A Bipyridine-Based Conjugated Microporous Polymer for the Ir-Catalyzed Dehydrogenation of Formic Acid

Article abstract of DOI:10.1021/acscatal.7b02425

Formic acid is considered a promising energy storage medium, and its selective dehydrogenation enables the generation of high-purity H₂. Herein we report a bipyridine-based conjugated microporous polymer (CMP) loaded with [Cp?IrCl₂]₂ for the base-free aqueous dehydrogenation of formic acid to H₂/CO₂. This catalyst exhibits high activity and selectivity at temperatures over 130 °C and with formic acid concentrations as high as 10 M. Recycling tests demonstrate a low Ir leaching and a gradual increase in the activity over six runs and a low CO content in the gas phase of about 138 ppm. TOFs of up to 123894 h^-1 were obtained using 0.1 wt % Ir loading.

Full text of DOI:10.1021/acscatal.7b02425

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Article
A Bipyridine-Based Conjugated Microporous Polymer
for the Ir-Catalyzed Dehydrogenation of Formic Acid
Cornelia Broicher, Severin Foit, Marcus Rose, Peter J.C. Hausoul, and Regina Palkovits
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02425 • Publication Date (Web): 31 Oct 2017
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Page 1 of 26
ACS Catalysis
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A Bipyridine-Based Conjugated Microporous Polymer for the Ir-
Catalyzed Dehydrogenation of Formic Acid
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a
b
a
a*
a*
Cornelia Broicher , Severin R. Foit , Marcus Rose , Peter J.C. Hausoul and Regina Palkovits
a
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen
(
b
Germany)
Forschungszentrum Jülich Institut für Energieꢀ und Klimaforschung Grundlagen der Elektrochemie (IEKꢀ9) 52425 Jülich
Germany)
(
Supporting Information Placeholder
ABSTRACT: Formic acid is considered a promising energy
storage medium and its selective dehydrogenation enables the
generation of high purity H . Herein we report a bipyridineꢀbased
2
conjugated microporous polymer (CMP) loaded with [Cp*IrCl2]2
for the baseꢀfree aqueous dehydrogenation of formic acid to
H /CO . This catalyst exhibits high activity and selectivity at
2
2
temperatures over 130°C and in formic acid concentrations as
high as 10 M. Recycling tests demonstrate a low Ir leaching and a
gradual increase in the activity over six runs and a low CO conꢀ
ꢀ
1
tent in the gas phase about 138 ppm. TOF’s up to 123 894 h
were obtained using 0.1wt% Ir loading.
Scheme 1. Hydrogen storage and delivery though FA production and
dehydrogenation
Keywords: formic acid, dehydrogenation , iridium, CMP,
heterogeneous
For example Beller et al. reported turnover numbers (TONs) up to
6
1
,000 000 for [RuCl (C H )] in the presence of DPPE. Recently,
2
6
6
2
1
. INTRODUCTION
Huang et al. reported
[(RuH(CO)(PN pincer)]. Iguchi et al. developed waterꢀsoluble
IrCp* complexes and reported TONs up to 320 000. Onishi et al.
a
TON of 1,100 000 for
3
7
Current energy production is mainly based on the combustion of
8
fossil fuels, which causes the release of large amounts of CO into
2
the atmosphere and contributes to global warming. To reduce
emissions and avert climate change, the use of sustainable and
achieved by the incorporation of a bidentate ligand that combines
an imidazoline and a pyridine moiety a TON of 2,000 000. Celaje
9
CO ꢀneutral energy sources such as wind and solar energy is
and coꢀworkers presented a TON up to 2., 60 000 for [Ir(cod) ꢀ
2
2
t
10
imperative. Nevertheless, due to the fluctuating nature of these
energy supplies, efficient storage and conversion strategies are
also necessary. A possible strategy is the conversion of excess
electrical energy to chemical energy via e.g. the electrolysis of
water to H /O . H is a promising energy carrier which can be
((diꢀ butylphosphino)methyl)pyridine]. Wang et al. achieved a
maximum TON of 2,500 000 for bifunctional cyclometalated
Ir(III) complexes with hydroxy substituted pyridineꢀazole and
11
pyrimidineꢀazole/azoline ligands. Li and coꢀworkers achieved a
ꢀ
1
maximum TOF of 147 000 h at 40°C with a bifunctional cyꢀ
2
2
2
12
used for combustion and for the chemical synthesis of liquid
clometalated Ir(III) complex. The [Ir(Cp*)Cl
2
(thbpym)] (Cp* =
1
fuels. Nevertheless, storage of gaseous H in compressed form is
pentamethylcyclopentadienyl) developed by Hull et al. showed a
2
ꢀ
1
13
energy intensive and introduces additional risks. Therefore, the
remarkable TOF of 228 000 h at 90°C. Recently, Fink et al.
ꢀ
1
conversion of H to a condensed state is preferred. In this regard,
reported a maximum TOF of 3300 h for [(Cp*)M(N–N’)Cl]
(M = Ir, Rh), which have been proven to be active in aqueous
media at 60°C. These studies demonstrate that particularly Ruꢀ
and Irꢀbased complexes with phosphorus and nitrogenꢀdonor
2
the direct hydrogenation of CO to formic acid (FA), formaldeꢀ
2
14
hyde/methane diol and methanol is of particular interest. Of these,
FA is highly interesting as it can be easily synthesized from
H /CO and decomposed back by transition metal catalysts
15
ligands provide the most active catalysts. The application of
2
2
1
6
(Scheme 1). In addition FA can also be obtained by oxidative
these catalysts in fuel cells has also been demonstrated. Howevꢀ
er from the point of view of continuous processing the use of
heterogeneous catalysts is clearly preferred.
biomass processing (e.g. the Biofine process for levulinic acid
2
production and OxFA process for the partial oxidation of bioꢀ
3
mass wastes to FA).
