Insight into the balancing effect of active Cu species for hydrogenation of carbon-oxygen bonds

Article abstract of DOI:10.1021/acscatal.5b01678

Hydrogenation of carbon-oxygen (C-O) bonds plays a significant role in organic synthesis. Cu-based catalysts have been extensively investigated because of their high selectivity in C-O hydrogenation. However, no consensus has been reached on the precise roles of Cu⁰ and Cu⁺ species for C-O hydrogenation reactions. Here we resolve this long-term dispute with a series of highly comparable Cu/SiO₂ catalysts. All catalysts represent the full-range distribution of the Cu species and have similar general morphologies, which are detected and mutually corroborated by multiple characterizations. The results demonstrate that, when the accessible metallic Cu surface area is below a certain value, the catalytic activity of hydrogenation linearly increases with increasing Cu⁰ surface area, whereas it is primarily affected by the Cu⁺ surface area. Furthermore, the balancing effect of these two active Cu sites on enhancing the catalytic performance is demonstrated: the Cu⁺ sites adsorb the methoxy and acyl species, while the Cu⁰ facilitates the H₂ decomposition. This insight into the precise roles of active species can lead to new possibilities in the rational design of catalysts for hydrogenation of C-O bonds.

Full text of DOI:10.1021/acscatal.5b01678

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Article
Insight into the Balancing Effect of Active Cu
Species for Hydrogenation of Carbon-Oxygen Bonds
Yue Wang, Yongli Shen, Yujun Zhao, Jing Lv, Shengping Wang, and Xinbin Ma
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01678 • Publication Date (Web): 14 Sep 2015
Downloaded from http://pubs.acs.org on September 15, 2015
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Insight into the Balancing Effect of Active Cu Species for
Hydrogenation of Carbon-Oxygen Bonds
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Yue Wang, Yongli Shen, Yujun Zhao, Jing Lv, Shengping Wang and Xinbin Ma*
Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative
Innovation Centre of Chemical Science and Engineering, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 30072, China
Corresponding author: Prof. Xinbin Ma
Eꢀmail: xbma@tju.edu.cn
Fax: +86ꢀ22ꢀ87401818
Tel: +86ꢀ22ꢀ27409248
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Abstract
Hydrogenation of carbonꢀoxygen (CꢀO) bonds plays a significant role in organic synthesis.
Cuꢀbased catalysts have been extensively investigated because of its high selectivity in CꢀO
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hydrogenation. However, no consensus has been reached on the precise roles of Cu and Cu
species for CꢀO hydrogenation reactions. Here we resolve this longꢀterm dispute with a series of
highly comparable Cu/SiO catalyst. All catalysts represent the fullꢀrange distribution of the Cu
2
species and have similar general morphologies, which are detected and mutually corroborated by
multiple characterizations. The results demonstrate that when the accessible metallic Cu surface
area is below a certain value, the catalytic activity of hydrogenation linearly increases with
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increasing Cu surface area, whereas it is primarily affected by the Cu surface area. Furthermore,
the balancing effect of these two active Cu sites on enhancing the catalytic performance is
+
0
demonstrated: the Cu sites adsorb the methoxy and acyl species, while the Cu facilitates the H
2
decomposition. This insight into the precise roles of active species can lead to new possibilities in
the rational design of catalysts for hydrogenation of CꢀO bonds.
0
+
Keywords: Cu catalyst, hydrogenation, Cu /Cu active sites, methyl acetate, alcohol, balancing
effect
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Introduction
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Hydrogenation of CꢀO bonds (e.g. esters, carboxylic acids, ethers, furfural and CO ) is one of
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ꢀ4
powerful tools in organic synthesis . It can generate a variety of attractive products with
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numerous commercial applications (chemical feedstocks, fuel alternatives or additives, polymers,
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etc) . However, in the hydrogenation process, hydrogenolysis occurs at both the CꢀO bond
(
forming alcohols) and the CꢀC bond (forming low molecular weight components such as CH4,
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ꢀ8
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CO and CO ). Comparing with other transition metals or oxides , Cu species are considered as
excellent catalytic active sites accounting for the selective hydrogenation of CꢀO bonds and
relatively inactive toward CꢀC bond hydrogenolysis, which have been widely used in vaporꢀphase
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chemoselective hydrogenation reactions
.
Significant advancements in preparing efficient and stable Cuꢀbased catalysts have been
achieved in the past decade, as well as the preliminary understanding of the catalytic essence of
the hydrogenation reactions. Regarding extensive investigations on silica carriers supported Cu
catalysts, one or both of the Cu dispersion and interaction between the Cu species and the support
or promoter are believed to be the pivotal in determining the catalytic activity and stability.
However, concerning the nature of active species, no consensus has been reached on the precise
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roles of Cu and Cu species for hydrogenation reactions. Several researchers have contradictory
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opinions about the proper surface distribution of Cu and Cu sites that most benefits the catalytic
performance.
