Study of structure-performance relationships in Meerwein-Ponndorf-Verley reduction of crotonaldehyde on several magnesium and zirconium-based systems

Article abstract of DOI:10.1016/j.cattod.2011.10.004

Several magnesia and zirconia systems were synthesized through the sol-gel process (calcination temperature in the 175-600°C range) and tested for liquid and gas-phase Meerwein-Ponndorf-Verley reduction of crotonaldehyde with 2-propanol. In the liquid pha

Full text of DOI:10.1016/j.cattod.2011.10.004

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Catalysis Today 187 (2012) 183–190
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Catalysis Today
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Study of structure–performance relationships in Meerwein–Ponndorf–Verley
reduction of crotonaldehyde on several magnesium and zirconium-based
systems
S. Axpuac, M.A. Aramendía, J. Hidalgo-Carrillo, A. Marinas∗, J.M. Marinas, V. Montes-Jiménez,
F.J. Urbano, V. Borau
Organic Chemistry Department, University of Córdoba, Campus de Excelencia Internacional Agroalimentario, ceiA3, Marie Curie Building E-14014, Córdoba, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2011
Received in revised form 2 October 2011
Accepted 3 October 2011
Available online 1 November 2011
Several magnesia and zirconia systems were synthesized through the sol–gel process (calcination temper-
ature in the 175–600 ◦C range) and tested for liquid and gas-phase Meerwein–Ponndorf–Verley reduction
of crotonaldehyde with 2-propanol. In the liquid phase, only zirconia systems were active, probably
because carbonates and water “poison” active sites in basic magnesia. Moreover, the more surface OH
groups were present in zirconia solids, the higher the activity exhibited. As far as reactions in the gas phase
are concerned, both zirconia and magnesia solids were active, the latter exhibiting higher conversions at
the same reaction temperatures. Furthermore, for MgO solids selectivity to crotyl alcohol increases with
the reaction temperature which suggests that either new active sites were “in situ” created or the existing
ones were unblocked and made accessible to the reactants. All in all, selectivities to crotyl alcohol of ca.
62% at 16% conversion were obtained for more active systems ZrO2-200 and ZrO2-250 in the liquid phase
whereas values above 85% at 50% conversion were achieved on MgO solids in the gas-phase.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Liquid and gas-phase
Meerwein–Ponndorf–Verley (MPV)
reaction
Crotonaldehyde
2-Propanol
Selective hydrogenation
␣,␤-Unsaturated compounds
1. Introduction
as the sacrificial alcohol though Ponndorf showed that secondary
alcohols were more readily oxidizable. The process was conducted
Chemoselective reduction of the C O group in the presence of
the C C bond is a challenge, since it is thermodynamically unfa-
vored [1]. Moreover, it leads to unsaturated alcohols, widely used in
the production of pharmaceuticals, agrochemicals and fragrances.
This makes the selected reaction interesting for both Industry and
Academia. There are two main approaches to the reaction using
heterogeneous catalysis. The first one consists in the hydrogena-
tion via hydrogen gas on supported metal systems [2]. There are
several examples in the literature on the use of monometallic cat-
alysts (Pd, Ni, Pt, Rh, Au) [3–6] to carry out this reaction, platinum
being the most widely used system [2,7]. Activity and selectivity
of the process is not only influenced by the type of active metal
but also by some other features such as the metal precursor [8,9],
solvent [10–13], support or metal particle size [10,14]. The second
approach, on which the present work is focused, is the reduction
of the C O bond through hydrogen transfer from an alcohol (the
so-called Meerwein–Ponndorf–Verley process) [15]. This elegant
method dates back to the 1920s [16–18]. Initially, ethanol was used
in the homogeneous phase using aluminium alkoxides as the cat-
alysts. The generally accepted mechanism (Fig. 1A) involves the
formation of a six-membered transition state in which both the
reducing alcohol and the carbonyl compound are coordinated to
the same metal center. As can be seen in Fig. 1A, the reverse reac-
tion known as Oppenauer oxidation is also possible. In order to shift
the equilibrium to the right (MPV process), an excess of the alco-
hol is used. The secondary alcohol of choice is typically 2-propanol.
This mechanism through a six-membered transition state has also
been proven for heterogeneous catalysts [19]. In fact, a wide range
of heterogeneous catalysts (e.g. magnesia [20–23], zirconia [15,24],
hydrotalcites [25], zeolites [26–30], mesoporous materials [31,32],
etc.) have been investigated for this reaction. For a review on the
heterogeneously catalyzed process see [33,34].
Despite the quite abundant literature on the topic, it is difficult
to compare the data as a result of the different reaction conditions
(e.g. liquid or gas phase, nature of substrate, or substrate to cata-
lyst ratio). In order to cast further light on the reaction mechanism,
in the present piece of research two series of catalysts (based on
zirconia or magnesia) were obtained through the application of
the sol–gel process and subsequent calcination at different tem-
peratures. The solids were tested for selective hydrogenation of
∗
Corresponding author. Tel.: +34 957211050; fax: +34 957212066.
E-mail address: alberto.marinas@uco.es (A. Marinas).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.10.004

