Selective hydrogenation of dimethyl maleate to tetrahydrofuran over Cu/SiO2 catalyst: Effect of Cu+ on the catalytic performance

Article abstract of DOI:10.1016/j.cclet.2015.04.031

Cu/SiO₂ catalysts prepared by different methods have been investigated focusing on the influence of Cu⁺ on the catalytic performance. The composition, structure and copper valence state were characterized by means of BET, XRD, XPS, FTIR, N₂O-titration. It was found that the Cu/SiO₂ prepared by ammonia-evaporation (AE) method had much higher TOF value than that prepared by wetness-impregnation (WI) with the same THF selectivity. The higher TOF value was attributed to the coexistence of Cu⁺ and Cu⁰ species in the activated AE-Cu/SiO₂, while only Cu⁰ species existing in the activated WI-Cu/SiO₂. Researches suggest that Cu⁺ can adsorb and polarize the C=O bond of DMM. It is concluded that Cu⁰ could be the main active site and the synergistic effect between Cu⁰ and Cu⁺ could contribute to hydrogenation of DMM to THF.

Full text of DOI:10.1016/j.cclet.2015.04.031

Accepted Manuscript
Title: Selective hydrogenation of dimethyl maleate to
tetrahydrofuran over Cu/SiO2 catalyst: Effect of Cu+ on the
catalytic performance
Author: Xue-Qing Han Qun-Feng Zhang Feng Feng
Chun-Shan Lu Lei Ma Xiao-Nian Li
PII:
S1001-8417(15)00164-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2015.04.031
CCLET 3298
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
3-3-2015
1-4-2015
14-4-2015
Please cite this article as: X.-Q. Han, Q.-F. Zhang, F. Feng, C.-S. Lu, L. Ma, X.-
N. Li, Selective hydrogenation of dimethyl maleate to tetrahydrofuran over Cu/SiO2
catalyst: Effect of Cu+ on the catalytic performance, Chinese Chemical Letters (2015),
http://dx.doi.org/10.1016/j.cclet.2015.04.031
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Graphical Abstract
Selective hydrogenation of dimethyl maleate to tetrahydrofuran over Cu/SiO2 catalyst: Effect of Cu+ on the
catalytic performance
Xue-Qing Han, Qun-Feng Zhang*, Feng Feng, Chun-Shan Lu, Lei Ma, Xiao-Nian Li
Institute of Industrial Catalysis of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry
Synthesis Technology, Hangzhou 310032, China
TOF = 6.57 h-1
WI-Cu/SiO2
O
O
Cu0
Cu0
Cu0
Cu0
Cu0
Cu+
Cu0
OCH3
OCH3
O
Cu+
TOF = 11.16 h-1
AE-Cu/SiO2
Cu/SiO2 catalyst prepared by ammonia-evaporation (AE) method has much higher TOF value than that prepared by wetness-impregnation (WI)
with the different active copper species, indicating that Cu0 could be the main active site and the synergistic effect between Cu+ and Cu0 would
further improve catalytic performance.
Page 1 of 7

