Preparation of an organophilic palladium montmorillonite catalyst in a micellar system

Article abstract of DOI:10.1039/a905321b

The cation-exchange reaction between sodium montmorillonite and tetradecyltrimethylammonium bromide serving as a stabilizing agent in a palladium hydrosol led to the formation of alkylammonium montmorillonite with simultaneous immobilization of the palladium nanoparticles in the organoclay host; the Pd-organoclay proved to be catalytically active in olefin hydrogenation in the liquid phase.

Full text of DOI:10.1039/a905321b

Preparation of an organophilic palladium montmorillonite catalyst in a
micellar system
a
a
b
c
a
Zoltán Király,* Bernadett Veisz, Ágnes Mastalir, Zsolt Rázga and Imre Dékány
a
Department of Colloid Chemistry, University of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary.
E-mail: zkiraly@chem.u-szeged.hu
Department of Organic Chemistry, University of Szeged, D o´ m t e´ r 8, H-6720 Szeged, Hungary
Laboratory for Electron Microscopy, University of Szeged, Kossuth L. sgt. 40, H-6725 Szeged, Hungary
b
c
Received (in Oxford, UK) 30th June 1999, Accepted 23rd August 1999
The cation-exchange reaction between sodium montmor-
illonite and tetradecyltrimethylammonium bromide serving
as a stabilizing agent in a palladium hydrosol led to the
formation of alkylammonium montmorillonite with simulta-
neous immobilization of the palladium nanoparticles in the
organoclay host; the Pd-organoclay proved to be catalyt-
ically active in olefin hydrogenation in the liquid phase.
The proposed mechanism for the successive formation of the
Pd hydrosol and the Pd-organoclay is outlined in Fig. 1.
Whereas water is a poor solvent, CHCl is a good solvent for
Pd(acac) . The apparent solubility of CHCl in water increases
considerably if a sufficient quantity of C14TAB surfactant is
present in the aqueous phase (c > cmc). This solubilization
effect allows the preparation of an aqueous colloidal solution of
3
2
3
Pd(acac)
(UVIKON 930 spectrophotometer) of Pd(acac)
shift in the absorbance maximum in the micellar solution (340
nm) as compared with that in pure CHCl solvent (327 nm).
This observation may be attributed to the change in the
microenvironment of Pd(acac) upon solubilization, or to the
formation of an adduct between the surfactant and the palladium
salt.8 In either case, the addition of an aqueous NH
NH
solution to the Pd(acac) –CHCl –C14TAB–water micellar sys-
2
previously dissolved in CHCl
3
. The UV-VIS spectra
The incorporation of Pd2+ ions, Pd(ii) complexes or metallic Pd
particles between the silicate layers of montmorillonite permits
liquid-phase hydrogenation reactions with improved catalytic
activity and specificity as compared with those involving
supported Pd catalysts prepared by conventional impregnation
routes, or with Pd(ii) catalysts used in homogeneous solution.
Pd(ii) acetate and Pd(ii) chloride have earlier been anchored to
internal surface sites of montmorillonite via bipyridyl, diph-
enylphosphine and other linkages to produce interlamellar
indicated a
2
3
2
2
2
2
3
tem resulted in the formation of nanoscale Pd particles, partly
sterically and partly electrostatically stabilized by the cationic
surfactant molecules adsorbed on the surface of the particles
(Fig. 1). The size distribution of the Pd particles in the hydrosol
