KJ/mole Figure 5. Framework stability as a function of Al-Al distance related to the location of Ni2+ ions in Ni-Aluminosilicate mordenite. Decreasing means increasing Ni2+ localization preference. The line is drawn as guide for the eye.vations. Firstly, the small differences in the relative lattice energies for Ni2+ located at the four different extra-framework sites indicate hardly any preferences on structural grounds. Secondly, the relative lattice energies of the Ni-alumina mordenites are observed to depend on the existence of specific Al-O-fSi-O^Al sequences in combination with a Ni2+ ion in close proximity (compare relative lattice energies in Tables III and IV). In particular, sequences with TV = 1 turned out to be very stable in comparison to sequences with TV > 1.Not only the value of TV but also the specific location of the Al-O-Si-O-Al sequences in the mordenite structure proves to be important. In this case (TV = 1), our calculations clearly show that Ni2+ is preferentially located in the 8-ring side pocket, followed by location in the 8-ring secondary pore system. Location of Ni2+ in the 12-ring main channel proves to be less favorable. These findings are illustrated in Figure 4.

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