Interactions of Illinois No. 6 Bituminous Coal with Solvents: A Review

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Review

The Interactions of Illinois No. 6 Bituminous Coal with Solvents: a Review of Solvent Swelling and Extraction Literature Jonathan P. Mathews, Caroline E. Burgess Clifford, and Paul C. Painter Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502548x • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Interactions of Illinois No. 6 Bituminous Coal with Solvents: a Review of Solvent Swelling and Extraction Literature Jonathan P. Mathews#$, Caroline Burgess-Clifford#$, Paul Painter#‡ The EMS Energy Institute#, John and Willie Leone Department of Energy & Mineral Engineering$, Department of Material Science and Engineering‡, 126 Hosler Building, The Pennsylvania State University, University Park, PA 16802 [email protected] Phone: 814 863 6213

Abstract There has been copious research exploring the interactions between coal and various solvents either for coal structural investigations using coal swelling or industrial consideration exploring extraction yields. Here the literature is reviewed for Illinois no. 6 coal. This is the most well studied coal in existence and so offers an opportunity to reveal a wealth of information with a wide variety of solvents and extractions. This coal swells extensively (raw) in a good solvent such as pyridine (Q-factors of 2.2) and lesser degrees in benzene (Q-factor of 1.1). However if first extracted the extent of swelling can be greater. The swelling process is typically slow, sometimes taking days with anisotropic swelling being evident on the first exposure but absent from subsequent exposures. Overshoot phenomena, similar to polymer swelling, is also observed for some particles. These behaviors are related to the complex interactions of solvent diffusion and coal “relaxation” that is related to the nature of the solvent and its interactions with coal. Extraction yields are similarly varied with values ~30 wt.% (daf basis) with pyridine although greater values are obtained with two-stage, higher temperature, and exhaustive extractions. With CS2/NMP/H2O a yield as high as 70% wt.% (daf basis) have been reported at 600 K. Using light cycle oil has yields between 20 and 70 wt.% depending on consitions at 633 K. The swelling evidence supports a cross-linked structure and solvent extraction supports a considerable amount of extractable material. These statements are not necessarily in conflict.

Table of Contents 1. Introduction ............................................................................................................2 2. Coal as a strained system and it preferential alignment ...........................................3 3. Solvent Swelling ......................................................................................................3 3.1 Extent of Swelling..............................................................................................................................................3 3.2 Kinetics of Swelling ..........................................................................................................................................6

4. Solvent Extraction ...................................................................................................7 4.1 Low Temperature Extract Yields ................................................................................................................7 4.2 High Temperature Extract Yields ...............................................................................................................9

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4.3 Variations with solvent and temperature ............................................................................................ 12 4.4 Nature of the Extracts................................................................................................................................... 12 4.4.1. Low Temperature Extracts .......................................................................................................................12 4.4.2 Higher Temperature Extracts ...................................................................................................................13

5. Mechanism of Swelling and Extraction .................................................................. 14 5.1 Thermodynamics of Coal Swelling and Extraction........................................................................... 14 5.2 Intermolecular Interactions in Coal........................................................................................................ 15 5.3 The Free Energy of Mixing.......................................................................................................................... 17 5.4 The Swelling of Coal ...................................................................................................................................... 21 5.5 Mechanical Properties of Swollen Coal Gels and Extracts............................................................. 22 5.6 Binary Solvents ............................................................................................................................................... 24 5.7 A Novel Class of Solvents – Ionic Liquids ............................................................................................. 25

6. Implications for Studies of Illinois no. 6 Coal .......................................................... 27 6.1 Swelling of Extracted vs. Unextracted coal .......................................................................................... 27 6.2 Why do Some Solvents Swell Coal more than Others? ................................................................... 27 6.3 Why do acetylated or methylated samples swell more in certain solvents? ......................... 28 6.4 Associations and Relaxation ...................................................................................................................... 28 6.5 Implications for Cross-Linked and Associated Structure .............................................................. 29

7. Acknowledgements ............................................................................................... 29 8. References ............................................................................................................ 29

1. Introduction The largest bituminous coal field in the United States is the Illinois basin. It is an industrially important basin for coal extraction, primarily for electricity generation but also potentially for coal-to-liquids production. A sample form the Illinois no. 6 seam was included in the Argonne Premium coal suite and thus the Illinois No. 6 Argonne Premium coal has been extensively studied1 and characterized chemically and physically, including evaluations of solvent swelling, behavior, kinetics, and the nature of the extracts. These topics are of broad interest for the evaluation of coal structure, direct liquefaction, and for novel coal treatments. Here we provide a review of the solvent swelling rate and extent, solvent extraction yield, nature of extracts, production of ultra-clean coal through extraction, and the application of ionic liquids to Illinois No. 6 bituminous coal. Argonne Premium Illinois No. 6 coal was collected from the Herrin seam in Illinois in 1985.2 The elemental composition, normalized to 100 carbon atoms, was C100H77.3O9.4N1.5S1.2 (dmmf) and maceral composition of 85% vitrinite, 10% inertinite, and 5% liptinite.2 The as-received coal had 8.0% moisture, 14.3% ash yield, 36.9% volatile matter, and 39.9% fixed carbon.2 It is a high-volatile bituminous coal. A literature review of the chemical and physical structure is available3 as is a paper discussing creation of an atomistic representation including a downloadable large-scale (~50,000 atoms) molecular model.4


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2. Coal as a strained system and it preferential alignment The exposure of bituminous coals to solvents commonly causes irreversible swelling due to partial extraction the coal although challenges in solvent removal are common that introduces uncertainty in the quantification of swelling.5 For Illinois coal thin slices there is reversible swelling if the solvent was removed quickly enough (50 seconds).6 For solvent swelling of individual chucks or particles a metastable state is sometimes observed.7 This “overshoot” of swelling8, 9 is only observed with the first exposure and is also found in polymeric systems.10 Over time there is a structural rearrangement causing a contraction and expulsion of solvent. Particle studies also observe anisotropy in solvent swelling with greater swelling observed perpendicular to the bedding plane (swelling ratios of ~1.21).7, 11 The swelling is irreversible however, it becomes reversible in subsequent exposures.11 Magnetic resonance imaging of the de-swelling process also shows swelling anisotropy, irreversible swelling and rapid increase in the solvent removal rate after 50% removal (PSU Sample Bank Illinois coal sample).12, 13 All coals have some preferential alignment commonly measured by X-ray diffraction14 or optical anisotropy (0.031 birefringence for Illinois No. 6),15 although many other analytical approaches have also found anisotropy.16 Optical anisotropy is more readily apparent in transparent rather than reflectance microscopy for the younger bituminous coals.17 For Illinois No. 6 coal with the addition of “drops of pyridine” the optical anisotropy disappears and the rubbery coal can have birefringence imparted again by the application of pressure.17 Other solvents also cause loss of the anisotropy, but for THF some anisotropy remained. Interestingly thin sections that are solvent swollen will transmit more light than the raw coal attributed to the reduction in light scattering from the pores.18 Changes in pore shape apparently occur with pyridine swelling according to both electron paramagnetic resonnance and Small Angle Neutron Scattering (SANS) (using Pittsburgh coal) the IL coal pores become more elongated and less spherical.19-21 This was attributed to the role of secondary interactions and anisotropy in hydrogen bond “densities”.19 Alternatively, preferential aromatic orientations are possible at least imparting this orientation of hydrogen bonding although this is expected to be small for a coal of this rank.16, 22 Thus, the nature of the coal is affected by the presence of solvent(s) this impacts the solvent swelling extent, rate, and the extraction yields.

