Noble metal nanoparticles stabilized by hyper-cross-linked polystyrene as effective catalysts in hydrogenation of arenes

Article abstract of DOI:10.3390/molecules26154687

This work is addressing the arenes’ hydrogenation—the processes of high importance for petrochemical, chemical and pharmaceutical industries. Noble metal (Pd, Pt, Ru) nanoparticles (NPs) stabilized in hyper-cross-linked polystyrene (HPS) were shown to be active and selective catalysts in hydrogenation of a wide range of arenes (monocyclic, condensed, substituted, etc.) in a batch mode. HPS effectively stabilized metal NPs during hydrogenation in different medium (water, organic solvents) and allowed multiple catalyst reuses.

Full text of DOI:10.3390/molecules26154687

molecules
Article
Noble Metal Nanoparticles Stabilized by Hyper-Cross-Linked
Polystyrene as Effective Catalysts in Hydrogenation of Arenes
Elena S. Bakhvalova 1, Arina O. Pinyukova 2, Alexey V. Mikheev 2, Galina N. Demidenko 2, Mikhail G. Sulman 2,
Alexey V. Bykov 2, Linda Z. Nikoshvili 2,
*
and Lioubov Kiwi-Minsker 1,3,
*
1
Regional Technological Centre, Tver State University, Zhelyabova Str., 33, 170100 Tver, Russia;
bakhvalova.es@mail.ru
2
Department of Biotechnology, Chemistry and Standardization, Tver State Technical University,
A.Nikitina Str., 22, 170026 Tver, Russia; yourchemist@mail.ru (A.O.P.); exmahina.leha@gmail.com (A.V.M.);
xt345@mail.ru (G.N.D.); sulmanmikhail@yandex.ru (M.G.S.); bykovav@yandex.ru (A.V.B.)
Department of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne (GGRC-ISIC-EPFL),
CH-1015 Lausanne, Switzerland
3
*
Correspondence: nlinda@science.tver.ru (L.Z.N.); lioubov.kiwi-minsker@epfl.ch (L.K.-M.);
Tel.: +7-904-005-7791 (L.Z.N.); +41-21-693-3182 (L.K.-M.)
Abstract: This work is addressing the arenes’ hydrogenation—the processes of high importance for
petrochemical, chemical and pharmaceutical industries. Noble metal (Pd, Pt, Ru) nanoparticles (NPs)
stabilized in hyper-cross-linked polystyrene (HPS) were shown to be active and selective catalysts in
hydrogenation of a wide range of arenes (monocyclic, condensed, substituted, etc.) in a batch mode.
HPS effectively stabilized metal NPs during hydrogenation in different medium (water, organic
solvents) and allowed multiple catalyst reuses.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Bakhvalova, E.S.;
Pinyukova, A.O.; Mikheev, A.V.;
Demidenko, G.N.; Sulman, M.G.;
Bykov, A.V.; Nikoshvili, L.Z.;
Kiwi-Minsker, L. Noble Metal
Nanoparticles Stabilized by
Hyper-Cross-Linked Polystyrene as
Effective Catalysts in Hydrogenation
of Arenes. Molecules 2021, 26, 4687.
https://doi.org/10.3390/
Keywords: polymeric catalysts; hydrogenation of arenes; noble metals; hyper-cross-linked
polystyrene; catalyst stability
1. Introduction
Hydrogenation of different arenes, mono- and polycyclic, and their mixtures are
highly used in modern industries: refining of fuels; hydrogenation of benzene (BZ) to
cyclohexane (CH) for the synthesis of caprolactam and toluene (TOL) to methylcyclohexane
(MCH); hydrogenation of naphthalene (NL) in the synthesis of tetralin (TL) and decalin
(industrial hydrogen donors and solvents used for extraction); hydrogenation of aniline
(AN) and its derivatives for the production of anticorrosive additives. All these reactions
are energy-intensive, large-capacity petrochemical processes that produce reagents for
chemical and pharmaceutical industries, and are used for reduction of residuals in fuels
and lubricants [1–4].
Most methods for the production of functionalized and nonfunctionalized cycloalka-
nes from BZ, NL, polyarenes, and their derivatives are based on gas-phase catalytic pro-
cesses. Such processes have a number of disadvantages: high temperatures of vapor–gas
mixture and nonuniform heat removal resulting in catalyst deactivation; catalyst abrasion
and metal loss in the case of fluidized bed reactors. Liquid-phase hydrogenation of arenes
is carried out at much lower temperatures using catalytic systems based mainly on nickel
or noble metals (Pt, Pd, Ru, Rh) deposited on inorganic supports, but is less studied [5–7].
The conditions for hydrogenation of aromatics are selected depending on the con-
molecules26154687
Academic Editor: Federico Cesano
Received: 30 June 2021
Accepted: 31 July 2021
Published: 3 August 2021
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centration of sulfur-containing impurities in the raw material [8]. If BZ is almost free of
sulfur compounds, the reaction is carried out in the presence of low-temperature catalysts
at low hydrogen pressure. The BZ containing sulfur impurities must be hydrogenated over
sulfur-resistant catalysts under more harsh conditions. Nickel catalysts, in comparison
with platinum group metals, are preferred for hydrogenation of sulfur-containing raw
materials since they are more resistant to sulfur poisoning.
Molecules 2021, 26, 4687. https://doi.org/10.3390/molecules26154687
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 4687
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The use of different inorganic supports (e.g., alumina, silica, zeolites) allows high
catalyst activity and selectivity; however, their acidic properties provoke carbon deposition.
Moreover, active metal phase deposited on inorganic supports is often unstable and un-
dergoes sintering, resulting in catalyst deactivation. The use of polymeric supports brings
a big advantage: the catalytically active phase is well stabilized within cavities distributed
over the entire volume of the polymer and becomes fully accessible to reactants when the
support swells. Moreover, polymeric supports are usually hydrophobic, and are suitable
for organic solvents.
In this work, for the first time, hydrogenation by molecular hydrogen of a wide
range of arenes (monocyclic and condensed, substituted and nonsubstituted) was studied
over noble metal (Pd, Pt, Ru) NPs supported on hyper-cross-linked polystyrene (HPS) of
two types: MN270 (nonfunctionalized) and MN100 (bearing tertiary amino groups and
designated as HPS-NR2).
It is noteworthy that, to date, there are only a few reports on the use of polymer-based
catalysts in hydrogenation of arenes. The most recent work, of Davankov et al. [9], was
devoted to the hydrogenation of BZ, TOL, TL, and phenol (PL) using Pd NPs supported
on the HPS of MN200-type (analogue of MN270, but with larger meso-/macro-pores with
a mean diameter of 70 nm). It was shown that, using supercritical CO2 or methylene
chloride as a solvent, Pd/MN200 successfully catalyzed hydrogenation of aromatics in
◦
the temperature range of 55–150 C. Moreover, Pd/HPS catalysts exhibited higher activity
compared to traditional catalysts containing an equivalent amount of Pd [10].
2. Results
First, the hydrogenation of BZ to cyclohexane (CH) was studied (the simplified reac-
tion scheme is shown in Figure 1a) at variation of solvent nature, BZ concentration, and
catalyst composition (polymer type, metal loading, and nature). It is noteworthy that
different side products can be found during BZ hydrogenation [5,7], but their formation
is associated with the acid–base properties of the supports used. In the case of polymeric
supports (in particular, HPS), 100% selectivity with respect to CH was observed irrespective
of the metal nature and reaction conditions (see Table 1).
(a)
(b)
Figure 1. Simplified schemes of selective hydrogenation of BZ to CH (a) and TOL to MCH (b).
◦
Most of BZ hydrogenations were carried out in hexane medium at 230 C, which was
found as optimal temperature during the reaction study with 1% Pt/HPS (see Figure 2
and Table 1, #1). As can be seen in Figure 2, the initial rate of BZ transformation gradually
increases with the temperature increase from 210 ◦C up to 230 ◦C. At 240 ◦C, the rate of
BZ hydrogenation attains the maximum, and the following increase of temperature up to
◦
260 C results in a sharp decrease of the reaction rate (note that the critical point of hexane
◦
is 234.5 C at 3.02 MPa). Recently, we have found that hydrogenation of NL to TL in hexane
can effectively proceed at 250 ◦C and at 6 MPa of overall pressure [11].
The strong decrease of hydrogenation rate while shifting to supercritical hexane, in
the case of BZ in contrast to NL, can be due to the difference in adsorption of mono- and
dicyclic arenes. In general, if the metal NPs are large enough, condensed arenes should
◦
adsorb stronger. Nevertheless, in the range of 210–230 C, the apparent activation energy of

