Origin of the activity of hydrogenation catalysts based on palladium complexes and primary phosphines

Article abstract of DOI:10.1023/B:RJAC.0000038809.37818.07

The activity of the catalytic system based on palladium bisacetylacetonate and phenylphosphine in hydrogenation catalysis was studied.

Full text of DOI:10.1023/B:RJAC.0000038809.37818.07

Russian Journal of Applied Chemistry, Vol. 77, No. 5, 2004, pp. 770 774. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 5,
004, pp. 774 778.
Original Russian Text Copyright
2
2004 by Belykh, Goremyka, Shmidt.
CATALYSIS
Origin of the Activity of Hydrogenation Catalysts
Based on Palladium Complexes and Primary Phosphines
L. B. Belykh, T. V. Goremyka, and F. K. Shmidt
Irkutsk State University, Irkutsk, Russia
Received December 3, 2003
Abstract The activity of the catalytic system based on palladium bisacetylacetonate and phenylphosphine
in hydrogenation catalysis was studied.
The unique capability of transition metals to cata-
lyze hydrogenation of unsaturated organic molecules
with gaseous hydrogen is known for a long time and
is used in numerous industrial processes [1 3].
Among Group VIII transition metals, much attention
is given to palladium-based catalysts. Modification of
palladium with nitrogen-containing compounds (qui-
noline, ethylenediamine, alkaloids) allows preparation
of highly selective catalysts for hydrogenation of
alkynes, chemoselective hydrogenation of unsaturated
aldehydes and ketones, enantioselective hydrogenation
of derivatives of , -unsaturated acids, etc. [1, 2].
Along with studies of heterogeneous supported palla-
dium catalysts, active efforts have been made in the
past three decades to elucidate factors responsible for
the activity of palladium complexes with phosphine
ligands as hydrogenation catalysts. Whereas the modi-
fying effect of tertiary [4 7] and secondary [8] phos-
phines on the properties of palladium complexes as
hydrogenation catalysts has been studied in detail,
primary phosphines have not yet been used in hydro-
genation catalysis. Interest in these systems is due to
the fact that primary phosphines exhibit simultaneous-
ly the properties of Lewis bases and Brønsted acids.
Furthermore, primary phosphines, like other three-
coordinate phosphorus compounds, are reductants.
Phenylphosphine was prepared by reaction of phen-
ylchlorophosphine with lithium aluminum hydride
[11] and isolated by vacuum distillation (38 C/5 mm
Hg). 31P NMR:
122 ppm ( J 200 Hz). The com-
1
PH
pound was stored in a sealed ampule under argon.
The catalyst based on Pd(acac) and PH Ph was
2
2
prepared as follows. To a solution of 0.00304 g
5
(1
10 mol) of Pd(acac) in DMF, prepared in a
2
duck-shaped vessel that had been evacuated and filled
with hydrogen, 1 ml of a phenylphosphine solution
was added; the P/Pd ratio was 0.1 2.0. The lemon-
yellow reaction mixture was stirred in a hydrogen
atmosphere at 80 C for 5 15 min. The resulting
black-brown solution was cooled to 30 C, and a
substrate to be hydrogenated was introduced with a
syringe. Hydrogenation was performed with vigorous
stirring at the initial hydrogen pressure of 1 atm. The
reaction progress was monitored volumetrically and
by GLC.
Acetylacetone and benzene in the catalyzate were
determined by GLC, after evaporation condensation
3
of the solution in a vacuum unit (20 C/1 10 mm
Hg), on a Chrom-5 chromatograph equipped with a
3.6-m packed column (stationary phase SE-30) and
flame ionization detector. The column temperature
was 100 C, and the carrier gas was nitrogen. The error
in determination of acetylacetone and benzene did not
exceed 10%.
The goal of this work was to gain insight into the
promoting effect of PH Ph in a microheterogeneous
2
hydrogenation catalyst based on Pd(acac)2.
The UV spectra were recorded on a Specord UV
VIS spectrometer in an all-sealed cell. The concentra-
EXPERIMENTAL
tion of Pd(acac) was calculated from the absorption
2
band at 330 nm ( 330 = 10630 l mol 1 cm ), and that
1
The solvents were purified by standard procedures
used in organometallic chemistry [9].
of acetylacetone, from the band at 290 nm [for Hacac,
1
1
= 5000, and for Pd(acac) , 3090 l mol cm ].
290
2
Palladium bisacetylacetonate was prepared accord-
ing to [10].
X-ray phase analysis of the catalyst samples was
performed on a DRON-3M diffractometer, CuK
1
070-4272/04/7705-0770 2004 MAIK Nauka/Interperiodica

