Dealkylation of Aromatics in Subcritical and Supercritical Water

Aug 25, 2016 - Under hydrothermal environments covering the subcritical and supercritical regions of water, the involvement of the carbonium mechanism...
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Dealkylation of Aromatics in Subcritical and Supercritical Water: Involvement of Carbonium Mechanism Yi Chen,† Kai Wang,† Jing-Yi Yang,‡ Pei-Qing Yuan,*,† Zhen-Min Cheng,† and Wei-Kang Yuan† †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Research Institute of Petroleum Processing, East China University of Science and Technology, Shanghai 200237, China

‡

ABSTRACT: Under hydrothermal environments covering the subcritical and supercritical regions of water, the involvement of the carbonium mechanism in the dealkylation of aromatics and its resulting influence on the pyrolysis of heavy oil were surveyed. αOlefin groups, as either a part of straight chain hydrocarbons or the terminal of alkyl substitutes of aromatics, are protonated spontaneously by hydronium ions into carboniums, followed by βscission with similar reaction kinetic characteristics. The probability of the protonation of α-olefins under hydrothermal environments depends on the ionic product of water, so the occurrence of the βscission in the carbonium mechanism is related to the thermodynamic state of water. With the aid of the carbonium mechanism at increasing water density, a recovered conversion rate and an increasing ratio of propene to ethylene in the product occur during the pyrolysis of a model α-olefin of dodecene under hydrothermal environments. Also, the dealkylation involved in the pyrolysis of maltenes is accelerated.

1. INTRODUCTION With the persistent demands for fuel oil but the exhausted reserves of light crude oil, the utilization of unconventional heavy oil becomes important to global energy security. Due to the high fractions of carbon residue and heteroatoms contained, it is increasingly difficult for refineries to deal with heavy oil with deteriorating quality. Seeing that water in its subcritical or supercritical state (Tc = 647 K, Pc = 22.1 MPa) is an environmentally benign alternative solvent for organics, an effective acid/base catalyst, and even a possible H-donor, the upgrading of heavy oil under hydrothermal environments covering mainly the supercritical region of water has been extensively investigated in academia since the 1990s.1−6 Pyrolysis is vital to the upgrading of heavy oil at high temperatures. One of the motivations for the upgrading of heavy oil under severe hydrothermal environments is that early studies considered supercritical water (SCW) as an efficient Hdonor for pyrolysis.7−10 Later experiments found that there is no essential difference in the yield of light oil fractions between the upgrading under SCW and N2 environments.11 The calculation based on quantum mechanism also confirmed that, even in the supercritical region of water, the H-abstraction between water molecules and hydrocarbon radicals is thermodynamically infeasible.12 By these considerations, recent work mostly recommended that SCW is an inert reaction medium for the pyrolysis, which is dominated by a free radical mechanism.11,13,14 According to visual observation, it was further proposed that the pyrolysis of heavy oil under hydrothermal environments follows the type IIIb or type II phase behavior defined by van Konynenburg and Scott.15,16 As a result, the upgrading of heavy oil can be run not only in the © 2016 American Chemical Society

traditional oil phase but also in the novel subcritical water (subCW) or SCW phase.17 The pyrolysis of heavy oil consists primarily of the condensation of aromatics and the dealkylation of aromatics. At a high water density and a high water to oil ratio, the pyrolysis under hydrothermal environments was reported to be faster than that under N2 environments.18 Such a phenomenon is reflected by the accelerated coke formation and by the reduced alkyl fraction of aromatics in the intermediate and ultimate condensation products. Since the diffusion coefficient of a solute in sub-CW or SCW is much higher than that in the oil phase by at least 1 order of magnitude, it was deduced that the improved diffusivity might be responsible for the promoted pyrolysis under hydrothermal environments.19,20 However, the reaction kinetics analysis indicated that in the sub-CW or SCW phase the apparent activation energies of the lumped reactions involved in pyrolysis actually are comparable with those of the lumped reactions occurring in the oil phase.21,22 A sole consideration on diffusivity therefore is far from adequate to account for the accelerated pyrolysis of heavy oil under hydrothermal environments. According to the latest molecular dynamics study, heavy aromatics contained in heavy oil have a repulsive interaction with the sub-CW or SCW solvent.23 Stimulated by the π−π attractive interaction between aromatic segments, the selfassembly of heavy aromatics occurs spontaneously and rapidly Received: Revised: Accepted: Published: 9578

June 15, 2016 August 24, 2016 August 25, 2016 August 25, 2016 DOI: 10.1021/acs.iecr.6b02323 Ind. Eng. Chem. Res. 2016, 55, 9578−9585

Article

Industrial & Engineering Chemistry Research

at temperature higher than 673 K. Subject to these restrictions, the theoretical and experimental studies on the pyrolysis of dodecene and maltenes in the presence of N2 or water were all performed at a compromising temperature of 683 K. 2.1. Theoretical Background. Based on density functional theory (DFT), the β-scission of a carbonium resulting from the protonation of α-olefin as the terminal of either straight chain hydrocarbons or the alkyl substitutes of aromatics was examined. All the calculations were performed with the Gaussian 03 program pack. Becke’s three-parameter exchange functional with the Lee−Yang−Parr correction was used to describe the interaction among particles, while the orbits of electrons were expanded with a moderate basis set of 631+G(d). A typical procedure of the characterization on β-scission is as follows. The optimization on and the frequency analysis of the geometries of the species participating in the reaction were applied first. On the basis of the QST2 algorithm, the transition state with a sole imaginary frequency then was located, followed by an intrinsic reaction path calculation to ensure that the transition state obtained connects the desired reactant and product. After the thermal correction to the energy of the reactant, transition state, and product, the reaction rate constant of β-scission was calculated according to the Eyring equation.

