Surface synergistic effect in well-dispersed Cu/MgO catalysts for highly efficient vapor-phase hydrogenation of carbonyl compounds

Article abstract of DOI:10.1039/c5cy00437c

The highly efficient vapor-phase selective hydrogenation of carbonyl compounds (e.g. furfural (FAL) and dimethyl 1,4-cyclohexane dicarboxylate (DMCD)) to corresponding alcohols was achieved excellently over well-dispersed MgO-supported copper catalysts (Cu/MgO), which were prepared by an alternative separate nucleation and aging step method. The characterization results revealed that the structure and catalytic performance of the as-formed Cu/MgO catalysts were profoundly affected by Cu loading. Especially, the results confirmed that the decrease in the Cu loading could lead to the improvement of metal dispersion and the formation of more surface strong Lewis basic sites. In the vapor-phase selective hydrogenation of FAL to furfuryl alcohol (FOL) and DMCD to 1,4-cyclohexane dimethanol (CHDM), two Cu/MgO catalysts with Cu loadings of 27.6 wt% and 70.9 wt% exhibited superior catalytic performance with higher conversions (>97.3%) and selectivities to alcohols (>96.0%) compared to the other supported ones. The high efficiency of the as-formed Cu/MgO catalysts was mainly attributed to the surface synergistic catalytic effect between the catalytically active metallic copper species and the Lewis basic sites, which held the key to the hydrogenation reaction related to the hydrogen dissociation and the activation of the carbonyl groups.

Full text of DOI:10.1039/c5cy00437c

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PAPER
Surface synergistic effect in well-dispersed Cu/MgO catalysts for highly
efficient vapor-phase hydrogenation of carbonyl compounds†
Hanwen Liu, Qi Hu, Guoli Fan, Lan Yang, Feng Li*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The highly efficient vaporꢀphase selective hydrogenation of carbonyl compounds (e.g. furfural
(FAL) and dimethyl 1,4ꢀcyclohexane dicarboxylate (DMCD)) to corresponding alcohols was
achieved excellently over wellꢀdispersed MgOꢀsupported copper catalysts (Cu/MgO), which were
prepared by an alternative separate nucleation and aging step method. The characterization results
revealed that the structure and catalytic performance of asꢀformed Cu/MgO catalysts were
profoundly affected by the Cu loading. Especially, the results confirmed that the decrease in the Cu
loading could lead to the improvement of metal dispersion and the formation of more surface strong
Lewis basic sites. In the vaporꢀphase selective hydrogenation of FAL to furfuryl alcohol (FOL) and
DMCD to 1,4ꢀcyclohexane dimethanol (CHDM), two Cu/MgO catalysts with the Cu loadings of 27.6
wt % and 70.9 wt % exhibited superior catalytic performance with the higher conversions (> 97.3%)
and selectivities to alcohols (>96.0 %) to other supported ones. The high efficiency of asꢀformed
Cu/MgO catalysts was mainly attributed to the surface synergistic catalytic effect between
catalytically active metallic copper species and Lewis basic sites, which held the key to
hydrogenation reaction related to the hydrogen dissociation and the activation of carbonyl groups.
alcohol (FOL),14 which is widely used for the production of a
great number of chemicals.15,16 In industry, these important
Introduction
alcohols usually are produced by the catalytic hydrogenation of
Nowadays, a series of hydrogenation processes of carbonyl
corresponding carbonyl compounds over Cr2O3 promoted
compounds to corresponding alcohols have been successfully
copperꢀbased (CuꢀCr2O3) catalysts with moderate activity and
developed in industry,1ꢀ6 due to their academic and commercial
selectivity at high temperatures and pressures.8,9, 17ꢀ20 The high
value. For instance, 1,4ꢀcyclohexane dimethanol (CHDM),
toxicity of Cr6+ ion, however, causes severe pollution to
which is preferred over ethylene glycol as a stepping stone for
environment and humans during the preparation and operation.
producing various highꢀquality and nontoxic resins and fibers in
Therefore, the development of nontoxic green Crꢀfree catalysts
the polymer industry,7 can be normally produced by catalytic
is crucial for the efficient production of alcohols from the
hydrogenation of dimethyl 1,4ꢀcyclohexane dicarboxylate
viewpoint of sustainable development of resources and
(DMCD).8,9 In addition, due to increasingly serious energy and
economy.
environmental issues, efficient utilization and conversion of
Over the past decade, a large number of new catalysts have
natural biomass resources are attracting considerable interest. In
been explored for selective hydrogenation of carbonyl
this respect, furfural (FAL), an important biomassꢀderived
compounds in either liquid phase or vapor phase. For example,
platform molecule, is employed extensively for the production
supported bimetallic RuSnꢀbased catalysts are found to be active
of a variety of nonꢀpetroleum based chemicals and biofuels.10–13
in liquidꢀphase hydrogenating DMCD into CHDM.21,22
Typically, catalytic hydrogenation conversion of FAL by using
Meanwhile, various supported metals (e.g. Cu, Ni, Co, Pt, Ru,
metalꢀbased catalysts can generate highꢀvalueꢀadded furfuryl
20, 23ꢀ30
Pd, and Rh)16,
and amorphous alloys31 catalysts are
employed to catalyze the hydrogenation of FAL. Especially,
lowꢀcost Cuꢀbased catalysts, as the most employed catalysts in
industry, can preferentially catalyze the hydrogenation of C=O
bond in carbonyl compounds. In some cases, surface Cu+ꢀCu0
synergy in Cuꢀbased catalysts can remarkably control the
State Key Laboratory of Chemical Resources Engineering, Beijing University of
Chemical Technology, Beijing 100029, China.
