Sol-gel-fluorination synthesis of amorphous magnesium fluoride

Article abstract of DOI:10.1016/j.jssc.2005.11.033

The sol-gel fluorination process is discussed for the reaction of magnesium alkoxides with HF in non-aqueous solvents to give X-ray amorphous nano-sized magnesium fluoride with high surface areas in the range of 150-350 m ²/g (HS-MgF₂). The H2 type hysteresis of nitrogen adsorption-desorption BET-isotherms is indicative for mesoporous solids. A highly distorted structure causes quite high Lewis acidity, shown by NH ₃ temperature-programmed desorption (NH₃-TPD) and catalytic test reactions. XPS data of amorphous and conventionally crystalline MgF₂ are compared, both show octahedral coordination at the metal site. Thermal analysis, F-MAS NMR- and IR-spectroscopy give information on composition and structure of the precursor intermediate as well as of the final metal fluoride. The preparation of complex fluorides, M⁺MgF ₃^-, by the sol-gel route is reported. From the magnesium fluoride gel of the above process thin films for optical application are obtained by, e.g., spin coating.

Full text of DOI:10.1016/j.jssc.2005.11.033

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 739–746
www.elsevier.com/locate/jssc
Sol–gel-fluorination synthesis of amorphous magnesium fluoride
Ã
J. Krishna Murthya, Udo GroXa, Stephan Rudigera, Erhard Kemnitza, , John M. Winfieldb
¨
aInstitute of Chemistry, Humboldt-University Berlin, Brook-Taylor-Str.2, 12489 Berlin, Germany
bDepartment of Chemistry, University of Glasgow, Glasgow, UK
Received 6 October 2005; received in revised form 10 November 2005; accepted 18 November 2005
Available online 4 January 2006
Dedicated to Professor Hans Georg von Schnering on occasion of his 75th birthday
Abstract
The sol–gel fluorination process is discussed for the reaction of magnesium alkoxides with HF in non-aqueous solvents to give X-ray
amorphous nano-sized magnesium fluoride with high surface areas in the range of 150–350 m2/g (HS-MgF2). The H2 type hysteresis of
nitrogen adsorption–desorption BET-isotherms is indicative for mesoporous solids. A highly distorted structure causes quite high Lewis
acidity, shown by NH3 temperature-programmed desorption (NH3-TPD) and catalytic test reactions. XPS data of amorphous and
conventionally crystalline MgF2 are compared, both show octahedral coordination at the metal site. Thermal analysis, F-MAS NMR-
and IR-spectroscopy give information on composition and structure of the precursor intermediate as well as of the final metal fluoride.
The preparation of complex fluorides, M+MgF3ꢀ, by the sol–gel route is reported. From the magnesium fluoride gel of the above process
thin films for optical application are obtained by, e.g., spin coating.
r 2005 Elsevier Inc. All rights reserved.
Keywords: High surface area fluorides; Sol–gel fluorination; Magnesium fluoride; Thin fluoride film
1. Introduction
Here, we report on a newly discovered genuine sol–gel
fluorination reaction where metal fluoride gels are inherent
particularities of the process and decisive for unexpected
properties and structures [2]. The procedure is similar to
the well-known sol–gel reaction of metal oxide preparation.
Starting from metal organic derivatives as, e.g., metal
alkoxides, and non-aqueous HF the reaction provides a
viable and low-cost method for high surface area metal
fluorides with high Lewis acidity as, for instance, in case of
AlF3 [3] as well as for thin film coating for optical materials
as it is shown for MgF2. Technological achievements are
projected as the latter may result in a strong growth
in the photonic market overcoming current technological
barriers [1a]. The characteristics of the process consist in
generating amorphous highly distorted metal fluorides with
unusually high surface area caused by a mesoporous
morphology and surface texture. The chemical inertness
and weak to moderate Lewis acidity of MgF2 make it
useful as support and/or host in mixed metal fluoride
catalyst systems [4]. Low refractive indices and wide ranges
Sol–gel synthesis of metal fluorides is rarely reported in
literature, describing a methodology combined of a first
step comprising hydrous metal oxide synthesis and a
second step comprising treatment with hydrogen fluoride
or a hydrolysable covalent fluoride to convert the hydrous
oxide to the fluoride [1a–c]. This arrangement of reactions
is obviously non-typical to a sol–gel methodology and the
designation as such is misleading. Other syntheses started
from magnesium alkoxide in non-aqueous solvent, in
alcohol, which was, however, reacted with aqueous
hydrofluoric acid [1d,e]. According to our experiences,
the presence of water results inevitably in formation of
magnesium oxyfluoride or mixtures of oxide and fluoride
leading to markedly higher surface area as reported in [1f].
