Surface Thermodynamics for Hydrocarbons on Wyodak Coals

Energy & Fuels 1996, 10, 797-805

797

Surface Thermodynamics for Hydrocarbons on Wyodak Coals Amy S. Glass* and Damon S. Stevenson Department of Chemistry, University of Dayton, Dayton, Ohio 45469-2357 Received November 7, 1995. Revised Manuscript Received January 22, 1996X

Isosteric adsorption enthalpies and dispersive surface tensions have been measured for hydrocarbons interacting with Wyodak coal surfaces. The results show that this coal’s intermolecular forces are complex. Saturated (n-alkane) adsorbates experience a low-energy Wyodak coal surface with a dispersive surface tension of ∼25 mJ/m2. Unsaturated (alkene and aromatic) adsorbates have more exothermic interactions with the coal. The interactions of all 1-alkenes studied decrease in exothermicity by ∼1.2 kcal/mol after heating the coal to 200 °C. Citric acid washing or HF-HCl demineralization causes the same increase (∼1.2 kcal/mol) in the 1-alkenes’ adsorption heats. These results demonstrate that unsaturated hydrocarbons interact specifically with the Wyodak coal surface. This specific interaction most likely involves the unsaturated portion of the hydrocarbon adsorbate and polar or ionic groups on the coal. Plots of adsorption heat vs polarizability have different slopes for alkenes and alkanes, demonstrating that the two types of adsorbates experience different nonspecific interactions with the coal as well. Demineralizing the coal has no effect on the plots, but heating to 250 °C, extracting in tetrahydrofuran, or alkylating the coal gives the same plot of adsorption heat vs polarizability for both types of adsorbates, demonstrating that these treatments result in a coal surface that has similar nonspecific interactions with saturated and unsaturated hydrocarbons. These treatments cause the coal’s dispersive surface tension to increase to ∼50 mJ/m2.

Introduction Many coal processing reactions depend on the interactions of reagents with coal surfaces. Coal surface interactions determine the outcomes of reactions in coal combustion, liquefaction, and in coal pretreatments including oxidation (weathering) and pyrolysis as well as coal cleaning (flotation, removal of mineral matter).1-6 Therefore, an understanding of coal surface chemistry is essential to understanding many of its important applications. In particular, knowledge of the strengths of interactions at coal surfaces (surface thermodynamics) would greatly improve our understanding of these processes. Due to the complex nature of coal, surface thermodynamic data are difficult if not impossible to obtain with classical techniques such as static adsorption and heats of wetting.7,8 Static adsorption and liquid phase techniques suffer from penetration of probe molecules into bulk coal so that a combination of bulk and surface properties is measured, making it difficult to interpret the data.9 The technique of inverse gas chromatography Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Van Krevelen, D. W. Coal, 3rd. ed.; Elsevier: Dordrecht, The Netherlands, 1993; p 733. (2) Dryden, I. G. C. In Chemistry of Coal Utilization; Supplementary Vol.; Lowry, H. H., Ed.; Wiley: New York, 1963; Chapter 6. (3) Berkowitz, N. An Introduction to Coal Technology; Academic: New York, 1979; Chapter 5. (4) Gavalas, G. R. Coal Pyrolysis, Coal Science and Technology 4; Elsevier: Dordrecht, The Netherlands, 1982; Chapter 5. (5) Silbernagel, B. G. In Interfacial Phenomena in Coal Technology; Botsaris, G. D., Glazman, Y. M., Eds.; Marcel Dekker: New York, 1989; Chapter 1. (6) Tsai, S. C. Fundamentals of Coal Beneficiation and Utilization; Coal Science and Technology 2; Elsevier: New York, 1982; Chapter 7. (7) Malherbe, P. Le R.; Carman, P. C. Fuel 1951, 31, 210-219. (8) Marsh, H. Carbon 1987, 25, 49-58. X

0887-0624/96/2510-0797$12.00/0

(IGC) solves this problem for coal surfaces because it is a dynamic technique (probe molecules contact the surface only briefly and are desorbed before bulk penetration begins).10 Inverse liquid chromatography and flow microcalorimetry are also dynamic techniques, but liquid carriers are used so that bulk penetration occurs and surface properties cannot be determined.11,12 At present IGC provides the only method to obtain accurate thermodynamic data for molecules interacting with coal surfaces. IGC studies have provided a picture of the surface interactions of the Argonne Premium Sample, Illinois No. 6 bituminous coal.10,13 The overall objective of these studies is to characterize surface forces of the wellcharacterized Argonne coals, a necessary prerequisite to understanding the relationship between coal reactions and surface chemistry. Before this method can be used to probe technologically important reactions, the surfaces of the original coals need to be characterized. The present study was undertaken to characterize a low-rank coal, the Argonne sample Wyodak subbituminous coal. Surface forces of low-rank coals are of interest from fundamental as well as practical viewpoints. IGC studies provide insight into the dependence of different coals’ behaviors on their intermolecular surface forces. Low-rank coals are abundant in the United States. Although these coals are less desirable as fuels and for coke production (their mineralogical compositions cause undesirable combustion properties (9) Hsieh, S.-T.; Duda, J. L. Fuel 1987, 66, 170(10) Glass, A. S.; Larsen, J. W. Energy Fuels 1993, 7, 994-1000. (11) Hayashi, J.; Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 1112-1117. (12) Fowkes, F. M.; Jones, K. L.; Li, G.; Lloyd, T. B. Energy Fuels 1989, 3, 97-105. (13) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 629-636.

