Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts During Diethyl Carbonate Synthesis from Ethyl Carbamate and Ethanol

Article abstract of DOI:10.1007/s10562-017-2097-2

Diethyl carbonate, an important member in the family of organic carbonates, is a fuel additive like dimethyl carbonate (DMC). It holds an extra edge of having better gasoline/water distribution coefficient than DMC, and also DEC is widely used as an electrolyte in lithium ion batteries. Ethanolysis of ethyl carbamate (EC) is the most economical and greener route for DEC synthesis. Zn–Al–M (M=Ca, La, Mg and Y) have been synthesized using two methods and their activity have been explored DEC systhesis from EC and ethanol. The catalysts were characterized using thermogravimetric analysis, Brunauer, Emmett and Teller surface area, N₂ adsorption–desorption textural analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, temperature-programmed desorption (TPD), atomic-force microscopy and Raman spectroscopy. Pure metal oxides were observed during the XRD analysis and Al₂O₃ was found to be in amorphous form. Third metal oxide prepared from impregnation method was found to be present on the surface as well as in impregnated form. CO₂-TPD analysis showed close correlation between the basicity and the DEC yield. Zn–Al–Mg, prepared from precipitation method, being most basic, was found to be most effective although the performances of Zn–Al–Ca and Zn–Al–La were good. The effect of precipitants was also studied by synthesizing Zn–Al–Mg using NaOH and liquid NH₃ as precipitants. DEC yield of 40.2% and turn over frequency of 1055?mg_DEC?g_cat^?1?h^?1 was obtained in 5?h at 190 °C using Zn–Al–Mg prepared from precipitation method. Effect of reaction conditions was also studied and equilibrium constant of the reaction was estimated using the Benson group contribution method. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2097-2

Catal Lett
DOI 10.1007/s10562-017-2097-2
Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals
Promotional Effects on Zn–Al Catalysts During Diethyl
Carbonate Synthesis from Ethyl Carbamate and Ethanol
Kartikeya Shukla · Vimal Chandra Srivastava1
1
Received: 10 March 2017 / Accepted: 29 May 2017
©
Springer Science+Business Media, LLC 2017
Abstract Diethyl carbonate, an important member in the
family of organic carbonates, is a fuel additive like dime-
thyl carbonate (DMC). It holds an extra edge of having
better gasoline/water distribution coefficient than DMC,
and also DEC is widely used as an electrolyte in lithium
ion batteries. Ethanolysis of ethyl carbamate (EC) is the
most economical and greener route for DEC synthesis.
Zn–Al–M (M=Ca, La, Mg and Y) have been synthesized
using two methods and their activity have been explored
DEC systhesis from EC and ethanol. The catalysts were
characterized using thermogravimetric analysis, Brunauer,
analysis and Al O was found to be in amorphous form.
2 3
Third metal oxide prepared from impregnation method was
found to be present on the surface as well as in impregnated
form. CO -TPD analysis showed close correlation between
2
the basicity and the DEC yield. Zn–Al–Mg, prepared from
precipitation method, being most basic, was found to be
most effective although the performances of Zn–Al–Ca
and Zn–Al–La were good. The effect of precipitants was
also studied by synthesizing Zn–Al–Mg using NaOH and
liquid NH as precipitants. DEC yield of 40.2% and turn
3
−
1
−1
over frequency of 1055 mg
g
h
was obtained in
DEC cat
Emmett and Teller surface area, N adsorption–desorption
5 h at 190°C using Zn–Al–Mg prepared from precipitation
method. Effect of reaction conditions was also studied and
equilibrium constant of the reaction was estimated using
the Benson group contribution method.
2
textural analysis, X-ray diffraction (XRD), Fourier trans-
form infrared spectroscopy, temperature-programmed des-
orption (TPD), atomic-force microscopy and Raman spec-
troscopy. Pure metal oxides were observed during the XRD
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2097-2) contains supplementary
material, which is available to authorized users.
*
Kartikeya Shukla
kartikeyashr@gmail.com
*
Vimal Chandra Srivastava
vimalcsr@yahoo.co.in; vimalfch@iitr.ac.in
1
Department of Chemical Engineering, Indian Institute
of Technology Roorkee, Roorkee, Uttarakhand 247667, India
Vol.:(0
1
123456789)
3

K. Shukla, V. C. Srivastava
Graphical Abstract
Keywords Diethyl carbonate · Ethyl carbamate ·
Synthesis methods · Catalysts · Precipitation ·
Characterization
higher valued chemicals is another window of CO2 utiliza-
tion [9].
Organic carbonates are green class of compounds with
a wide range of applications [10]. Dimethyl carbonate
(DMC) and diethyl carbonate (DEC) are well known sim-
ple carbonates in the family [11]. They also hold important
properties that of a fuel additive like commonly known
methyl t-butyl ether (MTBE) and ethyl tert butyl ether
(ETBE) [12, 13]. DEC, a colorless transparent liquid,
besides being a fuel additive like DMC holds an extra edge
of possessing better gasoline/water distribution coefficient.
