One-pot synthesis of ordered mesoporous zirconium oxophosphate with high thermostability and acidic properties

Article abstract of DOI:10.1039/c3cy00085k

A series of mesoporous zirconium oxophosphate (M-ZrPO) with different P/Zr molar ratios (0-1.25) has been prepared via a facile one-pot evaporation-induced self-assembly (EISA) strategy. After removing the structure-directing agents, the M-ZrPO with large specific surface area (160 m² g^-1), big pore volume (0.26 cm³ g^-1) and narrow pore size distribution (5.64 nm) has been obtained. Small-angle X-ray diffraction (SXRD) and transmission electron microscopy (TEM) results showed that these materials had ordered mesoporous structure. With the increase of P/Zr, the textural properties of M-ZrPO could be improved. Moreover, the ordered mesostructure could be maintained even when treated at 800°C, indicating the M-ZrPO had attractive thermal stability. NH₃-TPD and pyridine-IR analyses showed the presence of abundant Broensted and Lewis acid sites in the material. The M-ZrPO has been used successfully as solid acid catalyst and showed excellent performance in the ketalization reaction.

Full text of DOI:10.1039/c3cy00085k

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One-pot synthesis of ordered mesoporous zirconium
oxophosphate with high thermostability and acidic
properties†
Cite this: DOI: 10.1039/c3cy00085k
ab
ab
ac
ab
ac
Zhichao Miao, Leilei Xu, Huanling Song, Huahua Zhao and Lingjun Chou*
A series of mesoporous zirconium oxophosphate (M-ZrPO) with different P/Zr molar ratios (0–1.25) has
been prepared via a facile one-pot evaporation-induced self-assembly (EISA) strategy. After removing
2
À1
the structure-directing agents, the M-ZrPO with large specific surface area (160 m
g
), big pore
3
À1
volume (0.26 cm
g
) and narrow pore size distribution (5.64 nm) has been obtained. Small-angle
X-ray diffraction (SXRD) and transmission electron microscopy (TEM) results showed that these materials
had ordered mesoporous structure. With the increase of P/Zr, the textural properties of M-ZrPO could
be improved. Moreover, the ordered mesostructure could be maintained even when treated at 800 1C,
Received 4th February 2013,
Accepted 28th March 2013
3
indicating the M-ZrPO had attractive thermal stability. NH -TPD and pyridine-IR analyses showed the
DOI: 10.1039/c3cy00085k
presence of abundant Br o¨ nsted and Lewis acid sites in the material. The M-ZrPO has been used
successfully as solid acid catalyst and showed excellent performance in the ketalization reaction.
www.rsc.org/catalysis
big pore volumes and uniform pore sizes, which could provide
abundant ‘‘accessible’’ active sites for the reactants.
Zirconium phosphate has attracted considerable attention among It is expected that the ordered mesoporous zirconium oxo-
1
. Introduction
2
7,29
1
–4
5,6
7–9
many fields, such as catalysis, support, adsorption, fuel phosphate (M-ZrPO) with these superiorities possess more
1
0–12
13
30
cell
and selective separation. In particular, it has been attractiveness in the field of catalysis. As Bhaumik et al.
1
4,15
used as solid acid catalyst in many reactions, like dehydration,
isomerization, ketalization and Friedel–Crafts reaction,
reported, M-ZrPO exhibited excellent catalytic activity in bio-
diesel reaction of the long chain fatty acids. In addition, the
1
6
17
18
due to the excellent acidic property and high water tolerance catalytic activity of M-ZrPO was much higher than the conven-
1
9
ability. However, the low effective surface area and small pore tional layered zirconium phosphate in the Claisen–Schmidt
size are the main drawbacks for its successful utilization reaction.31
in these reactions. Since the discovery of mesoporous silica
To date, most of the reports about synthesizing M-ZrPO were
2
0,21
MCM-41,
different kinds of ordered mesoporous materials, concentrating on employing a two-step method with cationic
2
2–24
especially many non-siliceous mesoporous materials,
have surfactants as structure-directing agents (SDAs) and post-
The treatment with phosphoric source. The M-ZrPO was first realized
2
5–28
been obtained and widely used in catalytic fields.
materials with ordered mesostructure had many advantageous by the group of Sch u¨ th with employing cetyltrimethylammonium
32–34
structural properties, such as large specific surface areas, bromide (CTAB) as SDAs and Zr(SO ) as zirconium source.
4
2
Following this work, Iwanoto and co-workers changed the times
of post-treatment to get M-ZrPO with different ratios of P/Zr.
a
35
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China. E-mail: ljchou@licp.cas.cn;
Fax: +86 0931 4968129; Tel: +86 0931 4968066
In addition, different kinds of cationic surfactants, like tri-
3
6
head group quaternary ammonium and gemini cationic
3
7
b
c
surfactants, were used as SDAs to get hexagonal and cubic
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
symmetries M-ZrPO. Furthermore, Panda et al. fabricated
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
Suzhou 215123, People’s Republic of China
M-ZrPO in basic medium using zirconium carbonate as the
31,38
source of zirconium.
