Synthesis of zirconia porous phosphate heterostructures (Zr-PPH) for Prins condensation

Article abstract of DOI:10.1016/j.catcom.2013.09.020

Porous phosphate heterostructure materials with zirconia galleries have been prepared. Their acid properties were studied via 2,6-di-tert-butyl-pyridine adsorption, infrared spectra of pyridine adsorption and potentiometric titration. The samples prepared in alcohol media are more thermal stable than those prepared in aqueous media. They are very active and selective for the Prins condensation of β-pinene with paraformaldehyde because of large amount of Lewis acid sites as well as proper L/B ratio. The selectivity towards Nopol can be further improved by ion-exchanging with sodium cations, due to the elimination of Br?nsted acid sites which suppresses the by-reactions such as isomerization of β-pinene.

Full text of DOI:10.1016/j.catcom.2013.09.020

Catalysis Communications 43 (2014) 97–101
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of zirconia porous phosphate heterostructures (Zr-PPH) for
Prins condensation
Xueyan Wang, Tao Wang, Weiming Hua, Yinghong Yue ⁎, Zi Gao
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 May 2013
Received in revised form 4 September 2013
Accepted 11 September 2013
Available online 2 October 2013
Porous phosphate heterostructure materials with zirconia galleries have been prepared. Their acid properties
were studied via 2,6-di-tert-butyl-pyridine adsorption, infrared spectra of pyridine adsorption and potentiomet-
ric titration. The samples prepared in alcohol media are more thermal stable than those prepared in aqueous
media. They are very active and selective for the Prins condensation of β-pinene with paraformaldehyde because
of large amount of Lewis acid sites as well as proper L/B ratio. The selectivity towards Nopol can be further im-
proved by ion-exchanging with sodium cations, due to the elimination of Brønsted acid sites which suppresses
the by-reactions such as isomerization of β-pinene.
Keywords:
Porous phosphate heterostructure
Zirconia gallery
Lewis acid
© 2013 Elsevier B.V. All rights reserved.
Prins condensation
Nopol
1
. Introduction
Heterogeneous solid acid catalysts are of increasing interest in fine
paraformaldehyde. In accordance with the concept of green chemistry,
heterogeneous catalysts especially metal-doping mesoporous materials
such as ZnMCM-41 [11,12], SnMCM-41 [12,13], SnSBA-15 [14,15],
ZrSBA-15 [16], ZnCl /montmorillonite [17] and mesoporous iron phos-
chemical reactions due to their environmentally benign nature with
respect to corrosiveness, safety, less waste and ease of separation and
recovery [1]. Among them, metal oxide pillared layered metal (IV)
phosphates are promising catalysts, due to their high surface area,
large pore volume and high stability. Furthermore, the pillared phos-
phates are more versatile in framework chemical variety and composi-
tion compared with other mesoporous materials, which are indeed of
importance to the heterogeneous catalysis. These materials are already
found catalytically active for various acid-catalyzed reactions, such as
dehydration of isopropanol [2,3], cumene cracking [3], and polymeriza-
tion of pyrrole [4].
2
phate [18], were explored and utilized in this reaction in the past decades.
In this work, zirconia galleries were inserted into the PPH materials due to
the excellent catalytic behaviors of zirconia which come from its unique
acid and base properties. The catalytic application in Prins condensation
of β-pinene with paraformaldehyde was conducted. The reusability and
regeneration of the catalyst were also studied.
2. Experimental
2.1. Sample preparation
Recently, a novel kind of porous phosphate heterostructure (PPH)
material with a layered structure has been synthesized by a co-
templated method [5,6]. These materials possess much larger surface
area and better porous structure as compared with the silica pillared zir-
conium phosphate by the traditional method [7]. Chemical properties of
these materials can be modified by doping or replacing the silica galler-
ies with different elements. Cu-PPHs and Ru-PPHs were reported in the
Cetyltrimethylammonium bromide expanded zirconium phos-
phates (CTAB-ZrP) were prepared following the procedures in the
literature [5,6]. Zirconia galleries were inserted in aqueous media
as follows: 1 g CTAB-ZrP (≥99%, Guoyao) was suspended in
100 ml water. Hexadecylamine (90%, Alfa) in n-propanol (35 g/l)
solution was added into the water with hexadecylamine/P molar
ratio of 0.4. After stirred for one day at 40 °C, zirconium (IV)
propoxide (70% Aldrich) with Zr/P molar ratio of 2 was added. This
suspension was stirred for three days at 40 °C. The solid obtained
was centrifuged, washed several times with ethanol. The surfactants
were removed by refluxing in 200 ml ethanol with 1.5 ml of concentrated
hydrochloric acid at 70 °C overnight, washed with ethanol, dried at 60 °C,
and finally calcined for 3 h. The inserting in alcohol media follows the
same procedures except that CTAB-ZrP was suspended in 100 ml
n-propanol at the beginning. The obtained materials in aqueous and
x
selective catalytic reduction of NO and hydrotreating of aromatic hy-
drocarbons, respectively [8,9]. Mixed silica/zirconia pillared and mixed
silica/titania pillared zirconium phosphates were also prepared as new
solid acids [10].