Email: lifeng@mail.buct.edu.cn;Fax:8610 64425385 ; Tel :8610 64451226
† Electronic Supplementary Information (ESI) available: Additional experimental
results. See DOI: 10.1039/x0xx00000x
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catalytic performance in the hydrogenation of carbonyl
compounds.29, 32, 33 However, the main disadvantages are that
most of the above catalysts cannot be reused due to severe
deactivation of active species and liquidꢀphase hydrogenation
requires high hydrogen pressures. In contrast, vaporꢀphase
hydrogenation of carbonyl compounds may be generally
conducted under more mild conditions, i.e., lower hydrogen
pressures and reaction temperatures. In addition, it is well
known that the surface acidꢀbase nature of catalysts may greatly
influence the way that the reactions go. Recently, we found
preliminarily that surface basic sites on copperꢀbased catalysts
derived from CuꢀMgꢀAl layered double hydroxide precursors
could promote the catalytic performance in the hydrogenation of
DMCD.34
On the other hand, although the preparation approaches for
supported metal catalysts (e.g. depositionꢀprecipitation and
incipient wetness impregnation) have been intensively explored
in order to make full use of their catalytic properties,35 it is
difficult to obtain wellꢀdispersed metal nanopaticles (NPs) with
uniform size and good thermal stability in most cases, especially
at high loadings, which mainly originates from inhomogeneous
distribution of active precursors on supports as well as weak
interactions between them. Therefore, various highꢀspecificꢀ
surfaceꢀarea supports, such as MgO 36ꢀ38, have been adopted to
disperse active metal sites and enhance interactions between
them. Up to now, however, highly active and stable simple
metal oxides supported Cuꢀbased catalysts suitable for industrial
applications have not been developed yet.
subsequently aged. In this way, all the particles precipitated out at
almost the same time, and the following growth time was the
same. As a result, metal ions precipitated more uniformly and
integrated better with each other to form uniform catalyst
precursor particles.
Fig.1A shows the XRD patterns of calcined catalyst
precursors. In the cases of the Cu loadings of < 55.0 wt%, the
XRD patterns of samples only exhibit the characteristic
diffractions of poorly crystalline MgOꢀlike phase. However, no
diffraction peaks related to crystalline CuO are found in the XRD
patterns, as a consequence of its high dispersion nature. As the
Cu loading increases to 55 wt%, the weak characteristic
diffractions of CuO (JCPDS No. 05ꢀ0661) begin to appear. The
diffraction intensities for CuO phase increase gradually with the
Cu loading, whereas those for MgO phase decrease. The above
results demonstrate that the wellꢀdispersed CuO phase is
successfully formed in calcined samples.
Inspired by the above results, in this work, new
environmentꢀfriendly wellꢀdispersed MgO supported Cu
nanocatalysts (Cu/MgO) for the vaporꢀphase selective
hydrogenation of representative carbonyl compounds (e.g. FAL
and DMCD) were prepared by an alternative separate nucleation
and aging step method, which was developed previously in our
group.39 In combination with systematic characterizations, the
effects of the copper loading on the metal dispersion, surface
basicity and catalytic performance of Cu/MgO catalysts were
elucidated. It was found that the high conversions (> 97.3%) and
selectivities to alcohols (>96.0 %) were achieved over two
Cu/MgO catalysts with a low Cu loading of 27.6 wt % or a high
Cu loading of 70.9 wt % in the vaporꢀphase selective
hydrogenation. This could be attributed to the surface
synergistic catalytic effect between metallic copper species and
Lewis basic sites, which mainly held the key to hydrogenation
reaction related to hydrogen dissociation and the activation of
carbonyl groups. More importantly, the obtained catalyst
achieved a stable catalytic performance up to 200 hours. To the
best our knowledge, such highly efficient and stable simple
metal oxide supported Cuꢀbased catalysts for the hydrogenation
of carbonyls have not been reported until now.
Fig. 1 XRD patterns of calcined samples (A) and reduced catalysts (B). Inset
in (B) shows the detail XRD patterns of Cuꢀ40 and Cuꢀ55 samples in the 2 θ
range of 38°–48°.
Fig.1B presents the XRD patterns of reduced Cu/MgO
samples. In the cases of Cuꢀ10 and Cuꢀ25 samples, only three
broad (111), (200), and (220) diffractions of crystalline MgO
phase are observed. As for Cuꢀ40 and Cuꢀ55 samples, the broad
asymmetric peak at about 43o can be deconvoluted into two peaks
with their maxima at about 42.5 and 43.2o (inset in Fig.2B),
corresponding to the (200) diffraction of MgO and the (111)
diffraction of the faceꢀcentered cubic (fcc) metallic copper
(JCPDS no. 04ꢀ0836), respectively. Noticeably, with the further
increase in the Cu loading, Cuꢀ70 and Cuꢀ90 samples present a
strong (111) diffraction peak of metallic copper at 2θ of 43.2°
along with the (200) one at 50.4°, besides weak diffractions of
MgO phase. However, the diffractions of metallic copper phase
can not be detected in the cases of Cuꢀ10 and Cuꢀ25 samples,
mainly due to the lowered crystallinity.
