Facile functionalization of HTC-derived carbon microspheres

Article abstract of DOI:10.1039/c2cc35552c

Surfaces of HTC-derived carbons were functionalized using dienophiles such as maleimide (MI), tetracyanoethylene (TCNE), and 4,5-dicyano-1,3-dithiol-2-one (DCDTO). DCDTO-functionalized carbons were then hydrolyzed to yield thiol-functionalized carbons, wh

Full text of DOI:10.1039/c2cc35552c

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COMMUNICATION
Facile functionalization of HTC-derived carbon microspheresw
Hiromitsu Urakami,* Markus Antonietti and Filipe Vilela
Received 31st July 2012, Accepted 11th September 2012
DOI: 10.1039/c2cc35552c
Surfaces of HTC-derived carbons were functionalized using
dienophiles such as maleimide (MI), tetracyanoethylene (TCNE),
and 4,5-dicyano-1,3-dithiol-2-one (DCDTO). DCDTO-functionalized
carbons were then hydrolyzed to yield thiol-functionalized carbons,
which were used to perform conjugate addition on the carbon surface.
in post-modification of HTC-derived carbon materials through
possible DA reaction.18 Along with the in situ functionalization,
the capability of post-modification of HTC materials should enable
further control over the desired properties of HTC carbons. For
this initial study, maleimide (MI), tetracyanoethylene (TCNE), and
4,5-dicyano-1,3-dithiol-2-one (DCDTO) were selected as model
DA dienophiles. To date, maleimide and TCNE have been used
as model DA dienophiles in reactions using carbon materials as a
diene.19–22 DCDTO was selected because the molecule could both
be a dienophile as well as a thiol-precursor. Herein, we report a
facile surface functionalization of carbons (HTC@180 and @550)
prepared by hydrothermal treatment of glucose at two different
temperatures of 180 1C and 550 1C.
The production of functional materials from renewable resources
has caught much attention due to the growing concern for
sustainable energy and resources. Among various methods, the
hydrothermal carbonization (HTC) process of biomass offers a
cheap, environmentally friendly, and atom economical method
to produce functional carbon materials.1–4 In addition, since
HTC operates with biomass, HTC is thought of as a sustainable
‘‘CO2-neutral’’ or even ‘‘CO2-negative’’ operation if microbial
degradation of biomass is taken into consideration.3,5 To date,
our laboratory has produced a variety of functional materials in
the fields of catalysis,6,7 lithium ion batteries,8,9 adsorption/heavy
metal chelation,10 and supercapacitors11 by utilizing the HTC
process.
The DA reactions were carried out simply by the addition
of respective dienophiles into HTC@180 or @550, and
were subsequently heated for a pre-determined time. TCNE-
functionalized HTC@180 showed an increase of N% from the
elemental analysis as well as the appearance of the CRN
band at B2200 cmÀ1 on FTIR (see Fig. S1 and Table S1,
ESIw). The diminished band at around 1750 cmÀ1 could be
attributed to hetero-DA between a,b-unsaturated carbonyls of
the carbon surface and TCNE. Although 50 1C was sufficient to
observe an addition of TCNE to HTC@180, higher temperatures
were found to be necessary to add TCNE onto HTC@550.
Slight increases in the nitrogen content were found from
elemental analysis as the temperature was raised from 50 -
75 - 100 1C. However, the change was too small to be seen
from FTIR measurements. The difference in the reactivity of
the carbon surface could arise from the difference in the furan
content. It has been reported that post-pyrolysis of the HTC
carbon leads to decreased content of the polar functional
groups as well as the furanic structure, rearranging to a more
aromatic structure, leading to increased chemical stability of
pyrolyzed carbon surfaces. However, with a highly reactive
molecule such as TCNE, it is possible that the reaction
not only proceeds through the proposed cycloadditions, but
reaction such as tricyanovinylation of electron rich aromatics
cannot be excluded.23–25
Hydrothermal treatment of pentoses and hexoses is known
to produce furfural and furfural-based (i.e. hydroxymethyl-
furfural (HMF)) products as intermediates.1,12,13 These furfural
and furfural-derivatives are reported to further undergo a
series of cycloadditions and condensation reactions to poly-
merize and to form the final HTC material.1,2,14 Furans and
their derivatives have been proposed to be involved in this
Diels–Alder (DA) type cycloaddition. The abundance of furans in
resulting HTC-derived carbon spheres has been determined using
advanced solid-state MAS 13C NMR.15 By taking advantage of the
HTC process, incorporation of dienophiles such as vinyl imidazole
and acrylic acid has been demonstrated.6,10 The downside with the