© 1996 American Chemical Society

798

Energy & Fuels, Vol. 10, No. 3, 1996

and they have low calorific values compared to higher rank coals), they have been used in liquefaction and have been coprocessed with higher rank coals for other uses.14-18 A strong desire to use this abundant resource would gain impetus from knowledge of their surface chemistry, providing information needed to develop improved cleaning and processing methods, and making low-rank coals more viable for fuel and nonfuel applications. Several workers have studied intermolecular forces in low-rank coals.19-24 These coals possess ionic (cation) mineral matter forms which are not present in higher rank coals because the (carboxylic) sites for cation binding are lost with increased metamorphosis.25 Studies of intermolecular forces in low-rank coals have mostly concentrated on their ionic forces.19-24 Schafer studied ion-exchange properties of low-rank coals and he quantified these groups in different coals.19,20 Nishioka has conducted swelling studies on original and alkylated Wyodak coal.21 He concluded that ionic forces are important for determining the macromolecular structures of these coals; i.e., ionic forces, at least in part, act to hold coal clusters together. Studies on demineralized coals are one approach to understanding mineral matter effects on coals’ intermolecular interactions. Ahsan et al. showed that when Wyodak coal was subjected to citric acid wash (CAW) treatment, which removes ion-exchange and carbonate minerals, changes occurred in the coal’s immersion heats in organic acids.22 Wyodak’s interactions with aromatic acids were stronger after demineralization while its interactions with aliphatic acids were weaker. Vorres measured the pH of Wyodak coal slurries washed with nitric acid and found that a range of different strength ion-exchange sites exist in this coal.23 Martinez-Tarazona et al. found that ion-exchange sites exist as both organic (carboxylate and phenolate) and inorganic (clay) components in low-rank coals.24 Na and K are associated with inorganic components (clay minerals) while Mg and Ca are associated with the organic components in low-rank Spanish coals. On the whole, studies have concentrated on the ionic properties of low-rank coals. They have not completely characterized the nature of the coals’ intermolecular surface forces. In the present work, adsorption heats were determined for saturated and unsaturated hydrocarbons interacting with Wyodak coal. The thermodynamic data provide a picture of the complex intermolecular forces at the surface of Wyodak coal. (14) Parks, B. C. In Chemistry of Coal Utilization, Supplementary Vol.; Lowry, H. H., Ed.; Wiley: New York, 1963; Chapter 1. (15) Van Krevelen, D. W. Coal, 3rd ed.; Elsevier: Dordrecht, The Netherlands, 1993; Chapter 24. (16) Berkowitz, N. An Introduction to Coal Technology; Academic: New York, 1979; Chapter 13. (17) Donath, E. E. In Chemistry of Coal Utilization, Supplementary Vol.; Lowry, H. H., Ed.; Wiley: New York, 1963; Chapter 22. (18) Renton, J. J. In Coal Structure; Meyers, R. A., Ed.; Academic: New York, 1982; p 320. (19) Schafer, H. N. S. Fuel 1970, 49, 197-213. (20) Schafer, H. N. S. Fuel 1970, 49, 271-280. (21) Nishioka, M. Fuel 1993, 72, 1725-1731. (22) Ahsan, T.; Wu, J. H.; Arnett, E. M. Fuel 1994, 73, 417-422. (23) Vorres, K. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 1045-1057. (24) Martinez-Tarazona, M. R.; Martinez-Alonso, A.; Tascon, J. M. D. Fuel 1990, 69, 362-367. (25) Fowkes, W. W. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic: New York, 1978; Vol. II, Chapter 27.

Glass and Stevenson

Experimental Section Isosteric adsorption enthalpies, qst, were determined for coal-adsorbate pairs using the IGC procedure developed previously.10 Wyodak-Anderson coal from the Argonne Premium Coal Sample Bank was the stationary phase.26 The coal was demineralized using citric acid wash (CAW) and HF-HCl demineralization procedures.27,28 Extracted coal was prepared by extracting coal of 40/60 mesh in tetrahydrofuran (THF) in a Soxhlet extractor for several days. Alkylated coal was prepared using the O-methylation procedure of Liotta et al. with methyl iodide as the alkylating agent and tetrabutylammonium hydroxide as the base catalyst.29 All coals were dried overnight in a vacuum oven at 115 °C, sieved to 40/60 mesh, and ∼3 g packed into a 1/8 in. o.d. stainless steel tube about 120 cm long. Each packed column was degassed in flowing He at 150 °C for at least 1 week before experiments were begun. For IGC experiments at higher temperatures, the columns were heated at 200, 225, or 250 °C in flowing He for at least a week before experiments were begun. After each day’s experiments, the column was heated overnight in flowing helium at the highest temperature to which it had been previously subjected, i.e., 150, 200, 225, or 250 °C. Adsorbate hydrocarbon liquids, THF, methyl iodide, tetrabutylammonium hydroxide, and kaolin were obtained from Aldrich at the highest purities available and used without further purification. Adsorbate hydrocarbon gases were obtained from Aldrich or from Matheson at purities of at least 99.9%. Liquid adsorbates were subjected to at least three freeze-pump-thaw cycles before being admitted to the vacuum manifold for injection. Flow rates for the IGC experiments ranged from 5 to 35 mL/min using He as the carrier gas. The column pressure drop was 1-3 atm. The eluted adsorbate was detected with a flame ionization detector. Van’t Hoff plots of the natural log of the uncorrected retention volume vs one over the column temperature were used to obtain isosteric adsorption heats for different hydrocarbon adsorbates on the different Wyodak coals. At least five injections were made at each column temperature over a temperature range of at least 30 °C. Data were collected every 5 or 10 deg. The five retention volumes at each temperature were averaged and the natural logs of the averages used to construct the van’t Hoff plots. Errors in the adsorption heats were calculated from errors in the slopes of the van’t Hoff plots. These errors represent less than 5% of the measured values for adsorbates on the 150 and 200 °C heated coals (Tables 1 and 3) and less than 8% for the 225 °C heated, 250 °C heated, extracted, and alkylated coals (Tables 2 and 4).

Results Isosteric adsorption enthalpies for adsorbates on original, demineralized, extracted, and alkylated Wyodak coals are listed in Tables 1-4. Tables 1 and 2 list adsorption heats for saturated and unsaturated hydrocarbons on original and on demineralized coals heated at 150 or at 200 °C. Tables 3 and 4 list adsorption heats for saturated and unsaturated hydrocarbons on original and on demineralized coals heated at 225 or 250 °C and for hydrocarbons on extracted and on alkylated coals heated at 150 °C. (26) Vorres, K. S. Energy Fuels 1990, 4, 420-426. (27) Silbernagel, B. G.; Gebhard, L. A.; Flowers, R. A.; Larsen, J. W. Energy Fuels 1991, 5, 561-568. (28) Bishop, M.; Ward, D. L. Fuel 1958, 37, 191-200. (29) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277283. (30) Lide, D. R., Ed.; Handbook of Chemistry and Physics; CRC: Boca Raton, FL, 1994. (31) Avgul, N. N.; Kiselev, A. V. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; pp 1-124.

