344
Energy & Fuels 1988,2,344-350
structure is not the equilibrium structure. Evidence for the alteration of coal gel structure by methanol and pentane absorption was published 30 years ago.26 Our data make clear that nonequilibrium states due to slow macromolecular relaxation can also be observed with polar solvents. Once the structure has relaxed (initial pyridine exposure), an equilibrium state is reached and further swelling shows no hysteresis. With nonpolar solvents, as observed by Duda, the equilibrium state is apparently never reached so hysteresis continues. Solvent swelling in nonpolar liquids shows no hysteresis.26 Apparently, Soxhlet extraction of the samples by pyridine allowed them (25)Fugaesi, P.;Hudson, R.; Ostapchenko, G. Fuel 1958,37,25-28. (26)Lareen, J. W.;Lee,D.; Shawver, S. E. Fuel Process. Technol. 1986, 12,51-62.
to obtain their equilibrium state. Hsieh and Duda used a similar extraction procedure, which should also have caused the coal structure to relax. We do not understand why the different experimental conditions with nonpolar solvents should yield such different results. The data contained in this paper support Duda’s picture of very complex absorption processes, and we echo his statements that one must be sure equilibrium has been reached before making quantitative use of solvent-swelling data. Coals are extremely complex macromolecular systems. They are clearly anisotropic. In further papers, we will explore the significance of these observations for the behavior of coking coals and further explore the anisotropic structure. Registry No. PhC1, 25167-80-0;tetrahydrofuran, 109-99-9; pyridine, 110-86-1.
Macromolecular Chemistry of Coalification. Molecular Weight Distribution of Pyridine Extracts John W. Larsen* and You-Ching Wei Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received July 1, 1987. Revised Manuscript Received December 28, 1987
A gel permeation chromatograph with a mass sensitive detector has been constructed, interfaced with a computer, calibrated with coal extracts, and tested. Because of the nature of the detector, the molecular weight distributions obtained underestimate the amount of very low molecular weight material present, but the weight-average molecular weights are only slightly underestimated. The molecular weight distributions for a series of lower Kitanning coal extracts were measured. All of the coal pyridine extracts studied contained materials in the 10000 molecular weight range. Both the amount of pyridine extract and its average molecular weight increase with rank up to a carbon content of 86%. Both of these trends are consistent with the coalification process over this rank range being the depolymerization of a cross-linked macromolecular network. Thus,the origin of the extractable portion of high-rank coals is the depolymerization of the network, and the extracts are primarily products of network fragmentation. The ratio of weight-average to number-average molecular weights decreases with increasing rank, and the possible origin of this behavior is discussed.
Introduction The vitrinite portion of coals is thought to originate largely from lignin, a complex three-dimensional macromolecular netw0rk.l The process by which lignin becomes coal is best treated by using the well-developed kinetic and thermodynamic models for macromolecular network systems. We concern ourselves here with the coalification of a set of lower Kitanning bituminous coals and seek to answer the question, “can their coalification be treated as reactions of a three-dimensionally cross-linked macromolecular network?” The approach used is to determine the molecular weight distributions for the pyridine extracts from a set of coals and to determine whether the changes in those distributions are consistent with those expected if the extracts are in equilibrium with a network, that is, constitute a sol. It was necessary to develop a method for determining the molecular weight distributions of coal extracts. A (1)Freudenberg, K.Science (Washington D.C.)1965,148,595.
Table I. Elemental Analyses of Coals Used (dmmf) %
mineral %C %H %N %S coal PSOC 1278 82.5 5.5 1.9 0.8 PSOC 1309 85.8 5.8 1.8 1.6 PSOC 1215 86.3 5.2 1.6 0.8
P S O C 1236 87.4 PSOC 1024 88.9 PSOC 1133 90.7
5.2 5.1 5.0
1.6 1.3 1.4
0.8 0.8 0.4
% O(diff).
