MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene Epoxidation with N2O

Article abstract of DOI:10.1007/s10562-018-2420-6

Abstract: Vanadium modified mesocellular silica foams (MCF) materials (V content ca. 3 and 5 wt%) prepared by the impregnation method show mainly isolated or low-polymeric VO_x species, which was confirmed by means of Raman spectroscopy and DR UV–Vis. Textural measurements, and also XRD and TEM results indicate that the characteristic mesocellular structural features of MCFs are preserved after vanadium incorporation. The MCF-supported vanadia catalysts exhibit much higher propene conversion and propene oxide productivity when compared to vanadium modified mesoporous silicas of 2D structure, demonstrating that apart from the presence of highly dispersed isolated vanadium species, internal molecular transport within three-dimensional ultra large pores of MCF materials also plays an important role in gas-phase propene epoxidation. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-018-2420-6

Catalysis Letters
https://doi.org/10.1007/s10562-018-2420-6
MCF Material as an Attractive Support for Vanadium Oxide Applied
as a Catalyst for Propene Epoxidation with N2O
Agnieszka Held1 · Jolanta Kowalska‑Kuś1 · Krystyna Nowińska1 · Kinga Góra‑Marek2
Received: 3 January 2018 / Accepted: 15 May 2018
© The Author(s) 2018
Abstract
Vanadium modified mesocellular silica foams (MCF) materials (V content ca. 3 and 5 wt%) prepared by the impregnation
method show mainly isolated or low-polymeric VOx species, which was confirmed by means of Raman spectroscopy and
DR UV–Vis. Textural measurements, and also XRD and TEM results indicate that the characteristic mesocellular structural
features of MCFs are preserved after vanadium incorporation. The MCF-supported vanadia catalysts exhibit much higher
propene conversion and propene oxide productivity when compared to vanadium modified mesoporous silicas of 2D struc-
ture, demonstrating that apart from the presence of highly dispersed isolated vanadium species, internal molecular transport
within three-dimensional ultra large pores of MCF materials also plays an important role in gas-phase propene epoxidation.
Graphical Abstract
o
+v
o
o
o
CH3-CH=CH2
+ N2O
O
o
o
o
CH2-CH-CH3
+N2
-
+
+
+v
o
v
N=N=O
o
o
o
o
V/MCF
Keywords Epoxidation · Supported vanadia catalyst · Mesocellular silica foam · Propene · Propene oxide · Nitrous oxide
1 Introduction
These processes are complex, expensive and not environ-
mentally friendly. Therefore, the search for a method of
direct propene epoxidation in a gas phase is one of the most
desirable and challenging goals of PO production. Consid-
ering this, molecular oxygen would be the most desirable
oxidant for propene epoxidation because of its availability
and low price. However, direct propene with oxygen oxida-
tion brings about a mainly unselective process. Nevertheless,
silica supported ruthenium-copper based catalyst modified
with alkaline ions indicated relatively high PO yields when
oxygen was used for propene oxidation [3]. Furthermore,
utilization of hydrogen peroxide synthesised in situ from a
hydrogen and oxygen mixture as an oxidant has been com-
mercialized [4].
Propene oxide (PO), applied for polyurethane synthesis and
for the production of many other consumer goods, belongs
to the most important organic intermediates the demand for
which is continually increasing [1, 2]. PO is produced by
two major technologies: the chlorohydrin process and the
process based on organic hydroperoxide application [3].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2420-6) contains
supplementary material, which is available to authorized users.
* Agnieszka Held
awaclaw@amu.edu.pl
Another indirect source of oxygen may be treated nitrous
oxide found to be an alternative oxidant for different hydro-
carbon oxidation performed in the presence of various cata-
lysts [5, 6]. Application of nitrous oxide for direct propane
1
Faculty of Chemistry, Adam Mickiewicz University,
Umultowska 89b, 61-614 Poznań, Poland
2
Faculty of Chemistry, Jagiellonian University in Kraków,
Gronostajowa 2, 30-387 Kraków, Poland
Vol.:(0123456789)
1 3

A. Held et al.
and propene oxidation performed over iron modified silicas
evidenced the usefulness of that oxidant for propane oxyde-
hydrogenation (ODH) [7] as well as for propene epoxidation
[8, 9]. Iron modified zeolites have also been used for the
new technology of benzene to phenol oxidation with nitrous
oxide as an oxidant [10, 11]. ODH of propane and ethylb-
enzene with nitrous oxide was performed not only over iron
catalysts but also over vanadium catalysts and resulted in
high selectivity towards propene or styrene, respectively [12,
13]. Taking into consideration the earlier effective applica-
tion of vanadium catalysts with nitrous oxide for an oxida-
tion reaction we have successfully applied this system for
propene epoxidation [14, 15]. Both propene conversion and
selectivity to PO were influenced by the nature of the sup-
port and it appeared that silica brings about the best results
for propene epoxidation [14]. It has been shown that high
vanadium oxide dispersion results in the formation of iso-
lated vanadium species in tetrahedral coordination which
favours propene epoxidation.
