Rapid in situ synthesis of MgAl-LDH on η-Al2O3 for efficient hydrolysis of urea in wastewater

Article abstract of DOI:10.1016/j.jcat.2020.12.024

A rapid and efficient synthesis strategy of MgAl-LDH was proposed. MgAl-LDH with high specific surface area was synthesized in situ by using η-Al₂O₃ as carrier, and the synthesis time was greatly shortened by both increasing temperature and introducing ethanol as co-solvent. Besides, the growth process of MgAl-LDH on the surface of η-Al₂O₃ is also revealed. Under optimized conditions, the specific surface area of the MgAl-LDH is as high as 172.4 m²/g, the crystalline size is as small as 12.82 nm, and the basicity can reach 1.795 mmol/g. The urea wastewater was degraded from 8000 mg/L to 6.85 mg/L over the catalyst synthesized by the rapid method, and the catalyst still maintains high activity after four uses. Also, it was found that there is a good linear relationship between the urea removal rate and the basicity of MgAl-LDH.

Full text of DOI:10.1016/j.jcat.2020.12.024

Journal of Catalysis 395 (2021) 54–62
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Rapid in situ synthesis of MgAl-LDH on g-Al O for efficient hydrolysis of
2
3
urea in wastewater
a
a,
⇑
,1
a
b
c
a
a
Chenyuan Guo , Shuguang Shen
Huajie Pan
a
, Meina Li , Ying Wang , Jing Li , Yuanquan Xing , Cui Wang ,
a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China
Taiyuan Central Hospital, Taiyuan 030024, People’s Republic of China
College of Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A rapid and efficient synthesis strategy of MgAl-LDH was proposed. MgAl-LDH with high specific surface
Received 21 October 2020
Revised 19 December 2020
Accepted 22 December 2020
Available online 31 December 2020
area was synthesized in situ by using
both increasing temperature and introducing ethanol as co-solvent. Besides, the growth process of MgAl-
LDH on the surface of -Al is also revealed. Under optimized conditions, the specific surface area of the
MgAl-LDH is as high as 172.4 m /g, the crystalline size is as small as 12.82 nm, and the basicity can reach
.795 mmol/g. The urea wastewater was degraded from 8000 mg/L to 6.85 mg/L over the catalyst synthe-
sized by the rapid method, and the catalyst still maintains high activity after four uses. Also, it was found
that there is a good linear relationship between the urea removal rate and the basicity of MgAl-LDH.
Ó 2020 Elsevier Inc. All rights reserved.
2 3
g-Al O as carrier, and the synthesis time was greatly shortened by
g
2 3
O
2
1
Keywords:
Catalysis
Hydrolysis
LDH
Rapid synthesis
Urea wastewater
1
. Introduction
urea hydrolysis. Liquid acid and base can homogeneously catalyze
the hydrolysis of urea wastewater, which may cause secondary
The global urea market demand reached 187.8 million tons in
019, and it will be about 211.5 million tons by 2025 [1]. This
means that about 105.8 million tons of wastewater containing
.5–2 wt% urea will be produced by 2025. The wastewater dis-
pollution due to the difficulty in recovering liquid acid and base.
Therefore, heterogeneous catalytic should be given more consider-
ation for treating urea wastewater.
2
0
Schell used a group of vanadium compounds (NaVO
3 2 5
, V O ,
charge not only can cause eutrophication of water bodies but also
resource waste. Therefore, the urea content of the wastewater from
urea plant must be less than 10 mg/L [2]. At present, the most pop-
ular method of treatment for urea wastewater is thermal hydroly-
sis, such as Stamicarbon process and Snamprogetti process [3].
Nevertheless, these processes are carried out at high temperature
and pressure, which require a large amount of medium pressure
steam and are considered to be costly.
Some beneficial explorations have been made in other methods,
include adsorption, bio-hydrolysis, decomposition by strong oxi-
dants, and electrochemical oxidation [4–7], which are still in the
exploratory stage and can not be applied to industrialization for
urea process wastewater at present. Catalysts can accelerate the
rate of chemical reaction and reduce the required temperature of
NH VO , VOSO ) as catalysts to treat wastewater containing
4
3
4
0.05–10 wt% urea [8,9]. This method takes a long time, and toxic
vanadium may cause secondary pollution of water. Ananiev stud-
ied the hydrolysis of urea in the solution containing nitric acid
and formic acid by using Pt supported on SiO
2
as a catalyst. How-
ever, the activity of Pt/SiO catalyst decreases reversibly with the
2
increase in the drying temperature during its regeneration [10].
