Monophenols separation from monosaccharides and acids by two-stage nanofiltration and reverse osmosis in hydrothermal liquefaction hydrolysates

• Large separation factors of acetic acid and 2,6-dimethoxyphenol by RO.
• Extremely high separation factors of 2,6-dimethoxyphenol and glucose by NF.
• NF-RO two-stage process separated hydrolysates into three usable fractions.

Through hydrothermal liquefaction (HTL), lignocellulosic biomass is directly hydrolyzed into small organics like monosaccharides, monophenols and acids, which can be used as valuable chemicals by further purification. This study aims at investigating the feasibility of simultaneous separation of monophenols from sugar and acids in model lignocellulosic hydrolysate solution through two-stage nanofiltration (NF) and reverse osmosis (RO) process. The effects of pressure, temperature and pH on the solute retentions and permeate flux were examined with an eight-solute sugar-monophenols-acids model solution. NF-RO (DK-SE) two-stage membrane process was performed to confirm the multistage separation performance of model hydrolysates under an optimal operation condition. Results showed that membranes with higher water permeability had better performance in separation of sugar from monophenols. The decrease in temperature promoted separation of both acetic acid and monophenols from sugar, while higher pressure and lower temperature favored separation of acetic acid from phenols. The pH should be kept at low values to maintain good monophenols simultaneous separation of from sugar and acids. The maximum separation factors of acetic acid over 2,6-dimethoxyphenol and 2,6-dimethoxyphenol over glucose were 99.59 by RO membrane SE and 29.99 by NF membrane DK, respectively. Two-stage membrane process (DK NF+SE RO) was proven to be a feasible way to fractionate model HTL hydrolysates into three parts: incomplete hydrolyzed biomass fragments, monophenols riched concentrate, and acetic acid permeate.