Delaminated layered double hydroxides as catalysts for the Meerwein-Ponndorf-Verley reaction

Article abstract of DOI:10.1016/j.apcata.2013.11.007

The Meerwein-Ponndorf-Verley (MPV) reaction involves the hydrogen-transfer reduction of a carbonyl compound by an alcohol used as hydrogen donor. The process is catalysed by both homogeneous and heterogeneous catalysts. The latter can be of the acid or basic type. In this work, we used delamination and ion-exchange in combination to obtain delaminated solids from layered double hydroxides (LDHs). Three Mg/Al LDHs (Mg/Al ratio = 2) containing carbonate, hydroxyl or dodecylbenzenesulphonate (DBS) as interlayer anion were prepared using a coprecipitation method or by rehydration of the solid obtained by calcinations of carbonate containing LDH. The X-ray diffraction patterns for LDHs exhibited the typical lines for LDHs intercalated with carbonate, hydroxyl or DBS ions. The DBS-containing LDH was delaminated by sonicating at 60 C a suspension of this solid in 1-butanol. The DBS ions in the delaminated LDH could be exchanged by nitrate or hydroxyl ions by stirring a suspension of this delaminated LDH in 1-butanol containing an appropriate amount of Ca(NO ₃)₃ of Ca(OH)₂, respectively. The resulting delaminated solids consisted of brucite-like nanolayers excess charge in which was countered by dodecylbenzenesulphonate, nitrate or hydroxyl ions. Some of these solids exhibited moderate catalytic activity in the MPV reaction of benzaldehyde with 2-propanol. The reaction was conducted at 82 C (0.06 mol of 2-propanol was treated with 0.003 mol of benzaldehyde in the presence of 1 g of catalyst). Calcining at 450 C the delaminated LDHs, however, provided solids with a high surface activity that made them highly effective catalysts for the reaction. The catalyst obtained by calcining delaminated LDH containing hydroxyl ions provided the best catalyst (100% conversion after 2 h of reaction) and has been used in the MPV reaction of other aldehydes with excellent results.

Full text of DOI:10.1016/j.apcata.2013.11.007

Applied Catalysis A: General 470 (2014) 311–317
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Delaminated layered double hydroxides as catalysts for the
Meerwein–Ponndorf–Verley reaction
Julia M. Hidalgo, César Jiménez-Sanchidrián, José Rafael Ruiz∗
Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, Carretera Nacional IV-A, km. 396, 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 8 July 2013
Received in revised form 25 October 2013
Accepted 4 November 2013
Available online 11 November 2013
The Meerwein–Ponndorf–Verley (MPV) reaction involves the hydrogen-transfer reduction of a carbonyl
compound by an alcohol used as hydrogen donor. The process is catalysed by both homogeneous and
heterogeneous catalysts. The latter can be of the acid or basic type. In this work, we used delamina-
tion and ion-exchange in combination to obtain delaminated solids from layered double hydroxides
(LDHs). Three Mg/Al LDHs (Mg/Al ratio = 2) containing carbonate, hydroxyl or dodecylbenzenesulphonate
(DBS) as interlayer anion were prepared using a coprecipitation method or by rehydration of the solid
obtained by calcinations of carbonate containing LDH. The X-ray diffraction patterns for LDHs exhibited
the typical lines for LDHs intercalated with carbonate, hydroxyl or DBS ions. The DBS-containing LDH was
delaminated by sonicating at 60 ◦C a suspension of this solid in 1-butanol. The DBS ions in the delami-
nated LDH could be exchanged by nitrate or hydroxyl ions by stirring a suspension of this delaminated
LDH in 1-butanol containing an appropriate amount of Ca(NO3)3 of Ca(OH)2, respectively. The resulting
delaminated solids consisted of brucite-like nanolayers excess charge in which was countered by dodecyl-
benzenesulphonate, nitrate or hydroxyl ions. Some of these solids exhibited moderate catalytic activity
in the MPV reaction of benzaldehyde with 2-propanol. The reaction was conducted at 82 ◦C (0.06 mol
of 2-propanol was treated with 0.003 mol of benzaldehyde in the presence of 1 g of catalyst). Calcining
at 450 ◦C the delaminated LDHs, however, provided solids with a high surface activity that made them
highly effective catalysts for the reaction. The catalyst obtained by calcining delaminated LDH containing
hydroxyl ions provided the best catalyst (100% conversion after 2 h of reaction) and has been used in the
MPV reaction of other aldehydes with excellent results.
