Pd-catalyzed ethylene methoxycarbonylation with Bronsted acid ionic liquids as promoter and phase-separable reaction media

Article abstract of DOI:10.1039/c3gc41380b

Bronsted acid ionic liquids (BAILs) were prepared and applied as combined acid promoters and reaction media in Pd-phosphine catalyzed methoxycarbonylation of ethylene to produce methyl propionate. The BAILs served as alternatives to common mineral acids required for the reaction, e.g. methanesulfonic acid or sulfuric acid, resulting in high catalytic activity and selectivity towards methyl propionate. In addition, the BAILs yielded a biphasic system with the product and provided stability to palladium intermediates avoiding the undesirable formation of palladium black after reaction. These special features enabled facile methyl propionate separation and recovery of the ionic liquid catalyst system, thus allowing its re-use up to 15 times without apparent loss of catalytic activity or selectivity.

Full text of DOI:10.1039/c3gc41380b

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Pd-catalyzed ethylene methoxycarbonylation with
Brønsted acid ionic liquids as promoter and
phase-separable reaction media†
Cite this: Green Chem., 2014, 16, 161
Eduardo J. García-Suárez, Santosh G. Khokarale, Olivier N. van Buu,
Rasmus Fehrmann and Anders Riisager*
Brønsted acid ionic liquids (BAILs) were prepared and applied as combined acid promoters and reaction
media in Pd–phosphine catalyzed methoxycarbonylation of ethylene to produce methyl propionate. The
BAILs served as alternatives to common mineral acids required for the reaction, e.g. methanesulfonic acid or
sulfuric acid, resulting in high catalytic activity and selectivity towards methyl propionate. In addition, the
BAILs yielded a biphasic system with the product and provided stability to palladium intermediates avoid-
ing the undesirable formation of palladium black after reaction. These special features enabled facile
methyl propionate separation and recovery of the ionic liquid catalyst system, thus allowing its re-use up
to 15 times without apparent loss of catalytic activity or selectivity.
Received 11th July 2013,
Accepted 12th September 2013
DOI: 10.1039/c3gc41380b
www.rsc.org/greenchem
p-toluenesulfonic acid (TSA) or sulfuric acid (SA)) is needed to
promote the reaction. The main roles of the acid are to pre-
1
. Introduction
6
Alkoxycarbonylation of olefins with carbon monoxide and alco- serve catalytic activity by facilitating protonation of catalytically
+
hols is a versatile and atom-efficient C–C bond forming reac- inactive Pd(0) species into active [Pd(II)–H] species, and to
tion (Scheme 1), which is applied industrially for the stabilize intermediate cationic Pd(II) species formed during the
1
–3
production of commodity alkyl esters and derivatives.
The catalytic cycle by weak coordination to the anions from the
6
a,7
methoxycarbonylation of ethylene (Scheme 1, R = H and R = acid. Major drawbacks of applying such acids are their
3
) to obtain methyl propionate (MP) is of particular interest, manipulation, corrosion of reaction equipment and fast phos-
1
2
CH
due to the importance of MP as an intermediate in the pro- phine alkylation when using monodentate phosphines. Conse-
duction of methyl methacrylate (MMA) as a monomer applied quently, some attempts to avoid these drawbacks have been
4
to make poly-methylmethacrylate (p-MMA) – a transparent made by the employment of alternative acid promoters such
thermoplastic polymer in high demand with many useful as, e.g., polymeric sulfonic acids, borate esters, and aluminium
8
applications such as alternative to glass, medical technologies, triflate, instead of using common mineral acids.
implants, etc.
The use of ionic liquids (ILs) as reaction media in liquid–
The methoxycarbonylation of ethylene is carried out liquid biphasic reactions makes in many cases the processes
efficiently under mild reaction conditions in the presence of greener than when using traditional organic solvents, due to
Pd–phosphine complex catalysts, which afford high catalytic the IL advantages such as low vapor pressure, good thermal
5
activity and product selectivity. Moreover, a rather strong stability, tunable solubility and acidity/coordination pro-
9
Brønsted acid with pK ≤ 4 (e.g. methanesulfonic acid (MSA), perties. Furthermore, ILs can also relatively easily be designed
a
to accommodate functional groups which can provide the ILs
1
0
with auxiliary reactivity like, e.g., Brønsted acidity. In line
with this, Brønsted acid ionic liquids (BAILs) have been used
successfully as alternative to mineral acids in many
1
1
reactions.
Scheme 1 Pd-catalyzed alkoxycarbonylation of olefins.
