Porous fluorinated aluminum and mixed gallium/aluminum oxide pillared tin phosphate materials with acid properties

Article abstract of DOI:10.1021/jp973168o

Fluorinated aluminum and mixed gallium/aluminum oligomers of high nuclearity have been intercalated into α-tin phosphate by refluxing fluorinated Al and mixed Ga/Al oligomeric solutions in the presence of colloidal tin phosphate. ³¹P and ²⁷Al MAS NMR spectroscopy has revealed the existence of strong interactions between the phosphate layers and the oligomeric guest species, conferring to these expanded materials a high thermal stability, as confirmed by XRD studies. Formation of Al and mixed Ga/Al oxide nanoparticle pillars in the interlayer region of tin phosphate, upon calcination at 400 °C, gave rise to porous materials with specific surface areas between 160 and 304 m² g^-1 and micropore volumes close to 0.1 cm³ g^-1. The pillared materials are highly acidic solids and behave as dehydrating catalysts for the decomposition reaction of isopropyl alcohol. The Lewis acid sites, believed to be mainly responsible for the catalytic activity, are associated with low coordination sites of Al ions present in the interlayer oxide pillars.

Full text of DOI:10.1021/jp973168o

1
672
J. Phys. Chem. B 1998, 102, 1672-1678
Porous Fluorinated Aluminum and Mixed Gallium/Aluminum Oxide Pillared Tin Phosphate
Materials with Acid Properties
Pilar Braos-Garc ´ı a, Enrique Rodr ´ı guez-Castell o´ n, Pedro Maireles-Torres,
Pascual Olivera-Pastor, and Antonio Jim e´ nez-L o´ pez*
Departamento de Qu ´ı mica Inorg a´ nica, Cristalograf ´ı a y Mineralog ´ı a, Facultad de Ciencias, UniVersidad de
M a´ laga, Campus de Teatinos, 29071-M a´ laga, Spain
ReceiVed: September 29, 1997; In Final Form: December 18, 1997
Fluorinated aluminum and mixed gallium/aluminum oligomers of high nuclearity have been intercalated into
R-tin phosphate by refluxing fluorinated Al and mixed Ga/Al oligomeric solutions in the presence of colloidal
tin phosphate. 31P and Al MAS NMR spectroscopy has revealed the existence of strong interactions between
the phosphate layers and the oligomeric guest species, conferring to these expanded materials a high thermal
stability, as confirmed by XRD studies. Formation of Al and mixed Ga/Al oxide nanoparticle pillars in the
interlayer region of tin phosphate, upon calcination at 400 °C, gave rise to porous materials with specific
27
2
-1
3
-1
surface areas between 160 and 304 m g and micropore volumes close to 0.1 cm g . The pillared materials
are highly acidic solids and behave as dehydrating catalysts for the decomposition reaction of isopropyl alcohol.
The Lewis acid sites, believed to be mainly responsible for the catalytic activity, are associated with low
coordination sites of Al ions present in the interlayer oxide pillars.
9
Introduction
of stuffed structures. These studies also revealed that the
-
incorporation of F ions into intercalated clusters was beneficial
A vast majority of works on pillared structured solids are
in order to improve porosity and thermal stability of the pillared
materials. The question arises whether M(IV) phosphates can
develop interlayer porosity by intercalation of large fluorinated
species, formed by not only aluminum but also mixed species,
such as Ga/Al. This method of intercalation may thereby open
new possibilities for preparation of a great number of aluminum-
based oxide pillared materials with tunable surface properties
by appropriate choice of the composition of mixed species and
experimental conditions.
1
devoted to the synthesis and characterization of clays, phos-
2
3
phates and other hosts pillared with the well-known aluminum
7+
7+
polyoxocation, [AlO4Al12(OH)12(H2O)24] , (Al13 ), which
consists of a central tetrahedral aluminum surrounded by 12
edge-sharing octahedral aluminums.4 The products obtained
exhibit properties, such as surface area, pore size, and acidity,
which can be tuned by adequate choice of reactants and
experimental conditions. The similarity between the hydrolysis
behavior of Al3 and Ga ions has also led to the study of
mixed Ga/Al oligomeric species. The most relevant finding
+
3+
The purpose of the work reported in this paper was to
synthesize porous materials based on aluminum and mixed
gallium/aluminum pillared R-tin phosphate intercalation com-
pounds, by using solutions containing fluorinated Al or mixed
Ga/Al clusters of high nuclearity. Characterization of the
intercalates and calcined materials was carried out by chemical
analyses, X-ray diffraction, TGA-DTA, XPS, P and Al solid-
state MAS NMR, N2 adsorption-desorption (at 77 K), tem-
perature-programmed desorption of NH3, and the catalytic
decomposition of isopropyl alcohol.
