Energy & Fuels 2000, 14, 1121-1122
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Viscoelastic Properties of Concentrated Solution of Coal with N-Methyl-2-pyrrolidinone Koyo Norinaga,* Masahiro Kuniya, and Masashi Iino Institute for Chemical Reaction Science (ICRS), Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received May 24, 2000. Revised Manuscript Received August 1, 2000 Coal-derived materials such as coal extracts and liquefaction products are known to readily associate and form aggregate, or micelles in organic solvents.1 Some coal molecules contain functional groups that have either donating or accepting capabilities.1 These groups form aggregates through hydrogen bonds, ionic attraction, interaction between aromatics, and forth. Therefore, it can be expected that the coal solution would be capable of forming a kind of network structure through these interactions as has been observed in many polymer solution systems. This type of network is called thermoreversible gel, since the associative, or noncovalent interactions forming the junctions where network chains join together, can break and recombine under thermal fluctuation. A systematic study by rheological methods is expected to be a most appropriate approach for getting useful information as to the coal-solvent and coal-coal interactions resulting from the nature of constituent molecular species. The thermoreversible gels exhibit simultaneous elastic and viscous behavior under most conditions, and can be treated as a viscoelastic material. To characterize such materials accurately, both elastic and viscous responses must be measured. Dynamic mechanical analysis is therefore a uniquely powerful method because it measures both properties simultaneously. The present communication reports on the preparation of thermoreversible gels from coal concentrated solution and the viscoelastic properties of the resultant gels. Argonne Premium Upper Freeport and Illinois No. 6 coals2 were dried in a vacuum at 353 K. Solvent extraction of the dried Upper Freeport and Illinois No. 6 coals (hereafter referred to as UF and IL, respectively) follows the procedure of Iino et al.3 UF was extracted with a 1:1 mixture (by volume) of N-methyl-2-pyrrolidinone (NMP) and carbon disulfide (CS2). The mixed solvent extract was extracted with acetone to remove NMP and CS2 that were strongly retained and the residue was further fractionated into pyridine solubles (UFPS) and insolubles. IL was extracted with pyridine and subsequently fractionated into acetone solubles and insolubles (ILPS). The yields of UFPS and ILPS were 18 and 15 wt % on a dry-ashfree basis, respectively, and were used for the gel preparations. A 200 mg of the coal extracts was introduced to glass tube and then a known amount of N-methyl-2pyrrolidinone (NMP) was charged into the glass tube. The tube was connected to a vacuum system and the content * To whom all correspondence should be addressed. Fax: +81-22-2175655. E-mail:
[email protected]. (1) Stenberg, V. I.; Baltisberger, R. J.; Patal, K. M.; Raman, K.; Woolsey, N. F. Coal Science; Gorbaty, M. L.; Larsen, J. W.; Wender, I., Eds.; Academic Press: New York, 1983: Vol. 2, p 125. (2) Vorres, K. S. User’s Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, Illinois, 1993. (3) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639.
was frozen with liquid nitrogen. The freeze-thaw cycles were repeated for several times to remove the reactive gas trapped in the mixture. After the freeze-thaw cycles, 13 kPa of nitrogen was introduced to the tube. The tube was sealed and placed in an air bath kept at 353 K for at least 72 h to ensure a through homogenization of the contents. After the annealing, the tube was cooled to ca. 253 K. The firm gel formation was confirmed by observing that the sample did not flow when the tube was inverted and that the sample was not macroscopically phase separated. The gel samples prepared from UFPS and ILPS are hereafter referred to as UFPS/NMP and ILPS/ NMP, respectively. Rheological measurements of the gel samples were performed using a controlled stress rheometer (Rheometric Scientific Inc., ARES-2KSTD).4 All measurements were made with a 7.85 mm parallel plate geometry. The thickness of the sample was 1.5-1.8 mm. The measurements were carried out over temperature range from 213 to 273 K in 10 K steps, to yield a data set suitable for analysis by time-temperature superposition. At each measurement temperature, the moduli were recorded under temperature equilibration as a function of frequency, in which a shear rate was selected within the linear-viscoelastic region while the frequency increased from 0.05 to 50 Hz in 10 logarithmic steps. The temperature was stable within 0.2 K over the range used in this study. Strain sweeps were previously performed to ensure that the viscoelastic response was linear and strain value of 0.05% was consequently chosen. The NMP to coal extracts mass ratio (S/C) of the sample was determined after the measurement from the mass change observed upon heating at 423 K in a vacuum until it attains a constant weight. The respective S/C for UFPS/ NMP and ILPS/NMP are 1.48 and 1.41. The measured dynamic modulus of rigidity was resolved into storage and loss components. The storage (elastic) modulus G′ represents the portion of the oscillation energy that is stored elastically, whereas the loss (viscous) modulus G′′ represents the energy dissipated by the system. Result of the frequency sweep experiment for ILPS/NMP is shown in the form of log-log plots of G′ and G′′ against frequency ω, in Figure 1. Generally the G′ and G′′ decreases with temperature and lower w. The w dependence of the moduli become more significant with increasing temperature. The data have been analyzed using the reduced variable time-temperature superposition procedure of Ferry5 to yield so-called master curves. Data are reduced to the midrange temperature of the measurements performed for the samples, namely, 243 K and plotted with respect to G′ in Figure 2, in which (4) Norinaga, K.; Iino, M. Energy Fuels 2000, 14, 929. (5) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1961.
