Energy & Fuels 1993, 7, 1026-1029
1026
Effects of Glass Formation by Solvents in Differential Scanning Calorimetry Investigations of Solvent Swollen Coals M. Mirari Antxustegi, Alexander J. Mackinnon, and Peter J. Hall* Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow GI IXL,Scotland Received March 24, 1993. Revised Manuscript Received July 15, 1 9 9 P
The behavior of solvents under conditions of rapid cooling is investigated. It is demonstrated that certain coal solvents form glasses on quenching a t 60 K/min whereas others form crystals. In swollen coals, most free solvents tend to go into glassy states. An important exception is pyridine. The formation of glasses by free solvents has two effects on differential scattering calorimetry results. At temperatures below the solvent melting point, exotherms are produced. This is probably due to cold recrystallization. At high solvent weight fractions, second-order phase transitions may occur, which are not a feature of the coal. It is concluded that investigations of phase transitions in swollen coals should preferably be performed at the "gel" point, with no free solvent present. It is shown that low-temperature second-order phase transitions for quenched solvents occur at the same temperature as in coal gels formed with the same solvents. This suggests that phase changes in the solvents determine the form and temperature of the glass transitions.
There has been considerable interest in coalliquefaction for a number of years, although progress toward a truly viable process has been s1ow.l Many of the problems of coal liquefaction come from the macromolecular structure of coal itself, which dictates the viscoelastic properties of coal under different circumstances. At room temperature, coals are glassy solids.2 The glassy nature of coal restricts the diffusion of potentially reactive materials through the coal macromolecule and reaction rates tend to be diffusion limited. Brennerz has shown that thin sections of Illinois No. 6 become rubbery at room temperature when swollen with pyridine. This may have important implications for coal processing because the diffusion of liquids through rubbers is several orders of magnitude faster than through the corresponding glass. Therefore, considerable attention has been paid to coal/solvent interactions1 and a number of experimental techniques common to polymer/solvent interaction investigation have been applied to this subject. Of particular importance here are differential scanning calorimetry (DSC) and solvent-induced swelling. Hall and Larsen3 have conducted DSC on Illinois No. 6 swollenin 1-methyl-2-pyrrolidinone(NMP)and pyridine. Second-order phase transitions were detected at 180 K for NMP and 140 K for pyridine. The second-order phase transitions were assigned to being glass to rubber transitions. Hall and Larsen4 also performed DSC on NMPswollen coals in which the amount of solvent was progressively reduced. A state was reached at which no free NMP was present in the system. In other words, all of the NMP was present in a bound form. The second-order phase transition persisted in the system, which suggested
that the transition was a feature of the solvent-swollen coal, rather than of the free solvent. DSC is potentially a powerful technique to assist in an understanding of coal solvent interactions and there is current interest in the detailed interpretation of such data.516 In low-temperature DSC, Hall and Larsen4 also noted exotherms for the NMP-swollen coal in which enough heat was evolved to make the recorded specific heat (C,) values become negative. This exotherm was greatly reduced, or absent, for pyridine-swollen Illinois No. 6 coal. Hall and Larsen4 was unsure of the origin of the exotherm. However, similar exotherms have been noted for certain rubbery materials that are quenched to below their glass transitions.' As the rubber is quenched into the glassy state, any high-temperature disorder is "frozen" into the structure. When the glass is heated and goes through ita glass transition, eventually there is enough energy for the rubber to reorganize into a more stable configuration. This is referred to a cold recrystallization. Although cold recrystallization provides a plausible explanation for the origin of these exotherms, this hypothesis needs to be tested. An alternative explanation for this phenomenon is that any free solvent goes into a glassy state which subsequently undergoes cold recrystallization. This would certainly provide a good explanation of the coincidental disappearance of the exotherm and the solvent melting endotherm. Glasses are broadly defined as noncrystalline materials. A number of organic liquids form glasses if cooled sufficiently quickly.8 These tend to be large molecules of low symmetry, such as o-terphenyl,8 rather than the relatively simple solvents in the Hall and Larsen4 work. It is important to decouple the effects of glass formation
Abstract published in Adoance ACS Abstracts, September 1,1993. (1)Lowry,H. H.,Ed. ChemistryofCoal Utilization;Wiley: New York, 1963. (2)Brenner, D.;Fuel 1984,63,1324-1328. (3)Hall, P.J.; Larsen, J. W. Energy Fuels 1991,5,228-229. (4)Hall, P.J.; Larsen, J. W. Energy Fuels 1993,7,47-51.