CH3CH2O– and –CO– groups present in DEC tend to make
it more reactive helping in synthesis important chemicals
[14]. Also DEC is widely used as a solvent in lithium ion
batteries [15–17].
1
Introduction
The sequestration of carbon dioxide (CO ) into mineral
2
carbonates has recently received much attention due to its
indirect role as environment pollutant [1–3]. Many pro-
cesses are being developed to transform CO into valuable
2
products such as syn gas by reforming of methane, synthe-
sis of methanol from CO and H , transformation of CO
2
2
2
to cyclic carbonates by cycloaddition with epoxides [4–7].
CO is most oxidized state of carbon, the activation of CO
Phosgenation of ethanol is the oldest method which is
discontinued owing to toxicity of phosgene. Various non-
phosgene routes have been developed in the past dec-
ade. These include carbonylation of ethanol using CO2
2
2
2
is the most difficult and stringent step in utilization of CO
[
8]. Urea production is the most useful method for CO fix-
2
ation on a wide scale. Hence, utilization of urea to produce
1
3

Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts…
or urea, oxidative carbonylation of ethanol, transesteri-
fication of ethanol with organic carbonates, using ethyl
nitrite route, etc. [12–26]. All the methods, various cata-
lysts used and other engineering aspects have been criti-
cally reviewed earlier [27, 28]. Carbonylation of ethanol
using CO suffers from difficult activation of CO which
2 Experimental
2.1 Materials
Magnesium nitrate hexahydrate, Mg(NO ) ·6H O; cerium
3
2
2
nitrate hexahydrate, (Ce(NO ) .6H O); and lanthanum
2
2
3 3
2
affects its thermodynamics. Trans-esterification of etha-
nol with DMC causes three azeotrope formation which
hampers its recovery from the mixture. Urea ethanolysis,
however, is an economical method due to its cheap raw
materials and also because of no water formation during
the process, and hence no azeotrope formation.
(III) nitrate hexahydrate, La(NO ) .6H O; yttrium nitrate
3 3 2
hexahydrate, Y(NO ) .6H O; and biphenyl of 99% purity
3
3
2
was purchased from Sigma Aldrich; zinc nitrate hexahy-
drate, (Zn(NO ) .6H O) and aluminium nitrate nonahydrate
3
2
2
Al(NO ) ·9H O of 99.5% purity was purchased from Hi
3
3
2
media Laboratories, Mumbai. Sodium carbonate (Na CO )
2
3
Ethanolysis of urea to synthesize DEC, is a two step
process (shown by Eqs. 1 and 2), which deals with the
formation of ethyl carbamate (EC) in first step followed
by formation of DEC in second step (Eqs. 1 and 2).
and sodium hydroxide (NaOH) was purchased from S.D
Fine Chemicals Limited, and ethanol was purchased from
Merck.
2
.2 Preparation of Catalyst
C H OH + NH CONH → C H OCONH + NH
2
2
5
2
2
2
5
3
(1)
Zinc nitrate hexahydrate and aluminium nitrate nonahy-
drate were dissolved in Millipore water and mixed drop
C H OH + C H OCONH → C H OCOOC H + NH
3
2
5
2
5
2
2
5
2
5
(2)
(
C H O) CO + C H OCONH → C H OCONHC H + C H OH + CO
2
(3)
2
5
2
2
5
2
2
5
2
5
2
5
The process starts with urea which gives isocyanic
wise in the molar ratio of 3:1. 3 M NaOH solution was
mixed drop wise to the mixed solution under vigorous mix-
ing at 10 pH. The formed precipitate was aged at 80°C at
4 h and further for 24 h at room temperature. The precipi-
tate was filtered and washed several times with deionised
water till the pH of filtrate reached till 7. The precipitate
was dried at 80°C for 24 h. The formed Zn–Al hydroxide
was prepared for calcined to get binary oxides.
acid, a very active compound, which reacts further with
ethanol to give EC. The first step doesn’t requires use
of any catalyst while second step requires further use of
catalyst. Formation of N-ethyl ethyl carbamate (NEEC)
is also seen in trace as a byproduct (Eq. 3). Hence many
studies have been reported on the synthesis of DEC
from EC itself while only few investigators have focused
on synthesis of DEC from urea. Various studies on the
synthesis of DEC from EC have been summarized in
Table S1 (supporting information) [29–38].
