Chemo- and regioselective Meerwein-Ponndorf-Verley and Oppenauer reactions catalyzed by Al-free Zr-zeolite beta

Article abstract of DOI:10.1016/j.jcat.2004.05.037

Al-free Zr-beta zeolite with Si/Zr up to 75 was synthesized in a fluoride medium. The incorporation of zirconium into zeolite beta induced the formation of increased amounts of polymorph B. Lewis acid sites were predominant in the Al-free Zr-beta. Zr-zeolite beta was an excellent catalyst in the Meerwein-Ponndorf-Verley (MPV) reduction of several alkyl- and aryl-substituted cyclohexanones, with high selectivity to the corresponding alcohols. The catalyst was reusable and no leaching was detected under the reaction conditions. A prominent feature of the Zr-zeolite beta catalyst was its ability to maintain activity even in the presence of rather significant amounts of water, ≤ 9 wt%. The high catalytic ability of Zr-beta zeolites for the MPV reaction was attributed to the presence of Lewis acid sites with appropriate acid strength and to the ease of ligand exchangeability of Zr. Zr-zeolite beta had predominantly Lewis acidity with higher Lewis acid strength than that of Ti- and Sn-zeolite beta, which enabled Zr-zeolite beta to bind the carbonyl group effectively. Regeneration of the catalyst after poisoning by benzoic acid can be effected by thorough washing with 2-propanol. The sample showed good tolerance to the presence of water and pyridine.

Full text of DOI:10.1016/j.jcat.2004.05.037

Journal of Catalysis 227 (2004) 1–10
www.elsevier.com/locate/jcat
Chemo- and regioselective Meerwein–Ponndorf–Verley and Oppenauer
reactions catalyzed by Al-free Zr-zeolite beta
Yongzhong Zhu, Gaikhuan Chuah, Stephan Jaenicke ∗
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 13 February 2004; revised 11 May 2004; accepted 21 May 2004
Available online 20 July 2004
Abstract
Al-free Zr-beta zeolite with Si/Zr up to 75 was synthesized in a fluoride medium. The incorporation of zirconium into zeolite beta induced
the formation of increased amounts of polymorph B. Lewis acid sites were predominant in the Al-free Zr-beta. Zr-zeolite beta was found to
be an excellent catalyst in the Meerwein–Ponndorf–Verley (MPV) reduction of several alkyl- and aryl-substituted cyclohexanones, with high
selectivity to the corresponding alcohols. The catalyst was reusable and no leaching was detected under the reaction conditions. A prominent
feature of the Zr-zeolite beta catalyst is its ability to maintain activity even in the presence of rather significant amounts of water, up to 9 wt%.
The activity was unaffected by the presence of pyridine but was decreased by added acids. However, the poisoning effect could be easily
reversed by washing. The excellent performance of Zr-zeolite beta in the MPVO reaction is due to an appropriate Lewis acidity and the ease
of ligand exchange at the Zr active sites within the zeolite beta pore channels.
2004 Elsevier Inc. All rights reserved.
Keywords: Meerwein–Ponndorf–Verley reduction; Oppenauer oxidation; Zr-zeolite beta; Fluoride-assisted synthesis; Solid Lewis acid; Water resistance
1. Introduction
oxides, such as Al2O3 [2], hydrous ZrO2 [3,4], magnesium
oxide or phosphates [5,6], and grafted alkoxides or alkyl
complexes [7–9].
The Meerwein–Ponndorf–Verley (MPV) reduction pro-
vides a highly selective reduction of the C=O functional
group in unsaturated carbonyl compounds using secondary
alcohols as hydrogen donors [1]. The reverse Oppenauer
oxidation of alcohols is carried out with oxidants such as
furfural, benzophenone, and cyclohexanone. Both reactions
(collectively denoted as MPVO) can be catalyzed using ho-
mogeneous catalysts such as metal alkoxides. The homoge-
nously catalyzed MPVO reaction has many advantages, such
as chemoselectivity, mild reaction conditions, and ready
adaptation both in the laboratory and on a large scale. How-
ever, the need for almost stoichiometric amounts of catalyst,
the moisture sensitivity, and problems with separation limit
the practical applications. Hence, heterogeneous catalysts
have been developed for the reaction. These include metal
Al-zeolite beta has been reported to be a highly ac-
tive and regioselective catalyst for the reduction of 4-tert-
butylcyclohexanone to the thermodynamically less stable
cis-4-tert-butylcyclohexanol [10]. The high selectivity to-
ward the cis-alcohol was explained by a restricted transition
state around a Lewis-acidic aluminum atom in the straight
channels of the zeolite beta pore system. Whilst the presence
of water was important for the activation of the catalyst prior
to reaction, moisture during the reaction severely decreased
the activity of the catalyst [11]. Corma et al. [12] recently
reported that Sn-zeolite beta showed excellent activity and
selectivity in the MPV reduction of several ketones. In addi-
tion, the Sn-zeolite beta was found to be more resistant to the
presence of water in the reaction media than Ti- or Al-zeolite
beta.
Zirconium is increasingly applied as a catalyst in many
reactions due to its moderate acidity and oxidizing capa-
bilities [13]. We have found that zirconium in the form
*
Corresponding author. Fax: (65) 6779 1691.
E-mail address: chmsj@nus.edu.sg (S. Jaenicke).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.05.037
2004 Elsevier Inc. All rights reserved.

2
Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
of hydrous zirconia and zirconium 1-propoxide grafted on
supports is a good catalyst for the MPV reaction [14,15].
