Nanoheterogeneous ruthenium-containing catalysts based on dendrimers in the hydrogenation of aromatic compounds under two-phase conditions

Article abstract of DOI:10.1134/S0965544116060037

Nanoheterogeneous catalysts based on ruthenium nanoparticles dispersed in crosslinked dendrimer matrixes with a size of polymer particles of 100–500 nm show high activity in the hydrogenation of aromatic compounds under two-phase conditions. The addition of water to the reaction medium exerts a strong promoting effect on the activity of the catalysts: The turnover frequency increases by a factor of 3–90 depending on the substrate. When bimetallic (PdRu) nanoparticles are incorporated into the catalyst composition, the rate of benzene hydrogenation increases while the rate of transformation of substituted benzenes decreases.

Full text of DOI:10.1134/S0965544116060037

ISSN 0965ꢀ5441, Petroleum Chemistry, 2016, Vol. 56, No. 6, pp. 491–502. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © E.A. Karakhanov, A.L. Maximov, A.V. Zolotukhina, M.V. Terenina, A.V. Vutolkina, 2016, published in Nanogeterogennyi Kataliz, 2016, Vol. 1, No. 1, pp. 31–42.
Nanoheterogeneous RutheniumꢀContaining Catalysts
Based on Dendrimers in the Hydrogenation of Aromatic Compounds
under TwoꢀPhase Conditions
a
,
a, b
a
a
a
E. A. Karakhanov *, A. L. Maximov , A. V. Zolotukhina , M. V. Terenina , and A. V. Vutolkina
a
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
b
Topchiev Institute of Petrochemical Synthesis, Moscow, 119991, Russia
*
eꢀmail: kar@petrol.chem.msu.ru
Received January 11, 2016
Abstract—Nanoheterogeneous catalysts based on ruthenium nanoparticles dispersed in crosslinked denꢀ
drimer matrixes with a size of polymer particles of 100–500 nm show high activity in the hydrogenation of
aromatic compounds under twoꢀphase conditions. The addition of water to the reaction medium exerts a
strong promoting effect on the activity of the catalysts: The turnover frequency increases by a factor of 3–90
depending on the substrate. When bimetallic (PdRu) nanoparticles are incorporated into the catalyst comꢀ
position, the rate of benzene hydrogenation increases while the rate of transformation of substituted benzenes
decreases.
Keywords: catalytic hydrogenation, aromatic compounds, twoꢀphase catalysis, bimetallic nanoparticles, denꢀ
drimers, ruthenium, palladium
DOI: 10.1134/S0965544116060037
The hydrogenation of aromatic compounds is one TOF values on the order of 4000 h–1 with the 100%
of the most important processes in the modern petroꢀ conversion of benzene [11].
chemical industry: Such compounds as cyclohexane,
cyclohexene, and naphthene hydrocarbons are
obtained via this process [1–2]. The process has
acquired a considerable role because of the toughenꢀ
ing of ecological requirements for gasoline and diesel
fuels [3–5].
A higher efficiency is exhibited by systems in which
nanoparticles are dispersed with the use of
amphiphilic polymers, such as polyvinylpyrrolidone
(
PVP) [11–15], polyethyleneimine [6], and dendrimꢀ
ers [16]. As an example, in the presence of Ru/PVP
catalysts the values of specific catalytic activity (in
terms of absorbed hydrogen) were as high as 45000 h
–1
Twoꢀphase catalysis, which combines such advanꢀ
tages as the ability to perform reactions under mild for benzene, 30000 h for ethylbenzene, 16000 h
conditions, the use of water as a “green solvent,” and for anisole [12], and 21000 h for phenol [13].
–1
–1
–1
the feasibility of catalyst reuse, is of special interest [6].
The main disadvantage of this approach is that the rate
of reaction is moderately high because of the poor solꢀ
ubility of aromatic compounds in water.
Catalysts based on metal nanoparticles dispersed in
the particles of crosslinked dendrimer matrixes with a
size of polymer particles of 100–500 nm were
described in [16–18]. Rutheniumꢀcontaining cataꢀ
lysts exhibited an extremely high activity with respect
to phenols in water: In their presence the turnover freꢀ
It is possible to employ amphiphilic nanosized
metal particles dispersed in water, particularly rutheꢀ
nium nanoparticles, stabilized by cyclodextrins [7–9],
–1
quency was as high as 2775 h for phenol and
–1
calixarenes [10], and quaternary ammonium salts [5]. 80727 h for resorcinol [16]. In this case, the polymer
In this case, the catalytic activity increases due to the matrix served as a dispersion medium for metal nanoꢀ
immediate approach of metal sites and substrate molꢀ particles. Simultaneously the matrix was highly perꢀ
ecules (including the formation of “guest–host” comꢀ meable for substrates in the presence of water and proꢀ
plexes [8]) and a large surface area of the dispersed vided a high activity of the system in the hydrogenaꢀ
catalytic system. However, as a rule, the turnover freꢀ tion of waterꢀsoluble molecules, especially diatomic
quency (TOF) does not exceed several hundreds of phenols. At the same time, in the absence of water
inverse hours [5, 7–8]. The use of conventional heterꢀ these catalysts demonstrated a rather low activity in
ogeneous catalysts based on ruthenium nanoparticles the hydrogenation of aromatic hydrocarbons [18].
stabilized by tetramethylguanidine immobilized in the
This study presents data that suggest that the activꢀ
pores of montmorillonite makes it possible to attain ity of such nanosized catalysts in the hydrogenation of
491

492
KARAKHANOV et al.
aromatic hydrocarbons may be increased considerably grade), toluene (Khimmed, specialꢀpurity grade),
in the presence of water in a twoꢀphase system.
ethylbenzene (Reakhim, highꢀpurity grade),
Reakhim, highꢀpurity grade), ꢀxylene (Khimmed,
analytical grade), ꢀbutylbenzene (Reakhim, highꢀ
purity grade), secꢀbutylbenzene (Reakhim, highꢀ
purity grade), tertꢀbutylbenzene (Reakhim, highꢀ
purity grade), 1ꢀethylꢀ4ꢀisopropylbenzene (Reakhim,
highꢀpurity grade), naphthalene (Reakhim, highꢀ
purity grade), 1ꢀmethylnaphthalene (Aldrich, 95%),
and tetralin (Aldrich, 97+%).