In this work, we introduce a versatile reaction concept for
Pd–phosphine catalyzed methoxycarbonylation of ethylene to
produce MP, where BAILs function as reaction media as well
as alternative acid promoters to the commonly used strong
acids (Fig. 1). The application of BAILs led to excellent results
in terms of both catalytic activity and selectivity. Furthermore,
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical
University of Denmark, DK-2800 Kgs. Lyngby, Denmark. E-mail: ar@kemi.dtu.dk;
Fax: (+45) 45883136
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc41380b
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The thermal stability of the BAILs was evaluated by thermal
gravimetric analysis (TGA) using a Mettler Toledo (TGA/DSC1
−
1
STARe System) instrument under nitrogen flow (50 mL min
)
(
ESI†). In a typical experiment the BAIL was heated from room
−
1
temperature to 120 °C with a heating rate of 10 °C min . The
sample was dried at this temperature for 2 h in order to elimi-
nate moisture. Then, the sample was heated from 120 °C to
00 °C with a ramp rate of 10 °C min−
1
.
6
The relative acidity of the BAILs (and other applied
Brønsted acids) was evaluated using a Cary 5000 UV-Vis spec-
trophotometer with 4-nitroaniline as the indicator according
Fig. 1 Schematic representation of ethylene methoxycarbonylation with
BAILs as reaction media with MP product separation and Pd–catalyst recycling.
1
4
to reported procedures. In a typical experiment an ethanolic
solution of 4-nitroaniline (0.1 mM) was added to a solution of
the corresponding acid (10 mM) and the mixture was stirred
overnight. Thereafter, the absorbance was measured and com-
pared with the absorbance of a reference 4-nitroaniline solu-
tion. The absorbance difference was correlated to the Brønsted
the employed BAILs provide highly efficient immobilization of
the palladium complex catalyst as well as a good stability of
the catalytic Pd-intermediates. These features avoid the for-
mation of palladium black and enable facile catalyst recovery
and reutilization.
acidity through the Hammett acidity function H
0
= pK(I) +
+
log[IH ]/[I], where pK(I) is the pK
a
value of the indicator
+
referred to an aqueous solution, and [I] and [IH ] are the
molar concentrations of the un-protonated and protonated
forms of the indicator, respectively.
2
. Experimental
2
.1 Materials
Triethylamine (≥99%), pyridine (≥99.8%), 1-methylimidazole
≥99%), methanesulfonic acid (MSA, ≥98%), p-toluenesulfonic
2.3 Methoxycarbonylation of ethylene with BAILs
(
Catalytic experiments were performed in a 50 mL stainless
steel Parr reactor equipped with a pressure transducer (Parr
acid (TSA, ≥98.5%), sulfuric acid (SA, 95–97%), triphenylphos-
phine (≥95%), 1,4-butanesultone (99.97%), methanol
4843). In a typical experiment palladium(II) acetate (11.2 mg,
(≥99.8%), palladium acetate (99.98%), 1,2-bis(di-tert-butylphos-
0.05 mmol, 0.3 mol% Pd), 1,2-bis(di-tert-butylphosphino-
phinomethane)benzene (DTBPMB, ≥98%) and 1-butyl-3-methyl-
methyl)benzene, DTBPMB (98.7 mg, 0.25 mmol, ligand/Pd
molar ratio of 5) and 6 mL of a solution of the corresponding
BAIL in MeOH (32 wt%) were introduced into the reactor
directly or after stirring for 2 h under Ar at 80 °C in order to
pre-activate the catalytic system. Afterwards, the reactor was
imidazolium methanesulfonate ([BMIm][MeSO ]) (≥95%) were
3
purchased from Sigma-Aldrich and used without further puri-
fication. A gas mixture with a molar composition of CO : C H :
2 4
Ar = 2 : 2 : 1 was purchased from AGA and used as received for
the methoxycarbonylation reactions.
2 4
flushed three times with the gas mixture of CO : C H : Ar,
pressurized to 20 bars and heated to 80 °C where the reac-
tion was carried out. The conversion of the reactants was fol-
lowed and correlated to the pressure drop of the reaction
mixture after pre-calibration of the pressure transducer. After
the reaction, the reactor was cooled down, depressurized and
the product was analyzed by GC-FID (Agilent, 6890N, DB-1
capillary column, 50 m × 0.320 mm) to confirm the high purity
of the formed MP.
For the recycling experiments the reactor was re-pressurized
with the gas mixture up to 20 bars after cooling and depressur-
izing, as described above. After every fifth reaction run the MP
phase was removed and fresh 5 mL MeOH was added.
