Size dependence in solvent-free aerobic oxidation of alcohols

catalyzed by zeolite-supported palladium nanoparticles

Feng Li, Qinghong Zhang, Ye Wang *

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and

Chemical Engineering, Xiamen University, Xiamen 361005, PR China

Received 31 July 2007; received in revised form 3 October 2007; accepted 7 October 2007

Available online 13 October 2007

www.elsevier.com/locate/apcata

Available online at www.sciencedirect.com

Applied Catalysis A: General 334 (2008) 217–226

Abstract

The size dependence in the palladium nanoparticle-catalyzed solvent-free aerobic oxidation of alcohols was studied. Palladium nanoparticles

with tunable mean sizes in a range of 2.0–10.5 nm were prepared over NaX zeolite by ion exchange of Na+ with an ionic Pd precursor followed by

calcination and H2 reduction. The calcination temperature was found to be a crucial factor in determining the mean size of Pd nanoparticles. Pd/

NaX catalysts with proper mean sizes of Pd could catalyze the solvent-free aerobic oxidation of various alcohols, and were particularly efficient for

the oxidation of benzylic alcohols without substituents in benzene ring. Detailed studies using the Pd/NaX catalysts with different mean sizes of Pd

revealed that the solvent-free aerobic oxidation of benzyl alcohol was structure-sensitive, and the intrinsic turnover frequency (TOF) reached a

maximum at a medium mean size of Pd (2.8 nm). On the other hand, for the oxidation of geraniol or 2-octanol without delocalized p-ring, the

reaction was structure-insensitive, and the intrinsic TOF did not change significantly with the mean size of Pd particles.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Aerobic oxidation; Alcohols; Palladium; Size dependence; Structure sensitivity

1. Introduction

The selective oxidation of alcohols to the corresponding

carbonyl compounds is one of the most essential transforma-

tions in synthetic chemistry. The development of efficient

catalysts for selective oxidation of alcohols using molecular

oxygen or air to replace the stoichiometric oxidizing reagent

such as dichromate or permanganate has become urgent from

the demand for establishing both environmentally benign and

economically practical synthetic routes. Because of obvious

advantages of heterogeneous catalysts in product isolation and

catalyst recycling use, the liquid-phase aerobic oxidation of

alcohols using solid catalysts has attracted much attention in

recent years [1].

Many recent studies showed that heterogeneous catalysts

with well-designed active phases especially noble metal

nanoparticles could catalyze the aerobic oxidation of alcohols

efficiently. Uozumi and Nakao [2] found that amphiphilic resin-

* Corresponding author. Tel.: +86 592 2186156; fax: +86 592 2183047.

E-mail address: wangye@xmu.edu.cn (Y. Wang).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.10.008

supported Pd nanoparticles with a mean diameter of �9 nm

catalyzed the aerobic oxidation of benzylic and allylic alcohols

efficiently. Polymer-dispersed or incarcerated Au nanoclusters

or Ru catalysts also exhibited good performances in the aerobic

oxidation of alcohols [3,4]. Several groups reported that

hydrotalcite and hydroxyapatite were excellent supports for the

immobilization of Ru and Pd catalysts for the aerobic oxidation

of alcohols [5–8]. Among these catalysts, a hydroxyapatite-

supported Pd catalyst containing Pd nanoparticles with a mean

size of 3.8 nm exhibited the highest activity for the aerobic

oxidation of benzylic alcohols (e.g., TOF for 1-phenylethanol,

�9800 h�1 at 433 K) [6]. Pd nanoparticles supported on

magnesia [9] and alumina [10], entrapped in aluminum

hydroxide [11], or located in the mesopores of SBA-15 [12]

also showed good performances for the aerobic oxidation of

alcohols. Alumina-supported Ru and zeolite-confined nano-

RuO2 clusters were found to be effective for the aerobic

oxidation of various alcohols [13,14]. Au nanoparticles (2–

5 nm) deposited on nanocrystalline CeO2 (�5 nm) exhibited

very high efficiency for the aerobic oxidation of alcohols under

solvent-free conditions, and the TOF for the conversion of 1-

phenylethanol could reach 12,500 h�1 at 433 K [15]. Recently,

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226218

Enache et al. [16] reported that a Au-Pd/TiO2 catalyst

containing Au–Pd nanocrystals made of a Au-rich core with

a Pd-rich shell showed excellent TOFs for the aerobic oxidation

of alcohols (e.g., TOF for 1-phenylethanol, �269,000 h�1 at

433 K). Further studies by the same group revealed that the use

of zeolite (e.g., zeolite beta) as the support of Au or Au-Pd

catalysts could give comparable TOFs to those observed with

TiO2 as the support for the oxidation of benzyl alcohol [17].

