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Effect of Ethanolysis on the Structure and Pyrolytic Reactivity of Zhaotong Lignite Zhan-Ku Li,†,‡ Xian-Yong Wei,*,† Hong-Lei Yan,†,‡ and Zhi-Min Zong† †

Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining and Technology, Xuzhou 221116, Jiangsu, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Coal Clean Conversion and Utilization, Anhui University of Technology, Ma’anshan 243002, Anhui, People’s Republic of China S Supporting Information *

ABSTRACT: Lignite ethanolysis is one of the efficient conversion processes. In our previous study, Zhaotong lignite (ZL) from Southwest China was subjected to ethanolysis to afford an ethanol-soluble portion and ethanolyzed residue (ER). The structural features of ZL and ER were investigated by ruthenium-ion-catalyzed oxidation (RICO) and Fourier transform infrared spectrometry. The pyrolytic reactivities of ZL and ER were examined with a thermogravimetric analyzer and Curie-point pyrolyzer−gas chromatograph/mass spectrometer. The results show that both ZL and ER are rich in −CH2CH2− and −CH2CH2CH2− bridged linkages connecting aromatic rings. In comparison to the RICO of ZL, the RICO of ER produced much less long-chain alkanoic and alkanedioic acids, suggesting that long alkylene bridges and alkyl side chains in ZL were largely cleaved via ethanolysis. Interestingly, ZL has a higher condensation degree than ER, which was confirmed by RICO and solidstate 13C nuclear magnetic resonance analysis. The result was explained by ethanolysis simulation of lignite-related model compounds using density functional theory. Thermogravimetric analysis of ZL and ER exhibits their different pyrolytic reactivities. According to analysis with a Curie-point pyrolyzer−gas chromatograph/mass spectrometer, significant differences in the distributions of the volatile species from the pyrolyses of ZL and ER were observed. Guaiacols and carbazoles are the most abundant group components from the pyrolyses of ZL and ER, respectively. ZL pyrolysis released much more alkanes and phenolic compounds than ER pyrolysis. The cleavage of Car−O bonds significantly proceeded during ZL ethanolysis.

1. INTRODUCTION Lignites are abundant coal resources with reserves of ca. 4 trillion tons over the world,1 but they are considered to be inferior fuels as a result of their high moisture content and low calorific value.2 On the other hand, more oxygen-containing moieties existing in lignites than in high-rank coals3 makes lignites possible as feedstock for producing value-added chemicals. As one of the efficient conversion techniques, alkanolyses (especially methanolysis and ethanolysis) of lignites have been extensively investigated.4 Lignites can be alkanolyzed into soluble and insoluble portions. The chemical composition of the soluble portion was well-revealed with many instruments, such as a Fourier transform infrared (FTIR) spectrometer,5 gas chromatograph/mass spectrometer (GC/MS),6 and Fourier transform ion cyclotron resonance mass spectrometer.7,8 However, few reports were issued on the compositional features of the insoluble portion and the structural differences between lignites and their insoluble portions. Ruthenium-ion-catalyzed oxidation (RICO) and subsequent analyses proved to be an effective method for characterizing alkyl side chains on aromatic rings, alkylene bridges connecting aromatic rings, and condensed aromatic rings in coals9−15 and other heavy carbon resources.16−19 It is usually accepted that arylalkanes, α,ω-diarylalkanes, triarylalkanes, and condensed aromatics could be selectively oxidized into alkanoic acids (AAs), alkanedioic acids (ADAs), alkanetricarboxylic acids (ATCAs), and benzenecarboxylic acids (BCAs), respectively, by RICO (Scheme S1 of the Supporting Information). © 2017 American Chemical Society

Most current coal utilization processes are related to pyrolysis. Therefore, it is necessary to investigate coal pyrolysis. Thermogravimetric (TG) analysis was widely used for examining coal pyrolysis behaviors.20−24 Another well-developed pyrolysis approach is pyrolysis−gas chromatography/ mass spectrometry,25−27 which can easily separate and identify the resulting volatiles from coal pyrolysis. In our recent studies,7,8 ethanolysis of Zhaotong lignite (ZL) was conducted to afford ethanol-soluble portion and ethanolinsoluble portion [i.e., ethanolyzed residue (ER)], and the ethanol-soluble portion was analyzed with a Fourier transform ion cyclotron resonance mass spectrometer. In this paper, we investigated structural features of ZL and ER by FTIR, solidstate 13C nuclear magnetic resonance (NMR), and RICO and pyrolytic reactivities of ZL and ER with a TG analyzer and Curie-point pyrolyzer−GC/MS.

