Lignin depolymerizes coal at 300 °C

Abstract

Liquefaction of coal requires thermal rupture of its carbon–carbon bonds to form free-radical fragments. Before these radicals can recombine or polymerize they must be stabilized by hydrogen atoms usually supplied by a hydrogen-donating solvent or by gaseous hydrogen. The conditions required for such liquefaction reactions are usually temperatures ≳400 °C and hydrogen pressures of several hundred atmospheres^1,2. Often catalysts can be used to accelerate the breaking of carbon–carbon bonds and the transfer of hydrogen. Ouchi et al.⁵ reported that p-toluenesulphonic acid catalysed the depolymerization of coal. Darlage and Bailey⁶ investigated various solvents and Friedel–Crafts catalysts and found that repolymerization of the coal radical fragments was a predominant reaction when coal liquefaction was conducted in non-phenolic solvents such as toluene, xylene or tetralin. Heating coals in phenol at 425 °C was reported⁷ to cause extensive depolymerization accompanied by rearrangement of hydrogen to form a hydrogen-rich soluble fraction and a hydrogen-poor insoluble fraction. We now report that bituminous coal can be depolymerized at 300 °C by using, instead of phenol, the abundant, renewable raw material lignin which is a major component of all vascular plants. When a mixture of coal and lignin is reacted in a solvent in the presence of an acid catalyst both the coal and the lignin depolymerize in relatively mild conditions to form a filterable liquid product. The presence of the lignin increases substantially the amount of the coal that can be so liquefied.