Cu-Ag/hydrotalcite catalysts for dehydrogenative cross-coupling of primary and secondary benzylic alcohols

Article abstract of DOI:10.1039/c5ra22542f

The development of new and inexpensive heterogeneous catalysts for direct C-C cross-coupling of primary and secondary alcohols is a challenging goal and has great importance in academic and industrial sectors. In this work Cu-Ag/hydrotalcite (Cu-Ag/HT) catalysts were prepared and tested for their impact on this cross-coupling. The effect of supports, including MgO, γ-Al₂O₃ and HT with different Mg : Al molar ratios, was investigated. It was found that the acidic or basic properties of the supports affected product selectivity. The roles of Cu and Ag sites in the cross-coupling were also investigated with the prepared Cu-Ag/HT catalyst demonstrating high activity and selectivity for the reaction. The yield-to-target product of β-phenylpropiophenone reached 99% after 1 h under optimum reaction conditions. The stability in air and reusability studies show that Cu-Ag/HT can be stored for 6 days and can be used five times without apparent deactivation, respectively.

Full text of DOI:10.1039/c5ra22542f

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Page 1 of 45
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DOI: 10.1039/C5RA22542F
CuꢀAg/Hydrotalcite Catalysts for Dehydrogenative CrossꢀCoupling
of Primary and Secondary Benzylic Alcohols
Jin Xu a,b, Hongmei Yue a,b, Sheng Liu a,b, Hanfei Wang a,b, Yuqun Du a,b,
Chunli Xu a,b,*, Wensheng Dong a,b, Chunling Liu a,b
a
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi
Normal University), Ministry of Education, Xi’an 710119, PR China
b
School of Chemistry and Chemical Engineering, Shaanxi Normal
University, Chang’an West Street 620, Xi’an 710119, PR China
* Corresponding author.
School of Chemistry and Chemical Engineering, Shaanxi Normal
University, 620 Chang’an West Street, Xi’an 710119, PR China
Tel: 86ꢀ29ꢀ81530779
Eꢀmail: xuchunli@snnu.edu.cn
1

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Abstract
The development of new and inexpensive heterogeneous catalysts
for direct CꢀC crossꢀcoupling of primary and secondary alcohols is a
challenging goal and has great importance in academic and industrial
sectors. In this work CuꢀAg/hydrotalcite (CuꢀAg/HT) catalysts were
prepared and tested for their impact on this crossꢀcoupling. Effect of
supports, including MgO, γꢀAl2O3 and HT with different Mg:Al molar
ratios, was investigated. It was found that the acidic or basic properties of
the supports affected product selectivity. The roles of Cu and Ag sites in
the crossꢀcoupling were also investigated with the prepared CuꢀAg/HT
catalyst demonstrating high activity and selectivity for the reaction. The
yieldꢀtoꢀtarget product of βꢀphenylpropiophenone reached 99% after 1 h
under optimum reaction conditions. Stability in air and reusability studies
finds that CuꢀAg/HT can be stored for 6 days and can be used five times
without apparent deactivation, respectively.
Keywords:
CuꢀAg
nanoparticles,
Hydrotalcite,
Alcohol
crossꢀcoupling, Acceptorless dehydrogenation.
1. Introduction
Acceptorless dehydrogenation reactions, in which hydrogen is
liberated and new prospective bonds are generated by further reactions of
the dehydrogenated products, are emerging as an atomꢀeconomical and
environmentally benign synthetic method, circumventing the need for
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stoichiometric oxidants or the prefunctionalization of substrates [1,2].
The selfꢀcoupling of an alcohol or the crossꢀcoupling of two different
alcohols constitutes a key transformation in reaction schemes targeting
higher yields of alcohols, ketones, and esters [3]. The homodimerization
of primary and secondary alcohols, the soꢀcalled Guerbet reaction, has
been known for more than a century [3,4]. However, the reaction has
recently been extended to the crossꢀcoupling (selective βꢀalkylation) of
secondary alcohols with primary alcohols (Scheme 1) [5].
Several reports have demonstrated the βꢀalkylation of secondary
alcohols with primary alcohols under the catalysis of Ru and Ir
complexes [5ꢀ8]. However, a number of limitations exist for these
processes. These limitations include low catalytic turnover numbers, the
requirement of a large amount of a strong base, and expensive catalysts,
which are also difficult to synthesize and unable to be reused [9]. These
limitations reinforce the need for a ligandꢀfree, reusable catalyst system
for this type of transformation. To date, there are three related examples
of heterogeneous catalysis [9ꢀ11]. Ag/Al2O3 [9] and Pd/C [10] have been
used as efficient catalysts for direct CꢀC crossꢀcoupling of secondary and
primary alcohols. However, a sacrificial hydrogen acceptor and/or a
considerable amount of inorganic homogeneous base was required to
achieve a high selectivity. AuꢀPd/hydrotalcite (AuꢀPd/HT) catalysts have
been shown to avoid the need for a hydrogen acceptor and homogeneous
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base as well as exhibiting high catalytic performance [11]. However, Au
and Pd are precious metals with relatively scarce global reserves. The
development of new and inexpensive heterogeneous catalysts for the
reaction is therefore a challenging goal and has great importance in
academic and industrial sectors [12].
