Interaction between Hydrogen-Donor and Nondonor Solvents in Direct

Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ... Publication Date (Web): October 26...
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Interaction between Hydrogen-Donor and Nondonor Solvents in Direct Liquefaction of Bulianta Coal Ben Niu, Lijun Jin, Yang Li, Zhiwei Shi, Hanxue Yan, and Haoquan Hu* State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: Solvent plays two very important roles in coal liquefaction: (1) Physically, it serves as a medium for coal transport and heat transfer, and (2) chemically, it serves as an important source of transferable hydrogen (hydrogen donor) and hydrogen shuttle between hydrogen and coal. In this work, eight types of non-hydrogen-donor solvents, namely, decalin, 1methylnaphthalene, naphthalene, fluorene, anthracene, phenanthrene, pyrene, and fluoranthene, were examined in combination with the hydrogen-donor solvent tetralin in the liquefaction of Bulianta coal to clarify the interactions between the hydrogendonor and non-hydrogen-donor solvents and the mechanism of hydrogen transfer. In the absence of a catalyst, the mixed solvents of tetralin with phenanthrene, pyrene, and fluoranthene showed favorable effects on coal conversion and oil yield compared with pure tetralin, regardless of whether the reactions were conducted in a H2 or N2 atmosphere. The reactions of nonhydrogen-donor solvents with tetralin showed that phenanthrene, pyrene, and fluoranthene can pick up hydrogen atoms from the donor to produce 9,10-dihydrophenanthrene, 4,5-dihydropyrene, and 1,2,3,10b-tetrahydrofluoranthene, respectively, which are known to be more reactive hydrogen donors than tetralin. In the presence of nanosized iron catalyst, the addition of phenanthrene, pyrene, or fluoranthene to tetralin was found to improve the liquefaction performance in a N2 atmosphere, whereas in a H2 atmosphere, almost the same coal conversion and oil yield were obtained for all types of mixed solvents. This suggests that the transfer mechanisms of hydrogen are different in the cases of N2 and H2 atmospheres in the presence of nanosized iron catalyst; that is, the primary hydrogen-transfer mechanism might be directly from H2 to coal rather than through hydrogen-donor solvents.

1. INTRODUCTION Direct coal liquefaction (DCL), believed to consist of a series of competitive and consecutive reactions, is additionally complicated by the fact that recycled oil, serving as a DCL solvent, is a mixture of many individual compounds.1−3 There is no reason to believe that different compounds will react with coal by the same mechanism, particularly taking into account the complexity of the composition of coal and the addition of different catalysts. To obtain some basic knowledge about coal liquefaction, one must start with studies of the mechanism of the reaction of coal in mixed solvents with the designated catalyst. Some non-hydrogen-donor polycyclic aromatic hydrocarbons (PAHs), which might be contained in the recycled solvent of the DCL process, can abstract hydrogen atoms from hydrogen donors (H2 and hydrogen-donor solvent) or by transfer of a labile hydrogen from coal and form, in situ, a new hydrogen donor with a higher hydrogen-donating ability.4−8 Ruberto et al.5 concluded that hydrogenated phenanthrene is an excellent solvent as a hydrogen donor and transfer agent in the presence of a cobalt molybdate-type liquefaction catalyst. Chiba et al.6 reported that mixtures of fluorene or pyrene and tetralin can shuttle hydrogen efficiently. Derbyshire et al.7 reported that pyrene acts as an excellent hydrogen shuttle to give a higher conversion to tetrahydrofuran- (THF-) soluble product in DCL than could be obtained with tetralin alone. Mochida and co-workers8 observed that the mixed solvent of pyrene and 1,2,3,10b-tetrahydrofluoranthene (4HFL) produces more oil than pure 4HFL. Although many studies on the interactions between non-hydrogen-donor and hydrogen-donor © XXXX American Chemical Society

