Energy & Fuels 1988,2, 121-124 erals would be expected to act as catalysts in such react i o n ~ .Figure ~ 3 shows the presence of l80in molecular ions after mild oxidation while Figure 4 shows reactive oxygen species on the coal surface associated with mineral matter. These results would be expected if a free-radical mechanism is involved in low-temperature oxidation.
Conclusions Detailed SIMS images can be obtained for ions differing in mass by as little as 1:3500. The uptake of l80and molecular ions containing l80can be successfully studied with SIMS after exposure of coal surfaces to an l8OZatmosphere at room temperature. XPS provides additional insights when used on coal surfaces nearly identical with those studied by SIMS. The SIMS results, while not in themselves definitive, provide circumstantial evidence for a free-radical oxidation
121
mechanism in which clay minerals act as catalysts. Indeed the generation of 180H- and the presence of l80on the surface are indicative of low-temperature oxidation while the detection of Os-, 02-, l80and , 180H- in regions rich in clay is suggestive of catalytic activity. These suggestions have been made elsewhere in the l i t e r a t ~ r e . ~ , ~ J ~ - l ~ In a similar fashion the SIMS images for the molecular ion CS-, in conjunction with XPS results showing the existence of C-S' and C-S-Ox, seem to imply that a t least some of the sulfur present in the organic regions adjacent to pyrite arise from sulfonation of the coal by the products of air oxidation of the pyrite. The same result would be expected from in situ sulfonation of coal. Such a mechanism would be entirely consistent with the course of pyrite oxidation6s7and the relative ease of sulfonation of specific
coal^.^*^ ~~~
(12)Cole, D. A.;Herman, R.G.; Simmons, G. W.; Klier, K. Fuel 1985, 64,303.
(13)Liotta, R.; Brons, G.; Isaccs, J. Fuel 1983,62, 781. (14)Holstein, W. L.; Boudart, M. Fuel 1983,62, 162. (15)Liu, K. H.; Johannes, A. H.; Hamrin, C. E. Fuel 1984,63, 18.
Heats of Immersion of Pocahontas No. 3 Coal in n-Alkanes and Oxidized Coal in Watert James B. Hollenhead, J. 0. Glanville, and J. P. Wightman* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received August 9, 1987. Revised Manuscript Received September 25, 1987 The heats of immersion of Pocahontas No. 3 coal powder have been determined in a series of n-alkanes (C8-Clg) with a Calvet MS-70 microcalorimeter. The results indicate a similar wetting behavior for the alkanes longer than dodecane; however, the higher heats of immersion of coal in c8+2 alkanes indicate the availability of more surface area to these shorter alkanes. The heats of immersion of Pocahontas No. 3 in water have also been studied as a function of oxidation time at 320 "C and as a function of extraction by pyridine and methanol prior to oxidation. A dramatic increase in the heat of immersion in water was observed as oxidation time increases. The heat of immersion of the oxidized coal in water surprisingly showed no dependence on prior extraction with either methanol or pyridine. However, the time of immersion of oxidized coal in water was reduced by either extraction procedure. X-ray photoelectron spectroscopy (XPS) was employed as a complementary technique to heat of immersion measurements. Oxygen/carbon ratios determined from the XPS spectra obtained for the oxidized coals paralleled the heat of immersion results.
Introduction The interactions that occur between liquids and coal surfaces may be studied by a variety of experimental methods including both thermodynamic and spectroscopic onea. This study incorporates microcalorimetry and X-ray photoelectron spectroscopy to ascertain the effects of nalkane chain length and the degree of oxidation on the heats of immersion of coal. An excellent but dated review of immersional calorimetry has been given by Zettlemoyer,' and a number of researchers have used this technique to examine various
carbon surfaces.2-'0 The magnitude of heat released upon the immersion of a bare solid in a liquid yields information regarding the nature of the interaction. Heat of immersion determinations are sensitive and easily distinguish between the interaction of a hydrophobic surface with a. nonpolar wetting liquid and the interaction of a hydrophilic surface with a polar wetting liquid. This sensitivity has prompted a study of the oxidation of a Virginia coal by measuring the heat of immersion in water? where the increasing heat of immersion in water parallels an increase in surface oxygen content. Several papers'lJ2 have demonstrated the
'Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987. (1)Zettlemoyer, A. C.In Chemistry and Physics of Interfaces;Rosa, S., Ed.; American Chemical Society: Washington, DC, 1965. (2)Robert, L.Bull. SOC.Chim. Fr. 1967,7 , 2309-2316. (3)Clint, H. J.; Clunie, J. S.; Goodman, J. R.; Tate, J. R. Nature (London) 1969,223,51~52. (4)Larsen, J. W.; Kuemmerle, E. W. Fuel 1978,57, 55.
(5)Glanville, J. 0.; Wightman, J. P. Fuel 1980,59, 557-562. (6) Widyani, E.;Wightman, J. P. Colloids Surf. 1982,4, 209-212. (7)Nordon, P.; Bainbridge, N. W. Fuel 1983,62, 619-621. (8)Phillips, K. M.;Glanville, J. 0.;Wightman, J. P. Colloids Surf. 1986,21, 1-8. (9)Glanville, J. 0.; Newcomb, K. L.; Wightman, J. P. Fuel 1986,65, 485-488. (10)Barton, S.;Boulton, G. L.; Dacey, J. R.; Evans, M. J. B.; Harrison, B. J. Colloid Interface Sci. 1973,44(1),50-56.
