Boosting trace benzene capture for high-efficiency air filtration over hierarchically porous UiO-66-NH2 via a template-free quasi-gelation freeze-drying strategy: Thermodynamic and kinetic insights.

Zirconium-based metalorganic frameworks (Zr-MOFs) have emerged as promising high-efficiency adsorbents for trace-level toxic chemical filtration. However, pristine microporous Zr-MOFs commonly suffer from limited mass transfer kinetics and inferior adsorption performance under humid conditions, which impedes their environmental and respiratory protective applications. Herein, we demonstrate enhanced thermodynamic and kinetic properties for trace benzene capture over a hierarchically porous UiO-66-NH2 architecture (HP-66-NH2-F) engineered via a facile template-free quasi-gelation freeze-drying strategy. A mixed-acid modulator system was incorporated to regulate crystal nucleation and growth balance to form a quasi-gel state, and tert-butanol was employed as an in-situ porogen during freeze-drying without any exogenous templates addition. HP-66-NH2-F features a nanocrystal-assembled hierarchical framework with intraparticle micropores and interparticle ordered hexagonal mesopores and macropores. Systematic thermodynamic and kinetic evaluations were conducted on four UiO-66-NH2 analogues, including HP-66-NH2-F, pristine microporous UiO-66-NH2, and two hierarchical UiO-66-NH2 variants. HP-66-NH2-F exhibited higher static benzene capacity, lower desorption activation energy, and superior benzene/water selectivity under both dry and humid conditions, together with significantly enhanced adsorption kinetics and mass transfer. The adsorption and diffusion rates of benzene within HP-66-NH2-F particles were 23 and 57 times higher than those of the pristine UiO-66-NH2 benchmark. At the fixed bed scale, HP-66-NH2-F achieved a 27-fold enhancement in mass transfer efficiency relative to the latter, leading to more efficient bed utilization and extended benzene breakthrough time, particularly under high-humidity conditions. Structureactivity relationship analysis attributes its excellent capture capability to the synergy of high mesoporosity, reduced particle size, weakened ligand steric hindrance, and unique hierarchical pore architecture.