World Journal of Nano Science and Engineering, 2012, 2, 134-141 Published Online September 2012 (
Keggin Structure and Su rfa ce Acidity of
12-Phosphotungstic Acid Grafted Zr-MCM-48 Mesoporous
Molecular Sieves
Zhiqi Wang1*, Juan Navarrete2
1The American School Foundation, Bondojito No. 21, Col. Las Americas, México D.F., Mexico
2Dirección de Investigación y Posgrado, Instituto Mexicano del Petróleo,
Eje Lázaro Cárdenas Norte No. 152, México D.F., Mexico
Email: *
Received April 1, 2012; revised May 21, 2012; accepted May 31, 2012
A zirconium modified MCM-48 mesoporous material was synthesized by surfactant-templated method. Surface graft-
ing Zr-MCM-48 with tungstophosphoric acid led to a great enhancement of both the number of the Brønsted acid sites
and acidity strength in comparison with the bare support. At 100˚C, the 30 wt% H3PW12O40/Zr-MCM-48 contained
174 mol/g Brønsted acid sites which were 21.7 times greater than that of Zr-MCM-48. The Keggin structure of the
grafted heteropolyacid was rather stable after calcination at 400˚C for 2 h, approximately 93.3% of Keggin structure in
the dispersed heteropolyacid were remained without destruction but slightly distorted in some degree, as evidenced by
FTIR characterization and 31P NMR-MAS analysis. This H3PW12O40/Zr-MCM-48 solid with three dimensional
mesoporous system, large surface area and very strong Brønsted acidity will be a promising catalyst for acid catalytic
Keywords: Zr-MCM-48; Surface Acidity; Heteropolyacid; Spectroscopic Characterization
1. Introduction
In the history of porous materials discovery, the synthesis
of M41S mesoporous materials reported by Mobil Com-
pany in 1992 occupies an important position because
they bridge microporous and macroporous materials [1].
The unique properties of these mesoporous materials like
large surface area, great porosity, ordered pore system
and narrow pore size distribution make them very attract-
tive in the fields like pharmaceutical and fine chemical
industries, petrochemical and refinery, adsorption and
separation processes and heterogeneous catalysis 2-6.
One member of this novel M41S family, Si based
cubic type MCM-48 solid, has a structure consisting of
separate three-dimensional channel systems [7,8]. It is
recognized that MCM-48 is superior to MCM-41 which
has a one-dimensional pore system in the catalytic
applications because the three-dimensional interwoven
channels in MCM-48 could allow reactants and products
faster diffusion, and thus enhance the mass transfer
kinetics and avoid pore blocking 9. However, the pure
siliceous mesoporous materials have some disadvantages
like low hydrothermal stability, neutral framework and
lack of active sites in the pore wall which limit their spe-
cial applications. It is impossible, for example, to directly
use Si-based MCM-48 as catalyst in the reactions like
alkylation and isomerization of alkanes owing to lack of
Brønsted acidity; it is impossible to use it as adsorbent
for wastewater treatment due to its hydrothermal ins-
tability in an aqueous solution.
The thermal and hydrothermal stability of Si-based
MCM-48 could be improved by framework modification.
Zhang and coworkers recently report that zirconium
modified MCM-48 could remain its cubic mesoporous
structure even after calcination at 800˚C for 4 h or after
hydrothermal treatment in water at 100˚C for 24 h [10].
Incorporations of rare metals such as La [11] and Ce [12]
and transition metals like Fe [12], Mn [13], Ti [14], Sn
[15], W [16], V [17], Cu [18] and Zn [18] etc. into the
framework of the MCM-48 have been also reported.
These incorporated metal ions may serve as active sites
or structural stabilizers of the MCM-48 solid.