Although the method of using cationic
†
Electronic supplementary information (ESI) available: The XRD patterns of
surfactants is widely used, the M-ZrPO gotten from this method
has a small pore size and the P/Zr ratios cannot be controlled
accurately. The reports about employing nonionic surfactants
M-ZrPO-0 treated at 600 and 800 1C, M-ZrPO-1.25-500 and the used M-ZrPO-0.75-
00; nitrogen adsorption and desorption isotherms of M-ZrPO-1.25-500 and used
5
M-ZrPO-0.75-500. See DOI: 10.1039/c3cy00085k
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as SDAs through the EISA strategy to prepare M-ZrPO are still with Cu Ka radiation (0.15406 nm) at room temperature from
2
4,39
very few and have potential to be explored.
Furthermore, 0.61 to 5.01 (small angle) and 10.01 to 80.01 (wide angle).
the fabrication of ordered M-ZrPO with large specific surface Measurements were conducted using a voltage of 40 kV, current
area, big volume, uniform pore size distribution and high setting of 20 mA for small angle XRD and 40 mA for wide angle
thermal stability is still a challenging work.
XRD, step size of 0.021, and count time of 4 s.
In this paper, a facile and direct one-pot evaporation-
The scanning electron microscopy (SEM) images were
induced self-assembly (EISA) method was employed to fabricate obtained on SUPRA 55 (ZEISS) high-resolution scanning
a series of M-ZrPO with different ratios of P/Zr (from 0 to 1.25). electron microscopy.
The P/Zr ratios could be accurately regulated through changing
the precursors. Zirconyl chloride octahydrate (ZrOCl
Á8H
Transmission electron microscopy (TEM) images, selected
O) area electron diffraction (SAED) and energy-dispersive X-ray
2
2
was not only used as metal precursors, but also employed as spectroscopy (EDX) measurements were performed on the
4
0,41
2
acid modifier, due to the absence of other mineral acids.
The excellent structural properties combined with the fascinating scopy under a working voltage of 200 kV.
thermal stability would largely promise the applications of
The nitrogen adsorption and desorption isotherms at À196 1C
ordered M-ZrPO in catalytic fields. NH -TPD and pyridine-IR were recorded on an Autosorb-iQ analyzer (Quantachrome
TECNAI G F20 high-resolution transmission electron micro-
3
measurements suggested the existence of excellent acidic nature Instruments U.S.). Prior to the tests, all the samples were
in the M-ZrPO. In addition, the M-ZrPO exhibited high catalytic degassed at 200 1C for 2 h. The specific surface areas were
activity in acid catalytic reaction.
calculated via the Brunauer–Emmett–Teller (BET) method in
the relative pressure range of 0.05–0.3; the single-point pore
volume was calculated from the adsorption isotherm at a
relative pressure of 0.990; pore size distributions were calculated
using adsorption branches of nitrogen adsorption–desorption
isotherms by Barrett–Joyner–Halenda (BJH) method.
2. Experimental section
2
.1 Synthesis of ordered mesoporous zirconium
oxophosphate
EO)106(PO) (EO)
(
triblock copolymer (Pluronic F127, Mav
106
=
Thermogravimetric-differential scanning calorimetry (TG-DSC)
70
1
2 600, Sigma-Aldrich), zirconyl chloride octahydrate (ZrOCl Á8H O, measurements were carried out on a NETZSCH STA 449F3
2
2
Z99%, Tianjin Fengyue Reagent Company), trimethyl phos- thermogravimetric analyzer from room temperature to 1000 1C
À1
phate (PO(OCH
Co. Ltd.), ethanol (Z99.7%, Sinopharm Chemical Reagent
Co. Ltd.). All the reagents were A.R. grade and used as received (XRF) spectrum analyzer on an Magix PW2403 (PANalytical,
3
)
3
, Z99%, Sinopharm Chemical Reagent with the rate of 10 1C min under air atmosphere.
The elemental analysis was detected by X-ray fluorescence
without further purification.
Inc.).
UV-visible diffuse reflectance spectra were recorded on a PE
Ordered mesoporous zirconium oxophosphate was synthesized
via an improved one-pot evaporation-induced self-assemble Lambda 650S in the range of 700–180 nm.
EISA) strategy through controlling the volatile process. In a Fourier transform infrared (FT-IR) spectra were recorded on
(
typical procedure of synthesizing M-ZrPO, 1.2 g of F127 was KBr pellets by a Nicolet (iS10) infrared spectrometer with the
À1
dissolved in 15 ml of anhydrous ethanol. As the SDAs were wave number from 4000 to 400 cm
completely dissolved, 5 mmol of ZrOCl O and 3.75 mmol of
Á8H
PO(OCH
(the molar ratio of P/Zr was 0.75) were added into the samples were performed on a Thermon Scientific ESCALAB250xi
above solution with vigorous stirring. The final solution was spectrometer.
covered with PE film and stirred for at least 6 h. Finally, the
Temperature programmed desorption of ammonia (NH
transparent solution mixture was transferred to a petri dish TPD) was performed on a Micromerictics Autochem II 2920.
d = 9 cm) to undergo the slow EISA process at 60 1C for 48 h, In a typical experiment for the TPD measurement, the M-ZrPO
.
