Investigation of NaY Zeolite with Adsorbed CO₂ by Neutron Powder Diffraction

The crystal structure of dehydrated NaY zeolite (Na-FAU structure type) with and without adsorbed CO₂ has been determined at 4 K and at room temperature (RT) using neutron powder diffraction techniques. The CO ₂-containing sample was prepared at 195 K and 0.1 MPa p_CO2 (dry ice sublimation conditions). Neutron 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)°), with both oxygen atoms coordinating to two symmetry-related Na10. Using DFT total energy calculations we found that the Na-CO₂ interaction slightly facilitates the bending motion for CO₂ by decreasing the energy cost for the 148.3(3)° bond angle by ≈0.2 eV/CO₂. However, this Na-CO₂ interaction is not enough to cause a 32° bond angle distortion in CO₂ (the energy cost of ≈0.66 eV/CO₂). We propose that rotational disorder plays a significant role in the appearance of the bent CO₂, while a small bending is possible. Our studies will help to provide a basis for interpreting CO₂ adsorption phenomena in NaY and related zeolites.