Ethyl and butyl acetate oxidation over manganese oxides

Article abstract of DOI:10.1016/S1872-2067(17)62986-3

Mangenese oxides were synthesized using two new methods, a novel solvent-free reaction and a reflux technique, that produced cryptomelane-type products (K-OMS-2). Oxides were also synthesized using conventional methods and all specimens were applied to the oxidation of ethyl acetate and butyl acetate, acting as models for the volatile organic compounds found in industrial emissions. The catalysts were also characterized using N₂ adsorption, X-ray diffraction, scanning electron microscopy, temperature programmed reduction and X-ray photoelectron spectroscopy. Each of the manganese oxides was found to be very active during the oxidation of both esters to CO₂, and the synthesis methodology evidently had a significant impact on catalytic performance. The K-OMS-2 nanorods synthesized by the solvent-free method showed higher activity than K-OMS-2 materials prepared by the reflux technique, and samples with cryptomelane were more active than those prepared by the conventional methods. The catalyst with the highest performance also exhibited good stability and allowed 90% conversion of ethyl and butyl acetate to CO₂ at 213 and 202 °C, respectively. Significant differences in the catalyst performance were observed, clearly indicating that K-OMS-2 nanorods prepared by the solvent-free reaction were better catalysts for the selected VOC oxidations than the mixtures of manganese oxides traditionally obtained with conventional synthesis methods. The superior performance of the K-OMS-2 catalysts might be related to the increased average oxidation state of the manganese in these structures. Significant correlations between the catalytic performance and the surface chemical properties were also identified, highlighting the K-OMS-2 properties associated with the enhanced catalytic performance of the materials.

Full text of DOI:10.1016/S1872-2067(17)62986-3

Chinese Journal of Catalysis 39 (2018) 27–36
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Ethyl and butyl acetate oxidation over manganese oxides
Olívia Salomé G. P. Soares, Raquel P. Rocha, José J.M. Órfão, Manuel F. R. Pereira,
José L. Figueiredo *
Laboratory of Separation and Reaction Engineering‐Laboratory of Catalysis and Materials (LSRE‐LCM), Faculty of Engineering, University of Porto, Rua
Dr. Roberto Frias s/n, 4200‐465 Porto, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history:
Mangenese oxides were synthesized using two new methods, a novel solvent‐free reaction and a
reflux technique, that produced cryptomelane‐type products (K‐OMS‐2). Oxides were also synthe‐
sized using conventional methods and all specimens were applied to the oxidation of ethyl acetate
and butyl acetate, acting as models for the volatile organic compounds found in industrial emis‐
sions. The catalysts were also characterized using N2 adsorption, X‐ray diffraction, scanning electron
microscopy, temperature programmed reduction and X‐ray photoelectron spectroscopy. Each of the
manganese oxides was found to be very active during the oxidation of both esters to CO2, and the
synthesis methodology evidently had a significant impact on catalytic performance. The K‐OMS‐2
nanorods synthesized by the solvent‐free method showed higher activity than K‐OMS‐2 materials
prepared by the reflux technique, and samples with cryptomelane were more active than those
prepared by the conventional methods. The catalyst with the highest performance also exhibited
good stability and allowed 90% conversion of ethyl and butyl acetate to CO2 at 213 and 202 °C,
respectively. Significant differences in the catalyst performance were observed, clearly indicating
that K‐OMS‐2 nanorods prepared by the solvent‐free reaction were better catalysts for the selected
VOC oxidations than the mixtures of manganese oxides traditionally obtained with conventional
synthesis methods. The superior performance of the K‐OMS‐2 catalysts might be related to the
increased average oxidation state of the manganese in these structures. Significant correlations
between the catalytic performance and the surface chemical properties were also identified, high‐
lighting the K‐OMS‐2 properties associated with the enhanced catalytic performance of the materi‐
als.
Received 7 November 2017
Accepted 21 November 2017
Published 5 January 2018
Keywords:
Volatile organic compound
Ethyl acetate
Butyl acetate
Manganese oxide
Catalytic oxidation
© 2018, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
ozone and photochemical smog. Because of concerns regarding
the continual increases in VOC emissions, their release into the
Volatile organic compounds (VOCs) are recognized as major
contributors to air pollution. These compounds are toxic and
can also act as precursors to the formation of ground level
environment is controlled by stringent regulations. In this con‐
text, catalytic oxidation is considered a promising technology
for the control of VOC emissions [1,2]. The efficiency of catalytic
* Corresponding author. Tel: +351‐225‐081998; Fax: +351‐225‐081449; E‐mail: jlfig@fe.up.pt
This work was supported by project “AIProcMat@N2020‐Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal
2020”, with the reference NORTE‐01‐0145‐FEDER‐000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the
Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and of Project POCI‐01‐0145‐FEDER‐006984 –
Associate Laboratory LSRE‐LCM funded by ERDF through COMPETE2020‐Programa Operacional Competitividade e Internacionalizacao (POCI) – and
by national funds through FCT‐Fundacao para a Ciencia e a Tecnologia. R.P. Rocha acknowledges FCT grant SFRH/BD/95411/2013.
