Apatite Compound Catalysts with Various Constituent Elements: Acid-base Solid Catalysis for Conversions of Ethanol into 1-Butanol and Lactic Acid into Acrylic Acid

Abstract

Hydroxyapatite (Ca-P HAP: Ca₁₀(PO₄)₆(OH)₂) has both high structural stability and strong affinity for organic substances. In addition, the constituent elements can be substituted, such as replacing Ca²⁺ with Sr²⁺ or Pb²⁺, and PO₄³⁺ with VO₄³⁺, to vary the composition ratio (Ca/P) within the range of 1.5-1.7. Highly crystalline apatite compound nanoparticles with various compositions were synthesized by a hydrothermal synthesis method, aiming at continuous control of acid-base catalytic properties. Excellent catalytic performance was demonstrated for the conversions of ethanol and lactic acid, which are representative biomass derivatives. Product selectivity for ethanol conversion was found to strongly depend on the constituent elements, and strontium hydroxyapatite (Sr–P HAP) with a high Sr/P ratio exhibited high selectivity toward 1-butanol. The Sr–P HAP catalyst presumably has moderate acid-base catalytic properties for dehydrogenation, condensation, and hydrogen transfer. In contrast, Ca–P HAP showed superior performance in lactic acid conversion, and in particular, partial substitution of Ca²⁺ with Na²⁺ improved the acrylic acid yield to about 80 C-%. This improvement is mainly attributed to reduction of the acidic sites on the surface, which suppressed the formation of the byproduct acetaldehyde and catalyst deactivation.