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0
In the earlier study, San et al. considered only Cu species to be active sites in the
hydrogenation reaction. However, the synergetic effect between Cu and the oxide components has
been proposed to enhance the catalytic performance toward the hydrogenation of methyl acetate
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(
MA) , dimethyl oxalate (DMO)
, furfural and CO2 , and has been supported numerous
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subsequent researches. Although the cooperation of Cu and Cu species in CꢀO hydrogenation
has been mostly confirmed, their respective contributions to the catalytic activity remain in
dispute.
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Some researchers have proposed that metallic Cu is the primary active site, because the
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catalytic activity has been frequently observed to be proportional to the Cu specific surface area
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18, 19
20
2
(
S
Cu(0)). For example, numerous works on Cu/SiO
2
, Cu/HMS
and Cu/SiO
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ꢀTiO
catalysts
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by Dai et al. supported this view and reported catalysts with the maximum ratio of Cu /Cu
performed the highest catalytic activity in DMO hydrogenation. Furthermore, our previous studies
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revealed a strong correlation between the density of surface Cu sites and the formation rate of
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14, 21
alcohols, which indicated that Cu was the active site.
Simultaneously, no clear relationship
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was observed between the activity and the number of Cu sites. On the contrary, Yuan and
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coworkers suggested that a high proportion of Cu /Cu should be a key prerequisite to obtaining
outstanding hydrogenation performance; this suggestion was also based on extensive experimental
data. The catalytic activity and stability of Cu/SiO
2
was remarkably increased after modified with
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B , La , Ag , and Au , which essentially minimized the reducibility of cuprous species or
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+
increased the Cu surface concentration. These studies demonstrated that Cu species acted as
efficient active sites to activate the ester groups. Thus, a deeper understanding of the contributions
of Cu species to the reaction is necessary for more rational catalyst design.
Comparing the available literatures, it is found that the catalysts were prepared using
0
different methods and exhibited different distributions of Cu species; in particular, the Cu surface
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17ꢀ20
areas varied in completely different ranges: one range of SCu(0) varies from 1.6 to 11.7 m /g,
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whereas another varies from 21.3 to 49.1 m /g.
Notably, this discrepancy in the Cu surface
areas is likely responsible for the two aforementioned contradictory conclusions on the
contributions of Cu species. Therefore, to settle the debate and comprehensively elucidate the
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roles and the balancing effect of Cu and Cu sites in catalytic hydrogenation, the controlled
preparation of catalysts with both similar general morphology and fullꢀrange distribution of
cupreous species is imperative.
The ammonia evaporation method can conveniently and effectively disperse Cu species on
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silica and form strong metalꢀsupport interactions, thereby guaranteeing the formation of both Cu
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and Cu species. Thus, this method was used in this work and we obtained a series of Cu/SiO
2
catalysts with both highly similar textures and substantially different distributions of Cu species,
which enable us to elucidate the precise roles of the active sites.
Experimental Section
Materials
All chemicals were handled in air and used as received. Cu(NO
ammonia solution (25ꢀ28 wt%) were purchased from Sinopharm Chemical Reagent Co. Silica sol
30 wt%) was purchased from Qingdao Grand Chemical Co. Deionized water (18.2 Mꢁ) was used
3 2 2
) ·3H O (99%) and aqueous
(
in all synthesis. Ethanol (99.5 wt%) used to wash the precipitate was purchased from Tianjin
Kemiou Chemical Reagent Co.
Catalyst synthesis
The Cu/SiO
2
catalysts were prepared by the ammonia evaporation method briefly described
as follows. For example, to prepare the 20 wt% Cu loading catalyst, 15.2 g Cu(NO ) ·3H O was
3
2
2
dissolved in 100 mL deionized water and 50 mL aqueous ammonia solution. After stirring for 15
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min, 45 mL of silica sol was added dropwise to the copperꢀammonia complex solution and stirred
for several hours at room temperature. This procedure made the raw materials interact with each
other (denoted as the aging process). The aging time became one of the significant factors in
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catalyst preparation. nCuꢀxh was denoted the Cu/SiO catalysts with n wt% Cu loading and x
hours aging time. After that, the suspension was heated in a 353 K water bath to start ammonia
evaporation. This process was terminated with the pH value reached 6ꢀ7. The precipitate was
separated through filtration, washed with water and ethanol, and then dried under vacuum at 353
K overnight. The resulting solid was calcined in static air at 673 K for 4 h, tableted, crushed and
sieved to 40ꢀ60 meshes.
Preparation for treating silica sol with ammonia solely as the aging process is showed as
follows. The silica sol was firstly stirred with aqueous ammonia solution at 303 K and 353 K,
3 2 2
respectively, for 4 hours as the aging procedure. After the 50 mL solution of Cu(NO ) ·3H O
added, the mixture was heated in a 353 K water bath to allow the evaporation of ammonia. The
following steps of preparation were the same as described previously. These two kinds of catalysts
were denoted as nCuꢀ303ꢀ4h and nCuꢀ353ꢀ4h correspondingly.
