Mesoporous MgO: Synthesis, physico-chemical, and catalytic properties

Article abstract of DOI:10.1134/S0036024416060108

Mesoporous MgO was obtained via the hydrothermal synthesis using both ionogenic and non-ionogenic surfactants as structure-directing templates. The materials prepared were characterized by SEM, BET-N₂, XRD, and TG-DTA techniques. MgO particles are spherical 20-μm aggregates of primary oxide particles well shaped as rectangular parallelepipeds. Magnesium oxide samples have the specific surface area of 290–400 m²/g and pore sizes of 3.3–4.1 nm. Their mesoporous structure remained unchanged after calcination up to 350°C. Catalytic activity of mesoporous MgO was studied in acetone condensation reaction.

Full text of DOI:10.1134/S0036024416060108

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 6, pp. 1212–1216. © Pleiades Publishing, Ltd., 2016.
Published in Russian in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 6, pp. 902–906.
PHYSICAL CHEMISTRY OF NANOCLUSTERS
AND NANOMATERIALS
Mesoporous MgO: Synthesis, Physico-Chemical,
1
and Catalytic Properties
A. A. Maerle, I. A. Kasyanov, I. F. Moskovskaya*, and B. V. Romanovsky
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
*
e-mail: IFMoskovskaya@phys.chem.msu.ru
Received July 2, 2015
Abstract—Mesoporous MgO was obtained via the hydrothermal synthesis using both ionogenic and non-ion-
ogenic surfactants as structure-directing templates. The materials prepared were characterized by SEM,
BET-N , XRD, and TG-DTA techniques. MgO particles are spherical 20-μm aggregates of primary oxide
2
particles well shaped as rectangular parallelepipeds. Magnesium oxide samples have the specific surface area
2
of 290–400 m /g and pore sizes of 3.3–4.1 nm. Their mesoporous structure remained unchanged after cal-
cination up to 350°C. Catalytic activity of mesoporous MgO was studied in acetone condensation reaction.
Keywords: mesoporous magnesium oxide, template synthesis, acetone condensation.
DOI: 10.1134/S0036024416060108
1
. INTRODUCTION
bromide (CTMABr) or triblock copolymer (P123),
respectively, simplifies substantially the technology to
produce mesoporous MgO and thus reduces produc-
tion costs.
Unlike the majority of mixed metal oxides, which
are acidic in nature, such as silica-alumina, molecular
sieves and the like, magnesium oxide catalysts and
supports belong to a few solids which are weakly basic.
In this regard, the aim of this work was to develop a
Magnesium oxide is commonly used in the oxidation simple and relatively inexpensive method of template
of ketones to lactones and dehydrogenation of pro- synthesis of mesoporous MgO with high surface area
pane over V O /MgO, in the manufacture of bio- and thermally stable mesostructure.
2
5
diesel, desulfurization of fuels using CoMo/MgO and
NiMo/MgO catalysts, in environmental catalysis for
the destruction of organophosphonates and chlorohy-
drocarbons, for the trapping of sulfur and carbon diox-
ides, and of azo dyes, as well [1–8].
2
. EXPERIMENTAL
2
.1. Mesoporous MgO Catalyst Preparation
Ordinary hydrothermal method of MgO produc-
Magnesium nitrate hexahydrate Mg(NO ) ⋅ 6H O,
tion through hydroxide precipitation affords a product
3 2
2
2
urea (NH ) CO, cetyltrimethylammonium bromide
2 2
with a rather low surface area (no more than 100 m /g)
(
CTMABr) and Pluronic 123 block-copolymer (P123)
which restricts severely its practical use. Meanwhile, a
template synthesis developed in the mid-1990s to pro-
duce mesoporous molecular sieves with a high surface
area and spatially ordered system of nanoscale pores,
has been later successfully extended to the synthesis of
non-silicate materials, including mesoporous magne-
sium oxide [9–12].
were used. Template synthesis with CTMABr was per-
formed as follows. 7.50 g of CTMABr was dissolved in
1
50 mL of water at 40°C. Then, 10.0 g of magnesium
nitrate and 2.34 g of urea were added with vigorous
stirring. The mixture was transferred to an autoclave
and heated at 110 or 150°C for 48 h. In a parallel syn-
thesis, 20.0 g of magnesium nitrate and 4.68 g of urea
were added to the same template solution, the result-
ing mixture being further treated as indicated above.
On applying the so called “hard” templates such as
SBA-15 and CMK-3 molecular sieves in the synthesis
of magnesium oxide, a structured mesoporous MgO
2
material having a surface area of 250 m /g could be
The way the synthesis performed using the non-
obtained [1]. However, such two-step synthesis seems ionogenic P123 was somewhat different. 8.00 g of P123
to be both a time-consuming and somewhat costly was dissolved in 250 mL of water at 40°C and the
one. In contrast, the use of “soft” ionogenic or non- resulting solution was sonicated in the ultrasonic bath
ionogenic templates such as cetyltrimethylammonium SUNKKO 3050A (50 W, 40 kHz) for 100 min. Then,
1
0.0 g of magnesium nitrate and 2.34 g of urea were
added, and the mixture was kept in an autoclave for
1
The article was translated by the authors.
1212

