Acidic medium synthesis of zeolites - an avenue to control the structure-directing power of organic templates.

This paper deals with the extension of the synthesis field of microporous zeolite-type materials and types of organic structure-directing agents (OSDA) that can be used to promote their crystallization. The highly hydrophilic hexamethylenetetramine (urotropine), with its C/N ratio = 1.5, which is unusual to act as a structure-directing agent in the crystallization of open-framework silica polymorphs, is used to exemplify the novelty of the employed approach. Namely, the protonation of urotropine in an acidic fluorine-containing medium transforms it into a very efficient OSDA that yields dodecasil 3C ( MTN -type). This novel synthesis also allows gaining insights into OSDA-framework interactions in the MTN -type structure. The comprehensive 29 Si and 19 F MAS NMR indicate a small number of point defects of the framework T sites and the multiple bonding of F - ions to Si in a disordered manner. Based on this finding, a unit cell model has been generated using Monte Carlo simulation and validated with Rietveld refinement using experimental powder X-ray diffraction data. In the model, protonated urotropine cations are located in the center of the big hexakaidecahedral [5 12 6 4 ] cages at full occupancy with random orientations. The charge balance is provided by the disordered F - ions.