Combining phosphotungstic acid pretreatment with mild alkaline extraction for selective separation of hemicelluloses from hardwood kraft pulp

Abstract In this study, a modified process, namely PTA-4%CCE, consisting of an initial phosphotungstic acid (PTA) catalytic hydrolysis, followed by a low-alkali cold caustic extraction (4%CCE) was investigated for upgrading a bleached hardwood kraft pulp to dissolving pulp via selectively removing hemicelluloses. The results showed that the PTA hydrolysis resulted in degraded hemicelluloses with low molecular weight and an increased fiber accessibility. These favorable changes of fiber morphologies facilitate the alkali penetration inward and hemicelluloses diffusion/removal outward even under a very low alkali concentration (4% NaOH) in the subsequent CCE stage (4%CCE), thus enabling a high hemicellulose removal selectivity (85.2%) while decreasing alkali usage. PTA also showed excellent recyclability/reusability with more than 86% recovery. The cellulose purity and reactivity of resultant pulp were 94.92% and 49.5%, respectively. This study reveals that the modified process provides a sustainable and effective strategy for hemicelluloses separation from cellulose fibers.