Structural Features of Extraction Residues from Supercritical

Jul 16, 2013 - *Telephone: +86-516-83884399. ... The results show that the ER from SL is richer in highly condensed aromatic species than that from HL...
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Structural Features of Extraction Residues from Supercritical Methanolysis of Two Chinese Lignites Yu-Gao Wang, Xian-Yong Wei,* Hong-Lei Yan, Da-Ling Shi, Fang-Jing Liu, Peng Li, Xing Fan, Yun-Peng Zhao, and Zhi-Min Zong Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou 221116, Jiangsu, People’s Republic of China S Supporting Information *

ABSTRACT: The methanol-insoluble portions from supercritical methanolysis of Shengli lignite (SL) and Huolinguole lignite (HL) were extracted with an isometric carbon disulfide/acetone mixed solvent under ultrasonic irradiation to afford extracts and extraction residues (ERs). The ERs were subjected to ruthenium-ion-catalyzed oxidation, and soluble portions were separated from the reaction mixture and esterified. The resulting products were analyzed with a gas chromatography/mass spectrometer and atmospheric solids analysis probe/time-of-flight mass spectrometer to reveal structural features of heavy species in the two lignites. The results show that the ER from SL is richer in highly condensed aromatic species than that from HL, while both ERs have the same carbon number range (C9−C24) of alkyl groups with the highest content at C15 on aromatic rings and the same distribution of alkylene bridges (C2−C20) connecting aromatic rings with a higher abundance of shorter linkages than that of longer linkages.

1. INTRODUCTION Lignites are regarded as inferior fuels because of their high ash yields, high contents of moisture and organic oxygen, and low calorific values.1 Both the high moisture content and low calorific value are closely related to the high content of organic oxygen in lignites. In comparison to higher rank coals, more oxygen-containing organic compounds (OCOCs) are included in lignites.2 Most of the OCOCs are value-added chemicals. Hence, non-fuel use of lignites is more reasonable than conventional use of lignites.3 For this purpose, understanding molecular composition (MC) of organic matter (OM) in lignites is crucially important.4 Many organic species in lignites can be extracted with organic solvents and analyzed with a gas chromatography/mass spectrometer (GC/MS),5−9 but the extractable species only account for a small portion of OM in lignites. In other words, OM in lignites mainly consists of macromolecular species (MMSs), including highly condensed aromatic species (HCASs),10 which are present in extraction residues (ERs). The involatility and insolubility of the MMSs lead to difficulty in understanding MC of OM in ERs. To solve the problem, selective degradation of OM in ERs is needed. Ruthenium-ion-catalyzed oxidation (RICO) has been widely used to characterize alkylene bridges [ABs, i.e., −(CH2)n−] and alkyl side chains (ASCs) in coals11−16 and other fossil resources17−21 combined with subsequent analyses of the soluble organic species (SOSs). The SOSs contain both volatile and highly involatile components. As a commonly used instrument, GC/ MS is effective for analyzing some volatile species but invalid for the detection of highly involatile species. Carboxylic acids (CAs) are considered to be characteristic products from RICO for characterizing ABs and ASCs originally present in reactants, such as coals. For example, because heptadecylarenes (HDAs) and 1,6-diarylhexanes (DAHs) are converted to stearic acid © 2013 American Chemical Society

(SA) and octanedioic acid (ODA), respectively, by RICO, the detection of SA and ODA from RICO of a reactant suggests the possible presence of HDAs and DAHs, respectively, in the reactant. However, some CAs may originally exist in the reactant; e.g., palmitic acid (PA) was detected in the carbondisulfide-extractable fraction (CDSEF) from Shenfu Dongsheng sub-bituminous coal (SDSBC), and both PA and SA were identified in CDSEF from SDSBC and Pingshuo bituminous coal.22 Therefore, detection of CAs in SOSs from RICO of coals does not necessarily mean the existence of corresponding alkylarenes and α,ω-diarylalkanes. Removing SOSs from a reactant in advance is needed for characterizing ABs and ASCs in the reactant according to subsequent RICO and analyses of the resulting SOSs and thereby ERs, in which SOSs were extracted, from raw coals are better reactants than raw coals for characterizing ABs and ASCs based on RICO and subsequent analyses of the resulting SOSs. Atmospheric pressure solids analysis probe (ASAP) is a relatively new analytical technique. It can broaden the GC/MS detection range because of its good response to large molecules (500−1000 Da) and direct analyses of samples without complex pretreatments. ASAP has been successfully applied in identifying drug molecules,23 steroids,24,25 contaminants,26 crude oils,27 and synthetic polymers.28 However, to the best of our knowledge, few studies on coals and their derivatives covered the application of this new technique. Both Huolinguole lignite (HL) and Shengli lignite (SL) are typical lignites in China and were extensively investigated.5,8,9,29−33 In the present study, we investigated structural Received: May 11, 2013 Revised: July 16, 2013 Published: July 16, 2013 4632

