A ruthenium porphyrin-based porous organic polymer for the hydrosilylative reduction of CO2 to formate

Article abstract of DOI:10.1039/c9cc02273b

A ruthenium porphyrin-based porous organic polymer (POP) was synthesized, characterized, and used to reduce CO₂ to a formate salt. We demonstrate that Ru-BBT-POP can be utilized to reduce CO₂ to a silyl formate and then converted to potassium formate with a respectable turnover number and frequency.

Full text of DOI:10.1039/c9cc02273b

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DOI: 10.1039/C9CC02273B
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A Ruthenium Porphyrin-Based Porous Organic Polymer for the
Hydrosilylative Reduction of CO to Formate
2
Received 00th January 20xx,
Accepted 00th January 20xx
Grace. M. Eder, David. A. Pyles, Eric Wolfson, and Psaras. L. McGrier*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A ruthenium porphyrin-based porous organic polymer (POP) was products throughout the catalytic process. The modular nature of
synthesized, characterized, and used to reduce CO
to a formate POPs permits the integration of various metalated organic linkers to
salt. We demonstrate that Ru-BBT-POP can be utilized to reduce efficiently convert CO into a value-added chemical or fuel. For
2
2
CO to a silyl formate and then converted to potassium formate instance, one common strategy often employed is the reaction of
2
with a respectable turnover number and frequency.
2
epoxides with CO to form cyclic carbonates in the presence of
metalated porphyrin- and bypyridine-based POPs.10-13 Recently,
Yoon and coworkers demonstrated that an amorphous covalent
triazine framework (CTF) decorated with Ir(III)-N-heterocyclic
2
The continuous accumulation of carbon dioxide (CO ) in the
atmosphere is believed to be one of the biggest contributors
1
to rising sea levels and climate change. As a consequence, CO
2
carbene complexes is very effective at converting CO
formate salt in the presence of an amine solution and H
2
into a stable
.14 Liu and
capture and conversion has become a desirable way to not
only convert this abundant C building block into a useful
2
1
coworkers have also shown that an amorphous microporous
organic polymer (MOP) containing Troger’s base and a Ru(III)
complex is also effective at performing this transformation under
similar reaction conditions.15 In contrast to these methods, the
feedstock for high-value commodity chemicals and fuels, but
2
also to reduce the worlds dependence on fossil fuels. For
2
example, the catalytic hydrogenation of CO to methanol or
formic acid has the potential to create alternative energy
reduction of CO
alternative route to functionalize CO
2
using hydrosilanes has also emerged as an
by trapping it as a silyl
3
sources. In particular, formic acid is of considerable interest
2
because it is a liquid at ambient temperature and has a high
volumetric hydrogen density of 53 g L-1 making it a viable
hydrogen storage material.4 In addition, formic acid also
exhibits low-toxicity, which makes it more environmentally
formate. In addition to being a favorable thermodynamic process,
silyl formates are advantageous because they can be converted
directly to formic acid/formate or an array of carbonyl compounds
simply by reacting them with the proper nucleophiles.16 Although
safe than petrochemicals. Although the hydrogenation of CO
2
1
7
many homogeneous catalytic systems using transition metals (Ru,
to formic acid has been achieved using a variety of
homogeneous catalysts composed of noble transition
metals,5-7 difficulties associated with recyclability and catalyst
separation are still common for most systems. This leaves
plenty of room to design heterogeneous catalytic systems for
Ir,18 Ni,19 Zr,20 Cu,21 and Zn ) have been reported for the
22
2
hydrosilylation of CO , examples of performing this transformation
under heterogeneous catalytic conditions has yet to be reported.
Herein, we present the synthesis, characterization, and catalytic
properties of a benzobisthiazole (BBT)-linked Ru(II) porphyrin-based
POP (Ru-BBT-POP) (Scheme 1). We demonstrate that Ru-BBT-POP
2
CO capture and conversion that are stable, recyclable, and
exhibit high efficiencies.