in the water phase. Accordingly, the accelerated condensation of aromatics under hydrothermal environments can be ascribed to the coke-like supramolecular structure formed. As for the simultaneously accelerated dealkylation of aromatics, unfortunately no convincing explanation on this issue can be found in the literature so far. Being an effective Brönsted acid, water in its subcritical and supercritical regions has an ionic product varying from 10−10 to 10−20. Some ion mechanism based reactions, such as Beckmann arrangements, hydrolysis, and Diels−Alder reaction, proceed readily under hydrothermal environments without the addition of acidic catalysts.24−28 When applying the pyrolysis of polyethylene in SCW, Moriya detected secondary alcohols in the product. These secondary alcohols were proposed to be produced via the hydration of α-olefins, which are the intermediate products of the pyrolysis of paraffins.29,30 The proposal above was confirmed by Tomita, who investigated the hydration of propene over MoO3/Al2O3 in sub-CW and SCW.31 It was found that the activity of the catalyst is closely related to the pKw of water. Even in the absence of MoO3/ Al2O3, still the hydration of propene occurs in the subcritical region of water. One may notice that water at ambient conditions with the pKa around 15.7 normally does not add to α-olefins. Through the ionization of strong acids in water, hydronium ions with the pKa of −1.74 are obtained, catalyzing readily the hydration of α-olefins in the carbonium mechanism. In terms of the results reported by Moriya and Tomita, the hydronium ions resulting from the autoionization of sub-CW or SCW, though at extremely low concentration, are capable of activating α-olefins into carboniums.30,31 During the pyrolysis of heavy oil, the alkyl substitutes of aromatics are shortened by free radical H-abstraction and βscission, leaving α-olefin groups at the terminal of the substitute. Similar to the pyrolysis of paraffins under hydrothermal environments, theoretically the α-olefin terminals of the alkyl substitute of aromatics can also be protonated into carboniums. In addition to hydration, there is a good chance that the carboniums formed decompose further through βscission. Consequently, the dealkylation of aromatics in subCW or SCW could be accomplished in a parallel way, that is, the β-scission in the carbonium mechanism. Hereby, under N2 environments and hydrothermal environments covering the subcritical and the supercritical regions of water, the pyrolysis of a model α-olefin of dodecene and maltenes was investigated. By theoretical and experimental characterization, the reaction behavior of dodecene under hydrothermal environments was examined first, on the basis of which the involvement of the β-scission of carboniums at increasing water density was confirmed. The possible approaches for the dealkylation of aromatics in the presence of sub-CW or SCW then was proposed, and verified subsequently by characterizing the alkyl substitutes of the asphaltenes resulting from the condensation of maltenes under different solvent environments.

k=

⎛ −ΔH ≠ ⎞ ⎛ ΔS ≠ ⎞ kbT exp⎜ ⎟ exp⎜ ⎟ h ⎝ RT ⎠ ⎝ R ⎠

(1)

where T and k are reaction temperature and rate constant. ΔS≠ and ΔH≠ are the activation entropy and activation enthalpy of β-scission. The definition of the constants involved is explained in the Notation section. 2.2. Experimental Procedures. 2.2.1. Apparatus and Reaction Runs. Under N2 or hydrothermal environments, the pyrolysis of dodecene and maltenes was applied in a Parr HPHT-4598 autoclave with a capacity of 100 mL. Analytically pure dodecene was purchased from Sinopharm Group Co. Ltd., and maltenes were separated from the heavy oil obtained from Sinopec Changling Branch. Some basic properties of maltenes and dodecene are listed in Table 1. The definition of the Table 1. Properties of Maltenes and Dodecene Used in Pyrolysis under Varied Medium Environments Dodecene 3

Density (g/cm )

Mw (g/mol)

0.76

168.3

Tc (K)

657.6 Maltenes

Pc (MPa)

Bp (K)

1.93

486

Structural parameters Density (g/cm3)

H/C

Mn (Da)

n

r

RN

RS

RA

0.97

1.5

1195

7.3

0.3

2.0

6.8

7.7

symbols used is explained in the Notation section. To ensure a maximum error of product distribution within ±5%, each reaction run under the same operating condition mentioned below was repeated at least 3 times. A typical procedure of the pyrolysis of dodecene or maltenes under hydrothermal environments is as follows. First, water and 10 g of reactant were loaded into the autoclave. The weight of water loaded varied from 5 to 25 g. After purging with N2 of high purity (>99.9 vol %), the reactor was sealed. By the difference in the amount of water loaded, N2 of high purity at

2. THEORETICAL AND EXPERIMENTAL METHODS Under hydrothermal environments, the prerequisite for the dealkylation of aromatics by the carbonium mechanism is that the α-olefin terminal of alkyl substitutes is protonated first by the hydronium ions provided by water. In the subcritical region of water, a relatively higher ionic product is obtained at the temperature around 573 K.32 Driven by entropy change, the pyrolysis of heavy oil, however, can only be effectively initiated 9579

