Nanostructured Supports for the Oxidation of Alcohols
FULL PAPER
[
a]
Table 3. Oxidation of benzyl alcohol: effect of benzyl alcohol/cyclohexane ratio.
by inductively coupled plasma
(ICP) measurements on both
the collected solution from re-
cycling and the used catalyst
[
b]
À1
[c]
Catalyst
Benzyl alcohol/
Cyclohexane
ratio
TOF [h
]
Selectivity
Toluene
Benzaldehyde
Benzoic acid
Benzyl benzoate
Pd/AC
Pd/CTF
Pd/N-CNTs
Pd/AC
Pd/CTF
Pd/N-CNTs
Pd/AC
Pd/CTF
50:50
50:50
50:50
25:75
25:75
25:75
10:90
10:90
10:90
2678
4917
4438
1592
4162
3392
1059
1453
1094
29
31
30
40
30
32
–
70
66
69
56
68
65
95
98
>99
1
1
1
1
1
–
–
–
–
–
1
–
2
–
–
–
–
–
(
see Experimental Section). To
confirm the absence of leached
Pd, the “hot filtration” test was
[49]
performed.
After 30 min of
reaction, the catalyst was isolat-
ed by filtration without cooling
the reaction mixture and ICP
measurements on the filtrate
excluded the presence of Pd.
–
–
Pd/N-CNTs
[
2
a] Reaction conditions: alcohol/metal: 5000, T=808C, pO =2 atm. [b] TOF calculated after 15 min of reac-
tion based on the total metal loading. [c] Selectivity calculated at 90% conversion.
TOF of Pd/CTF with those of Pd/N-CNTs and Pd/AC
(
Table 3), we observed that the three catalysts behaved dif-
ferently even though all of them decreased in activity upon
increasing the amount of solvent. Pd on CTF always showed
the highest activity and Pd/AC the lowest. On the contrary,
Pd/N-CNTs behaved similarly to CTF when the organic sol-
vent was 50 vol%, but similarly to AC when the solvent
reached 90 vol%. When a large amount of cyclohexane was
present (90%), the difference among the three activities de-
À1
creased (1453, 1094, and 1059 h for Pd/CTF, Pd/N-CNTs,
and Pd/AC, respectively). AC with respect to CTF is a less
hydrophobic support due to the presence of carboxylic and
hydroxylic groups on the surface. Thus, in these two cases, it
is not surprising that by decreasing the polarity of the
medium (i.e., increasing the amount of cyclohexane) CTF
could interact with a polar reactant better than AC. On the
contrary, the behavior of N-CNTs highlights the double
nature of this support, in which N functionalities such as
Figure 4. Recycling test with Pd/CTF (dark gray) and Pd/N-CNT (light
gray): benzyl alcohol/cyclohexane 50:50, alcohol/metal: 5000, T=808C,
pO =2 atm.
2
Moreover, the filtrate was maintained under the reaction
conditions (T=808C, pO =2 atm, stirring) and samples
2
[45]
pyridine, pyrrole, and pyridone coexist with typical hydro-
philic functionalities of AC (carboxylic acid, phenols, etc.).
The capability of N functionalities to enhance the stability
of Pd NPs during catalytic reaction is of particular impor-
tance from an industrial point of view. We have already
shown that Pd/CTF is a much more durable catalyst than
were analyzed after an additional hour to check any changes
in the product distribution. No changes occurred. To defi-
II
nitely exclude a possible contribution of soluble Pd species
to catalysis, an additional test was performed with Pd ACHTUNGTRENNUNG( OAc)
2
(Ac=acetyl) as the catalyst under the same reaction condi-
tions as those used with Pd/CTF. No conversion of benzyl
alcohol was detected after one hour.
[42]
Pd/AC for glycerol oxidation in water.
In addition, we
also showed that Pd/AC deactivates rapidly in the oxidation
of benzyl alcohol in cyclohexane, probably because of a con-
[48]
sistent leaching of metal.
Herein, the durability of Pd/
Conclusion
CTF was investigated and compared with that of Pd/N-
CNTs by performing several reactions successively (recy-
cling). Each run was carried out under the same conditions
By the use of a CTF, a triazine-rich material, as the support
for Pd NPs, it was established that a large number of
N groups (9 wt%) with a controlled morphology greatly in-
creased the catalyst performance in alcohol oxidation in cy-
clohexane relative to AC and CNTs. Pd/CTF showed an ini-
tial activity similar to that of Pd on N-doped CNTs and this
confirmed the beneficial effect on catalytic activity of the
presence of N functionalities. Pd NPs similarly prepared
(from PVA sol) revealed a better stability during the immo-
bilization on CTF and N-CNTs than AC and CNTs. This
property allows a better catalytic performance in alcohol ox-
idation due to a higher dispersion of Pd NPs. However, Pd
on the CTF resulted in a really resistant catalyst (tested in
(
alcohol/metal: 5000, 808C, 2 atm O , 1250 rpm, cyclohexane
2
50 vol%) for two hours.
The catalyst was reused after separation from the solution
without any other treatment. Pd/CTF showed a similar con-
version (about 100%) along the nine runs and the selectivity
to benzaldehyde (66% taken at 90% conversion) did not
change (Figure 4). On the contrary, Pd/N-CNT deactivated
[48]
faster than Pd/CTF as observed for Pd/AC. Similarly to
the case of Pd/AC, the deactivation of Pd/N-CNTs is ex-
plained by consistent leaching of Pd (5% on the used cata-
lyst). In the case of Pd/CTF, no leaching of Pd was observed
Chem. Eur. J. 2011, 17, 1052 – 1057
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1055