Document [original]

Ionic Liquids as Surfactants in Aqueous

Multiphase Systems for the Pd-Catalyzed

Hydrocarboxylation

Ariane Weber*, Philipp Isbru

¨cker, Marcel Schmidt, and Reinhard Schoma¨cker

DOI: 10.1002/cite.202000165

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any

medium, provided the original work is properly cited.

Dedicated to Prof. Dr.-Ing. Matthias Kraume on the occasion of his 65th birthday

The suitability of ionic liquids (alkylmethylimidazolium bromides) as surfactants in an aqueous multiphase system for the

Pd-catalyzed hydrocarboxylation of 1-dodecene is discussed. Attention is paid to the influence of these ionic liquids on

the reaction performance and the phase separation. All tested ionic liquids are suitable as surfactants, but their concentra-

tion has a strong impact on the reaction performance. The resulting product concentrations are crucial for the phase

behavior and so for the phase separation. In recycling experiments, the Pd-catalyst is successfully separated and reused.

Keywords: Catalyst recycling, Homogeneous catalysis, Ionic liquids, Multiphase systems

Received: July 29, 2020; revised: December 02, 2020; accepted: December 02, 2020

1 Introduction

For industrial applications of homogeneous catalysts, cata-

lyst recycling is becoming increasingly important in addi-

tion to the key factors such as yield, selectivity, and operat-

ing costs. Furthermore, future processes must be designed

to be sustainable and environmentally friendly, based on the

twelve principles of green chemistry by Paul Anastas et al.

[1]. Some of these principles refer to the prevention of

waste, the use of less hazardous chemicals, atom-efficient

synthesis methods, and if possible, the use of a catalyst, no

matter whether a heterogeneous or a homogeneous one.

The commonly known problem of homogeneous catalysis

is the challenging separation of products and homogenously

dissolved catalyst, which makes a catalyst recycling difficult

to implement. Numerous solutions for catalyst recovery

from carbonylation reactions like hydroformylation or

alkoxycarbonylation of alkenes are discussed in the litera-

ture. For example, a further liquid phase, such as an aque-

ous phase, can be introduced and a so-called multiphase

system is formed. In this multiphase system, the catalyst

and the products are located in different phases and can

easily be separated. Lower alkenes can be converted in such

an aqueous biphasic system like it is done on an industrial

scale using the Ruhrchemie/Rhone-Poulenc process. The

rhodium catalyst is dissolved in the polar aqueous phase

due to its water-soluble TPPTS ligand and a nonpolar prod-

uct phase (butanal) is formed during the hydroformylation

reaction of vaporous propene [2]. The solubility of higher

alkenes decreases with increasing chain length. This causes

mass transport limitations in aqueous biphasic systems.

Nevertheless, higher alkenes (up to 1-octene) can be hydro-

formylated in such a biphasic system by using a packed

tubular reactor equipped with static mixers and a pump for

circulating the liquid phases [3]. This is associated with a

high expenditure of time, considerable costs, and a limita-

tion of suitable feedstocks.

An effective method to increase the solubility of alkenes

is the use of additives. Lower alcohols can be utilized as co-

solvents for the hydroformylation of 1-octene [4]. Cyclo-

dextrins can be used as promoters for the hydroformylation

[5, 6] as well as for the hydrocarboxylation of 1-decene [6].

The most known and studied solubilizers are surfactants.

By adding a surfactant into an aqueous biphasic system,

micelles, and different types of emulsions (so-called micro-

emulsions [7]) are formed. Depending on surfactant con-

centration, oil-water ratio, and temperature a distinction is

made between an oil-in-water emulsion (o/w), a water-in-

oil emulsion (w/o), and a bicontinuous structure.

The o/w-microemulsion contains surfactant micelles that

solubilize the nonpolar oil phase in the aqueous phase. The

microemulsion is in equilibrium with the excess oil phase.

The w/o-microemulsion contains inverse micelles, which

Chem. Ing. Tech. 2021,93, No. 1–2, 201–207 ª2020 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH www.cit-journal.com

–

Ariane Weber, Philipp Isbru

¨cker, Marcel Schmidt,

Prof. Dr. Reinhard Schoma¨cker

[email protected]

Technische Universita¨t Berlin, Department of Chemistry, Straße

des 17. Juni 124, 10623 Berlin, Germany.

