An Efficient and Practical System for the Synthesis of N,N-Dimethylformamide by CO2 Hydrogenation using a Heterogeneous Ru Catalyst: From Batch to Continuous Flow

Article abstract of DOI:10.1002/cssc.201903364

In the context of CO₂ utilization, a number of CO₂ conversion methods have been identified in laboratory-scale research; however, only a very few transformations have been successfully scaled up and implemented industrially. The main bottleneck in realizing industrial application of these CO₂ conversions is the lack of industrially viable catalytic systems and the need for practically implementable process developments. In this study, a simple, highly efficient and recyclable ruthenium-grafted bisphosphine-based porous organic polymer (Ru@PP-POP) catalyst has been developed for the hydrogenation of CO₂ to N,N-dimethylformamide, which affords a highest ever turnover number of 160 000 and an initial turnover frequency of 29 000 h^?1 in a batch process. The catalyst is successfully applied in a trickle-bed reactor and utilized in an industrially feasible continuous-flow process with an excellent durability and productivity of 915 mmol h^?1 g_Ru^?1.

Full text of DOI:10.1002/cssc.201903364

Accepted Article
Title: An Efficient and Practical System for the Synthesis of N,N-
Dimethylformamide by CO2 Hydrogenation using a Ru-based
Heterogeneous Catalyst: From Batch to Continuous Flow
Authors: Gunniya Hariyanandam Gunasekar, Sudakar Padmanaban,
Kwangho Park, Kwang-Deog Jung, and Sungho Yoon
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Link to VoR: http://dx.doi.org/10.1002/cssc.201903364
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
An Efficient and Practical System for the Synthesis of N,N-
Dimethylformamide by CO2 Hydrogenation using a Ru-based
Heterogeneous Catalyst: From Batch to Continuous Flow
Gunniya Hariyanandam Gunasekar,[b]† Sudakar Padmanaban,[a]† Kwangho Park,[c]† Kwang-Deog
Jung,[b] and Sungho Yoon*[a]
Abstract: In the context of CO2 utilization, a number of CO2
conversion methods have been identified in laboratory-scale
research; however, only a very few transformations have been
successfully scaled up and implemented industrially. The main
bottleneck in realizing industrial application of these CO2 conversions
is the lack of industrially viable catalytic systems and the need of
practically implementable process developments. Herein, we
developed a simple, highly efficient and recyclable ruthenium-grafted
bisphosphine-based porous organic polymer (Ru@PP-POP) catalyst
for the hydrogenation of CO2 to N,N-dimethylformamide, and obtained
best ever turnover number (TON) of 160,000 and initial turnover
frequency (TOF) of 29,000 h-1 in a batch process. The catalyst was
successfully applied in a trickle bed reactor and utilized in an
a
batch catalytic process.[6,7] However, advancing the
homogeneous hydrogenation towards the practical realization
remains infeasible owing to the serious obstacles in terms of
separation of the catalysts from the highly polar product DMF and
recycling of the catalyst. Nevertheless, Vorholt and co-workers
recently proposed a continuous stirred tank reactor as feasible for
the homogeneous hydrogenation of CO2 to DMF using multiphase
liquid–liquid extraction to separate DMF from the homogeneous
catalyst.[6d-g] However, this multiphase extraction process may not
be preferable and suitable in the industry particularly for a
chemical like DMF, the production scale of which is relatively large,
because this process is highly laborious, and most importantly,
leaching of Ru metal is thus far unavoidable, which in turn reduces
the economic viability of the system.
industrially feasible continuous-flow process for the first time with an
-1
excellent durability and productivity of 915 mmol h-1 gRu
.
Thus, scale up of the hydrogenation process using
heterogeneous catalytic systems would offer prodigious
advantages over simple continuous operations and separations
in industry. Unfortunately, the efficiency (TONs and TOFs) and
stability of reported heterogeneous catalysts are far below the
practical requirements, and sometimes non-recyclable acid
additives are required for improved activity (Table S1).[7] In
addition, hydrogenation catalyzed by heterogeneous catalysts in
a batch-type reactor process alone has been investigated, which
may not be reflected in the commercial process. Consequently,
there is an imperative to develop highly efficient, selective and
durable catalytic systems for the titled hydrogenation along with
simple and practically viable continuous production systems.
