Fabrication of NiFe layered double hydroxides using urea hydrolysis - Control of interlayer anion and investigation on their catalytic performance

Article abstract of DOI:10.1016/j.catcom.2014.02.024

NiFe layered double hydroxides (NiFe-LDHs) intercalated with nitrate and carbonate anion were synthesized by urea hydrolysis. The aging time and the molar ratio of NO₃^-/urea were varied in order to identify suitable parameters, which

Full text of DOI:10.1016/j.catcom.2014.02.024

Catalysis Communications 50 (2014) 44–48
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Catalysis Communications
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Short Communication
Fabrication of NiFe layered double hydroxides using urea
hydrolysis—Control of interlayer anion and investigation on their
catalytic performance
Xu Wu, Yali Du, Xia An, Xianmei Xie ⁎
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
NiFe layered double hydroxides (NiFe-LDHs) intercalated with nitrate and carbonate anion were synthesized by
Received 14 January 2014
Received in revised form 23 February 2014
Accepted 25 February 2014
Available online 4 March 2014
−
urea hydrolysis. The aging time and the molar ratio of NO
3
/urea were varied in order to identify suitable parameters,
which control the interlayer anion (nitrate or carbonate) and crystal structure of the final products. The prepared
samples were applied to the one-pot synthesis of benzoin ethyl ether from benzaldehyde and ethanol. NiFe–NO
LDH presented excellent catalytic activity, different from NiFe–CO -LDH which showed none catalytic activity at all.
2014 Elsevier B.V. All rights reserved.
3
-
3
©
Keywords:
Urea hydrolysis
NiFeNO
NiFeCO
3
-LDH
-LDH
3
Catalysis
Benzoin ethyl ether
1
. Introduction
exploration on synthesis procedure of NiFe-LDHs [21,22]. For exam-
ple, Saiah et al. reported NiFe–CO -LDH synthesized by co-
3
Layered double hydroxides (LDHs), also known as hydrotalcite-
like compounds, are a family of anionic clays with three-
dimensional networks. The structure of the LDHs is very similar
precipitation method [23]; Duan et al. prepared NiFe layered double
hydroxide by decomposition of ammonium carbonate [24]; Liu et al.
obtained NiFe–CO
hexagon using urea as hydrolysis agent and trisodium citrate (C
Na ·H O) as chelating reagent [25]; Forano et al. obtained layered
3
-LDH with high crystallinity and well-defined
to that of brucite Mg(OH)
2
, in which each magnesium cation is oc-
6 5-
H
tahedrally surrounded by hydroxyls. The resulting octahedron
shares edges to form infinite sheets having no net charge [1,2]. In
recent years, based on these structural characteristics, LDHs have
attracted much attention in the development of new environment
friendly catalysts [3–8].
Among all the studies covering this field, as a common feature, Al-
based LDHs are always the mainly concerns [9–13]. Nowadays, with
the research of layered double hydroxides in depth, as one member of
LDH family, the catalytic performances of NiFe-LDHs have been drawing
attentions of investigators [14–19].
As is well-known, catalytic activity is often ascribed to the pres-
ence of defect surface sites with unusually low coordination number,
or ensembles of contiguous surface sites [20]. To trace the origin of
these defects, the preparation method closely related with the cata-
lytic behavior of the catalysts is an essential influencing factor. In
view of this, numerous researchers have been endeavoring in
3
O
7
2
double hydroxide by enzymatic decomposition of urea [26]. Al-
though NiFe-LDHs with different structures were prepared by di-
verse synthesis process, however, until now, little discussion was
focused on interlayer ions in NiFe-LDHs. Furthermore, whether dif-
ferent interlayer ions of NiFe-LDHs will have effect on their catalytic
performances have not been involved in the relevant reports yet.
Given the abovementioned, in the present work, we successfully
prepared NiFe-LDHs by urea hydrolysis with seriously monitoring
the preparation process. Particularly worth mentioning, NiFe–NO
LDH (nitrate as the interlayer anions), Ni(HCO and NiFe–CO
3
-
-
3 2
)
3
LDH (carbonate as the interlayer anions) were acquired orderly
along with pH value and aging time altering. Especially, as environ-
mentally friendly heterogeneous catalysts, NiFe-LDHs with different
structures were introduced to the one-step synthesis of benzoin
ethyl ether from benzaldehyde and ethanol to evaluate their catalyt-
ic performances [27,28]. Noticeably, these two kinds intercalated
NiFe-LDHs presented totally different catalytic activities. Further-
more, the reason causing this distinct catalytic property of NiFe–
⁎
Corresponding author. Tel.: +86 351 6018528.
E-mail addresses: s2003707@163.com, wuxu@tyut.edu.cn (X. Xie).
3 3
NO -LDH and NiFe–CO -LDH was also discussed.
http://dx.doi.org/10.1016/j.catcom.2014.02.024
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

