Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Oxidation of Alcohols Using...

9

OXIDATION OF ALCOHOLSUSING NANOCATALYSTS

Takato Mitsudome and Kiyotomi Kaneda

INTRODUCTION

Oxidation of alcohols is widely recognized as a fundamental and important transfor-mation in both laboratory and industrial chemistry, because the obtained carbonyl com-pounds serve as versatile solvents, polymer precursors, fragrances, and intermediates forfine chemicals such as pharmaceuticals.1–5 Heavy metal salts, including permanganateand dichromate, have traditionally been employed as oxidizing reagents to achieve thistransformation.6–8 These stoichiometric oxidants, however, have serious drawbacks; theyare expensive and/or toxic and produce large amounts of waste.

From the standpoint of atom economy and environmental demand for chemicalreactions, much attention has been directed toward the development of promising het-erogeneous catalysts employing molecular oxygen (O2) as a primary oxidant.9–14 In thisoxidation system, use of harmful oxidants can be avoided and only water is produced asthe sole coproduct. Moreover, heterogeneous catalysts are readily available and reusable.

The aim of this review is to provide an overview of the heterogeneous catalyststhat have been developed for the aerobic oxidation of alcohols. Since several reviewsand many reports on heterogeneous catalysts for the aerobic oxidation of alcoholshave appeared to date,15–18 we focus on the recently advanced and highly efficientheterogeneous catalysts using inorganic material- or polymer-supported active metalsof Ru, Pd, and Au for alcohol oxidation under liquid-phase conditions (developments

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

287

288 OXIDATION OF ALCOHOLS USING NANOCATALYSTS

in the past 10 years). This review not only provides good understanding of this researchfield but also highlights the issues that remain and need to be addressed in the future.

RUTHENIUM-CATALYZED ALCOHOL OXIDATION

Supported RuOx Species

Some homogeneous Ru complexes have achieved high catalytic activity and selectivityfor the aerobic oxidation of alcohols.19 Chang and coworkers found that the combinationof [RuCl2(p-cymene)]2 with Cs2CO3 was an effective homogeneous catalyst system forthe aerobic oxidation of various alcohols.20, 21 They also succeeded in the heteroge-nization of [RuCl2(p-cymene)]2 by adsorption of the Ru species on activated carbon.22

The carbon-supported [RuCl2(p-cymene)]2 worked as a heterogeneous catalyst for theaerobic oxidation of alcohols such as benzylic alcohols and aliphatic secondary alcoholsin the presence of Cs2CO3 (5 mol%) (Scheme 9.1).

The [RuCl2(p-cymene)]2 on carbon was also reused for up to nine cycles with only aslight loss of activity (Table 9.1). During oxidation, only slight leaching of Ru occurred.For example, 2.2 wt% Ru in [RuCl2(p-cymene)]2 on carbon as a fresh catalyst decreasedto 1.5 wt% Ru after nine cycles.

Zhan et al. successfully incorporated RuO2 nanoparticles (NPs) (1.3 ± 0.2 nm)into the supercages of faujasite zeolite (RuO2-FAU).23 X-ray absorption fine-structure(XAFS) analysis reveals that the RuO2 nanoclusters anchored in the zeolite are struc-turally similar to hydrous RuO2; RuO6 species are connected together by two shared

OH

R1 R2

O

O

OO

CO2Et

R1 R2

O

O

92 80 80Isolated yield (%)

[RuCl2(p-cymene)]2/C

(2.2 wt%, 0.3 g)

Cs2CO3 (5 mol%), toluene, 110 oC, O2 (1 atm)

Time (h) 3 6 6

7

96 95

9

Isolated yield (%)

Time (h)

Scheme 9.1. Aerobic oxidation of alcohols using [RuCl2(p-cymene)]2/C.

RUTHENIUM-CATALYZED ALCOHOL OXIDATION 289

TABLE 9.1. Reusability of the heterogeneous catalyst [RuCl2(p-cymene)]2/C for the aerobicoxidation of 4-bromobenzyl alcohola,b

Run 1 2 3 4 5 6 7 8 9

Time (h) 6 7 8 8 8 8 8 8 9Yield (%)c 89 92 97 92 94 89 93 85 81

aA solution of alcohol (1.0 mmol) in toluene (3.3 ml) was reacted under 1 atm of O2 in the presence of[RuCl2(p-cymene)]2/C (initially 2.2 wt%, 0.3 g) at 110 ◦C.bCs2CO3 (3 mol%) was added in each cycle from the second run.cIndicates isolated yield after chromatography.

oxygen atoms (Figure 9.1). Various activated (benzylic and allylic) and unactivated(aliphatic) alcohols, except for alicyclic alcohols, were converted to the correspond-ing aldehydes and ketones without the formation of carboxylic acids as overoxidizedproducts (Scheme 9.2) using the RuO2-FAU as a catalyst.

It is noted that a quantitative yield of heptanal was successfully obtained from1-heptanol in the presence of RuO2-FAU as a catalyst. RuO2-FAU could be reused in

RuO

Sizeo

Sizeo

Hydrous RuO2 RuO2-FAU

2.51 Å

1.93 Å 1.91 Å

2.57 Å

3.05 Å3.15 Å Ru

O

Si

Figure 9.1. Structure of RuO2-FAU. Ru, larger six-coordinate spheres; O, smaller spheres;

Si, two spheres.


OH

R1 R2

RuO2-FAU

OO

O O

R1 R2

O

O

O

1.569>99 4

443

4>99

O2 (1 atm)

93 20

4 95

5 4Yield (%)Time (h)

Yield (%)Time (h)

Scheme 9.2. Aerobic alcohol oxidation using RuO2-FAU.

recycling experiments, and the physically trapped RuO2 nanoclusters did not leach fromthe narrow channels and pores of the FAU during the catalytic process.

Hydrotalcite (HT, Mg6Al2(OH)16CO3·nH2O) is a layered anionic clay consistingof a positively charged two-dimensional brucite layer with anionic species, such ashydroxide and carbonate, located in the interlayer24–26 (Figure 9.2). Transition metalsof various types can easily be introduced into the brucite-like layer or the interlayerspace or onto the surface of hydrotalcite. Kaneda and coworkers demonstrated that Ruspecies fixed on the framework of the Brucite layer showed high catalytic activity forthe aerobic oxidation of benzylic and allylic alcohols to the corresponding aldehydesand ketones. In most cases, primary alcohols were oxidized faster than secondary ones(Scheme 9.3).27

Among the hydrotalcite clays containingRu species in theBrucite layer and differentanions within the interlayer space, the most effective anion was found to be the carbonateion. The same group also found that the introduction of Co species in the Brucite layerof Ru-hydrotalcite improved the catalytic activity; that is, Ru-Co-hydrotalcite exhibitedthree times higher catalytic activity than the above-described Ru-hydrotalcite withoutthe added Co species (Table 9.2).28 Although the original Ru-hydrotalcite-catalyzed

Surface

Brucite layer Mg

Al

OHInterlayer space

OH–

OH–

HCO3–

CO32–H2O CO3

2–

HCO3–

H2O

Figure 9.2. Structure of hydrotalcite. (See color insert.)


CCR1 CH

OHR3

R2

Ru-hydrotalcite

Toluene, O2

CCR1 C

O

R3

R2

CHOH

R4

Ru-hydrotalcite

Toluene, O2

C

O

R4

a: R1 = H, R2 = H, R3 = H, b: R1 = H, R2 = CH3, R3 = H,

c: R1 = Ph, R2 = H, R3 = H, d: R1 = H, R2 = H, R3 = CH3

e: R4 = H, X = H, f: R4 = H, X = OCH3, g: R4 = H, X = i-Pr,

h: R4 = H, X = Cl, i: R4 = CH3, X = H, j: R4 = Ph, X = H

X

Scheme 9.3. Ru-hydrotalcite-catalyzed oxidation of benzylic and allylic alcohols.

oxidation of chlorobenzyl alcohol and pyridinemethanol gave moderate yields of thealdehyde products, the Ru-Co-hydrotalcite gave quantitative yields of aldehydes undersimilar reaction conditions.

Other multicomponent systems for the aerobic oxidation of alcohols, such as par-tially substituted ferrite spinel MnFe1.5Ru0.35Cu0.15O4,29,30 the combination of a Rucation with cobalt hydroxide and cerium oxide (Ru-Co(OH)2-CeO2),31,32 and het-erotrimetallic RuMn2 species on the surface of hydrotalcite (RuMn2/HT),33 were alsodeveloped by Kaneda’s group. MnFe1.5Ru0.35Cu0.15O4 promoted the aerobic oxidationof various alcohols to the corresponding aldehydes and ketones at room temperature

TABLE 9.2. Oxidation of cinnamyl alcohol with various Ru-hydrotalcitesa

Conversion Yieldb Heat ofEntry Catalyst Time (%) (%) adsorptionc/J g−1

1 Ru-Co-Al-CO3 HT 40 min 100 94 13.42 Ru-Mn-Al-CO3 HT 40 min 99 92 13.33 Ru-Fe-Al-CO3 HT 40 min 64 50 12.94 Ru-Zn-Al-CO3 HT 40 min 23 23 6.65 Ru-Mg-Al-CO3 HT 40 min 31 20 32.1

8 h 100 956d Co-Al-CO3 HT +

Ru-Mg-Al-CO3 HT40 min 33 22

aReaction conditions: cinnamyl alcohol (2.0 mmol), catalyst (0.30 g), toluene (5 ml), 60 ◦C, O2 atmosphere.bYields of cinnamaldehyde were determined by gas chlomatography (GC) analysis using internal standardsbased on cinnamyl alcohol.cThe basicity of the hydrotalcites was estimated by calorimetric heats of benzoic acid adsorption.dA physical mixture of the two catalysts, which contained the same amounts of Ru and Co as entry 1, wasused.


