Synthesis of dimethyl carbonate from methanol and urea over zinc-strontia mixed oxide catalysts

Article abstract of DOI:10.1016/j.catcom.2019.01.005

A series of ZnO-SrO mixed oxide catalysts were prepared by co-precipitation with different compositions and tested for dimethyl carbonate synthesis from urea and methanol. The catalysts were characterized by BET surface area, XRD, NH₃-TPD and CO₂-TPD. The catalyst with 1:1 mol ratio exhibited high DMC yield of 35%. The uniform distributions of moderate to strong basic sites along with considerable number of acidic sites are accountable for high activity. Different reaction parameters were also screened and best possible conditions were established. The catalyst was easily recovered and reused with consistent activity.

Full text of DOI:10.1016/j.catcom.2019.01.005

Accepted Manuscript
Synthesis of dimethyl carbonate from methanol and urea over
zinc-strontia mixed oxide catalysts
D. Dhana Lakshmi, B. Srinivasa Rao, N. Lingaiah
PII:
S1566-7367(19)30005-6
DOI:
https://doi.org/10.1016/j.catcom.2019.01.005
CATCOM 5589
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
24 October 2018
3 January 2019
7 January 2019
Please cite this article as: D. Dhana Lakshmi, B. Srinivasa Rao, N. Lingaiah , Synthesis
of dimethyl carbonate from methanol and urea over zinc-strontia mixed oxide catalysts.
Catcom (2018), https://doi.org/10.1016/j.catcom.2019.01.005
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ACCEPTED MANUSCRIPT
Synthesis of dimethyl carbonate from methanol and urea over zinc-strontia mixed oxide
catalysts‡
a,b
a,b
a,b
D. Dhana Lakshmi , B. Srinivasa Rao , N. Lingaiah
*
a
Catalysis and Fine chemicals Division,
CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, India
CSIR-Academy of Scientific and Innovative Research (CSIR-AcSIR), New Delhi, India.
b
Abstract
A series of ZnO-SrO mixed oxide catalysts were prepared by co-precipitation with
different compositions and tested for dimethyl carbonate synthesis from urea and methanol.
The catalysts were characterized by BET surface area, XRD, NH3-TPD and CO2-TPD. The
catalyst with 1:1 mole ratio exhibited high DMC yield of 35%. The uniform distributions of
moderate to strong basic sites along with considerable number of acidic sites are accountable
for high activity. Different reaction parameters were also screened and best possible
conditions were established. The catalyst was easily recovered and reused with consistent
activity.
Keywords: Dimethyl carbonate; Urea; Methanolysis; Zinc oxide; Strontium oxide.
*
Corresponding author. Tel.: +91 40 27191722; fax: +91 40 27160921
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah)
‡
Communication Number- IICT/Pubs./2018/367

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1
. Introduction
Dimethyl carbonate (DMC) an environmentally affable building block in chemical
industry [1]. DMC have been received much attention in recent years [2], as this molecule
contains active functional groups, negligible toxicity and good biodegradability. It can be
utilized as a methylating and carbonylating agent to replace toxic substances such as methyl
halides, phosgene and dimethyl sulphate [3]. Since it has elevated oxygen content (53%) and
high octane value than methyl ter-butyl ether or ter-amyl methyl ether, it can be used as good
additive of gasoline. Additionally, DMC can be used as an electrolyte in secondary lithium
ion battery owing to its high dielectric constant [4-5]. Conventionally DMC is obtained
majorly from phosgenation of methanol, carbonylation of methanol and transesterification of
cyclic carbonate with methanol. All these processes have their own environmental
disadvantages. Readily available carbon dioxide can be used as a raw material for DMC
synthesis. However, there are some restrictions to use CO2 as its activation is difficult [6-8].
In order to stay away from utilizing poisonous, corrosive and expensive raw materials,
the production of DMC from alcoholysis of urea is considered as indirect synthesis of DMC
from CO2 and methanol since the released NH3 can be recycled to produce urea by reaction
with CO2 [9-10]. This process has advantageous as it uses low cost urea and methanol. The
DMC synthesis proceeds in two intermediate steps. Isocyanic acid forms as an intermediate
molecule in the first step, which reacts with methanol to produce methyl carbamate. Then
methyl carbamate and methanol yields DMC. It may further react with methyl carbamate
(MC) to form N-methyl methylcarbamate (NMMC).
Different catalysts have been tested for the synthesis of DMC by methanolysis of urea.
Yuhan Sun et. al. reported DMC synthesis over different metal oxides and among them ZnO
showed superior yield [11]. Later many catalysts such as bases, organic tin, mixed metal
oxides have been employed [12-15]. Among these, ZnO and its salts are highly active toward

