Nanosized noble metals intercalated in clay as catalysts for selective hydrogenation

Article abstract of DOI:10.1016/S1872-2067(10)60112-X

The use of clay minerals in the synthesis of nanosized noble metal particles to give increased catalytic activity was investigated. Nanosized platinum and ruthenium catalysts intercalated in clay (montmorillonite/ hectorite) were synthesised and their catalytic activity was evaluated for the selective hydrogenation of three different α,β-unsaturated aldehydes, namely, crotonaldehyde, cinnamaldehyde, and citral, in a gas phase microreactor. The metal nano-sol was prepared by the chemical reduction of its precursor by the micellar technique in the presence of cetyl trimethyl ammonium bromide (CTAB), and the micelle stabilized metal particles were intercalated in the clay mineral by ion exchange. TEM micrographs of the catalyst particles showed that the metal particles were in the nanometre range. The average diameters of the particles were 1-25 nm. The effects of temperature, metal loading, and hydrogen flow rate on the catalytic activity and selectivity for α,β-unsaturated alcohol were studied. The results were correlated with the structural properties of the catalysts. The products formed in each reaction over the different catalysts showed that the catalysts were very active for hydrogenation, and the selectivity for the desired product, namely, α,β-unsaturated alcohol, was good. The metal catalysts intercalated in montmorillonite showed better selectivity than that in hectorite because of its higher acidity. Increased selectivity for α,β-unsaturated alcohol was observed with increased flow rate of hydrogen.

Full text of DOI:10.1016/S1872-2067(10)60112-X

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 10, 2010
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin J Catal, 2010, 31: 1200–1208.
Nanosized Noble Metals Intercalated in Clay as Catalysts for
Selective Hydrogenation
Manikandan DHANAGOPAL1,2, DURAISWAMI Divakar1,3, Rupa A. VALENTINE1,
Mangalaraja RAMALINGA VISWANATHAN2, Sivakumar THIRIPURANTHAGAN1,*
1Catalysis Laboratory, Department of Chemical Engineering, A.C. College of Technology, Anna University, Chennai 600 025, India
2Advanced Ceramic and Nanotechnology Laboratory, Department of Materials Engineering, Faculty of Engineering, University of Concepcion,
Concepcion 407-0417, Chile
3Department of Chemical Engineering, Faculty of Science and Technology, University of Basque Country, 48940 Leioa, Bizkaia, Spain
Abstract: The use of clay minerals in the synthesis of nanosized noble metal particles to give increased catalytic activity was investigated.
Nanosized platinum and ruthenium catalysts intercalated in clay (montmorillonite/hectorite) were synthesised and their catalytic activity
was evaluated for the selective hydrogenation of three different α,β-unsaturated aldehydes, namely, crotonaldehyde, cinnamaldehyde, and
citral, in a gas phase microreactor. The metal nano-sol was prepared by the chemical reduction of its precursor by the micellar technique in
the presence of cetyl trimethyl ammonium bromide (CTAB), and the micelle stabilized metal particles were intercalated in the clay mineral
by ion exchange. TEM micrographs of the catalyst particles showed that the metal particles were in the nanometre range. The average
diameters of the particles were 1–25 nm. The effects of temperature, metal loading, and hydrogen flow rate on the catalytic activity and
selectivity for α,β- unsaturated alcohol were studied. The results were correlated with the structural properties of the catalysts. The products
formed in each reaction over the different catalysts showed that the catalysts were very active for hydrogenation, and the selectivity for the
desired product, namely, α,β-unsaturated alcohol, was good. The metal catalysts intercalated in montmorillonite showed better selectivity
than that in hectorite because of its higher acidity. Increased selectivity for α,β-unsaturated alcohol was observed with increased flow rate of
hydrogen.
