Hydrothermal synthesis of uniform and well-shaped ITQ-29 crystals

Synthesis conditions for the preparation of uniform and well-shaped ITQ-29 crystals, i.e. synthesis time and the chemical composition of the batch such as the contents of water, structure-directing agent, hydrofluoric acid and germanium, were investigated at 423 K. The gels were prepared with molar compositions of (1 - x)SiO₂: xGeO₂: yROH: (0.5 - y)TMAOH: zHF: wH₂O, where x was varied from 0 to 1, y from 0 to 0.5, z from 0 to 0.5, and w from 7 to 80. The crystal morphology and size, phase purity and Si/Ge ratio of the crystals were characterized by SEM, XRD and EDXS, respectively. For Corma's standard recipe, a crystallization time of 5 days is found to be optimum for the synthesis of ITQ-29 crystals in this study. The crystal size increases with increasing dilution; crystal sizes of 1, 7, 12 and 30 μm are obtained for water contents w = 7, 30, 40 and 80, respectively. An appropriate amount of ROH and TMAOH is required to form uniform and phase pure ITQ-29 crystals. High quality ITQ-29 crystals can be formed not only at neutral but also at alkaline and acidic conditions although the crystals morphology and size are different. Without germanium or without silicon in the starting gel, no ITQ-29 crystals can be obtained. Without silicon in the batch, germanium oxide crystallizes as AST crystals. The Si/Ge ratio of the synthesis batch is found to be reflected by the ITQ-29 and AST crystals.