The relationship between the surface oxygen species and the acidic properties of mesoporous metal oxides and their effects on propane oxidation

Article abstract of DOI:10.1039/c4cy01231c

The V, Cu and Mo catalysts change the surface oxygen species of meso-Mn₂O₃ and these have been used in the oxidation of propane. The oxygen species on the different catalysts were studied in detail, including in situ IR of propane under aerobic and anaerobic conditions, X-ray photoelectron spectroscopy (XPS) and in situ IR of CO₂ adsorption. The in situ IR experiments of NH₃ showed changes in the Lewis and Bronsted acids on the catalysts. We discuss the relationship between the acidic properties and the oxygen species. The results showed that the performance of the catalysts was significantly affected by the different oxygen species.

Full text of DOI:10.1039/c4cy01231c

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The relationship between the surface oxygen
species and the acidic properties of mesoporous
metal oxides and their effects on propane
oxidation†
Cite this: DOI: 10.1039/c4cy01231c
Shu Chen, Yanhua Li, Fei Ma, Fang Chen and Weimin Lu*
2 3
The V, Cu and Mo catalysts change the surface oxygen species of meso-Mn O and these have been used
in the oxidation of propane. The oxygen species on the different catalysts were studied in detail, including
Received 22nd September 2014,
Accepted 4th November 2014
in situ IR of propane under aerobic and anaerobic conditions, X-ray photoelectron spectroscopy (XPS) and
2 3
in situ IR of CO adsorption. The in situ IR experiments of NH showed changes in the Lewis and Brønsted
acids on the catalysts. We discuss the relationship between the acidic properties and the oxygen species.
The results showed that the performance of the catalysts was significantly affected by the different oxygen
species.
DOI: 10.1039/c4cy01231c
www.rsc.org/catalysis
about the role of acidity. For propane oxidation, many
authors believe that the acid property is a key factor for the
1
Introduction
Effective catalysts for deep oxidation (combustion) and selec-
tive oxidation are urgently needed because of environmental
and industrial concerns. The deep oxidation catalysts mainly
include manganese oxides, such as Mn O , which are used
catalytic activity. In the selective oxidation of propane to pro-
pylene, some authors reported that the amount of acid
influenced the propane conversion, and the Brønsted acid
affected the selectivity of propylene. Many reports consid-
ered the acid properties to be significant and used high sur-
1
4
2
3
1–6
for the total oxidation of organic compounds owing to their
7–9
high activity and high mobility of the lattice oxygen.
Recently, the modified Mn O catalysts have gradually been
2 3
face area supports like SBA-15 and meso-Al O to load several
different elements, including Mo and V, to adjust the acidity
2
3
applied to the selective oxidation of light alkanes due to their
of the catalysts, but ignored the reasons behind the acid
1
0
15,16
surface oxygen properties.
The surface chemistry of Mn O materials, especially their
changes.
Therefore, it is favorable to enhance catalytic
properties by adjusting catalyst acidity.
2
3
surface oxygen species, is a very important factor that affects
their utility as absorbents, catalysts and/or catalyst supports.
Catalyst selectivity is largely governed by the nature of the
The synthesis and properties of bulk-Mn O materials
have been intensively studied. Our group synthesized meso-
2
3
Mn O to improve the dispersion of some elements and to
2
3
1
1–13
surface oxygen species.
These same authors explained that
adjust the acidity of the catalysts and thus the distribution of
the surface oxygen species. We loaded V, Cu and Mo on
meso-Mn O because V activates the α-H of propane and is
the catalytic reaction occurs mainly due to the effect of the
1
lattice oxygen and adsorbed oxygen. Other authors considered
2
3
that active surface oxygen species could equally be nucleophilic
acidic, Cu is an element that undergoes total oxidation, and
Mo also activates the α-H of propane but is not as acidic as V.
Here we study the influence of the different acidic properties
of the catalysts on the oxidation of propane and explore the
relationship between the acid properties and the different
oxygen species. Our goal is to understand propane oxidation
by using the different catalysts.
2−
<2−
(
O ) or electrophilic (O ) and suggested the use of Lewis
+
acid sites (Mn ) to initiate the activation of C–H bonds
2
−
and the involvement of nucleophilic O surface species in
total oxidation reactions. Thus far, the study of the role of
the surface oxygen species in catalytic oxidation reactions is
still controversial.
The acid properties are also taken seriously in the catalytic
reaction like the oxygen species, but there is also a debate
2
. Experimental
2
.1 Catalyst preparation
Ordered mesoporous silica with cubic Ia3d symmetry (KIT-6)
was prepared from TEOS, P IJEO PO EO , MW = 5800),
Institute of Catalysis, Zhejiang University (Xixi Campus), Hangzhou 310028,
PR China. E-mail: luweimin@zju.edu.cn
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
123
7
20
70
20
1
c4cy01231c
butanol, and HCl at 308 K.