DOI: 10.1016/S1872‐2067(17)62986‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 39, No. 1, January 2018

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Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
oxidation depends greatly on the type of catalyst employed,
and the most effective catalysts will be determined by the
properties of the effluent to be treated, including chemical
composition, flow rate, presence of poisons or/and inhibitors,
and by inlet temperature constraints.
2.1. Catalyst synthesis
Manganese oxide catalysts were synthesized by different
routes. A cryptomelane type (sample Mn1) was prepared using
a reflux method [21,22]. Briefly, 11 g of Mn(CH3COOH) was
dissolved in 40 mL of water and the pH of this solution was
adjusted with concentrated nitric acid to a value of 3.5. Next,
6.5 g of KMnO4 was dissolved in 150 mL of water and this solu‐
tion was slowly added to the previous mixture. A black precip‐
itate formed upon stirring and the mixture was refluxed at 100
°C for 24 h. The solid was filtered, washed with distilled water,
dried in an oven and calcined at 450 °C in air for 4.5 h. Other
samples were prepared by a novel solvent‐free solid‐state re‐
action adapted from the procedure described by Ding et al.
[23]. The KMnO4 and Mn(CH3COOH)2 were combined at a stoi‐
chiometric molar ratio of 2:3 and ball‐milled (Mixer Mill MM
200, Retsch, Haan, Germany) for 1 h at either 5 vibration/s
(sample Mn2) or 10 vibration/s (sample Mn3). In each case, the
resulting black solid was transferred to a capped bottle and
heated at 80 °C for 4 h. The product was then washed with wa‐
ter until the wash water was neutral, dried at 100 °C overnight
and finally calcined at 450 °C for 4.5 h. Traditional methods
were also used to synthesize four additional samples via the
direct calcination of the Mn(CH3COOH) at 450 °C for 4.5 h in
two different ovens (vertical (sample MnCV) or horizontal
(sample MnCH)) and by precipitation with NaOH using
Mn(CH₃COO)2 (sample MnPA) or Mn(NO3)2 (sample MnPN) as
the precursor, based on a procedure described elsewhere [24].
The most commonly used catalysts for the catalytic oxida‐
tion of VOCs are supported noble metals (Pt or Pd) [3,4], which
are generally considered to be more active than metal oxides
[5–8]. However, Co, Cu, Ni and especially Mn oxides have also
demonstrated high catalytic activities [9–11]. These oxides
have the additional advantages of lower cost and higher re‐
sistance to deactivation by poisoning in comparison to sup‐
ported noble metals. Manganese oxides (α‐MnO2, β‐MnO2,
γ‐MnO2, Mn3O4, Mn2O3) have been extensively studied as cata‐
lysts for the oxidation of many pollutants. The catalytic per‐
formance of MnOx compounds is attributed to the ability of
manganese to form oxides with different oxidation states hav‐
ing high oxygen storage capacities [2,12]. Although the nature
of the active sites associated with catalytic oxidation is not well
understood, several authors attribute the high activity of these
oxides to the mixed valence state of the manganese framework
and to the higher mobility of lattice oxygen in these com‐
pounds, suggesting that lattice oxygen participates in the oxi‐
dation process [13,14]. Moreover, the presence of structural
defects such as cationic vacancies and chemical defects have
also been found to improve catalyst performance [15,16].
Among the manganese oxides, cryptomelane (K‐OMS‐2) is
of particular interest due to its open tunnel structure, mixed
manganese valence states and highly mobile lattice oxygen.
Cryptomelane‐type manganese oxide (KMn8O16) was found to
be very active during the oxidation of several types of VOCs,
and its performance is evidently significantly affected by the
presence of other phases, namely Mn2O3 and Mn3O4 [17]. The
latter improves catalytic performance by increasing the reac‐
tivity and mobility of lattice oxygen, while the former has the
opposite effect.
The development of fast, simple and low cost methods to
produce highly active catalysts for VOC oxidation is still a chal‐
lenge, considering that this would require the scaled‐up pro‐
duction of various materials. The aim of the present work was
to prepare stable, efficient manganese oxides for use as cata‐
lysts for VOC oxidation. These materials were synthesized us‐
ing new techniques involving a novel solvent‐free reaction or a
reflux method to synthesize cryptomelane‐type manganese
oxides, but also by conventional methods. The goal of this study
was to assess the effects of the oxide surface area, morphology,
surface reducibility and redox properties on the catalytic per‐
formance. These catalysts were tested based on the oxidation
of two esters representative of those found in industrial emis‐
sions: ethyl and butyl acetate. These esters were selected be‐
cause the former is among the most studied catalytic oxidation
model compounds while there have been few reports of the
catalytic oxidation of the latter [18,19]. In addition, esters are
generally reported to be the most difficult organic solvents to
oxidize completely [20].
2.2. Catalyst characterization
Nitrogen adsorption isotherms were acquired at –196 °C
with a Quantachrome NOVA 4200e multi‐station apparatus to
evaluate the textural properties of the manganese oxide sam‐
ples. The surface areas of the oxides (SBET) were determined
according to the Brunauer‐Emmett‐Teller method. The struc‐
ture and phase purity of each prepared material was analysed
by X‐ray diffraction (XRD) with a Philips X’Pert MPD diffrac‐
tometer (Cu‐Kα, λ = 0.15406 nm). Data were collected over the
range of 2θ = 20°–80°. The crystallite sizes in the catalysts were
determined by Rietveld refinement.
