Sulfur-containing species in the extraction residue from Xianfeng lignite characterized by X-ray photoelectron spectrometry and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Abstract

Understanding the chemical composition of sulfur-containing species (SCSs) in coals is important because of their negative impact on the environment during coal utilization. In this study, SCSs in the extraction residue (ER) from Xianfeng lignite were characterized through methanolysis combined with X-ray photoelectron spectrometry (XPS) and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) analyses. The results show that the SCSs on the ER surface include pyrite, aliphatic sulfur, aromatic sulfur, sulfoxide, sulfone, and sulfate. The S₁O_x (x = 0–5), S₂O_x (x = 0–5), and N₃S₁ class species are the main SCSs in the soluble portion from ER methanolysis according to ESI FT-ICR MS analysis. Henicosane-1-thiol and alkylhydroxythiophenecarboxylic acids are dominant in the S₁O_x class species. The S₂O_x class species have double bond equivalent values of 1–14 and carbon numbers of 16–33. Sulfur in the S₂O_x class species is mainly present in thiol groups or S-heterocyclic rings (especially thiophene ring). Methanol can thermally break the –C–O– and –C–S– bonds connected between SCSs and macromolecular moieties in the ER matrix, leading to the release of SCSs.

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1. Introduction

Although sulfur contributes a small portion to the mass of coals, the environmental pollution, especially acid rain, resulting from the emission of SO_X during coal combustion, is still an urgent problem.^1–3 Moreover, great attention should also be paid to the catalyst poisoning⁴ and equipment corrosion caused by sulfur during coal conversion. Therefore, in order to reduce the environmental impacts from SO_X emission, removing sulfur in coals to an acceptable level prior to conversion should be a very important procedure for clean coal technology. Coal desulfurization with different methods^5–9 has been extensively investigated for decades but still remains technical challenges and economic burdens, largely due to the lack of knowledge on the detailed composition of sulfur-containing species (SCSs) in coals. A full understanding of the chemical forms, thermal reactivity and transformation of SCSs in coals can not only provide important information on the origin of sulfur but also have profound implications for removing sulfur from coals.

Most of the SCSs in coals are believed to be incorporated into coals and transformed during early diagenesis and maturation.¹⁰ In general, SCSs in coals are mainly present in three forms: pyrite, sulfate, and organic sulfur.^11,12 Different from pyrite and sulfate, most of the organic sulfur is covalently bonded to the coal matrix and hence not easily characterized. As non-destructive techniques, X-ray photoelectron spectrometry (XPS)^13–17 and X-ray absorption near edge structure spectrometry^{14,15,17–19} were widely used to characterize the occurrence forms of SCSs in coals. However, the results obtained by such characterizations cannot provide detailed information on the molecular structures of SCSs in coals. Severe destructive techniques, such as pyrolysis and catalytic hydrotreatment, in combination with subsequent analyses by gas chromatography (GC) and GC/mass spectrometry (GC/MS) provided information on only some moieties of organic sulfur in coals at the molecular level.^17,20–24 Moreover, some SCSs with polar functional groups released from coals via degradation cannot be detected by GC/MS due to their strong polarity.

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), which possesses high resolving power and high mass accuracy, was used for characterizing oxygen- and nitrogen-containing species in extracts,^25,26 liquefaction products,²⁷ tar,^28–31 and pyrolysis liquid³² from coals. However, identification of SCSs in coals and coal-derived liquids using FT-ICR MS was rarely reported. Since FT-ICR MS was successfully used for characterizing SCSs in petroleum-derived liquid,^33–36 heavy oil,³⁷ vacuum gas oil,³⁸ and oilsands bitumen,³⁹ appropriate degradation methods combined with FT-ICR MS analysis could also be applicable to the characterization of SCSs in coals. In our recent investigation,⁴⁰ electrospray ionization (ESI) FT-ICR MS proved to be an effective technique for characterizing oxygen-containing species in methanolysis products of the extraction residue (ER) from Xianfeng lignite (XL). The SCSs in XL could be released from the ER through methanolysis. In this study, we focus on the characterization of SCSs in the ER from XL using XPS and ESI FT-ICR MS, aiming at getting insights into the occurrence forms of sulfur in lignites to facilitate the sulfur removal.