X-ray photoelectron spectroscopy (XPS) analyses of the
2
2
3 3
)
3
-
(
then 100 1C for 24 h in a drying oven. The obtained colorless and (0.1 g) was pretreated at 300 1C for 2 h in flowing He to remove
transparent xerogel was calcined by slowly increasing tempera- any moisture and absorbed impurities. After cooling to 100 1C,
À1
ture (1 1C min ramping rate) to 500 1C and was kept at the final 10 mol% NH
–He was introduced until complete saturation.
was then removed by desorbing it in
3
temperature for 5 h to remove the SDAs. The obtained template- Physically absorbed NH
3
free self-assembled mesoporous zirconium oxophosphate was He at 100 1C for 40 min. TPD experiment was then carried out
denoted as M-ZrPO-x-500, where x (1.25, 1, 0.75, 0.5, 0.25, 0) by raising the temperature of the sample in a programmed
À1
stands for the molar ratio of P/Zr. The samples gotten through manner (10 1C min ) to 900 1C. The same sample without
treating at high temperature (600–900 1C) were denoted as introduction of NH
3
was used as blank control, no TCD signals
M-ZrPO-x-y, where y stands for the final calcination temperature. were observed in the programmed process from 100 to 900 1C.
For instance, M-ZrPO-0.75-800 refers to the sample with the ratio
of P/Zr 0.75 and calcined at 800 1C for 5 h in air.
The infrared spectra of pyridine (pyridine-IR) at different
desorption temperatures were recorded on Nicolet 510P
spectrometer. The sample (B10 mg) was pressed to a pellet
2.2 Characterization
(
B10 mm). The sample was activated at 350 1C for 2 h under
À3
Powder X-ray diffraction (XRD) measurements were performed 5 Â 10 Pa, then cooled to 100 1C and kept for 10 min, and
using an X’Pert Pro Multipurpose diffractometer (PANalytical, Inc.) pyridine was adsorbed for 30 s. The sample was raised to a
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desired temperature and held at that temperature for 10 min
À3
under 5 Â 10 Pa, after which the spectrum was recorded. The
blank experiments were operated under the same conditions
and used as the background to insure the accuracy of the
infrared spectra of pyridine. The calculation of Br o¨ nsted and
Lewis acidity were determined using the method proposed by
4
2
Emeis.
2.3 Catalytic reaction
The acid catalytic activity of M-ZrPO was probed by the liquid-
phase ketalization reaction. The apparatus and procedures for
the reaction were similar to that reported by Ganeshpure and
1
7
co-workers. A typical procedure for the ketalization of cyclo-
hexanone was as follows: a mixture of cyclohexanone (0.2 mol),
ethylene glycol (0.26 mol), cyclohexane (15 ml, used as water
carrying agent), dodecane (6.5 ml, used as an internal standard)
and catalyst (M-ZrPO-0.75-500) were taken in a round bottom
flask placed in an oil bath and fitted with a Dean–Stark
apparatus to remove the water continuously from the reaction
mixture. The yield was calculated from the GC analysis. All the
products were confirmed by GC-MS. After the completion of the
reaction, the catalyst was separated by centrifugation and
activated at 300 1C for 3 h for regeneration.
3. Results and discussion
3.1 Ordered mesostructure and thermostability
The small angle X-ray diffraction (SXRD) has been widely used
2
0,21
in the study of the ordered mesoporous materials.
The
SXRD patterns of M-ZrPO with different ratios of P/Zr are shown
in Fig. 1(1). Each sample displayed an obvious diffraction peak
around 0.9–1.21 and one poorly resolved diffraction peak
around 1.91 for M-ZrPO-0.75-500, which could be indexed as
Fig. 1 Small-angle X-ray diffraction (1) and wide-angle X-ray diffraction (2) patterns
of the M-ZrPO-x-500: (a) M-ZrPO-1-500, (b) M-ZrPO-0.75-500, (c) M-ZrPO-0.5-500,
(
d) M-ZrPO-0.25-500, (e) M-ZrPO-0-500.
(
100) and (110) reflections of p6mm two-dimensional hexagonal
4
3,44
structure, respectively.
These diffraction peaks in SXRD
Table 1 Textural properties of the M-ZrPO-x-y and the used M-ZrPO-0.75-500,
patterns indicated the presence of ordered mesostructure,
which also could be further confirmed from the SEM and
TEM images. With the increase of P/Zr (from e to a), the
diffraction peaks became stronger and narrower, indicating
the tendency of the mesostructure improvement. Meanwhile,
the low angle reflection peaks shifted to lower 2y region (from
derived from XRD and nitrogen adsorption and desorption data
Nitrogen adsorption and
desorption data
SXRD
Specific
surface Pore Pore
P/Zr
ratios 2y(1) (100) (nm) (m g ) (nm) (cm g
d-spacing area
size volume
1.171 to 0.971) with the increase of phosphorus content, corre-
2
À1
3
À1
Samples
)
spondingly, the d-spacing (100) changed to higher values (from
M-ZrPO-1.25-500 1.25 0.86 10.26
123.4
149.3
160.0
138.8
118.5
96.8
142.4
121.2
117.3
7.6
6.55 0.20
5.63 0.21
5.64 0.26
4.92 0.16
4.30 0.11
3.40 0.12
5.65 0.21
5.61 0.19
5.61 0.20
3.06 0.07
5.63 0.22
7
.52 nm to 9.11 nm) (shown in Table 1). This indicated a larger
M-ZrPO-1-500 0.97 9.11
M-ZrPO-0.75-500 0.75 0.99 8.92
M-ZrPO-0.5-500 0.5 1.02 8.67
M-ZrPO-0.25-500 0.25 1.06 8.32
M-ZrPO-0-500 1.17 7.52
1
4
5,46
pore size and unit cell parameter at a higher ratio of P/Zr.
