32
Energy & Fuels 1988,2, 32-36
Size Exclusion Chromatography of Coal Extracts Using Pyridine as a Mobile Phase David H. Buchanan,* Linda C. Warfel, Shawn Bailey, and Douglas Lucas Chemistry Department, Eastern Illinois University, Charleston, Illinois 61920
Received July 13,1987. Revised Manuscript Received October 5, 1987 Pyridine, which dissolves a wide variety of coal extracts and process liquids, has been shown to be an effective mobile phase for size exclusion chromatography (SEC) of these extracts. Calibration of a cross-linked polystyrene-divinylbenzene gel column train with polystyrene standards produced a linear correlation in the 30000-100-Da region for the relation log (MW) = 7.283 - O.213Vr. Coal extracts and related model compounds have smaller retention volumes (V,) than polymer standards of the same molecular weight. Recalibration of the column train with vapor pressure osmometer ( W O ) molecular weights of preparative SEC fractions and sequential solvent extracts from several Illinois coals also gave a h e a r log correlation in the 3000-100-Da region for the expression log (MW) = 5.377 - O.133Vr. Refractive index detector response and SEC retention volumes for a variety of model compounds were found to depend upon the functional groups present as well as the molecular weight. V, values for compounds containing phenol, ether, and extended aromatic systems were closer to those predicted by the coal fraction than those predicted by the polystyrene calibration.
Introduction Size exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), is a useful method for the characterization of material isolated from coal and other solid fuels by solvent extraction or chemical reactions leading to It has the advantage of providing a molecular size distribution rather than an average molecular weight and can be applied to molecules too large or too polar for gas chromatography or ionization methods, such as field ionization mass spectrometry. The limits of the method are also well-known,3chiefly lack of calibration standards for coal-derived materials, limited solubility of samples in typical SEC solvents, and incompatibility of solvents and SEC stationary phase^.^ This report is an evaluation of the use of pyridine as a SEC solvent for coal extracts. Pyridine is an excellent solvent for coal extracts and process liquids. It has the advantage of dissolving molecules too large, too polar, or too strongly attached by hydrogen bonding to the macromolecular network to dissolve in common SEC solvents such as chloroform, toluene, or THF.We have recently shown5that pyridine-soluble coal fractions can be isolated and maintained free of colloids by either membrane filtration or ultracentrifugation if they are carefully protected from air oxidation. Extracts so treated redissolve completely in pyridine. By careful attention to clarification of sample solutions and the mobile phase, plus dedication of the gel columns to a single solvent, SEC analyses of coal extracts in pyridine mobile phase have been run for over 1year on the same column train with no increase in back pressure or change in re(1) (a) Wong, J. L. In Coal Science and Chemistry; Volborth, A., Ed.; Elsevier: Amsterdam, 1987; p 461. (b) Lynch, A. W.; Thomas, M. G. Fuel Process. Technol. 1983,8, 13. (c) Baltisberger, R. J.; Wagner, S. E.; b o , S. P.; Schwan, J. F.; Jones, M. B. Zbid. 1985, 11, 213. (2) (a) Richards, D. G.; Snape, C. E.; Bartle, K. D.; Gibson, C.; Mulligan, M. J.; Taylor, N. Fuel 1983, 62, 724. (b) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Ibid. 1984, 63, 1556. (c) Evans, N.; Haley, T. M.; Mulligan, M. J.; Thomas, K. M. Ibid. 1986,65, 694. (3) (a) Bartle, K. D.; Mills, D. G.; Mulligan, M. J.; Amaechina, I. 0.; Taylor, N. Anal. Chem. 1986,58, 2403. (b) White, C. M.; Perry, M. B.; Schmidt, C. E.; Douglas, L. J. Energy Fuels, 1986, 1, 99. (4) Adame, J. R.; Bicking, M. K. L. Anal. Chem. 1985,57, 2844. (5) Buchanan, D. H.; Warfel, L. C.; Mai, W.; Lucas, D. Prep. PapAm. Chem. Soc., Diu. Fuel Chem. 1987, 32(1), 146.
