Formation, nature of activity, and hydrogenation catalysis by nickel bis(acetylacetonate)-lithium tetrahydroaluminate systems

Article abstract of DOI:10.1134/S1070427210110030

A new approach to synthesis of nickel catalysts under the action of lithium tetrahydroaluminate was proposed which allows preparation of high-performance nanosized catalytic systems with well-reproducible properties. The major stages of formation and the

Full text of DOI:10.1134/S1070427210110030

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 11, pp. 1911−1918. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © L.B. Belykh, Yu.Yu. Titova, A.V. Rokhin, F.K. Shmidt, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 11, pp. 1778−1786.
INORGANIC SYNTHESIS AND INDUSTRIAL
INORGANIC CHEMISTRY
Formation, Nature of Activity, and Hydrogenation Catalysis
by Nickel Bis(Acetylacetonate)–Lithium
Tetrahydroaluminate Systems
L. B. Belykh, Yu. Yu. Titova, A. V. Rokhin, and F. K. Shmidt
Irkutsk State University, Irkutsk, Russia
Received February 17, 2010
Abstract—A new approach to synthesis of nickel catalysts under the action of lithium tetrahydroaluminate was
proposed which allows preparation of high-performance nanosized catalytic systems with well-reproducible
properties. The major stages of formation and the nature of catalytically active species and inhibitors formed in
the Ni(acac) –LiAlH system were determined. The catalytic properties of the nickel nanoclusters were studied in
2
4
relation to the nature and concentration of the proton-containing compounds. Factors responsible for the promoting
action exhibited by these compounds were analyzed.
DOI: 10.1134/S1070427210110030
Ziegler-type catalytic systems comprised of transition
metal complexes and organic (or hydride) compounds
of I–III Group nontransition metals rank among the
major commercial hydrogenation (in particular, polymer
hydrogenation) catalysts. They have been the subject
of investigation for 40 years. One of the central tasks
in catalysis is to elucidate the formation mechanism,
composition, and structure of catalytically active species,
as well as the factors responsible for their deactivation, in
order to control the synthesis and optimize the application
of these systems for hydrogenation catalysis. Relevant
studies showed that, depending on the composition of
Ziegler-type catalytic systems, it is possible to prepare
truly homogeneous and microheterogeneous (nanosized)
catalysts (for details on the nature and properties, see [1]).
Thenanosizednatureofthecatalyticallyactivespecieswas
established for Ziegler-type systems based on iron, cobalt,
nickel, and palladium acetylacetonates as combined with
aluminum organic derivatives [2–5]. It was shown that
the function of organometal compounds in Ziegler-type
systems is not limited to reduction of transition metal
compounds: Organoaluminum compounds and their
A special place among hydrogenation catalysts is
occupied by systems in which the function of reducing
agent is accomplished by more readily accessible
complex hydrides, in particular, LiAlH and NaBH4
4
and their derivatives [7–16]. The nature of nickel
boride catalysts is well understood [11], while data on
hydrogenation catalysts formed under the action of
LiAlH and on their properties are disconnected and
4
frequently contradictory [7, 8, 12–16]. For example,
Matyushin et al. [13, 14] studied hydrogenation of
polycyclic arenes over a nickel catalyst prepared by
the reaction of nickel bis(acetylacetonate) with LiAlH4.
That study showed that the activity of the catalyst
is determined not only by the LiAlH to Ni(acac)2
4
ratio but also by the aggregation state of lithium
tetrahydroaluminate introduced into the system in the
catalyst formation stage. Those researchers succeeded
in preparation of a catalyst with a satisfactorily
reproducible activity only in the case when powdered
LiAlH was added to a solution of Ni(acac) in a mixed
4
2
benzene : tetrahydrofuran = 5 : 1 solution, but did not
analyze the nature of the catalyst. At the same time,
Takegami et al. [7] found that, surprisingly, the catalytic
properties of MX –LiAlH catalytic system were
transformation products [e.g., AlR (acac)] contribute to
2
stabilization of metal nanoclusters [3–5]. Excessive AlR3
compounds are catalytic poisons [5, 6].
n
4
significantly affected by even insignificant change in its
1
911

1912
BELYKH et al.
composition.
stored under argon in sealed ampules over 4A molecular
sieves. Tetrahydrofuran, after removal of peroxides,
Ambiguous data were also obtained in experiments
utilizing not only lithium tetrahydroaluminate but
also its alkoxy hydride derivatives as reducing agents.