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Figure 1. Decomposition of dodecene in free radical or carbonium mechanism in the presence sub-CW or SCW.

spontaneously in sub-CW or SCW, the carboniums formed might be converted through hydration or β-scission, as illustrated in Figure 1. The hydration of α-olefins with a reducing molecular number is exothermic. When hydration occurs, a high temperature is extremely unfavorable for its thermodynamic equilibrium. The production of a small quantity of secondary alcohols in sub-CW or SCW can be ascribed to the contribution of high pressure to equilibrium. Therefore, only the β-scission of the carbonium C+12H25 was characterized with the theoretical calculation. Through a transition state with the sole imaginary frequency of −675.21 cm−1, the carbonium C+12H25 is converted into a shorter carbonium C+9 H19, releasing a propene molecule at the same time. In the middle of the formation of the transition state as shown in Figure 2, an internal H-transfer from the secondary

different pressures was charged into the reactor. At a slope of 15 K/min, the reactor then was heated from ambient temperature to 683 K. Owning to the supplement of N2, the pyrolysis at different water densities had an approximately equivalent initial pressure around 30 MPa. After up to 180 min of pyrolysis, the autoclave was subjected to forced air cooling so as to terminate the reaction. At the preheating and pyrolysis stages, the stirring rate was always maintained at 800 rpm. Basically, the procedure of the pyrolysis of dodecene or maltenes under N2 environments was similar to that under hydrothermal environments. First, 10 g of reactant was loaded into the autoclave. After purging with N2 of high purity, the reactor was charged with N2 at the pressure of 11.0 MPa. The following operations were just the same as those under hydrothermal environments. 2.2.2. Analytical Procedures. After the pyrolysis of dodecene, the gas product was collected first, followed by a thorough wash of the reactor with toluene to get liquid products. The molar ratio of propene to ethylene in the gas product was determined on a GC 2060 equipped with a HP Plot-Q column, while the conversion of dodecene was measured on a HP 6890 GC. As for the pyrolysis of maltenes, the cooled reactor was washed thoroughly with toluene to collect products. Following the Industrial Standard of Chinese Petrochemical NB/SH/T 0509-2010, the product collected was separated sequentially into coke, asphaltenes, and maltenes. Asphaltenes resulting from the condensation of maltenes were applied for various structural characterizations. The H/C ratio of asphaltenes was measured on a Vario EL III element analyzer. The numberaverage molecular weight (Mn) of asphaltenes was measured on a vapor pressure osmometer (VPO, Knauer K7000) based on the Industrial Standard of Chinese Petrochemical SH/T 058394. The Fourier transform infrared (FT-IR) spectra of asphaltenes were characterized on a Nicolet Magna 550 IR spectrometer, averaged from 64 scans at a resolution of 4 cm−1. The nuclear magnetic resonance (NMR) spectra of asphaltenes were analyzed on a Bruker Avance 400 MHz NMR spectrometer. CDCl3 and tetramethylsilane were used as the solvent and the internal standard for calibrating chemical shift.

Figure 2. Geometry of the transition state of β-scission of C+12H25 carbonium.

C atom to the terminal primary C atom occurs, by which a secondary carbonium is produced eventually. The subsequent β-scission of C+9 H19 follows exactly the same method of the βscission of C+12H25. For the β-scission of C+12H25 to C+9 H19 and C+9 H19 to C+6 H13 in series, no abrupt change in potential energy in both the forward and the backward directions can be observed. Refer to Figure 3. The activation enthalpies of the β-scission in the forward and backward directions vary around 130.0 and 60.0 kJ/mol, respectively. A reaction barrier as high as 130.0 kJ/mol suggests that the β-scission of straight chain carboniums will be effectively promoted by increasing temperature. After incorporating the influence of activation entropy, the reaction rate constants of the successive β-scission of C+12H25 were evaluated, with the results listed in Table 2.

3. RESULTS AND DISCUSSION 3.1. Reaction Kinetics of β-Scission of Straight Chain Carboniums. According to DFT calculation, the protonation between dodecene and hydronium ions has a negative Gibbs free energy change of −133.3 kJ/mol. Once protonated 9580

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Figure 3. Potential energy profile of successive β-scission of C+12H25; temperature of 683 K.

At the temperature 683 K, the rate constants of the β-scission of C+12H25 and C+9 H19 in the forward direction are much larger than those in the backward direction by about 3 orders of magnitude. As a result, the β-scission of carboniums at high temperatures is approximately irreversible. In parallel with the traditional free radical mechanism, the conversion of straight chain α-olefins under hydrothermal environments through the carbonium mechanism based β-scission thus is viable, both thermodynamically and kinetically. 3.2. Pyrolysis of Dodecene under N2 and Hydrothermal Environments. To validate calculation results, the pyrolysis of dodecene under hydrothermal environments was applied at the temperature 683 K and at water densities ranging from 0.05 to 0.25 g/cm3. The experiments at the water densities 0.05 and 0.10 g/cm3 fall into the subcritical region of water, while those at the water densities 0.15, 0.20, and 0.25 g/ cm3 are in the supercritical region of water. For comparison purposes, the reaction was also run in the presence of N2 at the same temperature. Under different medium environments, the conversion of dodecene at extended reaction time is illustrated in Figure 4. For the pyrolysis under N2 environments, the conversion of dodecene increases with the extension of reaction time, approaching 100% at the reaction time 30 min. The H-balance of the pyrolysis of dodecene relies on the production of a large amount of α-olefins and diolefins of lower molecular weight as well as a trace amount of coke. When water at the density of 0.05 g/cm3 was introduced, the conversion of dodecene is significantly retarded, especially at the initial reaction stage. As the data presented in Figure 4 show, the conversion rate of dodecene at the reaction time of 5 min is only 20%, lower than the corresponding value of 33% under N2 environments. Nevertheless, a complete conversion of dodecene still can be achieved within 30 min. With the increase in water density to 0.15 g/cm3 or higher, the conversion of dodecene recovers steadily. At the water density of 0.25 g/cm3, it has already been able to be comparable with that under N2 environments.

