Molecules and Dimers in Zeolites

For adsorption of a series of structurally related molecules in the same zeolite, 

In conclusion, due to its high proton affinity (858 kJ moH) ammonia always is protonated in any H-zeolite. The hypothetical neutral adsorption complex, stabilized by hydrogen bonds, would be about 30 kJ moH less stable [13] as measured by AEp-pdefined in Eq. (22.4). 

The proton affinities (Table 22.1) of methanol (754 kJ moTi) and water (691 kJ moTi) are significantly lower than that of ammonia (854 kJ mol i) and whether or not these molecules are protonated in H-zeolites has created lively debates in the literature. 

Only for methanol adsorption in H-FER [19, 22] did the two DFT studies not lead to a consistent picture. Both simulations find a neutral adsorption complex of methanol in the 10-ring channel. In addition, Stich et al. [22] find a neutral adsorption structure for methanol in the 8-ring channel of FER. However in a dynamics run at 300 K proton transfer occurs and the average structure corresponds to the CH3OH2+ ion with two almost equal distances [22]. It is presently not dear 

All these studies tell us is that it is not enough to look at the stationary points on the PES. Because of the flatness of the potential energy surface and similar energies of neutral and ion-pair adsorption structures, the dynamics of the system at realistic temperatures needs to be considered. The average structures obtained under these conditions may deviate significantly from the equilibrium structures. We will come back to this point in the water adsorption section. 

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