Molecules 2021, 26, 4687
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BZ hydrogenation was calculated as Ea,app = 81
with the data reported elsewhere [12
±
5 kJ/mol (Figure 2), which is in agreement
13]. It is noteworthy that for calculation of Ea,app, the
,
initial reaction rate R0 was obtained as a tangent of the slope on the kinetic curve related
to the metal concentration (see Section 4.3). We can suppose that the coverage of metal
surface by hydrogen is low, which is typical for small conversions where concentration
of substrate is still high [14]. Thus, with high selectivity to target product and without
any influence of diffusion, the irreversible addition of hydrogen atoms to the adsorbed
substrate can be considered as a rate-determining step.
Table 1. Data of catalytic testing of HPS-based catalysts in hydrogenation of nonpolar aromatics (CS,0 = 0.28 mol/L, catalyst
◦
loading 100 mg, temperature 230 C, overall pressure 5 MPa, reaction duration 60 min).
N
Catalyst
Substrate
Product
Solvent
X (S), %
R0, molsub/(molMe × s)
1
2
3
4
1% Pt/HPS
1% Pt/HPS (1)
hexane
hexane
hexane
96 (100)
98 (100)
84 (100)
38 (100)
1.37
0.77
0.43
0.47
1% Pt/HPS 2nd use (1)
1% Pt/HPS-NR2
dodecane
5
6
7
i-PrOH
dodecane
hexane
90 (100)
92 (100)
100 (100)
0.90
0.89
1.66
(CH)
2% Pt/HPS-NR2
(BZ)
8
9
2% Ru/HPS-NR2
2% Pd/HPS-NR2
hexane
hexane
89 (100)
100 (100)
0.40
0.82
(2)
10
11
12
hexane [11]
dodecane
dodecane
94 (88)
82 (88)
80 (100)
0.15
0.85
1.41
1% Pt/HPS-NR2
(NL)
(TL)
1% Pt/HPS-NR2
1% Pt/HPS-NR2
(ANC)
(9,10-DANC)
(MCH)
(TOL)
(1)
CBZ,0 = 0.14 mol/L. (2) Temperature 240 ◦C.
Figure 2. Effect of temperature on hydrogenation of BZ to CH over 1% Pt/HPS (5 MPa, catalyst
loading 100 mg); inset shows the Arrhenius plot for initial reaction rate, R0.

