CO2-switchable Pickering emulsions: Efficient and tunable interfacial catalysis for alcohol oxidation in biphasic systems

Article abstract of DOI:10.1039/c9cc04947a

CO₂-responsive Pickering emulsions were fabricated on the basis of polymeric nanoaggregates with adjustable surface wettability. The static Pickering emulsion system provides an efficient and sustainable platform for in situ separation and reuse of catalysts in biphasic reactions.

Full text of DOI:10.1039/c9cc04947a

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DOI: 10.1039/C9CC04947A
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CO2-Switchable Pickering Emulsion: Efficient and Tunable
Interfacial Catalysis for Alcohol Oxidation in Biphasic System
Jun Tang,a Shixiong Caoa and Jianli Wang a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
CO2-responsive Pickering emulsions were fabricated on the basis of size will further affect the interfacial contact area and mass
polymeric nanoaggregates with adjustable surfacial wettability. transfer distance of substrate, which directly determine the
The static Pickering emulsion system provides an efficient and reactivity.
sustainable platform for in-situ separation and reuse of catalysts in The high activity and simple recovery of catalysts have always
biphasic reaction.
been the goal of researchers. Pickering emulsion with good
stability ensures the enhancement effect of microreactor, but it
brings difficulties to demulsification.[10] The emergence of smart
nanoparticles makes it possible to achieve a delicate balance
between reaction intensification and catalyst recycling.[11] The
nanoparticles can detach from oil-water interface under
stimulation, leading to phase separation of emulsion. Among
the stimuli, CO2 has attracted considerable interest of
researchers due to its advantages of green, pollution-free,
cheap and good biocompatibility.[12] Compared with acid/base
regulation, the introduction of CO2 will not change the
concentration of components in the system. And it can be
removed by importing inert gas, and will not react with products
in the system.[13] Therefore, the CO2-sensitive Pickering
emulsion can circulate the switch infinitely, which is an ideal
reversible medium for biphasic reaction.
The development of green and sustainable chemistry requires
us to innovate the traditional chemical processes, reduce the
utilization of toxic substances and improve production
efficiency.[1] As a green solvent, water has attracted widespread
concern owing to its low cost, easy availability, non-
flammability and non-toxicity.[2] Thus, water-organic biphasic
system has been widely employed in academic research and
industry-scale production.[3] Moreover, the use of water can
easily recycle catalysts from products, which avoid the
complicated separation and purification process.
However, the immiscibility of liquids, especially the two-phase
reaction involving solid catalysts, make it necessary to utilize
vigorous stirring, ultrasound, microwave and hydraulic
cavitation.[4] The application of these energy-intensive methods
promotes the full mixing of two-phase solvents and solid
catalysts and then accelerate the reaction rate, although it is
difficult to fundamentally resolve the limitations of mass
transfer.[5] Pickering emulsion stabilized by nanoparticles with
appropriate surface wettability can divide aqueous or oil phase
into highly dispersed droplets.[6] The reagents in oil and water
phase reacted with the catalyst at large interface to boost the
activity.[7] In general, the reaction system does not appear
phase separation and maintain huge interface area because of
extremely high stability of droplets. Binks and coworkers found
that the adjustment of properties of particles would affect the
droplet size of emulsion.[8] Driven by innovative application of
Pickering interfacial catalysts (PICs) , the surface modification of
nanoparticles has received tremendous attention.[9] The
nanoparticles with different wettability can form oil-in-water
(O/W) or water-in-oil (W/O) Pickering emulsion. The droplet
Herein, CO2-switchable Pickering emulsion system was
developed to reinforce the biphasic reaction rate and simplify
the recovery of catalyst (Scheme 1). Firstly, 2,2,6,6-tetramethyl-
4-piperidinylmethacrylate (TMPM, catalyst precursor), methyl
methacrylate
(MMA,
hydrophobic
part)
and
2-
(dimethylamino)ethyl methacrylate (DMA, hydrophilic and CO2-
responsive part) were copolymerized to fabricate amphiphilic
block polymer P(TMPM-co-DMA)-b-MMA) or P(TMPM-b-MM-
a. State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
Zhejiang Province Key Laboratory of Biofuel, Biodiesel Laboratory of China
Petroleum and Chemical Industry Federation, College of Chemical Engineering，
Zhejiang University of Technology, Hangzhou 310014, P. R. China.
b. E-mail: wangjl@zjut.edu.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 1. The illustration of CO2-responsive Pickering emulsion for biphasic
system.
