Striking dual functionality of a novel Pd?Eu-MOF nanocatalyst in C(sp2)-C(sp2) bond-forming and CO2fixation reactions

Article abstract of DOI:10.1039/d0dt00770f

Pd nanoparticles were immobilized on a highly porous, hydrothermally stable Eu-MOFviasolution impregnation and H₂reduction to yield a novel Pd?Eu-MOF nanocatalyst. This composite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS). Unprecedentedly, the Pd?Eu-MOF nanocatalyst could be applied with excellent results in two strikingly different, mechanistically distinct, reactionsi.e., Suzuki-Miyaura cross-coupling and cycloaddition of CO₂to a range of epoxides. Under the best reaction conditions, 98-99% yields have been attained in both catalytic processes. Moreover, in either case the heterogeneous catalyst was easily recovered and efficiently reused for more than four cycles, indicating its high stability and reproducibility. PXRD, TEM and XPS measurements on the recycled catalyst confirmed that it maintained its original structure and morphology; no Pd NP agglomeration was observed.

Full text of DOI:10.1039/d0dt00770f

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DOI: 10.1039/D0DT00770F
ARTICLE
Striking dual functionality of the novel Pd@Eu-MOF nanocatalyst
2
2
in C(sp )-C(sp ) bond-forming and CO fixation reactions
2
a
a
a
a
b
b
Li-Xin You, Shan-Xin Yao, Bai-Bei Zhao, Gang Xiong, Ileana Dragutan, Valerian Dragutan,* Xue-
Received 00th January 20xx,
Accepted 00th January 20xx
c
a
a
Gui Liu, Fu Ding and Ya-Guang Sun*
DOI: 10.1039/x0xx00000x
Pd nanoparticles were immobilized on a highly porous, hydrothermally stable Eu-MOF via solution impregnation
and H reduction to yield a novel Pd@Eu-MOF nanocatalyst. This composite was characterized by scanning electron
2
microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), inductive
coupled plasma optical emission spectroscopy (ICP), powder X-ray diffraction (PXRD) and X-ray photoelectron
spectroscopy (XPS). Unprecedentedly, the Pd@Eu-MOF nanocatalyst could be applied with excellent results in two
strikingly different, mechanistically distinct, reactions i.e., Suzuki-Miyaura cross-coupling and cycloaddition of CO
2
to a range epoxides. Under best reaction conditions, 98-99% yields have been attained in both catalytic processes.
Moreover, in either case the heterogeneous catalyst was easily recovered and efficiently reused for more than four
cycles indicating its high stability and reproducible activity. PXRD, TEM and XPS measurements on the recycled
catalyst confirmed that it has maintained its original structure and morphology; no Pd NPs agglomeration was
evidenced.
greatly limited in MOFs, so the stability and activity of the
9
a-d
9e-h
Introduction
Metal organic frameworks (MOFs), also known as porous
coordination polymers, are well-ordered crystalline materials
assembled from metal nodes and organic linkers. MOFs have
attracted a substantial interest in recent years due not only to
their easy synthesis and versatility in structure, but also to their
catalyst are preserved. Recently, our group
and others
have successfully applied the Pd-based MOFs in Suzuki-Miyaura
cross-coupling reaction of aryl halides with arylboronic acids, as
one of the most widely employed methods for the construction
of C(sp )-C(sp ) bonds because of its broad substrate scope,
high level of functional group tolerance and high turnover
1
2
2
9
i,j
rates.
Development of recoverable and recyclable
2
application profile in different fields such as energy storage,
heterogeneous catalysts is still a serious challenge, when
compared with the homogeneous palladium catalysts. In
addition, the chemical fixation and conversion of carbon dioxide
3
4
5
gas adsorption and separation, luminescence, magnetism
and catalysis.6 and even integration on protective layers
a-c
6
d
against dangerous agents.
