Comparative study of gas-phase "dehydration" of alkyl lactates and lactic acid for acrylic acid production over hydroxyapatite catalysts

Gas-phase "dehydration" of methyl (ML) and ethyl lactates (EL) for acrylic acid (AA) production was studied and compared with the dehydration of lactic acid (LA) over serial hydroxyapatite catalysts of varying compositions (HAP(m)-360, molar Ca/P ratio m = 1.58-1.68, calcination temperature: 360 degrees C). The product distribution from both lactic esters was found very similar to that from LA; AA but not methyl (MA) or ethyl acrylate (EA) also appeared as the prevailing product (> 50 mol% by selectivity) from both ML and EL. Compared with the reaction of LA, both esters showed much lower reactivity but higher AA selectivity. Among the two esters EL was less reactive but produced a higher selectivity for AA formation. Correlating the AA selectivity or yield with the catalyst surface acid-base property uncovers that a suitable balance between the surface acidity and basicity (or acidity/basicity ratio) is the key to the lactic ester-to-AA reactions. However, the optimum surface acidity/basicity ratio for producing the highest AA selectivity from both esters was higher than that from LA. These results suggest that the lactic ester-to-AA reactions proceeded by catalytic tandem processes involving a formation of LA (or adsorbed lactate) as the key intermediate at the catalyst surface. Acid-base catalysis responsible for these observations is discussed.