Original article
Selective hydrogenation of dimethyl maleate to tetrahydrofuran over Cu/SiO2 catalyst:
Effect of Cu+ on the catalytic performance
Xue-Qing Han, Qun-Feng Zhang , Feng Feng, Chun-Shan Lu, Lei Ma, Xiao-Nian Li
Institute of Industrial Catalysis of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,
Hangzhou 310032, China
ARTICLE INFO
ABSTRACT
Article history:
Cu/SiO2 catalysts prepared by different methods have been investigated focusing on the
influence of Cu+ on the catalytic performance. The composition, structure and copper valence
state were characterized by means of BET, XRD, XPS, FTIR, N2O-titration. It was found that
the Cu/SiO2 prepared by ammonia-evaporation (AE) method had much higher TOF value than
that prepared by wetness-impregnation (WI) with the same THF selectivity. The higher TOF
value was attributed to the coexistence of Cu+ and Cu0 species in the activated AE-Cu/SiO2,
while only Cu0 species existing in the activated WI-Cu/SiO2. Researches suggest that Cu+ can
adsorb and polarize the C=O bond of DMM. It is concluded that Cu0 could be the main active
site and the synergistic effect between Cu0 and Cu+ could contribute to hydrogenation of DMM
to THF.
Received 3 March 2015
Received in revised form 1 April 2015
Accepted 14 April 2015
Available online
Keywords:
Dimethyl maleate
Tetrahydrofuran
Copper phyllosilicate
Silica-supported copper
Copper valence state
1. Introduction
Tetrahydrofuran (THF) is an important chemical intermediate. It is widely used both as a precursor to manufacture pyrrolidone,
polyurethanes (PU), polytetramethylene ether glycol (PTMEG) and as a universal solvent in many fields. With the fast growing market
for PTMEG, the demand for THF is further enhanced as a polymer monomer [1]. At present, THF is produced through the dehydration
of 1,4-butanediol (BDO) catalyzed by inorganic acid in industry [2]. The raw material BDO is mainly synthesized from Reppe process
which needs stringent reaction conditions and high energy consumption [3]. Gas phase hydrogenation of dimethyl maleate (DMM) is a
promising method to produce THF under much milder reaction conditions than those of Reppe process [1,4]. Moreover, it has more
advantages over the economic costs in consideration of the raw materials price. It is reported that maleic anhydride as the upstream
product of dimethyl maleate can be obtained from direct oxidation of n-butane [5].
The whole reaction scheme of hydrogenation of DMM is shown in Scheme 1. The first hydrogenation of DMM occurs in C=C,
producing dimethyl succinate (DMS). Subsequent hydrogenation of C=O of DMS yields to γ-butyrolactone (GBL), 1,4-butanediol
(BDO) and THF. Dehydration of BDO to THF needs a weak acidic site such as SiO2. Recently, the studies on the Cu/SiO2 catalysts
have gradually attracted more and more attentions in hydrogenation of ester due to the excellent catalytic performance and low-cost
copper resource. The reported researches of Cu/SiO2 catalysts are mainly about the preparation methods [6-8], the addition of
promoters [9-11], the modification of supports [12,13] and the copper valence state [14-16], aiming at obtaining a high dispersed
Cu/SiO2 catalysts and finding out the active site in the hydrogenation of dimethyl oxalate (DMO). However, the systematic study about
the copper valence state of the Cu/SiO2 catalyst has not yet been reported in hydrogenation of DMM.
In this paper, a series of Cu/SiO2 catalysts were prepared by different methods for the purpose of forming different copper valence
states. The catalytic activity was valued via gas phase hydrogenation of DMM in a continuous flow unit with a fix-bed reactor. The
surfaces copper valence state of Cu/SiO2 was discussed in detail and correlated with the turnover frequency (TOF) and characterization
data.
2. Experimental
2.1. Catalyst preparation
Cu/SiO2 catalyst prepared by the wetness impregnation (WI) is as follows: 3.78 g Cu(NO3)2·3H2O was dissolved in 20 mL
distilled water. and 4 g silica was then added into copper aqueous solution. After impregnated for 12 h, the precursor was dried at 393
K for 8 h, and subsequently calcined at 723 K for 4 h. The obtained catalyst is denoted as WI-Cu/SiO2.
Cu/SiO2 catalyst prepared by the ammonia-evaporation (AE) [6] is as follows: 3.78 g Cu(NO3)2·3H2O was dissolved in 40 mL
distilled water and 11.5 mL concentrated ammonia (28 wt%) aqueous solution was then added to the copper aqueous solution under
Corresponding author.
E-mail address: zhangqf@zjut.edu.cn (Q.-F. Zhang)
Page 2 of 7