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DMC synthesis owing to its weak acidity/basicity, which is effectively helps for urea
activation [16]. The key disadvantage of ZnO based catalysts related to its solubility in
methanol [17]. The catalysts containing acidic and basic sites together are suitable for the
synthesis of DMC. In this respect mixed oxide which potentially acidic oxide like ZnO and
base oxide like SrO may suitable for urea methylation to DMC.
In this work, ZnO-SrO mixed oxide catalysts were prepared by varying the molar ratios
of Zn to Sr and calcination temperatures. These catalysts were used for urea methylation to
DMC. The activity results of the catalysts were supported with their characterization results.
The catalysts were characterized by XRD, BET surface area, and TPD of NH3, CO2.
2
. Experimental
2
. 1. Catalyst preparation
The ZnO-SrO mixed oxide catalysts were synthesized by co-precipitation method with
different molar ratios ranging from 3:1, 2:1, 1:1, 1:2, and 1:3. The samples were named as ZS
1, 21, 11, 12, 13, where the alphabets represented the metal ions and numbers represent the
3
mole ratios. Details regarding catalysts synthesis procedure, their characterization and
activity studies are reported in the supplementary data (Appendix A).
3
. Results and discussion
3
.1. Catalyst characterizations
BET surface areas of the catalysts are presented in Table 1. Surface areas of the catalysts
2
varied from 3 to 11 m /g depending on the ZnO and SrO molar ratio. The margin of error for
these low surface area materials is ±1. The catalyst surface area is increased with raise in
2
ZnO content. The catalyst with high SrO content (ZS13) showed low surface area (3 m /g)
due to the aggregation of crystals.

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Table 1. Textural and surface properties of the catalysts.
Entry Catalyst
SBET
2
Total basicity
Total acidity
(
m /g)
2
2
(mmol/g)
(mmol/m ) (mmol/g) (mmol/m )
1
2
3
4
5
ZS31
ZS21
ZS11
ZS12
ZS13
12.3
10.3
8.7
0.195
0.176
0.322
0.318
0.340
0.015
0.017
0.036
0.081
0.110
0.366
0.359
0.326
0.136
0.108
0.029
0.034
0.037
0.034
0.035
3.9
3.0
Fig.1 shows the XRD patterns of the ZnO-SrO catalysts. ZnO exhibited peaks at 31.8°, 34.5°,
6.3°, 47.6°, 56.6°, 62.9°, 67.9° and 69.2° which are characteristic peaks of hexagonal
wurtzite structure (JCPDS No. 36-1451). The diffraction lines at 2θ of 25.3°, 26°, 29.7°,
3
3
3
2.8°, 44.3°, 45.9°, and 48° were related to SrCO3 (JCPDS 52-1526). The peaks at 31.6°,
7.6°, and 52.1° are corresponding to the diffractograms of SrO. The catalyst with ZnO and
SrO with 1:1 molar ratio showed the diffractograms related to ZnO and SrO along with
SrCO3 phase. When SrO loading was increased in the catalysts, the intensities of
diffractograms related to SrCO3 also increased. The formation of SrCO3 may be due the
absorption of CO2 by SrO as the basic sites of SrO attract the acidic CO2. One can not
eliminate the formation of it during the preparation of the catalysts from K2CO3, which is
used as a precipitation agent.