Nopol, widely used in manufacturing household products, is
commercially synthesized by the condensation of β-pinene and
⁎
Corresponding author. Tel.: +86 21 65642409; fax: +86 21 65641740.
E-mail address: yhyue@fudan.edu.cn (Y. Yue).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.09.020

98
X. Wang et al. / Catalysis Communications 43 (2014) 97–101
n-propanol media were denoted as ZrO
respectively, where T is the calcination temperature.
The sodium ion-exchanged samples were obtained by ion-exchanging
ZrO –ZrP-a-T three times with 1 M NaCl (1 g/10 ml) at 80 °C for 4 h, and
designated as Na–ZrO –ZrP-a-T.
2 2
–ZrP-w-T and ZrO –ZrP-a-T,
2
2
2
.2. Catalyst characterization
The N
2
adsorption/desorption isotherms were measured on a Tristar
temperature. Surface areas of the samples
3
000 porosimeter at liquid N
2
were calculated by BET method, and the pore size distributions by Cran-
ston and Inkley method [19]. Scanning electron microscopic (SEM) im-
ages were obtained on a Philips XL-30 scanning electron microscope.
DTBPy adsorption in liquid phase was tested by adding 20 mg cata-
lyst into 2 ml DTBPy/xylene solution (0.004 mol/l). Trace amount of
tetradecane was utilized as internal standard. The mixture was stirred
for 5 h at room temperature. The remaining DTBPy was analyzed by a
gas chromatograph equipped with a SE-30 capillary column (30 m ×
0
3
6
9
12
15
d (nm)
Fig. 2. The isotherms of the pore size distribution for (▲) ZrO
2
2
–ZrP-w-150, (▼) ZrO –ZrP-a-
1
50, (♦) ZrO –ZrP-w-350, (◄) ZrO –ZrP-a-350, (►) ZrO –ZrP-w-550, (●) ZrO
2
2
2
2
–ZrP-a-550.
0.25 mm × 0.3 μm).
Py-IR spectra were recorded on a Nicolet Nexus 470 FT-IR spec-
trometer at 100 °C. Quantitative determination of Brønsted and
Lewis acid sites was derived from the integrated areas of the IR
2.4. Catalyst poisoning by DTBPy
−
1
bands at 1540 and 1450 cm , respectively. Extinction coefficients
= 0.73 cm/μmol and E = 1.11 cm/μmol) were obtained from previ-
ous work of Dakta et al. [20].
Catalyst poisoning was done by adding calculated amount of DTBPy
to the mixture of catalysts and solvent (toluene) prior to the addition of
reactants. The above mixture was stirred under room temperature over-
night. After that, catalytic tests were done as described in Section 2.3.
(E
B
L
The acidity was measured by means of potentiometric titration
21,22]. The 0.05 g catalyst was suspended in 10 ml acetonitrile, and
[
titrated with 0.1 mol/l butylamine in acetonitrile. The electrode potential
variation was recorded with a Mettler Toledo FE20 potentiometer.
The acid strength can be classified according to the following scale:
3. Results and discussion
3.1. Textural properties of the catalysts
E
−
i
N 100 mV (very strong sites), 0 b E
100 b E b0 mV (weak sites) [21,22].
i
b 100 mV (strong sites) and
i
The N2 adsorption–desorption isotherms of porous phosphate
heterostructure materials with zirconia galleries are shown in Fig. 1.
