Five-Coordinate Silicon in High-Silica Zeolites

Abstract

Pentacoordinated silicon units, SiO4/2F-, were found by solid-state NMR experiments in various as-made high-silica zeolites (Beta, SSZ-23, ITQ-3, ITQ-4, ZSM-12, Silicalite-1) that are prepared in the presence of fluoride ions as mineralizing agents. The SiO4/2F- units are part of the framework, being connected, through sharing of the four O atoms, with four neighboring tetracoordinated SiO4/2 units. These five-coordinate silicon units balance the charge of the cationic organic structure-directing agent. The 29Si chemical shift of these sites is between −140 and −150 ppm, and the 19F chemical shifts are between −56 and −78 ppm. The zeolites SSZ-23, ITQ-4, and Silicalite-1 show a dynamic motion of the fluoride ions at room temperature which is frozen out at a temperature of 130−140 K. In the case of fluoride motion, the 29Si chemical shifts are between −120 and −150 ppm, indicating an exchange between four- and five-coordinate silicon, which means that the fluoride ions exchange between different framework silicon sites. The rigid SiO4/2F- units show a characteristic parameter of the 19F chemical shift anisotropy: the span value, Ω = δ11 − δ33, is between 80 and 87 ppm.

Computational Study of 19F NMR Spectra of Double Four Ring‐Containing Si/Ge‐Zeolites

“…The experimental 19 F span values were used by Koller et al to study the nearest environment and the dynamic motion of the fluoride ion in pure silica zeolites. [13] We think that, analogously to Si-zeolites, the calculated 19 F span and chemical shift anisotropy in Si,Ge,F-zeolites could help to clarify the fluoride ion environment, if these theoretical values are compared to experimental results that may be obtained in forthcoming studies.…”

“…Second, it works as a catalyst in the condensation reactions that lead to the formation of SiÀOÀSi bonds. [5,9] Computational techniques, [10,11] NMR spectroscopy [12][13][14][15][16] and X-ray diffraction [9,[17][18][19] have been used to find the location of the fluoride in zeolites. So far, three general fluoride environments have been reported in pure silica zeolites: 1) Price et al [20] localise F À ions in close proximity to the charged structure-directing agent (SDA) as an ion pair in the main void volume of zeolite, although this result is subject to some controversy, [21] 2) the F À ions are separated from the SDA and are located inside small cages in the zeolite framework, but are not coordinated to any framework atom, [22] and 3) inside small cages but coordinated to a Si atom forming a pentacoordinated [SiO 4/2 F] À unit.…”

Synthesis of Core-shell Structured Porous Materials and Applications as Catalysts and Precursors for Hollow Porous Materials

“…If the mineralizing agent is changed from hydroxyl ion to fluoride ion, reaction gives large particles without defects 15), 16) . Large crystals are thought to be formed if nucleation and crystal growth occur at slower rates over longer times 16) , and occlusion of fluoride ion results in charge balance of the structure-directing agent without requiring the formation of Si _ O -defects 17) . Therefore, the use of fluoride ion provides a simple method for obtaining core zeolite with minimal defects.…”

Towards Inorganic Porous Materials by Design: Looking for New Architectures

“…[ 17 ] When D4R are absent fl uoride is found inside other larger cavities, typically coordinated to a Si of the framework giving rise to pentacoordinated [SiO 4/2 F] − units (sometimes fl uoride is found to jump between different such sites, which thus change coordination between 4 and 5). [ 18 ] The absence of D4R units in pure silica zeolites obtained by the hydroxide route, and its frequent existence in materials prepared by the fl uoride route, with fl uoride occluded in the D4R unit, suggested the hypothesis that fl uoride could exert a "structure-direction" effect towards structures featuring these units. [ 19 ] However, the rationale behind this effect and the apparent paradox that fl uoride would exert it precisely in cases in which there is less evidence for strong Si-F interactions in the fi nal material, i.e., when F is occluded in D4R, remained unexplained until the very recent works by Zicovich-Wilson, Camblor and coworkers.…”