0887-0624/88/2502-0032$01.50/0
tention volumes of the molecular weight standards. A variety of problems make quantitative interpretation of SEC data in all solvents difficult for coal fractions. Polymer calibration standards do not have the same shape or functionality as the coal molecules1cand detector response factors are largely unknown for these mixture^.^ SEC calibration methods, analyses of pyridine soluble coal extracts, and the effects on SEC analyses of functional groups such as ether and phenolic OH groups in both model compounds and coal fractions are discussed in this paper.
Experimental Section Coal extracts were isolated by exhaustive Soxhlet extraction as described previou~ly.~SEC analyses were performed on a three-column train of AS1 Ultragel size exclusion columns, 10 pm polystyenedivinylbenzene copolymer, 250 mm X 7.8 mm, of 100, 500, and 1000 A nominal pore size, respectively. HPLC grade pyridine (Aldrich) was continuously recycled through the system by a Beckman Model llOB solvent pump at 0.2 mL/min when not in use. For analyses, the system was slowly brought to 1.8 d / m i n flow and the column effluent diverted to waste collection by use of an Altex slider valve. Sample solutions, 6.0 mg/mL, in pyridine were centrifuged and/or filtered through a 0.45 pm Nylon 66 or P T F E membrane filter prior to injection onto the column through a 100-pL loop in a Rheodyne Model 7125 injector. Knauer Model 98.00 refractive index detector signals were sent via a Cyborg Isaac interface to an Apple IIe data station, recorded, and analyzed with Appligration I1 software from Dynamic Solutions, Inc. The measured column train efficiency was 7150 for toluene. Narrow molecular mass range polystyrene standards, purchased from Polysciences, Inc., were dissolved in HPLC grade pyridine for calibration. Ethylbenzene was used to extend the calibration to lower mass values. Reagent grade model compounds, commercially available or prepared for this project, were dissolved in pyridine and centrifuged before injection under the same conditions used for coal analyses. Preparative scale SEC separations of 0.4-g coal extract fractions from an Illinois No. 5 coal were performed on SX-1 and SX-2 Bio-Beads with pyridine mobile phase. The 200-400 mesh BioBeads were swollen in pyridine and slurry packed into 11 mm i.d. X 60 cm glass columns as described by Taylor? A coal fraction dissolved in 5 mL of pyridine was adsorbed on the column and (6) Hausler, D. W.; Hellgeth, J. W.; McNair, H. M.; Taylor, L. T. J. Chromatogr. Sci. 1979, 17, 617.
0 1988 American Chemical Society
Size Exclusion Chromatography of Coal Extracts COAL
Energy &Fuels, Vol. 2, No. 1, 1988 33 Table I. Calibration Standards
0930
Polystyrene MW log (MW) Vr MwIMn 310000 exc1u ded 12.60 1.05 34100 4.538 13.45 1.05 10200 4.009 15.35 1.07 3550 3.550 17.46 1.05 2000 3.301 18.21 1.06 lo00 3.000 19.85 1.3 500 2.699 21.46 1.03 106" 2.025 25.17 1 log (MW) .= 7.2832 - 0.2128( V,) r2 = 0.995
I
I
l
l
Exc 3K ZK
I
I
I
I
IK
.5K
ZK
.IK
I
Figure 1. SEC traces of sequential solvent extracts from Argonne Premium Coal No. 3 taken in the order (a) toluene, (b) THF, (c)
DMF, and (d) pyridine. Molecular size calibration values refer to eq 2, see text.
eluted with degassed pyridine at a flow rate of 0.2 mL/min. Thirty 2-mL fractions were collected and analyzed by SEC as described above, and fractions with similar mass distributions were combined. The combined fractions were isolated by removal of pyridine in vacuum, washing with 80% methanollwater, and drying to constant weight at 100 "C (0.05 Torr)? Number-average molecular weight values (M,) of the extracts were determined on a Knauer Model 11.00 vapor pressure osmometer in pyridine at 90 "C. Experimental values at three or four concentrations were extrapolated to infinite dilution by linear least-squares regression.
