Silicate ionic liquid synthesis of zeolite merlinoite: Crystal size control from crystalline nanoaggregates to micron-sized single-crystals

• Hydrated silicate ionic liquid is obtained by a two-steps procedure from TEOS.
• Coacervation was observed in highly alkaline solution.
• Phase separation by decantation allows extraction of silicate phase.
• Synthesis of purely mineral merlinoite was achieved from clear liquid.
• Large single-crystals are formed in low Al and water content precursors.

A new procedure for zeolite synthesis based on a silicate ionic liquid is reported. The method consists of a two-step precursor preparation. In the first step, a liquid silicate is prepared by hydrolyzing tetraethylorthosilicate (TEOS) with KOH–H2O mixture, which induces coacervation resolved into full phase demixing and subsequent separation from a water–ethanol phase by simple decantation. In the second step, an appropriate amount of an alkaline aluminate is mixed with the first liquid silicate, avoiding therefore precipitation. To the best of our knowledge, this represents the first example of homogeneous liquid synthesis, and not a clear sol or a gel, of an Al-rich zeolite, free of organic template. The approach is exemplified by the synthesis of merlinoite with potassium hydroxide as mineral base and could be extended to other zeolitic systems by varying the chemical composition and the nature of the alkali cation. The synthesis yield depends on the Al content and the crystalline products showed a very large range of particle sizes varying from nanometer scale to about a hundred microns when increasing the Al concentration. Crystallization proceeds most likely through nucleation from a liquid where nucleation and growth directly affects the ultimate crystal size and morphology.

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