Catalyst characterization
2
N adsorption/desorption analysis was conducted at 77 K using a Micromeritics Tristar 3000
apparatus. The specific surface area was calculated from the isotherms by BrunauerꢀEmmettꢀTeller
(BET) method and the pore size distribution was calculated by BarrettꢀJoynerꢀHalenda (BJH)
method from the desorption isotherms.
Inductively coupled plasma optical emission spectrometer (ICPꢀOES, Varian VistaꢀMPX)
was used to analyze the Cu content of samples. The peak at 324.754 nm was used as the Cu
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characteristic wavelength to identify the Cu concentration of samples.
Transmission electron microscopy (TEM) images were obtained on the Philips TECNAI G2
F20 system electron microscope equipped with a field emission gun. The samples were made by
dropping catalysts powers in ethanol solution after ground and ultrasonic dispersion onto the
carbonꢀcoated molybdenum grids.
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Xꢀray diffraction (XRD) patterns for the catalyst were obtained on Rigaku C/maxꢀ2500
diffractometer employing the graphiteꢀfiltered Cu Kα radiation (λ=1.5406 Å) at room temperature.
o
o
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Data were acquired by scanning at the rate of 8 /min from 2θ = 10 to 90 . To obtain accurate
information, the samples reduced just before XRD analysis and sealed into the glass bottles filled
with Ar. The Scherrer equation was used to calculate the particle size of Cu species.
Xꢀray photoelectron spectra (XPS) and Auger electron spectroscopy (XAES) were carried out
on PHI 1600 ESCA (PE Company) equipped with an Al Kα Xꢀray radiation source (hν = 1486.6
eV). To obtain accurate information of the surface states of Cu species in reduced catalysts, the
samples were collected in following procedures. The catalyst were pressed to thin disks and
2
reduced in a flow of H at 623 K for 4 h. After cooled to room temperature, the samples were
placed into centrifuge tubes and sealed with Ar. Then the samples were transferred to the holder in
less than 5 minutes and outgassed in the chamber. The analyses were taken under the vacuum of
ꢀ8
1
×10 Torr. The binding energies were calibrated using the Si 2p peak at 103.2 eV as the reference.
The experimental errors were within ±0.2 eV.
The Cu dispersion and the specific surface area of metallic Cu species was measured by the
N O chemisorption and hydrogen pulse reduction method (N O titration) using a Micromeritics
2
2
Autochem II 2920 apparatus. Briefly, after pretreatment with He at 473K for one hour, 50 mg
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catalysts were reduced in the flow of H at 623 K for 2 h and cooled to 363K. The pure N O was
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induced at a rate of 30 mL/min for an hour, ensuring that surface Cu atoms were completely
oxidized to Cu O. Then hydrogen pulse reduction of surface Cu O to metallic Cu was conducted
at 623 K and repeated until the pulse area no longer changed. The consumed amount of hydrogen
was the value obtained by subtracting the amount of H residual calculated from the areas shown
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in TCD signal from the total amount of H pulsed. From that, the surface metallic Cu areas of
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catalysts could be calculated. Combined with the ICP results, we could obtain the metallic Cu
dispersion as well.
The in situ FTIR of CO adsorption was performed to determine the amount of active surface
Cu(I) species of samples. Nicolet 6700 spectrometer equipped with a MCT/A detector and a
vacuum system. The samples were selfꢀsupporting discs which formed by pressing 20 mg of
finely ground dried catalysts at 8 MPa. Then the disc was weighted again before settled into the in
situ cell which allowed the possibility of high temperature experiments and was connected with a
vacuumꢀadsorption system. The sample disc was reduced and then was cooled to 303 K in
highꢀpurity He flow. After background collection, the sample was exposed and scanned to the flow
of CO at 303 K for 30 minutes. Then the sample was vacuumed and scanned. The last spectrum
was confirmed to be no differences with the previous one. For normalization, the intensity of
spectrum was enlarged with the same multiple as the disc weight used to be enlarged to 1 g.
Therefore the areas under the peaks in the normalized spectra could give a qualitative estimation
of the amount of certain species, since the extinction coefficients of the corresponding IR bands
are not known.
The in situ FTIR of methanol adsorption was also carried out by Nicolet 6700 spectrometer.
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The sample discs after reduction were exposed to a gas mixture of He/CH OH at 300 K for 30
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minutes. To guarantee methanol was monolayer adsorbed on the surface active sites of catalysts,
the desorption progresses were performed under vacuum at 300 K and the spectra were
programmed to be scanned. After the desorption spectra no longer changed, the temperature was
raised up to 488 K, which is the reaction temperature, to obtain another desorption spectra which
would help to distinguish the different methanol adsorption strength on different surface active
sites of catalysts. Deconvolution of the final spectrums allowed us to qualitative estimate the
amount of certain species by integral of the areas under corresponding IR peaks.