2. EXPERIMENTAL SECTION 2.1. Materials. ZL was collected from Zhaotong, Yunnan, China, and ground to pass through a 200-mesh sieve followed by desiccation in a vacuum at 80 °C for 24 h prior to use. As Table 1 lists, ZL has relatively high volatiles and oxygen content. ER was derived from ZL ethanolysis, as described in the Supporting Information and shown in Figure S1 of the Supporting Information.7 Functional groups of ZL Received: July 5, 2017 Revised: September 12, 2017 Published: September 13, 2017 10768

DOI: 10.1021/acs.energyfuels.7b01927 Energy Fuels 2017, 31, 10768−10774

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Energy & Fuels Table 1. Proximate and Ultimate Analyses (wt %) of ZLa proximate analysis

7890/5975 GC/MS. The analytical method was widely reported elsewhere and described in the Supporting Information. 2.4. Pyrolyses of ZL and ER. TG analysis was performed on a Mettler Toledo TGA/SDTA851e TG analyzer referring to a previous study.28 ZL or ER (ca. 15 mg) was placed in a ceramic crucible, heated from 30 to 110 °C at 10 °C min−1, maintained at the temperature for 0.5 h to remove moisture, and then heated from 110 to 900 °C at the same heating rate using argon as the carrier gas at 60 mL min−1. The differential thermogravimetric (DTG) curve was derived from the firstorder derivative of the TG curve by Origin 8.5. In addition, ZL and ER were also pyrolyzed using a JHP-5 Curie-point pyrolyzer, and the resulting volatiles were analyzed online with the GC/MS. About 0.5 mg of ZL or ER was wrapped in a ferromagnetic foil and pyrolyzed at 500 °C for 3 s. The analytical conditions for GC/MS were the same as that for analyzing the oxidation products.

ultimate analysis (daf)

Mad

Ad

VMdaf

C

H

N

Ob

St,d

11.6

21.0

53.6

52.5

3.3

1.0

>41.8

1.4

a

daf, dry and ash-free basis; Mad, moisture (air-dried basis); Ad, ash (dry basis, i.e., moisture-free basis); VMdaf, volatile matter (dry and ash-free basis); and St,d, total sulfur (dry basis). bBy difference. and ER were determined with a Nicolet Magna IR-560 FTIR spectrometer. Carbon skeleton structures of ZL and ER were analyzed with a Bruker Avance III solid-state 13C NMR in our previous studies.28,29 However, the difference in carbon skeleton structures of ZL and ER was not discussed previously. RuCl3, NaIO4, CCl4, CH3CN, CH2Cl2, CH2N2, distilled water, and anhydrous MgSO4 used in the experiments are analytical reagents. 2.2. Ethanolysis Simulation of Lignite-Related Model Compounds. 2-(Benzyloxy)naphthalene and 2-(phenoxymethyl)naphthalene selected as lignite-related model compounds were subjected to ethanolysis using density functional theory according to our previous report.30 Briefly, the method, functional, and basis set were set to generalized gradient approximation, Becke and Perdew functional, and double numerical plus polarization, respectively. 2.3. RICOs of ZL and ER and Subsequent Treatments. RICOs of ZL and ER were performed referring to previous studies.14,31 As Figure 1 shows, a mixture of 0.4 g of sample (ZL or ER), 20 mg of

3. RESULTS AND DISCUSSION 3.1. FTIR Analyses of ZL and ER. As demonstrated in Figure 2, strong absorbances from bound −OH (3421 cm−1),

