A thermo-responsive polymer-tethered and Pd NP loaded UiO-66 NMOF for biphasic CB dechlorination

Article abstract of DOI:10.1039/c8gc03651a

Herein, we report a poly(N-isopropylacrylamide) sulfide (PNIPAM-S-) brush-tethered and Pd nanoparticle (NP) loaded UiO-66 type NMOF, namely Pd@UiO-66-S-PNIPAM, based on a NMOF-polymer covalent composite approach. The obtained multifunctional composite material can be a highly efficient Pickering emulsifier to stabilize the toluene-in-water emulsion; furthermore, it promotes the biphasic chlorobenzene (CB) dechlorination at the toluene-water interface in excellent yields at room temperature. More importantly, the Pd@UiO-66-S-PNIPAM emulsifier is thermo-responsive and can trigger emulsification at room temperature (25 °C) and demulsification at an elevated temperature (45 °C) of the toluene droplets, which makes product-catalyst in situ separation and recycling feasible. This temperature switchable Pickering emulsion catalytic system can be reused at least five times without any loss of its catalytic activity.

Full text of DOI:10.1039/c8gc03651a

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DOI: 10.1039/C8GC03651A
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Thermo-responsive polymer-tethered and Pd NPs loaded UiO-66
NMOF for biphasic CBs dechlorination
Bing-Jian Yao,1a Qi-Juan Fu,1a Ai-Xiang Li,b Xiao-Man Zhang,a Yan-An Li,a and Yu-Bin Dong,*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Herein, we report a poly(N-isopropyl acrylamide) sulfide (PNIPAM-S-) brushes-tethered and Pd nanoparticles (NPs) loaded
UiO-66 type NMOF, namely Pd@UiO-66-S-PNIPAM, based on a NMOF-polymer covalently composite approach. The
obtained multifunctional composite material can be a highly efficient Pickering emulsifier to stabilize the toluene-in-water
emulsion, furthermore, promotes the biphasic chlorobenzenes (CBs) dechlorination at the toluene-water interface in
excellent yields at room temperature. More importantly, Pd@UiO-66-S-PNIPAM emulsifier is thermo-responsive and can
trigger the emulsification at room temperature (25 °C) and demulsification at elevated temperature (45 °C) of the toluene
droplets, which makes product-catalyst in situ separation and recycling available. This temperature switchable Pickering
emulsion catalytic system can be reused at least five times without loss of its catalytic activity.
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loss due to the filter blocking or low centrifugation efficiency
during the recycling process, especially when the catalyst sizes
are in the nanometer range. This limitation urges the
development of NMOFs-based catalytic materials that can
make their recycling process convenient and feasible,
moreover, meet the burgeoning demands of green and
sustainable chemistry.
1. Introduction
Metal-organic frameworks (MOFs),1,2 as one of the typical
porous materials, are promising candidates for application in
diverse fields, such as gas storage and separation,3,4 sensing,5
bio-application6 and catalysis.7,8 The distinct advantages of
MOFs are their controllable synthesis and various post-
Recently, stimuli-responsive Pickering emulsions are
emerging as an attractive class of catalytic systems due to their
modification
options,9–11
which
provide
numerous
opportunities to achieve MOF-based materials with different
functionalities. Very recently, we and others developed an
alternative approach to construct composite functional MOF-
based materials, in which the nanosized MOF particles (MOF
NPs) are covalently composited with organic polymers
(oligomers).12–19 Based on this MOF-polymer covalently
composite approach, the compatibility of organic polymer with
MOF NPs is largely enhanced, furthermore, a broad range of
potential applications are endowed, especially in separation
and catalysis.
As it is known, nanosized MOFs (NMOFs)-based catalysts are
highly efficient,7,8 but their application, especially their cyclic
utilization, is often limited by a fussy separation procedure
from reaction systems. The traditional separation methods
such as filtration and centrifugation are somewhat time and
energy consumption, which would often result in the catalyst
good catalytic performance, recyclability and
a facile
separation.20-24 For example, the Pickering emulsifiers with
surface decorated stimuli-responsive polymer brushes can
activate or deactivate in response to external stimuli such as
light, pH and temperature.25–27 In doing so, the recovery and
reuse of Pickering emulsifiers would be realized, furthermore,
to make the sustainable operation available. Among various
solid particles, the MOF NPs with well-defined morphology and
narrow size distribution would be the desirable candidates for
Pickering emulsifiers.14,28,29 Hopefully, the covalent
incorporation of MOF NPs with functionalized polymer brushes
is expected to access new types of stimuli-responsive Pickering
emulsion systems. In addition, as porous solid particles,
NMOFs are known to be able to load and stabilize the active
catalytic species such as metal NPs to highly promote organic
transformations in heterogeneous way.30,31 Together with the
stimuli-responsive polymer brushes, the active catalytic
species loaded NMOFs should be qualified for Pickering
interfacial catalysis, thus, the highly biphasic catalytic
efficiency and their cyclic utilization could be realized based on
the stimuli-responsive MOF-based Pickering emulsifiers.
a. College of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education,
Shandong Normal University, Jinan 250014, P. R. China.
b. School of Materials Science and Engineering, Shandong University of Technology,
Zibo, 255049, P. R. China.