Results and Discussion Of the solvents commonly employed for coal extracts and coal liquids, THF is most often used for SEC analyses. For the combination of THF with cross-linked polystyrene gel columns, the separation mechanism has been shown to be size exclusion only, in contrast to toluene and other less polar solvents in which solute-gel adsorption contributes to the separation m e ~ h a n i s m .However, ~ THF dissolves only a portion of the material extractable from most coals. For example, when an Illinois No. 6 coal (Argonne Premium Sample No. 3) was exhaustively extracted with various solvents, 15% was soluble in THF and 26.8% in pyridine. Sequential extraction of coals in the order toluene, THF, DMF, and pyridine separates the extracts into four fractions that differ in elemental composition, FT-IR spectra, and molecular size distribution as shown by SEC analyses.s The use of pyridine as mobile phase for SEC analyses permits these fractions to be compared, as illustrated in Figure 1, in which SEC traces for the four sequential extracts from Argonne No. 3 coal are superimposed. Qualitative differences in molecula size distribution are apparent, but care is needed in quantitative interpretation of this SEC data. Calibration of the polystyrene-divinylbenzene gel column train with polystyrene standards, coal fractions, and model compounds in a pyridine mobile phase is discussed below. Polystyrene Calibration. SEC retention volumes (V,) of seven polystyrene calibration samples plus ethylbenzene are listed in Table I along with the molecular weights and polydispersity values (M,/M,) provided by the supplier. V, for ethylbenzene, the repeating unit of polystyrene, is twice the exclusion volume of the column train, as expected from SEC theory.',* A linear least-squares regression of the polystyrene plus ethylbenzene data in the 34000(7) (a) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Liquid Chromatography;Wiley: New York, 1979;pp 204-226. (b) Janca, J.; Ed. Steric Exclusion Liquid Chromatography of Polymers; Marcel Dekker: New York, 1984. (8) Hoechst, U. Eur. Polym. J. 1982, 18, 273.
sample no. 4520-1 3804 7615 4520-2 3802 9120 7610
Coal Fractions log (MW) V, Mw/Mn M; 3.358 15.12 6.87 1120 3.148 17.61 1.79 1000 2.774 18.54 1.92 830 2.740 19.26 1.24 690 19.65 2.902 1.51 600 19.98 2.715 2.47 450 2.597 20.88 1.90 420 log Mn = 5.3770 - 0.1328(Vr) r2 = 0.934
Mnb 2280 1406 594 550 799 519 395
M : 2340 1090 820 660 580 520 420
OEthylbenzene. bDetermined by VPO. Geometric mean MW = (MWMn)'/*. dApparent MW of peak maximum for V, using calibration eq 2.
1
I
1
(9) (a) Larsen, J. W.; Mohammadi, M.; Yiginsu, I.; Kovac, J. Geochim. Cosmochim. Acta 1984,48, 135. (b) Unger, P. E.; Suuberg, E. M. Fuel 1984, 63, 606.
34 Energy & Fuels, Vol. 2, No. 1, 1988
Buchanan et al.
Table 11. SEC Parameters for Model Compounds MW v, PS" MW coalb MW
no.
compd
1 2
pyridine polystyrene polystyrene polystyrene tetraphenylporphyrin a-methylnaphthyl @-naphthylether o-(benzy1oxy)phenol 1-hydroxyanthraquinone 2-hydroxybenzyl alcohol 2-naphthol @-lactose p-anisaldehyde benzyl alcohol m-cresol TMS ether m-cresol p-cresol benzaldehyde toluene benzylacetate ethylbenzene m-cresyl-2,5-dioxahexane (MEM ether)d
3
4 5 6 7
8 9 10 11 12 13
14 15 16 17 18 19 20 21
Calculated from eq 1. Calculated from eq 2.
79 310000 34500 2000 614 284 200 224 124 144 342 136 108 180 108 108 106 92 150 106 172
solvent 12.60 13.45 18.21 22.26 23.49 22.55 24.62 22.23 23.49 19.68 24.35 23.44 24.16 23.71 23.49 25.11 25.92 24.25 25.17 23.48
... exclusn 26300 2550 350 192 304 110 355 192 1240 126 196 138 172 192 87 58 132 84 192
...
exclusn 3898 909 264 181 241 128 266 181 580 139 184 147 169 181 110 86 143 108 181
DR/pM"
...
