1260
Langmuir 2006, 22, 1260-1267
Application of Porous Phosphate Heterostructure Materials for Gas
Separation
G. Aguilar-Armenta,*,† M. E. Patiño-Iglesias,† J. Jiménez-Jiménez,‡
E. Rodrı́guez-Castellón,‡ and A. Jiménez-López‡
Centro de InVestigación, Facultad de Ciencias Quı́micas, Benemérita UniVersidad Autónoma de Puebla,
14 Sur y AV. San Claudio, Ciudad UniVersitaria, 72570 Puebla, Pue., México, and Departamento de
Quı́mica Inorgánica, Cristalografı́a y Mineralogı́a, Unidad Asociada al ICP-CSIC, Facultad de Ciencias,
UniVersidad de Málaga, 29071 Málaga, Spain
ReceiVed September 1, 2005. In Final Form: October 28, 2005
In this study, the adsorbents Cu+-3SiPPH723 and Cu2+-3SiPPH723 were prepared starting from a silica-expanded
zirconium phosphate heterostructure, 3SiPPH(0.2), which was subjected to an ion exchange with Cu(I) and Cu(II).
These materials were characterized using powder X-ray diffraction, X-ray photoelectron spectroscopy, ammonia
thermally programmed desorption, hydrogen temperature-programmed reduction, and N2 adsorption (77 K). The
equilibria and kinetics of adsorption of pure propylene (C3H6) and propane (C3H8) were studied using a conventional
glass high-vacuum volumetric device, equipped with grease-free valves, in the temperature range of 273-393 K. The
starting material, 3SiPPH(0.2), presented a high acidity and irreversible chemisorption of the olefin, which increases
with temperature. Unlike the support, the irreversible adsorption of the olefin on the Cu+-3SiPPH723 and Cu2+3SiPPH723 samples decreases with increasing temperature and disappears at 393 K, showing a very high selectivity
toward propylene. The C3H8 adsorption in all the samples was always reversible. On the basis of the results of this
study, both Cu+-3SiPPH723 and Cu2+-3SiPPH723 samples can be efficiently applied in the separation of a C3H6/
C3H8 mixture at 393 K. Cu+-3SiPPH723 would have the highest efficiency, because its capacity for C3H6 adsorption
was higher than that for the Cu2+-3SiPPH723 sample.
Introduction
al.1
Ten years ago, Galarneau et
reported a novel process of
synthesis of a new family of porous materials called porous clay
heterostructures (PCHs), where the surfactant molecules are
templates of inorganic arrays in the clay interlayer space. In
these materials, cationic surfactant molecules are placed in the
interlayer space of a clay host by means of a cationic exchange
process. For this purpose, clays, with a high cationic exchange
capacity (CEC), such as fluorohectorite,1-3 montmorillonite, or
saponite have been used as hosts.4-7 In all cases, a supplementary
neutral surfactant is necessary as cosurfactant for the correct
formation of silica galleries by the hydrolysis and condensation
of tetraethylorthosilicate (TEOS), used as the silica source. In
addition, extensive research has been devoted to the preparation
of mesoporous silica of hexagonal (MCM-41), cubic (MCM48), or lamellar (MCM-50) geometry, by reacting surfactant
micelles with different inorganic arrays.8,9 In the case of solids
of the MCM-41 or MCM-48 type, porous materials with high
specific surface areas and well-defined porous diameters are
* To whom correspondence should be addressed. E-mail: geaguila@
siu.buap.mx.
† Benemérita Universidad Autónoma de Puebla.
‡ Universidad de Málaga.
(1) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529.
(2) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Chem. Commun. 1997,
1661.
(3) Mercier. L.; Pinnavaia, T. J. Microporous Mesoporous Mater. 1998, 20,
101.
(4) Polverejan, M.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 2698.
(5) Ahenach, J.; Cool, P.; Vasant, E. F. Phys. Chem. Chem. Phys. 2000, 2,
5750.
(6) Benjelloun, M.; Cool, P.; Linssen, T.; Vasant, E. F. Microporous Mesoporous
Mater. 2001, 49, 83.
(7) Pichowicz, M.; Mokaya, R. Chem. Commun. 2001, 2100.
(8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.
Nature 1992, 359, 710.
(9) Corma, A. Chem. ReV. 1997, 97, 2373. (b) Ciesla, U.; Schüth, F. Microporous
Mesoporous Mater. 1999, 27, 131.
obtained after removal of surfactant molecules, generally by
calcination. However, in the case of solids of the MCM50 type,
nonporous materials were obtained upon calcination.
On the other hand, we synthesized a lamellar zirconium
phosphate, of MCM-50 type, expanded with cationic surfactant
molecules. This phosphate was used as the starting material for
the insertion of gallium oxide into its interlayer space by cationic
exchange of the surfactant guest with oligomeric gallium species,
obtaining a porous structure as in the case of PLS materials.10
Very recently, we have combined the strategies for obtaining
mesoporous materials, such as the formation of pillared layer
structures (PLSs) and MCM-41, for the preparation of porous
phosphate heterostructure (PPH) solids consisting of silica
galleries in the interlayer space of zirconium phosphate with
high specific surface area and acidity,11 where the obtained solids
present a strong Brönsted acidity as deduced by the formation
of iso derivatives in the 1-butene isomerization test.
