Effect of leaching temperature on structure and performance of Raney Cu catalysts for hydrogenation of dimethyl oxalate

Article abstract of DOI:10.1016/j.apcata.2015.10.029

As-synthesized Raney Cu catalysts through selective leaching of Cu-Al (50 wt%Cu-50 wt%Al) alloy pellets in sodium hydroxide solution have been systematically investigated during the catalyst preparation process. Results reveal that the microstructures, physicochemical and surface properties of the Raney Cu catalysts depend profoundly on the leaching temperature. In gas-phase hydrogenation of dimethyl oxalate (DMO), the Raney Cu catalysts prepared at the mild leaching conditions tend to deactivate rapidly, the formation of methyl glycolate (MG). As-prepared catalysts at higher leaching temperature exhibit excellent stability, favoring the synthesis of ethylene glycol (EG). Especially, Raney Cu catalyst obtained at 40°C shows the superior stability and MG selectivity of 95.0% in DMO hydrogenation. Furthermore, the reasons for deactivation of Raney Cu catalyst in DMO hydrogenation have been discussed in detail.

Full text of DOI:10.1016/j.apcata.2015.10.029

Accepted Manuscript
Title: Effect of leaching temperature on structure and
performance of Raney Cu catalysts for hydrogenation of
dimethyl oxalate
Author: Xiangpeng Kong Cailian Ma Juan Zhang Jiaqiang
Sun Jiangang Chen Kefeng Liu
PII:
S0926-860X(15)30206-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.029
APCATA 15605
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Applied Catalysis A: General
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Revised date:
Accepted date:
5-4-2015
18-10-2015
19-10-2015
Please cite this article as: Xiangpeng Kong, Cailian Ma, Juan Zhang, Jiaqiang
Sun, Jiangang Chen, Kefeng Liu, Effect of leaching temperature on structure and
performance of Raney Cu catalysts for hydrogenation of dimethyl oxalate, Applied
Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.10.029
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1
Effect of leaching temperature on structure and performance of Raney Cu catalysts for
hydrogenation of dimethyl oxalate
Xiangpeng Kong a,b, Cailian Ma a,b, Juan Zhang a, Jiaqiang Sun a, Jiangang Chen a,*,
Kefeng Liuc
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, Shanxi, P. R. China
b University of Chinese Academy of Sciences, Beijing, 100049, P. R. China.
c Petro China Petrochemical Research Institute, Beijing, 100195, P. R. China
Graphical abstract
* Corresponding author.
E-mail address: chenjg19@163.com
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Highlights



Raney Cu catalysts were prepared by selective leaching of Al in Cu-Al alloy.
The structure of Raney Cu can be modulated by changing of leaching temperature.
The main products can be controlled by tuning the structure of the Raney Cu
catalysts.

