Structural insights into aluminum chlorofluoride (ACF)

Article abstract of DOI:10.1021/ic034106h

The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlCl_xF_3-x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, ¹⁹F MAS NMR, and ²⁷Al SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of α-AlF₃·3H₂O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF₃ prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF₆ octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AlF₃. Terminal fluorine atoms were detected in ACF by ¹⁹F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl₃ phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.

Full text of DOI:10.1021/ic034106h

Inorg. Chem. 2003, 42, 6474−6483
Structural Insights into Aluminum Chlorofluoride (ACF)
,†
Thoralf Krahl,† Reinhard Sto1sser,† Erhard Kemnitz,* Gudrun Scholz,† Michael Feist,† Gilles Silly,‡ and
Jean-Yves Buzare´‡
Institute of Chemistry, Humboldt UniVersity of Berlin, Berlin, Germany, and Laboratoire de l’Etat
de Physique Condense´, UMR CNRS no. 6087, UniVersite´ du Maine, Le Mans, France
Received January 31, 2003
The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlCl F3-x, x ) 0.05−0.3) was
x
studied by IR, ESR, Cl K XANES, 19F MAS NMR, and 27Al SATRAS NMR spectroscopic methods and compared
with amorphous aluminum fluoride conventionally prepared by dehydration of R-AlF3‚3H O. The thermal behavior
2
of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF3 prepared in a
conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which
requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF6 octahedra forming
a random network. The degree of disorder in ACF is higher than in amorphous AlF3. Terminal fluorine atoms were
detected in ACF by 19F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl3 phase. Additionally,
chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable
at room temperature but do not take part in the catalytic reaction.
1. Introduction
a self-generating atmosphere.7 Although the overall composi-
tion of ACF and am-AlF3 is similar, high Lewis acidity has
not been observed for the latter. Therefore, the study of the
differences between am-AlF3 and ACF is of interest.
Despite known preparation routes for ACF and the testing
of ACF for many catalytic reactions, only very little is known
and understood about its physical and chemical nature. It is
well-known from heterogeneous catalysis that the physical,
the chemical, and especially the catalytic properties of a
material are determined by the chemical nature of the
components, by their inner and outer surface areas, and by
the degree of disorder of the material. These aspects have
not been considered in connection with ACF until now. It is
therefore the aim of the present work to show that the
combined action of those effects will be regarded as the main
reason for the outstanding physical and chemical properties
of ACF.
The samples investigated here are aluminum chlorofluoride
(ACF), amorphous aluminum fluoride (am-AlF3), and par-
tially fluorinated aluminum chloride (AlClxF3-x with x )
0.3-3.0). The latter were investigated to determine the
influence of the chlorine content on the properties of ACF.
The samples were synthesized, characterized, and compared
with the crystalline phases R- and â-AlF3. Since most of the
Aluminum chlorofluoride (ACF: AlClxF3-x, x ) 0.05-
0.30) was developed in 1992 by DuPont.1 It is a noncrystal-
line material which is formed during treatment of AlCl3 with
chlorofluorocarbons, such as CFCl3, according to the equa-
tion
AlCl3 + (3-x)CFCl3 f AlClxF3-x + (3 - x)CCl4
ACF behaves like a very strong Lewis acid; it catalyzes the
same reactions as antimony pentafluoride, and in some cases,
it acts as an even stronger Lewis acid than the latter one.2-6
Like all strong Lewis acids, it is moisture sensitive and must
be stored under dry inert gas.
Another noncrystalline phase of AlF3 (am-AlF3) is known
to emerge during heating of R-AlF3‚3H2O to 200 °C under
* To whom correspondence should be addressed. E-mail:
Erhard)Kemnitz@chemie.hu.berlin.de.
† Humboldt University of Berlin.
‡ Universite´ du Maine.
(1) Sievert, A. C.; Krespan, C. G.; Weigert, F. J. (DuPont Co.) U.S. Patent
5,157,171, 1992.
(2) Krespan, G. C.; Petrov, V. A. Chem. ReV. 1996, 96, 3269-3301.
(3) Krespan, G. C.; Dixon, D. A. J. Fluorine Chem. 1996, 77, 117-126.
(4) Petrov, V. A.; Krespan, G. C.; Smart, B. E. J. Fluorine Chem. 1996,
77, 139-142.
(5) Petrov, V. A.; Krespan, G. C.; Smart, B. E. J. Fluorine Chem. 1998,
89, 125-130.
(6) Petrov, V. A.; Krespan, G. C. J. Fluorine Chem. 2000, 102, 199-
204.
(7) Scholz, G.; Sto¨sser, R.; Sebastian, S.; Kemnitz, E.; Bartoll, J. J. Phys.
Chem. Solids 1999, 60, 153-162.
6474 Inorganic Chemistry, Vol. 42, No. 20, 2003
10.1021/ic034106h CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/03/2003

Structure of Aluminum Chlorofluoride
Table 1. Catalytic Activity in the Isomerization of
1,2-Dibromohexafluoropropane and Molar AlCl3 Content of the
Investigated Compounds Extracted from the Simulation of the 27Al
MAS SATRAS NMR Spectra
samples studied are noncrystalline, methods other than X-ray
diffraction had to be used for structural characterization.
Hence, the samples were investigated by IR and ESR
spectroscopy, 19F and 27Al MAS NMR, and X-ray absorption
spectra of the Cl K edge for the chlorine-containing species.
The BET surface area of ACF, the average pore sizes, and
the pore volumes were also determined.
catalytic
activity [%]
AlCl3 content
[%]
compd
AlCl3
AlCl2.39F0.61
AlCl1.75F1.25
AlCl1.25F1.75
AlCl0.80F2.20
ACF-013
<0.1
0.1
9.3
20.7
41.6
>90.0
100
85
66
24
21
0
There is also special interest in the crystallization behavior
of ACF and am-AlF3. The thermal behavior was studied by
DTA followed by X-ray diffraction of the bulk phase. In
addition, organic compounds evolved during heating were
determined by gas-phase IR spectroscopy.