Besides good activity, both types of catalysts could be eas-
ily handled in the ambient environment without a need for
moisture-free conditions. In the homogeneous form, zir-
conium 1-propoxide easily undergoes hydrolysis but after
grafting on a support, the catalyst was not deactivated by
the presence of water and showed good stability even af-
ter exposure to the ambient for 48 h [15]. In contrast,
grafted aluminum 2-propoxide required stringent moisture-
free conditions for activity and was deactivated when ex-
posed to air. In the present paper, we report on the incor-
poration of zirconium into zeolite beta, combining the prop-
erties of zirconium with the shape selectivity and possibly
higher acidity offered by the zeolite. In an earlier commu-
nication [16], we have reported on the successful synthesis
of Al-free Zr-zeolite beta and showed that Zr-zeolite beta
was an excellent catalyst for the MPV reduction of 4-tert-
butylcyclohexanone. Here, we present the results of a thor-
ough characterization of Al-free Zr-beta and its application
to MPV reduction of a number of ketones and Oppenauer
oxidation. The robustness of the catalyst is tested by poi-
soning experiments, ease of regeneration, and activity after
successive cycles.
the mixture was stirred until the ethanol formed upon hy-
drolysis of TEOS was evaporated. HF was added to the
clear solution and a thick paste was formed. Finally, an
aqueous suspension of the dealuminated zeolite beta seeds
was added. The final gel composition was 1.0 SiO2:0.02–
0.005 ZrO2:0.56 TEAOH:6–10 H2O:0.56 HF. Crystalliza-
tion was carried out in a Teflon-lined stainless steel autoclave
at 140 ◦C for 10, 20, and 25 days for samples with Si/Zr
200, 100, and 75, respectively. The solid product obtained
was filtered, washed with deionized water, dried at 100 ◦C
and calcined at 580 ◦C for 4 h. The Zr-zeolites are desig-
nated Zrn, where n = Si/Zr of 75, 100, and 200.
Pure silica zeolite beta (henceforth referred to as Si-beta)
was prepared as above but without the addition of ZrOCl2.
The gel, of composition 1.0 SiO2:0.56 TEAOH:7.50 H2O:
0.56 HF, was crystallized at 140 ◦C for 10 days. After calci-
nation at 580 ◦C for 4 h, the Si-beta was added to a solution
of ZrOCl2 to give a Si/Zr ratio of 95, stirred for 4 h at
room temperature, followed by the evaporation of water. The
sample, Zr-beta (im), was dried at 100 ◦C and recalcined at
580 ◦C for 4 h.
2.3. Synthesis of Al-, Ti-, and Sn-zeolite beta
For the purpose of comparison, Al-zeolite beta (Si/Al
100), Ti-zeolite beta (Si/Ti 100), and Sn-zeolite beta (Si/Sn
125) were prepared in a fluoride medium following proce-
dures reported in Refs. [12,18,19], respectively. These sam-
ples are referred to as Al100, Ti100, and Sn125 in sub-
sequent text. A high Al-zeolite beta (Si/Al = 12.5) with
extraframework aluminum was synthesized according to
Wadlinger et. al. [20]. The sample was ion-exchanged with
1 M aqueous ammonium nitrate at 80 ◦C for 24 h. The H-
form of the zeolite was obtained by calcining at 500 ◦C for
6 h (referred to as Al-beta-500). A portion was further cal-
cined at 700 ◦C for 2 h (Al-beta-700).
2. Experimental
2.1. Preparation of zeolite beta seeds
Nanocrystalline zeolite beta seeds were synthesized fol-
lowing the procedure described in Ref. [17]. A quantity of
0.216 g of metallic Al (Goodfellow) was dissolved in 41.23 g
of tetraethylammonium hydroxide (TEAOH) (40 wt% aque-
ous solution), and 29.26 g of deionized water and 12 g
of fumed silica were added and stirred for 2 h. The mo-
lar composition of the final gel mixture was 1.0 SiO2:0.56
TEAOH:0.02 Al2O3:15 H2O. The mixture was placed in a
Teflon-lined stainless steel autoclave and kept at 140 ◦C for
72 h under autogeneous pressure. The product was sepa-
rated by centrifugation, washed with deionized water, and
dried in air at 100 ◦C. One gram of the as-made sample
was treated with 50 ml of 6 M HNO3 at 80 ◦C for 24 h to
remove the aluminum. The solid was recovered by centrifu-
gation, washed with deionized water, and dried at 100 ◦C.
ICP analysis showed the Si/Al ratio of the resulting dealu-
minated zeolite beta to be higher than 500.
2.4. Characterization of the catalysts
The surface area and pore volume were determined by
nitrogen adsorption (Quantachrome NOVA 2000). The crys-
talline phase of all samples was determined by powder X-
ray diffraction. The powder patterns were recorded on a
Siemens D5005 diffractometer (Cu anode operated at 40 kV
and 40 mA) equipped with variable slits. The diffractograms
were measured from 5 to 50◦ (2θ) using a step size of 0.02◦
and a dwell time of 1 s/step.
Infrared spectra were recorded on a Biorad Excalibur
spectrometer with a resolution of 2 cm−1. Typically, sam-
ples were pressed into self-supported wafers of 8–10 mg.
The wafer was mounted in a Pyrex IR cell with NaCl win-
dows and dried by evacuating under vacuum (10−3 mbar)
for 2 h at 300 ◦C. After cooling to room temperature, a back-
ground spectrum was recorded. Pyridine was introduced for
15 min before the system was evacuated for an hour and
the spectrum was measured at room temperature. Further
2.2. Synthesis of Zr-zeolite beta
Al-free Zr-zeolite beta with Si/Zr 75, 100, and 200
was synthesized in a fluoride medium. Tetraethylorthosil-
icate (TEOS) was hydrolyzed in an aqueous solution of
40% tetraethylammonium hydroxide (TEAOH) under stir-
ring. A solution of ZrOCl2 · 8H2O in water was added and

Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
3
IR measurements were made after evacuation at 100 and
200 ◦C.