stirring for 30 min. And subsequently 4 g silica was added under continuous stirring. After stirred for 4 h at room temperature, the
mixture was rapidly heated to 363 K for evaporation of the ammonia until the pH value decreased to 6.5. The precipitate was filtered,
washed, dried at 393 K for 8 h, and calcined at 723 K for 4 h. The obtained catalyst is denoted as AE-Cu/SiO2.
2.2. Catalyst characterization
The surface area and porosity analysis of the catalysts were measured with N2 physisorption at 77 K on a Micromeritics ASAP
2020. The exposed metallic Cu surface area was determined by N2O titration [17] using a thermal conductivity detector (TCD). The
calculation of metallic Cu surface areas was based on the assumption that the shape of the copper particles was spherical and there
were 1.46 × 1019 copper atoms per square meter. The Powder X-ray diffraction (XRD) patterns were collected on a PANalytical X’Pert
diffractometer equipped with a Cu Kα radiation (λ = 0.1541 nm) with a scanning range of 10°-80°. The operating voltage was 40 kV,
and the current was 40 mA. Fourier transform infrared (FT-IR) spectra of the calcined catalysts and CO species adsorbed on the
activated catalysts were recorded on a Nicolet 6700 spectrometer. Before the adsorption of CO, the calcined catalysts were activated at
573 K in H2 atmosphere. The spectral resolution was 4 cm-1 and the scan time was 64. The surface copper species were analyzed by
X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra DLD spectrometer. The spectrum was detected with Al Kα line as
the radiation source. The binding energy was referenced to the C1s peak at 284.6 eV.
2.3. Activity test and product analysis
The catalytic activity tests were valued in a fix-bed continuous flow reactor with a stainless steel tube (i.d. = 10 mm). 2 mL of
catalyst (20-40 meshes) was loaded in the middle of the fix-bed, both sides packed with silica sand to ensure the flow well-distributed.
The calcined catalyst was activated in pure hydrogen gas flow at 573 K for 4 h. After the temperature cold down to the reaction
temperature of 513 K, 15 vol% DMM/CH3OH was transported to a vaporizer by a high-pressure pump, mixed with hydrogen and
flowed into the reaction tube at a system pressure of 5 MPa. The molar ratio of H2/DMM was 60, and the liquid hourly space velocity
(LHSV) was 0.9 h-1. At last, the products were analyzed by Agilent GC 7890A fitted with a 30m DB-1 capillary column equipped a
flame ionization detector (FID).
3. Results and discussion
The physical properties of the Cu/SiO2 catalysts and SiO2 support are listed in Table 1. It was found that both the surface area and
the pore volume of Cu/SiO2 catalysts decreased after the active component Cu was loaded into SiO2. Interestingly, the average pore
size of AE-Cu/SiO2 was much larger than that of the support SiO2. The reason is that the silica wall was dissolved in the alkaline
solution and then the dissolved silicate reacted with the neutral copper complex Cu(OH)2(H2O)4 to form copper phyllosilicate [18]. The
exposed Cu surface area determined by N2O titration suggests that the AE-Cu/SiO2 has a better copper dispersion than that of
WI-Cu/SiO2, and the similar result will be presented later.
XRD patterns of the calcined and reduced Cu/SiO2 catalysts are shown in Fig. 1. The broad diffraction peak at around 2θ = 22.5°
is assigned to the amorphous SiO2 in all samples. In the calcined catalysts, the colors of WI-Cu/SiO2 and AE-Cu/SiO2 were gray and
dark green respectively, implying the two catalysts contained different copper species. Actually, only CuO (JCPDS 80-1916) species
was detected at 2θ = 35.5, 38.7, 48.6° in the calcined Cu/SiO2 catalysts. In the reduced catalysts, the sharp diffraction peak of metal Cu
(JCPDS 89-2838) at 2θ = 43.3, 50.5, 74.1° was observed in the reduced WI-Cu/SiO2, and the weak diffraction peak of metal Cu at 2θ =
43.3° was also observed in the reduced AE-Cu/SiO2. However, a new weak diffraction peak appeared at 2θ = 36.4° in the reduced
AE-Cu/SiO2 which was corresponding to Cu2O (JCPDS 05-0667). According to the different colors and the new diffraction peak, there
should exist a new copper species (copper phyllosilicate) in calcined AE-Cu/SiO2 except for CuO, and the new copper species finally
was converted to Cu2O after reduction [14]. The presence of copper phyllosilicate will be verified via more information given below.
In addition, it is clear that AE-Cu/SiO2 has a better copper dispersion than WI-Cu/SiO2 because the diffraction peaks of former catalyst
were obviously lower and broader than that of latter.
In order to confirm the existence of copper phyllosilicate, the FT-IR spectra of calcined catalysts and pure SiO2 are illustrated in
Fig. S1 in Supporting information. The new peak appearing in the spectrum of AE-Cu/SiO2 at 670 cm-1 is absent both in the spectrum
of WI-Cu/SiO2 and pure SiO2, which is a characteristic band of copper phyllosilicate [18]. In addition, in situ FT-IR spectra of CO
species adsorbed on the reduced catalysts was used to identify the chemical adsorption state, and the results are shown in Fig. 2. The
intense absorption bands near 2122 cm-1 were observed both in WI-Cu/SiO2 and AE-Cu/SiO2 before evacuation, which was assigned to
CO adsorbed on Cu species (CO-Cu0 and CO-Cu+). However, the band near 2122 cm-1 in WI-Cu/SiO2 was disappeared whereas a new
band formed near 2128 cm-1 after evacuation for 20 min. The new band at 2128 cm-1 was attributed to CO-Cu+ because CO could be
strongly adsorbed on Cu+ while CO was weakly adsorbed on Cu0 during evacuation [19,20].
The XPS and Auger spectra (AES) of the Cu/SiO2 catalysts were carried out to identify surface valence state of copper species. As
shown in Fig. 3, two obvious and intensive photoelectron peaks appearing at 952.6 eV and 932.7 eV are assigned to the Cu 2p1/2 and
Cu 2p3/2 binding energy [6], respectively. The results suggest that the high valence state Cu2+ species in calcined WI-Cu/SiO2 and
AE-Cu/SiO2 are effectively reduced to Cu+ and Cu0 because the characteristic satellite peaks of Cu2+ species at about 11 eV higher than
932.7 eV (Cu 2p3/2) have disappeared and the peaks at 933.3 eV are shifted to 932.7 eV [21,22], which is consistent with the results of
XRD. Considering that the Cu+ and Cu0 have almost the same binding energy of Cu 2p3/2, Cu LMM XAES is traditionally used to
distinguish these two valence states by different kinetic energy [23]. According to the Fig. 4, an asymmetric and broad kinetic energy
peak is observed in the reduced AE-Cu/SiO2, indicating the existence of more than one copper species. In contrast, the reduced
WI-Cu/SiO2 catalyst only possesses one symmetric kinetic energy peak which is assigned to Cu0 species. As is reported in paper, the
Page 3 of 7