3. Solvent Swelling 3.1 Extent of Swelling Bituminous coals can, in certain solvents, swell to a remarkable degree. This is typically explored through the conventional method of height gain for pulverized coal with an excess23 of solvent in a tube soaking overnight or longer (days) depending on solvent and particle size. The height gain to original height (Q) (now commonly determined after centrifugation) determines the extent of swelling.24 Both raw and previously extracted coals are examined in this manner, although it should be kept in mind that for thermodynamic reasons raw coals cannot swell to their maximum theoretical limit. Alternatively, solvent vapor swelling on particles25-27 or polished coal surface swelling have also been performed.6 Single particle swelling techniques9, 28, 29 have also been used


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but not for Illinois No. 6 coal. There is also more accurate (swelling extent) work using dilatometry30, 31 and some magnetic resonance imaging work.12, 13 Table 1 shows that multiple solvents have been evaluated. With “good” common solvents, the swelling extent is ~1.8 for THF and 2.2 for pyridine. Benzene, its methylated forms, methanol and other alcohols swell the coal to Q factors around 1.3 (Table 1). Larsen et al. compared the swelling in the pyridine-extracted coal to the unextracted coal for multiple solvents.32 A greater extent of swelling is observed with pyridine-extracted coal (increase for benzene from approximately 1.1 raw coal to 1.5). The swelling extent was even greater for extracted and unextracted coal if the coals OH groups are acetylated.32 The soluble extracted fractions also display swelling behavior. For THF, benzene, and methanol greater swelling was observed for pyridine-soluble and acetone-insoluble (PS) and pyridine-insoluble (PI) CS2/NMP Illinois coal extracts.33 The extent of swelling is not very sensitive to temperature (room to 60°C, non-Illinois coals examined).34 Thermal drying and other coal drying approaches impact swelling extent: microwave, thermal, and chemical drying imparted slight differences in swelling extent in 1,4-dioxane.35 Although not a commonly used solvent, a remarkable Q factor of 4 was recorded with the dilatometry approach for tetrabutyl ammonia hydroxide solution.36


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Table 1. Extent of solvent swelling for Illinois No. 6 coal with various solvents and treatments

*Data is reported or estimated from graphical representations using pixel-counting approaches. Sources:5, 11, 30, 31, 37


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3.2 Kinetics of Swelling Solvent swelling is typically a slow process for even crushed coal: with equilibrium swelling typically occurs over several days or longer at room temperatures for some solvents. The process is likely controlled by the relaxation of the coal network structure. Diffusion of pyridine and THF (determined by swelling) follows non-Fickian diffusion and is termed case-II diffusional behavior.38 In contrast to the swelling-extent, swellingkinetics is strongly correlated to temperature and proceeds faster at higher temperatures.30, 34 Presumably this is due to a combination of the more rapid solvent diffusion and coal “relaxation”. Solvent kinetics are not easily followed with the standard swelling approach but can be adequately followed using either dilatometry,30, 39 coal mass gain (with solvent vapor) as a swelling proxy,26, 40 single particle time-lapse videography,9 or microscopy observations of particles swelling.28, 29, 41 The kinetic evaluations are complicated by the dual nature of the process: dissolution and swelling25 as well as by metastable states that are often observed in coal particle or coal piece swelling experiments.7, 9, 28, 42 Unfortunately, little work in this area has utilized Illinois coal. One exception was the novel magnetic resonance imaging work of single large (~5 mm) coal particle by Hou et al.13 Case-II diffusion was found for pyridine swelling, but Fickian diffusion for deswelling. Vapor evaluations for Illinois No. 6 determined pyridine sorption isotherms in which pyridine was described by two populations: one dissolved into the coal (highpressure region), the second occupying unrelaxed free volume (low pressure region) that were well modeled with Langmuir-Henry dual mode sorption equations.25-27 The size of the penetrant also influences the kinetics of swelling31, 43 as well as the extent31 (non-APC used). Larsen et al. state that, “It is clear that Illinois No. 6 coal recognizes the bulkiness of the penetrant molecules. The rate of retardation can be increased by almost a thousand fold simply by changing the alkyl substituent from normal to tertiary” (butylbenzene isomers).31 The shape of the penetrant is also important with flat molecules diffusing faster. Single particle work shows the formation of a metastable state for bituminous coals: the particle swells reaching a maximum swelling extent before contracting to reach a lower equilibrium swelling extent. This overshoot also occurs with glassy polymer systems.44 It is not observed in every particle and climbing type swelling are also typically observed.9 For a non-APC Illinois coal Cody et al.7 determined that overshoot was observed only on the first swelling. The overshoot, in one sample was 15% (155% to 140%) contraction, from maximum swelling extent, for the perpendicular direction to the bedding plane.7 Similar behavior was observed for the parallel to the bedding plane direction but to a lesser extent.7 The metastable state can significantly prolong the equilibrium swelling time.


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4. Solvent Extraction Solvent extraction has been quantified using a variety of solvents and conditions. In some cases, the conditions used were similar to those in coal liquefaction. Here solvent extraction using single or binary solvent combinations are considered at the boiling point, room temperature, and in some cases heated to up to 653 K. The nature of the material extracted is also evaluated. Liquefaction conditions where thermolysis and hydrogen transfer occur are not considered to be extractions.

4.1 Low Temperature Extract Yields A variety of conditions and solvents have been evaluated to extract coal.45-49 The four common methods of extraction are: 1) exhaustive Soxhlet extraction of the coal at the solvent boiling point, 2) ultrasonic irradiation of solvent-coal mixture followed by filtration, 3) mixing/soaking of the solvent-coal with centrifuging to separate the solvent, and 4) heating the solvent-coal mixture in a reactor, followed by filtration. The common solvents used are pyridine, CS2, NMP, CS2/NMP, CS2/NMP/H2O, benzene/methanol, and THF. Table 2 shows a summary of the extraction yields and conditions used. It is thought that the solvents that extract >20% of the coal are disrupting the hydrogen bonding within the coal, but that no covalent bonds are broken.49, 50


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Table 2: Extraction yields of Illinois No. 6 coal Lead author

Coal Source PSU APC APC

Preextracted -

CS2/NMP CS2/NMP CS2/NMP

Li 2000 52

APC APC APC APC APC APC APC APC

-

CS2/NMP/N2 CS2/NMP CS2/NMP CS2/NMP CS2/NMP CS2/NMP/H2O CS2/NMP/H2O NMP/HHA

Nishioka 199149

APC

-

pyridine

298 298 298 500 600 500 600 175300 389

APC

2 steps

pyridine

389

Fletcher 1993 45

APC

-

pyridine

389

Nishioka 1994 47

APC APC

Yes Yes, dried Yes Yes, dried -

pyridine pyridine

Iino 1988 46 Takanohashi 1990 48 Ishizuka 1993 50 Yoshida 2002 Iino 2004 51

APC APC Carlson 1992

APC

Solvent

Temp (K) 298 298 298

Approach Ultrasonic Ultrasonic Ultrasonic

Extr Yield (wt% daf) 20.2 32.1 33.1

Ultrasonic Ultrasonic Ultrasonic Heating Heating Heating Heating

31.2 34.5 32.1 28.1 31.0 28.0 70.2

42.0

298 298

Exhaustive Soxhlet extraction Exhaustive Soxhlet extraction Exhaustive Soxhlet extraction Mixing/centrifuge Mixing/centrifuge

THF THF

298 298

Mixing/centrifuge Mixing/centrifuge

26.0 24.0

THF

339

50.0 27.9 34.0 24.0

Exhaustive 16.8 Soxhlet extraction Xia 1987 53 APC Benzene/ 330 Exhaustive 4.3 methanol Soxhlet extraction PSU is the Pennsylvania State University Coal Sample Bank sample and APC is the Argonne Premium Coal sample

Generally, the extraction yields vary from 20 to 34 wt.% (daf basis), although some higher and lower yields were reported. The lowest extraction yield was obtained using the binary solvent mixture of benzene/methanol, with only 4.3 wt%.53 Using pyridine, with exhaustive Soxhlet extraction and mixing/centrifugation, the yield was 24 to 34 wt%. There were no significant differences in yield between pyridine, THF, or CS2/NMP, even at 500 to 600 K. However, there were two cases where extraction yields were >34% (Table 2). Multi-step extraction with pyridine produces higher yields, from an extraction yield of 42 to 50 wt.%.49 Water addition/removal alters this yield either by coal drying (before extraction) or in the solvent system CS2/NMP/H2O, were there are differences in extraction yield.47, 51 Thus, water plays a role in solvent extraction for this bituminous coal. When removing water, extraction yield is at the lower end, 24 wt.%. When water is added to the solvent system, particularly at 600 K, the extraction yield increases


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dramatically (70.2 wt.%). It is possible the water may play a role in bond breaking not seen in any of the other extraction conditions. Acetylated coal however has a much higher extractability.54 For example for a non-APC coal the Soxhlet extraction yield improved dramatically from 0.7 to 14.7 wt.% dmmf with benzene, the improvement with a good solvent such THF was less significant (14.4 to 22.1 wt.%) with methylation but not accounting for the 4.2 wt.% increase in mass gain.54 Others have also demonstrated extraction increases55 presumably due to a combination of both increased solubility of the molecules that were alkylatated and relaxation of the structure allowing an improved transportation of molecules into the solvent.

