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

Ab initio molecular orbital calculations (HF/6-31G*) are used to obtain thermodynamical information on the possibility to form NH-bridging or NH2-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 NH2-terminal groups or an oxygen atom substituted by a NH bridging unit. The Bronsted 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 NH2 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 NH2-terminal groups and the NH-bridging will be formed when increasing the temperature. The calculated proton affinities indicate the following order for Bronsted basicity of the nitrogen systems: dimethylamine > H3SiOSiH2NH2 > H3SiNHSiH3 > H3AlOPH2NH2, 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 NH2-terminal groups because the NH stretching on substituted NH-bridging and NH2-terminal Si-O-Si and Al-O-P linkages differ by 100 cm(-1) and 50 cm(-1), respectively. (C) 1998 Elsevier Science B.V. All rights reserved.