Applied Catalysis A: General 352 (2009) 259–264
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Microwave assisted in situ synthesis of USY-encapsulated heteropoly acid
(HPW-USY) catalysts
Dingfeng Jin a, Jing Gao a, Zhaoyin Hou a,*, Yan Guo a, Xiuyang Lu b, Yinghong Zhu c, Xiaoming Zheng a
a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (Xixi Campus), Hangzhou, Zhejiang 310028, China
Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China
c
College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou, Zhejiang 310012, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 18 June 2008
Received in revised form 9 October 2008
Accepted 9 October 2008
Available online 22 October 2008
Under microwave irradiation, 12-tungstophosphoric acid (HPW) could be successfully synthesized in
situ and encapsulated in the supercage of ultra stable Y (USY) in several minutes. But the framework of
USY collapsed easily in traditional hydrothermal synthesis routine. 31P MAS NMR, transmission electron
microscopy (HR-TEM), N2 adsorption, inductively coupled plasma (ICP) and X-ray diffraction (XRD)
characterizations found that the formed HPW molecule located separately in the supercage of USY.
Temperature-programmed desorption of NH3 (NH3-TPD) showed that HPW-USY exhibited stronger
acidity than that of pure USY, and adsorbed pyridine infrared (Py-IR) disclosed that the concentration of
Brönsted acid sites was enhanced. This hybrid solid acid exhibited higher activity in the synthesis of 4,40 dimethyldiphenylmethane via toluene and formaldehyde and could be utilized as a solid acid catalyst in
aqueous solutions.
ß 2008 Elsevier B.V. All rights reserved.
Keywords:
12-Tungstophosphoric acid
USY
Encapsulation
Microwave irradiation
1. Introduction
Substitution of strong liquid acids (such as HCl and sulfuric
acid) by solid acids has attracted much attention in past years. In
published papers, heteropoly acids with Keggin structure (abbreviated as HPA) were popularly reported for their strong acidity,
ease in separation and possible utilization in recycling [1–4].
However, the high solubility of HPA in polarity solvent brings
about a serious loss in reactant, which hindered the application of
HPA. Many efforts have been put into avoiding the loss of HPA, such
as immobilization of HPA on a support material and partial
substitution of proton by Cs+ ions [5].
As the Keggin heteropoly anions is larger (ca. 11–12 Å) than the
pore diameter of zeolite Y, HPA anions cannot enter its pore
channels and a severe leaching of heteropoly anions into water was
observed of the supported PW catalysts, which led to their poor
reusability [6–7].
Sulikowski et al. proposed that this problem could be solved by
encapsulating HPA in the supercage of zeolitic matrix [8]. It is well
known that ultra stable Y (abbreviated as USY) zeolite consists
of almost spherical 13 Å cavities interconnected tetrahedrally
through smaller apertures of 7.4 Å diameter [9]. And a Keggin
* Corresponding author. Tel.: +86 571 88273283; fax: +86 571 88273283.
E-mail address: zyhou@zju.edu.cn (Z. Hou).
0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2008.10.020
structured HPA encapsulated in a supercage of USY cannot escape
from its smaller apertures.
Several attempts were reported in order to encage HPW and
12-molydophosphoric acid (HPMo) in supercage of USY by a ‘‘ship
in the bottle’’ method via a hydrothermal routine [10–14].
However, major drawbacks of the reported hydrothermal routine
are long time (3 h), high temperature (90–95 8C) and strong
acidity of the aqueous medium. The zeolitic matrix collapses
easily under these conditions and amorphous SiO2 derived from
collapsed zeolite would enwrap the formed HPAs, which hindered
the contact of acid sites with reactants. And the final product has a
lower activity than that of pure USY [10]. It is of great practical
interest to overcome the collapse of the zeolitic matrix during the
preparation.