Keywords:
Hydrotalcite
Layered double hydroxide
Delamination
Hydrogen transfer
Meerwein–Ponndorf–Verley
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
region – which can vary widely in nature and be either inor-
ganic or organic. x, which denotes the ratio M(II)/[M(II) + M(III)],
Layered double hydroxides (LDHs) are compounds belong-
ing to the anionic clay family. All are structurally identical with
hydrotalcite [Mg6Al2(OH)16CO3·4H2O]. This mineral in turn is sim-
ilar to brucite [Mg(OH)2], where Mg2+ ions occupy octahedral
positions and form infinite layers connected by hydrogen bonds.
In hydrotalcite, some Mg2+ ions have been substituted by Al3+
ions, which introduces a charge deficiency that is offset by car-
bonate ions present in the interlayer region between octahedral
layers [1]. However, the magnesium can also be substituted by
other trivalent ions or even divalent ones and the interlayer ion
can differ in nature. Therefore, the general formula of LDHs is
typically ranges from 0.17 to 0.33 [1]. Calcining LDHs at 450 ◦C
produces mixed oxides – of the Mg(Al)O type from hydrotalcite –
that are often basic and useful as catalysts for condensation [2–4],
Baeyer–Villiger [5–8] or reduction reactions [9–11], and also as
metal supports [12–15]. In recent years, LDHs have also been used
to obtain discrete brucite-like layers by delamination. One of the
purposes of delaminating an LDH is to increase the accessibility of
species to its brucite-like layers, which is highly restricted by the
small size of the interlayer cavities. Delamination provides molec-
ularly thick nanolayers [16] with ample free space for attack, which
expands the catalytic scope of the solids. The procedure is usu-
ally applied to LDHs intercalated with an organic anion such as
dodecylsulphate and involves attack with an appropriate solvent
[17–20].
[M(II)1−xM(III)x(OH)2]x+[Ax/m
]
m−·nH2O, where M(II) and M(III) are
a divalent and a trivalent metal, respectively, lying at octahedral
positions of Mg2+ in brucite-like layers and A is the interlayer
The reduction of unsaturated organic compounds with an
organic molecule instead of hydrogen gas or a metal hydride as
hydrogen donor, which is known as “hydrogen transfer”, is widely
documented [21]. Such is the case with the catalytic transfer of
∗
Corresponding author. Tel.: +34 957 218638; fax: +34 957 212066.
E-mail address: qo1ruarj@uco.es (J.R. Ruiz).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.11.007

312
J.M. Hidalgo et al. / Applied Catalysis A: General 470 (2014) 311–317
Centrifugation of the translucent solution provided the solid named
MgAl-DBS-DEL.
Al(iPrO)3
CH3
CH
OH
CH3
+
R
R
R´
R´
CH
OH
C
O
2.3. Ion-exchange of MgAl-DBS-DEL
CH3
C
O
CH3
DBS ions in the delaminated LDH were replaced by treating
the above-described translucent solution with one of Ca(NO3)2 or
Ca(OH)2 to introduce nitrate or hydroxyl ions, respectively, in 1-
butanol. In a typical run, an amount of 1 g of undelaminated LDH
containing DBS anion (i.e. solid MgAl-DBS) was dispersed in 100 mL
of 1-butanol and heated at 70 ◦C under sonication for 2 days. Then,
the dispersion was allowed to cool and supplied with 20 mL of 1-
butanol containing 0.085 mol of Ca(NO3)2 or Ca(OH)2. The mixture
was kept under vigorous stirring at room temperature for 24 h,
which caused the formation of a precipitated solid that was iden-
tified as the calcium salt of DBS – therefore, Ca2+ ions acted as
scavengers for DBS ions and facilitated their exchange with nitrate
or hydroxyl ions in octahedral layers. The remainder was a translu-
cent solution that remained stable over time. Finally, the solution
was decanted and centrifuged to isolate the solid. The resulting
solids were designated MgAl-NO3-DEL and MgAl-OH-DEL.
