Zwitterion enhanced performance in palladium-phosphine catalyzed ethylene methoxycarbonylation

Article abstract of DOI:10.1016/j.catcom.2013.09.005

Zwitterions were used for the first time as promoters in ethylene methoxycarbonylation for the production of methyl propionate. They were found to improve the catalytic performance of the Pd-phosphine system. The presence of zwitterions could contribute to stabilize transition states and active catalytic Pd intermediates. The beneficial effect of the zwitterions was found to be most pronounced, when low amount of a strong acid (MeSO₃H) was used with respect to palladium (below 2 equiv.). Under these conditions, phosphine ligand alkylation and reaction vessel corrosion are also anticipated to be less severe.

Full text of DOI:10.1016/j.catcom.2013.09.005

Catalysis Communications 44 (2014) 73–75
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Zwitterion enhanced performance in palladium–phosphine catalyzed
ethylene methoxycarbonylation
S.G. Khokarale, E.J. García-Suárez, J. Xiong, U.V. Mentzel, R. Fehrmann, A. Riisager ⁎
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 April 2013
Received in revised form 16 August 2013
Accepted 3 September 2013
Available online 11 September 2013
Zwitterions were used for the first time as promoters in ethylene methoxycarbonylation for the production of
methyl propionate. They were found to improve the catalytic performance of the Pd–phosphine system. The
presence of zwitterions could contribute to stabilize transition states and active catalytic Pd intermediates. The
beneficial effect of the zwitterions was found to be most pronounced, when low amount of a strong acid
(
MeSO
3
H) was used with respect to palladium (below 2 equiv.). Under these conditions, phosphine ligand
alkylation and reaction vessel corrosion are also anticipated to be less severe.
Keywords:
Ethylene
©
2013 Elsevier B.V. All rights reserved.
Methoxycarbonylation
Methyl propionate
Palladium
Triphenylphosphine
Zwitterions
1
. Introduction
issue in the production of MP is to reduce the overall reaction cost. This
issue could be addressed by replacing the DTBPMB ligand with the
cheaper and readily available triphenylphosphine (TPP). Furthermore, a
challenge in the industrial production of MP is the use of large amounts
of strong acids. The acid can affect the process negatively due to corrosion
of metal containers and protonation of the monophosphine which facili-
tate alkylation [1a], thus decreasing the stability of the catalytic system
and consequently its activity.
Zwitterions (ZIs) are neutral, polar molecules which are often consid-
ered to be viable feedstocks for use in ionic liquid synthesis. Zwitterions
have been employed successfully in different fields from electrochemistry
to catalysis [10]. In addition, they can act as weak coordinating anions able
to interact with transition metal complexes [11]. In this work, we present
highly active, stable and selective catalyst systems for making MP by
ethylene methoxycarbonylation comprising a Pd–TPP complex and
four different ZIs in the presence of small amounts of acids. The small
amount of acid makes the system less prone for corrosion and phosphine
alkylation, thus affording MP in high yield and selectivity with the appli-
cation of TPP.
The formation of new carbon–carbon bonds by the carbonylation of
unsatured substrates is of increasing interest [1]. Alkoxycarbonylation of
olefins has been a well-known reaction for the production of commodity
chemicals since the early 1990s [2]. Among the carbonylation reactions,
the ethylene methoxycarbonylation (Scheme 1) to obtain methyl propi-
onate (MP) is especially interesting due to the importance of MP as inter-
mediate in the production of methyl methacrylate (MMA). MMA is a
monomer for the production of poly-methylmethacrylate (PMMA) that
is a highly demanded transparent thermoplastic with many useful appli-
cations. The ethylene methoxycarbonylation is efficiently carried out in
methanol at moderate temperature in the presence of a catalyst system
formed by a palladium salt, monodentate or bidentate phosphine and a
strong acid [3]. The initial perception that monodentate phosphines
were the best ligands for making MP was changed, when Shell reported
that bidentate ligands with a 1,3-propanediyl bridge gave high selectivity
and very fast reaction to MP [4–6]. So far, the best catalytic system for MP
synthesis is developed by Lucite [4] (now Mitsubishi Rayon) and includes
the rather expensive 1,2-bis(di-tert-butylphosphinomethyl)benzene
(
DTBPMB) ligand.
