Silica-supported silver-containing OMS-2 catalysts for ethanol oxidative dehydrogenation

Article abstract of DOI:10.1016/j.cattod.2016.05.058

The silver-modified cryptomelane-type crystalline and silica-supported OMS-2 catalysts (Ag/OMS-2 and Ag/OMS-2/SiO₂, respectively) were prepared by impregnation, co-precipitation, and consecutive impregnation methods and tested in selective oxidation of ethanol to acetaldehyde. Modifying OMS-2 and OMS-2/SiO₂ catalysts by silver was shown to improve the redox properties of the catalysts and increase the ethanol oxidation rate. However, selectivity to acetaldehyde decreased with increased reaction temperature due to intensification of total oxidation on the Ag-MnO_x sites. Preparation method of the catalyst affected the Ag localization and catalytic activity. Introducing silver into the OMS-2 channels provided a higher ethanol oxidation rate. The use of silica support for Ag/OMS-2 composites improved the selectivity towards acetaldehyde at higher temperatures and, therefore, increased the acetaldehyde yield in comparison with crystalline samples.

Full text of DOI:10.1016/j.cattod.2016.05.058

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Silica-supported silver-containing OMS-2 catalysts for ethanol
oxidative dehydrogenation
Valerii V. Dutov , Grigory V. Mamontov , Vladimir I. Soboleva,b, Olga V. Vodyankinaa,∗
a
a
a
Tomsk State University, 36 Lenin Ave., Tomsk, 634050, Russia
Boreskov Institute of Catalysis SB RAS, 5 Lavrentiev Ave., Novosibirsk, 630090, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The silver-modified cryptomelane-type crystalline and silica-supported OMS-2 catalysts (Ag/OMS-2 and
Ag/OMS-2/SiO2, respectively) were prepared by impregnation, co-precipitation, and consecutive impreg-
nation methods and tested in selective oxidation of ethanol to acetaldehyde. Modifying OMS-2 and
OMS-2/SiO2 catalysts by silver was shown to improve the redox properties of the catalysts and increase
the ethanol oxidation rate. However, selectivity to acetaldehyde decreased with increased reaction tem-
perature due to intensification of total oxidation on the Ag-MnOx sites. Preparation method of the catalyst
affected the Ag localization and catalytic activity. Introducing silver into the OMS-2 channels provided
a higher ethanol oxidation rate. The use of silica support for Ag/OMS-2 composites improved the selec-
tivity towards acetaldehyde at higher temperatures and, therefore, increased the acetaldehyde yield in
comparison with crystalline samples.
Received 31 December 2015
Received in revised form 14 May 2016
Accepted 20 May 2016
Available online xxx
Keywords:
Manganese dioxide
Octahedral molecular sieves
Ethanol
Selective oxidation
Silver
© 2016 Elsevier B.V. All rights reserved.
1
. Introduction
Manganese oxides possessing high oxidative activity and ability
to reoxidize at low temperatures are good candidates to produce
Because bioethanol is one of the main potential sources for pro-
ducing hydrogen and industrial chemicals such as acetaldehyde
which in turn is an intermediate in producing acetic acid, ethyl
effective oxidation catalysts operating under ambient conditions
[6,7]. However, appropriate reaction conditions must be chosen to
achieve high selectivity toward aldehyde because of the high total
oxidation activity of these catalysts. For instance, gas-phase selec-
tive oxidation of ethanol to acetaldehyde over OMS-2 catalysts is
commonly carried out in reaction mixtures containing low con-
centrations of oxygen (1–3%) [8]. Oxygen-rich mixtures (air) can
be used in gas-phase selective oxidation of aliphatic alcohols with
low reaction ability (such as octanol-1) and aromatic alcohols (e.g.,
benzyl alcohol) [7,9]. Another way to increase the selectivity is to
carry out liquid-phase oxidation reactions under mild conditions
[10,11].
(
acetate, butyraldehyde, crotonaldehyde, and n-butanol), develop-
ing new catalysts for ethanol conversion is of importance [1]. The
main advantages of gas-phase selective ethanol oxidation com-
pared to the liquid-phase processes are the use of air as a “green”
oxidant, atmospheric pressure, and simple evacuation of acetalde-
hyde from reaction products without time-consuming treatment
◦
of solvent. However, elevated temperatures (500–650 C) are usu-
ally used in industrial gas-phase Ag-catalyzed selective ethanol
oxidation [2]. The ethanol conversion and selectivity toward
acetaldehyde are 30–70% and 90–99% per pass, respectively. The
main byproduct is acetic acid, but small amounts of formic acid,
ethyl acetate, CO, and CO2 can also be formed.
In the present work we assume that modification of manganese
oxides by silver can enhance the selectivity towards acetaldehyde
while maintaining the total conversion of ethanol under ambi-
ent conditions. The use of air as an oxidant due to change in the
Combining silver and other IB subgroup metals with transi-
tion metal oxides in a catalyst composition allows significantly
decreasing temperature of the process while maintaining high
selectivity toward acetaldehyde [3]. Earlier we showed the key role
of iron additives in Si N -supported Ag catalysts in the efficiency
4+
3+
Mn /Mn ratio in the catalyst structure is also considered.
Octahedral molecular sieve materials with a cryptomelane
structure (OMS-2) attract special attention due to unique adsorp-
tive, ion-exchanging, semi-conductive, and catalytic properties.
Nanostructured materials based on cryptomelane-type manganese
oxide have been extensively investigated as supercapacitors
3
4
of selective oxidation of ethylene glycol and ethanol to glyoxal and
acetaldehyde, respectively [4,5].
[
12,13], sorbents for adsorptive desulfurization of fuel gas [14,15],
H S consumption [16], CO2 capture [17], catalysts for synthesis of
2
∗ Corresponding author.
E-mail address: vodyankina o@mail.ru (O.V. Vodyankina).
pharmaceutical substances [18], and valuable organic compounds
[19–22]. Moreover, the OMS-2 materials are environmentally
http://dx.doi.org/10.1016/j.cattod.2016.05.058
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
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2
friendly catalysts for selective oxidation of alcohols to correspond-
ing aldehydes [6,7,9,23], in particular, for liquid phase alcohol
oxidation [10,24].
chemical properties with special attention to surface acidity and
redox properties.
Cryptomelane
with
the
gross
formula
of
2
. Experimental
KxMn4+ –xMn xO16·nH O is the analog of the natural mineral
3+
8
2
hollandite (BaMn O ), consisting of double chains of edge-
8
16
2.1. Sample preparation
sharing MnO6 octahedra linked at vertices to form (1 × 1) and
(
2 × 2) channels. The (2 × 2) channels in the cryptomelane phase
Silver-containing OMS-2 crystalline catalysts were prepared
+
with a size of 4.6 Å are occupied by K ions and water molecules
to stabilize the structure. The cryptomelane-type materials can be
by impregnation (Impr) and co-precipitation (CP) methods. In
the impregnation method, the OMS-2 materials were primar-
ily prepared by a hydrothermal method similar to the one
described in [15,43,44]. A solution of KMnO4 (5.89 g) in dis-
tilled water (100 ml) was added gradually to an acidic solution of
Mn(NO ) *6H O (14.92 g) in distilled water (30 ml) with vigorous
stirring. The obtained dark brown precipitate with mother solution
was hydrothermally treated at 120 C for 24 h. Then the precipitate
obtained was filtered, washed, and dried at 120 C overnight. The
prepared via reaction between Mn2+ and MnO4 in acid solution
according to both a reflux method [18,25,26] and hydrothermal
synthesis [15,27]. The presence of potassium ions in the solution
is an essential condition for crystallization of OMS-2 structure.
