Compressed CO2 accelerated the synthesis of mesoporous heteroatom-substituted aluminophosphates for enhanced catalytic activity

Article abstract of DOI:10.1002/ejic.201402060

AFI-structured zeolites of aluminophosphates AlPO-5 and heteroatom-substituted aluminophosphates MeAPO-5 (Me = Fe, Co) have been synthesized in the presence of compressed CO₂, the synthesized materials exhibiting mesoporosity without the use of

Full text of DOI:10.1002/ejic.201402060

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DOI:10.1002/ejic.201402060
Compressed CO2 Accelerated the Synthesis of Mesoporous
Heteroatom-Substituted Aluminophosphates for Enhanced
Catalytic Activity
Fangxiao Wang,[a] Lin Liang,[b,c] Jun Ma,[b] Lei Shi,[a] and
Jianmin Sun*[a,b]
Keywords: Heterogeneous catalysis / Mesoporous materials / Aluminium / Zeolites / Oxidation
AFI-structured zeolites of aluminophosphates AlPO-5 and
heteroatom-substituted aluminophosphates MeAPO-5 (Me =
Fe, Co) have been synthesized in the presence of compressed
CO2, the synthesized materials exhibiting mesoporosity with-
out the use of additional mesotemplates. The mesoporosity
was introduced by CO2-in-water emulsions acting as meso-
template during the crystallization of the zeolites. In addition,
the crystallization time was shortened in compressed CO2
compared with the conventional synthesis in the absence of
CO2. The synthesized aluminophosphates MeAPO-5 (Me =
Fe, Co) displayed much higher activities in the oxidation of
styrene than conventional aluminophosphates MeAPO-5
(Me = Fe, Co) due to the presence of mesopores in the former
catalysts, which favor the diffusion of reactants and products.
The compressed CO2 method provides a simple, green, and
economic route to hierarchical zeolites, which are of great
importance for industrial applications.
commonly been used. The templates are then removed by
calcination after hydrothermal treatment to produce meso-
pores within the zeolite crystals. For example, Srivastava et
al. synthesized nanocrystalline MFI zeolites with intracrys-
tal mesopores by using alkyltriethoxysilane as template.[12]
Yang and Zhou and their co-workers reported the synthesis
of hierarchical and metal-substituted AlPO-5 by using gluc-
ose as the mesoporous template.[13,14] However, the high
cost of the organic templates for the formation of meso-
porosity and the complexity of the synthetic procedures li-
mit the far-ranging applications of these mesoporous zeo-
lites on an industrial scale. Therefore, the challenges associ-
ated with the synthesis of hierarchical porous materials by
facile and economic routes continue to be of great impor-
tance.
Supercritical or compressed CO2 has attracted consider-
able attention in recent years because of its availability, low
cost, nonflammability, and low toxicity, and it has been
widely used in different fields, such as extraction and frac-
tionation,[15] chemical reactions,[16–19] and materials sci-
ence,[20,21] among others.[22,23] To date, there have been a
few successful examples of the preparation of porous mate-
rials in the presence of compressed CO2.[24–28] Ming et al.
