Effect of particle restructuring during reduction processes over polydopamine-supported Pd nanoparticles

Article abstract of DOI:10.1166/jnn.2019.15770

The effect of catalyst restructuring on the polydopamine-supported Pd catalyzed transfer hydrogenation of ethyl 4-nitrobenzoate and the catalytic hydrogenation of (E)-2-methyl-2-butenoic acid is reported. Transmission electron microscopy investigation of different catalyst pre-treatment and reaction conditions revealed high catalytic activity in both reactions unless drastic aggregation of the active metal occurred. In the transfer hydrogenation reaction aggregation was primarily dependent on the H-source used, while in the catalytic hydrogenation additives in combination with the reductive environment led to extensive Pd aggregation and thus decreased catalytic activity. The enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid showed increased enantioselectivity and decreased conversion with increased particle size.

Full text of DOI:10.1166/jnn.2019.15770

Article
Journal of
Nanoscience and Nanotechnology
Vol. 19, 484–491, 2019
Copyright © 2019 American Scientific Publishers
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Printed in the United States of America
Effect of Particle Restructuring During Reduction
Processes Over Polydopamine-Supported
Pd Nanoparticles
Tamás Gazdag1ꢀ†, Ádám Baróthi1ꢀ†, Koppány Levente Juhász2ꢀ†, Attila Kunfi1ꢀ3, Péter Németh4,
2
2ꢀ5
2ꢀ6
7ꢀ∗
1ꢀ∗
˝
András Sápi , Ákos Kukovecz , Zoltán Kónya , Kornél Szori , and Gábor London
1Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences,
Budapest, Magyar tudósok körútja 2, 1117, Hungary
2Department of Applied and Environmental Chemistry, University of Szeged, Szeged, Rerrich Béla tér 1, 6720, Hungary
3Department of Organic Chemistry, University of Szeged, Szeged, Dóm tér 8, 6720, Hungary
4Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences,
Magyar tudósok körútja 2, 1117 Budapest, Hungary
5MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Szeged, Rerrich Béla tér 1, 6720, Hungary
6MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Szeged, Rerrich Béla tér 1, 6720, Hungary
7MTA-SZTE Stereochemistry Research Group, Szeged, Dóm tér 8, 6720, Hungary
The effect of catalyst restructuring on the polydopamine-supported Pd catalyzed transfer hydro-
IP: 185.223.160.40 On: Sun, 09 Dec 2018 17:28:04
genation of ethyl 4-nitrobenzoate and the catalytic hydrogenation of (E)-2-methyl-2-butenoic acid
Copyright: American Scientific Publishers
is reported. Transmission electron microscopy investigation of different catalyst pre-treatment and
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reaction conditions revealed high catalytic activity in both reactions unless drastic aggregation of
the active metal occurred. In the transfer hydrogenation reaction aggregation was primarily depen-
dent on the H-source used, while in the catalytic hydrogenation additives in combination with the
reductive environment led to extensive Pd aggregation and thus decreased catalytic activity. The
enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid showed increased enantioselectivity
and decreased conversion with increased particle size.
Keywords: Polydopamine, Palladium, Heterogeneous Catalysis, Transfer-Hydrogenation,
Enantioselective Hydrogenation, Transmission Electron Microscopy.
1. INTRODUCTION
sintering in order to minimize their surface energies which
could be induced by high reaction temperature and the
reactants can play a role as well.
Metal nanoparticles are increasingly becoming the cata-
lysts of choice in many organic transformations.1–3 Such
systems are not only efficiently promoting a great variety
of reactions but also part of the toolbox of green chemistry
efforts.4–6 Supported metal particles are robust, compatible
with a range of reaction conditions (solvents, pressure and
temperature, microwave and ultrasound) and, ideally, could
be reused several times with preserved catalytic activity.
An important aspect of nanoparticle catalysis, both in
colloidal7–10 and supported11–16 systems, is the restructur-
ing (aggregation, shape and size alteration) of the active
metal particles and its influence on the efficiency of
the catalyst.17–23 The catalytic nanoparticles are prone to
Recently, among others,24–35 we have reported on the
application of mussel-inspired polydopamine as a sup-
port for Pd nanoparticles and its use in catalytic transfer-
hydrogenation reactions.36 Polydopamine,37 a widely
explored functional polymer,38 can be easily prepared
through the auto-polymerization of dopamine hydrochlo-
ride in basic aqueous medium (Fig. 1(a)). Pd nanopar-
ticles of 1–3 nm could be obtained on the surface of
polydopamine (Pd/PDA) by making use of its redox active
interfacial catechol moieties (Fig. 1(b)).36
We have prepared core–shell Fe3O4/Pd/PDA magnetic
particles also in order to facilitate the recycling of the
catalyst in transfer hydrogenation of aromatic nitro groups.
