Ammonium in zeolites: coordination and solvation effects

Abstract

Proton transfer from a zeolitic cluster to NH3 and subsequent coordination of the ammonium cation onto the zeolitic cluster are studied by using ab initio quantum chemical cluster calculations. Proton transfer from the zeolite cluster to NH3 is favorable if, after proton transfer, the resulting NH4+ cation is coordinated to the zeolitic cluster with two or three hydrogen bonds. These structures are referred to as 2H and 3H, respectively. Their adsorption energies the energy needed for the process of proton transfer followed by the binding of the NH4+ cation, are calculated to be -1 14 and -1 13 kJ/mol, respectively. The geometries were optimized at the SCF level and the adsorption energies were calculated at the second-order Mprller-Plesset perturbation theory level (MP2), using the counterpoise correction (CPC) to avoid the basis set superposition error (BSSE). The basis set is the 6-3 1 l+G(d,p)/STO-3G one, which has previously been shown to give proper binding and proton transfer energies. The calculated heats of adsorption compare well with experimental heats of desorption. Proton transfer also occurs when another NH3 molecule is coadsorbed. However, the process of coadsorption is energetically less favorable than the 2H and 3H structures: the adsorption energy per NHs molecule is only -30 kJ/mol. For the clusters the N-H stretching frequencies have been calculated at the SCF level in the harmonic approach. They have been compared with experimental spectra of the NH4+ forms of some zeolites. The N-H stretching region of these spectra can be explained as a superposition of the spectra of the 2H and 3H structures. By comparison of the adsorption energy on a geometry optimized cluster and on a fixed geometry cluster, it was found that the choice of the geometry is important. On enlarging the fixed geometry cluster the adsorption energy remained constant.