Effect of solvent in the hydrogenation of acetophenone catalyzed by Pd/S-DVB

Article abstract of DOI:10.1039/d1nj00219h

A solvent effect was found in the hydrogenation of acetophenone catalyzed by a new Pd/S-DVB catalyst, immobilized on a styrene (S)/divinylbenzene (DVB) copolymer containing phosphinic groups. The porous structure of the catalyst was characterized by a specific surface area of 94.7 m²g^?1. The presence of Pd(ii) and Pd(0) in Pd/S-DVB was evidenced by XPS and TEM. Pd/S-DVB catalyzes the hydrogenation of acetophenone (APh) to 1-phenylethanol (PhE) and ethylbenzene (EtB). The highest conversion of APh was obtained in methanol (MeOH) and in 2-propanol (2-PrOH), while in water it was lower. The conversion of APh correlates well with the hydrogen-bond-acceptance (HBA) capacity of the solvent. However, in all binary mixtures of alcohol and water the APh conversion and the yield of products significantly decreased. The observed inhibiting effect can be explained by the microheterogeneity of these mixtures and the blocking of the catalyst surface restricting access of the substrates to the Pd centers.

Full text of DOI:10.1039/d1nj00219h

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Effect of solvent in the hydrogenation of
acetophenone catalyzed by Pd/S-DVB†
Cite this: New J. Chem., 2021,
45, 5023
b
Tomasz Bereta,a Ewa Mieczynska,a Sylwia Ronka,
Włodzimierz Tylusc and
´
a
Anna M. Trzeciak
*
A solvent effect was found in the hydrogenation of acetophenone catalyzed by a new Pd/S-DVB
catalyst, immobilized on a styrene (S)/divinylbenzene (DVB) copolymer containing phosphinic groups.
The porous structure of the catalyst was characterized by a specific surface area of 94.7 m2
g
À1. The
presence of Pd(II) and Pd(0) in Pd/S-DVB was evidenced by XPS and TEM. Pd/S-DVB catalyzes the
hydrogenation of acetophenone (APh) to 1-phenylethanol (PhE) and ethylbenzene (EtB). The highest
conversion of APh was obtained in methanol (MeOH) and in 2-propanol (2-PrOH), while in water it was
lower. The conversion of APh correlates well with the hydrogen-bond-acceptance (HBA) capacity of the
solvent. However, in all binary mixtures of alcohol and water the APh conversion and the yield of
products significantly decreased. The observed inhibiting effect can be explained by the
microheterogeneity of these mixtures and the blocking of the catalyst surface restricting access of the
substrates to the Pd centers.
Received 14th January 2021,
Accepted 21st February 2021
DOI: 10.1039/d1nj00219h
rsc.li/njc
carbon (Pd/C)17,18 and inorganic oxides, Al2O3,19 TiO2,20 SiO2,
and AlPO4,21,22 have been explored as supports. Pd loaded on a
1. Introduction
Hydrogenation of aromatic ketones is a process of great impor- hierarchical FAU nanozeolite membrane catalyzed transfer
tance because of the broad practical application of its hydrogenation using water as a solvent and sodium formate
products.1,2 Aromatic ketones can be efficiently hydrogenated as the hydrogen and electron donor.25 In contrast, organic
using different metal catalysts.3–7 In hydrogenation studies, polymers were scarcely used (Fig. 1).23,24
acetophenone (APh) is often used as a model substrate. The
Selective formation of PhE was achieved using Pd/POP2
first product of APh hydrogenation is 1-phenylethanol (PhE), (POP2 = spirofluorene-based porous organic polymer) at
used in the cosmetic and pharmaceutical industry.8–10 Further 25 bar of H2.26 The catalytic activity of Pd/PVP (PVP = poly-4-
hydrogenation of PhE can also occur, leading to ethylbenzene vinylpyridine) was found to be comparable to that of Pd/Al2O3,
(EtB).10–14 In this reaction, the C–OH bond is converted to a while lower activity and selectivity were noted for Pd supported
C–H bond, which is a transformation of practical importance in on electroactive PPY (PPY = polypyrrole).23 Enantioselective APh
organic synthesis, in particular in biomass conversion.15,16 hydrogenation was studied with cinchonidine [26] or proline
Moreover, hydrogenolysis of PhE plays an important role in modified catalysts.27,28
the manufacturing of propylene oxide in which PhE is an
Hydrogenation of APh with immobilized palladium catalysts
undesired waste product, while EtB can be recycled back into was studied in various solvents, including water. By using Pd/C
the process.12
in water, high conversion of APh was obtained, which was
Immobilized palladium catalysts were tested with good further enhanced in the presence of CO2.18 A similar positive
results for the hydrogenation of APh. In most cases, activated
a Faculty of Chemistry, University of Wrocław, F. Joliot-Curie St. 14 St.