Type IV isotherms can be observed for all the samples prepared, show-
ing that mesopores were generated after the zirconia inserting. This was
proved by the pore size distribution curves of the above materials,
as shown in Fig. 2. These obtained samples exhibit high BET surface
area and large pore volume as compared with the calcined CTAB-ZrP
(Table 1), indicating the presence of zirconia galleries between the
interlayer spaces of the phosphate, which resulted in a porous structure
2
.3. Catalytic tests
The Prins condensation of β-pinene with paraformaldehyde was car-
ried out in a three-necked flask equipped with a condenser. Typically,
0 mg catalyst, 1 mmol (0.136 g) β-pinene, 2 mmol (0.06 g) paraformal-
dehyde and 5 ml toluene were added with magnetic stirring at 80 °C for
h. The products were analyzed by a gas chromatograph equipped with a
SE-30 capillary column (30 m × 0.25 mm × 0.3 μm).
5
4
accessible to the N molecules. The surface area and pore volume decrease
2
with the increasing of the calcination temperature, since the pore struc-
ture may shrink or partially collapse due to self-condensation of P-OH
groups [23]. This decrement changes with the preparative media. The
samples obtained from alcohol media are more thermal stable than
those from aqueous one. After calcinations at 550 °C, the variation of
pore volume is 36% for alcohol media sample. While for aqueous media
Table 1
Textural properties of the samples.
Catalysts
BET surface
Pore
Average pore
diameter
(nm)
Adsorbed
DTBPy
(mmol/g)
c
area
volume
2
3
(
m /g)
(cm /g)
CTAB-ZrPa
24
0.03
0.45
0.34
0.26
0.42
0.27
0.27
0.49
0.33
–b
–b
ZrO
ZrO
ZrO
ZrO
ZrO
ZrO
2
–ZrP-w-150
2
–ZrP-w-350
2
–ZrP-w-550
2
–ZrP-a-150
2
–ZrP-a-350
2
–ZrP-a-550
455
378
265
543
426
342
483
310
4.0
3.6
3.9
3.1
2.5
3.2
4.1
4.3
0.12
0.04
0.04
0.25
0.20
0.08
b
0.0
0.2
0.4
0.6
0.8
1.0
Na–ZrO
Na–ZrO
2
–ZrP-a-150
–ZrP-a-350
–
b
2
–
Relative Pressure(P/Po)
a
b
c
Calcined at 550 °C for 6 h.
Not detected.
Obtained by 4 V/A.
Fig. 1. N
◄) ZrO
2
2
adsorption/desorption isotherms for (▼) ZrO –ZrP-a-150, (▲) ZrO
–ZrP-a-350, (♦) ZrO –ZrP-w-350, (●) ZrO
2
–ZrP-w-150,
–ZrP-w-550.
(
2
2
2
–ZrP-a-550, (►) ZrO
2

X. Wang et al. / Catalysis Communications 43 (2014) 97–101
99
a
b
c
d
2 2 2 2
Fig. 3. SEM images for (a) ZrO –ZrP-a-150, (b) ZrO –ZrP-w-150, (c) ZrO –ZrP-a-350, (d) ZrO –ZrP-w-350.
one, the value is 42%. Furthermore, the surface area of the catalyst pre-
pared in n-propanol is higher than that of the sample prepared in water
under the same calcination temperature.
3.2. Acidity measurements
The acidity of the catalysts was characterized by means of potentio-
metric titration. The acid sites with different acid strengths can be seen
in Table 2. The total amount of acid sites decreases markedly after calci-
nation, which may be caused by the condensation of free P-OH groups
[23]. Ion-exchanging by sodium cations also reduces the amount of
acid sites, since the Brønsted acid sites have been ion-exchanged. The
Liquid phase DTBPy adsorption reflects the capability of catalysts for
bulky molecular reactants since DTBPy was a moderate base with rela-
tively large size (~0.79 nm in diameter). As seen in Table 1, the amount
of adsorbed DTBPy decreased with the increasing of the calcination
temperature, from 0.12 mmol/g to 0.04 mmol/g for aqueous media
sample and from 0.25 mmol/g to 0.08 mmol/g for alcohol media one,
indicating the decrement of the possible active sites for the catalytic reac-
tions. However, the DTBPy amount of the catalyst prepared in propanol
was always higher than that of the sample prepared in aqueous media
under the same calcination temperature, which is in agreement with
the BET results.
acid sites for the four catalysts has the order of ZrO –ZrP-a-150 N Na–
2
ZrO –ZrP-a-150 ≈ ZrO –ZrP-a-350 N Na–ZrO –ZrP-a-350.