Figure 2. FTIR spectra of ZL and ER.

aliphatic moieties (2928 and 2871 cm−1), and aromatic rings (1614 cm−1) appear in both FTIR spectra of ZL and ER and there is no obvious difference in such absorbances between ZL and ER. The result suggests that ZL and ER have similar aromaticity indexes, which is in agreement with 13C solid-state NMR analysis (Table 2) of ZL28 and ER.29 It is noteworthy Table 2. Structural Parameters in ZL and ER Determined by Solid-State 13C NMR Analysisa structural parameter value (%) sample

fa

fal

fCa

χb

σ

ZL ER

42.2 42.0

52.3 52.3

5.5 5.7

22 12

60 40

a

Figure 1. Procedure for RICOs of ZL and ER, subsequent treatments, and product analysis.

o2 H b s o3 Aromaticity index, fa = fo1 a + fa + fa + fa + fa + fa ; aliphaticity index, fal o2 o3 = f1al + faal + f2al + f3al+ f4al + fo1 + f + f ratio of carbonyl carbon, fCa = fC1 al a al a C2 + fa ; molar fraction of aromatic bridgehead carbon, χb = fba /fa; o1 o2 s o3 substituted degree of the aromatic ring, σ = ( fa + fa + fa + fa )/fa.

RuCl3, 20 mL of CH3CN, 20 mL of CCl4, and 30 mL of distilled water were magnetically stirred at ambient temperature for 0.5 h and 8 g of NaIO4 was subsequently added to the mixture. After stirring at 35 °C for 48 h, the reaction mixture was filtered to obtain filtrate 1 and filter cake. Filtrate 1 was isolated to gain organic phase (OP) and aqueous phase (AP). Both the filter cake and AP were extracted with CH2Cl2 to afford residue and extraction solution 1 (ES1) and extraction solution 2 (ES2) and inextractable solution (IES), respectively. ES1, OP, and ES2 were incorporated, dried over anhydrous MgSO4, and filtered to acquire filtrate 2. The filtrate 2 and IES were concentrated, followed by esterification with CH2N2, to obtain methyl esterified extractable portion (MEEP) and methyl esterified inextractable portion (MEIEP), respectively. Both MEEP and MEIEP were analyzed with an Agilent

that the absorbance of aryl ethers around 1268 cm−1 from ZL is much stronger than that from ER, indicating that Car−O bonds in ZL were largely cleaved via ethanolysis. A more remarkable difference in the absorbances near 3696, 3619, 1033, 532, and 471 cm−1 attributed to mineral matter (such as kaolinite and quartz)32−36 between ZL and ER can be clearly observed, showing that inorganic species in ZL were enriched in ER. 3.2. Solid-State 13C NMR Analyses of ZL and ER. Solidstate 13C NMR spectra and carbon types of ZL and ER were reported in our previous investigations28,29 and shown in Figures S2 and S3 along with Tables S1 and S2 of the 10769

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Figure 3. Distribution of carboxylic acids from the RICOs of ZL and ER.

Supporting Information. In this paper, we focused on the differences in carbon skeleton structures of ZL and ER. As displayed in Table 2, the aromaticity and aliphaticity indexes have almost no differences before and after ethanolysis, demonstrating that nearly isometric aromatic and aliphatic species were dissolved during ethanolysis. Interestingly, the molar fraction of aromatic bridgehead carbon (χb) significantly decreased after ethanolysis. In other words, ZL has a higher condensation degree than ER. The result indicates that bridged bonds connecting highly condensed aromatic rings are easily cleaved via ethanolysis. Ethanolysis simulation of lignite-related model compounds implies that the activation energy of either 2-(benzyloxy)naphthalene ethanolysis (157.5 kJ mol−1) or 2(phenoxymethyl)naphthalene ethanolysis (158.8 kJ mol−1) is lower than that of benzyloxybenzene ethanolysis (163.0 kJ mol−1). This should be responsible for the decrease in the condensation degree. 3.3. Structural Features of ZL and ER through RICO. Both ZL and ER were almost converted into soluble portions or CO2. The total yield (82.1%) of the soluble portion from the RICO of ZL is higher than that (50.3%) from the RICO of ER, suggesting that more CO2 was released from the RICO of ER than from the RICO of ZL. A total of 99 carboxylic acids were confirmed on the basis of analysis of MEEP and MEIEP with GC/MS. They can be classified into normal alkanoic acids (NAAs), branched alkanoic acids (BAAs), oxoalkanoic acids (OAAs), ethoxyoxoalkanoic acids (EOAAs), normal alkanedioic acids (NADAs), branched alkanedioic acids (BADAs), oxoalkanedioic acids (OADAs), ATCAs, phenylalkanoic acids (PAAs), BCAs, and other carboxylic acids (OCAs), as summarized in Figures S4−S7 and Tables S3−S13 of the Supporting Information. As Figure 3 exhibits, ADAs are the most abundant products from the RICO of either ZL or ER, which is similar to the RICOs of Shengli lignite37 and Huolinguole lignite (HL).14