Commonly, the methoxycarbonylation reaction requires a protic
acid with pK equal or lower than 4 such as, e.g. methanesulfonic acid
MSA), p-toluenesulfonic acid (p-TSA) or sulfuric acid (SA) to achieve
2. Experimental
a
In a typical catalytic reaction, methanol (0.15 mol) was added into a
50 mL autoclave (Parr 4843) followed by the corresponding amounts of
ZI and 1,4-butanesultone or MSA, palladium acetate (0.05 mmol) and
TPP (0.5 mmol). Afterwards, the reactor was flushed three times with
the reactant gas mixture of CO/ethylene/Ar and pressurized to 20 bar.
The reaction was started by heating the autoclave from RT to 80 or
100 °C. After the desired reaction time the autoclave was rapidly cooled,
(
high reaction rates [7]. The main role of the acid is to form and to prevent
the deactivation of the active Pd(II) catalytic species [8,9]. An important
⁎
Corresponding author. Fax: +45 45883136.
E-mail address: ar@kemi.dtu.dk (A. Riisager).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.09.005

74
S.G. Khokarale et al. / Catalysis Communications 44 (2014) 73–75
Table 1
a
Methoxycarbonylation of ethylene in the presence of TPP and SBTA zwitterion.
Entry SBTA:Pd Sultone:Pd MSA:Pd Reaction time TPP:Pd Conversion
ratio
ratio
ratio
(min)
ratio
(%)
Scheme 1. Ethylene methoxycarbonylation.
b
b
b
b
b
b
b
c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
60
60
60
20
10
5
0
5
0
5
5
5
5
0
5
5
5
5
5
5
5
5
5
0
0
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
30
30
30
30
30
30
30
90
90
90
90
90
90
150
90
90
90
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
2
98
98
94
82
69
65
b1
84
73
59
53
34
98
74
65
b1
0
60
60
40
20
10
0
60
60
60
60
c
c
0
1
2
3
4
5
6
7
c
c
c
c
c
c
20
0
c
a
Reaction conditions: MeOH (6 mL), CO:ethylene:Ar gas mixture (2:2:1) (20 bar),
b
c
2
Pd(OAc) (0.05 mmol); 100 °C; 80 °C.
Fig. 1. The zwitterions employed in the ethylene methoxycarbonylation reaction.
When the ZIs were added to the catalytic system both stability and
activity were increased significantly. The catalytic reactions were firstly
carried out at 100 °C for 30 min with different molar ratios of the SBTA
ZI (Table 1, entries 1–7). Without added ZI 65% conversion to MP was
achieved, while the conversion reached 98% when the SBTA was
added. For comparison, only 2% conversion was achieved in reference
reactions with SBTA without 1,4-butanesultone addition. The effect
of the ZI amount on the catalytic activity was also studied by
employing different amounts of SBTA. As shown in Table 1, high con-
version to MP of 98 and 94% was obtained with 60 and 20 equiv. of
SBTA, respectively, while conversion decreased further to 82 and
depressurized and the products analyzed by GC-FID (DB-1 column,
0 m × 0.320 mm) using toluene (0.7 mL) as internal standard. The se-
lectivity was in all reactions ≥99% to MP thus allowing the conversion
%) to MP to be calculated as (nMP/nsubstrate)∙100, where nMP is the prod-
uct amount measured by the GC analysis and nsubstrate is the initial
amount of CO or ethylene in the gas mixture assuming ideal gas behavior.
5
(
6
9%, respectively, when using 10 or 5 equiv. of ZI.
After obtaining excellent results with the ZI at 100 °C and 30 min re-
3
. Results and discussion
action time, the reaction was examined at a lower reaction temperature
of 80 °C, which may contribute to save both energy and limit phosphine
alkylation (Table 1, entries 8–17). At 80 °C up to 84% conversion to MP
were attained after 90 min reaction and 98% after 150 min by
employing 60 equiv. of the same ZI. In comparison, only 34% was
obtained in the absence of SBTA after 90 min, thus clearly demonstrating
the beneficial effect of the ZI on the catalytic performance also at 80 °C.
When lower amounts of SBTA corresponding to 40, 20 or 10 equiv.
were employed, the conversion was reduced to 73, 59 and 53%, respec-
tively. Under these conditions the influence of the TPP ligand ratio was
also studied as can be seen in Fig. 2. The optimal TPP:Pd molar ratio
was found to be 10. With a ratio lower than 10 formation of Pd black
Four different ZIs were prepared using slightly modified reported
procedures (Fig. 1) [12], and the purity confirmed by NMR spectros-
copy and TG/DSC analysis (supporting information). The TG profiles
showed a major weight loss due to ZI decomposition in the temper-
ature range 310–370 °C with a stability order corresponding to
SBPP N SBMI N SBP N SBTA. Two endothermic peaks for the ZIs SBP,
SBIM and SBPP were observed in the DSC profiles; the first sharp
peaks are associated to the melting points while the other peaks re-
late to the decomposition processes of the ZIs. In the case of SBTA,
only one peak was observed related to the decomposition process,
meaning that SBTA decompose immediately after melting.