X. Wu et al. / Catalysis Communications 50 (2014) 44–48
45
Scheme 1.
2
. Experimental
2.2. Typical procedure for synthesis of benzoin ethyl ether
2
.1. Preparation and characterization of the samples
A three-neck flask installed with a condenser and a thermometer
was fixed in DF-101S magnetic stirred water bath, and then definite
dosage of benzaldehyde, ethanol and NiFe-LDH catalyst (pretreatment
NiFe-LDHs were prepared by urea hydrolysis. The corresponding
ratios of urea solution and the mixed solution with 1.0 mol/L
Ni(NO ·6H O and 0.5 mol/L Fe(NO ·9H O were added into a bea-
ker. The mixtures were stirred for 30 min and then hydrothermally
treated at 110 °C for 3–48 h, followed by cooling down the autoclave
to room temperature. After measuring the pH value, the samples were
dried at 80 °C in an air oven overnight.
2
under the condition of 423 K with N flashing for 2 h) was added.
)
3 2
2
)
3 3
2
After the reaction proceeded for a certain time at a constant tempera-
ture and normal pressure, the reaction mixture was analyzed using an
HP (C6890A = 5973MSD) gas chromatograph equipped with a FFAP
column (30 m ∗ 0.32 mm ∗ 0.5 μm) and an FID detector [9]. The main in-
volved reaction formula is shown as Scheme 1.
Powder X-ray diffraction (XRD) patterns of the synthesized products
were recorded on Rigaku D/max-2500 instrument (40 kV, 100 mA)
using Cu Kα radiation at a scanning speed of 8° (2θ)/min, with the scan-
ning territory 5–65°.
3. Results and discussion
3.1. Catalyst characteristics
NiFe-CO -LDH
Ni(HCO )
NiFe-NO -LDH
In urea hydrolysis method, the synthesis process and the products
are susceptible to the pH value. However, the final pH value of the syn-
3
3
2
3
aging time=24h
thesis mixture can be influenced by modulating the aging time and the
_
3
−
NO /urea
initial molar ratio of NO
3
/urea [29]. Therefore, we focused on the im-
final pH
pact of these two factors on the products, using XRD to analyze the crys-
tal structure and phase composition of the obtained products.
1
:5
:4
9.37
9.14
8.92
8.43
7.61
6.70
6.13
5.46
1
−
1:3
3.1.1. Effects of NO
3
/urea molar ratio
The XRD patterns for different initial concentrations of urea with
aging time being 24 h were displayed in Fig. 1. As explicitly revealed
in Fig. 1, three species gradually appeared with pH value varying. No-
1:2
2:1
3:1
ticeably, if the urea concentration is too low (higher molar ratio of
−
4:1
NO
3
/urea) to result in final pH value below 6, the NiFe-LDHs cannot
be obtained. The possible factors related to this may be as follows: gen-
erally, amorphous Fe(OH) initially formed in the aqueous solution,
with further addition of the base, Fe(OH) gradually converted to
NiFe-HTLcs resultantly. However, the very low solubility of Fe(OH) like-
to NiFe-HTLcs
5:1
3
0
10
20
30
40
50
60
70
3
2
/
3
ly made the transformation of pH value from Fe(OH)
close to 6 whereat Ni2 ion deposits in the solution.
3
+
Fig. 1. XRD patterns of products with different starting concentrations of urea.
When the concentrations of urea were relatively high (lower molar
−
ratio of NO
3
/urea) to make final pH value around 9, NiFe–CO
3
-LDH ap-
peared; however, when at too low concentrations of reactants and pH
value nearby 6.5, it is completely another case that the outcome became
_
NO /urea = 0.33
3
NiFe–NO
3
-LDH. That is, when the concentrations of urea shifting from
-LDH to NiFe–CO -LDH
during the mediate conversion
low to high, the product altered from NiFe–NO
3
3
NiFe-CO -LDH
Ni(HCO )
3
2
3
3 2
along with the arising of Ni(HCO )
final pH
aging time
48h
6h
4h
2h
8.86
8.90
8.92
8.63
8.50
8.38
8.15
_
NO /urea = 3
3
NiFe-NO -LDH
3
3
final pH
6.66
aging time
48h
2
1
6.70
24h
9h
6.62
12h
6h
6.43
6h
3h
3h
6.20
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
2
/
2 /
Fig. 2. XRD patterns of products when NO−
/urea = 0.33.
Fig. 3. XRD patterns of products when NO−
/urea = 3.
3
3

