A ZrO2-RGO composite as a support enhanced the performance of a Cu-based catalyst in dehydrogenation of diethanolamine

Article abstract of DOI:10.1039/c9ra05458h

The sintering resistance of supported Cu nanoparticle (NP) catalysts is crucial to their practical application in the dehydrogenation of diethanolamine (DEA). In this paper, co-precipitation, hydrothermal synthesis, and sol-gel condensation are used to form a new support material through chemical bonding between graphene oxide and ZrO₂. The composite carriers prepared by the three methods are mixed with copper nitrate and ground using a ball mill. A series of Cu/ZrO₂-reduced graphene oxide (RGO) composites were prepared by calcination under nitrogen at 450 °C for 3 h and hydrogen reduction at 250 °C for 4 h. The conversion of DEA to iminodiacetic acid (IDA) reached 96% with the Cu/ZrO₂-RGO catalyst prepared by hydrothermal synthesis. The conversion rate of DEA is more than 80% following the reuse of the CZG-2 catalyst for twelve cycles. The various physicochemical characterization techniques show that the Cu/ZrO₂-RGO layered and wrinkled nanostructures can improve catalytic stability and suppress the sintering of the supported Cu NPs during the catalytic dehydrogenation of diethanolamine. A synergistic effect between the RGO and the Cu nanoparticles is observed. The Cu nanoparticles with RGO have a better dispersibility, and a new nano-environment is created, which is the key to improving the efficiency of diethanolamine dehydrogenation. These new Cu/ZrO₂-RGO catalysts show increased durability compared to commercially produced Cu/ZrO₂ catalysts and show promise for practical applications involving diethanolamine dehydrogenation.

Full text of DOI:10.1039/c9ra05458h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A ZrO2-RGO composite as a support enhanced the
performance of a Cu-based catalyst in
dehydrogenation of diethanolamine†
Cite this: RSC Adv., 2019, 9, 30439
ab
ab
a
Yongsheng Wang,
Weixiang Xu,
Zhenzhen Zhao,
Li Chen,ab Dongjie Guoab and Zhengkang Duan
Yunlu Zhao,ab Xiaolin Lan,
ab
ab
*
The sintering resistance of supported Cu nanoparticle (NP) catalysts is crucial to their practical application in
the dehydrogenation of diethanolamine (DEA). In this paper, co-precipitation, hydrothermal synthesis, and
sol–gel condensation are used to form a new support material through chemical bonding between
graphene oxide and ZrO2. The composite carriers prepared by the three methods are mixed with copper
nitrate and ground using a ball mill. A series of Cu/ZrO2-reduced graphene oxide (RGO) composites
were prepared by calcination under nitrogen at 450 ꢀC for 3 h and hydrogen reduction at 250 ꢀC for 4 h.
The conversion of DEA to iminodiacetic acid (IDA) reached 96% with the Cu/ZrO2-RGO catalyst
prepared by hydrothermal synthesis. The conversion rate of DEA is more than 80% following the reuse
of the CZG-2 catalyst for twelve cycles. The various physicochemical characterization techniques show
that the Cu/ZrO2-RGO layered and wrinkled nanostructures can improve catalytic stability and suppress
the sintering of the supported Cu NPs during the catalytic dehydrogenation of diethanolamine. A
synergistic effect between the RGO and the Cu nanoparticles is observed. The Cu nanoparticles with
RGO have a better dispersibility, and a new nano-environment is created, which is the key to improving
the efficiency of diethanolamine dehydrogenation. These new Cu/ZrO2-RGO catalysts show increased
durability compared to commercially produced Cu/ZrO2 catalysts and show promise for practical
applications involving diethanolamine dehydrogenation.
Received 16th July 2019
Accepted 16th September 2019
DOI: 10.1039/c9ra05458h
rsc.li/rsc-advances
It is necessary to introduce a carrier to enhance the stability of
the catalyst.3
Introduction
For enhancement in the stabilization of supported Cu NPs,
the sintering mechanism must be understood, which is derived
from the Ostwald ripening process and Brownian-like motion of
Cu NPs on the surface of the oxide support.4 ZrO2 has weak
acidity and alkalinity, good thermal stability, and high
mechanical strength, therefore ZrO2 is widely used as a carrier
material in the eld of catalysts. ZrO2 can act as a charge buffer
for the Cu particles which are losing electrons in the catalytic
reaction, which can effectively improve the activity of copper-
based catalysts.5 ZrO2 has high mechanical strength and good
thermal stability, which can also prevent the sintering of copper
nanoparticles and improve the service lifetime of the catalyst.6
Graphene is a two-dimensional crystal material with the
theoretical thickness of a single-layer being only 0.335 nano-
meters, making it the thinnest two-dimensional material ever
discovered.7 It shows an extremely high mechanical strength.8
The Young's modulus of single-layer graphene can reach 1.0
TPa and the tensile strength may reach up to 130 GPa.9 These
excellent compression and tensile properties make graphene
a support material which can protect metal particles from wear.