50-383 Wrocław, Poland. E-mail: anna.trzeciak@chem.uni.wroc.pl
b Department of Engineering and Technology of Polymers, Faculty of Chemistry,
˙
´
Wrocław University of Science and Technology, Wybrzeze Wyspianskiego 27,
50-370 Wrocław, Poland
c Department of Advanced Material Technologies, Faculty of Chemistry,
˙
´
Wrocław University of Science and Technology, Wybrzeze Wyspianskiego 27 St.,
50-370 Wrocław, Poland
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00219h
Fig. 1 Products of acetophenone hydrogenation.
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effect of CO2 was also noted in MeOH.18 Water was found to be
a superior solvent in APh hydrogenation using Pd/Al2O3 and
Pd/C.19 The APh hydrogenation efficiency of these catalysts was
correlated with the hydrogen-bond-acceptance (HBA) capacity
(b) and hydrogen-bond-donor (HBD) capacity (a), depending on
the support.19,29 An enhancement of the hydrogenation activity
of the Ru/FCN (FCN = methacrylate-styrene resin) catalyst in
water/MeOH and water/i-PrOH systems was rationalized by
increased solvation of the reaction product and its accelerated
desorption from the catalyst surface in these media.30
Herein, we present the application of a new palladium
catalyst immobilized on a porous P-functionalized polymer
for APh hydrogenation. Considering the results cited above,
we expected a solvent effect on the hydrogenation yield and
selectivity. In particular, it was interesting to study mixed
solvents of the ROH/water type. Such mixtures, which significantly
deviate from the ideal solutions,31,32 have not yet been applied
in APh hydrogenation with Pd catalysts. In fact, we found for
the first time significant inhibition of the catalytic reaction in
MeOH/water and i-PrOH/water binary mixtures.
Fig. 2 Schematic representation of the polymer S-DVB synthesis.
2.2. Methods
The acid capacity in the polymer support was measured by
immersing a known amount of the centrifuged polymer (in
protonated form) in 50 mL of 0.1 M sodium hydroxide solution
for 24 h and titrating the resultant solution with 0.1 M hydro-
chloric acid. The water regain of the resin was investigated
using the centrifugation method.
2. Experimental
2.1. Preparation of the Pd catalyst
The phosphorus content in the polymer support and in the
catalytic material was determined by digesting a sample (ca. 20 mg)
in hot concentrated sulfuric acid followed by analysis of the
solution using the standard molybdate blue method. Readings
were taken at 700 nm.
The palladium concentration was measured using the
atomic absorption spectrometry method (AAS) on a GBC Avanta
instrument with the wavelength set at 244.8 nm for Pd, and a
slit of 0.2 nm.
The morphological features of the synthesized catalyst were
obtained using a scanning electron microscope (JEOL, JSM-
6610LVnx).
The XPS spectra were measured on a SPECS UHV/XPS/AES
system with a dual Mg/Al X-ray source and a hemispherical
PHOIBOS 100 analyzer operated in fixed analyzed transmission
(FAT) mode. The spectra were processed using the CasaXPS
v.2.2.19 program. The background was determined by the
Schirley method.
The porous structure of the synthesized material examined
by nitrogen adsorption at liquid nitrogen temperature gave no
measurable surface area. Therefore, the pore size and surface
area were characterized by benzene and carbon dioxide adsorp-
tion. A high-vacuum gravimetric apparatus with McBain–Bakr
quartz spring balances was used. Adsorption isotherms were
taken at 25 1C. Each sample was degassed at 250 1C for 3 h in a
vacuum before starting the measurement. The micropore
volume was determined by applying the Dubinin–Radushkevich
(DR) equation. The values of the specific surface area (SBET) were
calculated on the basis of the Brunauer, Emmett and Teller
(BET) equation from the adsorption branches of the benzene
isotherms.
