"Solvent-free" synthesis of thermally stable and hierarchically porous aluminophosphates (SF-APOs) and heteroatom-substituted aluminophosphates (SF-MAPOs)

Article abstract of DOI:10.1039/c1jm11451d

Hierarchically porous aluminophosphates (SF-APOs) and metal substituted aluminophosphates (SF-MAPOs, M = Co, Fe, Cr) have been synthesized via simple grinding and heating in the absence of solvent. Characterization results show that these mesoporous aluminophosphates have a hierarchically microporous/mesoporous structure. In addition, metal atoms can be efficiently incorporated into the walls of mesoporous aluminophosphates, and the SF-CoAPO sample shows high catalytic activity in cyclohexene oxidation compared with microporous samples. Special features of the "solvent-free" synthesis route, such as increasing product yield, saving energy, elimination of pollution, and convenience for incorporation of heterogeneous atoms, ensure its great potential in the synthesis of porous materials. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1jm11451d

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PAPER
‘‘Solvent-free’’ synthesis of thermally stable and hierarchically porous
aluminophosphates (SF-APOs) and heteroatom-substituted
aluminophosphates (SF-MAPOs)†
b
Pengling Zhang, Liang Wang, Limin Ren, Longfeng Zhu, Qi Sun, Jian Zhang, Xiangju Meng
b
b
b
a
b
a
*
a
*
and Feng-Shou Xiao
Received 6th April 2011, Accepted 24th May 2011
DOI: 10.1039/c1jm11451d
Hierarchically porous aluminophosphates (SF-APOs) and metal substituted aluminophosphates
(SF-MAPOs, M ¼ Co, Fe, Cr) have been synthesized via simple grinding and heating in the absence of
solvent. Characterization results show that these mesoporous aluminophosphates have a hierarchically
microporous/mesoporous structure. In addition, metal atoms can be efficiently incorporated into the
walls of mesoporous aluminophosphates, and the SF-CoAPO sample shows high catalytic activity in
cyclohexene oxidation compared with microporous samples. Special features of the ‘‘solvent-free’’
synthesis route, such as increasing product yield, saving energy, elimination of pollution, and
convenience for incorporation of heterogeneous atoms, ensure its great potential in the synthesis of
porous materials.
still a great challenge to prepare hierarchically porous alumi-
nophosphates from a single organotemplate.
Introduction
Since the discovery of the M41S family, series of ordered meso-
structured materials with versatile compositions, such as silica,
alumina, aluminosilicates, aluminophosphates, and transition
metal oxides, have been successfully fabricated from supramo-
lecular assemblies of inorganic species with surfactant micelles in
aqueous or alcoholic solution.1–5 Compared with mesoporous
silicas, most mesoporous aluminophosphates and heteroatom-
substituted aluminophophates synthesized in the early stage
exhibit relatively low thermal stability, which strongly hinders
their wide applications in catalysis and adsorption. To overcome
this problem, many approaches such as adjusting surfactants,6–9
preparing inorganic precursors,10,11 using ‘‘acid–base pair’’
mechanism12 etc, have been employed to successfully synthesize
thermally stable mesoporous aluminophosphates under hydro-
thermal and solvothermal conditions.
It is particularly emphasized that ordered mesostructured
materials including ordered mesoporous aluminophosphates are
usually assembled in the presence of solvents such as water and
alcohols, and the use of the solvent always produces liquid wastes
for the environment. Recently, we have reported a solvent-free
route for preparing nanosized sulfated zirconia catalysts,15 and
we demonstrate here a ‘‘solvent-free’’ synthesis of thermally
stable and hierarchically porous aluminophosphates and
heteroatom-substituted aluminophosphates from a mixture of
raw materials for the first time. This unique approach not only
saves solvent and energy in the synthesis, but also creates hier-
archical pores including micropores and mesopores from a single
surfactant template. Catalytic tests show that hierarchically
porous cobalt-substituted aluminophosphate is very active in the
catalytic conversion of cyclohexene.