Surface Thermodynamics for Hydrocarbons on Wyodak Coals

Energy & Fuels, Vol. 10, No. 3, 1996 799

Table 1. Isosteric Adsorption Heats (qst) for n-Alkanes on Original and Demineralized Wyodak Coals Heated at 150 °C and 200 °C, Adsorbate Volume Electronic Polarizabilities (r0′),a and Adsorption Heats for n-Alkanes on Graphitized Carbon Black (Q0)b qst (coals) (kcal/mol) R0′ (Å3)

adsorbate methane ethane propane n-butane

original 150 °C, 200 °C 2.8 ( 0.1c 4.8 ( 0.3c 5.3 ( 0.5c 6.2 ( 0.3c 5.8 ( 0.3d 7.6 ( 0.2c 8.1 ( 0.3d 8.6 ( 0.6d

2.60 4.50 6.32 8.12

n-pentane

10.0

n-hexane n-heptane n-octane cyclohexane neopentane neohexane

11.9 13.7 15.5 11.0 9.95 11.9

CAW 150 °C, 200 °C

HF-HCl 150 °C

6.2 ( 0.2c 7.3 ( 0.4c 7.6 ( 0.2d 8.1 ( 0.6c 9.2 ( 0.3c 10.2 ( 0.3c

3.7 ( 0.2 6.2 ( 0.2 8.3 ( 0.2 9.8 ( 0.3

HF-HCl 200 °C

Q0 (carbon) (kcal/mol)

7.2 ( 0.1

3.0 4.3 7.3 8.6

7.7 ( 0.6

9.1

9.1 ( 0.2 10.5 ( 0.2 12.7 ( 0.4

11.2 12.5 13.4 8.7 7.0 8.0

5.1 ( 0.2c

a Adsorbate electronic volume polarizabilities calculated from R ′ ) {(3M/FN)[(n2 - 1)/(n2 + 2)]}/4π, where M is the adsorbate molecular 0 weight, F is the density, N is Avogadro’s number, and n is the refractive index using densities and refractive indices from ref 30. b Adsorption c heats on graphitized carbon black from ref 30. Data obtained for coals heated at 150 °C. d Data obtained for coals heated at 200 °C.

Table 2. Isosteric Adsorption Heats (qst) for Alkenes and Aromatic Adsorbates on Original and Demineralized Wyodak Coals Heated at 150 °C and 200 °C, Adsorbate Volume Electronic Polarizabilities (r0′),a Adsorbate Volume Molecular Polarizabilities (r′),b and Adsorption Heats for n-Alkanes on Graphitized Carbon Black (Q0)c qst (coals) (kcal/mol) adsorbate

R0′, R′ (Å3)

propene 1-butene 1-pentene 1-hexene cyclohexene 1,3-cyclohexadiene benzene

7.0, 8.8 9.0, 10.8 9.9, 10.9 11.6, 13.1 10.7, 11.6 10.7, 13.6 10.4, 10.6

original 150 °C 200 °C 8.9 ( 0.2 10.8 ( 0.1 11.6 ( 0.7

CAW 150 °C

7.7 ( 0.2 9.5 ( 0.2 10.4 ( 0.4 11.9 ( 0.3 10.4 ( 0.3 14.0 ( 0.3

5.8 ( 0.3 6.4 ( 0.2 7.5 ( 0.2 9.5 ( 0.4 8.5 ( 0.2 10.5 ( 0.4 7.0 ( 0.4

200 °C

HF-HCl 150 °C 200 °C

9.1 ( 0.3 11.2 ( 0.5 9.5 ( 0.4

6.0 ( 0.2 8.2 ( 0.2 6.3 ( 0.2

13.1 ( 0.5

8.3 ( 0.3

6.2 ( 0.3 7.6 ( 0.2 9.9 ( 0.4 11.5 ( 0.3 12.3 ( 0.3

Q0 (carbon) (kcal/mol) 7.0d 8.7d 9.5d 11.0d 9.1c 10.2d 9.8c

Adsorbate electronic volume polarizabilities calculated from R0′ ) {(3M/FN) [(n2 - 1)/(n2 + 2)]}/4π, where M is the adsorbate molecular weight, F is the density, N is Avogadro’s number, and n is the refractive index using densities and refractive indices from ref 30. b Adsorbate molecular volume polarizabilities calculated from R′ ) {(3M/FN)[( - 1/ + 2)]}/4π, where is the permittivity of the adsorbate using densities, refractive indices, and permittivities from ref 30. c Adsorption heats on graphitized carbon from ref 31. d Adsorption heats on graphitized carbon determined from the adsorbate electronic polarizabilities using the linear regression line from a plot of R0′ vs Q0 for n-alkanes on graphitized carbon.31 a

The thermodynamic data for the various coals have been plotted vs adsorbate polarizabilities. We have used the volume electronic polarizabilities and volume molecular polarizabilities, R0′ and R′, respectively, which are related to the electronic and molecular polarizabilities, R0 and R, respectively, by R0′ ) R0/4π0 and R′ ) R/4π0 where 0 is the vacuum permittivity.32 The difference between the electronic and molecular polarizabilities can be seen from a comparison of the Clausius-Mossotti equation (eq 1) and the Debye equation (eq 2):

r - 1 n2 - 1 NR0 ) ) r - 2 n2 + 2 30

[ ( )]

r - 1 N µ2 ) R0 + r + 2 30 3kT

)

(1) NR 30

(2)

where r is the relative permittivity, N is Avogadro’s number, R0 is the electronic polarizability, µ is the dipole moment, k is Boltzmann’s constant, and n is the refractive index. The second equation for polarizability contains the dipole moment of the molecule, µ, while (32) Atkins, P. W. Physical Chemistry, 5th ed.; Freeman: New York, 1994; pp 754-761.

the first equation does not. The dipole moment is present in the expression for polarizability when the permittivity is obtained at low or static electric field frequencies because under these conditions the molecule has time to reorient in the field, causing its orientation, and hence its dipole moment, to contribute to the polarizability. The polarizability represented by eq 2 is therefore called the molecular polarizability. In contrast, the expression for the molecule’s electronic polarizability (eq 1) does not contain the dipole moment; it is only dependent on the permittivity of the electrons (n, electronic distribution of the molecule). In this case the permittivity is measured at high electric field frequencies (refractive index, n) where the molecule cannot follow the field. Thus R0 (electronic polarizability) represents the nonpolar component of polarizability while R (molecular polarizability) contains both nonpolar and polar contributions. For alkane adsorbates, which possess no dipole moment, only the electronic polarizability, R0′, is listed in Tables 1 and 3. Unsaturated adsorbates possess small dipole moments so that their values of electronic and molecular polarizability will differ slightly (see Tables 2 and 4). Both values, R0′ and R′, are listed in Tables 2 and 4 for these adsorbates.