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In a typical synthesis of ordered mesoporous Mn O3
For the IR spectra of NH3 or CO , all catalysts were
2
2
IJmeso-Mn O ), 1 g of KIT-6 was added in 50 mL of acetone.
pretreated under vacuum at 300 °C for 1 h and chemisorbed
2
3
After stirring at room temperature for 2 h, 2 mL of MnIJNO
3
)
2
3 2
under NH or CO when the sample was cooled to room tem-
−
1
solution (C = 5.2446 mol L ) was added slowly under stir-
ring. After stirring overnight, filtering and drying at room
temperature, a dry powder was obtained, followed by calcina-
tion at 873 K for 3 h and removal of silica using 2 M NaOH
solution in water. After washing with water and drying at
perature. Adsorption of NH was performed at room tempera-
3
ture (rt), followed by evacuation to remove the adsorbed NH3.
Adsorption of CO was performed at room temperature for
2
1 h, followed by increasing the temperature to 323, 373, 423,
473, 523 and 573 K. Evacuation and stabilization times at
1
8
3
33 K, meso-Mn
2
O
3
was finally obtained.
each temperature were 20 min. The signal of NH
desorption was analyzed by IR spectroscopy.
3 2
or CO
For comparison, bulk-Mn O was synthesized by thermal
2
3
decomposition of MnCO . In a typical run, an equal volume
and concentration of Na CO and MnCl were mixed and
2 3 2
X-ray photoelectron spectroscopy (XPS) experiments
used a Thermo ESCALAB 250 system with Al Kα radiation
(1486.6 eV). The X-ray anode operated at 150 W, and the
binding energies were calibrated using the C1s line as a stan-
dard (284.8 eV).
3
stirred for 30 min. After washing and filtering, the precipitate
was dried at 373 K, and the resulting white powder (MnCO3)
was then calcined at 873 K in air for 4 h with a ramping rate
−
1
of 2 K min .
The introduction of various elements such as V, Mo, and
2.3. Catalytic performance
Cu in the meso-Mn
2 3 X 2 3
O samples IJMO –Mn O ) was performed
by impregnation of meso-Mn O with ammonium meta-
The oxidation of propane was carried out in a tubular, fixed
bed flow quartz reactor (i.d. 7.4 mm, 270 mm long) under
atmospheric pressure. 0.2 g of the catalyst was used to test
2
3
vanadate, ammonium molybdate, and copper nitrate as the
precursor, respectively. After stirring overnight, the catalyst
was dried at 338 K for 12 h and calcined at 623 K in air for
the catalytic performance. The feedstock C H /O had a
3 8 2
−
1
−1
3
h with a ramping rate of 2 K min .
molar ratio of 1.0 at GHSV = 4500 ml (g h) and reaction
temperature of 480 °C. The products and reactants were ana-
lyzed by a dual channel on-line gas chromatograph equipped
with both TCD and FID and Porapak QS (4.0 m × 1/8 in) and
TDX-01 (2.0 m × 1/8 in) columns.
2
.2. Catalyst characterization
The N sorption isotherms were recorded on a TriStar II 3020
2
instrument at 77 K. Before measurement, the samples were
evacuated at 473 K for 2 h. The specific surface area was
determined by the Brunauer–Emmett–Teller (BET) method.
For high-resolution transmission electron microscopy
3
Results and discussion
(
HRTEM), a JEOL-135 2010F TEM at an accelerating voltage
3.1. Meso-Mn
Fig. 1A shows a TEM image of KIT-6 exhibiting a very ordered
mesostructure. Fig. 1B shows a TEM image of meso-Mn
also exhibiting an ordered mesostructure. These results
2 3
O characterization
of 200 kV was used. To prepare the TEM samples, a dilute
particle–ethanol colloidal mixture was ultrasonicated for
3
coated Cu TEM grid. Powder XRD patterns of the samples
were obtained with an X-ray diffractometer (XRD, Rigaku-D/
Max-B automated powder X-ray diffractometer) operating at
2 3
O
0 min and a drop of the solution was placed on a carbon-
4
5 kV and 40 mA using Cu Kα radiation (λ = 0.15418 nm).
The crystal phase was obtained by wide-angle scanning in the
range 10–80° and low-angle scanning in the range 0.5–10°.
IR spectra were recorded on a BioRad, FTS-40A FT-IR spec-
trophotometer with an IR cell equipped with CaF windows
2
connected to a vacuum dosing system. The catalyst powder
was pressed into a self-supporting disc and activated at
200 °C under vacuum for 1 h before the adsorption experiments.
