Mobilization of the Hydroxyls in a Brown Coal withSolvent-Induced Swelling Evaluated by Pulsed 1H NMR

Koyo Norinaga,*,† Jun-ichiro Hayashi, Ryota Kato, and Tadatoshi Chiba

Center for Advanced Research of Energy Technology (CARET), Hokkaido University,N13, W8, Kita-ku, Sapporo 060-8628, Japan

Received October 12, 1999. Revised Manuscript Received December 21, 1999

The amount of hydroxylic hydrogen mobilized with the solvent-induced swelling of a browncoal was measured using a pulsed 1H NMR technique. Yallourn brown coal (YL), and deuteratedYL (YL-d), prepared by selectively deuterating all the hydroxyls in YL, were swollen in binarysolvents, including different pyridine-d5/benzene-d6 and dimethyl sulfoxide(DMSO)-d6/benzene-d6 mixtures, at 303 K. The volumetric swelling ratio (Q) and the amount of mobile hydrogenwere measured as a function of the composition of the solvent. The amount of hydroxylic hydrogenremaining immobile, fIM(OH), was also determined experimentally by comparing the amounts ofmobile hydrogen in YL and YL-d. The fIM(OH) of YL in DMSO-benzene mixtures was found to besmaller than that in pyridine-benzene mixtures over the entire range of Q from 1 to 3.5,demonstrating that DMSO reduces the hydrogen bonding associations between coal macromol-ecules more extensively than pyridine does for the swelling with equivalent Q. The effects ofthese polar solvents on the relationship between fIM(OH) and Q can be attributed to the differencesin their hydrogen bonding abilities.

Introduction

It is widely recognized that hydrogen bonds occurringbetween the hydroxyls attached to the macromoleculesin coal strongly influence its characteristics of solvent-induced swelling. Larsen et al.1-4 thoroughly docu-mented the effects of hydrogen bonding interaction onthe swelling. One of their most interesting observationswas that when even a very small amount of hydrogenbond acceptors, such as pyridine and tetrahydrofuran,are added to a nonpolar solvent like chlorobenzene, theswelling ratio of bituminous coals increases remark-ably.4 They found that a plot of the swelling ratio versusthe hydrogen bond acceptor concentration is similar toa titration curve, and also observed that methylationof the hydroxyl groups permitted full swelling in theabsence of hydrogen bond acceptors. The swelling of coalis greatly enhanced when hydrogen bonding of coalmacromolecules is replaced by hydrogen bonding be-tween coal and solvent molecules, or when the hydroxylsare hindered. Although the roles of hydrogen bondingassociations in coal swelling have been well verifiedqualitatively, there is little quantitative informationavailable on the contribution of breakage of the hydro-gen bonds between coal macromolecules to the volu-metric swelling, and this forms the subject matter ofthe present study.

Since most hydrogen bonds in a coal matrix involvehydroxyls, the number of hydrogen bonds is approxi-mated by the number of hydroxyls participating in inter-macromolecular hydrogen bonds and in hydrogen bondsbetween macromolecules and solvent molecules. Toquantify the number of hydroxyls that are hydrogen-bonded to solvent molecules, to distinguish them fromthose involved in inter-macromolecular hydrogen bonds,we focused on the mobility of the hydroxylic hydrogens,which can be evaluated with a 1H NMR technique. Themeasured transverse relaxation can distinguish molec-ular structures and lattices on the basis of the molecularreorientation rate. When the rate is below approxi-mately 105 Hz, the molecular structures are deemedrigid; otherwise they are considered mobile. The mobileprotons experience exponential decay in the observedfree induction decay curve, while rigid protons undergoGaussian decay. 1H NMR techniques have been exten-sively applied to characterize the macromolecular struc-ture of solvent swollen coal and can yield quantitativeinformation about the relative abundance of hydrogenatoms that exist in significantly different physical andchemical environments in coals.5-10 Since solvent-swell-ing causes the self-associated molecular segments incoal to gain significant configurational freedom, thehydrogens in these segments would be detected asmobile hydrogens through 1H NMR transverse relax-

* Author to whom all correspondence should be addressed.† Present address: Institute for Chemical Reaction Science, Tohoku

University, Katahira, Aoba-ku, Sendai, 980-8577, Japan. Fax: +81-22-217-5655. E-mail: norinaga@hisui.icrs.tohoku.ac.jp.

