CO2 adsorption by nanosized CHA-type zeolite

International audienceThere is an increasing demand for porous materials able to separate the components of natural gas, mainly CH4 and CO2. Flexible small pore zeolites are interesting candidates due to their high sorption capacity and selectivity of molecules with similar sizes. This selectivity comes from their small pores, large cavities and shapes which can be control by the introduction of different extra-framework cations such as K+, Na+, Cs+. Indeed, the extra-framework cations in specific locations selectively provide access of molecules to pores and cages of the zeolite, depending on the nature of the molecules. The present work reports the characterisation of nanosized low silica CHA-type zeolite during the CO2 adsorption process. It was found that the amount and location of cations combined with the small crystallite sizes had significant influence on the sorption properties of CHA zeolite. Additionally, the in situ FTIR results provide information for the amount of physisorbed vs. chemisorbed CO2 in the CHA samples. Simultaneously the CH4sorption analysis was performed and no evidence for CH4 adsorbed into the CHA cage was found. To structurally elucidate how CO2 are adsorbed in this CHA, X-ray powder diffraction (XRPD) measurements were carried out on dehydrated and CO2-loaded CHA samples at 623K and 298K. Different amount of CO2 was delivered to a high temperature cell, and the change in the intensity of the Bragg peaks was followed; Rietveld analysis for each particular case was performed. The position of cations in the as prepared CHA samples and position of CO2 molecules in the samples subjected to controlled adsorption and desorption were determined.Further CO2 adsorption cycles on the CHA zeolite were carried out at 298 K. The capacity of the CHA zeolite under 10 continuous cycles was fully preserved; no loss of CO2 capacity was measured by TG. Moreover, the crystallinity is totally preserved, thus proving that the CHA zeolite has excellent stability