Reversible CO2 adsorption in preformed solid semiclathrates within porous silica gels

Abstract

Hydrate-based gas separation (HBGS) has emerged as a promising technology for CO2 capture. However, conventional HBGS approaches, which rely on temperature-triggered phase transitions for gas capture and recovery, face challenges due to their irreversible nature and associated inefficiencies. An innovative approach was adopted in the present study to overcome these limitations, which involved preforming solid tetra-n-butylammonium chloride (TBAC) semiclathrates within porous silica gels to enable reversible CO2 encapsulation and release via pressure-driven gas diffusion. A series of analytical techniques confirmed the successful formation of TBAC semiclathrates and demonstrated their effectiveness in pre-combustion CO2 capture. Cryo-scanning electron microscopy provided direct visual evidence of the solid semiclathrate structures occupying the internal pores of the silica gels, as opposed to interstitial spaces. A differential scanning calorimetry analysis revealed no significant heat flow variations during gas adsorption and desorption, indicating the absence of phase transitions and supporting the diffusion-based mechanism. CO2 selectivity was demonstrated with a concentration of approximately 90% within the TBAC semiclathrates, highlighting the preferential occupation of CO2. In situ Raman spectroscopy further validated CO2 encapsulation, as distinct peaks appeared upon gas injection and disappeared following gas ejection. The reversibility of the process was confirmed through 10 consecutive cycles of gas adsorption and desorption; consistent performance was observed across all cycles, which underscored the stability of the semiclathrate structures. These findings offer new possibilities for advancements in HBGS and provide a practical foundation for next-generation CO2 capture solutions.