Energy & Fuels 1991,5, 57-59
mation of Fe304caused the deactivation of catalyst.
Conclusions The use of Mossbauer and EXAFS techniques is quite effective in revealing the chemical form of iron species formed during the steam gasification of Fe-loaded brown coal. The main species observed were highly dispersed ultrafine FeOOH and aggregated FeBOl crystallite. The ratio between them was affected not only by char conversion but also by iron loading. The ultrafine FeOOH was predominant with low-loadingsamples and at low char conversions. This species seems to be responsible for the catalytic activity. Acknowledgment. The partial financial support of a Grant-in-Acid for Scientific Research on Priority Areas
57
from the Ministry of Education, Science and Culture, Japan (63603012),is acknowledged. Coal Corporation of Victoria, Australia, kindly supplied the raw brown coal. The X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee (87-139). We thank Prof. M. Nomura of KEK-PF for his advice in measuring EXAFS spectra. Thanks are due to Dr. A. Kunii of the Faculty of Science, Prof. M. Fujioka, and the staff of the Cyclotron and Radioisotope Center at Tohoku University for Mossbauer spectroscopy measurements. Helpful discussions with Prof. Ohtsuka of Chemical Research Institute of NonAqueous Solutions at Tohoku University are appreciated. Registry No. Fe(N03)3,10421-48-4;Fe304,1317-61-9;Fe02H, 20344-49-4.
Solvent Extraction of Coals during Analytical Solvent Swelling. A Potential Source of Error John W. Larsen,* Jane C. Cheng, and Cheng-Sheng Pan Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received April 16, 1990. Revised Manuscript Received October 23, 1990
Bruceton coal (Pittsburgh No. 8) was exhaustively extracted with pyridine, and the residue was swollen with toluene, chlorobenzene,and pyridine. The solvent volume used to swell a constant amount of coal was varied. The coal swelling in chlorobenzene and pyridine increased as the volume of the solvent used to swell the coal increased. Vapor pressure osmometry was used to measure the concentration of materials dissolved in the swelling solvents contacting the coal. Both chlorobenzene and pyridine extracted enough material from the coal residue to promote solvent activity and reduce solvent swelling. The concentration of extracted material decreased as the volume of the swelling solvent increased. The use of high solventxoal ratios in solvent swelling experiments is recommended and long exposure of the coal to the solvent after equilibrium has been reached should be avoided.
Introduction The swelling of cross-linked macromolecular networks (gels) in solvents is a classical and often-used technique for investigating their structure. In recent years, this technique has been increasingly applied to coals by a number of groups, and the results have provided insight both into coal structure and into changes in coal struct ~ r e . ' - ~These ~ measurements can be carried out gravi(1) For a review of the literature through 1985, see: Quinga, E. M. Y.; Larsen, J. W. In New Trends in Coal Science, Yurum, Y., Ed.; Kluwer: New York; 1988; pp 85-116. (2) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4 , 42-54. (3) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4 , 74-77. (4) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4 , 101-106. (5) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4 , 107-110. (6) Amemiya, K.; Kodama, M.; Esumi, K.; Meguro, K.; Honda, H. Energy Fuels 1989, 3, 55-59. (7) Goslar, J.; Kispert, L. D. Energy Fuels 1989, 3, 589-594. (8) Brandes, S. D.; Graff, R. A,, Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494-498. (9) Larsen, J. W.; Pan, C.-S; Shawver, S. Energy Fuels 1990, 3, 557-561. (10) Painter, P. C.; Park, Y.; Coleman, M. Energy Fuel 1988, 2, 693-702. (11) Cody, G. D., Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1988,2, 340-344. (12) Bockrath, B. C.; Illig, E. G.; Waasell-Bridger, W. D. Energy Fuels 1987. 1. 72-75. (13) 'Goslar, J.; Couray, L. S.;Kispert, L. D. Fuel 1989,68,1402-1407.