46
X. Wu et al. / Catalysis Communications 50 (2014) 44–48
3
Fig. 4. Chromatogram analysis of the reaction mixture with NiFe–NO -LDH as the catalyst.
−
process. Briefly speaking, different products appeared under different
reaction conditions. The reason for the appearance of this phenomenon
lied in the transforming of urea concentrations which caused variation
of pH value. Initially, at lower pH value, few carbonate could exist in
the synthesis mixture so that nitrate would be the main compound
available as interlayer anions. As carbonate ions possessing priority to
nitrate ions in entering the interlayer, with pH value rising, as the
concentration of urea permitted the abundant existence of HCO
meeting the conditions to form Ni(HCO ; secondly, when urea con-
centration was high, hydroxyl, deriving from the urea decomposition,
3
till
3 2
)
3 2
would interfere with the solubility equilibrium of Ni(HCO ) by
2
−
−
ratio of CO
force to exclude nitrate from the reaction products. Thus, NiFe–CO
began to emerge. Continually lifting of pH value brought about the
3
/NO
3
reached a threshold level to create sufficient driving
A) NiFe-NO3-LDH
150
3
-LDH
0.4
2
−
mushrooming of CO
product. Furthermore, in comparison with NiFe–CO
fraction peak of NiFe–NO -LDH in Fig. 1 moved slightly to lower angle
3
and thus NiFe–CO
3
-LDH became the unique
120
0.3
2
SBET=90.56(cm /g)
3
-LDH, the main dif-
0
0
.2
.1
3
9
6
0
0
direction, indicating the widening of interlayer space. All these
abovementioned traits of the products might have an effect on their cat-
alytic performances.
0.0
1
0
Pore Width (nm)
3
.1.2. Effects of aging time
30
0
The XRD patterns of the products with different hydrothermal treat-
−
ment times at NO
3
/urea being 0.33 were shown in Fig. 2. After aging for
, agreeing well with
standard powder diffraction patterns (JCPDS#:1520782). Ranging
from 6 h to 48 h, the diffraction peaks of Ni(HCO became lower and
3
3 2
h, the diffraction peaks are indexed to Ni(HCO )
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
3 2
)
lower till disappearing, followed by a new diffraction peak appearing
gradually and becoming sharper and sharper. A set of reflection
appeared at 2θ angles of 11.5°,23.1°,34.4°, 38.9°, 46.3°, 59.9° and
B) NiFe-CO -LDH
3
6
0
0
0
0.18
6
(
1.2°, which can be well indexed to (003), (006), (012), (015), (018),
110) and (113) of NiFe–CO -LDH, respectively. Obviously, diffraction
-LDH are narrow and sharp, exhibiting excellent
5
4
3
0.12
2
peaks of NiFe–CO
3
SBET=25.01(cm /g)
symmetry. No other crystalline phases were detected, suggesting that
NiFe–CO -LDH with unique phase was obtained after hydrothermally
0.06
3
treatment for 48 h at 110 °C. These phenomena evidently manifest
that the composition of the products kept varying with the reaction pro-
30
20
0.00
2
4
6
8
ceeding and shifting from Ni(HCO
3
)
2
3
to NiFe–CO -LDH in the synthesis
Pore Width(nm)
process. The reason causing this shift may be as follows: firstly, low
10
Table 1
0
Catalytic performance of NiFe–CO
3 3
-LDH and NiFe–NO -LDH.
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Sample
Conversion
Selectivity
NiFe–CO
NiFe–NO
3
-LDH
-LDH
0%
51.5%
0%
100%
2
Fig. 5. N adsorption–desorption isotherms and pore size distributions (inset) of NiFe-
LDH.
3