Graphene oxide (GO) has oxygen-containing functional groups
incorporated into the carbon structure. Due to the presence of
Iminodiacetic acid (IDA) is an ne chemical intermediate widely
used in agricultural and pharmaceutical water treatment. IDA is
used most commonly as a synthetic raw material for producing
the herbicide, glyphosate.1 The dehydrogenation process of
diethanolamine to prepare IDA has the advantages of environ-
mental protection and high efficiency. Cu-based catalysts are
commonly used in the dehydrogenation of diethanolamine.
Copper-based nanomaterials have good electrical conductivity,
corrosion resistance, high pressure resistance, and low cost.
They are widely used in the elds of catalytic organic conver-
sion, electrocatalysis, and photocatalysis.2 However, catalyti-
cally active copper nanoparticles (Cu NPs) are likely to
agglomerate and are easy to sinter during the catalytic reaction.
aCollege of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan,
China. E-mail: dzk0607@163.com; Tel: +8618907325698
bHunan Collaborative Innovation Center of New Chemical Technologies for
Environmental Benignity and Efficient Resource Utilization, Xiangtan 411105,
Hunan, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05458h
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 30439–30447 | 30439

View Article Online
RSC Advances
Paper
these oxygen-containing functional groups, it is an ideal sup- 1384 cmÀ1 are associated with the O–H bending vibrations in
porting material for metal oxides.10,11 GO has become a wide H2O and GO, respectively. Peaks at 1225 cmÀ1 and 1054 cmÀ1
ranging research interest for scientists and engineers, because are identied as the C–O stretches in the carboxyl groups of GO.
of its unique properties, versatile functionalities, and promising The absorption peak at 590 cmÀ1 may be assigned to the
applications in electronics, optoelectronics, energy storage, and asymmetric stretch vibration of the ZrO2 adsorbed on the GO. In
conversion.12 The composite materials of TiO2/GO and ZnO/GO the composite materials, the characteristic C]O peak from the
have been reported as exhibiting extraordinary photocatalytic GO disappeared while a new, broader peak is formed near
properties.13–17 In heterogeneous catalytic reactions, As an 1160 cmÀ1, which belongs to C–O–Zr. Another characteristic
electrode material, MnO2-GO has demonstrated excellent elec- peak at 1558 cmÀ1 is observed in the composite materials,
trochemical performance.18 RuPd@GO catalysts have shown which may be related to the C]C bonds formed by sp2-
high catalytic activity, good durability, and high dis- hybridization between adjacent carbon atoms in the lamellar
persibility.19–22 Thus, GO was chosen as the rst precursor fol- structure of graphene and reduced graphene oxide.23
lowed by the ZrO2 in order to improve the catalytic anti-
The samples were further analyzed by BET to determine the
sintering and dispersibility of the Cu-based catalyst. The novel properties of the porous structure, which are shown in Table 1
composite support was prepared through chemical reactions below. The specic surface area of the prepared GO is 721.6 m2
between the ZrO2 and the carboxyl and hydroxyl groups on the gÀ1. According to Table 1, ZrO2-RGO composites prepared by
GO.
the three different methods all exhibit higher specic areas
In this study, a new ZrO2-RGO composite support was than pure ZrO2. This can be rationalized by a uniform disper-
prepared by co-precipitation, hydrothermal synthesis, and sol– sion of ZrO2 particles on the surface and in the pores of RGO.
gel methods. Scanning electron microscopy, X-ray diffraction, We note that the specic surface area, pore volume, and average
Fourier-transform infrared spectroscopy, N2 adsorption– pore size of catalysts supported on Cu are slightly smaller
desorption, and H2-temperature programmed reduction were compared with the ZrO2-RGO support. This could be caused by
employed to characterize these catalysts. The inuence of the the blocking of pores in the ZrO2-RGO by the added Cu parti-
methods of adding GO on the material properties and structure cles, which can be seen by XRD.24 Among the ve copper con-
were investigated. Due to the increased performance of previous taining catalysts, CZG-1 has the largest specic surface area
catalysts which incorporated GO, the effect of GO in the struc- with 143.3 m2 gÀ1. This indicates that most of the Cu particles
ture should be identied. In order to do this, we compared the are dispersed homogeneously on the surface of the catalyst
catalytic activity of materials both with and without GO during rather than inside the pores, which would decrease the specic
the formation of IDA from DEA.
surface area.
