Ag-Cu nanoparticles as efficient catalysts for transesterification of β-keto esters under acid/base-free conditions

Article abstract of DOI:10.1039/c6ra00467a

Transesterification of β-keto esters and alcohols are traditionally catalyzed by acid or basic catalysts. However, these traditional catalysts do not always meet the requirements of modern synthetic chemistry which need to be highly efficient, selective, and environmentally friendly. In this work, Ag-Cu metal sites were first introduced as transesterification catalysts. The effect of the support, Ag:Cu molar ratio, and reaction conditions were investigated. The Ag-Cu metal sites were proved to be active in the β-ketoester transesterification with various alcohols, having yields comparable to the conventional acid- or base-catalysts.

Full text of DOI:10.1039/c6ra00467a

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Page 1 of 43
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Ag-Cu nanoparticles as efficient catalysts for transesterification of
β-keto esters under acid/base-free conditions
Hongmei Yue a, b, Hao Yu a, b, Sheng Liu a, b, Chunli Xu a, b,
*
a
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi
Normal University), Ministry of Education, Xi’an 710119, PR China.
E-mail: xuchunli@snnu.edu.cn
b
School of Chemistry and Chemical Engineering, Shaanxi Normal
University, Chang’an West Street 620, Xi’an 710119, PR China
* Corresponding author. E-mail: xuchunli@snnu.edu.cn
Address for correspondence: School of Chemistry and Chemical
Engineering
Shaanxi Normal University
620 Chang’an West Street
Xi’an 710119, P. R. China
Tel: 86-29-81530779
E-mail: xuchunli@snnu.edu.cn
1

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Abstract
Transesterification of β-keto esters and alcohols are traditionally
catalyzed by acid or basic catalysts. However, this traditional catalysts do
not always meet requirements of modern synthetic chemistry which need
to be highly efficient, selective, and environmentally friendly. In this
work, Ag-Cu metal sites were firstly introduced as transesterification
catalysts. The effect of support, Ag:Cu molar ratio, and reaction
conditions were investigated. The Ag-Cu metal sites were proved to be
active in the β-ketoester transesterification with various alcohols, having
yields comparable to the conventionally acid- or base-catalysts.
Keywords: β-keto esters, Transesterification, Acid-base free, Ag-Cu,
Hydrotalcite.
1. Introduction
The β-keto esters have been given much attention because of their
broad use, especially in the organic synthesis of a number of complex
biologically active natural products of pharmaceuticals, chemicals and
polymers.1 Several synthetic methods have been reported on preparation
of these β-keto esters, including Claisen condensation,2 carbonylation of
aryl halides,3 acylation of Meldrum's acid,4 and transesterification.5 In
this regard, the transesterification reaction of β-ketoesters has been
recognized as one of the most significant processes in producing other
β-ketoesters,6 because it allows the preparation of more complex products
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from more easily accessible synthon (Scheme 1).7
Transesterification is the process of exchanging the organic group R2
of an ester with the organic group R1 of an alcohol (Scheme 1). It is an
equilibrium-driven process.8 It has long been known that the reaction is
accelerated by acid or base catalysts.9 Because of their versatility, acid- or
base-catalyzed reactions were the subjects of extensive investigation. A
variety of acid or basic catalysts have been reported for transesterification
of β-ketoesters in the literature, such as homogeneous Brönsted acids and
bases (hydrochloric acids,10 sulfuric acids, phosphoric acids, sodium
alkoxides, potassium alkoxides,11 and cesium carbonate12), homogeneous
Lewis acids and bases (ZnCl2, FeCl3·6H2O,10 LiClO4,13 InI3,14
NH2SO3H,15 Yttria-Zirconia,16 Sodium perborate,17 and KCN11), solid
acid catalysts (montmorillonite K-10,18 SG-NH2-Bpy-Zn,19 Mo-ZrO2,20
Sulphated SnO2,21 B2O3/ZrO2,22 and minerals Clay23), solid basic catalysts
(Zinc(II) oxide,24 Mg–Al–O–t-butyl hydrotalcite,25 and basic silica26).
Homogeneous acid/basic catalysts are generally corrosive to equipment
and difficult to recycle, giving rise to serious environmental and
corrosion problems. The use of heterogeneous acid/basic catalysts
simplifies greatly the technological process by facilitating the separation
of the post-reaction. Unfortunately, the heterogeneous acid/basic catalysts
usually showed low activity.11 One kind of substitute to acid/basic
catalysts are enzymes. However, enzymes are easily deactivated.