The separation of mixtures of olefin/paraffin is a subject of
current interest, and several types of zeolites (NaX, ZSM-5)
exchanged with Co(II), Cu(II), and Zn(II) have been tested.12-14
Other porous solids, such as γ-Al2O3 and SiO2, ion-exchanged
or impregnated with monovalent cations proved to be efficient
in the separation process.15,16 MCM-41 ion-exchanged with Cu(II) and Ag(I),16 mesoporous Cu(II)- and Ag(I)-derivatized
(10) Jiménez-Jiménez, J.; Maireles-Torres, P.; Olivera-Pastor, P.; Rodrı́guezCastellón, E.; Jiménez-López, A. Langmuir 1997, 13, 2857.
(11) Jiménez-Jiménez, J.; Rubio-Alonso, M.; Eliche-Quesada, D.; Rodrı́guezCastellón, E.; Jiménez-López, A. J. Mater. Chem. 2005, 15, 3466.
(12) Khelia, A.; Derriche, Z.; Bengueddach, A. Appl. Catal., A 1999, 178, 61.
(13) Corma, A.; Ray, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004,
431, 287.
(14) Ivanov, A. V.; Graham, G. W.; Shelef, M. Appl. Catal., B 1999, 21, 243.
(15) Masuda, T.; Okubo, Y.; Mukai, S. R.; Kawase, M.; Hashimoto, K.; Shichi,
A.; Satsuma, A.; Hattori, T.; Kiyozumi, Y. Chem. Eng. Sci. 2001, 56, 889.
(16) Blas, F. J.; Vega, L. F.; Gubbins, K. E. Fluid Phase Equilib. 1998, 150151, 117.
10.1021/la052390e CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/22/2005
Application of PPH Materials for Gas Separation
aluminosilicate materials,17 mesoporous Ag(I)-impregnated SBA15,18 and SBA-15 ion-exchanged with Ag(I) and K+ natural
erionite19 have also been used, where the idea is the separation
of propylene/propane mixtures via π-complexation. With the
same mechanism, very recently, mesoporous γ-Al2O3 and
microporous SiO2 membranes impregnated with Ag(I) have been
successfully tested in this separation process at 313 K.20
In this study we propose the use of a silica-expanded zirconium
phosphate heterostructure, which will be ion-exchanged with
Cu(II) and Cu(I) in the separation of propylene (C3H6)/propane
(C3H8) mixtures.
Experimental Section
Materials. A cetyltrimethylammonium (CTMA)-expanded zirconium phosphate was prepared from a solution of (CTMA)Br
(Aldrich) in 1-propanol (Rectapur), to which H3PO4 (85%, BDH)
and zirconium(IV) propoxide (70%, Aldrich) were loaded according
to previously reported procedures.11 The obtained solid (CTMAZrP) was suspended in water (10 g L-1), and a solution of
hexadecylamine (Aldrich) in 1-propanol (35 g L-1) was added as
cosurfactant. After 1 day under stirring, a solution (50%, v/v) of
TEOS (Aldrich) in 1-propanol was added, and this suspension was
stirred at room temperature for 3 days. The solid obtained was then
centrifuged, washed with ethanol, and dried at 333 K in air. This
precursor material was calcined in air at 823 K for 5 h (1.5 K min-1
heating rate). The obtained solid 3SiPPH(0.2), with a d001 ) 40 Å,
a specific surface area (BET) of 619 m2 g-1, and a total pore volume
of 0.552 cm3 g-1, was used as the support for this study.11
The ionic exchange capacity of the support (1.73 mequiv g-1)
was determined by CNH elemental analysis of a sample exposed
under an atmosphere of NH3 for 5 min and then kept for 24 h in a
desiccator with concentrated H3PO4. This value was similar to that
obtained by ammonia thermally programmed desorption (NH3-TPD)
(1.66 mequiv g-1). Cu(II) was exchanged by adding an amount
equivalent to 5 times the cationic exchange capacity using a 0.1 M
(1:1, v/v, ethanol/water) solution of copper(II) acetate monohydrate
to the support, 3SiPPH(0.2), and stirring for 1 day at 298 K. The
solid was centrifuged, washed with ethanol, and dried at 333 K
(sample Cu2+-3SiPPH). This sample then was calcined at 723 K
for 5 h with a heating rate of 1.5 K min-1 (sample Cu2+-3SiPPH723).
The sample Cu+-3SiPPH723, ion-exchanged with Cu(I), was
prepared in a similar way using an ethanolic solution of copper(I)
chloride, but adding an amount equivalent to the cationic exchange
capacity.
Characterization of Samples. X-ray diffraction (XRD) powder
patterns were recorded on a Siemens D501 diffractometer (Cu KR
radiation) provided with a graphite monochromator. X-ray photoelectron spectroscopy (XPS) analyses were recorded using a Physical
Electronics PHI 5700 spectrometer with nonmonochromatic Mg
KR radiation (300 W, 15 kV, 1253.6 eV) as the excitation source.