MESOPOROUS MgO: SYNTHESIS, PHYSICO-CHEMICAL, AND CATALYTIC PROPERTIES
1213
a
b
c
10
20
30
40
50
60
70
80
2
θ, deg
Fig. 1. XRD patterns of MgO samples obtained using P123 at 110°C (a) and 150°C (b), and CTMABr at 110°C (c).
4
8 h at 110°C; in a parallel synthesis, the same mixture
3. RESULTS AND DISCUSSION
XRD pattern for the synthesized MgO samples are
was heated at 150°C.
As a reference sample, magnesium oxide was syn- shown in Fig. 1. Two peaks at 2θ = 43° and 62° indi-
thesized without template.
cate the formation of MgO phase. Also, the reflex at
θ = 21° accounts for the presence of Mg(OH) phase,
2
2
The resulting precipitates were filtered, washed
several times with hot distilled water and dried at
however its low intensity suggests MgO being a pre-
dominant phase in the prepared material. Besides, the
broadened reflections indicate small particle size of
the synthesized material.
1
10°C for 24 h. Template was then removed by the cal-
cination of air-dry samples at 350 or 550°C for 3 h.
Noteworthy, the positions of main peaks in XRD
patterns for the three samples of magnesium oxide are
similar to those for the periclase-type phase. This also
evidences that the use of templates of different in
nature does not affect the final phase composition of
MgO material.
2
.2. Catalyst Characterization
X-ray diffraction analysis was performed on a dif-
fractometer Stoe Stadi P (λ = 1.5418 Å) in the range of
θ = 15°–75° with the steps of 0.05°. Standard data
2
processing according to the WINXPOW program
included smoothing and background subtraction fol-
lowed by determination of the reflex positions and rel-
evant spacings. Adsorption measurements were per-
Figure 2 shows the low-temperature N adsorp-
2
tion-desorption isotherms obtained for MgO samples.
SEM micrographs of these samples are given in Fig. 3,
and their textural and morphological characteristics
formed on automatic analyzer ASAP 2000N are summarized in table.
(
Micromeritics). Before the measurement, sample was
The identifying features of N adsorption isotherms
–3
2
evacuated at 350°C and a pressure of 10 mm Hg for
on magnesium oxide synthesized at two different tem-
peratures using both CTMABr and P123 templates are
characteristic of type IV mesoporous materials
according to IUPAC classification. All samples exhibit
the wide hysteretic loops of E-type by de Boer’s clas-
sification [13] in 0.4–0.8 range of the relative pressure
2
h. The thermal stability of the resulting samples was
studied with the use of SDT Q600 TA Instruments
derivatograph. 20 mg of the air-dry sample was placed
in a corundum crucible and heated in the air stream.
DSC-TG curves were recorded at the linear tempera-
ture rise of 10 K/min in the range 20–800°C. SEM
micrographs were obtained with an electron micro-
scope MIRA3 TESCAN.
(
a–d adsorption–desorption isotherms in Fig. 2).
This type of hysteresis is due to the bottle-shape pores
with narrow equisized openings but with the different
diameters of pores in their wide part.
The catalytic activity of the mesoporous sample of
magnesium oxide calcined at 500°C was evaluated in a
On lowering the synthesis temperature down to
gas phase conversion of acetone into isophorone. It 110°C, another narrow hysteresis loop of C-type
was carried out in a flow system at 500°C, 0.101 MPa, appears on the adsorption-desorption isotherms in the
–1
and acetone WHSV of 2 h . The composition of the range of p/p from 0.8 to 1.0 for all samples (isotherm
0
reaction products was determined by GLC on a Kristal a and b). Such a loop is characteristic of slit-shaped
2000M apparatus.
pores. As to the reference MgO sample which was syn-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 6 2016

