Selective oxidation of: N -buthanol to butyraldehyde over MnCo2O4spinel oxides

Article abstract of DOI:10.1039/d0ra04738d

Partial oxidation of n-butanol to butyraldehyde, propionaldehyde and acetaldehyde over MnCo2O4 spinel oxides has been investigated. Physicochemical characteristics of samples, prepared by co-precipitation with different amounts of precipitating agent, were studied by XRD, N2 adsorption-desorption isotherms, FT-IR, SEM and XPS. The ratio between the precipitating agent and the precursors has a considerable influence both on the structure, which is evidenced by XRD, due to switching from a crystalline structure to an amorphous one and on the surface (XPS) by an obvious change in the ratio Co3+/Co2+ and Mn4+/Mn3+ and in the content of oxygen vacancies. The reaction rate is not influenced by the oxygen pressure, emphasizing that n-butanol oxidation occurs through the Mars van Krevelen mechanism. The conversion of n-butanol and yield of butyraldehyde are directly proportional to the cobalt content on the surface, while the propionaldehyde yield is proportional to the Mn4+/Mn3+ ratio.

Full text of DOI:10.1039/d0ra04738d

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Selective oxidation of n-buthanol to butyraldehyde
over MnCo2O4 spinel oxides
Cite this: RSC Adv., 2020, 10, 25125
a
b
b
*
*
Gheorghita Mitran,
Shaojiang Chen
and Dong-Kyun Seo
Partial oxidation of n-butanol to butyraldehyde, propionaldehyde and acetaldehyde over MnCo2O4 spinel
oxides has been investigated. Physicochemical characteristics of samples, prepared by co-precipitation
with different amounts of precipitating agent, were studied by XRD, N2 adsorption–desorption isotherms,
FT-IR, SEM and XPS. The ratio between the precipitating agent and the precursors has a considerable
influence both on the structure, which is evidenced by XRD, due to switching from a crystalline structure
to an amorphous one and on the surface (XPS) by an obvious change in the ratio Co3+/Co2+ and Mn4+
/
Received 29th May 2020
Accepted 25th June 2020
Mn3+ and in the content of oxygen vacancies. The reaction rate is not influenced by the oxygen
pressure, emphasizing that n-butanol oxidation occurs through the Mars van Krevelen mechanism. The
conversion of n-butanol and yield of butyraldehyde are directly proportional to the cobalt content on
the surface, while the propionaldehyde yield is proportional to the Mn4+/Mn3+ ratio.
DOI: 10.1039/d0ra04738d
rsc.li/rsc-advances
environmentally friendly and sustainable processes for selective
oxidation of alcohols.9–11
1 Introduction
The partial oxidation of n-butanol, in the gas phase, was
studied over Cu and Ru catalysts with good yields to butyral-
dehyde,12,13 over Au and Pd supported on titania,9 over LDH-
derived Ni-based catalysts14 and over platinum supported mes-
oporous silica CMI-1.15
Manganese based catalysts supported on Al2O3, CeO2, ZrO2,
SiO2 were studied for selective oxidation of 2-butanol to methyl-
ethyl-ketone16 revealing that, the presence of Mn3O4 phase is
responsible for catalytic activity. Co3O4 catalyst has been
studied for the total oxidation of n-butanol17 showing that Co3+/
Co2+ couple represents the active site for the oxidation reactions
by a redox mechanism.
Aer our knowledge, spinel type MnCo2O4 oxides have not so
far been studied in the partial oxidation of n-butanol. Among
other spinel oxides, those of cobalt–manganese are particularly
interesting because they contain metals with more than one
oxidation state, which could be placed in any of the two sites
(tetrahedral or octahedral), modifying the structural and
respectively chemical properties. The difficulty for this type of
oxides is represented by the identication of the oxidation state
and the cations distribution in tetrahedral or octahedral sites.18
The cobalt and manganese cations in superior oxidation states,
Co3+, Mn3+ and Mn4+ are usually placed in octahedral positions,
while the cations in inferior oxidation state (Co2+, Mn2+) have no
preference.
There is currently a particular interest for oxidation of higher
alcohols (chains of more than 4 carbon atoms), due to their
ability to convert relatively easy into valuable products such as
aldehydes, esters, ethers and oxygenated fuel additives.1–3 n-
Butanol is of special interest being considered a good alterna-
tive to gasoline compared to ethanol4 since it diminishes the
energy crisis and reduces hydrocarbon emissions.