(
CO
attention in last decades since CO
inexpensive C1 building block from the green chemistry and
2
) to structurally diverse compounds has received much
MOFs have been directly used in the realm of
heterogeneous catalysis owing to their assets i.e. large specific
surface area, controllable pore size, easily accessible catalytic
2
is abundant, renewable and
1
0
atom economy viewpoints. Cyclic carbonates resulted from
the cycloaddition of CO and epoxides are considered as one of
7
sites, high stability, and recoverability. In addition, MOFs can
2
be used as novel supports to incorporate metal nanoparticles
the most significant types of targeted substances due to the
relatively high yields and broad applications in fine chemicals,
8
(
NPs). The NPs hosted inside MOF can act as active sites, while
the pores of the MOF stabilize the NPs size and impede their
leaching and growth during operation of the catalytic systems.
To be used as hosts, MOFs should be sufficiently robust to
withstand the reaction conditions. The migration of NPs is
1
1
pharmaceuticals and medication syntheses.
In this paper, Pd NPs have been loaded on the matrix of a
highly porous and hydrothermally stable Eu-MOF, namely
1
2a
{
[Eu
carboxy-1H-benzimidazole-2-yl)benzene ligand (H
impregnation method followed by H reduction was chosen to
2 3
(L) •(H
2
O)
2 2
•(DMF) ]•16H
2
O}
n
,
based on the 1,4-bis(5-
2
L), A solution
a. Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province,
2
Shenyang University of Chemical Technology, Shenyang 110142, China. E-mail:
sunyaguang@syuct.edu.cn.
Institute of Organic Chemistry, Romanian Academy, Bucharest, P. O. Box 35-108,
obtain a new Pd@Eu-MOF catalyst with Pd NPs sizes ranging
between 2-5 nm. Moreover, valorisation of this new catalyst in
recyclable Suzuki-Miyaura cross-coupling reactions between
aryl halides and phenylboronic acids, as well as in the chemical
b.
0
60023, Romania. E-mail: vdragutan@yahoo.com.
c.
Institute of Functional Molecules, Shenyang University of Chemical Technology,
Shenyang 110142, China
†
Electronic Supplementary Information (ESI) available: Representation of the PXRD fixation of CO
to epoxides yielding cyclic carbonates were
2
1
diagrams, EDS spectrum, H NMR data and GC-yield Standard Curves for the
products. See DOI:10.1039/x0xx00000x
studied.
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Journal Name
Synthesis of Pd@Eu-MOF The activated Eu-MOF (0.393 g, 0.2
mmol) and PdCl (0.035 g, 0.2 mmol), were suspended in
2
methanol (100.0 mL), and stirred for 24 h. The mixture was
Experimental
Synthesis of the catalyst
View Article Online
DOI: 10.1039/D0DT00770F
All starting materials and solvents were obtained from filtered and washed 3 times with fresh methanol (20.0 mL) and
commercial suppliers and used without further purification. The deionized water (20.0 mL). The solid (Pd/Eu-MOF) was dried
overall synthesis route of Pd@ Eu-MOF is shown in Scheme 1.
Synthesis of Eu-MOF Eu-MOF was obtained according to in a flow of H /N (10/40) mL min-1. The product was washed
under vacuum for 24 h at 45
o
C, and reduced at 60 C, for 12h,
o
2
2
literature by reacting H
2
L (0.0398 g, 1.0 mmol) with europium with methanol and dried for 24 h at 45
C to obtain the Pd@Eu-
o
nitrate (0.0446 g, 1.0 mmol) in a mixture of DMF (5.0 mL) and MOF catalyst.
1
2a
2
H O (5.0 mL) at 120 °C for 72 h. Yellow crystals were collected
by filtration, washed with water and dried in the air.
