Acrylic acid synthesis from lactic acid over hydroxyapatite catalysts with various cations and anions

• We synthesized substituted hydroxyapatite catalysts by the hydrothermal method.
• Acrylic acid production from lactic acid needed the both acid and basic catalysis.
• The substituted hydroxyapatite catalysts gave acrylic acid from lactic acid.
• M-phosphate hydroxyapatite (M = Ca, Sr) catalysts showed high acrylic acid yields.

We prepared substituted hydroxyapatite catalysts, such as Ca10(PO4)6(OH)2, Sr10(PO4)6(OH)2, Pb10(PO4)6(OH)2, Ca10(VO4)6(OH)2, and Sr10(VO4)6(OH)2, and carried out conversions of lactic acid over the prepared catalysts. The hydroxyapatite catalysts had stabilities for the reaction media under flowing gas including lactic acid and water at 623 K. The hydroxyapatite catalysts exhibited markedly higher acrylic acid yields than those of P2O5/SiO2 and MgO catalysts. In the lactic acid conversions, the relatively acidic catalysts, such as P2O5/SiO2, Ca10(VO4)6(OH)2 and Sr10(VO4)6(OH)2 catalysts, accelerated the formations of acetaldehyde and propionic acid, whereas the relatively basic catalysts, such as MgO and Pb10(PO4)6(OH)2 catalysts, gave large amounts of unidentified products. On the other hand, the moderate acid–base catalysts, such as Ca10(PO4)6(OH)2 and Sr10(PO4)6(OH)2 catalysts, selectively accelerated the dehydration of lactic acid into acrylic acid.

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