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Research Article
pubs.acs.org/acscatalysis

Rational and Statistical Approaches in Enhancing the Yield of
Ethylene Carbonate in Urea Transesterification with Ethylene Glycol
over Metal Oxides
Dina Fakhrnasova,†,‡ Ricardo J. Chimentao,‡,§ Francisco Medina,‡ and Atsushi Urakawa*,†
̃
†

Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
Departament d’Enginyeria Quı ́mica, Universitat Rovira i Virgili, 43007 Tarragona, Spain
§
School of Chemistry, Yachay Tech, Yachay City of Knowledge, 100119 Uruqui, Ecuador
‡

S
* Supporting Information

ABSTRACT: This work employs (i) a rational approach to improve the material
properties of catalysts for urea transesterification with ethylene glycol (EG) to ethylene
carbonate (EC) over metal oxides and (ii) a statistical approach to maximize the desired
product. For the rational approach, single- and mixed-metal oxides with different
elemental combinations (Zn, Mg, Al, Fe) with a variety of acid−base properties were
synthesized and evaluated for the reaction. The roles of acidity and basicity in the
identified reaction paths were clarified on the basis of product selectivities and kinetic
parameters extracted from the concentration profiles of reactants and products in every
reaction path by means of in situ IR monitoring and subsequent multivariate analysis. The
paths toward EC are favorably catalyzed by acidic sites, while basic sites catalyze all paths
toward undesired products. However, the surface sites are blocked when the acidity is too
high and there exists an optimum value for the ratio of total acidic and basic sites to be an
efficient catalyst in the targeted reaction. A mixed-metal oxide consisting of Zn and Fe at a
3:1 atomic ratio was found to be the optimum catalyst with a well-balanced acid−base
property. Furthermore, a design of experiments (DoE) approach was used to statistically identify critical reaction parameters and
optimize them for the best Zn- and Fe-containing catalysts. These approaches successfully gave insights into the determining
material factors for the reaction and afforded excellent EC selectivity (up to 99.6%) with high yield.
KEYWORDS: mixed-metal oxides, transesterification, urea, ethylene glycol, ethylene carbonate, acid−base property,
reaction mechanism, multivariate curve resolution, kinetics, DoE

1. INTRODUCTION
Organic carbonates are a family of commercially important
chemicals widely employed as intermediates1 for a variety of
synthetic and industrial applications and also as solvents.2
Ethylene carbonate (EC) is a reactive starting and intermediate
chemical3 for selective alkoxylation,4 carbamate formation,5 and
transesterification6 to produce glycerol carbonate (GC)7 and
linear carbonates such as dimethyl carbonate (DMC),8 which
has drawn great attention as a green solvent, a phosgene
replacement, and a fuel additive.9
DMC can be obtained in high yield via the transesterification
reaction of EC with methanol. In this reaction, an equimolar
amount of ethylene glycol (EG) is produced along with DMC
(Scheme 1). This feature could be disadvantageous because of
the different market demands for these two products. One
approach to balance the amount of the two products at a
desired ratio is to convert EG back to EC via transesterification
of EG with urea, where NH3 is coproduced. The EC synthesis
in this route is advantageous for its mild operating pressure, low
toxicity, and ease of handling of starting chemicals in addition
to its high EC yield and selectivity, in comparison to the
commercialized path of EC production by cycloaddition of
© XXXX American Chemical Society

Scheme 1. Urea Transesterification with EG To Produce EC
and To Close the Cycle of DMC Synthesis

CO2 with ethylene oxide.10 In addition, the transesterification
of EG with urea, synthesized from CO2 and NH3, can be
considered as one path for indirect CO2 utilization (Scheme 1).
Received: July 25, 2015
Revised: September 13, 2015

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with deionized water, and dried at 373 K overnight. The
resultant dried materials with the hydrotalcite structure were
calcined at 723 K for 12 h to obtain the corresponding mixedmetal oxides. The synthesis of the single-metal oxides (ZnO,
Al2O3, MgO, Fe2O3) followed the same protocol but used the
aqueous solution of the respective nitrate precursor. Concerning the synthesis of the Zn2+-Fe3+ mixed-metal oxide, an
aqueous solution of Na2CO3 (2 M) was used as the second
solution because the resulting material after drying did not form
a hydrotalcite structure when the NaOH solution was used.
The precipitated solid was washed, dried, and calcined as
described above. Hereafter, Mg2+-Al3+, Mg2+-Fe3+, Zn2+-Al3+,
and Zn2+-Fe3+ mixed-metal oxides after calcination of the
corresponding materials with hydrotalcite structures are
denoted simply Mg−Al, Mg−Fe, Zn−Al, and Zn−Fe,
respectively, or with “mixed oxide” after the simplified notation.
Sodium hydroxide (Alfa Aesar, 97%), zinc nitrate hexahydrate (Sigma-Aldrich, 98%), magnesium nitrate hexahydrate
(Sigma-Aldrich, 98.0−102.0%), aluminum nitrate nonahydrate
(Sigma-Aldrich, 98%), iron(III) nitrate nonahydrate (Alfa
Aesar, 98.0−101.0%), sodium carbonate (Sigma-Aldrich,
99%), ethylene glycol (Alfa Aesar, 99%), urea (Alfa Aesar,
99−100.5%), 2-hydroxyethyl carbamate (Angene, 95%), ethylene carbonate (Sigma-Aldrich, 98%), and 2-oxazolidinone
(Sigma-Aldrich, 98%) were used as received.
2.2. Catalyst Characterization. The surface area of the
synthesized metal oxides was determined by N2 physisortion
(77 K) with the BET method using Autosorb 1-MP
(Quantachrome) equipment. Acid−base properties of the
materials were characterized by temperature-programmed
desorption (TPD) of adsorbed CO2 and NH3, and the
procedure was as follows. A sample (100 mg) was pretreated
for 2 h at 723 K in He, cooled to 353 K, saturated with a gas
containing the probe molecule (4.5% CO2/5% NH3 in He),
and flushed afterward with He for 20 min. Subsequently, TPD
measurements were performed at a heating rate of 20 K min−1
under He flow (20 mL min−1) and the quantity of desorbed
gases was recorded by a TCD detector in TPDRO 1100
(Thermo Fisher Scientific). The amount of basic and acidic
sites was calculated from the CO2 and NH3 peaks, respectively,
after deconvolution of comprising desorption peaks using the
PeakFit v4 software. It should be noted that single- and mixedmetal oxides are generally exposed to the reactive components
of air after the synthesis and during the storage. For instance,
Mg-containing materials readily adsorb and react with H2O and
CO2 to form magnesium hydroxide and carbonate, affecting the
acid−base properties of the materials.16 Therefore, it was
important to treat the samples under a controlled atmosphere
before the reaction and characterization (in our case hightemperature treatment at 723 K for 2 h in He) to remove the
majority of the surface carbonates, as discussed later.
Powder X-ray diffraction (XRD) experiments were performed on a Bruker AXS D8 Advance diffractometer equipped
with a Cu tube, a Ge (111) incident beam monochromator (λ
1.5406 Å), and a Vantec-1 PSD operated in transmission mode.
Identification of the crystal phases was made by comparing with
the reference data available in the Joint Committee on Powder
Diffraction Standards (JCPDS).
2.3. Catalytic Test and in Situ IR Monitoring. Transesterification of urea with EG as well as reference reactions
starting from the intermediates and products identified in the
transesterification reaction were performed in a 25 mL threenecked flask equipped with a magnetic stirrer and a reflux