synthesized porous, hollow biphase γ-/α-Fe2O3 nanopar-
ticles in CO2-expanded ethanol solution,[27] mesoporous sil-
ica hollow spheres were prepared from CO2-in-water emul-
sion-templating in the presence of non-ionic block copoly-
mers,[28] and our group reported the synthesis of MFI zeo-
lites ZSM-5 in compressed CO2 for the first time. Moreover,
mesoporosity has been generated in crystalline MFI zeolites
Introduction
Zeolites are extensively employed in the fields of adsorp-
tion, separation, and heterogeneous catalysis because of
their intrinsic properties such as high surface area, narrow
micropore distribution, high thermal stability, and capacity
for being doped by different heteroatom ions, which gener-
ates heterogeneous active sites within structures with shape-
selective ability.[1] However, their relatively small and uni-
form micropores strongly influence the mass transfer of re-
actants and products, reducing the catalytic conversion of
bulky molecules. Recently, the synthesis of mesoporous zeo-
lites, which possess both the advantages of mesoporous ma-
terials and zeolites, has been the focus of attention for solv-
ing the existing shortcomings of zeolites. Successful exam-
ples of the synthesis of mesoporous zeolites are the use of
a mixture of various mesoscale templates with a zeolite pre-
cursor. Mesoscale templates such as mesoporous car-
bon,[2–7] organic/inorganic hybrid surfactants,[8] silylated
polymers,[9] and mesoscale cationic polymers[10,11] have
[a] Natural Science Research Center, The Academy of
Fundamental and Interdisciplinary Science, Harbin Institute of
Technology,
Harbin 150080, China
[b] State Key Laboratory of Urban Water Resources and
Environment, Harbin Institute of Technology,
Harbin 150090, China
E-mail: sunjm@hit.edu.cn
http://homepage.hit.edu.cn/pages/sunjianmin
[c] School of Life Sciences and Technology, Harbin Institute of
Technology,
Harbin 150080, China
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without the use of any mesoscale organotemplate, and the
mesopore volume could be adjusted by changing the pres-
sure of CO2.[29] Thus, the use of compressed CO2 has led
to a new route for the simple and economic synthesis of
mesoporous zeolites. In our continuing efforts to prepare
porous materials by novel methods, we report herein the
successful synthesis of AFI-structured aluminophosphates
AlPO-5 and heteroatom-substituted aluminophosphates
MeAPO-5 with improved crystallization rates and meso-
porosity in compressed CO2 without the use of any meso-
scale templates. As a result of the presence of mesopores in
Fe- and Co-doped AlPO-5, the increased rate of mass trans-
fer of reactants and products has led to an improvement in
the catalytic activity and selectivity of these materials in the
oxidation of styrene compared with ordinary Fe- and Co-
doped AlPO-5 materials synthesized in the absence of CO2.
The formation of mesoporosity in aluminophosphates syn-
thesized in compressed CO2 is apparently related to the
presence of CO2-in-water emulsions as mesotemplate, which
was confirmed by the absence of mesoporosity in AlPO-5
synthesized in the presence of N2 and in the absence of
compressed CO2 in control experiments.
Figure 1. XRD patterns of (A) as-synthesized and (B) calcined
samples of (a) C-AlPO-5, (b) AlPO-8-28, and (c) AlPO-10-15
(AlPO-x-y: x and y represent the CO2 pressure and crystallization
time).
Results and Discussion
The powder XRD patterns of the as-synthesized AlPO-
5 samples are shown in Figure 1A. All the AlPO-5 samples
exhibited diffraction peaks at 2θ values of 7.5, 12.9, 15.0,
19.8, 21.0, and 22.5°, which were assigned to the AFI struc-
ture of AlPO-5.[30] No peaks from other crystalline phases
were observed, which indicates that AlPO-5 zeolites were
successfully synthesized in compressed CO2. Moreover, the
diffraction peaks did not change after calcination at 550 °C
(Figure 1B), which suggests that the AlPO-5 samples syn-
thesized in compressed CO2 possess high thermal stability.
Noticeably, it only took 28 h for the crystallization of
AlPO-5 samples at a CO2 pressure of 8 MPa. This crystalli-
zation time is markedly shorter than the 48 h required for
the crystallization of conventional AlPO-5 in the absence of
compressed CO2. Furthermore, the crystallization time was
further reduced to 15 h when the CO2 pressure was in-
creased to 10 MPa, which indicates that compressed CO2
favors the fast crystallization of AFI zeolites. Similar en-
hancements of crystallization rates have been observed in
the synthesis of MFI zeolites.[29] The pH of the reaction gel
played a crucial role in the synthesis of the AFI zeolites.