800


Glass and Stevenson

Table 3. Isosteric Adsorption Heats (qst) for n-Alkanes on Original and Demineralized Wyodak Coals Heated at 250 °C, and on Extracted and Alkylated Coals Heated at 150 °C, Adsorbate Volume Electronic Polarizabilities (r0′),a and Adsorption Heats for n-Alkanes on Graphitized Carbon Black (Q0)b qst (coals) (kcal/mol) adsorbate

original 250 °C

R0′ (Å3)

methane ethane propane n-butane n-pentane n-hexane n-heptane

2.60 4.50 6.32 8.12 10.0 11.9 13.7

n-octane cyclohexane neohexane

15.5 11.0 11.9

2.5 ( 0.2 5.3 ( 0.3 6.6 ( 0.2 7.8 ( 0.1 10.4 ( 0.5 12.5 ( 0.5

CAW 250 °C 4.2 ( 0.2 5.8 ( 0.2 7.6 ( 0.3 9.9 ( 0.2 11.6 ( 0.5 13.3 ( 0.4

HF-HCl 225 °C

extracted 150 °C

6.0 ( 0.1 8.1 ( 0.1 9.7 ( 0.4

alkylated 150 °C 5.0 ( 0.2 6.7 ( 0.3 8.7 ( 0.2

10.6 ( 0.3 13.5 ( 0.4 13.2 ( 0.2c 15.5 ( 0.3

Q0 (carbon) (kcal/mol) 3.0 4.3 7.3 8.6 9.1 11.2 12.5 13.4 8.7 8.0

8.6 ( 0.2 6.1 ( 0.3

a Adsorbate electronic volume polarizabilities calculated from R ′ ) {(3M/FN)*(n2 - 1)/(n2 + 2)]}/4π, where M is the adsorbate molecular 0 weight, F is the density, N is Avogadro’s number, and n is the refractive index using densities and refractive indices from ref 30. b Adsorption heats on graphitized carbon black from ref 31. c Data obtained for HF-HCl demineralized coal heated at 250 °C.

Table 4. Isosteric Adsorption Heats (qst) for Alkenes and Aromatic Adsorbates on Original and Demineralized Wyodak Coals Heated at 250 or 225 °C and on Extracted and Alkylated Coals Heated at 150 °C, Adsorbate Volume Electronic Polarizabilities (r0′),a Adsorbate Volume Molecular Polarizabilities (r′),b and Adsorption Heats for n-Alkanes on Graphitized Carbon Black (Q0)c qst (coals) (kcal/mol) adsorbate

R0′, R′ (Å3)

propene 1-butene cis-2-butene 1-pentene 1-hexene 1-heptene cyclohexene 1,3-cyclohexadiene benzene toluene tert-butylbenzene

7.0, 8.8 9.0, 10.8 8.6, 8.7 9.9, 10.9 11.6, 13.1 13.5, 14.9 10.7, 11.6 10.7, 13.6 10.4, 10.6 12.3, 13.3 17.8, 19.1

original 250 °C 7.7 ( 0.3 9.5 ( 0.2 11.4 ( 0.2 13.1 ( 0.4 14.8 ( 0.7

CAW 250 °C 9.8 ( 0.2 9.2 ( 0.4 11.5 ( 0.5 13.5 ( 0.3

HF-HCl 225 °C

9.2 ( 0.2 12.1 ( 0.4 14.1 ( 0.2

11.8 ( 0.1 14.5 ( 0.6 13.1 ( 0.7 13.5 ( 0.8

13.9 ( 0.4 14.4 ( 0.3 15.7 ( 0.2

15.0 ( 0.3 14.1 ( 0.2

extracted 150 °C

alkylated 150 °C

Q0 (carbon) (kcal/mol)

8.2 ( 0.3 10.1 ( 0.4

8.6 ( 0.2 10.3 ( 0.3

7.0d 8.7d 8.4d 9.5d 11.0d 12.6d 9.1 10.2d 9.8 11.6 13.8

a Adsorbate electronic volume polarizabilities calculated from R ′ ) {(3M/FN)[(n2 - 1)/(n2 + 2)]}/4π, where M is the adsorbate molecular 0 weight, F is the density, N is Avogadro’s number, and n is the refractive index using densities and refractive indices from ref 30. b Adsorbate molecular volume polarizabilities calculated from R′ ) {(3M/FN)[( - 1/ + 2)]}/4π, where is the permittivity of the adsorbate using densities, refractive indices, and permittivities from ref 31. c Adsorption heats on graphitized carbon black from ref 31. d Adsorption heats on graphitized carbon determined from the adsorbate electronic polarizabilities using the linear regression line from a plot of R0′ vs Q0 for n-alkanes on graphitized carbon.31

Discussion Specific and Nonspecific Interactions for Hydrocarbons on Wyodak Coal Surfaces. Figures 1 and 2 show isosteric adsorption heats for n-alkanes and for unsaturated hydrocarbons (alkenes and benzene) on original Wyodak coal heated at 150 and 200 °C. In Figure 1 the adsorption heats are plotted vs electronic volume polarizability of the adsorbates, R0′, while in Figure 2 they are plotted vs molecular volume polarizability of the adsorbates, R′. The lower plot (n-alkanes data) in the figures shows the expected linear dependence of adsorption heat vs adsorbate polarizability for n-alkanes.10 These nonpolar adsorbates can interact with other species by nonspecific van der Waals forces only.33 The adsorption heats for alkanes are characteristic of the nonspecific component (van der Waals dispersion force) of the coal’s intermolecular surface forces. The adsorption heats for the 1-alkenes in Figures 1 and 2 also show a linear dependence on polarizability, with all of the points for the unsaturated adsorbates (33) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1985; Chapter 6.