was determined by using a Philips C-10 transmission electron
microscope (TEM) at 100 kV, assisted by the UTHSCSA Image
Tool program. The present preparation method yielded spher-
ical, monodispersed particles with a good control of size in the
range 1.5–6 nm. We found that the particle size in the Pd
1,2
functionalized montmorillonite Pd(ii) catalysts. Reaction of
+
2+
ion-exchanging Na -montmorillonite with [Pd(NCMe)
4
]
and
subsequent reduction of the Pd2 in methanol afforded highly-
dispersed metallic Pd particles in the clay galleries. Ethanol is
both a solvent and a reducing agent for Pd(ii) acetate. The
+
3
4
preferential adsorption of ethanol from dilute hydrocarbon
solutions in the interlamellar spaces of alkylammonium mont-
morillonite5 and alumina-pillared montmorillonite provided a
suitable environment for in situ reduction of the Pd(ii) acetate
precursor in the interlamellar spaces of the clay host. The
present work describes a new pathway for the production of
organophilic Pd-montmorillonite (Pd-M) with a good control
over the size of the Pd nanoparticles and the Pd content of the
clay. The method involves the synthesis of a monodispersed Pd
hydrosol with subsequent deposition of the Pd particles onto the
clay lamellae.
,6
7
2
hydrosol decreases with a decrease of the Pd(acac) concentra-
tion and, in particular, with an increase of the C14TAB
concentration.
The formation of Pd-M was readily achieved by mixing the
hydrosol with a dilute montmorillonite suspension. Reaction of
+
the ion-exchanging Na -montmorillonite with the stabilizing
(and in part with free) cationic surfactant molecules resulted in
the formation of organophilic alkylammonium montmorillonite
with the simultaneous release and subsequent deposition of the
Pd nanoparticles onto the surface of the silicate layers (Fig. 1).
The preparation of the Pd hydrosol and a systematic study
relating to the control over the Pd particle size were performed
under the following experimental conditions. 25–200 mL of
Pd(ii) acetylacetonate [Pd(acac)
CHCl
2
] solution (0.5–3 w/v% in
) were added to 10 mL of tetradecyltrimethylammonium
3
bromide (C14TAB) surfactant solution [5–50 times the critical
micelle concentration (cmc = 3.9 mM at 298 K in water)]. An
excess of NH NH solution (55 w/w% in water) was then
2 2
introduced into the micellar system, which was left under
vigorous stirring overnight to produce ultrafine Pd particles
stabilized by the cationic surfactant.
The preparation of Pd-M and a systematic study concerning
the control over the Pd content of the Pd-organoclay included
the following experimental conditions. 50–200 mL of Pd
hydrosol (stabilized with C14TAB, 5–50 times the cmc) were
+
added to 100–500 mL Na -montmorillonite suspension (0.1–2
w/w% in water) and the system was mixed vigorously
overnight. The surfactant molecules rendered the clay surface
hydrophobic, in parallel with the adsorption of the Pd particles
on the silicate surface. The Pd-M material was purified by
several centrifugation/redispersion cycles in ethanol and finally
dried in the oven.
Fig. 1 Schematic illustration of the proposed mechanism for the successive
formation of Pd hydrosol and Pd-organoclay in a micellar system.
Chem. Commun., 1999, 1925–1926
This journal is © The Royal Society of Chemistry 1999
1925