9.2 5.0 6.2 4.9 3.8 2.4
matter 15.7 8.6 7.3 7.7 13.7 22.2
number of groups have successfully used gel permeation chromatography for this, but most have been hindered by the lack of a mass-sensitive detector and the choice of the proper calibration standard is also a A (2)Unger, P.E.;Suuberg, E.M. Fuel 1984,63,606-611. ( 3 ) Strachan, M.G.; Johns, R. B. J. Chromatogr. 1985,329,65-80. (4)Reerink, H.; Lijzenga, J. A n d . Chem. 1975,47,2160-2167. (5)Khan, M.M.Fuel 1982,61,553. (6)Wong, J. L.; Gladstone, C. M. Fuel 1983,62,870-872.
0887-0624/88/2502-0344$01.50/00 1988 American Chemical Society
Energy 13Fuels, Vol. 2, No. 3, 1988 345
Macromolecular Chemistry of Coalification Table 11. Maceral Composition of Coals Used (vol %, dmmf)" coal vitrinite inertinite liptinite PSOC 1278 88.2 9.0 2.5 PSOC 1309 92.6 5.3 2.1 PSOC 1215 90.6 7.8 1.6 PSOC 1236 91.3 0.6 8.1 PSOC 1024 81.3 18.7 0.0 PSOC 1133 88.7 11.4 0.0 'Data obtained by the Penn State Sample Bank. thorough analysisof the errors encountered in determinii molecular weight distributions of coal-derived materials by gel permeation chromatography has been published.8 We decided to use gel permeation chromatagraphy and have successfully used a mass-sensitivedetectd interfaced with a microcomputer for data acquisition and manipulation. This paper is divided into two parts. The first part contains a description of the apparatus, ita calibration, and a comparison of its output with independent data. The second part of the paper presents the molecular weight distributions for a set of lower Kitanning coals, discusses their meaning, and generalizes the discussion to other bituminous coals.
Experimental Section Coals. The lower Kittanning coals used were supplied by The Pennsylvania State University Coal Bank. Their elemental analyses are given in Table I and their maceral compositions in Table 11. Extractions. The coals were dried to constant weight under vacuum at 110 OC. They were extracted with dry pyridine (Burdick & Jackson) under a small positive pressure of dry N2 until the solvent was clear. The extract was stored in tightly capped containers in a moisture- and oxygen-free glovebox (Vacuum Atmospheres). Preparative Gel Permeation Chromatography. Pyridine extract from Ill. No. 6 coal was preparatively fractionated on a medium-pressuresystem consisting of the following components: pump, FMI Model RPSY with an FMI pulse damper; collector, ISCO Model 1220 with a Model 405 volumeter, column,glass 2.5 x 50 cm; packing, Biobeads S-X1; flow rate, 1mL/min; solvent, pyridine, Burdick & Jackson HPLC grade. The concentration of extract in each fraction was determined by evaporatinga 2.5mL aliquot to dryness under a flow of dry Nz and drying to constant weight under vacuum at 110 "C. Chromatograms obtained with a refractive index detector revealed that no water was present. Analytical Gel Permeation Chromatography. The instrument consisted of a Waters ALC 201 high-pressure liquid chromatograph equippedwith an evaporativemass analyzer made by Applied ChromatographySystems (Model 750/14). The output from the mass analyzer was fed to an A/D converter (Data Translation Model DT 2805) having a resolution of 12 bits. The data acquisition was programmed by using the ASYST software package (Macmillan Software Co.), which was also used to write the routines for integration and calculating number and weight average molecular weights. Three p-styragel columns (100,500, and 1000 A) were used in series with pyridine solvent (Burdick & Jackson HPLC grade) at a flow rate of 1.0 mL/min. The exclusion limit of this column combination is higher than the largest molecules encounteredin this work. For high-temperature operation, the columns were wrapped with heating tape and insulated with glass wool. Temperature measurement was by thermocouples in contact with the column exteriors. The mass detector operates by evaporating the solvent and measuring the nonvolatile material remaining. The flow from (7) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984,63,1556-1560. (8) Bartle, K. D.; Mills, D. G.; Mulligan, M. J.; Amaechina, I. 0.; Taylor, N. Anal. Chem. 1986,58, 2403-2408. (9) Bartle, K.D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson,C. Fuel 1983,62,1181-1185.