4161 571 21 6.88 4.76 1.95 1.25 1.14 1.13 0.84 0.76 0.67 0.65 0.61 0.53 0.39 -0.12 -0.18 -0.27 -0.37
nB 1.5095
1.5906
1.5713 1.5396 1.5438 1.5312 1.5463 1.4961 1.5232 1.4959
DR = detector response per micromole injected. dReference 13.
fractions for calibration is difficult, and unless the standards have very narrow molecular weight distributions (M,/Mn < l.l), the calibration methods used with polymer standards are not strictly applicable to the coal fraction calibrati~n.~~~~ For SEC systems calibrated with monodisperse standards, as in eq 1,the molecular weight corresponding to V, a t the detector peak maximum of a polydisperse sample, such as a coal extract, is not the number-average molecular weight (M,) but is close to the weight-average molecular weight For polydisperse samples eluting in the linear portion of the logarithmic calibration curve, V, a t peak maximum is the geometric mean of the molecular weight distribution, Mg = (M&n)1/2. These averages can be calculated from the SEC traces by a numerical integration pro~edure.l*J',~ Use of polydisperse coal fractions as calibration standards, as usually done, assumes that V, a t peak maximum corresponds to M,, leading to errors in molecular weights derived from SEC traces. In principle, polydisperse coal fraction calibration curves could be improved through an iterative procedure. An initial calibration curve, log M , vs V,, would be determined and numerical integration of the calibration SEC traces used to calculate M,, M,, and M gvalues, which would be compared to the VPO M, value. Coefficients in the calibration equation would then be adjusted to improve the fit, average molecular weights recalculated, and calibration coefficients readjusted until self-consistent averages were obtained. Calculation of molecular weight averages by integration of SEC traces requires detector response to be constant or vary with V, in a known manner for the standards and samples used. As shown by others3 and by the data reported here, this is not the case for coal samples or simple model compounds. Under these circumstances the extra effort for the iterative method is not justified and in fact may lend a false impression of accuracy to the calibration. The coal fraction calibration discussed below employed the simple correlation of W O M , with V, used by others and should be evaluated within these limitations. Several of our T H F and DMF sequential extracts from Illinois coals had symmetrical SEC molecular weight distributions and appeared to be suitable candidates for calibration fractions. VPO molecular weights of these fractions in pyridine were determined. To extend the calibration to a higher mass range, the sequential DMF
extract of an Illinois No. 5 coal was further separated by preparative SEC chromatography! and the VPO molecular weights of two fractions were determined. SEC and VPO data for the seven coal fractions used in the calibration are listed in Table I. Because of difficulty in separating low molecular weight material from heavier coal fractions, several fractions have molecular weight distributions broader than desirable, but the entire set still gave a usable calibration. A linear least-squares regression on the data for the seven coal fractions gave a regression coefficient of 0.934 for eq 2. log M , = 5.377 - 0.0133(V,) (2) The M,/M,, and M g values in Table I were calculated by numerical integration of the calibration traces using eq 2. Because the RI detector response is greater for equal concentrations of material a t short V, than a t long V , (discussed below), calculated M , values are overestimated and M , values underestimated for these standards. The true M,/M, ratios must be smaller than those reported in Table 11, and the use of these standards for calibration is thus not unreasonable. Equation 2 is plotted with the coal extract calibration points from Table I as solid circles on Figure 2. Geometric mean molecular weight values (Mg) and apparent molecular weights of the coal calibration fractions calculated from peak maximum V, values (M,) are also tabulated in Table I. Even though the M values are subject to the errors discussed above, Mp is doser to M than to either M , or M,, as predicted.8 hodel Compounds. Differential refractive index detectors are often used for SEC analyses of coal fractions, especially with solvents such as pyridine. However, peak height or area on the resulting SEC curves, e.g. Figure 1, is not an accurate measure of the relative amount of material present at a given V,. Refractive index, and hence detector response, is a function of molecular mass in the mass range (
