Hydrothermal conversion of macroalgae-derived alginate to lactic acid catalyzed by metal oxides†
Alginate derived from macroalgae was evaluated as a biomass feedstock for the production of lactic acid using metal oxides as solid base catalysts under hydrothermal conditions. The CaO catalyst exhibited the highest catalytic performance, yielding about 13% lactic acid at 200 °C for 1 h, while other metal oxide catalysts exhibited little activity. The hydration of CaO to Ca(OH)2 provides Brønsted bases (OH−) in an aqueous medium. The lactic acid yields were proportional to the number of Brønsted bases. The CaO catalyst demonstrated nearly similar activity when it was used for the second time and the spent catalyst was successfully regenerated by calcination. The deactivation of the CaO catalyst during subsequent repeated uses arises from the loss of the available basic sites. Plausible reaction pathways for the catalytic conversion of alginate to lactic acid over CaO are also discussed.