The wide angle X-ray diffraction (WXRD) patterns of these
samples are shown in Fig. 1(2). M-ZrPO-0-500 showed typical
0
diffraction peaks of ZrO
2
with tetragonal phase (t-ZrO
2
) (JCPDS M-ZrPO-0.75-600 0.75 0.98 9.03
M-ZrPO-0.75-700 0.75 0.97 9.11
card no. 80-0784). However, the other four patterns only
showed two broad peaks and the width of the peaks became
much broader with the increase of P/Zr. No individual crystal- M-ZrPO-0.75-500- 0.75 0.98 9.03
line peaks of zirconium phosphate or zirconia were observed,
indicating the wall of the samples became amorphous with the
introduction of phosphorous.
M-ZrPO-0.75-800 0.75 0.99 8.92
M-ZrPO-0.75-900 0.75
—
—
156.3
used
M-ZrPO-0-800
0
—
—
8.6
12.27 0.02
In order to investigate the thermal stability of the M-ZrPO, were selected as the typical representatives and studied. Their
the as-synthesized M-ZrPO-0.75 calcined at different temperatures SXRD patterns are shown in Fig. 2(1). Similar to the M-ZrPO
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Fig. 3 SEM images of M-ZrPO-0.75-500 along [001] and [110] direction.
the partial phase transition from tetragonal phase (t-ZrO ) to
2
monoclinic phase (m-ZrO ) took place (shown in Fig. S3, ESI†).
2
In order to observe the mesostructure, the SEM and TEM
images were used to characterize the structure of M-ZrPO. The
SEM images of M-ZrPO-0.75-500 are shown in Fig. 3.
The ordered hexagonal arrangement of pores along [001]
Fig. 2 Small-angle X-ray diffraction (1) and wide-angle X-ray diffraction (2)
patterns of the M-ZrPO-0.75 treated at different temperatures: (a) M-ZrPO- direction (Fig. 3a) and the alignment of cylindrical pores along
.75-500, (b) M-ZrPO-0.75-600, (c) M-ZrPO-0.75-700, (d) M-ZrPO-0.75-800, [110] direction (Fig. 3b) were distinctly observed. The TEM
0
(
e) M-ZrPO-0.75-900.
images (Fig. 4a–d) further confirmed the presence of long-
range pore channels in the samples. These SEM and TEM
images were in good agreement with the characterization
with different ratios of P/Zr, obvious diffraction peaks could be results of the SXRD, which predicted the presence of ordered
observed in the samples treated at the temperature as high as mesopores. In addition, the corresponding SAED pattern of
800 1C, indicating the existence of ordered mesoporous struc- M-ZrPO-0.75-500 (inset of Fig. 4b) showed diffuse rings, rather
ture. These facts implied that the M-ZrPO had favorable than obvious diffraction rings, indicating the walls of the
thermostability. Upon further increase of the temperature to mesoporous materials were amorphous in structure. These
900 1C, no diffraction peak could be observed in the SXRD results of SAED were consistent with those of the WXRD.
pattern, demonstrating the absence of the ordered mesostruc- However, the high-resolution TEM (HRTEM) image (Fig. 4e)
ture. Fig. 2(2) presents the WXRD patterns of the above- showed that the wall of the sample M-ZrPO-0-500 was highly
mentioned materials. When the calcination temperature was crystalline with a lattice spacing of 0.292 nm, agreeing well with
2
lower than 800 1C, the patterns of the samples were very similar the value of 0.295 nm for the (101) planes of t-ZrO in WXRD.
to each other and no obvious diffraction peaks could be This conclusion also could be gotten from SAED (inset of
observed, indicating no phase transition of the amorphous Fig. 4d), which showed obvious diffraction rings. Meanwhile,
mesostructure happened. However, typical diffraction peaks the TEM image of M-ZrPO-0.75-800 is shown in Fig. 4f, showing
of t-ZrO2 appeared in the WXRD pattern after treated at the ordered mesostructure with amorphous walls could be
900 1C, implying that the amorphous wall transferred to crystal- maintained even treated at 800 1C, indicating the M-ZrPO had
line frameworks (t-ZrO ). It was the phase transition that led superior thermostability. The EDS of M-ZrPO-0.75-500 is shown
2
to the collapse of ordered mesostructure. For the sample in Fig. 4g, where the exclusive peaks of Zr, P and O elements
M-ZrPO-0, its SXRD peaks disappeared when calcined at a were obviously observed, implying that all the elements were
temperature as low as 600 1C. Further, based on the WXRD patterns, successfully introduced into the wall of the pores and the
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Fig. 4 TEM and SAED images of (a) M-ZrPO-1-500, (b) M-ZrPO-0.75-500, (c) M-ZrPO-0.5-500, (d) M-ZrPO-0-500; (e) HRTEM image of M-ZrPO-0-500; (f) TEM and
SAED images of M-ZrPO-0.75-800; (g) EDX measurement of M-ZrPO-0.75-500.
atomic ratio of P/Zr (0.73) obtained from EDS was close to the the presence of cylindrical mesopores with excellent uniformity
theoretical value (0.75).