To date several studies concerning the use of supported metal
1
7
The dehydrogenation of FA to H /CO is well known and was
catalysts have been reported and mainly systems based on Au,
2
2
18
19
20
already reported in 1967 by Coffey, who revealed that transition4
metal complexes based on Ir and Ru are active for the reaction.
Over the past decades, several highly active catalysts have been
Pd, and bimetallic Au/Pd, and Pd/Ag, show the best activiꢀ
21
ties. For example, a Ag
Pd
/SBAꢀ15ꢀNR
catalyst prepared by
1
9
3
Wan et al. exhibited 100% H
964 h at 70°C.
selectivity and an initial TOF of
2
5
ꢀ1
22
identified.
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C 1s
4
000
1
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200
150
100
3 eV
Ir(III)
Ir 4f
3000
50
6
8
66 64 62 60
Binding energy / eV
2
1
000 N 1s
Cl 2p Br 3p
000
0
Ir 4f
0
1
2
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0
Scheme 2. Synthesis of CMP and Ir@CMP; Conditions: i) 0.2 eq (1), 1.0 eq
4
00
300
200
100
0
(
2), 0.8 eq. (3), 10 mL DMF, 10mL NEt
3
, 0.35 mol% CuI, 3.5 mol%
*
[Pd(PPh
3
)
4
], 70°C, 24 h; ii) [Ir(Cp )Cl
2
]
2
, MeOH, RT, 24 h.
Binding energy / eV
A Schiff base modified Au catalyst by Liu et al. showed a TOF of
Figure 1. XPS survey spectrum of 4 wt.% Ir@CMP, spectra
of C 1s; Ir 4f spectra, N 1s and O 1s spectra.
ꢀ
1
17b
ꢀ1
4
368 h at 50°C. Bi et al. reported a TOF of 5530 h for Pd on
1
7e
Nꢀdoped carbon at 25°C.
This demonstrates that supported
The adsorption isotherm exhibits a hysteresis which, based on
DFT pore size distribution, can be attributed to micropores and
mesopores (c.a. 5 nm). This is comparable to the data of Cooper et
al. who reported a BET surface area of 822 m /g and similar
isotherms. TGA shows that the polymer is thermally stable up
to 350°C (Figure S1) and the XRD pattern shows very broad and
weak reflexes at 25° and 45° 2θ, suggesting that the polymer is
amorphous (Figure S2). ICP analysis of the material shows that
ca. 0.1wt% Cu and 0.1 wt% Pd from the crossꢀcoupling procedure
are still present in the polymer matrix. The polymers were washed
several times with strong acids (such as glacial acetic acid and
1 M HCl) however ICP confirmed that Cu and Pd still remained
in the polymers. The CMP was loaded with different Ir concentraꢀ
metal catalysts, although active, generally exhibit lower activities
than homogeneous catalysts. To combine the advantages of heterꢀ
ogeneous and homogeneous catalysis, metal complexes immobiꢀ
2
lized on solid supports (e.g. SiO , polystyrene (PS)) have been
30a
2
proposed. Immobilized catalysts should retain the high activity
and selectivity of metal complexes and allow facile separation and
2
3
recycling. Typically Nꢀ and Pꢀdonor based ligands are tethered
to the solid support and loaded with metal complexes via coordiꢀ
nation. For example Laurenzcy et al investigated Ru/TPPTS
24
immobilized on PPh ꢀPS, DOWEX and NaꢀBEA. An initial
3
ꢀ1
TOF of 427 h at 90°C was reported for Ru/TPPTS@PPh ꢀPS.
3
ꢀ
1
Gan et al. reported
a
TOF of 2750
h
at 90°C for
2
5
Ru/TPPTS@PPh ꢀMCM41. Deng et al. reported TOF`s of 7357
3
ꢀ
1
ꢀ1
ꢀ1
tions (0.1wt% ꢀ 4.0 wt.%) by stirring with [IrCp*Cl ] in methanol
over 2 days at room temperature. The Irꢀloaded polymer (1 wt%
Ir@CMP) possesses a significantly reduced specific surface area
2 2
h , 3295 h and 1428 h for RuCl supported on SiO ꢀSH, SiO ꢀ
3
2
2
2
6
PPh and SiO ꢀNH , respectively. The development of functionꢀ
3
2
2
al porous polymers opened up new opportunities in heterogeneous
2
2
7
(482 m /g, Figure S3) which is attributed to the partial pore fillꢀ
catalysis. Tailor made porous organic polymers (POPs) featurꢀ
ing high surface areas and extraordinary chemical and thermal
stability have attracted considerable attention. A recent study by
us on Pꢀcontaining porous polymers demonstrated that they are
promising catalysts for onꢀsite hydrogen production in the bioreꢀ
ing. TEM image (Figure S4) shows that the polymers are comꢀ
posed of polydisperse particles. Ir nanoparticles were not obꢀ
served suggesting a molecular dispersion of metal complex. The
13
C SSꢀNMR spectrum (Figure S5) exhibits signals at 150 ppm
ꢀ
1
for the C=N of bipyridine, at 125ꢀ135ppm for the aromatic groups
and at 90 ppm for the C≡C of ethynyl, confirming the proposed
structure. Xꢀray photoelectron spectroscopy (XPS) of 4wt.%
Ir@CMP revealed the presence of Ir, O, C, Br, Cl and N (Figꢀ
ure 1). The presence of Br is attributed to incomplete conversion
of ArꢀBr end groups of the CMP polymer. Signals corresponding
to Ir 4f3/2 and Ir 4f1/2 are observed at 62.5 and 65.5 eV (spinꢀ
energy separation: 3 eV), indicating the presence of Ir(III) speꢀ
finery. The highest TOF (22900h ) could be obtained using
2
8
Ru@pDPPE at 160°C. Nevertheless, contrary to Pꢀbased ligꢀ
ands, Nꢀbased ligands are insensitive to oxidation and allow hanꢀ
dling and storage of the catalyst in air. In addition, Ir systems are
more efficient in FA dehydrogenation compared to Ru systems.