N2 adsorption–desorption isotherms of ZrO2-RGO compos-
ites are shown in Fig. 2a. There are obvious adsorption hyster-
esis loops in the curves of the materials containing ZrO2. Since
the adsorption and desorption isotherms are separated at
relatively low pressure (0.4 < P/P0 < 0.8), this indicates that the
pore size of the samples is small.25 Fig. 2b presents the pore size
distributions of the ZrO2-RGO composites, which gives a better
understanding of the internal structure of these composite
materials. The ZrO2-RGO composites consist mainly of
Results and discussion
FTIR spectra of the ZrO2-RGO materials prepared by different
methods, as well as the original components ZrO2 and RGO, are
shown in Fig. 1 below. All of the samples have similar charac-
teristic peaks. The broad absorption peak at 3407 cmÀ1 in the
spectrum of GO is attributed to the O–H stretch of H2O, which
indicates that oxide groups exist in GO and H2O molecules can
adsorb to these oxide groups. The band at 1717 cmÀ1 is
assigned to the C]O stretch in carbonyl groups which may be
found on the edges of GO sheets. Peaks at 1624 cmÀ1 and
Table 1 Structural properties of ZrO2-RGO, Cu/ZrO2-RGO, Cu/ZrO2
and Cu/RGO composite materials
Specic surface Pore volume Average pore
Sample
area (m2 gÀ1
)
(cm3 gÀ1
)
size (nm)
DCua (nm)
ZG-1
ZG-2
ZG-3
ZrO2
150.6
130.4
54.5
0.16
0.11
0.25
0.10
0.91
0.14
0.08
0.22
0.05
0.07
3.8
3.8
3.9
3.8
2.2
3.4
3.4
3.5
3.4
3.8
—
—
—
—
38.3
GO
721.6
143.3
129.0
50.2
31.4
40.3
—
CZG-1
CZG-2
CZG-3
Cu/ZrO2
Cu/RGO
13.3
7.1
11.2
10.8
8.3
a
Average crystallite size of metallic copper particles based on XRD
pattern.
Fig. 1 FTIR spectra of ZrO2, GO, and ZrO2-RGO composites.
30440 | RSC Adv., 2019, 9, 30439–30447
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 2 N2 adsorption–desorption isotherms (a) and BJH pore size distributions (b) of ZrO2-RGO composite materials as well as the GO and ZrO2
precursors.
mesopores, with diameters ranging from 2 to 10 nm. Pores in agglomeration. This indicates that hydrothermally prepared
the range of 2–5 nm pores comprise the largest portion of materials have better chemical and physical properties than the
observed pore sizes in the composite material. It is also shown other two methods tested. These results will be conrmed by
that there are very few pores smaller than 2 nm, known as other characterization methods.
micropores, in ZG-3, but there are small amounts of micropores
observed in ZG-1 and ZG-2. This may be part of the reason why TEM to determine their particle size distribution and structural
ZG-1 and ZG-2 have larger specic surface areas than ZG-3. morphologies. The Cu nanoparticles and ZrO2 are distributed
The monodisperse CZG-2 samples were also analyzed by
The SEM images of the GO in Fig. 3a reveal typical layered on the pleated layered RGO surface as shown in Fig. 4a. The
structures, similar to the layered structure of pure graphene in lattice spacings are 0.21 and 0.29 nm, respectively, corre-
terms of regularity. Fig. 3b–d show the morphological differ- sponding to the crystal plane of Cu (111) and t-ZrO2 in Fig. 4b.
ences of CZG-1, CZG-2, and CZG-3 by SEM. The RGO in CZG-1 Elemental scanning of the catalyst in Fig. 4c and d shows
appears to have the expected sheet-like structure, but the uniform distribution of Cu, Zr, C, and O in the catalyst.
sheets are stacked together. In CZG-2, the RGO shows a two-
Further analysis with Raman spectroscopy on the CZG-2
dimensional structure, exhibiting a yarn-like sheet, and wrin- sample showed obvious D (breathing mode of sp2-hybridized
kles in the two-dimensional layer can be seen. Cu/ZrO2 is carbon) and G (graphitic sp2-hybridized carbon) bands, as seen
uniformly dispersed on the wrinkled layer RGO surface. CZG-3 in Fig. 5. Raman spectra veried that the composites also con-
does not show a two-dimensional layer structure similar to sisted of amorphous carbon, from the G-band and D-band at
that seen GO. The two-dimensional wrinkles layer structure 1601 and 1349 cmÀ1, with an intensity ratio of 0.91. The ratio of
improves the dispersibility of Cu nanoparticles and effectively peak intensities for D and G bands (ID/IG) reects the extent of
prevents the sintering of Cu. The RGO in CZG-1 is stacked, so defects on graphene materials.
the dispersibility of Cu cannot be improved, and there is no
XRD patterns of the composite materials prepared by
obvious morphology of RGO in CZG-3 and the catalyst exhibits different methods are shown in Fig. 6a. The peaks in the
spectrum of ZrO2 are consistent with the (011), (112), (121),
(220), and (031) facets, revealing that the sample has tetragonal
symmetry. The characteristic diffraction peak of GO at 2q of 10ꢀ
is not observed in the ZrO2-RGO composites. However, there is
a weak peak at 2q of 23.9ꢀ in the composites, which belongs to
RGO. This shows that GO has been successfully transformed
into RGO. The peaks in the ZrO2-RGO composites are similar to
the pure ZrO2 sample, but the peaks have smaller intensities
and larger full widths at half maximum. According to the
Scherrer formula, we know that a larger FWHM leads to smaller
crystallite sizes in the composite, compared to the pure ZrO2. It
can be seen from the Fig. 6 that ZG-2 has the highest intensity
ZrO2 characteristic peak and the largest half slit width in the ZG
sample. This shows that the ZG carrier prepared by hydro-
thermal method has the smallest size.
