Polycondensation of boron- and nitrogen-codoped holey graphene monoliths from molecules: Carbocatalysts for selective oxidation

Article abstract of DOI:10.1002/anie.201209320

A simple but powerful chemical process - the copolymerization of biomass (glucose) and boric acid as templated by dicyandiamide (see picture) - was used to fabricate high-quality doped graphene monoliths with through-plane nanopores. The holey graphene mo

Full text of DOI:10.1002/anie.201209320

Angewandte
Chemie
DOI: 10.1002/anie.201209320
Carbocatalysis
Polycondensation of Boron- and Nitrogen-Codoped Holey Graphene
Monoliths from Molecules: Carbocatalysts for Selective Oxidation**
Xin-Hao Li* and Markus Antonietti
The development of low-cost sustainable catalysts with high
stability and excellent catalytic activity under mild conditions
remains at the heart of modern materials chemistry and green
In view of the two-dimensional (2D) structure of gra-
phene with its very large aspect ratio, a slight aggregation of
graphene layers during and/or after catalytic reactions is
inevitable and significantly decreases the number of catalyti-
cally active sites exposed to substrates in the following
reaction. Mass transfer is a standard problem of lamellar
materials, as organic molecules cannot transfer through
sheets, and even barrier properties are built up. In principle,
the generation of nanoholes in the graphene sheets is
[1–5]
chemistry.
Carbocatalysts (metal-free and heterogeneous
carbon materials) show great potential as sustainable catalysts
both for organic synthesis and as electrocatalysts in fuel cells
owing to their relatively low cost and their comparable
[
8]
[2–4]
activity to that of commercial platinum catalysts.
Among
the carbon allotropes, the high surface area and acceptable
stability of modified graphenes make them good candidates
favorable, as it not only increases the amount of active
[
4]
[8b,c]
for carbon catalysis. More importantly, the relative versa-
tility possible in the chemical modification of graphene sheets
affords them tunable electronic properties; this simple but
efficient approach for tuning their catalytic activity on the
edges, but also improves mass transfer during catalysis.
A
powerful synthesis therefore includes the possibility of
generating through-plane nanopores in doped or pristine
graphene sheets.
[
4–6]
atomic level is promising.
Herein, we report a one-step chemical approach to the
direct synthesis of gram quantities of doped graphene
monoliths with nanopores. By this method, we could simulta-
neously introduce high concentrations of heteroatoms and
through-plane nanopores into the graphene structure.
Through the copolymerization of glucose and boric acid
within the interlayers of lamellar carbon nitrides in a simple
thermal pyrolytic process (Figure 1), a boron- and nitrogen-
Engineering of the hexagonal crystal lattice of graphene
sheets through substitutional doping is thereby a direct way to
control the electronic structure/transport properties of gra-
phenes and expands their application in catalysis and
[
4–6]
electronic devices remarkably.
The insertion of hetero-
atoms into the graphene lattice to tune the position of the
valence band or conduction band of the doped carbon
materials results in altered function and a slightly opened
band gap; for this purpose, boron was been widely used as an
electron acceptor, and nitrogen and phosphorus as electron
[
5,6]
donors. Current methods for the mass production of doped
graphenes are limited to the chemical modification of layered
graphite oxides (GOs). These approaches usually lead to only
2
À1
a low surface area (< 300 m g ) and a low dopant content of
the resulting chemically modified graphenes (CMGs).
Despite tremendous efforts, the larger-scale production of
homogeneously doped graphene lattices with higher concen-
trations of these dopants still remains a great challenge for
materials scientists. The bottom-up chemical synthesis of
doped graphenes from small precursor molecules seems to be
the most suitable approach to the tuning of their composition
and morphology through the choice of appropriate precur-
[
7]
sors.
Figure 1. Proposed protocol for the synthesis of BNHG.
[*] Dr. X. H. Li, Prof. M. Antonietti
Department of Colloid Chemistry
Max Planck Institute of Colloids and Interfaces
Research Campus Golm, 14476 Potsdam (Germany)
E-mail: xin-hao.li@mpikg.mpg.de
codoped holey graphene assembly (BNHG) with a high
2
À1
surface area (978 m g ) was obtained. A synergistic effect of
N and B on the stability, dopant concentration, and catalytic
activity of the BNHG was observed. Even after the calcina-
tion of the precursor at 10008C, the as-formed BNHG still
had a very high dopant concentration exceeding 15 atom%.