The pH of the starting gel was 6.70 and decreased to 5.62
after crystallization in compressed CO2 at 8 MPa, which is
in the range of the optimum pH for the synthesis of AlPO-
5 zeolites. The pH decreased as a result of the dissolution of
CO2 in water and its partial ionization to form the acid.[29]
Figure 2 shows the nitrogen isotherms of the calcined
AlPO-5 samples. A steep increase in the isotherms can be
seen at relative pressures P/P0 in the range of 0.4–1.0 in the
samples of AlPO-5 synthesized in compressed CO2, which
is characteristic of the presence of mesopores. In contrast,
sample synthesized in the absence of compressed CO2 and
the N-AlPO-5 sample synthesized in N2. These results indi-
cate that the formation of mesoporosity in AlPO-5 zeolites
is strongly related to the use of compressed CO2. The textu-
ral properties of the calcined AlPO-5 samples are summa-
rized in Table 1. Notably, the mesoporous volume is signifi-
cantly influenced by CO2 pressure, as it increases remark-
ably from 0.03 to 0.15 cm3/g with CO2 pressure increasing
from 8 to 10 MPa. However, the microporous volume de-
creased markedly, further confirming that compressed CO2
is a key factor for the formation of mesoporosity. Because
of the decrease in microporosity, the BET surface area de-
creased from 104.5 to 66.2 m2/g with increasing CO2 pres-
sure. Correspondingly, the mesopore size distributions were
estimated to be around 30–50 nm by applying the BJH
method. Before the use of CO2 pressure, micelles were
Figure 2. Nitrogen isotherms of calcined samples of (a) C-AlPO-5,
(b) N-AlPO-5, (c) AlPO-8-28, and (d) AlPO-10-15. The isotherms
for (b), (c), and (d) are offset by 20, 40, and 40 cm3/g, respectively,
there were no obvious hysteresis loops in the C-AlPO-5 along the vertical axis for clarity.
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formed by using the protonated triethylamine (TEA) tem- shown in Figure 4. All the AlPO-5 samples were fully hy-
plate, which provided steric support for the microporous drated and display two peaks at δ = 37 and –11 ppm. The
channel of AFI zeolites through their specific molecular vi- intense signal at δ ≈ 37 ppm has been ascribed to the tetra-
brations in the early stages.[31] When the autoclave was hedral Al species in the AFI structure and the resonance
purged with compressed CO2, the gas quickly spread and peak at at δ ≈ –11 ppm has been assigned to hexacoordinate
partly dissolved in the water of the gel around the TEA Al, which indicates that the AlPO-5 sample is composed of
micelles to form CO2-in-water emulsions. Moreover, the AlO4 and AlO6 units.[34,35]
CO2-in-water emulsions were stabilized by the TEA micro-
porous-structured micelles. The existence of CO2-in-water
emulsions was proved by the phase behavior of the gel ob-
served in our previous work.[29] The introduction of meso-
pores possibly results from the CO2-in-water emulsions,
which act as mesoporous templates with TEA micelles act-
ing as microporous templates. After crystallization, TEA
was removed by calcination, and CO2 was depressurized,
and thus mesopores were introduced into the zeolite crystal.
Porous materials synthesized from such CO2-in-water emul-
sions have also been reported.[32,33]
Figure 4. Solid-state 27Al MAS NMR spectra of calcined samples
of (a) C-AlPO-5, (b) AlPO-8-28, and (c) AlPO-10-15.
Table 1. Textural parameters of the calcined AlPO-5 samples.
[a]
[b]
[c]
[d]
Sample
SBET
Vmeso
Vmicro
Dmeso
Compared with AlPO-5, heteroatom-substituted meso-
porous aluminophosphates generate catalytic redox or acid
sites.[14] The powder XRD patterns of calcined FeAPO-5
and CoAPO-5 samples are shown in Figure 5. All the dif-
fraction peaks of the samples can be assigned to AFI-struc-
tured AlPO-5 with no specific signals of metal oxide com-
pounds being observed, which indicates that the samples
are thermally stable, and the doped metals do not change
the structure of AlPO-5. The FeAPO-5 and CoAPO-5 sam-
ples crystallized within 28 h at 8 MPa; however, it took 48 h
for the crystallization of conventional MeAPO-5, which
suggests that the compressed CO2 favors the fast crystalli-
zation process.