Fig. 6b presents the XRD patterns of Cu/ZrO2-RGO. Three
characteristic diffraction peaks of Cu(111), (200), and (220) at
Fig. 3 The SEM images of GO (a), CZG-1 (b), CZG-2 (c), CZG-3 (d).
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 30439–30447 | 30441

View Article Online
RSC Advances
Paper
Fig. 4 The TEM (a and b) images of CZG-2 (inset: particle size distribution); high-magnification STEM image (c) with energy dispersive spec-
trometry (EDS) mappings of Cu (d1), Zr (d2), C (d3), and O (d4) elements.
approximately 2q values of 43.3ꢀ, 50.4ꢀ, 74.1ꢀ can be seen of the catalyst. Notably, the reduction peaks of CuO/ZrO2-RGO
ꢀ
distinctly. There are no peaks from CuO observed in any of the composites, observed between 50–200 C, differ from those of
CZG composites. This does not mean that CuO is not present in CuO/ZrO2 or CuO/RGO. These differences illustrate that strong
these materials. CuO may be present, but is in a highly interactions between CuO and ZrO2-RGO accelerate the electron
dispersed amorphous state, or are found in smaller crystallites, transfer between the active component (CuO) and the support
which may not be able to be detected by XRD. It is also noted (ZrO2-RGO) and enhances the oxidation-reduction ability of the
that the peaks in Cu/ZrO2-RGO aren't as sharp as the original Cu catalyst.26 Preliminary conclusions suggest that the
or ZrO2 samples. This likely occurs because of the strong
interactions between the Cu and ZrO2-RGO support. The half-
value width of the diffraction peak of Cu in the CZG-2 sample
is the largest compared with CZG-1 and CZG-3, indicating that
the Cu crystal particles formed are smaller. Because the ZG-2
carrier has a special, wrinkled layer, the interaction between
the ZG-2 support and Cu reduces the particle size of the Cu
nanoparticles.
In order to further investigate the interaction between ZrO2-
RGO composites and any CuO which may be present in the
composite, H2-TPR was performed on the unreduced precursor
to the composite catalyst and the results are shown in Fig. 7.
Two distinct reduction peaks are observed in the range from
30 ꢀC to 500 ꢀC in all ve materials tested, but the peaks occur at
different temperatures for each material. This seems to suggest
that there are two types of CuO in the composite system. The
main reduction peaks at higher temperatures correspond to
bulk CuO, whereas the shoulder reduction peaks at lower
temperatures are ascribed to well-dispersed CuO on the surface
Fig. 5 Raman spectra of CZG-2.
30442 | RSC Adv., 2019, 9, 30439–30447
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 6 XRD of the ZrO2-RGO samples, GO, and ZrO2 (a); Cu/ZrO2-RGO, Cu/ZrO2 and Cu/RGO (b).
the XPS spectrum of Cu 2p in the catalysts. The binding energy
of Cu0 observed around 933.4–935.6 eV, and the binding energy
of Cu2+ is found between 934.8–937.6 eV. Each catalyst showed
a broad satellite peak in the range of 940–948 eV, which repre-
sents Cu2+ in the catalyst. The CZG-2 catalyst has the highest
satellite peak intensity and it will be shown below that the
catalytic activity was also the highest of the three samples. This
suggests that Cu2+ is an important component of the catalytic
mechanism for the dehydrogenation of diethanolamine. Fig. 8b
shows the XPS spectrum of Zr 3d in the catalysts. Peak tting
shows two species of zirconium, representing Zr 3d5/2 and Zr
3d3/2, are present with a gap between the two of about 2.7 eV.
The relative intensity ratio of Zr 3d5/2 : Zr 3d3/2 is 1.5,27
demonstrating the presence of ZrO2. The rst is at a lower
binding energy, 182.3–183.8 eV (denoted as Zr1), and the second
is at a higher binding energy, 184.7–185.9 eV (Zr2). In each of the
three catalysts, the proportion of Zr1 is larger than Zr2. The Zr1
peaks in the catalyst samples are similar to Zr4+ (182.6 eV) in
pure ZrO2, but these peaks are at higher binding energies,
especially CZG-3 (183.8 eV) because of more oxygen vacancies at
the surface in this sample.28 Zr2, which is at higher binding
energies, is more electron attractive and related to partial Zrd+
site reduction.29
Fig. 7 H2-TPR patterns for the three CuO/ZrO2-RGO composites, as
well as CuO/ZrO2, and CuO/RGO.
hydrothermal synthesis improves the dispersion of CuO and
lowers the catalyst reduction temperature. Combined with SEM,
XRD and catalytic performance results, it can be seen that the
surface of CZG-2 catalyst sample uniformly disperses Cu
nanoparticles with smaller particle size, which can improve the
performance and stability of the catalyst.