Schuldt et al. [15] also noted that the activity and
stability of the catalyst synthesized by the reaction
was distilled successively from sodium, LiAlH , and
4
benzophenone ketyl and stored under argon in sealed
ampules. The water concentration was estimated by the
–
3
–3
Fischer method [18] at 1.1 × 10 and 6 × 10 M for
benzene and THF, respectively.
of CoBr2 and Li[AlH(O–tert-Bu) ] solutions in
3
tetrahydrofuran is largely determined by its preparation
procedure. Surprisingly, out of naphthalene, anthracene,
and phenanthrene substrates tested in that study
under identical conditions, only anthracene exhibited
significant conversions in hydrogenation with the
above-mentioned catalytic system. By contrast, the
conversions of the two remaining aromatic hydrocarbons
did not exceed several per cent even after a prolonged
period. The factors responsible for such different types
of behavior displayed by the catalyst in hydrogenation
of naphthalene, anthracene, and phenanthrene were not
analyzed. Based on the amount of hydrogen evolved in
the reaction between the CoBr –Li[AlH(O-tert-Bu) ]
Nickel bis(acetylacetonate) was synthesized by the
procedure described in [19].
Lithium tetrahydroaluminate (LiAlH , commercial)
4
was used without preliminary recrystallization.
A weighed potion of LiAlH was dissolved in THF, and
4
the resulting solution was filtered in an inert atmosphere
through a glass filter. The LiAlH concentration was
4
estimated from the amount of hydrogen evolved in
hydrolysis at 95–98% of the theoretical value. The
27
Al NMR spectrum (δ, ppm, solvent THF): 98 quintet
JAl–H 173 Hz).
(1
The interaction between nickel bis(acetylacetonate)
2
3
and LiAlH at different reactant ratios was studied under
4
system components, it was concluded [15] that CoBr2
is reduced to Со(I), and the active complex comprises
unchanged reducing agent, Li[AlH(O–tert-Bu)3].
dry deoxygenated argon in a thermostated vessel whose
design presumes preliminary evacuation and filling
with argon. The reaction products for the components
of the Ni(аcаc) –LiAlH system were analyzed
At the same time, studies of the hydrogenating
properties of the catalytic systems based on Pd(acac)2
showed that the catalytically active species are nanosized
in nature and that, upon incomplete hydrolysis and
alcoholysis of LiAlH [16] and AlEt [5, 6] prior to their
2
4
spectroscopically (27Al NMR, IR, UV, and ESR) and
volumetrically.
For quantitative analysis of the molecular
hydrogen evolved during interaction of the catalytic
system components we used a special thermostated
vessel provided with an internal “pocket” for the
second component. For example, into a preliminarily
evacuated argon-filled thermostated vessel connected
to the volumetric system, a solution of 0.02534 g
4
3
use in reduction of Pd(II) compounds, the activity of the
palladium catalysts sharply increased, and the inhibitory
action of the excess of these compounds in the catalyst
preparation stage was diminished.
Here, we studied the interaction between the
components of the Ni(acac) –LiAlH catalytic system
–
3
(1 × 10 mol) Ni(аcаc) in 12 ml of benzene was poured,
2
4
2
in solution by various chemical and physical methods
and elucidated conditions conducive to formation of
nickel catalysts exhibiting reproducible properties in
hydrogenation reactions.
and an aliqout of a LiAlH solution in THF, into the
internal “pocket” of the vessel. The vessel was plugged
4
with a Teflon stopper, and the LiAlH and Ni(аcаc)2
4
solutions were mixed together under vigorous shaking.
The resulting hydrogen was analyzed volumetrically,
and the solution, by UV spectroscopy. The Ni(acac)2
conversion was calculated by the iteration method using
the Bouguer–Lambert–Beer equation. In calculation of
the distribution of acetylacetonate ligands among Ni,
Li, and Al, based on the optical-spectroscopic data, we
took into account the fact that the resulting Li(acac) is
virtually insoluble in benzene and used the following
molarabsorptioncoefficients:Ni(acac)2 ε300=15100and
ε288 = 11 800; Al(acac) ε = 33 460 and ε288 = 43 600.
EXPERIMENTAL
The solvents (benzene, THF), substrates (styrene,
phenylacetylene, nitrobenzene, naphthalene), and
reagents (ethanol, n-propanol, tert-butanol, iso-butanol,
n-pentanol) were purified by standard procedures used
for manipulations with organometal substances [17].
