Lipophilic antioxidant dodecyl caffeate preparation by the esterification of caffeic acid with dodecanol using ionic liquid [Hnmp]HSO4 as a catalyst

Abstract

Caffeic acid (CA) is widely found in nature, and has a broad spectrum of biological activities. However, the low hydrophilicity and lipophilicity of CA limited its application. Dodecyl caffeate (DC) is the lipophilic ester of caffeic acid (CA), and also has high antioxidant activity. In this work, CA, used as a substrate, and three ionic liquids with different acidities and H₂SO₄ were used as economic catalysts for DC preparation. The effects of variables on DC yield were investigated and optimized by response surface methodology (RSM). And the kinetic and thermodynamic parameters of the esterification of CA and dodecanol were evaluated. Results showed that lipophilic DC was successfully synthesized using ionic liquid ([Hnmp]HSO₄) as a catalyst. And the optimal conditions by RSM were substrate ratio of 10.2 : 1, IL dosage of 9.8% at 87 °C for 118 min. Under the optional conditions, the maximum DC yield was 94.67 ± 1.32%. The k₀, E_a, ΔH, ΔS, and ΔG were 7.18 × 10⁷ mol (L min)⁻¹, 65.77 kJ mol⁻¹, 63.10 kJ (mol K)⁻¹, −103.80 J (mol K)⁻¹, and 99.78 kJ mol⁻¹ at 363 K, respectively. DC prepared in this work showed a good DPPH radical scavenging activity, which indicated that DC can be used as a potential antioxidant in food and cosmetics.