pꢀxylene
(
o
In addition, the effect of an additional metal (palꢀ
ladium) incorporated into the active phase on the effiꢀ
ciency of such catalysts was investigated. The use of
two metals in an alloy frequently facilitates a gain in
the activity of the catalyst and an increase in selectivity
for certain products [19]. One of the earliest publicaꢀ
tions concerning the use of bimetallic alloys in catalyꢀ
sis applies to the monomerization of dicyclopentadiꢀ
ene and subsequent hydrogenation of the formed
n
The G1−HMDI−Ru catalyst (a catalyst based on
cyclopentadiene in the presence of the PdRu memꢀ the firstꢀgeneration dendrimer DAB(NH ) (G1)
2
4
brane catalyst [20]. It was shown that a bimetallic crosslinked by hexamethylene diisocyanate (HMDI)
membrane catalyst that contains 9.8% Ru in the alloy and G3
HMDI Ru catalyst (a catalyst based on the
−
−
is the most efficient in the monomerization and subseꢀ thirdꢀgeneration dendrimer DAB(NH2)16 (G3)
quent hydrogenation of dicyclopentadiene and that
cyclopentene with a yield of up to 90% prevails in the
reaction products.
Heterogeneous PdRh catalysts were active in the
hydrogenation of both benzene [21] and polysubstiꢀ
tuted (diphenylꢀ and triphenylbenzenes) and conꢀ
crosslinked by hexamethylene diisocyanate) were synꢀ
thesized as described in [16] and [18].
The bimetallic PdRu catalyst was synthesized using
the following compounds: ruthenium(III) chloride
(
(
(
Aldrich, 45–55% Ru content), palladium(II) acetate
Aldrich, 99.9+%), and sodium borohydride
Aldrich, 98%). Support G3−HMDI (thirdꢀgeneraꢀ
densed
aromatic
compounds
(naphthalene,
≥
anthracene, terphenylene, etc.) [22]. In both cases,
efficiency was achieved through the interaction of palꢀ
ladium nanoparticles that were preliminarily encapsuꢀ
lated inside pores with semimobile rhodium comꢀ
plexes immobilized in the same place and the spillover
of adsorbed hydrogen atoms that occur between them
tion dendrimer DAB(NH ) crosslinked by hexameꢀ
2
16
thylene diisocyanate) was synthesized as described in
16] and [18].
[
Instruments and Methods
Transmission electron microscopy (TEM) studies
[
21]. The reduction of complexes and the formation of
nanoparticles occurred in this case either directly durꢀ
ing hydrogenation of aromatic substrates or in the
course of preliminary activation of the catalyst.
were performed on a LEO912 AB OMEGA electron
microscope at a voltage of the electron tube of 100 kV.
Highꢀresolution TEM analysis was conducted on a
JEMꢀ2100F (Jeol Ltd., Japan) instrument at a voltage
on the electron tube of 200 kV. The average size of the
particles was calculated via treatment of the obtained
micrographs with the Image J program.
Xꢀray photoelectron spectroscopy (XPS) studies
were performed on an LASꢀ3000 instrument equipped
with a photoelectron analyzer with the OPXꢀ150
retarding potential. Photoelectrons were excited by
Heterogeneous PdRu catalysts supported on mesoꢀ
porous SiO of the SBAꢀ15 type manifested high activꢀ
2
ity in the hydrogenation of benzene, cyclooctadiene,
norbornene, and cyclododecatriene [23]; when supꢀ
ported on mesoporous ꢀAl O they were active in the
θ
2 3,
hydrogenation of dimethyl terephthalate [24]. In both
cases, the highest efficiency was exhibited by the cataꢀ
lysts that contain palladium and ruthenium at equal
molar ratios. Note that PdRu/ ꢀAl O was found to be
θ
2 3
Xꢀray radiation of the aluminum anode (AlK =
α
the most selective with respect to the main product,
viz., cyclohexanedicarboxylic acid dimethyl ester [24].
On the other hand, in the hydrogenation of electronꢀ
deficient aromatic substrates (benzoic acid
and terephthalic acid dimethyl ester), activity was
exhibited by RuPt catalysts supported on mesoporous
1
486.6 eV) with a voltage on the tube of 12 kV and an
emission current of 20 mA. Photoelectron peaks were
calibrated relative to the 1s line of carbon with a bond
energy of 285 eV.
The amount of palladium was determined by the
method of inductively coupled plasma atomic emisꢀ
sion spectroscopy (ICPꢀAES) on an IRIS Interpid
II XPL instrument (Thermo Electron Corp., United
States) under radial and axial observations at waveꢀ
lengths of 310 and 95.5 nm.
The amount of ruthenium in the samples was meaꢀ
sured by UVꢀvisible spectroscopy on an Agilent UVꢀ
Vis 8453 instrument [25]. For this purpose, a series of
aqueous calibration solutions of RuCl (10ꢀmL each
volume) with concentrations of 0.02, 0.03, 0.05, and
SiO [23].
2
In this study, the behaviors of nanosized Ru and
PdRu particles dispersed in a network of crosslinked
PPI dendrimers were compared in the twoꢀphase
hydrogenation of various aromatic hydrocarbons.