symmetric peaks at 915.2 eV and 917.7 eV are attributed to Cu+ and Cu0 [24], respectively. Through deconvolution of Cu LMM curve
for the reduced AE-Cu/SiO2, the result is shown in Fig. 4 and the intensity ratio of Cu+/Cu0 is given in Table 1.
The catalytic performances of Cu/SiO2 catalysts prepared by different methods are illustrated in Fig. 5. It is clear that the DMM
conversion of AE-Cu/SiO2 is much higher than that of WI-Cu/SiO2 with the same THF selectivity of 100%, the THF yield is listed in
Table 1. The corresponding explanation is due to the AE-Cu/SiO2 has higher exposed Cu surface area listed in Table 1. Meanwhile, it
is found that the WI-Cu/SiO2 (only Cu0) can catalyze the reaction individually, which means Cu0 is the active site. TOF value is an
important standard to measure the performance of catalysts, and it refers to the reactant molecules consumed per mol active surface
copper per hour. To further get insight the effect of new species of Cu+ formed in activated AE-Cu/SiO2, TOF value is calculated to
eliminate the influence of the dispersion of copper and the results are listed in Table 1. Obviously, TOF value of AE-Cu/SiO2 is higher
than that of WI-Cu/SiO2. The wide difference of TOF values can be attributed to the formation of Cu+ in AE-Cu/SiO2, while only Cu0
species is present at WI-Cu/SiO2. Based on the above FT-IR spectra of chemisorbed CO, the Cu+ can stabilize carbonyl compound via
CO-Cu+. Moreover, it may also function as Lewis acid sites to polarize the C=O [6]. Thus, the Cu+ can improve the catalytic
performance as another active site. However, researches reported that the catalytic activity would come down when the ratio of
Cu+/Cu0 exceeded an appropriate value [25-27], indicating that Cu+ isn’t the main active site. Based on above analysis, we concluded
that Cu0 could be the main active site and the synergistic effect between Cu0 and Cu+ could further improve the activity.
4. Conclusion
The influence of Cu+ in Cu/SiO2 catalysts was investigated by the comparison of TOF value of the WI-Cu/SiO2 and AE-Cu/SiO2.
The Cu+ and Cu0 species were generated in the activated AE-Cu/SiO2, while only Cu0 was formed in the activated WI-Cu/SiO2. The
characterization suggested that the two species Cu+ and Cu0 were originated from the copper phyllosilicate and CuO during the
calcination process, respectively. The dispersion of copper species affects the catalytic performance significantly, which was
remarkably different between WI-Cu/SiO2 and AE-Cu/SiO2 illustrated in XRD graph. To have a single investigation of the effect of
Cu+ species, the TOF values of two catalysts were calculated for eliminating the interference of the dispersion of copper species.
Activated AE-Cu/SiO2 (contain Cu+ and Cu0) gave a higher TOF value than that of activated WI-Cu/SiO2 (only Cu0), which was
attributed to strong adsorption between Cu+ and the C=O bond of DMM. The correlation between surface valence state of copper
species and TOF values suggested that Cu0 was the main active site and the synergistic effect between Cu+ and Cu0 could further
improve the activity.
Acknowledgment
Funding for the present study from the National Key Basic Research Program of China (973 Program, No. 2011CB710800) and
National Natural Science Foundation of China (No. NSFC-21406199).
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O
O
O
O
O
+2H2
+2H2
OCH3 +H2
OCH3
OCH3
OCH3
-H2O
Acid
HO
O
O
OH
THF
GBL
BDO
DMM
DMS
Scheme 1. The whole reaction scheme of hydrogenation of DMM
Table 1 Physicochemical properties of the Cu/SiO2 catalysts and SiO2 support
Sample
Surface area
(m2 g-1)
167
Pore volume
(m3 g-1)
0.50
Average pore
size (nm)
11.8
SCu
0
SCu
+
Cu+/Cu0
THF yield
(%)
TOF
(h-1) d
-
(m2 g-1
)
(m2 g-1
)
cat
(%)c
-
a
b
cat
SiO2
-
-
-
WI-Cu/SiO2
AE-Cu/SiO2
113
117
0.33
0.42
11.5
14.4
9.91
18.88
0
0
1.53
11.87
83.65
6.57
11.16
28.89
a Exposed metallic Cu surface area (SCu
0
) determined by N2O titration method.
+ 0
b The Cu+ surface area (SCu and the ratio of Cu+/Cu0 assuming that Cu+ has the same surface area as Cu0.
) calculated by the SCu
c The ratio of Cu+/Cu0 based on the deconvolution Cu LMM XAES spectra.
d Turnover frequency (TOF) value expressed by the DMM molecules consumed per mol active surface copper per hour.
e
The parameters (Surface area, Pore volume, Average pore size) were determined based on the calcined Cu/SiO2 catalysts, the remainder parameters were
determined based on the reduced Cu/SiO2 catalysts.
♥
SiO2
♣
♦
CuO
Cu
Cu2O
♣
♠
♣
♦
♦
♥
♣
(d)
♦
♦
♦
♦
♦
♦
♦
♦
♦
(c)
(b)
♠
♣
♦
♦
(a)
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 1. XRD patterns of the calcined and reduced Cu/SiO2 catalysts: (a) the calcined AE-Cu/SiO2; (b) the reduced AE-Cu/SiO2; (c) the calcined WI-Cu/SiO2; (d)
the reduced WI-Cu/SiO2.
Page 5 of 7

