Pd/Mg(OH)2 Heterogeneous Nanocatalysts Synthesized by a Facile One-Pot Hydrothermal Method for CO Direct Esterification to Dimethyl Oxalate

Article abstract of DOI:10.1007/s10562-021-03559-y

Pd-based heterogeneous nanocatalysts have wide application in chemical industry. However, the traditional synthesis process contains multi-steps such as impregnation, dry, calcination and reduction. The pre-synthesis nanoparticles process can reduce the steps, but need to remove the surfactants, which are added in the synthesis process. In this work, a facile one-pot hydrothermal synthesis process named as single molecular precursor method was successfully developed to prepare Pd/Mg(OH)₂ heterogeneous nanocatalysts with clean surface. The as-synthesized Pd/Mg(OH)₂ heterogeneous nanocatalysts show excellent performance for CO direct esterification to dimethyl oxalate (DMO). The WTY (weight time yield) of DMO can reach the high-performance of 2544?g?kg_cat.^?1?h^?1, while the conversion of CO is 62.6% and selectivity to DMO is 90.8%. The single molecular precursor method developed by this work can be extended to other supported noble metal nanocatalysts. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-021-03559-y

Catalysis Letters
https://doi.org/10.1007/s10562-021-03559-y
Pd/Mg(OH)2 Heterogeneous Nanocatalysts Synthesized by a Facile
One‑Pot Hydrothermal Method for CO Direct Esterifcation to Dimethyl
Oxalate
Xiao‑Qi Lin1,2 · Zhi‑Qiao Wang1 · Zhong‑Ning Xu1 · Guo‑Cong Guo1
Received: 7 January 2021 / Accepted: 5 February 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
Pd-based heterogeneous nanocatalysts have wide application in chemical industry. However, the traditional synthesis pro-
cess contains multi-steps such as impregnation, dry, calcination and reduction. The pre-synthesis nanoparticles process can
reduce the steps, but need to remove the surfactants, which are added in the synthesis process. In this work, a facile one-pot
hydrothermal synthesis process named as single molecular precursor method was successfully developed to prepare Pd/
Mg(OH)2 heterogeneous nanocatalysts with clean surface. The as-synthesized Pd/Mg(OH)2 heterogeneous nanocatalysts
show excellent performance for CO direct esterifcation to dimethyl oxalate (DMO). The WTY (weight time yield) of DMO
can reach the high-performance of 2544 g kgcat.−1 h−1, while the conversion of CO is 62.6% and selectivity to DMO is 90.8%.
The single molecular precursor method developed by this work can be extended to other supported noble metal nanocatalysts.
* Zhi-Qiao Wang
842966248@qq.com
* Guo-Cong Guo
gcguo@firsm.ac.cn
1
State Key Laboratory of Structural Chemistry, Fujian
Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, Fujian,
People’s Republic of China
2
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
Vol.:(0123456789)
1 3

X.-Q. Lin et al.
Graphic Abstract
Keywords Heterogeneous nanocatalysts · Single molecular precursor method · CO direct esterifcation · Dimethyl oxalate
easily lead to particles aggregation at elevated reaction
temperatures [9]. Because of this, much work engaged
in new synthetic strategies. Studies have shown that the
introduction of organic compounds as surfactant agents or
structure guide agents (such as cetyltrimethyl ammonium
bromidecan (CTAB), poly-vinylpyrrolidone (PVP), poly-
ethylene glycol (PEG), etc.) can control efectively the size
and dispersion of Pd nanoparticles, namely pre-synthesis
nanoparticles process [11–14]. Nevertheless, stabilizers
that adsorbed strongly on the surface of active metal have
been extremely limited in application due to contamination
inevitably of the surface and resistance to reaction difu-
sion [12]. It is worth mentioning that the removal of the
stabilizers is often accompanied by harsh post-treatments,
which is extremely detrimental to the catalytic activity.
What’s worse, complete removal is almost impossible
[14]. Recently, atomic layer deposition (ALD) has been
demonstrated as an efective pathway for synthesizing Pd-
based nanocatalysts without the use of surfactants [8, 15].
