Communications
Pd@ZIF-8 hollow nanospheres, which is in agreement
with the pore size of ZIF-8.[32]
Table 1. Size-selective hydrogenation of olefins of different sizes.[a]
Conversion for different catalysts[b] [%]
Considering that the particular structure of
Pd@ZIF-8 hollow nanospheres, in which the Pd nano-
particles totally reside inside and ZIF-8 shell, has mo-
lecular sieving capabilities, it can be used as an ideal
nanoreactor to investigate catalytic activity and size
selectivity in liquid-phase solution. We chose the hy-
drogenation of olefins as a model reaction to probe
the catalytic performance. For comparison, the hy-
drogenation was also performed with commercially
available Pd on carbon (Pd/C), Pd nanoparticles im-
mobilized on the outer surfaces of ZIF-8 (denoted
Pd/ZIF-8), and Pd/CPS@ZIF-8.
Olefin
Pd/C
Pd/ZIF-8
CPS/Pd@ZIF-8
nanospheres
Pd@ZIF-8 hollow
nanospheres
100
100
100
0
100
0
100
100
100
100
0
0
0
0
First, the kinetic curves for the hydrogenation of 1-
hexene over the above four catalysts were examined,
and the turnover frequencies (TOFs) are summarized
in Figure S2. Pd/C and Pd/ZIF-8 showed much higher
activity than Pd/CPS@ZIF-8 and Pd@ZIF-8 hollow
nanospheres, which can be ascribed to the diffusion
barrier of the ZIF-8 shells for mass transformation of
1-hexene.
100
100
[a] Reaction conditions: ethanol (15 mL), 258C, H2 (1.0 MPa), catalyst/substrate molar
ratio of 1:100 (10 mmol scale), 24 h. [b] Conversion was determined by GC, and the
identity was ascertained by GC–MS.
We also found a confinement effect for catalysis on
Pd@ZIF-8 hollow nanospheres. The TOF of the Pd/
CPS@ZIF-8 core–shell composite nanospheres was
310 hÀ1, which is only approximately 15% of that of
Pd@ZIF-8 hollow nanospheres (2150 hÀ1). For Pd/
CPS@ZIF-8, in which the polystyrene template is not removed,
the reactants are confined in the micropores, which makes it
difficult for them to reach the active sites. After etching the
polystyrene spheres, the cavities can be used as reactors for
the reactants, which are confined in the cavities for conversion.
Moreover, the catalytic performance of Pd@ZIF-8 hollow nano-
spheres was much better than that of ZIF-8 encapsulated with
polyvinylpyrrolidone-protected Pd nanoparticles, as previously
reported.[33–35]
marked decrease in activity, even after 10 consecutive recycling
runs for the hydrogenation of 1-hexene. The TEM image show
that there was only slight aggregation of the Pd nanoparticles
after reuse (Figure S4). Evidently, the yolk@shell Pd@ZIF-8
hollow nanospheres are highly stable against migration and
sintering of the Pd nanoparticles and exhibit excellent activity
and stability for the hydrogenation of olefins.
In summary, Pd nanoparticles encapsulated in ZIF-8 hollow
nanospheres were synthesized by nucleation of ZIF-8 nano-
structures around unprotected Pd nanoparticles supported on
carboxylate-terminated polystyrene nanospheres. No further
reduction or removal of extra capping agents was needed, and
the resulting composites have a precisely controlled overall
structure. The obtained Pd@ZIF-8 hollow nanospheres exhibit-
ed size-selective catalytic properties for the hydrogenation of
olefins in liquid-phase solution. It is expected that other surfac-
tant-free noble-metal nanoparticles could also be employed to
fabricate NPs@MOFs with the same strategy.
Substrates with different sizes, including 1-hexene (1.9ꢁ
8.2 ꢀ), cis-cyclooctene (5.3ꢁ5.5 ꢀ), trans-stilbene (4.2ꢁ11.3 ꢀ),
and triphenylethylene (9.1ꢁ9.2 ꢀ), were then employed to
study the size selectivity of Pd@ZIF-8 hollow nanospheres. As
shown in Table 1, 1-hexene was converted into the corre-
sponding product with all catalysts. On the contrary, larger
sized molecules (i.e., cis-cyclooctene, trans-stilbene, and triphe-
nylethylene) were only converted on Pd/C and Pd/ZIF-8. No
products were detected on Pd/CPS@ZIF-8 and Pd@ZIF-8
hollow nanospheres, even after 24 h. The sharp difference in
reactivity suggests size selectivity of Pd@ZIF-8 hollow nano-
spheres in liquid-phase solution. ZIF-8 has large pores of 11.6 ꢀ
and small apertures of 3.4 ꢀ, which are bigger than the size of
1-hexene (1.9 ꢀ), and thus, mass transformation of 1-hexene in
liquid solution is facile. On the contrary, the molecular widths
of cis-cyclooctene, trans-stilbene, and triphenylethylene exceed
the size of the apertures in ZIF-8. Thus, they cannot diffuse
through the shell to reach the encapsulated Pd nanoparticles,
and consequently, no conversion is observed.
Experimental Section
Preparation of carboxylate-terminated polystyrene nano-
spheres
Polystyrene nanospheres were prepared according to a method re-
ported elsewhere. In a typical synthesis, a mixture of styrene
(21 mL), methyl methacrylate (1.1 mL), acrylic acid (0.92 mL), and
NH4HCO3 (0.49 g) was added to deionized water (100 mL) with me-
chanical stirring. Upon increasing the temperature to 708C, ammo-
nium persulfate (0.53 g) was added, and the mixture was allowed
to react for 12 h at 808C. The resulting product was separated by
Finally, the catalytic stability of Pd@ZIF-8 hollow nano-
spheres was investigated. As shown in Figure S3, there was no
ChemCatChem 2016, 8, 1 – 6
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