Microwave irradiation is an alternative thermal energy source
to conventional heating thanks to its rapid rise in temperature and
higher energy efficiency [15–20]. In a reaction irradiated with
microwaves, reaction time could be reduced greatly and the
procedure would be helpful to avoid the collapse of zeolitic matrix
during the in situ synthesis of HPW.
In this contribution, we want to report an efficient synthesis
routine of HPW encapsulated in a supercage of USY (abbreviated as
HPW-USY) under microwave irradiation. The synthesized HPWUSY was characterized by 31P MAS NMR, X-ray diffraction (XRD),
Fourier transform infrared (FT-IR), nitrogen adsorption-desorption
isotherms and inductively coupled plasma studies (ICP). And the
260
D. Jin et al. / Applied Catalysis A: General 352 (2009) 259–264
location of the formed HPW was detected by transmission electron
microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS),
adsorbed pyridine infrared (Py-IR) spectroscopy and temperatureprogrammed desorption of NH3 (NH3-TPD). This hybrid solid acid
was utilized in the synthesis of 4,40 -dimethyldiphenylmethane
(abbreviated as 4,40 - DMDPM) via toluene and formaldehyde, it
exhibited higher activity than pure USY.
2. Experimental
Sodium hydrogen phosphate (Na2HPO4), sodium tungstate
(NaWO4H2O) and hydrochloric acid (HCl) were purchased from
Shanghai Chemical Reagent Co. (China). USY zeolite was kindly
supplied by Huahua Catalysis Co. (Wenzhou, China). All chemicals
were used as received without further purification.
Encapsulation of HPW in zeolitic matrix was carried out via in
situ synthesis under microwave irradiation. USY was first calcined
at 550 8C for 3 h, and then put into an aqueous solution of well
mixed Na2HPO4 and NaWO4H2O with a controlled P/W atom ratio
(1/6). A stoichiometric amount of HCl was added dropwise into the
solution until pH 1.0. The flask containing the reaction mixture was
placed into a microwave oven with a working frequency of
2.45 GHz (Nanjing Robot Co., LWMC-201, 650 W). Microwave
irradiation was performed from 3 to 9 min. The formed HPW-USY
(x) (where, x denoted the microwave irradiation time in min) was
separated by filtration and washed 20 times with hot distilled
water (80 8C) in order to remove the HPW anions formed on the
external surface of USY completely. Finally, HPW-USY (x) were
dried at 100 8C overnight and calcined at 250 8C for 3 h.
At the same time, a reference sample was also prepared by a
hydrothermal method according to reference [12]. USY zeolite was
added to a solution of Na2HPO4 in deionized water and the mixture
was stirred for 2 h at room temperature. Then a solution containing
a known amount of NaWO4H2O was added dropwise. After 1.5 h of
stirring, a stoichiometric amount of HCl was added dropwise at
90 8C. This suspension was stirred for 3 h at 90 8C. The synthesized
sample (PW-USY (hydrothermal)) was separated and washed
intensively with hot distilled water, dried at 100 8C overnight and
finally calcined at 250 8C for 3 h.
Solid-state 31P MAS-NMR spectra were obtained at room
temperature with a 300 MHz spectrometer (Avance 300, BRUKER).
For 31P spectra, 908 pulse (2 ms) and 60 s delay were used to
acquire about 200–300 transients of 2k complex data.
XPS analyses of the fresh PW-USY (7) and in situ Ar ionic
sputtered PW-USY (7) were conducted on a PHI 5600 spectrometer
(Physical Electronics) under 1010 Torr, Al-Ka source (1486.6 eV).
The analyzed area was 8000 Å in diameter and the binding energies
were calibrated by contaminated carbon (C1s at 285.0 eV). Ar ionic
sputtering was performed with a beam energy of 4 keV and a
sputtering rate of 25 Å min1 for 4 minutes.
In situ pyridine adsorption was carried out by FT-IR spectroscopy (Nikolet 560, USA). The sample was first degassed
(103 mbar, 200 8C for 4 h) in an IR cell and then the spectra of
adsorbed pyridine were recorded.