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Fig.1. Powder XRD patterns of the catalysts. SrCO3 (•), SrO (♥), ZnO (ꞷ ).
Basicity of the catalysts was measured by TPD of CO2 and the profiles are revealed in
Fig. 2. All the synthesized catalysts showed an intense band around 600 °C, which indicates
the presence of strong basic sites in the catalysts. Bulk SrO exhibited strong basic sites along
with moderate sites, while pure ZnO did not show any considerable amount of basicity. The
basicity of the catalyst varied with increase in SrO content. The high SrO containing catalyst
(ZS13) showed high temperature desorption peak. This might be due to the presence of
SrCO3. The desorption peak relates strong basic sites shifted to low temperature with increase
in the content of ZnO. This is mainly due to the acidic nature of ZnO. The ZS11 catalyst
showed the presence of both moderate and strong basic sites compared other catalysts. The
coprecipitation of SrO with ZnO enhanced the strength of basicity. It might be due to the
enhanced dispersion of SrO by the interaction with ZnO. The basicity values are calculated
and shown in Table 2. The alkalinity of the catalysts increased with increase in Sr content and
decreased with increase in Zn content. The decrease in alkalinity for the catalyst with high Zn

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content is due the acidic nature of ZnO. The alkalinity of ZS11 is slightly more than ZS12.
This is may be due to well dispersion of SrO on ZnO at equimolar composition. In order to
know if any desorption of CO2 associated with SrCO3 as the catalysts showed the presence of
SrCO3 diffraction peaks in XRD, a TPD of CO2 experiment is carried without adsorption of
CO2. The TPD profile without CO2 adsorption is also shown in Fig. 2. The ZS11catalyst
without CO2 adsorption showed two peaks in 600-700 °C range, indicating that CO2 was
desorbed from strontium carbonate present in the catalyst. Desorption peaks in the catalysts
except the catalyst with high SrO (ZS13) are below 600 °C, which are related to basic sites
associated to SrO, but not SrCO3. The strong basic sites observed for the catalyst with high
SrO (ZS13) mainly due to the presence of SrCO3 phase.
ZS11 with out CO2
ZnO
ZS31
ZS21
ZS11
ZS12
ZS13
SrO
2
00
400
600
800
Temperature (C)
Fig.2. CO2-TPD profiles of the catalysts

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The acidity of the catalysts was measured by ammonia TPD and the patterns are shown
in Fig. S3. The acidity values of the catalysts are placed in Table 1. Fig. S3 indicates that the
presence of more amount of ZnO in catalysts led to the generation of strong acidic sites.
3
.2. Activity results
Alcoholysis of urea with methanol for the synthesis of DMC was carried over ZS
catalysts and the results are summarized in Table 2. SrO a base catalyst exhibited lower DMC
yield (15 %), while ZnO possessing predominantly acidic sites showed 21 % within 6 h of
reaction time. The catalysts ZS11, ZS12 and ZS13 showed 35, 24 and 19 % of DMC yield
respectively. High ZnO containing ZS21 and ZS31 catalysts exhibited about 18 and 16% of
DMC yields respectively. Aforementioned results suggesting that the catalyst with high Sr
content led to more NMMC yield and the one with high Zn content led to more MC yield.
The difference between the activities of various ZS catalysts was explained based on
their acid and base properties. ZnO which contain both acidic and basic sites showed better
activity than SrO which possess mainly basic sites. The ZnO-SrO mixed oxide catalysts
showed high activity than single oxides. This indicates that alcoholysis reaction is favourable
with the catalysts having both acid and base sites. ZS mixed oxide catalyst with high SrO
content has strong basicity and showed low DMC yield. This is due to the easy
decomposition of DMC that formed on strong basic sites and the decomposed DMC might
react with MC to yield NMMC. High ZnO containing catalysts showed comparatively low
DMC yield, as these catalysts have more acidity than basicity and these sites were not
sufficient to convert initially formed MC to DMC. The catalyst with equi molar (ZS11)
content of ZnO and SrO showed relatively high amount of DMC than other catalysts as it has
both basic and acidic sites in relatively equal ratios. Therefore, the presences of both acidic
and basic sites are responsible for the high catalytic activity of ZS11 catalyst towards DMC.

