Ionic liquid-functionalized amphiphilic Janus nanosheets afford highly accessible interface for asymmetric catalysis in water

Article abstract of DOI:10.1016/j.jcat.2021.01.022

High oil/water interfacial area together with accessible interfaces for regents is the key to achieving efficient asymmetric catalysis in water. Herein, by taking advantage of the excellent interfacial activity of Janus nanosheets (JNS), as well as the unique compatibility of imidazolium ionic liquid (IL), we developed a series of IL-functionalized amphiphilic Janus mesosilica nanosheets which afford highly accessible reaction interfaces for highly enantioselective sulfoxidation in water. The JNS-typed chiral salen Ti^IV catalysts were prepared by selectively decorating hydrophobic chiral salen Ti^IV complex on one side of Janus mesosilica nanosheets through the imidazolium-based IL linker. Benefiting from the two-dimensional porous Janus structure, as well as the compatible IL linker, the IL-tagged JNS catalysts afforded high oil/water interfacial areas and highly accessible reaction interface for sulfides and H₂O₂, significantly accelerating asymmetric sulfoxidation in water using H₂O₂ as an oxidant. In addition, they can be facilely recovered for stable reuse by simple centrifugation.

Full text of DOI:10.1016/j.jcat.2021.01.022

Journal of Catalysis 395 (2021) 236–245
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Ionic liquid-functionalized amphiphilic Janus nanosheets afford highly
accessible interface for asymmetric catalysis in water
⇑
Chaoping Li, Su Liu, Yibing Pi, Jingwen Feng, Zewei Liu, Shiye Li, Rong Tan
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education), College of Chemistry & Chemical Engineering, Hunan Normal University, No. 36, South Lushan Road, Changsha, Hunan 410081,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
High oil/water interfacial area together with accessible interfaces for regents is the key to achieving effi-
cient asymmetric catalysis in water. Herein, by taking advantage of the excellent interfacial activity of
Janus nanosheets (JNS), as well as the unique compatibility of imidazolium ionic liquid (IL), we developed
a series of IL-functionalized amphiphilic Janus mesosilica nanosheets which afford highly accessible reac-
tion interfaces for highly enantioselective sulfoxidation in water. The JNS-typed chiral salen TiIV catalysts
were prepared by selectively decorating hydrophobic chiral salen TiIV complex on one side of Janus
mesosilica nanosheets through the imidazolium-based IL linker. Benefiting from the two-dimensional
porous Janus structure, as well as the compatible IL linker, the IL-tagged JNS catalysts afforded high
oil/water interfacial areas and highly accessible reaction interface for sulfides and H2O2, significantly
accelerating asymmetric sulfoxidation in water using H2O2 as an oxidant. In addition, they can be facilely
recovered for stable reuse by simple centrifugation.
Received 20 October 2020
Revised 17 January 2021
Accepted 18 January 2021
Available online 28 January 2021
Keywords:
Aqueous asymmetric catalysis
Ionic liquid
Janus mesosilica nanosheets
Pickering emulsion
Asymmetric sulfoxidation
Ó 2021 Elsevier Inc. All rights reserved.
1. Introduction
Amphiphilic JNS with asymmetric wettability on opposing sides
have recently received increasing concerns in the Pickering interfa-
Asymmetric catalysis in water is of great interest in chemical
industry not only due to the green and sustainable process, but also
because water often exerts a positive effect on chemical reactivity
and selectivity [1,2]. However, they often suffered from low reac-
tion rate because of limited interfacial area and huge mass transfer
resistance. Surfactant-like catalysts can enhance the mass transfer
between oil/water phases through emulsification, but are not
favorable for industrial use due to the troublesome separation
and reusability [3]. Janus colloids which combine the unique fea-
tures of both traditional surfactant (e.g., amphiphilicity) and inor-
ganic materials (e.g., heterogeneous properties) have emerged as
the alternative interfacial catalysts for aqueous catalysis [4–6].
Compared with traditional surfactants, the Janus colloids offer a
higher interfacial activity, forming more stable Pickering emulsion
with maximal oil/water interfacial area [7–9]. Moreover, the inter-
facial reaction relying on solid particles allows for facile separation
and recycling of catalysts. Organic products should be in situ sep-
arated from the aqueous systems due to the difference in water–oil
solubility, which avoids the complicated separation and purifica-
tion process [10].
cial catalysis, due to their excellent interfacial activity and highly
anisotropic shape [11–13]. Their excellent interfacial activity
enables them to be steadily adhered on the oil/water interface, giv-
ing stable Pickering emulsion with maximal oil/water interfacial
area. Highly anisotropic shape greatly restricted their rotation of
the JNS at the oil/water interface, thus making the emulsion more
stable [14,15]. More interestingly, two-dimensional sheet-shaped
materials assembled at oil/water interfaces provide a short diffu-
sion distance between oil and water, which is critical for the mass
transfer on the interfaces [16–18]. However, with a strong interfa-
cial adhesion, JNS themselves may serve as a physical barrier
between the oil/water interfaces, making the reaction interfaces
inaccessible. Yang et al. synthesized mesoporous JNS and loaded
ultrafine Pd nanoparticles in the mesochannels. The abundant
mesochannels present on JNS provided efficient passageway for
reagents, ensuring accessibility of the catalytically active interface
for nitroarene hydrogenation in water [19]. Nonetheless, this strat-
egy only allows ultrafine catalytic species within the pores. If bulk
chiral metal complex is involved, it inevitably obstructs the diffu-
sion of reactants. More importantly, the nanosized pores may
restrain the conformational freedom of embedded chiral catalytic
species, which is undesirable for asymmetric catalysis [20,21]. If
bulk catalytic active species is selectively decorated on one side
⇑
Corresponding author.
E-mail address: yiyangtanrong@126.com (R. Tan).
https://doi.org/10.1016/j.jcat.2021.01.022
0021-9517/Ó 2021 Elsevier Inc. All rights reserved.

