Composites of covalently linked lignin and cellulose from one feedstock.

Future biorefineries must upgrade all constituents of lignocellulosic feedstocks to high-value products. To curb land-use pressure, technologies that upgrade existing side-streams from forestry and agriculture into substitutes for high-impact, fossil-derived materials are urgently needed. Prevailing biomass valorization typically prioritizes either cellulose pulp or lignin; true co-valorization remains uncommon. Here we report a metal-free, ambient-pressure reactive fractionation that concurrently yields high-quality cellulose and a functionalized lignin. The isolated lignin is etherified and subsequently covalently coupled to cellulose via epoxide ring-opening, producing a composite. The materials display mechanical performance equivalent to common packaging materials, with retention under wet conditions, overcoming the intrinsic limitations of hydrogen-bonded cellulose networks. By integrating mild fractionation with chemical upgrading, this strategy simplifies processing, broadens the product slate accessible from residual biomass side streams and advances the substitution of problematic packaging materials. These findings establish a scalable route to whole-biomass co-valorization and wet-tolerant bio-based packaging from residual streams.