Energy & Fuels 1997, 11, 1003-1005
1003
Einstein Specific Heat Modeling of the Effects of Chlorobenzene Refluxing on the Macromolecular Structure of Pittsburgh No. 8 Coal Peter J. Hall Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland Received January 13, 1997X
An interpretation of specific heat data for Pittsburgh No. 8 coal refluxed in chlorobenzene for varying time in terms of a two-component Einstein model is given. It is shown that in the initial stages of refluxing (up to 8 days) there is increased macromolecular association in the bedding plane of the coal but that further refluxing leads to macromolecular association perpendicular to the bedding plane. Second-order phase transitions in the coal are also observed, and the variation of Einstein temperatures below and above these transitions are discussed.
Introduction Recently, Larsen et al.1 have produced a comprehensive study of the structural arrangements that result from refluxing Pittsburgh No. 8 coal in chlorobenzene for varying time. They used a wide range of techniques to demonstrate that Pittsburgh No. 8 coal undergoes a transition to a more highly associated configuration. Part of the work was a differential scanning calorimetry (DSC) investigation of the treated coals. The results are reproduced in Figure 1. The general decrease in Cp with increasing reflux time in chlorobenzene is indicative of an increase in macromolecular association. Larsen et al.1 did not attempt to quantify this information, but this is possible because of modeling work by Merrick2 and more recently by Hall and Larsen.3 Following earlier work by Merrick2, Hall and Larsen3 have recently used two-component Einstein specific heat models to monitor changes in the macromolecular structure of three Argonne coals following pyridine extraction. The Einstein specific heat model predicts the variation of specific heat capacity (Cv). Essentially, experimentally measured specific heat capacity data (Cp) was fitted to the following function,
( )[ ( )
( )]
2 θ2 R 1 θ1 g Cp ) + g2 a 3 1 T 3 T
exp(θ/T)(θ/T)2 (exp(θ/T) - 1)2
θ)
hν k
n
1 a
fi
∑ i)1µ
(1)
(2)
θ is the Einstein temperature (K) and is defined by Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Larsen, J. W.; Flowers, R. A; Hall. P. J.; Carlson, G. Energy Fuels 1997, 11, 998-1002. (2) Merrick, D. Fuel 1983, 62, 540. (3) Hall, P. J.; Larsen, J. W. Energy Fuels 1993, 7, 42-46. X
S0887-0624(97)00008-X CCC: $14.00
(3)
where h and k have their usual meanings and ν is the frequency of vibration. For coals Cp ≈ Cv3. The mean atomic weight in eq 1 is defined by
)
where R is the gas constant, a is the mean atomic weight, and g(θ/T) is the Einstein function defined by
g(θ/T) )
Figure 1. Differential scanning calorimetry measured at 10 K min-1 for Pittsburgh No. 8 coal refluxed in chlorobenzene for varying time.
(4)
i
where fi is the mass fraction of element i and µi is its atomic weight. The Einstein specific heat model has been applied to a wide variety of materials, including polymers that have Einstein temperatures similar to that of coal.4 The 1/3, 2/3 weighting in eq 1 reflects the number of degrees of freedom of atomic vibrations in and out of the plane of aromatic clusters. The real value in evaluating Einstein temperatures is that it allows quantitative evaluation of how the intermolecular forces change in coal structure following treatment. Equation 3 shows that Einstein temperatures are proportional to the vibrational frequency, ν, which is in turn propor(4) Wunderlich, B.; Jones, L. D. J. Macromol. Sci., Phys. 1969, B3 (1), 67-79.
© 1997 American Chemical Society
1004 Energy & Fuels, Vol. 11, No. 5, 1997
Hall
tional to the square root of the force constant governing the vibration. Wunderlich and Jones4 evaluated Einstein temperatures for a variety of linear polymers. They demonstrated, for example, that the molecular forces governing the Einstein temperatures for poly(ethylene) were greater than for poly(tetrafluoroethylene) because the Einstein temperatures were greater for poly(ethylene). To date, the Einstein theory has not been applied to understanding changes resulting from chlorobenzene treatment. Experimental Section Full experimental details have been given by Larsen et al.,1 but the essential information is as follows. A Mettler DSC System 30 was used with a high sensitivity glass sensor. Standard aluminum pans were used with two pinholes to allow evaporation of water. The sample size was ∼10 mg, and DSC was performed at 10 K min-1 with a nitrogen carrier. Mackinnon and Hall5,6 have also used a Mettler DSC System 30 to investigate second-order phase transitions in a wide variety of coals. The instrument used in this investigation was based at Lehigh University and was therefore a similar, but different, instrument. Pittsburgh No. 8 coal was obtained from the Argonne Premium Coal Sample program. Chlorobenzene refluxing was in a Soxhlet apparatus under a slight positive pressure of nitrogen to prevent oxidation. The chlorobenzene was removed under vacuum in a rotary evaporator. Chlorine analysis of the dried coals showed no increase in chlorine content, suggesting that all the chlorobenzene could be removed by this procedure. Prior to measurement of Cp the coals were dried at 373 K for 20 min in the sample holders in a nitrogen carrier. The samples were then thermally relaxed by heating at 10 K min-1 to 473 K in the DSC cell. The samples were then reweighed. The resulting DSC traces were tested for reproducibility by cycling up and down in temperature between 323 and 408 K five times. The reported Cp values are the mean from experiments on three separate samples and have an accuracy of (2%. The mean atomic weight for Pittsburgh No. 8 coal (“a” in eq 1) was calculated from its elemental analysis and determined to be 7.72. This value does not change following chlorobenzene treatment.
