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Applied Catalysis A: General 312 (2006) 86–94

Baeyer–Villiger oxidation of cyclohexanone with hydrogen

peroxide/benzonitrile over hydrotalcites as catalysts

Cesar Jimenez-Sanchidrian *, Julia Marıa Hidalgo, Rafael Llamas, Jose Rafael Ruiz **

Departamento de Quımica Organica,Universidad de Cordoba, Campus de Rabanales, Edificio Marie Curie,

Carretera Nacional IV-A, km. 396, 14014 Cordoba, Spain

Received 7 March 2006; received in revised form 20 June 2006; accepted 20 June 2006

Available online 25 July 2006

Abstract

Various Mg/Al, Mg/Al/Sn and Mg/Al/Zr hydrotalcite-like compounds (HTs) were prepared for use as catalysts in the Baeyer–Villiger (BV)

oxidation of cyclohexanone with H2O2/benzonitrile as oxidant and benzonitrile itself as the solvent. The reaction gave water and benzamide as

byproducts, the latter can be recycled in an appropiate process. Analysis of the solids revealed that tin and zirconium are in fact incorporated into

the HT structure. The solids containing Mg/Al and Mg/Al/Sn and their calcination products (viz. Mg/Al and Mg/Al/Sn mixed oxides) were found

to be effective catalysts for the BVoxidation of cyclohexanone. The Sn solids exhibited a higher activity than a more conventional catalyst obtained

from an HT containing Mg and Al only (17% in the best case). Also, the catalyst containing tin was active when we used H2O2/acetonitrile system

as oxidant. The solids containing Zr were found to promote the decomposition of the hydrogen peroxide and hence to adversely influence the

oxidation reaction. A mechanism accounting for the experimental results is proposed. The reaction was conducted under very mild conditions (viz.

atmospheric pressure and a temperature of 70 8C), and conversions obtained were higher than 80% in some cases and 100% selectivity after 6 h.

The most active catalyst, which was that containing the largest amount of tin, retained full activity after three reuses.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Hydrotalcite; LDH; Baeyer–Villiger oxidation; Cyclohexanone; Tin; Zirconium

1. Introduction

Hydrotalcite-like compounds (HTs), also known as layered

double hydroxides, have aroused much debate over their

potential uses in various scientific areas including catalysed

organic synthesis [1–3]. The parent compound, a natural mineral

called hydrotalcite [4], is a magnesium-aluminium hydroxy-

carbonate of formula Mg6Al2(OH)16CO3�4H2O the structure of

which is similar to that of brucite except that some Mg2+ ions are

substituted by Al3+; this substitution produces layers bearing

positive charges that are neutralized by carbonate ions present in

the interlayer spacings. By replacing the magnesium, aluminium

or both cations, and the carbonate, by other ions, a large family of

compounds known as hydrotalcite-like (HT) or layered double

hydroxides (LDHs) of general formula [M(II)1�xM(III)x(OH)2]x+

(An�)x/n�mH2O can be obtained where M(II) is a divalent cation

* Corresponding author.

** Corresponding author. Tel.: +34 957 218638; fax: +34 957 212066.

E-mail address: qolruarj@uco.es (J.R. Ruiz).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.06.031

such as Mg, Cu, Ni, Co or Zn [3], M(III) a trivalent cation such as

Al, Fe, Cr, V, Mn, Ga or In [3,5,6] and An� the charge balancing

anion, which can be an inorganic or organic species of widely

variable nature [3].

There are two main types of HTs. One consists of divalent

and trivalent cations plus various interlayer anions; the other,

contains lithium and aluminium (i.e. a univalent metal and a

trivalent one) [7,8]. However, some HTs reported in the last

decade contain tetravalent metals such as tin [9], titanium [10]

or zirconium [11]; in any case, they require the presence of a

trivalent metal in addition to the divalent one as high

concentrations of the tetravalent metal cause the formation

of various phases (particularly mixed hydroxides of the di- and

tetravalent metal) in addition to the HT.