High-resolution spectra were recorded at a 45° takeoff angle by
using a concentric hemispherical analyzer operating in the constant
pass energy mode at 29.35 eV and an analysis area 720 µm in
diameter. Under these conditions, the Au 4f7/2 line was recorded
with 1.16 eV fwhm at a binding energy of 84.0 eV. The spectrometer
energy scale was calibrated by using the Cu 2p3/2, Ag 3d5/2, and Au
4f7/2 photoelectron lines at 932.7, 368.3, and 84.0 eV, respectively.
Cu 2p core level spectra were first recorded by irradiation for 10
min to avoid the photoreduction of Cu(II) to Cu(I). N2 adsorptiondesorption isotherms at 77 K were obtained using a conventional
volumetric device, after outgassing of samples at 413 K for 24 h,
or using a Micromeritics ASAP2020 apparatus. NH3-TPD was used
(17) Kargol, M.; Zajac, J.; Jones, D. J.; Steriotis, Th.; Rozière, J.; Vitse, P.
Chem. Mater. 2004, 16, 3911.
(18) Grande, C.; Araujo, J. D. P.; Cavenati, S.; Firpo, F.; Basaldella, E.;
Rodrigues, A. E. Langmuir 2004, 20, 5291.
(19) Aguilar-Armenta, G.; Patiño-Iglesias M. E. Langmuir 2002, 18, 7456.
(20) Stoitsas, K. A.; Gotzias, A.; Kikkinides, E. S.; Steriotis, Th. A.;
Kanellopoulos, N. K.; Stoukides, M.; Zaspalis, V. T. Microporous Mesoporous
Mater. 2005, 78, 235.
Langmuir, Vol. 22, No. 3, 2006 1261
to determine the total acidity of the samples. Before the adsorption
of ammonia at 373 K, the samples were heated at 773 K in a He
flow. NH3-TPD was performed between 373 and 773 K, at a heating
rate of 10 K min-1, using a thermal conductivity detector (TCD) for
the analysis. Hydrogen temperature-programmed reduction (H2-TPR)
experiments were carried out between 323 and 823 K, using a flow
of 10% H2/Ar (48 mL min-1) and a heating rate of 14 K min-1.
Water produced in the reduction reaction was eliminated by passing
the gas flow through a coldfinger (193 K). The H2 consumption was
controlled by an on-line gas chromatograph (Shimadzu GC-14A)
provided with a TCD. CHN elemental analysis was used to determine
the surfactant content and was carried out with a LECO instrument;
ion-exchanged metals were determined by atomic absorption (AA)
with a Perkin-Elmer AA420 spectrometer, while phosphorus was
determined colorimetrically.
The C3H6 and C3H8 adsorption capacity of the samples was
measured in a conventional glass high-vacuum volumetric device,
equipped with grease-free valves. Pressures were registered with
two types of pressure transducers (Balzers) of different ranges: TPR
017 (0.01333-666.61 Pa) and APR 011 (0.13332-133.322 kPa).
Before the measurements, the samples were dehydrated in situ at
693 K in an oven with a residual pressure of 0.01333 Pa and kept
in these conditions for a period of 3 h. After sample dehydration,
the temperature was lowered to the desired point, and the sample
was allowed to stabilize for at least 1.5 h before the measurements
were started. The adsorbed amount of the gases was referred to 1
g of dehydrated adsorbent. Weight loss of the adsorbent was
previously assessed by heating the sample at 623 K under atmospheric
pressure in a conventional oven. The adsorption uptake of gases as
a function of time, t, was obtained on the basis of the difference
between the initial amount of gas introduced into the cell and the
amount of gas remaining in the dead space of the cell at any given
time t, from t ) 0 up to teq (equilibrium). During the kinetic
measurements, the decrease of pressure in the system was measured
automatically with a custom acquisition data card, which allows
simultaneous monitoring and recording of time and pressure. In the
period from 0 to 3 min, pressure was monitored five times per second.
Afterward, in the period from 3 to 13 min, pressure was registered
once per second, and pressure was recorded once every 10 s in the
period from 13 min to teq. In all experiments, the initial pressure was
59.995 kPa.
Since the adsorbents usually work continuously in a given
adsorption process, as for instance in pressure-swing adsorption
(PSA) cycles, it is necessary to evaluate the extent of irreversible
adsorption, i.e., the amount of adsorbate which cannot be desorbed
in a vacuum at the given experimental temperature. The experimental
procedure to assess irreversible adsorption was described in detail
in a previous paper.21
Results and Discussion
Structural Properties of Samples. The formation of silica
galleries in the interlayer space of the layered zirconium phosphate
is the key to generate high surface areas, and in Scheme 1 the
mechanism of formation of the used support is described. Table
1 shows the structural and textural properties of the studied
solids.11
The basal spacing is modified upon impregnation with copper,
but this modification is very ambiguous due to the broadness of
the d001 reflection line. As an example, Figure 1 shows the powder
patterns of sample Cu2+-3SiPPH723, where a maximum centered
at 48 Å close to several shoulders at a higher angle is observed.
