One-pot synthesis of C8 aldehydes/alcohols from propylene using eco-friendly hydrotalcite supported HRhCO(PPh3)3 catalyst

Article abstract of DOI:10.1039/b616977e

A multi-functional catalyst [HF/HT] containing a rhodium complex, HRh(CO)(PPh₃)₃ [HF] and a solid base, hydrotalcite Mg _1-xAl_x(OH₂)^x+(CO ₃^2-)_x/n·mH₂O [HT], synthesized by impregnation of [HF] onto the surface of [HT], was investigated for the one-pot synthesis of C₈ aldol derivatives (aldehydes or alcohols) from propylene. The catalyst was found to be efficient to carry out hydroformylation, aldol condensation and hydrogenation reactions in one pot. The catalytic activity of [HF/HT(X)] was studied in detail as functions of Mg/Al molar ratio (X) of [HT], amount of [HF] complex and [HT], and reaction temperature. The selectivity for 2-ethylhexanal was observed to increase upon increasing X and amount of [HT]. The highest selectivity for 2-ethylhexanol was observed for [HT] Mg/Al molar ratio of 3.5 at 250°C. The kinetic profiles of the various products obtained were in agreement with the reaction pathway proposed to understand the role of the [HF/HT] catalyst on the formation of C₈ aldol derivatives. Thermal stability of the [HF/HT] catalyst system was also investigated. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b616977e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/njc | New Journal of Chemistry
One-pot synthesis of C8 aldehydes/alcohols from propylene using
eco-friendly hydrotalcite supported HRhCO(PPh3)3 catalystw
Sumeet K. Sharma,ab Vivek K. Srivastava,a Ram S. Shukla,a Parimal A. Parikhb
and Raksh V. Jasra*a
Received (in Montpellier, France) 20th November 2006, Accepted 12th December 2006
First published as an Advance Article on the web 11th January 2007
DOI: 10.1039/b616977e
A multi-functional catalyst [HF/HT] containing a rhodium complex, HRh(CO)(PPh3)3 [HF] and a
solid base, hydrotalcite Mg1ÀxAlx(OH2)x+(CO3
2À
)
x/n Á mH2O [HT], synthesized by impregnation
of [HF] onto the surface of [HT], was investigated for the one-pot synthesis of C8 aldol
derivatives (aldehydes or alcohols) from propylene. The catalyst was found to be efficient to carry
out hydroformylation, aldol condensation and hydrogenation reactions in one pot. The catalytic
activity of [HF/HT(X)] was studied in detail as functions of Mg/Al molar ratio (X) of [HT],
amount of [HF] complex and [HT], and reaction temperature. The selectivity for 2-ethylhexanal
was observed to increase upon increasing X and amount of [HT]. The highest selectivity for
2-ethylhexanol was observed for [HT] Mg/Al molar ratio of 3.5 at 250 1C. The kinetic profiles of
the various products obtained were in agreement with the reaction pathway proposed to
understand the role of the [HF/HT] catalyst on the formation of C8 aldol derivatives. Thermal
stability of the [HF/HT] catalyst system was also investigated.
involve multi-step processes using stoichiometric amounts of
Introduction
hazardous base solutions and require post synthesis work-up
for the separation of spent KOH/NaOH from the products’
mixtures. It is estimated that approximately, 1.0–1.5 tons of
spent base solutions are generated for every 10 tons of product
formed in homogeneous aldol condensation. In fact, 30% of
the selling price of C8 aldol derivatives is estimated to be
contributed by product purification, recovery and waste
treatment.9
Industrially, oxo products such as aldehydes and alcohols are
synthesized by hydroformylation of alkenes with synthesis gas
in the presence of a catalyst. Nearly 86% of the total hydro-
formylation production capacity (8.8 million tons/year) is
based on propylene hydroformylation to give butanals
(n- and iso-) and butanols (n- and iso-) as major products.1
n-Butanal is largely used for the production of C8-aldol
products such as 2-ethylhexanal and 2-ethylhexanol, which
are valuable intermediates for the production of dioctylphtha-
late, other plasticizers, coatings, adhesives, lubricants, alkyd
resins and fine chemicals.
Research efforts to develop catalytic processes alternative to
environmentally and economically inefficient stoichiometric
chemical processes are in progress. For example, the one-pot
synthesis of methyl isobutyl ketone (MIBK) from acetone has
been reported in the literature using Pd/Mg(Al)O catalyst.10
The aldol condensation process during production of 2-ethyl-
hexanol has also been modified by Mitsubishi Chemicals by
converting n-butanal into 2-ethylhexenal in the second step in
the presence of a basic ion-exchanger at 80–100 1C.11 The third
hydrogenation step is carried out under vapor phase condi-
tions over either Ni or Cu catalysts. After a three-step
distillation process, pure products are obtained. Shell and
Exxon both have developed a single-step process, known as
the ‘Aldox’ process, to produce ethylhexanol directly from
propylene by adding co-catalysts such as compounds of Ti, Sn,
Zn, Al, or Cu or KOH, to the original hydroformylation
catalyst.12 However, these processes still have disadvantages
such as the use of strong base solutions for the aldol con-
densation of n-butanal, low selectivity for the C8 aldol deri-
vatives and relatively low liquid space velocity in the
hydroformylation.