29
ꢀ1
Recently, Bavykina et al. reported an initial TOF of 27 000 h at
0 °C using a [IrCp*(OH)](OTf) loaded covalent triazine frameꢀ
8
2
work (CTF). This shows that Ir/N based porous polymers have a
high potential for FA dehydrogenation. Besides the cyclotrimeriꢀ
zation method used for CTF materials, Nꢀdonor ligands can also
be incorporated into porous polymers using crossꢀcoupling reacꢀ
tions. E.g. conjugated microporous polymers (CMP) are a wellꢀ
known class of materials and can be prepared via the Soꢀ
31
cies. This is comparable to the Ir 4f signals observed for the
complex [[IrCp*Cl ] ] (Figure S6). This together with the Cl
2
2
signals confirms that the [IrCp*Cl ] is successfully immobilizaꢀ
tion onto the CMP support without reduction.
.2 Dehydrogenation of FA
2
2
2
3
0
nogashiraꢀHagihara reaction. Here we propose the use of a
bipyridineꢀbased CMP loaded with [IrCp*Cl ] as catalyst for the
The Ir@CMP material as well as CMP, Ir/C, [IrCp*Cl ] and the
homogeneous analog [IrCp Cl(2,2ꢀbipy)]Cl were tested in the
2 2
2
2
*
aqueous, baseꢀfree dehydrogenation of FA.
dehydrogenation of aqueous FA (10 wt%, 2.17 M) at 160°C. The
results are summarized in Figure 2 and Table 1. A blank test gave
only 2.2 % conversion after 60 minutes, indicating that thermal
decomposition is negligible. Commercial Ir/C gave a similar
conversion confirming that Ir nanoparticles are not active in the
reaction. ICP analysis revealed that ca. 15 % of the introduced Ir
had leached into the reaction solution. Also the precatalyst
IrCp*Cl ] gave only 4.1 % conversion, demonstrating the need
for bipyridine ligands to achieve high activity. [IrCp*Cl /2,2ꢀ
bipy] reached full conversion within 25 minutes and a TOF of
43051h could be obtained.
2
. RESULTS AND DISCUSSION
2
.1 Catalyst synthesis and characterization
CMP was prepared using the monomers, 5,5‘ꢀDibromoꢀ2,2ꢀ
bipyridine (1), 1,3,5ꢀtriethynylbenzene (2), 1,4ꢀdibromobenzene
(
3) (Scheme 2). Polymerization is catalyzed by [Pd(PPh ) ] and
3 4
[
2 2
CuI in a mixture of N,Nꢀdimethylformamide and NEt overnight
3
2
at 70°C. After washing with CHCl , water, MeOH and acetone,
3
CMP was obtained as a yellowꢀbrown powder. N physisorption
2
ꢀ1
2
ꢀ1
of the CMP revealed a BET surface area of 706 m g .
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-
-
-
8.8
2
9.3
.25 2.3 2.35 2.4 2.45 2.5
1
2
3
4
5
6
7
8
9
9.8
-
-
-
10.3
10.8
11.3
y = -10.827x + 15.645
R² = 0.993
-11.8
1
000/T (K-1)
Figure 3. Reactor pressure profiles at different temperatures
left) and Eyring plot (right)
10
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58
59
60
(
Figure 2 Reactor pressure profiles of Ir-catalyzed dehydro-
genation of aqueous formic acid at 160°C
Table 2. Influence of reaction temperature on 1wt%
Ir@CMP catalyzed FA dehydrogenation between 120 -
[
a]
1
60°C.
Table 1. Ir-catalyzed dehydrogenation of aqueous formic
acid at 160°C.[a]
Temp.
(°C)
160
FA conv.
[%]
TOF
[CO](g)
Ir-loss
[%]
1.1
-
1
[h ]
[ppm]
Catalyst
FA
conv.
TOF
[CO](g) Ir
99.8
35246 117
13756 125
-1
loss
[h ]
[ppm]
150
99.8
1.4
[
%]
[%]
1
1
40
30
99.9
7488
3959
0
144
152
ꢀ
1.7
[b]
Blank
2.2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
99.9
5.2
[
c]
Ir/C
[IrCp*Cl2]2
[IrCp*Cl ] /2,2ꢀbipy
2.2
15.0
120
0.0
ꢀ
[
d]
4.1
ꢀ
[b]
1
10
98.9
3309
148
1.00
[
e]
99.9
13.1
99.8
99.8
43051 388
n.d. n.d.
ꢀ
2
2
[
a] Conditions: 15 mL 10 wt% FA soln. (32.6 mmol FA), 50
[f]
CMP
ꢀ
mg 1 wt% Ir@CMP, 0.008 mol% Ir, 160°C. [b] 3.24 mmol
HCOONa, 50 mg Ir@CMP, 0.008 mol% Ir.
