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 degrees 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 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 degrees 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 degrees 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.