Energy & Fuels 1994,8, 1370-1378
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Solution Structure of Coal Macromolecules in Pyridine: Small-Angle Neutron Scattering Analysis of Untreated and O-Methylated Coal Extracts? George D. Cody,* Pappannan Thiyagarajan,$ Robert E. Botto, Jerry E. Hunt, and Randall E. Winans Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received June 17, 1994@
Small-angle neutron scattering (SANS) has been applied to investigate the solution state characteristics of pyridine extracts obtained from three Argonne Premium coals: APCS No. 3 , 2 , and 8. In order to investigate the role of specific solvent-solute interactions, both untreated and O-methylated extracts were investigated. SANS analysis reveals that the solution structure of lower rank coal extracts exist as small particles, with radii -80 A, which loosely aggregate into topologically complex solution structures. Details on the structure of the aggregates are obtained from the scattering behavior at low Q by assuming a pair correlation function which follows a power law decay from the center to the perimeter of the aggregate. The dimensionality of the aggregates indicates mass fractal topology. Changes in the solution state due to O-methylation are manifested in densification of the extended solution structure. The size and shape of the elemental particles appear unaffected by the derivatization chemistry.
Introduction Interest in the physics and chemistry of coal extracts has extended over 100 years. It had been recognized very early on that pyridine was a particularly good solvent for coal, capable of extracting greater than 20 wt % for many coa1s.l Research into the physical properties of coal extracts, early in this century, however, led to a consensus that these mixtures are colloidal in nature; i.e., they constituted dispersions rather than true solutions, with thermodynamics governed principally by surface potentials rather than mixing potent i a k 2 During the 1940s and 1950s an alternative view evolved that coal extract-solvent mixtures were instead “molecular”solutions. These assertions were supported by analytical techniques such as viscometry and vapor pressure o ~ m o m e t r y . ~ This , ~ was an important intellectual shift and set the stage for the modern paradigm of coal structure, Le., the macromolecular network theory of coal structure. This model of the structure of bituminous coals considered the occurrence of two fundamental phases: a solvent-extractible fraction and a nonextractible covalently cross-linked network fraction. Recently, however, there has been mounting evidence that challenges this model of coal structure. For example, the work of Iino and co-workers4 has demonstrated that large extractibilities, in some cases greatly exceeding those possible using pyridine, are attainable + This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Enygy, under contract No. W-31-109-ENG-38. Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, I1 60439. Abstract published in Advance ACS Abstracts, September 15,1994. (1)van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1960. (2) Hinshelwood, C. N The Structure of Physical Chemistry, Clarendon Press: Oxford, U.K., 1952. (3) Dormans, N. N. M.; van Krevelen, D. W. Fuel 1960,39, 273. (4) Sanakawa, Y.; Takanohashi, T.; Iino, M. Fuel 1990,69,1577. 7
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given a judicious selection of solvents and complexing agents. Recent work by Nishioka has also questioned the case for structural limits in extractibility by demonstrating the role of strong associations in the structural characteristics of low- and medium-rank coals.5 Finally, a recent investigation by Cody et al.,6,7into the viscoelastic behavior of a selection of pyridine-dilated bituminous coals, has revealed behavior which is inconsistent with the macromolecular structure of the pyridine-insoluble portion of coal being a covalently cross-linked network. Several questions arise in light of these recent results. Perhaps the most compelling is the following: presumably lacking a covalent network structure, what limits the extractibility of coal in pyridine? It may be that pyridine is not capable of solvating all potentially extractible material. The potential of noncovalently bound, yet nonextractible components existing in coal has been suggested p r e v i o u ~ l y . ~ , ~ This question may be addressed through an analysis of the extract fractions directly. Unfortunately, coal extracts have proven to be difficult to characterize. For example, there have been numerous studies of the mass distribution within pyridine extracts utilizing a very broad range of analytical techniques.8,10-18 Neverthe(5) Nishioka, M. Fuel 1993,72, 1719. (6) Cody, G. D.; Davis, A.; Hatcher, P. G. Energy Fuels 1993,7,455. (7) Cody, G. D.; Davis, A,; Hatcher, P. G. Energy Fuels 1993,7,463. (8)Larsen, J. W.; Wei, Y-C. Energy Fuels 1988,2,344. (9)Painter, P. C.; Park, Y.; Coleman, M. Energy Fuels 1988,2,693. (10) Khan, M. M. Fuel 1982,61, 553. (11)Wong, J. L.; Gladstone, C. M. Fuel 1983,62, 870. (12) Unger, P. E.; Suuberg, E. M. Fuel 1984,63, 606. (13)Buchanan, D. H.; Warfel, L. C.; Bailey, S.; Lucas, D. Energy Fuels 1988,2, 32. (14) Hombach, H.-P. Fuel 1982,61, 215. (15) Hombach, H.-P. Fuel 1981,60, 663. (16) Unpublished results. (17) Larsen, J. W. Mohammadi, M.; Yiginsu, I.; Kovac, J. Geochim. Cosmochim. Acta. 1984,48, 135. (18)Schulten, H.-R.; Marzec, A. Fuel 1986,65, 855.