In this contribution, we report, the first of its kind, a thermo-
responsive poly(N-isopropylacrylamide) sulfide (PNIPAM-S-)
decorated and UiO-66 NMOF-based Pickering emulsifier,
namely UiO-66-S-PNIPAM. After Pd NPs loading, the obtained
1 These authors contributed equally to this work.
†
Electronic Supplementary Information (ESI) available: Synthesis and
characterization of PNIPAM-CTA and PNIPAM-SH; additional characterization for
UiO-66-based materials and Pickering emulsion; characterization for the
dechlorination reaction. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Pd@UiO-66-S-PNIPAM can not only stabilize the toluene-in- specimen films with a 90° to the electron beam determined
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water Pickering emulsion, but also highly promote the biphasic using 300W Al Kα radiation. IFT was meaDsuOrI:e1d0.u10s3in9g/Ca8GsCp0in3n65in1Ag
interfacial chlorobenzenes (CBs) dechlorination in the drop tensiometer (TX-500C), using toluene as the oil droplet
Pickering droplet “micro-reactors” at room temperature. Once with a rotated speed of 7000 r min-1. For the aqueous phase,
the reaction finished, this Pickering emulsion system can be NMOF samples were suspended into DI water at
a
demulsified upon simply adjusting the temperature, and then concentration of 5 mg mL-1. (For PNIPAM-SH, a diluted solution
macroscopic organic and aqueous phases separation can be of 1 mg mL-1 was used). The temperature was set to the target
easily achieved. In this way, the organic phase with target temperature and held for 20 min before the values were
product is commodiously isolated via simple liquid transfer, recorded. The powders of NMOFs were compressed into
meanwhile the aqueous phase with Pd@UiO-66-S-PNIPAM circular tablet using a table press (Shimadzu Press) for contact
catalyst can be reused directly for the next catalytic cycle.
angle measurement. Contact angle of DI water on circular
tablet was measured with sessile drop method by
a
commercial contact angle analyzer (Tracker, France). A droplet
(2 μL) of DI water was deposited on the tablet in air using a
precision syringe. The image was captured immediately after
droplet deposited on the sample. The image at 0 s was
captured and used in our experiments. Profile of the droplet
was fitted to the Laplace-Young equation.
2. Experimental
2.1 Materials and instruments
UiO-66-NH2 and UiO-66-Met were synthesized according to
our reported methods.12 Sulfhydryl-terminated poly(N-
isopropylacrylamide) (PNIPAM-SH) was prepared according to
the modified literature method.32 The initiator of 2,2’-
azobis(isobutyronitrile) (AIBN, 99%, Tianjin Kemiou Chemical
Reagent Co.,) was recrystallized twice from methanol prior to
use. Other chemicals were used as received. The monomer of
N-isopropylacrylamide (NIPAM, ≥ 99 %, Acros Organics) was
purified by recrystallization in an acetone-n-hexane mixture
(50: 50 v/v). Deionized water was used in the present study.
The other chemical reagents were of analytical grade and were
used as received. Proton nuclear magnetic resonance (1H-
NMR) data were obtained on a Bruker Avance-400 HD
spectrometer with tetramethylsilane as the internal tagging.
Fourier transform infrared (FT-IR) spectra was conducted on a
Bruker ALPHA spectrometer in the 400-4000 cm-1 range.
Transmission electron microscopy (TEM) was performed on
with an accelerating voltage of 120 kV. Scanning electron
microscopy (SEM) was conducted on a Gemini Zeiss SUPRA
scanning electron microscope equipped with an energy-
dispersive X-ray detector. The X-ray diffraction pattern was
measured on a D8 Advance X-ray powder diffractometer with
the Cu Kα radiation (λ = 1.5405 Å). Confocal fluorescence
microscopy was performed on Leica Microsystems CMS GmbH
(TCS SP5 II type) confocal microscopy. The water phase was
stained with FITC prior to test. The emulsion was excited by a
488 nm laser. UV-visible spectroscopy was performed on
Shimadzu UV-2500 spectrophotometer. Dynamic light
scattering was conducted on Malvern Zetasizer Nano ZS90
instrument. The hydrodynamic diameter of the emulsion
droplets was calculated from the autocorrelation function
which were analyzed with the CONTIN method. Inductively
coupled plasma (ICP) measurement was performed on an IRIS
Intrepid (II) XSP and Nu AttoM. Gel permeation
chromatography (GPC) equipped with a Waters e2695 system,
a 2410 RI detector, and three Styragel gel columns were
calibrated with narrow-molecular-weight polystyrene (PS)
standards for molecular weight determination. X-ray
photoelectron spectroscopy (XPS) data was obtained with a
PHI 5000 Versaprobe-II electron spectrometer from Ulvac-Phi
by analyzing the elements present in the air-facing side of the
2.2 Synthesis of UiO-66-S-PNIPAM
The as-synthesized UiO-66-Met nanoparticles (0.3 g, 1 mmol)
was mixed with the as-synthesized PNIPAM-SH (3.6 g, 1.5
mmol) in 10 mL of THF with the photoinitiator 2,2-dimethoxy-
2-phenylacetophenone (30 mg), and then fully dispersed via
stirring. The reaction mixture was irradiated with UV-light (100
W, 365 nm) for 1 h. Then, the unattached polymer species was
removed via centrifugation and the crude product was
completely washed with THF and water for several times to
yield the UiO-66-S-PNIPAM. FTIR (KBr, cm-1): 3296 (s), 2974
(m), 2877 (m), 1741 (m), 1667(m), 1611 (s), 1575 (s), 1509 (m),
1459 (s), 1386 (m), 1065 (m), 768 (m), 660 (m).