For more exhaustive drying, benzene was additionally
distilled from LiAlH on a distillation column and
4
3 300
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FORMATION, NATURE OF ACTIVITY, AND HYDROGENATION CATALYSIS
1913
The UV spectra were recorded on a Specord UV-VIS
spectrometer at 280–430 nm in an all-soldered cell.
naphthalene), as well as of double and triple bonds
in unsaturated compounds (1-hexene, styrene,
phenylacetylene, tolan) and of nitro and carbonyl groups
The IR spectra of the solutions were measured on
a Specord 75-IR spectrometer (solvent benzene–THF)
in a 0.2-mm-thick KRS cell preliminary blown through
with argon. The NMR spectra were recorded on a VXR-
(nitrobenzene, benzaldehyde). Upon replacement of the
solutions of lithium tetrahydroaluminate in THF by
its alkoxy hydride derivatives [LiAlH (OR) , where
4–x
x
x = 1, 2, 3; R = n-Bu, tert-Bu], the situation remained
unchanged, by contrast to palladium catalysts [20]. The
catalytic inertness in hydrogenation processes, exhibited
by the nickel-based systems prepared under the action of
5
00S Varian pulse spectrometer. The chemical shifts of
27
the Alsignalsweremeasuredrelativeto0.1MAl(NO )
3
3
solution (external standard); to prevent hydrolysis,
the analyzed solution was poured into a preliminarily
evacuated argon-filled quartz ampule 5 mm in diameter,
which was further sealed. The transmission electron
microscopic examinations were carried out with the use
of a Philips EM-410 microscope.
LiAlH solutions in THF, was rather a surprising result,
4
because the available published data prove the contrary
[7, 8, 12].
Addition of the LiAlH solution in THF to the
4
Ni(acac) solution in benzene led to a change in the color
2
The experiments on hydrogenation of unsaturated
compounds were carried out in a duck-shaped
thermostated vessel at 35°С and hydrogen pressure of
of the solution from green to dark brown and to evolution
of hydrogen. Volumetric analysis of hydrogen formation
in time for the Ni(acac) –LiAlH system revealed two
2
4
1
gauge atm (0.20265 MPa) under vigorous stirring,
stages: first, rapid stage which was complete within
–2 min, and the second stage which proceeded at
which precluded transition of the reaction to the
1
diffusion region. A weighed portion of Ni(acac) (from
2
a nearly three orders of magnitude lower rate. Hydrogen
formation in the second stage was observed for at least
1
× 10–5 to 2 × 10–4 mol) and benzene (10 ml) were
successively charged in a hydrogen stream into the
preliminarily evacuated hydrogen-filled thermostated
duck-shaped vessel, and the resulting mixture was
stirred until complete dissolution. Next, the substrate
6
h. Notably, the hydrogen yield even in the first stage
–
1
significantlyexceeds1mol[Ni(acac) mol] asestimated
2
from the stoichiometric equation describing the most
frequently observed case of reduction of transition-metal
compounds with lithium tetrahydroaluminate [21]:
and a solution of LiAlH in THF were poured to the
4
resulting light-green solution. The vessel was plugged
with a Teflon stopper supplied with a rubber gasket (for
taking samples), whereupon excess hydrogen pressure
Ni(acac) + 2LiAlH → Niquant + 2AlH3
2
4
(1 gauge atm) was produced, and hydrogenation was
+
H + 2Li(acac).
(1)
2
carried out. We monitored the reaction by volumetric
and gas-liquid chromatographic methods. If no catalytic
activity was observed after the first 20 min of the process,
the catalyst was activated by introduction of proton-
containing compounds (alcohols, water, phenol, acetic
acid). The styrene hydrogenation products, aromatic
compounds, phenylacetylene, and nitrobenzene were
analyzed on a KhROM-5 chromatograph with a flame-
ionization detector (carrier gas nitrogen; 3.6 m × 3 mm
packed column; Carbowax-20M phase). The heating
program was as follows: 50°С (4 min), 100°С (12 min),
The amount of hydrogen evolved was ≥ 8 mol per Ni
mol at LiAlH /Ni(acac) ≥ 6 (Fig. 1).
4
2
The optical-spectroscopic examination showed that
the conversion of Ni(acac) to Ni(0) depends on the
2
LiAlH /Ni(acac) molar ratio. Quantitative analysis
4
2
of the UV spectra for the Ni(acac) –LiAlH reaction
2
4
system, taken after 5 min of the reaction, with the aim
to elucidate the distribution of acetylacetonate ligands
among Ni, Li, and Al, showed that Ni(acac) was
2
converted quantitatively at the molar ratio LiAlH4/
1
40°С (4 min).