2. Experimental section

2.1. Material and methods

The ER was generated from XL through sequential ultrasonic extraction with five different solvents.⁴¹ The proximate and ultimate analyses of XL and the ER were exhibited elsewhere.⁴⁰ Detailed experimental procedures for the ER methanolysis and subsequent separation were described in ESI† and our recent investigation.⁴⁰

2.2. XPS analysis

Thermo Fisher ESCALAB 250Xi XPS was used to determine the occurrence forms of sulfur on the ER surface. It was equipped with a monochromatized Al Kα X-ray source operated at 150 W. XPS analysis was run at a fixed analyzer transmission mode, and calibration for binding energy of sulfur was referred to the main C 1s peak at 284.8 eV. The XPS PeakFit software was used for peak fitting and semi-quantitation of XPS S 2p spectrum. The corresponding S 2p spectrum was curve resolved using 50% Lorentzian–Gaussian line shape.

2.3. ESI FT-ICR MS analysis and data processing

The SCSs in extracts 1–4 (E₁–E₄) were analyzed with a Bruker apex-ultra ESI FT-ICR MS, which is equipped with a 9.4 T superconducting magnet and was operated in negative-ion mode. The sample solution was infused through an Apollo II ESI source with a syringe pump. The voltages at the emitter, capillary column front end, and capillary column end in ESI were set to −4.0 kV, −4.5 kV, and −320 V, respectively. The data with size of 4 M were acquired, and a total of 64 scans were accumulated. Data processing was performed according to our recent investigation⁴⁰ and elsewhere.^28,42 The number of S atom in each molecular formula was limited up to 2 when mass peaks were assigned.

3. Results and discussion

3.1. XPS analysis

The binding energy of S 2p peaks for different sulfur forms are 162.6 eV for pyrite, 163.5 eV for aliphatic sulfur, 164.2 eV for aromatic sulfur, 165.4 eV for sulfoxide, 168.5 eV for sulfone, and 169.5 eV for sulfate according to the literatures.^43,44 XPS was considered to be a surface-sensitive technique revealing the chemical forms of elements within the outermost 5 nm of the solid sample.¹⁰ Therefore, it is noteworthy that the sulfur composition on a sample surface determined by XPS could be significantly different from that in the bulk of the sample. As demonstrated in Fig. 1 and Table 1, sulfone is the most abundant sulfur form on the ER surface from XPS analysis, accounting for 44.5% of all the sulfur forms, followed by pyrite, aromatic sulfur, sulfoxide, sulfate, and aliphatic sulfur. The aromatic sulfur may include thiophene and its derivatives (e.g., benzothiophene, dibenzothiophene, and benzonaphthothiophene), and the aliphatic sulfur may include thiol, sulfide, and disulfide.^11,12 The molar content of sulfone is significantly higher than that of either aromatic or aliphatic sulfur on the ER surface perhaps due to the oxidation of the aromatic and aliphatic sulfur species to the corresponding sulfoxide and sulfone⁴⁵ when the sample was exposed to the air during transportation and storage. The oxidation of aliphatic sulfur is usually easier than that of aromatic sulfur in air.¹⁵

Fig. 1 XPS S 2p spectrum and its fitting curve of the ER.

