26
L. Cheng et al. / Catalysis Communications 57 (2014) 23–28
OH
OH
O
OH
Pd catalysts
H2
Pd catalysts
H2
Fig. 3. Phenol hydrogenation over Pd catalysts.
conversions and selectivity are listed in Table 1. With Pd catalysts
under the given conditions, we could readily achieve selectivities
towards cyclohexanone in the range 96–99%, with a conversion of
phenol of 86–100% within 6 h at 333 K and atmospheric pressure of
hydrogen. Pd catalysts exhibit higher activity in water than that in
cyclohexanone, while the selectivity is not dependent on solvent,
but on conversion. If the phenol is not converted completely, the for-
mation of cyclohexanol is almost not observed. The highest activity
in aqueous liquid is obtained over Pd/Al2O3-3 catalyst (Table 1),
under 333 K, atmospheric pressure of hydrogen and Pd/phenol =
0.023 with almost 100% phenol conversion and 99% cyclohexanone
selectivity in 3 h.
For kinetic consideration, the effect of phenol concentration in
cyclohexane and water was investigated (Table 1). At 0.213 mol/L
phenol, a similar reaction result was observed in water and cyclohexane
over Pd/Al2O3 catalysts (Table 1 entries 1, 2, 7 and 8). However, with the
decrease in phenol concentration down to 0.085 mol/L, the difference of
activity in water and cyclohexane was observed (Table 1 entries 3, 4, 9
and 10). For Pd/Al2O3-3 catalyst, the activity in water is higher than
that in cyclohexane, while the activity of Pd/Al2O3-13 catalyst in water
is lower than that in cyclohexane. The dependence of reaction rate on
phenol concentration is affected through complex modes. In water,
the reaction rate at low conversion is lower than that at high conversion.
Probable reason is that the concentration of cyclohexanone increases
with phenol conversion, and the adsorption of cyclohexanone would
be favorable to reaction. In cyclohexane, the conversion is proportional
to time, i.e., the reaction is independent of phenol concentration over
Pd/Al2O3-3 catalyst, while the pseudo-first-order can be obtained over
Pd/Al2O3-13 catalyst, where the effect of phenol concentration is indeed
more pronounced, showing lower reaction rates at low concentrations.
The reaction rates in water were estimated based on the data obtain-
ed at 333 K, atmospheric pressure of hydrogen and Pd/phenol = 0.02
(Fig. S2). Based on the converted mole of phenol per mole of surface
From the results shown in Table 1, the hydrogenation of phenol
in aqueous liquid over Pd/Al2O3 and Pd/SiO2 catalysts show high selec-
tivity of cyclohexanone, reaching at least 95% (Table 1 entries 1–6 and
11–12). Cyclohexanone and cyclohexanol are only reaction products
observed over the entire range of conditions studied. The hydrogenation
of phenol is going stepwise with cyclohexanone as the “intermediate”
(Fig. 3). The similar results were obtained over Pd/mpg-C3N4 under
338 K, 0.1 MPa hydrogen pressure and Pd/phenol = 0.05 [19]. It can
be thought that phenol is easily absorbed on the catalyst surface,
and H2 is activated by Pd species. The aromatic ring of phenol is then
partially hydrogenated to the enol, which, in turn, can isomerize rapidly
to give cyclohexanone. There is only a weaker H-bridge donor, and the
cyclohexanone leaves the surface of the catalyst quickly, being replaced
by a more strongly binding new phenol molecule and avoiding further
hydrogenation to cyclohexanol. Others [24,25] also found that the
hydrophilic support was favor to selective phenol hydrogenation to
cyclohexanone in aqueous liquid. The adsorption of phenol occurs easily
on hydrophilic support and cyclohexanone tends to leave the surface.
Over Pd/Al2O3 and Pd/SiO2 catalysts, Al–OH or Si–OH would interact
with phenol to form H-bridge while a weaker interaction between
cyclohexanone and catalyst (Fig. 4). On the other hand, we choose 2
or 4-tert-butyl-phenol (2-t-BPH or 4-t-BPH) as a comparative reactant
for phenol hydrogenation, as it adsorbs on the active site only via the
formation of σ-c because of a bulky tert-butyl group on the aromatic
ring. In the hydrogenation of 2-t-BPH or 4-t-BPH, much lower rates
were obtained over Pd/Al2O3 and Pd/SiO2 catalysts with 26% or 42.7%
conversion and 17.9% or 30.3% conversion for 2-t-BPH or 4-t-BPH within
6 h, respectively (Table S3). These results directly confirm that the
aromatic ring of phenol will interact with Pd through the formation
of π-complex. In this work, it can be concluded that the adsorption of
phenol on Pd catalysts occurs through both the interaction of hydroxyl
groups of phenol and catalysts and the formation of π-complex between
ring and Pd (Fig. 4).
Pd atoms, TOFs can be calculated to be 27.1 and 12.2 mmolphenol molsur
−1
s−1 over Pd/Al2O3-13 and Pd/SiO2-10 catalysts respectively
Pd
(Table S3). However, TOFs over Pd/Al2O3-3 and Pd/SiO2-3 were 7.4
−1
and 3.2 mmolphenol mol
s−1, respectively (Table S2). As known,
sur Pd
Pd nanoparticles ≥4 nm behave like Pd (111) which is favor to dissoci-
ate adsorption of H2 molecules and Pd nanoparticles b4 nm behave like
Pd (110) which is favor to absorb subtracts [27,28]. Higher TOFs of Pd
catalysts with larger Pd particles can be ascribed to highly effective
dissociation of H2 molecules which can be critical for phenol hydroge-
nation. In fact, in another experiment, the increase in H2 pressure
(the increase in hydrogen concentration) really promoted the phenol
hydrogenation (Fig S4). Based on the apparent kinetics, the rate over
−1
Pd/Al2O3-3 is 0.66 mmol g h−1, higher than that over Pd/Al2O3-13
cat
(0.53 mmol g−cat1 h−1), ascribed to much larger number of active sites
and interface area between Pd and Al2O3. For Pd/SiO2 catalysts, the
activity is lower whether in aqueous liquid or cyclohexane (Table 1
entries 11–14), probably due to lower interaction between the hydroxyl
group of phenol and Si–OH.
To illustrate the general applicability of Pd/Al2O3-3, the method was
extended to ring hydrogenation of other hydroxylated aromatics
compounds. Table 2 shows the results of these hydrogenations. As can
be seen, 99% conversion is achieved in all cases, and the selectivities
towards substituted cyclohexanone are excellent (99%). Hydrogenation
of pyrocatechol (Table 2, entry 5) and hydroquinone (Table 2, entry 6)
O
O
H
O
H
O
H
H2
H2
H
H
Pd
Pd
Pd
Al2O3
Al2O3
Al2O3
Fig. 4. Possible reaction mechanism of phenol over Pd/Al2O3 catalysts.