Catalytic hydrothermal liquefaction of sewage sludge over alumina-based and attapulgite-based heterogeneous catalysts

In this study, Co/γ-Al₂O₃, Co-Mo/γ-Al₂O₃, Co/attapulgite (ATP) and Co-Mo/ATP catalysts were prepared and applied for hydrothermal liquefaction (HTL) of sewage sludge (SS) to produce bio-oil. The properties of the catalysts were characterized by various techniques (N₂ physisorption, ICP, XRD and XPS). The mesoporous nature of synthesized catalysts was conformed. Beside, Co²⁺ and Mo⁶⁺ particles were responsible as the major active species for catalytic HTL reactions. The effect of carrier and metal type, loading of catalyst on product yields and composition of bio-oil was studied. The highest bio-oil yield of 31.36 wt% was achieved in the presence of 10 wt% catalyst loading. The main components of bio-oil were phenols, N-containing compounds and hydrocarbons, and the increased selectivity to hydrocarbons with Co-Mo/ATP was observed. Meanwhile, TGA results showed that Co-Mo/ATP increased the low boiling point fraction (<410 °C) of bio-oil to 81.19 wt%. Additionally, catalytic HTL of SS resulted in higher carbon recovery, increasing from 55.61 to 92.24 wt%. Finally, the potential reaction mechanism of the catalysts on HTL of SS was proposed. Overall, this study provides valuable information for bio-oil production from sewage plant and further HTL implementation with ATP based catalyst, improving energy efficiency and alleviating the energy shortage.