The prepared ZIs were employed in the ethylene methoxy-
carbonylation to improve the catalytic performance of a system com-
prising palladium acetate, TPP and MSA or 1,4-butanesultone. From
our knowledge this is the first time that 1,4-butanesultone has been
employed as acid promoter in methoxycarbonylation. The conversion
to MP obtained using 1,4-butanesultone as acid promoter was lower
than what was found using the traditional strong acids MSA or SA. We
100
8
6
0
0
speculate that the lower activity is attributed to the lower acidity (pK
a
of 1.7) of the corresponding acid formed, when the 1,4-butanesultone
ring is opened by MeOH under the reaction conditions (Scheme 2),
than MSA and SA having pK
a
values of −1.9 and −3, respectively.
40
2
0
0
0
5
10
15
20
PPh :Pd ratio
3
Scheme 2. 1,4-Butanesultone ring opening by methanol.
3
Fig. 2. Influence of the PPh amounts.

S.G. Khokarale et al. / Catalysis Communications 44 (2014) 73–75
75
Table 2
Table 3
a
a
Methoxycarbonylation of ethylene in the presence of TPP and SBP, SBPP or SBIM zwitterion.
Methoxycarbonylation of ethylene in the presence of TPP and SBTA zwitterion.
Entry
ZI:Pd ratio
Temperature (°C)
Sultone:Pd ratio
Conversion (%)
Entry
SBTA:Pd ratio
MSA:Pd ratio
Reaction time (min)
Conversion (%)
1
2
3
4
5
6
7
8
9
SBP (60)
SBP (40)
SBP (20)
SBPP (60)
SBPP (40)
SBPP (20)
SBMI (60)
SBMI (40)
SBMI (20)
SBP (60)
80
80
80
80
80
80
80
80
80
80
80
80
5
5
5
5
5
5
5
5
5
0
0
0
83
70
50
63
63
62
66
65
49
2
1
2
3
4
5
6
7
8
0
60
0
40
0
5
40
60
5
5
2
2
1
1
1
1
30
30
30
30
90
90
90
90
98
98
97
96
43
42
55
60
a
Reaction conditions: MeOH (6 mL), CO:ethylene:Ar gas mixture (2:2:1) (20 bar),
Pd(OAc) (0.05 mmol); temperature 80 °C; TPP (0.5 mmol).
1
1
1
0
1
2
2
SBPP (60)
SBMI (60)
3
b1
a
Reaction conditions: MeOH (6 mL), CO:ethylene:Ar gas mixture (2:2:1) (20 bar),
Pd(OAc) (0.05 mmol); TPP (0.5 mmol); reaction time 90 min.
2
4. Conclusions
Zwitterions (ZIs) were found to enhance the catalytic performance
of a Pd–TPP complex catalyst system in ethylene methoxycarbonylation
yielding 98% of MP at 100 °C in 30 min in the presence of 1,4-
butanesultone or MSA. Both 1,4-butanesultone and ZI were used for
the first time as acid promoter and stabilizer, respectively. The use of
ZI diminished considerably the amount of strong acid (MSA or others)
required for the catalytic reaction, facilitated use of the cheap and
ready available TPP ligand while lowering possible corrosion issues in
up-scaled applications. Further investigations are currently being
carried out in order to understand the role of both ZIs and 1,4-
butanesultone in methoxycarbonylation in more detail.
occurred towards the end of the reaction, i.e. the Pd–TPP complex catalyst
was not stable with low concentration of ligand. At higher TPP concentra-
tion (TPP:Pd = 20) the rate of the reaction was slowed down due to
competitive coordination of TPP with the reactants CO and ethylene.
Table 2 shows the catalytic results obtained for the remaining three
prepared ZIs (i.e. SBP, SBPP and SBMI), when applied at 80 °C in different
ratios corresponding to 20, 40 or 60 equiv. during 90 min reaction
(
Table 2, entries 1–9). When 60 equiv. of the ZIs were applied the best
results were obtained yielding conversions to MP of SBP (83%), SBMI
66%) and SBPP (62%), respectively. At lower ZI ratios the conversion
(
decreased for all three ZIs, however not in a gradual manner as found
for SBTA. A likely explanation for this difference could be related to the
solubility of the ZIs in MeOH, which was found to follow the same gener-
al order as the activity, namely SBPP b SBMI b SBP ~ SBTA. Hence, it sug-
gests that the obtainable promoting effect of the individual ZIs to a large
extent is determined by their solubility and less by their structural
difference.