1500 2100 Time (rec)
900
300
2700
Figure 1. Typical gel permeation elution curve for 111. No. 6 pyridine extract at 25 "C.
3 . 4
I
I
I
15
30
I
I
I
I
45 60 75 90 Concentration(Nkg)
I
I
105 120
Figure 2. Relationship between the concentration of Ill. No. 6 pyridine extract and the peak area of the elution curve. Table 111. Typical Preparative Fractionation of Ill. No. 6 Pyridine Eztract fraction no. fraction vol, mL conc, d k a M; 1 2
7.24 14.48 7.24 14.48 21.72 21.72 14.48
0.199 12.61 9.94 4.50 4.38 4.14 1.64
5050 3690 2370 750 470 260
'Measured by vapor-pressure osmometry in pyridine at 60 "C. the chromatograph emerges from a very small tube into a larger tube where it is mixed with a rapid 15 psig flow of dry NP This is fed into an atomker where the liquid is converted by a venturi jet into a uniform dispersion of droplets. This passes downward through a heated tube in an Nzflow where the solvent is evaporated. The resulting particles pass through a photometric detector where refracted light i s detected at an angle of 120" to the incident light beam. The signal from the photomultiplier is amplified and fed to the A/D converter or a strip chart recorder. Linear mass response was demonstrated by using Ill. No. 6 coal extracts. The detector response was sampled at 1.24 intervals for the 60-min duration of each run. A program for time synchronization was written with ASYST,and a plot of detector signal vs retention time was produced directly. A typical plot is shown in Figure 1. A plot of peak area vs sample concentration for Ill. No. 6 extract is shown in Figure 2. Linearity is good over a concentration range exceeding that used in our experiments. '€he system showed excellent reproducibility, which was checked often during this set of experiments. Vapor-Pressure Osmometry. A Knauer osmometer was used at 60 "C with pyridine solvent. Experimental procedures and data treatment have been previously described.1°
Results GPC Calibration. It was necessary to calibrate the gel permeation chromatograph by using molecular weight standards appropriate for use with coal extracts. It is well (10) Larsen, J. W.;
Choudhury,P. J. Org. Chem. 1979,442856-2859.
Larsen and Wei
346 Energy & Fuels, Vol. 2, No. 3, 1988
17 18 19 20 21 22 23 24 25 26 27 28 29 3031 Retention Volume (mP)
Figure 3. Column calibration curve obtained at 25 "C with the fractionated pyridine extract of Ill. No. 6 coal. The symbols refer
to independent determinations of the calibration curve.
2,000
8,000 10,000 14,000 18,000 Molecular Weight
Figure 5. Molecular weight distribution for Ill. No. 6 pyridine extract at 25 OC.
4
i 1
3 2 1 I
I
I
I
I
I
I
I
I
I
17 18 19 20 21 22 23 24 25 28 : Retentlon Volume (mg
2,000
10,000 14,000 18,000 Molecular Welght
6,000
Figure 4. Column calibration at 25 O C with polymer s t a n d a h
Figure 6. Molecular weight distribution for Ill. No. 6 pyridine extract at 65 "C.
recognized that most commercially available polymer molecular weight standards are The use of fractionated coal extracts as standards is growing and seems to provide a good solution to the calibration problem.2*gA pyridine extract from Ill. No. 6 coal was fractionated preparatively by size using a Biobeads column with pyridine solvent. The volume of _each fraction and its number-average molecular weight (M,) determined by vapor pressure osmometry in pyridine are given in Table 111. A 2.5-mL aliquot of each fraction was dried to constant weight to determine the concentration of the extract. Since some pyridine is retained, these concentrations will be overestimated slightly. This is not significant since M, calculated from the individual fraction molecular weights always agreed with that measured for the whole extract by VPO. Without drying, these fractions were used for vapor pressure osmometry and to calibrate the p-styragel columns. The calibration curve used in this work is shown in Figure 3. It contains points from three separate fractionations and demonstrates the good reproducibility of the chromatograph. Calibration lines for several other polymer standards are given in Figure 4. They are all unsuitable for use with coals. Having a calibrated instrument, our first task was to check the validity of the calibration. This was done by determining the molecular weight distribution of a pyridine extract from Ill. No. 6 coal by GPC, calculating the number average molecular weight from the GPC curve, and comparing this with the number-average weight obtained by using vapor pressure osmometry at 60 "C. Ill. No. 6 coal extracts are probably not the best standard to use. A set of fractionated extracts from coals of varying rank would be superior. Time and financial constraints limited us to one coal, and we chose Ill. No. 6 because others had measured extract molecular weights providing an external check on our data, because we were familiar with it, and because we had access to high-quality samples. The relevant molecular weight distribution is shown in Figure 5.
