Energy & Fuels 1993, 7, 1001-1005
1001
EPR-Spin Probe Studies of Model Polymers: Separation of Covalent Cross-Linking Effects from Hydrogen Bonding Effects in Swelled Argonne Premium Coal Samples D. R. Spears,?W. Sady,t D. Tucker,% and L. D. Kispert'95 Chemistry Department, The University of Alabama, Tuscaloosa, Alabama 35487 Received November 16, 1992. Revised Manuscript Received August 12, 1993"
The swelling behavior of 2-12 % cross-linked polystyrene-divinylbenzene (PSDVB) copolymers was examined by an EPR-spin probe technique. It was observed that the mechanism of spin probe inclusion was the intercalation into the matrix rather than diffusion into the pores. The disruption of van der Waals forces between adjacent aromatic rings appeared to be the primary mechanism for pyridine swelling of PSDVB. By comparing the data to results from coal swelling studies it was also inferred that the extent of hydrogen bonding in coal will have a much greater impact on its swelling properties than its covalently cross-linked character.
Introduction Molecular accessibility of catalysts is a critical problem to be addressed when considering the task of converting coal to usable chemicals. To attain the best results, a catalyst must be intercalated in the coal macromolecular structure so that hydrogenation catalysis can occur from within as well as normal surface catalysis. The molecular accessibility process can be studied using electron paramagnetic resonance (EPR)14 by swelling coal in a solvent solution of guest molecules, such as the nitroxyl free radical, followed by removal of the solvent. Previously, the EPR technique developed in this lab has addressed the variation in coal porosity as a function of rank upon swelling using nitroxides of varying size.24 The importance of hydrogen bonding on molecular accessibility in coal structure as a function of rank and swelling solvent was examined using nitroxides containing substituents with strong acid or base character that tend to hydrogen bond.215-8 By appropriate use of reactive substituents in the nitroxide radicals, it is also possible to monitor the variation in molecular accessibility with functionality and heteroatom content, although so far this aspect has not been studied. Unfortunately, molecular accessibility as a function of aromaticity cannot be monitored with the spin probe technique. However, molecular accessibility as a function of cross-link character in coal hamot been examined but can bestudied by EPR t Current address: U S . Bureau of Mines, P.O. Box L, Tuscaloosa, AL 35486-9777.
t Current address: Maria Curie-SkldowskaUniversity,Lublin, Poland. I Chemistry Department, The University of Alabama, Tuscaloosa, AL 35487.
Abstract published in Advance ACS Abstracts, October 1, 1993. (1) Silbernagel, B. G.; Ebert, L. B.;Schlosberg, R. H.; Long, R. B. Adu. Chem. Ser. 1979,192,23. (2) Spears, R.; Goslar, J.; Kispert, L. D. Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry 229; American Chemical Society: Washington, DC, 1993; p 467-482. (3) Spears, D. R.; Kispert, L. D.; Piekara-Sady,L. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36, 29. (4) Sady, W.; Kispert, L. D.; Spears, D. R. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1992, 37, 1151. (5) Spears, R.; Kispert, L. D.; Piekara-Sady,L. Fuel 1992, 71, 1003. (6) Spears, R.; Kispert,L. D.; Sady, W. Prepr. Pap-Am. Chem. SOC., Diu.Fuel Chem. 1991,36, 1277. 0
(7) Spears,D.R. Ph.D.Diesertation,UniversityofAlabama,Tuscaloosa, 1991.
(8)Spears, D. R.; Kispert, L. D.; Sady, W. Fuel 1993, 72, 1225.