Interestingly, metal particles can be deposited directly on
1 Introduction
Owing to its unique properties and broad applications, a
research theme about noble-metal materials (e.g. Pd, Pt, Au)
has been for decades [1]. It has played an important role
in catalytic feld, especially in heterogeneous catalytic pro-
cesses, such as oxidation, hydrogenation/dehydrogenation,
carbonylation, etc. [2–4]. Initially, Pd nanoparticles with
uniform geometries or high-index facets were extensively
studied [1, 5]. However, Pd nanoparticles with the small
particle size and the large surface energy as catalysts are
often unstable during catalytic application, leading to activ-
ity rapidly decays [3]. Several works have shown that this
problem can be overcome by anchoring and highly dispersed
the nanoparticles on the support as Pd-based heterogeneous
nanocatalysts [6].
Increasing attention has been paid to the fabrication
of Pd-based heterogeneous nanocatalysts, various syn-
thetic methods have been proposed [7–10]. Among that,
conventional wet impregnation and co-precipitation can
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Pd/Mg(OH)2 Heterogeneous Nanocatalysts Synthesized by a Facile One-Pot Hydrothermal Method…
the support through ALD and no post-treatment is encoun-
tered. Unfortunately, the chemisorption kinetics of the
ALD metal precursor are relatively slow, easily leading
to the sintering of metal species, thus afecting catalytic
activity [8].
2 Experimental
2.1 Catalysts Preparation
C3 catalyst (denoted as C3) was prepared by one-pot
single molecular precursor method. 0.0753 g palladium
acetylacetone [Pd(acac)2] was dissolved in 200 mL deion-
ized water under stirred and ultrasound. 5 g of MgO was
added in the palladium precursor solution with ultrasound
for another 5 min. After that, the mixture was transferred
to a 500 mL Tefon-lined steel autoclave and maintained
at 180 °C for 12 h. After cooling the mixture to room tem-
perature, the precipitate was centrifuged and washed with
ultrapure water and ethyl alcohol several times and then
dried at 80 °C for 12 h under vacuum. Meanwhile, C1 and
C2 catalyst were also obtained by heat treatment during
synthesis for 3 h and 5 h, respectively. For comparison,
C0-TI catalyst (denoted as C0-TI) was prepared by tradi-
tional impregnation reduction method using Mg(OH)2 as
support and Pd(acac)2 as palladium precursor. Details of
the preparation procedures are present in the supplemen-
tary information (SI).
Dimethyl oxalate (DMO) is an important raw chemi-
cal, which usually is used for synthesizing oxalic acid
and oxamide, etc. [15]. In the coal to ethylene glycol
(EG) technology, EG is obtained from the hydrogena-
tion of DMO, while DMO is synthesized from the CO
direct esterifcation reaction [17]. CO direct esterifcation
involves processes using CO as the starting material and
ester chemicals (DMO, dimethyl carbonate (DMC) and
methyl formate (MF), etc.) as products [16]. In traditional,
it is reported that DMO can be obtained from the reaction
of oxalic acid and alcohol [18]. However, the long produc-
tion cycle limits the application. Compared with the con-
ventional esterifcation method, CO direct esterifcation
to DMO (2CO+2CH3ONO=(COOCH3)2 +2NO, NO can
be reused to prepare CH3ONO) has drawn more and more
attentions from scientists and researchers. As an important
route to realize the transformation from inorganic carbon
to organic carbon, CO direct esterifcation to DMO has
been studied extensively. In recent years, Pd-based cata-
lysts have been widely applied for CO direct esterifcation
to DMO [13, 19]. Tremendous eforts have been devoted
to the design of Pd-based heterogeneous nanocatalysts.
However, several steps are needed for preparation, and
surfactants and structural guide agents are introducing
in the synthesis process, which is difcult to obtain Pd-
based catalysts with clean surface. Based on this concept,
a facile approach to prepare high-performance Pd-based
heterogeneous nanocatalysts with clean surface is still a
great challenge.
2.2 Catalysts Characterization
X-ray difraction (XRD) patterns were recorded on a glass
wafer by a Rigaku MiniFlex 600 difractometer. Induc-
tively Coupled Plasma (ICP) elemental analysis measure-
ments were carried out on an Ultima2 spectrometer. The
Brunauere–Emmette–Teller (BET) surface area is per-
formed on ASAP 2020 instrument (Micromeritics). Sam-
ples for transmission electron microscopy (TEM) and high
resolution TEM (HRTEM) observations were examined
by a TEM (JEM-2010). X-ray photoelectron spectroscopy
(XPS) measurements were taken with a VG Escalab 250
spectrometer equipped with an Al (Al-Kα = 1486.7 eV)
anode and an Mg anode (Mg-Kα = 1253.6 eV). In situ dif-
fuse refectance infrared spectroscopy (DRIRS) measure-
ments were performed on a Nicolet 6700 difuse refec-
tance infrared spectrometer equipped with a stainless steel
in situ IR fow cell.
