Interaction between Hydrogen-Donor and Nondonor Solvents in Direct Liquefaction of Bulianta Coal

Solvent plays two very important roles in coal liquefaction: (1) Physically, it serves as a medium for coal transport and heat transfer, and (2) chemically, it serves as an important source of transferable hydrogen (hydrogen donor) and hydrogen shuttle between hydrogen and coal. In this work, eight types of non-hydrogen-donor solvents, namely, decalin, 1-methylnaphthalene, naphthalene, fluorene, anthracene, phenanthrene, pyrene, and fluoranthene, were examined in combination with the hydrogen-donor solvent tetralin in the liquefaction of Bulianta coal to clarify the interactions between the hydrogen-donor and non-hydrogen-donor solvents and the mechanism of hydrogen transfer. In the absence of a catalyst, the mixed solvents of tetralin with phenanthrene, pyrene, and fluoranthene showed favorable effects on coal conversion and oil yield compared with pure tetralin, regardless of whether the reactions were conducted in a H2 or N2 atmosphere. The reactions of non-hydrogen-donor solvents with tetralin showed that phenanthrene, pyrene, and fluoranthene can pick up hydrogen atoms from the donor to produce 9,10-dihydrophenanthrene, 4,5-dihydropyrene, and 1,2,3,10b-tetrahydrofluoranthene, respectively, which are known to be more reactive hydrogen donors than tetralin. In the presence of nanosized iron catalyst, the addition of phenanthrene, pyrene, or fluoranthene to tetralin was found to improve the liquefaction performance in a N2 atmosphere, whereas in a H2 atmosphere, almost the same coal conversion and oil yield were obtained for all types of mixed solvents. This suggests that the transfer mechanisms of hydrogen are different in the cases of N2 and H2 atmospheres in the presence of nanosized iron catalyst; that is, the primary hydrogen-transfer mechanism might be directly from H2 to coal rather than through hydrogen-donor solvents.