95.0 % yield of MG and 98.0 % EG selectivity could be obtained from DMO
hydrogenation.
Abstract
As-synthesized Raney Cu catalysts through selective leaching of Cu-Al (50wt%Cu-
50wt%Al) alloy pellets in sodium hydroxide solution have been systematically investigated
during the catalyst preparation process. Results reveal that the microstructures,
physicochemical and surface properties of the Raney Cu catalysts depend profoundly on the
leaching temperature. In gas-phase hydrogenation of dimethyl oxalate (DMO), the Raney Cu
catalysts prepared at the mild leaching conditions tend to deactivate rapidly, the formation of
methyl glycolate (MG). As-prepared catalysts at higher leaching temperature exhibit excellent
stability, favoring the synthesis of ethylene glycol (EG). Especially, Raney Cu catalyst
o
obtained at 40 C shows the superior stability and MG selectivity of 95.0% in DMO
hydrogenation. Furthermore, the reasons for deactivation of Raney Cu catalyst in DMO
hydrogenation have been discussed in detail.
Keywords: Raney Cu, hydrogenation, dimethyl oxalate, ethylene glycol, methyl glycolate.
1. Introduction
Methyl glycolate (MG) and Ethylene glycol (EG), as essential organic compounds and
intermediates, are widely applied in various chemical fields [1, 2]. To date, varieties of
synthesis approaches of MG and EG have been researched and developed, such as, selective
oxidation of primary alcohol [3], hydrolysis of ethylene oxide [4], condensation of
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formaldehyde and methyl formate [5], and carbonylation of HCHO [6]. Nevertheless, there
are several disadvantages (e.g. harsh reaction conditions, low target product yield, shrinking
of the crude oil resource, high cost and excessive energy consumption) of these routes.
Therefore, it is essential to research and develop a novel and practical route. Recently, the
synthesis of EG or MG from syngas rather than crude oil resource，has attracted considerable
interests in the both academic and business circles owing to abundance and low-price of
syngas [7, 8]. Remarkably, an indirect technology from syngas to EG and MG, including the
coupling of CO with nitrite esters to dimethyl oxalate (DMO) and subsequently selective
hydrogenation to MG or EG, was successfully demonstrated by a pilot plant, accelerating
greatly the commercialization process [9-11]. However, there are still plenty of unsolved
technical problems. Wherein, one of the most vital problems is the poor performance of the
DMO selective hydrogenation catalyst in the terms of selectivity, activity and stability [12].
Being aware of the difficulties in the catalyst-product separation for homogeneous catalysts,
Cu and Ag based catalysts have been considered as the potential candidate for commercial
production of MG or EG by selective hydrogenation of DMO [13-23]. Nevertheless, the
selective hydrogenation of DMO to MG over the Cu based catalysts is facing with the great
challenge of the overabundant hydrogenation ability of Cu species, which results in the
behavior of MG excessive hydrogenation to EG [24]. In addition, the local overheating from
exothermic DMO hydrogenation and poor thermal conductance of the metallic oxide support
results in serious sintering of Cu nanoparticle [25]. For the Ag based catalysts, the high price
and low efficiency limit their further applications in the industrial field. As a result, it is urgent
to develop an economic and efficient non-noble metal based catalyst.
Raney Cu catalysts have been widely used in several hydrogenation and dehydrogenation
processes [26-28]. It is well known that the mesoporous structure, high Cu0 surface area and
thermal conductivity features of Raney Cu catalyst endow it with excellent hydrogenation
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performance [29, 30]. Previous studies proposed that the performance of Raney Cu catalysts
in ester hydrogenation was comparable to that of the commercial Cu-Cr and co-precipitated
Cu-Zn-Al catalysts [31]. Thomas et al. [32] reported the Raney Cu catalyst showed excellent
performance in oxalate ester hydrogenation, but suffered from severe deactivation. However,
the close relationship between the essential structure features and stability of Raney Cu
catalysts in the hydrogenation reaction has not been systematically investigated up to now,
which is of great importance for designing the Raney Cu catalyst.
In this work, Raney Cu catalysts with different structure features were prepared by
controlling the leaching temperature in the preparation process. Moreover, an intrinsic
relationship between microcosmic structures and macroscopical performance in DMO
hydrogenation for Raney Cu catalysts was investigated. Our results provided the essential
information for the research and development of Raney Cu catalysts with excellent
performance in DMO hydrogenation process.
2. Experimental
2.1 Preparation of catalysts
Raney Cu catalysts were prepared from crushed and screened Cu-Al alloy (50wt% Cu,
50wt% Al) by selective leaching of aluminum in aqueous sodium hydroxide. In order to
control the rate of highly exothermic leaching reaction, the alloy (40.0 g) was firstly placed in
a beaker containing 100 mL of distilled water inside a thermostated water bath at different
temperatures (20, 30, 40, 50, and 60 oC). Then 80 mL of NaOH solution (40 wt%) was added
dropwise into the solution during 90 min period, and the alloy was allowed to digest for further
90 min. After that, the catalyst was washed several times with deionized water to remove any
residual Al compound until the pH value of washed water is 7.0. Finally, the fresh Raney Cu
catalysts were passivated in a continuous-flow fixed-bed microreactor using nitrous oxide at
200 oC for 1 h, which has been described in detail elsewhere [33, 34]. The Raney Cu catalysts
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after passivation were denoted as Ro-T (T is the leaching temperature), and corresponding
activated Raney Cu catalysts were denoted as R-T.
2.2 Catalyst characterizations
The Brunauer-Emmett-Teller (BET) surface area (SBET), pore volume (Vp) and average
pore size (Dp) were tested via N2 physical adsorption at -196oC using a Micromeritics ASAP
2420 instrument. All the samples were degassed under vacuum at 90oC for 1 h and then 350oC
for 8 h before measurement. The number of surface metallic copper sites exposed was
determined by N2O oxidation and followed H2 titration using the procedure described by the
previous report [35]. X-ray powder diffraction (XRD) patterns of as-prepared catalysts were
collected on an advanced X-ray diffractometer (Bruker AXS D8, Germany) with
monochromated Cu Kα radiation, and the crystal Cu2O size was calculated using the Scherrer
equation. The morphology of catalysts was investigated by scanning electron microscopy
(SEM) (JSM-6701F, Japan). Transmission electron microscopy (TEM) images of the catalysts
were obtained by using a JEM 2010 microscope operated at 200 kV. The catalysts were
dispersed in ethanol and mounted on a carbon foil supported on a copper grid. H2 temperature
programmed reduction (H2-TPR) was carried out in a conventional atmospheric quartz reactor
(5 mm i. d.). 30 mg of the calcined sample was outgassed at 100oC under Ar for 1 h. After
cooling to room temperature under Ar, the gas was switched to 5% H2/Ar (volume ratio), and
the sample was heated to 600oC at a ramping rate of 10 oC/min. The amount of H2 consumed
was monitored by a thermal conductivity detector (TCD). The hydrogen adsorption properties
of catalyst were measured by H2 temperature-programmed desorption (H2-TPD). In each H2
temperature programmed desorption (H2-TPD), 0.3 g of as-prepared calcined sample was
loaded into the quartz tube flow reactor and reduced with 5% H2/Ar (50 mL/min, volume ratio)
at 300oC for 4 h. Then the reduced 10 sample was cooled to 80oC and switched to Ar until the
baseline of H2 signal leveled off. Afterward, the TPD experiment was started with a heating
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rate of 15 oC/min under Ar flow (40 mL/min), and the desorbed H2 was monitored by a TCD
of effluent gas. Thermal decomposition of the catalyst precursors was studied by the
thermogravimetric (TG) method using a NETZSCH TG209F1 thermal analyzer.
Measurements were performed in the temperature range 50-750oC with a linear temperature
o
program b=10 C/min in a continuous flow of Ar atmosphere (30 mL/min). Temperature
Programmed Oxidation (TPO) was carried out in a conventional atmospheric quartz reactor
(5 mm i. d.). 300 mg of the spent sample was outgassed at 100oC under Ar for 1 h. After
cooling to room temperature under He, the gas was switched to 5 % O2/He (volume ratio), and
the sample was heated to 700oC at a ramping rate of 15 oC/min. The amount of O2 consumed
was monitored by mass spectra.
2.3 Evaluations of catalytic performance
The DMO hydrogenation reaction was conducted in continuous flow mode in a stainless
steel tubular reactor. Typically, 3.0 g of passivated Raney copper catalyst (40-60 meshes) was
loaded into the center of the reactor, and both sides of the catalyst bed were packed with quartz
powders (40-60 meshes). Before the test, the inert samples were firstly activated in 5% H2/95%
o
N2 (volume ratio) atmosphere at 200oC for 4 h with a ramping rate of 2 C /min. After that,