Herein, we provide a one-pot hydrothermal synthesis pro-
cess named as single molecular precursor synthesis strategy
to obtain Pd/Mg(OH)2 heterogeneous nanocatalysts with
clean surface. It analyzes through a variety of characteriza-
tions such as XRD, BET, XPS, HRTEM and in situ DRIRS.
Under high temperature (180 °C) hydrothermal conditions,
Mg(OH)2 has been formed through reaction of MgO and
boiling water. At the same time, Pd nanoparticles as active
species could be easily introduced into the sample and obtain
Pd/Mg(OH)2 heterogeneous nanocatalysts, which takes pal-
ladium acetylacetone as single molecular precursor. The as-
synthesized Pd/Mg(OH)2 nanocatalysts show excellent per-
formance for CO direct esterifcation to DMO. The WTY of
DMO can reach the high-performance of 2544 g kgcat.−1 h−1,
while the conversion of CO is 62.6% and selectivity to DMO
is 90.8%. Its preparation process is much facile compared to
traditional impregnation-reduction method containing multi-
steps. This work will provide a facile and efective way to
preparation of high-performance supported nanocatalysts.
2.3 Catalytic Activity Measurements
The catalytic activities of catalysts for CO direct esterifca-
tion to DMO were evaluated on a fxed-bed quartz tubular
reactor with 100 mg catalysts. The reactant gases include
28% CO, 20% CH3ONO, 4% Ar as internal standard and
48% N2 as the balance gas (weight hour space velocity
(WHSV) about 6000 mL g−1 h−1). The activity of catalyst
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X.-Q. Lin et al.
(CO conversion and DMO selectivity, see more details
for SI) was evaluated under atmospheric pressure by an
online Shimadzu GC-2014 gas chromatograph equipped
with a thermal conductivity detector and a fame ioniza-
tion detector.
well with Mg(OH)2 phase (JCPDS: No. 07–0239), indicat-
ing that the MgO reacts with the boiling water in the system
(MgO+H2O=Mg(OH)2 at 453 K). For comparison, C0-TI
was synthesized by using the traditional wet-impregnation
method with commercial Mg(OH)2 support. It was found
in Fig. 1 that the XRD patterns of C3 and C0-TI catalysts
could be attributed to the same crystalline phase. There is
no obvious difraction peak of Pd in all catalysts (Fig. 1 and
Fig. S1), which suggests that the particle size of palladium
is very small or very low crystallinity. By the ICP measure-
ment, the Pd content in C1, C2, C3 and C0-TI catalysts
are 0.300%, 0.380%, 0.368%, 0.381%, respectively, which
further confrms the above conjecture.
3 Results and Discussions
We prepared C1, C2 and C3 catalysts by a one-pot single
molecular precursor synthesis strategy with diferent heating
time. As shown in Fig. S1, all the difraction peaks matched
It is worthy to mention that the morphologies of the syn-
thetic and commercial samples are diferent, which can be
revealed by the TEM images (Fig. S2). It can be seen that the
commercial Mg(OH)2 sample shows a lumpy shape, while
C3 is the thickness of Mg(OH)2 nanosheets and it stacks to
form a cluster of fowers, which may ofer a high surface
area for dispersing the metal. Physicochemical properties
of catalysts are shown in Table S1. The surface area of C1,
C2 and C3 catalysts was far higher than that of commer-
cial Mg(OH)2 (~80 vs.>4 m2 g−1, respectively, Table S1),
ofering the prospect of a higher density of anchor sites to
immobilize metal atoms. The BET surface area of C3 is
higher than that of C1, C2 and C0-TI, which is conducive to
the full contact between reactants and catalysts and improves
the catalytic performance.
HRTEM images presented in Fig. 2a and b reveal that
the Pd nanoparticles of C3 are highly dispersed on the
Mg(OH)2 support, while the Pd nanoparticles of C0-TI
are somewhat aggregated into large nanoparticles during
Fig. 1 PXRD patterns of C3 and C0-TI catalysts. Vertical bars at the
bottom denote the standard data for a brucite structure of Mg(OH)2
(JCPDS: No. 07-0239)
Fig. 2 HRTEM images of C3 (a) and C0-TI catalysts (b), respectively
1 3

Pd/Mg(OH)2 Heterogeneous Nanocatalysts Synthesized by a Facile One-Pot Hydrothermal Method…
the catalyst preparation process. On the basis of the size
distribution histograms shown in the insets of Fig. S3, Pd
nanoparticles of C3 revealed small Pd nanoparticles about
1.3 nm and a little Pd nanoparticles about 6 nm, which is
much smaller than that of C0-TI (4.0–6.2 nm) as a whole.