2.2.1 Method A
Previously reported studies on synthesis of DEC from
EC include use of Zn and Pb based catalysts. However,
no ternary oxides have been reported even for the synthe-
sis of urea or EC. Hence, an attempt has been made in the
present study to synthesize and characterize Zn–Al–M
Twelve gram of prepared hydroxide was dispersed in deion-
ised water. Similarly, other metal nitrates which include
magnesium, yttrium, calcium and lanthanum nitrate were
dispersed in deionised water such that the metal weight
was 1 g. Both the solutions were mixed drop wise and the
pH was adjusted to ten with the help of drop wise addi-
tion of 3 M NaOH solution. The precipitate was washed
with deionised water till the pH of the filtrate became 7.
The precipitate was aged at 80°C and dried at 110°C for
24 h followed by calcination to remove other type of impu-
rities and convert all types of hydroxyl groups to oxides.
The catalysts prepared by method A are represented by
Zn–Al–M–A (M=Ca, Mg, La and Y).
(
M=Ca, La, Mg and Y) catalysts. The study is focused on
understanding the effect of introducing third metal oxide
into the fixed amount of Zn–Al hydroxides. The catalysts
have been synthesized using precipitation and impregna-
tion methods. Further, two different precipitants namely
NaOH and liquid ammonia have been compared for their
affect on the best performing catalysts. The catalysts have
been characterized using various sophisticated instru-
ments. These catalysts have been further used to syn-
thesize DEC and study the effect on reaction conditions
2.2.2 Method B
(
temperature, time, catalysts wt% with respect to EC and
ethanol/EC ratio) on DEC yield, EC conversion and turn
over frequency (TOF).
Prepared hydroxide already was calcined at 700°C to get
the final form of oxides. 1 g of prepared Zn–Al oxides were
1
3

K. Shukla, V. C. Srivastava
impregnated with the third nitrate solutions and the solu-
tion was stirred vigorously for 24 h. The third nitrate solu-
tion was taken in such a way that the metal weighed ≈0.5 g.
The solutions were dried at 110°C for 24 h and were fur-
ther calcined at 700°C. The catalyst prepared by method B
were represented by Zn–Al–M–B (M=Ca, Mg, La and Y).
Further Zn–Al–Mg–C catalyst was also prepared by first
method but using liquid ammonia as precipitant. Firstly
Zn–Al hydroxides were prepared and then the third element
was precipitated using liquid ammonia as precipitant.
Fourier transform infrared (FTIR) spectroscopy was
used to find out the surface functional groups of the
fresh, regenerated catalysts, catalyst interacted with EC,
and the reaction solution by using a FTIR spectropho-
tometer (Thermo Nicolet, Model Magna 760). Catalyst
interacted with EC was prepared by blending EC (1.8 g)
with the catalyst (0.18 g) and heating the pellet for 3 h
at specified temperature (100–140 °C). The solid catalyst
was recovered by centrifuging the reaction solution. The
catalyst was further washed with de-ionised water and
ethanol three times followed by drying at 110 °C over-
night. Thereafter the catalyst was further activated by cal-
cination 700 °C.
2
.3 Catalyst Characterization
The structural morphology of fresh and regenerated
catalyst was also studied using atomic-force microscopy
(AFM) of M/s molecular tools and devices for nanotech-
nology (NT-MDT) coupled with NOVA software for
image analysis. The samples were prepared in ethanol in
which 20 mg of sample dispersed in 10 mL ethanol solu-
tion, which was sonicated for 120 min followed by small
dispersion of solution on a glass plate. The plate was
dried, at room temperature, overnight. This glass plate
was then, used for AFM analysis.
Thermogravimetric analysis (TGA) of the synthesized cata-
lysts was carried out with SII 6300 EXSTAR analyzer. The
temperature range was set in the range of 30–1000°C with
a heating rate of 10 K/min.
The prepared catalysts were characterized by X-ray dif-
fraction (XRD) (Bruker D8 ADVANCE) using Cu-K radi-
α
ation (l=0154 nm). This was done with X-ray gun oper-
ated at 40 kV and 30 mA. The 2θ values were varied in the
range of 5–90° with step size of 0.02°. PANalytical Xpert
high score software along with the Joint committee on
powder diffraction international centre for diffraction data
(
JCPDS-ICDD) library was used for identification of crys-
2.4 DEC Synthesis from Ethanol and EC
talline phases. Scherrer equation was used for the calcula-
tion of average crystalline size.
DEC synthesis reaction was carried out in 50 mL high
pressure autoclave. 1.8 g of EC (0.02 moles), 12 mL
ethanol and the catalyst (0.18 g) were charged into the
autoclave. The autoclave was purged with nitrogen so as
to remove any air present in the reactor. The reactor was
pressurized up to 4 Mpa and heated to the required tem-
perature. The ammonia was removed at regular interval
which was produced during the synthesis. The reactor
was cooled to room temperature after the completion of
the reaction followed by separation of catalyst using cen-
trifuge. The liquid mixture was analyzed using gas chro-
matograph (NETEL Michro-9100) equipped with Optima
Wax capillary column having length 30 m and internal
diameter 0.25 mm. DEC yields of products was calcu-
lated using the following formula:
To observe the textural properties such as surface
area and pore volume, liquid nitrogen (−195°C) adsorp-
tion–desorption method was used. Data were gener-
ated using Micromeritics ASAP 2020 instrument.