Figure 4. Effect of medium environments on conversion of dodecene at extending reaction time; temperature of 683 K.

An analysis of the molar ratio of propene to ethylene in the gaseous product collected at the reaction time of 120 min was applied subsequently, with the results illustrated in Figure 5.

Figure 5. Effect of water density on molar ratio of propene to ethylene in the gaseous product of pyrolysis of dodecene under hydrothermal or N2 environments; temperature 683 K and reaction time 120 min.

Under N2 environments or hydrothermal environments at water densities lower than 0.10 g/cm3, the molar ratio of propene to ethylene in the gaseous product basically is maintained at 1:1. With the increase in water density to 0.15 g/cm3 or higher, the corresponding value increases monotonically. At the water density 0.25 g/cm3, a molar ratio of propene to ethylene up to 4.0 can be observed. The neat pyrolysis of straight chain α-olefins follows the Kossiakoff−Rice (KR) or Fabuss−Satterfield−Smith (FSS) mechanism.33−35 The former mechanism involving mainly

Table 2. Reaction Kinetic Parameters of Successive β-Scission of Carbonium C+12H25; Temperature of 683 K ΔH‡ (kJ/mol)

ΔS‡ (kJ/mol)

k (s−1; mol−1·L·s−1)

Reaction

Forward

Backward

Forward

Backward

Forward

backward

C12+H25 ↔ C3H6 + C9+H19

1.29 × 1002

6.29 × 1001

1.62 × 10−04

−1.27 × 10−01

9.08 × 1004

1.18 × 1001

C9+H19 ↔ C3H6 + C6+H13

1.30 × 1002

5.86 × 1001

3.55 × 10−02

−1.31 × 10−01

1.20 × 1005

1.61 × 1001

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Figure 6. Possible reaction mechanisms involved in dealkylation of aromatics under hydrothermal environments.

Table 3. Reaction Kinetic Parameters of β-Scission of C6H5 − C+7 H14 and C6H5 − C•6H12a ΔH‡ (kJ/mol) Reactions

C6H5 −

C7+H14

→ C6H5 −

Forward

C4+H8

+ C3H6

C6H5 − C6· H12 → C6H5 − C4· H8 + C2H4 a

ΔS‡ (kJ/mol)

Backward

Forward −02

1.36 × 10

7.00 × 10

4.21 × 10

1.15 × 1002

3.26 × 1001

1.30 × 10−02

02

01

k (s−1; mol−1·L·s−1)

Backward −01

Forward

backward

−1.23 × 10

9.67 × 10

04

2.43 × 1001

−1.63 × 10−01

1.16 × 1005

1.47 × 1002

Temperature 683 K.

monomolecular β-scission is suitable for the gaseous phase reaction in which ethylene is the major product. The latter mechanism, consisting primarily of bimolecular H-abstraction, usually is applied to the liquid phase reaction, characterized by the nearly equimolar ratio of α-olefins to n-alkanes in the product. Miller once suggested that the pyrolysis of straight chain α-olefins may proceed through a six-ring transition state, by which propene is produced.36 By the phase structure calculation based on the Peng−Robinson equation of state, at the temperature of 683 K applied in this work, the pyrolysis of dodecene in the presence of N2 or water is run in a single phase whose densities lie between the gaseous and liquid phases. Accordingly, the KR and FSS mechanisms both have influence on the pyrolysis of dodecene under either N2 or hydrothermal environments. The pyrolysis of dodecene is initiated by the random C−C cleavage at high temperatures. The primary C radicals formed may release ethylene through β-scission, or be saturated by the H-abstraction with dodecene nearby. On the condition that sub-CW or SCW is an inert medium for pyrolysis, the unfavorable cage effect on the bimolecular H-abstraction between C radicals and dodecene should be responsible for the retarded conversion of dodecene observed at the water density of 0.05 g/cm3. Even if the conversion of dodecene is accomplished through the mechanism suggested by Miller, the presence of reaction media may result in steric hindrance to the formation of the six-ring transition state. With the increase in water density, the KR rather than the FSS mechanism will dominate the pyrolysis kinetics gradually and a monotonically decreasing conversion rate of dodecene is supposed to be observed. However, the experiments run at water densities higher than 0.15 g/cm3 present a widely different tendency. On the basis of theoretical calculation, dodecene can be protonated spontaneously by hydronium ions into carboniums, followed by nearly irreversible β-scission. At the temperature 683 K, the pKw of water decreases from 22 to 14.8 along with