spin probe techniques upon swelling and is the subject of this paper. The accessibility of guest (catalyst) molecules into a coal structure as a function of different temperature and solvent does vary with coal rank and a separation of the influence of cross-link character from other properties of coal upon swelling is of interest. To carry out an EPR spin probe study of solvent swelling as a function of crosslink character, a synthetically available polymer that exists as a 2 , 3 , 4 , 8 , or 12 5% cross-linked polymer possessing no hydrogen bonding, functional group, or heteroatom substituent will be examined. BET measurements of the polymer beads also show that the beads are relatively nonporous, making it easier to study the dependence of cross-link character on molecular accessibility. Although considerable evidence exists that coal is highly cross-linked, the nature and extent of covalent cross-links in coal is very uncertain. Cross-links can be of two types, covalent bridges and noncovalent interactions (i.e., charge transfer, van der Waals, and hydrogen bond associations). Coal becomes more and more aromatic as coalification progresses (Le., as rank increase^)^ and becomes steadily more close packed up to the low-volatile bituminous range (88% carbon, dmmf).1° This suggests that coal becomes more cross-linked. However, the cross-links are not hydrogen bonds at high ranks, but rather van der Waals or covalent bonds. Nishioka and Larsen, in a follow-up to the work of Larsen and Mohammadi" have shown that London dispersion forces increase in importance as rank increases12due to an increase in the concentration of the aromatic units. Covalent bridges are believed to consist predominantly of ether linkages13and methylene bridges.14J5 Ignasiak and Gawlak estimated that roughly half of all covalent cross-links might be ether linkages.16 Because of the limited similarities and significant differences between coal and polymers, considerable insight into the molecular accessibilityof coals as a function of cross-link character during swelling can be gained by (9) Schafer, H. N. S.Fuel 1970,49, 197. (10) Diamond, R. Acta Crystallogr. 1957, 10, 359. (11) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990,4, 107. (12) Nishioka, M.; Larsen, J. W. Energy Fuels 1990,4,100. (13) Ruberto, R. G.; Cronauer, D. C.; Jewell, P. M.; Seshadri, K. Fuel 1977,56, 25. (14) Ouichi, K.; Brooke, J. D. Fuel 1967, 46, 367. (15) Quichi, K.; Imuta, K. Fuel 1973,52, 171. (16) Ignasiak, B.; Gawlak, M. Fuel 1977,56, 216.
0887-0624/93/2507-1001$04.00/00 1993 American Chemical Society
S.
Spears et al.
1002 Energy & Fuels, Vol. 7, No. 6,1993
studying the swelling of polymers. Swelling studies have shown that coal behaves like a glassy polymer below ita glass transition temperature (T,).l7 Whether coal actually has aglass transition temperature is stillopen to question.18 Polymers have also been used to mocbldvent diffueien in ~0a1.'9~~0 These studies have been used to corroborate the assertion that coals behave like a glassy solid. More importantly, there is considerable evidence to show that coal is cross-linked covalently as well as by hydrogen bonding. If coal were entirely cross-linked by hydrogen bonding, coal, in theory, would be completely solubilized by repeated extraction with a strong base.21*22However, coal solubility and swellability decreases with rank.23 Further, although phenolic oxygen is primarily responsible for hydrogen bonding in coals above the subituminous rank, the hydroxyl content decreases with rank. This is because the oxygen substituents on the side rings have been converted to ether/furan (covalent) linkages. Since covalent bridges are not affected by solvents in any known way,17 it is not surprising that coal can only be partially solubilized. The polystyrene-divinylbenzenecopolymers used in this study, however, do not exhibit any hydrogen bonding or similar polar interactions, thereby eliminating some of the more prominent interactions in coal. In addition to this, the polymers have specific extents of covalent cross-linking. So, since these copolymers can be highly cross-linked with covalent interactions, without the presence of hydrogen bonding or other properties, they are valuable models for studying the molecular accessibility as a function of covalently cross-linked character in coal. The use of the nitroxide stable free radicals as guest molecules was first developed by Silbernagel et al. for studying the porosity of coal by EPR methods.' These radicals, whose general structure is given below, have a
nitrogen singly bound to oxygen and have four methyl groups on the a' ring positions to sterically stabilize the radical. The unpaired electron density resides 40% on nitrogen and 60% on oxygen. The nitroxyl group (NO) has little effect on the reactivity of any molecule to which it is attached. Consequently, nitroxides are very stable compounds, even at room temperature. The R group represents various functionalities which may be attached to the nitroxyl. The nitroxide radicals used for this study and generally referred to as spin probes are shown in Figure 1. For spin probe I, R = C-OH (spherical shape and an hydrogen bonding site), the molecular volume is approximately l U A 3 . For spin probe VIII,24R = C-H (spherical shape and no hydrogen bonding site) the molecular volume is about 132 A3. For spin probe X I I I p R = C-NH(CH2)eCH3 and the molecular volume is about 238 A3. (17) Green, T.; Kovac, J.;Brenner, D.;Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic: New York, 1982. (18) Fisher, C. H.; Eisner, A. Ind. Eng. Chem. 1937,29, 1371. (19) Barr-Howell, B. D.;Peppas, N. J. Appl. Polym. Sci. 1986,31,39. (20) Milewskaduda, J. Fuel 1987,66, 1570. (21) Painter, P. C.; Park, Y.; Coleman, M. M. Energy Fuels 1988, 2, 693. (22) Hall, P. J.; Marsh, H.; Thomas, K. M. Fuel 1988,67, 863. (23) Berkowitz, N. An Introduction t o Coal Technology; Academic: New York, 1975. (24) Cooray, L. S. M.S. Thesis, University of Alabama, 1988.
