Thermal Stability and Decomposition Kinetics of 1,3

Article pubs.acs.org/EF

Thermal Stability and Decomposition Kinetics of 1,3Dimethyladamantane Xiaomei Qin, Lei Yue, Jianzhou Wu, Yongsheng Guo,* Li Xu, and Wenjun Fang* Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: For a comprehensive understanding of the properties of 1,3-dimethyladamantane (1,3-DMA) as a candidate of high energy-density hydrocarbon fuels, thermal stability of 1,3-DMA under different conditions is investigated. The thermal decomposition kinetics in the batch reactor between 693 and 743 K has been determined, with the rate constants ranging from 4.00 × 10−7 s−1 at 693 K to 35.19 × 10−7 s−1 at 743 K, along with the Arrhenius parameters of A = 2.39 × 107 s−1 and activation energy Ea = 183 kJ·mol−1. The rate constants for the thermal decomposition of 1,3-DMA are observed to be smaller than those of some typical model fuels, decalin, propylcyclohexane, butylcylohexane, and n-dodecane, demonstrating that the thermal stability of 1,3-DMA is satisfactory. The thermal decomposition of 1,3-DMA in the flowing reactor at temperatures from 873 to 973 K and pressures from 0.1 to 5.0 MPa is further performed. It can be observed that the conversion of 1,3-DMA and the yield of gaseous products increase clearly with the rise of temperature or pressure. The residence time is the main factor for the change of decomposition depth. Methane and hydrogen are the major gaseous products that are produced through demethylation and dehydrogenation. In the liquid residues, toluene and xylene are observed and quantified by GC-MS, HPLC, and NMR as the main aromatics produced. On the basis of component analysis, a hypothetical mechanism of thermal decomposition of 1,3-DMA is proposed to explain the product distribution. It is shown that the different products are mainly obtained through a combination of isomerization, hydrogen transfer, β-scission, and dehydrogenation. The results are expected to provide experimental information for the search of new high energy-density hydrocarbon fuels.

1. INTRODUCTION High energy-density hydrocarbon fuels have been attracting wide attention due to their high densities and volumetric heating values.1−3 The higher the volumetric heating value of a fuel is, the greater energy per unit volume can be provided, which is beneficial for improving the performance of the aircraft with restricted volume. High energy-density hydrocarbon fuels can be broadly classified as kerosene with large specific gravity, polycyclic or cage-like hydrocarbons with high density. A high density hydrocarbon fuel usually refers to that with a density higher than 0.8 g·cm−3, either a pure compound or a blended hydrocarbon fuel.4,5 An adamantane derivative has a unique cage-like structure, and it can store energy through its strained cyclic geometry. Adamantane has high melt point (542 K),6 poor combustion performance and low solubility in the fuel. 1,3-Dimethyladamantane (1,3-DMA, shown in Figure 1), as an alkyl derivative of adamantane, has tetrahedral cage-like structure. The density of 1,3-DMA is 0.9016 g·cm−3 (293 K),7 and its lowtemperature property is adequate for many applications. The freeze point is 243 K, and the viscometric property is satisfactory (3.779 mPa·s, 293 K).7,8 Hence, 1,3-DMA has been expected to be used as one attractive component of high density fuels or fuel additives.9 Investigation on the thermal stability of 1,3-DMA should be a fundamental work to develop hydrocarbon fuels with high energy-density. Under practical application conditions, high energy-density fuels could be used to cool system structures of aircrafts regeneratively to appropriate temperatures in engines. Unfortunately, thermal stability of hydrocarbons usually limits the © XXXX American Chemical Society

Figure 1. Molecular structure of 1,3-DMA.

use of fuels as coolants.10,11 Hydrocarbon fuels are circulated in aircraft subsystems and they generally undergo oxidation at temperatures below 533 K and thermal decomposition above Received: May 21, 2014 Revised: September 10, 2014

A

dx.doi.org/10.1021/ef5011644 | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Table 1. Physical Properties of 1,3-DMA Tfa (K) 243 a

Tflash (K) 332.2

Tb (K) b

476.53

Tc (K)

pc (MPa)

b

704 708 ± 1c

2.75

b

ρ (g·cm−3) (298.15 K)

η (mPa·s) (298.15 K)

0.9016d

3.779d

Ref 8. bRef 24. cRef 25. dRef 7.

673 K.12−15 The thermal decomposition of fuels leads to the production of light species, then olefins, cyclic hydrocarbons, and aromatic compounds. Polycyclic aromatic compounds lead to the formation of solid deposits finally.16 The solid deposits might result in fouling of lines in aircraft.17−19 As the intermediate products of fuel decomposition, the produced aromatic compounds affect significantly the performance and stability of the hydrocarbon fuels.20−23 In the present work, the thermal stability of 1,3-DMA under different temperature−volume−pressure conditions is investigated. The kinetic results of thermal decomposition of 1,3DMA are obtained to reflect the thermal stability by performing static experiments in a batch reactor at a temperature range from 693 to 743 K. The thermal decomposition processes of 1,3-DMA at 873−973 K and different pressures in a flowing reactor, as a simulation of microchannel for the practical application, are further investigated. The effects of temperature, pressure, and residence time on the product distribution, especially the production of aromatics in the liquid products, are discussed. Toluene and xylene are observed as the main produced aromatics, and they are quantified precisely by GCMS, HPLC, and NMR. On the basis of composition analysis and quantum chemistry calculation, a probable mechanism is proposed to give a clue to the thermal decomposition of 1,3DMA and to explain the product distribution.