solvent have been published, more details still remain to be understood. The hydrogen-transfer mechanism has been discussed by many researchers,9−11 but there are still some arguments to be settled as to whether the principal route is direct transfer from H2 or indirect transfer through a solvent. Vernon9 studied the behavior of model compounds containing carbon−carbon bonds in the presence of tetralin and H2 at 450 °C and observed that H2 can promote hydrocracking reactions that do not occur to an appreciable extent in the presence of tetralin alone. McMillen et al.10 proposed that hydrogen-donor solvents can cleave some strong carbon−carbon bonds. Wei and coworkers11−14 reported that hydrogen-donor solvents hardly cleave diarylmethanes in the absence of a catalyst but do inhibit 1,2-di(1-naphthyl)ethane and 1,3-diphenylpropane thermolysis and also that hydrogen-donor solvents retard the hydrocracking of diarylmethanes in the presence of FeS2. Ouchi and coworkers15−18 investigated the hydrogen-transfer mechanism using PAHs and suggested that the larger the aromatic nucleus of the solvent, the greater the role of indirect hydrogenation through the solvent. Kabe and co-workers19−24 reported the mechanism of hydrogen transfer during coal liquefaction as determined using a tritium tracer method. They found that, in the primary stage of liquefaction, coal is thermally decomposed and hydrogenated by hydrogen atoms from hydrogen-donor Received: September 2, 2016 Revised: October 25, 2016 Published: October 26, 2016 A

DOI: 10.1021/acs.energyfuels.6b02223 Energy Fuels XXXX, XXX, XXX−XXX

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process had been maintained at 445 °C for 60 min, the reaction was quenched by cooling the reactor to room temperature with cooling water. The reported coal conversions and product yields for DCL are the average values of at least two equivalent experiments, and the error bars are indicated in Figures 1 and 2. The experimental absolute error was within ±1%, indicating good reproducibility. 2.3. Separation and Analysis of Products. The gaseous products were collected in an aluminum foil bag and analyzed by gas chromatography (GC). The solid and liquid products were recovered by washing with n-hexane and separated into oil (including water), asphaltene (A, toluene-soluble but n-hexane-insoluble), preasphaltene (PA, THF-soluble but toluene-insoluble), and residues (THF-insoluble) using an FOSS ST310 automatic Soxhlet extractor with n-hexane, toluene, and THF, respectively. The n-hexane solution was then poured into a 250 mL volumetric flask, supplemented with nhexane to 250 mL, and then analyzed by GC and GC/mass spectrometry (MS). Solvent hydrogen consumption was calculated from the amount of naphthalene in the recovered solvent as determined by GC analysis of the n-hexane solution. The N2, H2, CH4, CO, CO2, and C2−C3 compositions in the gases were analyzed by GC with thermal conductivity detector (5A molecular sieve packed column) and flame ionization detector (GDX502 packed column). The compositions of the recovered solvents were qualitatively and quantitatively analyzed by GC/MS and GC, respectively. In this case, GC analysis was carried out on a Shimadzu GC-2014 instrument equipped with a BPX-5 capillary column (25 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID) using He as the carrier gas at a flow rate of 1 mL/min. The injector and detector temperatures were set to 280 and 300 °C, respectively. The GC column was heated from 50 to 280 °C at 10 °C/ min and maintained at 280 °C for 3 min. The same n-hexane solution was analyzed by GC/MS using an Agilent 6890N gas chromatograph coupled with an Agilent 5975 mass detector, using the same GC operating conditions as used in the GC analysis and the following MS conditions: ionization mode, electron ionization (EI); solvent delay, 6 min; interface temperature, 250 °C; electron impact ion source temperature, 230 °C; quadruple spectrometer temperature, 150 °C; and scan mass fraction, from 50 to 550. The chromatogram from GC had the same peak distribution as that from GC/MS. The identification of each compound was achieved by the match of its mass spectrum to that in the spectral library (NIST 2000), and the content of each compound was determined by GC using an area normalization method. The coal conversion (Conv); the yields of asphaltene (yA), preasphaltene (yPA), gas (ygas), and oil (yoil); and the H2 consumption (ηH2) on a dry, ash-free (daf) basis during DCL were calculated according to the following equations

solvents and then the products from the primary stage of DCL are hydrocracked on the catalyst by hydrogen atoms mainly from H2. They also reported that the hydrogen exchange between the solvent and H2 during DCL is minimal even in the presence of a catalyst. Our studies have aimed at elucidating the interaction between hydrogen-donor and non-hydrogen-donor solvents and the mechanism of hydrogen transfer. In this work, nonhydrogen-donor solvents such as decalin (D), 1-methylnaphthalene (MN), naphthalene (Nap), anthracene (AN), phenanthrene (PN), fluorene (FN), pyrene (Py), and fluoranthene (FL), which can be contained in the recycled solvent of the DCL process, were mixed with the representative hydrogendonor solvent tetralin (T), and the mixtures were used as liquefaction solvents. The experiments were performed in N2 and H2 atmospheres with and without a catalyst.

