Selective production and characterization of aromatic carboxylic acids from Xianfeng lignite-derived residue by mild oxidation in aqueous H2O2 solution

Fuel Processing Technology 181 (2018) 91–96

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Selective production and characterization of aromatic carboxylic acids from Xianfeng lignite-derived residue by mild oxidation in aqueous H2O2 solution

T

Fang-Jing Liua, Zhi-Min Zonga, , Juan Guia, Xiang-Nan Zhub, Xian-Yong Weia,c, Lei Baid ⁎

a

Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China c State Key Laboratory of High-efficiency Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, Ningxia, China d Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, USA b

ARTICLE INFO

ABSTRACT

Keywords: Lignite derived residue Mild oxidation H2O2 Benzene carboxylic acids Polycyclic aromatic carboxylic acids

Lignites show great potential as feedstock for the production of valuable carboxylic acids by mild oxidation. In this study, valuable aromatic polycarboxylic acids were produced from Xianfeng lignite-derived residue (XLDR) through mild oxidation in green aqueous H2O2 solution. The results show that XLDR oxidation can produce benzene carboxylic acids (BCAs) and polycyclic aromatic carboxylic acids (PCACAs) in high selectivity. The BCAs are dominated by mellitic acid and benzenepentacarboxylic acid with 55.5% and 13.3% in the total yield of all group components based on the analysis with gas chromatography/mass spectrometer. Moreover, three series of PCACAs with an aliphatic ring were identified by direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer. The BCAs and PCACAs could be generated from different kinds of aromatic structures via oxidation by %OH produced from H2O2.

1. Introduction Lignites are low-rank coals and contain abundant oxygen-functional moieties compared with subbituminous coals, bituminous coals, and anthracites, resulting in low calorific values. Therefore, lignites are unfavorable for gasification and liquefaction or as clean solid fuels for power generation. However, this characteristic makes lignites with inherent advantages for producing value-added oxygenated chemicals, especially carboxylic acids (CAs) [1]. Liquid-phase oxidation with various oxidants is an attractive method for manufacturing valuable small-molecular fatty acids (SMFAs) and benzene carboxylic acids (BCAs) from lignites and offering specific insights into macromolecular structure of lignites [1,2]. The prevailing oxidants for lignite oxidation (LO) mainly consist of O2 [3–6], RuO4 (RuCl3 as precursor) [7–10], NaOCl [9,11,12], and H2O2 [9,13–16]. Catalytic oxidation of lignites in alkali or acidic aqueous solution using O2 as the oxidant has been employed to produce SMFAs and BCAs in recent years [3–6]. However, the practical application of this method is limited by employing high temperature, high pressure, and strong alkalis or acids. Although ruthenium ion-catalyzed oxidation (RICO) is a powerful way for simultaneously generating SMFAs and BCAs, and evaluating the alkyl side chains and methylene bridged linkages in lignites [7–10], developing RICO of lignites is limited by the high cost

⁎

of RuCl3. Aqueous NaOCl and H2O2 solutions have been considered to be cheap, clean and ecofriendly oxidants for LO to manufacture SMFAs and BCAs [1,12]. As for LO in aqueous NaOCl solution, a lot of chlorosubstituted compounds were generated accompanied with SMFAs and BCAs because chlorine in NaOCl participated in the reaction [9,11,12], causing problems in separating target CAs from the by-products as well as structure evaluation of lignites based on the resulting CAs. Lignite oxidation in H2O2 aqueous solution can avoid this drawback without other elements involved except H and O, and H2O2 presented good reactivity toward lignites for the production of SMFAs like malonic and succinic acids [9,13–16], along with relatively low yields of BCAs [9,16]. BCAs are more valuable than SMFAs but more difficultly produced from lignite oxidation [17]. Soluble organic species inherently present in lignites or released from the cleavage of weak covalent bonds have been extensively studied [18–21]. Lignite-derived residues (LDRs) are much simpler in chemical compositions than lignites themselves and contain highly condensed aromatic rings (ARs) which could be converted into valuable BCAs or polycyclic aromatic CAs (PCACAs, with ARs > 1) through RICO [22,23]. Therefore, it is feasible to produce valuable aromatic CAs (ACAs) from LDRs by oxidation in aqueous H2O2 solution. In this study, Xianfeng lignite-derived residue (XLDR) was subjected to mild

Corresponding author. E-mail address: [email protected] (Z.-M. Zong).

https://doi.org/10.1016/j.fuproc.2018.09.006 Received 7 July 2018; Received in revised form 8 September 2018; Accepted 8 September 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.