the increase in water density from 0.05 to 0.25 g/cm3. Refer to Figure 5. With the increase in the concentration of hydronium ions by several orders of magnitude, α-olefins, no matter dodecene or intermediate pyrolysis products, are more likely to be protonated into carboniums. Consequently, the conversion of dodecene retarded by the cage effect is compensated by the involvement of carbonium mechanism based β-scission. Although the pyrolysis at increasing water density tends to follow the KR mechanism, still a high molar ratio of propene to ethylene in the gas product up to 4 is observed owing to the promoted conversion of α-olefins in the carbonium mechanism. 3.3. Approaches for Dealkylation of Aromatics under Hydrothermal Environments. In terms of the pyrolysis of dodecene under different solvent environments, the possible approaches for the dealkylation of aromatics under hydrothermal environments as schematically illustrated in Figure 6 are proposed. Similar to the pyrolysis of straight chain hydrocarbons, secondary C radicals on the alkyl substitutes of aromatics are formed by the H-abstraction with free radicals nearby. The alkyl substitute then decomposes through free radical β-scission, producing a shortened substitute with either a primary C radical or an α-olefin terminal. In the presence of dense subCW or SCW with a higher concentration of hydronium ions, the α-olefin terminal of aromatics then is protonated, followed by the β-scission in the carbonium mechanism. Supposing that R′ shown in Figure 6 is the simplest benzene ring, the β-scission of the carbonium or free radical terminal illustrated in the figure was characterized by DFT calculation. The reaction kinetic parameters obtained are listed in Table 3. When β-scission occurs at the carbonium terminal of the substitute of aromatics, the transition state involved is much similar in structure to that involved in the β-scission of straight chain carboniums. An internal H-transfer from the secondary C atom to the primary terminal C atom also occurs, by which a secondary carbonium terminal finally is formed. A comparison 9582

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raw maltenes is 7.7, while the corresponding value of the asphaltenes formed under different medium environments all exceeds 30.0. The huge difference in RA between maltenes and asphaltenes indicates that it is condensation that dominates the pyrolysis of maltenes. To examine further the possible involvement of the carbonium mechanism in the dealkylation of aromatics, a parameter Lalkyl is defined here as the product of n and Rs, whose definition is explained in the Notation section. Lalkyl reflects the average total length of alkyl substitutes contained in asphaltenes. Along with the extension of reaction time, the variation of Lalkyl of the asphaltenes formed under N2 and hydrothermal environments is illustrated in Figure 8.

between the data listed in Tables 2 and 3 shows that the remaining part of carboniums, no matter condensed rings or alkyl chains, only has a minor influence on the reaction kinetics of β-scission. Besides, the dealkylation of aromatics by βscission, in either free radical or carbonium mechanism, is accomplished with similar reaction kinetic characteristics. By the fact that the ionic product of water is strongly temperature and density dependent, the involvement of the carbonium mechanism in the dealkylation of aromatics depends definitely on the thermodynamic state of water. 3.4. Pyrolysis of Maltenes under N2 and Hydrothermal Environments. To verify whether the dealkylation of aromatics in sub-CW or SCW might proceed in the alternative carbonium mechanism, the pyrolysis of maltenes was applied under N2 and hydrothermal environments. For the reaction run under hydrothermal environments, the density of water was varied at three levels: 0.05, 0.15, and 0.25 g/cm3. Asphaltenes, the intermediate product of the condensation of maltenes, were separated from the liquid product collected at the reaction times 60, 120, and 180 min. The H-balance of the pyrolysis of maltenes relies on the production of asphaltenes and coke with lower H/C ratios. Typical FT-IR spectra of asphaltenes formed under N2 environments and hydrothermal environments at the water density 0.25 g/cm3 are illustrated in Figure 7. Regardless of the

Figure 8. Effect of water density on the dealkylation rate of aromatics under hydrothermal environments; temperature of 683 K.

No matter whether the pyrolysis of maltenes is applied under N2 or hydrothermal environments, the value of Lalkyl decreases monotonically with reaction time, suggesting the occurrence of dealkylation during pyrolysis. When the pyrolysis is transferred from N2 to hydrothermal environments, a regular variation of Lalkyl can be observed with the increase in water density. Compared with the pyrolysis under N2 environments, at any reaction time the value of Lalkyl decreases slightly at the water densities of 0.05 and 0.15 g/cm3 but drastically at the water density of 0.25 g/cm3. Take the pyrolysis at the reaction time of 180 min as an example. The values of Lalkyl at the water densities of 0.0, 0.05, 0.15, and 0.25 g/cm3 are 42, 38, 36, and 14, respectively. A significantly promoted dealkylation of aromatics under dense hydrothermal environments thus is confirmed, in accordance with the FT-IR data presented in Figure 7. In practice, the substitutes of aromatics in heavy oil can be straight or branched alkyl chains, containing sulfur in the form of thioether sometimes. During the pyrolysis of maltenes under hydrothermal environments, the decomposition of thioether, the cyclization, and the β-scission in either the free radical or carbonium mechanism all may contribute to dealkylation. The decomposition of alkyl sulfides in sub-CW or SCW follows the free radical mechanism.41,42 Therefore, the removal of sulfur containing alkyl substitutes under hydrothermal environments depends more on reaction temperature rather than on water density. In fact, even at the water density of 0.15 g/cm3, alkyl sulfides may decompose readily in SCW.41 Based on the DFT calculation, the alkyl substitutes containing 2 to 4 carbon atoms may cyclize into naphthenic rings with a reaction barrier lower

Figure 7. Effect of reaction media on the FT-IR spectra of asphaltenes derived from condensation of maltenes; reaction time of 60 min and reaction temperature of 683 K.

