Preparation of high-performance nanosized palladium hydrogenation catalysts using white phosphorus and phosphine

Article abstract of DOI:10.1134/S1070427207090157

The nature and properties of nanosized palladium hydrogenation catalysts modified with elemental phosphorus and phosphine (PH₃) were studied.

Full text of DOI:10.1134/S1070427207090157

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 9, pp. 1523 1528. Pleiades Publishing, Ltd., 2007.
Original Russian Text L.B. Belykh, N.I. Skripov, L.N. Belonogova, V.A. Umanets, F.K. Shmidt, 2007, published in Zhurnal Prikladnoi Khimii, 2007,
Vol. 80, No. 9, pp. 1489 1494.
CATALYSIS
Preparation of High-Performance Nanosized Palladium
Hydrogenation Catalysts Using White Phosphorus
and Phosphine
L. B. Belykh, N. I. Skripov, L. N. Belonogova, V. A. Umanets, and F. K. Shmidt
Irkutsk State University, Irkutsk, Russia
Received November 20, 2006
Abstract The nature and properties of nanosized palladium hydrogenation catalysts modified with elemental
phosphorus and phosphine (PH ) were studied.
3
DOI: 10.1134/S1070427207090157
Previously were studied the nature of particles
active in hydrogenation that were formed by treatment
with different reducing agents of palladium(II) com-
plexes with organic phosphines [1 4]. Microhetero-
geneous nature of these catalysts was revealed, and
the key steps of their formation was determined. In
particular, the main reactions of palladium(II) phos-
phine complexes with hydrogen are reduction of
Pd(II) to Pd(0) by hydrogenolysis of the Pd X bonds
EXPERIMENTAL
The solvents and chemicals were purified by stan-
dard procedures used in handling of organometallic
compounds [5]. Benzene was additionally dried by
distillation from LiAlH on a column and was stored
4
in an argon atmosphere in sealed ampules over 4A
molecular sieves.
To remove water and amine impurities, dimethyl-
formamide was kept over anhydrous copper sulfate
until a green solution was formed. Then the solvent
was double-distilled in a vacuum (3 mm Hg) at tem-
perature no higher than 42 C.
(X is an acido ligand); degradation of phosphine
ligands in the coordination sphere of Pd(0) by oxida-
tive addition of organophosphorus ligands to Pd(0),
followed by hydrogenolysis of the Pd C bond with
hydrogen and formation of polynuclear palladium
phosphine and phosphinidene complexes (in the limit-
Palladium bisacetylacetonate was prepared by
the procedure described in patent [6]. 1H NMR:
ing case, palladium phosphides, Pd P and Pd4.8P);
(CH) = 5.04 ppm (s,1H), (CH ) = 1.76 ppm (s,6H).
6
3
and formation of Pd(0) clusters [2, 3]. Active in hy-
drogenation are Pd(0) clusters or cluster palladium
To prepare phosphine, a solution of 25 g of KOH
in 25 ml of water was added dropwise to a suspension
of 10 g of red phosphorus in 30 ml of toluene, pre-
heated to 110 C [7]. The liberating gas was succes-
sively passed through a Tishchenko bottle filled with
hydrides [Pd H ] immobilized on polynucliear palla-
x
y
dium complexes with phosphide and posphinidene
ligands or on palladium phosphides. Since formation
of Pd(II) phosphine complexes involves stepwise
degradation of organophosphorus ligands to form
a heterophase system containing in the limiting case
palladium phosphides of different composition and
a 50% solution of an alkali in an H O + DMSO mix-
2
ture to remove diphosphine impurities and then
through columns packed with NaOH and P O to re-
2
5
move water traces. Phosphine was collected by dis-
solution in benzene placed in a finger-like vessel.
The phosphine concentration in the benzene solution
was determined by 31P NMR spectroscopy using
Pd(0) clusters, we suggested that phosphine (PH ) and
3
white phosphorus (P ) can be used to modify the cata-
4
lyst. This approach can simplify preparation of nano-
sized palladium hydrogenation catalysts. The aim of
this study was to examine the nature and catalytic
properties of the systems prepared by treatment with
molecular hydrogen of mixtures of palladium bis-
acetylacetonate with white phosphorus or phosphine.
31
8
5% H PO as external reference; P NMR:
=
3
4
P
2
38 ppm (q, J
= 188 Hz).
PH
To remove surface oxidation products of white
phosphorus, it was washed with anhydrous benzene.
A benzene solution of white phosphorus was prepared
1523