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Catalytic activity evaluation and product analysis
The catalytic activity evaluations were carried out on a continuousꢀflow heterogeneous
reactor. Samples of 0.5 g catalysts (40~60 meshes) were placed carefully into a certain part of the
tube where could maintain a constant temperature and ensured to be smooth with a low ratio of
voids. The reaction tests was carried out at 488 K on 2.5 M Pa with 80 as the chargeꢀin molar ratio
2
of H to MA after the catalysts were reduced in the flow rate of 100 mL/min hydrogen at 623 K
for 4 hours. The products were condensed and analyzed on an Agilent Micro GC 6820 with an
HPꢀINNOWAX capillary column (Hewlett Packard Company, 30 m × 0.32mm × 0.50 ꢂm). Main
byproducts include methanol, ethyl acetate, nꢀpropanol, 2ꢀbutanol, acetaldehyde etc. The TOFCu(0)
+
were calculated based on metallic Cu dispersion. Assuming that Cu ion occupies the same area as
0
that of the Cu atom, the TOFCu(I) were calculated by dispersion based on SCu(I)
.
Results
Textural and morphology properties of the catalysts
To investigate the contributions of Cu species to the reaction, other factors (e.g., pore
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structure, size of the metal particles) should first be excluded. As shown in Table 1, all catalysts
have relatively high BrunauerꢀEmmettꢀTeller (BET) specific surface areas and parallel
mesoporous structures. Moreover, the transmission electron microscopy (TEM) images (Figure 1),
indicate that all of the reduced catalysts have similar morphologies and that the Cu is well
dispersed. The observed metallic Cu grains on the surface are subꢀ5 nm, even those on the surface
of catalyst with up to 40 wt % Cu. This grain size is also confirmed by the Xꢀray diffraction (XRD)
patterns of the reduced catalysts (Figure 2), which show extremely weak and board diffraction
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peaks at 2θ angles of 43.3 and 36.4 ; these peaks are attributed to the metallic Cu (111) (JCPDS
4ꢀ0836) and Cu O (111) planes (JCPDS 65ꢀ3288), respectively. No obvious diffraction peaks of
other crystal faces are observed, which strongly implies that the Cu species is highly dispersed on
0
2
SiO supports. Meanwhile, the sizes of Cu nanoparticles calculated using the Scherrer formula are
2
also notably small, consistent with the results from the TEM images (listed in Table 1). In addition,
+
the XRD peaks assigned to the Cu
2
O (111) plane suggest that a portion of Cu exists as Cu after
reduction. Given the above results, the textural and physicochemical structures of the Cu/SiO
2
catalysts are so similar that these factors are negligible in the following discussion, which enables
the further exploration of the active species contributions.
Cu species characterization
0
+
To investigate the reaction, the surface amounts of Cu and Cu sites must be qualitatively
and quantitatively measured sequentially. Thus, all of the reduced Cu/SiO catalysts were
2
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characterized by N O titration, Xꢀray photoelectron spectroscopy (XPS), Xꢀray Auger electron
spectroscopy (XAES), and in situ Fourierꢀtransform infrared spectroscopy (FTIR) of CO
0
absorption. The Cu surface areas that were measured using N
2
O titration are listed in Table 2.
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ꢀ1
With the increase in Cu loading, the surface area significantly varies from 5.0 to 44.2 m g . For
the catalysts with identical Cu contents, SCu(0) also significantly changes because of the modified
aging procedures used during preparation. The XPS data (Figure 3a) show only two peaks, which
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+
0
are centered at binding energies of 932.0 and 951.8 eV, conventionally assigned to Cu or Cu
2
+
species. The absence of the Cu 2p satellite peak at 942ꢀ944 eV strongly demonstrates that all Cu
species had been reduced to a low valence state of +1 or 0 after the reduction. Furthermore, the
appearance of two obvious overlapping peaks at 569.9 eV and 573.0 eV in the Cu LMM Auger
0
+
electron spectra (Figure 3b) can be employed to distinguish between Cu and Cu species. From
+
0
+
the deconvolution results, the molar ratios of the surface Cu /(Cu + Cu ), which is denoted as
+
X
Cu(I), are apparently affected by changes in the preparation conditions. Assuming that the Cu
0
+
ions and Cu atoms occupy identical areas, and have identical atomic sensitivity factors, the Cu
0
26
surface area can be estimated on the basis of Cu surface area and XCu(I)
.
This value was
calculated and listed in Table 2 as SCu(I), which obviously changes in contrast with SCu(0)
.
To verify the specific surface areas of Cu species using a third method, the Cu/SiO
2
catalysts
were characterized by in situ FTIR of CO adsorption (Figure 3c). Considering the weak
0
2+
2+
interaction between CO and Cu or Cu species and it has been proved that no Cu species exist
ꢀ1
after reduction by both XPS and XRD data, the obtained bands at 2100ꢀ2200 cm should be
+
27, 28
ꢀ1
attributed to CO adsorbed on Cu species.