This reaction has been reported to proceed in two steps
(Scheme 1).11 In the first step, urea reacts with EG to yield 2hydroxyethyl carbamate (2-HC), and in the second step EC is
formed by the elimination of NH3 from 2-HC. The major
challenge is to maximize the selectivity toward EC by avoiding
side reactions such as the dehydration of 2-HC, forming the
dominant side product, 2-oxazolidone (2-Ox).
Various solid catalysts have been investigated in the
transesterification of EG with urea. Su and Speranza12 described
reactions of alkylene glycols and urea to synthesize alkylene
carbonates using Sn-containing catalysts. More recently, a
variety of metal oxides have been investigated in the reaction.
Li13 has tested CaO, La2O3, MgO, ZnO, ZrO2, and Al2O3 and
found that ZnO is the most active and selective catalyst. An
identical conclusion was drawn by Bhanage.14 Bhadauria15
studied Cr-containing zinc oxide materials, while Zhao3
investigated Fe-containing materials. In both studies the
formation of the spinel phases (ZnCr2O4 and ZnFe2O4) was
found to be important, promoting EC yield. The previous
works agreed that a proper balance in the amount and nature of
acidic and basic sites is important; however, their roles in each
reaction step and in the overall reaction performance, i.e. yield
of EC, have been unclarified.
To gain precise insights into the roles of acidic and basic sites
in the reaction, we investigated the catalytic performance of
single- and mixed-metal oxide materials consisting of Zn, Mg,
Al, and/or Fe with distinct acid−base properties. Binary mixedmetal oxides were prepared via hydrotalcite precursors to
achieve high surface area and homogeneous mixing of the
resulting, typically nanosized metal oxide phases after
calcination treatment. The reaction network influencing the
yield of EC was identified, and every reaction path in the
network was separately studied. The catalytic tests were
performed with simultaneous monitoring of concentration
evolutions of chemical species by means of a dip-in IR probe
and multivariate analysis to extract kinetic information on every
reaction path. The kinetic parameters, the yield of products,
and acid−base properties of the materials determined by NH3-/
CO2-TPD, respectively, were holistically evaluated to elucidate
the function of acid−base sites on the identified reaction paths,
giving a comprehensive view of the strategies to rationally
maximize EC yield. In addition, optimization of the reaction
conditions was attempted with the best-performing catalyst by
applying a contrasting method, namely a statistical method, the
design of experiments (DoE). The results emphasize the
usefulness of such mathematical approaches to maximize the
selectivity and/or the yield of the target product influenced by
each path in the complex reaction network.

2. EXPERIMENTAL SECTION
2.1. Catalysts and Materials. Mixed-metal oxides with the
general formula Mb2+-Mt3+ containing Mg2+ or Zn2+ as a
bivalent cation (Mb2+) and Al3+ or Fe3+ as a trivalent cation
(Mt3+) with constant molar ratio Mb2+/Mt3+ = 3 were
synthesized by a coprecipitation method. Mg2+-Al3+, Mg2+Fe3+, and Zn2+-Al3+ mixed-metal oxides were synthesized as
follows. An aqueous nitrate solution (100 mL) with an
appropriate amount of the bivalent and trivalent cations was
added dropwise into a beaker containing 100 mL of deionized
water under vigorous stirring at room temperature. The pH of
the solution was kept constant at 10 by adding a second
aqueous solution of NaOH (2 M). The resultant slurry was
aged for 18 h at room temperature, filtered, thoroughly washed
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3. RESULTS AND DISCUSSION
3.1. Catalyst Characterization. 3.1.1. XRD. The ordered
layer structure characteristic of hydrotalcite was confirmed for
all precursors of the mixed-metal oxides (Mg-Al, Mg-Fe, Zn-Al,
Zn-Fe) examined in this work (XRD patterns are presented in
Figure S1 in the Supporting Information). After calcination at
723 K in air, the hydrotalcite structure was destroyed and new
metal oxide phases were formed (Figure S2 in the Supporting
Information). The calcined materials showed the presence of
oxides of bivalent cations (Zn2+ and Mg2+) as the major
crystalline phase (hexagonal wurtzite or cubic periclase
structure, respectively; Table 1) due to a ratio of 3 for Mb2+/

condenser. For the urea transesterification with EG, the reactor
was initially charged with 89.51 mmol of EG and heated to a
reaction temperature of 413 K. Subsequently, 8.33 mmol of
urea and 0.16 g of catalyst (equivalent to 3 wt % of EG) were
added to the solution and this point was considered as the
starting point of the reaction. Catalyst material was pretreated
at 723 K for 2 h under an He flow prior to the catalytic testing,
and it was readily taken to the reactor to minimize exposure to
the air. All reactions were performed for 6 h under an N2 flow
(0.5 L min−1) to remove formed gaseous ammonia, which
negatively affects the reaction performance. After 6 h of the
reaction, the reactor was cooled to room temperature, the solid
catalyst was separated from the solution by filtration, and the
products in the solution were analyzed by GC (Agilent 6890N)
equipped with an FID detector and an HP-35 capillary column
and GC-MS (Agilent 6890N (GC) 5973 (MSD)) equipped
with an HP-5 capillary column. In the case of the reactions
starting from intermediates and products, 4.28 mmol of 2-HC,
5.11 mmol of EC, or 5.17 mmol of 2-Ox was charged into the
reactor instead of urea, and the reactions were performed under
identical conditions.
Product selectivity was calculated without taking into account
the amount of 2-HC because other products are derived from
2-HC and neglecting 2-HC selectivity helps understand the
preferred paths of the reactions. For the reaction of urea
transesterification with EG as well as the reaction of 2-HC in
EG, product selectivity was determined by the formula