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solubilize the polar aqueous phase in the oil phase. It is in

equilibrium with the excess aqueous phase. Both micro-

emulsion types form a two-phase system with their excess

phase, while the bicontinuous structure appears in the

so-called three-phase system. It consists of the two excess

phases of water and oil and a surfactant-rich bicontinuous

middle phase [8]. Numerous examples of multiphasic recy-

clable carbonylation reactions are given in the literature.

The hydroformylation of 1-dodecene can be promoted

using the ionic surfactant cetyltrimethylammonium bro-

mide (CTAB) [9] whereas nonionic surfactants (e.g., alkyl

polyethylene glycol ethers) are successfully used for the

hydroformylation and hydrocarboxylation of 1-dodecene

[10, 11]. Porada et al. described the amphiphilic character of

ionic liquids like alkylmethylimidazolium salts and their

ability to form the described microemulsions [12]. There-

fore, ionic liquids can act as solubilizers like it is already

described for the hydrofomylation of 1-octene [13].

In this study, the Pd-catalyzed hydrocarboxylation (also

called hydroxycarbonylation) of 1-dodecene is investigated

(Fig. 1). The co-catalyst is necessary for the formation of

active Pd-hydride species and prevents the deactivation of it

[14]. Mostly sulfonic acids are used as co-catalysts due to

their non-coordinating anions. The reaction is carried out

in a micellar aqueous biphasic system. For improving the

solubility of the long-chain alkene and circumventing of

mass transport limitations, different ionic liquids as surfac-

tants were tested. The influence of their alkyl chain length

and their concentration on the reaction performance and

phase separation is described.

2 Experimental

2.1 Chemicals

The substrate 1-dodecene (94 %) and the co-solvent decane

(94 %) were purchased from Merck. The co-solvents octane

(99 %) and dodecane (99 %), the co-catalyst p-toluene

sulfonic acid (pTsOH, 98 %), and the catalyst precursor

palladium acetate (Pd(OAc)

, 99.9 %) were obtained from

Sigma-Aldrich. The ligand SulfoXantPhos (SX) was donated

by Molisa GmbH. HPLC grade water was acquired from

VWR. Carbon monoxide (99.9 %) was obtained from Air

Liquide. The ionic liquids 1-octyl-3-methylimidazolium

bromide (OMIM, 99 %), 1-decyl-3-methylimidazolium bro-

mide (DecMIM, 99 %) and 1-dodecyl-3-methylimidazolium

bromide (DodecMIM, 99 %) were purchased from abcr. All

chemicals were used without further purification.

2.2 Determination of the Critical Micelle

Concentration

The critical micelle concentration (CMC) of different sur-

factants was determined with the bubble pressure tensiome-

ter BP50 from Kru

¨ss and with the conductometer LF539

from WTW. Aqueous solutions of the surfactant with differ-

ent concentrations were prepared for this purpose. The

dynamic surface tension as a function of the surface age was

measured at 25 C using the bubble pressure tensiometer.

The conductivity as a function of surfactant concentration

was measured at 25 C using a conductivity cell with a cell

constant of 0.45 cm

–1

2.3 Phase Behavior Experiments

To determine the phase behavior various reaction mixtures

with different substrate and product concentrations were

prepared in a 10-mL test tube. The substrate 1-dodecene,

the co-solvent octane, water, and surfactant are weighted

into the test tube, flushed with argon and the prepared cata-

lyst solution (see Sect. 2.4) was injected by a syringe. The

content of the co-solvent and the ratio of substrate to the

product was varied. Immediately after preparation, the phase

behavior of the mixtures at room temperature was observed.

Subsequently, the tubes were heated up to 80 C (reaction

temperature) by a heated water bath and after thermal equi-

librium is reached the phase behavior is observed.

2.4 Preparation of the

Catalyst Solutions

For catalysis experiments, the

metal precursor Pd(OAc)

(0.05 mmol, 1 eq.) and the ligand

SX (0.2 mmol. 4 eq.) were added

in a Schlenk tube and evacuated

and flushed with argon three

times. 4 mL of degassed water

were added to the Schlenk tube.

The catalyst solution was ob-

tained after stirring (800 rpm,

room temperature) overnight.

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Figure 1. Hydrocarboxylation of 1-dodecene to the corresponding linear acid (tridecanoic acid)

and to the branched acid (2-methyldodecanoic acid), the chemical structure of the ligand Sul-

foXantPhos, and possible side reactions.