Scheme 1 Synthesis route of Pd@Eu-MOF.
o
Structural characterization of Pd@Eu-MOF. GC analysis
MPa. After heating at 80 C for 24 h, the reactor was cooled in
an ice bath, and excess CO was carefully vented off. The
2
Powder X-ray diffraction (PXRD) of the sample was acquired on
a BRUKER D8 ADVANCE X-ray diffractometer. N gas sorption
2
catalyst was separated by centrifugation, and the products
were analyzed by GC. For recyclability tests of Pd@Eu-MOF, the
catalyst was recovered by filtration and washed with hot
methanol after each cycle.
experiments were carried out on a V-Sorb 2800 TP volumetric
gas sorption instrument. X-ray photoelectron spectroscopy (XPS)
data were obtained on Axis Ultra DLD devices. Inductive
coupled plasma optical emission spectroscopy (ICP) was
accomplished on Thermo IRIS Advantage. Scanning
electronmicroscope (SEM) images were taken with a German
Results and Discussion
ZEISS-SUPRA55 high resolution field emission scanning electron Eu-MOF was readily synthesized by the solvothermal reaction
1
2a
microscope. Energy-dispersive X-ray spectroscopy (EDS) was according to the literature method. The PXRD patterns of the
1
2
achieved on a German ZEISS-SUPRA55 high resolution field resulted solid matched well the calculated and literature
emission scanning electron microscope with an Oxford-AztecX- patterns (Fig. 1). They revealed the crystalline properties of the
Max80 X-ray energy spectrometer. Transmission electron synthesized Eu-MOF to be properly utilized as a matrix for
microscopy (TEM) was measured by a TECNAI F30 imaging immobilization of Pd nanoparticles. The framework of Eu-MOF
1
was chemically stable under aqueous sodium hydroxide and
spectrometer; H NMR spectra were recorded on a Bruker
BioSpin GmbH AVANCE III 500 MHz spectrometer. Gas hydrochloric acid conditions (pH = 1-14) and in common organic
chromatography (GC) analyses were performed on an Agilent solvents, as confirmed by the PXRD pattern collected for the
Technologies 7890A gas chromatograph.
respective samples (Fig. S1, 2). During the preparation of
Pd@Eu-MOF there was no apparent loss of crystallinity and no
identifiable peaks attributable to metal NPs, indicating that the
Eu-MOF framework had maintained its structural integrity and
that only very small NPs had been present in the Eu-MOF.
Moreover, the stability of Pd@Eu-MOF was confirmed by PXRD
after the reactions (Fig. S3).
Catalytic activity tests
Suzuki-Miyaura coupling reaction A typical experimental
procedure for the Suzuki-Miyaura coupling reactions is as
follows. A mixture of aryl halide (1.0 mmol), boronic acid(1.2
mmol), base (2.0 mmol, the indicated species), solvent (6.0 mL,
as designated) and catalyst (the given amount) were placed in a
Schlenk tube and stirred in air, at the appropriate temperature
for the specified time (Tables 1, 2). After completion of the
reaction the mixture was extracted with ethyl acetate. The
product yield was determined by GC analysis using hexadecane
as the internal standard.
2
The porosity of Pd@Eu-MOF was investigated by N sorption
measurement at 77 K. As indicated in Fig. 2, Eu-MOF, Pd/Eu-
MOF and Pd@Eu-MOF show the typical type-I sorption
isotherms, which reveal their microporous properties (Fig. S4).
Pd@Eu-MOF displayed a slight decrease in the amount of
2
adsorbed N , as compared to Eu-MOF and Pd/Eu-MOF. In
addition, the Brunauer-Emmet-Teller (BET) surface area
Cycloaddition of CO
2
to epoxides yielding cyclic carbonates The
2
-1
reduced from 1361 to 706 m g after Pd nanoparticles had
been loaded (Table S1).
epoxide (34.5 mmol) and catalyst (50.0 mg) were introduced
into a stainless-steel autoclave. After sealing, the autoclave was
2
purged 3 times with CO . Then, the pressure was adjusted to 2.0
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0DT00770F
Fig. 4 SEM image (a) and elemental mappings of Pd@Eu-MOF shows the
presence of Eu, N, O, Pd and C atoms (b-f).
Fig. 1 PXRD patterns of different samples.
