Temperature effect on the photo-epoxidation of propylene over V-Ti/MCM-41 photocatalyst

Article abstract of DOI:10.1016/j.catcom.2012.12.024

The effect of reaction temperature on the gas-phase photocatalytic epoxidation of propylene was systematically studied over V-Ti/MCM-41 photocatalyst at different temperatures (298-393 K). Temperature significantly influences the product distribution of photo-epoxidation. Raising temperature is favorable towards the formation of propionaldehyde (PA) due to inhibition of PA transformation to propionic acid (C₂H₅COOH), the intermediate product of acetaldehyde (AA). Results indicated that the reaction temperature has a dual-action to the reaction rate: (1) improving the desorption of reaction products, which provides more active sites for reaction and (2) reducing the adsorption of propylene on the photocatalyst, which declines the reaction efficiency.

Full text of DOI:10.1016/j.catcom.2012.12.024

Catalysis Communications 33 (2013) 57–60
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Temperature effect on the photo-epoxidation of propylene over
V–Ti/MCM-41 photocatalyst
b
Van-Huy Nguyen a, Jeffrey C.S. Wu a, , Hsunling Bai
⁎
a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
b
Institute of Environmental Engineering, National Chiao Tung University, Hsin Chu 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of reaction temperature on the gas-phase photocatalytic epoxidation of propylene was systematically
studied over V–Ti/MCM-41 photocatalyst at different temperatures (298–393 K). Temperature significantly influ-
ences the product distribution of photo-epoxidation. Raising temperature is favorable towards the formation of
propionaldehyde (PA) due to inhibition of PA transformation to propionic acid (C2H5COOH), the intermedi-
ate product of acetaldehyde (AA). Results indicated that the reaction temperature has a dual-action to the
reaction rate: (1) improving the desorption of reaction products, which provides more active sites for reaction
and (2) reducing the adsorption of propylene on the photocatalyst, which declines the reaction efficiency.
© 2012 Elsevier B.V. All rights reserved.
Received 19 November 2012
Received in revised form 14 December 2012
Accepted 21 December 2012
Available online 4 January 2013
Keywords:
Epoxidation
Photocatalysis
Propylene oxide
Temperature effect
1. Introduction
heterogeneous photocatalysis [6]. To date, only few studies have inves-
tigated the temperature effect in gas-phase photocatalytic reactions, es-
One of the most important challenges that scientists faced today
is how to generate useful epoxides from photoreaction of alkenes,
i.e., photo-epoxidation of propylene to propylene oxide (PO) [1,2].
Despite photocatalytic epoxidation has several advantages such as low
energy consumption and the generation of eco-friendly products,
their efficiency is still low, making the process far from commercializa-
tion [3]. Therefore, enhancing photo-efficiency has become an increas-
ingly important task in chemical production as well as environmental
protection.
Among a variety of metal oxides, V2O5 and TiO2 with low loadings
onto silica-supported photocatalysts could achieve steady-state PO for-
mation with high selectivity (>40%) [4,5]. Recently, we have screened a
series of photocatalysts such as SiO2, TiO2, V–Ti/MCM-41, V2O5/SiO2,
Au/TiO2 and TS-1 [2]. V and Ti substituted MCM-41 material has
pecially in epoxidation [7–10]. In 1979, P. Pichat et al. reported that
temperature had a negative effect on the photo-epoxidation activity
over the following series of UV irradiated oxides: TiO2, ZrO2, V2O5,
ZnO, SnO2, Sb2O4, CeO2, WO3, and a Sn–O–Sb mixed oxide [7]. In
2007, H.J. Zhan et al. suggested that in the epoxidation of styrene, the
styrene conversion increased with temperature in the range of
343–383 K. However, the selectivity of epoxide first increased from
71.7% at 343 K to 82.3% at 353 K, then drastically dropped to approxi-
mately 61.7% at 383 K [8]. It must be emphasized that photocatalytic re-
actions can be conducted at the ambient temperature and do not
require heating since the reaction can be activated by photons. Even
though raising temperature can accelerate the surface reaction, it also
reduces the adsorption of reactant on the photocatalyst surface.
Hence, the effect of temperature on photo-epoxidation is relatively
complicated.
shown the highest possibility of PO production with PO formation rate
−1
and PO selectivity achieved are 114 μmol g
h−1 and 47%, respective-
To the best of our knowledge, the particular role of reaction tempera-
ture on photocatalytic epoxidation of propylene has not been systemati-
cally study yet. Therefore, the major purpose of this study is to
evaluate the direct photo-epoxidation in order to elucidate the
roles of reaction temperature in photocatalytic epoxidation propyl-
ene over V–Ti/MCM-41 photocatalyst under UV irradiation.
cat
ly. It seems to be generally agreed that the isolated tetra-coordinated
species, such as V and Ti, in the ligand-to-metal charge-transferred
state is the key structure to be favorable for promoting selective
photo-epoxidation. Additionally, it is interesting to observe the most ef-
ficient reaction temperature in the range of 313–333 K. However, there
is still lack of information on the particular role of reaction temperature
on the photocatalytic epoxidation of propylene to propylene oxide.
Generally, although reaction temperature has a significant impact
on gas–solid heterogeneous reactions, it is not very sensitive on
2. Experimental
2.1. Preparation and characterization of photocatalyst
The procedures for the preparation of V–Ti/MCM-41 photocatalyst
⁎
Corresponding author. Tel.: +886 2 23631994; fax: +886 2 23623040.
E-mail address: cswu@ntu.edu.tw (J.C.S. Wu).
were mentioned in detail in our previous work [2]. First, 21.2 g of
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.12.024