2. EXPERIMENTAL SECTION

Figure 2. (a) The schematic diagram of the stainless-steel static reactor: 1. furnace; 2. reactor; 3. temperature measurement tube; 4. liquid phase tube; 5. reactor cover; 6. needle valve; 7. manometer; 8. thermal couple; 9. safety valve; 10. fixed bolts; (b) The schematic diagram of tubular reactor: 1. feed; 2. pump; 3. needle valve; 4. nitrogen; 5. preheater; 6. reaction furnace; 7. condenser; 8. filter; 9. separator; 10. gas chromatography; 11. thermal couple; 12. proportional controller; 13. manometer; 14. pressure control valve.

2.1. Chemicals. 1,3-DMA (mass fraction purity > 0.99) is obtained from Bangcheng Chemical. Co., Ltd., Shanghai, China. The sample of 1,3-DMA is characterized by an Agilent 7890/5975 C gas chromatograph−mass spectrometry (GC-MS). The concentrations of 1,3-DMA and endo-tricyclo[5.2.1.02.6]decane are detected to be 99.75% and 0.25% by mass, respectively. Some physical properties such as freeze point (Tf), flash point (Tflash), boiling point (Tb), critical temperature (Tc), critical pressure (pc), density (ρ), and viscosity (η) of 1,3-DMA are listed in Table 1.8,24,25 The Tflash value is determined with an SYD261A automatic flash point tester. 2.2. Apparatus and Procedure. The thermal decomposition kinetics of 1,3-DMA in the batch reactor is investigated in a 316 stainless steel reactor with a total volume of 100 mL (shown in Figure 2a). The apparatus has been described previously.7 The operation temperature is controlled with an accuracy of 0.1 K in the range from 693 to 743 K. The sample with a definite mass of 25.0 g is injected into the reactor. After the reactor is purged with N2 to remove the air, the sample is then heated in the furnace to a certain temperature. The initial cold pressure of the system is set as atmospheric pressure. The pressure of the system during the thermal process is monitored by a manometer. After the thermal decomposition for a given time, the reactor is removed from the furnace and cooled down quickly to the room temperature. The volume of the gaseous products is quantified by the water displacement method. The liquid residue is analyzed by GC-MS. The thermal decomposition experiments in the flowing reactor are carried out with GH3128 alloy tube (3 mm o.d. × 1 mm i.d. × 800 mm in heated length). The schematic diagram of the apparatus is shown in Figure 2b. The fluid is fed with preset velocity using a dosing pump. It is preheated to 573 K in a preheater and then injected to the reactor with the constant temperature length of 400 mm. The thermal decomposition reactions of 1,3-DMA are performed under near and

super critical pressures. Prior to each run, the reactor is purged with N2 to exhaust the air inside, then heated up to the required temperature and kept for 1 h in order to maintain a constant temperature. The fuel sample is pumped into the reactor with a rate of 1.0 mL·min−1 under different pressures. The reaction effluent is quenched by passing through a condenser and then separated by a gas−liquid separator. 2.3. Product Analysis. The gaseous products were identified by a FULI (China) 9790 gas chromatograph (GC). The content of hydrogen was determined by a thermal conductivity detector (TCD) equipped with a stainless steel column. The column temperature was kept constant at 90 °C. The hydrocarbon gases were determined by a flame ionization detector (FID) equipped with a capillary column. The temperature of the oven for GC-FID was programmed from 50 to 120 °C at a heating rate of 5 °C·min−1 with an initial isothermal period of 3 min. The liquid products were determined by an Agilent 7890/5975C GC-MS. The GC was equipped with a capillary column, programmed from 50 to 260 °C at a heating rate of 10 °C·min−1 with an initial isothermal period of 2 min. The ion source temperature was 230 °C. The transfer line temperature was 250 °C, and the quadrupole temperature was kept at 150 °C. The mass range is 35−350 amu. The products were analyzed quantitatively by corrected area normalization method. In order to quantify precisely the produced aromatics in the liquid residues, both high-performance liquid chromatography (HPLC) and 1 H nuclear magnetic resonance spectroscopy (1H NMR) methods were employed. The HPLC analyses were carried out on a SHIMADZU LC-20AT with RID-10A refractive index detector. After filteration through a membrane filter, the liquid residue (10 μL) was injected into the chromatographic system with an aminobonded silica column (4.6 mm × 250 mm, 5 μm) and n-heptane as the B


Energy & Fuels

Article

mobile phase. The mobile phase was degassed and also filtered before each run of analysis. The flow rate of mobile phase was maintained at 1.0 mL·min−1 during the analysis. The temperatures of the column oven and detector were both set at 40 °C. The peak areas were compared with those obtained from calibration standards to calculate the aromatic contents. The 1H NMR experiments were performed on a Bruker DMX-400 NMR spectrometer with a frequency of 400 MHz. The samples of the liquid residues with the mass fraction of 5.0 wt % in CDCl3 containing 0.03 wt % tetramethylsilane (TMS) as internal standard were used.

The yield of gaseous products is expressed as gas yield = mg /m0

where mg is the total mass of gaseous products determined by GC. The values of gas yield for thermal decomposition of 1,3DMA under temperatures from 693 to 743 K are shown in Figure 3b. There are no gaseous products detected at 693 K because the conversion of thermal decomposition of 1,3-DMA is very low. The yield of gaseous products increases obviously with prolonging time at higher temperatures. To a certain extent, these changes reflect the depth of the thermal decomposition of 1,3-DMA. 3.1.2. Kinetics. The first-order reaction is used to describe the thermal decomposition of 1,3-DMA. The rate constant and half-lifetime can be determined from the following expressions.

