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Insights into physicochemical changes of Yinggemajianfeng lignite in co-solvents of ionic liquids and methanol Sheng-Kang Wang, Xian-Yong Wei, Sheng Li, and Zhi-Min Zong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03956 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019
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Energy & Fuels
1
Insights into physicochemical changes of Yinggemajianfeng lignite in
2
co-solvents of ionic liquids and methanol
3
Sheng-Kang Wang a, Xian-Yong Wei a,b,, Sheng Li a, and Zhi-Min Zong a
4
a
5
University of Mining & Technology, Xuzhou 221116, Jiangsu, China
6
b
7
Ningxia University, Yinchuan 750021, Ningxia, China
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China
State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering,
Abstract: co-solvent
Yinggemajianfeng lignite (YL) was subjected to thermal dissolution (TD) in a of
an
ionic
liquid
(IL,
1-butyl-3-methyl-imidazolium
chloride,
1-butyl-3-
methylimidazolium tetrafluoroborate, or 1-ethyl-3-methylimidazolium acetate) and methanol at 70 oC.
YL and treated YL (TYL) samples were analyzed with a scanning electron microscope (SEM),
thermogravimetric analyzer, solid-state
13C
nuclear magnetic resonance, and Fourier transform
infrared spectrometer. The results show that the yield of soluble portion from each TYL with a co-solvent is much higher than that with an IL alone due to the decrease in the solvent viscosity and increase in the interactions between the IL and YL. The particles in the TYL samples are smaller than those in YL based on the observation with the SEM and the TYL samples have more mass loss than YL during pyrolysis, suggesting that the TD destroyed the texture and structure of YL to some extent. Compared to YL, the TYL samples have more aliphatic carbons. The physicochemical changes of TYL samples indicate that the co-solvents are effective for treating coals and TYL samples are more appropriate for liquefaction and pyrolysis. 8
Keywords: Co-solvent; Ionic liquid; Lignite; Physicochemical changes
9
1. INTRODUCTION
10
Coal extraction is a traditional and useful way to obtain value-added chemicals and insight into
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structure
of
coals
[1-3].
Traditional
organic
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11
the
solvents,
such
as
pyridine
and
12
N-methyl-2-pyrrolidinone, are regarded as environmentally unfriendly solvents because of their
13
virulence and high boiling points [4,5]. Moreover, common organic solvents with low boiling point,
14
such as methanol, petroleum ether, and acetone, were widely applied in the extraction of coals and
15
their derivates [3,6,7], but the extract yields of raw materials with these solvents are low under mild
16
conditions.
17
Ionic liquids (ILs) possess relative low vapour pressure, high chemical and thermal stability, and
18
the ability to be designed by altering their cation-anion combination [8,9]. They were used in
19
investigating the structural features of biomass by disrupting hydrogen bonds in biomass [10-13]. In
20
contrast, the investigations on the interactions between coals and ILs are still in initial step [8,9,14].
21
Lei
22
1-butyl-3-methyl-imidazolium chloride (ILa). They found that ILa could break non-covalent
23
interactions in coals. Cummings et al. [9] selected 4 ILs to deal with 2 lignites. Their results
24
indicate that the lignite particle sizes decreased by treatment with the ILs via fragmentation and
25
swelling of the lignites.
26
[8]
et
al.
investigated
the
extraction
behaviour
of
Xianfeng
lignite
with
In this work, we designed a co-solvent system consisting of ILs and common solvents to solve
27
the
problem
of
ILs
high
viscosity.
Yinggemajianfeng
lignite
(YL),
ILa,
28
1-butyl-3-methylimidazolium tetrafluoroborate (ILb), 1-ethyl-3-methylimidazolium acetate (ILc),
29
and methanol, were selected to explore the physical and chemical changes of YL after treatment
30
with the co-solvent system.