HTs containing tetravalent metals are finding interesting

uses in Catalysis. This led us to prepare HTs containing Mg, Al

and Sn or Mg, Al and Zr in variable Mg/Al/M(IV) proportions

that were characterized by using a variety of instrumental

techniques including X-ray diffraction, IR and NMR spectro-

scopies, and nitrogen porosimetry. Once synthesized and

characterized, the different solids were used as catalysts in the

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–94 87

Scheme 1. General mechanism for the Baeyer–Villiger oxidation of ketones with organic peracids.

Baeyer–Villiger oxidation of cyclohexanone. This reaction was

first reported by Baeyer and Villiger in 1899 and provides a

method for converting ketones into esters [12]; since then, it has

become a very important organic synthetic pathway, as

reflected in a vast number of books and papers including not

only specific reviews [13–17], but also articles dealing with

organic oxidation reactions at large [18,19]. At present, the

Baeyer–Villiger reaction involves the oxidation of a ketone

with an organic peroxyacid, hydrogen peroxide or an alkyl

hydroperoxide to give an ester or lactone (or an alcohol or acid

derivative). Similar reactions involving the oxidation of

aldehydes to formates or their hydrolysis products are also

included in this designation. The general mechanism for the

process involves two steps (see Scheme 1). In the first, the

peroxyacid adds to the carbonyl compound to form a so-called

Criegee adduct (1), which undergoes rearrangement to the end-

product (2) in the second. The migration of group R0 is

concerted with the cleavage of the O–O bond. Bases favour

rearrangement of the adduct by facilitating the release of the

hydroxyl proton in the intermediate. The Baeyer–Villiger

oxidation of ketones provides a number of advantages

including (a) a high tolerance of a variety of functional groups

(e.g. when the double bond is conjugated with the carbonyl

group the selectivity is towards BV oxidation, but when the

C C bond is not conjugated the selectivity depends on the

substitution of this double bond [14]), (b) the regiochemistry

can usually be controlled via the migration ability of the

different groups – in some cases (e.g. with bicyclic systems),

however, the regioselectivity of the oxygen insertion process

can be influenced by various stereoelectronic factors – , and (c)

the reaction allows the use of a wide variety of oxidants.

Usually, the BV reaction is conducted with homogeneous

catalysts; however, an increasing number of heterogeneous

catalysts has been used for this purpose over the past decade.

Also, the growing environmental concern of recent years has

led to the replacement of many organic oxidants with more

environmentally benign agents such as hydrogen peroxide. This

has indeed been the case with the Baeyer–Villiger reaction,

where a number of heterogeneous catalysts have been

effectively used in conjunction with hydrogen peroxide.

Ti-Silicalite (TS-1) was one of the catalysts that revolutio-

nized the field of organic oxidations. This solid consists of a

zeolite structure (silicalite) into which titanium is incorporated

[20]. Its high oxidizing power lies in the ability of titanium

metal sites to form Ti-peroxy species that can activate hydrogen

peroxide in various oxidation reactions including epoxidations,

ammonoxidations and CH oxidations [21,22]. These catalysts

have been further developed by inserting new oxidizing metals

into zeolite structures, albeit with poorer results than those

originally obtained with titanium. However, Corma et al.

developed a catalyst consisting of Sn incorporated into a beta

zeolite [23]. In subsequent work [24–29], Corma et al.

conducted extensive research into Baeyer–Villiger oxidation

reactions using their Sn-beta zeolite catalysts. Recently, they

proposed a mechanism for the underlying process from a

combination of theoretical and experimental data [30].