3. RESULTS AND DISCUSSION 3.1. Thermal Stability in the Batch Reactor. 3.1.1. Conversion. Thermal decomposition of 1,3-DMA in the batch reactor was performed under temperatures from 693 to 743 K, along with the pressure given in Table 2 when the reactor Table 2. Rate Constants and Half-Life for Thermal Decomposition of 1,3-DMA in the Batch Reactor T (K)

693

713

723

733

743

p (MPa) k × 10−7 (s−1) t0.5 (h)

2.8 4.00 481

3.0 11.08 174

3.1 14.64 132

3.2 22.78 84.5

3.3 35.19 54.7

(2)

k = 1/t ln[1/(1 − x)]

(3)

t0.5 = ln 2/k

(4)

where x represents the conversion, t is the time, k is the rate constant, and t0.5 is the half-lifetime. The plots of ln[1/(1 − x)] against time for the thermal decomposition of 1,3-DMA in the experimental temperature range are shown in Figure 4. The rate

reached each investigated temperature. The thermal decomposition reactions should be under near and supercritical pressures. During the reaction, it was observed that the colorless liquid changed gradually to pale yellow at low temperatures and changed to brown at high temperatures. To minimize the influence of the time required for preheating the reactor on the kinetic data, the results at the zero reaction time when the reactor is just heated to the specified temperature were also detected. The conversion of the fuel is defined as conversion = (m0 − ml c)/m0

(1)

where m0 is the initial mass of 1,3-DMA, ml is the mass of liquid residues after reaction, and c is the mass fraction of 1,3-DMA in the liquid residues. The plots of conversion versus time for thermal decomposition of 1,3-DMA under different temperatures are shown in Figure 3a. It can be clearly seen that the decomposition percentage of 1,3-DMA increased with an increase of the reaction temperature or time. The conversion of thermal decomposition for 8 h is only less than 2% at 693 K and 13.0% at 743 K, respectively. As the extent of thermal decomposition of 1,3-DMA is quite low, the thermal stability of 1,3-DMA is satisfactory during this temperature range.

Figure 4. First-order kinetics for thermal decomposition of 1,3-DMA in the batch reactor at different temperatures.

constants were determined by the least-squares method with each linear correlation coefficient (r) near 0.999, indicating that the first-order kinetic assumption is appropriate to represent the decomposition of 1,3-DMA. The rate constants at the investigated temperatures, along with the values of t0.5, are

Figure 3. Plots of conversion of 1,3-DMA (a) and yield of gaseous products (b) versus time for thermal decomposition of 1,3-DMA in the batch reactor at different temperatures. C


Energy & Fuels

Article

presented in Table 2. The values of t0.5 show that 1,3-DMA is quite stable under these temperatures. According to the rate constants at different temperatures, the apparent activation energy, Ea, can be determined using the Arrhenius law k = A exp( −Ea /RT )

(5)

where A is the pre-exponential term, Ea is the activation energy, R is the gas constant, and T is the temperature. The Arrhenius plot of the first-order rate constants for the thermal decomposition of 1,3-DMA is shown in Figure 5. The Arrhenius parameters determined from a linear regression of the data are A = 2.39 × 107 s−1 and Ea = 183 kJ·mol−1.

Figure 6. Comparison of Arrhenius plots for thermal decomposition of different kinds of hydrocarbon compounds: (a) ref 28; (b) ref 29; (c) ref 30; (d) ref 31; (e) ref 32; (f) this work.

the gaseous products from thermal decomposition at 713−743 K are shown in Figure 7. It can be clearly seen that the major products in the gaseous phase are alkanes and hydrogen, and methane has the highest concentration. The content of hydrogen increased slightly with the increasing time, and those of ethane, ethane, propane, propene, butane and butane are quite low and changed little. The high content of methane in the gaseous phase is ascribed to the structure of 1,3-DMA, which contains methyl groups. The thermal decomposition occurs easily on the bonds between the methyl groups and cage-like structure. Methyl radicals can accept hydrogen atoms and change to methane easily. As an example, the components in the liquid residues of decomposition reaction at 743 K for 8 h are listed in Table 3. It can be seen that the major liquid products are adamantane derivatives, and only a small amount of aromatics is produced even after a long reaction time. It indicates that the thermal stability of 1,3-DMA is quite satisfactory under this experimental condition. 3.2. Thermal Stability in the Flowing Reactor. As discussed above, the kinetics and product distribution for the thermal decomposition of 1,3-DMA in the flowing reactor for a long time reflect the thermal stability of 1,3-DMA to a certain extent. In order to further investigate the case under conditions close to practical application, the thermal decomposition of 1,3DMA in the flowing reactor and high temperatures is studied in the following section. 3.2.1. Thermal Decomposition and Product Analysis. Thermal decomposition of 1,3-DMA is performed under temperatures from 873 to 973 K and pressures from 0.1 to 5.0 MPa. The residence times (ranging from 0.043 to 2.833 s) for 1,3-DMA under thermal decomposition at various pressures and temperatures are evaluated and given in Table S1 of Supporting Information. The residence time is evaluated as

Figure 5. Arrhenius plot for thermal decomposition of 1,3-DMA in the batch reactor.