The average crystallite sizes of metallic copper particles
calculated by the Scherrer formula according to the (111)
diffraction are listed in Table 1. It is noted that the crystallite size
of copper particles increases gradually from about 5.2 to 33.8 nm
when the Cu loading increases from 38.6 to 82.7 wt %. As shown
in Table 1, the copper dispersion measured by H2ꢀN2O titration
decreases monotonically from 0.283 to 0.016 with the Cu loading,
well consistent with the change in the average size of copper
Results and discussion
Structural analysis
In the course of synthesis for Cu/MgO catalyst precursors,
two solutions of metal salts and bases were rapidly mixed and
nucleation process could occur in a colloid mill within a quite
short time of 3 min. The resultant hydroxide slurry was
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particles determined by the XRD patterns. As a result, the
introduction of a large amount of Cu is not helpful for retarding
the growth of metallic copper crystallites. Usually, the high
specific surface area can result in the good metal dispersion.40,41
In the present catalyst system, the BET specific surface area of
Cu/MgO samples decreases gradually with the Cu loading (Table
1), which is associated mainly with the decrease in the component
of MgO support, as well as the enlarged sizes of Cu particles.
Meanwhile, Cu surface areas exhibit an increasing trend with the
Cu loading from 8.5 to 70.9 wt %, indicative of the increase in the
surface metallic copper sites. However, Cuꢀ90 sample has a
smallest Cu surface area (8.2 m2gꢀ1), which should be ascribed to
the agglomeration of large Cu particles.
due to poor contrast between metal and support. Further, the
HAADFꢀSTEM image of Cuꢀ25 sample (Fig.S1†) depicts a large
quantity of small bright spots in the image corresponding to Cu
NPs supported on the matrix. The presence and distribution of the
Mg and Cu elements within several particles were further verified
by means of the STEMꢀEDS line scan spectrum. Noticeably, the
intensity of Mg element in the EDS line spectrum appears to be
higher than that of Cu element, suggesting that Cu NPs are
probably surrounded by the MgO phase, thus leading to the
formation of a strong interfacial contact between Cu NPs and
MgO phase. In the case of Cuꢀ70 sample, a large number of
approximately spherical Cu NPs with the average diameter of
about 15 nm undergo a little agglomeration, due to its high Cu
loading of 70.9 wt% (Fig.2c). This is in good agreement with the
above N2O titration results. Furthermore, HRTEM images of Cuꢀ
25 and Cuꢀ70 depict the lattice fringes with an interplanar spacing
of approximately 0.21 nm within a single nanoparticle (Fig.2b, d)
corresponding to the (111) plane of the metallic Cu phase. In the
present synthetic system for Cu/MgO samples, homogeneous
precipitation of all metal ions can guarantee the formation of
uniform catalyst precursor particles, which provides a favourable
dispersing effect for copper species thus preventing agglomeration
of metallic copper particles, although the Cu loadings are pretty
high in some cases.
Table 1 Structural and textural data of different Cu/MgO
samples.
b
c
d
e
Sample
Cu a
SBET
D111
SCu
Ddis
Basic sites f SB sites g
(wt%)
(m2/g)
(nm)
ꢀꢀ
(m2/g)
8.3
(%)
(mmol/g)
1.136
1.188
0.856
0.719
0.439
0.075
(mmol/g)
0.424
0.686
0.405
0.242
0.168
0
Cuꢀ10
Cuꢀ25
Cuꢀ40
Cuꢀ55
Cuꢀ70
Cuꢀ90
8.5
74
28.3
27.6
38.6
48.5
70.9
82.7
71
ꢀꢀ
13.9 15.8
63
5.2
6.0
18.8
33.8
16.4
16.4
17.5
8.2
8.1
6.9
5.0
1.6
56
54
44
a
b
Redox behaviors
determined by ICPꢀAES; specific surface area calculated by the BET
c
method; average crystallite size of metallic copper particles based on XRD
d
e
patterns; Metallic copper surface area determined by H2ꢀN2O titration;
Copper dispersion degree; f the density of total basic sites determined by CO2ꢀ
TPD; g the density of strong basic (SB) sites determined by CO2ꢀTPD.
Fig. 3 H2ꢀTPR profiles of calcined samples.
H2ꢀTPR measurement was carried out to study the redox
properties of calcined samples and the interaction between the
copper species and the support. As shown in Fig. 3, the total
hydrogen consumption corresponds to the reduction of Cu2+
species. Obviously, two kinds of copper species exist in the Cuꢀ10
sample. The broad lowꢀtemperature reduction peak at ca. 235 °C is
assigned to the reduction of isolated highlyꢀdispersed CuO species,
42 while the small highꢀtemperature one at ca. 310 °C is associated
with the reduction of Cu2+ species having a strong interaction with
the matrix.43 With the Cu loading increasing to 27.6 %, two
reduction peaks all shift to the lower temperatures. Further, as the
Cu loading increases, the highꢀtemperature peak disappears; while
the lowꢀtemperature one continues to shift to the lower
temperatures and becomes broader. In the case of Cuꢀ90, besides
Fig. 2 TEM and HRTEM images of Cuꢀ25 (a,b) and Cuꢀ70 (c,d) samples.
