Extraction and isolation of hydrophobic compounds and tannins from Pinus radiata bark using switchable hydrophilicity solvents

Abstract

Pinus radiata is the dominant species of tree in the New Zealand forestry industry, with 38.3 million m3 of P. radiata trees harvested annually. Bark is removed from these trees when they are processed into logs, resulting in bark waste that is currently burnt to provide energy and heat or used as garden mulch despite its potential as a source of biorenewable chemicals such as waxes and polyphenols. To date, work to extract and isolate these components as part of a bark biorefinery has been affected by extraction selectivity and the energy efficiency of the extraction and isolation procedure, particularly the need to distill the solvent to recover the extract. Switchable hydrophilicity solvents N,N-dimethylcyclohexylamine (DMCHA) and N-methylcyclohexylamine (MCHA) have been explored for the extraction of hydrophobic compounds and tannins from P. radiata bark to enable the near ambient temperature recovery and isolation of these fractions. Fatty acids and alcohols were extracted by DMCHA and MCHA in their hydrophobic form and recovered by switching their polarity using CO2 and water in yields ranging from 1.1 – 4.5% relative to bark mass under optimised conditions. Compared with the use of supercritical CO2 extractions, DMCHA and MCHA showed greater selectivity toward fatty acids compared with resin acids. In their hydrophilic form, DMCHA showed greater selectivity than MCHA towards the extraction and separation of tannins compared with lignin and carbohydrates. Tannins were recovered with DMCHA by the reversible switching process in yields of 18% relative to total bark mass under optimised conditions compared with 5% for MCHA. The ability to recycle the solvents was demonstrated, with consistent results observed for DMCHA.