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The basicity per unit surface area values of ZS catalysts calcined at 500 °C was increased
with decrease in Zn/Sr mole ratio and there is no such variation was observed in case of
acidity values. The high activity of ZS-11 catalyst is mainly due to the presence of acid/base
sites per unit surface area.
Table 2. Activity of ZS catalysts for the synthesis of DMC
Catalyst
MC
DMC
NMMC
Total basicity Total acidity
Yield (%)
Yield (%)
Yield (%)
(mmol/g)
0.195
0.176
0.322
0.318
0.340
(mmol/g)
0.366
0.359
0.326
0.136
0.108
ZS31
ZS21
ZS11
ZS12
ZS13
72
63
42
45
44
16
18
35
24
19
12
19
23
31
37
Reaction conditions: methanol/urea mole ratio: 15, catalyst wt%: 2.6, time: 6h, temperature:
70 °C.
The present catalyst activity was compared with other reported catalysts and the details
are shown in Table S1. M. Wang et al. studied DMC synthesis over ZnO with 29% DMC
11]. However, its reusability is very poor. In another study Zn(NO3)2-SiO2 catalyst showed
1
[
only 4.5% yield [18]. F. Xiao et. al. used ZnO-Al2O3 catalyst and they got about 36% DMC
at relatively high methanol to urea ratio (20) in 10 h [19]. ZnO-CaO catalysts was showed
maximum amount of 41% DMC yield with in 10 h of reaction at a high methanol to urea
mole ratio of 20 [20]. The present reported catalyst is comparatively better than the other
catalysts in the way of low methanol to urea mole ratio and high DMC yield in less reaction
time.

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3
.3. Effect of calcination temperature
The catalyst activity is not only dependent on the molar ratio of the constituent
elements but also on their surface, structural properties. These properties also depend on the
calcination temperature of the catalysts. As the ZS11 catalyst showed high activity, this
catalyst was subjected to different calcination temperatures varying from 400-800 °C. The
effect of calcination temperature was studied and the results are mentioned in Table 3. The
catalysts calcined at 400-600 °C showed more DMC yield compared to the catalysts calcined
at 700 and 800 °C. Among these the catalyst calcined at 500 °C was shown high DMC yield.
The high temperature calcined catalysts showed less activity towards DMC.
Table 3. Activity of different calcined ZS11 catalyst.
Catalyst
MC
DMC
NMMC
Yield (%)
Yield (%)
Yield (%)
ZS11-400
ZS11-500
ZS11-600
ZS11-700
ZS11-800
55
42
38
80
85
28
35
29
12
9
17
23
33
8
7
Reaction conditions: methanol/urea mole ratio: 15, catalyst wt%: 2.6, time: 6h, temperature:
70 °C.
With the intention to know the alteration in activity of different calcined catalysts, these
1
catalysts were characterized further by XRD and CO2-TPD techniques. The XRD patterns of
different calcined temperatures (Fig. S1) indicates the decomposition of SrCO3 to SrO at high
calcination temperatures. CO2-TPD results as mentioned in Fig. S2 suggested that the basicity
of the catalysts was increased with increase in calcination temperature from 400 to 600 °C
and further increase in calcination temperature the strength of basicity was decreased sharply.

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The decrease in basicity might be due to the absence of the SrCO3 phase as it was stable up to
6
00 °C and decomposed to SrO when calcined at above 600 °C. The catalysts calcined at 700
and 800 °C showed less DMC yields as the basicity associated with SrCO3 was absent. As a
result of this, the activity towards DMC was decreased in case of ZS-11 catalyst calcined at
high temperature. These results suggest the presence of SrCO3 contribute to increase the
DMC yield by facilitating the decomposition of urea on the basic sites.
The presence of basic sites along with acidic sites enhanced the catalytic activity of ZS-
1
1 catalyst calcined at 500 °C.
3
.4 Optimization of reaction conditions
The impact of reaction temperature on DMC yield was shown in Table S2. The DMC
yield increased as the temperature increased in the range of 150-170 °C and then decreased
beyond 170 °C. Nevertheless, it was known that urea begins to decompose into NH3 and
isocyanic acid (HNCO) at a temperature of 135 °C under ambient pressure. At an equivalent
time, the speedy decomposition of urea at high temperature can lead to the reaction of
methanol with isocyanic acid to generate intermediate MC and the formed DMC reacts with
MC to produce NMMC. Thus, the rapid decline in the DMC yield was observed above 190
°C and this could be attributed to the formation of NMMC.
The effect of methanol/urea molar ratio on DMC synthesis is presented in Table S3.
The results demonstrate that the DMC yield increased when methanol/urea molar ratio is in
the range of 10 to 15. Further increase in the ratio up to 20 a decrease in activity was noticed.
When the methanol/urea molar ratio was 15, the DMC yield reached a maximum of 35%.
While the methanol/urea molar ratio was lower than 15, the urea concentration was high, the
side reaction N-methylation of urea with DMC would occur easily resulting a lower DMC
yield. Theoretically, higher methanol/urea molar ratio could promote the reaction equilibrium

