Br?nsted acidic ionic liquid as an efficient and recyclable promoter for hydroesterification of olefins catalyzed by a triphenylphosphine-palladium complex

Article abstract of DOI:10.1016/j.catcom.2010.05.025

Several Br?nsted acidic ionic liquids (BAILs) with different acidic scales were synthesized and employed as acid promoters for the hydroesterification of olefins catalyzed by a triphenylphosphine-palladium complex. The results showed that catalysts with BAIL promoters resulted inmoderate to high conversions to the ester product with excellent selectivity, depending on the acidity of BAIL used. The reaction conversion reached a plateau when the catalyst was promoted using BAIL with a Hammett acidity function less than or equal to-0.11. This catalyst system could be reused several times for the hydroesterification of 1-hexene without a considerable decrease in the catalytic performance.

Full text of DOI:10.1016/j.catcom.2010.05.025

Catalysis Communications 11 (2010) 1200–1204
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Brønsted acidic ionic liquid as an efficient and recyclable promoter for
hydroesterification of olefins catalyzed by a triphenylphosphine–palladium complex
⁎
Jing Yang, Hancheng Zhou, Xinning Lu, Youzhu Yuan
State Key Laboratory of Physical Chemistry of Solid Surfaces and National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several Brønsted acidic ionic liquids (BAILs) with different acidic scales were synthesized and employed as acid
promoters for the hydroesterification of olefins catalyzed by a triphenylphosphine–palladium complex. The
results showed that catalysts with BAIL promoters resulted in moderate to high conversions to the ester product
with excellent selectivity, depending on the acidity of BAIL used. The reaction conversion reached a plateau when
the catalyst was promoted using BAIL with a Hammett acidity function less than or equal to −0.11. This catalyst
system could be reused several times for the hydroesterification of 1-hexene without a considerable decrease in
the catalytic performance.
Received 26 March 2010
Received in revised form 23 May 2010
Accepted 31 May 2010
Available online 1 July 2010
Keywords:
Hydroesterification
Triphenylphosphine–palladium complex
Brønsted acidic ionic liquid
Acidity
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Room temperature ionic liquid (IL), regarded as green solvent
and catalyst, has gained wide attention due to its adjustable physical
The hydroesterification of olefins using CO and alcohols, known as
a Reppe carbonylation, is an emerging technology for producing
industrially valuable carboxylic acid esters [1–3]. Of the catalytic
systems investigated, phosphine–palladium complexes [e.g., triphe-
nylphosphine–palladium complex (PPh3–Pd)] promoted with a
strong protic acid have received particular attention [2]. A conven-
tional Brønsted protic acid can serve as a hydride source for the
formation of catalytic active species, considerably accelerating the
reaction [4,5]. Several studies have reported the use of other acidic
promoters for the reaction. Ooka et al. [6] used acidic resin with a
SO3H group as a promoter for the hydroesterification of styrene and
vinyl acetate. Ferreira et al. [7] used salicylborates, a type of Lewis
acid, to promote the hydroesterification of ethylene. Williams et al. [8]
also found that some Lewis acids like Al(OTf)3 (OTf=triflate) could
effectively promote the Pd complex-catalyzed hydroesterification of
styrene and 1-pentene. Recently, we have synthesized several
Lewis acids with methanesulfonic (CH3SO−3 ) or para-toluenesulfonic
(p-TsO−) anion for the hydroesterification of styrene catalyzed by a
PPh3–Pd complex and discussed the relationship between the acidity
of the Lewis acid and the promoting effect [9]. The results indicated
that the use of a Lewis acid with an apparent pH similar to p-TsOH
could bring about marked enhancement in the reaction rate and
considerable depression in PPh3 consumption.
and chemical properties, which make them advantageous for
extensive applications in catalytic and non-catalytic reactions.