29Si MAS NMR spectra of the samples were measured
on a Bruker DRX-400 widebore solid state spectrometer
operating at a resonance frequency of 79.46 MHz with a
spinning rate of 12 kHz, a pulse length of 3 µs, and a recyc-
ling time of 20 s. Rotors with diameter 4 mm were used and
the 29Si chemical shifts are reported relative to TMS. The
13C CP MAS NMR spectra were acquired at 100.6 MHz,
with a pulse width of 5 µs, repetition time 4 s, contact time
5 ms, and a spinning rate of 8 kHz. The carbon content of
the samples was evaluated using thermogravimetric analy-
sis (Dupont 2960). About 10 mg of the sample was heated
at 10 ◦C min−1 in 100 cm3 min−1 air flow. The elemental
composition of the samples was determined by ICP-AES
analysis after dissolution of the sample in HF. A JEOL JSM-
5200 scanning electron microscope was used to determine
size and morphology of the crystals.
XPS spectra were collected on an AXIS-His 165 Ul-
tra (Kratos Analysis) spectrometer using an AlKα X-ray
source (1486.71 eV, 400 W) at constant analyzer pass en-
ergy of 20.0 eV. Curve-fitting was carried out using nonlin-
ear (Shirley-type) least-squares fitting software (XPS-PEAK
41) to separate the overlapping peaks. Due to the charging
of the insulating samples, the binding-energy values were
referenced to the C1s line at 284.6 eV (arising from the in-
advertent carbon contamination).
Fig. 1. XRD of (a) Si-beta, (b) Zr200, (c) Zr100, and (d) Zr75 zeolite beta.
acids and bases was investigated by adding 0.37–0.52 mmol
of acetic acid, benzoic acid, or pyridine to the reaction mix-
ture.
3. Results and discussion
2.5. Catalytic reactions
3.1. Catalyst characterization
The MPV reduction was carried out in a 25 cm3 round-
bottomed flask equipped with a septum port, reflux con-
denser, and a guard tube. The reaction mixture containing
1.3 mmol of the ketone substrate and 5 g (83 mmol) of
2-propanol were placed in the flask and heated to 82 ◦C.
A quantity of 100 mg of catalyst, dried at 120 ◦C, was added
to the reaction mixture. Aliquots were removed at different
reaction times and the products were analyzed by gas chro-
matography. An internal standard, o-xylene, was added for
most of the reactions. When no internal standard was used,
the carbon mass balance was checked and found to agree to
within 2%. The identity of the products was verified by com-
paring the retention times and GC-MS spectra with that of
authentic samples. Where no authentic samples were avail-
able, NMR analysis was performed. Besides 2-propanol,
other secondary alcohols such as 2-butanol, 2-pentanol, cy-
clopentanol, cyclohexanol, and 4-methyl-2-pentanol were
used.
The Oppenauer oxidation of 4-tert-butylcyclohexanol
(cis:trans ≈ 27:73) was carried out using 2-butanone as ox-
idant. Samples of 1.3 mmol of 4-tert-butylcyclohexanol and
83 mmol of 2-butanone were reacted at 82 ◦C in the presence
of 100 mg of the dried Zr-zeolite beta catalyst.
To test the stability and activity of the catalyst upon ex-
posure to water, different amounts of water, from 0.6 to
9.1 wt%, were added to the reaction mixture. Poisoning by
The physical properties of the catalysts used are sum-
marized in Table 1. All the samples possess large specific
surface area, > 400 m2/g. The crystal size of the unseeded
pure Si-beta was in the range of 10–15 µm, while the seeded
samples were much smaller (1–2 µm). It was difficult to de-
termine the crystal size for the Al-zeolite beta (Si/Al 12.5)
synthesized according to Wadlinger et al. [20] due to aggre-
gation of particles and the rather small crystal size.
Table 1
Physical properties of catalysts
Catalyst
Si/
Surface Crystal Microporous
Total pore
volume
(cm /g)
metal area
size
(m /g) (µm)
pore volume
(cm /g)
2
3
3
Si-beta
Zr-beta (im)
Zr75
Zr100
Zr200
–
95
84
107
194
125
100
100
487
308
499
490
474
500
451
468
10–15
10–15
∼ 1
∼ 1
1–2
1–2
∼ 1
∼ 1
–
0.23
0.13
0.24
0.23
0.22
0.23
0.22
0.22
0.25
0.25
0.26
0.18
0.27
0.27
0.26
0.31
0.24
0.25
0.27
0.27
a
Sn125
Al100
Ti100
b
b
Al-beta-500
Al-beta-700
a
12.5 522
12.5 508
–
ZrOCl · 8H O impregnated on Si-zeolite beta.
2
2
b
◦
Ion-exchanged with NH NO and calcined at 500 and 700 C, respec-
4
3
tively.

4
Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
29
13
Fig. 2. Si NMR spectra of (a) Si-beta, (b) Zr200, (c) Zr100, and (d) Zr75.
Fig. 3. C NMR spectrum of Zr100.