0887-0624/91/2505-0057$02.50/0
metrically or volumetrically, and it has been shown that the two techniques are equivalent, with the volumetric method being preferred because it does not require a correction for the pore volume of the coal.30 A recent paper reports the observation of a dependence of coal swelling on the volume of the solvent used to swell the c0al.3~ In an ideal system (an insoluble network containing (14) Otake, Y.; Suuberg, E. Fuel 1989,68, 1609-1612. (15) Deng, C.-R.; Nio, T.; Sanada, Y.; Chiba, T. Fuel 1989, 68, 1134-1 139. (16) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla,J.; Khan, R. Fuel 1988,67, 719-726. (17) Rincan, J. M.; Cruz, S. Fuel 1988,67, 1162-1163. (18) Lucht, L. M.; Peppas, N. A. Fuel 1988,66,803-809. (19) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379-1387. (20) Green, T. K.; West, T. A. Fuel 1986,65, 298-299. (21) Stacy, W. 0.; Jones, J. C. Fuel 1986,65, 1171-1173. (22) Barr-Howell, B.; Peppas, N. A. Thermochim. Acta 1987,116,153. (23) Otake, Y.; Suuberg, E. M. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1988, 33,898. (24) Barr-Howell, B. D.; Peppas, N. A.; Winslow, D. N. Chem. Eng. Commun. 1986, 43, 301. (25) Aida, T.; Squires, T. G. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30,95. (26) Hsieh, S. T.; Dudah, J. L. Fuel 1987, 66, 170. (27) Matturro. M. G.: Liotta, R.: Isaccs. J. J. J . Ora. Chem. 1985,50, 5560. (28) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305-307. (29) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987,1,72-75. (30) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63,935.
0 1991 American Chemical Society
Larsen et al.
58 Energy & Fuels, Vol. 5, No.1, 1991
no extractable material), this result is thermodynamically impossible. Accordingly,we have explored the dependence of solvent swelling on the volume of the solvent, have observed a dependence of swelling ratio on solvent volume in a number of cases, and have demonstrated that the dependence is due to extraction of material from coal by the swelling solvent. The solvent swelling technique is an easy one to carry out. The height of a coal column in a glass tube is measured before and after the coal is swollen by a solvent. Alternatively, one can measure the mass of solvent vapors absorbed by the coal. If sufficient time is allowed to reach equilibrium, the measurements should be straightforward. The time required to reach equilibrium can be very long, and this is one source of erroramAnother potential source of error, which has been well recognized in the past, is the presence of extractable material in the coal.' In a solvent swelling experiment, material is extracted by the solvent activity when it is dissolved. The coal swelling experiment depends on reaching an equilibrium between the elastic restoring force of the network and the driving force for penetration of the coal by the solvent. A reduction in solvent activity will reduce the driving force for penetration of the network by the solvent, and this will reduce the coal swelling. So it is very difficult to obtain interpretable solvent swelling data using unextracted coal because material from the coal dissolves in the solvent, lowering the solvent activity and thereby reducing the amount by which the coal is swollen. Given the apparent ease with which these measurements can be made, reproducibility from group to group is expected. A recent review of the literature reveals that the data are badly scattered and various sets of measurements are not in the least comparable.' In this research group, we have developed swelling techniques which have given reproducible results on the same coals for more than 10 years. The precision of the measurements can also be quite high if one takes proper care. Reproducibility across laboratories has also been observed. However, repetitive swellings of the same sample do not always give identical results.26 The large scatter in the literature data is disturbing. Turpin, Ellis, and Rand recently reported their observations of a dependence on the solvent swelling ratio on the volume of solvent used to swell the coal.31 If the measurements are equilibrium swelling measurements and the system is ideal, this is a thermodynamically impossible result since the activity of a solvent is an intensive and not an extensive thermodynamic property. As long as there is any liquid which remains free and not dissolved in the network, the swelling ratio should be independent of the amount of solvent. However, their measurements utilized unextracted coals. As the ratio of solvent to coal decreases, the swelling also significantly decreases. This can be rationalized easily if the solvent is, as expected, extracting material from the coals. With the smaller amount of solvent, the solution will be more concentrated and its activity will be correspondingly reduced. They have provided an experimental demonstration of the often-mentioned caveat that extracted coals must be used. We decided to extend their work somewhat and measured the solvent swelling of Pittsburgh No. 8 (Bruceton) coal in a series of solvents as a function of solvent-to-coal ratio after extracting the coal with pyridine. In some of the systems, we observed a dependence of the solvent (31) Turpin, M.; Ellis, B.; Rand, B. In International Conference on Coal Science; Moulijn, J. A., Nater, K. A.; Chermin, H. A. G., Eds.; Elsevier: New York, 1987; pp 85-88.