TABLE 9.3. Aerobic oxidation of various primary alcohols catalyzed by Ru-Co(OH)2-CeO2a

CH2OH

HOH2C

OH

4 >99 89 (80)

5 >99 82 (76)

954 79

5 >99 64

Conversion (%)bSubstrate Time Yield (%)b, c

4 >99 97 (90)d

Product

3

CH2OH5

CH2OH

CH2OH

CH2OH

CH2OH

CH2OH

CH2OH

CH2OH

CH2OH

15

4 >99 85 (80)

6 >99 77

6 >99 87

1

3

4e

5

Entry

2

6

8g

9f

7f 6 >99 83

CO2H

O

O

O O

O O

3

CO2H5

CO2H

CO2H

CO2H

CO2H

15

aReaction conditions: alcohol (2 mmol), catalyst (0.3 g, Ru 0.2 mmol), benzotrifluoride (5 ml), O2 flow,60 ◦C.bDetermined by GC using an internal standard technique.cValues in parentheses are isolated yields.dAlcohol (20 mmol).eWater (0.2 ml) was added.fAlcohol (1 mmol), 80 ◦C.gAlcohol (1 mmol).

without any additives. In the Ru-Co(OH)2-CeO2 catalyst system, primary aliphaticalcohols were successfully oxidized to the corresponding carboxylic acids (Table 9.3).RuMn2/HT showed a high catalytic activity compared with previously reported Ru cat-alysts; the initial turnover frequency (TOF) based on Ru reached as high as 140 h−1 inthe aerobic oxidation of benzyl alcohol.

Kaneda and coworkers also developed a new strategy for the design of high-performance heterogeneous catalysts utilizing hydroxyapatite as a macroligand for cat-alytically active centers,34 and an efficient hydroxyapatite-bound Ru catalyst (RuHAP)was developed for the selective oxidation of various alcohols using O2.35

Apatites and related compounds, most notably hydroxyapatite (HAP,Ca5(PO4)3(OH)), are of considerable interest due to their potential as biomaterials,adsorbents, and ion-exchange materials.36 Various kinds of transition metal cations thathave a high potential for functioning as a catalytic-active center can be readily accom-modated into the apatite framework due to its large cation exchange ability. The useof hydroxyapatites as catalyst supports has the following advantages: (i) well-defined


Ru

Cl

OO

OO

P P

1.97 Å2.32 Å

Figure 9.3. Fine structure of the Ru species in RuHAP.

monomeric active species can be immobilized on the surface based on multiple func-tionalities, such as, cation exchange ability, adsorption capacity, and nonstoichiometry;(ii) their hydrophilic character allows smooth reactions under aqueous conditions; and(iii) due to their robust structure, no leaching of metals occurs.

RuHAP was simply prepared by the treatment of a stoichiometric HAP,Ca5(PO4)3(OH), with RuCl3·nH2O. Evaluation by powder X-ray diffraction (XRD)analysis, X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis, infraredspectroscopy, and Ru K-edge XAFS analysis showed that a monomeric Ru phosphatespecies was created on the HAP surface (Figure 9.3 and Table 9.4).

Oxidation of alcohols using the RuHAP catalyst proceeded efficiently at 80 ◦Cunder an O2 atmosphere to give the corresponding carbonyl compounds without theuse of additives. The most important catalytic property of RuHAP is its applicabil-ity to a wide range of alcohols. For example, the nonactivated alcohol 1-octanol wassmoothly oxidized to afford octanal without any formation of the corresponding car-boxylic acid or ester. Moreover, this catalyst system was useful for the oxidation ofheterocyclic alcohols containing nitrogen and sulfur atoms, such as 2-pyridinemethanoland 2-thiophenemethanol, giving the corresponding aldehydes in high yields (Table 9.5).

Evenwhen air was used in place of pureO2, the above oxidation reactions proceededsmoothly. The proposed catalytic cycle of this reaction is shown in Figure 9.4. Oxidationis initiated by ligand exchange between the alcohol and the Cl ligand of RuHAP to give aRu-alcoholate species that undergoes β-hydride elimination to produce the correspond-ing carbonyl compound and a Ru-hydride. The reaction of the hydride with O2 affords aRu-hydroperoxide, followed by ligand exchange to regenerate the Ru-alcoholate speciestogether with the formation of O2 and H2O from H2O2.

TABLE 9.4. Curve-fitting analysis for RuHAP catalysta

Shell Coordination number Interatomic distance (A) � �b (A)

Ru-O(1) 4.1 1.97 0.0067Ru-O(2) 2.1 2.28 0.0008Ru-O(3) 1.7 2.62 −0.0054Ru-Cl 1.2 2.32 0.0010

aThe region of 0.8–2.8 A was inversely Fourier transformed.b� � is the difference between the Debye–Waller factor of RuHAP and that of the reference sample.


TABLE 9.5. RuHAP-catalyzed oxidation of alcoholsa

R1 R1R2 R2

RuHAPOH

OH

OH

OH

ClOH

H3COOH

OH

OH

OH

OH

OHOH

OH

OHOH

NOH

SOH

H3CO

O

O

O

ClO

O

O

O

O

O

OO

O

OO

NO

SO

O

Toluene, 80 oC, O2 (1 atm)

Yield (%)bAlcohol Time (h) Conversion (%)bEntry Product

1

2

3

4

5

6

7

8

9

10

11

c12

13

d14

15

16

3

3

3

2

2

1

2

4

3

5

6

16

6

4

10

2

>99

>99

>99

>99

>99

>99

>99

99

90

93

83

95

96

>99

>99

>99

>99

>99

92

98

>99

99

95

99

85

91

80

94

96

95

>99

94

aReaction conditions: RuHAP (0.2 g), alcohol (2 mmol), toluene (5 ml), 80 ◦C, O2 (1 atm).bDetermined by GC using an internal standard technique. Isolated yields are shown in parentheses.c60 ◦C. dRuHAP (0.1 g), alcohol (1 mmol).


2

H

R2 OH

R1

R R

O

RuO

OO

OPP

Cl

3+

1

2

Ru3+O

OO

OPP

H

O2

H2O + 1/2O2 H2O2

Surface of RuHAP

HCl

R1 R2

O

H

R2 OH

R1

3+

RuO

OO

OPP

OH

R1

R2

RuO

OO

OPP

Ru3+O

OO

OPP

OOH

Figure 9.4. Plausible reaction mechanism for the aerobic oxidation of alcohols using RuHAP.

Recently, highly functionalized RuHAP materials with magnetic properties havealso been developed37; that is, magnetic � -Fe2O3 nanocrystallites dispersed in a hydrox-yapatite matrix (HAP-� -Fe2O3) have been synthesized as a new catalyst support. Thecation exchange ability of the external HAP surface enabled equimolar substitution ofRu for Ca to form a catalytically active center (RuHAP-� -Fe2O3). Characterization byseveral spectroscopic methods showed the formation of � -Fe2O3 nanocrystallites with amean diameter of 8 nmwithin the HAPmatrix, and a monomeric Ru species on the HAPsurface. RuHAP-� -Fe2O3 exhibited superior catalytic activity in the oxidation of variousalcohols to the corresponding carbonyl compounds using O2. The magnetic propertiesof RuHAP-� -Fe2O3 provided a convenient route for separation of the catalyst from thereaction mixture by application of an external permanent magnet, and the spent catalystcould be recycled without appreciable loss of catalytic activity (Figure 9.5).

The RuHAP-� -Fe2O3 catalyst was also found to be applicable to oxidation of ster-ically bulky alcohols. For instance, 3,5-dibenzyloxybenzyl alcohol, cholestanol, and 7-hexadecyn-1-olwere successfully converted into the corresponding carbonyl compoundswith excellent yields in the presence of RuHAP-� -Fe2O3 as a catalyst (Scheme 9.4). Animportant advantage of this catalyst system over other systems is its smooth oxidationof alcohols even at room temperature (Table 9.6). Benzylic and secondary alcohols wereefficiently oxidized to give the corresponding carbonyl compounds in high yields.


Magnet

H

OH

R1

R1 R2

R2

O

γ-Fe2O3

HAP matrix

+ 1/2O2

+ H2O

Ru

Ru

Ru

Ru

Ru Ru

Ru

Ru

Magnet

H

OH Ru

Ru

Ru

Ru

Ru Ru

Ru

Ru

Magnet

H

OH Ru

RuRu

RuRu

RuRu

RuRu RuRu

RuRu

RuRu

Figure 9.5. Catalytic and magnetic properties of RuHAP-γ-Fe2O3. (See color insert.)

Another highly efficient ruthenium-based heterogeneous catalyst system for the aer-obic oxidation of alcohols was reported by Yamaguchi and Mizuno.38 Highly dispersedRu(III) hydroxide supported on alumina (Ru/Al2O3) was found to have a large substratescope. Ru/Al2O3 has one of the highest catalytic activities among reported Ru catalystsystems; in the oxidation of 2-octanol and 1-phenylethanol under solvent-free conditionsat 150 ◦C, TOF values of 300 h−1 and 340 h−1 and turnover numbers (TON) of 950 and980, respectively, were obtained (Scheme 9.5). The RuHAP39 and Ru/Al2O340 catalystswere also able to oxidize primary amines to nitriles in high yield using O2 as an oxidant.

HO

OO

OH

O2, toluene, 90 oC, 24 h

O2, toluene, 90 oC, 24 h

O2, toluene, 90 oC, 10 hHO

OHC

O

OO

CHO

RuHAP-γ -Fe2O3 (2.5 mol%)

>99% yield


>99% yield


>99% yield

7 7

Scheme 9.4. Oxidation of bulky alcohols using RuHAP-γ-Fe2O3.


TABLE 9.6. Oxidation of various alcohols catalyzed by RuHAP-� -Fe2O3 at room temperature

OH

OH

OH

1

3

4

5

1.25 98

1.4 89

1 98

OH2 99

Substrate mol% Yield (%)bConversion (%)bEntry

OHO

O2 4

>99

94

99

>99

>99 >99

aAlcohol (0.5–1 mmol), RuHAP-� -Fe2O3 (1–2 mol%), toluene (5 ml),O2 flow, 24 h.bDetermined by GC using an internal standard technique.

Supported Perruthenate Species

Tetra-n-propylammoniumperruthenate (TPAP) is known as a versatile homogeneous cat-alyst for the oxidative dehydrogenation of various types of alcohols with O2 (1 atm).41,42

Hinzen and coworkers attempted to heterogenize TPAP, and then synthesized polymer-supported perruthenate (PSP).43 The oxidation of some alcohols using PSP (10 mol%)afforded the corresponding aldehydes without overoxidation to the carboxylic acids.However, the recycling of PSP was not conducted, which might be due to oxidativedegradation of the polystyrene support.

