated using the ideal gas equation (n = PV/RT), where P and T are the measured values as shown in Figure 2a and b; R = 8.314 J·mol−1·k−1 and

(3) C

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temperature. The same trend was observed for the pressure profiles. However, n showed different behavior; n was gas moles in the cell. The behavior of n at 70 and 85 °C was as follows: first, n slowly decreased and, then, slowly increased. The gas moles were reduced because ETBE absorbed oxygen and hydroperoxides formed. The gas increased due to thermal decomposition. Oxidation was not significant at 70 and 85 °C; ETBE was stable at low temperature. However, at 110 °C, first, n was reduced because of the marked oxidation reaction and the formation of hydroperoxides. Then, n increased rapidly because unstable hydroperoxides caused rapid thermal decomposition and produced many oxidation products. And then, n was reduced again, because the products of decomposition underwent an oxidation reaction to produce relatively large molecules. Therefore, the ETBE oxidation reaction was very rapid at high temperature. Therefore, the oxidation process of ETBE can be described by three steps: first, the formation of hydroperoxide (slow oxidation reaction), followed by the thermal decomposition of hydroperoxide, H atom abstraction reaction, and free-radical displacement on O−O bond33 if the temperature exceed the T0 of hydroperoxide and subsequent rapid oxidation reaction initiated by reactive free radicals. The oxidation results in the formation of hydroperoxide which increases the thermal instability of ETBE. Furthermore, increasing the temperature can accelerate the oxidation reaction, which would decrease the quality of ETBE. ETBE was thermally unstable to heat in the presence of oxygen due to the formation of hydroperoxide. The (O−C−O−O) structure in hydroperoxide can also easily break and lead to decomposition and explosive reactions. The initial formation of hydroperoxide was a slow oxidation reaction so that reaction heat cannot be detected at low temperatures. Runaway or explosive reactions would occur if the sample mass was too large or the operating temperature was increased. 3.2. Unknown Hydroperoxides. 3.2.1. Species of Hydroperoxides Formed at Low Temperatures. TLC analysis was applied to figure out the species of hydroperoxide considering its thermal instability and hazards in a high temperature environment. As shown in Figure 3, only one kind of hydroperoxide was generated at 70 and 85 °C within 8 h reaction time. The TLC results of hydroperoxide obtained at 70 and 85 °C are the same. TLC analysis suggested that only one kind (or a majority) of hydroperoxide was formed at the reaction temperatures of 70 and 85 °C. Therefore, an assumption can be made that the hydroperoxide was the initial product in the ETBE oxidation reaction at low temperatures. To accumulate the sample, we repeated the experiments several times and collected the oxidation products at 85 °C. This hydroperoxide is the initial oxidation product, and it is possible to separate. The separation

Figure 6. Autoxidation mechanism of organic compound.

Figure 7. Proposed mechanism of hydroperoxide formation of ETBE oxidation.

reactor volume V = 10 mL. The calculated n−t curves are present in Figure 2c. The oxygen consumption representing the oxidation degree was calculated by the difference between the n values at the beginning and ending of the reaction. Figure 2a showed that the temperature of the 110 °C reaction rose sharply from 110 to 135 °C, resulting in a temperature increase of 25 °C for the reaction system; however, this phenomenon cannot be detected at 70 and 85 °C reaction. The initial exothermic temperature (110 °C) of ETBE oxidation reaction was determined by ARC in the mode of “heat-wait-search” (HWS mode) based on the literature.14 However, the time for exothermic reaction in isothermal condition was only 6 h less than that detected by ARC (10 h). The explanation for this phenomenon was that high temperature was the major factor in the formation of hydroperoxide, which was also demonstrated by the quantitative analysis of hydroperoxide using the iodimetry method. Table 1 presents the oxygen consumption of ETBE oxidation at different temperatures. The results indicate that oxygen consumption increased rapidly with the reaction

Figure 8. Mass spectrum of TBEHP by Thermo Fisher LCQ Fleet in positive ionization mode. D

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. 1H and 13C NMR spectra of ETBE and TBEHP.

by iodimetry.25 In the test, sample (about 100.00 mg) obtained directly from an autoclave was added to aqueous potassium iodide, followed by the titration of sodium hyposulfite solution. Total hydroperoxide concentrations were 53 302 ppm for 110 °C, 24 810 ppm for 85 °C, and 3024 ppm for 70 °C, respectively.

of the hydroperoxide was implemented by column chromatography. 3.2.2. Hydroperoxide Concentration at Different Temperatures. In the study, the total molar concentration of hydroperoxide in the ETBE oxidation process was determined E

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 10. FTIR spectra of ETBE and TBEHP.