Potassium cation has a suitable ionic radius compared to other
monovalent ions and plays the role of a template. It can be partially
−
3
2
2
◦
substituted by other ions (Ag , Cu , Zn , Mg , Co , Ni , NH4+
+
2+
2+
2+
2+
2+
,
◦
+
+
H , Na , etc.) using two main methods, namely, the ion-exchange
technique (after cryptomelane phase synthesis) and the refluxing
method (addition of metal salt before cryptomelane synthesis)
OMS-2 sample obtained was then impregnated with an aqueous
solution of AgNO (0.49 M), dried overnight at 80 C, and calcined at
◦
3
◦
5
00 C in air. The catalyst prepared was denoted “Ag/OMS-2-Impr”.
[
28].
The features of the OMS-2 materials, including their catalytic
To investigate the effect of calcination temperature on the phase
composition of the OMS-2, the material obtained was calcined at
properties, depend on the nature and concentration of counter
cation in the channels [29], the average oxidation state of Mn and
lattice oxygen mobility [30], acid-base properties [31,32], and mor-
phology and textural properties [33]. The average oxidation state
of Mn in the OMS-2 materials may be changed from 3.4 to 3.99
◦
3
00, 400, and 500 C. During co-precipitation, AgNO3 (0.68 g) was
added to Mn(NO ) *6H O solution with the same concentration
3
2
2
(as for OMS-2 synthesis) before redox reaction and subsequent
hydrothermal treatment (HT) under the same conditions. The pre-
cipitate obtained was calcined at 500 C in air. The sample was
denoted “Ag/OMS-2-CP”.
◦
due to simultaneous presence of Mn2+, Mn , and Mn ions in
the framework [14,16,34]. The effect of temperature and pH on
the manganese oxide crystallization into a cryptomelane structure
was described in [35]. Amorphous manganese dioxide can only
be obtained in a pH range from 5 to 7 and a temperature range
3+
4+
Silica-supported OMS-2 catalysts were prepared by the
consecutive impregnation (CI) and co-precipitation meth-
ods.
DBJH = 9.8 nm, VBJH = 0.74 cm /g, grain sizes of 1–2 mm) was
used as a support for the catalysts. Prior to introduction of
active components, the silica was subjected to HT in ammonia
2
A
commercial mesoporous silica (SBET = 245 m /g,
3
◦
of 50–200 C. The hollandite-type manganese oxide may be syn-
thesized in a strongly acidic medium at pH = 1–2 in a temperature
range from 80 to 120 C.
Good catalytic performance (high TOF) of OMS-2 materials in
the oxidation reactions is commonly associated with high mobility
◦
◦
◦
solution at 120 C for 3 h, then dried at 120 C and calcined at
◦
5
00 C for 5 h. Initially a KMnO /SiO sample was obtained by
4
2
impregnation of silica support (8.5 g) with a solution containing
4+
3+
of lattice oxygen caused by a reversible Mn /Mn redox cycle and
one-dimensional micropores adsorbing small organic molecules, as
well as high surface acidity. Oxidation of organic compounds over
OMS-2 catalysts follows the Mars-van Krevelen mechanism [7,36].
From this point of view, it is interesting to investigate the influence
of a reoxidation rate on the catalytic activity. The effect of acid-base
properties on the catalytic activity of OMS-2 catalysts in oxidation
reactions, especially in selective alcohol oxidation, has not been
thoroughly investigated. The effect of catalyst acidity on its catalytic
properties was investigated in a liquid-phase cyclohexane oxida-
tion to cyclohexanone and cyclohexanol for H-exchanged OMS-2
catalysts [31]. Increased acidity was shown to increase the cyclo-
hexane conversion, while selectivity toward cyclohexanol was not
changed, and selectivity toward cyclohexanone increased.
One of the ways to control the redox and acid-base properties of
transition metal oxide catalysts is the use of various supports (sil-
ica, alumina, etc.). However, supported OMS-2 catalysts have not
been thoroughly investigated, while other supported MnOx cata-
lysts are widely presented in the literature [37–40]. The alumina-
and silica-supported OMS-2 catalysts were prepared and tested
in total oxidation of ethanol [41] and ethyl acetate [42], respec-
tively. To the best of our knowledge, supported OMS-2 catalysts for
vapor-phase selective oxidation of alcohols are not described in the
literature.
KMnO4 (1.20 g) in distilled water (6.8 ml) followed by drying at
◦
6
0 C. Then KMnO /SiO was added to acidic aqueous solution of
4
2
Mn(NO ) *6H O (3.04 g) in 30 ml of distilled water under stirring.
3
2
2
The precipitate obtained in the mother solution was placed in a
◦
Teflon autoclave and exposed at 120 C for 24 h. Then the pre-
cipitate was filtered, washed with distilled water, and dried at
◦
1
20 C. The OMS-2/SiO2 sample obtained was impregnated with
◦
aqueous solution of AgNO3 (0.69 M), dried overnight at 80 C,
and calcined at 500 C in air. The catalyst prepared was denoted
◦
“
Ag/OMS-2/SiO -CI”.
2
In the co-precipitation method, KMnO /SiO2 was added under
4
stirring to an acidic aqueous solution containing the same amount
of Mn(NO ) *6H O and 0.787 g of AgNO . The precipitate obtained
3
2
2
3
was hydrothermally treated in mother solution under the same
◦
conditions, dried, and calcined at 500 C in air. The catalyst pre-
pared was denoted “Ag/OMS-2/SiO -CP”. The Ag/SiO2 catalyst was
2
prepared by impregnation of silica support with aqueous solution
◦
of AgNO (0.63 M) followed by drying at 80 C and calcination in air
3
◦
at 500 C. Theoretical silver loading in all prepared catalysts was
5
wt%.
2.2. Sample characterization
Therefore, the aim of the present research work is to design
a preparation method to synthesize highly efficient supported
OMS-2 catalysts for gas-phase selective oxidation of ethanol under
ambient conditions (low temperature and atmospheric pressure
and using air as an oxidant) and to investigate their physical-
The catalysts prepared were examined by N2 adsorption
◦
at −196 C, atomic emission spectroscopy (AES ICP), pow-
der X-ray diffraction (XRD), temperature-programmed reduction
(TPR), temperature-programmed oxidation (TPO), temperature-
programmed surface reaction of adsorbed ethanol (TPSR-C H5OH),
2
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Table 1
3
NH pulse adsorption, thermogravimetric analysis, and differential
3
Textural properties of the prepared catalysts.
scanning calorimetry coupled with mass spectrometry (TGA-DSC-
MS).
Ssp, m /g Dp, nmb Vp, cm /gc Smicro, m /gd dMnO2 , nme
2
a
3
2
Catalyst
The surface area and pore diameter of the samples were mea-
OMS-2
29.5
27.0
37.6
15.7
17.0
13.7
14.8
15.1
15.4
15.3
13.4
0.14
0.12
0.14
0.68
0.67
0.67
0.67
0.70
19.5
12.9
18.7
21.8
17.6
22.8
15.0
26.8
19.5
19.0
17.4
13.9
14.1
14.3
–
◦
sured by nitrogen sorption at −196 C using the TriStar II 3020
Ag/OMS-2-Impr
Ag/OMS-2-CP
OMS-2/SiO2
Ag/OMS-2/SiO2-CI 154.6
Ag/OMS-2/SiO2-CP 150.4
analyzer (Micromeritics, USA). Prior to experiments, all samples
−2
◦
160.5
were outgassed (10 Torr) at 200 C for 2 h. The specific sur-
face area was determined by the BET method. The pore size
distribution was calculated from the desorption branch of the
adsorption-desorption isotherm according to the Barrett-Joyner-
Halenda (BJH) method. Micropore area was calculated by means of
the MP method.