Q I.
b
b
Vlll
I
Xlll Figure 1. Molecular structure of spin probes I, VIII,and XIII.
Silbernagel's method has been further developed and considerably expanded in our laboratory. The use of the EPR spin probes has proved to be a veryvaluable technique in gathering information about molecular inclusion and intercalation in coals, coal micropore structure, and surface Our results showed that swellingis a function of solvent polarity. Coal contains spherically shaped pores which disappear upon swelling. With polar solvents like pyridine, the pores elongate in a direction parallel to the bedding plane. Pore elongation takes place by gradual disruption of hydrogen bonds perpendicular to the bedding plane, resulting in an unzipping of the coal macromolecular structure. Small increases in temperature did not greatly affect the swelling ability of toluene, unlike polar solvents such as pyridine and nitrobenzene. Coal swellingdecreases with rank, due to both a decrease in porosity and in the hydrogen bond cross-link density. A great deal of this information provided insight in to the hydrogen bond crosslinking in coal and the effects of swelling solvents on these cross-links. However,the question of covalent bonding in coal and the relationship of coal to organic polymers is still uncertain. In an attempt to gain further insight into the swellingof coal, a series of model polymers,with specific extents of covalent cross-linking which do not exhibit hydrogen bonding between monomer units, will be investigated for swellingproperties and spin probe retention.
Experimental Section Model Polymers. Biorad Molecular Sieve Beads SX-2, SX3, SX-4, SX-8,and SX-12were used as the model polymers. These are polystyrene-divinyl benzene copolymer beads with a radius of approximately 3.2 X 1W m (or about 400 mesh). The percent cross-linking in these polystyrene-divinylbenzene (PSDVB) polymers is indicated by the numerical suffix. An approximate molecular structure is shown in Figure 2. They exhibit no hydrogen bonding and only covalent cross-linking. Spin Probes. Three spin probes were used in this study (Figure 1) and were numbered in a manner consistent with previously published work from this laboratory.2B Spin probes I (TEMPOL,or 4-hydroxy-2,2,6,6-tetra~nethylpiperidine-l-oxyl) and VI11 (TEMPO,or 2,2,6,6-tetramethylpiperidine-l-oxyl) were obtained from Aldrich Chemicals. Spin probe XIII (4-nonylamino-2,2,6,6-tetramethylpiperidine-l-oxyl) was obtained from Molecular Probes of Eugene, OR. Swelling of Polymerswith Spin Probes. Theexperimental method has been published in detail elsewhere.6 Briefly, 30 mg of the chosen polymer was swelled under argon with 2 mL of 1 mM solutions of spin probes at either 298 or 333 K. Toluene and pyridinewere used as solvents. After 18h, the solventwas filtered
EPR-Spin Probe Studies of Model Polymers
Energy &Fuels, Vol. 7, No. 6, 1993 1003
+-y
Retention of Spin Probe VI11 in PSDVB
Q
I
$1 10
CH&H-CH$H
X
8
Ef(d
h
6
k
4
0
a
Figure 2. Approximate molecular structure of polystyrenedivinylbenzene. 0.14 /
/
h
-
"E
Q
4 8
f
\
/
0.1
\
4
\
/
\
4
0.06
I
,
,
,
,
0
2
4
6
8 1 0 1 2 1 4
,
,
I
( 0 )toluene at 333 K; (v)pyridine at 298 K (v)pyridine at 333 K. The vertical lines indicate the reproducibility of the experiments.
\
/
0.08
O
% Crosslinking Figure 4. Spin probe VI11 concentration in swelled polymers vs toluene at 298 K; cross-link density for polymers swelled in (0)
\
4
0.12
e
.*\
--
-64
W
\
-
12
sf
10
b
0.04
Retention of Spin Probe I in PSDVB
I
u)
0.02
X
a
A . f - i .. -_.. -_ a..........