Ã
Corresponding author. Fax: +49 30 2093 7277.
E-mail address: erhard.kemnitz@chemie.hu-berlin.de (E. Kemnitz).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.11.033

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J. Krishna Murthy et al. / Journal of Solid State Chemistry 179 (2006) 739–746
740
of light transmission make some of the metal fluorides
preferred candidates in optical as well as protective coating
forming fluoride glasses.
instrument at 77 K. Before each measurement, the
samples were degassed at 4 ꢂ 10ꢀ3 Torr and 150 1C for
10 h. The surface area was calculated according to the BET
method.
2. Experimental
Temperature-programmed desorption of ammonia
(NH3-TPD) was employed to determine the strength of
acid sites and its distribution. The sample (about 0.2 g) was
first heated under nitrogen up to 300 1C, then at 120 1C
exposed to NH3. After flushing the excess NH3 at 120 1C
with N2 for 1 h and cooling to 80 1C the TPD programme
was started (101/min up to 500 1C, keeping for 30 min).
Desorbed NH3 was monitored continuously via IR
spectroscopy (FT-IR System 2000, Perkin-Elmer).
Highly resolved X-ray photoelectron spectra (XPS) and
X-ray excited Auger electron spectra (XAES) were
acquired using a VG Scientific ESCALAB 200X electron
spectrometer as described in [6] with non-monochroma-
tized Al Ka excitation (hn ¼ 1486:6 eV) operated at 15 kV
and 20 mA and in CAE 10 mode. Before recording the
spectra the samples were stored overnight in the VG
Extended PrepLock chamber of the ESCALAB in a
vacuum better than 10ꢀ6 mbar in order to degas. During
analysis the vacuum in the spectrometer was better than
10ꢀ8 mbar. Binding or kinetic energy data were referenced
to the aliphatic C 1s peak at 284.8 eV.
Thermal analysis was done with a NETZSCH thermo-
analyser STA 409 C Skimmers system equipped with a
quadrupole mass spectrometer BALZERS QMG 421 as
described in [2a] recording the thermoanalytical curves
(T, TG, DTA) together with the ionic current (IC) curves in
the multiple ion detection (MID) mode.
Solid-state F MAS NMR experiments were done on a
Bruker AVANCE 400 spectrometer using a 2.5 mm magic
angle spinning probe. Spectra were measured at 376.5 MHz
with ultrafast spinning of 30–36 kHz. Chemical shifts were
referenced to CFCl3.
2.1. Syntheses
HS-MgF2: The synthesis procedure followed basically
that given in [2]. Mg turnings (99.98%, Aldrich) were
reacted with excessive, thoroughly dried CH3OH under
reflux conditions. The Mg(OMe)2 ꢁ 2MeOH was not
separated but after cooling reacted further under stirring
with stoichiometric amounts of HF dissolved in MeOH or
ether. A clear gel or a sol, the viscosity of which depends on
concentration was formed. Low concentrated sols of low
viscosity were used for coating experiments. The wet
viscous gel was dried under vacuum at temperatures up
to 70 1C yielding a white solid containing some organics, a
result which corresponds to that observed in course of HS-
AlF3 synthesis [2a], although in the latter system the
remaining organics measured as carbon content was
substantially higher. By further fluorination with N2
diluted gaseous HF or CCl2F2 or CHClF2 at temperatures
up to 250 1C in a tube-type flow reactor HS-MgF2 was
obtained as fine white powder.
