Efficient and Selective Methane Borylation Through Pore Size Tuning of Hybrid Porous Organic-Polymer-Based Iridium Catalysts

Article abstract of DOI:10.1002/anie.201906350

As a new energy source that could replace petroleum, the global reserves of methane hydrate (combustible ice) are estimated to be approximately 20 000 trillion cubic meters. A large amount of methane hydrate has been found under the seabed, but the transportation and storage of methane gas far from coastlines are technically unfeasible and expensive. The direct conversion of methane into value-added chemicals and liquid fuels is highly desirable but remains challenging. Herein, we prepare a series of iridium complexes based on porous polycarbazoles with high specific areas and good thermochemical stabilities. Through structure tuning we optimized their catalytic activities for the selective monoborylation of methane. One of these catalysts (CAL-3-Ir) can produce methyl boronic acid pinacol ester (CH₃Bpin) in 29 % yield in 9 h with a turnover frequency (TOF) of approximately 14 h^?1. Because its pore sizes favor monoborylated products, it has a high chemoselectivity for monoborylation (CH₃Bpin:CH₂(Bpin)₂=16:1).

Full text of DOI:10.1002/anie.201906350

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Efficient and Selective Methane Borylation Through Porosity
Tuning of Hybrid Iridium Catalysts Based on Porous Organic
Polymers
Authors: Qi Chen, Anwang Dong, Dongxu Wang, Lu Qiu, Chunxin Ma,
Yihui Yuan, Yunpeng Zhao, Na Jia, Zhanhu Guo, and Ning
Wang
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Efficient and Selective Methane Borylation Through Porosity
Tuning of Hybrid Iridium Catalysts Based on Porous Organic
Polymers
Qi Chen, Anwang Dong, Dongxu Wang, Lu Qiu, Chunxin Ma, Yihui Yuan, Yunpeng Zhao, Na Jia,
Zhanhu Guo, and Ning Wang*
Dr. Q. Chen, A. Dong, D. Wang, L. Qiu, Dr. C. Ma, Dr. Y. Yuan, Prof. Dr. N. Wang
State Key Laboratory of Marine Resource Utilization in South China Sea
Hainan University
Haikou, 570228, P. R. China
E-mail: wangn02@foxmail.com (N. Wang)
Prof. Dr. Y. Zhao
State Key Laboratory of Coastal and Offshore Engineering
Dalian University of Technology
Dalian, 116024, P. R. China
Prof. Dr. N. Jia
Petroleum Systems Engineering, Faculty of Engineering and Applied Science
University of Regina
Regina, Saskatchewan S4S 0A2, Canada
Prof. Dr. Z. Guo
Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering
University of Tennessee
Knoxville, TN, 37996, USA
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Abstract
As a new energy source that could potentially replace petroleum, the global reserves of methane
hydrate (combustible ice) are estimated to be approximately 20,000 trillion cubic meters. A large
amount of methane hydrate has been found under the seabed, but the transportation and storage of
methane gas far from coastlines are technically unfeasible and expensive. The direct conversion of
methane to value-added chemicals and liquid fuels is highly desirable but remains a challenging task
because of its inert chemical properties. Herein, we prepared a series of iridium complexes based on
porous polycarbazoles with high specific areas and good thermochemical stabilities, and we optimized
their catalytic activities for the selective monoborylation of methane through structure tuning. One of
porous iridium catalysts (CAL-3-Ir) with high porosity can produce methyl boronic acid pinacol ester
(CH3Bpin) with a 29 % yield in 9 h and exhibits the best turnover frequency (TOF) of approximately 14
h-1 among all the reported iridium-based catalysts. Meanwhile, because it possesses pore sizes that favor
monoborylated products, CAL-3-Ir has a high chemoselectivity for monoborylation (CH3Bpin:CH2-
(Bpin)2 = 16:1). These results provide a new platform for exploring efficient and selective catalysts for
the direct conversion of methane.
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Methane hydrate, also named combustible ice, is regarded as a new energy source that could
potentially replace petroleum in the future.[1,2] The global reserve of methane hydrate is estimated to be
approximately 20,000 trillion cubic meters, which is twice the carbon content of conventional fossil fuel
supplies.[3] The amount of methane hydrate resources under the seabed is far greater than those found in
the permafrost.[4] Since methane hydrates were successfully drilled in deep sea areas, people have
realized that the transportation and storage of methane gas far from coastlines is too difficult and costly.
