L. Yi-chuan et al. / Catalysis Today 212 (2013) 169–174
171
Table 2
Instrumental and operating conditions for ICP-AES measurements.
100
Parameter
Type or amount
R.F. power (kw)
1.15
0.6
1.0
14
30
100
15
20
Carrier gas (Ar) flow rate (L min−1
)
90
80
70
60
Auxiliary gas (Ar) flow rate (L min−1
)
Coolant gas (Ar) flow rate (L min−1
Nebulizer flow (psi)
)
Pump rate (r min−1
)
X
H2O2
Observation height (mm)
Integration time (s) on-axis
off-axis
S
PO
5
Wavelength (nm)
Fe 259.940
content of H2O2, respectively. The nid represent the mole content
of PO that all the H2O2 fully react with C3H6 would produce.
PO
0
20
40
60
80
100
120
140
2.4. Characterization of TS-1
Reaction time (h)
The crystal structure was identified by X-ray diffraction (XRD)
(Rigaku D/max- 2550VB/PC) analyses using Cu-K␣ radiation. Spec-
tra were collected from 2ꢀ = 5◦ to 50◦ in 0.02◦ steps. The crystal
morphology was examined by a scanning electron microscopy
(SEM) (JEOL Model JSM-6360 LV, Japan). Framework IR spectra
were recorded on a Nicolet Magna-IR550, using KBr wafers.
Inductively coupled plasma atomic emission spectrometry(ICP-
AES) was used for Fe determination by an IRIS Advantage ER/S
inductively coupled plasma spectrometer (TJA, USA). The opera-
tion conditions and the wavelengths were summarized in Table 2.
The sample of TS-1 was pre-dissolved by aqua regia.
Fig. 2. Long-run test of the catalyst for propylene epoxidation.
catalyst effectively catalyzes the epoxidation of propylene as both
the H2O2 conversion and PO selectivity are maintained at about 90%
during 140 h on steam, and therefore the stability is impressively
good.
3.2. PGME effect on catalytic performances
PGME is the overriding by-product of propylene epoxidation
[16], formed by consecutive reactions of target reaction. Propylene
epoxidation was operated in the reactor and PGME was added to
The micro-structure of TS-1 was analyzed on
a Micro-
meritics ASAP 2020 including single- and multipoint BET
(Brunauer–Emmett–Teller) surface area, Langmuir surface area,
pore volume and pore-length distributions in the mesopore ranges
by the BJH (Barrett–Joyner– Halenda) algorithm, and the distribu-
tions of micropore volume and pore size by the methods of H–K
(Horvath-Kawazoe) with Saito & Foley model for cylindrical pores
as well.
H2-temperature-programmed reduction (H2-TPR) was per-
formed on a Micromeritics AutoChem II 2920 chemical adsorption
instrument. Using mixture gas H2–Ar (9.92% H2) as reduction gas.
Typically, the sample (0.2 g) was first pretreated in a quartz reactor
with a high-purity He gas flow at 120 ◦C for 1 h. After the sample was
cooled to room temperature, the H2–Ar mixture was introduced
into the reactor at a rate of 5 cm3 min, and the temperature was
raised to 700 ◦C at a rate of 10 ◦C/min. The consumption of H2 was
monitored by a thermal conductivity detector (TCD).
NH3-temperature-programmed desorption (NH3-TPD) was also
carried out with the Micromeritics AutoChem II 2920 chemical
adsorption instrument. Typically, the sample (0.2 g) loaded in the
quartz tube was first pretreated with a high-purity He gas flow at
250 ◦C for 1 h. The adsorption of NH3 was performed in an NH3–He
(9.93% NH3) mixture for 0.5 h, and then the remaining or weakly
adsorbed NH3 was purged by high-purity He. TPD was performed
in the He flow by raising the temperature to 800 ◦C at a rate of
10 ◦C/min. The desorbed NH3 was detected by the mass spectrom-
eter.
the raw material (0.5 wt%) continuously. From the trend of XH
,
O
SPO, and YPO shown in Fig. 3, it can be seen that the PO selectivit2y,
H2O2 conversion and yield of PO are monotonous at about 85%, 68%
and 57% in several, and they are distinctly stable along the reaction
time with PGME injection.
2
Since the by-product formation is due largely to strong acid
sites [3], the pH value of feed solution is adjusted to 5.5 for forc-
ing the rate of PO selectivity to be reduced at about 80%, in order
to further investigate the effect on catalyst of PGME. The reaction
status is exhibited in Fig. 4, it is observed that during the reaction
of approximately 80 h, the H2O2 conversion persists at 95% stably
as before and the PO selectivity fluctuates slightly near 80% all the
90
80
X
H2O2
S
PO
YPO
70
60
50
40
3. 2 Results and discussion
3.1. Catalytic performances
In order to prevent the formation of by-products, pH value of the
feed solution in the tests using chemically pure methanol as solvent
was adjusted to about 7 by ammonia solution (0.1 mol/L) [16]. The
level of catalytic reaction is shown in Fig. 2, it is clearly that the TS-1
0
20
40
60
80
Reaction time (h)
Fig. 3. Effect of PGME on catalytic performance for propylene epoxidation.