Mg(OH,F)-materials: The preparation is a two-step
process described in [5]. Magnesium hydroxofluorides with
F/Mg molar ratios between 0 and 2 were synthesized from,
e.g., 0.2 mol magnesium methoxide in dry methanol
and then fluorinated with less than two equivalents of a
HF/Et2O solution. The remaining –OCH3 groups (Eq. (1),
product I) were hydrolysed by addition of the equivalent
amount of water to give the magnesium hydroxofluoride of
the general formula Mg(OH)2ꢀx Fx.
Complex magnesium fluorides: 30 mmol of each magne-
sium methoxide and potassium methoxide in 30 ml
methanol were mixed and reacted with 0.1 mol of
HF/MeOH at room temperature to give KMgF3 after
drying under vacuum. Similar reactions were carried out
with varying the K:Mg ratio between 1 and 3. The
amorphous or low crystalline material was calcined at
300 1C under inert carrier gas.
FT-IR measurements were done of wafers of KBr and
CsI. Spectra were recorded between 200 and 4000 cmꢀ1 on
a Perkin-Elmer 2000 spectrometer.
3. Results and discussion
3.1. Synthesis and reaction mechanism
MgF2 coatings: Thoroughly purified and dried small
plates of SiO2 or Si were placed on a spin-coater (KW-4A,
Chemat), covered with low viscous sols of MgF2 in MeOH,
and spun for 40 s at 5000 rpm. After drying at 100 1C the
coating was repeated or the layer was immediately calcined
at 300 or 500 1C.
The sol–gel fluorination reaction results in complex
development of a polymer-like network of gel structures,
essentially determined by the strength of formed
H-bonding and of van der Waals interaction forces,
causing also formation of solvates by Mg?O-bonding.
The latter depends on the partial charge at the metal.
Therefore, in this respect sol–gel reactions and gel stability
are very sensitive to solvent properties such as dielectric
constant DK, ability to hydrogen bonding, polarity
and geometric size of the molecule. Generally, the
state of the gel is irreversible and permanent. The fresh
wet gel morphology is weak and cannot survive harsh
drying treatments at elevated temperatures leading to a
2.2. Characterisation
X-ray powder diffraction (XRD) characterisation was
carried out on a XRD-7 Seiffert-FPM with Cu-Ka
radiation.
Surface area of the catalysts was determined using
N2 adsorption by means of a Micromeritics ASAP 2000

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741
dense amorphous product. Structurally, the metal sites
coordinated by solvent are continuously and irreversibly
replaced by corner shared fluorine atoms, bridging the
octrahedral units. Thus, stable metal fluoride polymorphs
are formed. Amorphicity of the solid is evident from XRD
pattern due to missing reflections or of sometimes very
broad diffraction peaks and, additionally, from the
exothermic peak in the DTA curve of thermal analysis.
This peak is observed at 392 1C because of phase
transformation to the crystalline magnesium fluoride.
Aging the wet gel over longer periods may also affect the
gel structure, sometimes liberating the corresponding
alcohol and lowering gel viscosity.
Nevertheless, the rate determining step of the fluorina-
tion reaction of the alkoxide is protonation of the alkoxide
oxygen by the reactive onium ion species CH3O+H2
(HFx)Fꢀ (formed by protolytic reaction of HF with the
solvent methanol) and subsequent nucleophilic substitution
by the weak nucleophile Fꢀ to the Mg–F bond, as
schematically represented by Eq. (1).
in crystalline structures. However, in the alcohol environ-
ment the magnesium ions are solvated by alcohol blocking
some of the coordinating sites at magnesium. Thus, a
polymorphic network is formed without condensation
reactions. Alcoholic solvents, involved in the framework
structure are preventing the material to crystallise and
preserve an amorphous nature, consequently forming open
structures.
Under non-aqueous conditions according to Eq. (1)
MgF2 is formed via the intermediate magnesium alkoxide
fluoride of different compositions (I). The usefulness of
these products and of defined magnesium hydroxofluorides
of the type [MgOxF6ꢀx]n obtainable by controlled hydro-
lysis reaction will be discussed in more detail in Section 3.5
under the aspect of a possible tuning of the acidic–basic
properties of these compounds.