The reaction proceeds by dehydrogenation of the alcohol to an
intermediate carbonyl compound. This is followed by an Aldol
condensation with the ketone to yield an α,βꢀunsaturated carbonyl
derivative, which is finally reduced by the metal hydride to give the
corresponding carbonyl compound (Scheme 2) [9]. A key concept in the
catalyst design in some of the systems is the multifunctionality of
metalꢀloaded acidic and/or basic metal oxides. Here, the acidic and/or
basic sites selectively catalyze the Aldol condensation reactions while the
metal sites catalyze the transfer dehydrogenation of alcohol and transfer
hydrogenation of the condensation products as intermediates [5,13].
Catalysts based on Cu have attracted much attention in heterogeneous
catalysis because of the metal’s relative cost advantage [14ꢀ16]. Such
catalysts have been reported to provide high activity in the
dehydrogenation of alcohols and hydrogen transfer [17,18]. The catalytic
performance of Cu was reportedly affected by the coexistence of Ag
nanoparticles [19,20]. HT showed high activity for the Aldol
condensation [11,21]. Therefore, CuꢀAg/HT may offer the potential for
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catalyzing the reaction, the investigation of which is the focus of this
work.
2. Experimental section
2.1 Catalyst preparation
2.1.1 Preparation of supports
The HT support was prepared using a standard aqueous
coꢀprecipitation method, which we have described previously [22]. An
aqueous solution (160 mL) of metal nitrates in different Mg2+:Al3+ molar
ratios with a total concentration of 1.5 M was mixed slowly by
continuous stirring with an alkaline solution of Na2CO3ꢀNaOH. The
molar ratio of Na2CO3 to Al3+ was 2:1. The pH of the mixture was
maintained between a value of 9 and 10 by adjusting the flow rate of the
alkaline solution. The temperature was maintained at 25 °C. Following
this addition, which resulted in the formation of heavy slurry, the mixture
was aged at 60 °C for 18 h by stirring to selectively facilitate the growth
of the precipitated HT phase. The slurry was then cooled to 25 °C, filtered,
and washed with water until the pH value of the filtrate was
approximately 7. The precipitate was dried at 110 °C for 12 h. Depending
on the initial conditions, the resulting material was HT with a Mg2+:Al3+
molar ratio of 1:1, 2:1, 3:1, or 5:1. Hereafter, unless noted otherwise, HT
describes a 2:1 molar ratio of Mg2+:Al3+.
A mesoporous MgO support for the CuꢀAg catalyst was prepared by
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calcining commercially available Mg(OH)2 (99%, Guangdong Shantou
Hongwei Chemical Plant, P.R. China) at 500 °C for 3 h in static air
(muffle oven, heating rate: 10 °C min−1). A γꢀAl2O3 support was prepared
by calcining a commercial sample of γꢀAl2O3 (Alfa Aesar) under the same
conditions.
2.1.2 Synthesis of the CuꢀAg supported catalyst
CuxAg1ꢀx/HT (where 0 ≤ x ≤ 1) catalysts were prepared by a
coꢀimpregnation method. A mixture of the support and an aqueous
solution of Ag(I) and Cu(II) nitrates was stirred at 80 °C for 2 h, followed
by drying at 120 °C for 12 h, calcination in air at 300 °C for 5 h, and
reduction in H2 at 300 °C for 2 h. Cu0.95Ag0.05/HT (x = 0.95) with a total
metal (Cu + Ag) loading of 1 wt% was used as the standard catalyst.
Using a similar method, CuxAg1ꢀx/γꢀAl2O3 and CuxAg1ꢀx/MgO were also
prepared.
2.2 Characterization of catalyst
The surface area and pore characteristics of the catalysts were
determined using a Micromeritics ASAP 2020 instrument (USA). The
sample was degassed at 110 °C for 6 h in N2 prior to carrying out surface
area measurements. Nitrogen adsorption and desorption isotherms were
measured at −196 °C. The specific surface areas of the catalysts were
determined by applying the Brunauer–Emmett–Teller (BET) method to
the nitrogen adsorption data obtained in the relative pressure range of
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0.06 to 0.30. The total pore volume of the samples was estimated from
the amount of nitrogen adsorbed at a relative pressure of 0.995. Pore
volume and pore size distributions were obtained from analysis of the
desorption branches of the nitrogen isotherms using the
Barrett–Joyner–Halenda method.
Xꢀray diffraction (XRD) patterns were recorded using a D/Maxꢀ3C
Xꢀray powder diffractometer (Rigaku Co, Japan), with a Cu–K source
fitted with an Inel CPS 120 hemispherical detector.
Scanning transmission electron microscopy (STEM) micrographs
were obtained using a FEI Tecnai G2 F20 instrument (USA).
Elemental analysis of HT was carried out by an FEI Quanta 200
(USA) scanning electron microscope equipped with energyꢀdispersive
Xꢀray spectroscopy.
