A theoretical study of amines adsorption in HMOR by using ONIOM2 method

The two-layered ONIOM method is used to study the interaction of amines (NH₃, MeNH₂, Me₂NH and Me₃N) with H-type mordenite (HMOR). For all the calculations, the high-layer is described by the B3LYP/6-31G(d,p) method, while the HF/3-21G method is used for the low-layer. In the adsorption complexes, proton transfer from the HMOR framework to amines is observed, and the protonated amines (R₃NH⁺) are stabilized by hydrogen bonding between the negatively charged zeolite framework and the N---H bonds. The strength of the OH---N hydrogen bonding is reflected by differences in the N---H stretching frequency of R₃NH⁺ between the adsorbed state and the gas phase. The relative order of the amine basicity on the basis of the computed adsorption energies agrees well with the experiments, but differs from those in the gas phase (proton affinity) and in solvents (pK_a).