EXPERIMENTAL
3
Compounds Used in This Study
The reagents used as initial and reference comꢀ 0.08 mg/mL were prepared. Rutheniumꢀcontaining
pounds were as follows: benzene (Irea 2000, analytical 2.5ꢀmg samples (G3–HMDI–Ru), 1ꢀmg samples
PETROLEUM CHEMISTRY Vol. 56
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2016

NANOHETEROGENEOUS RUTHENIUMꢀCONTAINING CATALYSTS
493
3
+
(
G1–HMDI–Ru), and 1.5ꢀmg samples (G3– ([Ru ]
3d ); 282.3 (C 1s, 1.6%, C
≡
Ru/Pd),
bound
5/2
HMDI–PdRu) were treated with the mixture of 2 mL
of concentrated HCl (SigmaTech, reagent grade) and
2
2
83.4 (C 1s, 2.2%, C=N), 284.7 (C 1s, 6.1%, C–C,
C–H), 286.2 (C 1s, 2.3%, C–N), 287.4 (C 1s, 1.5%,
(=O)–NH–), 288.5 (C 1s, 0.9%, –NH–
(=O)–NH– (=O)–NH–), 289.6 (C 1s, 0.3%,
NH– (=O)–NH– (=O)–NH– or NH–
(=O)–OH); 284.3 (Ru 3d ), 285.0 (Ru/RuO
mL of 30% H O (Aldrich). The volumes of the –NH–
C
2
2
resulting solutions were adjusted to 10 mL With disꢀ
tilled water. Then, 10 mL of 0.015 Ì aqueous solution
of 1,10ꢀphenanthroline (Lachema, 98%) was added to
each calibration solution and sample solution. The
solutions acquired a maizeꢀyellow color. To prepare
complexes of Ru , 0.4–0.6 g of hydroxylamine
hydrochloride (Reakhim, highꢀpurity grade) was
added to each solution. When the solutions were
heated on a water bath at 100°C for 2 h, their color
C
⎯
C
C
C
⎯
0
C
3/2
x
3
d ), 285.9 (RuO or RuCl 3d ), 286.7 (RuCl
H O or [Ru ]
35.1 (Pd 3d , 0.15%), 336.8 (PdO 3d , 0.32%),
38.4 (Pd 3d , 0.50%), 339.7 ([Pd ]
×
3/2
2
3
3/2
3
2+
3+
x
3
3
0
3d ), 287.6 ([Ru ]
3d );
bound 3/2
2
bound
3/2
2+
0
5
/2
x
5/2
2+
2+
3d ,
5/2
5
/2
bound
2+
0
.33%), 340.4 (Pd 3d , 0.16%), 340.9 (Pd 3d ,
3/2
5/2
+
changed to orange–yellow. The amount of ruthenium sat, 0.21%), 341.8 (PdO
3d3/2, 0.40%), 343.4 (Pd2
x
2
+
was determined from the intensity of the absorption 3d , 0.55), 345.15 ([Pd ]
3d , 0.44%), 346.8
5/2
2+
3/2
bound.
–4
–1
–1
2+
band at 444–450 nm (
corresponded to the [Ru(phen) ] complex [26]).
ε
= 1.5
×
10 M cm ), which (Pd 3d , sat, 0.23%), 348.0 ([Pd ]
3d3 , sat,
bound 5/2
3/2
2+
3
0.10%); 396.6 (N 1s, 3.7%, N
.3%, –C=N–), 399.6
NCH CH CH N–, –CH –NH–C(=O)–NH–
≡
Pd/Ru), 398.3 (N 1s,
6
⎯
(N 1s, 1.2%,
The products of catalytic experiments were anaꢀ
2
2
2
2
lyzed by gasꢀliquid chromatography with
ChromPack CP9001 gas chromatograph equipped
with a flameꢀionization detector and a 30 m 0.2 mm
capillary column packed with a SEꢀ30 grafted phase. NCH CH CH N
a
+
+
CH –), 401.0 (N 1s, 6.5%, NH R ,
⎯
C=NH –,
2
3
×
⎯NH–C(=O)–NH–C(=O)–NH–,
n+
→
Pd/Ru ), 403.3 (N 1s, 2.7%,
2
2
2
Chromatograms were analyzed with the Maestro 1.4
program. Conversion was determined from a change
in the relative areas under the peaks of the substrate
and reaction products.
+
R N+
–
NR4,
NCH CH CH N–, –NH–C(=O)–NH–, sat);
4
→
O ), 405.3 (N 1s, 0.6%,
3
⎯
2
2
2
0
59.6 (Ru 3p , 0.35%), 461.6 (Ru/RuO 3p ,
3/2 x 3/2
0
.65%), 463.3 (RuO or RuCl 3p , 1.21%), 464.8
2
3
3/2
2+
(
(
RuCl
[Ru ]
×
x
H O or [Ru ]
3p , 0.60%), 466.1
3
2
bound
3/2
3
+
0
Synthesis of the Bimetallic
Palladium–Ruthenium Catalyst
3p , 0.18%); 528.3 (O 1s, 3.5%, OꢀPd ),
bound
3/2
II
5
⎯
7
29.8 (O 1s, 10.9%, OꢀPd ), 531.5 (O 1s, 32.4%,
CH –NH–C(=O)–NH–CH –), 533.20 (O 1s,
A oneꢀneck roundꢀbottom 100ꢀmL flask equipped
with a magnetic stirrer and a reflux condenser was
loaded with the G3–HMDI (300 mg) support and disꢀ C(=O)–NH–), 540.2 (O 1s, sat, 0.8%); 532.2 (Pd
tilled water (75 mL). After the addition of rutheꢀ 3p ), 533.1 (PdO 3p ), 534.0 (Pd 3p ), 534.9
nium(III) chloride (92 mg, 0.45 mmol) and pallaꢀ
dium(II) acetate (100 mg, 0.45 mmol) to the resulting
suspension, its color rapidly changed to black. The
reaction was conducted for 12 h at room temperature
and the reaction mixture was evaporated to dryness on
a rotor evaporator. The asꢀobtained black powder preꢀ
cipitate (400 mg, a yield of the first stage of 81%) was
placed in a 50ꢀmL roundꢀbottom flask equipped with
a magnetic stirrer and a reflux condenser and susꢀ
pended in a mixture of ethanol (35 mL) and water
2
2
.8%, H O), 534.5 (O 1s, 2.3%, H2O…–NH–
2
0
2+
3
/2
x
3/2
3/2
2+
2+
(
[Pd ]
3p ), 535.8 (Pd 3p , sat).
bound 3/2 3/2
The contents of metals were found to be Ru, 8%
and Pd, 10%.