Metal oxides, metallic simple substance and non-metallic mate-
rials were also used to promote the reaction of 10–50 wt% urea
solution to generate ammonia, which reduces the nitrogen oxides
from power plants and engine exhaust [11–18]. However, the urea
concentration of residual after reaction was far higher than the
effluent standard of urea wastewater. For example, Gangadharan’s
research results show that the maximum degradation rate of urea
was 77.26% when the initial concentration of urea was 10 wt% [14].
7
0% urea conversion was achieved when 50 wt% urea solution was
⇑
Corresponding author.
E-mail address: shenshuguang@tyut.edu.cn (S. Shen).
Chenyuan Guo and Shuguang Shen contributed equally to this work. Chenyuan
hydrolyzed in the research of Goffe [17]. 10–30 wt% urea solution
was hydrolyzed in Sahu’s work, the urea concentration of residual
solution still reached 1000 mg/L [13]. It is impossible to judge
1
Guo and Shuguang Shen should be considered co-first authors.
https://doi.org/10.1016/j.jcat.2020.12.024
0
021-9517/Ó 2020 Elsevier Inc. All rights reserved.

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
whether these catalysts can be competent for treating lower con-
centration urea wastewater from nitrogenous fertilizer plants.
In addition to the above studies, there is also some literature on
the heterogeneous catalytic hydrolysis of low concentration urea
to be calcined at 550 °C for 4 h to remove surface water and gas
before use, which was from Zibo Jiezhong New Material Co., Ltd.
2.2. Preparation of samples
2 2 3 2 3 2 2
wastewater. MnO , Al O , MgO, ZnO, Fe O , SiO , TiO and MgAl-
LDH were used as catalysts to hydrolyze low concentration urea
wastewater [19–22], in which MgAl-LDH exhibited better hydroly-
sis performance.
The solution containing Mg(NO
3
)
2
ꢀ6H
2
O and urea was prepared
2
+
with a molar ratio of c(urea)/c(Mg ) equal to 2 and an initial c
2
+
(Mg ) of 2 mol/L, in which the solvent is deionized water. Then
5 mL of the above solution was placed into an autoclave containing
The general formula of layered double hydroxide (LDH) is
2
+
3+
x+ n-
[
M
1-x
M
x
(OH)
metal ions and anions, respectively. Its structure is similar to brucite
Mg(OH) , in which magnesium ions and hydroxyl ions coordinate to
form an octahedral structure. In the structure of LDH, part of M is
2
]
[Ax/n]ꢀmH
2
O, where M and A represent different
2 3
2.5 g g-Al O . The autoclave was sealed and maintained at x °C for
y min after ultrasonic vibration for 30 min. The resultant was
washed with deionized water to neutral pH and dried at 110 °C,
and recorded as MLx-y.
In the preparation of EMgAl-LDH, the solvent was deionized
water and ethanol. EMgAl-LDH is prepared by dissolving Mg
2
2
+
3
+
replaced by M to form a positively charged layer, and anions and
water molecules in the middle of the layer are used to maintain
charge balance. With the variability of cations, the controllability
of M /M and the exchangeability of anions, LDH and calcined
LDH are widely used in ion-exchangers, adsorbents, flame retar-
dants, clay-modified electrodes, additives and pharmaceuticals
(NO
3
)
2
ꢀ6H
2
O and urea in a mixed solution with different volume
2
+
3+
ratios of 0%, 10%, 20%, 30%, 40%, 50% ethanol, denoted as EML0,
EML10, EML20, EML30, EML40, EML50, respectively. And the auto-
clave for producing EMgAl-LDH was sealed and maintained at
170 °C for 30 min. The remaining steps are the same as the synthe-
sis of MLx-y.