Asnotedearlier, in this workwe used HT-based heterogeneous

catalysts containing metals such as Sn and Zr in the Baeyer–

Villiger reaction of cyclohexanone with hydrogen peroxide and

benzonitrile as oxidant. Our group previously found HTs to be

effective catalysts for reactions requiring the assistance of a base

(e.g. the epoxidation of limonenewith hydrogen peroxide [31,32]

where the peroxycarboximidic acid is also the intermediate or the

Meerwein–Ponndorf–Verley reaction [33–36]). Hydrotalcite-

like compounds have also been explored by others authors in the

BV reaction, but using a mixture of molecular oxygen and

benzaldehyde [37–39] or peroxyacid as oxidant [40]. Also, a

hydrotalcite-supported SnO2 catalyst was found to effect the BV

reaction with hydrogen peroxide as oxidant in the presence of a

nitrile as oxygen transfer agent [41]. However, these authors used

Mg/Al hydrotalcite as a support for SnO2, which was the actual

catalyst, and obtained zero conversion with hydrotalcite contain-

ing no SnO2. In previous work [32], we showed the formation of

the peroxycarboximidic intermediate to require the presence of a

nitrile containing no a protons (e.g. benzonitrile). Otherwise (e.g.

with acetonitrile or butyronitrile), the epoxidation of limonene –

the first step of which (viz. the formation of a peroxycarbox-

ymidic intermediate) is similar to that for the Baeyer–Villiger

oxidation – developed to a smaller extent than it did in the

presence of benzonitrile (the conversion at 8 h was 90% with

benzonitrile, and only 20% and 22% with acetonitrile and

butyronitrile, respectively). We ascribed this result to the

formation of carbanionic species from the nitriles possessing a

protons and to the retention of such species on the hydrotalcite

surface hindering further reaction development.

2. Experimental

2.1. Preparation of hydrotalcite-like compounds

The HT containing only magnesium and aluminium

was prepared from solutions of Mg(NO3)2�6H2O and

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–9488

Table 1

Metal ratios and chemical formulae of the HTs

Catalyst Mg/[Al + M(IV)]a Al/M(IV)a Chemical formulaeb

HT-Mg/Al 1.8 – Mg0.631Al0.369(OH)2(CO3)0.185�0.65H2O

HT-Sn-1 1.8 30.8 Mg0.650Al0.339Sn0.011(OH)2(CO3)0.125�0.66H2O

HT-Sn-2 1.8 13.0 Mg0.650Al0.325Sn0.025(OH)2(CO3)0.115�0.65H2O

HT-Sn3 – – Hydrotalcite + magnesium hydroxystannate

HT-Zr1 2.2 7.7 Mg0.684Al0.280Zr0.036(OH)2(CO3)0.160�0.80H2O

HT-Zr2 2.4 3.0 Mg0.710Al0.217Zr0.073(OH)2(CO3)0.145�1.20H2O

HT-Zr3 – – Hydrotalcite + brucite

a Metal ratios as determined by ICP-MS.b Crystallization water was determined thermogravimetrically.

Al(NO3)3�9H2O in Mg(II)/Al(III) ratio = 2, using a coprecipi-

tation method described elsewhere [5]. In a typical synthetic

run, a solution containing 0.3 mol of Mg(NO3)2�6H2O and

0.15 mol of Al(NO3)3�9H2O in 250 mL of de-ionized water was

used. This solution was slowly dropped over 500 mL of a

Na2CO3 solution at pH 10 at 60 8C under vigorous stirring. The

pH was kept constant by adding appropriate volumes of 1 M

NaOH during precipitation. The suspension thus obtained was

kept at 80 8C for 24 h, after which the solid was filtered and

washed with 2 L of de-ionized water. The magnesium,

aluminium and tin or zirconium HTs with Mg(II)/[Al(III) + -

M(IV)] = 2 was obtained by following the same procedure,

using appropriate amounts of the Mg(II) and Al(III) nitrates and

Sn(IV) chloride or Zr(IV) oxychloride.

The HTs thus prepared were ion-exchanged with

carbonate to remove ions intercalated between layers. The

procedure involved suspending the solids in a solution

containing 0.345 g of Na2CO3 in 50 mL of bidistilled, de-

ionized water per gram of HT at 100 8C for 2 h. Then, each

solid was filtered off in vacuo and washed with 200 mL of

bidistilled, de-ionized water. The HTs thus obtained were

subjected to a second ion-exchange operation under the same

conditions. The exchanged Mg/Al solid was named HT-Mg/

Al and its Mg/Al/Sn or Mg/Al/Zr counterparts HT-Sn1, HT-

Sn2, HT-Sn3 and HT-Zr1, HT-Zr2 and HT-Zr3 (the former

being that with the lower Sn or Zr content) (see Table 1).