To our horror the VPO molecular weight was little more than half the chromatographicmolecular weight. The fact that a similar discrepancy had previously been observed by others with petroleum asphaltenes provided small ~ o m f o r t .We ~ proceeded to search for the source of the discrepancy. One of the first candidates was the difference in temperature between the VPO measurements (60"C) and the chromatography column temperature (25 "C). Association at the lower temperature would explain our results. The gel permeation chromatograph was therefore operated at 60 "C, this temperature being achieved by wrapping the columns and lines from the injector and to the detector with heating tape. An example of the curves obtained is shown in Figure 6. It is very similar to those obtained at room temperature. This was not the source of the difference. Very roughly, the temperature effect on retention volume suggests an average interaction enthalpy between the solute (pyridine solution) and polystyrene column packing of about 1 kcal/mol. A n additional test for association of the coal extracts was performed. The chromatograms were obtained at a series M. The meaof concentrations extending to about sured molecular weights did not depend on concentration so associative interactions are not responsible for the observed difference. There is an obvious possible error in our calibration method. The number-average molecular weights of a series of fractions are measured by VPO and combined with their peak retention volumes to calibrate the instrument. This is fine if the fractions are monodisperse, but they are not. To correct this, a calibration curve was calculated by utilizing number-average retention volumes, which can be easily calculated from the elution curves. This shifted the calibration curve by a bit less than 10%. We believe the origin of the discrepancy is in the operation of the mass analyzer and that it misses enough of the lowest molecular weight material to cause a large error in A& and a smaller error in M,. The detector works by evaporating the nebulized column effluent in a rapid N2
( 0 )polystyrene; (A)poly(a-methylstyrene): (W) poly(ethy1ene glycol).
Energy & Fuels, Vol. 2, No. 3,1988 347
Macromolecular Chemistry of Coalification
T
ooot
I
2,000
,
-
l\,-l
2,000
8,000 10,000 14,000 18,000 Molecular Weight
8,000
10,000 14,000 18,000
Molecular Weight
000 l
2,000
2,000
8,000 10,000 14,000 18,000 Molecular Weight
I
I
I
6,000
1
I
I
I
l
10,000 14,000 18,000
Molecular Weight
400 480
1
F
3 300
: g 200 -.-
=1 360 t-
-
240
8 100
a 120
000 000 2,000
2,000
6,000 10,000 14,000 18,000 Molecular Welght
6,000
10,000 14,000 18,000 Molecular Weight
Figure 7. Molecular weight distributions for pyridine extracts from a series of lower Kittanning coals: (a) PSOC 1278; (b) PSOC 1309;(c) PSOC 1215; (d) PSOC 1236; (e) PSOC 1024;(f) PSOC 1133.