2. EXPERIMENTAL SECTION 2.1. Materials. Bulianta (BLT) coal and a nanosized iron (ferric oxide hydrate) catalyst dispersed on the BLT coal sample were supplied by China Shenhua Coal to Liquid and Chemical Co., Ltd. The procedures for preparing nanosized iron catalyst were described in a patent for public disclosure.25 The Fe content in the catalyst, which was determined using an Optima 2000 DV inductively coupled plasma atomic emission spectrometer, was 6.08 wt %. Sulfur was used as a cocatalyst. The coal sample was crushed to a particle size below 100 mesh ( phenanthrene/tetralin > fluorene/tetralin ≈ tetralin > 1-methylnaphthalene/tetralin > decalin/tetralin ≈ anthracene/tetralin > naphthalene/tetralin. Figure 1b shows the DCL results with the nanosized iron catalyst. It can be seen that a tendency similar to that obtained without the catalyst in N2 atmosphere was observed. Specifically, the addition of phenanthrene, pyrene, or fluoranthene to tetralin improved the coal conversion and oil yield. Based on these results, it can be concluded that replacement of 25 wt % of the hydrogen-donor solvent tetralin with an equal amount of one of the non-hydrogen-donor solvents phenanthrene, pyrene, and fluoranthene in the starting liquefaction solvent can improve the liquefaction performance with or without a catalyst in N2 atmosphere. Compared with noncatalytic liquefaction, the conversions and oil yields of coal liquefaction with the pyrene/tetralin and fluoranthene/tetralin mixtures as solvents decreased slightly in the presence of the catalyst. It was reported that the mechanism of hydrogen transfer is that coal free radicals directly abstract hydrogen atoms from the hydrogen donor29 and that the different coal free radicals can be capped by different types of active hydrogen. The iron-based catalyst can promote the cracking reaction of coal.30 The coal free radicals generated by catalyst activation can recombine to form stable high-molecular-weight products in the absence of matched active hydrogen. 3.2. DCL with Non-Hydrogen Donor/Tetralin in H2. The conversions and product yields of coal liquefaction in H2 atmosphere without a catalyst are presented in Figure 2a. From Figure 2a, it can be seen that, when phenanthrene, pyrene, and fluoranthene were added to tetralin as solvents, the conversions and oil yields were higher than those obtained with pure tetralin. The hydrogen-shuttling ability of pyrene is well-known as the reaction in which pyrene acts as an agent for the transfer of hydrogen to coal either from tetralin or from H2.7 In DCL with only tetralin as the solvent, the conversion of coal, especially a low-rank coal such as BLT coal, is very sensitive to the hydrogen transferred from tetralin.31 This means that the acceleration of hydrogen transfer from the solvent is directly related to an improvement in coal conversion. Therefore, the

3. RESULTS AND DISCUSSION 3.1. DCL with Non-Hydrogen Donor/Tetralin in N2. Figure 1 presents the conversions and product yields of coal

Figure 1. Conversions and yields of DCL in N2 atmosphere (a) without and (b) with the nanosized iron catalyst.

liquefaction in N2 atmosphere with and without a catalyst. According to Figure 1a, when phenanthrene, pyrene, or fluoranthene was added without the catalyst, the conversion and oil yield were higher than those obtained with pure tetralin, indicating the favorable effects of these solvents on DCL. It is possible to accelerate the hydrogen transfer from tetralin by phenanthrene, pyrene, and fluoranthene because of their higher physical affinities and solubilities with coal.6 The solubility parameters, δ, of phenanthrene, pyrene, and fluoranthene are 10.5, 10.6, and 10.5 (cal/cm3)1/2,26 respectively, which are larger than that of tetralin [9.5 (cal/cm3)1/2]. The solubility parameter of BLT coal calculated by the method of van Krevelen is 14.6.27,28 The difference in δ between phenanthrene, pyrene, or fluoranthene and BLT coal is smaller than that between tetralin and BLT coal. Therefore, it appears that the physical solvent abilities of BLT coal in phenanthrene, pyrene, and fluoranthene are better than that in tetralin. Meanwhile, phenanthrene, pyrene, and fluoranthene can pick up hydrogen atoms from the hydrogen-donor solvent to produce 9,10-dihydrophenanthrene (2HPN), 4,5-dihydropyrene (2HPy), and 4HFL, respectively, which are known to be more reactive hydrogen donors than tetralin.5−7 The addition C