Key words: selective hydrogenation; nano-particle; platinum; ruthenium; intercalated montmorillonite or hectorite; vapor phase reaction
The synthesis of metal nano-particles was first reported by
Boutonnet et al. [1] in the early eighties. Metal nano-particles
are of wide interest in the catalysis industry because of their
large surface area, and they have some interesting properties
that are different from those of bulk metal solids [2,3]. Among
these, increased catalytic activity has been reported with
nano-particles, which was correlated with their sizes and
shapes [4,5]. As the size of the particle approaches a few
nanometres, the surface area to volume ratio dramatically
increases. The increased surface area of the nano-catalyst re-
sults in more interaction between the catalyst and reactants,
which leads to a higher conversion. Surface atoms that are in
corners and edges also dramatically increase when the size is
decreased to below a few nanometres.
A promising method to prepare metal nano-clusters is by
chemical synthesis in a solution with a dispersing agent to
produce fine metal particles. Nano-particles are formed upon
reduction of the salts of the corresponding metals in the pres-
ence of substrates that stabilize the colloidal particles in the
solutions. The methods generally use reduction by radiation or
chemical reduction [6] in an aqueous solution with [7] and
without stabilizing polymers [8–10], and the chemical or
photo-reduction of surfactant micelles [11–13]. The colloidal
stability, particle size, and catalytic properties of the
nano-particles are strongly dependent on the specific method
and experimental conditions used.
Polymer stabilized metal particles with a narrow particle size
distribution have been used as catalysts in selective hydro-
Received date: 29 March 2010.
*Corresponding author. Tel: +91-44-22203534 Extn. 24; E-mail: sivakumar@annauniv.edu
Foundation item: Supported by FONDECYT, Chile-Post Doctorate-2010 Project (3100010).
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60112-X

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
genation. Pt catalysts stabilized by poly-(N-vinyl-
drogenation of α,β-unsaturated aldehydes with different
2-pyrrolidone) with a particle size of 1.1 nm was reported as
highly active and selective in the hydrogenation of cinnamal-
dehyde [14]. A polymer resin stabilized Pt catalyst was also
found to be a good catalyst for the hydrogenation of citral [15].
Although these catalysts were active and selective for the hy-
drogenation of unsaturated aldehydes, their industrial use was
limited because of the difficulty in separating the catalysts for
recycling. Therefore, attempts were made to immobilize these
colloidal metal particles from the aqueous solution onto the
surface of solid supports [16–18]. The interlayer space avail-
able in clay minerals was shown to be a very good phase that
hosted finely dispersed metal nano-catalysts without aggrega-
tion [19–24]. The use of such catalysts in selective hydrogena-
tion reactions was beneficial in terms of both activity and
selectivity [25,26].
structures such as crotonaldehyde, cinnamaldehyde, and citral.
1 Experimental
1.1 Synthesis of the catalysts
Aknown amount of montmorillonite was first ion exchanged
by stirring in 2 mol/L NaCl solution to ensure that all the ex-
changeable cations were Na+. Dried Na-montmorillonite or
Li-hectorite (3 g) as received from Wako Pure Chemical In-
dustries Ltd., Japan were separately dispersed in 200 ml water
and allowed to swell for about 6 h with continuous stirring.
Platinum or ruthenium nanosol was prepared by adding the
metal precursors (hexachloroplatinic acid or hexamminer-
uthenium chloride, respectively) containing 1% or 2% metal in
water in the presence of cetyltrimethylammonium bromide and
stirred for 2 h. The precursor in the micelles was reduced to the
metallic state by the dropwise addition of an aqueous solution
of NaBH4 (0.05 g in 10 ml) and stirred for about 3 h. Then the
black colored metal nano-sol was added to the aqueous sus-
pension of the clay and kept under vigorous stirring for about
24 h. The ion exchange reaction of Na+ in the case of mont-
morillonite and Li+ in the case of hectorite with the cetyl-
trimethylammonium cation (CTA+) resulted in the formation of
an organo-clay complex with the simultaneous intercalation of
the Pt or Ru particles, respectively, into the internal of the
(interlamellar) clay material [35,36]. The organo-clay samples
(black in color) were washed thoroughly with ethanol and
toluene to remove excess surfactant and air-dried. The different
catalysts synthesized were named Pt-CTA-MM 1, Pt-CTA-
MM 2, Ru-CTA-MM 1, Ru-CTA-MM 2, Pt-CTA-Hec 1,
Pt-CTA-Hec 2, Ru-CTA-Hec 1, and Ru-CTA-Hec 2, where
MM referred to montmorillonite, Hec referred to hectorite,
CTA referred to cetyl trimethyl ammonium cation, and the
number 1 and 2 indicated the metal loading in wt%.