stream and measuring the amount of solid passing into an optical detector. Any low molecular weight molecules volatilized under these conditions will not be detected and the average molecular weight will be overestimated. Additionally, any loss of material when drying coal extract samples for the VPO measurements used to establish the calibration curve would have a similar effect. Since the VPO data are internally consistent, we do not believe this to be a problem. Any error due to lo!s of material in the maas analyzer will be much larger for M,, than for Iffw This can be shown most easily by using a simple calculation. For a mixture containing 10% (by weight) material having molecular weight 300 and 90% having molecular weight 2000, M,, and M, are 1280 and 1830,respectively. Removing the molecular weight 300 component leaves material of molecular weight 2000. For @, this is a 56% increase compared to a 9% increase in M,. The detector being used, because of its design, is apparently missing a significant amount of low molecular weight material. That such low molecular weight material is present in significant amounts of coal extracts has been shown by several mass spectroscopic studies." We believe the molecular weight
profile to be accurate above 600. Because of the missing low molecular weight material and its heavy representation this quantity is seriously in error. The effect of the in M,,, missing material on M, is much smaller, and this quantity will not be seriously in error. The ideal method for determining molecular weight distributions for coals has not been found, but this technique does appear to give reliable distributions for the higher molecular weight materials. Coal Extract Molecular Weight Distributions. Table IV contains the number- and weight-average molecular weights measured for a set of lower Kitanning bituminous coals together with the pyridine extractability and daf carbon content of the coals. The molecular weight distributions are shown in Figure 7. There is one other set of data with which our data can be compared. The molecular weight distributions of several pyridine coal extracts were determined by gel permeation chromatog(11) Marzec, A. J. Anal. Appl. Pyrolysis 1985,8, 241-254. Schulten, H-R. Fuel 1982,61,670-676. Drake, J. A. G.; Jones, D. W.; Games, D. E.; Gower, J. L. Fuel 1984,63,634-639. Schulten, H-R.;Simmleit, N.; Muller, R. Fresenius Z . Anal. Chem. 1986,323,450-454.
Larsen and Wei
348 Energy & Fuels, Vol. 2, No. 3, 1988 Table IV. Average Molecular Weights for Pyridine Extracts of Lower Kittanning Coals extract, w t % n;r,(VPO)" A?n(GPC)b &(GPC)b
coal PSOC 1278 PSOC 1309 PSOC 1215 PSOC 1236 PSOC 1024 PSOC 1133
" Reproducibility *100.
17 32 34 30 15 4
675 1020 1130 1620 870 590
amt rejected, coal PSOC 1236 PSOC 1215
%
~~~
3 7
4010 5060 4700 4830 3830 3540
%/fin
5.9 5.0 4.2 3.0 4.4 6.0
Reproducibility f150.
Table V. Amount" of Material (an = 150) Evaporated in Detectorb
~
1520 2280 2120 2180 1740 1480
amt rejected, coal PSOC 1309 PSOC 1278
%
9 14
"Wt % of extract. bAesuming all the difference between the
VPO and GPC molecular weights is due to this source. raphy.12 For sev_encoals having carbon contents between 70% and 94%, M, ranged from 600 to 760. These values are anomalously low, and there may be several reasons for this. The use of polystyrene to calibrate the columns may contribute. The chief problem is probably the assumption that "the height (intensity) of the refractive index curve is proportional to the number of molecules of a specific molecular weight". I t is well-known that the response of an RI detector is very sensitive to temperature changes and somewhat sensitive to the molecular weight of the solute and that it is extremely difficult to use an RI detector quantitatively in gel permeation studies.13 There is a clear break in the amount of extract and its molecular weight between coals of carbon contents 87% and 89%. We believe that this is due to the failure of pyridine to extract all of the material soluble in it. Two groups have recently observed extractability increases in high-rank coals after derivatization reactions that presumably broke no u bonds but disrupted stacking intera c t i o n ~ . ~ ~If. 'this ~ is so, then pyridine does not extract all the pyridine-soluble material from high-rank coals because some is held in the coal by strong interactions between the insoluble portion of the coal and the potentially extractable material. On the basis of this consideration and the sudden decrease in both the amount of extract and its molecular weight, we conclude that pyridine is not extracting all of the pyridine-soluble material from these coals. The pyridine extracta for PSOC 1024 and 1133 are therefore not representative of the whole extract and cannot be compared with the others. This fact will become very important in our data analysis because we must assume that the coal extraction has removed all of the material that is not part of the insoluble network. Not surprisingly, it appears as if lower molecular weight material is preferentially extracted. For reasons already made clear, A?fn measured by VPO is more reliable than when measured chromatographically. The ATwvalues are probably somewhat low but close to the real value. To put this into perspective, the amount of the = 150 that would have to be missed by extract having the GPC detector to cause the difference in the VPO and GPC number average molecular weights has been calculated and is presented in Table V. (12)Hill-Lievense, M.E.;Lucht, L. M.; Peppas, N. A. Angew. Makromol. Chem. 1986,134,73-95. (13)Yau, W. W.; Kirkland, J. J.; Bly, D.D.Modern Size-Exclusion Liquid Chromatography; Wiley: New York 1979. (14)Stock, L. M.;Mallya, N. Fuel 1986,65,736-738. (15)Quinga, E.M.Y.;Larsen, J. W. Energy Fuels 1987,1, 300-304.
