Heterogeneous hydroformylation of long-chain alkenes in IL-in-oil Pickering emulsion

Article abstract of DOI:10.1039/c7gc02574b

An efficient heterogeneous catalytic system for hydroformylation of long-chain alkenes is highly desirable for both academy and industry. In this study, an IL-in-oil Pickering emulsion system was employed for heterogeneous hydroformylation of 1-dodecene with Rh-sulfoxantphos as the catalyst and surface modified dendritic mesoporous silica nanospheres (DMSN) as the stabilizer. The IL-in-oil Pickering emulsion system outperformed IL-oil biphase, water-in-oil Pickering emulsion and IL-oil micelle system under similar reaction conditions to afford n/b ratio of 98:2, chemoselectivity of 94% and TOF of 413 h^-1, among the highest ever reported for IL-oil biphase hydroformylation of long-chain alkenes. The high efficiency of IL-in-oil Pickering emulsion was primarily attributed to the increased interface area and unique properties of ILs. Studies also revealed that solid stabilizers with large and open pore channels could greatly increase the reaction rate of Pickering emulsion systems by accelerating the diffusion rate. The recyclable IL-in-oil Pickering emulsion is promising not only for hydroformylation of long-chain alkenes but also for catalytic reactions with immiscible liquids.

Full text of DOI:10.1039/c7gc02574b

Green Chemistry
View Article Online
View Journal
PAPER
Heterogeneous hydroformylation of long-chain
alkenes in IL-in-oil Pickering emulsion†
Cite this: DOI: 10.1039/c7gc02574b
Lin Tao,a,b Mingmei Zhong,a,b Jian Chen,a,b Sanjeevi Jayakumar,a,b Lina Liu,a,b He Lia
a
and Qihua Yang
*
An efficient heterogeneous catalytic system for hydroformylation of long-chain alkenes is highly desirable
for both academy and industry. In this study, an IL-in-oil Pickering emulsion system was employed for
heterogeneous hydroformylation of 1-dodecene with Rh-sulfoxantphos as the catalyst and surface
modified dendritic mesoporous silica nanospheres (DMSN) as the stabilizer. The IL-in-oil Pickering emul-
sion system outperformed IL-oil biphase, water-in-oil Pickering emulsion and IL-oil micelle system under
similar reaction conditions to afford n/b ratio of 98 : 2, chemoselectivity of 94% and TOF of 413 h−1
,
among the highest ever reported for IL-oil biphase hydroformylation of long-chain alkenes. The high
efficiency of IL-in-oil Pickering emulsion was primarily attributed to the increased interface area and
unique properties of ILs. Studies also revealed that solid stabilizers with large and open pore channels
could greatly increase the reaction rate of Pickering emulsion systems by accelerating the diffusion rate.
The recyclable IL-in-oil Pickering emulsion is promising not only for hydroformylation of long-chain
alkenes but also for catalytic reactions with immiscible liquids.
Received 22nd August 2017,
Accepted 22nd November 2017
DOI: 10.1039/c7gc02574b
rsc.li/greenchem
different lipophilicity of reactants, products and catalysts.
Pickering emulsions, with solid particles residing at the inter-
Introduction
Aldehydes are important intermediates that can be converted face of droplets and bubbles against coalescence or fusion,
to many final products such as plasticizers, detergents, alco- has been recently used as a platform for the design of efficient
hols, esters and amines.1–4 “Oxo”-hydroformylation reaction catalytic systems.9 Compared with traditional surfactant emul-
developed by Otte Roelen in 1938 has been widely used for sified microemulsions, the Pickering emulsion system has the
production of aldehydes from alkenes in industry.5 The tra- advantages of facile emulsion properties tuning by particle
ditional hydroformylation reaction proceeding homogeneously surface wettability and easy emulsion breaking by filtration or
in oil phase confronts difficulties related to recycling of cata- centrifugation, which are very important for adjusting catalytic
lysts and separation of products.6 Thus, the oil–water reaction performance of the emulsion system, recycling of catalysts and
system has been applied in the hydroformylation of C3–C4 purification of products.10 Recently, several water-in-oil
olefins industrially with Rh-tris(m-sulfonatophenyl) phosphine Pickering emulsion systems have been constructed for
(Rh-TPPTS) complexes as catalysts.7 Oil-immiscible catalysts rhodium-catalysed hydroformylation of long-chain alkenes.
dissolved in the water phase make the recycling of catalysts The solid particles employed for the abovementioned
possible, leading to the reduction of high cost of expensive Rh Pickering emulsion systems include cyclodextrin,11,12 cyclodex-
metal.8 The biphase system is very successful in hydroformyla- trin with PEG as the supramolecular hydrogel,13,14 polymers15
tion of light alkenes, but not efficient for hydroformylation of and mesoporous silica nanospheres.16 Obvious increases in
long-chain alkenes due to the mass transfer resistance of lipo- both catalytic activity and chemoselectivity are reported in
philic substrates in the water phase.
these Pickering emulsion systems.
The emulsion catalytic system is a good choice for reactions
The reported Pickering emulsions are mainly water-in-oil or
facing problems related to diffusion resistance caused by oil-in-water emulsion systems.17,18 Though water is regarded
as a kind of green solvent, there are still some limitations,
such as low solubility for lipophilic substrates, damage to
water-sensible compounds, and low boiling point. The exten-
sion of Pickering emulsions to other green solvents will be of
great significance in but not limited to catalysis. Ionic liquids
(ILs) have some environmentally benign properties, for
example, extremely low vapor pressure, chemical and thermal
aState Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
E-mail: yangqh@dicp.ac.cn; Fax: +86-411-84694447; Tel: +86-411-84379552
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
†Electronic supplementary information (ESI) available: The additional figures
and tables. See DOI: 10.1039/c7gc02574b
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Paper
Green Chemistry
stability, high ionic conductivity and good solvent properties
towards both ionic and covalent compounds.19,20 In the past
decades, there have been many studies on the use of ILs in
biphase hydroformylation for increasing the solubility of lipo-
philic substrates in two-phase reaction systems.21,22 However,
the rate acceleration effect is still not very promising, possibly
due to the diffusion limitation of reactants in highly viscous
ILs. The introduction of Pickering emulsions into ILs will
divide bulk ILs into numerous droplets on the micrometer
scale, which may increase the interphase area and benefit fast
diffusion of reactants and products during the catalytic
process. However, the preparation of IL-in-oil/oil-in-IL
Pickering emulsions for hydroformylation reactions has been
rarely reported.