Table 1 Distribution of sulfur forms in the ER from XPS analysis

Binding energy (eV)	Sulfur form	Molar content (%)
162.6	Pyrite	15.2
163.5	Aliphatic sulfur	4.4
164.2	Aromatic sulfur	15.1
165.4	Sulfoxide	12.7
168.5	Sulfone	44.5
169.5	Sulfate	8.1

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3.2. SCS distributions in E₁–E₄ from negative-ion ESI FT-ICR MS analysis

Because XPS analysis only revealed the sulfur forms on the ER surface, methanolysis along with ESI FT-ICR MS analysis of the resulting soluble portions (i.e., E₁–E₄) was used for characterizing the SCSs in the ER. Since all of the detected ions from ESI FT-ICR MS are singly charged, each ion is denoted by its mass in u rather than its mass-to-charge ratio, i.e., m/z. Broadband mass spectra of negative-ion ESI FT-ICR MS for E₁–E₄ were illustrated in a previous investigation,⁴⁰ so they will not be discussed here. The relative contents of different class species in each spectrum were defined as the sum of the relative abundances (RAs) of all the species in a given class (number of heteroatoms) divided by the sum of the RAs of all the identified peaks in the mass spectrum. As Fig. 2 shows, the SCSs in E₁–E₄ detected by ESI FT-ICR MS mainly include S₁O_x (x = 0–5), S₂O_x (x = 0–5), and N₃S₁ class species. Obviously, the SCSs in E₁–E₃ from ESI FT-ICR MS analysis are dominated by the S₁O_x class species, especially the S₁ and S₁O₃ class species, while the S₂O_x (x = 0–4) and N₃S₁ class species were mainly detected in E₄. The S₂O₁ class species have the highest RA among the S₂O_x class species in E₄.

Fig. 2 RAs of SCSs in E₁–E₄ from the ER by negative-ion ESI FT-ICR MS analysis.

To facilitate identification of the homologous series, the measured masses in International Union of Pure and Applied Chemistry (IUPAC) mass scale were converted to Kendrick mass scale according to the equation:^42,46

Kendrick mass = IUPAC mass × (14/14.01565).

As a result, the mass of CH₂ (14.01565) in IUPAC scale was converted to exactly 14. The Kendrick mass defect (KMD) was defined as the following equation:

KMD = Nominal Kendrick mass − Kendrick mass.

Thus, the homologous series with the same class and same type (double bond equivalent, denoted as DBE) but different numbers of CH₂ groups have the same KMD. The plot of KMD vs. nominal Kendrick mass (rounding the Kendrick mass to the nearest integer) for S₂O_x (x = 0–4) in ESI FT-ICR mass spectrum of E₄ was drawn. As displayed in Fig. 3, for a given class, each homologous series fall on a single horizontal row with the same KMD and the equally spaced points result from the successive increase in CH₂ groups. For compounds of the same class but different type, each additional ring or double bond moves the horizontal row upward successively by the difference in KMD of two hydrogen atoms. Fig. 3 also shows that most of the S₂O_x class species in E₄ center between 325 u and 475 u in molecular mass.

Fig. 3 Plot of KMD vs. nominal Kendrick mass for S₂O_x (x = 0–4) in ESI FT-ICR mass spectrum of E₄.

Mass scale-expanded segment of ESI FT-ICR mass spectrum for E₄ from 396 to 417 u (Fig. 4, bottom) revealed a series of S₂O_x class species differing by multiple 2.0157 u (i.e., mass of H₂), corresponding to an additional DBE value. Further scale-expanded segment around 401 u (Fig. 4, top) resolved 13 distinct SCSs within 0.5 ppm mass accuracy, including S₁O_x (x = 1–2) and S₂O_x (x = 0–5) class species as listed in Table 2. The identified compounds have DBE values ranging from 0 to 14 (Table 2). As demonstrated in Fig. 4 (top), C₂₄H₃₄O₁S₂ (peak 8) is the most abundant, which is consistent with the result in Fig. 2 that the S₂O₁ class species are predominant SCSs in E₄.

Fig. 4 Mass scale-expanded segments from 396 u to 417 u (bottom) and around 401 u (top) in ESI FT-ICR mass spectrum of E₄.