Molecules 2021, 26, 4687
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For the catalyst 1% Pt/HPS, the effect of reuse was also studied (Table 1, #2,3). It was
found that nearly double a decrease of the initial reaction rate (R0) takes place at the
second reaction run. The polymeric matrix of HPS remained stable during the experiments.
Low-temperature nitrogen physisorption revealed that the specific surface area (SSA)
determined according to the BET model was nearly the same for both the initial (934 m2/g)
and the used (955 m2/g) catalyst. Using the XPS method, it was found that there is a slight
change in the ratio between different forms of Pt on the catalyst surface: the increase of
the content Pt0 (binding energy (BE) of Pt 4f7/2 is 71.5 eV) was observed (from 77 at.%
up to 87 at.%) with concomitant decrease of the content of PtO (BE of Pt 4f7/2 is 72.9 eV)
from 23 at.% to 13 at.%; however, these changes cannot explain the significant decrease in
catalytic activity for the second use. The reason can be due to the formation of carbonaceous
deposits on the surface of Pt NPs, which often causes the catalyst deactivation during
the hydrogenation of arenes [13,15]. However, Pt content (determined by the XPS) on
the catalyst surface even increases after the catalytic experiment from about 0.3 at.% up
to 0.5 at.%. Thus, we can suppose that the observed drop in catalytic activity is likely
due to the changes in morphology of the catalytically active phase, i.e., agglomeration
of NPs. It is known that the sizes of Pt NPs have a crucial effect on the kinetics of BZ
hydrogenation [16].
The effect of solvent nature was studied using 2% Pt/HPS-NR2 as catalyst (see Figure 3
and Table 1, #5–7). As can be seen from Figure 3, the use of hexane as a solvent provides the
highest rate of BZ transformation, although at the chosen temperature (230 ◦C), the vapor
pressure of hexane is rather high—about 2.7 MPa. In the case of i-PrOH and dodecane, the
reaction rate was noticeably lower, compared to hexane. It is noteworthy that at 230 ◦C,
i-PrOH has vapor pressure higher than 4 MPa, which drastically decreases hydrogen
partial pressure (overall pressure in the reactor was 5 MPa). Nevertheless, the rate of BZ
hydrogenation in i-PrOH medium was comparable with those in dodecane as a solvent,
possibly due to the ability of i-PrOH to serve as an effective hydrogen donor [17].
Figure 3. Solvent effect on BZ hydrogenation to CH over 2% Pt/HPS-NR2 (230 ◦C, 5 MPa,
CBZ,0 = 0.28 mol/L, catalyst loading 100 mg).
◦
At the chosen temperature (230 C) in hexane medium, the effect of metal nature
was studied (Table 1, #7–9). It was found that Pt is the most effective metal as it provides
a 3–6-fold higher initial reaction rate compared to Pd and Ru (note that RuO2 is an active
phase of 2% Ru/HPS-NR2).
In contrast to BZ, TOL was three-fold faster hydrogenated exclusively with MCH
(Figure 1b) (Table 1, #4 vs. 12) using 1% Pt/HPS-NR2 in dodecane. This result is un-
usual, since, according to other reports, the rates of BZ and TOL hydrogenation are often
comparable [18,19].
Hydrogenation of naphthalene (NL) and anthracene (ANC) was carried out over 1%
Pt/HPS-NR2. NL was found to be selectively hydrogenated to target TL (Figure 4), while
in the case of ANC, 9,10-dihydroanthracene (9,10-DANC) was the main reaction product
(Figure 5). Pt-containing catalysts were shown to be stable in four consecutive runs of NL

