Enhancing esterification of small molecular acids with alcohols by molten salt hydrates

A novel approach is reported for efficient esterifying low molecular acids with alcohols in molten salt hydrate (MSH) without additional catalyst. A series of MSHs (LiCl, CaCl 2 , ZnCl 2 and ZnBr 2 ) were evaluated for catalytic esterification of chloroacetic acid (CA) with methanol to methyl chloroacetate (MC). The effects of coordination water number also examined. It was found that appropriate coordination water number not only can promote the protonation of the reactant acids, but also enhance the activity of the protons as well as esterification equilibrium towards ester formation. A MC yield of 98.1% with concentration of 92.1% in the organic-phase could be readily achieved after reacting at 80 °C for 2.0 h in CaCl 2 ·4H 2 O. MSH also demonstrated excellent performance in esterification of other acids (acetic, mercaptoacetic, propionic, n -butyric and levulinic acids) with alcohols (ethanol, n-propanol and n-butanol). After removing portion of the water from MSH, it can be reused without any difficulty. The promotion effects of MSH on the esterification reaction can be categorized to three aspects: (1) accelerating the reaction rate due to the enhanced protonation of the reactant acids and reduction in the hydration degree of the protons; (2) facilitating phase-splitting by virtue of the poor solubility in MSH and thus expelling the formed esters into organic-phase, as a result, boosting up esterification conversions; and (3) promoting the esterification reaction equilibrium owing to the strong hygroscopicity of MSH. Economic analysis shows, compared with the conventional esterification of CA with ethanol, in which organic solvents is used as the water-carrying agent, that esterification in MSH could not only accelerate the reaction rate and strengthen the equilibrium conversion, but also save energy input. • Molten salt hydrates (MSHs) are excellent reaction media for esterification of small molecular acids and alcohols. • Enhancing the protonation of the reactant acids and reducing the hydration degree of the generated protons. • Expelling the esters into organic-phase due to phase-splitting, and thus intensifying equilibrium conversion. • Promoting the esterification conversion due to strong hygroscopicity of MSHs. • Saving energy input when esterification in MSH and averting the waste effluent.