Scanning electron microscopy (SEM)/energy dispersive
spectroscopy (EDS) observations were performed using a high
resolution (Schottky) environmental SEM instrument in con‐
junction with X‐Ray microanalysis and electron backscattered
diffraction (Quanta 400 FEG ESEM/EDAX Genesis X4M).
The reactivities of the oxygen species present in the man‐
ganese oxides were assessed based on temperature pro‐
grammed reduction (TPR) data obtained with a fully automat‐
ed AMI‐200 Catalyst Characterization apparatus (Altamira In‐
struments) equipped with a thermal conductivity detector
(TCD) to determine the hydrogen consumption during the re‐
duction step. In a typical run, 0.050 g of the sample was placed
in a U‐shaped quartz tube located inside an electrical furnace
and heated from room temperature to 600 °C at 5 °C min–1 un‐
der a flow of 5 vol%. H2/Ar at 30 cm3 min–1, while the hydrogen
consumption was monitored using the TCD. At the end of each
2. Experimental

Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
29
analysis, hydrogen calibration was carried out at 50 °C.
Table 1
The X‐ray photoelectron spectroscopy (XPS) analyses were
performed using a Kratos AXIS Ultra HAS. Data were acquired
with a monochromatic Al Kα X‐ray source (1486.7 eV) operat‐
ing at 15 kV (90 W) in the fixed analyser transmission (FAT)
mode with a pass energy of 40 eV for regions of interest and 80
eV for surveying.
Data from the characterization of manganese oxide by nitrogen adsorp‐
tion, XRD and TPR.
Crystalline
phases
(XRD)
Crystallite H2 consumption Tpeakmax
SBET
Sample
Mn1
sizes (nm)
(XRD)
(μmol g–1)
(TPR)
(°C)
(TPR)
(m2 g–1)
22
KMn8O16 (74%)
α‐MnO2 (26%)
KMn8O16
29
39
18
10
16
30
23
34
34
31
27
43
28
41
43
698
350
Mn2
Mn3
96
1003
604
320
305
2.3. Experimental procedures
120 KMn8O16 (82%)
α‐MnO2 (18%)
The catalytic oxidation of the VOCs was performed under
atmospheric pressure in a fixed‐bed reactor consisting of a
U‐shaped quartz reactor (6 mm internal diameter) inside a
temperature‐controlled electric furnace. In preparation for
each run, 50 mg of catalyst was mixed with an inert material
(carborundum) of the same particle size as the catalyst (0.2‒0.5
mm) to minimize thermal effects. Prior to each trial, the cata‐
lyst was pre‐treated in air at 400 °C for 1 h. The oxidation ex‐
periment was then initiated using a VOC concentration of 1000
mg cm–3 and a space velocity of 53,050 h–1 in conjunction with a
total feed flow rate of 500 cm3 min–1. The reaction was carried
out in two cycles consisting of increasing and decreasing tem‐
perature at a heating/cooling rate of 2.5 °C min–1.
MnCH
MnCV
10
Mn3O4(68%)
Mn5O8(22%)
Mn2O3 (10%)
Mn3O4 (66%)
Mn2O3 (30%)
Mn5O8 (4%)
Mn3O4 (81%)
Mn5O8 (19%)
Mn3O4 (95%)
Mn5O8 (5%)
351
333
430
413
11
MnPA
MnPN
22
16
321
339
455
420
ysis, and the associated data are summarized in Table 1, while
Fig. 1 presents XRD patterns of the materials. Significant dif‐
ferences are evident between these patterns, indicating the
presence of different manganese oxide phases. As expected,
cryptomelane phases were obtained in the specimens prepared
by reflux (Mn1) and solid state reaction methods (Mn2 and
Mn3). The Mn2 consisted of a pure cryptomelane phase
(KMn8O16), whereas the Mn1 and Mn3 were mixtures of
KMn8O16 and α‐MnO2. Some unidentified peaks in these pat‐
terns can possibly be ascribed to the presence of K [25]. Com‐
paring the XRD patterns of samples the Mn1, Mn2 and Mn3, the
latter exhibits more peak broadening, implying smaller crystal‐
lite sizes in this specimen. The crystallite domain sizes in the
cryptomelane‐type materials were in fact quite different; sam‐
ples Mn1, Mn2 and Mn3 had crystallite sizes (as calculated by
Rietveld refinement using the [211] plane) of 29, 18 and 10 nm,
The composition of the gas stream was determined by gas
chromatography, using a Master GC Dani equipped with a flame
ionization detector and by a CO2 non‐dispersive infrared
(NDIR) sensor (Vaisala GMT220). The conversion to CO2 (XCO2
)
was calculated as XCO2 = FCO2/(v·FVOC,in), where FVOC,in is the inlet
molar flow rate of the VOC, FCO2 is the outlet molar flow rate of
CO2 and v is the number of carbon atoms in the VOC molecule
(v = 4 for ethyl acetate and v = 6 for butyl acetate). Selected
experiments were carried out in duplicate and the results were
founded to be reproducible with a maximum error of 1%.