Results and Discussion Figure 1 shows the variation of Cp for Pittsburgh No. 8 coal with temperature following different times of chlorobenzene refluxing from Larsen et al.1 The rapid increase in Cp between 380 and 390 K has been previously observed by Mackinnon and Hall.5,6 It is evident for all the chlorobenzene-treated coals, and the temperature of the transition decreases with increasing time of chlorobenzene reflux. Figure 1 also shows that the Cp values decrease with increasing time of chlorobenzene reflux. Larsen et al.1 have shown that the reflux does not change the elemental analysis and functional group distribution of the coal. Therefore, it may be inferred that the decrease in Cp is due to some physical change in the structure. To investigate this further, the Cp data were fitted to the two-component Einstein model described in eq 1. It was found that good fits were obtained for all the data sets. To illustrate this, the fit to the 4-day reflux data will be considered in more detail. (5) Mackinnon, A. J.; Hall, P. J. Energy Fuels 1995, 9, 25-32. (6) Mackinnon, A. J.; Hall, P. J. Fuel 1996, 75, 85-88
Figure 2. Two-component Einstein model fits to specific heat capacity data for Pittsburgh No. 8 coal that had been refluxed in chlorobenzene for 4 days. Table 1. Experimental and Theoretical Values for Cp temp (K)
exptl Cp (J g-1 K-1)
theoret Cp (J g-1 K-1)
% difference
328 333 338 343 348 353 358 363 368 373
1.72 1.74 1.76 1.79 1.82 1.84 1.85 1.88 1.89 1.90
1.71 1.73 1.76 1.78 1.81 1.83 1.85 1.87 1.90 1.92
0.68 0.39 0.12 0.44 0.78 0.58 -0.12 0.27 -0.38 -1.00
Figure 2 shows the experimental data together with the two-component Einstein model fits for the 4-day refluxed sample. Two regions are evident: below and above the second-order phase transition. Below the transition θ1 ) 380 K and θ2 ) 1230 K; above the transition θ1 ) 380 K and θ2 ) 1170 K. Table 1 shows the percent difference between the experimental Cp values and the values from the Einstein theory. It can be seen that the differences are all θ1 for all the samples. This is indicative of stronger bonding in the plane of the aromatic clusters. Figure 3 also shows that θ2 increases with increasing reflux time. An increase in θ2 was postulated by Hall and Larsen3 to explain changes in Cp following pyridine extraction, although their DSC was not performed to
Energy & Fuels, Vol. 11, No. 5, 1997 1005
temperatures that were high enough to enable a new value of θ2 to be evaluated. The results presented here confirm this hypothesis and show that chlorobenzene reflux induces similar structural changes as pyridine extraction in Pittsburgh No. 8 coal. As mentioned above, the increase in θ2 is not caused by a change in the chemical nature of the coal (since there are none). Rather, it is caused by an increase in the strength of the intermolecular forces in the plane of the aromatics during reflux. There is a growing body of evidence to suggest that in its natural, as-mined state, coal is not in an energy-minimized configuration and is kept in this state because of the glassy nature of coal. Heat treating the coal above its glass transition or swelling in a basic solvent can induce changes in the macromolecular configuration to produce coal in a more energy-minimized state. The boiling point of chlorobenzene is 132 °C and in the thimble section of the Soxhlet is probably close to this value (it was not measured). Therefore, during the reflux the coal is close to its glass transition and, together with any plasticization effects of the liquid, the coal may be relaxed into an energy-minimized configuration. This is probably due to increased molecular association that restricts atomic vibrations. Therefore, up to 8 days of reflux, the principal changes appear to be an increase in the intermolecular forces in the plane of the aromatic clusters. Significant molecular association out of this plane only starts after 8 days and does not occur to the same extent. Hall et al.7 have also performed small angle neutron scattering on these samples. These results show an increase in the amount of microporosity. Therefore, the effects of chlorobenzene refluxing appear to be the creation of highly associated regions in the coal, which causes an increase in the Einstein temperatures as well as an increase in voidage. EF9700083 (7) Hall, P. J.; Antxustegi, M. M.; Mackinnon, A. J.; Burchill, P.; Winans R. E.; Thiyagarajan, P. Energy Fuels 1994, 8, 1526-1527.