The N2 adsorption-desorption isotherms at 77 K of the copperimpregnated solids are type IV isotherms corresponding to
mesoporous materials and similar to that observed in the case
of the support, 3SiPPH(0.2).11 However, the surface BET values
are smaller due to the formation of a surface of copper species
(21) Patiño-Iglesias, M. E.; Aguilar-Armenta, G.; Jiménez-López, A.; Rodrı́guez-Castellón, E. Colloids Surf., A 2004, 237, 73.
1262 Langmuir, Vol. 22, No. 3, 2006
Aguilar-Armenta et al.
Scheme 1. Mechanism of Support Formation
Table 1. Structural and Textural Properties of the Support and
Exchanged Samples
sample
d001
(Å)
SBET
(m2 g-1)
Vp
(cm3 g-1)
rp
(Å)
3SiPPH(0.2)
Cu2+-3SiPPH723
Cu+-3SiPPH723
41
48
43
619
409
325
0.552
0.397
0.316
16.3
17.2
17.2
Table 2. XPS Data of the Samples
sample
3SiPPH(0.2)
Cu2+-3SiPPH
Cu2+-3SiPPH723
Cu+-3SiPPH723
Cu 2p3/2
935.8
935.2
933.0 (64%)
935.4 (36%)
P 2p
Zr 3d5/2
Si 2p
O 1s
133.8
133.7
133.3
133.5
183.1
183.1
182.8
183.1
102.9
102.9
102.9
102.9
532.3
532.1
531.9
532.0
which block some pores, avoiding free access of N2 molecules,
whereas their average pore radius (rp) is similar to that of the
support (Table 1). That is, the structure of the support is preserved
upon impregnation with copper and calcination at 723 K.
XPS data show (Table 2) that (i) the binding energy values
of the P 2p, Zr 3d5/2, Si 2p, and O 1s core level peaks of the
Cu-impregnated samples are similar to those of the support,
evidencing again the preservation of the structure upon impregnation and calcination, (ii) the Cu 2p3/2 photoemission of the
samples Cu2+-3SiPPH and Cu2+-3SiPPH723 correspond to
the exclusive existence of Cu(II) species (Figure 2), but the
intensity of the Cu 2p3/2 photoemission is much higher in the
case of the calcined sample (Cu2+-3SiPPH723), probably due
to the formation of CuO on the surface upon calcination in addition
to Cu(II) located in exchange positions, (iii) the Cu 2p core level
spectrum (Figure 3) of the Cu+-3SiPPH723 sample shows two
types of copper species, Cu(I) in a higher proportion at a lower
binding energy of 933.0 eV and Cu(II) with a peak at 935.4 eV,
Table 3. XPS Data for the Support and the Copper-Exchanged
Samples (atom %)
sample
Cu
P
Zr
3SiPPH(0.2)
7.97 3.77
Cu2+-3SiPPH
2.82 9.08 5.06
Cu2+-3SiPPH723 5.71 8.93 4.36
Cu+-3SiPPH723 2.27 9.82 4.84
Si
O
18.20
14.48
12.57
12.44
70.05
68.56
68.44
70.62
P/Zr Cu/Si
2.11
1.79
2.05
2.03
0.19
0.45
0.18
and (iv) the chemical composition of the samples, expressed in
atomic concentration percentages (Table 3), also supports some
of the above asseverations since the P:Zr molar ratio is close to
the theoretical value of 2.00 and the Cu:Si molar ratio of sample
Cu2+-3SiPPH723 is higher than that of sample Cu2+-3SiPPH
due to the surface segregation of CuO.
The acidity of the samples (Figure 4) shows that the total
acidity of the support was higher than that for the other samples
(1659 µmol of NH3 g-1) and that upon impregnation with Cu(I)
and Cu(II) the acidity was slightly (1623 µmol of NH3 g-1) and
noticeably (956 µmol of NH3 g-1) reduced, respectively. The
main feature of these results is that an important proportion of
ammonia is desorbed at higher temperatures (>673 K) in the
case of the support, as compared with the copper-impregnated
samples, indicating a higher proportion of strong Brönsted acid
sites in the support in comparison with the copper-impregnated
samples.
H2-TPR is helpful in obtaining more information about copper
species present in the copper-impregnated structures. The H2
consumption curve for sample Cu2+-3SiPPH723 (Figure 5a)
shows a sharp maximum centered at 510 K assigned to the
Figure 3. Cu 2p core level spectrum of the Cu+-3SiPPH723 sample.
Figure 1. XRD patterns of sample Cu2+-3SiPPH723.
Figure 2. XPS patterns of the Cu2+-3SiPPH723 and Cu2+-3SiPPH
samples.
Figure 4. Acidity of the samples (NH3-TPD).