R = Hydrocarbon chain
R´= Hydrocarbon chain or Hydrogen
Scheme 1. General scheme for the Meerwein–Ponndorf–Verley reduction using
Al(iPrO)3 as catalyst.
hydrogen known as the Meerwein–Ponndorf–Verley (MPV) reac-
tion [22]. The MPV reaction allows the highly selective reduction
of aldehydes and ketones under mild conditions in the presence of
a metal alkoxide such as aluminium isopropoxide as catalyst (see
Scheme 1). However, the catalyst must be used in a large excess
in order to obtain acceptable yields and removing excess alkoxide
once the reaction has completed can be rather difficult. In recent
years, a variety of acid and basic heterogeneous catalysts have been
successfully used to avoid the need to remove the alkoxide from
the reaction mass. Especially prominent among such catalysts are
magnesium oxide [23–27] and calcined layered double hydroxides
[28–32].
2.4. Calcination of LDHs
In this work, we delaminated an LDH intercalated with dode-
cylbenzenesulphonate (DBS) ion and subsequently exchanged DBS
with nitrate or hydroxyl ions to obtain catalysts for use in the MPV
reaction. The starting solid was an Mg/Al LDH with a metal ratio
of 2 – which was previously found to be that providing the most
active catalysts for this reaction [33]. The results obtained with
the delaminated solids were compared with those provided by
undelaminated LDHs containing carbonate or hydroxyl interlayer
anions, calcination of which gave Mg(Al)O mixed oxides that were
successfully used in MPV reactions in previous work [28,29,31–33].
Selected laminated and delaminated solids were calcined at
450 ◦C in the air for 8 h, using a temperature gradient of 1 ◦C/min.
The resulting catalysts were named by adding 450 to the des-
ignation of the precursor; thus, the solid obtained by calcining
MgAl-CO3 was designated MgAl-CO3-450.
2.5. Characterization techniques
The LDHs and their calcination products were characterized by
using various instrumental techniques.
Elemental analysis of the solids was carried out by inductively
coupled plasma-mass spectrometry on a Perkin–Elmer ICP-MS
instrument under standard conditions.
All solids were subjected to X-ray diffraction analysis in order to
check formation of LDH phases. Powder patterns were recorded on
a Siemens D-5000 diffractometer using CuK␣ radiation. Scans were
performed over the 2Â range 5–70◦, using a resolution of 0.02◦ and
a count time of 2 s at each point.
2. Experimental
2.1. Preparation of layered double hydroxides
Three Mg/Al LDHs containing carbonate, hydroxyl or dodecyl-
benzenesulphonate (DBS) as interlayer ion were prepared. All three
had an Mg/Al ratio of 2. The carbonate- and DBS-containing LDHs
were prepared by using a coprecipitation method described else-
where [34]. The method involves the dropwise addition, in nitrogen
atmosphere, under vigorous stirring of an aqueous solution con-
taining 0.2 mol of Mg(NO3)2·9H2O and 0.1 mol of Al(NO3)3·9H2O to
another, aqueous solution of sodium carbonate or dodecylbenzene-
sulphonate. The pH of the mixture was kept at 10 by addition of 1 M
NaOH. The resulting suspensions were heated at 80 ◦C for 24 h, fil-
tered and washed with distilled, decarbonated water several times.