Herein, we report a simple, highly efficient and recyclable
ruthenium-grafted phosphine-based porous organic polymer (1)
catalyst for the titled hydrogenation. The catalyst 1 shows very
promising catalytic ability for hydrogenation in batch process with
a maximum TON of 160,000 and an initial TOF of 29,000 h-1. The
catalyst was successfully applied in a trickle bed reactor and
utilized in an industrially feasible continuous-flow process for the
first time with an excellent productivity of 915 mmol h-1 gRu-1. The
efficiency of the catalyst is well maintained over successive runs
in the batch process and the catalyst is highly durable over a long
period in the continuous-flow reaction. Therefore, these results,
collectively make the overall catalytic process practical in
commercial operations.
For the past few decades, a number of CO2 transformations have
been identified in the field of CO2 utilization; however, most of the
reported catalytic processes are not desirable for technical
realizations in industry, and thereby remain at the laboratory
scale.[1-3] Therefore, the design and development of practically
auspicious catalytic process systems is a key task to ensure CO2
transformations in the chemical industry. In this regard, one
conceptually challenging transformation is the synthesis of N,N-
dimethylformamide (DMF) via CO2 hydrogenation.[4] DMF is
widely used in various areas, such as textile coatings, production
of polyurethane products, acrylic fibers, electronic components,
pharmaceutical products and organic synthesis. Its current
industrial production is mainly from CO, i.e., through continuous-
flow carbonylation of dimethylamine (DMA) using an alkali
alkoxide catalyst under severe reaction conditions (150-350 °C
and 30–100 MPa).[5] To surrogate this hazardous and less-
sustainable CO-based route, the synthesis of DMF via CO2
hydrogenation has been introduced in the year of 1970, and
intensively investigated from then with homogeneous catalysts in
[a]
[b]
Dr. S. Padmanaban and Prof. S. Yoon,
Department of Chemistry, Chung-Ang university,
84 Heukseok-ro, Dongjak-gu, Seoul, Republic of Korea.
E-mail: sunghoyoon@cau.ac.kr
Dr. G. H. Gunasekar and Dr. K.-D. Jung.
Clean Energy Research Center, Korea Institute of Science and
Technology, P. O. Box 131, Cheongryang, Seoul 136-791,
Republic of Korea.
To date, porous organic polymers (POPs) have drawn
considerable interest in the field of catalysis due to their large
surface area, tunable pore size, and high chemical stability.[8]
Most importantly, the integration of organic functional ligands into
porous polymer backbone permits the successful immobilization
of coordination complexes and thereby offers the design and
development of efficient and selective solid catalysts for various
catalytic applications. In this regard, porous polymer with
bisphosphine backbone inspired a great deal of attention for the
immobilization of various phosphine-based metal complexes
[c]
K. Park,
Department of Applied Chemistry, Kookmin university
77, Jeongneung-ro, Seongbuk-gu, Seoul, Republic of Korea
These authors contributed equally to this work
†
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
Scheme 1. Synthesis of 1
including PdCl2, RhCl(PPh3)3, IrCp*, Ru(p-cymene).[9] In this
(a)
(b)
context, herein we explore
a
ruthenium-coordinated
bisphosphine-based POP for the synthesis of DMF by CO2
hydrogenation.
The preparation of catalyst 1 is outlined in Scheme 1. The
phosphine-based porous organic polymer support (PP-POP) was
prepared
in
a
one-pot
process
using
tetrakis(4-
bromophenyl)methane and 1,2-bis(dichlorophosphino)ethane by
following a reported procedure.[9a] Post-synthetic metalation was
performed by adding a methanolic solution of RuCl3 to a
suspension of PP-POP in methanol under N2 atmosphere. The
generated ivory solid 1 is insoluble in nearly all organic solvents
and water. Scanning transmission electron microscopy (STEM)
and energy dispersive X-ray spectroscopy (EDX) measurements
revealed the uniform metalation of Ru throughout the solid matrix
with a Ru/Cl ratio of 1:3 (Figures 1a-b and S1, and Table S2). The
amount of Ru loaded into PP-POP was 0.81 wt%, as determined
by inductively coupled plasma optical emission spectrometry
Ru 3p3/2
462.2 eV
=
(c)
Ru 3p1/2
485.0 eV
(d)
600
400
200
0
=
BJH plot
0
40
80
dp(nm)
120
160
0.0
0.2
0.4
p/p0
0.6
0.8
1.0
490 485 480 475 470 465 460
Binding Energy (eV)
(ICP-OES).