In situ IR spectra of propane, propylene, and propane/O gaseous
2
species were recorded under vacuum or 5% O /He at elevated
2
temperature after the gas phase components in the IR cell
were removed by evacuation. The spectrum of the adsorbed
species was referenced to the background spectrum of the
pretreated catalyst recorded under vacuum at the same
temperature before the introduction of adsorbents. Because
propane is inert, a slightly different protocol was used. A gas
mixture of propane/oxygen was added to the catalyst at 323,
3
73, 423, 473 and 523 K. We then recorded the spectrum at
Fig. 1 TEM images of (A) KIT-6 and (B) meso-Mn
2 3
O . HRTEM image of
each temperature.
(C) meso-Mn and low-angle XRD patterns of (D) meso-Mn O .
2
O
3
2 3
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suggest that the mesostructure of meso-Mn O is well
2
3
templated from the mesostructure of KIT-6. The HRTEM
image (Fig. 1C) of the sample clearly exhibits hexagonally
cubic close-packed mesopores with crystalline walls. The low-
angle powder X-ray diffraction pattern (Fig. 1D) of the sample
shows one sharp peak at ∼0.9° and a broad peak at 1.7–1.8°,
in good agreement with those of the Ia ¯3 d space group. These
results confirm that the meso-Mn O crystals have an ordered
2
3
mesostructure.
Fig. 2 shows the wide-angle XRD patterns of meso-Mn O ,
2
3
exhibiting characteristic peaks associated with α-Mn O
2
3
(
JCPDS no. 73-1826), where MnIJIII) cations are surrounded by
1
9
six oxygen atoms forming a distorted octahedron. Similarly,
bulk-Mn O is also crystalline α-Mn O . In addition, any
2
3
2
3
Fig. 3
N
2
sorption isotherms and average pore size distribution of
metal oxide phase, except for α-Mn
2
O
3
, cannot be observed
(A) KIT-6 and (B) catalysts: (a) meso-Mn
2 3 2 3 2 3
O , (b) Cu–Mn O , (c) Mo–Mn O ,
in Cu–Mn O , V–Mn O and Mo–Mn O , even if the metal
2
3
2
3
2 3
(d) V–Mn O .
2
3
loading is at 10%. This phenomenon suggests that the metal
species are well dispersed on meso-Mn . Furthermore, the
2 3
O
Raman spectra of the samples (Fig. S1†) show that there is a
little oligomer of V and Mo species, while Cu species are
highly dispersed. HAADF-STEM and elemental mapping
images (Fig. S2A and S2B†) prove the good dispersion of V
3
.2 Catalytic tests in propane oxidation
Table 2 (Table S1†) presents the catalytic data for propane
oxidation reaction performance of the different catalysts.
Notably, they have similar conversion, but their product
selectivities are quite distinguishable. Propane on meso-
Mn O underwent a deep oxidation reaction, yielding CO
2 3 2
(sel. 58.2%) as the main product as well as ethylene
(sel. 20.6%). All of the loaded catalysts were compared with
meso-Mn O in the following. After loading 10% CuO, the
2 3
ethylene selectivity decreased (12.2%), while propylene, as
the selective oxidation product of propane, selectivity increased
2 3
and Mo on meso-Mn O .
Fig. 3 shows the nitrogen sorption isotherms of the sam-
ples, showing a typical type IV adsorption. The Brunauer–
2
−1
Emmett–Teller (BET) surface area of KIT-6 is 614 m g , but
2
−1
2 3
the surface area of meso-Mn O is much smaller (81 m g ).
After the introduction of various elements, such as V, Mo,
and Cu, the surface areas of the samples are further reduced,
as summarized in Table 1. The pore size distributions, calcu-
lated from the desorption isotherms, exhibit a narrow distri-
bution centered at 6.6 nm for mesoporous KIT-6 and a bigger
(
1
29.5%) and CO
0% MoO , the propylene selectivity (37.2%) increased, and
the cleavage product ethylene decreased, while CO is predomi-
2
selectivity remained 50%. After loading
3
2
pore size (8.2 nm) for meso-Mn
supported catalysts is centered near 7 nm, which is smaller
than that of meso-Mn
2 3
O . The pore size of the
2 5
nant and little CO was noted. The 10% V O underwent selec-
tive oxidation of propane and gave propylene as the primary
product (sel. 51%). Because of these different products, we
suggest that the catalysts have different reaction pathways.
The catalytic performance was measured at V loading
levels of 1‰ to 15% to study the influence of the different
loading levels on the catalytic activity. The propane conver-
sion is ranging from 24.4 to 28.8%. Increasing the V loading
has no impact, except when the V loading level is at 15%.
The selectivity of propylene increases from 17.4 to 51.0% with
2 3
O .
10% V. Concurrently, the CO
2
selectivity drops greatly and
CO rises.
3.3 In situ IR analysis
The catalysts exhibited different catalytic activities after the
different elements were loaded. To understand the possible
reaction pathways for the oxidation of propane, the surface
species formed by adsorption of the reactant (propane) and
the reaction products (propylene), as well as the transforma-
tion of these species on the catalysts were studied by IR spec-
troscopy using both vacuum and different reaction atmo-
spheres as well as temperature change.