The loading of CuꢀAg was determined by inductively coupled
plasma mass spectrometry (Bruker, M90; Germany).
Xꢀray photoelectron spectroscopy (XPS) was used to characterize
the valence of catalysts. XPS data was obtained in an Axis Ultra
spectrometer (Kratos Analytical Ltd., Japan) using AlKα (hυ = 1486.6 eV)
Xꢀray radiation at an energy of 150 W.
NH3ꢀtemperatureꢀprogrammed desorption (TPD) and CO2ꢀTPD were
performed on a Micromeritics AutoChem 2920 II instrument. Typically,
the sample loaded in a quartz reactor was pretreated with highꢀpurity He
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at 350 °C for 1 h. After cooling the sample to 100 °C, CO2 adsorption was
performed by switching the He flow to a CO2ꢀAr (5 vol % CO2) gas
mixture and then maintaining the sample at 100 °C for 3 h. The gasꢀphase
(and/or weakly adsorbed) CO2, was purged by highꢀpurity He at the same
temperature. CO2ꢀTPD was then performed in the He flow by raising the
temperature to 800 °C at a rate of 10 °C⋅minꢀ1. The desorbed CO2
molecules were detected by using a MKS Cirrus 2 mass spectrometer.
NH3ꢀTPD was performed by using a similar procedure
.
2.3 Reaction procedure
The crossꢀcoupling of primary and secondary alcohols was carried
out using a 50 mL, threeꢀnecked, roundꢀbottomed flask with a
waterꢀcooled condenser. Typically, the reactor was charged with 0.1 g of
catalyst, 1.0 mmol of benzyl alcohol, 1.0 mmol of 1ꢀphenylethanol, and 3
mL of oꢀxylene. This mixture was then heated at 150 °C for 1 h under N2
at 1 atm. After cooling to room temperature, the resulting mixture was
diluted to a constant volume with toluene in a 15 mL volumetric flask.
The reaction products were quantitatively analyzed by a Shimadzu
GCꢀ2014 (Rtx®ꢀ5, 30 m × 0.32 mm × 0.25 ꢁm) gas chromatograph
equipped with a flame ionization detector. Nꢀdodecane (1 mmol) was
used as the internal standard for analysis. The products were identified by
1H NMR as well as GCꢀMS.
3. Results and discussion
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3.1 Catalyst characterization
3.1.1 BET surface area and pore size
As shown in Table 1 (Entries 2 to 5), the surface area of CuꢀAg/HT
was approximately 200 m2 g−1, which was irrespective of the Mg:Al ratio
(from 1:1 to 5:1). The surface area of CuꢀAg/γꢀAl2O3 (Table 1, Entry 1)
was similar to that of CuꢀAg/HT. CuꢀAg/MgO had the lowest surface area
of 125 m2 g−1. The isotherms of the six samples exhibited the
characteristic Type IV shape, which, together with the pore size
distribution results, suggested that the samples were mesoporous
materials.
3.1.2 XRD analysis
Figures 2–4 display the XRD patterns of the supports and CuꢀAg
catalysts. Results for HT supports with 1:1 molar ratio of Mg:Al showed
major diffraction peaks for HT and minor diffraction peaks for Al2O3 (Fig.
2a). For HT supports with molar ratios of 2:1, 3:1, and 5:1, the results
were the same as those for HT, with characteristic diffraction peaks at
11.4, 23.0, and 34.9° (Fig. 2b–d) [22]. CuꢀAg/HT results showed the
diffraction peaks characteristic of MgO, which were observed irrespective
of the Mg:Al molar ratio and distinct from the corresponding peaks from
their HT supports (Fig. 2e). In the XRD spectra of CuꢀAg/HT, a diffuse
diffraction diagram of MgO only was observed, and Al2O3 formed a
separate amorphous phase without diffraction pattern in XRD [23ꢀ25].
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This indicated that the HT supports were transformed into mixed oxide in
the process of preparing CuꢀAg/HT catalysts. Results for γꢀAl2O3 and
CuꢀAg/γꢀAl2O3 were similar, showing the typical features of γꢀAl2O3 (Fig.
3) [22]. This indicated that the crystalline structure of the γꢀAl2O3
supports did not change after supporting the CuꢀAg phase. Results for
MgO and CuꢀAg/MgO were also similar to each other. No reflections
corresponding to Cu or Ag were observed for catalysts loaded with 1 wt%
of catalyst because of the low loadings (Figs. 2 and 3). XRD patterns of
10 wt% CuꢀAg/HT were also determined (Fig. 4) and show reflections
corresponding to Cu because of the increased catalyst loading.
3.1.3 XPS analysis
XPS results for 1 wt% CuꢀAg/HT did not clearly show the XPS
spectra of Ag because of its low loading. Therefore, the total loading
amount of CuꢀAg was increased to 10 wt%. XPS spectra of the Cu 2p and
Ag 3d regions for 10 wt% CuꢀAg/HT are given in Fig. 5. All of the
spectra of the Cu 2p region could be satisfactorily fitted with maximum
peaks at binding energies of 932.9 and 953.1 eV, characteristic of metallic
Cu. No Cuδ+ was detected, suggesting that Cu had been reduced
completely. All of the spectra of the Ag 3d region could be satisfactorily
fitted with maximum peaks at binding energies of 367.8 and 374.0 eV,
characteristic of metallic Ag. This indicated that Ag had been reduced
completely.
