Lewis-base-promoted copper-based catalyst for highly efficient hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate

Article abstract of DOI:10.1039/c3gc40658j

The gas-phase hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate to 1,4-cyclohexane dimethanol was performed on a well-dispersed Cr-free supported copper-based catalyst derived from a Cu-Mg-Al layered double hydroxide precursor, and achieved a lasting 100% conversion with 99.8% selectivity up to 200 hours. The unprecedented catalytic performance is ascribed to the synergistic effect between surface active Cu⁰ sites and Lewis base sites.

Full text of DOI:10.1039/c3gc40658j

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Lewis-base-promoted copper-based catalyst for highly efficient
hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate †
Shaoyan Zhang, Guoli Fan, Feng Li*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
1
The gas-phase hydrogenation of dimethyl 1,4-cyclohexane
dicarboxylate to 1,4-cyclohexane dimethanol was performed
on a well-dispersed Cr-free supported copper-based catalyst
derived from Cu-Mg-Al layered double hydroxide precursor,
and achieved a lasting 100 % conversion with 99.8 %
selectivity up to 200 hours. The unprecedented catalytic
performance is ascribed to the synergistic effect between
unique catalytic properties. In most cases, however, it is difficult
to obtain well-dispersed metal nanopaticles (NPs) with uniform
size and good thermal stability, especially at high loadings, which
originates from inhomogeneous distribution of active precursors
on supports as well as weak interaction between them.
4
0
1
0
2
+
3+
x+
Layered double hydroxides (LDHs, [M1−x
M
x
(OH)
2
]
n−
[Ax/n] ·mH O) belong to a family of highly ordered two-
2
2+
3+
dimensional layered materials, where different M and M
0
surface active Cu sites and Lewis base sites.
metal cations uniformly distribute and orderly prearrange in the
4
4
5
brucite-like sheets. Correspondingly, well-dispersed supported
Nowadays, 1,4-cyclohexane dimethanol (CHDM),
commercially important diol, is preferred over ethylene glycol as
a stepping stone in the production of high-quality and nontoxic
polyester fiber materials with high melting point and excellent
transparency for extensive use in photography and in other 50 reactions. In comparison with other metal-supported catalysts,
applications involving polycarbonates and polyurethanes. In
industry, CHDM is produced normally by the catalytic
a
metal catalysts can be obtained by reducing calcined LDHs with
1
2
2
5
0
5
5
desired active metal species. This kind of structural
transformation endows LDH materials with extraordinary
capability as catalyst precursors in various metal-catalyzed
6
1
LDH-derived supported metal catalysts have two advantages: (i)
a uniform distribution of active metal cations on the atomic level
without a segregation of “lakes”of separate cations within the
brucite-like layers, which ensures the high homogeneity in a LDH
precursor, facilitates the formation of highly dispersed metal
species upon calcination; (ii) metal nanoparticles with tunable
particle size can be formed in a controllable manner. Recently,
considerable attempts have been made on using LDHs containing
Cu, Zn and Al with different metal ratios as catalyst precursors
for a variety of industrially important reactions including
hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate
2
(
DMCD) over traditional CuCr-based catalysts. In spite of the
5
5
high yield of desired product, the toxicity of such type of Cr-
containing catalysts, however, can cause quite serious
environmental pollutions, and thus limits their large-scale
practical applications. Therefore, it is imperative to look for Cr-
free and highly efficient catalysts for the hydrogenation of DMCD.
It is well known that supported copper-based catalysts
exhibited excellent activity and selectivity in some vapour-phase
6
0
7
methanol steam reforming, methane synthesis, and so on.
Herein, we conduct a facile single-source precursor route to
achieve green, well-dispersed supported Cr-free Cu-based
catalysts from CuMgAl-LDH precursors and use them for the
vapour-phase hydrogenation of DMCD. It is found that, in our
3
3
0
hydrogenation reactions. The preparation optimization of
catalytically active copper-based catalysts, however, is far more
advanced than the fundamental understanding of their catalytic
activity. Therefore, approaches to prepare Cu-based catalysts (e.g.
deposition-precipitation and incipient wetness impregnation)
have been intensively explored in order to make full use of their
6
7
7
8
5
0
5
0
0
case, the synergistic effect between surface active Cu sites
and Lewis base (LB) sites controls the hydrogenating
+
0
capability, unlike the Cu -Cu synergy reported in the literature
3
5
that mainly hold the key to hydrogenation reactions related to
8
ester groups. More importantly, the obtained catalyst achieves a
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, P. O. BOX 98, Beijing, 100029, P. R. China.