Normally, ordered mesoporous aluminophosphates have
single mesopores, and recent results confirm that hierarchically
porous materials are very helpful for practical applications in
catalysis and adsorption.13 To create hierarchically porous alu-
minophosphate zeolites, both small amines and bulky organo-
silane surfactants have been successfully used.14 However, it is
Experimental section
Synthesis
The reagents with AR purity used in this work were aluminum
triisopropoxide [Al(OiPr)3], ammonium dihydrogen phosphate
(NH4H2PO4), tetramethyl ammonium hydroxide pentahydrate
(TMAOH$5H2O), hexadecyl trimethyl ammonium bromide
(CTAB), ferric nitrate nonahydrate [Fe(NO3)3$9H2O], chromic
nitrate nonahydrate [Cr(NO3)2$9H2O], and cobalt nitrate hexa-
hydrate [Co(NO3)2$6H2O].
aKey Lab of Applied Chemistry of Zhejiang Province, Department of
Chemistry, Zhejiang University (XiXi Campus), Hangzhou, 310007,
China. E-mail: mengxj@zju.edu.cn; fsxiao@mail.jlu.edu.cn
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
Jilin University, Changchun, 130012, China
As a typical run for synthesis of hierarchically porous alumi-
nophosphates, solid raw materials of 0.60 g of Al(OiPr)3, 0.32 g
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm11451d
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of NH4H2PO4, 0.40–0.60
g of CTAB, and 0.40 g of
TMAOH$5H2O, were firstly mixed together with the molar
composition at Al2O3:P2O5:(0.80–1.20)CTAB:TMAOH:5H2O.
After grinding at room temperature for 4–5 min, the mixture
became viscous. Continuous grinding for another 10 min led to
the formation of dry powder again, which was transferred into
a crucible. After heating in an oven (RT-150 ꢀC) for 5–27 h, the
sample was washed and collected by filtration, dried at 60 ꢀC,
ꢀ
and calcined at 500 C for 6 h. The sample was designated as
SF-APOx, where x stands for the reaction temperature.
The preparation process of heteroatom-substituted alumi-
nophosphates was similar to that of SF-APOx, but in the
presence of the source of heteroatoms such as Fe(NO3)3$9H2O,
Cr(NO3)2$9H2O, or Co(NO3)2$6H2O. The molar composition of
the final mixture was 0.9Al2O3:0.1M (M ¼ Fe, Co, Cr):
P2O5:0.6CTAB:TMAOH:(5.6–5.9)H2O. The sample was
designed as SF-MAPO.
For
comparison,
conventional
mesoporous
alumi-
nophosphates (C-APO) were hydrothermally synthesized at
room temperature.4a
Characterizations
Fig. 1 Small angle XRD patterns for (A) as-synthesized and (B) calcined
(a) SF-APORT, (b) SF-APO100 and (c) SF-APO150 samples. (P) The
presence of CTAB.
X-ray powder diffraction (XRD) patterns were measured with
ꢀ
a Rigaku X-ray diffractometer using Cu-Ka (l ¼ 1.54 A) radi-
ation. Transmission electron microscopy (TEM) experiments
were performed on a JEM-3010 electron microscope (JEOL,
Japan) with an acceleration voltage of 300 kV. The nitrogen
adsorption and desorption isotherms at the temperature of liquid
nitrogen were measured using Micromeritics ASAP 2010M and
Tristar system. The sample compositions were determined by
inductively coupled plasma (ICP) with a Perkin-Elmer plasma
40 emission spectrometer. Solid-state NMR experiments were
performed with magic angle spinning (MAS) on a JEOL JNM-
A-400WB spectrometer operating at frequencies of 104.17 and
161.83 MHz for 27Al and 31P, respectively. Chemical shifts were
good agreement with the mesostructured aluminophosphates
reported previously.2 After calcination at 500 C, these samples
ꢀ
still showed a strong peak at ca. 1.9ꢀ (Fig. 1B), indicating that
their mesostructure was thermally stable. For comparison,
conventional mesostructured aluminophosphate (C-APO)
completely lost its mesostructure after calcination at the same
ꢀ
temperature (500 C, Fig. S1, ESI†).