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Table 1. Proximate and Ultimate Analyses (wt %) of the ERs proximate analysis

a

ultimate analysis (daf)

sample

Mad

Ad

Vdaf

C

H

N

S

Oa

H/C

ERHL ERSL

4.66 6.98

27.28 22.80

31.19 33.94

78.88 79.66

6.16 5.89

1.56 1.35

0.99 0.98

12.41 12.11

0.9306 0.8811

By difference.

features of ERs from the two lignites by RICO and subsequent analyses of the resulting SOSs.

2. EXPERIMENTAL SECTION 2.1. Thermal Extraction. HL and SL were thermally extracted, respectively, in methanol at 310 °C for 2 h in a 1.2 L stainless-steel, magnetically stirred autoclave with a metallic membrane filter at the bottom of the autoclave.9 The reaction mixture was separated into ER and extract by hot filtration through the filter and exhaustive extraction with isometric carbon disulfide/acetone mixed solvent under ultrasonic irradiation. For convenience in description, the ERs from HL and SL are abbreviated as ERHL and ERSL, respectively. Table 1 lists their proximate and ultimate analyses. 2.2. RICO of the ERs and Subsequent Treatments. As Figure 1 displays, a mixture of an ER (0.4 g), RuCl3 (20 mg), CH3CN (20 mL),

Figure 2. FTIR spectra of the ERs.

Table 2. AAsa Produced from RICO of the ERs

a

Figure 1. Procedure for RICO of the ERs, subsequent treatments, and product analyses.

peak

parent compound

10 14 19 25 30 34 36 42 45 47 52 54 56 60 61 64 66

6-oxoheptanoic acid decanoic acid undecanoic acid dodecanoic acid tridecanoic acid tetradecanoic acid pentadecanoic acid PA heptadecanoic acid SA nonadecanoic acid icosanoic acid henicosanoic acid docosanoic acid tricosanoic acid tetracosanoic acid pentacosanoic acid

Corresponding methyl alkanoates were detected in EDCMEPs.

which was esterified with CH2N2 in diethyl ether (DEE) to afford esterified DCMEP (EDCMEP). The DCMIES was evaporated under reduced pressure to afford DCM-inextractable portion (DCMIEP), which was esterified with CH2N2 in DEE to afford esterified DCMIEP (EDCMIEP). 2.3. Analytical Methods. Dehydrated ERHL and ERSL were analyzed with a Nicolet Magna IR-560 Fourier transform infrared (FTIR) spectrometer by collecting 50 scans at a resolution of 8 cm−1 in reflectance mode with a measuring region of 4000−400 cm−1. Both EDCMEPs and EDCMIEPs were analyzed with a HewlettPackard 6890/5973 GC/MS, which is equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, and 0.25 μm film thickness) and a quadrupole analyzer with a m/z range from 33 to 500 and operated

CCl4 (20 mL), and distilled water (30 mL) was magnetically stirred for 0.5 h at room temperature followed by the addition of NaIO4 (8 g) and rapid agitation at 35 °C for 48 h. Then, the reaction mixture was filtrated to afford filtrate 1 (F1) and filter cake (FC). The FC was separated to residue and extraction solution 1 (ES1) by extraction with dichloromethane (DCM). The F1 was separated to organic phase (OP) and aqueous phase (AP) with a separatory funnel. The AP was extracted with DCM to afford extraction solution 2 (ES2) and DCM-inextractable solution (DCMIES). The ES1, OP, and ES2 were incorporated as DCM-extractable solution (DCMES), which was dried over anhydrous magnesium sulfate (AHMS) and filtrated. The filtrate (F2) was evaporated to afford DCM-extractable portion (DCMEP), 4633