Table 1. BET Surface Area, Amount of Acidic and Basic
Sites, and Identified Crystalline Phases of the Single- and
Mixed-Metal Oxides
catalyst

BET surface
area/m2 g−1

CO2 uptake/
μmol g−1

NH3 uptake/
μmol g−1

Al2O3
ZnO
MgO
Fe2O3
Zn-Al
Zn-Fe

294
4
165
25
36
21

2.88
0.31
6.27
1.00
1.40
0.44

0.61
0.01
0.02
0.01
0.07
0.06

Mg-Al
Mg-Fe

173
113

6.22
3.22

0.22
0.15

crystalline phase
γ-Al2O3
wurtzite
periclase
hematite
ZnO (wurtzite)
ZnO (wurtzite),
ZnFe2O4
MgO (periclase)
MgO (periclase)

Sproduct = [product]/([EC] + [2‐Ox] + [DEG] + [TEA]

Mt3+ in the hydrotalcite structure. The Zn−Fe mixed oxide
exhibits an additional phase of zinc franklinite (ZnFe2O4) as
reported by Zhao et al.3 They observed the phase for Zn−Fe
mixed oxides prepared by mechanical mixing of the
corresponding metal precursors prior to a calcination treatment.
3.1.2. Textural Property. Specific surface areas of the metal
oxides are presented in Table 1. The order of the values was
Al2O3 > MgO > Fe2O3 > ZnO for the single-metal oxides. For
the mixed-metal oxides, the major contributions made by the
bivalent ions Zn2+ and Mg2+ to the surface areas were obvious.
The trends in lowering or enhancing surface area given by the
trivalent ions Al3+ and Fe3+ were also clearly visible and
consistent with those observed for the corresponding singlemetal oxides.
3.1.3. CO2- and NH3-TPD. The strength and the total
amount of basic and acidic sites of the single- and mixed-metal
oxides were characterized by TPD of CO2 and NH3,
respectively. The amount of desorbed gases, corresponding to
the amount of basic and acidic sites, is reflected in the peak
area, whereas the desorption peak temperature indicates the
strength of acidic and basic sites. Figure 1 presents the
desorption profiles of CO2 and NH3 from the thermally treated
catalysts (in He at 723 K). The quantified results are
summarized in Table 1. TPD was performed up to 723 K,
which is the same temperature as that used in the pretreatment
before catalytic testing in order to characterize the relevant
acidity and basicity of the catalysts in the reaction.
All metal oxides showed CO2 desorption peaks with maxima
at ca. 453 K. The Mg-containing materials MgO, Mg-Al, and
Mg−Fe, exhibited a broad CO2 desorption peak over a wide
temperature range of 373−673 K with a maximum at 473 K,
indicating the presence of a large number of basic sites with
different strengths (weak, moderate and strong sites).18 Zn−Al,

+ [3‐(2‐EtOH)‐2‐Ox]) × 100

In case of the reaction of 2-Ox with EG
Sproduct = [product]/([2‐Ox] + [TEA]
+ [3‐(2‐EtOH)‐2‐Ox]) × 100

was used to obtain product selectivity. The yield of EC was
expressed by the molar amount of EC produced per gram of
catalyst due to inaccurate quantification of EG and urea caused
by the excess of EG used and possible decomposition of urea
into NH3 and CO2 during the reaction.
In all of the reactions examined in this work, the reaction
mixture was also monitored with an in situ dip-in attenuated
total reflection infrared (ATR-IR) spectroscopic probe (Mettler
Toledo React-IR 4000 equipped with a diamond ATR crystal)
by immersing the probe into the reaction solution. Spectra were
recorded every 120 s at 4 cm−1 resolution.
Furthermore, a multivariate analysis method, multivariate
curve resolution (MCR),17 was employed for the analysis of the
time-resolved IR data. MCR is a very powerful blind source and
spectral separation method and allows disentangling overlapping spectral features on the basis of the kinetic resolution of
comprising underlying spectral bands. The results of MCR
analysis are the kinetically pure component spectra and their
concentration profiles. This way of spectral analysis is
significantly more precise in evaluating concentration profiles
and kinetics in comparison to taking such information from the
absorption band maxima because the latter method suffers from
the overlaps of bands, which is common in IR spectroscopy and
often leads to inaccurate extraction of kinetic parameters and
incorrect data interpretations. The detailed procedure of the
MCR and kinetic analysis used in this work is described in the
Supporting Information.
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Figure 1. (a) CO2-TPD and (b) NH3-TPD profiles of the metal oxides.

Table 2. Product Selectivity in Urea Transesterification with Ethylene Glycola
catalyst

YEC/(mmol gcat−1)

SEC/%

S2‑Ox/%

SDEG/%

S3‑(2‑EtOH)‑2‑Ox/%

STEA/%

blank
Al2O3
ZnO
MgO
Fe2O3
Zn-Al
Zn-Fe
Mg-Al
Mg-Fe

8.6
8.6
18.4
19.4
8.3
20.1
25.9
1.5
11.1

86.5
96.4
73.7
83.2
88.1
82.0
91.5
12.1
64.9

9.5
2.9
15.9
8.3
4.6
9.7
5.1
48.8
15.3

2.6
0
5.1
4.1
7.3
4.0
3.5
21.8
13.3

1.5
0
3.5
3.0
0
2.8
0
9.1
4.3

0
0
1.8
1.5
0
1.6
0
8.2
2.3

a

Only products formed after 2-HC in the reaction paths have been considered (see Scheme 2; thus, the amount of 2-HC was not taken into account
in the selectivity calculation). Reactions were performed at 413 K for 6 h starting with 89.51 mmol of EG, 8.33 mmol of urea, and 0.16 g of catalyst
or without catalyst (blank) under N2 flow (0.5 L min−1).

