Double-ring silicate anions in tetraalkyl-ammonium hydroxide/silicate solutions; their possible role in the synthesis of silicon-rich zeolites

The Solution Chemistry of Zeolite Precursors

“…Nevertheless, Van Santen et al [78] have reported several observations compatible with the idea of D5R species acting as precursors to the formation of ZSM-5. They note that ZSM-5 is usually grown from solutions for which the OH-/Si ratio is less than 0.5, and that under these conditions significant concentrations of D5R are observed [67]. Consistent with this, Mostowicz and Sand [79] have reported a reduction in the induction time and an increase in the crystallization rate for ZSM-5 formation with decreasing OH-/% ratio.…”

“…Of these three structures, the D4R and the D5R are the most fragile. Groenen et al [67] argue that as the negative charge on these anions increases with increasing pH of the solution, the anions can interact more strongly with the cations in solution. Such interactions produce a high degree of polarization in the anions, and thereby contribute to destabilization of the anions.…”

Effect of Surfactant/Silica Molar Ratios on the Formation of Mesoporous Molecular Sieves: Inorganic Mimicry of Surfactant Liquid-Crystal Phases and Mechanistic Implications

“…There has been speculation that these double fourring, D4R, silicate structures are precursors or secondary building units, SBU, for the formation of zeolites. [33][34][35][36][37][38] However, their hydrothermal stability is poor. For example, the tetramethylammonium cubic octamer is unstable above about 80 °C,28>34'35 suggesting it is probably not an intermediate in the formation of crystalline materials synthesized at temperatures >100 °C and at relatively high concentrations of alkali metal ions.39 Because milder synthetic conditions are required for the formation of M41S materials, the cetyltrimethylammonium octamer version could be an intermediate in the formation of these mesoporous molecular sieves.…”

Hydrothermal Zeolite Synthesis

“…The formation of polysilicate species in close contact with the TPA template, and already containing characteristics of the silicalite-1 framework topology, was proposed long ago [88][89][90][91]. Key to the confirmation of the aggregation model for silicalite-1 formation was the use of a so-called clear solution [92], which is a solution of TPA silicate that is spectroscopically much better accessible than hydrogel systems.…”