Acknowledgments
The Danish Council for Independent Research - Technology and
Production Sciences (project no. 11–106979) has supported the work.
References
Since MSA is the acid the Mitsubishi Rayon company uses in the in-
dustrial production of MP, catalytic test were also performed with this
acid in the presence of the SBTA ZI at 80 °C using TPP as ligand
[
1] P.W.N.M. van Leeuwen, Homogeneous Catalysis - Understanding the Art, ed, Kluwer
Academic Publishers, Dordrecht, The Netherlands, 2004;
Catalytic carbonylation reactions, in: M. Beller (Ed.), Topics in Organometallic
Chemistry, Springer, Berlin, 2006.
(Table 3, entries 1–8). No promoting effect on the catalytic activity
[
2] E. Drent, P.H.M. Budzelaar, Chem. Rev. 96 (1996) 663;
was measured of the ZI when 5 equiv. of MSA were employed (98%
MP formed). Assuming that the role of the ZI in the catalytic cycle
could be to act as a stabilizing weak coordinating anion to the catalytically
active Pd(II) species, the amount of acid was gradually reduced to facili-
tate the coordination of the ZI relative to the acid anion. No apparent
activity difference was measured when decreasing the amount of MSA
R.A.M. Robertson, D.J. Cole-Hamilton, Coord. Chem. Rev. 225 (2002) 67;
C. Godard, B.K. Muñoz, A. Ruiz, C. Claver, Dalton Trans. (2008) 853.
3] W.G. Reman, G.B.J. De Boer, S.A.J. Van Langen, A. Nahuijsen, Eur. Pat. Appl. (1989) EP
[
411 721 A3 (to Shell).
[4] W. Clegg, G.R. Eastman, M.R.J. Elsegood, R.P. Tozze, X.L. Wang, K. Whiston, Chem.
Commun. (1999) 1877.
5] E. Drent, E. Kragtwijk, Eur. Pat. Appl. (1991)(EP 495 548 to Shell).
6] R.I. Pugh, E. Drent, P.G. Pringle, Chem. Commun. (2001) 1476.
[7] G.P.C.M. Dekker, C.J. Elsevier, K. Vrieze, P.W.N.M. van Leeuwen, C.F. Roobeek,
J. Organomet. Chem. 430 (1992) 357.
[
[
to 2 equiv. and 96–97 % MP formed in 30 min, confirming that enough
−
weak coordinating ions (MeSO
3
) were available to stabilize the Pd spe-
[
8] C.J. Rodriguez, D.F. Foster, G.R. Eastham, D.J. Cole-Hamilton, Chem. Commun. 15 (2004)
720.
cies during the catalytic cycle. However, when using only 1 equiv. of
MSA a significant activity difference and ZI effect were observed, as the
conversion increased from 43% without added ZI to 60% with added ZI
1
[
9] C. Bianchini, A. Meli, Coord. Chem. Rev. 225 (2002) 35;
M.A. Zuideveld, P.C.J. Kramer, P.W.N.M. van Leeuwen, P.A.A. Klusener, H.A. Stil, C.F.
Roobek, J. Am. Chem. Soc. 120 (1998) 7977.
10] C. Tiyapiboonchaiya, J.M. Pringle, J. Sun, N. Byrne, P.C. Howlett, D.R. MacFarlane, M.
Forsyth, Nat. Mater. 3 (2004) 29;
(60 equiv.) after 90 min reaction. Also here it was confirmed, that a
[
high concentration of the SBTA ZI resulted in an increased reaction
rate and consequently enhanced catalytic activity. In summary, the
use of neutral, non-toxic ZIs can limit the use of corrosive and harmful
strong acids necessary to run the catalytic reaction thus making the
overall process greener.
D. Kundu, A. Majee, A. Hajra, Catal. Commun. 11 (2010) 1157.
11] E. Drinkel, F.D. Souza, H.D. Fiedler, F. Nome, Curr. Opin. Colloid Interface Sci 18
[
[
(2013) 26.
12] Z. Du, Z. Li, Y. Deng, Synth. Commun. 35 (2005) 1343;
Y. Zhao, F. Deng, C. Xia, J. Peng, Catal. Commun. 10 (2009) 732.