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To study the resistance to oxidation of CuꢀAg bimetallic
nanoparticles, XPS spectra of CuꢀAg/HT were taken after samples had
been stored for 2 or 3 months (Fig. 6). Spectra of Cu 2p for CuꢀAg/HT
samples that had been stored showed the characteristics of metallic Cu
and were similar to the results from the fresh catalysts. In contrast, the
XPS spectra of Cu 2p for Cu/HT samples that had been stored exhibited
peaks associated with Cu2+ alongside the characteristic peaks of metallic
Cu [20, 26]. This indicated that part of the Cu nanoparticles on Cu/HT
had been oxidized to Cu2+ by air. This also suggested that the Cu atoms of
the CuꢀAg bimetallic nanoparticles were less oxidized than those in the
pure Cu nanoparticles. The enhanced resistance to oxidation of CuꢀAg
bimetallic nanoparticles was ascribed to the addition of Ag [20]. The
structure and resistance to oxidation of CuꢀAg bimetallic nanoparticles
have been investigated in the literature [20,27ꢀ30]. The results from the
literature about CuꢀAg nanoparticles indicated that Cu nanoparicles are
easily oxidized, and the addition of Ag to Cu enhanced the resistance of
Cu to oxidation. The enhanced resistance was ascribed to the smaller
number of surfaceꢀexposed Cu atoms compared to the entire Cu atoms in
this AgꢀCu bimetallic nanoparticles and electronic interaction between Ag
and Cu atoms in the AgꢀCu bimetallic nanoparticles.
3.1.4 TEM analysis
The morphological and structural properties of CuꢀAg/HT were
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further characterized by STEM, selectedꢀarea electron diffraction (SAED),
and highꢀresolution TEM (HRꢀTEM), as shown in Fig. 7. The CuꢀAg
nanoparticles that were calcined and reduced at 300 °C were too small to
see in STEM images (Fig. 7a1) while for the process at 600 °C they were
large enough to be easily observed (Fig. 7b1). The higher calcination
temperature could have resulted in the observed increase in the size of the
nanoparticles [31,32].
The SAED patterns indicated that the CuꢀAg nanoparticle was
polycrystalline. Each ring of the SAED image is a reflection of planes
with different interplanar spacings [20]. Using the length of scale (L) and
the radius of the diffraction ring (R) in the equations L/5=R/N and X=1/N
(where N is the reciprocal spacing and X is the spacing), X values of 2.47,
1.79, and 1.33 Å were obtained. The spacing values of nanoparticles in
the samples were consistent with those of lattice planes (111), (200) and
(220) of Cu nanoparticles. Combined with the XPS and XRD analysis, it
can be concluded that both Cu and Ag nanoparticles were in a metallic
state.
3.1.5 Acidity and basicity of supports
The acidity and basicity of the supports were determined by TPD,
and the results are shown in Fig. 8. The CO2ꢀTPD profiles of HT supports
shows two big desorption peaks at 138 °C and 520 °C, respectively, as
well as two small desorption peaks at 288 °C and 662 °C, respectively.
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MgO had two main desorption peaks at 158 °C and 562 °C, respectively.
The CO2ꢀTPD profiles of γꢀAl2O3 supports only shows one small
desorption peaks at 138 °C. From the temperature and intensity of the
CO2 desorption peaks, it can be found that HT and MgO possessed
relatively strong basicity, while γꢀAl2O3 had weak basicity. The NH3ꢀTPD
profiles of HT supports shows two big desorption peaks at 166 °C and
441 °C, respectively. γꢀAl2O3 had one broad peak at 200 °C. In case of
MgO, no desorption peak was observed. From the temperature and
intensity of the NH3 desorption peak, we can see that HT and γꢀAl2O3
possessed relatively strong acidity, while MgO had no significant acidity.
3.2 Effect of supports
Analysis of γꢀAl2O3, HT with different Mg2+:Al3+ molar ratios, and
MgO supports (Fig. 9) revealed that the supports affected both the
activity and selectivity of CuꢀAg catalysts. When γꢀAl2O3 was used as
support, the conversion of benzyl alcohol was 75% (Fig. 9a). In the case
of HT supports, the conversion of benzyl alcohol varied with the
Mg2+:Al3+ molar ratio. At a molar ratio of 1:1 the conversion was 50%,
but this increased to a maximum of 88% when the molar ratio was 2:1.
Above 2:1, the conversion decreased as Mg2+:Al3+ was increased. When
pure MgO was used as the support the conversion of benzyl alcohol was
64%.