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hydrogen-transfer mechanisms in H2 and N2 atmospheres might be similar; that is, the predominant pathway of hydrogen transfer to coal might be through indirect reaction of solvent hydrogenation in the absence of a catalyst. The coal conversions and yields of DCL with the nanosized iron catalyst in a H2 atmosphere are presented in Figure 2b. It can be seen that the DCL with the nanosized iron catalyst in H2 atmosphere showed almost the same coal conversion and oil yield for all types of mixed solvents. These results are completely different from those obtained during DCL without a catalyst, in which higher coal conversions and oil yields could be obtained when mixtures of phenanthrene, pyrene, and fluoranthene with tetralin, which has a greater hydrogendonating ability, were used as solvents. These results suggest that H2 might play an important role in DCL with the nanosized iron catalyst and that the transfer mechanism of hydrogen is different from that without a catalyst; that is, the major hydrogen-transfer mechanism might be directly from H2 to coal rather than through hydrogen-donor solvents in the presence of a catalyst. When pure tetralin was used as the solvent, the coal conversion and oil yield in the presence of the catalyst were obviously higher than those in the absence of the catalyst, indicating that the catalyst can effectively improve the liquefaction performance. From Figure 2b, it can be seen that the coal conversion and oil yield during DCL with mixed nonhydrogen-donor/tetralin as the solvent were lower than those obtain with pure tetralin as the solvent. This can be explained from the competitive reaction between hydrogenation of the non-hydrogen-donor solvent and hydrogen abstraction of coal free radicals from the dissociated hydrogen atoms over the catalyst. In such a competitive environment, some coal radicals might not be immediately capped by activated hydrogen and can recombine to form stable high-molecular-weight products. 3.3. Hydrogen-Transfer Mechanism in DCL. Figure 3a shows the hydrogen consumptions during DCL with and without the nanosized iron catalyst under a N2 atmosphere. The total hydrogen consumption during DCL under N2 atmosphere was defined as the difference between the hydrogen consumption from the solvent and the generated H2. It can be seen that the hydrogen consumption from the solvent (sum of the total hydrogen consumption and the generated H2) increased when the catalyst was used, whereas the total hydrogen consumption decreased slightly no matter which solvents were used. As discussed in section 3.1, the conversions and oil yields of coal liquefaction in N2 atmosphere with the mixtures of pyrene/tetralin and fluoranthene/tetralin as the solvents in the presence of catalyst were lower than those in the absence of catalyst. From these results, it can be deduced that the coal free radicals cannot be effectively capped by hydrogen atoms from the solvent in the presence of catalyst. The hydrogen consumptions during DCL with and without the nanosized iron catalyst under a H2 atmosphere are presented in Figure 3b. The total hydrogen consumption of DCL in H2 atmosphere was defined as the sum of the hydrogen consumptions from the solvent and from H2. From Figure 3b, in the absence of the catalyst, about 65% of the total hydrogen consumption was from solvent; however, about 65% of the total hydrogen consumption was from H2 in the presence of catalyst. The addition of the catalyst can obviously improve the hydrogen consumption from H2 with a slight increase in the total hydrogen consumption. This suggests that the addition of the catalyst can effectively promote the formation of activated hydrogen. From Figure 3b, the hydrogen consumptions in 1-

Figure 2. Conversions and yields of DCL in H2 atmosphere (a) without and (b) with the nanosized iron catalyst.

role of the hydrogen shuttle is very important for coal liquefaction. The results in Figure 2a suggest that phenanthrene, pyrene, and fluoranthene are good hydrogen shuttles for noncatalytic DCL. The maximum conversion and oil yield, which were 74.8 and 44.3 wt %, respectively, were obtained when the mixed solvent of pyrene and tetralin was used, indicating that pyrene is a good shuttle for the transfer of hydrogen from H2 to coal. The addition of 1-methylnaphthalene to tetralin resulted in a higher preasphaltene yield but lower asphaltene and oil yields, indicating that the conversion of preasphaltene to asphaltene and oil is inhibited. In comparison to that in N2 atmosphere, the coal conversion and oil yield of DCL increased in H2 atmosphere, whereas the preasphaltene yield decreased, suggesting that H2 could take part in the reaction to stabilize the coal free-radical fragments and promote the hydrocracking of preasphaltene. Studies on model coal structures have provided evidence that H2 can directly participate in free-radical reactions under liquefaction conditions and promote some hydrocracking reactions during DCL.9 Meanwhile, the beneficial effects of H2 on DCL have also been verified by experimental results for coal.31−33 When 1-methylnaphthalene was added to tetralin as the solvent, the gas yield was higher than that in pure tetralin, and the content of CH4 in the gaseous product was higher than for the other non-hydrogen-donor/tetralin mixtures as the solvent. This indicates that the hydrogenolysis of 1-methylnaphthalene into CH4 and naphthalene occurs during DCL. This point also offers evidence that H2 can promote some hydrocracking reactions during DCL. The variation tendency in H 2 atmosphere is similar to that in N2, indicating that the primary D