2.3 Synthesis of Pd@UiO-66-S-PNIPAM
A mixture of UiO-66-S-PNIPAM (100 mg) and palladium acetate (65
mg, 0.29 mmol) in toluene (30 mL) was allowed to stand at room
temperature for 6 h. The resulting MOFs were washed with fresh
toluene (8 mL, 3 times) and dried in air. The obtained Pd (II) loaded
MOF was soaked in an aqueous solution (20 mL) of NaBH4 (20 mg,
0.52 mmol) for 1.0 h. The resulting powder was washed with
deionized water (10 mL, 3 times) and dried in air to afford Pd@UiO-
66-S-PNIPAM as dark brown crystalline powders. ICP measurement
indicated that the encapsulated amount of Pd NPs is 3.81 wt %.
FTIR (KBr, cm-1): 3296 (s), 2974 (m), 2877 (m), 1741 (m), 1667(m),
1611 (s), 1575 (s), 1509 (m), 1459 (s), 1386 (m), 1065 (m), 768 (m),
660 (m).
2.4 Emulsification and temperature-triggered demulsification
To prepare the MOF-based Pickering emulsion, toluene and
deionized water are used as the incompatible oil and water
phase, respectively. In a typical procedure, Pd@UiO-66-S-
PNIPAM (58 mg) were added into a glass vial containing
toluene-water (2.4 mL/0.6 mL, 4 : 1 in v/v) mixed solvent
system. The system was sonicated at 5000 rpm for 60s to
generate a Pickering emulsion with 2.0 wt % of Pd@UiO-66-S-
PNIPAM. The other types of emulsions with different MOF
contents and oil/water ratios were prepared in the similar
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way. Stabilization of the emulsions was assessed by visual system was cooled down to 25°C, along with a completely
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DOI: 10.1039/C8GC03651A
inspection and confocal images before and after long standing. migration of MOF NPs from oil phase into the aqueous phase.
To investigate the temperature-triggered demulsification Finally, the target organic product in the upper toluene phase
behaviour of the as-synthesized Pickering emulsions, the was isolated, while the bottom aqueous phase containing
temperature was controlled by a thermostat water bath. The Pd@UiO-66-S-PNIPAM was collected and reused for the next
effect of temperature variation, oil-to-water ratio, and the catalytic runs after re-homogenization. Pd@UiO-66-S-PNIPAM
content of emulsifiers on the emulsification and could be reused for at least five catalytic runs for the model
demulsification was characterized by both visual inspection reaction.
and confocal microscopy.
2.5.3 Leaching test. The solid catalyst of Pd@UiO-66-S-
PNIPAM was separated from the catalytic Pickering emulsion
right after reaction for the given time (1.0 h) by centrifugation.
2.5 Pickering interfacial catalytic decontamination
2.5.1 Reaction condition. The biphasic catalytic CBs The reaction was continued with the filtrate without the aid of
dechlorination was carried out in the microreactor constructed Pd@UiO-66-S-PNIPAM. No further increase in the yield of the
by Pd@UiO-66-S-PNIPAM-stabilized Pickering emulsion. target product was detected, demonstrating that the active
Typically, a mixture of ArCl (1 mmol) and HCO2NH4 (10 mmol) sites for the catalytic dechlorination were located on Pd@UiO-
was stirred at room temperature in the Pickering emulsion 66-S-PNIPAM.
consisting of toluene (2.25 mL)-water (0.75 mL) with Pd@UiO-
66-S-PNIPAM (58 mg, 2.0 wt%, 2.2 mg of Pd NPs, 2.0 mol%
equiv). The reaction process was monitored by GC and further
confirmed by 1H NMR spectra.