Technique of Catalytic Experiments
The catalyst and substrate were placed at the
desired ratio in a steel thermostatically controlled
autoclave equipped with a stirrer. In experiments perꢀ
formed in the presence of water, the latter was added at
a ratio of 1 mL of water per mL of substrate. The autoꢀ
clave was sealed, filled with hydrogen to a pressure of
(
10 mL). Sodium borohydride (182 mg, 4.8 mmol)
was then added in portions to the resulting suspension.
The reaction mixture acquired a saturated black color
and the intense evolution of gas bubbles was observed.
The reaction was conducted for 12 h at room temperꢀ
ature, and the precipitate was separated from the soluꢀ
tion that contained the residues of the intact sodium
borohydride and the formed borax via centrifugation,
washed three times with water and ethanol, and dried
1
0 or 30 atm, and connected to a thermostat. The
reaction was conducted at 85 or 90°C. The autoclave
was then cooled below room temperature and
unsealed. The reaction products were analyzed by gasꢀ
liquid chromatography.
The activity of catalysts (TOF) was estimated as the
in air. The yield of the resulting material was 325 mg amount of converted substrate per Ru mol per time unit:
(
82.5%).
ν
× ω
substr
TOF =
,
0
XPS data (eV) was as follows: 279.4 (Ru 3d ),
5/2
ν
× t
Ru
2
80.3 (Ru/RuO 3d ), 281.1 (RuO or RuCl 3d ),
x
5/2
2
3
5/2
where
ω
is the conversion of substrate expressed in
2+
281.7 (RuCl3
×
xH O or [Ru ]
2 bound
3d ), 282.2 unity fractions.
5/2
PETROLEUM CHEMISTRY Vol. 56
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494
KARAKHANOV et al.
Table 1. The physicochemical characteristics of the synthesized catalysts [16]
Surface atomic concentrations, %
Average size
of particles,
nm
Metal
content, %
Ru 3p3/2, % (eV)
Catalyst
C
N
O
Ru
RuO2,
RuCl3
Ru0
9.9
[Ru ]
3+
bound
2
4
48.8
(463.5)
21.3
(465.9)
G1–HMDI–Ru
G3–HMDI–Ru
34.6
9.4
3.8 ± 1.1
0.9 ± 0.2
26.1
18.9
11.9
26.8
41.6
45.6
20.4
8.7
61.5)
15.0
58.5
(463.4)
26.4
(466.1)
(
461.0)
Table 2. The physicochemical characteristics of the G3–HMDI–PdRu catalyst
Surface atomic concentrations, %
Content
of Pd, %
Content
of Ru, %
d
, nm
C
N
O
Pd
Ru
3.0
10
8
0.8 ± 0.1
14.9
21.0
57.7
3.4
Ru 3p3/2, at % (eV)
RuO or RuCl
RuCl3
×
x
H O
2
Ru0
Ru/RuOx
[Ru ]
bound
3+
2
3
2+
or [Ru ]
bound
11.9 (459.6)
21.9 (461.6)
40.3 (463.3)
20.7 (464.8)
6.1 (466.1)
Pd 3d5/2, at % (eV)
Pd2+
Pd0
PdOx
2+
[Pd ]
bound
Pd , sat
2+
9.9 (335.1)
21.1 (336.8)
33.0 (338.4)
21.7 (339.7)
14.2 (340.9)
RESULTS AND DICUSSION
In accordance with the TEM data, G3–HMDI–
PdRu is a highly dispersed material with a narrow parꢀ
ticle size distribution (Fig. 1). The average diameter is
Synthesis of Hybrid Catalysts Based on Ruthenium
Nanoparticles and Alloys Encapsulated in Crosslinked
Dendrimer Networks
0.8–1.0 nm. This value is close to that obtained for the
previously synthesized monometallic catalyst G3–
HMDI–Ru [16]. Note that G3–HMDI–PdRu also
features high density and uniform distribution of parꢀ
ticles on the support surface because of the strong
complexing ability of the support.
The synthesized materials, which consisted of
ruthenium nanoparticles dispersed within crosslinked
matrixes based on firstꢀ and thirdꢀgeneration denꢀ
drimers [16, 18], were characterized by the high denꢀ
sity and uniform distribution of particles on the supꢀ
port surface. The average sizes of metal particles were
As shown by XPS (Fig. 2), both metals occur on the
catalyst surface at an approximately equal proportion
3
.8 and 0.9 nm for the catalysts based on firstꢀ and
(
3.4% for Pd and 3.0% for Ru), which correlates with
their content in the catalyst volume (10% for Pd and
% for Ru). For both metals, oxidized and complex
thirdꢀgeneration dendrimers, respectively. The
detailed characteristics of the samples are summarized
in Table 1 [16]. The size of the matrix itself, in accorꢀ
dance with the TEM data, was from 100 nm to 2 μm.
8
forms prevail in the sample. The ratio of reduced and
oxidized forms was approximately 1 : 2 for ruthenium
A similar synthesis technique was employed for the
synthesis of the G3–HMDI–PdRu bimetallic cataꢀ
lyst. As a support, a network based on the thirdꢀgenerꢀ
ation dendrimers crosslinked by hexamethylene diisoꢀ
0
3+
(
[Ru + Ru/RuO ]/[RuO + Ru ]) and 1 : 9 for palꢀ
x
2
2+
0
ladium (Pd /[PdO + Pd ]). The higher content of
x
oxidized forms for palladium, despite a high affinity of
ruthenium to oxygen [27, 28], may be explained by the
incomplete dissolution of palladium in the form of
palladium acetate in the water that was used as a reacꢀ
tion medium during synthesis of the catalyst. As a conꢀ
cyanate (NCO : NH = 1 : 1) was used. The alloy was
2
created via the cocomplexation method according to
the technique that was previously used for the syntheꢀ
sis of monometallic ruthenium catalysts [18] based on
dendrimers at a nominal PdꢀtoꢀRu ratio of 1 : 1. The sequence, the ions of palladium (in the form of correꢀ
physicochemical characteristics of the sample are sponding complexes) predominantly concentrated in
listed in Table 2.
the volume of the amphiphilic support; as a result,
PETROLEUM CHEMISTRY Vol. 56
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NANOHETEROGENEOUS RUTHENIUMꢀCONTAINING CATALYSTS
a) (b)
495
(
50
40
30
20
10
0.8 ± 0.1 nm
0
1
2
3
4
5
20 nm
d, nm
Fig. 1. The G3–HMDI–PdRu catalyst: (a) TEM micrograph and (b) size distribution of particles.
their further reduction by sodium borohydride was
hindered.
