Dehydration of lactic acid to acrylic acid over alkaline earth phosphates
Résumé
Acrylic acid (AA) is a platform molecule produced at 4Mt/year by selective oxidation of propene, a petrochemical resource whose price is growing quickly because of increasing demand and rarefaction of petroleum. An alternative route consists in dehydration of lactic acid (LA) that can be yielded both by dehydrogenation of glycerol1 or fermentation of sugars2. However, AA is rarely obtained selectively from LA because of easy decarbonylation leading to acetaldehyde and CO. High yields to acrylic acid were obtained using modified zeolithes3,4 but they suffered from coking and probably hydrothermal instability. In the present work, the gas phase conversion of LA over different phosphates was investigated. A complete analysis of the products was obtained by GC completed by GC-MS and GC-2D data. Good catalytic performances were obtained for alkaline earth ortho, pyrophosphates and hydroxyapatites with selectivities to AA reaching 50% near complete conversion. The main by-products were acetaldehyde and CO. These catalysts were stable for at least 24 hours and only small amounts of carbonates and carboxylates were evidenced after reaction. The optimized temperature of 380°C was shown to favour AA desorption without thermal degradation. The presence of residual Na+ cations at the surface of catalysts inhibited the carbonylation route to the benefit of decarboxylation leading to by-producing of H2 that reacts with AA to form propionic acid. Finally, good catalytic performances were also obtained for alkaline earth phosphates using methyl lactate as reactant that yielded mainly methyl acrylate. Furthermore, NH3 and CO2-TPD curves achieved to characterize the acid-base properties revealed that the most efficient catalysts owned only weak and moderated sites. As very similar profiles of the curves were observed (Fig.1a) with molar acid/base balance around 2, it suggested the presence of high proportion of amphoteric sites that could be the active and selective sites for LA dehydration. The surface characterization obtained both from XPS, 1H-31P CP MAS NMR spectra (Fig.1b, c) evidenced the presence of hydroxylated phosphorous rich phase that is XRD amorphous. Its composition was very different to the bulk one with O/P molar ratio close to 3 and a proportion of oxygen involved in hydroxyl groups of 20%. This phase could correspond to polyphosphates in interaction with alkaline earth cations.