5
was that GaAl12 possessed a much higher thermal stability than
Al13 and Ga13 homologues and presented a similar Keggin-type
structure with Ga ions in the tetrahedral sites, as demonstrated
by X-ray diffraction and NMR spectroscopy. Indeed, the
intercalation of GaAl12 species in clays has provided pillared
solids with basal spacings close to that obtained with Al13
oligomers and specific surface areas which were preserved at
temperatures where alumina pillared clays collapsed. These
studies have demonstrated, therefore, the high thermal and
hydrothermal stability of GaAl12-pillared clays and, moreover,
the presence of strong acid sites in these materials which results,
for instance, in better selectivity for the isomerization of
n-heptane.6
31
27
Experimental Section
Materials. Colloidal suspensions of tetramethylammonium-
tin phosphate (TMA-SnP) were synthesized as follows: 5 g
of R-SnP, prepared according to the Costantino and Gasperoni
Likewise, layered metal(IV) phosphates are, by their proper-
ties as ion exchangers and catalysts, adequate candidates for
intercalating many different oligomeric species.7 The synthesis,
characterization, and applications of nanostructured inorganically
pillared layered metal(IV) phosphates have been very recently
1
0
method, was dispersed in 355 mL of a 0.06 M solution of
n-propylamine (NPA) in n-hexane (corresponding to the 70%
of the cation exchange capacity of R-tin phosphate), and the
solution was stirred for 2 h at room temperature. The solid,
recovered by centrifugation and air-dried, was then added to
122 mL of a 2.5 M aqueous solution of tetramethylammonium
chloride (TMA-Cl), maintained for 1 day under stirring at room
temperature, and finally the suspension was centrifuged and the
solid washed with water and air-dried at 60 °C.
8
reviewed. Precedent studies have demonstrated that species
larger than the Al137 ion are needed for generating interlayer
porosity in R-zirconium phosphate, in order to balance the strong
interaction established between the phosphate layer and the
oxide nanoparticles acting as pillars, which favors the formation
+
S1089-5647(97)03168-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/14/1998

Al and Ga/Al Oxide Pillared Materials
J. Phys. Chem. B, Vol. 102, No. 10, 1998 1673
TABLE 1: Surface (XPS) Atomic Ratios, d00l Spacings, and Chemical Formulas for the Studied Precursor Samples
d
00l/Å
a
a
chemical formulasb
(OH)46(H
sample
Al-SnP
surface F/M
surface Ga/Al
rt
400 °C
0.45
0.50
0.36
0.13
23.4
23.0
29.3
23.4
17.5
16.7
21.6
16.3
Sn[Al24
O
8
2
O)18
]
0.14
H0.6(PO
4
)
2
2
‚7.3H O
GaAl13-SnP
1/9
3/8
6/1
Sn[Ga1.75Al22.25
Sn[Ga6.02Al17.98
Sn[Ga17.66Al6.34
O
8
O
8
O
8
(OH)46(H
(OH)46(H
(OH)46(H
2
2
2
O)18
O)18
O)18
]
]
]
0.13
H
0.16
H
0.12
H
0.7(PO
0.4(PO
0.8(PO
4
)
2
2
2
‚4.6H
2
2
2
O
O
O
Ga
Ga
3
Al11-SnP
Al -SnP
4
)
)
‚7.9H
‚7.3H
6
7
4
a
From XPS analysis (M ) Al + Ga). b Al and Ga/Al expressed as the [Al24-xGa O)18]10+
x 8 2
O (OH)46(H .