The textural structure of HPW-USY was measured by nitrogen
adsorption at 196 8C using an OMNISORP 100CX system (COULTER
Co., USA). All samples were pretreated at 250 8C in vacuum, and the
pore structure was calculated from adsorption isotherms. The
micropore diameter in a range of 5.5–18 Å was calculated by the
Horvath–Kavazoe (HK) method and a mesopore size in a range
of 30–1000 Å was calculated by Barrett–Joyner–Halenda (BJH)
method.
The element contents were measured using ICP in IRIS Intrepid
II XSP (Thermo Fisher Scientific, USA) after leaching the metal
ions with HF and diluting with distilled water (for aluminum,
phosphorus and tungsten) or very dilute aqueous NaOH solution (for
silicon) to specific volumes.
HR-TEM images were obtained using an accelerating voltage of
200 kV in JEM 2010HR (JEOL Ltd., Japan). Samples were first ground
to powder, suspended in tetrachloromethane under supersonic
shaking and finally dispersed on a copper grid. Energy dispersive Xray analysis (EDX) was performed with an INCA Energy 300
(Oxford, UK).
XRD patterns were obtained on an D8 ADVANCE (BRUKER,
Germany) instrument using nickel-filtered Cu Ka radiation at
40 kV and 40 mA. Diffraction data were recorded with a rate of
0.018 s1.
The acidity of synthesized HPW-USY catalysts was performed via
NH3-TPD. Each sample was first treated at 300 8C in Ar flow of
30 ml min1 for 1 h, and then cooled to room temperature, exposed
to 20% NH3/Ar for 30 min, and purged by Ar at 100 8C for 5 h in order
to eliminate the physical adsorbed ammonia. Temperature programmed desorption was conducted by ramping to 750 8C at
10 8C min1 and NH3 (m/e = 16) in effluent was detected and
recorded as a function of temperature by a quadrupole mass
spectrometer (OmniStarTM, GSD301, Switzerland).
Condensation between toluene and formaldehyde over these
HPW-USY catalysts was carried out in a custom-designed 150 cm3
stainless autoclave (Lanzhou, China). A mixture of toluene
(450 mmol), paraformaldehyde (30 mmol of HCHO) and nitrobenzene (20 mmol, internal standard) was poured into the
catalyst. The suspension was vigorously stirred with a magnetic
stirrer at 140 8C (4 h). After the reaction, the reactor was cooled to
room temperature, solid catalyst was separated by centrifuging,
and organic reactant solution was diluted with ethanol and
analyzed using an FID gas chromatography–mass spectrometer
equipped with a 25 m capillary column of a cross-linked 5%
phenylmethylsilicone (HP5988A, USA).
3. Results and discussion
The detected 31P MAS NMR spectrum of PW-USY (7) is a broader
(ca. 20.0 ppm) peak with a chemical shift at 15.0 ppm from H3PO4
(Fig. 1); this profile was wider than that of bulk HPW (a sharp
signal at 15.3 ppm with a width of ca. 1 ppm) [8,21]. This broader
spectrum could be due to the fact that the encapsulated Keggin
unit located separately in the supercage of USY and its electronic
environment would be disturbed by the surrounding Si, Al and O of
the zeolitic matrix.
Fig. 1. Solid-state
31
P MAS NMR spectra of HPW-USY (7).