Typical TEM images of representative Cu/MgO samples (Cuꢀ
25 and Cuꢀ70) are presented in Fig. 2. It can be found that small
Cu clusters without agglomeration of particles are well dispersed
on the support in the Cuꢀ25. However, the clear interface between
the Cu phase and the support matrix is difficult to be identified,
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the main peak at 183 °C, a shoulder peak related to the reduction of
large bulk CuO particles appears at 197 °C. The above results
demonstrate that the increase in the Cu loading leads to the
weakened interactions between the Cuꢀcontaining species and the
MgO matrix and thus the improved reducibility of copper species,
in spite of the enlarged particle size.
Cuꢀ25 sample.
Further, the reduced samples were characterized by XPS to
obtain more information about the surface chemical states of
copper (Fig.5). In all cases, the Cu 2p3/2 signal at ca. 932.6 eV is
due to the presence of metallic copper species.47, The solid
48
evidence of the copper reduction stems from the absence of XPS
47
signals of Cu2+ at 933–935 eV.
Clearly, the appearance of a
Surface chemical states
single peak at kinetic energy of 918.2 eV in the Cu LMM XAES
depicts the sole presence of Cu0 species on the surface of Cuꢀ25
sample.49 Further analysis on the surface copper species was
performed by in situ FTꢀIR spectra of CO absorption on the Cuꢀ
25. Usually, the band of CO absorption on the surface Cu2+ site is
above 2140 cmꢀ1, while the bands of CO absorption on surface
Cu0 and Cu+ sites are below 2110 cmꢀ1 and between 2110 and
2135 cmꢀ1 respectively.50 As shown in the inset in Fig. 6, a single
symmetric peak is found at ca. 2100 cmꢀ1 after saturating the Cuꢀ
25 with CO at room temperature. Upon heating, the adsorbed CO
is totally removed at 100 oC, which is a typical feature of weakly
bound CO on Cu0 sites.51 Therefore, it can be deduced that there
are only metallic Cu0 species in reduced catalysts.
Fig. 4 Cu 2p XPS of calcined samples.
It is well known that the catalytic performance of catalysts is
greatly related to their surface chemical states. The surface/nearꢀ
surface chemical states of calcined catalyst precursors were
analyzed by XPS (Fig. 4). Usually, the binding energy (BE) for
the main Cu 2p3/2 peak related to Cu2+ species is about in the
44,45
range of 932.0–935.0 eV,
whereas its shakeꢀup satellite line
lies between 940 and 945 eV. As for Cꢀ10 sample, the peak at
about 933.6 eV is attributed to highly dispersed Cu2+ species
present in metal oxide (CuA2+), while the one at about 935.0 eV
may be associated with covalent Cu2+ꢀO bond polarized by Mg2+
ions (CuB2+) thereby decreasing the effective charge of Cu2+
2+
species.46 Noticeably, the increase in the Cu loading favors CuA
Fig. 6 In situ FTꢀIR spectra of CO absorption on the Cuꢀ25 sample. CO under
500 Pa (a), followed by evacuation at 30 oC (b) and 100 oC (c)
2+
species rather than CuB species, due to the formation of more
CuO phase, indicative of the weakened interactions between Cuꢀ
Mg species.
Surface basicity
As for asꢀformed Cu/MgO samples, basic sites are mainly
related to O2ꢀ ions on the surface.52 CO2ꢀTPD measurement was
used to probe surface basicity of Cu/MgO samples. As shown in
Fig.7, except for Cuꢀ90 sample, all samples present three CO2
desorption peaks including weak Brønsted basic sites centered at
o
83ꢀ142 C, mediumꢀstrength Lewis basic sites centered at 196ꢀ
o
o
265 C, and strong Lewis basic sites centered at 509ꢀ566 C,53
which are related to surface OHꢀ groups, MgꢀOꢀMg groups or
Mg2+ꢀO2ꢀ pairs, and coordinatively unsaturated O2ꢀ ions,
respectively. However, Cuꢀ90 sample does not shows
a
desorption peak above 350 oC, indicative of the absence of strong
basic (SB) sites. According to a quantitative analysis on the
densities of basic sites on Cu/MgO samples (Table 1), the
densities of total basic sites and SB sites increase from 1.136 and
0.424 for Cuꢀ10 to 1.188 and 0.686 and mmol/g for Cuꢀ25, then
drop gradually with the continuous increase in the Cu loading.
Among them, Cuꢀ25 sample possesses the highest densities of
Fig. 5 Cu 2p XPS of reduced Cu/MgO samples. Inset is the Cu LMM XAES of
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total basic sites and SB sites.
