Controlled synthesis of mesocrystal magnesium oxide parallelogram and its catalytic performance

Article abstract of DOI:10.1039/c5ce00136f

Mesocrystal magnesium oxide (MgO) parallelogram with lengths of diagonal line in the range of 7-9 μm has been synthesized through a facile precipitation method by using phosphate species as the morphology regulator and Mg(NO₃)₂ and N

Full text of DOI:10.1039/c5ce00136f

CrystEngComm
PAPER
Controlled synthesis of mesocrystal magnesium
oxide parallelogram and its catalytic performance
Cite this: CrystEngComm, 2015, 17,
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Xiaoling Zhang, Yajun Zheng, Haijun Yang, Qian Wang and Zhiping Zhang
Mesocrystal magnesium oxide (MgO) parallelogram with lengths of diagonal line in the range of 7–9 μm
has been synthesized through a facile precipitation method by using phosphate species as the morphology
regulator and MgIJNO3)2 and Na2C2O4 as the inorganic sources. Scanning electronic microscope
examination revealed that there is a canyon in the middle of the diagonal line of the as-synthesized
products. The canyon resulted from the self-assembly of layer-like structures with a thickness of around
100–150 nm on both sides. To investigate the effect of reaction conditions on the morphologies and com-
ponents of the resulting products, the types and addition amount of phosphate species and the reaction
temperature were studied in detail. The results demonstrated that the introduction of trace amounts of
phosphate species in the reaction system played a crucial role in determining the morphologies of the
obtained products with variation of reaction temperatures, whereas their components have little been
influenced after analyses by Fourier-transform infrared spectroscopy and X-ray diffraction. When as-
synthesized mesocrystal MgO was employed as a catalyst in the Meerwein–Ponndorf–Verley (MPV) reaction
between benzaldehyde and ethanol, it demonstrated superior performance to MgO particles without the
presence of phosphate species.
Received 21st January 2015,
Accepted 12th February 2015
DOI: 10.1039/c5ce00136f
www.rsc.org/crystengcomm
large amounts of step edges and corners, low coordinated
sites and lattice defects. MgO nanoflakes demonstrated high
1. Introduction
Controlled synthesis of inorganic materials with desirable
morphologies plays an important role in the development of
new materials due to the strong relationship between physical
properties and morphologies.1,2 Magnesium oxide, a typical
wide band gap insulator with a band gap of 7.8 eV, has been
extensively used as a magnetic tunnel junction for semicon-
ductor spintronics, as an additive in refractory experiments
due to its high thermal stability, and as a component of high
temperature superconductor products and catalysts.3–5 MgO
has also demonstrated its promise in the adsorption of many
pollutants.6,7 In recent years, numerous studies have demon-
strated that the performance of MgO is highly influenced by
its morphologies.5,8–11 For example, many reports have
evidenced that the catalytic performance of MgO was closely
related to its shape, illustrating an apparent morphology-
dependent phenomenon.9 Sutradhar et al. observed that the
extent of the catalytic properties of MgO was highly controlled
by its morphology.8 Namely, the shapes of flowers and house-
of-cards were highly active for the condensation reaction of
benzaldehyde with acetophenone owing to the presence of
reactivity in the condensation reaction between benzaldehyde
and acetophenone resulting from their high surface area.
Wang et al. reported that MgO nanosheets and nanodisks
showed much higher reaction rates than nanofibers because
of their large numbers of surface basic sites.9 Therefore, the
catalytic, adsorption, electronic, optical, and magnetic proper-
ties of MgO can be tailored by controlling its morphology.
Up to now, various methods, including template synthe-
sis,12 hydrothermal method,11 chemical vapor deposition,13
precipitation method,14,15 precursor decomposition,16 sol–gel
technique17 and so on,18,19 have been developed for the prep-
aration of different morphologies of MgO. The reported mor-
phologies of MgO are mainly composed of spherical struc-
tures,14 nanowires, nanotubes or nanobelts,20 nanoflowers,21
nanosheets,22 cubes,23 whiskers,24 stacks of plates,25 rectan-
gular parallelepiped structures,26 columnar structures27 and
coralline.28 In this study, MgO with a unique morphology of
parallelogram has been synthesized via facile precipitation.