X-ray
photoelectron
spectroscopic
(XPS)
Figure 1. Characterization of 1. (a) Scanning transmission electron microscopy
(STEM) image. (b) Energy-dispersive spectroscopic (EDS) mapping of the Ru
atoms. (c) Deconvoluted X-ray photoelectron spectrum of Ru-3p core level. (d)
N2 sorption measurement.
measurement of Ru-3p core level suggests (Ru-3p3/2 = 462.2 eV)
that the valence state of Ru in 1 was +3 (Figure 1c and S2).[10a]
Notably, the electron density of Ru in 1 was increased significantly,
compared to that in the Ru precursor (Ru-3p3/2 = 463.5 eV),[10a,b]
indicating the donation of electron density from the bidentate
phosphine ligands to the Ru site and successful complexation of
Ru with PP-POP as depicted in scheme 1. Powder X-ray
diffraction measurement (PXRD) revealed that the Ru species are
present in a highly dispersed state, as there was no apparent
change in the diffraction pattern of the parent polymer after Ru
complexation (Figure S3). N2 sorption analysis revealed that the
structure comprised micro- and mesopores (mean pore diameter
= 9.7 nm) (Figure 1d), with a Brunauer–Emmett–Teller (BET)
surface area of 469 m2 g−1 and total pore volume of 0.475 cm3 g−1.
Such hierarchical pores in 1 could gently expose the Ru sites to
the reactive species and improve the mass transfer during
catalysis.
Prior to evaluating the catalytic ability of 1 in a continuous-
flow trickle-bed reactor, catalyst 1 was tested in a batch process
to preliminarily assess the influence of various reaction
parameters such as the solvent, temperature, pressure, durability,
and mechanistic pathway. To select an optimal solvent, the
hydrogenation was performed with various commercially
available solvents in which dimethylamine (DMA) exhibits
solvency ([DMA] = 2.0 M) and the results are shown in Figure 2a.
Methanol was thus selected as the optimal solvent for the titled
reaction as it presented the highest conversion of DMA. Along
with the yield, the relatively low energy requirement for methanol
distillation over water solvent during DMF isolation made
methanol as the preferable solvent for the hydrogenation.
Along with DMF, as shown in Figure 2a, the generation of
dimethylammonium formate (FA) was observed in all cases, albeit
in various ratios. It is commonly considered that the synthesis of
DMF by CO2 hydrogenation is accompanied by FA generation,
followed by dehydration, as shown in Figure 2b. However, there
is another possible route that involves the generation of CO via
the reverse water gas shift reaction, followed by DMA
carbonylation (Figure 2b).[6g] To acquire insight into the pathway,
a series of experiments was carried out. While performing time-
dependent kinetic studies with 1, FA was detected as a major
intermediate during the hydrogenation as the initial rapid
generation of FA declined steadily and the formation of DMF
increased constantly with respect to the reaction time (Figure 2c).
In line with this result, the hydrogenation with FA added to the
starting solution gave a higher yield than that without FA (Table
S3). Moreover, no CO was detected by gas chromatographic
analysis of the gas mixtures (Figure S4). CO2 hydrogenation in
the presence of CO produced a significantly lower yield of the
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
(a)
(c)
(b)
100
75
100
75
100
100
DMF
FA
75
75
DMF
FA
50
50
25
0
50
50
25
25
0
25
0
0
6
9
12
15
3
Time (h)
Figure 2. (a) 20.0 mg of 1 in 2.0 M DMA in corresponding solvent (20.0 mL); T = 140 °C; p(CO2)/p(H2) = 2/2 MPa; (b) Possible pathways for production of DMF
from CO2/H2. (c) 10.0 mg of 1 in 2.0 M DMA in methanol, T = 100 °C; p(CO2)/p(H2) = 2/2 MPa.
