Studies on the Role of Fluoride Ion vs Reaction Concentration in Zeolite Synthesis

Abstract

This work is an experimental response to an intriguing paper recently published by Catlow and co-workers, which looked at the computational feasibility of fluorine location in three different all-silica zeotypes (Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708). The materials were chosen as representative of three unique host locations. Our present work examined the synthesis of zeotypes AST, IFR, and MTT using organo-cations with a strong preference for crystallizing these structures. We studied the effect of reaction time and the H(2)O/SiO(2) reactant ratio. The latter is probably the most important function in these zeolite crystallizations that use HF. As reaction conditions became more dilute, AST gave way to SGT and IFR to MTW as host structures, while the MTT synthesis was invariant. Our reactions were studied in terms of product yield vs time, product organo-cation content, fluorine content, and the representative (29)Si and (19)F NMR spectra for certain samples. A single crystal study was carried out for a sample of MTT. Our results showed that, consistent with other recent studies, low H(2)O/SiO(2) reactant ratios lead to more open framework host structures (i.e., IFR vs MTW), and there is typically a higher uptake of organo-cation and fluorine. The structure may well contain a higher population of 4-rings within the silicate substructure. While MTT that contains no 4-rings was chosen as the best possible candidate to achieve an ion-pair for the organo-cation and fluoride anion within the silicate host, both NMR and single crystal work confirm that fluoride is bonded to a 5-coordinate silica center within the lattice.

The chemistry of phase selectivity in the synthesis of high-silica zeolites

“…In a recent study [79], Zones and coworkers investigated the effect of H 2 O / SiO 2 ratio on the fluoride content of synthesis products using molecule 28. This molecule produces IFR and MTW phases at conditions of low and high dilution, respectively.…”

Syntheses of high-silica zeolites in urea/choline chloride deep eutectic solvent

“…The chemical shift for SiF 6 2-has been reported in several studies and usually falls in the range of -120 to -130 ppm. [24][25][26] According to a previous study, the resonance at -68.5 ppm corresponds to the fluoride that is covalently bonded to Si(9) of the ZSM-5 framework. 27 There is also a weak signal at -78.6 ppm, which is not usually seen in the high-temperature hydrothermally synthesized F-MFI.…”

“…There is also a very weak D r a f t 11 signal at -126.7 ppm that is due to the SiF 6 2-impurity. [24][25][26] The small peak at -81.1 ppm is assigned to F -inside the framework. SEM images show the crystals often aggregate together (Figure 6e).…”

“…The ppm. [24][25][26] DECDMPI (Figure 1b) was also used as SDA to produce ZSM-11 in the same DES. This molecule is known to have the ability of producing phase-pure MEL under hydrothermal and dense gel conditions.…”

“…impurity. [24][25][26] The SEM images (Figures 6k, 6l) of the as-synthesized sample show that the crystals of ZSM-11 are also long needles. Furthermore, there is a greater size distribution in this case: Some crystals are larger than the others (Figure 6k), which is probably due to the use of seeds.…”