The surface acidity of mesoporous materials can be
effectively enhanced through multifunctionalization ap-
proach. It is reported that by grafting some strong acid
sites like sulfate ions 19 or heteropolyacids 20,21,
onto the surface of mesoporous materials MCM-41, its
*Corresponding author.
opyright © 2012 SciRes. WJNSE
surface Brønsted acidity can be generated and the total
number of acid sites is significantly increased. Salas et al.
once reported the modification of thermal stability and
surface acidity of MCM-41 by using surface sulfating
and zirconium substitution in the framework [22]. How-
ever, their results show that the surface acidity enhan-
cement seems not significant. Late on, Wang and cowo-
rkers simultaneously introduced Zr4+ ions into the frame-
work and grafted heteropoly acid on the surface of MCM-
41, both thermal stability and surface acidity were
improved [23]. Unfortunately, investigation on surface
acidity of the mesoporous materials was tensely con-
centrated on the MCM-41. Up to date, little attention was
paid on the surface acidic property of pure or modified
MCM-48 [16]. Owing to the importance of the acidity of
MCM-48 material in the acid catalyzed reactions, studies
on the enhancement of surface acidity of MCM-48 will
be of interesting.
In the present work, we describe an approach for
double modifications of Si-based MCM-48 by frame-
work substitution by Zr ions and surface grafting with
12-phosphotungstic acid. Introducing zirconium ions into
the framework of MCM-48 aims to enhancing its hydro-
thermal stability, while surface grafting with heteropoly
acid is for creating acid sites in Zr-MCM-48 for acidic
catalysis reactions. Because the hydrothermal stability of
the Zr-MCM-48 has been studied by other researchers
[10], herein we will mainly focus our attention on the
Keggin structure and surface acidity properties of
H3PW12O40/Zr-MCM-48 materials.
2. Experimental
2.1. Synthesis of Zr-MCM-48 and
A Zr-incorporated MCM-48 mesoporous molecular sie-
ves was synthesized through a surfactant-templated me-
thod using tetraethyl orthosilicate (TEOS) as Si precursor
and zirconyl chloride octahydrate (>99%) as Zr source,
along with cetyltrimethylammonium chloride (CTACl)
(25 wt% solution in water) as synthesis template. The
molar composition of the starting materials was con-
trolled as:
1TEOS:0.1ZrOCl2:0 .75CTACl:0.5NaOH:120H2O.
According to this composition, a given amount of
CATCl surfactant, NaOH and water was mixed at 50˚C
with magnetic stirring for 0.5 h. Then TEOS and zirconyl
chloride octahydrate were slowly added into the above
solution with vigorous agitation for 2 h. The product was
aged at 110˚C for 72 h in a sealed Teflon bottle under
autogenous pressure. The resultant slurry, containing a
white precipitate, was filtered and extensively washed 4
times with 500 ml deionized water, then dried at ambient
temperature for 12 h. Finally, the resultant solid was cal-
cined at 600˚C for 6 h in a flowing air (50 ml/min). Care
was taken in the calcination by controlling the tempera-
ture increasing rate at 1˚C/min in order to avoid any col-
lapsing of the mesoporous structure of the products due
to the temperature rapid increasing.
The 30 wt% H3PW12O40/Zr-MCM-48 catalyst was pre-
pared by impregnating the Zr-MCM-48 sample with a
proper amount of H3PW12O40 (noted as HPW) in 50 ml
methanol. The solvent was evaporated at 40˚C in a rotary.
The dried solids were calcined in air at 400˚C in air for 2
2.2. Characterization
The X-ray diffraction patterns of the calcined sample
were measured in a D-500 SIEMENS diffractometer with
a monochromatic CuK
radiation. The scanning was
made for a 2
angle from 1.5˚ to 10˚, with a step size of
0.02˚ and a step time of 2 s. Position correction was
made using the NIST standard reference material 675.
The surface area, pore volume and pore size distri-
bution of the samples were measured with a Digisorb
2600 using N2 adsorption-desorption isotherms method.
Before the measurement, the sample was treated at 300˚C
under vacuum to remove the surface impurities. The sur-
face area was computed by using the multi-point Brun-
auer-Emmett-Teller (BET) method based on the adsorp-
tion data in the partial pressure P/Po range from 0.01 to
0.2. The value of 0.1620 nm2 was taken for the
cross-section area of the adsorbed N2 molecule. The pore
diameters were determined by using the (BJH) method.