As shown in Table 1, the studies on the temperature and
total pressure dependence revealed a proportional relationship
Table 1. CO2 hydrogenation in batch process[a]
with the yield, indicating that the chemical equilibrium is more
[d]
Entry
T (°C)
P
YieldFA+DMF
(%)[c]
TONFA+DMF
readily shifted towards product side at higher temperature and
pressure (Table 1, entries 1-8). The initial TOF and maximum
TON for the catalyst were 29,000 h-1 (entries 8 and 9) and up to
160,000, respectively, under the optimized reaction conditions
without any additives. Surprisingly, these are the best values
among those reported for heterogeneous catalysts. Notably, the
product yield with respect to the CO2 feed (15.6 g, 0.354 mol, for
entry 8) was calculated to be 11.1%, which is comparable to that
of the current process for CO2 hydrogenation to methanol
(∼15%),[11] highlighting the industrial viability of 1 for scale up of
the hydrogenation.
(MPa)[b]
1
2
3
4
5
6
7
8
80
2
2
2
2
3
4
6
8
10
19
27
36
56
65
84
98
5,000
9,400
100
120
140
140
140
140
140
13,600
18,250
27,300
31,700
41,300
Before checking the recyclability in the batch process, a
filtration test was conducted to evaluate the heterogeneous
48,250
18
(a)
(29,000)[e]
15
12
9
9[f]
140
140
8
8
80
2
160,000
700
10[g]
6
[a]Reaction conditions: 10.0 mg of 1 in 2.0 M DMA in methanol, time = 15 h.
[b]Total pressure at room temperature with p(CO2)/p(H2) = 1. [c]Calculated
based on 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an
internal standard, and the yields were with respect to DMA. [d]TON =
3
0
1
2
3
4
5
(molFA+DMF)/molRu.
[e]Value in parenthesis represents the initial TOF, which
was calculated after 15 min of the reaction. [f]2.5 mg of 1 for 40 h;
[g]p(CO2)=3.9 MPa + p(H2) = 3.9 MPa + p(CO) = 0.2 MPa
Number of cycle
1250
1000
750
500
250
0
100
75
50
25
0
(b)
products, indicating that 1 is very resistant to carbonylation (table
1, entry 10). Therefore, these results collectively indicate that the
formation of DMF is accompanied by FA generation and the
pathway related to carbonylation is very unlikely in 1 catalyzed
hydrogenation. Notably, there was no discernible distinction in the
point of selectivity for FA versus DMF during the heterogeneous
hydrogenation, because FA, if present, in the reaction medium,
can be converted to DMF during thermal treatment in the DMF
isolation step, which has also been reported by Vorholt and co-
workers during distillation of the solvents in the DMF isolation
step.[6d-g] Henceforth, the word “yield” in this manuscript indicates
the combined FA and DMF yields.
0
10
20
30
40
50
60
Time (h)
Figure 3. (a) Recyclability of 1 in batch process. (b) CO2 hydrogenation in
continuous-flow process using 1.
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
Figure 4. Plausible mechanism for the synthesis of DMF over 1.
nature of 1. The test results clearly show that 1 functions in a
purely heterogeneous fashion as the filtered solution failed to
catalyse the hydrogenation even after a prolonged reaction time
(Figure S5). Moreover, ICP-OES analysis of the filtered solution
revealed that no Ru metal were leached out during the
hydrogenation, indicating the strong binding of the Ru ion to the
PP-sites in 1. To further understand the robustness, catalyst 1
was reused for multiple runs via simple filtration after each cycle.
Figure 3a shows that the catalytic efficiency of 1 was well
maintained over five consecutive cycles, and the cumulative TON
was ~ 80,000 during these runs. As shown in Figures S6 and S7,
the characterization of recovered catalyst by STEM-EDX and
XPS analysis revealed that the Ru is still intact to the support
without any change in its binding energy (Ru3p3/2 = 462.2 eV).
However, the presence of Cl element was reduced to below the
detection limit of the SEM-EDX measurement, suggesting the
inactive role of Cl in the catalysis (Table S4). These results
indicating that the original motif of Ru is well intact with PP-POP
albeit with slight changes in its coordination environment.
reactor under the same condition, the simple design and
operation have practical advantages over the stirred tank reactor.
Additionally, no requirement of the recovery of catalysts during
the reaction makes this type of reactor feasible for
commercialization.
A schematic diagram of the setup is presented in Figure S8.
A stainless steel tubular reactor with an internal diameter of 7 mm
and length of 460 mm was positioned vertically and the flow
stream was consistently set from top to bottom. The setup can
maintain simultaneous flow of the liquids (DMA in methanol) and
gases (CO2 and H2) to the reactor. DMA in methanol and CO2
were delivered using a HPLC pump and H2 was delivered through
a mass flow controller. The gases and liquids were temperature
equilibrated and thoroughly mixed in the preheating system
before entering into the reactor. The liquid in the collecting vessel
1
could be removed periodically to analyze the solution through H
NMR analysis.