Figure 1. Isosteric adsorption heat (qst) vs adsorbate volume electronic polarizability (R0′) for n-alkanes, alkenes, and benzene on original Wyodak coal heated at 150 °C and at 200 °C in helium: (O) n-alkanes (methane, ethane, propane, n-butane, and n-pentane) on 150 °C heated coal; (b) n-alkanes (n-butane, n-pentane, n-hexane) on 200 °C heated coal; (4) 1-alkenes on 150 °C heated coal; (2) alkenes and benzene on 200 °C heated coal.

falling above the n-alkanes line. The adsorption heats for 1-alkenes are ∼2-4 kcal/mol more exothermic than the n-alkanes adsorption heats (Figure 1). Benzene has an even more exothermic interaction with the 200 °C


Figure 2. Isosteric adsorption heat (qst) vs adsorbate volume molecular polarizability (R′) for n-alkanes, alkenes, and benzene on original Wyodak coal heated at 150 °C and at 200 °C in helium: (O) n-alkanes (methane, ethane, propane, n-butane, and n-pentane) on 150 °C heated coal; (b) n-alkanes (n-butane, n-pentane, n-hexane) on 200 °C heated coal; (4) 1-alkenes on 150 °C heated coal; (2) alkenes and benzene on 200 °C heated coal.

heated coal. An adsorption heat could not be measured for benzene on the 150 °C heated coal. Higher (>150 °C) column temperatures are needed to obtain useful peaks for this adsorbate on Wyodak coal. As seen by comparing Figures 1 and 2, plotting the data vs R′ instead of vs R0′ gives points for unsaturated hydrocarbons which still fall above the alkanes line, demonstrating that the small dipole moments of unsaturated hydrocarbons are not responsible for the more exothermic adsorption heats of unsaturated compared to saturated hydrocarbons on this coal. Intermolecular surface forces may be divided into specific and nonspecific forces.34 Nonspecific forces are “whole-molecule” forces in the sense that their strengths depend on the overall electron distribution of the entire adsorbate molecule.35 Since electronic polarizability, R0, characterizes a molecule’s electron distribution, nonspecific adsorbate-surface interactions depend on the adsorbate’s electronic polarizability. Electronic polarizability increases linearly with increasing molecular size for nonpolar molecules in a given class (e.g., n-alkanes, cycloalkanes, etc.) so that a linear increase in adsorption enthalpy with adsorbate size is characteristic of nonspecific surface forces.36 In contrast, the strength of the specific intermolecular interaction between an adsorbate and a surface depends on the electronic properties of a specific portion of the adsorbate molecule.35,37 These types of interactions do not depend on the adsorbate’s size. Specific surface forces include acid-base forces or any type of interaction where a specific functional group in the adsorbate molecule interacts with a specific functional group on a surface.38 For Wyodak coal, plots of adsorption heat vs polarizability differ for unsaturated hydrocarbons and for saturated hydrocarbons. (See Figures 1 and 2.) In contrast, on Illinois No. 6 coal, both types of adsorbates give the same plot of adsorption heat vs polarizability.10 (34) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538-2541. (35) Kiselev, A. V.; Yashin, Y. I. Gas-Adsorption Chromatography; Plenum: New York, 1969; Chapter 2. (36) Littlewood, A. B.; Gas Chromatography, 2nd ed.; Academic: New York, 1970; Chapter 3. (37) Good, R. J.; Van Oss, C. J. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum: New York, 1991; pp 1-27. (38) Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes, 3rd ed.; Dover: New York, 1964; Chapter 11.


The different plots for alkanes and alkenes on Wyodak coal may be the result of different specific forces, different nonspecific forces, or different specific and nonspecific forces experienced by these two types of hydrocarbons. Other thermodynamic data discussed below demonstrate that while saturated hydrocarbons interact only nonspecifically, unsaturated hydrocarbons most likely interact both specifically and nonspecifically with Wyodak coal. The data support the idea that nonspecific interactions on Wyodak coal differ for saturated and for unsaturated hydrocarbons. The results demonstrate the complexity of the intermolecular forces between hydrocarbons and Wyodak coal. As seen from Figures 1 and 2, heating Wyodak coal to 200 °C causes the adsorption heats for 1-alkenes to become less exothermic. Heating to 200 °C caused the same quantitative change in adsorption enthalpy (∼1.2 kcal/mol) for all 1-alkenes studied. These results suggest that a specific interaction may be responsible for the more exothermic interactions of alkenes on the 150 °C heated compared to the 200 °C heated coal. All 1-alkenes possess a single double bond site. If unsaturated hydrocarbons are interacting specifically with Wyodak coal, this portion of the molecule should produce the same interaction with the surface for all 1-alkenes, independent of the size of the 1-alkene adsorbate molecule. The fact that a constant decrease in adsorption heat occurs for all 1-alkenes after heating the coal to 200 °C suggests that a specific interaction between the double bond of the 1-alkenes and the coal surface is lost as a result of heating. Since the adsorption heats for 1-alkenes in Figures 1 and 2 all decrease by ∼1.2 kcal/mol, the data suggest that there is a 1.2 kcal/mol specific interaction between 1-alkenes and Wyodak coal. This is a relatively small specific interaction. There are two possible types of functional groups on the coal that could interact specifically with unsaturated hydrocarbons. Cations chelated to carboxylates on the coal could interact specifically with unsaturated hydrocarbons by a “cation-π” interaction.39 Cation-π interactions involve the excess electron density in an unsaturated bond interacting with the positive charge of a small metal cation at a surface.35 This type of interaction is found in biological systems and at the surfaces of inorganic solids such as clay minerals and zeolites.35,39 It typically has strengths of 1-5 kcal/mol.35 It is known that removal of carboxylates leads to loss of metal cations and loss of ion-exchange properties from coals.18 Higher rank coals do not possess ion-exchange properties because they have no carboxylate groups that can chelate to metal ions.18 As the coal is heated, carboxylates are lost, which would be expected to lead to a loss of the cation-π interaction.40 It is expected that the virgin (unheated) coal would have even stronger cation-π interactions than the 150 °C heated coal. If cation-π interactions are responsible for the specific interactions between unsaturated hydrocarbons and Wyodak coal, then this technique provides a way to quantitatively assess ionic forces at low-rank coal (39) Dougherty, D. A.; Kearney, P. C.; Mizoue, L. S.; Kumpf, R. A.; Forman, J. E.; McCurdy, A. In Computational Approaches in Supramolecular Chemistry; NATO ASI Ser., Ser. C; 1994, Vol. 426, pp 301-309. (40) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Gravel, D.; Baillargeon, M.; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 319-333.