Alternatively, a heterogeneous catalyst based on perruthenate tethered to MCM-41(a mesoporous silicate material, first reported by Mobil Co.) was developed by the samegroup.44 In TPAP-tethered MCM-41, the RuO4− group of potassium perruthenate wasintroduced to primary ammonium groups (Si(CH2)3NH3+) within the pores by ion-pairformation (Figure 9.6).

OH

OH

O

O

Ru/Al2O3 (Ru: 0.1 mol%)

150 oC, O2 (1 atm)

(solvent-free)

Yield 98%

TOF 340 h–1

TON 980

Yield 95%

TOF 300 h–1

TON 950

Scheme 9.5. Aerobic oxidation of 1-phenylethanol and 2-octanol using Ru/Al2O3 with high

activity.


O

Si OO

Ph Ph

Si

Si O

Ph Ph

O Si

O

Ph Ph

O

Si

Si

O

Ph Ph

O

MCM-41

SiOO

O

SiO

O

SiO

Si

OOO

XX XX

1

2 X = NH3+

3 X = NH3+RuO4

-

4 X = Br

5 X = NMe3+RuO4

-

6 X = NEt3+RuO4

-

Figure 9.6. Catalyst structure of TPAP-tethered MCM-41.

TPAP-tethered MCM-41 (10 mol%) catalyzed the aerobic oxidation of various ben-zylic and allylic alcohols to the corresponding aldehydes under an atmospheric pressureof O2 without overoxidation of the aldehydes to the carboxylic acids. Furthermore,TPAP-tethered MCM-41 could also be reused up to 12 times without any loss of activ-ity. Ru leaching from MCM-41 did not take place. This was confirmed by allowing theoxidation to proceed to 50% conversion, and filtering and separating the solid catalyst.The filtrate was then heated at 80 ◦C under O2, which gave no further oxidation.

Pagliaro and Ciriminna found that organically modified silica doped with TPAPprepared via the sol–gel method acted as a reusable heterogeneous catalyst for theaerobic oxidation of alcohols.45, 46 The perruthenate encapsulated in the sol–gel matrixwas more active than the unsupported TPAP. It is notable that the use of the encapsulatedperruthenate gave a high conversion of primary aliphatic alcohols; for example, 1-octanolwas transformed to octanal in 96% yield. “Hot filtration” of the catalyst revealed that theencapsulated perruthenate was truly a heterogeneous catalyst. The authors mentionedthat the hydrophobicity of the sol–gel matrix cage was crucially important for thesuppression of leaching of the Ru species, which was observed to occur at a negligiblelevel, although they did not describe how the Ru species was fixed in the organicallymodified silica.

Perruthenates encapsulated in fluorinated silica,47 quaternary ammonium cations,48

and ionic liquid-supported silica49 have also been synthesized and applied to the aerobicoxidation of benzyl alcohols in supercritical carbon dioxide. The use of supercriticalcarbon dioxide in such reactions can help avoiding the use of large amounts of organicsolvent. In perruthenate-doped supported ionic liquids or quaternary ammonium cations(Figure 9.7), the RuO4− anionic species were fixed in the sol–gel matrix by an ion–pairinteraction with an imidazolium cation. No Ru leaching from the catalyst was observed(Ru detection limit �1 ppb) during the oxidation of alcohols. In a typical example, the


O

SiOHO

NN+

O

SiO

HON

N+

RuO4-

RuO4-

S

S

S

P

P

P

O2

CO2

O

SiOHO

NN+

O

SiO

HON

N+

4-

4-

Sol-gel S

S

S

P

P

P

(a)

(b)

(c)

Dense-phaseCO2

Figure 9.7. Illustration of the transport, reaction, and adsorption/desorption steps of sup-

ported ionic liquid catalyst in scCO2. The dense-phase CO2 dissolving the substrate (a) spills its

content into the catalyst tethered to the ionic liquid moiety where (b) the catalytic process

takes place, and then (c) the highly diffusible dense-phase CO2 carries the product back into

solution. (See color insert.)

oxidation of benzyl alcohol using 10 mol% of the catalyst under 220 bar of scCO2 at75 ◦C gave benzaldehyde in approximately 90% yield.

Organic Polymer-Supported Ruthenium Catalysts

There have been many reports of inorganic material-supported Ru catalysts used inthe aerobic oxidation of alcohols. On the other hand, polymer-supported Ru catalystsfor such oxidation reactions have not yet been widely studied. As described above,early work on polymer-supported perruthenate (PSP) was conducted by Hinzen andcoworkers.43 Dalal and Ram found that a Ru(III)–salen complex (Scheme 9.6) wassufficiently heterogenized using cross-linked styrene–divinylbenzene resin.50 The sup-ported Ru(III)–salen complex catalyzed the oxidation of benzyl alcohol. However, thelevel of Ru leaching and the reusability of the heterogeneous catalyst were not described.

A polymer-incarcerated Ru catalyst (PI-Ru) was synthesized from an epoxide-containing copolymer and RuCl2(PPh3)3 using microencapsulation and cross-linking.51

PN

H

CN H

ORu

Cl

ClH2O

Scheme 9.6. Schematic structure of polymer-supported Ru(III)—salen complex.


The PI-Ru catalyst showed activity for the oxidation of several alcohols, includingaliphatic primary alcohols, using N-methylmorpholine-N-oxide as an oxidant in thepresence of molecular sieves. PI-Ru could be reused and applied to a flow system,where benzyl alcohol was converted to benzaldehyde in high yield passing through acolumn containing the PI-Ru and MgSO4. Careful choice of micelle-forming conditionsand polymer components improved the catalytic activity.52 In a similar way, PI-Rusynthesized from an epoxide-containing copolymer and RuCl3·nH2O (5 mol%) wasfound to be effective for the aerobic oxidation of alcohols in the presence of 15 mol%2,2,6,6-tetramethylpiperidine N-oxide.53 Benzylic, aliphatic, and heterocyclic alcoholswere transformed to the corresponding aldehydes and ketones, although leaching of smallamounts of theRu species in the PI-Ru occurred. The catalytic activity of PI-Ru graduallydecreased during the recycling, and a prolonged reaction time was necessary to obtaina quantitative yield of the product. The same group also reported an organic–inorganichybrid ruthenium catalyst (HB Ru) synthesized from a polystyrene-based copolymercontaining a trimethoxysilyl functionality and RuCl2(PPh3)3.54 The characterization ofHB Ru showed no chlorine atoms and a small amount of phosphorous atoms. HB Rucatalyzed the oxidation of various alcohols without any additives. The leaching levelwas below 0.010% Ru in most cases, except for some primary alcohols. Although thecatalytic activity of HB Ru decreased in recycling experiments, the activity of HB Ruwas recovered by pretreating with 1 M K2CO3 aq, and HB Ru could be reused at leastfive times without loss of activity.

PALLADIUM-CATALYZED ALCOHOL OXIDATION

Supported Pd-NPs

Kaneda and coworkers first reported that Pd-NPs showed catalytic activity for theoxidation of alcohols using molecular oxygen. Pd-NPs of Pd4Phen2(CO)(OAc)455

and Pd561Phen60·(OAc)180,56 which were synthesized from Pd4(CO)4(OAc)2AcOH and1,10-phenanthroline, promoted the oxidation of cinnamyl alcohol under an atmosphericpressure of O2. Various allylic alcohols were also converted to the corresponding �,β-unsaturated aldehydes, accompanied by formation of a small amount of hydrogenatedproducts (Scheme 9.7).

Aiming at heterogenization of the Pd-NPs, TiO2-supported Pd561Phen60(OAc)180,57

and the eight-shell Pd cluster Pd2060(NO3)360(OAc)360O8058 were developed. These Pd-NPs could be uniformly dispersed on the TiO2 surface while maintaining their original

Pd4Phen2(CO)(OAc)4

or

Pd561Phen60(OAc)180R1

R2

OH R1

R2

O

50–90 oC, O2(1 atm)

Scheme 9.7. Oxidation of α,β-unsaturated alcohols using Pd4Phen2(CO)(OAc)4 or

Pd561Phen60(OAc)180.

PALLADIUM-CATALYZED ALCOHOL OXIDATION 301

4040 Å

O

P

O

P

Pd2+

Cl Cl

2.01 Å2.36 Å

Hydroxyapatite

OPOO

PO

Ca2+

PdCl2(PhCN)2

-2PhCN

PdHAP

Alcohol

Figure 9.8. Preparation of PdHAP.

structure. The TiO2-supported Pd-NPs showed a similar activity as that observed forthe unsupported Pd-NPs and were reusable. The size of the Pd-NPs on the TiO2 afterrecycling was similar to that of the fresh Pd-NPs, as confirmed by XAFS analysis.

There have been many reports of the use of Pd-NPs immobilized on the surface ofinorganic solid supports for the aerobic oxidation of alcohols. Although some progresshas been made on the preparation of heterogeneous Pd catalysts such as Pd on pumice,59

Pd-NPs/Al2O3,60,61 and Pd-NPs/MgO,62 unfortunately, most of these heterogeneous Pdsystems have suffered from low catalytic activities, a limited substrate scope, leachingof the Pd species from the supports, and/or low reusabilities.

To overcome these issues, a palladium-grafted hydroxyapatite (PdHAP) was devel-oped by Kaneda and coworkers.63,64 PdHAP was synthesized by treatment of a stoi-chiometric hydroxyapatite with PdCl2(PhCN)2. Analysis by several spectroscopic meth-ods revealed that a monomeric PdCl2 species was chemisorbed onto the HAP surfaceand was readily transformed into Pd nanoclusters during the oxidation of alcohols(Figure 9.8).

PdHAP catalyzed the oxidation of a wide range of alcohols without any need foradditives to complete the catalytic cycle. PdHAP worked effectively in the aerobic oxi-dation of 1-phenylethanol under solvent-free conditions on a 250 mmol scale, showinga remarkably high TON of up to 236,000, with an excellent TOF of approximately9800 h−1 (Scheme 9.8). The diameter of the generated Pd nanoclusters could be con-trolled by the choice of the alcohol molecule. Calculations on the palladium crystallitesshowed that the oxidation of alcohols occurred primarily at low-coordination sites withina regular arrangement of the Pd-NPs (Figure 9.9).