In the previous investigation,14 we found that the morphol-
ogy of spherical MgO could be well tailored by using trace
amounts of phosphate species as the morphology regulator
and commercial Mg5IJCO3)4IJOH)2Ĵ4H2O solution as the seed
in the reaction system of MgIJNO3)2 and K2CO3. Using this
method, a suture in the middle of the spherical-like MgO pre-
cursor, Mg5IJCO3)4IJOH)2Ĵ4H2O, can be elaborately smoothed
a School of Chemistry and Chemical Engineering, Xi'an Shiyou University, Xi'an
710065, China. E-mail: zhangzp0304@gmail.com; Fax: +86 29 8838 2693;
Tel: +86 29 8838 2694
b Department of Chemistry, Tsinghua University, Beijing 100084, China
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by addition of phosphate species, and the formation rate of
spherical Mg5IJCO3)4IJOH)2Ĵ4H2O can be accelerated by addi-
tion of trace amounts of Mg5IJCO3)4IJOH)2Ĵ4H2O seeds. How-
ever, the effect of phosphate species on the morphologies
and properties of the resulting products in other reaction sys-
tems is still unclear. To gain further insight into the capaci-
ties of phosphate species, this study systematically investi-
gated the evolution of the MgO precursor magnesium oxalate
with the addition of phosphate species in the reaction
between MgIJNO3)2 and Na2C2O4. The influence of the types
of phosphate species (e.g., sodium phosphate, sodium
tripolyphosphate and sodium polyphosphate), the addition
amount of sodium tripolyphosphate and the reaction temper-
ature on the morphologies of the obtained products have
been examined in detail. Also, a comparison has been made
between the catalytic performance of the obtained products
without and in the presence of sodium tripolyphosphate for
the MPV reaction of benzaldehyde and ethanol.
2.3 Catalytic evaluation
The catalysts were evaluated by using the MPV reaction
between ethanol and benzaldehyde. In a typical reaction, eth-
anol (7.0 mL, 120 mmol), benzaldehyde (0.60 mL, 6 mmol),
and as-synthesized MgO (1.0 g) were transferred into a 50 mL
round-bottom flask fitted with a reflux condenser. The reac-
tion mixture was heated to the reflux temperature (351 K)
under magnetic stirring. The reaction solution was with-
drawn in different intervals by using a micropipette during
the reaction. The collected products were analyzed with an
Agilent GC-7890B gas chromatograph equipped with a HP-
INNOWax capillary column (30 m × 0.32 mm I.D., 0.25 μm
film thickness).
3. Results and discussion
SEM and TEM observation and powder XRD
The uniform mesocrystal MgO parallelogram was synthesized
by using phosphate species (sodium tripolyphosphate or
sodium polyphosphate) as the morphology regulator of
MgC2O4Ĵ2H2O, and MgIJNO3)2 and Na2C2O4 as the inorganic
sources. The reaction mixture was stirred vigorously for 1
min and then maintained under static conditions for reac-
tion. Fig. 1 shows the typical SEM and TEM images and XRD
pattern of parallelogrammic MgO obtained by calcination of
their precursors, which were prepared by directly pouring
Na2C2O4 into MgIJNO3)2 solution in the presence of 0.05 g of
sodium tripolyphosphate. Fig. 1a demonstrates the lower-
magnification SEM image of the as-synthesized monodis-
perse parallelogram. It is obvious that the particles are
parallelogrammic products with lengths of diagonal line in
the range of 7–9 μm, and the average length is about 8.6 μm.
Fig. 1b presents the magnified view of an individual parallel-
ogram. It has been found by careful observation that there is
a canyon in the middle of the diagonal line of the as-
synthesized product. The canyon resulted from the self-
assembly of layer-like structures with a thickness of ca. 100–
150 nm on both sides, and the detailed structures of the can-
yon and the layer-like structures on both sides of the canyon
are shown in Fig. 1c and d. However, on both ends of the
diagonal line of the as-synthesized product, it is interesting
that the corresponding regions become relatively flat (Fig. 1b).