31
2. RESULTS AND DISCUSSION
32
2.1. Extraction behaviors under different conditions. As Table 2 lists, the recovery of
33
each TYL with an IL is higher than that from TYL only with methanol (TYLM) and the recoveries ACS Paragon Plus Environment
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Energy & Fuels
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of TYL samples with ILa (TYLILa/M) and ILc (TYLILc/M) are over 100%, implying that ILa and ILc
35
remain in the TYL samples to some extent. Although the recovery of TYL with ILb (TYLILb/M) is
36
less than 100%, the remaining of ILb in TYLILb/M is inevitable. As demonstrated in Figure 1, the
37
yields of soluble portion (SP) from TYL samples with the ILs are much higher than that of YL and
38
TYLM, suggesting that the co-solvent system can improve YL dissolution, which can be ascribed to
39
the capture of strong hydrogen bonds among some species in YL to release the species into SP. For
40
TYLILc/M, SP yield increases with increasing methanol volume (MV) from 0 to 20 mL and reach a
41
peak at 20 mL with the yield of 13.7%, then decreases to 12.3% at MV of 40 mL. With increasing
42
MV from 10 to 40 mL, SP yield from TYLILb/M increases gradually. Interestingly, SP yield from
43
TYLILa/M decreases with increasing MV from 10 to 40 mL. SP yields from TYLIL/M increase in the
44
order: ILa (9.7%) < ILb (10.8%) < ILc (13.7%). In addition, more ILc remain in TYL samples
45
compared to ILb, indicating that ILb could be a suitable IL to process YL.
46
2.2. Change in particle size. SEM was adopted to view the morphological changes in TYL
47
samples [15]. As Figure 2 shows, the particles in TYL samples are clearly smaller than those in YL,
48
indicating that the particles in TY were appreciably broken the treatment with co-solvents. The
49
particle sizes in TY, TYLM, and TYLIL/M primarily range from 50 to 110 m, 30-100 m, and 20-90
50
m, respectively, making TYL samples more suitable for liquefaction and pyrolysis than YL.
51
Furthermore, the surface of particles in TYL samples is rougher than that in YL, indicating that the
52
pore structures of TYL samples become larger as displayed in Figure 2.
53
2.3. Effect of the co-solvent system on the thermochemical behavior of the TYL
54
samples. As shown in Figure 3, the total mass losses of the 5 samples at 900 oC increase in the
55
order: YL (55.9%) < TYLM (57.3%) < TYLILa/M (57.5%) < TYLILb/M (58.1%) < TYLILc/M (61.2%).
56
Referring to previous reports [16,17], the mass losses before 330
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oC
are mainly from
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decarboxylation and the release of bound water and some small molecules with weak bonds.
58
Notably, the remaining mass of each TYL at temperatures lower than 250 oC is more than that in
59
YL, which could be due to the strong hydrogen bonds generated from residual ILs with water or
60
with heteroatom-containing organic species (HACOSs) in TYL samples. In contrast, much more
61
masses of TYL samples lost than YL at temperatures higher than 320 oC.
62
The DTG curves in Figure 3 exhibit that YL pyrolysis was significantly enhanced at 441 oC due
63
to both the cleavage of relatively weak covalent bonds and the dehydroxylation of kaolinite [17]. At
64
315-325 oC, the DTG curves of TYL samples with ILs show a peak owing to the decomposition of
65
residual IL in the TYL samples at temperatures > 250 oC [15,18] and the breakage of interaction
66
between ILs and some macromolecules in TYL samples. Then, the massess of TYLILa/M, TYLILb/M,
67
and TYLILc/M decrease rapidly with raising temperature to 437, 436, and 435 oC, respectively, which
68
is similar to YL and TYLM.
69
2.4. Effect of the co-solvent system on TYL composition. The 13C NMR spectra of coals
70
are usually classified into 3 main chemical regions: aliphatic carbons (0-90 ppm), aromatic carbons
71
(90-170 ppm), and carbonyl carbons (170-220 ppm) [19-21]. As displayed in Tables 3 and 4, 12
72
carbon types are differentiated and on the whole, there is little difference among the TYL samples
73
compared to YL. The differences in relative content (RC) of aromatic, aliphatic, and carbonyl
74
carbons between YL and TYL samples are not over 1.5%, 3.5%, and 2.5%, respectively (Table 4).