Considering that the origin of basic sites is related to the
surface oxygen species, O1s XPS of Cu/MgO samples also were
analysed. From the O 1s spectra (Fig.8), it is seen that there are
four fitted peaks in all cases, except for Cuꢀ90 sample,
representing four different kinds of surface oxygen species. The
peak with the BE around 531.7 eV belongs most likely to
oxygen species of hydroxyl groups on the surface, while the
peaks at lower BE of about 530.6 and 529.3 eV are the
characteristic of lattice oxygen bound to metal cations of the
structure, i.e., MgꢀOꢀMg groups and Mg2+ꢀO2ꢀ pairs.54,55 In
addition, the peak located at 528.2 eV can be attributed to the
coordinatively unsaturated O2ꢀ species strongly bound to Mg2+
cations (Oδꢀ), which should be related to the reduction of the Cuꢀ
OꢀMg parts in calcined precursors during the catalyst activation
procedure, leading to the formation of Cu NPs in strong
interfacial contact with MgO phase. As for Cuꢀ90 sample,
however, there are not Oδꢀ species on the surface. It indicates the
presence of SB sites on the surface of Cu/MgO samples (not
including Cuꢀ90 sample), in good agreement with the CO2ꢀTPD
results. Furthermore, it is worthy to note that the proportion of
Oδꢀ species in the total oxygen species presents a decreasing
trend with the Cu loading, indicative of the decrease in the
proportion of surface SB sites.
Fig. 7 CO2ꢀTPD profiles of diferent reduced samples. (a) Cuꢀ10, (b) Cuꢀ25, (c)
Cuꢀ40, (d) Cuꢀ55, (e) Cuꢀ70 and (f) Cuꢀ90.
Vapor-phase hydrogenation
Firstly, the catalytic performance of Cu/MgO catalysts was
investigated in the vaporꢀphase hydrogenation of DMCD. In the
course of hydrogenation, CHDM product can be further
dehydrated to 4ꢀmethylꢀcyclohexanemethanol (MCHM). In
addition, methyl 4ꢀhydroxymethylcyclohexane carboxylate
(MHMCC) intermediate and incomplete hydrogenation product,
4ꢀ(hydroxymethyl)ꢀ1ꢀcyclohexanecarboxaldehyde (HMCCA),
can be detected.
The catalytic activity and selectivity for DMCD
hydrogenation on different Cu/MgO catalysts are listed in Table
2. Interestingly, the catalytic activity and selectivity to CHDM
present a bimodal distribution with the Cu loading under the
given reaction conditions. That is, Cuꢀ25 and Cuꢀ70 catalysts
yield higher DMCD conversions (>97.0%) with the higher
selectivities to CHDM (>96%), compared with other catalysts.
In contrast, Cuꢀ10 and Cuꢀ90 catalysts show the worst catalytic
performance for the DMCD hydrogenation. Additionally,
different oxides (MgO, TiO2, SiO2, Al2O3, CaO) supported Cuꢀ
based catalysts prepared by the conventional impregnation
method were used here as comparison catalysts. Note from
Table 2 that their catalytic efficiency is markedly lower than
that of Cuꢀ25 with the similar Cu loading.
In addition, the reactions under different LHSV values were
conducted (Fig.9). The DMCD conversions over Cuꢀ25 and Cuꢀ
70 catalysts remain above 97% between LHSVs of DMCD from
1.0 to 3.0 hꢀ1, then decrease slowly with LHSV increasing from
3.0 to 4.0 hꢀ1. Subsequently, the conversions over other four
Cu/MgO catalysts decline continuously at a LHSV of 1.0 hꢀ1.
Despite the gradual decrease in selectivity to CHDM with the
further increase in LHSV, Cuꢀ25 and Cuꢀ70 catalysts continue
to show higher selectivity than other catalysts. As a result, the
Fig. 8 O1s XPS of reduced Cu/MgO samples. (a) Cuꢀ10, (b) Cuꢀ25, (c) Cuꢀ40,
(d) Cuꢀ5, (e) Cuꢀ70, (f) Cuꢀ90.
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best catalytic performance, as well as high tolerance to the
LHSV of DMCD, is achieved over Cuꢀ25 and Cuꢀ70 catalysts.
DMCD (hꢀ1) over different catalysts. Reaction conditions: P=6.0 Mpa, T=220
oC, H2/DMCD=220 (mol molꢀ1).
As to hydrogenation catalyzed by metallic copper, one
important key to high performance is a large accessible Cu
surface area, because higher copper surface area can supply
more catalytically active sits for hydrogen dissociation and thus
higher conversion.56 However, in the present catalyst system,
the change in the activity of the catalysts does not change
monotonically with the metallic copper surface area. Cuꢀ70
catalyst with the largest active Cu surface area exhibits superior
catalytic performance to others. Interestingly, when Cuꢀ25
catalyst with the largest amounts of total basic sites and SB sites
is used to catalyze the hydrogenation, similar conversion and
selectivity to those over the Cuꢀ70 catalyst are obtained,
although Cuꢀ25 catalyst presents lower copper surface area.
Furthermore, Cuꢀ25 exhibits higher catalytic activity than Cuꢀ40
and Cuꢀ55 catalysts with the larger Cu surface areas. Therefore,
in order to rigorously compare the intrinsic nature of
catalytically active copper sites on different Cu/MgO catalysts,
the DMCD conversion data limited to less than 30 % by
adjusting the LHSV of DMCD were used to calculate the initial
turnover frequency (TOF), according to the moles of DMCD
converted by each mole of the surface copper on the catalyst
surface in the initial 1h. As shown in Table 2, the TOF value
reaches the maximum (49.1 hꢀ1) in the case of Cuꢀ25. Cuꢀ70,
however, possesses a relatively small TOF of 39.6 hꢀ1. The
aforementioned results indicate that the Cu surface area alone
cannot explain the differences in catalytic performance of
catalysts, and some complicated interactions on catalytically
active sites for the hydrogenation should occur. It is suggested
that the surface basic nature of asꢀformed Cu/MgO catalysts
probably plays an important role in the hydrogenation of
DMCD. In this regard, the fact that acidic TiO2 and SiO2
supported Cuꢀbased catalysts showed much lower catalytic
hydrogenation performance than basic MgO and CaO supported
ones (See Table 2) also suggests the promotional effect of
surface basicity on the catalytic performance.
