Infrared Study of the Surface Properties of HTB-Type Al-, Cr-, Fe-Hydroxyfluorides

Article abstract of DOI:10.1021/jp036496z

The surface properties of iron, chromium, and aluminum fluorides in their hexagonal tungsten bronze (HTB) form have been investigated by infrared spectroscopy. The presence of hydroxyls is clearly observed. H/D exchange experiments with different deuterated probe molecules having various molecular sizes show that they are localized inside of the channels of the structure. The v(OH) bands in the infrared spectra of these materials are downward shifted compared to those of corresponding metal oxides, whereas the δ(OH) in-plane bending mode presents an unusual high wavenumber. These spectral features are compared to those observed on zeolites, for which hydroxyl environment and bridged conformation are similar. HTB compounds exhibit both strong Lewis and Br?nsted acid sites. The use of two basic probes with a different size (pyridine and ammonia) allows one to localize and to quantify these two kinds of acidity. Br?nsted acidity is related to the presence of hydroxy groups into the microporous channels and to chemisorbed HF, whereas Lewis acidity is due to defect sites both on the outer surface of crystallites and in the channels. The strength of acid sites is unambiguously found stronger than that reported for Al, Cr, and Fe oxides. This result is discussed taking into account the electronegativity of fluorine but also the bridged conformation of OH groups, as in the case of zeolites.

Full text of DOI:10.1021/jp036496z

3246
J. Phys. Chem. B 2004, 108, 3246-3255
Infrared Study of the Surface Properties of HTB-Type Al-, Cr-, Fe-Hydroxyfluorides
Alexandre Vimont,† Jean-Claude Lavalley,† Lo1ıc Francke,‡ Alain Demourgues,‡
Alain Tressaud,‡ and Marco Daturi*,†
Laboratoire de Catalyse et Spectrochimie, UMR 6506 CNRS-ENSICaen, 6, bd Mare´chal Juin,
F-14050 Caen Cedex, France, and Institut de Chimie de la Matie`re Condense´e de Bordeaux, CNRS, 87,
aV Docteur A. Schweitzer, F-33608 Pessac Cedex, France
ReceiVed: August 21, 2003; In Final Form: December 23, 2003
The surface properties of iron, chromium, and aluminum fluorides in their hexagonal tungsten bronze (HTB)
form have been investigated by infrared spectroscopy. The presence of hydroxyls is clearly observed. H/D
exchange experiments with different deuterated probe molecules having various molecular sizes show that
they are localized inside of the channels of the structure. The ν(OH) bands in the infrared spectra of these
materials are downward shifted compared to those of corresponding metal oxides, whereas the δ(OH) in-
plane bending mode presents an unusual high wavenumber. These spectral features are compared to those
observed on zeolites, for which hydroxyl environment and bridged conformation are similar. HTB compounds
exhibit both strong Lewis and Brønsted acid sites. The use of two basic probes with a different size (pyridine
and ammonia) allows one to localize and to quantify these two kinds of acidity. Brønsted acidity is related
to the presence of hydroxy groups into the microporous channels and to chemisorbed HF, whereas Lewis
acidity is due to defect sites both on the outer surface of crystallites and in the channels. The strength of acid
sites is unambiguously found stronger than that reported for Al, Cr, and Fe oxides. This result is discussed
taking into account the electronegativity of fluorine but also the bridged conformation of OH groups, as in
the case of zeolites.
Introduction
on H/D exchange using deuterated probe molecules such as
tertiobutanol-OD, D2O, and even D2 have been performed. As
for the acidity determination, two basic probe molecules,
pyridine and ammonia, have been used. These probes allow us
to distinguish between Brønsted and Lewis acidity and to
quantify them. The ammonia molecule (σ ) 0.12 nm2) is smaller
than pyridine (σ ) 0.28 nm2) and equivalent to water (σ )
0.109 nm2). Therefore, NH3 can better diffuse inside the
channels of the HTB structure and therefore reaches a higher
proportion of acidic sites than in the case of pyridine. The results
are compared to those related for the parent metal oxides, e.g.,
Fe2O3, Cr2O3, and Al2O3. Up to now, no surface characterization
by infrared spectroscopy has been reported for FeF3, whereas
those concerning aluminum10,11 and chromium compounds12 are
not numerous and have not taken into account the accessibility
factor.
Fluorinated and hydroxyfluoride compounds are interesting
materials because they are good catalysts for many important
industrial reactions as aromatic alkylation1,2 or halogen exchange
in CFCs and HFCs.3 However, the knowledge of their surface
properties, necessary to understand the key parameters governing
their catalytic activities, is relatively limited with respect to the
parent oxides. In fact it is well-known that fluorination or
chlorination of the surface of numerous oxides affects their
catalytic activity,1-4 but the influence of halogenation on the
surface properties has been clearly investigated only for
alumina,5 for which it has been observed that fluorination
increases the surface acidity and modifies the nature of the
hydroxy groups.5,6 The results appear to depend on the fluorina-
tion method used and the fluorine content. To avoid such
experimental features, in the present work, we focus on well
crystallized fluorinated materials. The crystalline structure of
these compounds is related to that of hexagonal tungsten bronzes
(HTB), like that of MxWO3 bronzes. In particular, it exhibits
one-dimensional channels of about 3-4 Å diameter, free from
cations, in which water molecules occupy well-defined crystal-
lographic sites.7,8 Preliminary results on the characterization of
HTB fluoride samples used in the present study have shown
the presence of hydroxy groups, as evidenced by infrared
spectroscopy.9 Such hydroxyls can play an important role in
catalysis and it is therefore important to localize them and to
measure their acidity. To determine whether the hydroxy groups
are on the outer surface or in the channels, experiments based
Experimental Section
As described in a previous paper,13 the different samples were
prepared by various routes of synthesis: either by thermal
degradation of the ammonium metal fluoride ((NH4)3MF6), by
dehydration of the trihydrated metal fluoride (MF3‚3H2O), or
by fluorination of the hydroxide M(OH)3 under anhydrous HF
in the case of chromium fluoride. HTB (â) AlF3-x(OH)x was
also obtained by thermal dehydration of AlF3‚3H2O. The starting
compound AlF3‚3H2O was prepared from an alcoholic alumi-
num(III) nitrate solution, which favored the precipitation of
AlF3‚3H2O which is less soluble in ethanol than in water.
Aluminum(III) nitrate nanohydrate was dissolved in a large
amount of (ethanol:water) ) (7:3) solution. A stoichiometric
aqueous solution of HF was slowly added. The precipitate was
filtered, washed with a large amount of water and ethanol, and
dried overnight at 363 K. HTB-AlF3 was then obtained by
* To whom correspondence should be addressed. E-mail: daturi@ismra.fr.