Compared to Pd/PDA, where particles between 1–3 nm
∗Authors to whom correspondence should be addressed.
†These three authors contributed equally to this work.
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doi:10.1166/jnn.2019.15770

Gazdag et al.
Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
(a)
(b)
H
N
H
N
O
O
O
O
O
m
n
m
n
Step 1
Step 2
H
N
H
N
OH
Pd(0)
HO
HO
O2
O
Pd(OAc)2
MeOH, rt
OH
OH
H
N
H
N
O
Pd(0)
NH2
HCl
H2O,
pH=8.5
rt
O
OH
OH
o
o
p
dopamine HCl
O
Pd(0)
NH2
NH2
O
OH
p
PDA
Pd/PDA
Figure 1. (a) Polymerization of dopamine hydrochloride to PDA (step 1) and Pd deposition onto the surface of PDA (step 2); (b) TEM image of
Pd/PDA.
size were present, the size of the Pd nanoparticles on
the surface of the magnetic system were somewhat big-
ger (5–6 nm), which had an influence on the catalytic
activity especially in the early stages of the reaction.
The conversion of ethyl 4-nitrobenzoate in 96% EtOH as
electron microscope at 100 kV accelerating voltage and a
FEI TECNAI G2 20 X-Twin high-resolution transmission
electron microscope operating at an accelerating voltage
of 200 kV.
ꢀ
solvent at 85 C using HCOONa as a hydrogen source
2.2. Catalyst Preparation
over Pd/PDA was found to be 10% to the corresponding
amine after 10 min, while this value was about 30% over
Fe3O4/Pd/PDA. The difference became smaller by time
and after 40 min the conversion was complete in both
2.2.1. Polymerization of Dopamine
Tris base (484 mg, 4.0 mmol) was dissolved in deionised
water (350 mL) and stirred for 30 min (pH = 8.5). A solu-
tion of dopamine hydrochloride (1.0 g, 5.27 mmol) in
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cases. However, post-reaction TEM showed Pd particle
water (50 mL) was added and the solution was stirred at
Copyright: American Scientific Publishers
aggregation in the case of Pd/PDA, while in the case of the
magnetic catalyst no pronounced changes were observed.
Based on these findings we became interested in the
effect of Pd particle size and morphology changes on the
outcome of reduction processes over PDA supported Pd
catalysts. We studied the effect of individual reactants on
the aggregation behavior of the active Pd in order to gain
deeper insights regarding the sensitivity of the system to
certain reactants and reaction conditions. This study is
aimed at better describing the applicability of Pd/PDA in
both transfer hydrogenation and catalytic hydrogenation
processes.
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room temperature for 30 hours. The black suspension of
polydopamine (PDA) was filtered, the residue was washed
with deionised water and dried under air overnight.
2.2.2. Preparation of Pd/PDA
PDA (200 mg) was dispersed in a solution of Pd(OAc)2
(21 mg, 0.094 mmol) in MeOH (50 mL) and stirred vigor-
ously overnight. The suspension was filtered, washed with
methanol (30 ml) and acetone (20 ml) and dried under
ambient conditions.
2.2.3. Preparation of Pd/PDA(L)
Pd/PDA(L), a catalyst bearing Pd particles that are larger
in size (10–16 nm) were prepared based on a method
reported in the literature:39 a suspension of PDA (200 mg)
in water (20 mL) was mixed with an acetone solution
(5 mL) of Pd(OAc)2 (70 mg) and stirred at 90 C for 1 h.
The solid was filtered, washed with acetone and methanol
and dried on air overnight.
2. EXPERIMENTAL DETAILS
2.1. General
Commercial reagents and solvents (Aldrich, Fluorochem,
VWR) were purchased as reagent-grade and used without
further purification.
GC-MS analysis was performed on a Shimadzu GCMS-
QP2010 Ultra System operated in Electron impact ioniza-
tion mode. GC analysis was performed with a YL6100 gas
chromatograph equipped with a flame ionization detector
using a HP-Chiral capillary column (J & W Scientific Inc.)
For transmission electron microscopy (TEM) investiga-
tion the samples were dispersed in ethanol and deposited
onto copper grids covered by carbon supporting films.