0 1994 American Chemical Society
Coal Macromolecules in Pyridine less, significant ambiguity regarding the true mass distribution within the pyridine-soluble extract still remains, In an effort to better understand the solution state characteristics of coal extract-solvent systems, smallangle neutron scattering (SANS) has been applied to characterize pyridine extracts of low- and intermediaterank coals dispersed in pyridine. The small-angle scattering of either X-rays or neutrons is a technique unique in its capability to probe solution structures in the macromolecular size region. The hydrogen-rich extracted coal macromolecules in deuterated pyridine yields high contrast and enables the detection of scattered neutrons from fairly dilute solutions. While there have been numerous applications of scattering methods to address the characteristics of fossil fuel derived organics in solution, these have predominantly focused on the solution structure of asphaltenes. If there exists any consensus among the SAXS and SANS analyses, it is that asphaltenes solubilized in apolar solvents constitute a polydisperse ensemble of small particles. Disagreement arises regarding the fundamental shape of the scattering entity. SAXS analysis reported by Herzog et al.19 and SANS analysis reported by Ravey et a1.20interpret asphaltene dispersions in terms of a distribution of flat, wide, and porous particles. A preliminary analysis by Overfield et aLZ1suggested cylindrical-shaped particles. Recent results by Senglet et a1.22were interpreted in terms a polydisperse distribution of spherical particles. A polydisperse ensemble of spherical particles is also favored by Storm et al.23using the internally consistent treatment by she^.^^ Scattering analyses of coal-derived liquids are less prevalent than those for asphaltenes. As noted previously, the best solvents for coals are highly polar; this fact coupled with the fact that enormous differences in molecular structure exist between petroleum-based asphaltenes and coal extracts suggests that the coal molecule-polar solvent mixtures will be significantly different than the asphaltene dispersions. An early SAXS analysis of the scattering behavior of coal-derived liquids dispersed in pyridine was interpreted as resulting principally from a trimodal distribu~ proposed that the tion of spherical p a r t i ~ l e s .It~ was particles were aggregates of smaller particles grouped in a hexagonal closest packed arrangement. The particle size distribution was interpreted to be a function of a stepwise association of micelles leading to progressively larger particles. The end result of this association pathway was the formation of very large particles with radii in the 1000 A range. The presence of these largest particles, however, was not detected directly via scattering. A slightly more recent SANS analysis of coal extracts dispersed in pyridine was plagued by sample-derived complications.26 The most serious of these was an (19) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988,67,245. (20) Ravey, J. C.; Ducouret, C.; Espinat, D. Fuel 1988,67,1560. (21) Overfield, R. E.; Sheu, E. Y.; Sinha, S. K.; Liang, K. S. Fuel Sci. Technol. Znt. 1989,7,611. (22) Senglet, N.; Williams, C.; Faure, D.; Des Courihres, T.; Guilard, R. Fuel 1990,69,72. (23) Storm, D. A,; Sheu, E. Y.; DeTar, M. M. Fuel 1993,72,977. (24) Sheu, E. Y. Phys. Rev. A 1992,45, 2428. (25) Ho, B.; Briggs, D. E. Colloids Surf: 1982,4 , 285. (26) Triolo, R.; Child, H. R. Fuel 1984,63, 274.