The partial oxidation of n-butanol to butyraldehyde has been
studied to a lesser extent, although it has particular industrial
importance due to its use for obtaining many high-value
chemicals.5,6 At the moment, the butyraldehyde production is
carried out from propene via hydroformylation (oxo-synthesis),
but this reaction presents a number of disadvantages such as
the thermodynamically favored product is alkane (from hydro-
genation of alkene), takes place at high pressures (200 bar), the
reaction is homogeneously catalyzed (carbonyl complexes of
cobalt or rhodium) and major products are branched aldehyde
not linear which are the desired ones.7,8
Considering the fact that biobutanol can be produced by the
biomass feedstock fermentation, its use as a raw material for
the production of butyric aldehydes is very attractive in agree-
ment with the current environmental policies. Withal, many
chemical industries are oriented towards the development of
Herein, we have evaluated the behavior of cobalt-manganese
spinel oxides, prepared by coprecipitation using different molar
ratio between the precipitation agent and Co + Mn, in selective
oxidation of n-butanol to butyraldehyde.
aLaboratory of Chemical Technology and Catalysis, Department of Organic Chemistry,
Biochemistry & Catalysis, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018
Bucharest, Romania. E-mail: geta_mitran@yahoo.com; geta.mitran@chimie.unibuc.
ro
bSchool of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA.
E-mail: DSeo@asu.edu
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washed for more times with distilled water, dried at 60 ꢀC for 24 h
and calcined at 500 ꢀC (5 ꢀC minꢁ1) for 6 h.
2 Experimental
2.1. Catalysts preparation
The spinel type oxides based on cobalt and manganese, with
formula MnCo2O4 were prepared by co-precipitation method. For
this purpose, the mixture of (CH3COO)2Co$4H2O (99.99% from
Merck) and (CH3COO)2Mn$4H2O (99.99% from Merck) solutions
in water has been homogenized by stirring for 30 min. Ammo-
nium carbonate (99.99% from Merck), dissolved in water, was
2.2. Catalysts characterization
The samples structure was analyzed with a PANalyticalX'Pert Pro
MRD X-ray diffractometer with a Cu Ka radiation (l ¼ 0.1514 nm).
Data were recorded in the 2q range of 5–80ꢀ with a step size of
0.0251ꢀ. The specic surface area (determined by the Brunauer–
Emmett–Teller (BET) methods, in the relative pressure region P/P0
0.06–0.2) and the porosity measurements (calculated by Barrett–
Joyner–Halenda (BJH) method) were performed from the nitrogen
adsorption–desorption isotherms, using a Micromeritics ASAP
2020. Scanning electron microscope (SEM) morphology and the
energy dispersive X-ray (EDX) composition of the samples were
+
chosen as a precipitating agent, with different (Co + Mn)/NH4
molar ratios (1 : 2, 1 : 3, 1 : 4), denoted MnCo2O4(2), MnCo2O4(3)
and respectively MnCo2O4(4). The precipitating agent was added
dropwise, with continuous stirring, over the precursor solution.
The obtained precipitates were separated through centrifugation,
Fig. 1 N2 adsorption–desorption isotherm and pore size distribution of MnCo2O4 samples: (a) MnCo2O4(2); (b) MnCo2O4(3); (c) MnCo2O4(4).
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collected on a XL-30 Environmental SEM. X-ray photoelectron adsorption–desorption isotherms at 77 K are presented in
spectra (XPS) were collected on a VG-220IXL spectrometer equip- Fig. 1. Likewise, the specic surface area, pore volume and
ped with a monochromated Al Ka radiation (1486.6 eV, line width average pore diameter are shown in Table 1. The isotherms
0.8 eV), and the energy resolution of 0.4 eV. The C1s peak (284.8 could be classied as type IV with a type H1 hysteresis loop,
eV) was used as a reference. Deconvolution of spectra was recorded characteristic for mesoporous structure. The average pore
with CASAXPS soware, using the Shirley background. Informa- diameter, from BJH desorption, is about 14.0 nm for sample 1,
tion on metal oxygen linkages was obtained by Fourier transform and respectively 12.1 nm for samples 2 and 3, conrming that
infrared (FT-IR) with a Bruker IFS 66 V/S spectrometer, equipped all the samples contain mesoscale pores. Also, the size distri-
with a diamond attenuated total reectance (ATR), with a resolu- bution is very narrow the majority of pores are in the range 9–
+
tion of 4 cmꢁ1, covering a spectral range of 400–4000 cmꢁ1
.
