Indian Journal of Chemistry
Vol. 49A, September 2010, pp. 1182-1188
Transesterification of propylene carbonate with methanol
using Mg-Al-CO3 hydrotalcite as solid base catalyst
C Murugan & H C Bajaj*
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute
Council of Scientific and Industrial Research (CSIR), G B Marg, Bhavnagar 364 021, Gujarat, India
Email: [email protected]
Received 18 June 2010; revised and accepted 17 August 2010
The catalytic activity of synthetic hydrotalcites of varying Mg/Al molar ratio from 2 to 5 has been investigated for the
transesterification of propylene carbonate with methanol. The hydrotalcite with Mg/Al molar ratio of 5 at different
calcination temperature has been characterized using FT-IR, P-XRD and surface area analyses. Under optimized conditions,
the catalyst with Mg/Al molar ratio 5 shows highest propylene carbonate conversion (72 %) with 97 % dimethyl carbonate
selectivity. The catalyst shows the highest TON (280 mmol DMC/gcat) ever reported with heterogeneous catalysts.
A plausible reaction mechanism has been proposed based on the results obtained.
Keywords: Catalysts, Transesterification, Hydrotalcites, Carbonates, Magnesium, Aluminum
IPC Code: Int. Cl.9 B01J21/02; B01J21/10; B01J27/232; C01F5/00; C01F7/00; C07B41/14
Dimethyl carbonate (DMC) finds diversified
applications in the chemical industries1,2. Though one
step synthesis of DMC from CO2 and CH3OH is
attractive, yield is relatively low due to high
thermodynamic stability and kinetic inertness of CO2.
Also, it leads to the deactivation of catalyst by water
formed during the process3. Recently, cyclo-addition
of carbon dioxide to epoxides under ambient
condition has been reported4,5 and needs more
emphasis on the transesterification step (Scheme 1) to
utilize CO2 directly in a one pot synthesis.
Several researchers have reported the formation of
DMC from cyclic carbonates using different
catalysts6-15. The equilibrium yield of DMC from
propylene carbonate (PC) is found to be three times
lower than that from ethylene carbonate (EC) in the
presence of Na3PO4 as solid base catalyst6. This has
been explained in terms of steric factor though both
the reactions have same activation energy6
(29 ± 2 kJ/mol).
Synthesis of dimethyl carbonate from ethylene
carbonate at atmospheric pressure9 and dialkyl
carbonates from cyclic carbonates at higher pressure
(25-30 bar)11 have been reported using Mg-Al
hydrotalcite as solid base catalyst. We have reported
the synthesis of dimethyl carbonate from propylene
carbonate and methanol using KF/Al2O3 as efficient
base catlyst16. Takagaki et al.17 have recently reported
the synthesis of glycerol carbonate from glycerol and
DMC using hydrotalcite. The ionic species from the
catalyst were found to play an important role in
overall reactivity of the reaction. The anion present in
the hydrotalcite gallery plays an important role in
determining the basic behavior of the materials
calcined below the decomposition temperature18. We
have studied the effect of N,N′-dimethylformamide in
cycloaddition reaction of carbon dioxide to propylene
oxide (step I in Scheme 1) using Mg/Al hydrotalcite19.
In the present study, we report the efficiency of
Mg-Al-CO3 hydrotalcite in transesterification of
propylene carbonate with methanol in atmospheric
pressure. The effect of calcination and reaction
temperatures, reactant ratio and catalyst amount has
also been studied.
MURUGAN & BAJAJ: CATALYTIC ACTIVITY OF Mg-Al-CO3 HYDROTALCITES
Materials and Methods
Propylene carbonate (PC), ethylene carbonate (EC)
and biphenyl were purchased from Acros Organics,
Belgium. Dimethyl carbonate, propylene glycol,
methanol, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3,
and NaOH were procured from SD Fine Chemicals,
India. Methanol was dried using standard procedure
and stored under activated molecular sieve 3A. Millipore deionized water was used for the synthesis of
hydrotalcite catalyst.