[
23,24]. Because the basic strength of LDH is basically consistent
with the divalent metal ion hydroxide on the laminate, LDH can be
used as an environmentally friendly catalyst for aldol condensations,
alkylation, isomerization, methanolysis of soybean oil and cycloaddi-
tion reactions of CO with epoxides [25–34]. And recent studies have
2
shown that this material can also be effectively applied to electrocat-
alytic degradation of urea [35–38].
However, the LDH prepared by the traditional method has a
small specific surface area, resulting in a small base amount [39],
which limits its application in catalysis to some extent. The LDH
synthesized in situ has a relatively large specific surface area,
3 2
P-MgAl-LDH was prepared by ex situ method. urea, Mg(NO ) -
2+
ꢀ6H
2
O and Al(NO
3
)
3
ꢀ9H
2
O are weighed to make c(urea)/c(Mg )/
3
+
c(Al ) = 10:2:1, then the mixture was dissolved in deionized water
2
+
and diluted to c(Mg ) = 2 mol/L. The solution was transferred to an
autoclave, which was sealed and maintained at 130 °C for 4 h. The
remaining steps are the same as the synthesis of ML x-y. The expla-
nation of the abbreviations involved in this work is shown in
Table 1.
which is prepared by using
a
large specific surface area
2.3. Hydrolysis of urea
aluminum-containing material as a carrier and an aluminum
source.
0.5 g catalyst and 10 mL solution with urea concentration of
8.0 g/L were added in a Teflon reactor and heated (165 °C) under
vigorous stirring at regular periods (30–150 min). After cooling
to room temperature, the filtrate was analyzed by the diacetyl
monoxime colorimetric method to determine the urea concentra-
tion remaining in the aqueous solution [47], and the separated cat-
alyst was washed with deionized water and dried for reuse.
Whether in situ or not, the synthesis temperature range is usu-
ally 80–130 °C and the synthesis time range is 4–12 h in the prepa-
ration of LDH [40–46]. However, the hydrolysis temperature of
catalytic urea process wastewater is required to reach 165 °C.
The service temperature of catalysts is higher than the synthesis
temperature, which has adverse effects on catalysts. Therefore,
we intend to raise the synthesis temperature of MgAl-LDH, which
is also beneficial to shorten the synthesis time.
Increasing synthesis temperature could improve the growth
rate of crystal according to the theory of crystal growth, it is also
possible to further shorten the preparation time of MgAl-LDH.
Therefore, we proposed a high-temperature and rapid synthesis
method of MgAl-LDH. A series of catalysts were synthesized
in situ by increasing the synthesis temperature to 170 °C. The
effects of synthesis conditions on the structure of MgAl-LDH were
investigated through some characterizations such as SEM, XRD,
C
0
ꢁ C
A
Urea degradation rate ð%Þ ¼
ꢂ 100
ð1Þ
C
0
Where C is the initial urea concentration (8.0 g/L) and C
0
A
is the final
urea concentration after hydrolysis reaction.
2.4. Characterization
X-ray diffraction (XRD) patterns of the samples were recorded
on Shimadzu XRD-6000 diffractometer to identify their crystal
ICP, XPS, FT-IR, TGA and N
2
adsorption. MgAl-LDH was applied to
structures. Cu Ka radiation (k = 0.15405 nm) was used with a
catalytic hydrolysis of urea and its effect on urea removal rate
was investigated. Further, catalysts were modified to improve the
stability of the catalyst by introducing ethanol in the process of
synthesis.
power setting of 40 kV and 30 mA. The samples were scanned 2h
values ranging from 5° to 70° with a scan speed of 8°/min. The unit
cell parameters were calculated from a = 2d110 and c = 1/3
(3d003 + 6d006 + 9d009) [48], and the mean crystalline size D003
was calculated from the width of the peak at half height of the
0
03 peaks using the Scherrer formula.
The Mg/Al ratio of the samples was determined by flame atomic
2
. Experiments
absorption spectroscopy (F-AAS) with a Varian AA240FS. X-ray
Photoelectron Spectroscopy (XPS, ESCALAB250) was used to char-
acterize the surface element composition and binding state of
samples.