Presently, the synthesis of C8 aldol derivatives from propy-
lene is a three-step process. In the first step, the hydroformyla-
tion of alkene is carried out to produce the aldehyde using Rh
or Co based catalysts.2 In the second step, the obtained
aldehyde undergoes an aldol condensation in the presence of
stoichiometric amount of a strong base, namely KOH or
NaOH, to produce unsaturated aldol derivatives.3,4 Hydro-
genation of unsaturated aldol derivatives is carried out using
Ni or Cu catalysts in the third step.5–8 Thus, existing com-
mercial strategies for the production of C8 aldol derivatives
a Silicates and Catalysis Discipline, Central Salt and Marine
Chemicals Research Institute (CSMCRI), G. B. Marg, Bhavnagar,
364 002, Gujarat, India. E-mail: rvjasra@csmcri.org; Fax: 91 278
2567562; Tel: 91 278 2471793
b Department of Chemical Engineering, S. V. National Institute of
Technology, Surat, 395 007, Gujarat, India
w Electronic supplementary information (ESI) available: Fig. S1: 31P
FT-NMR of [HF] complex in C6D6. Fig. S2: P-XRD patterns of [HF]
complex, [HT(3.5)] and [HF/HT(X)]. Fig. S3: SEM images of
[HT(3.5)] and [HF/HT(3.5)]. Fig. S4: TGA of [HT(3.5)], [HF] complex
and [HF/HT(3.5)]. See DOI: 10.1039/b616977e
The present study reports a novel approach13 for the one-
pot synthesis of C8 aldol derivatives (aldehydes or alcohols)
from propylene (Scheme 1) employing a multi-functional
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 277–286 | 277

View Article Online
samples with Mg/Al molar ratios varying from 1.5 to 3.5.16
Typically, an aqueous solution of Mg(NO3)2 Á 6H2O (0.22 mol)
and Al(NO3)3 Á 9H2O (0.088 mol) in 200 ml double-distilled
deionized water was prepared. A second solution (200 ml)
containing NaOH (0.72 mol) and Na2CO3 (0.21 mol) was
prepared and was slowly added to the first solution over
around 2 h in a 1 L round bottom flask under vigorous stirring
at room temperature. The content was aged at 65 1C for 16 h.
The precipitate formed was filtered off and washed with hot
distilled water until pH of the filtrate was neutral. The
precipitate was dried in an oven at 80 1C for 12 h to give a
hydrotalcite sample with Mg/Al molar ratio = 2.5.
Synthesis of multi-functional catalyst [HF/HT(X)]. Typi-
cally,
a
10 ml toluene solution of [HF] complex
Scheme 1 One-pot synthesis of C8 aldol derivatives from propylene
using [HF/HT].
HRh(CO)(PPh3)3 (500 mg) and triphenylphosphine (1050 mg)
was poured into a flask containing 3.5 g [HT(X)] (where X =
Mg/Al ratio). The slurry was stirred for 32 h at room
temperature under inert atmosphere (N2). After 32 h, toluene
was removed under vacuum. The final product obtained was a
free-flowing light yellow powder and was stored under inert
atmosphere at room temperature.
heterogeneous catalyst [HF/HT] prepared by impregnation of
a rhodium complex, HRh(CO)(PPh3)3 [HF], on the surface of
2À
a
solid base, hydrotalcite Mg1ÀxAlx(OH2)x+(CO3
)
x/n Á
mH2O [HT].
Characterization of the catalyst
The FT-NMR characterization of the [HF] complex was
carried out by Bruker Avance DPX 200 MHz FT-NMR
system. The C, H and N elemental analysis of the [HF]
complex was done by Perkin Elmer CHNS/O 2400 analyzer.
The FT-IR spectra of the [HF] complex, [HT(X)] and [HF/
HT(X)] of different Mg/Al ratio (X) were recorded with a
Perkin–Elmer Spectrum GX Fourier transform infrared spec-
trophotometer (FT-IR) system in the region of 400 to
4000 cmÀ1 using KBr pellets. The powder X-ray diffraction
patterns of [HT(X)] and [HF/HT(X)] were recorded with
Phillips X’Pert MPD system equipped with XRK 900 reaction
chamber, using Cu-Ka radiation (l = 1.5405 A). Scanning
electron microscopy images of [HT(X)] and [HF/HT(X)] were
measured on a microscope (Leo Series VP1430, Germany)
having silicon detector equipped with EDX facility (Oxford
instruments). The samples were coated with gold using sputter
coating to avoid charging. Analyses were carried out at an
accelerating voltage of 18 kV and probe current of 102 pA.
Thermogravimetric analysis (TGA) of [HF] complex, [HT(X)]
and [HF/HT(X)] was carried out using a Mettler TGA/SDTA
851e equipment in flowing N2 (flow rate = 50 ml minÀ1), at a
heating rate of 10 1C minÀ1 and data were processed using
Stare software.
Experimental
Materials
Propylene (99.6%), carbon monoxide (CO, 99.8%) and
hydrogen (H2, 99.98%) were procured from Alchemie Gases
and Chemicals Private Limited, India. The rhodium metal
precursors RhCl3 Á 3H2O, triphenylphosphine (PPh3), sodium
borohydride (NaBH4, 99.98%) and formaldehyde (HCHO,
34%) were purchased from Sigma-Aldrich, USA for the
synthesis of HRh(CO)(PPh3)3 [HF] complex. Magnesium
nitrate (Mg(NO3)2 Á 6H2O, 99.99%), aluminium nitrate
(Al(NO3)3 Á 9H2O, 99.99%), sodium carbonate (Na2CO3,
99.99%) and sodium hydroxide (NaOH, 99.99%) were pur-
chased from s.d. Fine Chemicals, India for synthesizing
2À
(Mg1ÀxAlx(OH2))x+(CO3
)
x/n Á mH2O) [HT]. Organic sol-
vents required for the synthesis of [HF] complex were pur-
chased from Rankem, India and were purified by reported
methods.14 Double-distilled milli-pore de-ionized water was
used throughout the present study.
Catalyst synthesis
Synthesis of HRh(CO)(PPh3)3 [HF]. The complex [HF] was
synthesized as per reported method.15
A solution of
RhCl3 Á 3H2O (7.6 mmol) in ethanol (70 ml) was added into
a refluxing solution of triphenylphosphine (46.0 mmol) in
ethanol (300 ml). After 2 min aqueous formaldehyde solution
(10 ml) was added dropwise and the resulted solution was
observed to turn yellow with the formation of trans-
RhCl(CO)(PPh3)2. Addition of the ethanolic solution of
sodium borohydride (2.0 g) into this hot mixture yielded the
yellow crystals of HRh(CO)(PPh3)3. The yellow crystals were
washed with ethanol to remove unreacted rhodium metal.