Brunauer–Emmett–Teller (BET) model was used for sur-
face area analysis.
Basicity measurements of the catalysts were obtained
by carrying out (CO ) temperature programmed desorp-
2
tion (TPD) analysis using Micromeritics ChemiSorb
2
720 instrument, USA. The catalysts were placed in
quartz U-tube reactor and degassed at 200°C under N
2
3
−1
flow (20 cm min ) for 2 h. For CO -TPD, Micromerit-
2
ics Chemisorb 2720 instrument with thermal conductiv-
ity detector (TCD) was used. The catalysts were placed
in quartz U-tube and pretreated at 200°C under N flow
2
Moles of product formed
3
−1
(
20 cm min ) for 2 h. CO adsorption for 1 h was fol-
Y =
× 100%
(4)
2
Moles of EC taken
lowed by desorption, whose profiles were recorded in the
range of 50–800°C with a heating rate of 10°C/min under
where, Y is the yield of DEC or NEEC.
3
−1
helium flow (20 cm min ) and the evolved CO was mon-
2
itored with a TCD.
Zn–Al–Mg–A and Zn–Al–Mg–C synthesized from first
method were also characterized by Raman spectroscopy
equipped with Olympus BX series optical microscope. The
−
1
catalysts were scanned within the range 0–1300 cm of
Raman line frequency.
1
3

Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts…
3
Results and Discussion
was also responsible for decrease in BET surface area in
the catalysts prepared by impregnation method.
3
.1 Characterization of Catalyst
Adsorption/desorption isotherm is shown in Fig. 2a. The
catalysts synthesized can be termed as mesoporous materi-
als and classified to follow type IV and V [42]. This con-
firms the presence of mesopores and absence of micropo-
res in the synthesized catalysts. The pores of Zn–Al–Mg
oxides synthesized using ammonia precipitant and NaOH
precipitant were found to possess H3 and H4 hysteresis
have slit-shaped pores with the latter having narrower one.
It can be seen that capillary condensation occurs within
3
.1.1 TG‑DTA
To analyze the nature of precursor of the catalysts, TG-
DTA study was conducted whose plot is given in Fig. S1
(
Supporting Information). TG profile seems to be quite
similar to that reported in the literature for ZnO and Al O
2
3
[
39, 40]. Weight loss below 250°C may be due to loss of
−
OH ions and dehydration of surface adsorbed water [41].
As the catalysts were thoroughly washed by double distilled
water, therefore, many ions were removed and hence, no
drastic loss in weight was observed as are usually observed
when the catalyst are prepared from sol–gel method. Usu-
ally the weight loss must be accompanied by endother-
mic peak, but the smoothness of TGA curve may be due
to crystallization of ZnO which is exothermic in nature.
No significantly weight lost was observed beyond 650°C.
Hence it was selected as the optimum calcination tempera-
ture in order to convert the precursors into their final forms.
micropores when relative pressure increases (P/P >0.65).
o
3.1.4 CO ‑TPD Analysis
2
CO -TPD curves of the catalysts are shown in Fig. 3a,
2
b. The catalysts were further categorized by their base
strength and basicities. Base strength is represented by the
desorption temperature while the quantity of CO desorbed
2
signifies the basicity of the catalyst [43]. Catalysts base
strength was categorized as weak sites (<200°C), moder-
ate sites (200°C<T<550°C) and strong sites (>550°C).
−
Weak sites are due to presence of OH groups, medium
+
−2
3
.1.2 XRD
sites are due to Mn –O while strong sites are due to
2
−
presence of O
[44, 45]. All the catalysts were found to
2
The XRD pattern of the catalysts prepared by method A
and B are shown in Fig. 1a, b. Al O was found to occur
possess strong basic sites, and high base strength, since
the quantity of Zn and Al was found to be greater than the
third metal oxide. Ternary oxides prepared by precipitation
method were found to be more basic as compared to the
catalysts prepared by impregnation method. This may be
due to blocking of some pores by third metal oxide during
impregnated into the catalyst which can also be observed
2
3
in amorphous form. The patterns were matched with ICDD
library and presence of pure phase was found to exist in
all the catalyst. Since Al O can be seen in EDX analysis
2
3
in Table 1, hence, it can be inferred that it was present in
amorphous form. Pure phases of ZnO, CaO, Y O and
2
3
La O were identified to be present in crystalline form
by witnessing shoulder peaks in CO -TPD profile of case of
2
3
2
in the catalysts (shown in Fig. 1) with the PDF-ICDD of
Zn–Al–M–B catalysts. Zn–Al–Mg was found to have maxi-
mum basicity among all ternary oxides in the range stated
above.