The styrene (S) and divinylbenzene (DVB) copolymer was prepared
by typical suspension polymerization using 0.5 wt% benzoyl
peroxide as an initiator. The nominal crosslinking degree of the
S-DVB copolymer was 20 wt%. Synthesis of the polymer was
carried out in the presence of nonsol and sol solvents, heptane/
toluene 1 : 9 v/v, in order to obtain a porous structure. The
obtained polymer beads were washed with hot and cold water
and acetone and dried. Then, the polymer was swollen in toluene
and heated with this solvent for 9 h in a Soxhlet apparatus.
The S-DVB copolymer was then modified by phosphorylation.
Thus, dry polymer beads were placed in a round bottom flask
together with an excess of PCl3 and swollen for 30 minutes.
Then, powdered aluminum chloride was added and the entire
mixture was refluxed for 6 h. After that, the beads were filtered,
washed briefly with dioxane and dropped into a beaker containing
ice and a solution of sodium chloride. After washing with
distilled water, the beads were transferred to a ca. 3 M NaOH
solution and left in it overnight followed by standard conditioning
with distilled water, 1 M HCl, distilled water, 1 M NaOH,
distilled water, 1 M HCl and finally with distilled water. The
synthesis of S-DVB with phosphinic acid groups is presented in
Fig. 2.
Palladium was introduced to the phosphorylated polymer
using an aqueous solution of K2[PdCl4] and applying a 2-fold
metal excess with respect to the phosphinic groups. The
palladium solution was put in contact with the phosphorylated
S-DVB copolymer for 5 days (shaking at 150 rpm at room
temperature). During this time, the color changed from yellow
to black due to the reduction of Pd(II) to Pd(0). The resulting
Pd/S-DVB material was washed with deionized water to remove
the palladium excess and dried.
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2.3. Hydrogenation experiments
The catalytic experiments were carried out in AMTECH corp.
personalized autoclaves with PID and magnetic stirring providing
precise control of the pressure and temperature.
0.00395 g of the palladium catalyst (containing 1 Â 10À5 mol Pd),
0.1 mL (8.57 Â 10À4 mol) of acetophenone and 3 mL of solvent
were added to a 15 mL stainless steel autoclave under an N2
atmosphere. The autoclave was pressurized to 15 bar of N2.
After pressure release, the autoclave was pressurized to 10 bar
H2 (in ca. 30 min), and then heated to 80 1C. After the reaction
was completed, the autoclave was cooled down over 30 min, the
pressure was released and 10 mL of methanol was added to the
mixture. Next, 0.1 mL of mesitylene was added as an internal
standard and the organic products were analyzed by the GC-FID
and GC-MS methods (Shimadzu QP 2010 SE).
Fig. 3 XPS spectra: Pd 3d core level spectrum of the sample before the
reaction (left) and Pd 3d core level spectrum of the sample after the
reaction (right).
The optimal fit was obtained for the presence of three palladium
3. Results and discussion
forms, namely Pd(0) with a binding energy of 335.1 eV for Pd 3d5/2
,
PdO with a binding energy of 336.6 eV and Pd(II) with a binding
energy 338.0 assigned to the non-reduced PdCl42À precursor.33 The
content of these palladium forms was 21.0, 57.4 and 21.6%,
respectively. The presence of PdO can be explained by the oxida-
tion of Pd NPs in air.
After hydrogenation, the Pd : P ratio decreased to 0.08,
indicating palladium leaching or its transfer inside the polymer
(Fig. 3). At the same time, the amount of Pd(0) on the surface
increased to 41.5% while the amount of PdO decreased to 8.9%
due to Pd reduction in the presence of H2.