E-mail: lifeng@mail.buct.edu.cn; Fax: +8610-64425385;
Tel.: +8610-64451226
lasting 100 % conversion of DMCD with 99.8 % selectivity to
CHDM up to 200 hours. To the best our knowledge, such
ultra highly active, efficient and stable Cr-free Cu-based
catalyst for hydrogenation of DMCD has not been reported
until now.
The preparation procedure of supported Cu-based catalysts
with different Cu/Mg/Al atomic ratios (Table 1) invoked the
formation of single-source CuMgAl-LDH precursors and
controllable calcination and reduction[see SI†]. CuMgAl-LDH
precursors were prepared by separate nucleation and aging steps
†
Electronic supplementary information (ESI) available: Experimental section,
XRD patterns of Cu-3 catalysts obtained at different calcination temperature
Fig.S1), XRD patterns of different Cu-based catalysts (Fig.S2), H -TPR
(
2
profiles of different calcined CuMgAl-LDH precursors (Fig. S3), TEM and
HRTEM images of Cu-3 sample (Fig.S4), the elemental mapping of Cu, Mg
and Al atoms over the Cu-3 sample (Fig. S5). See DOI: 10.1039/b000000x
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9
5
method developed in our laboratory. The powder XRD patterns
of the representative LDH precursor with the Cu/Mg/Al atomic
ratio of 6:6:6 (Fig. 1a) show the characteristic diffractions of
cations in the brucite-like layers of LDH precursor, the resulting
MgO and amorphous Al
2 3
O phases may function as supports or
dispersing agents between Cu-containing phases to form porous
2
−1
hydrotalcite-like materials (JCPDS no. 37-0630) with good 35 aggregates exposing a large Cu surface area of up to ~35 m g .
crystallinity. After calcination at 600 °C, a broad diffraction
observed at 2θ of about 36 is obviously different from those of
5
0
5
0
5
o
2 3
crystalline simple metal oxides (e.g. CuO, MgO and Al O ),
suggestive of the formation of mixed metal oxide solid-solution
phase. Addtionally, diffraction peaks belonging to MgO and CuO
with poor crystallinity are detectable in the XRD patterns. After
1
1
2
2
o
reduction, a strong diffraction appears at 2θ of about 43.3 along
o
with a weak one at 50.4 , which is characteristic of fcc copper
phase (JCPDS 04-0836). With the increasing calcination
temperature, the diffraction intensities of metallic Cu and MgO
phases are enhanced gradually (Fig.S1†).No diffractions related
to alumina phase are observed, indicative of the presence of
amorphous aluminia. With the increasing Mg content, the broader
and weaker characteristic diffractions of metallic copper (Fig. S2†)
can be attributed to the dilution effect of MgO component in
reduced samples, thus leading to the reduced particle size of Cu
NPs (Table 1). H -TPR results (Fig. S3†) further confirm that the
2
addition of Mg can improve the the reducibility of Cu species in
the calcined CuMgAl-LDHs, due to the increase in the metal
dispersion degree (Table 1).
TEM images give a direct insight into the microstructure of
representative Cu-3 sample. As shown in Fig.1b-c and Fig.S4†, a
large number of approximately spherical Cu NPs of a size around
Fig. 1 XRD patterns (a) of different samples, TEM (b,c) and
HRTEM (d) images of representative Cu-3 sample.
4-6 nm are well-dispersed on the support, in good agreement with
the elemental mapping observation of relatively homogeneous
distributions of Cu, Mg and Al atoms over the sample (Fig. S5†).
It is believed that benefiting from the uniformly distributed
3
4
0
0
Table 1 Physico-chemical properties and catalytic hydrogenation performance of different Cu-based catalysts
Atomic
Cu
Specific
Total LB sites
DMCD
Selectivity (%)
b
c
d
e
S
BET
2
S
Cu
2
D
i
D
Cu
a
f
f
Catalysts
ratios of loadings
Cu/Mg/Al (wt.%)
basicity
(SLB sites)
conversion
(
m /g)
(m /g)
(%)
(nm)
CHDM MHMCC MCHM
g
(μmolCO
2
/g)
(μmolCO
2
/g)
(%)
Cu-1
Cu-2
Cu-3
Cu-4
12:0:6
9:3:6
6:6:6
4:8:6
70.5
56.8
40.3
27.9
29.6
51.3
67.3
76.6
21.9
35.7
34.9
4.6
9.3
14
7
30.8
23.1(0)
92.3
96.5
100
86.2
93.1
99.8
94.3
6.6
3.2
0.0
1.9
2.3
1.2
39.5
26.8(1.8)
12.8
12.8
6
108.9
76.1
96.2(44.1)
0.2
24.2
4
63.8(14.1)
96.9
0.2
a
b
c
d
e
determined by ICP-AES analysis. BET surface area. Cu surface area determined by N
2
O titration . Cu dispersion degree determined by N
2
O titration.
f
g
-1
2
crystal size of Cu NPs estimated by XRD patterns. determined by CO -TPD. reaction conditions: liquid hour space velocity (LHSV) = 1.0 h ,
o
H
2
/DMCD = 203, reaction temperature = 220 C and reaction pressure = 6 MPa.