Molecules 2021, 26, 4687
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and ANC hydrogenation without any noticeable loss of activity and selectivity under the
chosen reaction conditions.
Figure 4. Simplified scheme of selective hydrogenation of NL to TL.
Figure 5. Simplified scheme of selective hydrogenation of ANC to 9,10-DANC.
It was also found that the initial hydrogenation rate (R0) increases in the order:
NL < BZ < ANC (Table 1, #4,10,11). In contrast to the data reported by Rautanen et al. [20],
who claimed that in condensed aromatics the first ring of diaromatic compounds (i.e., NL)
is more reactive than the second one, in the case of Pt/HPS catalysts, NL was less reactive
compared to BZ.
At the same time, in the ANC hydrogenation, the main side product was
1,2,3,4-tetrahydro-ANC (about 9% was accumulated during 60 min), and no compounds
with higher degree of saturation were found. This result indicates that the rate of side-ring
hydrogenation is low and can be due to the peculiarities of arenes adsorption on Pt NPs.
Higher reactivity of ANC in comparison with BZ is likely due to the fact that C9 and C10
atoms of the central ring in ANC have partially higher electron density due to the influence
of two side rings, making these carbon atoms more active for hydrogenation.
Hydrogenation of different substituted arenes was also studied, taking AN, PL,
phenylacethylene (PHA), and propiophenone (PP) as models. In the selective hydro-
genation of AN to cyclohexylamine (CHA) (see Figure 6), HPS-supported Pt NPs were the
most effective noble metal-containing catalysts independent of the polymer type and metal
loading (Figure 7 and Table 2, #1–5). In spite of the fact that numerous products can be
formed during the AN hydrogenation [5], we observed only two side products depicted
in Figure 6.
Figure 6. Simplified scheme of selective hydrogenation of AN to CHA.

Molecules 2021, 26, 4687
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◦
Figure 7. CHA accumulation using different noble metal NPs supported on HPS-NR2 (150 C, 3 MPa,
CAN,0 = 0.41 mol/L, catalyst loading 100 mg).
Table 2. Data of catalytic testing of HPS-based catalysts in hydrogenation of substituted aromatics.
R0, molsub
(molMe × s)
/
◦
N
Catalyst
Substrate
Product
Solvent
T, C
P, MPa
X (S), %
100 (55) 1
100 (56) 1
41 (51) 2
54 (87) 2
58 (60) 2
1
2
3
4
5
1% Pt/HPS
hexane
hexane
hexane
hexane
hexane
150
150
150
150
150
2
2
3
3
3
0.32
0.21
0.32
0.05
0.07
1% Pt/HPS 2nd use
2% Pt/HPS-NR2
2% Ru/HPS-NR2
2% Pd/HPS-NR2
(CHA)
(AN)
96 (40) 3
41 (60) 3
53 (64) 3
100 (88) 4
100 (78) 4
100 (69) 4
6
7
8
water
EtOH
BuOH
150
150
150
2
2
2
0.24
0.07
0.12
2% Pd/HPS-NR2
1% Pd/HPS-NR2
(CHN)
(STY)
(PL)
9
10
11
TOL
EtOH
i-PrOH
90
65
70
ambient
ambient
ambient
6.70
1.25
2.25
(PHA)
92 (98) 5
12
1% Pd/HPS-NR2
hexane
80
0.25
0.13
(PP)
(PPL)
1
2
CAN,0 = 0.14 mol/L, catalyst loading 100 mg, values of conversion (X) and selectivity (S) are indicated for the reaction time 120 min.
3
CAN,0 = 0.41 mol/L, catalyst loading 100 mg, values of X and S are indicated for the reaction time 150 min. CPL,0 = 0.21 mol/L, catalyst
4
loading 50 mg, 2 MPa is partial hydrogen pressure, values of X and S are indicated for the reaction time 90 min. CPHA,0 = 0.48 mol/L,
catalyst loading 30 mg, maximum values of X and corresponding S are indicated. CPP,0 = 0.09 mol/L, values of X and S indicated for the
5
reaction time 150 min.
However, in spite of the high initial reaction rate, Pt-containing HPS catalysts deacti-
vated, especially at high initial concentrations of AN, which is seen from the shape of kinetic
curves (see Figure 8a). At the same time, the apparent activation energy was 53 ± 5 kJ/mol
(Figure 8b), which is comparable with literature data [21]. Moreover, it was found that
1% Pt/HPS is rather stable at the reuse (Table 2, #1,2); although the initial reaction rate
slightly decreases, conversion and selectivity achieved in 120 min of the reaction remains
the same. Observed loss of activity during the same reaction run can be due to strong
adsorption of AN and products on the metal surface, acting similar to poisons [22
,23], but
this effect seems to take place only in situ under the reaction conditions.
Hydrogenation of PL, PHA, and PP traditionally is carried out using Pd-containing
catalysts. In the case of PL hydrogenation (Figure 9), different reaction conditions (sol-
vents, temperatures, and pressures) can be used [24]. Earlier, we showed that PL can
be successfully hydrogenated in a gas phase in a fixed-bed reactor over 0.5% Pd/HPS
catalyst [25]. ◦In this work, an autoclave-type reactor was used, and three solvents (Table 2
,
#6–8) at 150 C and hydrogen partial pressure of 2 MPa (comparable with some other
works) [26] were tested. As can be seen from Table 2 (#6–8), water is the best solvent,