3.2. Urea Transesterification with EG. The chemical
components of the reaction mixture were identified by means
of GC-MS and quantified by GC analysis. Table 2 gives the
product selectivity in the absence of a catalyst (blank) and in
the presence of the metal oxides. In addition to the major
reaction paths and products generally reported3,13,14 and
described in Scheme 1, there were three other products
identified. The first one was diethylene glycol (DEG), formed
via the secondary ring-opening reaction of EC with EG
accompanying a release of CO2 (box B in Scheme 2). The
other two products found in this work were 3-(2hydroxyethyl)-2-oxazolidinone (3-(2-EtOH)-2-Ox) and triethanolamine (TEA) via the reaction of 2-Ox with EG (box C in
Scheme 2). Bhanage14 reported secondary product formation
possibly from reactions of EC and 2-Ox with EG, yielding 1-(2hydroxyethyl)-2-imidazolidone and 3-(2-EtOH)-2-Ox. In addition, in a rare case a minor amount of ethylene urea (2imidazolidone) was also found.14 In this work, 1-(2hydroxyethyl)-2-imidazolidone and ethylene urea were not
detected, and instead, DEG and TEA were identified for the
first time to the best of our knowledge. As shown later, the
reaction paths toward DEG from EC (box B) and 3-(2-EtOH)2-Ox and TEA from 2-Ox (box C) shown in Scheme 2 were
verified by testing the reactions starting from EC and 2-Ox with
EG, respectively.

ZnO, and Al2O3 showed narrower desorption peaks, compared
to the Mg-containing materials, with the maxima at slightly
lower temperatures of ca. 443 K. The differences indicate
weaker and less types of available surface basic sites for Zn−Al,
ZnO, and Al2O3. In contrast, CO2 desorption profiles of Zn−Fe
and more notably of Fe2O3 showed an additional peak at a
higher temperature of ca. 683 K, implying the presence of
relatively strong surface basic sites in addition to the weaker
sites for these two Fe-containing materials.
An NH3-TPD study of these materials clearly showed that
Al2O3, with the largest and broadest peaks of all samples
examined, has highly acidic character with different (weak and
strong) types of acidic sites. In contrast, other single-metal
oxides (MgO, ZnO, and Fe2O3) showed very small or
negligible NH3 desorption peaks, indicating poor availability
of surface acidic sites. However, it is interesting to observe that
when Al3+ or Fe3+ cations are introduced to ZnO (i.e., Zn-Al,
Zn-Fe), the acidity and basicity became more prominent, as
confirmed by a larger peak of NH3 and CO2 desorption (Figure
1 and Table 1). Similar enhanced acidity and basicity by
incorporation of foreign metal ions into the metal oxide
structure have been reported.19 Also, the total amount of acidic
sites of MgO was improved by incorporation of Al3+ and Fe3+
cations (i.e., Mg-Al, Mg-Fe). These results show that
introducing metal cation(s) to a single metal oxide can
enhance surface acid−base properties.
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Scheme 2. Urea Transesterification with Ethylene Glycol: Identified Reaction Pathways

Figure 2. In situ IR monitoring of urea transesterification with EG and subsequent MCR analysis.

tested with a clearer product distribution: only 2-Ox as the
single side product.
The best catalytic performance in terms of EC yield was
obtained for Zn-Fe, showing 25.9 mmol gcat−1. The material
effectively suppressed the formation of 2-Ox (5.1%) and the
secondary reaction products, DEG, 3-(2-EtOH)-2-Ox, and
TEA (the formation of the last two products was fully
suppressed). The other Zn-containing mixed-metal oxide, ZnAl, also showed high EC yield, but there was a noticeable
increase in selectivity toward 2-Ox as well as secondary reaction
products.
Furthermore, in situ IR monitoring of the reaction solution
together with GC and MCR spectral analyses (Figure 2) was
employed to investigate the reaction pathways and kinetics.
The MCR analysis facilitated the extraction of comprehensive
and quantitative concentration profiles of the major reactants,
intermediates, and products during the course of the reaction
for different catalyst materials. Performing the IR monitoring
with the help of secondary reaction studies (testing the reaction
from intermediate products) and product identification

It is important to note that the reaction could proceed
without catalyst to a moderate extent with a relatively high EC
selectivity of 86.5% with 8.6 mmol gcat−1 EC yield and low but
considerable (ca. 10%) selectivity to 2-Ox in addition to minor
formation of DEG and 3-(2-EtOH)-2-Ox under the reaction
conditions of this work after 6 h (Table 2). In contrast, Li13
reported that without catalyst no 2-Ox was found and the EC
yield was relatively low after 3 h of the reaction at 423 K under
vacuum (11 kPa) starting from 0.75 mol of EG and 0.5 mol of
urea with 0.9 g of catalyst.
The type of metals in the catalyst and how the metal ions are
combined in the mixed-metal oxides drastically affected both
EC yield and product selectivity (Table 2). Regarding EC
selectivity, most materials showed relatively high values except
for Mg-containing mixed-metal oxides (Mg-Al and Mg-Fe). For
these oxides, the selectivity to all undesired side products was
very high, reaching even 48.8% 2-Ox for Mg-Al. In contrast,
MgO showed high EC selectivity (83.2%). This indicates that
Mg is not the sole factor to lower the EC selectivity. Al2O3
showed the highest EC selectivity (96.4%) of all materials
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of metal oxides) via coordination of O atoms of the carbonyl
group. Through such strong interactions of surface acidic sites
and urea, surface catalytic sites might be poisoned for further
reactions. This well explains the observed lower reaction rate of
urea (Figure 3) when the catalyst contains more and stronger
acidic sites.
3.3. Reaction of EC with EG. As described earlier, the
desired target product (EC) reacts with EG, forming DEG
accompanying a release of CO2. This reaction takes place even
in the absence of catalyst (Table 2) and should be avoided.
Although the excess of EG present in the system as solvent
makes prevention of this reaction path difficult, it is possible to
fully suppress the formation of DEG as observed for Al2O3
(Table 2). In order to understand the nature of the reaction in
light of the acidity and basicity of the catalysts, we have
investigated the reaction path (box B, Scheme 2) by means of
catalytic testing combined with in situ IR monitoring for all the
catalysts.
The kinetic analysis of the EC concentration clarifies that EC
conversion is apparently a first-order reaction and the rate
constant was calculated assuming the reaction order (Figure S5
and Table S5 in the Supporting Information). Among the
different possibilities, good correlation between EC conversion
and its rate constant k and the amount of basic sites was found.
Figure 4 shows the plots of (a) EC conversion and (b) EC
conversion rate constant against the total amount of basic sites
determined by CO2-TPD.
The EC conversion and its rate showed similar trends, as
expected. Clearly, the reactivity of EC with EG increases when
more basic sites are available on the catalyst surface. The
materials with a small number of basic sites, e.g. Zn-containing
materials, showed low activity in the reaction, whereas strongly
basic Mg-containing materials exhibited high reactivity, even
above 90% EC conversion to DEG using MgO. It is interesting
to note that Fe2O3 showed high activity in the reaction,
although it has a small amount of basic sites. The origin of the
high activity might be attributed to the presence of strong basic
sites, as demonstrated by CO2-TPD analysis (Figure 1a). Also,
Al2O3 showed small but some activity in the reaction, although
DEG was not observed for the reaction starting from EG and
urea as mentioned above. This may be attributed to the
modification of acidity−basicity by the adsorption of urea and
other products on the catalyst surface in urea transesterification
with EG.
In the course of careful investigation on factors influencing
EC conversion and DEG yield, we observed that catalyst
pretreatment conditions can drastically affect the reactivity of
the catalyst materials. Notably, DEG formation can be
effectively suppressed by thermally treating catalysts under an
inert atmosphere (He flow). Figure 5 illustrates the DEG
selectivity in the urea transesterification with EG over the metal
oxides thermally treated at 723 K in He and also in air.
Evidently, thermal treatment under an inert atmosphere led to
the suppression of DEG formation. Carlson reported that the
reaction between EC and alcohols (particularly glycols) or
thioalcohols can be promoted in the presence of basic materials
such as alkali carbonates.22 Therefore, it is probable that the
thermal treatment in air permitted CO2 in the atmosphere to
react with the activated catalyst surface, forming surface
carbonates with basic character which catalyzed the formation
of DEG. Although the decomposition temperature of bulk
inorganic carbonates is considerably higher (773−1173 K) than
the pretreatment temperature used in this work (723 K),23 the

