Position and configuration of the guest organic molecules within the framework of the ZSM-5 and ZSM-11 zeolites

Abstract

Thermogravimetry (t.g.) and high resolution n.m.r. spectroscopy ( HRNMR), using cross-polarization ( CP) and magic angle spinning ( MAS), enable the characterization of the configuration and conformation of tetrapropylammonium (TPA) and tetrabutylammonium (TBA) or tetrabutylphosphonium (TBP) species within the ZSM-5 and ZSM-11 frameworks respectively. It is observed by thermal analysis that 3.3 to 3.8 TPA entities are normally present per unit cell of the ZSM-5 precursor while for ZSM-11 the corresponding value is 2.6 for TBA and TBP. HRNMR shows a splitting of the terminal methyl groups resonance for all organic species. Differences in chemical shifts are accounted for by different environments, referring to the known structures of the zeolites, and locating these cations at their channel intersections. Specific interactions are suggested, which explain in addition the variations which occur in the other n.m.r. parameters (linewidths and relative intensities). Whilst each channel intersection in zeolite ZSM-5 tends to be occupied by one TPA entity during synthesis, the largest intersections in the ZSM-11 framework (two per unit cell) are preferentially occupied by the bigger TBA and TBP cations.