Paper Sludge as a Prospective Feedstock for Furan Production: A Review of Sludge Characteristics and Pure-Sugar Dehydration

Paper sludge, a major industrial byproduct of pulp and papermaking processes, represents an abundant, low-cost lignocellulosic feedstock for biorefineries.However, its substantial inorganic ash fraction, high moisture content, and batch-to-batch compositional variability significantly hinder its direct chemical valorization.This review evaluates paper sludge as a source of glucose and xylose for subsequent conversion into furan platform chemicals, specifically 5-hydroxymethylfurfural and furfural.As direct studies on the catalytic dehydration of paper sludge remain limited, this review integrates two complementary areas: established studies on ash removal and enzymatic hydrolysis, and the broader literature on catalytic dehydration using pure glucose and xylose as model substrates.Paper sludge characteristics and ash removal strategies are reviewed, focusing on the effects of inorganic components on enzymatic hydrolysis.Studies on catalytic dehydration are examined to establish mechanistic and process-design principles relevant to future applications of paper sludge, with emphasis on Lewis and Brønsted acid catalysts, solvent effects, side reactions, and currently debated dehydration pathways.This review links established upstream sugar recovery studies with mechanistic insights derived mainly from model sugar systems and identifies key knowledge gaps and research priorities for systematically validating paper sludge as a feedstock for future furan production in chemical biorefineries.