among the framework of M-ZrPO materials. These conclusions
The textural properties of the M-ZrPO with different ratios of could also be deduced from the characterization results of SXRD
P/Zr were characterized by nitrogen physisorption. The nitro- and TEM. From M-ZrPO-0-500 to M-ZrPO-0.75-500, the capillary
gen adsorption and desorption isotherms as well as their pore condensation steps of the resulting isotherm shifted to higher
size distributions (PSD) are shown in Fig. 5. As displayed in relative pressures, indicating the pore size of the samples
4
8,49
Fig. 5(1), the isotherms could be classified as type IV isotherms increased with the increasing of P/Zr.
with H1-shaped hysteresis loops,
This conclusion
which were the significant was in good agreement with the SXRD and the pore size
characteristics of ordered mesoporous structures. The H1-shaped calculated by the BJH method (shown in Table 1). Meanwhile,
hysteresis loops and the steep condensation steps suggested there was little nitrogen adsorption around P/P of 0.75–0.99,
4
1,47
0
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Fig. 5 Isotherm (1) and pore size distributions (2) of the M-ZrPO-x-500: (a) M-ZrPO-1-500, (b) M-ZrPO-0.75-500, (c) M-ZrPO-0.5-500, (d) M-ZrPO-0.25-500,
e) M-ZrPO-0-500.
(
1
indicating almost no larger mesopores among the ZrPO parti- the 500–800 1C displayed typical type IV isotherms with H -type
cles existed in the materials. This also could be deduced from hysteresis loops and uniform pore size distributions, indicating
the PSD of M-ZrPO. As shown in Fig. 5(2), all the curves of PSD the presence of ordered mesostructure when treated as high as
were extremely narrow and the position was located between 800 1C. However, the isotherms and the PSD of the sample
3–7 nm, once again reflecting the presence of uniform pore calcined at 900 1C were very different to the aforementioned
structure in the mesoporous materials. Additionally, the samples. No hysteresis loop and narrow pore size distribution
textural parameters of all these samples are given in Table 1. could be observed, indicating the disappearance of the meso-
With the increase of P/Zr from 0 to 0.75, the specific surface porous structure at 900 1C. The structural parameters of these
2
À1
2
À1
area became larger (from 96.8 m g to 160.0 m g ). The samples are given in Table 1. With the temperature rising from
similar trends were also observed in the alteration of pore size 500 1C to 800 1C, the pore size and volume suffered little
3
À1
(
from 3.40 nm to 5.64 nm) and pore volume (from 0.12 cm g
change, the specific surface area decreased gradually. Although
3
À1
to 0.26 cm
g ). Therefore, the mesostructure could be the specific surface areas suffered some damage during the increas-
improved with the increase of P/Zr ratios. Further increase ing of treating temperature up to 800 1C, the ordered mesostructure
the P/Zr up to 1.25, the pore size increased, but the specific and the uniform pore size distribution were successfully pre-
surface area began to decrease.
served, which was in good agreement with the characterization
The nitrogen adsorption and desorption isotherms and the of SXRD and TEM images. After treating at 900 1C, the specific
pore size distributions of M-ZrPO-0.75 calcined at different surface area and the pore volume became very small, showing
temperatures are displayed in Fig. 6. The samples treated at the thorough disappearance of the mesostructure.
Fig. 6 Isotherm (1) and pore size distributions (2) of the M-ZrPO-0.75 treated at different temperatures: (a) M-ZrPO-0.75-500, (b) M-ZrPO-0.75-600, (c) M-ZrPO-0.75-700,
(
d) M-ZrPO-0.75-800, (e) M-ZrPO-0.75-900.
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3.2 Chemical composition and bonds
X-ray fluorescence (XRF) characterization was used to determine
the molar ratios of P/Zr in the samples. The P/Zr ratios of the
theoretical values, as-synthesized and calcined M-ZrPO are shown
in Table 2. The results showed that the ratios of P/Zr remained
unchanged after removing the SDAs, meanwhile, both of them
were very close to the ratios of precursors, indicating the P and Zr
elements did not suffer great loss during the whole process of
synthesizing M-ZrPO. Therefore, the ratios of P/Zr in the samples
could be accurately controlled through changing the precursors.
UV-vis spectroscopy is used to characterize the coordination
modes presented in M-ZrPO. Fig. 8(1) showed UV-vis spectra of
Table 2 XRF analysis of the molar ratios of P/Zr in the M-ZrPO-x-500
Theoretical
values
As-synthesized
M-ZrPO
Calcined
M-ZrPO
Samples
M-ZrPO-1-500
1
0.94
0.70
0.48
0.25
0
0.98
0.71
0.49
0.27
0
M-ZrPO-0.75-500
M-ZrPO-0.5-500
M-ZrPO-0.25-500
M-ZrPO-0-500
0.75
0.5
0.25
0
Fig. 7 TG-DSC curves of the as-synthesized M-ZrPO-x: (1) M-ZrPO-0.75, (2) M-ZrPO-0.
Thermogravimetric-differential scanning calorimetry (TG-DSC)
analysis profiles of the as-synthesized sample M-ZrPO-0.75 are
given in Fig. 7(1). With the increase of temperature from room
temperature to 100 1C, the sample suffered gradual weight loss
of about 3% due to the desorption of physisorbed water,
corresponding to a weak endothermic peak in DSC curve.