Table 2 Identification of singly charged ions around 401 u from negative-ion ESI FT-ICR MS analysis of E₄

Peak	Formula [M − H]^−	Mass (u)		Error (ppm)	DBE
		Measured	Theoretical
1	C21H21O4S2	401.08880	401.08867	−0.3	11
2	C22H25O3S2	401.12486	401.12506	0.5	10
3	C19H29O5S2	401.14622	401.14619	−0.1	5
4	C26H25O2S1	401.15786	401.15807	0.5	14
5	C23H29O2S2	401.16134	401.16145	0.3	9
6	C20H33O4S2	401.18239	401.18257	0.5	4
7	C27H29O1S1	401.19463	401.19446	−0.4	13
8	C24H33O1S2	401.19768	401.19783	0.4	8
9	C21H37O3S2	401.21881	401.21896	0.4	3
10	C25H37S2	401.23407	401.23422	0.4	7
11	C22H41O2S2	401.25530	401.25535	0.1	2
12	C23H45O1S2	401.29161	401.29173	0.3	1
13	C24H49S2	401.32810	401.32812	0.0	0

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3.3. DBE versus carbon numbers (CNs) for SCSs

As illustrated in Fig. 5, although DBE values of S₁ class species in E₁–E₄ range from 0 to 13, the species with DBE of 0 corresponding to alkylsulfanes or alkylthiols are predominant, among which a compound with molecular formula C₂₁H₄₄S (quasimolecular ion at 327.309 u) has the highest RA. Because alkylthiols are easier to be ionized in negative-ion mode, the molecule of C₂₁H₄₄S could be assigned to henicosane-1-thiol rather than its sulfane isomer. As Fig. 6 demonstrates, the S₁O₃ class species have DBE values from 4 to 14 with the highest RA at DBE of 4. The species with DBE of 4 are most likely to be alkylhydroxythiophenecarboxylic acids. They contain CNs from 9 to 19 and center between 16 and 19 corresponding to CNs of alkyl side chains between 11 and 14.

Fig. 5 RA of S₁ vs. DBE values in E₁–E₄ from the ER by negative-ion ESI FT-ICR MS analysis.

Fig. 6 RA of S₁O₃ vs. DBE values in E₁–E₄ from the ER by negative-ion ESI FT-ICR MS analysis.

As Fig. 7 displays, most of the S₂ class species in E₃ have 4–14 DBE values and 23–33 CNs, while those in E₄ have smaller DBE values (1–11) and CNs (19–29). The size of the circle in the figure corresponds to the RA of the individual compound in the spectrum. For compounds in a given homologous series, the RA decreases with increasing CNs. There are no S₂ class species with DBE of 0 detected in E₃ and E₄, indicating that alkyldisulfanes do not exist or have very low content. The species with DBE of 5 in the S₂ class species could be assigned to alkyldihydrobenzo[b]thiophenethiols as shown in Fig. 7. Since an additional benzene ring attached to the original aromatic core leads to the increase of three DBE units, the S₂ class species with DBE values at 6, 9, and 12 could be alkylbenzo[b]thiophenethiols, alkyldibenzo[b,d]thiophenethiols, and alkylbenzo[d]naphtho[2,3-b]thiophenethiols, with CNs of alkyl side chains from 15 to 23, 8 to 20, and 10 to 18, respectively.

Fig. 7 Iso-abundance plots of DBE versus CN distributions of S₂ class species in E₃ and E₄ from the ER.

As demonstrated in Fig. 2 and discussed above, the S₂O_x (x = 1–4) class species were mainly concentrated in E₄. As Fig. 8 shows, these class species in E₄ have DBE values from 1 to 11 and CNs from 16 to 29. The sulfur forms in the S₂O_x class species with DBE < 6 are likely present in the cyclic disulfides and thiophenic compounds, while S₂O_x class species with DBE ≥6 should contain one or more benzene rings. The oxygen in the S₂O_x class species could be hydroxy, carboxyl, and/or carbonyl (present in sulfoxide and sulfone) groups. Some core structures for homologous series in the S₂O_x class species were speculated according to their DBE and CN distributions as demonstrated in Fig. 8.