D. Jin et al. / Applied Catalysis A: General 352 (2009) 259–264
XPS spectra of HPW-USY (7) are shown in Fig. 2. On the surface
of fresh HPW-USY (7), the detected binding energies of Al2p, Si2p
and O1s locate at 74.9, 103.9 and 533.5 eV. Neither phosphorous
nor tungsten was detected (curve a in Fig. 2), which indicated that
the HPW that formed on external surface of USY was removed
completely during preparation. After Ar ionic sputtering, contaminated carbon was removed completely and the detected
binding energy of Al2p, Si2p, O1s remained constant (curve b in
Fig. 2). But phosphorous with a binding energy at 134.4 eV (of P2p
in Fig. 3) and tungsten with binding energies at 35.4 and 38.4 eV
(of W4f 72 in Fig. 4) were detected. These binding energy levels
fit well with those of P and W in a Keggin anion [22]. These
observations confirmed that phosphorous and tungsten in the
matrix of USY combined into Keggin anions. And these formed
HPW might be located in the supercage of USY (about 13 Å in
diameter) because the size of a spherical HPW molecule is about
11–12 Å (diameter). HPW molecules encapsulated in the supercages of USY cannot escape from its smaller apertures of 7.4 Å
diameters.
The spectra of Py-IR of pure USY zeolite and HPW-USY (7)
catalysts are illustrated in Fig. 5. A typical absorption band
assigned to the vibration modes of adsorbed pyridine, forming
Lewis-type adducts (Py-L, Py-M) with bands at 1610 cm1 and
1450 cm1, and to the mode of pyridine in interaction with
Brönsted acid sites (Py-B) as a band at 1540 cm1 were detected.
And a band at 1490 cm1 is assigned to pyridine adsorbed on both
Lewis and Brönsted acid sites [23,24]. It can be found that HPWUSY (7) has an enhanced concentration of Brönsted acid sites over
that of pure USY. One can conclude that such enhanced Brönsted
acid sites contributed to the protonic acidity of synthesized HPW.
As XPS analysis has confirmed that the HPW that formed on
external surface of USY was removed completely during preparation, the enhanced Brönsted acid sites are from these of HPW in
supercage of USY.
Nitrogen adsorption isotherms disclosed that the amount of
adsorbed nitrogen of USY before P/Po = 0.3 was higher than that of
HPW-USY (7), which indicated that HPW molecule encapsulated in
supercage of USY would fill part of the micropores. Calculated
structure data of USY and synthesized HPW-USY are summarized in
Table 1. It can be found that the surface area of HPW-USY decreased
from 693.6 (of pure USY) to 331.4 m2 g1 (of HPW-USY(9)) and the
pore volume also decreased from 0.25 to 0.11 ml g1 with the
Fig. 2. High-resolution XPS spectra of the entire region in HPW-USY (7): (a) surface
of HPW-USY and (b) down to 100 Å below the surface of HPW-USY.
261
Fig. 3. High-resolution XPS spectra of the P2p region in HPW-USY (7).
increasing microwave irradiation time because part each of supercage was occupied by a synthesized HPW molecule.
The percentages of element content in pure USY and HPW-USY
(7) determined by ICP are presented in Table 2. It shows that the
atomic ratio between Si and Al in USY is lower than that of HPWUSY (7), which means that some aluminium could be dissolved.
Phosphorus and tungsten were detected in HPW-USY (7) and the
atom ratio between P and W was in accord with HPW due to the
presence of HPW in the super cages of the USY zeolite. The
calculated amount of formed HPW encapsulated in USY is about
5.6 wt% of the parent USY.
HR-TEM of PW-USY (7) reveals a random distribution of dark
regions (marked by smaller circles) in a zeolitic matrix (Fig. 6). The
circled diameter of this region is 20 Å, in which HPW molecules
(about 10 Å) could be identified clearly. EDX analyse around this
area (marked by bigger circles) indicated that both P and W with
atom ratio around 1/12 were detected. This result is the most
promising image of the encapsulated HPW in USY. It is the first
time that the location of HPW has clearly been identified.
As a contrast, a sample in which the zeolitic matrix partly
collapsed (confirmed by XRD analysis) and in which HPW crystals
Fig. 4. High-resolution XPS spectra of the W4f
72
region in HPW-USY (7).