One must have in mind that educt as well as product of
the reaction sequence of Scheme 1 exist in more complex
geometric structures as for the starting material in a
tetrameric magnesium–oxygen heterocubane structure with
oxygen octahedral coordination of the metal [8] while the
product MgF2 reveals infinite chains of edge-shared
octahedra, where the chains are connected to each other
via corners, of rutile-like structure with known 6/3
coordination. In the course of reaction the octahedral
coordination of magnesium is not changed.
MgðOCH3Þ2 þ HF ! Mg-OCH3 þFꢀ ! MgðOCH3ÞF
ðIÞ
j
HOþCH3
(1)
HF
þCH3OH ꢀ! MgF2 þCH3OH;
ðIIÞ
If water is also present a competition reaction as
according to Eq. (2) may take place forming hydroxyl
groups, and subsequent condensation yields metal–
oxygen–metal bonds and metal oxide fluorides. To prevent
side reactions of such type exclusion of moisture is
necessary, otherwise product mixtures are obtained as
found by use of aqueous HF [1d–f] as fluorinating agent.
Thus, magnesium hydroxyfluorides and magnesium hydro-
xide and oxide, respectively, the latter are formed after
drying or calcination procedure in a temperature range
from 150 to 700 1C, contribute to very high surface areas
[1f] not being observed for MgF2 under non-aqueous
reaction conditions. Partly amorphous Mg(OH)2 not easily
detectable in XRD pattern exhibit surface area up to
700 m2/g [7], whereas surface areas of MgO dried gels
amount to 1011 m2/g [9].
MgðOCH3Þ2 þ H2O ! ½H2O ! MgðOCH3Þ2ꢃ !
(2)
MgðOHÞOCH3 ! MgðOHÞ2:
The observations of the gel formation process during HF
fluorination show a partial similarity to the hydrolysis of
magnesium methoxide (Eq. (2)) as reported by Ranjit and
Klabunde [7]. This is not surprising because both reactions
are nucleophilic substitution reactions where solvent effects
are of importance, and subsequent formation of large
polymorphic entities is a known fact. Concerning hydro-
lysis, condensation of formed hydroxyl groups under
release of alcohol or water is the dominating process
following the nucleophilic attack, while in case of sol–gel
fluorination the fluoride ions formed upon reaction of
alkoxide with HF cannot undergo a condensation reaction.
The fluoride ions are principally able to become bridges,
linking the magnesium ions, which should eventually result
According to Scheme 1 distorted magnesium fluoride is
obtained, sometimes containing residual methoxo groups
CH3
O
H3C
H
F
F
Mg
F
Mg
H3C
O
F
F
+HF
O
Mg
O
O
+HF
Mg(F,OCH3)(gelous)
Mg
F
F
F
O
H3C
-CH3OH
CH3
F
F
Mg
O
F
H
CH3
Mg
H3C
Scheme 1. Schematic representation of the fluorination process from Mg(OCH3)2 ꢁ 2CH3OH to MgF2 via a gel state.

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742
DTG /(%/min)
Mg(OCH3)2 in CH3OH
DTA /uV Ion Current *10-10 /A
[2]
+HF in ether or methanol
(Liquid phase fluorination)
0.20
30
TG
6.0
100
95
90
85
80
75
-5.80
0.10
0
20
10
0
DTG
5.0
4.0
3.0
2.0
.
MgFx(OCH3)2-x n CH3OH
DTA
176
-0.10
-0.20
-0.30
-0.40
C = ≤ 18%
exo
↑
Gas-phase fluorination
with HF
Gas-phase fluorination
with CCl2F2
-10
-20
-30
m18
120°C
MgF2
SA: 218m2/g; C=<0.1%
350°C
m31(x50)
MgF2
100 200 300 400 500 600 700 800
SA: 190m2/g; C=0.5%
Temperature /°C
Fig. 1. TA/MS curves of HS-MgF2 with the IC curves for m=z 18 (H2O+
and m=z 31 (CH3O+).