The bands at 2122 cm should be assigned to the
+
29, 30
ꢀ1
associated Cu species.
As shown in Figure 3c, the intensity of bands at 2122 cm generally
increased with the increasing Cu loadings. Moreover, the proportion of integral areas under the
ꢀ1
peak at 2122 cm displayed an opposite trend with the amounts of copper phyllosilicate in
calcined catalysts (see detail in Figure S1), which has been proved helpful in Cu dispersion. In this
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ꢀ
1
+
case, the broad bands around 2190 cm should be attributed to CO adsorbed isolated Cu species.
ꢀ1
Deconvolution of it allows us to see that there are two peaks at around 2197 and 2173 cm . It has
+
been reported that Cu possesses two coordinative vacancies and can coordinate one or two CO
0
1
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molecules to form monocarbonyls or dicarbonyls, which explain the two peaks.
The bands at
ꢀ1
+
2
197 and 2173 cm are attributed to the isolated Cu species in dicarbonyls and monocarbonyls
29, 32
+
respectively.
The integral areas of the Cu ꢀCO peaks after normalization are listed in Table 2;
these integral areas are consistent with the SCu(I) (Figure S2). This consistency check confirms that
0
+
the qualitatively and quantitatively measurements of the Cu and Cu surface areas are accurate
and reliable, which establishes a solid foundation for further investigations.
Catalytic performance
To find the proper distribution of Cu species that enhances the catalytic performance, the
vaporꢀphase hydrogenation of MA was performed as a probe reaction over these reduced Cu/SiO
2
catalysts. Correlating the MA conversion and the distribution of Cu species, we observed several
interesting results. For example, in the case of the 20Cuꢀxh catalysts, the catalytic activities
+
perfectly changed according to the number of Cu sites, while the SCu(0) remained approximately
constant. This result is agreed with recent reports from Yuan and coꢀworkers, who suggested that
cuprous species acted as efficient active sites to activate acyl groups and the maximum amount of
+
14, 21ꢀ23, 30
Cu led to the greatest catalytic activities (see examples in Figure S3a).
However, in the
case of the catalysts with 10 wt% Cu loadings and the 5Cuꢀ303ꢀ4h sample, all of which exhibited
+
0
similar surface areas of Cu species but significant differences in Cu surface areas, a different
trend was observed. The catalytic activities no longer remained consistent with SCu(I) but primarily
0
increased with the Cu surface area, which is also supported by numerous studies (see examples in
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4, 17, 20, 33
Figure S3b).
On the basis of this discussion, the catalytic activity of these controllable
prepared Cu/SiO
2
catalysts is concluded to undergo complex changes in the hydrogenation
reactions. Therefore, it is necessary to establish a fullꢀrange relationship between the catalytic
performance and Cu species.
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All of the results indicate that the catalytic performance of hydrogenation establishes a linear
0
+
correlation with the Cu and Cu surface areas in different segments. For the convenience of
discussion, the results are divided to two different parts according to different relationships
between the surface area of the Cu species and the catalytic performance (Figure 4). Part I consists
0
of the data for the 5 wt% and 10 wt% Cuꢀloaded catalysts, whose the Cu surface areas are less
2
than 20 m /g. The catalytic activities scale linearly with the accessible metallic Cu surface areas
+
but independently with Cu surface areas. Part II consists of the results from the remaining
2
+
catalysts, whose SCu(0) is 22.1ꢀ44.2 m /g and the catalytic activities and Cu surface areas exhibit a
good linear correlation. The scatters indicate that neither SCu(0) nor SCu(I) can solely explain the
0
+
changes in catalytic performance. In general, both of Cu and Cu active sites play indispensable
roles in the hydrogenation; however, they apparently compete to exert a dominant effect on the
catalytic ability, which is responsible for the contradictory opinions from different investigators.
Thus, further exploration of the precise roles of Cu species in the reaction is essential to
understanding the balancing effect of Cu sites on the enhancement of hydrogenation.
0
+
Roles of Cu and Cu species in hydrogenation reactions
13
0
+
2
Poels and Brands have been suggested that the Cu species activated H while the Cu
species adsorbs methoxy and acyl species in ester hydrogenation; however, no convincing
evidence has been presented. The debate has been focused on the essentiality of cuprous species
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functions. Because the adsorption of ethyl acetate on Cu under reduction conditions has been
34
hypothesized to proceed via the cleavage of the CꢀO bond adjacent to the carbonyl group, we
propose that the adsorption of aliphatic esters also proceeds in this manner. In the case of MA, the
0
1
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0
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2
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0
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9
0
3
products of the reaction consist of CH C*O and CH
3
O* species, which indicates that the methanol
4
dissociative adsorption likely reflects the esters dissociative adsorption. Herein, the in situ FTIR
+
of methanol adsorption was further applied to validate the important role of Cu species on the
ꢀ1
ester activation. It has been demonstrated that the IR bands at 2813 and 2924cm are attributable
0
35
3
to the asymmetric and symmetric CꢀH stretching vibrations of Cu ꢀOCH , respectively.