Table 2 The catalytic performance of different catalysts in vaporꢀ
phase hydrogenation of DMCD a.
Catalyst
TOF b
(hꢀ1)
28.8
49.1
36.7
38.6
39.6
27.0
ꢀꢀ
Conversion
(%)
Selectivity (%)
CHDM MHMCC
MCHM HMCCA
Cuꢀ10
Cuꢀ25
33.9
97.3
85.3
90.2
98.7
31.8
67.4
1.4
30.1
96.1
82.6
87.8
98.2
27.4
75.1
15.1
32.0
8.7
1.3
0.3
0.2
0.3
0.2
0.7
0
53.6
2.5
15.0
1.1
4.7
3.1
0.9
8.0
1.5
1.6
0.7
1.2
0.5
Cuꢀ40
12.5
8.8
Cuꢀ55
Cuꢀ70
0.7
Cuꢀ90
Cu/MgO c
63.9
23.4
83.3
67.3
90.1
39.5
c
Cu/TiO2
ꢀꢀ
0
c
Cu/Al2O3
ꢀꢀ
23.5
0.4
0
c
Cu/SiO2
ꢀꢀ
0
Cu/CaO c
ꢀꢀ
45.3
60.0
0
o
a Reaction conditions: temperature, 220 C; hydrogen pressure: 6 MPa, LHSV
of DMCD, 3.0 hꢀ1; H2/DMCD = 220 (mol molꢀ1). b calculated according to the
moles of DMCD converted in the initial 1 h per mole surface metallic copper. c
prepared by the conventional impregnation.
Table 3 The catalytic performance of Cu/MgO catalysts in
vaporꢀphase hydrogenation of FAL a.
Catalyst
Conversion
(%)
Selectivity (%)
FOL
1ꢀMF
3.8
Cuꢀ10
Cuꢀ25
Cuꢀ40
Cuꢀ55
Cuꢀ70
Cuꢀ90
45.7
96.2
97.4
95.6
96.0
97.6
94.3
97.2
2.6
95.5
4.4
92.5
4.0
98.1
2.4
36.7
5.3
a
Reaction conditions: temperature, 180 oC; LHSV of FAL, 1.0 hꢀ1; H2/FAL =
16 (mol molꢀ1); atmospheric pressure.
Secondly, the vaporꢀphase hydrogenation of FAL to FOL
was performed on asꢀformed Cu/MgO catalysts. Here, a side
reaction, hydrogenolysis of FOL to 2ꢀmethylfuran (MF), also
can occur. As shown in Table 3, Cuꢀ25 and Cuꢀ70 catalysts also
show superior catalytic performance to other catalysts,
Fig. 9 DMCD conversion (A) and CHDM selectivity (B) versus LHSV of
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indicating that Cu/MgO catalysts follow a similar regulation of
catalytic performance to that in the hydrogenation of DMCD. It
means that as for asꢀformed Cu/MgO catalysts, the catalytic
mechanism is the same in two kinds of hydrogenation reactions.
further confirm that no obvious structural change of Cuꢀ25
catalyst is observed after 200 h of hydrogenation reaction
(Fig.S2†). In addition, a weak diffraction related to quartz (JCPDS
No. 46ꢀ1045) appears at 26.6o, because
a small amount
of quartz filled in the reactor is dragged into the catalyst during
the long time reaction. After 200 h of catalytic test, no obvious
agglomeration and accumulation of the Cu NPs on the support
surface can be observed from HRTEM images of used Cuꢀ25
catalyst (Fig.S3†), and the crystallite size of used Cu NPs is about
4.5 nm. Due to the confinement effect of MgO phase, active
copper species are surrounded by the MgO phase and stabilized
on the support surface, thereby leading to the stable activity of asꢀ
formed Cu/MgO catalyst.
Scheme 1 Proposed possible mechanism of the hydrogenation of aldehyde
over asꢀformed Cu/MgO catalyst.
At last, based on the above experimental results, it can be
deduced that the Cu0 species should be the effective catalytically
active centres for dissociating hydrogen in the hydrogenations of
DMCD and FAL. Additionally, surface basic sites, especially SB
sites in intimate contact with metallic Cu0 sites, can easily interact
with the π* acceptor orbital of the C=O group, thus improving the
reactivity of the carbonyl group in substrate molecules.57
Therefore, a synergistic effect mechanism for hydrogenation
reaction between surface Cu0 and basic sites over asꢀformed
Cu/MgO catalysts is tentatively proposed to rationalize the
experimentally observed structureꢀperformance relation (Scheme
1). Here, the C=O group may form a zwitterionic, tetrahedral
intermediate with enhanced nucleophilic reactivity at the oxygen
atom on the surface basic sites of catalysts. Consequently, the πꢀ
bond of C=O activated by basic sites become easy to be attacked
by dissociative hydrogen on the surface of Cu0 particles.