As observed in the high-resolution TEM image, the morphology
of Pd nanoparticles was spherical and completely dispersed and
deposited inside the Eu-MOF, the particle size of Pd nanoparticles
was found to be about 2-5 nm (Fig. 5a).
Fig. 5 TEM images of Pd@Eu-MOF before reaction (a), after Suzuki-Miyaura
2
coupling reaction(b) and cycloaddition of CO with epoxide (c).
2
Fig. 2 N adsorption/desorption isotherms of Eu-MOF, Pd/Eu-MOF and
Pd@Eu-MOF.
In our work pore sizes between 0.7-2.7 nm have been
evidenced for Pd@Eu-MOF (Fig. S4); this range is complying
The nanocomposite structure was investigated by EDS with the pore dimensions found for Eu-MOF( 19x 12 A2).
analysis, SEM and mapping images. The signals recorded in Therefore, and as shown in Scheme 1 and confirmed by TEM
energy-dispersive X-ray spectroscopic measurements (EDS) experiments (Fig. 5), we infer that the majority of Pd NPs are
identified the elements of Eu, Pd, Cl, C, N and O to be suitably located within the pores of Eu-MOF. These findings parallel
12a
1
2b
displayed in the Pd@Eu-MOF nanocatalyst (Fig. 3).
those recently obtained by W.-Y. Sun for Ptnanoparticlesinside
a Zr-MOF.
Noteworthy, the TEM images of the recovered Pd@Eu-MOF
after use in the Suzuki-Miyaura and CO
2
cycloaddition reaction
(Fig. 5b and 5c, respectively), indicated that no agglomeration
of the particles occurred and the morphology and size of the
particles have not changed much in the recovered catalyst.
These results proved that essentially no modification in the
structure of Pd@Eu-MOF took place during the reaction.
The XPS spectra of Pd@Eu-MOF determined the oxidation
states of Pd (Fig. 6). The peaks for binding energy of 334.97 and
3
40.22 eV demonstrated that Pd is present only in its reduced
0
form, Pd . In addition, the valence of Pd species is restored to
Pd after the Suzuki-Miyaura (peaks at 335.09 and 340.31) and
Fig. 3 EDS spectrum of Pd@Eu-MOF.
0
2
CO cycloaddition (peaks at 335.28 and 340.64) reactions,
Additionally, SEM along with elemental mapping evidenced the
adequate distribution of the constituent elements in the catalyst.
Elemental maps indicated that Pd NPs were prepared in a regular and
uniform manner, evidencing the presence of Eu, Pd, C, N and O in the
respective samples (Fig. 4).
respectively. ICP-AES was measured on IRIS Advantage; after
calculation and analysis, the mass percentage of the Pd element
in the catalyst Pd@Eu-MOF was 0.61 wt%.
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reaction temperature and time affected notably the reaction
View Article Online
DOI: 10.1039/D0DT00770F
yield (Entry 9-14). In conclusion, the optimum catalytic
conditions for the Suzuki-Miyaura cross-coupling reaction of
bromobenzene and phenylboronic acid are as follows:
anhydrous ethanol as solvent, potassium carbonate as base,
o
catalyst loading 11.0 mg, reaction time 6 h, temperature 80 C.
On carrying out the cross-coupling of bromobenzene with
phenylboronic acid, under the above optimum reaction conditions
but using Pd@Eu-MOF, Pd/Eu-MOF and Eu-MOF as different
promoters, quite interesting data were recorded (Table 2).
Table 2 Catalytic activity of Pd@Eu-MOF, Pd/Eu-MOF and Eu-MOF in Suzuki-
Miyaura cross-coupling of bromobenzene with phenylboronic acid.
Catalyst
Br
+
B(OH)2
Base, Solvent
Entry
Halide (R
Pd@Eu-MOF
Pd/Eu-MOF
1
/X)
Yield (%)
>99
1
2
81
Fig. 6 XPS spectra of Pd@Eu-MOF before reaction(a); Pd 2p before reaction
Eu-MOF
3
<5
(
b), after Suzuki-Miyaura coupling reaction(c) and cycloaddition of CO
2
with Reaction conditions: aryl halide (1.0 mmol), arylboronicacid (1.2 mmol),
o
epoxide (d).