2
2
2
Py-IR was taken to study the nature of acid sites [24]. The character-
istic bands of pyridine coordinately bonded to Lewis acid sites at ca.
−
1
−1
1450 cm , as well as the bands at ca. 1540 cm assigned to proton-
ation of pyridine by Brønsted acid sites can be observed on the catalysts.
The amount of Brønsted and Lewis acid sites was quantified and sum-
marized in Table 3. The total number of acid sites has the sequence of
ZrO –ZrP-a-150 N ZrO –ZrP-a-350 N Na–ZrO –ZrP-a-150 N Na–ZrO –
2 2 2
The SEM images of the ZrO –ZrP-a-150, ZrO –ZrP-w-150, ZrO –ZrP-
a-350 and ZrO –ZrP-w-350 are shown in Fig. 3. The lamellar nature of
2
the obtained materials prepared in propanol (Fig. 3a and c) can be ob-
served from their platelike crystal morphology while similar structure
is not so clear for the materials obtained in aqueous medium (Fig. 3b
and d), which is due to the quick hydrolysis of zirconium(IV) propoxide
in water than in propanol. The particles of the four samples are all
nonuniform.
2
2
2
2
+
ZrP-a-350. Calcining or ion-exchanging with Na mainly reduced the
Brønsted acid sites while keeping the Lewis acid sites almost unchanged,
resulting the great increase of L/B ratio.
3.3. Catalytic activity
The activities of the prepared zirconia pillared zirconium phosphate
catalysts for the reaction were investigated and the results are listed in
Table 4. All catalysts were very active for this condensation, higher than
Table 2
Acidic properties of the catalysts obtained by titration.
Catalysts
Amount of acid sites (mmol/g)
Table 3
Strong acida
Mild acidb
Weak acidc
Total
Acidity of the catalysts determined by pyridine absorbed FT-IR.
ZrO
Na–ZrO
ZrO –ZrP-a-350
2
–ZrP-a-150
1.60
0.13
0.90
0.57
0.60
1.20
0.20
0.25
0.70
0.60
0.69
0.28
2.9
1.9
1.8
1.1
Catalysts
Brønsted acid s
ites (mmol/g)
Lewis acid
sites (mmol/g)
Total acid
sites (mmol/g)
L/B
ratio
2
–ZrP-a-150
2
2
Na–ZrO –ZrP-a-350
ZrO
Na–ZrO
ZrO –ZrP–a-350
Na–ZrO –ZrP-a-350
2
–ZrP-a-150
0.72
0.36
0.60
0.08
0.60
0.68
0.69
0.64
1.32
1.04
1.29
0.72
0.83
1.9
1.1
2
–ZrP-a-150
a
E
1
0
i
N 100 mV.
00 mV N E
mV N E N −100 mV.
b
c
2
i
N 0 mV.
2
8.5
i

100
X. Wang et al. / Catalysis Communications 43 (2014) 97–101
Table 4
Activities of the catalysts for Prins condensation.
Table 5
a
2
Reusability of Na–ZrO –ZrP-a-350 in the Prins condensation.
Catalysts
Conv. Selectivity (%)
%)
Yield of
Nopol (%)
Catalysts
Conversion of
β-pinene (%)
Selectivity of
Nopol (%)
Yield of
Nopol (%)
(
Nopol Camphene Limonene Othersb
Fresh
86
79
73
56
83
87
89
88
91
82
75
70
64
51
68
ZrO
Na–ZrO
ZrO
2
–ZrP-a-150
98
79
82
77
87
9
6
9
5
5
3
5
3
7
9
9
5
77
75
68
75
1
2
3
st reuse
nd reuse
rd reuse
2
–ZrP-a-150 92
2
–ZrP-a-350
89
Na–ZrO
2
–ZrP-a-350 86
a
After regeneration
a
Reaction condition: 50 mg catalyst, 1 mmol β-pinene and 2 mmol paraformaldehyde,
a
Catalyst was washed with acetone and recalcined at 350 °C for 3 h before use.
5
ml toluene as solvent, 80 °C for 4 h.
b
Including terpinolenes, terpinenes and α-pinene.
