Keggin Structure and Surface Acidity of 12-Phosphotungstic Acid Grafted Zr-MCM-48 Mesoporous Molecular Sieves

doi:10.4236/wjnse.2012.23017

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World Journal of Nano Science and Engineering, 2012, 2, 134-141

http://dx.doi.org/10.4236/wjnse.2012.23017 Published Online September 2012 (http://www.SciRP.org/journal/wjnse)

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: *[email protected]

Received April 1, 2012; revised May 21, 2012; accepted May 31, 2012

ABSTRACT

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

reactions.

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.

Z. Q. WANG, J. NAVARRETE 135

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

H3PW12O40/Zr-MCM-48

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

Z. Q. WANG, J. NAVARRETE

136

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 cm−1 and 1540 cm−1, respectively [24,25].

The areas of the IR absorption bands at 1450 and 1540

cm−1 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

section.

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

H3PW12O40/Zr-MCM-48

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

Z. Q. WANG, J. NAVARRETE 137

Figure 2. High Resolution TEM micrographs (A): Zr-

MCM-48; (B): H3PW12O40/Zr-MCM-48 solid.

cm−1 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 cm−1. The normal W = Od

stretching mode is indicated by IR band at 982 cm−1. The

bands at 896 and 819 cm−1 are associated with the

stretching motion of W-O-W bridges. The band at 896

cm−1 is assigned to the W-Ob-W stretching mode (inter-

bridges between corner sharing octahedral) and the band

at 819 cm−1 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.

Z. Q. WANG, J. NAVARRETE

138

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+)

(21240

) as indicated by the NMR resonance at

−13.48 ppm 29.

M-OH

HPW

12 40

HPWO



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

-20-18-16-14-12-10

(ppm)

-14.93ppm

HPW

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 cm−1 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 cm−1 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 cm−1. The band at 1640 cm−1 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.

Z. Q. WANG, J. NAVARRETE 139

Figure 7. A set of FTIR spectra of pyridine adsorption-de-

sorption of H3PW12O40/Zr-MCM-48 at differrent tempera-

tures.

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 cm−1 (L), 1610 cm−1 (L), 1540 cm−1 (B)

and 1637 cm−1 (B). The bands at 1595 and 1578 cm−1 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-

48.

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-

peratures.

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

temperatures.

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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