0
0
1-076-0704, 00-023-1009, 00-048-1467, 01-083-0927 and
1-083-1348, respectively. Most intensive peak was identi-
fied and used for the calculation of crystallite size. Crystal
size of the catalysts decreased with the insertion of third
element in the Zn–Al oxide catalyst. CaO, MgO and Y O
3.1.5 Raman Spectroscopy
2
3
were found to be in crystalline form in the catalysts pre-
pared by first method. Zn–Al–La was found to be mostly
in amorphous state as compared to others. Zn–Al–La pre-
pared from impregnation method was observed to contain
both Al O and La O in amorphous and impregnated
Raman spectra of ternary oxides of Zn–Al–Mg–A and
Zn–Al–Mg–C prepared using two different precipitating
agents (NaOH and NH ) is shown in Fig. 4. The spectrum
3
Zn–Al–Mg–A revealed two high intensity peaks at 99 and
−
1
437 cm corresponding to the E2L and E2H modes of
hexagonal ZnO, respectively. The Raman peak observed
2
3
2
3
form.
−
1
at 1065 cm is attributed to the A1(TO)+E1(TO)+E2L
multi photon scattering mode. In addition, as-prepared ter-
nary oxide also displayed few weak intensity peaks at 150,
3
.1.3 Textural Properties
−
1
Table 1 summarizes the BET surface areas of the synthe-
sized catalysts. Ternary oxides synthesized by first method
were found to possess more surface area than that pos-
sessed by catalysts prepared from second method. The
plugging of pores of Zn–Al oxide with the third element
330, 480, and 607 cm which can be assigned to 2E2L,
3E2H–E2L, E1(TO)+E2L and 2(E2H–E2L) mode, respec-
tively [46]. The presence of oxide of aluminum is evident
−
1
from a low intensity shoulder peak at 375 cm [47]. The
Raman spectrum of Zn–Al–Mg–C is similar to that of
1
3

K. Shukla, V. C. Srivastava
Fig. 1 XRD patterns of cata-
lysts prepared by a precipitation
$
$
(
Zn–Al–M–A), and b impregna-
$
*
*
$
$
* *
$
$
$
$
$
*
*
* *
*
*
tion (Zn–Al–M–B) methods
Zn-Al-Y
~
*
*
~
*
*
^
*
~
~
*
*
Zn-Al-Mg
&
&
*
* *
*
&
&
*
*
*
Zn-Al-La
*
* #
#
*
^
^
*
*
#
*
* #
*
*
^
^
^
*
Zn-Al-Ca
*
*
*
*
*
^
^
*
^
*
Zn-Al
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2
θ
*-ZnO, ^-Al
2
O
3
, #-CaO, &-La
2
O
3
, ~-MgO, $-Y
2
O
3
(a)
$
$
$
$
*
$
*
*
$
$
$
$
*
*
*
*
*
**
~
Zn-Al-Y
*
*
~
*
*
~
~
*
*
Zn-Al-Mg
*
*
*
*
*
*
*
*
*
*
* *
Zn-Al-La
#
#
#
*
*
*#
*
*
^
^
#
*
*
*
*
Zn-Al-Ca
*
*
*
*
*
*
^
^
*
Zn-Al
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
*-ZnO, ^-Al
2
O
3
, #-CaO, &-La
2
O
3
, ~-MgO, $-Y
2
O
3
(b)
−
1
Zn–Al–Mg–A except that the peak observed for multi pho-
around 2320 cm signifies the formation of Zn(NCO)
2
−1
ton scattering mode is observed at 1080 cm instead of
which is not witnessed in the regenerated catalyst. This
might have formed from the reactions 5 and 6 as suggested
by An et al. [31].
−1
1
065 cm which can be due to the difference in the nature
of precipitating agent.
NH OCOC H → HNCO + C H OH
5
2
2
5
2
(5)
3
.1.6 FTIR Analysis
ZnO + HNCO → Zn(NCO) + H O
2
2
(6)
FTIR spectra of fresh, recovered and EC interacted cata-
lysts are shown in the Fig. 5a. Peaks at 3461, 1642, 1386
The same peak is also witnessed in the reaction solu-
tion emphasizing the formation of isocyanate during the
−1
and 1115 cm as present in the fresh catalyst are also seen
in the regenerated catalysts. The regenerated catalyst also
−
1
reaction (Fig. 5b). The peak at ≈1273 cm is due to the
C–O–C stretch of dialkyl group in the reaction solution.
−1
shows peaks around 2320 and 1580 cm which are due to
isocyanate, N=C=O stretch and N–H bend of amide. Peak
−
1
The peak at ≈1033 cm shows the C–O stretch of ethanol.