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A) through reversible addition-fragmentation chain transfer
(RAFT) polymerization. Then these Intermediates were
transformed to P(TMA-co-DMA)-b-MMA) or P(TMA-b-MMA) by
oxidation (Scheme 2 and Table S1-2). The feeding of monomers
was regulated to receive PICs with adjustable surfacial
wettability. The type and droplet size of Pickering emulsion
were well organized for the specific demands of reactions.
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DOI: 10.1039/C9CC04947A
Finally,
2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO)
immobilized-PIC were introduced in a typical biphasic reaction
(Anelli system, NaClO/NaBr/TEMPO) to estimate the catalytic
activity of alcohol oxidation with stirring and non-stirring. After
the reaction finished, the fast demulsification were achieved by
bubbling CO2. Subsequently, the PIC was deprotonated and
directly used in new reaction system. This polymer-based
catalyst realize highly activity and facile separation for the
requirement of sustainable development and growing
challenges of industrial applications.
Fig. 1 (a) FTIR spectra of P(TMPM-co-DMA)-b-MMA)-4 and P(TMA-co-DMA)-b-
MMA)-4; (b) CA of P(TMA-b-MMA) and P(TMA-co-DMA)-b-MMA)-n (n = 1 - 5);
TEM images of (c) P(TMA-b-MMA) and (d) P(TMA-co-DMA)-b-MMA)-3.
(N-H). The evidences indicated the formation of TEMPO.
Based on nitrogen content of element analysis (EA), the
loadings of TEMPO and DMA on polymers were estimated by
Equation S1-2. In Table S3, TEMPO and DMA loading of samples
were coincide with the results obtained by 1H NMR. The above
results further implied the successful preparation of polymer-
based catalysts.
The P(TMA-b-MMA) and P(TMA-co-DMA)-b-MMA)-n (n = 1 - 5)
were dispersed in CH2Cl2 under ultrasound. These amphiphilic
polymers could be well-dispersed in organic solvent and self-
assembled into nanoaggregates with uniform size.[14] As shown
in Fig. S5, P(TMA-b-MMA) and P(TMA-co-DMA)-b-MMA)-n (n =
1 - 5) have average size about 24, 38, 44, 65, 78, 89 nm as
measured by digital light scattering (DLS). The size of
nanoaggregates in organic solvent gradually increases with
more DMA loading, resulting in poor dispersibility in solvents.
The morphology and sizes of solid nanoshperes were proved by
transmission electron microscope (TEM). In Fig. 1(c)-(d), the
particle size of P(TMA-co-DMA)-b-MMA)-3 was obviously larger
than that of P(TMA-b-MMA), and their diameters were in
agreement with the DLS results.
1H NMR spectra showed that the peaks of 6 samples were well
assigned, and the chemical structures were confirmed (Fig. S1-
2). The molecular characteristics of P(TMA-b-MMA) and P(TMA-
co-DMA)-b-MMA)-n (n = 1 - 5) were listed in Table 1. The
feeding of monomers to initiator were tuned to afford versatile
polymers with different degree of polymerization. The molar
ratio of monomers to initiator of the samples, which were
calculated by the proton integral ratios of methylene (3.2 ppm)
in 4-cyan-4-(dodecylsulfanylthiocarbonyl) sulfanylpentanoic
acid (DTTCP) to the methyne (5.1 ppm), methylene (4.1 ppm)
and methyl (3.6 ppm), respectively. Furthermore, the TEMPO
loading of P(TMA-b-MMA) and P(TMA-co-DMA)-b-MMA)-n (n =
1 - 5) were 2.76, 2.65, 2.69, 2.55, 2.38 and 2.26 mmol·g-1,
respectively (Table 1). The DMA loading of P(TMA-b-MMA) and
P(TMA- co-DMA)-b-MMA)-n (n = 1 - 5) (0 - 2.30 mmol·g-1)
growing with increasing DMA addition.
