Energy & Fuels 1988,2,295-300 active a t temperatures as low as 200 "C. Because of the disparity in the method of catalyst preparation and reaction conditions, any explanation of differences must await future results. The loss of activity of the ruthenium when exposed to hydrogen sulfide is disappointing, since sulfur tolerance is essential for HDN of crude petroelum and coal liquids. Moreover, compared to commercial catalysts such as CoMo and NiMo, ruthenium consumes excessive amounts of hydrogen and, under our conditions, requires complete hydrogenation prior to C-N bond cleavage. The purpose of thiswork, however, was to s w e y candidate metals that may provide some potential to develop more selective HDN catalysts. In concurrent work we have been using ruthenium with other metals and have found that appropriate metal combinations can provide enhanced HDN activity and low hydrogen consumption and overcome the problem of sulfur tolerance.'lJ*
Experimental Section Analytical Procedures. Product analyses for all the kinetic studies were performed on a Hewlett-Packard 5890 GC equipped with FID and a 30-mDB-1column. GC-mass spectral analyses were performed on a LKB-d9000 or a Ribermag R 10-10 mass spectrometer.
295
Preparation of Catalysts. Ruthenium powder (Stem Chemicals) was activated by heating a t 400 "C under flowing oxygen for 12 h. Molybdenum powder (Alfa, 250 mesh), nickel powder (Alfa, 100 mesh), platinum powder (Alfa, 24 m2/g), rhenium powder (Alfa 325 mesh), osmium powder (Strem), and rhodium black (Alfa) were activated in flowing hydrogen for 12 h a t 400 "C and stored in a dry box prior to use. Bulk metal surface area was measured by standard BET methods (Omicron Technology, Inc.). Surface areas, +lo% m2/g: Ru, 2.0; Ni, 0.12; Mo, 0.24; Co, 0.30; Os, 1.04; Re, 0.30; Rh, 4.06; Pt, 0.30. Standard HDN Reaction Procedures. Under nitrogen, the catalyst (0.100 g), a glass stir bar, and 10 mL of 0.151 M THQ and 0.098 M n-dodecane (as internal standard) in n-hexadecane were placed in a quartz liner, in a 45-mL Parr bomb. The bomb was then purged, and pressurized with 500 psig of hydrogen and heated a t the desired temperature and time. The Parr bomb requires less than 5 min to reach 250 "C.
Acknowledgment. We would like to thank the DOE Pittsburgh Energy and Technology Center for support of this work through Grants No. DE-FG22-83PC60781 and DE-FG22-85PC80906 and the NSF for partial support through Chem. Eng. Grant No. 82-19541. Registry No. THQ, 25448-04-8; MeCHA, 100-60-7;EtCHA, 5459-93-8;PrCHA, 3592-81-2; Ru, 7440-18-8;Rh, 7440-16-6;Pt, 7440-06-4; Ni, 7440-02-0; Mo, 7439-98-7; Re, 7440-15-5; Os, 7440-04-2; Nz,7727-37-9.
Thermochemical Comparisons of Six Argonne Premium Coal Samples Michael Gumkowski,t Qitao Liu, and Edward M. Arnett* Department of Chemistry, Duke University, Durham, North Carolina 27706 Received September 4,1987. Revised Manuscript Received December 2, 1987
Two thermochemical methods were used to determine the interaction of six Argonne premium coals (Wyodak, Illinois No. 6, Pittsburgh No. 8, Pocahontas No. 3, Upper Freeport, North Dakota Beul a h - % ~lignite) with 12 carefully chosen organic solvents. The heats evolved are compared with those from a previously published study from this laboratory of Wyoming subbituminous, Illinois No. 6, and Texas lignite and also with thermochemicalmeasurements using a sulfonic acid resin as a prototype Brernsted acid, silica as a prototype hydrogen-bondingacid, and a graphitized carbon black (Carbopack F) as a prototype physical adsorbent for dispersion force interaction. A comparison of the use of these solid prototypes versus homogeneous analogues (e.g. Taft-Kamlet parameters) for a limited group of only five basic solvents shows that homogeneous and heterogeneous parameters are about equally good for correlation. However, when correlations using eight solvents and homogeneous parameters are compared to correlations using 10 solvents and solid prototypes, the latter are clearly superior.
Introduction Thermochemical methods based on various types of calorimetry are a powerful tool for comparing acid-base interactions in both homogeneous and heterogeneous systems. Previous reports from this laboratory have described the thermochemical method for comparing solid acids with their homogeneous analogues in response to interactions with a variety of basic liquids. We have attempted to find appropriate solid prototypes for Brernsted acidity,' hydrogen-bonding acidity? and dispersion force 'Present address: Pfizer Central Research, Groton, CT 06340.
interaction^.^ These could be used as standards for comparison in classifying more complex solid acids such as coals. Prototype systems are commonly used in homogeneous solution, for example; the pK,'s of benzoic acids in aqueous solution are the Brernsted acid prototype, which is the standard for linear free energy relationship^.^^^ (1) Amett, E. M.; Haaksma, R. A,; Chawla, B.; Healy, M. H. J. Am. Chem. SOC.1986,108, 4888. (2) Arnett, E. M.; Cassidy, K. F., submitted for publication in Reu. React. Intermed. (3) Hutchinson, B. J.; Healy, M.; Amett, E. M., manuscript in preparation.