1
214
MAERLE et al.
the solutions of an ionogenic template which depends
3
N
2
volume adsorbed, cm /g
not only on surfactant concentration but also on the
temperature of reaction. This assumption is confirmed
by the textural data obtained for the sample synthe-
sized in the presence of the same CTMABr template
but at a temperature of 110°C (isotherm c in Fig. 2).
250
200
150
100
c
b
Indeed, this sample has the largest surface area of ca.
2
4
00 m /g and two types of pores, i.e., both bottle and
а
slit types.
Apart from the synthesis temperature and the type of
surfactant used as a template, the starting concentration
of components represent not less important factor for
the formation of MgO mesoporous structure. Thus, on
doubling the amount of reagents used, the specific sur-
face area of the synthesized material increases almost by
d
5
0
e
4
times (samples 4 and 5 in table), the shape of adsorp-
tion-desorption isotherm being the same.
0
0.2
0.4
0.6
0.8
1.0
This very fact is evident to reflect the strong influ-
ence of reactant starting concentrations on the rate of
hydroxide precipitation and as a consequence on the
number of nucleation centers for the metal hydroxide
deposition which leads to a reduction in the size of its
primary particles.
p/p
0
Fig. 2. N adsorption-desorption isotherms of the samples
2
prepared using P123 (a, b) at 110°C (a), 150°C (b),
CTMABr (c, d) at 110°C (c), 150°C (d), and of the refer-
ence sample (e).
There is no doubt, a crucial factor that determines
thesized without template, it has no mesopores at all ultimately the most important textural characteristics
(
isotherm e in Fig. 2).
of a prepared material–specific surface area and pore
size, are the conditions of the final stage, i.e., its
annealing to remove the spent template from of the as-
synthesized porous solid. The textural characteristics
of sample 3 (table) calcined at two different tempera-
tures are as follows:
Thus, on performing the hydrothermal synthesis at
50°C and using non-ionogenic P123 template, the
1
magnesium oxide with specific surface area of
2
3
00 m /g and with equisized mesopores could be pre-
pared (table).
On the other hand, the hydrothermal synthesis at
2
Dpore, nm
Temperature, °C
50
550
S, m /g
1
50°C in the presence of ionogenic CTMABr template
3
395
130
3.8
5.5
yields a material with rather low specific surface area
isotherm d) which is very close to that for the refer-
ence sample prepared without template: 23 and
(
2
2
2 m /g, respectively (table). This result seems to be
It is evident, the more severe conditions of anneal-
connected with the features of micelle formation in ing the stronger degradation of mesoporous MgO tex-
5
μm
20 μm
(а)
(b)
Fig. 3. SEM images of mesoporous MgO prepared at 110°C using CTMABr template.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90
No. 6
2016