The precursors and samples were testified by fourier transform
infrared spectroscopy (FTIR). In Fig. 1 (a), the typical peak at
1720 cm-1 was ascribed to C=O bond of TMPM, DMA and MMA.
And the peak appeared at 1168 cm-1 was attributed to the C-N
of TMPM and DMA. Then H2O2 was added as oxidant into the
mixture, and the color of reaction liquid changed to orange in 1
h (Fig. S3). Correspondingly, the FTIR spectra of P(TMA-co-
DMA)-b-MMA)-n (n = 1 - 5) (Fig. 1(a) and S4) appeared new peak
of N-O· at 1360 cm-1 and completely disappeared at 1558 cm-1
The surface wettability of polymeric interfacial catalysts were
evaluated by contact angle (CA), which played an important role
in the stabilization of Pickering emulsion.[15] As shown in Fig.
1(b), the contact angles of water on P(TMA-b-MMA) and
P(TMA-co-DMA)-b-MMA)-n (n = 1 - 5) were decreased (102 -
80°) since the increasing load of relatively hydrophilic DMA. We
investigated the emulsifying ability of P(TMA-b-MMA) and
Table 1. The preparation of P(TMA-b-MMA)/P(TMA-co-DMA)-b-MMA)-n (n = 1 - 5)
TEMPO loading
(mmol·g-1) [b]
DMA loading
(mmol·g-1) [b]
Samples
Feed
ratio[a]
P(TMA-b-MMA)
80/0/120/1
80/10/110/1
80/20/100/1
80/40/80/1
80/60/60/1
80/80/40/1
2.76
2.65
2.69
2.55
2.38
2.26
0
P(TMA-co-DMA)-b-MMA)-1
P(TMA-co-DMA)-b-MMA)-2
P(TMA-co-DMA)-b-MMA)-3
P(TMA-co-DMA)-b-MMA)-4
P(TMA-co-DMA)-b-MMA)-5
0.29
0.66
1.25
1.75
2.30
Scheme 2. The synthesis of (a) P(TMA-co-DMA)-b-MMA) and (b) P(TMA-b-MMA).
[a] Feed molar ratio of TMA/DMA/MMA/DTTCP; [b] calculated from 1H NMR.
2 | J. Name., 2012, 00, 1-3
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P(TMA-co-DMA)-b-MMA)-n (n = 1 - 5) in biphasic system. The catalyzed by P(TMA-co-DMA)-b-MMA)-3 reached V1i0ew0A%rticinle O3n0linse,
DOI: 10.1039/C9CC04947A
samples (4 mg) were dispersed in organic solvent (CH2Cl2, and its catalytic efficiency was four times higher than that of
toluene and ethyl acetate), and the same volume H2O were TEMPO. This enhancement effect probably originate from the
added in the dispersion to afford Pickering emulsion after formation of emulsion, which divide the two-phases into
vigorous stirring. Fig. 2 showed the appearances of emulsion considerable number of microreactors and greatly improve the
system and their optical micrographs of Pickering emulsion. contact interface area of reaction and shorten the mass transfer
According to the droplet test, the relatively hydrophobic distance.[16] It is noteworthy that this type of polymeric
P(TMA-b-MMA) and P(TMA-co-DMA)-b-MMA)-n (n = 1 - 4) can interfacial catalyst showed superior catalytic activity (TOF)
form W/O Pickering emulsion. In contrast, the P(TMA-co-DMA)- compared with previous studies (Fig. S10, Table S5).[17]
b-MMA)-5 stabilize O/W Pickering emulsion. The diameters of We used microscope to observe Pickering emulsions droplets,
these Pickering emulsion droplets were within the range of 50 - and the results were shown in Fig. 3(b) and S11. The average
150 μm (Fig. S6-8). These polymeric nanoaggregates have been size of emulsions stabilized by P(TMA-b-MMA) and P(TMA-co-
proved to be effective in emulsifying a wide range of oil-water DMA)-b-MMA)-n (n = 1 - 5) were 100 ± 35 μm, 95 ±39, 80 ± 44,
systems, and to realize the type and size of emulsions by 69 ± 40, 103 ± 42 and 99 ± 43 μm, respectively. As the surface
controlling the surfacial properties.