Corresponding to the large weight loss stage between 300 1C
and 500 1C, a remarkable exothermic peak could be observed in
DSC curve. This was attributed to the combustion of SDAs. The
major weight-loss stages were observed below 500 1C with a total
weight loss about 60 wt%. After 500 1C, there was no weight loss
in the TG profile, indicating the complete removal of SDAs.
During 500 1C to 800 1C, there was no observed change in the
TG-DSC profiles, indicating no phase transition happened in the
structure of the M-ZrPO. Approximately at 900 1C, there was an
exothermic peak, implying the phase transition of the structure.
This conclusion was in agreement with the WXRD patterns of the
sample calcined at 500–900 1C. Compared with the M-ZrPO-0.75,
the M-ZrPO-0 (shown in Fig. 7(2)) had an exothermic peak at
550 1C, indicating the partial transition of the pore wall from t-ZrO2
to m-ZrO . This could be further confirmed by the characterization
2
Fig. 8 UV-vis diffuse reflectance spectra (1) of the M-ZrPO-x-500: (a) M-ZrPO-1-500,
b) M-ZrPO-0.75-500, (c) M-ZrPO-0.5-500, (d) M-ZrPO-0.25-500, (e) M-ZrPO-0-500;
2) of the M-ZrPO-0.75 treated at different temperatures: (a) M-ZrPO-0.75-500, (b)
of WXRD in Fig. S3 (ESI†). The introduction of phosphorus could
delay the occurrence of phase transition from 550 1C to 900 1C and
largely improved the thermal stability of M-ZrPO.
(
(
M-ZrPO-0.75-600, (c) M-ZrPO-0.75-700, (d) M-ZrPO-0.75-800, (e) M-ZrPO-0.75-900.
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M-ZrPO samples with different ratios of P/Zr. The samples from
M-ZrPO-0.25-500 to M-ZrPO-1-500 showed one absorption
peak at 205–210 nm, which was attributed to the ligand to
metal charge transfer transition (LMCT) from the outermost
2
À
4+
p electrons of oxygen (2p orbital of O ) to an isolated Zr (4d)
5
0,51
ion in a tetrahedral configuration of Zr–O–P coordination.
Further, the peaks in the long wavelength region (B280 nm)
could be due to the interaction of Zr cations with the
phosphate counteranions in the mesoporous framework.
The sample M-ZrPO-0-500 showed a broad peak at about
20 nm, this could be attributed to the oxygen-to-metal charge
transfer transition of Zr (Zr–O–Zr). The spectra of the
samples treated at different temperatures are shown in
Fig. 8(2). When the calcination temperature was lower than
4
+
3
0
2
4
+
52
800 1C, the patterns remained consistent with each other.
However a broad peak at 220 nm could be observed in the
sample M-ZrPO-0.75-900, indicating that Zr–O–Zr coordination
modes appeared after being treated at 900 1C. This conclusion
also could be obtained from WXRD pattern.
The FTIR spectra of the as-synthesized and M-ZrPO-0.75-500
are shown in Fig. 9(1). Compared with the as-synthesized
À1
À1
sample, the absorption bands at 2800–3000 cm , 1460 cm
1
,
À1
350 cm , which were assigned to the C–H stretching vibra-
tion of F127, disappeared after treating at 500 1C, indicating the
complete removal of SDAs. This conclusion agreed quite well
with the characterization by TG-DSC. The FTIR spectra of
M-ZrPO with different ratios of P/Zr are shown in Fig. 9(2).
À1
À1
The bands at 3450 cm
and 1634 cm
were assigned to
the asymmetric OH stretching and bending of the water
5
3
À1
molecule. The broad peak between 1000–1200 cm , corre-
3
5
sponded to the P–O stretching and bending vibration. The
weak peak around 758 cm could be ascribed to the P–O–P
À1
Fig. 9 FT-IR spectra (1) of as-synthesized M-ZrPO-0.75 and the M-ZrPO-0.75
treated at 500 1C; (2) of the M-ZrPO-x-500: (a) M-ZrPO-1-500, (b) M-ZrPO-0.75-
deforming vibrations, indicating the existence of P–O–P bond
À1
À1
in M-ZrPO-1-500. The bands 2400 cm and 528 cm was due
5
00, (c) M-ZrPO-0.5-500, (d) M-ZrPO-0.25-500, (e) M-ZrPO-0-500.
5
3
to (P)–O–H stretching and deformation vibration. With the
À1
decrease of P/Zr, the bands of 758 cm gradually disappeared,
indicating the structure of P–O–P vanished and the phosphorus
element was mainly existing as P–O bonds.
of M-ZrPO-0.75-500 showed peaks at 530.7 eV for Zr–O–P and
1
3,57
532.8 eV for P–O–H,
indicating that the main coordination
High-resolution XPS spectra were used to investigate the
chemical state of the surface elements in the M-ZrPO. Fig. 10
shows the spectra of Zr, P and O taken on the surface of
M-ZrPO. In Fig. 10(2), the Zr 3d signal of M-ZrPO-0.75-500
was composed of two signal peaks located at 182.2 eV for Zr
modes transformed from Zr–O–Zr to Zr–O–P with the introduc-
tion of phosphorus.
3.3 The roles of Zr–O–P bonds in ordered mesostructure and
thermostability
3
d5/2 and 184.6 for Zr 3d3/2, which were the characteristic
4
+ 54,55
of Zr .