In the current work, an IL-in-oil Pickering emulsion system
for hydroformylation of long-chain alkenes was constructed
with Rh-sulfo-xantphos as the catalyst and surface modified
dendritic mesoporous silica nanospheres as the solid stabil-
izers. The overall catalytic performance of the IL-in-oil
Pickering emulsion system was superior to that of IL-oil
biphase and water-in-oil Pickering emulsion catalytic systems,
implying that the combination of the Pickering emulsion with
ILs resulted in better outcomes than that of other systems.
Fig. 1 TEM images of (a) DMSN and (b) DMSN-C18N-0.8, (c) N2 adsorp-
tion/desorption isotherms and (d) pore size distribution curves of DMSN
and DMSN-C18N-0.8.
of DMSN-C18N-X (X = 0 to 1.2) decreased gradually from
813 m2 g−1 to 339 m2 g−1 as the amount of C18N increased
(Table 1 and Fig. 1d). The sharp decrease in pore diameter
suggested that the C18N group was mainly grafted in the
mesopores of DMSN.
The FT-IR spectrum of DMSN-C18N-0.8 clearly showed
characteristic C–H stretching vibration at 2926 cm−1 and
2856 cm−1 and C–H bending vibration at 1468 cm−1, confirm-
ing the successful grafting of C18N onto DMSN (Fig. 2a).16 The
thermogravimetric (TG) curves of DMSN-C18N-X exhibited
three obvious weight loss steps (Fig. 2b). The first one at
around 50 to 200 °C was related to physically adsorbed water.
The second and third ones in the range of 200 to 700 °C could
be attributed to the decomposition of C18N. The TG analysis
demonstrated that DMSN-C18N-X samples were stable in air
up to 200 °C. The stability was sufficient for hydroformylation
of long-chain alkenes, generally performed at temperature
Results and discussion
Characterization of interfacially active nanoparticles
Interfacially active nanoparticles are very important for the
preparation of Pickering emulsions.9 Silica nanoparticles with
tunable surface wettability are generally used as stabilizers for
Pickering emulsions.23,24 In this work, the surface wettability
of silica nanospheres was modified with C18N (dimethyl-
octadecyl[3-(trimethoxysilyl)propyl]ammonium
chloride)
group considering its enrichment properties for negatively
charged Rh-sulfo-xantphos through electrostatic interactions.
For nonporous silica nanospheres, the interphase area
depends mainly on the particle size of nanospheres.25 With
mesoporous silica as the stabilizer, the porous structure and
pore diameter may affect the diffusion rates of the substrates
and products in the Pickering emulsion. Thus, MCM-41 nano-
spheres and dendritic mesoporous silica nanospheres (DMSN)
with similar particle size but different pore structure and pore
size were chosen as stabilizers for comparison.
All DMSN-C18N-X (X = 0 to 1.2, X refers to mmol of C18N
per gram of DMSN used during the grafting process) samples
were composed of monodispersed nanospheres with uniform
particle size of ∼100 nm based on transmission electron
microscopy (TEM) images (Fig. 1a, b and Fig. S1†). Center-
radial mesopore arrangement could be clearly observed in the
TEM images before and after grafting, revealing the structure
robustness of DMSN in the grafting process. Nitrogen adsorp-
tion–desorption isotherms of DMSN-C18N-X (X = 0 to 1.2)
showed a typical type IV isotherm pattern (Fig. 1c and
Fig. S2†), which are characteristic of mesoporous materials.
Compared with DMSN, the BET surface area and pore volume
Table 1 Textural parameters, content of organic groups and water
contact angle of DMSN, DMSN-C18N-X and MCM-C18N-1.8
BET
Pore
surface
area
volume
Pore
size
Content of Water
organic contact
(cm3
Sample
DMSN
DMSN-C18N-0.5 387
DMSN-C18N-0.8 345
DMSN-C18N-1.2 339
MCM-C18N-1.8 203
(m2 g−1
)
g−1
)
(nm) groupsa (%) angle (°)
813
1.43
0.78
0.68
0.93
0.25
6.2
5.4
5.1
4.4
2.3
0
35
91
100
112
101
17.1
21.3
27.9
33.1
a Calculated by weight loss in the range of 200 to 800 °C.
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Paper
Fig. 2 (a) FT-IR spectrum of DMSN-C18N-0.8 and (b) TGA curves of
DMSN-C18N-X.
below 150 °C. The content of C18N (calculated by weight loss
between 200 and 800 °C) increased from 17.1 wt% to 27.9 wt%
with increase in initial amount of C18N added, indicating that
the content of the organic group on DMSN-C18N-X could be
facilely adjusted by the grafting method.
Fig. 3 Photographs and microscopic images of emulsions with
DMSN-C18N-X and MCM-C18N-1.8 as stabilizers. Emulsion formation:
60 mg of silica nanospheres, 1 mL of [BMIM][BF4] (including Rh 3.0 ×
10−3 mmol, P/Rh = 15) and 1 mL of 1-dodecene. The emulsions were
kept under static conditions for different time intervals, (a) newly
formed, (b) after 1 day, (c) after 3 days, (d) after 10 days. Scale bar is
200 μm.