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
of mesoporous JNS, leaving the channels free, it would be possible
to achieve highly accessible reaction interfaces with excellent
asymmetric induction. However, the JNS-based chiral catalysts
have rarely been reported for aqueous asymmetric catalysis to
date.
according to the described procedure in Ref. [28]. Polystyrene
(PS) microsphere was synthesized according to the procedure in
Ref. [29]. PS@mSiO2 where porous silica layer was coated on PS
microsphere was synthesized according to the procedure in Ref.
[19]. PS@SiO2 where non-porous silica layer was coated on PS
microsphere was synthesized according to the procedure in
Ref. [30].
Bulk chiral metallosalen complexes are privileged catalysts, as
demonstrated by their successful application in a wide variety of
challenging field ranging from materials chemistry to organic syn-
thesis [22,23]. Herein, we selectively grafted hydrophobic chiral
salen TiIV complex on one side surface of hydrophilic mesosilica
nanosheet, to develop the amphiphilic JNS-type Ti(salen) catalyst
for asymmetric sulfoxidation in water. An imidazolium IL was
introduced as a flexible linker between the bulk Ti(salen) units
and mesosilica surface. Apart from ensuring the necessary confor-
mation freedom of Ti(salen) groups, the IL modifier with unique
compatibility will fine-tune the surface properties of mesosilica
and transfer the ILs’ properties to material, which thus makes the
reaction interface more accessible for reactants [24,25]. Benefiting
from the two-dimensional porous Janus structure, as well as the
unique IL linker, our JNS-type Ti(salen) catalysts not only displayed
an excellent interfacial activity but also provide a highly accessible
reaction interface for organic sulfides and aqueous H2O2 in asym-
metric sulfoxidation in water. Extremely high activity (conversion
of 92–98%), chemo- (95–99%) and enantioselectivity (88–99%)
were achieved for a wide range of sulfides, while traditional chiral
salen TiIV complex was far less efficient. Furthermore, the catalysts
can be quantitatively recovered from the aqueous system by cen-
trifugation for steady reuse. Such IL-functionalized JNS materials
combine the benefits of chiral phase-transfer and heterogeneous
catalyst, and avoid the tedious procedures of separation, which is
a benefit for energy-saving and industrial applications.
2.2. Analytical methods
Fourier-transform infrared spectroscopy (FT-IR) spectra were
recorded on AVATAR 370 Thermo Nicolet spectrophotometer using
potassium bromide pellets in the range of 400–4000 cmꢀ1. Nuclear
magnetic resonance (NMR) spectra were represented by a BRUKER
AVANCE-500 spectrometer. Titanium contents of samples were
determined by inductively coupled optical emission spectra (ICP-
OES) on an Agilent 5110. Transmission electron microscopy
(TEM) micrographs were obtained on a Microscope Tecnai F20 at
an accelerating voltage of 200 kV. Scanning electron microscopy
(SEM) was performed with a HITACHI S-4800 apparatus operated
at an accelerating voltage of 15 kV. N2 adsorption–desorption iso-
therms were obtained on TriStar 3000-Micromeritics. The compos-
ites were degassed at 120 °C for 6 h before measuring the
isotherms. Water contact angles (WCA) of the samples were mea-
sured by the static sessile drop method on a Theon TX500TM (Kono
Corp.) instrument. Samples were directly compressed without the
aid of binder. A drop of deionized water was then placed on it and
imaged by camera. Microscope images of sulfide-in-water emul-
sions were obtained on BX53M (Olympus Corp.). The conversion
and chemoselectivity to chiral sulfoxides were analyzed by a
6890N gas chromatograph (Agilent Co.) using equipped with a cap-
illary column (HP19091G-B213, 30 m ꢁ 0.32 mm ꢁ 0.25
lm) and a
FID detector. Enantioselectivity (Ee value) of chiral sulfoxides was
2. Experimental
determined by HPLC analysis using Daicel Chiralpak AD column.
2.1. Materials and reagents
2.3. Preparation of the IL-functionalized JNS catalysts (denoted as JNS-
IL/Ti(salen)x)
Poly(vinyl pyrrolidone) (PVP, Mw = 40000), 2,2-azobis(2-methyl
propionitrile) (AIBN), N-[3-(triethoxysilyl)propyl]-4,5-dihydroimi
dazole and 2,20-azobis[2-methylpropionamidine] dihydrochloride
(AIBA) were obtained by Macklin. 3-Aminopropyltriethoxysilane
(APTES) and lysine were obtained from Aladdin. Tetraethoxysilane
(TEOS), phenyl methyl sulphide, phenyl ethyl sulphide and
chloroauric acid was obtained from Aldrich. 4-Methoxyphenyl
methyl sulfide, 2-methoxyphenyl methyl sulfide, and 4-
bromophenyl methyl sulfide were obtained from J&K. 2-tert-
Butyl phenol was purchased from Alfa Aesar. Other commercially
available chemicals and solvents were obtained from local suppli-
ers by standard procedure without further purification.
Phenyl n-butyl sulfide and phenyl n-hexyl sulfide were synthe-
sized according to Ref. [26]. Alkoxysilane reagents of (C2H5O)3Si-IL/
Ti(salen) and (C2H5O)3Si-Ti(salen) (Chart 1) were synthesized as
described procedure in Ref. [27]. [(R,R’)-[N,N’-(3,5-di-tert-
butylsalicylidene)-1,2- cyclohexanediaminato]titanium(IV) di-
isopropyl (denoted as neat complex, Chart 2) was synthesized
JNS-IL/Ti(salen)x were prepared via simply crushing hollow
mesosilica microspheres with two distinct surfaces, as shown in
Scheme 1.
Synthesis of PS@mSiO2. The PS@mSiO2 where a silica layer with
perpendicular mesochannels was coated on the PS microspheres
was synthesized via a sol–gel method [19]. PS microsphere
(1.92 g) and cetyltrimethyl ammonium bromide (CTAB, 2.6 mmol,
0.96 g) were added into mixed solvent of ethanol (120 mL) and
water (100 mL). After stirring for 30 min, TEOS (2.7 mmol,
0.6 mL) was quickly added. The obtained mixture was stirred at
room temperature for 6 h, and then kept undisturbed at 80 °C over-
night. After being centrifuged, PS@mSiO2 was washed with deion-
ized water, and subsequently dried under vacuum at room
temperature for 12 h.