12.5wt.% DMO (purity 99.9%) in methanol and H2 were fed into the reactor at a H2/DMO
molar ratio of 100. The products were analyzed with a GC-920 gas chromatograph fitted with
a DB-FFAP capillary column (30.00 m×0.45 mm×0.85 μm) and a flame ionization detector
with relative standard deviation less than 2%. Reaction conditions: T=210oC, P=2.5 MPa, and
Liquid Hourly Space Velocity (LHSV)=2.0 h-1.
3. Results
3.1 Textural Properties of Catalysts
The physicochemical property of the as-prepared Raney Cu samples are summarized in
Table 1. The analysis results of chemical compositions reveals that the Al contents are
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21.4wt% and 20.9wt% at the leaching temperatures of 20oC and 30oC, respectively. With the
leaching temperature increasing, the residual Al compounds decreased gradually, and the least
Al compositions remained in the Ro-50 and Ro-60. Besides, the data of BET surface area,
pore volume, and metallic Cu specific area (SCu) indicate that the SBET of the Ro-T decreases
from 49 m2/g to 21 m2/g when the leaching temperature increasing from 30oC to 50oC, while
the Vp raises from 0.051 cm3/g to 0.136 cm3/g. Further elevating the leaching temperature to
60oC causes the increase of SBET adversely to 33 m2/g and decline of Vp to 0.102 cm3/g.
Additionally, there is the same change trend for the metallic Cu specific area (SCu) and SBET
of R-T with the varying leaching temperature. The pore size distribution curves derived from
the desorption branch using the BJH algorithm (Fig. 1) show that the Ro-20 and Ro-30 samples
have intense peak at about 4.0 nm and 4.5 nm, respectively. While the Ro-40 and Ro-50 were
mainly composed of well-developed macroporous structure (20-40 nm). Nevertheless,
increasing the leaching temperature to 60oC results in the decreasing pore size of Ro-60
sample. The rearrangement of the resultant skeletal Cu structure, due to over loss of Al
remained in the core in the Raney Cu pellets at the excessively high temperature, was reported
to be responsible for the adverse structure change of Ro-60 sample [36]. Therefore, it is
inferred that the leaching temperature has remarkable effects on the textural properties and the
chemical compositions of the Raney Cu catalysts.
3.2 Crystallite Structure of Catalysts
Fig. 2(A) gives the XRD patterns of the Cu-Al alloy precursor and passivated Raney Cu
samples. In the aspect of the Cu-Al alloy, there are a series of diffraction peaks assigned to
crystalline CuAl2 (JCPDS 65-2695). Besides, three weakened diffraction peaks from Al
(JCPDS 04-0787) and Cu9Al4 (JCPDS 24-0003) can be observed, confirming that CuAl2 phase
is the main chemical constituent in the precursor [37]. As for the as-prepared Raney Cu
catalysts, it is found the passivated Raney Cu catalysts exhibits main diffractions peaks
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characteristic of Cu2O (JCPDS 05-0677), suggesting the resultant samples should be mainly
composed of Cu2O phase. Moreover, the intensity of the diffraction peaks strengthened
gradually with the leaching temperature increasing from 20oC to 50oC, implying that Cu
crystals in Raney Cu catalysts grow larger gradually. Nevertheless, the higher leaching
temperature (60oC) will lead to the intensity of the Cu2O diffractions peaks weakens,
indicating the smaller Cu crystals in the R-60. In additional, CuO phase (JCPDS 48-1548)
could be found in both the Ro-50 and Ro-60 catalysts, which should be related to the severe
preparation conditions. Furthermore, with the leaching temperature increasing, the intensity
of the diffraction peaks assigned to CuAl2 weakened gradually and vanished at the leaching
temperature higher than 50oC, indicating that crystalline alloy precursor was leached
completely.
It is from the XRD patterns (Fig.2(B)) of the activated Raney Cu samples found that there
are only three diffractions peaks positing at the 2θ of 43.2o, 51.5o and 75.8o ascribed to Cu
(JCPDS 04-0836) and one weakened characteristic diffractions peak ascribed to Cu9Al4
(JCPDS 24-0003), indicating that CuxO (x=1 or 2) species could be completely restored at the
present conditions. Moreover, the characteristic diffractions peaks of Cu at the 2θ of 43.2o
exhibited the similar trend with the Cu2O phase in the Ro-T samples with the increasing
leaching temperature, demonstrating that this passivation and reactivating method is an
effective route for the storage of the Raney Cu catalysts. Additionally, the diffraction peaks
characteristic of Cu9Al4 could still be detected for all the Ro-T and R-T samples, confirming
that Cu9Al4 phase was immune to alkali liquor [38].
3.3 SEM and TEM Results
The surface morphology of the Ro-T catalysts via the leaching temperature can be directly
observed in Fig. 3. It is obvious that all the Ro-T samples are consisted of large plates, and
there exists no obvious cracks on the surface of the Ro-20 and Ro-30 catalysts. Nevertheless,
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the higher leaching temperature caused the surface of catalysts much more coarse, and the
more obvious parallel cracks. Especially, the Ro-60 became more fragile. Additionally, there
exist uniform network of short interconnected copper rods in the cracks of the Ro-50 and Ro-
60 catalysts, and the intervals reach up to 20-50 nm, which is well accordant with the size
distribution of the larger mesoporous. The cracks and network formation could be explained
in terms of faster dissolution of the elemental Al from the alloy precursor, which contributed
to the mesoporous structure and the large SBET [38]. In the leaching process, excessive loss of
Al could aggravate structure rearrangement of the samples, resulting in the collapse of their
network structures at the severe leaching conditions (Fig. 3e) [39]. Thus, the obtained results
are in good agreement with the degeneration of the mesoporous structure (Fig. 1).
Fig. 4 displays the TEM images of the Cu-Al alloy precursor and R-T samples. Form the
TEM image of Cu-Al alloy (Fig. 4a), it is noted that crystalline metallic Cu particles are highly
dispersed by interacting with the disordered crystalline Al phase. For the Raney Cu samples,
the crystalline metallic Cu phase with platelet-like particles becomes the primary phase,
confirming the occurring of coarsening of the metallic Cu rims in the leaching process (Fig.
4b-4f). Whereas, the platelet-like Cu particles in the Raney Cu samples prepared at 50oC and
60oC became accumulational and disordered, indicting the existent of over-arrangement at the
severe leaching conditions. Additionally, the Cu aggregates were surrounded by continuous
pore space in the range from 5 nm to 10 nm, corresponding to the small mesoporous structure
in Raney Cu samples.
3.4 Reduction behavior of catalysts
The H2-TPR technique has been adopted to determine the reducibility of the Cu-Al alloy
and passivated samples, as shown in Fig. 5. Notably, there exists no chemical hydrogen
consumption signal in the Cu-Al alloy sample, illustrating the high stability of Cu compounds
in Cu-Al alloy. For the Ro-20 sample, there are the main broad reduction peak at 216oC and
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another weak shoulder peak locating at the higher reduction temperature, suggesting the
appearance of two different Cu species. When the leaching temperature increased to 40oC, the
main reduction peaks shift to 265oC and the width become narrower, suggesting that the Cu
particle size distribution becomes more symmetrical. Whereas, the shoulder peaks locating at
lower reduction temperature reemerge again and the peak intensity continue to increase over
50oC, implying the much more isolated Cu species released from alloy. Combined with the
results of XRD (Fig. 2) and TEM (Fig. 4), the hydrogen consumption peaks should be
associated with the reduction of highly dispersed CuxO with different particle sizes.
Furthermore, the areas of the reduction peaks at different temperatures from TPR results can
indirectly reflect the distribution of the Cu species in the resultant Raney Cu catalysts (Table
2). Thus the leaching temperatures can determine the size distribution of the Cu species in the
Raney Cu catalysts.
3.5 Hydrogen adsorption properties
Temperature-programmed desorption of H2 (H2-TPD) has been considered as a useful tool
for ensuring the hydrogen desorption features of the surface of the catalysts [40]. The H2-TPD
profiles of the Cu-Al precursor and R-T samples have been plotted in Fig. 6. No hydrogen
desorption signal could be detected below the desorption temperature of 450oC for Cu-Al
alloy, implying that Cu species in the form of CuAl2 phase are inactive in hydrogen molecules.
However, the weak desorption peak appears over 450oC, which mainly origins from the
surface hydroxy existing on the interface of the Cu-Al alloy. As for the Raney Cu samples,
there are three distinct hydrogen desorption peaks locating at different temperature regions:
low temperature peak (LT: 80-200oC), medium temperature peak (MT: 250-575oC), and high
temperature peak (HT: >575oC). The LT desorption peaks corresponded to the H*desorption
on the Cu0 sites, and the intensity is closely related to the quantity of exposed active Cu0 sites
[41]. Particularly, the R-50 sample exhibits the weakest H* desorption intensity, indicating
M10