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Therefore, the attention has to be diverted to discover new catalysts.27
In the transesterification mechanism, the carbonyl carbon of the
starting ester (RCOOR2) undergoes nucleophilic attack by the incoming
alkoxide (R1O−) to give a tetrahedral intermediate, which proceeds to the
transesterified product (RCOOR1). Alkoxide (R1O−) is generated by
removing a proton from the starting alcohol (R1OH). It can be accelerated
by activating the O-H bond in the alcohol on the surface of catalyst. Since
the forming rate of alkoxide (R1O−) decides the reaction rate of
transesterification,28 it is essential to find an efficient catalyst which have
the ability to activate the O-H bond in the alcohol. Metal catalysts, such
as Au, Pt, Pd, Ni, Cu, and Ag, are one important type of catalysts in
chemistry and chemical engineering.29-31 The metal sites could activate
the O-H bond in alcohol.32 In other sides, the metal catalysts have vacant
orbitals. The vacant orbitals of metals could coordinate to the oxygen of
β-keto esters. The coordination increases the electrophilicity of the
carbonyl carbon. The increase in electrophilicity of esters could also
accelerate the rate of transesterification.33,34 Therefore, metal catalysts
may offer the potential for catalyzing the reaction, the investigation of
which is the focus of this work. Catalysts based on Cu or Ag have
attracted much attention in heterogeneous catalysis because of the metal’s
relative cost advantage. Besides, bimetallic catalysts frequently exhibit
better catalytic performance than their monometallic counterparts and
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have therefore attracted increasing attention from both academic and
industrial fields.35 In this work, Hydrotalcite-like materials supported
Ag-Cu catalysts (Ag-Cu/HTs) were prepared and tested in the
transesterification of β-ketoester and alcohols.
2. Experimental section
2.1 Catalyst preparation
2.1.1 Preparation of supports
HTs support was prepared by a standard aqueous co-precipitation
method as described in our previous report.36 In brief, an aqueous solution
(120 mL) with a total concentration of 1.5 Mol/L containing
Mg(NO3)2·6H2O and Al(NO3)3·9H2O with a Mg2+:Al3+molar ratio of 5:1
and another aqueous solution (240 mL) containing NaOH (1.5 Mol/L)
were slowly dropped into Na2CO3 (40 mL, 1.5 Mol/L) aqueous solution
with continuous stirring at room temperature. Afterwards, a white
suspension was gained and the pH value of mixture was controlled
between 9 and 10 with the alkaline solution. Then, the suspension was
aged at 60 °C for 18 h with constant stirring to generate the
hydrotalcite-like structure. The resulting precipitate was filtered with the
deionized water until the pH value of mixture was about 7 after the
suspension has been cooled to room temperature. Finally mixture was
dried at 110 °C for 12 h in air. The resulting material was Mg-Al
hydrotalcite-like support. Finally, the support having a nominal molar
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ratio of Mg:Al = 5:1 was used to study the catalytic activity, unless
otherwise noted.
γ-Al2O3 was prepared by calcining γ-Al2O3 (a commercial sample,
Alfa Aesar Chemicals Co., Ltd.) at 400 °C for 5 h in air. Similarly, MgO
was prepared by calcining MgO sample (AR, 98.5%, Sinopharm
Chemical Reagent Co., Ltd.) at 500 °C for 3 h in air.
The activated-carbon (AC) support was prepared by treating
commercial charcoal active powder (AR, Tianli Chemical Reagent Co.,
Ltd.) with HNO3 (6 mol/L, 60 mL) in a three-necked batch reactor fitted
with a reflux condenser, oil bath and magnetic stirrer at 100 °C for 3 h.
The slurry was then cooled to room temperature, filtered, and washed
with deionized water until the pH value of filtrate was about 7. The
carbon was dried at 120 °C for 3 h and then calcined at 300 °C for 3 h.
2.1.2 Synthesis of Ag-Cu supported catalyst
We prepared a series of supported Ag-Cu catalyst (Ag:Cu molar
ratio = 1:0, 1.8:1, 1.2:1, 0.59:1, 0.29:1, or 0:1) with different Ag-Cu
content (1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 7wt%, 8wt%) by a
sol-immobilisation method.37 Cu(NO3)2・3H2O (AR, 99%, Sinopharm
Chemical Reagent Co., Ltd) and AgNO3 (99%, Reagent No.1 Factory Of
Shanghai Chemical Reagent Co., Ltd.) of the desired concentration were
used as metal precursors and were added simultaneously, drop by drop, to
a beaker containing deionized water to form an aqueous mixture. Then
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the poly (vinyl alcohol) (PVA) dispersion (1 mL, 1 mg·mL-1) as the
stabilizer was added (PVA/(Ag + Cu) (wt/wt) = 0.125) ) with vigorous
stirring for 2 min. After that, NaBH4 prepared freshly (0.1 Mol/L aqueous
solution, NaBH4/metal mole fraction 0.24) was rapidly dropped into the
mixture solution as the reducing agent. The dispersion was stirred for 30
min, and followed by the introduction of the desired support materials.
After 2 h of stirring, the mixture was filtered, washed with distilled water
and dried at 110 °C in air overnight. The resulted materials were Ag-Cu
supported catalysts. Hereafter, if there is no other special statement, 5wt%
Ag-Cu/HTs (1.2:1 of Ag/Cu molar ratio, and 5:1 of Mg:Al molar ratio)
was used.
2.2 Characterization of supports and catalysts
Scanning transmission electron micrographs (STEM) were obtained
using a FEI Tecnai G2 F20 instrument (USA).
X-ray photoelectron spectroscopy (XPS) was used to characterize
the elements valence present in the catalysts. XPS data was obtained in an
Axis Ultra spectrometer (Kratos Analytical Ltd., Japan) with Al Kα X-ray
source (1486.6 eV, 15 kV, 5 mA) at an energy of 150 W.
Elemental analysis of HTs was carried out by an FEI Quanta 200
(USA) scanning electron microscope equipped with energy-dispersive
X-ray spectroscopy.