The surface wettability of the nanospheres is one of the key
factors to get a stable emulsion.26 Water contact angle experi-
ments were used to test the surface wettability of
DMSN-C18N-X (Table 1). DMSN with a water contact angle of
35° has a hydrophilic surface. After modification with C18N,
all DMSN-C18N-X materials had a hydrophobic surface with
water contact angles larger than 90°. From DMSN-C18N-0.5 to
DMSN-C18N-1.2, the water contact angle slightly increased
from 91° to 112°, suggesting that grafting is an efficient
method for precisely adjusting the surface wettability of silica
nanoparticles. A control sample, MCM-C18N-1.8 with water
contact angle of 101°, was also prepared using a method
similar to that used for DMSN-C18N-X with the exception that
MCM-41 nanospheres (particle size: ∼100 nm) were used
instead of DMSN (Table 1 and Fig. S1†). MCM-C18N-1.8 with a
similar water contact angle as DMSN-C18N-0.8 had much
lower BET surface area, pore volume and pore diameter than
the latter.
Fig. 4 CLSM images of IL-in-oil emulsions formed with different
amounts of DMSN-C18N-0.8, (a) 40 mg, (b) 60 mg and (c) 80 mg by
dying [BMIM][BF4] with rhodamine. Scale bar is 100 μm.
Fig. S3†). The emulsion volume formed under identical con-
ditions increased in the order of DMSN-C18N-1.2 (0.4 mL) <
DMSN-C18N-0.5 (0.6 mL) < DMSN-C18N-0.8 (2 mL), suggesting
that DMSN-C18N-0.8 had optimized surface wettability for the
formation of the IL-in-oil emulsion. For fresh emulsions, the
Preparation of IL-in-oil emulsion
In our previous work, C18N modified silica nanoparticles were droplet size was strongly related to the wettability of the
used to stabilize oil-in-water Pickering emulsions.16 The IL DMSN-C18N-X samples. Based on the microscopic images, the
1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), droplet size of emulsions with DMSN-C18N-0.5 and
possesses different polarity and viscosity compared to water. DMSN-C18N-1.2 was 20–100 μm and 30–50 μm, respectively.
Therefore, emulsion formation abilities of DMSN-C18N-X (X = Relatively large droplet size of 50–230 μm was observed for the
0.5, 0.8, 1.2) were first investigated with 1-dodecene as the oil emulsion with DMSN-C18N-0.8 as the stabilizer.
phase and [BMIM][BF4] as the IL phase in the presence of
The emulsion with DMSN-C18N-0.8 was very stable and no
hydroformylation catalyst Rh-sulfo-xantphos (Fig. 3). The for- obvious change in droplet size and emulsion volume could be
mation of emulsions could be clearly observed with all observed even after 10 days, indicating that DMSN-C18N-0.8 is
DMSN-C18N-X samples at room temperature. For identifying a good stabilizer for oil-IL emulsions. However, emulsions
the formation of IL-in-oil or oil-in-IL emulsion, confocal laser with DMSN-C18N-0.5 and DMSN-C18N-1.2 separated gradually
scanning microscopy (CLSM) experiments were performed with time increasing, and broke completely within 10 days.
with rhodamine 6G, which can dissolve in the IL but not in The control sample, MCM-C18N-1.8, also acted as an efficient
1-dodecene. CLSM images of emulsions with DMSN-C18N-0.5, stabilizer to form a stable emulsion in a manner similar to
DMSN-C18N-0.8 and DMSN-C18N-1.2 clearly showed the DMSN-C18N-0.8 (Fig. 3).
uniform distribution of red colour inside the droplet,
The water contact angle of DMSN-C18N-X only varied
suggesting the formation of IL-in-oil emulsions (Fig. 4 and slightly from 91 to 112° with X varying from 0.5 to 1.2.
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Paper
Green Chemistry
Nevertheless, the emulsion quality and stability were quite possibly due to the high mass transfer resistance. In the IL-in-
different. In fact, all DMSN-C18N-X samples could form stable oil Pickering emulsion system, the conversion and aldehyde
water-in-oil emulsions with 1-dodecene as the oil phase and selectivity were improved dramatically to 93% and 94%,
Rh-sulfo-xantphos as the catalyst under identical conditions respectively. Under similar reaction conditions, the TOF of the
(Fig. S4†). This reveals that the formation of IL-in-oil emul- IL-in-oil Pickering emulsion system was up to 413 h−1, two
sions requires more accurate control of surface properties of times that of the biphase system. This suggests that the
silica nanospheres.
Pickering emulsion could efficiently improve both the reaction
rate and chemoselectivity.
Surfactants were often used as the phase transfer reagent in
reaction systems involving water and oil.29 Thus, CTAB with a
functional group similar to C18N was chosen as the phase
transfer reagent in hydroformylation of olefins. In the pres-
ence of CTAB, the conversion and chemoselectivity were
similar to those obtained for the Pickering emulsion system.
But the TOF of Pickering emulsion system is much higher
than that of the IL-oil-CTAB micelle system. Notably, it was
very difficult to separate and recycle the catalyst for the IL-oil-
CTAB micelle system. The Pickering emulsion system could
not only give better catalytic performance than the biphasic or
micelle system under the same conditions, but also be separ-
ated and recycled facilely by centrifugation (described later).
The IL-in-oil Pickering emulsion catalytic system was also
efficient for hydroformylation of 1-decene for the production
of 1-undecanal, an important intermediate in manufacturing
of perfumes (Table 2). Up to 92% of conversion with chemo-
selectivity of 68% and n/b ratio of 92 : 8 could be obtained,
proving the generality of the IL-in-oil Pickering emulsion cata-
lytic system.
