Preparation, characterisation and catalytic hydrogenation properties of palladium supported on C60

Article abstract of DOI:10.1039/a700804j

A C₆₀-supported Pd catalyst has been prepared by reaction between C₆₀ and Pd(OAc)₂(PPh₃)₂ in toluene, to give the complex C₆₀[Pd(OAc)₂(PPh₃)]₃, followed by

Full text of DOI:10.1039/a700804j

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Preparation, characterisation and catalytic hydrogenation
properties of palladium supported on C
60
Rongqing Yu,a Qiping Liu,a Kuang-Lee Tan,b Guo-Qin Xu,a* Siu Choon Ng,a
Hardy S. O. Chana and T. S. Andy Hora*
Departments of Chemistrya and Physicsb, Faculty of Science, National University of Singapore,
Kent Ridge, Singapore 119260
A C -supported Pd catalyst has been prepared by reaction between C and Pd(OAc) (PPh ) in toluene, to give the complex
60
60
2
3 2
C
[Pd(OAc) (PPh )] , followed by H treatment at 523 K for 4 h. Catalytic quantities (1 mol%) promote hydrogenation of
60
2
3 3
2
diphenylacetylene, phenylacetylene, cyclohexene and hex-1-ene to give 100% conversion to 1,2-diphenylethane, phenylethane,
cyclohexane and hexane within 18, 13, 21 and 12 min, respectively. Hydrogenation of the same substrates under similar condi-
tions using Pd on activated charcoal (10%) as catalyst gives similar yields but at a longer time (20, 18, 27 and 15 min,
respectively). Both the Pd-C catalyst and its precursor were characterised by thermogravimetry (TG), FTIR, mass spectrom-
60
etry (MS), powder XRD, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).
Since the discovery and subsequent large-scale production of
as a new allotrope of carbon, there has been great activ-
(OAc~ \ CH CO ~) (0.033 g, 0.15 mmol) was dissolved in
3
2
C
toluene (15 ml) and triphenylphosphine (PPh ) (0.0786 g, 0.3
60
3
ity in the development of the science and technology of this
new material.1h7 In line with our on-going work on the syn-
thesis and catalytic applications of organometallic complexes8
mmol) was added under a nitrogen atmosphere. After 30 min,
a toluene solution (35 ml) of C (0.036 g, 0.05 mmol) was
60
added to the resultant yellow solution. Upon stirring, the
and C -doped polymeric materials,9 we are especially inter-
mixture darkened and a black precipitate formed. This pre-
cipitate was isolated by ültration and washed with toluene
(2 ] 15 ml). The resulting black product (1) was dried in
vacuo for 5 h. 1 was then reduced under a dynamic ýow (20
60
ested in the development of metallobuckminsterfullerenes,
(ML ) C , and their catalytic applications. In recent years,
x n 60
there has been increasing interest in this area of research.10h14
The technological signiücance of catalytic hydrogenation and
the established hydrogenation ability of many organometallic
complexes in catalytic quantities prompted us to focus our
investigations in this direction. Palladium on activated carbon
in the form of charcoal is an established hydrogenation cata-
lyst and is one of the current catalysts-of-choice.15 Its multi-
tude of applications in organic synthesis, materials science
and industrial operations reýects its scientiüc and technologi-
cal signiücance. The use of this heterogeneous catalyst,
however, has several notable problems: (1) the preparative
procedure usually involves heterogeneous deposition, such as
ion exchange or impregnation, whereby the conformity and
uniformity of the dispersion are critical as they aþect the cata-
lyst performance; (2) commercial samples are commonly
available in the form of 1–10% Pd on charcoal. There is little
room for regio- or chemioselectivity except by changing the
concentration of Pd on the support; (3) despite the wide uses
of Pd on charcoal, the catalytic mechanism in many cases is
still poorly understood. This is traced to the difficulty in the
study of surface structure and behaviour of impregnated Pd
solids in solution. In an attempt to overcome these problems,
we chose to study the molecular interaction of palladium
cm3 min~1) of H gas while the temperature was raised from
2
298 to 523 K at 3 K min~1 and then held at 523 K for 4 h.
The gas ýow was then changed to helium and the catalyst 2
was obtained after cooling the sample.
Microanalysis, FTIR, MS and TG were used to character-
ise 1 and 2. Powder XRD was measured with a Philips 1700
diþractometer with Cu-Ka radiation. XPS analysis for the Pd
state in 1 and 2 was carried out on a VG ESCALAB MKII
spectrometer using an Mg-Ka X-ray source (1253.6 eV, 120 W)
at a constant analyser. TEM observation was made on a
JEOL-100 CX II transmission electron microscope at 100 keV
accelerating voltage. A conventional carbon-supported palla-
dium catalyst (Pd/C) with 10% Pd content was obtained from
Aldrich Chemical Inc.
Catalytic hydrogenation over the Pd-C catalyst and Pd/C
60
was carried out in methanol under 1 atm H at room tem-
2
perature
(rt).
Four
substrates
were
investigated:
diphenylacetylene, phenylacetylene, cyclohexene and hex-1-
ene. In a typical reaction, Pd in the form of 2 or commercial
Pd/C (0.01 mmol) was mixed with the substrate (1 mmol) in
methanol (15 ml) under a H atmosphere. Conversion was
2
monitored by gas chromatographic (GC) analysis using a
complexes on C , where we can aþord a leverage of control
on the nature of the catalyst used. This molecular control is
Hewlett-Packard 5890 GC equipped with a ýame ionisation
detector and methyl silicone capillary column.
60
facilitated by adjusting the nature and number of the ligands
on the metal, or the number of metal fragments on the spher-
ical buckyball. It is somewhat surprising that the catalytic use
of Pd complexes on C as a molecular model for Pd on char-
60
coal is ill developed in the current literature.12 Here, we
report some of our initial ündings and establish a means of
Results and Discussion
hydrogenation of a Pd-C complex to give a catalyst which
shows comparable, or better hydrogenation activity than con-
ventional Pd on charcoal catalysts.
60
A catalyst precursor 1 was synthesised as a near-black powder
from Pd(OAc) (PPh ) , generated in situ from Pd(OAc) and
2
3 2
2
PPh (1 : 2),16 and
C
in toluene. Microanalysis of
3
60
C
[Pd(OAc) (PPh )] gives: C, 69.06; H, 2.69; Pd, 15.40; P,
60
2
3 3
126 63 3 12
4.11%. Calc. for C
H P O Pd : C, 69.38; H, 2.89; Pd,
3
14.65; P, 4.26%. Its insolubility in all common organic sol-
vents precludes the growth of suitable crystals for X-ray
single-crystal diþraction analysis. It was characterised by
FTIR, MS, TG, XRD, TEM and XPS.
Experimental
Preparation of palladium-C catalyst (2) and its precursor,
60
C
[Pd(OAc) (PPh )] (1) was as follows: Pd(OAc)
60
2
3 3
2
J. Chem. Soc., Faraday T rans., 1997, 93(12), 2207È2210
2207