One-pot synthesis of C8 aldol derivatives
Reactions for the one-pot synthesis of C8 aldol derivatives
were carried out in 100 ml EZE-Seal Stirred Reactor supplied
by Autoclave Engineers, USA, equipped with a controlling
unit.17,18 The requisite amount of [HF/HT(X)] was added into
the autoclave reactor having 50 ml toluene as a solvent. The
autoclave was flushed twice with N2 gas prior to introducing
desired amount of propylene (10 atm). The reactor was then
brought to 60 1C temperature (T1) for hydroformylation
reaction. The CO and H2 (1 : 3) gases were introduced in the
reactor. The reaction was then initiated by starting the
Synthesis of hydrotalcites [HT]. Co-precipitation method at
constant pH was employed to synthesize the hydrotalcite [HT]
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
278 | New J. Chem., 2007, 31, 277–286

View Article Online
magnetic stirrer at 1000 rpm. The reaction was kept at 60 1C
temperature for 3 h (t1) following which, the reaction tem-
perature was raised to T2 1C to initiate aldol condensation
reaction. The reaction temperature was kept for 9 h (t2) at
T2 1C. The reaction was continued at constant pressure by
supplying H2 from the reservoir vessel. After 12 h total
reaction time (t), the reactor was cooled to room temperature
under flowing water. The product mixture was then analyzed
using Gas Chromatography (GC). For kinetic studies liquid
samples were withdrawn for GC analysis by a sampling valve
at different time intervals during the experiments.17
Analysis of the product mixture was carried out by GC-MS
(Shimadzu, GCMS-QP2010) and GC (Shimadzu 17A, Japan)
equipped with 5% diphenyl and 95% dimethyl siloxane uni-
versal capillary column (60 m length and 0.25 mm diameter)
and a flame ionization detector (FID). The GC oven tempera-
ture was programmed from 40 to 200 1C at the rate of
10 1C minÀ1. N2 was used as the carrier gas. The temperature
of injection port and FID was kept constant at 200 1C. The
retention times of different compounds were determined by
injecting pure compounds under the identical conditions. The
conversions were determined in terms of consumption of
propylene and were found in the range of 95–99%. The
consumption of propylene was calculated by reported meth-
Fig. 1 P-XRD patterns of [HF] complex, [HT(3.5)] and [HF/HT(3.5)]
system.
sharp, intense and symmetric peaks at lower diffraction angles
(2y = 10–25) and broad asymmetric reflections at higher
diffraction angles (2y = 30–50), which are characteristics of
hydrotalcite.21 The sharp peaks at 2y = 8.5 and 20 were
observed in the P-XRD pattern of the [HF] complex and were
also present in the P-XRD patterns of the [HF/HT(X)] cata-
lyst. The P-XRD patterns of the [HF/HT] catalyst showed that
the characteristic planes of the [HT] are retained after impreg-
nation of [HF] complex. The intensity of the 00l planes of the
[HF/HT] catalyst, which is related to the crystallinity of [HT],
decreased on the impregnation of the [HF] complex. With
increase in Mg/Al ratio of [HT] from 1.5 to 3.5, a slight shift
towards lower 2y values for the peaks corresponding to (003)
and (006) planes was observed due to concurrent decrease of
the positive charge of the layers of [HT]. These small shifts in
the peaks of the (003) and (006) planes were also observed in
the P-XRD patterns of the [HF/HT(X)] catalyst systems.
However, lack of broadening of the 00l plane peak of
[HF/HT] compared to P-XRD patterns of the starting [HT]
sample, showed the absence of intercalation of [HF] complex
into the [HT] interlayer space.
od,19,20 using formula: Cp
=
Cpo(1
À
Xp); where
Xp = conversion of propylene, Cpo = initial concentration
of propylene and calculated as Cpo = pp/RT; where pp is the
partial pressure of propylene. The percentage selectivity for
each product was calculated by the method reported earlier.20
To ensure the reproducibility of the reaction, experiments were
repeated under the identical reaction conditions. The conver-
sions and selectivities were reproducible in the range of 5%
variation.
In a catalyst thermal stability experiment, a known amount
of [HF/HT(3.5)] catalyst and solvent toluene (50 ml) were
charged into the reactor and the reactor was flushed with N2
before introducing CO and H2 at desired pressure. The CO
and H2 (1 : 3) were fed into the reactor at 20 atm. The reactor
was then brought to set temperature. After 12 h reaction time,
the reaction mixture was cooled to room temperature and the
catalyst was filtered off. The obtained solid material was dried
under vacuum at room temperature. The dry solid material
was characterized by the powder X-ray diffraction (P-XRD)
and FT-IR analysis.
The appearance of a band at 2036 cmÀ1 for n(Rh–H) and at
1965 cmÀ1 for n(CQO) in the FT-IR spectrum of the [HF]
complex (Fig. 2), confirmed the formation of the [HF] com-
plex. The FT-IR spectra of [HF/HT(X)] prepared using dif-
ferent Mg/Al molar ratios of HT(X) were quite similar,
though, some difference was observed in the intensity and
the broadness of the bands. The absorption at 3500–3600
cmÀ1, present in [HT(X)] is attributed to the H-bonding
stretching vibrations of the OH group in the brucite-like layer.
The maximum of this band is shifted depending upon the
Mg/Al molar ratio of [HT].21 The hydrogen stretching and
bending frequencies in [HT] were increased with increase in the
Mg/Al ratio from 2.0 to 3.5. In the FT-IR spectra of [HT(X)],
Results and discussion
Characterisation of the complexes
The appearance of the doublet at 56.34 and 58.31 ppm
[J(Rh–P) = 160 Hz] in 31P NMR spectra of [HF] complex
showed that all the three phosphorus atoms possess the same
environment and are in the equatorial position. The hydride
(HÀ) and CO axial positions showed trigonal bipyramidal
structure in the complex. The %C and %H for the [HF]
the appearance of the shoulders at 1647 and 1435 cmÀ1
,
2À
characteristic bands of H2O and CO3 confirmed the forma-
,
tion of Mg–Al hydrotalcites. The shoulder present at 3000 cmÀ1
complex are: calculated (found): %C
%H = 5.0 (5.1).
= 71.9 (71.6);
is attributed to the hydrogen bonding between water molecules
2À
and the interlayer CO3 anions. The vibration of carbonates
(asymmetric stretching, n3) appeared at 1350–1380 cmÀ1 and
P-XRD patterns of [HT(X)], [HF] complex and [HF/HT(X)]
are shown in Fig. 1. The P-XRD patterns of [HT(X)] showed
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 277–286 | 279

View Article Online
Fig. 2 FT-IR spectra of [HF] complex, [HT(3.5)] and [HF/HT(X)] systems.
could be assigned to interlayer carbonates (chelating or brid-
ging bidentate). The lower wavenumber region showed a band
at about 541 cmÀ1, corresponding to the translation modes of
hydroxyl groups, influenced by Al3+ cations. The peak at the
664 cmÀ1 (n4) is assigned to the in-plane carbonate bending.22
The SEM images were recorded to observe the effect of the
impregnation of [HF] complex on the morphology of the [HT].
