Structure-Directing Effects in Zeolite Synthesis:  A Single-Crystal X-ray Diffraction, ²⁹Si MAS NMR, and Computational Study of the Competitive Formation of Siliceous Ferrierite and Dodecasil-3C (ZSM-39)

The competitive formation of the zeolites ferrierite and dodecasil-3C has been studied under a variety of synthetic conditions involving HF/pyridine as a solvent together with other amines. Single crystal X-ray diffraction measurements on the siliceous zeolite ferrierite product (Si 18 O 36 ·1.9C 5 H 5 N·0.1C 3 H 9 N, space group Pmnn, a = 18.8273(6) Å, b = 14.095(1) Å, c = 7.4318(7) Å, V = 1972.2(3) Å 3, Z = 2, R = 3.1%) reveal that it contains pyridine in both channels. Small amounts of propylamine are found in the main channel. The dodecasil-3C, by contrast, contains pyridine in the larger cavities and propylamine in the smaller ones (Si 17 O 34 ·0.8C 5 H 5 N·0.25C 3 H 9 N, space group I42d, a = 13.639(2) Å, c = 19.512(4) Å, V = 3629.67(6) Å 3, Z = 4, R = 6.8%). Both the synthetic observations and our computer simulations indicate that the dodecasil-3C is thermodynamically more stable than the ferrierite product, which forms under kinetic control at lower temperatures or shorter reaction times. An additional phase, identified as a propylammonium hexafluorosilicate, is also present during the course of the reaction. The propylammonium cation appears to play a dual role: acting as a pH buffer and nucleating the ferrierite phase. The dodecasil-3C phase is probably nucleated by pyridinium ions.