PdHAP-0 (Pd: 4 × 10–4 mol%),

150 oC, O2 (1 atm), 24 h(solvent-free)

OH O

Yield 94% TOF 9800 h–1

TON 236,000

250 mmol

Scheme 9.8. Aerobic oxidation of 1-phenylethanol using PdHAP under solvent-free

conditions.


CHOPd2+

O2

H2O2

Pd0

Edge atom

Face atom

H2O + 1/2O2

OH

O

H

H

H

Figure 9.9. Proposed reaction mechanism for the aerobic oxidation of alcohols catalyzed by

PdHAP.

Wu and coworkers found that Pd-NPs on Al2O3 prepared via an adsorption method(Pd/Al2O3-ads) were muchmore effective in the aerobic oxidation of alcohols than thoseprepared via an impregnation method (Pd/Al2O3-imp) (Table 9.7).65

The absorption method provided highly dispersed mononuclear or oligonuclearPd(II) species stabilized on the surface of the Al2O3. The adsorbed Pd(II) species weretransformed into small Pd-NPs with a diameter of 5 nm during oxidation, while theimpregnation method gave much larger Pd-NPs with a diameter of 20 nm. Pd/Al2O3-ads efficiently catalyzed the oxidation of benzyl alcohol under solvent-free conditions,

TABLE 9.7. Comparison of the catalytic properties of Pd/Al2O3-ads and Pd/Al2O3-imp forthe solvent-free oxidation of benzyl alcohol with O2

a

Temperature Time Conversion PhCHO TOFCatalyst (◦C) (h) (%) selectivity (%)b (h−1)c

Pd/Al2O3-ads 88 8 97 96 1952100 4 83 97 3300

Pd/Al2O3-imp 88 8 8 96 161100 4 60 90 2415

aThe reactions were carried out under the following conditions: Pd amount in each catalyst, 3 μmol; benzylalcohol, 48.5 mmol; O2 flow rate, 3 ml min−1.bThe by-product was toluene.cTOF is based on the number of moles of the substrate converted per mole of Pd in the catalyst per hour.


Pd(PPh 3)4

+

4

+

(sec-BuO)3Al/BuOH

Air

120 oC

10 h

120 oC

30 min

(1) Filtration(2) Wash with acetone

(3) Dry1HO -- CH2CH2O --H

H2O

Scheme 9.9. Preparation of aluminum hydroxide-enwrapped Pd-NPs.

giving a TOF of 3300 h−1. The same group also found that Pd/SiO2-Al2O3 prepared viathe adsorption method showed a higher catalytic activity than Pd/Al2O3. The size of thePd-NPs, which ranged from 2.2 to 10 nm, could be controlled by changing the Si/Alratio, with the Pd-NP size decreasing as the Si/Al ratio decreased. A maximum catalyticactivity was observed for Pd/SiO2-Al2O3 with a mean diameter of 3.6–4.3 nm.66 Asimilar size dependency was also observed for zeolite-supported Pd-NPs used for theoxidation of benzyl alcohols.67

Inorganic material-supported Pd-NP catalysts often suffer from palladium leachingand aggregation of the Pd-NPs to larger inactive nanoparticles or Pd black duringoxidation. To solve this problem, Kwon and coworkers reported that small Pd-NPswith a diameter of 2–3 nm that were enwrapped in an aluminum hydroxide matrix(Scheme 9.9) showed high catalytic activity and durability in the oxidation of alcohols.68

For example, the oxidation of 1-phenylethanol under solvent-free conditions proceededsmoothly in the presence of 0.005 mol% aluminum hydroxide-enwrapped Pds at 150 ◦C,giving acetophenone in 98% yield. This Pd-NP catalyst was also reused at least 10 timeswithout activity loss.

Karimi and coworkers reported another method for preparing highly dispersedPd-NPs using SBA-15-type ordered mesoporous silica, and investigated their use inthe catalysis of the aerobic oxidation of alcohols.69 SBA-15 was functionalized witha bipyridyl amide ligand followed by complexation with Pd(OAc)2 to give Pd(II)-immobilized SBA-15 (Pd/SBA-15) (Figure 9.10). Pd/SBA-15 (0.4 mol%) was found tobe an efficient heterogeneous catalyst for the oxidation of benzylic and allylic alcoholsunder pure O2 or air conditions, as shown in Table 9.8.

Primary alcohols were quantitatively converted to the corresponding esters. Theoxidation of 2-substituted benzylic alcohols gave the aldehydes in low yields due to thesteric hindrance resulting from the quasi-two-dimensional surface of the nanoparticles.

O SiO

O N

O

N

N

Pd

OAc

OAcSBA

-15

Figure 9.10. Structure of Pd/SBA-15.


TABLE 9.8. Substrate scope of the Pd/SBA-15-catalyzed aerobic oxidation of alcohols

R1 R1R2 R2

OHPd@SBA-15 (0.4 mol%)

1 equiv. of K2CO3, toluene, 80 oC, O2

O

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

Ph

4-MeC6H4

2-MeC6H4

2-ClC6H4

4-MeOC6H4

4-ClC6H4

4-NO2C6H4

Ph

Ph

PhCO

Ph

PhCH–CH–

PhCH–CH–

(CH3)2C–CHCH–

CH3(CH2)4

CH3(CH2)3

CH3(CH2)5

cyclohexanol

PhCH2CH2

PhCH2CH2CH2

4-phenylcyclohexanol

4-tert-butylcyclohexanol

Entry R1 Yield (%)a, b, cTime (h)

3.5

3.5

8

12

2.5

12

14

15

15

12

20

5

6

6

6

7

24

24

24

24

16

16

>99 (83)

>99

25

35

>99

>99

>99

>99 (91)

>99

>99 (95)

>99

98

>95

>99

93

>95

>99d

>99d

>99d

>99d

53

45

–

–

–

R2

H

H

H

H

H

H

H

Me

Et

Ph

Ph

H

Me

H

CH–CH2

H

H

H

H

–

aGC yield is based on an internal standard method unless otherwise stated.bYields in parentheses refer to isolated pure products.cThe molar ratios of substrate/metal are 1 : 0.004.dConversions refer to the corresponding esters.

A transmission electron microscopic (TEM) image of the catalyst after the oxidationreaction revealed that highly dispersed Pd-NPs with a diameter of 7 nm were formedinside the regular mesoporous channels, and the channels of the SBA-15 remainedopen. The recovered catalyst was successfully used in 12 subsequent reactions andexhibited consistent catalytic activity. The bipyridyl ligands played an important role


CH2Cl225 oC, 1 h

Pd(OAc)2

(0.10 M)

N

OO

N

Me

Si

7

N

O O

N

Me

Si

7

Si

Pd

PdOAcAcO

OAcAcO

Si-BOX-Pd

N

OO

N

Me

7

BOX M)(0.33

+

Si(1 1 1)-H

H

Si

H

Si

H

Si

H

Si

N

O O

N

Me

Si

7

N

OO

N

Me

Si

7

Si

Mesitylene160 oC, 22 h

Si-BOX

Figure 9.11. Preparation of Si-BOX-Pd.

in uniformly distributing the mononuclear Pd species throughout the solid support, giv-ing highly dispersed Pd-NPs formed inside the mesoporous channels of the SBA-15.SBA-15-immobilized Pd(II) species without bipyridyl ligands did not exhibit durabil-ity in the recycling experiments, and Pd black was formed on the outer surface of theSBA-15. Following this research, Li and coworkers demonstrated a simple method forthe preparation of SBA-15-encapsulated Pd-NPs via the adsorption of [Pd(NH3)4]2+

into SBA-15.70 Pd-NPs with a diameter of 3.3–3.9 nm were formed inside the cylindri-cal porous channels. These SBA-15-encapsulated Pd-NPs prepared by the adsorptionmethod worked well under solvent-free conditions without any additives.

Hara and coworkers reported a bisoxazoline Pd(II) species anchored onto the surfaceof a single-crystal silicon (Si-BOX-Pd) (Figure 9.11).71

Interestingly, the oxidation of benzyl alcohol proceeded efficiently in the presenceof Si-BOX-Pd without stirring the reaction mixture, affording benzaldehyde together


with the formation of small amounts of benzyl benzoate and benzoic acid. The TONreached as high as 780,000 (65% yield of benzaldehyde). Si-BOX also showed highcatalytic activity for some benzylic alcohols; however, aliphatic primary alcohols werenot converted. In the recycling experiments, the catalytic activity of Si-BOX-Pd wasmaintained in the second run, but decreased significantly in the third and fourth runs.The Pd(II) species coordinated with Si-BOX turned into a Pd(0) species after the firstrun. The Pd(0) species may be a mononuclear bisoxazoline-Pd(0) species or Pd clusterswith a diameter less than 1.5 nm. It is still not clear how the Pd(0) cluster is bound tothe Si-BOX.

Most Pd-NP catalysts have required high temperatures (65–150 ◦C) for alcoholoxidation. Recently, Layek and coworkers reported the aerobic oxidation of alcohols atroom temperature under air conditions using nanocrystalline magnesium oxide (NAP-MgO)-supported Pd-NPs (NAP-Mg-Pd(0)) in the presence of 120 mol% K2CO3.72

NAP-MgO has a high surface area (590 m2 g−1) due to its three-dimensional structure,thus enabling stabilized Pd-NPs with a diameter of 5–7 nm to be highly dispersed on thesurface. NAP-Mg-Pd(0) showed a wide substrate scope; benzylic, allylic, aliphatic andalicyclic alcohols were oxidized at room temperature under air, giving the correspondingcarbonyl compounds from good to excellent yields. It is noteworthy that nonactivatedprimary aliphatic alcohols were successfully converted to the corresponding aldehydesin 68–72% yields with more than 95% selectivities. NAP-Mg-Pd(0) could also berecycled four timeswithout loss of catalytic efficiency. The proposed reactionmechanisminvolved the facile formation of a metal-alcoholate species through a cooperative effectbetween the basic support and the active metal nanoparticles.

Chemical reactions using water as a solvent have received considerable attentionin organic synthesis, from both a practical and an economic standpoint.73–76 The useof water in organic reactions has the following significant advantages: low cost, abun-dance, safety (nonexplosive, nonflammable, and nontoxic), easily controlled reactiontemperatures due to the high heat capacity of water, and ease of phase separation whenwater-insoluble products are formed.

