ARTICLE pubs.acs.org/IECR
Study on Extraction Asphaltenes from Direct Coal Liquefaction Residue with Ionic Liquids Yi Nie,† Lu Bai,† Yi Li,† Haifeng Dong,† Xiangping Zhang,*,† and Suojiang Zhang*,† †
State Key Lab of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Direct coal liquefaction residue (DCLR) contains about 25% asphaltenes which are proved to be important precursors for preparing high value-added carbon materials. In this work, ionic liquids (ILs) were used as potential solvents to extract asphaltenes from DCLR, and a series of dialkylphosphate ILs, i.e., imidazolium-based, pyridinium-based, and ammonium-based, were synthesized and used to extract asphaltenes from DCLR. The influences of extractive time, extractive temperature, and mass ratio of ILs to DCLR on extraction efficiency of asphaltenes were investigated and the optimized conditions were determined. In order to understand the mechanism of extraction asphaltenes with ILs, the extracts were characterized by elemental analysis, FT-IR, 13 CNMR, and so on. The results show that it is feasible to extract asphaltenes from DCLR with dialkylphosphate ILs. The structure and size of anion and cation of ILs probably are the main factors that influence the extraction yield and the physicochemical characteristics of extracted asphaltenes, such as atomic ratio of H/C, the structure and aromatic cluster size, and so on.
’ INTRODUCTION Direct coal liquefaction (DCL) refers to conversion of coal into liquids fuels and chemicals at 400470 °C and an elevated H2 pressure. DCL residue (DCLR) formed as byproduct usually accounts for 2030 wt % coal that fed into the liquefaction reactor1 and contains about 30 wt % heavy oil, 25 wt % asphaltenes2 which are a highly aromatic, polydisperse mixture.3 DCLR has been frequently designated as a feed stoke for gasification or combustion, which is low of market value and causes difficulties to the feed system due to the presence of asphaltenes.4 Methods have been developed for high-value use of DCLR, including direct use as asphalt or asphalt modifiers,5 preparation of advanced carbon materials,6,7 and production of marketable fuels through hydrogenation.1 DCLR is abundant of polycyclic aromatics of asphaltenes and resource of fragrant carbon and might be used effectively as a value-added carbon resource from the viewpoints of the resource conservation and economy. Therefore, the efficient separation of asphaltenes from DCLR is necessary. Some traditional polar hydrocarbon solvents, such as tetrahydrofuran, pyridinium, N,N-dimethylacetamide (DMA), and N-methyl pyrrolidinone, have the ability to dissolve DCLR and enable the extraction of soluble materials due to their acceptor groups that form strong hydrogen bonds, similar to how these solvents are used to swell and dissolve the nonanthracitic coal.8,9 However, these traditional solvents are volatile, toxic, and difficult to recycle. Hence, it is significant to exploit new solvents with better extractive performance for asphaltenes. Ionic liquids (ILs) have been used as potential solvents for a wider variety of organic and inorganic chemical compounds because of their low vapor pressure, unique dissolubility, and good thermostability.1012 In addition, ILs are “designable” because structural modifications in both the cation and anion permit the tuning of properties, e.g., miscibility with organic solvents, melting point, and viscosity.13 Previous studies showed that the cations and anions connect to each other to form a hydrogen-bonded network in ILs, and it has been recognized that r 2011 American Chemical Society
the hydrogen bonds have a significant influence on the physical properties of ionic liquids.14 Therefore, to design or select reasonable ILs for extraction asphaltenes from DCLR is feasible. Dialkylphosphate ILs have been proven to interact with aromatic sulfur compounds selectively.15,16 Benzene and many other aromatic compounds are remarkably soluble in but rarely completely miscible with ionic liquids.17 Considering the aromatic structure characteristic of asphaltenes, in this study, a series of dialkylphosphate ILs were synthesized and investigated to dissolve and extract asphaltenes from DCLR. In order to have a better insight into the structurefunction relationship of this series of ILs with respect to the extractive performance, imidazolium, pyridinium, and ammonium as the IL cations were selected in determining a suitable IL for the extraction asphaltenes from DCLR. Extracts were characterized via many analytical methods to determine their components and structures for furthermore modification requiring a thorough understanding of physicochemical characteristics of asphaltenes. The research indicates that the structure and size of anion and cation of ILs probably are the main factors to influence the physicochemical characteristics of asphaltenes. This study may be referred for the efficient separation of asphaltenes from DCLR.
