Laboratoire de Matériaux Minéraux
ESA CNRS no. 7016
Ecole Nationale Supérieure de Chimie de Mulhouse
Untversité de Haute Alsace, Mulhouse, France
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 .
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
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 .
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
. 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
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 .
 F. Hoffner-Marcucdilli, Thesis, Universit~ de Haute Alsace, Mulhouse, 1992
 A. Kuperman, S. Nadinti, S. Oliver, G. A. Ozin, J. M. Garcés, M. M.
Olken, Nature, 365 (1993) 239
 Y. Xu, P. J. Maddox, J. M. Couves, J. Chem. Soc., Faraday Trans., 86 (1990)
 H. Kessler, J. Patarin, C. Schott-Darie, Stud. Surf. Sci. & Catal., Vol.
85 (1994) 75-113