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Table 3. ADAs Produced from RICO of the ERs ERHL peak 1 2 7 11 15 21 28 33 35 38 44 46 53 58 63 65 67 3 5 6 8 9 12 13 16 17 23 29 55

parent name NADAs oxalic acid succinic acid glutaric acid adipic acid heptanedioic acid ODA nonanedioic acid decanedioic acid undecanedioic acid dodecanedioic acid tridecanedioic acid tetradecanedioic acid hexadecanedioic acid octadecanedioic acid icosanedioic acid henicosanedioic acid docosanedioic acid BADAs 2-methylsuccinic acid 4-methoxy-4-oxobutanoic acid 2,3-dimethylsuccinic acid 2-methylpentanedioic acid 3-methylpentanedioic acid 2-methylhexanedioic acid 3-methylhexanedioic acid 2-methylheptanedioic acid 3-methylheptanedioic acid 4-oxoheptanedioic acid 4-oxooctanedioic acid 3-methylhexadecanedioic acid

ERSL

EDCMEP

EDCMIEP

× × × × × × × × × × × × × × × ×

× × × ×

×

× × × × ×

× × × × × × × × ×

in electron impact (70 eV) mode. The capillary column was heated at a rate of 5 °C min−1 from 60 to 300 °C and held at 300 °C for 5 min. Data acquired were processed using Chemstation software. Compounds were identified by comparing mass spectra to NIST05a library data. A series of esters, such as methyl butyrate, methyl palmitate, dimethyl oxalate, dimethyl succinate, and dimethyl phthalate, were used as external standards for quantitative analysis of corresponding or similar species. The quantitative analysis of each ester was converted to that of corresponding CA. The yields of CAs (i.e., YCA) based on the organic carbon content in the ERs were calculated according to the following formula:

EDCMEP

× × × × × × × × × × × × × × ×

EDCMIEP × × × × ×

×

× × × × × × × × ×

× × ×

that are subsequently analyzed. The operation was run in positive mode, and the m/z range was from 100 to 1000. Mass spectral data were processed using MassHunter software.

3. RESULTS AND DISCUSSION 3.1. FTIR Analysis. As Figure 2 shows, the absorbances of >C−O− vibration around 1096 and 1033 cm−1 from ERHL are much stronger than those from ERSL, while the absorbance of −OH vibration around 3411 cm−1 from ERSL is significantly stronger than that from ERHL, suggesting the significant difference in the oxygen-containing group between the two ERs. The stronger absorbances around 728 and 1451 cm−1 from ERHL than those from ERSL indicate the existence of more ABs and ASCs in ERHL than in ERSL. The characteristic absorbance of aromatic rings is around 1598 cm−1.34 Such absorbance from ERSL is significantly stronger than that from ERHL, which is in agreement with a lower H/C of ERSL than that of ERHL, as shown in Table 1. 3.2. GC/MS Analysis. In total, 67 CAs were confirmed according to GC/MS analysis of EDCMIEP and EDCMIEP from RICO of the ERs, as summarized in Figures SI1−SI5 of the Supporting Information and Tables 2−4. They can be classified into alkanoic acids (AAs), alkanedioic acids (ADAs), alkanetricarboxylic acids (ATCAs), and benzene polycarboxylic acids (BPCAs). AAs only existed in DCMEPs (Table 2), and most of ADAs existed in DCMEPs (Table 3), whereas most of ATCAs and BPCAs appeared in DCMIEPs (Table 4). AAs,