Energy &Fuels, Vol. 8, No. 6,1994 1371 Table 1. Samples and Elemental Characteristics sample seam rank %Ca % H % O % S b APCS No. 3 Illinois No. 6 HVb-C 78 5.0 14 4.8 APCSNo.2 Wyodak-And 8ub-C 75 5.4 18 0.6 APCSNo. 8 Beulah-Zap lignite 73 4.8 20 0.8 a Moisture free and ash free basis. Dry basis. ~
~~~~
apparent sample dependence on the scattering behavior which could not be accounted for by simple variations in solute concentration. Notwithstanding these admitted difficulties, the authors modeled their data using the Debye approximation and concluded that their scattering data could be interpreted by a single correlation length on the order of 50 A. Regardless of the problems with the specific samples, the results of their experiment demonstrated the feasibility of SANS for the analysis of coal macromolecules in solution. In the present report, SANS is applied to probe the role of strong interactions on solution structure of pyridine-soluble coal extracts dispersed in pyridine. Three relatively low rank coals were selected as they have the highest natural abundance of acidic functional groups capable of specifically interacting with pyridine; these are a lignite, a subbituminous B coal, and a highvolatile C bituminous coal. The scattering behavior of both neat and 0-methylated extracts was studied. In addition to SANS, laser desorption mass spectrometry (LDMS), high-resolution electron impact mass spectrometry (HREIMS), and solution-state NMR were applied to help characterize the pyridine extractible fractions. Finally, the connection between our experimental data and the solution thermodynamics of coalsolvent systems is considered.
Experimental Section 0-Methylation Procedure. Pyridine-extractible fractions were obtained by exhaustive Soxhlet refluxing of coal samples Illinois No. 6 (APCS No. 31, Wyodak (APCSNo. 2), and North Dakota Lignite (APCS No. 8) (Table 1). In all these samples, 7.5 mmoVg of reactive sites were assumed, corresponding to essentially one acid site per aromatic ring. Alkylation of the extracts was accomplished using the procedure devised by LiottaZ7with minor modification. Approximately 0.5 g of dry extract was added to a mixture of 3 mL of THF and 0.7 g of 40% KOH (aqueous). The mixture was capped under Nz and stirred for 1-2 h. A n excess of CH3I (approximately 1g) was added and the solution was stirred for 12-24 h. THF and residual CH3I were evaporated from the solution by blowing dry nitrogen across the solution. The remaining aqueous solution was brought to neutral pH by adding dilute HC1; the precipitated coal extract was then washed with distilled water, centrifuged, decanted, and rewashed with distilled water for at least four cycles. The final product was dried to constant weight in a vacuum oven set at 60 "C. The degree of alkylation was assessed by solution-state lH NMR and 13CCPMAS solid-state NMR. In the case of carbon NMR, the presence of a substantial absorption at approximately 50 ppm revealed the presence of methoxy carbons added to each of the coals. Small downfield (high frequency) shifts on the order of 2-3 ppm of a prominent shoulder (near 150 ppm) of the main aromatic carbon resonance are consistent with alkylation of phenol. In the case of both the lower rank coals a slight downfield (high frequency) shift of the carboxylic carbon resonance near 175 ppm indicates the formation of some methyl esters in addition to the ubiquitous methyl aryl ethers. Evidence of substantial addition of methyl groups t o (27) Liotta, R. Fuel 1979,58, 724.
Cody et al.