Uozumi and Nakao synthesized polymer-supported Pd-NPs with a mean diamete of9 nm (ARP-Pd) via the reduction of a polystyrene-polyethylene glycol resin-supportedbipyridyl–Pd(II) complex (Scheme 9.10).77

The ARP-Pd catalyst worked well in the aerobic oxidation of alcohols in aque-ous media. The concept behind using ARP-Pd in aqueous media was that the organicsubstrate (alcohol) would diffuse into the hydrophobic ARP-Pd matrix, giving a highlyconcentrated reaction field, where the active Pd-NPs in the polymer matrix wouldefficiently oxidize the trapped alcohols. The oxidation of benzylic alcohols to the corre-sponding aldehydes and ketones, and primary aliphatic alcohols to the carbxylic acids,did proceed, although the catalytic activity of ARP-Pd was not so high.

Supported Pd(II) Species

Uemura and coworkers found that the aerobic oxidation of alcohols proceededusing a catalytic amount of Pd(OAc)2, pyridine, and 3 A molecular sieves (M3A)(Pd(OAc)2/pyridine/MS3A) under an atmospheric pressure of pure O2.78,79 His group


PhCH2OHN

N

N

Pd0

3

N

N

N

Pd(OAc)2

TolueneRT, 1 h

Pd(OAc)2PS NCHNOO

N

N

O

n

1

2

Releasing Pd0

precipitation

nano-

Pd

ARP-Pd (4) =Amphiphilic resin-dispersion ofnanoparticles of palladium

Scheme 9.10. Preparation of amphiphilic resin-dispersed palladium nanoparticles (ARP-Pd).

attempted heterogenization of this homogeneous catalyst system for constructing a“greener” oxidation system by developing a Pd(II) species adsorbed on the surface ofhydrotalcite (Pd(II)-hydrotalcite).80–82 The oxidation of various benzylic and aliphaticalcohols, except for epoxy alcohols and heterocyclic alcohols including nitrogen andsulfur atoms, was promoted by 5 mol% Pd(II)-hydrotalcite, even under an atmosphericpressure of air, although the addition of at least 4 mol equivalents of pyridine relative toPd was essential for obtaining the products quantitatively. In these reactions, the reactiontemperatures and concentrations of the alcohol substrate, as well as the amounts of pyri-dine, significantly affected the catalytic efficiency. When the oxidation of 1-dodecanolwas carried out at 80 ◦C for 13 h, a 70% yield of dodecanal was obtained. During oxida-tion, the color of the catalyst turned fromwhite-yellow to black, indicating the formationof Pd black. On the other hand, a higher yield (91%) of dodecanal was obtained at alower temperature (65 ◦C) for 10 h, and Pd black was not formed at all. These resultsshowed that the active species of Pd-hydrotalcite was a divalent Pd species, and the for-mation of Pd black made the reaction slow. In a similar fashion, when the oxidation wascarried out with a higher concentration of benzyl alcohol (0.1 M), Pd black was formedand the yield of benzaldehyde decreased. The use of Pd(II)-hydrotalcite with pyridinegave higher activity and selectivity than the homogeneous Pd(OAc)2/pyridine/MS3Acatalytic system in the oxidation of allylic alcohols, although the reason for the superiorcatalytic activity of the heterogeneous version was not clear (Table 9.9).

Pd(II)-hydrotalcite was shown to be reusable at least three times, but its cat-alytic activity gradually decreased. The deactivation of Pd(II)-hydrotalcite was dueto the leaching of 14% of the Pd salt from the hydrotalcite support. When modified


TABLE 9.9. Catalytic oxidation of geraniol and nerol using Pd(II)-hydrotalcite andPd(OAc)2/pyridine/MS3Aa

OH CHO

OH CHO

Entry Substrate Product

Pd(II)-hydrotalciteb

Time (h) Isolated yield (%)d

Pd(OAc)2/pyridine MS3Ac

Time (h) Isolated yield (%)d

1e

2

12

4.5

45 (58)E : Z = 95 : 591 (98) 15 56 (76)

3 4.5 89 (100)E : Z = 6 : 94

E : Z = 96 : 4 E : Z = 63 : 37

E : Z = 31 : 6915 39 (71)

5 E : Z = 98 : 2 6

7 E : Z = 2 : 98 8

aReaction conditions: alcohol (1.0 mmol), pyridine (5.0 mmol), O2, 80 ◦C.bPd(II)-hydrotalcite (1.56 mmol g−1 Pd: 300 mg, 0.05 mmol Pd).cPd(OAc)2 (0.05 mmol), MS3A (500 mg).dThe value in parentheses is the conversion of alcohol (%). E : Z ratio was determined by 1H-NMR.ePyridine (0.2 mmol) was used.

Pd(II)-hydrotalcite(m), which had half the amount of Pd content compared with Pd(II)-hydrotalcite, was prepared and tested in the recycling experiment, the reusability wasimproved and the Pd leaching amounts decreased to 0.8%.82 Uemura proposed a plau-sible reaction mechanism for the oxidation of alcohols, as illustrated in Scheme 9.11.The pathway is intrinsically the same as that proposed for the homogeneous system.

O2

R1 R2

O

Pd(II)HS

Pd(II)S OOH

Pd(II)S O

H

R2

R1

HHO

R2R1

HHO

R2R1

H2O2 H2O2

H2O + 1 / 2 O2

AcOH

HO

H

R2

R1

Pd(II)OAcS

S = hydrotalcite

β-Hydrideelimination

Scheme 9.11. Plausible reaction pathway for the Pd(II)-hydrotalcite-catalyzed aerobic oxida-

tion of alcohols.


R1 R2

OH

NH2

2

NN

NPd

OAc

OAc

NN

O

NN

NPd

OAc

OAc

1

NN

N

O2 (1 atm), dry toluene, K2CO3 (1 mmol), 75–80 oC R1 R2

O

Dry toluene, reflux, 18 h(azeotropic removal of water)

Pd(OAc)2

dry acetone, RT

24 h

(4 mol%)

Scheme 9.12. Preparation of a silica-anchored bipyridyl—Pd complex.

A bipyridyl–Pd(II) complex covalently anchored onto a silica surface was synthe-sized as a heterogeneous catalyst by Karimi’s group (Scheme 9.12).83

This silica-anchored bipyridyl–Pd complex exhibited activity for benzylic andaliphatic alcohols, giving the corresponding carbonyl compounds in high yields. How-ever, the addition of 100 mol% K2CO3 relative to the alcohol was necessary to achieveefficient oxidation; lower amounts of K2CO3 led to a reduced yield of the producttogether with the rapid formation of Pd black. In the case of the oxidation of allylicalcohols, Pd black was easily formed, even in the presence of more K2CO3. With thiscatalyst, the oxidation of primary benzylic alcohols proceeded under an atmosphericpressure of air instead of pure O2, although nonbenzylic primary and secondary alco-hols were not suitable for this system. The silica-anchored bipyridyl–Pd complex couldbe reused at least three times. In the fourth run, however, the yield of products decreasedeither due to the deactivation of the active centers resulting from complexation with bothstarting materials and products, or due to microscopic changes in the structure of thecatalyst.


C.S.=

8.4

Å= OH-

= Ni2+

= Zn2+

= OH–

= Ni2+

= Zn2+

H2O

H2O

CH3

O OCH3

OO

2.01Å

8.40 Å

NiZn sheet

Pd

OHHO

OHHO

Zn

Zn

2-

(a) (b)

Figure 9.12. (a) Structure of the Ni—Zn mixed basic salt (NiZn) and (b) proposed structure

around the Pd(II) center of the Pd/NiZn(0.02) catalysts.

Lee and coworkers found, using several X-ray techniques, that surface PdOx speciesof supported Pd catalysts were the truly active species in the selective oxidation ofderivatives of cinnamyl and crotyl alcohol. The high dispersion of the Pd species and thehigh concentration of the surface PdOx species played crucial roles in the oxidation.84–87

Partially oxidized Pd species were also important in the oxidation of benzyl alcoholsusing Pd/Al2O3 in supercritical carbon dioxide.88,89

Hara and coworkers reported that anionic Pd hydroxide species could be immobi-lized in the interlayer of a Ni–Zn mixed basic salt (Pd/NiZn), which is classified as ahydroxyl double salt based on the anion-exchange ability of the Ni–Zn (Figure 9.12).90

The Pd/NiZn catalyst efficiently promoted the aerobic oxidation of alcohols. It isnoteworthy that this catalyst has high durability as it is successfully reused withoutany loss of activity, and its divalent Pd species in the interlayer of the NiZn maintaintheir original monomeric structure without any organic ligands, even during oxidationat 100 ◦C, which is confirmed by XAFS analysis. The high stability of the divalent Pdspecies was determined to be derived from the strong electrostatic interactions betweenthe anionic [Pd(OH)4]2− species and the layered NiZn host. The most attractive cat-alytic feature of Pd/NiZn was its high efficiency in the oxidation of aliphatic primaryalcohols; for example, 1-octanol was converted to octanal in more than 99% yield.Recently, the same group succeeded in improving the catalytic activity of Pd/NiZnby introducing d-valine and PO43−.91 The structure of the anionic Pd species wasdetermined, as shown in Scheme 9.13, using XAFS analysis. The modified Pd/NiZn([d-valine–Pd(OH)2]−/PO43−/NiZn) catalyst had both a higher durability and a superiorcatalytic activity as compared to Pd/NiZn. For example, 2-adamantanol was quantita-tively converted to 2-adamantanone using [d-valine–Pd(OH)2]−/PO43−/NiZn, but noreaction occurred in the presence of Pd/NiZn (Table 9.10). The high catalytic activityof [d-valine–Pd(OH)2]−/PO43−/NiZn can be explained by a concerted effect betweenthe PO43− and [d-valine–Pd(OH)2]−; that is, the PO43− abstracts the proton from analcohol, promoting the facile formation of a Pd-alcoholate species. The Pd-alcoholate

Au-CATALYZED ALCOHOL OXIDATION 311

PdCl

Cl

NH2

O O

PdOH

OH

NH2

O O

0.198 nm

0.230 nm-

(a)

0.199 nm

0.199 nm-

(b)

Scheme 9.13. Proposed structure of (a) an anionic (D-valine—PdCl2)− complex and (b) an

anionic [D-valine—Pd(OH)2]− complex in a NiZn interlayer.

species initiates the catalytic cycle involving a β-hydride elimination, as described inScheme 9.11.