0887-0624/88/2502-0295$01.50/00 1988 American Chemical Society
296 Energy & Fuels, Vol. 2, No. 3, 1988
Correspondingly, p-fluorophenol is the standard hydrogen-bonding acid used as a standard of comparison by Taft and Kamlet for their multiparameter solvation model.s Again, Gutman's well-known donicity scale uses SbC15as a standard Lewis acid in homogeneous solution.' Finally, aliphatic hydrocarbons are the commonly chosen standard for completely nonpolar interactions in liquid systems. Much of the recent Iiterature%l9on the thermochemistry of adsorption onto coals has focused on their interactions with water or alkanols 80 that pretreatment conditions,*'l coal particle size: temperature,12J3chain length of the a l k a n ~ l , ~ ~ Jsurface ~ * ' ~ Jtension ~ of the immersional liquid,14 rate of wetting?l0J4 outgassing temperature,l0 oxygen content," moisture c ~ n t e n tand , ~ mineral ~ ~ ~ ~content8 have been examined with respect to their influence of the resulting heat of interaction. Some studies have examined other types of interacting compounds, such as amines, pyridines, and alkanes.12-14JsJ9 The present report compares six carefully classified coals from the Argonne National Laboratory Premium Coal bank by two calorimetric methods (heats of immersion and thermometric titration) using a series of 12 solvents chosen especially to bring out the differences between Bransted acidity, hydrogen bonding, and dispersion force interactions. Three years ago, we published a report'@on heats of immersion of three types of coal (Wyoming Rawhide, 11linois No. 6, and Texas Big Brown lignite), which had been supplied to us by Exxon Laboratories and stored and handled under nitrogen in a controlled drybox environment. Since then, the Argonne National Laboratory has established a premium coal bank which supplies samples that have been collected, stored, and classified under very carefully controlled conditions so that they may serve as primary standards for coal chemistry. The present report describes results for the thermochemical comparison of the premium coal samples from Argonne with our previous results on the three Exxon coals and also with our prototype solid acids: Dowex resin, silica, and graphitized carbon black. As of this writing, we still do not have an appropriate solid standard for Lewis acidity.
Experimental Section The liquid bases were all obtained from commercial suppliers and were purified by standard methods.'* Samples of premium coals were obtained from Argonne National Laboratory where they had been treated in the following manner.21 The original (4)Hammett, L. P. Physical Organic Chemistry;McGraw-Hill: New York. ~-~ , 1940. ~(5)Advances in Linear Free Energy Relationships; Chapman, N. B., Shorter, J., Eds.; Plenum: London, 1972. (6) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. B o g . Phys. Org. Chem. 1981,13,485. (7)Gutman, V. The Donor-Acceptor Approach to Molecular Znteractions; Plenum: London, 1978. (8) Fuller, E. L., Jr. J. Colloid Interface Sci. 1980, 75, 577. (9)Senkan, S.M.;Fuller, E. ., Jr. Fuel 1979,58,729. Newcomb, K. L.; Wightman, J. P. Fuel 1986,65, (10)Glanville, J. 0.; 485. (11)Phillips, K. M.; Glanville, J. 0.; Wightman, J. P. Colloids Surf. 1986,21, 1. (12)Brooks, D.;Finch, A.; Gardner, P. J.;Harington, R. Fuel 1986,65, 1760. (13)Larsen, J. W.; Kaemmerle, E. W. Fuel 1978,57,59. (14)Glanville, J. 0.; Wightman, J. P. Fuel 1980,59,557. (15)Widyani, E.; Wightman, J. R. Colloids Surf. 1982,4,209. (16) Glanville, J. 0.; et al. Fuel 1986,65,647. (17)Nordon, P.; Bainbridge, N. W. Fuel 1983,62,619. (18)Larsen, J. W.; Kennard, L.;Kuemmerle, E. W. Fuel 1978,57,309. (19)Chawla, B.;Arnett, E. M. J. Org. Chem. 1984,49,3054. (20)Perrin, D.D.;Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergammon: New York 1983. (21)Announcement of premium coal sample program: Chem. Eng. News 1986,(July 14),28. ~
Gumkowski et al. Table I. Analysis of Premium Coal Samples Obtained Argonne Laboratoriesd % comvosition H" 0" S" moistureC C" Wyodak 74 5.1 19 0.5 27 77 9 Illinois No. 6 5.7 LO 5.4 Pittsburgh No. 8 83 5.8 8 1.6 1.8 4.7 3 0.9 0.6 Pocahontas No. 3 91 87 4 2.8 0.8 Upper Freeport 5.5 North Dakota 73 5.3 21 0.8 34 Wyoming Rawhideb 66.85 4.79 19.5 0.53 Texasb 64.79 5.03 19.6 1.15 67.85 4.78 9.2 4.22 Illinois No. 6b
from
ash 8 16 9 5 13 6
Moisture- and ash-free composition. Nonpremium coal (supplied by Exxon).lD 'Reference 21. Unless indicated otherwise, these data are from the May 18,1987, announcement from Argonne National Laboratory.