difference in reaction media, the asphaltenes resulting from the condensation of maltenes all share a similar FT-IR pattern. The absorption peaks at the wavenumbers 2921 and1455 cm−1 correspond to the methyl groups of the alkyl substitutes of aromatics, while those at 2851 and 1375 cm−1 result from the symmetrical bending and stretching vibrations of methyl groups.37−39 Evidently, the intensity of the peaks relating to the alkyl substitutes of the asphaltenes formed under hydrothermal environments is much weaker than that of the asphaltenes formed under N2 environments, suggesting that the dealkylation of aromatics under hydrothermal environments is significantly accelerated. Based on VPO and NMR data, the average structure of asphaltenes was characterized according to the Brown−Ladner equation.40 The average number of aromatic rings (RA) of the 9583

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Industrial & Engineering Chemistry Research than 31.5 kJ/mol.43 The presence of sub-CW or SCW media results in steric hindrance for the formation of the transition state involved in cyclization, going against the reaction kinetics of cyclization. As for the monomolecular β-scission of the alkyl substitute in the free radical mechanism, the presence of reaction media has no substantial influence on its reaction behavior. Among the dealkylation approaches mentioned above, only the β-scission in the carbonium mechanism has a sensitive and positive response to the increase in water density, since the probability of the protonation of the α-olefin terminal of alkyl substitutes is strongly related to the pKw of water. Benefiting from the effective protonation, an accelerated dealkylation in the carbonium mechanism during the pyrolysis of maltenes at the water density of 0.25 g/cm3 as shown in Figure 8 thus can be observed. 3.5. Effect of Carbonium Mechanism on Pyrolysis of Heavy Oil under Hydrothermal Environments. The pyrolysis of heavy oil is composed of the simultaneous occurrence of the condensation of aromatics and the dealkylation of aromatics. Essentially, the condensation of aromatics is accomplished by the 2-dimensional extension and the 3-dimensional stacking of aromatic segments. Due to the spontaneous coke-like supramolecular stacking of aromatic segments in the sub-CW or SCW phase, the sequential condensation of maltenes to asphaltenes and the condensation of asphaltenes to coke are accelerated.19,21,44 Nevertheless, the existence of alkyl substitutes of aromatics interferes with the structured stacking of aromatics. By the simultaneous involvement of free radical and carbonium mechanisms under dense hydrothermal environments, the magnitude of the alkyl substitute of aromatics can be rapidly reduced, which in turn accelerates the condensation of aromatics in the water phase. The effective protonation of the α-olefin terminal of the alkyl substitute of aromatics is the prerequisite for the involvement of the carbonium mechanism in the dealkylation of aromatics. Only in the subcritical region of water, that is, temperature around 573 K and water density higher than 0.20 g/cm3, can a relatively higher ionic product of water be obtained. The upgrading of heavy oil under severe hydrothermal environments reported in the literature mostly was run at high temperatures up to 723 K but low water densities even to 0.05 g/cm3. At that time, the carbonium mechanism has a marginal influence on the dealkylation of aromatics and the dealkylation is still dominated by the traditional free radical mechanism. As for the pyrolysis of light model aromatics such as alkylbenzene, the presence of sub-CW or SCW at a low water density merely results in unfavorable steric hindrance to the reaction kinetics of dealkylation, which is similar in mechanism to the retarded conversion of dodecene shown in Figure 4.2

thermal environments the dealkylation of aromatics involved in pyrolysis of heavy oil is significantly accelerated.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +862164253529. Fax: +862164253528. E-mail address: [email protected] (P.Q. Yuan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE-13C02) and the National Natural Science Foundation of China (Grant No. 21376075; Grant No. 21676084).

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NOTATION Bp = boiling point of species; K g = gas constant; J·mol−1·K−1 h = Planck constant; 6.63 × 10−34 J·s−1 k = rate constant of β-scission in the forward or reverse direction; s−1; mol−1·L·s−1 kb = Boltzmann constant; 1.38 × 10−23 J·K−1 Lalkyl = average total length of alkyl substitutes contained in asphaltenes Mw = molecular weight of species; g/mol Mn = numerical average weight of oil fractions; Da n = average number of carbon atoms per alkyl substituent Pc = critical pressure of species; MPa r = number of naphthene rings per substituent R = gas constant; 8.314 J·mol−1·K−1 RA = average number of aromatic rings per average molecule RN = average number of naphthenic rings per average molecule RS = average number of alkyl substituents per average molecule T = reaction temperature; K Tc = critical temperature of species; K ΔH≠ = enthalpy change of reaction; kJ·mol−1 ΔS≠ = entropy change of reaction; J·mol−1 REFERENCES