Most of the above studies indicated that the size of the active

phase (usually metal nanoparticles) was a key factor in

determining the catalytic activity in the aerobic oxidation of

alcohols. The elucidation of size effect would be helpful in

understanding catalysis mechanisms and in designing more

efficient catalysts [18]. However, to date, only very few reports

have been contributed to the study of particle size effect in the

aerobic oxidation of alcohols. Mori et al. [6] compared the

intrinsic activities of two hydroxyapatite-supported Pd catalysts

containing Pd particles with mean diameters of 3.8 and 7.8 nm

for the oxidative conversions of 1-phenylethanol and benzhy-

drol, and found that the catalyst with the smaller mean size of

Pd showed higher activities per surface Pd atom. For better

understanding the size-dependence in the Pd nanoparticle-

catalyzed aerobic oxidation of alcohols, further investigations

using catalysts with Pd particle sizes variable in a wider range

for various kinds of alcohols are needed.

Zeolites with ordered porous structures are expected to be

good matrices for the preparation of Pd nanoparticles.

Generally, zeolite-supported Pd nanoclusters or nanoparticles

can be easily prepared by ion exchange to introduce the ionic Pd

precursor followed by calcination and reduction [19–23].

Sachtler and co-workers [20–22] studied the transformation of

a Pd precursor, i.e. [Pd(NH3)4]2+, exchanged in NaY zeolite,

and found that the temperature used for calcination affected the

dispersion of metallic Pd after the subsequent reduction with

H2. Okumura et al. [23] strengthened the important roles of the

acid sites of the H-type zeolites in the dispersion of Pd during

the calcination and reduction processes, and found the

formation of Pd6 and Pd13 small clusters inside H-ZSM-5

and H-Y zeolites, respectively, after the reduction by H2.

Recently, we succeeded in preparing NaX zeolite-supported Pd

nanoparticles with tunable mean sizes from 2.0 to 10.5 nm. The

present paper reports our recent studies on the size dependence

in the Pd-catalyzed solvent-free aerobic oxidation of various

kinds of alcohols using thus prepared Pd/NaX catalysts.

2. Experimental

Pd/NaX catalysts were prepared by an ion exchange method

followed by calcination and reduction. NaX zeolite with a Si/Al

ratio of 1.3 was added into an aqueous solution of

[Pd(NH3)4]Cl2, and the ion exchange between Na+ and

[Pd(NH3)4]2+ was carried out at room temperature with

continuous stirring for 24 h. After the ion exchange, the

powdery solid was separated by filtration and washed with

deionized water. The catalyst precursor was then calcined in air

at 393–773 K and was finally reduced by H2 at 573 K. The Pd

content in the final product was characterized by ICP analysis

and was found to be dependent on the concentration of

[Pd(NH3)4]2+ aqueous solution. Most of our studies were

carried out using the Pd/NaX samples with a Pd content of

1.35 wt%, and the concentration of [Pd(NH3)4]2+ used for

preparation was 2.8 mmol dm�3.

The Pd/NaX catalysts were characterized by XRD, XPS,

TEM, H2-TPR, and CO chemisorption. XRD patterns were

recorded on a Panalytical X’pert Pro Super X-ray diffractometer

with Cu-Ka radiation (40 kV and 30 mA). XPS measurements

were carried out with a PHI Quantum 2000 Scanning ESCA

Microprobe (Physical Electronics) using monochromatic Al-Ka

radiation (1846.6 eV) as X-ray source. TEM was taken on a FEI

Tecnai 30 electron microscope (Phillips Analytical) operated at

an acceleration voltage of 300 kV. Samples for TEM observa-

tions were suspended in ethanol and dispersed ultrasonically.

Drops of suspensions were applied on a copper grid coated with

carbon. Chemisorption of CO was performed using a Micro-

meritics ASAP2010C. In a typical experiment, the catalyst after

H2 reduction at 573 K was evacuated at 373 K, and then the

temperature was decreased to 308 K under vacuum. Subse-

quently, CO was introduced, and the first isotherm (total CO

uptake) was measured. After the first isotherm, the sample was

evacuated for�10 min, and then the second isotherm (reversible

CO uptake) was measured. Using the difference between the total

and the reversible adsorbed CO, the amount of the chemisorbed

CO was calculated. H2-TPR was performed for the samples

before reduction using a Micromeritics AutoChem II 2920

instrument. Typically, the sample (0.1 g) was first pretreated in a

quartz reactor with a gas flow containing O2 and N2 at 823 K for

1 h, followed by purging with high-purity N2. After the sample

was cooled to 303 K, a H2-Ar (5 vol% H2) mixture was

introduced into the reactor and the temperature was raised to

1173 K at a rate of 10 K min�1. The consumption of H2 was

monitored by a thermal conductivity detector (TCD).