The droplet size of emulsion determines the interface area
of the Pickering emulsion system, which may influence the
catalytic performance.30 Therefore, catalytic performances of
Pickering emulsion systems with different droplet size were
investigated using DMSN-C18N-0.8 as the model stabilizer
(Table 2). In order to accurately measure the droplet size of the
emulsion, the CLSM technique was used by dying the IL phase
with rhodamine 6G as discussed before (Fig. 4). With the
amount of DMSN-C18N-0.8 increasing from 40 mg to 60 mg,
the droplet size gradually decreased from 150–400 μm to
50–200 μm. Further increasing DMSN-C18N-0.8 content to
80 mg, the droplet size decreased to 30–150 μm.
Hydroformylation of 1-dodecene
Based on the catalytic cycle, two possible isomers, linear and
branched aldehydes, can be produced from alkenes with more
than two carbon atoms. In industry, linear aldehydes are more
desirable.27 Consequently, regioselectivity of hydroformylation
should be considered in addition to activity and chemo-
selectivity. Xantphos based on xanthene backbones with an
appropriate “natural” bite angle can improve the n/b ratio of
aldehydes via controlling the olefin insertion step.28 Thus, sul-
fonated xantphos ligand (sulfo-xantphos) was chosen as the
IL/water-soluble ligand in this work.
To make a comparison of the catalytic performance of the
IL-oil biphase system, IL-oil micelle system and IL-in-oil
Pickering emulsion system, the hydroformylation of 1-dode-
cene was performed under identical conditions (Table 2).
Taking stability into consideration, the IL-in-oil emulsion
system with DMSN-C18N-0.8 as the stabilizer was selected to
study the catalytic performance of the Pickering emulsion.
Because of the high regioselectivity of sulfonated xantphos
ligand, 1-dodecene could be converted to the corresponding
aldehyde with an n/b ratio varying in the range of 96 : 4 to
98 : 2 in all the abovementioned catalytic systems, but big
differences in activity and chemoselectivity were observed. The
IL-oil biphase system could only afford 47% conversion with
aldehyde selectivity of 89%. The by-products were isomeric
olefins and dodecane generated from isomerization and hydro-
genation of 1-dodecene. The reason for low conversion was
Table 2 Catalytic performances of Rh-sulfo-xantphos in biphase,
micelle and Pickering emulsion systems in hydroformylation of 1-dode-
cene and 1-decenea
The n/b ratio of the Pickering emulsion system with
different stabilizer amount was very high and only varied
slightly from 97 : 3 to 94 : 6, irrespective of the droplet size
(Table 2). However, the conversion and chemoselectivity
depended strongly on the droplet size. As the droplet size
decreased from 150–400 μm to 50–200 μm, the conversion
P/Rh
ratio
Conv.
(%)
Sel.b
(%)
TOFd
Solid material
n/b c
(h−1
)
No
CTAB
15
15
15
15
15
5
25
15
15
47
91
93
54
81
95
46
92
62
89
90
94
77
79
87
73
68
81
98 : 2
97 : 3
98 : 2
97 : 3
94 : 6
95 : 5
98 : 2
92 : 8
93 : 6
45
315
413
49
144
260
127
250
243
DMSN-C18N-0.8
DMSN-C18N-0.8e
DMSN-C18N-0.8f
DMSN-C18N-0.8
DMSN-C18N-0.8
DMSN-C18N-0.8g
MCM-C18N-1.8
(from 54% to 93%) and TOF (from 49 h−1 to 413 h−1
)
increased sharply accompanied by an increase in chemo-
selectivity from 77% to 94%. Further decreasing the droplet
size decreased both activity and chemoselectivity. This is poss-
ibly due to the existence of excessive stabilizers, which may
hinder the mass transfer during the catalytic process.
In previous research, we found that the Pickering emulsion
system formed with MCM-41 nanospheres afforded higher
activity than that formed with nonporous silica nanopsheres
in the hydroformylation of 1-octene.16 Therefore, the influence
a Reaction conditions: Rh 4.5 × 10−3 mmol, 60 mg of solid materials or
30 mg of CTAB, 1.0 mL of [BMIM][BF4], 1.0 mL of 1-dodecene (tetra-
decane as the internal standard), S/C = 1000, 110 °C, 20 bar CO/H2
(1 : 1), 1000 rpm, 12 h, 25 mL reactor. b Selectivity to aldehyde.
c Normal/branched. d TOF (mmol 1-dodecene per mmol Rh) was calcu-
lated with conversion less than 30%. e 40 mg of DMSN-C18N-0.8.
f 80 mg of DMSN-C18N-0.8. g 1-Decene was used as the substrate.
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Paper
of the porous structure of the solid stabilizer on the catalytic stained 1-dodecene could not reach the bottom of the column
performances of the Pickering emulsion was investigated even after 120 h. The diffusion rate of 1-dodecene in the
using DMSN with dendritic mesoporous structure and Pickering emulsion with DMSN-C18N-0.8 and MCM-C18N-1.8
MCM-41 with 2-D hexagonal mesoporous structure as as stabilizers was 1.2 mm h−1 and 0.8 mm h−1, respectively.
model for comparison (Table 2). DMSN-C18N-0.8 and The faster diffusion rate of 1-dodecene could be the main
MCM-C18N-1.8 had similar surface wettability and particle reason for the higher activity and chemoselectivity of the
size, but different textural parameters (Table 1). The IL-in-oil Pickering emulsion with DMSN-C18N-0.8 as the stabilizer. In a
emulsion could be efficiently formed with MCM-C18N-1.8 and simplified model, the interface area was defined as the cross
the droplet size and stability of the emulsion were comparable section of nanoparticles floated on the oil-IL interface.32 This
with emulsion that formed with DMSN-C18N-0.8 (Fig. 3 and is true for nonporous nanoparticles. In case of porous nano-
Fig. S4†). Pickering emulsion with DMSN-C18N-0.8 as the particles, their nanochannels can also contribute to the inter-
stabilizer afforded much higher TOF (413 versus 213 h−1), con- face area. This field is still in its infancy and more theoretical
version (93% versus 62%) and selectivity to aldehyde (94% studies are needed.
versus 81%) than that with MCM-C18N-1.8 as the stabilizer.