0887-0624/88/2502-Ol21$01.50/00 1988 American Chemical Society
Hollenhead et al.
122 Energy & Fuels, Vol. 2, No. 2, 1988
effect of moisture content of coals on the heat of immersion in water. Nordon and Bainbridgel have studied Australian coals and found that the heat of immersion in water decreases with increasing moisture content. In addition, they determined that the heat of wetting of a normal moist coal (65 % relative humidity) does not contribute significantly to self-heating. The time of immersion determined from microcalorimetry provides another means by which to examine the interaction of coal surfaces with liquids. The time of immersion is defined as the time required for total heat release to occur. In porous solids, the limiting factor in the rate of heat release may be the ease with which the molecules of the liquid can access the solid. This kinetic effect has been used to study the effect of molecular chain length and bulkiness on the immersion of various coals. Widyani and Wightman have studied the immersion of Pocahontas No. 3 coal in a series of n-alkanok6 While the heats of immersion decrease nonlinearly with increasing carbon chain length, the time of immersion reaches a maximum for n-butanol, indicating the difficulty with which n-butanol negotiates the microporous structure of the coal. Glanville et al.9 have examined the times of immersion of a subbituminous coal in butanol isomers. The times of immersion were found to increase with increasing bulkiness of the liquid, in the series n-butanol > sec-butanol > tert-butanol. This result was mirrored by a concomitant decrease in the heat of immersion for the same series of liquids. The oxidation of coal has important consequences in processes such as coal transportation and gasification.13J4 Heat of immersion measurements have already been used by Phillips et aL3 to study the oxidation of coal as a function of oxidation temperature. Their results demonstrate a 50-fold increase in the heat of immersion in water for coal heated to 320 O C for 24 h over the unoxidized coal. The oxidation of coal may also be followed with X-ray photoelectron spectroscopy (XPS), which yields information about the structure and bonding of surface atoms as opposed to those in the bulk. Clark and Wilson15 have evaluated the application of XPS on several bituminous coals, kerogen, and pitch. The surface variations studied included the oxidation of carbon and sulfur components. Comparisons were also made between bulk and XPS analysis of oxygen, sulfur, aluminum, and silicon, and the findings indicate that changes from the bulk elemental analysis were detectable with XPS. The objective of this work was to examine the relationship between the heats and times of immersion for Pocahontas No. 3 coal and chain length of a series of nalkanes (C3-Ci9). A secondary objective was to demonstrate the complemetary use of microcalorimetry and XPS in following the oxidation of coal a t 320 O C as a function of time.
Experimental Section Materials and Reagents. The n-alkanes used in this study were obtained from Aldrich Chemical Co. and were used without further purification. Deionized water was prepared in the laboratory. The coal studied in this investigation was Pocahontas No. ~
~~
~~
~
~~
(11)Sondreal, E.A.; Ellman, R. C. Rep. Invest.-U.S. Bur. Mines 1974,No. 7887. (12)Glanville, J. 0.; Hall, S. T.; Messick, D. L.; Newcomb, K. L.; Phillips, K. M.; Webster, F.; Wightman, J. P. Fuel, 1986,65, 647-649. (13)Riley, J. T.;Reasoner, J. W.; Fatemi, S. M.; Yates, G. S. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel. Chem. 1986,32(1), 162-170. (14)Rozelle, P.L.;Scaroni, A. W. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel. Chem. 1986,32(1), 330-336. (15)Clark, D.T.;Wilson, R. Fuel 1983,62, 1034-1040.
Table I. Proximate and Ultimate Analysis of Pocahontas No. 3 Coal Proximate Analysis (wt % ) (As Received) 1.04 moisture ash 5.0 volatile matter 17.0 fixed carbon 77.0 Ultimate Analysis (wt carbon hydrogen nitrogen chlorine sulfur ash moisture oxygen (by diff)
%)
(As Received) 84.75 4.6 0.97 0.08 0.61 5.0 1.04 2.95
3 coal, a low-volatile bituminous coal from McDowell County, WV. The coal had been crushed and sieved to -325 mesh. The asreceived proximate and ultimate analyses of Pocahontas No. 3 coal are given in Table I. Heat of Immersion Measurements. The heat/time of immersion determinations were made with a Calvet MS-70 microcalorimeter2J6operated at 36.5 "C. Electrical calibration of each calorimeter cell was carried out prior to sample determination. The determination of the heat of immersion in a specific liquid consisted of preparing four coal samples each of nearly identical mass. For the immersion of coal in the n-alkanes, 50-mg samples were used. For the immersion of oxidized coal in water, only 25-mg samples were prepared for each oxidized coal, due to the long immersion times required. The samples were placed in small custom Pyrex bulbs (ca. 3 X 0.75 cm) made with break-off tips. The bulbs were then connected to a vacuum line and evacuated at approximately 1 X lob Torr for 2 h at room temperature. After being sealed under vacuum, separate bulbs were attached to four calorimeter insertion rods via modified Swagelok fittings. A stainless-steel cylinder containing 5 cm3of the wetting liquid was placed around each sample bulb, forming a sealed unit at the bottom of the insertion rod. These four assembled units were then placed in the microcalorimeter and allowed to equilibrate overnight. After attaining thermal equilibrium,the first sample was broken by remote mechanical action. The liberated heat was detected by a thermopile and the signal amplified and integrated to obtain a value proportional to the heat produced. When the calorimeter regained the steady state, the next sample was broken and so on. Steady state is defined as the condition of no detectable heat evolution over a given time period. The practical criterion of steady state was taken as a heat release of