For hydrogenation in the trickle bed reactor systems,
catalyst 1 was uniformly mixed with silicon carbide and loaded into
the reactor with glass wool plug, and fitted into the rig. Before
starting the experiment, the system was thoroughly flushed with
N2 to remove oxygen from the setup. Based on the results of the
batch process, continuous hydrogenation was carried out at
140 °C at a total pressure of 12 MPa using 0.5 g of catalyst. During
this continuous process, the liquids were periodically collected
from the rig for the product analysis. As shown in Figure 3b, the
generation of products (FA+DMF) over the time stream was well
maintained for a long period (60 h) with an excellent productivity
of 915 mmol h-1 gactive metal-1, indicating that the catalytic system is
highly durable. Notably, this value is significantly high compared
Based on our experience and previous literature,[6g, 12]
a
plausible mechanism for the synthesis of DMF via formate
pathway by CO2 hydrogenation over 1 is proposed (Figure 4). The
pathway involves the generation of Ru-H intermediate through
substitution of Cl ligand with H2 followed by CO2 insertion into Ru-
H intermediate to generate Ru-formate species, which then
releases out the formate from the catalytic cycle by the oxidative
addition of H2 and regenerates the Ru-H species. The formate
extruded from the catalytic cycle finally undergoes dehydration
and produce DMF.
Building on the promising results obtained with 1,
continuous-flow CO2 hydrogenation was performed in a specially
designed trickle-bed reactor (TBR) system, where liquid and gas
substances are continuously introduced into a fixed (packed) bed
of catalyst particles with simple operations. TBRs are widely used
in industry particularly for the three-phase reaction (solid, liquid,
and gas),[13] The reactor system can offer auspicious advantages
in terms of mixing, heat management, scalability, energy
efficiency, and sustainability, etc., during the reaction.[13] Although
this type of reactor may show a lower TOF than the stirred batch
to industrial catalyst of methanol synthesis by CO2 hydrogenation
-1 [11]
(5.45 mmol h-1 gactive
)
emphasizing that the developed
metal
catalytic process is potentially economical. Therefore, this
catalytic process is scalable, simple, and has the potential viability
for DMF synthesis via CO2 hydrogenation in the industry.
In summary, a practically viable catalytic process for the
synthesis of DMF by CO2 hydrogenation was demonstrated using
a ruthenium grafted phosphine-based porous organic polymer
catalyst. The catalytic system shows best ever activity in the batch
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
(initial TOF of 29,000 h-1 and TON of 160,000) and continuous-
flow process (productivity of 915 mmol h-1 gRu-1). The catalyst
shows excellent stability during both hydrogenation processes,
with good selectivity. The simplicity of the catalytic process, with
high efficacy and durability, thus provides a promising venue for
realizing industrial-scale hydrogenation of CO2 to DMF.
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Acknowledgements
We acknowledge the financial support provided by the Korea
CCS R&D Centre (KCRC) grant funded by the Korean
government (Ministry of Science, ICT and Future Planning) (no.
2014M1A8A1049300).
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Keywords: DMF synthesis via CO2 hydrogenation • Phosphine-
based heterogeneous catalyst • Continuous flow CO2
hydrogenation • Porous organic polymer • Heterogeneous CO2
hydrogenation.
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10.1002/cssc.201903364
ChemSusChem
COMMUNICATION
COMMUNICATION
A simple and continuous
process has been
developed using a highly
durable and efficient
heterogeneous Ru
catalyst.
Gunniya Hariyanandam
Gunasekar, Sudakar
Padmanaban, Kwangho
Park, Kwang-Deog Jung and
Sungho Yoon*
O
CO2
+
H2
NMe2
Me2NH
H
+ H2O
Page No. 1– Page No. 5
 Simple & Continuous Process
 Productivity: 915 mmol h-1 gRu
 Highly Durable
An Efficient and Practical
System for the Synthesis
of N,N-Dimethylformamide
by CO2 Hydrogenation
using Ru-based
Ru
-1
Heterogeneous Catalyst:
From Batch to Continuous
Flow
This article is protected by copyright. All rights reserved.