Y. Lei et al. / Journal of Catalysis 321 (2015) 100–112
101
0
0
Cu(Cu )-based catalysts, probably due to the Cu transforming into
discussed, and the role of Ti modification in enhancing the catalytic
activity for PO formation will also be discussed in depth.
I
II
oxidized copper species (Cu or Cu ) [18,31]. On the other hand,
copper oxide (CuO )-based catalysts have been reported to show
superior performance in the epoxidation of C
x
3
H
6
+
by O
2
, especially
+
when modified with alkali metal ions such as K or Cs . However,
2. Experimental
a PO selectivity of ꢀ30–60% can be obtained at C
3 6
H conversions
of ꢀ1–10% over the above catalysts. It has been revealed that the
2.1. Catalyst preparation
presence of alkali metal ion can decrease the acidity arising from
CuO
is favorable to PO selectivity [19,21,32]. Recent studies disclosed
that binary metal catalysts such as RuO –CuO /SiO , RuO –CuO
NaCl/SiO , and VO –CuO could enhance the activity and selectivity
for PO formation [33,34]. But the C conversion hardly exceeds
0% even with PO selectivities of ꢀ30–50%.
Furthermore, a few supported molybdenum oxide catalysts
have been reported for the gas-phase epoxidation of C by O
The catalyst MoO /SiO prepared by a simple impregnation method
was highly effective for the epoxidation of C , and a C con-
version of 17.6% and a PO selectivity of 43.6% were obtained. It
was found that the active species of the MoO /SiO samples are
crystalline MoO species, which are effective in abstracting a
x
and increase the dispersion of copper oxide species, which
Bi SiO /SiO (Si/Bi = 50), a typical mesoporous material with a
2
5
2
2
ꢁ1
surface area of 492 m g and a pore diameter of 2.5 nm, was
synthesized by a procedure described elsewhere, using hexadecylt-
rimethylammonium bromide (CTAB, Sinopharm Chemical Reagent
Co., Ltd., AR) as the surfactant template and tetraethyl orthosilicate
(TEOS) as the silicon source [37]. Typically, a homogeneous mix-
ture composed of CTAB as the template, Bi(NO ) ꢂ5H O as the
2
x
2
2
x
–
2
x
x
3
H
6
2
3
3
2
3
H
6
2
.
source of bismuth, and TEOS as the source of silicon in water
was stirred in an ice-water bath for 3 h, and the obtained milky
suspension was further treated in an autoclave at 353 K for 20 h.
The resultant solid product was then collected and washed with
deionized water three times by centrifugation. The obtained solid
precipitate was dried at 383 K for 12 h in air. The organic template
was finally removed via calcination in air by heating from ambient
x
2
H
3 6
3 6
H
x
2
3
hydrogen atom from propylene but not in inserting lattice oxygen.
Contrary to previous studies that reported that molybdenum oxide
was a sink for radicals, in this study it was found that molybdenum
oxide was the most effective of the oxides studied in generating
ꢁ1
temperature to 823 K at a rate of 2 K min and keeping it at 823 K
for 8 h.
The MoO –Bi SiO /SiO and Ti-modified MoO –Bi SiO /SiO
2
3
2
5
2
3
2
5
radicals [35]. However, monoclinic nanosized MoO
a diameter of less than 10 nm, stabilized in a nanorod matrix, have
been found to be promising catalysts for the epoxidation of C
with N O [36]. Previous studies have revealed that environmen-
tally friendly materials such as Bi SiO /SiO [37] and Bi–Si meso-
porous zeolites [38] exhibit highly selective oxidation and
photocatalytic performance. Moreover, the Bi-containing materials
are also widely used in the gas-phase epoxidation of alkene by
molecular oxygen. The activation of molecular oxygen by bismuth
oxide cluster cations has been observed in the presence of unsatu-
rated hydrocarbon molecules [39]. Recently, we found that the
2
particles with
catalysts were prepared by an impregnation method. Briefly, after
undergoing calcination at 823 K, powdery Bi SiO /SiO was
immersed in an aqueous solution of ammonium molybdate
2
5
2
3
H
6
2
((NH ) Mo O ꢂ4H O, Shanghai Colloid Chemical Plant, AR) and
4
6
7
24
2
2
5
2
stirred for about 30 min in an ice-water bath. Then a quantity of
tetrabutyl titanate was added in anhydrous ethanol and stirred
evenly. The mixed alcohol solution was dropped into the suspen-
sion solution above, which was kept in the ice-water bath with
stirring for 3 h. Subsequently, the slurry was evaporated to dryness
overnight at 383 K. The powdery samples were calcined in air at
different temperatures for 8 h to gain the catalyst. The Ti-modified
Bi SiO /SiO catalyst was prepared by the same method without
2 5 2
modification of Bi-containing mesoporous materials Bi SiO /SiO
2
5
2
with molybdenum oxides could greatly enhance their activity
and selectivity for PO formation. The epoxidation occurred effi-
ciently under relatively mild conditions over our catalysts, and
adding Mo species. The MoO3 was obtained by calcination of
ammonium molybdate in air at 823 K. The Bi Mo O was
prepared via precipitation. In brief, a homogeneous mixture con-
2
3 12
obtained a PO selectivity of 55.14% at 21.99% C
Favorable synergistic effects between Mo and Bi species have been
found with a Mo/Bi molar ratio of 5 [40].
