Atmospheric hydrogenation of Α Β-unsaturated ketones catalyzed by highly efficient and recyclable Pd nanocatalyst

Article abstract of DOI:10.1016/j.catcom.2019.03.008

A thermoregulated phase-transfer Pd nanocatalyst was explored firstly and shown to be highly efficient and recyclable in the atmospheric hydrogenation of α β-unsaturated ketones. Under optimized reaction conditions, the conversion of chalcone and the selectivity of dihydrochalcone were 99% and 98%, respectively. The catalyst can be easily separated from the product and used directly for four times without evident loss in activity and selectivity. The turnover frequency (TOF) for the atmospheric hydrogenation of chalcone was 870 h^?1, which to the best of our knowledge was the highest value ever reported among transition metal nanocatalysts.

Full text of DOI:10.1016/j.catcom.2019.03.008

CatalysisCommunications125(2019)10–14
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Short communication
Atmospheric hydrogenation of α, β-unsaturated ketones catalyzed by highly
efficient and recyclable Pd nanocatalyst
Pu Chen, Wenjiang Li, Yanhua Wang⁎
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A thermoregulated phase-transfer Pd nanocatalyst was explored firstly and shown to be highly efficient and
recyclable in the atmospheric hydrogenation of α, β-unsaturated ketones. Under optimized reaction conditions,
the conversion of chalcone and the selectivity of dihydrochalcone were 99% and 98%, respectively. The catalyst
can be easily separated from the product and used directly for four times without evident loss in activity and
selectivity. The turnover frequency (TOF) for the atmospheric hydrogenation of chalcone was 870 h−1, which to
the best of our knowledge was the highest value ever reported among transition metal nanocatalysts.
Atmospheric H2 pressure
Thermoregulated phase-transfer catalysis
Pd nanocatalyst
Selective hydrogenation
α, β-unsaturated ketones
1. Introduction
the hydrogenation of chalcone, affording the C]C bond hydrogenation
product in lower yields, and the TOF was only 23.8 h−1 and 73.8 h−1
,
Selective hydrogenation of α, β-unsaturated ketones is an important
catalytic process in organic synthesis and industrial production [1–4].
And the selective hydrogenation of the C]C bond functional group is a
significant method for the production of saturated ketones, which are
key intermediates for synthesizing biologically active compounds such
as HIV-I protease inhibitors [5,6]. In recent years, although non-noble
transition metal like copper was investigated as an economical catalyst
alternative [7], much more studies have still used different noble
transition metal catalysts for this kind reaction, such as Ru [8,9], Pt
[10,11], Ir [12,13], Pt/Re [14], Au [15,16], and Pd [4,17–20]. How-
ever, most experiments were conducted under high H2 pressure in order
to obtain high catalytic activity. Disappointingly, till now only a few
attempts for this catalytic process were carried out under atmospheric
H2 pressure [4,18–20]. For example, J. E. Bäckvall and co-workers used
supported Pd nanoparticles as catalysts for the selective hydrogenation
of the C]C bond of α, β-unsaturated carbonyl compounds at 1 atm H2
pressure, the TOF was 200 h−1 for benzylideneacetone [18], and one of
the catalysts has also been used for mild hydrogenation of α, β-un-
saturated carbonyl compounds in flow [21]. Ganji et al. reported the
selective hydrogenation of chalcone at 1 atm H2 pressure with im-
mobilized Pd nanoparticle catalyst on functionalized SBA-15, and the
TOF value was 27 h−1 [4]. Morimoto et al. applied Pd-supported gra-
phene oxide as catalyst precursor for the selective hydrogenation of
chalcone at 1 atm H2 pressure. But the reaction went on for 24 h and the
TOF value was 42 h−1 [19]. In addition, Wei et al. found that the oxide-
supported Pd nanoparticles (Pd/TiO2, Pd/MgO) were not effective for
respectively [20]. Therefore, it is still a challenge to further improve the
TOF of α, β-unsaturated ketones atmospheric hydrogenation.
Soluble transition metal nanocatalysts have received much attention
in several areas of science and industry [22,23]. In our previous work, a
thermoregulated phase-transfer nanoparticle catalysis based on the
cloud point (Cp) of the thermoregulated ligands (Ph2P(CH2CH2O)nCH3,
n = 16 or 22) was developed [24]. The nanocatalysts (Rh, Pt, Ir, etc.)
stabilized by the thermoregulated ligand were prepared, and accom-
plished the thermoregulated phase-transfer process in an aqueous/al-
coholic biphasic system. The character of this system can be depicted as
follows (Fig. S1): the system was composed of the upper organic phase
and lower water phase. Before reaction, the nanocatalyst was in the
water phase. Afterward, when the system was heated gradually to a
higher temperature than the Cp, the nanocatalyst would transfer into
the upper organic phase, where the reaction proceeded homogeneously.
At the end of the reaction, the nanocatalyst would return to the water
phase when the temperature dropped to room temperature. Therefore,
the nanocatalyst could be separated from products easily and reused
directly. Until now, the thermoregulated phase-transfer nanocatalysts
have been used for hydroformylation of olefins catalyzed by Rh nano-
catalyst [24], selective hydrogenation of quinolines/isoquinolines cat-
alyzed by Pt nanocatalyst and selective hydrogenation of α, β-un-
saturated aldehydes/ketones catalyzed by Ir nanocatalyst with high
activity, selectivity and reusability [25–29]. However, the application
of thermoregulated phase-transfer Pd nanocatalyst has not been stu-
died.
⁎ Corresponding author.
E-mail address: yhuawang@dlut.edu.cn (Y. Wang).
https://doi.org/10.1016/j.catcom.2019.03.008
Received 15 December 2018; Received in revised form 18 February 2019; Accepted 7 March 2019
Availableonline13March2019
1566-7367/©2019ElsevierB.V.Allrightsreserved.