The catalytic oxidation of alcohols was carried out using a

batch-type reaction vessel with a reflux condenser with the

following procedure. The powdery catalyst (typically 0.1 g)

was added into the alcohol pre-charged in the reaction vessel,

and then, the mixture was heated to the reaction temperature

with stirring. Subsequently, an O2 flow was bubbled into the

mixture to start the reaction. After the reaction, the catalyst was

separated by centrifugation, and the liquid products were

analyzed by a gas chromatograph (Shimazu GC-14 B) after the

addition of an internal standard.

3. Results and discussion

3.1. Characterizations of Pd nanoparticles prepared over

zeolite NaX

Fig. 1 shows the XRD patterns of NaX zeolite and a typical

Pd/NaX sample (Pd, 1.35 wt%) prepared by the ion exchange

with [Pd(NH3)4]2+ followed by calcination and reduction both

at 573 K. The comparison suggests that the crystalline structure

of NaX zeolite does not undergo significant changes after the

supporting of Pd. From XRD patterns, we could not obtain any

information about Pd species because of the low content of Pd.

Fig. 1. XRD patterns. (a) NaX, (b) 1.35 wt% Pd/NaX prepared by ion exchange

followed by calcination and H2 reduction both at 573 K.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226 219

Fig. 2 shows the typical TEM micrographs for the Pd/NaX

samples prepared by ion exchange followed by calcination at

different temperatures and reduction with H2 at 573 K. The

corresponding particle size distributions derived from the TEM

micrographs by counting ca. 150–200 particles are also plotted

in Fig. 2. For the sample calcined at a low temperature (393 K),

the size distribution was relatively broad. We could observe Pd

particles with diameters from �6 to �19 nm, and the mean

particle size of Pd was 10.5 nm. Relatively narrow size

distributions were observed in other cases. The calculated mean

sizes of Pd particles for the samples calcined at different

temperatures are summarized in Table 1. The increase in the

calcination temperature from 393 to 773 K decreased the mean

size of Pd particles from 10.5 to 2.0 nm.

We attempted to understand why the higher calcination

temperature caused the formation of smaller Pd nanoparticles.

Zhang and Sachtler [22] clarified that the [Pd(NH3)4]2+ species

exchanged in NaY zeolite underwent gradual changes to

[Pd(NH3)2]2+(OZ)2 (OZ = lattice oxygen of zeolite) and then to

Pd2+(OZ)4 with a rise in calcination temperature from 423 to

773 K through UV–vis and EXAFS investigations. This

suggests that the interaction between the ionic Pd species

Fig. 2. TEM micrographs and particle size distributions for the 1.35 wt% Pd/

NaX samples prepared by ion exchange followed by calcination at different

temperatures and then by H2 reduction at 573 K. (a) 393 K, (b) 473 K, (c)

523 K, (d) 573 K, (e) 673 K, and (f) 773 K.

Table 1

Mean sizes and dispersions of Pd nanoparticles in Pd/NaX catalystsa

Calcination

temperature (K)

Mean size

of Pd (nm)

Pd dispersion

from CO

chemisorptionb

Pd dispersion

from mean

size of Pdc

393 10.5 0.06 0.11

473 5.4 0.18 0.21

523 2.9 0.31 0.39

573 2.8 0.31 0.40

673 2.6 0.35 0.43

773 2.0 0.57 0.56

a The catalysts with a Pd content of 1.35 wt% were prepared by ion exchange

followed by calcination at different temperatures and finally by H2 reduction at

573 K.b Expressed using the moles of chemisorbed CO per mole of Pd.c Estimated using the following equation: Pd dispersion = 1.12/Pd diameter

(nm) [24].

Fig. 3. H2-TPR profiles for the 1.35 wt% Pd/NaX samples calcined at different

temperatures. Calcination temperature: (a) 393 K, (b) 473 K, (c) 523 K, (d)

573 K, (d) 673 K, and (f) 773 K.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226220

and the anionic zeolite framework becomes stronger at higher

calcination temperatures. Since such changes might be

reflected in the reduction behavior of the ionic Pd species,

we performed H2-TPR studies for the Pd/NaX samples after

calcination at different temperatures. The quantification of the

reduction peaks in Fig. 3 for each sample revealed that these

peaks corresponded to the reduction of Pd(II) to Pd(0), and

most of the Pd(II) species (ca. 70–80%) could be reduced to

Pd(0) at 573 K for all of the samples shown in Fig. 3. XPS

measurements showed that the binding energy values for the

Pd3d5/2 after H2 reduction at 573 K were 334.7–335.1 eV for all

of these samples, confirming that palladium was in metallic,

i.e., Pd(0) state in each sample. Fig. 3 shows that the main peak

for the reduction of Pd species shifts to a higher temperature

with increasing the calcination temperature from 393 to 573 K.