Table I. Swelling of Extracted Bruceton Coal in Pyridine at 26 OC wt ratio solventxoal 12.55 12.48 12.78 12.59 volumetric swelling (Q) 2.35 2.45 2.45 2.45 Table 11. Elemental Analysesa %Ot S %
Bruceton coal residue from pyridine extraction
C
H
%N
5.5 4.7
1.4 1.8
%
75.6 73.0
(daff) 9.7 11.5
% ash
7.8 9.0
Galbraith Laboratories.
~
14
12
10
6
8
4
Welght Ratlo C,H,CI/Coal
Figure 1. Replicate determinations of volumetric solvent swelling (Q) (A,0)in toluene and the concentration (0, 0) of material extracted into the solvent measured by vapor pressure osmometry.
2.5
14
12
10
6
4
Weight Ratio C,H,CI/Coal
Figure 2. Replicate determinations of volumetric solvent swelling (Q)(A,0)in chlorobenzene and the concentration (0,0) of material extracted into the solvent measured by vapor pressure osmometry.
swelling ratio on the volume used and have been able to assign this to further extractions of the already extracted coal by the swelling solvent.
Experimental Section The techniques for solvent swelling of coals have been described in the literature.g0 We used our normal volumetric technique using 100-mg samples of -100 mesh coal in 6-mmglass tubes. Concentrations of extracted material in the swelling solvent were measured with a Knauer vapor pressure osmometer and techniques which have been described in the literature.= The pyridine extraction was carried out under dry Nzand proceeded until the extracting solvent was clear and water white. The residue was dried under vacuum for 3 days a t 110 OC. Analyses of the coal and the insoluble residue after exhaustive Soxhlet extraction are given in Table I. The extraction was carried out in an apparatus connected to an N2cylinder and an oil-filled bubbler, ensuring a slight (5 cm oil) positive pressure of Nzin the system at all times. Soxhlet extraction with pyridine normally removes material having a slightly higher H/C ratio than the coal, leaving a residue depleted in H and enhanced in 0 (32) Larsen, J. W.; Choudhury, P. J. Org. Chem. 1979,44,2856-2859 and references therein.
Energy & Fuels, Vol. 5, No. 1, 1991 59
Solvent Extraction of Coals
1.9
1
t
3.0 cane 12.5
0
0
1.4 I
14
12
,
10
8
,
I
6
0.5
0
A I
/
I
,
4
Weight Ratio C,H,CH,/Coai
Figure 3. Replicate determinations of volumetric solvent swelling (Q) (A, 0 ) in pyridine and the concentration (0, 0 )of material extracted into the solvent measured by vapor pressure osmometry. compared to the starting coal. This is observed here, but the increase in 0 + S is larger than anticipated. Precautions were taken to guard against the ready oxidation of coals which occurs in pyridineF3 but these data suggest some oxidation may have occurred. The reported increase in 0 + S may also be due to the unfortunate summing of analytical errors, all of which reside in 0 + S determined by difference.