Therefore, the carbonyl group gets two H atoms from Cu0 and
parts from basic sites.
As a result, in our case, a uniform distribution of Cu and Mg
species in catalyst precursors facilitates high dispersion of active
Cu species, as well as the formation of strong interfacial contact
between active copper NPs and MgO support. Correspondingly,
the conversion rate of carbonyl compounds increases significantly
owing to the facile dissociation of H2 molecules on catalytically
active Cu0 sites and activation of carbonyl groups by adjacent
basic sites. In particular, such formed surface synergistic catalytic
effect between metallic copper species and basic sites in the Cuꢀ
25 catalyst with the highest amount of basic sites and the
appropriate amount of Cu0 sites can highly efficiently promote the
hydrogenation reaction related to the carbonyl groups. As for Cuꢀ
Fig. 10 Catalytic performance as a function of reaction time over Cuꢀ25
catalyst: (A) hydrogenation of DMCD, P = 6 MPa, reaction temperature = 220
oC, H2/DMCD = 220 (mol molꢀ1), LHSV = 3.0 hꢀ1 and (B) hydrogenation of
FAL, reaction temperature = 180 oC, LHSV = 1.0 hꢀ1 ,H2/FAL = 16 (mol molꢀ
1), atmospheric pressure.
Thirdly, the longꢀterm stability of catalysts is very significant
for practical applications. Generally, the activity loss for metalꢀ
based catalysts can be attributed to surface transmigration and
aggregation of metal NPs. Fig.10 presents the catalytic
hydrogenation performance as a function of time on stream over
Cuꢀ25 catalyst. It is noted that Cuꢀ25 exhibits high stability for the
hydrogenation of DMCD and FAL up to 200 hours and no
decrease in either the conversions of DMCD and FAL or the
selectivities to CHDM and FOL can be observed. XRD analyses
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70 catalyst, the high efficiency mainly relies on its largest copper
surface area, due to relatively weaker surface basic nature.
In general, the current study can be more advantageous over
previous studies because of the two following merits. Firstly, the
synthesis of catalyst precursors was performed by a separate
nucleation and aging step method, which resulted in excellent
stability of the resultant Cu/MgO catalysts due to the strong
confinement effect of MgO phase. Secondly, unlike the Cu+–Cu0
synergy reported in the literature that mainly holds the key to
hydrogenation reactions and is easily weakened due to deep
reduction of Cu+ species under real reaction conditions, in our
case, Lewis basic sites synergistically promote the hydrogenation
activity of Cu0 sites, which results in significantly enhanced
catalytic performance in the hydrogenation of carbonyl
compounds.
Xꢀray diffraction (XRD) data were collected at room
temperature on Shimadzu XRDꢀ6000 diffractometer with
graphiteꢀfiltered Cu Kα source (λ= 0.15418 nm), 40 kV and 40
mA.
Elemental analysis was carried out using a Shimadzu ICPSꢀ
7500 inductively coupled plasma atomic emission spectroscopy
(ICPꢀAES) after the samples were dissolved in dilute hydrochloric
acid (1:1).
Transmission electron microscopy (TEM) and highꢀresolution
TEM (HRTEM) were performed on a JEOL 2100 operated at an
accelerating voltage of 200 kV.
Highꢀangle annular darkꢀfield scanning TEMꢀenergyꢀ
dispersive Xꢀray spectroscopy (HAADFꢀSTEM) image was
recorded on a JEOL2010F instrument combined with Xꢀray
energy dispersive spectroscopy (EDS) system.
N2 adsorptionꢀdesorption isotherms of samples were obtained
on a Micromeritics ASAP 2020 sorptometer apparatus at ꢀ196 oC.
The specific surface areas were obtained by the multipoint
Brunauer–Emmett–Teller (BET) method.
Conclusions
In summary, wellꢀdispersed Cu/MgO catalysts were prepared
by an alternative separate nucleation and aging step method. Asꢀ
formed Cu/MgO catalysts with the largest amount of basic sites or
the largest amount of copper surface area exhibited superior
catalytic performance in the vaporꢀphase hydrogenation of DMCD
and FAL, which could be tentatively ascribed to the synergistic
effect between the metallic copper species and surface basic sites
(especially SB sites). The Cu0 sites contributed to the activation of
H2 molecule, while surface basic sites favored the activation of
substrate molecules. From both academic and industrial
viewpoints, this work is very meaningful to guide the preparation
of lowꢀcost and highlyꢀefficient supported Cuꢀbased catalysts for
the selective hydrogenation of carbonyls to alcohols.
Xꢀray photoelectron spectroscopy (XPS) and Xꢀray induced
Auger spectra (XAES) were recorded on
a Thermo VG
ESCALAB250 Xꢀray photoelectron spectrometer at a base
pressure of 2×10ꢀ9 Pa using Al Kα Xꢀray radiation (1486.6 eV
photons).
Hydrogen temperatureꢀprogrammed reduction (H2ꢀTPR) was
performed using Micromeritics ChemiSorb 2720 instrument.
Sample (0.1g) was placed in a quartz Uꢀtube reactor and degassed
o
at 200 C for 2 h under argon flow (40 mL/min). Then TPR was
conducted in a stream of 10% v/v H2/Ar (40 mL/min) with a
heating rate of 5 oC /min up to 800 °C. The effluent gas was
analyzed by a thermal conductivity detector (TCD).