Phone: +33-231452730. Fax: +33-231452822.
† Laboratoire de Catalyse et Spectrochimie.
‡ Institut de Chimie de la Matie`re Condense´e de Bordeaux.
10.1021/jp036496z CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/13/2004

Surface Properties of HTB-Type Hydroxyfluorides
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3247
Figure 1. Infrared spectra of FeF3-x(OH)x, after calcination at 473 K under vacuum (10-4 Pa) (spectrum a), then introduction of the equilibrium
pressure Pe ) 666 Pa of tertiobutanol-OD at room temperature and evacuation under vacuum at 373 K (spectrum b); then introduction of Pe ) 1333
Pa of D2O at 373 K and evacuation under vacuum at 473 K (spectrum c). Inset: infrared range of the ν+δ(OH) combination band.
dehydration of AlF3‚3H2O under vacuum at 493 K during 5 h
followed by a thermal treatment under argon atmosphere during
14 h at 723 K.
HTB-FeF3-x(OH)x‚δH2O and HTB-Fe0.8Cr0.2F3-x(OH)x‚
δH2O were prepared from their homologous trihydrated fluo-
rides, under “self-generated” atmosphere at 493 K during 16 h.
The starting materials MF3‚3H2O was obtained from an
alcoholic M(III) nitrate solution, as described above.
the top of the cell for thermal treatments. Spectra were recorded
at room temperature after quenching the sample from the thermal
treatment temperature. The cell was connected to a vacuum line
for evacuation and calcination steps (Presidual ) 10-3-10-4 Pa).
Addition of accurately known increments of probe molecules
in the cell was possible via a calibrated volume connected to a
pressure gauge and a control of the probe pressure by a gauge.
A Nicolet Magna 750 IR spectrometer equipped by a DTGS
detector and an XT-KBr beam splitter was used for the
acquisition of spectra recorded in the 500-5600 cm-1 range.
Samples were activated by calcination under vacuum (10-4
Pa) by heating (1 K/min) up to a limit temperature nonexceeding
that of sample decomposition (i.e., fluorine departure). Accord-
ing to the thermogravimetric analysis coupled with mass
spectrometer,13 these temperatures are 613 K for AlF3-x(OH)x
prepared by decomposition of trihydrated fluoride, 473 K for
FeF3-x(OH)x and Fe0.8Cr0.2F3-x(OH)x, and 373 K for CrF3-x(OH)x.
For instance in the case of AlF3-x(OH)x prepared by decomposi-
tion of trihydrated fluoride, the OH concentration determined
by chemical analysis13 is high and the x value is equal to 0.8.
Due to the tendency of chromium salts to form pyrochlore
hydroxyfluoride phases, a CrIII-based HTB phase could not
successfully been synthesized by thermal dehydration of CrF3,-
3H2O. Therefore the chosen route was the thermal decomposi-
tion of (NH4)3CrF6 under a “self-generated” atmosphere.
Moreover, to avoid the presence of any ammonium group which
would generate basic properties in the final material, another
route was developed. Cr(OH)3‚3H2O reactant was prepared by
precipitation from an aqueous chromium(III) nitrate solution.
Chromium(III) nitrate nanohydrate was dissolved in a large
amount of water. A stoichiometric amount of a 30% NH4OH
aqueous solution was then slowly added. The green precipitate
was filtered, washed with a large amount of water, and dried
overnight at 363 K. The chromium(III) trihydrated hydroxide
was then dehydrated at 373 K during 14 h under an argon flow
and then fluorinated under an anhydrous HF flow at 323 K
during 8 h. The as-prepared amorphous solid was then placed
in a platinum tube sealed under argon atmosphere and heated
at 498 K during 16 h. In this latter process, the ammonium
content remained smaller than in the case of the thermal
decomposition of (NH4)3CrF6. In a previous study,9 F- and OH-
amounts in the different samples have been evaluated: in the
Results
Hydroxy Groups: Nature and Accessibility Determina-
tion. FeF3-x(OH)x HTB and Fe0.8Cr0.2F3-x(OH)x HTB.
FeF3-x(OH)x and Fe0.8Cr0.2F3-x(OH)x HTB samples show strong
absorption bands in the 3200-4000 cm-1 ν(OH) range (Figures
1 and 2, spectrum a). A strong and slightly asymmetric band is
observed at 3595 cm-1, and weaker features at 3505 cm-1
(FeF3-x(OH)x) and 3520 cm-1 (Fe0.8Cr0.2F3-x(OH)x) are also
detected.
case of Al compounds, the estimated formula is AlF2.20(OH)0.80
0.29H2O for AlF3‚3H2O route.
‚
In both spectra, a broad absorption band is detected between
2800 and 3500 cm-1 and another at 1640 cm-1, characterizing
traces of water strongly bonded to the surface.
Powders were first characterized by X-ray diffraction and
chemical analysis.9,13 The HTB structural features of these
phases were clearly confirmed by X-ray9 and neutron diffraction
experiments; no spurious phases or compounds were detected.
The thermal decomposition was also investigated.13 For FTIR
spectroscopy, samples were pressed (107-109 Pa) into self-
supported disks (2 cm2 area, 7-20 mg cm-2). They were placed
in a quartz cell equipped with KBr windows. A movable quartz
sample holder permits the pellet to be adjusted in the infrared
beam for spectra acquisition and to displace it into a furnace at
IR spectra b in Figures 1 and 2 are recorded after tertiobutanol
OD treatment (see caption for experimental details). After
introduction of tertiobutanol OD, no significative decrease of
ν(OH) band intensities is observed; consequently, only a very
weak band at 2652 cm-1 is detected in the 2700-2600 cm-1
ν(OD) stretching range . Therefore, it is concluded that the
amount of OH groups accessible to tertiobutanol is negligible.
By contrast, after D2O exchange (spectrum c, Figures 1 and
2, see caption for experimental details), ν(OH) band intensities

3248 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Vimont et al.
Figure 2. Infrared spectra of Fe0.8Cr0.2F3-x(OH)x, after calcination at 473 K under vacuum (10-4 Pa) (spectrum a), then introduction of Pe ) 666
Pa of tertiobutanol-OD at room temperature and evacuation under vacuum at 373 K (spectrum b); then introduction of Pe ) 1333 Pa of D2O at 373
K and evacuation under vacuum at 473 K (spectrum c). Inset: infrared range of the ν+δ(OH) combination band.