2
.2 Synthesis and characterization of Brønsted acid ionic
liquids (BAILs)
The employed BAILs were synthesized in two reaction steps fol-
lowing a slightly different procedure to that previously
reported in the literature. The first reaction step involved the
synthesis of zwitterions which were subsequently converted
to BAILs by reaction with an equimolar amount of the corres-
ponding acid (Scheme 2, see ESI† for details).
1
2
1
3
1
31
1
H and P{ H} NMR spectra of the synthesized BAILs were
recorded using either a Varian Mercury 300 MHz or a Varian
Unity Inova 500 MHz spectrometer (ESI†).
3
. Results and discussion
3.1 Synthesis and characterization of BAILs
Six different BAILs consisting of different cations bearing an
alkylsulfonated moiety and anions were prepared following a
1
2
12,13
slightly modified reported procedure (Fig. 2). The purity of
the prepared BAILs was confirmed by NMR spectroscopy and
the thermal stability was measured by TGA (see ESI†). In
Scheme 2 Synthesis route of the BAILs.
pyridine, triethylamine or triphenylphosphine; HX = methanesulfonic acid
MSA), p-toluenesulfonic acid (TSA) or sulfuric acid (SA).
3
YR = 1-methylimidazole,
(
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contrast, the non-functionalized IL 1-butyl-3-methylimidazo-
lium methanesulfonate ([BMIm][MeSO ]) revealed an H of
3
0
3
.22 with an Amax value of 1.64, which was very close to the
absorbance measured for the indicator 4-nitroaniline alone
1.65). This showed that the non-functionalized IL possessed
very poor acidity, thus confirming the acidity of the BAILs to
(
3
be correlated to the –SO H group functionalization, as also
expected.
3.2 Methoxycarbonylation of ethylene with BAILs
Fig. 2 Overview of the synthesized BAILs.
The prepared BAILs 1–6 were tested as combined acid promo-
ters and reaction media in the methoxycarbonylation of ethy-
lene for producing MP (32 wt% in methanol). For comparison
the non-functionalized IL [BMIm][MeSO ] was also tested as
3
reaction medium to demonstrate the decisive role of acid
addition, calculation of the Hammett acidity function of
the BAILs was carried out in order to validate their acidic
properties.
The TGA profiles confirmed that the synthesized BAILs
were thermally stable up to their decomposition temperature
of 280–310 °C. No direct relationship was found between the
decomposition temperature and the cation and/or anion com-
position. However, the phosphonium based BAIL [SBPP]-
functionalization of the ILs (i.e. –SO H group) in their success-
ful application in the reaction. Similarly, the mineral acid
3
MeSO H was used as an acid promoter in catalytic amounts
3
instead of the BAILs to benchmark the performance of
the BAILs. In all reactions Pd(OAc) was selected as the catalyst
2
[
p-TsO] (5) proved to be more thermally stable (T
d
= 310 °C)
precursor in combination with the diphosphine ligand
DTBPMB, which has been reported to result in highly selective
than the nitrogen based analogues, as also reported in the
1
5
literature.
5a
and active methoxycarbonylation systems for MP production.
The Hammett method consists of the determination of
acidity functions using UV-Vis spectroscopy, where a basic
The obtained results are compiled in Table 2.
Firstly, methoxycarbonylation of ethylene was performed
3
using MeSO H (5 equivalents) as an acid promoter in the pres-
ence of 1.2 or 5 equivalents of DTBPMB ligand (Table 2,
entries 1 and 2). In both cases very high catalytic activity was
obtained and a conversion of about 99% was achieved after
1
4
indicator is used to trap the dissociative proton. In this work,
-nitroaniline was selected as an indicator and ethanol was
4
used as a solvent since most of the prepared BAILs were
soluble herein. A maximum absorbance (Amax) of 1.65 was
observed at λmax = 370 nm in ethanol for the un-protonated
form of 4-nitroaniline. This absorbance decreased gradually
when the concentration of the BAILs was increased, thus allow-
10 min of reaction with no apparent difference in reactivity
pattern. However, a significant difference in the visual appear-
ance of the post-reaction mixtures was indeed observed, as
depicted in Fig. 3. When only 1.2 equivalents of the ligand
were used a large amount of Pd-black was obviously formed,
thus confirming the ligand amount to be insufficient to stabi-
lize catalytically active palladium species. On the other hand,
0
ing the Hammett acidity function (H ) to be calculated from
the ratio of the measured absorbances of the unprotonated
+
(
[I]) and protonated forms ([IH ]) of 4-nitroaniline (Table 1).