The reaction produced two types of products (Scheme 3). Ether was
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formed from the direct etherification of primary and secondary benzylic
alcohols. Alternatively, βꢀphenylpropiophenone was formed by
dehydrogenative crossꢀcoupling of primary and secondary benzylic
alcohols. When γꢀAl2O3 was used as the support, ethers were produced
(Fig. 9b). In contrast, when HT or MgO were the support only
βꢀphenylpropiophenone was found. Furthermore, the yield of
βꢀphenylpropiophenone varied with ratio of Mg2+:Al3+; at 1:1 the yield
was 21%, but this increased to the maximum of 76% at 2:1. Above 2:1,
the yield decreased as Mg2+:Al3+ was increased. When pure MgO was
used as the support, the yield of βꢀphenylpropiophenone decreased to
35%. Notably, α,β,ꢀunsaturated carbonyl derivatives, for example, the
intermediate 1,3ꢀdiphenylꢀacrylketone, were not observed in the presence
of CuꢀAg/HT or CuꢀAg/MgO.
The effect of the supports on the reaction selectivity was ascribed to
whether the supports were acidic or basic. Acidic sites promoted the
etherification of primary and secondary benzylic alcohols [33].
Conversely, strongly basic sites catalyzed the Aldol condensation step in
the dehydrogenative crossꢀcoupling of primary and secondary benzylic
alcohols [5,13]. γꢀAl2O3 includes weakly acidic sites [34], while HT and
MgO have strong basic sites [35]. This explains why the main product
was ether for CuꢀAg/γꢀAl2O3, and βꢀphenylpropiophenone for CuꢀAg/HT
and CuꢀAg/MgO.
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The activity of pure, undoped γꢀAl2O3 was also tested for the
reaction between primary and secondary benzylic alcohols (Table 2).
Here, as with CuꢀAg/γꢀAl2O3, the products were also ethers, indicating
that either Cu or Ag failed to function with the γꢀAl2O3 support.
3.3 Effect of Cu:Ag mass ratio
Figure 10 shows that the conversion of benzyl alcohol and yield of
βꢀphenylpropiophenone were lowest for the reaction using pure Ag, the
latter being almost zero. Increasing the Cu content increased both the
conversion and the yield with respective maximums of 88 and 76%
observed after 0.5 h when the content of Cu was 95%. Using pure Cu
caused the conversion and yield after 0.5 h to fall to 68% and 53%,
respectively. These results suggested that the main active site was Cu, but
that its activity was promoted by the presence of Ag. As shown in Fig. 6,
the addition of Ag enhanced the resistance to oxidation of Cu. The role of
Ag in the activity of CuꢀAg may be related to the increase in the
resistance to oxidation of Cu. Besides, the presence of Ag may change the
electronic and morphological structure of Cu, which affected the activity
of Cu. The role of Ag will be studied in the future work.
3.4 Catalytic performance
The effect of calcination temperature and reaction conditions was
studied (Figures S1ꢀS3, and Tables S1 and S2). The results indicated that
the optimum reaction conditions involved using 0.1 g of catalyst in a
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reaction at a temperature of 150 °C that lasted 1 h. Under these conditions
both the conversion of benzyl alcohol and the yield of
βꢀphenylpropiophenone approached 99%.
Liu et al. [11] reported that AuꢀPd/HT catalysts showed high
catalytic performance for the reaction studied here. Indeed, they reported
that both conversion and yield neared 97% after 5 h for a reaction
involving primary alcohol (1 mmol), secondary alcohol (1 mmol),
pꢀxylene (3 mL), and the AuꢀPd/HT catalyst (0.38 g) in a N2 atmosphere
at 120 °C and 1 bar. Results for similar reaction conditions using
CuꢀAg/HT catalyst showed that both the conversion of benzyl alcohol
and the yield of βꢀphenylpropiophenone were similar to those obtained
with AuꢀPd/HT. Therefore, and given that Cu and Ag are more widely
available and less expensive than Au and Pd, we suggest that
CuꢀAg/HT is a better choice of catalyst for the dehydrogenation
crossꢀcoupling reaction of primary and secondary alcohols.
3.5 Coupling with various alcohols
With suitable reaction conditions established, the scope of substrates
was then investigated (Table 3). Very high yields (90–100%) of the
corresponding
crossꢀcoupled
products
were
obtained
from
1ꢀphenylethanol and various substituted benzyl alcohols, including both
electronꢀrich and electronꢀdeficient examples (Table 3, Entries 1–5).
Notably, electronꢀdonating substituents on the primary alcohol appeared
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to play a significant role, shortening the reaction time required to achieve
completion (Entries 1–2). Conversely, mꢀsubstituted alcohols appeared to
exhibit lower reaction rates and yields than pꢀsubstituted alcohols, likely
because of steric effects (Entries 3–4). Similarly, excellent yields
(97–99%) of the corresponding crossꢀcoupled products were also
obtained when using various substituted 1ꢀphenylꢀethanol and benzyl
alcohols, again including both electron–rich and electronꢀdeficient
examples (Entries 6–7).