This material shows great potential as a carbocatalyst for
organic synthesis. For example, BNHG can efficiently acti-
vate oxygen (1 atm) at very low temperatures for the
[
**] This research was supported by the L2H project of the BMBF, the
EnerChem project, and the UNICAT project. X.H.L. thanks Prof.
X. C. Wang and J. S. Zhang of Fuzhou University for TEM
measurements and Dr. Q. S. Gao of the MPIKG for fruitful
discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209320.
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oxidative coupling of amines with high selectivity, compati-
bility, and reusability.
As depicted in Figure 1, BNHG was typically prepared
simply by heating a mixture of dicyandiamide (DCDA, 40 g),
glucose (1 g), and boric acid (0.4 g) above 8008C under the
men. Holes with a size between several nanometers and
hundreds of nanometers were observed (Figure S2b,d). The
synthesis was not optimized to control the pore size. We
proved that boric acid was the dominating factor in inducing
the formation of holes by using borazane, borazine, and other
boranes as additives; these alternative additives caused no
obvious hole formation (data not shown). The acid character
of boric acid (which tends to form strong contacts through an
electrostatic interaction with ammonia groups of carbon
nitride) and its layered structure in the solid phase make boric
acid a suitable “template” to generate carbon-poor areas
(shown schematically in red in Figure 1) between the as-
formed aromatic carbon intermediates (highlighted in black
in Figure 1) in the interlayer space of layered carbon nitride.
Subsequent thermolysis of the hybrid solid liberated the
graphenes and opened the through-plane pores.
HRTEM images (Figure 2b; see also Figure S4) revealed
the formation of only very small graphene domains with a size
of a few nanometers, which matched well with the very broad
G and D peaks found by Raman analysis (see Figure S5). The
SAED pattern of the same area (Figure 2c) revealed the
typical pattern observed for multicrystalline materials. The
broad XRD peaks of BNHG1000 further indicated only
restricted stacking in the c direction (see Figure S3). The
lamellar stacking distance of BNHG1000 was estimated to be
protection of flowing N gas. The doping level with B and N
2
atoms was controlled by varying the heating temperature. The
samples obtained by heating at 800, 900, and 10008C were
named BNHG800, BNHG900, and BNHG1000, respectively.
Large-area scanning electron microscopy (SEM) images
(
see Figure S1a in the Supporting Information) revealed that
the as-obtained BNHG materials are assemblies of continu-
ous, flexible, wrinkled sheets. The holey structure of BNHG
becomes obvious under higher magnification (see Fig-
ure S1b,c). Further transmission electron microscopy (TEM)
analysis (Figure 2a; see also Figure S2) and a dark-field TEM
image (Figure 3, left) confirmed the uniform 2D structure of
the layered carbon with through-plane holes and excluded the
presence of condensed particles or a condensed bulk speci-
0
2
.32 nm on the basis of the very broad (002) peak centered at
7.28, which is smaller than that of ideal graphite (0.34 nm).
This result speaks for the homogeneous inclusion of B and N
atoms in the graphene planes and secondary acid–base
interactions between the layers.
X-ray photoemission spectroscopy (XPS) analysis unam-
biguously excluded the presence of metal components in the
BNHG samples (see Figure S6). The introduction of B and N
atoms into the graphene framework was also demonstrated by
elemental-mapping images (Figure 3) and XPS analysis (see
Figure S6) of the as-obtained BNHG monoliths, whereas the
UV/Vis absorption spectra of BNHG (see Figure S7) without
an obvious absorption edge at 218 nm excluded the formation
of pristine boron nitride domains. Homogeneous heteroatom
doping of graphene was shown by elemental mapping of any
selected area (Figure 3; see also Figure S8). The C 1s XPS
spectrum of the BNHG1000 sample (see Figure S9a) indi-
Figure 2. a) TEM image of BNHG1000; b) HRTEM image of the edge
area; c) selected area electron diffraction (SAED) pattern of the edge
area.
2
cated significant modifications of the sp carbon atom with
heteroatoms through the formation of CÀB, CÀC, and CÀN
[6]
bonds. The N 1s XPS peak (see Figure S9b) demonstrates
that the N atoms are inserted into the graphene lattice mainly
in the form of “pyridinic” and “graphitic” N atoms and NÀB
[6,7c]
bonds.
