An insight into the Meerwein-Ponndorf-Verley reduction of α,β-unsaturated carbonyl compounds: Tuning the acid-base properties of modified zirconia catalysts

Article abstract of DOI:10.1016/j.jcat.2009.09.005

A series of catalysts consisting of ZrO₂ in pure form or modified by doping with boron or an alkaline-earth metal to enhance their acid and basic properties, respectively, were prepared and used in the Meerwein-Ponndorf-Verley reduction of cinnamaldehyde with 2-propanol, where they exhibited moderate activity and a high selectivity (up to 98%) towards cinnamyl alcohol. The catalyst preparation procedure and its modification (viz. doping with boron or an alkaline-earth metal) were found to delay crystallization and increase the number of hydroxyl groups present on the catalyst surface, and hence its catalytic activity. The surface acid and basic properties of the catalysts were determined by pyridine and carbon dioxide chemisorption, respectively. The most active sites in the studied reaction are seemingly proton (Bronsted) acid sites of medium-high strength formed by modification of the ZrO₂ with boron; however, the increased activity thus obtained is accompanied by a loss of selectivity towards cinnamyl alcohol. Modifying ZrO₂ with an alkaline-earth metal enhances its basicity, thereby reducing its catalytic activity and increasing its selectivity for the unsaturated alcohol.

Full text of DOI:10.1016/j.jcat.2009.09.005

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Journal of Catalysis 268 (2009) 79–88
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
An insight into the Meerwein–Ponndorf–Verley reduction of a,b-unsaturated
carbonyl compounds: Tuning the acid–base properties of modified zirconia catalysts
*
Francisco J. Urbano , María A. Aramendía, Alberto Marinas, José M. Marinas
Department of Organic Chemistry, University of Córdoba, Campus de Rabanales, Marie Curie Building (Annex), 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:
A series of catalysts consisting of ZrO2 in pure form or modified by doping with boron or an alkaline-earth
metal to enhance their acid and basic properties, respectively, were prepared and used in the Meerwein–
Ponndorf–Verley reduction of cinnamaldehyde with 2-propanol, where they exhibited moderate activity
and a high selectivity (up to 98%) towards cinnamyl alcohol. The catalyst preparation procedure and its
modification (viz. doping with boron or an alkaline-earth metal) were found to delay crystallization and
increase the number of hydroxyl groups present on the catalyst surface, and hence its catalytic activity.
The surface acid and basic properties of the catalysts were determined by pyridine and carbon dioxide
chemisorption, respectively. The most active sites in the studied reaction are seemingly proton (Bron-
sted) acid sites of medium–high strength formed by modification of the ZrO2 with boron; however, the
increased activity thus obtained is accompanied by a loss of selectivity towards cinnamyl alcohol. Mod-
ifying ZrO2 with an alkaline-earth metal enhances its basicity, thereby reducing its catalytic activity and
increasing its selectivity for the unsaturated alcohol.
Received 20 May 2009
Revised 26 August 2009
Accepted 6 September 2009
Available online 9 October 2009
Keywords:
Zirconia catalysts
Zirconia–Boria
Zirconia–alkaline-earth
Surface acidity
Surface basicity
Meerwein–Ponndorf–Verley reduction
Cinnamaldehyde
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
have seen a rise in research aimed at facilitating conduct of the
process in a heterogeneous phase on account of the major advan-
The reduction of carbonyl compounds by hydrogen transfer
from an alcohol is known as the ‘‘Meerwein–Ponndorf–Verley
reaction” or ‘‘MPV reaction” in Organic Chemistry. The presence
tages of operating in this way in large-scale processes [4].