The TEM (Fig. S3a) images of C1 showed that the large Pd
nanoparticles are obvious with an average size approximate
8.7 nm. Similarly, the TEM (Fig. S3b) images of C2 show
that the average Pd size is about 6.6 nm, which is larger than
that of C0-TI (4.0–6.2 nm). Previous reports have shown
that the smaller size of the Pd active component facilitates
the CO direct esterifcation reaction [13]. It illustrates that
the catalytic activity of C0-TI is slightly higher than that
of C1 and C2, but lower than that of C3 catalysts. Unfor-
tunately, it is easily for Mg(OH)2 support to be hydrolyzed
by electron breakdown, leading to the observation of the
Mg(OH)2 support and the metal is not clear by TEM images.
Fig. 3 Pd 3d XPS (Mg Kα source) spectra of C3 and C0-TI catalysts
It was necessary to employ HRTEM and HRTEM-mapping
in order to characterize the dispersion of the supported Pd
on the Mg(OH)2 support. Active component Pd could be
clearly identifed in the HRTEM image of C3, and the Pd
nanoparticles were highly dispersed in the mapping results
(Fig. 2). As a contrast, Pd particle size of C0-TI is obviously
much bigger than that. Therefore, C3 with much smaller
particle size and much higher dispersion of Pd nanoparticles
can easily account for the higher catalytic activity.
process [20]. Furthermore, the oxidized species increase
with the extension of heat treatment time. Figure 3 ofers
ancillary evidence of the electronic state of Pd, revealing
identical Pd 3d5/2 binding energies of 336.0 eV for the C3,
characteristic of Pdδ+ (0<δ<2) species while Pd 3d5/2 bind-
ing energies of 337.0 eV for the C3, characteristic of Pd2+
species. Moreover, one possible cause of producing partial
Pd2+ is the surfaces of the catalysts are partially oxidized
by the air during the characterization process [22]. Surface
Pdδ+ species for CO direct esterifcation reaction are easily
reduced to Pd0 active component by CO, which further leads
to the formation of DMO as main products [16]. Besides, the
appearance of the two remaining peaks with binding ener-
gies of C3 prepared by single molecular precursor method
shifts higher than 340.7 and 335.5 eV of the C0-TI (Fig. 3).
Based on the above experimental facts, we can infer that the
introduction of a single molecular precursor method into
the reaction system has a great infuence on the interaction
between metal and support, signifcantly increases the dis-
persion of active component Pd, and it also decreases the
ensemble size of Pd nanoparticles over the C3.
Figure S4 provides the XPS overview spectrum (Al Kα
source) for C3. It can be seen from the fgure that the posi-
tion of the Mg 2p peak at 49.2 eV is attributed to the pres-
ence of Mg(OH)2 [20]. This is consistent with the structure
of XRD as mentioned above. Furthermore, the signal of Pd
peak is difcult to detect as Mg KLL peak (300–350 eV)
overlaps in the region of Pd 3d5/2 peak and Pd loading is
low. Hence, we further probe the Pd 3d peak using an Mg
Kα radiation source, which can eliminate the efect of Mg
KLL peak. In general, the peak around 335.0 eV assigns
to Pd0, while one around 337.0 eV assigns to Pd2+ [13].
When the heat treatment is only 3 h, the electron state of
Pd over C1 catalyst is still biased towards Pd0 as two peaks
at around 340.4 and 335.2 eV (Fig. S5), suggesting that the
oxidation state of the Pd is mainly Pd0. Pd(II) complexes
with acetylacetonate as ligand have the characteristics of
easy decomposition and good difusion performance due
to the introduction of acetylacetonate group [23]. During
preparation via the single molecular precursor method, pal-
ladium acetylacetone as single molecular precursor is rap-
idly decomposed by heat to form Pd nanoparticles, which
are deposited on the support [24]. While another two small
peaks located at 342.1 and 336.0 eV are attributed to Pdδ+
(0 < δ < 2). Indeed, it is likely that the obtained Pd nano-
particles showed characteristic of Pdδ+ species is due to the
re-oxidation by adsorbing oxygen or hydroxyl groups on
the surface of Mg(OH)2 during the hydrothermal reaction
Moreover, the interaction between the metal and sup-
port increases with the extension of the heat treatment time,
which can improve CO adsorption and thus enhance cat-
alytic activity [13]. CO adsorption can be further proved
by in situ DRIRS of CO adsorption. As shown in Fig. 4,
the peaks of 2019 cm−1 and 1925 cm−1 over C3 catalyst
are attributed to the bridge and linear adsorption of CO on
the surface of the Pd species, respectively [21]. It can be
observed that the broad peak of C3 is still slightly higher
than that of C1 and C2 from in situ DRIRS spectra of CO
adsorption on C1, C2, C3 catalysts (Fig. S6). CO adsorption
peak of C3 is the largest while there is only a small amount
CO adsorbed in the C0-TI catalyst after N2 sweeping. It
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X.-Q. Lin et al.