orthosilicate (TEOS, Aldrich, 98%) as a source of silicon,
hydrochloric acid (Chempur, 35–38%), and NH4F (POCh).
A total of 4.0 g of Pluronic P123 was dissolved in 150 mL
of aqueous solution of HCl (1.6 M) at 313 K. After 2 h
of vigorous stirring, NH4F (46.7 mg) and 1,3,5-trimethylb-
enzene (mesitylene, 2.0 g) were added and stirred for 1 h.
Then, tetraethyl orthosilicate (TEOS, 9.14 cm3) was added.
The obtained suspension was stirred for 20 h at 313 K. Sub-
sequently, the slurry was transferred to an autoclave and
aged at 373 K for 24 h. The obtained precipitate was fil-
tered, washed with distilled water, and dried in air. In order
to remove the template, the resulting solid was calcined at
823 K for 8 h with a heating rate of 1 K/min.
The supported catalysts were prepared by an impregna-
tion technique using an aqueous solution of VOSO4 (97%,
Aldrich). The amount of vanadyl sulfate was adjusted to
obtain a transition metal content in the catalysts equal to 3
and 5 wt%. After drying at 373 K overnight, the obtained
samples were calcined at 823 K in air under static condi-
tions for 1 h.
Vanadium oxide supported on mesoporous silicas char-
acterized with a 2D structure (SBA-3, SBA-15 and MCM-
41) shows a very high surface area and creates conditions
for the formation of isolated vanadium species, catalytically
active for PO formation. It seems, however, that product
penetration along the channels of the 2D structure may be
responsible for further PO oxidation and, as a consequence,
in relatively high selectivity to COx (CO and CO2).
2.2 Characterization Techniques
As prepared non-modified mesoporous matrices and vana-
dium modified samples were characterized by powder
X-ray diffraction (XRD) measurements performed on a
Bruker AXS D8 Advance diffractometer, CuKα radiation
(λ=0.154 nm) in the range of 2θ equal to 0.3°−6°.
Therefore, in the presented paper we report the develop-
ment of the new efficient mesoporous V-containing MCF
silica catalyst featuring a well-defined 3D, ultra-large
mesopore structure that exhibits much higher propene oxide
productivity than the previously studied V/silica systems.
The influence of the nature of vanadium species on the cata-
lytic performance of the V-containing materials is discussed
in the light of a detailed characterization of the physico-
chemical properties of the catalysts by low-temperature N2
adsorption/desorption isotherms, low-angle XRD, TEM,
SEM, DR UV–Vis, Raman spectroscopy, and FT-IR spectra
of adsorbed pyridine.
The textural parameters of the as-prepared non-mod-
ified mesoporous matrices and vanadium modified final
catalysts were determined by N2 sorption at 77 K using a
Quantachrome Nova 1000e sorptometer after outgassing the
materials under vacuum at 573 K for 16 h.
UV–Vis diffuse reflectance analysis was carried out on
a Varian Cary 100 spectrophotometer. The UV–Vis spec-
tra were recorded at room temperature (RT) using BaSO4
as a reference material. Diffuse reflectance spectra were
taken in the range of 200–800 nm for the dehydrated sam-
ples (calcined at 673 K before UV–Vis spectra recording).
The absorption edge energies of the UV–Vis spectra were
determined by finding the intercept of the straight line in
the low-energy rise of a plot of [F(R∞)hν]1/2 against hν and,
where F(R∞) is a Kubelka–Munk function, hν is the energy
of the incident photon [17].
2 Experimental
2.1 Catalyst Preparation
The nature of acid sites was investigated using pyridine
as the probe molecule. All the samples were pressed into the
form of self-supporting discs (ca. 5 mg/cm2) and subjected
to heating in a quartz IR cell at 623 K under vacuum for
1 h. Excess pyridine vapour sufficient to neutralize all acid
sites was adsorbed at 443 K under static conditions, fol-
lowed by an evacuation at the same temperature to remove
the gaseous and physisorbed pyridine molecules, which had
been tracked by the recording of spectra. Subsequently, the
Mesocellular silica foam (MCF) was used as a support for
the vanadium oxide active phase. The synthesis of MCF was
performed using a procedure described by Kuśtrowski et al.
