Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks

Article abstract of DOI:10.1016/j.chempr.2020.01.011

Porous graphitic framework (PGF) is a two-dimensional (2D) material that has emerging energy applications. An archetype contains stacked 2D layers, the structure of which features a fully annulated aromatic skeleton with embedded heteroatoms and periodic pores. Due to the lack of a rational approach in establishing in-plane order under mild synthetic conditions, the structural integrity of PGF has remained elusive and ultimately limited its material performance. Here, we report the discovery of the unusual dynamic character of the C=N bonds in the aromatic pyrazine ring system under basic aqueous conditions, which enables the successful synthesis of a crystalline porous nitrogenous graphitic framework with remarkable in-plane order, as evidenced by powder X-ray diffraction studies and direct visualization using high-resolution transmission electron microscopy. The crystalline framework displays superior performance as a cathode material for lithium-ion batteries, outperforming the amorphous counterparts in terms of capacity and cycle stability. Insertion of well-defined, evenly spaced nanoscale pores into the two-dimensional (2D) layers of graphene invokes exciting properties due to the modulation of its electronic band gaps and surface functionalities. A bottom-up synthesis approach to such porous graphitic frameworks (PGFs) is appealing but also remains a great challenge. The current methods of building covalent organic frameworks rely on a small collection of thermodynamically reversible reactions. Such reactions are, however, inadequate in generating a fully annulated aromatic skeleton in PGFs. With the discovery of dynamic pyrazine formation, we succeeded in applying this linking chemistry to obtain a crystalline PGF material, which has displayed high electrical conductivity and remarkable performance as a cathode material for lithium-ion batteries. We envision that the demonstrated success will open the door to a wide array of fully annulated 2D porous frameworks, which hold immense potential for clean energy applications. We report the unusual dynamic characteristics of the C=N bonds in the pyrazine ring promoted under basic aqueous conditions, which enables the successful synthesis of two-dimensional porous graphitic frameworks (PGFs) featuring fully annulated aromatic skeletons and periodic pores. The PGF displayed high electrical conductivity and remarkable performance as a cathode material for lithium-ion batteries, far outperforming the amorphous counterparts in terms of capacity and cycle stability.

Full text of DOI:10.1016/j.chempr.2020.01.011

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Article
Dynamic Covalent Synthesis of Crystalline
Porous Graphitic Frameworks
Xinle Li, Hongxia Wang, Hao
Chen, ..., Emory M. Chan, Jian
Zhang, Yi Liu
jianzhang@lbl.gov (J.Z.)
yliu@lbl.gov (Y.L.)
HIGHLIGHTS
The dynamic characteristics of
C=N bonds in aromatic pyrazine
rings has been reported
The dynamics enabled the
synthesis of a crystalline porous
2D graphitic framework
The periodic feature was directly
visualized using transmission
electron microscopy
The framework displays
outstanding performance as a
cathode for lithium-ion batteries
We report the unusual dynamic characteristics of the C=N bonds in the pyrazine
ring promoted under basic aqueous conditions, which enables the successful
synthesis of two-dimensional porous graphitic frameworks (PGFs) featuring fully
annulated aromatic skeletons and periodic pores. The PGF displayed high
electrical conductivity and remarkable performance as a cathode material for
lithium-ion batteries, far outperforming the amorphous counterparts in terms of
capacity and cycle stability.
Li et al., Chem 6, 1–12
April 9, 2020 ª 2020 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2020.01.011