α,β-unsaturated alcohols find applications in the chemical,
pharmaceutical, food flavors, and fragrance industries. It will
be useful to find new catalysts for the selective hydrogenation
of α,β-unsaturated aldehydes (namely cinnamaldehyde, cro-
tonaldehyde, citral, etc.) that can give very high conversion and
selectivity for the corresponding α,β-unsaturated alcohols.
Among the various parameters which govern the selectivity,
the structure of the reactant is one of the more important [27].
Small molecules such as acrolein, crotonaldehyde, etc. yield
their corresponding saturated aldehydes as final products be-
cause of the easy hydrogenation of the C=C bond due to fa-
vorable thermodynamic and kinetic factors. However, in the
case of larger unsaturated aldehydes, the steric effect due to the
presence of substituents on the C=C bond can influence prod-
uct selectivity. A variety of catalytic systems have been re-
ported for the selective hydrogenation of α,β-unsaturated al-
dehydes in the liquid phase [15,28,29]. We have reported the
selective hydrogenation of cinnamaldehyde and citral over
noble metals catalysts intercalated in clay [30,31]. The liquid
phase reaction has inherent limitations such as difficulty in
handling high pressure, corrosive nature of the solvents, sepa-
ration of the catalysts, lack of reusability, large amount of
catalysts used, and disposal of the reaction wastes. Therefore,
the chemical and pharmaceutical industries prefer vapor phase
reactions. Thus, it is necessary to find a suitable catalyst system
with high selectivity for the vapor phase hydrogenation of
α,β-unsaturated aldehydes. The literature on the vapour phase
hydrogenation of α,β-unsaturated aldehydes is scarce [32–34]
because the high temperature in these hydrogenation reactions
favour C=C hydrogenation rather than C=O hydrogenation. In
this context, the investigation of selective hydrogenation of
α,β-unsaturated aldehydes in the vapor phase using a pulse
reactor and metal nano-particle catalysts loaded in clay is im-
portant.
1.2 Physicochemical characterization
All the reduced catalysts were characterized by X-ray dif-
fraction (XRD) in the region of 2θ = 1.0° to 50° (Philips X'pert
X-ray diffractometer using Cu Kα radiation of wave length λ =
0.154 nm). For the transmission microscope analysis, the in-
tercalated catalysts (black powder after reduction) were dis-
persed in isopropanol, and a drop of this dispersion was placed
on a copper grid and TEM photographs were taken using a
JEOL JEM-2010F to determine the size of the noble metal
particles. The catalysts were also characterized by atomic
absorption spectrophotometry (AAS) (Perkin Elmer 2380).
The specific surface area of representative samples were de-
termined by the BET method using nitrogen as adsorbate with a
Quantachrome Nova 4200 e surface area analyser. The samples
were degassed at 250 °C under vacuum (< 0.1 Pa) for 2 h prior
In this paper, we report the synthesis and characterization of
the noble metals of Pt and Ru intercalated in montmorillonite
and hectorite and their catalytic activity for the selective hy-

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
to the adsorption measurement.
1.3 Catalytic activity in vapor phase hydrogenation
(3)
(2)
Catalytic reaction experiments were carried out in the vapor
phase with a flow microreactor equipped with an automated
online analysis system described below.
