Combined Production of Polymeric Birch Xylan and Paper Pulp by Alkaline Pre-extraction Followed by Alkaline Cooking

Abstract

Alkaline pre-extraction of birch wood was performed to isolate polymeric xylan and subsequently produce a paper-grade pulp. At 95 °C and 2.5 mol/L NaOH, 7% of wood was transferred to the E-lye as polymeric xylan with an anhydroxylose-lignin ratio of 6.5. Xylan with a weight-average molar mass of 20 kDa was quantitatively precipitated from the solution previously concentrated from 7.4 to 37 g/L. The anhydroxylose-lignin ratio in the carbohydrate fraction increased to 29 g/g upon precipitation. Enzymatic hydrolysis of the commercial birch xylan with Pentopan Mono PG resulted in a uniform xylooligosaccharide product with low xylose content at a yield of 61%. The pre-extracted pulp had excellent papermaking properties but its yield was 4.9% units lower than that of the reference pulp. Commercial potential of the modified process was discussed.