This supports the idea that the higher calcination temperature

would enhance the interaction of the exchanged ionic Pd

species with the zeolite framework. At temperatures �523 K,

the main reduction peak became broader and asymmetric,

indicating that several types of Pd species with different

coordinations with the zeolite framework might co-exist in

these samples. It can be expected that the size of the metal

particles is dominated mainly by the rate of nucleation and the

rate of nuclei growth. The Pd species having weaker

interactions with the framework may have higher mobility,

and the rate of nuclei growth would be larger in this case,

leading to the formation of larger Pd particles. On the other

hand, the stronger interactions would cause the formation of

smaller Pd nanoparticles.The dispersion of Pd, i.e., the fraction

of surface Pd atoms in all of Pd atoms in each sample was

evaluated via CO chemisorption measurements by assuming a

chemisorption stoichiometry of CO to surface Pd atom of 1.

The result summarized in Table 1 shows that the Pd dispersion

thus evaluated increases significantly with the calcination

temperature. This trend coincides with that of the decrease of

the mean size of Pd particles with increasing calcination

temperature. Actually, by assuming spherical particles, the

dispersion of Pd can also be simply estimated using the

diameter of Pd particles by the following equation [24],

Pd dispersion ¼ 1:12

diameter of Pd particleðnmÞ

The values of Pd dispersion estimated using this equation are

also listed in Table 1. The values evaluated by the two methods

are roughly in agreement with each other. For most of the

samples, the dispersion evaluated from CO chemisorption is

slightly lower than that estimated using the diameter of Pd

particles. This is probably because a small part of ionic Pd

species cannot be reduced at 573 K (Fig. 3). This small part of

Pd species may not contribute to CO chemisorption and are not

observable by TEM, leading to higher estimation of Pd dis-

persion using the mean diameter of Pd particles observed by

TEM. This small proportion of Pd species might be located

inside the small cages of zeolite [20,21]. On the other hand,

TEM observations (Fig. 2) suggest that most of the Pd nano-

particles with mean sizes from 2.0 to 10.5 nm are located

outside the cages of NaX zeolite. This is in agreement with

the fact that the size of the largest cage (supercage) of NaX

zeolite is only �1.3 nm.

3.2. Catalytic behaviors of NaX-supported Pd

nanoparticles for solvent-free aerobic oxidation of alcohols

Table 2 shows the catalytic performances of the Pd/NaX

catalyst calcined and reduced both at 573 K (mean size of Pd,

2.8 nm) for the solvent-free aerobic oxidation of various

alcohols to their corresponding carbonyl compounds. To the

best of our knowledge, although many reports have shown that

Pd-based heterogeneous catalysts can catalyze the aerobic

oxidation of a wide scope of alcohols in the presence of an

organic solvent such as toluene or trifluorotoluene (PhCF3) [6,9–

12], the studies on the substrate scope under solvent-free

conditions are very few. Our result in Table 2 reveals for the first

time that various kinds of alcohols could be oxidized selectively

to their corresponding carbonyl compounds by O2 under

solvent-free conditions over the Pd/NaX catalyst. As expected,

different substrates exhibited quite different reactivities.

Comparison of TOFs (as generally adopted in literature, TOF

here was calculated by the moles of substrate converted per mole

of Pd in the whole catalyst) shows that the Pd/NaX catalyst is

particularly efficient for the oxidation of benzyl alcohol and 1-

phenylethanol, both of which possess benzene ring. In addition

to benzaldehyde and acetophenone, small amounts of toluene

and benzaldehyde were also formed in the conversions of benzyl

alcohol and 1-phenylethanol, respectively. The present catalyst

was also applicable to the solvent-free aerobic oxidation of 2-

thiophenemethanol, and relatively better performance was

obtained for this heteroatom-containing alcohol (entry 11),

whereas monomeric complex catalysts were generally not useful

for this kind of alcohol because of the strong coordination of

substrate to the metal center. The oxidation of geraniol (entry

12), an allylic alcohol, proceeded relatively slower than

the alcohols with conjugate p-ring, i.e., the aromatic alcohols

and 2-thiophenemethanol. The solvent-free aerobic oxidation of

Table 2

Solvent-free aerobic oxidation of various alcohols catalyzed by a Pd/NaX catalysta

Entry Substrate Time (h) Temperature (K) Conv. (%) Carbonyl compound select. (%) TOFb (h�1)

1 4 373 66 97c 626

2d 4 373 97 100 38

3 4 373 20 100 161

4d 4 373 81 100 32

5 96 423 19 100 8.0

6 4 373 50 100 403

7 24 373 97 100 159

8 96 423 93 100 38

9 96 423 78 100 32

10 4 423 98 91e 967

11 8 423 81 100 397

12 24 373 37 100 71

13 96 423 70 100 29

14 96 423 81 100 33

a Reaction conditions: Pd/NaX (Pd content, 1.35 wt%; mean Pd size, 2.8 nm), 0.1 g (Pd, 12.7 mmol); substrate, 50 mmol (48.5 mmol for benzyl alcohol); O2,