7 .*
*O 18 5 O
E
.
311967
3123 67 531167
16-
P
0 00
0 10 midpoint mmoles ethylenediamiie / g coal
0 20
F i g u r e 1. Titrametric enthalpies for interaction of ethylenediamine in acetonitrile with Wyodak subbituminous premium coal. samples were taken from an active mine and removed with a hand pick from the face of the seam onto plastic sheets on the mine floor. From there they were placed in double plastic bags, transferred to stainless-steel drums, which were purged with argon of oxygen content less than 100 ppm. The samples were transferred by a refrigerated truck at 42 OF to the Argonne National Laboratory where they were handled in a large glovebox under argon a t a relative humidity of 80-100%. They were crushed, pulverized and screened to 20 or 100 mesh and kept in borosilicate glass carboys under argon for long-term storage. In our laboratory all of the coals were stored and handled in a nitrogen-filled drybox. Due to their high and varied moisture contents,21 Wyodak and Illinois No. 6 premium coals were first dried at 110 "Cunder vacuum for 24 h; North Dakota Beulah-Zap lignite was dried at 8&90 "C. Dry handling allowed for intercomparisons based upon the moisture-free elemental analyses provided by Argonne National Laboratory (Table I). The particle size distribution of the coal samples was -100 mesh except for Wyodak, which was -20 mesh. Texas Big Brown lignite obtained from Exxon was used as received. Dowex Mp50 sulfonic acid resin was treated as described previously.' Calorimetry. l b o types of thermochemical experiments were conducted: heats of immersion were performed as described previously1gby using the Seteram C-80 calorimeter at 75 "C rather than at 80 "C as in our previous report. These measurements involved the release of the coal sample into a large excess of the purified basic solvent. These runs usually required less than an hour for thermal equilibration. Heat that might be generated by the slow swelling of coal such as that observed to occur over many daysn could not be detected by our equipment. Since there is no means for determining how many moles of base interacted with the coal in this type of experiment, the results are expressed in terms of calories/gram of coal. Results are expressed in terms of the total amount of coal sample used after drying. Titrametric heats were determined at 25 "C, as before, on a Tronac Model 450 titration calorimeter.' In this experiment the coal was slurried with acetonitrile for an hour, until thermal equilibration was reached, whereupon a solution (0.2-0.3 M) of (22) Green, T. K.; West, T. A. Fuel, 1986,65, 298.
Thermochemical Comparisons of Coal Samples 4 m a
.
31
0.00
0.10 0.20 0.30 cumulative mmols ethylenediamine added
0.40
Figure 2. Cumulative enthalpies of adsorption of ethylenediamine on Wyodak premium coal versus the cumulative millimoles of base added. the purified base dissolved in acetonitrilewas titrated into the coal slurry. Not surprisingly,the heat evolved per increment of basic solution decreased steadily as the titration proceeded. Such behavior is to be expected for the acid-base titration of a solid that contains a wide variety of acid sites of different acidity and accessibility.l2S Figure 1 presents the results of a typical series of measurements from the titration of an acetonitrilesolution of ethylenediamineinto Wycdak subbituminous coal. The experiment was repeated on three different days over a period of 2 weeks. Since all parte of the thermogram were curved,the resulta are expressed in terms of kilocalories per mole of added base plotted at the midpoint of each successive added increment of basic solution. Figure 2 shows the gradual decrease (relative to linear behavior) in calories evolved as the quantity of added ethylenediamine increases. All titrations behaved in this way. Thermometric titrations were carried out at 25 O C because the Tronac calorimeter is not designed to operate conveniently at higher temperatures. An important problem is choosing conditions for thermometrictitration of coal, and many other solids,is that of diffusion of the base into various types of less accessible acid sites. The slow diffusion of pure basea into unswelled coal samples was the principle reason why heats of immersion were not conducted with the Seteram C-80at room temperature. Surface Area Measurements. Multipoint BET surface area measurements were performed by adsorbing N2 at 77 K with a Quantasorb apparatus. The samples were outgassed with heating for 2 h at the temperature at which they had been dried or by repetitive adsorption and desorption of N2
Rssults Heats of immersion of the six premium coal samples, three coals from our previous, and three prototype solids (Dowex, silica, and Carbopack F)into 12 carefully chosen solvents at 75 "C are listed in Table 11. The values reported are averages of two or three measurements, along with the standard deviation. Heats of immersion from our previous study were determined a t 80 "C, but with few exceptions (e.g., Cmethylpyridine with Wyodak coals) the trends from the two sets of measurements fall reasonably close to each other although the samples were treated quite differently and may very well come from different seams, if not different mines, of the same coal field.lg The most glaring discrepency in this table is the very exothermic AH- for Dowex in cyclohexanone. We speculate that this is the result of acid-catalyzed polymerization of this ketone by aldol condensation. Table 111provides titrametric heats in ethylenediamine and n-butylamine for the five coals and also the Exxon sample of Texas lignite, which had been stored in a sealed container in room air since the previous study 3ll2 years ago. Comparison is also made with heats of immersion from Table 11. (23)Zimmerman, R.;Schneider, H.A.; Wolf, G.Thermochim. Acta 1985, 92, 317.
