The crystal structure of dehydrated NaY zeolite (Na-FAU structure type) with and without adsorbed CO2 has been determined at 4K and at room temperature (RT) using neutron powder diffraction techniques. The CO2-containing sample was prepared at -78 °C and 0.1 MPa pCO2 (dry ice sublimation conditions). Our diffraction data provides direct evidence that adsorption of CO2 results in significant migration of the extraframework Na cations in the zeolite structure. At 4 K, 45 of the apparent 76 CO2/cell were located in two crystallographically independent sites bonding to the Na cations (Na10) in the supercage site II. While the CO2 molecule in the first site has a linear configuration interacting with Na10 via one terminal oxygen, the CO2 molecule in the second site appears to have a bent O-C-O configuration (148.3(3)deg), with both oxygen atoms coordinating to two symmetry-related Na10. Using DFT total energy calculations we found that the presence of Na10while slightly facilitates the bending motion for CO2 by decreasing the energy cost for the O-C-O bond angle of 148.3(3)deg by approx. 0.2 eV/CO2. However, this Na-CO2 interaction is not strong enough to cause a 32 bond angle distortion in CO2. It is concluded that the rotational disorder plays a significant role in the appearance of the bent CO2, while a small bending is still possible. Our studies will help to provide a basis for interpreting CO2 adsorption phenomena in NaY and related zeolites.
Citation: Microporous and Mesoporous Materials
Pub Type: Journals
CO2 mitigation, NaY zeolite, crystal structure with CO2, migration of the extraframework Na cations