Synergistic steam explosion-catalytic hydrogenation: A biorefinery strategy for bio-based polyurethane coated fertilizers

The development of controlled-release fertilizers using straw-based polyurethane reduces the fertilizer industry's dependence on petrochemical feedstocks, while enhancing environmental sustainability and cost-effectiveness. This study addresses the dual challenges of low lignocellulose conversion efficiency and poor film-forming capacity in straw-based controlled-release fertilizers through the innovative integration steam explosion (SE) pretreatment with Fe 3 O 4 @SiO 2 -catalyzed hydrogenation. SE modifies the structure of maize straw via high-pressure steam, reducing cellulose crystallinity by disrupting hydrogen bonds and leaching film-formation-inhibiting ions (K + , Ca 2+ , Na + ). Concurrent autohydrolysis of hemicellulose generated acetic acid, driving glycosidic bond cleavage. The Fe 3 O 4 @SiO 2 core-shell catalyst, featuring acid-resistant SiO 2 encapsulation and preserved Fe 3 O 4 active sites, hydrogenates carbonyl intermediates (aldehydes/ketones) into hydroxyl-rich polyols. This process elevates the hydroxyl value from 83.58 to 197.20 mg KOH·g −1 and reduces the residue rate to 5.08 %. The synergistic combination of steam explosion and catalytic hydrogenation enhances the crosslinking density between polyols and isocyanates, extending the nutrient release period of the prepared controlled-release urea from 10 to 56 days. This breakthrough addresses the long-standing challenge of short release periods in bio-based coating materials. Additionally, the substitution of traditional sulfuric acid with phosphoric acid in liquefaction effectively circumvents oxidative side reactions of primary/tertiary alcohols, further densifying the coating structures. This work establishes a sustainable biorefinery paradigm for valorizing agricultural residues into high-performance fertilizer coating materials, while simultaneously addressing agricultural non-point source pollution and reducing dependence on petrochemical resources. • The synergistic pretreatment-hydrogenation strategy effectively overcame lignocellulose recalcitrance. • The Fe 3 O 4 @SiO 2 core-shell catalyst hydrogenated aldehydes/ketones into hydroxyl-rich polyols. • The nutrient release period of bio-based polyurethane-coated urea represents a 4.6-fold improvement.