Hydrogenation of Aromatic Compounds
in the Presence of DendrimerꢀBased Ru Catalysts
The above suggestion is confirmed by the XPS
spectrum of the 1s line of nitrogen (Fig. 2b), in which PPI dendrimers crosslinked by hexamethylene diisoꢀ
the predominant signals are as follows: a signal that cyanate that were used in the hydrogenation of pheꢀ
Materials based on ruthenium nanoparticles and
corresponds to the imine complexes of zeroꢀvalent nols [16] were tested in the hydrogenation of aromatic
0
compounds. Hydrogenation was conducted at presꢀ
sures of 5, 10, and 30 atm and temperatures of 85–
metals (398.3 eV, –CH –CH=NH
→
Pd/Ru ), which
2
formed as a result of reduction of the latter by the
amino groups of dendrimers [29], and a signal that
corresponds to the amine complexes of bound metal
90
°
C both in the presence of water and in its absence.
As in the case of phenols, the activity of both cataꢀ
n
+
ions (401.0 eV, –CH –CH–NH2
→
Pd/Ru ) [30]. It lysts depended on such factors as substrate size and
2
should be emphasized that the observed picture is not geometry, dendrimer generation, and the electronic
effects of substituents. The main products were cycloꢀ
hexane and its derivatives with a selectivity of 97–
typical for the catalysts based on dendrimer networks
that contain metals in the reduced or partially reduced
state, where the main fraction of nitrogen that occurs
on the surface is due to the inner amino groups of denꢀ
drimers and urethane fragments [16]. In this case, the
shift in bond energies on passage from the support to
1
00%. The experimental data are summarized in
Tables 3–6. Some of these results were reported
in [18].
As is seen from Tables 3 and 4, the ruthenium cataꢀ
the catalyst is as a rule practically invisible because of lysts based on dendrimers efficiently hydrogenate aroꢀ
the similarity of their values for the –CH –CH– matic
compounds
under
twoꢀphase
2
0
conditions, including reduced pressures (Table 3). As
an example, the turnover frequency for the catalyst
NH2
→
Pd/Ru [31, 32] and –CH –NH–C(=O)–
2
NH–CH – [33–35] fragments.
2
G1–HMDI–Ru in the hydrogenation of benzene
0
3+
–1
A higher ([Ru + Ru/RuO ]/[RuO + Ru ]) ratio attains even 5000 h ; quantitative conversions
x
2
become attainable at high substrate/Ru ratios and
pressures and smaller reaction times (Table 3).
is also noteworthy for the bimetallic catalyst (1 : 2) relꢀ
ative to that of the monometallic catalyst (1 : 5.7) on
the analogous support. Against the background of a
wellꢀdefined dominance of the oxidized forms for palꢀ
ladium in G3–HMDI–PdRu, it may be assumed that
the latter, along with sodium borohydride, is involved
As in the case of phenols [18], the substrate size and
the negative dendrite effect [37] still retained their
impacts on the activity of the catalyst; however, the
latter value does not decline as abruptly upon replaceꢀ
in the reduction of ruthenium and probably decelerꢀ ment of benzene by its alkylꢀsubstituted derivatives,
ates the oxidation process during storage in air, as is such as toluene and ethylbenzene (Fig. 3). Note that
typical for ruthenium nanocatalysts [36].
now the electronic effects of substituents insignifiꢀ
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KARAKHANOV et al.
(a)
1–Ru0
0
4000
1
2
2
–Ru /RuOx
3
G1–HMDI–Ru
G3–HMDI–Ru
10
3–RuO or RuCl3
2
2000
4–RuCl3 · H2O or
8
6
4
2
0
2+
[Ru ]bond
2
–[Ru3+]bond
400
5
4
3
00
00
2
1
5
100
0
455
460
465
470
Bond energy, eV
Fig. 3. The hydrogenation of aromatic compounds in the
(b)
presence of Ru catalysts based on dendrimers in an aqueꢀ
0
ous medium. Reaction conditions: 85
°C and p(H ) =
2
1
2
3
4
–Pd 3d5/2
–PdOx 3d5/2
–Pd2 3d5/2
1
4
30 atm.
+
12
2+
–[Pd ]bond 3d5/2
2
+
10
5–Pd 3d5/2, sat
the Ruꢀcatalyzed hydrolysis of the crosslinked denꢀ
drimer matrix, which leads to a reduction in steric hinꢀ
drances around catalytic sites and, as a consequence,
facilitates the substrate access to them [16]. In other
words, we obtain waterꢀdispersed fragments of the
polymer matrix that function as peculiar microreacꢀ
tors that contain inside ruthenium nanoparticles and
are able to capture substrate hydrophobic molecules
8
0
3
6
–Pd 3d3/2
7
9
8
7–PdO 3d
x
3/2
+
8
9
4 1
–Pd2 3d3/2
2
4
6
2+
–[Pd ]bond 3d3/2
10
0—Pd2+
3d3/2, sat
5
6
2
0
1
[
14]. Nevertheless, if in the case of linear polyviꢀ
nylpyrrolidone, an increase in chain length and, as a
result, in the molecular mass of the polymer leads to a
gain in the stability and activity of the catalytic system
3
25
330
335
Bond energy, eV
c)
340
345
350
(
[
12], then in the case of branched polypropyleneimine
2
0
1
2
3
—N≡Pd/Ru
dendrimers, in contrast, the activity decreases as
before because steric hindrances and negative dendrite
effect increase with the increasing molecular mass and
generation of the dendrimer.