2.1. Materials
Urea, Mg(NO
3
)
2
ꢀ6H
2
O, Al(NO
3
)
3
ꢀ9H
2
O, MgCO
3
, absolute ethyl
Scanning Electron Microscope (SEM) tests were performed on a
JSM-6700F microscope (JEOL, Japan) with the Au-sputtered sample
operated at 10 keV to observe the morphology of the samples.
alcohol, indicators were purchased from Tianjin Kermel Chemical
Reagent Co., Ltd. Benzoic acid was acquired from Xi’an Chemical
Reagent Factory. Diacetyl monoxime and thiosemicarbazide were
N
2
adsorption experiments were carried out using an automatic
2 3
obtained from Aladdin Chemistry Co., Ltd. g-Al O (99.6%) needs
surface analyzer (F-sorb 3400, Gold APP Instrument, China). The
55

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
Table 1
Explanation of abbreviation and its preparation method.
Abbreviation Explanation
Preparation
method
LDH
MgAl-LDH
Layered double hydroxides.
Magnesium aluminum layered double
hydroxides.
–
–
P-MgAl-LDH Pure MgAl-LDH, which does not use
carrier and aluminum source.
2 3
g-Al O as Ex situ
MLx-y
MgAl-LDH, which was synthesized at ꢂ °C for In situ
y min. ( -Al was used as carrier and
g
2 3
O
aluminum source)
EMgAl-LDH
MgAl-LDH, which was synthesized at different In situ
volume ratios of ethanol. (
2 3
g-Al O was used as
carrier and aluminum source)
samples were heated in vacuum at 110 °C for 3 h before
measurement.
Fourier Transform Infrared spectroscopy(FT-IR) was recorded
using a Shimadzu 8400 Fourier transform spectrometer (resolution
Fig. 1. XRD patterns of catalysts for different synthesis time at 170 °C.
ꢁ
1
4
cm ), samples were pressed into disks with KBr.
Thermogravimetric analysis (TGA) was conducted using a Met-
It is common knowledge that the synthesis temperature has a
great influence on the growth rate of the crystal. The crystal struc-
ture parameters of the catalysts were calculated from XRD pat-
terns, as shown in Table 2. Except for ML170-10, the others
satisfy the relationship of d003 = 2d006 = 3d009, which reveals highly
packed stacks of brucite-like layers. The crystalline size (D003) and
the ratio of Mg/Al gradually increase, which shows that MgAl-LDH
tler Toledo TGA/DSC 3 + at 10 °C/min under N
ature range from 25 to 300 °C.
2
flow in the temper-
A Hammett indicator procedure was used for the characteriza-
tion of the base strength and basicity of catalysts [32]. 150 mg of
the catalysts, 1 mL of a solution of the indicators and 5 mL ethanol
were mixed and the mixture was ultrasonic vibration for 30 min.
The color of the catalyst surface was recorded after 1.5 h. The indi-
formed in situ on the surface of
thesis time. In particular, the relatively low ratio of Mg/Al is attrib-
uted to the fact that the carrier is -Al O in this work.
2 3
g-Al O with the increase of syn-
cators were methyl red (H
ymol blue (H = 7.2), phenolphthalein (H
= 15.0). The basic strengths are quoted as being stronger than
-
= 4.8), neutral red (H
-
= 6.8), bromoth-
-
-
= 9.8), 2, 4-dinitroaniline
g
2
3
(
H
-
Basicity and basic strength of MgAl-LDH are very important for
the performance of hydrolyzing urea, so the Hammett indicator
method was used to test the catalysts and the results are shown
the weakest indicator that exhibits a color change, but weaker than
the strongest indicator that produces no color change. 0.02 mol/L
benzoic acid absolute ethanol solution was used to titrate the
basicity of the catalyst. The above solution was slowly added until
the catalyst surface reaches the endpoint of indicator discoloration,
and the volume consumed was recorded.
in Table 3. The basic strength range of
P-MgAl-LDH (without -Al O ) synthesized ex situ is in the range
of 7.2–15. It can be seen that the difference in basic strength
between -Al O and P-MgAl-LDH is distinct. The basic strength
of 7.2–15 is shown in all the catalysts synthesized in situ, indicat-
ing that MgAl-LDH formed on the surface of the -Al . It is worth
noting that ML170-10 exhibits not only the basic strength of MgAl-
LDH but also that of -Al 3, although the peak of MgAl-LDH was
not observed on the XRD pattern. This indicates that a small
2 3
g-Al O is 4.8–7.2, while
g
2
3
g
2
3
CV
m
g
2 3
O
Basicity ðmmol=gÞ ¼
ð2Þ
g
2
O
Where C (mol/L) is the concentration of the benzoic acid solution, V
mL) is the volume of benzoic acid absolute ethanol solution con-
sumed, and m (g) is the mass of the catalyst.