The results also suggest that aliphatic primary alcohols are not
suitable coupling partners. For example, when 2ꢀphenylethanol was
employed as the substrate, complete conversion and complete yield were
obtained after 8 h (Entry 8). However, when cinnamyl primary alcohol
was used as the substrate, the corresponding yield decreased to 58% after
the same reaction period (Entry 9). Unfortunately, after 8 h none of the
desired crossꢀcoupled product was detected if aliphatic primary alcohols
without benzene substituents, such as allyl alcohol or propyl alcohol,
were employed as substrates (Entries 10–11). Instead these cases
produced 1,3ꢀdiphenylꢀ1ꢀbutanone, which resulted from the selfꢀcoupling
of 1ꢀphenylethanol.
For the reactions of Entries 10–11 (Table 3), allyl alcohol or propyl
alcohol kept unchanged when 1ꢀphenylethanol occurred the
dehydrogenative selfꢀcoupling. In the absences of allyl alcohol or propyl
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alcohol, 1ꢀphenylethanol still occurred the dehydrogenative selfꢀcoupling
in similar yield. It was found that the crossꢀcoupling and selfꢀcoupling
competed with each other in the primary and secondary mixed alcohol
system. As shown in Scheme 2, the first step of the reaction proceeds by
dehydrogenation of the alcohol to an intermediate carbonyl compound.
The dehydrogenation of aliphatic primary alcohols may go slowly in the
presence of CuꢀAg/HT catalysts [36,37], whereas the rate of
dehydrogenation of 1ꢀphenylethanol was high. The low rate in
dehydrogenation of aliphatic primary alcohols decreased the rate of
crossꢀcoupling between aliphatic primary alcohol and 1ꢀphenylethanol,
but did not affect the dehydrogenative selfꢀcoupling of 1ꢀphenylethanol.
This explained the reactions of Entries 10–11. However, for reactions of
Entries 1–9, the detected products were mainly from the crossꢀcoupling
of primary and secondary alcohol, but not the selfꢀcoupling of alcohol.
Furthermore, the yield to selfꢀcoupling of alcohol was very low and
negligible. The high yield to crossꢀcoupling of primary and secondary
alcohol in Entries 1ꢀ9 was mainly ascribed to the property of the involved
primary alcohol, which was various substituted benzyl alcohols. The
various substituted benzyl alcohols was easily dehydrogenated to form
the corresponding aldehyde [36,37], accordingly, easily crossꢀcoupled
with the secondary alcohol.
3.6 Stability in air and reusability of catalysts
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3.6.1 Stability in air
Fresh CuꢀAg/HT and Cu/HT catalysts were stored in a desiccator for
a number of days with their activity then tested to investigate the stability
in air of CuꢀAg/HT. As shown in Fig. 11, the yield of
βꢀphenylpropiophenone did not appear to change for catalysts stored for
up to 6 days. However, after 9 days the yield decreased by 4% and fell by
18% after a total of 76 days. Cu/HT catalysts showed a faster decrease in
yield than that of CuꢀAg/HT catalysts with the yield from reactions
involving Cu/HT falling by 10 and 31% for 9 and 76 days, respectively.
XPS analysis indicated that it was the addition of Ag, which enhanced the
resistance to oxidation of CuꢀAg bimetallic nanoparticles, and was
responsible for the extra longevity.
3.6.2 Reusability
CuꢀAg/HT catalysts were found to be usable five times with no
apparent loss of activity (Table 4) with activity then declining to zero
after the catalyst was used more than nine times. The deactivation of
CuꢀAg/HT may be ascribed to structural change, especially the leaching
of active sites of Cu and basic sites of Mg2+. Indeed, the 0.95% loading of
Cu in fresh catalyst decreased to 0.5% after it had been used nine times.
Similarly, the molar ratio of Mg2+ to Al3+ in fresh CuꢀAg/HT was 2:1, but
this decreased to almost 1.2:1 after nine uses. The decrease in loading
amount of Cu and Mg2+:Al3+ molar ratio showed that both Cu and Mg2+
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could leach into the reaction system. Further work will therefore focus on
the prevention of the observed leaching of Cu and Mg2+ into the reaction
system.
4 Conclusions
Acidic/basic properties of catalyst supports affected the activity and
selectivity of CuꢀAg/HT in the reactions studied here. Acidic supports
promoted the etherification of primary and secondary benzylic alcohols,
whereas basic supports advanced the dehydrogenative crossꢀcoupling of
primary and secondary benzylic alcohols. CuꢀAg/HT with a 2:1 molar
ratio of Mg to Al showed the highest activity and demonstrated it to be an
active catalyst for the CꢀC crossꢀcoupling of secondary and primary
alcohols. The active sites of CuꢀAg/HT were Cu, with Ag involved in
promoting the activity of Cu sites as well as hindering their oxidation.
The yield of βꢀphenylpropiophenone reached 99% after 1 h under
optimum reaction conditions. The catalyst also displayed reusability, with
no loss of activity after five cycles, and resistance to oxidation, being able
to be stored for 6 days without deactivation. However, the CuꢀAg/HT
catalyst was completely deactivated after more than nine uses because of
leaching of Cu and Mg2+ into the reaction system.
Acknowledgments
This work was supported by projects funded by the Natural Science
Basic Research Plan in Shaanxi Province of China (Program No.
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2013JM2003) and by the Fundamental Research Funds for the Central
Universities (Program No. GK201505002).