These solids were calcined at 450 8C in the air for 8 h, using

a temperature gradient of 1 8C/min.

2.2. Experimental techniques

The HTs and their calcination products obtained from them

were characterized by using various instrumental techniques.

Thus, the metal contents in the catalysts were determined by

inductively coupled plasma-mass spectrometry on a Perkin-

Elmer ICP-MS instrument under standard conditions.

X-ray diffraction (XRD) analysis was performed in all

catalysts in order to check for crystallinity. Powder patterns

were recorded on a Siemens D-5000 diffractometer using Cu

Ka radiation. Scans were performed over the 2u ranges from 58to 708, using a resolution of 0.028 and a count time of 2 s at each

point.

Fourier transform infrared (FT-IR) spectra for the HTs were

recorded over the wavenumber range from 400 to 4000 cm�1

on a Bomen MB-100 FT-IR spectrophotometer. Samples were

prepared by mixing the powdered solids with KBr (the blank).

Solid-state 27Al MAS NMR spectra were recorded at room

temperature on a Bruker ACP-400 spectrometer. A resonance

frequency of 104.21 MHz, a recycle delay of 0.1 s, and short

3 ms pulses (458 pulses of 3 ms) and a spin rate of 12 kHz were

applied with an accumulation of 3500 scans. Signals were

referred to Al(H2O)63+ (0 ppm).

Thermogravimetric analysis were performed on a Setaram

Setsys 12 instrument by heating in an argon atmosphere from

25 to 800 8C at a 10 8C/min rate.

BET surface areas, pore radii and pore volumes were

calculated from nitrogen adsorption–desorption isotherms that

were obtained at �196 8C using a Micromeritics ASAP 2010

instrument. Samples were outgassed in vacuo at 100 8C for 12 h

prior to use.

2.3. Reaction conditions

Baeyer–Villiger oxidations runs were performed at 70 8C in

a two-necks flask containing 0.012 mol of carbonyl compound,

0.098 mol of benzonitrile, 0.10 mol of hydrogen peroxide (30%

(v/v) aqueous solution) and 0.25 g of catalyst. One of the flask

necks was fitted with a reflux condenser and the other was used

for sampling at regular intervals. The system was stirred

throughout the process. Products were identified from their

retention times as measured by GC–MS analysis on an HP 5890

GC instrument furnished with a Supelcowax 30 m � 0.32 mm

column and an HP 5971 MSD instrument.

3. Results and discussion

3.1. Characterization of catalysts

3.1.1. Elemental analysis

Table 1 shows the chemical composition and empirical

formula of each solid as determined by ICP-MS. The

differences between the theoretical and experimental values

were all within the range of experimental error.

3.1.2. XRD results

Fig. 1 shows the XRD pattern for solid HT-Mg/Al. The

signals are typical of hydrotalcite [42], with tall, narrow,

symmetric peaks in the planes (0 0 3), (0 0 6), (1 1 0) and

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–94 89

Fig. 1. XRD patterns for catalyst HT-Mg/Al.

Fig. 2. XRD patterns for catalysts containing tin. Symbol (^) denotes signals

for MgSn(OH)6.

(1 1 3) and broad, somewhat less symmetric peaks in the planes

(0 0 9), (0 1 5) and (0 1 6) that are typical of layered clay

minerals. Table 2 shows the values of the lattice parameters a

and c as determined from the XRD patterns [a = 2d(1 1 0) and

c = 3d(0 0 3)].

As noted in Section 2, the Sn-containing HTs were prepared

from different amounts of tetravalent cation. As can be seen

from Fig. 2, only two such HTs could be prepared because the

solids obtained at Al/Sn ratios above 0.85:0.15 contained

various hydrotalcite phases (specifically, a cubic phase

corresponding to magnesium hydroxoystannate [43]). The

XRD patterns for the other two Sn HTs were typical of

hydrotalcite [42]. As can be seen in Fig. 2, peaks decreased with

increasing tin content; also, the bands were broader than those

for solid HT-Mg/Al, which suggests higher disorder in the

brucite layers. That tin is incorporated into the brucite layers is

supported by the fact that parameter a increased with increasing

tin content (Table 2).