Fig. 2 shows TEM images of calcined SF-APOx samples,
giving direct evidence for worm-like mesopores in the samples,
which are well consistent with the results obtained from XRD
patterns. Fig. 3A shows nitrogen isotherms of calcined SF-APOx
samples, and their textural parameters are summarized in
Table 1. Notably, the isotherms for all samples appeared to be
a transition from type IV for mesoporous materials to type I for
microporous materials, exhibiting well-defined capillary
condensation at relative pressure (P/Po) of 0.01–0.25. Similar
phenomena have been observed in the isotherms of other
supermicroporous materials reported previously.16 Correspond-
ingly, they gave BJH pore size distribution at 1.7–1.8 nm
(Fig. 3B) and DFT pore size distribution at 2.7–2.8 nm (Table 1).
Particularly, HK pore distribution curves of these samples
showed additional pore size distribution at 0.60–0.74 nm, which
was associated with the presence of micropores (Fig. 3C). These
results suggested that the SF-APOx samples contained both
mesopores and micropores. Furthermore, it was observed that
the BET surface area and pore volume of the samples are
significantly influenced by the preparation temperature. For
example, the BET surface area and pore volume of calcined
SF-APO150 (710 m2 gꢁ1 and 0.44 cm3 gꢁ1, respectively) synthe-
sized at high temperature were much higher than those of
calcined SF-APORT (438 m2 gꢁ1 and 0.26 cm3 gꢁ1, respectively)
synthesized at low temperature.
referenced to an external standard of Al(H2O)6 for 27Al and
3+
85% H3PO4 for 31P. The sample was spun at 10 and 5.2 kHz for
27Al and 31P, respectively. A Perkin-Elmer TGA 7 unit was used
to carry out thermogravimetric analysis (TGA) in air at a heating
rate of 10 ꢀC minꢁ1
.
Catalytic tests
Cyclohexene oxidation was carried out in a 50 ml glass reactor
and stirred with a magnetic stirrer. Substrate, solvent, and
catalyst were mixed in the reactor and heated to the reaction
temperature. After reaction, the product was taken out from the
reaction system and analyzed by gas chromatography (GC-14C,
Shimadzu, using a flame ionization detector) with a flexible
quartz capillary column coated with OV-17.
Results and discussion
Hierarchically porous aluminophosphates (SF-APOs)
Fig. 1 shows small-angle XRD patterns of SF-APOx samples
prepared via the ‘‘solvent-free’’ route using CTAB surfactant as
a template before and after calcination. All samples exhibited
one broad peak at ca. 1.9ꢀ, suggesting worm-like mesopores, in
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Fig. 3 (A) N2 adsorption and desorption isotherms, (B) BJH, and (C)
HK pore size distribution for (a) SF-APORT, (b) SF-APO100 and (c)
SF-APO150 samples. The isotherms for (b), and (c) were offset by 100,
200 cm3 gꢁ1 along the vertical axis for clarity, respectively.
Fig. 2 TEM images for (a) and (b) SF-APORT, (c) SF-APO100 and (d)
SF-APO150 samples.
Fig. 4 shows 27Al and 31P MAS NMR spectra of the SF-APOx
samples. In the 27Al spectra, the samples exhibited two peaks at 7
and 42 ppm, assigned to 6- and 4-coordinated Al species,
respectively.17 In contrast, the 31P spectra exhibited only one
broad peak centered at about ꢁ10 ppm, associated with
4-coordinated P species.5b These results suggested that the
SF-APOx samples were composed of AlO4, AlO6, and PO4 units,
in good agreement with mesostructured aluminophosphates
synthesized in the presence of solvents.14,17
Fig. 5 shows TG-DTA curves of the SF-APOx samples. All
samples had two steps for weight loss. The first step occurs at
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Table 1 Textural parameters for SF-APOx samples synthesized at different temperatures (RT–150 ꢀC)
Sample
P/Al
BET/m2 gꢁ1
BJH pore size/nm
DFT/nm
HK pore size/nm
Pore volume/cm3 gꢁ1
Micropore volume/cm3 gꢁ1
SF-AlPORT
SF-AlPO100
SF-AlPO150
0.46 : 1
0.43 : 1
0.43 : 1
438
508
710
1.8
1.7
1.7
2.8
2.7
2.7
0.74
0.68
0.60
0.26
0.28
0.44
0.21
0.25
0.34
Fig. 4 (A) 27Al and (B) 31P MAS NMR spectra for (a) SF-APORT, (b)
Fig. 5 TG and DTA curves for (A) SF-APORT, (B) SF-APO100 and (C)
SF-APO100 and (c) SF-APO150 samples.
