Non-empirical quantum chemical study of the siting and pairing of aluminium in the MFI framework

Abstract

Ab initio molecular orbital calculations were performed on two types of pentameric model clusters with geometry derived from the known structure of zeolite ZSM—5. The present results confirm earlier conclusions regarding the preferential siting of aluminium at sites T 12 and T 2 in the MFI framework. Random siting of aluminium on such sites may lead to the formation of Al-pairs (Al 12OSiOAl 2); the data suggest further that pairing may be favoured when it occurs along the boundary of the channel parallel to [010]. The average acid strength of isolated aluminium sites is found to be comparable to that of paired Al-sites, in agreement with the observation that acidic sites in the true H-form of ZSM—5 have the same average catalytic activity, independently of SiO 2 Al 2O 3 ratio. Finally, charge distribution analyses indicate the highly covalent nature of zeolite frameworks. The anionic framework behaves as a weak but soft base, which contributes to explaining the high acid strength of zeolites as well as their strong affinity for large and polarizable cations.