TEM investigations were performed on a Morgagni 268D
ꢀ
2.3. Catalytic Transfer Hydrogenation Reaction of
Ethyl 4-Nitrobenzoate to Ethyl 4-Aminobenzoate
2.3.1. General Procedure for the Catalytic Transfer
Hydrogenation (CTH) Reaction
Pd/PDA (10 mg, 0.57 mol% Pd), ethyl 4-nitrobenzoate
(0.5 mmol), H-donor (2.0 mmol) and 96% EtOH (2 mL)
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Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
Gazdag et al.
Table I. Effect of H-donors in the catalytic transfer hydrogenation reac-
tion of ethyl 4-nitrobenzoate.
2.4.2. Enantioselective Hydrogenation
Pd/PDA (15 mg), cinchonidine (CD) (0.05 mmol), benzy-
lamine (BA) (1 mmol) and toluene (10 mL) were placed
in a glass tube, which was placed in a stainless steel auto-
clave and one of the following pre-treatments were applied
(for details see text): (1) no pre-treatment, (2) stirring the
catalyst in toluene for 30 min or (3) stirring the catalyst
in toluene under a H2 atmosphere (50 bar) for 30 min.
Following the pre-treatment TA (1 mmol) was added to
the mixture and the reaction was stirred under a H2 atmo-
sphere (50 bar) at rt for 90 min. Subsequently, the catalyst
was filtered, washed with toluene and MeOH then dried.
The filtrate was concentrated under reduced pressure and
analyzed by GC-FID on a HP-Chiral capillary column.
Analysis conditions: head pressure—140 kPa, column
Pd/PDA (0.57 mol% Pd)
hydrogen source (4 eq)
COOEt
COOEt
EtOH (96%)
85 ºC, 30 min
H2N
O2N
Entry
Hydrogen source
Yield (%)
1
2
3
4
5
HCOONa
THN
Terpinene
59
No conversion
12
24
30
HCOONa+THN
HCOONa+terpinene
were placed in a sealed vial and the mixture was stirred at
85 C for 30 min. The reaction mixture was diluted with
EtOAc, filtered through a pad of silica and concentrated
under vacuum and analysed by GC-MS.
ꢀ
ꢀ
temperature—85 C. Retention times: (S)-2-methylbutyric
acid—13.2 min, (R)-2-methylbutyric acid 14.1 min, (E)-2-
methyl-2-butenoic acid 22.0 min.
2.4. Catalytic Hydrogenation of TA to
2-Methylbutyric Acid
3. RESULTS AND DISCUSSION
2.4.1. Racemic Hydrogenation
3.1. Transfer Hydrogenation of Ethyl 4-Nitrobenzoate
We have investigated the reduction of ethyl 4-nitro-
benzoate to ethyl 4-aminobenzoate (Table I), also known
as benzocaine, which is a compound of pharmaceutical
interest due to its use as a local anesthetic.
Pd/PDA (15 mg) and toluene (10 mL) was placed in a
glass tube, which was placed in a stainless steel autoclave
and one of the following pre-treatments were applied (for
details see text): (1) no pre-treatment, (2) stirring the cat-
alyst in toluene for 30 min or (3) stirring the catalyst in
toluene under a H2 atmosphere (50 bar) for 30 min. Fol-
lowing the pre-treatment TA (1 mmol) was added to the
mixture and the reaction was stirred under a H2 atmo-
sphere (50 bar) at rt for 90 min. Subsequently, the catalyst
was filtered, washed with toluene and MeOH then dried.
The filtrate was concentrated under reduced pressure and
analyzed by GC-FID on a HP-Chiral capillary column.
Among the tested H-donors, HCOONa was found to
be the best reagent as after 30 min reaction, the prod-
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uct aniline derivative was isolated in 59% yield while
the conversion reached completion after 40 min (Table I,
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entry 1). 1,2,3,4-Tetrahydronaphthalene (THN) was an
inactive H-donor under the reaction conditions (Table I
entry 2) which is in agreement with previous litera-
ture reports,40ꢀ41 while ꢁ-terpinene, although in a similar
Figure 2. TEM images of Pd/PDA catalysts under different CTH conditions. Effect of the H-donors HCOONa (a), THN (b) and terpinene (c) on the
aggregation of the Pd particles in the absence of the nitro compound and images following the reduction of ethyl 4-nitrobenzoate in the presence of
HCOONa (d), THN (e) and terpinene (f).
