Mesoporous Mn-Zr composite oxides with a crystalline wall: Synthesis, characterization and application

Article abstract of DOI:10.1039/c4dt03835e

Mesoporous Mn-Zr composite oxides (M-MnZr) with a crystalline wall were designed and achieved by a facile one-pot evaporation-induced self-assembly (EISA) strategy. As proved by XRD, HRTEM and SAED characterization, the wall of the obtained mesoporous materials exhibited a typical tetragonal phase of ZrO₂. In addition, the introduced manganese species were homogeneously dispersed in the mesoporous skeleton. N₂-physisorption and TEM results showed that all the final materials possessed an obvious mesoporous structure accompanied by a large specific surface area (～120 m² g^-1), big pore volume (～0.2 cm³ g^-1) and uniform pore size (～4.9 nm). In addition, the liquid phase oxidation was chosen as the test reaction and the excellent catalytic performance of M-MnZr demonstrated their potential applications in oxidation reactions. This journal is

Full text of DOI:10.1039/c4dt03835e

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Mesoporous Mn–Zr composite oxides with a
crystalline wall: synthesis, characterization and
Cite this: Dalton Trans., 2015, 44,
2997
application†
a,b
Received 14th December 2014,
Accepted 6th January 2015
a
a
a
a,c
Zhichao Miao, Huahua Zhao, Jian Yang, Jun Zhao, Huanling Song and
a,c
DOI: 10.1039/c4dt03835e
Lingjun Chou*
www.rsc.org/dalton
Mesoporous Mn–Zr composite oxides (M–MnZr) with a crystalline The non-siliceous mesoporous manganese based materials
wall were designed and achieved by a facile one-pot evaporation- with a crystalline wall have potential to be investigated.
induced self-assembly (EISA) strategy. As proved by XRD, HRTEM
and SAED characterization, the wall of the obtained mesoporous thesize mesoporous Mn–Zr composite oxides (M–MnZr) with
materials exhibited a typical tetragonal phase of ZrO . In addition, different contents of manganese (0–15%). After removing the
the introduced manganese species were homogeneously dispersed SDAs at 550 °C, the M–MnZr materials were obtained. The
in the mesoporous skeleton. N -physisorption and TEM results XRD, HRTEM and SAED characterization techniques were
Herein, we employed a facile one-pot EISA strategy to syn-
2
2
showed that all the final materials possessed an obvious meso- employed to investigate the crystalline framework of the pore
porous structure accompanied by a large specific surface area wall and the dispersion of manganese species. N -physisorp-
2
2
3
(
∼120 m g−1), big pore volume (∼0.2 cm g−1) and uniform pore tion and TEM characterization methods were used to clarify
size (∼4.9 nm). In addition, the liquid phase oxidation was chosen the mesoporous properties of materials. In addition, the
as the test reaction and the excellent catalytic performance of resulting materials were tested as catalysts in liquid phase oxi-
M–MnZr demonstrated their potential applications in oxidation dation of ethylbenzene. More details would be described
reactions.
specifically in the main text.
2
. Experimental section
1
. Introduction
(
M
(
EO)106(PO)70(EO)106 triblock copolymer (Pluronic F127,
av = 12 600, Sigma-Aldrich), zirconyl chloride octahydrate
ZrOCl ·8H O, ≥99.5%, Tianjin Fengyue Reagent Company),
Since the discovery of the M41S family in 1992, mesoporous
materials have been widely investigated, owing to their
superior textural properties, such as high surface areas,
2
2
1
–4 manganese chloride tetrahydrate (MnCl ·4H O, ≥99.0%, Sino-
2
2
regular pore frameworks with a large pore size and volume.
2 5
pharm Chemical Reagent Co. Ltd), ethanol (C H OH, ≥99.7%,
On account of the excellent sorption, catalytic, magnetic, elec-
trochemical, optical and biosensing properties of manganese
species, its mesoporous materials attract continuous attention
Sinopharm Chemical Reagent Co. Ltd). All the reagents are of
A.R. grade and used as received without further purification.
M–MnZr was synthesized via a one-pot evaporation-induced
5
–8
in these fields.
However, most reports about mesoporous
1
1
self-assembly (EISA) strategy.