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S. Axpuac et al. / Catalysis Today 187 (2012) 183–190
Fig. 1. (A) Mechanism generally accepted for Meerwein–Ponndorf–Verley reaction in the homogeneous phase, on the example of the use of an aluminium alkoxide as the
catalyst. (B) Reaction network for the reduction of crotonaldehyde, emphasizing the importance of 2-butenol.
crotonaldehyde to crotyl alcohol (2-butenol, Fig. 1B) both in the liq-
uid and the gas phase using identical reaction conditions for both
MgO and ZrO2 solids.
FT-IR spectra were recorded over a wavenumber range
400–4000 cm−1 on a Bomen MB-100 FT-IR spectrophotometer.
The pellets were prepared by mixing the powdered solid with
KBr in a 5:95 (w/w) ratio. In the case of MgO-600 solid, different
DRIFT spectra were performed on the powder at several tem-
peratures in order to monitor the release of adsorbed carbonate
species.
2. Experimental
2.1. Catalyst synthesis
75 mL of zirconium propoxide (Aldrich Art. 333972) were added
to 150 mL of 1-propanol. Next, NH4OH 5N was added dropwise
until pH 10 was achieved. The gel was aged for 6 h and then fil-
tered and washed with Milli-Q water. The resulting solid was then
placed in an oven at 110 ◦C for 8 h, ground and sifted (particle size
<0.149 mm) thus obtaining the solid labelled as ZrO2-gel.
The system labelled as MgO-gel was synthesized in a similar way
but starting from a mixture of 125 mL ethanol and 6 g of magnesium
ethoxide (Fluka Art. 63055).
Calcination of ZrO2-gel or MgO-gel for 6 h in a synthetic air flow
(25 mL min−1) at different temperatures in the 175–600 ◦C range
led to the solids labelled as ZrO2-x or MgO-y where x or y denotes
the calcination temperature.
2.3. Acid–base characterization of the solids
TPD-MS experiments were carried out in an on-line device
described elsewhere [37] but using a more modern quadrupole
mass spectrometer (Thermoscientific VG Prolab Benchtop). CO2
(m/z = 44) and pyridine (m/z = 79) were the probe molecules for
basicity and acidity, respectively.
For both acidity and basicity, the experimental procedure is
similar, following the conventional steps of catalyst cleaning-
saturation with the probe molecule-cleaning of physisorbed probe
molecule and temperature ramp (monitoring the desorption
through mass spectrometry).
Therefore, prior to the adsorption of the probe molecule, the cat-
alyst (100 mg) was cleaned by passing an Ar stream (56 mL min−1
)
2.2. Textural and structural characterization of the catalysts
up to the corresponding calcination temperature of the gel (at
10 ◦C min−1) and cooling down in Ar to 50 ◦C (for basicity) or
30 ◦C (acidity). The solids were then saturated with the probe
molecule. In the case of basicity, a CO2/Ar (5% in Ar) stream
(56 mL min−1) at 50 ◦C was used whereas for acidity determina-
tion, an Ar flow of 56 mL min−1 saturated with pyridine at 30 ◦C
was utilised. Saturation of the catalyst lasted 30 min. Subsequently,
a pure Ar stream (56 mL min−1) was passed at the saturation
temperature for 30 min in order to remove any physisorbed
molecules. Once a stable line had been obtained, chemisorbed
CO2 or pyridine was desorbed by heating from saturation tem-
perature up to the corresponding calcination temperature for each
gel in a programmed fashion, at a rate of 10 ◦C min−1. The final
temperature was kept for 30 min. Selected peaks were moni-
tored through the whole process. Quantification was based on
Gels were subjected to thermogravimetric and differential
thermal analysis on a Setaram Setsys 12 system, using Air at
40 mL min−1 as carrier gas, ␣-Al2O3 as reference material and a
Pt/Pt-Rh (10%) thermocouple. The heating rate was 10 ◦C min−1 and
the temperature range 30–1000 ◦C. The amount of gel used in each
test was ca. 20 mg.
The textural properties of solids (specific surface area, pore
volume, and mean pore radius) were determined from nitrogen
adsorption–desorption isotherms at liquid nitrogen temperature
by using a Micromeritics ASAP-2010 instrument. Surface areas
were calculated by the BET method [35], while pore distributions
were determined by the BJH method [36]. Prior to measurements,
all samples were degassed to 0.1 Pa at 110 ◦C.