0
,
I
2.0
4.0
6.0
'
"
,
'
'
'
8.0
I
'
10.0
'
1
12.0
1
% Crosslinking
Figure 3. Surface area (mz/g) versus % cross-linked PSDVB copolymes from (+) Nz BET surface data and (A)by assuming a theoretical smooth surface.
Ei(d k
8
k
4
0
a
I
T
: 2
.d
a
off and the polymer vacuum-dried at room temperature for 2 h. The polymer was then washed in cyclohexaneto remove any spin probes on the polymer surface. This procedure also removes any spin probes from the macro or mesopores. The cyclohexanewas then removed under vacuum. Finally, the polymer was placed in an EPR tube and evacuated. The concentration of spin probes retained in the polymer was then determined by electron paramagnetic resonance spectroscopy. In previous studies? the optimum concentration of spin probe to be used for this method was determined to be 1 mM. A t spin-probe concentrations above 10-2 M, more than one probe could become trapped in an accessiblepore upon swelling, giving rise to a difficulty to interpret a single spin exchange EPR line. At concentrations below l(r M, the concentration of trapped radical in a coal matrix was so low to make study difficult. In all studies, the amount of spin probe in solution was in far excess of what was found to become trapped in the studied matrix. For instance, approximately 20-30% of the spin probes in solution was found on or in the polymer bead after removing the solvent. After washing with cyclohexaneto remove the spin probes on the surface of the beads or in the macropores, a maximum of only 2% of the probes in solution were trapped in the polymer. Determining Surface Areas. The surface areas of the polymers were determined by N2 BET surface analysis using a Leeds & Northrup Instruments Surface Area Analyzer Model 4200. A graph showing the Nz, BET surface area as a function of percent cross-linking is shown in Figure 3. Figure 3 also indicates the approximate surface area of the beads alone, assuming that they are nonporous. It was determined that 150 beads weighed 0.00056 g. So with each bead having a radius of 3.21 X 106 m, the surface area for a nonporous bead would be 1.2 x 10-8 mz/sphere or 0.003 m2/g. It can be easily seen that these polymers are porous but have porosities which are at least 1order of magnitude lower than coal which has average Nz BET surfaces of 1-10 m2/g. A swelling experiment also revealed that pyridine was a much better swelling solvent than toluene; the volume increase was 2-fold greater after 18 h of swelling.
8
m
o 0
2
4
6
8 1 0 1 2 1 4
% Crosslinking Figure 5. Spin probe I concentration in swelled polymers vs
toluene at 298 K; cross-link density for polymers swelled in (0) ( 0 )toluene at 333 K; (v) pyridine at 298 K, (v)pyridine at 333 K. The vertical lines indicate the reproducibility of the experiments.
Results The retention of spin probe VI11 in various swelling solvents at different temperatures is shown as a function of percent cross-linking in Figure 4. There seemed to be a local minimum in the curves at 3% cross-linking for toluene and at 4% cross-linking for pyridine, although all curves increase overall as percent cross-linking increased. The effects of swelling temperature had negligible effect on the swelling of the copolymers in pyridiune or below 4 % cross-linking for toluene. However, an increase in swelling temperature resulted in increased retention of spin probe VI11 in toluene for the more highly cross-linked polymers. Overall, toluene appeared to be a better solvent for retention of spin probe VI11 into the copolymer structure following the cyclohexane wash. The retention of spin probe I in various swelling solvents at different temperatures is shown as a function of crosslinking in the PSDVB copolymers in Figure 5. Like spin probe VIII, there is a local minimum at 298 K for toluene a t 3% cross-linking and for pyridine a t 4 % cross-linking. Upon raising the swelling temperature to 333 K, the local minimum at 298 K for 3 5% cross-linking becomes a local maximum for toluene as a solvent. It is very clear that swelling temperature has little or no effect for pyridine; however a measurable increase is observed for spin probe
1004 Energy & Fuels, Vol. 7,No. 6,1993 Retention of Spin Probe XI11 in PSDVB
Ll
t
n
0
2
l.O
i
m
d
.3
a
0.0
4
6
8 1 0 1 2 1 4
% Crosslinking
Figure 6. Spin probe XI11 concentration in swelled polymers vs cross-link density for polymer swelled in (0) toluene at 298 K; ( 0 )toluene at 333 K, (v)pyridine at 298 K; (v)pyridine at 333 K. The vertical lines indicate the reproducibility of the experiments.