3 mL min�1.b The turnover frequency here was calculated by the moles of substrate converted per mole of Pd in the whole catalyst per hour.c The remaining product was toluene.d Substrate, 2 mmol; PhCF3 solvent, 5 mL.e The remaining product was benzaldehyde.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226 221

nonactivated alcohols such as cyclooctanol and 2-octanol

(entries 13 and 14) could also proceed with the present Pd/

NaX catalyst, producing the corresponding ketones with yields

of 70% and 81% after 96 h of reactions.

It is quite unexpected that all the substituted benzyl alcohols

investigated here exhibit lower reactivities than benzyl alcohol.

For homogeneous Pd complex catalysts, it is typical that the

conversions of substituted benzyl alcohols with electron-

donating substituents in benzene ring proceed faster than that of

benzyl alcohol, whereas the presence of an electron-with-

drawing substituent in benzene ring decreases the reaction rate

[25,26]. The same phenomenon has also been observed for

some heterogeneous catalysts such as Pd/MgO and Ru/Al2O3

[9,13]. The unique observation that benzyl alcohol exhibits the

highest reactivity over the present Pd/NaX catalyst will be

discussed later.

We further investigated the solvent-free aerobic oxidation of

1-phenylethanol using the Pd/NaX catalysts with different Pd

contents. As shown in Table 3, even the catalyst with a very low

Pd content (0.069 wt%) could catalyze the aerobic oxidation of

1-phenylethanol efficiently. Under the optimized Pd content

(0.31 wt%) and Pd amount (0.58 mmol), the TOF based on Pd

in the whole catalyst could reach 18,800 h�1 at 423 K. This

value is higher than those reported over most heterogeneous

catalysts including Pd/hydroxyapatite and Au/CeO2, which

provides TOFs of 9800 h�1 and 12,500 h�1, respectively for the

oxidation of 1-phenylethanol at 433 K [6,15]. To the best of our

knowledge, only the Au-Pd/TiO2 catalyst showed a higher TOF

(269,000 h�1) for this reaction [16].

We have carried out recycling uses of the Pd/NaX catalyst

with a Pd content of 0.31 wt% for the solvent-free aerobic

oxidation of 1-phenylethanol at 423 K. Fig. 4 shows that the

conversion, acetophenone selectivity, and TOF do not undergo

significant changes during the repeated uses for five cycles. The

Pd content in the catalyst after the fifth recycling use was

measured by ICP, and the measured Pd content was 0.32 wt%,

Table 3

Solvent-free aerobic oxidation of 1-phenylethanol catalyzed by the Pd/NaX catalysts with different Pd contents and amountsa

Entry Pd content (wt%) Pd amountb (mmol) Conv. (%) Acetophenone select.c (%) TOFd (h�1)

1 0.069 0.65 93 90 17900

2e 0.069 0.32 84 95 16200

3 0.31 0.58 87 95 18800

4 0.79 0.74 68 83 11500

a Reaction conditions: temperature, 423 K; substrate, 50 mmol; O2, 3 mL min�1; time, 4 h. NaX alone was inactive under these reaction conditions.b Pd amount was regulated by varying the catalyst amount.c The byproduct was benzaldehyde.d The TOF here was calculated by the moles of 1-phenylethanol converted per mole of Pd in the whole catalyst per hour.e Time, 8 h.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226222

confirming that there was no leaching of Pd from the Pd/NaX

catalyst during the aerobic oxidation of 1-phenylethanol under

the reaction conditions used. Therefore, the present catalyst

was fully recyclable.

3.3. Size dependence in the Pd/NaX-catalyzed aerobic

oxidation of alcohols

We have investigated the effect of Pd particle size on

catalytic performances of the Pd/NaX catalysts in the solvent-

free aerobic oxidation of alcohols. We chose benzyl alcohol,

geraniol, and 2-octanol as the model substrates of aromatic

alcohols, allylic alcohols, and nonactivated alcohols, respec-

tively. Table 4 compares the catalytic behaviors of the Pd/NaX

catalysts with different mean sizes of Pd for the oxidation of

these alcohols. It is of interest that different alcohols exhibit

different tendencies in the change of alcohol conversions as the

Pd particle size is changed. For the oxidation of benzyl alcohol,

the catalyst with a medium mean size of Pd (2.8 nm) exhibited

the highest conversion, while the catalysts with both larger and

smaller Pd particles showed lower conversions. On the other

hand, for the oxidation of geraniol or 2-octanol, the conversion

decreased monotonically with increasing the mean size of Pd

particles. In other words, the smaller Pd particles exhibited

higher substrate conversions for the oxidation of geraniol or

2-octanol.

