Understanding the demetallization of nitrogen-rich hydrothermal liquefaction biocrudes by FTICR mass spectrometry: Recalcitrant effect of metalloporphyrins and basic nitrogenates

Abstract

Higher metallic content in nitrogen-rich hydrothermal liquefaction (HTL) biocrudes is a bottleneck during downstream hydroprocessing, which is necessary for producing drop-in biofuels from urban residues and algal feedstocks. Therefore, this work explores a non-catalytic pathway for effective demetallization of nitrogen-rich HTL biocrudes obtained from sewage sludge and cyanobacteria (generally referred as Spirulina algae). Herein, we utilized five different organic/inorganic acids and comprehensively documented the effect of acid washing on basic nitrogen containing compounds (NCCs), oxygenates and organometallics. HTL biocrudes before and after acid washing were characterized by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS). FTICR MS showed the presence of iron porphyrins in cyanobacteria biocrude, whereas no sign of metalloporphyrins was observed in sludge biocrude. Moreover, cyanobacteria biocrude mostly contains basic NCCs (N1 and N2 species), whereas sludge biocrude apart from basic NCCs mostly contain fatty acids (O2 and O4 species). Due to the presence and absence of metalloporphyrins, acid washing (0.1 M H2SO4) showed ineffective demetallization (3.2 %) for cyanobacteria and effective demetallization 89.2 % for sludge biocrude. For cyanobacteria, 66.3 % demetallization was achieved at 10 M H2SO4 which not only degrades the biocrude but also accounts for 35 % organic loss. However, O1-O4 species in sludge biocrude represent acidic groups such as carboxyl groups; thereby, 20.1 % organic loss is encountered even at 0.1 M H2SO4. For both biocrudes denitrogenation via acid washing remains unfeasible due to high organic loss. Present results indicate that a priori information about organometallics, NCCs, and oxygenates in a given biocrude could provide guidance to select and further optimize the demetallization process.