Please cite this article in press as: Li et al., Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks, Chem (2020), https://doi.org/
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10.1016/j.chempr.2020.01.011
Article
Dynamic Covalent Synthesis
of Crystalline Porous
Graphitic Frameworks
Xinle Li,1,11 Hongxia Wang,4,11 Hao Chen,4,11 Qi Zheng,2,11 Qiubo Zhang,2 Haiyan Mao,3,5,10 Yawei Liu,1
Songliang Cai,1,7 Bing Sun,1,8 Chaochao Dun,1 Madeleine P. Gordon,1 Haimei Zheng,2,6
Jeffrey A. Reimer,3,5 Jeffrey J. Urban,1 Jim Ciston,1 Tianwei Tan,9 Emory M. Chan,1 Jian Zhang,1,
*
and Yi Liu1,12,
*
SUMMARY
The Bigger Picture
Insertion of well-defined, evenly
spaced nanoscale pores into the
two-dimensional (2D) layers of
graphene invokes exciting
Porous graphitic framework (PGF) is a two-dimensional (2D) material that has
emerging energy applications. An archetype contains stacked 2D layers, the
structure of which features a fully annulated aromatic skeleton with embedded
heteroatoms and periodic pores. Due to the lack of a rational approach in estab-
lishing in-plane order under mild synthetic conditions, the structural integrity of
PGF has remained elusive and ultimately limited its material performance. Here,
we report the discovery of the unusual dynamic character of the C=N bonds in
the aromatic pyrazine ring system under basic aqueous conditions, which en-
ables the successful synthesis of a crystalline porous nitrogenous graphitic
framework with remarkable in-plane order, as evidenced by powder X-ray
diffraction studies and direct visualization using high-resolution transmission
electron microscopy. The crystalline framework displays superior performance
as a cathode material for lithium-ion batteries, outperforming the amorphous
counterparts in terms of capacity and cycle stability.
properties due to the modulation
of its electronic band gaps and
surface functionalities. A bottom-
up synthesis approach to such
porous graphitic frameworks
(PGFs) is appealing but also
remains a great challenge. The
current methods of building
covalent organic frameworks rely
on a small collection of
thermodynamically reversible
reactions. Such reactions are,
however, inadequate in
INTRODUCTION
generating a fully annulated
aromatic skeleton in PGFs. With
the discovery of dynamic pyrazine
formation, we succeeded in
applying this linking chemistry to
obtain a crystalline PGF material,
which has displayed high
The discovery of graphene as a two-dimensional (2D) sheet of fully conjugated carbons
has led to a surge of research interest in diversifying its structure and properties.1–4
One appealing approach is to introduce heteroatoms and nanometer-sized pores into
the periodic 2D graphene layers that further stack into porous graphitic frameworks
(PGFs). Such in-plane structural modification can effectively tune the band structure,
introduce catalytic sites, and create guest transporting channels in three-dimensionally
stacked layers,5–7 which opens up a wide range of applications, including nanoelec-
tronics, catalysis, separation, and energy storage.8–10 As a representative PGF structure,
PGF-1 (Figure 1) features a fully fused, nitrogen-rich aromatic 2D framework with periodic
pores connected via hexaazatrinaphthalene (HATN) nodes. On account of the high lith-
iation capacity of HATN,11,12 such a periodic framework with high-density HATN units
arranged in well-defined nanochannels is especially promising for applications in
lithium-ion batteries (LIBs). However, to date, the ideal PGF-1 that exhibits a high level
of structural precision has not been realized as prior synthetic attempts only resulted in
amorphous crosslinked microporous polymers.13–16
electrical conductivity and
remarkable performance as a
cathode material for lithium-ion
batteries. We envision that the
demonstrated success will open
the door to a wide array of fully
annulated 2D porous frameworks,
which hold immense potential for
clean energy applications.
The great challenge of synthesizing crystalline PGF-1 lies in the lack of effective
chemical tools to covalently assemble annulated aromatic units into periodic 2D
Chem 6, 1–12, April 9, 2020 ª 2020 Elsevier Inc.
1