The micrographs of the [HT(3.5)] and [HF/HT(3.5)] catalysts
showed a well developed layered structure and possessed
platelet structures. However, due to overlapping of such
platelets spongy type structure was observed.
water and carbon dioxide.21 The weight loss in the TGA for
the [HF] complex at 150 1C is observed to be less than 4%. The
62% weight loss of the [HF] complex is observed in the range
170–450 1C due to thermal decomposition of the complex. The
weight loss observed for [HF/HT(3.5)] occurred in three steps.
In the first step, only 5% weight loss was observed in tem-
perature range 160–200 1C that increased up to 33% on
increasing the temperature from 220 to 400 1C in the second
step. However, on further increase in the temperature from
400 to 650 1C, a weight loss of 5% was observed. The weight
losses for [HF/HT(3.5)] in the second and third steps are
attributed to the removal of condensed water molecules
(hydroxyl group) and carbon dioxide from the carbonate
anion present in the brucite layer of [HT] and the thermal
decomposition of the [HF] complex.
The TGA of [HT(3.5)] showed two stages of weight loss
accompanied by an endothermic transformation (Fig. 3). The
first weight change (15%) was observed in the range 200–220 1C
due to the loss of interlayer water molecules, without collapse
of the structure and this step is reversible.21 The second weight
loss (29%) was observed in the range 250–450 1C and is
attributed to the removal of condensed water molecules
(hydroxyl group) and carbon dioxide from the carbonate
anion present in the brucite layer. The second weight loss
usually appeared broad because of the simultaneous loss of
Mechanistic considerations
To understand the function of each component of the multi-
functional catalysts [HF/HT] during the synthesis of C8 aldol
derivatives from propylene, the proposed reaction pathway is
shown in Scheme 2. The reaction gets initiated by formation of
n-butanal via hydroformylation of propylene (step 1) by [HF]
complex of the catalyst. This is followed by condensation step
wherein two molecules of n-butanal undergo condensation
reaction in the presence of solid base [HT] to give 2-ethyl-3-
hydroxyhexanal (b-hydroxyaldehyde) and subsequently
2-ethylhexenal after removal of one molecule of water. The
condensation of n-butanal is a base catalyzed reaction and
dehydration of b-hydroxyaldehyde is catalyzed by the Lewis
basic sites of the catalyst.22,23 The [HT] component of the
[HF/HT] catalyst facilitates the steps 2 and 3. The step 4 is a
hydrogenation step, which includes hydrogenation of 2-ethyl-
hexenal to 2-ethylhexanal. As per the stoichiometry, this step
requires a second hydrogen molecule. The [HF] complex under
the employed reaction conditions assists this hydrogenation
step. The final step 5 is again a hydrogenation step, that
involves reduction of 2-ethylhexanal to 2-ethylhexanol cata-
lyzed by the [HF] complex. Stoichiometrically, the third
Fig. 3 TGA of [HT(3.5)], [HF] complex and [HF/HT(3.5)] system.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
280 | New J. Chem., 2007, 31, 277–286

View Article Online
Fig. 4 Effect of Mg/Al ratio of [HT] in [HF/HT] at 150 1C aldol
temperature at partial pressure of propylene = 10 atm, partial
pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HF/HT]
= 700 mg, HT/HF ratio = 7, T1 = 60 1C, t1 = 3 h, T2 = 150 1C,
t2 = 9 h, total time (t) = 12 h.
Scheme 2 Mechanistic pathways for the formation of 2-ethylhexanol
from propylene in one step.
2-ethylhexanal was observed to increase linearly up to 45%
on increasing the Mg/Al molar ratio up to 3.5 with decrease in
the selectivity for butanals.
hydrogen molecule is utilized in the step 5. Thus, the [HF]
complex of the [HF/HT] catalyst plays role in three reactions,
once in the hydroformylation and twice in the hydrogenation
reactions during the one-step preparation of 2-ethylhexanol
from propylene.
Conducting the experiments at higher aldol condensation
temperature (T2) 250 1C showed (Fig. 5) formation of sig-
nificant amount of 2-ethylhexanol within 12 h by hydrogena-
tion of 2-ethylhexanal (step 5, Scheme 2). The selectivity for
2-ethylhexanol increased from 11 to 21% on increasing the
Mg/Al molar ratio of [HT] from 1.5 to 3.5. On increasing the
total reaction time to 24 h, the selectivity for 2-ethylhexanol
was observed to increase up to 27% at the Mg/Al molar ratio
of 1.5 of [HT] (entry 6, Table 3). The higher selectivity for C4
and C8 aldehydes was observed at 150 1C as compared to
250 1C (T2). The lower selectivity for C4 and C8 aldehydes at
250 1C (T2) was observed due to continuous hydrogenation of
aldehydes to the respective alcohols at employed reaction
conditions, thereby, butanols and 2-ethylhexanol were formed
as major products at higher T2.
However, the possibilities of side reactions involved in a
one-step preparation of 2-ethylhexanol from propylene under
the hydroformylation conditions cannot be ignored. The most
probable possibility is the reduction of butanals (n- and iso-) to
their corresponding butanols (step 6 and 7) under the studied
reaction conditions.
The [HF] complex can catalyze the reduction of butanals in
the presence of hydrogen and this was confirmed by carrying
out hydrogenation of n-butanal in a separate experiment by
taking 2 g n-butanal, 20 mg [HF] complex in 50 ml solvent
(toluene) at 30 atm (hydrogen) and 150 1C. Therefore, to avoid
this competitive reaction, the amount of solid base [HT] in the
[HF/HT] catalyst and the reaction conditions, i.e., aldol reac-
tion temperature (T2), are to be optimized to divert the
reaction to step 2 rather than steps 6 and 7. The other side
reactions, cross aldol condensation of n- and iso-butanal were
not observed in the present study.