The promotion effect of Brønsted acid can be also proved by the fact
the recent results over Zr-SBA-15 [16]. The sequence of the activity
is as follows: ZrO –ZrP-a-150 N Na–ZrO –ZrP-a-150 N ZrO –ZrP-a-
50 N Na–ZrO –ZrP-a-350, which is the same as that of the total
that the yield of Nopol over ZrO
over Na–ZrO –ZrP-a-350 with same amount of DTBPy added, since
Py-IR results showed that the amount of Brønsted acid sites of ZrO
ZrP-a-150 was much more than that of Na–ZrO –ZrP-a-350 while the
amount of Lewis acid sites was nearly equal. Thus, it can be concluded
that the amount of Lewis acid sites as well as proper L/B ratio plays an
important role in Nopol production through condensation.
The reusability of the catalyst was also studied and the results are
listed in Table 5. After reaction, the catalyst was recovered by filtration,
washed with acetone for several times, dried at 80 °C and then reused.
An obvious deactivation is observed. After three runs, the conversion
drops from 86% to 56%. Tar or polymeric compound deposition on the
catalyst is probably the cause for the reduction in catalytic activity.
Hence, the catalyst was recalcined at 350 °C for 3 h after four runs.
The catalytic activity was recovered mostly. The conversion over the
recalcined sample was 83% which is comparable to that of the fresh cat-
alyst (Table 5).
2
–ZrP-a-150 is always higher than that
2
2
2
2
3
2
2
–
amount of acid sites by titration, since the reaction is thought to be cata-
lyzed by acid sites. However, the selectivity towards Nopol has an oppo-
site trend. It was reported that the existence of Brønsted acid sites is
also favorable to form monoisomers through pinene isomerization rather
than Nopol through condensation [16]. The formation of isomers is irre-
versible so that if isomerization is predominant, less β-pinene is available
for condensation. The selectivity can be improved by eliminating the
Brønsted acid sites through ion-exchanging with sodium ions. This is
the reason why the catalyst Na–ZrO –ZrP-a-350 with the lowest Brønsted
2
acid sites has the highest selectivity to Nopol.
To further study the influence of the acidity on the Prins condensa-
tion, the activities of the catalysts poisoned by DTBPy have been inves-
tigated. The relevance of the DTBPy amount added and the Nopol yield
2
2 2
over ZrO –ZrP-a-150 and Na–ZrO –ZrP-a-350 was illustrated in Fig. 4.
The yield decreases slowly with the rise of DTBPy amount over both cat-
alysts. When the DTBPy amount is 2.0 mmol/g, the yields on both cata-
lysts are still as high as 80% of the original ones. Since DTBPy molecule
was thought to adsorb mainly on the accessible Brønsted acid sites
4. Conclusions
The zirconia pillared zirconium phosphate has been synthesized
successfully. The samples prepared in n-propanol are more thermal sta-
ble and higher acidic than that prepared in water. These materials
showed good activity and selectivity in the Prins condensation of
β-pinene and paraformaldehyde. A Nopol yield of 75% with 87% selec-
tivity was obtained over Na–ZrO –ZrP-a-350. The catalysts can be
reused just by recalcination. The amount of Lewis acid sites as well as
proper L/B ratio plays an important role in selective synthesis of Nopol.
[25], the above results indicate that the uncovered Lewis acid sites can
motivate the condensation efficiently without Brønsted acid sites. How-
ever, the changing tendencies for the two catalysts are different. The
yield of Nopol over Na–ZrO
of poisoning test, and then changes little afterwards. Since the Brønsted
acid sites on Na–ZrO –ZrP-a-350 are few, which are easy to be poisoned
by small amount of DTBPy added, the abrupt reduction between 0 and
.50 mmol/g shows the promotion effect of Brønsted acid sites on the
condensation reaction. Similar promotion effect can be observed over
ZrO –ZrP-a-150. The yield decreases with the reduction of Brønsted acid
sites. But this is decreasing more gradually between 0 and 2.0 mmol/g,
since the amount of Brønsted acid sites on ZrO –ZrP-a-150 is much larger,
they were difficult to be totally poisoned by the addition of DTBPy.
2
–ZrP-a-350 drops quickly in the beginning
2
2
0
Acknowledgments
2
This work was supported by the National Natural Science Foun-
dation of China (20633030, 20773027, 20773028 and 21273043)
and the Science & Technology Commission of Shanghai Municipality
2
(
08DZ2270500).
100
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