The incorporation of Zr into the silica framework was
deduced from powder XRD and XPS measurements. The
powder XRD patterns of the pure Si-zeolite beta and Zr-
zeolite beta with Si/Zr of 75, 100, and 200 are typical for
well-crystallized zeolite beta (Fig. 1). There is a loss of crys-
tallinity with Zr substitution together with a change in the
line profile at 2θ = 7◦–9◦. The asymmetry in this peak in-
dicates the presence of two isostructures of zeolite beta. In
the pure Si-zeolite beta, the bigger maximum at ∼ 8◦ with
a shoulder at ∼ 7◦ indicates that the polymorph A consti-
tutes more than 60% of the phase [21]. With increasing Zr-
substitution, the change in the line profile indicates the pref-
erential formation of polymorph B. No peaks due to ZrO2
or any other crystalline impurity phases are seen. From the
XPS measurements, the observed binding energy of Zr3d5/2
(183 eV) is significantly higher than that of ZrO2 (182.2 eV),
but close to that of Zr in ZrSiO4 (183.3 eV). The position of
the lines is very similar to that observed for Zr in the MFI
structure [22], where it had been shown that the Zr is incor-
porated into the framework of the zeolite structure.
Thermogravimetric analysis and 13C CP MAS NMR
were performed to determine the chemical state of organic
materials filling the pores. The 13C CP MAS NMR spectrum
of the as-synthesised Zr100 is given in Fig. 3. Two peaks
were observed, one at 51.9 ppm assigned to the methylene
groups and the other at 6.5 ppm due to the methyl groups of
the TEA cations. A comparison of the chemical shifts with
those of aqueous TEAOH confirms the presence of TEA
cations in the zeolite pores. The TGA curves for Zr-zeolite
beta show three main weight losses occurring in the fol-
lowing temperature ranges: (i) 185–320◦C, (ii) 320–380◦C,
(iii) 380–500 ◦C (Fig. 4). A similar decomposition pattern
has been observed by zeolite beta synthesized in fluoride me-
dia [18,24]. The first two weight losses can be attributed to
the degradation of TEA+ balancing the F− ions and the oxi-
dation of the organic materials. The third weight loss can be
assigned to TEA+ balancing framework Zr charges. Indeed,
it is found that as the Zr content increased, the weight loss at
the high temperature range also increased proportionally.
The acidic properties of the metal-containing zeolites
were determined by IR spectroscopy following adsorption
of pyridine at room temperature and desorption at 100 ◦C
(Fig. 5). The pyridine adsorption spectra showed the pres-
ence of only Lewis acid sites on Sn- and Ti-zeolite beta.
Due to the very low metal content, the IR spectra of these
zeolites were very similar to that of silica. However, the
band at ∼ 1442 cm−1, indicative of H-bonded and Lewis-
acid bonded pyridine, was shifted to higher wavelength for
Zr100 as compared to Ti- and Sn-zeolite beta. In addition
to the bands at 1445 cm−1 and 1490 cm−1 (the latter is as-
signed to coordinately bonded pyridine and pyridinium ion),
a small band at 1545 cm−1 indicates the presence of some
Brønsted acid sites [25].
29Si MAS NMR spectra of the calcined pure Si- and
Zr-zeolite beta (Fig. 2) show resolved signals at −111.5,
112.3, 113.0, and 115.8 ppm, all in the range of Q4 (Si
with four O–Si connectivities). No Q3-resonance indicative
of (OH)Si(O–Si)3 or structural defects of connectivity was
observed. This is further proof of the high quality of the
material with very few broken connectivities in the frame-
work. The lines broaden as the amount of Zr in the zeolite
increases, which resulted in a loss of resolution. It has been
reported that for pure siliceous zeolite beta synthesized in
fluoride medium, only the resonance lines corresponding to
Q4 species occupying different crystallographic sites were
detected in the 29Si MAS NMR spectrum [18]. In compar-
ison, zeolite beta synthesized in basic medium show two
resonances, one at −110.9 ppm assigned to Q4 and the other,
assigned to Q3, at −101.6 ppm [23].

Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
5
Table 2
MPV reduction of 4-tert-butylcyclohexanone over various zeolite beta cat-
alysts
a
b
Catalyst
Conversion (%)
Selectivity (%)
cis:trans
Si-beta
0
–
–
Zr-beta (im)
Zr75
Zr100
Zr200
Sn125
Ti100
Al100
Al-beta-500
Al-beta-700
0
–
–
97.3
95.2
72.8
70.6
2.9
< 0.5
> 99
> 99
> 99
> 99
> 99
–
99:1
99:1
98:2
98:2
100:0
–
c
c
c
16.0
65.4
98
84:16
89:11
c
> 99
Reaction conditions: 5.2 mmol 4-tert-butylcyclohexanone, 83 mmol 2-
◦
propanol, 100 mg catalyst, under reflux and stirring at 82 C.
a
Conversion after 60 min.
Selectivity to the corresponding alcohol.
b
c
1.3 mmol 4-tert-butylcyclohexanone used.
3.2. Catalytic activity
Zr-zeolite beta was highly active in the MPV reduction
of 4-tert-butylcyclohexanone (Table 2). The only product
formed was the 4-tert-butylcyclohexanol. After 60 min,
> 95% conversion was achieved over Zr75 and Zr100.