58
V.-H. Nguyen et al. / Catalysis Communications 33 (2013) 57–60
0.30
sodium metasilicate monohydrate dissolved in 100 ml DI water was
combined with appropriate amount of titanium oxysulfate hydrate
and vanadyl sulfate hydrate (dissolved in 20 ml of 2 M H2SO4) to
form a uniform gel. Next, 7.28 g of CTAB dissolved in 25 ml of DI
water was added slowly into the mixture. After stirring for 3 h, the gel
mixture was transferred into autoclave and kept in an oven at 418 K
for 36 h. Then, the resulting solid was filtered, washed and dried at
383 K for 8 h. Finally, the solid was calcined at 823 K for 10 h.
Powder X-ray diffractometer (XRD, Xray-M03XHF, Ultima IV) was
used to verify the crystalline structure of the photocatalyst. All peaks
identified by XRD were checked and assigned to known crystalline
phases. The light absorption of photocatalyst was fully characterized
by ultraviolet–visible light (UV–vis) spectroscopy (Varian Cary-100).
The adsorption/desorption isotherm and pore size distribution of
catalysts were characterized by nitrogen adsorption (ASAP2010
Micromeritics) at 77 K. Before analysis, the catalyst was degassed
at 100 °C for 12 h to remove adsorbed vapors or gases.
0.25
0.20
0.15
0.10
0.05
0.00
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. UV–vis absorption spectrum of the photocatalyst.
3. Results and discussion
3.1. Characteristics of photocatalyst
2.2. Photocatalytic epoxidation of propylene
The experimental system, which was conducted continuously in the
reactant gas mixture of C3H6:O2:N2=1:1:18 (GHSV=6000 h−1) under
atmospheric pressure, can be found in our previous study [2]. First,
20 mg of photocatalyst was evenly spread in a photo-reactor
(0.55 cm3) equipped with a quartz window on the top for transmis-
The nitrogen adsorption/desorption isotherms and a clear single and
narrow pore size distribution of V–Ti/MCM-41 are depicted in Fig. 1.
The BET surface area of V–Ti/MCM-41 is approximately 790 m2·g−1
while BJH average pore diameter is around 2.9 nm which was estimat-
ed by the BJH method from the desorption branches. It suggests that the
photocatalyst obtained has a very uniform pore structure.
Fig. 2 shows the UV–vis spectrum of the photocatalyst. A strong band
centered at about 220 nm is generally attributed to the transition of
ligand-to-metal charge transfer involving tetra-coordinated both of tita-
nium species (Ti+4 state) and vanadium species (V+5 state) in the frame-
work of MCM-41 photocatalyst. A weaker band at around 340 nm is also
attributed to the charge transfer on tetra-coordinated V-oxide species
[11].
Powder low-angle X-ray diffraction (XRD) pattern of V–Ti/MCM-41
photocatalyst is shown in Fig. 3 which clearly indicates the charac-
teristic low-angle peaks of mesoporous materials, featuring pattern
with narrow (1 0 0) peaks. There are no peaks corresponding to
metal oxides observed in this study, suggesting that metal ions are
atomically dispersed in the framework of mesoporous MCM-41. Ad-
ditionally, (1 1 0) and (2 0 0) reflection peaks essentially merge into
one broad peak, indicating that the mesostructure is changed by
metal incorporated. Karthik et al., who synthesized Si, Cu and Al in-
corporated MCM-41 materials, also observed this phenomenon
[12]. They suggested that it might also be due to the partial loss of
long-range crystallographic order and the formation of metal oxide
species by calcination at 823 K.
sion of light. Then, light from Mercury-Arc-lamp (0.2 mW·cm−2
,
320–500 nm) was guided by an optical fiber and irradiated on the
photocatalyst. A hot plate was used to control the reaction tempera-
ture in this study.
The stream was analyzed online periodically using a gas chromato-
graph (Young Lin, YL6100 GC) equipped with a flame ionization detector
(FID) and a thermal conductivity detector (TCD). Carbon dioxide (CO2)
and oxygenated products (propylene oxide (PO), propionaldehyde (PA),
acetone (AC), acetaldehyde (AA) and ethanol (ROH)) were separated pe-
riodically using a Porapak-N packed column and analyzed by FID (except
CO2 via TCD), while the lighter gases (O2 and N2) were separated by a mo-
lecular sieve-5A capillary column and analyzed by TCD. The propylene
conversion and the selectivity of products in the reaction were calculated
directly by Eqs. (1) and (2) as defined below.
C3H6 conv: rate ¼ ∑rate of all productsconvertedtoC3
ð1Þ
Product selectivity
¼ ðproduct converted to C3=all products converted to C3Þ ꢀ 100%:
ð2Þ
500
400
300
200
100
Adsorption
Desorption
6000
5000
4000
3000
2000
1000
0
1.0
0.8
0.6
0.4
0.2
0.0
2
4
6
8
10 12 14 16
Pore Diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
0
2
4
6
8
10
12
2θ (degrees)
Fig. 1. The nitrogen adsorption/desorption isotherms of the photocatalyst and its pore
size distribution curve (inset).
Fig. 3. Low angle X-ray diffraction pattern of the photocatalyst.

