Theoretical Study of the Adsorption of Alkylamines in H-Mordenite: The Role of Noncovalent Interactions

The adsorption of NH3 and alkylamines (MeNH2, Me2NH, and Me3N) on H-mordenite was studied using density functional theory (DFT) and the ONIOM method. The adsorption of the probe molecules was investigated with a set of functionals (B3LYP, B3LYP-D3, CAM-B3LYP, LC-omega PBE, and omega B97X-D) properly chosen to verify the importance of noncovalent interaction on the adsorption process. The adsorption energy increases with the size of the amine and with the type of functional used, according to the trend B3LYP-D3, omega B97X-D > CAM-B3LYP, LC-?PBE > B3LYP. The noncovalent contribution to the adsorption energy is higher for Me3N, reaching 92 kJ/mol when B3LYP-D3 is used and 70 kJ/mol for omega B97X-D. Although the adsorption energies for each amine span a wide range of values depending on the functional used, the geometries of equilibrium are quite similar. The adsorption energies obtained by full geometry optimization of the complexes do not differ appreciably from those of single-point calculations using the geometries optimized at the B3LYP:UFF level of calculation, suggesting that for these systems the single-point strategy may be a reasonable alternative to reduce the computational cost of including noncovalent corrections in DFT results. A comparison was also made with the experimental differential heats of adsorption. At low coverage the computed Eads at the T-4 site corrected for the basis set superposition (BSSE) and zero-point vibration energy errors exhibit the same trend of the experimental Delta H-ads with the degree of methylation observed by Lee et al. (J. Am. Chem. Soc.1996, 118, 3262-3268) as opposed to the one by Chen et al. (J. Catal.1994, 146, 257-267). Also, the results obtained at the B3LYP-D3:UFF level of calculation including BSSE corrections strongly suggest that the falloff observed on the Delta H-ads x coverage diagram, normally attributed to site heterogeneity, may be due to a combination of two factors: the number of molecules adsorbed on a given site and the acid site local configuration.