The loading amount of Ag-Cu was determined by inductively
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coupled plasma mass spectrometry (ICP-MS; M90, Bruker, Germany)
and Atomic Absorption Spectroscopy (AAS; ZA3000, Hitachi, Japan).
X-ray diffraction analysis (XRD) was collected on X-ray
diffractometer of Rigaku Co. Japan (D/Max2550VB+/PC) with Cu-Ka
radiation at 40 Kv and 450 mA.
Micromeritics ASAP 2020 instrument (America) was used to
determine the surface area, average pore diameter, as well as pore
distribution of Ag-Cu/HTs using Brunauer-Emmett-Teller (BET) and
Barrett-Joyner-Halenda (BJH) method. The sample was degassed at 110
°C for 4 h in N2 prior to carrying out surface area measurements.
2.3 General experimental procedure
In our experimental procedure, unless otherwise noted, all reagents
were used as received without further purification. The reaction tests were
performed in a 50 mL flask equipped with oil bath and magnetic stirrer at
110 °C for 6 h. The cinnamyl alcohol (11 mmol, 9 Dingchem Co., Ltd)
and ethyl acetoacetate (11 mmol, Sinopharm Chemical Reagent Co., Ltd)
were added into the flask with DMF (1.5 mL, Sinopharm Chemical
Reagent Co., Ltd). After the completion of reaction, the mixture was
cooled to room temperature and the catalyst was removed by
centrifugation. The DMF solvent was removed by extraction. The yield
was characterized by the 1H NMR spectrum of the crude reaction mixture.
The crude reaction mixture was further purified by column
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chromatography using hexane and ethyl acetate as eluents (hexane:ethyl
acetate 10:1-5:1) on silica gel to obtain the pure products.
3. Results and discussion
3.1 The characterization of catalysts
3.1.1 BET analysis
The surface area of HTs and Ag-Cu/HTs catalyst is displayed in
Table 1, which were 35 m2·g−1 and 29 m2·g−1, respectively. They had no
obviously difference in pore volume and average pore diameter. Fig. S1
shows the isotherms of the two samples. They had the characteristic Type
IV shape. The hysteresis loop could be further specified as type H3.38,39
The data of pore size distribution indicted that neat HTs and Ag-Cu/HTs
were mesoporous materials.
3.1.2 XRD analysis
Fig. 1 shows XRD patterns of neat HTs, and Ag-Cu/HTs (fresh and
after used for six times). The diffraction of neat HTs displayed reflections
at (003), (006), (009), (015), (018), (110), and (113), which were
characteristic of HTs.40 The XRD pattern of Ag-Cu/HTs showed, in
addition to HTs, a series of new diffraction. Among them, the reflections
at (111) may correspond to Cu and the reflections at (220) and (222)
could be associated with Ag (Fig. 1b). The XRD pattern of Ag-Cu/HTs
after used for six times were similar with that of fresh Ag-Cu/HTs,
suggesting that there was no apparent change in the structure of
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Ag-Cu/HTs after used for six times.
3.1.3 The stabilization in air of Ag-Cu bimetallic nanoparticles
XPS spectra of the Cu 2p and Ag 3d regions for Cu/HTs, Ag-Cu/HTs,
and Ag/HTs are given in Fig. 2. The XPS spectra of Cu 2p for fresh and
stored Cu/HTs samples exhibited satellite peaks associated with Cu2+
alongside the characteristic peaks of metallic Cu (Fig. 2a and 2b) The
difference in their XPS spectra was the relative peak area of Cu2+ to
metallic Cu. The stored Cu/HTs showed higher peak area of Cu2+. This
indicated that part of the Cu nanoparticles on stored Cu/HTs had been
oxidized to Cu2+ by air. In contrast, spectra of Cu 2p for Ag-Cu/HTs
samples that had been stored showed the characteristics of metallic Cu
and were similar to the results from the fresh catalysts (Fig. 2c and 2d)
Furthermore, both the fresh and the stored Ag-Cu/HTs showed the similar
Ag 3d spectra, which were characteristics of metallic Ag (Fig. 2g and 2h).
This suggested that the Cu atoms of the Ag-Cu bimetallic nanoparticles
were less oxidized than those in the pure Cu nanoparticles. The enhanced
resistance to oxidation of Ag-Cu bimetallic nanoparticles was ascribed to
the addition of Ag and the electron transfer from Cu to Ag.41 Spectra of
Ag 3d for Ag/HTs samples that had been stored also showed the
characteristics of metallic Ag and was similar to the results from the fresh
catalysts (Fig. 2e and 2f).
3.1.4 The nanostructures and size of Ag-Cu/HTs catalyst
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The morphology and structural properties of Ag-Cu/HTs were
further characterized by STEM, selected-area electron diffraction (SAED),
and high-resolution TEM (HR-TEM), as shown in Fig. 3. The mean size
of Ag-Cu nanoparticles was 4.23 nm. Fig. 3c reveals the HR-TEM image
of a Ag-Cu nanoparticle. The observed latticespacing values match the
expected lattice parameters obtained from XRD analysis (i.e., dAg (111)
=
0.235 nm and dCu (220) = 0.127 nm), which indicated nanoparticles are
Ag-Cu mixture structure loaded on supports. The SAED patterns
indicated that the Ag-Cu nanoparticle was polycrystalline (Fig. 3d).
