40
T. Zhang et al. / Catalysis Communications 66 (2015) 38–41
Table 1
Table 2
Texture structure and catalytic performance over different catalysts.
2
The CO diffraction peak intensity of the ZnO–MgO catalysts.
Zn/Mg(a)
S
BET (m /g)
2
PC yield(%)
Catalyst
Amount of basic site (CO
stripping peak area/10−9
)
Entry
Catalyst
2
1
2
3
4
5
6
7
8
9
ZM4
ZM2
ZM1
ZM0.5
ZM0.25
ZM0.167
ZM0.1
ZnO
12.56
12.47
6.71
3.02
0.75
–
3.49
5.20
6.96
8.74
15.06
10.19
3.83
6.2
58.2
64.7
74.8
77.1
94.8
93.5
80.4
55.2
72.9
65.4
19.3
T
max (b473 K) max (473–773 K)
T
T
max (N773 K)
ZM1
ZM4
ZM0.167
ZnO
MgO
3.21
1.54
0.52
0.18
2.18
0.83
0.41
1.37
0.29
5.06
0.12
0.43
0.27
0.05
0.37
–
∞
0
0.75
MgO
21.0
–
1
1
0
1
ZM0.75m
None
3
3
.2. Catalytic activity
aExperiment Zn/Mg atomic ratio by ICP and m: ZnO/MgO mechanical mixing.
Reaction conditions: 443 K, 0.5 h, 300 mm Hg, PG: urea = 1.5 (the quality of urea 30 g),
and catalyst: 0.6 g.
.2.1. Effect of Zn/Mg ratio
Table 1 showed the catalytic performance for the prepared ZMx cat-
alysts. Entries 1–5 disclosed that the Zn/Mg molar ratio of the catalysts
was higher than that of the feed, which was caused by the greater affin-
ity of Zn ions to the precipitation than Mg ions [24,25]. As a result,
Mg was partially retained in the mother solutions during preparation.
The ZnO and MgO produced PC at the yields of 55.2% and 72.9%, respec-
tively, which were lower than that of the mixed metal oxide. The PC
yield increased from 58.2% for ZM4 to 94.8% for ZM0.25 with the in-
creasing magnesium content. The increased catalytic activity of the bi-
nary oxide system might attribute to the modification of the electronic
properties and porosity, and which could lead to the increase in basicity
and specific surface areas [21]. However, the decrease of catalyst activity
was observed for ZM0.1 (80.4%) because of the catalyst possessing the
lower specific surface area.
2
+
2+
mostly MgO, as shown in Fig. S1(D). However, the presence of gathered
MgO may contribute to the smaller specific surface areas.
Specific surface areas of the different materials were obtained using
the BET methodology, seen in Table 2. The BET surface areas of catalyst
increased from 3.49 m /g for ZM4 to 15.06 m /g for ZM0.25 since Mg
suppressed the coalescence of ZnO (ZM4–0.25) effectively. The increase
in MgO content with a porous structure increased the surface areas of a
catalyst. However, when the Zn/Mg mole ratio decreased to 0.1, the BET
2
+
2
2
2
surface area significantly decreased to 3.83 m /g due to the aggregation
of crystals. As mentioned by Olutoye et al. [21], the MgO clusters signif-
icantly increased the surface area, altered the surface reactivity and led
to the high activity. Generally, these results indicated that the surface
areas of the catalyst by using BET were consistent well with the SEM.
3
.2.2. Correlation between the strong basicity of unit specific surface area
and PC yield
Fig. 3 illustrated the relationship between strong alkaline sites of
unit surface and the PC yield. The amount of the alkaline sites was calcu-
lated by integrating the CO -TPD curves, shown in Table 2. A well
established linear relationship (R = 0.99) was obtained, which demon-
strated that the catalytic activity depended on the base strength and
density of the catalyst. Higher density of the strong basic sites was help-
ful to initiate the formation of isocyanate [14] and lead to better catalytic
rate.
2
The CO -TPD measurements were carried out to determine the total
basicity and base strength distribution of the catalyst, shown in Fig. 2.
The TPD profiles suggested that several alkaline sites were available
on ZnO–MgO catalysts. The desorption peak was found at Tmax ranging
from 298 to 473 K, which was attributed to the interaction of CO with
2
the weak alkaline sites present in the catalyst. While, another desorp-
tion peak found at Tmax varying from 473 to 773 K was due to the inter-
2
2
2
action of CO with the medium alkaline sites of the catalyst. In addition,
the ZnO–MgO exhibited the desorption peak at temperatures N773 K,
which was ascribed to the strong alkaline sites that correspond to isolat-
ed O2 anions located on a particular position on the mixed oxide cata-
lyst surface [22]. With the increase of the content of Mg, the strength of
the weak alkaline sites increased, while the amount of the weak alkaline
sites decreased. The middle strong basic sites also increased. These re-
sults were in agreement with Olutoye's studies [21], in which the pres-
ence of a synergetic effect between Mg and Zn increased the catalytic
basicity [20,23].
−
3.3. Regeneration and reusability of ZnO–MgO catalysts
Reusability is one of the most important features of a heterogeneous
catalyst. In a typical regeneration procedure: the used catalysts were
washed with ethanol for three times, followed by calcination at 873 K
for 5 h. It was found that the ZM0.25 catalyst could be reused for up to
5
times with less changed PC yield in Fig. S2 (Supporting information
S.2).
4. Conclusion
The mixed metal oxide of Zn–Mg prepared by the urea–precipitation
method was used as a solid catalyst for the synthesis of PC by using urea
and PG. ZnO–MgO binary system was superior to the individual bulk
oxide of ZnO and MgO. ZM0.25 had high specific surface area and stron-
ger alkaline density, which led to the highest PC yield. The PC yield was
found to be related to the strong alkaline sites of unit and specific sur-
face areas. Moreover, the ZM0.25 catalyst could be reused up to 5
times upon the removal of residual organics.
Acknowledgments
This work is financially supported by Qinghai Province High and
Fig. 3. The relationship between strong basic sites of unit surface and PC yield.
New Technology Research and Development Project of China (2014-