ꢀ1
Additionally, the bands at 2856 and 2957 cm have been assigned to the asymmetric and
35
symmetric CꢀH stretching vibrations of methoxy species chemisorbed on silica support.
However, we propose these two peaks also include the CꢀH stretching vibration of methoxy
+
ꢀ1
species chemisorbed on Cu species. As shown in Figure 5, the peaks at 2813 and 2924 cm
disappear when the desorption temperature is increased to 488 K, whereas the intensities of the
ꢀ1
bands at 2856 and 2957 cm decrease only slightly. The bands at higher wavenumbers can be
+
reasonably assigned to a stronger methoxy adsorption, which is presumed to be the Cu ꢀOCH
3
species. Under this assumption, we integrated and normalized the areas of different peaks for
0
+
+
comparison with the surface areas of both Cu and Cu species to determine whether the Cu
species act as the methoxy adsorption sites. The correlations in Figure S4 establish an excellent
linear fit between the integral areas and the surface areas of the corresponding active sites.
ꢀ1
Moreover, the changes in intercepts of the fittings of the peaks at 2856 and 2957 cm can be
explained by a blank test with the SiO support, which shows that the methoxy species are
2
chemisorbed onto silica at 300 K but only slightly at 488 K. These results mutually confirmed the
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validity of the bands ascription and Cu function for ester activation. Hence, the Cu sites
contribute to the adsorption of methoxy and acyl species of esters during the hydrogenation
0
reactions, while Cu species cannot adsorb methoxy at the experimental reaction temperature.
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A series of density functional theory (DFT) calculations was employed here as well to further
0
+
identify the active roles of Cu and Cu species (see SI for details). In the DFT calculations, the
δ+
+
optimized Cu /SiO structures were modeled as Cu sites because the accommodation and
2
stabilization of cuprous species benefit from the strong interaction between Cu and supports.
o
Owing to the diffraction peak at a 2θ angle of 43.3 in the XRD pattern and the observed lattice
fringes in TEM images, which are attributed to metallic Cu (111), the Cu (111) plane structure was
0
selected to represent the Cu species. The processes of MA adsorption and H
2
decomposition were
0
+
calculated on both models of Cu and Cu species, respectively. For the MA adsorption, the results
3
show that the formation of CH Oꢀ and activation of the acyl group of MA are facilitated on
δ+
Cu /SiO
2
structures (see Table S2), whereas the activation can almost impossible happen on the
2
Cu (111) plane (see Table S3). For the H decomposition, the activation energy calculation results
indicate that decomposition occurs much more easily on Cu (111) (see Figure S10). Thus,
+
according to the DFT calculation results, the Cu sties likely adsorb methoxy and acyl species
0
since the ester is difficult to be activated on the metallic Cu species, while Cu facilitates the
2
decomposition of H . These inferences are consistent with the deductions from the in situ FTIR of
methanol adsorption.
Discussion
0
+
According to the obtained results above, both of Cu and Cu active sites play indispensable
roles in the hydrogenation and they compete against each other to exert a dominant effect on the
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+
catalytic ability. Thus, there is a equilibrium problem between the amouts of Cu and Cu species
for best interest of the catalytic performance. Here, we combine the TOFs of each Cu species and
the precise roles analyzed above to elucidate the balancing effect of the active sites in
hydrogenation reactions.
0
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0
Analyzing the catalytic performance on the surface area of each Cu species will provide
0
+
further understandings of the contributions of Cu and Cu species toward the hydrogenation
reactions. The TOFCu(0) and TOFCu(I) were calculated; the results are shown in Figure 6. The
samples were divided into two parts as the same as those in Figure 4. As shown in Part I, the
+
catalytic performance independently changes with the Cu surface area but linearly increases with
0
increasing number of Cu sites when the metallic Cu surface area is below a certain value
2
(
approximately 20 m /g). This result is consistent with the hypothesis that the maximum ratio of
0
+
Cu /Cu exhibits the best performance, where the experiments were performed using catalysts
2
0
with low metallic Cu surface areas (1.6ꢀ11.7 m /g). Given the role of Cu sites, this relationship
may originate from insufficient H
2
decomposition, which limits the reaction rates. On the other
+
hand, the catalytic activity linearly increases with increasing Cu surface area in Part II, where the
0
samples have relative high Cu surface areas. This trend is in agreement with the reports that the
+
maximum number of Cu corresponds to the highest catalytic activity, where a series of catalysts
2
were prepared using various methods and exhibited high SCu(0) (21.3ꢀ49.1 m /g). In this case, we
+
deduce that the effect of Cu on the catalysts starts to emerge when the H
2
decomposition is
+
sufficient. In addition, the catalytic performance increases with both the Cu surface area and the
0
Cu surface area (Figure 6b, Part II). We inferred that the relative space position of the two types
of active Cu sites may play an important role in the catalytic performance. Increasing the
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accessible surface area of cuprous species undoubtedly improves the amount of active sites
adjacent to metallic Cu species, which may result in the sustained growth of TOFCu(I). Therefore,
0
when the amount of Cu species present is insufficient, the catalytic activity of hydrogenation is
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2
determined by the activity of H , whereas it is most strongly affected by the adsorption of
methoxy and acyl species. The two types of Cu species demonstrate a balancing effect on
enhancing the catalytic performance in the CꢀO hydrogenation reactions.