the least exposed active Cu0 sites, which is in good accordance with the Cu0 dispersion data
on the basis of N2O titration. Both the MT and HT peaks originate from H* adsorbed from the
hydroxy on the interface between Cu and Cu-Al alloy phase. The intensity of the desorption
peaks at various temperature range can directly reflect the surface composition of the as-
prepared samples.
3.6 Catalytic performance of Raney Cu catalysts
The catalytic behavior of Raney Cu catalysts prepared at different leaching temperatures in
gas-phase hydrogenation of DMO was investigated, as shown in Fig. 7(A). All the catalysts
exhibit 100% DMO conversion in the initial period. After the reaction time of 8 h, DMO
conversion of the R-20 and R-30 drops rapidly to less than 50%. In contrast, there is no loss
of DMO conversion efficiency for R-40, R-50 and R-60 catalysts after 14 h. Therefore, as-
prepared R-40, R-50 and R-60 samples have the superior stability.
It is from Fig. 7(A) found that there is no loss of EG selectivity over the R-50 during 20 h,
and the EG selectivity of other catalysts decreases sharply with the increasing MG selectivity
during 4 h, indicating that R-50 Raney Cu catalysts exhibit the best stability. Moreover, Fig.
7(B) gives the catalytic performance of the R-40 catalyst. The DMO conversion drops
gradually to 80% after 20 h reaction time, and the MG selectivity increases sharply to 80% at
the expense of the conversion of DMO. Interestingly, the selectivity of MG can actually reach
up to 95.0 % after 100 h reaction time, which is the highest value on the copper based catalysts
until now [20].
4. Discussions
4.1 The evolution of structural and surface properties
Our results can directly reflect the evolutions of microstructures, physicochemical and
surface properties of Raney Cu catalysts correlative to the leaching temperature. In the alloy
leaching process, the phase constitution of the precursor alloy and kinetics of the leaching
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reaction should be responsible for the resultant morphology of the Raney Cu catalyst [42].
Meanwhile, coarsening of the Cu rims occurred in the dissolution and re-precipitation process
of the Cu-Al alloy by the Oswald ripening mechanism, leading to the crystalline Cu formation
[43, 44].
Based on the H2-TPR profiles (Fig. 5) and crystalline Cu size versus the leaching
temperatures (Fig. 8), it is obvious that the particle size of the crystalline Cu in the Raney Cu
catalysts grows with the increasing leaching temperatures, indicating that the Cu particle size
is closely related to the leaching rate determined by the leaching temperatures. While the
adverse decreasing of the crystalline Cu size at 60oC suggests that the leaching process should
be mainly controlled by the liquid phase diffusion, where the leaching depth approaches the
core in the Cu-Al alloy pellets. Thus, it is confirmed that, the diffusing effect should be another
important factor determining the resultant Raney Cu structure in the leaching process, except
for the leaching temperature. What is more, the leaching conditions could modify the chemical
state of the Cu elements and surface properties. The residual Al (Table 1) in the Raney Cu
catalysts and H2-TPD profiles (Fig. 6) confirms that elevating leaching temperature induces
surface enrichment of metallic Cu and decreases the rest of CuAl2 content phase. While the
Cu9Al4 phase is independent of the leaching temperature, and still remains in the Raney Cu
catalysts. Nevertheless, over-rearrangement of the Raney Cu catalysts at the excessive
leaching conditions will cause noticeable change of the structural feature [37,38], due to the
excessive loss of the remained Al component, as shown in the Fig. 1, Fig. 3 and Fig. 4.
Scheme 1 gives the main structural evolution diagram of the Raney Cu catalyst upon the
leaching temperature. In the initial leaching depth, the crystalline Cu size increases with the
leaching temperatures increasing, indicating that the leaching rate should be responsible for
the leaching process. After that, at the relative leaching depth before forming the core in the
pellets (60oC), the SBET of the resultant Raney Cu pellet reaches the minimum, where the
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leaching process is mainly controlled by the liquid phase diffusion in the product layer until
the alloy was leached completely [45].
4. 2 The structure-performance relationships
The evolutions of structural and surface properties of the resultant Raney Cu catalysts
depend on the leaching temperature, and consequently, determine the catalytic hydrogenation
performance. The Raney Cu catalyst prepared at the mild leaching conditions with smaller
pore structure has the highest metallic Cu dispersion that decreases with the increasing
leaching temperature. The R-50 and R-60 catalysts, obtained from the harsh leaching
conditions, exhibit the less metallic Cu dispersion and larger pore structure, and possess
excellent performance and stability, which well determines the resultant main product (Fig.
7(A)). However, the R-20, R-30 and R-40 suffer rapid deactivation in the DMO hydrogenation
process. In order to further explore the relation between the structure and catalytic
performance of the Raney Cu catalysts in the DMO hydrogenation process, the spent Raney
Cu catalysts are investigated and illustrated in Fig.9. It is found that there are three diffraction
peaks at 2θ of 43.3o, 50.5o and 74.0o assigned to Cu0 (JCPDS 04-0836) in the spent Raney Cu
catalysts. Remarkably, there is no obvious change of the Cu0 particle size in the spent sample
compared to the catalyst without reaction based on the Scherrer equation, suggesting that no
change of the Cu nanopatciles occurred in the DMO hydrogenation process. Additionally, the
diffraction peaks at 2θ around 22o and 44.1o assigned to quartz and Cu9Al4 (JCPDS 24-0003),
respectively, could still be detected for all the spent catalysts. Moreover, there is a diffraction
peak with strong intensity characteristic of carbonaceous (JCPDS 26-1076) for the spent R-
50, R-60 and R-70, indicating the crystalline carbonaceous deposited in the reaction.
Previous studies proposed that sintering of Cu nanoparticles is the mainly reason for the
deactivation of the Cu based catalysts [46]. Weight loss of ca. 7% in the thermogravimetric
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analysis (Fig. 10) of the spent catalyst argue against strong chemiorption of a poison. Besides,
XRD patterns reveal that Cu sintering is not the reason for the spent Raney Cu catalyst in the
DMO hydrogenation reaction. It is well known that the Raney Cu catalysts, is not only highly
active in hydrogenation reaction, but an excellent candidate for the methanol dehydrogenation
reaction. Notably, a trace of formaldehyde detected in the product can afford evidence that
dehydrogenation of feedstock solvent occurred, which can cause the polymer deposition over
the Raney Cu catalyst in the present reaction conditions. Moreover, the intermediate MG used
to be an easy polymerization unit, which can polymerize on the active Cu0 sites, accelerating
the deactivation [32]. Thus, it induced that the polymer formation on the active sites and pore
structure, resulting in the active sites being covered and pore structure being blocked, should
be another reason for the deactivation of the Raney Cu catalysts [47]. What is more, TPO was
also conducted to further investigate polymer deposited in the spent Raney Cu catalyst. As
shown in Fig. 11, the spent R-50 catalyst displayed a broad oxygen consumption in the range
from 275oC to 700oC, corresponding to combustion of amorphous carbonaceous adsorbed on
the metal Cu0 sites and crystalline coke deposited on the surface of the catalysts [48]. These
results demonstrate that carbonaceous residues deposit on the surface of the Raney Cu catalyst
in the DMO hydrogenation process.
In the preparation process, coarsening of rim Cu resulted in large pore size and low Cu0
dispersion that will exhibit an advantage of reducing the activity extent of fouling by polymer
by-products in the DMO hydrogenation process, which will be beneficial to excellent stability
on the expense of the partial activity [48]. Excessive exposed active Cu density over the
surface of the Raney Cu catalyst facilitates the dehydrogenation reaction, but it will has passive
influence on the lifetime of the Raney Cu catalysts. Deposition of fouling with polymer in the
Raney Cu catalysts can block the pore structure and cover the active sites exposed, reducing
the catalytic hydrogenation activity [49]. As a result, the intermediate product of MG from
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DMO partially hydrogenation becomes the main product. Moreover, long-term catalytic
stability is a necessary feature for industrial catalysts of DMO hydrogenation to EG, and
synthesis of highly stable copper catalysts remains a significant challenge.
5. Conclusions
In summary, the leaching temperature has the significant effect on the microstructures,
physicochemical and surface properties of the resultant Raney Cu catalysts, and the catalytic
performance in the gas-phase hydrogenation of DMO. Raney Cu catalyst with lower Cu0
dispersion and larger macroporous structure exhibits better stability. The high EG yield of
98.0% was obtained over R-50 catalyst, and the selectivity of MG could reach up to 95.0 %
over R-40 Cu catalyst. Our analysis results confirm that the main reason for the deactivation
of the Raney Cu catalyst is the carbonaceous deposition rather than the sintering of the Cu
nanoparticles in the DMO hydrogenation process. These studies should provide fundamental
knowledge for the development of DMO hydrogenation technology.
Acknowledgments
Financial support from Natural Science Foundation of China (Grant No. 21373254) are
gratefully acknowledged.
Referances
[1] A. Y. Yin, C. Wen, W. L. Dai, K. N. Fan, Appl. Catal., B 108-109 (2011) 90-99.
[2] H. R. Yue, Y. J. Zhao, X. B. Ma and J. L. Gong, Chem. Soc. Rev., 41 (2012) 4218-4244.
[3] T. Hayashi, T. Inagaki, N. Itayama, H. Baba, Cata. Today 117 (2006) 210-213.
[4] A. Y. Yin, C. Wen, X. Y. Guo, W. L. Dai, K. N. Fan, J. Catal., 280 (2011) 77-88.
[5] D. He, W. Huang, J. Liu, Q. Zhu, Catal. Today 51 (1999) 127-134.
[6] Y. Sun, H. Wang, J. Shen, H. Liu, Z. Liu, Catal. Commun., 10 (2009) 678-681.
M15