Fig. 1e shows the TEM image of an individual parallelogram,
further illustrating the detailed structure of the synthesized
parallelogram particle. Although the MgO parallelogram dem-
onstrates a uniform structure (Fig. 1b–e), the 3D hierarchical
structure illustrates that it is structurally polycrystalline in
nature (Fig. 1f) and is made of domains of single crystalline
nanoparticles and amorphous particles as indicated by the
HRTEM images (Fig. 1g–i), and increasing the reaction tem-
perature and aging time favors the growth of crystalline
nanoparticles as shown in Fig. 1i. The HRTEM dark-field
image (Fig. 1j) clearly demonstrates that the single crystalline
nanoparticles are distributed in the as-prepared MgO bulk
particles. These results revealed that the as-prepared MgO
micrometer-sized particles have the characteristic of
2. Experimental
2.1 Synthesis of MgO parallelogram
In a typical procedure, 10.26 g of MgIJNO3)2Ĵ6H2O was
dissolved into 25 mL of double-deionized water, and the solu-
tion pH value was 6.6. Then, the MgIJNO3)2 solution was
transferred into a 250 mL three-necked flask and heated to
323 K. Subsequently, 0.05 g of sodium tripolyphosphate was
added into 125 mL of 0.4 M Na2C2O4 solution, and the mix-
ture was also heated to 323 K. Under vigorous stirring (ca.
800 rpm), the Na2C2O4 mixture solution with a pH value of
7.1 was poured into the MgIJNO3)2 solution in 4–5 s. The mix-
ture was further stirred for 1 min and then maintained at a
temperature of 323 K under static conditions for 1 h. After
that, a white precipitate was collected, filtered off, and
washed with double-deionized water and ethanol several
times. The MgO sample was prepared by calcination of the
MgC2O4Ĵ2H2O parallelogram in air from room temperature to
573 K with a rate of 2 K min−1 for 2 h. After that, the product
was calcined from room temperature to 823 K in a muffle fur-
nace and then maintained at that temperature for 6 h.
2.2 Characterization
The crystal structures of the as-synthesized products were
characterized by X-ray diffraction (XRD) on a XRD-6000 dif-
fractometer using Cu Kα radiation. The operation voltage
was 40 kV, and the current was 30 mA. The morphology and
size of the obtained particles were examined by a JEOL JSM-
6390A scanning electron microscope (SEM) and a FEI Tecnai
G2 F20 transmission electron microscope (TEM). The IR spec-
tra of the obtained samples were recorded with a Bruker ver-
tex 70 series FT-IR spectrometer in transmission mode in the
range of 4000–400 cm−1. The resolution was 4 cm−1 and eight
scans were signal-averaged in each interferogram. Nitrogen
adsorption–desorption isotherms were obtained using
a
Micrometrics ASAP 2020HD88 instrument at 77 K.
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Fig. 1 Typical SEM images of parallelogrammic MgO synthesized by the calcination of its precursor which was obtained by pouring Na2C2O4 into
MgIJNO3)2 solution in the presence of 0.05 g of sodium tripolyphosphate at 323 K followed by a period of 1 h aging: (a) panoramic morphologies,
(b) individual parallelogram, (c) and (d) detailed view of the surface structure of an individual particle, (e) typical TEM image of parallelogrammic
MgO, (f) selected area electron diffraction pattern of MgO, (g) HRTEM image showing the fine structure of the MgO parallelogram, (h) and (i)
HRTEM image of single crystalline domains of MgO, (j) HRTEM dark-field image and (k) XRD pattern of as-synthesized MgO. Note: (1) the MgO par-
ticles for (i) were prepared by pouring Na2C2O4 into MgIJNO3)2 solution in the presence of 0.05 g of sodium tripolyphosphate at 323 K followed by
1 h aging, and then the aging temperature was increased up to 373 K; (2) the inset in (h) is the enlarged crystallite surface used for measuring the d
spacing between two consecutive lattice planes.