1372 Energy & Fuels, Vol. 8,No. 6, 1994
I
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measured in SANS is I(&>, defined as the differential cross section per volume, given as
where N is the number of particles and A&> is the particle form factor given as
A&> describes the angular dependence on the magnitude of Figure 1. Diagram depicting scattering geometry. Incident neutrons scattering off two different regions of a particle separated by a correlation length, R. The scattered intensity is measured as a function of the Bragg angle 8/2. each of the extracts is also clearly evident using solution-state proton NMR. Pyridine extracts were resolubilized in deuteropyridine a t concentrations of 1,3,and 5 w t %. Each solution was mildly heated and sonicated for a period of 1 h in order that the macromolecules are solubilized. Each sample sat for approximately 6 h prior to analysis. All of the solvent-extract mixtures exhibited stability well beyond the time they were formed; there was no indication of precipitation evident even after weeks of storage. Mass Spectrometry. The mass distribution within each extract was determined through mass spectrometry. Using laser desorption mass spectrometry, LDMS, the molecular constituents within each sample are simultaneously desorbed and ionized using a pulsed nitrogen laser. Mass detection is achieved through time of flight. LDMS is capable of detecting an extremely broad mass range with masses up to 150 000 Da readily accessible.% The minimal amount of fragmentation during ionization typically attained using LDMS indicates that this method should yield an accurate indication of the molecular mass distribution in the extracts. However, coal extracts exist in a glassy state at room temperature. The extent to which this might interfere with the desorption phase of LDMS is unknown. A parallel, thermal desorption experiment with temperature ramped up to 700 “C using a Kratos MS50 high-resolution mass spectrometer and electron impact ionization, therefore, was also performed on each of the samples to provide an additional assessment of the mass distributions. Small-AngleNeutron Scattering. Small-angle neutron scattering was performed using the small-angledifiactometer (SAD)a t the Intense Pulsed Neutron Source (IPNS) a t Argonne National Laboratory. This instrument uses neutrons produced in pulses by spallation due to the deposition of 450 MeV protons on a depleted uranium target, followed by moderation by a solid methane moderator (22K)yielding a wavelength range of 0.5-14 A . Detection of scattered neutrons was accomplished with a 128 x 128 array area sensitive, gas-filled proportional counter, while the wavelength of the scattered neutrons was determined by their time of flight. Data were corrected for both scattering from the cell and incoherent scattering. Analysis of SANS Data. The interaction of neutrons of wavelength A with a small particle is illustrated in Figure 1. The fundamental variable of all scattering experiments is the scattering vector Q given by the difference in initial, h,and final wave vectors, k,
where 8 is half the Bragg scattering angle. The accessible Q range using SAD is from 0.005 to 0.25 A-1. The parameter ~~
(28)Chan, T-W.D.; Colburn, A. W.; Derrick, P. J. J. Org. Mass. Spectrom. 1992,27, 53.
the phase shift of neutrons scattered from one region versus another off a given particle, measured at a large distance relative to the position of the sample, and averaged over all orientations of the particle. The magnitude of AQ) is scaled by the contrast factor AQ, the difference in the scattering length density of the solvent and solute, respectively. SCQ) is an interparticle structure factor which takes into account scattering resulting from correlations between the centers of individual particles. In simple, dilute solutions S(Q> is safely considered to be 1. However, in concentrated solutions or dilute solutions containing aggregated or, otherwise, associated particles, contributions from S(Q) cannot be dismissed. Assuming a monodisperse, dilute, solution of spherical particles, the neutron scattering data may be analyzed using the Guinier appr0ximation,2~where
(4) The scattering intensity at Q = 0, I(O), is given as a function of the scattering length difference of the solvent and extract, A@, the number of scatterers, N,and their volume, V. The radius of gyration, R,, is the root mean squared distance of all of the atoms to the centroid of the particle and is obtained from the slope of a line in the In I(&) vs Q2 plot in the Q region whereR& 5 1.0. The data can alternatively be analyzed in terms of a form factor derived from the structural elements of a freely jointed Gaussian coil. Such an analysis is highly appropriate for dilute solutions of flexible polymers in good solvents. Individual scattering elements, Ni, each length L,are linked to form a flexible linear chain where the joints between each element are capable of completely free rotation. Under such circumstances the conformation of the chain is given by a three-dimensional random walk where the most probable length distribution of vectors connecting segment Ni with segment Nj is given by a Gaussian distribution function.30The form factor, A&), was determined by D e b ~ e : ~ ~
A&) = (l/Z2)(e-’ - 1 + 2)
(5)
where Z = Q2R,2. In many systems the assumption of monodisperse particle distributions is not justified. Solutions of asphaltenes dispersed in toluene, for example, have been shown to be p ~ l y d i s p e r s e . ~ l *An ~ ~effective *~~ and consistent means to analyze such systems has been discussed in detail by Sheu,% whereby the choice of the optimum particle geometry and size distribution function is constrained through the invariance of a justification parameter which is also a function of particle geometry, size distribution, and concentration. In many complex liquid-phase systems there exist random solution structures which exhibit scattering behavior resulting from self similar, or fractal, topology. For example, in the (29) Feigin, L.A.; Svergun, D.I. Structure Analysis by Small Angle X-ray and Neutron Scattering, Plenum Press: New York, 1987. (30) Flory, P. J. Statistical Mechanics of Chain Molecules; Hanser Pub.: New York, 1989. (31) Debye, P. J. Phys. Colloid. Chem. 1947,51, 18. (32) Sheu, E. Y.; De Tar,M. M.; Storm, D.A., Fuel Sci. Technol. Int. 1992,10,607.