The reduction of ,b-unsaturated carbonyl compounds by
a
hydrogen transfer under heterogeneous catalysis has so far been
studied in the presence of a variety of catalysts including magne-
sium hydroxides [6], magnesium oxides [7–9], hydrous and cal-
cined zirconia [7,10,11] and a wide range of active components
supported on zeolitic [3,12], mesoporous [13,14] and various other
materials [15].
of a C@C double bond conjugated with the C@O group in an
a,b-
unsaturated carbonyl compound introduces an additional dimen-
sion in the process: the chemoselective reduction of the C@O group
in the presence of the C@C bond, which leads to the formation of an
a,b-unsaturated alcohol. This selective synthesis for primary and
secondary alcohols is an important process in pharmaceutical, fra-
grance and food flavouring industries. Their preparation by cata-
lytic hydrogenation with a metal-supported catalyst is rather
difficult owing to the high reactivity of the C@C bond relative to
the carbonyl group [1] although some interesting results have been
reported [2]. However, the MPV reaction has been successfully
Although the mechanism behind these heterogeneous hydrogen
transfer processes is seemingly quite clear, there remains some
uncertainty as to the respective roles of surface acid (Lewis or
Bronsted) and/or basic sites in the catalysts. Rather than a unified
mechanism, researchers have proposed a number of them depen-
dent on the particular catalyst used in the heterogenous MPV reac-
tion. Thus, the reduction of carbonyl compounds by hydrogen
transfer has been hypothesized to occur at Lewis acid sites, basic
sites and acid–base pairs [8,16]. In fact, some suitably activated
zeolites have proved highly efficient in the hydrogen-transfer
reduction of carbonyl compounds, the mechanism behind which,
consistent with the decreased activity observed upon addition of
pyridine to the reaction medium, appears to involve Lewis acid
sites [3]. On the other hand, activity in the MPV reaction in the
presence of basic zeolites, magnesium oxides and other, essentially
basic catalysts, has been found to decrease upon addition of CO2
and benzoic acid, which suggests that the process takes place at
basic sites [17,18].
used for the selective reduction of the C@O bond in
a,b-unsatu-
rated carbonyl compounds to the corresponding unsaturated
alcohols.
Traditionally, the MPV reaction has been conducted by using a
metal (Al, Zr) alkoxide as catalyst in a homogeneous process. The
reaction mechanism involves the formation of a six-membered
cyclic intermediate where both reactants coordinate to the same
metal site in the alkoxide [3–5]. The past two decades, however,
* Corresponding author. Fax: +34 957212066.
E-mail address: FJ.Urbano@uco.es (F.J. Urbano).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.09.005

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F.J. Urbano et al. / Journal of Catalysis 268 (2009) 79–88
However, a recent study revealed that the catalytic activity in
trimethylborate) or alkaline-earth metal precursor (N for the
nitrate).
the MPV reaction decreases with increasing calcination tempera-
ture (i.e. with increasing loss of surface hydroxyl groups) in hy-
drous zirconia; this underlines the significance of proton
(Bronsted) sites in the process [11,10]. In any case, the presence
of very strong acid sites appears to reduce the selectivity via side
reactions [10,11]. In this respect, surface-modifying MgO with
chloroform has been found to seemingly block the Lewis acid sites
leading to poisoning of the catalyst and producing acid enough sur-
face OH groups (viz. Bronsted acid sites) for the reaction [19–21].
The loss of selectivity by effect of the reduction of the C@C dou-
2.2. Catalyst characterization
Gels were subjected to thermogravimetric and differential ther-
mal analysis on a Setaram Setsys 12 system, using Air at 40 mL/min
as carrier gas,
a-Al2O3 as reference material and a Pt/Pt–Rh(10%)
thermocouple. The heating rate was 10 °C/min and the tempera-
ture range 30–800 °C. The amount of gel used in each test was
ca. 17 mg.
ble bond has been ascribed to adsorption of the
carbonyl compound via the above-mentioned double bond fa-
voured by the presence of strong Lewis acids [8].
a
,b-unsaturated
The elemental analysis of the catalysts was performed on a Per-
kin–Elmer ELAN DRC-e ICP-MS instrument following digestion of
the samples in a 1:1:1 mixture of HF, HNO3 and H2O, and dilution
in 3% HNO3. Calibration samples were prepared from appropriate
atomic spectroscopy standards (PE Pure Plus, Perkin–Elmer) in
HNO3 (10 lg/mL of each metal). Calibration curves were con-
structed over the concentration range 1–100 ppb and the results
were included for a blank.
The textural properties of the solids were determined from
nitrogen adsorption–desorption isotherms obtained at liquid nitro-
gen temperature on a Micromeritics ASAP-2010 instrument. All
samples were degassed to 0.1 Pa at 110 °C prior to measurement.