+
Besides Cu species, numerous kinds of Lewis acidity sites have been reported recently
exhibiting promoting effect on the homogeneous and heterogeneous catalysis in hydrogenation of
36ꢀ39
36
3+
amides and other derivatives of carboxylic acids.
Hardacre and coꢀworkers suggested Ti
Lewis acid species weakened the C=O of the carbonyl oxygen of the acid and promoted
hydrogenation and carbonꢀoxygen bond cleavage in TiO ꢀsupported PtꢀRe catalysts for
2
3
7
hydrogenation of carboxylic acids to fatty alcohols. Beller et al reported the Lewis acid and
water for activation of the catalyst in toluene and the novel selective CꢀOH bond cleavage of
hemiacetal intermediates. The concepts described in all these papers for very different systems are
essentially identical with our results. Actually, the Lewis acids activation effects on esters and
other carbonyl moieties are quite general phenomena. The balancing effect of active species
reported here have potential to contribute to understanding the catalytic essence of hydrogenation
of polar moieties in both homogeneous and heterogeneous catalysis.
Conclusion
In summary, using multiple and mutually corroborated characterizations we observed
fourteen Cu/SiO
2
catalysts with high comparability due to their fullꢀrange distribution of Cu
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species and similar general morphologies. This comparability enabled us to trace the differences in
0
+
activity of the samples to the contributions of both Cu and Cu species in CꢀO hydrogenation
reactions. When the accessible metallic Cu surface area was below a certain value, the catalytic
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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1
2
3
4
5
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9
0
0
activity of hydrogenation linearly increased with increasing Cu surface area, whereas it was
+
primarily affected by the Cu surface area. The obtained results are consistent with the available
literatures and present a complete picture of the balancing effect of these two active Cu sites on
+
enhancing the catalytic performance, where the Cu sties adsorb methoxy and acyl species and
0
0
2
Cu facilitates the decomposition of H . It is indicated that when the amount of Cu species is
2
insufficient, the catalytic activity of hydrogenation is determined by the activity of H , whereas it
is most affected by the adsorption of methoxy and acyl species. The controversy regarding the
precise roles of active Cu sites in the hydrogenation is resolved, and the potential to make new
progress in studies of the CꢀO hydrogenation mechanism is demonstrated. Furthermore, new
possibilities have been introduced for the rational design of catalyst through the modular approach
with relevant functional chemicals according to a deep understanding of the contribution of active
sites toward the reaction.
Associated Content
Supporting Information
The catalytic performance; the supplementary analysis and the details for DFT calculations. This
material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgement
This research was supported by the National Natural Science Foundation of China (91434127,
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1325626, 21276186).
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9. Hadjiivanov, K.; Kantcheva, M.; Klissurski, D. J. Chem. Soc. Faraday Trans. 1996, 92,
4595ꢀ4600.
0. Lin, H.; Zheng, X.; He, Z.; Zheng, J.; Duan, X.; Yuan, Y. Appl.Catal. A: Gen. 2012, 445,
2
87ꢀ296.
1. Kuroda, Y.; Yoshikawa, Y.; Kumashiro, R.; Nagao, M. J. Phys. Chem. B 1997, 101,
497ꢀ6503.
32. Hadjiivanov, K.; Klissurski, D. React. Kinet. Catal. Lett. 1991, 44, 229ꢀ235.
3. Yin, A.; Guo, X.; Fan, K.; Dai, W. L. Appl.Catal. A: Gen. 2010, 377, 128ꢀ133.
34. Kohler, M. A., Cant, N. W., Wainwright, M. S., Trimm, D. L. Chem. Inst. Can. 1988, 1043.
6
3
3
5. Zhang, R.; Sun, Y.; Peng, S. Fuel 2002, 81, 1619ꢀ1624.
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Manyar, H. G.; Paun, C.; Pilus, R.; Rooney, D. W.; Thompson, J. M.; Hardacre, C. Chem.
Commun. 2010, 46, 6279ꢀ6281.
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7. Li, Y.; Topf, C.; Cui, X.; Junge, K.; Beller, M. Angew. Chem. Int. Ed. 2015, 54, 5196ꢀ5200.
38. Zhang, Y.; MacIntosh, A. D.; Wong, J. L.; Bielinski, E. A.; Williard, P. G.; Mercado, B. Q.;
Hazari, N.; Bernskoetter, W. H. Chem. Sci. 2015, 6, 4291ꢀ4299.