AlCl0.13F2.87
am-AlF3
inactive
δiso. Hexafluorobenzene13 and an aqueous 1 M solution of aluminum
nitrate were used as reference for the 19F and 27Al chemical shifts,
respectively, at room temperature and are accurate to (1 ppm.
The solution 19F NMR spectra were recorded on a Bruker DPX
300 spectrometer in CDCl3 with an internal standard of C6F613 (set
to 0 ppm) at a resonance frequency of 282.4 MHz.
2. Experimental Section
2.1 Methods. X-ray diffraction (XRD) measurements were
performed on a Seiffert RD7 (Freiberg, Germany) with Cu KR
radiation. Moisture sensitive samples were prepared in a drybox,
covered with a special X-ray amorphous polystyrene foil, and sealed
with Kel-F grease (Roth, Germany). Phases were identified by
comparison with the ICSD powder diffraction file.8
Measurements of X-ray absorption spectra were carried out at
HASYLAB on the beamline E4 (DESY, Hamburg, Germany) in
transmission mode. The edge energy of the Cl K shell was 2822.0
eV; photon energies are accurate to (0.2 eV. Samples were
prepared in a drybox.
FT-IR spectra of CsI pellets were recorded on a Perkin-Elmer
2000 spectrometer in transmission mode. About 250 mg of CsI
(for optical properties, Fluka, Germany) was pressed with 1.5-2.0
mg of the sample inside the drybox, and then the samples were
measured in the regions 4000-400 and 700-200 cm-1. The gas
phase spectra were measured using a 10 cm long Teflon cell
equipped with KBr windows.
Specific BET surface areas9 and pore size distributions were
determined by low temperature nitrogen adsorption using a Micro-
meritics ASAP 2000. Samples were degassed in a vacuum at 200
°C before measurement.
2.2 Sample Preparation. If not mentioned otherwise, all
preparations were consequently carried out under standard Schlenk
conditions. Solvents were dried before using and stored over
molecular sieves. Abbreviations for the samples used in this study
are given in Table 1.
Aluminum Chlorofluoride (ACF-013). ACF was prepared from
aluminum chloride (99.99%, Aldrich) containing 10 ppm Fe. About
10-20 g of AlCl3 was filled into a round-bottomed flask equipped
with a dry ice condenser. The flask was cooled with ice. CFCl3 (5
equiv, 99.5%, Fluka, Germany) was added slowly over 15 min.
The suspension was stirred at 0 °C for 1 h and then for 3 h at room
temperature. The heat of the reaction refluxed the solvent (bp 23.8
°C) without further heating. After removing the solvent in a vacuum,
quantitative amounts of the fine yellow powder ACF were obtained
and stored in the glovebox. The sample used for investigation here
correlated with the formula AlCl0.13F2.87 and contained 0.44 wt %
carbon. This sample will be referred to as ACF-013.
The ESR measurements were carried out on a spectrometer ERS
300 (X-band (9 GHz), ZWG Berlin-Adlershof, Germany). For the
data acquisition as well as computer control of the spectrometer,
the control unit of the Magnettech Co. (Germany) was used. The
spectra were simulated using the programs COMPAR10 and ESR-
MAKRO.11 Spectra were recorded at room temperature, if not
mentioned otherwise.
Solid state NMR experiments were done on a Bruker AVANCE
300 spectrometer using a 2.5 mm double-bearing magic angle
spinning (MAS) probe with a decoupling channel optimized for
19F observation. The samples were characterized at room temper-
Samples with a lower fluorine content were synthesized using
less CFCl3. If less than 1.5 molar equiv was used, AlCl3 was first
suspended in perfluorohexane to obtain a stirrable liquid. The
composition of each sample was determined by Cl and F analysis
(see Table 1).
ature by measurements of 19F (I ) /2, 282.4 MHz) and 27Al (I )
1
5/2, 78.2 MHz) nuclei. 19F MAS NMR was performed at a spinning
speed of 35 kHz; the measured parameter was the isotropic chemical
shift δiso. 27Al MAS satellite transition spectroscopy (SATRAS)
experiments were performed at two spinning rates, 25 and 3 kHz,
as described in ref 12. The measured NMR parameters were the
quadrupolar frequency νQ ) 3eQVzz/2I(2I - 1)h, the asymmetry
parameter ηQ ) (Vxx - Vyy)/Vzz, and the isotropic chemical shift
Noncrystalline AlF3 (am-AlF3). R-AlF3‚3H2O was annealed at
200 °C in a special platinum Q-crucible under self-generated
atmosphere (quasi-isobar conditions14) for 2 h. This sample is further
named am-AlF3 (see Table 1).
r-AlF3. R-AlF3 was prepared by thermal decomposition of
R-NH4AlF4 in an open corundum crucible in flowing nitrogen by
heating to 700 °C at 5 K/min and holding this temperature for 4 h.
The phase purity was confirmed by XRD.8
(8) Powder Diffraction File, ICPDS-ICDD 2001, PDF numbers used: 44-
231 (R-AlF3), 84-1672 (â-AlF3), 47-1659 (θ-AlF3), 43-436 (R-AlF3‚
3H2O), 35-827 (â-AlF3‚3H2O), 76-56 (R-NH4AlF4), 22-10 (AlCl3),
44-1473 (AlCl3‚6H2O), 41-380 (AlF2OH, isomorphous with η-AlF3).
(9) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60,
309-319.
â-AlF3. â-AlF3 was prepared by thermal decomposition of
R-AlF3‚3H2O under self-generating atmosphere by heating to 400
°C at 5 K/min and holding this temperature for 2 h. The phase
purity was confirmed by XRD.8
(10) Program COMPAR: Software of the Spectrometer ERS-300; Zentrum
fu¨r wissenschaftlichen Gera¨tebau: Berlin, 1990.
(11) Scholz, G.; Sto¨sser, R.; Krossner, M.; Klein, J. Appl. Magn. Reson.
2001, 21, 105-123.
