Insight into Deactivation Behavior and Determination of Generation Time of the Hydroxyapatite Catalyst in the Dehydration of Lactic Acid to Acrylic Acid

For the purpose of industrial applications, deactivation behavior of the hydroxyapatite catalyst was studied in the dehydration of lactic acid to acrylic acid. The hydroxyapatite catalyst shows stable acrylic acid selectivity with 140 h on stream. Obvious increases in acetaldehyde and lactide selectivities were observed from 140 to 250 h, which resulted in a decrease of sharp acrylic acid selectivity. The fresh and used hydroxyapatite catalysts were further characterized by techniques including N₂ adsorption–desorption, X-ray diffraction, transmission electron microscopy, ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance, thermal gravity analysis, and temperature-programmed desorption of CO₂ and NH₃. These results indicate that the mechanism of coke deactivation consists of two steps. One is the formation of aliphatic carbon by the deposition of low carbon products with 140 h; the other is the formation of aromatic coke by the condensation of lactide. Also, the best generated time of 140 h was determined.