Application of PPH Materials for Gas Separation
Figure 5. H2-TPR curves for samples with Cu: (a) Cu2+3SiPPH723 and (b) Cu+-3SiPPH723.
reduction of Cu(II) ions to Cu(0). This reduction temperature is
higher than that of the bulk CuO,22 and near but higher than those
observed in some Cu(II)-exchanged zeolites.23 This is indicative
of the strong interaction of Cu(II) with the strong acid sites of
the support. The presence of the surface CuO was not confirmed,
perhaps because this species is only located on the external surface
of the support. The H2-TPR curve for sample Cu+-3SiPPH723
(Figure 5b) shows two maxima. The first sharply centered at 507
K is very similar to that observed for sample Cu2+-3SiPPH723
and also assigned to the reduction of Cu(II) species, and the
second one has a maximum at 603 K with a shoulder at 564 K.
This peak is assigned to the reduction of Cu(I) to Cu(0).24
Kinetics and Equilibrium of Adsorption. Support 3SiPPH(0.2). The kinetic curves of the total (reversible + irreversible)
and reversible adsorption of C3H6 on the support at 273, 293,
313, and 333 K for short, t < 160 s, and long, t < 15000 s,
gas-adsorbent contact times, respectively, are shown in parts a
and b of Figure 6.
Clearly, the irreversible adsorption (total - reversible) uptake
tends to increase with increasing temperature. The total adsorbed
amount at equilibrium (neq) decreases with increasing temperature
in the range between 273 and 313 K, but when the temperature
was increased to 333 K, neq was higher (1.2192 mmol g-1) than
that at 313 K (1.1250 mmol g-1).
It was established that the total adsorption capacity for C3H6
at 273, 293, and 313 K occurred rapidly during the first 20 s
(Figure 7a), achieving a fractional uptake nt/neq ) 0.8, where nt
is the adsorbed amount at time t, and at 333 K this fractional
uptake was reached for a contact time of 7500 s; that is, a slow
adsorption for temperatures higher than 313 K was observed.
The adsorption equilibrium time (teq), i.e., the time required
to achieve the total adsorption capacity nt/neq ) 1, increased in
the order of 900, 1750, 7900, and 21500 s for temperatures of
273, 293, 313, and 333 K, respectively (Figure 7b). These results
(22) Torre-Abreu, C.; Henriques, C.; Ribeiro, F. R.; Delahay, G.; Ribeiro, M.
F. Catal. Today 1999, 54, 407.
(23) Moreno-Tost, R.; Santamarı́a-González, J.; Rodrı́guez-Castellón, E.;
Jiménez-López, A.; Autié, M. A.; González, E.; Carreras Glacial, M.; Delas
Pozas, C. Appl. Catal., B 2004, 50, 279.
(24) Coq, B.; Tachon, F.; Figueras, F.; Mabillon, G.; Prigent, N. Appl. Catal.,
B 1995, 6, 271.
Langmuir, Vol. 22, No. 3, 2006 1263
Figure 6. Kinetic curves of total (full symbols) and reversible (empty
symbols) adsorption of propylene on 3SiPPH(0.2) at various
temperatures: (b, O) 273 K; (2, 4) 293 K; (9, 0) 313 K; ([, ])
333 K.
Figure 7. Fractional adsorption uptake for propylene on 3SiPPH(0.2) at various temperatures.
indicate that the adsorption process of C3H6 exhibits some general
features (activated, slow, and irreversible) of chemisorption which
involves the formation of a chemical bond between the olefin
molecule and the surface of the sorbent.
In view of these results, it was interesting to carry out adsorption
kinetic measurements at a higher temperature (423 K). The kinetic
curves of the total adsorption of C3H6 are shown in parts a and
b of Figure 8 for short and long t, respectively, at 273, 293, 313,
333, and 423 K. The adsorption uptake at 423 K for short t was
too small and increased continuously with time, reaching not
only the adsorption uptake at 333 K, but even the one at 273 K.
Apparently, the propylene is chemisorbed, mainly for temperatures higher than 313 K, on the Brönsted acidic centers,
leading probably to a cationic polymerization. To support this
supposition, the chemical composition of the surface after C3H6
adsorption at 423 K was assessed. After adsorption at 423 K,
part of the organic phase of the solid was extracted with n-heptane
and then analyzed by GC-MS. Many hydrocarbons were
detected, but two intense peaks with molecular weights of 168
1264 Langmuir, Vol. 22, No. 3, 2006
Aguilar-Armenta et al.
Figure 10. Isosteric heat of adsorption as a function of the adsorbed
amount. QL ) latent heat of condensation.
Figure 8. Adsorption kinetic curves of propylene on 3SiPPH(0.2)
at various temperatures.
Figure 11. Kinetic curves of total (full symbols) and reversible
(empty symbols) adsorption of C3H6 on Cu+-3SiPPH723 at various
temperatures (K). (b, O) 293; (2, 4) 333; ([, ]) 393, (0) 393,
C3H8.
Figure 9. Adsorption equilibrium isotherms of C3H6 (full symbols)
and C3H8 (empty symbols) on 3SiPPH(0.2) at 273 K (b, O) and 293
K (2, 4). Lines are the Freundlich model fits.
and 210 were observed, which correspond to C12H24 and C15H30
olefins. Unlike the C3H6 adsorption, the C3H8 adsorption on the
3SiPPH(0.2) sample was reversible at all the studied temperatures.