(1) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538.(2) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50,

4729.(3) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4, 74.(4) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy

Fuels 1996, 10, 1269.

(5) Jurkiewicz, A.; Marzec, A.; Idziak, S. Fuel 1981, 60, 1167.(6) Jurkiewicz, A.; Marzec, A.; Pislewski, N. Fuel 1982, 61, 647.(7) Barton, W. A.; Lynch, L. J.; Webster, D. S. Fuel 1984, 63, 1262.(8) Kamienski, B.; Pruski, M.; Gerstein, B. C.; Given, P. H. Energy

Fuels 1987, 1, 45.(9) Yang, X.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1993,

7, 439.(10) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Energy Fuels 1994,

8, 266.

503Energy & Fuels 2000, 14, 503-508

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ation measurements. Although the wide line NMRtechnique cannot identify which structural units in coalcontribute to an individual component of the multipledephasing components observed, the amounts of mobilehydroxylic hydrogen can be quantified by using areference sample prepared by selectively deuterating allthe hydroxylic hydrogens in coal.11 A comparison ofnondeuterated and deuterated coal samples makes itpossible to quantify the mobile hydroxylic hydrogen.However, one may suppose that the hydroxylic hydro-gens in the swollen coal are less mobile than those inthe nonswollen coal since the hydroxyl groups in theswollen coal form stronger hydrogen bonds than in thenonswollen state. The 1H NMR transverse relaxationrate depends on the strength of the interaction of thenuclear magnetic dipoles. Hence, if the mobilized coalsegments include the hydroxyl groups, the hydroxylichydrogens can be detected as mobile hydrogens, as faras we use deuterated solvent. Because there never existsthe interaction of the nuclear magnetic dipoles betweencoal hydroxyls and deuterated swelling solvent.

In this study, the swelling of a brown coal is studiedin which the hydrogen bonding associations are domi-nant because of its large abundance of oxygen-contain-ing functional groups. Nondeuterated and deuteratedbrown coals were swollen in binary solvents, such asperdeuterated pyridine-benzene or dimethyl sulfoxide-benzene mixtures. The use of a binary solvent has anadvantage, in that changing the solvent compositioncontinuously and arbitrarily varies the solvent proper-ties, such as solubility parameter. This allows us toprepare coal samples with different degrees of swellingwithout changing the combination of nonpolar and polarsolvents. The number of mobile hydroxyls is quantifiedby comparative NMR measurements of nondeuteratedand deuterated coal samples. The contributions of themobilization of hydroxylic and non-hydroxylic hydrogento the volumetric swelling are quantitatively evaluated.The effect of the type of polar solvent on the contributionis also discussed.

Experimental Section

As-received Yallourn brown coal was dried under a pressureof less than 1 Pa at 333 K for 48 h, which was long enough forit to attain a constant weight. The elemental composition ofthe dried Yallourn coal (hereafter referred to as YL) was C )65.0 wt %, H ) 4.6 wt %, N ) 0.6 wt %, S ) 0.2 wt %, and O) 29.6 wt % on a dry-ash-free basis. Deuterated YL (YL-d)was prepared by selectively and completely deuterating thehydroxylic hydrogen in YL. The procedure used for deuterationhas been reported elsewhere.11 A 200 mg sample of YL or YL-dwith particle sizes finer than 150 µm was transferred to asample tube with a 10 mm o.d. This tube was charged with 1mL of deuterated benzene and a known amount of a deuter-ated polar solvent such as methanol, dimethyl sulfoxide, orpyridine. The deuterated solvents had isotopic purities of99.9% or higher. The deuterated benzene, methanol, dimethylsulfoxide, and pyridine are hereafter referred to as benzene-d6, methanol-d4, DMSO-d6, and pyridine-d5, respectively. Theamount of polar solvent is referred to as Ms using moles perkilogram of coal as the units. When Ms exceeded 14, theamount of benzene was reduced so that the total volume ofthe binary solvent was 1 mL. The swollen samples were