(12) Scholz, G.; Sto¨sser, R.; Klein, J.; Silly, G.; Buzare´, J. Y.; Laligant,
Y.; Ziemer, B. J. Phys.: Condens. Matter 2002, 14, 2101-2117.
(13) δF(C6F6) vs CFCl3 -162.2 ppm.
(14) Menz, D. H.; Bentrup, U. Z. Anorg. Allg. Chem. 1989, 576, 186-
196.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6475

Krahl et al.
Figure 1. IR spectra of (a) R-AlF3, (b) â-AlF3, (c) ACF-013, and (d)
am-AlF3.
Irradiation of CCl4. Tetrachloromethane was irradiated at 77
K with ultraviolet light of 254 nm wavelength inside a quartz dewar
and then transferred together with liquid nitrogen into the dewar
of the ESR cavity. This procedure was applied to avoid the spectral
inference of E′ centers formed in quartz by the short wavelength
of UV radiation.
Test of Catalytic Activity. About 20 mg of the corresponding
sample was filled into a 10 mL V-shaped flask and closed with a
septum inside the glovebox. The 1,2-dibromohexafluoropropane (10
µL per mg sample, 99%, FluoroChem, U.K.) was added, and the
suspension was stirred at room temperature for 2 h. After that the
catalyst was hydrolyzed with water, the organic phase was
investigated by 19F NMR.5 When the organic phase solidified during
the reaction (more then 75% conversion), the flask was slightly
heated during hydrolysis of the catalyst. The degree of conversion
to 2,2-dibromohexafluoropropane (mp 52 °C) was determined by
integration of the signals in the NMR spectrum. 19F NMR (282
MHz, CDCl3): reactant, δ 28.9 (m, 1F, CFBr), 87.9 (m, 3F, CF3),
Figure 2. X-ray absorption spectra of the Cl K edge of (a) ACF-013, (b)
AlCl0.80F2.20, (c) AlCl1.75F1.25, (d) AlCl2.39F0.61, and (e) AlCl3.
The catalytic activity of ACF-013 for the isomerization
of 1,2-dibromohexafluoropropane is high (80-90% conver-
sion), whereas AlCl3 has conversion rates below 0.1% under
the conditions applied here. The samples of fluorinated AlCl3
yield lower catalytic activities than ACF-013 (see Table 1).
After being exposed to air for 1 day, ACF-013 is no longer
catalytically active. As expected, am-AlF3 is inactive in this
reaction.
The specific BET surface area of ACF-013 is 101.1 (
0.6 m2/g. The total pore volume is 0.081 cm3/g, and the
average pore size is 31.9 Å. The BET surface area of am-
AlF3 could not be determined. No equilibrium could be
achieved during low temperature N2 adsorption, due to
remaining water content. As observed before, the surface
areas of the polycrystalline R- and â-AlF3 are in the range
of 40 m2/g.15
2
103.0 (dm, 1F, CFFBr, JFF ) 176 Hz), 105.0 (dm, 1F, CFFBr,
2JFF ) 176 Hz), ppm; product, δ 90.1 (s, CF3) ppm.
3. Results
3.1 Spectroscopic Investigation of ACF and am-AlF3.
XRD, IR, Catalysis, and BET. ACF-013 and am-AlF3 do
not show any distinct reflections in the X-ray powder
diffractograms, whereas the samples of fluorinated AlClxF3-x
with x ) 0.8-2.6 show more or less distinct reflections of
AlCl3.
The IR spectra of ACF-013 and am-AlF3, and of the
R-AlF3 and â-AlF3 crystalline phases, are given in Figure 1.
Both R-AlF3 and â-AlF3 show two strong bands in regions
of 660 and 350 cm-1. These crystalline phases have
additional, weaker bands at 607 and 544 cm-1, respectively.
The spectra of ACF-013 and am-AlF3 differ somehow from
the spectra of the crystalline phases. They show two very
broad signals around 660 and 350 cm-1 without any further
structuring. The spectra of partially fluorinated AlCl3 samples
can be interpreted as a superposition of those of AlCl3 and
ACF-013.
XANES. Normalized X-ray absorption spectra of the Cl
K edge (2822.0 eV) of AlCl3, ACF-013, and partially
fluorinated AlCl3 are shown in Figure 2. None of the spectra
(15) Kemnitz, E.; Menz, D. H. Prog. Solid State Chem. 1998, 26, 97-
153.
6476 Inorganic Chemistry, Vol. 42, No. 20, 2003

Structure of Aluminum Chlorofluoride
Figure 3. X-band ESR spectra: (a) experimental spectrum of AlCl3, (b)
simulated spectrum for AlCl3:Fe3+ with the parameters S ) 5/2, g ) 2.0, D
) -5670.0 MHz, E ) 48.3 MHz, a ) -6.6 MHz, F ) -62.0 MHz,16 (c)
experimental spectrum of ACF-013, the intensive signal at g’ ∼ 2 was
discriminated, and (d) experimental spectrum of am-AlF3.
shows a pre-edge absorption. AlCl3 exhibits a sharp reso-
nance at 2824.4 eV and a broader one at 2828.2 eV. During
the process of fluorination, the resonance at 2824.4 eV loses
intensity, and the broader resonance at 2828.2 eV also loses
intensity and is shifted to higher energy values (2830.0 eV
in ACF-013). With increasing fluorine content of the sample,
a new characteristic and sharp resonance at 2826.0 eV
appears. Spectra of samples with medium fluorine content
can be recognized as a superposition of the spectra of AlCl3
and ACF-013.
Figure 4. Experimental X-band ESR spectra of (a) ACF-013, 293 K, (b)
ACF-013, 77 K, (c) AlCl2.39F0.61, 77 K, (d) irradiated CCl4, 77 K, and (e)
irradiated CCl4, 220 K. Asterisk (*) indicates internal standard MgO:Cr3+
.
are involved in the synthesis of ACF-013, the formation of
such radicals can be expected. For comparison, a corre-
sponding radical was generated photochemically in a tetra-
chloromethane matrix (77 K, 254 nm), and its spectrum is
presented in Figure 4d,e. Comparison of these spectra with
that of ACF-013 shows the expected similarities between
the systems, keeping in mind that in both cases a superposi-
tion of different chlorine containing radicals appeared.