4.2 High Temperature Extract Yields The extractions discussed so far are mainly to determine how much coal can be extracted at either room temperature or the solvent boiling point in Soxhlet extraction – the information gleaned provides insight into the nature of the coal structure, primarily without covalent bond breaking. Higher extraction yields were obtained at higher temperatures, typically >350°C. Japanese researchers have developed a thermal extraction process, with the goal of producing a very low mineral matter/ash product (HyperCoal) that may be directly fired in gas turbines.56 At these higher temperatures covalent cross-links may be broken. These conditions compared to Solvent Refined Coal (SRC) process are shown in Table 3.57-60 For the SRC process the temperatures are higher, there is application of high hydrogen pressure, and the inclusion of a hydrogen donor solvent. These factors make SRC more expensive than HyperCoal production. Kobe Steel has developed a commercial process to produce HyperCoal – a description of the pilot-scale unit and experiments is detailed in Okuyama et al.57 Smaller scale experiments have used a wide variety of coals with various solvents – tetralin, 1methylnaphthalene (MN), dimethylnaphthalene (DMN), and petroleum refinery products from the distillate of fluid catalytic cracking processing: light cycle oil (LCO) and decant oil (DO).57-63 Here the focus is on the thermal extraction of coal under those specific conditions, as it is not technically coal liquefaction and the conditions are such to keep the overall costs low. The extraction yields are expected to be >50 wt.% daf for Illinois No. 6 coal. Table 3: Comparison of conditions for the SRC Process and HyperCoal SRC HyperCoal Temperature (°C) 400 to 450 < 400 Pressure (gas) High (H2) Low (N2) Solvent Hydrogen donor Non-hydrogen donor Extraction yield (%) > 60 > 60 Ash in extract (%) < 0.3 < 0.02 State of extract Pitch-like Powder (coal-like) Modified from Yoshida et al.59 by Burgess-Clifford 201061 Table 4 shows the extract yield for higher temperatue extractions with solvents and petroleum refinery products. Early extraction yields using the HyperCoal conditions59


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with Illinois No. 6 and LCO or DMN solvents were similar to extraction results using CS2/NMP (Table 2, around 35%), being on the order of 31 to 40 wt.%, although when using a different brand of DMN, the conversion increased to 54 wt.%. However, more recent results57, 58, 60, 62, 63 using a flow reactor and/or hot filtration, thermal solvent extraction rose from 51.2 to 74.6 wt.% depending on solvent; these results met the expectation of >50% conversion, and by using a petroleum-generated solvent, the expensive solvent separation may not be required.62, 63 Researchers suggest that bond breaking is not taking place at 633K, but that the coal structure is “relaxing” with higher temperatures to release trapped molecules.58, 60 The contribution of bond cleavage to enhanced extraction yields at temperatures above 350 ˚C is not entirely clear. In older work it was acknowledged that thermal decomposition might play a role, but it was concluded that any such effect was minor.64 However, Xu and Kandiyoti noted that 350 ˚C is not a precise dividing line and that there is a characteristic temperature band that may shift tens of degrees as a function of individual coal properties.65 They proposed a two-stage model, with the first stage occurring below a characteristic temperature, which they identified to be near 350 ˚C for Illinois No. 6 coal. The model did not identify the physical and chemical processes that occur at lower temperatures, but clearly at temperatures in the 350 to 360 ˚C range there is the onset of processes that give measureable bond cleavage. In this regard Shi et al. have recently discussed the role of bond breakage in coal pyrolysis and noted on the basis of bond energies that Cal–O and Cal–S bonds would cleave at temperatures near 350 ˚C.66 There would also be a relatively slow cleavage of Cal–Cal bonds at these temperatures. Although the peak rate of cleavage of these latter bonds is near 450 ˚C, the band describing the rate of cleavage has a tail that extends below 350 ˚C. Also the contribution of solvent degradation products to measured extraction yields needs to be considered. Li et al. showed that at 350 ˚C tetralin, quinoline and NMP gave degradation products that contaminated the pentane insoluble coal extracts.67 Polymerization products were particularly prominent for NMP and this would adversely affect reported yields.67


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Table 4: Extraction yields for various conditions for thermal solvent extraction (low ash coal product) for Illinois No. 6 coal. Lead Author/Year

Coal

Solvent

Yoshida 2004 Okuyama 2004 Takanohashi 2008 Zhang 2008 Zhang 2008 Yoshida 2002

APC APC APC

MN MN MN

APC APC APC

MN MN/IN DMN

Yoshida 2004 Miura 2001 Yoshida 2002 Yoshida 2004

APC APC APC APC

DMN Tetralin LCO LCO

Yoshida 2004

APC

CMNO

Carlson 1992

APC

KOH/ methanol

Temp (K)

Approach

10:1

633 633 633

flow solvent extr 60 min static solvent extr 60 min flow solvent extr 60 min

10:1 10:1 10:1

633 633 633

static solvent extr 60 min static solvent extr 60 min static solvent extr 60 min

633 623 633 633

Extr Yield (wt% daf) 53.7 59.4 62.4 54.3 70.3 39.2, 54.4 69.3 63.0 31.1 55.8

10:1

563

flow solvent extr 60 min flow solvent extr 60 min static solvent extr 60 min flow solvent extr 60 min hot flow solvent extr 60 min hot HT-BCD, 60 min

LCO LCO LCO LCO LCO LCO

10:1 10:1 10:1 10:1 10:1 10:1

473 573 623 633 643 653

static solvent extr 60 min static solvent extr 60 min static solvent extr 60 min static solvent extr 60 min static solvent extr 60 min static solvent extr 60 min

0.3 13.5 21.7 31.1 32.0 33.0

LCO

10:1

623

static solvent extr 60 min hot

51.2 55.0

LCO

5:1

623

37.6

PSU

LCO

3:1

623

PSU

LCO

10:1

623

PSU

LCO

10:1

623

PSU

LCO

10:1

623

static solvent extr 60 min hot static solvent extr 60 min hot flow solvent extr 60 min hot, 3 reactors in seriesstage 1 flow solvent extr 60 min hot, 3 reactors in series, stage 2 flow solvent extr 60 min hot, 3 reactors in series stage 3

Temperature Variations Yoshida 2002 APC APC APC APC APC APC Solvent/Coal Ratio Variations Griffith, 2009; PSU Burgess Clifford 2008 PSU

Burgess Clifford 2008

Solvent Coal Ratio

10:1

633

74.6 58.3

20.0 63.9

71.2

73.7

LCO is light cycle oil.


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4.3 Variations with Solvent and Temperature The coal to solvent ratio can also affect the extraction yield. The solvents that yielded the highest coal extraction yields were 1-methylnaphthalene (MN), dimethylnaphthalene, tetralin, a binary solvent of MN/indole (indole being 20%), and the industrial solvent CM–yields ranged from 60 to 75 wt.%,68 similar to when the mixed solvent system CS2/NMP/H2O extraction yield at 600 K (70.2 wt.%)51 Yoshida et al. (2002)59 determined the best temperature for coal conversion to HyperCoal – the conversion increases to ~30% in early research at ~360°C. Griffith et al. worked to reduce the solvent: coal ratio from 10:1 to 5:1 and 3:1, but determined coal conversion decreased and mechanical problems at the lower solvent: coal ratios.61-63 Compared to other coals, Illinois No. 6 yielded higher conversions; the high fluidity in Illinois No. 6 may aid in the higher conversions.

4.4 Nature of the Extracts 4.4.1. Low Temperature Extracts Fletcher at al. extracted coals using pyridine and characterized the extract and residue with 13C solid state CP/MAS NMR;45 the data for Illinois No. 6 coal, extract, and residue are shown in Table 5. The extract and residue are not significantly different from the raw coal (H/C basis), but the authors found that the main differences in the extract were the number of cross-links was reduced and the number of aromatic ring substituents were lower than in the residues.45 These data are also consistent with the concept that the extracted material is not extensively incorporated by means of covalent bonds into the macromolecular structure. This data is also consistent with data obtained from CS2/NMP extracted material.48 Table 6 also shows the N/C atomic ratios are higher for residue. As is discussed in the subsequent section, when coals are solvent extracted with thermal treatments (typically >350°C) the extraction yield increases and there are changes in the H/C and N/C ratios. In those cases, the higher temperature either helps to open up/relax the macromolecular structure or possibly the start of thermolysis for this bituminous coal. Table 5: 13C NMR of Illinois #6 coal, pyridine extract, and extract residues. Sample fa fa’ faC faH faN faP faS faB fal falH fal* falO C σ+1 p0 MW χb Illinois #6 72 72 0 26 46 6 18 22 28 13 11 4 0.31 15 5.0 0.63 316 Raw Illinois #6 72 68 4 23 42 7 18 17 28 19 9 5 0.25 12 4.4 0.64 270 Py Extract Illinois #6 67 62 5 23 39 7 16 16 33 24 9 7 0.25 12 4.4 0.61 300 Py Residue Data taken from Solum et al. (coal) and Fletcher et al. (extract and residue) fa = total sp2-hybridized carbon; fa’ = aromatic carbon; faC = carbonyl, δ > 165 ppm; faH = aromatic with proton attachment; faN = nonprotonated aromatic; faP = phenolic or phenolic ether; faS = alkylated aromatic , δ = 135-150 ppm; faB = aromatic bridgehead; fal = aliphatic carbon; falH = CH or CH2; fal* = CH3 or nonprotonated; falO = bonded to oxygen, δ=50-90 ppm; χb = fraction of aromatic bridgehead carbons; C = number of aromatic carbons per cluster; σ+1 = number of attachments per cluster MW = total molecular weight per cluster.