2
is an effective catalyst for the reduction of CO to silyl formate using
Porous organic polymers (POPs)8,9 have emerged as an
exceptional class of amorphous porous materials suitable for
heterogeneous catalytic applications due to their superior chemical
stability, high surface areas, and tunable pore sizes. These
characteristics enable the prompt diffusion of substrates and
hydrosilanes. The silyl formate was then efficiently converted to
potassium formate with a respectable turnover number and
frequency.
Ru-BBT-POP was synthesized by reacting premetalated Ru(II)-
tetrakis(4-formylphenyl)porphyrin (Ru-TFPP) with 2,5-diamino-1,4-
benzenedithiol dihydrochloride (DABTD) in a 1:1 (v/v) mixture of
a. Department of Chemistry & Biochemistry, The Ohio State University, Columbus,
Ohio 43210, United States
DMF and o-xylenes at -78 °C for 5 h to avoid prompt precipitation
of the imine-linked intermediate. Afterwards, the reaction mixture
was gradually brought to room temperature overnight, and
†
Electronic supplementary information (ESI) available: Synthetic procedures, FT-
1
3
IR, solid-state C NMR, TGA, PXRD, and SEM. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
2
2
1
1
50
00
50
00
View Article Online
DOI: 10.1039/C9CC02273B
5
0
0
0
0.2
0.4
0.6
0.8
1
Scheme 1 Synthesis of Ru-BBT-POP 1.
P/P0
______________________________________________________________
0
.06
0.05
.04
0.03
then stirred under air at 130 °C for 96 h. Ru-BBT-POP was obtained
by filtration and washed with acetone to produce dark red solids.
Ru-BBT-POP was purified by soaking the polymer in methanol and
dichloromethane for two days to remove unreacted monomers and
then dried under vacuum. Powder X-ray diffraction (PXRD) analysis
revealed that Ru-BBT-POP was amorphous and exhibited no long-
range order (Fig. S5, ESI).
3
.3 nm
1
.8 nm
0
5
.1 nm
0
0
.02
.01
0
Ru-BBT-POP was characterized by Fourier transform (FT-IR) and
13C
cross-polarization magic angle spinning (CP-MAS)
spectroscopies. The FT-IR spectra revealed stretching modes at
1
2
3
4
5
6
7
-1
1
654 (C=N) and 713 cm (C-S), which is indicative of the formation
of the benzothiazole ring (Fig. S3, ESI). The strong resonance at
938 cm-1 is attributed to the carbonyl ligand bound to Ru(II)
Pore Width (nm)
Fig 2. Nitrogen adsorption/desorption isotherm (top) and NLDFT
pore size distribution (bottom) for Ru-BBT-POP.
1
center. The FT-IR peaks corresponding to the phenyl and pyrrole
substituents were also confirmed (Fig. S3 and Table S3, ESI). The
connectivity of Ru-BBT-POP was verified by 13C CP-MAS exhibiting a
resonance at 167.0 ppm that corresponds to the central carbon on
the benzothiazole ring, and resonances at 124.2 and 119.9 ppm,
which correspond to the pyrrole substituents (Fig. S6, ESI). Diffuse
reflectance UV-vis spectroscopy was also used to verify the
presence of the porphyrin units of Ru-BBT-POP (Fig. S20, ESI).
Thermogravimetric analysis (TGA) indicated that Ru-BBT-POP
retained ~ 90 % of its weight up to 380 °C (Fig. S7, ESI). Scanning
electron microscopy (SEM) images revealed a cloud-like
morphology for the material (Figures 1a and b).
X-ray photoelectron spectroscopy (XPS) of Ru-BBT-POP
further confirmed the presence of Ru on the polymer surface
(Fig. 1c and d). The broad signals at 281.8 and 285.4 eV
correspond to the core energy levels of Ru 3d5/2 and Ru 3d3/2
,
respectively. The broad peak at 281.8 eV is typical for Ru(II)
species bound to nitrogen-based ligands, and further confirms
that Ru(II)-porphyrin complexes are embedded within the
POP.23,24 The presence of the carbon, nitrogen, and oxygen
atoms of Ru-BBT-POP were also confirmed by XPS. Inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
analysis indicated that ~ 6.32 wt% (0.75 mol%) of Ru was
incorporated into Ru-BBT-POP.