[
g]
1
1
.0 wt% IrCl @CMP
19568 75
1.00
1.10
3
[
a]
.0 wt% Ir@CMP
35246 117
Interestingly at 120°C no pressure build up was observed indicatꢀ
ing that the reaction does not proceed or only very slowly. Using
[
a] Conditions: 15 mL 10 wt% FA soln. (32.6 mmol FA),
‡
the Eyring equation the enthalpy of activation (ꢁH ) was deterꢀ
0
.008 mol% Ir, 160°C. [b] 90 min. [c] 50 mg Ir/C (1 wt%,
mined by linear regression from the plot of ln(k/T) vs 1000/T
0.008 mol% Ir). [d] 2.25 mg, [e] 1.0 mg [IrCp*Cl ] , 6.0 mg bipy,
2
2
(
Figure 3, right). The data indicate an apparent activation barrier
0.008 mol% Ir. [f] 50 mg. [g] 50 mg IrCl @CMP 0.008 mol% Ir,
‡
3
of 90.0 kJ/mol with an entropy of activation (ꢁS ) of ꢀ67.5
J/(molꢂK). At a temperature of ca. 418 K this leads to an apparent
free energy of activation (ꢁG ) of 118.2 kJ/mol. The activation
1
60°C.
‡
Surprisingly the CMP material without Ir showed activity and
resulted in 13.1 % conversion after 50 min. Figure 2 shows that
the reactor pressure increases to 12 bar in 34 min, after which the
pressure remains constant. As discussed above ICP data of the
CMP materials showed traces of Cu and Pd. Hence it is proposed
enthalpy found by for Ir/CMP is approx. 12 kJ/mol higher than
+
.
11
that reported for [IrCp*(pyrazol pyrimidine)]
The free energy
of activation is in agreement with the energy spans calculated for
+
11
[IrCp*(pyrazol pyrimidine)] : 111.4ꢀ132.7 kJ/mol
and
from
1
8
+
32
that the activity of CMP is likely caused by Pd. In contrast,
Ir@CMP (1wt.% Ir) reached near full conversion (TON 12475)
[IrCp*(4,4ꢀdihydroxyꢀ2,2,ꢀbipyridine)] : 101.3 kJ/mol
reported DFT data. This suggests that Ir/CMP catalyzed FA dehyꢀ
drogenation occurs via the same mechanism as the homogeneous
analogs. It is known that the catalytic dehydrogenation of FA can
ꢀ1
within 27 minutes with a maximum TOF of 35246 h . ICP of the
reaction solution revealed that 1.1% Ir is leached into solution. A
test with the filtered solution, to which fresh formic acid was
added, gave no conversion, confirming that the leached species
5
be improved by the addition of formate. When the reaction at 130
°C is performed in the presence of 0.1 eq. sodium formate, the
ꢀ
1
are inactive. GC analysis of the gas phase showed a high H /CO2
activity is increased by a factor of 1.8, yielding a TOF of 7291 h
(Figure S7). As such the temperature could be lowered to 110°C
2
selectivity with a CO impurity of 117 ppm. [IrCp*Cl /2,2ꢀbipy]
2
ꢀ
1
demonstrated
a
similar catalyst performance compared to
resulting a TOF of 3309 h with 148 ppm CO in the the gas
phase. At 100 °C, no pressure increase could be observed within
the monitored time frame, again indicating a very slow reaction.
Ir@CMP (1wt.% Ir). Compared to Pꢀcontaining porous polyꢀ
28
mers, the CO content of the gasphase was 8 times higher; howꢀ
ꢀ1
ever Ir@CMP showed a higher maximum TOF of 35246 h . To
The influence of FA concentration was tested while keeping the
catalyst amount constant. The data (Figure S8 and Table 3) conꢀ
firm an approximately linear dependence of the TOF on FA conꢀ
centration between 10ꢀ50%. Interestingly, a low amount of CO
was produced at 50% FAꢀsolution (140 ppm) and a low Irꢀcatalyst
(<1.1%) leaching was observed at higher formic acid concentraꢀ
tions between 25ꢀ50%. The experiment with 100% FA gave inꢀ
test if Ir(III) salts are also suitable for the reaction, we have loaded
CMP with IrCl (IrCl @CMP (1wt.% Ir). Also for this catalyst,
3
3
the reaction reached full conversion (TON 12475) within 42
ꢀ
1
minutes with a maximum TOF of 19568 h .These results demonꢀ
strate that Ir@CMP is a highly active catalyst for FA dehydroꢀ
genation. The influence of reaction temperature was studied beꢀ
tween 120 – 160 °C. The results are summarized in Figure 3 and
Table 2.When the temperature is lowered to 130°C the TOF deꢀ
complete conversion (91.9%) within 234 minutes with a TOF of
ꢀ1
5
034 h and 365 ppm CO was observed. This indicates a decreasꢀ
ꢀ1
creases to 3959 h . The CO concentration slightly increases to
ing catalyst performance in pure FA.
1
52 ppm whereas the loss of Ir increases to 5.2%.
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Table 3. Influence of formic acid concentration on
2500
Page 4 of 26
650
600
Ir(III)
C 1s
[
a]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3 eV Ir(0)
3 eV
Ir@CMP catalyzed FA dehydrogenation.