The oligomeric aluminum solution was prepared according
to the procedure described by Fu et al.11 Al foil was dissolved
in an AlCl3 aqueous solution under reflux for 2 days (final
concentration of Al was equal to 1 M). In the case of mixed
oligomeric aluminum-gallium solutions, Al foil was treated
with a mixture of AlCl3 and GaCl3, to achieve final Ga/Al molar
ratios of 1/12, 3/10, and 6/7, under reflux for 1 day (final
concentration of (Al + Ga) ) 0.4 M). In all cases, the GaCl3
solutions were prepared by treating metallic gallium in a
concentrated 3:1 HCl/HNO3 mixture (HCl/Ga molar ratio being
distributions were calculated by the use of the Cranston and
Inkley method for cylindrical pores.
The total acidity of the calcined samples was determined by
12
temperature-programmed desorption of ammonia (NH TPD).
3
Before the adsorption of ammonia at 100 °C, samples were
treated under helium at 400 °C for 1 h. The desorption of
-
1
ammonia was run between 100 and 400 °C, at 10 °C min ,
and analyzed in an on-line gas chromatograph (Shimadzu 6C-
14A) provided with a thermal conductivity detector.
Catalytic Activity Measurements. The catalytic activity of
the pillared solids in the decomposition of isopropyl alcohol
was tested in a fixed-bed tubular glass reactor at atmospheric
pressure and 220 °C, with a catalyst charge of 20 mg without
dilution. The feed was a mixture of isopropyl alcohol in He
obtained by passing a flow of He through isopropyl alcohol
3
:1). A tetramethylammonium fluoride solution was finally
-
3+
3+
added to each solution to attain F /(Al + Ga ) molar ratios
lower than 0.5. Aliquots of these oligomeric solutions, corre-
sponding to 6.90 mmol of M2O3/g of SnP2O7 (M ) Al, Ga),
were added to a colloidal aqueous suspension of TMA-SnP (2
wt %). The resulting suspensions were refluxed for 1 day,
centrifuged, water-washed, and air-dried. Intercalation com-
pounds (precursors), hereafter designated as Al-SnP, GaAl13-
SnP, Ga3Al11-SnP, and Ga6Al7-SnP, were finally calcined in
air at 400 °C during 6 h.
(
chromatographic purity grade) held in a saturator-condenser
-7
3
-1
kept at 30 °C. A constant total flow of 4.17 × 10 m s
with an alcohol concentration of 11.3 vol % was used, with a
-1 -1
spatial velocity of 83.5 µmol s g . Catalysts did not show
diffusional restrictions. Prior to catalytic tests, carrier gas was
passed through a molecular sieve, and the samples were
pretreated under a He flow at 220 °C for 2 h. The reaction
products were analyzed in an on-line gas chromatograph with
a flame ionization detector and fused silica column SPB1.
Chemical Analyses and Characterization. Chemical analy-
ses of the synthesized materials were carried out after digestion
in 60% HNO3 for 1 day and hot 1 M NaOH for 4 days. The
concentrations of Al and Ga were determined by atomic
absorption with a Perkin-Elmer 400 A.A. spectrometer.
XRD of samples as cast films was recorded on a Siemens
D501 diffractometer using Cu KR radiation and a graphite
monochromator. Thermogravimetric (TG) and differential
thermal analyses (DTA) were carried out with a Rigaku
Thermoflex TG 3110 instrument (calcined Al2O3 as a reference
Results and Discussion
Intercalation of Al and Mixed Ga/Al Oligomers. Previous
studies have demonstrated that the colloidal intermediate phase
of tetramethylammonium-tin phosphate (TMA-SnP) is ap-
propiate for the intercalation of metal oligomeric species.13 The
preparation of this colloidal phase requires a previous intercala-
tion of n-propylamine and then to exchange the interlayer
n-propylammonium ions formed by tetramethylammonium. In
the present work, n-hexane was used as solvent for intercalating
n-propylamine in order to minimize the hydrolysis of tin
phosphate, which is a major drawback in neutral and basic
-
1
and 10 °C min heating rate).
X-ray photoelectron spectra (XPS) were recorded with a
Physical Electronics 5700 spectrometer equipped with a dual
X-ray excitation source (Mg KR, hν ) 1253.6 eV, and Al KR,
hν ) 1486.6 eV) and multichannel hemispherical electron
analyzer. The samples were turbopumped to ca. 10- Torr
before they were moved into the analysis chamber. The residual
pressure in this ion-pumped chamber was maintained below
9
14
aqueous media.