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towards DMC. Nevertheless, too high methanol/urea molar ratio would lead to the reduction
of urea concentration. Consequently, some active sites of the catalyst for activating urea
would be occupied by a larger amount of methanol. Thus, the reaction rate might decrease,
resulting in a lower DMC yield. In the present case, from a practical point of view, the
suitable methanol/urea molar ratio is 15.
The influence of catalyst quantity was investigated over ZS11 catalyst calcined at
5
00 °C. As shown in Fig. 3, the amount of DMC produced was elevated from 14 % to 35 %
when catalyst amount varied from 0.6 to 3.9 wt%. The DMC yield was declined with further
increment in catalyst amount, owning to the decomposition of DMC on available basic sites.
The DMC yield reached maximum (35 %) at catalyst content of 2.6 wt%.
Fig.3. Effect of catalyst loading on reaction, Reaction conditions: methanol/urea mole ratio:
1
5, catalyst loading wt%: 0.6-3.9, time: 6h, temperature: 170 °C.
The was also studied Fig. 4 shows the profile of reaction time on DMC synthesis. The
maximum DMC yield of 35% was achieved at 6 h of reaction time. Further elevation in
reaction time led to decrease in DMC yield. When the reaction time exceeded 6 h, the yield
was sharply decreased due to subsequent reaction of DMC with MC to produce NMMC as by

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product. The optimum operational conditions based on above studies were obtained for ZS
catalysts are; reaction temperature 170 °C, reaction time 6 h, CH3OH/urea molar ratio 15 and
the catalyst content of 2.6 wt%.
Fig.4. Influence of time on urea methanolysis, Reaction conditions: methanol/urea mole ratio:
1
5, catalyst wt%: 2.6, time: 2-8h, temperature: 170 °C.
.4. Stability of catalyst
3
The stability and reusability of the catalyst was also studied. The catalyst was removed
by centrifugation after reaction. After the separation, the catalyst was oven dried for 2 h and
reused for the next run under the same conditions. The recycling was carried for four times
and the results are shown in Fig. S4. The results indicate that the activity of the catalyst was
almost similar even after fourth run. The used catalyst was characterized by XRD and the
profiles are presented in Fig. S5. It was found that there is no much changes their structural
features in used catalyst compared to the fresh catalyst. These results indicating that the
catalyst was stable and active for repeated use.

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4
. Conclusions
The ZnO-SrO catalysts with both the acid and base properties were prepared by
coprecipitation method. These are active catalysts for urea methanolysis to produce DMC.
The catalyst with ZnO and SrO equimolar ratio and calcined at 500 °C showed high yield
towards DMC. The catalyst with both acidic and basic sites is necessary to get high DMC
yield. The surface characteristics of the catalysts are depends on the molar ratio of ZnO/SrO
and treatment temperature. The presence of well dispersed ZnO and SrO/SrCO3 phases were
favours the urea alcoholysis to DMC. The catalyst is easy to separate and reusable with no
substantial loss in activity.
Acknowledgement
D. Dhana Lakshmi thanks to University Grants Commission (UGC), New Delhi, India for the
financial support in the form of junior research fellowship.
Appendix A. Supplementary data
Supplementary data to this article can be found online
5
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[
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The research highlights:


Zn and Sr mixed oxide are active catalysts for synthesis of DMC with high yield
The activity of the catalysts depends on the mole ratio and calcinations
temperature.

The presence of both acidity and basic sites directing the DMC yield.