5
50
2000
1500
FA conc.
wt%] / [M]
t
FA conv. TOF
[CO](g)
[ppm]
Ir
loss
500
450
400
Ir 4f
-1
[
[min] [%]
[h ]
N 1s
1
000
[
%]
3
50
300
50
Br 3p
200
5
00
0
Ir 4f
1
2
0 / 2.17
5 / 5.42
34
99.8
99.9
99.8
91.9
35246 117
51518 128
73095 140
1.1
0.6
1.1
1.2
2
59
68
66
64
62
60
400
300
100
0
Binding energy / eV
Binding energy / eV
50 / 10.85
00 / 2.17
70
1
234
5034
365
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
a] conditions: 15 mL aqueous FA soln., 50 mg 1 wt%
Ir@CMP, 160°C.
Table 4. Recycling of Ir@CMP using Start-stop and solvent-
[
a]
change procedures.
Recycling catalyst mol%
amount Ir
Run TOF
[CO](g) Ir
loss
-1
[h ]
[ppm]
[
mg]
[
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
%]
5
0
0.0080
0.0080
1
2
3
4
5
6
29026 128
29235 138
29549 126
29315 134
30408 132
30300 125
Figure 4. XPS survey spectrum (left) and XPS iridium spec-
0
0
0
0
.0080
.0080
.0080
.0080
1
3
Startꢀstop
trum (right) of Ir@CMP-rec-M1. Overlay of C SS-NMR of
CMP (red) and 1 wt.% Ir@CMP-M1 (black) (bottom)(* =
spinning side-band)
1.0
The XPS survey spectrum (Figure 2) of Ir@CMPꢀrecꢀM1 reveals
the presence of O, N, C, Br and Ir. Interestingly, compared to the
fresh catalyst the Br 3p signal is now much less intense. This
suggests the occurrence of reductive dehalogenation during reacꢀ
tion. Next to the Ir 4f signals of the original Ir(III) complex, addiꢀ
tional signals are observed at 61.5 eV and 64.5 eV originating
from Ir(0) species. This is in line with the formation of nanopartiꢀ
cles, but also confirms that still a significant portion of the metal
remains molecularly dispersed on the polymer. In addition, the Cl
1
10
0.0175
0.0150
0.0123
0.0111
0.0098
0.0086
1
2
3
4
5
6
18981 127
22212 87
25623 100
26694 87
30407 116
35016 104
0.59
0.58
0.45
0.27
0.19
0.19
94
77
70
Solvent
Change
6
5
2
4
ꢀ
[a] conditions: 15 mL aqueous FA soln., 1 wt% Ir@CMP,
60°C.
2p signals are absent suggesting that the Cl ligands are completeꢀ
ly exchanged during the reaction.
1
13
A comparison of the C CPꢀMAS NMR spectra of the asꢀ
synthesized CMP material and Ir@CMPꢀrecꢀM1 is shown Figꢀ
ure 4 (right).The spectrum of the spent catalyst shows significant
changes in the aromatic range (140ꢀ110 ppm) i.e., two additional
peaks (Stilbene) are observed at 137 ppm and 129 ppm. Also the
signal corresponding to the ethynyl moieties (90 ppm) is reduced
in intensity. It is proposed that either the Ir complexes or the
nanoparticles catalyze the partial hydrogenation of ethynyl groups
In contrast, the Ru/P catalysts showed significantly increased CO
production and leaching at FA concentrations over 30 wt.% FA
This shows that Ir/Nꢀbased systems are better suited for converꢀ
sion of conc. FA solutions.
2
.3 Recycling
Catalyst recycling was performed in two ways. The first series of
experiments was performed using a startꢀstop procedure (RecꢀM1)
in which the decomposed FA was reꢀadded after each run. The
data (Table 4) illustrate a minor increase of the TOF. The CO
content remains stable in the range between 125ꢀ138 ppm. At the
end of the series 1% of the introduced Ir had leached into solution.
The second recycling method is based on a solventꢀchange proceꢀ
dure (RecꢀM2), wherein after each run the catalyst was filtered
and dried and reꢀintroduced into a fresh FA solution. Due to miꢀ
nor losses after each run, the catalyst amount decreased from
110 mg to 54 mg in the last run. Interestingly the data show that
the TOF steadily increases over the runs. The CO concentrations
33
in the polymer backbone. Signals corresponding to complete
hydrogenation were not observed.
34
Finally the effect of metal loading was studied. To this end a
series of catalysts with decreasing metal content was prepared and
tested at 160°C. The data (Figure 5 and Table 5) reveal that with
decreasing metal loading on the polymer and thus decreasing
metal content in the reaction, the reaction remains fast and full
conversion is obtained after 20 – 30 min. Only in the case of
0.1 wt% Ir@CMP, the rate of the reaction decreases and full
conversion is reached after 120 min. As a result, the TOF increasꢀ
es from 13 794 h for 4 wt% Ir@CMP to 123 894 h for 0.1 wt%
Ir@CMP. The data suggest that most of the metal remains molecꢀ
ularly dispersed on the polymer. We suggest that only a limited
amount of actives sites are available and this could explain why a
higher catalyst loading does not lead to much faster reaction. In
further research we will focus on the limiting factors of these
reaction and application of the catalyst in a fuel cell.
ꢀ
1
ꢀ1
(87ꢀ128 ppm) are stable and comparable to the previous experiꢀ
ments. It is also noteworthy that leaching of Ir is low (0.59 %) and
gradually decreases with each run. However, a cumulative amount
of 2.3 % was lost after the last run.
TEM images of the spent catalysts (Ir@CMPꢀRecꢀM1 and
Ir@CMPꢀRecꢀM2, Figure S9 and S10) reveal the formation of
large amounts of small nanoparticles (ca. 5 nm) and a few isolated
big nanoparticles.
