Ammonia-Solvated Ammonium Species in the NH4-ZSM-5 Zeolite

Interaction of gaseous ammonia with a NH4-ZSM-5 zeolite (Si/Al=-11.5) was studied by means of infrared (IR) spectroscopy both at constant ambient temperature and in the temperature range 373-573 K. H-bonding of NH3 molecules to the NH4+ species takes place. The interaction is weak and reversible, resembling a solvation process. Spectral evidence shows that only one N-H moiety is actually available, indicating that ammonium ions are tricoordinated to the zeolite inner surface. H-bonded NH3 has an absorption band at 1712 cm(-1), which grows with increasing pressure in two steps: a monosolvated ammonium species is initially formed, evolving to a disolvated species for pressures above 5 mbar. Coordination of the second NH3 molecule takes place at the already coordinated NH3 molecule and not at the ammonium cation. From the changes in intensity of the 1712 cm-1 band with changing temperature under a moderate NH3 equilibrium pressure, the calculated standard enthalpy and entropy of the monosolvation reaction were Delta H-0 = 34(+/- 5) kJ mol(-1) and Delta S-0 = -88(+/- 10) J mol(-1) K-1, respectively. The enthalpy of the second solvation step was calculated from the corresponding equilibrium constant under the assumption of (nearly) the same entropy change for both solvation processes. In agreement with the overall picture, this enthalpy change is small (-15 kJ mol(-1) at the most). Since in a previous work (M. Armandi, B. Bonelli, I. Bottero, C.O. Arean, E. Garrone, J. Phys. Chem. C 2010, 114, 6658) the thermodynamic features of the reaction between bare Bronsted acid sites and NH3 yielding NH4+ species were determined, the data reported herein allow the study of the coexistence of different species in the NH3/H-ZSM-5 zeolite system: 1) unreacted acid Bronsted sites, 2) bare ammonium ions, and 3) variously solvated ammonium species. The relevant description is particularly simple when the overall average coverage is one molecule per site.