Molecules 2021, 26, 4687
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allowing the highest hydrogenation rate, while the selectivity over 2% Pd/HPS-NR2 with
respect to cyclohexanone (CHN) is moderate (91% to CHL at 50% of PL conversion) and
needs further improvement.
(a)
(b)
Figure 8. Effect of AN initial concentration (
a
) and temperature (
b
) on selective hydrogenation of
AN to CHA using 1 wt.% Pt/HPS (2 MPa, catalyst loading 100 mg).
Figure 9. Scheme of selective hydrogenation of PL to CHN.
PHA can be easily hydrogenated to styrene (STY) (Figure 10) under mild reaction
conditions (low temperatures and ambient hydrogen pressure). Nevertheless, it is very
difficult to provide high selectivity with respect to STY [27] due to exceptionally high
reactivity of triple and double carbon–carbon bonds, when one of the hydrogen atoms is
substituted in a benzene ring. In the case of 1 wt.% Pd/HPS-NR2, TOL was found to be
the best solvent, providing exceptionally high activity (Figure 11) and 88% selectivity with
respect to STY at close to 100% PHA conversion (Table 2, #9–11), which is comparable with
other reports [28]. It is noteworthy that different temperatures were tested which were
about 10–15 ◦C lower than the boiling point of the corresponding solvent. The higher the
temperature, the higher the selectivity to olefinic product; however, hydrogenation rate
usually goes through a maximum, which is 90 ◦C for TOL [29].
Figure 10. Scheme of selective hydrogenation of PHA to STY.

Molecules 2021, 26, 4687
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Figure 11. Solvent effect on PHA hydrogenation to STY over 1 wt.% Pd/HPS-NR2 (ambient H2
pressure, CPHA,0 = 0.48 mol/L, catalyst loading 30 mg).
PP was also hydrogenated using Pd-containing catalyst (1 wt.% Pd/HPS-NR2). Phenyl-
propanol (PPL) was found to be the main reaction product (Figure 12).
Figure 12. Scheme of selective hydrogenation of PP to PPL.
Variation of overall pressure (Figure 13a) allowed determining the formal kinetic
parameter with respect to hydrogen, which was close to the unit. The study of the tem-
perature effect was carried out at 0.25 MPa (Figure 13b). It was found◦that the reaction
◦
rate increased with the increase of temperature from 60 C up to 80 C (in this range,
Ea,app = 75 ± 5 kJ/mol, calculated taking into account the same assumptions as in the case
of BZ hydrogenation). Further increase of temperature resulted in noticeable decrease of
the PP hydrogenation rate due to the high vapor pressure of the solvent (hexane). Thus,
◦
80 C can be considered as the optimal temperature (see also Table 2, #12), which provided
98% conversion of PP (for 240 min) and exclusive formation of PPL (100% selectivity).
(a)
(b)
Figure 13. Effect of overall pressure (a) and temperature (
b
) on selective hydrogenation of PP to PPL
using 1 wt.% Pd/HPS-NR2 (CPP,0 = 0.09 mol/L, catalyst loading 100 mg).