Au-CATALYZED ALCOHOL OXIDATION

Inorganic Material-Supported Au-NPs

Since the discovery of the high catalytic activity of Au-NPs for the oxidation of COat low temperature,92 Au has been recognized as an active metal in aerobic oxidationreactions. Pioneering work on the liquid-phase oxidation of alcohols was done by Pratiand Rossi. They found that Au/C was effective in the aerobic oxidation of vicinaldiols in an alkaline solution. For example, ethane-1,2-diol and propane-1,2-diol weresuccessfully oxidized to the corresponding �-hydroxy carboxylates in high yields. Au/Cshowed higher activity, selectivity, and reusability compared with commonly used Pd/C

TABLE 9.10. Aerobic oxidation of 2-adamantanol with various catalystsa

OH O NiZn catalyst

80 oC, 3 h, air flow

none

0.44

0.83

0.44

0.83

0.83

0.44

-

98

20

57

trace

trace

trace

trace

98

20

57

trace

trace

trace

trace

1

2

3

4

5d

6d

7

Entry Catalyst C.S. (nm)b Conversion (%)c Yield (%)c

[1-Pd(OH)2]–/PO43-/NiZn

[1-Pd(OH)2]–/CH3COO-/NiZn

[Pd(OH)4]2–/PO43–/NiZn

[Pd(OH)4]2–/CH3COO-/NiZn

CH3COO–/NiZn

PO43–/NiZn

a2-Adamantanol (0.5mmol), Pd catalyst (Pd 1mol%), trifluorotoluene (5ml), 80 ◦C, 3 h, air flow (20ml/min).bC.S. was determined from the XRD pattern; C.S. = d001 − thickness of the layer (0.46 nm).cDetermined by GC using an internal standard technique.dNiZn (0.1 g) was used as a catalyst.


TABLE 9.11. Comparison of MgAl2O4-supported Au, Pt, and Pd catalysts for the aerobicoxidation of aqueous ethanol to acetic acida

Temperature Pressure Time Conversion Yield STYb

Catalyst (K) (MPa) (h) (%) (%) (mol h–1 l–1)

Auc 453 3 4 97 83 0.21Pt 453 3 4 82 16 0.047Pd 453 3 4 93 60 0.15

aConditions: 150 mg catalyst, 1 wt% of metal, 10 ml of 5 wt% aqueous ethanol.bSpace time yield.cCorresponding to 0.07 mol% Au.

and Pt/C.93 Interestingly, the catalytic performance of Au/C was much influenced by thesize and content of the Au particles on the support, and it was found that Au/C with Au-NPs having a diameter of 7.5 nm exhibited themaximum activity in the aerobic oxidationof glycols, which is different from the results obtained for Au/TiO2 and Au/Al2O3. Theactivities of Au/TiO2 and Au/Al2O3 increase with a decrease in the size of the Au-NPs.94

Au/C was also found to be applicable to the oxidation of simple alcohols and sugarsto the corresponding acids.95 Similarly, the oxidation of glycerol to glycerate usingAu/C was demonstrated by Carrettin and coworkers.96 Biella and Rossi also found thatAu/SiO2 could promote highly selective oxidations of primary and secondary aliphaticalcohols to the corresponding aldehydes and ketones in the gas phase.97

Christensen and coworkers found that MgAl2O4-supported Au-NPs with a meandiameter of 5 nm prepared by a deposition–precipitation method were highly effectivein the oxidation of ethanol to acetic acid in aqueous acidic media under 0.6 MPaO2 at 150 ◦C.98 Au/MgAl2O4 showed a distinguished catalytic activity and selectivitycompared to MgAl2O4-supported Pd and Pt-NPs in the oxidation of ethanol, providingacetic acid in 90% yield accompanied by small amounts of CO2 (Table 9.11). This directsynthetic protocol for the production of acetic acid from ethanol using an Au catalystcould be considered a green process because acetic acid is one of the significantlyvaluable products that can be produced from bioethanol.

Among Au-NPs immobilized on inorganic oxides,99–103 activated carbon, or poly-mers,104 the most efficient and powerful heterogeneous catalyst is probably developedby Abad and coworkers,105,106 which consists of Au-NPs with a 2–5 nm diameter com-bined with nanocrystalline cerium oxide (Au/npCeO2). The Au/npCeO2 catalyst showedextremely high performance in the selective oxidation of a wide range of primary andsecondary alcohols without requiring the use of solvents, additional bases, or high oxy-gen pressure (Table 9.12). Au/npCeO2 also exhibited a high TOF of 12,500 h−1 in theoxidation of 1-phenylethanol, and the total TON reached 250,000 after three recyclingexperiments (Scheme 9.14).

For the reuse ofAu/npCeO2 in the oxidation of benzylic alcohol, Au/npCeO2 neededto be washed with an aqueous solution of NaOH to maintain the catalytic activity. Thisrequirement was because small amounts of carboxylic acids produced in the reactionstrongly adsorbed onto the surface of the catalyst, poisoning the catalytic activity. The


TABLE 9.12. Aerobic alcohol oxidation catalyzed by Au/npCeO2a

Entry Substrate Time (h) Conversion (%) Product Selectivity (%)

lb 3-Octanol 3.5 97 3-Octanone �992b 1-Phenylethanol 2.5 92 Acetophenone 973b Cinnamyl alcohol 7 66 Cinnamaldehyde 734c Vanillin alcohol 2 96 Vanillin 985c Cinnamyl alcohol 3 �99 Cinnamic acid 986d n-Hexanol 10 �99 Hexanoic acid �99

aConversion and selectivity were determined by GC using nonane and nitrobenzene as external standards.bSubstrate (4.85 mmol), Au/npCeO2 (0.5 mol%), 80 ◦C, O2 (1 atm, flow: 25 ml min−1).cSubstrate (0.4 mmol), Au/npCeO2 (0.66 mol%), H2O (5 ml), Na2CO3 (0.3 g), 50 ◦C, O2 (1 atm, flow;25 ml min−1).dSubstrate (1 mmol), Au/npCeO2 (0.25 mol%), H2O (5 ml), Na2CO3 (0.71 g), 100 ◦C, Air (25 atm).

basic washing removed the carboxylic acids adsorbed on the active site of the catalyst.The catalytic activity of Au/npCeO2 was significantly dependent on the particle sizeof the Au and the Au content on the npCeO2; namely, the activity correlated linearlywith the total number of external Au atoms, and with the surface coverage of thesupport. Au/npCeO2 exhibited a high chemoselectivity compared with the use of Pd-NP catalysts in the oxidation of allylic alcohols to the corresponding �,β-unsaturatedcarbonyl compounds under solvent-free conditions107; a low chemoselectivity for the�,β-unsaturated carbonyl compounds was observed in the case of Pd catalysts due to theformation of other by-products arising from isomerization and hydrogenation of C Cbond and/or polymerization (Scheme 9.15).

The difference in the chemoselectivity between Au-NPs and Pd-NPsmay be derivedfrom the different stabilities and steady-state concentrations of the metal hydrides (Au-Hand Pd-H). The Au-H species generated during oxidation is much more reactive withO2 than the Pd-H, preventing C C bond hydrogenation or isomerization.

Although several Au catalyst systems for aerobic oxidation have been reported,most of them have been carried out above 100 ◦C. Su and coworkers demonstratedthat a spinel type of mesostructured Ga-Al mixed oxide prepared by an alcoholic sol–gel method might be a highly promising support for the effective aerobic oxidation ofalcohols using Au-NPs under mild conditions (80 ◦C or ambient temperature, 1 atm

Au/npCeO2

160 oC, O2 (1 atm), 24 h

(solvent-free)

OH O

Selectivity >99 %TOF 12,500 h –1

TON 250,000

250 mmol

Scheme 9.14. Aerobic oxidation of 1-phenylethanol using Au/npCeO2 under solvent-free

conditions.


OH

O

O

Isomerization or hydrogenation

Polymeric material

O2, 120 oC

Supported Pd catalyst

Supported Au catalyst

Scheme 9.15. Differences in product distribution for the aerobic oxidation of allylic alcohols

under solvent-free conditions catalyzed by Au and Pd catalysts.

of O2) without the addition of bases.108 The catalytic activity of Au/Ga3Al3O9 wassuperior to that of Au/Ga2O3 and Au/Al2O3 and similar to that of the efficient PdHAPcatalyst in the aerobic oxidation of benzyl alcohol. Au/Ga3Al3O9 was highly active forvarious alcohols, but, in a similar fashion to other Au catalysts reported, the oxidationof primary aliphatic alcohols to the corresponding aldehydes did not proceed efficiently.Au/Ga3Al3O9 could be reused in five runs, and the leaching amount of the Au wasnegligible.

Mitsudome and coworkers found that Au-NPs with a diameter of 2.7 nm could beuniformly dispersed on hydrotalcite (Au/HT) using a deposition–precipitation method.Au/HT acted as a highly active solid catalyst for both the oxidation of various alcoholsand the oxidative lactonization of�,�-diols under the followingmild reaction conditions:(i) no requirement for additives, (ii) air atmosphere, and (iii) ambient temperature.Au/HT oxidized a wide range of alcohols to the corresponding carbonyl compounds inhigh yields under mild reaction conditions (Table 9.13).109

In particular, the Au/HT catalyst was effective toward less reactive cyclohex-anol derivatives, producing the corresponding cyclohexanones in excellent yields. TheAu/HT catalyst was also applicable under neat conditions; for example, 1-phenylethanol(30 mmol, 3.7 g) was oxidized under solvent-free conditions to give a 93% yield of ace-tophenone (3.4 g) with 99% selectivity after 24 h, with TON and TOF of 200,000 and8300 h−1, respectively. The Au/HT catalyst could be reused without any loss of its activ-ity or selectivity. TEM images of the reused Au/HT catalyst showed that the averagesize and distribution of the Au-NPs were not significantly altered, and that no aggregatesof Au-NPs were observed. The investigation of the effect of supports on the oxidationactivity reveals that a concerted catalysis between the base sites of the inorganic supportand the Au-NPs is essential for achieving high catalytic activity in the oxidation ofalcohols. The base sites of HT may extract a proton from the hydroxyl group in thealcohol, promoting the facile formation of an Au-alcoholate species. The Au-alcoholatespecies then undergoes β-hydride elimination, yielding the carbonyl product and theAu-hydride species. The catalytic cycle is complete after reaction of the Au-hydridespecies with O2 (Figure 9.13). The in situ generated Au-hydride species and the protonat the interface between the Au-NPs and HT was applicable in the deoxygenation ofepoxides to alkenes.110


TABLE 9.13. Oxidation of various alcohols using Au/HTa

R1 R1R2 R2

OH OAu/HT

Toluene, 40 oC, air (1 atm)

Yield (%)b

+ H2O

Alcohol Time (h) Conversion (%)bEntry

3

3

6

1.5

15

12

36

24

36

8

24

8

6

24

24

72

24

99

99

98

99

91

83

93

90

93

95

85

99

95

60

81

60

36

99(97)

99(97)

98

94(94)

90

83

93

88

93

94

78

81

85

60

81

41

17

1

2

3

4

5

6

7c

8

9c

10

11d

12d

13d

14

15

16d

17d

+ 1/2 O2

OH

OH

OH

OH

OH

OH

OH

OHO

O

NOH

O

OH

OH

OH

SOH

OH

OH

Cl

OH

OH

O

O

O

O

O

Product

O

O

OO

O

NO

O

OO

SO

O

O

O

Cl

O

O

aReaction conditions: Au/HT (0.1 g, Au 0.45 mol%), alcohol (1 mmol), toluene (5 ml).bDetermined by GC or LC (liquid chlomatography). Using internal standard technique. Isolated yields areshown in parentheses.cAlcohol (0.5 mmol).dEster was formed as a by-product.


