Palladium hydrogenation catalysts modified with aluminum- and phosphorus-containing compounds and with alcohols: Effect of modifiers

Article abstract of DOI:10.1134/S1070427206080106

The catalytic properties of nanosize catalysts derived from Pd(acac) ₂ and triethylaluminum in hydrogenation of organic substrates were studied, and the optimal conditions for the catalyst preparation were found. The maximum observed on the plot of the catalyst specific activity vs. Al/Pd ratio was explained by experiments.

Full text of DOI:10.1134/S1070427206080106

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 8, pp. 1271 1277.
Pleiades Publishing, Inc., 2006.
Original Russian Text
No. 8, pp. 1285 1291.
L.B. Belykh, Yu.Yu. Titova, V.A. Umanets, F.K. Shmidt, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79,
CATALYSIS
Palladium Hydrogenation Catalysts Modified
with Aluminum- and Phosphorus-containing
Compounds and with Alcohols:
Effect of Modifiers
L. B. Belykh, Yu. Yu. Titova, V. A. Umanets, and F. K. Shmidt
Irkutsk State University, Irkutsk, Russia
Received March 14, 2006
Abstract The catalytic properties of nanosize catalysts derived from Pd(acac)2 and triethylaluminum in
hydrogenation of organic substrates were studied, and the optimal conditions for the catalyst preparation
were found. The maximum observed on the plot of the catalyst specific activity vs. Al/Pd ratio was explained
by experiments.
DOI: 10.1134/S1070427206080106
Transition metals and their complexes catalyze
hydrogenation of unsaturated organic compounds with
gaseous hydrogen; the process has numerous industri-
al applications [1, 2]. A particular place is occupied
by palladium catalysts, whose properties significantly
differ from those of catalysts containing Rh, Pt, Ru,
Ni, or Cu [2]. Palladium is one of the most active
metals in the hydrogenation of double bonds conju-
fated with an aromatic ring, such as Ar C=C, Ar C=O,
or Ar C=NR. Furthermore, Pd is one of the most
selective metal catalysts in the hydrogenation of triple
and conjugated double bonds and the most active
metal in hydrogenolysis, i.e., reductive cleavage of
C X bonds. However, at low temperatures Pd is vir-
tually inactive in the hydrogenation of the majority
of aromatic rings and shows low activity in the hy-
drogenation of aliphatic ketones and aldehydes [2].
These characteristics determine the applications of
palladium-based catalysts in chemo- and stereo-selec-
tive hydrogenation, hydrogenolysis, and dehydrogena-
tion of various organic compounds.
complexes of transition metals of the first row [4 10],
and also the synthesis of metal nanoparticles by treat-
ment with organoaluminum compounds [11 16].
The problems of the formation mechanism, phase
state, and nature of hydrogenation-active species in
Ziegler-type catalytic systems based on Pd(II) com-
plexes and the properties of these systems have been
studied to a lesser extent [17, 18] and are the subject
of this paper.
EXPERIMENTAL
Solvents (toluene, benzene, hexane) were purified
by standard procedures required for manipulations
with organometallic compounds [19]. For more ex-
haustive drying, the solvents were additionally dis-
tilled from LiAlH4 on a distillation column and were
stored under Ar in sealed ampules over 4 molecular
sieves.
Palladium bis(acetylacetonate) Pd(acac)2 was pre-
pared according to [20]. 1H NMR, , ppm: 5.04 s (2H,
CH), 1.76 s (12H, CH3).
Along with heterogeneous catalysts, hydrogenation
catalysts based on transition metal complexes has at-
tracted growing researchers’ attention since the second
half of the XXth century with the development of co-
ordination catalysis. A particular place among such
catalysts is occupied by Ziegler-type catalytic systems
[3]. The majority of studies in this field concerned the
nature and properties of hydrogenation catalysts based
on -diketonate, carboxylate, and cyclopentadienyl
Commercial-grade triethylaluminum AlEt3 was
distilled in a vacuum; the fraction boiling at 48 49 C
1
(1 mm Hg) was collected. H NMR, , ppm: 1.22 t
3
3
(9H, CH3, J = 8.24 Hz), 0.45 q (6H, J = 8.24 Hz).