The [HF/HT] catalyst was investigated in detail for the one-
pot synthesis of C8 aldol derivatives from propylene under
varied reaction parameters namely changing the amount of
[HT] and [HF] complex, Mg/Al molar ratio (X) of [HT], aldol
condensation temperature (T2) and partial pressure of CO and
H2 at a fixed hydroformylation temperature (T1).
Effect of Mg/Al molar ratio (X) of [HT] in the [HF/HT(X)]
catalysts
The effect of Mg/Al molar ratio (X) from 1.5 to 3.5 of [HT] in
the [HF/HT(X)] catalyst was studied at 150 and 250 1C (Fig. 4
and 5) aldol condensation temperatures (T2) while keeping the
hydroformylation temperature (T1) constant at 60 1C. At
150 1C aldol condensation temperature, the selectivity for
Fig. 5 Effect of Mg/Al ratio of [HT] in [HF/HT] at 250 1C aldol
temperature at partial pressure of propylene = 10 atm, partial
pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HF/HT]
= 700 mg, HT/HF ratio = 7, T1 = 60 1C, t1 = 3 h, T2 = 250 1C,
t2 = 9 h, total time (t) = 12 h.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 277–286 | 281

View Article Online
Table 1 Effect of [HT(3.5)] in [HF/HT(3.5)] on the selectivity for C8 aldol derivatives
Selectivity (%)
Entry
[HT(3.5)] (mg)
2-Ethylhexanal
2-Ethylhexanel
Butanals
Butanols
1
2
3
4
5
100
300
500
700
7
24
43
45
48
0
8
6
9
13
43
50
45
40
39
50
18
6
6
0
1000
Reaction conditions: Partial pressure of propylene = 10 atm, partial pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HF] complex =
100 mg, T1 = 60 1C, t1 = 3 h, T2 = 150 1C, t2 = 9 h, total time (t) = 12 h.
Table 2 Effect of [HF] complex in [HF/HT(3.5)] on selectivity for C8 aldol derivatives
Selectivity (%)
Entry
[HF] complex (mg)
2-Ethylhexanal
2-Ethylhexenal
Butanals
Butanols
1
2
3
4
5
6
15
30
50
100
125
160
24
29
37
45
43
27
10
12
20
9
22
16
51
41
31
40
14
17
15
18
12
6
21
40
Reaction conditions: Partial pressure of propylene = 10 atm, partial pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HT(3.5)] =
700 mg, T1 = 60 1C, t1 = 3 h, T2 = 150 1C, t2 = 9 h, total time (t) = 12 h.
Table 3 Effect of aldol reaction temperature (T2) on the selectivity for C8 aldol derivatives using [HF/HT(3.5)]
Selectivity (%)
Entry
T2/1C
2-Ethylhexanol
2-Ethylhexanal
2-Ethylhexenal
Butanals
Butanols
1
2
3
120
150
200
250
150
250
—
—
—
18
—
27
14
45
61
4
54
4
2
9
3
2
8
5
73
40
9
1
21
1
11
6
27
75
17
63
4
5a
6b
Reaction conditions: Partial pressure of propylene = 10 atm, partial pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HF/HT(3.5)] =
b
700 mg, HT/HF = 7, T1 = 60 1C, t1 = 3 h, t2 = 9 h, total time (t) = 12 h. a Single reaction temperature (T) = 150 1C for 12 h. Total time (t) =
24 h, Mg/Al molar ratio of [HT] = 1.5.
The increase in the selectivity for C8 aldol derivatives with
increasing Mg-content in [HT] is explained in terms of
enhanced basicity of [HT] which aids condensation reaction.
It is known that the basic strength of a hydrotalcite increases
with increasing the Mg/Al molar ratio.24–27 The basicity of the
hydrotalcite resulting from surface hydroxyl groups and two
types of basic sites are identified for hydrotalcite, one weaker
Brønsted OHÀ and second, stronger Lewis O2À sites.23 The
Brønsted OHÀ sites are responsible for the aldol condensation
of n-butanal to b-hydroxyaldehyde and Lewis O2À sites are
responsible for dehydration of b-hydroxyaldehyde.22,23
2-Ethylhexenal is produced by the dehydration of aldol con-
densation dimer by Lewis O2À sites present on the hydrotalcite
surface of [HF/HT] catalysts.
(Table 1). The selectivity for 2-ethylhexenal was observed to
increase on increasing the amount of the [HT(3.5)]. Decrease
in selectivity for butanals and butanols shows that condensa-
tion reactions become predominant with increasing amount of
[HT] in the [HF/HT(3.5)] catalyst.
The amount of [HF] complex was varied from 15 to 160 mg
in the [HF/HT(3.5)] catalyst by keeping the amount of the
[HT(3.5)] constant. On increasing the amount of [HF] complex
from 15 to 100 mg, the selectivity for 2-ethylhexanal increased
(entries 1–4, Table 2) and was found to be 45% at the 100 mg
weight of [HF] complex (entry 4, Table 2). On further increasing
the amount of [HF] complex, the selectivity for 2-ethylhexanal
was observed to decrease at the expense of increase in the
selectivity for butanols (entries 5 and 6, Table 2).
Higher selectivity for butanols observed at lower amount of
[HT(3.5)] (entries 1 and 2, Table 1), could be due to the
insufficient amount of solid base in the [HF/HT(3.5)] catalyst
to affect aldol condensation reaction. The active catalyst for
the aldol condensation of n-butanal possesses basic sites on
their surface which are responsible for the condensation
Effect of ratio of [HT(3.5)] and [HF] complex
The effect of the variation of the amount of [HT(3.5)] in the
[HF/HT(3.5)] catalyst on the selectivity was studied with
varying the amount of [HT(3.5)] from 100 mg to 1000 mg.