Ag/SiO2
SiO2
148
177.3
–
a
Specific surface area calculated by applying the BET method.
Average pore diameter calculated from BJH method as 4 V /Sext.
Total pore volume.
Surface area of micropores calculated by applying the MP method.
Crystallite size calculated using Scherrer’s equation.
b
c
p
Chemical composition of the samples was analyzed using the
100 MP-AES EuroEA 3000 analyzer (Agilent Technologies, USA).
d
e
4
The OMS-2 samples were dissolved in a concentrated HNO3 in the
presence of sulphite-anion. Silica-supported OMS-2/SiO2 catalysts
were dissolved in the same mixture, but with the addition of HF.
X-ray diffraction (XRD) patterns were recorded on the Mini-
Flex 600 (Rigaku, Japan) diffractometer using monochromatized
Cu-K␣ radiation (␭ = 1.5418 Å) with a power setting of 40 kV and
from the reactor were analyzed by an online gas chromatograph.
The catalysts were treated in 8 vol%O2/He at 500 C for 1 h before
the catalytic experiments.
◦
◦
1
5 mA. The XRD patterns were recorded in the range of 2ꢀ = 10–90
3. Results and discussion
◦
with a scanning rate of 2 /min. The phase identification was made
according to the PCPDFWIN database.
3.1. Textural properties of the samples
The redox properties of catalysts were investigated by TPR and
TPO using the AutoChem HP chemisorption analyzer (Micromerit-
ics, USA) equipped with a thermal conductivity detector (TCD).
The TPR experiments were performed in a temperature range from
N2 adsorption-desorption isotherms for crystalline (namely,
OMS-2 support, Ag/OMS-2-Impr, and Ag/OMS-2-CP catalysts) and
supported (OMS-2/SiO2, Ag/OMS-2/SiO2-CI, and Ag/OMS-2/SiO2-
CP) samples are shown in Fig. 1(a, b). It is clearly seen that the
isotherms for both series of samples may be attributed to IV type
isotherm with H1 hysteresis loop according to IUPAC classifica-
tion. Table 1 shows textural properties of the catalysts. One can
see that the average pore diameter for crystalline and supported
catalysts differs insignificantly; however, the porous structure of
these catalysts is different. The specific surface area of crystalline
◦
◦
room temperature to 700 C at a heating rate of 10 C/min using a 10
vol%H /Ar mixture (flow rate of 20 ml/min). The TPO experiments
2
were performed in the temperature range from room temperature
◦
о
to 500 C at a heating rate of 10 C/min using a 5 vol%O /He mix-
2
ture (flow rate of 20 ml/min). Before TPO experiments the samples
◦
were reduced in a TPR regime to 500 C.
To investigate the interaction of C H5OH molecule with the cat-
2
2
alyst surface TPSR-C H5OH, experiments were performed using the
and supported samples is ∼30 and ∼150 m /g, respectively. The
2
AutoChem HP chemisorption analyzer coupled with the UGA 300
quadrupole mass spectrometer (Stanford Research Systems, USA).
surface of crystalline OMS-2 materials consists of a micropore sur-
face area (from 48 to 66% of the total area) and the external surface
of the crystallites. The micropore area for the Ag/OMS-2-Impr cat-
alyst is less than that of OMS-2 and Ag/OMS-2-CP. The decreased
micropore area indicates partial blocking of porous structure by
silver or silver oxide particles when the samples were prepared
by the wetness impregnation method. In the samples synthesized
by the CP method, silver ions substitute the potassium ions in the
channels of OMS-2 without blocking the porous structure. Surface
area for the Ag/OMS-2-CP catalyst is higher compared to OMS-2
and Ag/OMS-2-Impr, which is associated with smaller crystallite
sizes (according to the XRD data).
◦
Prior to experiments the catalyst samples were heated to 400 C
◦
in a 5 vol%O /He mixture. Then ethanol was adsorbed at 80 C in a
2
pulse mode using syringe (each impulse volume was 1 ␮l). Doses
of ethanol were injected into Ar flow (30 ml/min) until peak areas
became identical. Ethanol desorption was performed in the tem-
◦
◦
perature range 80–600 C with a heating rate of 10 C/min in Ar
flow (30 ml/min).
Pulse experiments using the AutoChem HP analyzer were per-
formed to evaluate the number of acid sites on the surface of
catalysts NH . Prior to the experiments the catalysts were heated
3
◦
◦
in He to up to 400 C. Then NH adsorption was performed at 100 C
The porosity of supported OMS-2 catalysts includes the meso-
porous structure of silica support and microporous structure of the
supported OMS-2 phase. Reduction of surface area for supported
OMS-2/SiO2, Ag/OMS-2/SiO2-CI, and OMS-2/SiO2-CP catalysts in
comparison with the initial silica support has resulted from distri-
bution of active components (manganese oxide and silver) inside
the pores of silica. Micropore areas are similar for SiO2, supported
OMS-2/SiO2, Ag/OMS-2/SiO2-CI, and Ag/OMS-2/SiO2-CP catalysts.
During the OMS-2 introduction onto silica support, the initial
micropores of silica were blocked, while new micropores appeared
due to formation of the OMS-2 structure.
3
in a He flow (30 ml/min) injecting 10 vol%NH /He mixture via loop
3
3
with V = 1 cm . The total amount of adsorbed NH was calculated
3
from peak areas using a calibration coefficient.
The TG-DSC-MS analysis of the samples was performed using
the STA 449 F1 Jupiter analyzer (Netzsch, Germany) coupled with
the quadrupole mass spectrometer QMS 403D Aëolos (Netzsch,
Germany). The samples (5–10 mg) were placed in an Al O cru-
2
3
◦
◦
cible and heated from 25 to 800 C at the rate of 10 C/min in
Ar atmosphere (70 ml/min) and Ar/air atmosphere (20 ml/min
Ar + 50 ml/min air).
2.3. Catalytic test
3.2. Phase and chemical composition
Catalytic activity of the samples in the ethanol oxidation was
The XRD patterns for OMS-2 samples calcined at different
temperatures are presented in Fig. 2. One can see that the as-
prepared OMS-2 sample (dried at 120 C) contains cryptomelane
tested in a flow fixed-bed reactor at atmospheric pressure using
.5 g of catalyst according to the procedure described in [45]. Gas
mixture containing 2 vol% C H5OH and 18 vol% O in He was passed
◦
0
KMn O , pyrolusite (␤-MnO ), and nsutite ␥-Mn(O,OH)2 phases.
2
2
8
16
2
3
◦
The main reflections at 2␪ = 12.7, 18.0, 28.6, 37.4, 49.9, and 60.2
through the reactor at a total flow rate of 60 cm /min. The effluents
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Fig. 1. The N2 adsorption-desorption isotherms for crystalline (a) and silica-supported (b) OMS-2 catalysts.
Fig. 3. XRD patterns for OMS-2, Ag/OMS-2-Impr and Ag/OMS-2-CP catalysts cal-
cined at 500 C.
Fig. 2. Influence of calcination temperature on the phase composition of OMS-2
sample.
◦
comparison with the peak for the OMS-2. Reduction of Mn O con-
2
3
are attributed to the (110), (200), (310), (211), (411), and (521)
planes of cryptomelane, respectively [43,46,47]. Reflections from
the pyrolusite phase at 2␪=28.6 and 37.4 are similar to those of
tent in silver-containing catalysts may be associated with increase
of thermal stability of the sample due to incorporation of sil-
ver ions in the channels of OMS-2 forming an Ag Mn O phase.