YCA = (NC in CAmCA /MCA )/(mC in ER,daf /MC) where NC in CA, MC, mCA, mC in ER, daf, and MCA denote the number of carbon atoms in individual CA, atomic mass of the carbon atom, mass of individual CA, mass of carbon in the ER, and molecular mass of individual CA, respectively. In addition, the EDCMIEPs were analyzed with an IonSense ASAP/ Agilent 6210 time-of-flight mass spectrometer (ASAP/TOF-MS), which is equipped with an atmosphere pressure chemical ionization ion source. The corona discharge (CDC) current and capillary voltage were set to 4.0 μA and 4000 V, respectively. The hot nitrogen stream and drying gas temperatures were set to 250 and 350 °C, respectively. Each EDCMIEP was dissolved into a proper amount of acetone and placed onto the tip of a capillary tube. Then, this tube was inserted into the ion source via a Teflon sample holder after removing excess samples from the tube with a tissue to prevent overloading and contamination of the ion source. The hot nitrogen gas from the electrospray probe directs molecules in the gas phase toward the CDC to create either protonated molecular ions or radical molecular cations 4634

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Table 4. ATCAs and BPCAs Produced from RICO of the ERs ERHL peak 18 27 31 32 22 24 26 37 39 40 41 43 48 49 50 51 57 59 62

parent name

ERSL

EDCMEP

EDCMIEP

EDCMEP

EDCMIEP

× ×

× × × ×

× ×

× × × ×

× × ×

×

× × ×

×

ATCAs propane-1,2,3-tricarboxylic acid butane-1,2,4-tricarboxylic acid 3-(carboxymethyl)hexanedioic acid 4-(formyloxy)heptanedioic acid BPCAs phthalic acid terephthalic acid isophthalic acid hemimellitic acid trimellitic acid benzene-1,3,5-tricarboxylic acid methylbenzenetricarboxylic acid methylbenzenetricarboxylic acid benzene-1,2,3,4-tetracarboxylic acid pyromellitic acid benzene-1,2,3,5-tetracarboxylic acid methylbenzenetetracarboxylic acid benzenepentacarboxylic acid methylbenzenepentacarboxylic acid benzenehexacarboxylic acid

× ×

× × × × × × × × × × ×

× × ×

× × × × × × × × × ×

ADAs, ATCAs, and BPCAs are considered to be generated from RICO of corresponding arylalkanes, α,ω-diarylalkanes, triarylalkanes, and condensed aromatics, respectively.10,14−16,35 Murata et al.14 confirmed the validity of the RICO method for qualitative and relatively quantitative analysis of many aliphatic structures, including ABs and ASCs, in bituminous and brown coals. Such a validity was also confirmed by Kidena et al.16 In comparison to the products from RICO of ERHL, the products from RICO of ERSL contain much less AAs, ADAs, and ATCAs but significantly more BPCAs, as exhibited in Figure 3a. This result is consistent with lower H/C of ERSL

Figure 4. NADAs and BADAs in the products from RICO of the ERs.

is the lowest among the TY of CAs, suggesting that ASCs in the ERs are not significant. As demonstrated in Figure 3b, the TY of benzenedicarboxylic acids from RICO of both ERs is almost the same, while only appreciably more benzenetricarboxylic acids were produced from RICO of ERHL than from RICO of ERSL; however, significantly more BPCAs with 4−6 carboxyl groups were generated from RICO of ERSL than from RICO of ERHL. The fact further proved that aromatic species (ASs), especially HCASs, are more abundant in ERSL than in ERHL. The carbon number in AAs from RICO of both ERs ranges from 10 to 25, while PA (peak 42) is predominantly more abundant than other CAs, as illustrated in Figure 3c, suggesting that pentadecyl is a predominant alkyl group on aromatic rings (ARs) in the ERs. Different from the distribution of AAs, the TY of ADAs from RICO of each ER basically tends to decrease with increasing carbon number from 4 to 22 (Figure 3d), implying that short ABs, especially −(CH2)2− and −(CH2)3−, are predominant in both ERs. Recent investigations also proved

Figure 3. Distributions of CAs in the products from RICO of the ERs.

than that of ERHL (Table 1), less ABs and ASCs, but a higher content of aromatic moieties in ERSL than in ERHL (Figure 2). The total yield (TY) of AAs from RICO of either ERHL or ERSL 4635

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Figure 5. Mass spectra of EDCMIEPs from RICO of the ERs by ASAP/TOF-MS analysis.

