1
90
M. Zhong et al. / Journal of Catalysis 338 (2016) 184–191
Fig. 6. (A) Recycling of CoIII(salen)-OTs@FDU-12 in the hydration of PO with and without activation. (B) UV–vis spectra of CoIII(salen)-OTs@FDU-12 (a) after 1 cycle in PO
hydration and (b) after activated by PTSA under air in EtOH solution.
As shown in Fig. 5A and B, a strong absorption band at 360 nm and
than homogeneous counterpart is related with the restricted
chemical reaction in nanocages. Similarly, the reversible counte-
III
at 406 nm was respectively observed for fresh Co (salen)-OTs and
III
II
III
Co (salen)-OAc in CH
2
Cl
2
. The inactive Co (salen) in CH
2
Cl
2
gives
rion addition process for Co (salen)-OTs was also restricted in
III
two bands at 355 and 415 nm. After 30 min, the UV–vis spectrum
nanocages. As a result, the deactivation of Co (salen)-OTs in nano-
III
of reaction system catalyzed by Co (salen)-OTs shows a weak
cages is faster than homogeneous counterpart.
III
II
III
band at 415 nm, indicating the reduction of Co to Co starts.
The intensity of the absorption band at 415 nm increases along
with the increase in the reaction time accompanied by the appear-
ance of the absorption band at 355 nm. After 90 min, the bands
Recycle ability of Co (salen)-OTs@FDU-12 was tested in PO
hydration (Fig. 6A). Without activation process, the sharp decrease
in PO conversion was observed for the second cycle due to the
III
deactivation of Co (salen)-OTs. The UV–vis spectrum of the recov-
II
assigned to Co (salen) dominate in the UV–vis spectrum, showing
ered catalyst after 1 cycle clearly shows the bands from Co(II)
(Fig. 6B). After activation, the characteristic bands assigned to Co
(III) appears again and PO conversion increases from 42% to 86%,
showing the Co(III) could be regenerated by activation process
(the solid catalyst filtered from the reaction mixture was regener-
ated by dispersing in EtOH containing PTSA under air for 1 h).
However, the gradual decrease in PO conversion could still be
observed even after catalyst regeneration. For clarifying the reason
III
III
the reduction of most Co (salen)-OTs. In the case of Co (salen)-
III
II
II
OAc, the reduction of Co to Co occurs in 10 min and Co species
dominates in the reaction system based on the UV–vis spectrum.
III
II
This suggests that the reduction rate of Co to Co is faster for
III
III
Co (salen)-OAc than Co (salen)-OTs. The kinetic curves for the
III
III
PO hydration catalyzed by Co (salen)-OAc and Co (salen)-OTs
give the similar tendency (Fig. 2). The higher anti-reduction ability
of Co (salen)-OTs may be related with its reversible counterion
III
III
for catalyst deactivation, the spent Co (salen)-OTs@FDU-12 was
addition.
fully characterized by nitrogen sorption, TGA, XRD and FT-IR
analysis.
III
As shown in Fig. 5C and D, fresh Co (salen)-OTs@FDU-12 and
III
Co (salen)-OAc@FDU-12-C3 in ethanol afford UV–vis band at
The decomposition of the salen ligand may result in the deacti-
vation of catalyst. Thus, the fresh and spent catalysts were charac-
terized by FT-IR (Fig. S1). The fresh catalyst displays the vibration
II
4
00 nm and 409 nm, respectively. The UV–vis bands of Co (salen)
III
@
FDU-12 appear at 360 and 415 nm in ethanol. For Co (salen)-
ꢀ1
OTs@FDU-12, the red-shift of the band at 400 nm was observed
in 10 min and the band at 360 nm appears in 20 min, showing
assigned to imine stretch at 1631 cm . Similar to the fresh cata-
lyst, the spent catalysts also give the characteristic imine band at
III
II
ꢀ1
the reduction of Co to Co . Along with the reaction time, the band
gradually red shift from 400 to 417 nm and the intensity of the
band at 360 nm becomes stronger. After 60 min, the bands of
1631 cm , suggesting that the metal complexes remained their
structure during the catalytic test and regeneration process.
III
The N
2
sorption isotherms of Co (salen)-OTs@FDU-12 after the
II
III
Co (salen) dominate. As for Co (salen)-OAc@FDU-12-C3, no obvi-
ous changes in the intensity and wavelength for the band assigned
first, the third and the sixth run in Fig. S2A show that all samples
exhibit type IV isotherm pattern with H3 hysteresis loop, charac-
teristic of mesoporous materials with cage-like mesostructure.
III
to Co could be observed after 30 min. For 70 min, the slight red
III
shift of the band from 409 to 418 was found. The characteristic
band of Co is still invisible even after 180 min. Under similar con-
The XRD patterns of Co (salen)-OTs@FDU-12 after the first, the
II
third and the sixth run show an intense diffraction peak similar
III
II
III
III
ditions, almost all Co was reduced to Co for Co (salen)-OAc,
to FDU-12 and fresh Co (salen)-OTs@FDU-12, showing that all
III
III
Co (salen)-OTs and Co (salen)-OTs@FDU-12. The above results
samples have the cubic Fm3m symmetry (Fig. S2B). The above
results suggest that the mesostructure of FDU-12 is preserved dur-
ing the catalytic process.
III
suggest that Co (salen)-OAc@FDU-12-C3 has the highest anti-
reduction ability among all catalysts investigated.
Previously, it was found that the confinement effect of the
nanoreactor could effectively suppress the formation of a dynamic
mixture of BINOLate/Ti species because the chemical reaction in
nanocages is more restricted than in bulk solution [23]. Lu reported
After the first run, the BET surface area and pore volume
decrease slightly compared with fresh catalyst (Table 5). After
regeneration, the BET surface area, the total pore volume and the
micropore volume increase and are even higher than fresh
III
III
that {CoIIIꢀOꢀCoIII} derived from a dimeric Co (salen)-OH com-
Co (salen)-OTs@FDU-12 (Table 5). Based on TG analysis, the
III
plex by a dehydration undergoes a disproportionation to produce
a Co(IV) species and a Co(II) species [24]. The higher anti-
weight loss in the range of 160–800 °C for Co (salen)-OTs@FDU-12
after the 1st run is larger than fresh sample. After regeneration, this
weight loss decreases obviously but is still higher than those of
III
reduction ability of Co (salen)-OAc encapsulated in nanocages