______________________________________________________________
(a)
(b)
2
μm
1 μm
The permanent porosity of Ru-BBT-POP was measured by
nitrogen gas adsorption at 77 K (Fig. 2). Ru-BBT-POP exhibited
a reversible type I isotherm with a noticeable hysteresis.
Application of the Brunauer-Emmett-Teller (BET) model over
3
2
2
1
1
0000
5000
0000
5000
0000
6000
5000
4000
3000
2000
1000
0
(c)
(d)
Ru 3d & C 1s
C 1s
84.7 eV
2
O 1s
Ru 3p
N 1s
the low-pressure region (0.001 < P/P
surface area of 655 m /g. Nonlocal density functional theory
0
< 0.069) provided a
Ru 3d3/2
2
85.4 eV
2
Ru 3d5/2
2
81.8 eV
5
000
0
(NLDFT) was used to estimate the pore size distribution of the
6
00
500
400
300
200
100
290
288
286
284
282
280
278
material yielding values of 1.8, 3.3, and 5.1 nm. This broad
Binding Energy (eV)
Binding Energy (eV)
pore size distribution is not uncommon for amorphous porous
polymers and is attributed to interparticle voids that can
mimic sizeable pores. The total pore volume was calculated
Fig. 1 SEM images at different magnifications (a and b) and XPS
spectra (c and d) of Ru-BBT-POP.
2
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Table 1 Catalytic Performance of Ru-POR and Ru-BBT-POP for the
reduction of CO to formate.
View Article Online
DOI: 10.1039/C9CC02273B
2
Scheme 2. Proposed mechanism for the hydrosilylative reduction of
2
CO to potassium formate.
from the single point value of P/P
of 0.312 cm g for Ru-BBT-POP. To assess the CO uptake
capacity of Ru-BBT-POP, we measured gas adsorption
isotherms at 273 and 298 K from 0 to 1.2 bar (Fig. S13, ESI).
0
= 0.901 to provide a value
best results, increasing the temperature to 80 °C did not
improve the efficiency of the reaction and resulted in a
decrease in yields (Entry 3 Table S7, ESI). It is worth noting that
reaction conditions in which KF was not used resulted in no
formation of potassium formate (Entry 2 Table 1).
3
-1
2
-1
Ru-BBT-POP displayed uptake capacities of 105 mg g and
-1
7
1 mg g at 273 and 298 K. Isoteric heat of adsorption (Qst)
With these results in hand, we then evaluated the
reactivity of Ru-BBT-POP under the optimal reaction conditions
-1
data revealed a high value of 48 kJ mol at zero coverage,
which is just below the lower limit for chemisorption (~ 50
kJ/mol). This indicates that the BBT-linkage and presence of
(
Entries 3 & 4 Table 1). At a catalyst loading of 0.5 mol %,
-1
Ru-BBT-POP afforded a TON of 67 and a TOF of 17 h within
h at 60 °C at 1 atm of CO (Entry 3 Table 1). Increasing the
Ru(II) significantly enhances the binding affinity for CO
2
4
2
2
5
due to a combination of Lewis acid/base and metal
catalyst loading of Ru-BBT-POP to 1 mol % under the same
reaction conditions improved the yield of the reaction but
resulted in a slight decrease in the TON (Entry 4 Table 1). The
TON values are on par with or higher than other homogeneous
Ru-based catalysts that have been reported for the
2
6
ionquadrupole interactions, respectively (Fig. S14, ESI).