At present, most methane (>90 %) is directly combusted as fuel, and the utilization of methane with a
higher economic value is a concern.[5,6] Moreover, the impact of methane, as a greenhouse gas,[7] on
global climate change is approximately 21 times greater than that of carbon dioxide.[8,9] Therefore, the
necessity and significance of converting methane to other value-added chemicals is growing[10] and is
increasingly capturing the attention of the academic community.[11-13]
In current industrial practices, various chemicals [2,14-16] have already been produced from methane
via the synthesis of syngas (CO/H2) followed by additional reaction processes. However, the high
energy requirements and capital inputs, as well as the multistep reactions, are disadvantages that need to
be addressed.[17,18] Direct conversion of methane, by contrast, is a promising alternative and has already
garnered much interest in the field.[14,19-21] Methane is the simplest hydrocarbon and exhibits rather inert
chemical properties, such as the strong C–H bond, low polarizability, and high ionization potential,
which render its direct transformation and functionalization unfavorable.[ 22 - 24 ] Moreover, once a
monofunctionalized product is produced to enable chemical activation, further C–H functionalization is
unavoidable under the same conditions. Therefore, selective chemical functionalization of methane is a
great challenge. It is worth noting that remarkable progress has recently been made regarding the
catalytic borylation of methane. Pioneering contributions by Mindiola,[25] Sanford,[26] and coworkers
demonstrated that certain transition metal (Ir, Rh, Ru) complexes are effective homogeneous catalysts
for the selective monoborylation of methane with diborane reagents. Research conducted by Baik and
Mindiola indicated that the use of a phosphine ligand (dmpe) to form iridium complexes could
significantly enhance homogeneous catalytic efficiencies.[ 27 ] More recently, a microporous metal
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organic framework containing an iridium complex as the active site was designed as a heterogeneous
catalyst by Farha and coworkers, and it demonstrated high chemoselectivity for methane
monoborylation due to the shape-selective concept.[28] Despite these innovations, the catalytic C–H
borylation of methane still warrants further investigation, especially the utilization of heterogeneous
catalysts and the study of the relationship between their porosities and catalytic efficiencies.
Scheme 1. Schematic representation for preparation of all the porous polymers.
Versatile porous organic polymers have been proven to be an effective matrix candidate for the
utilization of heterogeneous catalysts,[29-33 ] which exhibit diverse structures, good stabilities, high
porosities, and flexible functionalization.[34,35] Motivated by the exploration of robust and efficient
heterogenous catalysts for the selective borylation of methane, herein, a series of porous polycarbazoles
containing iridium complexes as catalytic sites were designed and prepared. All of the composite
materials that were produced possess different porosities due to their structural tunability. To prove the
feasibility of assisting in the C–H functionalization of methane, the catalytic performance of all the
porous materials for methane monoborylation were studied systematically. The corresponding effects of
the porosity on catalytic efficiency were also investigated. These are the first heterogenous catalysts for
direct methane conversion based on porous organic polymers. Moreover, the efficient catalysts can be
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screened through structure tuning.
13
Figure 1. (a) C NMR spectra of the building block DCP, the polymers of CAL-1and CAL-1-Ir
(asterisks denote spinning sidebands). (b) The representative XPS spectra of CAL-1-Ir. (c and d) SEM
and TEM images of polymer CAL-1-Ir.
Our previous works show that functional porous polycarbazoles, via oxidative coupling
polymerization, can either be heterogeneous catalysts or the matrix in heterogeneous catalysis. Inspired
by diimine-supported iridium complexes as homogeneous catalysts for methane borylation, in our
strategy shown in Scheme 1, a building block DCP [36] including carbazole groups and phenanthroline
ligands was coordinated with a [Ir(COD)(μ-Cl)]2 complex under argon atmosphere to form Ir (I)
complex, followed further oxidation by exposure to air to produce an intermediate DCP-Ir (III), in
which the OH groups were derived from air oxidation (Scheme S1).[28,37] Polymerization of the DCP-Ir,
via a carbazole-based oxidative coupling reaction using a FeCl3 catalyst at room temperature,[38,39]
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formed a homopolymer (CAL-1-Ir) with a built-in iridium complex. To tune the porosity of the
polymers, other monomers (CZ-2~4) [32,40] with varying structures and different connecting arm lengths
were selected to copolymerize with DCP-Ir to produce three copolymers (CAL-2~4-Ir). Polymer CAL-1
(Scheme S1) with a free base was also obtained under the same conditions and employed as the control
for structure characterization in the following catalytic studies.
Figure 2. The representative TEM-EDX elemental mapping images (C, N, O, Cl, and Ir) of CAL-1-Ir.