TABLE 1: Recapitulative Table of OH(D) Bands of Hydroxyls Observed on the Spectra of the HTB Compounds after
Activation
hydroxyl bands
compound
ν
(OH)/ν(OD) (cm-1
)
δ
(OH)/δ(OD) (cm-1
)
(ν+δ)OH (cm-1
)
assignments
Fe0.8Cr0.2F3-x(OH)x
3595/2652
3595/2652
1005/783
1038/800
1070/830
1005/783
1060/831
1070
4580
4614
bridged OH group (Fe site)
bridged OH (closed to Cr site)
FeF3-x(OH)x
3595/2652
4581/3422
bridged OH group
CrF3-x(OH)x
AlF3-x(OH)x
∼3600 (awaited)
4650
4772
bridged OH group
bridged Al-OH group
3665/2701
1128/879
strongly decrease. Simultaneously, ν(OD) bands are clearly
observed at 2652 and 2582 cm-1 (FeF3-x(OH)x) and at 2652
and 2598 cm-1 (Fe0.8Cr0.2F3-x(OH)x), corresponding (according
to the ratio ν(OH)/ν(OD) equal to 1.355) to OD groups having
replaced hydroxy groups initially observed at 3595 and 3505
cm-1 and 3595 and 3520 cm-1, respectively. For the sake of
clarity, OH and OD band wavenumbers have been summarized
in Table 1. Intensity measurements of ν(OH) bands before and
after exchange show that almost all of the hydroxy groups are
accessible to D2O since at least 85% of them have been
exchanged.
Before H/D exchange, the FeF3-x(OH)x spectrum displays a
sharp IR band at 1005 cm-1 with a shoulder at 1060 cm-1. In
the case of the Fe0.8Cr0.2F3-x(OH)x material, three bands are
observed at 1005, 1038, and 1070 cm-1 (shoulder). They are
not sensitive to deuterated tertiobutanol treatment, whereas after
D2O treatment, they disappear while new bands are observed
at 783 and 831 cm-1 for FeF3 and at 783, 800, and 830 cm-1
for Fe0.8Cr0.2F3-x(OH)x. Therefore, due to this sensitivity to
deuterium exchange, these bands are not due to Fe-F vibration
modes, fundamentals, or overtones but to hydroxy groups
deformation modes. The isotopic effect 1060/831 and 1005/
783 cm-1 equal to 1.28 suggests an in-plane deformation mode
(δ(OH)) character as it will be discussed below.
treatment, a weak band is observed at 3422 cm-1 (Figure 1c)
and corresponds to the (ν+δ)OD combination band. Note that,
in the case of the Fe0.8Cr0.2F3-x(OH)x spectrum, two bands are
observed between 4650 and 4500 cm-1. Comparison with
FeF3-x(OH)x strongly suggests that the supplementary band at
4614 cm-1 is certainly associated to the supplementary δ(OH)
band observed at 1038 cm-1. Taking into account the anhar-
monicity coefficient of about 20 cm-1, we deduce that the 4614
cm-1 band corresponds to the combination 3595+1038 cm-1
.
This shows that the δ(OH) and the combination (ν+δ)OH
ranges are more sensitive than the ν(OH) range to the
heterogeneity of the OH-groups.
CrF3-x(OH)x HTB. Figure 3, spectrum a, represents the
infrared spectrum of the CrF3-x(OH)x sample after outgassing
at 373 K during 24 h. The low signal/noise ratio is due to the
necessity of using a high quantity of material (30 mg) to prepare
a self-supported disk. This high quantity partly explains why
the sample is not transparent to the IR beam in the 3600-2500
cm-1 range. Moreover, after evacuation at low temperature,
ammonia and water remain strongly absorbing in this range.
The presence of a strong band at 1610 cm-1 characterizes water
retained onto the sample. In addition to a band at 5239 cm-1
due to the (ν+δ)H2O combination band, a band is detected at
4650 cm-1, associated to the (ν+δ)OH combination. Taking
into account the strong δ(OH) band detected at 1070 cm-1 and
considering an anharmonic coefficient closed to 20 cm-1 (as
observed in the case of FeF3-x(OH)x) the frequency of the ν-
(OH) band at about 3600 cm-1 is deduced.
The NIR region of the unexchanged sample spectra gives
information about the vibrational modes associated with the
above-mentioned bands: the FeF3-x(OH)x spectrum shows a
band at 4581 cm-1 assigned to a combination band involving
those at 3595 and 1005 cm-1. Such a combination band confirms
that the 1005 cm-1 feature has a δ(OH) character. The
anharmonicity coefficient is equal to 19 cm-1. After D2O
A strong band at 1427 cm-1 is also detected, typical of the
δa(NH4) vibration of ammonium ions, probably formed on
Brønsted acid sites by ammonia introduced during the synthesis

Surface Properties of HTB-Type Hydroxyfluorides
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3249
Figure 3. Infrared spectra of CrF3-x(OH)x, after calcination at 373 K under vacuum (10-4 Pa) (spectrum a) and after calcination at 473 K under
vacuum (spectrum b).
Figure 4. Infrared spectra of AlF3-x(OH)x, after calcination at 613 K under vacuum (10-4 Pa) (spectrum a), then introduction of Pe ) 666 Pa of
tertiobutanol-OD at room temperature and evacuation under vacuum at 373 K (spectrum b); then introduction of Pe ) 1333 Pa of D2O at 373 K
and evacuation under vacuum at 613 K (spectrum c), after heating at 573 K under Pe ) 1.33 × 105 Pa of D2 (spectrum d). Left and right insets:
zoom of the ν(OH) and ν+δ(OH) regions, respectively.
step of the material. Outgassing CrF3-x(OH)x in more severe
conditions (473 K) permits the elimination of the bands at 5239
and 1427 cm-1 (Figure 3b), showing that water and ammonia,
respectively, have been desorbed from the surface. However,
new bands at 1280 and 1630 cm-1 are observed and assigned
to coordinated ammonia on Lewis acid sites created at the
expense of ammonium species. Therefore, it appears that
residual ammonia from CrF3-x(OH)x synthesis is chemisorbed
as ammonium ions in the presence of Brønsted acid sites and
converted to ammonia coordinated onto Lewis sites upon
thermal treatment. D2O and terbutanol OD do not produce any
isotopic exchange of the free hydroxy groups, characterized by
bands at 4650 and 1070 cm-1, suggesting that micropores are
certainly blocked by ammonia present onto the surface.
band at 2701 cm-1 after D2O exchange (Figure 4c) shows that
the OH groups are slightly accessible to heavy water molecules.