confirmed that urea transesterification with EG to EC is a
consecutive two-step process and also EC as well as 2-Ox can
react further with EG (boxes B and C in Scheme 2)
To rationalize the factors influencing the reactivity and
product selectivity at each reaction step, we divided and looked
into the overall reaction with a wider and narrower scope of
comprising reactions (Scheme 2): (A) overall reaction of urea
transesterification, (B) EC decomposition with EG, (C) 2-Ox
reaction with EG, and (D) 2-HC reaction in EG in the absence
of urea. Reactions A−D were all monitored by the dip-in IR
probe, and products were identified and quantified by GC.
Thus, obtained concentration profiles of urea and products
were used in the determination of reaction orders, reaction rate
constants (k), and kinetic modeling of transesterification steps
and overall process. The details of the kinetic modeling method
and associated assumptions are described in the Supporting
Information. Furthermore, the k values calculated for the single
reaction steps were used for the rationalization of catalytic
performance in relation to the amount of acidic and basic sites
and also for a comparison with the k values estimated from the
overall reaction, where the presence of other reactants and
products may influence the kinetics of specific reaction paths.
In the overall reaction (box A), the urea conversion rate was
found to apparently be second order (Figure S3 and Table S3
in the Supporting Information). We searched for the catalyst
properties showing the best correlation to the reaction rate
constant, and we identified that the total amount of acidic sites
shows a clear trend with it (Figure 3). The reaction rate

Figure 3. Urea reaction rate constant vs the total amount of acidic sites
in urea transesterification with EG. The definition of k′urea can be
found in the Supporting Information. Dashed lines only serve to guide
the eyes.

increases when the surface of the catalyst contains more acidic
sites; however, if the catalyst surface possesses too many and/or
too strong acidic sites, the urea reaction rate decreases, as
observed for Mg-Al and Al2O3. Al2O3 showed the lowest urea
reaction rate due to the high concentration of acidic sites and
their overstrong character. Li et al.20 concluded that the
reaction between diols and urea was inhibited over acidic oxides
to some extent, due to the formation of steady coordination of
O and N atoms of urea to metal cations, which is in good
agreement with our observations. Urea can be considered as a
donor of electron pairs and a weakly basic molecule.21 As will
be reported separately, urea interacts with and can adsorb
strongly on the acidic sites of the catalyst (surface metal atoms
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Figure 4. (a) EC conversion (X) and (b) EC reaction rate constant vs the total amount of basic sites. Reactions were performed for 6 h at 413 K.
Dashed lines serve only to guide the eyes.

formation from EC and EG, there was a general trend observed
between the reactivity and the amount of basic sites (Figure 6).
The relation of 2-Ox reactivity with the amount of basic sites
was similar to that of EC reactivity (Figure 4), showing lower
and higher 2-Ox reactivity for Zn- and Mg-containing materials,
respectively. In addition, Fe2O3 promoted the reaction
significantly as in the reaction between EC and EG likely due
to the presence of strong basic sites (Figure 1a). It is worth
noting that Zn-Fe with one of the lowest amount of basic sites
(Table 1) converted only 14% of 2-Ox, whereas the reaction
without catalyst (blank) results in 18% of 2-Ox conversion. On
the basis of the higher initial reaction rate of 2-Ox using Zn-Fe
in comparison to a few other materials (Figure 6b), the lowest
2-Ox conversion (Figure 6a) implies that the active sites of ZnFe were poisoned in the course of the reaction. This poisoning
(more precisely selective site blocking) effect seems very
effective starting from urea and EG, where no formation of
TEA and 3-(2-EtOH)-2-Ox was observed using Zn-Fe (Table
2). Bhanage14 mentioned that catalysts with strong basic
character such as CaO and La2O3 are highly active in the
formation of 2-Ox and its further reaction products such as 3(2-EtOH)-2-Ox, which is in accordance with the finding of this
work. Hence, these undesired side reactions of 2-Ox are likely
catalyzed by basic sites and are possibly prevented by reducing
the number and/or weakening the strength of basic sites.
3.5. Reaction of 2-HC in EG. Furthermore, the reaction
starting from the intermediate (2-HC) was investigated
(Scheme 2, box D) to uncover the roles of acidic and basic
sites toward the cyclization and the formation of EC or 2-Ox in
the absence of urea in the reaction mixture. As observed in the
overall reaction (Scheme 2A), EC and all other side products
were detected with somewhat different product selectivity
(Table S1 in the Supporting Information). In situ IR
monitoring clarified that the EC and 2-Ox concentration rose
at the expense of exponential decay in the 2-HC concentration
and that the reaction is first order on 2-HC concentration
(Figure S3 and Table S3 in the Supporting Information). With
the aim of maximizing EC selectivity, factors such as the acidic
and basic nature of catalysts influencing the cyclization
selectivity to EC or 2-Ox were examined. After an extensive