Several reports have found improvements in performance and
catalyst–product separation when ILs were used as the reaction
media for the hydroesterification of olefins. Rangits and Kollár
[10,11] studied a palladium complex catalyst for the hydroethox-
ycarbonylation of styrene in imidazolium-based IL with [PF6]− and
[BF4]− counterions. Klingshirn et al. [12] reported a homogeneous
system using imidazolium- and pyridinium-type ILs for the hydro-
esterification of styrene derivatives catalyzed by (PPh3)2PdCl2. The
IL/catalyst phase could be recycled several times. Balázs et al. [13]
also used molten salts as reaction media for the hydroesterification
of olefin with a Pd-based catalyst for the purpose of improving
product isolation. However, most of these catalyst systems require
conventional promoters, such as p-TsOH and CuCl2, to enhance their
activities.
The functions of ILs vary considerably depending on their
structural properties. One of the recent trends in the field of IL
chemistry is to design task-specific ILs for catalytic applications [14].
For instance, Brønsted acid ILs (BAILs) are considered to have the
dual function of being a solvent and a catalyst, making them useful
for several classical reactions [15,16]. In fact, different combinations
of organic cations with suitable anions can adjust the acidity and
recyclability of BAILs. In this communication, we report on the
promoting effect of several BAILs (Scheme 1) with different acidi-
ties for the hydroesterificaton of olefins catalyzed by a PPh3–Pd
complex.
⁎
Corresponding author. Tel.: +86 592 2181659; fax: +86 592 2183047.
E-mail address: yzyuan@xmu.edu.cn (Y. Yuan).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.05.025

J. Yang et al. / Catalysis Communications 11 (2010) 1200–1204
1201
Scheme 1. Structures of synthesized BAILs.
2. Experimental
down and slowly degassed. The unreacted olefin, n-dodecane, and
products were extracted by cyclohexane and separated from the layer
of catalyst–BAIL–methanol. The cyclohexane phase was analyzed
using a gas chromatograph-flame ionization detector (GC-FID)
equipped with a HP-5 column (30 m×0.25 mm×0.25 μm). The
reaction conversion was calculated with a corrected area normaliza-
tion method. The Pd content in the cyclohexane phase was
determined by inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) on a Thermo Elemental IRIS Intrepid II XSP.
2.1. Chemicals
1-Methylimidazole was freshly distilled twice just before use.
Styrene was passed through a column filled with neutral Al2O3 to
remove the inhibitor. Alcohols were dried according to standard
procedure. All other chemicals were commercially available and used
as received.
2.2. General preparation of BAILs
3. Results and discussion
The BAILs were synthesized according to the procedures described
in literature (Scheme 1) [17,18]. Their structures were confirmed by
proton and carbon nuclear magnetic resonance (1H NMR and 13C
NMR) spectroscopy using a Brüker AV-400 spectrometer.
3.1. Thermal stability and acidity of BAILs
The single decomposition peak in the TGA profiles (not shown)
implied that the synthesized BAILs were not mixed compounds,
which coincided with the NMR data. The thermal stability of these
BAILs was characterized by the thermal decomposition temperature
(Td), which ranged from 218 °C to 310 °C. The Td value was related to
the cation and anion structures of the BAILs, and the structure of the
anion of BAIL had a stronger impact on the thermal stability than the
cation. Specifically, [MIMPs][H2PO4] and [MIMPs][BF4] displayed the
highest and lowest Td, respectively. The BAILs with the TsO− anion
possessed a similar Td value regardless of the cation structure.
The Hammett method is an effective method of evaluating the
acidity of chemicals. The representative UV-vis spectra of H0 values
are shown in Fig. 1. The maximal absorbance of the unprotonated
form of NPA in ethanol appeared at 403 nm (curve a in Fig. 1.). Thus,
the [I]/[IH+] ratio was determined by measuring the absorbance
differences of the indicator before and after adding BAIL (curves b and
c in Fig. 1.). In this way, the H0 values of several BAILs from Eq. (1) are
incorporated into Table 1. The [MIMPs][HSO4] and [MIMPs][H2PO4]
BAILs have H0 values of −0.26 and −0.01, respectively, representing
the strongest and weakest acidity of the BAILs with SO3H group in the
cation structure.
The acidity of BAILs without SO3H group in the cation structure like
[MIM][TsO] was so weak that hardly any protonation of the indicator
was observed. But its H0 value may be possibly determined if a
stronger base indicator was employed [17]. Moreover, the H0 value of
[PyPs][TsO] can be measured at higher temperature [16]. For
conciseness, we then used another simple and convenient method,
so-called potentiometric titration analysis, to compare the relative
acidity strength of these synthesized BAILs. The changes of potential
in the process of titration of the synthesized BAILs are shown in Fig. 2.