By contrast, after D2 treatment at 553 K (Figure 4d), the intensity
of the ν(OD) band at 2701 cm-1 is significant, whereas that of
bands at 3665 and 4772 cm-1 has decreased. It can be deduced
that about half of the hydroxy groups are exchangeable by D2.
In Figure 4a, a sharp band is situated at 1128 cm-1 and shifts
to 879 cm-1 upon H/D exchange. These bands are assigned to
the δ(OH)/δ(OD) vibration mode of OH groups giving rise to
the stretching frequency at 3665/2701 cm-1. In agreement, the
(ν+δ)OH combination mode at 4772 cm-1 (3665+1128 cm-1
,
with an anharmonicity coefficient of 21 cm-1) is clearly
observed. The position of the different hydroxyl bands for all
of the studied compounds is summarized in Table 1.
AlF3-x(OH)x HTB. The IR spectrum of AlF3-x(OH)x out-
gassed at 613 K (Figure 4a) shows a well defined ν(OH) band
at 3665 cm-1. In the NIR range, a narrow band is observed at
4772 cm-1. These bands are not sensitive to tertiobutanol OD
exposure (Figure 4b). The occurrence of a very weak ν(OD)
Acidity Measurements by Basic Probe Molecules: Pyri-
dine and NH3. Pyridine Adsorption. Acidity of materials has
been first probed by pyridine adsorption. This probe molecule
clearly permits the nature of the acidic sites to be identified.
Infrared spectra of the different samples after introduction of

3250 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Vimont et al.
TABLE 2: Significative Coordinated Pyridine and Pyridinium Ion Vibrations on the Different HTB Compounds
sensitive modes of coordinated pyridine on Lewis acid sites
pyridinium
ν
19b at 1545 cm-1
compounds
ν
8a (cm-1
)
ν
19b (cm-1
)
FeF3-x(OH)x
1609 (r.t.)
1613 (473 K)
1609 (r.t.)
1613 (473 K)
1612 (r.t.)
1620 (r.t.)
1447 (r.t.)
1449 (473 K)
1447 (r.t.)
1449 (473 K)
1448 (r.t.)
1454 (r.t.)
yes (r.t.)
no (373 K)
Fe0.8Cr0.2F3-x(OH)x
no
no
no
CrF3-x(OH)x
AlF3-x(OH)x.
1627 (573 K)
1456 (573 K)
species are formed, we deduce from the ν19b band intensity a
value close to 100 µmol g-1 for the coordinatively unsaturated
sites.
Ammonia Adsorption on FeF3-x(OH)x and Fe0.8Cr0.2F3 HTB.
Ammonia is often used as a probe molecule to distinguish
between Brønsted and Lewis acid sites: the infrared spectrum
of coordinated ammonia presents characteristic δs(NH3) and δa-
(NH3) bands at about 1200-1300 and 1620 cm-1, respectively,
whereas a band at about 1400-1450 cm-1 characterizes the δa-
(NH4) bending mode of protonated species. H-bonded species
give rise to a δs(NH3) band at about 1000-1100 cm-1 16
.
Ammonia has been introduced quantitatively in the IR cell
at 423 K to facilitate its diffusion and to prevent the formation
of weak adsorbed species. Infrared spectra of FeF3-x(OH)x and
Fe0.8Cr0.2F3-x(OH)x samples during ammonia adsorption and
desorption are presented in Figure 6. In the 700-1700 cm-1
range, spectra are similar: they show a strong band at 1425
cm-1 characteristic of ammonium species formation. Two other
weaker features at 1612 and 1260 cm-1 are assigned to
coordinated NH3 species. Another band at about 840 cm-1 is
also detected; its intensity seems to vary similarly to that at
1425 cm-1. In the 4000-2800 cm-1 range, ammonia adsorption
provokes the appearance of a complex massif between 2500
and 3400 cm-1, assigned to the ν(N-H) bands of ammonia and
ammonium species. The decrease in intensity of the ν(OH)
bands at 3595 cm-1 is clearly observed and is correlated with
the decrease of those at about 1000-1060 cm-1, in agreement
with their assignment to the δ(OH) vibrations of the same
hydroxyl. After introduction of Pe ) 400 Pa of ammonia into
the cell, the intensity of the 3595 cm-1 has strongly decreased,
becoming analogous to that observed after D2O treatment. It
shows that a large amount of hydroxy groups are accessible to
NH3 and D2O molecules and that these groups are acidic since
they give rise to ammonium species. Evacuation at 373 K and
then 473 K (spectra 1g and 1h) partly eliminates ammonium
species, whereas bands characterizing free hydroxy groups are
not restored at all in the case of FeF3-x(OH)x. The same feature
occurs in the case of Fe0.8Cr0.2F3-x(OH)x when outgassing from
373 to 423 K (Figure 6, spectra 2g and 2i). By contrast, when
Figure 5. Integrated intensity of ν19b band of coordinated pyridine at
about 1450 cm-1 versus concentration of pyridine introduced into the
cell at 423 K.
the equilibrium pressure Pe ) 133 Pa of pyridine followed by
evacuation at room temperature have been previously presented
(Figure 8 in ref 9) excepted for CrF3-x(OH)x. The characteristic
adsorbed pyridine bands in the 1400-1700 cm-1 range are
summarized in Table 2. The formation of coordinated species
on the surface is detected for all of the samples but with a low
concentration in the case of CrF3-xOHx, because of the lower
activation temperature of the sample. An additional band at 1545
cm-1 in the spectrum of FeF3-x(OH)x indicates the formation
of pyridinium species that easily desorbs after outgassing at
473 K.
For quantitative measurements, some experiments have been
performed by adding successive small doses (10-20 µmol g-1
)
of pyridine on the activated samples. In Figure 5, the variation
of the integrated area of the ν19b band, situated at about 1456-
1447 cm-1, has been reported versus the amount of pyridine
introduced. A linear plot, almost identical for both compounds
studied (Fe0.8Cr0.2F3-x(OH)x and AlF3-x(OH)x), is initially
observed suggesting that the first doses of pyridine introduced
are totally chemisorbed as coordinated species. The change of
the slope is well explained by the formation of physisorbed
species (band at 990 cm-1, not shown here). The slope of the
straight line allows us to deduce the apparent extinction
+
outgassing at 473 K, a large amount of NH4 species are
desorbed from that sample, leading to a partial restoration of
OH groups, mainly those characterized by the 1038 cm-1 δ-
(OH) band. The different behavior of the two types of hydroxy
groups is confirmed when NH3 is added: Figure 6, spectra 2a
to 2g clearly evidence a preferential perturbation of OH groups
characterized by the δ(OH) band at 1005 cm-1. This suggests
an acidity difference of the two kinds of OH groups, those
characterized by the 1005 cm-1 band being more acidic.
