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

Hydroxyapatites (HAP m - 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, CO 2 -TPD, and NH 3 -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 HAP m - T sample were affected little by variation of m, its surface acidity decreased but basicity increased with the increase in m . The HAP m - 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 HAP m - T catalyst; the selectivity for AA maximized at m = 1.62 but decreased steadily with the T increase. The HAP 1.62 -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, WHSV LA = 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 HAP m - T catalysts became more or less deactivated after reaction, but the reacted ones could be fully regenerated by in situ treatment with flowing air.