Separation and Purification of Vanillin and p-Hydroxybenzaldehyde from Grass Lignins

Abstract

Lignin from renewable sources such as corn residue and grasses can produce a diverse range of products from food grade cellulose to fine chemicals such as vanillin and p-hydroxybenzaldehyde (pHB). Isolating the individual components from a depolymerized lignin mixture is a difficult task. The objective of this work is to experimentally demonstrate one route to isolate and crystallize vanillin and pHB utilizing chromatography followed by a supercritical antisolvent (SAS) process. For method development, simulated mixtures of vanillin and pHB that are typical of those obtained from fast ozonolysis of lignin solution were employed. Such mixtures were loaded onto a chromatography column comprised of a 500 mL silica gel bed and eluted using acetic acid (AcOH) in toluene solvent gradient. At optimal conditions (a solvent gradient of 15 to 45% v/v % AcOH in toluene and flow rate of 15.1 ± 0.1 mL/min), 98.5 ± 2.0% of vanillin and 91.1 ± 1.3% of pHB were recovered in separate fractions. To demonstrate the formation of dry powders of vanillin and pHB using SAS process, model solutions of vanillin or pHB in ethyl acetate (EtOAc) solution were sprayed into a high-pressure vessel containing a supercritical fluid (SCF). The SCF selectively removes the solvent from the solution droplets, precipitating the solute which is retained in the vessel by a fine filter. At the end of spraying, the solvent is eluted from the vessel using a flowing stream of pure SCF. The crystallized solids were then recovered from the vessel and analyzed using Differential Scanning Calorimetry (DSC). Three SCFs, CO2 (Pc = 7.382 MPa, Tc = 31.04°C), ethane (Pc = 4.872 MPa, Tc = 32.17°C), and ethylene (Pc = 5.041 MPa, Tc = 9.19°C) were tested. Predicted critical loci of the binary mixture of each of these SCFs with ethyl acetate displayed a maximum in the mixture critical temperature. By operating the chamber above this maximum, a single fluid phase in the chamber (in addition to the solid phase) is ensured. While all three SCFs were successful in crystallizing vanillin and pHB, the solid yields were highest in the case of ethylene. At the higher supercritical operating temperatures required for CO2 and ethane, the aromatic aldehydes increasingly partition into the SCF+solvent phase decreasing the solid yields. Ethylene with the lowest critical temperature among the tested SCFs, enabled operation at room temperature and milder pressures of 6.9 MPa and yielded the highest recoveries of vanillin (74.8 ± 0.9 %) and pHB (50.8 ± 1.4%) from the vessel. The solute that escapes from the vessel in the SCF phase is partly recovered (~20%) in the condensed solvent. DSC melting point profiles of the starting materials and precipitated solids overlap, confirming that the precipitated solids are pure. These results establish that column chromatography is a viable method to separate vanillin and pHB as separate solvent streams and that the SAS technique is a fast process to obtain dry crystalline powders of pHB and vanillin from their solutions. The results also pave the way for further process optimization and scaleup.