Fig. S3 (ESI†) presents the P 2p spectra of three Pd/S-DVB
samples before and after hydrogenation. The binding energies,
BE 133.4 eV and BE 134.36 eV, were attributed to P 2p3/2 and
P 2p1/2, respectively, and they are characteristic of phosphonate
groups.34,35 The shape and position of the peaks were the same
for all three measurements, indicating that the polymer was not
changed during the reaction.
The presence of Pd(0) in the studied catalyst was additionally
evidenced by XRD measurements before and after hydrogenation
(Fig. 4). The XRD pattern presented four peaks at 401, 46.71, 681,
and 821, corresponding to Pd(0) crystallized in the Fm3m space
group (JCPDS card number 5-681).
3.1. Synthesis and characterization of Pd/S-DVB
The obtained Pd/S-DVB catalyst was characterized by benzene
and carbon dioxide adsorption–desorption experiments. The
results indicate that the synthesized polymer support is pre-
dominantly submicroporous (pores with diameters between
0.4 and 0.7 nm). The mesopore structure of the investigated
product is poorly developed. Its specific surface area and
the volume of pores are 94.7 m2 gÀ1 and 0.059 cm3 gÀ1
respectively. The Pd to P ratio is close to 1 and the experimental
data are comparable with the theoretical P and Pd atom content
(2.5–2.6 mmol gÀ1) (Table 1).
,
XPS analysis of the obtained Pd/S-DVB catalyst evidenced the
presence of Pd(II) and Pd(0) attached to the polymer. Thus,
Pd(II) was in part reduced to Pd(0) in the reaction with the
polymeric support and simultaneously the phosphinic groups
on the polymer surface were oxidized to phosphonate ones.
Both forms of Pd are stabilized on the surface of the polymer by
interactions with P = O and OH groups. Due to the small
diameter of the pores, the bonding of Pd inside the pores is
less likely.
Fig. 3 shows the Pd 3d spectra of the surface of the fresh
catalyst. The Pd : P ratio was estimated as 0.26, significantly
lower than in the AAS analysis. It can be assumed that part of
the palladium was localized inside the polymer, not on its
surface. Before analysis, the surface of the catalyst sample was
cleaned with an Ar+ beam (Ar sputtering) using minimal energy
over a short time period (1 keV, 1 min, 1.3 mA cmÀ2).
The presence of Pd NPs in the Pd/S-DVB analyzed after the
hydrogenation reaction was confirmed by TEM. The size of the
Pd NPs varied from 12 to 26 nm, with an average of ca. 17 nm,
Table 1 Characteristics of the Pd/S-DVB catalyst
Pd content [mmol gÀ1
]
2.6
2.5
Phosphorus content [mmol gÀ1
]
a
St (SBET
)
[m2 gÀ1
]
94.7 (5.3)
0.059 (0.002)
a
Vt (VBET
) ]
[cm3 gÀ1
a
St/Vt – the total surface area/volume of the pores calculated as the sum
of the surface areas of submicropores, micropores and mesopores;
SBET/VBET – the BET surface area/volume of pores calculated from
benzene adsorption isotherms.
Fig. 4 XRD patterns of the as prepared Pd/S-DVB.
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Table 2 Effect of solvents on the hydrogenation of acetophenone
indicating some aggregation under the reaction conditions
(Fig. 5).
Fig. 6 presents the SEM micrograph of the Pd/S-DVB catalyst,
which has a regular spherical shape. Mapping of the catalyst
surface showed a uniform distribution of palladium and phos-
phorus (Fig. 6).
EtB
PhE
Solvent (mL)
Xi H2O APh conv. (%) yield (%) yield (%) b29
DMF
MeOH
69
91
24
18
33
62
66
90
61
68
33
87
80
48
28
72
18
12
23
56
52
81
53
54
18
77
65
18
41
14
6
3
8
6
4
7
7
11
15
10
16
28
0.69
0.66
1.5 H2O/1.5 MeOH
0.25 H2O/2.75 MeOH 0.17
0.5 H2O/2.5 MeOH 0.31
2.75 H2O/0.25 MeOH 0.96
H2O
i-PrOH
0.25 H2O/2.75 i-PrOH 0.28
0.5 H2O/2.5 i-PrOH
1.5 H2O/1.5 i-PrOH
BuOH
0.25 H2O/2.75 BuOH 0.32
Toluene
0.69
3.2. Catalytic activity
0.18
0.95
3.2.1. Effect of solvents. The immobilized Pd/S-DVB
catalyst was employed in the hydrogenation of acetophenone
(APh) with H2 (10 bar) in various solvents (Table 2). The reaction
performed in DMF produced a mixture of two products, EtB and
PhE, with the latter being the main one. Similar low selectivity
was obtained in toluene, with 28% PhE and 18% EtB. In MeOH
and in BuOH, APh was hydrogenated mainly to EtB with yields of
72% and 77%, respectively. An even higher amount of EtB was
produced in i-PrOH (81%). Hydrogenation of APh in water
proceeded with lower efficiency, and 66% conversion was
achieved after 4 h with 52% EtB (Table 2).