In order to identify the catalytic activity of the composite
materials, the dehydrogenation of diethanolamine (DEA) was
XPS was used to investigate the surface electronic states of
Cu and Zr in the different Cu/ZrO2-RGO catalysts. Fig. 8a shows
Fig. 8 XPS of the Cu 2p (a) and Zr 3d (b) for Cu/ZrO2-RGO samples.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 30439–30447 | 30443

View Article Online
RSC Advances
Paper
performed. For all samples, catalytic experiments are carried
out under similar conditions. Potentiometric titration was
chosen to analyze the product iminodiacetic acid (IDA) and the
conversion ratio of the DEA. The catalyst test results are shown
in Table 2.
Through previous studies of the mechanism of DEA dehy-
drogenation,31 it is known that DEA is adsorbed on surface of
the catalyst. Therefore, it is assumed that the larger the amount
of reduced Cu particles on the surface of the composite, the
better the catalytic properties of the material will be. Interest-
ingly, the catalytic activity improved when GO was added to the
catalyst. The experimental results agree with previous literature
results. When adding GO into the catalyst, the specic surface
area of catalysts increases and the dispersion of the active
component, Cu in this study, can be improved. The reducibility
of GO can protect Cu from being oxidized in air. For example,
CZG-1 and CZG-2 have larger specic surface areas than other
composite catalysts, and better activity is also found for these
materials. CZG-1 has the largest specic surface area of the
composites, but CZG-2 has the best catalytic performance.
According to Table 2, the DEA conversion rate of CZG-2 reached
96% and the IDA selectivity reached 92%, respectively. This
suggests that specic surface area is not the dominant factor
inuencing the performance of the DEA dehydrogenation
reaction. The interactions between Cu and ZrO2-RGO as well as
the pore structure appear to have an important inuence on the
performance of the catalyst. The porous nature of the composite
can help present more active sites which may lead to stronger
interactions between Cu and the ZrO2-RGO support and better
catalytic activity of the composite material. Comparing the
sample CZG-2 with the commercially produced RANEY® Cu, the
reaction time was greatly shortened, and the reaction time was
the fastest and the reuse rate was higher than other catalysts in
the literature, such as CZ@CN30 and Cu–Cr.3
Fig. 9 The reusability of Cu/ZrO2 and CZG-2 composite in the
dehydrogenation of diethanolamine.
Fig. 9 shows the reusability of the catalysts. Aer 8 catalytic
cycles with CZG-2, DEA conversion rate remains above 90%, and
the conversion rate decreases slowly until the 12th iteration,
where the conversion rate of DEA is still above 80%. These
results show that the CZG-2 catalyst has good stability. It is
noted that the color of the reaction solution gradually changes
from colorless to blue, with increasing cycle repetition. The
presence of Cu in the reaction solution was detected by induc-
tively coupled plasma (ICP). This shows that Cu leaches from
the composite catalyst during the reaction process. The leach-
ing of Cu is correlated with the reduction of the catalyst activity.
Further testing is needed to determine if this Cu leaching is in
fact the cause of the catalyst deactivation.
It is generally believed that the catalytic mechanism of
catalytic dehydrogenation of diethanolamine, to produce imi-
nodiacetic acid, is similar to the catalytic dehydrogenation of
monoethanolamine, to produce glycine.32 This suggests that
diethanolamine acts as a sub-catalyst under the Cu-based
catalyst. Two molecules of aldehyde are converted to sodium
iminodiacetic acid in a strong base environment. In the Cu/
ZrO2-RGO catalysts, the hydroxyl group on diethanolamine
interacts with the Cu-based catalyst basic sites to capture
protons, thus forming an alkoxide intermediate with the Cu
species. Alkaline solutions have been shown to accelerate the
Catalyst reusability is oen a problem because of material
instability and leaching of the active sites from the catalyst.
Therefore, a reusability experiment was performed to test these
composite catalysts. The CZG-2 catalyst was chosen, as it had
the best performance of the composite materials for repeated
experiments. Aer each cycle, the catalyst was washed to remove
any reaction solution still interacting with the catalyst, followed
ꢀ
by drying at 50 C for 12 hours. A total of 12 cycles were per-
formed, each using the ltered catalyst from the previous run.