X. Wu et al. / Catalysis Communications 50 (2014) 44–48
47
−
2−
consuming HCO
growing speed of pH value slowed down, Ni(HCO
formed to NiFe−CO -LDH. All the involved reactions were given in se-
quence as follows:
3
and thus generating CO
3
. With the time going, the
3.2. Catalytic behaviors
3 2
)
gradually trans-
3
3.2.1. Catalytic results
Based on the above method, NiFe–NO
3
-LDH and NiFe–CO
3
-LDH
were obtained under the most optimal preparation conditions and
used in the synthesis of benzoin ethyl ether via benzaldehyde and eth-
anol to evaluate their catalytic performance. The conditions of the reac-
tion were as follows: the dosage of NiFe-LDH catalyst being (after
þ
2−
COðNH Þ þ 2H O→2NH þ CO3 ðNH Þ CO →2NH ↑ þ CO ↑ þ H O
2
2
2
4
4
2
3
3
2
2
pretreatment with N
0 mL ethanol; reaction temperature 60 °C and reaction time 60 min.
Fig. 4 exhibited the chromatogram analysis of the reaction mixture
when the reaction performed for 45 min with NiFe–NO -LDH as the cat-
2
blowing for 2 h) 0.1 g, 3 mL benzaldehyde and
þ
−
3
NH þ H O↔NH OH↔NH þ OH
3
2
4
4
3
alyst. As it can be seen from the chromatogram, the constituent peaks
can be respectively ascribed to ethanol, benzaldehyde and benzoin
ethyl ether from left to right. It can be easily confirmed that benzoin
ethyl ether was synthesized efficiently from benzaldehyde and ethanol.
þ
−
þ
2−
CO þ H O↔H CO ↔H þ HCO ↔2H þ CO3
2
2
2
3
3
What's more, the NiFe–NO
times and showed stable activity without structural change. However,
when with NiFe–CO -LDH as catalyst, the peaks of benzoin ethyl ether
3
-LDH catalyst could be recycled several
3
þ
−
2þ
−
Fe þ 3OH →FeðOHÞ ↓
Ni þ 2HCO →NiðHCO Þ ↓
3
3
3
2
3
did not appear, which witnessed the failed synthesis of benzoin ethyl
ether. The catalytic behaviors of these two materials were listed in
Table 1. The results evidenced that the synthesis conditions of the cata-
lyst had a profound effect on its corresponding performances.
Â
Ã
−
2−
FeðOHÞ þ 3NiðHCO Þ þ 6OH → Ni FeðOHÞ ðCO Þ ·6H O þ 3CO
:
3
3
2
3
3
3
3
2
3
Fig. 3 exhibited the XRD patterns of the products with different hy-
−
drothermal treatment times at the ratio of NO
3
/urea of 3. As being
3.2.2. Plausible mechanism
clearly shown in Fig. 3, from 3 h to 48 h, an imperfect structure of
NiFe–NO -LDH was the unique product and a set of characteristic dif-
After investigation, it has been shown that synthesis reaction of
benzoin ethyl ether belongs to the acid catalyzed reaction. The reason
3
fraction peaks appeared at 2θ angles of 9.9°, 19.9° and 34.2°. Here, the
absence of other relevant diffraction peaks implies the existence of crys-
why benzaldehyde and ethanol, catalyzed by NiFe–NO
generate benzoin ethyl ether may be of appropriate porous structure
and L acid sites of the catalyst. N adsorption/desorption test was per-
formed and the results were given in Fig. 5. The specific surface area
3
-LDH, can
3
tal defect. Nevertheless, these defects of NiFe–NO -LDH may be benefi-
2
cial to its catalytic performance.
Scheme 2. Plausible mechanism.

48
X. Wu et al. / Catalysis Communications 50 (2014) 44–48
2
3
of NiFe–NO
3
-LDH is 90.56 m /g with pore volume of 0.21 cm /g.
Appendix A. Supplementary data
Comparatively, NiFe–CO
3
-LDH has relatively smaller surface area
2
3
(
25.01 m /g) and pore volume hovering around 0.06 cm /g. Generally,
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.02.024.
larger surface area and pore volume of porous solids in catalytic applica-
tion are favored to expose the activity as much as possible. Meanwhile,
3
acidic conditions for the synthesis of NiFe–NO -LDH will make more un-
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