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185
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
5
0
B
A
435
ZrO2
MgO
0
-5
0.5
exo
-5
0.0
330
85
-10
-15
-20
-25
-30
-0.5
-1.0
-1.5
-10
-15
-20
113
200
380
400
600
800
1000
200
400
600
800
1000
Temperature (ºC)
Temperature (ºC)
Fig. 2. Thermal study of precursor gel of ZrO2 (A) and MgO (B) systems by TGA–DTA.
the 5% (v/v) CO2/Ar standard (basicity) or by injecting pulses
of variable size of a pyridine solution in cyclohexane (acid-
ity).
exotherm) is attributed to the crystallization of zirconia [38,39]. In
the case of MgO-gel, the TGA profile shows the typical endother-
mal peak centered at 380 ◦C due to the transformation of Mg(OH)2
into MgO [40]. In this case, no exothermal peaks due to the organic
residues coming from the precursor were detected, evidencing a
better removal of ethoxide groups during the hydrolysis and sub-
sequent washing of the gel with Milli-Q water. The fact that the
loss of weight (28%) is below the value expected for the Mg(OH)2
to MgO conversion (31%) suggests that there is no complete dehy-
droxylation of Mg(OH)2 during the calcination. On the other hand,
the weight loss in Zr-gel (ca. 18.7%) is much higher than the typi-
cally found for zirconia gels (12.7% maximum corresponding to the
change from ZrO(OH)2 to ZrO2) as a result of the relatively high
quantity of organic residues in the gel coming from the precursor
[41].
Fig. 3 represents the evolution of surface area (as determined
by the BET method) as a function of the calcination temperature of
the gels. Results are consistent with the above-mentioned TG–DTA
profiles. Therefore, starting with the ZrO2-x systems there is no
substantial change in surface area between 175 ◦C and 300 ◦C. For
higher temperatures (300–600 ◦C) there is a progressive decrease
in surface area, though the decrease is sharper between 400 and
500 ◦C as a consequence of the crystallization of the gel. In the case
of MgO-y solids, there is a maximum in surface area at 400 ◦C which
2.4. Meerwein–Ponndorf–Verley reaction
The MPV reaction was conducted in both the liquid and the gas
phase. Tests in the liquid phase were performed in a two-mouthed,
round-bottom flask one mouth of which was used to introduce a
0.5 M solution of crotonaldehyde in isopropyl alcohol and 0.5 g of
catalyst. The flask was fitted with a reflux condenser and placed in
an ethyleneglycol bath that was kept at 100 ◦C throughout the reac-
tion. The reaction medium was shaken in a continuous manner for
8 h and 0.2 mL aliquots were withdrawn from it at different times
during the process and passed through a nylon filter of 0.45 ␮m
pore size prior to analysis.
Tests in the gas phase involved using 100 mg of catalyst in a
cylindrical reactor 10 mm in diameter that was placed in a tubular
oven equipped with a 6-segment temperature controller.
Prior to reaction, the gel was in situ calcined at the desired
temperature (300–600 ◦C) in a synthetic air stream (25 mL min−1).
Temperature was ramped from 20 ◦C up to the calcination temper-
ature which was finally kept for 6 h. Then, the catalyst was allowed
to cool down to 125 ◦C. The reaction was started by replacing the
synthetic air stream with a nitrogen stream flowing at 50 mL min−1
and carrying a 0.5 M solution of crotonaldehyde in isopropyl alco-
hol. The solution was injected at a rate of 2 g/h via a Bronkhorst
High-Tech liquid mass flow controller and evaporated at 130 ◦C in a
CEM mixer/evaporator (Bronkhorst High-Tech). Reaction was per-
formed between 125 ◦C and the calcination temperature of each gel
400
Maximum weight loss of
MgO gel at 380ºC (from TG-DTA)
350
ZrO2
MgO
at a heating rate of 1.56 ◦C min−1
.
300
250
200
150
100
50
Product analysis was carried out on a Fisons Instruments
GC 8000 Series gas chromatograph furnished with a 30 m long,
0.53 mm i.d. Supelcowax-10 semi-capillary column and fitted to a
flame ionization detector (FID). A calibration plot for each product
was constructed from commercially available standards. Moreover,
reaction products were confirmed by GC–MS.
Glow exotherm
at 435ºC (from
TG-DTA)
3. Results and discussion
TGA–DTA profiles of ZrO2-gel and MgO-gel are represented in
Fig. 2A and B, respectively. In the case of ZrO2-gel there are three
peaks. The first endothermic peak centered at 113 ◦C corresponds to
the loss of water and is followed by an exothermic peak with a max-
imum at 330 ◦C which can be assigned to the oxidation of organic
residues (i.e. propoxide coming from the zirconium precursor).
Finally, the sharp exothermal peak at 435 ◦C (the so-called glow
0
150 200 250 300 350 400 450 500 550 600 650
Calcination temperature (ºC)
Fig. 3. Evolution of BET surface area of ZrO2-x and MgO-y systems as a function of
calcination temperature.

