3D printed mullite monoliths with triply periodic minimal surface (TPMS) architectures functionalized with HKUST-1 for CO2 capture

Abstract

Ceramic porous scaffolds functionalized with Metal Organic Frameworks (MOFs) are promising systems for carbon capture, providing a valuable strategy to decrease CO₂ atmospheric concentration and mitigating the dramatic issues related to global warming. Thus, the present work focuses on the combination of a highly microporous CO₂ adsorbent HKUST-1 coating with porous and interconnected mullite (3Al₂O₃⋅2SiO₂) substrates obtained by a combination of additive manufacturing and impregnation techniques, before a complete characterization of their CO₂-sorption properties. Two triply periodic minimal surface (TPMS) architectures, Schwartz Primitive and gyroid, were fabricated with high resolution and accuracy by Digital Light Processing, using two mullite powders, labelled Mc and Mf, presenting different compositions and particle size distribution. Mullite monoliths were functionalized with a continuous HKUST-1 (Cu₃(BTC)₂) coating. The impact of the type of architecture on the amount of deposited HKUST-1 and the sorption capacity were monitored. MOFs mass intakes reached 4.2 and 3.9 wt% for Mc Schwartz primitive and gyroid respectively. The textural properties and CO₂ sorption capacity of the materials were studied by N₂ and CO₂ sorption at 77 K and 298 K respectively. CO₂ gas chromatography was performed at different temperatures (32 °C–80 °C) and gas flows (10–40 mL/min) using a filled column with the different materials. TPMS monoliths were compared to traditional adsorbent powder bed in terms of pressure drops, permeability, gas speed and retention time normalized by MOFs amount, highlighting the advantages of the shaping approaches with respect to powder beds. High permeabilities were reached (Darcy's coefficient k ≈ 10 x10⁻¹³ m² for Mc Schwartz). Monoliths also promoted CO₂/adsorbent contact time, lowering the gas speed below 1.5 cm/s, compared to 2–5 cm/s, in the case of powder bed. HKUST-1 functionalized TPMS monoliths drastically enhanced the CO₂ retention time normalized by MOFs amount, with values increased by a factor 6, from 7 s/g for the powder bed to 30 s/g and 20 s/g for gyroid and Schwartz primitive scaffolds respectively. This work represents a crucial step forward in the development of hierarchically porous and geometrically complex carbon capture and storage systems. Indeed, the current work goes beyond our previous studies by producing and comparing different TPMS designs and introducing for the first time gas chromatography to demonstrate the advantages of TPMS scaffolds in enhancing CO₂ adsorption efficiency.