In order to find the best reactions conditions suitable for
the reduction of CO
hydrosilanes and KF, we first examined the reactivity of CO
2
to formate using inexpensive
in
2
the presence of various hydrosilanes at temperatures ranging
from 45-80 °C (Table S6 & S7, ESI) using Ru-POR as a
homogenous catalyst (Scheme S4, ESI). We found that the
.27,28 In an effort to examine the
hydrosilylation of CO
2
recyclability of Ru-BBT-POP after each reaction, the dark red
solid was recovered by filtration, washed with MeCN, and used
directly in the next reaction. Fig 3 demonstrates that Ru-BBT-
POP exhibited a minor reduction in catalytic efficiency at 0.5
mol %, and virtually no reduction in performance at 1 mol %
after three cycles (Tables S8 & S9, ESI). FT-IR spectra revealed
that the resonance from the carbonyl ligand bound to the
Ru(II) center displayed no subtle changes (Fig. S4, ESI), which
suggests that the coordination environment around the Ru(II)-
porphyrin center remains intact after the reaction. In addition,
XPS data taken after catalysis also revealed no apparent
changes in the oxidation state of the Ru (Fig. S27, ESI).
However, a nitrogen isotherm of Ru-BBT-POP after catalysis
revealed a reduction in the surface area and pore volume of
the material (Fig. S10 & S11, Table S5, ESI). This reduction
could be attributed to the build-up of residual KF inside the
pores of Ru-BBT-POP.
reactions that were carried out using Me
2
PhSiH in MeCN at
0°C and 0.5 mol % Ru-POR for 4h at an initial CO pressure of
atm provided the best results yielding a TON of 110 and a
6
1
2
-1
2
TOF of 28 h (Entry 1 Table 1). While Me PhSiH provided the
1
Surprisingly, H NMR of the crude reaction mixture prior to
addition of KF revealed very little formation of the
intermediate dimethylphenylsilyl formate product and a
significant amount of a dimethylphenylsilyl ether by-product
with a ratio of 1:19, respectively (Fig. S28, ESI). However, the
formation of silyl ether by-products during the hydrosilylative
2
Fig 3. Recyclability of 0.5 (blue) and 1.0 (red) mol % Ru-BBT-POP at 60 reduction of CO is not uncommon and has been observed for
Ni- and other Ru-based29 catalytic systems. This could explain
19
°
2 2
C in the presence KF, Me PhSiH, and 1 atm of CO .
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the low TON and TOF values for this particular system. Based
on the experimental data collected, we propose the following
1
DOI: 10.1039/C9CC02273B
mechanism for the hydrosilylative reduction of CO
potassium formate using Ru-BBT-POP (Scheme 2). Initially, CO
coordinates to Ru-BBT-POP (A) to generate the activated
species (B). Then, CO undergoes σ-bond metathesis with
Me PhSiH (C) to produce the dimethylphenylsilyl formate (D).
2
to
1
2
9 L. González-Sebastián, M. Flores-Alamo and J. J. García,
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012, 14, 2642-2645.
From here, the dissociated fluoride ion is able to cleave the
dimethylphenyl silyl group to generate the product potassium
formate (E).
In summary, a Ru-BBT-POP was constructed and utilized for
2
the hydrosilylative reduction of CO to potassium formate. This
work provides a rare example of utilizing a Ru-based POP to
perform this chemical transformation. The proof-of-principle is
important as POPs metalated with cheap earth-abundant
metals (i.e. Cu, Co, Fe, etc.) could be utilized as sustainable
catalysts to produce formate/formic acid for energy storage
applications. Such investigations are underway in our
laboratory and will be reported in the near future.
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P.L.M. acknowledges the National Science Foundation (NSF) and
Georgia Tech Facilitating Academic Careers in Engineering and
Science (GT-FACES) for a Career Initiation Grant and funding from
the Center for Emergent Materials (DMR-1420451) at The Ohio
State University.
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Table of Contents Entry
R3Si-H + KF
O
O
N
N
N
N
N
S
S
N
N
N
N
N
O
Ru
Ru
H O K
N
S
S
N
S
N
N
S
O
O
N
N
N
N
N
N
S
S
N
Ru
Ru
N
N
N
A ruthenium-based porous organic polymer is constructed and used to reduce CO to potassium
2
formate.