Synthesis of JNS-IL/Ti(salen)x. PS@mSiO2 (1.0 g) was dispersed
in ethanol (50 mL) by ultrasound. Alkoxysilane reagent of
Chart 1. The structures of (C2H5O)3Si-IL/Ti(salen) and (C2H5O)3Si-Ti(salen).
237

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
Chart 2. Their representative structures of neat complex, JNS-IL/Ti(salen)x, JNS-Ti(salen), HNS-IL/Ti(salen), and nJNS-IL/Ti(salen).
Scheme 1. Synthesis of JNS-IL/Ti(salen)x (x = 0.38, 0.48, 0.60) and HNS-IL/Ti(salen).
(C2H5O)3Si-IL-Ti(salen) (0.4 mmol for JNS-IL/Ti(salen)0.38, 0.6 mmol
for JNS-IL/Ti(salen)0.48, and 1.0 mmol for JNS-IL/Ti(salen)0.60) was
then added to the dispersion, and the resulting mixture was
refluxed at 80 °C for 12 h. After centrifugation, the as-made
PS@mSiO2-IL/Ti(salen) sample was treated with THF (75 mL) at
25 °C for 12 h, to remove templates of CTAB and PS completely.
IL/Ti(salen)-coated hollow mesosilica microsphere was subse-
quently collected by centrifugation (10000 rpm, 10 min), washed
with ethanol for several times, and then dried under vacuum.
Crushing the IL/Ti(salen)-coated hollow mesosilica microspheres
gave Janus silica nanosheets of JNS-IL/Ti(salen)x as light yellow
powder (where x represents the titanium content in samples, x
= 0.38, 0.48, and 0.60 mmolꢂgꢀ1). IL/Ti(salen) and hydroxyl groups
are distinctly located on the corresponding sides of the mesosilica
0.38 mmolꢂgꢀ1. JNS-IL/Ti(salen)0.48: FT-IR:
c
max/cmꢀ1: 3451, 2954,
2924, 2871, 1719, 1652, 1631, 1551, 1466, 1384, 1315, 1187, 1089,
807, 692, 622, 569, 464. Titanium content: 0.48 mmolꢂgꢀ1. JNS-IL/
Ti(salen)0.60: FT-IR:
c
max/cmꢀ1: 3451, 2951, 2924, 2924, 2856,
1651, 1551, 1466, 1384, 1315, 1187, 1090, 808, 692, 622, 564,
470. Titanium content: 0.60 mmolꢂgꢀ1
.
2.4. Preparation of the IL-free JNS counterpart of JNS-Ti(salen)
An IL-free counterpart of JNS-Ti(salen) was also prepared
according to a similar preparation procedure to that of JNS-IL/Ti
(salen)x. During the procedure, the organosiloxane of (C2H5O)3Si-
Ti(salen) was used instead of the (C2H5O)3Si-IL-Ti(salen) to modify
the external surface of PS@mSiO2. After complete removal of CTAB
and PS templates, the obtained Ti(salen)-coated hollow mesosilica
microspheres were crushed into JNS-Ti(salen), where Ti(salen)
was directly decorated on one side surface of mesosilica
nanosheets, as shown in Chart 2. JNS-IL/Ti(salen)0.38: FT-IR: cmax
/
cmꢀ1: 3451, 2950, 2927, 2868, 1715, 1655, 1632, 1551, 1466,
1384, 1315, 1191, 1090, 807, 693, 618, 568, 466. Titanium content:
238

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
nanosheets through an alkyl linker. The representative structure of
3. Results and discussion
JNS-Ti(salen) was shown in Chart 2. FT-IR: c
max/cmꢀ1 3451, 3081,
3061, 3026, 2926, 1623, 1439, 1452, 1384, 1313, 1186, 1083, 813,
3.1. Preparation of catalysts
757, 697, 568, 466. Titanium content: 0.48 mmolꢂgꢀ1
.
Pickering emulsions offers an efficient platform for the asym-
metric catalysis in water, because it provides a high interfacial
reaction area per unit volume, improves the compatibility between
hydrophilic and hydrophobic reagents, and facilitates the mass
transfer [31]. Amphiphilic JNS with mesoporous structure are very
suitable as Pickering interfacial catalysts for the aqueous catalysis.
On one hand, the two-dimensional materials with highly anisotro-
pic shapes and surface chemistry can steadily adhere to immiscible
oil/water interface, forming stable Pickering emulsions and thus
providing larger interfacial areas. On the other hand, the abundant
mesoporous channels present at the interface benefit the contact
between the reactants inside and outside the droplets [27,32,33].
With those points in mind, we decided to selectively graft chiral
salen TiIV complex on one side of mesosilica nanosheet to develop
the amphiphilic JNS-type chiral salen TiIV catalyst for efficient
asymmetric sulfoxidations in water. Imidazolium-based IL was
used as a flexible linker between chiral salen TiIV complex and
mesosilica surface. Apart from ensuring conformational freedom
of Ti(salen), the IL linker also stabilizes the formed metallosalen
active intermediates, thereby further enhancing the catalytic effi-
ciency of the chiral metallosalen catalyst [34]. More importantly,
such an IL moiety with unique solvent power may transfer the
compatible properties to JNS material, which is expected to further
improve the accessibility of catalytically active interface [35].
The preparation of IL-functionalized amphiphilic JNS catalysts
was shown in Scheme 1. First, silica shell with perpendicular
mesochannels was coated on PS microspheres via a sol–gel method
by using TEOS as a silica source and CTAB as a soft template. The
obtained PS@mSiO2 was surface-modified with organosiloxane of
(C2H3O)3Si-IL/Ti(salen) through silylation. Since CTAB still occu-
pied the mesochannels, the bulk organosiloxane are difficult to dif-
fuse into mesochannels and internal surface of the mesosilica shell.