Fig. 8 Iso-abundance plots of DBE versus CN distributions of S₂O_x (x = 1–4) class species in E₄ from the ER.

3.4. Possible mechanisms for the release of SCSs from the ER methanolysis

The release of SCSs trapped in the ER matrix through strong covalent bonds is difficult by solvent extraction. The –C–O– bonds in lignites, including –C_aryl–O–, –C_acyl–O–, and/or –C_alkyl–O– bonds, can be thermally broken via methanolysis to produce soluble oxygen-containing species.⁴⁷ NaOH facilitated the breakage of –C–O– bonds in the ER during methanolysis and hence increased the yields of soluble portions.⁴⁸ Since most of the SCSs released from the ER methanolysis contain oxygen-functional groups (e.g., hydroxy and carboxyl groups) demonstrated by ESI FT-ICR MS analysis, part of SCSs could result from the cleavage of –C–O– bonds in the ER matrix. On the other hand, the cleavage of –C–S– bonds in the ER matrix to form SCSs during methanolysis should also be considered. As illustrated in Scheme 1, during methanolysis, the oxygen in methanol can attack the –C_acyl–O– bond connected between SCSs and a macromolecular moiety in the ER matrix, leading to the formation of alkylhydroxythiophenecarboxylic acids (S₁O₃ class) and alkylbenzo[d]dithiolecarboxylic acids (S₂O₂ class). In the same way, alkylbenzo[d]naphtho[2,3-b]thiophenethiols (S₂ class) and alkylmercaptobenzo[b]thiophenols (S₂O₁ class) could result from the cleavage of –C–S– bonds via methanolysis (Scheme 2).

Scheme 1 Reaction pathway for the formation of alkylhydroxythiophenecarboxylic acids and alkylbenzo[d]dithiolecarboxylic acids from the ER methanolysis.

Scheme 2 Reaction pathway for the formation of alkylbenzo[d]naphtho[2,3-b]thiophenethiols and alkylmercaptobenzo[b]thiophenols from the ER methanolysis.

4. Conclusions

Sulfur forms on the ER surface include pyrite, aliphatic sulfur, aromatic sulfur, sulfoxide, sulfone, and sulfate according to XPS analysis, among which sulfone is the most abundant and could result from the oxidation of aromatic and aliphatic sulfur when the sample is exposed to the air. ESI FT-ICR MS analysis shows that the S₁O_x (x = 0–5), S₂O_x (x = 0–5), and N₃S₁ class species are the major SCSs in E₁–E₄ from the ER methanolysis. The SCSs in E₁–E₃ are dominated by the S₁O_x class species, especially the S₁ and S₁O₃ class species, while the S₂O_x (x = 0–4) and N₃S₁ class species mainly exist in E₄. Distributions of DBE versus CNs for SCSs reveal the molecular core structures of different sulfur class species and their alkyl side chains. The SCSs released from the ER could be ascribed to the cleavage of –C–O– and –C–S– bonds in the ER matrix via methanolysis. Characterizing the chemical composition of SCSs in the ER and speculating their reaction pathways during methanolysis will be beneficial to sulfur removal from lignites and thereby facilitate clean utilization of lignites.

Acknowledgements

This work was subsidized by National Basic Research Program of China (Grant 2011CB201302), 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 Fundamental Research Fund for the Doctoral Program of Higher Education (Grant 20120095110006), the Research and Innovation Project for College Graduates of Jiangsu Province (Grant CXZZ13_0942), Strategic Chinese-Japanese Joint Research Program (Grant 2013DFG60060), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) availables. See DOI: 10.1039/c4ra12435a

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