A DFT-D study of the interaction of methane, carbon monoxide, and nitrogen with cation-exchanged SAPO-34

Abstract

Density-functional theory calculations including a semi-empirical dispersion correction (DFT-D) are employed to study the interaction of small guest molecules (CH 4 , CO, N 2 ) with the cation sites in the silicoaluminophosphate SAPO-34. Eight different cations from three different groups (alkali cations, alkaline earth cations, transition metals) are included in the study. For each case, the total interaction energy as well as the non-dispersive contribution to the interaction are analysed. Electron density difference plots are used to investigate the nature of this non-dispersive contribution in more detail. Despite a non-negligible contribution of polarisation interactions, the total interaction remains moderate in systems containing main group cations. In SAPOs exchanged with transition metals, orbital interactions between the cations and CO and N 2 lead to a very strong interaction, which makes these systems attractive as adsorbents for the selective adsorption of these species. A critical comparison with experimental heats of adsorption shows reasonable quantitative agreement for CO and N 2 , but a pronounced overestimation of the interaction strength for methane. While this does not affect the conclusions regarding the suitability of TM-exchanged SAPO-34 materials for gas separations, more elaborate computational approaches may be needed to improve the quantitative accuracy for this guest molecule.

First-Principles-Derived Force Fields for CH4 Adsorption and Diffusion in Siliceous Zeolites

“…The most accurate description is achieved at the RPA level with Hartree–Fock (HF) exchange energies (RPA-HF) . Later, they found similar trends for the small alkanes in protonated CHA. − The overestimation of adsorption energies using PBE-D2 or -D3 and a variety of vdW-DF approaches have also been observed for CH 4 in silicalite, HZSM-5, MOR, ZSM-12, and SAPO-34 zeolites. − …”

“…We chose adsorption of CH 4 in Si-CHA ( R 3̅ m space group) as the benchmark system because the computational cost for this material using its primitive unit cell is lower than that of many other zeolites. For this reason, CHA zeolites have been often used for the benchmarking of QM methods. − Among these studies, Göltl et al investigated the adsorption of CH 4 and other small alkanes in purely siliceous and Na-exchanged CHA at different levels of theory, including the standard density functional Perdew–Burke–Ernzerhof (PBE), dispersion-corrected PBE-D2, van der Waals (vdW)-DF, and the random phase approximation (RPA) in combination with the adiabatic-connection fluctuation-dissipation theorem using wave-functions . They found that the PBE method underestimates the strength of the interactions between these small alkanes and zeolites, whereas PBE-D2 and vdW-DF strongly overestimate the adsorption energies.…”

Exploring Cage Effects of Alkane Adsorption on SSZ-39 (AEI) and SSZ-13 (CHA) Zeolites: A Comparative Study

“…In Na-CHA, a significant portion of the cages is occupied by cations, which reduces the effective cage size and also results in a more negative adsorption enthalpy. 36,41,69,70 In contrast, in Na-SSZ-13 and Si-CHA, the cages are predominantly made from silica tetrahedra, mostly without alumina tetrahedra and their associated cations.…”