The chemical structures of all the polymers were verified by the 13C CP/MAS NMR spectrum. For
example, CAL-1-Ir (Figures 1 a and S1), like most of the well-defined polycarbazoles reported
previously, [41-43] has three different signal peaks between 100 and 150 ppm, indicating the presence of
an aromatic carbon atom linked with a nitrogen atom (148 or 143 ppm), a carbon atom linked with a
hydrogen atom (110 ppm), and a carbon atom linked with other atoms (127 or 128 ppm). In addition,
two new signal peaks were found in CAL-1-Ir at 67 and 29 ppm, which indicate the presence of
13
methylene carbon derived from COD groups.[44] A similar C NMR signal distribution was found for
other copolymers and is shown in Figure S2. The different chemical elements in the polymers were
characterized and proven by X-ray photoelectron spectroscopy (XPS) measurements. As shown in
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Figures 1b and S3, the Ir 4f signal peaks of all the polymers with built-in iridium complexes are
comprised of Ir 4f5/2 (65.1 eV) and Ir 4f7/2 (62.1 eV), indicating that the oxidative state of Ir in the
materials is +3, which is different from the iridium complex source [Ir(COD)(μ-Cl)]2. The binding
energy of Cl 2p is about 197.9 eV, proving the presence of an outer-sphere chlorine atom (chlorine
anion) not an inner-sphere chlorine atom such as the bridging chlorine in [Ir(COD)(μ-Cl)]2, which
usually possesses the higher binding energy at about 199.0 eV.[45] The peaks corresponding to O 1s, Cl
2p, and Ir 4f of the materials demonstrate that the formed new iridium complex active sites are bound to
the matrixes as compared with those of the control polymer CAL-1 (Figure S3). TEM and SEM
observation show that the prepared materials consist of agglomerated nanoparticles of irregular sizes
(Figures 1c−d and S4−5). The corresponding EDXS elemental mapping images are shown in Figures 2
and S6−8. It can be seen that all the elements of the composite materials, such as C, N, O, Cl, and Ir,
were present as expected and uniformly distributed across the matrix. The results also confirm that
polymers with built-in iridium complexes have successfully been obtained.
Figure 3. (a) Nitrogen adsorption–desorption isotherms of all the polymers measured at 77 K (For
clarity, the isotherms of CAL-3-Ir and CAL-4-Ir were shifted vertically by 100 and 200, respectively).
(b) The PSD profiles of them calculated by NLDFT.
The thermodynamic stabilities of all the materials were measured via thermogravimetric analysis
(TGA). Less than 5 % mass loss of the prepared polymers is found up to 220 °C (Figure S9), indicating
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thermodynamic stability under normal catalytic conditions (~150 °C). Due to the cross-linked molecular
structures, not all of the polymers are soluble in common solvents and therefore can be used in
heterogeneous systems. The porosities of the polymers were analyzed by nitrogen sorption and
desorption isotherms measured at 77 K, and the corresponding results are shown in Figure 3a. We can
see that the adsorbed volumes of all four polymers have linear increase at the relative pressure (P/P0)
from 0 to 0.01, followed by flat growth until the relative pressure approaches 1. Hysteresis loops were
also found for all the materials due to the porosity and irregular pore sizes of the polymers.[46,47] The key
porosity data are listed in Table 1. The BET specific surface area of homopolymer CAL-1-Ir is
approximately 467 m2 g-1, and its dominant pore size distribution (PSD, Figure 3b) analyzed via the
NLDFT model is 0.51 nm. Regarding the copolymers CAL-2~4-Ir, their specific surface areas are larger
and range from 860−1210 m2 g-1. The dominant PSD of these copolymers (Figure 3b) are also changed
accordingly. For example, CAL-2-Ir averages 0.80 nm and CAL-3-Ir is approximately 0.77−1.10 nm.
Mesopore size distribution (~2.0 nm) was also found for CAL-4-Ir, along with the microporous PSD of
approximately 0.75−1.05 nm. These data indicate that structure tuning of the building blocks has
significant influence on the porosities of the prepared porous polymers. The methane uptake capacities
of the materials were measured at 273 K with pressures up to 1.0 bar. As shown in Table 1, CAL-3-Ir,
which has the largest BET specific surface area (1210 m2 g-1) and pore volume (0.66 cm3 g–1), possesses
the highest methane uptake of approximately 8.5 wt% among the four prepared polymers. Studies on
porosities of all the materials will be helpful to gain insight into their catalytic performance as
heterogeneous catalysts.[48,49] We also studied the adsorption performance of prepared materials CAL-
2~4-Ir for other gases (H2, N2, and CO2) and toluene as shown in Figures S10a-e. Compared the
sorption properties of the materials with some reported state-of-art materials possessing high specific
surface areas,[32,50] the gas uptake performance of the CAL-2~4-Ir is relatively moderate or even lower.