’ EXPERIMENTAL SECTION Properties of DCLR. DCLR samples used in the present study were supplied by the Shenhua Group of China. The true density of DCLR is 1.59 g/cm3, and the softening point is 148 °C. The content of C, H, N, S, O element is 84.1, 6.40, 0.91, 3.07, 4.97%. The DCLR was ground to sizes smaller than 0.5 mm before dissolution. Received: June 3, 2011 Accepted: August 2, 2011 Revised: July 17, 2011 Published: August 02, 2011 10278
dx.doi.org/10.1021/ie201187m | Ind. Eng. Chem. Res. 2011, 50, 10278–10282
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Scheme 1. Chemical Structures of the Dialkylphosphate ILs Studied
Preparation of Ionic Liquids. All dialkylphosphate ionic liquids (their structures are illustrated in Scheme 1) were prepared by reacting N-methylimidazole (3-picoline, triethylamine) and the corresponding trialkyl phosphate at 423 K for 12 h according to the published procedure.15,16 The resulting viscous liquids were washed several times with diethyl ether at room temperature followed by rotary evaporation under reduced pressure for 12 h to remove all volatile residues (e.g., the reactants unreacted and diethyl ether). The purity and structure of these ILs were identified by HNMR and electronic spray mass spectrum. Extraction Asphaltenes from DCLR. A known weight of DCLR was added into the ILs in a conical flask and heated at 3050 °C under vigorous magnetic stirring for 1030 min. Then, the insoluble materials were separated by vacuum filter through a nylon filter membrane (particle retention size was 2.5 μm). Soluble matter can easily be precipitated from the IL solution using water as a back-extractant. The precipitation occurs when a 10-fold amount deionized water was added into the filtrate. The solid/liquid mixture was separated using vacuum filter and washed thoroughly with the water to remove the ILs adhered to the extracts. The extracts were dried overnight at 70 °C. Sample Characterization. The extracts were characterized for physical and chemical properties using a series of analytical tools, including Elementary Analysis, Fourier Transfer Infrared Spectroscopy (FT-IR), Solid 13C-Nuclear Magnetic Resonance Spectroscopy (13CNMR), Thermo Gravimetric Analysis (TGA), and Optical Microscopy (OM).
’ RESULTS AND DISSCUSSION The physical and chemical properties of asphaltenes are critically important to extractive performance of ILs and further modification of asphaltenes. Therefore, H/C, functional groups, aromaticity, aromatic cluster size, quinoline insolubles, softening point, and so on were determined as follows. 1. Determination of Extraction Conditions. Asphaltenes were extracted from DCLR with dialkylphosphate ILs through extracting and re-extracting. The extraction conditions, such as time, temperature, and mass ratio of ILs to DCLR were investigated with 1-ethyl-3-methylimidazolium diethylphosphate [EMIM]DEP. The optimized conditions were determined by comparison of yield and atomic ratio of H/C. Elemental analyses of the extracts were carried out with a microanalyzer LECO
Table 1. Yield and H/C of Asphaltenes at Different Conditionsc temp
time
(°C)
(min) yield (%) C (%) H (%) N (%) O (%) S (%) H/C
50
10
16.43
85.08
6.08
1.90
6.83
0.11
0.858
50
20
17.08
87.62
6.05
1.64
4.51
0.18
0.829
50 30
30 20
17.84 14.82
87.40 85.74
6.01 6.05
1.56 1.74
4.89 6.19
0.14 0.28
0.825 0.847
50a
20
13.07
83.29
5.85
1.73
8.97
0.16
0.843
50b
20
16.68
85.42
6.09
1.88
6.48
0.13
0.856
a
IL:DCLR = 6:1. b IL:DCLR = 8:1. c Conditions: [EMIM]DEP;IL: DCLR = 10:1; water:IL = 10:1.