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Table 6: Atomic H/C ratio of coal, coal extract, and residue with CS2/NMP Solvent H/C N/C Ash Yield% Raw coal 0.86 0.021 15.0 Extract CS2/NMP 0.86 0.020 Residue CS2/NMP 0.87 0.028 48 Data from solvent extraction yield 33 wt.% daf. 4.4.2 Higher Temperature Extracts Yoshida et al.59 performed proximate and ultimate analyses on the raw coal, the extract, and the residue (Table 6). For Illinois No. 6 using DMN-T as solvent at 360°C for 60 min, the atomic H/C ratio of the original coal was 0.86, while the extract was 0.91 and the residue 0.72. Other solvents produced similar and somewhat higher H/C ratios for the extract ~0.87-0.93. However, in a recent publication by Takanohashi et al., as higher extraction yield was achieved using MN (62.4%), and the H/C ratio of the extract was lower, 0.79.60 Miura et al.69 examined at the effect of coal extraction at 623 K (350°C) using two-ring flowing solvents such as MN and tetralin. At 350°C, the extraction yield is higher than room temperature extraction, but the authors noted that they had two “parts” to the extraction, a deposit that precipitated out as the reactants cooled (36 wt.%) and the extract in the solvent (27 wt.%). The overall extraction yield was 63 wt.%.69 The residue H/C ratio was 0.72, lower than the original coal 0.77, while the extract and deposit 0.80 were higher. They also ran solid state 13C NMR and matrix assisted laser desoprtion ionizationtime of flight mass spectroscopy (MALDI-TOFMS), for a Pittsburgh coal. There were differences in aromaticity and MALDI-TOFMS for extract, residue and deposit. The residue and deposit were more aromatic and the soluble extract less aromatic than the raw coal. The MALDI-TOFMS showed the soluble portion were lighter in molecular weight, and that the soluble, deposit, and residue portions were fractions of the raw coal and added together could produce the raw coal spectrum, supporting their “extraction” terminology even at this temperature/pressure for these bituminous coals.69 The goal of Griffith et al. was to increase the 2- and 3-ring aromatic concentration in the liquid extracts using the solvent LCO; the extract would then be hydrotreated to produce a thermally stable jet fuel similar to JP-900.62, 63 Griffith et al. characterized the extracts from one of the coals reacted – the extract from Pittsburgh coal after reaction in LCO at 350°C for 60 min. The main result was that the coal contributed additional 2- and 3-ring aromatics to the product, with coal conversions ranging from 20-73% depending on the conditions used (Table 7). The extracts and residues from low temperature extraction are similar to each other and to the original coal, but clearly, under higher temperature conditions, the nature of the extracts are different from the coal and the residues, which strongly suggests covalent bond breaking, contrary to HyperCoal data.


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Table 7: Illinois No. 6 atomic H/C and N/C ratios for raw coal, HyperCoal, and HyperCoal extract and residues Sample

Solvent

Raw Coal59 HyperCoal59 Residue59 HyperCoal59 HyperCoal59 HyperCoal60 Raw Coal69 Extract69 Deposit69 Residue69

-DMN-T DMN-T 1-MN LCO 1-MN -tetralin tetralin tetralin

Extraction Yield

54.4 54.4 n.d. 31.1 62.4 27 36 Total 63.0

H/C

N/C

0.86 0.91 0.72 0.87 0.93 0.79 0.77 0.80 0.80 0.72

0.021 0.036 0.026 0.012 0.016 0.019 0.015 0.008 0.019 0.015

Ash Yield 15.0 0.4 ---0.09 15.4 0.3 0.1 32.1

Other “extractions” have been carried out on Illinois No. 6 and other Illinois coals, but the reaction conditions are more typical of coal liquefaction than solvent extraction or even thermal solvent extraction;70 the changes include the addition of tetralin and sulfur compounds to the reactor, with conditions at 300 to 400°C, and the products were separated from the reacted coal using sequential Soxhlet extraction and solvents of hexane, toluene, and pyridine. Stock et al. showed that conversion increased with addition of the sulfur compounds;70 sulfur compounds are known to increase liquefaction yields. There are many publications that discuss variations in coal liquefaction and the effect of temperature, solvents, and catalysts on conversion of Illinois No. 6 coal; these efforts are not reviewed here. However, the limited work on inorganics into extracts of HyperCoal are relevant. Extraction with NMP under nitrogen pressure at 360°C produced HyperCoal with 822 ppm inorganic elements, with the most prevalent being Fe (440 ppm) with abundant transition metals also (ppb levels): Cr (most prevalent at 9800 ppb), Ni, Mn, Co, Cu, and Zn.71 Inorganics are undesirable in HyperCoal due to turbine blades fouling, trace elements are undesirable due to corrosion in combustion systems, and also issues in refinery operations in coal-to-liquids and coal-to-chemicals. Submicron discrete inorganics particle were also present in the HyperCoal.71 Similar work followed boron partitioning (around 320 µg/g in the raw coal) and found it mostly to be in the residue rather than the HyperCoal.72

5. Mechanism of Swelling and Extraction 5.1 Thermodynamics of Coal Swelling and Extraction Most coal solubility and swelling evaluations rely on theories developed to describe the phase behavior of polymer solutions and swelling of macromolecular networks. There are various observations that support treating coal in this manner. Coals, and covalently cross-linked polymer networks, swell in good solvents but (with one or two exceptions) only a fraction of the coal dissolves and can be extracted. In the polymer literature the extractable material is described as the “sol” (soluble), while the insoluble component is


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called the “gel” (insoluble) fraction. In synthetic networks, the proportion of these two components varies systematically with the cross-link density of the network. With coals, the swelling and extraction yields obtained with solvents increases systematically with rank (until the low-volatile bituminous rank), suggesting that cross-link density changes in a corresponding fashion. Alternatively there is the assertion that coal can be regarded as an associated structure, largely held together by secondary forces such as hydrogen bonds, rather than a covalent network. Much of the evidence for this has come from the seminal work of Iino and colleagues, who found that there is an enhanced solubility of certain coals in the mixed solvent system N-methyl-2-pyrrolidinone/carbon disulfide (NMP/CS2), relative to the individual solvents and “good” solvents like pyridine.33, 46, 48, 50, 73-77 The significance of this assertion is that many coals could be dissolved if the right solvent (or combination of solvents) and conditions could be elucidated. Accordingly here we review of the types of interactions found in coals, particularly bituminous coals such as Illinois No. 6, and also a review of the thermodynamics of solubility and swelling.