The selectivity for 2-ethylhexanal increased from 7 to 48%
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
282 | New J. Chem., 2007, 31, 277–286

View Article Online
reaction.28 At lower HT/HF ratios, a higher amount of the
hydroformylation complex is on the surface of [HT]. The
impregnated [HF] complex on the surface could be blocking
the basic sites of [HT], and thereby, suppressing the condensa-
tion reaction. Therefore, at higher amount of [HF] complex,
the hydrogenation of aldehydes is observed to be more pro-
nounced than the aldol condensation of n-butanal. The max-
imum selectivities for C8 aldol derivatives (entries 1–5, Table
1) were observed at higher amount of [HT] due to the avail-
ability of more basic sites for the condensation reaction on the
surface of the catalyst.
carrying out the experiment at a single temperature (T =
150 1C) rather than in two stages with temperatures at 60 and
150 1C (entries 2 and 5, Table 3). One of the possible reasons
for higher selectivity on carrying the reaction at a single
temperature (150 1C) could be the increased activity of solid
base component [HT(3.5)] for condensation reaction at the
beginning of the reaction.
Effect of the partial pressures of CO and H2
The effect of partial pressures of CO and H2 (1 : 3), from 12.5
to 80 atm on product distribution of C8 aldol derivatives was
studied using the [HF/HT(3.5)] catalyst (Table 4). The partial
pressure of CO and H2 was varied with the CO : H2 ratio kept
at 1 : 3. For example, on increasing the partial pressure of CO
and H2 from 12.5 to 40 atm, the selectivity for 2-ethylhexanal
increased from 38 to 47% (entries 1–3, Table 4). After that, the
selectivity decreased from 47 to 40% on increasing the partial
pressure of CO and H2 from 40 to 60 atm (entries 3 and 4,
Table 4). However, the selectivity for 2-ethylhexanal was
found to increase from 40 to 47% on increasing the partial
pressure of CO and H2 from 60 to 80 atm (entries 4 and 5,
Table 4). The formation of butanols was observed with
butanols selectivity being more at higher partial pressure of
CO and H2.
Effect of the aldol temperature (T2)
The results given in Fig. 4 and 5 show that the reaction is
significantly influenced by the aldol condensation temperature
(T2). The effect of T2 on the selectivity for C8 aldol derivatives
was studied at different T2 (120 to 250 1C) using the [HF/
HT(3.5)] catalyst (Table 3). The selectivity for 2-ethylhexanal
was found to increase from 14 to 61% on increasing T2 from
120 to 200 1C (entries 1–3, Table 3) and formation of
2-ethylhexanol was not observed. On further increasing the
temperature T2 from 200 to 250 1C, the selectivity for
2-ethylhexanal decreased from 61 to 4% and the selectivity
for 2-ethylhexanol was observed to be 18%. At 250 1C, the
selectivity for the butanols sharply increased from 27%
(at 200 1C) to 75%. The selectivity for butanals was found
to decrease from 73 to 1% on increasing the T2 from 120 1C to
250 1C. The rapid decrease in selectivity for butanals on
increasing T2 shows the consumption of butanals either for
the aldol condensation or reduction to butanols.
Kinetic profiles for one-pot synthesis of C8 aldol derivatives
In order to calculate the rate constants for hydroformylation
of propylene, aldol condensation of n-butanal and hydrogena-
tion of 2-ethylhexenal, separate experiments were conducted
by taking propylene, n-butanal and 2-ethylhexenal as a reac-
tant under the reaction conditions identical to those used for
one-step synthesis of 2-ethylhexanal from propylene. The rate
constants for the hydroformylation of the propylene, aldol
condensation of the n-butanal and hydrogenation of 2-ethyl-
hexenal were found to be 0.50, 0.07 and 0.20 hÀ1, respectively.
These rate constant values showed that the hydroformylation
of propylene for synthesis of butanals is the fastest reaction.
Aldol condensation of n-butanal for the production of the
2-ethylhexenal is the slowest reaction among the three subse-
quent reactions. The rate of the formation of the 2-ethylhex-
enal is dependent on the rate of aldol condensation which is
catalyzed by [HT] of the [HF/HT] catalyst. The rate of
hydrogenation of 2-ethylhexenal is also slower compared to
hydroformylation of propylene. After the hydroformylation of
propylene, butanals can either undergo aldol condensation or
hydrogenation depending on the reaction conditions. The rate
It is observed from the results (Table 3), that the higher
temperature favoured butanals reduction that is catalyzed by
the [HF] complex present in the [HF/HT(3.5)] catalyst. It is
clear from the TGA data that the [HF] present in the [HF/HT]
catalyst was stable up to 150 1C, and after that decomposition
was observed. The decomposition of the [HF] complex may
result into a rhodium species which could be more active for
the hydrogenation of aldehydes. To confirm this assumption,
one separate experiment was carried out by taking n-butanal
(2 g) as reactant in 50 ml of toluene as solvent at 250 1C and at
30 atm of H2 with 700 mg [HF/HT(3.5)] for 12 h reaction time.
After 12 h, 72% selectivity for butanols was observed which
are in accordance with the results observed in the case of
propylene as a reactant (entry 4, Table 3).
An important observation that was made regarding the
selectivity for 2-ethylhexanal increased from 45 to 54% on
Table 4 Effect of the partial pressures of CO and H2 on the selectivity of for C8 aldol derivatives using [HF/HT(3.5)]
Pressure/atm
CO
Selectivity (%)
2-Ethylhexanal
Entry
H2
2-Ethylhexenal
Butanals
Butanols
1
2
3
4
5
2.5
5
10
15
20
10
15
30
45
60
38
45
47
40
47
12
9
7
7
9
42
40
38
39
31
8
6
8
14
13
Reaction conditions: Partial pressure of propylene = 10 atm, [HF/HT(3.5)] = 700 mg, HT/HF = 7, T1 = 60 1C, t1 = 3 h, T2 = 150 1C, t2 = 9 h,
total time (t) = 12 h.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 277–286 | 283

View Article Online
Fig. 6 Kinetics profiles for synthesis of C8 aldol derivatives from
propylene in one step using [HF/HT(3.5)] catalytic system at partial
pressure of propylene = 10 atm, partial pressure of CO = 5 atm,
partial pressure of H2 = 15 atm, [HF/HT(3.5)] = 700 mg, HT/HF
ratio = 7, T1 = 60 1C, t1 = 3 h, T2 = 150 1C, t2 = 9 h, t = 12 h.
Fig. 7 P-XRD Patterns for study of thermal stability of the [HF/
HT(3.5)] system.