Energy & Fuels, Vol. 2, No. 3, 1988 297
Discussion An important goal of this project is to see whether acid-base interactions of complex solids such as coals can be characterized thermochemically in the same manner that has been successful for characterizing acid-base interactions of homogeneous systems. A number of years ago, we demonstrated that there was a clear difference between the thermochemical order for interaction of a series of bases with the strong Brolnsted acid fluorosulfuric acid as compared with the hydrogen-bonding acid pfluor~phenol!*~~The 12 basic solvents listed in Table I1 were chosen primarily to discriminate between surface sites that hydrogen bond and those that were capable of Brolnsted acid interactions. For example, dimethyl sulfoxide is a strong hydrogen-bond acceptor although it is a relatively weak proton acceptor from Brolnsted acids in solution.24 Comparison of Premium Coals with Each Other. Heats of immersion data for six coals listed in Table I1 were subjected to linear correlation analysis with the results shown in Table IV. By heat of immersion, the greatest similarity is between Illinois No. 6 and Pittsburgh No. 8 and between Wyodak and N. Dakota lignite. The biggest difference is between Pittsburgh No. 8 and Pocahontas No. 3. Comparison with Earlier Work. The premium Wyodak coal sample (taken from the Gillette strip mine and treated as described above) may be compared to the 4 year old sample of Wyoming Rawhide coal obtained from Exxon and kept dry under nitrogen. Comparison of heats of immersion in 10 solvents (see Table 11) gives a correlation coefficient of 0.96. A similar correlation for the Exxon sample of Illinois No. 6 as compared to the Argonne premium coal, using only six bases, has an r value of 0.97. Finally, with a sample of only five bases, correlation of the old data for Texas Big Brown lignite with the premium sample of North Dakota lignite gives an r value of 0.97. Comparison with Standard Solid Acids. Heats of immersion of Dowex sulfonic acid resin, the prototype Brolnsted acid, and of silica, the prototype solid hydrogen-bonding acid, can be compared with heats of immersion of the five premium coals by using data in 10 bases: pyridine, dimethyl sulfoxide, 4-methylpyridine, toluene, cyclohexanone, 2,6-dimethylpyridine, 2,4,6-trimethylpyridine, n-butylamine, propylene carbonate and nhexylamine as shown by the correlations in Table V. It is clear that by themselves neither Dowex, silica, nor graphitized carbon provides good models for the interaction of basic liquids with these coals. When two-parameter equations are used to include contributions from both Brolnsted acidity and hydrogen bonding, there is considerable improvement. As might be expected, the introduction of yet another correlation parameter for dispersion forces improves things even more. Recent work in this laboratory indicates that Carbopack F is a better model than graphite for nonspecific physical adsorption. Regression equations using heats of immersion of Dowex and silica and van't Hoff heats of adsorption determined by gas chromatography on Carbopack F as parameters to describe the heats of immersion of five premium coals in 10 liquids are also shown in Table V. Table VI analyzes the percentage contributions of Brernsted acidity (Dowex),hydrogen bonding (Silica), and dispersion force interactions (Carbopack) to the heats of immersion for each coal in 10 bases by the method of (24) Arnett, E.M.; Mitchell, E.J.; Murty, T.S. S. R. J. Am. Chem. SOC.1974,96, 3875.
298 Energy & Fuels, Vol. 2, No.3, 1988
Gumkowski et al.