On both samples, the intensity of the ammonia coordinated
band at about 1260 cm-1 is very weak compared to that of the
ammonium band at 1425 cm-1, suggesting that ammonia
adsorption mainly occurs under the form of ammonium species.
We have plotted in Figure 7 the variation of the 1425 cm-1
band area versus the amount of ammonia introduced. A linear
coefficient of the ν19b band: it is close to 1.7-1.9 µmol cm-1
,
a value similar to that reported on metal oxides.14 After
introduction of Pe ) 133 Pa of pyridine, outgassing at room-
temperature desorbs physisorbed species. From the intensity of
the ν19b band, the amount of pyridine coordinated after saturation
can be deduced. It is about 125 µmol g-1 on Fe0.8Cr0.2F3-x(OH)x
and 35 µmol g-1 on AlF3-x(OH)x. Taking into account the
surface area of these samples, 55 and 20 m2/g, respectively,
the number of molecules of pyridine coordinated can be
estimated to 1.2-1.3 per nm2. Such a value is in agreement
with that reported for alumina activated at 573 K (1.65 per nm2
15). As for FeF3-x(OH)x, on which pyridinium and coordinated

Surface Properties of HTB-Type Hydroxyfluorides
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3251
Figure 6. Infrared spectra of FeF3-x(OH)x and Fe0.8Cr0.2F3-x(OH)x during introduction of NH3 into the IR cell and then desorption. FeF3-x(OH)x:
spectrum 1a before NH3 adsorption; spectra 1b-f: after introduction into the cell at 423 K of 120, 240, 360, and 750 µmol g-1 and then Pe ) 666
Pa of NH3, respectively. Spectra 1g and 1h: after evacuation under vacuum at 373 and 473 K, respectively. Fe0.8Cr0.2F3-x(OH)x: spectrum 2a
before NH3 adsorption; spectra 2b to 2g respectively after introduction into the cell at 423 K of 140, 260, 380, 630, and 920 µmol and Pe ) 666
Pa of NH3; spectra 2h, 2i, and 2j respectively after evacuation under vacuum at 373, 423, and 473 K.
that OH groups are well involved in the NH4+ species formation.
However data related to desorption experiments at 373, 423,
and 473 K does not fit the curve, showing that other acidic
centers have also to be taken into account, as it will be discussed
below.
Infrared difference spectrum of CrF3-x(OH)x after introduction
of ammonia into the cell is shown in Figure 9. Ammonia
adsorption on CrF3-x(OH)x mainly gives rise to ammonium
species (band at 1440 cm-1) and formation of coordinated
species, in a smaller amount (band at 1284 cm-1). Note that
the 1440 cm-1 band in Figure 9 adds to that already present at
the same wavenumber in the spectrum of the activated sample.
The perturbation of the δ(OH) band at 1070 cm-1 (negative
band in Figure 9) shows that ammonia protonation involves free
hydroxy groups. On AlF3-x(OH)x, only a band at a relatively
high wavenumber (1312 cm-1) is detected and characterizes
NH3 species strongly coordinated on Lewis acid sites (spectrum
not shown).
Figure 7. Integrated intensity of ammonium band (δa(NH4)) at
1425 cm-1 versus concentration of ammonia introduced into the cell
at 423 K.
variation is observed. Its slope leads to an apparent extinc-
tion coefficient of about 8 µmol cm-1, slightly lower than
that reported on zeolites (11 µmol cm-1).17 Such a difference
is well explained by the occurrence on FeF3-x(OH)x and
Fe0.8Cr0.2F3-x(OH)x samples of a small amount of coordinated
species. From these results, we can estimate the number of
Brønsted acid sites to about 1100 and 600 µmol g-1 on
FeF3-x(OH)x and Fe0.8Cr0.2F3-x(OH)x, respectively.
Discussion
IR Spectra of OH Groups. MF3-x(OH)x compounds are
constituted by MIII(F,OH)6 corner sharing octahedra. The
resulting framework is related to that of hexagonal tungsten
bronzes. Hexagonal cavities are formed and lead to one-
dimensional channels along the c axis with a six membered
ring7,8 (Scheme 1). The maximum diameter of these channels
is 3.30 Å for AlF3-x(OH)x and 3.60 Å for FeF3-x(OH)x.
Infrared spectra of the different HTB materials clearly show
OH bands characterizing the occurrence of hydroxyls. For
divided metal oxides, the origin of OH groups is due to the
necessity to complete the coordinence of cations in the exposed
To determine the nature of Brønsted acid sites involved in
+
NH4 species formation, we have studied the effect of NH3
adsorption on the intensity of bands due to hydroxy groups.
Since the (ν+δ)OH combination band is better resolved than
the ν(OH) one, we have determined the variation of the
combination band intensity versus the NH3 amount introduced.
Figure 8 shows a linear trend for both studied samples, showing

3252 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Vimont et al.
SCHEME 1: Conformation of Bridged Hydroxyls into a
Truncate Fragment of the HTB Structure (left) and
Representation of the Channel Aperture into the
Structure (right).
at about 3600 cm-1 for CrF3-x(OH)x is therefore assigned to
these lattice hydroxy groups.
H/D exchange experiments show that the accessibility of these
hydroxy groups depends on the size of the deuterated probe
molecule used. In general, tertiobutanol does not have access
to hydroxy groups since no isotopic exchange is observed. On
the contrary, hydroxy groups are exchanged by D2O, the
AlF3-x(OH)x sample excepted. In this latter case, the acces-
sibility is very low: only D2 partly exchanges the OH groups,
in agreement with the lower channel diameter of AlF3-x(OH)x.
We conclude that these hydroxy groups not exchanged by
deuterated tertiobutanol but by D2O or D2 are localized into
the channels of the HTB structure.
Another important point is the relative low frequency of the
ν(OH) band observed in the spectra of the HTB materials: about
3595 cm-1 for iron and chromium HTB material, whereas OH
groups are clearly pointed above 3600 cm-1 in the case of R-
and γ-Fe2O318 and R-Cr2O3.19 Furthermore, the ν(OH) band of
AlF3-x(OH)x HTB is observed at a lower wavenumber (3665
cm-1) than those detected on alumina (between 3680 and 3800
cm-1). It has been observed on zeolites having small pores (six-
and eight-membered ring) such as chabbazzite and erionite, an
unusual downward shift of the ν(OH) frequency of hydroxy
groups compared to zeolites with larger pores.20 This structural
effect has been explained by electrostatic interactions favored
by the small channel size. The channels of the HTB structure
are comparable to those presented by zeolites with small pores.