The solids thus obtained from the carbonate- and DBS-containing
LDHs were designated MgAl-CO3 and MgAl-DBS, respectively. The
hydroxyl-containing LDH was prepared from MgAl-CO3. Calcining a
carbonate-containing LDH at 450 ◦C destroys its layered structure;
the process, however, can be reversed by rehydration because LDHs
have a memory effect [1]. Our MgAl-CO3 was calcined at 450 ◦C in
a nitrogen atmosphere and then rehydrated at 100 ◦C by passing a
nitrogen stream containing water vapour through it for 72 h. The
resulting solid was designated MgAl-OH.
Thermogravimetric analysis was performed on a Setaram Setsys
12 instrument by heating in an argon atmosphere from 25 to 800 ◦C
at 10 ◦C/min.
BET surface areas were calculated from nitrogen
adsorption–desorption isotherms obtained at −196 ◦C on
a
Micromeritics ASAP 2010 instrument. Samples were outgassed
in vacuo at 100 ◦C for 12 h prior to use. The amount of CO2
chemisorbed on each solid was measured on a Micromeritics
2900 TPD/TPR analyser. Prior to analysis, samples were heated
at 450 ◦C in argon stream for 1 h. Measurements were made at
room temperature by alternately passing argon an the same gas
containing 5% CO2 over each sample; the amount of chemisorbed
CO2 was calculated as the difference between the first adsorption
peak (physisorbed plus chemisorbed CO2) and the arithmetic mean
of the adsorption and desorption peaks. Basicity was assessed
under the assumption that one molecule of CO2 was adsorbed at
one basic site. The number of basic sites found was thus a measure
of basicity.
2.2. Delamination of MgAl-DBS
2.6. Reaction conditions
The DBS-containing LDH was delaminated by sonicating at 60 ◦C
a suspension containing 1 g of solid in 250 mL of 1-butanol until it
acquired a translucent appearance that remained stable over time;
at that point, an LDH is seemingly completely delaminated [35,36].
The Meerwein–Ponndorf–Verley reaction was conducted in a
two-necked flask furnished with a condenser and a magnetic stir-
rer. 2-Propanol (60 mmol) was treated with 3 mmol of the aldehyde

J.M. Hidalgo et al. / Applied Catalysis A: General 470 (2014) 311–317
313
Table 1
Metal ratios and chemical formulae of the HTs.
LDH
Mg/Ala
Chemical formulaeb
MgAl-CO3
MgAl-DBS
MgAl-OH
2.13
1.97
2.13
Mg0.681Al0.319(OH)2(CO3)0.160·0.62H2O
Mg0.663Al0.327(OH)2(C12H23C6H4SO3)0.168·0.15H2O
Mg0.681Al0.319(OH)2(OH)0.320·0.71H2O
a
Metal ratios as determined by ICP-MS.
Crystallization water was determined thermogravimetrically.
b
and the reaction mixture heated at reflux temperature with stirring
(1000 rpm). The reaction was started by introducing 1 g of freshly
calcined catalyst. The reaction products were analysed by GC–MS,
using an HP 5890 gas chromatograph furnished with a Supelcowax
30 m × 0.32 mm column and equipped with an HP 5971 MSD instru-
ment using helium as carrier gas and an injector temperature of
250 ◦C. Each chromatographic run was performed with an amount
of sample of 0.1 ␮L and the following temperature gradient: (1)
80 ◦C for 5 min, (2) 80–150 ◦C ramp at 10 ◦C/min and (3) 150 ◦C for
20 min. Under these conditions, the retention time for benzalde-
hyde and benzyl alcohol was 6.84 and 5.18 min, respectively. To
present the results we have used the conversion and selectivity
values defined as:
mol of converted aldehyde
mol of initial aldehyde
Conversion (%) =
Selectivity (%) =
× 100
mol of obtained alcohol
mol of obtained alcohol + mol of subproducts
× 100
3. Results and discussion
3.1. Characterization of catalysts
Table 1 shows the metal ratios and empirical formulae of
the three synthetic LDHs. As can be seen, the experimental
ratios were quite consistent with the theoretical value [2]. The
calcination–rehydration process had no effect on the metal ratio,
which was thus identical for MgAl-OH and MgAl-CO3.
