Anthropogenic emissions of green-house gases and increasing CO₂ atmospheric concentration are considered the major cause of global warming and ocean acidification. Carbon capture and sequestration turned out to be a valuable strategy to solve these problems, making it urgent to develop novel materials able to selectively capture CO₂. Recently, intriguing new methods and materials have been proposed for CO₂ capture, storage and utilization. In particular, Metal Organic Frameworks (MOFs) show a great potential in various gas separation processes due to their high interaction with guest molecules and adsorption capacity. MOFs exhibit a robust and modular 3D structure characterized by high surface area, low density and unique pore characteristics. Current challenges related to the use of these materials mainly deal with the production of functionalized coatings by inducing a controlled MOFs growth on specific surfaces acting as structural support.
Due to their chemical and thermal stability, porous ceramics attracted the attention as suitable growth substrates. In particular, mullite (3Al₂O₃⋅2SiO₂) is an ideal ceramic refractory material due to its durability, high mechanical strength, creep resistance, low thermal expansion and high thermal shock resistance. Porous mullite structures with tailored porosity, mechanical strength and permeability can be shaped by additive manufacturing techniques. Among the additive technologies used for mullite shaping, Digital Light Processing (DLP) stands out due to its high resolution and dimensional precision in realizing customized tiny complex geometries with superior surface quality.
The object of the present experimental study is the development of a new system for CO₂ capture, based on a mullite substrate fabricated by DLP and properly functionalized with MOFs. HKUST-1 (Cu₃(BTC)₂), one of the most widely studied MOF, was selected due to the characteristic paddlewheel structure able to generate potential binding sites for small molecules. Printable ceramic pastes were obtained by mixing mullite powders, photocurable commercial resin, dispersant and sintering additive in proper amounts to optimize the rheological behaviour, printability and solid loading. The fine-tuning of curing parameters, i.e. layer thickness, LED power, exposure time, was a crucial step to convey samples quality and integrity. Different geometries have been shaped, including bars, pellet, lattice monoliths with two structures, honeycomb and Schwartz primitive triply periodic minimal surface (TPMS). After debinding and sintering of the as-printed samples, mullite substrates were successfully functionalized with MOFs crystals by a two-step solvothermal synthesis process based on crystal nucleation and subsequent growth, until a homogeneous coating was obtained. HKUST-1 coated lattice monoliths were tested in a catalytic bench reactor with different regeneration conditions, gas flows and CO₂ percentages: samples showed an efficient CO₂ adsorption capacity, and the regeneration efficiency leads to reusable and durable systems. Preliminary results showed that the TPMS structure is more efficient in capturing CO₂ because of its higher surface area. Thus, this study demonstrates how the combination between additive manufacturing and MOFs technologies could set the stage for the manufacturing of efficient engineered systems for CO₂ capture.