In a typical procedure of
manganese based materials are based on the mesoporous sili-
9
,10 synthesizing M–MnZr, 1.2 g of F127 was employed as struc-
ture-directing agents (SDAs) and dissolved in 15 mL of anhy-
drous ethanol. As the SDAs were completely dissolved, a mmol
ceous materials, such as MCM-41, SBA-15 and KIT-6.
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
2 2 2 2
of ZrOCl ·8H O and b mmol of MnCl ·4H O (a + b = 5 mmol)
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
were added into the above solution and stirred for 6 h at least.
Finally, the solution mixture was transferred to a petri dish
(d = 9 cm) to undergo the slow EISA process at 60 °C for 48 h,
then at 100 °C for 24 h in a drying oven. The obtained xerogel
was calcined at 550 °C for 6 h to remove the SDAs. The
obtained template-free self-assembled M–MnZr was denoted
as M–XMnZr and the nominal molar ratio X% = (b/(a + b) ×
100%).
b
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China
c
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
Suzhou 215123, People’s Republic of China
†
Electronic supplementary information (ESI) available: The reaction formula of
liquid phase oxidation of ethylbenzene; N -physisorption and TEM characteriz-
2
ation of the used catalyst. See DOI: 10.1039/c4dt03835e
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Thermogravimetric-differential
scanning
calorimetry
(TG-DSC) measurements were carried out on a NETZSCH STA
4
1
49F3 thermogravimetric analyzer from room temperature to
000 °C with the rate of 10 °C min under an air atmosphere.
−1
Powder X-ray diffraction (XRD) measurements were performed
using an X’Pert Pro Multipurpose diffractometer (PANalytical,
Inc.) with Cu Kα radiation (0.15406 nm) at room temperature
from 10.0° to 80.0°. The nitrogen adsorption and desorption
isotherms at −196 °C were recorded on an Autosorb-iQ analy-
zer (Quantachrome Instruments U.S.). The specific surface
areas were 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 iso-
therm at a relative pressure of 0.990. Pore size distributions
were calculated using adsorption branches of nitrogen adsorp-
tion–desorption isotherms by the Barrett–Joyner–Halenda Fig. 1 TG-DSC profiles of the as-synthesized M–10MnZr.
BJH) method. Transmission electron microscopy (TEM), high-
(
resolution transmission electron microscopy (HRTEM),
selected area electron diffraction (SAED) and energy-dispersive
X-ray spectroscopy (EDX) measurements were performed on
(
F127). The last weight loss step (∼10 wt%) and an exothermic
peak at 350–510 °C might be related to the dehydroxylation of
OH during the formation of the crystalline structure. After
10 °C, there was no obvious weight loss in the TG profile,
2
a TECNAI G F20 high-resolution transmission electron
–
microscope under a working voltage of 200 kV. The element
compositions were analyzed by X-ray Fluorescence (XRF) spec-
trometry on a Magix PW2403 (PANalytical, Inc.).
The liquid phase oxidation of ethylbenzene was carried out
in a round bottom flask fitted with a water condenser. Initially,
M–XMnZr (200 mg), acetonitrile (15 mL), ethylbenzene
5
implying the SDAs were completely removed.
The crystalline structure of M–MnZr after removing the
SDAs was investigated by the XRD technology. As shown in
Fig. 2, obvious diffraction peaks could be clearly observed in
these XRD patterns, indicating the nanocrystalline nature of
the obtained M–MnZr. For the M–2MnZr sample, there existed
two groups of diffraction peaks. The strong diffraction peaks at
(9 mmol) and tert-butyl hydroperoxide (36 mmol) were added
into a 50 mL round bottom flask. Then the reaction mixture
was stirred at 80 °C for 12 h and the products were centrifuged
and extracted with ether. The conversion of ethylbenzene and
the selectivity of products were monitored using a gas chrom-
atography (GC) instrument (Agilent-7890A; equipped with a
flame ionization detector (FID) and HP-5 column (30 m ×
3
0.2°, 35.0°, 50.3°, 60.2° and 74.8° (JCPDS card no. 80-0784)
belonged to the ZrO with the tetragonal phase (t-ZrO ). The
weak diffraction peaks which belonged to the ZrO
2
2
2
with the
monoclinic phase (m-ZrO
5.5° (JCPDS card no. 83-0944). This indicated that the pore
wall mainly existed as the type of t-ZrO and still existed as a
part of m-ZrO in the M–2MnZr sample. Further increasing the
2
) appeared at 24.1°, 28.3°, 40.9° and
5
0
.32 mm × 0.25 μm)) using octane as the external reference.