Discussion We seek here an explanation for the origin of the extractable material present in bituminous coals and an understanding of the way its quantity changes with coal rank. We use the approach pioneered by van Krevelen: coals are three-dimensionalmacromolecular networks, and the extractable materials are the ~ 0 1 . ' If ~ coals ~ ~ are macromolecular networks, there will exist relationships between the progress of the coalification process and the amount of extract and its molecular weight distribution, relationships whose form is controlled by the nature of the network.18J9 These coals were selected because they are a set of high-vitrinite coals with similar depositional environments and histories. The presence of significant amounts of material in the extracts from macerals other than vitrinite could render the results misleading. The coals studied all contain 90 f 2% vitrinite. Since the chemical process occurring is coalification and we are studying its effect, the samples used should be as nearly identical in origin as possible with the only variable being the extent of coalification. We judged this set of lower Kittanning coals the best set available to us. Our approach involves comparing experimental results with the qualitative predictions resulting from a statistical kinetic treatment of a model three-dimensional macromolecular network. It is necessary to delineate the model network structure used and the essence of the kinetic treatment. The model used is one developed for a related cross-linked macromolecular network, lignin.20 The statistical kinetics of ita depolymerization in the post/gel stage have been extensively The treatment has its origin in the general approach to macromolecular networks taken by Flory and Stockmayer and the statistical kinetic treatment of Whittle.24,25 Since only qualitative comparisons between prediction and experiment will be presented here, the model network and the predictions based on it will only be presented qualitatively. Used quantitatively, this model and analysis provide a new very fundamental approach to coal depolymerization kinetics. This approach is being explored and applied to both coal conversion and coalifcation and will be the subject of other papers. Lignin is a three-dimensionally cross-linked macromolecular network. Its transformation to coal necessarily involves many transformations of that network. The model used here aasumes the existence of a single monomer (16)van Krevelen, D.W.Coal; Elsevier: New York, 1981. (17)Dormans, H.N. M.; van Krevelen, D.W. Fuel 1960,39,273-292. (18)van Krevelen, D.W.Fuel 1965,44,229-242. (19)Flory, P. J. Principles of Polymer Chemistry; Comell University Preee: Ithaca, NY,1963. (20)Bolker, H. I.; Brenner, H . S. Science (Washington, D.C.) 1970, 170,173-176. (21)Yan, J. F.;Johnson, D.C. J. Agric. Food Chem. 1980,28,850-855. (22)Yan, J. F. Macromolecules 1981,14, 1438-1445. (23)Yan, J. F.;Johnson, D. C. J. Appl. _ _ Polym. Sci. 1981, 26, 1623-1635. (24)Stockmayer, W. H. J. Chem. Phys. 1943,11,45-55. (25)Whittle, P. Proc. Cambridge Philos. SOC.1965,61,475-495.
