NH4+ in zeolites : coordination and solvation effects

Proton transfer from a zeolitic cluster to NH3 and subsequent coordination of NH4+ on the zeolitic cluster were studied by using ab initio quantum chem. cluster calcns. 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 2 or 3 H bonds. These structures are referred to as 2H and 3H, resp. Their adsorption energies (the energy needed for the proton transfer process followed by the binding of the NH4+ cation) were calcd. to be -114 and -113 kJ/mol, resp. The geometries were optimized at the SCF level and the adsorption energies were calcd. at the 2nd order Moeller-Plesset perturbation theory level (MP2) by using the counterpoise correction (CPC) to avoid the basis set superposition error (BSSE). The basis set is 6-311 + G(d,p)/STO-3G, which has previously been shown to give proper binding and proton transfer energies. The calcd. heats of adsorption compare well with exptl. heats of desorption. Proton transfer also occurs when another NH3 mol. is coadsorbed. However, the process of coadsorption is energetically less favorable than the 2H and 3H structures; the adsorption energy per NH3 mol. is only -30 kJ/mol. The N-H stretching frequencies were calcd. for the clusters at the SCF level in the harmonic approach. They are compared with exptl. 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. Comparison of the adsorption energy on a geometry optimized cluster and on a fixed geometry cluster showed that the choice of geometry is important. On enlarging the fixed geometry cluster, the adsorption energy remained const