Table 5. Comparison of [M − 31]+/z Data Obtained by GC/MS and ASAP/TOF-MS Analyses for PMBPCs from EDCMIEPs of the ERs [M − 31]+/z with ASAP/TOF-MS PMBPC

formula ([M − 31]+/z)

with GC/MS

ERHL

ERSL

dimethyl phthalate trimethyl benzenetricarboxylate tetramethyl benzenetetracarboxylate pentamethyl benzenepentacarboxylate hexamethyl benzenehexacarboxylate

C9H7O3 C11H9O5 C13H11O7 C15H13O9 C17H15O11

163.1 221.1 279.1 337.1 395.1

163.03891 221.04251 279.04711 337.05567 395.08022

163.03720 221.04251 279.04788 337.05343 395.08082

Scheme 1. Possible Pathways for Fragmentations of Tetramethyl Benzene-1,2,3,4-tetracarboxylate during GC/MS and ASAP/ TOF-MS Analyses

that −(CH2)2− is one of main bridged linkages connecting ARs in lignites.32,36 The −CH2−CH2− bonds in 1,2-diarylethanes (DAEs) are typical weak bonds in coals.37 The reactivities of DAEs toward thermolysis significantly depend upon the types of ARs. For example, in decalin under pressurized nitrogen at

400 °C, only 21.7% bibenzyl was converted for 10 h,38 whereas 1,2-di(1-naphthyl)ethane conversion reached 45.8% only for 1 h.39 These facts indicate that −(CH2)2− connecting more condensed ARs in coals is more reactive toward thermolysis. The ADAs include normal ADAs (NADAs) and branched 4636

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Table 6. Identification of Macromolecular CAs in EDCMIEP from ERSL with ASAP/TOF-MS formula ([M − 31]+/z)

measured mass

theoretical mass

error (ppm)

corresponding CA

C33H23O9 C35H25O11 C37H27O13 C39H29O15 C41H31O17 C43H33O19

563.13439 621.13984 679.14475 737.14954 795.15596 853.16315

563.13421 621.13969 679.14517 737.15065 795.15612 853.16160

0.32 0.24 −0.62 −1.51 −0.20 1.82

C24H11(COOH)5 C24H10(COOH)6 C24H9(COOH)7 C24H8(COOH)8 C24H7(COOH)9 C24H6(COOH)10

Article

ASSOCIATED CONTENT

S Supporting Information *

Total ion chromatograms of EDCMEP and EDCMIEP from the RICO of the ERs and mass spectra of PMBPCs detected in EDCMEP and EDCMIEP from RICO of the ERs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