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Gazdag et al.
Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
14
12
10
8
6
4
2
0
8
10
12
14
16
18
Particle diameter (nm)
Figure 3. TEM image and particle size distribution of Pd/PDA(L).
system was found to be an efficient H-donor,42 deliv-
ered only 12% conversion after 30 min reaction (Table I,
entry 3).
First, we examined the effect of the H-donors in the
absence of the nitro compound on the aggregation behav-
ior of the Pd particles. TEM imꢀaging of the samples after
stirring Pd/PDA in EtOH at 85 C for 30 min in the pres-
ence of each H-donor, revealed different effects on Pd
aggregation. Although aggregation was clearly induced in
the presence of HCOONa as particle sizes up to 5 nm
were observed (Fig. 2(a)), it occurred to a lesser extent
compared to THN (Fig. 2(b)) and terpinene (Fig. 2(c))
where particles sizes up to 9 nm were measured. Similar
trend was observed upon post-reaction imaging of the cat-
alyst after the reduction of the nitro compound was carried
out. In the reduction performed using HCOONa the aggre-
gation was comparable to that occurred in the absence
of the nitro compound (Fig. 2(d)). When both the nitro
IP: 185.223.160.40 On: Sun, 09 Dec 2018 17:28:04
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Figure 4. TEM images of Pd/PDA catalysts after different pre-treatment and catalytic hydrogenation conditions of TA. Effect of pre-treatment on
the aggregation of the Pd particles: (a) After stirring the catalyst in toluene for 30 min and (b) after pre-hydrogenation of the catalyst (50 bar H2ꢂ in
toluene for 30 min. TEM images of differently pre-treated catalysts following the hydrogenation of TA: (c) No pre-treatment, (d) stirring the catalyst
in toluene for 30 min and (e) pre-hydrogenation of the catalyst (50 bar H2ꢂ in toluene for 30 min. (f) TEM image after stirring the catalyst in the
presence of TA (1 mmol) in toluene at rt for 30 min.
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Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
Gazdag et al.
compound and THN (Fig. 2(e)) or terpinene (Fig. 2(f))
H-donor was present in the reaction mixture, the Pd aggre-
gation was more pronounced. These results point towards
the contribution of the H-donors to the aggregation of the
Pd particles, furthermore, suggest the effect of particle size
on the reaction efficiency.
catalyzed by HCOONa. To test this latter speculation, we
prepared Pd/PDA catalyst (Pd/PDA(L)) with larger Pd par-
ticle sizes39 in the 10–16 nm range (Fig. 3). Performing the
reaction in the presence of this catalyst using HCOONa as
H-source the conversion was only 9% after 30 min reac-
tion indicating a clear size dependent catalytic activity.
To further test this possibility we performed control
experiments where HCOONa/THN (Table I, entry 4) and
HCOONa/terpinene (Table I, entry 5) mixed H-sources
were used. In the former case, 24%, while in the latter case
30% conversion of the nitro compound was detected under
3.2. Hydrogenation of Tiglic Acid Over Pd/PDA
We also aimed at extending the Pd/PDA catalyzed reduc-
tion processes to catalytic hydrogenation of C C double
bonds and studied the hydrogenation of (E)-2-methyl-2-
butenoic acid (tiglic acid, TA) (Table II).
Initially, we examined the effect of pre-treatments on the
aggregation behavior of the catalyst. Two different room
temperature pre-treatments were applied: (A) stirring the
catalyst in toluene for 30 min prior to adding the sub-
strate (Fig. 4(a)) and (B) pre-hydrogenation of the cata-
lyst (50 bar H2ꢂ in toluene for 30 min prior to adding
the substrate (Fig. 4(b)). None of the pre-treatments had a
pronounced effect on the Pd morphology, apart from the
appearance of few somewhat larger particles on the PDA
surface.
ꢀ
otherwise identical reaction conditions (85 C, 30 min).