[m2/g]
[cm3/g]
[cm3/g]
[nm]
C-AlPO-5
N-AlPO-5
AlPO-8-28
AlPO-10-15
113.8
21.4
104.5
66.2
0.002
0.007
0.034
0.150
0.055
0.010
0.044
0.015
–
–
30–50
30–50
[a] BET surface area. [b] Mesoporous volume. [c] Microporous vol-
ume. [d] Mesopore size distribution calculated from the desorption
branch by the BJH method.
The presence of mesopores in the AlPO-5 sample is di-
rectly evidenced by bright parts in the TEM image (Fig-
ure 3). Previously reported mesoporous AlPO-5 zeolites
were synthesized by using mesoscale templates.[7,13] How-
ever, in this work, we show the synthesis of mesoporous
AlPO-5 zeolites in pressurized CO2 without the use of any
mesoscale organotemplate. As organic mesoporous tem-
plates are not used during the synthesis, this novel synthetic
process is attractive for its cost-effective and environmen-
tally benign features.
Figure 3. TEM image of the AlPO-5 sample.
Figure 5. XRD patterns of calcined samples of (A) FeAPO-5 and
(B) CoAPO-5. (a) C-MeAPO-5, (b) MeAPO-5 (0.04), (c) MeAPO-
5 (0.08), and (d) MeAPO-5 (0.12). For the meaning of the numbers
Solid-state NMR measurements provided convincing evi-
dence for the coordination environment of aluminium. The
27Al MAS NMR spectra of calcined AlPO-5 samples are in parentheses, see footnotes [c] and [d] in Table 2.
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Figure 6 shows the nitrogen isotherms of the MeAPO-5 and CoAPO-5 samples give clear evidence for the existence
samples. The MeAPO-5 samples synthesized in compressed of mesopores (Figure 7), consistent with the results of the
CO2 exhibit typical type IV adsorption curves. A steep in- nitrogen isotherms.
crease can be seen at relative pressures P/P0 in the range of
0.4–1.0 in the samples of FeAPO-5, ascribed to the presence
of mesopores. In contrast, conventional FeAPO-5 displays
no hysteresis loop in this range. The BET surface area of
the FeAPO-5 sample synthesized in compressed CO2 is
76 m2/g, and its mesoporous volume is 0.20 cm3/g (Table 2).
The reduced surface area is related to the loss of micropo-
rosity compared with conventional FeAPO-5. Similarly,
CoAPO-5, which also possesses mesopores, shows charac-
teristic type IV isotherms, and its BET surface area was re-
duced to 120 m2/g due to the presence of mesopores, as
compared with 260 m2/g for conventional CoAPO-5, and
the mesoporous volume is 0.30 cm3/g (Table 2). Corre-
spondingly, the mesopore size distributions were estimated
to be around 20–60 nm by applying the BJH method. The
bright parts of the TEM images of the calcined FeAPO-5
Figure 7. TEM images of calcined (A) FeAPO-5 and (B) CoAPO-
5 samples.
UV/Vis spectroscopy is a powerful tool for characterizing
the substitution of heteroatoms in aluminophosphates. Fig-
ure 8 shows the UV/Vis spectra of the as-synthesized and
calcined Fe- and Co-substituted aluminophosphates. The
spectra of the as-synthesized FeAPO-5 samples show a
strong broad peak at 280 nm and a weak shoulder at
470 nm [Figure 8A (a) and (b)]. The peak at 280 nm has
been assigned to Fe3+ species in a tetrahedral network, and
the peak at 470 nm has been attributed to octahedral iron
species.[36,37] The spectra of the calcined FeAPO-5 samples
Figure 6. Nitrogen isotherms of calcined samples of (A) FeAPO-5
and (B) CoAPO-5. (a) C-MeAPO-5, (b) MeAPO-5 (0.04), (c) Me-
show no obvious difference to those of the as-synthesized
APO-5 (0.08), and (d) MeAPO-5 (0.12). The isotherms for (b), (c),
samples. The as-synthesized CoAPO-5 samples show four
absorption peaks at 480, 540, 580, and 626 nm [Figure 8B
(a) and (b)]. The peak at 626 nm has been assigned to tetra-
and (d) are offset by 30, 60, and 90 cm3/g, respectively, along the
vertical axis for clarity. For the meaning of the numbers in paren-
theses, see footnotes [c] and [d] in Table 2.