[7] E. Jin, Y. Y. Zhang, L. L. He, H. G. Harris, B. T. Teng, M. H. Fan, Appl. Catal., A 476
(2014) 158-174.
[8] R. L. Pruett, science 211 (1981) 11-16.
[9] S. Zhao, H. Yue, Y. Zhao, B. Wang, Y. Geng, J. Lv, S. Wang, J. Gong, X. Ma, J. Catal.,
297 (2013) 142-150.
[10] X. Zheng, H. Lin, J. Zheng, X. Duan, Y. Yuan, ACS Catal., 3 (2013) 2738-2749.
[11] Y. Yamamoto, Catal. Surv. Asia, 14 (2010) 103-110.
[12] G. H. Xu, Y. C. Li, Z. H. Li, H. J. Wang, Ind. Eng. Chem. Res., 24 (1996) 2371-2378.
[13] J. L. Gong, H. R. Yue, Y. J. Zhao, S. Zhao, L. Zhao, J. Lv, S. P. Wang, X. B. Ma, J.
Am. Chem. Soc., 134 (2012) 13922-13925.
[14] Z. He, H. Q. Lin, P. He, Y. Z. Yuan, J. Catal., 277 (2011) 54-63.
[15] A. Yin, C. Wen, X. Guo, W. L. Dai, K. Fan, J. Catal., 280 (2011) 77-88.
[16] J. Zheng, H. Lin, Y. Wang, X. Zheng, X. Duan, Y. Yuan, J. Catal., 297 (2013) 110-118.
[17] L. F. Chen, P. J. Guo, M. H. Qiao, S. R. Yan, H. X. Li, W. Shen, H. L. Xu, K. N. Fan, J.
Catal., 257 (2008) 172-180
[18] A. Yin, X Guo, W. L. Dai, H. Li, K Fan, Appl. Catal., A 349 (2008) 91-99.
[19] J. Zheng, H. Lin, X. Zheng, X. Duan, Y. Yuan, Catal. Commun., 40 (2013) 129-133.
[20] C. Wen, Y. Cui, X. Chen, B. Zong, W. L. Dai, Appl. Catal., B 162 (2015) 483-493.
[21] A. Yin, C. Wen, W. L. Dai, K. Fan, Appl. Catal., B 108-109 (2011) 90-99.
[22] A. Yin, X. Guo, W. Dai, K. Fan, Chem. Commun., 2010,46, 4348-4350.
[23] A. Y. Yin, X. Y. Guo, W. L. Dai, K. N. Fan, Chem. Commun., 46 (2010) 4348-4350.
[24] C. Wen, A. Y. Yin, Y. Y. Cui, X. L. Yang, W. L. Dai, Appl. Catal., A 458 (2013) 82-
89.
[25] Y. Zhu, X. Kong, D. B. Cao, J. Cui, Y. Zhu, Y. W. Li, ACS Catal., 4 (2014) 3675-3681.
[26] L. Ma, B. Gong , T. Tran, M. S. Wainwright, Catal. Today 63 (2000) 499-505.
M16