Surface areas were calculated by using the Brunauer–Emmett–
Teller (BET) method [38], while pore size distribution (PSD), mean
pore diameter (DBJH) and cumulative pore volume (VBJH) were all
determined with the Barrett–Joyner–Halenda (BJH) method [39];
the adsorption branch, the amount of cylindrical pores open on
one end only and the adsorbed layer thickness were calculated
using the Halsey method.
Zirconia has proved a highly effective choice among heteroge-
neous catalysts used in the MPV reaction [11]. Zirconium oxide is
a solid with a high thermal stability and corrosion resistance in
addition to a strong amphoteric character [22]. Its textural and
acid–base properties depend largely on its synthetic procedure
and calcination temperature. Adjusting its acid–base properties is
possible by modifying its surface with sulphate ions [23–25], phos-
phate ions [26] and mixtures of other oxides; this has proved a
highly effective method for tailoring the activity of zirconia to-
wards many organic processes. Boron oxide is one of the most
widely used compounds for altering the textural and acid–base
properties of metal oxides such as Al2O3 [27], AlPO4 [28], TiO2
[29], ZrO2 [30,31] and MgO [32], which have their acid sites grown
at the expense of basic sites. On the other hand, the presence of
alkaline-earth metals in metal oxides and mixed oxides has been
found to endow the final catalyst with a basic character [33–37].
This piece of research deals with the selective reduction of cin-
namaldehyde by hydrogen transfer from 2-propanol in the pres-
ence of various solids consisting of ZrO2 in pure form or modified
to alter its acid (ZrO2–B2O3) or basic properties (ZrO2–alkaline-
earth metal). Altering the surface chemical properties of the start-
ing ZrO2 allowed us to examine the influence of acid (Bronsted vs.
Lewis) sites and basic sites on activity and selectivity in the selec-
X-ray diffraction patterns were obtained on a Siemens D5000
diffractometer equipped with a graphite monochromator and
using Cu K radiation. The 2h angle was scanned from 5° to 85°
a
with a step size of 0.02°. The average diameter of the zirconia crys-
tallites, d, was calculated from the full width at half maximum
(FWHM) of the X-ray reflection at 2h = 30.3 (tetragonal phase),
using the Scherrer equation.
tive reduction of
gen transfer from 2-propanol.
a
,b-unsaturated carbonyl compounds by hydro-
FT-Raman spectra were obtained on a Perkin–Elmer 2000 NIR
FT-Raman system equipped with a diode-pumped NdYAG laser
(9394.69 cmꢀ1) that was operated at 300 mW laser power and a
resolution of 4 cmꢀ1 throughout the 3500–200 cmꢀ1 range in order
to gather 64 scans.
2. Experimental
Surface acidity in the catalysts was determined from the FT-Ra-
man spectra for chemisorbed pyridine. The most sensitive Raman
2.1. Catalyst synthesis
vibration of pyridine is its symmetric ring breathing (mCCN) (vs,
The catalysts were obtained by thermal treatment of a hydroxy-
gel prepared by alkaline hydrolysis of the corresponding zirconium
precursors: zirconyl chloride (Merck) and zirconium propoxide
(Fluka); boron (boric acid and trimethylborate from Fluka); and
the alkaline-earth metals (magnesium, calcium, strontium and bar-
ium carbonates from Merck). The proportions of each other were
calculated for an atomic ratio Zr/B = 10 or Zr/alkaline-earth = 13
in the final catalyst.
v1, A1), which appears at 991 cmꢀ1 in liquid pyridine. The interac-
tion of pyridine with acid sites induces a shift of this band to a
higher wavenumber. Therefore, the position of the skeletal vibra-
tion band can be used to detect interactions between pyridine
and protonic weak acid sites through hydrogen bonds (996–
1008 cmꢀ1
) or its chemisorption at strong Bronsted (1007–
1015 cmꢀ1) and/or Lewis acid sites (1018–1028 cmꢀ1) on a solid
surface [40–42].
