Fuel 111 (2013) 211–215

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Fuel

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Oxidation of Shengli lignite with aqueous sodium hypochloritepromoted by pretreatment with aqueous hydrogen peroxide

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.04.041

⇑ Corresponding author. Tel.: +86 516 83885951; fax: +86 516 83884399.E-mail address: wei_xianyong@163.com (X.-Y. Wei).

Fang-Jing Liu, Xian-Yong Wei ⇑, Ying Zhu, Yu-Gao Wang, Peng Li, Xing Fan, Yun-Peng Zhao,Zhi-Min Zong, Wei Zhao, Yan-Bin WeiKey Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou 221116, Jiangsu, China

h i g h l i g h t s

� SL was pretreated with H2O2 and then oxidized in NaOCl aqueous solution.� The pretreatment significantly increased the yields of soluble species.� The pretreatment enhanced the formation of compounds with m/z 300 to 500.� The formation of chloro-substituted alkanoic acids was inhibited by the pretreatment.� Increase in yields of soluble species is ascribed to introduction of �COOH and �OH.

a r t i c l e i n f o

Article history:Received 20 March 2013Received in revised form 16 April 2013Accepted 18 April 2013Available online 3 May 2013

Keywords:OxidationPretreatmentH2O2

NaOClBenzene carboxylic acids

a b s t r a c t

Shengli lignite (SL) was pretreated in H2O2 aqueous solution at 40 �C for 4 h to obtain pretreated SL(PTSL). Both the SL and PTSL were oxidized in NaOCl aqueous solution at 30 oC for 5 h to investigatethe effect of pretreatment (PT) on SL oxidation with NaOCl. Gas chromatography/mass spectrometryanalysis shows that the PT with H2O2 significantly increased the yields of alkanoic acids, alkanedioicacids, and benzene carboxylic acids (BCAs), but substantially suppressed the formation of chloro-substi-tuted alkanoic acids (CSAAs). The total yield (daf) of BCAs was 7.4% from SL and increased to 10.1% fromPTSL, while that of CSAAs decreased from 5.4% for SL to 0.8% for PTSL. Direct analysis in real time ioniza-tion source coupled to mass spectrometry analysis exhibits that the PT with H2O2 enhanced the forma-tion of compounds with molecular from m/z 300 to 500. The possible mechanisms for the PT andsubsequent oxidation are discussed according to the experimental results. The cleavage of covalent bondsand introduction of oxygen functional groups such as �COOH and �OH by the PT could be responsible forthe increase in the yields of soluble species.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Coal oxidation has been paid much attention as an effectivemethod for obtaining organic acids and for understanding coalstructures [1–6]. The dominant products from coal oxidation arealkanoic acids (AAs), alkanedioic acids (ADAs), and benzene car-boxylic acids (BCAs), which are useful for synthesizing functionalmaterials [7], medicines [8], and aircraft fuel [9]. H2O2 and NaOClwere widely used as oxidants for coal oxidation [1,2,10–17] dueto their low price, easy availability, and eco-friendliness. Becauseof its relatively low reactivity toward coal degradation, H2O2 seemsto be suitable only for low-rank coals to obtain value-added chem-icals (VACs). NaOCl has higher reactivity toward coal oxidativedegradation than H2O2, which made it more attractive for coal

oxidation to produce organic acids in recent years [18,19]. How-ever, the attempts to obtain organic acids by coal oxidation withNaOCl were not successful, because the formation of large amountsof chloro-substituted species (CSSs) led to the difficulties both ininsight into coal structures and in the product separation. Greatcare should be taken to optimize the reaction conditions or developa new process in order to minimize the yields of CSSs and increasethe yields of important chemicals such as AAs, ADAs, and BCAs.

Since lignites are rich in oxygen functional groups (OFGs), pro-duction of VACs especially organic acids through oxidation couldbe one of promising methods for lignite utilization. According toour recent investigation [20], SL is rich in condensed aromatic spe-cies and hydroxyl-, methoxyl-, and/or methyl-substituted benzenerings, and �CH2CH2� is dominant bridged linkage connecting aro-matic rings in SL. Most of organic matter in SL was converted intosoluble fractions in either H2O2 or NaOCl aqueous solution. How-ever, the product composition from SL oxidation with H2O2 was

4

6

8

10

Yiel

d (w

t%, d

af)

PTSLSL

212 F.-J. Liu et al. / Fuel 111 (2013) 211–215

more complicated than that with NaOCl, but products from SL oxi-dation with NaOCl contained CSSs. Moreover, SL oxidation withH2O2 was much slower than that with NaOCl. Therefore, develop-ing an improved approach is needed to overcome such drawbacks.

