Direct liquefaction of lower-rank coals and biocoals with magnetically separable catalysts as a sustainable route to fuels

• For the first time: magnetically recoverable catalysts for sulfur-free and sulfur-containing coals.
• The higher the metal dispersion, the better the liquefaction activity of the DCL catalysts for low-rank coals.
• Environmental sustainability: improved biocoal liquefaction.

Direct coal liquefaction (DCL) encompasses the catalytic conversion of coal by hydrogen and high temperatures in presence of a solvent and can be used to produce transportation fuels on a commercial scale. DCL economy suffers from its severe reaction conditions (up to 20 MPa hydrogen pressure and reaction temperatures up to 728 K). Industrial catalysts are of low activity since only cheap disposable iron-based catalysts are used. We tested two groups of highly active low-rank coals, one “bitumen-rich” brown coal with a high aliphatic content and a biocoal from the hydrothermal carbonization (HTC) of biomass employing novel catalysts. The oil yield (up to 71 wt.%) and the oil quality from the direct brown coal liquefaction with a small catalyst amount of nanostructured Co/SiO2 or Co/SiO2/Fe3O4 showed promising results for further upscaling. The liquefaction of the biocoal with low amounts of Ni/TiO2 (oil yields up to 42 wt.%) proved to be a meaningful perspective especially for environmental concerns. Furthermore, Co/SiO2/Fe3O4 and Ni/TiO2 could be recovered and recycled by magnetic gradient, with only small decreases in their catalytic activity.