Table 1. Porosity data of all the porous materials.
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[c]
[a]
[b]
CH4 uptake [d]
Dpore
SBET
Vtotal
(m2 g–1)
(cm3 g–1)
(wt %)
Polymers
CAL-1-Ir
CAL-2-Ir
CAL-3-Ir
CAL-4-Ir
(nm)
467
860
0.42
0.51
4.1
0.55
0.80
6.2
1210
1100
0.66
0.77−1.10
8.5
0.61
0.75−1.05,1.6−2.1
7.3
[a] Specific surface area calculated via the BET method. [b] Total pore volume at P/P0=0.99. [c] Data
calculated with the NLDFT method. [d] Data were obtained at 1.0 bar and 273 K.
The borylation of methane is difficult to accomplish due to the inherent chemical stability of
methane, which is unreactive even in comparison with the corresponding conversion products.
Motivated by pioneering work with homogeneous catalytic systems under the optimized “Mindiola
process” conditions [25], the prepared catalysts were tested for methane borylation, which was performed
at 150 °C and 3.4 MPa of methane pressure. Cyclohexane was replaced by dodecane as the reaction
solvent, due to the consideration that C−H activation of cyclohexane was found to form borylated
byproducts, and dodecane shows better solubility than methane and provides high chemoselectivity for
monoborylated methane as the main product. Since all the catalysts are insoluble in the liquid reactants
and solvents, the catalytic studies were studied under heterogeneous systems. The catalytic
performances of the prepared catalysts were screened and evaluated with various amounts (0.5−1.5 mg)
of the catalysts with different reaction times. As shown in Tables 2 and S1, the starting material B2Pin2
cannot be completely converted using CAL-1-Ir and CAL-2-Ir as the catalysts. Both the yield (16−18 %)
and the selectivity of CH3Bpin are lower than those found in other reported works. Under the same
reaction conditions, CAL-3-Ir and CAL-4-Ir can fully convert B2Pin2 with higher yields and shorter
reaction times (9−12 h). However, as either homogeneous or heterogeneous catalyst, it has previously
been reported to require 14-16 h. For CAL-3-Ir, the best yield of CH3Bpin (29 %) and selectivity were
found with a reaction time of 9 h. With a longer reaction time of 12 h, both the yield and selectivity of
CH3Bpin are apparently decreased. The experimental optimization for iridium catalyst screening
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indicates that CAL-3-Ir exhibits the best turnover frequency (TOF) of approximately 14 h-1, which is
higher than that of the reported homogeneous iridium catalysts, such as Ir-Phen and Ir-dmpe complexes
and heterogeneous porous iridium catalyst UiO-67-Ir based on MOFs (Figure 4).
Table 2. Summaries to catalytic performance of iridium catalysts for methane borylation.[a]
Yield of
CH3Bpin
(%)
Time
(h)
Conversion
(%)
CH3Bpin:
CH2(Bpin)2
TOF
(h-1)
Catalysts
Refs
CAL-1
CAL-1-Ir
14
14
12
9
< 1
65
≈ 0
16
−
3
−
1
This work
This work
This work
This work
[25]
CAL-2-Ir
81
18
6
4
CAL-3-Ir
100
100
−
29
16
9
14
7
CAL-4-Ir
12
16
14
16
14
22
Ir-dmpe-CyH [b]
Ir-Phen [c]
52
3
~7
1
−
45
4
[26]
Ir-dmpe-CyO [d]
UiO-67-Ir [e]
38.2
100
31
9
10.5
~5
[27]
19.5
15
[28]
[a] for CAL-1~4-Ir, reaction condition details were shown in the Supporting Information. [b] 0.5 mol%
in cyclohexane (CyH) at 150 °C under 3447 kPa of CH4, the ration of ligand Me2PCH2CH2PMe2 (dmpe)
to the Ir precatalyst [Ir(COD)(μ-Cl)]2 is 2/1. [c] 3 mol% in CyH at 150 °C under 2800 kPa of CH4,
ligand 3,4,7,8-tetramethyl-1,10-phenanthroline (Phen) to [Ir(COD)(μ-Cl)]2 is 1/1. [d] 0.2 mol% in
cyclooctane (CyO) at 150 °C under 3500 kPa of CH4, the ration of dmpe to [Ir(COD)(μ-Cl)]2 is 2/1. [e]
1.8 w/w% iridium (weight of catalyst UiO-67-Ir) in dry dodecane at 150 °C under 34 atm of CH4.