—N=C–
—NCH2CH2CH2N,
CH2CH2N→Pd/Ru0,
2
4
1
5
CH2–NH–C(=O)–NH–CH2
3
4
—NH R+, –CH2CH2N→Pd/Run+
,
Another factor that contributes to an increase in
the catalytic activity in the presence of water is the
1
0
–NH–C(=O)–NH–C(=O)–NH–
–
NHR +3 , R N→Ο
5—
1
3
“
washingꢀoff” of adsorbed water and other oxide layꢀ
6
—CH2–NH–C(=O)–NH–CH2,
NCH2CH2CH2N– (sat)
5
ers from the surface of ruthenium nanoparticles,
which prevents the competitive adsorption of nonpoꢀ
lar substrates [38]. Hence, the presence of water in the
reaction medium assists in adsorption and, at the same
time, accelerates the hydrogenation of aromatic comꢀ
pounds.
5
3
5
6
0
3
80
385
390
395
400
405
410
Bond energy, eV
In addition, note that the increase in the activity in
the presence of water is more pronounced for the nonꢀ
polar aromatic compound than that for more polar
phenols [18]. This finding may be explained by the fact
that the dendrimer network acts as an amphiphilic disꢀ
persion medium that simultaneously interacts with
water and the aromatic substrate [14]. The combinaꢀ
Fig. 2. The XPS spectra of G3–HMDI–PdRu: (a) 3p line
of ruthenium, (b) 3d line of palladium, and (c) 1s line of
nitrogen.
cantly affect the activity of the catalyst but meet the tion of these factors facilitates both the desorption of
water during activation of the surface of nanoparticles
and the subsequent adsorption of nonpolar aromatic
substrates. As a consequence, the rate of their hydroꢀ
steric demand solely.
The effect of water on the rate of hydrogenation of
arylꢀcontaining substrates was previously explained by genation increases considerably.
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497
Table 3. Hydrogenation of aromatic compounds in the presence of the G1–HMDI–Ru catalyst under twoꢀphase conditions
Substrate
Substrate/Ru, mol/mol
t
, h
Conversion, %
TOF, h–1
1
60
6
100
100
100
100
95*
97.5
52
27
80
2
800
2
400
1600
1510
4675
4975
22
Benzene
0.5
0
.5
2
400
795
0.5
0.5
6
4
135
100
100
100
100
47.5
100
100
100
100
29
2
67
Toluene
670
010
2
400
1610
2295
19.5
58
0
.5
2
0.5
6
1
16
2
Ethylbenzene
580
750
2
396.5
1320
1380
19.5
58.5
390
207
19.5
58
0
.5
1
0.5
6
117
585
116
580
200
100
100
98
2
o
p
ꢀXylene
ꢀXylene
2
0
6
2
2
0
6
2
.5
13
100
100
100
22
396
350
8.5
.5
25
Butylbenzene
15.5
15.5
Reaction conditions: 85°C, 30 atm of H , and substrate/water = 1 : 1 (vol/vol).
2
*
10 atm of H2.
The use of the catalyst based on the thirdꢀgeneraꢀ
Such a sharp change in the behavior of the catalyst
tion dendrimer, G3–HMDI–Ru, caused a marked may be associated with not only a very strong negative
reduction in the rate of hydrogenation (Table 4) relaꢀ dendrite effect [37], as observed on passage from the
tive to that of G1–HMDI–Ru (Table 3). The activiꢀ support based on the firstꢀgeneration dendrimer to
ties of both catalysts are compared in Fig. 3. As an that based on the thirdꢀgeneration dendrimer (Fig. 3),
example, in the case of G3–HMDI–Ru quantitative but also with a change in the hydrophobicity of
conversions are attainable only for benzene (Table 3) the support itself. As an example, in the case of G1–
even for a rather long time (6 h) and the observed TOF HMDI–Ru, the end amino groups of the dendrimer
values are much lower than those previously obtained next to the center are now involved in bonding with
in the presence of the same catalyst for phenols [16].
hydrophobic diisocyanates and, as a result, form a
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498
KARAKHANOV et al.
Table 4. Hydrogenation of aromatic compounds in the agents are converted into carbamate ones. As a conseꢀ
presence of the G3–HMDI–Ru catalyst under twoꢀphase
conditions
quence, despite partial hydrolysis of the dendrimer
matrix and reduction in steric hindrances surrounding
catalytic sites, the hydrophobic aromatic substrate
Substrate/Ru,
mol/mol
Converꢀ
sion, %
Substrate
Benzene
t
, h
TOF, h–1
cannot be efficiently involved in the microreactors
formed by hydrophilic residues of the aboveꢀmenꢀ
tioned dendrimer matrix; therefore, the rate of reacꢀ
tion declines.
As in the case of phenols [16], when the reaction is
performed in the absence of water, the activities of
both catalysts decrease appreciably (Table 5), although
the reaction is conducted at a higher temperature
170
6
2
0
2
6
2
0
2
6
2
0
2
6
2
6
2
6
2
0
2
100
94
28
80
.5
.5
.5
56.5
55
190
230
13
845
(
90
°
C). As an example, the turnover frequency for
–1
G1–HMDI–Ru now does not exceed 1500 h with
Toluene
170
47
–1
respect to benzene (instead of 5000 h in presence of
water). Increase in the size of substituents and in their
amount caused a decrease in the conversion and turnꢀ
over frequency, especially at high substrate/Ru ratios
and smaller reaction times. Reduction in the pressure
of hydrogen to 5 atm also led to a decrease in the catꢀ
alytic activity. Nevertheless, conversions close to 100%
could be obtained for a wide range of substrates.
44
38
22
75
850
19.5
61
83
Ethylbenzene
168
17
57
48
Similar tendencies were observed when the reacꢀ
tion was performed in the absence of water with the
use of the catalyst based on the thirdꢀgeneration denꢀ
drimer (Table 6). A decrease in the pressure of hydroꢀ
gen resulted in a more distinct decline in the rate of
reaction than that in the case of G1–HMDI–Ru [18].
In this case, increase in the reaction time and the catꢀ
alyst concentration slightly affected the attained level
of substrate conversion.
Hence, it may be assumed that in the presence of
highꢀgeneration dendrimers used as stabilizing ligands
both the adsorption of the aromatic substrate on the
surface of ruthenium nanoparticles and the subseꢀ
quent addition of hydrogen are rateꢀlimiting stages.