The samples were stored in a glass ampule under Ar.
Solutions of triethylaluminum in hexane were pre-
pared in a Schlenk vessel under Ar.
1271

1272
BELYKH et al.
A TEM examination of the samples was per-
formed with a BS-300 microscope (Czechia). A drop
of a solution was applied onto a supporting grid
coated with a carbon film and was dried in an argon
atmosphere. The recording conditions excluded melt-
ing and decomposition of the samples under electron
beam.
The IR spectra of the precipitates were recorded on
a Specord 75-IR spectrometer in the range 4000
1
400 cm using KBr windows. Samples for recording
the IR spectra were prepared in a box with an inert
atmosphere as mulls in mineral oil preliminarily heat-
treated to remove traces of moisture.
Fig. 1. Hydrogenation of styrene in the presence of
3
the catalytic system Pd(acac) nAlEt . c = 5 10 M,
2
3
Pd
The catalytic properties of the Ziegler-type systems
based on Pd(acac)2 and AlEt3 in the hydrogenation
depend on numerous factors, in particular, on the
Al/Pd molar ratio. The dependence of the specific
activity in the hydrogenation of alkenes on the com-
ponent ratio passes through a maximum at Al/Pd = 4
(Fig. 1).
n
= 9 mmol, T = 30 C, P = 1 atm, solvent toluene;
styrene
H2
the same for Fig. 3. (W ) Specific activity of the system.
The hydrogenation of the substrates was performed
in a temperature-controlled glass vessel at 30 C and
initial hydrogen pressure of 1 atm in the presence of
the catalytic system formed in situ. To a solution of
5
A high activity of the Pd(acac)2 AlEt3 system at
Al/Pd = 4 was also observed in the hydrogenation of
other organic substrates: -alkenes, alkynes, nitro and
carbonyl compounds (Table 1). In particular, the spe-
cific activity W in hydrogenation of 1-hexene was
0.01522 g (5 10 mol) of Pd(acac)2 in toluene,
placed in a temperature-controlled duck-shaped vessel,
we added dropwise in a hydrogen flow a solution of
AlEt3 in hexane from a Schlenk vessel; the mixture
was stirred for 5 min. The molar ratio AlEt3/Pd(acac)2
was varied from 2 to 16. The total solution volume
was 10 ml. To the resulting brown-black mixture, we
added a substrate whose hydrogenation was performed
with vigorous stirring at the initial hydrogen pressure
of 1 atm. The reaction progress was monitored volu-
metrically and by GLC. Experiments involving modif-
ication of the catalyst with triphenylphosphine and
ethanol were performed similarly.
1
112 (mol H2) (mol Pd) 1 min . The hydrogenation
was accompanied by isomerization of 1-hexene into
2-hexene (cis + trans). At 99% conversion of 1-hex-
ene, the reaction system contained 59% hexane, 33%
trans-2-hexene, and 7% cis-2-hexene. The rates of
hydrogenation of the terminal and internal double
bonds differed by an order of magnitude; however,
even under these conditions, 2-hexene fairly rapidly
and quantitatively transformed into hexane under mild
The NMR spectra were recorded on a Varian VXR-
500S pulse spectrometer. The 31P chemical shifts are
given relative to 85% H3PO4.
conditions (30 C, PH = 1 atm). The high rate of
the positional isomerization of the double bond was
observed in the hydrogenation of 1,5-hexadiene whose
2
Table 1. Hydrogenation of organic substrates in the presence of the system Pd(acac)2 4AlEt3 (cPd = 5 10 3 M, 30 C,
PH = 1 atm, [substrate] : [Pd] = 182, solvent toluene)
2
W,
Product, %
Substrate
Styrene
1
(mol H2) (mol Pd) 1 min
145
112
77
117
39
3
Ethylbenzene 100
Hexane 100
1-Hexene
1,5-Hexadiene
Phenylacetylene
Nitrobenzene
Benzaldehyde
Naphthalene
1-Hexene 9.1, trans-2-hexene 43.7, cis-2-hexene 23.7, hexane 11.3*
Ethylbenzene 100
Aniline 97
Benzyl alcohol 98, toluene 0.1**
0
**
* At 99% conversion of 1,5-hexadiene.