The adsorption equilibrium isotherms of both hydrocarbons
at 273 and 293 K (Figure 9) are nonrectangular in shape and
were described fairly well by the Freundlich empirical equation
with a correlation coefficient R2 > 0.995.
To assess the strength of the bonding between the adsorbate
and the surface, the isosteric heat of adsorption (Qst) with gas
loading was computed from equilibrium data (Figure 9) using
the Clausius-Clapeyron equation, which at constant adsorbate
loading results in
-∆H ) Qst ) R
[
]()
T1T2
P2
ln
T1 - T 2
P1
(1)
where P2 and P1 are the equilibrium adsorption pressures at
temperatures T2 and T1, respectively, at a specific adsorbate
loading.
According to the obtained results (Figure 10), the isosteric
heat of adsorption decreases and tends to the latent heat of
condensation (QL) with gas loading for both hydrocarbons, which
is a characteristic of highly heterogeneous adsorbents with a
wide distribution of gas-solid interaction energies.25,26 The Qst
values at very low loading (0.1 mmol g-1) for propylene and
propane were 2.4 and 1.6 times higher than the latent heat of
evaporation, respectively. By virtue of the adsorption of nonpolar
(25) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A.
L. Langmuir 1996, 12, 5888.
(26) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir
1996, 12, 5896.
molecules such as propane only presenting nonspecific interactions, which are largely determined by the polarizability of the
adsorbate, the contribution of π electrons to the total adsorbateadsorbent interactions can be assessed by the difference ∆Qst )
Qst(C3H6) - Qst(C3H8) as a function of loading. The small
irreversible adsorption observed at 273 and 293 K (Figure 6a),
which increases with increasing temperature, can be due to the
chemisorption of propylene beginning to be apparent in the
Brönsted acidic adsorption sites on the surface.
Cu+-3SiPPH723. The kinetic curves of the total and reversible
adsorption of C3H6 were measured at 293, 313, 333, 373, and
393 K. For clarity, Figure 11 presents only the kinetic curves of
C3H6 at 293, 333, and 393 K. In contrast to that on the support,
irreversible adsorption of C3H6 decreases with increasing
temperature and disappears at 393 K. The irreversible adsorption
observed at T < 333 K is not due to irreversible polymerization
of the olefin because the adsorbed gas could be entirely removed
with the aid of heat at vacuum. Figure 11 includes the adsorption
kinetic curve for C3H8 at 393 K for comparison. The adsorption
of propane, as on the support (393 K), was always reversible.
Because of the reversible adsorption of propylene and the
insignificant adsorption of propane observed at 393 K, this solid
could be used for separating mixtures of these hydrocarbons at
this temperature.
To study the adsorption behavior at equilibrium, the adsorption
isotherms of C3H6 and C3H8 were obtained at 293, 313, 333, and
373 K (Figure 12). For clarity, in this figure only the C3H8
adsorption isotherms measured at 293 and 313 K are included.
The adsorption isotherms for propylene show a sharp increase
at low pressures (<2.666 kPa), and subsequently, an increase is
observed, which is less and less pronounced with increasing
temperature. This behavior could be explained assuming that the
C3H6 molecules are adsorbed in two different adsorption sites
with different adsorption potentials following a monolayer
mechanism at low pressures and a multilayer mechanism as the
pressure is increased. Interestingly, the adsorption isotherms were
fairly well described (R2 ) 0.998) by the classical Langmuir
Application of PPH Materials for Gas Separation
Langmuir, Vol. 22, No. 3, 2006 1265
Figure 12. Adsorption equilibrium isotherms of propylene (full
symbols) and propane (empty symbols) at different temperatures
(K): (b, O) 293; (2, 4) 313; (9) 333; ([) 373, on Cu+-SiPPH723.
Lines are for C3H6 dual Langmuir (293, 313, and 333 K), C3H6
Langmuir-Freundlich (373 K), and C3H8 Freundlich model fittings.
Figure 13. Kinetic curves of total (full symbols) and reversible
(empty) adsorption of propylene on Cu2+-SiPPH723 at various
temperatures (K): (b, O) 293; (2, 4) 313; ([, ]) 333; (9, 0) 393;
(•, total - reversible) 423.
Table 4. Optimal Parameters for the Dual Langmuir Equation
in Fitting C3H6 Adsorption Data on Cu+-3SiPPH723
293 K
313 K
333 K
param number 1
2
1
2
1
2
b (kPa-1)
26.9775 0.0195 21.1395 0.0165 15.7657 0.0112
-1
am (mmol g ) 0.6683 1.5109 0.5810 1.3225 0.5123 1.3472
model, eq 2, only in a narrow pressure range (0.0066 < Peq <
3.3325 kPa). The difference in enthalpy between adsorbed and
gaseous phases of the sorbate was evaluated (-∆H ) 17 kJ
mol-1) from the b values, using the van’t Hoff equation b ) b0
exp({-∆H}/{RT}).
