Hierarchically Porous Cellulose–Lignin–Chitosan Aerogels Reinforced with APP@SiO₂–MEL Hybrids for High-Performance Flame Retardancy

Developing lightweight, eco-friendly, and fire-safe materials is essential for sustainable engineering applications. Here, we report a structurally engineered biomass aerogel constructed from a cellulose–lignin–chitosan skeletal framework reinforced with nanoencapsulated P/N–Si flame-retardant hybrids. Ammonium polyphosphate(APP) was encapsulated within a silica shell and combined with melamine (MEL) to form a multifunctional flame-retardant phase uniformly anchored onto the bio-derived network. The synergistic structural–chemical design yielded a robust, highly porous microarchitecture with enhanced hydrophobicity (water contact angle up to 73.4°), improved thermal stability (T_max increased from 195°C to 234°C), and outstanding flame retardancy (> 74% reduction in peak heat release rate), while maintaining low bulk density. Mechanistic studies reveal that during combustion, APP@SiO₂–MEL promotes phosphorylation–dehydration reactions, triggering early char nucleation and catalytic graphitization in the condensed phase. The resulting P/N-rich, SiO₂-reinforced intumescent char forms a cohesive barrier with an “indirect path” effect, significantly slowing down the transport of heat and flammable volatiles. Simultaneously, released P = O• radicals quench flame-propagating radicals in the gas phase, further suppressing combustion. This dual-phase synergistic mechanism effectively preserves structural integrity under fire exposure. This work provides a scalable, environmentally friendly strategy for producing high-performance, fire-safe biomass aerogels, offering significant potential for applications in thermal insulation, construction safety, and energy storage protection. The combination of renewable bio-based matrices and advanced flame-retardant nanotechnology represents a promising pathway toward next-generation sustainable fire-protection materials.