Both are lower than using HCOONa alone (59%). When
ꢀ
the catalyst was stirred at 85 C for 30 min in the pres-
ence of THN or terpinene prior to the addition of the
nitro compound and HCOONa as H-donor the difference
increased. The THN treated catalyst provided 17% con-
version while the one treated with terpinene led to only
12% conversion of the nitro compound to the correspond-
ing aniline. These results could be explained by the poi-
soning of the Pd by the THN and terpinene H-sources,
however, could also point towards the size dependent cat-
alytic activity of the Pd in the reaction. The catalysts that
were treated with THN and terpinene contained larger Pd
particles, which could be less active in the nitro reduction
Following the different pre-treatments the hydrogenation
was complete in 90 min (Table II, entry 1 and 2). Simi-
lar result was obtained when no pre-treatment was applied
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Figure 5. TEM images of Pd/PDA catalysts after different pre-treatment and catalytic enantioselective hydrogenation conditions of TA in the presence
of CD. Effect of pre-treatment on the aggregation of the Pd particles: (a) After stirring the catalyst and CD (0.05 mmol) in toluene for 30 min and
(b) after pre-hydrogenation of the catalyst and CD (0.05 mmol) (50 bar H2ꢂ in toluene for 30 min. TEM images of differently pre-treated catalysts
following the enantioselective hydrogenation of TA: (c) No pre-treatment, (d) stirring the catalyst and CD (0.05 mmol) in toluene for 30 min and
(e) pre-hydrogenation of the catalyst (50 bar H2ꢂ in toluene for 30 min.
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Gazdag et al.
Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
Table II. Catalytic hydrogenation of tiglic acid over Pd/PDA catalyst.
We also examined the possibility of the enantioselective
hydrogenation of TA43–51 over cinchonidine (CD) modified
Pd/PDA as chiral short-chain carboxylic acids and esters52
are industrially relevant as building blocks in the synthesis
of pharmaceuticals and fragrances.53–57
Pd/PDA (0.57 mol% Pd)
H2 (50 bar)
O
O
Additive
OH
OH
Toluene, 90 min, rt
Pre-treatment of the catalyst in the presence of CD had
no particular effect on the Pd particles (Figs. 5(a, b)) com-
pared to the as-prepared Pd/PDA.
Entry Pre-treatment
Additive
Conversion (%) Ee (%, S)
1
2
3
4
5
6
7
A
B
–
A
B
–
–
–
–
100
100
100
95
97
100
53
–
–
–
12
32
19
33
When TA was added to the reaction mixture, stronger
aggregation was observed, regardless of the pre-treatment
(Figs. 5(c–e)), however, it was only slightly affecting the
conversion, which was virtually complete in each case
(Table II, entries 4–6). Particle sizes in the range of
6–8 nm, regardless of their shape, were previously found
to be ideal in this transformation.46ꢀ58 On the other hand,
the pre-treatment was influencing the obtained enantiose-
lectivity of the reaction. While no pre-treatment or stirring
the catalyst in toluene resulted in 12% and 19% ee, respec-
tively, pre-hydrogenation was more beneficial, as an ee of
32% was obtained in this case.
CD (0.05 mmol)
CD (0.05 mmol)
CD (0.05 mmol)
CD (0.05 mmol)
BA (1 mmol)
CD (0.05 mmol)
BA (1 mmol)
CD (0.05 mmol)
BA (1 mmol)
–
8
9
A
B
39
11
27
34
Notes: A: Stirring the catalyst in toluene for 30 min prior to adding the substrate
and additives. B: Pre-hydrogenation of the catalyst (50 bar H2ꢂ in toluene prior to
adding the substrate and additives.
As it has been reported in the literature that the pres-
ence of benzylamine (BA) is beneficial in enantioselective
hydrogenations over CD modified Pd catalyst,48–51ꢀ59ꢀ60
we tested the effect of the added amine (Table II,
entries 2–9). Interestingly, in this case, already the pre-
treatments showed pronounced differences in the catalyst
(Table II, entry 3). Compared to the pre-treated cata-
lyst, post-reaction imaging revealed stronger aggregation
of the active metal upon contact with TA (Figs. 4(c–e)),
although TA itself had no pronounced effect on the catalyst
(Fig. 4(f)).
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Figure 6. TEM images of Pd/PDA catalysts after different pre-treatment and catalytic enantioselective hydrogenation conditions of TA in the presence
of CD and BA. Effect of pre-treatment on the aggregation of the Pd particles: (a) After stirring the catalyst, CD (0.05 mmol) and BA (1 mmol)
in toluene for 30 min and (b) after pre-hydrogenation the mixture of the catalyst, CD (0.05 mmol) and BA (1 mmol) (50 bar H2ꢂ in toluene for
30 min. TEM images of differently pre-treated catalysts following the enantioselective hydrogenation of TA in the presence of CD and BA: (c) No
pre-treatment, (d) stirring the catalyst, CD (0.05 mmol) and BA (1 mmol) in toluene for 30 min and (e) pre-hydrogenation (50 bar H2ꢂ of the catalyst,
CD (0.5 mmol) and BA (1 mmol) in toluene for 30 min.
