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X. Wu et al. / Applied Catalysis A: General 473 (2014) 13–20
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Table 1
The composition, BET surface area and DMC yields of ZAO catalysts.
Entry
Catalyst
Zn/Al molar ratio
Surface area (m2/g)
MC yield (%)
DMC yield (%)
Total yield (%)
1
2
3
4
5
6
7
8
None
ZnO
–
–
0.48
–
0.21
0.37
2.24
2.41
2.47
2.47
2.18
–
–
–
49.5
–
142.0
76.4
26.7
34.0
55.7
27.7
9.8
84.8
60.9
82.3
65.2
85.8
85.5
60.8
62.7
66.6
55.1
40.2
69.9
6.5
29.5
7.7
22.1
9.3
91.3
90.4
90.0
87.3
95.1
95.4
92.2
92.8
98.0
91.6
76.2
89.0
ZnAl2O4
ZnO + ZnAl2O4
ZAO-0.25(1073K)
ZAO-0.5(1073K)
ZAO-3(1073K)
ZAO-4(625K)
a
9.9
31.4
30.1
31.4
36.5
36.0
19.1
9
ZAO-4(873K)
10
11
12
ZAO-4(1073K)
ZAO-4(1273K)
ZAO-6(1073K)
–
Reaction condition: Urea, 0.1 mol; methanol, 64 g; catalyst amount, 0.5 g; reaction time, 10 h; reaction temp. 453 K.
a
Metal oxides were physically mixed and Zn/Al molar ratio was 2.47.
samples with different Zn/Al molar ratios were further evaluated
(Fig. 1b). The XRD patterns were quite different with respect to
changed Zn/Al molar ratios. The catalyst consisted of only ZnAl2O4
spinel as the molar ratio of Zn/Al was less than 0.5, or ZnO and
ZnAl2O4 spinel yielded together. The results revealed that low Zn
content and high calcination temperature benefited the formation
of ZnAl2O4 spinel, which was favorable to stabilize the ZAO cata-
lysts.
Table 1 displayed the tuned Zn/Al molar ratios and BET surface
ratio of the catalysts were less than that of feed ratio, which was
caused by the greater affinity of Al3+ ions to the precipitation than
Zn2+ ions. As a result, Zn2+ was partially retained in the mother solu-
tions during preparation of precursors [17]. The BET surface area of
catalyst was raised from 34.0 m2/g of ZAO-4(623K) to 55.7 m2/g of
ZAO-4(873K) along with an increase from 623 to 873 K. It was owed
to the formation of loose catalyst structure, which was attributed
to the destruction of precursor lamellar shape. However, BET sur-
face area significantly decreased to 9.8 m2/g when the temperature
reached to 1273 K due to the aggregation of crystals. Remarkably,
BET surface area of catalyst was up to 142 m2/g with a Zn/Al molar
ratio of 0.21 since Al effectively suppressed the coalescence of ZnO
(Entries 5–7). After removal of ZnO from ZAO-3(1073K), ZnAl2O4
showed a higher BET surface area of 49.5 m2/g, exposing more
acidic–basic active sites.
As shown in Table 2, acidic catalysts such as ␥-ZrP and Zn-␥-ZrP
showed poor catalytic activity (Entries 2 and 3). Analogously, MgO
and CaO as basic catalysts (Entries 4 and 5) also exhibited lower
DMC yield. However, ZnO and ZnCl2 (Entries 6 and 7) possessing
both acidic and basic activity sites have highly enhanced perfor-
mance up to 29.5% and 21.7%, implying a synergy effect between
acidic and basic activity sites. Unfortunately, the homogeneous dis-
solution of ZnO and ZnCl2 in reaction solution brought about serious
problem for the separation of DMC and the recycling of catalysts.
Regarding the crucial demand for stable and efficient catalysts,
ZAO-4(1073K) was prepared and addressed better DMC yield up to
36.5% far more than single acidic or basic catalysts. Meaningfully,
ZAO-4(1073K) as a heterogeneous catalyst also displayed recycling
stability that was feasible to be applied in fixed-bed reactor.
3.3. Catalyst stability
The regeneration of catalyst was performed by washing the
used ZAO-4(1073K) sample with methanol, followed by calcina-
tion at 1073 K for 2 h. The catalytic activity of ZAO-4(1073K) could
be recovered despite a little decrease of DMC yield, which was
caused by the little leaching of ZnO during the reaction (Fig. 2).
It was noticed from XRD patterns (Fig. 3) that ZnO diffraction peaks
decreased after several repeated ZAO-4(1073K) catalyst, while
ZnAl2O4 spinel diffraction peaks remained unchanged. Herein,
ZAO-4(1073K) catalyst showed better catalytic stability and DMC
yield referred to the pure ZnO sample.
3.2. Catalytic performance
3.4. Synergistic effect between ZnO and ZnAl2O4 spinel
determined by TPD
The prepared ZAO catalysts were utilized to catalyze DMC syn-
thesis (Table 1). Heterogeneous ZAO catalysts exhibited enhanced
catalytic performance and ZAO-4(1073K) sample presented the
best DMC yield (36.5%, Entry 10). Noticeably, catalysts with Zn/Al
molar ratio less than 0.5 displayed poor activity toward DMC
synthesis (Entries 5 and 6), owing to the formation of stable
ZnAl2O4, consistent with XRD characterization. The catalytic activ-
ity remarkably increased when the Zn/Al molar ratio was more than
0.5, ascribing to the synergistic effect between ZnO and ZnAl2O4 in
ZAO catalysts. Furthermore, the DMC yield reached to the highest
level at calcination temperature of 1073 K. According to previous
reports, ZnO as active component generally enhanced the pro-
duction of DMC. However, pure ZnO catalyst exhibited a well
performance (∼29.5%, Entry 2) owing to its homogeneous dissolu-
tion. Obviously, ZAO-6(1073K) catalyst presented lower DMC yield
than that of ZAO-4(1073K) (Entries 10 and 12). The phenomenon
also illuminated that the capability of ZnAl2O4 spinel for stabiliz-
ing ZnO led to an optional ratio of Zn/Al mixed oxide catalysts,
attributing to the synergistic effect between ZnO and ZnAl2O4.
In order to further identify the catalytic activity of prepared
catalysts, a series of comparative experiments were conducted.
The surface acidity of ZAO samples was investigated through
NH3-TPD analysis (Fig. 4(A)). Firstly, the high temperature with ZnO
Table 2
DMC synthesis over various catalysts.
Entry Catalyst
Catalyst calcination Regeneration DMC yield (%)
temp. (K)
1
2
3
4
5
6
7
8
None
␥-ZrPa
Zn-␥-ZrPb
MgO
CaO
ZnO
–
–
–
1073
1073
1073
–
–
Yes
Yes
Partially
Partially
No
No
Yes
6.5
4.6
5.2
5.6
15.8
29.5
21.7
36.5
ZnCl2
ZAO-4(1073K) 1073
Reaction condition: Urea, 0.1 mol; methanol, 64 g; reaction time, 10 h; reaction temp.
453 K; catalyst amount, 0.5 g.
a
The molecular formula of ␥-ZrP: Zr(PO4)(H2PO4)·2H2O. Ref. [18].
b
Ref. [19].