Solvent benzene.
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PALLADIUM HYDROGENATION CATALYSTS MODIFIED WITH ALUMINUM-CONTAINING COMPOUNDS 1273
reduction is usually regarded as a model reaction of
the hydrogenation of polyene compounds.
ously, when studying the reaction of Pd(acac)2 with
AlEt3 in an inert atmosphere by H NMR spectros-
1
copy and electron microscopy, we have revealed three
major facts [22]. First, the reduction of Pd(acac)2 with
triethylaluminum is quantitative at the ratio Al/Pd
3.5, rather than at Al/Pd = 2 as could be expected
from the stoichiometric equation
The hydrogenation of phenylacetylene in the pres-
ence of the catalytic system Pd(acac)2 AlEt3 occurs
not only at a high rate, but also with a fairly high
selectivity: 97% with respect to styrene at 96% con-
version of phenylacetylene. No side reaction of di-
and oligomerization of phenylacetylene was observed.
The high selectivity of palladium catalysts in the hy-
drogenation of -alkynes is due to a thermodynamic
factor: considerably higher equilibrium constant of the
coordination of -alkyne, compared to alkene [21],
with the active center. The Pd(acac)2 4AlEt3 system
also shows high selectivity in the hydrogenation of
benzaldehyde to benzyl alcohol, which is usually fol-
lowed by hydrogenolysis of the C6H5CH2 OH bond.
Pd(acac)2 + 2AlEt3 = 1/xPd(O)x + 2Al(acac)Et2
+ C2H4 + C2H6.
This is apparently due to side catalytic decomposi-
tion of triethylaluminum in the presence of Pd(0)
nanoparticles. At Al/Pd 3.5, unchanged triethylalu-
minum remains in the reaction system. Second, the re-
duction of Pd(acac)2 yields Pd(0) nanoparticles sta-
bilized with AlEt2(acac) (1) and/or with adduct of
The observed dependence of the specific activity
on the Al/Pd ratio is caused by several factors. Previ-
AlEt2(acac) with AlEt3 at Al/Pd
3.5 (2):
Third, the mean size of the Pd(0) nanoparticles
depends on the Al/Pd ratio and decreases with an
increase in the triethylaluminum concentration from
nanoparticles (Al/Pd > 4) suggests the appearance of
a catalytic poison in the system. Unchanged AlEt3 can
act as such a poison. To confirm the inhibiting effect
of AlEt3 at high Al/Pd ratios, we studied the effect of
4.5 (Al/Pd = 4) to 1 2 nm at Al/Pd
8.
Thus, the rise in the specific activity of the
Pd(acac)2 AlEt3 system in the initial portion of the
curve (Fig. 1) is caused by an increase in the fraction
of reduced palladium and in the dispersity of the mi-
croheterogeneous system. The nonlinear dependence
of the styrene hydrogenation rate on the palladium
concentration and a decrease in the reaction rate at Pd
concentrations exceeding 10 mM suggest association
of the active particles into less active aggregates and
confirm the microheterogeneous nature of the catalyst
(Fig. 2).
The hydrogenation of alkenes is a structure-insensi-
tive reaction [23, 24]. Therefore, a decrease in the
catalytic activity with a decrease in the size of Pd
Fig. 2. Styrene hydrogenation rate w vs. the Pd concen-
tration c. Al/Pd = 4, n
= 9 mM, 30 C, P = 1 atm,
styrene
H2
solvent toluene.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 8 2006

1274
BELYKH et al.