75
However, methine carbons were not detected in YL and TYLM and the RCs of oxy-methylene
76
carbons in TYLIL/M samples increased compared to YL (Table 3). Usually, IL consists of cation and
77
anion which have large difference in charge density. Therefore, IL can disrupt the cross linked
78
network of coals [22-24] and release more organic species, especially HACOSs. Moreover, the RC
79
of carbonyl carbons in each TYLIL/M is lower than that in YL, suggesting that carbonyl-containing
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Energy & Fuels
compounds could form strong hydrogen bonds with ILs or water so that they are easily released. The RC of protonated aromatic carbons in each TYLIL/M is more than that in YL, while other 3 aromatic carbons in each TYLIL/M are less than those in YL. The molar fraction of aromatic bridgehead carbon (χb) of YL is 0.29 (Table 4), similar to anthracene or phenanthrene, indicating that the average number of aromatic rings (ARs) per cluster in YL is 3. The average carbon number in methylene chain (Cn) of YL is 2.34, implying that lots of short bridged bonds or side-chains exist in the carbon skeletal structure of YL [25,26]. Since the average substituted degree of aromatic ring (σ) in YL is 0.38, the average number of substituents on each aromatic ring is 2.28. Interestingly, the χb in each TYLIL/M is ca. 0.2, which matches with naphthalene. Also, the Cn of each TYLIL/M is higher than that of YL and the σ of each TYLIL/M is lower than that in YL, implying that the treatment of YL with each co-solvent could release many species with less substituents and/or aromatic bridgehead bonds with longer methylene chains, further proving that the RCs of protonated aromatic carbons in the TYL samples increase. As Figure 4 illustrates, the strong absorbances around 3556, 3485, 3412, and 3235 cm-1 indicate that rich existence of -OH and hydrogen bonds in YL and TYL samples [17,22]. The absorbances around 2924 and 2852 cm-1 from aliphatic moieties (AMs) in TYL samples are slightly stronger than those in YL, which is consistent with the change in RC of aliphaticity index (fal) (Table 4). The strong absorbances around 1638 and 1617 cm-1 result from aromatic moieties. The absorbance from -CH2 bending vibration around 1449 cm-1 is enhanced in TYLM, TYLILb/M, and TYLILc/M compared to YL. The weak absorbance from >C-O- moieties around 1036 cm-1 in each TYL is appreciably stronger than that in YL, corresponding to the SS 13C NMR analysis. The weak absorbance around 1165 cm-1 results from >C=O groups, while the strong absorbances around 621 and 467 cm-1 could be ascribed to mineral matter.
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2.5. The analysis of recovered ILs. The average recoveries of TYLILa/M, TYLILb/M, and
104
TYLILc/M are 72%, 79%, and 84%, respectively, suggesting the loss of different ILs to different
105
extents. Because the treatment was conducted at 70 oC, the fresh ILs are almost the same as the
106
corresponding recovered ILs, as shown in Figure S6. The results indicate that the ILs can be
107
effectively recovered.
108
3. CONCLUSIONS
109
The particles in each TYL are obviously smaller than those in YL. The co-solvent treatment
110
destroy some cross linked networks in YL to release more SP. Each TYL is more reactive than YL
111
toward pyrolysis. Each cluster in YL has 3 ARs on average and each AR has ca. 2 substituents.
112
Overall, the approach with the co-solvent system, which is superior to the system only with IL, is
113
effective for coal treatment.
114
ASSOCIATED CONTENT
115
Supporting Information
116
Including the SS
13C
NMR spectra and their fitting curves of YL and TYL samples (Figures
117
S1-S5) and FTIR spectra of ILs and recovered ILs (Figure 6). This material is available free of
118
charge via the Internet at http://pubs.acs.org.
119
AUTHOR INFORMATION
120
Corresponding Author
121
*Telephone: +86 (516)83884399 (X.-Y.W.). E-mail:
[email protected] (X.-Y.W.).
122
Notes
123
The authors declare no competing financial interest.
124
ACKNOWLEDGEMENTS
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This work was supported by the Fundamental Research Funds for the Central Universities (Grant
126
2017BSCXB27) and the Research and the Postgraduate Research & Practice Innovation Program of
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Jiangsu Province (Grant KYCX17_1507).
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200, 282-289. (26) Wang, S. K.; Wei, X. Y.; Zong, Z. M. Insight into the structural features of organic species in Fushun oil shale via thermal dissolution. Chin. J. Chem. Eng. 2018, 26 (10), 2162-2168. Table 1. Proximate and Ultimate Analyses (wt%) of YL proximate analysis ultimate analysis (daf) VMdaf C H N S Mad Ad 5.45 9.74 55.84 59.90 4.81 0.86 0.55
H/C
Oa 33.88
0.96
198 199
daf: dry and ash-free base; Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); VMdaf: volatile matter (dry and ash-free base); a by difference.