Another determining factor that could influence both the
For further understanding the difference in catalytic per- activity and selectivity of the hydroformylation reaction is the
formance, the diffusion rate of 1-dodecene in the above two molar ratio of Rh/ligand.33 Hence, the influence of P/Rh ratio
Pickering emulsion systems was measured in a column on the catalytic performance of the IL-in-oil Pickering emul-
(Fig. 5).31 First, the IL-in-oil Pickering emulsion was trans- sion was investigated (Table 2). As the P/Rh ratio increased
ferred in a column. Then, 1-dodecene stained with Sudan I from 5 to 25, the n/b ratio increased from 95 : 5 to 98 : 2. The
was packed carefully on the top of the Pickering emulsion. For TOF and chemoselectivity reached the highest at P/Rh ratio of
the Pickering emulsion with DMSN-C18N-0.8 as the stabilizer, 15. Lower activity and chemoselectivity were obtained with P/
the stained 1-dodecene gradually passed through the emulsion Rh ratio of 25. The influence of P/Rh ratio on the catalytic per-
to elute out and the oil phase in the emulsion was totally formance is possibly due to the variation in the dynamic
replaced by sustained 1-dodecene within 84 h. But for the balance state of the coordination of Rh and ligands.34
Pickering emulsion with MCM-C18N-1.8 as the stabilizer, the
Under optimized reaction conditions, the IL-in-oil
Pickering emulsion showed TOF of 413 h−1, chemoselectivity
of 94% and n/b ratio of 98 : 2 in the hydroformylation of
1-dodecene. Table S1† lists the catalytic performance of
biphase systems reported previously (with Rh-sulfo-xantphos
as ligand) in hydroformylation of long-chain alkenes. For pre-
viously reported oil–water or oil-IL biphase systems, the TOF
for the hydroformylation of long-chain alkenes is less than
40 h−1, far less than that of the IL-in-oil Pickering emulsion
reported in this work. The catalytic activity of the IL-in-oil
Pickering emulsion approaches that of the water-in-oil micro-
emulsion (TOF of 642 h−1, nonionic surfactant as emulsifier),
verifying the high efficiency of the current oil-IL Pickering
emulsion system.
Hydroformylation of 1-dodecene in water-in-oil and IL-in-oil
Pickering emulsion
Catalytic performances of water-in-oil and IL-in-oil Pickering
emulsions were compared with that using DMSN-C18N-0.8 as
the stabilizer in the hydroformylation of 1-dodecene (Tables 2
and 3). The water-in-oil emulsion exhibited similar conversion
and n/b ratio as the IL-in-oil emulsion. But the latter exhibited
much higher chemoselectivity (94% versus 69%). Compared
with water, ILs have higher solubility towards lipophilic sub-
strates35 and even toward CO.36 The unique properties of ILs
prompted the reaction to go through hydroformylation path-
ways for enhancing chemoselectivity to aldehydes.
It is well known that triphenylphosphine (TPP) and xant-
phos have different bite angles, which greatly affects the n/b
ratio (Scheme 1).37 Xantphos/sulfo-xantphos gave much higher
n/b ratio and chemoselectivity than TPP/TPPTS did (Table 3).
Unexpectedly, the IL-in-oil emulsion using the mixture of
Fig. 5 Photographs of a flow Pickering emulsion (5 mL of [BMIM][BF4],
5 mL of 1-dodecane and 0.30 g of particle stabilizer (DMSN-C18N-0.8
or MCM-C18N-1.8)) column with 1-dodecene stained Sudan
mg mL−1) on the top, 1.05 cm in the column diameter.
I (0.1
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Paper
Green Chemistry
Table 3 Catalytic performance of Rh-TPPTS/sulfo-xantphos in different reaction systemsa
Reaction system
Ligand
Conv. (%)
Sel.b (%)
n/bc
IL-in-oil Pickering emulsion
IL-in-oil Pickering emulsiond
Water-in-oil Pickering emulsiond
Homogeneous reactione
TPPTS
67
54
88
>99
>99
46
21
53
37
97
43
99
99
35
78
30
69
69 : 31
98 : 2
TPPTS/sulfo-xantphos
TPPTS/sulfo-xantphos
TPP
TPP/xantphos
TPPTS
TPPTS/sulfo-xantphos
TPPTS
Sulfo-xantphos
80 : 20
76 : 24
74 : 26
74 : 26
90 : 10
74 : 26
98 : 2
Homogeneous reactione
IL-oil biphase reaction
IL-oil biphase reaction
Water-in-oil Pickering emulsion
Water-in-oil Pickering emulsion
94
a Reaction conditions: Rh 4.5 × 10−3 mmol, S/C = 1000, P/Rh = 15, 110 °C, 20 bar CO/H2 (1 : 1), 1000 rpm, 12 h, 25 mL reactor. b Selectivity to alde-
hyde. c Normal/branched. d TPPTS/ sulfo-xantphos = 13/1. e Homogeneous system with toluene (2 mL) as solvent.
drance around the phosphorus atom, which may cause
difficulty in the coordination of Rh with TPPTS. It has little
effect on the coordination of sulfo-xantphos as the sulfonic
group is attached at the 2 and 7 positions of xantphos.
Therefore, Rh-sulfo-xantphos complexes are the main active
species during the catalytic process of ILs even in the presence
of a large amount of TPPTS. For these reasons, a high n/b ratio
and chemoselectivity could be obtained. In water or toluene,
Scheme 1 Molecular structure of sulfo-xantphos, TPPTS and ILs used
in this work.
there are no such strong interactions. As a result, Rh-TPPTS
complexes are the main active species and n/b ratio and
chemoselectivity could not be improved sharply in the pres-
ence of a small amount of sulfo-xantphos.