(1) Timko, M. T.; Ghoniem, A. F.; Green, W. H. Upgrading and Desulfurization of Heavy Oils by Supercritical Water. J. Supercrit. Fluids 2015, 96, 114. (2) Carr, A. G.; Class, C. A.; Lai, L.; Kida, Y.; Monrose, T.; Green, W. H. Supercritical Water Treatment of Crude Oil and Hexylbenzene: An Experimental and Mechanistic Study on Alkylbenzene Decomposition. Energy Fuels 2015, 29, 5290. (3) Li, N.; Yan, B.; Xiao, X. M. A Review of Laboratory-Scale Research on Upgrading Heavy Oil in Supercritical Water. Energies 2015, 8, 8962. (4) Sato, T.; Sumita, T.; Itoh, N. Effect of CO Addition on Upgrading Bitumen in Supercritical Water. J. Supercrit. Fluids 2015, 104, 171. (5) Hosseinpour, M.; Fatemi, S.; Ahmadi, S. J. Catalytic Cracking of Petroleum Vacuum Residue in Supercritical Water Media: Impact of Alpha-Fe2O3 in the Form of Free Nanoparticles and Silica-supported Granules. Fuel 2015, 159, 538. (6) Gai, X. K.; Arano, H.; Lu, P.; Mao, J. W.; Yoneyama, Y.; Lu, C. X.; Yang, R. Q.; Tsubaki, N. Catalytic Bitumen Cracking in Sub- and Supercritical water. Fuel Process. Technol. 2016, 142, 315. (7) Dente, M. E.; Ranzi, E. M. Pyrolysis: Theory and Industrial Practice; Academic Press: New York, Chapter 7, 1983.

4. CONCLUSIONS The ionic product of water depends strongly on its thermodynamic state, so a moderate temperature and a high water density facilitate the spontaneous protonation of α-olefins dissolved in sub-CW or SCW to carboniums. The carbonium formed, as either the terminal of the alkyl substitute of aromatics or one part of straight chain hydrocarbons, may decompose through β-scission with similar reaction kinetic behavior. With the effective involvement of the carbonium mechanism, a recovered conversion rate together with an increasing molar ratio of propene to ethylene in the gaseous product is observed during the pyrolysis of dodecene under dense hydrothermal environments. Also, under dense hydro9584

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Article

Industrial & Engineering Chemistry Research (8) Sato, T.; Adschiri, T.; Arai, K.; Rempel, G. L.; Ng, F. T. T. Upgrading of Asphalt with and Without Partial Oxidation in Supercritical Water. Fuel 2003, 82, 1231. (9) Han, L.; Zhang, R.; Bi, J. Upgrading of Coal-tar Pitch in Supercritical Water. J. Fuel Chem. Technol. 2008, 36, 1. (10) Henrikson, J. T.; Grice, C. R.; Savage, P. E. Effect of Water Density on Methanol Oxidation Kinetics in Supercritical Water. J. Phys. Chem. A 2006, 110, 3627−3632. (11) Morimoto, M.; Sugimoto, Y.; Saotome, Y.; Sato, S.; Takanohashi, T. Effect of Supercritical Water on Upgrading Reaction of Oil Sand Bitumen. J. Supercrit. Fluids 2010, 55, 223. (12) Zhu, C. C.; Ren, C.; Tan, X. C.; Chen, G.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Initiated Pyrolysis of Heavy Oil in the Presence of Near-critical Water. Fuel Process. Technol. 2013, 111, 111. (13) Cheng, Z. M.; Ding, Y.; Zhao, L. Q.; Yuan, P. Q.; Yuan, W. K. Effects of Supercritical Water in Vacuum Residue Upgrading. Energy Fuels 2009, 23, 3178−3183. (14) Morimoto, M.; Sugimoto, Y.; Sato, S.; Takanohashi, T. Solvent Effect of Water on Supercritical Water Treatment of Heavy Oil. J. Jpn. Pet. Inst. 2014, 57, 11. (15) Amani, M. J.; Gray, M. R.; Shaw, J. M. Phase Behavior of Athabasca Bitumen+water Mixtures at High Temperature and Pressure. J. Supercrit. Fluids 2013, 77, 142. (16) Amani, M. J.; Gray, M. R.; Shaw, J. M. The Phase Behavior of Athabasca Bitumen+toluene+water Ternary Mixtures. Fluid Phase Equilib. 2014, 370, 75. (17) Watanabe, M.; Kato, S. N.; Ishizeki, S.; Inomata, H.; Smith, R. L., Jr. Heavy Oil Upgrading in the Presence of High Density Water: Basic Study. J. Supercrit. Fluids 2010, 53, 48. (18) Liu, Y.; Bai, F.; Zhu, C. C.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Upgrading of Residual Oil in Sub- and Supercritical Water: An Experimental Study. Fuel Process. Technol. 2013, 106, 281. (19) Liu, Q. K.; Zhu, D. Q.; Tan, X. C.; Yang, J. Y.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Lumped Reaction Kinetic Models for Pyrolysis of Heavy Oil in the Presence of Supercritical Water. AIChE J. 2016, 62, 207. (20) Alomair, O.; Elsharkawy, A.; Alkandari, H. A Viscosity Prediction Model for Kuwaiti Heavy Crude Oils at Elevated Temperatures. J. Pet. Sci. Eng. 2014, 120, 102. (21) Tan, X. C.; Liu, Q. K.; Zhu, D. Q.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Pyrolysis of Heavy Oil in the Presence of Supercritical Water: The Reaction Kinetics in Different Phases. AIChE J. 2015, 61, 857. (22) Kozhevnikov, I. V.; Nuzhdin, A. L.; Martyanov, O. N. Transformation of Petroleum Asphaltenes in Supercritical Water. J. Supercrit. Fluids 2010, 55, 217. (23) Xin, S. M.; Liu, Q. K.; Wang, K.; Chen, Y.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Solvation of Asphaltenes in Supercritical Water: A Molecular Dynamics Study. Chem. Eng. Sci. 2016, 146, 115. (24) Boero, M.; Ikeshoji, T.; Liew, C. C.; Terakura, K.; Parrinello, M. Hydrogen Bond Driven Chemical Reactions: Beckmann Rearrangement of Cyclohexanone Oxime into Epsilon-Caprolactam in Supercritical Water. J. Am. Chem. Soc. 2004, 126, 6280. (25) Li, Y.; Wang, K.; Qin, K.; Wang, T. Beckmann Rearrangement Reaction of Cyclohexanone Oxime in Sub/supercritical Water: Byproduct and Selectivity. RSC Adv. 2015, 5, 25365. (26) Ikeshoji, T. Supercritical Water Properties and Beckmann Rearrangement Reaction: First Principles Molecular Dynamics Study. J. Jpn. Pet. Inst. 2011, 54, 159. (27) Cantero, D. A.; Bermejo, M. D.; Cocero, M. J. Governing Chemistry of Cellulose Hydrolysis in Supercritical Water. ChemSusChem 2015, 8, 1026. (28) Kang, K.; Azargohar, R.; Dalai, A. K.; Wang, H. Hydrogen Production from Lignin, Cellulose and Waste Biomass via Supercritical Water Gasification: Catalyst Activity and Process Optimization Study. Energy Convers. Manage. 2016, 117, 528. (29) Moriya, T.; Enomoto, H. Role of Water in Conversion of Polyethylene to Oils Through Supercritical Water Cracking. Kagaku Kogaku Ronbunshu 1999, 25, 940.