3. Results and discussion
3.1 Synthesis and characterization of Pd@UiO-66-S-PNIPAM
2.5.2 Catalyst recovery. Once the reaction finished, the
Pd@UiO-66-S-PNIPAM was prepared by covalently composite
reaction system temperature was adjusted from 25 to 45°C
of UiO-66-NH2 NPs and PNIPAM-SH polymer brushes under UV
with mild stirring. After the complete phase separation, the
irradiation via thiol-ene click reaction.
Scheme 1 Synthesis of Pd@UiO-66-S-PNIPAM Pickering emulsifier. The photographs of UiO-66-S-PNIPAM and Pd@UiO-66-S-PNIPAM samples are inserted.
The trithiocarbonate-containing polymer of PNIPAM-CTA its precursor of PNIPAM-CTA completely disappeared (Fig. 1b).
was prepared in 83 % yield by polymerization of N-isopropyl The PNIPAM-S- decorated nano UiO-66-S-PNIPAM was
acrylamide monomer (NIPAM) using CTA as RAFT agent at 70 generated as yellow crystalline solids by post-synthetic grafting
oC in dioxane.33 By controlling the molar ratio of [NIPAM]0: of PNIPAM-SH onto the UiO-66-Met NPs through
a
[CTA]0: [AIBN]0 at 1000 : 20 : 1, PNIPAM-CTA with the Mn, photoinduced thiol-ene reaction (365 nm, 100 W, THF, 1 h)
of 4824 Da in a narrow distribution (PDI = 1.21) was (Scheme 1). The grafting rate was calculated to be 56 % based
obtained, which is well consistent with that of calculated value on H NMR analysis (Fig. S2†). As shown in Fig. 1a, the GPC
experimental
1
(Mn, theoretical = 4956 Da) (Table S1†). As shown in Scheme 1, measurement performed on the released polymer, which was
sulfhydryl-terminated PNIPAM-SH was prepared in 88 % yield obtained by digesting UiO-66-S-PNIPAM by concentrated
via aminolysis of PNIPAM-CTA with the aid of reducing agent H2SO4, and followed by precipitation in cold ether, gave an
PBu3.32 The GPC analysis (Table S1†) and 13C spectrum (Fig. S1†) average molecular weight Mn = 2551 Da with a narrow
indicated that the number-average molecular weight of distribution (PDI = 1.24) (Table S1†). This can be a direct
PNIPAM-SH was halve to 2416 Da (PDI = 1.23) after the evidence for the formation of polymer brushes on the MOF
aminolysis and no PNIPAM-SH-based disulfide homo-coupling NPs. Pd NPs loaded Pd@UiO-66-S-PNIPAM was prepared as
species was detected (Fig. 1(a)). Meanwhile the UV adsorption deep brown crystalline solids by the reduction (NaBH4, H2O, 1
peak at 310 nm associated with C=S (n→π transition) group in h, r.t.) of Pd(II)@UiO-66-S-PNIPAM that was obtained by
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impregnating UiO-66-S-PNIPAM in a toluene solution of
palladium acetate at room temperature for 6 h (Scheme 1). (1741 cm ), acylamino (1667 cm ), phenyl (1611 and 1459 cm
Inductively coupled plasma (ICP) measurement indicated that 1) and sulfide (1065 cm-1) in UiO-66-S-PNIPAM and Pd@UiO-
the loading amount of Pd is up to 3.81 wt %, and the Zr 66-S-PNIPAM was unambiguously confirmed by their FT-IR
Journal Name
The existence of alkyl (2974, 2877 and 1386 cm-1), carboxyl
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-
DOI: 10.1039/C8GC03651A
-1
-1
content is 17.68 wt %, respectively.
spectra (Fig. 1c). In contrast, no sulfide absorption peak was
found in the IR spectra of UiO-66-Met and PNIPAM-SH.
Fig. 1 a) GPC traces of PNIPAM-CTA, PNIPAM-SH, and the freed PNIPAM chains obtained by digesting UiO-66-S-PNIPAM with concentrated H2SO4. b) UV-vis spectra of
PNIPAM-CTA and PNIPAM-SH. c) FT-IR spectra of PNIPAM-SH, UiO-66-NH2, UiO-66-Met, UiO-66-S-PNIPAM, and Pd@UiO-66-S-PNIPAM.
Fig. 2 a) PXRD patterns and SEM images of UiO-66-NH2, UiO-66-Met, UiO-66-S-PNIPAM, and Pd@UiO-66-S-PNIPAM. b) Corresponding DLS results. c) HRTEM images
and SEM-EDX elemental mapping of Pd@UiO-66-S-PNIPAM. The insert is the lattice fringes of Pd NPs.
