Investigation of NaY Zeolite with Adsorbed CO2 by Neutron Powder Diffraction

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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 CO2-containing sample was prepared at 195 K and 0.1 MPa pCO2 (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.