A pyrex reactor (20 cm length and 0.5 cm i.d.) was placed in
a vertical furnace which can be heated to high temperature in a
controlled manner. A thermocouple was inserted to give the
temperature of the catalyst bed. The pulse reactor contained an
injector port covered with a ribbon type heater to prevent the
condensation of the reactant and also to serve as a preheating
zone. The ribbon type heaters were also used to cover both the
inlet and outlet of the reactor. The entire tubing was insulated
with asbestos tape. The stainless steel tubings on both sides of
the pulse reactor and the injection valve were heated to 270 oC
to prevent condensation. The outlet of the reactor was con-
nected to the inlet of a chromatographic column ApiezonT of
the GC (Shimadzu 17A).
(1)
0
10
20
30
40
50
2θ/(o )
Fig. 1. XRD patterns of Na-MM (1), Pt-CTA-MM 1 (2), and
Pt-CTA-MM 2 (3).
ability of the montmorillonite due to its higher CEC value than
the other members in the smectite family. A small peak at 2θ =
40º indicated the presence of platinum particles (Pt(111)), and
the intensity of this peak increased when the loading of plati-
num was increased (Fig. 1(b) and (c)). Furthermore, there was
no shift in any other peak, which indicated that the exchange of
Na+ with CTA+ along with the incorporation of platinum parti-
cles in the interlayer surface did not alter the basic lattice
structure of montmorillonite. The ruthenium intercalated in
montmorillonite catalysts also showed similar XRD patterns
(Fig. 2) to that of the platinum intercalated in montmorillonite
catalysts. A shift from 2θ = 6.1º (d spacing 1.447 nm) to 2θ =
1.5º (d spacing 5.925 nm) and the (111) peak of Ru observed at
2θ = 44.5º confirmed the presence of ruthenium intercalated in
the montmorillonite.
The reduced catalysts (0.1 g) placed in the microreactor were
o
pretreated in flowing hydrogen (40 ml/min) at 200 C for 1 h
prior to catalytic reaction. The catalytic activity was evaluated
by injecting 1.0 μl of the reactant (crotonaldehyde or cinna-
maldehyde or citral) at different temperatures from 100 to 200
o
oC in steps of 25 C with different flow rates of hydrogen
(20–80 ml/min). The products were separated and identified by
online gas chromatographic analysis. The column temperature
was kept at 200 °C. Reagents and products were identified by
comparison with standard samples. All the experiments were
repeated at least twice, and the reproducibility was within ±
3%. Blank experiments showed no catalytic activity due to the
support under these conditions.
The XRD patterns of Li-hectorite is shown in Fig. 3, in
which the first order reflection appeared at 2θ = 7.4° (1.19 nm).
The same peak after the modification with the cationic sur-
factant (CTAB) was shifted to 2θ = 4.6° (1.93 nm). The in-
crease in the basal spacing in hectorite was less when compared
to montmorillonite. This was because of the lower CEC value
of hectorite as compared to montmorillonite. The CEC value
2 Results and discussion
2.1 Characterization studies
2.1.1 XRD analysis
The XRD patterns of Na-montmorillonite and Pt intercalated
in montmorillonite (Pt-CTA-MM 1 and Pt-CTA-MM 2) pre-
viously reported by us [37] showed that the d(001) reflection of
Na-montmorillonite appeared at 2θ = 6.1°, which corresponded
to a d spacing of 1.447 nm, and this 2θ value was shifted to 2θ =
1.5° for the CTA+ exchanged Pt montmorillonite catalysts,
which corresponded to the d spacing of 5.925 nm (Fig. 1).
The shift in the 2θ value of the d(001) reflection to a lower
value revealed that the basal spacing was increased by the
incorporation of a bigger organic moiety in the interlamellar
space that replaced M+. The remarkable increase in the d
spacing value to 5.925 nm after the intercalation of the CTA+
cation into the interlayer space indicated the better swelling
(3)
(2)
(1)
0
10
20
30
40
50
2θ /(o )
Fig. 2. XRD patterns of Na-MM (1), Ru-CTA-MM 1 (2), and
Ru-CTA-MM 2 (3).