SF-APO150 samples.
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80–150 ꢀC associated with desorption of water. Correspondingly,
in the DTA curves there was an endothermic peak at 80–150 ꢀC.
ꢀ
The second step ranged at 200–550 C due to decomposition of
organic compounds such as TMA+, CTA+ cations or dehydra-
tion of the samples.6c,18 Notably, the samples synthesizedꢀ at
various temperatures have different weight loss at 200–550 C.
For example, SF-APORT, SF-APO100, and SF-APO150 exhibited
the weight loss at 65, 57, and 45%, respectively. The relatively low
weight loss for SF-APO150 might be related to a partial decom-
position of CTAB surfactant during the preparation process
at 150 C.19
ꢀ
Hierarchically porous heteroatom-substituted
aluminophosphates (SF-MAPOs)
To obtain the materials with catalytically active sites, the
‘‘solvent-free’’ route is applied to synthesize hierarchically porous
heteroatom-substituted aluminophosphates (SF-MAPOs). Fig. 6
shows small angle XRD patterns of SF-MAPOs (M ¼ Cr, Co,
Fe) synthesized at room temperature using CTAB surfactant as
a template before and after calcination. They exhibited a broad
peak at 1.3–1.5ꢀ, indicating the presence of worm-like meso-
pores. Fig. 7 shows nitrogen isotherms of SF-MAPOs samples,
and their textural parameters are summarized in Table 2. Simi-
larly, these samples exhibited typical adsorption curves of type
IV plus type I. A steep increase occurred in the curve at a relative
pressure less than 0.01, associated with the filling of micropores,
and another step can be identified in the adsorption curve at
a relative pressure (P/Po) of 0.2–0.5, which was due to the pres-
ence of mesopores. HK and BJH pore size distribution showed
that these samples were typically bimodal porous materials,
giving values of 0.6–0.7 and 2.1–2.6 nm, respectively. However, it
is notable that these hierarchically porous heteratom-substituted
Fig. 7 (A) N2 adsorption and desorption isotherms, (B) BJH and (C)
HK pore size distribution for (a) SF-CrAPO, (b) SF-CoAPO and (c)
SF-FeAPO samples. The isotherms for (b) and (c) were offset by 100,
180 cm3 gꢁ1 along the vertical axis for clarity, respectively.
aluminophosphates had relatively low surface areas, compared
with the aluminophosphate samples. This phenomenon was
possibly related to a loss of microporosity in the samples when
the heteroatoms are introduced. For example, SF-CrAPO had
a surface area of 160 m2 gꢁ1, which mainly results from the
mesoporosity (0.36 cm3 gꢁ1), and the contribution of the micro-
porosity (0.04 cm3 gꢁ1) was little.
Fig. 6 Small angle XRD patterns for (A) as-synthesized and (B) calcined
(a) SF-CrAPO, (b) SF-CoAPO and (c) SF-FeAPO samples. (P) The
presence of CTAB.
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Table 2 Textural parameters for SF-MAPO samples synthesized at room temperature
Sample P/Al/M
BET/m2 gꢁ1 BJH pore size/nm DFT/nm HK pore size/nm Pore volume/cm3 gꢁ1 Micropore volume/cm3 gꢁ1
SF-FeAPO 0.77 : 1 : 0.15 399
SF-CoAPO 0.67 : 1 : 0.08 202
SF-CrAPO 0.79 : 1 : 0.17 160
2.1
2.6
2.5
3.0
3.4
3.0
0.60
0.65
0.70
0.38
0.22
0.36
0.19
0.12
0.04
UV-Vis spectroscopy is a powerful tool for characterizing the
substitution of heteroatoms in aluminophosphates. Fig. 8 shows
UV-visible spectra of as-synthesized and calcined Fe, Co and Cr
substituted aluminophosphates (SF-MAPO). As-synthesized
SF-FeAPO (Fig. 8a) showed a strong broad peak at 280 nm and
a weak shoulder peak at 470 nm.20a The peak at 270 nm was
assigned to Fe3+ species in a tetrahedral network, and the peak at
470 nm was attributed to octahedral iron species in the samples.