The mean crystallite size for the solids was determined by

applying the Debye–Scherrer equation [44] to the (0 0 3) and

(0 0 6) diffraction lines. As can be seen from Table 2, crystallite

size decreased with increasing Sn content in the sample. The

decreased crystallinity and crystallite size observed on

increasing the proportion of tin in the solid can be ascribed

to isomorphic substitution of some Al3+ (ionic radius 0.53 A)

with Sn4+ (ionic radius 0.69 A), which distorts the brucite

layers and reduces crystallinity as a result.

Fig. 3 shows the XRD patterns for the Zr-containing HTs. As

with the Sn HTs, crystallinity decreased with increasing

proportion of Zr in the HT (Table 2) to a point where the two

phases (hydrotalcite and brucite) were resolved. Likewise,

Table 2

Lattice parameters and crystallite size for the HTs

Catalyst a (A)a c (A)b t (A)c

HT-Mg/Al 3.046 21.972 166

HT-Sn1 3.050 22.703 82

HT-Sn2 3.057 22.866 73

HT-Zr1 3.053 22.978 114

HT-Zr2 3.056 23.925 107

a Lattice parameters a and c were calculated from d(1 1 0) and d(0 0 3) planes,

respectively.b Average crystallite size as calculated from d(0 0 3) and d(0 0 6) planes,

respectively.c Crystallite size.

Fig. 3. XRD patterns for catalysts containing zirconium. Symbol (^) denotes

signals for Mg(OH)2.

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–9490

Fig. 4. XRD patterns for calcined catalysts.

Table 3

Lattice parameter for the MgO solid solution obtained by calcining the HTs at

450 8C.

Catalyst Lattice parameter of

MgO solid solution (A)

HT-Mg/Al-450 4.155

HT-Sn1-450 4.153

HT-Sn2-450 4.174

HT-Zr-1-450 4.148

HT-Zr2-450 4.193

MgO purea 4.211a

a From Ref. [38].

crystallite size decreased with increasing Zr content; this,

together with the increase in a, suggests that Zr is in fact

incorporated into the brucite layers.

Calcining Mg/Al hydrotalcite at 450 8C is reported to cause

deep structural changes leading to the formation of perfectly

crystalline periclase MgO phases [5,35] the a values for which

are smaller than those for pure MgO; this is ascribed to the

dissolution of some Al3+ in the MgO lattice to form solid

solutions. Fig. 4 shows the X-ray diffraction patterns for the

solids containing a hydrotalcite phase alone. In all cases,

periclase MgO was the sole crystal phase observed [45], the

solids differing only in line height and width, and in the value of

parameter a (Table 3).

3.1.3. FT-IR spectroscopy results

Fig. 5 shows the FT-IR spectra for solids HT-Mg/Al, HT-Sn1

and HT-Zr1—those for the other solids were similar and are not

shown. All spectra exhibited bands typical of HTs with

intercalated carbonate [46–48], i.e. a strong, broad band at ca.

3500 cm�1 assigned to torsional vibrations in physisorbed

water, structural OH group vibrations, valence vibrations

typical of OH–HO bonds and/or torsional vibrations of M–OH

bonds in hydroxycarbonates. An additional band was observed

at 3050 cm�1 that was assigned to the vibration of the hydrogen

bonds between carbonate groups and water. The band at

1630 cm�1 is typical of the deformation of water in the

interlayer region. The bands for carbonate appeared at ca.

1370 cm�1. Finally, the region below 1000 cm�1 exhibited

bands corresponding to backbone vibrations of M–O bonds.

Fig. 5. FT-IR spectra for catalysts HT-Mg/Al (a), HT-Sn1 (b) and HT-Zr1 (c).

