Liquefaction of coal under mild conditions: Catalysis by strong acids, iodine and their combination

Abstract

The concept of using an acid coal depolymerization catalyst with a balancing hydrogenation/ hydrogenolysis function to prevent retrogressive reactions forms the basis of this work. A catalytic system based on combining the superacid CF 3SO 3H (triflic acid) with iodine was examined in coal liquefaction under mild reaction conditions. This system was shown to be effective for three types of coal. For example, CF 3SO 3H (25 wt%) I 2(0.5 wt%) in tetralin as solvent gave 79 ± 2% conversion of a Wyodak coal to 54% asphaltenes and 37% oils after 2 h at 300 °C under 6.8 MPa of H 2 pressure using a batch microreactor. Under identical conditions the acid alone gave only 46% conversion, while iodine alone gave 58%. The combined catalytic system also performed well with toluene as solvent; with the higher rank Illinois no. 6 and Pittsburgh seam coals, best results were obtained with 10 wt% acid 0.5 wt% I 2 . Weaker acids such as p-toluenesulphonic acid and sulphuric acid (98%) were less effective than triflic acid in this catalytic coal conversion. From the dependence of product selectivity on the catalytic system and reaction conditions, and from elemental analyses of liquids and residues and 1H and 13C n.m.r. measurements, it is concluded that the main function of the acid is to enhance coal depolymerization to asphaltenes while the (major) role of iodine in the combined catalytic system is to hydrogenate and hydrocrack the asphaltenes to oils. It is important to note that each catalyst alone, triflic acid as well as iodine, can utilize molecular H 2 to liquefy coal. Finally, the combined catalytic system removed more than 50% of the nitrogen and over 90% of the sulphur of the original coal, as shown for Illinois no. 6 and Pittsburgh seam samples.