15 nm. The (Co + Mn)/NH4 molar ratios did not affect the
volume of mesoporous, keeping it at 0.27 cm3 gꢁ1 irrespective of
the amount of ammonium used. Conversely, the specic
surface area increases at ammonium amount increasing from
2.3. Catalytic activity test
The partial oxidation of n-butanol has been carried out in
a continuous-ow xed bed reactor (8 mm i.d.) at atmospheric
pressure. The reaction temperature, monitored by electronic
control, was varied in the range 100–250 ꢀC. The mixture of
reaction has been achieved by passing air through a stainless
steel saturator with n-butanol, enabling an n-butanol concen-
tration of 400 ppmv in air. The saturated vapor pressure of
butanol was determined from Antoine equation and used for
calculation of ppmv concentration. The amount of catalyst used
was 0.2 g. The space velocity, expressed as Freactants/Wcat, was
71 to 86 m2 gꢁ1
.
Diffraction patterns of samples are shown in Fig. 2. The
diffraction peaks of the rst two catalysts correspond to meso-
porous MnCo2O4 spinel oxide phases (Card PDF 01-084-0482)
and show better crystallinity than the third catalyst. The purity
of all prepared samples was evidenced by the absence of other
peaks than those corresponding to MnCo2O4 spinel. The cell
˚
˚
parameter “a” increases from 8.11 A for MnCo2O4(2), to 8.12 A
for MnCo2O4(3) and respectively 8.16 A for MnCo2O4(4) catalyst.
˚
The cell parameter “a” has been determined from line corre-
sponding to 2q ¼ 42.5ꢀ and Miller indices (hkl-311). The stan-
varied between 9000 and 21 000 mL gꢁ1 hꢁ1
.
The analysis of reactants mixture and the reaction products
(aer a stabilization time of 30 min) was performed with a GC
K072320 Thermo-Quest gas chromatograph equipped with an FID
detector. The n-butanol conversion and the products yields were
determined from three successive measurements with formulas:
˚
dard deviation for cell parameters is ꢂ0.005 A. A peak
broadening has been highlighted for the (311) plane of all
samples. Usually, a peak broadening is correlated with the
decreasing of crystallite size and stress/strains induced in the
nanoparticles.19 The average crystallite size (d) was calculated
with the Scherrer formula:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
n
X
u
u
t
2
ðxi ꢁ xÞ
i¼1
d ¼ 0.9l/b cos q
SD ¼
n
where l is the X-ray wavelength, b is the full-width at half
maximum (FWHM), and q is the diffraction angle. The average
crystallite size, calculated for 2q ¼ 36, 43 and 52ꢀ, has been
n
X
1
x ¼
xi
n
i¼1
ꢀ
where SD – standard deviation, n – number of measurements, x
– the mean of xi
3 Results and discussion
3.1. Characterization of catalysts
In order to investigate the specic surface areas and pore-size
distribution of MnCo2O4 catalysts, Brunauer–Emmett–Teller
(BET) gas-sorption measurements were performed. N2
Table 1 Textural properties of Co–Mn mixed oxides
a
SBET
Vmicro
Vmeso
dmeso
(nm)
Sample
(m2 gꢁ1
)
(cm3 gꢁ1
)
(cm3 gꢁ1
)
MnCo2O4(2)
MnCo2O4(3)
MnCo2O4(4)
71
81
86
0.0001
0.0001
0.0002
0.27
0.27
0.27
14.1
12.1
12.1
a
BJH desorption average pore diameter.
Fig. 2 XRD patterns of MnCo2O4 spinel oxides.
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positions of the spinel.20 The peaks corresponding to octahedral
positions are shied towards higher wavenumber from rst to
third catalyst, emphasizing that the interactions between Co
and Mn are different in the three samples.
The bulk composition of MnCo2O4 nanoparticles was
determined by EDX analysis and is shown in Table 2 and Fig. 4,
and has been observed that Co, Mn and O are the main detected
elements. The atomic compositions determined by EDX
emphasize that the ratio between Co and Mn is very closed to 2,
while the ratio between O and Co and between O and Mn is
lower than the one calculated (2) suggesting the existence of
oxygen vacancies in the prepared samples.
The morphology of synthesized MnCo2O4 samples (Fig. 5)
investigated by SEM evidenced the presence of microspheres that
are formed by the aggregation of nanoparticles, with a hierar-
chical structure well-dened and well-dispersed like a ower.
