Structural incorporation of nitrogen into zeolites, and alpos: ab initio molecular orbital calculations on stability and basicity

Abstract

Ab initio molecular orbital calculations (HF/6-31G*) are used to obtain thermodynamical information on the possibility to form NH-bridging or NH 2-terminal groups in amorphous and crystalline materials containing Si–O–Si and Al–O–P structures, such as for instance, zeolites and aluminophosphates. We have employed dimeric model clusters Si–O–Si and Al–O–P which contain NH 2-terminal groups or an oxygen atom substituted by a NH bridging unit. The Brönsted basicity and the softness–hardness of these structures have been determined using the proton affinity and the HOMO energy, respectively. The obtained results indicate that for both Si–O–Si and Al–O–P linkages and from a thermodynamic standpoint, the formation of a NH 2 terminal group is the most favorable process being exothermic by 6–7 kcal mol −1, whereas the substitution of a bridging oxygen by a NH group is endothermic by 27–28 kcal mol −1. Therefore, it appears that at lower temperatures one should preferentially form NH 2-terminal groups and the NH-bridging will be formed when increasing the temperature. The calculated proton affinities indicate the following order for Brönsted basicity of the nitrogen systems: dimethylamine>H 3SiOSiH 2NH 2>H 3SiNHSiH 3>H 3AlOPH 2NH 2, and that the bridged and terminal systems can be classified as strong bases. The calculation of NH stretching frequencies allows to distinguish between NH-bridging will be formed and NH 2-terminal groups because the NH stretching on substituted NH-bridging and NH 2-terminal Si–O–Si and Al–O–P linkages differ by 100 cm −1 and 50 cm −1, respectively.