Gels were obtained by adding 5 M ammonia to a solution of
each precursor in 250 mL of milliQ water up to pH 10 under vigor-
ous stirring. The mixture was then refluxed for 24 h, vacuum-fil-
tered and washed with milliQ water to remove all chloride ions
as checked with the AgNO3 test. The solid residue was air-dried
for 24 h, suspended in isopropanol, rotated for 5 h and the solvent
removed by vacuum evaporation at 50 °C. The final texture of solid
thus obtained was a fine dust instead of a rock type solid. The
resulting hydroxygels were vacuum-calcined at 400 °C for 5 h to
obtain the final catalysts.
Chemisorbed pyridine FT-Raman measurements were made in
a Ventacon H4 environmental cell coupled to the FT-Raman instru-
ment. A nitrogen stream at 50 mL/min was saturated with pyridine
at room temperature and then flowed through a catalytic bed at
50 °C for 1 h. Then, physisorbed pyridine was removed by flowing
nitrogen at 40 mL/min until complete disappearance of the peak
for physisorbed pyridine at 991 cmꢀ1. Spectra were recorded using
a resolution of 4 cmꢀ1, a laser power of 300 mW and 64 accumula-
tions. The spectra obtained after clean-up were subsequently pro-
cessed with the software PeakFit v. 4.11 in order to determine the
components for physisorbed and chemisorbed pyridine in their
three variants (hydrogen-bonding interactions, Bronsted sites and
Lewis sites).
The name of each catalyst includes the elements forming the so-
lid and information about the zirconium precursor (O for Zr oxy-
chloride and P for Zr propoxide), boron (A for boric acid and T for

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F.J. Urbano et al. / Journal of Catalysis 268 (2009) 79–88
81
Catalyst basicity was determined using a pulse method on a
Micromeritics TDP/TPR 2900 instrument equipped with a thermal
conductivity detector. Carbon dioxide (10% in argon) was em-
ployed as the probe molecule. Before titration, the catalyst
(50 mg) was cleaned in an Ar stream at 50 ml/min at 400 °C for
30 min. This was followed by cooling to the working temperature
(50 °C). The amount of chemisorbed CO2 was determined by inject-
3. Results
3.1. Catalytic systems
3.1.1. Thermal analysis and chemical composition of the catalysts
Fig. 1 shows the FT-Raman spectra for the precursor gels of the
studied catalysts. As can be seen, all except those for Zr–O and
ZrCa–PN exhibited the typical bands for isopropanol incompletely
removed during rotaevaporation and drying, and retained in the
gels as a result. Such bands included those for the stretching vibra-
tion of C–H bonds in saturated chains below 3000 cmꢀ1 and chain
ing 100-ll CO2 pulses containing 10% argon until a constant area
signalling saturation of the catalyst surface was obtained.
2.3. Catalytic tests
vibrations for aliphatic compounds at ca. 821 cmꢀ1
.
On the other hand, by way of example, Fig. 2 shows the TGA–
DTA–DTG profiles for the solids Zr–O and Zr–P. As can be seen, they
exhibited an endothermic peak centred at ca. 115 °C that was as-
signed to the loss of water and, for the gels containing alkaline-
earth metals, also to that of CO2. In addition, the spectrum for
Zr–P exhibited an exothermic peak centred at 287 °C due to the
combustion of isopropanol retained in the gel, which, as noted ear-
lier, reflected in the FT-Raman spectra for all gels except Zr–O and
ZrCa–PN. Finally, all gels exhibited an exothermic peak centred at
ca. 440–475 °C for the Zr gels and at 500–600 °C for the Zr solids
modified with boron or an alkaline-earth metal. Such a peak, which
is known as the ‘‘glow exotherm” in the literature, was assigned to
crystallization of the gel in the monoclinic or tetragonal system,
which resulted in internal structural rearrangement.