Sn-zeolite beta (Si/Sn 125) was also active with 70.6%
conversion. However, both Ti-zeolite beta (Si/Ti 100) and
Al-zeolite beta (Si/Al 100) were rather inactive, with con-
versions of 2.9% and < 0.5%, respectively. Al-zeolite beta
with a higher Al content (Si/Al 12.5) was also not very
active; the conversion being 16% for the 500 ◦C-calcined
sample and 65.4% for the 700 ◦C sample. Of the two pos-
sible isomers for the product, the thermodynamically less
favored cis-4-tert-butylcyclohexanol was formed with very
high selectivity. Over Zr-zeolite beta, the cis:trans alcohol
was 99:1, while over Al-zeolite beta, the ratio was slightly
lower. The high regioselectivity of the reduction proves that
the reaction proceeds in the channels of the zeolite struc-
ture, where steric constraints force the reaction to proceed
via the less bulky transition state, as proposed by Creyghton
et al. [10] for Al-zeolite beta. The presence of the metal
atom is important as Si-zeolite beta was inactive for the re-
action. Impregnation of the Si-zeolite beta with zirconium
oxychloride to give Si/Zr 100 did not result in an active
material, indicating that isolated Zr atoms in the framework
of zeolite beta are important for activity. Further support
comes from a comparison of the activity of Zr-zeolite beta
with that of grafted Zr propoxide/SBA-15 containing non-
monomeric Zr species [15]. Despite the higher Zr loading,
10 wt%, for the grafted catalyst, only 60% conversion of
4-tert-butylcyclohexanone was achieved after 30 min as
compared to 5 min over Zr-zeolite beta.
Fig. 4. TGA of (a) Zr75, (b) Zr100, and (c) Zr200.
Corma et al. [12] demonstrated that the Sn atoms in the
zeolitic framework were most likely the catalytically ac-
tive sites in Sn-zeolite beta as they were able to coordinate
cyclohexanone and polarize the carbonyl group. Likewise,
using 4-methylcyclohexanone as probing molecule, the IR
◦
Fig. 5. IR spectra after pyridine desorption at 100 C over (a) Sn125,
(b) Ti100, (c) Zr100, and (d) Zr75.

6
Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
Table 3
Table 4
Activity of Zr-zeolite beta after 8 cycles of MPV reduction
Reduction of 4-tert-butylcyclohexanone over Zr100 with various secondary
alcohols
Catalyst
Si/Zr
Conversion
(%)
cis:trans
a
Alcohol
Conversion (%)
Selectivity (%)
cis:trans
Before
After
2-Propanol
2-Butanol
2-Pentanol
Cyclopentanol
Cyclohexanol
4-Methyl-2-pentanol
98.8
98.5
90.5
27.3
32.4
52.9
> 99
> 99
> 99
98.2
82.8
> 99
99:1
99:1
99:1
99:1
100:0
100:0
Zr100
Zr100
107
108
108
110
98.3
99.0
99:1
99:1
a
b
Zr100
107
110
95.6
98.2
Reaction conditions: 1.3 mmol 4-tert-butylcyclohexanone, 83 mmol 2-pro-
◦
panol, 50 mg catalyst, under reflux and stirring at 82 C.
a
◦
Regenerated by calcination at 580 C.
Washed with 2-propanol and reused without calcination.
Reaction conditions: 1.3 mmol 4-tert-butylcyclohexanone, 83 mmol alco-
b
◦
hol, 100 mg catalyst, under reflux and stirring at 82 C.
a
After 30 min reaction.
spectrum of adsorbed 4-methylcyclohexanone on Zr100 re-
vealed a strong interaction with the substrate [16]. Besides
the unperturbed carbonyl vibration at 1719 cm−1, a second
signal appeared at 1675 cm−1. Upon desorption at 200 ◦C,
the former signal disappeared completely, whereas the lat-
ter still had substantial intensity, indicating a strong dative
bond from the carbonyl oxygen to the Zr-centers in the ze-
olite. The shift of 45 cm−1 is comparable to that reported
for Sn-zeolite beta [12] and is considerably larger than that
observed for Ti-zeolite beta.
of these alcohols in the pores of the zeolite as well as steric
hindrance in the formation of the transition state between
4-tert-butylcyclohexanone and the alcohol.
The position of the methyl substituent in cyclohexanone
has considerable influence on the rate of MPV reduction.
The rate of reaction was fastest for cyclohexanone, fol-
lowed by 4-methylcyclohexanone, 3-methylcyclohexanone,
and 2-methylcyclohexanone. As with Sn-zeolite beta [12],
Zr-zeolite beta was able to catalyze the reduction of
2-methylcyclohexanone,unlike Al-zeolite beta, where it was
observed that the reaction did not proceed [10]. This may be
due to the bigger size of the Zr4+ and Sn4+ ions as com-
pared to Al3+, leading to greater “exposure” of the ions
in oxygen framework atoms. The variation in reaction rate
with position of the methyl group on the cyclohexanone ring
may be explained by steric hindrance posed by the methyl
group in the coordination of the carbonyl to the active site.
It was expected that when cyclohexanone has a smaller sub-
stituent group such as methyl instead of tert-butyl, the pore
constraints limiting the formation of the axially oriented
transition state would be minimized. However, the results
showed that 4-methylcyclohexanone is reduced almost ex-
clusively to the cis-alcohol (cis:trans > 99:1). This has been
observed for Ti-zeolite beta [24] and Sn-zeolite beta [12].
The results suggest that the solvent may participate in the
intracrystalline pore channels, restricting the volume so that
the more aligned cis-transition state is favored. The use of
zeolite catalysts in the liquid phase leads to confinement of
molecules in the intracrystalline volume, a subject that has
been covered well by Derouane et al. [26,27].
3.3. Reuse of Zr-zeolite beta
The Zr-zeolite beta retained good activity over several
rounds of catalytic testing (Table 3). The used catalyst was
isolated from the reaction mixture by filtration and reacti-
vated at 580 ◦C. The activity of the catalyst was found to be
slightly improved after reactivation, which can be explained
by the exposure of some active sites originally being blocked
by coke. Interestingly, the catalyst could be regenerated by
just thorough washing with 2-propanol. The conversion to
4-tert-butylcycohexanolwas still as high as 95.6% even after
eight recycle runs. From the weight of the recovered cata-
lyst after the testing, the slight drop in activity is not due
to the leaching of the zirconium active sites but the loss of
the catalyst during washing process. ICP analysis showed
that the Si/Zr remained the same as in the original catalyst,
within the experimental uncertainty. Furthermore, the filtrate
obtained from refluxing Zr100 in 2-propanol was inactive.