(30) Moriya, T.; Enomoto, H. Conversion of Polyethylene to Oil Using Supercritical Water and Donation of Hydrogen in Supercritical Water. Kobunshi Ronbunshu 2001, 58, 661. (31) Tomita, K.; Koda, S.; Oshima, Y. Catalytic Hydration of Propylene with MoO3/Al2O3 in Supercritical Water. Ind. Eng. Chem. Res. 2002, 41, 3341. (32) Bandura, A. V.; Lvov, S. N. The Ionization Constant of Water over Wide Ranges of Temperature and Density. J. Phys. Chem. Ref. Data 2006, 35, 15. (33) Kossiakoff, A.; Rice, F. O. Thermal Decomposition of Hydrocarbons: Resonance Stabilization and Isomerization of Free Radicals. J. Am. Chem. Soc. 1943, 65, 590. (34) Fabuss, B. M.; Smith, J. O.; Satterfield, C. N. Thermal Cracking of Pure Saturated Hydrocarbons. Adv. Pet. Chem. Refin. 1964, 9, 157. (35) Fabuss, B. M.; Smith, J. O.; Lait, R. I.; Borsanyi, A. S.; Satterfield, C. N. Rapid Thermal Cracking of n-Hexadecane at Elevated Pressures. Ind. Eng. Chem. Process Des. Dev. 1962, 1, 293. (36) Miller, D. B. High Alpha, Omega-Dienes in Paraffin and Olefin Pyrolyzates. Ind. Eng. Chem. Prod. Res. Dev. 1963, 2, 220. (37) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Structural Characterization of Asphaltenes of Different Origins. Energy Fuels 1995, 9, 225. (38) Poveda, J. C.; Molina, D.; Martinez, H.; Florez, O.; Campillo, B. Molecular Changes in Asphaltenes Within H2 Plasma. Energy Fuels 2014, 28, 735. (39) Wang, Z. C.; Hu, J. C.; Shui, H. F.; Ren, S. B.; Wei, C.; Pan, C. X.; Lei, Z. P.; Cui, X. P. Study on the Structure and Association of Asphaltene Derived from Liquefaction of Lignite by Fluorescence Spectroscopy. Fuel 2013, 109, 94. (40) Clutter, D. R.; Petrakis, L.; Stenger, Jr.; Jensen, R. K. Nuclear Magnetic Resonance Spectrometry of Petroleum Fractions: Carbon-13 and Proton Nuclear Magnetic Resonance Characterizations in Terms of Average Molecule Parameters. Anal. Chem. 1972, 44, 1395. (41) Kida, Y.; Class, C. A.; Concepcion, A. J.; Timko, M. T.; Gree, W. H. Combining Experiment and Theory to Elucidate the Role of Supercritical Water in Sulfide Decomposition. Phys. Chem. Chem. Phys. 2014, 16, 9220. (42) Patwardhan, P. R.; Timko, M. T.; Class, C. A.; Bonomi, R. E.; Kida, Y.; Hernandez, H. H.; Tester, J. W.; Green, W. H. Supercritical Water Desulfurization of Organic Sulfides Is Consistent with FreeRadical Kinetics. Energy Fuels 2013, 27, 6108. (43) Van Speybroeck, V.; Hemelsoet, K.; Waroquier, M.; Marin, G. B. Reactivity and Aromaticity of Polyaromatics in Radical Cyclization Reactions. Int. J. Quantum Chem. 2004, 96, 568. (44) Zhu, D. Q.; Liu, Q. K.; Tan, X. C.; Yang, J. Y.; Yuan, P. Q.; Cheng, Z. M.; Yuan, W. K. Structural Characteristics of Asphaltenes Derived from Condensation of Maltenes in Supercritical Water. Energy Fuels 2015, 29, 7807.

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DOI: 10.1021/acs.iecr.6b02323 Ind. Eng. Chem. Res. 2016, 55, 9578−9585