As shown in Fig. 2a, the covalently composite of UiO-66-Met cubic Pd NPs.34 The uniform texture of Pd@UiO-66-S-PNIPAM
NPs with PNIPAM-SH did not cause its crystallinity and was further confirmed by the SEM-EDX mapping, which
structural integrity change. In addition, the scanning electron showed a homogeneous distribution of Zr, Pd, S, and C
microscopy (SEM) image showed that the UiO-66-NH2 NPs was elements in the covalently composite nanocomposite (Fig. 2c).
uniformly distributed, their sizes were centred at ca. 199 nm in XPS spectrum of Pd@UiO-66-S-PNIPAM demonstrated the
diameter (Fig. 2a-b). After successive post decoration, the sizes successful reduction of palladium from Pd(II) to Pd (0) based
of MOF NPs were slightly increased, which is well supported by on the observation of Pd d5/2 and d3/2 peaks at 335.5 and 340.9
the dynamic light scattering (DLS) (Fig. 2b). Their morphology, eV, respectively (Fig. S4†).35
however, was well maintained (Fig. 2a), which was further
evidenced by the high-resolution transmission electron
3.2 Thermosensitivity
microscopy (HRTEM) (Fig. S3†). In addition, HRTEM image The thermosensitivity of the as-synthesized UiO-66-S-PNIPAM
showed that the loaded Pd NPs were well dispersed in UiO-66- NPs was firstly characterized by measuring its hydrodynamic
S-PNIPAM matrix with an average particles size of ca. 6 nm, diameters in dilute aqueous solution depending on the
and the atomic lattice fringes had an interplanar spacing of temperature. As indicated in Fig. 3a, the diameter of UiO-66-S-
0.24 nm belonging to the 1/3 (422) fringes of face-centered PNIPAM in the swollen state at 25 °C and shrunken state at 45
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°C are 238 and 193 nm respectively, which should be cycles in the range of 20 to 50 °C was conducted,ViinewdiAcrtaictlienOgnliintes
attributed to the volume phase transition of the PNIPAM good thermo-responsive reversibility D(OFIi:g1.0.1303b9)/.C8LGoCg0i3c6a5ll1yA,
brushes attached on the MOF NPs surface upon heating. At Pd@UiO-66-S-PNIPAM showed the same thermal sensitive
lower temperature, the PNIPAM brushes were hydrophilic and property as UiO-66-S-PNIPAM (Fig. S5†). In contrast, the size
stretched freely in water.36,37 When the temperature was of UiO-66-NH2 or UiO-66-Met without decorated polymer
higher than the LCST (ca. 33 °C), the PNIPAM brushes turned to brushes was almost unchanged in the given temperature
be hydrophobic and began to compacted on the surface of the range, implying that they are not temperature-responsive (Fig.
MOF NPs, leading them to shrinking in size quickly.36,38 The S6†).
hydrodynamic diameters as a function of five heating-cooling
Fig. 3 a) Hydrodynamic diameters of UiO-66-S-PNIPAM in dilute aqueous solution at different temperatures. b) Five heating-cooling cycles in the range of 20 to 50 °C.
c) IFT data of UiO-66-S-PNIPAM and Pd@UiO-66-S-PNIPAM. d) Water contact angles (WCAs) of UiO-66-NH2, UiO-66-Met, UiO-66-S-PNIPAM, Pd@UiO-66-S-PNIPAM
measured at different temperatures.
To better understand the thermal sensitive behaviour of mN m-1). As the temperature increased further, the IFT
UiO-66-S-PNIPAM and Pd@UiO-66-S-PNIPAM, their interfacial abruptly increased, and then kept increasing with
tension (IFT) between toluene and water was measured by temperature, which was similar to the reported results.28
spinning drop tensiometer between 25 and 50 °C (Fig. 3c). The These observations would be helpful to demonstrate why
IFT value for pure toluene/water reduced from 33 to 31 mN m- PNIPAM-S- decorated MOF NPs behaved well in terms of
1
with the temperature increase (Fig. S7†). Upon addition of emulsification.
UiO-66-S-PNIPAM, the IFT was down to 7.36 mN m-1 at 25 °C,
In addition, the water contact angle (WCA) of the MOF NPs
and then accompanied with a slightly decreasing process. IFT, was measured at different temperatures. Fig. 3d showed the
however, went through a sharp increase from 7.55 to 8.10 mN photographs of water-drop profiles on the NMOF tablets at 25
m-1 in a range of 30 - 35 °C, and then gradually increased to 8.3 and 45 °C, UiO-66-S-PNIPAM exhibited a hydrophilic surface
mN m-1 within 35 - 40 °C. From 40 to 50 °C, the IFT steadily with the WCA of 22°±2° at 25 °C. When the temperature rose
decreased to 8.14 mN m-1. As shown in Fig. 3c, the IFT of up to 45 °C, the WCA increased to 99°±2°. Similarly, the WCA
Pd@UiO-66-S-PNIPAM showed
a
similar temperature- of Pd@UiO-66-S-PNIPAM increased from 34±2° to 105°±2°
dependent change to that of UiO-66-S-PNIPAM but with an upon temperature increasing from 25 to 45 °C. This indicated
even lower ITF of 6.7 mN m-1 at 25 °C. In addition, the IFT of an undoubted phase transition or entropic change of the
UiO-66-NH2 and UiO-66-Met suspensions was more than those PNIPAM-S- brushes within 25-45 °C temperature range. In
of UiO-66-S-PNIPAM and Pd@UiO-66-S-PNIPAM under the contrast, the WCAs of UiO-66-NH2 and UiO-66-Met without
same conditions (Fig. S8†), implying that the PNIPAM-S- the PNIPAM-S- brushes were immune to temperature
decoration made the MOF NPs more hydrophobic. The IFT of alternation.