Molecules 2021, 26, 4687
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It is noteworthy that, across the whole range of chosen temperatures, 98% selectivity
with respect to PPL was found at 0.25 MPa overall pressure, which is a very promising
result in comparison with other reported data [30,31]. A slight decrease of selectivity (at
99% of PP conversion) was observed at higher pressures: 97% at 5 MPa, and about 95% at
10 MPa.
3. Discussion
The results of our work reported here demonstrate that noble metal NPs are effectively
stabilized within polymeric networks of HPSs. HPSs of different types and properties (SSA,
hydrophobicity, functional groups, etc.) are commercially available at a relatively low price
while being chemically and mechanically stable. Moreover, contrary to most inorganic
solids (such as alumina, silica, zeolites, etc.), HPSs have excellent wettability in organic
solvents and do not present surface acidity, which is known to be responsible for the
formation of a number of byproducts and carbon deposits, leading to catalyst deactivation.
HPS-based catalysts containing NPs of different metals (Pt, Pd, Ru) were prepared via
simple wet-impregnation and thoroughly characterized by different physical and chem-
ical methods. Pd NPs had mean diameter of 2.6
±
0.4 nm and were evenly distributed
through the polymeric network irrespective of the polymer type, palladium loading, or
reduction method. The 2% Pd/HPS-NR2 with hexane as solvent demonstrated extraor-
dinary catalytic properties in BZ to CH hydrogenation, showing 100% CH selectivity at
close to 100% conversion of BZ. Pd NPs stabilized by HPS also showed excellent results in
PP hydrogenation, giving almost exclusively the target PPL. Pt NPs had slightly bigger
mean diameter (3.9 ± 1.7 nm) compared to Pd and also some aggregates with diameters of
>11 nm. The 2% Pt/HPS-NR2 also demonstrated 100% CH selectivity at up to full BZ con-
version, but outperformed the Pd-based catalyst in terms of a twofold higher reaction rate
under the same reaction conditions. The 2% Ru/HPS-NR2 contained small NPs of RuO2 of
about 2–4 nm in diameter, which were distributed nonuniformly, closer to the outer surface
of the polymer granule, and showed much lower performance in hydrogenation of BZ,
compared to Pd and Pt. In contrast, it outperformed Pt- and Pd-based HPS in production
of CHA during AN hydrogenation.
Synthesized Pt-, Pd-, and Ru-containing HPS-based catalysts were shown to be highly
active and selective in hydrogenation of both nonsubstituted and substituted arenes. Cat-
alytic properties of synthesized samples are promising in comparison with some other
reported results [32]. For example, hydrogenation of BZ and TL proceeded with 100% se-
lectivity to CH and MCH, respectively, while using Pt NPs supported on HPS or HPS-NR2.
For comparison, in the case of zeolite-supported catalysts, isomerization took place, result-
ing in the formation of methylcyclopentane as the main product [32]. Hydrogenation of
NL to TL over Pt/HPS-NR2 proceeded with the selectivity of 88% at 94% of NL conversion,
which is comparable with some reported results [32], or, in some cases, even higher [33].
Finally, we would like to underline that hyper-cross-linked polymers with various
functionalities and cross-linking degrees should be used as catalytic supports for different
metal NPs and should be tested in liquid-phase hydrogenations of arenes, giving much
attention to catalyst stability in view of their multiple reuse. This work is in progress and
will be reported elsewhere.
4. Materials and Methods
4.1. Materials
HPS (Purolite Int., Llantrisant, UK), used as catalyst support, was of two types: hyper-
cross-linked nonfunctionalized polystyrene, Macronet MN270 (BET SSA 1337 m2/g), and
HPS containing tertiary amino groups, MN100 (BET SSA 724 m2/g), designated here as
HPS-NR2. Both supports were washed with distilled water and acetone and then dried
under vacuum as described elsewhere [34]. Benzene (BZ, 99.8%), toluene (TOL, 99.8%),
naphthalene (NL, 99%), aniline (AN, 99%), anthracene (ANC, 97%), phenylacethylene
(PHA, 98%), phenol (PL,
≥
99%), propiophenone (PP, 98%), diphenylamine (DPA, 99%),

Molecules 2021, 26, 4687
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tetrahydrofuran (THF,
peroxide (H2O2, 35%), isopropanol (i-PrOH,
99%), and dodecane ( 99%) were obtained from Sigma-Aldrich. Chloroplatinic acid
hydrate (H2PtCl6 6H2O, Pt content 38.41%), palladium acetate (Pd(CH3COO)2, Pd content
≥
99.9%), methanol (MeOH, 99.8%), acetone (
≥
99.5%), hydrogen
≥
99.5%), butanol (BuOH,
≥
99%), hexane
(
≥
≥
·
47.68%), and ruthenium hydroxychloride (Ru(OH)Cl3, Ru content 45.05%) were purchased
from JSC “Aurat” (Moscow, Russia). Sodium hydroxide (NaOH) was obtained from
Reakhim (Moscow, Russia). Reagent-grade hydrogen of 99.999% purity was received from
AGA (Tver, Russia). All chemicals were used as received. Distilled water was purified with
an Elsi-Aqua water purification system.
4.2. Catalyst Synthesis and Characterization
Pt-, Pd-, and Ru-containing HPS-based catalysts were synthesized via wet-impregnation
according to procedure, which is described elsewhere [34
1 g of pretreated, dried, and crushed (<63 m) granules of HPS were impregnated with
2.8 mL of the THF solution of precursor (H2PtCl6 6H2O or Pd(CH3COO)2) of a chosen
–36]. In a typical experiment,
µ
·
concentration. The HPSs impregnated with Pt or Pd were dried, washed with distilled
water until reaching a neutral pH, and dried again at 70 ◦C until the constant weight was
achieved. In the case of Ru/HPS, a complex solvent (THF, MeOH, and water in a volu-
metric ratio of 7:1:1) with dissolved therein calculated amount of Ru(OH)Cl3 was used for
impregnation. The Ru-containing HPS was dried at 70 ◦C for 1 h, the dried catalyst was
boiled in aqueous solution of NaOH (concentration of 0.1 mol/L) at continuous stirring,
and then H2O2 was added. The resulting catalyst was washed with distilled water until
neutral pH was reached, and dried again at 70 ◦C.
Thus, the catalysts Pt/HPS, Pt/HPS-NR2, Pd/HPS-NR2, and Ru/HPS-NR2 were
synthesized containing either 1 wt.% or 2 wt.% of noble metals. Before the experiments,
◦
all the catalyst samples were activated in a hydrogen flow (100 mL/min) at 300 C for
3 h. Such activation has no influence on the HPS structure [11] (see also Figure S1) and
results in formation of NPs of metallic Pd or Pt and RuO2 (see the data of XPS below) in
a micro–mesoporous polymeric environment (BET SSA for all the samples was in the range
700–1000 m2/g).
Figure 14 shows STEM data using the example of the 1% Pt/HPS-NR2, 2 wt.%
Pd/HPS-NR2, and 2% Ru/HPS-NR2 catalysts. It was found that among these three
samples, Pd NPs were the most evenly distributed (Figure 14b) and had mean diameter
of 2.6 ± 0.4 nm, which is typical for HPS impregnated with Pd acetate, irrespective of
the polymer type, palladium loading, and reduction method [37,38]. Pt NPs had mean
diameter of 3.9 ± 1.7 nm, but also a lot of aggregates with diameters up to 11 nm and
higher (Figure 14a). The 2 wt.% Ru/HPS-NR2 contained small NPs of RuO2 of about
2–4 nm in diameter (Figure 14c), which were located nonuniformly, closer to the outer
surface of the polymer granule [39]. See also Figure 14d, where light regions correspond to
RuO2 (the average content of elements calculated via EDX is the following: carbon 34%,
oxygen 41%, ruthenium 25%).
Metal state was confirmed by the XPS analysis. It was shown that palladium is present
in the form of metallic NPs (BE of Pd 3d5/2 is 335.0
for small Pd clusters) and PdO (BE is 337.2
±
0.1 eV for Pd0 NPs and 336.0 ± 0.1 eV
±
0.1 eV [40]). The metallic form Pd0 was
predominant on the catalysts’ surface (Figure S2c). The existence of oxide can be explained
by the fact that, after the activation in H2 flow, the catalysts were stored in air. In the
case of 1 wt.% Pt/HPS (Figure S2a) and 2 wt.% Pt/HPS-NR2 (Figure S2b), the following
values of BE (
0.1 eV) of Pt 4f7/2 were found: 71.5 eV (Pt0) and 72.9 eV (PtO) [40]. For
±
1% Pt/HPS-NR2 [11], BE of Pt 4f7/2 was equal to 71.4
±
0.1 eV, which corresponds to
Pt0 [40]. In the case of 2 wt.% Ru/HPS-NR2 (Figure S2d), ruthenium was in the form of
RuO2 (BE is 281.2 ± 0.1 eV) and RuO2*nH2O (BE is 283.0 ± 0.1 eV).