As a result, most of Ti(salen) groups were grafted on the external
surface of PS@mSiO2 through an IL linker. The Ti(salen) loading
could be fine tuned by changing the organosiloxane concentration.
Removal of the CTAB and PS affords the Janus hollow mesosilica
sphere which is covered by IL/Ti(salen) group. The hollow micro-
2.5. Preparation of homogeneous counterpart of HNS-IL/Ti(salen)
To investigate the ‘‘Janus effect” of JNS-IL/Ti(salen)x, a homoge-
neous counterpart of HNS-IL/Ti(salen) (Chart 2) where IL/Ti(salen)
groups were uniformly dispersed on both sides of the mesosilica
nanosheets was prepared as a control catalyst, as shown in
Scheme 1. PS@mSiO2 (1.0 g) was treated with THF (75 mL) at
25 °C for 12 h to remove CTAB and PS completely. The obtained
hollow silica microspheres were crushed into silica nanosheets,
and then modified with (C2H5O)3Si-IL/Ti(salen) (0.6 mmol,
0.59 g) in toluene (50 mL) at 80 °C for 12 h. Removal of the unre-
acted (C2H3O)3Si-IL-Ti(salen) afforded the HNS-IL/Ti(salen) as light
yellow powder. FT-IR (KBr):
c
max/cmꢀ1: 3451, 2953, 2924, 2875,
1718, 1656, 1632, 1551, 1466, 1385, 1319, 1090, 964, 808, 692,
619, 568, 466. Titanium content: 0.48 mmolꢂgꢀ1
.
2.6. Preparation of the nonporous JNS counterpart of nJNS-IL/Ti
(salen)
Furthermore, a nonporous counterpart of nJNS-IL/Ti(salen)
(Chart 2) was also prepared as the control catalyst to investigate
the ‘‘passageway effect” of mesochannels. The preparation proce-
dure was similar to that of JNS-IL/Ti(salen). PS@SiO2 where non-
porous silica layer was coated on PS microsphere [30] was used
instead of PS@mSiO2 during the procedure. The abundant surface
hydroxyl groups (AOH) present on PS@SiO2 were readily reacted
with the alkoxysilane reagent of (C2H5O)3Si-IL/Ti(salen) through
silylation, giving the IL/Ti(salen)-coated PS@SiO2 microspheres.
After removal of PS template, the obtained hollow spheres were
crushed into the nonporous nJNS-IL/Ti(salen). FT-IR (KBr): cmax
/
cmꢀ1: 3451, 2952, 2926 2870, 1715, 1651, 1628, 1551, 1466,
1384, 1315, 1187, 1090, 807, 692, 623, 569, 466. Titanium content:
0.49 mmolꢂgꢀ1
.
spheres were finally crushed by
a simple grind, giving IL-
functionalized amphiphilic JNS catalysts of JNS-IL/Ti(salen)x.
Hydrophobic IL/Ti(salen) and hydrophilic Si-OH groups are dis-
tinctly terminated onto the corresponding sides of JNS-IL/Ti
(salen)x. Their representative structure is shown in Chart 2.
2.7. General procedure for asymmetric sulfoxidation in water
The selected catalyst (0.9 mol% of substrate, based on titanium
content) was stirred with sulfides (0.5 mmol) in deionized water
(1 mL) at 25 °C. H2O2 (30 wt%, 0.6 mmol) was then added dropwise
over 15 min. The resulting mixture was stirred at 25 °C. Reaction
progress was monitored by TLC. After the reaction, the catalytic
systems were subjected to demulsification through high speed
centrifugation (18000 r/min) for 15 min. Catalyst was completely
precipitated from the aqueous system. Recovered catalyst was
washed with ethanol, dried in a vacuum, and finally recharged
with fresh substrate and oxidant for the next catalytic cycle.
Organic product in aqueous phase was extracted with ethyl acetate
(3 ꢁ 4 mL). Notably, the extraction process should be excluded in
large-scale industrial processes, in which the oily product phase
can be directly separated from water after demulsification. Com-
bined organic layer was dried over anhydrous sodium sulfate,
and then purified by chromatography on silica gel (petroleum
ether/ethyl acetate, 5: 1). The depurated chiral sulfoxides have
been identified by 1H NMR spectra. Ee values of the products were
determined by HPLC analysis using the Daicel chiralpak AD col-
umns. Detailed NMR and HPLC analyses for the sulfoxides are
available in ESI.
3.2. Characterization of samples
3.2.1. FT-IR
The successful decoration of IL/Ti(salen) moiety on mesosilica
nanosheet was verified by FT-IR. Fig. 1 shows the FT-IR spectra of
typical JNS-IL/Ti(salen)0.48, JNS-Ti(salen), as well as pristine
mesosilica nanosheet and neat complex for comparison. Clearly,
pristine mesosilica nanosheet shows distinct characteristic bands
at 3451, 1090 and 807 cmꢀ1, which are assigned to the skeletal
vibrations of OAH, SiAOASi and SiAOH groups, respectively
(Fig. 1a) [28]. Upon silylation with (C2H3O)3Si-IL-Ti(salen), the
characteristic bands of OAH and SiAOH groups significantly
weaken, although they are also present in the FT-IR spectrum of
typical JNS-IL/Ti(salen)0.48 (Fig. 1b). It provided convincing evi-
dence that partial surface hydroxyl groups on mesosilica
nanosheet have participated in silylation with the IL/Ti(salen)-
containing organosiloxane. Indeed, the FT-IR spectrum of JNS-IL/
Ti(salen)0.48 exhibited the stretching vibrations of ACH2A (in the
239

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
distinctly exhibits a perpendicular pattern of the mesochannels
with the size of about 3.8 nm (the inset in Fig. 1d). Such a meso-
porous structures should provide enough passageway to ensure
mass accessibility of the interfacial catalysis. JNS-Ti(salen) and
HNS-IL/Ti(salen) display similar 2D sheet-like mesoporous mor-
phology (Fig. 2e and 2f vs. 2c), while, any nanopores cannot be
observed in the nanosheets of nJNS-IL/Ti(salen) (Fig. 2g), confirm-
ing the nonporous morphology of nJNS-IL/Ti(salen). To prove Janus
character of JNS-IL/Ti(salen)x, Pd NPs were used to label JNS
instead of IL/Ti(salen) via electrostatic interaction (detailed proce-
dure is available in section S1 of ESI). Clearly, most of Pd NPs are
selectively labeled on one side of the mesosilica nanosheet, leaving
the other side smooth (Fig. 2h). This region-selective modification
is agree with the distinct compartmentalization of hydrophilic Si-
OH and hydrophobic IL/Ti(salen) groups onto both sides of JNS-
IL/Ti(salen)x, respectively.