The initial decomposition mechanism of 1,3-DMA probably involves scission of the carbon−carbon bond between the hydrocarbon cage and an methyl group. However, the measured activation energy is much smaller than the bond energy (373 kJ·mol−1)26 of a C−C bond. It suggests that other kinds of processes or reactions occur, such as chain processes.27 Accordingly, each of the reported rate constants should be an overall or global one, and it is not a rate constant for the initial reaction. Since the thermal stability of 1,3-DMA is not available in the literature, the results of thermal decomposition of 1,3DMA are compared with those of n-dodecane, butylcylohexane, decalin, and propylcyclohexane in Figure 6. Thermal decomposition experiments of n-dodecane, butylcylohexane, and decalin were carried out in a pyrex glass tube reactor with a total volume of 45−50 μL (a fixed loading ratio of sample was 0.36), in some cases, in a 316 stainless steel tubing bomb reactor.28−30 A stainless steel thermostated block with several 316L stainless steel reactors was used to perform the thermal decomposition of propylcyclohexane. Sample masses were typically on the order of 0.06 g and varied depending on the experimental temperature and cell volume.31 It can be seen clearly that the decomposition rate constant of 1,3-DMA is the smallest in the temperature range investigated. That is to say, 1,3-DMA is the most stable component among these hydrocarbons, and the thermal stability of decalin is after that of 1,3-DMA. 3.1.3. Products. Gaseous products of decomposition of 1,3DMA such as hydrogen, methane, ethane, ethene, propane, propene, and butane, butene were detected by GC. As no gaseous products were observed at 693 K, the mole fractions of

τ = Vr /v

(6) 3 −1

where v is the actual flow velocity (m ·s ) of the fluid at the specified temperature, and Vr is the volume (m3) of the thermostatic reaction tube. The properties of the fluid are assumed to be consistent with those of the feed (no change due to product formation) for the estimation of residence time. The constant furnace temperature is used to calculate the residence D


Energy & Fuels

Article

Figure 7. Distribution of the gaseous products for thermal decomposition of 1,3-DMA in the batch reactor: (a) 713 K; (b) 723 K; (c) 733 K; (d) 743 K.

From the data of residence time and conversion, the rate constants at different temperatures can be obtained. An Arrhenius plot of the pseudo-first-order rate constants for thermal decomposition of 1,3-DMA in the flowing reactor is shown in Figure 6. The Arrhenius parameters determined from a linear regression are A = 1.58 × 109 s−1 and Ea = 180 kJ· mol−1. The calculated activation energy for thermal decomposition in the flowing reactor is consistent with the value (Ea = 183 kJ·mol−1) in the batch reactor. The pre-exponential factor, mainly reflecting the entropy effect of the reactions, for thermal decomposition of 1,3-DMA in the flowing reactor is significantly higher than that (A = 2.39 × 107 s−1) in the batch reactor, which should be consistent with the cracking conditions. In Figure 6, the Arrhenius analysis is also compared with that of JP-10. The cracking experiments of JP-10 were performed in the annular tubular reactor at atmospheric pressure with the following range of variables: temperature, 903−968 K, and flow rate, 0.4−2.3 g/min, and thus a space time, 0.68−6.4 s.32 Overall, these two compounds exhibit similar trends of decomposition kinetics. The decomposition of 1,3-DMA in the flowing reactor is observed to have the same kinds of gaseous products (hydrogen, methane, ethane, ethene, propane, propene, butane, and butane) as that in the batch reactor. The major components in the gaseous phase are also methane and hydrogen. The change of mole fraction of gaseous products is compared in Table S1 of Supporting Information. Overall, the mole fraction of methane decreases with increasing the pressure at the temperatures from 873 to 973 K. The mole fraction of hydrogen increases obviously with increasing temperature under atmospheric pressure. Under pressurized pressures, no significant changes with the pressure at each temperature are observed. Variations in the composition of the liquid residues of 1,3DMA are listed in Tables S3−S7 of Supporting Information. In

Table 3. Components in the Liquid Residues for Thermal Decomposition of 1,3-DMA in the Batch Reactor at 743 K for 8 h component

mole fraction (×102)

isobutane toluene m-xylene benzene, 1,3-dimethylbenzene, 1,3,5-trimethylbenzene, 1,2,4-trimethylcis-8-ethyl-bicyclo[4.3.0]non-3-ene adamantane, 1,3-dimethyladamantane, 1,3,6-trimethyl1H-indene, 2,3-dihydro-5-methyladamantane, cis-1,4-dimethyladamantane, 1,2,3-trimethyladamantane, 1,3,5-trimethyl1H-indene, 2,3-dihydro-4,7-dimethyladamantane, 1,3-dimethyl-5-ethyladamantane, 1,3-dimethyl-5-n-propyl-

0.53 0.55 1.35 0.20 0.44 0.25 0.71 88.18 0.41 0.18 5.68 0.28 0.26 0.19 0.47 0.35

time for data representation, though the fluid is non-isothermal in the entire flowing reactor. The variations of conversion with pressure for thermal decomposition of 1,3-DMA under various temperatures are shown in Figure 8a, and the corresponding yields of total gaseous products are given in Figure 8b. It can be observed that the values of conversion are quite low at atmospheric pressure and increase rapidly from 0.1 to 3.0 MPa. The yield of gaseous products increases with the increasing temperature and pressure, which is similar to the profile of conversion. Hence, increasing pressure increases the residence time, and it is the main factor that leads to the increase of conversion and gas yield. E


Energy & Fuels

Article

Figure 8. Plots of conversion of 1,3-DMA (a) and yield of gaseous products (b) versus pressure for thermal decomposition of 1,3-DMA in the flowing reactor at different temperatures.

