Birch fractionation in γ-valerolactone with the emphasis on pulp properties: prehydrolysis, acid-catalyzed, and alkaline-catalyzed concept

Organosolv pulping represents an alternative to the traditional production of pulp, cellulosic fibrous material, allowing the utilization of other pulping streams. Pulping of resistant wood species such as Finnish silver birch (Betula pendula) is generally challenging for an uncatalyzed organosolv system. In this study, we investigate two concepts of organosolv γ-valerolactone (GVL) pulping for the production of dissolving pulp. Silver birch sawdust was fractionated in both a one-step process with sulfuric acid or sodium hydroxide-catalyzed liquor and a two-step process with a prehydrolysis step followed by an alkaline GVL digestion step. The selectivity of delignification and hemicellulose removal in one-stage alkaline-GVL pulping was less efficient compared to uncatalyzed GVL pulping due to the partial consumption of sodium hydroxide solution in the GVL pulping liquor, which hydrolyzed to 4-hydroxyvaleric acid (4-HVA) under alkaline conditions according to the equilibrium conditions. Acid-catalyzed GVL pulping at higher temperature (180 °C) resulted in a pure cellulose fraction with a low amount of lignin and hemicelluloses (<5%), but at the expense of pulp yield. Prehydrolysis removed 50% of the original hemicellulose content, but the overall performance of two-stage pulping did not improve pulping selectivity and thus pulp qualities. Optimum conditions for one-stage acid-catalyzed GVL pulping were identified as 150 °C, 120 min, and 5–10 kg H₂SO₄ per t for the production of dissolving pulp with specifications and macromolecular properties comparable to those of an acid sulfite dissolving pulp.