10.1021/ef000109s CCC: $19.00 © 2000 American Chemical Society Published on Web 08/26/2000
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Figure 1. Storage (G′, top) and loss (G′′, bottom) moduli of ILPS/ NMP gel (S/C ) 1.41) as a function of both the temperature and the oscillation frequency ω.
Figure 2. Master curves of storage moduli at reduced temperature of 243 K for UFPS/NMP gel and ILPS/NMP gel.
the reduced frequency scales extend from 10-6 to 103105 Hz. Though the application of the time-temperature superposition rule to the concentrated solution of the complex coal-derived materials is questionable at present, a single composite curve can empirically be drawn. The temperature dependence of the shift factor could be well described by Arrhenius equation. The curves have no rubbery plateau regions that are observed in permanently cross-linked polymer or highly entangled polymer melt or concentrated solution. In the frequency or time scales accessible by the present conditions, the gel samples respond differently with varying the oscillation frequency. The weight-averaged molecular masses of UFPS and ILPS are estimated to be 1500 and 1200, respectively, by laser desorption mass spectrometry.4 Coal is known to consist of various types of aromatic rings linked by methylene or ether oxygen groups. As suggested by Painter et al.,6 even though the aromatic rings are linked by small sequences of flexible units, these structure would still not be particularly flexible, since conformational changes in the flexible sequences would require displace(6) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 393.
Communications
ments of the rigid aromatic rings. Thus, the small molecular mass of the extracts and low flexible character of coal molecules make the highly chain entanglement impossible. The associative interactions among coal molecules themselves and between coal molecule and solvent would be key factors for the transient network formations in the coal concentrated solutions. G′ range from 104 to 107 Pa for UFPS/NMP and 106 to 108 Pa for ILPS/NMP, respectively at reduced temperature of 243 K. It is noted that the moduli of the gel formed from ILPS are always larger than that from UFPS at equivalent temperatures and frequencies, though the S/C of both gels are almost equivalent. IR spectra of the gels showed that peaks arisen from stretching vibrations of carbonyls in NMP and from stretching vibrations of hydroxyls in coal extracts shift to lower frequency regions than those arisen from pure NMP and the dried extract samples. This reveals that the hydrogen bonds are formed between coal extracts and NMP. Green and Tobolosky7 proposed the simplest theory of transient networks in which the network junctions can break and recombine by thermal motion of the polymers and/or under applied deformation. Their theory predicts G′∞ ) νeffkBT, where G′∞ is the high-frequency storage modulus, νeff the number of elastically effective chains in the network, kB the Boltzmann constant, and T the absolute temperature. Following this theory, the modulus is proportional to νeff at same temperature. G′ of the ILPS/NMP is approximately an order of magnitude larger than G′ of the UFPS/ NMP at high-frequency regions, suggesting that ILPS/ NMP has more elastically effective chains in the transient network than UFPS/NMP does. The numbers of phenolic hydroxyls in the extracts samples were quantified following the method proposed by Blom et al.8 and to be 0.9 and 3.5 mol/kg extracts for UFPS and ILPS, respectively. The large abundance of hydroxyls in ILPS implies the large probability of hydrogen bonding interactions between coal molecules and/or between coal molecules and NMP: one of the reasons why ILPS/NMP has more elastically effective chains and shows higher stability than does UFPS/NMP. The stable gels that are not macroscopically phase separated can be prepared from the coal extracts and NMP mixtures. Dynamic mechanical responses of the gels over a wide range of the time scales, ca. 10-6-105 Hz, were obtained by the application of time-temperature superposition rule. Theoretical analysis of these viscoelastic behaviors will give information on the macroscopic stress relaxation time. The macroscopic relaxation time can lead to the microscopic lifetime of a temporary junctions which formed by hydrogen bonds or other noncovalent interactions. Further experimental works including the examinations of the effect of O-methylation and hydrogenation of aromatic rings of the extracts on the dynamic modulus of the gels are now in progress and will give new insights into the dynamics of coal-coal and coal-solvent associative interactions. Acknowledgment. This work was supported in part by a “Research for the Future Project” grant from the Japan Society for the Promotion of Science (JSPS), through the 148 Committee on Coal Utilization Technology. EF000109S (7) Green, M. S.; Tobolsky, A. V. J. Chem. Phys. 1946, 14, 80. (8) Blom, L.; Edelhausen, L.; van Krevelen, D. W. Fuel 1957, 36, 135.