The copper surface areas and copper dispersions were
determined according to H2ꢀN2O titration.59 Firstly, calcined
sample underwent a H2ꢀTPR process in 10% H2/Ar mixture from
50 to 400 °C and held the temperature until no more H2
consumption. After cooling down, the gas was switched to 10%
N2O/N2, and the sample was oxidized at a flow rate of 50 ml/min
at 60 °C for 1 h, followed by Ar purging and cooling the sample
bed down to room temperature. Finally, H2ꢀTPR was carried out
again with a gas mixture of 10 % H2/Ar to 400 °C. Copper
dispersion was calculated by dividing the amount of surface
copper sites by the total number of Cu atoms; copper surface area
was calculated by assuming spherical shape of the Cu metal
particles and a surface concentration of 1.47 × 1019 Cu atoms/m2.
Temperatureꢀprogrammed desorption of CO2 (CO2ꢀTPD) was
conducted on a ChemiSorb 2720 instrument equipped with a
TCD. 0.1 g sample was heated at a rate of 10 °C/min to 600 °C
under He flow (40ml/min) and maintained at this temperature for
1 h in order to remove the surface impurities. Then the sample
was reduced with 10% H2/Ar mixture at 300 °C for 1 h. After
cooling down to room temperature under He flow, the sample was
exposed to pure CO2 (20 ml/min) for 1 h. Subsequently, the
sample was purged with He (40 ml/min) for 1 h and then heated to
800 °C at a rate of 5 oC /min.
Experimental
Synthesis of catalysts
Supported Cu/MgO catalyst precursors were prepared by a
separate nucleation and aging step method developed in our
group.39 Solution A: Cu(NO3)2·3H2O and Mg(NO3)2·6H2O salts
with the theoretical Cu/(Cu+MgO) mass ratios of x in wt.% (x
=10, 25, 40, 55, 70 and 90) were dissolved in 100 mL of deionized
water to give a solution with a total cationic concentration of 0.4
M. Solution B: NaOH and Na2CO3 were dissolved in 100 mL of
deionized water to form a mixed base solution ([CO32−] = 0.4 M,
[OH−] =0.8M). Solutions A and B were simultaneously added to a
colloid mill with the rotor speed set at 6000 rpm and mixed for 3
min. The resulting suspension was washed several times with
deionized water, aged at 60 °C for 12 h, and finally dried at 70 °C
over night. The catalyst precursor was calcined in static air at 450
oC for 6 h, pelletized, crushed, sieved to 40ꢀ60 mesh, and denoted
as Cꢀx. Before the hydrogenation reaction, the calcined samples
o
were in situ reduced in 10 % H2/N2 atmosphere at 300 C for 2 h
at a ramp rate of 2 C min−1, and the obtained reduced catalysts
o
were denoted as Cuꢀx.
For comparison, different commercial supports (MgO, CaO,
Al2O3, TiO2, SiO2) also were used to synthesize supported Cuꢀ
based catalysts with the same Cu loading of about 25.0 wt.% by
impregnation method.58
In situ FTꢀIR spectra of CO adsorption were recorded on a
Thermo Nicolet 380 FTꢀIR spectrometer. The sample powder was
pressed into a selfꢀsupporting wafer (50 mg) and mounted into the
IR cell. The sample was first reduced with H2 (10 ml/min) at 300
Characterizations
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o
6
7
8
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°C for 1 h, followed by evacuating for 1 h at 400 C and then
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Then the cell was evacuated and heated from room temperature to
100 °C. The FTꢀIR spectra of CO adsorption on the catalyst were
obtained by subtracting the background spectrum.
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Catalytic hydrogenation
The hydrogenation of DMCD was carried out in a stainlessꢀ
steel fixedꢀbed tubular reactor with the inner diameter of 12 mm.
The calcined Cuꢀbased sample (2.0 g) was loaded into tubular
reactor with quartz powders packed in both sides of the catalyst
bed, and then were reduced in a 10% H2/N2 atmosphere at 300 oC
for 2 h at a heating rate of 2 oC min−1. After activation of catalysts,
20.0 wt. % DMCD (purity >99.9 %) in methanol and H2 were fed
into the reactor at a H2/DMCD molar ratio of 220 at a certain
temperature. During hydrogenation, the total pressure was kept at
6.0 MPa, and the roomꢀtemperature liquid hourly space velocity
(LHSV) of DMCD changed from 1.0 to 4.0 h−1. In addition, the
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o
o
increased from 150 to 180 C with a ramp rate of 10 C/min and
then to 240 oC with a ramp rate of 20 C/min. As for products
o
from FAL hydrogenation, the column temperature was increased
o
o
from 55 to 100 C with a ramp rate of 20 C/min and then to 230
oC with a ramp rate of 30 oC/min. The conversions were
calculated by the change in molar number before and after the
reaction. The selectivities of all products were calculated by the
equation: selectivity (mol.%) = (moles of product)/(the sum of the
moles of all products) × 100 %. The average conversions and
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We gratefully thank the financial support from 973 Program
(2011CBA00506), the National Natural Science Foundation of
China, the Specialized Research Fund for the Doctoral Program
of Higher Education (20120010110012), and Project from
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