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
Table 1 Physico-chemical properties of the catalysts
Material
Metal content (wt%)
A
BET/(m2/g) dave/nm
Na-Montmorillonite
CTAM
208.7
90.5
(3)
Pt-CTA-MM 1
Pt-CTA-MM 2
Ru-CTA-MM 1
Ru-CTA-MM 2
Li-Hec
0.98
1.97
0.89
1.98
—
128.9
—
2.7
6.9
112.5
—
11.2
21.7
—
(2)
(1)
47.13
34.21
40.68
—
CTA-Hec
—
—
Pt-CTA-Hec 1
Pt-CTA- Hec 2
Ru-CTA- Hec 1
Ru-CTA- Hec 2
0.92
1.68
0.93
1.85
2.3
9.5
41.81
—
21.3
24.5
0
10
20
30
40
50
(2), and
2θ/(o )
XRD patterns of Li-Hec (1), Pt-CTA-Hec
Pt-CTA-Hec 2 (3).
Fig. 3.
1
introduction of metal particles and the surfactant micelle.
2.1.2 TEM analysis
(87 meq/100 g and 49 meq/100 g for montmorillonite and
hectorite, respectively) determines the amount of exchange of
the surfactant cation (CTA+), which in turn causes the increase
in basal spacing. More exchange of the bulky organic cation
gives a larger basal spacing. The XRD data also revealed that
the lattice structure of the clay minerals was not affected by the
TEM images of platinum and ruthenium intercalated in
montmorillonite are shown in Fig. 4. A good and uniform
dispersion was obtained with almost all the catalysts, which
was previously reported by us [37,38]. The average diameters
of the particles are reported in Table 1. The diameters of the
(a)
(a)
(b)
(b)
20 nm
50 nm
200.0 kV × 300k
200.0 kV × 150k
(d)
(c)
(c)
(d)
20 nm
50 nm
200.0 kV × 200k
200.0 kV × 100k
Fig. 4. Transmission electron micrographs of Pt-CTA-MM 1 (a), Pt-CTA-MM 2 (b), Ru-CTA-MM 1 (c), and Ru-CTA-MM 2 (d).

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
particles were in the nm range, which may be due to the pres-
as support materials and the metal loaded clay catalysts are
given in Table 1. It was observed that the surface area de-
creased when the clay minerals were intercalated with the
bulky surfactant cations. This was because of the inaccessibil-
ity of the adsorbate nitrogen molecules into the interlamellar
space due to the presence of the bulky organic cations [36]. The
increase in the surface area of the metal loaded catalysts was
due to the adsorption of nitrogen molecules on metallic sites
also.
ence of the surfactant in the interlamellar space. The surfactant
cations swell the layer spacing so that the particles have more
chance to disperse over a wide space and also prevented ag-
glomeration.
TEM of platinum intercalated in hectorite (Pt-CTA-Hec 1
and Pt-CTA-Hec 2) are shown in Fig. 5. This figure indicated
that the particle size was similar to that of Pt intercalated in
montmorillonite but the dispersion of the particles was poor
when compared to the Pt-CTA-MM catalysts. TEM images
(Fig. 5) showed that the particle size of Ru in the Ru-hectorite
catalysts was larger than in the Ru-montmorillonite catalysts.
This increase in particle size may be due to the lesser swelling
of hectorite compared to montmorillonite.