b1P
a ) am,1
1 + b1 P
(2)
It was established that, over the entire studied pressure range
(0.0066 < Peq < 66.65 kPa), the dual-site Langmuir model, eq
3, which implicitly takes into account interactions between
adsorbed molecules, was better for describing the adsorption
isotherms of C3H6 measured at 293, 313, and 333 K, and the Sips
(Langmuir-Freundlich) model, eq 4, described fairly well the
adsorption isotherm measured at 373 K. The fitting parameters
were determined (Table 4) using a computational program written
in Matlab version.27
b1P
b2P
+ am,2
a ) am,1
1 + b1P
1 + b 2P
(3)
(bP)n
a ) am
1 + (bP)n
(4)
As is known, the Sips equation, even though it is similar in
form to the Freundlich equation, has a finite limit when the
pressure is sufficiently high. Unlike the other isotherms, the one
measured at 373 K presents a clear tendency to a limit at high
pressures. The adsorption isotherms of C3H8 were described by
the conventional Freundlich equation. Because the isotherms of
propylene and propane at 293 and 313 K (Figure 12) are similar
in form to each other for pressures Peq > 2.666 kPa, it can be
assumed that, once the copper cations Cu(I) are saturated by
C3H6 molecules at low equilibrium pressures (Peq < 2.666 kPa),
the adsorption process of this hydrocarbon takes place by a
mechanism of nonspecific interactions (attraction forces +
repulsion + polarization), that is, following the usual mechanism
in the adsorption of C3H8.
(27) Duong, D. D. Adsorption Analysis: Equilibria and Kinetics; Series on
Chemical Engineering, Vol. 2; Imperial College Press: London, 1998.
Figure 14. Adsorption equilibrium isotherms of propylene on Cu2+SiPPH723 at various temperatures (K): (b) 293; (2) 313; ([) 333;
(9) 373; (O) 423; (4) 443.
Cu2+-3SiPPH723. Figure 13 depicts the total and reversible
adsorption kinetic curves of propylene at 293, 313, 333, 393, and
423 K for low contact times (<60 s). The adsorption process
occurs almost instantaneously. This figure shows that the total
adsorption uptake decreases, but the irreversible adsorption passes
through a maximum (313 K), with increasing temperature. For
example, for a contact time of 20 s, the values of irreversible
adsorption (mmol g-1) change as follows: 0, 0.17, 0.22, 0.18,
0.04, and 0 for 293, 313, 333, 393, and 423 K, respectively.
Since the irreversible adsorption begins to decrease starting at
313 K, the increase of irreversible adsorption of C3H6 on the
Cu2+-3SiPPH723 sample can be due to chemisorption of
molecules on weak acidic sites. Unlike that on the 3SiPPH(0.2)
sample, the chemisorption on Cu2+-3SiPPH723 did not lead to
polymerization reaction of the olefin.
The C3H6 adsorption equilibrium isotherms at 293, 313, 333,
373, 423, and 443 K are shown in Figure 14. These isotherms
have a behavior, with increasing temperature, similar to that for
the Cu+-SiPPP723 sample (Figure 12); that is, as the temperature
is increased, the limit of adsorption becomes more and more
clear. The adsorption isotherms of Figure 14 were fitted into
three models: the Freundlich equation described the isotherm
at 293 K, the ones measured at 313, 333, and 373 K were fitted
with the dual Langmuir model (eq 3), and the other two isotherms
were described with the classical Langmuir model (eq 2).
Moreover, Figure 14b shows that at low pressures (<3 kPa) the
adsorption capacity of the adsorbent at 313 and 333 K is greater
1266 Langmuir, Vol. 22, No. 3, 2006
Aguilar-Armenta et al.
Figure 17. Isosteric heat of adsorption of C3H6 (full symbols) and
C3H8 (empty symbols) on the samples: (b, O) 3SiPPH(0.2); (9, 0)
Cu2+-3SiPPH723; (2, 4) Cu+-3SiPPH723. QL ) latent heat of
condensation.
Figure 15. Adsorption equilibrium isotherms of C3H6 (full symbols)
and C3H8 (empty symbols) on the samples at 293 K: (b, O) 3SiPPH(0.2); (9, 0) Cu2+-3SiPPH723; (2, 4) Cu+-3SiPPH723.
Figure 18. Adsorption kinetic behavior for propylene (full symbols)
and propane (empty symbols) at 393 K: (2, 4) Cu+-3SiPPH723;
(9, 0) Cu2+-3SiPPH723.
Figure 16. Irreversible adsorption of propylene as a function of
temperature: (b) 3SiPPH(0.2); (0) Cu2+-3SiPPH723; (4) Cu+3SiPPH723;
than that measured at 293 K. This can be attributed to
chemisorption, which was also observed in the adsorption kinetic
study (Figure 13).
Comparison of the Adsorption Behaviors of the Gases. To
compare the adsorption behaviors of C3H6 and C3H8 in the three
studied adsorbents, the adsorption isotherms (293 K) for low and
high pressures are shown in parts a and b of Figure 15,
respectively. These isotherms show that the cations of Cu+ and
Cu2+ influenced the ability of the samples to adsorb C3H6 and
C3H8.