mesocrystal materials.29–31 Although up to now different
mesocrystal materials, including biomimetic materials,32,33
metal oxides,34–37 metal particles38–41 and other inorganic
materials,42–44 have been extensively studied, little investiga-
tion on MgO mesocrystals has been reported.45 This study
presents one example of the preparation of an MgO meso-
crystal. The detailed surface structure illustrates that the crys-
tallite size varies in the range of 0.5–5 nm (Fig. 1g), and the d
spacing between two consecutive lattice planes is 0.21 nm
(Fig. 1h), suggesting the preferential growth of thermodynam-
ically stable (200) planes. The crystal structures of the as-
synthesized products were further confirmed by XRD. As
shown in Fig. 1k, all diffraction peaks in the XRD pattern can
be readily indexed to the cubic phase of MgO with a lattice
constant of a = 4.213, in good agreement with the values
reported in the literature (JCPDS card: 4-829). Also, no other
impurities were detected by XRD (Fig. 1k), indicating that the
presence of trace amounts of sodium tripolyphosphate
almost has no effects on the phase of the final product.
sodium phosphate and sodium polyphosphate) on the mor-
phologies of the products was investigated. Fig. 2 shows the
Influence of phosphate species
To better understand the formation of parallelogrammic
MgO, the influence of reaction conditions on the morphol-
ogies of MgO precursors was examined in detail. In the previ-
ous report of Zhang et al.,14 the addition of trace amounts of
phosphate species had a great influence on the morphology
of Mg5IJCO3)4IJOH)2Ĵ4H2O during the precipitation of magne-
sium nitrate and potassium carbonate. The added phosphate
species can erase an obvious suture in the middle of
spherical-like particles due to the strong complexing interac-
tion between Mg2+ and phosphate ions absorbed on the sur-
face of the crystals.14,46,47 The influence of phosphates on the
morphologies of the products increased with the increase in
the polymerization degree of phosphates.14,47 In the present
study, the influence of various phosphate species (e.g.,
Fig. 2 SEM images of MgO precursors obtained by adding different
types of phosphate species at 323 K: (a) and (b) without addition of any
phosphate species; (c) and (d) 0.05 g of sodium phosphate; (e) and (f)
0.05 g of sodium tripolyphosphate; and (g) and (h) 0.05 g of sodium
polyphosphate. (b), (d) (f) and (h) are the corresponding representative
particles in (a), (c), (e) and (g).
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effect of phosphate species on the morphology of MgO pre-
cursors. Apparently, the addition of phosphate species has a
pronounced effect on their morphologies. When there is no
phosphate salt in the reaction system, the product is mainly
composed of trapezoidal particles (Fig. 2a and b), similar to
the recent investigation done by Zhu et al.48 With the addi-
tion of 0.05 g of sodium phosphate, the morphology of the
obtained product changed from a relatively regular trapezoid
to a trapezoid-like structure containing different layers as
shown in Fig. 2c and d. With the increase in the polymeriza-
tion degree of phosphate salt from sodium phosphate to
sodium tripolyphosphate, the product demonstrates a mono-
disperse parallelogram composed of layer-like structures
(Fig. 2e). As discussed above, there is a canyon in the middle
of the diagonal line of the as-synthesized product (Fig. 2f).
Similar results were obtained for the reaction system containing
0.05 g of sodium polyphosphate as shown in Fig. 2g and h. It
is clear that the influence of phosphate species on the mor-
phologies of the obtained products increased with the poly-
merization degree of phosphates, which is in good agreement
with previous reports.14,47 However, different from the effect
of phosphates on the morphology of Mg5IJCO3)4IJOH)2Ĵ4H2O,
addition of phosphates into the reaction system of
MgC2O4Ĵ2H2O makes the layer-like structures of the obtained
product more obvious, even changing its basic morphology
from a trapezoid to a parallelogram. These results suggest
that with the variation of reaction systems, the complexing
effect between Mg2+ and phosphates will change the shape of
the final product into different morphologies.