However, it is noteworthy that the Bi–Mo binary catalyst is also
known as a representative catalyst for the allylic oxidation of C
3
H
6
conversion.
taining ammonium molybdate ((NH ) Mo O ꢂ4H O) and bismuth
4
6
7
24
2
nitrate (Bi(NO ) ꢂ5H O) in glycerol solution was first stirred at
3
3
2
room temperature for 30 min, and then the solution pH was
adjusted to 1 with aqueous ammonia (25%). The solution was aged
for 24 h at 453 K. The obtained solid was washed three times with
distilled water and then dried in air at 353 K to get Bi Mo O12.
3 6
H
to acrolein by O [41]. It is generally believed that competition
2
2
3
exists between the allylic oxidation and the epoxidation. However,
little work have been devoted to discussing the selectivity control
in the gas-phase epoxidation of C
3
H
6
over MoO
3
-based catalysts.
2.2. Catalytic reaction
Therefore, it is of great significance to study the factors controlling
the reaction route for the formation of acrolein or PO. Our previous
works have demonstrated that the synergistic effects between
The catalytic reaction was carried out using a tubular fixed-bed
reactor under a pressure of 0.15 MPa at 673 K with a flow rate of
25 mL min . The catalyst (typically 0.1 g) was mixed with quartz
sand (typically 2.0 g, 24–50 mesh) and placed in the central zone of
ꢁ1
nanoparticulate MoO
able to the epoxidation of C
the MoO -modified catalysts in our work are very different from
3
and bismuth oxide cluster cations are favor-
3
H
6
by O to form PO. This suggests that
2
3
the reactor. The reactant gas mixture of C
3 6 2
H and O diluted with
the previously reported bismuth molybdate catalysts used in the
selective oxidation of propylene to acrolein.
N was introduced by mass-flow controllers to start the reaction
at flow rates of C H /O /N = 2/4/19 mL min . The products were
3 6 2 2
2
ꢁ
1
The present work is a continuation of our previous studies on
analyzed by on-line gas chromatography. All of the lines and valves
between the exit of the reactor and the gas chromatograph were
heated to 403 K to prevent the condensation of organic products.
the selective oxidation of C
3 6 2 3
H by O over MoO -based catalysts.
Although the MoO -based catalysts in our previous studies showed
3
good epoxidation performance, we did not pay much attention to
reaction routes and the Mo loading. Here in the present paper,
C
3
H
6
conversion was calculated on a carbon basis from the concen-
trations of the products detected (PO, acrolein, allyl alcohol, CO
and the remaining C . PO selectivity was evaluated by the
x
)
we report a Ti-modified MoO
catalyze the epoxidation of C
a lower Mo loading. The catalytic behavior and the structural char-
acteristics of the Ti-modified MoO –Bi SiO /SiO catalyst will be
3
–Bi
2
SiO
5
/SiO
2
catalyst, which can
3 6
H
3
H
6
by O
2
much more efficiently with
amount (in mol) of PO as a fraction of the total amount (in mol)
of all the products detected; the difference in carbon number in
each product was considered.
3
2
5
2