200
Table 2. Recoveries of TYL under Different Conditions IL methanol (mL) YL (g) none 10 0.9996 ILa 10 1.0017 ILa 20 1.0022 ILa 40 1.0022 ILb 0 0.9989 ILb 10 1.0020 ILb 20 1.0025 ILb 40 1.0019 ILc 0 1.0005 ILc 10 1.0005 ILc 20 0.9980 ILc 40 1.0014
TYL (g) 0.9784 1.0152 1.0231 1.0110 0.9937 0.9921 0.9822 1.0022 1.0538 1.0911 1.0793 1.0793
recovery (%) 97.9 101.3 102.1 100.9 99.5 99.0 98.0 100.0 105.3 109.1 108.1 107.8
201
ILa 10 g, ILb 10 mL, ILc 10 mL
202 203
Table 3. Distribution of different carbon types in YL and TYLs determined by SS analysis molar content (%) chemical shift (ppm) carbon type YL TYLM TYLILa TYLILb aliphatic 11-13 f 1 al 5.33 5.55 6.35 5.80 19-21 f a al 9.35 9.38 9.76 9.84 29-30 f 2 al 22.39 23.13 21.01 21.41 38-39 f 3 al 5.78 4.18 41-45 f 4 al 6.00 5.60 1.95 2.88 49-58 f o al 1.84 1.75 3.48 3.84 aromatic 102-125 fHa 17.13 15.04 26.28 26.67 128-134 fba 14.77 15.88 10.47 10.12 140-144 fsa 9.56 9.74 6.90 6.88 151-166 foa 9.97 9.41 6.54 7.08 carbonyl 171-180 f C1 a 2.44 3.11 1.48 1.30 206-219 f C2 a 1.22 1.41
204 205 206
13C
NMR
TYLILc 6.73 9.77 20.16 5.94 1.96 3.73 26.10 10.57 6.66 6.73 1.65
f 1 al: aliphatic CH3; f a al: aromatic CH3; f 2 al: methylene; f 3 al: methine; f 4 al: quaternary; f o al: oxy-methylene; f H a: protonated aromatic carbon; f b a: aromatic bridgehead; f s a: aromatic branched; f o a:
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oxygen-substituted aromatic; f C1 a: carbonyl in carboxyl and ester; f C2 a: carbonyl in ketone and aldehyde.
208 209
Table 4. Structural Parameter Values (%) of YL and TYLs Determined by SS Analysis sample fa fal fCa χb Cn YL 51.43 44.91 3.66 29 234 TYLM 50.07 45.41 4.52 32 237 TYLILa 50.19 48.33 1.48 21 304 TYLILb 50.75 47.95 1.30 20 311 TYLILc 50.06 48.29 1.65 21 303
210 211 212 213
13C
NMR
σ 38 38 27 28 27
Aromaticity index fa = f H a+ f b a+ f s a+ f o a; aliphaticity index fal = f 1 al+ f a al+ f 2 al+ f 3 al+ f 4 al+ f o al; ratio of carbonyl carbon f C a= f C1 a+ f C2 a; molar fraction of aromatic bridgehead carbon χb = f b a/fa; average carbon number in methylene chain Cn = f 2 al/f s a; average substituted degree of aromatic ring σ = (f o a + f s a)/fa.
214
14 TYLILc/M
SP yield (wt%, daf)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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12
8
216 217
TYLILb/M
6 4 1:0:10
215
TYLILa/M
10
1:10:0
1:10:10 1:10:20 YL:IL:methanol
1:10:40
Figure 1. The yields of SP in YL and TYLs extracted by methanol (YL: g; IL: g or mL; methanol: mL).
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Figure 2. Scanning electron micrographs of YL and TYLs (TYLM: YL only treated with methanol; TYLILa/M, TYLILb/M, and TYLILc/M represent the TYLs (YL:IL:methanol = 1:10:20)). ACS Paragon Plus Environment
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TYLM
YL
TYLILa/M
TYLILb/M
TYLILc/M
TG (%)
87 74 61 48 35 -0.1 -0.4 DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 222 44 223 45 224 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
-0.7 -1.0 -1.3 -1.6 -1.9 120
250
380 510 640 Temperature (oC)
770
900
Figure 3. TG/DTG curves of YL and the TYL samples (TYLM: YL only treated with methanol; TYLILa/M, TYLILb/M, and TYLILc/M represent the TYLs (YL:IL:methanol = 1:10:20)).
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TYLILc/M
TYLILb/M
Transmittance
TYLILa/M
621
467
1036
1165
1449
YL 1638 1617
2924 2852
3235
TYLM
3556 3485 3412
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
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4000 226 227 228
3600
3200
2800
2400 2000 Wavenumber (cm-1)
1600
1200
800
Figure 4. FTIR spectra of YL and TYLs (TYLM: YL only treated with methanol; TYLILa/M, TYLILb/M, and TYLILc/M represent the TYLs (YL:IL:methanol = 1:10:20)). ACS Paragon Plus Environment
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