Coal Macromolecules in Pyridine
Energy & Fuels, Vol. 8, No. 6, 1994 1373
Q (A-’) Figure 2. Intensity of the scattered beam, Z(Q), as a function of the magnitude of the momentum vector, Q. APCS No. 2 at three concentrations, 1, 3, and 5 wt % in deuteropyridine. Z(Q) normalized for concentration. analysis of scattering behavior it is often useful to investigate the relationship ofl(Q) to Q in terms of a power law, e.g., Z(Q) = Qd. The magnitude of d is indicative of the gross topology of the scattering particle. For homogeneous spheroidal particles, d = 3 (in the region QRB >> l), corresponding to the Euclidean, third dimension, e.g., as in the relationship between particle mass and radius, M = R.3 Mass fractal particles or solution structures, however, exhibit the relationship whereby M = Rd and d < 3,33indicative of a structure whose density decreases moving from its center t o its edge. An elegant theory has been formulated by Freltoft et al.34 to account for the scattering behavior of mass fractal solution structures. In the simplest case, they describe the situation where an ensemble of monodisperse, spherical particles aggregate t o form topologically random solution structures. The form factor is given as that of spherical, elemental particle. The structure factor, however, deviates appreciably from 1.0, rather it is governed by a interparticle pair correlation function, C(r), which exhibits a power law relationship with the radial distance from the center of the aggregate, i.e., C(r) = ra,where a = D - d (Dis the Euclidean dimension). At low values of Q, the scattering behavior will be governed by the topology of the mass fractal aggregate; at high Q the limit of scattering is governed by the radius of the elemental particle.34 Finally, scattering of neutrons off of the surface of very large particles with fractal surface topology is also indicated through power law scattering behavior, however, in this case the exponent d lies between 3 and 4.35 In the case of d = 4,the scattering corresponds to classical Porod type behavior at the asymptotic high Q limit indicating scattering off of smooth particle surfaces.29
Results and Discussion Mass Spectrometry. LDMS analysis of both extracts, alkylated and neat, indicates a predominance of relatively low mass material, with M , on the order 300 and a mass envelope which tails off around a thousand (33) Feder, J. Fractals; Plenum Press: New York, 1988. (34)Freltoft, T.; Kjems, J. K.; Sinha, S.IC,Phys. Reu. B 1986,33, 269
(35)Bale, H. D.; Schmidt, P. W. Phys. Rev. Lett. 1984,53,596.