Figure 9. Distribution of the liquid products for thermal decomposition of 1,3-DMA in the flowing reactor: (a) 873 K; (b) 898 K; (c) 923 K; (d) 948 K; (e) 973 K.

C12H 20 = nl liquid + ng gas

order to ensure the product distribution, the balances of the C number and H number before and after the reaction are listed in Table S8 of Supporting Information. The total ratio of C/H for the gaseous products and liquid residues under different conditions are close to that of 1,3-DMA (12:20), which means that the material balance is satisfactory. The decomposition of 1,3-DMA can be expressed as

(7)

which means that 1 mol of 1,3-DMA decomposes to nl mol liquid residues and ng mol gaseous products. The average molecular weights of the liquid residues and the gaseous products can be obtained from the detailed product distribution. The values of nl and ng are then calculated from F


Energy & Fuels

Article

the corresponding average molecular weights and masses. As an example, the values are nl = 0.97 and ng = 1.02 for the decomposition of 1,3-DMA at 973 K and 4.0 MPa. So, it can be expressed as C12H20 = 0.97C10.83H17.02 + 1.02C1.38H4.38, and the total composition after decomposition is C11.92H20.97. The thermal decomposition products of 1,3-DMA are visually shown in Figure 9. It indicates that the depth of the thermal decomposition under atmospheric pressure is still quite low even at the temperatures from 873 to 973 K. At 873 K, the major components of the liquid products are isomers of 1,3DMA (C12 hydrocarbons) such as 1,4-dimethyladamantane, and a small amount of toluene (C7 hydrocarbon) and xylene (C8 hydrocarbons) begins to occur with increasing pressure. With increasing temperature or pressure, the contents of the isomers of 1,3-DMA and aromatics increase gradually. The bicyclic aromatic, i.e., 2-methylnaphthalene, begins to occur at 923 K and 5.0 MPa. When the temperature continues to increase, the contents of bicyclic aromatics begin to increase. When the decomposition temperature is higher than 873 K, xylenes (C8 hydrocarbons) are the major products. At the same temperature below 973 K, the contents of C12 hydrocarbons increase gradually with rising pressure. At 973 K, the content of C12 hydrocarbons exhibits a decrease trend at pressures higher than 3.0 MPa. These phenomena indicate that the consumption of 1,3-DMA isomers increases, and more cage-like structures are destroyed with the extent increase of thermal decomposition. As a result, small-carbon-number hydrocarbons are generated, such as C6, C7, and C8 hydrocarbons. Similarly, Wohlwend et al.33 has reported the thermal stability of JP-10 under supercritical conditions (between 473 and 923 K, 3.4 MPa, residence time = 1.8 s at 473 K) and the aromatic yields in terms of “percent yield relative to parent response”. The experiments were performed with a flow rate of 0.5 mL·min−1. JP-10 started to fragment slightly at 723 K and decomposed readily at 873 K. The formation of benzene, toluene, and naphthalene was observed between 823 and 873 K. In comparison, the bicyclic aromatics from the decomposition of 1,3-DMA only begins to occur at higher temperature and with longer residence time (923 K and 5.0 MPa, residence time = 2.522 s). Hence, the thermal stability of 1,3-DMA should be satisfactory. 3.2.2. Aromatics Determined by HPLC and NMR. Because the produced aromatic compounds reflect significantly the performance of the fuel, their contents in the corresponding liquid residues are quantified by both HPLC and 1H NMR methods. The aromatic contents determined by HPLC are shown in Figure 10. The aromatics cannot be determined distinctly in the samples picked up at atmospheric pressure, and they are clearly observed with increasing reaction temperature at higher pressures. Relatively, there is a rapid increase of aromatic content at elevated reaction pressure at 948−973 K. The aromatic content is determined to be near 20% at 973 K and 4.0 MPa, while it is less than 9% at 923 K and 4.0 MPa (residence time = 2.014 s). However, the aromatic content (benzene, toluene, and naphthalene) from thermal decomposition of JP-10 at 923 K and 3.4 MPa (the residence time should be less than 1.8 s) was observed to be more than 12%.33 Similar results were reported by Kunzru et al.32 who investigated the cracking of JP-10 and defined the percent yields of benzene and toluene as “the kg of product formed per kg of JP-10 fed to reactor”. At the temperatures of 923 and 948 K, the total yields of benzene and toluene were 1.2% and 1.4% (residence time = 1.3 and 0.8 s), respectively.

Figure 10. Aromatic contents from thermal decomposition of 1,3DMA determined by HPLC (■, 873 K; ●, 898 K; ▲, 923 K; ▼, 948 K; left-pointing solid triangle, 973 K) and 1H NMR (□, 873 K; ○, 898 K; Δ, 923 K; ▽, 948 K; left-pointing open triangle, 973 K).

