Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bangalore-562164, Karnataka State, India
Email-address: manjunathanp@poornaprajna.org, sanjeevpm@poornaprajna.org
*Corresponding author: shanbhag@poornaprajna.org
Tel: +91(080)27408552; Fax: +91 23619034.
1.
Introduction
There has been an increasing interest in recent years in developing novel solid catalysts for the synthesis of fine and specialty chemicals. Among them, metal oxides including mixed oxides and composites are widely used in industry as well as in academia which constitute the largest family of heterogeneous catalysts for the selective organic transformations. Bimetal oxide composite is the combination of two metal cations resulting in mixed phases of individual oxides and bimetallic oxide. Due to the presence of M 2+ O 2- ion pairs in mixed oxides, it is found to exhibit acid-base and redox properties. Composite metal oxides exhibit better catalytic activity than the individual oxides due to increase of surface active sites with increase of surface area.
2.
Materials and methods
2.1 Catalyst preparation: Three different methods were adopted for the synthesis of Zn-Sn composite viz., co-precipitation, solid state and evaporation.
2.2 Catalyst characterization : The catalysts physico-chemical properties were derived from XRD, nitrogen sorption, NH
3
-TPD,
CO
2
-TPD and SEM techniques.
3.
Significant Results and Discussion
The XRD pattern of synthesized catalysts showed mixed phase of ZnO, SnO
2
and Zn
2
SnO
4
crystalline phases. The
XRD patterns of the Zn-rich Zn-Sn composites prepared from co-precipitation method clearly showed the increase of wurtzite structured ZnO with increasing of Zn/Sn molar ratio in the range of 1 to 3. Moreover, the formation of ZnO has not affected the presence of SnO
2
and Zn
2
SnO
4
. The result clearly indicates that the catalysts exhibit a complex phase with the combination of
Zn
2
SnO
4
spinel, ZnO and SnO
2
.
SEM image of Zn-Sn composite synthesized via different methods showed different morphology with particle size ranging from 0.5 to 30
Zn
2
Sn-CoPre possessed cuboid shape with the particle size ranging from 0.8 to 2.5 µm and the average particle size was found to be 1.6 µm.
The catalytic activity of Zn-Sn composite was evaluated for the glycerol carbonate synthesis via carbonylation of glycerol using urea shown in Table-1.

Table-1 : Catalytic activities of different catalysts for glycerol carbonylation reaction a
Catalyst Acidity
(µmol
NH
3
/g) a
Basicity
(µmol
CO
2
/g) a
Glycerol conversion
(wt%)
Glycerol carbonate selectivity
(wt%)
Glycerol carbonate yield
(wt%)
Blank
ZnO
-
60
-
12
32
66.4
95.8
98.1
30.6
65.1
SnO2
Zn
1
Sn-CoPre-600
Zn
1
Sn-SS-600
Zn
1
Sn-Evp-600
30
235
216
181
4
52
38
35
39.6
88
79.5
76.1
99
99.2
99.4
99.2
39.2
87.3
79
75.5
Zn
2
Sn-CoPre-600
Zn
3
Sn-CoPre-600
268
256
61
48
96
90.7
99.6
98.7
95.6
89.5
Zn
2
Sn-Mechem-600 223
Zn
2
Sn-SS-600 210
45
48
84
80
99.2
99
83.3
79.2 a Reaction condition: Glycerol = 2 g, Urea = 1.31 g, catalyst = 0.33g, reaction temperature = 155 °C, under nitrogen bubbling, b From TPD analysis
4. Conclusions
Zinc-Tin composite catalysts were synthesized by three different methods and evaluated for the synthesis of glycerol carbonate. These catalysts showed high activity towards the selective synthesis of glycerol carbonate. Zn-Sn composites prepared from co-precipitation showed excellent catalytic performance compared to other methods. The superior catalytic activity of ZnSn-CoPre catalyst is mainly attributed to the presence of high amount of acidic and basic sites. The better catalytic activity was obtained over the sample of Zn/Sn mole ratio of 2 calcined at 600 °C. A good correlation between the catalytic activity and the amount of acidic and basic sites were observed. Thus, the bi-functional Zn
2
Sn-CoPre-600 composite catalyst is found to be more active than other catalysts due to its high acid-base property.
References
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