Ni(acac) > 5 (Fig. 2). At lower reactant ratios, there
2
Despite partial or quantitative reduction of nickel
was unchanged nickel bis(acetylacetonate) remaining in
the reaction system. At LiAlH /Ni(acac) = 2, Ni(acac)
bis(acetylacetonate), the Ni(acac) –LiAlH catalytic
2
4
4
2
2
system resulted from interaction of the solutions of
these components in benzene and THF, respectively
was converted by 40–45%, with ~85% acetylacetonate
groups bound to lithium ion as Li(acac), and 14%, to
aluminum. At LiAlH /Ni(acac) = 5, there was 96–98%
[
LiAlH /Ni(acac) = 1–20], was catalytically inactive
4 2
4
2
in hydrogenation of aromatic compounds (benzene,
acetylacetonate ligands bound to lithium, and only
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

1914
BELYKH et al.
Ni(acac) was derived from the 27Al NMR and IR
2
spectroscopic data.
The 27Al NMR spectrum of lithium tetrahydro-
aluminate contains a resonance signal at 98 ppm
represented by a quintet characterized by the spin-spin
1
coupling constant J
(
173 Hz and halfwidth Δν1/2
Al–H
27Al) of 47 Hz (Fig. 3a). A multiplet structure of the
spectrum is due to scalar spin-spin coupling of equivalent
protons with the aluminum isotope and suggests that the
–1
Fig. 1. Yield of molecular hydrogen Y, mol [Ni(acac) mol] ,
2
(a)
vs. reactant ratio in the Ni(acac) –LiAlH system. Solvent
2
4
benzene:THF = 6 : 1, сNi = 0.01 M, solution volume 12 ml;
the same for Fig. 2.
(b)
Fig. 2. Conversion of Ni(acac) α,%, vs. reactant ratio in the
2
Ni(acac) –LiAlH system.
2
4
2
–4%, to aluminum, and at LiAlH /Ni(acac) = 8–10
4 2
(c)
all the acetylacetonate groups passed from nickel to
lithium.
Nonquantitative conversion of Ni(acac)2 under
the action of lithium tetrahydroaluminate at LiAlH4/
Ni(acac) = 2 in accordance with the stoichiometric
2
equation (1) proved to be rather a surprising result.
It should be noted that Pd(acac) was exhaustively
2
converted in reaction with LiAlH even at LiAlH /Pd = 1
4
4
[16]. The reason is that palladium bis(acetylacetonate)
was also reduced by Al, AlH , the redox transformation
3
Fig. 3. 27Al NMR spectra of (a) solution of LiAlH in THF
products of LiAlH , which appeared in the reaction
4
4
and (b, c) Ni(acac) –6LiAlH system (b) before and (c) after
2
4
system.
activation with tert-BuOH. Solvent benzene : THF = 5 : 1,
сNi = 0.03 M, [tert-BuOH]/[LiAlH4] = 4. (δ) Chemical shift,
ppm.
The information on conversion of LiAlH and on
4
the products of its transformation in the reaction with
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

FORMATION, NATURE OF ACTIVITY, AND HYDROGENATION CATALYSIS
1915
–
AlH anion preserves its tetrahedral configuration in
These data suggest that the Ni(0) clusters catalyze
decomposition of not only alane but of lithium
tetrahydroaluminate as well.
4
a THF solution [22, 23]. At the same time, there were
no resonance signals from AlH and its transformation
products in the Al NMR spectrum of the Ni(acac)2–
LiAlH reaction system at the molar ratio of reactants
LiAlH /Ni(acac) ≤ 10 (Fig. 3b). A resonance signal
from AlH at 98 ppm was recorded in the Al NMR
spectrum at LiAlH /Ni ≥ 20 solely.
–
4
2
7
At the present time, the LiAlH decomposition
4
4
products still remain to be identified, but Li AlH is
3
6
4
2
commonly recognized as one of the stable products of
–
27
4
thermal decomposition of LiAlH [26]. A presumption
4
4
that lithium tetrahydroaluminate is decomposed under
the action of Ni(0) via formation of Li AlH leads to
The NMR data for the Ni(acac) –LiAlH system
agree with the IR spectroscopic findings. The IR
spectra suggests a decrease in intensity at LiAlH4/
Ni(acac) = 2 for the absorption bands associated
with ν(С=О) + ν(С=С) (1585 and 1510 cm )
stretching vibrations in О,О-chelated acetylacetonate
ligand in Ni(acac) , as well as with stretching
ν(Al–H) = 1690 cm and bending δ(Al–H) = 765 cm
vibrations of LiAlH [24]. Upon completion of the
2
4
3
6
conclusion that this process, along with Li AlH , should
3
6
additionally yield alane:
2
–
1
Ni
Ni
Ni
LiAlH4
LiH + AlH3
(4)
5)
Li AlH + 2AlH , (6)
2
1
(
LiAlH + 2LiH
Li AlH
3 6
–
–1
4
4
3
LiAlH4
reaction the Ni(acac) concentration in the Ni(аcac) –
3
6
3
2
2
LiAlH reaction system [LiAlH /Ni(acac) = 2] was
4
4
2
close to 50%. It should be noted that the IR spectra did
not contain absorption bands from tetrahydroaluminate
anion [ν(Al–H)=1690 cm , δ(Al–H) = 765 cm ] and
whose catalytic decomposition results in additional
hydrogen evolution. Low conversion of Ni(acac) at
2
–
1
–1
the stoichiometric reactant ratio may be associated
from alane which has a characteristic broad absorption
specifically with catalytic decomposition of LiAlH by
4
–
1
–1
band at 1600–1800 cm , peaked at 1750 cm [25].
