Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Synthesis of high-silica zeolites and phosphate-based materials in the presence of fluoride

Henri Kessler
Laboratoire de Matériaux Minéraux
ESA CNRS no. 7016
Ecole Nationale Supérieure de Chimie de Mulhouse
Untversité de Haute Alsace, Mulhouse, France

1. Introduction

The most common mineralizer for silica-based zeolites is the hydroxide ion OH-. The alkaline pH is generally adjusted by addition of an inorganic base or an organic base, especially when an organic species is used as a template. The replacement of the hydroxide anions by fluoride anions as mineralizers makes it possible to obtain zeolites even in slightiy acidic media (pH 5). At such pH values the solubility of silica, for example, increases significantly in the presence of fluoride because of the formation of hexafluorosilicate SiF62- species. Such species were observed in particular in the mother liquor of fluoride Silicalite-1 by 19F and 29Si liquid NMR It can be assumed that the hydrolysis of fluorosilicate anions yields polycondensable hydroxylated species whose condensation leads to the crystalline material. In the synthesis of Borosilicalite-1 the presence of the hydroxyfluoroborate anions BF3OH- and BF2(OH)2 in addition to BF4- and SiF62- was indeed evidenced by 19F NMR [1].

2. Synthesis of silica-based zeolites - Usual synthesis conditions with fluoride

Typically the synthesis mixture is prepared by adding a silica source (such as fume silica, colloidal, precipitated silica), a source of framework elements if necessary (for example, B, Al, Fe, Ga, Ti), an organic species and a fluoride source. The temperatures of crystallization are similar to those used in the synthesis without F- but the crystallization time is generally longer.

The crystals are usually of good quality and the size generally exceeds the values obtained in alkaline type synthesis.

When the crystallization is carried out in the presence of an organic cation, as in the case of silica-rich zeolites, fluoride is generally occluded in the pores of the solid as a compensating negative charge, in addition to the negative framework charge, of the organic cations. Fluoride is essentially completely removed on calcination.

The most common and preferred fluoride sources are NH4F, NH4HF2 or HF. Fluoride may also be combined with the source of framework elements such as in (NH4)2SiF6 or AlF3. H2O, and be released on hydrolysis. The calcination of the as-synthesized material then leads directly to the H form of the zeolite. However, in the case of aluminum-rich starting gels, the sparingly soluble salts NH4AlF4 and (NH4)3AlF6 may be present in the as synthesized solid. They can be dissolved by washing with an aqueous alkaline dimethylamine solution.

Most of the syntheses employing the fluoride route have been carried out in aqueous medium. However, an essentially non-aqueous fluoride route has been developed for the synthesis of large crystals in the mm range using HF-pyridine or HF-alkylamines as mineralizers [2].

3. Synthesis of phosphate-based materials - Usual synthesis conditions with fluoride

In contrast to the alkaline pH values in the conventional synthesis of silica-based zeolites, the usual pH of the reaction mixture for the synthesis of phosphate-based materials such as aluminophosphates and gallophosphates is slightly acidic to slightly alkaline (typically, starting pH = 3-10). Therefore the pH conditions for the synthesis of phosphate-based materials in the presence of fluoride are close to those that would be used in its absence.

Nevertheless, various beneficial effects are observed in the presence of fluoride. The crystallization times are generally shorter and the crystals usually larger and well formed. Thus,in the synthesis of the CHA-type materials SAPO-34 and CoAPSO-34, it was observed that, when F was present, the induction time was smaller (divided by about 3), but the rate of crystal growth was smaller than when it was absent [3]. It was assumed that the presence of fluoride favors the fast production of fewer nuclei, after which crystal growth consumes preferentially but slowly the precursors. The stability of the fluorocomplexes must not be so high that further reaction involving them is inhibited.

Another beneficial effect of the presence of fluoride is the production of a number of phases which do not form in a fluoride-free medium, thus showing a structure-directing role of the fluoride ion. For such phases, fluoride is generally part of the framework bonded to Al or Ga atoms as terminal or bridging species, or even trapped in double-four-ring units.

The preferred fluoride source is HF, the pH being adjusted by addition of an organic base. The presence of NH4+ or Na+ cations is undesired in that case since ammonium or sodium aluminophosphates may be produced, for example AlPO4-l5, NH4Al2OH(PO4)2⋅ 2 H2O, may be produced. On calcination fluoride is removed together with the organic template.

For more information the reader is referred to the article entitled "The Opportunities of the Fluoride Route in the Synthesis of Microporous Materials" by Kessler, Patarmn and Schott-Darie [4].

4. References

[1] F. Hoffner-Marcucdilli, Thesis, Universit~ de Haute Alsace, Mulhouse, 1992
[2] A. Kuperman, S. Nadinti, S. Oliver, G. A. Ozin, J. M. Garcés, M. M. Olken, Nature, 365 (1993) 239
[3] Y. Xu, P. J. Maddox, J. M. Couves, J. Chem. Soc., Faraday Trans., 86 (1990) 425
[4] H. Kessler, J. Patarin, C. Schott-Darie, Stud. Surf. Sci. & Catal., Vol. 85 (1994) 75-113