Energy-dispersive X-ray spectroscopy (EDS) analysis was also
carried out. The variations of composition across the red line in STEM
image (Fig. 3e) are present in Fig. 3f. The line scan shows that Ag and Cu
were mixed well in the nanoparticles. Combined with the XPS and XRD
analysis, it can be concluded that both Cu and Ag nanoparticles were in a
metallic state and mixed well.
The structure of the used Ag-Cu/HTs was also determined. HR-TEM
image reveals that the structural properties of Ag-Cu nanoparticle did not
change after four times used (Fig. 3g). After six times used, a little part of
Ag-Cu nanoparticle were found to be agglomerated (Fig. 3h).
3.2 The investigation of active sites
Mg:Al molar ratio may affect the acid/basic property of HTs.11,25
Because the acid/basic property of catalysts may affect the reaction rate
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of transesterification,11 the effect of Mg:Al molar ratio on neat HTs and
Ag-Cu/HTs were studied. As shown in Fig. 4, the yield to cinnamyl
acetoacetate changed with Mg:Al molar ratio for both neat HTs and
Ag-Cu/HTs. In the presence of neat HTs, the yield increased with Mg:Al
molar ratio. At 4:1 of Mg:Al molar ratio, the yield reached the plateau of
maximum. Above 6:1, the yield decreased with Mg:Al molar ratio. In the
presence of Ag-Cu/HTs, the yield also increased with Mg:Al molar ratio.
At 5:1 of Mg:Al molar ratio, the yield reached the maximum. Above 5:1,
the yield decreased with Mg:Al molar ratio. This indicated that Mg:Al
molar ratio affected the catalytic activity of both neat HTs and
Ag-Cu/HTs. The effect may be due to the change in acid/basic property of
catalysts ascribing from the change in Mg:Al molar ratio.11, 25
The catalytic activity of neat HTs and Ag-Cu/HTs was also
compared each other. As shown in Fig. 4, the yield to cinnamyl
acetoacetate in the presence of Ag-Cu/HTs was far higher than that in the
presence of neat HTs. For example, the yield was 50% for neat HTs at 5:1
of Mg:Al molar ratio, whereas the yield was 97% for Ag-Cu/HTs at the
same Mg:Al molar ratio. This indicated that Ag-Cu/HTs was more active
than neat HTs. Considering the difference of neat HTs and Ag-Cu/HTs in
composition, it could be deduced that Ag-Cu promoted the activity of
Ag-Cu/HTs. For neat HTs, the active sites were obviously the acid/basic
sites. However, for Ag-Cu/HTs, it was difficult to point out the main
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active sites since Ag-Cu metal sites and acid/basic sites were coexisted. In
order to clarify the role of Ag-Cu sites, other three supports, i.e., AC,
MgO, and γ-Al2O3, were investigated. As shown in Table 2, the supported
Ag-Cu catalysts were more active than their corresponding neat supports,
especially for AC supports. The yield to cinnamyl acetoacetate was 18%
for neat AC, whereas the yield was 85% for Ag-Cu/AC. There were little
acid/basic sites on neat AC, which explained its low activity. This
indicated that Ag-Cu were the main active sites of Ag-Cu/AC. This also
suggested that metal sites of Ag-Cu could catalyze transesterification in
the absence of acid/basic sites.
3.3 The influence of Ag:Cu molar ratio
The effect of Ag:Cu molar ratio was investigated. As shown in Fig. 5,
Ag:Cu molar ratio affected the yield to cinnamyl acetoacetate. When
monometallic Cu/HTs was used as catalyst, the yield was 72%. With the
increase of Ag:Cu molar ratio, the yield was increased. When Ag content
was 55%, i.e., 2:1 of Ag:Cu molar ratio, the yield reached the maximum
of 94%. Above 55% of Ag content, the yield decreased. It went down to
86% when monometallic Ag/HTs was used as catalysts. This indicated
that bimetallic AgCu nanoparticles were more active than monometallic
Ag or Cu nanoparticles. The high activity of bimetallic Ag-Cu
nanoparticles may be ascribed to the interaction between Cu and Ag
nanoparticles.
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The structure and property of Ag-Cu bimetallic nanoparticles have
been reported in literature. Chen et al. synthesized Ag-Cu nanoparticles in
the range of 12–30 nm with the assistance of microwave irradiation.42
Czaplinska et al. studied the effect of metal loading on bimetallic
AgCu/SBA-15 system.43 Chattopadhyay et al. have synthesized Ag-Cu
alloy nanoparticles of four different compositions by using the laser
ablation technique. Following this, they reported a morphological
transition in the nanoparticles from a normal two-phase microstructure to
a structure with random segregation and finally a core−shell structure at
small sizes as a function of Cu concentration.44 Lee group investigated
the enhanced resistance to oxidation by a combined experimental and
theoretical studied.41 They calculated the atomic charge of the Ag-Cu
bimetallic nanoparticles using Mulliken charge analysis, and found that
there was electron transfer from Cu to Ag within these bimetallic
nanoparticles. The electron transfer from Cu to Ag within these bimetallic
nanoparticles allows far better resistance to oxidation than monometallic
Cu nanoparticles. On the basis of the Gibbs free energy model, Peng et al.
modeled the phase stability of janus, core−shell, and alloyed Ag-Cu
nanoparticles.45 The results from the literature about Ag-Cu nanoparticles
indicated that the structure of Ag-Cu nanoparticles were complicated,
which ranged from Janus, core-shell, to alloy, and were affected by the
size of nanoparticles, metal loading, and temperature. The results from
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the above literature also showed that the structure of Ag-Cu nanoparticles
could not be fully characterized using conventional techniques. Owing to
the complication in the structure of Ag-Cu nanoparticles, effect of the
interaction between Cu and Ag nanoparticles on their catalytic
performance will be further investigated in our future work.