The pretreatment (PT) with H2O2 was proved to be an effectiveapproach for promoting hydrogenation [21], hydrogenolysis [22],extraction [23], desulfurization [24], and flash pyrolysis [25,26]of coals. Such a technique could be also effective for improving coaloxidation with NaOCl. Taking this expectation into account, weinvestigated the effect of the PT with H2O2 on SL oxidation in NaO-Cl aqueous solution.

2. Materials and methods

2.1. Materials

SL was collected from Shengli Coal Mine in Xilinhaote, InnerMongolia Autonomous Region, China. It was pulverized to passthrough a 200-mesh sieve (particle size of <75 lm) followed bydesiccation in a vacuum at 80 �C for 24 h before use. Table 1 liststhe proximate and ultimate analyses of SL. Ethyl acetate (EA),diethyl ether (DEE), diazomethane (DAM), hydrochloric acid(HCA, 36% of HCl), H2O2 aqueous solution (30%), and NaOCl aque-ous solution (6% available chlorine) used in the experiments areanalytical reagents, and all the organic solvents were distilled be-fore use.

2.2. PT of SL with H2O2

SL (3.0 g) was mixed with 30 mL of H2O2 aqueous solution in a250 mL spherical flask with magnetical agitation at 40 oC for 4 h.The reaction mixture was filtered and the filter cake, i.e., pretreatedSL (PTSL), was dried in a vacuum at 80 oC for 24 h. After cooling toroom temperature, the sample was transferred into a desiccator forsubsequent oxidation in NaOCl aqueous solution.

2.3. Oxidation of SL and PTSL with NaOCl

According to our recent investigation [20], 1.0 g SL or PTSL and80 mL NaOCl aqueous solution were added to a 250 mL sphericalflask and fully mixed by magnetical agitation at 30 oC for 5 h. Thenthe reaction mixture was separated to filter cake 1 (FC1) and filtrate1 (F1) by filtration through a polytetrafluoroethylene membranefilter with 0.45 lm of pore size. F1 was acidized with HCA and fil-trated to obtain filter cake 2 (FC2) and filtrate 2 (F2). F2 was sequen-tially extracted with DEE and EA followed by solvent evaporationto acquire extract 1 (E1), extract 2 (E2), and inextractable fraction(IEF). E1, E2, and IEF were esterified with DAM in DEE to obtainmethyl esterified E1 (MEE1), methyl esterified E2 (MEE2), andmethyl esterified IEF (MEIEF), correspondingly.

2.4. Sample analyses

Both SL and PTSL were analyzed with a Nicolet Magna IR-560Fourier transform infrared (FTIR) spectrometer by collecting 50scans at a resolution of 8 cm�1 in reflectance mode with measuringregions of 4000–400 cm�1. MEE1, MEE2, and MEIEF were analyzed

Table 1Proximate and ultimate analyses (wt.%) of SL.

Proximate analysis Ultimate analysis (daf)

Mad Ad VMdaf C H N S Oa

13.74 7.51 46.40 70.84 5.05 0.88 1.32 21.91

A By difference.

with a Hewlett–Packard 6890/5973 gas chromatography/massspectrometer (GC/MS) and direct analysis in real time ionizationsource coupled to mass spectrometer (DART–MS). The GC/MS isequipped with a capillary column coated with HP-5 (cross-link 5%PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 lm filmthickness) and a quadrupole analyzer and operated in electron im-pact (70 eV) mode. Compounds were identified by comparing massspectra with NIST05a library data. The DART (IonSense Company,Saugus, USA) is interfaced to ion trap mass spectrometer using he-lium (He) as the discharge gas and nitrogen (N2) as an alternativegas with a flow rate of 2 L min�1 and operated at 450 �C. Quantita-tive analysis was also conducted on the basis of parent SL samplemass with GC/MS using a series of authentic compounds as externalstandards, e.g., methyl caproate for AAs, dimethyl adipate for ADAsand alkanetricarboxylic acids (ATCAs), and dimethyl phthalate forBCAs. All the yields were calculated on dried and ash-free mass of SL.

3. Results and discussion

3.1. Yields of FC1 and FC2

After oxidation in NaOCl aqueous solution for 5 h, the color ofreaction mixture from SL and PTSL changed from black to yellow.FC2 was defined as black acids [17] which were insoluble in acidsolution but soluble in acetone and ethanol. The yields of FC1 andFC2 were 10.1% and 9.4% from SL but they appreciably decreasedto 9.0% and 3.3% for PTSL, respectively. These results indicate thatmore organic matter in PTSL than that in SL was converted towater-soluble species. In other words, the PT with H2O2 enhancedSL oxidation with NaOCl.