2
K CO
3
(2.0 mmol), EtOH (6.0 mL), catalyst (11.0 mg), temperature (80 C),
time (6 h).
Table
1
Optimization of Suzuki-Miyaura cross-coupling of
bromobenzene with phenylboronic acid in presence of Pd@Eu-MOF.
The higher yield obtained with Pd@Eu-MOF (Table 2, Entry 1) could
be rationalized if we take into account that the major part of Pd NPs
is distributed inside the pores of Eu-MOF, the latter conferring, under
such a protective confinement, superior activity and stability to the
catalytic sites, preventing the Pd NPs aggregation and Pd leaching. It
is noteworthy that the high yield in biaryls (>99% ), attained in this
work with Pd@Eu-MOF, is comparable to that reported recently by
us in the Suzuki-Miyaura cross-coupling reaction using the
Pd@Eu-MOF
Br
+
B(OH)2
Base, Solvent
Time
h)
Temperature
( C)
Entry Solvent
Base
Yield
o
(
1
2
3
4
5
6
7
DMF
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
6
6
6
6
6
6
6
80
80
80
80
80
80
80
36
<5
56
99
36
16
56
Toluene
2
H O
3 4
nanostructured catalyst Fe O
@La-MOF-Schiff base-Pd8e as well as in
EtOH
EtOH
EtOH
EtOH
3
cross-couplings induced by Ln/Pd MOF catalysts 9a-d, 13b
Cs
CO
2 3
Table 3 Suzuki-Miyaura cross-coupling of various aryl halides with
NaOH
Et
arylboronic acids.a
3
N
NaOC
t
4 9
H -
Pd@Eu-MOF
8
EtOH
6
80
74
R1
X + R2
B(OH)2
R1
R2
2 3
K CO , EtOH
9
1
1
1
1
1
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
K
K
K
K
K
K
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
6
6
6
5
4
2
70
60
50
80
80
80
89
50
32
92
86
70
Entry
Halide (R /X)
Arylboronic acid (R2)
Yield (%)
99
1
1
2
H/I
H
H
0
1
2
3
4
3
3
3
3
3
H/Br
99
3
4
5
6
CH
3
/Br
H
H
90
99
87
99
COCH
H/Br
H/Br
3
/Br
COCH
3
CH
3
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid
7
8b
COCH
COCH
3
/Cl
/Cl
H
H
14
39
(
1.2 mmol), Pd@Eu-MOF (11.0 mg), base (2.0 mmol), solvent (6.0 mL).
3
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.2
Optimization of Suzuki-Miyaura cross-coupling reaction with
Pd@Eu-MOF has been performed employing different bases
and solvents (Table 1). It can be observed that the best
combination leading to highest yield (>99%) was to use K
as the base and anhydrous ethanol as the solvent (Entry 4).
When N,N-dimethylformamide (DMF), toluene and H O were
used as a solvent instead of ethanol (Entry 1, 2, 3) or Cs CO
NaOH, sodium t-butoxide and triethylamine (Et N) as a base
Entry 5-8), the yield dropped significantly. Furthermore,
mmol), K CO (2.0 mmol), EtOH (6 mL), Pd@Eu-MOF (11.0 mg),
2
3
o
b
temperature (80 C), time (6 h). Reaction conditions: reaction time: 12
h.