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–94 91

Fig. 6. FT-IR spectra for catalysts HT-Mg/Al-450 (a), HT-Sn1-450 (b) and HT-

Zr1-450 (c).

Fig. 7. 27Al MAS NMR spectra for catalysts HT-Mg/Al (a) and HT-Mg/Al-450

(b).

Table 4

Specific surface area, pore volume and average pore diameter of the synthesized

solids and their calcination products

Catalyst SBET (m2/g)a Vp (mL/g)b dp (A)c

HT-Mg/Al 71 0.21 117

HT-Sn1 72 0.20 104

HT-Sn2 75 0.26 117

HT-Zr-1 123 0.30 95

HT-Zr2 150 0.24 76

HT-Mg/Al-450 107 0.29 106

HT-Sn1-450 128 0.37 95

HT-Sn2-450 145 0.38 98

HT-Zr-1-450 223 0.42 75

HT-Zr2-450 241 0.46 76

a Specific surface area.b Pore volume.c Average pore diameter.

As shown by the XRD patterns, calcination of the HTs at

450 8C destroyed the layered structure and gave a mixed metal

oxide. This resulted in rather different FT-IR spectra. Thus, as

can be seen in Fig. 6, the solids exhibited a band at ca.

1400 cm�1 typical of O–C–O vibrations in adsorbed (non-

interlayer) carbonate on the surface of the mixed oxide. The

broadband between 3800 and 3000 cm�1 is due to HO–OH2

and H2O–OH2 valence vibrations. The 1000–400 cm�1 region

is also considerably different in the calcined solids; in fact, it

exhibits the typical vibrations of the mixed oxide [48,49],

which differ from those of the hydroxide.

3.1.4. 27Al MAS NMR spectroscopy results

Fig. 7 shows the 27Al MAS NMR spectra for solid HT-Mg/

Al and the product of its calcination at 450 8C—the results for

the other HTs were similar and are not shown. The chemical

shifts obtained exclude the requirement for second-order

quadrupole correction. As can be seen from Fig. 7a, the

spectrum for solid HT-Mg/Al exhibits a single signal at ca.

9 ppm. This suggests that Al occupies octahedral sites (AlOh)

preferentially. The destruction of the layered structure and the

resulting hydroxylation by effect of calcination causes some Al

atoms to occupy tetrahedral sites (AlTd), the signal for which

appears at ca. 74 ppm (Fig. 7b).

3.1.5. Textural properties

Table 4 shows the results of the nitrogen porosimetry tests

(viz. specific surface areas, pore volumes and average pore

diameters) for the hydrotalcites and their calcination products.

The presence of zirconium cations in the solids strongly

influenced their specific surface area, which ranged from 71 to

150 m2/g for the HTs, and from 107 to 241 m2/g for their

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–9492

Fig. 8. Conversion to e-caprolactone and H2O2 uptake in the Baeyer–Villiger

oxidation of cyclohexanone using HT-Sn2 as catalyst.

calcination products. Obviously, the increased specific surface

areas obtained upon calcination are related to an increased pore

volume and a decreased average pore diameter. This is

consistent with previous results for other HTs containing metals

such as Ga or In [5,6].

