1
20
Y. Goto et al. / Applied Catalysis A: General 509 (2016) 118–122
Table 2
Ammoxidation of 3-picoline.a
◦
Catalyst
Cat. wt. (g)
T ( C)
Conversion (%)
PIC
Selectivity (%)
CP
O2
NAc
NA
Pyr
CO
CO2
WO3
1.00
0.38
0.75
0.85
383
389
386
381
0.0
100
100
11.5
47.3
51.7
59.8
0.0
98.0
99.5
91.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.7
0.0
0.7
0.2
3.8
0.0
0.1
0.0
0.2
0.0
1.2
0.3
4.3
VOx/WO3
W–V–O
V2O5
100
a
Conditions: gas composition, PIC/H2O/NH3/O2/He = 1/8/6/4.4/19.6.
1
5
0
5
0
The rod-shaped crystal is due to stacking of the layers along the
c-axis by sharing the apex oxygen. The microporosity is suggested
to be due to the heptagonal channel structure of the a–b plane.
The following results of XRD, STEM, N adsorption suggest that the
2
1
present W–V–O catalyst has the same structural characteristics as
the porous and layered structure illustrated in Fig. 1. The XRD pat-
◦
tern of W–V–O (Fig. 2) showed two sharp diffraction lines at 22.7
◦
and 46.5 . The XRD pattern of VOx/WO3 showed diffraction lines
attributed to VO , V O5 and WO .
2
2
3
The STEM image of W–V–O (Fig. 3) shows rod shaped crystals.
The N -adsorption isotherm of W–V–O (Fig. 4) shows N adsorption
2
2
−
6
feature at low relative pressure (P/P < 10 ) which is characteristic
0
1
0-8
10-6
10-4
P / P0
10-2
100
to microporos materials. The micropore volume estimated by the
−3
−1
.
t-plot method is 7.7 × 10 mL g
Fig. 4. N2-adsorption isotherm of W–V–O.
3.2. Catalytic performance
2
.3. Catalytic testing
First, four catalysts (WO3, V2O5, VOx/WO3, W–V–O) were tested
◦
for the ammoxidation of PIC at 383–389 C. Table 2 lists the con-
versions of PIC and O2 and selectivities to cyanopyridine (CP),
nicotinamide (NA), nicotinic acid (NAc), pyridine (Pyr), CO, and CO2.
As previously reported [4], V2O5 showed good selectivity of CP at
high conversion level (99.2%), while WO3 showed no conversion
of CP. This indicates that vanadium is an indispensable element for
this catalytic system. The W-added vanadium oxides, VOx/WO3 and
W–V–O, were tested for the reaction under the conditions of full CP
conversion. VOx/WO3 and W–V–O show higher CP selectivities than
V2O5, which indicates that tungsten oxides as co-catalysts increase
the CP selectivity of V-based ammoxidation catalysts. Especially,
the W–V–O catalyst showed the highest CP selectivity of 99.5%.
The catalytic performances of the V-based catalysts, including
a well established VOx/TiO2 catalyst for this reaction [8], were
Ammoxidation of 3-picoline was carried out at atmospheric
pressure using a fixed-bed flow reactor (Pyrex glass tube) with an
inner diameter of 9 mm. Catalyst powders were pressed to pellets,
crushed, and sieved. Catalyst pellets (0.25–0.50 mm size), diluted
with quartz (0.2–0.4 mm) in a volumetric ratio of 1:6, were set in the
reactor. The reaction temperature was measured inside the catalyst
bed by a thermocouple. The gas stream (NH /O /He) was fed to the
reactor with mass flow controller. The aqueous solution of 11 mol%
3
from a syringe pump with a micro-feeder. The volumetric com-
position of reaction gas was PIC:H O:NH :O :He = 1:8:4:4.4:19.6
3
2
◦
-picoline (PIC) was fed continuously into the gas stream at 150 C
2
3
2
(
PIC/H O/NH /O /He = 2.7%/21.6%/10.8%/11.9%/53.0%). Total flow
2 3 2
−
1
rate was 37 mL min . CH4 gas was fed into outlet gas as exter-
nal standard. The gas phase products (CO and CO ) in the outlet gas
◦
further compared under different contact time at 380 C. From
2
were collected in a gas bag and analyzed by TCD-GC (GL science
GC-3200, 6 m SHINCARBON-ST packed column). Organic products,
trapped in ethanol at 0 C, followed by adding n-octane as exter-
the slope of the curve in the plot of CP conversion versus con-
tact time (Fig. 5), the order of the catalytic activity is as follows:
VOx/WO3 > VOx/TiO2 > W–V–O ≈ V2O5. The selectivities of the main
product (CP) and byproducts (Pyr and CO2) are plotted as a function
◦
nal standard, were analyzed with FID-GC (Shimadzu GC-14A, 30 m
0.32 mm TC-5 capillary column). Note that carbon balance and
oxygen balance for the catalytic results were 100.1 ± 0.6% and
100
9
7.5 ± 2.6%, respectively.
80
60
40
3
. Result and discussion
3.1. Catalyst characterization
Previously, we have reported a series of studies on the
hydrothermal synthesis of metal oxides consisted of group 5 and 6
elements. A group of the synthesized metal oxides have micropore
due to heptagonal channel and layered structure as illustrated in
Fig. 1, which is characterized by the common structural results as
20
◦
◦
0
0.5
1
1.5
follows: (i) two sharp diffractions around 23 and 46 along with
-
1
◦
◦
Contact time (g s mL )
broad diffraction peaks around 8 and 27 observed by XRD, (ii)
long rod-shaped crystal morphology, (iii) the presence of microp-
Fig. 5. Effects of contact time on PIC conversion for ammoxidation of PIC over
◦
◦
ore. The diffractions around 23 and 46 have been attributed to the
0 0 1) and (0 0 2) planes of the layered structure in c-axis direction.
(
ꢀ)W–V–O, (ꢁ) VOx/WO3, (᭹) V2O5, and (ꢁ) VOx/TiO2. Conditions: catalyst
◦
(
amount = 0.5 g, T = 380 C, gas composition, PIC/H2O/NH3/O2/He = 1/8/6/4.4/19.6.