The recycling stability of water-in-oil and IL-in-oil Pickering
emulsion systems was investigated (Fig. 6). In the reaction
TPPTS and sulfo-xantphos as the ligand (sulfo-xantphos/
TPPTS ratio as low as 1/13) gave an n/b ratio and chemo-
selectivity similar to those obtained using sulfo-xantphos as
the ligand, though the conversion was less. However, the n/b
ratio (80 : 20 versus 74 : 26) and chemoselectivity (43% versus
30%) of the water-in-oil emulsion using the mixture of TPPTS
and sulfo-xantphos as the ligand were only slightly higher
than that using TPPTS as the ligand. It revealed that a small
amount of sulfo-xantphos in the IL-in-oil emulsion could
greatly enhance both the n/b ratio and chemoselectivity. In
order to further understand this unique phenomenon, hydro-
formylation of 1-dodecene was performed in a homogeneous
system with toluene as the solvent and in the IL-oil biphase
system with different ligands (Table 3). No obvious difference
was observed in the conversion and n/b ratio between TPP and
the mixture of TPP and xantphos in the homogeneous reaction
system, suggesting that a small amount of xantphos has little
effect on the n/b and chemoselectivity with toluene as the
solvent. The results of the IL-oil biphase system were similar to
those of the IL-in-oil Pickering emulsion system in that the n/b
ratio and chemoselectivity could be greatly increased with a
small amount of sulfo-xantphos. Consequently, the enhance-
ment effect of n/b and chemoselectivity in the presence of a
small amount of sulfo-xantphos could be possibly related with Fig. 6 Recycling stability of (a) IL-in-oil Pickering emulsion system and
(b) water-in-oil Pickering emulsion system in the hydroformylation of
1-dodecene (with DMSN-C18N-0.8 as the stabilizer, the reaction con-
ditions are the same as those in Table 2), (c) TEM image of the
DMSN-C18N-0.8 sample after 6 cycles, (d) photograph and microscopic
ILs. It is well known that ILs and TPPTS/sulfo-xantphos are all
composed of cations and anions (Scheme 1), so that anions of
TPPTS/sulfo-xantphos could interact with cations of ILs via
electrostatic interactions.38 The interaction of the sulfonic
image of the Pickering emulsion formed with DMSN-C18N-0.8 at the
group of TPPTS and ILs tended to increase the steric hin- beginning of the fifth cycle (scale bar, 200 μm).
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Paper
system, only 1-dodecene was used as the oil phase. After reac- Pickering emulsion may have potential applications in other
tion, up to 93% of 1-dodecene was converted to the corres- liquid–liquid biphase reactions involving diffusion problems.
ponding aldehyde under optimized conditions. At the end of
the reaction, the emulsion was destroyed on account of change
in polarity. The influence of the concentration of tridecyl alde-
hyde on the emulsion stability is shown in Fig. S5.† With
increasing aldehyde concentration, the emulsion volume
gradually decreased. The existence of the emulsion could still
Tetraethyl orthosilicate (TEOS, AR), hexadecyltrimethyl-
be observed in 1-dodecene conversion less than 80%,
suggesting that hydroformylation occurs mainly in the emul-
cene and cyclohexane were purchased from Shanghai
sion system. The silica nanospheres were very stable and the
emulsions could be easily formed by adding 1-dodecene after
removing the oil phase at the end of the reaction (Fig. 6d and
Fig. S6†).
After each cycle, the oil phase was separated after centrifu-
gation, and a fresh oil phase was added again into the mixture
of solid and IL or water. The Pickering emulsion regenerated
by vigorous shaking was directly used for the next run. For the
water-in-oil Pickering emulsion system, the obvious decreases
in conversion, aldehyde selectivity and n/b ratio were observed
for the third cycle, while for the IL-in-oil Pickering emulsion
system, within six cycles, no obvious decreases in conversion,
selectivity and n/b ratio could be observed, showing high re-
cycling stability. After each cycle, the Rh content in the oil
phase was analyzed by ICP analysis. For six cycles, the total Rh
leaching in the oil phase was about 0.33%, indicating that the
IL-in-oil Pickering emulsion was an efficient and stable reac-
tion system for hydroformylation of long-chain alkenes. The
TEM image of the DMSN-C18N-0.8 sample after six cycles was
almost identical to that of fresh one, showing that the stabil-
izer was stable enough in the hydroformylation process
(Fig. 6c).
Experimental section
Chemicals and reagents
All chemicals were used without further purification.
ammonium bromide (CTAB), toluene, trimethylamine, 1-dode-
Chemical Reagent Company of the Chinese Medicine Group.
Metal precursor Rh(acac)(CO)2 (acac
=
acetylacetonato)
was purchased from the Energy Chemical Company.
Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chlor-
ide (C18N, 60% in MeOH) was purchased from Acros
Company. 1-Dodecene, dodecane, undecanal and triphenyl-
phosphine were purchased from TCI Company. 9,9-Dimethyl-
4,5-bis(diphenylphosphino)xanthene (xantphos) was pur-
chased from Tianjin Heowns Biochemical Technology
Company. Tris(m-sulfonatophenyl) phosphine and tridecanal
were purchased from J&K Chemicals. 1-Butyl-3-methyl-
imidazolium tetrafluoroborate ([BMIM][BF4]) was purchased
from Ark Pharm Company. Rhodamine 6G was purchased
from Aladdin Company. 9,9-Dimethyl-2,7-bissulfonato-4,5-bis
(diphenylphosphino)xanthene sodium salt (sulfo-xantphos)
was prepared according to a reported method.39 MCM-41
nanospheres40 and dendritic mesoporous silica nanospheres41
with a diameter of ∼100 nm were synthesized according to a
reported method.
Characterization
Transmission electron microscopy (TEM) was performed using
an FEI Tecnai G2 Spirit at an acceleration voltage of 120 kV.