So, an analogous confinement effect well explains the low ν-
(OH) band frequency of the hydroxyls in HTB materials. This
interpretation is in agreement with the higher frequency position
of the ν(OH) bands of hydroxyls on R-AlF3 (bands at 3680 and
3702 cm-1),9 a structure for which a very small channel with a
diameter lower than 0.2 nm occurs (ReO3-type structure deriving
from perovskite). However, ν(OH) bands observed on R-AlF3
and partially fluorinated alumina are positioned also at a lower
wavenumber than ν(OH) on alumina, suggesting that fluorine
inductive effects can also influence the ν(OH) band position.
The in-plane bending vibrational mode of the hydroxyls is
unambiguously well detected in the 1000-1200 cm-1 range in
the HTB infrared spectra. On metal oxides, these vibrational
modes are generally indirectly detected via the (ν+δ)OH
combination band in the NIR range.21 Lavalley et al. have
studied the low-frequency IR spectrum range of different metal
oxides.22 They have observed that the frequency of the δ(OH)
band of OH groups is often situated close to the cutoff of the
samples. They have proposed to explain this phenomenon by a
coupling between the δ(OH) mode and the metal-oxygen
ν(M-OH) mode. The ratio δ(OH)/δ(OD) on HTB materials,
deduced by H/D exchange experiments, is equal to 1.28 for
FeF3-x(OH)x and AlF3-x(OH)x. It is slightly lower than the
theoretical value for a pure in plane bending mode (1.36). Such
Figure 8. Variation of the area of the ν+δ(OH) combination band
versus the NH3 amount introduced into the cell and after outgassing at
different temperatures (indicate on the graph).
Figure 9. Infrared difference spectrum for CrF3-x(OH)x after introduc-
tion into the cell of Pe ) 666 Pa of NH3 at 373 K (the reference
spectrum is that of the activated sample).
crystallographic surface planes. For HTB fluorinated com-
pounds, in principle no OH groups are expected. The partial
substitution of OH- groups for F- anions occurs during the
decomposition of hydrated precursors. From the structure of
the samples, the hydroxy groups share vertexes of two octahedral
cations and can be considered as type II according to their
coordination number. The sharp ν(OH) band observed at 3665
cm-1 for AlF3-x(OH)x, 3595 cm-1 for FeF3-x(OH)x, and awaited

Surface Properties of HTB-Type Hydroxyfluorides
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3253
TABLE 3: Comparative Position of the Sensitive Bands of
Basic Probe Molecules Coordinated onto the Surface of
Different Oxides and HTB Materials
observe that the ν8a band of coordinated pyridine on HTB
compounds is located at about 3 or 4 wavenumbers higher than
on the corresponding oxides. This feature expresses the stronger
Lewis acidity of HTB materials. A confirmation is given by
the position of the characteristic band of coordinated am-
monia: a blue shift of several tens of cm-1 is observed for δs-
(NH3) referring to metal oxides: about 20 cm-1 for AlF3-x(OH)x,
40 cm-1 for FeF3-x(OH)x, and more than 50 cm-1 in the case
of CrF3-x(OH)x.
sensitive mode of acidic probe molecules
ammonia
pyridine
compounds
δs(NH3)
ν8a
â-FeF3
1260 cm-1
1609 cm-1 (r.t.)
1613 cm-1 (473 K)
1608 cm-1
18
R-Fe2O3
1180, 1220 cm-1
1608 cm-1
28
γ-Fe2O3
H/D exchange experiments show that surface sites into the
channels have a low accessibility to bulk molecules, and the
diameter of these channels (inferior to 4 Å) probably induces a
steric hindrance in the case of pyridine molecules. So, at least
for pyridine, the detected Lewis acid sites probably characterize
only some defects localized onto the outer surface of the
crystallites. Theses defects are certainly associated to the
presence of M3+ coordinatively unsaturated (cus) ions with
coordinative vacancies due to fluorine departure. On γ-Al2O3,
two ν8a bands of coordinated pyridine and 3 δs(NH3) bands of
coordinated ammonia have been reported, assigned to different
configurations of surface cus Al3+, as expected by the presence
of both tetraAl3+ and octaAl3+ in the spinel structure.27,30 The
observation of only one ν8a pyridine band and one δs(NH3) band
in the spectra related to AlF3-x(OH)x suggests the presence of
only one type of cus Al3+ cations on its surface. On R-alumina
samples, where from structural considerations Al3+ cations in
octahedral position are only expected, it has been reported that
the Lewis acidity of such coordinatively unsaturated cations is
very low, as shown by pyridine adsorption giving rise to a ν8a
â-CrF3
1284 cm-1
1220 cm-1
1312 cm-1
1612 cm-1 (r.t.)
1608 cm-1
27
R-Cr2O3
â-AlF3
1620 cm-1 (r.t.)
1627 cm-1 (473 K)
1625, 1615 cm-1
27
γ-Al2O3
1295 cm-1
1265, 1220 cm-1
a difference has also been observed for silica (1.26) for which
a coupling of the δ(OH) and ν(M-O) modes has been
reported.23 The existence of such a coupling likely occurs in
HTB since a mass effect of the cation on the δ(OH) band
position is observed: 1128 cm-1 for Al, 1070 cm-1 for Cr, and
1005 cm-1 for Fe homologous compounds. Another important
feature is the relatively high frequency of the δ(OH) mode. For
corresponding metal oxides, it is detected at lower wavenum-
24
bers: about 860-900 cm-1 for δ-FeOOH
and below 900
cm-1 on alumina.25 By contrast, in zeolites, the δ(OH) mode
of ≡Si-OH-Al≡ bridged hydroxy groups is often observed
above 1000 cm-1, whereas for unbridged isolated ≡Si-OH
groups, it is detected at about 760 cm-1 in silica.26 Thus, the
blue shift of the δ(OH) band observed on HTB spectra could
be explained again by a “structural effect” induced by a strong
bridged conformation of hydroxyls in the HTB lattice (Scheme
1); moreover confinement effect may also occur. Note that, for
all of the compounds studied, the (ν+δ)OH combination bands
are clearly observed and perfectly fit the expected values,
considering an anharmonicity coefficient of about 20 cm-1. In
the case of mixed iron and chromium HTB compounds, the
presence of two δ(OH) bands and two corresponding (ν+δ)-
OH combination bands can be explained considering that some
OH groups are bridged to two Fe cations (the OH bands are
then those observed for FeF3-x(OH)x), the others being bridged
either to an Fe and Cr cation or even to two Cr cations. The
δ(OH) frequency of the latter compound is higher than that
observed for FeF3-x(OH)x. This effect can be related to the
difference of chemical bond strengths mainly due to the high
crystal field stabilization in the case of trivalent chromium, even
if it is difficult to conclude since the bending mode is influenced
by different factors.