References
[1] C. Gunanathan and D. Milstein, Science, 2013, 341, 1229712.
[2] X. Liu, L. He, Y. Liu and Y. Cao, Acc. Chem. Res., 2014, 47, 793.
[3] I.S. Makarov and R. Madsen, J. Org. Chem., 2013, 78, 6593.
[4] J.T. Kozlowski and R.J. Davis, ACS Catal., 2013, 3, 1588.
[5] K. Shimizu, Catal. Sci. Technol., 2015, 5, 1412.
[6] S. Musa, L. Ackermann and D. Gelman, Adv. Synth. Catal., 2013, 355,
3077.
[7] D. Gnanamgari, C.H. Leung, N.D. Schley, S.T. Hilton and R.H.
Crabtree, Org. Biomol. Chem., 2008, 6, 4442.
[8] D. Wang, K. Zhao, X. Yu, H. Miao and Y. Ding, RSC Adv., 2014, 4,
42924.
[9] K. Shimizu, R. Sato and A. Satsuma, Angew. Chem., 2009, 121, 4042.
[10] C.S. Cho, W.X. Ren and S.C. Shim, Bull. Korean Chem. Soc.,
2005, 26, 1611.
[11] X. Liu, R. Ding, L. He, Y. Liu, Y. Cao, H. He and K. Fan,
ChemSusChem, 2013, 6, 604.
[12] L. Zhang, A. Wang , W. Wang ,Y. Huang , X. Liu, S. Miao , J. Liu
and T. Zhang, ACS Catal., 2015, 5, 6563.
[13] J.D. Lewis, S.V. Vyver and Y. RománꢀLeshkov, Angew. Chem. Int.
21

RSC Advances
Page 22 of 45
View Article Online
DOI: 10.1039/C5RA22542F
Ed., 2015, 54, 9835.
[14] P. Fakhri, B. Jaleh and M. Nasrollahzadeh, J. Mol. Catal. A:
Chem., 2014, 383– 384, 17.
[15] G. Corro, S. Cebada, U. Pal, J.L.G. Fierro and J. Alvarado, Appl.
Catal. B, 2015, 165, 555.
[16] Y. Zhu, X. Kong, S. Zhu, F. Dong, H. Zheng, Y. Zhu and Y. Li,
Appl. Catal. B, 2015, 166–167, 551.
[17] D. Damodara, R. Arundhathi and P.R. Likhar, Adv. Synth. Catal.,
2014, 356, 189.
[18] F. Santoro, R. Psaro, N. Ravasio and F. Zaccheria, RSC Adv., 2014,
4, 2596.
[19] K. Shimizu, K. Shimura, M. Nishimura and A. Satsuma, RSC Adv.,
2011, 1, 1310.
[20] N.R. Kim, K. Shin, I. Jung, M. Shim and H.M. Lee, J. Phys.
Chem. C, 2014, 118, 26324.
[21] Z. Wang, P. Fongarland, G. Lu and N. Essayem, J. Catal., 2014,
318, 108.
[22] C. Xu, J. Sun, B. Zhao and Q. Liu, Appl. Catal. B, 2010, 99, 111.
[23] J.M.L. Nieto, A. Dejoz and M.I. Vazquez, Appl. Catal. A, 1995,
132, 41.
[24] Y.Z. Chen, C.M. Hwang and C.W. Liaw, Appl. Catal. A, 1998,
169, 207.
22

Page 23 of 45
RSC Advances
View Article Online
DOI: 10.1039/C5RA22542F
[25] A. Villa, A. Gaiassi, I. Rossetti, C.L. Bianchi, K. Benthem, G.M.
Veith and L. Prati, J. Catal., 2010, 275, 108.
[26] J.P. Espinós, J. Morales, A. Barranco, A. Caballero , J.P. Holgado
and A.R. GonzálezꢀElipe, J. Phys. Chem. B, 2002, 106, 6921.
[27] Z. Chen, D. Mochizuki, M.M Maitani and Y. Wada,
Nanotechnology, 2013, 24, 265602.
[28] J. Czaplinska, I. Sobczak and M. Ziolek, J. Phys. Chem. C, 2014,
118, 12796.
[29] K.D. Malviya and K. Chattopadhyay, J. Phys. Chem. C, 2014, 118,
13228.
[30] H. Peng, W. Qi, S. Li and W. Ji, J. Phys. Chem. C, 2015, 119,
2186.
[31] A. Cao, R. Lu and G. Veser, Phys. Chem. Chem. Phys., 2010, 12,
13499.
[32] Y. Wang, J. He, C. Liu, W. Chong and H. Chen, Angew. Chem. Int.
Ed., 2015, 54, 2022.
[33] E.R. Sacia, M. Balakrishnan and A.T. Bell, J. Catal., 2014, 313,
70.
[34] J. Luo, J. Yu, R.J. Gorte, E. Mahmoud, D.G. Vlachos and M.A.
Smith, Catal. Sci. Technol., 2014, 4, 3074.
[35] C. Xu, Y. Du, C. Li, J. Yang and G. Yang, Appl. Catal. B, 2015,
164, 334.