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2.5 Experimental Procedure

In a typical hydrocarboxylation experiment the substrate

1-dodecene, the co-solvent dodecane, degassed water, the

co-catalyst, and the surfactant were added into a 100-mL

stainless-steel autoclave, equipped with a gas dispersion stir-

rer and baffles. After evacuating and flushing with nitrogen,

the prepared catalyst solution was added to the autoclave

under nitrogen counterflow. The reactor was pressurized

with reaction gas, heated up to reaction temperature (80 or

85 C), and stirred at 1200 rpm. During the reaction, the

pressure was kept constant and the gas consumption was

measured via a mass flow controller. After the reaction time,

a homogenous sample was taken and analyzed by GC with

a Shimadzu GC2010 Plus (FID, column: Restek RTX5-MS,

30 m ·0.25 mm ·0.25 mm).

The catalyst activity can be quantified by the turnover

frequency (Eq. (1)) and the turnover number (Eq. (2)):

TOF ¼n01dodeceneðÞYtðÞ

nPdðÞt(1)

TON ¼nacidðÞ

nPdðÞ (2)

2.6 Recycling Experiments

For the recycling experiments 1-dodecene (17.8 mmol),

pTsOH (2 mmol, 40 eq.), OMIM (1.65 g), octane (9 g,

co-solvent) and the prepared catalyst solution (0.05 mmol

metal precursor (1 eq.) and 0.2 mmol SX (4 eq.) in 12 g

water) were added into the autoclave and the reaction was

started as described in the experimental procedure. After

reaction time the reactor was depressurized and cooled to

25 C. After phase separation (10 min) the whole nonpolar

phase was removed by a syringe. A sample was taken from

this nonpolar phase. A new organic phase (17.8 mmol

1-dodecene and 9 g octane) was added to the reactor under

nitrogen counterflow. The autoclave was heated up to reac-

tion temperature and pressurized with reaction gas (30 bar).

This procedure was repeated for further recycling runs. The

samples were analyzed by GC.

2.7 Determination of Palladium and Phosphorous

Leaching

After the reaction, the nonpolar phase was transferred into

a round bottom flask and evaporated under reduced pres-

sure (5 mbar, 180C). 1 ml nitric acid (65%), 3 ml hydro-

chloric acid (37%), and 2 ml sulfuric acid (96%) were added

to the residue and filled up to 20 ml with water (HPLC

grade). The solution was analyzed by ICP-OES for palladi-

um and phosphorous with a Varian 715-ES.

3 Results

3.1 Determination of the CMC

For significant reaction rates, surfactant concentrations

above the critical micelle concentration are necessary.

Therefore, the CMC of different ionic liquids is determined

by conductivity and surface tension measurements and are

shown in Tab. 1. The chain length of the nonpolar hydro-

carbon chain is crucial for the CMC value since longer

chain length leads to an increasing hydrophobicity. The

highest CMC can be found for the 1-octyl-3-methyl

imidazolium bromide (OMIM) with a C8-alkyl chain

(146.3 mmol L

–1

). With the increasing length of the hydro-

carbon chain, the CMC value decreases rapidly (Dodec-

MIM: 9.5 mmol L

–1

, C12-alkyl chain). This is a well-known

phenomenon and is described for similar cationic surfac-

tants like alkyl trimethylammonium bromides [15]. The

deviation of the determined CMC values from literature

values increases with increasing CMC but corresponds with

the literature approximately.

3.2 Hydrocarboxylation Experiments

3.2.1 Variation of Surfactant Concentration and

Chain Length of the Surfactant

The determined CMC values confirm the assumed surface

activity of the examined ionic liquids and so the ionic

liquids may behave like a surfactant. As known from the

literature, the surfactant concentration is crucial for the

Chem. Ing. Tech. 2021,93, No. 1–2, 201–207 ª2020 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH www.cit-journal.com

Table 1. Critical micelle concentration of different ionic surfactants and alkyl imidazolium salts at 25 C.

Surfactant CMC from bubble pressure tensiometer

[mmol L

–1

]

CMC from conductivity measurement

[mmol L

–1

]

CMC average

[mmol L

–1

]

CMC from literature

[mmol L

–1

]

OMIM 135.3 157.3 146.3 150.0

b,c

[16]

DecMIM 30.0 35.3 32.7 29.3

[17], 32.9

[17]

DodecMIM 11.0 8.0 9.5 8.5

[17], 10.9

[17]

a) For a constant surface age of 14 000 ms; b) conductivity measurement; c) surface tension measurement.

