Interaction of ammonia with a zeolitic proton: ab initio quantum-chemical cluster calculations

Abstract

The interaction of NH3 and a zeolitic cluster as well as the protonation of NH3 by zeolitic protons are studied by quantum-chemical calculations on small clusters at different levels of approximation. The focus of the paper is on a comparison of results obtained by the different methods. The clusters are studied at the SCF level as well as at the correlated level. Electron correlation is included through second-order Moller-Plesset perturbation theory. The basis-set superposition error (BSSE) was avoided by using the counterpoise scheme. Monodentate singly bonded NH3, that is NH3 being attached to one oxygen atom, forms a strong hydrogen bond with the zeolitic OH group. This bond has a strength of 60 or 67 kJ/mol, depending on the geometry of the zeolitic cluster. This value is approximately half the experimentally found heat of desorption. For this case, the O⋯N distance is found to be very short (2.74 or 2.73 A) and the intermolecular O-H-N stretching frequency is calculated to be 185 or 193 cm-1. The latter values agree reasonably with experimental data. Upon complexation with NH3, the OH stretching frequency shows a red shift of 551 cm-1. Proton transfer from the zeolitic cluster to NH3 is calculated to be unfavorable by 52 kJ/mol, as long as NH4+ is considered to be monodentate coordinated. The description of the hydrogen-bonded form is only slightly dependent on the basis set used. However, the proton-transfer energy does strongly depend on the basis set used. Electron correlation makes the proton transfer more favorable. The BSSE has a large influence on the description of the structures, especially if electron correlation is included. Although electron correlation has a nonnegligible effect on the proton-transfer energy, some conclusions can be drawn from SCF calculations on doubly and triply coordinated NH4+. The computed energy of adsorption now is approximately twice that computed for the hydrogen-bonded and singly coordinated NH3 and close to experimentally observed values of ammonia adsorption. From these results, it follows that these adsorption modes are prefered over the singly bonded form. These forms are preferred because of the favorable electrostatic stabilization of NH4+ when bonded to the cluster by two or three hydrogen bonds.