It should be noted that the radical signal does not undergo
changes while performing the isomerization reaction of 1,2-
dibromohexafluoropropane inside the cavity of the ESR
spectrometer.
19F MAS NMR. The 19F MAS NMR spectrum of ACF-
013 is presented in Figure 5. Three types of components are
evidenced corresponding to three different ranges of 19F
isotropic chemical shift δiso values. The main one (type 1)
has a δiso value close to 0 ppm as in the various AlF3 phases.
The second contribution (type 2) is related to unusual
negative δiso values of about -40 ppm. The third type
consists of very small and narrow lines in the 50-100 ppm
range, which corresponds to organic fluorine compounds.
Figure 5 shows the best reconstruction of the experimental
spectrum with two lines for both the type 1 and type 2
components. The corresponding parameters are given in the
Figure 5 caption. The deconvolution was achieved using the
DMFIT program17 assuming anisotropic chemical shifts and
Gaussian line shapes. 19F dipolar coupling was not explicitly
taken into account.
ESR. Aluminum chloride used in the preparation of ACF-
013 has few signals in the X-band ESR spectrum originating
essentially from Fe3+ ions (Figure 3a). This powder spectrum
was simulated using the parameters obtained from single
crystal measurements of Fe3+ in an AlCl3 matrix16 (Figure
3b). In addition, a symmetric signal of unknown origin
appeared at g′ ∼ 2.
The ESR spectra of ACF-013 and am-AlF3 recorded in
X-band in the whole spectral range are shown in Figure 3c,d,
respectively. They exhibit two broad resonances of low
intensity at g′ ∼ 4.3 and g′ ∼ 2.0. ACF-013 has an additional,
intensive signal at g′ ∼ 2.
The latter, measured at room temperature and 77 K, is
shown magnified in Figure 4a,b, respectively. With increas-
ing fluorine content, the signal gains in intensity, while
simultaneously the components are resolved worse (compare
Figure 4b,c). The resulting spectral pattern including the
g-shift to higher values led to the tentative assignment of
radicals derived from CCl4. Since CCl4 and its precursors
(16) Bang, G.; Sperlich, G.; Becker, W.-J. Phys. Status Solidi 1970, 41,
369-374.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6477

Krahl et al.
Figure 6. 19F MAS NMR spectra recorded at 35 kHz for different phases
of AlF3: (a) ACF-013, (b) am-AlF3, (c) AlCl2.39F0.61, (d) hydrolyzed ACF-
013, (e) R-AlF3, (f) â-AlF3, (g) R-AlF3‚3H2O, and (h) â-AlF3‚3H2O.
Figure 5. Experimental and calculated 19F MAS NMR spectra (35 kHz
spinning speed). The calculated spectrum is the sum of five components.
Type 1 19F: (1a) δiso ) 3.5 ppm; width, 24 ppm; rel int, 71%; (1b) δiso
)
-4 ppm; width, 16 ppm; rel int, 23%. Type 2 19F: (2a) δiso ) -33 ppm;
width, 14 ppm; rel int, 5%; (2b) δiso ) -42 ppm; width, 6 ppm; rel int,
1%. Type 3 19F: (3) δiso ) 44 ppm; width, 3 ppm; rel int, ≈0%.
The 25 kHz spectrum is presented in Figure 7. The shape of
the central transition obtained for the highest speed (see
insert) is characteristic of a disordered material with a
distribution of quadrupolar frequencies.
We used the same theoretical treatment of the SATRAS
NMR spectra18-20 as in a previous study of nanostructured
alumina.12 Since the solid is amorphous, the spectra cannot
be explained using single values for the quadrupolar param-
eters, νQ and ηQ. The broad envelope of the spinning
sidebands of the satellite transition is reproduced well by
simulation using a two-dimensional Czjzek distribution12,21,22
P(νQ,ηQ)
Samples of partially fluorinated AlCl3 show very similar
spectra. The relative intensities of the 19F type 2 lines increase
with higher fluorine content. This is shown by comparing
the spectra of ACF-013 and AlCl2.39F0.61 (Figure 6a,c). The
spectrum of am-AlF3 is shown in Figure 6b. It exhibits only
one line of type 1 with a δiso value of 2 ppm.
Experiments were also done on compounds which were
hydrolyzed after exposure to air for 6 h, but which remained
in a powder state afterward (ACF-013, AlCl0.80F2.20, and
AlCl1.25F1.75). Only one intense signal of type 1 was observed
in these spectra. Compared to ACF-013 and crystalline AlF3,
the lines are shifted to higher value by about 6 ppm (Figure
6d for ACF-13).
For comparison, 19F NMR spectra recorded for various
crystalline AlF3 phases (R-AlF3, â-AlF3, R-AlF3‚3H2O,
â-AlF3‚3H2O) are included in Figure 6. Note that δiso shifts
toward higher values (about 20 ppm) from the pure AlF3
phases to the corresponding hydrated ones.
27Al Solid State NMR. 27Al SATRAS NMR spectra of
ACF-013 were recorded at 25 and 3 kHz spinning speeds.
d-1
2
νQ
νQ
2
2
P
d,σ(νQ, ηQ) )
ηQ(1 - ηQ /9)exp -
(1 + ηQ /3)
{
}
x
2σ2
d
2πσ
with d ) 3 and σ ) (440 ( 40) kHz (Figure 7). The δiso
parameter was -13 ( 1 ppm. The individual line width was
assigned equal to 1.2 kHz. For a fine reconstruction of the
(18) Skibsted, J.; Vosegaard, T.; Bildsoe, H.; Jakobsen, H. J. J. Phys. Chem.
1996, 100, 14872-14881.
(19) Skibsted, J.; Nielsen, N. C.; Bildsoe, H.; Jakobsen, H. J. J. Magn.