The results imply that both ZL and ER are rich in alkylene bridges. Most ADAs from the RICOs of both ZL and ER are NADAs. The carbon numbers in NADAs from the RICOs of ZL and ER range from 2 to 22 (except 3) and from 2 to 12 (except 3), respectively, and most of NADAs from the RICOs of both ZL and ER are low-carbon ADAs, especially succinic and glutaric acids. The results suggest that ZL has long alkylene bridges [−(CH2)n−, where n > 10], with most of them cracked via ethanolysis, and short alkylene chains, especially −CH2CH2− and −CH2CH2CH2−, are dominant bridged linkages connecting aromatic rings in both ZL and ER. Such bridged linkages are labile to catalytic hydrocracking.38 The range of carbon numbers in NADAs from the RICO of ZL is much wider than that of HL.14 Additionally, oxalic acid was not detected in the products from the RICO of HL, implying that biphenyl-type aromatic rings exist in ZL rather than in HL. Similar to ADAs, NAAs account for most of the AAs from the RICOs of both ZL and ER. The ranges of carbon number in NAAs from the RICOs of ZL and ER are from 9 to 29 and from 5 to 10, respectively (Figure 3). In other words, the carbon numbers in alkyl side chains on aromatic rings in ZL and ER range from 8 to 28 and from 4 to 9, respectively. The result indicates that the length of alkyl side chains on aromatic rings in ZL was largely reduced via ethanolysis. Only two ATCAs (propane-1,2,3-tricarboxylic acid and butane-1,2,4tricarboxylic acid) were identified in the products from the RICOs of ZL and ER, indicating the presence of alkylene linkages connecting three aromatic rings in ZL and ER. According to the RICO mechanism (Scheme S1 of the Supporting Information), the numbers of carboxy groups on benzene rings in the resulting BCAs can reflect the condensation degree of aromatic clusters. As Figure 3 demonstrates, relative contents (RCs) of BCAs from the RICOs of ZL and ER are 17.9 and 13.9%, respectively. Benzenetricarboxylic and benzenetetracarboxylic acids are 10770

DOI: 10.1021/acs.energyfuels.7b01927 Energy Fuels 2017, 31, 10768−10774

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Energy & Fuels predominant in BCAs from the RICOs of both ZL and ER, suggesting a lower condensation degree of aromatic clusters in both ZL and ER than that in Shengli lignite.37 Unexpectedly, more BCAs with 4−6 carboxy groups were generated from the RICO of ZL than from the RICO of ER. The result implies that the condensation degree of aromatic clusters in ZL is higher than that in ER, which is well in agreement with the solid-state 13 C NMR analysis. 3.4. Pyrolytic Reactivities of ZL and ER. From the TG curves (Figure 4), the mass loss of ZL is higher than that of ER