ORIGIN OF THE ACTIVITY OF HYDROGENATION CATALYSTS
771
radiation. The procedure for determination of Pd(0)
was described in [12].
Table 1. Hydrogenation of styrene in the presence of Pd
black formed from Pd(acac) . Solvent DMF, 30 C, P
=
H
2
2
3
1
atm, substrate concentration csub = 8.7 10
M
We showed previously that palladium bisacetylace-
tonate, like palladium(II) acetate [13], is not reduced
with hydrogen in aprotic solvents (benzene, toluene)
below 80 C in the absence of water. We found that
Pd(Acac) ,*
W,
Styrene con-
version, %
2
mM
mol H /(mol Pd min)
2
water breaks down the oligomeric species [Pd(acac)2]n
0.45
2
6
12
9
11**
19**
16**
12**
3
25
62
29**
61**
100**
100**
preserved upon dissolution in benzene and facilitates
0
.90
heterolytic cleavage of the hydrogen molecule. The
2.27
4.54
hydrogenolysis of Pd(acac) in DMF occurs under
100
2
milder conditions. At 30 C, it takes no less than 2 h,
whereas at 80 C it is complete in 5 15 min depending
on the Pd(II) concentration.
* Reduction of Pd(acac) with hydrogen was performed at
2
80 C.
*
* Reduction of Pd(acac) was performed with addition of
2
The initial rate of styrene hydrogenation in the
presence of the thus generated Pd black is relatively
low, and the maximal specific activity W does not
exceed 19 mol of styrene per mole of Pd per minute.
1 wt % H O to the solvent.
2
aniline on palladium catalysts. The advantage of our
catalyst is the high reaction rate. The specific activity
of our catalyst in reduction of the nitro group exceeds
by a factor of 3.5 that of Pd nanoparticles immobi-
lized on the styrene divinylbenzene copolymer [14]
but is lower than that of the Pd catalyst with diphenyl-
phosphine [8].
At low initial concentrations of Pd(acac) , the catalyst
2
undergoes rapid deactivation in the course of hydro-
genation (Table 1). Introduction of water enhances the
performance of Pd black.
To obtain a more effective catalytic system, we
used PH Ph as modifier. Catalytic hydrogenation
To gain insight into the promoting effect of PH Ph,
2
2
of styrene in the presence of the system based on
we studied in more detail the system with P/Pd = 0.3.
Pd(acac) + nPH Ph (P/Pd = 0.3, 1.0, 2.0) at 30 C
2
2
W, mol H /(mol Pd min)
in DMF is characterized by a long induction period
40 min) and low reaction rate. The catalyst that is the
2
(
most active in hydrogenation is formed after prelimi-
nary reaction of the system components, Pd(acac) +
2
nPH Ph, with H in DMF at 80 C for 5 15 min. The
2
2
catalytic system exhibits the highest performance at
the PH Ph : Pd(acac) ratio of 0.3 (Fig. 1).
2
2
The specific activity of the system Pd(acac) +
2
0
.3PH Ph does not change upon hydrogenation of no
2
less than 3000 mol of styrene per mole of Pd, after
which the activity gradually decreases with formation
of a precipitate. The reaction rate as a function of the
catalyst concentration nonlinearly grows in the range
Fig. 1. Hydrogenation of styrene in the presence of the
catalytic system Pd(acac) + nPH Ph. (W) Specific activity
2
2
and (P/Pd) ratio of the starting reactants. Solvent DMF,
= 0.9 mM, 30 C, P = 1 atm.
0
.5 2 mM and starts to decrease at cPd > 1.4 mM
c
(
Fig. 2).
Pd
H
2
In hydrogenation of other unsaturated hydrocarbons
W, mmol H2/min
and of carbonyl and nitro compounds, the system
under consideration also shows high activity and
selectivity (Table 2).
In reduction of benzaldehyde on palladium cata-
lysts (Pd/C), the C O bond in the forming benzyl al-
cohol usually undergoes further hydrogenolysis, with
the rates of both steps being comparable [2]. At the
c, mM
Fig. 2. Rate of styrene hydrogenation W in the presence of
the catalytic system Pd(acac) + 0.3PH Ph as a function of
same time, the selectivity of the system Pd(acac) +
2
2
2
0
9
.3PH Ph in benzaldehyde hydrogenation reaches
the Pd(acac) concentration c. Solvent DMF, 30 C, P
=
2
2
H
3
2
2%. Nitrobenzene is usually selectively reduced to
1 atm, c
= 8.7 10 M.
sub
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 5 2004

