Hydrothermal liquefaction of woody biomass in hot-compressed water: Catalyst screening and comprehensive characterization of bio-crude oils

Hydrothermal liquefaction of woody biomass (birchwood sawdust) with and without catalyst was investigated at 300 °C for 30 min. The activities of KOH, FeSO4·7H2O, K2CO3, MgO, synthetic hydrotalcite (HT), and ground colemanite (calcium borate mineral) as catalysts were compared. The alkaline catalysts (KOH, K2CO3 and colemanite) showed the best performance considering the oil yield and solid residue yield. The bio-crude oil yield with KOH was increased to around 40 wt%, more than double the yield of the un-catalyzed operation (∼18 wt%). It also reduced the solid residue yield from approx. 33 to 12 wt%. Among all catalysts tested, the least active catalysts for bio-crude oil production are FeSO4 and MgO. The bio-crude oil products were comprehensively characterized using an elemental analyzer, GC-MS, FT-IR, GPC and TGA. Occurrence of phenol derivatives (mainly 2-methoxy-phenol) and aliphatic compounds increased significantly in presence of catalysts, especially the alkaline ones such as HT and KOH. The GPC results indicate that the oils produced in the presence of catalysts have very similar molecular weights and distribution, which are slightly greater than the oil produced in absence of any catalyst, suggesting that the presence of a catalyst promoted certain condensation/polymerization of the reaction intermediates during the HTL process. The TGA results show that all bio-crude oils are similar with respect to thermal stability, irrespective of the presence or type of catalyst.