Valorization of red macroalgae biomass via hydrothermal liquefaction using homogeneous catalysts

[Display omitted] •Use of alkali catalyst produced 62% deoxygenation in bio-crude from Kappaphycus alvarezii.•99% energy recovery was observed in bio-crude and biochar from Eucheuma denticulatum.•Catalytic treatment resulted in high bio-crude yield (∼16 wt%) from both macroalgae.•High selectivity to cyclic oxygenates (84%) was observed in the bio-crude with K2CO3.•The use of catalysts enhanced the release of sulphates, chlorides, and cations like Na+, K+. Hydrothermal liquefaction of red macroalgae species, Kappaphycus alvarezii (KA) and Eucheuma denticulatum (ED), was performed at 350 °C in the presence of 5 wt% neutral and alkali catalysts like Na2CO3, K2CO3, CaCO3, Na2SO4, NaOH, and KOH. The maximum bio-crude yield of 26.7 wt% and 18.5 wt%, on a dry ash-free basis, was obtained from Na2CO3 treatment of KA and KOH treatment of ED, respectively. The bio-crude from both feedstocks mainly consisted of cyclic oxygenates, whose selectivities were maximum in K2CO3 and CaCO3 treatments. The calorific value of the bio-crude was 38.5 MJ/kg from KA and 30.8 MJ/kg from ED, while that of biochars was 20–24 MJ/kg. A high degree of deoxygenation (64.2%) was observed in bio-crude produced from Na2SO4 treatment of KA biomass. Salts of Cl–, SO42– and K+ constituted the major inorganic portion of the aqueous phase. Maximum energy recovery (99%) was observed from the Na2CO3 treatment of ED.