The temperature of the ‘‘glow exotherm”, Tglow, depends on the
composition of the particular gel as presented in Table 1. Thus,
the presence of additives such as boron or an alkaline-earth metal
delayed crystallization of the gel and shifted Tglow to higher lev-
els: 473 and 442 °C for Zr–O and Zr–P, respectively, and 512,
539, 588 and 598 °C for ZrB–OA, ZrB–OT, ZrB–PA and ZrB–PT,
respectively. This was also the case with the gels containing an
alkaline-earth metal, which exhibited a markedly higher Tglow
value than those consisting for pure zirconium oxide: 506 °C for
The catalytic hydrogen-transfer reaction was conducted in a
two-mouthed flask furnished with a magnetic stirrer and a con-
denser. An amount of 0.09 mol of isopropyl alcohol was supplied
with 0.003 mol of carbonyl compound and the mixture was re-
fluxed with stirring at 1000 rpm. The reaction was started by feed-
ing 0.5 or 1 g of freshly calcined catalyst to the mixture. Several
aliquots of the mixture were withdrawn during the course of the
reaction for analysis on a Fisons Instruments GC 8000 Series gas
chromatograph furnished with a 30 m long, 0.53 mm i.d. Supelco-
wax-10 semi-capillary column and fitted to a flame ionization
detector (FID).
The reaction products obtained for the cinnamaldehyde (UAL)
reduction were the cinnamyl alcohol (unsaturated alcohol, UOL),
3-phenylpropanol (saturated alcohol, SOL) and 3-phenylpropanal
(saturated aldehyde, SAL). The initial reduction rate of cinnamalde-
hyde was calculated from the initial slope of the substrate reaction
profile at conversions below 10% and expressed in terms of catalyst
weight (g) and surface area (m2). The selectivity towards the unsat-
urated alcohol at variable conversion levels was calculated from
the following expression:
SUOL
¼ ðmol cinnamyl alcohol=mol cinnamaldehyde convertedÞ ꢁ 100:
Zr-O
Zr-P
Zr-P
ZrMg-PN
ZrB-OA
ZrB-PA
ZrCa-PN
ZrSr-PN
ZrB-OT
ZrB-PT
ZrBa-PN
3500 3000 2500 2000 1500 1000 500
3500 3000 2500 2000 1500 1000 500
Raman Shift (cm -1)
Raman Shift / cm-1
Fig. 1. FT-Raman spectra for the precursor gels of the synthesized catalysts.

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F.J. Urbano et al. / Journal of Catalysis 268 (2009) 79–88
3.1.2. Textural and structural characterization
DTG %/min (___)
1,0
Table 1 also summarizes the specific surface area (SBET, m2/g) of
the catalysts obtained from their nitrogen adsorption–desorption
isotherm. Such isotherms for all zirconium oxides studied, both
pure and modified, were type IV in the classification of Brunauer
et al. [38] and hence typical of mesoporous materials. This indi-
cates that modifying the solids with boron oxide or an alkaline-
earth metal introduces no textural changes in the resulting cata-
lyst. As can be seen from the table, solids ZrB–PA and ZrB–PT
exhibited a markedly greater surface area than all others while
ZrB–OA and Zr–P were the solids having the lowest SBET values.
Fig. 3 shows the XRD patterns for the catalysts. Using the Scher-
rer equation allowed the average crystallite size for the catalysts
exhibiting some crystallinity to be calculated (see Table 1). Both
pure zirconium oxides (Zr–O and Zr–P) crystallized in the tetrago-
nal system (2h = 30) and exhibited no bands for the monoclinic
system (2h = 29 or 2h = 30). Among the boron-containing solids,
those obtained from zirconium oxychloride (ZrB–OA and ZrB–OT)
crystallized in the tetragonal system (2h = 30), whereas those pre-
pared from zirconium propoxide (ZrB-PA and ZrB-PT) were amor-
phous. Finally, the solids containing an alkaline-earth metal were
crystalline (tetragonal) in the case of ZrMg–PN and ZrCa–PN and
essentially amorphous in the case of ZrSr–PN and, especially,
ZrBa–PN. Therefore, crystallinity in the solids decreased in
descending order of the elements in group II, from Mg to Ba.
