The energy-minimum conformation calculated by molecular mechanics−molecular dynamics simulation for the asphaltene obtained from the vacuum residue of Khafji crude oils showed that structures aggregated through several noncovalent interactions are the most stable. Changes induced in aggregated structures by heating were investigated using molecular dynamics calculations. The simulation showed that the hydrogen bond between asphaltene molecules dissociated at 523 K, while aromatic−aromatic stacking interactions appeared to be stable. At 673 K, however, some stacking interactions could be disrupted, but some stable aggregates remained even at this high temperature where some decomposition reactions would be expected to occur. Simulations on two model compounds were carried out to investigate the effects of aliphatic chains and polar functional groups on the stability of asphaltene aggregates during heating. Aliphatic chains and polar functional groups contributed to the stability of aggregates; in simulations of “imaginary” structures in which the original structure was modified by removing the aliphatic side chains and then replacing heteroatoms with carbon, dissociation occurred at lower temperatures at to greater extents than for the original structure; van der Waals interactions between aliphatic chains acted cooperatively to stabilize the asphaltene aggregates.

Solvents have two main roles in coal dissolution: (1) donating hydrogen to stabilize free radicals generated upon heating of coal, (2) rendering coal molecules soluble. Both roles have been studied in the commonly used solvent 1,2,3,4-tetrahydroquinoline (THQ), 1-4 which is a highly efficient solvent for dissolving coal molecules and which provides a ready supply of hydrogen to stabilize radicals. However, it has been found difficult to examine these two roles independently; also, their inter-relationship is not well understood.In our previous work, coals were heat-treated in various solvents at temperatures up to 300°C. The yields following dissolution in various hydrogen-donating solvents, such as 1,4,5,8,9,10-hexahydroanthracene (HHA), were found to correlate well with the total amount of hydrogen donated by each solvent. 5,6 After heat treatment in HHA at 300°C, the free radical concentration was found to be much lower than in the raw coal, 7 with hydrogen donation from HHA to the free radicals proving to be a key factor in promoting coal dissolution even at such a low temperature. We also found that a highly polar solvent N-methyl-2-pyrrolidinone (NMP) gave a higher dissolution yield in some low-rank coals than did hydrogen-donating HHA. Furthermore, a mixture of NMP with HHA gave an even higher dissolution yield for some coals than either HHA or NMP alone. 7 The dissolution mechanism in NMP would appear to be quite different from that in HHA, since NMP possesses a limited ability to donate hydrogen under the conditions of dissolution. Instead, NMP appears able to release noncovalent bonds via strong interactions at polar sites in coals. 7 Moreover, NMP interacts with free radicals and thus prevents them from recombining or taking part in further reactions. 8 However, the precise nature of the effect that NMP exercises on hydrogen-donating solvent to donate hydrogen is quite unclear. The goal of the present study was to clarify this solvent's role in hydrogen donation from hydrogen-donating solvent to coals and/or coal free radicals.Banko (Indonesia) coal, with a dry ash-free analysis of C%: 71.3; H%: 5.4%; N%: 1.2%; S%: 0.5%; O%: 21.6% (ash%: 2.4%), was used as the experimental sample, which was ground to <74 µm and dried in vacuo at 80°C for 24 h before use. The solvents employed in heat treatment were nonpolar 1-methylnaphthalene (1-MN), polar NMP, and hydrogen-donating 9,10-dihydroanthracene (DHA); they are from Wako Pure Chemical Industries, Ltd., Japan. Their purity is >97% for 1-MN and NMP, and >95% for DHA. Heat treatment of each coal sample was carried out in a 50-mL autoclave at 300°C for 1 h, as described before. 7 In each run, 1 g of coal and 5 g of solvent (either each independent solvent or various NMP/DHA mixtures) were charged into the autoclave, which was purged with nitrogen 3-5 times and finally pressurized with nitrogen to 5.0 MPa at room temperature. The autoclave was heated to 300°C within one to two minutes. Following heat treatment, the autoclave was cooled in an ice-water bath to roo...