802


Figure 3. Isosteric adsorption heats (qst) vs adsorbate volume electronic polarizability (R0′) for n-alkanes, alkenes, and benzene on CAW Wyodak coal heated at 200 °C in helium: (O) n-alkanes (n-butane, n-pentane, n-hexane, n-heptane, n-octane) on 200 °C heated CAW coal; (2) alkenes and benzene on 200 °C heated CAW coal.

Figure 4. Isosteric adsorption heat (qst) vs adsorbate electronic polarizability (R0′) for n-alkanes, alkenes, and benzene on HF-HCl demineralized Wyodak coal heated at 200 °C in helium: (double triangles) alkenes and benzene on 200 °C heated HF-HCl demineralized coal; (triangles) n-alkanes on 200 °C heated HF-HCl demineralized coal. Adsorbates from left to right are n-pentane, n-hexane, n-heptane, n-octane.

surfaces. Another possible type of specific force that could be responsible for the interaction between unsaturated hydrocarbons and Wyodak coal could involve polar functional groups such as carbonyls on the coal surface. Polar groups are responsible for the more exothermic adsorption heats for alkenes compared to alkanes on oxidized carbon surfaces.41 This force would involve a specific interaction between the double bond of the adsorbate with polar functional groups on the coal. The data do not enable us to determine which of these two types of interaction is responsible for the specific force operating between unsaturated hydrocarbons and Wyodak coal. Isosteric adsorption heats for n-alkanes and for unsaturated hydrocarbons on demineralized Wyodak coals heated at 200 °C are shown in Figures 3 and 4. Figure 3 shows data for CAW Wyodak coal heated at 200 °C while Figure 4 shows data for HF-HCl demineralized coal heated at 200 °C. It is seen that the adsorption heats for 1-alkenes on these demineralized coals are more exothermic by about 1-4 kcal/mol than adsorption heats for n-alkanes. In Figure 5 the adsorp(41) Elkington, P. A.; Curthoys, G. J. Phys. Chem. 1969, 73, 23212323.

Glass and Stevenson

Figure 5. Isosteric adsorption heat (qst) vs adsorbate volume electronic polarizability (R0′) for 1-alkenes on original, CAW, and HF-HCl demineralized Wyodak coals heated at 200 °C: (b) 1-alkenes on original Wyodak coal heated at 200 °C; (2) 1-alkenes on CAW Wyodak coal heated at 200 °C; (4) 1-alkenes on HF-HCl demineralized Wyodak coal heated at 200 °C.

tion heats for 1-alkenes and for benzene on the three 200 °C heated coals (original, CAW, and HF-HCl demineralized) are replotted. As seen from Figure 5, 1-alkenes have approximately the same adsorption heats with all three of these coals. As discussed above, the 200 °C heated original coal has most likely lost some of its specific interaction with unsaturated hydrocarbons. CAW and HF-HCl treatments result in a loss of mineral matter from low-rank coals, but these treatments do not change the oxygen content of the coals.42 Data obtained by others show that heating Wyodak coal at 200 °C leads to a loss of carboxyl groups.43 Comparison of others’ CO and CO2 yields for acid washed and original Wyodak coals shows that a substantial amount of carboxyl groups continue to decompose to CO2 at temperatures up to 700 K (∼400 °C).43 These observations suggest that while some polar groups have been lost as a result of 200 °C heating, causing a loss of some specific interactions between alkenes and the coal, polar groups still remain on Wyodak coal which has been heated above 200 °C. The interactions of alkenes on 200 °C heated Wyodak coal are likely due to specific and/or nonspecific interactions with these remaining functional groups. Since the strength of the nonspecific force is proportional to the size of the adsorbate molecule, the slopes of the plots in Figures 1-4 provide information about the nature of the nonspecific force(s) on the coal. As seen from Figures 1-4, the slopes of the plots for 1-alkenes on original and demineralized Wyodak coals heated at 150 or 200 °C differ from the slopes for n-alkanes on these coals. The slopes of the plots for alkenes are steeper than those for alkanes, demonstrating that unsaturated hydrocarbons experience stronger dispersive van der Waals forces than saturated hydrocarbons at the surface of Wyodak coal. It is unknown what functional group(s) on the coal could be responsible for the different nonspecific interactions of unsaturated and saturated hydrocarbons with this coal. On Illinois No. 6 bituminous coal, alkanes and alkenes show an identical dependence of adsorption heat on adsorbate (42) Larsen, J. W.; Pan, C.-S.; Shawver, S. Energy Fuels 1989, 3, 557-561. (43) Franklin, H. D.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 39-42.


Figure 6. Isosteric adsorption heats (qst) vs adsorbate volume electronic polarizability (R0′) for n-alkanes, alkenes, and benzene on CAW Wyodak coal heated at 150 °C and at 200 °C in helium: (O) n-alkanes (n-pentane, n-hexane, n-heptane, and n-octane) on 150 °C heated CAW Wyodak coal; (b) n-alkanes (n-butane, n-pentane) on 200 °C heated CAW coal; (4) alkenes on 150 °C heated CAW coal; (2) alkenes and benzene on 200 °C heated CAW coal.

