S. Zhang et al. / Catalysis Communications 71 (2015) 61–64
63
indicating the incomplete hydrolysis of TiO
2
precursor [16].
Table 1
Acetone conversion and product distribution over various catalysts.
a
Fig. 2(d) presented the FT-IR patterns of T500 powder and T500/
−
1
Cor. The broad band at about 3500 cm
1
and the little band at
642 cm and 1625 cm could be attributed to the –OH stretching
vibrations and –OH bending, respectively [17]. As for T500, the broad
Catalyst
Conv. (%)
Selectivity (%)
−
1
−1
Otherj
MIBK
DIBK
MO
IPA
T500/Cord
7.6
14.4
39.7
59.5
69.0
71.0
69.3
68.2
59.0
4.8
0.89
88.2
82.0
75.2
65.3
65.8
65.0
68.2
34.2
92.9
0
6.1
17.0
5.7
5.6
4.1
7.9
11.1
12.0
14.3
59.2
4.5
82.1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
−
1
band at 478.95 cm reflected the Ti–O–Ti bond vibration [18]. When
it was coated on cordierite, the characterization band of Ti–O–Ti net-
b
c
T500/Cor&Pd/Cor
T500/Cor&Pd/Cor
T500/Cor&Pd/Cor
T500/Cor&Pd/Cor
12.4
20.7
26.8
23.1
23.0
17.5
6.6
d
e
work [19] was also observed indicating the presence of TiO
2
coating.
−
1
−1
Besides the two strong peaks at 1181 m and 956 cm were due to
the Si–O–Si bending and stretching, respectively, which originated
from the cordierite substrate [20].
T500/Cor&Pd/Corf
T500/Cor&Pd/Corg
h
T500/Cor&Pd/Cor
T500/Cor&Pd/Cor
0.7% Pd/T500/Cor
It is difficult to evaluate the acid/base property of T500/Cor, since the
i
surface coating is extremely thin and the content of TiO
dierite is determined to be 2.33 wt.% by XRF (X Ray Fluorescence) meth-
od. The NH -TPD and CO -TPD analysis results for T500 powder were
shown in Fig. 3. As shown in Fig. 3(a) of NH -TPD, the peaks at 570 °C
and 640 °C could be attributed to the moderately strong acid sites
21], and the overall amount of acid sites on T500 was calculated to be
6 μmol/g. As for CO -TPD profile in Fig. 3(b), two peaks were observed
2
coated on cor-
2.6
0
a
2 2
Reaction conditions: 1.5 g TX/Cor & 1.5 g Pd/Cor, 15 ml/min H flow, 120 ml/min N
−
1
3
2
flow, GHSV = 2700 h
.
b
Reaction temperature = 100 °C.
Reaction temperature = 150 °C.
Reaction temperature = 200 °C.
Reaction temperature = 250 °C.
Reaction temperature = 275 °C.
Reaction temperature = 290 °C.
Reaction temperature = 300 °C.
Reaction temperature = 350 °C.
Others mainly C9+ condensation products and mesitylene.
3
c
d
e
f
[
7
2
g
h
i
at 142 °C and 650 °C, respectively. The weak and moderately strong base
amounts were evaluated to be 204 μmol/g and 34 μmol/g, respectively.
It is indicated that TiO powder is dominated by weak base sites. Mean-
2
while, moderate amount of strong acid sites and strong base sites pre-
sents on T500.
j
3
.2. Catalyst tests
that the selectivity of the IPA was related to the Pd particle size. The
larger the size the Pd particles are, the lesser IPA will be produced.
This is probably related to the absorption model of C_O and C_C
on Pd particles with different sizes [23]. As revealed by TEM result of
Pd/Cor in Fig. S3, the Pd particle size ranged about 10–20 nm. In addi-
tion, it was found that the activity of T500/Cor&Pd/Cor was almost ten
The representative gas-phase reaction results were shown in
Table 1. When only T500/Cor was used as catalyst, the acetone conver-
sion was 7.6% and mainly MO was produced. Some data suggested that
the condensation/dehydration of acetone to MO was limited to about
2
0% conversion at 140 °C [22]. The value of acetone conversion is even
times higher than that of 0.7% Pd/TiO
same Pd loading. The bad performance of the latter could be attributed
to the change of acid/base property of TiO caused by Pd loading.
2
/Cor, although they had the
higher than that catalyzed by MgO [10]. This means that T500/Cor has
the ability to catalyze the first two steps, namely condensation and de-
hydration, due to the dominant base sites. We refer that the presence of
large amount of base sites is greatly beneficial to the first step of acetone
condensation, which is commonly regarded as the rate-controlling step
in the overall one-pot process from acetone to MIBK. When Pd/Cor was
mixed with T500/Cor, a much higher acetone conversion of 59.5% and
MIBK selectivity of 75.2% were observed. This confirms that MO hydro-
genation was readily catalyzed by Pd/Cor, which disturbs the equilibri-
um limitation and greatly enhances the acetone conversion. Except for
MIBK, the byproducts included DIBK and C9+. No intermediate prod-
uct of MO was observed, indicating that the Pd/Cor amount is suffi-
cient. In addition, it was noteworthy that no IPA was presented in
all catalytic tests, which is the product of acetone hydrogenation. Re-
2
A series of runs at different reaction temperatures was carried out
using T500/Cor&Pd/Cor bifunctional catalyst as shown in Table 1. The
reaction temperature presented a significant effect on acetone conver-
sion, increasing from 14.4% at 100 °C to 65% at 200 °C. With the increas-
ing reaction temperature below 200 °C, a significant increase of DIBK
selectivity was observed, whereas MIBK selectivity decreased mono-
tonically. This may be explained by the enhanced formation of the
over-condensation byproduct at higher reaction temperature [22]. The
increase of acetone conversion suggests that the kinetics of acetone con-
densation also plays an important role. The acetone conversion and
MIBK selectivity had slight fluctuations between 200 °C and 300 °C.
When temperature was higher than 300 °C, the remarkable decline of
MIBK selectivity was observed.
2 2 2
search on Pd/SiO –ZrO /HZSM-5 [4] and Pd-MgO/SiO [10] indicates
2 3
Fig. 3. CO -TPD and NH -TPD of T500 powder.