◦
2
8
16
cryptomelane, therefore, the presence of a pyrolusite phase is diffi-
cult to prove unambiguously. However, additional reflections with
low intensity at 2␪ = 42.7, 56.6, and 59.3 may correspond to the
No visible diffraction peaks for silver-containing phases (metallic
Ag or Ag O) are observed for Ag/OMS-2-Impr and Ag/OMS-2-CP
2
◦
catalysts, which is associated with a highly dispersed Ag state. Sim-
ilar results were obtained in [46,49]. In the Ag/OMS-2-CP catalyst
(
111), (211), and (220) planes of a pyrolusite phase, respectively.
◦
Reflections at 2␪ = 37.1 and 22.2 may be attributed to the (220)
and (101) planes of nsutite. The nsutite and pyrolusite phases may
be formed in the course of the HT as impurity phases [6,48].
formation of silver-substituted cryptomelane phase, Ag Mn O
2 8 16
occurs via partial substitution of potassium in the channels of
OMS-2. However, this cannot be proved by means of XRD due to
the similarity of cryptomelane and Ag Mn O structures. Forma-
◦
The increase of calcination temperature to 400 C results in
2
8
16
disappearance of nsutite reflections accompanied by increase of
reflection intensity for cryptomelane and pyrolusite phases caused
by dehydration. Increase of the calcination temperature to 500 C
tion of an Ag Mn O phase in the Ag/OMS-2-Impr sample is also
2 8 16
possible. Silver ions are able to substitute water molecules in the
channels of the OMS-2 and occupy the vacant sites inaccessible for
potassium cations because potassium cations have a slightly larger
ionic radius (0.24 and 0.28 nm for Ag+ and K , respectively) [50]. A
◦
leads to partial decomposition of manganese dioxide (pyrolusite)
and OMS-2 with formation of a bixbyite phase (Mn O ). Reflections
+
2
3
◦
at 2␪ = 23.0, 32.9, 38.2, 55.2, and 65.8 can be attributed to the (211),
similar problem about identification of an Ag Mn O phase in the
2 8 16
(
222), (400), (440), and (622) planes of Mn O , respectively.
presence of MnO2 was mentioned in [51]. Here we attempted to
prove the presence of an Ag Mn O phase using thermal analysis
data.
2
3
Fig. 3 shows the XRD patterns for Ag/OMS-2-Impr, Ag/OMS-
2
8
16
◦
2
-CP, and OMS-2 catalysts calcined at 500 C. The phases of
cryptomelane, pyrolusite, and bixbyite were observed for all sam-
Thermal stability of the samples was investigated by the
TGA-DSC-MS. Fig. 4 (a, b) shows thermal analysis data obtained
for OMS-2, Ag/OMS-2-Impr, and Ag/OMS-2-CP catalysts in inert
and oxygen-containing atmospheres. Decomposition of pyrolusite,
ples. It is clearly seen that intensity of the diffraction peak at
◦
2
ꢀ = 32.9 , attributed to the Mn O phase, is much lower for
2
3
Ag-containing samples (Ag/OMS-2-Impr and Ag/OMS-2-CP) in
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V.V. Dutov et al. / Catalysis Today xxx (2016) xxx–xxx
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Fig. 4. TG-DSC data for as-prepared (dried) OMS-2, Ag/OMS-2-Impr and Ag/OMS-2-CP catalysts in Ar (a) and Ar/air (b) atmospheres.
◦
2
ꢀ = 33 was observed for the Ag/SiO2 sample. This peak may be
attributed to highly dispersed Ag O [52]. Pyrolusite and cryptome-
2
lane phases are observed for all Mn-containing samples. However,
◦
characteristic reflexes of the OMS-2 phase at 2ꢀ = 12.7 and 18.0
cannot be detected because of the wide halo of amorphous silica
and low amount of manganese in the catalysts. No visible diffrac-
tion peaks of Mn O are observed, probably due to low Mn content.
2
3
◦
Weak reflection from metallic silver at 2ꢀ = 38.0 is observed only
for the Ag/OMS-2/SiO -CI catalyst. The absence of any diffraction
2
peaks of the metallic silver for Ag/OMS-2/SiO -CP is connected with
2
low loading of silver in this sample (see Table 2) as well as Ag
incorporation in the OMS-2 structure.
Table 2 shows the chemical composition of the prepared crys-
talline and supported OMS-2 catalysts. One can see that silver
loading in the catalysts prepared by the CP method is lower than the
theoretical amount. The decrease of silver in the catalysts prepared
by the CP method may be associated with leaching of silver during
the HT. Loading of potassium in the Ag/OMS-2-Impr and Ag/OMS-
2-CP catalysts is less than that in OMS-2 due to partial substitution
of potassium ions in the channels of the OMS-2. Silver occupies the
Fig. 5. XRD patterns for Ag/SiO2, OMS-2/SiO2, Ag/OMS-2/SiO2-CI, Ag/OMS-2/SiO2-
CP catalysts calcined at 500 C.
◦
cryptomelane, and Ag Mn O was assumed to occur according to
2
8
16
the following reactions:
2
MnO2 → Mn2O3 + 0.5O2(theoretical relative mass change was −9.19%),
(0 0 1/2) positions in the OMS-2 channels, while potassium occu-
pies (0 0 0) positions [49]. Therefore, an additional silver atom per
unit cell appears during the potassium substitution. When silver is
2
KMn8O16 → 8Mn2O3 + K2O + 3.5O2(theoretical relative mass change was −7.62%),
3
+
introduced into the OMS-2 structure, additional Mn ions appear
to maintain the charge balance, and oxygen vacancies are formed in
the framework, playing a crucial role in diffusion of lattice oxygen.
Acid-base and redox properties of the catalyst surface play a
crucial role in alcohol oxidation reactions. Alcohol molecules as
Lewis bases are adsorbed on the Lewis acid sites of the catalyst
forming ethoxide species. According to data in the literature, the
OMS-2 catalysts are characterized by strong Lewis acidity and weak
Brønsted acid sites [32,53]. Basic properties of the OMS-2 cata-
lysts are poor, which is confirmed by our CO2 adsorption data.
No detectable adsorption of CO2 is observed in a pulse adsorption
Ag2Mn8O16 → 2Ag + 4Mn2O3 + 2O2(theoretical relative mass change was −7.02%).