3. Results and discussion
3.1. Characterization
3.1.1. Specific surface area
The BET surface areas (SBET) of the manganese oxide cata‐
lysts are provided in Table 1. The nitrogen adsorp‐
tion‐desorption isotherms of the materials prepared by the
same methodology were found to be similar, and the materials
had surface areas between 10 and 120 m2 g–1. The samples
prepared by solid‐state reaction (Mn2 and Mn3) had the high‐
est surface areas, especially the Mn3, which was prepared using
the highest milling vibrational rate, while all the other samples
had very low surface areas. Ding et al. [23] studied K‐OMS‐2
specimens prepared by a solvent‐free method and found that
these materials had SBET much higher than those prepared by
reflux. The samples prepared by calcination of the precursor
(MnCV and MnCH) or by precipitation (MnPN and MnPA) had
the lowest surface areas.
KMn8O16
x
Mn2O3
Mn3O4
o Mn5O8
+
x
 - MnO2
*
x
x
x
o
o
MnPN
x
o
x
x
x
o
x
o
x
o
x
x
x
x
MnPA
MnCV
x
x
x
+
x
o
o
o
o
o
x
o
x
+
+
x
+
x
o
x
o
x
MnCH
*
*
*
*
*
*
*
*
Mn3
Mn2
*
*
*
*
*
*
*
*
Mn1
*
*
20 25 30 35 40 45 50 55 60 65 70 75 80
2/( o )
3.1.2. Structure and morphology
The effect of the synthesis methodology on the crystalliza‐
tion of the manganese oxides was assessed based on XRD anal‐
Fig. 1. XRD patterns of the synthesized materials.

30
Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
Mn1
Mn2
Mn3
MnPN
MnCV
MnPA
MnCH
Fig. 2. SEM micrographs of the manganese oxide catalysts.
respectively. These values are in good agreement with data
reported by Schurz et al. [26], who also obtained crystal sizes
between 10 and 30 nm for cryptomelane samples. It is clear
that the crystal size decreases with increases in the milling
vibration process, as smaller crystals were obtained in the Mn3.
The samples prepared by thermal decomposition of the pre‐
cursor (MnCH and MnCV) evidently contained varying amounts
of Mn3O4, Mn5O8 and α‐Mn2O3, with Mn3O4 present in the high‐
est concentrations. This phase was also predominant in the
samples prepared by precipitation with sodium hydroxide
(MnPA and MnPN), although only Mn3O4 and Mn5O8 were pre‐
sent in those materials as additional phases. Crystal sizes in the
range of 30 to 40 nm were observed for these samples.
Fig. 2 shows SEM micrographs of the materials. The samples
synthesized by the solid‐state reaction method exhibit homo‐
geneous nanorod morphologies. These nanorods are smaller in
the Mn3 compared with the material obtained using the reflux
method (Mn1), which instead shows a fibrous morphology,
typical of cryptomelane prepared by this method. The diame‐
ters of these nanorods are in the range of 10‒20 nm, in agree‐
ment with the particle sizes (10 and 18 nm for Mn3 and Mn2,
respectively) calculated from the XRD patterns. These nano‐
rods were therefore much smaller than those in the sample
prepared by the reflux approach (Mn1). In these SEM images,
Mn 2p
O KLL
O 1s
Mn Auger peaks
Mn 3p
Mn 3s
K 2s
K 2p
C 1s
1000
800
600
400
200
0
B.E. (eV)
(b)
(c)
(d)
O 1s
Mn 2p3/2
Mn 2p1/2
Mn 3s(1)
Mn 3s(2)
660 655 650 645 640 635
B.E. (eV)
95
90
85
B.E. (eV)
80
75
538 536 534 532 530 528 526
B.E. (eV)
Fig. 3. XPS spectra obtained from the Mn1 sample. (a) survey spectrum; (b) Mn 2p; (c) Mn 3s; (d) O 1s.

Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
31
the structure of the Mn3 is finer than that of the Mn2. This is
Table 2
consistent with the BET measurements, because smaller parti‐
cle sizes will usually result in higher surface areas. The samples
prepared with KMnO4 (Mn1, Mn2 and Mn3) show different
morphologies from the others. Samples MnCH or MnCV have
similar homogeneous surface morphologies with pris‐
matic‐shaped particles, while MnPA and MnPN, which were
prepared by the precipitation method, present different surface
morphologies. These results demonstrate that the precursor
used in the precipitation method affects the morphology and
the structure of the resulting manganese oxide. MnPA has a
very heterogeneous surface with prismatic particles having
widely different sizes, while the MnPN shows a more homoge‐
neous surface.
Data from the surface characterization of the manganese oxides as
determined by XPS.
C 1s
(at%) (at%)
O 1s
K 2p1+2p3 Mn 2p3
Sample
Mn/O
AOS
(at%)
(at%)
Mn1
Mn2
Mn3
MnCH
MnCV
MnPA
MnPN
16.4
21.0
18.5
25.6
25.2
24.7
24.8
52.0
50.3
52.0
47.7
48.1
48.6
48.3
4.6
4.2
3.4
0.0
0.0
0.0
0.0
27.0
24.5
26.1
26.7
26.7
26.7
26.9
0.52
0.49
0.50
0.56
0.56
0.55
0.56
3.7
3.7
3.6
2.8
2.7
2.7
2.6
gen (O22– or O–), OH groups and oxygen vacancies (531.2‒531.5
eV), and adsorbed molecular water (532.0‒532.5 eV) is indi‐
cated by two shoulders [17,27].