(
amount of MgAl-LDH formed on the surface of the
g-Al
2
O
3
for
ML170-10, which was not enough to cover the surface of
2 3
g-Al O
completely.
In the catalysts synthesized in situ at 170 °C, the basicity
increases with the increase of time within 30 min (Table 3). The
reason is that the amount of MgAl-LDH produced was increasing,
which can be proved by the increase in the ratio of Mg/Al (Table 2).
After 30 min, the Mg/Al ratio of catalysts still increases slightly
while the basicity decreases with the increase of time. One reason
is that the increase in crystalline size leads to the decrease in Mg-
OH at the edge of laminate [19]; the other is that the generated
3
. Results and discussion
3.1. MgAl-LDH prepared at high temperature
A series of catalysts under different synthesis time were pre-
pared by raising the synthesis temperature to170 °C (higher than
urea hydrolysis temperature). XRD patterns are shown in Fig. 1,
ML130-240 as a control experiment. The diffraction diagram for
ML170-10 has four reflections at 37.6°, 39.5°, 45.8°, and 67.0°,
MgCO
.8–15, covers some basic sites of MgAl-LDH. The results show that
except for ML170-10, the surface of -Al in the other catalysts is
completely covered by MgAl-LDH. In short, MgAl-LDH gradually
3
, which does not exhibit basic strength in the range of
4
which belong to
Al , there are also four reflections at 11.6°, 23.5°, 35.0°, and
0.8° for ML170-20 and ML170-30, corresponding to (003), (006),
009), and (110) crystal planes of MgAl-LDH respectively [50].
The XRD results show that MgAl-LDH formed in situ on the surface
of -Al when the synthesis time was 20 min or more. Except for
2 3
g-Al O [49]. In addition to the reflection of g-
g
2 3
O
2 3
O
6
(
formed and grew up with the increase of time, and MgCO
3
began
to form and covered some basic sites after 30 min.
XPS was employed to characterize the surface elemental com-
position of the catalysts and binding state of the elements, the test
results are shown in Fig. 2. There are Mg, Al, C and O elements on
the surface of the catalysts from the XPS full scan spectrum
g
2 3
O
the above several peaks, ML170-40 and ML170-50 exhibit three
weak diffraction peaks at 32.4°, 42.82° and 53.54°, revealing the
existence of MgCO . We speculated that the production of MgCO
3 3
results from excess CO
thesis time.
(Fig. S1). It can be seen from Fig. 2a that for ML170-10, the peak
2
of urea hydrolysis with the increase of syn-
of Al 2p could be deconvoluted into two peaks of Al-OH and Al-
56

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
Table 2
The ratios of Mg/Al and XRD data of MgAl-LDH catalysts.
Catalyst
The ratios of Mg/Al
d (nm)
03
a (nm)
c (nm)
D 003 (nm)
0
006
009
110
ML130-240
ML170-10
ML170-20
ML170-30
ML170-40
ML170-50
0.154
0.083
0.186
0.248
0.253
0.262
0.760
–
0.766
0.758
0.760
0.765
0.382
–
0.382
0.380
0.381
0.379
0.257
–
0.256
0.257
0.257
0.257
0.152
–
0.152
0.152
0.152
0.152
0.304
–
0.304
0.304
0.305
0.304
2.301
–
2.299
2.273
2.283
2.295
13.56
–
13.56
15.92
17.52
17.66
Table 3
Basic strength and basicity of catalysts.
Catalyst
H
-
(mmol/g)
4
.8–6.8
6.8–7.2
7.2–9.8
9.8–15
7.2–15
g
-Al
2
O
3
0.080
0.031
–
–
–
P-MgAl-LDH
ML130-240
ML170-10
ML170-20
ML170-30
ML170-40
ML170-50
–
–
–
–
0.044
0.636
0.027
0.308
0.559
0.451
0.435
–
0.022
0.967
0.081
0.675
0.804
0.773
0.743
–
0.066
1.603
0.108
0.983
1.363
1.224
1.178
–
0.059
0.014
–
–
–
–
–
–
–
–
–
–
MgCO
3
Fig. 2. High-resolution XPS spectra of Al 2p (a) and Mg 2p (b).