Table 11. Heats of Immersion (AH,) A&Ilm
bases vvridine dimethyl sulfoxide 4-methylpyridine toluene cyclohexanone n-hexylamine 2,6-dimethylpyridine 2,4,64rimethylpyridine n-butylamine propylene carbonate ethylenediamine acetonitrile
WYOdak, cal/g -35.98 f 2.78 -41.70 f 0.70 -17.96 f 1.35 -3.44 f 0.86 -1.58 f 1.18 -59.06 f 2.61 -4.52 f 0.25 -2.38 f 0.43 -101.66 f 3.10 -1.23 f 0.71 -101.74 f 0.88 -13.27 f 1.28
~
i
l
p
Wyommp Rawhide, cal/g -32.14 f 0.90 -41.05 f 0.37 -35.15 f 0.10 -2.08 -88.60 -8.70 -2.47
f 0.59
f 3.27 f 0.60 f 0.19
+2.52 f 1.20 -105.50 f 1.96 -3.25 f 0.76
of Five Premium Coals, Three Coals Reported in a Previous
w"
Illinois No. 6, cal/g -30.49 f 0.80 -26.43 f 1.67 -32.76 f 1.21 -6.69 f 0.90 -6.83 f 1.10 -82.42 f 1.17 -26.15 f 1.80 -17.05 f 0.45 -70.55 f 0.35 -6.67 f 0.95 -66.26 f 3.07 -12.20 f 0.74
AHim
Illinois No. 6," cal/g -41.30 f 0.40 -35.90 f 1.00 -43.70 f 1.17
Mimm
Texas: cd/g -29.46 f 0.17 -36.81 f 0.17 -21.27 f 0.74
-11.30 f 0.30
-86.11 +0.92
f f
1.98 0.55
-93.35 f 1.42 -7.33 f 0.48
~ i m m
Pittsburgh No. 8, cal/g -20.28 f 1.09 -17.47 f 1.13 -26.11 f 0.62 -0.86 f 0.89 +0.96 f 2.17 -49.09 f 0.86 -9.92 f 0.007 -2.95 f 1.44 -54.90 f 2.51 -0.27 f 0.04 -34.13 f 2.60 -3.15 f 1.01
Reference 19. *Obtained by K. F. Cassidy. Obtained by B. J. Hutchinson.
Table 111. Comparison of Titrametric Heats of Coals by Solutions of Bases in Acetonitrile at 25 OC with Heats of Immersion in Pure Bases at 75 OC (from Table 1) North Dakota Texas Wyodak Illinois Pittsburgh Pocahontas base and type of measurement ligniteb limite subbit No. 6 No. 8 No. 3 ethylenediamine init titrametric heat in acetonitrile at 25 "C, kcal/mol -17.3 f 0.7 -17.8 k 0.5c -17.5 k 1.2c -16.9 f 2.2 -5.1 f 2.6 -4.6 f 1.2 heat of immersion at 75 OC, cal/g -103.66 -93.35 -101.74 -66.26 -34.13 -12.89 n-butylamine init titrametric heat in acetonitrile at 25 OC, kcal/mol -13.5 f 0.2 -13.1d -12.7 f 1.8 -12.9 f 0.4 heat of immersion at 75 "C, cal/g -94.35 -50.3P -101.66 -70.55 ~
OOctylamine a t 80 OC.le
as received. cThree titration runs performed.
Table IV. Linear Regression Correlation Coefficients for Heats of Immersion of Five Premium Coals in 12 Bases" Illinois Pittsburgh Pocahontas North No. 3 Dakota Wsodak No. 6 No. 8 Wyodak 1.0 Illinois 0.88 1.0 No. 6 Pittsburgh 0.87 0.96 1.0 No. 8 Pocahontas 0.71 0.51 0.46 1.0 No. 3 0.94 0.68 0.69 0.73 1.0 North Dakota Bases: pyridine, dimethyl sulfoxide, 4-methylpyridine, toluene, cyclohexanone, n-hexylamine, 2,6-dimethylpyridine, 2,4,6-trimethylpyridine, n-butylamine, propylene carbonate, ethylenediamine, and acetonitrile. Table V. Regression of Premium Coal Immersion Values against Those for Dowex and Silica and Carbopack-F for 10 Bases (See Table 11)" -10.833 O.754AHhe, + 4.44OAH.3, AHw,.& 7.218mCarbap.ck-F r = 0.962; rhwex = -0.546; r13, = 0.886; rC=bpa&.F = 0.468 MH, = 61.806 - 0 . 3 1 2 A . H ~+~3.625AH8a, ~~ 1.126Affcmbpack.~ r 0.947; r h e x 0.662; r.3, 0.942; rCmbpack.F 0.078
-5.084 - 0.085AH~,, + 0.202AH,3, O*519AHCubapack-F r 0.658; rDowe1 = 0.156; r.3, = 0.466; rCubpmk.F = 0.453 , ~3.481AH8u3, ~~ AHH, -52.691 - 0 . 9 0 5 A H ~ ,+ 9.556MClrbpack-F r = 0.893; rhwex= 0.375; r.3, = 0.715; rC&pmk.F = 0.596 AHpd
10 bases as listed in Table 11. Key to coals: IL, Illinois No. 6; Pitts, Pittsburgh No. 8; Pocah, Pocahontas No. 3; ND, North Dakota.
titration run.