Thermogravimetric analysis (TGA) was performed under air
atmosphere with a heating rate of 5 °C min−1 to 900 °C using a
NETZSCHSTA-449F3 thermogravimetric analyzer. FT-IR spectra
were collected on a Nicolet Nexus 470 IR spectrometer with
KBr pellets. Nitrogen physical adsorption measurements were
carried out on Micromeritics ASAP2020 volumetric adsorption
analyzer. Before the measurements, the samples were
degassed at 393 K for 6 h. The BET surface area was evaluated
from the data in the relative pressure range P/P0 of 0.05 to
0.25. The total pore volume was estimated from the amount
adsorbed at the P/P0 value of 0.99. The pore size distribution
was derived from the adsorption branch using the BJH
method. The confocal laser scanning microscopy images of
the emulsions were taken on Andor Spinning Disk Evolution
WD microscope. The content of rhodium in the organic phase
was determined by ICP-MS analysis.
Conclusions
In this work, an IL-in-oil Pickering emulsion was prepared for
catalyzing the hydroformylation of 1-dodecene. The catalytic
activity of the IL-in-oil Pickering emulsion was much higher
than that of IL-oil biphase and micelle systems, revealing the
role of the unique properties of the Pickering emulsion in
accelerating reaction rate. The diffusion rate of the hydrofor-
mylation of 1-dodecene through the Pickering emulsion with
DMSN-C18N-0.8 as the stabilizer was much faster than that
with MCM-C18N-1.8 as the stabilizer, which was possibly
attributed to the large pore diameter and open porous struc-
ture of DMSN. The IL-in-oil Pickering emulsion afforded
higher chemoselectivity than the water-in-oil Pickering emul-
sion owing to the high solubility of ILs towards lipophilic sub-
strates and CO. In ILs, Rh preferred to coordinate with sulfo-
xantphos even in the presence of a large amount of TPPTS,
Surface modification of DMSN16
probably because of the increased steric hindrance of TPPTS After degassing of dendritic mesoporous silica nanospheres
induced by its electrostatic interaction with ILs. The IL-in-oil (DMSN, 1 g) in a round bottom flask at 120 °C for 3 h, toluene
Pickering emulsion could be recycled at least 5 times without (30 ml) and Et3N (1.2 ml) were added under N2 atmosphere.
any obvious loss of conversion and selectivity. The stable IL-oil Then C18N in the MeOH solution (0.76 mL) was added and
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Paper
Green Chemistry
the mixture was stirred under reflux conditions at 110 °C for sion systems was calculated briefly with diffusion distant
12 h. After cooling down to room temperature, the solid divided by time.
materials were filtered, washed with methanol and toluene,
and subsequently dried under vacuum at room temperature.
The materials were denoted as DMSN-C18N-X, in which X Conflicts of interest
refers to the mmol amount of C18N per gram of DMSN used
There are no conflicts to declare.
during the grafting process. For example, DMSN-C18N-0.8 rep-
resents the material synthesized with 1.0 g of DMSN and
0.8 mmol of C18N.
Acknowledgements
Construction of Pickering emulsion system for catalyzing the
hydroformylation reaction
This work was supported by the National Key Research and
Development Program of China (No. 2017YFB0702800), the
NSFC (No. 21232008, 21621063) and the Strategic Priority
Research Program of the Chinese Academy of Sciences Grant
(No. XDB17020200).
IL-in-oil Pickering catalytic emulsion system: a mixture of
Rh(acac)(CO)2 (11.67 mg, 4.5 mmol) and desired amount of
phosphine ligand were dissolved in 2 mL of degassed methanol
for 0.5 h. After addition of 10 ml of [BMIM][BF4], the solution
was stirred under N2 atmosphere overnight. The catalyst solution
was obtained after removing methanol under vacuum at 40 °C.
DMSN-C18N-X, 1-dodecene (1 mL) and tetradecane (0.3 mL
as internal standard) were added to a glass vial. After ultra-
sonication for 1 min, the catalyst solution prepared above
(1 mL) was added. Subsequently the mixture was vigorously
shaken for about 15 min to form the IL-in-oil emulsion. Then
the emulsion was transferred to an autoclave (25 mL). Before
the reaction, the autoclave was purged three times with CO/H2
(molar ratio = 1 : 1). Then, the pressure was raised to 20 bar
and the autoclave was stirred in an oil bath at 110 °C for a
desired time interval. After the autoclave was cooled down to
room temperature, the gas was carefully released. The organic
layer was separated by centrifugation, diluted with toluene and
analyzed by gas chromatography on a HP-5 capillary column
(30 m × 0.32 mm × 0.25 mm).
For the catalyst recycling, the IL phase containing catalyst
and solid materials were obtained after centrifugation and
washed with toluene 3 times. Then the substrate and internal
standard were added for the next catalytic reaction.
Water-in-oil Pickering catalytic emulsion system: the con-
struction of the water-in-oil Pickering emulsion system is
similar to that of the IL-in-oil Pickering emulsion system with
the exception that water is used instead of [BMIM][BF4].
The IL-oil biphasic catalytic reaction system was prepared
in a method similar to that of the IL-in-oil Pickering emulsion
system with the exception that no DMSN was used.
Notes and references
1 R. Franke, D. Selent and A. Borner, Chem. Rev., 2012, 112,
5675.
2 F. Hebrard and P. Kalck, Chem. Rev., 2009, 109, 4272.
3 F. Agbossou, J. F. Carpentier and A. Mortreaux, Chem. Rev.,
1995, 95, 2485.
4 X. M. Zhang, S. M. Lu, M. M. Zhong, Y. P. Zhao and
Q. H. Yang, Chin. J. Catal., 2015, 36, 168.
5 H. Adkins and G. Krsek, J. Am. Chem. Soc., 1949, 7, 3051.
6 S. M. Lu and H. Alper, J. Am. Chem. Soc., 2003, 125, 13126.
7 P. W. N. M. van Leeuwen and C. Claver, Rhodium Catalysed
Hydroformylation, Kluwer, Dordrecht, 2000.