The pyrolysis became dramatic with further raising the temperature to 432 and 468 °C for ZL and ER, respectively, owing to the breakage of weak covalent bonds in organic matter and dehydroxylation of kaolinite. The less mass loss and higher temperature at a maximum weight loss rate of ER than those of ZL are ascribed to the release of the ethanol-soluble portion from ZL mainly by destructing non-covalent bonds and cleaving weak covalent bonds. As exhibited in Figure 4, the mass losses of ZL and ER became slow at temperatures higher than 500 °C. Consequently, the pyrolyses of ZL and ER at 500 °C and subsequent online analysis were examined with the Curie-point pyrolyzer−GC/MS to investigate the differences in the product distribution. In total, 129 organic compounds were detected in the volatiles. They can be categorized into normal alkanes (NAs), branched alkanes (BAs), alk-1-enes, other alkenes (OAs), arenes, phenol and alkylphenols (P & APs), catechols, guaiacols, polymethyl-2-(4,8,12-trimethyltridecyl)chroman-6ols (PMTMTDCs), benzaldehydes, ketones, NAAs, dialkyl phthalates (DAPs), pyridines, carbazoles, nitriles, amides, and other compounds (OCs), as summarized in Figures S8 and S9 and Tables S14−S31 of the Supporting Information. Obvious differences can be observed in the volatiles from pyrolyses of ZL and ER (Figure 5). The RCs of the group components detected in the volatiles from ZL pyrolysis decrease in the order: guaiacols ≫ NAs > P & APs > NAAs > arenes > OAs > alk-1-enes > ketones > OCs > PMTMTDCs > catechols > BAs > pyridines > benzaldehydes, while those from ER pyrolysis decrease in distinct order: carbazoles ≫ arenes > NAAs > P & APs > OAs > NAs > DAPs > nitriles > amides > benzaldehydes > OCs > alk-1-enes. The NAs and alk-1-enes released are basically characterized as the straight-chain forms in pairs (Figure 6 and Tables S14

Figure 4. TG/DTG curves of ZL and ER.

at the same temperature and the total mass losses of ZL and ER recorded at 900 °C are 45.1 and 34.3%, respectively. At temperatures up to 330 °C, the mass losses are in the range of 6−13%, which mainly originated from the release of bound water, decarboxylation, and cleavage of some weak bonds.21

Figure 5. Distribution of group components released from the pyrolyses of ZL and ER. 10771

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Figure 6. Selective ion chromatograms of alkanes (m/z 57, black) and alkenes (m/z 97, green) released from the pyrolyses of ZL and ER.

from ER pyrolysis. The result implies that most guaiacols and syringols were released as the soluble portion from ZL ethanolysis. As listed in Table S9 of the Supporting Information, six ketones were released from ZL pyrolysis, while no ketones were produced from ER pyrolysis. Noteworthily, ER pyrolysis released a large number of nitrogen-containing organic compounds, among which carbazoles is dominant, but only two pyridines (pyridine and 2-methylpyridine) were released from ZL pyrolysis. The results further indicate the difference in molecular structures and pyrolytic reactivities of ZL and ER.

and S16 of the Supporting Information), which was also observed in volatiles from Xiaolongtan lignite pyrolysis.39 The NAs released from the pyrolyses of ZL and ER demonstrate bimodal distributions ranging from C13 to C33 and from C14 to C27, respectively. The alkanes may come from either desorption of inherent hydrocarbons or decarboxylation, while some of the alkenes probably originated from the cracking of alkyl side chains on aromatic rings.40 Because most of the alkanes can be easily dissolved during ZL ethanolysis, both the number and amount of alkanes released from ZL pyrolysis are much more than those from ER pyrolysis. The arenes released from ZL pyrolysis are dominated by alkylbenzenes, while the RC of alkylbenzenes from ER pyrolysis significantly decreased. The result suggests that alkylbenzenes were thermally dissolved during ZL ethanolysis. Among the arenes, toluene and fluoranthene are the most abundant from the pyrolyses of ZL and ER, respectively. Phenolic compounds are the important constituents released from the pyrolyses of coals, especially lignites. Most of them are generated via the breakage of aryl ethers during coal pyrolysis.41,42 As illustrated in Figure 5, ZL pyrolysis produced much more phenolic compounds than ER pyrolysis, further proving the dissociation of Car−O bonds in ZL via ethanolysis. Guaiacols and syringols are the two typical structural units of lignin.43 The most abundant phenolic compounds released from ZL pyrolysis are guaiacols (e.g., 4-methylguaiacol and guaiacol), followed by P & APs (e.g., phenol and p-cresol), suggesting that lignin-like structures exist in ZL and lignites retain relatively integrate macromolecular structures of coalforming plants. Only one syringol, i.e., 4-allylsyringol, was released from ZL pyrolysis. The much less kinds and amounts of syringol than guaiacols could be related to the fact that the syringol-type structure readily loses a methoxy group to form a guaiacol-type structure during ZL formation. Quite different from ZL pyrolysis, no guaiacols and syringols were released