23

RSC Advances
Page 24 of 45
View Article Online
DOI: 10.1039/C5RA22542F
[36] T. Mitsudome, Y. Mikami, K. Ebata, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Chem. Commun., 2008, 4804.
[37] T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Angew. Chem., 2008, 120, 144.
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Table captions
Table 1 Nitrogen physisorption data of CuꢀAg catalysts
Table 2 Etherification of benzyl alcohol and 1ꢀphenyl ethanol catalyzed
a
by γꢀAl2O3
Table 3 Dehydrogenation crossꢀcoupling reaction of other primary and
secondary alcohols
Table 4 Reusability of CuꢀAg/HT a
Figure captions
Fig. 1 Nitrogen physisorption isotherms (■, adsorption; ●, desorption)
and Barrett–Joyner–Halenda (dV–dlogD) pore size distribution (inset) for
CuꢀAg catalysts
Fig. 2 XRD patterns of HT supports and CuꢀAg/HT catalysts
Fig. 3 XRD patterns of MgO and γꢀAl2O3 supports, and their supported
CuꢀAg catalysts
Fig. 4 XRD patterns of 1 wt% CuꢀAg/HT and 10 wt% CuꢀAg/HT
Fig. 5 XPS spectra of Cu 2p and Ag 3d for 10 wt% CuꢀAg/HT
Fig. 6 Cu 2p XPS spectra of Cu 2p for catalysts stored for two or three
months for (a) CuꢀAg/HT and (b) Cu/HT
Fig. 7 STEM images and SAED patterns of CuꢀAg/HT catalysts calcined
and reduced at (a) 300 °C and (b) 600 °C. Inset: HRꢀTEM images of the
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selected area.
Fig. 8 (A) CO2ꢀTPD and (B) NH3ꢀTPD profiles of supports: (a) HT, (b)
MgO, and (c) γꢀAl2O3.
Fig. 9 Effect of Mg:Al ratio on activity and selectivity of CuꢀAg catalysts.
Reaction conditions: benzyl alcohol (1.0 mmol), 1ꢀphenylethanol (1.0
mmol), catalyst (0.1 g), oꢀxylene (3 mL), 150 °C, 1 bar N2, reaction time
of 0.5 h.
Fig. 10 Effect of Cu:Ag mass ratio on catalytic activity of CuꢀAg/HT in
the dehydrogenation crossꢀcoupling reaction. Reaction conditions: benzyl
alcohol (1.0 mmol), 1ꢀphenylethanol (1.0 mmol), CuꢀAg/HT (0.1 g),
oꢀxylene (3 mL), 150 °C, 1 bar N2.
Fig. 11 Catalytic activity versus standing time for (a) CuꢀAg/HT and (b)
Cu/HT catalysts. Reaction conditions: primary alcohol (1.0 mmol),
secondary alcohol (1.0 mmol), oꢀxylene (3mL), catalyst (0.1 g), 150 °C, 1
bar N2, reaction time of 1 h.
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Table 1 Nitrogen physisorption data of CuꢀAg catalysts
Metal ratio (Mg:Al)a
Surface
Pore
Pore
Entry Samples
area
(m2g˗1)b
200
volume
(cm3g˗1)c
0.58
diameter
(Å)
Prepared/determined
78
1
2
3
4
5
6
CuꢀAg/γꢀAl2O3 0:1/0:1
CuꢀAg/HT
CuꢀAg/HT
CuꢀAg/HT
CuꢀAg/HT
CuꢀAg/MgO
1:1/1.25:1
2:1/2.08:1
3:1/2.96:1
5:1/5.63:1
229
1.12
127
102
96
198
0.60
209
0.66
202
0.49
73
1:0/1:0
125
0.44
105
a
b
c
Determined by energyꢀdispersive Xꢀray spectroscopy. Calculated by the BET method. Calculated by the
Barrett–Joyner–Halenda method from the desorption isotherm.
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a
Table 2 Etherification of benzyl alcohol and1ꢀphenyl ethanol catalyzed by γꢀAl2O3
Yields (%)
Conversion of
benzyl alcohol
(%)
Reaction
time
Entry
Dibenzyl ether 1ꢀmethyl benzyl ether 1,1′ꢀdiphenyl ether
1
2
3
45 min
1 h
81
87
88
99
0.2
0.4
0.3
0.5
49
58
69
95
2.9
3.6
4.0
2.6
1.5 h
2 h
4
a
Reaction conditions: benzyl alcohol (1.0 mmol), 1ꢀphenylethanol, (1.0 mmol), catalyst (0.1 g),
oꢀxylene (3 mL), 150 °C, 1 bar N2.
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Table 3 Dehydrogenation crossꢀcoupling reaction of other primary and secondary
alcohol
Reaction Conversion Yield to
Entry
a
b
c
time (h)
2
of a (%)
100
c (%) a
1
100
2
3
100
100
92
98
100
90
93
99
97
100
58
46
39
3
8
4
8
5
8
100
100
100
100
80
6
12
8
7
8
8
9
8
10
11
8
85
8
93
a Conversion and yield based on alcohol consumption.
29