ACS Paragon Plus Environment

Page 5 of 26
ACS Catalysis
tained from TCI. 5ꢀBromoꢀ2ꢀiodopyridine, NEt3, CuI and
[IrCp*Cl ] were purchased from Sigma Aldrich. [Pd(PPh ) ] was
1
2
3
4
5
6
7
8
9
2
2
3 4
obtained from Abcr. Formic acid (99ꢀ100%) and MeOH were
obtained from Chem Solute. Et O, Toluene, CHCl , dimethylforꢀ
2
3
mamide and acetone was purchased from Applichem Panreac.
Xꢀray diffraction (XRD) was performed on a Siemens D5000 with
a Cu κꢀalpha, 0.15418 nm radiation source. Thermogravimetric
analysis (TGA) was performed on a STA 409 cell from Netzsch.
N ꢀphysisorption was performed on a Asap 2000 from Micromerꢀ
2
tics (the sample was degassed in vacuo at 200°C for 24 h). XPS
(Xꢀray Photoelectron Spectroscopy) measurements were carried
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
out in an Ultra AxisTM spectrometer, from Kratos Analytical,
Manchester UK. The samples were irradiated with monoenergetic
Al K*1,2 radiation (1486.6 eV) and the spectra were taken at a
power of 144 W (12 kV x 12 mA). The aromatic carbon (C=C) at
a binding energy of 285 eV (C 1s photoline) was used to deterꢀ
Figure 5. Pressure profiles of Ir@CMP-catalyzed dehydro-
genation of formic acid with different metal loadings.
1
3
mine the charging. The solid state magic angle spinning C NMR
spectra were recorded on Bruker avance III, 500 Mhz (4 mm
1
13
Table 5. Effect of metal loading on Ir@CMP-catalyzed dehy-
MAS rotor, 11 kHz spinning, RT, Hꢀ C Cross Polarization).
HPLC analysis was performed on a Shimadzu 2020 with an Orꢀ
ganic Acid Resin column (2 mM Trifluoroacetic acid, 1 mL/min,
40°C). Gas chromatography (GC) was performed on a Agilent HP
6890 with a Shin Carbon ST micropacked GC column, (35ꢀ
100°C, TCD, He). Inductively coupled plasma (ICP) was measꢀ
ured on a SPECTROFLAME by SPECTRO Analytical Instruꢀ
ments.
[
a]
drogenation of formic acid.
Catalyst
[Mol%] FA
conv.
%]
TOF
[h-1]
[CO](g) Ir-
loss
[ppm]
[
[%]
1.1
0.8
1.1
0.1
0.1
0.0
4.0wt%
0.032
0.016
0.008
0.004
0.002
0.001
99.9
99.8
99.8
99.9
99.8
99.9
13794
25496
35246
52880
118
121
117
87
2
1
0
.0wt%
.0wt%
.5wt%
5
,5-dibromo-2,2’-bipyridine (1) was prepared according to a
35
literature procedure. 5ꢀBromoꢀ2ꢀiodopyridine (6.06 g, 21.35
mmol) was dissolved in toluene (120 mL). Pd(PPh ) (0.66 g, 0.57
3 4
n
mmol) and Bu Sn (6,65g, 11.46 mmol) were added to the soluꢀ
6
2
0.2wt%
101576 71
123894 68
tion. After 5 min. a color change from light yellow to red was
observed. The stirred mixture was refluxed for 5 days. The cooled
reaction mixture was diluted with diethylether (200 mL), resulting
in a suspension. The brown precipitate was filtered over a glass
filter and washed with distilled water (200 mL) and diethyl ether
0
.1wt%
[
a] conditions: 15 mL 10 wt% FA solution (32.6 mmol FA),
1
3
60°C.
(200 mL). The product was dissolved in chloroform and extracted
. CONCLUSION
three times with brine and dried with MgSO . The crude product
4
In this study, we successfully utilized a bipyridineꢀbased conjuꢀ
gated microporous polymer (CMP) for the Irꢀcatalyzed dehydroꢀ
genation of formic acid to H and CO in aqueous media. The
was recrystallized from methanol giving a colorless solid (2,47 g
1
74%). H NMR (500 Mhz, CDCl
, 25°C): δ(ppm)= 8.68 (d, J =
3
2.4 Hz, 2H, H6/6’) 8.24 (d, J = 8.5 Hz. 2H, H4/4’), 7.89 (dd, J = 8.5,
2.4 Hz, 2H, H3/3’) 13C NMR (101 MHz, CDCl , 25°C) δ(ppm) =
2
2
results clearly demonstrate that the bipyridine moieties of the
3
CMP enable a facile immobilization of the [IrCp*Cl ] complex,
152.64 (2C, C2/2’), 149.26 (2C, C6/6’) 138.59 (2C, C4/4’) 121.21
(2C, C3/3’), 120.43 (2C, C5/5’).
2
leading to a highly active heterogeneous catalyst. A comparison
with the homogeneous analog shows that the activity of Ir@CMP
is only mildly affected by heterogenization. Notably the Ir@CMP
6
5
CMP was synthesized according to a literature procedure. .