Table 2. Textural parameters of calcined MeAPO-5 samples.
SBET [m2/g]
Vmeso [cm3/g]
SMe/Al/P (molar ratio)
FMe/Al/P (molar ratio)
[a]
[b]
[c]
[d]
Sample
C-FeAlPO-5
FeAPO-5 (0.04)
FeAPO-5 (0.08)
FeAPO-5 (0.12)
C-CoAPO-5
CoAPO-5 (0.04)
CoAPO-5 (0.08)
CoAPO-5 (0.12)
88
76
135
27
260
120
125
19
0.004
0.20
0.17
0.14
0.006
0.30
0.09
0.13
0.04:0.96:1
0.04:0.96:1
0.08:0.92:1
0.12:0.88:1
0.04:0.96:1
0.04:0.96:1
0.08:0.92:1
0.12:0.88:1
0.03:0.92:1
0.04:0.84:1
0.08:0.90:1
0.11:0.89:1
0.03:0.91:1
0.04:0.85:1
0.04:0.91:1
0.13:0.89:1
[a] BET surface area. [b] Mesoporous volume. [c] Me/Al/P (molar ratio) in the starting gels. [d] Me/Al/P (molar ratio) in the final products.
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hedral coordination, and the peak at around 540 nm has perature, around 150 °C, which suggests that their acid
been attributed to the octahedral Co2+ species. The peak at strengths are higher than those of the C-MeAPO-5 samples
480 nm indicates the presence of octahedrally coordinated synthesized in the absence of CO2.[40]
extra-framework Co2+ species.[36] However, the calcined
Moreover, to determine the surface chemical states of the
CoAPO-5 samples show one broad peak between 450 and metals in the MeAPO-5 samples, XPS measurements were
700 nm, which also suggests the existence of tetrahedral and carried out. Peaks ascribed to Fe 2P3/2 and Fe 2P1/2 are ob-
octahedral coordinated cobalt.[38]
served at binding energies of 712.8 and 726.4 eV, respec-
tively (Figure 10A). The Fe 2P3/2–Fe 2P1/2 peak separation
is approximately 13.6 eV, which indicates the presence of
Fe3+ species in the FeAPO-5 sample.[41] In the case of Co-
APO-5, the incorporation of Co2+ was confirmed by the
typical characteristic peaks of Co 2P3/2 and Co 2P1/2 at
782.7 and 796.6 eV, respectively (Figure 10B). These results
are in good agreement with the results of the UV/Vis char-
acterizations.[42]
Figure 8. UV/Vis spectra for samples of (A) FeAPO-5 and
(B) CoAPO-5. (a) As-synthesized C-MeAPO-5, (b) as-synthesized
MeAPO-5 (0.04), (c) calcined C-MeAPO-5, (d) calcined MeAPO-
5 (0.04), (e) calcined MeAPO-5 (0.08), and (f) calcined MeAPO-5
(0.12).
Temperature-programmed desorption of ammonia
(NH3-TPD) was used to investigate the acid properties of
the samples (Figure 9). For AlPO-5, a desorption peak can
be seen at around 100 °C, which corresponds to the weak
acid site caused by Al3+ in the framework.[39] In the C-Me-
APO-5 sample, the low-temperature peaks at around 100 °C
are stronger, which implies that the introduction of metal
atoms into the AlPO-5 framework leads to an increase in
the number of weak acid sites. In the case of FeAPO-5 and
CoAPO-5, synthesized in compressed CO2, the peaks aris-
ing from the weak acid sites tend to shift to a higher tem-
Figure 10. (A) Fe 2P and (B) Co 2P XPS spectra of the MeAPO-5
(0.04) samples.
It is well known that Fe- and Co-substituted zeolites are
active in catalytic oxidation reactions. In this paper, the oxi-
dation of styrene by tert-butyl hydroperoxide (TBHP),
which is of great scientific and technological importance,
was employed as a model reaction to test the catalytic prop-
erties of these zeolites. The results are presented in Table 3.