carefully prepared in a dry-N2-filled box since preliminaryobservations indicated that the deuterium in YL-d is easilyexchanged with hydrogen from water in the atmosphere. Thetube was sealed under a pressure of less than 2 Pa while thecontents were frozen in liquid nitrogen. The coal-solventmixture was stored at 303 K for at least 1 month prior to theexperiments. The volumetric swelling ratio (Q) was determinedfollowing the method reported by Green et al. 12 The deutera-tion of YL had no effect on the observed Q values. Therelaxation measurements were carried out at 303 K using aproton-pulsed NMR spectrometer (JEOL Mu-25) operating at25 MHz. The solid-echo pulse sequence,13 90°x-τ-90°y (90° phaseshift) provided an approximation to the complete free inductiondecay (FID). Typical values for the pulse width, pulse spacing,repetition time, and number of scans were 2.0 µs, 8.0 µs, 6 s,and 64 s, respectively.

Results and Discussion

Effect of the Polar Solvent on Swelling Ratio.Figure 1 plots the Q of YL against Ms for methanol-d4,pyridine-d5, and DMSO-d6. Q is seen to increase withincreasing Ms from 1.0 to 1.5, 1.6, and 1.9 at Ms ) 14for methanol-d4, pyridine-d5, and DMSO-d6, respec-tively. Assuming that the YL completely absorbed thepolar solvents in the binary mixtures, the increase inthe volume of YL at Ms ) 0.5 is approximately 5 timeslarger than the volume of the polar solvent. Larsen etal.4 explained how this volume increase occurs. Theincorporation of benzene-d6 results from the disappear-ance of hydrogen bonding interactions between coalmacromolecules induced by absorption of the polarsolvent. However, our plots were not similar to thetitration curves reported by Larsen et al. 4 regardlessof the type of polar solvent. They observed that addinga very small amount of pyridine to chlorobenzene causeda remarkable increase in the swelling ratio of IllinoisNo. 6 coal, which leveled off when the coal absorbed 0.6mol-pyridine/kg coal. Although Larsen et al.4 do not

(11) Norinaga, K.; Kumagai, H.; Hayashi, J.-i.; Chiba, T. EnergyFuels 1998, 12, 1013.

(12) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935.(13) Powles, J. G.; Mansfield, P. Phys. Lett. 1962, 2, 58.

Figure 1. Plots of Q versus Ms for YL in the binary solventsmethanol-d4/benzene-d6, pyridine-d5/benzene-d6, and DMSO-d6/benzene-d6.

504 Energy & Fuels, Vol. 14, No. 2, 2000 Norinaga et al.

mention the amount of chlorobenzene, the amount waslikely much larger than in our case. In their case, theadded amount of polar solvent is negligible, so theproperties of the binary solvent, such as the solubilityparameters, are almost the same as those of chloroben-zene. The results of Green and Larsen1 support thisspeculation. They examined the effect of the compositionof a pyridine-chlorobenzene binary solvent on theswelling ratio of pyridine-extracted Illinois No. 6 coal.They used 400 mg of coal and 2 mL of the binarysolvent. Despite a rapid increase in the swelling ratiowith increasing pyridine concentration, the swelling didnot level off, even when the coal absorbed 7.4 molpyridine/kg coal. Hence, the amount of nonpolar solventper mass of coal seems to be a crucial factor influencingthe results.