PNIPAM-SH was also measured by the same method (Fig. S9†).
Its IFT was also greatly reduced by PNIPAM-SH at 25 °C (12. 1
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3.3 Fabrication and characterization of Pd@UiO-66-S-PNIPAM
stabilized Pickering emulsion
Pd@UiO-66-S-PNIPAM in a fixed total volume ofViteww oArtipclheaOsnelinse.
The obtained Pickering emulsions were characterized by visual
DOI: 10.1039/C8GC03651A
inspection and confocal microscopy. For easy observation, the
aqueous phase was stained with FITC prior to the test, which
could give a green emission colour when excited by laser at
488 nm.
As shown in Fig. 4, Pd@UiO-66-S-PNIPAM stabilized Pickering
emulsion with different toluene-to-water ratios and MOF
contents was formed by homogenization using a homogenizer
at 5000 rpm for 60 s. Each contains a given amount of
Fig. 4 a) Visual and confocal microscopy images of Pd@UiO-66-S-PNIPAM stabilized Pickering emulsions with different toluene-to-water ratios (1 : 1 to 4 : 1). b)
Corresponding images with different contents of Pd@UiO-66-S-PNIPAM (0.5 wt% to 3.0 wt%) in a fixed toluene-to-water ratio of 3:1, v/v. c) Photographs of
temperature-responsive cycle of the Pickering emulsion stabilized by Pd@UiO-66-S-PNIPAM (2.0 wt%, toluene-to-water = 3:1, v/v). For the confocal microscopy
images, the aqueous phase was stained with FITC prior to the test, and the emulsion was excited by laser at 488 nm.
The effect of toluene-to-water ratio on the Pickering indicated that the emulsion was assuredly stabilized by
emulsion was firstly investigated. All the samples showed a NMOFs, which was in accordance with the observation in
highly viscous toothpaste-like gel emulsion upon other reported Pickering emulsions.14 The obtained emulsion
homogenization. As shown in Fig. 4a, the toluene-in-water at 3 : 1 of toluene-to-water ratio with 2.0 wt% Pd@UiO-66-S-
type of emulsion was first confirmed by the drop test, where a PNIPAM was very stable and no observable phase separation
drop of emulsion was added to pure water, and for all the was detected after storage at room temperature for two
emulsions with toluene-to-water ratios from 1:1 to 4:1, a quick weeks (Fig. S10†), indicating that Pd@UiO-66-S-PNIPAM is a
dispersibility was observed. On the other hand, oil-in-water highly effective emulsifier to stabilize the toluene-in-water
type emulsion was also tested by the confocal microscopy Pickering emulsion.
image, in which the FITC-labeled aqueous phase in all the
The thermal-stimuli switchable behaviour of the prepared
samples showed a continuous green phase upon irradiation Pickering emulsion was examined. As shown in Fig. 4c, the
with 488 nm light (Fig. 4a and b). Notably, only the sample Pickering emulsion generated from a toluene-to-water (2.25
with the toluene-to-water ratio of 3:1 was suitable to form the mL/0.75 mL) mixed solvent system with 2.0 wt % Pd@UiO-66-
most stable and complete emulsion. Thus, the oil-to-water S-PNIPAM was very stable at 25 °C, and it became unstable
ratio was fixed as 3:1 in volume for further investigation. In with the temperature increase under gentle stirring. When the
addition, the effect of Pd@UiO-66-S-PNIPAM content on the temperature reached 45 °C, the Pickering emulsion was
emulsion was also explored. As indicated in Fig. 4b, the smaller broken and a complete phase separation occurred quickly, the
droplets coalesced to produce large ones in the emulsion with resultant separated toluene and aqueous phases were clear,
low Pd@UiO-66-S-PNIPAM loading at 0.5 wt %. With the and ICP analysis showed that no Zr species existed in the
Pd@UiO-66-S-PNIPAM mass fraction increased from 1.0 to 2.0 aqueous phase, indicating that all the MOF NPs stayed in the
wt%, coalescence was greatly suppressed as the inter-phase of top toluene phase. As shown above, the attached PNIPAM
toluene drops were effectively stabilized with sufficient nano polymeric brushes compacted on the MOF NPs surface above
MOF emulsifier. For a 3.0 wt% emulsion, it was stable but too LCST (> 33 °C) and turned to be hydrophobic, which led
viscous to use. Thus, the optimal amount of Pd@UiO-66-S- Pd@UiO-66-S-PNIPAM to be incompatible with the aqueous
PNIPAM herein was determined to be 2.0 wt%. The decrease phase due to the highly dehydrated state of the attached
of inter-phase droplet size with the increase of NMOF content polymer brushes under the given conditions (45 °C). When the
6 | J. Name., 2012, 00, 1-3
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system was cooled down to 25 °C, Pd@UiO-66-S-PNIPAM 3.4 Catalytic performance
View Article Online
Chlorobenzenes (CBs) have been consiDdOerI:e1d0.1a0s39t/Ch8eGCp0ri3o6r5i1tAy
pollutants because of their high toxicity, persistence, and
bioaccumulate.39–41 Among various CBs dechlorination
methods, Pd NPs-based catalytic hydrogenation is regarded as
a powerful, economic, and highly efficient approach whereby
the inert C-Cl bond cleavage in CBs can be realized under mild
conditions with lower net impact on the environment.42,43
With Pd@UiO-66-S-PNIPAM in hand, we chosen the p-
chloroacetophenone dehalogenation as the model reaction to
examine biphasic catalytic activity of Pd@UiO-66-S-PNIPAM in
a toluene-water Pickering emulsion system, furthermore,
tested its temperature-driven catalyst-product separation and
cyclic utilization.