Figure 5. Selectivity to DEG in urea transesterification with EG for the
catalysts calcined in the air and in He. Reactions were performed for 6
h at 413 K.

pretreatment is effective in keeping the surface less basic, most
likely due to the lower decomposition/desorption temperature
of CO2 from the catalyst surface as observed in CO2-TPD
(Figure 1a). Figure 5 clearly shows the importance of catalyst
pretreatment under an inert atmosphere prior to catalytic
testing. Therefore, we have employed the inert thermal
treatment for all catalytic testing in this work.
3.4. Reaction of 2-Ox with EG. In a similar fashion, we
have investigated one of the major undesired side reactions, i.e.
reactions of 2-Ox with EG (Scheme 2, box C), producing TEA
as the major product (ca. 90−95%) and also 3-(2-EtOH)-2-Ox
as another byproduct (Table S2 in the Supporting
Information). In situ IR monitoring clarified that the reactions
between 2-Ox and EG apparently are a first order in 2-Ox
concentration (Figure S6 and Table S6 in the Supporting
Information).
Although the correlation between 2-Ox conversion or
reaction rate and a specific characteristic of the catalysts was
less obvious in comparison to the previous case of DEG
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Figure 6. (a) 2-Ox conversion and (b) 2-Ox reaction rate constant vs the total amount of basic sites. Reactions were performed for 6 h at 413 K.
Dashed lines serve only to guide the eyes.

Figure 7. (a) 2-HC conversion and (b) EC selectivity vs the ratio between the total amount of acidic and basic sites (A/B). Reactions were
performed for 6 h at 413 K. Dashed lines serve only to guide the eyes.

Fe, have a comparably higher amount of acidic sites and this
property seems important to guide the reaction from 2-HC to
the EC path (Scheme 2B) and not to 2-Ox path (Scheme 2C).
Metal oxides with lower A/B values, i.e. more basic materials,
showed deteriorated EC selectivity, guiding the reaction toward
the 2-Ox path and also secondary reaction paths (Table S1). A
high A/B value is important in guiding the reaction path toward
EC formation as well as in suppressing the secondary reactions
catalyzed by basic sites as previously clarified (3.3 and 3.4).
When it comes to EC yield, the two factors2-HC conversion
as well as EC selectivitydirectly contribute and the presence
of well-balanced acidic and basic sites (A/B) is expected to be
crucial.
3.6. Role of Acidic and Basic Sites in Urea Transesterification of EG. From the studies above, the variation of
constituting elements in single-metal and mixed-metal oxides
was found to drastically alter the acid−base properties of the
materials and consequently influence reactivity, reaction paths,
and EC yield. Fine tuning of acid−base properties and a

search, no obvious relation was found between 2-HC
conversion or reaction rate and the number of acidic or basic
sites (the total number of sites or site density calculated by
scaling the total site number by the surface area of the
materials). Hence, a more integral parameter representing the
integrated acid−base properties of materials by taking the ratio
between total amount of acidic and basic sites (A/B) was
evaluated.
Figure 7 presents the plots of (a) 2-HC conversion and (b)
EC selectivity (Table S1 in the Supporting Information) against
A/B. A trend identical with 2-HC conversion was observed for
the 2-HC reaction rate, and therefore, it is not shown for the
sake of brevity. The metal oxides with a lower ratio of A/B, i.e.
more basic material, exhibited very high conversion of 2-HC,
reaching nearly full conversion as observed for Mg-containing
materials (Figure 7a), with the exception of Fe2O3 showing low
2-HC conversion despite the low A/B value. In contrast, metal
oxides with higher A/B values exhibit nearly full selectivity
toward EC (Figure 7b). These materials, such as Al2O3 and Zn6291

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reaction rate constants of the two reactions (i.e., starting from
urea and EG or from 2-HC in EG). The reaction rate constants
for 2-HC conversion in the urea + EG case were obtained from
a fitting of the concentration profiles determined by in situ IR
monitoring by varying rate constants. The details of the fitting
procedure are described in the Supporting Information and the
comparison of all rate constants is shown in Table S7 in the
Supporting Information. The major difference lies in the rate
constant for 2-HC conversion, which is given in Table 3.

balanced proportion of surface acidic and basic sites are
suggested to be critical for high EC yield with high selectivity
on the basis of the reaction starting from 2-HC (Figure 7). The
same conclusion is valid when the reaction starts from urea and
EG (Scheme 2A). Figure 8 shows the EC yield in urea

Table 3. Reaction Rate Constants of 2-HC Conversion
Starting from 2-HC in EG and from Urea and EG in the
Presence and Absence of the Metal Oxidesa
k2‑HC/103 min−1
catalyst

Figure 8. EC yield vs A/B ratio. Reactions were performed for 6 h at
413 K. Dashed lines serve only to guide the eyes.