Reson. 1991, 95, 88-117.
(20) Ding, S.; McDowell, C. A. Chem. Phys. Lett. 2001, 333, 413-418.
(21) Czjzek, G.; Fink, J.; Go¨tz, F.; Schmidt, H.; Coey, J. M. D.; Rebouillat,
J. P.; Lienard, A. Phys. ReV. B 1981, 23, 2513-2530.
(22) Czjzek, G. Phys. ReV. B 1982, 25, 4908-4910.
(17) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve´, S.; Alonso,
B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson.
Chem. 2002, 40, 70-76.
6478 Inorganic Chemistry, Vol. 42, No. 20, 2003

Structure of Aluminum Chlorofluoride
Figure 8. Experimental 27Al MAS SATRAS NMR spectra (25 kHz
spinning speed): (a) ACF-013 and (b) am-AlF3. Details of the central line
shown in the insert.
Figure 7. Experimental and calculated 27Al MAS SATRAS NMR spectra
(25 kHz spinning speed). The calculated spectrum is the sum of two
components: (a) contribution calculated with a Czjzek distribution of
quadrupolar parameters (σ ) 440 kHz, d ) 3); (b) contribution calculated
with νQ ) 11 kHz. Details of the central line shown in the insert.
central transition, a second contribution evidenced on the 3
kHz spectrum with νQ ) (11(2) kHz, ηQ ) 0.5(0.5, and
δiso ) (-19(1) ppm with the same line width has to be
taken into consideration. It corresponds to 10% of the Al
nuclei.
In Figure 8, the 27Al SATRAS NMR spectra of ACF-013
and am-AlF3 measured at 25 kHz are compared. The latter
exhibits a narrower central line and spinning sideband
envelope. This is in agreement with the smaller Czjzek
parameter σ ) (360 ( 40) kHz deduced from the simulation
of the spectrum. Both distributions are compared in Figure
9. The corresponding parameters in R-AlF3 measured here
are νQ ) (32(4) kHz, ηQ ) 0, and δiso ) (-19(1) ppm.23,24
Samples of partially fluorinated AlCl3 were also investi-
gated at two spinning speeds. All the spectra recorded at 25
kHz give evidence of two main contributions: a crystalline
one and a disordered one like ACF-013. The reconstruction
of these spectra recorded at 3 kHz spinning speed demon-
strated that the crystalline phase corresponds to a superposi-
tion of the main component with νQ ) (65 ( 5) kHz, ηQ )
0.27 ( 0.05, δiso ) (-2.5 ( 1) ppm, and a line width equal
to 0.3 kHz (crystalline AlCl3 phase25) in addition to a small
one (8%) with νQ ) (8(2) kHz and the same isotropic
1
Figure 9. 1D Czjzek distributions R(νQ) ) ∫0 Pd,σ(νQ, ηQ) dνQ with d )
3 for (a) ACF-013, σ ) 440 kHz, and (b) am-AlF3, σ ) 360 kHz. For
comparison, the quadrupolar frequencies including the error bars are shown
for (c) R-AlF3 and (d) â-AlF3.22 Note that cQ ) 2/3νQI(2I - 1).
chemical shift. The amount of crystalline AlCl3 extracted
from the simulation of the spectra at 25 kHz is given in Table
1 and Figure 10.
The spectra of the hydrolyzed compounds studied,
AlCl0.80F2.20 and AlCl1.25F1.75, show that the AlCl3 spectrum
is reduced to a contribution with νQ ) (4 ( 2) kHz and
δiso) (-2.5 ( 1) ppm, whereas the ACF-013 part seems to
remain almost unaffected.
3.2 Thermal Behavior of ACF and am-AlF3. Thermal
analysis. The thermal behavior of ACF-013 was studied by
DTA measurements in sealed quartz ampules. In order to
distinguish between reversible and irreversible transitions,
a temperature program including multiple heating and cooling
runs and different maximum temperatures was applied.
(23) Chupas, P. J.; Ciraolo, M. F.; Hanson, J. C.; Grey, C. P. J. Am. Chem.
Soc. 2001, 123, 1694-1702.
(24) Delattre, J. L.; Chupas, P. L.; Grey, C. P.; Stacy, A. M. J. Am. Chem.
Soc. 2001, 123, 5364-5365.
(25) Ding, S.; McDowell, C. A. J. Magn. Reson. A 1995, 112, 36-42.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6479

Krahl et al.
The X-ray diffractogram of the remaining phase recorded
in an open sample holder showed â-AlF3 and a small amount
of AlCl3‚6H2O. The hydrate was formed by the uptake of
water by AlCl3 during the diffraction measurement.
The thermal behavior of am-AlF3 is completely different
from that of ACF-013. Its DTA curves are shown in Figure
11b. They exhibit a weak, broad endothermic region with a
maximum at about 120 °C. It is followed by a sharp
endothermic effect with an onset temperature of 243 °C and
a peak temperature of 248 °C, and a broad exothermic peak
with a maximum at 427 °C. The first weak, broad endo-
thermic effect is irreversible, whereas the second, sharp
endothermic peak is reversible. In contrast to ACF-013, the
phase remaining after thermal analysis here was pure R-AlF3.
It is noteworthy that the exothermic peak at 427 °C is spread
over a temperature interval of more than 200 °C, whereas
for ACF-013 a very sharp effect was observed.
Figure 10. Molar AlCl3 content of AlClxF3-x versus x from Table 1. The
straight line corresponds to a linear regression for x > 0.3.
XRD and Catalysis. For a better understanding of the
DTA curves obtained for ACF-013, the nature of the phases
occurring during the heating of ACF was investigated by
isochronous annealing of ACF-013 sealed in a quartz ampule
for 20 min at temperatures of 300, 400, 500, and 600 °C,
and recording the X-ray diffraction patterns8 at room
temperature. After annealing at 300 °C, the sample did not
show any reflections. After annealing at 400 °C, intensive
reflections of η-AlF3 and weak ones of AlCl3 occurred. After
annealing at 500 °C, large reflections of θ-AlF3 appeared
together with a small ones of â-AlF3; after annealing at 600
°C, â-AlF3 was found along with a small amount of θ-AlF3.