These results indicate that zirconium did not leach out dur-
ing reaction.
The cis- and trans-isomers of 3-methylcyclohexanol
could not be separated by gas chromatography. To ascer-
tain if only one or both isomers were formed, H NMR of
3.4. Effect of different substrates and reducing agents
1
Different reducing agents were tested using the MPV re-
duction of 4-tert-butylcyclohexanone over Zr100 (Table 4).
The linear alcohols were more effective than branched
or cyclic alcohols. The rate of reaction was fastest using
2-propanol, followed by 2-butanol and 2-pentanol. The rate
of reaction was slower with cyclopentanol and cyclohexanol.
In the latter case, some cyclohexene, formed by dehydration
of cyclohexanol, was detected. The lower activity achieved
with bulkier alcohols such as cyclopentanol, cyclohexanol,
and 4-methyl-2-pentanol may be due to hindered diffusion
the products was measured. The results show that trans-3-
methylcyclohexanol was the major isomer, with a cis:trans
ratio of 29:71. Trans-3-methylcyclohexanol, with axial hy-
droxyl, is more linear and can align better with the pore
channels of zeolite beta than the 3-cis isomer. However,
cis-3-methylcyclohexanol with both hydroxyl and methyl
groups in equatorial position is the thermodynamically sta-
ble isomer. More trans-3-methylcyclohexanol was formed
(cis:trans 21:79) when a bulkier alcohol such as 4-methyl-
2-pentanol was used as the reductant instead of 2-propanol,

Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
7
Table 5
Table 6
MPV reduction of alkyl cyclohexanones with 2-propanol over Zr100 and
grafted Zr propoxide/SBA-15
MPV reduction of different substrates over Zr100
a
Entry Substrate
Time (h) Conversion (%) Selectivity (%)
a
c
Substrate
Catalyst
Zr100
Conversion (%) cis:trans
Cyclohexanone
94.5
–
1
6
92.1
0
> 99
–
Zr(OPr)/SBA-15 59.5
Zr100 6.1
Zr(OPr)/SBA-15 3.6
Zr100 54.4
Zr(OPr)/SBA-15 94.1
Zr100 81.7
Zr(OPr)/SBA-15 98.7
4-tert-Butylcyclohexanone Zr100 98.8
Zr(OPr)/SBA-15 59.8
–
Cyclopentanone
2-Methylcyclohexanone
3-Methylcyclohexanone
4-Methylcyclohexanone
55:45
42:58
29:71
75:25
99:1
19:81
99:1
16:84
b
b
b
2
3
4
6
2-Cyclopen-1-one
Cyclohexanone
0.5
6
94.5
31.2
> 99
Reaction conditions: 1.3 mmol ketone, 83 mmol 2-propanol, 50 mg catalyst,
56.5
◦
under reflux and stirring at 82 C.
2-Cyclohexen-1-one
a
After 30 min reaction time.
After 360 min reaction time.
Only product was corresponding alcohol.
b
c
5
6
0.5
98.8
> 99
> 99
4-tert-Butylcyclohexanone
again suggesting that the size of the channels in microporous
materials plays a very important role in the stereoselectiv-
ity of the reaction. For 2-methylcyclohexanone, where the
rate of reaction was the slowest of the substituted cyclo-
hexanones, more of the thermodynamically stable trans-
alcohol was formed, with a cis:trans ratio of 45:55.
6
90
1,4-Benzoquinone
8
9
24
0
–
Dihydrocarvone
Acetophenone
For comparison, the MPV reduction of methylcyclo-
hexanones was carried out over grafted Zr(1-propoxide)/
SBA-15 containing 10 wt% Zr. Details of this catalyst
have been reported previously [15]. The grafted Zr cata-
lyst was less active than Zr-zeolite beta despite its higher
Zr loading (Table 5). As the mean pore diameter in this
catalyst is ∼ 6.5 nm, the effect of pore constraints af-
fecting the stereoselectivity of the products is expected
to be small so that the thermodynamically stable isomer
should be formed in each case. Indeed, the main iso-
mer for the MPV reduction of 4-methylcyclohexanol over
grafted Zr(propoxide)/SBA-15 is the thermodynamically
stable trans-4-methylcyclohexanone with a cis:trans ratio
of 19:81. The reduction of 3-methylcyclohexanone also led
a higher proportion of the thermodynamically stable isomer,
i.e., cis-3-methylcyclohexanol(cis:trans ratio of 75:25). Fur-
thermore, in the MPV reduction of 2-methylcyclohexanone,
slightly more trans-2-methylcyclohexanol was formed than
cis-alcohol as compared with Zr-zeolite beta.
24
24
24
67.8
84.1
> 99
10
97.6
Benzyl methyl ketone
11
89.1
> 99
4-Chloroacetophenone
b
12
13
24
24
81.2
63.1
66.9
4-Methylacetophenone
4-Methoxyacetophenone
b
< 1
14
24
0
–
The regiospecificity over Zr-zeolite beta was also ob-
served in the Oppenauer oxidation of 4-tert-butylcyclohexa-
nol using 2-butanone as the oxidant. Starting with a mix-
ture of cis- and trans-4-tert-butylcyclohexanol (cis:trans ≈
27:73), the cis-isomer was almost exclusively converted to
the corresponding ketone within 1 h, while the trans-isomer
remained essentially unreacted even after 2 h (Fig. 6). This
again indicates that the constrained pore space of Zr-zeolite
beta hinders the formation of the transition state involving
the trans-alcohol.