Prior to the intercalation reaction, the oligomeric solutions
were aged at reflux conditions, and subsequently a tetramethyl-
ammonium fluoride aqueous solution was added at room
temperature in order to form large oligomeric species. This
addition of fluoride was necessary to improve the thermal
-
9
1
0
Torr during data acquisition. The binding energies were
obtained with an accuracy of (0.1 eV and by charge referencing
to the adventitious C 1s peak at 284.8 eV.
The MAS NMR spectra were recorded on a Bruker AMX
3
00 spectrometer equipped with a multinuclear probe. Powder
stability of intercalated oligomers and avoid competition of
aluminum monomers, by removing them as AlF6
27
samples were packed in zirconia rotors. For Al, samples were
spun at 3.0 kHz, and spectra recorded at 78.23 MHz, using a
pulse width of 3 µs, a spectral width of 50 kHz, a recycle delay
of 0.3 s, 10 000 scans, and 10 Hz exponential line broadening.
3- 9
.
The
fluorinated Al- and Ga/Al-tin phosphate precursor solids are
obtained by slow addition of aliquots of fluorinated oligomeric
solutions to colloidal TMA-SnP and refluxing the mixture for
Chemical shifts are reported in ppm from H3PO4 and
1
day. Chemical compositions of the precursor Al- and Ga/
+
[
Al(H2O)6]3 solutions.
Al-SnP materials are shown in Table 1. Bulk Ga/Al analyses
are close to nominal values, except for Ga6Al7-SnP solid in
which a very low uptake of Al is detected. In this sample, the
Adsorption-desorption isotherms of N2 at 77 K of calcined
samples were obtained in a conventional volumetric apparatus,
-
4
-1
after outgassing of samples at 200 °C (10 Torr overnight).
total uptake is the lowest (4.84 mmol of M2O3 g of SnP2O7),
whereas the other samples exhibit values similar to that of
2
BET specific surface areas were evaluated using 0.162 nm as
the cross-sectional area of the adsorbed N2 molecule. Pore size
fluorinated Al-SnP material. A comparison of the bulk and

1674 J. Phys. Chem. B, Vol. 102, No. 10, 1998
Braos-Garc ´ı a et al.
Figure 1. XRD patterns of GaAl13-SnP at different temperatures: (a)
room temperature, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C.
Figure 2. Evolution of the basal spacing, d00l, with temperature for
fluorinated Al- and Ga/Al-SnP intercalates: (a) Al-SnP, (b) GaAl13
-
SnP, (c) Ga Al11-SnP, (d) Ga Al -SnP, and (e) Chlorhydrol-SnP.
3
6
7
surface (XPS) Ga/Al ratios with the added Ga/Al ratios reveals
that, at least, for Ga/Al ratios e3/10 the composition of the
intercalated oligomeric species does not deviate appreciably
from the initial composition of the oligomeric solution, and
moreover, mixed oligomers are inserted into the phosphate
interlayer region, as no significant variations between the bulk
and surface ratios are found. However, upon increasing the
relative concentration of Ga in solution, the composition of the
intercalated oligomers is strongly enriched in Ga. This may
be due to mixed oligomers with intermediate compositions
having a very low stability, as a consequence of the difference
of size between Ga3 and Al ions, which must cause strong
distortions in the mixed oligomeric structure. In any case, the
affinity of the phosphate layers seems to be higher for mixed
species with high Ga content, which may be related to the trend
of aluminum to bond preferentially to fluoride ions. This is
confirmed by the fact that the highest fluoride content corre-
sponds to the intercalated mixed oligomers with the highest Al
content (Table 1).
+
3+
3
Figure 3. TG and DTA curves of fluorinated Al-SnP and Ga Al11-
SnP materials.
The thermal behavior of these intercalation compounds is
significantly different from that of the Chlorhydrol-SnP mate-
rial, prepared by adding a Chlorhydrol solution to colloidal tin
1
3a
phosphate at room temperature.