[16]. The following chemicals were used in the synthesis
of the support: triblock copolymer, poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol)
(Aldrich) as a template, 1,3,5-trimethylbenzene (mesi-
tylene, Aldrich) as the organic swelling agent, tetraethyl
1 3

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene…
FTIR spectrum was taken at 403 K. The band intensities in
this spectrum were used to calculate the total concentra-
tion of Brønsted and Lewis sites. The total concentration of
Brønsted and Lewis sites was calculated using the intensi-
ties of the 1545 cm−1 band of pyridinium ions (Py+) and
the 1450 cm−1 band of pyridine coordinatively bonded to
Lewis sites (PyL), respectively, by applying their respective
extinction coefficients [18]. The acid strength distribution
was determined on the basis of pyridine desorption stud-
ies at elevated temperatures under high vacuum conditions.
The conservation of the 1545 cm−1 (Brønsted sites) and
1450 cm−1 (Lewis sites) bands under the desorption treat-
ment at elevated temperature (503 K) was taken as a measure
of the strength of the acid sites (Ades./A0). All the spectra
were recorded with a resolution of 2 cm−1 using a Bruker
Vertex 70 spectrometer equipped with an MCT detector.
The microRaman measurements of dehydrated samples
were performed with a Renishaw InVia dispersive spec-
trometer equipped with a CCD detector and integrated with
a Leica DMLM confocal microscope. Prior to the meas-
urements the catalyst had been dehydrated by evacuation
at 673 K using a home-made cell. The 514 nm line of an
argon ion laser was used as exciting light. The spectra were
recorded at ambient conditions with a resolution of 2 cm−1.
The Raman scattered light was collected with a 50× Olym-
pus objective in the spectral range of 100–1500 cm−1.
Surface morphology and structural properties of the
MCF support, as well as vanadium doped samples, were
studied by both transmission electron microscopy (TEM,
Hitachi HT7700 operating at 100 kV) and scanning electron
microscopy (SEM, Hitachi SU3500). For TEM analysis the
specimens were prepared by dry dispersing of the catalyst
powder on standard copper grids coated with carbon film.
Transmission electron microscope (TEM) images were also
used for elemental analysis with the help of energy disper-
sive X-ray spectroscopy TEM/EDS.
x=62, 40, 24, and 12.5 cm3/min, related to different con-
tact times (1.5, 2.1, 3.0 and 4.2, respectively). The flow of
the gases was precisely controlled by mass flow controllers
(MFC Brooks).
The composition of the reactor outlet was analysed by
on-line gas chromatography. The feed and the products
were analysed by two GCs equipped with TCD (Porapak
QS packed column) and FID (WCOT Fused Silica capillary
column) detectors with automatic injection systems. The
valves and the lines connecting the output of the reactor with
the gas chromatographs were heated to 393 K to prevent
condensation of the products. The results after 30 min on
stream are presented and used for discussion, unless stated
otherwise.
The catalytic activity calculation has been described else-
where [19]. It was evaluated from the concentrations of the
products detected (i.e., propene oxide, acrolein, acetone, pro-
pionaldehyde, CO, and CO2) and the remaining propene.
TOF was evaluated on the basis of the amount of propene
(in moles) transformed to propene oxide related to surface
vanadium species (expressed in moles) per second [20]. The
space–time yield (STY) of propene oxide was defined as the
number of PO molecules produced at the reactor output per
catalyst mass per hour.
3 Results
3.1 Catalyst Characterization
The surface area of MCF support was 676 m2/g and it
decreased after vanadium incorporation to about 500 m2/g
for the samples comprising 3 wt% of vanadium and to
443 m2/g for the samples with 5 wt% of vanadium (Table 1).
Both non modified and vanadium modified samples show
the adsorption/desorption isotherms of IV type, typical for
a three-dimensional structure with H1 shape, with a steep
hysteresis curve recorded at a high p/p0 value (0.6–0.7)
(Fig. 1A). On the basis of adsorption and desorption
branches of isotherms, the cell size (Dc) and the window
size (Dw) characteristic for MCF support and also 3V/MCF
and 5V/MCF were calculated according to BJH theory. Their
values are presented in Table 1.
2.3 Reaction Test
Catalytic performance tests were made under atmospheric
pressure in a glass reactor (10 mm i.d.) using a continu-
ous-flow system in the temperature range of 593–673 K.