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Table 3. Contents of Non-Hydrogen-Donor Solvents and Their Derivatives in the Solvents Recovered after Reactions of Non-Hydrogen-Donor Solvents with Tetralin with or without Coal N2 (wt %) compounda

without coal

2HAN 8HAN 4HAN AN other

7.3 − 2.8 15 0.5

2HPN 4HPN PN other

0.6 0.5 24.9 −

2HPy Py other

1.3 21.4 0.3

4HFL FL other

1.2 20.7 −

with coal AN/T Solvent 2.4 0.2 5.7 3.8 3.9 PN/T Solvent 1.1 0.5 16.5 2.1 Py/T Solvent 1.1 16.4 5.2 FL/T Solvent 0.5 16.5 1.2

H2 (wt %) without coal

with coal

7.9 0.6 8.7 2.6 2.1

2.2 0.9 10.2 1.8 6.5

2.9 0.5 16.1 1.6

1.8 1.0 16.2 3.7

3.9 17.5 0.5

1.7 14.7 3.9

5 15.7 0.9

1.9 15.5 5.0

a

AN, anthracene; 2HAN, 9,10-dihydroanthracene; 4HAN, 1,2,3,4tetrahydroanthracene, 8HAN, 1,2,3,4,5,6,7,8-octahydroanthracene; FL, fluoranthene; 4HFL, 1,2,3,10b-tetrahydrofluoranthene; PN, phenanthrene; 2HPN, 9,10-dihydrophenanthrene; 4HPN, 1,2,3,4-tetrahydrophenanthrene; Py, pyrene; 2HPy, 4,5-dihydropyrene.

Figure 3. Hydrogen consumption of DCL in N2 and H2 atmospheres.

the direct abstraction of hydrogen atoms from the hydrogen donor by coal free radicals. The preceding results suggest that there might be several different pathways affecting hydrogen transfer during DCL with mixed phenanthrene/tetralin, pyrene/tetralin, and fluoranthene/tetralin solvents in the absence of a catalyst. Apart from the traditional hydrogen-transfer routes from tetralin or H2 to coal free radicals, there are at least three other possible pathways for hydrogen transfer, as shown in Figure 4. Reactions 1 and 2 can take place thermally; reaction 3 is the process of hydrogen transfer from a hydrogen donor to coal free radicals. Based on these reactions, the observed synergism in coal liquefaction performance can be explained. Reactions 1 and 2 can occur at 300−400 °C and form 2HPN, 2HPy, and 4HFL,34,35 which have higher hydrogen-donating abilities than tetralin.4,5 Then, 2HPN, 2HPy, and 4HFL, acting as hydrogen donors, can transfer hydrogen effectively to coal. Meanwhile, hydrogen transfer to the ipso positions of linkages to aromatic systems in the coal structure can engender the cleavage of strong bonds and improve the coal liquefaction performance.36 However, this process requires both a hydrogen-donor species and a hydrogen-acceptor species to form an active hydrogentransfer intermediate, a cyclohexadienyl-type radical carrier, which can either give free hydrogen atoms or transfer hydrogen directly, to engender cleavage of even strong bonds in the coal structure.37 Phenanthrene, pyrene, and fluoranthene are good hydrogen acceptors, and the compounds newly formed through reactions 1 and 2 have higher hydrogen-donating abilities . Therefore, the addition of phenanthrene, pyrene, and fluoranthene in tetralin can improve the coal liquefaction performance.

