Esterification of Lactic Acid by Catalytic Extractive Reaction: An Efficient Way to Produce a Biosolvent Composition

Abstract

A biosolvent composition containing ethyl lactate and biodiesel was directly obtained by catalytic extractive esterification of lactic acid. Since the esterification of organic acids with alcohols is a thermodynamically limited reaction, the methodology consists in conducting the esterification and the ester extraction simultaneously thereby shifting the equilibrium towards more esters. The acid catalyzed esterification was performed in a biphasic solvent system composed of (i) a reactive polar phase which contains the esterification constituents, lactic acid, ethanol and an acid catalyst (ii) an extractive solvent selective of the ester, fatty acid methyl ester (biodiesel). This solvents system increases the ethyl lactate yield of more than 30 %. During the reaction progress, the selective solvent, fatty acid methyl ester, is progressively blended with ethyl lactate providing a mixture of esters, which can be directly used as an efficient composition of biosolvents without further purification. The solid acidic potassium salt of 12-phosphotungstic acid, K_2.5H_0.5PW₁₂O₄₀, was shown to be more efficient than the conventional esterification catalysts, H₂SO₄ or Amberlyst 15. K_2.5H_0.5PW₁₂O₄₀ gives a yield in ethyl lactate higher than 80 mol% after 2 h of reaction in the biphasic solvent system, using a molar ratio ethanol/lactic acid of 3.3 only. Moreover, K_2.5H_0.5PW₁₂O₄₀ shows a remarkable higher activity per protonic sites (factor 30) and is recyclable at least two times without apparent loss of activity. These excellent properties were ascribed to its low density of strong Brønsted acid sites and its adequate hydrophobicity which would make this acid more water tolerant than the conventional esterification catalysts, H₂SO₄ or Amberlyst 15.

Graphical Abstract

Acid catalyzed esterification of lactic acid with ethanol was performed in a biphasic system where biodiesel acts as an extractive solvent of ethyl lactate. This extractive reaction produces directly a composition of biosolvent: biodiesel blended with ethyl lactate.