wettability of polymeric particles turned from relative
CH2Cl2-H2O system was chosen as a model to evaluate the CO2- hydrophobicity to hydrophilicity, P(TMA-co-DMA)-b-MMA)-3
responsive behavior of the PIC. CO2 flow was imported in to with appropriate surface wettability (CA = 90°, Fig. 1(b))
emulsion stabilized by P(TMA-b-MMA) and P(TMA-co-DMA)-b- minimized diameter of droplet and lead to highest
MMA)-n (n = 1 - 5). P(TMA-b-MMA) without DMA cannot realize reinforcement effect forbiphasic reaction.[18] In addition, the
demulsification under CO2 bubbling. In comparison, the type of emulsion also plays an important role in biphasic
Pickering emulsion stabilized by P(TMA-co-DMA)-b-MMA)-n (n reaction. Compared with others, the activity of P(TMA-co-
= 1 - 5) would be broken at 16, 13, 12, 10 and 8 min, respectively DMA)-b-MMA)-5 was weaker, even though the interfacial area
(Fig. S9). The faster response rate was ascribed to the increasing of the stabilized emulsion was larger. It may be that the internal
DMA loading. Besiedes, the increase of zeta potentials also circulation in the W/O emulsion can greatly intensify the mass
indicated the protonation on the surface of nanoparticles, transfer of the substrate.[16] This result also provides us with
resulting in the instability of emulsion (Table S4). This green new enlightenment to realize directional strengthening of
trigger-responsive Pickering emulsion system not only biphasic reactions by adjusting surfacial wettability.[19]
effectively boost the oxidation rate, but also avoid the We found the Pickering emulsion stabilized by P(TMA-co-DMA)-
production of redundant byproducts and complicated recycling. b-MMA)-n (n = 1 - 3) remained stability after placing one week
Subsequently, the Pickering emulsions stabilized by P(TMA-co- (Fig. S12). By contrast, P(TMA-b-MMA), P(TMA-co-DMA)-b-
DMA)-b-MMA)-n (n = 1 - 5) were removed CO2 by introducing MMA)-4 and P(TMA-co-DMA)-b-MMA)-5 were much
N2 for 0.5 h. After 5 cycles, the formation of emulsion revealed hydrophobic or hydrophilic, and difficult to maintain stability for
the great switchability of the polymeric interfacial catalysts.
a long time. The surface wettability of P(TMA-co-DMA)-b-
The TEMPO-immobilized PICs were used to catalyze the MMA)-n (n = 1 - 3) were in an appropriate range for emulsion
reaction in Anelli system, which was a green and environment- stabilization. This static emulsion system without stirring can be
friendly platform for alcohol oxidation. The catalytic activity of regarded as efficient and energy-saving microreactor for
conventional biphasic system and Pickering emulsion system intensification reactions.[20] The oxidation reactions without
were investigated by adding benzyl alcohol as the substrate stirring were proceed. In Fig. 3(c), P(TMA-co-DMA)-b-MMA)-3
inoil-water system. In Fig. 3(a), free TEMPO achieved full
conversion of benzyl alcohol at 120 s, and its selectivity was >
99%. In contrast, P(TMA-b-MMA) and P(TMA-co-DMA)-b-
MMA)-n (n = 1 - 5) all can convert benzyl alcohol completely into
benzaldehyde in 90 s. Particularly, the conversion of alcohol
Fig. 3 (a) The catalytic performance of TEMPO, P(TMA-b-MMA) and P(TMA-co-
DMA)-b-MMA)-n (n = 1 - 5) in Anelli system; (b) the droplet size and interface area
of Pickering emulsions stabilized by P(TMA-b-MMA) and P(TMA-co-DMA)-b-
MMA)-n (n = 1 - 5) ( scale bar: 200 μm); (c) the catalytic performance of TEMPO
and P(TMA-co-DMA)-b-MMA)-n (n = 1 - 3) without stirring at 30 s; (d) the
recyclability of P(TMA-co-DMA)-b-MMA)-3 at 30 s.