The former display much
higher thermal stability than the latter, despite the basal spacing
at room temperature of this solid being compatible with the
The XRD data for the intercalated compounds at room
temperature reveal that oligomers with similar size were
intercalated in samples Al-, GaAl13-, and Ga6Al7-SnP. The
d00l basal spacings of these samples, 23-23.4 Å, are slightly
7+
presence of a bilayer of Al13 ions. In particular, the free height
of sample Ga3Al11-SnP is about 10 Å higher than that of
Chlorhydrol-SnP at 500 °C. The high thermal stability showed
by fluorinated Al- and mixed Ga/Al oligomer intercalated
materials as compared with Chlorhydrol-SnP may be attributed
to (i) the presence of species larger than Al13, which might be
formed by condensation of incomplete units derived from the
9
higher than that of the analogous fluorinated Al-ZrP material,
which means that species with similar nuclearity are intercalated
in both phosphates using the same intercalation method. This
difference could be a consequence of the hydrothermal treatment
(200 °C) of the latter. The corresponding gallery height values,
11
-
Keggin ion framework, and (ii) the partial replacement of OH
obtained by subtracting the layer thickness (∼6.5 Å) to the d00l
-
groups by F ions in the oligomeric framework, which
diminishes the possibility of condensation of M-OH groups
from the oligomers with P-OH groups from the phosphate layer
and, thus, retards spreading of the intercalated species throughout
the phosphate interlayer region.
spacing, are in the range 16.5-16.9 Å, much higher than that
7+
expected for the intercalation of a monolayer of the Al13 ions,
4
which have a prolate form with a major axis of ∼9.5 Å. The
sample Ga3Al11-SnP exhibits the highest basal expansion yet
observed for intercalation compounds of metal(IV) phosphates
containing aluminum species. Upon heating, the intercalated
materials undergo a gradual decrease of the d00l spacing and a
progressive amorphization (Figures 1 and 2), but high basal
expansions are still maintained at 400 °C (Table 1) and the
crystallinity is completely lost beyond 500 °C. In any case,
XRD data suggest that both Al and mixed Ga/Al species
intercalated into the layered tin phosphate show higher nuclearity
than the tridecameric species, usually found in the interlayer
region of pillared smectites. This is mainly attributed to the
fact that the reflux method used favors coalescence of the
Thermogravimetric and differential thermal analyses (TG-
DTA) of fluorinated Al- and Ga/Al-SnP materials show
transitions at similar temperatures (Figure 3). The first endo-
thermic effect in the DTA curve centered at 69-76 °C is
assigned to the loss of waters of hydration, and the second one
at 108-125 °C could correspond to the removal of coordinated
water molecules. However, these two phenomena are not well
differentiated in the TG curves, where a continuous weight loss
is observed, extending from room temperature to ca. 700 °C
which may also include the dehydroxylation of the oligomers
and condensation with the phosphate layers. A characteristic
exothermic effect, at temperatures higher than 800 °C, corre-
sponding to the phase transition from layered to cubic tin
1
h
-
Keggin ion. In addition, the presence of F prevents any
competition of low nuclearity species.

Al and Ga/Al Oxide Pillared Materials
J. Phys. Chem. B, Vol. 102, No. 10, 1998 1675
Figure 4. 27Al MAS NMR spectra of fluorinated precursor materials:
3 6 7
(a) Al-SnP, (b) GaAl13-SnP, (c) Ga Al11-SnP, and (d) Ga Al -SnP.
Figure 5. 31P MAS NMR spectra of fluorinated precursor materials:
(
a) Al-SnP, (b) GaAl13-SnP, and (c) Ga Al11-SnP.
3
pyrophosphate is not observed in any case. This is attributed
to the existence of strong interactions between metal oxide
pillars and phosphate groups in the interlayer region, thus
avoiding collapse of the layers. Hydration and coordination
water range between 27 and 31 wt %, and in all cases, a small
proportion of organic matter is still present.
observed in the spectra of n-propylamine metal(IV) phosphate
intercalates, where the interaction between P-O and NH3-R
groups was postulated. The presence of Ga atoms in the mixed
-
+
18
31
oligomeric species only leads to slight changes in the P MAS
NMR spectra which suggest that the intercalated mixed Ga/Al
oligomers are structurally analogous to the Al species, with the
gallium ions first occupying the central tetrahedral position and
then the octahedral sites. It seems, therefore, that the presence
of the phosphate induces the condensation of GaAl12 units,
present initially in solution, to form more bulky species which
are intercalated in the interlayer region.