2 3
CO
Significantly, the results obtained on the effect of
substituents of arylhalides (Table 3) are in full agreement with
the well-known data in this regard. The bromide and iodide
derivatives react easier with phenylboronic acid to give higher
yields (Entry 1-4). As expected, the yield of the chloride
derivative was low (Entry 7), and only when the reaction time
2
2
3
,
3
(
4
| J. Name., 2012, 00, 1-3
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ARTICLE
was prolonged to 12 hours, the yield increased to 39% (Entry 8). recycling of the Pd@Eu-MOF catalyst in SuVziuewkiA-rMticlieyOanulirnae
These results are rationale taking into account the high reaction are shown in Fig. 7. The catalyticDrOeIs: u10lt.1s0i3n9d/Dic0aDtTe0d0t7h70aFt
activation energy needed for the chlorine derivative as the Pd@Eu-MOF catalyst can be efficiently recycled showing
compared to readily ionisable iodine and bromine counterparts, almost no loss in activity for at least four successive runs.
the last two involving a more electron-rich anionic [Pd(0)-X]-
A second catalytic ability of our Pd@Eu-MOF was also
species in the rate-determining oxidative addition step of the investigated, namely the technically and environmentally important
1
3a
14a,b,c
palladium catalytic cycle (Scheme 2).
CO
2
capture and conversion into value-added products
such as
the cyclic carbonates. Coupling with epoxides to yield cyclic
carbonates (production: 40,000 tons/year) is in fact the major
R X
R R'
1
5
industrial catalytic utilization of CO
2
.
2
Chemical fixation of CO by
Oxidative Addition
Reductive Elimination
Pd0
cycloaddition to an epoxide proceeds through new C-O bond
formation and is most frequently carried out heterogeneously with a
MOF-supported metal catalyst. CO capture by the epoxide is
2
Base
R X
(
X = I, Br, Cl)
affected by its interaction with the MOF adsorbent. A prerequisite
for successful catalysis is the occurrence on the MOF of densely
dispersed Lewis acid and Lewis base sites (even as ILs, e.g.
PolyILs@MIL-101), as well as the presence of a co-catalyst, all of
which playing essential roles in the different reaction stages.
Palladium, though since long applied in this reaction17 is not
II
II
Y
B
Y
R-Pd -R' Pd@Eu-MOF R-Pd -X
OH
OH
OH
1
6
Base
Transmetalation
II
Boron Substituent
R'-Pd -OH
R'
X
often encountered among the host of metals used as catalysts.
Y
B
Y
OH
Moreover, Pd nanoparticles (supported on mesoporous TiO
Pd@MTiO ) have only recently been communicated as catalysts in
cyclic carbonate synthesis.
2
,
B
R'
OH
Y
Y
2
1
8
Scheme 2 Palladium catalytic cycle for C-C cross-coupling with Pd@Eu-MOF.
In our case, the catalyst consists of Pd nanoparticles
embedded in a microporous Eu-based MOF. To establish
optimum reaction conditions with this new heterogeneous
Tests with electron-donating (4-Me)-substituents (Entry 6)
and electron-withdrawing substituents (4-COMe) (Entry 5) at
the phenylboronic acid gave higher yields with 4-Me (> 99%)
than with 4-COMe (87%), indicating that the catalyst is more
prone to catalyze the reaction of phenylboronic acid with an
electron donating substituent. This outcome is understandable
if we consider the activating effect of the electron donating
group on the ligand exchange rate within the boron complex
promoter, the solvent-free CO
was selected as a model. As mandatory, a co-catalyst (n-Bu
was also used. It was proved that Pd@Eu-MOF can convert CO
2
cycloaddition to epichlorohydrin
4
NBr)
2
and epichlorohydrin to chloropropene carbonate in almost
o
quantitative yields (98%), at 80 C, 2.0 MPa CO
2
and 24 h
reaction time, in the absence of any solvent (Table 4, entry 1).
As indicated by previous research on cyclic carbonates synthesis
using different catalysts, conversion is favoured by a higher
operation temperature and pressure, the latter ensuring an
increased solubility of CO2;19 in comparison to these data, a
during the transmetalation step of the pallacycle
mechanism.13b-f
9
9
99
9
6
95
8
3
more advantageous molar ratio epoxide: catalyst (and
epoxide:co-catalyst) could be established in this work.