[27] X. Huang, L. Ma, M. S. Wainwright, Appl. Catal. A 257 (2004) 235-243.
[28] J. Toyir, M. Saito, I. Yamauchi, S. Luo, J. Wu, I. Takahara, M. Takeuchi M, Stud. Surf.
Sci. Catal., 114 (1998) 267-272.
[29] A. D. Tomsett, M. S. Wainwright, D. J. Young Appl. Catal., 12 (1984) 43-48.
[30] A. J. Smith, M. S. Wainwright, Handbook of Heterogeneous Catalysis. 2008. 92-99.
[31] J. W. Evans, N. W. Cant, D. L. Trimm, M.S. Wainwright, Appl. Catal. 6 (1983) 355-
362.
[32] D. J. Thomas, J. Y. Wehrli, S. Wainwright, D. L. Trimm, Appl. Catal. A 86 (1992) 101-
114.
[33] C. Z. Fan, M. S. Wainwright, D. L. Trimm, Appl. Catal. 54 (1989) 53-58.
[34] A. D. Tomsett, M. S. Wainwright, D. J. Young, Appl. Catal. 12 (1984) 43-48.
[35] Z. Yuan, L. Wang, J. Wang, S. Xia, P. Chen, Z. Hou, X. Zheng, Appl. Catal. B
101(2011) 431-440.
[36] H. E. Curry-Hyde, M. S.Wainwright, D. J. Young, Appl. Catal. 77 (1991) 89-94.
[37] J. R. Mellor, N. J. Covflle, A. C. Sofianos, R. G. Copperthwaite, Appl. Catal. A 164
(1997) 171-183.
[38] U. Birkenstock, R. Holm, B. Reinfandt, S. Storp, J. Catal. 93(1985) 55-67.
[39] T. Tanabe, S. Kameoka, A. P. Tsai, Catal. Today. 111 (2006) 153-157.
[40] M. Muhler, L. P. Nielsen, E. Törnqvist, B. S. Clausen, H. Topsøe, Catal. Lett. 14 (1992)
241-249.
[41] T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) 137-141.
[42] B. J. Friedrich, M. S. Wainwright, D. J. Young, I. F. Boag, Chern. Eng. Commun., 26
(1984) 63-171.
[43] A. D. Tomsett, D. J. Young, M. S. Wainwright, Appl. Catal. 35 (1987) 321-328.
[44] P. W. Voorhees, M. E. Glicksman. Metall. Trans. A, 15(A) 1081-1088.
M17