The lowest apparent pH value was given by [MIMPs][HSO4] BAIL,
2.3. Acidity measurement
Measurement of the acidic scale of BAILs was conducted using a
Cary 5000 UV-visible spectrophotometer with a basic indicator
according to previously reported procedures [19,20]. The 2-nitrophe-
nylamine (NPA) indicator and BAILs were dissolved in ethanol at
concentrations of 0.43 and 72 mM, respectively. The Hammett acidity
function (H0) can effectively express the relative acidity strength of
BAILs. The H0 value can be calculated using Eq. (1):
ꢀ
h
iꢁ
H0 = pKðIÞ + log ½Iꢀ = IHþ
ð1Þ
where pK(I) is the pKa value of the indicator referred to an aqueous
solution, and [I] and [IH+] are the molar concentrations of the
unprotonated and protonated forms of the indicator in a solvent,
respectively. The acidity of BAILs was also determined through
potentiometric titration analysis using an operating procedure
described elsewhere [9].
2.4. Hydroesterification
Calculated amounts of Pd(OAc)2, PPh3, olefin, BAIL, alcohol, and n-
dodecane (internal standard) were charged into a 60 mL stainless
steel autoclave with a magnetic stirrer. The reactor was purged with
CO three times and heated up to the appropriate temperature. Then,
CO was added to adjust the pressure, and the reaction started with a
stirring speed of 700 rpm. After the reaction, the reactor was cooled

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J. Yang et al. / Catalysis Communications 11 (2010) 1200–1204
Fig. 2. Titration curves of different BAILs (0.015 M) by 0.1 M n-butylamine in ethanol.
(1) [MIMPs][HSO4], (2) [MIMPs][BF4], (3) [PyPs][TsO], (4) [MIMPs][TsO], (5) [MIMPs]
[H2PO4], (6) [MIM][TsO].
Fig. 1. NPA absorption spectra before and after the addition of different BAILs in ethanol.
(a) blank, (b) [MIMPs][H2PO4], (c) [MIMPs][HSO4].
while the highest one was obtained with [MIM][TsO]. The presence of
a SO3H group in the cation of BAIL might have resulted in higher
acidity. Specifically, the acidity of [MIMPs][TsO] was slightly weaker
than that of [PyPs][TsO]. This phenomenon could be attributed to the
electronic effect of the methyl group attached to N atom of the
imidazole ring.
Therefore, both the results of the Hammett method and potenti-
ometric titration analysis indicated that the anion and cation
structures had decisive influences on the acidity scale of BAILs. The
acidity order of the synthesized BAILs was as follows: [MIMPs]
[HSO4]N[MIMPs][BF4] N [PyPs][TsO] N [MIMPs][TsO] N [1,2-DiMIMPs]
[TsO] N [MIMPs][H2PO4] N [MIM][TsO].
of fact, the present catalyst system containing BAIL was immiscible
with the olefin substrate and ester product. However, due to the
presence of considerable amount of methanol, the reaction system
became not truly biphasic. Therefore, several factors that would
influence the reaction rate could not be excluded, such as the acidity
of BAIL promoter, the immiscibility of ester product with the catalyst
system, and the CO solubility in the catalyst system. In other words,
the correlation between the reaction rates and the acidic properties of
the ILs can be counter balanced by the solubility properties of the IL-
containing systems. In particular, we presumed that the solubility of
CO in the IL-containing system might be important for the reaction
rate since the IL with same [BMIM] cation but containing [BF4] anion
would possess the poorest solubility to CO [12,22]. Nevertheless, more
detailed studies are required to clarify the weaker promoting effect of
[MIMPs][BF4]. The correlation between the Hammett acidity function
(H0) and the ester yield of styrene hydroesterification is depicted in
Fig. 3. As can be seen, the ester yield increased with a decrease of the
H0 value and reached a plateau when the H0 value was less than or
equal to −0.11.