Figure 7. Isosteric adsorption heat (qst), vs molecular volume polarizability, R′, for alkenes on original and CAW Wyodak coals and for n-alkanes on CAW and original Wyodak coals: × n-alkanes on original and CAW Wyodak coals heated at 150 and 200 °C; (O) alkenes (propene, 1-butene) on original Wyodak coal heated at 150 °C; (b) alkenes (propene, 1-butene, cyclohexene, 1-hexene) on 200 °C heated original Wyodak coal; (2) alkenes (1-pentene, cyclohexene, 1-hexene) on 200 °C heated CAW Wyodak coal; (4) alkenes on 150 °C heated CAW Wyodak coal (propene, benzene, 1-pentene, cyclohexene, 1-hexene, 1,3-cyclohexadiene).

polarizability.10 Low-rank coals have higher concentrations of oxygenated functional groups than high-rank coals.44 It is possible that the nonspecific interaction of unsaturated hydrocarbons with polar functional groups such as carbonyls at the coal surface leads to more exothermic nonspecific adsorption heats for these types of adsorbates.41 Figure 6 shows isosteric adsorption heats for saturated and unsaturated hydrocarbons on CAW Wyodak coal which has been heated at 150 and 200 °C. It is seen that the adsorption heats for alkenes are less exothermic on the 150 °C heated CAW coal than on the 200 °C heated CAW coal. This is the opposite result from that found for the original coal. (See Figures 1 and 2.) In Figure 7 the data from Figure 6 are replotted along with adsorption heats for 1-alkenes on 150 and 200 °C heated original Wyodak coal. As seen from (44) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193.


Figure 8. Isosteric adsorption heat (qst) vs adsorbate molecular volume polarizability (R′) for n-alkanes and unsaturated hydrocarbons on Wyodak coal heated at 250 °C: (b) n-alkanes on Wyodak coal heated at 250 °C. Adsorbates in order of increasing exothermicity are: ethane, propane, n-butane, n-pentane, n-hexane, and n-heptane; (O) alkenes and aromatics on Wyodak coal heated at 250 °C.

Figure 7, adsorption heats for alkenes on the 150 °C heated CAW coal are even less exothermic than adsorption heats for some of the n-alkanes with this coal. However, after heating the coal to 200 °C, the adsorption heats for alkenes have returned almost to their values on the 200 °C heated original coal. These results suggest that citric acid washing suppresses intermolecular interactions between unsaturated hydrocarbons and Wyodak coal. The adsorption of organic ions at mineral surfaces has been found to suppress forces at these surfaces.45,46 It is possible that, after citric acid washing, organic material adsorbs on the coal, suppressing its surface forces. This material may be removed when the coal is heated to 200 °C. These results have implications for studies using the CAW demineralization procedure. Figure 8 shows data for n-alkanes, alkenes, and benzene on original Wyodak coal heated at 250 °C. Heating the demineralized Wyodak coals (CAW or HFHCl demineralized) at 250 °C gave the same result; i.e., after these treatments the same plot of adsorption heat vs polarizability was found for alkanes and alkenes (data not plotted). Alkylating or extracting the coal and heating at 150 °C gave the same result as well (i.e., the same plot for alkanes and alkenes; data not plotted). The fact that identical plots are found for alkanes and alkenes on original Wyodak coal heated at 250 °C, on the demineralized coals heated at 250 °C, and on the extracted and alkylated coals heated at 150 °C shows that, on these coals, adsorption heats for alkanes and alkenes depend on polarizability in the same way. These observations can be explained if dispersive van der Waals forces are the only types of forces contributing to the adsorption heats of both types of hydrocarbons on these treated coals. On graphitized carbons which have only nonspecific van der Waals forces, the same dependence of adsorption heat on polarizability is found for alkanes and alkenes.35 However, unlike graphitized carbon, Wyodak coal heated at 250 °C contains oxygenated functional groups.40 An alternate explanation for the data in Figure 8 is that different forces are experienced by alkanes and alkenes on these treated coals but (45) Mokaya, R.; Jones, W.; Davies, M. E.; Whittle, M. E. J. Solid State Chem. 1994, 111, 157-163. (46) Dutta, P. K.; Robins, D. S. Langmuir 1994, 10, 4682-4687.

804


Glass and Stevenson

Table 5. Dispersive Solid Surface Tensions (γSd)a for Original and Modified Wyodak Coals Heated at Various Temperatures coal, temperatureb

γSd (mJ/m2)

original, 150 °C, 200 °C CAW, 150 °C HF-HCl, 150 °C HF-HCl, 200 °C original, 250 °C CAW, 250 °C HF-HCl, 225 °C extracted, 150 °C alkylated, 150 °C

25.2 24.9 18.6 33.4 46.7 52.2 49.1 50.2 53.8

a γ d calculated from the equation RT ln V ) 2Na(γ dγ d)1/2, S N S L where R is the gas constant, T is the temperature, N is Avogadro’s number, a is the adsorbate area at the surface, and γLd is the dispersive surface tension of the adsorbate.47 VNs determined at 90 °C or extrapolated to 90 °C using the linear regression line from the van’t Hoff plots. b Coals heated in helium at the specified temperature.

these forces have the same dependence on adsorbate polarizability for both types of adsorbates. The unusually exothermic adsorption heat for benzene in Figure 8 supports this second explanation. If only van der Waals dispersive forces were operating, the point for benzene would fall on the line in Figure 8. Furthermore, the adsorption heats for alkenes and benzene have not changed from their values on the 200 °C heated coal; compare data in Tables 2 and 4. It is possible that these types of adsorbates interact with oxygenated functional groups on both 200 °C heated and 250 °C heated coal surfaces. On the other hand, the increased exothermicities of the adsorption heats for alkanes on the 250 °C heated coal compared to the 200 °C heated coal can be explained by a change (increase) in exothermicity of the van der Waals dispersive surface force as a result of heating the coal to 250 °C. Dispersive Surface Tensions for Wyodak Coal Surfaces. Table 5 lists dispersive surface tensions, γSd, for various Wyodak coal surfaces.47 These values represent the nonspecific dispersive component of the surface force experienced by an adsorbed molecule per unit area of surface occupied. They are determined from the retention volumes for n-alkanes interacting with the coal. The dispersive surface tension is normalized to adsorbate size (area occupied by the adsorbate at the surface). Since adsorbate size is proportional to adsorbate polarizability, the dispersive surface tension provides essentially the same information as the slopes of the plots of adsorption heat vs polarizability (for nalkanes). This information is also known as the “Traube effect” and is commonly expressed as the change in free energy of adsorption per CH2 group in the adsorbate. The symbol QCH2 is used by Kiselev to represent the increment in adsorption heat per CH2 group in the (alkane) adsorbate.35 These parameters all characterize the strength of the van der Waals force at a surface. The higher surface tension for 250 °C heated Wyodak coal (∼50 mJ/m2) vs 150 °C heated Wyodak coal (∼25 mJ/m2) is reflected in the steeper slope of the plot for n-alkanes on 250 °C heated coal compared to 150 °C heated coal (Figures 1 and 8). Both of these parameters (the slope and the dispersive surface tension) show that Wyodak coal which has been heated to 250 °C has a (47) Dorris, G. M.; Gray, D. G. J. Colloid Interface Sci. 1980, 77, 353-362.