Results The reproducibility of the swelling measurements is shown by the data in Table 11. Our results are shown in Figures 1, 2, and 3, each containing a duplicate series of experiments in which the insoluble residue from a pyridine extraction of Bruceton coal is swollen in a solvent. The first swelling solvent used is toluene. The coals were swollen to equilibrium in toluene, and then the supernatant toluene was separated and the molar concentration of any molecules dissolved in it was measured by using vapor pressure osmometry. The vapor pressure osmometer was carefully calibrated by using benzyl. Since the molecular weights of the dissolved molecules are not known, we cannot convert from molarity to a weight concentration scale. The data are rather scattered for toluene but show no significant dependence of swelling volume on solvent volume down to a so1vent:coal ratio of 5. Below this value, swelling appears to decrease. The two series of repeated VPO measurements disagree. One shows small increases in dissolved material with increasing volume while the other shows small decreases. The changes are small. If the two runs are averaged, the concentration change is zero. While the scatter in the data is troublesome and not unexpected at the low concentrations being studied, it is clear that there is little material extracted and essentially no dependence of swelling on solvent volume. In Figure 2, similar data are shown for chlorobenzene. With this solvent, the dependence of swelling ratio on the volumetric ratio of solvent to coal is quite apparent; the two series of experiments agree well and show a decrease in swelling ratio from about 1.8 to 1.4. The concentration of dissolved material increases from approximately 0.5 to about 2.5 pM over the same concentration range. We were astounded to discover that chlorobenzene at room tem(33) Solash, J.; Hazlett, R. N.; Bumett, J. C.; Climenson, P. A,; Levine,
J. R.Fuel
1980,59,667-669.
perature could extract significant amounts of material from a coal which had been exhaustively Soxhlet-extracted with pyridine, but the data are incontrovertible. This is indeed happening. A similar situation exists with pyridine as shown in Figure 3. Solvent swelling decreases from 2.5 to 2.0, a smaller change than that observed with chlorobenzene. The data are significantly more scattered than with chlorobenzene. Pyridine is a hygroscopic, difficult solvent, and the concentrations are low. Despite the scatter, the overall trends are clear: decreasing swelling with increasing concentration of dissolved material. The concentration of material dissolved in the pyridine is greater than with chlorobenzene, but the change in concentration with volume is smaller. Discussion There is one obvious explanation for these results. Even after exhaustive Soxhlet extraction, soluble material remains in the coal and can be slowly extracted even at room temperature. If a finite amount of material is present to be extracted, its concentration in the extraction solvent will decrease with increasing solvent volume. Removal of soluble material from coals by solvent extraction is very slow. Diffusion through a macromolecular network is required, and the rate-determining step is probably the motion of the coal macromolecular chain segments necessary to let pass the extractable molecules. Some soluble material apparently remaining in the coal even after 5 days of Soxhlet extraction emerges during the swelling experiment. It is clear from these results that one must be very cautious in carrying out solvent swelling measurements even on extracted coals. Our recommendations are to use a very high ratio of solvent to coal (at least 1O:l) and to follow the swelling to equilibrium in an expeditious manner. It is not wise to leave the coal and solvent together for extended periods of time once they have reached equilibrium before remeasuring the height of the coal column. The problem exists with both polar and moderately polar solvents, so it is wise to be cautious with all solvents. We cannot evaluate the role that this phenomenon has played in the solvent swelling data which have been published. Our published swelling data in nonpolar solvents have proven to be reproducible. This is not surprising since this is a set of solvents for which this difficulty should be minimized. Another potential problem for solvent swelling measurements is the occurrence of internal conformational rearrangements of swollen coals resulting in the formation of new noncovalent intera~tions.~?~ These interactions change the swelling behavior of coals, leading to nonreproducible successive swellings of the same sample~.'.~J~a Acknowledgment. We are grateful to the US. Department of Energy and to the Exxon Education Foundation for support of this work. Registry No. Pyridine, 110-86-1;toluene, 108-88-3;chlorobenzene, 108-90-7.