Benzoylcyclohexane
a
Selectivity to alcohol.
Corresponding isopropyl ether was the main byproduct.
b
less easily reduced than cyclohexanone. Conjugation be-
tween the carbonyl bond and the C=C double bond in
2-cyclohexene-1-one and 2-cyclopenten-1-one makes these
compounds more difficult to reduce than saturated ketones.
An aryl substituent as in acetophenone slowed down the
reduction compared to ketones with alkyl substituents. In
benzyl methyl ketone, a CH2 group separates the benzene
ring from the carbonyl group; this substrate was more easily
In addition to alkyl-substituted cyclohexanones, a vari-
ety of other ketones were also tested over Zr-beta with 2-
propanol as reducing agent (Table 6). Cyclopentanone is

8
Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
reduced than acetophenone. The increased flexibility leads
to a reduction of steric constraints around the carbonyl. Sub-
stituents in the aromatic ring have an effect on the conversion
and selectivity of the reaction due to inductive and reso-
nance effects. The incorporation of an electron-withdrawing
group with lone pair electrons such as Cl in the benzene ring
resulted in an enhanced rate of reaction compared to the un-
substituted acetophenone. The only product was the reduced
alcohol, 1-(4-chlorophenyl)ethanol. Although the rate of re-
duction of 4-methoxyacetophenone was similar to that for
acetophenone, very little of the alcohol was detected and the
major product was 1-(1-isopropoxylethyl)-4-methoxy ben-
zene, which resulted from the etherification of the formed
alcohol with 2-propanol. For the electron-donating methyl
substituent, the rate of reaction was enhanced over ace-
tophenone but the selectivity to the alcohol was only 68%
with the other products being due to etherification of 1-(4-
methylphenyl) ethanol and 2-propanol and dehydration of
the formed alcohol. The formation of ethers and dehydration
products can be attributed to the more acidic nature of Zr-
zeolite beta as similar products were not observed over (Zr
propoxide)/SBA-15 catalyst [15].
Fig. 6. Oppenauer oxidation of 4-tert-butylcyclohexanol with 2-butanone
over Zr100: (1) trans-4-tert-butylcyclohexanol, (2) cis-4-tert-butyl-
cyclohexanol, and (") 4-tert-butylcyclohexanone.
Dihydrocarvone and benzoylcyclohexane were not re-
duced over Zr-zeolite beta which may be due to the rigid
and bulky structure of the molecules posing steric hindrance
in the formation of the transition state.
3.5. Influence of acid, base, and moisture on the catalyst
activity
The activity of Zr-zeolite beta for the MPV reduction of
4-tert-butylcyclohexanonedecreased by 30% in the presence
of benzoic acid but was only slightly suppressed when pyri-
dine was added to the reaction medium (Fig. 7). Acetic acid
also decreased the activity of the catalyst. A similar effect
had been observed over zirconium 1-propoxide grafted on
SBA-15 where the activity decreased to one-ninth that of the
fresh catalyst [15]. However, unlike the grafted catalyst, the
effect of poisoning was completely reversible over Zr-zeolite
beta, and full activity was recovered after washing the cata-
lyst with 2-propanol. These results indicate that Zr-beta is
very stable as neither base nor acid causes the leaching of
zirconium active sites.
Fig. 7. Conversion of 4-tert-butylcyclohexanone with added base and acids:
(!) pure reactants, (") 0.37 mmol pyridine, (2) 0.52 mmol acetic acid,
(Q) 0.37 mmol benzoic acid, and (P) after benzoic acid and washing with
2-propanol.
In addition, the Zr-zeolite beta was found to show good
resistance to the presence of water (Fig. 8a) up to 9.1 wt%.
Although the rate of reaction was decreased, the conversion
of 4-tert-butylcyclohexanone was ∼ 98% after 60 min. The
turnover numbers calculated from the first 5 min of reac-
tion showed that Zr-zeolite beta retained almost 50% of its
activity in the presence of as much as 9.1% water. In con-
trast, Al- and Ti-zeolite beta were completely inactivated by
the presence of water. The activity of Sn-zeolite beta (Si/Sn
125) was more adversely affected by the presence of wa-
ter (Fig. 8b). With 9.1 wt% water in the reaction mixture,
the conversion was ∼ 10% after 60 min as compared to
over 95% under moisture-free conditions. This observation
agrees with the result of Corma et al. [12] who found that
the turnover number dropped from 109 to 3.8 upon exposure
to about 10% water content. The authors reported that the
water resistance of their Sn catalyst could be considerably
improved by hydrophobizing the surface in a postsynthe-
sis silylation step with hexamethyldisilazane. The activity
of the modified material at 10% water content was 48 mol
per mol h, or about 45% of the activity under dry conditions.
The Zr-zeolite beta synthesized for this study retained this
level of activity in the presence of water, without requiring
additional surface modification.

Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
9
4. Conclusion
Well-crystallized Al-free Zr-zeolite beta can be obtained
in a fluoride medium using nanocrystalline zeolite seeds.
Framework substitution of Zr in the zeolite beta structure
is possible up to about 1.3% (Si/Zr 75) in a seeded synthe-
sis in fluoride medium. The incorporation of Zr into zeolite
beta resulted in an active catalyst for the MPV reduction of
several ketones. The high catalytic ability of Zr-beta zeo-
lite for the MPV reaction can be attributed to the presence
of Lewis acid sites with appropriate acid strength and to the
ease of ligand exchangeability of Zr. Zr-zeolite beta has pre-
dominantly Lewis acidity with higher Lewis acid strength
than that of Ti- and Sn-zeolite beta. This appropriate medium
Lewis acidity enables Zr-zeolite beta to bind the carbonyl
group effectively. An insight into the ease of ligand ex-
changeability of Zr-zeolite beta may be drawn from the re-
tention of activity with addition of acids, base or moisture.