reactive site; (2) the formation of peroxy radicals with the presence molecular oxygen and the formation of hydroperoxide by extracted hydrogen from carbon; (3) the termination of a chain reaction if two free radicals contact with each other.34 According to the autoxidation mechanism, the mechanism of hydroperoxide formation of ETBE is proposed, as shown in Figure 7. To the best of our knowledge, the secondary hydrogen possesses stronger reactivity than primary hydrogen and primary hydrogen has stronger reactivity than that from the tertiary butyl group. These would be responsible for the formation of the hydroperoxide. 3.2.4. Molecular Weight of Hydroperoxide. To obtain the molecular weight of the hydroperoxide, Thermo Fisher LCQ Fleet analysis was applied. Figure 8 shows a mass spectrum with a signal at m/z 135.081 representing the M + H molecular ion peak of TBEHP. Therefore, the molecular weight of TBEHP is about 134. Based on this result, the molecular weight increased from 102 (ETBE) to 134 (TBEHP), due to the combination of ETBE and oxygen. 3.2.5. Confirmation of Molecular Structure of TBEHP by 1H and 13C NMR Spectra. Nuclear magnetic resonance is a powerful technique for the identification of molecular structure and the reactive site by comparing the spectra of reactants and products. The 13C and 1H spectra of ETBE and TBEHP were presented in Figure 9. In Figure 9b, 1H NMR exhibited a single peak at ppm = 8.03 and a peak at ppm = 5.25. Compared to Figure 9a, the conclusion can be made that the reactive site of ETBE is secondary hydrogen (−CH2−) and only one of them takes part in the initial oxidation reaction. The chemical shift value changed from 55.76 to 99.84 indicated the formation of C−O bond as shown in Figure 9c and d. In Figure 9a and b, the peaks at 1.67 and 1.69 ppm were the impurity (water) in solvent (CDCl3). It suggested that all alkanes, ethers, and alcohols reacted with oxygen to form corresponding hydroperoxides via free radical mechanism and the only difference was the hydroperoxide yields.11,12,16,35 However, the reactivity of ethers was enhanced by the oxygen atoms in the ethers and the active site was the carbon connected to the oxygen atom. This was confirmed by the bond strength calculation. The secondary hydrogen has the C− H bond strength of 87.82 kcal·mol−1, which is weaker than that for the primary hydrogen (C−H bond for the primary hydrogen of the tertiary butyl group: 100.02 kcal·mol−1 and C−H bond for

Figure 11. Heat flow versus temperature of TBEHP by DSC at 3 °C· min−1 heating rate.

The yield of hydroperoxide increases rapidly as the reaction temperature increases, as shown in Figure 4. Therefore, a large amount of hydroperoxide would be formed at system temperatures exceeding 110 °C, which may cause runaway reactions or accidents. Therefore, it is necessary to control the formation of hydroperoxide or to prevent it from contacting with oxygen in ETBE usage. 3.2.3. Molecular Structure of Hydroperoxide. The oxygen− oxygen chemical bond of hydroperoxide is unstable and easily splits into reactive radicals that will trigger sophisticated oxidation or explosive reaction via hemolytic cleavage. In this regard, it is necessary to achieve the specific spectrometric evidence of hydroperoxide. An assumed molecular structure of hydroperoxide was shown in Figure 5, which was further confirmed by MS spectrum, 1H and 13C NMR spectrum and FTIR spectrum. The separated hydroperoxide named 1-tertbutoxy-ethyl hydroperoxide (mass 95%) is colorless and oily in a liquid state. The formation of hydroperoxides is impeded by the primary oxidation, or autoxidation, mechanism as shown in Figure 6. The process can be thought of as three steps: (1) the formation of a carbon free radical by H-abstraction reaction from

Table 2. Thermal Parameters of TBEHP by DSC at 3 °C· min−1 Heating Rate sample mass/mg

T0/°C

Toffset/°C

Tp/°C

TMEA/°C

MEA/W·min−1

QDSC/J·g−1

1.09

99.12

120.63

115.47

114.01

122.20

1523.89

F

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 12. Aftermath of ETBE oxidation reaction in amplification experiment at 140 °C. (a) Round bottom flask. (b) Broken flask after explosion.