CHNS-932 to determine C, H, N, and S contents. O was calculated by difference. The results were shown in Table 1. The results indicate that for [EMIM]DEP the asphaltenes yield increases with increasing extractive temperature, time, and mass ratio of IL to DCLR. H/C changes with the change of temperature, time, and mass ratio. The yield at 50 °C is a bit higher than that at 30 °C, and the yield for 20 min is close to that for 30 min, higher than that for 10 min. The yield is higher at mass ratio of IL to DCLR being 10:1. At last, extractive conditions, including 50 °C, 20 min, and 10:1 of mass ratio, were determined considering viscosities of ILs, quick separation using vacuum filter, yield, and H/C of the extracts. The regeneration of [EMIM]DEP can be performed by evaporating water from the final filtrate. The asphaltenes H/C (being 0.823) of recycled ILs remains at the same level as that of a fresh IL sample. 2. Sample Characterization. 2.1. Yield and Elemental Analysis. Extraction yields of 6 ILs for DCLR were listed in Table 2 and are lower than yields of DMA (50.12%, DMA:DCLR = 4:1, 60 min, 120 °C) and furfural (46.12%, furfural:DCLR = 4:1, 60 min, 20 °C).8 Elemental analyses of the different extracts obtained from DCLR using different ILs were determined. The H/C ratio which is a good indication of the degree of the aromaticity of asphaltenes was calculated. The data in Table 2 show that sulfur content in extracts is lower than 0.5%. The result indicates most of the sulfur compounds in DCLR can almost be removed after extraction with ILs. Sulfur removal can avoid crystal swelling in 10279
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Table 2. Yields and Elemental Analysis of DCLR Extractsa ILs
a
yield (%) C (%) H (%) N (%) O (%) S (%) H/C
[MMIM]DMP
3.63
85.56
5.94
1.97
6.40
0.13
0.833
[EMIM]DEP
17.08
87.62
6.05
1.64
4.51
0.18
0.829
[BMIM]DBP
22.46
84.64
6.36
1.75
7.09
0.16
0.902
[M3MPy]DMP
23.74
81.38
5.79
2.17
10.31 0.36
0.854
[E3MPy]DEP
29.80
84.54
6.11
1.96
7.25
0.14
0.867
[MTEtA]DMP
23.76
86.32
5.81
1.58
6.20
0.094 0.808
Conditions: IL:DCLR = 10:1; Water:IL = 10:1; 20 min; 50 °C.
preparation of carbon materials, which meets with the requirements of raw materials for modification. Structure and size of cation and anion of ILs has effects on the yield and H/C of asphaltenes. For imidazolium-based and pyridinium-based ILs, the bigger the cation and anion is, the higher the yield and H/C is. For the same anion [DMP], ammonium-based IL has the higher extraction yield and has the lower H/C. Considering viscosities and price of ILs, yield, and H/C of extracts, [MTEtA]DMP was selected to be promising ILs to extracts asphaltenes from DCLR. As coal-derived asphaltenes, aside from polycyclic aromatic hydrocarbon rings, the hydrogen bonds in coal, involving phenolic OH, carboxylic acids, and other groups containing heteroatoms,9 are still maintained or enriched because of the hydroprocessing of coal. Since they possess both hydrogen bond donor and acceptor sites, asphaltenes can readily bond two or several molecules.18 ILs are proved to be strong acceptors of hydrogen bonds.18,19 The above properties of asphaltenes and ILs should be taken into account in the analysis of the mechanism of interactions of ILs with asphaltenes or asphaltenes aggregates. The ability of dialkylphosphate ILs to dissolve asphaltenes might be related to the competitive intermolecular interactions of different strength between the IL cation and anion and asphaltenes molecules, such as hydrogen bonds, π-cation interactions, which can disrupt interactions between asphaltenes molecules.9,18,2023 The problem of hydrogen bonding is crucial for understanding the solvation of dissolved particles, because the interaction of a solvating ion with the solute has to compete with the interaction with the counterions. In particular, it seems possible to control the solvation capability of ions by varying the counterions.21 The strength and extent of the interactions between asphaltenes and ILs will vary with the nature and size of cation and anion. Bulky IL group plays a role in the inhibition the aggregation of asphaltenes. For imidazolium-based, pyridinium-based ILs, on one hand, as the increase of the cation size, the Coulombic interaction between cation and anion decreases, and π-cation interaction between asphaltenes and the imidazolium ring of ILs increases. On the other hand, alkyl is an electrons donating group to the aromatic π system,24 as the size of cation increases, π-cation interaction between asphaltenes and the imidazolium ring of ILs increases. It should be pointed out that the mechanism of the interactions between asphaltenes and ILs is complex and should be studied furthermore for designing and selecting excellent ILs in extraction asphaltenes from DCLR. 2.2. FT-IR. The functional groups of the extracts were examined by FT-IR using the standard procedure, and spectra were shown in Figure 1. FT-IR spectra of extracts with different ILs are similar. The bands near 3050 cm1 and 1600 cm1 are due to
Figure 1. FT-IR spectra of extracts from DCLR.