daltons. No statistically significant differences in the mass distributions were observed between the alkylated and untreated extract of any of the coals. LDMS is capable of ionizing and detecting high molecular weight material with minimal fragmentation. An important question arises. If there were high-mass material in these extracts, would they be detectable by this technique? If the mass distribution is extremely polydisperse, it may be difficult to detect the high-mass material above background as the contribution a t any mass interval may be low even if the integrated contribution t o the total mass over a wide mass range, e.g., (0-1) x lo5 Da, is high. LDMS is sensitive to M,; a low value of M,, however, does not preclude the presence of high-mass material as discussed in detail by Larsen and Wei.* High-resolution electron impact mass spectrometry yields results comparable with LDMS. Although the accessible mass range scanned was limited to less than 900 Da, the mass envelope tailed off well before this maximum. It is well-known that electron impact ionization is prone t o inducing fragmentation, therefore, it is conceivable that high-mass material intrinsic to the extract does not yield an abundance of molecular ions. Fragmentation would account for the slight disparity at the high mass end of the mass envelopes observed using LDMS and EIMS, respectively. The numberaveraged molecular weights derived in both cases are on the order of 300. Similarly to the LDMS results, no significant differences between the alkylated and untreated experiments were observed. Small-Angle Neutron Scattering. Prior to discussing the results of each pair of samples, it is necessary to first establish the reproducibility and quality of the data. Figure 2 presents the scattering data for one of the extracts, APCS No. 2 untreated, with the scattering intensities normalized to concentration, 1, 3, and 5%, respectively. The scattering behaviors are exactly the same within the error of the technique. In these
1374 Energy & Fuels, Vol. 8, No. 6, 1994 l L
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samples and all the others there was no evidence of concentration dependent scattering behavior. In Figure 3, the scattering behavior of APCS No. 3 and its 0-methylated derivative is compared. At high Q, both the alkylated and untreated scattering behaviors are comparable, exhibiting a power law scattering behavior on the order of 4. This behavior corresponds to asymptotic Porod type behavior at the high-Q limit indicating scattering off a smooth surface. In both the treated and untreated extract, therefore, there exists a lower bound in the scattering behavior, which corresponds to a similarly sized particle common to the untreated and derivatized extract. At low Q the scattering behavior of the untreated and derivatized extract diverges. The untreated extract exhibits an upper bound in its scattering behavior at
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low Q as evident by the roll over in I(Q)with decreasing Q. Preliminary analysis using the Guinier approximation (eq 4)suggests scattering from particles with R, = 41.4 & 3 A , I ( 0 ) = 0.01 mg-l cm2 (Figure 4). In principle, the molecular weight of the particle could also be determined from I(0). However, this requires an exact scattering length density for the coal macromolecule, which is currently not known. In the future, contrast variation measurements will be made to address these issues. Alternatively, the scattering data from the untreated APCS No. 3 extract can be fit using the Debye equation ( 6 )(Figure 5). Assuming Gaussian coil characteristics for coal extracts in solution is not totally unreasonable as solvent-dilated coals have been shown to exhibit viscoelastic behavior similar t o that of natural rubbers
Coal Macromolecules in Pyridine
1
Energy & Fuels, Vol. 8, No. 6, 1994 1375
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9 Figure 5. Debye analysis of APCS No. 3 untreated extract in deuteropyridine. Table 2. Scattering Exponents and Elemental Particle Radii
treatment
samDle AFkS No. 3
untreated
fractal dimension, d
NA
R (& 72 & 6
0-methylated 1.70 f 0.04 72 f 6 APCS No. 2 untreated 1.79 f 0.01 81 f6 0-methylated 2.46 f 0.01 9 1 f6 APCS No. 8 untreated 2.13 f 0.03 85 f 6 0-methylated 2.29 f 0.04 85 f 6 a Estimated using the theory of FreltoR, Kjems, and Sinha (ref 34).
when swollen in ~ y r i d i n e . ~Furthermore, a~~ the stressstrain behavior of swollen coals has been well described using the statistical theory of rubber elasticity,6 which applies Gaussian statistics explicitly. The quality of the fit is good, yielding a value for R , = 44.2 f 1A; however, this fit may be fortuitous. Known polydisperse systems can be fit to mondisperse scattering curves;24i.e., a good fit is not necessarily unique. Moreover, considering the polyfunctional nature of fragments identified through pyrolysis mass ~ p e c t r o m e t r yit, ~would ~ be surprising if the macromolecules which contribute t o the coal extracts have a linear structure. Far more likely are highly branched macrom~lecules.~ The magnitudes of R, obtained from the two different techniques are comparable, each suggesting that the presence of a relatively large particle may be responsible for the scattering. In the case of the 0-methylated APCS No. 3 sample (Figure 3) both the Guinier and Debye analyses fail to describe the scattering behavior . The scattering intensity, I(
