Hierarchically Structured MIL-100(Al) Beads for Diffusion-Optimized Amine Sorbents in Direct Air Capture

Abstract

Direct air capture (DAC) demands sorbents that integrate high affinity for CO₂ at ultradilute concentrations with efficient mass transport and structural compatibility with practical contactors. However, most amine-functionalized metal-organic frameworks (MOFs) are evaluated as fine powders, where shaping into macroscopic bodies introduces transport limitations that suppress amine accessibility and cyclic performance. Here, we report a morphology-engineered strategy to construct hierarchically porous MIL-100(Al) beads that simultaneously enhance amine loading and mitigate intraparticle diffusion resistance.Macroporous γ-alumina beads are sacrificially converted into fully intergrown MIL-100(Al) architectures, followed by diethylenetriamine (DETA) incorporation throughout the macromicroporous network. The resulting beads achieve an amine loading of 2.46 mmol g⁻¹ (1.5 DETA per Al₃ cluster), exceeding that of the powder analogue, while exhibiting approximately two-fold reduced diffusion resistance. At 400 ppm CO₂ and 298 K, the beads deliver a CO₂ uptake of 0.51 mmol g⁻¹ and a six-fold enhancement in dynamic breakthrough capacity relative to DETA-functionalized MIL-100(Al) powder. The partial mass-transfer zone is significantly narrowed, leading to nearly doubled cyclic CO₂ productivity (19.8 vs. 9.5 mg g⁻¹ day⁻¹).Importantly, the hierarchical architecture preserves structural integrity and maintains over 94% capacity after repeated adsorption-desorption cycles. These findings demonstrate that morphology engineering at the synthesis stage provides a viable pathway to reconcile adsorption thermodynamics with transport kinetics, establishing a general design principle for diffusion-optimized, process-ready MOF sorbents for DAC and related separations.