2 3 2 3
Fig. 2 Wide-angle XRD patterns of bulk-Mn O , meso-Mn O and
supported catalysts.
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Table 1 Surface areas and average pore width (nm) of catalysts and KIT-6
Catalyst
KIT-6
Meso-Mn O
2 3
V–Mn O
2 3
2
Cu–Mn O
3
2 3
Mo–Mn O
2
−1
Area (m g
)
614
6.6
81
8.2
46
6.8
65
7.6
59
7.5
Average pore width (nm)
3
.3.1 In situ IR spectra collected in the reaction atmosphere.
both on V–Mn O and Mo–Mn O , but the acetone peak
2 3 2 3
(1715 cm ) is only seen on V–Mn O . These differences illus-
2 3
−
1
The FT-IR spectra of the catalysts under the reaction atmo-
sphere (propane : O = 1 : 1) were recorded from 373 to 573 K
trate that the oxidation of propane on each catalyst under the
reaction atmosphere occurs through two routes. One is
2
−1
(
Fig. S3†). For meso-Mn O , the 1699 cm band at 373 K red-
2 3
−
1
shifted to 1710 cm when the temperature increased but
vanished at 573 K. This band can be assigned to the υCO of
acetone coordinated with different Lewis acid sites on the
mostly complete oxidation to give CO
x
, such as that on meso-
Mn O and Cu–Mn O . The other is primarily selective oxida-
2
3
2 3
tion to achieve olefins. The isopropanol salt intermediate is
further oxidized to acetone and propylene on V–Mn dur-
2
0
−1
catalyst surface. A new vibration band at 1290 cm appears
with increasing temperature but disappeared at 573 K. This
peak is assigned to carbonate adsorption on the surface of
2 3
O
ing the reaction, but is further oxidized to propylene and CO2
on Mo–Mn O . The Π-bonded propylene was not detected on
2
3
the catalyst and is considered to be a precursor salt of CO
2
.
2 3 2 3
the meso-Mn O and Cu–Mn O catalysts, but gaseous olefins
−
1
Unlike meso-Mn O , the Cu–Mn O catalyst showed low reac-
at 1630 cm were detected on Cu–Mn O . This is inconsis-
2
3
2
3
2 3
tivity at low temperature (373–473 K). A strong band at
tent with the fact that the propylene selectivity is as high as
51% in 10% V–Mn . We next studied the adsorption and
−
1
1
259 cm is detected above 473 K, which means that a large
2 3
O
−
1
number of carbonate exists. The band at 1650 cm is
assigned to the physical adsorption of alkenes, indicating
desorption reaction of propylene on the catalysts to illustrate
the huge differences in selectivity between the catalysts.
3.3.2 In situ IR of propylene adsorption. Fig. 5 shows the
IR spectra of the catalysts after adsorption of propylene at
323 K and their evolution at 373 K. Propylene adsorption on
that the reaction of propane on Cu–Mn
2 3
O produced propyl-
ene or ethylene. The 10% V–Mn O and 10% Mo–Mn O cata-
2
3
2 3
lysts exhibit similar absorption spectra and have two weak
−
1
bands near 1600 cm belonging to the Π-bonded propylene
meso-Mn
2
O
3
−
was detected at 323 K. The broad vibration peak
1
species interacting with the Lewis acid sites on the surface of
at 1600 cm belongs to the Π-bonded propylene combined
with a Lewis acid site, and the shoulder peak at 1630 cm is
due to the physically adsorbed propylene. A new peak at
1670 cm appeared at 373 K, and the intensity of the
−
1
−1
the catalysts. The peak at 1255 cm is from the precursor of
the carbon oxides. The difference between the V–Mn and
the Mo–Mn O catalyst is that a vibration peak at 1715 cm
2
2
2
O
3
−
1
−1
2
3
−
1
appears with increasing temperature for 10% V–Mn
2
O
3
. This
Π-bonded propene at 1600 cm decreased. The peak of
−
1
implies that the catalytic reaction goes through the acetone
1670 cm is regarded as the CO in aldehyde. This is attrib-
uted to the reaction of the Π-bonded propene with the oxide
21,22
step.
Fig. 4 compares the FT-IR spectra of the catalysts at 573 K.