Fig. 3 (a) The typical SEM image of selected micrometer-size particle
used for EDS analysis, (b) the corresponding EDS analysis result, and (c)
the different atom percentages (e.g., C, O, Mg and P atom) in one
micrometer-sized particle with the variation of phosphate species
(sodium phosphate, sodium tripolyphosphate and sodium poly-
phosphate) and the number of phosphate or tripolyphosphate mole-
cules in a MgC2O4 molecule which is based on the different atom per-
centages in the selected particle. Note: (1) the different atom
percentages are the average value from five different micrometer-size
5−
particles; (2) P3O10 is used to calculate the number of phosphate
species in a MgC2O4 molecule due to the unavailable formula structure
of sodium polyphosphate; (3) the Al peak in Fig. 3(b) is from the back-
ground matrix.
that in the reaction systems of sodium tripolyphosphate
and sodium polyphosphate, the phosphate species are prone
to interact with the MgC2O4 molecule and participate in the
formation of the MgC2O4 precipitate, thus leading to the
disruption of the growth of MgC2O4 extended structures
(Fig. 2e–f).
FT-IR spectroscopy is a useful tool to understand the func-
tional groups of inorganic materials. This study investigated
the spectra of the products from the reaction between
Na2C2O4 and MgIJNO3)2 in the presence of different types of
phosphate species. With the variation in the polymerization
degree of the phosphate species from sodium phosphate to
sodium polyphosphate, the IR spectra (data not shown) were
kept constant and had the characteristic adsorption bands of
MgC2O4Ĵ2H2O.16,49 These results suggest that the addition of
trace amounts of phosphate species has almost no effects on
the IR adsorption bands of the obtained products.
For the purpose of investigating the relationship between
the crystallite structures of the obtained products and the
phosphate species, the XRD patterns of MgO precursors
obtained by adding different types of phosphate species at
323 K were examined (Fig. 4). It is obvious that the XRD pat-
terns are very similar, and can be indexed to magnesium
oxalate dihydrate IJMgC2O4Ĵ2H2O, JCPDS card: 28-0625). No
characteristic peaks of impurities such as phosphate salts or
other by-products were observed, indicating that the addition
of phosphate species has no influence on the components
of the resulting products, which is consistent with the IR
analysis above. However, by careful examination of these
spectra, it can be found that the peak intensity (e.g., (200),
In order to gain insight into the effect of different phos-
phate species on the morphologies of the resulting products,
energy dispersive spectroscopy (EDS) analysis was carried out
to determine the content of different phosphate species in a
micrometer-sized particle. Fig. 3a and b show the typical
micrometer-size particle used for EDS analysis and the corre-
sponding analysis results. After analyzing the percentages of
different atoms such as C, O, Mg and P in the selected parti-
cles, it can be seen from the list in Fig. 3c that the contents
of C, O and Mg atoms were kept almost constant, whereas
the percentage of P atom in the resulting particles gradually
increased ranging from sodium phosphate to sodium tripoly-
phosphate, and then to sodium polyphosphate. These results
indicate that the level of phosphate in the obtained particle
3−
is very low (0.006 0.007%, namely, 0.0005 PO4 molecule
per MgC2O4 molecule), which is below the limit of detection
of EDS analysis (0.1%). This case to a certain degree reveals
the reason why the morphology of the obtained product in
the presence of 0.05 g of sodium phosphate (Fig. 2c and d) is
similar to that of the particles without the addition of any
phosphate species (Fig. 2a and b). On the contrary, the con-
tent of tripolyphosphate in the particles from the reaction
systems in the presence of 0.05 g of sodium tripolyphosphate
and sodium polyphosphate is high enough, and the values
5−
are 0.192
0.032% (0.0047 P3O10 molecule per MgC2O4
5−
molecule) and 0.260 0.032% (0.0070 P3O10 molecule per
MgC2O4 molecule), respectively. This information suggests
¯
¯
(402) and (602)) gradually decreased with an increase in the
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a very little amount of P atom was determined in the
micrometer-sized particles in the presence of 0.05 g of sodium
phosphate, which further confirms that the Mg3IJPO4)2 precipi-
tate does not participate in the nucleation and self-assembly
of MgC2O4 particles. As a result, little effect was observed in
the XRD pattern (Fig. 4b) of the obtained product. On the con-
trary, Mg2+ can form a stable soluble complex compound with
sodium tripolyphosphate or polyphosphate. The resulting
complex compound, on the one hand, would decrease the
supersaturation degree of Mg2+ in aqueous solution, and pre-
vent the growth of MgC2O4Ĵ2H2O.50 More importantly, the
polyphosphate ions with high charge density will interact with
Mg2+ at the surface of the formed MgC2O4Ĵ2H2O, thus
blocking the active growth sites of MgC2O4Ĵ2H2O.51 The higher
content of P atom in the obtained particles (Fig. 3c) also sug-
gests that tripolyphosphate or polyphosphate participates in
the reaction between Mg2+ and C2O42−. Accordingly, the peak
intensity in the XRD patterns became weaker in contrast to
those of the products without phosphate species (Fig. 4).