O, which indicates that the surface composition of ML170-10 is
MgAl-LDH and Al . However, only the peak of Al-OH was
detected in ML170-30 and ML170-50, which may be due to the fact
that MgAl-LDH covers the entire surface of -Al . As shown in
2 3
O
g
2 3
O
Fig. 2b, the peak of Mg-OH and Mg-O appeared at 48.86 eV in
the Mg 2p spectra, revealing the generation of MgAl-LDH on the
surface of
the two peaks at 48.86 eV and 52.52 eV, assigned to Mg-OH and
MgCO respectively [51]. This phenomenon shows that MgAl-
LDH and MgCO exist on the surface of ML170-50. The XPS results
confirm that MgAl-LDH gradually formed with the increase of time
and MgCO began to form after 30 min, which is consistent with
2 3
g-Al O . Only ML170-50 could be deconvoluted into
3
3
3
the XRD results and the alkaline tests.
Fig. 3 shows the urea removal rate and the basicity of different
catalysts. It can be seen that the urea removal rate and the basicity
show the same trend, revealing that the active site of urea hydrol-
ysis is related to the basic sites of MgAl-LDH, namely Mg-OH at the
edge of laminate. Among the catalysts prepared at 170 °C, ML170-
Fig. 3. Effect of synthesis time on the basicity and urea removal rate of catalyst
(Catalyst dosage: 50 g/L, reaction temperature: 165 °C, time: 30 min).
3
0 exhibits the highest urea removal rate and the highest basicity,
57

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
Table 5
BET areas, basicity and Mg/Al ratios of EMgAl-LDH.
2
Catalyst
BET area (m /g)
Basicity (mmol/g)
The ratios of Mg/Al
EML0
145.7
151.9
156.7
164.1
169.0
172.4
1.363
1.445
1.529
1.622
1.702
1.795
0.248
0.235
0.225
0.211
0.184
0.172
EML10
EML20
EML30
EML40
EML50
Fig. 4. XRD patterns of EMgAl-LDH.
but it is still significantly lower than ML130-240. Therefore, we
need to improve the synthesis method to expose more basic sites.
3.2. MgAl-LDH prepared at different volume ratios of ethanol (EMgAl-
LDH)
Adding organic solvents in the preparation of LDH can effec-
tively reduce the crystalline size and increase the specific surface
area [52–55]. As far as we know, the preparation of LDH by adding
organic solvents in the process of in situ synthesis has not been
reported. Because ethanol is relatively inexpensive, widely avail-
able and non-toxic, ethanol was added to assist the in situ synthe-
sis of MgAl-LDH on the surface of g-Al O at the optimal synthetic
2 3
conditions (170 °C, 30 min) in the following.
Fig. 5. FT-IR spectra of EMgAl–LDH.
which is smaller than not only EML0 but also ML130-240. In this
work, the synthesis process takes only 30 min and the smallest
crystalline size could be as low as 12.82 nm.
As can be seen from data in Table 5, the specific surface area
increases gradually with the increase of ethanol ratio. The specific
The crystal structure of EMgAl-LDH was characterized by XRD,
as shown in Fig. 4. The reflections of g-Al O and MgAl-LDH are
2 3
2
surface area of EML50 is 172.4 m /g, which is much higher than
that of ML130-240. It is also found that the basicity exhibits the
same trend as the specific surface area. The basicity of EML50
reaches a maximum value of 1.795 mmol/g, which is also higher
than that of ML130-240. The increase in the basicity of EMgAl-
LDH is attributed to the increase in specific surface area and the
decrease of crystalline size. The ratio of Mg/Al decreased from
observed in all the catalysts, which indicates that MgAl-LDH could
be synthesized in situ in the presence of ethanol. The intensity
decreases and the shape broadens for diffraction peaks of EMgAl-
LDH, suggesting that the crystalline size of MgAl-LDH gradually
decreases with the increase of ethanol ratio.
