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

Hydroxyapatites (HAP_<i>m</i>-<i>T</i>) of varying molar Ca/P ratios <i>m</i> (1.58–1.69) and calcination temperatures <i>T</i> (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 HAP_<i>m</i>-<i>T</i> sample were affected little by variation of <i>m</i>, its surface acidity decreased but basicity increased with the increase in <i>m</i>. The HAP_<i>m</i>-<i>T</i> sample with a higher <i>T</i> showed a higher crystallinity but lower surface area, acidity, and basicity. The conversion of LA decreased with increasing either <i>m</i> or <i>T</i> of the HAP_<i>m</i>-<i>T</i> catalyst; the selectivity for AA maximized at <i>m</i> = 1.62 but decreased steadily with the <i>T</i> increase. The HAP_1.62-360 sample (<i>m</i> = 1.62, <i>T</i> = 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_<i>m</i>-<i>T</i> catalysts became more or less deactivated after reaction, but the reacted ones could be fully regenerated by in situ treatment with flowing air