The binding energy of the B 1 s orbital (see
Figure S9c) of around 190.8 eV indicated the formation of
[
6]
CÀB and NÀB bonds. A minor amount of BÀO bonds were
also observed; these bonds terminate the layers, as was
further confirmed by solid-state NMR spectroscopic analysis
(see Figure S10). The total heteroatom content of BNHG1000
is 14 atom% and 16 atom% for B and N, respectively,
according to XPS analysis (see Figure S6 and Table S1 in
the Supporting Information). The heating temperature had
a significant effect on the concentration of B and N atoms;
higher temperatures resulted in lower dopant concentrations.
Moreover, a certain amount of oxygen was present in the
Figure 3. Dark-field TEM image of the BNHG1000 monolith (left), and
the corresponding boron, carbon, and nitrogen elemental-mapping
images of the selected area in the square.
2
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BNHG samples, mainly in the form of O=CÀO or HÀOÀH
(Table 1). GO offered good conversion and slightly lower
[
8]
bonds (see Figure S9d), and could not be changed signifi-
cantly by varying the heating temperature. We attribute the
presence of oxygen to strongly chemisorbed oxygen-contain-
selectivity for the imine even in the absence of O (Table 1,
2
entries 2 and 3), which indicated that GO underwent a chem-
[4d]
ical reaction with the substrate under mild conditions. No
[
8]
ing components, presumably CO and H O. This phenom-
2
2
enon is general for high-surface-area aromatic carbons, which
show super-adsorption toward gases (as seen by a spontaneous
mass increase after their release from a vacuum); on the other
hand, this property facilitates their application as catalysts to
activate gases (especially polarizable gases, such as O , CO ,
[a]
Table 1: Study of reaction conditions.
2
2
[b]
[b]
Entry
Catalyst
Conversion [%]
Selectivity [%]
and NO) and organic molecules for catalytic conversion.
1
2
3
4
5
6
7
8
9
–
GO
–
–
95
97
88
98
>99
>99
–
The specific surface area of the BNHG samples increased
with the heating temperature; in contrast, a higher heating
temperature only decreased the specific surface area of
pristine or N-doped graphenes, as demonstrated by us in
72
46
17
45
48
91
–
10
91
91
91
89
GO (N2)
NGr
BNHG800
BNHG900
BNHG1000
[
7c]
a previous study.
The Brunauer–Emmett–Teller (BET)
2
À1
surface area of BNHG1000 was determined to be 978 m g
BNHG1000 (N2)
by nitrogen-adsorption–desorption analysis (see Figure S11).
This value corresponds to a calculated average number of
layers of 3, as averaged over the complete sample. Simulta-
neously, the synergetic effect of B and N enhanced not only
the chemical stability of the dopant atoms (to remain in the
structure) but also the thermal stability of the BNHG
monoliths as such. BNHG1000 has a higher surface area as
RGO
BNHG1000
BNHG1000 (2nd cycle)
BNHG1000 (3rd cycle)
BNHG1000 (4th cycle)
91
[
c]
10
11
12
>99
>99
>99
>99
[d]
[d]
[
d]
13
[
a] Standard reaction conditions: acetonitrile (5 mL), substrate
1 mmol), catalyst (30 mg), O balloon (1 atm), 4 h, 858C. [b] The
conversion and selectivity were determined by GC–MS by using aniline
as an internal standard. [c] The free-radical scavenger butylated
hydroxytoluene (BHT; 10 mol%) was added. [d] The BNHG1000 catalyst
(
2
2
À1
compared with pristine graphenes (821 m g ) obtained
[7c]
under the same conditions without boric acid,
and the
surface areas of both are higher than that of B- and N-
codoped graphenes obtained by the post-treatment of GO
(
30 mg) was reused for multiple cycles.
2
À1 [5]
(
< 300 m g ). Thus, our copolymerization method has
advantages for the bottom-up preparation of doped graphe-
nes.
conversion was observed without a catalyst (Table 1, entry 1).
BNHG could activate O for the transformation of the amine
2
A number of carbon or carbon nitride nanoallotropes
have already shown their great potential as metal-free
into an imine with very high selectivity (> 99%); its catalytic
activity depended on the condensation temperature of the
material and thus on its surface area and composition
(Table 1, entries 5–7). No reaction was observed in the
catalysts for the activation of O or air for selective oxidation,
2
[
2–4,9]
which is of great importance in organic synthesis.
As
a graphene-based carbocatalyst, GO with its high surface area
and its myriad of oxide functionalities has been a benchmark
carbocatalyst with fascinating catalytic properties for selec-
absence of O (Table 1, entry 8); this result underlines the
2
occurrence of a catalytic reaction over BNHG. Without any
cocatalysts, BNHG1000 gave the best conversion (91%) into
the corresponding imine within 4 h under quite mild con-
ditions. Thus, BNHG1000 showed outstanding potential as
a sustainable carbocatalyst for O2 activation or air-based
catalytic oxidation. Also, both the conversion and selectivity
over BNHG1000 were much higher than those observed with
GO under mild conditions.