Table VI. Relative Importance of Three Independent Actions As Assessed by the Method of Swain and L u ~ t o n ~ ' 7 ' 0 importance Bronsted hydrogen disversion acid -bod force Wyodak 14.6 62.2 23.2 Illhois No. 6 10.0 84.0 6.0 Pittsburgh No. 8 14.1 78.3 7.6 Pocahontas No. 3 30.1 44.0 25.3 North Dakota 18.1 50.3 31.6
Swain and L ~ p t o n .It~ is ~ interesting to see the variation of these contributions from one type of coal to another and the relatively large role of hydrogen bonding. This supports the proposal of L a s e d 6 for the role of this type of interaction to the swelling and solubilization of coal. This treatment has the advantage of expressing the results of three types of actions that are presumed to affect an interaction (such as that between a solid and liquid) in percentage terms. However, its shortcoming is that the results are assumed to be completely determined by these actions; that is, they add up to 100%. This in turn implies that a perfect fit should be obtained with three parameters. This is clearly far from the case. Table V shows that our fundamental strategy of trying to dissect the interactions of a complex solid, such as a coal, with a series of solvents into contributions that are modeled by prototype 'kimpler" solids has had only modest success. Although the correlation coefficients for Wyodak, Illinois, and Pittsburgh coals are "fair" by accepted statistical norms,2' our three-parameter treatment for the North Dakota coal is poor and that for Pocahontas is Swain, C. G.; Lupton, E. C. J. Am. Chem. SOC.1968, 90,4328. (26) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (27) Shorter, J. Correlation Analysis of Organic Reactiuity;Research Studies Press: New York, 1982; p 15. (25)
Energy & Fuels, Vol. 2, No. 3, 1988 299
Thermochemical Comparisons of Coal Samples
stud^.'^ and Three Prototype Solids (Dowex, Silica, and Carbopack F)in 12 Liquid Bases Lvii" A H h of AH- North AH- Upper Dowex MP-50, AHh, of Pocahontas No. 3, c d / a -7.30 f 0.65 -4.i7 f 1.86 -4.23 f 0.56 -2.05 f 0.48 -0.74 f 1.42 -3.83 f 0.69 -0.68 f 0.12 -1.36 f 0.42 -4.24 f 0.40 -0.90 f 0.16 -12.89 f 0.50 -6.01 f 1.13
Dakota, cal/g -24.99 0.62 -44.80 f 1.11 -5.48 f 0.88 -2.05 f 0.94 +0.63 f 0.56 -16.14 f 1.38 -1.30 f 0.57 +0.41 f 0.31 -94.35 f 2.46 -0.98 f 0.24 -103.60 f 0.94 -14.94 f 0.70
*
Freeport, cal/g -6.82 f 0.34 -3.47 f 0.99 -0.45 f 0.62
kcd/equiv of acid -15.99 f 0.003 -9.87 f 0.19 -16.62 f 0.09 -1.51 f 0.13 -27.63 f 0.21 -28.07 f 0.20 -17.06 f 0.003 -17.23 f 0.038 -29.27 f 0.25 -3.36 f 0.04
silica,bcd/g -24.95 f 0.48 -22.81 f 1.40 -23.02 f 0.52 -12.27 1.20 -18.41 f 1.08 -34.18 f 0.63 -23.67 f 1.02 -21.89 f 0.70 -37.76 0.85 -17.48 f 1.16
* *
GC ad. heats of Carbopack-F,' kcal/mol -8.92 0.08 -9.69 f 0.28 -11.29 f 0.25 -10.49 f 0.28 -9.65 f 0.19 -11.62 f 0.29 -12.02 f 0.10 -13.51 f 0.32 -8.56 f 0.14 -9.96 f 0.48
*
-14.33 f 2.82
Table VII. BET Multipoint Analysis for Six Premium Coals in the ConcentrationRange 0.05 IP / P , 5 0.35 for Nitrogen Adsorption wt of specific BET ea Darams surface surface area, monolayer of sample wt, g dope intercept cor1 coeff y coal NZ, g area, m2 m2/g 1096.94 0.9995 8.9194 X lo4 24.22 3.11 6.20 Wyodak (-20 mesh) 0.5013 270.82 4.04 0.9998 3.6381 X 12.68 23.53 Illinois No. 6 (-100 mesh) 0.5388 2128.38 0.9968 104.74 4.4780 X lo4 1.56 2.38 Pittsburgh No. 8 (-100 mesh) 0.6559 4.1892 X 10"' 1.46 2.77 2294.58 0.9993 Pocahontas No. 3 (-100 mesh) 92.53 0.5276 3.5561 X lo4 2716.96 0.9997 North Dakota (-100 mesh) 95.12 0.6690 1.24 1.85 5.6746 X lo4 1697.35 64.89 0.9994 0.6978 1.98 2.48 Upper Freeport (-100 mesh) O
a8 .m
-20
a
I
-1204
0
10
20 %
30
oxygen
Figure 3. Enthalpies of immersion of premium coals as a function of the oxygen content of the coals.