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S. Axpuac et al. / Catalysis Today 187 (2012) 183–190
B
A
MgO
ZrO2
MgO- 300
MgO- 400
MgO- 500
MgO- 600
ZrO-300
2
ZrO-400
2
ZrO-500
2
ZrO-600
2
4000 3500 3000 2500 2000 1500 1000
Wavenumbers (cm-1)
4000 3500 3000 2500 2000 1500 1000
Wavenumbers (cm-1)
Fig. 4. FT-IR spectra of ZrO2-x (A) and MgO-y (B) solids.
corresponds to the maximum weight loss in TGA profile (Fig. 2B).
From that temperature, surface area drops again.
Fig. 4 allows us to evidence the progressive loss of surface OH
groups in both MgO and ZrO2 gels as the calcinations temperature
rises (see the decrease in the band associated to bridging OH groups
at ca. 3400 cm−1 in both systems and the isolated OH groups at ca.
3700 cm−1 in MgO solids). Such a loss of surface hydroxyl groups is
much more significant in the case of ZrO2-x solids as compared to
MgO-y ones (compare ZrO2-600 and MgO-600 solids). Again, this is
consistent with TGA–DTA results which suggests that in the case of
MgO gel the transformation of Mg(OH)2 into MgO was not complete
at 600 ◦C.
Furthermore, Fig. 4 also shows the presence of adsorbed CO2 in
the systems. In the case of ZrO2-x solids there are three significant
bands centered at ca. 1610, 1551 and 1412 cm−1, respectively. The
first two bands could be related to coordinated water due to the
“scissor” bending mode of molecular water, and to an atmospheric
constituent CO2 adsorbed on the gel forming bicarbonate species.
The third band could be attributed to bidentate carbonates formed
by the “side-on” coordination of atmospheric constituent CO2 on
coordinatively unsaturated O−2–Zr4+ pairs [42]. The fact that all
three bands decrease in intensity with calcinations temperature
confirms their nature (adsorbed water and/or CO2).
PyZrO2400
PyMgO400
CO2MgO400
CO2ZrO2400
Py-ZrO2
CO2-MgO
CO2-ZrO2
Py-MgO
100
200
300
400
400
temperature (ºC)
Fig. 5. TPD-MSprofiles of preadsorbedCO2 (basicity) and pyridine (acidity) for MgO-
400 and ZrO2-400.
In the case of MgO systems, two main bands, centered at 1432
20
15
10
5
80
70
60
50
40
30
20
10
0
and 1632 cm−1 can be observed in the region below 2000 cm−1
.
ZrO2
Mg–O stretching is known to appear at 1400–1441 cm−1 which
could account for the former band whereas the latter one could
be assigned to bidentate carbonate. In fact, such species shows
a symmetric O–C–O stretching at 1320–1340 cm−1 and an asym-
metric O–C–O stretching at 1610–1630 cm−1. Nevertheless, given
the width of such bands the contribution of some other carbonate
species to the spectra (e.g. unidentate carbonate and bicarbon-
ates) cannot be ruled out [43–45]. It is also worth noting that
bands of carbonate species are more prominent in MgO solids as
compared to ZrO2 systems (especially at high calcinations temper-
atures) which suggests the higher basicity of the former systems.
Results obtained for acid–base characterization of the systems
calcined in the 300–600 ◦C range expressed in terms of total acidity
and basicity are summarized in Table 1. ZrO2-x solids are predom-
inantly acid solids (the density of acid sites is greater than that of
basic ones) whereas the opposite holds true for MgO-y systems. In
the case of ZrO2-x solids the density of acid sites decreases as the
temperature rises whereas the maximum basicity corresponds to
ZrO2-400. As for MgO-y systems, the solid exhibiting the highest
density of acid and basic sites is MgO-400 solid.
molar conversion t=8h(%)
selectivity to crotyl alcohol t=8h(%)
0
175
200
250
300
350
400
calcination temperature (ºC)
Fig. 6. Results obtained for liquid-phase selective hydrogenation of crotonalde-
hyde to crotyl alcohol on ZrO2-x systems. Reaction conditions: 0.5 M solution of
crotonaldehyde in isopropyl alcohol and 0.5 g of catalyst. Reflux temperature.