Synthesis of the catalyst
The hydrotalcite samples with Mg/Al molar ratio
varying from 2-5 were synthesized by co-precipitation
method19,20. The catalysts obtained were powdered,
dried at 120 °C for 4 h and stored in a vacuum
desiccator to avoid contamination by atmospheric
moisture. The catalysts were coded as HT-X with X
representing the molar ratio of Mg/Al salts used for
synthesis. The catalyst (HT-5) was calcined separately
at 200 °C and 450 °C for 4 h to evaluate the effect of
calcination temperature on the title reaction.
Characterization of the catalyst
Powder X-ray diffraction (P-XRD) patterns of the
hydrotalcite samples were recorded with Phillips
X’Pert MPD system equipped with XRK 900 reaction
chamber, using Ni-filtered Cu-Kα radiation
(λ = 1.5405 Å) over a 2θ range of 2–70°. The Fourier
transform infra red (FT-IR) spectra of the hydrotalcite
samples were recorded with Perkin–Elmer (Spectrum
GX) FT-IR system using KBr pellets. The surface
area was computed using N2 adsorption data
measured at 77 K on a Micromeritics, ASAP 2010
USA system. The BET isotherm model was used to
calculate surface area while the HK model was used
to calculate pore volume and pore radius. The samples
were degassed at 120 °C for 4 h under vacuum
(5 × 10−2 mm Hg) prior to N2 adsorption
measurement. Hammett indicator method was used to
determine the basic strength of the catalyst21.
Phenolphthalein (H ≥ 9.8), bromothymol (H ≥ 7.2)
and methyl red (H ≥ 4.2) were used as indicators for
different basic strengths. In a typical measurement,
50 mg of the catalyst in 5 mL dry methanol was
stirred for 2 h under nitrogen atmosphere and the
resultant solution was titrated against standard acid
(0.02 mol/L benzene carboxylic acid in dry methanol)
solution. The end points, i.e., pink to colorless with
phenolphthalein indicator, blue to yellow with
bromothymol and yellow to red with methyl red
1183
indicator, were measured separately and the results
are given under three different strengths.
Catalytic reactions
In a typical catalytic experiment, the oven dried
25 mL round bottom flask was fed with 2.04 g
(20 mmol) propylene carbonate, 6.4 g (200 mmol)
methanol, 80 mg biphenyl (internal standard) and
50 mg catalyst. The reaction was carried out under N2
atmosphere using N2 filled balloon and long spiral
condenser setup. The reaction setup was placed in a
preheated oil bath (±2 °C temperature variation) and
the reaction was initiated by stirring (500 rpm). After
4 h, the product mass was filtered to remove the
catalyst and analyzed by gas chromatography
(Shimadzu 17A) equipped with a flame ionization
detector (FID). The products were further confirmed
by GC–MS (Shimadzu GCMS, QP 2010); 5 %
diphenyl- and 95 % dimethylsiloxane universal
capillary column (60 m length, 0.25 mm diameter)
was employed in both GC and GC-MS analyses.
Propylene glycol was co-produced in all the reactions
equimolar to the DMC formed and thus it has been
exempted from the discussion. The conversion and
selectivity were calculated using the following
relationship in response to the internal standard:
% Conversion of PC =
initial moles of PC − final moles of PC
×100
initial moles of PC
moles of DMC formed
% Selectivity of DMC =
×100
moles of PC reacted
Results and Discussion
Catalyst characterization
The X-ray powder diffraction patterns of the
synthesized samples with the reflection planes (003),
(006), (012), (015), (018), (110), (113) and (116)
matched perfectly with the literature report22. No
additional impurity phase was observed in any of the
synthesized catalysts (see Supplementary Data). The
basal spacing d(003) of HT-CO3 at ~7.65 Å was in
good agreement with the literature value and
increased with decrease in the Al content (Table 1) as
also reported by Miyata23. The sample calcined at
200 °C showed decreased intensity of the HT peaks
(003 and 006 reflections) and that at 450 °C showed
the complete decomposition of HT phase into Mg-Al
mixed oxide (Fig. 1). The intensity of 003 reflection
of the reused catalyst after the 4th cycle was found to
decrease as compared to the fresh catalyst, showing
some crystallinity loss.
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INDIAN J CHEM, SEC A, SEPTEMBER 2010
Table 1 – Physical characteristic values of HT samples with
varying Mg/Al ratio
Catalyst
d spacing of
(003) plane
(Å)
BET surface
surface area
(m2/g)
Pore
vol.
