Synthesis of all-silica zeolites from highly concentrated gels containing hexamethonium cations

A pure and highly crystalline all-silica EU-1 zeolite has been obtained from the crystallization of gels containing very low water contents in the presence of hexamethonium cations. Decreasing the water content in the gel down to H2O/Si < 1 inhibited the formation of ZSM-48, which is usually observed under more diluted standard crystallization conditions. Moreover, addition of NH4F to the synthesis led to the formation of “half-fluorinated” ITQ-13 in which fluoride anions occupied only the center of D4R cages. In larger cages, the charge of the template was compensated by framework connectivity defects, clearly demonstrating once more the essential role of F− in the formation of D4R units. The formation of such hybrid (F,OH) is particularly interesting from a synthesis point of view, particularly for understanding the respective roles of fluoride and hydroxide anions in the crystallization process.

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► Pure silica EU-1 zeolite has been prepared in the absence of seeds.
► Water plays a key role in the synthesis of EU-1.
► Half-fluorinated ITQ-13 zeolite can be prepared by direct synthesis.