2.2 Selective hydrogenation of crotonaldehyde,
cinnamaldehyde, and citral
Catalytic reaction experiments were carried out in a micro-
reactor (vapor phase) equipped with an online gas chroma-
tographic system. In each experiment 0.1 g of reduced catalyst
(Pt-CTA-MM 1, Pt-CTA-MM 2, Ru-CTA-MM 1, Ru-CTA-
MM 2, Pt-CTA-Hec 1, Pt-CTA-Hec 2, Ru-CTA-Hec 1, or
Ru-CTA-Hec 2) was preactivated in flowing hydrogen (40
ml/min) at 200 °C for 1 h. The catalytic activity was evaluated
by injecting 1.0 μl of crotonaldehyde or cinnamaldehyde or
citral at different temperatures. The comparison of the catalytic
activity of the intercalated montmorillonite and hectorite
catalysts was made. It showed the effect of host material on
conversion and selectivity towards the corresponding
2.1.3 AAS analysis
The catalysts were subjected to AAS analysis to determine
the actual metal content in the clay minerals. The results re-
ported in Table 1 indicated that the actual metal contents of the
catalysts were almost equal to the nominal amounts.
2.1.4 BET surface area analysis
The surface area of the different smectite clay minerals used
(b)
(b)
(a)
(a)
200.0 kV × 200k
20 nm
20 nm
200.0 kV × 200k
(c)
(d)
(d)
(c)
100nm
200.0 kV × 80k
100nm
200.0 kV × 50k
Fig. 5. Transmission electron micrographs of Pt-CTA-Hec 1 (a), Pt-CTA- Hec 2 (b), Ru-CTA- Hec 1 (c), and Ru-CTA- Hec 2 (d).

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
α,β-unsaturated alcohols.
nium catalyst (Ru-CTA-MM 1) also exhibited the same trend
of catalytic activity (Fig. 6(b)) to that of the intercalated Pt
catalysts. The conversion at 100 °C was about 19%, which
increased linearly as the temperature increased and reached a
maximum of 87% at 200 °C. The selectivity for crotyl alcohol
over the intercalated Ru catalysts was comparatively low when
compared to Pt-CTA-MM 1. This may be due to the adsorption
at the C=C bond followed by hydrogenation, which is more
favorable over the ruthenium particles than C=O hydrogena-
tion because of its smaller d orbital and the larger particle size
of ruthenium compared to platinum [41].
2.2.1 Effect of temperature
The reaction temperature was varied from 100 °C to 200 °C
in steps of 25 °C at a constant flow rate of hydrogen (40
ml/min) over 0.1 g of Pt-CTA-MM 1 and Ru-CTA-MM 1
catalysts. The conversion of crotonaldehyde (CRAL) increased
as the temperature of the reaction increased (Fig. 6 (a) and (b)).
This was because metallic sites at higher temperatures gave
more adsorption of the reactant, namely, crotonaldehyde,
which resulted in increased conversion. As the temperature
increased, the selectivity for crotyl alcohol (CROH) decreased
over Pt-CTA-MM 1 (Fig. 6(a)). A similar observation was
made by Englisch et al. [39]. This decrease in selectivity with
temperature may be due to either the isomerization of crotyl
alcohol into butyraldehyde or the consecutive hydrogenation of
crotyl alcohol to yield butyl alcohol at higher temperatures.
Also, the observed selectivity (31%) of crotyl alcohol over
Pt-CTA-MM 1 was better than that with the same amount (1
wt%) of Pt catalysts loaded on CeO2 and reduced at 473 K,
which showed no more than 5% selectivity [40]. Even with the
very low metal content, the preferred formation of Pt(111)
particles in the Pt-CTA-MM 1 catalyst may make the catalyst
more selective towards C=O hydrogenation, which is less
favored in ceria supported catalysts. The intercalated ruthe-
As the selectivity is influenced by the presence of substitu-
ents on the C=C bond, studies were conducted with a reactant
containing a phenyl group on the C=C bond, that is, cinna-
maldehyde (CAL). The hydrogenation of cinnamaldehyde was
carried out over all the catalysts using the same microreactor
(vapor phase) connected to a gas chromatograph. Data ob-
tained in the hydrogenation of cinnamaldehyde over
Pt-CTA-MM 1 and Ru-CTA-MM 1 catalysts are shown in Figs.