At the latter two temperatures, AlCl3 was not seen but could
not be excluded, since its reflections overlap with either the
â-AlF3 or θ-AlF3 ones.
Samples of ACF-013 annealed below 400 °C were as
catalytically active as ACF-013. Samples annealed above 400
°C were inactive.
Evolution of Volatile Compounds. Since the sample for
DTA analysis was sealed in an ampule, the evolved gases
could not be analyzed simultaneously. Therefore, ACF-013
was heated with 10 K/min in flowing argon in a quartz
reactor up to 600 °C, and the gas stream was analyzed by
FTIR spectroscopy. It shows clearly that carbon containing
species are evolved. Wavenumbers of the vibrations are given
in parentheses.
Up to 250 °C, CCl4 (795 cm-1) is evolved with a
maximum at 180 °C; COCl2 (1830, 850 cm-1) and HCl (2886
cm-1) are released between 200 and 350 °C. A sudden sharp
pulse of these gases and further of CO2 (2350 cm-1), CF4
(1282 cm-1), and SiF4 (1032 cm-1) is observed at the
crystallization point. COCl2, CO2, HCl, and HF are produced
during pyrohydrolysis of the sample. HF reacts with the
quartz reactor and forms SiF4.
Figure 11. DTA measurements of (a) ACF-013 and (b) am-AlF3 in quartz
ampules. 1, 2, and 3 indicate the number of the heating cycle.
Figure 11a shows the heating curves for ACF-013. During
the first heating, a sharp and intensive exothermic signal with
an onset temperature of 400 °C and a peak temperature of
401 °C occurs, indicating an irreversible process. Therefore,
the second heating shows thermal effects of a new phase
composition characterized by a weak, reversible endothermic
effect at 174 °C and an irreversible exothermic effect at 592
°C; the latter exclusively occurred during the second heating.
Consequently, the third heating showed only the weak
endothermic signal at 174 °C.
4. Discussion
FTIR Spectra. All crystalline phases of AlF3 known so
far are built up of corner-sharing AlF6 octahedra linked to a
3
three-dimensional network (∞ [AlF6/2]).26 The strong bands
around 660 and 360 cm-1 in the crystalline phases (Figure
1) can be assigned to stretching and bending vibrations of
6480 Inorganic Chemistry, Vol. 42, No. 20, 2003

Structure of Aluminum Chlorofluoride
the corner-sharing AlF6 octahedra. The Al-F bond lengths
in both of these phases are almost equal.27,28 So, the force
constants of the vibrations and the reduced masses can
expected to be almost equal. Therefore, the vibrational
frequencies in both crystalline phases are very similar. An
additional weak band originates from an Al-F-Al bending
vibration.29 Since the angles between the octahedra are
different in both crystalline modifications, both bands have
different frequencies.
nuclei shared by two AlF6 octahedra (bridging fluorine). The
contribution of type 2 signals with δiso around -40 ppm can
be explained according to the so-called superposition model
for the 19F chemical shift.31 This low value is consistent with
F atoms with only one Al atom as the nearest neighbor
(terminal or unshared fluorine) (Figure 5).
Hydrolysis results in the disappearance of the signals of
the terminal 19F atoms and a shift in the type 1 line to higher
positive values. This shift is also observed going from water-
free crystalline AlF3 phases to the corresponding hydrates
(Figure 6). Following that, it may be related to the presence
of water molecules.
Furthermore, fluorine atoms shared between two AlF6
octahedra that are responsible of the type 1 contribution could
be classified in the following way: The contribution at δiso)
-4.0 ppm (around 25%, see Figure 5) may be related to
fluorine atoms in a 3D almost regular AlF6 octahedra network
as in R-AlF3, while the one at δiso ) +3.5 ppm (around 75%,
see Figure 5) could be assigned to fluorine atoms in a more
disordered network or at the surface.
From the 19F measurements, except the lines related to
the organic fluoride molecules, the main difference between
ACF-013 and am-AlF3 is the existence of these unusual low
δiso values at about -40 ppm (compare Figure 6a,b). Thus,
in ACF-013, it could be assumed that a small amount (4-
5%) of unshared fluorides is present.
In the case of ACF-013 and am-AlF3, two strong bands
are also observed at wavenumbers similar to those of R- and
â-AlF3 (Figure 1c,d). For these noncrystalline samples, the
bands are significantly broader than those of the crystalline
samples. The bands of ACF-013 are even broader than those
of am-AlF3. Therefore, it can be assumed that ACF-013 and
am-AlF3 are also built up of corner-sharing AlF6 octahedra.
The broader bands of ACF-013 indicate that the degree of
disorder in this compound is higher than in am-AlF3.
The main difference between the crystalline and the
amorphous samples is that additional weak bands are not
seen in the latter. This can be explained with the help of a
simplified structure model for am-AlF3 and ACF-013, in
which the bond lengths, and especially the bond angles, are
statistically distributed around a mean value. Bond lengths
are usually very restricted, so their distribution is compara-
tively narrow. Since the distribution of the Al-F bond
lengths causes a broadening of the bands of the AlF6
stretching and bending vibrations, it does not shift the peak
maxima significantly. Contrary to this, the Al-F-Al bond
angle for corner-sharing octahedra can vary from about 132°
to 180°.30 Since the frequency of the Al-F-Al bending
vibration strongly depends on this angle and there is no
corresponding band visible in the IR spectrum, it can be
concluded that these angles are distributed over a compara-
tively wide range. The bands of these bending vibrations
become very broad and are completely superposed by the
other two strong bands.
