Communication
ChemComm
Although pH-responsive polymer emulsifiers are available in to 7–8, the system with SN–ON rapidly restored Pickering
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the literature, inorganic particles are preferred because of emulsion. Moreover, this reversible switch behaviour was
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its robustness. As extension to our previous protocol, we here observed in other biphase systems such as toluene-, benzene-,
used a significantly increased molar fraction of pH sensitive ether-, dichloromethane- and trichloromethane–water (Fig. S5,
(
MeO) SiCH CH CH (NHCH CH ) NH (10%, in its mixture ESI†). More impressively, the SN–ON-stabilized Pickering emul-
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2
2
2
2
2 2
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with hydrophobic (MeO)
3
Si(CH
2
)
7
CH
3
) to modify smaller silica sion could be reversibly switched on and off at least ten times
nanospheres. The obtained material is denoted as SN–ON. For (Fig. S6, ESI†). The significantly different behaviour of these
comparison, we also prepared triamine-monofunctionalized materials is attributed to the difference in surface chemistry.
and octyl-monofunctionalized silica microspheres, denoted as SN–O and SN–N are too hydrophobic or too hydrophilic
SN–N and SN–O, respectively.
to stabilize Pickering emulsion, whereas SN–ON is not only
The TEM image shows that SN–ON consists of monodisperse moderately hydrophobic but also pH-responsive. After the
spheres with diameters of around 50–60 nm (Fig. S1a, ESI†). It is SN–ON surface triamines are protonated, their surfaces become
2
À1
almost non-porous since its specific surface area is only 37 m g
Fig. S2, ESI†). The electron energy loss energy (EELS) confirms
too hydrophilic to stabilize emulsion.
(
Next, hydrogenation was chosen to evaluate the catalytic
that triamine and octyl groups are both uniformly distributed on efficiency and recyclability. We prepared a Pd/SN–ON by load-
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silica nanospheres (Fig. S1b, ESI†). The solid state C CP-MAS ing Pd nanoparticles on SN–ON (Pd loading is 1 wt%). The TEM
NMR spectrum exhibits C signals, which can be assigned to octyl image shows that Pd nanoparticles (1–2 nm in size) are homo-
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and triamine groups (Fig. S3a, ESI†). In the solid state Si geneously distributed on the surface (Fig. S7a, ESI†). X-ray
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2
CP-MAS NMR spectrum (Fig. S3b, ESI†), T [SiR(OSi) ] and T
(EDX) spectroscopy (EDS) confirms the presence of Pd besides
3
[
SiR(OSi) (OH)] bands appear, indicating that these function- N, C, O and Si elements (Fig. S7b, ESI†).
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alities are linked to the SiO2 surface through Si–O–Si bonds.
We compared Pd/SN–ON catalytic efficiency in the Pickering
Elemental analysis gives quantitative results (Table S1, ESI†): octyl emulsion system and in the conventional biphasic system that
À1
and triamine loadings on SN–ON are 0.31 and 0.12 mmol g
,
was obtained by adding isopropyl alchohol as the demulsifier.
À1
respectively; the octyl loading on SN–O 0.31 mmol g ; triamine Based on H
loading on SN–N 0.72 mmol g . These results are broadly these two systems proceeded at remarkably different rates.
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consumption rates (Fig. 3a), one can find that
À1
supported by the TG measurement result (Fig. S4, ESI†).
Fig. 3b quantitatively compares their catalytic efficiency (CE,
After mixtures of silica nanospheres, ethyl acetate and water see the footnote of Fig. 3). CE in the Pickering emulsion is
were stirred for 3 min (800 rpm), different phenomena were 5.14 times higher than that in the conventional biphase. The
observed (Fig. 2). For SN–O and SN–N, the systems consist of significantly enhanced catalytic efficiency is attributed to the
two phases with SN–O and SN–N distributed in the upper oil presence of emulsion droplets that create a large reaction
phase and the lower water phase, and emulsion droplets were interface area. At the end of reaction, the Pickering emulsion
not found using an optical microscope. Different from SN–O and was demulsified after lowering the pH to 3–4, allowing the
SN–N, SN–ON led to a Pickering emulsion phase at the bottom organic product to separate through a simple liquid decanta-
since droplets were observed. The drop test confirmed that it was tion (Fig. 3c and Fig. S8, ESI†). In the subsequent reaction
of W/O type. Interestingly, after adding a few drops of HCl cycles, Pickering emulsion was obtained again by increasing the
solution, the systems with SN–O and SN–N had no obvious pH to 7–8. As shown in Fig. 3e, ethylbenzene yield is up to 83%
changes, while the SN–ON-stabilized Pickering emulsion was in the first reaction, which is higher than those obtained with
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demulsified, as observed from the optical micrographs. When a our previous Pickering emulsion systems. From the second to
few drops of NaOH solution were added and the pH was tuned fifth cycle, all isolated yields are more than 90%. The slight
activity loss is due to the Pd particle aggregation and slight Pd
leaching (Fig. S9, ESI†).
This smart system worked well for hydrogenation of other
unsaturated compounds. Table 1 lists the results for the first
and second reaction cycles. For all the investigated substrates,
Pickering emulsion systems gave more than 98% conversions
within 1.5–3 h, and 77–82% yields can be achieved with a little
amount of product sacrificed at the phase boundary (Fig. S7,
ESI†). In the second reaction cycle, 99% conversions were
afforded and the yields increased up to 97–99%. The results
further justify the importance of our smart Pickering emulsion
strategy.
In summary, through appropriate surface modification we
first demonstrate a pH-responsive Pickering emulsion system
for organic/aqueous biphasic catalysis. Such a system exhibits
Fig. 2 Appearance of ethyl acetate–water mixture in the presence of
different silica nanospheres (photographs taken after standing for 0.5 h).
Every vial contains 4 mL of ethyl acetate, 4 mL of water, and 0.04 g of silica
fivefold reaction rate enhancement effects in comparison to
using HCl; state 3: pH is adjusted to 7–8 using NaOH (1 M). Scale bar is 200 mm. the conventional biphasic reaction. Its demulsification on
nanospheres. State 1: before adding HCl (1 M); state 2: pH is adjusted to 3–4
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015
Huang, Jianping
