Direct separation of acetate and furfural from xylose by nanofiltration of birch pretreated liquor: Effect of process conditions and separation mechanism

Lignocellulosic biomass from wood is a potentially rich source of glucose and xylose, which may be used either directly as monosaccharides or function as monomeric building blocks for a range of bioeconomy processes. In this paper we present a forthright separation of monosaccharides from other compounds present in the liquor after thermal biomass pretreatment. The main focus is on separation of xylose from acetate, formate and furans (hydroxymethyl furfural and furfural) by direct nanofiltration of sulphuric acid-hydrothermally pretreated birch liquor (a process optimized for xylose liberation from the biomass). In the study, the performance of eight commercially available flat-sheet membranes with different molecular weight cut-off was compared at 20 and 40 bars of pressure for efficient separation, focusing mainly on xylose retention. The nanofiltration membranes type TS40 (polypiperazine amide) and NF90 (polyamide) with cut-off of around 200 Da were found to be the most suitable for such direct detoxification of the liquor providing xylose retention over 98% and less than 20% retention of the accompanying thermal carbohydrate degradation compounds. The fouling mechanism during separation of the pretreated birch liquor appeared to be mainly cake-layer fouling on the surface of the membrane. Employment of diananofiltration resulted in particularly high purity (over 90%) and recovery (over 99%) of xylose after three diafiltration rounds. Moreover, the selected membranes exhibited great reusability, as the retention of monosaccharides was almost unaltered after five repeated nanofiltration cycles. The data provide new insight into the factors and mechanisms affecting membrane-separation of complex biomass liquors and are of significance for refining of lignocellulosic biomass.