)
Scheme 2. HS-MgF2 syntheses procedures.
because of incomplete fluorination. Although the nucleo-
philic substitution reaction is thermodynamically favoured,
the reaction is kinetically controlled because of the gel
viscosity (see also [2a]). Obviously, this does not hold for
very diluted systems of low viscosity as used for thin film
preparation. The presence of alkoxo groups or solvent
solvates is indicated by the carbon content in elemental
analysis and by thermal analysis as well as by spectroscopic
methods. The carbon content of the ‘‘precursor’’ com-
pound, i.e. the dried gel, is in the range of 5–15%
depending on reaction conditions and the alcohol used.
However, MgF2 obtained from very diluted sol, as used for
thin films, by evaporation the solvent had a carbon content
corresponding to the limit of determination, i.e., p0.5% C.
Mild post-fluorination of the product in reaction (1) with
CCl2F2 replaces the methoxo groups by fluorine preserving
the distorted structure. Alternatively, post-fluorination of
the precursor compound with HF at lower temperatures
preserves a high surface area of the fluoride of, e.g.,
218 m2/g and a strongly reduced carbon content of o0.1%
(Scheme 2). After post-fluorination HS-MgF2 is not longer
a gel and can not be transformed into a gel.
The principal reaction route to magnesium fluoride
according to Eq. (1) and Schemes 1 and 2 is also confirmed
by DTG/MS. The major weight loss to be found in the TG
curve (not shown) of a precursor of HS-MgF2 is 18.8%
caused by the liberated methanol. At elevated temperatures
remaining methoxo groups are decomposed releasing
ethylene (m=z 27; C2H+3 ) according to the well-known
thermal decomposition pathway [8]. At 392 1C the exother-
mic crystallisation point occurs revealing the transition
of amorphous to crystalline material. Post-fluorination
of the precursor results in HS-MgF2, the thermal analysis
of which is shown in Fig. 1. The weight loss is only
5.8% mainly due to loss of adsorbed water identified in
MS by its mass number of 18. The DTG peak of the
water loss expectedly corresponds with an endothermic
peak in the DTA curve. The loss of methanol is minor
as shown by the mass number m=z 31(CH3O+) in
correspondence with the carbon content ofo0.1%. Besides
magnesium methoxide as starting material, also the
ethoxide and n-butoxide were used with principally similar
results.
adsorption
desorption
120
100
80
60
40
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/po)
Fig. 2. BET-N2 adsorption–desorption isotherms and pore size distribu-
tion (insert) of a HS-MgF2 of 218 m2/g surface area.
3.2. Morphology, structural and surface properties
Morphology and texture as well as physico-chemical
properties including surface behaviour of crystalline MgF2
are strikingly different from that of amorphous HS-MgF2
obtained from the sol–gel fluorination. SEM micrographs
of the amorphous material present a porous material with a
high total pore volume, while conventionally prepared
fluoride shows a clear-cut morphology of well-sized
crystallites [6]. The porous structure results in unusually
high surface area ranging from 150 to 350 m2/g, compared
with expected surface areas of crystalline MgF2 in the
range of 1–44 m2/g [10]. The commercially available
crystalline magnesium fluoride (Aldrich) has a surface area
of 0.4 m2/g. Nitrogen adsorption–desorption in Fig. 2
exhibit a broad hysteresis typical of adsorbents possessing
mesopores. The shape of the hysteresis loop and the steep
desorption branch classify it as of the H2 type, associated
with a more complex bottle neck pore structure in which
network effects are of importance [11]. The relative
uniformity of pores is shown with the distribution curve
inserted in Fig. 2.
The octahedral coordination of magnesium by
fluorine as known from X-ray structure determination of
crystalline MgF2 holds also for the X-ray amorphous
sol–gel fluoride. From XPS data presented as chemical

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743
1179
1178
1177
1176
1175
1174
2482
655
1339
1338
1337
1336
1335
1334
HS-MgF2
Com. MgF2
HS-MgF2
Com. MgF2
2481
2480
2479
2478
2477
654
653
652
651
650
1308 1307 1306 1305 1304 1303
689
688
687
686
685
684
(a)
(b)
Binding Energy Mg 1s (eV)
Binding Energy F 1s (eV)
Fig. 3. (a) Chemical state plot for Mg of MgF2. (b) Chemical state plot for F of MgF2.