3.4 Effect of reaction conditions
The effect of reaction conditions, including loading amount of
Ag-Cu, catalyst amount, reaction temperature and time, and solvents, was
investigated. As shown in Figures 6, the optimum reaction conditions
involved using DMF solvent and 0.05 g of 5wt% Ag-Cu/HTs catalyst in a
reaction at a temperature of 110 °C that lasted 6 h. Under these conditions
the yield to cinnamyl acetoacetate approached 98% for the
transesterification of cinnamyl alcohol and ethyl acetoacetate.
3.5 The reusability of Ag-Cu/HTs
In order to verify whether the Ag-Cu/HTs catalysts were truly
heterogeneous, a heterogeneity test was conducted. The catalyst was
removed by filtration at half the specified reaction time period. Then, the
filtrate was allowed to react under same conditions. The yield to cinnamyl
acetoacetate was 17%, which was similar with that of the blank (Table 2,
entry 1). This suggested that nature of Ag-Cu/HTs catalysts was truly
heterogeneous.
As shown in Fig. 7, the reusability of neat HTs and Ag-Cu/HTs were
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studied for transesterification of cinnamyl alcohol and ethyl acetoacetate.
The catalysts were recovered after each run by centrifugation and
washing with DMF for five times. Using Ag-Cu/HTs as catalyst, the yield
to cinnamyl acetoacetate decreased slowly with number of cycles. The
yield decrease from 99% for the fresh to 90% for the used for four times.
In the presence of neat HTs, the yield did not apparently change with
number of cycles. This indicates that the deactivation of Ag-Cu/HTs may
be due to the change in the property of Ag-Cu nanoparticles. The
leaching of Ag-Cu nanoparticles was investigated by AAS after each run.
Negligible leaching was observed in the reaction solution. The observed
leaching of Ag-Cu nanoparticles in reaction solution was 0.0015wt% of
its total amount. However, leaching was found in washing slovents. The
leaching in the washing slovents may be due to the high rotation speed
(3000 rpm) of centrifuge. After each run, the catalyst was washed by
DMF solvents and seperated by centrifuge for 5 times. The determined
leaching of Ag-Cu nanoparticles for each washing was 0.43wt% of its
total amount. XRD analysis shows that there was no apparent change in
the structure of Ag-Cu/HTs after used for six times. However, TEM
analysis shows that a little part of used Ag-Cu nanoparticle agglomerated.
Therefore, the deactivation of Ag-Cu/HTs may be due to agglomeration
of part Ag-Cu nanoparticle and the loss of Ag-Cu nanoparticle in washing
solvents.
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3.6 Transesterification between various alcohols and different esters
With the suitable reaction conditions established, the scope of
substrates was then investigated (Tables 3 and 4). Very high yields (97%)
of the corresponding products of transesterification were obtained from
ethyl acetoacetate and aromatic benzyl alcohols (Table 3, entry 1). It was
also found that the yield for allylic alcohol was far higher than that for the
corresponding aliphatic alcohols. For example, the yield was 98% at 6 h
of reaction time for cinnamyl alcohol (Table 3, entry 2), while the yield
was 89% at 24 h for 3-phenylpropanol (Table 3, entry 3). For the other
example, the yield for allyl alcohol was 88% (Table 3, entry 6), whereas
the yield was 66% for n-propanol (Table 3, entry 7). The relation between
yield in transesterifcation and the structure of alcohol was in accordance
with that between yield in dehydrogenation and the structure of alcohol in
the presence of Cu catalyst.46 This demonstrated the role of Cu sites in
activating the O-H bond in the alcohol.
Notably, the yield for β-ketoesters was far higher than that for the
corresponding esters without the β-keto substituent (Table 4). For
example, the yield was 97% at 5 h of reaction time for transesterification
between ethyl acetoacetate (β-ketoester) and benzyl alcohols (Table 3,
entry 1), whereas the yield only reached 80% at 24 h for the
corresponding transesterification between ethyl acetate (ester without the
β-keto substituent) and benzyl alcohols (Table 4, entry 1). Ag-Cu has
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vacant orbitals, and β-ketoesters have two carbonyl group. The Ag-Cu
may react with β-ketoesters to form a cyclic intermediate through the
interaction between the vacant orbits and two carbonyl oxygen atoms.9
This interaction may explain the high conversion of β-ketoesters. Based
on the role of Ag-Cu nanoparticles and property of β-ketoesters, the
reaction mechanism involved in the transesterification are proposed and
shown in Scheme 2.