Table 2 Activity test results of Cu/ZrO2-RGO composites, as well as Cu/RGO and Cu/ZrO2, which is the current catalyst used in the production
of IDA from DEA
Samples
Time (min)
DEA conversion (%)
IDA selectivity (%)
Yield (%)
Reuse times
CZG-1
CZG-2
CZG-3
Cu/ZrO2
Cu/RGO
CZ@CN30
RANEY® Cu3
Cu–Cr3
240
75
240
150
240
90
41
96
38
91
36
—
—
—
83
92
80
89
82
—
—
—
34
90
30
81
29
92
98
51
—
12
—
4
—
8
600
510
—
—
30444 | RSC Adv., 2019, 9, 30439–30447
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
rate of deprotonation.33 The alkoxide intermediate is adsorbed LTD., USA); NICOLET 380 Fourier infrared spectrometer (FTIR,
on the Cu active sites, and then another active proton is Niccoli, USA); Dmax-2500/PC X-ray polycrystalline powder
removed by a-H cleavage during catalysis by Cu, eventually diffuser (XRD, Kishi Corporation, Japan); model 1200 high
forming an aldehyde. The reaction process for preparing an performance liquid chromatography (Agilent, USA).
ester from an aldehyde may proceed through two reaction
routes: (1) the two aldehyde molecules undergo the Cannizzaro Preparation of graphene oxide (GO)
reaction or (2) the aldehyde and the alcohol undergo a nucleo-
Graphene oxide (GO) was synthesized from NG akes using
philic addition reaction. However, the observed ester cannot be
Hummers' method.35 The prepared graphene oxide was
obtained by the rst route when an a-H is present in the alde-
uniformly dispersed in water using ultrasonication,36 to obtain
hyde, as the a-H will be removed by the base in the reaction to
produce aldol condensation. Therefore, the ester formation
reaction does not proceed through the Cannizzaro route. The
a solution of 1 g LÀ1 concentration. The dispersed solution was
used as-prepared for further experiments.
second route proceeds as follows: the base captures the a-H on
the intermediate aldehyde, thus causing the electrophilic a-C to
appear, thus having a nucleophilic addition. The active alkoxide
Preparation of ZrO2-RGO by co-precipitation
200 mL of a 0.1 M ZrO(NO3)2$5H2O solution was slowly added to
300 mL of the dispersed GO solution. The solution was stirred
reacts to form an ester, which is decomposed in an alkaline
rapidly for 1 h at room temperature. Then, ammonia was added
environment to give the iminodiacetic acid salt.34
as the precipitating agent, until the pH of the mixture reached
approximately 11. The solution was then heated at 70 ꢀC for 4 h.
The deposits were washed with water, until the pH of the
Experimental
Materials
resulting solution was found to be 7. The solution and precip-
itate were held at 60 C for about 12 h in order to remove the
ꢀ
Natural graphite (NG) with an average particle size of 125 mm,
CAS 7782-42-5 was obtained from Aladdin. Zirconium oxynitrate
hydrate (ZrO(NO3)2$5H2O), 99.5% purity, was obtained from
Shanghai Macklin of China. Cupric nitrate (Cu(NO3$3H2O)) and
sodium hydroxide (NaOH) powder were supplied by Tianjin
Chemical Industry Co. Ltd., China. Hydrochloric acid (HCl)
36%–38%, and sulfuric acid (H2SO4) 98%, were obtained from
Hengyang Chemical Industry Co. Ltd., China. Diethanolamine
(A.R. > 99.0%) was provided by Tianjin Hengxing Chemical
Industry of China. 25% Ammonia solution (NH3$H2O),
CAS1336-21-6, and citric acid powder (C6H8O7), CAS77-92-9,
were from Hengyang Kaixin Chemical Industry Co. Ltd.,
China. All chemical reagents are analytical grade, and ultrapure
water was used throughout the experiments.
solvent and fully dry the precipitate. Finally, the product,
denoted as ZG-1, was fabricated by hot pressing using a tubular
furnace under a nitrogen atmosphere at 450 C for 3 h.