band at a wavenumber lower than 1600 cm-1 31
. In a recent paper
devoted to the acidity of R-gallia (a compound expected to also
present only cations in octahedral position), it has been however
reported that surface reconstruction occurs, leading to surface
cations in tetrahedral position, explaining the high surface Lewis
acidity of R-gallia.32 The same phenomenon could occur in the
case of AlF3-x(OH)x; 27Al NMR studies are planned to check
such a hypothesis.
The size of the NH3 molecule is close to that of water. So,
NH3 probes the acidic sites on the outer surface and those into
the channels, the latter containing the hydroxyls exchangeable
by D2O. Moreover, on the surface, the protonation of ammonia
is generally easier than that of pyridine and therefore NH3 can
probe weaker acidic sites on oxide surfaces.27 Coordinated NH3
gives rise to the two bands situated near 1300 (δs(NH3)) and
1610 cm-1 (δa(NH3)). Datka et al. in a study devoted to
zeolites33 determined the apparent molar extinction coefficient
of the band near 1610 cm-1 (0,022 cm2 µmol-1). Extending it
to HTB materials, and taking into account the intensity of the
1610 cm-1 band, a total number of NH3 coordinated species of
about 300 and 450 µmol g-1 can be estimated for FeF3-x(OH)x
and Fe0.8Cr0.2F3-x(OH)x, respectively. Though not very accurate
due the weakness of the intensity of the δa(NH3) band, such
values are much higher than those determined from pyridine
adsorption (100-125 µmol g-1). This strongly suggests the
presence of Lewis acid sites in the channels of the HTB
materials considered. This could be related to the relatively high
temperature used to activate these samples, a temperature close
to that for which fluorine departure has been evidenced.13
Fluorine Effect on the Acidic Properties of HTB Materials.
Acidic sites of HTB material have been probed by pyridine and
ammonia. These two probes, quite basic, do not give concordant
results relative to hydroxyls acidity: NH3 generally shows that
they are acidic since ammonium species are formed. By contrast,
pyridine hardly affects them, even when a excess of pyridine is
introduced, before evacuation (spectra not shown). This is well
explained considering that hydroxy groups are situated in
micropores, not accessible to pyridine.
Pyridine and ammonia adsorption on HTB materials give rise
to coordinated species. The strength of the acidic sites can be
estimated by the position of the ν8a and δs(NH3) bands in the
spectrum of coordinated pyridine and ammonia, respectively.27
The higher the position of these bands, the stronger the acidic
Lewis sites are. In Table 3, we have compared band positions
observed on Fe, Cr, and Al HTB materials with those reported
for Al2O3, Fe2O3, and Cr2O3 oxides.28-30 In all of the cases, we
Except for FeF3-x(OH)x, no pyridinium species have been
detected on the materials studied. Moreover, under 133 Pa of
pyridine at equilibrium pressure, no band at about 1595 cm-1
characterizing H-bonded species has been observed. We con-
clude that the detection of Brønsted acid sites by NH3 only is
not due to its different basicity respect to pyridine but to its

3254 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Vimont et al.
smaller size, allowing us to situate these sites only in the
channels. The sharpness of the 1425 cm-1 band and the absence
of the corresponding δa(NH4) one near 1680 cm-1 show that
the NH4 species formed have a high symmetry and that the sites
are homogeneous. Curves reported in Figure 8 show a linear
decrease of the ν+δ(OH) band intensity versus the amount of
ammonia introduced. This unambiguously evidences that OH
groups in the channels participate to ammonium formation.
Note that on CrF3-x(OH)x no pyridinium formation is
observed and only a weak pyridinium signal is detected on
FeF3-x(OH)x, easily eliminated by evacuation. So, the larger
part of Brønsted acidic sites is probably localized inside the
channels and only perturbed by ammonia.
On corresponding metal oxides, ammonium bands are not
detected onto the surface of hematite after NH3 adsorption.18
This absence of strong Brønsted acidic sites on iron oxide is
confirmed by pyridine adsorption on R- and γ-Fe2O3:28,18 no
pyridinium species are reported, only coordinated and H-bonded
species are evidenced. On R-Cr2O3, Brønsted acidity of OH
groups probed by CO has been studied by Kno¨zinger:19 the
perturbation of hydroxy groups by CO adsorption at low
temperature is very weak (∆ν(OH) < 75 cm-1), thus showing
the absence of strong Brønsted acid sites. So, stronger Brønsted
acidity of HTB materials is observed compared to that of metal
oxides containing the same metal. This phenomenon can be
explained involving the high electronegativity of fluorine,
leading to the strengthening of the M-OH bond corresponding
to adjacent hydroxyls and to the weakening of the O-H bond.
This fluorine effect is often invoked to explain the creation of
strong Brønsted sites on alumina after fluorination.1
fluorinated materials. Using basic probes with different sizes
(pyridine and ammonia), it was possible to determine the
localization of Brønsted and Lewis acid sites, their strength in
the case of Lewis acidity, and their amount. To specify the
strength of hydroxy group Brønsted acidity in the channels,
experiments using CO adsorption at low temperature are
planned.
The catalytic activity studies of these materials for synthesis
of CFCs and HFCs molecules show that the Lewis acid sites
seem to be the active sites, whereas the presence of Brønsted
acid sites hinders the reaction. Therefore, the formation of OH
groups should be avoided in such compounds. By contrast, it
is well-known that fluorinated materials are better catalysts than
metal oxides for hydrocarbon conversion due to the presence
of strong Brønsted acid sites, suggesting the promoting effect
of hydroxyls in the latter case. So another important point is
the determination of the role and the accessibility of OH groups
in HTB fluorinated compounds for catalyst experiments by
adjusting the F-/OH- ratio by water addition. IR spectroscopy
appears to be a powerful technique for these kinds of studies.
Acknowledgment. This work has been supported by Rhodia
Recherches (Centre de Recherches d’Aubervilliers, France).
References and Notes
(1) Ghost, A. K.; Kydd, R. A. Catal. ReV. Sci. Eng. 1985, 27, 539.