retention with temperature for toluene. Again there was an overall increase in spin probe I retention with increasing percent cross-linking, but the trend was not as strong as for spin probe VIII. Clearly, toluene is the better solvent for the inclusion of spin probe I into the copolymer structure following the cyclohexane wash. The retention of spin probe XI11 in various swelling solvents at different temperatures is shown as a function of percent cross-linking in the PSDVB copolymers in Figure 6. The overall retention of spin probe XI11 was much less than either spin probe VI11 or I. Unlike spin probes VI11 or I, the retention of spin probe XI11 in the copolymers decreased with increasing percent crosslinking. The swelling temperature had no effect for those polymers swelled in toluene except for SX-2 (2% crosslinking) which showed poorer retention at a higher swelling temperature. The reverse was true for pyridine which showed a dramatic increase in spin probe retention with an increase in the swelling temperature. There was not much difference between toluene and pyridine as solvents for the intercalation of the nitroxide radicals until the swellingtemperature was increased, at which point pyridine improved and toluene actually got worse. The uncertainties in the spin/gram values presented in Figures 4-6 are indicated by the error bars. It is noteworthy that the variation in the concentration of the nitroxide spin probe in the polymer does not vary strongly with percent cross-link. However, it is noted that there is a higher retention of all labels at 1 2 % cross-linkcharacter for label VIII, higher retentions for both VI11 and I in toluene at 333 K while comparable retentioff of VIII and I for all other conditions. There is a much lower (order of magnitude) level of inclusion of XI11 in all cases except pyridine at 333 K does produce a relatively higher retention level. Discussion
It is generally accepted that, as coals increase in rank, they become more highly covalently cross-linked. In similar swelling experiments used to determine the molecular accessibility of spin probes to coal structure, it was determined for spin probes I and VI11 that spin probe retention decreased with increasing cross-linking (rank) (decreasing hydrogen bonding); from an order of magnitude more for Beulah-Zap lignite (74% C) to 2 orders of
Spears et al.
magnitude less for upper Freeport (88%C).s97 This trend is not observed for the polystyrene-divinyl benzene copolymers. In fact, it seemed that spin probe retention of spin probes I and VI11 increased slightly with increasing cross-linking. This result indicates that the mechanism for the inclusion of spin probes in a coal matrix dominated by covalently cross-links operates under a different mechanism than when hydrogen bonding dominates. It is also shown that the better swelling solvent is not necessarily the best solvent for providing molecular accessibility of the spin probe to the cross-linked structure of system followinga cycohexanewash. Although pyridine was clearly a better swelling solvent for coal, it did not allow for as much intercalation of spin probes I and VI11 in the polymer structure as toluene. The same has been observed for coals under certain circumstances.7 In coal this is attributed to a opening of the structure to such an extent that the smaller spin probes are not trapped in the coal matrix after removal of the solvent and are removed from the porous surface by the nonswelling cyclohexane wash? It is possible that the same mechanism is occurring in the copolymers since there are higher retention of the larger spin probe (XIII) in pyridine, especially at higher swellingtemperature. This is consistent with observations made for coals. No real difference between the absolute values for the retention of spin probes I and VI11 was observed. This was to be expected since the copolymers exhibit no hydrogen bonding character, and the functionality of the hydroxyl group would have no effect. The only real difference between spin probes I and VI11would therefore be the size difference between the two molecules, which is really negligible. It is important to consider the method by which the spin probes are intercalated into the structure of the copolymers. The spin probes could be trapped in preexisting pores when the solvent is removed, or they could be trapped in accessible regions created in the macromolecular structure of the polymer as they are dissolved into it. The surface area of the polymer beads was determined to be about 0.1 m2/gwhich was a t least 1 order of magnitude higher than the approximated surface area of the beads alone. Since the spin probes were held trapped in the macromolecular structure and are not freely rotating in the enclosed pore, let us assume that all of the surface area measured by the Nz BET analysis is due to pores which are the same approximate size as the spin probes (about 140 A3) I and VIII. In coals, the isotropic signal of a freely rotating spin probe would be completely hidden under the EPR signal produced by the coal, which is 100 times more intense. However, the EPR spectra obtained for both the polymers and coals are those of a powder pattern for the nitroxyl spin probe, indicating that the spin probes are trapped in pores which are close to their own molecular volume. If all of the porosity of the polymers was due to pores which have an approximate volume of 140A3, then each bead could contain a maximum of 61 spin probes. This would then provide us with a maximum of 1.3 X 107 spins/g, which is about 8 orders of magnitude less than the observed retention of spin probes I and VIII. This leads us to conclude that either most of the pore structure is not accessible to Nz BET surface analysis or, more likely, spin probes of this size are trapped in regions of the polymer created when the solvent dissolves into the macro molecular structure.