The crystal structure parameters of EMgAl-LDH were calculated
by XRD patterns, as shown in Table 4. For all EMgAl-LDH, the rela-
tionship d003 = 2d006 = 3d009 is satisfied. Based on the results of
quantitative calculation, it can be more clearly seen that the crys-
talline size decreases gradually with the increase of ethanol ratio.
On one hand, this may be because ethanol effectively inhibits
stacking of the layers as a polar solvent [56]. On the other hand,
0
.248 (EML0) to 0.172 (EML50) with the increase of ethanol ratio,
which indicates that ethanol does slow down the growth rate of
MgAl-LDH and reduces the stack of the layers. The basic strength
of EMgAl-LDH is also 7.2–15 after the modification of ethanol, indi-
cating that
2 3
g-Al O was well coated by the generated MgAl-LDH.
Fig. 5 shows the FT-IR spectra of EMgAl-LDH with a wavenum-
ꢁ
1
–
ber range of 400–4000 cm . The adsorption bands of EMgAl-LDH
are observed at about 440, 570, 670, 875, 1370, 1640 and
the addition of ethanol slows down the rate of ionization of OH
from weak alkaline solution and the release of Al3+, which is equiv-
ꢁ
1
3
470 cm , which are the typical bands of MgAl-LDH [55,57–59].
alent to reducing the concentration of reactants, thus slowing the
growth rate of crystal. EML50 has the smallest crystalline size,
ꢁ1
It is noteworthy that there is a blue shift at 3470 cm correspond-
Table 4
XRD data of EMgAl-LDH.
Catalyst
d (nm)
03
a (nm)
c (nm)
D003 (nm)
0
006
009
110
EML0 (ML170-30)
EML10
EML20
EML30
EML40
0.758
0.769
0.776
0.772
0.775
0.782
0.380
0.383
0.385
0.384
0.386
0.387
0.257
0.256
0.255
0.254
0.254
0.255
0.152
0.152
0.152
0.152
0.151
0.151
0.304
0.304
0.304
0.304
0.303
0.302
2.273
2.302
2.311
2.301
2.309
2.322
15.92
15.36
15.11
14.58
13.86
12.82
EML50
58

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
Fig. 6. High-resolution XPS spectra of EML50: Mg 2p (a) and Al 2p (b).
ing to O-H stretching vibrations of interlayer water molecules and
physically absorbed water, which may be due to differences in
hydration.
TGA was used to determine whether there are hydration differ-
ences and the TG curves of EML0 and EML50 are shown in Fig. S2.
The mass loss below 170 °C is attributed to the evaporation of
physically adsorbed water molecules, and that in the range of
adsorption. In the range of 170–250 °C, the mass loss of EML50 is
lower than that of EML0, indicating that the addition of ethanol
will lead to the difference of interlayer water content. The differ-
ence would result in a blue shift at 3470 cm in the FT-IR spectra.
This phenomenon may be due to the hydrogen bonding of ethanol
ꢁ1
6
and M(OH) octahedra that limits the diffusion of water molecules
to the interlayer galleries during the stacking growth of brucite
layers, which can also indirectly affect the growth rate of LDH.
Besides, Fig. 6 was the high-resolution XPS spectra of EML50. It
can be seen that only Mg-OH is detected on the surface of EML50 in
Fig. 6a and only Al-OH is detected in Fig. 6b. This shows that the
1
70–250 °C is due to the evaporation of interlayer water molecules
54,55]. The mass loss of EML50 is higher than that of EML0 below
70 °C. This is because EML50 has a small crystalline size and a
large specific surface area, which provides more places for water
[
1
Fig. 7. SEM images of EML50.
59

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
2 3
Fig. 8. The plot of basicity versus urea removal rate for EMgAl-LDH (a) and all catalysts (b) (Red line: all catalysts; Blue line: catalysts except g-Al O and ML170-10).
composition of the EML50 surface is mainly MgAl-LDH, not
and MgCO3.
g-Al
2
O
3
rest of the catalysts except for
2 3
g-Al O and ML170-10. The basic
strength of -Al is in the range of 4.8–7.2 and the basic strength
g
2 3
O
The morphology was characterized by SEM, as shown in Fig. 7.