5.2 Intermolecular Interactions in Coal Although there are numerous structural models that depict coal structure78 it is safe to say that one can view bituminous coals as a largely aromatic hydrocarbon where aromatic and hydroaromatic fused rings of various size are linked together by aliphatic and ether groups. Aromaticity increases systematically with rank (and carbon content). Conversely, the oxygen content of coals decreases with increasing rank. In a bituminous coal, such as Illinois No. 6, oxygen is largely present as phenolic hydroxyl (OH) groups (with perhaps some alkyl OH), ethers and carboxylic acid groups.79 Mineral matter is also present, some of it being bound to the organic matter (discussed later). These elements of structure serve to define the types of intermolecular interactions that dominate in bituminous coals. Starting with the weakest intermolecular interactions, one would anticipate that dispersion forces play a major role in coals cohesion and solubility. They are a major component of π–π interactions between aromatic rings.80 Although binding energies between pairs of benzene molecules are quite weak, they become significantly larger in polycyclic aromatic units. These molecules become increasingly insoluble in common organic solvents as the size of the aromatic unit increases. The presence of heteroatoms imply that dipole forces and possibly charge-transfer complexes also play a role. However, perhaps the strongest individual intermolecular interactions present in Illinois No. 6 coal involve hydrogen bonds, both OH/OH interactions between phenolic groups and hydrogen bonds between OH and ether oxygen groups and between OH and basic nitrogen groups.81 In low-rank coals there are interactions between cations and organic groups: we will return to a consideration of these later, as unlike other interactions these involve multiple groups and can act as cross-links and affect solubility and swelling as a result of a microphase separation into “junction zones”,82, 83 defined below. In studies that regard coal as a largely associated structure, it has been argued that hydrogen bonds and other secondary interactions (e.g., π–π interactions) act as crosslinks.33, 46, 48, 50, 73-77 This seems unlikely, for various reasons,84, 85 but perhaps the


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arguments also involve semantics. Anthracite coals, for example, are insoluble because of their more graphitic-like structure. Are the interactions between the aromatic units then cross-links, or is their insolubility simply a result of the extensive planar ring systems, which through the summing of weaker forces over a large area result in very strong intermolecular interactions? The insolubility of these coals is then a matter of solution thermodynamics. They simply do not dissolve in solvents because the balance of interactions is unfavorable. Accordingly, we will regard a cross-link as a connection between well defined points on separate large molecules, as in covalent cross-links between polymer chains, or involving a localized, junction zone of units that are strongly bonded in a particular fashion e.g., through ionic interactions. These types of interactions correspond to those observed in ethylene-methacrylic acid copolymers (e.g., Surlyn), where strongly polar carboxylate salts essentially phase separate from the non-polar part of the material to form ionic microdomains that act as cross-links or, more accurately, junction zones, even though the concentration of carboxylic acid groups is often small (~5%). Cross-linking behavior then depends on two related factors. One is the strength of the interaction or bond relative to thermal energy (RT), while the second is the lifetime of the interaction relative to the time scale of the experiment or phenomenon under investigation. Thus, at ordinary temperatures and in the absence of chemical reactions, the lifetime of a covalent bond spans geological time periods, as calculated by the probability that a bond would break solely as a consequence of thermal motion. At the other end of the scale, contacts between small hydrocarbon molecules interacting through simple dispersion forces last about 10-12 seconds in the liquid state.86 Hydrogen bonds, with strengths in the range of 4-7 kcal/mole for the phenolic groups found in Illinois No. 6 coal, have transient lifetimes in the liquid state, ranging from 10-11 seconds in water to about 10-5 to 10-6 seconds in urazole modified elastomers.87 Donor-acceptor type interactions have strengths in the range 4-10 kcal/mole, so that these would presumably also be transient in the liquid state. Nevertheless, hydrogen bond “lifetimes” are long compared to other molecular processes, such as the frequency of collisions in the liquid state, so that properties such as diffusion and viscosity are affected, because hydrogen bonded molecules will diffuse or translate considerable distances (relative to molecular dimensions) before breaking and reforming with other molecules. However, if the time frame of the experiment is significantly longer than the time frame of hydrogen bond relaxations, as in a deformation experiment, for example, then these interactions will not act as cross-links, because they have sufficient time to break and reform with new partners. Accordingly, in experiments such as swelling, the hydrogen bonds and π–π interactions in coal are not cross-links. Coals do not swell significantly in non-hydrogen bonding solvents because the mixtures are grossly phase separated, not because their hydrogen bonds are cross-links. Although interactions between pairs of functional groups may be individually too weak or transient to act as a cross-link, junction zones can form as a result of a microphase separation, as mentioned above. It has been demonstrated that washing with weak acids significantly enhances the pyridine extraction yields obtained from certain coals, as


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illustrated for the Argonne Premium Coal Sample (APCS) set in Figure 1.82, 83 The pyridine extraction yield obtained for Illinois No. 6 coal increased from 34% to 46%, on a dry ash-free basis. It was proposed that in coals with significant carboxylic acid contents, such as lignites and subbituminous coals, metal cations are exchanged for protons thus reforming carboxylic acids from their salts. In many bituminous rank coals π-cation interactions between metal ions and the π orbitals of aromatic structures appear to be crucial. For bituminous coals such as Illinois No. 6, both cross-linking mechanisms probably play a role in swelling and solubility.

Ex traction Yield (wt % daf)

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60

50

40

30

20

10

0 70

80

90

100

%C

Figure 1. Extraction yields (in pyridine) before (triangles) and after (diamonds) acid washing of the raw coals.83 As a result of this work on ionic interactions, it has been proposed that cross-linking in coal should be thought of in a different way and classified into two types; first, “permanent” covalent linkages that cleave only at elevated temperatures or through chemical reaction; second, “reversible” cross-links, largely associated with ionic structures such as carboxylate salts and π-cation complexes. The enhanced solubility of certain coals in the mixed solvent system NMP/CS2 may be a result of their ability to complex cations and could also explain the unusual results obtained when mixing Illinois No. 6 coal with a novel class of solvents, ionic liquids, described later.

5.3 The Free Energy of Mixing For solubility and phase behavior, the thermodynamic criterion for two materials to spontaneously mix and form a miscible mixture is the inexorable second law of


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thermodynamics, which, in this context, is that the change in entropy upon mixing must be positive. It is important to recall that this is the total entropy change, because mixing could result in heat being emitted to or absorbed from the surroundings, also changing entropy. However, this criterion for mixing can be formulated simply in terms of changes within the system by defining the change in free energy as ∆G = –T∆Stotal where ∆Stotal is the total change in entropy upon mixing. If the entropy change in the surroundings (assuming a reversible process) is now expressed in terms of the heat in or out of the mixture, ∆Ssurroundings = –∆Hsystem/T, (at constant pressure), we then obtain the usual expression for the Gibbs free energy of mixing: ∆ = ∆ − ∆

(1)

This means that when the entropy change is positive, the change in free energy is negative. Now, it would seem, our only problem is to determine whether or not the free energy change is negative for a given mixing problem. This requires that we relate entropy and enthalpy changes to molecular quantities. It does get a little more complicated, in that a negative free energy change is a necessary condition for mixing, but not a sufficient one. The second derivative of the free energy with respect to composition also has to be positive. But for the most part in this review we will not have to deal with such complexities. The enthalpy and entropy of mixing in the equation for the free energy can be related to the energy of interaction between the molecules and the number of arrangements available to the system, respectively (or, more precisely, the change in these quantities upon mixing) using statistical mechanics. These thermodynamic quantities are to some degree inter-related. If molecules of component “A” in a mixture strongly interact with those of component “B”, then one would expect that on average there would be more arrangements where A’s would be next to B’s than in a system where they did not attract as strongly. It’s a bit more complicated than this, in that the number of arrangements will also depend upon the strength of the interactions relative to thermal energy, RT, a measure of the average kinetic energy of the molecules (per mole). In other words, if the interaction energies between the molecules are relatively weak, say just dispersion forces and weak polar forces, then at ambient temperatures one would expect that the disordering effect of thermal motion would result in essentially random mixing. This leads to the most widely used approximation in mixing theories, the mean field assumption, where each molecule is assumed to be acted upon by a potential that is an average taken over all the interactions in the system, rather than one determined by local composition. This allows an approach where the entropy and enthalpy changes upon mixing are treated as separate and additive quantities. We have included this somewhat lengthy preamble on basic solution thermodynamics because it is important to grasp up front the nature of the assumptions that go into the most widely used model for dealing with coal solubility (and with additional terms, swelling), the Flory-Huggins theory. Essentially, it is assumed that interactions are weak and there are random contacts between the molecules or segments of molecules (for


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polymers) in a mixture. With this assumption, Flory and Huggins independently derived an equation for the free energy of mixing components A and B that can be written as: ∆