As-synthesized SF-CoAPO (Fig. 8b) showed three adsorption
peaks at 540, 580 and 626 nm, respectively. The peak at 626 nm
can be assigned to the tetrahedral coordinated Co2+ species, and
the peak around 540 nm was attributed to the octahedral Co2+
species.20b Compared with the as-synthesized samples, calcined
SF-FeAPO and SF-CoAPO samples (Fig. 8d and 8e) had no
obvious difference in the spectra. In contrast, as-synthesized and
calcined SF-CrAPO samples were quite different (Fig. 8c and 8f).
The as-synthesized sample showed peaks at 221, 420, and
600 nm. The peak at 221 nm could be attributed to O / Cr6+
charge transfers of a chromate species; the peaks at 420 and
600 nm were assigned to d–d electron transitions from octahedral
Cr3+ described as A2g / T2g and A2g / T1g transitions,
respectively.20c Compared with the as-synthesized sample,
calcined SF-CrAPO had purple shifts, giving 252, 456, and
610 nm. In addition, a new peak at 355 nm appears, which is
usually assigned to a charge-transfer transition of O / Cr4+.20c
These results indicated that calcination of the as-synthesized SF-
CrAPO leads to partial transformation from Cr3+ to Cr4+ in the
sample.
4
4
4
4
It is well known that Fe, Co and Cr substituted zeolites are
active for catalytic oxidations. In this work, cyclohexene oxida-
tion was employed to test the catalytic properties of calcined
mesoporous Fe, Co and Cr substituted aluminophosphates
(SF-MAPO), as presented in Table 3. By using H2O2 as an
oxidant, the SF-FeAPO sample showed a low activity of 5% with
a major product of cyclohexanone (99%), and the SF-CrAPO
sample exhibited a relatively high catalytic activity of 24% with
cyclohexanone selectivity at 62%. Interestingly, although the
SF-CoAPO sample gave a relatively low catalytic activity (7%)
by using H2O2 as oxidant, this sample exhibited a very high
activity (60.0%) by using molecular oxygen as oxidant, which
was much higher even than that (23%) of microporous CoAPO-5
crystals under the same reaction conditions. Apparently, the
combination of using environmentally benign and highly effec-
tive molecular oxygen as an oxidant and high activity in catalytic
oxidations is potentially important for the production of fine
chemicals by green routes.21
Mechanism on the formation of hierarchically porous
aluminophosphates and heteroatom-substituted
aluminophosphates
To understand the mechanism of the ‘‘solvent-free’’ synthesis of
hierarchically porous aluminophosphates and hetroatom-
substituted aluminophosphates, in situ XRD patterns for
synthesizing SF-APORT were measured, as shown in Fig. 9.
When the raw materials of Al(OiPr)3, NH4H2PO4,
TMAOH$5H2O, and CTAB were mixed, the mixture exhibited
typical XRD peaks of each compound (Fig. 9B-a, Fig. S2†). By
increasing the aging time to 105 min at room temperature, the
wide-angle XRD peak intensity associated with raw materials
gradually reduced (Fig. 9B-b–f), and these peaks were very weak
at the aging time of 105 min (Fig. 9B-f). These results suggest that
the crystalline structure of the raw materials was gradually lost,
giving the amorphous nature of AlPO4, confirmed by 27Al and 31
P
MAS NMR spectra (Fig. 4). This phenomenon was in good
agreement with the spontaneous dispersion of solid inorganic
salts due to the increase of entropy (DS).22 In the meantime,
small-angle XRD patterns of the samples aged at 60 min
(Fig. 9A-c) showed a small and new peak at about 2.3ꢀ, and this
peak intensity increased with aging time. The appearance of the
Fig. 8 UV-Vis spectra for as-synthesized (a) SF-FeAPO, (b) SF-
CoAPO, (c) SF-CrAPO and calcined (d) SF-FeAPO, (e) SF-CoAPO, (f)
SF-CrAPO samples.