(cm3/g)
Pore
radius
(nm)
HT-2.0
7.60
86
0.45
10.6
HT-3.0
7.69
65
0.38
11.7
HT-4.0
7.92
90
0.39
8.6
HT-5.0
7.95
93
0.46
9.9
HT-5.0a
7.55
82
0.40
9.6
-
229
0.72
6.3
HT-5.0
a
b
o
Calcined at 200 C.
Calcined at 450 oC.
b
Fig. 2 ― FT-IR profile of HT-5 catalyst [(a) as synthesized;
(b) calcined at 200 °C; (c) after first cycle; (d) after fourth cycle].
Fig. 1 ― P-XRD patterns of HT-5 catalyst. [(a) as synthesized;
(b) calcined at 200 °C; (c) calcined at 450 °C; (d) after first cycle;
(e) after fourth cycle].
The FT-IR spectra of hydrotalcite samples matched
well with the literature data24,25. The FT-IR peak
at ~1650 cm−1 showed the presence of monodentate
carbonate species with δO-C-OH asymmetric mode
in as synthesized catalyst. The sample calcined at
200 °C showed the splitting of carbonate peak
into two peaks at around 1530 and 1380 cm−1 (Fig. 2).
This is assigned to the change in the coordination of
the carbonate ion upon loss of interlayer
water molecules, as also noted by Cabrera et al.26
Similar observation, though less pronounced was
noticed in the reused catalyst (d in Fig. 2) indicating
some loss (~ 11.7 %) in catalytic activity after
four cycles.
The BET surface, pore volume and pore radius
values decreased on calcination at 200 °C as
compared to those of the as synthesized catalyst. This
may be due to the loss of interlayer water followed by
decreased d-spacing of (003) plane (7.95 Å to
7.55 Å). On calcination at 450 °C, the catalyst
completely loss its interlayer anions and surface
hydroxyl groups resulting in the formation of Mg-Al
mixed oxides. This resulted in increased surface area
and pore volume but decreased pore radius.
The hydrotalcite samples were found to have
different basicity at different basic strengths
(Table 2). The basicity values in the basic strength
between 7.2 and 9.8 increased with increase in Mg/Al
ratio with as synthesized catalysts and decreased with
calcination temperature of HT-5. The basicity of
stronger basic sites (H ≥ 9.8) decreased with increase
in Mg/Al ratio and increased with calcination
temperature of HT-5. The total number of basic sites
presented in the calcined catalysts may not be the
actual one, since the adsorption depends on the
number of accessible sites which in turn depends on
the pore size of the catalyst.
Screening of the catalyst
The catalytic activity of hydrotalcite samples dried
at 120 °C was studied for the transesterification
reaction and the data are given in Table 2. There was
no conversion of propylene carbonate at the studied
reaction temperature (130 °C) in the absence of
catalyst. The conversion of PC increased with
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MURUGAN & BAJAJ: CATALYTIC ACTIVITY OF Mg-Al-CO3 HYDROTALCITES
Table 2 – Effect of Mg/Al molar ratio on transesterification of PC with methanol. {React. cond.: Methanol/PC ratio 10:1 (mol/mol);
Catalyst amt: 50 mg; Temp.: 130 °C ; Time 4 h}
Catalyst
Conv. of PC
(%)
Selectivity of DMC
(%)
4.2 <H ≤ 7.2
Basicity (µmol/gcat)a
7.2 <H ≤ 9.8
H ≥ 9.8
Total
HT-2.0
61.3
90.2
44
126
14
184
HT-3.0
63.8
95.0
46
136
14
196
HT-4.0
67.8
97.9
42
159
11
212
HT-5.0
72.2
97.1
36
190
10
236
b
54.5
95.4
40
144
24
208
c
52.8
76.1
20
104
44
168
HT-5.0
HT-5.0
a
Values are within ±5 % error limit.
Calcined at 200 °C.
c
Calcined at 450 °C.
b
increase in Mg/Al molar ratio reaching a maximum of
72.2 % with HT-5. The selectivity of DMC also
increased with increase in Mg/Al molar ratio and
reached a maximum of 97 % with HT-5. Considering
the catalytic efficiency of the catalyst in terms of both
conversion and selectivity, HT-5 was selected for
further detailed investigations.
