Klemola and Tuovinen [170] have developed the technology of supercritical extraction applied to the vanillin production process in order to replace extraction with organic solvents and reextraction to aqueous solution. After the air oxidation of lignin under alkaline conditions, the resulting solution is submitted to a supercritical carbon dioxide flow in the range of operation 75-400 bars and 303-373 K extracting vanillin and other chemically related compounds. The vanillin dissolved in CO2 can be recovered by passing the gas flow into a receiver with suitable pressure and temperature conditions. The supercritical extraction can also be associated to the bisulfite treatment for vanillin recovery [171]. These solutions are treated with supercritical CO2 and then the gas flow passes through an aqueous bisulfite solution that dissolves vanillin and liberates the CO2 for reuse. Subsequently, the aqueous solution containing vanillin-bisulfite adducts is acidi­fied with sulfuric acid and heated up to 90°C. After the breakage of adducts by acidification, the aqueous solution is cooled off and the vanillin crystallizes reaching to an appreciable purity.

The final product with up to 85-90% of vanillin can be further purified by successive crystallization and dissolution steps in methanol:water [172], fractional precipitation using magnesium or zinc salts [164] successive liquid-liquid (co-) extractions in alkaline solutions and n-butanol and vacuum distillation with or without an inert [173]. The final purification represents a difficult task because the phenolic impurities have very similar chemical and physical properties to vanillin, such that conventional fractionation techniques are inadequate and only multistage crystallization could lead to a final product of the desired high purity [173]. The main impurities of vanillin obtained from lignin processes mainly consist of vanillin-related species as o-vanillin, 5-formyl vanillin, vanillin acid, and aceto — vanillone. Apart from multiple water-methanol crystallization process, the purified vanillin can be obtained also by one or more crystallizations from water, using charcoal to adsorb last traces of impurities [172, 174]. Ibrahim et al. [136] reported the separation of vanillin from soda lignin, (from the black liquor of oil palm empty fruit bunches) by crystallization based on the solubility of vanillin in ace­tone. Afterwards, they developed the molecular imprinting polymer technique that allowed removing additional impurities in the sample.