6(a) and (b). Again, the temperature played a vital role in both
the activity and selectivity for cinnamyl alcohol (COL). As the
temperature increased, the conversion increased linearly. A
remarkable conversion of 84% was obtained with the
Pt-CTA-MM 2 catalyst at 200 °C. This was because of the
higher reactivity of hydrogen on the platinum surface than on
the other noble metals [42]. The steric effect due to the bulky
(a)
(b)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
100
80
60
40
20
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
0
(c)
(d)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
100
80
60
40
20
0
100
125
150
Temeprature (oC)
175
200
100
125
150
Temperature (oC)
175
200
Fig. 6. Effect of temperature on conversion and selectivity over Pt-CTA-MM 1 (a), Ru-CTA-MM 1 (b), Pt-CTA-MM 2 (c), and Ru-CTA-MM 2 (d) cata-
lysts. Mass of catalyst: 0.1 g; hydrogen flow rate: 40 ml/min.

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
phenyl group and the temperature of the reaction altered the
montmorillonite showed better selectivity than the metal
loaded in hectorite. This difference in selectivity may be due to
the presence of higher acidity in montmorillonite compared to
hectorite. The higher selectivity was due to Lewis acid sites
located in the vicinity of the metal [20]. The carbonyl oxygen is
able to adsorb on the Lewis acid sites, so that the simultaneous
adsorption of C=C and C=O does not occur. Also, the reactant
structure and presence of substituents in the C=C bond may
have some influence on the selectivity of the catalyst for the
formation of corresponding unsaturated alcohol.
selectivity significantly. Even though the bulky phenyl group
was present in cinnamaldehyde, a high temperature favored the
consecutive hydrogenation and resulted in some amount of
hydrocinnamyl alcohol and less selectivity for cinnamyl al-
cohol.
The conversion of citral into the hydrogenated products was
found to be comparatively higher than the conversion obtained
for crotonaldehyde or cinnamaldehyde hydrogenation over
both Pt-CTA-MM 1 and Ru-CTA-MM 1. This was due to the
presence of more reducible functionalities in the citral mole-
cule. At high temperature, less selectivity for the desired
products, namely, geraniol and nerol (G+N), was obtained over
these catalysts due to consecutive hydrogenation and isom-
erization.
2.2.2 Effect of metal loading
The data obtained in the hydrogenation of crotonaldehyde,
cinnamaldehyde, and citral at different temperatures over
Pt-CTA-MM 2, Ru-CTA-MM 2, and Pt-CTA-Hec 2,
Ru-CTA-Hec 2 are plotted in Fig. 6(c), (d), and Fig. 7(c), (d),
respectively. The results indicated that as the metal loading
increased, conversion increased due to the availability of more
catalytic sites. However a slight decrease in selectivity was
observed with 2 wt% metal loaded catalysts, which may be due
to the presence of larger sized metal particles.
The effect of temperature on the catalytic activity and se-
lectivity of Pt and Ru intercalated in hectorite (Pt-CTA-Hec 1
and Ru-CTA-Hec 1) was also evaluated with the hydrogenation
of the three different α,β-unsaturated aldehydes. The results
are reported in Figs. 7(a) and (b). As observed with the other
catalysts, the conversion of the different aldehydes into hy-
drogenated products over Pt and Ru intercalated in hectorite
increased and the selectivity for the corresponding unsaturated
alcohols decreased when the temperature was increased.