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Table 3 Catalytic properties for SF-MAPO samples in cyclohexene oxidation
Selectivity(%)
Entry
Catalyst
Conversion
cyclo-
-ol
-one
-diol
Others
1
2
3
4
5
SF-FeAPOa
5
24
7
60
23
–
6
–
17
9
1
19
1
20
32
99
62
99
52
56
–
13
–
8
3
–
–
–
3
SF-CrAPOa
SF-CoAPOa
SF-CoAPOa
Conventional CoAPO-5b
a
10 mmol of substrate, 10 mol of H2O2, 10 ml of MeCN as solvent, 80 ꢀC, 4 h. b 10 mmol of substrate, 1 atm O2, 10 ml of MeCN as solvent, 80 ꢀC, 24 h.
use of a solvent, which can significantly reduce the pollutants in
the production process of porous materials. (2) A simple and
convenient procedure. Normally, porous materials are synthe-
sized by a hydrothermal route, which includes the preparation of
gels and hydrothermal treatment. In contrast, the ‘‘solvent-free’’
route is only grinding and aging raw materials. (3) Saving energy.
The ‘‘solvent-free’’ route could be performed at room tempera-
ture, which greatly saves energy, compared with hydrothermal
synthesis at relatively high temperatures. (4) High product effi-
ciency. Generally, under hydrothermal conditions, the solvent
occupies a large space in reactors and partial nutrients such as
aluminates and phosphates still exist in the solvent, thus giving
a low yield of solid porous materials. However, the ‘‘solvent-free’’
route effectively saves space in the reactors and avoids dissolu-
tion of the nutrients in the solvent. Therefore, it is believed that
the ‘‘solvent-free’’ synthesis will be of great importance in
industrial processes in the future.
Conclusion
Fig. 9 (A) Small angle and (B) wide angle XRD patterns for (a) SF-
Thermally stable and hierarchically porous aluminophosphates
(SF-APO) and heteroatom-substituted aluminophosphates
(SF-MAPO, M ¼ Fe, Cr, Co) have been synthesized via
APORT-omin, (b) SF-APORT-45min, (c) SF-APORT-60min, (d) SF-APORT-
70min, (e) SF-APORT-90min, (f) SF-APORT-105min, (g) SF-APORT-2h and (h)
SF-APORT-27h, respectively.
2.3ꢀ peak was well consistent with CTAB micelles in amorphous
silicas and aluminophosphates.23 When the aging time was at
27 h, there was a series of very weak peaks in the wide-angle
XRD patterns, due to residual raw materials (Fig. S2†). After
washing with water, these weak peaks completely disappear
(Fig. S3†). It was difficult to observe the peak at 3.4ꢀ assigned to
CTAB raw material in the small-angle XRD patterns at 27h,
suggesting complete dispersion of CTAB in the samples.
All above results suggested that CTAB existed in the samples
in two forms: One was mono-dispersed, and another was as
aggregated micelles. After calcination at 500 ꢀC for 6 h, the
mono-dispersed CTAB molecules created the microporosity
(0.60–0.74 nm), and the aggregated CTAB molecules templated
the mesoporosity (1.7–2.6 nm), as proposed in Scheme 1.
Notably, the synthesis of hierarchically porous alumi-
nophosphates and heteroatom-substituted aluminophosphates
under the ‘‘solvent-free’’ route exhibits the following obvious
features: (1) A significant reduction of pollutants. Convention-
ally, a solvent is necessary for the synthesis of porous materials,
which always forms a large amount of waste liquor, which is
harmful for the environment. This work completely avoids the
Scheme 1 Schematic drawing for the formation of hierarchically porous
aluminophosphates (SF-APOs).
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a ‘‘solvent-free’’ route, and SF-CoMPO exhibits very high
activity in cyclohexene oxidation with molecular oxygen,
compared with Co-APO-5 zeolite. Compared with hydrothermal
synthesis, the ‘‘solvent-free’’ route has obvious advantages
including a significant reduction of pollutants, a simple and
convenient procedure, saving of energy, and a high product yield,
which are potentially important for the industrial production of
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This work is supported by the National Natural Science Foun-
dation of China (20973079 and 21003107), State Basic Research
Project of China (2009CB623507) and Fundamental Research
Funds for the Central Universities (2010QNA3035).
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