MAS NMR spectroscopy has been utilized in order to
elucidate the chemical nature of the intercalated Al- and Ga/
Al oligomers and the types of interactions between the phosphate
groups and the guest species. A study by 27Al NMR and powder
X-ray diffraction on kinetics of formation in solution of Al13,
hybrid MAl12 species, and larger oligomers has been very
recently reported.15 At high temperature, the polynuclear
species, so-called AlP2, which is thought to be formed by
condensation of two Al12 units, resulting from the loss of an
Al(H2O)63 monomer in the tridecameric structure, was found
Pillared Materials. The Al- and Ga/Al oxide pillared
materials were obtained by calcining the precursor solids at 400
°
C in air. Transformation of intercalated oligomers into oxide
+
nanoparticles causes, as mentioned above, a partial shrinkage
of the interlayer region, but these materials still maintain large
spacings between 10.2 and 15.1 Å (Table 1). The absence of
reflection lines corresponding to Al or mixed Ga/Al oxides in
the X-ray diffraction patterns of fluorinated Al- and Ga/Al-
SnP materials indicates that no precipitation of the metal
hydroxides took place during the intercalation process of the
metallic oligomers.
The binding energy (BE) values of P(2p) and Sn(3d5/2) for
these materials range between 133.6-134.5 and 486.5-487.1
eV, respectively. They are similar to those found in chromia-
pillared SnP materials. The O(1s) BE values occur between
5
eV) and Al2O3 (531.8 eV) but significantly lower than those
reported for aluminum phosphates (532.8 eV); thus, the forma-
tion of a segregated AlPO4 phase during the pillaring process
may be ruled out. On the other hand, F is an anion of very
strong affinity for Al and thus may cause a break of the Al-
OH bond and act as a ligand in the oligomeric framework by
partially substituting OH groups. The F(1s) BE (684.4-684.8
eV) of pillared materials is consistent with this partial substitu-
tion of OH for F , as the values found were intermediate
between those corresponding to ionic (e.g., 684.5 eV for NaF)
and covalent (e.g., 685.5 eV for AlF6 ) fluorides. Although
Al displays a higher affinity for F than Ga , the existence
of Ga-F bonds in the mixed Ga/Al oligomers cannot be ruled
out since the BE of gallium fluoride compounds is also close
1
1
in solution, confirming previous work reported by others.
However, it seems that the presence of Ga3+ in Al solution
3+
favors the formation of GaAl12 species against polymerization.
Figure 4 shows the 27Al MAS NMR spectra of fluorinated
Al- and Ga/Al-SnP intercalates. In the spectrum of the
fluorinated Al-SnP solid, a resonance band appears at 72.4
ppm, practically in the same position as that of tetrahedral
11
aluminum (AlIV) in AlP2 clusters. As gallium is added, this
signal disappears probably due to the tendency of Ga to occupy
first the tetrahedral positions in the Keggin ion framework,
forming the most thermodynamically favored GaAl12 species.
In the region corresponding to octahedral aluminum (AlVI), two
maxima at -6.2 to 2.4 and 6.0 to 10.9 ppm can be distinguished.
This splitting of the AlVI signal is probably a consequence of
the interaction of the intercalated oligomers with the phosphate
layer.
19
30.9 and 531.8 eV and are comparable to those of R-SnP (531.8
20
-
3
1
3+
P MAS NMR spectra of Al- and Ga/Al-SnP intercalates
31
are displayed in Figure 5. The P chemical shift (δ), which
appears at -12.5 ppm in R-SnP,16 is split into two peaks upon
intercalation of the oligomeric species of aluminum, the most
intense centered at -15.0 ppm and the second at -9.3 ppm.
The first resonance, according to previous work,17 is assigned
to phosphate groups interacting with aluminum oligomers. The
shift toward higher field positions may be attributed to the
increase of cation charge around the phosphorus tetrahedra upon
interaction with Al oligomers. On the other hand, the peak at
lower field, between -9.3 and -9.9 ppm, may correspond to
deprotonated phosphate groups which are more distant from the
interlayer oligomeric species. This signal has been also
-
-
-
3
-
3
+
-
3+
2
0
to 685 eV.
Concerning the 27Al MAS NMR spectra of pillared materials
(Figure 6), the most relevant feature is the presence of a new

1676 J. Phys. Chem. B, Vol. 102, No. 10, 1998
Braos-Garc ´ı a et al.
Figure 6. 27Al MAS NMR spectra of fluorinated calcined materials:
a)-(d) as in Figure 4.
