Gate-Opening CO2 Adsorption in Cs+-Type CHA/PHI Composite Zeolites Synthesized via OSDA-Free Steam-Assisted Conversion

Zeolites are a group of crystalline, microporous aluminosilicates that can occur both naturally and as synthetic materials. They have unique structure of three-dimensional framework with precisely sized pores and channels. One of the most notable features of zeolites is their ability to act as molecular sieves. This means they can selectively adsorb molecules based on size and shape, making them highly useful in a range of industrial and environmental applications. The uniformity of the pore size in zeolites is critical for this selective adsorption, which can be used for gas separation, water purification, and as catalysts in various chemical reactions. The unique ability of zeolites to adsorb CO₂ has garnered significant attention, especially in carbon capture and storage and reducing greenhouse gas emissions. This property is particularly valuable in efforts to mitigate climate change by capturing CO₂ from industrial processes and power generation before it is released into the atmosphere. The mechanism behind CO₂ adsorption in zeolites involves the interaction between the CO₂ molecules and the internal surface of the zeolite pores. The efficiency of CO₂ adsorption depends on several factors, including the pore size and structure of the zeolite, the presence of metal ions within the zeolite framework, and the temperature and pressure conditions of the adsorption process. The unique CO₂ adsorption capabilities of zeolites and zeolite-like materials make them a subject of intense research, particularly in developing technologies for CO₂ capture and sequestration.  The ability to modify zeolite frameworks through various synthesis methods, including organic structure-directing agents (OSDAs)-free approaches, has expanded their utility and efficiency.  A new study published in ACS Applied Materials & Interfaces led by Professor Shunsuke Tanaka from Kansai University and conducted by Dr. Yuto Higuchi, Sana Miyagawa, Associate Professor Yasunori Oumi, and Associate Professor Satoshi Inagaki, the researchers synthesized and characterized Cs⁺-type CHA/PHI composite zeolites and investigate their CO₂ adsorption behaviors and thermal stability. Their findings offer significant insights into the potential of these composite zeolites for CO₂/N₂ separation applications.

The authors used steam-assisted conversion (SAC) and Cs+ ion exchange, avoiding organic structure-directing agents (OSDAs), to transform FAU-type zeolite into Cs+-type CHA/PHI composite zeolite. They demonstrated the SAC method to enable the successful synthesis of CHA/PHI composite zeolites. The process was efficient, environmentally friendly, and conducive to producing materials with desirable properties for CO₂ capture. The team employed in situ powder X-ray diffraction (PXRD) and Fourier-transform infrared (FTIR) spectroscopy to characterize the synthesized zeolites. The characterization confirmed the successful formation of the Cs⁺-type CHA/PHI composite framework. PXRD analysis revealed the structural transition associated with CO₂ adsorption, and FTIR spectroscopy provided insights into the interactions between CO₂ and the zeolite framework. Moreover, the authors conducted in situ PXRD of CO₂ adsorption to investigate the mechanism of gate-opening adsorption behavior. Additionally, used ideal adsorbed solution theory (IAST) to calculate CO₂/N₂ separation selectivity at different temperatures. The Cs+-type CHA/PHI composite zeolite exhibited gate-opening CO₂ adsorption behavior, a novel feature that enhances its suitability for CO₂ capture applications. The CO₂ adsorption measurements and in situ PXRD analysis suggested that the unique adsorption behavior was due to framework transition or Cs⁺ ions movement within the frameworks, enhancing CO₂ selectivity and adsorption capacity. The CO₂/N₂ separation coefficient was found to be >10,000, significantly higher than that of other zeolites, showcasing the composite’s exceptional selectivity for CO₂. The researchers evaluated the thermal stability of the synthesized zeolites through thermal gravimetric analysis. The Cs+-type CHA/PHI composite zeolite demonstrated good thermal stability, a critical attribute for its potential industrial application in gas separation processes under varying temperatures. They used CO₂ in situ HR-PXRD and FT-IR spectroscopy to explore the mechanism behind the gate-opening CO₂ adsorption. The studies provided evidence for the transition of the PHI framework or movement of Cs+ ions as potential mechanisms for the observed gate-opening adsorption behavior. The findings suggest that the Cs+ ions and the unique framework structure play crucial roles in facilitating the selective adsorption of CO₂ molecules.

 In conclusion, the detailed experimental work led by Professor Shunsuke Tanaka and his team revealed that the Cs⁺-type CHA/PHI composite zeolites synthesized via an innovative OSDA-free steam-assisted conversion method exhibit unique CO₂ adsorption behaviors, high CO₂/N₂ separation selectivity, and good thermal stability. These characteristics make them highly promising materials for applications in CO₂ capture and separation technologies. The research advances our understanding of zeolite framework transitions and adsorption mechanisms and highlights the potential for developing more efficient and environmentally friendly materials for addressing the challenges of CO₂ emissions.