(2) Rodriguez, L. M.; Alcaraz, J.; Hernandez, M.; Dufaux, M.; Ben
Taaˆrit, Y.; Vrinat, M. Appl. Catal. A: General 1999, 189, 53.
(3) Kemnitz, E.; Menz, D.-H. Prog. Solid State Chem. 1998, 26, 97.
(4) Badri, A.; Binet, C.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans.
1997, 93, 2121.
From a quantitative point of view, we have evaluated the
density of Brønsted acid sites by calculating the amount of
ammonium species formed after ammonia introduction on
FeF3-x(OH)x and Fe0.8Cr0.2F3-x(OH)x compounds. Assuming that
one ammonium species corresponds to one hydroxy group (that
is realistic, considering the narrowness of the channels), and
taking into account that we have 12 cations per unit cell, we
deduce that 1.8 and 1 OH groups per unit cell are awaited,
respectively.
However, the following observation strongly suggests that
hydroxyls are not the sole Brønsted acid sites interacting with
ammonia: during ammonia evacuation at room temperature and
up to 473 K, the decrease of the ammonium bands intensity at
about 3200 and 1425 cm-1 is observed (Figure 8) while no ν-
(OH) bands are clearly restored (spectra 1h, 1g, 2h, 2i, and 2j,
Figure 6). This shows that some protonated species desorb at
low temperature and do not involve hydroxy groups. Clet et al.
reported on chlorinated alumina the presence of protonated
species after pyridine or lutidine adsorption, easily desorbed
by vacuum at low temperature.34 They suggest that Brønsted
sites are not necessarily weakly acid and explain the phenom-
enon by the presence of HCl loosely bound onto the surface,
giving rise to labile Cl-pyH+ species. In the present case, we
propose as the origin of these easily desorbed ammonium species
F-NH4+ complex formation due to some residual presence of
chemisorbed HF molecules onto the surface. Such HF species
could be either in pores (easiness of NH4 desorption) or on the
surface of FeF3-x(OH)x, since pyridinium species formation does
not apparently involve hydroxyls.
(5) Kerkhov, F. P. J. M.; Oudejans, J. C.; Moulijn, J. A.; Matulewicz,
E. R. A. J. Colloid Interface Sci. 1980, 120, 77.
(6) Scokart, P. O.; Selim, S. A.; Damon, J. P.; Rouxhet, P. G. J. Colloid
Interface Sci. 1979, 70, 209.
(7) Le Bail, A.; Jacoboni, C.; Leblanc, M.; de Pape, R.; Duroy, H.;
Fourquet, J. L. J. Solid State Chem. 1988, 77, 96.
(8) Leblanc, M.; Ferey, G.; Chevalier, P.; Calage, Y.; de Pape, R. J.
Solid State Chem. 1983, 47, 53.
(9) Francke, L.; Durand, E.; Demourgues, A.; Vimont, A.; Daturi, M.;
Tressaud, A. J. Mater. Chem. 2003, 13, 2330.
(10) Hess, A.; Kemnitz, E. J. Catal. 1994, 149, 449.
(11) Morterra, C.; Cerrato, G.; Cuzzato, P.; Masiero, A.; Padovan, M.
J. Chem. Soc., Faraday. Trans. 1992, 88, 2239.
(12) Kemnitz, E.; Hess, A.; Rother, G.; Troyanov, S. J. Catal. 1996,
159, 332.
(13) Demourgues, A.; Francke, L.; Durand, E.; Tressaud, A. J. Fluorine
Chem. 2002, 114, 229.
(14) Khabtou, S.; Chevreau, T.; Lavalley, J.-C. Micro. Mater. 1994, 3,
133.
(15) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc.,
Faraday. Trans. 1979, 75, 27.
(16) Davydov, A. A. In Infrared spectroscopy of adsorbed species on
the surface of transition metal oxides; Rochester, C. H., Ed.; J. Wiley &
Sons Ltd.: New York, 1990.
(17) Saussey, J. Private communication
(18) Lorenzelli, V.; Busca, G. Mater. Chem. Phys. 1985, 13, 201.
(19) Zaki, M. I.; Kno¨zinger, H. Mater. Chem. Phys. 1987, 17, 201.
(20) Jacobs, P. A.; Nortier, W. J. Zeolites 1982, 2, 226.
(21) Tsyganenko, A. A.; Lamotte, J.; Saussey, J.; Lavalley J.-C. J. Chem.
Soc., Faraday Trans. 1. 1989, 85, 2397
(22) Lavalley, J.-C.; Bensitel, M.; Gallas, J.-P.; Lamotte, J.; Busca, G.;
Lorenzelli, V. J. Mol. Struct. 1988, 175, 453.
(23) Burneau, A.; Gallas, J.-P. In The surface properties of silicas;
Legrand, A. P., Ed.; J. Wiley & Sons Ltd.: New York, 1998; Chapter 3A.
(24) Ishikawa, T.; Cai, W. Y.; Kandori, K. J. Chem. Soc., Faraday Trans
1992, 88, 1173.
Conclusions
(25) Mariette, L.; Hemidy, J.-F.; Cornet, D. Stud. Surf. Sci. 1985, 21,
263; Che, M., Bond, G. C, Eds.
(26) Kustov, L. M.; Borovkov, V. Y.; Kazansky, V. B. J. Catal. 1981,
72, 149.
Thanks to the presence of hydroxy groups as “impurities”
substituting fluorine into the samples, infrared spectroscopy
appears a suitable method to study the acid properties of HTB
(27) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723.

Surface Properties of HTB-Type Hydroxyfluorides
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3255
(28) Harrouche, N.; Batis, H.; Ghorbel, A. J. Chim. Phys. 1984, 81,
267.
(29) Zecchina, A.; Coluccia, E.; Guglielminotti, S.; Ghiotti, G. J. Phys.
Chem. 1971, 75, 2774.
(30) Morterra, C.; Chiorino, A.; Ghiotti, G., Garrone, E. J. Chem. Soc.,
Faraday Trans. 1 1979, 75, 271.
(32) Lavalley, C.; Daturi, M.; Montouillout, V.; Clet, G.; Otero Arean,
C.; Rodriguez Delgado, M.; Sahibed-dine, A. Phys. Chem. Chem. Phys.
2003, 5, 1301.
(33) Datka, J.; Gil, B.; Kubacka, A. Zeolites 1995, 15, 501
(34) Clet, G.; Goupil, J.-M.; Cornet, D. Bull. Soc. Chim. Fr. 1997, 134,
(31) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497.
223.