3.2.3. Nitrogen adsorption–desorption
Porous structure of JNS-IL/Ti(salen)x (x = 0.38, 0.48, 0.60) was
further investigated by nitrogen adsorption–desorption, as shown
in Fig. 3. Different from nJNS-IL/Ti(salen) which is nonporous
(Fig. 3A), typical JNS-IL/Ti(salen)0.48 exhibit a hysteresis loop at
P/P0 > 0.45 in its sorption isotherm, featuring mesoporous material
(Fig. 3B) [36]. The pore diameter is centered at 3.81 nm in pore size
distributions curve, which is consistent with the size observed in
TEM image. Notably, pore size of JNS-IL/Ti(salen)0.48 (3.81 nm) is
approximate to that of pristine mesosilica nanosheet (3.83 nm)
(Fig. 3B vs. 3C). It is the convincing evidence that IL/Ti(salen)
groups are grafted just on mesosilica surface, rather than being
entrapped into the channel. For this reason, it is not surprised that
the pore size of JNS-IL/Ti(salen)x remains almost constant as the Ti
(salen) loading varied from 0.38 to 0.60 mmolꢂgꢀ1 (Fig. S2 in ESI).
Furthermore, homogeneous HNS-IL/Ti(salen) exhibit similar textu-
ral parameters to JNS-IL/Ti(salen)x, due to the similar surface mod-
ification (Fig. 3D). The unhindered channels may ensure diffusion
and mass accessibility of the interfacial catalysis, giving rise to
prominent catalytic activity.
Fig. 1. FT-IR spectra of pristine mesosilica nanosheet (a), typical JNS-IL/Ti(salen)0.48
(b), the recovered JNS-IL/Ti(salen)0.48 after the 7th reuse (b’), JNS-Ti(salen) (c), and
neat complex (d).
range of 2821–3000 cmꢀ1) and Ph-H (at 1466 cmꢀ1) in Ti(salen)
and imidazole ring in IL moiety (at 1551 cmꢀ1) [28]. Notably, the
CTAB molecules still occupied the mesochannels during the silyla-
tion process, bulk organosilane of (C2H3O)3Si-IL-Ti(salen) is diffi-
cult to diffuse into the inner surface of mesosilica shell. We can
thus envision that most chiral salen TiIV complexes are just located
on external surface of the mesosilica shell, leaving the inner sur-
face free. The region-selective modification not only makes the
JNS-IL/Ti(salen)x catalytically active, but also endows them unique
Janus structure and excellent interfacial activity, which are crucial
in Pickering interfacial catalysis. The IL-free counterpart of JNS-Ti
(salen) exhibited similar FT-IR spectrum to JNS-IL/Ti(salen)0.48
(Fig. 1c vs. 1b), except for the absence of characteristic stretching
vibrations of the imidazole ring (near 1551 cmꢀ1). The difference
is related to the inexistence of imidazolium-based IL linker in the
framework of JNS-Ti(salen). Actually, compared with JNS-Ti
(salen), a significant feature observed for JNS-IL/Ti(salen)0.48 is
the presence of an imidazolium-based IL linker that positively
affects catalytic performance in the aqueous catalysis. Notably,
all active species of Ti(salen) are intact during the grafting as its
characteristic bands in silica-supported samples are identical to
those in neat complex (Fig. 1b and 1c vs. 1d).
3.2.4. Surface wettability determination
WCA was used to evaluate the surface wettability of JNS-IL/Ti
(salen)x, which is crucial in dictating the type of formed Pickering
emulsions. Pristine mSiO2 sheet exhibits a WCA of 26°, suggesting
its highly hydrophilic surface (Fig. 4a). The WCA value increases
upon the hydrophobic modification of IL/Ti(salen) on mesosilica
surface, and can be fine tuned by tailoring the IL/Ti(salen) loading.
When the content of IL/Ti(salen) increased from 0.38 to
0.60 mmolꢂgꢀ1, WAC of IL-functionalized JNS is controlled from
60 to 82°, accordingly (Fig. 4b–d). These trends agree well with
the tendency of its hydrophobic/hydrophilic balance. With similar
titanium loading, HNS-IL/Ti(salen) and nJNS-IL/Ti(salen) exhibits
the WCA values close to JNS-IL/Ti(salen)0.48 (76°) (Fig. 4e and 4 g
vs. 4c), while, IL-free JNS-Ti(salen) gives much higher WCA value
(84°) than JNS-IL/Ti(salen)0.48 (76°) (Fig. 4f vs. 4c). It suggests that
the imidazolium-IL moiety also controls the surface wettability of
JNS-IL/Ti(salen)x. Notably, WCA values for all the catalytic
nanosheets were lower than 90°. It suggests the relatively hydro-
philic property and preferred oil-in-water (o/w) Pickering emul-
sions [37], which is favorable for aqueous catalysis.
3.2.2. SEM and TEM
SEM and TEM images provide information for the morphology
of as-prepared JNS-IL/Ti(salen)x. As shown in Fig. 2, spherical par-
ticles with an average diameter of ca. 2.8 mm are observed on the
SEM image of PS@mSiO2 (Fig. 2a). Surface modification and fol-
lowed template removal give Janus hollow silica microsphere with
a shell of 78 nm in thickness (Fig. 2b). After being milled, almost all
the Janus hollow microspheres have been crushed into Janus
mesosilica nanosheets, as shown in the SEM image (Fig. 2c).