2128
2122
0.5
(d)
(c)
(b)
(a)
2200
2150
2100
2050
2000
Wavenumber (cm-1)
Fig. 2. In situ FT-IR spectra of CO species adsorbed on the reduced catalysts: (a) WI-Cu/SiO2: before evacuation; (b) WI-Cu/SiO2: after evacuation for 20 min;
(c) AE-Cu/SiO2: before evacuation; (d) AE-Cu/SiO2: after evacuation for 20 min.
Cu 2p
932.7
933.3
952.6
(d)
satellite
(c)
(b)
satellite
(a)
970
965
960
955
950
945
940
935
930
Binding energy (eV)
Fig. 3. XPS spectra of the Cu/SiO2 catalysts: (a) the calcined WI-Cu/SiO2; (b) the reduced WI-Cu/SiO2; (c) the calcined AE-Cu/SiO2; (d) the reduced
AE-Cu/SiO2.
Cu LMM
Cu0
Cu+
(b)
(a)
906 908 910 912 914 916 918 920 922 924 926
Kinetic energy (eV)
Fig. 4. Cu LMM Auger spectra of the reduced Cu/SiO2 catalysts: (a) AE-Cu/SiO2; (b) WI-Cu/SiO2.
Page 6 of 7

120
100
80
60
40
20
0
DMM conversion(%)
THF selectivity (%)
WI-Cu/SiO2
AE-Cu/SiO2
Fig. 5. The catalytic performances of Cu/SiO2 catalysts prepared by different methods at 513 K, 5 MPa, LHSV of 0.9 h-1 and H2/DMM molar ratio of 60.
Page 7 of 7