Sustainable Production of Acrylic Acid: Catalytic Performance of Hydroxyapatites for Gas-Phase Dehydration of Lactic Acid

Abstract

Hydroxyapatites (HAPm-T) of varying molar Ca/P ratios m (1.58–1.69) and calcination temperatures T (360–700 °C) were prepared and comprehensively characterized by nitrogen adsorption, TG, XPS, XRD, CO2-TPD, and NH3-TPD and were employed to catalyze the gas-phase dehydration of lactic acid (LA) to produce acrylic acid (AA). While the texture and crystallinity of the HAPm-T sample were affected little by variation of m, its surface acidity decreased but basicity increased with the increase in m. The HAPm-T sample with a higher T showed a higher crystallinity but lower surface area, acidity, and basicity. The conversion of LA decreased with increasing either m or T of the HAPm-T catalyst; the selectivity for AA maximized at m = 1.62 but decreased steadily with the T increase. The HAP1.62-360 sample (m = 1.62, T = 360 °C) was identified as the most efficient catalyst, offering an AA yield as high as 50–62% for longer than 8 h (AA selectivity: 71–74 mol %) under optimized reaction conditions (360 °C, WHSVLA= 1.4–2.1 h–1). Correlating the catalyst performance with its surface acidity and basicity disclosed that the LA consumption rate increased linearly with the acidity/basicity ratio, but volcano-type dependence appeared between the AA production rate and the acidity/basicity ratio, which reveals a kind of cooperative acid–base catalysis for selective AA production. The HAPm-T catalysts became more or less deactivated after reaction, but the reacted ones could be fully regenerated by in situ treatment with flowing air.