The identication of the chemical states and the composition
of species on the surface were carried out using XPS spectra of all
samples that are shown in Fig. 6. The peak of C 1s, which is used
as standard, was evidenced at 284.8 eV. The Co 2p3/2 spectra
conrm the presence of Co2+ and Co3+ on the surface. The
binding energies of Co2+ species are located in the region 780.6–
781.5 eV, the value of binding energy decreasing continuously
from the rst to the third catalyst. The Co3+ species present peaks
in the region 778.8–780 eV, the binding energy decreasing in the
same order, which indicates a decrease in the interaction between
Co and Mn in the same order. The satellite peak at 784.6–785.3 eV
is ascribed to Co2+ in tetrahedral oxygen coordination while the
satellite peak from 789.1 eV is characteristic of Co3+ species in
octahedral coordination, according to literature data.21
Fig.
3 FT-IR spectra of MnCo2O4 nanoparticles synthesized by
coprecipitation with different (Co + Mn)/NH4+ molar ratios (1 : 2, 1 : 3,
1 : 4).
estimated at 20.4 nm for MnCo2O4(2), 19.2 nm for MnCo2O4(3)
and 7 nm for MnCo2O4(4).
The FT-IR spectra for all samples are shown in Fig. 3. Three
distinctive bands are observed in the spectrum, at 655 cmꢁ1
corresponding to the stretching vibration of Co2+–O bond
located in tetrahedral units of the spinel structure, at 551 cmꢁ1
corresponding to octahedral units of Co3+–O and at 500 cmꢁ1
which corresponds of Mn3+(Mn4+)–O from the octahedral
Table 2 The EDX composition of MnCo2O4 spinel oxides
Wt (%)
Co
At (%)
Co
Catalyst
Mn
O
Mn
O
Co/Mn
O/Co
O/Mn
MnCo2O4(2)
MnCo2O4(3)
MnCo2O4(4)
53.69
52.21
55.72
23.9
23.58
23.95
22.41
24.21
20.32
33.17
31.32
35.66
15.84
15.18
16.44
50.99
53.50
47.90
2.09
2.06
2.16
1.53
1.70
1.34
3.2
3.5
2.9
Fig. 4 EDX analysis of the MnCo2O4 spinel oxides.
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Fig. 5 SEM images of MnCo2O4 spinel oxides: (a and b) MnCo2O4(2), (c and d) MnCo2O4(3), (e and f) MnCo2O4(4).
The ratio between Co3+/Co2+ also decreases from rst to last vacancies formation, therefore it is expected that by increasing
catalyst (Table 3) from 1.53 to 1.41 and respectively to 0.73. The of Co3+ composition to increase the percentage of oxygen
species of cobalt with lower valence do not contribute to oxygen vacancies by the process: 2Co3+ + O2ꢁ / 2Co2+ + , + 1/2O2.
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+
Fig. 6 XPS spectra of MnCo2O4 spinel oxides with different ratios (Co + Mn)/NH4
.
Table 3 The elemental surface composition from XPS
Ata (%)
Co/Mn
Catalyst
Co
Mn
O
Co3+/Co2+
Mn4+/Mn3+
OL1 (%)
OL2 (%)
OV (%)
XPS
MnCo2O4(2)
MnCo2O4(3)
MnCo2O4(4)
26.7
26.2
24.5
12.7
17.4
17.0
48.7
47.8
49
1.53
1.41
0.73
0.59
0.61
0.83
40.09
38.99
38.60
20.82
23.76
24.53
39.08
37.26
36.87
2.09
1.51
1.44
a
The difference is represented by C percent.
The spectra of Mn 2p3/2 conrm the presence of Mn3+ and decreasing at NH4+/(Co + Mn) molar ratio in the catalysts
Mn4+ as the major manganese species. The peak corresponding preparation increasing. The peak at 642.3–643.6 eV corresponds
to Mn3+ is located at 640.5–641.6 eV22 the binding energy values to Mn4+ cations being shied in the same order as that of Mn3+
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Fig. 7 The catalytic performance of MnCo2O4 samples for n-butanol selective oxidation (a), butyraldehyde (b), acetaldehyde (c) and propio-
naldehyde (d) yield (0.2 g catalyst, 400 ppmv n-butanol).