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software. Quantitative analysis of individual compounds was calculated based on the dry and ash-free mass (mdaf) of XL using different types of compounds as external standards. The total yield of a given group component is equal to the sum of yield of all the detected individual compounds in the group.

Table 1 Proximate and ultimate analyses (wt%) of XL and XLDR a. Sample

XL XLDR

Proximate analysis

Ultimate analysis (daf)

Mad

Ad

Vdaf

C

H

N

Odiff

25.67 14.56

18.45 37.93

36.52

63.07 70.09

6.01 3.54

1.79 3.05

28.73 22.55

St,d

H/C

0.40 0.77

1.1355 0.6018

3. Results and discussion

a

daf: dry and ash-free base; Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); diff: by difference; St,d: total sulfur (dry base).

3.1. Functional group distributions of XLDR Different bands in the FTIR spectrum were assigned to corresponding functional groups according to literatures [24–26]. As shown in Fig. S2, strong absorbance of aromatic > C=C < stretching vibration can be observed around 1593 cm−1, suggesting that XLDR is rich in ARs which has low H/C atomic ratio as confirmed by ultimate analysis (Table 1). The -CH3 & > CH2 stretching vibration around 2920 and 2867 cm−1 and -CH3 & -CH2- bending vibration around 1438, 1382, and 1355 cm−1 indicate the presence of aliphatic moieties in XLDR. The broad -OH stretching vibration around 3183 cm−1 and aromatic or aliphatic > C-OH vibration around 1273 cm−1 suggests the presence of OH-containing moieties in XLDR. The aryl or alkyl ether moieties are present in XLDR as demonstrated by the > C-O-C < vibration around 1166, 1118, and 1035 cm−1. The C 1s spectrum from the analysis with XRPES was adopted to illustrate the oxygen-functional groups in coals [21,27]. As shown in Fig. S3 and Table S1, it was divided into 4 peaks assigned to aliphatic or aromatic carbon, C-O or > C-OH, > C=O, and -COOH, respectively. Aliphatic and aromatic carbons are predominant in XLDR which is consistent with FTIR analysis. XLDR has relatively lower contents of oxygen-functional groups than XL (Table 1). The > C-OH or > C-Ogroups could originate from arenols, alkanols, or ethers, and the > C=O could be present in esters and ketones. The -COOH also exists in XLDR.

oxidation in aqueous H2O2 solution for selectively producing valuable ACAs followed by characterization with 2 mass spectrometers. The structural characteristics of the ARs in XLDR were evaluated based on the resulting ACAs. 2. Materials and methods 2.1. Coal sample and reagents Xianfeng lignite (XL) was collected from Xianfeng Coal Mine in Yunnan Province, China and grinded to powders with particle sizes smaller than 74 μm followed by dessication in a vacuum oven. XLDR was obtained from XL through sequential ultrasonic extraction followed by sequential thermal dissolution [23]. The yield of XLDR is 46.2 wt% on the dry and ash-free basis of XL. XLDR is rich in condensed or polycyclic ARs which are precursors of ACAs. The proximate and ultimate analyses of XL and XLDR are presented in Table 1. Aqueous H2O2 solution (30%), CS2, acetone, anhydrous MgSO4, FeSO4, and CH2N2/ diethyl ether solution used in the experiment are analytical reagents. 2.2. Experimental procedure As presented in Fig. S1, XLDR (0.5 g) and aqueous H2O2 solution (30 mL) were loaded into a spherical flask (250 mL) and stirred at 30 °C for 24 h. The excessive H2O2 was decomposed by adding 1 g FeSO4 into the reaction mixture, which was then filtered using a sintered disc filter funnel. The water in the filtrate was removed by evaporation followed by extracting soluble species (SSs) from the filtrate with isometric CS2/ acetone mixed solvent to obtain the extract solution, which was dried over anhydrous MgSO4 followed by solvent evaporation to obtain the extract. The extract then was underwent methyl esterification with CH2N2/diethyl ether solution to obtain the methyl esterified extract (MEE).