(
Figure 8. (a) N
distribution for sample Ga Al11-SnP.
2
adsorption-desorption isotherm and (b) pore size
3
TABLE 2: Textural Parameters of Fluorinated Al- and
Ga/Al Pillared Tin Phosphate Materials
a
b
b
(av)b
(Å)
c
S
BET
2
∑Sac
∑V
p
d
p
V
µ
sample
Al-SnP
(m g-1) (m g-1) (cm g-1)
2
3
(cm g-1)
3
304
187
243
160
190
400
264
300
198
141
0.478
0.332
0.357
0.241
0.295
48
50
48
49
84
0.104
0.075
0.093
0.064
0.019
GaAl13-SnP
Ga
Ga
3
Al11-SnP
Al
6
7
-SnP
Figure 7. 31P MAS NMR spectra of fluorinated calcined materials:
a)-(c) as in Figure 5.
d
Chlor-SnP
(
a
BET specific surface area. b Accumulated surface area (∑Sac), pore
), and average pore diameter (d (av)) calculated by
volume (∑V
p
p
broad band, especially intense in the fluorinated Al-SnP sample,
at 44.2 ppm which cannot be unambiguously assigned to
distorted tetrahedral or pentacoordinated aluminum, but in any
case, it would mean that cross-linking between oxide pillars
and the phosphate layers causes a decrease in the coordination
c
Cranston and Inkley’s method. Micropore volume, calculated by
Dubinin-Radushkevich’s equation. d From ref 13a.
cation intercalates, where segregated phases of Cr2O3 and cubic
SnP2O7 were detected,
1
3b
the strong interaction between the
9
,17
number of the aluminum ions directly bonded to the layer.
aluminum-containing species and the phosphate groups leads
finally to the breakup of the layered tin phosphate structure and
subsequent formation of aluminum and/or gallium phosphate
upon calcination at 1000 °C. The corresponding XRD patterns
show peaks characteristic of SnO2 (3.35, 2.64, and 2.37 Å) and
gallium/aluminum phosphates (4.13, 3.57, 2.53, and 2.16 Å).
Textural properties of fluorinated aluminum- and gallium/
aluminum oxide pillared tin phosphate materials have been
evaluated by N2 adsorption-desorption at 77 K. The corre-
sponding isotherms (Figure 8) are typical of mesoporous solids
with a certain contribution of micropores. The textural param-
eters are summarized in Table 2, as well as those of Chlorhy-
drol-SnP calcined at 400 °C, for comparison. Although R-SnP
itself is a nonporous solid, with a BET specific surface area
This evolution of the 27Al MAS NMR spectra with temperature
is similar to that reported for the AlP2 species, where the signals
of AlIV and AlVI vanish at the same time as a new resonance,
attributed to pentacoordinated aluminum, appears at 35 ppm,
21,22
becoming dominant at higher temperature.
On the other
hand, Mortlock et al. have observed that the signal of penta-
coordinated aluminum shifts downfield, near 45 ppm, upon
linking to phosphate ions.23 Calcination of hydroxy-Al pillared
montmorillonite and beidellite also gives rise to the appearance
of a signal near 35 ppm.6 The existence of Al ions in low
coordination environments is important from a catalytic view-
point, because they act as Lewis acid sites.
c
3
1
The P MAS NMR spectra of calcined materials are shown
in Figure 7. After calcination at 400 °C, the resonance at -9.6
ppm disappears, whereas the signal at higher field is broadened
and simultaneously moves to upfield and is now centered
between -17.0 and -18.6 ppm. These changes are in line with
a strong interaction between the intercalated species and the
phosphate layers, possibly established through P-O-Al bonds.
Unlike what happens upon calcining Cr(III) polyhydroxo-
2
-1
close to 10 m g , the pillared materials are porous materials
having SBET values between 160 and 304 m g . The most
porous material is Al-SnP with a SBET of 304 m g and a
pore volume of 0.478 cm g , which are higher than those
2
-1
2
-1
3
-1
2
-1
found for pillared Chlor-SnP (SBET < 230 m g ) and
2
-1
fluorinated pillared Al-ZrP solids (SBET < 185 m g and ∑Vp
3
-1
< 0.27 cm g ). The mesoporous character of the Al-SnP

Al and Ga/Al Oxide Pillared Materials
J. Phys. Chem. B, Vol. 102, No. 10, 1998 1677
Figure 10. Catalytic activity of fluorinated pillared materials in the
decomposition of 2-propanol as a function of time on stream: (a)-(d)
as in Figure 9. Experimental conditions: atmospheric pressure; T )
Figure 9. Total acidity of fluorinated calcined solids from NH
3
TPD:
(
a) Al-SnP, (b) GaAl13-SnP, (c) Ga
3
Al11-SnP, and (d) Ga Al -SnP.