Fig. 1. XRD patterns for synthesized LDHs.
delaminated, the suspension becomes translucent, which can be
checked by centrifuging it and analysing the solid by XRD spec-
troscopy. Fig. 2 shows the XRD pattern for the centrifuged solid,
which was named MgAl-DBS-DEL. As can be seen, the solid exhib-
ited no baseline reflection, which confirms that the LDH structure
was completely destroyed. There was, however, a halo at low 2Â
values that was assigned to dispersion of disordered aggregates of
exfoliated nanolayers [35] in addition to a signal at 2Â = 60◦ due to
the 11 side of a hexagonal two-dimensional unit cell ca. 30.4 nm in
size suggesting that two-dimensional layers in the LDH preserved
their crystal order after delamination.
Once the procedure was checked to completely delaminate the
DBS-containing LDH, we exchanged it with nitrate and hydroxyl
ions. Fig. 2 shows the XRD patterns for the two new solids, named
MgAl-NO3-DEL and MgAl-OH-DEL, respectively. As can be seen,
ion-exchange caused no relamination of the materials. A qualitative
elemental analysis of the exfoliated solids revealed the complete
absence of organic matter and sulphur, and hence that they were
completely ion-exchanged.
As noted earlier, calcining an Mg/Al LDH provides a basic
Mg(Al)O mixed oxide. Fig. 3 shows the XRD patterns for the solids
MgAl-CO3-450, MgAl-OH-450, Mg/Al-OH-DEL-450 and Mg/Al-
NO3-DEL-450. All exhibited the typical signals for a periclase MgO
phase [1].
The solids were characterized in textural and surface chemical
terms via N2 physisorption and CO2 chemisorption tests. Table 2
shows the specific surface area as determined with the BET method,
and the number of basic sites, for each calcined solid. As can
Fig. 1 shows the XRD patterns for the LDHs. That for MgAl-CO3
exhibited the typical lines for an LDH with intercalated carbon-
ate ions and hence for natural hydrotalcite [37]. Calcination at
450 ◦C destroyed the layered structure and led to the formation
of an Mg(Al)O mixed oxide structurally identical with periclase
MgO [1]. Rehydration of the oxide under the above-described con-
ditions restored the original LDH structure, albeit with hydroxyl as
interlayer ion. The XRD pattern for this solid (MgAl-OH) was also
similar to that for natural hydrotalcite; lines, however, were taller
and narrower than those for MgAl-CO3 by effect of its increased
crystallinity. Finally, the pattern for MgAl-DBS was similar to that
reported in the literature [38]. The shift to higher 2Â values in the
(0 0 l) reflections was assigned to the presence of DBS ions in the
interlayer region. The 2Â value for the (0 0 3) reflection was used
to calculate the lattice distance (viz. the sum of one brucite-like
layer and one interlayer spacing), which was obviously dependent
on the nature and orientation of the interlayer ion and the degree of
hydration of the LDH [1]. The distance was 0.762 nm for MgAl-CO3,
0.768 nm for MgAl-OH and 3.12 nm for MgAl-DBS.
As noted under Experimental, all LDHs were delaminated in 1-
butanol. This solvent was chosen on the grounds of its increased
boiling point relative to water and hence of its efficiently remov-
ing all interlayer water molecules hindering delamination of the
LDHs [16,17]. This is seemingly the key to effective exfoliation of
LDHs. 1-Butanol molecules gradually enter the interlayer region
with the aid of ultrasound and cause it to swell to a point where
discrete layers are separated. Once the LDH has been thoroughly

314
J.M. Hidalgo et al. / Applied Catalysis A: General 470 (2014) 311–317
MgAl-NO3-450-DEL
MgAl-OH-450-DEL
MgAl-OH-450
MgAl-CO3-450
20
40
60
2θ (º)
Fig. 3. XRD patterns for calcined LDHs.
low specific surface area relative to them. Consistent with reported
results [28], calcination at 450 ◦C invariably increased surface areas.