Table 2. Influence of the modifier on the specific activity
of equimolar amounts of OPPh3 and AlEt3 does not
contain the signal from OPPh3 ( = 25 ppm), but
of the Pd(acac)2 nAlEt3 system in the hydrogenation
of styrene (cPd = 5 10
M, 30 C, PH = 1 atm,
contains a signal at
= 39 ppPm, which can be
3
P
2
[substrate]/[Pd] = 182, solvent toluene)
assigned to the adduct Ph3PO AlEt3 formed by
the donor acceptor interaction.
W1
W2
AlEt3
X
As follows from Table 2, addition of ethanol or tri-
phenylphoshine oxide to the Pd(acac)2 AlEt3 catalytic
system prepared in advance makes higher the hydro-
genation rate. In particular, in the presence of OPPh3
the rate of styrene hydrogenation increases by a factor
of approximately 10. With additions of ethanol, even
the performance of the most active catalytic system
(Al/Pd = 4) is surpassed. Hence, the role of triethyl-
aluminum is not restricted to the reduction of Pd(II)
compounds. When in excess, AlEt3 acts as a catalytic
poison. The poisoning effect of AlEt3 on the catalyst
may be due to formation of surface compounds by
the reaction of AlEt3 with hydride 3 or alkyl 4 inter-
mediates of the catalytic cycle:
Modifier X
OPPh3
(mol H2)(mol Pd) 1 min
1
Pd(acac)2
Pd
15
4
2
2
2
145
140
147
5
19
116
106
37
4
4
4
10
10
20
30
60
15
15
15
15
15
15
51
C2H5OH
4
71
3
172
4
18**
138
90**
.
.
*
H
R
.
.
(W , W ) Specific activity before and after introduction of
the modifier, respectively.
.
.
.
AlEt2
1
2
.
AlEt2
Pd
Pd
.
.
.
.
**
.
.
.
.
Hydrogenation of phenylacetylene.
Et
3
Et
4
Table 3. Hydrogenation of phenylacetylene (PA) in
the presence of the Pd(acac)2 nAlEt3 system (cPd
5
As noted above, the donor power of -alkynes is
higher than that of alkenes. Therefore, the poisoning
effect of triethylaluminum in the hydrogenation of
phenylacetylene was observed at higher Al/Pd ratios
(Table 3). However, the hydrogenation of -alkynes
is, in contrast to that of alkenes, a structure-sensitive
reaction [24], and a question naturally arises: Can the
observed drastic decrease in the rate of phenylacetyl-
ene hydrogenation at Al/Pd > 10 be attributed to the
too strong adsorption of the substrate on small palla-
dium particles? Addition of ethanol to the Pd(acac)2
15AlEt3 catalytic system prepared in advance led to
an increase in the specific activity in the hydrogena-
tion of phenylacetylene by a factor of 5. Hence, as in
the case of alkenes, the major factor responsible for
the decrease in the catalytic activity of the Pd(acac)2
AlEt3 system in the hydrogenation of phenylacetylene
at high Al/Pd ratios is the inhibiting effect of triethyl-
aluminum.
=
3
10 M, 30 C, PH = 1 atm, [substrate]/[Pd] = 182,
2
solvent toluene)
1
W, (mol H2) (mol Pd) 1 min
Al/Pd
C C
C=C
2
4
6
8
10
16
39
117
121
124
113
15
34
100
122
124
85
3
various modifying additives capable of binding AlEt3
(Table 2).