3.2. Promoting effect of BAILs
The promoting effect of BAILs for the hydroesterification of styrene
is summarized in Table 1. The reaction hardly occurred in the
presence of [BMIM][BF4], while the reaction conversion was enhanced
to 26.3–97.1% when BAILs with a SO3H group in the cation were used
as promoters. The catalytic activity of styrene hydroesterification was
almost in agreement with the acidity order of the BAILs. As an
exception, [MIMPs][BF4] showed a relatively weaker promoting effect
than [MIMPs][HSO4], although its acidity was only a little smaller than
the latter. We noticed that most of BAILs synthesized were miscible
with polar organic solvents such as methanol, acetonitrile, and acetic
acid but insoluble or partly soluble in non-polar organic solvents such
as benzene, ethyl ether, and n-butyl acetate. The solubility of ILs in
reaction mixture can be measured according to the procedure
described in literature using UV-visible spectroscopy [21]. As a matter
The [MIMPs][TsO] BAIL with a H0 value of −0.11 was selected for
the following experiments. Table 2 lists the hydroesterification of
other olefins catalyzed by PPh3–Pd in the presence of [MIMPs][TsO].
When the carbon atoms of the alcohol molecule increased, the
reaction conversion and product l/b ratio decreased, except in the case
of n-butanol (Entries 1–4). A higher ester product yield was also
Table 1
Hydroesterification of styrene catalyzed by PPh3–Pd complex in the presence of several
BAILsa.
Entry BAIL
Amax [I] (%) H0
Conv. (%) Selec. (%)b l/b
c
1
2
3
4
5
6
7
8
[BMIM][BF4]
N.D.
0.88
1.05
N.D.
0.90
1.16
N.D.
N.D.
46
55
N.D.
48
61
N.D.
Trace
–
–
[MIMPs][HSO4]
[MIMPs][TsO]
[MIM][TsO]
[MIMPs][BF4]
[MIMPs][H2PO4]
[PyPs][TsO]
−0.26 97.1
−0.11 93.3
N.D.
−0.23 75.9
−0.01 26.3
N.D.
98.2
99.1
100
100
100
98.8
99.2
2.21
1.91
2.17
2.49
1.27
1.96
1.60
4.6
N.D.
58
95.9
[1,2-DiMIMPs][TsO] 1.11
−0.06 76.6
a
Pd/PPh3/styrene=1/4/480 (molar ratio), Pd=0.0183 mmol, BAIL=0.92 mmol,
alkene=8.784 mmol, CH3OH=2.0 mL, P(CO)=2.0 MPa, T=343 K, t=100 min.
b
In some cases, there existed minor products due to the addition reaction of
Fig. 3. Correlation between Hammett acidity function (H0) and ester yield in the
hydroesterification of styrene. The reaction conditions are the same as those presented
in Table 1.
methanol with alkene.
c
N.D., not determined or without any protonation under present conditions.

J. Yang et al. / Catalysis Communications 11 (2010) 1200–1204
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Table 2
Results of hydroesterification for different alcohols and olefins catalyzed by PPh3–Pd in
the presence of [MIMPs][TsO]a.
Entry
Olefin
Alcohol
Conv. (%)
Selec. (%)
l/b
1
2
3
4
5
6
7
Styrene
Styrene
Styrene
Styrene
1-Hexene
1-Dodecene
cis-Cyclooctene
Ethanol
75.2
73.4
34.5
70.4
82.6
15.6
9.4
99.5
100
100
100
94.7
1.58
1.50
2.60
1.46
2.16
3.05
n-Propanol
i-Propanol
n-Butanol
Methanol
Methanol
Methanol
b
100
100
c
/
a
The reaction conditions are the same as those presented in Table 1. The products
were analyzed by GC-MS.
b
Other products were ascribed to the hydroesterification of 3-hexene.
Only methyl cyclooctenecarboxylate was produced.
c
obtained for the hydroesterification of 1-hexene (Entry 5). However,
the hydoesterification of 1-dodecene and cis-cyclooctene produced
lower ester yields under identical conditions (Entries 6 and 7). The l/b
ratio of the esters increased with an increase of the carbon chain
length, being essentially dependent on the steric hindrance of alkene.