reaction (6), catalyzed by nickel clusters resulted from
According to the IR spectroscopic data, Ni(acac) is
2
Ni(acac) reduction.
2
quantitatively converted at LiAlH /Ni(acac) = 6.
4
2
At the same tine, the above-discussed facts do
not disclose reasons for catalytic inertness of nickel
systems. As mentioned above, the ESR data suggest
that the reaction of Ni(acac) with LiAlH at various
An ESR spectroscopic study of the Ni(acac)2–
LiAlH system [LiAlH /Ni(acac) = 5, 10, 20] revealed
4
4
2
formation of ferromagnetic nickel clusters (g = 2.2,
ΔH = 600 Oe).
2
4
reactant ratios yields ferromagnetic nickel particles.
A transmission electron microscopic examination
showed that, at LiAlH /Ni(acac) = 10, the nanoparticles
Thus, our experiments revealed incomplete reduction
of Ni(acac) at LiAlH /Ni(acac) < 6, formation of up to
2
4
2
4
2
–1
8
.5 mol (Ni mol) hydrogen, and lack of free LiAlH4
measured ca. 2 nm in size. The stability of the resulting
disperse systems varies with the molar ratio of the
reactants. The disperse phase particles formed in the
Ni(acac) –LiAlH system at LiAlH /Ni(acac) = 1–2
at LiAlH /Ni(acac) < 20. Combined, these findings
4
2
suggest that LiAlH participates not only in a redox
4
process involving Ni(acac)2.
2
4
4
2
As known, highly dispersed powders of transition
metals catalyze decomposition of alane into components
under mild conditions [16, 21]:
precipitated already after 1 h, by contrast to LiAlH4/
Ni(acac) = 10, 20, in which case the particles remained
2
stable for several weeks. The ESR spectroscopic
and electron microscopic data suggest formation of
reduced nickel nanoparticles, but their isolation from
the dispersion medium proved to be problematic.
They were essentially reversible colloids; transition to
dissolved state after complete removal of the solvent
in a vacuum was achieved under the action of not only
THF, benzene, and diethyl ether but of hexane as well.
Precipitation from Ni(acac) –10LiAlH was observed
Ni
2
AlH → 2Al + 3H .
(2)
3
2
However, the total amount of molecular hydrogen in
the Ni(acac) –LiAlH system, formed by reactions (1)
2
4
–
1
and (2), cannot exceed 4 mol [Ni(acac) mol] even if
2
alane is exhaustively decomposed into elements:
2
4
Ni(acac) + 2LiAlH → Ni + 2Al + 4H .
(3)
2
4
2
only after contact with air and formation, according to
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

1916
BELYKH et al.
the XPA data, of nickel oxide.
alcohols, acids, and phenol) upon reduction of Ni(acac)2
with LiAlH excess caused appearance of a catalytic
As known, there exist electrostatic, steric, and
electrosteric stabilizers of metal nanoparticles; mere
solvents (in our case, benzene, THF) are unable of metal
nanocluster stabilization [27]. The ability of complex
hydrides to form bridging bonds with the surface atoms of
transition metals [28] suggests that tetrahydroaluminate
anions can contribute to stabilization of nickel
4
activity within a brief (1–2 min) period. The catalytic
activity level depends on the nature and concentration
of proton-donating compounds (see table). Among the
compounds that we tested, the strongest promoting effect
was exhibited by n-butanol. Weaker promoting powers
of tert- and iso-butanol are evidently due to their lower
reactivities. In fact, adding n-butanol to the Ni(acac)2–
nanoparticles. Also, formation of Ni–AlH (x = 0, 1, 2)
x
1
0LiAlH catalytic system activated by tert-butanol,
groups on the surface of the nickel nanoparticles (in
catalytic decomposition of alane) cannot be ruled out.
Hence, catalytic inertness of the nickel catalysts formed
4
caused its catalytic activity in styrene hydrogenation to
increase sixfold.
under the action of LiAlH can be associated with
It should be noted that acetic acid, when tested
as promoter, caused a 400-fold deceleration of the
reaction even when in a twofold excess. The reason is
that acetic acid reacts not only with catalytic poisons
but also with nickel and converts it to the oxidized
state. A promoting effect is exhibited by water as well.