P. Chen, et al.
CatalysisCommunications125(2019)10–14
Fig. 1. Atmospheric hydrogenation of α, β-un-
saturated ketones using Pdnano as catalyst.
Fig. 2. TEM images and particle size histograms of Pd nanoparticle catalyst (A) freshly prepared and (B) after three cycles in the reusability experiments.
Considering that only a few hydrogenations of α, β-unsaturated
ketones were conducted at atmospheric H2 pressure, and the activity
was very low [4,19,20]. Therefore, in this paper, the thermoregulated
phase-transfer Pd nanocatalyst was explored firstly for the atmospheric
hydrogenation of α, β-unsaturated ketones (Fig.1) with the aim to
achieve high activity, selectivity and recyclability of catalyst.
on a Tianmei 7890 GC equipped with a 50 m OV-101 column and an
FID detector. GC–MS measurement was performed on a HP 6890 GC/
5973 MSD instrument.
2.2. Preparation of the thermoregulated phase-transfer Pd nanocatalyst
The aqueous solution of Na2PdCl4·xH2O (4 mL, containing
2.82 × 10−6 mol of Pd), thermoregulated ligand (6.70 mg,
5.64 × 10−6 mol), 1-pentanol (4 mL) were added into a 75 mL teflon-
lined stainless-steel autoclave, which was stirred under hydrogen
(4 MPa) at 110 °C for 5 h. Then the autoclave was cooled to room
temperature and depressurized. The color of the aqueous phase
changed from colorless to brown, indicating the formation of Pd na-
nocatalyst.
2. Experimental
2.1. Materials and analyses
Sodium tetrachloropalladate (II) hydrate (Na2PdCl4·xH2O, 99.95%,
metals basis, Pd 30%), chalcone and other substrates were purchased
from Alfa Aesar. 1-Pentanol and n-decane were purchased from Kermel.
All these chemical agents were analytical reagents. Thermoregulated
ligand Ph2P(CH2CH2O)nCH3 (n = 22) was prepared according to the
reported literature [30]. The UV–vis instrument was a Ruili double
beam UV–vis spectrophotometer. TEM analyses were performed on a
Philips Tecnai G2 20 transmission electron microscope operating at
200 kV. Inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analyses of metal element were carried out on Optima 2000DV
(Perkin Elmer, USA). Gas chromatography analyses were accomplished
2.3. The atmospheric hydrogenation of α, β-unsaturated ketones
The selective hydrogenation of chalcone was used as
a re-
presentative: The Pd nanocatalyst, 1-pentanol, n-decane (internal
standard), and chalcone were added into a 75 mL teflon-lined standard
stainless steel autoclave, which was flushed three times with 1.0 MPa
H2, then inflated with H2 under atmospheric pressure (balloon) and
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CatalysisCommunications125(2019)10–14
Table 1
Atmospheric hydrogenation of chalcone using the as-prepared Pd nanocatalyst.a
f
Entry
Temperature (°C)
Time (min)
Conversion (%)b
Selectivity (%)b,c
TOF (h−1
)
1
2
3
4
5
6
7
8
50
60
70
80
90
90
90
90
90
90
90
90
90
90
90
90
30
45
60
75
30
30
55
66
73
86
99
35
39
47
69
40
87
> 99
> 99
> 99
98
183
220
243
287
330
350
260
235
276
400
870
98
> 99
> 99
> 99
> 99
> 99
99
9
10d
11e
a
Reaction conditions: 1-pentanol 4 mL, water 4 mL containing 2.8 × 10−3 mmol Pd, Ph2P(CH2CH2O)22CH3/Pd = 2 (molar ratio), 50 mg of n-decane as internal
standard, chalcone/Pd = 500 (molar ratio), balloon (H2).
b
Determined by GC and GC–MS.
Selectivity for the C]C bond. The main byproduct is 1, 3-diphenylpropan-1-ol.
Stirring the catalysts and solvents for 30 min under N2 atmosphere at 90 °C, then the substrate was added immediately.
Stirring the catalysts and solvents for 30 min under H2 atmosphere at 90 °C, then the substrate was added immediately. TOF: calculated as the moles of
c
d
e
f
converted chalcone per mol of Pd per hour based on the total Pd amount at the whole stage of reaction.
Table 2
Evaluation of Pd leaching in the reusability experiments.
Cycle number
1
1.4
2
1.0
3
0.3
4
Pd leaching (wt%a)
Nob
a
wt% was calculated using the following formula: wt% = (the amount of
leached Pd in organic phase)/(the total amount of Pd).
b
No: the leaching of Pd were lower than the detection limit (3 μg/L).
phase-transfer Pd nanocatalyst by using chalcone as a model substrate.
At first, the effect of the reaction temperature was investigated in the
range from 50 °C to 90 °C under atmospheric H2 pressure. As shown in
Table 1, when the temperature increased from 50 °C to 90 °C, the con-
version of chalcone increased evidently until the 99% conversion was
achieved at 90 °C (Table 1, entries 1–5). While the selectivity of dihy-
drochalcone almost unaffected by the temperature variation and re-
mained ≥98%. Subsequently, we studied the effect of the reaction time
at 90 °C (Table 1, entries 5–9). When the reaction time was 30 min, the
conversion of chalcone was 35%. And prolonging the reaction time to
60 min, the conversion was only increased to 47%. But the reaction rate
was obviously accelerated from 60 min to 90 min (Table 1, entries 5, 8,
9). It was clear from the results that the Pd nanocatalyst underwent an
induction period for the atmospheric hydrogenation of chalcone.
Hence, we investigated the effects of different pretreatment methods on
the induction period by carrying out the following operations: the
catalysts and solvents were stirred for 30 min under N2 atmosphere
(Table 1, entry 10) and H2 atmosphere (Table 1, entry 11) at 90 °C,
respectively. And then the substrate was added immediately stirring for
30 min under atmospheric H2 pressure. Compared with the experiment
without pretreatment, the result showed that the pretreatment under N2
atmosphere only had slight influence on the induction period (Table 1,
entry 6 vs. entry 10). However, the pretreatment under H2 atmosphere
can evidently shorten the induction period (Table 1, entry 11). The
reason of this phenomenon may be related to the formation of hydride
active species [32,33]. Under the optimized reaction conditions, the
TOF for chalcone atmospheric hydrogenation is up to 870 h−1 (Table 1,
entry 11), which is significantly higher than the TOF reported in the
literatures [4,19,20].
Fig. 3. The reusability of the thermoregulated phase-transfer Pd nanocatalyst in
the hydrogenation of chalcone.
held at the prearranged temperature with magnetic stirring for an ap-
propriate reaction time. After reaction, the reactor was cooled to room
temperature. The upper organic phase was carefully removed from the
lower water phase and analyzed by GC and GC–MS.
3. Results and discussion
We initially explored the characterization of the as-prepared Pd
nanocatalyst. The UV–vis absorption spectra of Na2PdCl4·xH2O and
thermoregulated ligand solution before and after reduction was dis-
played in Fig. S2. Obviously, the Pd (II) has a characteristic absorption
peak at 346 nm due to the charge-transfer between Pd ions and ther-
moregulated ligand. After reduction, the characteristic absorption peak
disappeared completely, which indicated the formation of Pd0 [31].
Additionally, the TEM analysis showed the formation of Pd nano-
particle catalyst with a mean diameter of 3.1
0.4 nm (Fig. 2A).
With the optimized reaction conditions in hand and considering the
Then, we explored the catalytic performance of the thermoregulated
12