Energy & Fuels, Vol. 2, No. 3, 1988 349
Macromolecular Chemistry of Coalification
rather than a distribution of "monomers" as actually occurs in coals. This is ita largest divergence from reality and one that will have little effect on the qualitative comparisons made. The network is comprised of divalent monomers linked together in chains with the chains interlinked by monomers that are trivalent, rather than divalent as most are. Here, valence refers to the number of other monomers to which a given monomer is bound. There are two kinds of bonds that can be broken in a depolymerization of the network. One is between a pair of divalent monomers, and the other will have a trivalent monomer (a branch point) as at least one of the units involved. Cleavage of these two different bonds will have different effects, and this is explicitly recognized in the kinetic treatment. Random bond breaking is assumed, and recombination of the fragments is an explicit part of the process. This structural model is a reasonable fit to coals with their divalent and higher valent clusters linked together by methylene chains, ethers, etc. Coals are obviously more complex, but this complexity will not alter the qualitative conclusions drawn. Whether bond breaking is random cannot be decided now, but this is the easiest place to begin. First let us consider the amount of extract present as a function of rank. For these coals,as for most, it increases with rank, reaching a maximum at about 8647% C, and then decreases precipitously.l8 Evidence exists that at least some of the precipitate decrease is due to the failure of pyridine to remove all of the soluble material from the coals because of increasingly strong coal-extract interaction~."*~ Both ~ of these papers reported increases in the amount of extract from high-rank coals after the coals had been derivatized to break up aromatic-aromatic interactions without cleaving u bonds. This demonstrates that Soxhlet extraction of these high-rank coals does not remove all of the pyridine soluble material. We believe the origin of the decrease in the amount of extract observed with PSOC 1024 and PSOC 1133 is bonding of otherwise soluble molecules to the network through multiple aromatic-aromatic interactions. Since their pyridine extracts are not representative, they cannot be compared with the others and will not be considered further. The kinetic model used requires knowledge of the molecular weight distribution of all the soluble material. The increase in amount of extract between 82% C and 86% C is most easily explained by a net depolymerization of the coal network, producing increasing amounts of extractable material. As bonds are broken, the network becomes less highly cross-linked and fragments are broken off. These fragments are soluble and so the amount of extract increases. PSOC 1236 is an anomalous coal. Its extractability is high but a bit less than some lower rank coals. This may be the start of the precipitous decline or just the normal scatter due to the complex nature of the material and processes involved in coalification. It may also be due to the presence of 8.1% liptinite in this coal, much more than in the other coals. We will treat this as part of the depolymerizing series while pointing out that it may belong with the higher rank coals instead of the lower rank ones. The increasing amount of extract has been taken as an indicator of the coalification process being a net depolymerization over this rank range. Clearly, coalification is a complex phenomenon with many chemical reactions occurring simultaneously. These include both cleavage and condensation reactions. We claim that the result of all of these reactions is a net depolymerization. To depolymerize, bonds must be broken. What are they? A good answer to this cannot be given. It is known, for
2 34
Figure 8. Coal extraction yields in pyridine at 115 "C vs rank reprinted with permission from van Krevelen, ref 16. Copyright 1981 Elsevier.
example, that ethers are lost with increasing rank. When a C-O ether bond is broken, is it replaced by another bond leaving no net change in the cross-link density or is there no replacement that results in a reduction in cross-link density? We will not speculate on this but will point out that studies of coalification chemistry have identified a number of bond types cleaved during this process.26 The depolymerization of a network should result in an increase in the average molecular weight of the extract as the amount of extract increases, that is as the coalification process proceeds. This counterintuitive notion is easy to visualize by considering random bond breaking in a cross-linked network. With only a few bonds broken the fragments produced will most probably be small. Small structural units will most probably be bound to the network at only a few points and may be released as soluble molecules at low degree of bond breaking. A high molecular weight fragment will most probably be bound to the network at many sites and will not be freed until many bonds have been broken. The clear prediction is that the molecular weight of the soluble materials produced by the depolymerization of a network will increase ivith the extent of the depolymerization. This idea has been expressed in and Mw will quantitative f ~ r m . ~ Put ~ " concretely, ~ increase with the amount of extract and with the progress of the depolymerizing coalification process. The data obtained with the series of lower Kittanning coals generally show the expected increase in extract molecular weight with extract amount, although there is some scatter in the &Iw values. This trend is shown most clearly in the VPO data, which we regard as the most reliable. We know of no way other than a network depolymerization to rationalize the increase in extract number-average molecular weight. This is impressive support for the idea that the coalification process is a depolymerization and that coals are cross-linked networks. This behavior seems to be general. Figures 8 and 9 are reproduced from van KrevelenlGand show the amount of pyridine extract and its molecular weight, the data being obtained by several different groups and the correlation lines drawn by van Krevelen. Between 80% C and 86% C, there is an increase in both the amount of extract and its number-average molecular weight. These coals too
a,,
(26) Given, P. H. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic: New York, 1984; Vol. 3.