the primary hydrogen of the ethoxy group: 100.22 kcal·mol−1).13 It is rational to conclude that the C−H bond from the secondary hydrogen possesses a stronger reactivity than that from the primary hydrogen. Therefore, the formation of hydroperoxide in the active site would be responsible for the hazard caused by ETBE in transportation, production, and ETBE blended gasoline usage. 3.2.6. FTIR Spectra of ETBE and TBEHP. FTIR spectroscopy is a powerful technique for the identification of organic groups in chemicals. In addition, to determine the reaction active site of ETBE, the FTIR spectra of ETBE and TBEHP were analyzed in comparison. As shown in Figure 10, three characteristics of hydroperoxides, at about 3375, 1260, and 842 cm−1 in the spectrum, were due to the stretching vibration of the O−H, C− O, and O−O bond.36 From the FTIR spectrum, the reaction active site of ETBE with oxygen was on the methylene carbon site connected with an oxygen atom. 3.3. Thermal Characteristics of TBEHP. Additional supporting evidence for hydroperoxide relies on the thermal decomposition heat of oxygen−oxygen chemical bond in organic hydroperoxide. Many organic hydroperoxides can initiate explosive polymerization in materials37 or be used directly as potential explosives for their large amount of thermal decomposition heat and low exothermic onset temperature. DSC experiments are necessary to give specific information on thermal characteristics of TBEHP.28b,38 The DSC curve could properly acquire the heat flow, and then, via experiment data, we know the T0, at which the hydroperoxide will thermally decompose and release enormous heat leading to runaway reactions, such as spontaneous combustion reaction in vehicle engine or explosive reaction during the storage, transport, usage, and production of ETBE.39 The DSC curve of TBEHP was shown in Figure 11. The DSC curve exhibited only one exothermic event which was attributed to the rupture of the O− O band. According to STARe analysis, the DSC curve indicated that the T0 of TBEHP was 99.12 °C. Hydroperoxides are usually regarded as inherently hazardous substance if the QDSC exceeds 250.00 J·g−1. The QDSC of TBEHP (mass 95%) was 1523.89 J·g−1 by DSC (β = 3 °C·min−1). Therefore, precautions must be taken to govern the concentration of TBEHP in ETBE usage or ETBE blended gasoline. The potential hazard of hydroperoxide can be also characterized by its ability to react by different mechanisms

of an explosion in the condensed phase, e.g., the thermal explosion or the detonation. According to the UN recommendations on the transport of dangerous goods (UNRTDG), TBEHP should be classified as the fifth type of dangerous oxidizing substances and organic peroxides.40 Table 2 presents T0, Te, Tp, MEA, TMEA, and QDSC of TBEHP that would offer a good understanding of TBEHP’s thermal stability and hazard properties. 3.5. Danger of ETBE Oxidation Process in Amplification Experiments. In the amplification experiment, no explosive reaction occurred at 85, 110, and 120 °C. However, when the temperature was 140 °C, oxygen pressure was 1.00 MPa, an explosive reaction took place when the reaction time was only 4 h. The round-bottom flask was broken into pieces. The rupture disc (withstand pressures of 10.00 MPa) of the autoclave and the piezometer were also broken due to the extensive explosion. As shown in Figure 12, the round-bottom flask broke into fragments after the explosive reaction. In addition, enough oxygen could exacerbate the explosion. Concerning the experimental safety, larger sample mass tests were not designed. Finally it should be pointed out that a detonation causes more destructive effects than the other types of explosions. ETBE and hydroperoxide-blend mixtures with higher energy content can undergo a transition from a thermal runaway to a detonation via deflagration.

4. CONCLUSION The oxidation reaction of ETBE with oxygen was studied using a custom-designed MCPVT. Results of T−t, P−t, and n−t curves suggest that ETBE has a significant oxidation reaction when the temperature is 110 °C. The hydroperoxide concentration in the oxidation process was analyzed by the iodimetry method. The hydroperoxide concentration increases rapidly with time at 110 °C. This is a promising experimental method to investigate the oxidation process of organic compounds. The most critical substance hydroperoxide was separated by column chromatography in the oxidation process. The molecular structure of the hydroperoxide was determined by MS, NMR, and FTIR and refers to 1-tert-butoxy-ethyl hydroperoxide. These results are supportive for the organic synthesis and oxidation mechanism of ETBE. G

DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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The thermal decomposition characteristics of 1-tert-butoxyethyl hydroperoxide were determined by DSC, the exothermic onset temperature (T0), and thermal decomposition heat (QDSC) were 99.12 °C and 1523.89 J·g−1, respectively. In addition, a high pressure reactor was used to evaluate the risk of the ETBE oxidation process, and the explosion occurred when the temperature was 140 °C. This is very important information for the safety of production, storage, transport, and applications of ETBE.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 138 7713 6730. E-mail: [email protected]. ORCID

Qiang Zhang: 0000-0001-6738-5775 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a Project from the National Natural Science Foundation of China (Project No. 11462001). The authors gratefully thank the Nanjing University of Science and Technology and Professor Yuejun Zhang and Wanghua Chen.

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NOMENCLATURE P = pressure T = temperature n = amount of substance t = time β = heating rate GLY = glyoxal MG = methylglyoxal T0 = exothermic onset temperature Tp = exothermic peak temperature Te = exothermic offset temperature QDSC = heat of thermal decomposition MEA = maximum exothermic acceleration TMEA = maximum exothermic acceleration temperature TBEHP = 1-tert-butoxy-ethyl hydroperoxide ARC = accelerating rate calorimeter

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DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b00607 Energy Fuels XXXX, XXX, XXX−XXX