772
BELYKH et al.
Table 2. Catalytic properties of the system Pd(acac) + 0.3PH Ph in hydrogenation. Solvent DMF, c = 0.9 mM,
2
2
Pd
3
0 C, PH2 = 1 atm, csub/cPd = 870
Substrate
W, mol H /(mol Pd min)
Conversion, %
Products, %
2
PhC CH
104 ( C C ), 112 ( C=C )
113 ( C=C ), 21 ( C=C )
100
100
100
70
Ethylbenzene 85, styrene 15
Diphenylethane 69, cis-stilbene 24, trans-stilbene 7
Ethylbenzene 100
Benzyl alcohol 92, toluene 8
Aniline 100
PhC CPh
PhCH=CH2
PhC(O)H*
PhNO2*
260
10
48
97
*
c
= 5 mM.
Pd
Table 3. Conversion of the catalytic system Pd(acac) + nPH Ph in hydrogen, according to UV and GLC data. Solvent
2
2
DMF, 80 C, cPd = 5 mM
Concentration, wt %
Reaction time,
PH Ph/Pd(Acac)
C H /PH Ph
2
2
6
6
2
min
Pd(Acac)2
HAcac
0
0
0
0
0
1
1
1
.3
.3
.3
.5
.5
.0
.0
.0
5
15
40
15
40
15
40
60
2
0
0
2
0
99
98
99
87
93
79
83
113
0.26
0.50
0.51
0.46
0.45
0.13
0.19
0.20
24
12
Phenylphosphine, in contrast to tertiary phosphines,
exhibits both base and acid properties. The previous
ratio of 0.3, upon ligand exchange, about 15% of Pd
appears in the form of polynuclear complexes, and
study of the reaction of Pd(acac) with PH Ph in
85%, in the form of Pd(acac) . The degree of conver-
2
2
2
benzene and DMF in an inert atmosphere [15] showed
that the reaction went beyond simple complexation
and was accompanied by fast exchange of acetylace-
tonate ligands for organophoshorus groups, yielding
acetylacetone and polynuclear palladium complexes
containing 3-PPh, -PHPh, and coordinated PH Ph
sion of Pd(acac) to the reaction products grew with
2
an increase in the P/Pd ratio. Quantitative formation
of acetylacetone was observed at P/Pd > 2.
Additional treatment of the system Pd(acac) +
2
nPH Ph with hydrogen results in quantitative forma-
2
2
2
tion of acetylacetone (Table 3), i.e., not only Pd(acac)2
ligands, and also O,O-chelate acetylacetonate ligands,
but also acetylacetonate ligands incorporated in poly-
like
nuclear Pd complexes are subject to hydrogenolysis.
The catalyzate from the reaction with H contained
2
not only acetylacetone but also benzene originating
from decomposition of the organophosphorus ligands.
The amount of benzene formed per mole of PH Ph
2
grows as the P/Pd ratio is decreased. The content of
Pd(acac) grows in the same direction. Similar trends
2
were observed with triphenylphosphine [5]. The ob-
served dephenylation is consistent with the previously
proposed mechanism according to which degradation
of organophosphorus ligands occurs in the coordina-
tion sphere of Pd(0) [8]; therefore, the mechanism of
benzene formation in the system can be tentatively
presented as follows.
where L = PH Ph.
2
The reaction is complete in 2 3 min. At the P/Pd
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 5 2004