From the 1H NMR spectra of liquid residues and the characteristic chemical shift region of δ = 6.1−8.0 ppm corresponding to the hydrogen of aromatics, the aromatics contents from the thermal decomposition of 1,3-DMA can be quantified from the following equations.34,35 aromatics (wt%) = (ml /(ml + mg )) × ρaromatics /ρsample × (A + C /3)0.97 × 102 /((A + C /3)0.97 + (D − 2B + E /2 + F /3)1.02 + 3.33B))

(8)

where ρsample is the density of the liquid residues, and ρaromatics is the density of the aromatics. An average density of the aromatics can be considered as 0.88 g·mL−1. A, B, C, D, E, and F are the integral intensities of different types of hydrogens, as listed in Table 4. Table 4. Assignment of 1H NMR Regions to Components and Structural Groups integral designation

type of hydrogen

chemical shift δ (ppm)

A B C D E F

aromatics olefin α-methyl methine (paraffins) methylene (paraffins) methyl (paraffins)

6.5−8.0 4.2−6.0 2.0−3.5 1.5−2.0 1.0−0.5 0.5−1.0

As examples, parts of 1H NMR spectra are visually shown in Figure S1 of Supporting Information. The aromatic contents obtained from the 1H NMR measurements for the samples from the thermal decomposition of 1,3-DMA are also shown in Figure 10. The aromatic contents at atmospheric pressure are nearly zero, consistent with the results of HPLC. Overall, the results obtained from 1H NMR method tend to be consistent with those from HPLC method. The higher aromatic contents indicate that the thermal stability becomes poorer to a certain extent. The acceptable deviation between the results of HPLC and 1H NMR is attributed to the different measurements. 3.3. Hypothetical Mechanism. The thermal decomposition in the batch reactor for a long time and that in the flowing reactor under several constant pressures at high temperatures has been performed, respectively. All of the products from G


Energy & Fuels

Article

Figure 11. Possible radicals obtained from the breaking of C−C and C−H bond of 1,3-DMA.

The hypothetical decomposition processes are proposed in Figure 12 to explain the formation of main products such as substituted benzene, adamantane derivative, and alkyl naphthalene, which are found by GC, GC-MS, HPLC, and NMR during the thermal decomposition of 1,3-DMA under different temperature−volume−pressure conditions. The breaking C−C bonds are labeled in red for easy observation. The primary decomposition mechanism includes the following elementary steps: (1) unimolecular initiations by bond fission, (2) propagations by hydrogen abstraction, or metathesis, via bimolecular reaction, (3) propagations by β-scission decomposition, (4) propagations by isomerization, and (5) terminations by a combination of two free radicals. The possible decomposition processes of these biradicals in routes 1−4 in Figure 12 indicate that the radicals mostly pass through hydrogen transfer, β-scission, and dehydrogenation to produce second radicals and smaller molecules.37 Relatively, the methyl radical is easily produced from the thermal decomposition. Methyl radicals can accept hydrogen atoms and change to methane. The substituted adamantane are mainly from the scission of radicals listed in routes 6, 9, and 10; indene and naphthalene are obtained from the isomerization of radicals and the detailed processes are listed in routes 7 and 10 in Figure 12. Radical R8 may pass through β-scission decomposition to form the same second radical as radical R7.

thermal decomposition in the batch reactor can be detected among the products obtained from that in the flowing reactor. On the basis of component analysis, a hypothetical mechanism of the thermal decomposition of 1,3-DMA is proposed. 1,3DMA is one of the cage-like compounds with polycyclic rings. On supposition of the thermal decomposition of 1,3-DMA with radical initiations, the possible biradicals produced from 1,3DMA are listed in Figure 11. Several single radicals can be produced by the loss of hydrogen atom from the molecule, which are also listed in Figure 11. The dissociation enthalpies of 1,3-DMA molecule to different free radicals are calculated using Gaussian09 software package36 (listed in Table 5). The initiation occurs by homolytic cleavage Table 5. Dissociation Enthalpies of 1,3-DMA Molecule to Several Free Radicals 1 2 3 4 5 6 7 8 9 10

C12H20 C12H20 C12H20 C12H20 C12H20 C12H20 C12H20 C12H20 C12H20 C12H20

→ → → → → → → → → →

reactions

ΔH (kcal·mol−1)

Radical Radical Radical Radical Radical Radical Radical Radical Radical Radical

23.6 30.3 29.0 21.7 74.3 100.6 98.7 98.4 98.2 96.6

R1 R2 R3 R4 R5 + CH3· R6 + H· R7 + H· R8 + H· R9 + H· R10 + H·

4. CONCLUSIONS The thermal stability of 1,3-DMA under different temperature− volume−pressure conditions has been investigated. The thermal decomposition kinetics of 1,3-DMA in the batch reactor are calculated quantitatively at temperatures from 693 to 743 K with the rate constants ranging from 4.00 × 10−7 s−1 at 693 K to 35.19 × 10−7 s−1 at 743 K, along with the Arrhenius parameters of A = 2.39 × 107 s−1 and activation energy Ea = 183 kJ·mol−1. The thermal decomposition of 1,3-DMA in the flowing reactor at temperatures from 873 to 973 K and pressures from 0.1 to 5.0 MPa is further performed. The

of the C−C bond to form biradicals. The smaller dissociation enthalpies of reactions 1−4 indicate that biradicals R1−R4 from the 1,3-DMA molecule are most likely to be produced. The biradicals could undergo further reactions to form new single radicals and unsaturated products, depending on the number of decomposition steps. The hydrogen-deficient molecules can be produced through β-scission and dehydrogenation. Hence, toluene and xylene as the major products can be observed under high temperatures and pressures. H


Energy & Fuels

Article

Figure 12. Hypothetical mechanism for thermal decomposition of 1,3-DMA.