0.46
0.81
0.88
0.11
Reaction conditions: APh 0.1 mL (8.3 Â 10À4 mol), Pd (1 Â 10À5 mol), 4
h, 80 1C, 10 bar H2.
To rationalize the effect of the solvent on the APh conver-
sion, different correlations were analyzed and the best one was
obtained for the HBA ability expressed by the b parameter.29
The b-scale presents a measure of the solvent ability to accept a
proton. Correlation of the APh conversion with the b parameter
was noted for Pd/Al2O3 and Rh/Al2O3 having OH groups on the
surface, but not for Pd/C and Rh/C.19,36 In our case, the
interaction of the solvent molecules with OH groups present
on the surface of the Pd/S-DVB catalyst influenced the catalytic
activity. Attraction of protons by the stronger HBA solvents
modified the surface, facilitating coordination of APh and its
hydrogenation. With the increase in the b value, the conversion
of APh also increased (Fig. 7). This trend is opposite to that
noted for Pd and Rh catalysts.19,36 On the other hand, attempts
Fig. 7 Dependence of the APh conversion on the b value of the solvents.
to correlate the APh conversion with the a parameter expressing
the HBD ability failed. Similarly, there is no correlation
between the conversion and H2 solubility in the studied
solvents.
3.2.4. Binary solvent mixtures. Next, binary mixtures of
the ROH/H2O type were investigated as solvents for APh hydro-
genation. A decrease in APh conversion was observed for
MeOH/H2O mixtures when compared to the pure solvents.
For example, 0.25 mL of water added to 2.75 mL of MeOH
decreased the conversion to only 18%. Interestingly, a further
increase in the amount of water caused a slow increase in the
conversion up to 62% in the presence of 2.75 mL of water.
A similar, non-linear dependence of the APh conversion on the
amount of water was also found for i-PrOH/H2O mixtures. The
minimal conversion in the studied series was 38% and in all
cases the conversion of APh was lower than in i-PrOH or water
only. Interestingly, in a mixed system of BuOH/H2O, the
inhibiting effect was much smaller and hydrogenation was
only slightly less efficient than in pure BuOH with the conver-
sion decreasing by 7%.
Fig. 5 TEM micrograph of Pd/S-DVB after the hydrogenation of APh.
Analysis of the product’s composition led to the conclusion
that the EtB/PhE ratio, illustrating the hydrogenation selectivity,
changed only slightly when the composition of the solvents
Fig. 6 (a) SEM micrograph of the Pd/S-DVB catalyst; (b) Pd mapping; and
(c) P mapping on the surface.
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changed. In general, the highest EtB/PhE value was noted in
pure water because of the low amount of PhE formed in this
solvent. In the binary mixtures, the EtB/PhE values were lower
than for solely water or solely alcohol.
The most intriguing feature of the studied system is the non-
linear dependence of the APh conversion on the amount of
water in the MeOH/H2O and i-PrOH/H2O mixtures (Fig. 8 and 9).
For an explanation, one can consider the microheterogeneous
structure of the ROH/H2O mixtures derived from the presence of
hydrated alcohol clusters, stabilized by hydrogen bonding.31,32
As a result, the ROH/H2O mixtures are separated at a molecular
level and deviate from the ideal mixture. The extent of this
phenomenon depends on the kind of alcohol. Thus, clusters of
BuOH are less hydrated than clusters of i-PrOH or MeOH.