Owing to the blocking effect of polyphosphate salts, the self-
assembly behavior of MgC2O4Ĵ2H2O also exhibits a great
change, and the final morphologies of the obtained products
have much difference from the systems without the presence
of polyphosphate salts as shown in Fig. 2.
Fig. 4 Typical XRD patterns of MgO precursors obtained by adding
different types of phosphate species at 323 K. (Note: no phosphate
species means that the reaction system did not contain any phosphate
species, and the product was obtained by pouring Na2C2O4 into
MgIJNO3)2 solution. Sodium phosphate, sodium tripolyphosphate and
sodium polyphosphate, respectively, denotes the products obtained by
pouring Na2C2O4 into MgIJNO3)2 solution in the presence of 0.05 g
sodium
phosphate,
sodium
tripolyphosphate
and
sodium
polyphosphate.)
Influence of the amount of tripolyphosphate
polymerization degree of phosphate salts, especially sodium
tripolyphosphate and sodium polyphosphate. As well known,
phosphate can form the undissolved compound Mg3IJPO4)2
with Mg2+ due to the small solubility product constant
IJKspIJMg3IJPO4)2) = 9.8 × 10−25). So when the reaction system
Fig. 5 shows the typical SEM images of the MgO precursors
obtained by pouring Na2C2O4 into MgIJNO3)2 solution in the
presence of various amounts of sodium tripolyphosphate
(STPP). When there is no STPP in the reaction system, the
product demonstrates a trapezoid-like structure (Fig. 5a).
With the addition of 0.05 g of STPP, it is interesting that
monodisperse parallelogram particles composed of layer-like
structures were obtained, and there is a canyon in the middle
of the diagonal line of the as-synthesized product (Fig. 5b).
Further increasing the amount of STPP from 0.1–0.15 g, the
canyon in the particle gradually disappeared, and the surface
of the product became relatively flat although it is still
3−
contains a little amount of sodium phosphate, PO4 will
interact with Mg2+ and more favorably produce a Mg3IJPO4)2
2−
precipitate than Mg2+ with C2O4
owing to its higher
KspIJMgC2O4) = 8.6 × 10−5. Accordingly, Mg3IJPO4)2 nuclei will
3−
first be produced, and with the exhaustion of PO4 ions in
the reaction system, C2O4 will interact with Mg2+ to form a
2−
MgC2O4 precipitate followed by self-assembly into a trapezoid-
like structure (Fig. 2c and d). As mentioned above (Fig. 3c),
Fig. 5 Typical SEM images of MgO precursors obtained by pouring Na2C2O4 into MgIJNO3)2 solution in the presence of various amounts of sodium
tripolyphosphate: (a) 0 g; (b) 0.05 g; (c) 0.1 g; (d) 0.15 g; (e) 0.2 g; (f) 0.3 g; and (g) 0.5 g at 323 K.