The TGA–DTA–DTG, nitrogen adsorption and XRD results are
mutually consistent and facilitate our understanding of the process
by which the precursor gel becomes a catalyst with a certain spe-
cific surface area and crystallinity. Thus, one can assume that the
loss of specific surface area will peak when an amorphous gel crys-
tallizes and rearranges itself internally. Also, the crystallization
temperature is governed by the glow exotherm point, Tglow, which
in turn is influenced by the presence of additives and their propor-
tion in the solid. Thus, an increased amount of additive helps the
gel to stabilize and shifts Tglow to much higher values than those
for the pure zirconium oxide gels (440–470 °C) approaching
600 °C in some cases. In this respect, the results of the chemical
analyses indicate that the amount of boron present in the catalysts
obtained from zirconium oxychloride is considerably smaller than
that present in the solids prepared from zirconium propoxide; this
is consistent with the decreased Tglow value for the former (512 °C
for ZrB–OA and 539 °C for ZrB–OT) relative to ZrB–PA and ZrB–PT
(588 and 598 °C, respectively). Although the catalysts containing
an alkaline-earth metal were prepared in the same metal/Zr atomic
ratio, the increase in atomic weight as one descends in their peri-
odic group led to the percent weights in the final solids increase
from Mg to Ba; this resulted in a gradual shift in the solid crystal-
lization temperature that reflected in Tglow (506 °C for ZrMg–PN,
513 °C for ZrCa–PN, 564 °C for ZrSr–PN and 590 °C for ZrBa–PN).
Since all catalysts were vacuum-calcined at 400 °C for 5 h, we
can assume that the gels crystallizing to some extent were those
0
-10
-20
-30
-40
Zr-O
473ºC
0
-20
-40
-60
-1,5
115ºC
-4,0
0
287ºC
40
20
0
0
-10
-20
-30
Zr-P
442ºC
-1
-2
-20
115ºC
200
400
Temperature / ºC
600
Fig. 2. TGA–DTA–DTG profiles for the precursor gels of the Zr–O and Zr–P catalysts.
ZrMg–PN, 513 °C for ZrCa–PN, 564 °C for ZrSr–PN and 590 °C for
ZrBa–PN.
Table 1 also shows the results of the chemical analysis of the
studied catalysts. All boron-containing solids exhibited a lower B
content than expected. However, the amount of boron contained
in the catalysts obtained from zirconium propoxide (about 1 wt%
B) was greater than that present in the catalysts prepared from zir-
conium oxychloride (about 0.4 wt% B). This is consistent with the
increased crystallization (glow exotherm, Tglow) temperature found
in the differential thermal analysis (DTA) of the catalysts ZrB–PA
and ZrB–PT relative to ZrB–OA and ZrB–OT, which contained less
boron.
The solids containing an alkaline-earth metal exhibited a pro-
portion of Mg slightly higher than its theoretical value. This was
not the case with the solids containing Ca, Sr or Ba, which exhibited
contents increasingly departing from their theoretical counter-
parts. This discrepancy can be explained in terms of the solubility
products for the corresponding hydroxides, which increase as one
moves down the alkaline-earth group. Even though the atomic pro-
portion of Ba was lower than that of Mg, the large difference in
atomic weight between the two metals resulted in an increased
proportion of Ba in the catalyst relative to Mg; as a result, the crys-
tallization temperature for ZrBa–PN was higher than that for the
other catalysts in our series.
Table 1
Thermal properties of the precursor gels, and chemical, textural and structural properties of the final catalysts.
Catalyst
Tglow (°C)a
Theoretical composition % w/w
ICP composition% w/w
SBET (m2 gꢀ1
)
Crystallite diameter from XRD (nm)
Crystalline phase
Zr–O
Zr–P
473
442
512
539
588
598
506
513
564
590
172
140
135
170
247
240
172
154
191
209
11
11
13
10
–
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Amorphous
Amorphous
Tetragonal
Tetragonal
Amorphous
Amorphous
ZrB–OA
ZrB–OT
ZrB–PA
ZrB–PT
ZrMg–PN
ZrCa–PN
ZrSr–PN
ZrBa–PN
0.85 (B)
0.85 (B)
1.20 (B)
1.20 (B)
1.45 (Mg)
2.36 (Ca)
5.00 (Sr)
7.66 (Ba)
0.39 (B)
0.37 (B)
1.02 (B)
1.05 (B)
1.80 (Mg)
2.22 (Ca)
3.70 (Sr)
4.70 (Ba)
–
12
12
–
–
a
Temperature of the ‘‘glow exotherm” obtained from the TGA–DTA–DTG analysis of the precursor gels of the catalysts.