27Al MAS SATRAS NMR Experiments. The simulation
of the MAS SATRAS NMR spectra clearly shows that the
distribution of quadrupolar frequencies is broader in ACF-
013 than in am-AlF3 (Figure 8). This is related to a larger
disorder in the former phase. The 1D distribution functions
1
R(νQ) ) ∫0 Pd,σ(νQ, ηQ) dηQ for both phases are given in
Figure 9. The larger distribution width σ shows clearly that
the degree of disorder in ACF-013 is higher than in am-
AlF3. The quadrupolar frequencies in the crystalline phases
are not distributed but are fixed values (R-AlF3, 35 ( 3 kHz;
â-AlF3, 120 ( 15 kHz23,24). For comparative purposes, they
are also given with their error bars in Figure 9.
Cl K Edge XANES. The shape of the XANES spectrum
of the Cl K edge in AlCl3 changes significantly during
fluorination with CFCl3 (Figure 2). The resonance at 2826.0
eV develops during fluorination. These spectra can be
regarded as a fingerprint for the local environment of
chlorine. It is obvious that the electronic structures of the
Cl atoms in AlCl3 and ACF-013 are different from each
other. Looking at the spectra of AlCl2.39F0.61 and AlCl1.75F1.25
(Figure 2), one can clearly see that these spectra are a
superposition of the spectra of AlCl3 and ACF-013. These
compounds contain two different chlorine species: a species
such as that in AlCl3 and a species such as that in ACF-013.
19F MAS NMR Experiments. The type 1 signals with
From the experiments on the partially fluorinated AlCl3,
it was demonstrated that an AlCl3 crystalline phase coexists
with a disordered ACF-013 phase. Linear regression of the
AlCl3 content versus the chlorine content x in AlClxF3-x
(Figure 10) shows that for x e 0.3 the sample does not
contain an AlCl3 phase any more.
ESR Spectra. Fe3+ is easily and doubtlessly detected in
aluminum chloride by its fine structure splitting (Figure 3a).
Fe3+ substitutes for Al3+ on a regular lattice position. Starting
from the corresponding single crystal data,16 the simulation
delivers a spectrum that acceptably agrees with the measured
spectrum (Figure 3b).
Already in the first phase of the reaction with CFCl3 at
about 25 °C, the discrete Fe3+ spectrum disappears and
changes into comparatively broad resonances in the g’ ∼
4.3 and g’ ∼ 2.0 regions and at g’ g 2 (Figure 3c). The
simultaneous appearance of signals at g’ ∼ 4.3 and g’ ∼
δ
iso at around 0 ppm as in the AlF3 phase correspond to 19
F
(26) Herron, N.; Thorn, D. L.; Harlow, R. L.; Jones, G. A.; Parise, J. B.;
Fernandez-Baca, J. A.; Vogt, T. Chem. Mater. 1995, 7, 75-83.
(27) Daniel, P.; Bulou, A.; Rousseau, M.; Nouet, J.; Fourquet, J. L.; Leblanc,
M.; Burriel, R. J. Phys.: Condens. Matter 1990, 2, 5663-5677.
(28) Le Bail, A.; Jacoboni, C.; Leblanc, M.; De Pape, R.; Duroy, H.;
Fourquet, J. L. J. Solid State Chem. 1988, 7, 96-101.
(29) Bondam, J. Acta Chem. Scand. 1971, 25, 3271-3276.
(30) Mu¨ller, U. Anorganische Strukturchemie; Teubner-Verlag: Stuttgart,
Germany, 1996; Chapter 15.
(31) Bureau, B.; Silly, G.; Buzare´, J. Y.; Emery, J. Chem. Phys. 1999,
249, 89-104.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6481

Krahl et al.
2.0 is typical for Fe3+ in amorphous fluoride matrixes (Figure
3c,d).11 From this, it can be concluded that some of the iron
impurities at this temperature are incorporated into the arising
amorphous ACF-013 phase. In addition, another part is
forced to aggregate. Evidence for this is given by the very
broad resonances at g’ g 2: one in the region below 100
mT and another between 200 and 400 mT. Similar aggrega-
tion phenomena were also observed in fluoride glasses.32
As shown in Figure 4a,b, the spectra of ACF-013 are
characterized by an intensive signal in the g’ ∼ 2 region. It
has already been mentioned that these signals are essentially
caused by chlorine-containing radicals, which are formed
during the synthesis of ACF-013. At this point, the com-
paratively large thermal stability of such a radical is
surprising. However, these observations are supported by the
fact that a partially amorphous AlF3 matrix is able to stabilize
organic radicals and even atomic hydrogen up to the
temperature region already mentioned.33 Hence, the occur-
rence of the these radicals is further proof of the amorphicity
of the ACF. The evolution of CCl4 and COCl2 during heating
clearly shows that significant amounts of chlorine containing
organic species are trapped in ACF-013. The radicals do not
take part in the catalytic reaction used here.
was heated to a temperature above 400 °C at least once, and
hence, AlCl3 has been formed. This gives further proof that
AlCl3 does not exist as a separate phase in ACF-013.
The nature of the phase transition is evidenced by two
more facts: (i) The catalytic activity of annealed samples
of ACF-013 is high, as long as the annealing temperature is
below 400 °C. (ii) Corresponding with the exothermic signal,
a bunch of volatile organic compounds (mainly CCl4, CF4,
and hydrolysis products) are evolved. So at this point, the
structure completely decomposes and releases all the trapped
species.
X-ray diffraction of ACF-013 annealed in an ampule up
to the transition point of 400 °C shows η-AlF3. At 500 °C,
the main phase is θ-AlF3; at 600 °C, it is â-AlF3. This
sequence is in good agreement with the DTA measurements,
which show a weak, irreversible exothermic transition at 592
°C. Thus, this peak can be assigned to the reaction θ-AlF3
f â-AlF3.