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composed of layer-like structures (Fig. 5c–d). When the
amount of STPP was 0.2 g, the obtained product was self-
assembled by obvious layer-like structures (Fig. 5e). With the
increase in the amount of STPP to 0.3 g, the product turned
into a square-like structure with the loss of two opposite cor-
ners (Fig. 5f). The obtained product demonstrates a regular
layer-like square structure when the amount of STPP is 0.5 g
as shown in Fig. 5g. These results suggest that in the reaction
system of MgC2O4Ĵ2H2O, the morphologies of the products
are highly dependent on the addition amount of STPP, which
could vary the morphologies from a trapezoid-like structure
to a parallelogram, then to a square-like structure. Besides
that, the basic composition unit of the particles, namely, the
layer-like structure, also demonstrates different self-assembly
forms with the variation of the amount of STPP. Although
the amount of STPP or other phosphate species has a great
influence on the morphologies of the obtained products in
the literature,14,52 the basic form of the particles was kept
almost constant. In the present study, it can be observed that
the morphologies of the resulting products significantly var-
ied with the addition of STPP, illustrating that phosphate
species play a more significant role in determining the final
morphologies of MgC2O4Ĵ2H2O than those of spherical
Mg5IJCO3)4IJOH)2Ĵ4H2O14 or spindle-type hematite particles.52
Although STPP has a great effect on the morphologies of
the final products, the chemical composition of the final pre-
cipitate formed in the reaction solution was kept almost con-
stant (Fig. 6), which is consistent with previous reports.14,52
Fig. 6a shows the XRD patterns of the obtained products
from the systems in the presence of different amounts of
STPP. As can be seen, all the XRD patterns can be indexed to
MgC2O4Ĵ2H2O (JCPDS card: 28-0625). By careful observation,
it can be found that with the increase in the addition amount
of STPP in the reaction system of Na2C2O4 and MgIJNO3)2, all
influence of STPP on the crystal structures of the final prod-
ucts is via the interaction between STPP and Mg2+ at the sur-
face of the formed MgC2O4Ĵ2H2O, which blocks the active
growth sites of MgC2O4Ĵ2H2O.51 Due to the blocking effect,
the growth speed of MgC2O4Ĵ2H2O became slower and slower
with the increase in the addition amount of STPP. Therefore,
the crystal structure gradually became poor, which is directly
reflected in the intensity of the diffraction peaks shown in
Fig. 6a. Also, we examined the calcined products from the
reaction system in the presence of different amounts of STPP,
as shown in Fig. 6b. Similar to the XRD patterns in Fig. 6a,
the intensity of all the diffraction peaks gradually decreased,
revealing that the crystallization degree became poorer rela-
tive to the reaction system without STPP. These results dem-
onstrate that the presence of STPP in the reaction system of
Na2C2O4 and MgIJNO3)2 can not only greatly vary the mor-
phologies of the obtained products, but can also tailor the
crystallization degree of the resulting products.
Influence of reaction temperature
In the previous study, it was found that the morphologies of
MgCO3ĴxH2O significantly varied with the reaction tempera-
ture.15,53 To investigate the effect of reaction temperature on
the morphologies of MgC2O4Ĵ2H2O, we systematically exam-
ined the variation in the morphologies of the obtained prod-
ucts from 303–363 K with an increase in the addition amount
of STPP. Fig. 7(a-1)–(g-1), (a-2)–(g-2) and (a-3)–(g-3) show the
variation of the products with the amount of STPP at temper-
atures of 303 K, 343 K and 363 K. Similar to the products at
323 K, as shown in Fig. 5, the morphologies of the particles
in products formed at 303 K and 343 K also varied from
trapezoid-like structures to parallelogram, then to square-like
structures. The difference between them is the variation of
the surface structure of the products. For example, for the
products with the addition of 0.1 g of STPP in the reaction
system, the morphologies changed from parallelogram-like to
¯
the diffraction peak intensities such as (200) and (625) gradu-
¯
ally decrease. Even more, some diffraction peaks (e.g., (312)
¯
and (425)) became much weaker. As discussed above, the
Fig. 6 Typical XRD patterns of (a) MgO precursors obtained by pouring Na2C2O4 into MgIJNO3)2 solution in the presence of various amounts of
sodium tripolyphosphate (0–0.5 g) at 323 K and (b) their corresponding calcined MgO products.