Fig. 2 Pickering emulsion stabilized by P(TMA-b-MMA) and P(TMA-co-DMA)-b-
MMA)-n (n = 1 - 5) with (a) CH2Cl2, (b) toluene and (c) ethyl acetate as oil phase
(standing for 30 min, scale bar: 200 μm).
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3
4
(a) H. Chen, H. Zou, Y. Hao and H. Yang, ChemSusChem, 2017,
10, 1989; (b) Z. Meng, M. Zhang andDHO.I:Y1a0n.1g0,3G9/rCe9eCnCC0h49e4m7A.,
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gave a conversion of 100% at 30 s, which was almost same with
reaction under stirring. The conventional biphasic system were
rapidly separated and represented low reactivity owing to high
mass resistance. This catalytic reaction results concluded that
Pickering emulsion system provides a green and prospective
model for the enhancement of catalytic rate in biphasic
reaction. We tested the applicability of Pickering emulsion
catalytic system for alcohol oxidation. In Table S6, the aromatic
alcohols and aliphatic alcohols catalyzed by P(TMA-co-DMA)-b-
MMA)-3 in Anelli system were all exhibited remarkable activity
and selectivity. When the reaction finished, the systems were
demulsified by CO2 bubbling and the polymer were then
deprotonated and dispersed in new reaction medium. In
Fig.3(d), the conversion of benzyl alcohol catalyzed by P(TMA-
co- DMA)-b-MMA)-3 was up to 99.3% over 5 runs with stirring.
In a non-agitating mode, the conversion of P(TMA-co- DMA)-b-
MMA)-3 was 99% after 5 recycling (Table S6 Entry 10).
View Article Online
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Furthermore, the results of H NMR spectra (Fig. S13), FTIR
spectra (Fig. S14) and element analysis (Table S7) illustrated
little loss of TEMPO, showed excellent recyclability of the PIC.
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In this study, we syntheszied a well-designed, renewable
polymer-based interfacial catalyst by RAFT polymerization and
oxidation reaction. The surfacial wettability of polymeric
nanoparticles were adjusted by the feeding of monomers. Such
a tuning is crucial for Pickering interfacial catalysis in biphasic
reactions, which can determine the catalytic activity of
reactions. In aqueous-oil system for alcohols oxidation, the
catalytic activty of P(TMA-co-DMA)-b-MMA)-3 is 4-folds than
the unemulsified one. Moreover, the static Pickering emulsions
can served as an efficient platform for enhancing biphasic
reaction, and its performance is basically the same as that under
stirring. The Pickering emulsion can be broken by bubbling CO2.
Subsequently, PICs were deprotonated by N2 and directly used
in new reaction media without complex recovery process. The
sustainable and effective polymeric interfacial catalyst could be
recovered for 5 cycles, and displayed great activity and
recyclability. These results highlight that the CO2-sensitive PIC
can be used as green and sustainable catalytic system, which
greatly promote the further studies of highperformance
interfacial catalysis in industrial applications.
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Conflicts of interest
There are no conflicts to declare.
18 M. J. Zhang, Z.Y. Tang, W. Q. Fu, W. Y. Wang, R. Tan and D. H.
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Table of content
CO2-responsive Pickering emulsion system were used as efficient platform for in-situ separation
and reuse of catalysts in biphasic reaction.