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Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
Gazdag et al.
Figure 7. The effect of BA under different conditions on the catalyst morphology. (a) After stirring the catalyst in the presence of BA in toluene at
rt for 30 min and (b) after stirring the catalyst in the presence of BA and H2 (50 bar) in toluene at rt for 30 min.
morphology (Figs. 6(a, b)). When the Pd/PDA was stirred
in toluene for 30 min in the presence of both CD and
BA, no extensive aggregation was observed. However,
when reductive pretreatment was applied (50 bar H2ꢂ TEM
imaging of the catalyst showed stronger aggregation of the
Pd particles. Such H2-induced aggregation of Pd nanopar-
ticles has been mentioned in the literature.59ꢀ61ꢀ62 To fur-
ther support the effect of reductive pre-treatment on the
catalyst morphology, we compared the effect of BA with
on the hydrogenation reaction. This is also supported by
literature data where decreased conversions and increased
ees were reported with increasing Pd particle sizes in the
catalytic enantioselective hydrogenation of TA over CD
modified Pd nanocube catalysts.58
4. CONCLUSIONS
In summary, we have shown that both transfer hydrogena-
tion and catalytic hydrogenation type reduction processes
and without H atmosphere (Figs. 7(a, b)). The images
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2
over polydopamine supported Pd catalyst are sensitive to
Copyright: American Scientific Publishers
reveal the large impact of the H2 atmosphere on the metal
the aggregation of Pd. In both cases decreased catalytic
activity was associated with pronounced Pd particle aggre-
gation. For transfer hydrogenation, the aggregation was
found to be dependent on the nature of the H-source, for
catalytic hydrogenation of a C C double bond the H2
atmosphere in combination with the substrate and different
additives (CD, BA) played a crucial role. In the latter case
the contribution of H2 induced structural changes of PDA
to particle aggregation cannot be ruled out.
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aggregation in the presence with BA. However, it has to be
noted that the reductive atmosphere in the presence of CD
influenced Pd aggregation much less (Fig. 5(b)). It could
be due to the adsorption of CD on the surface of Pd which
hinders the restructuring of the metal.
The pre-treatment in the case of the CD + BA system
strongly influenced the outcome of the catalytic hydro-
genation also (Table II, entries 2–9). The reduction of TA
in the presence of CD and BA, when no catalyst pre-
treatment was applied, resulted in a moderate 53% conver-
sion and 33% ee. When the pre-treatment involved stirring
the catalyst in toluene, a decreased conversion of 39% and
ee of 27% was obtained. Strikingly, when reductive pre-
treatment was applied, the conversion was diminished to
11% and an ee of 34% was obtained. Additionally, all
these decreased conversions were associated with consid-
erable Pd aggregation (Figs. 6(c–e)). Although the poison-
ing effect of BA on the catalytic hydrogenation of TA has
been considered in the literature,59 furthermore the reac-
tion rate lowering effect of the CD has been described;47
the observed decreasing tendency of the conversion cannot
be solely attributed to these effect. In all three reactions
both TA, CD and BA are present and they only differ in
the pre-treatment method. Thus, it is suggested that these
features together with the contribution of the reagents to
strong Pd aggregation, which is also facilitated by the
reductive pre-treatment59 has an overall hindering effect
Acknowledgments: Financial support by the János
Bolyai Research Scholarship of the Hungarian Academy of
Sciences (AS, GL), the ÚNKP-ÚNKP-16-4 New National
Excellence Program of the Ministry of Human Capaci-
ties (AS) and the National Research, Development and
Innovation Office, Hungary (NKFIH OTKA Grants PD
120877 (AS), PD 115436 (GL) and K 109278 (KS)) is
gratefully acknowledged. This collaborative research was
partially supported by the “Széchenyi 2020” program in
the framework of GINOP-2.3.2-15-2016-00013 “Intelli-
gent materials based on functional surfaces—from syn-
theses to applications” project and the NKFIH (OTKA)
K112531 grant. The Chemical Biology Research Group
and the Organocatalysis Research Group (both at the Insti-
tute of Organic Chemistry Research Centre for Natural
Sciences, Hungarian Academy of Sciences) are acknowl-
edged for support.
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Gazdag et al.
Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles
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Received: 6 September 2017. Accepted: 9 November 2017.
J. Nanosci. Nanotechnol. 19, 484–491, 2019
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