D. Jin et al. / Applied Catalysis A: General 352 (2009) 259–264
262
Fig. 5. The infrared spectra of pyridine adsorbed infrared over USY and HPW-USY (7)
after degassing at 200 8C: (a) USY and (b) HPW-USY (7).
assembled into big clusters (PW-USY (9)) is shown in Fig. 7. It can
be found that one part of bigger-sized HPW crystals are assembled
in the external surface of USY and another part of the HPW crystals
are enwrapped by the collapsed zeolitic matrix.
Fig. 8 shows the XRD patterns of pure HPW, USY, HPW-USY (7)
and HPW-USY (hydrothermal). Traces a and b are of HPW and
faujasite zeolite, respectively. A perfect zeolite crystalline of USY
was retained in HPW-USY (7). However, the structure of the
faujasite collapsed in HPW-USY (hydrothermal), which was caused
by dissolution of Al in the aqueous HCl solution during the
hydrothermal process [25].
The XRD patterns of HPW-USY prepared at different microwave
irradiation time also confirmed that no HPW crystal on the external
surface of zeolite was detected as the formed HPW molecule
dispersed highly and located separately in the supercage of the
zeolitic matrix. However, longer irradiation time has a striking effect
on the structure of the zeolitic matrix and amorphous silicon dioxide
can be detected in HPW-USY (9). Amorphous silicon dioxide caused
the formed HPW clusters to be enwrapped (also confirmed in HRTEM, Fig. 7) and so they cannot contact with reactant. We suppose
that this is the major objection of traditional hydrothermal synthesis
routine, which caused the final product to have a lower activity than
that of pure USY [10].
The acidity of USY, HPW-USY (7) and HPW-USY (hydrothermal)
detected by NH3-TPD is shown in Fig. 9. A desorption peak at about
200 8C could be assigned to weakly adsorbed ammonia on zeolite
Fig. 6. The HR-TEM image of HPW-USY (7).
matrix and the peak at about 450 8C could be ascribed to ammonia
desorbed from those strong acid sites of zeolitic matrix [26–29].
When HPW was encapsulated in supercages of USY zeolite, the
desorption temperature of NH3 on the strong acid site rose to
520 8C and the total amount of desorbed NH3 increased.
The desorbed amount of NH3 from these acid sites was
calculated and is summarized in Table 3. It can be found that
the acid sites from zeolite matrix decreased slightly with the
increasing irradiation time from 3 to 7 min. When the irradiation
time prolonged to 9 min, this part of acid sites decreased quickly
from 1.05 mmol g1 (of pure USY) to 0.19 mmol g1. The stronger
acid sites of zeolitic matrix reduced from 1.59 to 1.28 mmol g1,
too. However, total acidity remained almost constant because of
the formation of HPW. It is found that HPW formed after 3 min
irradiation with a microwave and that the amount of HPW
increased with irradiation time. Because HPW that formed on
external surface of zeolitic matrix was removed completely during
Table 1
The structure of all samples irradiated with microwaves for different times.
Catalysts
USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
(0 min)
(3 min)
(5 min)
(7 min)
(9 min)
SBET (m2 g1)
Pore volume (ml g1)
693.6
680.2
542.8
383.2
324.5
331.4
0.25
0.28
0.22
0.13
0.11
0.10
Table 2
Chemical compositions of USY and HPW-USY (7 min).
USY
HPW-USY (7 min)
Al%
Si%
P%
W%
Si/Al
P/W
3.34
2.68
30.09
30.10
–
0.05
–
2.99
8.69
10.83
–
11.91
Fig. 7. The HR-TEM image of HPW-USY (9).
D. Jin et al. / Applied Catalysis A: General 352 (2009) 259–264
263
Table 3
The acid strength distribution of the catalysts calculated by the results of NH3-TPD.