(5)Lucht, L. M.;Larson J. M.; Peppas, N. A. Energy Fuels 1987,I , 56-58. (6)Yun, Y.;Suuberg, E. M. Energy Fuels 1992,6,328-330. (7)Billmeyer, F.W. Text Book ofPolymer Science; Wiley: New York, 1966. (8)Greet, R. J.; Turnbull, D. J. Chem. Phys. 1967,46,1243-1251.
Introduction
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0887-0624/93/2507-lO26$04.00/00 1993 American Chemical Society
Effects of Glass Formation by Solvents
Energy & Fuels, Vol. 7, No. 6,1993 1027
Table I. Swelling Ratios of Coals in Solvents solvent Illinois No. 6 Pittsburgh No. 8 NMP 3.0 3.2 pyridine 2.4 2.4 3-chloropyridine 2.8 2.6 2-chloropyridine 2.9 1.6 2-fluorop yr idine 1.7 1.3
by free solvent from any phase changes in coal structure during DSC investigations of solvent-swollen coals. One objective of the current paper is to report DSC performed on a number of commonly used coal solvents following quenching from room temperature. Experiments will also be performed on coals swollen by these solvents under similar conditions and with varying solvent content. Solvent-induced swelling of coals has become a standard technique to investigate coal/solvent interactions. The amount of swelling being determined by the strength of interaction between the solvent and the coal (e.g., the extent to which it disrupts coal hydrogen bonding) and the mean cross-link density of the coal.
Experimental Section Pittsburgh No. 8 and Illinois No. 6 coal were selected from the Argonne Premium Coal Sample Program. A Mettler System 30 instrument was used. Solvents were used as obtained from Aldrich Chemicals. The experimental procedure for the solvent-induced swelling as well as solvent characteristics has been described by Hall et al? For DSC on the pure solvents approximately 10mg were placed in an aluminum container. The sealed lid of the container was pierced and the sample was quenched at 60 K/min from room temperature to 113 K. DSC was performed at 10 K/min to 273 K. The procedure for DSC on the solvent-swollencoal mixtures and for reducing the amount of free solvent has been described previously.' The only significant departure from this procedure was that the coal was mixed with excess solvent at room temperature and allowed to come to swelling equilibrium for several days, rather than exhaustive Sohxlet extraction by the solvent. It was found that the gel point for NMP and Illinois No. 6 was the same for the whole coal and the exhaustively extracted coal. The form of the swollen coal for the DSC experiments was that of a thick paste, swollen coal held in the paste by free solvent. The procedure to reduce the amount of solvent in the sample was as follows: DSC commenced at 113 K and continued at 10 K/min until an endotherm corresponding to solvent evaporation was observed. The DSC experiment was terminated by quenching to room temperature after the endotherm had indicated a certain amount of solvent loss. In this way, the amount of solvent was reduced by 2 or 3 wt % during each DSC scan. The sample holder and coal were weighed and the procedure repeated. To determine the mass of coal, the final DSC experiment was performed until the evaporation endotherm had returned to the baseline, thus indicating no further evaporation of solvent. The amount of solvent present in the mixture was characterized by its weight fraction, W,, defined by,
W,= M,/M, + M, where M eis the mass of coal and M,is the amount of solvent. Peak integration and glass transition intensities were performed using algorithms supplied in the Mettler DSC controller.
Results and Discussion Table I shows the results of the solvent-induced swelling experiments. The swelling ratios are broadly in line with (9) Hell, P. J.; Thomas, K.M.;Marsh, H. Fuel 1988,67,863.