3.1.3. Manganese valency and reducibility
XPS data were acquired to investigate the manganese oxida‐
tion state on the oxide surfaces. Fig. 3(a) presents the XPS sur‐
vey spectrum acquired using the Mn1 specimen (which was
similar to those obtained from the other samples) over the
binding energy range from 0 to 1000 eV. Characteristic Mn 2p,
Mn 3s, Mn 3p and O 1s core level signals are observed, as well
as Auger signals from Mn and O. The Mn:O atomic ratios on the
specimen surfaces (Table 2) varied between 0.49 and 0.56,
with the samples prepared by the solvent‐free reaction and
reflux techniques (Mn1, Mn2 and Mn3) having lower values.
The shape of the O 1s spectra suggests the presence of overlap‐
ping peaks generated by different oxygen species (Fig. 3(d)). In
the spectrum of each sample, the most intense peak appears in
the binding energy range between 529.9 and 530.3 eV, typically
associated with lattice oxygen (O2–). This is therefore believed
to be the predominant O species in these manganese oxides. At
higher binding energies, the presence of surface adsorbed oxy‐
The oxidation states and relative amounts of various Mn
species in each oxide were ascertained by analysing Mn 2p and
Mn 3s X‐ray core level photoelectron spectra. Fig. 3(b) and (c)
present the Mn 2p and Mn 3s spectra of the Mn1. The Mn 2p3/2
and Mn 2p1/2 centroid between 642.6 and 654.4 eV provides
evidence for the formation of the cryptomelane phase, in
agreement with reported values [28,29]. Although it was diffi‐
cult to identify the oxidation states, the binding energy shifts in
the Mn 2p and Mn 3s regions can provide some information. In
particular, the magnitude of the Mn3s multiplet splitting can be
used to calculate the average oxidation state (AOS) of manga‐
nese, according to the relationship AOS = 8.95‒1.13 ∆Es (eV)
where ∆Es represents the extent of multiplet splitting (the dis‐
tance between the main peak and its satellite) [30]. Fig. 4(b)
shows the Mn 3s spectra for the prepared manganese oxides
highlighting the ∆Es parameter. Table 2 summarizes the corre‐
sponding AOS values. It is evident that samples prepared by the
ΔES
(a)
(b)
Mn1
Mn1
Mn2
Mn3
Mn2
Mn3
MnCH
MnCV
MnCH
MnCV
MnPA
MnPN
MnPA
MnPN
660
655
650
B. E. (eV)
645
640
635
95
90
85
80
75
B. E. (eV)
Fig. 4. The Mn 2p (a) and Mn 3s (b) XPS spectra of the manganese oxides.

32
Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
solvent‐free reaction and reflux methods (Mn1, Mn2 and Mn3)
had higher AOS values (3.6‒3.7) while the oxides obtained us‐
ing the conventional methods had lower values (2.6‒2.8). All
specimens generated similar Mn 2p core level spectra (Fig.
4(a)). Based on the theoretical oxidation numbers of manga‐
nese stabilized as KMn8O12, MnO2, Mn3O4, Mn5O8 and Mn2O3
(the crystalline phases identified by XRD) and the relative pro‐
portions of these different phases as reported in Table 1, the
average bulk oxidation states for the manganese samples as
determined by XRD show a maximum deviation of 8% relative
to the AOS values calculated from XPS data. Thus there is good
agreement between the results obtained from both techniques.
As an example, the MnPN was found to contain 95% Mn3O4 and
5% Mn5O8, corresponding to an average bulk oxidation state of
2.7 [95% × (+2.67) + 5% × (+3.2)], while an AOS of 2.6 was
calculated using the XPS results.
H2‐TPR experiments were performed to evaluate the reac‐
tivity of O species in these materials with hydrogen (that is, the
reducibility of the catalysts). Fig. 5 provides the TPR profiles of
the different materials, each of which is characterized by the
presence of overlapping components between 200 and 500 °C.
However, there are evident differences between the Mn1, Mn2
and Mn3 (samples prepared by the solvent‐free and reflux
methods) and MnCH, MnCV, MnPA and MnPN (produced by
conventional methods). In the first group, the TPR profiles are
narrower, with hydrogen consumption between 200 and 375
°C, and exhibit intense peaks with maxima in the range of
300‒350 °C. The remaining samples (MnCH, MnCV, MnPA and
MnPN) produced TPR profiles that are wider and in which the
major peaks appear at higher temperatures. In keeping with
the results of the XRD and XPS analyses, these data suggest that
the oxides contain a variety of crystalline phases that signifi‐
cantly affect their reducibility. Furthermore, the data in Table 1
demonstrate that both the onset of reduction and the maximum
temperature of the most intense reduction peak increase as the
average oxidation state decreases (Table 2). Table 1 also shows
that a greater degree of hydrogen consumption was exhibited
by the Mn1, Mn2 and Mn3.
The low specific surface area of the Mn1 may explain the rela‐
tively high temperature of the first reduction step [31]. These
results clearly indicate that the Mn2 and Mn3 were reduced
more readily than the Mn1.