3.2. ACAs produced from the oxidation As displayed in Fig. 1, the strong absorbances around 1741 cm−1 for > C=O and around 1233 cm−1 for > C-O-C < may result from -COOCH3 in the methyl esters in MEE derived from CAs, while relatively weak absorbance of -OH around 3340 cm−1 can be observed, indicating that most of -COOH groups on CAs in the extract were esterified by CH2N2 [9]. Most of the –OH groups may attach on ARs. The absorbances around 2926 and 2858 cm−1 could be assigned to (AGs) with -CH3 in -COOCH3 or other alkyl groups with > CH2 and -CH3. The absorbances of aromatic > C=C < around 1660 and 1551 cm−1 could be derived from the ARs in the ACAs, which are much weaker than those in XLDR (Fig. S2), suggesting that highly condensed ARs in XLDR were degraded into ACAs during the oxidation. Small-molecular CAs in the extract are easily detected with GC/MS after methyl esterification by CH2N2 into corresponding methyl esters [9,22]. Hereinafter, CAs rather than methyl esters were used for convenient description. The yield of MEE is 10.7 wt% based on the dry and ash-free mass of XL. As exhibited in Figs. 2 and 3, along with Tables S2–S16 in the Supplementary information, the GC/MS detectable compounds in the extract are subdivided into normal alkanes (NAs), alkanols, nitroanisole (NAs'), N,N-dimethylalkan-1-amines (DMAAs), esters, alkanoic acids (AAs), alkenoic acids (AAs'), alkanetricarboxylic acids (ATCAs), hydroxyalkancarboxylic acids (HACAs), phthalic acids (PAs), benzenetricarboxylic acids (BTCAs), benzenetetracarboxylic acids (BTCAs'), benzenepentacarboxylic acids (BPCAs), other benzene ring-containing carboxylic acids (OBRCCAs), and other compounds (OCs). As Fig. 3 illustrates, BCAs (including PAs, BTCAs, BTCAs', BPCAs, and mellitic acid) have predominance of total yield over the other group components, accounting for 78.3% of all the group components.

2.3. Sample characterizations Fourier transform infrared (FTIR) spectra of XLDR and MEE were obtained by collecting 32 scans at a 4 cm−1 resolution in the 4000–400 cm−1 wavenumber on a Nicolet Magna IR-560 FTIR spectrometer. XLDR was analyzed with a Thermo Fisher ESCALAB 250Xi Xray photoelectron spectrometer (XRPES) and the acquired spectrum was processed with PeakFit software. MEE was analyzed with a Hewlett-Packard 6890/5973 gas chromatography/mass spectrometer (GC/ MS) with a HP-5 MS capillary column (60 m length × 0.25 mm inner diameter × 0.25 μm film thickness) and an IonSense/Agilent 6210 direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer (DARTIS/TOFMS). The GC oven was heated using a temperature program from 60 to 300 °C at 6 °C/min−1 and kept at 300 °C for 10 min during the sample analysis. Compounds assignments were performed by matching the measured mass spectra with NIST11 library database using MSD ChemStation software. The analysis with DARTIS/TOFMS was operated in the positive mode at 450 °C. Helium and nitrogen gases were used as the discharge gas and the alternative gas, respectively. The mass spectrum was processed with MassHunter 92

Fuel Processing Technology 181 (2018) 91–96

988 856 766

1117

3340

2858

1660 1551 1445 1362

2926

Absorbance

1233

1741

F.-J. Liu et al.

