Evaluation of Drying Induced Changes in the Molecular Mobility of Coal by Means of Pulsed Proton NMR Koyo Norinaga,* Haruo Kumagai, Jun-ichiro Hayashi, and Tadatoshi Chiba Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13, W8, Kita-ku, Sapporo 060-8628, Japan Received April 20, 1998. Revised Manuscript Received July 16, 1998 Drying induced changes in the molecular properties of six different as-received coals with water contents ranging from 8 to 60 wt % of their wet weight were investigated on the basis of the mobility of the coal hydrogen and the distribution of different types of water. When dried at 303 K, a brown coal releases water in the following order: free water identical to bulk water, bound water that froze at around 226 K, finally, nonfreezable water that never froze even at 123 K. According to 1 H NMR criteria, a portion of the coal hydrogen was found to be mobile. The amount of the mobile coal hydrogen (C MH ) varied inversely with the amount of the nonfreezable water, while the release of the free and bound water had little effect on the reduction of C MH . For coals with water contents of up to 32 wt %, C MH in the as-received samples agreed well with the hydroxylic hydrogen content, C DH , which was determined by a hydrogen-deuterium exchange technique. However, in coals with higher water content, C MH was approximately twice as great as C DH . Introduction During desorption of water from the bed moist state the coals shrink and on readsorption of water they swell. 1-3 The drying induced shrinkage would ac- company the collapse of the gel-like structure of coal, and thus, it could limit the accessibility of organic solvents 4 and mass transfer into coal matrix in aqueous media. 5 However, there have been few studies that examined the changes in the macromolecular structure of coal that are induced by drying. Moreover, there has been little information available on the relationship between the gel structure and the properties of water within the coal. Generally, water sorbed in or on solid materials, such as coal, have properties that differ from those of bulk water in its normal thermodynamic states. 6-11 The authors 11 classified water sorbed in coals (ranging from brown to bituminous coals) on the basis of its congela- tion characteristics, which were evaluated by a combi- nation of differential scanning calorimetry (DSC) and proton magnetic resonance ( 1 H NMR) techniques. Two different types of freezable water were observed; free water identical to bulk water and bound water that froze at around 226 K. These two types of water account for only 35-78% of the total water content, leaving non- freezable water. Bound water has a lower freezing point and congelation enthalpy than bulk water. The differ- ences in the properties of bulk and bound water would be directly related to the size of a cluster of water molecules, that is, the size of the space in which they are condensed. Since nonfreezable water molecules occur in clusters smaller than the critical size for freezing, 12 this water is likely to be dispersed on a molecular scale. It is expected to be condensed in micropores or bound to specific sites via specified interactions such as hydrogen bonds. Hence, changes in the properties of nonfreezable water must be ex- plained not only in terms of macroscopic phenomena, such as porosity, 9,10 but also molecular interactions between water and coal matrix. 6 The 1 H NMR tech- nique may help to determine the nature of these molecular interactions. 13 The measured transverse relaxation can distinguish molecular structures/lattices on the basis of whether the molecular reorientation rates are below or above approximately 10 5 Hz. In the former and latter cases, the molecular structures are termed rigid or mobile, respectively. If the rate is below approximately 10 5 Hz, the molecular structures are deemed rigid, otherwise they are considered mobile. The * To whom all correspondence should be addressed. Fax: +81-11- 726-0731. E-mail: norinaga@carbon.caret.hokudai.ac.jp. (1) Evans, D. G. Fuel 1973, 52, 186. (2) Deevi, S. C.; Suuberg, E. M. Fuel 1987, 66, 454. (3) Woskoboenko, F.; Stacy, W. O.; Raisbeck, D. The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth-Heinemann, Ltd.: Oxford, 1991; p 152. (4) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384. (5) Gorbaty, M. L. Fuel 1978, 57, 796. (6) Lynch, L. J.; Barton, W. A.; Webster, D. S. Proceedings of 16th Biennial Low-Rank Fuels Symposium; Groenewold, G. H., Ed.; Energy and Environmental Research Center: Montana, 1991; p 187. (7) Lynch, L. J.; Webster, D. S. Fuel 1979, 58, 429. (8) Barton, W. A.; Lynch, L. J. Proceedings of 6th Australian Coal Science Conference, Newcastle, Australia, 1994; p 65. (9) Mraw, S. C.; Naas-O’Rourke, D. F. Science 1979, 205, 901. (10) Mraw, S. C.; Naas-O’Rourke, D. F. J. Colloid Interface Sci. 1982, 89, 268. (11) Norinaga, K.; Kumagai, H.; Hayashi, J. i.; Chiba, T. Energy Fuels 1998, 12, 574. (12) Sheng, P.; Cohen, R. W.; Schrieffer, J. R. J. Phys. C: Solid State Phys. 1981, 14, 565. (13) Lynch, L. J. Magnetic Resonance and Biology; Cohen, J. S., Ed.: John Wiley & Sons: New York, 1983; p 248. 1013 Energy & Fuels 1998, 12, 1013-1019 S0887-0624(98)00087-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/25/1998