Fig. 4. Recycling uses of the 0.31 wt% Pd/NaX catalyst for the solvent-free

aerobic oxidation of 1-phenylethanol. Reaction conditions: Pd, 0.58 mmol;

temperature, 423 K; substrate, 50 mmol; O2, 3 mL min�1; time, 4 h.

To gain information about the intrinsic reaction rate over the

catalysts with different mean sizes of Pd, we have monitored

the change of activity with reaction time over each catalyst.

Fig. 5 shows the time courses for the benzyl alcohol oxidation

over the Pd/NaX catalysts with mean sizes of Pd from 2.0 to

10.5 nm. The conversion of benzyl alcohol over NaX alone was

zero even at a longer reaction time, further confirming that the

supported Pd nanoparticles were responsible for the oxidation

of benzyl alcohol. Over all of the Pd/NaX catalysts shown in

Fig. 5, the conversion of benzyl alcohol increased linearly with

time in the initial 3 h. The selectivity of benzaldehyde was

generally higher than 90% over these catalysts, and toluene was

observed as the main byproduct. We also monitored the O2

consumption at the same time for the catalyst with a mean Pd

size of 2.8 nm, and found that the consumption of O2 also rose

linearly with time in the initial stage and the molar ratio of O2

consumed to benzaldehyde yield was�0.5. These observations

clearly indicate that the oxidation proceeds steadily without any

induction period. This is quite different from that observed for

the Pd/hydroxyapatite and the Pd/Al2O3 catalysts [6,10], where

the samples without pre-reduction were used as catalysts. The

ionic Pd species underwent reduction by alcohols during the

reaction, forming Pd nanoparticles, which functioned as the

true active phases, and thus an induction period was observed

over these catalysts. Recently, an in situ X-ray absorption

spectroscopy study with Pd/Al2O3 catalyst confirmed that the

metallic palladium species mainly accounted for the aerobic

oxidation of benzyl alcohol and the palladium oxide species

was less active [27]. We observed the steady-state conversion of

benzyl alcohol because we applied Pd/NaX samples after H2

reduction to catalytic reactions.

We further carried out TEM observations for the recovered

Pd/NaX catalysts after the oxidation of benzyl alcohol at 373 K.

As shown in Fig. 6, the mean size of Pd particles did not

undergo significant changes after the reaction. Therefore, we

can discuss the effect of Pd particle size on the catalytic activity

in the solvent-free aerobic oxidation of benzyl alcohol directly

using the initial reaction rate calculated from Fig. 5 and the

mean size of Pd particles shown in Table 1. The initial rates for

benzyl alcohol oxidation calculated from Fig. 5 are summarized

in Table 5. Since it is generally accepted that only the surface Pd

atoms can contribute to the catalytic reactions, we have

calculated the intrinsic TOFs (i.e., mols of substrate converted

at the initial stage per mole of surface Pd per hour) using the

Table 4

Solvent-free aerobic oxidation of benzyl alcohol, geraniol, and 2-octanol over the Pd/NaX catalysts with different mean sizes of Pd particlesa

Entry Substrate Mean size of Pd (nm) Conv. (%) Carbonyl compound select. (%)

1 Benzyl alcohol 2.0 36 95b

2 Benzyl alcohol 2.6 55 97b

3 Benzyl alcohol 2.8 66 97b

4 Benzyl alcohol 2.9 63 92b

5 Benzyl alcohol 5.4 25 94b

6 Benzyl alcohol 10.5 7.7 97b

7 Geraniol 2.0 58 100

8 Geraniol 2.6 41 100

9 Geraniol 2.8 37 100

10 Geraniol 2.9 36 100

11 Geraniol 5.4 23 100

12 Geraniol 10.5 8.7 100

13 2-octanol 2.0 74 100

14 2-octanol 2.6 51 100

15 2-octanol 2.8 43 100

16 2-octanol 2.9 41 100

17 2-octanol 5.4 28 100

18 2-octanol 10.5 11 100

a Reaction conditions: catalyst, 0.1 g (Pd, 12.7 mmol); substrate, 50 mmol (for benzyl alcohol, 48.5 mmol); O2, 3 mL min�1; temperature, 373 K for benzyl alcohol

and geraniol, 423 K for 2-octanol; time, 4 h for benzyl alcohol, 24 h for geraniol, 48 h for 2-octanol.b The remaining product was toluene.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226 223

initial conversion rate and the Pd dispersion evaluated by CO

chemisorption. It becomes clear that the intrinsic TOF depends

significantly on the mean size of Pd particles, and the catalyst

with a mean size of Pd of 2.8 nm exhibits the highest intrinsic

TOF. Therefore, the solvent-free aerobic oxidation of benzyl

alcohol over the present catalysts is a structure-sensitive

reaction. The phenomenon of optimum particle size has been

reported in several catalytic systems such as Au/TiO2 catalysts

for CO oxidation and for propylene epoxidation with H2–O2

gas mixture [28,29]. However, for the aerobic oxidation of

alcohols, the reports on the optimum particle size phenomenon

are very scarce.