Cross-sectional SEM image reveals that the thickness of nanosheet
is approximately 78 nm (the inset in Fig. 2c), which is consistent
with the shell thickness of hollow microsphere. Furthermore,
abundant nanopores (3–5 nm) are found to be dispersed through-
3.2.5. Emulsifying capacity determination
Interfacial activity of JNS-IL/Ti(salen)x was examined by testing
their emulsifying capacity in a phenyl methyl sulfide/water sys-
tem. Benefiting from the Janus characteristic, all JNS-IL/Ti(salen)x
give the highly dispersed Pickering emulsions, and the emulsions
are stable for at least 24 h with no obvious change in physical
appearance (Fig. 5A–C). While, emulsion stabilized by HNS-IL/Ti
out on the resultant JNS-IL/Ti(salen)x (x
= 0.38, 0.48, and
0.60 mmolꢂgꢀ1) (see TEM images of Fig. 2d and Fig. S1 in ESI),
which are generated from the removal of CTAB species occupying
the frameworks of silica shell. HRTEM image of JNS-IL/Ti(salen)0.48
240

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Journal of Catalysis 395 (2021) 236–245
Fig. 2. SEM images of PS@mSiO2 microspheres (a), JNS-IL/Ti(salen)0.48 (c) and Pd-labeled JNS (h), TEM images of JNS-IL/Ti(salen)0.48 before being crushed (b), JNS-IL/Ti
(salen)0.48 (d, the inset is HRTEM image), JNS-Ti(salen) (e), HNS-IL/Ti(salen) (f), and nJNS-IL/Ti(salen) (g).
Fig. 3. Nitrogen adsorption–desorption isotherms and pore size distributions curves of nJNS-IL/Ti(salen) (A), JNS-IL/Ti(salen)0.48 (B), pristine mesosilica nanosheet (C) and
HNS-IL/Ti(salen) (D).
241

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
Fig. 4. WCA of pristine mSiO2 nanosheet (a), JNS-IL/Ti(salen)0.38 (b), JNS-IL/Ti(salen)0.48 (c), JNS-IL/Ti(salen)0.60 (d), HNS-IL/Ti(salen) (e), JNS-Ti(salen) (f), and nJNS-IL/Ti
(salen) (g).
Fig. 5. Optical microscopy image of Pickering emulsions stabilized by JNS-IL/Ti(salen)0.38 (A), JNS-IL/Ti(salen)0.48 (B), JNS-IL/Ti(salen)0.60 (C), HNS-IL/Ti(salen) (D), JNS-Ti
(salen) (E), and nJNS-IL/Ti(salen) (F). The inset shows the emulsion photos after standing for different times (a: 0 h; b: 24 h).
(salen) is unstable, and shows obvious phase separation after
standing for 24 h (Fig. 5D). It is not surprised that the absorption
energy of a Janus colloid at an oil/water interface is about 3 times
than that of their counterpart uniform colloids [38], which thus
stabilizes the Pickering emulsions. With the anisotropic surface
wettability, JNS-Ti(salen) and nJNS-IL/Ti(salen) also give stable
Pickering emulsions with the droplet sizes in the range of 2–
the phenyl methyl sulfide is confined by surrounding water. This
result demonstrates that the type of the resulting Pickering emul-
sion is o/w. Notably, the oil droplet size depends strongly upon the
content of Ti(salen) group in the JNS emulsifier (Fig. 5A-C and
Fig. S3). JNS-IL/Ti(salen)0.48 gives the smallest emulsion droplets
with diameter of ca. 1.0 lm (Fig. 5B). It is accompanied by a max-
imal interfacial contact area, which favors the interfacial mass
transfer in interfacial catalysis. The stable sulfide/water Pickering
emulsions, together with tunable interfacial contact area, makes
JNS-IL/Ti(salen)x the desirable PIC for asymmetric sulfoxidation
in water.
10 lm (Fig. 5E and 5F). To infer the emulsion type, coumarin dye
was added to label the oil phase of JNS-IL/Ti(salen)0.48-stablized
Pickering emulsion. Blue oil droplets are clearly observed in the
fluorescence microscope image (inserted in Fig. 5B), indicating that
242

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
3.3. Catalytic performances
centration of reactants around the active sites. Dense chiral salen
TiIV complex confined in the oil phase may enforce a cooperative
reaction pathway resulting in an enhanced reaction rate and higher
selectivity [40]. Furthermore, H2O which is the reduction product
of H2O2 should rapidly diffuse out of oil droplet through the
mesochannels, which further accelerated the aqueous asymmetric
sulfoxidation. For these reasons, it is not surprised that the uniform
HNS-IL/Ti(salen), which gave less stable emulsions, showed much
lower catalytic activity and selectivity under identical conditions
(Table 1, entry 5).
Short and unhindered channels present on JNS should make the
reaction interface highly accessible, and then improved the reac-
tion rate of aqueous reactions. Indeed, nonporous nJNS-IL/Ti
(salen) gave only 19% of conversion with lower chemoselectivity
(86%) within 1 h, albeit with stable Pickering emulsion (Table 1,
entry 6), and the catalytic efficiency had hardly increased even
after 2 h (Table 1, entry 7 vs. 6). This is probably arisen from the
nonporous structure of nJNS-IL/Ti(salen), which cannot offer effi-
cient passageway for the mass transfer of reactants on the inter-
faces. As a consequence, the nJNS-IL/Ti(salen) serves as physical
barrier between the oil/water interfaces, making the reaction inter-
faces inaccessible. The results demonstrated the passageway effect
of unhindered mesochannels in JNS-IL/Ti(salen)x, which enables
the rapid mass transfer of regents inside and outside the emulsion
droplets.