ꢀ
Preparation of ZrO2-RGO by hydrothermal synthesis
The ZrO2-RGO solution is the same as used in the co-
precipitation method, but a different precipitating agent was
chosen. 0.5 M NaOH was added slowly to the ZrO2-RGO solution
until the solution was pH 9. The solution was transferred ꢀto
a hydrothermal reactor with a Teon-lined autoclave at 170 C
for 12 h. Aer repeated washing with distilled water, the
product, denoted as ZG-2, was heated for an additional 12 h at
ꢀ
60 C to remove all solvent.
Catalyst characterizations
Preparation of ZrO2-RGO sol–gel
FEI Tecnai G2 F20 transmission electron microscope (TEM, The ZrO2-RGO solution was prepared as in the other two
FEI); SU8200 eld emission scanning electron microscope synthetic methods. 200 mL of a 1 M citric acid solution was
(SEM, Hitachi, Japan); TriStar II 3020 specic surface area and slowly pumped into the ZrO2-RGO solution over 1 h with stir-
aperture analyzer (BET, McMuratick Instrument Co., LTD., ring. Aer 30 min of additional stirring, ammonia was added
USA); AutoChem II 2920 (H2-TPR McMuratick Instrument Co., dropwise to the mixture until the solution reached pH 5. The
Fig. 10 Schematic diagram of Cu/ZrO2-RGO catalyst.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 30439–30447 | 30445

View Article Online
RSC Advances
Paper
solution was heated in a water bath at 80 ꢀC for 4 h. The
resulting product, denoted as ZG-3, was dried in air and
calcined under nitrogen at 400 C for 3 h.
Conflicts of interest
ꢀ
There are no conicts of interest to declare.
Preparation of Cu/ZrO2-RGO
Acknowledgements
The three ZrO2-RGO products, ZG-1, ZG-2, and ZG-3, (6.44 g)
were each mixed separately with Cu(NO3)2$3H2O (4.84 g) and
ground together using a ball mill. Cu/ZrO2-RGO composites,
independently named as CZG-1, CZG-2, CZG-3, were prepared
This work was supported by the National Natural Science
Foundation of China (21576229).
ꢀ
by calcination under nitrogen at 450 C for 3 h and hydrogen
Notes and references
reduction at 250 ꢀC for 4 h. To highlight the importance of the
structure and composition of CZG, the corresponding ZrO2, Cu/
ZrO2, and Cu/RGO materials were made and compared to the
composite materials. The preparation procedure for the ZrO2,
Cu/ZrO2, and Cu/RGO materials can be found in the ESI.† The
schematic diagram of Cu/ZrO2-RGO catalyst is shown in Fig. 10.
1 G. H. Lu, X. M. Hua, J. Cheng, Y. L. Zhu, G. H. Wang,
Y. J. Pang, R. W. Yang, L. Zhang, H. Shou, X. M. Wang,
J. Qi and Y. H. Yang, Curr. Genomics, 2018, 19, 36–49.
2 Y. Zhu, X. Kong, X. Li, G. Ding, Y. Zhu and Y. W. Li, ACS
Catal., 2014, 4, 3612–3620.
3 D. A. Hickman, K. Mosner and J. W. Ringer, Chem. Eng. J.,
2015, 278, 447–453.
4 Z. Liu, R. Che, A. A. Elzatahry and D. Zhao, ACS Nano, 2014, 8,
10455–10460.
5 Z. K. Duan, S. Li, F. Xie, J. H. Yan and T. Zhang, Chem. Res.
Appl., 2015, 27, 417–423.
Catalytic evaluation
Cu/ZrO2-RGO was used as a catalyst for the dehydrogenation of
diethanolamine. Diethanolamine and sodium hydroxide (1 : 2.5
mol : mol) were reacted with 4.5 g catalyst at 160 ꢀC for 4 h. The
content of sodium iminodiacetic acid (DSIDA) in the reaction
solution was analyzed by high-performance liquid chromatog-
raphy (HPLC).
´
´
6 K. Samson, M. Sliwa, R. P. Socha, K. Gora-Marek, D. Mucha,
D. Rutkowska-Zbik, J. F. Paul, M. Ruggiero-Mikołajczyk,
´
R. Grabowski and J. Słoczynsk, ACS Catal., 2016, 4, 3730–
3741.
`
7 P. Y. Chen and A. Alu, ACS Nano, 2011, 5, 5855–5863.
Conclusions
8 A. C. Neto and A. Geim, New Sci., 2012, 214, iv–v.
9 J. Hwang, T. Yoon, S. H. Jin, J. Lee, T. S. Kim, S. H. Hong and
S. Jeon, Adv. Mater., 2013, 25, 6724–6729.
In this paper, ZrO2-RGO composite carriers successfully incor-
porated copper-based catalysts in order to obtain high catalytic
dehydrogenation activity, good stability, and good dis- 10 A. M. Dimiev and J. M. Tour, ACS Nano, 2014, 8, 3060–3068.
persibility. The ZrO2-RGO composite carriers prepared by 11 A. Malas and C. K. Das, Composites, Part B, 2015, 79, 639–648.
hydrothermal, sol–gel, and coprecipitation methods were used 12 X. Cai, Y. Luo, B. Liu and H. M. Cheng, Chem. Soc. Rev., 2018,
as a raw material to synthesize Cu/ZrO2-RGO catalysts. RGO
with a folded layer structure was introduced as the second 13 M. Kumar, Z. Gholamvand, A. Morrissey, K. Nolan,
carrier for the catalyst to improve the dispersion and sintering M. Ulbricht and J. Lawler, J. Membr. Sci., 2016, 506, 38–49.