Comparing the TPR profiles of the MnCH, MnCV, MnPA and
MnPN, the temperature for the onset of reduction is similar for
each. In addition, the temperature of the most intense reduc‐
tion peak is much higher in these samples, while their hydro‐
gen consumption values are significantly lower.
3.2. Catalytic tests
Both ethyl and butyl acetate were oxidized over the manga‐
nese oxide materials to evaluate the effects of the oxide physi‐
cal and chemical properties on catalytic performance. Temper‐
ature programmed experiments (so‐called “light‐off curves”)
were employed for rapid screening. In this type of test, the cat‐
alysts with the highest performance can be identified based on
achieving a specific conversion value at a lower temperature.
These trials also demonstrated that each of the oxides was
highly stable. Each test consisted of two cycles increasing and
decreasing temperature at a heating/cooling rate of 2.5 °C
min–1 and the catalytic performance was very similar during
the increasing and decreasing temperature steps. The oxide
catalysts were compared based on the results obtained during
the decreasing temperature cycle, and the catalytic perfor‐
mance is plotted as functions of reaction temperature in Figs. 6
and 7. Table 3 gives the temperatures required to achieve 50%
(T50) and 90% (T90) conversions of ethyl acetate and butyl ace‐
tate to CO2. Each of the materials exhibited activity during the
VOC oxidation reactions, although the catalytic performance
varied depending on the preparation method. The following
trend was observed with regard to decreasing performance:
Mn2 ≈ Mn3 ˃ Mn1 ˃ MnPA > MnPN > MnCV ≈ Mn
CH. The data show that the T90 values were between 213 and
265 °C, and between 202 and 265 °C for ethyl acetate and butyl
acetate, respectively.
The catalytic activity of manganese oxides during VOC oxi‐
dation is typically determined by the manganese oxidation
The hydrogen consumption of the Mn1 begins at approxi‐
mately 250 °C, while the consumption onset associated with
the Mn2 and Mn3 is shifted to lower temperatures (< 200 °C).
1.0
Mn1
Mn2
Mn3
MnPA
MnPN
MnCH
MnCV
0.0015
0.8
0.6
0.4
0.2
0.0
Mn1
Mn2
Mn3
MnCH
MnCV
PA
0.0012
0.0009
PN
0.0006
0.0003
0.0000
100 120 140 160 180 200 220 240 260 280 300
T/oC
0
100
200
300
400
500
600
Temperature (oC)
Fig. 6. Conversion to CO2 as a function of temperature during ethyl
acetate oxidation.
Fig. 5. TPR profiles of the manganese oxides.

Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
33
der such conditions, this demonstrates the involvement of lat‐
tice oxygen. The highest CO2 releases during these experiments,
for both VOCs, were obtained using samples prepared by the
solvent‐free and reflux approaches. In the case of ethyl acetate
oxidation, the CO2 production varied between 576 and 705
µmol g–1 when using the Mn1, Mn2 and Mn3, compared with
values of 120 to 499 µmol g–1 when using the MnPA, MnPN,
MnCH and MnCV. The oxidation of butyl acetate gave the same
trend, with 320 to 541 µmol g–1 CO2 formed using the Mn1,
Mn2 and Mn3 and only between 59 and 151 µmol g–1 resulting
from the MnPA, MnPN, MnCH and MnCV.
These experiments also confirmed that the oxides with the
highest oxidation states exhibit the best oxidation activities. In
fact, a trend can be observed in Fig. 8(a) and (b) in which the
Mn1, Mn2 and Mn3 (having the highest AOS values of 3.6‒3.7)
are the most active and thus show the lowest T90 values for
ethyl and butyl acetate. Conversely, the MnPA, MnPN, MnCH
and MnCV (with lower AOS values of 2.6‒2.8) require higher
temperatures for the complete oxidation of the esters. This is
closely related to the two different types of samples prepared:
one with and another without cryptomelane‐type manganese
oxides. Due to the close AOS values of the two groups of sam‐
ples, a clear differentiation of their performances based exclu‐
sively on this parameter is not possible. For instance, samples
Mn1 and Mn2 have the same AOS but slightly different catalytic
performance. Therefore, it can be concluded that the perfor‐
mance of these samples must also be related to the presence of
small amounts of α‐MnO2, as detected by XRD analyses of the
Mn1 and Mn3, in contrast with the Mn2, which was purely
cryptomelane.
In a previous study, Luo et al. [21] attributed the superior
catalytic performance of cryptomelane materials to their strong
affinity for organic compounds. In a later work, Chen et al. [32]
suggested that the elevated activity of cryptomelane was due to
the tunnel structure of this phase, which promotes the adsorp‐
tion of organic compounds and the subsequent reaction. How‐
ever, the presence of other phases significantly affects the
cryptomelane performance. As an example, Santos et al. [17]
observed that the presence of Mn3O4 improves the catalytic
performance of cryptomelane due to the increased the mobility
of lattice oxygen, while the Mn2O3 phase has the opposite effect.
