Energy & Fuels 1998, 12, 115-119
115
Depolymerization of Subbituminous Coal under Mild Conditions in the Presence of Aromatic Hydrocarbon with Recycable Superacid HF/BF3 Kiyoyuki Shimizu* and Ikuo Saito Energy Resources Department, National Institute for Resources and Environment, AIST, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan Received June 16, 1997. Revised Manuscript Received September 15, 1997X
Hydrogen fluoride, boron trifluoride, and a mixture of them were examined as acid catalysts for solubilization of coal under milder reaction conditions. Hydrogen fluoride or boron trifluoride alone accelerated solubilization of subbituminous coal with toluene at 150 °C to some extent. Mixtures of hydrogen fluoride and boron trifluoride were found to solubilize coal considerably in the presence of toluene at 150 °C for 3 h under autogenous pressure of as low as 2.4 MPa without gaseous hydrogen. Coal solubilization depended on acidity in the reaction, reaction temperature and solvent. High extractability of the treated coal with HF/toluene depended on the cleavage of methylene, and the higher effectiveness of the HF/BF3/toluene system for the depolymerization of coal could be ascribed to more cleavage of ether groups as well as of methylene bridges.
1. Introduction Conventional coal liquefaction which relies basically on the thermal cracking of internuclear bonds requires severe reaction conditions: high temperature (400-500 °C), high hydrogen pressure (15-20 MPa), and a sophisticated hydrogen donor solvent to prevent the produced radicals from recombination leading to retrogressive reactions. Such severe conditions raise the costs of facilities and operation. Acid-catalyzed coal depolymerization has been widely studied as a way to liquefy coal under milder conditions.1-8 However, it is usually very difficult to recover completely the acid catalyst from solids or products with high viscosity. Anhydrous hydrogen fluoride (HF) is a frequently utilized alkylation catalyst for converting light olefins to gasoline in the petroleum chemical industry. HF/BF3, which has been recognized as a Bro¨nsted/Lewis superacid catalyst for Friedel-Crafts reactions: isomerization and separation of m-xylene, and formylation of aromatic compounds on an industrial scale (Mitsubishi Gas Chemical Co., Ltd.). HF/BF3 mixtures are fully recoverable from the product by distillation only, and can be reused, because their boiling points are very low (HF 19.9 °C, BF3 -101 °C). Olah studied coal liquefaction using the HF-BF3-H2 * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Heredy, L. A.; Neuworth, M. B. Fuel 1962, 41, 21. (2) Ouchi, K.; Imuta, K.; Yamashita, Y. Fuel 1965, 44, 205. (3) Larsen, J. W.; Kuemmerle, E. W. Fuel 1976, 55, 162. (4) Shabtai, J.; Oblad, H. B.; Katayama, Y.; Saito, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30 (3), 495. (5) Kumagai, H.; Shimomura, M.; Sanada, Y. Fuel Process. Technol. 1986, 13, 97. (6) Farcasiu, M. Fuel Process. Technol. 1986, 14, 161. (7) Shimizu, K.; Karamatus, H.; Inaba, A.; Suganuma, A.; Saito, I. Fuel 1995, 74, 853. (8) Shimizu, K.; Karamatus, H.; Iwami, Y.; Inaba, A.; Suganuma, A.; Saito, I. Fuel Process. Technol. 1995, 45, 85.
system and HF-BF3-isopentane.9 Gaseous hydrogen and aliphatic compounds do not make complex with HF/ BF3; in contrast, weak basic alkylated aromatic hydrocarbons form complexes with HF and HF/BF3.10,11
toluene + HF + BF3 h [toluene*H ] + BF4Alkylated aromatic hydrocarbons can significantly enhance the acid-catalyzed transalkylation reaction of coal.12 In the present study, solubilization of coal was carried out under lower reaction temperature and lower reaction pressure without gaseous hydrogen and a sophisticated hydrogen donor solvent in order to evaluate recyclable superacid HF/BF3 as catalyst for efficient coal solubilization with aromatic hydrocarbon via an ionic reaction. Chemical structure and behaviors of oxygencontaining functional groups of coal in the acid-catalyzed reaction were studied, applying several analyses. 2. Experimental Section A subbituminous coal, Taiheiyo (C 73.3, H 6.4, N 1.2, O 19.1 wt %, daf, and ash 13.7 wt %) was used, ground to