Effect of calcination temperature of the catalyst (HT-5)
The catalyst HT-5 dried at 120 °C showed higher
activity as compared to the samples calcined at higher
temperatures (Entries 5 and 6 in Table 2). The sample
calcined at 200 °C resulted in 54.5 % conversion with
95.4 % selectivity. The drop in conversion (~18.5 %)
may be due to the change in coordination of carbonate
species from interlayer water to interlayer OH upon
losing the interlayer water (Fig. 2). The presence of
H-bonding between interlayer OH (Lewis basic sites)
and carbonate ion gave selectivity greater than 95 %,
while the absence of interlayer water brought the
conversion down to 54.5 %. The presence of
interlayer water was found to accelerate the activation
of methanol via hydrogen bonded carbonate ion. The
weak and medium basicity values of the hydrotalcite
samples were matched well with the observed
conversion of PC (Table 2). The sample calcined at
450 °C showed lower selectivity (76.1 %) of DMC.
This might be due to the presence of high stronger
basic sites (Brønsted basic sites) present in Mg-Al
mixed oxide leading to the partial decomposition of
PC. A similar lower selectivity has been observed
with metal oxide catalyst in transesterification of
ethylene carbonate with methanol15. Watanabe et al.9
have reported that the ethylene carbonate gets
activated at the edge of the HT layer during the
synthesis of dimethyl carbonate. Based on this
concept and the selectivity data obtained with the
samples calcined at high temperature, one can say that
PC can be activated at the surface of the catalyst and
methanol by the hydrogen bonded carbonate ions
present in the interlayer of the catalyst. Thus, the
overall reactivity of Mg-Al-CO3 hydrotalcite samples
in this transesterification reaction was assigned to the
surface basicity and carbonate coordination with
interlayer water/OH.
Effect of temperature and amount of catalyst
The increase in temperature increased the
formation of DMC and reached equilibrium at 130 °C
within 4 h (Fig. 3a). Reaction temperature >130 °C
resulted in lower selectivity of DMC, shifting the
equilibrium back to the reactant side. Wei et al.27 have
reported similar reversing of equilibrium at higher
temperature. The cyclic carbonate with no sterically
hindered methyl group, i.e., ethylene carbonate,
attained optimum conversion (entry 11 in Table 3) at
80 °C in 4 h. This is in accordance with the difference
in the frequency factor between EC→DMC and
PC→DMC reaction as explained by Filippis et al.6
The conversion of PC was significantly increased
with increase in the reactant ratio up to 10 and was
less significant beyond the ratio 10 (Fig. 3b). This
might be due to the formation of methanol-DMC
azeotrope which is subsequently involved in deciding
the equilibrium shift. The variation of amount of
catalyst shows that the conversion of PC increased
with increase in amount of catalyst, keeping the
selectivity greater than 97% (Fig. 4). Above 50 mg,
the amount of catalyst was not significant in the
optimum conversion (72 %) of 20 mmol PC.
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INDIAN J CHEM, SEC A, SEPTEMBER 2010
Fig. 3 ― Effect of reaction temperature at methanol/PC ratio 10:1 (mol/mol) (a) and reactant ratio at reaction temperature 130 °C (b).
[React. cond.: catalyst (HT-5): 50 mg; time 4 h. 1, Conversion of PC (%); 2, Selectivity of DMC (%)].
Fig. 4 ― Effect of catalyst amount on [React. cond.: methanol/PC
ratio: 10:1 (mol/mol); catalyst HT-5; temp. 130 °C; time 4 h.
1, Conversion of PC (%); 2, Selectivity of DMC (%)].
Efficiency of the catalyst in terms of TON
The catalytic efficiency of Mg-Al-CO3 used in this
study was compared with the data for catalysts
reported in literature8,14,15,23 (Table 3). Since the
basicity values are not available for the reported
catalysts, TON (TON = mmol of product formed per
gram catalyst) was employed as a unit of comparison.