To clarify whether the conversion of other kinds of alcohols

also possesses a similar structure-sensitive feature, we have

further investigated the effect of the mean size of Pd particles on

the reaction rates in the solvent-free oxidative conversions of

Fig. 5. Conversions of benzyl alcohol versus reaction time over the 1.35 wt%

Pd/NaX catalysts with different mean sizes of Pd particles. The mean size of Pd

(in nm) was shown in the parenthesis. Reaction conditions: catalyst, 0.1 g (Pd,

12.7 mmol); temperature, 373 K; substrate, 48.5 mmol; O2, 3 mL min�1.

geraniol and 2-octanol by O2. Using a similar procedure, we

measured the initial conversion rates and calculated the

intrinsic TOFs for these two alcohols. The results for these

two alcohols are plotted in Fig. 7 together with that for benzyl

alcohol. Unexpectedly, the intrinsic TOFs for the conversions

of geraniol and 2-octanol are almost independent of the mean

size of Pd particles. Thus, the solvent-free conversions of

geraniol and 2-octanol by O2 are structure-insensitive reactions.

We have also investigated the influence of the presence of a

solvent (PhCF3) on the size effect for the Pd/NaX-catalyzed

oxidative conversions of benzyl alcohol and geraniol by O2. As

shown in Fig. 8, although the intrinsic TOF values become

lower in the presence of PhCF3 probably because of the lower

concentration of the reactant, the trends of the variation of the

intrinsic TOF with the mean size of Pd particles for the two

alcohols observed here are very similar to those observed under

Table 5

Initial conversion rates and intrinsic TOFs of the Pd/NaX catalysts (Pd content,

1.35 wt%) with different mean sizes of Pd particles for solvent-free aerobic

oxidation of benzyl alcohola

Mean size

of Pd (nm)

Initial conversion

rateb (mmol h�1)

Intrinsic

TOFc (h�1)

2.0 4.56 630

2.6 7.61 1720

2.8 11.70 2970

2.9 10.70 2730

5.4 3.54 1550

10.5 1.21 1590

a See Fig. 5 for reaction conditions.b Calculated from Fig. 5.c Evaluated from the initial conversion rates per Pd atom on the catalyst

surface (Pd dispersion from CO chemisorption in Table 1 was used for

calculation).

Fig. 6. TEM micrographs and particle size distributions for the 1.35 wt% Pd/NaX catalysts with mean sizes of 2.0 nm (a), 2.8 nm (b) and 5.4 nm (c) after the aerobic

oxidation of alcohols under the reaction conditions in Fig. 5.

Fig. 7. Intrinsic TOFs for solvent-free aerobic oxidation of benzyl alcohol,

geraniol, and 2-octanol as a function of the mean size of Pd particles. Reaction

conditions: catalyst, 1.35 wt% Pd/NaX, 0.1 g (Pd, 12.7 mmol); temperature,

373 K (423 K for 2-octanol); substrate, 50 mmol (48.5 mmol for benzyl alco-

hol); O2, 3 mL min�1.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226224

solvent-free conditions (Fig. 7). Thus, the observation that

different alcohols exhibit different size-dependent features over

the Pd/NaX catalysts is not a special phenomenon for the

solvent-free aerobic oxidation of alcohols.

It is known that there are different kinds of surface sites, i.e.,

terrace, edge, and corner atoms on a Pd particle, and generally,

the relative fraction of the low-coordinated sites (edge and

corner atoms) to the terrace atoms increases as the particle size

decreases [30]. For many Pd nanoparticles-catalyzed organic

reactions, e.g., hydrogenation, Heck and Suzuki couplings, and

vinyl acetate synthesis, the smaller Pd particles containing

more coordinately unsaturated Pd sites are generally believed to

be more active [31–34]. For the aerobic oxidation of alcohols,

as described above, Mori et al. [6] also proposed that the low-

coordinated Pd atoms were responsible for the oxidation of 1-

phenylethanol or benzhydrol via a comparison between Pd/

hydroxyapatite catalysts with Pd sizes of 3.8 and 7.8 nm. Ferri

et al. [35] recently reported that the dehydrogenation of benzyl

Fig. 8. Intrinsic TOFs for aerobic oxidation of benzyl alcohol and geraniol in

the presence of PhCF3 solvent as a function of the mean size of Pd particles.