higher dispersive van der Waals surface energy than Wyodak coal that has been heated to 150 or 200 °C. A solid’s dispersive surface tension reflects underlying structural and chemical characteristics of the surface.35 The dispersive surface tension increases with an increase in concentration of surface atoms and/or with increasing polarizability of the atoms at the surface.35,41 The surface tension may also be a reflection of the surface porosity.35,48 However, porous solids typically have much higher surface tension values than listed in Table 5.35 It has been shown that surface porosity does not likely determine the dispersive surface tensions determined by IGC for Illinois No. 6 coal.13 The surface tensions for 150 °C heated CAW and original coals in Table 5 are similar to values found for surfaces such as saturated hydrocarbons and polyethylene.49 These materials have low-energy surfaces compared to solids such as graphite, metals, and inorganic solids which have γSd > 100 mJ/m2.50 Drelich et al. reported values of ∼20-30 mJ/m2 for surface tensions of bitumens using contact angle methods.51 Bitumens are hydrocarbon oils which are associated with source rock in the earth. They consist of high molecular weight aromatic and saturated compounds. The fact that Wyodak coal heated at 150 °C has similar surface tensions to bitumens suggests that n-alkanes experience a low-energy aromatic and/or aliphatic organic Wyodak coal surface. As seen from the data in Table 5, when mineral matter is removed from the coal by CAW treatment, the dispersive surface tension is unaffected. However, removal of mineral matter by HF-HCl treatment decreases the surface tension to 18.6 mJ/m2. Removing minerals such as silicates that contain polarizable atoms and/or a higher concentration of atoms than the coal may lead to a lower energy coal surface. The dispersive surface tension of HF-HCl demineralized coal heated to 200 °C (33.4 mJ/m2) is slightly higher than that of the original coal heated to 200 °C (25 mJ/m2). As discussed previously, heating to 250 °C, extracting, or alkylating Wyodak coal causes its adsorption heats with alkanes to become more exothermic. See Figure 8. This is reflected in the increase in the dispersive surface tension to 50 mJ/m2 which results from heating the coal to 250 °C. These treatments were found to have a similar effect on Illinois No. 6 coal; i.e., heating Illinois No. 6 coal to 250 °C or extracting it with THF caused its dispersive surface tension to change (in this case to decrease) to the same value of 50 mJ/m2.13 The surface tension value of ∼50 mJ/m2 is characteristic of flat carbonaceous surfaces.52 Data obtained from other techniques demonstrate that heating coal to ∼250 °C or extracting with a good solvent leads to an irreversible structural rearrangement of the bulk coal.53,54 This bulk (48) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Chromatographia 1992, 33, 441-444. (49) Zisman, W. A. In Contact Angle, Wettability, and Adhesion; Adv. Chem. Ser. No. 43; American Chemical Society: Washington, DC, 1964; pp 1-51. (50) Fowkes, F. M. In Contact Angle, Wettability, and Adhesion; ACS Adv. Chem. Ser. No. 43; American Chemical Society: Washington, DC, 1965; pp 99-111. (51) Drelich, J.; Bukka, K.; Miller, J. D.; Hanson, F. V. Energy Fuels 1994, 8, 700-704. (52) Lopez-Garzon, F. J.; Pyda, M.; Domingo-Garcia, M. Langmuir 1993, 9, 531-536.


structural rearrangement may be responsible for the increased exothermicities of the adsorption heats for alkanes on the 250 °C heated coal. The fact that heating to 250 °C or extracting Wyodak or Illinois No. 6 coal results in a dispersive surface tension of 50 mJ/m2 suggests that coals subjected to these treatments have atom densities and polarizabilities that are similar to those found on flat carbon surfaces. Summary The nonspecific interactions of saturated hydrocarbons with 150 °C heated Wyodak coal are characteristic of a low-energy surface (γSd ∼ 25 mJ/m2). Heating the coal to 250 °C, alkylating, or extracting it leads to a higher dispersive surface energy (γSd ∼ 50 mJ/m2) and hence stronger interactions with alkane adsorbates. In (53) (a) Brenner, D. Fuel 1984, 63, 1324-1328. (b) Cody, G. D., Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340-344. (c) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107-110. (54) Yun, Y.; Otake, Y.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 1314-1324 (ref 55). (55) In ref 10 we incorrectly stated that Yun et al. had reported a low-temperature (∼150 °C) DSC transition for Illinois No. 6 coal. In actuality, no such transition was reported.


contrast, unsaturated hydrocarbons experience a stronger interaction with the 150 °C heated coal which most likely results from stronger (more exothermic) nonspecific forces compared to those of alkanes as well as additional specific interactions. The specific interaction of unsaturated hydrocarbons decreases when the coal is heated at 200 °C and/or is demineralized. These results suggest that this specific interaction most likely involves a cation-π interaction between the unsaturated portion of the adsorbate and alkali cations on the coal surface. The stronger nonspecific interactions of unsaturated hydrocarbons compared to saturated hydrocarbons on the 150 °C heated coal may result at least in part from the presence of polar groups on the coal surface. On 250 °C heated Wyodak coal, both types of adsorbates experience the same (nonspecific) interaction. Acknowledgment. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the ACS, for support of this research. We are grateful for the helpful comments of Prof. John Larsen. EF950224G

Surface Thermodynamics for Hydrocarbons on Wyodak Coals

Recommend Documents