MESOPOROUS MgO: SYNTHESIS, PHYSICO-CHEMICAL, AND CATALYTIC PROPERTIES
1215
Characteristics of mesoporous MgO samples
Synthesis
Crystal morphologyа
S, m /g
2
No.
Template
Dpore, nm
conditions
1
2
3
4
5
6
P123
P123
CTMABr
CTMABr
CTMABr
110°С, USb
150°С, US
110°С
150°С
150°С
Fine white spheres
Fine white spheres
Fine white spheres
Fine white spheres
Large white spheres
White powder
4.1
3.4
3.8
3.3
3.7
–
336
338
395
23
88
22
c
–
150°С
а
b
c
Annealing at 350°C, sonicated template solution, double amounts of both magnesium nitrate and urea used.
ture: pore size increased and surface area decreased template, and decomposition of pure organic tem-
sharply, although it did remain considerable.
plate, respectively.
Thus, the results of thermal analysis and adsorp-
tion measurements showed that the P123 non-iono-
genic template seems to be preferred for the synthesis
of mesoporous MgO at 150°C: the obtained solid has
high surface area and uniform pores; besides, the sur-
factant applied in the synthesis could be removed at a
lower temperature than in the case of CTMABr iono-
genic template.
The catalytic activity in the condensation of ace-
tone was evaluated using the sample 3 of mesoporous
MgO (table). The catalytic conversion of acetone is
well documented to be a multistep process [15]. Diac-
etone alcohol as a primary product of acetone self-
condensation then successively converts into second-
ary mesityl oxide, wherein both steps occur upon both
the acidic and basic active centers. Furthermore, mes-
ityl oxide gives isobutylene and acetic acid on acidic
active sites.
In the presence of the basic centers, acetone con-
version occurs to give a large variety of cyclization
products, in particular, isophorone and methyl phe-
nols. As it was shown by GLC analysis, at about 40%
acetone conversion on mesoporous MgO isophorone
and methylphenol were formed with a selectivity of
about 33% while the selectivity to isobutylene is 3%
only. This ratio of product selectivity enables one to
conclude that the surface of mesoporous magnesium
oxide possesses sufficient number of strong basic cen-
ters and at the same time a small amount of aprotic
acidic centers.
The results of thermogravimetric study on the mes-
oporous MgO samples are shown in Fig. 4. In the ref-
erence sample obtained without template (Fig. 4, pro-
file a'), endoeffect in the range 100–130°C is due to
water removal; two endopeaks at 375 and 425°C are
owing to conversion of Mg(OH) and decomposition
2
of MgCO impurity, respectively [14].
3
TG-DSC study was performed with MgO samples
synthesized applying with both P123 (Fig. 4, curve b')
and CTMABr (Fig. 4, curve c') templates. In both
cases, the endothermic peaks at 400–440°C originate
from the decomposition of magnesium hydroxide and
carbonate mixture. In addition to these peaks, the exo-
thermic effects at 490 and 525°C are also present and
correspond to complete combustion of P123 and
CTMABr templates, respectively. In the case of P123
template used, exopeak at 230°C and endopeak at
2
85°C are present at TG-DSC profiles. They are
probably due to the partial oxidation of pure organic
100
4.5
3
2
1
0
.5
.5
.5
.5
80
60
40
20
0
c
b
а
4. CONCLUSION
c'
−
0.5
1.5
Thus, the nature of template, the synthesis tem-
perature, the concentration of starting reactants and
conditions to remove the spent templates are all key
factors to obtain mesoporous MgO catalysts having a
−
b'
а'
330
−2.5
2
130
230
430
530
surface area of about 400 m /g. Morphology of mag-
T, °C
nesium oxide particles is seemed to be very peculiar:
they are the 20-μm spherical aggregates of primary
nanoparticles composed of primary oxide species in
the form of rectangular parallelepipeds. The prepared
Fig. 4. DSC-TG profiles of MgO samples prepared without
template (a, a') and using P123 (b, b') and CTMABr (c, c').
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 6 2016

1
216
MAERLE et al.
6. C. Gao, W. Zhang, H. Li, et al., Cryst. Growth Des. 8,
mesoporous MgO showed good activity and selectivity
in the catalytic conversion of acetone to isophorone
and other products that require the presence of both
basic and acidic active sites.
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8
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ACKNOWLEDGMENTS
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no. 14-23-00094).
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