methylnaphthaltene/tetralin and anthracene/tetralin were higher than that in pure tetralin, indicating that 1-methylnaphthaltene and anthracene can easily pick up hydrogen atoms from H2. To elucidate the mechanism of hydrogen transfer during DCL with mixed solvents, the solvents recovered from the nhexane extract after DCL were analyzed by GC and GC/MS. Tables 3 and 4 report the contents of compounds detected in the recovered solvents with different mixed solvents after DCL. 2HPN, 2HPy, and 4HFL were detected with mixed phenanthrene/tetralin, pyrene/tetralin, and fluoranthene/tetralin, respectively, as the solvent, and their contents in H2 atmosphere were higher than those in N2 atmosphere. This suggests that hydrogen transfer between the hydrogen donor and phenanthrene, pyrene, and fluoranthene occurs during DCL. To further support this conclusion, the reactions of anthracene, phenanthrene, pyrene, and fluoranthene with tetralin in the absence of coal were carried out under the same conditions as used for DCL. The contents of the compounds detected in the recovered solvents after reaction are also presented in Tables 3 and 4. The results show that 2HPN, 2HPy, and 4HFL were formed in the reactions of phenanthrene, pyrene, and fluoranthene, respectively, with tetralin, and their contents in H2 atmosphere were also higher than those in N2 atmosphere; that is, hydrogen transfer between the hydrogen donor and phenanthrene, pyrene, and fluoranthene really occurs. From Table 4, it can be seen that the content of naphthalene in the case with coal was significant higher than that without coal no matter what solvent was used, suggesting that the mechanism of hydrogen transfer involves E

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hydrogenated derivatives, resulting in a difference in liquefaction performance when phenanthrene/tetralin and anthracene/tetralin as liquefaction solvents. Therefore, the equilibrium compositions of 2HPN/4HPN/phenanthrene/H2 and 2HAN/4HAN/anthracene/H2 reaction systems at 14 MPa were calculated using Outokumpu HSC Chemistry (the molar ratios of phenanthrene to H2 and anthracene to H2 were both 2:3). Figure 5 shows the tendencies of the equilibrium

Table 4. Contents of Tetralin and Its Derivatives in the Solvents Recovered after Reaction of Non-Hydrogen-Donor Solvents with Tetralin with or without Coal N2 (wt %) compound

a

a

without coal

D 1-ML T Nap other

0.5 4.0 59.4 8.8 1.7

D 1-ML T Nap other

0.3 2.0 69.7 1.5 0.2

D 1-ML T Nap other

0.4 2.8 71.7 1.9 0.2

D 1-ML T Nap other

0.6 2.9 70.9 3.2 0.5

with coal AN/T Solvent 0.7 2.2 29.5 50.4 1.2 PN/T Solvent 0.9 1.9 35 40.5 1.5 Py/T Solvent 1.0 2.1 33.1 39.0 2.1 FL/T Solvent 0.8 2.3 31.2 45.7 1.8

H2 (wt %) without coal

with coal

1.7 3.3 66.0 6.9 0.5

1.4 2.2 38 34.9 1.9

2.1 4.2 67.6 4.7 0.3

1.8 2.2 42.3 29.3 1.7

1.7 2.2 72.7 1.1 0.4

2.0 2.3 44.2 29.1 2.1

2.0 2.4 70.7 3.0 0.3

1.8 2.3 42.3 29.2 2.0

D, decalin; 1-ML, 1-methylindan; T, tetralin; Nap, naphthalene.

Figure 5. Calculated equilibrium compositions of the (a) 2HAN/ 4HAN/AN and (b) 2HPN/4HPN/PN systems.

Figure 4. Hydrogen-transfer routes of DCL without a catalyst in phenanthrene/tetralin, pyrene/tetralin, and fluoranthene/tetralin mixtures.

compositions in the reaction system as a function of reaction temperature. As indicated by these calculation results, it is clear that the anthracene conversion (88.1 wt %) of the experiments under liquefaction conditions is almost the same as the theoretical equilibrium conversion, whereas the phenanthrene conversion (23.7 wt %) for the experiments under liquefaction conditions is much less than expected from the theoretical calculations. This indicates that the hydrogenation reaction of anthracene occurs easily and that the reaction of phenanthrene is dynamically limited under liquefaction conditions. That is, anthracene easily picks up hydrogen atoms from H2 and inhibits the hydrogenation reaction of coal.