V.-H. Nguyen et al. / Catalysis Communications 33 (2013) 57–60
59
250
240
230
220
210
200
190
180
170
adsorption on a photocatalyst, which declines the reaction efficiency. To
the end, the reaction temperature has a dual-action to the reaction rate
in this study.
Additionally to PO, other products such as PA, AC, AA and ROH were
detected and their selectivity was calculated. The selectivity of products
on stream as shown in Fig. 5 is very stable and it shows very similar and
stable time-dependent behavior for the photocatalytic epoxidation.
Moreover, it is noted that temperature has a significant effect on prod-
uct distribution. For instance, epoxidation selectivity slowly increases
with the rising of temperature. However, at temperature higher than
323 K, a slightly loss of selectivity towards PO is observed, which is
due to the competition of multiple reactions towards different products.
Generally, when photocatalyst is irradiated with UV-light, a series
of reactions occurs to generate the active species: oxygen radicals
280
300
320
340
360
380
400
Reaction temp. (K)
Fig. 4. The effect of reaction temperature on photocatalytic epoxidation rate. The C3H6
conversion rate was obtained on stream after 4 h, calculated by Eq. (1).
photocatalyst þ hv→e– þ hþ
ð3Þ
ð4Þ
−
O2ðgÞ þ e−→O2
ðadsÞ
3.2. Effect of reaction temperature on photo-epoxidation
−
þ
ꢁ
O2 ðadsÞ þ h →2 O
:
ð5Þ
ðadsÞ
From the epoxidation result, no activity was observed either in the
absence of photocatalyst with UV illuminated or with the photocatalyst
in the absence of UV irradiation, meaning that the epoxidation over
V–Ti/MCM-41 photocatalyst is mainly photocatalytic epoxidation.
It also suggests that temperature can affect the photo-epoxidation
reaction by altering the adsorption of propylene and the desorption
of the reaction products on the photocatalyst. Adsorption is an exo-
thermic process; therefore, increase of temperature tends to inhibit
the process. Alternatively, high temperature is beneficial to desorp-
tion process since it is an endothermic process. In order to elucidate
the effect of reaction temperature, experiments were carried out at
298, 323, 348 and 393 K.
With the humidity in the gas stream, the water molecules could
trap holes (h+) to form hydroxyl radicals OH*(ads)
þ
H2O þ h →OH ðadsÞ þ Hþ:
ð6Þ
ꢁ
Following is the competing reaction pathways of photo-epoxidation
of propylene via atomic oxyradicals O*(ads) and hydroxyl radicals OH*(ads)
.
Fig. 4 shows that there is no monotonic relationship in the epoxida-
tion rate and the reaction temperature. Generally, the photo-reaction
does not require heat due to the photonic activation. Kim and Lee men-
tioned that the true activation energy Et is nil in photo-reaction; as a re-
sult, the activation energy is the apparent activation energy Ea which is
very small [13]. Hence, the rate on desorption of the final product
should become the limiting step and Ea tends to the heat of adsorption
of the product in the medium temperature range. In other words, rais-
ing temperature improves the desorption of the photocatalytic reaction
products, which enhances the reaction efficiency by releasing more ac-
tive sites for reaction. The result in Fig. 4 confirms the reaction efficiency
increases in the range of 300–330 K which passes or overlaps the boiling
points of main products (Table S.2) and consists of above explanation. In
contrast, raising temperature might cause the difficulty of propylene
ð7Þ
In the previous study, we have mentioned that the surface atomic
oxyradicals O*(ads) can react with propylene to yield three isomerization
products via the angle of H-atom abstraction from the propylene attacked
by O*(ads) [2]. Hence, temperature should not play important roles for the
change of the selectivity of products. Interestingly, however, high temper-
ature inhibits the water molecules migrate to the photocatalyst surface
where the hydroxyl radicals OH*(ads) intermediates can be generated.
Therefore, it will inhibit PA transform to propionic acid (C2H5COOH) —
d) 393 K
a) 298 K
b) 323 K
c) 348 K
50
40
30
20
10
0
PA
PO
PO
PO
PA
PO
AA
PA
AA
PA
AC
AA
AC
AA
AC
AC
ROH
ROH
0
60
120 180 240 0
60
120 180 240 0
60
120 180 240
0
60
120 180 240
Time on stream (min)
Fig. 5. The selectivity of epoxide products on stream calculated by Eq. (1) for different temperature conditions: (a) 298 K; (b) 323 K; (c) 348 K and (d) 393 K.