resistance of the catalyst. The performance of the Cu/ZrO2-RGO 14 M. Rashid, P. K. Mondal, S. Q. Usmani and S. Sabir, Journal
catalyst prepared by hydrothermal synthesis of ZrO2-RGO of Industrial Research & Technology, 2013, 3, 72–76.
carrier is best compared with the commercial RANEY® Cu or 15 E. Rokhsat and O. Akhavan, Appl. Surf. Sci., 2016, 371, 590–
the literature,3,29 and the reaction time is the shortest. This is
595.
because Cu and ZrO2 are uniformly distributed on the surface of 16 H. Yang, J. Jiang, W. Zhou, L. Lei, L. Xi, Y. M. Lam, Z. Shen,
the pleated, layered RGO, and the Cu nanoparticles have higher B. Khezri and T. Yu, Nanoscale Res. Lett., 2011, 6, 531–538.
dispersibility. This provides a new method for the preparation 17 Y. Mithilesh, R. Y. Kyong, S. J. Park and D. Hui, Composites,
of copper-based catalysts with excellent dispersibility and sin- Part B, 2014, 66, 89–96.
tering resistance. Catalyst cycling tests show that the CZG-2 18 X. Huang, K. Shi, J. Yang, G. Mao and J. Chen, J. Power
catalyst can still dehydrogenate diethanolamine aer 12 cycles Sources, 2017, 356, 72–79.
of reaction. The conversion to IDA is still 80% aer the 12th 19 T. Demirci, B. Çelik, Y. Yıldız, S. Eri¸s, M. Arslan, F. Sen and
cycle, indicating that the hydrothermally prepared ZrO2-RGO B. Kilbas, RSC Adv., 2016, 80, 76948–76956.
carrier can prevent the sintering and agglomeration of Cu 20 G. Haydar, Y. Yıldız, B. Çelik, M. Yazıcı, B. Kilbas and F. Sen,
nanoparticles and improve the stability of the catalyst. The ChemistrySelect, 2016, 1, 953–958.
Cannizzaro reaction in the dehydrogenation of diethanolamine 21 G. Haydar, Y. Yıldız, B. Çelik, M. Yazıcı, B. Kilbas and F. Sen,
to prepare sodium iminodiacetic acid was negated, and the Catal. Sci. Technol., 2016, 6, 2318–2324.
nucleophilic addition reaction of aldehyde and alcohol was 22 B. ¸Sen, E. H. Akdere, A. ¸Savk, E. Gultekin, O. Paralı, H. Goksu
proposed for the reaction mechanism. However, the leaching and F. ¸Sen, Appl. Catal., B, 2018, 225, 148–153.
problem of Cu in the catalyst needs further study in order to 23 K. Krishnamoorthy, M. Veerapandian, K. Yun and S. J. Kim,
47, 6224–6266.
¨
¨
¨
improve the cycling lifetime of the catalyst.
Carbon, 2013, 53, 38–49.
30446 | RSC Adv., 2019, 9, 30439–30447
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
24 Y. Q. Song, D. H. He and B. Q. Xu, Appl. Catal., A, 2008, 337, 31 G. Carotenuto, R. Tesser, M. D. Serio and E. Santacesaria,
19–28. Catal. Today, 2013, 203, 202–210.
25 X. M. Liu, H. X. Xue, X. Li and Z. F. Yan, Catal. Today, 2010, 32 Y. Yang, Z. Duan and W. Liu, Chem. React. Eng. Technol.,
158, 446–451. 2001, 17, 210–215.
26 L. C. Wang, Q. Liu, M. Chen, Y. M. Liu, Y. Cao, H. Y. He and 33 E. Balaraman, E. Khaskin, G. Leitus and D. Milstein, Nat.
K. N. Fan, J. Phys. Chem. C, 2007, 111, 16549–16557. Chem., 2013, 5, 122–125.
27 J. Wan, P. Yang, X. L. Guo and R. X. Zhou, Chin. J. Catal., 34 M. Neurock, Z. Tao, A. Chemburkar, D. D. Hibbitts and
2019, 40, 1100–1108. E. Lglesia, Faraday Discuss., 2017, 197, 59–86.
28 A. G. Sato, D. P. Volanti, D. M. Meira, S. Damyanova and 35 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskill,
J. M. C. Bueno, J. Catal., 2013, 307, 1–17.
29 S. Ardizzone and C. L. Bianchi, Surf. Interface Anal., 2015, 30,
77–80.
Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,
ACS Nano, 2010, 4, 4806–4814.
˜
36 Y. Liu, H. V. Babu, J. Zhao, A. Goni-Urtiaga, R. Sainz,
30 Y. Wang, Y. Zhao, Z. Zhao, X. Lan, J. Xu, W. Xu and Z. Duan,
Acta Chim. Sin., 2019, 71, 661–668.
R. Ferritto, M. Pita and D. Wang, Composites, Part B, 2016,
89, 108–116.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 30439–30447 | 30447