The increase was especially marked in the mixed oxides obtained
from the delaminated solids. As regards basic sites, delamination
and subsequent calcination led to the formation of more basic solids
(particularly MgAl-OH-450).
Fig. 2. XRD patterns for delaminated LDHs.
Table 2
Specific surface area, of the synthetized solids and their calcination products and
number of basic sites of the calcined solids.
3.2. Meerwein–Ponndorf–Verley reaction
Solid
SBET (m2/g)a
nb (␮mol CO2)b
All synthetic LDHs and their calcination products were tested as
catalysts in the MPV reaction of benzaldehyde with 2-propanol as
hydrogen donor. In previous work [28], we found the MPV reactions
of aldehydes and ketones in the presence of mixed oxides obtained
from Mg/Al hydrotalcites to exhibit a linear correlation between the
natural logarithm of the concentration of the carbonyl compound
and the reaction time (i.e. the reaction to be first-order in the alde-
hyde or ketone concentration). Fig. 4 shows the temporal variation
of the conversion to benzyl alcohol with the four most active cata-
lysts. As can clearly be seen, the slopes of the curves for the catalysts
obtained from delaminated solids were greater than those for the
catalysts obtained by calcining undelaminated solids. Table 3 shows
the initial catalytic activity in mmol benzyl alcohol per hour per
gram of catalyst and the rate constant, in addition to conversion
and selectivity data. As noted earlier, the initial catalytic activity
was greater for the solids obtained by calcining delaminated solids
than for those from calcined undelaminated solids. The selectivity
was 100% or very close to this value with all catalysts.
Mg/Al-CO3
Mg/Al-OH
Mg/Al-DBS
Mg/Al-DBS-DEL
Mg/Al-OH-DEL
Mg/Al-NO3-DEL
Mg/Al-CO3-450
Mg/Al-OH-450
Mg/Al-OH-DEL-450
Mg/Al-NO3-DEL-450
71
59
3
5
67
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
348
361
403
372
66
107
149
198
176
a
Specific surface area.
Number of basic sites (n.d. = non determined).
b
be seen, intercalating DBS ions dramatically reduced the specific
surface area of the solid by effect of the organic molecules cover-
ing the surface of brucite-like layers and hindering access of any
others. The specific surface area of the LDH containing interlayer
carbonate was consistent with reported values for similar solids
[28]. The calcination–rehydration process caused a slight decrease
in specific surface area. On the other hand, delamination increased
the area of MgAl-OH, albeit only slightly. MgAl-NO3 was similar to
MgAl-OH in this respect, but the DBS-containing LDH had a very
The reaction develops via a concerted process involving both
benzaldehyde and the donor (alcohol), and the formation of a
six-membered cyclic intermediate adsorbed on the catalyst sur-
face (see Scheme 2) [22,39,40]. This mechanism is similar to that

J.M. Hidalgo et al. / Applied Catalysis A: General 470 (2014) 311–317
315
Table 3
Catalytic activity, rate constant and conversion to benzylalcohol obtained in the Meerwein–Ponndorf–Verley reaction of benzaldehyde with 2-propanol.a
b
Solid
ra
kc
Conversion (%)d
Selectivity (%)e
t (h)f
Mg/Al-CO3
Mg/Al-OH
Mg/Al-DBS
Mg/Al-DBS-DEL
Mg/Al-OH-DEL
Mg/Al-NO3-DEL
Mg/Al-CO3-450
Mg/Al-OH-450
Mg/Al-OH-DEL-450
Mg/Al-NO3-DEL-450
–
0.08
–
–
0
65
0
–
96
–
24.0
24.0
24.0
24.0
24.0
24.0
3.5
3.5
2.0
2.0
0.002
–
–
0.009
0.012
0.869
1.034
2.142
1.633
–
0
–
0.17
0.19
1.32
1.89
5.40
4.73
79
82
96
98
100
97
95
96
98
100
100
100
a
Reaction conditions: T = 82 ◦C; 0.003 mol of benzaldehyde; 0.06 mol of 2-propanol; 1 g of catalyst.