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Temperature (K) 1. Differential scanning calorimetry at 10 K/min for-
2-fluoropyridine and 3-chloropyridine that had been quenched at 60 K/min from room temperature.
swelling ratios noted by Hall et aL9with the more basic solvents tending to swell coals to a greater extent. NMP swells coals to a much greater extent than the other solvents. The rest of this section will be divided into two parts. To start, the origin of exotherms in DSC of solvents and solvent-swollen coals will be considered. The effect of free solvent on second-order phase transition will then be discussed. Figure 1 shows DSC for 3-chloropyridine and 2-fluoropyridine. Although structurally similar, the two solvents exhibit quite distinct DSC behavior following quenching. The 2-fluoropyridine simply goes through its normal melting endotherm. This is consistent with the liquid forming a crystal state on cooling. The 3-chloropyridine exhibited a second-order phase transition at 155K and an exotherm centered at 195 K. The most reasonable explanation for the evolution of heat under these conditions is a phase change in the system. The formation of a crystal from a disordered glass, cold recrystallization, is the most probable phase change. The interpretation of the 3-chloropyridine is as follows. On quenching from the liquid phase there is not enough time for the liquid to form an ordered crystalline state; rather, it forms a disordered glass. The second-order phase transition represents some change in ordering of the glass. The exotherms may represent transformation of the glassy state into the crystalline state. This is cold recrystallization. The melting endotherm is as expected. Although we have not as yet performed DSC at slower cooling rates, if such experiments were to be performed then the exotherm may be absent. Figure 2 shows the behavior of pyridine and NMP. As with the 2-fluoropyridine, there is no evidence of secondorder phase transitions or exotherms for pyridine. Even on the rapid quenching of the DSC experiments it goes into a crystalline state. This does not exclude the possibility that at higher cooling rates pyridine may form a glass. The NMP seems to exhibit intermediate behavior between t h e pyridine and t h e 3-chloropyridine. Asecondorder phase transition is evident at 170 K but no exotherms occur. Prima facie, there seems to be a contradiction between ~ free Figure 2 and the results of Hall and L a r ~ e n .When NMP was present in the NMP-swollen coals, clear exotherms were observed, in contradiction to Figure 2. It may be the case that when the solvent is quenched in the swollen macromolecular network of the coals the solvent
Antxustegi et al.
1028 Energy & Fuels, Vol. 7,No. 6,1993 14 ’I
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Temperature (K)
Figure 2. Differential scanning calorimetry at 10 K/min for
NMP and pyridine that had been quenched at 60 K/min from room temperature.
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Figure 3. Differential scanning calorimetry at 10 K/min for Pittsburgh No. 8 and 2-fluoropyridine for weight fractions of 0.868 and 0.636 in the mixture.
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Integrated Endotherm (Jig) Figure 5. Integrated exotherms and melting endotherms for
Pittsburgh No. 8 coal mixed with different weight fractions of 2-fluoropyridine.
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is not able to attain the crystalline state. Conversely, the solvent on its own goes into a partially glassy state. A further example of this is 2-fluoropyridine. Figures 3 and 4 show the results of DSC performed on different weight fractions of 2-fluoropyridine with Pittsburgh No. 8 coal. The higher weight fractions shown in Figure 3 show welldefined endotherms absent from Figure 1. A possible explanation is analogous to that for NMP above; Le., when the solvent is in the swollenmacromoleculethere is a higher propensity for it to form a glass when quenched. Figure 4 shows that at a weight fraction of 0.600 of 2-fluoropyridine
there is no free solvent present; i.e., the gel state has been achieved. This is similar behavior to NMP observed by Hall and L a r ~ e n .Note ~ that, as with NMP, the melting endotherms and exotherms disappear identically. More explicit evidence for this is shown in Figure 5 where the integrated exotherms are plotted against the integrated melting endotherms for the 2-fluoropyridine reduction experiments. It can be seen that there is a linear relationship. Therefore, it is argued that the most reasonable explanation for the origin of exotherms in DSC on solvent-swollen coals is the presence of free solvent. We have found no case in which exotherms occur without associated melting endotherms. Most solvents seem to go into the glassy state under the conditions of rapid quenching employed in our DSC experiments. The principal exception seems to be pyridine. We have found well-defined exotherms in certain experiments but they are not easily reproducible, even for different samples of the same coal/solvent system. This suggests that pyridine has a greater propensity to form a crystalline state. It is to be noted that pyridine is of a higher symmetry than the substituted pyridines and NMP. In summary to this part of the discussion, exotherms in DSC are signatures that the free solvent is freezing