4. CONCLUSION According to FTIR analysis, Car−O bonds in ZL were largely cleaved during ethanolysis. The carboxylic acids produced from the RICOs of ZL and ER include AAs, ADAs, ATCAs, and BCAs, among which ADAs, especially succinic and glutaric acids, are the main products based on the analysis of methyl esterified products with GC/MS. The result suggests that −CH2CH2− and −CH2CH2CH2− are predominant alkylene bridges in both ZL and ER. More BCAs with 4−6 carboxy groups generated from the RICO of ZL than from that of ER implies that ER has a lower condensation degree than ZL, which is well consistent with the solid-state 13C NMR analysis. The result could be ascribed to the lower activation energies of 2-(benzyloxy)naphthalene ethanolysis (157.5 kJ mol−1) and 2(phenoxymethyl)naphthalene ethanolysis (158.8 kJ mol−1) than that of benzyloxybenzene ethanolysis (163.0 kJ mol−1). The most abundant group components released from the pyrolyses of ZL and ER at 500 °C are guaiacols and carbazoles, respectively. The dissolution of alkanes and significant cleavage of Car−O bonds in ZL during ethanolysis should be responsible for the much lower amounts of alkanes and phenolic compounds released from ER pyrolysis than from ZL pyrolysis. 10772

DOI: 10.1021/acs.energyfuels.7b01927 Energy Fuels 2017, 31, 10768−10774

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ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01927. Origin of ER, analysis with GC/MS, Scheme S1, Figures S1−S9, and Tables S1−S31 (PDF)

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ZL = Zhaotong lignite

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-516-83884399. E-mail: wei_xianyong@163. com. ORCID

Xian-Yong Wei: 0000-0001-7106-4624 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the key project of the Joint Fund from the National Natural Science Foundation of China and the Government of Xinjiang Uygur Autonomous Region (Grant U1503293) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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NOMENCLATURE AA = alkanoic acid ADA = alkanedioic acid AP = aqueous phase ATCA = alkane tricarboxylic acid BA = branched alkane BAA = branched alkanoic acid BADA = branched alkanedioic acid BCA = benzenecarboxylic acid DAP = dialkyl phthalate DTG = differential thermogravimetric EOAA = ethoxyoxoalkanoic acid ER = ethanolyzed residue from ZL ES = extraction solution FTIR = Fourier transform infrared GC/MS = gas chromatograph/mass spectrometer HL = Huolinguole lignite MEEP = methyl esterified extractable portion MEIEP = methyl esterified inextractable portion NA = normal alkane NAA = normal alkanoic acid NADA = normal alkanedioic acid NMR = nuclear magnetic resonance OA = other alkane OAA = oxoalkanoic acid OADA = oxoalkanedioic acid OC = other compound OCA = other carboxylic acid OP = organic phase PAA = phenylalkanoic acid PMTMTDC = polymethyl-2-(4,8,12-trimethyltridecyl)chroman-6-ol P & AP = phenol and alkylphenol RC = relative content RICO = ruthenium-ion-catalyzed oxidation TG = thermogravimetric 10773

DOI: 10.1021/acs.energyfuels.7b01927 Energy Fuels 2017, 31, 10768−10774

Article

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DOI: 10.1021/acs.energyfuels.7b01927 Energy Fuels 2017, 31, 10768−10774