Table 1 Characterization of organophilic Pd-montmorillonite catalysts
1023r a
(styrene)
1023
r
i
a
1023
r
i
a
Catalyst
code number
Pd (w/w%)
(UV–VIS)
Pd (w/w%)
(ICP–AES)
i
d(TEM)/nm
(hex-1-ene) (cyclohexene)
Pd-M1
Pd-M2
Pd-M3
2.42
3.25
5.21
0.09
1.15
1.68
0.11
1.32
1.90
275.1
34.8
1.8
101.8
26.5
1.1
9.5
5.8
0.8
a
i 2
Initial rate of hydrogenation, r [mL H
(g Pd min)21], in toluene.
TEM images indicated that the particles are more or less
uniformly distributed on the external surface sites. The Pd
contents of the Pd-M samples were determined indirectly by
that the consecutive (isomerization plus hydrogenation) reac-
tions hex-1-ene?(trans- and cis-hex-2-ene)?n-hexane are
superimposed on the hydrogenation reaction hex-1-ene?n-
hexane. While the simultaneous hydrogenation and isomeriza-
tion of hex-1-ene were found to be fast processes, as was the
hydrogenation of styrene, the rate of conversion to n-hexane
was moderated by the appearance of the hex-2-ene isomers and
the kinetic curve resembled that of cyclohexene (internal
olefin).
It is anticipated that the basic principles of the present
preparation method can be applied to other systems where the
metallic particles are stabilized by ionic surfactants and the
solid support is an oppositely charged ion exchanger. For
example, if transition metal particles (M) stabilized by anionic
surfactants are mixed with positively charged layered double
hydroxides (LDH), organophilic M-LDH will be formed, which
can be used in contact catalytic reactions in organic solvents.
Experimental work on this system is currently being under-
taken.
9
measuring the absorbance of the supernatant at 600 nm during
adsorptive removal of the Pd colloids by montmorillonite.
Further, the Pd content was measured directly by inductively
coupled plasma atomic emission spectroscopy at 229.7 nm7
(
ICP-AES, Jobin Yvon 24). It was established that the Pd
content of the organoclay increases with decreasing solid to
liquid ratio and, in particular, with decreasing surfactant
concentration. X-Ray diffraction measurements on dry Pd-M
samples were made with a Philips 1820 diffractometer (40 kV,
3
1
5 mA, Cu-Ka radiation) in the diffraction angle range 2q =
–10°. Each material gave a broad d001 reflection, with an
interlayer spacing substantially smaller than the mean diameter
of the Pd particles as determined by TEM. This finding is
consistent with a disordered deposition of layer packets
encapsulating the Pd particles (Fig. 1). A regular pattern of
intercalation of the Pd species between the layers is unlikely.
Pd-M can be dispersed in organic solvents (ethanol, benzene,
acetone, etc.) that are generally used in contact catalytic
hydrogenation reactions in the liquid phase. Three Pd-M
samples were selected for catalytic test reactions (Table 1).
Toluene was used as the reaction medium: this solvent readily
wets and swells the organoclay and provides access to the Pd
particles for the reactant molecules. The rates of styrene, hex-
Z. K. and Á. M. gratefully acknowledge the financial support
of the Hungarian Scientific Foundation through grants OTKA
T025002 and T026430.
Notes and references
1
2
3
K. Ravikumar, B. M. Choudary and Zafir Jamil, J. Chem. Soc., Chem.
Commun., 1986, 130.
B. M. Choudary, G. V. M. Sharma and P. Barathi, Angew. Chem., Int. Ed.
Engl., 1989, 28, 465.
1
-ene and cyclohexene hydrogenation were recorded by using
an automated vibration reactor at atmospheric pressure at 303
K. Typically, the reaction vessel contained 1 mL of toluene, 0.1
mL of substrate and a calculated amount of catalyst loaded with
M. Crocker, J. G. Buglass and R. H. M. Herold, Chem. Mater., 1993, 5,
2
5 mg of Pd. A Hewlett Packard GC–MS apparatus (HP 5890;
1
05.
HP 5970) was used for product analysis. Close correlations
were found between the catalytic activity and the position of the
CNC double bond of the substrate, and the catalytic activity and
the degree of dispersion of the Pd particles (Table 1). For a
4 K. Esumi, T. Itakura and K. Torigoe, Colloids Surf., 1994, 82, 111.
5 Z. Király, I. Dékány, Á. Mastalir and M. Bartók, J. Catal., 1996, 161,
401.
6
Á. Mastalir, F. Notheisz, Z. Király, M. Bartók and I. Dékány, Stud. Surf.
Sci. Catal., 1997, 108, 477.
given substrate, the initial rate of hydrogenation, r
appreciably with decreasing particle size. For a given Pd-M
catalyst, r increases in the sequence styrene > hex-1-ene >
i
, increases
7
A. Szücs, Z. Király, F. Berger and I. Dékány, Colloids Surf., 1998, 139,
1
09.
i
8
9
N. Arul Dhas and A. Gedanken, J. Mater. Chem., 1998, 8, 445.
D. N. Furlong, A. Launikonis, H. F. Sasse and J. V. Sanders, J. Chem.
Soc., Faraday Trans. 1, 1984, 80, 571.
cyclohexene. The CNC bond in conjugation with the aromatic
ring displays the highest affinity toward the uptake of hydrogen,
and the reactivity is higher for the terminal olefin than for the
internal one. GC–MS analysis at intermediate stages indicated
Communication 9/05321B
1926
Chem. Commun., 1999, 1925–1926