The capacity of the samples for adsorption of C3H6 at low
pressures increases in the following order: 3SiPPH(0.2) < Cu2+3SiPPH723 < Cu+-3SiPPH723. However, the adsorption
capacity of both propylene and propane at high pressures decreases
as the acidity of the samples (Figure 4) decreases in the sequence
3SiPPH(0.2) > Cu+-3SiPPH723 > Cu2+-3SiPPH723. These
results may be explained assuming that the C3H6 molecules are
adsorbed in two different adsorption sites with different adsorption
potentials, following a monolayer mechanism at low pressures,
where Cu+ and Cu2+ cations are the specific adsorption sites for
propylene, and when the pressure is increased, the adsorption
process follows a multilayer mechanism.
Figure 16 shows that the irreversible adsorbed amount of
propylene at 293 K increases in the order 3SiPPH(0.2) < Cu2+3SiPPH723 < Cu+-3SiPPH723. This means that, in the samples
with copper, the irreversible adsorption was mainly due to the
π-complexation bonding28 between the adsorbate and Cu+.
(28) Yang, R. T. Adsorbents, Fundamentals and Applications; John Wiley and
Sons: Hoboken, NJ, 2003.
Although the Cu2+-3SiPPH723 sample has a higher amount of
copper (Table 3), the fact that the irreversible adsorbed amount
of propylene was higher for the Cu+-3SiPPH723 sample can
be due to the π-back-donation of d electrons of Cu+ to the
antibonding π-orbitals of the olefin being more easily achieved.
The maximum of irreversible adsorption of C3H6 (Figure 16),
observed for the Cu2+-3SiPPH723 sample, can be due to
chemisorption of some molecules on weak acidic sites, since
irreversible adsorption begins to decrease starting at 313 K and
does not lead to polymerization reaction of the olefin. Unlike
that on the support, the fact that the irreversible adsorbed amount
on the samples Cu2+-3SiPPH723 and Cu+-3SiPPH723 decreases with temperature, disappearing at 393 K, indicates that
the bond between the olefin and the adsorbent is not strong
enough to cause a chemical reaction on the surface.
The variation of the isosteric heat of adsorption (Qiso), as a
function of the adsorbed amount of C3H6 and C3H8, is shown
in Figure 17. Unlike that for the support, the heat of adsorption
profiles of C3H6 for the samples with copper show a maximum,
indicating that the gas-solid interaction energy is fairly constant
at low coverage and the contributions of the collateral interactions
(C3H6 T C3H6) on the surface are dominant while the monolayer
has not been completed. These results indicate that the copper
cations occupied the stronger acidic sites over the support surface,
converting it into an energetically more homogeneous adsorbent.
When the monolayer has been completed, that is, when the copper
cations are saturated, the gas-adsorbent interactions are predominant and the adsorption process occurs on an energetically
heterogeneous surface. As compared to the support, the samples
with copper show a similar slight increase of the isosteric heat
of adsorption for C3H8, because this molecule is not able to
adsorb specifically.
Figure 18 compares the adsorption kinetic behavior for
propylene and propane in the Cu+-3SiPPH723 and Cu2+3SiPPH723 samples at 393 K. Taking into account the considerable and insignificant adsorption of propylene and propane,
respectively, at 393 K, these samples present a high selectivity
toward olefin. Thus, if the C3H6/C3H8 mixtures were contacted
with any of the two studied samples, C3H6 would be adsorbed
Application of PPH Materials for Gas Separation
preferentially, leading to a separation of these gases. Figure 18
shows that Cu+-3SiPPH723 would have the highest efficiency,
because its ability to adsorb C3H6 was higher than that for the
Cu2+-3SiPPH723 sample.
Conclusions
In this study, a silica-expanded zirconium phosphate heterostructure has been used as a support, 3SiPPH(0.2), to prepare
two mesoporous adsorbents, Cu+-3SiPPH723 and Cu2+3SiPPH723, by ion exchange with Cu(I) and Cu(II) for separation
of propylene/propane mixtures. Adsorption equilibrium and
kinetic measurements of pure C3H6 and C3H8 in the three samples
have been performed at different temperatures in the range
between 273 and 393 K. A marked influence of Cu cations on
the adsorption behavior of the olefin was established. It was also
established that the adsorption process of propylene in the support
occurs by a mechanism similar to that of chemisorption, leading
to a polymerization reaction at 423 K. This is attributable to the
Langmuir, Vol. 22, No. 3, 2006 1267
high acidity of the adsorbent. Unlike that on the support, 3SiPPH(0.2), the irreversible adsorption of the olefin on the samples
with Cu decreases with increasing temperature and disappears
at 393 K, showing a high selectivity toward propylene. The C3H8
adsorption was always reversible. On the basis of the results of
this study, the Cu+-3SiPPH723 and Cu2+-3SiPPH723 samples
can be applied in the separation of a C3H6/C3H8 mixture at 393
K.
Acknowledgment. We gratefully acknowledge the Ministerio
de Ciencia y Tecnologı́a (Spain) (Project MAT2003-02986) and
CYTED Project V.8, “Clean Technology for the Separation of
Light Olefins”, for funding this work. We also thank the Consejo
Nacional de Ciencia y Tecnologı́a (CONACYT; México) for
financial support via a scholarship (Grant No. 144883) for
M.E.P.-I.
LA052390E