1524
BELYKH et al.
and stored under an inert atmosphere in a finger-like
vessel. The design of this vessel allows evacuation
diffraction on a DRON-3M diffractometer using CuK
radiation. The NMR spectra were recorded on a
and filling with argon. 31P NMR:
= 523 ppm.
VXR-500S Varian pulse spectrometer. The P chemi-
31
P
cal shifts are given relative to 85% phosphoric acid.
Catalytic hydrogenation was performed in a duck-
shaped vessel thermostated at 30 C at the initial
hydrogen pressure of 1 atm in the presence of the
catalytic system formed in situ. To a solution of
The samples were studied by transmission electron
microscopy (TEM) on a BS-300 microscope (Czech
Republic). A drop of the solution was applied onto a
gauze coated with a carbon film and was dried in an
argon atmosphere. The electron micrographs were
taken under conditions excluding melting and degrada-
tion of the samples on exposure to an electron beam.
5
Pd(acac) (0.00304 g, 1 10 mol) in DMF (9 ml)
2
placed in a duck-shaped vessel, a solution of phos-
phine (0.3 10 5 mol) in benzene (1 ml) was added
dropwise in a hydrogen stream. The mixture was
stirred for 5 min at room temperature. Then the sys-
tem was heated to 80 C with stirring in a hydrogen
stream. The resulting black-brown solution was
cooled to 30 C. A substrate was injected with a sy-
ringe. It was hydrogenated with vigorous stirring to
exclude the diffusion control of the reaction. The reac-
tion course was monitored by GLC. The experiments
at other P/Pd ratios and preparation of the palladium
catalyst modified with white phosphorus were per-
formed similarly. The optimal time required for pre-
paring Pd(acac) nP catalytic system at 80 C was
The procedure for determining Pd(0) is described
in [9].
Previously we showed [1 4, 10] that palladium(II)
-diketonate phosphine complexes became catalytical-
ly active in hydrogenation reactions only after their
activation with a reducing agent, e.g., with molecular
hydrogen. To compare the catalytic properties of the
catalysts derived from palladium bisacetylacetonate
[Pd(acac) ] modified with white phosphorus and phos-
2
phine with those of palladium(II) complexes with
organic phosphines (PPh , PHPh , PH Ph), these
2
4
2
5 30 min.
3
3
2
catalysts were prepared similarly in a hydrogen at-
mosphere at 80 C. It should be noted that at a lower
The palladium catalysts modified with white phos-
phorus were prepared in a temperature-controlled
temperature the rate of Pd(acac) reduction is lower
duck-shaped glass vessel in hydrogen at 80 C. To
2
a solution of Pd(acac)2 (0.761 g, 2.5 10 3 mol)
(Table 1). The properties of palladium catalysts modi-
fied with phosphine or white phosphorus were studied
in relation to different parameters. It was found that
the dependences of the specific activity in styrene hy-
drogenation on the P/Pd ratio passed through maxima.
The most active catalysts are formed at similar ratio
P/Pd = 0.3 (Fig. 1). We observed the similar depen-
in DMF (90 ml), a solution of phosphine (0.75
3
1
0
mol) (P/Pd = 0.3) in benzene (25 ml) was added
drop-wise in a hydrogen stream. The mixture was
stirred at a hydrogen pressure of 1 atm and 80 C for
5 min. The solution color changed to dark brown in
5 min. After reaction completion, the black suspen-
sion was cooled to room temperature and was trans-
ferred under an inert atmosphere into a finger-like
vessel. Then the solvent was removed in a vacuum
1
2
dence for the Pd(acac) PH Ph system [10]. The in-
2
2
fluence of organophosphorus compounds on the cata-
lytic activity of Pd(acac) decreases in the following
2
(
2/3 of the volume). Ether was added until a precipi-
order: P4 PH Ph [10] > PH > PHPh [11] > PPh
2
3
2
3
tate was formed. The precipitate was washed with
ether in an argon atmosphere and was dried in a
vacuum (50 C/1 mm Hg). Yield 0.235 g. Elemental
analysis, %: Pd 87.88; P 3.59; C 0.99; H 0.25. IR: no
[3].
The most active promoters are white phosphorus
and phenylphosphine, all catalytic reaction conditions
being the same; the phosphine is slightly less effec-
tive; the palladium bisacetylacetonate + triphenyl-
phosphine system exhibits the lowest catalytic activity
in alkene hydrogenation (Fig. 2).
1
absorption in the range 4000 400 cm .
Analysis for Pd and P was performed as follows.
A weighed portion of the sample (about 0.01 g) was
dissolved in 2 ml of boiling HNO . The resulting solu-
3
It should be noted that the specific activity of the
palladium catalysts modified with phosphine (PH3)
increases with decreasing palladium concentration
and reaches a maximum [330 (mol styrene) (g-at.
tion was diluted with water to 50 ml in a volumetric
flask. The palladium concentration was determined by
atomic absorption spectroscopy on a Perkin Elmer
403 spectrometer in a propane air flame. The phos-
Pd) min ] at C = 5 10 4 M. Nonlinear depen-
1
1
Pd
phorus content was determined photocolorimetrically
by the standard procedure [8].
dence of the reaction rate on the Pd(acac) concentra-
2
tion in the case of the catalyst modification with both
phosphine and phosphorus (Fig. 3) suggests associa-
The catalyst samples were studied by powder X-ray
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