A qualitative evaluation of the present catalytic results con‐
firms that samples with a cryptomelane structure (Mn1, Mn2
1.0
0.8
0.6
0.4
0.2
0.0
Mn1
Mn2
Mn3
MnPA
MnPN
MnCH
MnCV
100 120 140 160 180 200 220 240 260 280 300
T/oC
Fig. 7. Conversion to CO2 as a function of temperature during butyl
acetate oxidation.
state, lattice oxygen content, degree of crystallinity, specific
surface area or even reducibility. However, the results obtained
in this study do not provide evidence for a clear correlation
between catalytic performance and the various properties of
the catalysts. As an example, the performance of these materi‐
als cannot be related to the surface area, as samples Mn2 and
Mn3 (both prepared by solid‐state reaction) show essentially
the same level of activity in spite of their different surface are‐
as. In addition, the Mn1 and MnPA have the same surface area
but significantly different catalytic abilities. Therefore, these
variations in performance must be related to the manganese
oxide species present on the catalyst surface, and consequently
to the AOS value. It is well established that the oxidation of
organic molecules over manganese oxide catalysts at low tem‐
peratures involves a Mars‐van‐Krevelen mechanism [5,10,14]
that includes a redox cycle on the catalyst surface with the par‐
ticipation of lattice oxygen. The involvement of lattice oxygen in
the VOC oxidation process can be evaluated by experiments
without oxygen in the feed. Thus, additional oxidation experi‐
ments were performed, in which the air was replaced with
nitrogen. In these trials, following the pre‐treatment step, the
reactor was purged with nitrogen to remove physisorbed oxy‐
gen. Subsequently, the reactions were performed using the
same VOC composition as in the oxidation reactions, and the
resulting CO2 was measured. If VOC oxidation is observed un‐
Table 3
Catalytic performances of the manganese oxides as indicated by the temperatures required to achieve 50% and 90% conversions of ethyl acetate
(EtAC) and butyl acetate (BtAc) to CO2 (T50 and T90).
EtAc
BtAc
Sample
X [CO2]
X [EtAc]
X [CO2]
X [BtAc]
T50 (°C)
213
T90 (°C)
219
T50 (°C)
213
T90 (°C)
219
T50 (°C)
210
T90 (°C)
228
T50 (°C)
190
T90 (°C)
207
Mn1
Mn2
200
213
198
213
188
202
175
196
Mn3
200
213
198
213
190
208
171
194
MnPA
MnPN
MnCH
MnCV
229
234
245
245
229
232
249
245
218
229
233
251
151
210
180
233
256
257
263
265
256
256
264
266
249
244
265
255
220
214
255
245

34
Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
300
250
200
150
300
250
200
150
300
250
200
150
(c)
(a)
(b)
without
KMn8O16
without
KMn8O16
MnCV
MnPN
MnCH
MnPA
Mn1
Mn2
Mn3
with
KMn8O16
with
KMn8O16
R2=0.803
2.0
2.5
3.0
3.5
4.0
0.70
0.75
0.80
0.85
0.90
2.0
2.5
3.0
3.5
4.0
AOS
AOS
Mn/O
300
250
200
150
300
250
200
150
300
(f)
(e)
(d)
MnCV
MnCV
MnPN
MnPN
MnCH
MnCH
MnCH
MnPN
MnPA
250
200
150
MnCV
Mn1
Mn1
MnPA
MnPA
Mn3
Mn2
Mn3
Mn1
Mn2
0.75
Mn3
Mn2
R2=0.813
0
10
20
30
40
50
0.70
0.80
0.85
0.90
0
10
20
30
40
50
TPeak Max / SBET (oC (m2 g-1)-1)
TPeak Max / SBET (oC (m2 g-1)-1)
Mn/O
Fig. 8. T90 values for the oxidation of ethyl acetate (a, c, e) and butyl acetate (b, d, f) as functions of AOS (a, b), the Mn/O ratio (c, d) and the tempera‐
ture of the most intense peak obtained from TPR normalized by the specific surface area (e, f).
and Mn3) had the best catalyst performance. Furthermore, it
appears that the presence of α‐MnO2 may negatively affect
performance. Comparing the samples with low AOS values
(MnCH, MnCV, MnPA and MnPN) and a mixture of phases, it can
be concluded that the presence of α‐Mn2O3 decreases the cata‐
lytic performance.
Nevertheless, a close correlation between the VOC oxidation
performance and the Mn/O ratios calculated from XPS data is
evident in Fig. 8(c) and (d), demonstrating the important role
of surface oxygen in the catalytic performance of the samples.
The oxide with the lowest Mn/O ratio (Mn2) had the highest
activity, while the materials prepared by the conventional
methods and having the highest Mn/O ratios produced the
highest T90 values (i.e., had the lowest activities).
method and reflux approach had higher amounts of surface
lattice oxygen (as indicated by the OI peak intensities) and
lower reduction temperatures compared with the oxides ob‐
tained using conventional methods. However, comparing the
Mn1 and Mn2, it can be concluded that the catalytic perfor‐
mance is not determined solely by the concentration of surface
lattice oxygen, because the Mn1 contained more of this type of
oxygen but showed slightly lower performance than the Mn2.
In summary, it is evident that the surface chemistry of the
two sets of samples was more important than the textural
properties in determining catalytic performance during VOC
oxidation, although no direct correlations can be shown for
each individual parameter. The data also show that an efficient
manganese oxide catalyst (Mn2) was prepared using the sol‐
vent‐free technique and could have applications as an alterna‐
tive to the noble metal catalysts more typically used in the oxi‐
dation of VOCs.