completely migrated to the aqueous phase because the
PNIPAM brushes were outspread and miscible with water
below LCST. This observation was well supported by its WCA
measurement at different temperatures. Upon re-
homogenization, a stable emulsion was formed again at 25 °C
(Fig.
4c).
This
temperature-driven
emulsification-
demulsification cycle can be repeated at least five times
without any loss in reversibility, suggesting that the Pd@UiO-
66-S-PNIPAM-stabilized emulsion was very thermal sensitive
and its reversible emulsification-demulsification process could
be easily controlled by simply adjusting the temperature
within a narrow range. Thus, the Pd NPs loaded MOF
emulsifier herein has laid a solid foundation for the Pd@UiO-
66-S-PNIPAM-based biphasic catalysis with facile separation
and good recyclability under mild conditions.
Fig. 5 p-Chloroacetophenone dehalogenation catalysed by Pd@UiO-66-S-PNIPAM-based Pickering emulsion. a) Reaction time examination (black line) and the
leaching test (red line). For leaching test, the solid catalyst was isolated from the reaction system after 1.0 h, whereas the filtrate was transferred to a new vial and
the reaction was carried out under the same conditions for an additional 2.0 h. b) Catalytic cycles. After each run, the catalyst was collected by a temperature-
induced phase separation. The yield for each catalytic run is run 1, >99 %; run 2, 99 %; run 3, 99 %, run 4, 98 %; run 5, 96 %; run 6, 96 %; run 7, 94 %; run 8, 93%. c)
The PXRD patterns of the Pd@UiO-66-S-PNIPAM before and after each catalytic run. Reaction conditions: p-chloroacetophenone (1.0 mmol), HCO2NH4 (10 mmol),
toluene-in-water (2.25 mL : 0.75 mL), Pickering emulsion with 58 mg of Pd@UiO-66-S-PNIPAM (2.2 mg Pd NPs, 2.0 mol% with respect to the ArCl), room
temperature, in air. The yield was determined by GC and confirmed by 1H NMR, respectively.
For safe and convenient reason, the nongaseous hydrogen rpm) over Pd@UiO-66-S-PNIPAM without homogenization and
source of HCO2NH4 was used instead of hydrogen as the
a monophasic toluene over Pd@UiO-66-NH2 were also
reducing agent, and the reaction was conducted at room examined, respectively. Each of them showed a poorer
temperature. After optimization, we were pleased to find that catalytic performance under the same conditions (Fig. S11†).
of p-chloroacetophenone (1.0 mmol) was completely turned The high activity of the Pd@UiO-66-S-PNIPAM-based emulsion
into acetophenone (>99 %, determined by determined by GC) was certainly ascribed to the highly dispersed Pd NPs as well as
within 2.5 h in a toluene-water Pickering emulsion (3 mL) with the larger specific surface area of the droplet microreactor,
2.0 wt % of Pd@UiO-66-S-PNIPAM (2.2 mg Pd NPs, 2.0 mol% which facilitated the mass transfer between the two phases. A
with respect to the ArCl) (Fig. 5a). Herein, the Pd loaded and leaching test was performed and indicated that no further
MOF-based particles not only effectively stabilized the reaction occurred without Pd@UiO-66-S-PNIPAM after
emulsion system, but also significantly facilitated the contact ignition of the reaction at 1.0 h, indicating that it featured a
between p-chloroacetophenone in toluene and HCO2NH4 in typical heterogenous catalytic nature (Fig. 5a).
aqueous phases, consequently, made the interfacial catalytic
In addition, the recyclability of the catalytic emulsion was
reaction between the two immiscible phases available. In examined. As shown in Scheme 2, after each catalytic run, the
addition, the catalytic activities of an equimolar amount of complete phase separation was achieved by heating up to 45
active sites in a simply mixed system of water/toluene °C. After being cooled to 25 °C, the Pd@UiO-66-S-PNIPAM
(obtained by magnetic stirring with a constant speed of 500 species completely moved to the bottom aqueous phase, and
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Journal Name
O
O
resulting upper toluene phase containing the reaction product
could be simply isolated. ICP analysis confirmed that all the
MOF emulsifier stayed in the bottom aqueous phase.