In the blank experiment, the conversion of styrene was low
at 10.6% without any catalyst. With C-FeAPO-5 and C-
CoAPO-5 as catalysts, the conversions increased to 51.4
and 59.8%, respectively, which indicates that Fe and Co are
the active sites in the oxidation reaction. Noticeably, the
conversion and selectivity to styrene oxide increased signifi-
cantly with mesoporous MeAPO-5 catalysts. The styrene
conversionincreasedto72.1%withFeAPO-5andto78.9%with
CoAPO-5. Correspondingly, the selectivity to styrene oxide
was enhanced to 24.9 and 26.1%, respectively, and the
selectivity to phenylacetaldehyde decreased. In addition to
the oxidation properties of the metal-doped AlPO-5 cata-
Figure 9. NH3-TPD profiles for calcined samples of (a) AlPO-5,
(b) C-FeAPO-5, (c) C-CoAPO-5, (d) FeAPO-5, and (e) CoAPO-5.
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lysts, the acidity of the catalysts also had effects on the ac- green, and economic procedure for the synthesis of mesopo-
tivity and product selectivity. The enhanced catalytic ac- rous zeolites, which are of great importance for industrial
tivity and selectivity in the oxidation of styrene to styrene applications of reactions involving bulky molecules.
oxide may be related to the greater acidity in the MeAPO-
5 samples compared with the C-MeAPO-5 samples, as ob-
Experimental Section
served by NH3-TPD, which favors the epoxidation of styr-
ene to styrene oxide and hinders the isomerization of styr-
ene oxide to phenylacetaldehyde (Scheme 1). Although the
oxidation of styrene is not a diffusion-controlled reaction,
considering the similar crystal sizes (within 0.5–1 μm), tex-
tural parameters, and crystallinity of MeAPO-5 to micro-
porous conventional MeAPO-5, the high catalytic activity
over MeAPO-5 has been assigned to the presence of hierar-
chical mesoporosity in the sample, which may accelerate the
mass transport of reactants and products in the oxidation
of styrene.
Preparation of AFI-Structured Mesoporous AlPO-5 Zeolites in
Compressed CO2: Aluminium triisopropoxide [Al(OiPr)3], ortho-
phosphoric acid (H3PO4), triethylamine (TEA), hydrofluoric acid
(HF), ferric nitrate nonahydrate [Fe(NO3)3·9H2O], and cobalt
nitrate hexahydrate [Co(NO3)2·6H2O] were of AR purity.
In a typical run for the synthesis of aluminophosphate AlPO-5 in
compressed CO2, first, Al(OiPr)3 (4.184 g) was dissolved in deion-
ized water (18 g), then dilute H3PO4 (0.196 g) and TEA (0.304 g)
were added dropwise, and the mixture was stirred at ambient tem-
perature for 2 h. Then dilute HF solution (8 mg) was added with
continuous stirring for a further 2 h, and subsequently the mixture
was transferred to a 50 mL stainless-steel autoclave. The molar
Table 3. Catalytic activity in styrene oxidation reactions over vari-
ous catalysts.[a]
composition of the starting gel was Al/P/TEA/HF/H2O
=
1.0:1.0:1.5:0.2:50. The 50 mL autoclave was heated to 170 °C, pres-
surized with CO2 up to 8 MPa, and crystallization occurred at
170 °C for 28 h. CO2 was introduced into the free volume over the
liquid gel, but during the reaction process, some CO2 dissolved in
the liquid gel to form a CO2-in-water emulsion as the mesoporous
template. Furthermore, the gel was not moved during the crystalli-
zation, and after the crystallization, the CO2 in and above the gel
was released by depressurization. Finally, the solid products were
separated by filtration, dried at room temperature, and calcined at
550 °C for 6 h. The samples were designated as AlPO-x-y, in which
x and y represent the CO2 pressure and crystallization time, respec-
tively.