3.2. Baeyer–Villiger oxidation

3.2.1. Catalytic activity

The Baeyer–Villiger oxidation of cyclohexanone is a

complex process under the reaction conditions used in this

work. It was therefore essential to identify the role played by

each reactant as a function of its nature and concentration. We

used a large excess of both benzonitrile – which additionally

acted as the solvent – and hydrogen peroxide with the sole

purpose of simplifying the kinetic equation for the process. In

these conditions the use of a low cyclohexanone/hydrogen

peroxide ratio ensured a zero-order kinetics in H2O2. A plot of

cyclohexanone concentration as a function of time at low e-

caprolactone conversions was a straight line the slope of which

was used to determine the activity of each catalyst tested. As

can be seen from Table 5, the most active catalysts were those

containing Sn. Also, the uncalcined solids were more active

than their calcined counterparts. Table 5 also shows the e-

caprolactone conversion obtained in the Baeyer–Villiger

oxidation of cyclohexanone with hydrogen peroxide and the

different catalysts studied. One immediate inference is that

introducing Sn in the hydrotalcite structure has a favourable

effect on conversion, whereas inserting Zr has the opposite

effect. In fact, Zr was found to promote the decomposition of

the hydrogen peroxide and hence to adversely influence the

oxidation reaction. Thus, using pure Zr(OH)4 resulted in a

violent H2O2 decomposition reaction. On the other hand, the

HT-Sn solids provided conversions near 100% within 24 h, and

solid HT-Mg/Al values in the region of 90% in the same time.

The selectivity towards e-caprolactone was 100% in all cases.

Table 5

Conversion to e-caprolactone in the Baeyer–Villiger oxidation of cyclohex-

anone

Catalyst Conversiona (%) ra (mmol/h)br0a ðmmol gcat=h m2

catÞc

HT-Mg/Al 71 3.0 0.042

HT-Sn1 74 3.2 0.044

HT-Sn2 83 3.6 0.048

HT-Zr-1 12 0.9 0.007

HT-Zr2 22 1.1 0.007

HT-Mg/Al-450 65 2.2 0.021

HT-Sn1-450 68 2.8 0.022

HT-Sn2-450 69 2.9 0.020

HT-Zr1-450 8 0.7 0.003

HT-Zr2-450 15 0.8 0.003

HT-Mg/Ald 35 0.9 0.013

HT-Sn2d 66 3.3 0.044

a Conversion to e-caprolactone (reaction time, 6 h); selectivity is 100% in all

cases.b Catalytic activity (reaction time, 6 h).c Catalytic activity by g/m2 of catalyst.d Reaction using acetonitrile instead of benzonitrile.

Table 5 shows the results provided by the calcined solids; as can

be seen, calcination resulted in decreased conversion.

Although our experimental conditions differed from those

used by Kaneda et al. [37–39], our conversion to lactone was

higher; as noted in the Introduction, however, these authors

used a benzaldehyde/O2 system as oxidant.

Based on the results of Pillai and Sahle-Demessie [41], for a

Baeyer–Villiger oxidation with H2O2 and acetonitrile to

develop to an adequate extent, the Mg/Al hydrotalcite must

contain some Sn on its surface; in fact, the conversion to lactone

in the absence of Sn was zero after 4 h of reaction. In order to

check beyond doubt that Sn in our catalysts had a decisive

influence on their activity in the Baeyer–Villiger reaction, we

conducted the oxidation with H2O2/acetonitrile as the oxidant.

The results thus obtained (Table 5) suggest that the use of an

Mg/Al hydrotalcite as catalyst results in substantially reduced

catalytic activity relative to one containing surface Sn. We can

therefore conclude that tin has a favourable effect on the

Baeyer–Villiger oxidation of cyclohexanone under the experi-

mental conditions used in this work.

We monitored the hydrogen peroxide uptake over a period of

24 h – which sufficed to have all cyclohexanone react – in one

reaction and found this reactant to not to be depleted.

Specifically, more than 60% of the starting H2O2 remained

in the reaction medium after 24 h, once all cyclohexanone had

been consumed (Fig. 8).

3.2.2. Catalyst reuse

We initially studied the reuse of the most active catalyst, HT-

Sn2, which was employed immediately after reacting,

following separation from the reaction mixture by filtration.

The conversion values thus obtained were rather poor, which

suggests that the active sites in the catalyst are poisoned by

products or intermediates involved in the reaction. In fact, we

used XRD spectroscopy to characterize the catalyst after the

reaction and found it to exhibit a markedly decreased

crystallinity – reflecting in broader lines – and an also

decreased specific surface area. Therefore, the reuse tests

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–94 93

Table 6

Recycling of the HT-Mg/Al and HT-Sn2 catalysts in the Baeyer–Villiger

oxidation of cyclohexanone

Recycle number Conversion (%)a

HT-Sn2 fresh 83

1b 12

1 81

2 80

3 81

a e-Caprolactone conversion (reaction time, 6 h); selectivity is 100% in all

cases.b Unactivated catalyst.