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Pd(OAc) (PPh ) is an established pre-catalyst in many
2
3 2
C–C coupling reactions, especially the Heck-type reactions.17
Its ability to undergo spontaneous decomposition to a Pd0
phosphine complex enables it to be used as a convenient cata-
lyst precursor.18,19 This complex is hence not isolated upon
formation. Its reaction with C is rapid and not sensitive to
60
the stoichiometry; use of molar ratios 1 : 1, 2 : 1, 3 : 1 and 5 : 1
for Pd(OAc) (PPh ) : C lead to the same product. These
2
3 2 60
reactions suggest that 1 is in a thermodynamically stable
form.
The IR spectrum of 1 shows, besides the usual absorption
peaks of coordinated acetate and PPh , some C absorptions
3
60
characterising metal-coordinated C . The absorption peaks
60
of its FTIR spectrum (KBr) are at 486(s), 524(vs), 565(w),
578(w), 692(s), 726(s), 1182(m), 1424(m), 1650(m, br) cm~1.
The doublet pattern of a weak band at ca. 576 cm~1 is indica-
tive of a lower symmetry of the C moiety upon metal
60
attachment.20 The mass spectrum conürms the presence of
the ligands, viz. PPh and CH CO ~, and their oxidised and
3
3
2
thermal fragments, such as OPPh , OPPh , Ph and CO .
3
2
2
The TG proüle also indicates the release of the ligands below
573 K and the C fragment at a higher temperature.
60
Although the molecular structure of 1 cannot be ascer-
tained, based on the above data, there is strong evidence that
it contains a palladium fragment bearing labile acetate and
phosphine ligands anchoring onto a spherical C ball. The
60
elimination of one phosphine from Pd(OAc) (PPh ) in the
2
3 2
formation of 1 is consistent with its replacement by an
electron-donating ethylene-type group at the 6 : 6 ring fusion
Fig. 1 TEMs of (a) the precursor,
showing its molecular aggregation, and (b) the catalyst, 2, showing
the Pd clusters formed during treatment with hydrogen
C
[Pd(OAc) (PPh )] , 1,
60
2
3 3
of C .21,22 This would maintain a 16-electron d8-PdII
60
complex. It is interesting to note the thermodynamic stability
of
been
a
3 : 1 Pd : C
found
complex. Similar observations have
60
in
a
similar
exo-complex,
a metalloful-
MC [Ru(C Me )(CH CN) ] N3`3X~,3 and
60 2 3
lerene solid, C Pd .23
Although complex 1 is catalytically active for the hydro-
genation of diphenylacetylene, the level of activity (17% yield
at rt after 3 h) is unsatisfactory. This is explained by the satu-
of both Pd0 metallic state and a higher oxidation state inter-
5
5
60
3
mediate between 0 and ]2. The higher E found in 1 is con-
3
b
sistent with the higher oxidation state of Pd (i.e. ]2) in 1.
The dominant peak of 2, with a lower E , is indicative of
b
metallic Pd clusters on the C support. As fullerenes are
60
ration of the PdII sphere. Treatment with H by heating the
known to be electron deücient,25 it is reasonable to suggest
2
complex under a dynamic gas ýow of H at a temperature of
2
523 K activates the catalyst by introducing coordination
unsaturation by ligand elimination with simultaneous
reduction by providing active hydrogenated sites. The cata-
lyst, 2, thus formed was similarly analysed by FTIR, XRD,
TEM and XPS.
The IR spectrum of 2 shows the characteristic coordinated
C
absorptions, but those of the co-ligands are signiücantly
60
diminished. The peaks are found at 487(s), 516(vs), 692(vs),
740(vs), 1093(m), 1182(m), 1433(s) cm~1. Powder XRD
reveals that both 1 and 2 are poorly crystalline, in contrast to