species on the surface of meso-Mn
propylene is prone to chemisorption on meso-Mn
resulting deep oxidation. Propylene adsorption on Cu–Mn O
2 3
2
O
3
. This is evidence that
−
1
The CO
2
vibration peak at 2350 cm is very strong for meso-
2 3
O
with
Mn O and the intensity is in the order: meso-Mn O > Cu–
2
3
2
3
Mn
2
O
3
> Mo–Mn
2
O
3
> V–Mn
2
O
3
. These results, combined
−
1
with the carbonate peak at 1250 cm , verified that propane
on Cu–Mn O and meso-Mn O produces more CO than that
2
3
2
3
2
on other catalysts. In addition, the intermediate isopropanol
−
1
−1
(
1065 cm ) and Π-bonded propene (1600 cm ) appeared
Table 2 The performance of the catalysts. Reaction conditions: temper-
−1
ature 480 °C, GHSV = 4500 mL (g h) , and C
3 8 2
H :O =1 : 1 (molar ratio)
Conversion (%) Selectivity (%)
Catalyst
Propane
Ethylene Propylene CO CO
2
Meso-Mn
2
O
3
24.3
21.1
22.2
19.7
25.7
24.4
25.6
28.8
11.9
20.6
30.6
12.0
12.2
27.5
35.8
27.8
9.0
18.6
22.8
37.2
29.5
17.4
28.7
31.0
51.0
41.6
0.0 58.2
Bulk-Mn
2
O
3
0.0 41.1
11.6 33.0
0.0 56.6
0.0 50.2
0.0 23.5
0.0 26.9
21.1 11.6
36.4 16.8
1
1
1
1
5
1
1
0% Mo–Mn
0% Cu–Mn
2 3
‰ V–Mn O
% V–Mn
% V–Mn
2 3
O
2 3
O
2
O
O
3
Fig. 4 FT-IR spectra of the reaction atmosphere (propane : O
adsorbed on 10% V–Mn , 10% Cu–Mn , 10% Mo–Mn
meso-Mn sample at 573 K.
2
=1 : 1)
O , and
2
3
0% V–Mn
5% V–Mn
2
O
O
3
3
2
O
3
2
O
3
2 3
2
4.9
2 3
O
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been studied by IR spectroscopy, with NH as a probe mole-
3
cule. The spectra arising from NH adsorption at room tem-
3
perature followed by evacuation at the same temperature are
shown in Fig. 6.
Adsorption of NH3 on meso-Mn O leads to the appear-
2
3
−
1
ance of two strong IR bands at 1425 and 1610 cm corre-
sponding to Brønsted IJNH4 deformation vibrations) and
Adsorption of NH on Cu–
3
Mn shows a strong band centered at 1600 cm and two
+
2
3–25
Lewis acid sites, respectively.
−
1
2 3
O
−
1
bands from 1350 to 1470 cm that could be related to the
inhomogeneity of the Brønsted acid sites or a splitting of the
+
δ
as vibration due to the adsorption symmetry of the NH
4
molecule with the catalyst surface. V–Mn O and Mo–Mn O
2
3
2
3
−
1
show a weak IR band at 1640 cm and a medium-strong
−
1
band at 1420 cm . This suggests very little Lewis acid con-
tent and dominant Brønsted acids on the surface of these
two catalysts. The amount of both Brønsted and Lewis acid
2 3 2 3
sites in the meso-Mn O and Cu–Mn O samples is larger
than those in the V–Mn O and Mo–Mn O samples: meso-
2
3
2
3
Mn
2
O
3
> Cu–Mn
2
O
3
> V–Mn
2
O
3
2 3
> Mo–Mn O . The amount
of Lewis acid changed more obviously as the dopant
changed.
The large number of Lewis acid on meso-Mn
Mn is liable to the adsorbed propylene which then con-
tinues to be oxidized to CO2 in the presence of a Brønsted
acid. This caused the selectivity for CO to be greater than
0% on both catalysts. The low amounts of Lewis acid on
V–Mn O and Mo–Mn O allowed propylene to be physically
2 3
O and Cu–
2 3
O
2
5
2 3
Fig. 5 FT-IR spectra of propylene adsorbed on meso-Mn O , 10%
2
3
2 3
V–Mn
2 3 2 3 2 3
O , 10% Mo–Mn O , and 10% Cu–Mn O from 323 to 373 K, and
adsorbed and then be desorbed. Thus, both catalysts have
higher selectivity for propylene. With increasing V loading,
the amount of Lewis acid is significantly reduced, and there-
fore the selectivity of propylene increased from 17.4 to 51%.
Simultaneously, the selectivity of CO is influenced. Too much
Lewis acid on meso-Mn O and Cu–Mn O and low loadings
the comparison of the catalysts at 323 K.
2 3
is similar to that on meso-Mn O at 323 K. The wide peak
−
1
around 1630 cm could include both the vibration of physi-
cal adsorption and Π-bonded propylene. After increasing the
2
3
2 3
−
1
temperature to 373 K, a sharp peak at 1600 cm for the
Π-bonded propylene appeared because propylene was easily
desorbed. In the spectra of V–Mn O and Mo–Mn O , only
the peak of the physically adsorbed propylene (1630 cm )
remained at 323 K. It is markedly weakened as the tempera-
ture increases. When comparing the different catalysts at
(1‰, 1%, and 5%) of V–Mn
2 3
O resulted in no CO produc-
tion. Increasing the V loadings reduced the Lewis acid
2
3
2 3
−
1
3
23 K (Fig. 5), we see that the intensity of propylene adsorp-
tion is of the order: meso-Mn > Cu–Mn > V–Mn
Mo–Mn O . The Π-bonded propylene is seen only on meso-
2
O
3
2
O
3
2 3
O >
2
3
Mn
Mn
2
2
O
O
3
3
and Cu–Mn
than on Cu–Mn
2 3
O , and its intensity is stronger on meso-
2 3
O .