As expected, the hydroesterification of cis-cyclooctene produced only
methyl cyclooctenecarboxylate.
3.3. Reusability of [MIMPs][TsO]
The organic reactant was readily separated by the extraction of
cyclohexane and the Pd catalyst retained in the IL–methanol layer.
However, the leaching of free PPh3 and O=PPh3 may occur in much
higher level than that of palladium complex during the process of
extraction [11,12]. Also, the ligand PPh3 would inevitably undergo
alkylations with alcohols and reactions with BAILs under the
hydroesterification conditions [8]. Thus, for each recycle run, a
quantitative amount of fresh PPh3 was added into the system to
render the Pd catalyst stable and complement the consumptions. The
recyclability of the [MIMPs][TsO]–containing catalyst system for the
hydroesterifications of styrene and 1-hexene are shown in Fig. 4. The
conversion of styrene decreased by about 20% after the 2nd recycle,
even if fresh PPh3 was added to the reaction (Fig. 4a). However, the
performance of such a catalyst system for the hydroesterification of 1-
hexene did not display considerable deterioration after several
recycling procedures when PPh3 was resupplied (Fig. 4b). The IL–
catalyst solution all maintained yellow in colour throughout the
recycling, suggesting that the catalyst might be structurally stable.
To investigate the factors that affected the catalyst recyclability,
we determined the coefficient partitions (K) of PPh3–Pd complex in
organic phase (OP) and IL–methanol phase (IP) for the styrene and 1-
hexene systems by UV-vis measurements (not shown), respectively.
The K(OP/IP) value for the 1-hexene system was only 0.36 and that was
3.8 for the styrene one. Moreover, the Pd content in the cyclohexane
extraction phase was analyzed by ICP-AES (Table 3). A higher Pd loss
occurred in the case of styrene hydroesterification in comparison to 1-
hexene, which may be related to the different solubilities and
coordinative abilities of the reactants with the Pd species. Since the
Pd species can easily be coordinated by the π electrons of an olefinic
group, a lower conversion is expected to result in a heavier Pd
leaching during the catalyst recycles. Indeed, the results showed that
the leaching of Pd decreased in the case of higher conversion
(Table 3), indicating that the π bond of the olefinic group might
have more chelatability than that of other groups.
Fig. 4. Recycling of
a
[MIMPs][TsO]-containing PPh3–Pd catalyst system for the
hydroesterification of (a) styrene and (b) 1-hexene. Reaction conditions: fresh PPh3
was added in each recycle with PPh3/alkene=1/120 (molar ratio), others are the same
as those presented in Table 1. No fresh PPh3 was added in this time of reuse and
abundant Pd black was formed.
⁎
significant promoting effect on the hydroesterification of olefins. The
ester yield attained a plateau when the catalyst was promoted with
BAIL that had a H0 value less than or equal to −0.11. The catalyst
system containing [MIMPs][TsO] BAIL, having a H0 value of −0.11,
could be reused several times without considerable loss of activity for
the hydroesterification of 1-hexene.
Acknowledgements
The authors wish to acknowledge the financial support provided by
the National Natural Science Foundation of China (20873108 and
20923004), National Basic Research Program of China (2009CB939804),
and Key Scientific Project of Fujian Province (2009HZ0002-1).
Table 3
ICP-AES results of the Pd content in the cyclohexane-extracted phasea.
Substrate
Pd content in the cyclohexane-extracted phase (×10−4 g/mL)
1st run
2nd run
3rd run
4th run
5th run
6th run
1-Hexene
0.12
0.95
3.22
0.51
1.43
1.90
0.18
1.26
1.40
0.35
/
/
0.30
/
/
0.06
/
/
4. Conclusion
b
Styrene
c
Styrene
Several BAILs with different acidic scales were synthesized. Their
acidity was influenced by the structures of both the anion and the
cation. In particular, the presence of a SO3H group in the cation of BAIL
might be responsible for a higher acidity, which brought about a
a
Reaction conditions are the same as those presented in Fig. 4.
Styrene conversion was higher than 70% in each run.
Styrene conversion was controlled to be less than 30% in each run by shorten the
b
c
reaction time.

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J. Yang et al. / Catalysis Communications 11 (2010) 1200–1204
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