[
4]
tive oxidation. As GO itself has only moderate stability
under conditions of photoirradiation or higher tempera-
[10]
ture,
an excess amount of the GO catalyst and highly
oxidative reaction conditions (high pressure of O and higher
2
reaction temperatures) have usually been required to avoid
the reduction of GO and thus to ensure catalytic activity.
Our BNHG monoliths have high thermal stability (the
samples were obtained at temperatures above 8008C), a high
surface area, a high dopant concentration, and good disper-
sibility in various solvents (see Figure S12) and are simple to
make in high yield at a low cost. We therefore tested the
catalytic reactivity of BNHG as a sustainable carbocatalyst
for catalytic oxidation.
However, a high surface area and a high dopant concen-
tration are not enough to ensure the catalytic activity of
graphene-based catalysts. The fact that NGr, with a similar N-
dopant concentration (15.4 atom%) and a very high surface
2
À1
area (916 m g ), could only offer very low conversion and
moderate selectivity for the imine product demonstrated the
importance of B atoms in enhancing the catalytic activity of
the BNHG materials. The introduction of doping atoms into
graphite-based materials can induce electron relocalization in
both the conduction band (CB) and the valence band (VB)
We focused our initial studies on the oxidative coupling of
amines to form imines, which are important intermediates in
[
4,9]
organic synthesis,
ature (< 1008C) and a low laboratory pressure of O (1 atm).
under mild conditions: at a low temper-
[4h]
and thus make them appropriate catalysts for oxidation.
2
[
7c]
BNHG, GO, N-doped graphene monoliths (NGr),
and
Similarly, B atoms (as electron acceptors) and N atoms (as
electron donors) could in principle lower the VB (or HOMO)
reduced GO (RGO) were compared as catalysts. All showed
good selectivity for the formation of the imine, but the
conversion varied under the standard reaction conditions
[5b,c]
and elevate the CB (or LUMO)
and thus induce electron
relocalization to activate the substrate and oxygen molecules
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for oxidation reactions. It is believed that the catalytic activity
and selectivity are derived from the electronic structure of the
HOMO of BNHG1000, as controlled by the N and B
heteroatoms, whereas the high surface area and holey
great potential as catalysts, electrochemical catalysts, or
electrode materials for further applications.
structure are beneficial for mass transfer. The failure of Experimental Section
a free-radical scavenger (BHT, 10 mol%) to quench the
oxidation of benzylamine over BNHG1000 (Table 1,
Typical synthesis of BNHG1000: Dicyandiamide (40 g), monohydrate
glucose (1 g), and boric acid (0.4 g) were dissolved in deionized water
(
200 mL) and stirred at 808C to remove water. The resulting white
entry 10) revealed that O is activated directly on the surface
2
À1
powder was transferred into a crucible, heated at a rate of 2.48Cmin
to 6008C, and then tempered at this temperature for another 2 h
under a flow of nitrogen. The material was then heated further at
of the catalyst for the oxidation reactions, without the
formation of free radicals (e.g. superoxide radicals) in the
solution. Since no overoxidation products of the amine, such
as aldehydes and ketones, were detected in the GC–MS
spectra of the products, we propose the formation of an
À1
a rate of 3.38C min to 10008C (or 8008C for BNHG800, or 9008C
for BNHG900) and maintained at that temperature for 1 h. The
sample was then allowed to cool naturally to room temperature under
the protection of N2.
aminal intermediate, which can readily lose NH to give the
3
[
9d]
imine coupling product (Scheme 1).
Received: November 21, 2012
Published online: && &&, &&&&
Keywords: aerobic oxidation · carbocatalysts · dopants ·
.
graphene · porous carbon
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[
[
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Carbocatalysis
A simple but powerful chemical pro-
cess—the copolymerization of biomass
(glucose) and boric acid as templated by
dicyandiamide (see picture)—was used
to fabricate high-quality doped graphene
monoliths with through-plane nanopores.
The holey graphene monoliths had a high
surface area and showed excellent per-
formance as metal-free carbocatalysts for
selective oxidation.
X. H. Li,* M. Antonietti
&&&& — &&&&
Polycondensation of Boron- and
Nitrogen-Codoped Holey Graphene
Monoliths from Molecules:
Carbocatalysts for Selective Oxidation
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!