1,3,5ꢀtriethynylbenzene (1126 mg, 7.5 mmol), 1,4ꢀ
exhibits low leaching and high selectivity to H /CO and recycling
2
2
dibromobenzene (1415 mg, 6.0 mmol) and 5,5‘ꢀDibromoꢀ2,2ꢀ
bipyridine (471 mg, 1.5 mmol) were dissolved in a mixture of
DMF (30 mL) and NEt3 (30 mL). To the stirred solution,
studies confirm that the catalyst activity and stability remain
stable for more than 6 runs. The data also show that the catalyst
performs well in FA concentrations up to 50 wt% (10.8 M). Charꢀ
acterization of the spent catalysts revealed the formation of signifꢀ
icant amounts of (inactive) nanoparticles, suggesting that the
observed activity is actually caused by a relatively low amount of
active sites with extremely high activity. This is supported by a
variation of the metal loading which resulted in calculated TOFs
[
Pd(PPh ) ] (90 mg, 0.09 mmol) and CuI (30 mg, 0.15 mmol)
3 4
were added and left to react overnight at 70°C. The precipitated
polymer was filtered and washed with chloroform (2x300 mL),
water (2x 300mL), methanol (2x 300 mL) and acetone (2x 300
mL). The polymer was further purified by soxhlet extraction with
methanol for 24 hours. Finally the brownꢀyellow solid was dried
in vacuo at 60°C giving 2433 mg CMP.
ꢀ
1
up to 123 894 h was confirmed by decreasing the metal loading.
This study demonstrates that Ir@CMP exhibits desirable features
such as facile preparation, base free operation, a superior catalytic
activity as well as high H /CO selectivity and is a promising
Ir@CMP. CMP (1 g) and [IrCp*Cl ] were stirred in methanol
2
2
(100 mL) for 2 days. The polymer was filtered over a glass filter
2
2
and dried under in vacuo at 60°C. The filtrate was concentrated in
vacuo and dissolved water and transferred into a volumetric flask
heterogeneous catalyst for the continuous hydrogen generation
from formic acid.
(25mL) and analyzed by ICP.
4
. Experimental Section
Formic acid dehydrogenation. Catalyst, water and formic acid
were charged into a Hastelloy autoclave (50 mL) equipped with a
pressure gauge and with a stirring bar. The autoclave was closed
All solvents were dried and degassed before use. Reactions were
performed under inert atmosphere using standard Schlenk techꢀ
niques. Hexa(nꢀbutyl)ditin and 1,4ꢀdibromobenzene were purꢀ
chased from Alfa Aesar and 1,3,5ꢀtriethynylbenzene were obꢀ
and flushed with H (3x 60 bar) and heated to the desired reaction
2
temperature while stirring. Time zero was set at the point where
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 26
the reaction reached 160°C. The pressure was periodically recordꢀ
2007, 107, 3952–3991. (c) Mori, K.; Dojo, M.; Yamashita, H. ACS Catal.
013, 3, 1114–1119.
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ed until no further change occurred. After completion of the reacꢀ
tion, the autoclave was cooled to room temperature and the gas
phase transferred to a gas bag for GCꢀanalysis. The catalyst was
filtered (milipore filter polyamide, 45/25, 0.45 ꢃm) and an aliquot
of the filtrate analyzed by HPLC and ICP. Catalysts screening and
temperature variation were performed twice and the data averꢀ
aged.
(
(
4) Coffey, R. S. Chem. Commun. 1967, 923
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Chen, H.; Anders, A.; Lai, Z.; Zheng, J.; Huang, K. W. Chem. Asian J.
2016, 11, 1357–1360.
Start-Stop recycling. After completion of the reaction, the autoꢀ
clave was cooled to r.t. The gas phase was transferred to a gas
bag, the autoclave was opened and a portion of pure formic acid
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
was added. The autoclave was closed, flushed 3x with 60 bar H2
and heated to reaction temperature. After the last run the catalyst
was filtered and the filtrate analyzed by ICP.
(
8) Iguchi, M.; Himeda, Y.; Manaka, Y.; Kawanami, H. ChemSusChem
016, 9, 2749–2753.
9) Onishi, N.; Ertem, M. Z.; Xu, S.; Tsurusaki, A.; Manaka, Y.;
Solvent change recycling. After each run, the autoclave was
cooled to r.t. and the gas phase transferred to a gas bag. The conꢀ
tents were transferred to a 25 mL volumetric flask and an aliquot
filtered for ICP and HPLC analysis. The catalyst in the volumetric
flask was recovered from the reaction products by filtration under
inert atmosphere and drying in high vacuum at 60°C.
2
(
Muckerman, J. T.; Fujuta, Y.; Himeda, Y. Catal. Sci. Technol. 2016, 6,
988–992.
(10) Celaje, J. J. A.; Lu, Z.; Kedzie, E. A.; Terrile, N. J.; Lo, J. N.;
Williams, T. J. Nat. Commun. 2016, 7, 1–6.
(11) Wang, W.; Ertem, M. Z.; Xu, S.; Onishi, N.; Manaka, Y.; Suna,
TOF calculation
Y.; Kambayashi, H.; Muckerman, J. T.; Fujita, E.; Himeda, Y. ACS Catal.
2015, 5, 5496–5504.
(12) Li, J.; Li, J.; Zhang, D.; Liu, C. ACS Catal. 2016, 6, 4746−4754.
TOF is calculated based on the assumptions that, full to near full
conversion is reached, that the dehydrogenation is highly selective
(13) Hull, J. F.; Himeda, Y.; Wang, W.ꢀH.; Hashiguchi, B.; Periana, R.;
(>99.9%) and a negligible solubility of gasses. Pressure diagrams
Szalda, D. J.; Muckerman, J. T.; Fujita, E. Nat. Chem. 2012, 4, 383–388.
(14) Fink, C.; Laurenczy, G. Dalton Trans. 2017, 46, 1670.
were converted to formic acid conversion diagrams using the
formula:
(
15) Matsumani, A.; Kuwata, S.; Kayaki, Y. ACS Catal. 2017, 7, 4479ꢀ
4484.
(16) (a) Liu, C.; Xie, J.; Tian, G.; Li, W.; Zhou, Q. Chem. Sci. 2015, 6,
FA Conv. = 1ꢀ(P /P(final))
(t)
ꢀ
1
The rate constants (k (min )) were determined by linear fitting.
The turnover frequency (TOF) was calculated using the formula:
2928–2931. (b) Czaun, M.; Kothandaraman, J.; Goeppert., A.; Yang, B.;
Greenberg, S.; May, R. B.; Olah, G. A.; Surya Prakash, G. K. ACS Catal.
2016, 6, 7475–7484.
ꢀ
1
TOF (h ) = k * 60 min * N / NIr
FA
(17) (a) Bi, Q.; Lin, J.; Liu, Y.; He, H.; Huang, F. J Power Sources
AUTHOR INFORMATION
2016, 328, 463–471.(b) Qinggang, L.; Yang, X.; Yanqiang, H.; Shutao,
X.; Su, X.; Pan, X.; Xu, J.; Wang, A.; Liang, C.; Xinkui, W.; Zhang, T.
Energy Environ. Sci. 2015, 8, 3204–3207. (c) Bi, Q.; Lin, J.; Liu, Y.;
Huang, F. Int. J. Hydrogen Energy 2016, 41, 21193–21202. (d) Ojeda, M.;
Iglesia, E. Angew. Chem. Int. Ed. 2009, 48, 4800–4803. (e) Bi, Q.; Du, X.;
Liu, Y.; Cao, Y.; He, H.; Fan, K. J. Am. Chem. Soc. 2012, 134,
8926−8933. (f) Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M. Chem.
Soc. Rev. 2016, 45, 3954–3988.
Corresponding Author
Palkovits@itmc.rwthꢀaachen.de
Hausoul@itmc.rwthꢀaachen.de
ASSOCIATED CONTENT
(18) (a) GarcíaꢀAguilar, J.; NavlaniꢀGarcía, M.; BerenguerꢀMurcia, A.;
Supporting Information
Kohsuke, M.; Kuwahara, Y.; Hiromi, Y.; AmorósꢀCazorla, D. RSC Adv.
2016, 2, 91768–91772. (b) NavlaniꢀGarcía, M.; Kohsuke, M.; Nozaki, A.;
Yasutaka, K.; Yamashita, H. ChemistrySelect 2016, 1, 1879–1886. (c)
Martis, M.; Lozanoꢀcastelló, D.; Cazorlaꢀamorós, D. Catal. Sci. Technol.
Characterization of CMP with and without metal loading and after
catalysis including XRD, TGA, N ꢀphysisorption, solid state
2
NMR, ICP, XPS and STEM(ꢀEDX); graphical representation of
the impact of FA concentration: This material is available free of
charge via the Internet at http://pubs.acs.org.
2
015, 5, 364–371. (d) NavlaniꢀGarcía, M.; Mori, K.; Nozaki, A.; Kuwahaꢀ
ra, Y.; Yamashita, H. Ind. Eng. Chem. Res. 2016, 55, 7612−7620. (e)
Chaoquan Hu, Jayasree K. Pulleri, SiuꢀWa Ting, K.ꢀY. C. Int. J.
Hydrogen Energy 2014, 39, 381–390. (f) Lv, Q.; Feng, L.; Hu, C.; Liu, C.;
Xing, W. Catal. Sci. Technol. 2015, 5, 2581–2584. (g) Jiang, K.; Xu, K.;
Zou, S.; Cai, W. J. Am. Chem. Soc. 2014, 136, 4861–4864.
(19) (a) Wang, Z.; Yan, J.; Wang, H.; Ping, Y.; Jiang, Q. J. Mater.
Chem. A 2013, 1, 12721–12725. (b) Yan, J.; Wang, Z.; Gu, L.; Li, S.;
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Ping, Y.; Yan, J.; Wang, H.; Jiang, Q. Int. J. Hydrogen Energy 2014, 39,
ACKNOWLEDGMENT
The authors acknowledge financial support by the European
Regional Development Fund (ERDF) and the state of Northꢀ
Rhine Westphalia, Germany under the operational program ‘’Reꢀ
gional Competitiveness and Employment’’ Project ‘’Sustainable
Chemical Synthesis’’ and the Federal Ministry of Education and
Research (BMBF) for funding this work with the MANGAN
research cluster BMBFꢀPTJ FKz 03SF0508.
4
850–4856. (d) Cheng, J.; Gu, X.; Liu, P.; Wang, T.; Su, H. J. Mater.
Chem. A 2016, 4, 16645–16652. (e) Zhou, X.; Huang, Y.; Liu, C.; Liao, J.;
Lu, T.; Xing, W. ChemSusChem 2010, 3, 1379–1382. (f) Gu, X.; Lu, Z.;
Jiang, H.; Akita, T.; Xu, Q. J. Am. Chem. Soc. 2011, 133, 11822–11825.
(g) Zhao, P.; Xu, W.; Yang, D.; Luo, W.; Cheng, G. ChemistrySelect
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ACS Catalysis
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A bipyridineꢀbased conjugated microporous polymer (CMP) loaded with [Cp*IrCl ] is proposed for the baseꢀfree aqueous
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dehydrogenation of formic acid to H /CO . This catalyst exhibits high activity and selectivity under a wide range of condiꢀ
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tions and Recycling tests demonstrate facile catalyst separation with low Ir leaching. TOF’s up to 123 894 hꢀ1 were obtained
using 0.1wt% Ir loading.
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