CO2 Hydrate Formation Kinetics Using Aqueous MOF Ink-Soaked Water-Absorbing Materials

Abstract

Although numerous studies have focused on gas hydrates using water-adsorbent materials, there is a lack of a detailed understanding regarding the role of water-absorbing materials in hydrate formation. This study tested everyday-purpose water-absorbing hygroscopic materials such as textile fabrics, bamboo wipe fibers, and baby diaper foam for their role in CO₂ hydrate formation. These materials are highly hydrophilic, readily available commercially, and affordable and exhibit high water retention capabilities. The kinetics of CO₂ hydrate formation using these water-soaked hygroscopic materials are investigated using a rocking cell reactor under constant ramping and isothermal temperature programs. The study evaluated the influence of material wetness, the presence and concentration of metal–organic frameworks (MOFs) (HKUST-1, MIL-53(Al), and MOF-303) in water, and the temperature on the nucleation temperature, induction time, water-to-hydrate conversion, and total mmol of CO₂ per gram of material. Results indicated that above temperatures exceeding 1 °C, chenille fabrics, bamboo wipes, and polyether polyurethane foam (PPU) did not exhibit significant nucleation temperatures or trends. Conversely, at temperatures below 0 °C, only PPU-based CO₂ hydrate studies demonstrated rapid pressure drops, confirming high water-to-hydrate conversion. PPU materials soaked in water-based MOF ink showed induction times lower than those in water or SDS solution. Among the water-based MOF inks, MOF-303 ink exhibited the best stability, CO₂ induction times, and total CO₂ captured in hydrates. PPU material performance was due to embedded superabsorbing polymers (SAP) into nonwoven fabrics, which improved the contact area between liquid and gas compared to those studies where SAP was used in powdered form. Furthermore, PPU materials demonstrated high water retention, even after multiple cycles of formation and dissociation. Comparative benchmarks against other wet solid porous materials showed that PPU achieved a maximum CO₂ uptake in hydrates of approximately 32 mmol per gram of material at an initial starting pressure of 30 bar and when temperature reached <0 °C, representing competitive mmol CO₂/gram with respect to other materials. The authors propose that PPU and similar high-performance hygroscopic materials embedded with SAP or similar water-absorbing materials could serve as the surface material of a moving bed. They proposed a conceptual layout for a novel moving bed reactor for continuous CO₂ capture and separation.