EPR-Spin Probe Studies of Model Polymers
Figures 4 and 5 show that swelling temperatures had little effect on spin probe retention for polymers swelled in pyridine, but a significant effect for polymers swelled in toluene, especially for spin probe I. It has been shown from previous research that spin probe retention in coals is increased with swelling temperature for good solvents and has little effect for poor solvents. Since these results are contradictory, it is apparent that the intercalation of guest molecules in the macromolecular structures during swellingis dominated by completely different mechanisms for PSDVB and coal. This is further indication that accessibility to covalently cross-linked structure and hydrogen-bonded structure is achieved in much different manners. Pyridine was shown to open up the polymer structure and permit more intercalation of the spin probes. This was further supported by the data shown in Figure 6. The results show that only small amounts of spin probe XI11 were retained in PSDVB when toluene was used as a swelling solvent, indicating that the structure was not opened enough to allow accessibility of the long cylindrically shaped spin probe. When pyridine was used as a swelling solvent at 333 K, the structure was opened enough to allow significant amounts of spin probe XI11 to be retained. This also further indicates that the smaller spin probes are not retained a t higher temperatures because during swellingthe structure is opened too much. In the low cross-link density PSDVB’s much of the tertiary structure is due to van der Waals forces between adjacent aromatic rings. Pyridine, which is also aromatic, is a flatter molecule than toluene, and is capable of disrupting interactions between adjacent aromatic rings. Unlike coal, PSDVB contains no hydrogen bonds, so pyridine does not swell PSDVB by breaking hydrogen bonds. This implies that breakage of hydrogen bonds is not the only mechanism through which pyridine swells coals.
Conclusion Molecular accessibility of spin probes to the structure of polystyrene-divinylbenzene copolymers is achieved by
Energy & Fuels, Vol. 7, No. 6,1993 1005
a much different mechanism than coal. Hydrogen bonding character seems to play a much more significant role in spin probe retention in coals than the covalently crosslinked character. Because the polymers contained no functional groups, they did not exhibit selective retention to TEMPOL as opposed to TEMPO. Pyridine was a much better swelling solvent for PSDVB than toluene, and enlarged the pores enough to permit access by larger spin probes at higher swelling temperatures. However, even though pyridine was a better swellingsolvent, toluene was a much better solvent for the intercalation of spin probes in the macromolecular structure of PSDVB copolymers following a cyclohexane wash. Pyridine causes accessible regions so large that the small spin probes are washed away with cyclohexane. It was demonstrated that most of the spin probe retention was due to intercalation of the guest molecules into the structure of the PSDVB copolymers, and not to any inclusion of spin probes in preexisting pores accessible from the surface of the polymer beads. The swelling mechanism for pyridine appeared to be the disruption of the van der W a d s force between adjacent aromatic rings, since no hydrogen bonds existed which could be disrupted, as would be the case for coals. The results imply that, although coal may have some characteristics similar to PSCVB copolymers, the presence of hydrogen bonds significantly alters the swelling properties of coal and is far more important in the molecular accessibility in coal than covalent cross-linking character. However, it was also shown that a significant amount of spin probe retention can be achieved in covalently cross-linked materials and that this information should be taken into consideration when attempting intercalation of guest molecules to coals with larger extents of covalent crosslinking.
Acknowledgment. This work was supported by the
U.S.Department of Energy, Pittsburgh Energy Center, University Coal Research Program Grant No. DE-FG2290PC90284. The U S . Bureau of Mines, Tuscaloosa, is thanked for use of their BET equipment.