4. Conclusions
Ag-Cu metal site was proved to be active in catalyzing the
transesterification between β-ketoester and various alcohols. The
bimetallic AgCu nanoparticles were more active that monometallic Ag or
Cu nanoparticles. The high activity of bimetallic AgCu nanoparticles may
be ascribed to the interaction between Cu and Ag nanoparticles. The role
of metal sites was ascribed to its activation to both the alcohol and
β-ketoester. This work provided a successful try of applying metal
catalysts to transesterification of β-ketoester and alcohols. Considering
the diversity in metal catalysts, it is potential to develop more efficient
catalysts through tuning the structure and types of metal catalysts. On one
side, this work extended the application of Ag-Cu catalysts; on the other
side, it introduced a new type of catalysts to transesterification.
Acknowledgments
This work was supported by projects funded by the Major Research
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Plan of National Natural Science Foundation of China (Program No.
91545130), the Natural Science Basic Research Plan in Shaanxi Province
of China (Program No. 2013JM2003), and by the Fundamental Research
Funds for the Central Universities (Program No. GK201505002).
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Characterization of the products
Benzyl 3-oxobutanoate (Table 2, entry 1)
1H-NMR (600 MHz, CDCl3): δ=7.28 - 7.43 (m, 5 H, Ph), δ=5.18 (s, 2 H, PhCH2),
δ=3.49 (s, 2 H, COCH2CO), δ=2.25 (s, 3 H, COCH3). 13C-NMR (600 MHz, CDCl3):
δ=30.25, δ=50.06, δ=66.92, δ=128.14, δ=128.65, δ=135.13, δ=166.95, δ=200.11.
Cinnamyl acetate (Table 2, entry 2)
1H-NMR (600 MHz, CDCl3): δ=7.22 - 7.42 (m, 5 H, Ph), δ=6.67 (d, J = 24.0 Hz, H,
PhCH), δ=6.27 (dt, J = 24.0 Hz, 9.6 Hz, H, CHCH2), δ=4.80 (dd, J = 9.6 Hz, 1.8 Hz, 2
H, CH2O), δ=3.49 (s, 2 H, COCH2CO), δ=2.27 (s, 3 H, COCH3). 13C-NMR (600 MHz,
CDCl3): δ=29.69, δ=49.49, δ=65.36, δ=122.36, δ=126.34, δ=127.85, δ=128.33,
δ=134.127, δ=135.78, δ=166.62, δ=200.25.
3-phenylpropyl 3-oxobutanoate (Table 2, entry 3)
1H-NMR (600 MHz, CDCl3): δ=7.13 - 7.29 (m, 5 H, Ph), δ=4.16 (m, 2 H, CH2O),
δ=3.46 (s, 2 H, COCH2CO), δ=2.69 (s, 2 H, PhCH2), δ=2.28 (s, 3 H, COCH3), δ=1.98
13
(m, 2 H, CH2CH2O). C-NMR (600 MHz, CDCl3): δ=30.16, δ=32.40, δ=50.05,
δ=67.15, δ=123.71, δ=128.59, δ=135.30, δ=166.92, δ=200.30.
1-phenyl-2-propanolyl 3-oxobutanoate (Table 2, entry 4)
1H-NMR (600 MHz, CDCl3): δ=7.17 - 7.30 (m, 5 H, Ph), δ=5.17 (s, H, CH), δ=3.41
(s, 2 H, COCH2CO), δ=2.90 (m, 2 H, PhCH2), δ=2.28 (s, 3 H, COCH3), δ=1.25 (m, 3
H, CHCH3). 13C-NMR (600 MHz, CDCl3): δ=21.94, δ=30.16, δ=43.56, δ=50.06,
δ=73.96, δ=128.65, δ=135.30, δ=166.89, δ=200.34.
1-phenylethyl 3-oxobutanoate (Table 2, entry 5)
1H-NMR (600 MHz, CDCl3): δ=7.19 - 7.41 (m, 5 H, Ph), δ=5.92 (dt, J = 10.80 Hz,
2.4 Hz, H, CH), δ=3.44 (s, 2 H, COCH2CO), δ=2.26 (s, 3 H, COCH3), δ=1.47 (s, 3 H,
CHCH3). 13C-NMR (600 MHz, CDCl3): δ=21.91, δ=30.20, δ=50.01, δ=73.92,
δ=128.64, δ=130.21, δ=135.31, δ=166.96, δ=200.32.
(E)-but-2-en-1-yl 3-oxobutanoate (Table 2, entry 6)
1H-NMR (600 MHz, CDCl3): δ=5.89 (m, H, CH3CH), δ=5.79 (m, H, CHCH2), δ=4.57
(s, 2 H, CH2O), δ=3.46 (s, 2 H, COCH2CO), δ=2.27 (s, 3 H, COCH3), δ=1.72 (d, J =
5.4 Hz, 3 H, CH3CH=CH). 13C-NMR (600 MHz, CDCl3): δ=17.16, δ=30.12, δ=49.97,
δ=65.94, δ=118.90, δ=131.53, δ=166.75, δ=200.33.
n-Butylacetoacetate (Table 2, entry 7)
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1H-NMR (600 MHz, CDCl3): δ=4.14 (m, 2 H, CH2O), δ=3.45 (s, 2 H, COCH2CO),
δ=2.27 (s, 3 H, COCH3), δ=1.62 (m, 2 H, CH2CH2O), δ=1.39 (m, 2 H, CH3CH2),
δ=0.94 (t, J = 7.2 Hz, 3 H, CH3CH2). 13C-NMR (600 MHz, CDCl3): δ=13.69, δ=18.80,
δ=30.07, δ=30.54, δ=50.06, δ=65.56, δ=167.18, δ=200.64.
