Production of synthetic vanillin from coconut husk lignin via alkaline nitrobenzene oxidation

Abstract

Vanillin is one of the essential constituents that provide the flavor and aroma characteristics to the vanilla extract. Due to the increasing market demands, the production of synthetic vanillin has gradually developed as an alternative to the complex production of natural vanillin from vanilla orchid beans. Recently, the chemical production of synthetic vanillin from biomass lignin has gained significant attention as the more environmental friendly alternative for vanillin production. Accordingly, lignin isolated from three sources of lignocellulosic biomass which are oil palm empty fruit bunch (OPEFB) fiber, coconut husk and kenaf fiber were studied and compared prior to be used in the vanillin derivation process. Among the biomass studied, the highest lignin fractions were recovered from the coconut husk which exhibits the best characteristics of lignin in term of structure, purity and thermal behavior. Therefore, only the coconut husk lignin was used for further study. Coconut husk lignin was subjected to an alkaline oxidation at 160 ºC for 2.5 hours, using 0.40 mL nitrobenzene as the oxidant. Approximately 2.83 % of vanillin was successfully produced and its presence in the oxidized mixture was identified by using the high performance liquid chromatography (HPLC) and gas chromatography (GC) method. Meanwhile, the crystalline structure of purified vanillin was verified using the Fourier transform infrared (FTIR) spectroscopy and proton nuclear magnetic resonance (H1NMR) method. In order to maximize the production yield of vanillin, several variables were considered for optimization using the response surface methodology (RSM) which includes oxidation temperature, oxidation time and volume of nitrobenzene. A maximum yield of 4.01 % vanillin was obtained from coconut husk lignin at 140 ºC for 2.8 hours using 0.44 mL nitrobenzene. The production yield of vanillin at these optimum conditions was compared with the oxidation of OPEFB and kenaf fibers lignin. The comparison shows the highest yield was obtained for the coconut husk lignin oxidation. As a conclusion, the yield of vanillin was improved at low oxidation temperature.