A gas chromatography-mass spectroscopy (GC-MS) instrument
5975c vl MSD with a triple-axis detector, GC Agilent-7890A)
2
2
(
was employed to identify the reaction products. The separated
catalyst was washed with ether and activated at 400 °C for 3 h
for regeneration.
3
. Results and discussion
TG-DSC profiles of the as-synthesized M–MnZr sample
recorded in air were first employed to investigate the calcina-
tion process. As shown in Fig. 1, the TG curve of the as-syn-
thesized M–10MnZr had three stages for the weight loss and
the DSC curve had one endothermal peak and three exother-
mic peaks at different temperatures. The first weight loss step
(
∼3 wt%) accompanied by an endothermal peak at 50–200 °C
was observed. This was attributed to the loss of physically
adsorbed water and other volatile species generated in the
EISA process. The second step represented the largest weight
loss (∼55 wt%) and two obvious exothermic peaks in the range
Fig. 2 XRD patterns of M–XMnZr: (a) M–2MnZr, (b) M–5MnZr, (c) M–
2
00–350 °C. This was caused by the decomposition of SDAs 7MnZr, (d) M–10MnZr, (e) M–15MnZr.
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Table 1 Textural properties of M–MnZr obtained from N
2
-physisorption, XRD and catalytic performance in liquid phase oxidation of
ethylbenzene
2
N -physisorption
XRD
Catalytic performance
Specific
surface
Pore
size
(nm)
Pore
Interplanar
spacing
of (101) (nm)
Average
crystalline
sizes (nm)
volume
Conversion
of EB (%)
Selectivity
of AP (%)
area (m g−1)
2
(cm g−1)
3
Samples
M–2MnZr
M–5MnZr
M–7MnZr
M–10MnZr
M–15MnZr
113.0
120.1
115.6
119.0
130.3
4.89
4.90
4.89
4.89
4.90
0.18
0.19
0.18
0.17
0.18
0.297
0.296
0.295
0.295
0.294
6.10
5.45
5.37
5.08
4.46
52.6
67.6
80.0
82.2
80.0
44.1
67.1
80.1
88.8
84.0
manganese contents to 5, 7, 10 and 15%, the peaks of m-ZrO2
became inconspicuous and completely vanished in the M–MnZr with different manganese contents were character-
M–10MnZr sample. This might be owing to that the intro- ized by N -physisorption. N adsorption–desorption isotherms
The mesoporous structure and textural properties of
2
2
duced manganese species were in favor of maintaining the and pore size distributions (PSD) are shown in Fig. 3. It was
crystalline phase and preventing the transformation of the observed that all the samples displayed the typical IV isotherm
pore wall from t-ZrO
peaks of t-ZrO2 became broader and the average crystalline Meanwhile, the hysteresis loops of all the samples were inter-
sizes calculated from the Scherrer equation (shown in Table 1) mediate between typical H1 and H2-type in the P/P range
2 2
to m-ZrO . In addition, the diffraction curve, which was the characteristic of mesoporous materials.
0
became smaller with the increasing manganese contents. from 0.4 to 0.8, suggesting the presence of cage-like mesopores
Moreover, the position of diffraction peaks slightly shifted to or mesopores with constriction. In addition, all the isotherms
higher 2θ values with the introduction of manganese species. exhibited steep capillary condensation steps, which indicated
This phenomenon implied that the interplanar spacing and the presence of uniform mesopores. This also could be further
cell parameters became smaller because of the isomorphous confirmed from the PSD and TEM images. As shown in
of zirconium with manganese species, which had a smaller Fig. 3(2), the pore size narrowly distributed around 5 nm, once
4
+
3+
2+
ionic radius (0.53, 0.64, 0.83 Å for Mn , Mn , Mn and again reflecting the presence of uniform mesopores and the
4
+ 12,13
0
.84 Å for Zr ).
were no diffraction peaks related to the manganese species in images. Moreover, the textural properties of M–MnZr obtained
the XRD patterns. This might be owing to that the introduced from N adsorption–desorption isotherms are given in Table 1.
manganese species were highly dispersed in the mesoporous All the samples showed excellent textural properties with a
In addition, it is noteworthy that there pore dimension is in good agreement with the following TEM
2
2
−1
skeleton of M–MnZr. The highly dispersed manganese species large specific surface area (∼120 m
might offer plentiful active sites for the reactants. (∼4.9 nm) and a large pore volume (∼0.2 cm g ). In addition,
g ), uniform pore size
3
−1
2
Fig. 3 (1) N -physisorption isotherms and (2) pore size distributions of M–XMnZr: (a) M–2MnZr, (b) M–5MnZr, (c) M–7MnZr, (d) M–10MnZr,
(
e) M–15MnZr.