H2O2

HHO

R2R1

R2R1

O

OH

R2

R1

HT

BS

H+

Au

HT

BS

Au

HT

BS

H+

Au

H

H2O + O2

O2

Figure 9.13. A plausible mechanism for the Au/HT-catalyzed aerobic oxidation of alcohols

involving participation of Au-NPs and a basic site of HT (represented by BS).

Recently, Au-NPs stabilized in inorganic matrices have been reported. Au-NPsof 5–13 nm immobilized in an AlO(OH) matrix (Au/AlO(OH)) demonstrated highcatalytic activity for alcohol oxidation under mild reaction conditions. The oxidation ofvarious alcohols, except for primary aliphatic alcohols, proceeded at room temperatureunder 1 atm of O2 in the presence of 300 mol% of Cs2CO3. Au/AlO(OH) was alsoapplied to the coupling of ketones with alcohols (Table 9.14).111 Au-NPs have also beensuccessfully confined in the walls of mesoporous silica112 and SBA-15,113 which showedhigh selectivity for the oxidation of benzyl alcohol to benzaldehyde.

Organic Polymer-Supported Au-NPs

Tsunoyama and coworkers fabricated small Au-NPs having a mean diameter of 1.3 nmwith a narrow size distribution that were stabilized by poly(N-vinyl-2-pyrrolidone)(labeled as Au/PVP(1.3)).114,115 They investigated the size effect of the Au-NPs inthe aerobic oxidation of benzylic alcohols in water at room temperature in the pres-ence of K2CO3. Smaller Au/PVP(1.3) exhibited much higher catalytic activity thanlarger Au-NPs with a diameter of 9.5 nm stabilized by PVP (labeled as Au/PVP(9.5)).Kinetic measurements reveal that C–H bond cleavage at the benzylic position is therate-determining step, and the activation energy associated with the C–H bond cleavageusing Au/PVP(1.3) is much lower than that for Au/PVP(9.5), as well as Pd-NPs stabi-lized by PVP. The size-specific catalytic activity of Au/PVP(1.3) and Au/PVP(9.5) can


TABLE 9.14. Coupling of various ketones with alcohols using Au/AlO(OH)a

O

R1R2 OH

O O

Cl

O O

n-C5H11 n-C5H11

O

Ph

O

O

MeO

O

Ph Ph

+

90 86 70

79c 78c Tracec–e Tracec–e

Yield (%)b

Time (h)

Yield (%)b

Time (h)

Au/AlO(OH) (Au:1 mol%)

3 equivalent of Cs2CO3

24 24 24

30 30 30 30

aThe reaction was performed using 1.0 mmol of ketone and 3.0 mmol of alcohol dissolved in 3.0 ml oftoluene with 1.0 mol% Au and Cs2CO3 (3 equivalent) at 25 ◦C under O2 balloon.bIsolated yield.cAlcohol (4.0 mmol) was used.dAu (3.0 mol%) was used.eThe ratio of 3-phenylpropanol and 3-phenylpropanoate was approximately 3 : 1 as determined by GC.

be explained in the efficient activation of O2 by the small-sized Au-NPs. The effect ofthe electronic structure of the Au clusters stabilized by PVP on alcohol oxidation wasalso reported.116 X-ray photoelectron spectroscopy, Fourier transform infrared spec-troscopy of the adsorbed CO, and X-ray absorption near-edge structure measurementsrevealed that the Au clusters were negatively charged by electron donation from thePVP, and the catalytic activity increased with increasing electron density on the Au core.Based on these results and gas-phase studies of the reactivity of Au-NPs with O2,117–119

a possible reaction mechanism was proposed (Figure 9.14). O2 may be activated byAu/PVP(1.3) through electron transfer from the negatively charged Au-NPs, giving asuperoxo or peroxo-like species (O2n−) adsorbed on Au/PVP(1.3). These O2n− speciesdirectly abstract the hydrogen from alcohol to form the corresponding aldehyde. Thisproposed mechanism is much different from that suggested for Pd-NP-catalyzed alco-hol oxidation, which involves Pd-hydride formation via β-hydride elimination from aPd-alcoholate species, as described in Figure 9.9.

The role of the polymer support for the Au-NPs in the oxidation of glycerol wasrecently reported by Villa and coworkers. They prepared Au-NPs stabilized by polyvinylalcohol (PVA) (Au/PVA,withAu-NPswith amean diameter of 2.45 nm), tetrakishydrox-ypropylphosphonium chloride (THPC) (Au/THPC, with Au-NPs with a mean diameterof 2.02 nm), and citrate (Au/citrate, with Au-NPs with a mean diameter of 9.76 nm)and tested their catalytic activities in the oxidation of glycerol. The catalytic activityincreased with decreasing particle size; the turnover frequencies for Au/THPC, Au/PVA,and Au/citrate were 2478 h−1, 715 h−1, and 160 h−1, respectively. However, the sta-bility of the Au-NPs on THPC was less than those on PVA and citrate, respectively;the THPC-stabilized Au-NPs increased in size during the oxidation, while the PVA andcitrate-stabilized Au-NPs maintained their particle sizes. There is a suitable compromise


O

N

O

O

O

N+

O

-O

RCHO

O

N+

O

HHR

O

O-

-OCH2R

ON+

n

n

--

n

n

Figure 9.14. A proposed mechanism for the aerobic oxidation of alcohols by Au/PVP based

on activation of molecular oxygen.

between stability and catalytic activity in aerobic oxidation using the polymer-stabilizedAu-NPs. Au/PVA was more stable but less active than Au/THPC and Au/citrate due tothe limited accessibility of the reactants through the pores of the polymers.120

Colloidal Au has the problem of being difficult to recover from the reaction mix-ture for reuse. Kobayashi and coworkers succeeded in heterogenization of colloidalAu through the development of insoluble cross-linked polymer-incarcerated Au-NPs(PI-Au) (Scheme 9.16).121,122 The aerobic oxidation of several alcohols proceeded atroom temperature using PI-Au, and no leaching of the Au was observed during oxida-tion. However, 300 mol% K2CO3 was required to obtain high yields of the products, andthe oxidation did not proceed at all in the absence of an additional base. The PI-Au cat-alyst was reused 10 times in the oxidation of 1-phenylethanol, although longer reactiontimes were required. PI-Au exhibited a high TOF value of 20,000 h−1 in the oxidationof 1-phenylethanol, but the yield of acetophenone was low (6 %). Similar high TOFscalculated at low alcohol conversions were also seen in the case of Au/β-MnO2123 andAu/Ga3Al3O9.108

Another approach for the facile separation of polymer-supported Au-NPs from thereaction mixture is the use of thermosensitive polymer-supported Au-NPs. Kanaoka andcoworkers synthesized a star polymer consisting of 2-(2-ethoxy)ethoxyethyl vinyl ether(EOEOVE)-supportedAu-NPswith a diameter less than 4 nm.124 TheAu-NPs supportedbyEOEOVEexhibitedLCST (lower critical solution temperature)-type phase-separationbehavior in water (Scheme 9.17). Namely, after the oxidation of benzyl alcohols to the

BIMETALLIC NANOPARTICLE-CATALYZED ALCOHOL OXIDATION 319

1 + NaBH4AuClPPh3 Et2O

PI-Au

Diglyme

Coacervation

Cross-linking

No solvent150 oC, 5 h

(1) Filtration(2) Wash (CH2Cl2, H2O)

(3) Crush(4) Dry

0.06–0.08mmol g–1

No solvent150 oC, 5 h

OO

OOH

4

x y z

(x / y / z 28:34:38)Polymer 1

Scheme 9.16. Synthesis of PI-Au catalyst.

corresponding benzoic acids in water at 27 ◦C in the presence of KOH, Au/EOEOVEwas collected by filtration at a temperature higher than the cloud point, such as at 60 ◦C.The recovered Au/EOEOVE was reusable at least six times without loss of activity.TEM analysis revealed that aggregation of the Au-NPs encapsulated in the EOEOVEdid not occur during oxidation.

BIMETALLIC NANOPARTICLE-CATALYZEDALCOHOL OXIDATION

Bimetallic nanoparticles have attracted enormous attention due to their unique syner-gistic catalytic properties that cannot be obtained in monometallic nanoparticles. In theoxidation of alcohols, the efficiency of Au-NP-catalyzed aerobic oxidation was foundto be greatly advanced by involving other noble metals such as Pt and Pd. Prati andcoworkers first studied the synergistic effect between Au and Pd or Pt in the oxidationof glycerol and sorbitol. In terms of activity, selectivity, and durability, the bimetallicAu–Pd/C and Au–Pt/C catalysts were superior to the monometallic of Pd/C, Au/C,and Pt/C catalysts (Table 9.15).125,126 To investigate the bimetallic effect, single-phasebimetallic Au–Pd supported on activated carbon was synthesized and characterizedusing TEM.127,128 The high catalytic activity in the oxidation of glycerol was attributedto the synergistic effects of the alloy.

