One Ca²⁺ Site in CaNaY Zeolite Can Attach Three CO₂ Molecules

Abstract

To design efficient CO2 capture materials, it is necessary to ensure a high adsorption capacity. We recently reported that one Na+ site in NaY zeolite can attach two CO2 molecules. However, the process is not suitable for practical use because it proceeds at a low temperature. Here, we present results on CO2 adsorption on CaNaY zeolites, demonstrating that one Ca2+ site can attach three CO2 molecules. The ν3(13CO2) mode arising from the natural 13C abundance allows for easy infrared monitoring of the processes: it appears at 2298, 2294, and 2291 cm–1 for the complexes with one, two, and three CO2 ligands, respectively. The 12CO2 molecules in the polyligand complexes interact vibrationally, leading to the split of the ν3(12CO2) modes. At ambient temperature, Ca2+(CO2)2 complexes predominate at >1 mbar CO2 and triligand species begin to form at 65 mbar. The obtained results show that CaY zeolites can be very effective CO2 capture materials.

Effect of Water on CO2 Adsorption on CaNaY Zeolite: Formation of Ca2+(H2O)(CO2), Ca2+(H2O)(CO2)2 and Ca2+(H2O)2(CO2) Complexes

“…In the pilot communication to this study [ 23 ], we reported that indeed three CO 2 molecules can be attached to one Ca 2+ site in CaNaY. The one-ligand Ca 2+ –OCO species are produced at very low equilibrium pressure.…”

“…The ν 3 ( 13 CO 2 ) satellite band is observed at 2298 cm −1 [ 37 ] and is shifted to 2293.5 cm −1 with coverage increase. In addition, we detected the (ν 1 + ν 3 )( 12 CO 2 ) and (2ν 2 + ν 3 )( 12 CO 2 ) combination modes [ 14 , 23 , 37 ], which at high coverage appeared at 3715 and 3604 cm −1 , respectively (see the upper inset in Figure 3 ). Two more bands were recorded at lower frequencies, at 1379 and 1270 cm −1 (see the lower inset in Figure 3 ).…”

“…The general picture of CO 2 adsorption on dehydrated CaNaY material was reported elsewhere [ 23 ]. However, in order to obtain optimal intensities of the bands, the work concerned mainly results in a sample with low-calcium content and having a large amount of residual sodium.…”

“…Two more bands were recorded at lower frequencies, at 1379 and 1270 cm −1 (see the lower inset in Figure 3 ). They are attributed to the IR forbidden ν 1 mode and the overtone of the ν 2 vibration, respectively [ 14 , 23 , 37 ]. No other bands were detected.…”

“…In this study, we report a comprehensive picture of CO 2 adsorption on CaNaY zeolites and how the process is affected by water vapor. In the pilot communication [ 23 ], we mainly reported the results with a CaNaY sample with a low calcium content in order to achieve an optimal intensity of the IR bands. Here we concentrate on a CaNaY sample with a high exchange degree that demonstrates a good adsorption capacity as well as examine the effect of water on the CO 2 adsorption process.…”