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CatalysisCommunications125(2019)10–14
Table 3
The atmospheric hydrogenation of different α, β-unsaturated ketones.a
c
Entry
Substrate
Product
Time (min)
10
Conversion (%)b
> 99
Selectivity (%)b
> 99
TOF (h-1
)
1
2970
2
3
4
20
25
25
> 99
> 99
> 99
> 99
> 99
> 99
1485
1188
1188
5
210
> 99
> 99
141
a
Reaction conditions: 1-pentanol 4 mL, water 4 mL containing 2.8 × 10−3 mmol Pd, Ph2P(CH2CH2O)22CH3/Pd = 2 (molar ratio), 50 mg of n-decane as internal
standard, substrate/Pd = 500 (molar ratio), T = 90 °C, balloon (H2).
b
Determined by GC and GC–MS.
c
TOF: calculated as the moles of converted substrate per mol of Pd per hour based on the total Pd amount at the whole stage of reaction.
lifetime of the nanocatalyst, we turned our attention to investigate the
reusability of the Pd nanocatalyst. After reaction, the autoclave was
cooled and depressurized. The lower aqueous phase containing the Pd
nanocatalyst was separated and reused directly in the next reaction
cycle. The results in Fig. 3 showed that the Pd nanocatalyst can be used
for four times with the selectivity remained almost the same, while the
conversion of the fourth cycle decreased from 98% in the third cycle to
91%. To investigate the factors, which resulted in the decrease of the
reaction conversion, we firstly detected Pd leaching in the upper 1-
pentanol phase after reaction. The results were obtained through ICP-
AES and summarized in Table 2. As can be seen, with the increase of the
cycle number, the Pd leaching gradually decreased, and after the third
reaction cycle, the Pd leaching was lower than the detection limit (3 μg/
L). Then, the average particle size of the Pd nanocatalyst after three
cycles was detected. And the result showed that the particle size
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21373039, 21173031).
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