Regeneration of the catalyst after poisoning by benzoic acid
can be effected by thorough washing with 2-propanol. The
sample showed good tolerance to the presence of water and
pyridine.
References
[1] C.F. de Graauw, J.A. Peters, H. van Bekkum, J. Huskens, Synthesis 10
(1994) 1007.
[2] V.A. Ivanov, J. Bachelier, F. Audry, J.C. Lavalley, J. Mol. Catal. 91
(1994) 45.
[3] H. Kuno, K. Takahashi, M. Shibagaki, K. Shimazaki, H. Matsushita,
Bull. Chem. Soc. Jpn. 64 (1991) 312.
[4] H. Kuno, K. Takahashi, M. Shibagaki, K. Shimazaki, H. Matsushita,
Bull. Chem. Soc. Jpn. 63 (1990) 312.
Fig. 8. Conversion of 4-tert-butylcyclohexanone over (a) Zr100 and (b)
Sn125—(!) without water and in the presence of added water: (2)
0.6 wt%, (Q) 2.9 wt%, and (") 9.1 wt%.
[5] G. Szöllösim, M. Bartok, Appl. Catal. A 169 (1998) 263.
[6] M.A. Aramendía, V. Borau, C. Jimenez, J.M. Marina, F.J. Romero,
Catal. Lett. 58 (1999) 53.
[7] R. Anwander, C. Palm, Stud. Surf. Sci. 117 (1999) 413–420.
[8] R. Anwander, C. Palm, G. Gerstberger, O. Groeger, G. Engelhardt,
Chem. Commun. (1998) 1811.
[9] P. Leyrit, C. McGill, F. Quignard, A. Choplin, J. Mol. Catal. A 112
(1996) 395.
Zeolites synthesized in the fluoride medium is reported
to be relatively free of connectivity defects, making it more
hydrophobic [19]. Furthermore, the high water tolerance of
Zr-beta may be due to the facile ligand exchange at the Zr
active center within the zeolite framework. The retention
of activity in the presence of water suggests that adsorbed
water at the Zr centers can be exchanged for 2-propanol un-
der reaction conditions so that the MPV activity is relatively
unaffected. Water is not irreversibly adsorbed at the Lewis
acidic Zr sites. Indeed, we have shown that hydroxyl groups
in hydrous zirconia play an important role in its activity
as an MPV catalyst [14]. In contrast, aluminum-containing
catalysts such as aluminum 2-propoxide, aluminum alkox-
ides grafted on MCM-41, and Al-zeolite beta are generally
moisture-sensitive. Even traces of water can dramatically
reduce their activity. This feature limits their practical ap-
plicability. Sn-zeolite beta is water-resistant to some extent,
indicating that the ligand exchange is not as facile as for Zr-
zeolite beta.
[10] E.J. Creyghton, S.D. Ganeshie, R.S. Downing, H. van Bekkum, J. Mol.
Catal. A 115 (1997) 457.
[11] P.J. Kunkeler, B.J. Zuurdeeg, J.C. van der Waal, J.A. van Bokhoven,
D.C. Koningsberger, H. van Bekkum, J. Catal. 180 (1998) 234–244.
[12] A. Corma, M.E. Domine, S. Valencia, J. Catal. 215 (2003) 294–304.
[13] K. Tanabe, T. Yamaguchi, Catal. Today 20 (1994) 185.
[14] S.H. Liu, S. Jaenicke, G.K. Chuah, J. Catal. 206 (2002) 321.
[15] Y. Zhu, S. Jaenicke, G.K. Chuah, J. Catal. 218 (2003) 396.
[16] Y. Zhu, S. Jaenicke, G.K. Chuah, Chem. Commun. (2003) 2734.
[17] M.A. Camblor, A. Corma, A. Mifsud, J. Pérez-Pariente, A. Valencia,
Stud. Surf. Sci. Catal. 105 (1997) 341.
[18] M.A. Camblor, A. Corma, S. Valencia, J. Mater. Chem. 8 (1998) 2137.
[19] T. Blasco, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A. Martinez,
J.A. Perdigón-Melón, S. Valencia, J. Phys. Chem. B 102 (1998) 75.
[20] R.L. Wadlinger, G.T. Kerr, E.J. Rosinski, US Patent 3,308,069 (1967).
[21] J.M. Newsam, J.M.J. Treacy, W.T. Koetsier, C.B. De Gruyter, Proc. R.
Soc. London Ser. A 420 (1998) 375.
[22] B. Rakshe, V. Ramaswamy, S.G. Hegde, R. Vetrivel, A.V. Ratnasamy,
Catal. Lett. 45 (1997) 41.

10
Y. Zhu et al. / Journal of Catalysis 227 (2004) 1–10
[23] J.C. van derWaal, M.S. Rigutto, H. van Bekkum, Chem. Commun.
(1994) 1241.
[24] J.E. Hazm, P. Caullet, J.L. Paillaud, M. Soulard, L. Delmotte, Micro-
por. Mesopor. Mater. 43 (2001) 11.
[25] E.P. Parry, J. Catal. 2 (1963) 371.
[26] E.G. Derouane, J. Mol. Catal. A 134 (1998) 29.
[27] E.G. Derouane, C.J. Dillon, D. Bethell, S.B. Derouane-Abd Hamid,
J. Catal. 187 (1999) 209.