In the presence of equimolar amounts of water with
4
poisoning of the active surface nickel sites by aluminum,
surface aluminum hydride (Ni–AlH ), and LiAlH . These
x
4
compounds form fairly strong bonds, thereby blocking
active sites of the catalyst. The lack of a resonance
signal from both free and coordinated LiAlH in the
4
27
Al NMR spectrum of the Ni(acac) –LiAlH [LiAlH /
2
4
4
Ni(acac) < 20] reaction system does not contradict the
respect to the initial LiAlH the catalytic activity of
2
4
above presumption: This fact may be due to broadening
of the resonance signal owing to formation of a dispersed
system and low concentration of stabilizers.
nickel-based systems in styrene hydrogenation reached
–
1
–1
70 styrene mol (mol Ni) min . These results allow
understanding the contradictions in the published data
to be associated with the experimental integrity, i.e., the
presence of various uncontrollable amounts of water in
the reaction system.
Considering the above-said, we proposed the
following strategy for synthesis of high-performance
nickel catalysts. First, Ni(acac) is to be reduced with
2
LiAlH excess in the presence of a substrate in order
The catalytic activity of Ni(acac) –LiAlH systems
4
2
4
to prepare highly dispersed systems, and this is to be
followed by introduction of a proton-donating compound
to chemically bind LiAlH4 and its transformation
products.Our experiments showed that, indeed, the
introduction of proton-donating compounds (water,
depends not only on the nature and concentration of
the promoter but also on the LiAlH /Ni(acac) ratio
and Ni(0) concentration (Fig. 4). The occurrence of
a maximum in the dependence of the catalyst activity
on the Ni(acac)2 concentration is consistent with the
4
2
Influence of the nature of the proton-containing compound on the catalytic activity of the Ni(аcac) –20 LiAlH system in styrene
2
4
hydrogenation с = 3.85 mM, [substrate]/[Ni] = 174, solvent benzene : THF = 10 : 2, solvent volume 12 ml, T = 35°С, P = 1 atm
Ni
H
2
Catalytic activity, substrate mol (Ni mol)–1 min , at indicated [ROH]/[LiAlH ], mol mol
–1
–1
4
ROH
1
2
4
8
11
–
H O
69
94
94
76
2
C H OH
47
100
35
41
82
–
123
151
38
130
220
50
164
131
45
49
123
–
–
131
49
2
5
n-C H OH
4
9
tert-C H OH
4
9
iso-C H OH
90
90
49
4
9
n-C H OH
192
–
123
–
130
37
5
11
C H OH
6
5
CH COOH
82
0.2
0.6
0
0
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

FORMATION, NATURE OF ACTIVITY, AND HYDROGENATION CATALYSIS
1917
nanosized nature of the catalytically active species.
A dramatic decline in activity with increasing Ni(acac)2
concentration is explained by aggregation of the catalyst
particles and decrease in the proportion of the active
surface sites. Enhancement of the catalytic activity with
increasing LiAlH concentration is due to formation of
4
a more highly dispersed nickel catalyst.
Introduction of promoters, e.g., alcohols, into the
Ni(acac) –LiAlH reaction system was accompanied
2
4
by an additional fast (within 0.5–1 min) evolution of
molecular hydrogen which may result from alcoholysis
of LiAlH , Li AlH , and/or surface compounds of the
4
3
6
–
3
Ni–AlH type. For example, adding a fourfold excess
cNi × 10 , M
x
of n-butanol to the Ni(acac) –LiAlH system caused
2
4
Fig. 4. Catalytic activity А, styrene mol (Ni mol) –1 min–1,
evolution of additional 2, 3, and 20 ml of hydrogen at
LiAlH /Ni(acac) of 2, 6, and 10, respectively. Formation
of the Ni(acac) –LiAlH system activated with n-butanol in
2
4
styrene hydrogenation vs. catalyst concentration c , M, at
4
2
Ni
the [LiAlH ]/[Ni(acac) ] ratio of (1) 5, (2) 10, and (3) 20.
of hydrogen was also observed upon introduction of other
proton-containing promoters into the catalytic system.
4
2
νstyrene = 8.7 × 10–3 M, V
=1 ml, T = 35°C, PH2 = 1 atm.