. = + + (2) The term on the left is the free energy normalized to a dimensionless form by dividing by RT. (The equation is also based on a statistical mechanical lattice model, which gives the Helmholtz free energy, but the difference in the Helmholtz and Gibbs free energies is negligible at ordinary temperatures and pressures.) The left hand side of the equation is also normalized by a factor Vr/V, where V is the total volume and Vr is a reference volume, the volume of a lattice site. In polymer solution theory the molar volume of the solvent is usually used to define this quantity. This normalization allows the combinatorial entropy of mixing, the first two terms on the right hand side of the equation, to be written in a particularly illuminating form. They are expressed in terms of the volume fractions, φA and φB and the quantities MA and MB, which can be thought of as the degrees of polymerization of the components, but is more precisely the molar volume of the molecule divided by the reference molar volume. The two terms are each negative, because they involve the logarithms of fractions and therefore contribute a quantity to the free energy that is favorable to mixing. It can be seen that if we are mixing small molecules that are of roughly equal molar volume, then MA ≈ MB ≈ 1 and the entropy of mixing is large and is favorable to mixing. If one of the components of a mixture is a solvent, say A, while the other is a high molecular weight polymer, i.e., MB is large, then the entropy of mixing is much less favorable to mixing. If this latter component is a cross-linked network, then MB is effectively infinite and only the first term on the right hand side of equation 2 contributes to the entropy of mixing. In the original Flory theory the third term on the right of equation 2 was related to the change in enthalpy upon mixing. (It is now often considered to be a free energy term to absorb factors not accounted for in the simple model). The product of φA and φB accounts for the number of AB contacts in a random mixture, while χ is related to the change in energy on replacing AA and BB contacts in the pure components with AB contacts in the mixture. Because the theory was developed for weak interactions (dispersion and some weal polar forces), χ is positive and unfavorable to mixing. This is more easily seen if the energy of interactions is expressed in terms of Hildebrand solubility parameters.88 Both the Flory χ parameter and solubility parameters are based on similar assumptions and are therefore related through the well-known relationship ( − ) + (3) where Vr is the reference volume (usually the molar volume of the solvent when dealing with solutions, as mentioned above), δA and δB are the solubility parameters of the A and B components, respectively, and β is an empirical parameter that partly accounts for approximations in the theory and for polymer solutions usually has a value of the order of 0.34. For small molecules, solubility parameters can be determined from measurements of the heat of vaporization, but for large molecules (where the heat of vaporization cannot =


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be measured) they are usually determined in one of two ways: from group contributions using, for example, the methods of van Krevelen,89 Hoy,90 or Small91 (see88 and citations therein) or from experimental swelling measurements. We have been at pains to point out that these theories only apply to the random mixing of molecules that interact through relatively weak interaction forces. Hydrogen bonds are different and cannot be dealt with by means of a simple approximation. Polymer segments and solvent molecules that interact in this manner are truly associated, and in the liquid state there is a dynamic equilibrium distribution of hydrogen-bonded species. There is a non-random arrangement of the hydrogen-bonding functional groups (relative to one another) and the interaction is a complex function of the distribution of charges on the molecules or segments involved and the specific arrangement of molecules in space. Furthermore, when two molecules or segments of molecules hydrogen bond, the vibrational and rotational degrees of freedom of each become seriously modified and there is an entropy term associated with this. A number of attempts have been made to incorporate hydrogen-bonding interactions into solution theory. One such attempt, where the solubility parameter is separated into nonpolar, polar, and hydrogen-bonding interactions, cannot be successful because of the assumptions underlying the solubility parameter approach, such as random contacts, the geometric mean assumption, which is used to relate the energy of interaction of AB contacts in terms of AA and BB contacts, and so on. One factor alone should make it clear that a solubility parameter approach to hydrogen bonding and other strong interactions is fatally flawed; such an approach cannot result in a negative, favorable contribution to the free energy of mixing (equation 3). It has been experimentally established that hydrogen bonding contributes a negative (favorable to mixing) term to the free energy of mixing materials with complementary functional groups, as in certain polymer blends. Various models have been developed to account for hydrogen bonding and other strong, specific interactions in mixtures. A particularly simple approach uses association models.88, 92, 93 This treats the associated complexes, most often chains of hydrogenbonded molecules or macromolecular chain segments, as new, distinguishable, and independent molecular species. The hydrogen-bonding interaction is treated separately from the forces involved in mixing and essentially defines the number and distribution of species present, typically through equilibrium constants that are related to the free energy change corresponding to the formation (or dissociation) of particular types of hydrogen bonds. The hydrogen-bonded species then mix and interact with one another or other chain segments through simple dispersion forces only, a process that can be readily handled by conventional lattice theories. A more versatile approach, originally described by Veytsman94 and further developed by Panayioutou and Sanchez,95 counts how hydrogen bonds are distributed between “donor” and “acceptor” groups. The two models can be shown to be equivalent,92, 93 and the theory results in a modified Flory-Huggins equation containing an additional term that accounts for the energy and constraints imposed by the specific interaction:


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∆ = ∆ + ∆ (4) where ∆GFH is the contribution of the combinatorial entropy and dispersion forces (χ term) to the free energy of mixing, while ∆Gsp accounts for the entropy and enthalpy changes imposed by interactions such as hydrogen bonds. Note that if solubility parameters are used to estimate χ, they must be based on group contributions where there is no contribution from hydrogen bonds or other specific interactions.88 Again, this somewhat lengthy review has a purpose in illuminating a specific aspect of the phase behavior of coal/solvent mixtures. For a material like Illinois no. 6 coal that hydrogen bonds to itself in the unmixed state, ∆Gsp is related to the free energy difference between the distribution of hydrogen bonds found in the pure state and those found in the mixture. If one attempts to mix a self-associating material such as phenol with a nonhydrogen bonding solvent such as cyclohexane, essentially all that can happen is that the number of hydrogen bonds decreases with increasing dilution (as it becomes more difficult for the phenol molecules to “find” one another). Because this is unfavorable to mixing (there are fewer hydrogen bonds in the mixture than in the parent coal), phenol has only limited solubility in cyclohexane. Essentially, only dilute single-phase mixtures can form, as the balance of terms in equation 4 only favor mixing at low concentrations. If one considers poly(vinyl phenol) instead of phenol, there is very little mixing, because the favorable entropy of mixing would be smaller in such mixtures, as discussed above. Similarly, if one tries to mix two materials that self-associate in the pure state, say a polymeric phenolic compound with an alcohol, there will be phase separation over most of the composition range. This is because hydrogen bonds in both the phenolic material and the alcohol have to be “broken” in order to form hydrogen bonds between unlike groups in the mixture. The most favorable contribution to mixing occurs when a selfassociating material, such as Illinois no. 6 coal, is mixed with a solvent that has a hydrogen bonding acceptor group, such as an ester, ether or basic nitrogen group (as in pyridine or NMP). Because the enthalpy of hydrogen bonding is greater between acidic phenolic OH groups and basic nitrogen groups than between OH groups and ethers (which in turn are stronger than OH-ester hydrogen bonds), then one would anticipate that solvents such as pyridine and NMP would be the most effective in swelling and dissolving the sol fraction of coal. But keep in mind that unfavorable dispersion interactions are also important. A solvent that has a solubility parameter that is close in value to that of the coal of interest, thus minimizing the value of χ, will be the most effective. It is a balance between all the interactions in the system that will determine phase behavior – the ability of a solvent to swell and dissolve portions of a coal such as Illinois no. 6.

5.4 The Swelling of Coal If one now considers a covalently cross-linked network, and we will assume that Illinois no. 6 coal is such, then the chains or units present cannot become randomly dispersed in a solvent, because the covalent linkages prevent this. All that can happen is that the network can swell, as solvent molecules are absorbed into the material forming a gel. The


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extent of swelling will depend upon the cross-link density. If there are long chains present and only a few cross-links per chain, then one can envisage the chains stretching out between cross-link points forming a highly swollen gel. If the cross-link density is increased, then the chain length between cross-link points will be shorter and they will not be able to stretch out as much, so that the network is correspondingly less swollen. The stretching of the chains affects the entropy of the system and will introduce an additional term into the free energy. The classical approach to the swelling of networks is embodied in the Flory-Rehner theory.96, 97 This assumes that the deformation of the elementary chains of the network is in some fashion affine with the macroscopic deformation of the sample (the chains stretch in proportion to the deformation of the sample). For the swelling of lightly crosslinked synthetic networks it is now clear that this assumption is unrealistic. Neutron scattering results have demonstrated that the chains in swollen networks have approximately the same dimensions as equivalent non-cross-linked chains in solutions of the same concentration. There is still chain expansion, but it is far less than would be predicted on the basis of affine deformation of the network. An alternative approach, based on a topological rearrangement of cross-link junctions and the c* blob models of de Gennes, provides a much better agreement with the experimental observations.98 This latter approach has been applied to coal, but results in a major internal inconsistency. Essentially, the model predicts, reasonably enough, that there are very few statistical segments between cross-link junctions, but this naturally violates the central assumption of the theory that the chains obey Gaussian statistics. A statistical-mechanical model of densely connected networks was then developed.99 Ironically, this approach resurrects the affine assumption and for small deformations of networks, where the number N of freely hinged segments between cross-links is greater or equal to 3, the simple Gaussian result is obtained. We will not go into the details of this treatment here, but simply observe that in treating the swelling of coal one derives an expression for the chemical potential of the solvent, ∆µs, which has the form: ∆" = ∆" + ∆"#$ (5) In this equation ∆µm is the contribution to the chemical potential from mixing terms, while ∆µel is the contribution from elastic terms. The equation is then solved by assuming that the swollen gel is in equilibrium with pure solvent, so that ∆µs in equation 5 can be put equal to zero and the equations solved to calculate cross-link density, or, equivalently, the molecular weight of the chains between cross-link points. It is important to note that if there is extracted material in the solvent in contact with the coal, then ∆µs ≠ 0 and the swelling will be less than when the swollen coal gel is in contact with pure solvent.