In Figure 2, the Q of YL in the binary solvents isplotted against the Hildebrand solubility parameter,14

which was calculated assuming that the deuteratedsolvents have parameter values (SP) identical to thoseof the corresponding nondeuterated solvents. For metha-nol-d4/benzene-d6, Q change with SP through a maxi-mum of around 25 J1/2/cm3/2. For the other binarysolvents, Q increases monotonically with SP and reachesmaxima at the SP of the individual polar solvents,specifically 21.7 for pyridine-d5 and 26.2 for DMSO-d6.Since the solubility parameter describes only nonspecificvan der Waals interactions, it is not surprising that Qas a function of SP depends appreciably on the type ofpolar solvent. Specific interactions, such as hydrogenbonds between YL and the polar solvents, should disruptthe inter-macromolecular hydrogen bonds, thereby en-hancing the swelling of the coal. However, experimentalevidence of this disruption has not been explored, andthis is why we are trying to quantify mobile hydroxylsbased on the molecular mobility, and to evaluate thecontribution of hydrogen bond breakage to the swelling.

Proton Transverse Relaxation Characteristics.The effect of Ms on the transverse relaxation signals forYL swollen in DMSO-d6/benzene-d6 are shown in Figure3. The slowly decaying components appear after 100 µs,and the amplitude increases with increasing Ms. Basedon the examinations on the deconvolution of the entirerelaxation decay into Gaussian and exponential func-tion, the following equation gave the best approximationand was used to fit the signal.

where I(t) and Ii(t) are the observed intensity at time t,and that attributed to component i, respectively. InFigure 4, the observed and fitted I(t) for YL swollen inDMSO-d6/benzene-d6 at Ms ) 6.85 were compared. Thestatistical errors of the fit were generally smaller than1%. Since the attenuation of the Gaussian componentis negligible under the present conditions,7,15,16 theintensity at 0 µs corresponds to the total amount ofhydrogen in the specimen. The amount of mobilehydrogen (CMH [mol/kg-coal]) yielding the exponentialdecay can be calculated using

where CH is the molar amount of hydrogen in YL. CMHwas found to be approximately 1 mol/kg-coal at Ms ) 0where YL was little swollen. Here, the net increase inthe amount of mobile hydrogen brought about by theaddition of polar solvent is defined as the differencebetween CMH at a given Ms and that at Ms ) 0, andreferred to as ∆CMH. ∆CMH is plotted against Ms inFigure 5. In both pyridine-d5/benzene-d6 and DMSO-

(14) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular andRelated Solutions; Van Nostrand Reinhold Co.: New York, 1970.

(15) Lynch, L. J.; Webster, D. S.; Barton, W. A. Adv. Magn. Reson.1988, 12, 385.

(16) Norinaga, K.; Kumagai, H.; Hayashi, J.-i.; Chiba, T. EnergyFuels 1998, 12, 574.

Figure 2. Plots of Q versus the solubility parameter for YLin the binary solvents methanol-d4/benzene-d6, pyridine-d5/benzene-d6, and DMSO-d6/benzene-d6.

Figure 3. Effect of Ms on the transverse relaxation signalsof YL swollen in the binary solvent DMSO-d6/benzene-d6.Reading from the bottom, Ms equals 0, 1.01, 2.91, 4.88, 6.85,9.78, and 14.0 mol/kg-YL.

I(t) ) IG1(0) exp[-t2/2T2G12] +

IG2(0) exp[-t2/2T2G22] + IL(0) exp[-t/T2L] (1)

CMH ) CHIL(0)/I(0) (2)

Mobilization of the Hydroxyls in a Brown Coal Energy & Fuels, Vol. 14, No. 2, 2000 505

d6/benzene-d6, the ∆CMH of YL increases with increasingMs. It is also noted that the ∆CMH for DMSO-d6/benzene-d6 is larger than that in pyridine-d5/benzene-d6. Asshown in Figure 6, the ∆CMH of YL increases with Q,indicating that greater expansion of the network furtherenhances mobilization of the organic structure of YL.The ∆CMH of YL swollen in DMSO-d6/benzene-d6 islarger than that in pyridine-d5/benzene-d6 for swellingwith equivalent Q. This indicates that ∆CMH is neverdetermined by the degree of swelling alone, suggestingthe effect of a polar solvent on the mechanism of theswelling-induced mobilization of hydrogen in YL.