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1) Fig. 1. FTIR spectrum of MEE from XLDR oxidation in aqueous H2O2 solution.

This is almost in accordance with the result from RICO of XLDR [23], whereas ADAs, such as malonic and succinic acids, are the predominant products from lignite oxidation in aqueous H2O2 solution [15,16], indicating that most of soluble organic matter in XL was released by ultrasonic extraction and subsequent thermal dissolution, remaining insoluble macromolecular ARs. As listed in Tables S11–S15, nonsubstituted BCAs with 2–6 carboxylic groups on the benzene ring are predominant along with some substituted BCAs. The substituent groups include methyl, hydroxy, methoxy, chloric, and nitro groups. Different types of BCAs are reported to be derived from different aromatic structures in coals through oxidation [28]. As reported previously, the possible precursors of various BCAs were divided into cata-condensed aromatics, polyaryls, and peri-condensed aromatics [28,29]. As Table 2 displays, the much higher yields of benzenepentacarboxylic (BPCA) and mellitic acid (MA) than other BCAs suggest that XLDR may contain abundant peri-condensed aromatics but low contents of cata-condensed aromatics and polyaryls, similar to the results of Jincheng anthracite oxidation in aqueous NaOCl solution [28]. As shown in Fig. 2, the yields of BPCA and MA account for 13.3% and 55.5% in the total yield of all the group components, respectively. Such high selectivity of MA was not reported from coal oxidation to our best knowledge, making it feasible to attain MA in high purity through subsequent separation and purification. Medium pressure liquid chromatography combined with column chromatography could be an effective technique for separating MA from the oxidation products. A series of valuable chemicals with high purity have been separated from coals using this technique [30]. BCAs are value-added chemicals with extensive applications in the fine chemical fields [17]. MA is a useful polydentate ligand and shows flexible ligand properties for synthesizing complexes with metal ions [31,32]. In addition, pendent polyimides derived from MA dianhydride were reported to be applied to aeronautics as well as other high technologies [33]. MA is 72.1 USD·g–1 for sale on the website of Sigma-Aldrich. It was prepared from mineral mellite (i.e., honeystone) by oxidizing graphite or mellitene with

KMnO4 or HNO3, or by electrochemically oxidizing graphite [34,35]. Considering LDRs as wastes, it could be a promising approach for turning the “wastes” into MA by oxidation in aqueous H2O2 solution. In addition, 3 PCACAs were detected in the extract and their possible molecular structures are presented in Fig. S4. DARTIS/TOFMS proved to be feasible for analyzing some ACAs which were difficultly identified with GC/MS [23]. As shown in Fig. 4, the species with m/z 234 + 14n (n = 0–5) are likely to be alkyldihydroindenedicarboxylic acids with 0–5 carbon numbers (CNs) in the AGs. The series of ions with m/z 340 + 14n (n = 0–4) could result from alkylfluorenetricarboxylic acids having 0–4 CNs in the AGs. However, the ion with m/z 368 could also result from BPCA which was confirmed by the analysis with GC/MS. The species with m/z 412 + 14n (n = 0–3) could be alkyldihydroanthracenetetracarboxylic acids with 0–3 CNs in the AGs. Similarly, the ion with m/z 426 could be related to MA. All the 3 series of PCACAs identified with DARTIS/TOFMS contain an aliphatic ring, indicating that XLDR contain condensed hydroaromatic rings. Miura et al. [13] supposed that H2O2 could simultaneously break weak > C-O- bonds and rupture ARs in lignites to generate small-molecular CAs, especially SMFAs. The weak > C-O- bonds in XL could be broken by thermal dissolution with alkanols to form SSs [23]. The condensation degree of ARs in XLDR should be much higher than XL. Therefore, H2O2 could partially rupture condensed ARs and keep some ARs, resulting in ACAs, especially BPCA and MA. Dalal et al. [36] found that surface Fe species in coal mine dust promoted the formation of HO· from H2O2 by Fenton reaction. XLDR with high ash content (Table 1) should contain the Fe species which can catalyze the formation of HO· from H2O2. According to Fenton reaction, Fe2+ can react with H2O2 in aqueous solution, leading to the formation of HO· and HO− [37], and meanwhile Fe2+ is converted to Fe3+.