Please cite this article in press as: Li et al., Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks, Chem (2020), https://doi.org/
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10.1016/j.chempr.2020.01.011
1The Molecular Foundry, Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA
Figure 1. Schematics of Crystalline PGF-1 and Amorphous Polymers AP-1–3
2Materials Sciences Division, Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA
(A) Condensation between BTA and HKH under acidic conditions leads to the formation of
amorphous polymers AP-1–3 with ill-defined structures. The red boxes highlight two mishaps of
3Environmental Energy Technologies Division,
Lawrence Berkeley National Laboratory,
(B) Rendering the system reversible under a basic hydrothermal condition allows the formation of
Berkeley, CA 94720, USA
connectivity that cause the loss of in-plane order.
PGF-1 with high in-plane order.
4Department of Materials and Engineering,
(C) Space-filling model of PGF-1 with layers arranged in an eclipsed stacking mode. Color code: C,
Stanford University, Stanford, CA 94305, USA
orange; N, blue; H, gray.
5Department of Chemical and Biomolecular
Engineering, University of CaliforniaÀBerkeley,
Berkeley, CA 94720, USA
sheets. Retrosynthetically, the formation of PGF-1 can be conceived by stitching the
6Department of Materials Science and
two monomeric units: C2-symmetric 1,2,4,5-benzenetetramine (BTA) tetrahydro-
Engineering, University of CaliforniaÀBerkeley,
chloride and C3-symmetric hexaketocyclohexane (HKH) octahydrate (Figure 1).
Berkeley, CA 94720, USA
Annulation occurs through the formation of the aromatic pyrazine bridging units
from the condensation reaction between vicinal diamines and vicinal diketones.17
The problem with this route is that the nonspecific reactive sites in BTA and HKH
can lead to mishaps of connectivity, such as the example illustrated in Figure 1A. Irre-
versible formation of such defects will prevent the propagation of shape-persistent
macrocyclic pores within the 2D plane and result in amorphous crosslinked poly-
mers. Indeed, pyrazine formation has been deemed as an irreversible synthetic
dead end because of aromatic stabilization. With few exceptions,18–22 previous im-
plementations of similar crosslinking chemistry only resulted in amorphous micropo-
rous polymers,23,24 as reflected in their featureless powder X-ray diffraction (PXRD)
patterns. If the condensation reaction can be rendered reversible, it may allow error-
checking and self-healing to facilitate the growth of crystalline frameworks. Such dy-
namic covalent reactivity is the basis for the growth of the closely related covalent
organic frameworks (COFs);25–27 however, the majority of the known dynamic
7School of Chemistry and Environment, South
China Normal University, Guangzhou 510006,
China
8School of Science, China University of
Geosciences (Beijing), Beijing 100083, China
9National Energy R&D Center for Biorefinery,
Beijing University of Chemical Technology,
Beijing 100029, China
10College of Materials Science and Engineering,
Nanjing Forestry University, Nanjing 210037,
China
11These authors contributed equally
12Lead Contact
*Correspondence: jianzhang@lbl.gov (J.Z.),
yliu@lbl.gov (Y.L.)
https://doi.org/10.1016/j.chempr.2020.01.011
2
Chem 6, 1–12, April 9, 2020

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10.1016/j.chempr.2020.01.011
Figure 2. Scrambling Experiments Illustrating the Dynamic Nature of Pyrazine Linkages in the
HATN Ring System
(A) Schematics of dynamic amine exchange of the pyrazine units in HATN-Me under basic
hydrothermal condition. Condition: HATN-Me (1 equiv), PDA (10 equiv), KOH (6 equiv), H2O, 120ꢀC
for 3 days, N2 atmosphere.
(B) Solution 1H NMR spectra (500 MHz, CDCl3, 298 K) of the model compound, HATN-Me (blue),
HATN (orange), and the products from reactions carried out in aqueous acetic acid (dark blue), pure
water (green), and aqueous KOH (purple) at 120ꢀC for 3 days.
covalent reactions are limited to non-annulated systems and are not applicable for
the synthesis of fully annulated COFs.28 Synthetic advances that can instill revers-
ibility in annulated ring systems may thus provide a viable solution. Here, we report
the discovery of reversible characteristics of the C=N bonds in the aza-fused HATN
unit under simple basic aqueous conditions, which enables the successful synthesis
of PGF-1 with significantly increased crystallinity compared with those in the
literature and may be generalized for more annulated frameworks. We further
demonstrate that when used as the cathode material in LIBs, PGF-1 significantly out-
performs the non-crystalline counterparts with excellent capacity, rate capability,
and cycle stability, thus highlighting their uncharted potential as high-capacity
organic cathodes for LIBs.
RESULTS
‘‘Scrambling’’ Experiments
We first conducted a series of ‘‘scrambling’’ experiments to validate whether amine
exchange would occur in a model reaction involving hexamethyl-substituted HATN
(HATN-Me) and 1,2-phenylenediamine (PDA) (Figure 2A). All the scrambling exper-
iments, or control experiments, were conducted in aqueous conditions. At the end
of the reaction, the reaction mixture was extracted by CDCl3, resulting in the com-
plete transfer of all the solid residue into the organic phase. Given that the products
of interest (HATN and HATN-Me) have poor solubility in aqueous phases, it is
reasonable to assume that mass transfer is approximate to completion from the
aqueous solution to the organic phase, and the observed ratio reflects the ratio of
the scrambling products from the heterogeneous reaction in water. The extract
was used directly for NMR spectroscopic studies unless noted otherwise (see Sup-
plemental Information for comparisons of results from different workup procedures).
No exchange product was detected when the mixture of HATN-Me and PDA at a
Chem 6, 1–12, April 9, 2020
3