However, the signal of M-ZrPO-0-500 showed After the EISA process, the colorless solution transformed to
2
2,39
Zr 3d5/2 at 181.4 eV and Zr 3d3/2 at 183.8 eV. From M-ZrPO-0- xerogel (shown in Fig. 11).
In the process of removing
500 to M-ZrPO-0.75-500, the Zr 3d changed to higher binding structure-directing agents (F127), the pore structure of M-ZrPO
energy and this indicated the electron density of Zr decreased partially shrunk due to thermal sintering and hence the specific
2
9,58
with the appearance of Zr–O–P linkages. In addition, the P 2p surface areas, pore sizes and volumes decreased.
Among
binding energy of M-ZrPO-0.75-500 was observed at 132.8 eV, the samples with different ratios of P/Zr, the specific surface
suggesting that phosphorus in the sample existed in a penta- areas, along with the pore sizes and volumes, increased from
5
+ 55
valent-oxidation state (P ). Furthermore, the O 1s signals are M-ZrPO-0-500 to M-ZrPO-0.75-500. As proved by UV-vis, FTIR
presented in Fig. 10(4). The signal of M-ZrPO-0-500 could and XPS characterizations, the main coordination modes of
be fitted as two peaks: the main peak at 529.4 eV was the the sample transformed from Zr–O–Zr to Zr–O–P due to the
Zr–O–Zr bond and the binding energy of 531.2 eV was the introduction of phosphorus. Therefore, we deduced that the
5
6
Zr–O–H on the surface of the sample. Meanwhile, the signal thermal sintering of the pore structure in the calcination
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Fig. 10 High-resolution XPS spectra (1) full spectrum, (2) Zr 3d, (3) P 2p, (4) O1s of the sample: (a) M-ZrPO-0.75-500, (b) M-ZrPO-0-500.
3.4 The acidic properties of M-ZrPO-0.75-500
3
NH -TPD was carried out to evaluate the acidic properties of the
calcined M-ZrPO-0.75-500. The characteristic TPD pattern of
M-ZrPO-0.75-500 (displayed in Fig. 12(1)) curve showed three
obvious peaks at different temperatures due to the existence of
weak, medium and strong acid sites. The result indicated
different acidic sites were present in the calcined M-ZrPO.
In addition, the adsorption of pyridine based on the solid
acid catalyst is one of the most important characterization
techniques to distinguish the Br o¨ nsted and Lewis acidity. The
pyridine-IR spectra of the M-ZrPO-0.75-500 (Fig. 12(2)) showed
À1
bands at 1540 and 1640 cm , which were the characteristic
Fig. 11 Schematic illustration of the difference of M-ZrPO-0 and M-ZrPO-0.75 in bands of pyridinium ion, indicating the presence of Br o¨ nsted
thermostability.
acid sites. In addition, the obvious bands at 1450 and
À1
1
610 cm were due to the pyridine attached to Lewis acid
À1
sites. The band at 1490 cm was a combination between two
separate bands at 1545 and 1450 cm , corresponding to
Br o¨ nsted and Lewis acid sites, respectively. The Br o¨ nsted
process could be prevented to some extent due to the existence
of Zr–O–P bonds. In addition, there was a decrease of the
specific surface area in the M-ZrPO when the ratio of P/Zr
exceeded 0.75. This might be owing to superfluous phosphorus
inhibited the formation of mesostructure in the EISA process.
Not only the textural property, but also the thermal stability of
M-ZrPO was improved with the introduction of phosphorus.
The ordered mesostructure of M-ZrPO-0 collapsed at the tem-
perature as low as 600 1C with the partial phase transition of
À1
59
and Lewis acidity of M-ZrPO-0.75-500 gotten from pyridine-IR
characterization at different desorption temperatures are
shown in Table 3. The acidity of Br o¨ nsted and Lewis gradually
decreased with the increase of the desorption temperature,
whereas, even treated at 350 1C, the Lewis acid sites still existed,
which indicated that the Lewis acidity was relatively strong
compared to the Br o¨ nsted acid sites.
the pore walls from t-ZrO2 to m-ZrO . However, the ordered
2
mesostructure could be maintained in the M-ZrPO-0.75 even
treated at 800 1C. It was the Zr–O–P bonds that delayed the
3.5 Catalytic study
occurrence of phase transition from 550 1C to 900 1C, therefore, The M-ZrPO will be promising solid acid catalysts toward many
the thermal stability of the sample was highly improved.
acid-catalyzed reactions due to their reasonable acid properties.
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Fig. 13 (1) Effect of catalyst amount on the ketal yield (reaction conditions:
cyclohexanone, 0.2 mol; ethylene glycol, 0.26 mol; 120 1C; 2 h); (2) Effect of
reaction temperature on the ketal yield (reaction conditions: cyclohexanone,
3
Fig. 12 NH -TPD profile (1) of the M-ZrPO-0.75-500; IR spectra (2) for
pyridine desorption on M-ZrPO-0.75-500 at different temperatures: (a) 150 1C,
b) 250 1C.
0
.2 mol; ethylene glycol, 0.26 mol; catalyst amount, 0.05 g; 2 h).
(
reaction in organic synthesis and the product ketal could be
used as fragrances, food additives and substances for CO
fixation.