PREPARATION OF HIGH-PERFORMANCE NANOSIZED PALLADIUM
1525
Table 1. Hydrogenation of styrene in the presence of palladium catalysts modified with phosphine and white phosphorus.
CPd = 1 mM, P/Pd = 0.3, [substrate]/Pd = 870, solvent DMF, T = 30 C, PH2 = 1 atm
Conditions of catalyst preparation in hydrogen
W, (mol H ) (mol Pd) 1 min 1
2
time, min
T,
C
Pd(acac) PH3
Pd(acac) P
2 4
2
Pd(acac) PH
Pd(acac) P4
2
3
2
3
5
7
0
0
0
15
15
15
25
25
25
0
0
166
0
0
0
4
25
25
0
181
280
137
8
9
0
0
15
15
190
56
Table 2. Specific hydrogenation activity of palladium catalysts modified with phosphorus and phosphine. CPd = 1 mM,
solvent DMF, VDMF = 10 ml, P/Pd = 0.3, T = 30 C, PH2 = 1 atm
W, (mol H ) (mol Pd) 1 min 1
Product yield, %
Pd(acac) P
4
2
Substrate, mol
Pd(acac) PH
Pd(acac) P4
Pd(acac) PH3
2
3
2
2
2
Styrene, 8.7
Phenylacetylene, 9.1
Nitrobenzene:
190
59
280
157
Ethylbenzene, 100
Ethylbenzene, 100
Ethylbenzene, 100
Ethylbenzene, 100
4
2
.9
.4
21
1*
64
128
15*
Aniline, 100
Aniline, 100
Aniline, 100
Benzyl alcohol, 94;
toluene ( )
Benzaldehyde, 9.8
Benzyl alcohol, 95;
toluene, 5
*
c
= 5 mM.
Pd
tion of reactive palladium species in less active ag-
gregates and microheterogeneity of the catalysts.
W, (mol H ) (mol Pd) min 1
1
2
Palladium species modified with white phosphorus
and phosphine are highly active and selective catalysts
of hydrogenation of acetylene derivatives and nitro-
and carbonyl groups (Table 2). For example, phenyl-
acetylene is successively hydrogenated to styrene and
ethylbenzene without its side di- or oligomerization.
W, (mol H ) (mol Pd) 1 min 1
2
Fig. 1. Hydrogenation of styrene in the presence of palla-
dium catalysts modified with (1) white phosphorus and
P4
(
2) phosphine. C = 1 mM, [substrate]/Pd(acac) = 870,
Pd 2
solvent DMF, T = 30 C, P = 1 atm. (W) Specific activity;
Fig. 2. Influence of the modifier on the specific activity
of palladium hydrogenation catalysts.
H
2
the same for Fig. 2.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