cations. The ratio between Mn4+/Mn3+ increases in order corresponding to cobalt oxide decrease at Co/Mn ratio
MnCo2O4(2) (0.59) < MnCo2O4(3) (0.61) < MnCo2O4(4) (0.83), in decreasing, while the percentage of lattice oxygen correspond-
the same way should increase the percentage of oxygen vacan- ing to manganese oxide increase at manganese composition on
cies. The ratio Co/Mn on the surface is 2 for the rst sample the surface increasing. The composition of oxygen defects, on
while for the second and the third sample is approximately 1.5. the surface of the rst sample, is with approximately 2–2.5%
The O 1s peaks are tted at 528.6–530.1 eV, typical for lattice more than for the others. Presence of oxygen vacancies can
oxygen in the metal–oxygen bond23 which is split into two peaks, promote Co3+ and respectively Mn4+ formation that contribute
one corresponding to oxygen lattice of cobalt oxide (OL1), and to active oxygen species apparition like peroxo (O22ꢁ) or
the other to oxygen lattice of manganese oxide (OL2); while the superoxo (O2ꢁ) on the catalyst surface. The standard deviation
peak situated at 530.2–531.0 eV is associated with oxygen for composition of oxygen defects has been of ꢂ5%.
defects in the subsurface. Percentage of lattice oxygen
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Fig. 8 The butanol conversion and butyraldehyde yield as function of Co/Mn and respectively Co3+/Co2+ ratio (a) and propionaldehyde yield as
function of Mn4+/Mn3+ ratio and respectively Co/Mn ratio (b) (0.2 g catalyst, 400 ppmv n-butanol, 200 ꢀC).
120 ꢀC, for the rst catalyst, at 148 ꢀC and respectively 165 ꢀC for
3.2. Catalytic activity
the second and third catalyst. The activity was improved by the
presence of spinel structure as could be observed from XRD
patterns.
The partial oxidation of n-butanol, in the gas phase, was studied
over MnCo2O4 catalysts. The n-butanol conversion and the
products yields as a function of reaction temperature, for three
The major products observed are butyraldehyde, propio-
successive measurements, over all studied catalysts are shown
naldehyde and acetaldehyde while formaldehyde was detected
in Fig. 7. The experimental errors are up to ꢂ5%, as shown in
in traces. The n-butyraldehyde yields increased with tempera-
ture increasing, reaching a maximum at 200 ꢀC then decreasing,
probably due to the secondary combustion reactions of butyr-
aldehyde, as observed by Jiang24 in partial oxidation reaction of
error bars from graphs. The conversion increased with
temperature increasing for all catalysts. The catalyst with ratio
+
(Co + Mn)/NH4 equal with 2 showed a higher conversion
compared to others, in the case of the other two catalysts the
butanol to butyraldehyde over a series of perovskites with Mn,
conversion decreases with increasing of the ammonium
Fe and Co. The highest yield in butyraldehyde (37%) was
content. The T50 temperature (50% n-butanol conversion) is at
Scheme 1 Selective oxidation of n-butanol over MnCo2O4 spinel.
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achieved on the rst samples with higher ratio Co/Mn and Co3+/ octahedral positions being more reactive than tetrahedral ones
Co2+ (from XPS). Our results are in agreement with those re- towards the oxidation reactions that occur by Mars–van-
ported by Xie25 in carbon monoxide oxidation observing that Krevelen mechanism.26
Co3+ represents the active site for reaction, while the Co2+ was
almost inactive.
The correlation between conversion of butanol, yield of
butanal, respectively propanal and Co/Mn, Co3+/Co2+, Mn4+/
The propionaldehyde and acetaldehyde yields, conversely, Mn3+ ratios is shown in Fig. 8, emphasizing that the conversion
increased with temperature increasing, in line with the increase and the butanal yield depend of Co/Mn ratio and Co3+/Co2+
of Mn4+/Mn3+ ratio, proving that manganese is more effective ratio, being directly proportional to them, while the propanal
for breaking of C–C bonds than cobalt. The presence in octa- yield increase to Mn loading and Mn4+/Mn3+ ratio increasing.
hedral positions of both cobalt and manganese trivalent and
Based on these ndings we propose the following scheme of
tetravalent cations leads to appearance of active sites, oxidation of n-butanol on the studied catalysts (Scheme 1).
Fig. 9 The reaction rate dependence on oxygen partial pressure.
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4 Conclusions
Partial oxidation of n-butanol over cobalt–manganese spinel
oxides, prepared by coprecipitation with different amounts of
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/
The oxygen vacancies presence ensures lattice oxygen
mobility, contributing to increasing the concentration of active
oxygen species and respectively to increase the catalytic activity.
Conflicts of interest
There are no conicts to declare.
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