The P-XRD patterns of the thermally treated catalyst
are divided into two categories. In the first category (T =
30–100 1C), sharp and intense peaks at low diffraction angles
(2y = 10–271) and broad reflections at high angles (2y =
30–671) were observed, the spectra being identical to the
original [HF/HT(3.5)] catalyst. In the second category, (T =
150–250 1C), significant changes in the P-XRD patterns were
observed. For example, the peaks corresponding to the 003
and 006 planes were shifted to higher angles, 13.2 and 26.61,
respectively, when compared to the original catalyst (11.3 and
22.71, respectively). Furthermore, as the treatment tempera-
ture was raised up to 250 1C, the intensity of all peaks
decreased. The maximum decrease in peak intensity was
observed in the 2y range 20–501. In contrast, the region
around 2y = 601 was less affected by the rise in temperature.
The temperature at which these changes appear (transition
temperature) in the P-XRD patterns was found to depend on
the Mg/Al molar ratio of [HT]. The transition temperature
was found to decrease with increasing the Mg/Al molar ratio
of [HT]. The observed changes are due to the reaction between
interlayer carbonates and interlayer water molecules, produ-
cing hydroxyl anions at the interlayer gallery of this phase.
The TGA of the [HF] complex confirmed the stability of
complex up to 150 1C, where decomposition of [HF] resulting
in the observed shifting of the peaks of the 003 and 006 planes
of [HF/HT]. The layered structure of [HF/HT(3.5)] was still
maintained above this temperature, but it contained hydroxyl
anions in the interlayer, which are less bulky than the carbo-
nate anions found in the interlayer of the original catalyst. The
observed P-XRD patterns of [HF/HT] are more comparable
to that of [HT] than to that of [HF]. Similar observations have
been reported in the literature for hydrotalcite at different
temperatures characterized by high temperature in situ powder
XRD (HTXRD).29,30
constant for hydrogenation of n-butanal was found to be 0.40
hÀ1 at higher amount of [HF] complex (HT/HF ratio = 2),
T2 = 250 1C, and 0.37 hÀ1 at HT/HF ratio = 1, T2 = 150 1C,
which are comparable with the calculated rate constant for
hydroformylation reaction. The kinetic profiles of the pro-
ducts formed during the one-pot synthesis of C8 aldol deriva-
tives give important information for different stages of
formation of various products with time to arrive at the actual
reaction pathway. The kinetic profile for the formation of
2-ethylhexanal from propylene in one step is shown in Fig. 6.
The formation of butanals via propylene hydroformylation at
60 1C catalyzed by the [HF] complex was observed up to 3 h.
As the T1 increased to T2, the formation of 2-ethylhexenal was
seen after 3.25 h via aldol condensation of n-butanal, which is
subsequently consumed into 2-ethylhexanal in the presence of
the hydrogen. As the reaction time increased, the selectivity for
2-ethylhexanal was found to increase up to 9 h, with decrease
in the selectivity for the 2-ethylhexenal, due to the simulta-
neous consumption of the 2-ethylhexenal into 2-ethylhexanal.
No significant change in the selectivity for 2-ethylhexanal and
2-ethylhexenal were observed after 9 h reaction time. The
decrease in the selectivity for butanals after 3 h indicated it’s
regular consumption in the formation of C8 aldol derivatives
and butanols. It was interesting to see that the formation of
butanols also started after 4 h, the time after which formation
of C8 aldol derivatives were observed.
Thermal stability of multi-functional [HF/HT] catalysts
The experiments were conducted at different temperatures to
study the thermal stability of the [HF/HT(3.5)] catalyst at
partial pressure of CO and H2 at 20 atm; 700 mg of [HF/
HT(3.5)]; 50 ml toluene and 12 h reaction time. The thermal
treatment temperature was varied in the range 30–250 1C by
keeping the other reaction parameters constant. The P-XRD
patterns of the original [HF/HT(3.5)] catalyst and thermally
treated catalyst are shown in Fig. 7.
All the peaks of the original [HF/HT(3.5)] catalyst were
found in the FT-IR spectra of the thermally treated catalysts
(Fig. 8).
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
284 | New J. Chem., 2007, 31, 277–286

View Article Online
[HF/HT] has been developed and their multi-functional
potential evaluated for the one-pot selective synthesis of C8
aldol derivatives (aldehydes/alcohols) from propylene. The
[HF/HT] catalyst showed catalytic activity for hydroformyla-
tion, aldol condensation and hydrogenation in one pot. The
Mg/Al ratio of [HT], amount of [HF] complex and [HT], and
reaction temperature showed pronounced effect on the selec-
tivity for C8 aldol derivatives. Aldol condensation temperature
T2 played a significant role in the formation of 2-ethylhexanol
in one pot. As the Mg/Al molar ratio and amount of [HT] was
increased, the selectivity for 2-ethylhexanal also increased due
to the enhancement in the basicity of the catalyst. The amount
of [HF] complex in the catalyst significantly influenced the
selectivity of 2-ethylhexanal. From the kinetic experiments, it
was observed that the rate of formation of 2-ethylhexanal is
dependent on the rate of aldol condensation which is catalyzed
by hydrotalcite in the catalyst. The reaction pathways and role
of each component of multi-functional catalysts [HF/HT] for
the synthesis of 2-ethylhexanol is discussed with the help of the
kinetic profiles of the reaction with time.
Fig. 8 FT-IR spectra for study of the thermal stability of the
[HF/HT(3.5)] system.
Table 5 Effect of the high temperature treatment on the activity of
the catalyst
Selectivity (%)
Acknowledgements
C4 aldehydes C4 alcohols C8 aldehydes/alcohols
We thank Dr P. K. Ghosh, Director, CSMCRI, Bhavnagar,
India, for encouraging this publication and Network Project
on Catalysis, Council of Scientific and Industrial Research
(CSIR), New Delhi, India for financial supports. S. K. S.
thanks CSIR, New Delhi, for the award of a Senior Research
Fellowship.
Fresh catalyst 40
6
71
76
77
54
o1
o1
o1
Second run
Third run
Fourth run
28
23
22
Reaction conditions: Partial pressure of propylene = 10 atm, partial
pressure of CO = 5 atm, partial pressure of H2 = 15 atm, [HF/HT
(3.5)] = 700 mg, HT/HF = 7, T1 = 60 1C, t1 = 3 h, T2 = 150 1C,
t2 = 9 h, total time (t) = 12 h.