Total acidity (mmol g1)
Catalysts
Acidity (mmol g1)a
Weak (T1)
USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
a
b
c
(0 min)
(3 min)
(5 min)
(7 min)
(9 min)
2.64
2.30
2.39
2.35
2.40
2.35
1.05
0.92
0.88
0.67
0.51
0.19
Strong (T2)
T2–1b
T2–2c
1.59
1.38
1.33
1.31
1.27
1.28
0
0
0.18
0.37
0.62
0.88
Desorption temperature: T1 = 150–300 8C, T2 = 300–700 8C.
Acidity supplied by USY zeolitic matrix.
Acidity supplied by HPW heteropoly acid.
Table 4
The conversion of paraformaldehyde and selectivity of DMDPMs product catalyzed
by USY zeolite and HPW-USY prepared with different methodsa.
Fig. 8. XRD patterns of pure HPW, USY, HPW-USY (7) and HPW-USY
(hydrothermal): (a) Pure HPW, (b) USY, (c) HPW-USY (7) and (d) HPW-USY
(hydrothermal).
preparation, this enhance acidity comes from those HPW molecules in the matrix of USY.
The catalytic properties of these samples have been examined
in the synthesis of DMDPMs from toluene and formaldehyde, and
the reaction results are summarized in Table 4. It was found that
pure USY zeolite was less active for this reaction (with a 20.1%
conversion of HCHO). On the other hand, the detected conversion
of HCHO increased to 30.8% on HPW-USY (7). This increased
activity contributed to the formation of more active sites as HPW
became encapsulated in supercage of USY. Though the detected
amount of formed HPW in HPW-USY (9) is larger than that of HPWUSY (7), its activity (with a HCHO conversion of 7.9%) is lower than
that of pure USY. These results were because HPW heteropoly acid
was covered by amorphous silicon derived from collapsed zeolitic
matrix that hindered the contact of acid sites with reactants. And
the lower activity of HPW-USY (hydrothermal) could be attributed
to the same reason.
According to previous experimentation, pure HPW exhibited
high activity in synthesis DMDPMs from toluene and paraformaldehyde, while the selectivity of 4,40 -DMDPM was low [30–32]. And
those zeolites with pore sizes large enough to allow diffusion of
large molecules could increase the selectivity of 4,40 - DMDPM [33].
After heteropoly acid was packed into the supercages of the zeolitic
Catalysts
USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
HPW-USY
Conversion (mol %)
(0 min)
(3 min)
(5 min)
(7 min)
(9 min)
(hydrothermal)
20.1
13.1
23.8
25.2
30.8
7.9
11.7
Selectivity (mol %)
2,20
2,40
4,40
1.5
1.8
2.2
2.3
2.3
1.1
1.7
33.6
36.2
42.1
40.4
42.6
31.0
36.8
64.9
62.5
55.7
57.3
55.1
67.9
61.5
a
Reaction conditions: toluene 450 mmol, paraformaldehyde 30 mmol, catalyst
1.0 g, reaction time 4 h, reaction temperature 140 8C.
matrix, the HPW-USY catalyst can also increase the selectivity of
4,40 - DMDPM more than pure HPW could.
4. Conclusions
In conclusion, we found that HPW could be synthesized
successfully in situ in supercages of USY without destruction of
the zeolitic matrix. 31P MAS NMR, HR-TEM, N2 adsorption, ICP and
XRD characterizations gave a promising image of the encapsulated
HPW in USY and the location of HPW is clearly identified. And both
the stronger acid sites and the concentration of Brönsted acid sites
were enhanced in HPW-USY. HPW-USY (7) exhibited higher
activity for the synthesis of DMDPM via toluene and formaldehyde
than pure USY.
Acknowledgements
This Project is supported by National Natural Science Foundation
of China (90610002, 20433030), Zhejiang Provincial Natural Science
Foundation (Z406142), the Ministry of Science and Technology of
China through the National Key Project of Fundamental Research
(Contract No. 2007CB210207) and the State Key Laboratory
Breeding Base of Green Chemistry-Synthesis Technology.
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