[45] A. J. Smith, D. L.Trimm, Annu. Rev. Mater. Res., 35 (2005) 127-142.
[46] J. Lin, X, Zhao, Y, Cui, H, Zhang, D, Liao, Chem. Commun., 48 (2012) 1177-1179.
[47] S. P. Tonner, D. L. Trlmm, M. S. Walnwrlgh, Ind. Eng. Chem. Prod. Res. Dev. 23
(1984) 384-388.
[48] J. Barbier, E. Churin, J.M. Parera, J. Riviere, React. Kinet. Catal. L. 29 (1985) 323-330.
[49] N. I. Onuoha, M. S. Wainwright, Chem. Eng. Commun. 29 (1984) 1-12.
M18

Table 1
Table 1 Physicochemical property of catalysts
Catalyst
Al (wt.%)a
SBET (m2/g)
Vp (cm3/g)
DA p (nm)d
Dc Cu (nm)
SCu (m2/g)b
Cu dispersion (%)b
Ro-20
Ro-30
21.4
20.9
45.7
49.0
0.051
0.071
4.7
5.2
8.6
2.4
2.5
32.7(±0.3%)
23.3(±0.3%)
12.4
Ro-40
Ro-50
Ro-60
12.2
5.5
29.9
20.8
33.6
0.136
0.121
0.102
19.9
19.8
1.8
14.5
17.6
15.6
2.4
2.3
4.0
17.8(±0.3%)
11.6(±0.3%)
23.8(±0.3%)
6.4
a Al element contents were determined by ICP-OES analysis.
b SCu and copper dispersion (DCu) were measured by N2O adsorption.
c dCu were calculated from the reflections of Cu (1 1 1) and CuO (1 1 1) planes in the XRD using the Scherrer equation.
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Table 2 Cu2O, CuO particle percent calculated from disintegration of H2-TPR peaks
R-20
205.9
61.5
R-30
331.0
28.4
R-40
342.8
23.9
R-50
552.4
627.3
46.8
R-60
607.1
563.5
51.9
LT Peak Area
HT Peak Area
LT Peak percent
(%)
77.0
92.1
93.5
M20