Catalyst
Conversion [%]
Selectivity [%]
BA PA
SO
Others
Without
C-FeAPO-5
10.6
51.4
72.1
78.5
82.4
59.8
78.9
81.8
85.4
5.9 28.1 0.9
15.9 52.6 13.5
24.9 65.1 2.6
31.6 52.1 4.4
56.4 39.5 2.2
18.7 58.8 11.4
26.1 63.2 2.1
35.6 48.3 5.2
59.9 32.6 3.4
65.1
18.0
7.4
11.9
1.9
11.1
8.6
10.9
4.1
FeAPO-5 (0.04)
FeAPO-5 (0.08)
FeAPO-5 (0.12)
C-CoAPO-5
CoAPO-5 (0.04)
CoAPO-5 (0.08)
CoAPO-5 (0.12)
[a] Reaction conditions: catalyst (90.1 g), styrene (10 mmol), TBHP
(15 mmol), 60 °C, 3 h. SO = styrene oxide; BA = benzaldehyde; PA
= phenylacetaldehyde; Others = acids etc.
Heteroatom-substituted aluminophosphates MeAPO-5 (Me = Fe,
Co) were synthesized similarly in the presence of Fe(NO3)3·9H2O
or Co(NO3)2·6H2O with starting gel molar compositions of Al/Me/
P/TEA/HF/H2O = (1 – n):n:1.0:1.5:0.2:50. The sample was desig-
nated as MeAPO-5 (n), in which n represents the amount of metal
added. The autoclave was pressurized with CO2 up to 8 MPa, and
the samples crystallized at 170 °C for 28 h. Finally, the solid prod-
ucts were separated, dried and calcined at 550 °C for 6 h.
For comparison, conventional syntheses of AlPO-5 and MeAPO-5
(Me = Fe, Co) in the absence of compressed CO2 were synthesized
as reported in the literature at 180 °C for 48 h[43,44] and designated
as C-AlPO-5 and C-MeAPO-5. To investigate the effect of atmo-
sphere on the structures of the zeolites, AlPO-5 was synthesized at
170 °C for 28 h under N2 instead of CO2 and designated as N-
AlPO-5.
Scheme 1. Proposed reaction routes in the epoxidation of styrene
over MeAPO-5.
Characterizations: Powder X-ray diffraction (PXRD) analysis was
performed with a Bruker D8 Advance X-ray powder diffractometer
with Cu-Kα radiation (40 kV, 40 mA). Nitrogen adsorption/desorp-
Conclusions
AFI-structured aluminophosphates and heteroatom-sub- tion isotherms were measured at –196 °C with an ASAP 2020 volu-
metric analyzer. Before analysis, the samples were outgassed at
200 °C under vacuum for 12 h. The surface areas were calculated
by using the Brunauer–Emmett–Teller (BET) method, and the total
pore volumes were determined from the amount of nitrogen ad-
sorbed at P/P0 ≈ 0.99. TEM experiments were performed with a
Tecnai G2 Spririt electron microscope with an acceleration voltage
of 120 kV. The sample compositions were determined by induc-
tively coupled plasma (ICP) using a Perkin–Elmer plasma 40 emis-
sion spectrometer. Solid-state NMR experiments were performed
with a Varian Infinity plus-400 spectrometer equipped with a 4 mm
stituted aluminophosphates with mesoporosity have been
successfully synthesized in the presence of compressed CO2.
The mesoporosity formed in these zeolites is related to the
presence of CO2-in-water emulsions acting as templates.
Moreover, the crystallization time was shortened in the
presence of compressed CO2. Owing to the existence of
mesopores, Fe- and Co-substituted AlPO-5 catalysts dis-
played higher activity in the oxidation of styrene than con-
ventional catalysts obtained in the absence of compressed
CO2. The compressed CO2 approach provides a simple, magic angle spinning (MAS) probe. 27Al MAS NMR spectra were
Eur. J. Inorg. Chem. 2014, 2934–2940
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
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Acknowledgments
We sincerely acknowledge the financial support from the National
Natural Science Foundation of China (21073049, 21373069), the
New Century Excellent Talents in University (NCET-10-0064), the
State Key Laboratory of Urban Water Resources and Environment
of the Harbin Institute of Technology (HIT2013TS01), the Open
Fund of the Key Laboratory of Functional Inorganic Materials
Chemistry (Heilongjiang University) and the Fundamental Re-
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Received: February 15, 2014
Published Online: May 20, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim