Palladium(II) Immobilized on Metal-Organic Frameworks for Catalytic Conversion of Carbon Dioxide to Formate

Article abstract of DOI:10.1021/acs.inorgchem.9b03654

In this work, we report the design of a two-dimensional (2D) isostructural metal-organic framework containing Pd(II) active sites, using a bipyridyl dicarboxylate linker (Mg(bpdc)(DMF)₂PdCl₂]_n (Pd?Mg:JMS-2) and [Mn(bpdc)(DMF)₂PdCl₂]_n(Pd?Mn:JMS-2)). The activated MOFs Pd?Mg:JMS-2a and Pd?Mn:JMS-2a were evaluated as heterogeneous catalysts for the hydrogenation of carbon dioxide (CO₂) to formate. Under optimal conditions, the MOFs exhibited impressive catalytic activity with formate turnover numbers of 7272 and 9808 for Pd?Mg:JMS-2a and Pd?Mn:JMS-2a, respectively, after 24 h. These catalysts exhibited higher catalytic activity when compared to its homogeneous counterpart that was used as a linker during MOF synthesis. Post-experimental characterization showed that the structural integrity of the MOFs was not altered after catalysis. This work demonstrates that the catalytic activity of homogeneous systems can be enhanced under heterogeneous conditions by anchoring them on MOFs.

Full text of DOI:10.1021/acs.inorgchem.9b03654

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Article
Palladium(II) Immobilized on Metal−Organic Frameworks for
Catalytic Conversion of Carbon Dioxide to Formate
Piwai Tshuma, Banothile C. E. Makhubela, Ndzondelelo Bingwa, and Gift Mehlana*
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ABSTRACT: In this work, we report the design of a two-
dimensional (2D) isostructural metal−organic framework containing
Pd(II) active sites, using a bipyridyl dicarboxylate linker (Mg(bpdc)-
(DMF)2 PdCl2 ]n (Pd@Mg:JMS-2) and [Mn(bpdc)-
(DMF)2PdCl2]n(Pd@Mn:JMS-2)). The activated MOFs Pd@
Mg:JMS-2a and Pd@Mn:JMS-2a were evaluated as heterogeneous
catalysts for the hydrogenation of carbon dioxide (CO2) to formate.
Under optimal conditions, the MOFs exhibited impressive catalytic
activity with formate turnover numbers of 7272 and 9808 for Pd@
Mg:JMS-2a and Pd@Mn:JMS-2a, respectively, after 24 h. These
catalysts exhibited higher catalytic activity when compared to its
homogeneous counterpart that was used as a linker during MOF
synthesis. Post-experimental characterization showed that the
structural integrity of the MOFs was not altered after catalysis. This work demonstrates that the catalytic activity of homogeneous
systems can be enhanced under heterogeneous conditions by anchoring them on MOFs.
reactor design, which reduce the costs for large-scale
production.13,15 However, the majority of heterogeneous
reactions are far from ideal, in that they require harsher
conditions than homogeneous catalysts.16,17 Hence, a potent
catalyst-design strategy that provides a pathway for both
enhanced catalytic activity under relatively mild conditions is
essential to develop industrially viable heterogeneous catalysts.
Metal organic frameworks (MOFs) are a class of crystalline
porous materials in which inorganic clusters are connected by
organic linker molecules.18 Because of their high porosity,
several studies have revealed that MOFs have remarkable H2
storage and CO2 capture ability.19−21 Recently, they have been
proven to be efficient catalysts in heterogeneous catalytic
reactions.22,23 Although some MOFs have been considered as
catalysts for CO2 conversion24,25 it is still necessary to
construct more effective MOFs, which can perform under
relatively mild conditions of temperature and pressure. Key to
this reaction design is the construction of catalytically active
MOF systems by encapsulation of homogeneous catalysts
inside MOF cavities during synthesis,26 functionalization of
linkers with catalytically active species,18,27 and utilization of
INTRODUCTION
■
Carbon dioxide (CO2) conversion to value-added products is
very important not only for the reduction of CO2 emissions,
which is a greenhouse gas and a major contributor to global
warming, but also for the efficient use of CO2 to achieve a
carbon neutral cycle.1−3 Among the several chemicals that can
be directly prepared from CO2, formic acid (FA) acid is widely
used as a silage agent, dyeing additive, and precursor for
synthesis.4 It also represents a convenient hydrogen storage
material, because of its relatively high H2 content (4.4 wt %),
low toxicity, and liquid state under ambient conditions.5 For
these reasons, the development of efficient catalysts for the
reduction of CO2 has been extensively investigated, yet still
remains a challenge, since CO2 is thermodynamically stable.6−8
Hydrogenation of CO2 to FA over homogeneous catalysts
has been extensively studied,8−11 and several catalyst design
approaches such as the use of proton responsive ligands7,12 and
second coordination sphere groups5 have been introduced to
develop efficient homogeneous catalysts. However, the
homogeneous route for CO2 reduction has shown some
disadvantages, such as difficulty in recycling of the precious-
metal catalyst and limited liquid phase solubility of H2. This
leads to significant mass-transfer limitations and restricts
regeneration of FA from the salts.13 Although base-metal
homogeneous catalysts have been introduced as less-expensive
alternatives to precious-metal-based catalysts, these typically
give low turnover numbers (TONs).14
Received: December 16, 2019
Alternatively, heterogeneous catalysts are preferable, in
terms of stability, separation, handling, and reuse, as well as
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the modified MOF nodes as the active sites.28 The linkers can
be functionalized with different chemical groups to enhance
gas adsorption29,30 or to mimic ligands in organometallic
complexes, thereby stabilizing homogeneous organometallic
complex catalysts in a solid matrix.18,26,31−33
Recently, a new approach to encapsulate a transition-metal
complex, (tBuPNP)Ru(CO)-HCl, within a MOF was devel-
oped that capitalizes on the existence of solvent-dependent,
aperture-opening events resulting from dissociative linker
exchange reactions in the MOF. ICP-OES confirmed 0.35
wt % Ru loading in the framework. The encapsulated catalyst,
Ru@UiO-66, was highly active for the hydrogenation of CO2
to formate at 27 °C to obtain a TON of 350 000 in 30 min,
which is the highest value reported to date for MOF
heterogeneous systems. The catalyst exhibited greater recycla-
bility over its homogeneous counterpart, slower bimolecular
deactivation, and resistance to catalyst poisoning.26
PdCl2 was purchased from Heraeus South Africa and used as received.
The metal precursor, PdCl2(CH3CN)2, was synthesized according to
a reported literature method.40
Fourier Transform Infrared (FTIR) Analysis. Fourier transform
infrared (FTIR) spectra of the samples were recorded in the range of
400−4000 cm−1 on a Perkin-Elmer FTIR spectrophotometer (Model
BX II) that was fitted with an attenuated total reflectance (ATR)
probe.
Thermal Analysis. Thermogravimetric analysis (TGA) was
performed using a TA Discovery Instrument TA-Q50 with a heating
rate of 10 °C min−1 within a temperature range of 25−500 °C under a
dry nitrogen purge gas flow of 50 mL min−1.
NMR Studies. 1H and 13C {1H} NMR spectra were acquired on a
Bruker Ultrashield 400 MHz spectrometer (for 1H, 400 MHz; for 13C,
100 Hz). All chemical shifts were recorded in units of ppm.
Powder X-ray Diffraction Studies. Powder X-ray diffraction
(PXRD) data were collected using an XPERT-PRO diffractometer
(Cu Kα radiation). X-rays were generated with a current flow of 40
mA and voltage of 40 kV.
Lin and co-workers incorporated molecular iridium
complexes into UiO-type MOF using 2,2′-bipyridine-5,5′
dicarboxylate ligands with hydroxyl groups. After activation,
the recyclable catalyst exhibited high activity toward the
hydrogenation CO2 to formate in the Soxhlet reflux
condensing setup, with a maximum TON of 6149, under
atmospheric pressure at 85 °C.33
Incorporation of palladium complexes into MOFs has been
reported without conclusive single-crystal X-ray structure
elucidation,34−38 except for one report by Wade and co-
workers, where the structure was elucidated from synchrotron
powder X-ray diffraction (PXRD) data.37 To the best of our
knowledge, this work present a first example of this type of
catalyst precursor that is being employed in CO2 hydro-
genation.
Herein, we report the design and synthesis of novel
isostructural MOFs containing catalytically active Pd(II) sites
using 2,2′-bipyridine-4,4′-dicarboxylate linker Pd@Mg:JMS-2a
and Pd@Mn:JMS-2a (where JMS denotes Johannesburg and
Midlands State). After activation, the Pd-immobilized MOF
catalysts exhibited high activity for CO2 hydrogenation to
formate. This work represents the first Pd catalysts anchored
on a MOF for hydrogenation of CO2 to formate. The majority
of work reported in the literature on Pd anchored on MOFs
mainly has been focused on CO oxidation.39 Our results show
that the catalytic activity of a palladium complex (homoge-
neous catalysis) is increased by 2-fold after incorporation in
two-dimensional (2D) MOFs (heterogeneous catalysis).
Homogeneous catalysts are known to be highly active,
compared to heterogeneous catalysts. In our findings, we
attribute the high catalytic activity of the MOF systems to the
presence of open metal sites that can concentrate the CO2 and
H2 gases before catalysis. These MOF catalysts are robust
under the optimal conditions employed in this work and could
be recycled four times without loss of structural integrity. Their
ease of preparation, high catalytic activity, and stability during
catalytic conditions represent a step forward in the develop-
ment of CO2 hydrogenation systems.
Inductively Coupled Plasma Optical Emission and Elemen-
tal Analysis. Inductively coupled plasma optical emission (ICP-
OES) analysis was performed on a Spectro Arcros instrument that was
calibrated using known standards of magnesium, manganese, and
palladium. Samples were microwave-digested in aqua regia (4 mL of
HNO3, 1 mL of HCl) and analyzed directly. Elemental analysis was
performed on a Scientific FLASH 2000 CHNS-O Analyzer.
Brunauer−Emmett−Teller (BET) Surface Area Analysis.
Nitrogen sorption studies were performed using a Micromeritics
ASAP 2460 surface and porosity analyzer at 77 K. Prior to analysis,
the samples were degassed with nitrogen gas at 150 °C to remove
strongly adsorbed water molecules and any other adsorbed species.
High-Resolution Transmission Electron Microscopy
(HRTEM) Analysis. The catalysts were analyzed using electron
microscopes. The morphological studies were performed on a JEOL
Model JEM 2100F electron transmission electron microscopy (TEM)
system operating at a voltage of 200 kV. Prior to analysis, the catalytic
samples were dispersed in methanol and sonicated for 30 min.
Thereafter, they were placed on a carbon-coated copper grid and
dried at room temperature.
Scanning Electron Microscopy−Energy-Dispersive (SEM-
EDX) Analysis. The samples were analyzed using the Tescan Vega
3LMH scanning electron microscopy (SEM) system. The samples
were placed on a sticky tape and carbon-coated using an Agar-Turbo
carbon sputter.
Synthesis and Characterization of Complex 1 (C1). 2,2′-
Bipyridine and 4,4′-dicarboxylic acid (H2 bpdc) (180 mg, 0.737
mmol) were dissolved in dimethyl formamide (DMF) (40 mL), and
the metal precursor, PdCl2(CH3CN)2 (249 mg, 0.737 mmol), was
added. The solution was left to stir at room temperature for 48 h. The
reaction mixture was filtered using a Buchner funnel, washed with
methanol, and dried under vacuum to afford a yellowish powder in
good yield. Yield: 278 mg (89.3%), 1H NMR (400 MHz, DMSO-d6):
(δ, ppm), 8.20 (d, 2H, CHarom, 3JH−H = 8.0 Hz), 9.00 (s, 2H, CHarom),
3
9.29 (d, 2H, CHarom, JH−H = 4.0 Hz); 13C NMR (DMSO-d6) (δ,
ppm): 164.55, 156.86, 150.83, 142.49, 126.74, 123.74. Elemental anal.
for C12H8N2O4PdCl2 (%): Calculated: 34.20 C, 1.91 H, 6.65 N.
Found: 33.86 C, 1.56 H, 6.83 N.
Synthesis of Mg(bpdc)(DMF)2PdCl2]n (Pd@Mg:JMS-2) and
[Mn (bpdc)(DMF)2PdCl2]n(Pd@Mn:JMS-2). The complex C1 (15
mg, 0.036 mmol) was dissolved in DMF (7 mL), and Mg(NO3)2·
6H2O (138 mg, 0.537 mmol) was added. The mixture was stirred for
10 min at room temperature. The resulting homogeneous mixture was
placed in a tightly sealed vial and then heated at 80 °C in an oven for
24 h. Yellowish block-shaped crystals of Pd@Mg:JMS-2, suitable for
single-crystal X-ray diffraction (XRD) data collection were obtained.
Following a similar procedure using MnCl2·4H2O (120 mg, 0,606
mmol), diamond-shaped orange crystals of Pd@Mn:JMS-2 were
obtained after 48 h at 85 °C. The compounds were characterized
using SCXRD, PXRD, FTIR, and TGA. Elemental analysis for Pd@
Mg:JMS-2 gave the following elemental analysis results (%):
EXPERIMENTAL SECTION
■
Materials and Methods. Magnesium nitrate hexahydrate,
manganese chloride, 2,2′-bipyridine 4,4′-dicarboxylic acid, potassium
hydroxide, sodium hydrogen carbonate, potassium carbonate,
methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide,
acetone, and triethyl amine were all purchased from Sigma−Aldrich
and used without further purification, unless otherwise mentioned.
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Scheme 1. Synthesis of (a) C1 and (b) Compounds Pd@Mg:JMS-2 and Pd@Mn:JMS-2
Figure 1. (a) Coordination environment around Mg(II) and Pd(II) centers in Pd@Mg:JMS-2. (Hydrogen atoms have been omitted for the sake of
clarity.) (b) Packing diagram viewed along the c-axis. (c) Polyhedra packing diagram (the DMF molecules have been omitted for the sake of
clarity). (d) Channels found in compound Pd@Mg:JMS-2 viewed along the b-axis (blue denotes the outside surface and red indicates the inside
surface).
Calculated: 36.64 C, 3.42 H, 9.50 N. Found: 37.17 C, 3.77 H, 9.11 N.
Elemental analysis for Pd@Mn:JMS-2 gave the following elemental
analysis results (%): Calculated: 34.83 C, 3.25 H, 9.03 N. Found:
35.07 C, 3.35 H, 8.77 N.
room temperature and the residual gas was carefully released in a
fume hood. The supernatant was analyzed by 1H NMR, using acetone
as an internal standard. The solid catalyst was washed several times
with ethanol and dried in an oven at 80 °C for recycling. All
experiments were performed in duplicate and the average recorded.
Crystallographic Data Collection and Refinement. Single-
crystal XRD data collections of Pd@Mg:JMS-2 and Pd@Mn:JMS-2
were recorded at 293 K on a Bruker KAPPA APEX II DUO
diffractometer that was equipped with graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å). Unit-cell refinement and data
reduction were performed using the program SAINT. Lorentz
polarization and absorption correction were applied. The structures
of the two compounds were solved by direct methods (program
SHELXS)41 and refined anisotropically on F2 full-matrix least-squares
with SHELXL41 within the X-SEED42 interface. Anisotropic thermal
parameters were applied to non-hydrogen atoms while all hydrogen
atoms were calculated and added at idealized positions. The crystal
data for Pd@Mg:JMS-2 and Pd@Mn:JMS-2 is given in Table S1 in
the Supporting Information. The structures were deposited at the
Cambridge Crystallographic Data Centre and allocated the numbers
CCDC 1943375 and 1943376 for Pd@Mg:JMS-2 and Pd@Mn:JMS-
2, respectively.
Catalytic Hydrogenation of CO2 to Formate. The hydro-
genation of CO2 was performed in a Teflon-lined stainless-steel
reactor that was equipped with a magnetic stirrer. In a typical run,
known amounts of catalyst and 5 mmol KOH in 8 mL of ethanol were
loaded into the reactor. The reactor was sealed and purged three
times with CO2, and then CO2 and H2 were charged to the desired
pressure at room temperature. The catalytic reactions were performed
within a temperature range of 90−120 °C while stirring was
maintained at 945 rpm. After reaction, the reactor was cooled to
RESULTS AND DISCUSSION
■
Synthesis and Characterization of C1, as Well as
Compounds Pd@Mg:JMS-2 and Pd@Mn:JMS-2. Complex
C1 was prepared by reacting the commercially available H2
bpdc ligand with [PdCl2(CH3CN)2] in DMF for 48 h. (See
Scheme 1a.) This complex is yellow and soluble in DMSO and
DMF upon slight heating.
1
The H NMR spectra of the free ligand shows aromatic
protons in the region from 8.915 ppm to 7.898 ppm.
1
Significant downfield shift in the H NMR signals of these
spectrum to the region from 9.29 ppm to 8.19 ppm, compared
to the free ligand, was observed, indicating the successful
formation of the complex (see Figure S1 in the Supporting
Information). 13C {H} NMR spectrum of the complex showed
the expected number of carbon signals (see Figure S2 in the
Supporting Information). Elemental analysis results obtained
for carbon, hydrogen, and nitrogen are within the general
accepted values of 0.40.
Pd@Mg:JMS-2 and Pd@Mn:JMS-2 were prepared by the
solvothermal reaction of C1 with Mg(NO3)2·6H2O and
MnCl2·4H2O, respectively, for 24−48 h (see Scheme 1b).
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Figure 2. (a) IR spectra of complex C1, as well as as-synthesized and activated phases (Pd@Mg:JMS-2a and Pd@Mn:JMS-2a); (b) PXRD patterns
of Pd@Mg:JMS-2 and Pd@Mn:JMS-2 (calculated, as-synthesized, and activated).
Figure 3. (a) FTIR and (b) PXRD of Pd@Mg:JMS-2a soaked in different bases dissolved in ethanol and solvents. (Conditions: time = 36 h, base
concentration = 5 mmol in 5 mL ethanol, and room temperature.)
Single-crystal XRD analysis revealed that compound Pd@
Mg:JMS-2 crystallizes in the monoclinic system and space
group C2/c. Pd@Mn:JMS-2 is isostructural to Pd@Mg:JMS-2.
The asymmetric unit of Pd@Mg:JMS-2 consists of one
crystallographically independent Mg(II) ion, one Pd(II) ion,
two Cl atoms, one deprotonated linker, and two coordinated
DMF molecules. Each Mg(II) center is coordinated to three
bpdc linkers: one in a bidentate chelating mode and the other
two in a bidentate bridging mode. (See Figure 1a.) The overall
geometry around the Mg(II) center is a distorted octahedral.
The two N atoms of the linker and two Cl ligands are
coordinated to Pd(II) to give a square planar geometry. Mg−
O, Pd−N, and Pd−Cl bond length ranges from 1.999 (2) Å to
2.2833(9) Å and are consistent with literature reports. The
bond angles around Mg(II) and Pd(II) centers are in the range
of 81.02(10)°−175.70(11)°.
Figure 1b illustrates the packing diagram of Pd@Mg:JMS-2.
The overall structure is 2D, with channels running along the b-
axis. These channels (Figures 1c and 1d) are occupied by DMF
molecules and account for 43.8% and 46.6% of the unit-cell
volume for Pd@Mg:JMS-2 and Pd@Mn:JMS-2, respectively,
as estimated in PLATON.43 No classic hydrogen bonding
occurs between DMF molecules. However, weak intermolec-
ular and intramolecular interactions such as π···π interactions
of the aromatic ring reinforce the geometry and impart stability
to the MOF.
The thermal behavior of the MOFs were examined by TGA.
Figure S3 in the Supporting Information illustrates the thermal
profile of Pd@Mg:JMS-2 and Pd@Mn:JMS-2. As illustrated in
Figure S3, Pd@Mg:JMS-2 has three stages of weight loss. The
initial two-stage weight loss of 24.1% (calculated 24.7%) that
occurs between 50 °C and 244 °C corresponds to the loss of
two DMF molecules per formula unit modeled in the structure.
Decomposition of the framework occurs above 400 °C. The
TGA of Pd@Mn:JMS-2 exhibits a total weight loss of 27.4%
between 160 °C and 270 °C, which corresponds to two DMF
molecules modeled in the asymmetric unit. Contrary to Pd@
Mg:JMS-2 that exhibit high thermal stability, compound Pd@
Mn:JMS-2 decomposes around 325 °C. Figure 2a depicts
FTIR studies performed on compound C1 Pd@Mg:JMS-2 and
Pd@Mn:JMS-2. A stretching frequency at 1728 cm−1
(COOH) in C1 shifts to 1600 cm−1 in Pd@Mg:JMS-2 and
Pd@Mn:JMS-2, which is evidence of coordination of Mg or
Mn to the carboxylate moiety. These results are consistent with
the single-crystal XRD data.
The as-made MOFs (Pd@Mg:JMS-2 and Pd@Mn:JMS-2)
were activated to give Pd@Mg:JMS-2a and Pd@Mn:JMS-2a,
respectively, by soaking in methanol at room temperature for
24 h, during which methanol was refreshed every 6 h, followed
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Figure 4. (a) FTIR and (b) PXRD of Pd@Mn:JMS-2a soaked in different bases and solvents. (Conditions: time = 36 h, base concentration = 5
mmol in 5 mL ethanol, and room temperature.)
by drying under vacuum for 24 h at 80 °C. The FTIR of Pd@
Mg:JMS-2 and Pd@Mn:JMS-2 (Figure 2a) show carbonyl
stretches at 1676 cm−1 attributed to the coordinated DMF
molecules. Successful activation is evidenced by the disappear-
ance of these bands in Pd@Mg:JMS-2a and Pd@Mn:JMS-2a.
The asymmetric and symmetric stretches of the carboxylate
were observed at 1600 and 1386 cm−1 in both Pd@Mg:JMS-2
and Pd@Mn:JMS-2. This position does not change after
activation, suggesting that the binding mode of the carboxylate
moiety is not affected. The stretches at 1552 cm−1 in all of the
compounds were assigned to the conjugated C−C of the
benzene rings.
As illustrated in Figure 2b, there is a good agreement
between the calculated and experimental PXRD patterns,
confirming the phase purity of the synthesized compounds.
PXRD studies also reveal that the structural integrity of the
MOFs is maintained after activation, as evidenced by a good
match between as-synthesized and activated phases.
Chemical Stability Tests. A stable MOF should have
strong coordination bonds to survive the attack by guest
molecules. In most MOFs, the chemically weak points are the
nodesmore specifically, the metal linker bonds.44 It
determines the thermodynamic stability of MOFs under
different operating environments. Activated MOFs Pd@
Mg:JMS-2a and Pd@Mn:JMS-2a were thus soaked in different
solvents and bases dissolved in dry ethanol for 36 h to evaluate
their chemical stability. Except for crystals soaked in water, the
FTIR studies (Figure 3a) of Pd@Mg:JMS-2a soaked in
different bases and solvents show that the position of
asymmetric and symmetric carboxylate stretches located at
1600 and 1386 cm−1, respectively, are not affected. This
suggests that the binding mode of the carboxylate moiety is
retained under different chemical environments. In addition,
several bands in the region of 1400 and 1600 cm−1 are
attributable to the vibrations of the CC and CN bonds of
the linker. PXRD studies in Figure 3b revealed that Pd@
Mg:JMS-2a is poisoned by water, as evidenced by a
transformation from crystalline to an amorphous phase. In
the presence of NaHCO3, a new phase is obtained. Notably,
the 2D network is maintained in KOH and K2CO3 dissolved in
ethanol and Et3N.
retained under various chemical conditions. This is in
agreement with PXRD patterns (Figure 4b), which shows
that the structural integrity is maintained. Notably, compound
Pd@Mn:JMS-2a is stable in water. The difference observed in
chemical stability of the two MOFs may be attributed to the
strength of the metal oxygen bond.
Sorption Studies. The specific surface area and pore
volume of Pd@MgJMS-2a and Pd@MnJMS-2a were evaluated
by N2 adsorption−desorption measurements at 77 K, as shown
in Figure S4 in the Supporting Information. From the sorption
isotherms, the appearance of type IV hysteresis loops (H-1
type) is indicative of the presence of porous channels, with a
BET surface areas of 0.72 and 5.8 m2/g for Pd@MgJMS-2a
and Pd@MnJMS-2a, respectively. The surface areas and pore
volumes are relatively low, because of partial occupancy by
palladium complexes immobilized in the MOFs during
synthesis.
Catalysis Studies. Pd@Mg:JMS-2a and Pd@Mn:JMS-2a
were screened as precatalysts for hydrogenation of CO2.
Scheme 2 illustrates the catalytic reaction for the hydro-
genation of CO2 to formate.
Scheme 2. Catalytic Reaction for the Hydrogenation of CO2
Hydrogenation to Formate
In our first attempt, the catalytic hydrogenation of CO2 by
the more stable compound Pd@Mn:JMS-2a was performed
under various conditions in order to establish optimum
reaction conditions (see Table 1, entries 1−11). The formate
formed was detected and quantified by 1H NMR spectroscopy
with acetone as an internal standard (see Figures S5 and S6 in
the Supporting Information). The effect of Et3N, KOH,
K2CO3, and NaHCO3 bases on the conversion of CO2 was
evaluated (see Table 1, entries 1−4). When strong inorganic
bases were used (see Table 1, entries 2−4), higher yields of
formate were observed, with KOH giving the highest turnover
frequency (TOF) of 204 h−1. In the presence of a weak organic
base, such as triethylamine (pKa = 18.8), a TOF of 102 h−1 was
recorded (Table 1, entry 1). Therefore, we opted to use KOH
as our preferred base for the reaction in all optimization
studies. In the absence of a base (Table 1, entry 5), no formate
was produced. In most reported catalyst systems, a base is
Similar chemical studies were conducted using Pd@
Mn:JMS-2a. FTIR studies (Figure 4a) of the recovered
samples in different bases and solvents indicate that the
binding mode of the carboxylate moiety to the metal center is
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a
Table 1. Hydrogenation of CO2 to Formate Using Pd@Mn:JMS-2a, Pd@Mg:JMS-2a, and C1
entry
catalyst
base
Et3N
temperature (°C)
pressure, CO2/H2 (bar)
turnover number, TON
turnover frequency, TOF (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
14*
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
none
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mn:JMS-2a
Pd@Mg:JMS-2a
C1
90
90
90
90
90
90
90
90
90
90
90
100
100
100
100
40 (1:3)
40 (1:3)
40 (1:3)
40 (1:3)
40 (1:3)
40 (1:3)
20 (1:1)
30 (1:2)
50 (1:4)
40 (0:4)
10 (1:0)
50 (1:4)
50 (1:4)
50 (1:4)
50 (1:4)
2448
4896
3672
484
0
102
204
153
121
0
0
73
120
312
0
KOH
NaHCO3
K2CO3
None
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
0
1752
2880
7488
0
0
0
9816
7272
4080
3217
409
303
170
134
C1
a
Conditions: time (24 h), ethanol (8 mL), and base (5 mmol) and TOFs are based on Pd (0.46 μmol). Yield is based on 1H NMR analysis, using
acetone as an internal standard.
Figure 5. Effect of temperature on the TON of formate produced (Pd@Mg:JMS-2a (blue) and Pd@Mn:JMS-2a (orange)). [Conditions: pressure
= 50 bar (CO2/H2 = 1:4), time = 24 h, TONs are based on Pd (0.46 μmol), 5 mmol KOH, and 8 mL of ethanol.]
needed to facilitate the hydrogenation of CO2 to formate,
because (i) the base deprotonates the generated formic acid,
thus thermodynamically shifting the equilibrium forward; and
(ii) the base has been found to accelerate the heterolysis of H2
during the catalytic cycle, thereby kinetically promoting the
catalytic activity.45
It is important to note that, in the absence of a catalyst
(Table 1, entry 6), no formate is formed, indicating the crucial
role of the catalyst in CO2 reduction. A TOF of 312 h−1 was
obtained (Table 1, entry 9) when the pressure of H2 was
increased to 40 bar. This might be due to the fact that
sufficient partial pressure of H2 is required to generate Pd-
hydride active necessary for CO2 hydrogenation.
Figure 5 depicts the results for CO2 reduction conducted at
different temperatures (90−120 °C) with other parameters
kept constant. The outcome suggests that, for both catalysts,
the turnover number (TON) increases as the temperature
increases. However, at 110 °C, a decrease in formate was
observed because the hydrogenation of CO2 to formate is an
exothermic reaction. Furthermore, high temperatures may
result in either the reverse-gas-shift reaction or decomposition
of the formate. We also noted that, for the selected conditions,
the more-stable catalyst Pd@Mn:JMS-2a performed better
than Pd@Mg:JMS-2a.
We further studied the effects of other parameters, such as
the effect of time and catalyst loading on the reaction at 100
°C, and the results are shown in Figure 6. The TON/time
profile of the CO2 hydrogenation to formate (Figure 6a) was
mapped out by performing batch reactions with different times
under identical conditions. The results show a progressive
increase in the product formed, with a maximum TON after 24
h. Catalyst loading based on Pd was varied from 0 to 0.55
μmol, keeping all parameters constant (see Figure 6b). In both
cases, an increase in catalyst load gave rise to an increase the
turnover number. Further increase in catalyst loading results in
no significant increase in product.
Subsequently, pressure variation was determined by
changing the partial pressure of H2 while keeping that of
F
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Figure 6. (a) Effect of time (conditions: pressure = 50 bar (CO2/H2 = 1:4), temperature = 100 °C, 0.46 μmol catalyst loading based on Pd, 5
mmol KOH, and 8 mL of ethanol); (b) effect of catalyst loading (conditions: pressure = 50 bar (CO2/H2 = 1:4), temperature = 100 °C, time = 24
h, 5 mmol KOH, and 8 mL of ethanol).
Figure 7. (a) Recyclability studies for compound Pd@Mg:JMS-2a and Pd@Mn:JMS-2a; (b) PXRD studies of the catalysts before and after the
fourth cycle of catalysis.
CO2 constant (see Figure S7 in the Supporting Information).
Upon applying a pressure totalling 20 bar of CO2 (10 bar) and
H2 (10 bar), with the other reaction parameters kept constant,
the observed TON was 1104 and 1896 for Pd@Mg:JMS-2a
and Pd@Mn:JMS-2a, respectively. As illustrated in Figure S7,
an increase in the partial pressure for hydrogen resulted in
more formate being produced, as evidenced by an increase in
the TON.
The performances of C1, Pd@Mn:JMS-2a, and Pd@JMS-2a
were compared under optimized conditions (Table 1, entries
12−14). The homogeneous counterpart, C1, exhibits a TOF of
170 h−1 (Table 1, entry 14) while Pd@Mn:JMS-2a and Pd@
Mg:JMS-2a MOFs recorded high TOF values of 409 and 303
h−1, respectively (Table 1, entries 12 and 13), which is ∼2
times greater than that of C1. This may be explained on the
basis that heterogeneous catalysts are often more stable and
degrade much slower than the homogeneous catalysts. The
metals and ligands are strongly bound within the framework, to
prevent its decomposition in solution.26,46 In addition to that,
after activation, MOFs contain coordinatively unsaturated
metal centers that can enhance the adsorption of CO2 and
H2,47,48 and conversion at the catalytic centers.33,49,50 Table S2
in the Supporting Information compares the catalytic activities
of systems reported in the literature, which confirms that, in
our work, milder reaction conditions were employed.
To illustrate that most of the active species is within the
pores rather than being bound to the surface, thiol poisoning
studies were performed using the most active catalyst, Pd@
MnJMS-2a as a model. Thiols are known to be incompatible
with many transition-metal catalysts,26 especially Pd and Pt.
Such poisons can be ascribed to the strong and intractable
thiol−Pd interactions that effectively seal off the coordination
and suppress the bonding lability that is crucial for catalytic
process.51 When bulky benzylmercaptan, which does not easily
penetrate the host network, was added to the catalytic reaction,
the TON was reduced by 19.8% (see Table S3 in the
G
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Figure 8. SEM images of (a) Pd@Mn:JMS-2a before catalysis, (b) Pd@Mn:JMS-2a after catalysis, (c) Pd@Mg:JMS-2a before catalysis, (d) Pd@
Mg:JMS-2a after catalysis, (e) elemental mapping for Pd@Mg:JMS-2a, and (f) Pd@Mn:JMS-2a after catalysis
Figure 9. TEM images of (a) Pd@Mg:JMS-2a before catalysis, (b) Pd@Mg:JMS-2a after catalysis, (c) Pd@Mn:JMS-2a before catalysis, and (d)
Pd@Mn:JMS-2a after catalysis.
Supporting Information). Appreciable inhibition was observed
(60.3%) when 2 mercapto-ethanol was used in the reaction
mixture. This is because 2 mercapto-ethanol can easily enter
the pores and poison the Pd sites. Figures S8 and S9 in the
Supporting Information shows the H NMR spectra of these
studies. These results are consistent with the bulk of the active
species being situated inside the MOF pores.
Leaching, Heterogeneity, and Recycling Studies. To
investigate whether Pd(II) in Pd@Mg:JMS-2a and Pd@
Mn:JMS-2a were actually working in a heterogeneous manner,
the supernatants from the catalysis reaction mixture were
reused for catalysis under optimized conditions. No additional
formate was detected, which proved the heterogeneous nature
of the catalytic systems. ICP-OES analysis of the solutions in
the reactor showed negligible metal leaching (∼0.0003% Pd).
To determine the reusability of the catalysts, they were
isolated after the reaction by simple filtration, followed by
washing several times with ethanol and then they were
vacuum-dried. The catalysts were recycled and reused for four
consecutive cycles without losing significant catalytic activity
(Figure 7a). Remarkably, the PXRD pattern of the recycled
Pd@Mn:JMS-2a shown in Figure 7b matches well with that of
the parent MOFs, suggesting retention of the original
framework. After catalysis, Pd@Mg:JMS-2a exhibits slight
changes in the PXRD pattern. In addition to the lower void
volume estimated in PLATON, the change in the diffraction
pattern Pd@Mg:JMS-2a may be attributable to the lower
1
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catalytic activity, compared to its counterpart Pd@Mn:JMS-2a.
FTIR studies illustrated in Figure S10 in the Supporting
Information indicate that the binding mode of the carboxylate
moiety in both MOFs to the metal center is retained after
catalysis. To verify that Pd is necessary for catalytic activity and
that catalysis is not promoted by Mg(II) or Mn(II) or other
moieties present in MOF, we performed catalysis using metal
salts, since we were unable to isolate the two isostructural
nonpalladated MOFs. No formate was detected, indicating the
role of Pd(II) in the formation of formate. SEM was used to
probe the surface morphology of the MOFs before and after
catalysis. The results illustrated in Figure 8 shows that the
morphology of Pd@Mn:JMS-2a and Pd@Mg:JMS-2a is not
affected during catalysis.
Figure 10. TGA results of the recovered dry catalysts after four cycles.
Elemental mapping presented for individual atoms in Figure
8e and 8f of the used catalysts after the fourth cycle shows that
the Pd atoms are fairly distributed in Pd@Mg:JMS-2a and
Pd@Mn:JMS-2a. TEM images were collected to check for the
formation of Pd(0) during catalysis. As depicted in Figure 9,
the TEM image of Pd@Mg:JMS-2a do not show significant
formation of nanoparticles after catalysis, this may also explain
the insignificant decrease in the catalytic activity of this system,
as presented in Figure 7a. Contrary to Pd@Mg:JMS-2a, Pd@
Mn:JMS-2a shows a significant formation of nanoparticles,
which are agglomerated after catalysis. These findings may help
explain the observed decrease in catalytic activity from ∼10
000 TON to ∼6000 TON over the four cycles (Figure 7a).
The energy-dispersive X-ray spectroscopy (EDX) analysis
data of Pd@Mn:JMS-2a and Pd@Mg:JMS-2a before and after
the multiple cycles are given in Figure S11 in the Supporting
Information and show the presence of Pd and Cl ions before
and after the multiple cycles.
In order to evaluate the extent to which palladium
nanoparticles formed during catalysis are involved in the
catalytic cycle, a mercury drop test was conducted using the
homogeneous counterpart C1 as a model. Hg(0) has been
known for quite some time to bind to metal surfaces, resulting
in poisoning catalytic reactivity for metals in their zero
oxidation state.52 It is not anticipated that Hg(0) will poison
metals bound to protective ligands or in higher oxidation
states.53,54 Thus, the Hg(0), was used to determine if a “naked”
Pd(0) species was involved in the catalytic cycle. As shown in
Table 1, entry 14*, in the presence of Hg(0), the TON values
decreased from 4078 to 3212. This is an indication that both
nanoparticles and molecular catalysts, which have been
referred to as a “cocktail” of catalysts,55−57 are responsible
for the catalytic activity. The added Hg(0) scavenges the
Pd(0) through amalgamation or adsorption to the active
species, thereby inhibiting their catalytic action, while residual
molecular Pd continue to catalyze the reaction. Several studies
have demonstrated that the formation of Pd(0) is a common
problem in the applicability of Pd-based catalysts in many
organic reactions, especially in the presence of a strong base at
elevated temperatures.56,51,58
Kinetics of Formate Formation from CO2 Hydro-
genation. The formation of formate follows a pseudo-first-
order kinetics (the rate of reaction is based on the conversion
of KOH to HCOOK). Accordingly, the rate of CO2
conversion is expressed as
d[CO2]
d[formate]
−r[CO2] = −
= k[CO2] =
(1)
dt
dt
Here, k represents the rate constant of KOH conversion to
HCOOH.
According to the Arrhenius equation,
E
RT
i
y
a
j
j
j
z
z
z
ln k = −
+ ln A
(2)
(3)
k
{
Substituting eq 1 into eq 2 gives
−ln(1 − x) = kt + C
where the amount of KOH is expressed in terms of x, t is the
reaction time, and C is a constant. The linear relationship of
the plot of ln(1 − x) versus time t confirms the assumption of
pseudo-first-order reaction kinetics for KOH to HCOOK (see
Figure S12 in the Supporting Information).
The rate constant k, at different temperatures, is obtained
from the plot in Figure S12. Kinetic studies reveals that Pd@
Mn:JMS-2a has high rate constants within a temperature range
of 70−100 °C, compared to that observed for Pd@Mg:JMS-2a
(see Table S4 in the Supporting Information). This may
explain why the TOF of formate in Pd@Mn:JMS-2a is higher
than that obtained using the Pd@Mg:JMS-2a MOF.
Mechanism for CO2 Hydrogenation. The insolubility of
the C1 complex in several NMR solvents precluded
mechanistic studies using high-pressure NMR tubes. Herein,
we propose the mechanistic pathway for the hydrogenation of
CO2 by Pd@Mg:JMS-2a and Pd@Mn:JMS-2a, based on
literature reports for related heterogeneous systems.11,33,59,60
Scheme 3 illustrates the proposed mechanism for the
hydrogenation of CO2 to formate at the Pd(II) site. The
metal center activates the reactants H2 and CO2; however,
because of the lower reactivity of CO2, H2 gets activated first to
form Pd-dihydride intermediate. Elimination of HCl from the
Pd-dihydride complex gives the active species Pd-hydride
complex. The insertion of CO2 via associative addition into the
Pd-hydride complex generates the formato complex that
readily dissociates the formate.
TGA curves of the recovered catalysts after four cycles are
plotted in Figure 10. The recovered catalysts were dried under
vacuum before thermal analysis. Interestingly, our results show
that the thermal stability of the MOFs is maintained as
evidenced by similar thermal decomposition temperature, in
comparison to their native structures (Figure S3 in the
Supporting Information). This may support that the structure
of the MOFs remains intact after catalysis.
I
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Scheme 3. Plausible Mechanism for the Hydrogenation of CO2 to Formate at the Pd(II) Site
CONCLUSION
ASSOCIATED CONTENT
■
■
sı
* Supporting Information
In summary, two novel isostructural Pd(II) containing MOFs
(Pd@Mg:JMS-2a and Pd@Mn:JMS-2a) were prepared and
evaluated for hydrogenation of CO2 to formate. Our design
strategy involves functionalization of the bipyridyl dicarbox-
ylate linker with catalytically active species (Pd) prior to MOF
synthesis. The tailored catalysts demonstrated excellent
catalytic activity in basic media and could be recycled up to
four times. Pd@Mnds:JMS-2a exhibited a significant loss in
catalytic activity, while Pd@Mg:JMS-2a exhibited a slight
change in the catalytic activity. PXRD studies of the MOFs
revealed that Pd@Mg:JMS-2a is poisoned by water, while Pd@
Mn:JMS-2a is stable in all of the solvents and bases normally
used for CO2 hydrogenation. The MOFs recorded highest
TOF values of 303 and 409 h−1 for Pd@Mg:JMS-2a and Pd@
Mn:JMS-2a, respectively, in 24 h, which greatly exceeds what is
obtained with the homogeneous counterpart, C1. The majority
of reports in the literature focus on Pd/Au or Pd/Ag@MOF
for the reduction of CO2 to formate.61 Our results demonstrate
that the Pd@MOF have superior catalytic performance.
Furthermore, we have shown that the catalytic performance
of homogeneous systems can be improved by incorporating
them in MOFs as organic linkers bearing catalytic sites. The
current systems show comparable to better catalytic activity
than Ir systems. The development of palladium-based catalysts
is of interest, because of its low cost, compared to Ru- and Ir-
based catalysts.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03654.
H NMR spectra, TGA, EDX, and reaction kinetics data
for Pd@Mn:JMS-2a and Pd@Mg:JMS-2a (PDF)
Accession Codes
CCDC 1943375 and 1943376 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Gift Mehlana − Faculty of Science and Technology, Department
of Chemical Technology, Midlands State University, Gweru,
Zimbabwe; orcid.org/0000-0003-1294-0895;
Email: mehlanag@staff.msu.ac.zw
Authors
Piwai Tshuma − Faculty of Science and Technology, Department
of Chemical Technology, Midlands State University, Gweru,
Zimbabwe; Center for Synthesis and Catalysis Department of
Chemical Sciences, Faculty of Science, University of
Johannesburg, Auckland Park 2006, South Africa
Banothile C. E. Makhubela − Center for Synthesis and
Catalysis Department of Chemical Sciences, Faculty of Science,
J
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(13) Li, Y.; Chan, S. H.; Sun, Q. Heterogeneous catalytic conversion
of CO2: A comprehensive theoretical review. Nanoscale 2015, 7,
8663−83.
University of Johannesburg, Auckland Park 2006, South Africa;
orcid.org/0000-0002-2292-7400
Ndzondelelo Bingwa − Center for Synthesis and Catalysis
Department of Chemical Sciences, Faculty of Science, University
of Johannesburg, Auckland Park 2006, South Africa
(14) Lane, E. M.; Zhang, Y.; Hazari, N.; Bernskoetter, W. H.
Sequential Hydrogenation of CO2 to Methanol Using a Pincer Iron
Catalyst. Organometallics 2019, 38, 3084−3091.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03654
(15) Li, Y.-N.; He, L.-N.; Liu, A.-H.; Lang, X.-D.; Yang, Z.-Z.; Yu, B.;
Luan, C.-R. In situ hydrogenation of captured CO2 to formate with
polyethyleneimine and Rh/monophosphine system. Green Chem.
2013, 15, 2825−2829.
(16) Gnanamani, M. K.; Jacobs, G.; Hamdeh, H. H.; Shafer, W. D.;
Liu, F.; Hopps, S. D.; Thomas, G. A.; Davis, B. H. Hydrogenation of
Carbon Dioxide over Co-Fe Bimetallic Catalysts. ACS Catal. 2016, 6,
913−927.
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Yamamoto, T.; Toriyama, T.; Matsumura, S.; Kitagawa, H. Charge
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank the Green Chemistry for Life
Program (PhosAgro/UNESCO/IUPAC), The Royal Society,
and the African Academy of Sciences (AAS) Future Leaders
Africa Independent Research (FLAIR) programme for funding
this project. P.T. is grateful for the financial support she
received from the Organization for Women in Science for the
Developing World. The authors are grateful to the University
of Johannesburg Centre for Synthesis and Catalysis Spectrum
availing their facilities.
(18) Mino, L.; Gallo, E.; Agostini, G.; Lamberti, C.; Borfecchia, E.;
Olsbye, U.; Lomachenko, K. A.; Lillerud, K. P.; Bordiga, S.; Øien, S.;
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Metal-Organic Frameworks. Chem. Mater. 2015, 27, 1042−1056.
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Applications of metal-organic frameworks in heterogeneous supra-
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