25
84
840
24
100
3.5
9
o
p
ꢀXylene
ꢀXylene
168
13
11
167.5
162
28.5
25
8
21
Butylbenzene
49.5
45.5
24
13.5
37
Because oꢀxylene, for which tetralin is the strucꢀ
tural analog, is hydrogenated to 100% in the presence
of G1–HMDI–Ru in an aqueous medium, it was supꢀ
posed that tetralin may be also hydrogenated under
milder conditions. It was found that the activity of the
.5
78
810
10
39.5
Reaction conditions: 85
°
C, 30 of atm H , and substrate/water = 1 : 1 dendrimerꢀbased ruthenium catalysts in the absence
2
(vol/vol).
of water grows significantly if the reaction is conꢀ
ducted at elevated temperatures (170 C) (Fig. 4).
°
Thus, for both catalysts the conversion of
benzene achieved 60–70% within 2 h. Tetralin was
hydrogenated by 95% over 6 h in the presence of the
G1–HMDI–Ru catalyst and by 81% in the presence
of G3–HMDI–Ru. In both cases, the dominant
product in the hydrogenation of tetralin was cisꢀdecaꢀ
lin, with a selectivity of 80–85%, as is typical for Ru
catalysts [39].
polyurethane network with an almost uniform distriꢀ
bution of hydrophobic and hydrophilic fragments.
In contrast, G3–HMDI–Ru is characterized by
the presence of more defined hydrophobic and hydroꢀ
philic regions because the thirdꢀgeneration dendrimꢀ
ers that comprise the support contain a large amount
of inner nitrogen atoms that are uninvolved in the forꢀ
mation of urethane groups with hexamethylene diisoꢀ
The replacement of tetralin with a mixture of
cyanate molecules and, therefore, are able to form naphthalene and 1ꢀmethylnaphthalene (Fig. 4)
hydrogen bonds. During hydrolysis of this network caused a marked drop in conversions (down to 5–12%
under the action of ruthenium nanoparticles end over 6 h). This effect may be associated with the size
amino groups of dendrimers are released from ureꢀ and rigid structure of condensed aromatic comꢀ
thane bonds; urethane groups from the side of binding pounds. As a consequence, the latter meet substantial
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NANOHETEROGENEOUS RUTHENIUMꢀCONTAINING CATALYSTS
499
Table 5. The effect of water on the activity of the G1–HMDI–Ru catalyst in the hydrogenation of aromatic compounds
Substrate
Hydrogenation in water, TOF, h–1
Hydrogenation without water TOF, h–1
4
975
1550
17* [18]
67
Benzene
1
510*
Toluene
2295
1380
390
Ethylbenzene
65
o
p
ꢀXylene
ꢀXylene
27
395
53.5
6
Butylbenzene
15.5
Reaction conditions: 85
without water.
°C, 30 atm of H , and substrate/water = 1 : 1 (vol/vol), for hydrogenation with water; for hydrogenation at 90°C
2
*
5–10 atm of H2.
barriers from the crosslinked dendrimer matrix main product is 5,6,7,8ꢀtetrahydroꢀ1ꢀmethylnaphꢀ
on their way to catalytic sites. Moreover, condensed thalene with a selectivity of up to 80%.
aromatic rings are subjected to the action of a weak
G3–HMDI–Ru, which is more sensitive to the
⎯
M effect from each other; this creates additional hinꢀ
size, geometry, and conformational rigidity of the subꢀ
strate, was tested only in the hydrogenation of tetralin.
The conversion of this compound was as low as
drances to their adsorption on the surface of rutheꢀ
nium nanoparticles. The lower turnover frequency for
the G1–HMDI–Ru catalyst may be explained by an
insufficiently high substrate/Ru ratio compared to 13% (TOF ~4 h ) in the presence of the aboveꢀmenꢀ
G3–HMDI–Ru.
–1
tioned catalyst (90°C, 40 atm of H ). This value is
2
comparable with the conversion in the hydrogenation
made it possible to attain quantitative conversions of of oꢀxylene (Table 4).
The addition of water to the reaction medium
tetralin even at a much lower temperature (90
against 170 C). As an example, in the presence of the
°
C
°
Table 6. The effect of water on the activity of the G3–
HMDI–Ru catalyst in the hydrogenation of aromatic comꢀ
pounds
G1–HMDI–Ru catalyst a conversion of 90% was
reached over 30 min and a conversion of 100% was
achieved over 2 h at substrate/Ru ~170 (Fig. 5). In this
–1
case, the turnover frequency was 305 h . The main
reaction product, as in the absence of water, was cis
Hydrogenation
Hydrogenation
ꢀ
Substrate
without water
TOF, h
–1
decalin with a selectivity of up 85%. When the subꢀ
strate/Ru ratio was changed to 850, the turnover freꢀ
in water, TOF, h
–1
–1
quency was increased 805 h ; however, quantitative
conversions were attainable only during a very long Benzene
period of time (Fig. 5).
At the same time, naphthalene and 1ꢀmethylnaphꢀ Toluene
thalene were hydrogenated in the presence of water at
300
83
130
9
56
3.5
<1
4.5
3
90 C, as before, over 6 h with conversions of 2 and 4%,
°
Ethylbenzene
respectively. It appears that the conformational rigidꢀ
ity and the –M effect play the key role in the hydrogeꢀ
nation of these substrates and thus hamper their
adsorption and diffusion even through a partially
hydrolyzed dendrimer matrix. A lower conversion of
naphthalene than that of 1ꢀmethylnaphthalene may
probably be rationalized by the ability of naphthalene
to sublime under the reaction conditions. The main
products of naphthalene hydrogenation are tetralin
with a selectivity of 65–70% and cisꢀdecalin with a
selectivity of 25–30%. For 1ꢀmethylnaphthalene, the
o
p
ꢀXylene
ꢀXylene
21
78
Butylbenzene
0.5
Reaction conditions: 85°C, 30 atm of H , and substrate/water = 1 : 1
2
(
vol/vol), for hydrogenation with water; at 90°C without water.
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500
KARAKHANOV et al.