Fe2 + + H2 O2

Fe3 + + OH· +HO

HO· can attack condensed ARs in XLDR followed by rupturing the condensed ARs to produce ACAs. The detailed mechanism for

Relative abundance (%)

100

HO O

60

O

O OH OH

O OH

OH

HO O

40

OH

HO O

80 HO O

O

OH O

O OH

55.5%

O

13.3% 20 0 5

10

15

20

25 30 Retention time (min)

35

40

45

Fig. 2. Total ion chromatogram of the MEE from the analysis with GC/MS.

93

50

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9

HO O

8

OH

HO O

7

Yield (mg⋅g , daf)

O

O OH OH O

O OH

6 5 4 3 2 1 0

NAs

alkanols

NAs'

DMAAs

esters

AAs

AAs'

ATCAs

HACAs

PAs

BTCAs

BTCAs'

BPCAs

OBRCCAs

OCs

Group component

Fig. 3. Yields of different group components in MEE from analysis with GC/MS.

producing ACAs could be complicated because many radicals are involved and thereby needs further investigation.

Table 2 Possible precursors and yields of the BCAs Precursor cata-Condensed aromatics

BCA

Yield (wt%, daf)

4. Conclusions

0.001

Valuable ACAs were selectively produced from XLDR oxidation in aqueous H2O2 solution under mild conditions. BCAs from XLDR oxidation account for 78.3% of all the group components detected with GC/MS and most of the BCAs are BPCA and MA, suggesting that XLDR contain highly condensed ARs. Three series of PCACAs containing an aliphatic ring were identified with DARTIS/TOFMS. Some Fe species in the ash of XLDR could catalyze the formation of HO· from H2O2. The resulting HO· could play an important role in producing ACAs from XLDR. Future work will focus on optimizing oxidation conditions to maximize the yield and selectivity of BCAs and studying the mechanisms for XLDR oxidation in aqueous H2O2 solution.

0.002

Polyaryls

0.007 0.001

Nomenclature

0.005

peri-Condensed aromatics

AAs alkanoic acids AAs' alkenoic acids ACAs aromatic carboxylic acids AGs alkyl groups ARs aromatic rings ATCAs alkanetricarboxylic acids BCAs benzene carboxylic acids BPCA benzenepentacarboxylic acid BPCAs benzenepentacarboxylic acids BTCAs benzenetricarboxylic acids BTCAs' benzenetetracarboxylic acids CAs carboxylic acids DARTIS/TOFMS direct analysis in a real-time ionization source coupled to a time-of-flight mass spectrometer DMAAs N,N-dimethylalkan-1-amines FTIR Fourier transform infrared GC/MS gas chromatograph/mass spectrometer HACAs hydroxyalkancarboxylic acids LDRs lignite-derived residues LO lignite oxidation

0.002

0.003

0.205

0.830

94

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O

n=3

n=2

n=1

n=0

368

100 O Cn

0 200

382

O Cn

354

304

234

396

340

250

300

350 m/z

400

412

426

4

n=3

n=2

290

n=1

O

n=0

276

2

n=5

n=3

n=2

248 262

40 20

n=1

60

O

n=4

80 n=0

Relative abundance (%)

3

n=4

O

Cn

440

454

450

500

Fig. 4. Mass spectrum of MEE from the analysis with DARTIS/TOFMS and possible molecular structures of the 3 series of ions.

MAs mellitic acid MEE methyl esterified extract NAs normal alkanes NAs' nitroanisoles OBRCCAs other benzene ring-containing carboxylic acids OCs other compounds PCACAs polycyclic aromatic carboxylic acids PAs phthalic acids RICO ruthenium ion-catalyzed oxidation SMFAs small-molecular fatty acids SSs soluble species XLDR Xianfeng lignite-derived residue XL Xianfeng lignite XRPES X-ray photoelectron spectrometer

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