Fig. 7a shows irregular sample particles with different sizes below
of ML170-10 is in the range of not only 7.2–15 but also 4.8–7.2,
while the rest of the catalysts exhibits the basic strength of 7.2–
2
1
0
l
m. The SEM image of a single sample particle can be seen in
15. The degree (R ) of linear fit of the red line is far lower than that
Fig. 7b, and Fig. 7c is an enlarged image of the selected area of
the red box in Fig. 7b. Fig. 7b and 7c show that the sheet structure
2 3
of the blue line, which may be due to the exposure of g-Al O in
ML170-10. Notably, the urea removal rate of EML50 can reach
73.02% and is higher than that of ML130-240 of 71.29% after
30 min, indicating that the rapid in situ synthesis method has
advantage over the conventional in-situ method.
is almost coated on the entire surface of
2 3
g-Al O . Besides, some
transparent particles are observed in Fig. 7b and 7c. Fig. S3 is an
enlarged image of a transparent particle of Fig. 7c, and the layered
structure of the transparent particle can still be observed, indicat-
ing that these particles are MgAl-LDH and not aluminum oxide.
This phenomenon was caused by that MgAl-LDH grew on some
The effect of hydrolysis time on urea removal rate was shown in
Fig. 9a. The urea removal rate with the catalyst is significantly
higher than that without any catalyst when the reaction time was
in the range of 30–90 min. After 120 min, the difference is still obvi-
ous after amplification. The urea concentration in the hydrolyzate
with EML50 as the catalyst was only 6.85 mg/L, which is lower than
that with ML130-240. The discharge standard can be achieved with
EML50 as the catalyst under the hydrolysis temperature of 165 °C,
which is lower than the general hydrolysis temperature (220 °C) in
the nitrogen fertilizer plant. This will greatly reduce energy con-
sumption and economic cost. Finally, the stability of EML50 was
investigated in Fig. 9b under the conditions of reaction temperature
of 165 °C, hydrolysis time of 60 min and catalyst dosage of 50 g/L,
shown in Fig. 9b. The urea removal rate of EML50 is still stable after
four uses, indicating that EML50 has good reusability.
small-sized aluminum oxide particles (<2
lm). Fig. 7d is an
enlarged image of the selected area of the red box in Fig. 7c, it
can be observed that the ‘‘desert rose” is composed of a plurality
of layered structures [60]. Those SEM images also confirm that
MgAl-LDH could be synthesized successfully in situ in the presence
of 50% ethanol.
The relationship between basicity and urea removal rate was
also plotted after linear fitting, as illustrated in Fig. 8. As shown
in Fig. 8a, the urea removal rate of EMgAl-LDH gradually increases
with the increase of ethanol ratio and there is a strong correlation
between basicity and urea removal rate for EMgAl-LDH. In Fig. 8b,
the red line is the fit line for all the catalysts and the blue is for the
Fig. 9. Effect of hydrolysis time on urea removal rate (a) and recycling of EML50 (b) (Catalyst dosage: 50 g/L, reaction temperature: 165 °C).
60

C. Guo, S. Shen, M. Li et al.
Journal of Catalysis 395 (2021) 54–62
Fig. 10. The possible mechanism diagram of MgAl-LDH catalytic degradation of urea.
This work shows that the Mg-OH at the edges of MgAl-LDH is
the active site for catalyzing the hydrolysis of urea, and its possible
mechanism is shown in Fig. 10. The Mg-OH first attacks the car-
bonyl group of urea, and then negatively charged Int.1 and posi-
Acknowledgments
Funding: This work was supported by The Shanxi Science and
Technology Department (project number: 20161004).
+
tively charged LDH are formed. Int.1 reacts with water to form
–
–
+
Int.2 and OH , and the OH combines with LDH to form LDH. Since
Int.2 is unstable with two hydroxy groups on one carbon, hydroxy
Appendix A. Supplementary data
hydrogen and –NH
NH COOH is also formed. NH
water to generate NH and H
CO and H O.
2
are combined to generate NH
COOH is unstable and reacts with
CO , which then decomposes into
3
. Meanwhile,
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2020.12.024.
2
2
3
2
3
2
2
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