Optical Investigations of Intergrowth Effects in the Zeolite Catalysts ZSM-5 and ZSM-8

The essential identity of the framework structures of ZSM-8 and ZSM-5

The crystal structures of TEA-ZSM-8 and its calcined template-free form have been studied by Rietveld analyses based on X-ray powder diffraction data. The results show that the aluminosilicate framework structure of ZSM-8 is essentially identical to that of ZSM-5. The tetraethylammonium molecule could be located in the difference Fourier analysis yielding positions similar to its TPA counterpart in ZSM-5. Lattice constants of ZSM-8 are in the same range as for ZSM-5. Contrary results are related to incorrectly indexed powder patterns. Optical investigations of ZSM-5 and ZSM-8 crystals showed that the zeolites studied here are both twinned exhibiting differently shaped twin individuals.

The plastic crystalline A15 phase of dimethylaminoalane, [N(CH3)2–AlH2]3

A plastic crystalline phase of dimethylaminoalane has been discovered at T > 332 K. The phase transitions solid - plastic phase - liquid are fully reversible. The plastic crystalline phase exhibits a cubic unit cell, space group Pm3[combining macron]n, in which the dimethylaminoalane molecules rotate and adopt a structural arrangement reminiscent of the A15 phase.

Pitfalls in the powder diffraction analysis of zeolites ZSM-5 and ZSM-8

Crystal structure analyses by the Rietveld method have shown that the framework structures of zeolites ZSM-5 and ZSM-8 are essentially identical. Therefore, it is difficult to distinguish the two phases especially when the template-free H forms are studied. In addition, some inconsistencies in publications on the two zeolites aggravate the correct interpretation of the powder diagrams. A powder pattern published for ZSM-8 which indicates significant differences between the lattice constants of ZSM-8 and ZSM-5 is shown to be incorrectly indexed. Correct reindexing gives lattice constants for ZSM-8 matching average ZSM-5 values. Peak splitting of a ZSM-8 reflection at 2θ≈23° (CuKα) has been u…