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Fig. 7 Typical SEM images of MgO precursors obtained by pouring Na2C2O4 into MgIJNO3)2 solution in the presence of various amounts of sodium
tripolyphosphate at different reaction temperatures: (a) 0 g; (b) 0.05 g; (c) 0.1 g; (d) 0.15 g; (e) 0.2 g; (f) 0.3 g; and (g) 0.5 g at 303 K; IJa-1)–IJg-1) at
303 K, IJa-2)–IJg-2) at 343 K, and IJa-3)–IJg-3) at 363 K.
rectangle-like structures (Fig. 7(c-1), (c-2), (c-3) and 5c). The
parallelogram-like products at a lower reaction temperature
(e.g., 303 K) demonstrate a relatively flat surface structure.
With the increase of reaction temperature, a canyon in the
middle of the diagonal line of the as-synthesized product was
self-assembled by some layer-like particles beside it. More
interestingly, the canyon gradually becomes obvious as
shown in Fig. 5c and 7(c-2). When 0.2 g of STPP was added
into the reaction system, the morphologies of the resulting
products changed from parallelogram-like structures assem-
bled by unobvious layer-like units (Fig. 7(e-1)) to particles
with obvious layer-like structures (Fig. 5e), then to products
with petaloid-like structures (Fig. 7(e-2)), and finally to
square-like structures with the loss of two opposite corners
(Fig. 7(e-3)). From the discussion above, it is obvious that for
the reaction system without STPP, the products demonstrate
a relatively smooth surface structure (Fig. 5a and 7(a-1)–(a-3)).
With the addition of STPP into the reaction solution, the
obtained particles tend to become layer-like structures. From
these results, it can be seen that the reaction temperature and
the amount of STPP in the reaction system have significant
effects on the self-assembly of the layer-like units, and finally
change their morphologies.
the reaction still proceeded for the parallelogram-like MgO
mesocrystal. When the reaction time was 24 h, the conver-
sion efficiency of benzaldehyde reached as high as 87%.
These results demonstrate that the performance of the
parallelogram-like MgO was superior to that of the trapezoid-
like particles, probably attributable to the higher specific sur-
face area (69.3 m2 g−1) or the unique surface structure, as
mentioned above, of the former compared to that of the lat-
ter (59.3 m2 g−1).
In the MPV reaction, another side reaction occurred,
which is accompanied by the conversion of benzaldehyde
Study of catalytic performance
To gain insight into the catalytic performance of the
parallelogram-like MgO mesocrystal in the presence of STPP
(Fig. 2e and f) and the trapezoid-like MgO without any STPP
(Fig. 2a and b), the MPV reaction between benzaldehyde and
ethanol was carried out. During the reaction, ethanol is oxi-
dized to acetaldehyde, and benzaldehyde is reduced to benzyl
alcohol. Fig. 8a compares the benzaldehyde conversion effi-
ciency between both MgO materials at 351 K. It is obvious
that the conversion efficiency of benzaldehyde gradually
increased over the reaction time. Although the conversion of
both MgO materials in the period of 0–8 h was close, much
difference occurred after a reaction time of 8 h. For the
trapezoid-like MgO particles, the value was kept almost con-
stant with a conversion efficiency of around 58%, whereas
Fig. 8 Comparison of (a) benzaldehyde conversion and (b) product
selectivity in the MPV reaction between benzaldehyde and ethanol at
351 K by using parallelogram-like MgO mesocrystal and trapezoid-like
MgO as catalysts.
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into benzyl alcohol. Namely, the resulting acetaldehyde from
ethanol will further react with benzaldehyde to produce
cinnamaldehyde.9 Fig. 8b compares the reaction selectivity
for benzyl alcohol and cinnamaldehyde. Apparently, the
selectivity for both products of the parallelogram-like MgO
was kept almost constant with values of 67% and 33%,
respectively, in the reaction period. But the trapezoid-like
MgO demonstrated a higher selectivity in the first 8 h
followed by a similar value to that of the parallelogram-like
one. The reaction selectivity between both materials illus-
trates that the as-synthesized parallelogram-like MgO meso-
crystal had more uniform surface properties than the
trapezoid-like one in the MPV reaction.
and Technology Program of Shaanxi Province of China (no.
2014K13-16), and the Xi'an Science and Technology Project in
China (no. CXY1434IJ6)).
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