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Figure 4. Summary on the turnover frequency (TOF/ h-1) and chemoselectivity (CH3Bpin: CH2(Bpin)2)
of the reported iridium catalysts (details to reaction conditions were shown in the Table 2).
In the control experiment, the porous polymer CAL-1 without the incorporated iridium complex
was used as the catalyst for methane monoborylation and no CH3Bpin was detected, indicating that the
iridium complex acts as the active site in the catalyst CAL-2~4-Ir. The proposed catalytic mechanism of
methane borylation was demonstrated in Figure S11.[25,28] The active intermediate (A) containing three
boryl groups CAL-1-[Ir(III)(boryl)3] is formed from the reaction of CAL-1-Ir and B2pin2 and plays a
key role in this reaction. Methane is reacted with this active intermediate, followed by transformation
steps to give intermediate (C), which furnishes the desired product CH3Bpin and intermediate (D).
Finally, intermediate (A) can be reproduced by the addition reaction of B2pin2 and intermediate (D). The
high catalytic efficiency in our system might be ascribed to the high specific surface areas and the pore
sizes, which are suitable for mass transfer and effective collision between the available active site
iridium complex and the reaction substrate.[51,52] Due to having the largest BET specific surface area
(1210 m2 g-1) and pore volume (0.66 cm3 g–1), CAL-3-Ir has the highest methane uptake of
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approximately 8.5 wt% and possesses the best TOF among the four prepared polymers. For the porous
catalysts, pore size has a critical role in the shape-selective reaction. Similar to reports of UiO-67-Ir, [28]
the dominant pore size of CAL-3-Ir is centered at 0.77−1.10 nm, which is not only suitable for B2Pin2 to
access and react with methane in the pore but is also favorable for the selective formation of the
monoborylated product. Therefore, high chemoselectivity for CH3Bpin was found for catalyst CAL-3-Ir,
which is comparable to UiO-67-Ir. However, in comparison, the bigger pore size (1.6−2.1 nm) of CAL-
4-Ir gives relatively low selectivity for monoborylation. For CAL-1-Ir and CAL-2-Ir, the pore sizes
(0.51 and 0.80 nm) are too small for B2Pin2 to access the inside of the pore and make contact with the
iridium complex active site. As a result, the corresponding TOF and chemoselectivity are not as high
when compared with CAL-3-Ir. We tested the catalytic activity of the recovered CAL-3-Ir and found
that there was a loss of activity after the recycling run. This result was likely induced by the degradation
of the active catalyst intermediates, which was also reported for the heterogeneous catalyst UiO-67-Ir.[28]
Because the active catalytic intermediate A shown in Figure S11 go through the IrIII-IrV-IrIII catalytic
cycle for methane borylation, degradation of the active iridium species has inevitably taken place during
the multistep catalytic cycles and the catalytic activity of the catalyst is decreasing. Compared with a
homogeneous catalyst, a heterogeneous catalyst can be separated quickly and conveniently from the
reaction system, which leads to the improvement of the efficiency of the reaction workup and product
purification.
In conclusion, iridium hybrid composites based on porous polymers were prepared as
heterogenous catalysts via a simple presynthetic functionalization strategy for the direct conversion of
methane. The obtained catalysts show high specific surface areas (467−1210 m2 g-1) and good stability.
Structure tuning of the building blocks has a significant influence on the porosities of the prepared
porous polymers. The catalytic performance of the polymers for C–H borylation of methane by a
diborane reagent was studied. CAL-3-Ir gives the best yield of CH3Bpin (29 %) with a reaction time of
9 h and exhibits the best turnover frequency (TOF) of approximately 14 h-1 among the reported iridium
catalysts. Additionally, the highest chemoselectivity for the monoborylation of methane was found for
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the catalyst CAL-3-Ir due to the suitable PSD favorable for selectively furnishing a monoborylated
product. These results not only broaden the application of porous polymers but also provide a new
platform for exploring efficient catalysts used in the direct transformation of methane.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
Q. Chen and A. Dong contributed equally to this work. The financial support of the Key Research and
Development Program of Hainan Province (No. ZDYF 2019016), the National Natural Science
Foundation of China (Grants 51873053, 21574031, 61761016 and 51775152), and the Start-up
Scientific Research Foundation of Hainan University (Grant KYQD(ZR)1812) is acknowledged.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: direct methane conversion, heterogeneous catalyst, hybrid composite, iridium complex,
porous organic polymer
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Table of Contents
A series of iridium complex based on porous polycarbazoles were prepared as the heterogenous
catalysts for selective monobolylation of methane. The catalyst with high efficiency and good
chemoselectivity can be screened through the structure tuning.
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