Meanwhile, there was no detectable CBs or dechlorinated
product in the additional toluene extract from aqueous phase
based on GC analysis, implying a complete catalyst-product
separation. After replenished the consumed HCO2NH4 and
fresh toluene phase with CBs, the next catalytic was carried
out under the same conditions. Remarkably, the Pd@UiO-66-
S-PNIPAM-based emulsion showed excellent activity and the
dehalogenation yield was still up to 93% even after eight
catalytic cycles (Fig. 5b). PXRD patterns showed that the
crystallinity and structural feature of the MOF were well
maintained during the reusable process (Fig. 5c). In addition,
the HRTEM image revealed that no significant aggregation of
Pd NPs occurred in the reused Pd@UiO-66-S-PNIPAM (Fig.
S12†), while XPS analysis indicated that no valence changes for
Pd species after recycle (Fig. S13†). ICP analysis showed that
basically no Pd (retained 3.72 wt %) and Zr (retained 17.01 wt
%) leaching even after eight catalytic runs (Table S2†). Thus,
this Pd@UiO-66-S-PNIPAM-stabilized thermal-responsive
demulsification-emulsification cycle provided an excellent
catalytic activity, recyclability with a facile catalyst-product
separation.
View Article Online
5
6
DOI:410.1039/C8GC03651A
99
Cl
O
O
Cl
O
O
4
97
Cl
Cl
7
8
4
4
99
98
Cl
a
Reaction conditions: aryl chloride (1.0 mmol), HCO2NH4 (10 mmol), 3.0 mL of the
UiO-66-S-PNIPAM-based Pickering emulsion (toluene-to-water ratio is 3 : 1, 2.2 mg
Pd, 2 mol % equiv), r. t., in air. b Yields were determined by GC.
After that, the scope of the dehalogenation reactions was
examined under the optimized conditions. Interestingly, the
dehalogenation reactions proceeded extraordinarily well with
different CBs, and the target products were easily isolated. As
shown in Table 1, all the CBs, including either electron-
withdrawing or electron-donating groups at different
substituted positions on CBs, gave the corresponding
dechlorinated products in excellent yields (94 - ≥99 %) within
2.5 - 4.0 h (Table 1, entries 1-8, Figs. S14-S23†), even for the
dichlorobenzene derivative (Table 1, entry 6). Besides, larger
sized CBs such as 1-chloronaphthalene (99 % yield, Table 1,
entry 7) and 4-chlorobiphenyl (98 % yield, Table 1, entry 8)
could also be dechlorinated in excellent yields within 4 h. Thus,
the UiO-66-S-PNIPAM-based Pickering emulsion could be
applicable to a wide range of CBs dehalogenation.
4. Conclusions
In summary, we reported a smart thermal-sensitive Pickering
emulsion catalytic system based on UiO-66 type NMOF and
PNIPAM polymer brushes via a covalently composite approach
and a simple Pd NPs loading step. The obtained Pd@UiO-66-S-
PNIPAM can stabilize toluene-in-water Pickering emulsion,
furthermore, display
a
smart temperature-triggered
emulsifying and demulsifying behaviour caused by the
covalently attached PNIPAM brushes. In addition, the as-
prepared Pickering emulsion can highly promote the CBs
dehalogenation at the biphasic oil/water interface. We expect
the presented approach might provide some guidance for the
construction of many more novel MOF-based Pickering
emulsion catalytic systems for various applications, especially
the growing global environmental issues.
Scheme 2 A schematic model for the temperature-driven catalytic cycle of the
Pd@UiO-66-S-PNIPAM-based Pickering catalytic system.
Table 1. Results of the CBs dechlorination promoted by Pd@UiO-66-S-PNIPAM-
stabilized Pickering emulsion. a
entr
y
substrate
product
t (h) yield (%)b
Conflicts of interest
There are no conflicts to declare.
O
O
1
2.5
2.5
≥99
≥99
Cl
Cl
O
O
2
Acknowledgements
This work was supported by NSFC (Grant Nos. 21671122,
21604049, 21475078 and 51303096), the Shandong Provincial
Natural Science Foundation (BS2015CL015), China, A Project of
Shandong Province Higher Educational Science and Technology
Cl
O
O
O
3
4
2.5
4
≥99
O
94
Cl
O
O
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DOI: 10.1039/C8GC03651A
For table content
As a thermo-responsive MOF-based emulsifier, Pd@UiO-66-S-PNIPAM can highly promote the biphasic chlorobenzenes
dechlorination at the toluene-water interface at room temperature.
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