9,10-Dihydroanthracene (2HAN) and 1,2,3,4-tetrahydroanthracene (4HAN) were detected in the recovered solvents with and without coal, but both have lower hydrogen-donating abilities than 2HPy, as reported by McMillen and coworkers.36,37 The conversion of anthracene without coal under H2 atmosphere was 88.1% and was obviously higher than the coal conversion (69.3%) obtained in pure tetralin under a H2 atmosphere. This indicates that anthracene is significantly more reactive than coal free radicals under the investigated reaction conditions. Therefore, anthracene can preferentially consume hydrogen donors and inhibit the coal hydrogenation reaction during DCL with anthracene/tetralin as the solvent. It is well-known that there is an equilibrium among decalin, tetralin, and naphthalene in H2 at high temperature.38 The same situation might exist among phenanthrene and its hydrogenated derivatives or among anthracene and its

4. CONCLUSIONS In the absence of a catalyst, the addition of phenanthrene, pyrene, or fluoranthene to tetralin can improve the liquefaction performance regardless of whether the reactions are conducted in a H2 or N2 atmosphere. These results are mainly ascribed to the higher physical dissolving capacities of phenanthrene, F

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Energy & Fuels pyrene, and fluoranthene and the higher hydrogen-donating abilities of 2HPN, 2HPy, and 4HFL, produced by the reactions of phenanthrene, pyrene, and fluoranthene, respectively, with tetralin or H2. In the presence of nanosized iron catalyst, the addition of phenanthrene, pyrene or fluoranthene to tetralin can improve the liquefaction performance in N2 atmosphere, whereas in H2 atmosphere, almost the same coal conversion and oil yield are obtained for all types of mixed solvents. This suggests that the transfer mechanism of hydrogen is difference in the cases of N2 and H2 atmosphere in the presence of nanosized iron catalyst; that is, the primary hydrogen-transfer mechanism might be directly from H2 to coal rather than through hydrogen-donor solvents in H2 atmosphere.

of H2 (g/mol); and yH2 is the mole fraction of H2 in the gaseous products. Z1 and Z2 are the compressibility factors before and after the reaction, respectively, and were calculated by eqs 3−13 using the method of iterative computations with an initial Z value of 1 and a deviation of 0.001. Tci and Tcj represent the critical temperatures of the ith and jth components, respectively, of the gaseous products; pci and pcj are the critical pressures of the ith and jth components, respectively, of the gaseous products; Vci and Vcj are the critical volumes of the ith and jth components, respectively, of the gaseous products; Zci and Zcj are the critical compressibility factors of the ith and jth components, respectively, of the gaseous products; yi and yj are the mole fractions of the ith and jth components, respectively, of the gaseous products; aii = ai; and aij = aji.

■

■

APPENDIX The process for calculating the weight of H2 is as follows: V (P1 + 0.1033) = Z1M H2 R(T1 + 273.15)

mH2

V (P2 + 0.1033) m H′ 2 = Z 2M H2yH 2 R(T2 + 273.15)

Notes

The authors declare no competing financial interest.

(3)

h=

bP ZRT

(4)

ai =

0.42748R2Tci 2.5 pci

(5)

aij =

ACKNOWLEDGMENTS This work was supported by the Key Program Project of the Joint Fund of Coal Research by NSFC and the Shenhua Group (No. 51134014) and the National Key Research and Development Program of China (2016YFB0600301). We thank China Shenhua Coal to Liquid and Chemical Co., Ltd., for providing the coal sample and the nanosized iron catalyst.

■

pcij

(6)

i

(7)

8

∑ ∑ (yyi j aij) j

(8)

8

b=

∑ yb i i i

(9)

Tcij = (TciTcj)0.5

(10)

⎛ V 1/3 + V 1/3 ⎞3 ci cj ⎟ Vcij = ⎜⎜ ⎟ 2 ⎝ ⎠

(11)

Zcij =

pcij =

Zci + Zcj 2

(12)

ZcijRTcij Vcij

REFERENCES

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*Tel./Fax: +86-411-84986157. E-mail: [email protected].

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AUTHOR INFORMATION

Corresponding Author

(13)

where V is the volume of the tubing-bomb microreactor (mL); P1, T1 and P2, T2 are the pressure (MPa) and temperature (°C) of the microreactor before and after reaction, respectively; R is the gas constant (8.314 Pa·m3/mol·K); MH2 is the molar mass G

DOI: 10.1021/acs.energyfuels.6b02223 Energy Fuels XXXX, XXX, XXX−XXX

Article

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H

DOI: 10.1021/acs.energyfuels.6b02223 Energy Fuels XXXX, XXX, XXX−XXX