useless. Table IV shows that Pocahontas correlates poorly with all the other coals, a fact that is consistent with the comparisions in Table V. It is reasonable to suppose that most, if not all, of the acid sites on the coals are derived from such organic oxygen functions as carboxylic acids, and phenolic groups and inorganic oxides. This notion is tested in Figure 3 where a crude relationship between the oxygen content and the exothermicity of heats of immersion is found for the five coals in three basic liquids. This does not hold for all bases (see Tables I and 11). There is some precedent for such a relation in the work of Phillips, Glanville, and Wightman,ll who found that the surface oxygen content (from ESCA spectra) of Virginia4 coal paralleled the total oxygen content and was proportional to AH- in water. The unusually high C/O ratio of the sample of Pocahontm coal clearly sets it apart from the other coals and is probably related to its singularly poor correlation by the three-parameter treatment noted above (Table V). Surface Sites Compared to Total Sites. Table I11 compares thermometric titration at 25 "C of the coals, suspended as slurries in acetonitrile, with their heats of immersion in the pure liquid bases. Thermometric titration was the technique used for our previous studies of Dowexl and silica.2 This technique fiist measures the heat
of interaction of the basic titration solution with the most acidic and most accessible acid sites. Figures 1and 2 show how the heat of interaction of ethylenediamine with Wyodak rapidly decreases as these sites are neutralized and weaker, less accessible sites are finally reached. In contrast, the heat of immersion simply gives a global value for the interaction of all acid sites with the base. Furthermore, the heat of immersion study involves no prior swelling with acetonitrile, and since an unknown amount of base is adsorbed, the results cannot be expressed in kilocalories per mole of base. In contrast, the heats of immersion can only be determined as calories evolved per gram of solid. Despite these differences, the trends in Table 111 show a rough proportionality between the two types of reactions between ethylenediamine and n-butylamine with the five coals. By this criterion, Pittsburgh No. 8 and Pocahontas No. 8 are considerably less acidic than the other coals. However, this difference is not to be related directly to their oxygen content nor to their general ranking with the other basic liquids. Finally, it may be asked whether accessibility or acid properties are strongly affected by the surface areas of the coals. These have been determined by BET analysis, and when the results in Table VI1 are compared with heats of immersion of titrametric heats, there is no indication that surface area is a significant factor. If it were, Illinois No. 6 would be widely separated from the others. This behavior is very different from heats of immersion of silicas in the same bases where surface area plays a key role.2 In all probability the difference lies in the fact that coals are readily swollen and penetrated by the basic solvents so that eventually most acid sites are reached in the open crosslinked gel network. In contrast silica is a relatively undeformable solid. Comparisons of Homogeneous and Heterogeneous Models for Acidic Properties of Coals. Our original proposal to test was that complex solid acids might be modeled better by combinations of prototype solid acids than by multiparameter treatments based on homogeneous prototypes. Unfortunately, we cannot give a definitive
300 Energy & Fuels, Vol. 2, No.3,1988 Table VIII. Regression of Premium Coal Immersion Values against Those of Dowex MP-SO, Silica and Carbopack-F4
A H p d = 0.583 - 0.005AFZ~wex + O.817AH,fim - 1.389AHcarbpack.~ r = 0.911; rhwx = 0.536; r,sm = 0.651; rCarbpack.F= 0.549 AHm = -219.043 - 3.236AH-, - 2.734AHBifi, 8.312mCubpack.F r = 0.995; rhwex = 0.776; resea = 0.399; rCarbpack.F= 0.547
Five bases: pyridine, DMSO, 4-Mepy, cyclohexanone, 2,6Melpy. Key to coals: IL, Illinois No. 6; Pitts, Pittsburgh No. 8; Pocah, Pocahontas No. 3; ND, North Dakota.