Quantification of Mobilized Hydroxylic Hydro-gen. Norinaga et al. 11 demonstrated that the hydroxylgroups of YL can be completely and exclusively deuter-ated when exposed to gaseous D2O at reduced pressures.

The effects of added amount of polar solvent on the∆CMH of YL-d swollen in DMSO-d6/benzene-d6 andpyridine-d5/benzene-d6 are shown in Figure 7, parts aand b, respectively. The plots of ∆CMH versus Ms for YL

Figure 4. 1H NMR transverse relaxation signal of YL swollenin DMSO-d6/benzene-d6 at Ms ) 6.85, approximated by twoGaussian components and a slowly decaying one.

Figure 5. Plots of ∆CMH versus Ms for YL in the binarysolvents pyridine-d5/benzene-d6 and DMSO-d6/benzene-d6.

Figure 6. Plots of ∆CMH versus Q for YL in the binarysolvents pyridine-d5/benzene-d6 and DMSO-d6/benzene-d6.

Figure 7. Plots of ∆CMH versus Ms for YL and YL-d. (a)DMSO-d6/benzene-d6, (b) pyridine-d5/benzene-d6.

506 Energy & Fuels, Vol. 14, No. 2, 2000 Norinaga et al.

are reproduced therein for comparison. Since the hy-droxyl groups of YL-d are undetectable by 1H NMR,∆CMH of YL-d represents the amount of mobilized non-hydroxylic hydrogen in YL. Therefore we can determinethe amount of mobile hydroxyls, ∆CM(OH), by subtracting∆CMH of YL-d from that of YL at a given Ms. As shownin Figure 8, ∆CM(OH) is rapidly increased by the additionof less than 5 mol of polar solvent/kg-coal and isgradually increased by adding more. DMSO-d6 seemsto be a more powerful solvent for mobilizing hydroxylsthan pyridine-d5. The total amount of hydroxylic hy-drogen in YL, COH, was determined to be 8.1 mol/kg-coal from the initial intensities of the transverse relax-ation signals of YL and YL-d.11 Although the hydroxylsare remarkably mobilized at the initial stage of swelling,∆CM(OH) never equals COH, even at Ms ) 14. The extentsof the mobilization of both hydroxylic hydrogen and non-hydroxylic hydrogens are larger for YL swollen inDMSO-d6/benzene-d6 than for YL swollen in pyridine-d5/benzene-d6. The SP of DMSO and pyridine are 26.2and 21.7 J1/2/cm3/2, respectively. Therefore, there is abetter match of the solubility parameter between DMSO-d6/benzene-d6 and YL than between pyridine-d5/benzene-d6 and YL at an equivalent Ms. Since the free energychanges of random mixing tend to be more favorable inDMSO-d6/benzene-d6, nonspecific or van der Waals typeinteractions are reduced, thereby producing more ex-tensive swelling and mobilizing non-hydroxylic hydro-gens. The fraction of hydroxylic hydrogen remainingimmobile, fIM(OH), compared to the total hydroxylichydrogen is given by

Figure 9 shows fIM(OH) as a function of the volumefraction of YL in the swollen gel, ΦC ()1/Q). Painter etal.17,18 considered this type of plot theoretically, and