Enache and coworkers developed a highly efficient heterogeneous catalyst, TiO2-supported Au–Pd (Au–Pd/TiO2) for solvent-free oxidation of primary alcohols to alde-hydes.129 The addition of Au to Pd-NPs significantly improved the activity and selec-tivity for the production of aldehydes via oxidation of primary alcohols. For example,


CH

O

O

O

C2H5

H2C200

H2C CHO 2 O

NaBH4

O

O

O

C2H5

CHCH2

Polymer synthesis

Living cationicpolymerization

EOEOVE Poly(EOEOVE)

1

Star poly(EOEOVE)

Preparation of Au-NPs

Au ion

Star poly(EOEOVE) with Au-NPs

Au-NP catalystStar poly(EOEOVE) with Au-NPs

Substrate

Aerobic alcohol oxidation

Thermosensitivephase separation

Product

Scheme 9.17. Preparation and catalytic use of EOEOVE-supported Au-NPs.

in the oxidation of benzyl alcohol at 100 ◦C under solvent-free conditions, the use ofAu–Pd/TiO2 gave full conversion of benzyl alcohol and high selectivity (�96%) forbenzaldehyde. In contrast, both monometallic Pd/TiO2 and Au/TiO2 catalysts providedmoderate conversions. In addition, Pd/TiO2 resulted in the production of toluene andbenzene as by-products, and the use of Au/TiO2 produced large amounts of an acetalproduct. The catalytic performance (the best compromise between activity and selectiv-ity) was significantly affected by the Au–Pd ratio in Au–Pd/TiO2, and 2.5% Au–2.5%Pd/TiO2 showed the best catalytic activity among the Au–Pd/TiO2 catalysts having var-ious ranges of Au–Pd ratios.130 TEM combined with X-ray photoelectron spectroscopyshowed that 2.5% Au–2.5% Pd/TiO2 was made up of an Au-rich core with a Pd-richshell, indicating that Au electronically influences the catalytic properties of the Pd.

Notably, Au–Pd/TiO2 showed a much higher TOF, reaching 270,000 h−1 for theoxidation of 1-phenylethanol under solvent-free conditions at 160 ◦C under 1 atm of

BIMETALLIC NANOPARTICLE-CATALYZED ALCOHOL OXIDATION 321

TABLE 9.15. Comparison of catalytic activity in the oxidation of d-sorbitola

Catalyst TOF (h−1) Selectivity for gluconic/gulonic acid (%)

Au–Pd/C 154 60Au–Pt/C 149 71Au/C 54 55Pd/C 23 60b

Pt/C 62 40

aReaction conditions: d-sorbitol (0.3 M) at 50 ◦C, pH 11, sorbitol/metal = 1000, flowing O2 at atmosphericpressure (25 ml min−1).bSelectivity at 20% conversion.

O2 (Scheme 9.18), compared to the most active PdHAP and Au/CeO2 catalysts, whichexhibited high TOFs of 9800 h−1 and 12,500 h−1, respectively.

Au–Pd/TiO2 also showed awide substrate scope with high TOFs (the TOFwasmea-sured after the first 0.5 h). However, it is still not clear whether high yields of aldehydesare obtained in the Au–Pd/TiO2-catalyzed oxidation of primary alcohols (except foractive primary benzylic alcohols). Recently, the same group further demonstrated thatthe choice ofmetal oxide support also influenced the product distribution in the oxidationof benzyl alcohol using the Au–Pd bimetallic system.131 The side reaction involving dis-proportionation of twomolecules of benzyl alcohol to produce benzaldehyde and tolueneoccurred in the case of TiO2, Nb2O5, and activated carbon-supported Au–Pd-NPs. MgO-and ZnO-supported Au–Pd-NPs were superior to the above catalysts for the selectiveoxidation of benzyl alcohol, giving benzaldehyde with over 99% selectivity.

Similar to this research, Kobayashi reported that polymer-incarcerated Au–Ptalloyed bimetallic nanoparticles (PI-Pt/Au) also exhibited higher catalytic activity thansingle-metal Au or Pt-NPs for the aerobic oxidation of alcohols.132 Although PI-Aurequired a large amount of K2CO3 to promote the oxidation, PI-Pt/Au worked well with-out additional bases in water at room temperature. In this system, water was indispens-able. Kobayashi supposed that water acted as a hydrogen transporter in the hydrophobicpolystyrene surroundings, thus helping the abstraction of the �-hydrogen. The samegroup also developed carbon black-stabilized polymer-incarcerated bimetallic catalysts(PI-CB catalysts).133 Notably, PI-CB containing Au and Pt-NP catalysts (PI-CB/Au–Pt)that contained 1 : 1 ratio of Au to Pt in the alloy cluster showed high catalytic activity andselectivity in the oxidation of primary aliphatic alcohols to the corresponding aldehydesunder mild reaction conditions. For example, 1-octanol was converted to octanal in 92%

OH2.5% Au–2.5% Pd/TiO2

160 oC, O2 (1 atm), 0.5 h

(solvent-free)

O

TOF = 270,000 h–1

Scheme 9.18. Oxidation of 1-phenylethanol using Au—Pd/TiO2 under solvent-free conditions.


PI-CB/Au–Pt (2 mol%)

BTF / H2O (1 / 1), O2 (1 atm),

RT, 9 h

OH O

PI-CB/Au–Pt (1 mol%)

OH

O

OHBTF / H2O (1 / 1), 3 equivalent K2CO3

O2 (1 atm), RT, 24 h

Scheme 9.19. Aerobic oxidation of 1-octanol catalyzed by PI-CB/Au—Pt.

OHR

Oxidation

R O

Hydration

H2O OHR

OH

OHR

OOxidation

Hemiacetal formation

R'OH R OR'

OHOxidation

R OR'

O

Scheme 9.20. Two reaction pathways for oxidation of alcohols.

yield in benzotrifluoride/H2O (1 : 1) at room temperature under an atmospheric pressureof O2 in the presence of 2 mol% PI-CB/Au–Pt. This high catalytic activity could notbe obtained using monometallic PI-CB catalysts of PI-CB/Au and PI-CB/Pt; these cata-lysts barely promoted the oxidation reaction. The reactivity and selectivity were stronglydependent on the combination of metals and the solvent system. Namely, the additionof the base K2CO3 in this PI-CB/Au–Pt catalyst system dramatically changed the mainproduct from octanal to octanoic acid (94% yield) (Scheme 9.19). It is notable thatoveroxidation of 1-octanol to octanoic acid was suppressed under neutral conditions.

Furthermore, PI-CB/Au–Pd showed completely different activity and selectivitycompared with PI-CB/Au–Pt; direct oxidative methyl ester formation catalyzed by PI-CB/Au–Pd proceeded in methanol/H2O in the presence of K2CO3. A possible reactionpathway is shown in Scheme 9.20. The oxidation to carboxylic acids or esters is depen-dent on the hydration of or hemiacetal formation from the aldehyde.

Bimetallic Au–Pd-NPs stabilized by PVP have also been reported. Bimetallic 1 : 3Au–Pd-NPs had a higher catalytic activity than Au and Pd-NPs in the oxidation ofbenzyl alcohol, 1-butanol, 2-butanol, 2-buten-1-ol, and 1,4-butanediol.134 Mertens andcoworkers also studied PVP-supported Au–Pd-NPs in the oxidation of allylic, aliphatic,and benzylic alcohols.135 The optimized ratio of Au–Pd was found to be 8 : 2. Theyalso succeeded in the heterogenization of the bimetallic 8 : 2 Au–Pd-NPs by supportingthem on high-surface-area BaAl2O4. This catalyst showed high catalytic activity for theoxidation of benzyl alcohol and 1-phenylethanol, with TOFs reaching 19,910 h−1 and58,020 h−1, respectively.

SUMMARY AND FUTURE OUTLOOK

In this review, recently developed high-performance heterogeneous catalysts for theaerobic oxidation of alcohols have been explored. Several advanced heterogeneous

REFERENCES 323

catalysts including Ru, Pd, and Au active species were shown to have high activity andselectivity for the oxidation of a wide range of alcohols under an atmospheric pressureof O2 or air. These catalysts were also proven to be reusable without any loss of activityor selectivity.

This review highlights not only the state of the art with regard to high-performanceheterogeneous catalysts but also future issues in the modern aerobic oxidation of alco-hols. With respect to reaction conditions, the oxidation reactions using Au catalystsproceed at lower temperatures compared to those using Pd and Ru systems. Pd and Rucatalysts work under neutral conditions, while most Au catalysts require basic supportsor additional bases to promote oxidation. In terms of catalytic activity, most of thePd and Au catalysts show higher activity than Ru catalysts. Ru, Pd(II), and bimetallicAu–Pt catalysts efficiently promote the oxidation of primary aliphatic alcohols to thecorresponding aldehydes. With regard to the supports, organic polymers used in thesesystems are potentially susceptible to oxidative degradation under aerobic oxidationconditions, thus restricting the reaction temperature and catalyst recovery over a longperiod. Some anchor-type catalysts that are heterogenized analogues of homogeneousmetal complexes are effective and useful for easy separation and reuse, but it seemsdifficult to exceed the activity of the homogeneous metal complex precursors and toshow unique heterogeneous catalysis.

Although the catalytic performance of heterogeneous catalysts has been greatlyadvanced in the last decade, the development of more efficient heterogeneous cata-lysts that promote oxidation using lower amounts of active metals under mild reactionconditions in air without any additives and solvents is still vital. In addition, the devel-opment of heterogeneous catalysts applicable to industrial-scale synthesis of carbonylcompounds in the oxidation of alcohols remains a challenging issue. In particular, highlyefficient heterogeneous catalysts for the selective oxidation of primary alcohols (exceptfor active primary benzylic alcohols) to the corresponding aldehydes with high yield andselectivity have been highly desired. In the field of fine chemistry, catalysts that enablethe selective oxidation of complicated alcohols containing easily oxidizable functionalgroups are also required. Moreover, these heterogeneous alcohol oxidation catalystsalso have high potential for application in other reactions such as the kinetic resolutionof racemic secondary alcohols136,137 and one-pot syntheses using alcohols as startingmaterials.138–140 We believe that advances in oxidation technology that address thesechallenges will enable the greener oxidation of alcohols.

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