Since chiral sulfoxides are very important synthetic intermedi-
ates for widespread applications [39], the catalytic performances of
JNS-IL/Ti(salen)x were evaluated in the asymmetric oxidation of
sulfides to enantiopure sulfoxides in water using aqueous H2O2
(30 wt%) as an oxidant. The results are summarized in Table 1.
Obviously, JNS-IL/Ti(salen)x (x = 0.38, 0.48, 0.60) significantly
accelerated asymmetric sulfoxidation with aqueous H2O2 in water
through the formation of stable sulfide/water emulsion droplets
(Table 1, entries 1–3). Kinetics study indicates that only 0.9 mol%
of JNS-IL/Ti(salen)0.48 was sufficient to give almost quantitative
yield (94%) of (R)-phenyl methyl sulfoxide with remarkable enan-
tioselectivity (>99% ee) in water within 60 min (see Fig. S4 in ESI).
In contrast, neat complex was inactive in the aqueous asymmetric
sulfoxidation due to limited reaction interfacial area and high mass
transport resistance, giving only 40% conversion of phenyl methyl
sulfide with low chemoselectivity (66%) and ee value (81%)
(Table 1, entry 4). There is no doubt that Pickering emulsions over
JNS-IL/Ti(salen)x should provide high oil/water interfacial areas
and highly accessible reaction interfaces for the aqueous asymmet-
ric oxidation of sulfides with H2O2. In fact, the interface-active
mesoporous materials carry catalytically active Ti(salen) com-
plexes to assemble at the oil/water interface, creating large oil
(sulfide)-water interfacial area. A smaller size of emulsion droplets
(JNS-IL/Ti(salen)0.48) formed is more favorable for the transforma-
tion due to the larger oil/water interfacial areas for asymmetric
catalysis (Table 1, entry 2 vs. entries 1 and 3). The formed emulsion
effectively isolated sulfide from the surrounding aqueous environ-
ment, creating a concentrated environment for confined catalysis.
Simultaneously, hydrophobic Ti(salen) side orients itself to oil (sul-
fide) phase at the oil/water interface, allowing for high local con-
In addition to the fascinating mesoporous structure, highly
accessible of reaction interface should also be related to the
imidazolium-based IL spacer. Clearly, IL-free analog of JNS-Ti
(salen) was far less active and selective than JNS-IL/Ti(salen)0.48
(Table 1, entry 8 vs. 2). Moderate conversion (69%) of phenyl
methyl sulfide with only 87% chemoselectivity and 92% ee was
obtained over JNS-Ti(salen) under identical conditions (Table 1,
Table 1
Results of asymmetric sulfoxidation over different chiral salen TiIV complexes in water.a
b
b
c
d
Entry
Catalysts
R1
H
R2
T (h)
Conv. (%)
Sel. (%)
Ee. (%)
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
JNS-IL/Ti(salen)0.38
JNS-IL/Ti(salen)0.48
JNS-IL/Ti(salen)0.60
neat complex
HNS-IL/Ti(salen)0.48
nJNS-IL/Ti(salen)0.48
nJNS-IL/Ti(salen)0.48
JNS-Ti(salen)0.48
HO-SiO2-IL-Ti(salen)0.07
JNS-IL/Ti(salen)0.48
neat complex
JNS-IL/Ti(salen)0.48
neat complex
JNS-IL/Ti(salen)0.48
neat complex
JNS-IL/Ti(salen)0.48
neat complex
JNS-IL/Ti(salen)0.48
neat complex
CH3
1
1
1
1
1
1
2
1
1
1
1
1.5
1.5
2
2
2
87
98
83
40
65
19
21
69
99
93
23
97
25
90
13
95
37
92
22
21
31
93
96
82
66
87
86
89
87
97
95
70
97
66
99
99
97
72
98
71
90
66
93 (R)
99 (R)
94 (R)
81 (R)
94 (R)
99 (R)
99 (R)
92 (R)
98 (R)
99 (R)
90 (R)
94 (R)
90 (R)
92 (R)
79 (R)
99 (R)
96 (R)
88 (R)
76 (R)
99 (R)
92 (R)
96.6
109
92.2
44.4
72.2
21.1
11.7
76.7
e
9
79.2
10
11
12
13
14
15
16
17
18
19
20
21
H
C2H5
n-C4H9
n-C6H13
CH3
103
25.6
71.9
18.5
50.0
7.8
51.1
20.6
94.4
24.4
18.9
34.4
H
H
2-OCH3
4-OCH3
4-Br
2
1
1
1
CH3
JNS-IL/Ti(salen)0.48
neat complex
CH3
1
a
b
c
Catalyst (0.9 mol% of substrate, based on titanium content), substrate (1.0 mmol), H2O2 (1.2 mmol, added in 15 min), H2O (1.0 mL), 60 min, 25 °C.
Determined by GC.
Determined by HPLC (Daicel chiralpak AD column).
d
e
Turn over frequency (TOF) is calculated by [product]/[catalyst] ꢁ time (hꢀ1).
HO-SiO2-IL-Ti(salen)0.07 (1.25 mol% of substrate, based on titanium content), phenyl methyl sulfide (1.0 mmol), H2O2 (1.2 mmol, added in 15 min), H2O (1.0 mL), 60 min,
25 °C [Ref. [28]].
243

C. Li, S. Liu, Y. Pi et al.
Journal of Catalysis 395 (2021) 236–245
Fig. 6. (A) Reuse of JNS-IL/Ti(salen)0.38 (a), JNS-IL/Ti(salen)0.48 (b) and JNS-IL/Ti(salen)0.60 (c) in asymmetric sulfoxiation of phenyl methyl sulfide in water, (B) kinetic curves
of the consecutive experimental runs 1–3 over JNS-IL/Ti(salen)0.48
.
entry 8). In fact, the IL moiety with unique solvent power [41] was
assembles at the oil/water interface along with Ti(salen) units. It
made the reaction interface more compatible with organic sulfides
and aqueous H2O2, increasing the concentrations of reactants at
the interface. Furthermore, the imidazolium-based IL linker places
the chiral salen TiIV complex away from the silica sheets, enabling
all bulk active sites to adapt their preferential conformation for
favorable cooperative action. High polarity and ionic properties
of ILs also had a positive effect on stabilizing the formed active
metallosalen intermediate, which in turn improves the catalytic
efficiency of JNS-IL/Ti(salen)x [42].