39. Jiang, Y.; Huang, W.; Schmalle, H. W.; Blacque, O.; Fox, T.; Berke, H. Organometallics 2013,
3
2, 7043ꢀ7052.
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Tables and Figures
2
Table 1 Textural and physicochemical properties of reduced Cu/SiO catalysts
Table 2 Characterization of Cu species on reduced Cu/SiO
Figure 1 TEM images of the reduced catalysts.
Figure 2 XRD patterns of reduced catalysts
2
catalysts.
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Figure 3 a) XPS spectra, b) XAES spectra and c) in situ FTIR of CO adsorption spectra of the
reduced catalysts.
0
+
Figure 4 Balancing effect of Cu and Cu on the catalytic performance: Correlation of the MA
0
+
conversions with a) Cu surface areas and b) Cu surface areas.
Figure 5 In situ FTIR spectra of methanol adsorption at 300 K and 488 K.
+
0
Figure 6 Effects of a) Cu active sites and b) Cu active sites on the catalytic performance
respectively.
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1
1
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Table 1 Textural and physicochemical properties of reduced Cu/SiO catalysts
Metal loading
S
BET
2
D
pore
Crystallite size (nm)
Catalyst
a
b
(
wt%)
4.9
(m /g)
254
250
263
309
302
294
381
382
409
414
406
421
480
558
(nm)
10.4
11.7
13.6
7.7
by XRD
by TEM
2.6
c
5
Cuꢀ303ꢀ4h
Cuꢀ353ꢀ4h
ND
5
5.0
ND
ND
ND
ND
ND
4.5
4.8
ND
4.5
4.7
ND
3.6
3.8
2.3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5Cuꢀ4h
5.0
3.0
1
0Cuꢀ303ꢀ4h
0Cuꢀ353ꢀ4h
9.8
2.9
1
9.9
12.4
15.7
6.8
2.8
10Cuꢀ4h
9.9
2.2
2
2
2
0Cuꢀ303ꢀ4h
0Cuꢀ353ꢀ4h
0Cuꢀ4h
19.7
19.6
19.5
19.6
19.5
19.6
29.0
37.2
3.0
10.7
10.8
11.0
9.9
3.5
2.8
20Cuꢀ16h
2.5
2
2
3
0Cuꢀ24h
0Cuꢀ40h
0Cuꢀ4h
3.1
10.5
8.6
3.3
3.0
40Cuꢀ4h
6.2
3.0
a
b
Metal loading was determined by ICPꢀOES. Diameter of Cu particles calculated from
c
the XRD data by Scherrer equation. ND: not detected.
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Table 2 Characterization of Cu species on reduced Cu/SiO
2
catalysts
ACu(I)
S
X
S
Cu(0)
2
Cu(I)
b
Cu(I)
2
Catalyst
a
c
d
(
m /g)
5.0
(%)
(m /g)
8.9
(area/g)
199.7
156.9
132.2
167.1
162.3
160.5
474.5
239.0
250.3
366.9
498.5
430.1
269.8
363.3
5
Cuꢀ303ꢀ4h
Cuꢀ353ꢀ4h
64.2
19.2
46.6
41.8
28.7
31.3
42.4
19.2
29.7
36.6
48.1
38.7
24.4
23.9
5
11.2
7.6
6.9
10
11
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32
33
34
35
36
37
38
39
40
41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5Cuꢀ4h
6.7
1
1
1
0Cuꢀ303ꢀ4h
0Cuꢀ353ꢀ4h
0Cuꢀ4h
12.0
19.9
18.6
23.1
39.6
24.4
23.9
22.1
23.5
37.6
44.2
8.6
8.0
8.5
20Cuꢀ303ꢀ4h
17.0
9.4
2
2
2
0Cuꢀ353ꢀ4h
0Cuꢀ4h
10.3
13.8
20.5
14.8
12.2
0Cuꢀ16h
20Cuꢀ24h
2
0Cuꢀ40h
0Cuꢀ4h
3
40Cuꢀ4h
a
13.9
b
+
+
0
Metallic Cu surface area determined by N O titration. Cu /(Cu + Cu ) calculated
2
c
from Cu LMM XAES spectra. Cu(I) surface area estimated on the basis of S
and
Cu(0)
+
0
X
Cu(I) under the assumption that the Cu ions and Cu atoms occupy identical areas
d
and have identical atomic sensitivity factors. Integral area under the peaks in the
FTIR spectra of CO adsorption on active Cu(I) on the surface of catalysts after
evacuation.
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Figure 1 TEM images of the reduced catalysts
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Figure 2 XRD patterns of reduced catalysts
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reduced catalysts
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Figure 4 Balancing effect of Cu and Cu on the catalytic performance: Correlation of the MA
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conversions with a) Cu surface areas and b) Cu surface areas.
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Figure 6 Effects of a) Cu active sites and b) Cu active sites on the catalytic performance
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