One can see that for all samples, TG curves obtained in Ar
atmosphere are characterized by two stages of weight loss. Accord-
ing to MS data (not shown) the first stage of weight loss in the
◦
temperature range from 25 to 360 C is connected with decom-
◦
position of HNO3 residuals (peak of NO at 160 C), desorption
◦
◦
of CO2 (peak at 200 C), and water (peak at 270 C). The sec-
ond stage of weight loss in the range from 360 to 680 C is
◦
associated with decomposition of OMS-2, MnO2, and Ag Mn O
2
8
16
phases to form Mn O . The weight loss is 7.13% for OMS-2
◦
2
3
experiment at 100 C. To investigate the acid properties of the sam-
and Ag/OMS-2 samples and 6.37% for Ag/OMS-2-CP catalyst. The
ples prepared, the NH3 pulse adsorption experiments were carried
out. Table 2 shows the number of acid sites per gram of the cata-
lyst and those normalized on the surface area of the samples. One
◦
area (enthalpy) of peak at 500–520 C corresponding to OMS-2
(
2
MnO ) → Mn O phase transition decreases in the order OMS-
2
2
3
> Ag/OMS-2-Impr > Ag/OMS-2-CP. One can see that weight loss
can see that for Ag/OMS-2-Impr, the concentration of the acid sites
for silver-containing catalysts occurs more slowly and in a wider
temperature range than for OMS-2 catalysts. Moreover, additional
high-temperature wide endothermic peaks and a corresponding
O2 desorption peak on the mass spectra are observed. Thus, the
per gram of the sample (or per nm2 of the surface area) is much
lower than that for OMS-2 and Ag/OMS-2-CP. This may be associ-
ated with a partial blocking of porous structure and acid sites by
silver-containing species. The number of acid sites per nm2 for the
Ag/OMS-2-CP sample was slightly lower compared to the number
for the OMS-2 sample. This may be associated with the changing
quantity of cryptomelane and/or MnO phases in the Ag-containing
2
catalysts decreases due to formation of the Ag Mn O phase. This
2
8
16
phase is characterized by higher thermal stability than MnO2 and
KMn O . The content of the Ag Mn O phase is higher for the
4+
3+
of the Mn /Mn ratio in the presence of silver. Supported OMS-
2/SiO , Ag/OMS-2/SiO -CI, and Ag/OMS-2/SiO -CP samples have
8
16
2
8
16
2
2
2
Ag/OMS-2-CP catalyst than for the Ag/OMS-2-Impr sample. TG-
DSC data obtained in oxygen-containing atmosphere (see Fig. 4b)
is similar to that obtained in Ar flow.
much lower acidity than the crystalline catalysts. It is clearly seen
that the number of acid sites is similar for all supported catalysts.
Thus, the use of silica as a support results in reducing the density
of acid sites and equalizing the acidity for the samples prepared by
different methods.
Fig. 5 shows the XRD patterns for Ag/SiO2 and supported
OMS-2/SiO , Ag/OMS-2/SiO -CI, and Ag/OMS-2/SiO -CP catalysts
2
2
2
◦
calcined at 500 C. A wide diffraction peak with low intensity at
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Table 2
Chemical composition and acid properties of the prepared catalysts.
Catalyst
Mn, wt.%
K, wt.%
Ag, wt.%
Ag/Mn molar ratio
Acidity, ␮mol-NH3/g
112
30
132
25
Acidity, N/nm2
OMS-2
58.9
56.5
56.8
3.4
3.3
7.2
3.20
2.30
2.40
0.17
0.16
0.09
–
–
2.3
Ag/OMS-2-Impr
Ag/OMS-2-CP
OMS-2/SiO2
Ag/OMS-2/SiO2-CI
Ag/OMS-2/SiO2-CP
5.00
3.70
–
5.39
0.95
0.045
0.033
–
0.832
0.067
0.68
2.11
0.09
0.12
0.11
30
29
Thus, from the data presented, one can conclude that the method
used to prepare the crystalline catalysts influences the localization
of silver. In samples prepared by CP, silver ions are predominantly
interaction of silver with manganese oxide [58]. Silver may inter-
act with the OMS-2 phase by means of the interface between AgOx
species and manganese oxide. Ag ions may be introduced into the
+
3+
located in the channels of the OMS-2 and partly substitute the K
channels of the OMS-2, providing formation of Mn and oxygen
due to similar ionic radius. However, the existence of highly dis-
persed silver species on the external surface of the OMS-2 cannot
be excluded. In samples prepared by Impr, silver predominantly
forms highly dispersed species (clusters) on the external surface of
the crystallites. However, the introduction of silver in the OMS-2
framework is also possible.
vacancies in the framework, which are crucial for lattice oxygen
diffusion. Application of different preparation methods allows con-
trolling the character of the silver-manganese oxide interaction and
catalytic properties of the catalysts.
According to the Mars-van Krevelen concept, both reoxidation
of the catalyst surface and restoration of lattice oxygen are of
importance for oxidation reactions over oxide catalysts. Obviously,
for total oxidation reactions high oxygen coverage of the catalyst
surface is required and provided by the high reoxidation rate of the
catalyst surface. We attempted to investigate the reoxidation of the
prepared catalysts under model conditions (TPO regime). Fig. 7(a,
b) shows the TPO profiles for crystalline (a) and supported samples
(b). One can see that reoxidation of crystalline catalysts occurs via
3.3. Characterization of active surface and catalytic properties
The ratio between reduction and reoxidation of the catalyst sur-
face influences the selectivity of the process of oxidation reaction.
From this point of view, it is interesting to study the reducing and
reoxidation abilities of the catalysts. Fig. 6(a, b) shows the TPR pro-
files for crystalline (a) and supported (b) catalysts. The reduction
of manganese dioxide is a multistep process [30]. For the OMS-2
sample, two peaks of hydrogen consumption are observed. The first
◦
two steps. Two overlapping peaks at 330 and 470 C are observed
for the OMS-2 and Ag/OMS-2-Impr catalysts, while for Ag/OMS-
◦
2-CP samples the reoxidation occurs at 330 and 400 C. Decreased
◦
peak at 316 C may be associated with reduction of MnO to form
reoxidation temperature may be associated with stronger interac-
tion of silver with manganese oxides in the Ag/OMS-2-CP catalyst
compared to Ag/OMS-2-Impr. However, no visible difference in the
reoxidation rates for crystalline catalysts was observed in the tem-
2
◦
Mn O , while the second peak at 358 C corresponds to reduction of
3
4
Mn O into MnO [54]. Three overlapping peaks at lower temper-
3
4
ature are observed for Ag-containing samples. Peaks at 195, 227,
◦
◦
and 265 C for Ag/OMS-2-Impr sample and peaks at 227, 250, and
2
perature range from room temperature to 250 C.
◦
81 C for Ag/OMS-2-CP may be attributed to consecutive reduction
A multistep reoxidation is observed for supported OMS-2 cat-
alysts at lower temperatures than for crystalline samples. This is
caused by higher dispersion of Mn oxide particles. The maximum
of manganese oxides strongly interacting with silver. The decrease
of reduction temperature of MnOx in the presence of silver is com-
monly attributed to spillover of hydrogen from silver clusters to
manganese oxide [55]. However, silver is characterized by weak
adsorption of hydrogen [56]. Therefore, the decrease in reduction
temperature is probably connected with oxygen spillover from the
Mn oxide surface to silver clusters.
◦
rate of reoxidation is observed at 220–240 C for all supported cat-
alysts. However, the reoxidation rate for the Ag/OMS-2/SiO2-CI
samples is higher than the rate for the OMS-2/SiO2. Thus, silver may
facilitate the reoxidation of manganese oxides. The higher reoxida-
tion rate for Ag/OMS-2/SiO2-CP catalyst than for Ag/OMS-2/SiO2-CI
may be associated with the higher amount of manganese in the
catalyst.
In silica-supported OMS-2/SiO2 catalyst, a one-step reduction
◦
occurs at 320 C due to higher dispersion of MnO . Addition of
2
silver results in the decrease in reduction temperature of man-
ganese dioxide and the appearance of several overlapping peaks.
The TPSR-C2H5OH experiments were performed to investigate
the interaction of an ethanol molecule with the catalyst surface.
Fig. 8(a, b) shows the desorption curves of products formed dur-
ing the TPSR of adsorbed ethanol over crystalline catalysts. Two
◦
High temperature peaks at 176, 219, and 287 C for Ag/OMS-2/SiO -
2
◦
CI and peaks at 160, 239, and 305 C for the Ag/OMS-2/SiO -CP
2
◦
catalyst may be attributed to a multistep reduction of manganese
CO2 evolution peaks at 175 and 230 C (Fig. 8a) assigned to differ-
dioxide interacting with silver. Low-temperature H consumption
peaks at 50 and 92 C connected with reduction of AgOx species
ent adsorption-reaction sites and two H2O desorption peaks at 136
and 281 C (Fig. 8b) are observed for all catalysts. A similar desorp-
2
◦
◦
on the silica surface [57,58] are observed only for the Ag/SiO2
tion profile consisting of two peaks of CO2 was observed in [30,41].