1
3

Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts…
Table 1 DEC yield and selectivity using various synthesized catalysts
Catalyst
B.E.T surface DEC yield (%) NEEC
EC con-
TOF
(mg
Crystallite Elemental analysis (EDX)
size (nm)
2
gcat−1 h−1
area (m /g)
yield (%) version
%)
)
DEC
Zn
Al
M
O
(
Zn–Al
Zn–Al–Ca–A
Zn–Al–La–A
36
28
30
20.1
32.5
23.2
40.2
30.2
21.2
19.4
28.2
25.2
35.6
0.2
0.4
0.4
0.6
0.2
0.1
0.2
0.1
0.2
0.4
26.4
42.6
28.2
50.8
38.1
28.8
24.5
36.7
31.8
48.8
526.6
852.2
507.4
1055.2
792.2
555.5
508.8
747.7
661.1
934.4
47.7
39.5
18.1
19.1
16
38.7
44
28
29
–
48.11 11.67
14.99 1.11
40.51 3.5
58.41 4.19
39.88 5.75
–
40.22
28.77 55.13
28.17 27.82
0.55
7.18
Zn–Al–Mg–A 31
36.85
47.19
Zn–Al–Y–A
Zn–Al–Ca–B
Zn–Al–La–B
Zn–Al–Mg–B 21
Zn–Al–Y–B 18
28
15
18
33.68 10.11 10.47 45.74
60.94 9.35
69.77 4.76
54.36 8.64
11.17 18.54
–
7.18
–
25.47
29.82
–
Zn–Al–Mg–C 28
–
–
Fig. 2 a Liquid nitrogen
0
.14
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
adsorption–desorption isotherm,
and b pore size distribution of
the Zn–Al–Mg catalysts
Zn-Al-Mg NN HH 33 pp rr ee cc i ipp i it taa nn tt
Zn-Al-Mg NaOH precipitant
Zn-Al-Mg NN HH 3 pp rr ee cc ii pp ii ttaa nn tt
3
0.12
Zn-Al-Mg NaOH precipitant
0
0
0
0
.10
.08
.06
.04
0.02
.00
0
0
0
.00
0.50
1.00
0
1000
Pore diameter (Å)
2000
3000
Relative Pressure (P/Po)
(a)
(b)
3
.1.7 Surface Morphology and Elemental Analysis
3.2 Catalytic Activity
The surface morphologies and elemental analysis were
studied using AFM technique. Surface elemental com-
positions of various catalysts are shown in Table 1.
Zn–Al–Mg–A was the best performing catalyst among
all the prepared catalysts. Hence, it was chosen to study
the surface morphology by AFM technique, the size of
fresh and regenerated Zn–Mg–Al–A catalyst after the
third reaction. The distribution of catalysts sizes were
studied over whole surface of the catalysts. It can be
seen from Fig. 6a, b, that the catalysts size was not as
evenly distributed in the recovered catalyst as it was in
fresh catalyst. Catalyst agglomeration was also slightly
observed after recovery of the catalyst for the third time
Basic strength and basicity have profound effect on DEC
yield and hence have been examined carefully. Basic
strength, a measure of temperature up to which a site can
retain CO molecule, and basicity, a measure of the basic
2
sites present on the catalyst, can be seen from Fig. 3 and
Table 2. All the catalysts possessed mild and strong basic
sites. The sites are mostly strong as they desorbed CO at
2
temperature greater than 400°C. Table 1 summarizes the
results of the screening study of the catalysts. DEC yields
obtained were seemed to be dependent on the basicity of
the catalysts. Zn–Al–M–A catalysts were found to give bet-
ter yield than the Zn–Al–M–B type catalysts. This may be
due to higher basicities of ‘A’ series of catalysts as com-
pared to ‘B’ type catalysts. Plugging of pores was found to
(
as seen in Fig. 6c).
occur in ‘B’ type catalysts. CO -TPD clearly shows shoul-
2
der peak which is attributed to the impregnation of third
metal oxide within the pores of the catalysts.
1
3

K. Shukla, V. C. Srivastava
ο
o
ο
ο
ο
ο
ο
1
50
C
462
C
522
C
666 C 722 C 817
C
929
C
Zn-Al-Y
Zn-Al-Mg NaOH precipitant
6
07
1
065
Zn-Al-Mg
Zn-Al-La
Zn-Al-Ca
9
9
Zn-Al-Mg NH3 precipitant
3
30 375
480
1
080
0
100
200
300
400
500
600
700
800
900
1000 1100 1200 1300
-
1
Raman line frequency (cm )
1
00 200 300 400 500 600 700 800 900
ο
Temperature ( C)
(
a)
Fig. 4 Raman spectra of Zn–Al–Mg catalysts synthesized by A
method using NaOH and liquid ammonia as precipitants
o
o
660 C 738
C
790
o
o
o
C
850
o
C
150
C
400
C
Zn-Al-Y
catalyst. ZnO and MgO were found mostly in crystalline
form as compared to Al O . Activation of ethanol occurs
Zn-Al-Mg
2
3
at the oxygen site of the oxides. Carbonyl group of ethyl
carbamate is adsorbed at the metal sites of the catalysts.