It is known that organoaluminum compounds form
stable 1 : 1 adducts with Lewis bases (e.g., ethers,
amines) and react, with release of hydrocarbons,
with water, alcohols, acids, and other proton-donor
compounds. Therefore, we tested as modifiers the sub-
stances that should, on the one hand, bind excess tri-
ethylaluminum and, on the other hand, be relatively
inert toward palladium. Among such compounds are
alcohols and triphenylphosphine oxide OPPh3. The
31P NMR spectrum of the mixture from the reaction
An opposite pattern was observed when ethanol was
added to the Pd(acac)2 4AlEt3 system, which is the
most active in the hydrogenation of styrene: The spe-
cific activity decreased with an increase in the alcohol
concentration (Table 2). On adding ethanol, the sys-
tem became aggregation-unstable, and a black precip-
itate formed. This fact indicates that the compounds
Al(OEt)2(acac) and/or Al(OEt)Et(acac) formed by
the reaction of AlEt2(acac) with ethanol exhibit weaker
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 8 2006

PALLADIUM HYDROGENATION CATALYSTS MODIFIED WITH ALUMINUM-CONTAINING COMPOUNDS 1275
stabilizing power than does AlEt2(acac), which re-
sults in the aggregation of the palladium nanopar-
ticles.
Taking into account the microheterogeneous nature
of the catalyst based on Pd(acac)2 and AlEt3, one
of the ways to enhance the catalytic activity is to
decrease the palladium particle size. However, in-
creasing the dispersity by adding more AlEt3 leads
to an opposite result because of the poisoning effect
of excess triethylaluminum. The dispersity can be
increased not only by taking the reductant in a higher
concentration, but also by performing the catalyst syn-
Fig. 3. Hydrogenation of styrene in the presence of the cat-
thesis by chemical condensation in the presence of
a stabilizer; as stabilizer we used triphenylphos-
phine.
alytic system Pd(acac) nPPh3 4AlEt3.
2
It is known that the complexation of triphenylphos-
phine with an equimolar amount of Pd(acac)2 leads
to the transformation of one of the acetylacetonate
ligands from the O,O-chelate to the C3-bound form,
with the formation of the complex Pd(acac)2PPh3
[25]. At P/Pd < 1, the reaction mixture contains
two complexes, Pd(acac)2 and Pd(acac)2PPh3, and
at P/Pd > 1, Pd(acac)2PPh3 and free triphenylphos-
phine.
Addition of triphenylphosphine to the Pd(acac)2
4AlEt3 before adding triethylaluminnum affects in a
complex fashion the catalytic properties in hydro-
genation. In particular, in the hydrogenation of sty-
rene, as the P/Pd ratio is raised, the specific activ-
ity first increases (P/Pd = 0.25) relative to the unmod-
ified catalyst, reaches a maximum at P/Pd = 1, and de-
creases to zero at P/Pd = 2 (Fig. 3). The catalyst pre-
pared at P/Pd = 1 [i.e., by the reduction of Pd(acac)2
PPh3 with AlEt3] is the most active; therefore, we
studied in more detail the catalyst based on Pd(acac)2
PPh3 AlEt3.
The 31P NMR spectrum of the Pd(acac)2PPh3
4AlEt3 system prepared in an inert atmosphere con-
tains broadened signals from free triphenylphosphine
Fig. 4. Electron micrograph of the Pd(acac) PPh 4AlEt
3
2
3
system and particle size distribution (inert atmosphere).
(A) Fraction of particles and (d) particle size; the same for
Fig. 5.
(
=
6 ppm) and from PPh3 coordinated to Pd in
P
lower oxidation states ( = 22, 16.4 ppm). A TEM
P
1
676, 450 cm ); this pattern indicates that both tri-
examination of the reaction mixture revealed the pres-
ence of high-contrast palladium particles, among
which particles 2.5 nm in diameter prevail (Fig. 4).
The IR spectrum of the brown powder isolated from
the reaction mixture contains bands of AlEt2(acac)
phenylphosphine and diethyl(acetylacetonato)alumi-
num molecules act as stabilizers of palladium nano-
particles.