A strong relationship between T90 and the reduction tem‐
perature of the most intense peak normalized by the surface
area is also apparent (Fig. 8(e) and (f)). This parameter,
TPeakMax/SBET, emphasizes the simultaneous importance of lat‐
tice oxygen (hence reducibility) and available surface area on
the catalytic oxidation of the VOCs. Accordingly, the samples
with the lowest reduction temperatures showed higher cata‐
lytic activities and required lower temperatures for the total
oxidation of ethyl and butyl acetate. Previous works [11,33,34]
have also demonstrated a relationship between the catalytic
performance and the reducibility of the catalysts, which is in
agreement with the results obtained in this work. These find‐
ings confirm that the oxidation of the VOCs proceeds via the
Mars‐van‐Krevelen mechanism, such that the surface lattice
oxygen of the catalysts is involved in the reaction with the VOC.
As a result, the reducibility of the material is an important fac‐
tor.
3.3. Stability tests
Prolonged use experiments were carried out with the Mn2
to examine the stability of the catalyst. This specimen was se‐
lected due to its high catalytic performance. The conversions of
ethyl acetate and butyl acetate at 210 and 200 °C as a function
of time are plotted in Fig. 9(a) and (b), respectively. These reac‐
tion temperatures were employed to achieve a high initial VOC
conversion of approximately 90%. The ethyl acetate conversion
evidently increased by 13% over a span of 150 h, while the
catalyst remained stable during butyl acetate oxidation over
approximately 180 h, showing a conversion of 90% over the
entire test. Similar stability behaviour was also observed when
assessing conversion to CO2.
The samples prepared in this work by the novel solvent‐free

Olívia Salomé G. P. Soares et al. / Chinese Journal of Catalysis 39 (2018) 27–36
35
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
0.8
(b)
0.6
0.4
0.2
0.0
0
30
60
90
120 150 180
0
30
60
90
120
150
t (h)
t (h)
Fig. 9. The VOC conversion as a function of time using the Mn2 during the oxidation of ethyl acetate at 210 °C (a) and butyl acetate at 200 °C (b).
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The preparation methodology was found to play an important
role in determining the catalytic performance, although each of
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tate or butyl acetate to CO2. The catalysts prepared by the re‐
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The authors thank Dr. Carlos Sá at CEMUP for assistance
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Graphical Abstract
Chin. J. Catal., 2018, 39: 27–36 doi: 10.1016/S1872‐2067(17)62986‐3
Ethyl and butyl acetate oxidation over manganese oxides
Olívia Salomé G. P. Soares, Raquel P. Rocha, José J. M. Órfão, Manuel F. R. Pereira, José L. Figueiredo *
University of Porto, Portugal
During trials involving the oxidation of ethyl acetate and butyl acetate over manganese oxide catalysts, samples containing a cryptome‐
lane phase and synthesized by a novel solvent‐free method showed higher catalytic performance.
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氧化镁上乙酸乙酯和乙酸丁酯氧化反应
Olívia Salomé G. P. Soares, Raquel P. Rocha, José J. M. Órfão, Manuel F. R. Pereira,
José L. Figueiredo *
波尔图大学工程学院分离与反应工程实验室-催化与材料实验室(LSRE-LCM), Rua Dr. Roberto Frias s/n,
波尔图4200-465, 葡萄牙
摘要: 采用新型无溶剂反应和回流的方法制得锰钾矿型氧化镁(K-OMS-2), 同时采用常规方法制得氧化镁, 并测试不同催
化剂对工业排放气中有机挥发性物质(VOCs)中的模型化合物––乙酸乙酯和乙酸丁酯的催化氧性能. 采用N2吸附-脱附、X
射线衍射、扫描电镜、程序升温还原和X射线光电子能谱等技术对催化剂进行了表征. 所有氧化镁样品均表现出很高的催
化乙酸乙酯和乙酸丁酯氧化生成CO2的活性, 且制备方法对催化剂性能起着重要作用. 新型无溶剂法制得的K-OMS-2纳米
棒样品比常规的回流法制得样品表现出更好的催化性能, 含锰钾矿型氧化镁的样品比常规方法制得样品表现出更高的活
性. 性能最好的催化剂也表现出较高的稳定性, 在213和202 oC条件下, 可分别使90%的乙酸乙酯和乙酸丁酯转化为CO2.
催化剂性能的显著差异清楚地表明, 对于所选VOCs氧化反应, 采用新型无溶剂法制得的K-OMS-2纳米棒样品比常规法制
备的氧化镁混合物更好, 这可能与样品结构中含有更高的Mn平均氧化态有关. 本文表明了催化剂性能与其表面化学性质
间存在显著的关联, 显示了K-OMS-2内在性质决定了其高的催化性能.
关键词: 挥发性有机化合物; 乙酸乙酯; 乙酸丁酯; 氧化镁; 催化氧化
收稿日期: 2017-11-07. 接受日期: 2017-11-21. 出版日期: 2018-01-05.
*通讯联系人. 电话: +351-225-081998; 传真: +351-225-081449; 电子信箱: jlfig@fe.up.pt
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).