Y. Zhang, et al.
MolecularCatalysis478(2019)110567
Table 4
adsorption-desorption analysis (Table 4), the specific surface area of the
catalyst was reduced from 1109 m2/g to 873 m2/g and pore volume is
decreased from 0.6 to 0.5 cm3/g, respectively. It was indicated that the
pores were clogged with organic compounds (e.g. reactants or pro-
ducts), or the Pd nanoparticles agglomeration. To confirm whether the
organic was absorbed in the catalyst, deionized water (500 mL) was
collected for washing the catalyst and extracted with ethyl acetate, then
analyzed by Agilent 6890 N GC (Fig. 7b). It is shown that the major
species in the solution were reactant phenol and product cyclohex-
anone, and slight cyclohexanol. The TEM image of the used catalyst was
shown in Fig. 7f, it obvious agglomeration. Then, by calculating the
that the percentage of Pd0 was 65.9% and 83.9% (Table 4), respec-
tively. Maybe partial Pd2+ species was reduced to Pd0 by hydrogen
during the reaction. It demonstrates that the change of Pd valence state
could not be responsible for the decrease in reaction. In addition,
comparing the Zr 3d spectra of fresh and used catalyst did not reveal
any changes (Fig. 7d). Therefore, the main deactivation reason of this
clogged with organic compounds which may partially block the active
sites, further study for the repeatability of this catalyst is still carried
out.
Physical properties of Pd/@-ZrO2/AC(500) catalyst before and after reaction.
Sample N2 absorption-desorption
Pd/B.E(eV)
Surface area(m2/
g)
V(cm3/g) Pd+2 Pd0
Pd2+/Pd0(%)
Fresh
Used
1109
873
0.6
0.5
336.8 341.2 334.6 34.1/65.9
336.5 340.3 334.7 16.1/83.9
AC and Pd/ZrO2(500) were applied as catalysts, the phenol conversion
were 31.8% and 7.4%, respectively. Surprisingly, the catalytic activity
was greatly enhanced by using Pd/@-ZrO2/AC(500) as the catalyst,
phenol was completely converted. Besides, when the ZrO2 loading de-
creased to 5 wt.% or 8 wt.%, the conversion was decreased. This result
means that the ZrO2 content was less than enough to cover the entire
surface of the AC, the Pd nanoparticles were major anchored on the AC
directly, the Pd/@-5%ZrO2/AC or Pd/@-8%ZrO2/AC consisted of
major Pd/AC and slight Pd/@-ZrO2/AC. Therefore, the Pd/@-ZrO2/AC
with low loading of ZrO2 shown low activity. Based on the above results
and characterizations, we could conclude that this hybrid nano-struc-
ture Pd/@-ZrO2/AC(500) catalyst exhibited the synergistic effect be-
tween Pd nanoparticles and ZrO2/AC support, which enhanced the
activity for the hydrogenation of phenol. Compared with the recent
works of the Pd catalysts, relative advantage can be obtained from our
Pd/@-ZrO2/AC(500) catalyst, and the turnover frequency (TOF) based
on surface Pd reached 222.3 h−1, which is higher than that of the 0.9%
Pd/Amberlyst-45(118 h−1).44
According to this synthesis strategy of catalyst, we prepared Pd/@
-CeO2/AC, Pd/@-La2O3/AC and Pd/@-TiO2/AC catalysts. These hybrid
nano-structure catalysts also provide superior activity and synergistic
catalysis in the hydrogenation of phenol (Fig. 8a) and benzaldehyde
study opens a new direction to develop a hybrid nano-structure catalyst
for heterogeneous catalytic hydrogenation reaction.
Then we studied the effect of reaction conditions on phenol con-
version over Pd/@-ZrO2/AC(500) catalyst. The reaction temperature
(Fig. 6a) had a considerable effect on the activity of the catalyst, the
phenol conversion of 100% was achieved at 0.7 MPa hydrogen pressure
and 80℃ for 3 h. However, even at a temperature of 60℃, 92% con-
version of phenol could be reached after 4 h. It was indicated that our
catalyst system could catalyze this reaction under the low reaction
temperature. Moreover, when gradually increasing the pressure of hy-
drogen from 0.3 to 0.8 MPa, the activity was correspondingly enhanced
(Fig. 6b), the selectivity slightly decreased, the increase of hydrogen
pressure should enhance the solubility of hydrogen in the liquid phase
and provide more active hydrogen on the surface of the catalyst, fa-
voring the hydrogenation of the phenol.
4. Conclusions
In summary, a hybrid nano-structure Pd catalyst was synthesized
through coating ZrO2 on the AC, then loaded Pd nanoparticles on the
ZrO2/AC by a photochemical route. The catalytic activity was closely
related to the calcination temperature of ZrO2/AC, the ZrO2/AC support
calcined at 500℃ with a minimum particle size, which effects Pd na-
noparticles supported on it with high dispersion, and exhibited stronger
interaction of Pd nanoparticles with the ZrO2/AC surface as compared
to other catalysts.The Pd/@-ZrO2/AC(500) catalyst exhibit highest ac-
tivity to the hydrogenation of phenol, which was much better than that
of Pd/@-ZrO2/AC(450, 550, 600) catalyst. In addition, this Pd hybrid
nano-structure catalyst exhibited synergistic catalysis in selective hy-
drogenation, outperforming both conventional Pd/metal oxide and Pd/
AC catalyst. This structure could be inferred as a feasible synthesis
strategy to obtain desired metal nano-catalysts for the heterogeneous
catalysis.
After the reaction, the recyclability of Pd/@-ZrO2/AC(500) catalyst
was investigated. The used catalyst was washed by deionized water
(500 mL). Even though the phenol conversion decreased to 69.3% with
the recycled catalyst, the activity of the used catalyst still higher than
that of Pd/ZrO2 and Pd/AC. Then the used catalyst was characterized
fraction peak of the used catalyst was stronger. Combined with N2
Fig. 8. Hydrogenation of phenol(a) and hy-
drogenation of benzaldehyde(b) on different
catalysts. Reaction conditions: c catalyst(0.1 g),
phenol(0.353 g), 70 °C, 3 h, 0.7 MPa H2 pres-
d
e
sure, H2O(20 ml); 2 h, 0.5 MPa H2 pressure;
catalyst(0.05 g),
benzaldehyde(1.88 mmol),
f
40℃, 1 h, 0.7 MPa H2 pressure, H2O(20 ml);
g
30 min; 80 min.
8