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Fig. 4 TEM images of M–XMnZr: (a) M–2MnZr, (b) M–5MnZr, (c) M–7MnZr, (d, f) M–10MnZr, (g) M–15MnZr; (e) HRTEM image; (h) SAED pattern
and (i) EDX measurement of M–10MnZr.
the textural parameters changed little with the increasing were gradually improved with the increasing manganese con-
manganese contents. This might be due to that the manga- tents and the optimal catalytic performance was obtained at
nese species were highly dispersed in the mesoporous walls 10% manganese content with 82.2% conversion of EB and
and had little influence on the mesoporous structure.
88.8% selectivity of AP. The excellent catalytic performance
TEM images were obtained to further confirm the presence might be owing to the highly dispersed manganese species,
of mesopores in M–MnZr. As shown in Fig. 4, all the samples which could offer sufficient active sites for the reactants.
with different manganese contents showed an obvious meso-
As for a promising heterogeneous catalyst, the heterogen-
porous structure along [110] and [001] orientations. This eity and recyclability of M–10MnZr were investigated. As
phenomenon further confirmed the prediction of N -physi- shown in Fig. 5(1), there was no obvious increase in the yield
2
sorption. In addition, as shown in Fig. 4e, a highly crystalline of AP after removing the catalyst from the reaction system at
mesoporous framework could be clearly observed and the 4 h. However, the reaction recovered and the yield of AP
lattice spacing obtained from the HRTEM image was increased when the catalyst was added again. This indicated
0
.296 nm, keeping consistent with the value of 0.295 nm for that the M–MnZr was truly a heterogeneous catalyst and the
the (101) planes of t-ZrO in the XRD patterns. As shown in reaction was only being possible in the presence of M–MnZr.
2
Fig. 4h, the SAED analysis of M–10MnZr, which exhibited Meanwhile, compared with the fresh catalyst (10.6%), the Mn
obvious diffraction rings, further confirmed that the pore walls contents showed little change in the used catalyst (10.8%),
of the obtained materials were made up of nanocrystalline demonstrating that no obvious manganese species leached
oxides. In addition, the d-spacing values obtained from SAED into the liquid phase in the reaction process.
diffraction rings are in good agreement with the (101), (110),
The M–10MnZr was recycled for five runs to evaluate the
(
112) and (211) planes of t-ZrO in the XRD patterns. In the recyclability of the catalyst and the results are shown in
2
EDX measurement of M–MnZr (Fig. 4i), the exclusive peaks of Fig. 5(2). Compared with the fresh catalyst, no significant
Mn and Zr elements were obviously observed in the pattern, decrease occurred in the catalytic performance even after five
showing that all the elements were successfully introduced runs, showing that the M–MnZr could be reused as a hetero-
into the framework of the mesostructure.
geneous catalyst in the liquid phase oxidation of EB. In
-phy-
The M–MnZr materials were employed as catalysts for the addition, the regenerated catalyst was characterized by N
2
liquid phase oxidation of ethylbenzene (as shown in Fig. S1†), sisorption and TEM (shown in Fig. S2 and S3†). The results
which was an important reaction in pharmaceutical and fine- manifested that the mesoporous structure with excellent tex-
1
4,15
2
−1
chemical industries.
As shown in Table 1, the conversion tural properties (specific surface area (99.9 m g ), pore size
3
−1
of ethylbenzene (EB) and the selectivity of acetophenone (AP) (4.90 nm) and volume (0.15 cm g )) and the crystalline pore
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catalytic performance in the liquid phase oxidation of
ethylbenzene.
Acknowledgements
The authors sincerely acknowledge the financial support from
the “Strategic Priority Research Program” of the Chinese
Academy of Sciences, Grant no. XDA09030101 and the
National Natural Science Foundation of China (no. 21133011,
21373244).
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Fig. 5 Liquid phase oxidation of ethylbenzene: (1) heterogeneity and (2)
recyclability of M–10MnZr.
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. Conclusion
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