The reaction of Pd(acac)2PPh3 with AlEt3 in a
hydrogen atmosphere yields benzene along with the
above-mentioned products. This fact indicates that the
catalyst formation is accompanied not only by the
reduction of Pd(II) to Pd(0), but also by decomposi-
tion of the phosphine ligands. At P/Pd = 1, about half
1
[ (C=O) + (C=C) 1585 and 1525 cm ; (C=C) +
1
(C CH3) 1284, 1218 cm ; (Al C H) + (C CH3)
1
1187, 1104, 1047, 1010 cm ] and bands characteris-
tic of out-of-plane bending vibrations of the C H and
C C bonds in the monosubstituted benzene ring (706,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 8 2006

1276
BELYKH et al.
Table 4. Formation of benzene in the course of preparation
Table 5. Hydrogenation of styrene in the presence of
of a catalyst based on the Pd(acac)2 AlEt3 system
the Pd(acac)2 4AlEt3 system modified with elemental P
3
3
modified with triphenylphosphine (cPd = 5 10
M,
(cPd = 5 10 M, 30 C, PH = 1 atm, [substrate]/[Pd] =
2
30 C, PH = 1 atm, solvent toluene)
182, solvent toluene)
2
1
C6H6/PPh3 in indicated time, min
W, (mol H2) (mol Pd) 1 min
PPh3/Pd(acac)2
P/Pd
1
5
Pd(acac)2 P4 4AlEt3
Pd(acac)2 4AlEt3 P4
0.25
0.5
1.0
2.0
2.9
2.3
0.7
0.8
2.9
2.9
1.4
0.8
0
145
147
155
31
145
155
163
49
0.2
0.3
0.5
1.0
0
0
of preparation of the Pd(acac)2 nPPh3 4AlEt3 cata-
lyst in hydrogen increased with a decrease in the P/Pd
ratio (Table 4). A similar trend has been observed
previously in reduction of Pd(acac)2 in the presence
of tertiary and primary phosphines [26, 27]. At P/Pd
0.25 or 0.5, triphenylphosphine decomposes virtually
completely, i.e., palladium phosphides are formed,
and the majority of Pd(0) passes into the form inactive
in the hydrogenation.
of the phenyl groups of triphenylphosphine trans-
formed into benzene (Table 4). The detection of
broadened signals with
= 22 and 16.4 ppm, which
P
were observed when the reaction was performed in an
inert atmosphere and were assigned to PPh3 coordi-
nated with Pd, indicates that a part of triphenylphos-
phine remained intact. The PPh2 and PPh fragments
formed by dephenylation of the phosphine ligands
readily undergo further degradation, ultimately yield-
ing palladium phosphides of various compositions.
According to electron microscopic examination, the
reaction of Pd(acac)2PPh3 with AlEt3 (Al/Pd = 4) in
a hydrogen atmosphere yields a polydisperse micro-
heterogeneous system in which particles 2.5 nm in
diameter prevail; the relative content of 1.2-nm parti-
cles increases (Fig. 5). Thus, on the one hand, modifi-
cation of the catalytic system with phosphine in a
hydrogen atmosphere leads to partial transformation
of Pd into an inactive form as a result of decompo-
sition of the phosphine ligands; on the other hand,
addition of phosphine favors formation of smaller
Pd(0) particles. A decrease in the palladium particle
size is one of the factors responsible for the promoting
effect of triphenylphosphine at P/Pd = 1.
It should be noted that the amount of benzene
formed per mole of triphenylphosphine in the course
Previously,we have put forward a hydrolysis, ac-
cording to which palladium phosphides show no cat-
alytic activity in the hydrogenation but act as a matrix
for Pd(0) clusters which are actual hydrogenation cata-
lysts [26]. To confirm this hypothesis, we performed
additional experiments on modification of the catalytic
system Pd(acac)2 4AlEt3 with elemental phosphorus.
Irrespective of the order of phosphorus addition (be-
fore or after adding triethylaluminum), we observed
at P/Pd 0.5 a decrease in the catalytic activity with
an increase in the concentration of elemental phos-
phorus (Table 5). A slight increase in the specific
activity at small additions of P may be due to changes
in the catalyst dispersity.