Therefore, water strongly influenced the catalytic reactions
carried out in i-PrOH and MeOH binary mixtures, while a weaker
effect was observed in BuOH.
Fig. 10 Hydrogenation of differently substituted acetophenones.
Table 3 Hydrogenation of differently substituted acetophenones (Fig. 10)
R=
R-APh conv. (%)
R-PhE (%)
R-EtB (%)
H
91
82
59
59
33
13
36
9
24
25
11
10
30
7
67
57
48
49
3
6
2
3-Me
2-Me
2,5-Me
3-NO2
2-NO2
3-NH2
2-NH2
34
7
2
The observed solvent effects differ from those reported for
other systems, most probably because of the specifity of the
S-DVB support. In our system, protic solvents of the highest
b values, such as alcohols, provided the best results.
3.2.5. Hot filtration test. In order to check the heteroge-
neous nature of the hydrogenation process the hot filtration test
was performed.37 The solid catalyst was removed after 1 h of the
catalytic reaction, and the remaining solution was subjected to
further heating (4 h). The GC analysis evidenced that the
composition of the reaction mixture did not change due to the
non-activity of the solution after catalyst separation. Thus,
hydrogenation occurred with an immobilized Pd/S-DVB catalyst.
3.2.6. Hydrogenation of different acetophenones. In the
next stage of the study, differently substituted acetophenones
were used as substrates in hydrogenation performed with Pd/
S-DVB for 1 h in MeOH (Table 3). By using a shorter reaction
time, we expected to see differences between the applied
substrates. The highest conversion was obtained for non-
substituted APh, while the presence of substituents, in general,
resulted in a decrease in the yields of the products. Considering
the electronic effect of the substituents, higher conversions
were obtained in the presence of Me groups of weaker donor
character than with the stronger donor NH2 substituent. It
should be noted that the products obtained from NO2 and NH2
substituted APh were the same, with the main one being
aminophenylethanol. This is because the NO2 group was fully
hydrogenated to the NH2 group under the applied conditions.
These experiments produced additional data concerning
hydrogenation selectivity. In the reaction of APh and its
Me-substituted derivatives, hydrogenolysis of R-PhE occurred
efficiently leading to the formation of R-EtB as a main product.
The stronger donor group NH2 increased the selectivity for
R-PhE and only a small amount of R-EtB was formed.
The addition of water to ROH changed the structure of the
solvent by forming hydrated clusters, in particular in the case of
short-chain ROH. As a result, the HBA capacity of the mixed
solvent system also decreased and the reaction slowed down.
The inhibiting effect observed in the ROH/H2O mixture can
be significantly reduced by adding a small amount of a-cyclo-
dextrin. Thus, the conversion of APh increased from 18% to
78% when 10 mol% a-cyclodextrin was added to the reaction
carried out in 2.75 MeOH/0.25 H2O. Cyclodextrin changed the
solvent structure and facilitated hydrogenation. In the analo-
gous experiment performed with addition of [nBu4N]Br, known
as a phase transfer agent, the conversion of APh only slightly
increased, to 24%.
Fig. 8 Conversion of APh (left) and the EtB/PhE ratio (right) at different
mole fractions of water for APh hydrogenation in MeOH.
The steric effect of the substituents was responsible for higher
conversion when the substituent was present in the 3-position
rather than the 2-position, promoting the smaller steric hindrance.
Conclusions
The new Pd/S-DVB catalyst immobilized on a P-functionalized
porous polymer efficiently catalyzed the hydrogenation of APh,
Fig. 9 Conversion of APh (left) and the EtB/PhE ratio (right) at different
mole fractions of water for APh hydrogenation in i-PrOH.
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protic solvents such as MeOH, i-PrOH and BuOH. Our results
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Acknowledgements
The financial support of the National Science Foundation (NCN)
with grant 2017/25/B/ST5/00394 is gratefully acknowledged. The
authors are grateful to Mr Marek Hojniak (Faculty of Chemistry,
University of Wroclaw) for GC and GC-MS analysis, and to Dr
Wojciech Gil (Faculty of Chemistry, University of Wroclaw) for
performing SEM measurements.
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5028 | New J. Chem., 2021, 45, 5023À5028
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2021