ORIGIN OF THE ACTIVITY OF HYDROGENATION CATALYSTS
773
phenylphosphine took no less than 24 h, with phenyl-
phosphine the formation time was as short as 5 min.
Our results suggest that, in the course of formation
of the catalytic system based on Pd(acac) at P/Pd =
2
0
.3, phenylphosphine acts as a structural promoter. Its
effect can be described as follows. The reaction of
PH Ph with Pd(acac) yields nanosized associates of
polynuclear palladium complexes with organophos-
phorus ligands. The associates of the polynuclear pal-
ladium complexes and palladium phosphides formed
2
2
Pd(0)
1/2xPd(0).
Pd(acac)2 is readily reduced with hydrogen in
DMF at 80 C to Pd(0). The Pd(0) atoms arising from
hydrogenolysis of Pd(acac) can both aggregate to
form palladium clusters and react with phenylphos-
phinylidene ligands in polynuclear palladium com-
plexes. Oxidative addition of Pd(0) to these complexes
with hydrogenolysis of the P C bond yield benzene
and, probably, palladium phosphides:
in an H atmosphere by decomposition of organo-
2
phosphorus ligands act as crystallization centers for
immobilization and growth of Pd(0) clusters. At high-
er P/Pd ratios, the content of Pd(I) and Pd(II) com-
plexes with phosphorus-containing ligands increases
relative to Pd(0), and the catalytic activity of the
system decreases.
2
PPh
Pd Pd
+1/2H2
C H
+
Pd(0)
Pd P .
CONCLUSIONS
x
6
6
(
1) A new high-performance catalyst based on
Elemental analysis of the black precipitate (sam-
Pd(acac) and PH Ph was suggested for hydrogena-
tion of unsaturated bonds and of carbonyl and nitro
groups.
2
2
ple 1) isolated from the reaction system Pd(acac) +
2
0
.3PH Ph + H corresponds to the empirical formula
2
2
Pd P C H , and the content of Pd(0) determined
8
1 1 1
chemically does not exceed 28%. According to X-ray
phase analysis, sample 1 is elemental palladium in the
dispersed state; the size of its coherent scattering
domain, as estimated from the shape of the Pd line
hkl 111, is 2 nm. We believe that the results of X-ray
phase and elemental analyses do not contradict each
other. Sample 1 may contain higher palladium phos-
phides in the X-ray amorphous state for which the
main reflections are observed in the same region as
for elemental palladium and overlap with the broad-
ened Pd(111) line. It should be noted that the products
formed by the components of the catalytic system in
an inert atmosphere, associates of polynuclear palla-
dium complexes, were X-ray amorphous, giving a
diffuse maximum (halo) at d/n = 13.556 ; the size
of coherent scattering domains was 2.7 nm. The ab-
sence of an amorphous halo at small diffraction angles
(2) Transformations of the catalytic system
Pd(acac)2 + nPH Ph in hydrogen yield nanosized
2
particles (2 nm) containing elemental palladium and,
probably, palladium phosphides. Associates of poly-
nuclear palladium complexes and palladium phos-
phides are crystallization centers for immobilization
and growth of Pd(0) clusters. Phenylphosphine acts as
a structural promoter.
REFERENCES
1
. Colquhoun, H.M., Holton, J., Thomson, D.J., and
Twigg, M.V., New Pathways of Organic Synthesis:
Practical Applications of Transition Metals, New
York: Plenum, 1984.
2
3
4
. Blaser, H.-U., Indolese, A., Schnyder, A., et al.,
J. Mol. Catal. A: Chem., 2001, vol. 173, pp. 3 18.
(
2 = 5 40 ) in the diffraction pattern of sample 1
. James, B.R., Homogeneous Hydrogenation, New
indicates that the structure of polynuclear palladium
complexes is broken down upon treatment of the cata-
lytic system with hydrogen.
York: Wiley Interscience, 1973.
. Collman, J.P., Hegedus, L.S., Norton, J.R., and Fin-
ke, R.G., Principles and Applications of Organotran-
sition Metal Chemistry, Mill Valley, California: Univ.
Science Books, 1987.
It was shown previously that the catalytic activity
of the system Pd(acac) + nPPh in hydrogenation of
2
3
phenylacetylene was maximal at the P/Pd ratio of 0.3
also [16], and the solid reaction products identified
5. Shmidt, F.K., Belykh, L.B., and Cherenkova, T.V.,
Kinet. Katal., 2001, vol. 42, no. 2, pp. 182 194.
were palladium phosphide Pd P and crystalline Pd.
However, whereas formation of the system with tri-
6. Cairns, G.R., Gross, R.J., and Stirling, D., J. Mol.
Catal. A: Chem., 2001, vol. 172, pp. 207 218.
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 5 2004

774
BELYKH et al.
7
8
9
. Moiseev, I.I. and Vargaftik, M.N., Zh. Obshch. Khim.,
002, vol. 72, no. 4, pp. 550 560.
13. Mironova, L.V., Levkovskii, Yu.S., Belykh, L.B.,
et al., Koord. Khim., 1985, vol. 2, no. 12, pp. 1689
693.
14. Klyuev, M.V. and Nasibulin, A.A., Kinet. Katal.,
996, vol. 37, no. 2, pp. 231 244.
2
1
. Shmidt, F.K., Belykh, L.B., and Cherenkova, T.V.,
Kinet. Katal., 2003, vol. 44, no. 5, pp. 683 691.
1
. Gordon, A.J. and Ford, R.A., The Chemist’s Compa-
nion. A Handbook of Practical Data, Techniques and
References, New York: Wiley Interscience, 1972.
1
5. Belykh, L.B., Goremyka, T.V., Antipina, D.V., and
Schmidt, F.K., Abstracts of Papers, Symp. Modern
Trends in Organometallic and Catalytic Chemistry,
Moscow, May 18 23, 2003, p. 145.
1
1
0. US Patent 3474464.
1. Kormachev, V.V. and Fedoseev, M.S., Preparativ-
naya khimiya fosfora (Preparative Chemistry of Phos-
phorus), Perm: Ural. Otd. Ross. Akad. Nauk, 1992.
1
6. Bakunina, T.I., Zinchenko, S.V., Khutoryanskii, V.A.,
et al., in Metallokompleksnyi kataliz: Sbornik nauch-
nykh statei (Coordination Catalysts: Coll. of Scientific
Papers), Shmidt, F.K., Ed., Irkutsk: Irkutsk. Gos.
Univ., 1989, p. 132.
1
2. Shmidt, A.F. and Mametova, L.V., Kinet. Katal.,
1
996, vol. 37, no. 3, pp. 431 433.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 5 2004