Fig. 5. Electron micrograph of the Pd(acac) PPh 4AlEt
3
2
3
system and particle size distribution (hydrogen atmosphere).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 8 2006

PALLADIUM HYDROGENATION CATALYSTS MODIFIED WITH ALUMINUM-CONTAINING COMPOUNDS 1277
CONCLUSIONS
6. Kalechits, I.V., Lipovich, V.G., and Shmidt, F.K.,
Neftekhimiya, 1966, vol. 6, pp. 813 816.
(1) Examination of the conditions for preparing
a Ziegler-type catalyst derived from palladium bis-
(acetylacetonate) and triethylaluminum and active in
hydrogenation of unsaturated compounds and carbonyl
and nitro groups showed that the role of triethylalu-
minum is not limited to the reduction of Pd(II).
The complex AlEt2(acac) formed by the reaction of
AlEt3 with Pd(acac)2 is a stabilizer of palladium nano-
particles. At high Al/Pd ratios, triethylaluminum acts
as a catalytic poison.
7. Lipovich, V.G., Shmidt, F.K., and Kalechits, I.V.,
Kinet. Katal., 1967, vol. 8, no. 4, pp. 812 815.
8. Lipovich, V.G., Shmidt, F.K., and Kalechits, I.V.,
Kinet. Katal., 1967, vol. 8, no. 6, pp. 1099 1103.
9. Shmidt, F.K., Kataliz kompleksami metallov pervogo
perekhodnogo ryada reaktsii gidrirovaniya i dimeriza-
tsii (Catalysis of Certain Hydrogenation and Dimer-
ization Reactions by Complexes of Transition Metals
of the First Row), Irkutsk: Irkutsk. Gos. Univ., 1986.
10. Savel’ev, S.R. and Noskova, N.F., Usp. Khim., 1994,
vol. 63, no. 11, pp. 995 1003.
(2) It was found that the modifying effect of etha-
nol depends on the Al/Pd ratio. At Al/Pd 4, ethanol
makes the catalytic activity lower as a result of bind-
ing of the stabilizer and, as a consequence, of aggrega-
tion of nanoparticles. At high Al/Pd ratios, the pro-
moting effect of ethanol is due to the adduct forma-
tion with the catalytic poison, AlEt3.
11. Active Metals: Preparation, Characterization, Ap-
plication, Furstner, A., Ed., Weinheim: VCH, 1995,
pp. 339 376.
12. Duteil, A., Schmid, G., and Meyer-Zaika, W.,
J. Chem. Soc., Chem. Commun., 1995, pp. 31 32.
13. Kozitsyna, N.Yu., Troitskii, S.Yu., Kryukova, G.N.,
et al., Mendeleev Commun., 2003, p. 1.
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Rev. Roum. Chim., 1999, vol. 44, nos. 11 12,
pp. 1003 1010.
15. Bonnemann, H., Waldofner, N., Yautbold, H.-G., and
Vad, T., Chem. Mater., 2002, vol. 14, no. 3,
pp. 1115 1120.
16. Shmidt, F.K., Nindakova, L.O., Shainyan, B.A.,
et al., J. Mol. Catal. A: Chemical, 2005, vol. 235,
pp. 161 172.
(3) The nature of the modifying effect of triphen-
ylphosphine on the catalytic properties of the Zieg-
ler-type system Pd(acac)2 AlEt3 was examined.
The formation of the hydrogenation catalyst in a hy-
drogen atmosphere is accompanied by decomposition
of the phosphine ligands; the extent of this process
increases with a decrease in the P/Pd ratio. The in-
hibiting or promoting effect of phosphine at P/Pd
1
depends on the ratio of two factors exerting opposite
effects on the catalyst activity: transformation of pal-
ladium particles into an inactive form as a result of
decomposition of phosphine ligands and decrease in
the size of Pd(0) particles upon stabilization with
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