The ethoxy group attacks the electrophilic site of carbon
Zn-Al-La
to replace NH group with it thereby producing DEC and
2
NH .
3
3
.2.1 Effect of Operating Parameters
Zn-Al-Ca
The effect of temperature (453–483 K) on DEC yield and
EC conversion was studied (Fig. 7a). The conversion of EC
and yield of DEC increased up to 463 K and remained the
same at 483 K. Theoretically equilibrium constants of DEC
synthesis from EC, varying with temperature, was also
studied. Standard molar entropy change of DEC and EC
was estimated using the Benson group contribution method
1
00 200 300 400 500 600 700 800 900
ο
Temperature ( C)
(
b)
Fig. 3 CO -TPD profiles of Zn–Al–M catalysts prepared by a pre-
2
cipitation and b impregnation method
[48]. The coefficients of heat capacity of the reaction and
Yield obtained with Zn–Al–Mg–C was close to
Zn–Al–Mg–A. It means the effect of precipitant doesn’t
affects the yield and DEC selectivity substantially. This can
also be seen from Fig. 4, which shows almost same Raman
profile suggesting almost equal defects created in the cata-
lysts. The decomposition of EC as suggested by Eqs. 5 and
products components were estimated using Ruzicka and
Zabranski model [49]. Equilibrium constant was found to
be increase abruptly after 150°C as shown in the Fig. S3a.
Hence it is advisable to conduct reaction beyond this tem-
perature. EC conversion was found to be increase with an
increase in temperature but DEC yield was almost the same
for 190 and 200°C as shown in the Fig. 7a. This may be
due to the increase in rates of side reaction leading to the
formation of N-EEC.
6
, which was also witnessed in the FTIR spectra, leads
to the formation of HNCO and ethanol. This contributes
to the loss in mass of raw material as undesired products
(
Table 1). Zn–Al–Mg–A was found to be the best after
The effect of time on the DEC yield was studied in the
range of 1–6 h and the results are shown in the Fig. 7b. The
yield of DEC and EC conversion increased up to 5 h and
thereafter they were constant. This may be attributed to the
reaction approaching equilibrium. Increasing the time will
deprive TOF of the reaction and hence 5 h was chosen to be
the optimum time for DEC synthesis.
screening among various catalysts. Hence it was chosen to
study the effect of different operating conditions.
The general mechanism of the reaction, proposed for
Zn–Al–Mg–A, is shown in Fig. S2 (supporting infor-
mation). It can be seen from Fig. 1a, all the three oxides
(
Zn, Al and Mg) were found to co-exist in the synthesized
1
3

Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts…
Fig. 5 FTIR spectra of a Zn–Al–Mg fresh, regenerated and interacted catalyst, b EC and reaction solution
The effect of catalyst wt (by weight% of EC) was studied
in the range of 5–15. Figure 7c shows that the conversion
of EC increased abruptly when catalyst concentration was
increased from 5 to 10 wt%. On further increasing the cata-
lyst concentration, change in conversion of EC and yield of
DEC was very low. This may be attributed to the reaction
1
3

K. Shukla, V. C. Srivastava
(
a)
(b)
2
2
2
1
1
1
1
500
250
000
750
500
250
000
Zn-Al-Mg regenerated
7
5
2
50
00
50
0
Zn-Al-Mg fresh
0
.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
Diameter (nm)
(
c)
Fig. 6 a AFM—image of Zn–Al–Mg precipitation fresh catalyst, b AFM—image of Zn–Al–Mg precipitation regenerated catalyst and c cata-
lysts size of fresh and regenerated Zn–Al–Mg precipitation catalyst
Table 2 CO -TPD classification of various catalysts
2
Catalysts
Basicity or amount desorbed CO (mmol g−1)
2
Weak sites (<200)°C
Medium sites (200–550)°C
Strong sites (>550)°C
Total basicity
Zn–Al–Ca–A
Zn–Al–La–A
Zn–Al–Mg–A
Zn–Al–Y–A
ZnAl–Ca–B
Zn–Al–La–B
Zn–Al–Mg–B
Zn–Al–Y–B
0.04 (150°C)
0.04 (150°C)
0.04 (150°C)
0.04 (150°C)
0.04 (150°C)
0.04 (150°C)
0.03 (150°C)
0.04 (150°C)
0.02 (462 and 522°C)
0.06 (462°C)
0.06 (522°C)
0.06 (522°C)
0.06 (400°C)
0.02 (400°C)
0.02 (400°C)
0.02 (400°C)
0.15 (666 and 817°C)
0.21
0.2
0.13 (666, 722, 929°C)
0.15 (666, 722, 817 and 929°C)
0.14 (666, 722, 817 and 929°C)
0.13 (660 and 850°C)
0.11 (738 and 790°C)
0.13 (660 and 790°C)
0.25
0.24
0.23
0.17
0.18
0.11
0.05 (660 and 790°C)
approaching thermodynamic or equilibrium conversion.