Table IX. Regression of Premium Coal Immersion Values against pKBB+#and Solvatochromic Parameters T* and 19 of Taft and Kamletn for Five Bases" AHw= 100.494 - 1.552(pKBH+)- 236.095~*+ 125.9878 r = 0.985; rpK= 0.239; re = 0.909; r8 = 0.383 m& -2.529 - 1.986(pKBH+)- 50.418~*+ 38.7208 r = 0.993; rpK = 0.932; re = 0.487; rg = 0.664 m p i m = 6.035 - 2*101(pKBH+) - 83.541~*+ 83.3288 r = 0.947; rpK = 0.760; r, = 0.487; ro = 0.664 mpd = -0.186 - 0.56O(pK~~+) - 33.620~*+ 39.9658 r = 0.987; rpx = 0.421; r,. = 0.547; r8 = 0.074 m m = 154.244 + o.o24(pK~~+) - 229.218~s+ 39.0808 r = 0.976; rpK= 0.112; r,. = 0.963; rp = 0.448 Bases: pyridine, DMSO, 4-Mepy, cyclohexanone, 2,6-Me2py. Key to coals: IL, Illinois No. 6; Pitta, Pittsburgh No. 8; Pocah, Pocahontas No. 3; ND, North Dakota.
statement on the merits of this notion a t this time. In order to do so we would need a full set of data for the interactions of p r o t o m homogeneous analoguesto Dowex MP-50, silica, and Carbopack F in the same 12 solvents used to study the solids listed in Table 11. Tables VI11 and IX compare the results for our solid acids with the aqueous pKBH+values28 and hydrogenbonding j3's and ** polarizability parameters of Taft, Kamlet, and their colleagues.m Although we obviously (28) (a) Organic-Soloents-PhysicalProperties and Methods of Purification; Riddick, J. k; Bunger, W.B., Weiesberger, A, Fda.; Interscience: New York, 1970; Vol. 2. (b) Smith, R. M.; Martell, A. E. Critical Stability Comtants; Plenum: New York, 1975; Vol. 2. (29) Kamlet, M. J.; Abbound, J. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983,48, 2877.
Gumkowski et al. Table X. Regression of Premium Coal Immersion Values against Those of Dowex MP-60" and the Solvatochromic Parameters T* and 8 of Taft and Kamlet" for Eight Basesb AHw,.- = 50.691 - 0.375AHhwex- 84.194~*- 5.5468 r = 0.718; rDowsx= 0.032; r,. = 0.697; r8 = 0.519 AH& = -19.346 - 0.574AH~.,~~~ + 38.654~*- 70.6978 r = 0.847; rhwex= 0.320; r,. = 0.619; r8 = 0.790 AHpitt. = 4.262 - 0.407AH~,, + 12.476~*- 46.3508 r = 0.726; rDower = 0.249; rr. = 0.599; ro = 0.686 A H p d = 4.304 - 0 . 0 2 5 A H ~ -~ 12.615~* ~ ~ ~ + 4.1858 r = 0.412; rDowex = 0.144; r,. = 0.302; r8 = 0.096 AHm = 61.407 - 0.509AHhex- 104.790~*+ 8.5lla r = 0.748; rD,, = 0.124; re = 0.673; ro = 0.405 Heats of immersion of Dowex were used in place of A H m A as a homogeneous model because there A H m A data were missing, but is directly proportional to AHDowex (1). bBases: pyridine, DMSO, 4-Mepy, toluene, cyclohexanone, 2,6-Me2py,propylene carbonate, acetonitrile. Key to coals: IL, Illinois No. 6; Pitta, Pittsburgh No. 8; Pocah, Pocahontas No. 3; ND, North Dakota.
have almost enough data for larger correlations using more solvents, Tables VI11 and IX are the only complete measured sets using exactly the same solvents. Unfortunately, cyclohexanone is part of the set. The anomalously high AH- of Dowex in this solvent is probably the result of an acid-catalyzed aldol reaction. If we remove this solvent from the comparison we me left with only four quite similar nitrogen bases, which provide little grounds for a good statistical test. On the basis of comparison of Tables VI11 and E, there is no cause for favoring either homogeneous or heterogeneous prototypes for the coals as solid acids, except for the greater accessibility of homogeneous solution data in the literature. A much stronger case can be made for the use of solid models if Table X, using homogeneous solution data for eight bases, is compared to Table V, using data for 10 liquid bases. Although the data sets do not match completely, the correlation coefficients using the solid models are so superior to the homogeneous ones as to make a very strong case for their use.
Acknowledgment. This work was supported by Contract No. DE-FG22-85PC80521from the U.S.Department of Energy for which we are most appreciative. We are glad to acknowledge helpful discussions with Dr. Karl Vorres of Argonne Laboratories, Dr. Birbal Chawla, M. H. Healy, and B. J. Hutchinson. Registry No. Dowex, 37199-22-7; silica, 7631-86-9; pyridine, 110-86-1; dimethyl sulfoxide, 67-68-5; rdmethylpyridine, 108-89-4; toluene, 108-88-3; cyclohexanone, 108-941; hexylamine, 111-26-2; 2,&dimethylpyridine, 108-48-5; 2,4,6-trimethylpyridine, 108-75-8; butylamine, 109-73-9; propylene carbonate, 108-32-7; ethylenediamine, 107-15-3; acetonitrile, 75-05-8.
