Y.Y. Titova, F.K. Schmidt
Applied Catalysis A, General 547 (2017) 105–114
this issue, for example [17–22]. In addition, there are review articles
distilled over needle column. Further, it was boiled and distilled over
sodium collecting the fraction, which boiled at 78 °C [34].
[
23,24], where examples of the investigation of Ziegler systems are
discussed among other things. Nevertheless, the fundamental research
of the nature of catalytically active forms in the catalysis of arenes
hydrogenation are virtually absent, with the exception of [18,21,22].
The authors of these papers presented convincing evidences of the na-
nosized nature of catalytically active centers. Quantitative data,
namely, values of TTO (100,000 total turnover) are presented in several
articles only.
The samples of nickel bis(acetylacetonate) (Ni(acac)
thesized according to the protocols described in [11,35]. The purity of
the Ni(acac) was determined by thermogravimetric analysis combined
with differential scanning calorimetry on a STA 449 F3 Jupiter deri-
vatograph (Netzch, Germany) under the following conditions: rate of
nitrogen feeding was 30 ml/min, rate of heating was 5 K/min.
2
) were syn-
2
Nevertheless, exactly the catalysts of the Ziegler type have found
widespread use in industrial processes such as the hydrogenation of
butadiene-styrene copolymers [25]. Thus, it can be assumed that the
Ziegler catalysts for arene hydrogenation could be used, for example, in
fine organic synthesis, where one of the steps is a benzene ring hy-
drogenation.
Moreover, the process of hydrogenation of benzene and its homo-
logs is important both from an industrial and environmental point of
view. It is sufficient to mention the benzene hydrogenation to cyclo-
hexane and cyclohexene and the phenol hydrogenation to cyclohexanol
2.2. Experimental protocols
2.2.1. Catalyst formation
The sample of Ni(асас)
2
(4 × 10−4mol, 0.1028 g), benzene or other
arene (36 ml) were sequentially placed into preliminarily vacuumed in
the thermostatically controlled flask filled with hydrogen. The reaction
mixture was stirred for 25 min until a transparent light-green solution
3 2
was formed. To the solution obtained, AlEt (or AlEt (OEt)) solution in
n-hexane (4 ml) were added, the varied ratios of Al/Ni being 1–10
(general volume of system is 40 ml). The reaction of catalyst formation
was monitored by gas evolution and UV-spectroscopy (method of se-
quential approximation).
[
26], or the issues related to the diesel fuel production with a low
content of aromatic hydrocarbons. The results published in the litera-
ture mainly concern the effect of the catalytic system composition on
the yield of the hydrogenation products [14,27–30]. However, quanti-
tative data about the TOF and TON values in the arene hydrogenation
reaction are virtually absent.
2.2.2. Hydrogenation experiments
The experiments on hydrogenation were carried out in an autoclave
(Reactor System M-FGA-ST-MDF-AC-R, “REXO Engineering”, S. Korea)
provide with mechanical stirrer, manometer, two gas lines, sampling
device and controller (Control System CS-1000, “REXO Engineering”, S.
Korea), at specified temperature (80–120 °С), initial hydrogen pressure
(15 bar) and vigorous stirring (500–750 turn/min) which provides the
process flow in the kinetic region.
The generated in situ reaction mixture with hydrogenated arenes
(view “Catalyst formation”) was placed into an autoclave preliminarily
filled with hydrogen. The hydrogenation reaction was monitored by
pressure drop on a monometer and the samples were analyzed by GLC
technique. The analysis was performed on a «Khromatek-Kristall
5000.2” instrument (“Khromatek”, Russia) equipped with flame-ioni-
zation detector using HP Carbowax 20 M capillary column (30 m
length, 0.53 mm diameter), and GCMS-QP-2010 mass spectrometer
The aim of this study was to determine the catalytic properties of
Ziegler systems based on Ni(acac) from a quantitative standpoint in
2
the hydrogenation of benzene and its methyl homologs, including the
hydrogenation under competitive conditions, and to establish the
nature of the particles that are responsible for the catalytic process. The
application of the competitive hydrogenation method to the Ziegler
hydrogenation catalysts have a special importance, both from the fun-
damental point of view, since it allowed to compare the reactivity of
arenes under strongly identical conditions, and with the applied point
of view, for those cases when the arene substrates contains of the
aromatic impurities.
2. Experimental
(
Shimadzu, Japan).
2.1. General procedures
TOF was estimated as the number of molecules reacted at nickel
mole per time (in min). Relative error of the TOF value that determined
from three to five parallel experiments is not more than 5%.
Benzene, toluene, xylenes (o-,m-, p-), n-hexane were purified by the
standard protocols applied for handling with organometallic com-
pounds [31]. To reach a deeper drying, benzene and toluene were ad-
ditionally distilled under LiAlH (xylenes under CaH ) on the rectifying
4 2
column and stored under argon in sealed ampoules over molecular
sieves 4 A. Concentration of water in the solvents, measured by Fischer
method [32], was about 1 mmol/l.
Argon was purified from moisture and oxygen by consecutive pas-
2 5
sing through the columns filled with P O , granulated alkali, molecular
sieves SAA and powder copper heated to 200 °C.
The TON values for the considered catalytic systems were not given
in the first variant of the article. This is due to the fact that the technical
capabilities of our autoclave allow us to carry out the hydrogenation
process within 3 ± 0.5 h. So we were not able to determine the
maximum lifetime of the catalyst (even with acceptably low catalyst
concentrations we were not able to achieve the complete deactivation
of the catalytic activity during the indicated time period). Accordingly,
to this, we refrained from comparing our TON values with previously
published parameters for heterogeneous catalytic systems.
Hydrogen (Brand 1, National standard No. P51673-2000) was pur-
ified from oxygen and water traces by consecutive passing through the
columns filled with nickel-chromium catalyst and SAA zeolites.
Triethylaluminum [33] was purified by distillation in vacuum at
2.3. Research methods
2.3.1. ESR-spectroscopy
4
8−49°С/1 Hg mm. It was stored in ampoules under argon atmo-
sphere. AlEt was diluted in Schlenk tube in hexane or in octane under
argon. Concentration of AlEt solution was determined by volumetric
analysis removing aliquot of the solution with water. Purity of AlEt
The ESR spectra were recorded on an ESP 70-03 XD/2 spectrometer
(KBST Enterprise, Republic of Belarus) at working frequency of
9.3 GHz; DFPH was used as reference.
3
3
3
1
was checked by the PMR method: δсн
δсн
2
= 0.45 ppm (q, J = 8.24 Hz),
2.3.2. UV-spectroscopy
The UV spectra were recorded on a SF 2000 (Russia) spectrometer
in the range 280–430 nm in a seamless cuvette (0.01 cm).
1
3
= 1.22 ppm (t, J = 8.24 Hz).
Diethyl aluminum ethoxide (AlEt
(OEt)) was obtained from AlEt
2 3
2 5
solution in hexane by dropping of the desired quantity of C H OH
under continuous cooling at T = −10 °C and stirred in an argon at-
mosphere.
2.3.3. Transmission electron microscopy
Samples were collected from the Ni(acac)
2
–5AlEt
3
(or Ni
Ethanol was refluxed over freshly calcinated CaO under argon and
(acac) –4AlEt (OEt)) reaction system both before and during benzene
2
2
106