reach up to 62.6%, close to the theoretical maximum value of
71.4%. It implies that methyl nitrite was almost completely
consumed, which can be verifed using gas chromatography.
What’s more, the duration lifetime test of C3 can be stable
for at least 100 h at 403 K (Fig. S7) when it was applied
to CO direct esterifcation to dimethyl oxalate. In addition,
XRD patterns of C3 before reaction and after reaction show
no obvious difraction peak of Pd species (Fig. S8). Even
after use in the reaction, no obvious aggregation of Pd spe-
cies was observed. This highly efcient and longlived nano-
catalysts prepared via a facile one-pot single molecular pre-
cursor method may have a potential industrial application.
4 Conclusion
Fig. 4 In situ DRIRS spectra of CO adsorption on C3 and C0-TI cat-
alysts at 293 K in the range from 2050 to 1900 cm−1
In summary, we developed a facile one-pot hydrothermal
synthesis process named as single molecular precursor
method to prepare Pd/Mg(OH)2 heterogeneous nanocatalysts
with clean surface. Notably, the as-synthesized Pd/Mg(OH)2
heterogeneous nanocatalysts exhibits excellent performance
when applied in CO direct esterifcation to DMO, and the
WTY of DMO is 2544 g kgcat.−1 h−1. The obtained catalysts
were fully characterized by the XRD, BET, XPS, HRTEM
and in situ DRIRS analyses. It was shown that, the single
molecular precursor method has obvious superiority to the
traditional impregnation-reduction method and pre-synthesis
nanoparticles process. This new method can be extended
to other supported noble metal nanocatalysts with clean
surface.
Table 1 The performances of C3 and C0-TI catalysts used for CO
direct esterifcation to DMO
Catalysts
Conversion of
CO (%)
Selectivity to
DMO (%)
WTYa
(g kgcat.−1 h−1
)
C3b
62.6
57.7
90.8
90.8
2544
2347
C0-TIc
Reaction conditions: 100 mg of catalyst, 6000 mL g−1 h−1 of weight
hour space velocity (WHSV), reactants CO/CH3NO volume ratio 1.4,
0.1 MPa, 403 K (for more details see SI)
aWTY represents the weight-time yield, grams of DMO per kilo-
grams of catalyst per hour (g kgcat−1 h−1
)
bC3 prepared with the heating time of 12 h
cC0-TI prepared with the traditional impregnation-reduction prepara-
Supplementary Information The online version of this article (https://
doi.org/10.1007/s10562-021-03559-y) contains supplementary mate-
rial, which is available to authorized users.
tion
Acknowledgements This work was supported by the National Key
Research and Development Program of China (2017YFA0206802,
2017YFA0700103, 2018YFA0704500), the Programs of the Chinese
Academy of Sciences (QYZDJ-SSW-SLH028), the Natural Science
Foundation of Fujian Province (2018J06005, 2018J05036).
indicates that C3 has a better CO adsorption and catalytic
activities, which is consistent with the results of catalytic
evaluation. Herein, we suggest that single molecular precur-
sor method is an efective access to strength the interaction
between metal and support and optimize the dispersion of
active component Pd.
Compliance with Ethical Standards
The evaluation of catalytic performance for CO direct
esterifcation to DMO was carried out in a home-made cat-
alytic evaluation device. C0-TI using traditional impreg-
nation-reduction method were evaluated under the same
reaction conditions as those that used a single molecular
precursor method. As shown in Table 1, when used for CO
direct esterifcation to DMO, the average WTY of DMO at
the same reaction conditions demonstrate that the activity
of C3 shows better activity than C0-TI. It’s noting worth
that the catalytic performance improves as the heat treat-
ment time increases (Table S2). CO conversion of C3 can
Conflict of interest The authors declare no confict of interest.
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