1526
BELYKH et al.
mined by X-ray diffraction, the precipitate is a hetero-
R, (mol H ) l min 1
1
2
phase mixture containing palladium phosphides of
various compositions: Pd P (d/n 2.696, 2.583, 2.368,
6
2
.291, 2.256, 2.232, 2.093, 1.998 ), Pd4.8P (d/n
2
.439, 2.327, 2.256, 2.232, 2.122, 1.998 ), Pd5P2
(d/n 2.734, 2.696, 2.439, 2.291, 2.256, 2.232, 2.122,
1
2
.993, 1.858, 1.432 ) and crystalline palladium (d/n
.256, 1.950, 1.375 ) [13]. Palladium phosphides
Pd P and Pd4.8P prevail in the sample. The fraction of
6
reduced palladium determined by the chemical meth-
od is 31%. Knowing the total content of palladium
and phosphorus, the fraction of reduced palladium,
and the composition of palladium phosphides, we can
describe the composition of sample I by the empirical
formula
cPd, mM
Fig. 3. Rate of styrene hydrogenation R in the presence of
catalysts formed by reduction of (1) Pd(acac) PH and
2) Pd(acac) P systems as a function of palladium con-
2
3
(
2 4
centration. Solvent DMF, P/Pd = 0.3, T = 30 C, P
=
H
3
2
1
atm, [substrate] = 8.7 10 mol.
[
{Pd(0)}2.33{Pd P}0.40{Pd4.8P}0.55{Pd P }
].
6
5 2 0.026
At 96% conversion of phenylacetylene, the selectivity
with respect to styrene is 94 95%. Nitrobenzene is
selectively reduced to aniline.
The carbon and hydrogen in the sample most likely
originate from the residual solvent (DMF). Hence,
the composition will be described by the formula
Thus, the activity of the palladium catalysts modi-
fied with white phosphorus and phosphine (PH ) is
[{Pd(0)}2.33{Pd P}0.40{Pd4.8P}0.55{Pd P }
{DMF}0.3].
3
6
5 2 0.026
not only similar to, but in some cases even higher
than that of the previously studied nanosized palla-
dium catalysts prepared from palladium complexes
with organic phosphines (PHPh , PPh ).
X-ray diffraction analysis of the catalyst prepared
in the presence of white phosphorus [Pd(acac) 0.3P]
2
shows that the sample consists of palladium phos-
2
3
phide Pd P, crystalline palladium, and palladium
6
The most active hydrogenation catalysts prepared
by treatment of Pd(acac) + 0.3PH (or P ) system
hydride PdH0.7 [13]. The fraction of reduced palla-
2
3
4
dium, determined by the chemical procedure, is 25%.
with molecular hydrogen were studied by X-ray dif-
fraction and TEM.
It should be noted that reduction of Pd(acac) with
2
hydrogen is an autocatalytic reaction, with the induc-
tion period in the presence of white phosphorus in-
creasing from 7 to 18 min. This fact can be explained
as follows. Palladium atoms formed in the first steps
of hydrogen reduction react with white phosphorus to
form palladium phosphides which are not capable to
activate molecular hydrogen. After the phosphorus
is completely bound into palladium phosphides,
As determined by TEM, the reaction of the
Pd(acac)2 + 0.3PH3 system with hydrogen yields
a microheterogeneous system containing mainly high-
contrast particles 7.2 7.8 nm in diameter (Fig. 4a).
When the time of hydrogen treatment increases from
1
5 to 30 min, the system becomes more structured
(
Fig. 4b). The loose and dendritic structure observed
in the micrographs is typical for fractal clusters [12].
These clusters are formed by self-aggregation of
nanoparticales of similar size in the absence of sta-
bilizing factors. It should be noted that, in the course
of formation of microheterogeneous hydrogenation
catalysts in the presence of phosphine ligands, fractal
clusters were observed for the first time. Palladium
particles formed by reduction of palladium(II) com-
plexes in the presence of tertiary [3], secondary [11],
or primary phosphines [10] are more stable to ag-
gregation.
Pd(acac) is autocatalytically reduced with hydrogen.
2
Thus, the general feature of formation of hydro-
genation catalysts in the system in question is forma-
tion of palladium phosphides of different composi-
tions and reduced palladium in the amount of about
25 30%. It should be noted that palladium phosphides
as modifiers of organic phosphines, in particular
phenyl- and triphenylphosphine, are formed after
almost complete degradation of the organophosphorus
ligand during treatment of the catalyst with molecular
hydrogen at the molar ratio P/Pd 0.5 [3, 10, 14].
However, a long time required for reduction of
Pd(acac) with hydrogen in the presence of PH and
The composition of the black precipitate (sample I)
isolated from Pd(acac) 0.3PH system is described
2
3
2
3
by the empirical formula Pd7
P
H2.1. As deter-
low stability of the resulting nanoparticles to aggrega-
.5 1.07
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