Solid state 31P MAS-NMR spectra were recorded with
a Bruker 400 MHz spectrometer at room temperature at a
frequency of 79.49 MHz, spinning of 7.5 kHz, emtting
pulses at 90 s intervals and using 4 mm zirconia rotors.
85% H3PO4 solution (0 ppm) was used as reference to
obtain the chemical shift. The spectral deconvolution was
performed with the Unity spectrometer software.
High Resolution TEM was carried out at room tem-
perature in a JEOL 4000 EX electron microscope equip-
ped with a pole piece having a spherical aberration co-
efficient of Cs = 1.00 mm. The powder sample was softly
grounded in an agate mortar and dispersed in isopropyl
alcohol within an ultrasonic bath for several minutes. A
few drops were then deposited on 200 mesh copper grids
covered with a carbon film. Mesostructures of the solid
were imaged directly by a conventional method.
The surface acidity of the samples was analyzed by
FTIR method of pyridine adsorption on a 170-SX FTIR
spectrometer in a temperature range between 25 and
400˚C. Before pyridine adsorption, the sample was heat-
ed to 400˚C under vacuum, then cooled down to room
temperature. Afterwards, the solid wafer was exposed to
Copyright © 2012 SciRes. WJNSE
pyridine by breaking, inside the spectrometer cell, a
capillary containing 50 l of liquid pyridine. The FTIR
spectra were recorded under various conditions by in-
creasing the IR cell temperature from 25˚C to 400˚C. The
quantitative calculation of Lewis and Brønsted acid sites
was made with respect to the area of the adsorption
bands at 1450 cm1 and 1540 cm1, respectively [24,25].
The areas of the IR absorption bands at 1450 and 1540
cm1 at different temperatures were integrated by using
the extinction coefficients EB = 1.0086 mol /cm2 for
Brønsted acid sites and EL = 0.9374 mol/cm2 for Lewis
acid sites. The acid strength was determined according to
the variation of the number of acid sites as a function of
the temperature.
3. Results and Discussion
3.1. XRD Analysis
The low angle XRD patterns of the calcined sample are
shown in Figure 1. Several peaks at approximately 2.2˚,
2.5˚, 3.7˚, 4.3˚ and 4.9˚ diffraction angles are observed,
these correspond to the reflections of the (211), (220),
(400), (321) and (420) planes of a typical MCM-48
mesostructure with Ia3d cubic symmetry (hereafter, de-
noted as Zr-MCM-48) 1.
3.2. Textural Properties
The textural characteristics of Zr-MCM-48 and H3PW12O40/
Zr-MCM-48 samples were determined from nitrogen
adsorption-desorption isotherms. The surface area, pore
volume and average pore diameter are reported in Table
1. Zr-MCM-48 has a surface area 797.8 m2/g, pore vol-
ume 0.75 cm3/g and an average pore diameter 3.8 nm.
After grafting with 30 wt% H3PW12O40, the surface area
diminishes to 674.1 m2/g, pore volume and average pore
diameter to 0.66 cm3/g and 3.2 nm, respectively. This is
due to the surface coverage and partial pore blocking by
Figure 1. Low angle XRD patterns of Zr-MCM-48 solid.
Table 1. Textural data of Zr-MCM-48 and HPW/ZrMCM-
48 solids.
Zr-MCM-48 HPW/Zr-MCM-48
Surface area (m2/g) 797.8 674.1
Pore diameter (nm) 3.8 3.2
Pore volume (cm3/g) 0.75 0.66
the deposited heteropolyacid which was confirmed by
high resolution TEM observation shown in the following
3.3. High-Resolution TEM Observation
Figure 2 shows the morphologies of the Zr-MCM-48
and H3PW12O40/Zr-MCM-48 solids observed by high
resolution TEM. The Zr-MCM-48, Figure 2(A), shows a
very ordered pore system with pore diameter between 2.5
and 4 nm. In some areas, surface structural defects are
observed. The structural imprefects may be formed in the
calcination procedure due to collapse of the walls be-
tween adjact pores resulted from the removal or combus-
tion of the synthetic template, herein the surfactant.