8 C. C. Miyagawa, J. Kupka and A. Schumpe, J. Mol. Catal. A:
Chem., 2005, 234, 9.
9 T. Ngai and S. A. F. Bon, Particle-stabilized emulsions and
colloids: formation and applications, Cambridge, UK, 2015.
10 M. Pera-Titus, L. Leclercq, J. M. Clacens, F. De Campo and
V. Nardello-Rataj, Angew. Chem., Int. Ed., 2015, 54, 2006.
11 F. Hapiot, S. Menuel, M. Ferreira, B. Léger, H. Bricout,
S. Tilloy and E. Monflier, ACS Sustainable Chem. Eng., 2017,
5, 3598.
12 M. Chevry, T. Vanbésien, S. Menuel, E. Monflier and
F. Hapiot, Catal. Sci. Technol., 2017, 7, 114.
13 J. Potier, S. Menuel, M. H. Chambrier, L. Burylo, J. F. Blach,
P. Woisel, E. Monflier and F. Hapiot, ACS Catal., 2013, 3,
1618.
14 J. Potier, S. Menuel, E. Monflier and F. Hapiot, ACS Catal.,
2014, 4, 2342.
15 K. Kunna, C. Muller, J. Loos and D. Vogt, Angew. Chem., Int.
Ed., 2006, 45, 7289.
General procedure for preparing continuous flow Pickering
emulsion31
A glass column with a sand filter and a valve at the bottom was
chosen as the column reactor. The inner diameter of the 16 Y. Zhao, X. Zhang, J. Sanjeevi and Q. H. Yang, J. Catal.,
column is 1.05 cm. The desired volume of the emulsion was 2016, 334, 52.
prepared as mentioned before. After one piece of membrane 17 H. Q. Yang, L. M. Fu, L. J. Wei, J. F. Liang and B. P. Binks,
(oil-permeable but water-impermeable) was spread out on the J. Am. Chem. Soc., 2015, 137, 1362.
sand filter, 3 g of silica was added. The prepared Pickering 18 J. P. Huang and H. Q. Yang, Chem. Commun., 2015, 51,
emulsion was poured carefully into this column; after that, 7333.
1-dodecene dyed with Sudan I was added on top of the emul- 19 L. Kollar and J. Dupont, Ionic Liquids (ILs) in Organometallic
sion. The diffusion rate of dyed 1-dodecene in different emul-
Catalysis, Verlag, Heidelberg, 2015.
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Paper
20 Q. H. Zhang, S. G. Zhang and Y. Q. Deng, Green Chem., 31 M. Zhang, L. Wei, H. Chen, Z. Du, B. P. Binks and
2011, 13, 2619. H. Q. Yang, J. Am. Chem. Soc., 2016, 138, 10173.
21 S. L. Desset, S. W. Reader and D. J. Cole-Hamilton, Green 32 B. P. Binks and J. H. Clint, Langmuir, 2002, 18, 1270.
Chem., 2009, 11, 630.
22 M. Haumann and A. Riisager, Chem. Rev., 2008, 108, 1474.
33 C. Li and Y. Liu, Bridging Heterogeneous and Homogeneous
Catalysis, Wiley-VCH, Weinheim, Germany, 2014.
23 W. J. Zhou, L. Fang, Z. Fan, B. Albela, L. Bonneviot, F. De 34 T. Hamerla, A. Rost, Y. Kasaka and R. Schomacker,
Campo, M. Pera-Titus and J. M. Clacens, J. Am. Chem. Soc.,
ChemCatChem, 2013, 5, 1854.
2014, 136, 4869.
24 M. Li, R. L. Harbron, J. V. Weaver, B. P. Binks and S. Mann,
35 P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis,
Wiley-VCH, Weinheim, Germany, 2007.
Nat. Chem., 2013, 5, 529.
25 E. Vignati, R. Piazza and T. P. Lockhart, Langmuir, 2003, 19,
36 C. A. Ohlin, P. J. Dyson and G. Laurenczy, Chem. Commun.,
2004, 1070.
6650.
37 M. Kranenburg, Y. E. M. Vanderburgt, P. C. J. Kamer,
P. W. N. M. van Leeuwen, K. Goubitz and J. Fraanje,
Organometallics, 1995, 14, 3081.
26 L. Davarpanah and F. Vahabzadeh, Starch-Starke, 2012, 64,
898.
27 A. Mann and M. Taddei, Hydroformylation for Organic 38 A. Riisager, R. Fehrmann, M. Haumann, B. S. K. Gorle and
Synthesis, Verlag, Heidelberg, 2013. P. Wasserscheid, Ind. Eng. Chem. Res., 2005, 44, 9853.
28 M. Kumar, R. V. Chaudhari, B. Subramaniam and 39 A. Osichow and S. Mecking, Chem. Commun., 2010, 46,
T. A. Jackson, Organometallics, 2015, 34, 1062. 4980.
29 G. La Sorella, G. Strukul and A. Scarso, Green Chem., 2015, 40 A. Firouzi, F. Atef, A. G. Oertli, G. D. Stucky and
17, 644. B. F. Chmelka, J. Am. Chem. Soc., 1997, 119, 3596.
30 F. Xue, Y. B. Zhang, F. W. Zhang, X. M. Ren and H. Q. Yang, 41 D. Shen, J. Yang, X. Li, L. Zhou, R. Zhang, W. Li, L. Chen,
ACS Appl. Mater. Interfaces, 2017, 9, 8403.
R. Wang, F. Zhang and D. Zhao, Nano Lett., 2014, 14, 923.
This journal is © The Royal Society of Chemistry 2017
Green Chem.