2-HC and EG

urea and EG

blank
Al2O3
ZnO
MgO
Fe2O3
Zn-Al
Zn-Fe
Mg-Al
Mg-Fe

0.2
0.4
1.3
2.4
0.5
0.8
0.6
2.4
2.4

1.0
1.1
13.9
7.1
1.1
8.2
10.4
6.2
18.2

a

transesterification with EG as a function of the A/B value. It
shows the highest EC yield for a balanced A/B material (ZnFe), whereas lower EC yields were found for the more acidic or
basic materials. The low basicity of Zn-Fe is important in
driving the reaction more selectively toward EC, while the
presence of weak acidity is important for urea activation
without being poisoned (Figure 3).
The importance of balanced acid−base properties of catalyst
materials in this or similar reactions has been concluded by
other studies. Ball et al. reported that the reaction between
primary and secondary alcohols with urea to form alkyl
carbonates can be improved using an adequate combination of
a weak Lewis acid and a Lewis base.11 More recently, Climent
et al.19a concluded that the high activity and selectivity of Zn-Al
mixed oxide in urea transesterification with glycerol can be
attributed to well-balanced acid−base properties of the catalyst.
Therein, cooperative effects between active sites created by
different metal oxides were suggested; weakly Lewis-acidic
metal sites activate the carbonyl group of the urea and Lewis
basic sites activate glycerol. Wang et al.24 investigated mixed
Zn-Y oxides and demonstrated the importance of binary metal
oxide phases, which resulted in catalytic activity higher than
that of ZnO or Y2O3 due to the unique acid−base property
given by the synergistic effects. Similar synergetic effects of ZnFe oxides were also reported by Zhao et al.3 They concluded
that the formation of a ZnFe2O4 phase, as observed in our
work, was responsible for the high catalytic performance. The
presence of a unique phase where Zn and Fe are well mixed on
an atomic scale in the oxide may be responsible for creating
well-balanced acid−base sites in close vicinity and are thus
favorable for the reaction.
Furthermore, we observed substantial differences in product
selectivity when we start the reaction from urea and EG
(Scheme 2A) or from 2-HC (Scheme 2D). The difference
likely stems from the acid−base character of urea and also the
modification of acidity and basicity upon its adsorption over the
metal oxide surface. To gain detailed insights and highlight the
influence of urea in the overall reaction paths, we compared the

Confidence intervals of the rate constants (urea and EG) obtained by
fitting are shown in Table S7 of the Supporting Information.

Clearly, the rate constants of 2-HC conversion starting from
urea and EG are much higher than those starting from 2-HC in
EG. For some catalysts such as ZnO and Mg-Fe, the 2-HC
reaction rate boost was particularly prominent. According to
the knowledge gained in this work (Figures 7 and 8), it is
speculated that urea adsorbs on the acidic sites and renders the
surface more basic, thus accelerating 2-HC conversion.
A higher 2-HC conversion rate is in principle advantageous
to enhance EC yield, but this is only true when the path to EC
is dominant in comparison to the path to 2-Ox (Scheme 2). To
clarify the effects of urea on the selectivity from 2-HC toward
EC or 2-Ox, we have compared the selectivity to the 2-Ox path
as calculated by
S(2‐Ox path) = [2‐Ox] + [TEA] + [2‐(1‐EtOH)‐2‐Ox]
/([EC] + [DEG] + [2‐Ox] + [TEA]
+ [2‐(1‐EtOH)‐2‐Ox]) × 100

Figure 9 shows the effect of the starting reaction conditions
on the 2-Ox path selectivity. Generally, we observed remarkably
higher selectivity toward the 2-Ox path when the reaction
started from urea and EG. In other words, selectivity to EC can
be drastically decreased in the presence of urea. These results
suggest, as speculated earlier, that the poisoning by urea
enhances the basic character of the surface, which drives the
reaction toward the undesired 2-Ox path. The excellent catalyst
Zn-Fe shows low 2-Ox path selectivity in the urea + EG
reaction, with a value even lower than that of the blank
reaction. This might be due to the fact that weakly acidic
materials may not strongly adsorb urea and thus the
enhancement of surface basicity is minimized, while in the
liquid phase urea itself likely functions as a basic catalyst.
3.7. Catalyst Optimization. According to the studies
above, we identified that a mixed oxide with a combination of
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Zn-Fe. This A/B value represents the optimum A/B value to
maximize EC yield. The addition of Mg or Al did not improve
the acidity and basicity favorably, overenhancing the basic
nature of the materials.
It is important to remark that the reaction performance in
terms of product selectivity may not be satisfactory even when
EC yields are high. Table S9 in the Supporting Information
shows the EC yields and product selectivity of the additionally
evaluated Zn-Fe-based materials. The best material in terms of
EC yield (i.e., Zn1-Fe2) exhibited lower selectivity to EC and
notably ca. 5% higher selectivity to DEG in comparison to Zn−
Fe, which makes Zn-Fe a more attractive catalyst considering
the slightly lower but comparable EC yield. The trimetallic
oxides containing Mg or Al showed lower EC yield in
comparison to the binary Zn-Fe family. Introduction of acidic
Al resulted in an increase of EC selectivity, whereas
introduction of Mg enhanced the basicity and consequently
negatively affected the selectivity, as clearly visible from the
high selectivity to DEG (42.6%). These effects are in full
agreement with the previously suggested roles of acidic and
basic sites.
This work has clarified that the acidity and basicity of catalyst
materials play decisive roles in guiding the reaction path to
specific and desired products. As a general observation, we
remark that the reactions accompanying the release of NH3 are
catalyzed by acidic sites, whereas those accompanying the
release of CO2 and/or H2O are catalyzed by basic sites. In
Scheme 2, the first two important reactions toward EC, namely
(i) urea + EG to 2-HC and (ii) 2-HC to EC are acid-catalyzed
reactions, although overly strong acidic sites seem to be blocked
by urea and therefore moderate acidity is important for urea
activation and for better product yields. The other reactions,
including the secondary reaction of EC to DEG and formation
of 2-Ox and its secondary reactions, are identified to be
catalyzed by basic sites. This simple and seemingly general
trend would be very valuable to tune the catalyst acidity and
basicity when reaction paths are identified from product
distribution.
Among all materials investigated in this work, Zn-Fe (i.e.,
Zn3-Fe1) with a well-balanced acid−base property showed the
best catalytic performance in terms of EC yield and selectivity.
This material was selected for further reaction parameter
optimization using a statistical approach presented in the
following section.
3.8. Design of Experiments (DoE). We have uncovered
the role of acidic and basic sites in the comprising reactions and
their integrating effects on EC yield and selectivity in urea
transesterification with EG. Apart from the important balance
of acidic and basic sites, a large number of experimental
parameters may influence the final outcome. To date, all studies
have been devoted to finding the optimum reaction conditions
for EC synthesis, where an examining-one-factor-at-a-time
approach is used.3,14,24 Although this common approach can
reveal how certain factors affect product yields, it explores the
space of experimental parameters very poorly and also reveals
little about the interaction (correlation) between these
parameters for better optimization of the targeted quantity
(here, EC yield and selectivity). With the aim of examining and
demonstrating the power of statistical methods to efficiently
identify experimental parameters affecting the targeted quantity
and to optimize them with as few experiments as possible, we
have employed a design of experiment (DoE)25 method. The
parameters considered as variables were reaction temperature,

Figure 9. Selectivity to the 2-Ox path (Scheme 2C) against the EC
path (Scheme 2B) starting from urea and EG (shadowed bar) or from
2-HC and EG (open bar). Reactions were performed for 6 h at 413 K.