ADAs (BADAs). As Figure 4 demonstrates, NADAs are predominant in the ADAs. 3.3. ASAP/TOF-MS Analysis. In EDCMIEP of the ERs, a series of species with m/z 163 + 58n (n = 0−4) was detected with ASAP/TOF-MS (Figure 5). The species could be ascribed to polymethyl benzenepolycarboxylates (PMBPCs) from dimethyl benzenedicarboxylates to hexamethyl benzenehexacarboxylate according to GC/MS analysis and corresponding ASAP/TOF-MS analysis, as listed in Table 5. Data [M − 31]+/z are attributed to the most stable fragmental ions (MSFIs, i.e., base peaks shown in Figure SI5 of the Supporting Information), which resulted from the loss of CH3O− from corresponding molecular ions during GC/MS analysis, while the formation of each MSFI could be related to the attack of H+ on the oxygen atom in the OCH3 group of a PMBPC and the subsequent loss of CH3OH during ASAP/ TOF-MS analysis, as depicted in Scheme 1. Different from EDCMIEP from RICO of ERHL, many heavy species (HSs) with m/z larger than 395.1 were detected in EDCMIEP from RICO of ERSL with ASAP/TOF-MS (Figure 5). The HSs include a series of species with m/z 563 + 58n and corresponding formula C24H11 − nCO(COOCH3)4 + n (n = 0−5), as listed in Table 6. Similar to the pathway for fragmentation of PMBPCs depicted in Scheme 1, the attack of H+ on the oxygen atom in the OCH3 group of the HSs and the subsequent loss of CH3OH during ASAP/TOF-MS analysis can also be considered. Such species were not detected with GC/MS because of their involatility. Their parent CAs could be C24H11 − n(COOH)5 + n (n = 0−5). Murata et al.14 detected a series of HSs with formula C12H10 − n(COOCH3)n (n = 6−10) in EDCMIEPs from RICO of two bituminous coals with a field desorption mass spectrometer and ascribed the HSs to polymethyl biphenylpolycarboxylates. By analogy with the structural ascription, we consider that the HSs that we detected could be polymethyl tetraphenylenepolycarboxylates. This result provided more convincing evidence that more ASs, especially HCASs, are contained in ERSL than in ERHL, although the detailed structures of the HSs and their parent species in ERSL need further investigation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was subsidized by National Basic Research Program of China (Grant 2011CB201302), National Natural Science Foundation of China (Grants 20936007, 50974121, 51074153, 21276268, and 21206187), the Fund from National Natural Science Foundation of China for Innovative Research Group (Grant 51221462), the Key Project of Coal Joint Fund from National Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant 51134021), the Strategic Chinese−Japanese Joint Research Program (2013DFG60060), the Fundamental Research Fund for the Doctoral Program of Higher Education (Grant 20120095110006) and the Central Universities (China University of Mining & Technology; Grants 2010LKHX09 and 2011QNA22), the China Postdoctoral Science Foundation Funded Project (Grants 2011M500975, 2012M511339, and 2012T50501), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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4. CONCLUSION RICO of ERs from coals along with subsequent treatments and analyses with GC/MS and ASAP/TOF-MS proved to be an effective approach for understanding macromolecular structures of coals. Both ERHL and ERSL contain smaller amounts of ASCs with C9−C24 on ARs but larger amounts of ABs with C2−C20 between ARs. The pentadecyl group is a predominant ASC, while short ABs, especially −(CH2)2− and −(CH2)3−, are predominant in both ERs. More ASs, especially HCASs, are contained in ERSL than in ERHL. Because no ABs exist in such HCASs, thermolysis of the HCASs is difficult. 4637

NOMENCLATURE HL = Huolinguole lignite SL = Shengli lignite ER = extraction residue OCOC = oxygen-containing organic compound MC = molecular composition OM = organic matter GC/MS = gas chromatography/mass spectrometer MMS = macromolecular species HCAS = highly condensed aromatic species RICO = ruthenium-ion-catalyzed oxidation AB = alkylene bridge ASC = alkyl side chain AR = aromatic ring SOS = soluble organic species CA = carboxylic acid HDA = heptadecylarene DAH = 1,6-diarylhexane SA = stearic acid ODA = octanedioic acid PA = palmitic acid CDSEF = carbon-disulfide-extractable fraction SDSBC = Shenfu Dongsheng sub-bituminous coal ASAP = atmospheric pressure solids analysis probe ERHL = ER from HL ERSL = ER from SL F1 = filtrate of the reaction mixture from RICO of an ER dx.doi.org/10.1021/ef400881m | Energy Fuels 2013, 27, 4632−4638

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FC = filtrate cake of the reaction mixture from RICO of an ER ES1 = extraction solution from FC with DCM DCM = dichloromethane OP = organic phase AP = aqueous phase ES2 = extraction solution from AP with DCM DCMIES = DCM-inextractable solution DCMES = DCM-extractable solution AHMS = anhydrous magnesium sulfate F2 = filtrate of dried DCMES DCMEP = DCM-extractable portion DEE = diethyl ether EDCMEP = esterified DCMEP DCMIEP = DCM-inextractable portion EDCMIEP = esterified DCMIEP FTIR = Fourier transform infrared ASAP/TOF-MS = ASAP/time-of-flight mass spectrometer CDC = corona discharge AA = alkanoic acid ADA = alkanedioic acid ATCA = alkanetricarboxylic acid BPCA = benzene polycarboxylic acid TY = total yield AS = aromatic species DAE = 1,2-diarylethane NADA = normal ADA BADA = branched ADA PMBPC = polymethyl benzenepolycarboxylate MSFI = most stable fragmental ion HS = heavy species

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dx.doi.org/10.1021/ef400881m | Energy Fuels 2013, 27, 4632−4638