Acetoacetic acid allyl ester (Table 2, entry 8)
1H-NMR (600 MHz, CDCl3): δ=5.93 (m, H, CCH3), δ=5.36 (m, H, trans-CH=CHH),
δ=5.26 (m, H, cis-CH=CHH), δ=4.64 (s, 2 H, CH2O), δ=3.50 (s, 2 H, COCH2CO),
δ=2.28 (s, 3 H, COCH3). 13C-NMR (600 MHz, CDCl3): δ=29.72, δ=49.83, δ=66.09,
δ=118.89, δ=131.37, δ=166.69, δ=200.11.
Propyl 3-oxobutanoate (Table 2, entry 9)
1H-NMR (600 MHz, CDCl3): δ=4.10 (m, 2 H, CH2O), δ=3.46 (s, 2 H, COCH2CO),
δ=2.28 (s, 3 H, COCH3), δ=1.68 (m, 2 H, CH3CH2), δ=0.95 (t, J = 7.8 Hz, 3 H,
CH3CH2). 13C-NMR (600 MHz, CDCl3): δ=10.14, δ=21.82, δ=29.95, δ=50.06,
δ=66.91, δ=166.98, δ=200.64.
2-methylallyl 3-oxobutanoate (Table 2, entry 10)
1H-NMR (600 MHz, CDCl3): δ=4.86 (m, 2 H, CH2=C), δ=4.57 (s, 2 H, CCH2),
δ=3.49 (s, 2 H, COCH2CO), δ=2.29 (s, 3 H, COCH3), δ=1.75 (m, 3 H, CCH3).
13C-NMR (600 MHz, CDCl3): δ=19.87, δ=30.14, δ=50.44, δ=101.74, δ=152.04,
δ=166.77, δ=200.32.
n-Hexyl acetylacetate (Table 2, entry 11)
1H-NMR (600 MHz, CDCl3): δ=4.14 (t, J = 6.6 Hz, 2 H, CH2O), δ=3.45 (s, 2 H,
COCH2CO), δ=2.28 (s, 3 H, COCH3), δ=1.96 (m, 2 H, CH2CH2O), δ=1.66 (m, 2 H,
CH2CH2CH2O), δ=1.34 (m, 2 H, CH2CH2CH2CH2O), δ=1.30 (s, 2 H, CH3CH2),
δ=0.89 (t, J = 6.6 Hz, 3 H, CH3CH2). 13C-NMR (600 MHz, CDCl3): δ=13.98, δ=22.69,
δ=25.41, δ=28.1, δ=29.99, δ=31.39, δ=49.88, δ=65.59, δ=167.23, δ=200.68.
Acetic acid benzyl ester (Table 3, entry 1)
1H-NMR (600 MHz, CDCl3): δ = 7.26-7.39 (m, 5 H, Ph), δ=5.10 (s, 2 H, PhCH2),
δ=2.09 (s, 3 H, COCH3). 13C-NMR (600 MHz, CDCl3): δ=20.99, δ=66.31, δ=128.28,
δ=128.59, δ=136.02, δ=170.84.
Cinnamyl acetate (Table 3, entry 2)
1H-NMR (600 MHz, CDCl3): δ = 7.21 - 7.39 (m, 5 H, Ph), δ=6.65 (d, J = 12.0 Hz, H,
PhCH), δ=6.28 (dt, J = 24.0 Hz, 6 Hz, H, CHCH2), δ=4.71 (dd, J = 6 Hz, 0.6 Hz, 2 H,
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CH2O), δ=2.09 (s, 3 H, COCH3). 13C-NMR (600 MHz, CDCl3): δ=20.99, δ=65.33,
δ=123.23, δ=126.78, δ=128.10, δ=128.63, δ=134.38, δ=136.25, δ=170.76.
Phenylpropyl acetate (Table 3, entry 3)
1H-NMR (600 MHz, CDCl3): δ=7.15 - 7.35 (m, 5 H, Ph), δ=4.08 (s, 2 H, CH2O),
δ=2.68 (m, 2 H, PhCH2), δ=2.04 (s, 3 H, COCH3), δ=1.95 (m, 2 H, CH2CH2O).
13C-NMR (600 MHz, CDCl3): δ=20.98, δ=29.95, δ=32.13, δ=63.68, δ=125.71,
δ=128.66, δ=141.16, δ=171.04.
Acetic acid 2-butenyl ester (Table 3, entry 4)
1H-NMR (600 MHz, CDCl3): δ=5.79 (m, H, CH3CH), δ=5.59 (m, H, CHCH2), δ=4.50
(d, J = 6.60 Hz, 2 H, CH2O), δ=2.05 (s, 3 H, COCH3), δ=1.73 (m, 3 H, CH3CH).