35
state plots in Figs. 3a and b the structural similarity of the
two is evident, because crystalline and amorphous HS-
MgF2 are positioned in the same grid of the graph.
Nevertheless, the binding energies (BE) and modified
Auger parameters (MAP ¼ BE+KE ¼ BE of core le-
vels+kinetic energies of Auger electrons) of the Mg 1s
and F 1s are different for the two fluorides. The BE of the
Mg 1s orbital of 1306.0 eV of the amorphous sample is
shifted to little higher values, indicative for a higher
positive charge, i.e., increased Lewis acidity, at the
magnesium.
pretreatment at 500°C
pretreatment at 300°C
pretreatment at 120°C
30
25
20
15
10
5
0
3.2.1. F MAS NMR spectra
Solid-state MAS NMR is the preferred analytical tool
for amorphous material, of which X-ray data are mostly
not available for structural elucidation. The amorphous
HS-MgF2 provides in F MAS spectra a symmetric
unresolved peak of Lorentzian line shape with a chemical
shift of 198.7 ppm. The samples were rotated at 30–36 kHz
resulting in strong line narrowing. Considering the
chemical shift parameter there is no remarkable difference
to that of crystalline MgF2 with 198.5 ppm, but indication
for identical coordination numbers. Otherwise, the line
width of the two peaks is different. Full-width at
half-height is 2210 kHz for HS-MgF2 and 1830 kHz for
MgF2 cryst. The larger line width results from structural
distortion and the related anisotropy of shielding. Quite
differently, the substitution of fluorine by other ligands,
e.g., OH groups in magnesium hydroxofluorides of the
Section 3.5, shifts the resonance to the lower field [5].
-5
0
100
200
300
400
500
const.
Temperature (°C)
Fig. 4. NH3-TPD of HS-MgF2.
In the precursor state of HS-MgF2 characteristic bands
of the remaining organic alkoxo groups were found. These
are in the wave number range at 1050–1130 cmꢀ1 assigned
to the C–O stretching mode and for C–H stretching at
2750–3100 cmꢀ1
.
3.2.3. Lewis acidity
According to Pearsons HSAB concept of hard and soft
acids and bases, Mg2+ ion is defined as a hard, i.e., strong
Lewis acid, possessing unoccupied orbitals that can accept
an electron pair from a donor molecule. Actually, highly
ordered crystalline MgF2 exhibits only little Lewis acidity
because of the complete shielding of the acidic metal site by
fluorine ligands in an octahedral lattice. Such weak surface
acidity is reported because of pyridine adsorption and
of moderate catalytic activity in dehydration reaction of
i-propanol at 320 1C [12].
If the magnesium fluoride becomes X-ray amorphous,
that is, lattice distortions occur with deviating –Mg–F–Mg–
angle from 1801 and probably metal coordination numbers
lower than 6 due to structure defects, then the acidic metal
site may be accessible for basic reactants. This agrees well
with the fact we have observed in the aluminium fluoride
system [2,13].
3.2.2. IR spectra
Because of the octahedral coordination of magnesium by
fluorine one has to expect Oh symmetry for an ordered
structure with two infrared active modes. In HS-MgF2 the
Mg–F stretch n1 is at 450 cmꢀ1 and the bending vibration q
at 260 cmꢀ1. An additional, weaker band occurs at
400 cmꢀ1. For the amorphous HS-MgF2 the bands are
significantly broader than those of the crystalline MgF2.
The structural disorder of the compound corresponds with
the broadness of absorption bands. These result from the
superimposition of all the existing structural deviations.