Figure 2.
13
CNMR spectra of extracts.
aromatic CH stretching vibrations, and the well-defined peak at ca. 2920 cm1 clearly arises from the aliphatic CH stretching vibrations of alkyl substituents.25 The band at 750 cm 1 is assigned to 1,2-substituted aromatic rings. The bands at 1054 cm1 and 1196 cm1 are ascribed to phenoxy and ether RCOCH3 stretching. The formation or enrichment mechanism of these O-containing groups is uncertain, but desired, for the modification of asphaltenes structure can be realized with different ILs. 2.3. 13CNMR. Among the six dialkylphosphate ILs, [MTEtA]DMP has the higher extraction yield and lower H/C. Therefore, the extracted materials with [MTEtA]DMP and [EMIM]DEP were selected as typical samples and characterized using the solid 13 C-nuclear magnetic resonance spectroscopy to speculate the structure and aromatic cluster size. 13CNMR spectra of extracts were listed in Figure 2. The twelve structural parameters for the two extracts are given in Table 3. These structural parameters are then used to estimate the aromatic cluster size. The mole fraction of aromatic bridgehead carbons xb was then calculated, which is important as it can be used to estimate the aromatic cluster size AC/CL. The total molecular weight (MW) of an average cluster was also calculated from the elemental analysis and f 0 a,26,27 and given in Table 4. There is a bit difference in parameters xb, f 0 a, and MW of asphaltenes extracted with [EMIM]DEP and [MTEtA]DMP. Both of the extracts have 21 aromatic carbons per cluster, which is relatively large. It is anticipated that such structural information will be valuable in modification of asphaltenes. 10280
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Table 3. Carbon Structural Distribution of the Extractsa IL
fa
fal
faC
f 0a
faH
faN
faP
faS
faB
falO
falH
fal*
Ha
[MTEtA]DMP
0.641
0.279
0.081
0.560
0.150
0.410
0.019
0.113
0.278
0.023
0.149
0.068
0.186
[EMIM]DEP
0.638
0.283
0.079
0.559
0.143
0.416
0.022
0.128
0.266
0.013
0.149
0.081
0.172
a Fractions of sp2-hybridized carbon: fa = total carbon; f 0 a = in an aromatic ring; faC = carbonyl,δ > 165ppm ; faH = protonated and aromatic; faN = nonprotonated and aromatic; faP = phenolic or phenolic ether, δ = 150165 ppm; faS = alkylated atomatic, δ = 135150 ppm; faB = aromatic bridgehead. Fractions of sp3-hybridized carbon: fal = total carbon; falH = CH or CH2; fal* = CH3 or nonprotonated; falO = bonded to oxygen, δ = 5090 ppm; Ha: fraction of hydrogen in an aromatic ring.