No other products, and in particular, no acetaldehyde and acetic
acid, are observed. The TPSR data correlate with the catalytic prop-
erties. Increased CO2 yield is observed for crystalline catalysts in a
and Ag/OMS-2/SiO -CI catalysts. Thus, according to the TPR and
2
XRD data, one can conclude that the method to prepare the sup-
ported catalyst also impacts the localization of silver. In sample
prepared by co-precipitation, silver interacts only with manganese
oxide and is located in the OMS-2 framework. Interaction of sil-
ver ions with silica surface is poor, especially at low pH, due to
low charge density of the silica surface [59]. Therefore, silver is
removed (leached) from the silica surface in the course of the HT of
◦
temperature range from 170 to 230 C.
The activity of the prepared catalysts in a selective ethanol
oxidation was examined by means of temperature-programmed
reaction under atmospheric pressure. Fig. 9 shows the tempera-
ture dependence of ethanol conversion (Fig. 9a), selectivity toward
acetaldehyde (Fig. 9b), and selectivity toward COx (Fig. 9c) for
crystalline catalyst. Acetaldehyde and CO2 are the main reaction
products and no other products, for example ethylene, acetic acid,
or ethyl acetate, are observed. The OMS-2, Ag/OMS-2-Impr, and
Ag/OMS-2-CP samples have high selectivity toward acetaldehyde
the Ag/OMS-2/SiO -CP catalyst. In sample prepared by consecutive
2
impregnation, silver-containing species interact with manganese
dioxide and silica surface, which is confirmed by TPR data.
To sum up, adding silver facilitates the reduction of manganese
oxides in both crystalline and supported samples due to strong
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Fig. 6. TPR-H2 profiles for crystalline OMS-2, Ag/OMS-2-Impr, Ag/OMS-2-CP (a) and supported Ag/SiO2, OMS-2/SiO2, Ag/OMS-2/SiO2-CP, Ag/OMS-2/SiO2-CI (b) catalysts.
Fig. 7. TPO profiles for crystalline OMS-2, Ag/OMS-2-Impr, Ag/OMS-2-CP (a) and supported OMS-2/SiO2, Ag/OMS-2/SiO2-CP, Ag/OMS-2/SiO2-CI (b) catalysts.
Fig. 8. Desorption curves of CO2 (a) and H2O (b) formed during TPSR of adsorbed ethanol for crystalline OMS-2, Ag/OMS-2-Impr and Ag/OMS-2-CP catalysts.
Fig. 9. Temperature dependences of ethanol conversion (a), selectivity toward acetaldehyde (b) and COx (c) for crystalline catalysts (Reaction conditions: m = 0.5 g,
−1
−1
H , Et/O2 = 1/9).
GHSV = 7200 ml g
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Table 3
Comparison of various Ag-containing catalysts from the literature and the present work for gas-phase ethanol oxidation.
Activity*10 2
−
,
TOF , h
a
−1
Reaction conditions
SAc, %
STY , h
b
−1
Reference
Catalyst
Ag, wt. %
mol-Et/g-kat*h
◦
Et/O2c
GHSV, ml/g*h
T,
C
Ag/OMS-2
Ag/OMS-2
Nanoporous Ag
Ag/Fe-Si3N4
Ag/MgCuCr2O4
OMS-2
Ag/OMS-2-CP
Ag/OMS-2-Impr
Ag/OMS-2-CP
Ag/OMS-2/SiO2-CP
OMS-2/SiO2
–
–
1.25
5.52
46.3
0.58
1.23
0.43
0.66
0.64
1.41
1.91
0.15
0.23
0.28
–
–
50
36000
36000
133333
7200
100000
7200
7200
7200
36000
36000
7200
190
230
250
283
225
170
170
170
200
230
170
170
170
1/2
1/2
2/1
1/9
1/3
1/9
1/9
1/9
1/9
1/9
1/9
1/9
1/9
98
95
95
96
99
98
97
96
58
85
98
98
98
–
26
21
4.9
65
[6]
[62]
[60]
[45]
100
4.9
0.89
–
3.7
5
3.7
0.95
–
5.39
0.95
12.7
149.2
–
19.3
13.8
41.3
217.6
–
[61]
–
This work
This work
This work
This work
This work
This work
This work
This work
7.7
5.3
9.8
75.4
–
Ag/OMS-2/SiO2-CI
Ag/OMS-2/SiO2-CP
4.6
32.1
7200
7200
1.81
12.8
a
−1
−1
TOF calculated as amount of ethanol (in mol) converted per mol of silver in the catalyst per hour (mol-Et*mol Ag*h ).
b
c
−1
−1
Space-time-yield in galdehyde*g silver*h
.
Molar ratio of ethanol to oxygen in the reaction mixture.
connected with total oxidation of ethanol on the Ag-MnOx interfa-
cial sites as well as on the MnOx sites. The amount of silver in the
Ag/OMS-2/SiO -CP catalyst is only 0.95 wt%. However, the activ-
2
◦
ity of this catalyst in the temperature range from 130 to 180 C is
slightly higher than for those for the Ag/OMS-2/SiO -CI and Ag/SiO2
2
samples containing 5 wt% of silver. Formation of oxygen vacancies
due to introduction of silver into the manganese oxide structure
and the appearance of Ag-MnOx sites may play a crucial role in
ethanol oxidation.
It is noteworthy that ethanol oxidation over supported OMS-
2
/SiO , Ag/OMS-2/SiO -CI, and Ag/OMS-2/SiO -CP catalysts can
2 2 2
be carried out at higher temperatures than over the crystalline
catalysts while maintaining high selectivity toward acetaldehyde.
Selectivity of 93–98% and conversion of ∼72 and 50% can be
◦
Fig. 10. Temperature dependences of ethanol conversion and selectivity toward
achieved at 200 C for Ag-containing samples and the OMS-2/SiO2,
◦
acetaldehyde for Ag/OMS-2-CP catalyst calcined at 400 and 500 C (Reaction condi-
respectively. Fig. 12 shows the stability test with 0.1 g of catalyst
−
1
−1
H , Et/O2 = 1/9).
tions: m = 0.1 g, GHSV = 36,000 ml g
for the crystalline Ag/OMS-2-CP and supported Ag/OMS-2/SiO -CP
2
samples. One can see that both catalysts give stable conversion and
selectivity toward acetaldehyde for at least 5 h. The supported cat-
alyst gives ∼65% ethanol conversion and ∼85% selectivity toward
(
>95%) and high conversion (∼53% for Ag-containing samples) at
◦
temperatures below 170 C. However, elevation of reaction tem-
perature results in increased total oxidation activity, and selectivity
toward acetaldehyde decreases sharply. Catalytic activity of the
crystalline Ag-containing samples normalized on the catalyst mass
is higher than that for the OMS-2 (Table 3). Increased reaction rates
for Ag-containing catalysts may be explained by increased reaction
ability of surface oxygen in the presence of silver, which is con-
firmed by the TPR-H2 data (Fig. 6a). Fig. 10 shows the influence
of the calcination temperature on the activity of the Ag/OMS-2-CP
◦
acetaldehyde at 230 C, while the crystalline sample gives 48% con-
◦
version and only ∼58% selectivity to acetaldehyde at 200 C. Thus,
the use of silica support for crystalline Ag/OMS-2 composites allows
controlling the catalytic properties in ethanol oxidation and sig-
nificantly improving the selectivity toward acetaldehyde under
selected conditions and an ethanol/O2 ratio of 1/9.