A mixture of propene, N2O, and helium as diluent was
passed through a fixed bed containing the catalyst sam-
ples. Before the reaction run, 0.50 g of the catalyst (sieve
fraction of 0.3–0.5 mm) was pre-treated in a helium flow
(12.5 cm3/min) at 673 K for 30 min. The feed gas composi-
tion of propene and N2O (propene 99.5 vol% from Linde
and N2O: 99.995 vol% from Messer) diluted with helium
(≥ 99.9999 vol%, Linde) was introduced into the reactor
to start the reaction after the catalyst bed had reached the
desired reaction temperature. The ratio of the reaction gas
components C3H6, N2O, and He was equal to 1:15:x, where
XRD patterns of the synthesised support (MCF) and the
related vanadium catalysts recorded in the range of low dif-
fraction angels 0.3–6° of 2θ showed signals characteristic of
regular wide mesopores (Fig. 1B), indicating the preserva-
tion of support structure after the introduction of vanadium
precursors.
FT-IR spectra recorded for bare MCF and vanadium mod-
ified MCF support indicate that vanadium species interact
with the hydroxyl groups characterized with an IR band at
3735 cm−1, which are considered as non-acidic. As a result
1 3

A. Held et al.
Table 1 Properties of vanadium
free and vanadium modified
MCF
Catalysts
Vanadium concentra- SBET (m2/g)
tion (wt%)a
Dc (nm)b
Dw (nm)c
Surface density
(V atoms/nm2)
MCF
3V/MCF
5V/MCF
–
676
502
443
20.3
18.2
17.5
13.7
12.0
10.0
–
0.66
1.30
2.8
4.9
aV content by ICP
bCell diameter (Dc) obtained from nitrogen sorption
cWindow diameter (Dw) obtained from nitrogen sorption
Fig. 1 Nitrogen adsorption/
desorption isotherms (A) and
XRD patterns at low diffrac-
tion angles (2θ in the range of
0.3°–6°) (B) on parent MCF
(a) and vanadium modified 3V/
MCF (b), and 5V/MCF (c)
B
A
2500
2000
c
1500
1000
c
b
b
500
a
a
0
0.0
0.2
0.4
0.6
0.8
1.0
1
2
3
4
5
6
Relative pressure (p/p0)
2 Theta
Fig. 2 The IR spectra in the
range of OH groups (A) and IR
spectra of pyridine adsorbed
(B) for parent MCF (a) and
vanadium modified 3V/MCF
(b), and 5V/MCF (c)
A
0.25
B
PyL
1448
0.025
PyH+
1545
c
b
c
b
a
a
3800 3600 3400 3200 3000
Wavenumber (cm-1)
1700
1600
1500
1400
Wavenumber (cm-1)
1 3

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene…
of the introduction of vanadium precursors the intensity of
these bands decreases (Fig. 2A). FT-IR spectra of adsorbed
pyridine show that non modified MCF support does not pos-
sess any Brønsted acidity and only extremely weak Lewis
acidic sites were detected. Introduction of vanadium species
results in the formation of both Brønsted and Lewis acidic
sites (Fig. 2B; Table 2). The number of acidic sites increases
with the amount of vanadium introduced. However, the num-
ber of Brønsted acidic sites is still low and it is comparable
to the acidity of zeolites with an Si/Al ratio equal to 500. The
number of Lewis acid sites is also low and is clearly lower
than the related values for aluminosilicate mesoporous mate-
rials with an Si/Al ratio from 1 to 10, calculated in a similar
way [21]. On the other hand, the strength of both Brønsted
and Lewis acid sites of vanadium modified MCF material
is low and is comparable to that recorded for mesoporous
aluminosilicates with an Si/Al equal to 10 (Table 2).
The TEM images recorded for MCF and vanadium cata-
lysts 3V/MCF and 5V/MCF reveal the presence of cylindri-
cal pores of about 20 nm diameter with a disordered arrange-
ment without any channels (Fig. 3B, D, F). The introduction
of vanadium modifier does not influence the MCF structure
and no crystalline V2O5 particles have been observed. TEM/
EDS mapping (Fig. 1S) indicates a homogeneous distribu-
tion of vanadium species on the support surface.
vanadium species are formed as a result of a small amount
of vanadium precursor impregnation on MCF support.