conversion of thermal decomposition of 1,3-DMA at atmospheric pressure has relatively low values and increases obviously with increasing pressure because of different residence times (0.043−2.833 s). The calculated activation energy (Ea = 180 kJ·mol−1) for thermal decomposition of 1,3DMA in the flowing reactor is consistent with the value (Ea = 183 kJ·mol−1) in the batch reactor. The relatively small rate constants for thermal decomposition of 1,3-DMA in the batch reactor indicate that its thermal stability is satisfactory at the given temperatures. The Arrhenius analysis for thermal decomposition of 1,3-DMA in the flowing reactor is also compared with that of JP-10 in the flowing reactor, and these

two compounds exhibit similar trends of decomposition kinetics. The residence time is the main factor for the changes of the decomposition depth at high temperatures. The major gaseous products are methane and hydrogen, which are produced through demethylation and dehydrogenation. The major components of the liquid products are the isomers of 1,3DMA at the primary stage and then aromatics increase gradually. The main produced aromatic species in the liquid residues, quantified by GC-MS, HPLC, and NMR, are detected to be toluene and xylene. This results from the continuous consumption of 1,3-DMA isomers and the destruction of cagelike structures. On the basis of component analysis, a I


Energy & Fuels

Article

(8) Scheider, A.; Warren, R. W.; Janovski, E. J. Formation of perhydrophenalenes and polyalkyladamantanes by isomerization of tricyclic perhydroaromatics. J. Am. Chem. Soc. 1964, 86, 5365−5367. (9) Grice, K.; Alexander, R.; Kagi, R. I. Diamondoid hydrocarbon ratios as indicators of biodegradation in australian crude oils. Org. Geochem. 2000, 31, 67−73. (10) Marren, D.; Lewis, M.; Maurice, L. Q. Experimentation, test, and evaluation requirements for future airbreathing hypersonic systems. J. Propul. Power 2001, 17, 1361−1365. (11) Edwards, T. Liquid fuels and propellants for aerospace propulsion: 1903−2003. J. Propul. Power 2003, 19, 1089−1107. (12) Li, D.; Fang, W. J.; Xing, Y.; Guo, Y. S.; Lin, R. S. Spectroscopic studies on thermal-oxidation stability of hydrocarbon fuels. Fuel 2008, 87, 3286−3291. (13) Park, S. H.; Kwon, C. H.; Kim, J.; Chun, B. H.; Kang, J. W.; Han, J. S.; Jeong, B. H.; Kim, S. H. Thermal stability and isomerization mechanism of exo-tetrahydrodicyclopentadiene: experimental study and molecular modeling. Ind. Eng. Chem. Res. 2010, 49, 8319−8324. (14) Wang, Z.; Guo, Y. S.; Lin, R. S. Pyrolysis of hydrocarbon fuel ZH-100 under different pressures. J. Anal. Appl. Pyrolysis 2009, 85, 534−538. (15) Xing, Y.; Fang, W. J.; Xie, W. J.; Guo, Y. S.; Lin, R. S. Thermal cracking of JP-10 under pressure. Ind. Eng. Chem. Res. 2008, 47, 10034−10040. (16) Kim, J.; Park, S. H.; Lee, C. H.; Chun, B. H.; Han, J. K.; Jeong, B. H.; Kim, S. H. Coke formation during thermal decomposition of methylcyclohexane by alkyl substituted C5 ring hydrocarbons under supercritical conditions. Energy Fuels 2012, 26, 5121−5134. (17) Meng, X. H.; Xu, C. H.; Li, L.; G, J. S. Cracking performance and feed characterization study of catalytic pyrolysis for light olefin production. Energy Fuels 2011, 25, 1357−1363. (18) Commodo, M.; Wong, O.; Fabris, I.; Groth, C. P. T.; Gülder, Ö . L. Spectroscopic study of aviation jet fuel thermal oxidative stability. Energy Fuels 2010, 24, 6437−6441. (19) Fumoto, E.; Matsumura, A.; Sato, S.; Takanohashi, T. Recovery of lighter fuels by cracking heavy oil with zirconia-alumina-iron oxide catalysts in a steam atmosphere. Energy Fuels 2009, 23, 1338−1341. (20) Andrésen, J. M.; Strohm, J. J.; Sun, L.; Song, C. S. Relationship between the Formation of Aromatic Compounds and solid deposition during thermal degradation of jet fuels in the pyrolytic regime. Energy Fuels 2001, 15, 714−723. (21) Dewitt, M. J.; Edwards, T.; Shafer, L.; Brooks, D.; Striebich, R.; Bagley, S. P.; Wornat, M. J. Effect of aviation fuel type on pyrolytic reactivity and deposition propensity under supercritical conditions. Ind. Eng. Chem. Res. 2011, 50, 10434−10451. (22) Edwards, T. Cracking and deposition behavior of supercritical hydrocarbon aviation fuels. Combust. Sci. Technol. 2006, 178, 307−334. (23) Huber, M. L.; Lemmon, E. W.; Diky, V.; Smith, B. L.; Bruno, T. J. Chemically authentic surrogate mixture model for the thermophysical properties of a coal-derived liquid fuel. Energy Fuels 2008, 22, 3249−3257. (24) Varushchenko, R. M.; Pashchenko, L. L.; Druzhinina, A. I.; Abramenkov, A. V.; Pimersin, A. A. Thermodynamics of vaporization of some alkyladamantanes. J. Chem. Thermodyn 2001, 33, 733−744. (25) Steele, W. V.; Chirico, R. D.; Knipmeyer, S. E.; Nguyen, A. Vapor pressure of acetophenone, (±)-1,2-butanediol, (±)-1,3butanediol, diethylene glycol monopropyl ether, 1,3-dimethyladamantane, 2-ethoxyethyl acetate, ethyl octyl sulfide, and pentyl acetate. J. Chem. Eng. Data 1996, 41, 1255−1268. (26) Lide, D. R. CRC Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, FL, 2002. (27) Gough, R. V.; Widegren, J. A.; Bruno, T. J. Thermal decomposition kinetics of 1,3,5-triisopropylcyclohexane. Ind. Eng. Chem. Res. 2013, 52, 8200−8205. (28) Yu, J.; Eser, S. Kinetics of supercritical-phase thermal decomposition of C10-C14 normal alkanes and their mixtures. Ind. Eng. Chem. Res. 1997, 36, 585−591. (29) Yu, J.; Eser, S. Thermal decomposition of jet fuel model compounds under near-critical and supercritical conditions. 1. n-