60
V.-H. Nguyen et al. / Catalysis Communications 33 (2013) 57–60
the intermediate product of AA, the resulting is PA increases and AA de-
creases with the temperature.
Acknowledgment
Additionally, in the photocatalytic epoxidation, it is very difficult to
distinguish the photo and thermal effects on the reaction because the
heating may enhance not only the desorption of products but also the
partial and/or total oxidation of intermediates. Even at room tempera-
ture, the thermal effect may not be negligible; therefore, reaction tem-
perature just above the boiling point of those products was selected in
order to reduce the thermal effect to reaction. It confirms that products
in this reaction are partially adsorbed and the desorption of these prod-
ucts requires heating. However, increase the reaction temperature also
changes the partial oxidation; as a result, products distribution will be
re-arranged. The results in this study show that high temperature is
favorable towards the formation of propionaldehyde, one of the most
reactive aldehydes. Yoshida et al. reported a re-arrangement of selectivity
over vanadium oxide supported on silica. They also found that tempera-
ture was favorable towards the formation of aldehydes [14].
The authors gratefully acknowledge the funding of the projects by
the National Science Council of Taiwan under contract numbers NSC
99-2923-E-002-002-MY2 and NSC 99-2221-E-009-037-MY3.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.12.024.
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The effect of temperature on the photocatalytic epoxidation of propyl-
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