However, the increase in vanadium concentration (from 3
to 5 wt%) results in the appearance of a weak band at about
1000 cm−1 indicating the formation of a small amount of
V2O5 crystals. A similar observation, showing the formation
of vanadia in V/MCF samples with a vanadium concentra-
tion of about 5 wt%, prepared by means of impregnation,
has been reported by Piumetti et al. [25] and also by Liu
et al. [26]. According to Wang and Wachs [23] and also
to Keller et al. [24] the bands in the range of 200–300 and
500–800 cm−1 should be attributed to V–O–V vibrations,
which indicate the presence of dimeric or polymeric tet-
rahedral vanadium species. Additionally, Raman spectra
recorded for all the studied samples revealed a broad band at
about 920–930 cm−1 and a weak band at 1070 cm−1. These
bands, according to Nguyen et al. [27], Magg et al. [28],
and also to Keller et al. [24] should be attributed to V–O–Si
out-of-phase vibrations and they confirm a high distribution
of vanadium species.
Additional information about the structure of vanadium
surface species was obtained on the basis of UV–Vis spec-
tra. In order to distinguish between different VOx species,
the presented spectra have been deconvoluted into several
Gaussian curves. For the dehydrated samples of V/MCF
catalysts, the spectra could be deconvoluted into three indi-
vidual components.
SEM micrographs of MCF support show the pres-
ence of mostly spherical silica particles, of about 1–3 µm
in size (Fig. 3A, C, E) [22]. The introduction of a vana-
dium precursor does not change the size and shape of MCF
agglomerates.
The maxima of the presented spectra were located in the
range of 250–360 nm (Fig. 5A, B). UV–Vis spectra of 3V/
MCF and 5V/MCF samples recorded under dehydrated con-
ditions showed the strongest band at about 250 nm, which
according to earlier literature is attributed to charge transfer
transitions of isolated V5+ species in tetrahedral coordination
[29, 30]. The weaker bands with maxima located at about
300 and 350 nm point to the presence of a small amount
of polymerized vanadium occurring in a square-piramidal
coordination. The UV–Vis spectra are in agreement with the
results of Raman spectra analysis indicating the presence
of mainly isolated vanadium species in tetrahedral coordi-
nation. The high vanadium species dispersion can be also
derived from the edge energy calculated for V/MCF samples
using Tauc law [17]. The edge energy was higher than 2.5
and it was equal to 3.8 and 3.6 for 3V/MCF and 5V/MCF
samples, respectively (Fig. 2S).
Raman spectra recorded for dehydrated 3V/MCF cata-
lyst indicate the bands at about 270–280, 415, 486, 699,
820, 930, 1034 cm−1, and a weak band at about 1070 cm−1
(Fig. 4). The Raman band at 1035 cm−1 is characteristic of
the stretching frequencies of a terminal V=O bond attributed
to the presence of isolated, tetrahedrally coordinated VO4
species [23, 24]. It confirms the high dispersion of supported
vanadium species (in accordance with TEM/EDS pictures).
This suggestion is also in agreement with DR UV–Vis spec-
tra, which point to the high dispersion of vanadium spe-
cies (Fig. 5A, B). It shows that mainly isolated tetrahedral
Table 2 The concentration and
Sample Concen- Strength
the strength of Brønsted (B) and
tration
of acid
Lewis (L) sites measured by
pyridine
of acid sites
3.2 Catalytic Tests in Propene Epoxidation
sites
(Ades./A0)
(µmol
g−1
B
)
The activities of the vanadium catalysts prepared by impreg-
nation of VOSO4 on mesocellular siliceous foams were
explored under steady-state conditions at 593–673 K with
a constant space velocity (GHSV −3.4 L g−1 h−1) and pro-
pene/N2O/helium ratio (1:15:12.5). The remarkable propene
epoxidation activity over V/MCF catalysts is indicated in
L
B
0
L
0
MCF
0
55
3V/MCF 15 160 0.35 0.80
5V/MCF 30 180 0.55 0.80
1 3

A. Held et al.
Fig. 3 Representative SEM (A, C, E) and TEM (B, D, F) images of parent MCF (A, B), and vanadium modified 3V/MCF (C, D), and 5V/MCF
(E, F)
Fig. 6. For comparison, the catalytic results of V-containing
mesoporous SBA-3, MCM-41, and SBA-15 catalysts, previ-
ously studied by our group [15, 16], are also presented in
Fig. 6.