Up to now, all metastable phases of AlF3 were reported
to transform directly into R-AlF3.26 However, these phases
seem to be relatively close to each other from a thermody-
namic point of view. Slight changes in the reaction conditions
(type and pressure of the gas phase) cause different phases
to emerge. Surprisingly, this is the first time that a consecu-
tive cascade of phase transformations is observed. In the
closed ampules, certain partial pressures of AlCl3 and AlF3
develop on decomposition of ACF-013. Phase transitions as
observed here seem to be very likely under conditions such
as those used in our case, since, e.g., â-AlF3 can be produced
from R-AlF3 in a chloride flux.29
The thermal behavior of am-AlF3 strongly differs from
that of ACF-013 (Figure 11b). As the first weak endothermic
effect is irreversible, it is attributed to small amounts of water
evolved by the solid that remained from the synthesis and
could not be removed in a vacuum. The endothermic process
occurring at about 250 °C seems to be closely related to the
subsequent crystallization processes, the nature of which
remains uncertain. It is noteworthy that the crystallization
region is spread over a temperature interval of more than
200 °C and delivers R-AlF3, whereas a very sharp effect is
observed for ACF-013 and the product is â-AlF3. Conse-
quently, the crystallization process of ACF-013 is completely
different from the one observed for am-AlF3.
The spectrum of the radical appears to be anisotropic, and
at lower temperature, it is split into hyperfine components
3
due to coupling with the chlorine nuclei (I ) /2 for 35Cl
and 37Cl). Several of these hyperfine components are quite
well resolved in the spectra of the samples with low fluorine
content (Figure 4c), but only poorly resolved in ACF-013
(Figure 4a,b). This loss of spectral resolution gives direct
evidence of the destruction of the crystalline structure during
the synthesis of ACF-013. The samples of partially fluori-
nated AlCl3 with a low fluorine content are mainly crystal-
line, which leads to comparatively sharp lines. The crystalline
regions disappear, and the solid becomes amorphous with
increasing amount of fluorine. This causes a statistical
distribution of the spectral parameters, in this case the
g-components (gj ) 2.017) and the components of the
hyperfine constants, and results in an effective line broaden-
ing in the spectrum.
Thermal Behavior. Heating up to about 350 °C does not
change the bulk structure of ACF-013 significantly. The
sample is still catalytically active and noncrystalline. At 400
°C, an irreversible, strongly exothermic phase separation
takes place (Figure 11a). Crystalline AlF3 and gaseous AlCl3
5. Conclusions
(i) The method published by DuPont for the synthesis of
ACF seems to play a key role in obtaining unique properties
of the resulting solid. Using low temperatures prevents the
formation of ordered AlF3 phases. Aluminum chloride and
organic compounds present during synthesis further disturb
the crystallization process. The latter can be detected in the
ACF formed. In contrast to ACF, the am-AlF3 phase does
not exhibit strong Lewis acidity; it is inactive in the
isomerization of 1,2-dibromohexafluoropropane.
3 - x
3
x
3
AlClxF3-x f
AlF3(cryst) + AlCl3(g)
are formed. After cooling, both solid phases are detected by
X-ray diffraction. Further evidence for the formation of AlCl3
is given by the observation of a reversible endothermic signal
at 174 °C. It correlates very well with the sublimation
temperature of AlCl3 at 70 kPa.34 It should be noted that the
sublimation of AlCl3 can only be observed when the sample
(ii) Although ACF and am-AlF3 are both X-ray amorphous
materials mainly composed of aluminum and fluorine, their
(32) Scholz, G.; Sto¨sser, R.; Grande, T.; Aasland, S. Ber. Bunsen-Ges. Phys.
Chem. 1997, 101, 1291-1296.
(33) Scholz, G.; Sto¨sser, R. Phys. Chem. Chem. Phys. 2002, 4, 5448-
5457.
(34) Gmelins Handbuch der Anorganischen Chemie, System-Nummer 35,
Aluminium, Teil B; Verlag Chemie GmbH: Germany, 1933; p 188.
6482 Inorganic Chemistry, Vol. 42, No. 20, 2003

Structure of Aluminum Chlorofluoride
local structures are different. ACF and am-AlF3 are built up
of corner-sharing AlF6 octahedra. This is made clear by IR
and NMR spectroscopy. These methods confirm a higher
degree of disorder in ACF than in am-AlF3. The X-band ESR
spectra of both materials are also typical for disordered
fluoride systems. 27Al SATRAS NMR shows that ACF is
the most distorted AlF3 solid known so far. However, small
amounts of nearly regular AlF6 octahedra are present in the
sample. Furthermore, terminal fluorine atoms, which are
bound to only one aluminum atom, are evidenced by 19F
MAS NMR. They are a unique feature of the ACF phase.
The fluorine atoms of am-AlF3 bridge two AlF6 octahedra;
no terminal fluorine atoms exist.
(iii) Paramagnetic chlorine-containing species are trapped
in cavities inside the amorphous ACF solid. These radicals
are similar to those generated in CCl4 by UV irradiation and
are the subject of a separate study. The trapped radicals are
stable up to 150 °C. They can be identified by their ESR
spectra and do not change upon performance of heteroge-
neous catalytic reactions.
released. The corresponding exothermic peak found by DTA
is very sharp and narrow; crystallization takes place in a small
temperature range of only 20 K. The thermal behavior of
am-AlF3 is completely different. Crystallization takes place
over a range of more than 200 K. The peak in the DTA is
broad resulting in the thermodynamically stable R-AlF3
phase. Furthermore, an endothermic effect of unknown origin
appears at around 250 °C.
(v) The high catalytic activity of ACF is related to the
absence of a crystalline AlCl3 phase. Residual chlorine in
AlClxF3-x cannot be attributed to a separate, crystalline phase
of AlCl3, when x e 0.3. NMR, XANES, ESR, and DTA
clearly prove this. This agrees with the values given by
DuPont of x ) 0.05-0.3 for the catalytically active ACF
phase.
Acknowledgment. We would like to thank Dr. S.
Schro¨der (Free University of Berlin) for the help in planning
and performing the X-ray absorption measurements and Dr.
A. Zehl (Humboldt University of Berlin) for ESR measure-
ments.
(iv) During heating, ACF forms crystalline metastable
phases of AlF3 and gaseous AlCl3 at 401 °C. At crystalliza-
tion, the volatile organic compounds trapped in cavities are
IC034106H
Inorganic Chemistry, Vol. 42, No. 20, 2003 6483