PREPARATION OF HIGH-PERFORMANCE NANOSIZED PALLADIUM
a) Fraction of particles, %
1527
(
50 nm
d, nm
(
b)
Fraction of particles, %
100 nm
d, nm
Fig. 4. Electron micrograph of the Pd(acac) 0.3PH system treated with hydrogen at 80 C and particle size distribution.
2
3
(
d) Particle size. Treatment time, (a) 15 and (b) 30.
tion lead to a sharp decrease in the activity of the cata-
lyst as compared to the catalysts prepared from
phine. Hence, formation of more finely dispersed sys-
tem can be one of the reasons determining the promot-
ing effect of phosphine. The assumption that the
promoting effect of phosphine is due to an increase in
the catalyst dispersity is well consistent with the
experimental data for the case when Pd(0) atoms are
located on the nanoparticle surface. Based on the
results obtained, we suggest that microheterogeneous
hydrogenation catalysts consist of nanoparticles whose
cores are formed by palladium phosphides with Pd(0)
clusters immobilized on the surface.
Pd(acac) and P , PH , and PH Ph.
2
4
3
2
To determine the mechanism of the promoting ef-
fect of the ogranophosphorus compounds on the cata-
lytic properties of the palladium hydrogenation cata-
lysts, we compared the fraction of the surface atoms
for the Pd(acac) 0.3PH catalytic system and that of
2
3
Pd black formed under the similar conditions from
Pd(acac)2.
The size of Pd black particles determined by TEM
is 25 30 nm, which is larger by a factor of almost 4
It should be noted that the high activity of nano-
sized palladium hydrogenation catalysts cannot be
solely due to a small particle size. In particular, the
particles of microheterogeneous palladium catalysts
prepared from palladium complexes with organic
phosphines are smaller (3 5 nm [3, 10]) than particles
of the examined systems, but, as noted above, their
catalytic activity is lower.
than that of particles formed in the Pd(acac) 0.3PH3
2
system. It is known that the fraction of the surface
atoms for metal particles 8 10 nm in diameter is
2
0 15%, and it decreases to 3 2% with increasing
particle size to 25 30 nm [15]. The increase in the
fraction of the surface atoms by a factor of 7 (on the
average) correlates with the promoting effect of phos-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

1528
BELYKH et al.
This is due to the fact that transformation of palla-
ACKNOWLEDGMENTS
dium phosphine complexes under the action of molec-
ular hydrogen is a complex multistep process.
The general feature of formation of hydrogenation
This study was financially supported by the Zama-
raev International Charitable Foundation and Irkutsk
State University grant no. III-020000/107.
catalysts from Pd(acac) and different phosphines in
2
hydrogen is not only formation of microheterogeneous
systems but also oxidation of palladium particles after
degradation of the phosphorus-containing ligands or
reaction of Pd(0) with elemental phosphorus. The
ratio of oxidized and reduced palladium changes
depending on the ratio of the reduction rates of Pd(II)
complexes, aggregation of reduced palladium par-
ticles, and degradation of phosphorus-containing
fragments in the coordination sphere of Pd(0). In addi-
tion, the electron density on Pd(0) clusters can change
after their immobilization on palladium phosphides.
The structure and catalytic properties of the palladium
nanoparticles depend on all of these factors. The par-
ticles formed after hydrogen treatment of Pd(acac)2
modified with phosphine and elemental phosphorus
have similar structure and a high content of reduced
palladium, and hence their catalytic properties in hy-
drogenation are similar.
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2
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CONCLUSIONS
9
1
(
1) White phosphorus and phosphine (PH ) were
3
10. Belykh, L.B., Goremyka, T.V., and Shmidt, F.K.,
used for the first time as modifiers in synthesis of ef-
fective palladium hydrogenation catalysts whose
catalytic activity is not only similar to, but in some
cases higher than that of nanosized palladium hydro-
genation catalysts prepared from palladium complexes
with organic phosphines.
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(
2) Reaction of Pd(acac) with hydrogen in the
2
1
1
presence of phosphine (PH ) and white phosphorus
3
yields Pd(0) and palladium phosphides of various
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(3) The promoting effect of phosphine is caused,
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2
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