4
5
5
0
As to hydrogenation catalyzed by metallic copper, the surface
nature of Cu species in the Cu-based catalysts was investigated
by XPS measurements. As shown in Fig. 2A, the disappearance
of the Cu 2p satellite peak at binding energy (BE) of 942-944 eV
in the XPS and the appearance of a single peak at kinetic energy
of 918.3 eV in the Cu LMM XAES dipict clearly the sole
5
5
0
10
presence of Cu species on the surface of Cu-3 sample. Further
evidence can be obtained from the in situ FT-IR spectrum of CO
absorption on the Cu-3 sample. As shown in Fig. 2B, a single
-1
symmetric peak is observed at about 2100 cm after saturating
the sample with CO at room temperature. Upon heating, the
6
0
o
Fig. 2 (A) Cu 2p XPS and Cu LMM XAES of Cu-3; (B) In situ
adsorbed CO is totally removed at 100 C, which is a typical
0
11
FT-IR spectra of CO absorption on Cu-3 sample. CO pressure of
feature of weakly bound CO on Cu sites.
CO -TPD measurements were used to probe surface basicity
o
o
5
00 Pa (a), followed by evacuation at 30 C (b) and 100 C (c).
2
2
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of the as-synthesized Cu-based catalysts. As shown in Fig.3, Cu-
sample presents four kinds of CO desorption peaks. The three
peaks at lower temperatures are assigned to weak Brønsted basic
3
2
It is well documented that the catalytic hydrogenation of
DMCD to CHDM usually goes through a consecutive multi-step
o
o
sites centered at 100 C, moderate LB sites centered at 150 C, 35 reaction pathway (Scheme 1), and that CHDM product can be
o
12
5
0
5
0
5
and strong LB sites centered at 400 C, respectively. The
further dehydrated to 4-methyl-cyclohexanemethanol (MCHM).
Meanwhile, the accumulation of the reaction intermediate,
methyl 4-hydroxymethylcyclohexane carboxylate (MHMCC),
can not be neglected.
o
intensive desorption peak centered at 510 C should be associated
with the large amount of super LB (SLB) sites existing on the
surface of Cu-3 sample. From the O 1s spectrum of Cu-3 sample
(
Fig.4), it is seen that there are four fitted peaks, representing four
40
1
1
2
2
different kinds of surface oxygen species. There is a general
agreement between the literature and the present results such that
the peak with the BE around 530.9 eV belongs most likely to
oxygen species of hydroxyl groups at surface, while the peaks at
lower BE of about 530.1 and 529.3 eV are the characteristic of
lattice oxygens bound to metal cations of the structure, i.e., M-O-
2-
13
M groups and Mg-O pairs. Interestingly, the peak located at
5
2
-
28.3 eV can be attributed to the coordinatively unsaturated O
2
+
δ-
species strongly bound to Mg cations (O ), which should be
related to the reduction of the Cu-O-Mg parts in mixed metal
oxide solid-solution phase during the catalyst activation
procedure, leading to the formation of Cu NPs in strong
interfacial contact with MgO phase. It also indicates the presence
of SLB sites on the surface of Cu-3 sample, in good agreement
with the CO -TPD result. As listed in Table 1, Cu-3 possesses the
largest amount of total LB sites as well as that of SLB sites
among the Cu-based samples.
Scheme 1 Reaction pathway for the hydrogenation of DMCD to
CHDM
4
5
5
6
6
7
7
8
5
0
5
0
5
0
5
0
Table 1 summarizes the results of catalytic hydrogenation of
DMCD over as-synthesized Cu-based catalysts with different
Cu/Mg/Al atomic ratios, which were obtained under industrial
conditions. The yield of CHDM decreases in the following order
for the catalysts: Cu-3 > Cu-4 ≈ Cu-2 > Cu-1. Especially, a
complete DMCD conversion with 99.8% selectivity was achieved
over the Cu-3 catalyst, which is the most efficient catalyst for this
reaction reported to date. As to hydrogenation catalyzed by
metallic copper, one important key to high performance is a large
accessible Cu surface area. The Cu-1 catalyst with the smallest
active Cu surface area exhibits lower catalytic activity than the
other catalysts, because of smaller copper surface area supplying
less catalytically active sits for hydrogen dissociation and thus
2
1
4
lower DMCD conversion. However, the change in the activity
of the catalysts does not change monotonically with the metallic
copper surface area. It is worthy to note that the Cu-3 catalyst
with the largest amount of surface LB sites exhibits superior
catalytic performance to others, in spite of slightly lower copper
surface area than that of the Cu-2 catalyst. Furthermore, when the
Cu-4 catalyst with relatively more surface LB sites is used to
catalyze the hydrogenation, similar DMCD conversion and
selectivity of CHDM to those of the Cu-2 catalyst are found,
though it presents much lower copper surface area than the Cu-2
catalyst. The aforementioned results indicate that the Cu surface
area alone cannot explain the differences in catalytic performance
for catalysts, and the surface basic nature also should play an
important role in the hydrogenation of DMCD. In contrast,
traditional Cu-Cr O catalyst with the Cu loading of 40.3 %
2
Fig. 3 CO -TPD profile of Cu-3 sample.