Table 4 Synthesis of cyclic carbonates under different catalysis
conditions.
O
O
Pd@Eu-MOF, n-Bu NBr
4
O
Cl
Entry
o
2
MPa CO , 80 C
O
2
Cl
Catalyst
Cocatalyst (mg)
Yield (%)
98
1
2
3
4
5
1
2
Pd@Eu-MOF
Eu-MOF
100
100
Cycle
38
Fig. 7 Recycling of Pd@Eu-MOF in Suzuki-Miyaura cross-coupling
3
4
Pd/Eu-MOF
-
100
100
42
46
reaction.
Reaction conditions: epichlorohydrin (34.5 mmol), catalyst (50.0 mg),
The recyclability of the heterogeneous catalyst is the most
important advantage for industrial application. Catalyst
recycling in Suzuki-Miyaura cross-coupling reaction of
bromobenzene and phenylboronic acid was carried out under
the optimum reaction conditions. After each reaction cycle, the
catalyst was separated by centrifugation, washed with absolute
4 2
cocatalyst (n-Bu NBr, 100.0 mg), CO pressure (2.0 MPa), temperature
o
(
80 C), time (24 h). Yield was determined from GC analysis.
The reaction takes place with 46% conversion (Entry 4) even with
just the co-catalyst (tetrabutylammonium bromide, TBAB) thus giving
proof for the synergistic effect of the quaternary ammonium salt
o
ethanol, vacuumed at 50 C and dried for the next run. Data on
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J. Name., 2013, 00, 1-3 | 5
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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Journal Name
2
during CO cycloaddition (Scheme 3, path A). At the working
temperature TBAB (m. p. 103 C) is likely dissolved in the epoxide
View Article Online
o
To conclude, the greatly enhanced caDtaOlyI:t1ic0.1p0e3r9f/oDr0mDaTn0c0e770oFf
phase enhancing the CO
2
uptake. Control experiments using Eu-MOF Pd@Eu-MOF, vs. Pd/Eu-MOF, results from the superior Lewis acid
and Pd/Eu-MOF, i.e. the MOF precursors of our catalyst, were also activity of the evenly distributed Pd nanoparticles (vs. Pd2+ ions), thus
conducted leading to, not surprisingly, much poorer results (38% and engendering an amplified coordination of the epoxide in the initial
4
2%, respectively; entries 2 and 3). Europium is oxophilic yet, being step of Path B. An unorthodox parallelism of the catalytic
relatively sparsely distributed and less exposed on the MOF surface, performance in the distinct reactions under study, Suzuki-Miyaura
has lesser chances to attach the epoxide at the O atom; the same
goes for Pd ions in the Pd/Eu-MOF.
and cyclic carbonate synthesis (Table 2 and Table 4), shows the same
trend for a decreasing activity in the range Pd@Eu-MOF > Pd/Eu-
MOF > Eu-MOF, again revealing the positive role of Pd nanoparticle
vs. Pd ions when either is incorporated in our catalytic MOF system.
NH4
R
R
Br
R
The catalytic capacity and substrate selectivity of Pd@Eu-
MOF were further explored using variously substituted
epoxides to synthesize the corresponding cyclic carbonates,
under the same reaction conditions. The size selectivity was
evident when comparing the yields obtained from substrates
with different substituents. The highest catalytic activity was
observed when the substrate was phenyloxirane (yield of 90%;
R = Ph, Entry 1, Table 5) or epichlorohydrin (yield 98%; R = ClCH ,
2
Entry 1, Table 4). Substituent effects, both steric and electronic,
are therefore at play: bulkier substituents sterically hinder
O
O
Br
O
Br
R
R
=
Pd@Eu-MOF
O
I
O
O
NH4- Br
O
R
O
Br
Br
R
O
O
C
O
O
O
CO2
Scheme 3 Proposed mechanistic pathway for CO
2
insertion in the access of the epoxide to the catalytic centers whereas electron-
withdrawing substituents facilitate nucleophilic attack of the
epoxide (Path A).
base during ring opening of the epoxide. Therefore, CO
To explain our results with Pd@Eu-MOF in the coupling reaction insertion is sensitive to the geometric and electronic
2
2
and epichlorohydrin we propose a two-pathway mechanism
environments of the epoxide, that govern the access of the
substrate to the NP active sites inside the MOF, a conclusion
shared by all previous reports on yield variation as a function of
2
0
2
1
the epoxide R substituent.