Table 4. Physicochemical Characteristics of the Extracts with ILsa IL
H/C
xb
AC/CL
MW
QI (%)
A (%)
density (g/cm3)
SP (°C)
[MTEtA]DMP
0.808
0.496
21
539.9
0
0.013
1.2239
95
[EMIM]DEP
0.829
0.476
21
553.3
0
0.32
1.2282
76
a
xb: mole fraction of aromatic bridgehead carbons; AC/CL: number of aromatic carbons per cluster; MW: average molecular weight; QI: quinolione insolubles; A: ash content; SP: softening point.
with [MTEtA]DMP is shown in Figure 3. The results in Table 4 show that ash content of sample extracted with [MTEtA]DMP is much lower than that with [EMIM]DEP. QI and ash content of extracts with [MTEtA]DMP are 0% and 0.013%, respectively,8 and much lower than that with DMA. Therefore, extraction DCLR with [MTEtA]DMP has an advantage over conventional solvent DMA. Softening points (SP) were determined using optical microscopy matched with heating stage. SP of asphaltenes extracted with [MTEtA]DMP is 95 °C and is higher than that with [EMIM]DEP. This trend is consistent with H/C results from the elemental analysis. Figure 3. Proximate analysis for sample extracted with [MTEtA]DMP by TGA.
2.4. Quinoline Insolubles, Ash Contents, and Softening Points. Referring to the evaluation methods of asphaltenes, some important physicochemical characteristics of the extracts were summarized in Table 4. Quinoline insolubles (QI) were quantitative evaluated according to the Chinese standard test method described in GB/T 2293-2008 because QI can delay the commencement of the sphere generation and expands the time period of the generation.28 One g extracts (accuracy 0.0002 g) were dissolved in 25 mL of quinoline and immersed into a homothermal water bath (75 ( 5 °C), and the mixture was stirred for 30 min. The mixture was filtered, and the filter paper was washed by heated quinoline and toluene successively until yellow was not seen. The filter paper was dried and weighed. The results show that there is no QI content in the extracts from DCLR. Ash contents were evaluated using Thermo Gravimetric Analysis under the following temperature program.29 (1) Purging the TGA system with ultrahigh-purity nitrogen at 30 °C for 10 min. (2) Taring and weighing a sample of 1620 mg. (3) Raising the temperature to 110 °C at a heating rate of 20 °C/min and holding for 5 min, the weight loss is defined as moisture. (4) Ramping the temperature to 950 °C at 40 °C/min and holding for 10 min, the weight loss is defined as volatile matter. (5) Introducing air at 950 °C and holding for 30 min, the weight loss is defined as fixed carbon and the remained weight is ash. The weight loss in each TGA step on the sample extracted
’ CONCLUSION It has been demonstrated that dialkylphosphate ILs can be used as effective solvents for extraction asphaltenes from DCLR to a remarkable extent. [MTEtA]DMP exhibits the better extractive performance due to its low viscosity, higher asphaltenes yield, and good characteristics of its extract, such as higher number of aromatic carbons per cluster, higher SP, and the advantage of lower ash content, no QI over DMA. It is postulated that in part this is a result of the ability of ILs to disrupt intermolecular interactions in these asphaltenes. Structure of ILs can be tuned by varying the ring and substituent groups and the nature of anion to increase the extractive performance. ILs might be successfully used to control the physicochemical characteristics of asphaltenes. Our findings may be helpful in separation asphaltenes from DCLR. The research of new ionic liquids that are efficient and economically justified extractants for extraction asphaltenes from DCLR continues in our laboratory. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +86-10-82627080. Fax: +86-10-82627080. E-mail: sjzhang@ home.ipe.ac.cn (S.Z.),
[email protected] (X.Z.).
’ ACKNOWLEDGMENT The authors gratefully acknowledge the support of the National Institute of Clean and Low Carbon Energy (NICE), 10281
dx.doi.org/10.1021/ie201187m |Ind. Eng. Chem. Res. 2011, 50, 10278–10282
Industrial & Engineering Chemistry Research Beijing, China, the National Natural Science Foundation of China (No. 21076113), and the Key Program of National Natural Science Foundation of China (No. 21036007).
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