3.2. Product distribution from SL and PTSL oxidation in NaOCl aqueoussolution

3.2.1. GC/MS analysisAs illustrated in Figs. SI1–SI6 and Tables SI1–SI5 (Supplemen-

tary information), in total, 103 organic acids were identified fromSL and PTSL oxidation by GC/MS analysis, including 13 AAs, 13chloro-substituted alkanoic acids (CSAAs), 29 ADAs, 9 ATCAs, and39 BCAs.

The distribution of different types of products from SL and PTSLoxidation was drawn according to data in Tables SI1–SI5. As exhib-ited in Fig. 1, total yield (TY) of AAs from PTSL is twice more thanthat from SL, while TY (0.8%) of CSAAs from PTSL is significantly

AAs CSAAs ADAs ATCAs BCAs0

2

Product type

Fig. 1. Distribution of different types of products from SL and PTSL oxidation withNaOCl.

F.-J. Liu et al. / Fuel 111 (2013) 211–215 213

lower than that (5.4%) from SL, suggesting that the PT with H2O2

significantly increased the yields of AAs but suppressed the forma-tion of CSAAs. CSAAs, the by-products, are considered to be phyto-toxic and some of them are presumably cancerogenic, which havebeen reported to be main chlorinated products of humic or fulvicacids [27–29] and phenolic moieties in Shenfu coal [18]. Further-more, TYs of ADAs and ATCAs from PTSL are twice more than thosefrom SL as shown in Fig. 1, indicating the PT with H2O2 also facil-itated the formation of ADAs and ATCAs.

As shown in Fig. 1, BCAs are main products from both SL andPTSL oxidation with NaOCl, and TY (10.1%) of BCAs from PTSL issignificantly higher than that (7.4%) from SL. Benzenetricarboxylicacids and benzenetetracarboxylic acids account for 46% and 32% ofBCAs from PTSL, respectively (Table SI5). In addition, TY of benzen-etricarboxylic acids from PTSL is nearly as twice as that from SL.BCAs and their derivatives are important chemicals and ligandsto synthesize metal complexes and polymers [30–35]. For instance,as an derivative from pyromellitic acid, pyromellitic dianhydride isa raw material for synthesizing advanced polyimides, plasticizers,epoxy resin curing agents, alkyd resin coating, and flatting agents[36].

m

87.5185.3

212.3270.4 328.4414.4

499.

100.4184.3 253.3

328.3 418.4462.4

83.4

116.3

198.2

238.3310.4

428.450

80.5

167.3210.3268.3

328.4 390.3

200 400

020406080

100

116.3

158.2

210.3282.3

328.4

410.4

020406080

100

020406080

100

020406080

100

020406080

100

020406080

100

Rel

ativ

e ab

unda

nce

(%)

116.4

210.3 328.4406.4466.4

100 300 500

Fig. 2. MM distributions of E1, E2, and IEF fr

3.2.2. Molecular mass distributions (MMDs) characterized by DART–MS

DART–MS was applied to characterize MMDs of reaction mix-tures from SL and PTSL oxidation with NaOCl. It was carried outin positive ionization mode. In this mode, the charged He formedmetastable He⁄ and reacted with water in atmosphere to produceH3O+. The proton in H3O+ was transferred to analyte, forming aprotonated quasimolecular ions [M + H]+. As Fig. 2 displays, theMMDs of both E1 and E2 from SL and PTSL are similar, rangingin molecular mass from m/z 100 to 800 with a peak around m/z450. In addition, most of compounds center between m/z 300and m/z 500. However, relative abundances of mass spectra ofE1 and E2 from PTSL are significantly higher than those from SL,correspondingly, implying that the PT with H2O2 facilitated theformation of soluble organic compounds with molecular mass(MM) from m/z 100 to 800. The MMDs of IEF from SL are broaderthan other soluble fractions, ranging in molecular mass from m/z100 to 900 and having a peak around 500. The MMDs of IEF fromPTSL range from m/z 100 to 800 with a peak in molecular massaround m/z 500 either. However, relative abundances of thecompounds with MM greater than m/z 500 in IEF from SL are

/z

4 606.5 664.4

722.4

0.4

600 800 1000

SL-E1

536.4

SL-E2

SL-IEF

PTSL-E1

536.4PTSL-E2

PTSL-IEF

700 900

om SL and PTSL oxidation with NaOCl.

C-O

H

SL

PTSL

Tran

smitt

ance

5004000 3500 3000 2500 2000 1500 1000

Wavenumbers (cm-1)

-OH

-CH

3& >

CH

2

>C=O

>C=C

<

C-O

-C

-OH

Fig. 3. FTIR spectra of SL and PTSL.

Scheme 1. Proposed pathway for PM conversion in SL during PT with H2O2.

Scheme 3. Proposed pathways for the oxidation of naphthalene and substitutednaphthalenes with NaOCl.