The results of the infrared spectra of propylene adsorption
on the catalysts show that there are more Lewis acid sites
that could easily produce a stable combination with propyl-
ene on the surface of Cu–Mn O and meso-Mn O . After load-
2
3
2 3
ing V and Mo, propylene adsorption is decreased. Consider-
ing the IR results of (propane + O ) adsorption, the acid
2
properties of the catalysts affected propylene adsorption–
desorption, which in turn determined the selectivity of pro-
x
pylene and CO production.
3
.3.3 In situ IR of NH adsorption. The nature of the acid
3
Fig.
6
FT-IR spectra of NH3 adsorbed and desorbed at room
, Cu–Mn , V–Mn , and Mo–Mn
sites (Lewis and Brønsted) on the surface of the catalysts has
temperature on meso-Mn
2
O
3
2
O
3
2
O
3
2 3
O .
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content and increased the CO selectivity to as high as 36.4%.
Thus, if a large number of Lewis acid sites exist on the sur-
face of the catalysts, deep oxidation of propane is favorable
and the selectivity of CO increases. That is, lower amounts
2
of Lewis acid increase the selectivity for propylene and CO.
3
.4 Surface oxygen species
.4.1 XPS and IR. The composition and oxidation state of
3
the catalysts were confirmed by XPS. The XPS results (Table 3)
account for the presence of charged oxygen atoms and
hydroxyl oxygen (–OH). On the surface of meso-Mn O , the
2 3
atomic percentage is 71.99% for O and 28.01% for Mn. These
results indicate that the O : Mn atomic ratio (2.57 : 1) is much
higher than expected (1.5 : 1) and the test sample exposes
oxygen-rich surfaces.
Fig. 7 FT-IR spectra of –OH on the surface of the samples.
The bulk-Mn O sample has a lower O : Mn atomic ratio
2
3
(2.35 : 1) than meso-Mn
2 3
O , which indicates the high oxygen
capacity of meso-Mn O . The peaks of Mn 2p and Mn 2p1/2
are shown at 642.0 and 653.78 eV, respectively, and the spin–
amounts of V, the Lewis and Brønsted acids decreased with
increasing loading, while the propylene selectivity increased
2
3
3/2
orbit splitting is 11.8 eV. These match well with the data of
2
from 17.4% to more than 40% and CO decreases signifi-
2
6
Mn O . Two peaks appeared near 531.5 and 529 eV in the
cantly. At 10% V, the products are mainly propylene and CO.
In short, the surface oxygen species changed the acid proper-
ties and thereby affected the reactive products of propane.
3.4.2 The action of oxygen species. To illustrate the role of
the oxygen species in the reaction, the adsorption of propane
on the catalysts under an oxygen-free atmosphere was mea-
sured by in situ IR (Fig. 8). Meso-Mn O and Cu–Mn O show
2
3
XPS profiles of the O 1s region (Fig. S4†) and are attributed
2
−
to the oxygen (O ) in the lattice of Mn–O–Mn or the low
2
−
2−
coordination O anions in the bulk (O ) and to the –OH in
2
7–30
the interlayers or on the surfaces, respectively.
of the two kinds of oxygen (–OH : O ) on the surface of meso-
The ratio
2
−
Mn O is the highest (0.774), while that on bulk-Mn O is
2
3
2
3
2
3
2 3
−
1
only 0.432. After loading the different elements, the ratio
decreases in the order of Cu–Mn > V–Mn > Mo–
Mn O , which is consistent with the changing amounts of
a CC vibration peak near 1580 cm at 423 K. The CO
peak at 2400 cm is attributed to CO
−
1
2
O
3
2
O
3
2
and increases as the
temperature increases. For the Cu–Mn O catalyst, a peak at
2
3
2 3
−
1
Brønsted acid. The ratio of the two kinds of oxygen decreased
upon increasing the amount of vanadium loading.
1722 cm belongs to acetone vibration and emerges at 373 K;
it also increases with increasing temperature. A weak peak of
Π-bonded propylene at 1591 cm is seen only on V–Mn O at
−
1
We used in situ IR to verify the change in surface –OH.
2
3
−
1
Fig. 7 shows a –OH vibration peak at 3700 cm in all the
samples. Meso-Mn has the maximum number of surface
OH, followed by Cu–Mn O , V–Mn O and Mo–Mn O . Meso-
elevated temperatures. There is very little CO
and Mo–Mn compared to meso-Mn
Mo–Mn O did not show any reaction intermediates of pro-
2
on V–Mn
2
O
3
2
O
3
2
O
3
2 3 2 3
O and Cu–Mn O .
