Hydrogenation of Coal and Coal-related Materials by Using Molten Salts as Catalysts

Abstract

The most important factors for the conversion of the coal to liquid products are the hydrocracking of its aromatic structures, the cleavage of the weak bonds of coal and the stabilization of its intermediate radicals which lead to the effective depolymerization of coal. Through the authors studies on the hydrocracking of several types of polycyclic aromatic compounds, zinc chloride was revealed that it had relatively low activity for hydrogenation as compared with its high cracking ability.
Based on these imformation, binary eutectic salt of ZnCl₂ and other transition metal chloride which was expected to add hydrogenation ability to ZnCl₂, were used for the liquefaction of Yubari coal under the following conditions: contents of eutectic salt 5 wt% temperature 400°C, duration of reaction 3 h, hydrogen pressure 100 kg/cm². The binary mixture of ZnCl₂-CuCl(4 : 1), ZnCl₂-CrCl₃(4 : 1) and ZnCl₂-MoCl₅(4 : 1) eutectics showed better performance than ZnCl₂ from the standpoint of coal conversion to oil fraction (hexane-soluble portion) (Table 1). Especially, ZnCl₂-MoCl₅(4 : 1) eutectics was the best among the molten salts tested in the experiments in decreasing both gas and coke yield and increasing oil yield. In the hydrocracking of phenanthrene, the highest yield of monocyclic aromatic compounds was obtained by the equimolar mixture of ZnCl₂-MoCl₅(Table 2). Massive amounts of ZnCl₂-KCl-NaCl(3 : 1 : 1) ternary eutectics showed good catalytic effect in affording higher oil yield than that of ZnCl₂ for Yubari coal liquefaction under the reaction condition described above (Table 3).
Referring to physical properties of molten salt along with their intrinsic hydrocracking abilities, discussions were done on the role of such melts in coal liquefaction process. Binary melts, ZnCl₂-KCl(3 : 2) also showed its superiority over ZnCl₂ melts, in suppressing both gas and coke formation and increasing oil yield for the hydrocracking of SRC derived from Taiheiyo coal (Table 4).