13C-NMR (600 MHz, CDCl3): δ=17.71, δ=20.95, δ=65.34, δ=125.09, δ=131.38,
δ=170.75.
n-Propyl acetate (Table 3, entry 5)
1H-NMR (600 MHz, CDCl3): δ=4.20 (t, J = 7.2 Hz, 2 H, CH2O), δ=2.05 (s, 3 H,
13
COCH3), δ=1.65 (m, 2 H, CH3CH2), δ=0.94 (t, J = 7.8 Hz, 3 H, CH3CH2). C-NMR
(600 MHz, CDCl3): δ=10.37, δ=20.47, δ=21.81, δ=65.84, δ=171.09.
n-Propyl acetate (Table 3, entry 6)
1H-NMR (600 MHz, CDCl3): δ=4.06 (t, J = 6.6 Hz, 2 H, CH2O), δ=2.05 (s, 3 H,
COCH3), δ=1.61 (m, 2 H, CH2CH2O), δ=1.39 (m, 2 H, CH3CH2), δ=0.94 (t, J = 7.2
Hz, 3 H, CH3CH2). 13C-NMR (600 MHz, CDCl3): δ=13.64, δ=19.09, δ=20.76,
δ=30.55, δ=63.98, δ=171.05.
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Table captions
Table 1 Nitrogen physisorption data of HTs and Ag-Cu/HTs.
Table 2 Transesterification over Ag-Cu doped on other supports.a
Table 3 Transesterification between β-ketoester and various alcohols.
Table 4 Transesterification between ethyl acetate and various alcohols.
Figure captions
Fig. 1 XRD patterns of (a) neat HTs, (b) fresh Ag-Cu/HTs, and (c)
Ag-Cu/HTs after used for six times.
Fig. 2 Cu 2p XPS spectra of (a) fresh Cu/HTs, (b) Cu/HTs stored for 40
days, (c) fresh Ag-Cu/HTs, (d) Ag-Cu/HTs stored 40 days and Ag 3d XPS
spectra of (e) fresh Ag/HTs, (f) Ag/HTs stored for 40 days, (g) fresh
Ag-Cu/HTs, (h) Ag-Cu/HTs stored for 40 days.
Fig. 3 The morphological and structural properties of Ag-Cu/HTs. (a)
STEM images of Ag-Cu/HTs catalyst, (b) The mean size graph of Ag-Cu
nanoparticles, (c) HR-TEM images of the selected area, (d) SAED
patterns of the Ag-Cu nanoparticles, (e) STEM image of Ag-Cu/HTs
catalyst; (f) intensity profiles of Ag(M) and Cu(M) components in the
nanoparticle obtained by a EDS line scan analysis(along the red line in (e)
from left to right). (g) STEM images of Ag-Cu/HTs catalyst after using
four times, and (h) STEM images of Ag-Cu/HTs catalyst after using six
times.
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Fig. 4 Catalytic activity of neat HTs and Ag-Cu/HTs on different Mg:Al
molar ratio. Reaction conditions: cinnamyl alcohol (11.0 mmol), ethyl
acetoacetate (11.0 mmol), DMF (1.5 mL ), catalyst (0.06 g), loading
amount (4wt%), 100 °C, reaction time of 24 h.
Fig. 5 Effect of Ag:Cu molar ratio on Ag-Cu/HTs catalysts. Reaction
conditions: cinnamyl alcohol (11.0 mmol), ethyl acetoacetate (11.0
mmol), DMF (1.5 mL ), catalyst (0.06 g), loading amount (4wt%), 100 °C,
reaction time (24 h).
Fig. 6 Effect of reaction conditions of (a) Ag-Cu loading amount,a (b)
amount of Ag-Cu/HTs catalyst,b (c) reaction temperature,c (d) reaction
time,d (e) solvents.e a Reaction conditions: cinnamyl alcohol (11.0 mmol),
ethyl acetoacetate (11.0 mmol), DMF (1.5 mL ), catalyst (0.06 g), 100 °C,
reaction time (24 h). b Reaction conditions: cinnamyl alcohol (11.0 mmol),
ethyl acetoacetate (11.0 mmol), DMF (1.5 mL), 100 °C, reaction time (24
h), loading amount (5wt%). c Reaction conditions: cinnamyl alcohol (11.0
mmol), ethyl acetoacetate (11.0 mmol), DMF (1.5 mL), catalyst (0.05 g),
d
loading amount (5wt%), reaction time of 24 h. Reaction conditions:
cinnamyl alcohol (11.0 mmol), ethyl acetoacetate (11.0 mmol), DMF (1.5
e
mL ), catalyst (0.05 g), loading amount (5wt%), 110 °C. Reaction
conditions: cinnamyl alcohol (11.0 mmol), ethyl acetoacetate (11.0
mmol), solvent (1.5 mL), catalyst (0.05 g), loading amount (5wt%),
110 °C, reaction time of 6 h.
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Fig. 7 The reusability of catalyst for transesterification of cinnamyl
alcohol and ethyl acetoacetate. Reaction conditions: cinnamyl alcohol
(11.0 mmol), ethyl acetoacetate (11.0 mmol), DMF (1.5 mL ), catalyst
(0.05 g), loading amount (5wt%), 110 °C, reaction time of 6 h.
29