formed on the studied catalysts. With a similar vanadium
concentration, the V/MCF sample exhibited much higher
C3H6 conversion and propene oxide yield, and also clearly
lower total oxidation than the V/SBA-3, V/SBA-15, and V/
MCM-41 samples. The highest propene oxide productiv-
ity, expressed as STY of PO, and calculated for vanadium
Propene oxide, propionaldehyde, acetone, acrolein,
and COx (CO and CO2) were the main reaction products
1 3

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene…
A
Conversion
S-PO
Y-PO
148
45
40
35
1037
30
25
1000
20
15
10
c
5
0
1034
5V/SBA-3 5V/MCM-41 5V/SBA-15
S-PA S-ACR S-ACT
3V/MCF
5V/MCF
b
B
S-COx
a
45
40
35
200
400
600
800
1000
1200
30
25
20
15
10
5
Raman shift (cm-1)
Fig. 4 Raman spectra of parent MCF (a) and vanadium modified 3V/
MCF (b), and 5V/MCF (c)
0
5V/SBA-3 5V/MCM-41 5V/SBA-15
3V/MCF
5V/MCF
A
B
250
260
2.0
1.0
Fig. 6 Propene selective oxidation over vanadium modified 5V/SBA-
3, 5V/MCM-41, 5V/SBA-15, 3V/MCF, and 5V/MCF: (A) propene
conversion, selectivity to propene oxide (PO), propene oxide yield,
(B) selectivity to oxygen-bearing products (propionaldehyde (PA),
acrolein (ACR), acetone (ACT), COx,(CO, CO2)); reaction tempera-
ture 653 K
310
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
300
343
360
200
400
600
800
200
400
600
800
Wavelength (nm)
Wavelength (nm)
Fig. 5 Deconvoluted UV–Vis reflectance spectra of dehydrated vana-
dium modified MCF: samples 3V/MCF (A) and 5V/MCF (B)
0.0
5V/SBA-3 5V/MCM-41 5V/SBA-15
3V/MCF
5V/MCF
modified mesoporous materials of 2D structure, has been
obtained for 5V/MCM-41 (9.5 gPO/kgcat/h). For 5V/MCF
catalyst, PO productivity was much higher and reached
almost 37 gPO/kgcat/h (Fig. 7).
Fig. 7 Space time yield of propene oxide over vanadium modified
mesoporous silica; reaction temperature 653 K
Not only propene conversion and selectivity to PO show
better performance on V/MCF when compared to 2D sili-
cas but also turnover frequency, indicating the exploitation
of supported vanadium species, shows higher value when
compare to V/SBA-3, V/SBA-15, and V/MCM-41 (Fig. 3S).
This confirms the suggestion that the MCF with a large
pore diameter and the interconnected 3D pore system is far
superior to two-dimensional hexagonally ordered SBA-3,
SBA-15, and MCM-41 as supports.
Figure 8 plots the epoxidation of propene on the V/
MCF catalyst as a function of the reaction temperature.
It has been shown that with an increase in reaction tem-
perature the propene conversion grows significantly, with
1 3

A. Held et al.
A
A
60
50
40
30
20
10
0
40
35
30
25
20
15
10
5
593
603
613
623
633
643
653
663
673
0
Temperature (K)
30
60
90
120 150 180 210 240 270 300
Time on stream (min.)
B
60
50
40
30
20
10
0
B
40
35
30
25
20
15
10
5
593
603
613
623
633
643
653
663
673
Temperature (K)
0
Fig. 8 Propene selective oxidation over vanadium modified 3V/MCF
as a function of reaction temperature; (A) black filled diamond pro-
pene conversion, green filled triangle propene oxide yield, red filled
square selectivity to propene oxide, (B) selectivity to: blue filled dia-
mond propionaldehyde, brown filled circle acrolein, purple filled tri-
angle acetone, and blue multiplication sign COx,(dashed line)
30
60
90
120 150 180 210 240 270 300
Time on stream (min.)
Fig. 9 Propene selective oxidation on 3V/MCF performed at 653 K
as a function of time on stream: (A) black filled diamond propene
conversion, red filled square selectivity to propene oxide; (B) selec-
tivity to oxygenates: blue filled diamond propionaldehyde, brown
filled circle acrolein, purple filled triangle acetone, and blue multipli-
cation sign COx,(dashed line)
a simultaneous enhancement of selectivity to PO of about
30% (from 20 to 26%) (Fig. 8A). With an increase in
the reaction temperature, a clear decrease in the selectiv-
ity to propionaldehyde and to acrolein with a concurrent
increase in the selectivity to acetone was also observed
(Fig. 8B). Independent of vanadium concentration in cata-
lysts supported on MCF material, the total oxidation was
very low and selectivity to COx did not exceed 3% in the
whole range of the applied reaction temperatures.