C H OH
4
9
According to the NMR spectroscopic data that we
obtained, introduction of tert-butanol into Ni(acac)2–
LiAlH (LiAlH /Ni = 6) catalytic system leads to
appearance in the Al spectra of three poorly resolved
signals in a weak field with chemical shifts at 53, 59,
and 69 ppm (Fig. 3c). The resonance shift at 53 ppm
to zero. Reactivation of the nickel catalyst with n-BuOH
allowed the catalytic activity not only to regain its initial
value but even to double. These data validate the poi-
soning action of lithium tetrahydroaluminate but do not
completely rule out the inhibitory action of its catalytic
decomposition products.
4
4
2
7
is characteristic for LiAl(tert-BuO) (δ = 52 ppm [29]),
4
Thus, the Ni(acac) –LiAlH system examined by
2
4
and that near 69 ppm can be most probably assigned to
us is distinguished from similar palladium catalysts
described in literature [16, 20] by the ability of the
resulting nickel nanoparticles to catalyze decomposition
of not only alane but also of lithium tetrahydroaluminate.
LiAlH(tert-BuO) [29]. The lack of hyperfine structure
3
is not surprising, since it is known that, if one of the
–
ligands in the AlH4 anion differs in nature from the
2
7
other ones, the multiplet structure in the Al spectrum
is not manifested [23]. A shift of this signal by 9 ppm
to stronger fields can be associated with distortion of
the tetrahedral structure [30] owing to interaction with
nickel clusters. The assignment of the third signal
The function of LiAlH is not limited to reduction of the
4
nickel precursor: The tetrahydroaluminate anions are
adsorbed on the surface and act as stabilizers of nickel
nanoparticles. Moreover, tetrahydroaluminate anions
and, probably, the catalytic decomposition products of
(δ = 59 ppm) still remains problematic.
LiAlH , occupy vacant surface sites and act as catalytic
4
The use of n-butanol as promoter for the Ni(acac)2–
poisons. The latter undergo chemical transformations
via reaction with proton-donating compounds, which
leads to nickel catalysts exhibiting high performance
characteristics in hydrogenation of alkenes. The strategy
that we proposed allows preparation of nanosized
nickel catalysts of alkene hydrogenation with well-
reproducible properties, which are considerably superior
in the activity not only to similar systems based on iron
triad [8] but also palladium catalysts [16, 20].
LiAlH system leads to appearance in the reaction
4
2
7
system of LiAl(n-BuO) , as follows from the Al NMR
4
spectroscopic data.
Our data suggests that alcoholysis of lithium
tetrahydroaluminate adsorbed on the nickel clusters
belongs to factors responsible for the promoting action
by alcohols (or other proton-containing compounds).
This presumption was additionally validated by our
kinetic experiments. When 1 or even 0.5 mol LiAlH
4
–1
CONCLUSIONS
[
Ni(acac) mol] is introduced into the catalytic system
2
during styrene hydrogenation, the catalytic activity dramat-
ically decreases from 70 (substrate mol) (Ni mol) min
–1
–1
(1) It was found that the Ni(acac) –LiAlH system
2 4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 11 2010

1918
BELYKH et al.
in a mixed benzene–tetrahydrofuran solvent does not
exhibit catalytic activity in hydrogenation of alkenes
and alkynes, as well as of nitro- and carbonyl-containing
and aromatic compounds.
Chem., 2008, vol. 285, nos. 1–2, pp. 29–35.
11. Geng, J., Jefferson, D.A., and Johnson, B.F.G., Chem.
Commun., 2007, no. 9, pp. 969–971.
1
2. Ashby, E.C. and Lin, J.J.J., J. Org. Chem., 1978, vol. 43,
(2) A synthesis strategy was proposed which is based
no. 13, pp. 2567–2572.
on reduction of Ni(acac) with LiAlH excess, followed
13. Matyushin, V.P., Chenets, V.V., Shmidt, F.K., and
2
4
by introduction into the system of a proton-donating
Kalechits, I.V., Khim. Tverd. Tela, 1987, no. 6, pp. 47–49.
promoter with a view to chemical conversion of LiAlH4
and itsc atalyticde compositionpr oducts.
14. Matyushin, V.P., Chenets, V.V., Shmidt, F.K., and
Kalechits, I.V., Zh. Vses. Khim. O–va. im. D.I. Men-
deleeva, 1986, vol. 31, no. 4, pp. 475–477.
(3) The catalytic properties of the nickel systems
were studied in relation to the nature and concentration
of proton-containing compounds (alcohols, acids,
water), among which the strongest promoting action
was exhibited by n-butanol.
15. Schuldt, B., Heller, D., and Camadeya, A., J. Mol. Catal.,
1993, vol. 81, no. 2, pp. 195–206.
16. Belykh, L.B., Titova, Yu.Yu., Rokhin, A.V., et al., Zh.
Prikl. Khim., 2008, vol. 81, no. 6, pp. 917–925.