Figure 2(B) shows high resolution TEM image of the
H3PW12O40/Zr-MCM-48 sample. Most of the regions are
covered with nanoparticles with diameter 2 - 3 nm, indi-
cating a high dispersion of the supported heteropolyacid
clusters. The diameter of a single H3PW12O40 molecule is
estimated to be 1 - 1.2 nm, which is smaller than the di-
mension of the pores (2.5 - 4 nm). Therefore, the heter-
opolyacid species can locate inside the internal pore of
the support. However, it is also possible that the
H3PW12O40 molecules forms clusters by assembling sev-
eral molecules, leading to most of particles having diam-
eter 2 - 3 nm, and some of these larger than the dimen-
sion of the pores. Therefore, heteropolyacid may locate
inside the internal pores, or disperse on the external sur-
face of the support. These TEM image reveals that het-
eropolyacids dispersion is very homogeneous and large
heteropolyacid aggregates with diameter larger than 6 nm
are not formed in the H3PW12O40/Zr-MCM-48 sample.
3.4. Keggin Structure Stability of the
In the H3PW12O40/Zr-MCM-48 solid, the dispersion and
structural stability of Keggin unit in the H3PW12O40 are
very important because they determine the surface acid-
ity and thermal stability of the solid. In this work, two
spectroscopic techniques, FTIR and 31P MAS-NMR,
were applied to characterize the dispersion and structural
stability of Keggin structurre in the grafted H3PW12O40.
Figure 3 shows the FTIR spectrum of the H3PW12O40/
Zr-MCM-48. Four bands around 1080, 982, 896 and 819
Copyright © 2012 SciRes. WJNSE
Figure 2. High Resolution TEM micrographs (A): Zr-
MCM-48; (B): H3PW12O40/Zr-MCM-48 solid.
cm1 are observed; these bands correspond to the IR ab-
sorption vibrations of the heteropolyanions and they are
very similar to the fingerprint absorption bands observed
in the pure H3PW 12O40 [26,27]. In the Keggin unit, the
P-O symmetric stretching is characterized by the vibra-
tional transition at 1080 cm1. The normal W = Od
stretching mode is indicated by IR band at 982 cm1. The
bands at 896 and 819 cm1 are associated with the
stretching motion of W-O-W bridges. The band at 896
cm1 is assigned to the W-Ob-W stretching mode (inter-
bridges between corner sharing octahedral) and the band
at 819 cm1 is attributed to W-Oc-W stretching mode
(intra-bridges between edge sharing octahedral) [26,27].
Appearance of these fingerprints of IR absorption bands
indicates that the primary structure of Keggin unit in the
H3PW12O40 is completely retained in the H3PW12O40/
Zr-MCM-48 solid after thermal treatment at 400˚C for 2
h. Figure 4 shows the spectrum of 31P MAS-NMR spec-
troscopy of H3PW12O40/Zr-MCM-48 solid. One strong
signal is present at approximately –13.48 ppm. In addi-
tion, two very small peaks are observed at 14.94 ppm
and 12.11 ppm, respectively. The 31P MAS-NMR spec-
troscopic features of H3PW12O40/Zr-MCM-48 clearly
differ from both the H3PW12O40/TiO2 28 which shows
four peaks at around 4, 8, 11 and 13 ppm, and that
shown in a H3PW12O40/Zr-MCM-41 sample where three
NMR peaks at approximately 12.2, 13.5 and 14.9
ppm were observed 29. The resonances observed in the
TiO2 and Zr-MCM-41 supported solids are assigned to
different species: the peak at 15 - 14 ppm to dispersed
H3PW12O40 particles with weak interaction with the sup-
port; the one around 14 - 12 ppm to the distorted Keg-
gin structure due to moderately strong interacting with
surface hydroxyl groups of the support; the one at 11 -
8 ppm to the defective Keggin species due to the partial
fragmentation of the Keggin structure caused by the
strong interaction with the support and the one around 4
ppm to phosphorus species derived from a highly frag-
mented Keggin unit, respectively 28,29.