into a glassy state. Exotherms give no information about the state of “gel” bound solvent. Turning now to the questions concerning the secondorder phase transitions. As mentioned in the Introduction, Hall and Larsen have established the existence of metastable coal/solvent gels. They reported that Illinois No. 6 swollen with a weight fraction of 0.654 of NMP had no melting endotherm for the NMP. In other words, all of the NMP was present in some bound, nonfreezable, form. As mentioned in the Introduction, this system as a whole exhibited a glass transition. Clearly, in this case, the glass transition was a property of the solvent-swollencoal system as a whole and could not have been due to any free solvent. Figures 3 and 4 show the results of a similar experiment performed on Pittsburgh No. 8 swollen with 2-fluoropyridine. Results for two weight fractions, W, = 0.868 and 0.636, are shown in Figure 3. The higher weight fraction exhibits a distinct second-order phase transition, whereas the curve with a weight fraction of 0.636 exhibits no secondorder phase transition. Figure 4 shows DSC for Pittsburgh No. 8 with a weight fraction of 0.600. There are no exotherms or melting endotherms for the solvent. This shows that the gel state, analogous to that of Hall and
Effects of Glass Formation by Solvents
Larsen4 with NMP, has been attained. It is notable that there is no second-order phase transition evident. These results suggest that the second-order phase transition in the W,= 0.868 sample is due to the free solvent. There is an obvious explanation for the absence of a secondorder phase transition at lower solvent concentrations: 2-fluoropyridine is a much weaker base than NMP, it does not swell coals significantly and it does not change the viscoelastic properties of coals significantly. This is consistent with swelling and diffusion data by Hall et al.9 The important implication of this is that the existence of low-temperature second-order phase transitions in DSC of swollen coals can be deceptive. Ideally, investigations of phase transitions of swollen coals need to be performed on coals without free solvent. Comparison of Figures 1and 2 with the DSC results of Hall and Larsen4 and Figure 3 of the present communication reveals that glass transitions for the quenched solvents occur a t the same temperature as for the swollen coals. The implication of this is that phase transitions in the glassy solvents may effectively determine the viscoelastic properties of the swollen coal gels. This is not surprising considering that the swollen NMP gel with Illinois No. 6 consisted of 65.4 wt 7% of solvent. It should be noted that there is an important difference between the low-temperature second-order phase transitions for pyridine and NMP. NMP has a sharp phase transition, which is presumably due to the solvent. Pyridine, which does not go into the glassy state on quenching is associated with relatively broad second-order phase transitions in swollen coals. We have found that on the occasions in which pyridine exhibits an exotherm during DSC, Le., when it goes into aglassy state, relatively sharp exotherms, similar to NMP, are seen. It may be significant that Hall and Larsen4 were unable to produce pyridine-swollen coal in a state in which there was no free pyridine present, although they were able to prove the presence of significant amounts of -bound” or “nonfreezable” pyridine. Although more experimental information is required, in the case of pyridine case it may be that the coal macromolecular structure that dominates the viscoelasticproperties of the swollen system to a greater extent than for NMP.
1.6 1A
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Energy & Fuels, Vol. 7,No. 6,1993 1029
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oa0.6
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OA: I - I I = I . I - I I = I . I = i 100 120 140 160 180 200 220 240 260 280 300
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Figure 6. Differential scanning calorimetry at 10 K/min for Pittsburgh No. 8 and Illinois No. 6 with 3-chloropyridine.
As further examples of sharp glass transitions, Figure 6 shows DSC for Illinois No. 6 and Pittsburgh No. 8 swollen in 3-chloropyridine. Even though there is no free solvent in the systems, second-order phase transitions are evident. Also, the close coincidence of the low-temperature phase transitions for the quenched solvent in Figure 1 (155 K) and the coal gels (148 K for Illinois No. 6 and 158 K for Pittsburgh No. 8) suggests that phase changes in the solvent determines the global viscoelastic properties of the swollen coal gels for both coals.
Conclusions It has been demonstrated that the presence of exotherms in DSC studies of solvent-swollen coals is due to the presence of free solvent. Even relatively simple solvents in swollen coals tend to form glasses rather than crystals when quenched. The formation of glasses by solvents may lead to misleading information about the presence of lowtemperature second-order phase transitions and ideally such transitions should be studied when the coal/solvent system is in the “gel” state. It appears that phase changes of solvents in swollen gels determine the global viscoelastic properties. Acknowledgment. This work was funded by SERC grant GR/H18821. M.M.A. is supported by a University of Strathclyde studentship.
