Promoting catalysis and high-value product streams by in situ hydroxyapatite crystallization during hydrothermal liquefaction of microalgae cultivated with reclaimed nutrients

Abstract

Although algal biofuels hold great potential for renewable energy, production costs limit widespread technology adoption. Co-producing high-value products can ensure economic viability. We have discovered subcritical water will simultaneously convert algae, grown with reclaimed nutrients, into pure-phase substituted hydroxyapatite nanocrystals and a petroleum-like biocrude. The hydroxyapatite contains substitutions of carbonate, silicate, and magnesium, and can be easily modified to produce varying ratios of hydroxyapatite and tricalcium phosphate. The crystallization process is shown to undergo a nanoscale hierarchical order from long hexagonal crystals which aggregate to bundles, sheets, and flower-like microstructures. The hydroxyapatite promotes in situ catalytic upgrading of the biocrude product, particularly, the dehydration of fatty acid amides. Overall, in situ oil upgrading provides a superior quality biocrude by reducing the oxygen content to <6.7% and resulting in >96% of the oil boiling below 600 °C. Major compounds found within the biocrude include phenolics and unsaturated hydrocarbons. In addition to heterogeneous catalysis, the hydroxyapatite product has significant promise for biomedical engineering applications. Herein, we demonstrate live cell-adhesion of human Wharton's jelly cells through extended filopodia on the hydroxyapatite product. This discovery establishes a new paradigm for water and nutrient reclamation systems and algae-based fuels and chemicals by producing a versatile high-value product, substituted hydroxyapatite, which can integrate into multiple markets and rapidly improve economic feasibility for algae biofuels.