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phase behavior and so for the local concentrations at the

interfacial area [11]. The influence of different ionic liquids

and their concentrations on the hydrocarboxylation of

1-dodecene and the subsequent phase separation of the

reaction mixture is shown in Fig. 2. The yield of the desired

acid and the conversion of 1-dodecene increases with

increasing surfactant concentration. As expected, a surfac-

tant concentration above the CMC is necessary to get note-

worthy yields. If the CMC is exceeded (surfactant concen-

tration above 0.15 mol L

–1

), the yield of acid can be

enhanced by a factor of about seven from 3 % to 22 %. A

further increase in surfactant concentration leads to yields

of up to 55 %. It is well known that the ligand SulfoXant-

Phos is surface-active itself [10, 17]. This indicates that the

catalyst complex is located at the water-oil interface. With

an increasing surfactant concentration, the interfacial area

and the local concentration of the catalyst complex also

increases resulting in higher reaction rates, yields, and con-

version. Further increasing OMIM concentration from

0.7 mol L

–1

to 1 mol L

–1

has no further impact on the forma-

tion of acid but the conversion of 1-dodecene increases.

Instead, this leads to a higher side reaction (isomerization

of the double bond). The concentration of ionic liquid does

not influence the regioselectivity (l:b = 90:10) of the reac-

tion.

Furthermore, the influence of the concentration of ionic

liquids with different alkyl chains on the hydrocarboxyla-

tion reaction was studied. The given surfactant concentra-

tions in Tab. 2, entry 1–3, correspond to the 5-fold CMC of

the alkyl-MIMs. The turnover frequency (TOF) and conver-

sion decrease with the increasing length of alkyl chains and

with decreasing CMC of the alkyl-MIM, respectively. A

higher CMC, e.g., 146.6 mmol L

–1

for OMIM, implies a

higher concentration of surfactant molecules required for

micelle formation. This leads to higher solubilization within

the micelles and higher reaction rates. At a constant surfac-

tant concentration of 0.5 mol L

–1

, the catalyst activity

increases with the increasing length of the alkyl chain

(Tab. 2, entry 4–6). The yield of acid enhances from 44 %

(OMIM) to 60 % (DodecMIM). At this concentration, the

number of initial surfactant molecules is the same for all al-

kyl-MIMs. Due to the different CMC values, the number of

surfactant molecules required for micelle formation differs

for the three ionic liquids. The surfactant concentration that

is efficient for the reaction can be classified in relation to

the corresponding CMC. A concentration of 0.5 mol L

–1

cor-

relates with a nearly 3-fold CMC of OMIM and a 55-fold

CMC of DodecMIM. By using DodecMIM, more surfactant

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Figure 2. Influence of surfactant concentration on the hydro-

carboxylation of 1-dodecene. Experimental conditions: a= 0.5,

0.05 mmol Pd(OAc)

, Pd:SX:pTsOH = 1:4:40, non-polar phase: 3 g

1-dodecene and 9 g octane (co-solvent), polar phase: 12 g water,

concentration of OMIM in polar phase, T=80C, p(CO) = 30 bar,

n= 1200 rpm, t

=20h.

Table 2. Influence of surfactant concentration of different alkyl imidazolium bromides on the hydrocarboxylation of 1-dodecene,

l:b = 90:10.

Entry Surfactant c(alkyl-MIM)

[mol L

–1

mic

[mol L

–1

] Yield (acid) [%] Conversion (1-dodecene) [%] TOF [h

–1

] Successful phase

separation at RT

1 OMIM 0.73

0.58 51.4 87.4 11.0 yes

2 DecMIM 0.17

0.14 46.0 61.8 11.9 yes

3 DodecMIM 0.05

0.041 16.9 20.1 3.3 yes

4 OMIM 0.5 0.35 44.0 73.4 4.8 yes

5 DecMIM 0.5 0.47 57.4 92.3 18.9 no

6 DodecMIM 0.5 0.49 60.1 95.1 23.6 no

7 OMIM 0.35 0.2 37.5 57.5 3.7 yes

8 DecMIM 0.23 0.2 53.0 80.8 17.9 no

9 DodecMIM 0.21 0.2 67.2 93.4 24.4 no

a) Experimental conditions: a= 0.5, 0.05 mmol Pd(OAc)

, Pd:SX:pTsOH:1-dodecene = 1:4:40:356, non-polar phase: 3 g 1-dodecene and 9 g

octane (co-solvent), polar phase: 12 g water, T=85C, p(CO) = 30 bar, n= 1200 rpm, t

= 20 h; b) surfactant concentration in aqueous

phase; c) five-fold CMC; d) micellar surfactant concentration c

mic

=c–CMC = 0.2 mol L

–1

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molecules are available for micelle formation,

higher solubilization is reached, resulting in

higher conversion and yields [9, 13, 19].