–
2
3
2
3
2
3
2 3
Mn O shows a sharp and strong peak of –OH, while the
2 3
pane with increasing temperature. Thus, Mo–Mn O does not
2
3
intensity is very weak in bulk-Mn
2
O
3
. The –OH difference in
have catalytic activity for propane under anaerobic conditions.
The reaction intermediate could be compared by the
adsorption of propane under aerobic and anaerobic atmo-
spheres on the different catalysts. This relates closely to the
different products of propane. The reaction intermediate on
meso- and bulk-Mn O was further confirmed by XPS. It is
2
3
clear that the changes in acidity in the supported catalysts
are due to oxygen species on the catalyst surface.
Thus, the Brønsted acids depend on the amount of –OH,
which in turn affects propylene selectivity. With differing
2 3 2
meso-Mn O is not detected except with significant CO under
2 3
Table 3 Surface chemical composition of pure Mn O
and loaded catalysts by XPSa
3
+
2−
x+
2−
3+
2−
Catalyst
Mn (%)
O
(%)
M
(%)
O
/Mn
–OH/O (area)
Meso-Mn
Bulk-Mn O
2 3
2
O
3
28.0
29.8
22.5
21.5
21.0
26.1
24.6
27.0
72.0
70.2
71.7
69.6
71.9
73.5
74.2
70.7
0
0
2.57
2.35
3.18
3.22
2.66
2.81
3.01
2.61
0.774
0.432
0.428
0.773
0.454
0.500
0.473
0.261
1
1
1
1
5
1
0% Mo–Mn
0% Cu–Mn
0% V–Mn
% V–Mn
% V–Mn
2
O
3
5.76
8.85
1.63
0.43
1.15
2.34
2 3
O
O
2
3
2
O
O
3
2
3
5% V–Mn
2
O
3
a
x+
M
: loaded metal elements.
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bicarbonate. The formation of unidentate carbonate requires
2
−
isolated surface O ions, i.e. low-coordination anions such
as those present in corners or edges. Bidentate carbonate
2
+
2−
forms on the Lewis acid–Brønsted base pairs IJM –O pair
site), and the formation of bicarbonate species involves sur-
face hydroxyl groups. The different CO adsorption modes
2
reveal that the surface of the catalysts may contain different
oxygen species.
On meso-Mn
mode at 1214 cm
stretching at 1677 cm . The intensity of the two peaks
2 3
O , bicarbonates show a C–OH bending
−
1
as well as an asymmetric O–C–O
−
1
decreased with increasing temperature. The C–OH bending
−
1
mode at 1214 cm disappeared at 523 K, indicating that
bicarbonate is prone to decomposition. Bidentate carbonate
shows symmetric and asymmetric O–C–O vibrations at 1313
−
1
−1
and 1627 cm , respectively. The intensity of the 1313 cm
peak decreases slightly with temperatures up to 573 K,
indicating that bidentate carbonate is more stable than bicar-
Fig. 8 FT-IR spectra of propane (in the absence of O
2
) adsorbed on
and (D) 10%
(
A) meso-Mn
2
O
3
, (B) 10% V–Mn
2
O
3
, (C) 10% Cu–Mn
2
O
3
bonates. Unidentate carbonate shows a vibration peak at
Mo–Mn , from 323 to 573 K.
2
O
3
−1
1
564 cm , which increased with increasing temperature. Among
2 3
the three adsorption situations for meso-Mn O , the formation
of bicarbonate is the most labile. Both the unidentate and
bidentate carbonates remain on the surface after evacuation
at 573 K. However, only the unidentate band is enhanced
with rising temperature. The stability of adsorption suggests
that the strength order for the surface sites is as follows:
an aerobic atmosphere. However, the surface oxygen species on
meso-Mn is involved in the reaction of propane. Here, ole-
fins are generated under anaerobic conditions. Olefins and
acetone appeared in the IR spectra of Cu–Mn under the
anaerobic atmosphere. On V–Mn , the reaction intermedi-
2 3
O
2 3
O
2
−
2 3
O
O
ions > oxygen in M–O pairs > –OH groups.
The adsorption on Cu–Mn O also has three carbonate
ates, propylene and acetone, and a trace of CO were detected
2
2 3
under both aerobic and anaerobic conditions. No olefins were
vibrations in the IR (Fig. 9). Bicarbonates show a C–OH bend-
ing mode at 1216 cm , as well as an asymmetric O–C–O
−
1
detected under anaerobic conditions on Mo–Mn
2
O
3
, same as
−
1
−1
under aerobic conditions. The difference in performance
under aerobic and anaerobic conditions suggested different
exchanges between surface-bound and free oxygen.
stretching at 1670 cm . This shifts to 1681 cm at 423 K.