In order to establish the effect of the host material on selec-
tivity for the unsaturated alcohol, the data obtained at 150 °C
for all the catalysts was compared. The metal loaded in
2.2.3 Effect of hydrogen flow rate
In this vapor phase hydrogenation reaction, hydrogen gas
was used as reactant as well as carrier gas. Therefore, a change
(b)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
(a)
100
80
60
40
20
0
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
(c)
(d)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
100
80
60
40
20
0
100
125
150
175
200
100
125
150
Temperature (oC)
175
200
Temperature (oC)
Fig. 7. Effect of temperature on conversion and selectivity over Pt-CTA-Hec 1 (a), Ru-CTA- Hec 1 (b), Pt-CTA- Hec 2 (c), and Ru-CTA- Hec 2 (d)
catalysts. Mass of catalyst: 0.1 g; hydrogen flow rate: 40 ml/min.

Manikandan DHANAGOPAL et al. / Chinese Journal of Catalysis, 2010, 31: 1200–1208
80
70
60
50
40
30
20
(a)
(b)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. forCROH
Sel. for COL
Sel. for G+N
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
10
80
(c)
(d)
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
Conv. of CRAL
Conv. of CAL
Conv. of Citral
Sel. for CROH
Sel. for COL
Sel. for G+N
70
60
50
40
30
20
10
20
30
40
50
60
70
80
20
30
40
50
60
70
80
Hydrogen flow rate (ml/min)
Hydrogen flow rate (ml/min)
Fig. 8. Effect of hydrogen flow rate on conversion and selectivity over Pt-CTA- MM 1 (a), Ru-CTA-MM 1 (b), Pt-CTA-Hec 1 (c), and Ru-CTA-Hec 1 (d)
catalysts. Mass of catalyst: 0.1 g; temperature: 150 °C.
in the flow rate of hydrogen alters the partial pressure of hy-
drogen and also the contact time of the reactant in the catalyst
bed, which may lead to a significant change in the yield and
selectivity. Englisch et al. [39] observed significant changes in
the conversion and selectivity in the vapor phase hydrogena-
tion of crotonaldehyde with different partial pressures of hy-
drogen. Thus, in this study, the hydrogenation of crotonalde-
hyde, cinnamaldehyde, and citral at different hydrogen flow
rates between 20–80 ml/min over representative catalysts,
namely, Pt-CTA-MM 1, Ru-CTA-MM 1, Pt-CTA-Hec 1, and
Ru-CTA-Hec 1, were carried out to find out the influence of
hydrogen flow rate on the conversion and selectivity. The
reactant (1.0 µl) was injected onto the catalyst in the microre-
actor at 150 °C. The data obtained in the hydrogenation of
various unsaturated aldehydes at different flow rates are shown
in Fig. 8.
3 Conclusions
Intercalation of nano-particles of noble metals in clay min-
erals by the micellar technique was used to disperse the
nano-particles in the interlamellar region of the clay material.
TEM micrographs of the intercalated metal catalysts confirmed
that the dispersion of the particles was good and had sizes in
the nm range (1–25 nm). Due to higher swelling capacity of
montmorillonite than hectorite, a better dispersion of metal
particles was obtained with the former. The vapor phase hy-
drogenation of unsaturated aldehydes showed that almost all
the catalysts showed comparable catalytic activity under the
same conditions. The increased selectivity of the metal cata-
lysts intercalated in montmorillonite than in hectorite was due
to the higher acidity of the former. The observed increase in
selectivity for α,β-unsaturated alcohol with increase in flow
rate of hydrogen showed that the selectivity was not only con-
trolled by the active sites but also by the partial pressure of
hydrogen. The hydrogenation of α,β-unsaturated aldehydes
over these synthesized catalysts indicated that they can be used
as the catalysts for many other hydrogenation reactions.
The flow rate of hydrogen has a strong influence on the
conversion of different unsaturated aldehydes and also on the
distribution of the products in the hydrogenation. The increase
in hydrogen flow rate increased the partial pressure of hydro-
gen and thus decreased the contact time which decreased
conversion. The selectivity for the corresponding unsaturated
alcohols increased when the flow rate of hydrogen increased.
This indicated that the selectivity was not only controlled by
the properties of the active sites but also by the pressure of
hydrogen [43,44].
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