showed that it demonstrates the contribution of thedecrease in the number of hydrogen bonding associa-tions between coal macromolecules to the volumetricswelling. They simulated coal swelling using a modifiedFlory-Huggins expression, involving a term that de-scribes the thermodynamic effect of hydrogen bondingon mixing, and estimated the effect based on an as-sociation model.19 Their calculation clearly demon-strated the effect of the hydrogen bonding ability of thesolvent on the fraction of molecular segments that areself-associated via hydrogen bonds for swelling in py-ridine and tetrahydrofuran. The hydrogen bondingabilities of these solvents were represented by theassociation equilibrium constants for the formation ofa hydrogen bond between phenol and solvent. Pyridineforms much stronger hydrogen bonds with phenolichydroxyl groups than tetrahydrofuran does. Painter etal. predicted that the number of coal/coal hydrogenbonds remaining in tetrahydrofuran is larger than thatin pyridine for swelling with equivalent swelling ratio.Figure 9 demonstrates that the fIM(OH) of YL in DMSO-d6/benzene-d6 is smaller than that in pyridine-d5/benzene-d6. Spencer et al.20-22 studied the hydrogen bonding ofphenol to DMSO or pyridine in benzene solutions as afunction of temperature by monitoring the change in thehydroxyl stretching frequencies, and evaluated theequilibrium constants at 298 K for the phenol-DMSOand phenol-pyridine complexes to be 106 and 25,respectively. This indicates that DMSO forms hydrogenbonds with phenolic hydroxyl groups more readily thanpyridine. Thus, the effects of the polar solvent on therelationship between fIM(OH) and ΦC can be attributed

(17) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. EnergyFuels 1990, 4, 384.

(18) Painter, P.; Shenoy, S. Energy Fuels 1995, 9, 364.

(19) Kretschmer, C. B.; Wiebe, R. J. Chem. Phys. 1954, 22, 1697.(20) Spencer, J. N.; Harner, R. S.; Penturelli, C. D. J. Phys. Chem

1975, 79, 2488.(21) Spencer, J. N.; Sweigart, J. R.; Brown, M. E.; Bensing, R. L.;

Hassinger, T. L.; Kelly, W.; Housel, D. L.; Reisinger, G. W. J. Phys.Chem 1976, 80, 811.

(22) Spencer, J. N.; Sweigart, J. R.; Brown, M. E.; Bensing, R. L.;Hassinger, T. L.; Kelly, W.; Housel, D. L.; Reisinger, G. W.; Reifsnyder,D. S.; Gleim, J. E.; Peiper, J. C. J. Phys. Chem. 1977, 81, 2237.

Figure 8. Plots of ∆CM(OH) versus Ms for YL in the binarysolvents pyridine-d5/benzene-d6 and DMSO-d6/benzene-d6.

fIM(OH) ) 1 - ∆CM(OH)/COH (3)

Figure 9. Changes in fIM(OH) with ΦC for YL swollen in thebinary solvents pyridine-d5/benzene-d6 and DMSO-d6/benzene-d6. The full lines are guides for the eye.

Mobilization of the Hydroxyls in a Brown Coal Energy & Fuels, Vol. 14, No. 2, 2000 507

to differences in their hydrogen bonding abilities. Theo-retical and quantitative assessments of these observa-tions are now in progress and will be reported in thefuture.

Conclusions

The mobilization of hydrogen in YL upon its swellingin binary solvents such as DMSO-d6/benzene-d6 andpyridine-d5/benzene-d6 was studied. The mobilities ofhydroxylic hydrogen can be successfully evaluated byusing reference brown coal samples, YL-d, with com-pletely deuterated hydroxyl groups that are undetect-able by 1H NMR. The fIM(OH) of YL in DMSO-benzenemixtures was found to be smaller than that in pyridine-benzene mixtures over the entire range of Q from 1 to3.5, demonstrating that DMSO decreases the number

of hydrogen bonding associations between coal macro-molecules more extensively than pyridine for the swell-ing with equivalent Q. The effect of these polar solventson the relationship between fIM(OH) and Q can beattributed to the difference in their hydrogen bondingabilities.

Acknowledgment. This work was supported in partby a “Research for the Future Project” grant from theJapan Society for the Promotion of Science (JSPS),through the 148 Committee on Coal Utilization Tech-nology. The authors are grateful to Drs. Tadashi Yoshi-da and Masahide Sasaki of the Hokkaido NationalIndustrial Research Institute for their useful advice onthe NMR measurements.

EF990212L

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