4
4
P. Unnikrishnan, D. Srinivas / Journal of Molecular Catalysis A: Chemical 398 (2015) 42–49
Table 1
Catalytic activity for transesterification of ethylene carbonate (EC) with methanol.
a
Catalyst
SBET
Basicity (CO2-TPD)
(mmol/g)
EC conversion
(mol%)
DMC selectivity
(mol%)
TON
2
(mol/m2)
(
m /g)
(mmol/g)
HT-C
MgO–La2O3
MgO
121
–
–
96
82
79
68
204
157
115
286
0.422
–
–
3.5
–
–
16.0
19.2
27.5
37.7
52.9
65.2
73.8
19.0
24.9
70.2
18.0
97.7
99.0
99.5
100
100
100
100
100
100
100
96.7
35
42
61
84
HT-2 La–C
HT-5 La–C
HT-8 La–C
HT-10 La–C
HT-8 Y–C
HT-8 Ce–C
HT-8 Pr–C
HT-8 Sm–C
0.481
0.489
0.521
0.446
0.418
0.251
0.266
0.354
5.0
5.9
6.6
6.6
2.0
1.6
2.3
1.2
118
145
164
42
55
156
39
a
◦
Reaction conditions: EC = 0.88 g (10 mmol), methanol = 3.2 g (100 mmol), catalyst = 5 wt% of EC, reaction temperature = 60 C, and reaction time = 4 h. Turnover number
TON) = mmol of DMC formed per gram of catalyst.
(
spectrophotometer in 400–4000 cm−1 region. Basicity of the cata-
lysts was determined by temperature programmed desorption of
To determine the kinetic parameters (rate of reaction, equilib-
rium constant, and activation energy), reactions were conducted at
◦
carbon dioxide (CO -TPD) technique. About 0.1 g of the catalyst was
four different temperatures 40, 60, 80, and 100 C (EC–methanol)
2
◦
◦
activated at 500 C under He (30 ml/min). The sample was cooled
and 110, 130, 150, and 170 C (PC–methanol) over a period of 0–6 h
◦
to 50 C and CO was adsorbed for 30 min. Desorption of CO2 was
using HT-8 La–C and HT-10 La–C catalysts. Initial rates were deter-
mined from the time versus conversion plots collected at different
temperatures. Activation energies were estimated from the Arrhe-
nius plots. Pseudo-first order kinetics was considered. Mass balance
was more than 98%.
2
◦
traced in the temperature range of 50–900 C by rising the temper-
ature at the ramp rate of 10 C/min.
◦
2.3. Reaction procedure
EC (10 mmol), alcohol (100 mmol), and catalyst (5 wt% of EC)
3
. Results and discussion
were taken in a Teflon-lined stainless-steel autoclave placed
in a rotating hydrothermal reactor (Hiro Co., Japan; rotation
◦
Calcination of rare-earth modified HTs led to formation of
speed = 50 rpm). Reactions were conducted at 40–100 C for
ternary oxides (XRD, Fig. 1). The calcined materials showed XRD
0
.5–6 h. After completion of the reaction, the autoclave was
◦
◦
◦
◦
pattern with peaks at 35.1 , 43.4 , and 62.6 due to (1 1 1), (2 0 0),
and (2 2 0) planes, respectively, of (Mg)Al oxide [22]. Incorporation
of a small amount of rare-earth ion in Mg(Al) oxide could not be
ruled out. Peaks corresponded to a separate rare-earth oxide phase
were also detected which were marked by asterisk (Fig. 1). These
were weak for HT-8 Y–C and HT-8 Sm–C. Other than lanthanum
oxide, formation of La O CO phase was also detected for HT La–C
catalysts with 8–10 mol% of La loading (JCPDS 48-1113). FTIR spec-
troscopy too confirmed the catalysts as ternary oxide phases [23].
cooled to 25 C, and the catalyst was separated by centrifuga-
tion/filtration. The liquid product was analyzed and quantified
by gas chromatography (GC, Varian 3800; CP-SIL 5 column;
5
(
0 m × 0.25 mm × 0.25 m). The influence of reaction parameters
reaction time, reaction temperature, catalyst amount, and type of
alcohol) on product yield was investigated. For comparison, exper-
iments were also conducted with PC instead of EC. Those runs were
2
2
3
◦
carried out at 80–170 C.
In a different set of reactions, dialkyl carbonates were syn-
thesized from DMC by reaction with alcohols. In a typical
reaction, DMC (10 mmol), ethanol/n-propanol/n-butanol/benzyl
alcohol (100 mmol), and catalyst (10 wt% of DMC) were loaded
on a Teflon-lined stainless-steel autoclave placed in a rotating
HT-C showed two CO -TPD peaks with maximum at 246 and
2
◦
6
53 C attributable to CO2 desorbed from weak and strong basic
sites, respectively (Fig. 2). Rare-earth modified HTs showed four
CO2 desorption peaks at 204, 349, 645, and 815 C (for HT-8
◦
◦
La–C) consistent with the presence of four types of basic cen-
ters. Intensity of the second and fourth peaks increased with
increasing La content. Based on this observation, the first and
third peaks were attributed to those arising from (Mg)Al oxides
while the second and fourth peaks were assigned to a segre-
gated rare-earth oxide phase. A change in peaks positions was
noted with a change in the loading of rare-earth oxide (Fig. 2).
With increase in La loading, peaks corresponding to rare-earth
oxide shifted to higher temperatures. This shift to higher temper-
atures could be a consequence of increase in the basic strength
and particle size of dopent oxide. The overall basicity of dif-
ferent modified catalysts is reported in Table 1. It decreased in
the order: HT-8 La–C (0.521 mmol/g) > HT-C (0.422 mmol/g) > HT-
hydrothermal reactor. Reactions were conducted at 80–150 C for
◦
6
h. After the reaction, the autoclave was cooled to 25 C, and the
catalyst was separated by centrifugation/filtration. Liquid product
was analyzed and quantified by GC. The dialkyl carbonates formed
from ethanol, n-propanol, n-butanol, and n-benzyl alcohol were
designated as DEC, DPC, DBC, and DBzC, respectively.
For quantitative analysis of dialkyl carbonates, different known
concentrations of standard analate (reactants and possible prod-
ucts) mixtures was prepared, injected into GC and peak areas of
each analate were noted. A graph of peak area versus concentration
of each analate was drawn, and response factor was determined,
which was then used in determining the exact concentration of the
products formed in the reaction mixture. Conversion and selectivity
were determined using the following equations:
8
Y–C (0.418 mmol/g) > HT-8 Sm–C (0.354 mmol/g) > HT-8 Pr–C
(0.266 mmol/g) > HT-8 Ce–C (0.251 mmol/g). In other words, dop-
%
Conversion
ing of Sm, Pr, and Ce ions had adverse effects on the basicity of these
oxides.
Transesterification of ethylene carbonate (EC) with different
alcohols yielded equimolar quantities of the dialkyl carbonate and
ethylene glycol (EG) (Scheme 1). Controlled experiments revealed
that this reaction does not occur in the absence of a catalyst. As-
= (
Initial moles of cyclic carbonate − final moles of cyclic carbonate) × 100
Initial moles of cyclic carbonate
Moles of product formed × 100
%
Product selectivity =
Moles of cyclic carbonate converted into products