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744
To provide evidence for Lewis acidity a catalytic test
Table 1
XPS results of MgF2 and different Mg(OH)2ꢀxFx
reaction and NH3-TPD were performed, the results of
which are shown in Fig. 4. The desorption peaks give
information on the strength and number of acidic sites of
the solid. The desorption behaviour of HS-MgF2 depends
strongly on the temperature of its pre-treatment. The
500 1C pre-treated sample did not show any NH3
desorption because at this temperature the sample became
crystalline and the ordered MgF2 structure is practically
not Lewis acidic. In full contrast, the dotted line curves,
pre-treatment at 120 and 300 1C, show desorption of NH3
because at these temperatures the structural disorder of the
amorphous MgF2 remains and the metal site is accessible
for the base as discussed for HS-AlF3 [13]. As expected,
strongest sites were found at 120 1C, i.e., the lowest
temperature of pre-treatment. The bimodal curve with
two maxima points to Lewis acidic sites of different
strengths. In going from 120 to 300 1C pre-treatment, the
structure of HS-MgF2 is partly affected, and especially the
strongest sites disappeared, because the temperature is
already near the phase transition temperature, which is
according to our thermal analysis investigations at 392 1C.
As catalytic test the dismutation of CHClF2, a reaction
known to depend on Lewis acidic catalyst was monitored
with increasing temperature. At 300 1C 3%, at 350 1C a
60% conversion of CHClF2 over HS-MgF2 could be
observed, which is indicative for weak to moderate Lewis
acidity. For comparison, the strong Lewis acidic solid
catalyst HS-AlF3 yields a 96% conversion at only 250 1C.
However, the test reaction needs rather high temperatures
at which HS-MgF2 becomes already partially deactivated.
Reactions proceeding at distinctly lower temperatures
should be much more efficiently catalysed by HS-MgF2.
The Lewis acidity of HS-MgF2 is further evidenced by the
observed formation of [MgF3]ꢀ entities, i.e., of complex
magnesium fluorides, acting as a weak Fꢀ ion acceptor
according to Eq. (3):
Sample
BE of Mg 1s (eV)
MgF2 (sol–gel, amorphous)
MgF2 (comm., crystalline)
Mg(OH)0.4F1.6
1306.0
1305.6
1304.0
1303.0
1302.8
1302.6
Mg(OH)0.8F1.2
Mg(OH)1.2
Mg(OH)1.6F0.4
F
0.8
600
500
400
K/Mg=1.0
KMgF3
after calcination
at 300 °C
300
200
100
0
5
10
15
20
25
30
35
40
45
50
55
60
65
Fig. 5. XRD diffraction of sol–gel prepared KMgF3 before and after
calcinations.
methanolic Mg(OCH3)2 results in only partial substitution
of the methoxo groups by fluoride, i.e., magnesium
methoxide fluoride according to Eq. (1) is formed as an
intermediate. The residual methoxo groups can than be
removed either thermally or by subsequent hydrolysis
adding a defined amount of water. This way followed by
the steps of drying and calcinations magnesium oxide/
hydroxyde fluorides of varied composition were prepared,
which showed surprisingly good catalytic properties [5].
The tendency of acidity/basicity changes with substitu-
tion of F by OH is demonstrated with the BE of the Mg 1s
orbitals in Table 1.
Decreasing the fluorine content in the magnesium
fluoride compounds shifts the BE of the Mg 1s orbital to
lower values synonymously with a decrease in positive
charge for Mg and the apparent Lewis acidity of the
sample.
MgF2 þ Fꢀ ! ½MgF3ꢃꢀ.
(3)
3.3. Mixed magnesium methoxide fluoride and magnesium
hydroxofluoride
Magnesium hydroxide as well as its oxide are basic
materials. Especially under the aspect of their use as
catalyst [5] a tuning of the acidic–basic behaviour is of
interest. This can be done by creating magnesium oxide/
hydroxide fluorides of tailored Mg-to-F ratio for which the
sol/gel synthesis route provides excellent preconditions.
The only one structurally known magnesium hydroxo-
fluoride, Mg(OH)F, is obtained by hydrothermal synthesis,
the structure of which was solved by powder methods from
X-ray data [14]. It exists as intermediate brucite sellaite
phase between Mg(OH)2 and MgF2 of Pnma space group.