Reaction conditions: catalyst, 1.35 wt% Pd/NaX, 0.1 g (Pd, 12.7 mmol); tem-

perature, 373 K; substrate, 2 mmol; PhCF3, 5 mL; O2, 3 mL min�1.

F. Li et al. / Applied Catalysis A: General 334 (2008) 217–226 225

alcohol to benzaldehyde could occur on all of the exposed Pd

faces on Pd/Al2O3 and was virtually structure-insensitive,

whereas the decarbonylation of benzaldehyde might proceed

preferentially on the hollow sites on Pd (1 1 1) faces. Therefore,

the experimental results over our present Pd/NaX catalysts

shown in Fig. 7 are very specific.

At this moment, we still do not have a satisfactory

explanation for the unique observations in Fig. 7. We speculate

that, for the oxidation of benzyl alcohol, the terrace Pd atoms

may contribute to the chemisorption of the substrate molecules

via interaction with the delocalized p-bond of benzene ring,

which facilitates the subsequent formation of Pd-alcoholate

species and the elimination of b-hydride occurring probably on

the nearby edge or corner Pd sites via a cooperative effect as

proposed by Mori et al. [6]. However, Mori et al. [6] argued that

the elimination of b-hydride limited the reaction rate. On the

other hand, the results obtained over our catalysts allow us to

speculate that the chemisorption of alcohol molecules on the

terrace Pd sites also plays a pivotal role in addition to the

elimination of b-hydride. It should be noted here that, over our

catalyst, the substituted benzyl alcohols all exhibited lower

activities than benzyl alcohol, and the one with the bulky

substituent (tert-butyl) showed an especially lower reactivity

(Table 2, entry 5). This trend is much different from that

observed for some Pd-catalyzed homogeneous aerobic oxida-

tion of alcohols, where the oxidation of benzylic alcohols with

electron-donating substituent in benzene ring proceeds more

rapidly than that of benzyl alcohol because the electron-

donating group may facilitate the elimination of b-hydride

[25,26]. The hindrance of the substituent observed in our case

may support our speculation about the importance of the

chemisorption of alcohol molecules. Thus, an appropriate ratio

of the low-coordinated sites to the terrace sites may be required

for the conversion of such kinds of alcohols, resulting in the

highest intrinsic TOF at a medium mean size of Pd particles

(2.8 nm).

On the other hand, for the oxidation of 2-octanol or geraniol,

which does not possess a conjugate p-ring, the substrate

molecule may be difficult to be adsorbed. We speculate that this

may have caused the significantly lower reactivity and the

structure-insensitive feature for such kinds of alcohols although

the reaction mechanism on this occasion is still unclear. Further

elucidations of the reaction mechanism and the nature of the

unique size-dependent features are definitely needed in the

future studies.

4. Conclusions

Palladium nanoparticles with tunable mean sizes were

successfully prepared over NaX zeolite by the ion exchange of

Na+ with [Pd(NH3)4]2+ followed by calcination and H2

reduction. The calcination temperature was a key in tuning

the mean size of Pd nanoparticles. The higher calcination

temperature resulted in smaller Pd particles, and the variation of

the calcination temperature from 393 to 773 K followed by H2

reduction at 573 K provided Pd/NaX samples with mean sizes

of Pd ranging from 10.5 to 2.0 nm. The Pd/NaX catalyst with a

proper mean size of Pd particles could catalyze the solvent-free

aerobic oxidation of various alcohols including benzylic

alcohols, heteroatom-containing alcohols, allylic alcohols,

and nonactivated alcohols. The catalyst exhibited particularly

high efficiency for the oxidation of benzylic alcohols without

substituents in benzene ring. A 0.31 wt% Pd/NaX catalyst gave

a turnover frequency as high as 18,800 h�1 for the oxidation of

1-phenylethanol, and the catalyst could be used repeatedly

without leaching of Pd. With a change in the mean size of Pd

particles from 2.0 to 10.5 nm, different tendencies in the change

of catalytic activity were observed for the oxidation of different

kinds of alcohols. The intrinsic turnover frequency for the

aerobic oxidation of benzyl alcohol reached a maximum at a

medium mean size of Pd of 2.8 nm. On the other hand, for the

oxidation of geraniol or 2-octanol, the intrinsic turnover

frequency was almost independent of the mean size of Pd

particles.

Acknowledgments

This work was supported by the National Natural Science

Foundation of China (Nos. 20625310 and 20433030),

the National Basic Research Program of China (grants

2003CB615803 and 2005CB221408), the Key Scientific

Project of Fujian Province of China (No. 2005HZ01-3), and

the Programs for New Century Excellent Talents in

University of China (No. NCET-04-0602, to Y.W.) and in

Fujian province (to Q.Z.).

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