Larsen and Wei
350 Energy & Fuels, Vol. 2, No. 3, 1988
c
show the behavior of depolymerizing macromolecular networks. On the basis of two criteria, the increase in the quantity of extractable material with rank and the increase in the molecular weight of the extracts with rank, the coalification of bituminous coals is a net depolymerization. This idea can be tested directly by measuring the cross-link density of coals using solvent swelling. We have pointed out that this technique requires the use of solvents that do not specifically interact with The effects of internal hydrogen-bond cross-links also need to be separated from those of covalent links. No solvent-swelling study of a broad rank range of bituminous coals meeting these criteria has been published, and so the solvent-swelling studies so far reported reveal little of the coalification process. Literature data on solvent swelling do reveal a precipitate decrease in swelling in all solvents occurs a t about 87% C, consistent with a sudden increase in cross-link density, which we believe is not due to covalent bonds but is due to aromatic-aromatic interactions. There is one feature of these data that is not comjstent with a simple depolymerizationprocess. The ratio Mw/Mn
contains information about the statistics of bond breaking. Assuming the described structure model and random bond breaking, this ratio should increase as the depolymerization and amount of extract increase. For VPO data, the ratio increases with coalifcation in opposition to the prediction. If this ratio is calculated by using only GPC data, it is constant at 2.2 except for one coal (PSOC1278). This is clearly easier to rationalize than the VPO-based ratio and is more consistent with the depolymerization process. Since we believe the VPO data to be more reliable, we concentrate on that. There are several possible explanations, all of which require further study. This quantity is sensitive to details of the model, and that sensitivity needa to be explored. A more disturbing possibility is that some high molecular weight material remains trapped in the coal, unextracted by pyridine. Since this trapping is expected to increase with molecular size, it is capable of explaining our data. Note that this would enhance the increase in extract molecular weight with extract amount. Finally, the assumption of random bond breaking may not be realistic. This point will be addressed more completely as this approach to the coalification process is developed in a quantitative fashion. Now, there is clearly a conflict between prediction based on a very simple model and experiment. There are several possible explanations that do not require discarding the idea that coals are macromolecular networks and their coalification can be treated as reactions of networks. Overall, we find the case for the network structure of coals convincing and think that the coalification process is best described using the models and concepts of polymer chemistry. All data do not fit a simple model assuming random bond breaking and we will be scrutinizing carefully those that do not fit. There is misinformation in the literature about the molecular weight distributions of coal extracts. The tendency is to find low molecular weights when techniques, such as mass spectroscopy, that are limited to low molecular weight materials are used. This tendency is somewhat reinforced by common use of number-average molecular weights, an averaging procedure that weights most heavily the smaller molecules. All of the extracts studied contain material having a molecular weight of 10 OOO and some extend to 15OOO. Pyridine coal extracts are soluble macromolecules.
(27) Larsen, J. W.; Green, T. K . ; Kovac, J. J. Org. Chem. 1985, 50, 4729-473s. (28) Quinga, E. M. Y.; Larsen, J. W. In Proceedings of the NATO
Acknowledgment. We are grateful to the U. S. Department of Energy for financial support. Helpful discussions with Dr. Jeff Kovac are gratefully acknowledged.
I
1200
B 0
.
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P
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800-
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400
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-88 wc d.aT Figure 9. Number-averagemolecular weights of pyridine extracts vs coal rank reprinted with permission from van Krevelen, ref 16. Copyright 1981 Elsevier.
Advanced Study Institute on New Trends in Coal Science: Yurum, Y., Ed., D. Reidel: Amsterdam, in press.
Registry No. Pyridine, 110-86-1.