The efficiency of various heterogeneous catalysts on
the title reaction was compared with the presently
studied catalyst (Table 3). The TON obtained
(280 mmol DMC/gcat) was much higher than values
reported in literature. The TON of Mg-Al mixed
oxide (HT-5 calcined at 450 °C) was also comparably
higher (160 mmol DMC/gcat) than that of metal oxides
reported in the literature14,15,27.
Reusability of the catalyst HT-5
The catalyst once used in the fresh run was
collected and thoroughly washed with methanol to
remove the various adsorbed molecules such as
internal standard, propylene carbonate propylene
glycol, etc. The washed catalyst was dried at 120 °C
for 4 h before use in the consecutive runs. The
catalyst showed 60.5 % conversion of PC with 92.4 %
DNC selectivity after the fourth cycle. This slight
drop in activity (11.7 % in conversion and 4.7 % in
selectivity) is due to the structural changes, i.e., loss
in the crystallinity observed in the used catalyst as
seen by the P-XRD (e in Fig. 1) and FT-IR studies
(d in Fig. 2).
Plausible reaction mechanism
Based on the above results, a possible reaction
mechanism is proposed (Scheme 2). In the first step,
the H-bonded carbonate ions present in the basic
gallery of the catalyst abstract proton from methanol
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MURUGAN & BAJAJ: CATALYTIC ACTIVITY OF Mg-Al-CO3 HYDROTALCITES
Table 3 – Comparative efficiency of Mg-Al-CO3 in transesterification of PC with methanol
Run
Catalyst
React. temp.
React. time
Conv. of PC Selectivity of DMC
(°C)
(h)
(%)
(%)
TONc
Ref.
1a
HT-5.0
130
4
72.2
97.1
280.4
This work
2
Au/CeO2 (1.5 wt%)
140
6
63
55
30.1
14
3
Fe-Zn double metal
cyanide
170
8
-
86.6
36.4
8
4
MgO
160
2
~ 40
~93
~21
23
5
MgO
150
4
28
100
14
15
6
ZrO2
150
4
11.8
100
5.9
15
7
CeO2
150
4
32.8
98.8
16.2
15
8
ZnO
150
4
23
100
12.5
15
9
CaO
150
4
25.6
100
12.8
15
Mg/Al mixed oxide
130
4
52.8
76.1
160.7
This work
HT-5.0
80
4
82.9
98.6
326.9
This work
10a
11
b
a
React. cond.: Methanol/PC ratio: 10:1 (mol/mol); catalyst amt.: 50 mg.
Ethylene carbonate was used instead of propylene carbonate.
c
Yield of DMC in mmol/g catalyst.
b
forming methoxy species (CH3O-) and the Lewis acid
site (active metal centre) present on the surface of HT
interacts with carbonyl oxygen of PC to generate
relatively charged carbonyl carbon. In the second
step, the methoxy ions attack the carbonyl carbon of
PC at the edge of the HT structure to produce DMC.
Simultaneously, the relative positive charge is
transferred into the gallery to release proton, which is
subsequently involved in the formation of propylene
glycol. The water molecule present in the gallery is
proposed to be involved in easier activation of
methanol followed by the proton transfer during
simultaneous formation of DMC and PG. In the
reaction with Mg-Al mixed oxide, both the reactants
are to be activated on the surface of the catalyst where
the transfer of methoxy species and proton is
supposed to be slower due to the strong basic nature
of the surface. This leads to the decomposition of PC
into CO2 and propylene oxide leading to decreased
selectivity of DMC.
Conclusions
The hydrotalcite materials, Mg-Al-CO3 with
different Mg/Al molar ratio from 2-5 have been
synthesized and characterized with FT-IR and P-XRD
techniques. The catalyst HT-5 dried at 120 °C showed
optimum activity at 130 °C. The catalyst calcined at
200 oC resulted in slightly lower conversion with
nearly the same selectivity while that at 450 °C
resulted in decreased conversion and selectivity. The
increase in methanol to propylene carbonate ratio
increased the conversion of propylene carbonate,
shifting the equilibrium to the product side. The
catalyst showed very high TON with moderate
recyclability even after four cycles.
Supplementary Data
P-XRD patterns of HT samples of varying Mg/Al
ratio may be obtained from the authors on request.
Acknowledgement
Authors are thankful to CSIR for financial support
under CSIR Network Programme NWP-0010.
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