Molecules 2021, 26, 4687
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(a)
(b)
(c)
Figure 14. HAADF-STEM images of activated samples ((
(d)
a
) 1% Pt/HPS-NR2 (scale 100 nm), (b) 2% Pd/HPS-NR2 (scale
50 nm), (c) 2% Ru/HPS-NR2 (scale 10 nm)) and SEM image (d) of 2% Ru/HPS-NR2 (scale 20 µm).
4.3. Reaction Procedure and Analysis of Reaction Mixture
Testing of Pt-, Pd-, and Ru-containing HPS-based samples in hydrogenation of differ-
ent arenes, with the exception of PHA, was carried out in a stainless-steel autoclave reactor
(Parr Instruments, Moline, IL, USA) having an internal volume of 80 mL. In a typical
experiment, 0.05 g (or 0.1 g) of preliminarily activated catalyst was placed in the reactor.
Then, substrate and 40 mL of solvent were added. After that, the reactor was sealed, purged
with nitrogen, and heated under stirring (1500 rpm) up to operating temperature. After
reaching working temperature, the first sample of the reaction mixture was taken. Then,
nitrogen was replaced with hydrogen, overall working pressure was attained (time “zero”
for the reaction), and the reaction started. Sampling of the reaction mixture was carried
out via long narrow capillary (internal diameter of 0.1 mm); the volume of each sample
was 200 µL.
In the case of PHA, hydrogenation was carried out in a 60 mL isothermal glass batch
reactor installed in a shaker at vigorous stirring (more than 800 two-sided shakings per
minute). The total volume of the liquid phase was 30 mL. A recirculating bath (LOIP LT 100,