Catalytic activity at 10% conversion, in mmol of benzylalcohol·h−1 g1.
b
c
d
e
f
Rate constant, in h−1
.
Conversion to benzyl alcohol.
Selectivity to benzylalcohol.
Reaction time.
the LDHs intercalated with carbonate and DBS were inactive in the
reaction (conversion after 24 h in their presence was zero). On the
other hand, the solid intercalated with hydroxyl ions was active
and exhibited 50% conversion after the same reaction time. One
plausible explanation for these results is the difference in accessi-
bility to basic sites between solids. Basicity in LDHs can be ascribed
to Brönsted sites (viz. hydroxyl groups at the edges of brucite-like
layers). According to Winter et al. [45], the basicity of carbonate-,
oxalate- and hydroxyl-containing LDHs is related to the accessi-
bility of reactant molecules to these hydroxyl groups. Calcining
and rehydrating a carbonate-containing LDH leads to a new, struc-
turally distorted LDH as reflected in its XRD pattern. The distortion
causes Brönsted basic sites at the edges of brucite-like layers to
be more readily accessed by reactant molecules than in carbonate-
or DBS-containing LDHs, which are much more crystalline. There-
fore, the solid MgAl-OH must possess a greater catalytic activity
than MgAl-CO3 and MgAl-DBS. The solid obtained by delaminating
and ion-exchanging MgAl-DBS (i.e. MgAl-DBS-DEL) continued to be
inactive in the reaction, probably because of the steric hindrance
posed by the long hydrocarbon chains bonded to the brucite-like
layers. With MgAl-OH-DEL and MgAl-NO3-DEL, conversion was
much higher than with undelaminated MgAl-OH, however. These
results can be ascribed to the solids consisting of discrete brucite
layers being more accessible by the reagents than the correspond-
ing layered solids. In addition, solids containing an organic anion
have a very low specific surface area. In any case, conversions were
rather poor as a result of the catalytic effect involving Lewis basic
sites. For this reason, we used the calcined LDHs and delaminated
solids, which were Mg(Al)O mixed oxides containing Lewis O2–
sites.
100
75
50
25
0
0
1
2
3
t (h)
Fig. 4. Temporal variation of the conversion to benzyl alcohol in the MPV reaction of
benzaldehyde with 2-propanol as catalysed by varios catalysts. Reaction conditions:
T = 82 ◦C; 0.003 mol of benzaldehyde; 0.06 mol of 2-propanol; 1 g of catalyst.
H
CH3
H
H3C
C
O
C
O
H
Table 3 shows the conversions to benzyl alcohol obtained with
the LDHs and delaminated solids calcined at 450 ◦C. Clearly, the
mixed oxides obtained by calcination of hydrotalcites are much
more effective catalysts for the MPV reaction than are their uncal-
cined precursors. This is consistent with the proposed reaction
mechanism [39,22]. Thus, based on Scheme 2, the hydrogen trans-
fer takes place via a concerted process where 2-propanol and the
carbonyl compound are absorbed onto the catalyst surface to form
a six-membered cyclic intermediate. The surface of these solids
can be assumed to contain pairs of strong Lewis basic sites (O2–
ions) and Lewis acid sites (coordinatively unsaturated Mg2+ and
Al3+ ions). Therefore, the ability to adsorb 2-propanol molecules
will increase with increasing number of sites; as a result, the solid
with the greatest number of basic sites can be expected to be the
best catalyst and that with the smallest the one giving the lowest
conversions. In previous work [46], our group found the MPV reduc-
tion of citral in the presence of catalysts consisting of mixed oxides
of magnesium and a trivalent metal to be directly related to the
δ+
Mg
δ−
O
Al
O
Scheme 2. Proposed mechanism for Meerwein–Ponndorf–Verley reaction of benz-
aldehyde with 2-propanol over calcined Mg/Al LDH.