Hence, 10 wt% was chosen to be the optimum catalyst con-
centration for DEC synthesis from EC.
to be better than others when DEC yield was considered.
This optimum ethanol/EC ratio was found to be similar
to that reported in the literature [16]. Lower ethanol/EC
ratio increases EC concentration and hence it increases
the N-ethylation of EC. By raising the ethanol/ EC above
The effect of ethanol/EC ratio was studied in the range
of 5–20 (Fig. 7d). Clearly, ethanol/EC ratio of 10 was found
1
3

Alkaline Earth (Ca, Mg) and Transition (La, Y) Metals Promotional Effects on Zn–Al Catalysts…
8
7
6
5
4
0
0
0
0
0
1200
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
1200
1
8
000
00
1
8
000
00
600
400
200
0
600
30
20
10
0
4
2
0
00
00
440
450
460
470
480
1
3
5
time (h)
Temperature (K)
(a)
(b)
8
0
1200
60
1200
1000
800
600
400
200
0
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1
8
000
00
50
40
30
20
10
0
600
4
2
0
00
00
2
7
12
Catalyst wt.
17
2
7
12
17
22
%
Ethanol/EC ratio
(c)
(d)
Fig. 7 Effect on operating parameters on DEC yield (blue diamond), EC conversion (red square) and TOF (green triangle) using Zn–Al–Mg–A
catalyst. a Temperature (443–473 K), b time (1–6 h), c catalyst weight (5–15 wt%), and d ethanol/EC ratio (5–20)
optimum value, the reaction will be pushed to the product
side by Le Chatelier principle. At too higher ethanol/EC
ratio, the catalytic sites get covered with ethanol and hence
can’t be accessed by EC.
be attributed to the loss of zinc from the catalyst which can
be observed from the FTIR of the regenerated catalyst.
4
Conclusion
3
.2.2 Catalysts Regeneration and its Reusability
Zn–Al–M (M=Ca, Mg, La and Y) based ternary oxides
were synthesized using two methods and their activity
was studied for synthesis of DEC from EC. BET surface
area of the catalysts prepared by the precipitation method
was found to be more than that prepared by the impregna-
tion method. This may be due to the plugging of pores of
Zn–Al oxide by third element in the impregnation method.
All the catalysts were found to posses strong basic sites.
Yield of DEC was found to be in relation to the basicity
of the catalysts. The ternary metal oxides prepared from
the precipitation method were found to be more active
Zn–Al–Mg–A was the best catalyst for the synthesis of
DEC from EC owing to its high basicity and high surface
area. The catalyst was successfully regenerated, even after
third time, which can be shown in Fig. S3b. Almost same
peaks, as that of fresh Zn–Al–Mg–A, were obtained in
XRD and FTIR. Hence its activity was studied after regen-
eration. The catalyst was observed to be reusable for three
−
1
−1
times with TOF of 1055.2, 1050 and 998 mg
g
h
DEC cat
in subsequent experimental runs. Little loss in activity can
1
3

K. Shukla, V. C. Srivastava
than the impregnation method. Zn–Al–Mg was found to
be most active catalyst owing to its highly basic character.
The activation of EC by catalyst was studied by carrying
out FTIR spectra of EC interacted catalyst. Zn–Al–Mg
18. Fan M, Zhang P, Ma X (2007) Fuel 86:902–905
1
9. Zhang P, Zhou Y, Fan M, Jiang P (2015) Appl Surf Sci
3
32:379–383
2
0. Zhang P, Zhou Y, Fan M, Jiang P (2014) Appl Surf Sci
295:50–53
catalyst prepared using liquid NH as precipitant was also
21. Ma X, Fan M, Zhang P (2004) Catal Commun 5:765–770
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2. Pimprom S, Sriboonkham K, Dittanet P, Föttinger K, Rupprech-
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Zn–Al–Mg prepared from precipitation method was found
to be reusable three times with only 5% decline in its activ-
ity. Overall, DEC yield of 40.2% and turn over frequency
2
24. Fan M, Zhang P (2007) Energy Fuel 21:633–635
2
5. Kumar P, With P, Srivastava VC, Shukla K, Gläser R, Mishra IM
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2016) RSC Adv 6:110235–110246
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−1
(
TOF) of 1055 mg
g
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