the pristine crystallinity of C . The diþraction patterns of 1
60
and 2 are similar and contain broad peaks. One notable dif-
ference between 1 and 2 is an additional diþraction peak in
the latter at 2h B 40¡, which could be assigned to the (111)
plane of metallic Pd clusters24 supported on the C . The
60
broadness of this peak signiües the small particle size of the
clusters. This conclusion is supported by the TEM studies
which indicate that 1 is a uniform amorphous powder and
that 2 contains black particles of metallic palladium dispersed
on the C supporting surface (see Fig. 1). These Pd clusters
60
are 5–15 nm in diameter and are not found in the TEM of the
precursor 1. This is understandable, since 1 resembles more a
molecular complex of PdII. Further support for this arises
from the XPS data shown in Fig. 2. Complex 1 gives a pair of
peaks with the binding energy (E ) of the Pd3d
state at
b
5@2
and 3d
338.1 eV and a spin–orbit splitting of the 3d
5@2
3@2
states of 5.3 eV. The spectrum of 2 gives a broad band which
can be curve-ütted into two pairs of peaks with E at 335.5
b
and 337.0 eV of Pd3d
(in a 7 : 3 ratio) with a spin–orbit
Fig. 2 XP spectra with computer-ütted peak of Pd 3d core level for
(a) the precursor, C [Pd(OAc) (PPh )] , 1 and (b) the catalyst, 2
5@2
splitting of ca. 5.3 eV. These peaks correspond to the presence
60
2
3 3
2208 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Table 1 Catalytic hydrogenation of alkynes and alkenes at rt in methanol
yield
(%)
time/
min
substrate
catalyst
product
diphenylacetylene
Pd-C
60
1,2-diphenylethane
1,2-diphenylethane
100
100
18
20
Pd/C
phenylacetylene
cyclohexene
hex-1-ene
Pd-C
60
phenylethane
phenylethane
100
100
13
18
Pd/C
Pd-C
60
cyclohexane
cyclohexane
100
100
21
27
Pd/C
Pd-C
60
hexane
hexane
100
100
12
15
Pd/C
that some interaction exists between Pd clusters and the C
support, whereby electron donation from the metal fragment
Pd on C and the interaction between Pd clusters and the
60
60
C
support probably contribute to the high hydrogenation
60
to C is achieved. The observed higher oxidation state in 2
activities in these systems.
60
with higher E can be attributed to this interaction. Similar
This study serves as a model for our future study of the
interaction of other metallic fragments with the spherical n-
b
behaviour has been observed in Pd dispersed on fullerene
soot.26
electron system of C . A study of catalytic dehalogenation
60
A series of experiments was carried out for the hydro-
genation of diphenylacetylene, phenylacetylene, cyclohexene
of polyhalogenated aromatic hydrocarbons (PHAHs) using
these metallofullerene catalysts is in progress.
and hex-1-ene under 1 atm of H in methanol, catalysed by 1
2
mol% of 2 at rt. Two typical reaction proüles are depicted in
The authors acknowledge the National University of Singa-
pore (NUS) for ünancial support (RP930631) and technical
assistance from our departments. R. Y. thanks NUS for a
research scholarship award.
Fig. 3 and the ünal results are listed in Table 1. All four sub-
strates indicate complete conversion (100% yields) within 13
to 21 min. For comparison, a commercial sample of 10%
Pd/C catalyst was also studied in these systems under the
same conditions. The results, which are summarised in Table
1, showed a similar conversion but at a slightly longer time
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2210
J. Chem. Soc., Faraday T rans., 1997, V ol. 93