reported for the same process with a Lewis acid catalyst such as beta
zeolite [41–43]. In this case, the first step of the process involves
chemisorption of the hydrogen-donor alcohol at a Lewis acid site
(Al3+ ion). This results in the formation of a surface alkoxide that is
assumed to be the activated hydrogen donor. Subsequent coordina-
tive interaction of the carbonyl compound with this acid site causes
the formation of a six-membered transition state similar to that of
Scheme 2. This mechanism was subsequently confirmed by Peters
et al. [44] using deuterated 2-propanol. Therefore, the activity of
a solid acting as a catalyst for the reaction will be closely related
to its surface chemical properties. As can be seen from Table 3,

316
J.M. Hidalgo et al. / Applied Catalysis A: General 470 (2014) 311–317
Table 4
Results obtained in the MPV reaction of aldehydes with 2-propanol using MgAl-OH-DES-450 and MgAl-OH-450 as catalysts.a
Entry
Reactant
MgAl-OH-DES-450
Conversion (%)b
MgAl-OH-DES-450
Conversion (%)b
t (h)c
t (h)c
O
1
2
100
100
2.0
98
96
3.5
H
O
3.0
1.9
4.8
3.3
H
H
3
O
100
97
98
96
O
O
H
4
5
3.0
4.2
H
100
98
3.0
3.0
100
98
5.0
5.0
O
H
6
O
a
Reaction conditions: T = 82 ◦C; 0.003 mol of aldehyde; 0.06 mol of 2-propanol; 1 g of catalyst.
Conversion to alcohol.
Reaction time.
b
c
surface basicity of the oxides. In fact, our most active catalyst was
the solids obtained from undelaminated LDHs containing carbonate
or hydroxyl interlayer anions, which were inactive in the reaction
and exhibited 50% conversion only after 24 h, respectively. These
results can be explained in terms of accessibility of hydroxyl groups
at layer edges, which are Brönsted basic sites. In highly crystalline
LDHs, accessibility to such sites is poor but increases when the crys-
tal lattice is distorted; hence, MgAl-OH, with a reduced crystallinity,
was active in the reaction, whereas MgAl-CO3 was completely inac-
tive. Delamination facilitated access of the reactant molecules to
hydroxyl ions at layer edges in the LDHs, thereby increasing con-
version in the MPV reaction. Calcining layered and delaminated
LDHs at 450 ◦C produced mixtures of Mg(Al)O double oxides with
a high surface basicity of the Lewis type. The calcined solids exhib-
ited excellent (quantitative or nearly quantitative) conversion after
only 2 h of reaction.
also the most basic. The catalytic activity and conversion results
of Table 3 are also strongly correlated with the basicity results of
Table 2: the most active solids were those containing the largest
populations of basic sites.
In order to check whether this catalytic behaviour applied to
all solids, we conducted the reaction by using other substrates and
MgAl-OH-DES-450 and MgAl-OH-450 as catalysts. Table 4 shows
the conversion obtained with the two catalysts after different reac-
tions times. Irrespective of the nature of the hydrocarbon chain of
the aldehyde, the catalyst obtained by calcining the delaminated
solid provided increased conversion values at shorter reaction
times than that obtained from the undelaminated solid.
Finally, we assessed catalyst regeneration and recycling by using
the most active catalyst, MgAl-OH-DES-450, in the reaction of ben-
zaldehye with 2-propanol. To this end, we initially used the catalyst
as isolated by filtering from the reaction mixture. The conversion
values thus obtained were very poor. This led us to regenerate it
by washing with methanol (25 mL/g catalyst) four times and cal-
cination at 450 ◦C for 2 h. Under these conditions, conversion in
successive reuses was similar to that obtained with fresh catalyst
(viz. 100% within 2.5 h).
Acknowledgments
The authors gratefully acknowledge funding by Spain’s Ministe-
rio de Educación y Ciencia (Project MAT-2010-18778), Feder Funds
and to the Consejería de Innovación, Ciencia y Empresa de la Junta
de Andalucía.
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