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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2022-08, Vol.51 (3), p.11499-1156
• Main Authors: Fu, Guangying, Dib, Eddy, Lang, Qiaolin, Zhao, Haonuan, Wang, Songxia, Ding, Ruiqin, Yang, Xiaobo, Valtchev, Valentin
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Crystal defects; Crystallization; Fluorine; Hexamethylenetetramine; Material chemistry; Monte Carlo simulation; NMR spectroscopy; Point defects; Protonation; Silicon; Synthesis; Unit cell; X ray powder diffraction; Zeolites
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d2dt01554d

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. The acidic-medium protonated urotropine acts as a structure-directing agent for crystallizing |HMTA + 8 ,F − ∼4 |[Si 136(1-2%) O 272 ]-dodecasil 3C- MTN and is located in the big [5 12 6 4 ] cage.