With respect to the pure H3PW12O40 where only a sharp
Figure 3. FTIR spectrum of the HPW/Zr-MCM-48.
Figure 4. 31P MAS-NMR spectrum of HPW/Zr-MCM-48.
Copyright © 2012 SciRes. WJNSE
peak appears at around 14.93 ppm, Figure 5, a peak
position shift (∆δ = 1.45 ppm) is observed in the
H3PW12O40/Zr-MCM-48 sample. The main peak position
shift indicates that the Keggin structure is deformed in
certain degree. Because no peaks around 4, 8, 11
ppm were observed in the H3PW12O40/Zr-MCM-48 sam-
ple, the formation of highly fragmented or a partially
fragmented or defective Keggin unit can be completely
excluded. The major peak at 13.48 ppm indicates that
the more than 93.3% of Keggin structure in the dispersed
heteropolyacid compound of H3PW12O4/Zr-MCM-48
were remained without destruction but slightly distorted.
The weak signal at 14.94 ppm (3.6%) can be assigned
to large H3PW12O40 particles similar to bulk 12-
phosphotungstic acid crystals having very weak inter-
action with MCM-48 support; while, the one at 12.11
ppm (3.1%) is due to the formation of the species
(2)2(), where M is Zr or Si ions, by
Keggin units interacting with surface hydroxyl groups,
which is slightly different from the species (M-OH+)
) as indicated by the NMR resonance at
13.48 ppm 29.
12 40
The results of FTIR and 31P MAS-NMR characteriza-
tions confirm that the Keggin structure of H3PW 12O40 is
rather stable; it is only slightly deformed after impregna-
tion and calcination at 400˚C. This conclusion is impor-
tant because the Keggin structure at 400˚C has a great
influence on the surface property, i.e. surface acidity of
the H3PW12O40/Zr-MCM-48.
3.5. Surface Acidity
Although the goal to introduce zirconium ion into the
framework of MCM-48 is to improve the thermal and
hydrothermal stability, it certainly influences the surface
acidity. In order to study the surface acidity property,
both Zr-MCM-48 and H3PW12O40/Zr-MCM-48 samples
were comparatively measured by in situ FT-IR spectro-
scopy of pyridine adsorption. Figure 6 shows a set of in
Figure 5. 31P MAS-NMR spectrum of pure H3PW12O40.
situ FTIR spectra of pyridine adsorption on the Zr-
MCM-48 support. The absorption band at 1445 cm1 in
the IR spectra corresponds to the pyridine associating
with Lewis acid sites (noted as L) of the solid 30,31.
Another band at 1540 cm1 was observed, indicating the
formation of Brönsted acid sites (noted as B) in the sam-
ple 31,32. In addition, an IR band corresponding to
pyridine vibrations associated with both, Lewis and
Brønsted acid sites (noted as B + L acid sites), was ob-
served at 1490 cm1. The band at 1640 cm1 was as-
signed to pyridine adsorbed on the hydroxyls ions. As the
thermal treatment temperature in the IR cell increased
from 50˚C to 100˚C under vacuum condition, the intensi-
ties of all the bands significantly lowered. After calcina-
tion at 200˚C, these bands were further reduced and fi-
nally they completely disappeared at 250˚C.
Due to the neutral framework of pure Si-based MCM-
48, it does not contain any Brønsted acid sites; therefore,
the formaton of the Brønsted acid sites in the Zr substi-
tuted MCM-48 should relate to the replacement of Si4+
by Zr4+ ions in the framework of the MCM-48. It is noted
that Zr4+ ion (r = 0.084 nm) has diameter three times
greater than that of Si4+ ion (r = 0.026 nm), thus the bond
length of Zr-O (0.21 nm) is longer than that of Si-O bond
(0.16 nm). When the smaller Si4+ ions are substituted by
the larger Zr4+ ions in the lattice cell of the solid frame-
work, the formed Zr-O-Si bond length is clearly different
from the one of Si-O-Si, and this must generate structural
microstrain within the lattice cell. Any change in the
electron density around Si nuclei, due to either charge
unbalance, or difference in electronegativity or local
structure deformation resulting from the introduction of
the Zr4+ ion into the vicinity of the hydroxyls carrying
silicon, may weaken the SiOZr-O-H bond 33, giving rise
to the formation of Brønsted acid sites in Zr-MCM-48.