Bidentate carbonate vibration appears at 1301 and
−
1
−1
1623 cm . The peak at 1301 cm is stable at 573 K, but the
peak at 1623 cm vanishes when the temperature rises.
Unidentate carbonate shows a vibration peak at 1538 cm
(asymmetric O–C–O stretching). The mode of the oxide
−
1
−1
The peak near 3800 cm is assigned to the –OH vibration
on the surface of the catalysts (Fig. S5†). The intensity of the
peak decreases gradually as the temperature rises, and the
trend is the same for all catalysts, except for Mo–Mn O .
−
1
2
3
Therefore, the number of –OH on the surface of the catalysts
has a great influence on the catalytic reaction of propane.
The formation of acetone during propane adsorption sug-
gests that propane is activated by removing a hydrogen atom
from the methylene group. This is followed by the bonding
of the oxygen species on the catalyst with the nearby second-
ary carbon of the isopropyl species. This leads to the forma-
tion of an isopropoxy species, as shown by Iglesia and co-
3
1
workers. Thus, when propane is adsorbed, different oxygen
species on the surface of the catalysts are prone to oxidation,
which is necessary for identification.
3
.4.3 CO -IR. Infrared spectra recorded following CO
2 2
adsorption on catalysts at RT and subsequent temperature-
programmed desorption are shown in Fig. 9. A number of IR
−
1
bands from 2000 to 1000 cm are observed when CO was
2
adsorbed on the catalysts. This was mainly due to the various
types of carbonate-like (–CO
3
) and bicarbonate (–HCO
3
)
Fig. 9 Infrared spectra of CO
2
adsorbed at 298 K on (A) meso-Mn
2 3
O ,
3
2–36
species.
There are three adsorption situations: the forma-
(
B) 10% V–Mn , (C) 10% Cu–Mn
2
O
3
2
O
3
and (D) 10% Mo–Mn taken as
2 3
O
tion of unidentate carbonate, bidentate carbonate and
a function of temperature during progressive heating under vacuum.
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species does not change on the surface of Cu–Mn O , com-
further influence the catalytic reaction pathways of the probe
molecule (propane). A large number of Brønsted acids (con-
2
3
pared to that on meso-Mn O , but the amount on each mode
2
3
does change. Bicarbonate species on Cu–Mn
2
O
3
is less than
trolled by –OH) exist on the surface of the catalysts IJmeso-Mn
2 3
O ,
that on meso-Mn O , while the unidentate carbonate is much
Cu–Mn O ). Brønsted acids mainly go through the deep oxi-
2
3
2 3
greater on Cu–Mn O than on meso-Mn O . Thus, Cu loading
increased the isolated surface O ions and decreased the
dation process during propane oxidation and are conducive
to the generation of CO . Lower amounts of Lewis acids are
2
3
2 3
2
−
2
–OH group.
advantageous to the selectivity of propylene and CO. The sur-
The CO adsorption intensity on V–Mn O is much weaker
face oxygen species were detected by XPS and CO -IR. A small
2
2
3
2
−1
than that on Cu–Mn
2
3
O . Two peaks appear at 1677 and 1637 cm
amount of the Lewis acid–base pairs decreases CO
2
genera-
at room temperature and nearly disappear at 373 K.
These are the asymmetric O–C–O stretching vibrations of
bicarbonates and bidentate carbonates, respectively. The
unidentate carbonate shows a weak peak at 1380 cm (sym-
metric O–C–O stretching) and achieves stability at 573 K. The
tion, but is advantageous for propylene. This implies that
3
+
2−
Mn –O could be considered to form the Lewis acid sites.
Different oxygen species play different roles in propane oxida-
tion as propane molecules could undergo selective oxidation
or complete oxidation reactions. Thus, these results provide a
basis for managing the complete or selective oxidation of
light alkanes by adjusting the surface oxygen species.
−
1
2 2 3
adsorption of CO on Mo–Mn O is the weakest in all the cat-
−
1
alysts. Weak bidentate carbonate appeared at 1643 cm and
vanished with increasing temperature. No bicarbonates and
unidentate carbonates were detected. These results also sug-
gest the following content order for OH groups: meso-Mn O >
Acknowledgements
2
3
2
−
Cu–Mn
2
O
3
> V–Mn
2
O
3
> Mo–Mn
> meso-Mn
2
O
3
. For O ion groups,
> V–Mn > Mo–
The authors are grateful for the financial support from the
National Natural Science Foundation of China (grant no.
21173186).
the trend is: Cu–Mn
2
O
3
2
O
3
2 3
O
Mn O , and for oxygen in M–O pairs: meso-Mn O > Cu–
2
3
2 3
Mn
2
O
3
> V–Mn
2
O
3
> Mo–Mn
2
O
3
. The order for –OH groups
is consistent with the results of XPS and IR.
Notes and references
In summary, the amount of –OH oxide species (–HCO ) is
3
the same as that of the Brønsted acid. This shows that the
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