5.5 Mechanical Properties of Swollen Coal Gels and Extracts The mechanical properties of swollen coal gels has been examined by a number of authors to evaluate structure and the nature of cross-links in these materials. Stress/strain measurements on such swollen coals were first reported in the seminal work of Brenner.6, 100 This author noted that thin coal sections had distinct rubbery characteristics, but because samples split easily when stretched only compression measurements were possible. Values of the modulus of 4.1 and 2.1 MPa were reported for Illinois No. 6 coal


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and Pittsburgh No. 8 coal, respectively, in the range characteristic of somewhat stiff synthetic elastomers. This work was continued and expanded upon considerably by Cody et al.,101, 102 who used a micro-dilatometer to make viscoelastic measurements on exhaustively extracted pyridine-swollen coal particles. Although the observed mechanical behavior of coals such as Illinois No. 6 had the characteristics of rubber-like materials, the long-term creep behavior of the samples suggested that coal might be a highly entangled network, as opposed to one that is primarily connected by covalent cross-links. These authors noted that the issue is complex, however, in that coal sections and particles are obviously fragile and there could be problems with bond breakage, internal microcrack formation, and so on. Furthermore, it is well-known that cross-linked elastomers also display significant long-term creep properties and compression, albeit at higher stresses than those used in these coal experiments. Subsequently, Cody and Painter103 reported compression measurements on individual Illinois No. 6 coal particles that had been exhaustively extracted and swollen in pyridine. The temperature dependence of the overall or equilibrium modulus of the swollen coal gel was found to be characteristic of a material whose response is primarily entropic, in other words the deformation of the network is accompanied by rearrangements. An associated structure would have given an energetic rather than entropic response. However, the elementary chains are relatively stiff, with an appreciable relaxation time, resulting in a viscoelastic response to an applied load. There was also an initial elastic response that was modeled in terms of the chain being treated as a stiff “bent wire” or frozen random walk, as over short time spans the linked aromatic clusters do not have time to relax, or change their conformational distributions. These results are consistent with the idea that coal is a macromolecular network and not an associated structure and for Illinois No. 6 coal a molecular weight of about 4900 between cross-link points was estimated.103 Not only do individual coal particles display viscoelastic behavior, so do suspensions of extracted coal particles and their extracts when mixed with certain solvents.104, 105 It was proposed that a physical network is formed by coal particle/NMP interactions in the framework of a solvated colloid model where the cross-links in the mixture are due to secondary bonds between the coal and NMP.104-106 However, it was argued above that most secondary interactions are too weak and transient to act as cross-links and we therefore favor an alternative interpretation of the origin of gelation and viscoelasticity in these suspensions. The physical gels formed from soluble extracts are formed from concentrated suspensions in poor solvents. As a result, the system is phase separated into solvent rich and solvent poor domains. Bicontinuous domains, where the solvent poor phase is glassy, would provide the cohesive properties necessary for elastic properties. However, gels formed from insoluble coal particles suspended in poor solvents most probably associate through forces common to many colloidal suspensions. In such systems, it is not the local pairwise interactions between molecules alone that is important, but the sum of all interactions between the molecules in each particle. This results in a large interaction energy that is proportional to particle size, resulting in an association of these particles to form a space filling network and a gel with viscolelastic properties.


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5.6 Binary Solvents There have been a number of very interesting studies involving mixed solvents. In older work, Green and Larsen107 examined the swelling of Illinois no. 6 coal in mixed solvents. They observed that the addition of very small amounts of pyridine leads to an enormous increase in the uptake of chlorobenzene by the coal. They proposed that sites that interact specifically with pyridine can be titrated by measuring the composition of the solvent absorbed by the coal. This was taken up in more detail in a subsequent paper108 where it was proposed that the data could be used to estimate the number of hydrogen-bond crosslinks in the coal. The basis for the calculations was the proposal that pyridine selectively disrupts hydrogen-bond cross-links in the coal, as opposed to hydrogen bonding to free coal OH groups. It was subsequently argued on thermodynamic grounds that this argument is flawed:109 free OH groups must always be the favored site of interaction, not inter-cluster hydrogen bonds. It makes no sense (in terms of statistical mechanics) to argue that certain types of OH groups become distinguishable in a swelling process and are targeted preferentially for disruption by solvent. All one can say is that there is a certain equilibrium distribution of hydrogen bonds in the original coal, a certain distribution of coal/coal and coal/solvent interactions in the swollen state, and a certain contribution to the overall change in the free energy from the difference in these two states. The most intriguing work concerning the use of mixed solvents to swell coal and extract soluble material is that of Iino and coworkers.46, 48, 50, 73-77 They found that NMP/CS2 mixed solvents can significantly increase the swelling of coals and the extraction yields of soluble material. However, much of this work is based on results obtained from a single coal, Upper Freeport, about 59% of which is soluble in NMP/CS2 mixtures compared to only 35% in pyridine. This coal appears to be anomalous. The NMP/CS2 solubility of Illinois no. 6 and other Argonne Premium coals are comparable to those obtained with pyridine before acid treatment. Acid treatment increases the yields even further as a result of a removal of the cations associated with ionic domains,82, 83 as discussed above. Nevertheless, the observations of Iino et al. remain intriguing. In this regard, it has been shown that mixed solvent NMP/CS2 can extract cations from various clays and form complexes with them that strongly absorb light in the visible region. The individual solvents cannot form such complexes. Takanohashi et al.73 observed that when certain salts are added to the NMP/CS2 mixed solvent system extraction yields are enhanced to an extent that is proportional to the size of the anion involved. Similarly, Chen et al.74 demonstrated that an anion formed from tetracyanoethylene also promotes solubility. These anions can associate with positively charged organic functional groups in certain coals or species such as cations that through cooperative interactions (e.g., πcation) act as cross-links in these coals. The ability of NMP/CS2 mixed solvents to form complexes with cations, while the parent solvents cannot, could therefore be at the heart of their ability to dissolve larger fractions of certain coals than other solvents. The role of ionic interactions in coal is a topic we will revisit below, where we consider ionic liquids.


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5.7 A Novel Class of Solvents – Ionic Liquids It was noted above that a remarkable swelling value of 4 was recorded for solutions of Illinois No. 6 in tetrabutyl ammonia hydroxide solutions.110 This is probably a consequence of the ionic liquid (IL) character of this solvent. Ionic liquids consist entirely of ionic species. They usually comprise large organic cations associated with various anions and melt at or below 100˚C as a result of their asymmetric structures. They have an unusual range of properties such as outstanding chemical and thermal stability. They are non-flammable and have a negligible vapor pressure (i.e., they are essentially non-volatile). In recent work it was shown that the IL 1-butyl-3-methylimidazolium chloride, [bmim][Cl], could dissolve and disperse Illinois No. 6 coal to a remarkable degree.111 This IL is a solid at room temperature, but apparently melts near 80˚C (the observed melting temperature depends on the water content of this hydrophilic material). It was found that if Illinois No. 6 coal is placed in contact with this IL and the powders are heated to 100˚C, a dark black solution/dispersion is obtained and the mixture remains a liquid on cooling to room temperature. The portion of the coal that dissolved could not be easily determined, because filtration stopped very early in the process, as a result of the formation of a highly viscous gel as the coal/solvent mixture became more concentrated. Centrifugation at ordinary speeds was also inefficient, because there are very fine, swollen coal particles in these mixtures. This can be seen in Figure 2, which shows a transmission optical micrograph (x200) of a thin smear of Illinois No. 6 coal dispersed in [bmim][Cl]. The original coal was characterized by numerous particles of the order of 75-100 µm in size. After mixing with the IL, there are a few residual large particles and the bulk of the sample appears to have become dispersed as small particles, many of the order of 10 µm or less in diameter.