References
1 V. K. Srivastava, D. U. Parmar and R. V. Jasra, Chem. Weekly,
July 8, 2003, 173–178, and July 15, 2003, 181–190; Oxo Chemical
Report, SRI International, January 2003.
2 D. Frohning and C. W. Kohlpaintner, in Applied Homogeneous
Catalysis with Organometallic Compounds, ed. B. Cornils and W. A.
Hermann, Wiley-VCH, Weinheim, 2000, vol. 1, ch. 2, pp. 29–104.
3 A. D. Godwin, R. H. Schlosberg, F. Hershkowitz, M. G Matturro,
G. Kiss, K. C. Nadler, P. L Buess, R. C Miller, P. W. Allen, H. W.
Deckman, R. Caers, E. J. Mozeleski, J. Edmund and R. P.
Reynolds, US Pat., 6,307,093, 2001.
High-temperature treatment in hydrogen may cluster the
[HF] complex into Rh(0) supported aggregates, which is an
efficient catalyst for hydrogenation, but quite unreactive in the
hydroformylation of propylene. Keeping this point in view, we
have checked the catalytic activity of [HF/HT(3.5)] after four
successive runs, using the same catalyst under identical reac-
tion conditions. The results are shown in Table 5.
As seen from the data in Table 5, no change of activity of
the [HF] complex was observed in the hydroformylation and
hydrogenation reaction after high temperature treatment in
the presence of excess hydrogen atmosphere (Table 5). There-
fore, it can be safely said that under the conditions employed
in our reaction, the formation of Rh(0) is less likely. However,
the activity of [HT] on the aldol condensation of the aldehyde
was influenced significantly with a loss of selectivity for
C8 aldehydes/alcohols. This can be attributed to the structural
changes of hydrotalcite as evidenced by X-ray diffraction
(Fig. 7). This clearly shows that the [HF/HT] catalyst is
thermally stable up to 150 1C and slow decomposition follows
up to 200 1C. This is consistent with our experimental data,
which showed that excellent selectivity with respect to C8 aldol
derivatives was observed in the temperature range 150–200 1C.
4 (a) F. J. Doering and G. F. Schaefer, J. Mol. Catal., 1987, 41,
313–328; (b) B. J. Arena and J. S. Holmgren, US Pat., 5,144,089,
1992.
5 G. Horn, C. D. Frohning, H. Liebern and W. Zgorzelski, US Pat.,
5,475,161, 1995.
6 W. Bueschken and J. Hummel, US Pat., 5,756,856, 1998.
7 H. G. Lueken, U. Tanger, W. Droste, G. Ludwig and D. Gubisch,
US Pat., 4,968,849, 1990.
8 T. Mori, K. Fujita and H. Hinoishi, US Pat., 5,550,302, 1996.
9 (a) J. J. Spivey and M. R. Gogate, Pollution Prevention in Industrial
Condensation Reactions, Research Triangle Institute, USEPA
Grant, 1996; (b) G. J. Kelly, F. King and M. Kett, Green Chem.,
2002, 4, 392–399.
10 D. Tichit and B. Coq, CATTECH, 2003, 7, 206.
11 M. Tomoyuki, O. Yasukazu, F. Koichi, E. Hiroki and T. Akio,
Jpn. Pat., 11,269,118, 1999.
12 C. R. Greene, US Pat., 3,278,612, 1966.
13 R. V. Jasra, V. K. Srivastava, R. S. Shukla, H. C. Bajaj and S. D.
Bhatt, US and PCT Pat., US Appl. No. 20060149101, 2006.
14 D. D. Perrin, W. L. F. Armarego and D. D. Perrin, Purification of
Laboratory Chemicals, Pegamon Press, Oxford, 2nd edn, 1980.
15 D. Evans, G. Yagupsky and G. Wilkinson, J. Chem. Soc. A, 1968,
2660–2665.
Conclusions
Rhodium complex; HRh(CO)(PPh3)3 [HF] and hydrotalcite
(Mg1ÀxAlx(OH2))x+(CO3
16 M. J. Climent, A. Corma, S. Iborra, K. Epping and A. Velty, J.
Catal., 2004, 225, 316–326.
2À
)
x/n Á mH2O) [HT] based catalyst
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 277–286 | 285

View Article Online
17 V. K. Srivastava, S. K. Sharma, R. S. Shukla, N. Subrahmanyam
and R. V. Jasra, Ind. Eng. Chem. Res., 2005, 44, 1764–1771.
18 V. K. Srivastava, R. S. Shukla, H. C. Bajaj and R. V. Jasra, Appl.
Catal. A: Gen., 2005, 282, 31–38.
19 O. Levenspiel, Chemical Reaction Engineering, Wiley, New York,
3rd edn, 1998.
20 V. K. Srivastava, S. K. Sharma, R. S. Shukla and R. V. Jasra,
Catal. Commun., 2006, 7, 881–886.
21 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11,
173–301.
23 H. Liu and E. Min, Green Chem., 2006, 8, 657–662.
24 F. Basile, G. Fornasari, V. Rosetti, F. Trifiro and A. Vaccari,
Catal. Today, 2004, 91–92, 293–297.
25 J. M. Herman, P. J. Van Den Berg and J. J. F. Scholten, Chem.
Eng. J., 1987, 35, 25–35.
26 S. Ueno, K. Ebitani, A. Ookubo and K. Kaneda, Appl. Surf. Sci.,
1997, 121–122, 366–371.
27 K. Kaneda, K. Yamaguchi, K. Mori, T. Mizugaki and K. Ebitani,
Catal. Surv. Jpn., 2000, 4, 31–38.
28 H. Tsugi, F. Yagi, H. Hattori and H. Kita, J. Catal., 1994, 148,
759–770.
29 E. Kanezaki, Inorg. Chem., 1998, 37, 2588–2590.
30 E. Kanezaki, Solid State Ionics, 1998, 106, 279–284.
22 S. Abello, F. Medina, D. Tichit, J. P. Ramirez, J. C. Groen,
J. E. Sueiras, P. Salagre and Y. Cesteros, Eur. Chem. J., 2005, 11,
728–739.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
286 | New J. Chem., 2007, 31, 277–286