The Hammett acidity functions (H ) of the examined BAILs
and 3–5 (2 and 6 were not soluble enough in ethanol to allow
0
1
formation of Pd-black seemed to be avoided when
5
the determination) were found to be quite similar, in the
range of 2.06–2.12, thus suggesting only a minor influence of
the anion and/or cation backbone structure on acidity. In
Table 2 Methoxycarbonylation of ethylene in the presence of BAILs,
a
3
MeSO H or non-functionalized IL
Table 1 Hammett acidity functions (H
acids
0
) of BAILs and other Brønsted
Conversion
(%)
MP selectivity
(%)
a
Entry
Acid promoter
H0
b
+
b,c
Entry
Material
A
max
[I] (%)
[IH ] (%)
H
0
1
MeSO H
2.06
2.06
2.12
—
2.11
2.06
2.11
—
99.2
98.7
95.3
99.2
99.1
99.2
98.3
98.7
<1
>99
>99
>99
>99
>99
>99
>99
>99
>99
3
b
2
MeSO
[SBMI][HSO
[SBMI][MeSO
3
3
H
1
2
3
4
5
6
7
8
9
—
1.65
1.52
1.51
1.49
1.54
1.53
1.52
1.53
1.64
100.0
92.2
91.5
90.9
93.2
93.0
92.1
93.0
99.4
0.0
7.8
8.5
9.1
6.8
7.0
7.9
7.0
0.6
—
3
4
5
6
7
8
4
] (1)
] (2)
MeSO
SO
3
H
2.06
2.02
1.99
2.12
2.11
2.06
2.11
3.22
H
2
4
[SBMI][p-TsO] (3)
[SBTA][p-TsO] (4)
[SBPP][p-TsO] (5)
[SBP][p-TsO] (6)
p-TsOH
[SBMI][HSO ] (1)
4
[SBMI][p-TsO] (3)
[SBTA][p-TsO] (4)
[SBPP][p-TsO] (5)
d
9
[BMIm][MeSO
3
]
3.22
a
[BMIm][MeSO
3
]
2
Reaction conditions: 0.05 mmol Pd(OAc) (0.3 mol% Pd), DTBPMB : Pd
mol ratio = 5 : 1, 6 mL of a 32 wt% solution of BAIL or IL in methanol,
a
+
b
H
0
= pK[I]aq + log([I]
s
/[IH ]
s
). 4-Nitroaniline and the BAILs were P(CO : C
dissolved in ethanol with 0.1 mM and 10 mM, respectively. Average equivalents of MeSO
absorbance at λ = 370 nm of three measurements.
2
H
4
: Ar = 2 : 2 : 1) = 22 bars, T = 80 °C, t = 20 min. With 5
H (acid : Pd mol ratio = 5 : 1) instead of the BAIL,
b
3
c
d
t = 10 min. With 1.2 equivalents of DTBPMB. t = 120 min.
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Fig. 3 Reaction mixtures after Pd–DTBPMB catalyzed ethylene meth-
3
oxycarbonylation with MeSO H as an acid promoter and DTBPMB : Pd
Fig. 4 Recycling experiments using the BAIL [SBMI][p-TsO] (3) as reac-
tion media and an acid promoter in the methoxycarbonylation of ethy-
ratio = 1.2 : 1 (left) and DTBPMB : Pd ratio = 5 : 1 (right).
lene (reaction conditions: 0.05 mmol Pd(OAc)
DTBPMB : Pd mol ratio = 5 : 1, 6 mL of a 32 wt% solution of BAIL in
methanol, P(CO : C : Ar = 2 : 2 : 1) = 22 bars, T = 80 °C, t = 20 min).
2
(0.3 mol% Pd),
equivalents of the ligand were used and a clear yellow solution
was obtained. Based on these findings, reaction conditions
with 5 equivalents of the diphosphine ligand were selected for
further experiments with the BAILs.
2 4
H
a pivotal influence on the catalytic performance of the system
In the reactions where the BAILs were used as acid promo- under the selected reaction conditions.
ters in place of MeSO H the reaction rates were somewhat
One of the most important issues – and a common chal-
lower, but excellent conversions of about 99% were still lenge in homogeneous catalysis – is the recovery and re-use of
3
1
6
achieved in only 20 min (Table 2, entries 3–8) and – very the catalytic system. In the reaction concept introduced in
importantly – the selectivity to MP remained higher than 99% this study the role of the applied BAILs was not only to act as
(
i.e. ≥98% MP yields). The observed activity difference cannot an acid promoter, but also to provide facile separation of the
be correlated to the Brønsted acidity of the BAILs and MP product by phase-separation and to preserve the catalyst
MeSO
H, which were almost identical. Instead, the lower reac- solvation (see Fig. 1).