The catalytic performance of V/MCF materials was
also tested with time on stream. Propene conversion
decreases slowly with time on stream (during the first
2 h), while selectivity to oxygenates changes slightly
(Fig. 9), which indicates the high stability of vanadium
supported MCF catalysts.
Selectivity to COx and to PA decreases with lowering of
propene conversion.
4 Discussion
Spectroscopic characterization of V/MCF catalysts using
Raman and UV–Vis spectroscopy, revealed the presence of
various VOx surface structures (monomers, polymers, and
crystals), however, isolated tetrahedral VO4 species predom-
inated (Figs. 4, 5). Previous investigations concerning the
use of vanadia-based silica supported catalysts for propene
epoxidation have shown that various parameters of the cata-
lysts, including aggregation state and reducibility of vana-
dium species, acid/base properties, and vanadium content,
need to be considered to account for the catalytic behavior
noted for the selective oxidation of propene [15, 19]. It has
been shown that catalysts with high vanadium dispersion
show better performance in a number of oxidation reactions
such as propane oxydehydrogenation [31], methanol partial
To discuss the probable reaction path of propene oxi-
dation the additional catalytic experiments at different
contact times (1.5, 2.1, 3.0, and 4.2 s) were performed.
Prolongation of contact time results in higher propene
conversion and also in some increase in selectivity to
PO and clear decrease in selectivity to ACR (Fig. 4S).
1 3

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene…
oxidation [32], and also in selective propene oxidation [15,
19, 33]. The presence of an isolated tetrahedral vanadium
oxide species is essential as it facilitates the interaction with
–Nδ−=Nδ+=O species, a mezomeric form of N2O, with the
formation of mildly electrophilic oxygen forms. Weakly
electrophilic oxygen interacts with a C=C bond of propene
with the subsequent formation of mainly PO, although PA
and ACT may also be products of this interaction [8]. The
high contribution of PA and ACT in reaction products is
also a result of the high reactivity of PO, which brings about
further transformation.
conversion was the lower the selectivity to PO. Conversly,
when vanadium catalysts were prepared over MCF silica of
3D structure, an increase in the reaction temperature resulted
not only in the growth of propene conversion but also in
higher selectivity to PO (Fig. 8). This may be explained by
the specific structure of MCF support. MCF is characterized
by the presence of spherical cells of about 20 nm diam-
eter interconnected with windows (of a diameter of about
10 nm) forming an open and easily accessible structure
(Fig. 3). Thanks to the open structure of the support, vana-
dium precursors may be spread uniformly on the surface, as
indicated in Raman spectra (Fig. 4) and also in TEM/EDS
mappins images (Fig. 1S), which results in the formation of
isolated monomeric vanadium species with V–O–Si bonds,
active for propene epoxidation. The open structure of the
V/MCF system also has an influence on the shortening of
the way necessary to release the products from the inner
space. It brings about a shortened contact of the reactive
propene oxide molecule with catalytic sites and limits its
further transformation. Very low selectivity to COx, at rela-
tively high propene conversion (53% at 673 K) also seems to
result from the easy release of oxygenates from the catalysts
inner space (Fig. 8).
The vanadium-supported catalysts, obtained by a simple
impregnation of an aqueous solution of VOSO4 on MCF
mesoporous material, characterised by high vanadium dis-
tribution, appeared to be highly effective in propene epoxi-
dation at temperatures below 673 K, offering propene oxide
yields much higher than those provided by previously stud-
ied vanadium containing catalysts [14, 15, 19] as presented
in Fig. 6. Interestingly, some specific changes in selectivity
to oxygenate by-products have also been noted. Distinctly
higher selectivity towards acetone with vanadium concentra-
tion increase and lowering in selectivity to propionaldehyde
was recorded. Analysis of acidity of the studied catalysts
seems to deliver some explanation. According to Annanieva
et al. [9] the presence of relatively strong acidic sites (both
Brönstedt and Lewis) results in the predominance of propi-
onaldehyde as a product of PO isomerization. It has been
found in an earlier study [15] that 5V/SBA-3 shows the
highest acidity among the studied catalysts, while the acid-
ity of 5V/SBA-15, 5V/MCM-41, and 5V/MCF is succes-
sively lower. Consequently, the amount of propionaldehyde,
formed as a result of PO isomerization over catalysts char-
acterized by lower acidity, decreases. This may explain the
changes in PA and ACT contribution in products of propene
oxidation recorded on different catalysts.