-
4
NBr) attacks the less-
hindered C-atom of the now coordinated epoxide opening the epoxy
ring and generating an O-anion [I]; the latter is subsequently
Table 5 Epoxide substrate in CO
2
fixation reactions catalyzed by Pd@Eu-
MOF.
attacked by CO
2
to yield a carbonate anion that ring-closes in the final
O
step (Scheme 3).
O
Pd@Eu-MOF, nBu NBr
4
O
The cooperative mechanism further implies participation of the
intermediate [I] in a second reaction cycle (Path B) where it
o
2
MPa CO , 24 h, 80 C
O
R
2
R
o
coordinates to Pd nanoparticles (from Pd@Eu-MOF) yielding an
Entry
Substrate
Product
Yield (%)
90
anionic species able to bind two CO
2
molecules. On demand of
O
-
O
ammonium cations, this species expels Br generating two cyclic
carbonate molecules coordinated to palladium; ultimately, the
reaction cycle B is closed by release of the cyclic carbonate in the
reaction medium and restoration of the catalyst (Scheme 4).
O
1
O
O
O
Br
R
2
3
4
O
57
59
34
R
O
2
O
2
O
O
O
O
O
O
O
I
R
R
Br
Br
O
O
Pd
O
O
R
R
O
O
O
O
O
O
O
O
Pd
=
Pd@Eu-MOF
CO2
Reaction conditions: epoxide (34.5 mmol), Pd@Eu-MOF (50.0 mg),
2
Br NH4
R
R
O
cocatalyst (n-Bu NBr, 100.0 mg), CO pressure (2.0 MPa), temperature
4
2
Br
Br
o
O
(80 C), time (24 h). Yield was determined from GC analysis.
O
O
O
O
Scheme 4. Role of Pd nanoparticles in cyclic carbonates synthesis (Path
2
In this work insertion of CO into epoxides was conducted as a
B).
solvent-free process which is an asset for a facile catalyst recovering
6
| J. Name., 2012, 00, 1-3
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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ARTICLE
and recycling. After completion of the reaction the Pd@Eu-MOF
catalyst could be efficiently separated from the mixture by
centrifugation and reused in subsequent experiments showing no
reduction of activity during the first 5 runs and just a slight decrease
Conflicts of interest
There are no conflicts of interest to declare.
View Article Online
DOI: 10.1039/D0DT00770F
(
to 93-94%) after the next two cycles (Fig. 8); therefore leaching of
Acknowledgements
the active catalytic species was negligible.
This work was supported by the National Natural Science
Foundation of China (21671139), the Distinguished Professor
Project of Liaoning province (2013204), the National Key
9
8
98
98
97
97
9
4
93
Research
and
Development
Program
of
China
(2018YFD0200102), the Scientific Research Project of Education
Department of Liaoning Province (LDF2017001), the Program
for Liaoning Innovative Research Team in University
(
LT2018012) and the Science and Technology Innovation
Program for Middle-aged and Youth Talents of Shenyang
RC180086). ID and VD acknowledge support from the Institute
(
of Organic Chemistry of the Romanian Academy.
1
2
3
4
5
6
7
Cycle
Notes and references
2
Fig. 8 Recycling of the Pd@Eu-MOF in the cycloaddition of CO to
epichlorohydrin.
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DOI: 10.1039/D0DT00770F
Graphical Abstract
Pd@Eu-MOF nanocatalyst was obtained via solution impregnation and H reduction and used in
2
recyclable Suzuki-Miyaura reactions and chemical fixation of CO to epoxides.
2