To ensure an identical number of surfactant

molecules for the micelle formation, the micellar

surfactant concentration c

mic

=c–CMC =

0.2 mol L

–1

is defined (Tab. 2, entry 7–9).

Increasing yield of acid and TOF can be

observed by increasing the length of the alkyl

chains. The TOF increases by a factor of 7, from

3.7 to 24.4 h

–1

. This leads to a different solubiliz-

ing capacity for the surfactants. The polarity of

micelles decreases with the increasing length of

the alkyl chain, which is already described for

the cationic alkyl trimethylammonium bromide

surfactants [20]. Due to the better solubilization

of substrate in the DodecMIM micelles, the acid

yield is higher.

In addition to the reaction performance, the

subsequent phase behavior and separation of the

reaction mixture plays a decisive role. In this

case, successful phase separation can be defined

as an entire separation of the reaction mixture

into at least two phases in which the catalyst and

the products must be in different phases. This applies to all

experiments with OMIM and the experiments with a 5-fold

CMC of DecMIM and DodecMIM as shown in Tab. 2. This

indicates a strong influence of the produced acid on the

phase behavior which is also discussed in the following sec-

tion. The length of the alkyl chain does not influence the l:b

regioselectivity (90:10) of the reaction.

3.2.2 Variation of Substrate Concentration

The used co-solvent octane dilutes the non-polar phase to

avoid possible disruptive effects due to higher substrate con-

centrations. One principle of green chemistry is to prevent

waste and to avoid solvents. By using the pure substrate as a

nonpolar phase without further co-solvent this principle

can be achieved. This leads to higher initial substrate con-

centrations and the effect on the hydrocarboxylation reac-

tion is investigated. Therefore, the composition of the oil

phase was varied as shown in Fig. 3.

Increasing the content of 1-dodecene results in increasing

TOF values and higher conversion of 1-dodecene. This

shows that the local substrate concentration at the interface

increases and more substrate molecules can be converted by

the catalyst, which can be specified by the turnover number

(TON). A low substrate concentration leads to a TON of 34,

whereas using the pure substrate as oil phase a TON of 816

is achieved. This indicates that a co-solvent is not necessary

for the high performance of the reaction. Interestingly, the

results of the subsequent phase separation of the reaction

mixtures (also shown in Fig. 3) show a phase inversion.

Lower substrate concentrations form an o/w-emulsion

where the catalyst is located in the water phase, while higher

substrate concentrations form a w/o-emulsion where the

catalyst is located in the oil phase. Therefore, a successful

phase separation and catalyst recycling is not possible at

higher substrate concentrations. The phase inversion with

higher substrate concentration can be a result of the phase

behavior of the reaction mixture and the impact of the sub-

strate. It is also possible that the formed acid influences the

phase behavior and phase inversion due to its weak surface

activity.

Therefore, the phase behavior of simulated reaction mix-

tures is investigated and summarized in Fig. 4. All mixtures

without acid form an o/w-emulsion both at room tempera-

ture and reaction temperature. The initial content of the

substrate does not influence the phase behavior. Increasing

yield of acid at lower substrate content (25 wt %) has also

no influence on the phase behavior. Phase inversion can be

observed for an initial substrate concentration of above

50 wt % and a product yield of more than 60 %. For an ini-

tial substrate content of 100 %, a w/o-emulsion is formed

above a product yield of 50 %. This indicates a strong influ-

ence of the formed acid on the phase behavior. In literature,

the corresponding salt showed surface-active properties

[21]. The formation of micelles from the produced acid is

possible and can result in different phase behavior. Further-

more, the separation temperature is also important. As seen

in Fig. 4 some mixtures do not show a phase separation at

room temperature. The separation time depends on various

parameters like density, temperature, and especially surface

tension [22].