The optimized reaction conditions for the ketalization of
cyclohexanone, such as the catalyst amount, reaction tempera-
ture and time, were investigated. Initially, the reaction was
carried out with different amounts of catalyst (shown in
Fig. 13(1)). The yield of ketal initially increased with the
increase of catalyst amount. The maximum yield of ketal
Table 3 The acidity of M-ZrPO-0.75-500 gotten from pyridine-IR at different
desorption temperatures
2
1
7,60
Desorption
temperature (1C)
Br o¨ nsted acidity
Lewis acidity
Total acidity
À1 a
À1 b
À1
(mmol g
)
(mmol g
)
(mmol g
)
1
2
2
3
50
00
50
50
62.5
44.3
0
324.3
239.1
117.1
44.0
386.8
283.4
117.1
44.0
0
a
2
b
C(pyridine on B sites) = 1.88IA(B)R /W. C(pyridine on L sites) =
2
À1
1
.42IA(L)R /W. C = concentration (mmol g
catalyst); IA(B, L) = (98%) was obtained with the catalyst amount up to 0.05 g.
À1
À1
integrated absorbance of B (at 1545 cm ) or L (at 1455 cm ) band
Moreover, the effect of reaction temperature over the catalytic
performance was investigated. With the increase of reaction
temperature from 90 1C to 120 1C, the yield of ketal increased
À1
(cm ); R = radius of catalyst disk (cm); W = weight of disk (mg).
From the characterizations of SXRD and nitrogen physisorp- from 50% to 98%. In addition, the kinetic plots of the reaction
tion, M-ZrPO-0.75-500 had the most uniform mesostructure are given in Fig. 14(1). The maximum yield was obtained at
and excellent textural properties with the largest specific sur- 120 min. Therefore, under the optimized reaction conditions,
face area and pore volume among all the samples, which might the molar ratio of cyclohexanone to M-ZrPO (based on Zr atom)
provide the abundant ‘‘accessible’’ active sites for the reactants. was 680 : 1 and the yield of ketal could reach 98% in 2 h at
Therefore, M-ZrPO-0.75-500 was chosen as a promising catalyst 120 1C. This catalytic activity was much higher than those
for the liquid-phase ketalization reaction. This is an important reported in Table 4.
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Table 4 Reaction between carbonyl compounds and ethylene glycol catalyzed
by M-ZrPO-0.75-500
a
Mole ratio of
Yield
Entry
1
Substrate
substrate : catalyst (%)
b
680 : 1
65 : 1
98.0
68.0
82.0
91.7
2
3
4
(ref. 17)
(ref. 61)
100 : 1
680 : 1
c
c
c
c
5
6
7
680 : 1
680 : 1
680 : 1
84.6
95.5
91.9
Fig. 14 (1) The kinetic plots of the ketalization of cyclohexanone with ethylene
glycol (reaction conditions: cyclohexanone, 0.2 mol; ethylene glycol, 0.26 mol;
catalyst amount, 0.05 g; 120 1C); (2) Reusability of the catalyst (reaction condi-
tions: cyclohexanone, 0.2 mol; ethylene glycol, 0.26 mol; catalyst amount, 0.05 g;
c
8
680 : 1
80.9
90.2
1
20 1C; 2 h).
d
9
1
340 : 1
340 : 1
In order to investigate the recyclability of the catalyst,
M-ZrPO-0.75-500 was recycled five times under the optimized
reaction conditions (shown in Fig. 14(2)). No significant decline
in catalytic activity was observed with respect to the fresh
catalyst. Moreover, as shown in Fig. S2 (ESI†), obvious diffrac-
tion peaks in SXRD pattern, typical H1-shaped hysteresis loops
and narrow pore size distribution of the used catalyst could be
observed, indicating the ordered mesostructure of the M-ZrPO-
d
d
0
85.2
11
340 : 1
80.1
a
b
Yield based on carbonyl compounds taken. Reaction conditions:
carbonyl compound, 0.2 mol; ethylene glycol, 0.26 mol; catalyst, 0.05 g;
0.75-500 was still maintained after five cycles. Meanwhile, the
c
specific surface area, pore size distribution and pore volume reaction temperature, 120 1C; reaction time, 2 h. Reaction conditions:
changed little, indicating the catalyst suffered little damage in
the acid-catalyzed reaction.
carbonyl compound, 0.2 mol; ethylene glycol, 0.26 mol; catalyst,
.05 g; reaction temperature, 120 1C; reaction time, 4 h. Reaction
conditions: carbonyl compound, 0.2 mol; ethylene glycol, 0.26 mol;
d
0
Moreover, different kinds of carbonyl compound, such as catalyst, 0.10 g; reaction temperature, 120 1C; reaction time, 4 h.
methyl ethyl ketone, cyclopentanone, cycloheptanone and benz-
aldehyde, were used as substrates to react with ethylene glycol
in this reaction (shown in Table 4). The M-ZrPO-0.75-500
showed excellent catalytic activity for the substrates. All the
4. Conclusion
ketal yields in the reaction were above 80%. Therefore, M-ZrPO A series of mesoporous zirconium oxophosphate was synthesized
was a series of ideal solid acid catalysts due to their excellent through a one-pot EISA strategy. The P/Zr ratios of the samples
catalytic activity.
could be accurately controlled through changing the precursors.
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