Benefiting from the high oil/water interfacial areas together
with highly accessible reaction interfaces, JNS-IL/Ti(salen)0.48
was also efficient for a wide range of aryl alkyl sulfides, such as
phenyl ethyl sulfide, phenyl n-butyl sulfide, phenyl n-hexyl sulfide,
p-methoxyphenyl methyl sulfide, and o-methoxyphenyl methyl
sulfide. Almost quantitative yields of the corresponding chiral sul-
foxides (92–98%) were obtained over an extremely low amount of
JNS-IL/Ti(salen)0.48 (0.9 mol%) in water (Table 1, entries 10, 12, 14,
16 and 18). In particular, in the case of phenyl n-butyl sulfide, the
yield of chiral phenyl n-butyl sulfoxide over JNS-IL/Ti(salen)0.48
increased even up to 7 times as compared with that of neat com-
plex (Table 1, entry 14 vs. 15). Despite still being aryl alkyl sulfide,
p-bromophenyl methyl sulfide underwent a slower oxidation (21%
of conversion) over JNS-IL/Ti(salen)0.48 in water, although the
chemo- (90%) and enantioselectivity (99%) were encouraging
(Table 1, entry 20). It is probably due to that, unlike the other liquid
sulfides, p-bromophenyl methyl sulfide is solid and unfavorable for
the formation of oil/water emulsions for efficient interfacial catal-
ysis. The observations provided further evidence for the Pickering
interfacial catalysis of asymmetric sulfoxidation in water.
Janus platelets at the oil/water interface to a great degree, thus
making the emulsion relatively more stable [14,15]. Such a two-
dimensional structure also extremely shortens the diffusion dis-
tance for sulfide and H2O2 at the oil/water interface, enhancing
the mass transfer of reactants on the interfaces [16–19]. Further-
more, the active sites on sheet-shaped materials are easily exposed
in comparison to those of the sphere-shaped particles, leading to
the large surface areas, which would be favorable for the catalysis
process to some degree [43,44].
Apart from the observed acceleration effect, another salient fea-
ture of the JNS-IL/Ti(salen)x is its facile recovery. After reaction, the
novel catalysts could be quantitatively recovered from the aqueous
system by simple centrifugation, and re-formed the stable Picker-
ing emulsion for the recycle experiment. Fig. 6A shows the
reusability of JNS-IL/Ti(salen)x in the asymmetric oxidation of phe-
nyl methyl sulfide in water. To our delight, all JNS-IL/Ti(salen)x
catalysts could be reused at least ten times without significant loss
in activity and selectivity. ICP-OES analysis gave titanium content
in recovered JNS-IL/Ti(salen)x of 0.376, 0.473, and 0.597 mmol gꢀ1
for x = 0.38, 0.48, and 0.60, respectively, which was almost identi-
cal to that of the corresponding fresh one. Indeed, negligible
amount of titanium was detected in the supernatant according to
ICP-OES analysis, which revealed the negligible leaching loss of
titanium species during the reaction. More importantly, all JNS-
IL/Ti(salen)x were stable to oxidative decomposition during
H2O2-based oxidation, as evident from the FT-IR spectra of typical
JNS-IL/Ti(salen)0.48 (Fig. 1b vs. 1b’). Kinetics study provided the
other substantial proof for the stability of JNS-IL/Ti(salen)x, based
on the fact that no significant change in the apparent rate con-
stant occurred in the consecutive experimental runs 1–3 over typ-
ical JNS-IL/Ti(salen)0.48 (Fig. 6B). The excellent stability of catalysts
should arise from the shielding of the chiral salen TiIV complex in
oil droplets, which potentially protected the Ti(salen) complex
from oxidative decomposition by aqueous H2O2 [45]. Furthermore,
the imidazolium IL moiety assembled at the oil/water interface
along with Ti(salen) units should also has a positive effect on sta-
bilizing the complex [42]. The facile recovery together with the
perfect stability makes JNS-IL/Ti(salen)x highly promising in sus-
tainable industrial applications.
Notably, the mesoporous JNS-IL/Ti(salen)0.48 was even more
efficient than our reported Janus HO-SiO2-IL-Ti(salen)0.07 nano-
spheres, although the latter also provided a highly accessible reac-
tion interface. As shown in Table 1, JNS-IL/Ti(salen)0.48 afforded
TOF value of 109 hꢀ1, which is 1.4 times higher than that of HO-
SiO2-IL-Ti(salen)0.07 (79.2 hꢀ1) (Table 1, entry 2 vs. 9). Higher effi-
ciency of JNS-IL/Ti(salen)0.48 should benefit from the two-
dimensional nanosheets structure which restrict the rotation of
244

C. Li, S. Liu, Y. Pi et al.
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4. Conclusions
A
novel IL-functionalized amphiphilic Janus mesosilica
nanosheet which contains Ti(salen) complex on one side has been
prepared via simply crushing hollow mesosilica microspheres with
two distinct surfaces. The two-dimensional porous Janus structure
together with the compatible IL modifier endowed the JNS cata-
lysts with excellent interfacial activity as well as highly accessible
reaction interface for reactants. As
a consequence, the IL-
functionalized JNS materials induced an efficient Pickering interfa-
cial catalysis approach for asymmetric sulfoxidation in water using
H2O2 as an oxidant, leading to significant rate acceleration and
remarkable high selectivity over a wide range of aryl alkyl sulfides.
In addition, they can be facilely recovered from the aqueous sys-
tem for efficient reuse by simple centrifugation. The novel catalytic
system offers several advantages, such as operational simplicity,
mild reaction conditions, high efficiency, effective separation of
final products, and facile reuse of catalyst. All these advantages
make it a benign protocol from the sustainability point of view,
and inspired us to design the other advanced interface-active cat-
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