hypothetical mechanism of the thermal decomposition of 1,3DMA is proposed to explain the product distribution, especially the production of toluene, xylene, and substituted adamantane. It is shown that the species are mainly produced through the processes of isomerization, hydrogen transfer, β-scission, and dehydrogenation. Investigation on the thermal stability of 1,3DMA is a fundamental work to develop hydrocarbon fuels. Although the study here is fundamental, it shows the potential of 1,3-DMA as a new component of high energy-density hydrocarbon fuels. The experimental results are hoped to provide reference information for the property optimization of new hydrocarbon fuels. Of course, if 1,3-DMA could be considered as a candidate, further engine testing under a broad range of conditions should be carried out.

■

ASSOCIATED CONTENT

S Supporting Information *

Mole fraction of gaseous products for thermal decomposition of 1,3-DMA in the flowing reactor. Components in liquid residues for thermal decomposition of 1,3-DMA in the flowing reactor. The carbon and hydrogen balance of products under different conditions. The 1H NMR spectra for 1,3-DMA and the liquid residues from thermal decomposition in the flowing reactor at 898 K. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*(W.F.) E-mail: [email protected]. Tel: +86 571 88981416. Fax: +86 571 88981416. *(Y.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Grant Nos. 21273201, 21173191, and J1210042.

■

REFERENCES

(1) Hudzik, J. M.; Asatryan, R.; Bozzelli, J. W. Thermochemical properties of exo-tricyclo[5.2.1.02,6]decane (JP-10 jet fuel) and derived tricyclodecyl radicals. J. Phys. Chem. A 2010, 114, 9545−9553. (2) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E. Designing high thermal stability jet fuels for the 21st century. Energy Conversion Engineering Conference, August 11−16, 1996, Washington, DC; Institute of Electrical and Electronic Engineers: Piscataway, NJ, 1996. (3) Chung, H. S.; H. Chen, C. S.; Kremer, R. A.; Boulton, J. R. Recent developments in high-energy density liquid hydrocarbon fuels. Energy Fuels 1999, 13, 641−649. (4) Azevedo, D. A.; Tamanqueira, J. B.; Dias, J. C. M.; Carmo, A. P. B.; Landau, L.; Gonçalves, F. T. T. Multivariate statistical analysis of diamondoid and biomarker data from brazilian basin oil samples. Fuel 2008, 87, 2122−2130. (5) Roan, M. A.; Boehman, A. L. The Effect of fuel composition and dissolved oxygen on deposit formation from potential JP-900 basestocks. Energy Fuels 2004, 18, 835−843. (6) Mair, B. J.; Shamaiengar, M.; Krouskop, N. C.; Rossini, F. D. Isolation of adamantane from petroleum. Anal. Chem. 1959, 32, 2082. (7) Qin, X. M.; Cao, X. F.; Guo, Y. S.; Xu, L.; Hu, S. L.; Fang, W. J. Density, viscosity, surface tension, and refractive index for binary mixtures of 1,3-dimethyladamantane with four C10 alkanes. J. Chem. Eng. Data 2014, 5, 775−783. J


Energy & Fuels

Article

butylbenzene and n-butylcyclohexane. Ind. Eng. Chem. Res. 1998, 37, 4591−4600. (30) Yu, J.; Eser, S. Thermal decomposition of jet fuel model compounds under near-critical and supercritical conditions. 2. decalin and tetralin. Ind. Eng. Chem. Res. 1998, 37, 4601−4608. (31) Widegren, J. A.; Bruno, T. J. Thermal decomposition kinetics of propylcyclohexane. Ind. Eng. Chem. Res. 2009, 48, 654−659. (32) Rao, P. N.; Kunzru, D. Thermal cracking of JP-10: Kinetics and product distribution. J. Anal. Appl. Pyrol. 2006, 76, 154−160. (33) Wohlwend, K.; Maurice, L. Q.; Edwards, T.; Striebich, R. C.; Vangsness, M.; Hill, A. S. Thermal stability of energetic hydrocarbon fuels for use in combined cycle engines. J. Propul. Power 2001, 17, 1258−1262. (34) Joo, H. S.; Guin, J. A. Continuous upgrading of a plastics pyrolysis liquid to an environmentally favorable gasoline range product. Fuel Process. Technol. 1998, 57, 25−40. (35) Qin, X. M.; Chi, H.; Fang, W. J.; Guo, Y. S.; Xu, L. Thermal stability characterization of n-alkanes from determination of produced aromatics. J. Anal. Appl. Pyrol. 2013, 104, 593−602. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E. Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1, Gaussian, Inc.: Wallingford, CT, 2009. (37) Olivier, H.; Baptiste, S.; Roda, B.; René, F.; Frédérique, B. L.; Gérard, S.; Marquaire, P. M. Primary mechanism of the thermal decomposition of tricyclodecane. J. Phys. Chem. A 2006, 110, 11298− 11314.

K


Thermal Stability and Decomposition Kinetics of 1,3

Recommend Documents