214 F.-J. Liu et al. / Fuel 111 (2013) 211–215

remarkably higher than those from PTSL, while relative abun-dances of the compounds with MM between m/z 300 and m/z500 in IEF from SL is lower than those from PTSL. These resultssuggest that the PT with H2O2 enhanced the formation of com-pounds with MM from m/z 300 to 500.

3.3. Possible mechanisms for the PT with H2O2 and subsequentoxidation with NaOCl

Mae et al. [23,26] supposed that the PT with H2O2 partly brokesome covalent bonds and simultaneously introduced OFGs such as�COOH and �OH into coals, and hence facilitated coal extractionand flash pyrolysis. Similar mechanism for the promotional effectof PT with H2O2 on SL oxidation with NaOCl can be considered.As Fig. 3 exhibits, the absorbance at 3610 cm�1 attributed to free�OH in PTSL is much stronger than that in SL, indicating that free�OH was readily introduced into lignite by PT with H2O2. Remark-ably stronger absorbance at 1705 cm�1 assigned to –C@ O bond ofcarboxyl groups in PTSL than that in SL proved that �COOH wasintroduced into SL by PT with H2O2. The absorbance of aromaticor alcoholic C–OH at 1215 cm�1 in PTSL is appreciably stronger

Scheme 2. Proposed pathway for the format

than that in SL, but the absorbance of C–O–C at 1035 cm�1 in PTSLis appreciably weaker than that in SL, implying that weak bridgedbonds like C–O–C connecting condensed aromatic rings was bro-ken by PT with H2O2 and led to the formation of aromatic or alco-holic C–OH. Both covalent bond cleavage and OFG introductionresulted in higher reactivity and hydrophilicity (HP) of PTSL thanthose of SL. The increase in HP improved interaction between SLparticles and NaOCl aqueous solution, and thereby promoted SLoxidation in NaOCl aqueous solution.

Phenolic moiety (PM) was reported to be an important precursorof CSAAs during coal oxidation with NaOCl [18]. As Scheme 1 illus-trates, PM in SL could be converted to carbonyl group-containingmoieties (CGCMs) during PT with H2O2. Such conversion preventedthe formation of some CSAAs, since CGCMs are inert toward chlori-nation. Because of their high HP, the resulting CGCMs could be read-ily converted to BCAs during subsequent oxidation with NaOCl.

BCAs proved to be products from oxidation of condensedaromatics (CAs) in coals with NaOCl [17,18]. High HP of OFGs suchas hydroxyl and carboxylic groups on the CAs facilitated the forma-tion of BCAs [16,37]. The PT with H2O2 could break weak covalentbonds and simultaneously convert OFGs on CAs to more hydrophilicones, facilitating subsequent oxidation with NaOCl to produce BCAs(Scheme 2). Both naphthalene and naphtha-1-ol were oxidized with

ion of BCAs promoted by PT with H2O2.

F.-J. Liu et al. / Fuel 111 (2013) 211–215 215

NaOCl to phthalic acid (PC), while benzene-1,2,3-tricarboxylic acidand benzene-1,2,4-tricarboxylic acid were produced in addition toPC from the oxidation of naphthalen-1-ylmethanol and 2-naphthoicacid with NaOCl, respectively (Scheme 3); naphthalene was muchless reactive than the 3 OFG-substituted naphthalenes toward oxi-dation with NaOCl [38,39]. These facts corroborate that the pro-posed mechanism for the increase of BCAs yields by PT is convictive.

4. Conclusions

The PT with H2O2 proved to be an effective approach forincreasing the yields of VACs from subsequent SL oxidation withNaOCl. More organic matter in PTSL than in SL was converted intosoluble species, including AAs, ADAs, ATCAs, and BCAs, by oxida-tion with NaOCl, while the formation of CSAAs was substantiallyinhibited by PT with H2O2. The PT with H2O2 also enhanced the for-mation of compounds with MM from m/z 300 to 500. The introduc-tion of OFGs such as �COOH and �OH into SL by the PT with H2O2

could be responsible for the increase in the yields of solublespecies.

Acknowledgments

This work was subsidized by National Basic Research Programof China (Grant 2011CB201302), National Natural Science Founda-tion of China (Grants 20936007, 50974121, 51074153, 21276268,and 21206188), the Fund from National Natural Science Founda-tion of China for Innovative Research Group (Grant 51221462),Strategic Chinese–Japanese Joint Research Program (Grant2013DFG60060), the Fundamental Research Fund for the DoctoralProgram of Higher Education (Grant 20120095110006) and theCentral Universities (China University of Mining & Technology,Grant 2010LKHX09), Research and Innovation Project for CollegeGraduates of Jiangsu Province (Grant CXLX12_0960), and a ProjectFunded by the Priority Academic Program Development of JiangsuHigher Education Institutions.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2013.04.041.

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