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Figure 2. Optical micrograph of a thin smear of an Illinois #6 coal dispersed in [bmim][Cl].111 Because of the difficulties encountered in attempting to filter and centrifuge the viscous dispersion, the IL/coal suspension was diluted with NMP. The dispersion could be filtered and the soluble material precipitated in water. It was found that more than 40% of the Illinois No. 6 coal had dissolved in this mixture, compared to about 16% of the coal in NMP alone.111 Similar large increases in solubility were obtained with IL/pyridine mixtures. It was proposed that the remarkable degree of fracturing that occurs in these mixtures is a consequence of the high stresses that can be generated by the swelling process. This has been observed in the swelling of glassy polymer networks as a result of a mismatch in the moduli of the solvent swollen rubbery domains and the remaining unswollen glassy core. In coals, a mismatch in the swelling tendencies of different types of macerals, the presence of mineral inclusions and various other heterogeneities that can lead to uneven swelling would play a part. The unusual degree of fracturing and dispersion would then be a consequence of a rapid and large build up of stresses when the coal is immersed in [bmim][Cl]. This dispersion is highly beneficial for coal-catalyst interactions where improved liquefaction yields have been obtained.112 The ability of certain ILs to disintegrate and disperse Illinois No. 6 coal must be related to the intermolecular interactions between the components of these mixtures. These cover a wide range, from the electrostatic (between ILs and mineral surfaces and chelated ions in the coals), ion-dipole and dipole-dipole interactions to in some cases even hydrogen bonding (the C2 hydrogen, the hydrogen attached to the carbon between the two nitrogen atoms in the imidazolium ring, hydrogen bonds). It is also well known that ILs can engage in π-cation interactions. It is possible that the replacement of metal cations with bulky IL groups plays a role in the fracturing and dispersion that was observed


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It is also interesting and important that ILs allow a far larger proportion of Illinois No. 6 coal to dissolve in NMP and (to a lesser extent) pyridine. It was proposed that this is because these coals consist of entities that are indeed cross-linked networks rather than associated structures. In rubber-like networks (i.e., lightly cross-linked) made up of flexible chains the “sol” fraction is usually readily extracted, the macromolecules being able to diffuse out by a reptation-type process. However, Aharoni and Edwards113 pointed out a number of years ago that if both the network and sol are highly branched and rigid, the latter would be trapped. They showed that solvent extraction of such networks fails to remove highly branched macromolecules. In such a densely connected network it is likely that a significant portion of soluble material can be obtained as a result of breaking just a few bonds, releasing high molecular weight but soluble material. These results indicate that there is a significant quantity of trapped, soluble material in Illinois No. 6 coal. Low-temperature treatment of Illinois coal with ionic liquds has also been explored demonstrating that at temperatures of 300°C or less a dramatic increase in pyridine solubility was obtained with 1-butyl-3-methylimidazolium tetrafluoromethanesulfonate and tetralin under hydrogen pressure.114 This choice of ionic liquid was made for reasons of thermal stability. Extensive fragmentation of Illinois no. 6 coal was observed. Unfortunately there was also considerable variability in yield with >90% extraction in one case.

6. Implications for Studies of Illinois no. 6 Coal With the theoretical background and a summary of the behavior of this coal we can now rationalize much of the swelling data (Table 1) and extraction yields previously discussed.

6.1 Swelling of Extracted vs. Unextracted coal This follows directly from the points made earlier. It also explains the observation that when an unextracted coal is placed in contact with a good solvent like pyridine, swelling increases initially with time, appears to reach a maximum value, then decreases. This is because the chemical potential of the solvent in contact with the coal changes as soluble material is extracted, resulting in a lower equilibrium swelling value relative to what is observed when extracted coal is in contact with pure solvent.

6.2 Why do Some Solvents Swell Coal more than Others? It is clear that non-hydrogen bonding solvents such as benzene or cyclohexane do not swell Illinois no. 6 coal to any significant extent. First, this is because they do not have acceptor groups that can hydrogen bond to the phenolic OH groups found in this coal (although hydrogen bonds between OH groups and the π-bonds of aromatic molecules do occur, these are relatively weak). Second, Illinois No. 6 probably has a solubility parameter in the range 11-12 (cal.cm–3)0.5.115 Benzene has a solubility parameter close to


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9.1 (cal.cm–3)0.5,115 while cyclohexane has a solubility parameter of about 8.1 (cal.cm– 3 0.5 89 ) . Accordingly, the value of χ for cyclohexane mixtures is large and unfavorable to mixing and is still appreciable for benzene mixtures. Pyridine and NMP, on the other hand not only have strong hydrogen bond acceptor groups (basic nitrogen), but also solubility parameters close to 11 (cal.cm–3)0.5,89 maximizing favorable hydrogen bonding interactions and minimizing unfavorable differences in dispersion and weak polar interactions between the pure components and mixtures. However, there is one surprising result that cannot be completely explained by the factors discussed so far: the unusual degree of swelling observed using tetrabutyl ammonium hydroxide. This solvent is not only a strong base, but has been classified as a member of a novel class of solvents called ionic liquids. These unusual solvents will be considered separately below.

6.3 Why do acetylated or methylated samples swell more in certain solvents? As described earlier, Illinois No. 6 coal strongly self-associates through hydrogen bonds between phenolic groups and other functional groups within the coal. If these OH groups are methylated or acetylated, then this factor is eliminated in the equations for the free energy and mixing with non-hydrogen bonding solvents that have a solubility parameter value close to that of the coal becomes more favorable. In this regard, Larsen et al.32 observed a significant increase in swelling in non-hydrogen bonding solvents upon acetylation. On the other hand, Nishioka and Larsen116 observed a decrease in the swelling of Illinois no. 6 coal upon reaction with maleic anhydride. This was surprising, in that it was thought that this reaction would introduce bulky groups that would disrupt π−π stacking. However, the reaction was conducted using chlorobenzene as a solvent and the authors noted that this reduction in swelling (and soluble extraction yields) could be related to a relaxation of the coal to a more stable configuration. They observed that coals that were heated in pyridine, chlorobenzene or toluene displayed a decrease in swelling and extraction yields. Nishioka et al.5 also observed a decease in solvent extraction yields upon soaking in various solvents, which they interpreted in terms of the formation of charge-transfer complexes.

6.4 Associations and Relaxation Larsen et al.11 investigated the effect of solvent soaking in more detail in subsequent work. They first noted that native coals are anisotropically strained and swell approximately twice as much perpendicular to the bedding plane as they do parallel to the bedding plane on their first exposure to solvents. Subsequent swelling is isotropic, however, indicating that the coal relaxes to a lower free energy structure upon initial exposure to solvent. They then determined that simply soaking various coals (including Illinois No. 6) in chlorobenzene (at 115 °C) for a few days resulted in a significant decrease in the amount of pyridine-extractable material. The swelling ratio of the gel also decreased. Soaking in pyridine for just 1 day had the same effect. (In work that preceded this, Takanohashi and Iino117 also reported that refluxing with pyridine greatly decreased the NMP/CS2 extractability of certain coals.) Larsen et al.11 also observed that there was no evidence for the formation of new covalent bonds, which would be unlikely under the conditions of their experiments. They suggested that relaxation to a more associated structure occurs. Subsequent work by Painter et al.82, 83 demonstrated that the increase in


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association is due to the formation of ionic complexes involving carboxylic acid groups and π-cation interactions, which could be removed by acid washing, restoring and even enhancing the degree of swelling and extraction yields. It was proposed that contact with solvent during soaking experiments plasticizes the coal matrix, allowing sufficient mobility for ion exchange (involving mineral matter) and a phase separation of ionic domains from the hydrocarbon matrix to occur. Because at least one of the ions pairs (e.g., COO– groups) is attached to the hydrocarbon coal matrix, steric factors limit the size of these ionic clusters, so that ionic microdomains are scattered throughout the material. These “junction zones” act as cross-links, because the cumulative strength of the ionic interactions in these clusters is much larger than RT. This is what occurs in the class of polymers known as ionomers (e.g., surlyn). Although the concentration of such groups is usually small, their presence has a disproportionate effect on behavior and this would explain the strong effect of solvent soaking on swelling and extraction yields.

6.5 Implications for Cross-Linked and Associated Structure Given high extraction yields with select bituminous coals with CS2/NMP the commonly held view of coal as a cross-linked macromolecular structure was challenged with an associated coal structure model being proposed.118, 119 Swelling evidence supports a cross-linked structure and solvent extraction supports a considerable amount of extractable material. These statements are not necessarily in conflict.

7. Acknowledgements This project was funded by the Illinois Clean Coal Institute with funds made available through the Office of Coal Development of the Illinois Department of Commerce and Economic Opportunity.

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Interactions of Illinois No. 6 Bituminous Coal with Solvents: A Review

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