2
3
exhibits much lower conversion of DMCD (89.0%) and
selectivity to CHDM (90.0 %), compared with the Cu-3 catalyst.
Based on the above experimental results, it is deduced that
0
Cu species should be the effective catalytic centres for the
hydrogenation of DMCD. The quantity of dissociative hydrogen
on the active sites directly determines the rate of the reaction.
Additionally, surface LB sites, especially SLB sites in intimate
0
3
0
contact with metallic Cu sites, can easily interact with the π*
acceptor orbital of the C=O group, thus improving the reactivity
of the ester group in DMCD molecule. Therefore, a synergistic
Fig.4 O 1s XPS of Cu-3 sample.
15
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effect mechanism for hydrogenation reaction between surface
Cu and LB sites over as-formed Cu-based catalysts is tentatively
proposed to rationalize the experimentally observed structure-
30
As a result, due to the highly dispersed nature of Cu
nanoparticles, metallic Cu may interact closely with neighboring
MgO component at the interface in the Cu-based catalysts. Such
interaction facilities the occurrence of synergistic effect between
0
performance relation (Scheme 2). The first step is the dissociation
0
0
5
of H on the surface of Cu particles and the activation of DMCD
2
surface active Cu sites and LB sites for hydrogenation of DMCD.
by surface LB sites. On the surface LB sites of catalyst, the C=O
group may form a zwitterionic, tetrahedral intermediate with
enhanced nucleophilic reactivity at the oxygen atom.
Consequently, the π-bond of C=O become easy to be attacked by
35
Remarkably, it is interesting to find that the excellent
catalytic performance of the most efficient Cu-3 catalyst in the
gas-phase hydrogenation of DMCD stays nearly unchanged over
a 200 h reaction time when the other reaction conditions are kept
1
0
dissociative hydrogen. Therefore, the ester carbonyl gets two H
atoms from Cu and parts from LB sites. Due to the 40 reaction temperature of 240 C, the DMCD conversion still
constant (Fig.5a). Furthermore, even though at the higher
0
o
hydrogenation of C=O bond, DMCD transforms gradually to
retains 100 % over the Cu-3 catalyst and the selectivity to CHDM
reaches as high as 98.0% (Fig. 5b). These results indicate that the
catalyst derived from CuMgAl-LDH precursor is very stable, due
to the stabilization effect of MgO and Al O phases in the
MHMCC by eliminating a CH OH, which can be confirmed by
3
the incompletely hydrogenation product. In the proposed
mechanism, only one of the ester group hydrogenation is shown,
while the other ester group is supposed to undergo the same
hydrogenation path. As a result, the extremely high and effective
hydrogenation activity of Cu-3 catalyst may rely on the active
1
5
2
3
45 catalyst preventing the Cu NPs from aggregation.
In summary, we have presented that the introduction of Mg
into Cu-based catalysts from LDH precursor provides
a
0
Cu particles and the surface LB nature.
significant route to modify the structural and surface properties of
catalysts. The Cu-3 catalyst possessing the largest amount of LB
sites and relatively high copper surface area exhibited the best
catalytic performance, which could be tentatively ascribed to the
synergistic effect between the metallic copper species and surface
LB sites (especially SLB sites). Furthermore, the Cu-based
catalyst showed a good stability with a lasting 200-hour ultrahigh
activity and selectivity to CHDM. We believe that such low-cost
and environment-friendly Cr-free Cu-based catalysts are of great
industrial importance for the gas-phase successive production of
CHDM, although further investigation of the reaction mechanism
and catalytic stability still remains to be done.
2
0
5
0
5
5
Scheme 2 Schematic representation of over as-synthesized Cu-
based catalyst.
6
0
Acknowledgment
We gratefully acknowledge the financial support from 973
Program (2011CBA00506) and the National Natural Science
Foundation of China.
Notes and references
6
7
7
8
8
5
0
5
0
5
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C3GC40658J
Graphical and textual abstract
Synthesis of 1,4-cyclohexane dimethanol was conducted over an efficient
lewis-base-promoted Cr-free supported copper-based catalyst.