It has been shown that increase in the reaction tempera-
ture results not only in higher propene conversion but also
in some increse in PO selectivity. However, even though
the yield of PO exceeded 10% (reaching 13.5%), oxygenates
such as PA, ACT and ACR constitute the majority of oxida-
tion products. Analysis of relationship of selectivity to spe-
cific oxidation products versus propene conversion (Fig. 4S)
indicates that selectivity to oxygenates (propene oxide, acr-
olein and acetone) extrapolated to zero value of propene
conversion show 0.26, 0.31, and 0.33 values, respectively.
These results suggest that PO, ACR, and ACT are formed
mainly directly form propene. Selectivity to PA decreases
with propene conversion lowering and at propene conver-
sion extrapolated to zero it shows a value of about 0.08. It
suggests that PA may be formed both as a primary product
from propene oxidation and also as an effect of PO trans-
formation. Selectivity of COx decreased to near zero value
when propene conversion was extrapolated to zero, which
indicates that COx is both a primary product of propene oxi-
dation and the secondary product resulting from consecutive
oxidation of propene oxide and other oxygenates. According
to Liu et al. [33], propene oxide characterised by relatively
high reactivity undergoes isomerisation to propionaldehyde
and acetone. In the presented results acetone seems to be
mainly a primary product of propene oxidation, although
formation of some ACT as a result of PO isomerization can-
not be excluded. According to Lifshitz and Tamburu [34] the
formation of PA and ACT as a result of PO isomerization is
a simple process involving C–O rapture and hydrogen shift.
However, according to Thömmas et al. [35] a very high rate
Polymeric vanadium species, observed for samples with
higher vanadium concentration, could favour undesired
combustion reaction pathways, leading to the formation of
carbon oxides and also high molecular products. V/MCF
catalysts were characterized by a relatively high selectiv-
ity to propene oxide, comparable with those observed for
previously reported V/SBA-3, V/SBA-15, and V/MCM-41
catalysts, where the predominance of isolated vanadium spe-
cies was also confirmed. Nevertheless, vanadium supported
mesocellular silica foams were characterized by much higher
propene conversion when compared to vanadium-supported
mesoporous silicas of 2D structures, which results in a
higher PO yield and a lower contribution of carbon oxides
on V/MCF catalysts.
Thömmes et al. [32] studying iron supported catalysts
for propene epoxidation indicated that because of the very
high reactivity of PO the selectivity to this main product
is limited and it was observed that the higher the propene
1 3

A. Held et al.
of PA conversion, especially in the presence of N2O, results
in a lowering of the PA contribution in the oxidation prod-
uct. Taking into account the high concentration of the active
centers on V/MCF catalysts it is possible to explain the low
selectivity to PA in propene oxidation recorded on the stud-
ied samples, especially with higher vanadium concentration.
It has also been shown by Li and Shen [36] that the
formation of acetone by means of propene oxidation over
supported vanadium catalysts depends both on vanadium
species dispersion and also on the number and strength of
Brønsted acids. The above mentioned authors indicate that
propene interacts with –V–O–H species, which are the Brøn-
sted acid sites, with the following formation of isopropoxy
species (–O–CH(CH3)2). The adsorbed isopropoxy species
are transformed to acetone. Acetone may be easily adsorbed
on the present Lewis acid sites. Considering that the studied
catalysts (V/MCF) are characterized by their low number
and the very low strength of Lewis acids (Table 2), acetone
is easily desorbed and its further transformation is limited.
This may explain the very high selectivity to acetone with
temperature increase (Fig. 8B) and also clarify the different
contribution of propionaldehyde and acetone depending on
differing amounts of vanadium in catalysts.
catalysts demonstrates the superior performance of the V/
MCF system in propene epoxidation. It has been shown that
a high concentration of isolated V species easily accessible
to the reactants is readily achievable on the surface of the
MCF support. Moreover, a much lower acidity of the surface
acid sites of V/MCF catalysts limits the transformation of
primary products. The superior performance of the V/MCF
catalysts in propene epoxidation has been attributed to the
well-defined 3D mesopore systems leading to favourable
conditions for internal mass transfer thanks to an open and
easily accessible structure.
Acknowledgements This work was supported by the National Science
Centre, Poland, Grant No. 2016/23/B/ST5/00615.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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Turnover frequency (TOF), defined as molPO molV
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A slightly lower TOF value calculated for 5/MCF when
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Raman and UV–Vis spectrscopies indicating a small amonut
of polymeric vanadium species on the catalyst with higher
vanadium concentration.
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