Molecules 2021, 26, 4687
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◦
1 C, with
Saint Petersburg, Russia) was used to stabilize the reaction temperature within
±
water as a heating medium. The reactor was connected to a gasometrical burette for online
hydrogen consumption control. At the beginning of each experiment, the temperature was
stabilized, the reactor was charged with catalyst, and the catalyst was kept under hydrogen
for 60 min. Then PHA was added, and reaction was started.
Samples of the reaction mixture were analyzed via GC (Kristallux 4000M) equipped
with FID and capillary column ZB-WAX (60 m
GC–MS (Shimadzu GCMS-QP2010S) equipped with a capillary column HP-1MS
(100 m 0.25 mm i.d., 0.50 m film thickness). Helium was used as a carrier gas. The con-
×
0.53 mm i.d., 1 µm film thickness) or
×
µ
centrations of the reaction mixture components were calculated using absolute calibration
method using chemically pure components (in the case of GC) or using the internal stan-
dard calibration method (DPA was used as an internal standard) (in the case of GC–MS).
Conversion of substrates was defined as
−1
X (%) = (Csub,0 − Csub,i) × Csub
× 100.
(1)
(2)
(3)
,0
Selectivity with respect to target product was given as
−1
S (%) = Cprod,i × (Csub,0 − Csub,i
)
× 100.
Initial reaction rate was designated as R0, [molsub·molMe−1·s−1 ]:
R0 = (Csub,0 − Csub,i) × (CMe × τi)−1
.
4.4. Catalyst Characterization Methods
HPS-based catalysts were characterized by liquid nitrogen physisorption, diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spec-
troscopy (XPS), and scanning transmission electron microscopy (STEM).
Liquid nitrogen physisorption was carried out using a Beckman Coulter SA 3100
(Coulter Corporation, Miami, FL, USA). Prior to the analysis, each sample was placed
in a quartz cell installed in the Becman Coulter SA-PREP. The samples were pretreated
over 60 min under nitrogen at 120 ◦C. Once the pretreatment was completed, the cell was
cooled and weighed, and then transferred to the analytical port. Analysis was performed
at
−
196 ◦C and at relative pressure of 0.9814 (for pores less than 100 nm in diameter) to
obtain a PSD (ADS) profile.
DRIFTS analysis was carried out using an IRPrestige-21 FTIR spectrometer (Shimadzu,
Kyoto, Japan) equipped with a DRS-8000 diffuse reflectance accessory (Shimadzu, Japan).
The background sample was a mirror of the material of the optical system of the DRS-8000
accessory. All spectra were recorded in the 4000–500 cm−1 range of wavenumbers at
a resolution of 4 cm−1
XPS data were obtained using Mg K
.
α
(hν = 1253.6 eV) radiation with an ES-2403
spectrometer (Institute for Analytic Instrumentation of RAS, Saint Petersburg, Russia)
equipped with an energy analyzer PHOIBOS 100-MCD5 (SPECS, Berlin, Germany) and
X-ray source XR-50 (SPECS, Berlin, Germany). All the data were acquired at X-ray power
of 250 W. Survey spectra were recorded at an energy step of 0.5 eV with an analyzer pass
energy of 40 eV, and high-resolution spectra were recorded at an energy step of 0.05 eV
with an analyzer pass energy of 7 eV. Samples were outgassed for 180 min before analysis
and were stable during the examination. The data analysis was performed via CasaXPS.
STEM characterization was carried out using an FEI Tecnai Osiris instrument (Thermo
Fisher Scientific, Waltham, MA, USA) operating at an accelerating voltage of 200 kV,
equipped with a high-angle annular dark field (HAADF) detector (Fischione, Export,
PA, USA) and an energy-dispersive X-ray (EDX) microanalysis spectrometer (EDAX,
Mahwah, NJ, USA). Samples were prepared by embedding the catalyst in epoxy resin
followed by microtomming (ca. 50 nm thick) at ambient temperature. For the image
processing, DigitalMicrograph (Gatan, Pleasanton, CA, USA) software and TIA (Thermo

Molecules 2021, 26, 4687
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Fisher Scientific, Waltham, MA, USA) were used. Holey carbon/Cu grid was used as
a sample support.
Supplementary Materials: The following are available online, Figure S1: DRIFT spectra of 2%
Pt/HPS-NR2 (a), 2% Pd/HPS-NR2 (b), and 2% Ru/HPS-NR2 (c): initial (black line) and after activa-
tion in hydrogen flow (red line), Figure S2: High-resolution XPS spectra of Pt 4f (a,b), Pd 3d (c), and
Ru 3d (d) in the activated samples: 1% Pt/HPS (a), 2% Pt/HPS-NR2 (b), 2% Pd/HPS-NR2 (c), and
2%-Ru/HPS-NR2 (d).
Author Contributions: Conceptualization, A.V.B. and L.K.-M.; methodology, A.V.B.; data curation,
A.V.B.; validation, L.K.-M. and L.Z.N.; investigation, E.S.B., A.V.M. and A.O.P.; formal analysis, E.S.B.
and A.O.P.; resources, M.G.S.; writing—original draft preparation, L.Z.N.; writing—review and
editing, L.K.-M.; visualization, G.N.D.; supervision, L.K.-M.; project administration, L.K.-M. and
L.Z.N.; funding acquisition, M.G.S. and L.K.-M. All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by the Russian Science Foundation, grant number 20-19-00386.
Data Availability Statement: Data sharing is not applicable to this article.
Acknowledgments: The authors highly appreciate the work of Irina Yu. Tiamina on the catalysts
preparation. The authors thank Alexander L. Vasiliev (National Research Centre “Kurchatov Insti-
tute”, Moscow, Russia) for help with STEM studies of the catalysts.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of the HPS-based catalysts are available from the authors.
Abbreviations
AN, aniline; ANC, anthracene; BZ, benzene; CH, cyclohexane; CHA, cyclohexy-
lamine; CHN, cyclohexanone; 9,10-DANC, 9,10-dihydroanthracene; diphenylamine, DPA;
HPS, hyper-cross-linked polystyrene; MCH, methylcyclohexane; NL, naphthalene;
NP, nanoparticle; PHA, phenylacethylene; PL, phenol; PP, propiophenone, PPL, phenyl-
propanol; STY, styrene; TL, tetralin; TOL, toluene.
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