Scheme 2. Proposed mechanism for the Baeyer–Villiger reaction of cyclohexanone over HTs.

Table 7

Influence of the H2O2:cyclohexanone molar ratio in the Baeyer–Villiger

oxidation of cyclohexanone with HT-Sn2 as catalyst

H2O2:cyclohexanonea Conversion (%)b ra (mmol/h)c

8.3 83 3.6

5 48 3.0

4 45 2.7

3 41 2.6

2 37 2.0

a Molar ratio.b Conversion to e-caprolactone (reaction time, 6 h); selectivity is 100% in all

cases.c Catalytic activity (reaction time, 6 h).

required previously separating the catalyst from the reaction

mass. Once isolated, the catalyst was activated by calcination at

450 8C and subsequent rehydration in a 5% Na2CO3 solution

prior to recycling. This process converted the hydrotalcite into a

mixture of oxides that regained the hydrotalcite structure – as

checked by XRD spectroscopy – upon subjection to an aqueous

treatment exploiting their memory effect. Table 6 shows the

results obtained in the three reuse cycles; as can be seen, the

catalyst lost no activity after them.

3.2.3. Mechanism of the process

Scheme 2 depicts a plausible mechanism for the reaction.

Initially, hydrogen peroxide attacks a Bronsted basic site at the

catalyst surface to form a hydroperoxide species that

subsequently attacks benzonitrile to give a peroxycarboximidic

acid intermediate [50]. In the second step of the process, the

previous intermediate attacked cyclohexanone adsorbed at an

acid site of the catalyst to form an intermediate equivalent to the

Criegee adduct in homogeneous catalysis processes; finally, the

intermediate undergoes rearrangement to e-caprolactone, and

the peroxycarboximidic acid intermediate is transformed in

benzamide. This benzamide can be recycled to benzonitrile

using an appropriate procedure [51–54]. This mechanism

explains why the catalyst containing Sn4+ provided higher

catalytic activity and conversion values; in fact, this cation is

more acidic than Mg2+ and Al3+, so it must favour adsorption of

cyclohexanone and hence result in increased conversion values

relative to the solid containing the latter two cations.

Once the mechanism for the process was elucidated, the

Baeyer–Villiger was examined under more ‘‘realistic’’ condi-

tions in order to extrapolate the results to a large-scale scenario.

To this end, the H2O2:cyclohexanone ratio was decreased to a

level were, as can be seen from Table 7, the initial catalytic

activity was not significantly decreased. However, conversion

levelled off through exhaustion of peroxide after some time

under these conditions. We thus adopted a H2O2:cyclohexanone

ratio of 4 as optimal for the reaction.

4. Conclusion

The method used in this work to prepare tin- and zirconium-

containing hydrotalcite-like compounds allows the metal to be

incorporated into the structure as confirmed by the XRD

patterns for the solids. Thermal treatment of the resulting

LDHs at 450 8C for several hours destroys their layered

structure and produces mixed oxides of the constituent metals.

All uncalcined and calcined solids thus obtained exhibit

acceptable conversion and selectivity towards e-caprolactone

in the Baeyer–Villiger oxidation of cyclohexanone; however,

the best catalysts were found to be the Mg/Al/Sn hydrotalcites,

the activity of which increased with increasing tin content.

These results are consistent with a mechanism involving the

adsorption of hydrogen peroxide at a Bronsted basic site of the

catalyst to attack benzonitrile and form a peroxycarboximidic

acid intermediate that transfers oxygen to cyclohexanone

C. Jimenez-Sanchidrian et al. / Applied Catalysis A: General 312 (2006) 86–9494

adsorbed at a Lewis acid site of the catalyst. The most active

catalyst was found to remain active after three reuses.

Acknowledgments

The authors gratefully acknowledge funding by Spain’s

Ministerio de Educacion y Ciencia, Feder Funds and to the

Consejerıa de Innovacion, Ciencia y Empresa de la Junta de

Andalucıa.

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