The sol–gel fluorination method opens a simple route to
more differently substituted hydroxofluorides of the above
type. Adding under-stoichiometric amounts of HF to
3.4. Complex magnesium fluorides
Complex magnesium fluorides of the general formula
M+MgFꢀ3 , isostructural with perovskite phases, have been
synthesised by high-temperature melting process or as
shown recently [15] by a hydrothermal reaction at
120–240 1C. Under the soft sol–gel conditions KMgF3 is
obtained from a 1:1 molar mixture of magnesium
methoxide and potassium methoxide in methanol solvent
fluorinated with 3 equivalents of HF/MeOH. The KMgF3

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745
Fig. 6. AFM micrograph of a spin-coated MgF2 layer on SiO2 calcined at 300 1C.
obtained this way
from 19 to 222 nm and correlated well with the MgF2
concentration of the sol and of the number of coatings
applied. The refractive index agreed very well with
literature data over the experimental wavelength range
from 300 to 1000 nm. AFM micrographs showed a
homogeneous layer formed of submicron particles as
shown in Fig. 6 for a layer prepared by threefold coating
and calcination at 300 1C. The MgF2 layers adhere strongly
to their support and are mechanically rigid. Altogether, the
new non-aqueous MgF2 sol/gel easily accessible by our new
route opens a broad field of applications as coating in,
preferably, optical applications [17].
(4)
is in the crystalline state according to XRD analysis,
calcinations at 300 1C did not alter the diffraction pattern
(Fig. 5). The BET surface area of the calcined material was
85 m2/g. With a K/Mg ratio of 2, the expected K2MgF4 was
not formed. Besides an unidentified amorphous solid broad
XRD reflexes indicate low crystalline KMgF3. Further
increase of the K/Mg ratio resulted in additional KHF2
formation.
4. Conclusions
3.5. Magnesium fluoride as host or support for catalyst
preparations
The newly developed anhydrous sol–gel methodology
can be applied for the preparation of magnesium fluoride
yielding an amorphous high surface area magnesium
fluoride (HS-MgF2) with outstanding properties. Thus,
although the arrangement of the next neighbours of
magnesium is in HS-MgF2 the same as in crystalline
MgF2, i.e., six F atoms octahedrally arranged, the
former exhibits markedly higher Lewis acidity, obviously
because of its high distortion in combination with
coordinatively unsaturated Mg surface sites like in
HS-AlF3 and ACF [13]. In addition to HS-MgF2 synthesis,
the sol–gel route opens multiple possibilities for the
preparation of, e.g., doped MgF2 or MgF2-supported
systems and magnesium oxide fluorides as high surface
area catalysts, MgF2 thin layers, and complex magnesium
fluorides.
The high surface area of HS-MgF2 and its easy
preparation makes it a promising host for doping with
higher valent metal ions such as, e.g., Cr3+ [4], Fe3+ [16],
Ga3+ and others [6] to be used as catalysts as already
reported by us. HS-MgF2 can further be an interesting
support for catalytically active materials such as, e.g., Pd or
Pt, as preliminary successful hydrodechlorination experi-
ments indicate.
3.6. Thin film magnesium fluoride
The reaction of magnesium methoxide in methanol with
stoichiometric amounts of a non-aqueous solution of HF
in, e.g., methanol results in a clear gel or a clear sol of low
viscosity, depending on the magnesium concentration used.
Thus, in terms of stoichiometry, a non-aqueous stable
‘‘solution’’ of MgF2 is obtained, which should be ideal for
depositing MgF2 layers on solids by established techniques.
To prove this Si as well as SiO2 plates were tentatively
coated with methanolic MgF2 sol by dip- or spin-coating.
This way after drying and subsequent calcination at 300 or
500 1C a clear transparent layer was formed. Thickness and
refraction index of these layers were determined by
ellipsometry. The thickness of these tentative layers ranged
Acknowledgments
The authors thank Dr. Zhijan Li for careful preparations
of hydroxofluorides and complex fluorides, H. Kruger for
¨
the AFM measurements, and Dr. Michael Feist for
carrying out the thermal analysis. This work was done
with the support of the EU in the FUNFLUOS Project,
Contract No. NMP3-CT-2004-5005575 within the 6th
Framework Programme.

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