Figure captions
Fig. 1 Profiles of pore size distribution of the passivated Raney Cu catalysts.
Fig. 2 XRD patterns of the Cu-Al alloy precursor and passivated Raney Cu samples (A),
the activated Raney Cu samples (B).
Fig. 3 SEM images of the passivated Raney Cu samples.
Fig. 4 TEM images of the Cu-Al alloy and reduced Raney Cu samples.
Fig. 5 H2-TPR profiles of the alloy and passivated Raney Cu samples.
Fig. 6 H2-TPD profiles of the alloy and activated Raney Cu samples.
Fig. 7 DMO Conversion, EG Selectivity and MG Selectivity of the Raney Cu catalysts
with the time on stream (A). Long time catalytic test over the R-40 catalyst (B).
Fig. 8 Cu2O particle sizes as a function of the leaching temperature.
Fig. 9 XRD patterns of the spent Raney Cu samples.
Fig. 10 Thermogravimetric analysis of the spent R-50 catalyst.
Fig. 11TPO profiles of the spent R-50 catalyst.
Scheme 1. Structural evolution over the Raney Cu catalyst upon increasing leaching
temperatures
M21

0.8
13.0
Ro-60
Ro-50
3.0
4.3
32.0
20.0
Ro-40
Ro-30
4.5
4.0
1.9
Ro-20
0
10
20
PoreDiameter(nm)
30
40
Fig. 1 Profiles of pore size distribution of the passivated Raney Cu catalysts.
(A)

CuAl2 Al  CuAl4  Cu o CuO  CuO
9
2








Ro-60
Ro-50
Ro-40
Ro-30
Ro-20













Alloy



10
20
30
40
50
60
70
80
2Theta(degree)
(B)
 Cu CuAl4 CuAl2
9




R-60
R-50
R-40
R-30
R-20


10
20
30
40 50
2Theta(degree)
60
70
80
Fig. 2 XRD patterns of the Cu-Al alloy precursor and passivated Raney Cu samples
(A), and the activated (B).
M22

Fig. 3 SEM images of the passivated Raney Cu samples: (a) Ro-20, (b) Ro-30, (c) Ro-40,
(d and e) Ro-50, and (f) Ro-60
Fig. 4 TEM images of the Cu-Al alloy and reduced R-T samples: (a) Alloy, (b) R-20, (c)
R-30, (d) R-40, (e) R-50, and f) R-60
M23

LTPeak
234
HTPeak
280
Ro-60
Ro-50
265
Ro-40
Ro-30
260
216
Ro-20
Alloy
150
200
250
300
350
400
o
Temperature(C)
Fig. 5 H2-TPR profiles of the alloy and Ro-T samples.
HTPeak
MTPeak
LTPeak
R-60
R-50
R-40
R-30
R-20
Alloy
50
150
250
350
450
550
650
Temperature(oC)
Fig. 6 H2-TPD profiles of the Cu-Al alloy and activated R-T samples.
M24

( A )
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
R-20
R-30
R-40
R-50
R-60
0
2
4
6
8
10
12
14
16
18
20
22
Stream time / h
(B) 100
80
60
40
20
0
DMO Conversion over R-40
MG Selectivity over R-40
20
40
60
80
100
Time on stream (h)
Fig. 7 DMO Conversion, EG Selectivity and MG Selectivity of the Raney Cu catalysts with
the stream time (A). Long time catalytic test over the R-40 catalyst (B).
M25

18
16
14
12
10
8
R-50
R-60
R-40
R-30
R-20
20
30
40
50
60
Leachingtemperature(oC)
Fig. 8 Cu particle sizes as a function of the leaching temperature.
o SiO  C  Cu Cu Al

2
9 4





SpentR-60
SpentR-50
SpentR-40
SpentR-30
SpentR-20
10
20
30
40 50
2Theta(degree)
60
70
80
Fig. 9 XRD patterns of the spent Raney Cu samples.
M26

100
99
98
97
96
95
94
93
92
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
50
150
250
350
450
550
650
750
Temperature (oC)
Fig. 10 Thermogravimetric analysis of the spent R-50 catalyst.
350
100
200
300
400
500
600
700
Temperature(oC)
Fig. 11 TPO profiles of the spent R-50 catalyst.
M27

Scheme 1. Structural evolution over the Raney Cu catalyst upon increasing leaching
temperatu
M28

M29