Role of tetrapropylammonium cations in gel-phase silicalite synthesis

Silicalite synthesis from tetrapropylammonium (TPA+) sodium silicate gels was studied by X-ray diffraction, elemental analysis, ion exchange, 29Si magic angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. Based on this information we confirm a hydrogel—solid transformation mechanism for silicalite crystallization. The initial synthesis gel is a highly articulated silicate network containing pockets of water with solvated Na+ and TPA+ cations. As the silica condenses and becomes more hydrophobic, water and solvated cations are expelled. The condensed silicate gel then encapsulates the hydrophobic TPA+ cations in cages resembling the channel intersections of silicalite before X-ray crystalline silicalite is observed. Crystallization occurs within the gel via rearrangement of the TPA+-occluded silicate cages by the breaking and reformation of siloxane bonds into the more stable silicalite structure. Rates of nucleation and crystallization both increase with increasing TPA+ gel content. The amount of silicalite which forms is limited by the amount of TPA+, which must be present in the ratio of one TPA+ per channel intersection.