5
4
3
2
00
00
00
00
4
3
2
00
00
00
G1–HMDI–PdRu
G3–HMDI–Ru
G1–HMDI–Ru
G3–HMDI–Ru
100
4
2
0
100
0
Fig. 6. Comparison of the activities of G3–HMDI–PdRu
Fig. 4. Hydrogenation of benzene and condensed aromatic
and G3–HMDI–Ru in the hydrogenation of aromatic
compounds in the presence of Ru catalysts based on denꢀ
compounds. Reaction conditions: 80
°C, 2 h, 30 atm of H ,
2
drimers. Reaction conditions: 170°C and p(H ) = 30 atm.
and substrate/water = 1 : 1 (vol/vol).
2
Hydrogenation of Aromatic Compounds
in the Presence of the Bimetallic Catalyst PdRu
nanoparticles [21, 39, 40]. A sharper (more than twoꢀ
fold) increase in activity relative to the monometallic
catalyst (Fig. 6) may be apparently attributed to the
synergistic effect of palladium and ruthenium in the
melt [19, 21].
The data on the hydrogenation of aromatic comꢀ
pounds in the presence of G3–HMDI–PdRu are preꢀ
sented in Table 7. As in the case of G3–HMDI–Ru,
for G3–HMDI–PdRu conversions and activities
decreased abruptly with the increasing substrate size.
This observation may be explained by the use of steriꢀ
cally hindered thirdꢀgeneration dendrimers and the
concomitant dendrite effect [37]. The presence
of products of incomplete hydrogenation (cyclohexꢀ
ene and its derivatives) and isomerization (in the case
Palladium, which does not catalyze the hydrogenaꢀ
tion of aromatic compounds under these conditions,
serves as an additional source of adsorbed hydrogen
that is further transferred on ruthenium atoms as a
result of spillover. The drop in activity in the case of
alkylꢀsubstituted benzenes, in particular butylbenꢀ
zene, may probably be explained as follows. For the
hydrogenation reaction to be advantageous, the size of
the area on the nanoparticle surface, which is comꢀ
posed of ruthenium atoms, should be sufficient to
of oꢀ and pꢀxylenes) may be due to the presence of palꢀ
ladium in the active phase and, as a consequence, to a
change in the electronic properties of the surface of
allow adsorption of the aromatic compound and the
π
simultaneous adsorption of hydrogen. As the size of
the substrate and the number of its substituents
increase, the size of the area should become accordꢀ
ingly larger. At the same time, the number of such
adsorption sites on bimetallic particles turns out to be
lower because of the surface “dilution” by palladium.
1
00
8
0
0
0
0
6
4
2
Materials based on ruthenium nanoparticles disꢀ
persed in crosslinked dendrimer matrixes manifested
themselves as highꢀactivity catalysts in the hydrogenaꢀ
tion of aromatic compounds in twoꢀphase systems
water/substrate. Water, as an additional dispersion
medium, exerted a strong promoting effect on the rate
of hydrogenation due to a change in the properties of
the dendrimer matrix, its partial hydrolysis, and facilꢀ
itated reduction of the catalyst surface. The introducꢀ
tion of an additional metal into the catalyst with the
analogous support (G3–HMDI–PdRu) leads to a
marked gain in the rate of hydrogenation of benzene
and a decrease in the rate of its transformation.
S/Ru = 170
S/Ru = 850
0
1
2
3
t, h
4
5
6
Fig. 5. Hydrogenation of tetralin in the presence of G1
HMDI Ru under twoꢀphase conditions. Reaction condiꢀ
tions: 90 C, 40 atm of H , and V(substrate) = V(H O).
−
−
°
2
2
PETROLEUM CHEMISTRY Vol. 56
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NANOHETEROGENEOUS RUTHENIUMꢀCONTAINING CATALYSTS
501
Table 7. Hydrogenation of aromatic compounds in water in the presence of the G3–HMDI–PdRu catalyst
Substrate/Ru,
Substrate
Conversion, %
TOF, h–1
Products
mol/mol
8
8
4
5
302
299
423
436
71.5
Cyclohexane 99% Cyclohexene 1%
Cyclohexane 99.5% Cyclohexene 0.5%
Cyclohexane 99% Cyclohexene 1%
Cyclohexane 98.5% Cyclohexene 1.5%
7
12
4*
7.5
Benzene
1780
4
9*
Toluene
715
723
20
Methylcyclohexane 99% Methylcyclohexene 1%
Ethylcyclohexane 99%
Ethylbenzene
18
65.0
Ethylcyclohexene 1%
Cisꢀ1,2ꢀDimethylcyclohexane 6%
Transꢀ1,2ꢀDimethylcyclohexane 65%
Cisꢀ1,4ꢀDimethylcyclohexane 4.5%
Transꢀ1,4ꢀDimethylcyclohexane 10.5%
Ethylcyclohexane 13.5%
o
ꢀXylene
734
1**
3.7
Cisꢀ1,2ꢀDimethylcyclohexane 30%
Transꢀ1,2ꢀDimethylcyclohexane 5%
Cisꢀ1,4ꢀDimethylcyclohexane 56%
Transꢀ1,4ꢀDimethylcyclohexane 9%
Butylcyclohexane 100
p
ꢀXylene
718
2
7.2
Butylbenzene
Tetralin
730
840
2
5
7.3
CisꢀDecalin 90%
21.0
Transꢀdecalin 10%
Reaction conditions: 80°C, 2 h, 30 atm of H , and substrate/water = 1 : 1 (vol/vol).
2
*
10 atm of H2.
** Isomerization of substrate, 8% (PhEt).
ACKNOWLEDGMENTS
8. Nowicki, A., Zhang Yo, Léger, B., Rolland, J.ꢀP., Briꢀ
cout, H., Monflier, E., and Roucoux, A., Chem. Comꢀ
mun., 2006, no. 3, p. 296.
This work was supported by the Russian Foundaꢀ
tion for Basic Research, no. 14ꢀ03ꢀ31531.
9. Tilloy, S., Bricout, H., and Monflier, E., Green Chem.
,
2002, vol. 4, no. 3, p. 188.
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Translated by T. Soboleva
PETROLEUM CHEMISTRY Vol. 56
No. 6
2016