Gas phase dehydration of lactic acid to acrylic acid over alkaline-earth phosphates catalysts

• Pure alkaline-earth ortho, pyrophosphates and hydroxyapatites were prepared.
• Selectivity to acrylic acid reaching 50% was obtained in dehydration of lactic acid.
• The catalysts contained high proportion of sites with same weak strength.
• Acid–base balance close to 1 favored C3 selectivity.
• Alkaline-earth lactates were shown to be adsorbed over the catalysts.

A series of alkaline-earth phosphates were prepared by co-precipitation method using sodium free or sodium containing precursors and evaluated for gas phase dehydration of lactic acid. The catalysts were characterized by BET measurements, X-ray diffraction, chemical analysis, XPS spectroscopy and both NH3 and CO2-TPD. After checking the stability of catalysts under feed, it was shown that selectivity to acrylic acid strongly depended on reaction temperature but not on contact time. At temperature of 380 °C, values ranging from 19 to 49% were measured for the different prepared catalysts. The highest value was reached with Ba3(PO4)2 (55% for C3 products) but selectivities rather close were obtained with different other phosphates suggesting kinetic limitation. Acid–base properties measurements revealed that alkaline-earth phosphates exhibited high proportion of acidic and basic sites with same weak strength. Furthermore, correlation between acrylic selectivity of alkaline earth phosphates and the acid–base balance were clearly established for the first time: selectivity was 50% for balance close to 1 and decreased by factor two increasing this parameter to 2. Finally, FTIR spectra of spent catalysts showed alkaline-earth lactates adsorbed over the catalysts which could be reaction intermediates for dehydration of lactic acid.

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