Figure 6. A set of FTIR spectra of pyridine adsorption-de-
sorption of Zr-MCM-48 sample at different temperatures.
Copyright © 2012 SciRes. WJNSE
Figure 7. A set of FTIR spectra of pyridine adsorption-de-
sorption of H3PW12O40/Zr-MCM-48 at differrent tempera-
As shown in Figure 6, the Brønsted acidity of the
Zr-MCM-48 is not sufficient and the acidity strength is
still week, enhancement of both the number and strength
of the Brønsted acid sites becomes necessary if Zr-
MCM-48 is considered as catalyst for acid catalyzed re-
actions like n-alkanes hydroisomerization.
The FTIR spectra of pyridine adsorption on the
H3PW12O40/Zr-MCM-48 catalyst are shown in Figure 7.
The quantitative acidity data derived from the FTIR
spectra of Figures 6 and 7 are reported in Tables 2 and 3,
respectively. Both Brønsted (B) and Lewis (L) acid sites
coexisted on the solid as indicated by the adsorption
bands at 1450 cm1 (L), 1610 cm1 (L), 1540 cm1 (B)
and 1637 cm1 (B). The bands at 1595 and 1578 cm1 are
assigned to the hydrogen-bonded pyridine and they were
very weak and rapidly disappeared after thermal treat-
ment at 200˚C 30. In comparison with the bare Zr-
MCM-48 support, the number of the Brønsted acid sites
of the heteropolyacid grafted H3PW12O40/Zr-MCM-48
catalyst was greatly enhanced by 21.7 times at 100˚C
(Tables 2 and 3). Also, the acidity strength was signifi-
cantly improved as evidenced by the acid nature even
after 400˚C of treatment, 36 mol/g B sites and 28
mol/g L sites still remain in the H3PW12O40/Zr-MCM-
The Brønsted acidity strength is closely correlate with
the Keggin sturcture and the dispersion of the loaded
heteropoly anions which are determined by the interact-
tion between heteropoly acid and the support. The strong
acidity of the H3PW 12O40/Zr-MCM-48 may relate to the
three dimensional pore system of the Zr-MCM-48 sup-
port. These hydroxyls may have a moderately strong in-
teraction with the heteropolyacid, producing a big num-
ber of Brønsted acid sites. This H3PW12O40/Zr-MCM-48
solid with three dimensional mesoporous system, large
surface area and very strong Brønsted acidity will be a
Table 2. Acid sites on Zr-MCM-48 solid at different tem-
Temperature (˚C) B (mol/g)L (mol/g) Total (mol/g)
25 14 968 982
100 8 94 106
200 0 7 7
250 0 0 0
Table 3. Acid sites on HPW/Zr-MCM-48 solid at different
Temperature (˚C) B (mol/g)L (mol/g) Total (mol/g)
100 174 164 338
200 142 56 198
250 118 52 170
300 91 39 130
350 59 36 95
400 36 28 6
promising catalyst for acid catalytic reactions. The cata-
lytic evaluation will be carried out in the near future.
4. Conclusion
Incorporation of zirconium ions into the framework of
the mesoporous silica generates Brønsted acidity which
would be remarkably enhanced by more than 21.7 times
after surface grafting with 30 wt% 12-tungstophosphoric
acid. The highly dispersed heteropolyacid was uniformly
distributed within the pores or pore openings of
Zr-MM-48 solid, and after thermal treatment at 400˚C for
2 h, the Keggin structure in the heteropoly anions of the
H3PW12O40/Zr-MCM-48 was only deformed in some
degree without destruction as evidenced by FTIR, TEM
and 31P MAS-NMR characterizations.
5. Acknowledgements
The authors thank Dr. P. Pérez Romo and Dr. A. Paz for
their help in the NMR and TEM characterization.
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