tion rates were likely an effect of the lower solubility of the
With this consideration in mind, the recyclability of the
reactant gases in the BAIL–MeOH systems compared to pure catalytic system with [SBMI][p-TSO] (3) was tested as a rep-
3
9
MeOH, as normally observed in biphasic IL reaction systems,
resentative example of all the BAIL systems. The recycling
or the interference of the BAILs with the catalyst system. experiments were carried out under the same reaction con-
However, no influence on the catalytic activity of the cation ditions used in the previous reactions, and the results are
structure and/or anion of the employed BAILs could directly be shown in Fig. 4.
confirmed under the studied reaction conditions.
As shown in Fig. 4, the catalytic system could be re-used
The non-functionalized IL [BMIm][MeSO ] was also tested four times with intermediate pressurizing of the reactor
3
as reaction medium (in the absence of acid promoter) under without any apparent loss of activity. However, after the fifth
comparable reaction conditions. As expected, almost no con- reaction run the activity was somewhat lowered due to
version was achieved after 120 min of reaction (Table 2, entry Pd-black formation (Fig. 5a). We believe that the interaction of
9
). This demonstrates clearly that functionalization of the ILs Pd(OAc) with the reactants – especially with CO which is a
2
with a strong acidic moiety, such as a sulfonic acid group, has known reducing agent for homogeneous catalysts leading to
Fig. 5 (a) Biphasic reaction mixture after five reaction runs using the in situ generated catalyst system (a large amount of Pd-black). (b) Biphasic
reaction mixture after five reaction runs using the preformed catalyst system (no Pd-black). (c) Biphasic reaction mixture after ten reaction runs using
the preformed catalyst system (no Pd-black). (d) Biphasic reaction mixture after fifteen reaction runs using the preformed catalyst system
(
no Pd-black). (e) Solution of the pre-formed catalytic system containing Pd(OAc)
2
, DTBPMB, MeOH and the BAIL (3). (f) The BAIL phase containing
the dissolved palladium catalyst after MP evaporation after the fifteenth reaction run (no Pd-black).
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provided a biphasic system with the MP product affording easy
product separation and catalyst recovery – two features which
are imperative for possible future industrial exploration.
Acknowledgements
The Danish Council for Independent Research – Technology
and Production Sciences (project no. 11-106979) has provided
support for this work.
Fig. 6 Recycling experiments using BAIL (3) (after pre-formation)
as reaction media and acid promoter in the methoxycarbonylation of References
ethylene with selectivity to MP > 99% (reaction conditions: 0.05 mmol
Pd(OAc)
solution of BAIL in methanol, P(CO : C
0 °C, t = 20 min). After reaction cycle 5, 10 and 15 the MP were de-
canted and excess of the organic phase was evaporated and the IL
phase containing the catalyst was recovered.
2
(0.3 mol% Pd), DTBPMB : Pd mol ratio = 5 : 1, 6 mL of a 32 wt%
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H
: Ar = 2 : 2 : 1) = 22 bars, T =
8
(
c) Catalytic Carbonylation Reactions, Topics in Organometal-
1
7
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enable Pd reduction or destabilization and further decompo-
sition yielding Pd-black during the in situ complex formation.
Notably, the reaction solution phase-separated after the fifth
run (when a considerable amount of MP was formed) into an
upper phase containing the MP and a lower phase containing
the catalyst system dissolved in the BAIL, thus confirming the
basis of the process concept to work.
Instead of performing the complex formation in situ in the
presence of the reactants, pre-formation of the catalytic system
2
by stirring the BAIL, Pd(OAc) and DTBPMB ligand in MeOH
under Ar for 2 h at 80 °C proved highly useful to avoid the for-
mation of Pd-black and thus improve the reusability of the
catalyst system. Hence, when the catalytic system was pre-
formed it maintained its excellent performance of >97% MP
yield during fifteen recycle experiments (Fig. 6), and after
every fifth reaction the BAIL-catalyst system was recovered
without the observation of any appreciable Pd-black (Fig. 5b–f)
2
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. Conclusions
4
Efficient and durable Pd–diphosphine catalyst systems were
prepared with BAILs and successfully applied in selective
methoxycarbonylation of ethylene to obtain MP. Excellent
results in terms of conversion and selectivity (>99% MP yield)
were achieved. The application of BAILs allowed re-using the
catalytic systems for fifteen times without any loss of perform-
ance, thus corroborating an efficient immobilization of the
palladium complex catalyst. In addition, the use of BAILs
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