(
4) The prepared nicked catalysts are considerably
17. Gordon, A. and Ford, R., The Chemist’s Companion: A
Handbook of Practical Data, Techniques and References,
New York: John Wiley&Sons, 1972.
superior in the activity displayed in hydrogenation of
alkenes not only to similar systems based on iron triad
but also to palladium catalysts.
18. Mitchell, J., Jr. and Smith, D.M., Aquametry, New York:
Wiley Interscience, 1977.
ACKNOWLEDGMENTS
19. Dzhemilev, U.M., Popod’ko, N.P., and Kozlova, E.V.,
Metallokompleksnyi kataliz v organicheskom sinteze:
Alitsiklicheskie soedineniya (Metal-Complex Catalysis
in Organic Synthesis: Alicyclic Compounds), Moscow:
Khimiya, 1999.
Thisstudywasfinanciallysupportedbythe“Scientific
and Scientific and Pedagogic Staff of Innovation Russia”
Federal Target Program (state contract no. P 1344).
2
0. Belykh, L.B., Titova, Yu.Yu., Rokhin, A.V., and
Shmidt, F.K., Zh. Prikl. Khim., 2008, vol. 81, no. 7,
pp. 1075–1081.
REFERENCES
2
1. Soloveichik, G.L. and Bulychev, B.M., Usp. Khim., 1983,
1
. Alley, W.M., Hamdemir, I.K., Johnson, K.A., and
Finke, R.G., J. Mol. Catal. A: Chem., 2010, vol. 315,
no. 1, pp. 1–27.
vol. 52, no. 1, pp. 72–103.
22. Tarasov, V.P., Bakum, S.I., Privalov, V.I., and Busla-
ev, Yu.A., Koordinats. Khim., 1981, vol. 7, no. 5, pp. 674–
2
3
4
. Schmidt, F.K., Ratovskii, G.V., Dmitrieva, T.V., et al.,
J. Organomet. Chem., 1983, vol. 256, pp. 309–329.
6
92.
2
2
3. Tarasov, V.P., Privalov, V.I., Gorbik, A.A., and
Bakum, S.I., Koordinats. Khim., 1985, vol. 11, no. 12,
pp. 1626–1634.
. Bonnemann, H., Brijoux, W., Brinkmann, R., et al., Rev.
Roum. Chim., 1999, vol. 44, nos. 11–12, pp. 1003–1010.
. Shmidt, F.K., Nindakova, L.O., Shainyan, B.A., et
al., J. Mol. Catal. A.: Chem., 2005, vol. 235, nos. 1–2,
pp. 161–172.
4. Ashby, E.C., Dobbs, F.R., and Horkins, H.P., J. Amer.
Chem. Soc., 1975, vol. 97, pp. 3158–3162.
5
6
. Belykh, L.B., Goremyka, T.V., Skripov, N.I., et al., Kinet.
Katal., 2006, vol. 47, no. 3, pp. 373–380.
25. Ashby, E.C., Jonh, J., Watkins, J., and Prasad, H.S., Inorg.
Chem., 1975, vol. 14, no. 3, pp. 583–588.
. Belykh, L.B., Titova, Yu.Yu., Umanets, V.A., and
Shmidt, F.K., Zh. Prikl. Khim., 2006, vol. 79, no. 8,
pp. 1285–1291.
26. Tarasov, V.P., Bakum, S.I., and Kuznetsova, S.F., Zh.
Neorg. Khim., 1997, vol. 42, no. 5. pp. 779–786.
2
2
2
3
7. Ott, L.S. and Finke, R.G., Coord. Chem. Rev., 2007,
7
8
9
. Takegami, I., Ueno, T., and Fugii, T., Bull. Chem. Soc.
Jpn., 1965, vol. 38, no. 8, pp. 1279–1285.
vol. 251, pp. 1075–1100.
8. Guella, G., Zanchetta, C., and Patton, B., J. Phys. Chem.
B, 2006, vol. 110, pp. 17024–17033.
. Ganem, B. and Osby, J., Chem. Rev., 1988, vol. 86,
pp. 763–780.
9. Horne, D.A., J. Amer. Chem. Soc., 1980, vol. 102, no. 19,
. Lee, S.P. and Chen, Y.W., J. Mol. Catal. A: Chem., 2000,
pp. 6011–6014.
vol. 152, nos. 1–2, pp. 213–223.
0. Haraguchi, H. and Fujiwara, S., J. Phys. Chem., 1969,
1
0. Hui, L., Qingfei, Zh., and Hexing, L., J. Mol. Catal. A:
vol. 73, no. 10, pp. 3467–3473.
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