Successive Vapor-Phase Guerbet Condensation of Ethanol and 1‑Butanol to 2‑Ethyl-1-hexanol over Hydroxyapatite Catalysts in a Flow Reactor

Calcium-containing hydroxyapatites including those modified with magnesium and strontium were prepared and tested as promising catalysts for the sustainable production of higher linear and branched alcohols from bioethanol. The hydroxyapatite catalysts were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy, and temperature-programed desorption of NH3 and CO2. The partial replacement of Ca2+ ions by Mg2+ and Sr2+ in the hydroxyapatite structure was found to cause the deformation of its crystal lattice with the possible formation of amorphous calcium phosphate species. Moreover, there is both a decrease in the number and strength of the surface base sites of the catalysts and a redistribution of the strength of acid sites. The catalysts showed high activity and selectivity during the vapor-phase condensation of both aqueous ethanol into 1-butanol and 1-butanol into 2-ethyl-1-hexanol; however, a gradual deactivation of the catalysts occurs as a result of the blocking of aldol condensation sites. The highest selectivity values for 1-butanol and C4+ oxygenates (57 and 79.2% at 573 K and 0.36 L·Lcat –1·h–1) and for 2-ethyl-1-hexanol (77.5% at 573 K and 0.11 L·Lcat –1·h–1) were achieved over the calcium-containing hydroxyapatite catalyst modified with magnesium, which also showed an advanced resistance to deactivation with increasing time-on-stream.