Zn and Fe was particularly favorable for efficient EC synthesis
by suppressing undesired side reactions due to its unique
balance of acidity and basicity. Learning that A/B is an
important parameter and that of Zn−Fe is optimal among the
materials tested, we have attempted to synthesize Zn-Fe-based
materials to cover A/B above and below the value of Zn-Fe
(0.13, Figure 8). For this purpose, we have varied the Zn/Fe
ratio (1/2, 6/1, and 9/1) or introduced a third metal (Mg or
Al). In the former case, these oxides were produced by an
identical method without formation of a hydrotalcite precursor.
In the latter case molar ratio Mb2+/Mt3+ = 3 was kept.
The syntheses were successful, as shown in Figure 10 (XRD
patterns are shown in Figure S9 in the Supporting Information,

Figure 10. EC yield against A/B ratio of Zn-Fe-based oxides with
different Zn/Fe molar ratios and the oxides with an additional third
metal (Al or Mg). Gray markers are the data points taken from Figure
8. Reactions were performed for 6 h at 413 K.

and the material properties are summarized in Table S8 in the
Supporting Information), mainly covering the lower A/B values
except the material with Zn/Fe = 6/1 (denoted as Zn6−Fe1),
which showed a slightly higher A/B value in comparison to that
of Zn-Fe. In terms of EC yield, all of the materials synthesized
followed the trends obtained previously. A slightly higher value
of EC yield was obtained for the Zn-Fe material with a slightly
lower A/B value (ca. 0.09; Zn1-Fe2) in comparison to that of
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reaction time, catalyst amount, and EG/urea ratio. The catalyst
used was the best one we found in this work, Zn-Fe. We have
used fractional factorial design in order to estimate the
significance of the main effects and the interactions among
various experimental parameters. Important variables were
further optimized by central composite design. A detailed
description of the DoE method is given in the Supporting
Information.
The main factors influencing the selectivity of EC, 2-Ox, and
DEG were found to be temperature and reaction time. This is
reasonable due to the generally high effects of temperature on
reactivity and product selectivity as well as to the fact that the
reaction is of a consecutive nature; thus, an optimum reaction
time is expected. Neither EG/urea ratio nor catalyst loading
seemed to have considerable effects on the product distribution
and EC yield, although the impacts of these parameters should
be taken into account when process feasibility is evaluated.
Additionally, interaction of the two parameters time and
temperature with the parameters of interest (EC selectivity and
yield) was found to be most significant. On the basis of
identified important variables, models predicting EC selectivity
(S(EC)) and yield (Y(EC)) in relation to reaction time and
temperature were generated. The models for Y(EC) and S(EC)
are displayed as response surfaces in Figures S11 and S12,
respectively, in the Supporting Information.
According to the response surfaces, two optimum sets of
reaction time and temperature were suggested to be advantageous to give high SEC and YEC values: (1) 12 h at 399 K and
(2) 2 h at 439 K. The reaction was performed under the two
conditions with the standard conditions for the rest of the
experimental parameters of this work using the Zn-Fe mixed
oxide.
Both, very distinct, reaction conditions were highly beneficial
for EC selectivity and yield. The first condition led to an
excellent EC selectivity (99.6%) with a good EC yield (16
mmol gcat−1), while in the second case, a very high EC yield
(28.59 mmol gcat−1) was obtained with a high EC selectivity
(93.3%). In comparison to the previously achieved values for
EC yield (25.9 mmol gcat−1) and selectivity (91.5%) at 423 K
and 6 h, there is a clear advantage of DoE to improve the
reaction parameters with a small number of tests, identifying
importantly the reaction conditions we would not test
otherwise. EC selectivity was maximized using the specific
reaction conditions at a lower temperature, and the yield was
maximized with the high reaction temperature but with the
much shorter reaction time. This DoE study shows clearly the
usefulness and effectiveness of statistical approaches to
maximize the target product selectivity and yield in the
complex, consecutive, and acid−base catalyzed reactions.

materials and the reaction performance or the kinetic
information (reaction rate constants).
We could generalize that the reactions accompanying NH3
release are acid-catalyzed and those accompanying CO2 or H2O
release are base-catalyzed. The target product (EC) can be
formed selectively over acidic sites; however, the surface
reactivity is hindered when the acidity is too strong by urea
adsorption (i.e., site blocking). The undesired product
formation is catalyzed by basic sites. For these reasons, wellbalanced amounts of acidic and basic sites are critical to achieve
high EC yield with high EC selectivity. Among all of the
materials tested, Zn-Fe material with 3/1 atomic ratio showed
the best overall performance toward EC production.
Furthermore, a statistical approach (DoE) was used for the
reaction where a number of experimental parameters influence
the final quantify of interest in a very complex and dependent
manner. This study demonstrated the usefulness and
effectiveness of DoE in this reaction and suggested the same
for complex acid−base catalyzed reactions in general. The DoE
approach successfully identified experimental conditions where
EC productivity and selectivity can be maximized with a small
number of experiments for optimization. In summary, this
study shows a holistic approach to improve the catalytic
performance of a complex acid−base reaction on the basis of a
combined rational (using structure−activity relationship) and
statistical approach to efficiently maximize the yield of desired
products.

■

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01575.
XRD patterns of the metal oxides and precursors when
they are derived from hydrotalcites, materials properties
of Zn-Fe-based materials, and the procedures for MCR
and kinetic analyses, fitting, and DoE (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for A.U.: aurakawa@iciq.es.
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
D.F. and A.U. acknowledge financial support from the ICIQ
Foundation and the MINECO (CTQ2012-34153) and also
thank the MINECO for support through Severo Ochoa
Excellence Accreditation 2014−2018 (SEV-2013-0319). R.J.C.
acknowledges the Universitat Rovira i Virgili (URV) and
Spanish Ministry of Science and Technology for financial
support through the Juan de la Cierva program (JCI-201007328).

4. CONCLUSIONS
A series of single- and mixed-metal oxides, mostly derived from
hydrotalcite precursors, consisting of four metal cations (Zn,
Mg, Al, Fe) were synthesized with the aim of fine-tuning the
acid−base properties. These materials were tested in urea
transesterification with EG and in every comprising reaction
pathway identified in this work. The concentration profiles of
the reactants/products and kinetics parameters were obtained
by means of IR monitoring using a dip-in ATR-IR probe with
subsequent multivariate analysis (MCR). The roles of acidity
and basicity at each reaction step have been unambiguously
clarified from the relations between acidity and basicity of the

■

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