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 Si-29 and F-19 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.

Automated summary

This paper explores the synthesis field of microporous zeolite-type materials and discusses the types of organic structure-directing agents (OSDA) that can be utilized for their crystallization. The study demonstrates the novelty of using hexamethylenetetramine (urotropine) as a structure-directing agent for open-framework silica polymorphs. By protonating urotropine in an acidic fluorine-containing medium, it becomes an efficient OSDA that leads to the formation of dodecasil 3C (MTN-type). The author also investigates OSDA-framework interactions in the MTN-type structure and proposes a unit cell model based on experimental data and Monte Carlo simulation.