Comparison of the activity and selectivity toward acetaldehyde
for the prepared catalysts with the literature is shown in Table 3.
One of the main parameters, which significantly affects the selec-
◦
sample calcined at 400 and 500 C. One can see that activity and
selectivity are similar for both samples due to localization of the
Mn O phase not on the surface, but inside the OMS-2 crystallites.
tivity toward acetaldehyde, is the ethanol/O molar ratio in the feed
2
2
3
flow [45]. In the literature, the silver-containing catalysts are com-
monly tested in the gas mixture with an ethanol/O2 molar ratio
ranging from 0.33 to 2 [60–62]. In industry, air is used as an oxidant.
This allows diluting the feed flow with N2 and increasing produc-
tivity of the process. The increase in oxygen content in the reactant
flow is accompanied by sharp growth of the yields of byproducts
such as acetic acid, ethyl acetate, and COx. The main advantage of
our supported catalysts is an operation in oxygen-rich gas mixtures
while maintaining high selectivity toward acetaldehyde. Thus, our
supported OMS-2 catalysts have comparable and higher space-
time yield (STY) of acetaldehyde in comparison with known silver
catalysts. Moreover, the main by-product of the reaction is CO2.
Therefore, acetaldehyde may be easily separated from the reaction
mixture.
Temperature dependences of ethanol conversion and selectivity
toward acetaldehyde and COx are shown in Fig. 11 (a–c) for sup-
ported OMS-2 catalysts and the Ag/SiO2 sample. It is clearly seen
that the activity of the Ag-containing catalysts is higher than that of
the OMS-2/SiO2 sample, which correlates with improving of redox
properties of the catalysts (Fig. 6b, Fig. 7b). Reduction temperature
of manganese oxides decreases, and reoxidation rate increases in
the presence of silver in the catalyst composition. The activities of
the Ag/SiO2 and Ag/OMS-2/SiO -CI samples are similar. However,
the Mn-containing sample is characterized by lower selectivity at
temperatures above 220 C. Thus, the active species participating
in selective ethanol oxidation are silver sites for both catalysts.
2
◦
◦
Increased CO2 yield above 220 C for Mn-containing samples is
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Fig. 11. Temperature dependences of ethanol conversion (a) and selectivity toward acetaldehyde (b) and COx (c) for supported OMS-2 catalysts and Ag/SiO2 sample (Reaction
−
1
−1
H , Et/O2 = 1/9).
conditions: m = 0.5 g, GHSV = 7200 ml g
over, the oxidation process could be carried out in an oxygen-rich
mixture with high selectivity toward acetaldehyde and only CO be
2
formed as a byproduct.
Acknowledgments
This work was supported by “The Tomsk State University
Academic D.I. Mendeleev Fund Program” grant (8.2.07.2015) in
2
015–2016. The authors thank Jean Kollantai, Tomsk State Univer-
sity, for style review.
References
[
[
1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411–2502.
2] J.V. Ochoa, F. Cavani, RSC Green Chem. 28 (2015) 203–230.
Fig. 12. Stability test of the crystalline Ag/OMS-2-CP and supported Ag/OMS-
2
-CP/SiO2-CP samples for ethanol oxidation (Reaction conditions: m = 0.1 g,
[3] V.I. Sobolev, K.Yu. Koltunov, ChemCatChem 3 (2011) 1143–1145.
[4] O.V. Vodyankina, A.S. Blokhina, I.A. Kurzina, V.I. Sobolev, K.Yu. Koltunov, L.N.
Chukhlomina, E.S. Dvilis, Catal. Today 203 (2013) 127–132.
−1
−1
H , Et/O2 = 1/9).
GHSV = 36,000 ml g
[
5] A.S. Blokhina, I.A. Kurzina, V.I. Sobolev, K.Yu. Koltunov, G.V. Mamontov, O.V.
Vodyankina, Kinet. Catal. 53 (2012) 477–481.
4
. Conclusion
Crystalline and silica-supported OMS-2 catalysts with and with-
[
6] C. Junli, L. Juan, L. Qiying, H. Xiumin, S. Wenjie, Chin. J. Catal. 28 (12) (2007)
1034–1036.
[7] G.D. Yadav, R.K. Mewada, Chem. Eng. Res. Des. 90 (2012) 86–97.
[
[
8] H. Zhou, J.Y. Wang, X. Chen, Microporous Mesoporous Mat. 21 (1998)
15–324.
out the addition of Ag were prepared using co-precipitation,
impregnation, and consecutive impregnation techniques and
tested in the gas-phase selective oxidation of ethanol to acetalde-
hyde. Investigation of the catalysts by physical-chemical methods
showed that the preparation method influenced the localization
of silver for both the crystalline and supported catalysts. Conse-
quently, the samples had different active sites. Co-precipitation
provided the introduction of silver ions into the OMS-2 frame-
work, while impregnation and consecutive impregnation resulted
in localization of silver species on the external surface of the OMS-
3
9] N.N. Opembe, C. Guild, C. King’ondu, N.C. Nelson, I.I. Slowing, S.L. Suib, Indian
Eng. Chem. Res. 53 (2014) 19044–19051.
[
[
[
10] G.D. Yadav, A.R. Yadav, J. Mol. Catal. A: Chem. 380 (2013) 70–77.
11] V.D. Makwana, L.J. Garces, J. Liu, Catal. Today 85 (2003) 225–233.
12] D.A. Tompsett, S.C. Parker, M.S. Islam, J. Mater. Chem. A 2 (2014)
15509–15518.
[
[
13] S. Navaladian, V. Balasubramanian, K.V. Thirukkallam, RSC Adv. 4 (2014)
33911–33922.
14] P.H. Ho, S.C. Lee, J. Kim, D. Lee, H.C. Woo, Fuel Process. Technol. 131 (2015)
238–246.
[15] P.H. Ho, S.C. Lee, J. Kim, D. Lee, H.C. Woo, Korean J. Chem. Eng. 32 (9) (2015)
766–1773.
16] M. Prabu, K. Ramalingam, RSC Adv. 5 (2015) 18554–18564.
17] L. Li, E. Cockayne, I. Williamson, L. Espinal, W. Wong-Ng, Chem. Phys. Lett. 580
(2013) 120–125.
18] X. Meng, C. Yu, G. Chen, P. Zhao, Cat. Sci. Technol. 5 (2015) 372–379.
19] K. Yamaguchi, K. Sakagami, Y. Miyamoto, X. Jin, N. Mizuno, Org. Biomol.
Chem. 12 (2014) 9200–9206.
1
2
or silica supports. Interaction of silver species with manganese
[
[
dioxide improved both the reducing and reoxidation abilities of the
samples, resulting in higher activity of the silver-containing cata-
lysts. However, the selectivity to acetaldehyde sharply decreased
with increased reaction temperature due to the total oxidation
[
[
process. Increased CO yield for the supported Mn-containing com-
[20] X. Meng, J. Zhang, G. Chen, B. Chen, P. Zhao, Catal. Commun. 69 (2015)
39–242.
2
2
posites compared to the Ag/SiO2 sample was connected with total
oxidation of ethanol on the Ag-MnOx sites and also on MnOx sites.