Chem. Ing. Tech. 2021,93, No. 1–2, 201–207 ª2020 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH www.cit-journal.com

Figure 3. Influence of initial substrate concentration on the hydrocarboxyla-

tion. Experimental conditions: 0.05 mmol Pd(OAc)

, Pd:SX:pTsOH = 1:4:40, non-

polar phase: xg 1-dodecene and 12xg octane (co-solvent), polar phase: 12 g

water, T=80C, OMIM as surfactant (0.5 mol L

–1

,p(CO) = 30 bar, n= 1200 rpm,

= 20 h, l:b = 90:10.

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3.2.3 Recycling Experiment

For a comprehensive evaluation of this multiphase system

in terms of the principles of green chemistry, a catalyst recy-

cling is necessary. For this purpose, 0.5 mol L

–1

OMIM as a

surfactant and an initial content of a substrate of 25 wt %

was chosen at which the phase separation is successful after

a reaction time of 20 h, regardless of conversion, yield, or

temperature. The results are shown in Fig. 5. In the first

run, 73 % of 1-dodecene are converted and a yield of 45 %

of acid was obtained. In the second run, the conversion of

substrate increased to 90 %, and 61 % of the acid is formed.

In the third and fourth run, the yields and conversions re-

mained almost the same. This

can be explained with the in situ

formation of the active catalyst

species and the resulting induc-

tion period. Only in the first re-

cycle run the active Pd-hydride

species is formed, which causes

an induction period and a lower

yield of product and conversion

of the substrate. Looking at the

leaching results, it is noticeable

that a relatively large amount of

palladium and phosphorous is

lost into the oil phase in the first

run (0.43 ppm Pd, 1.69 ppm P).

The subsequent runs showed

lower leaching of 0.01 ppm Pd

and 0.22 ppm P. The linear to

branched ratio is constant with

90:10 and demonstrates the high

stability of the catalyst complex.

4 Conclusion

The investigated ionic liquids are surface-active amphi-

philes that can be used as a solubilizer for the homogene-

ously catalyzed hydrocarboxylation of 1-dodecene in an

aqueous multiphase system, as demonstrated by the results.

The selected multiphase systems showed satisfactory reac-

tion performances. The catalysis takes place at the internal

interface of the emulsion and is controlled by the surfactant

concentration. The choice of ionic liquid and its concentra-

tion is crucial for the performance of the reaction, but not

for the regioselectivity (l:b = 90:10). For a successful catalyst

recycling, complete phase separation is indispensable. The

catalyst system could successfully be separated and reused

for three more experiments. The loss of metal and ligand is

rather low. Phase behavior studies indicate the high impact

of the produced acid. Higher acid concentrations lead to a

phase inversion at which the catalyst cannot be separated

from the product. For further investigations, e.g., a continu-

ous process, it is necessary to operate at steady-state condi-

tions that circumvent this phase inversion.

Gefo¨rdert durch die Deutsche Forschungsgemeinschaft

(DFG) – TRR 63 ,,Integrierte chemische Prozesse in flu

¨s-

sigen Mehrphasensystemen‘‘ (Teilprojekt A2) –

56091768. Funded by the Deutsche Forschungsgemein-

schaft (DFG, German Research Foundation) – TRR 63

‘‘Integrated Chemical Processes in Liquid Multiphase

Systems’’ (subprojects A2) – 56091768. Open access

funding enabled and organized by Projekt DEAL.

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Figure 4. Phase behavior of simulated reaction mixtures and the influence of initial substrate

concentration and product concentration. Test conditions: a= 0.5, 0.05 mmol Pd(OAc)

, Pd:SX =

1:4, octane as co-solvent.

Figure 5. Catalyst recycling for the hydrocarboxylation of 1-do-

decene. Experimental conditions: 0.05 mmol Pd(OAc)2,

Pd:SX:pTsOH = 1:4:40, non-polar phase: 3 g 1-dodecene and 9 g

octane (co-solvent), polar phase: 12 g water, T=80C, OMIM as

surfactant (0.5 mol L

–1

), p(CO) = 30 bar, n= 1200 rpm, t

=20h,

l:b = 90:10.

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Symbols used

CMC [mmol L

–1

] critical micelle concentration

TOF [h

–1

] turnover frequency

TON [–] turnover number

Abbreviations

CTAB cetyltrimethylammonium bromide

DecMIM 1-decyl-3-methylimidazolium bromide

DodecMIM 1-dodecyl-3-methylimidazolium bromide

l:b linear:branched ratio

OMIM 1-octyl-3-methylimidazolium bromide

o/w oil-in-water microemulsion

SX SulfoXantPhos

w/o water-in-oil microemulsion

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