6 7
220 °C; feed mixture, isopropyl alcohol/He; alcohol concentration, 11.3
solid is attributed to platelet stacking or end-end particle
interactions, while the microporosity comes from interlayer
cavities induced by pillaring. The increase in specific surface
area of fluorinated Al- and Ga/Al-SnP solids in comparison
to Chlorhydrol-SnP is related to the presence of oligomeric
species with higher nuclearity and stability in the interlayer
region, whereas the differences with the fluorinated Al-ZrP
materials are rather related to the strength of interaction between
the phosphate layer and the intercalated species. The presence
vol %.
catalysts is not only dependent on the chemical nature of the
materials but also on the textural characteristics, since a good
relationship between catalytic activity and pore volume is found,
indicating that the confinement effect in pores influences the
catalytic decomposition of isopropyl alcohol molecules.
Conclusions
of larger species in the interlayer results in a greater mi-
croporosity (Vµ ) 0.064-0.104 cm3 g-1) for the pillared
Fluorinated aluminum and mixed gallium/aluminum oligo-
7
+
3
-1
mers of higher nuclearity than the species Al13 have been
intercalated into colloidal R-tin phosphate. These intercalated
materials are thermally more stable and lead to fluorinated
aluminum and mixed gallium/aluminum oxide pillared materials
with higher free heights, and hence higher interlayer porosity,
materials, reaching a micropore volume of 0.1 cm g , close
to those reported for pillared clays.1
e
Typical pore size
distributions, determined by the Cranston and Inkley method,
are also shown in Figure 8. Narrow distributions were found
with most of the pores having radii in the range 16-22 Å.
The acidity of the pillared materials has been evaluated by
temperature-programmed desorption of ammonia between 100
and 400 °C. Values of total acid sites range between 1, for
7+
31
27
than those prepared from solutions of Al13
.
P and Al MAS
NMR studies have revealed that strong interactions between the
phosphate layer and the intercalated species are established upon
calcination. The appearance of low coordination sites of
aluminum in calcined materials points to aluminum and mixed
gallium/aluminum oxide nanoparticles that are cross-linked to
the phosphate layer, through Al-O-P bonds. These sites may
act as Lewis acid centers and could be mainly responsible for
the high acidity displayed by the oxide pillared materials, which
behave exclusively as dehydrating catalysts for the decomposi-
tion of isopropyl alcohol.
-1
Al-SnP, and 2 mmol NH3 g , for Ga6Al7-SnP; i.e., the acidity
increases with Ga content (Figure 9). The substitution of Al
by Ga in octahedral sites of the clusters increases, therefore,
the acidity of the oxide pillars. Ammonia is mainly desorbed
between 200 and 300 °C, which is indicative of the presence
of acid sites with medium strength in all materials. In addition,
strong acid sites (ammonia desorption between 300 and 400
°
C) are also detected for Ga6Al7-SnP. The total acidity of these
24
materials is higher than that of pillared clays but lower than
Acknowledgment. This research was supported by the
CICYT (Spain) Project MAT 97-906. P.B.-G. thanks the Junta
de Andaluc ´ı a for a fellowship. The authors thank Dr. M. D.
Alba-Carranza for recording MAS NMR spectra.
that of fluorinated Al-ZrP.9
The reaction of decomposition of isopropyl alcohol has been
used in order to evaluate the acid properties of the catalytic
sites of the fluorinated Al- and Ga/Al oxide pillared tin
phosphate solids. Figure 10 shows the catalytic activity of
pillared solids as a function of time on stream. In all cases, the
catalytic activity was almost stable for at least 20 h of reaction
time, and propylene (dehydration reaction) was the only reaction
product, demonstrating the exclusive presence of acid sites in
these solids. Strikingly, the Al-SnP material exhibits an activity
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