On the other hand, formation of Ag-MnOx sites in the supported cat-
alysts by introducing of silver ions into the channels of the OMS-2
was favorable for the oxidation rate, while formation of Ag/MnOx
interfaces did not improve conversion of ethanol compared to the
Ag/SiO2 catalyst. The use of silica support for the Ag/OMS-2 com-
posites allowed improving the selectivity toward acetaldehyde in
an extended temperature range and, therefore, increasing the yield
of acetaldehyde in comparison with the crystalline samples. More-
[
[
21] X. Yang, C. Yin, J. Han, Y. Wu, React. Kinet. Mech. Catal. 117 (2016) 269–281.
22] M. Ousmane, G. Perrussel, Z. Yan, J.-M. Clacens, F. De Campo, M. Pera-Titus, J.
Catal. 309 (2014) 439–452.
[
[
23] H. Zhou, J.Y. Wang, X. Chen, Microporous Mesoporous Mat. 21 (1998)
315–324.
24] V. Mahdavi, S. Soleimani, Mater. Res. Bull. 51 (2014) 153–160.
[25] L. Duan, B. Sun, M. Wei, S. Luo, F. Pan, A. Xu, X. Li, J. Hazard. Mater. 285 (2015)
56–365.
26] S. Luo, L. Duan, B. Sun, M. Wei, X. Li, A. Xu, Appl. Catal. B: Environ. 164 (2015)
2–99.
3
[
9
[27] G. Qiu, H. Huang, S. Dharmarathna, E. Benbow, L. Stafford, S.L. Suib, Chem.
Mater. 23 (2011) 3892–3901.
Please cite this article in press as: V.V. Dutov, et al., Silica-supported silver-containing OMS-2 catalysts for ethanol oxidative dehydro-
genation, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.058

G Model
CATTOD-10309; No. of Pages10
ARTICLE IN PRESS
V.V. Dutov et al. / Catalysis Today xxx (2016) xxx–xxx
1
0
[
[
[
[
[
[
[
[
28] G.G. Xia, Y.G. Yin, W.S. Willis, J.Y. Wang, S.L. Suib, J. Catal. 185 (1999) 91–105.
29] J. Liu, V. Makwana, J. Cai, S.L. Suib, J. Phys. Chem. B 107 (2003) 9185–9194.
30] V.P. Santos, M.F.R. Pereira, Appl. Catal. B: Environ. 99 (2010) 353–363.
31] R. Kumar, S. Sithambaram, S.L. Suib, J. Catal. 262 (2009) 304–313.
32] R. Ghosh, X. Shen, J.C. Villegas, J. Phys. Chem. B 110 (2006) 7592–7599.
33] H. Tian, J. He, L. Liu, Ceram. Int. 39 (2013) 315–321.
34] C. Almquist, M. Krekeler, L. Jiang, Chem. Eng. J. 252 (2014) 249–262.
35] Y.F. Shen, R.P. Zerger, R.N. DeGuzman, S.L. Suib, L. McCurdy, D.I. Potter, C.L.
O’Young, Science 260 (1993) 511–515.
[48] R.N. DeGuzman, Y.-F. Shen, E.J. Neth, S.L. Suib, C.-L. O’Young, S. Levine, J.M.
Newsam, Chem. Mater. 6 (1994) 815–821.
[49] M. Özacar, A.S. Poyraz, H.C. Genuino, C.-H. Kuo, Y. Meng, S.L. Suib, Appl. Catal.
A: Gen. 463–463 (2013) 64–74.
[50] L. Wu, F. Xu, Y. Zhu, A.B. Brady, J. Huang, J.L. Durham, E. Dooryhee, A.C.
Marschilok, E.S. Takeuchi, K.J. Takeuchi, ACS Nano 9 (2015) 8430–8439.
[51] Z. Qu, Y. Bu, Y. Qin, Y. Wang, Q. Fu, Appl. Cat. B: Environ. 132–133 (2013)
353–362.
[52] X. Zhang, Z. Qu, X. Li, M. Wen, X. Quan, D. Ma, J. Wu, Sep. Purif. Technol. 72
(2010) 395–400.
[
[
36] V.D. Makwana, Y.C. Son, A.R. Howel, J. Catal. 210 (2002) 46–52.
37] W. Zhong, S.R. Kirk, D. Yin, Y. Li, R. Zou, L. Mao, G. Zou, Chem. Eng. J. 280
[53] A.R. Gandhe, J.S. Rebello, J.L. Figueiredo, Appl. Catal. B: Environ. 72 (2007)
129–135.
(
2015) 737–747.
[
[
38] V. Mahdavi, M. Mardani, Mater. Chem. Phys. 155 (2015) 136–146.
39] J.M. Zamaro, A.V. Boix, A. Martinez-Hernandez, Catal. Commun. 69 (2015)
[54] R. Hu, C. Yan, L. Xie, Int. J. Hydrogen Energy 36 (2011) 64–71.
[55] R. Xu, X. Wang, D. Wang, K. Zhou, Y. Li, J. Catal. 237 (2006) 426–430.
[56] M. Bowker, M.A. Barteau, R.J. Madix, Surf. Sci. 92 (1980) 528–548.
[57] G.V. Mamontov, O.V. Magaev, A.S. Knyazev, O.V. Vodyankina, Catal. Today 203
(2013) 122–126.
[58] G.V. Mamontov, V.V. Dutov, V.I. Sobolev, O.V. Vodyankina, Kinet. Catal. 54
(2013) 487–491.
[59] A.S. Savel’eva, O.V. Vodyankina, Rus. J. Phys. Chem. A 88 (2014) 2203–2208.
[60] Z. Li, J. Xu, X. Gu, K. Wang, W. Wang, X. Zhang, Z. Zhang, Y. Ding, Chem. Cat.
Chem. 5 (2013) 1705–1708.
2
12–216.
40] H. Yang, J. Deng, S. Xie, Y. Jiang, H. Dai, C.T. Au, Appl. Catal. A: Gen. 507 (2015)
39–148.
[
[
[
1
41] M.A. Peluso, E. Pronsato, J.E. Sambeth, H.J. Thomas, G. Busca, Appl. Catal. B:
Environ. 78 (2008) 73–79.
42] H. Perez, P. Navarro, G. Torres, O. Sanz, M. Montes, Catal. Today 212 (2013)
1
49–156.
[
[
[
43] H. Sun, Z. Liu, S. Chen, X. Quan, Chem. Eng. J. 270 (2015) 58–65.
44] Y. Yang, J. Huang, S. Zhang, Appl. Catal. B: Environ. 150–151 (2014) 167–178.
45] O.V. Vodyankina, A.S. Blokhina, I.A. Kurzina, V.I. Sobolev, K.Yu. Koltunov, L.N.
Chukhlomina, E.S. Dvilis, Catal. Today 203 (2013) 127–132.
[61] P. Liu, X. Zhu, S. Yang, T. Li, E.J.M. Hensen, J. Catal. 331 (2015) 138–146.
[62] J. Chen, X. Tang, J. Liu, E. Zhan, J. Li, X. Huang, W. Shen, Chem. Mater. 19 (2007)
4292–4299.
[
[
46] R. Hu, L. Xie, S. Ding, J. Hou, Y. Cheng, D. Wang, Catal. Today 131 (2008)
5
13–519.
47] M.F.L. Haris, C.-Y. Yin, Z.-T. Jiang, B.-M. Goh, X. Chen, W.A. Al-Masry, A.M.
Abukhalaf, M. El-Harbawi, N.M. Huang, H.N. Lim, Ceram. Int. 40 (2013)
1
245–1250.
Please cite this article in press as: V.V. Dutov, et al., Silica-supported silver-containing OMS-2 catalysts for ethanol oxidative dehydro-
genation, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.058