in situ functionalization method are the HTC conditions themselves
(>180 1C, water as a solvent), where structures of many of the
conventional small molecules could be altered.16,17 In this regard,
post-modification of the pre-formed HTC material compared to
in situ functionalization could offer a milder and controlled
functionalization process. Due to the presence of furans in the
carbons formed via the HTC process, we also became interested
In this regard, though MIs are known as an active dieno-
phile as well as a Michael acceptor, substitutions will not occur
as TCNE. For both HTC@180 and @550, DA reactions with
MI lead to a moderate increase in the nitrogen content. FTIR
for both MI-functionalized HTC@180 and @550 exhibited an
increased signal at B1700 cmÀ1 which is most likely attributed
to the additional CQO stretch from the grafted maleimide
(Fig. S3, ESIw). Elemental analysis of MI-functionalized carbons
Department of Colloid Chemistry, Max-Planck-Institute of Colloids
and Interfaces, D-14476 Potsdam, Germany.
E-mail: hiromitsu.urakami@mpikg.mpg.de; Tel: +49-331-567-9513
w Electronic supplementary information (ESI) available: Synthesis of
surface modified HTC as well as anthracene acrylate. Additional
characterization information (FTIR, elemental analysis, 1H and 13C
NMR, SEM, fluorescence microscopy). See DOI: 10.1039/c2cc35552c
c
10984 Chem. Commun., 2012, 48, 10984–10986
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analysis results (Table 1) demonstrated the higher reactivity of
HTC@180 towards DCDTO than HTC@550, @750. Typical
sulfur contents found for DCDTO-functionalized HTC@180
from elemental analysis were 7–10%, inferring a surface sulfur
density of such carbons to be approximately 6–7 sulfur molecules
per nm2. For DCDTO-functionalized HTC@550, the sulfur
density on carbon surfaces was determined to be roughly 1–2
sulfur molecules per nm2.
Fig. 1 Proposed addition of DCDTO onto HTC@180 and @550.
As with the addition of TCNE, FTIR of DCDTO-functionalized
HTC@180 shows the appearance of the CRN absorption
band at 2200 cmÀ1 (Fig. S2, ESIw). For DCDTO-functionalized
HTC@550, new stretch attributed to the carbonyl group of
DCDTO at B 1730 cmÀ1 was observed. Nitrile peaks were not
observed for the HTC@550 samples due to their low degree of
functionalization and inherent weak intensity of the CRN
stretch. On the other hand, the CQO stretch from the
dithiocarbonyl moiety was only observed for HTC@550
samples. Most likely, the CQO stretch dithiocarbonyl band
becomes more apparent for HTC@550 due to the reduced
carbonyl content in the unfunctionalized HTC@550 compared to
HTC@180. DCDTO-functionalized HTC samples were then
hydrolyzed in an attempt to obtain thiol-functionalized
HTC@180 and @550 (Fig. 2a). Out of the few different cleavage
conditions (acid treatment, base treatment, and NaSMe–MeOH),
reflux in 0.3 N HCl in ethanol was determined to be the optimal
condition for our materials. As shown in Fig. 2b, FTIR showed
the disappearance of the dithiocarbonyl band at 1730 cmÀ1
upon acid-catalyzed alcoholysis. Elemental analysis indicated
no significant change in the percent composition of the
element between the HTC samples before and after the acid
treatment, implying the absence of undesired side reactions.
showed a small increase (2–3%) in N%. Nonetheless, the
reaction between HTC carbons and TCNE/MI confirmed
the ability of dienophiles to modify the carbon surface.
Similarly to the addition of TCNE and MI, HTC@180 and
@550 were functionalized with DCDTO by simply mixing
the respective starting materials at 100 1C for 48 h (Fig. 1).
Functionalized samples were washed repeatedly to ensure the
removal of all physisorbed DCDTO. As with TCNE, elemental
Table 1 Elemental analysis found for DCDTO-functionalized
HTC@180 and @550
DCDTO/HTC
(weight eq.)
HTC@
Solvent
C%a
H%
N%
S%
180
180
180
180
550
550
750
6.4
6.4
5.0
5.0
12.0
6.4
6.4
DMF
p-Xylene
DMF
ODCB
p-Xylene
p-Xylene
ODCB
57.2
63.0
59.1
64.9
85.0
80.5
85.6
3.4
4.1
3.6
4.4
2.9
3.1
1.8
6.0
3.3
5.1
2.1
2.6
2.8
0.6
9.4
2.9
7.8
2.2
2.6
3.0
1.8
a
Typical C% for unmodified HTC@180: 64–66%. Typical C% for
unmodified HTC@550: 84–88%.
Fig. 2 (a) Proposed reaction scheme of the alcoholysis and the subsequent conjugate addition. (b) FTIR before (A) and after (B) the acid
catalyzed alcoholysis. (c) Fluorescent image of anthracene-functionalized HTC@180.
c
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Chem. Commun., 2012, 48, 10984–10986 10985

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Table 2 Elemental analysis found for HTC@180 and @550 before (A)
and after the hydrolysis (B) and the conjugate addition of PEG 480 (C)
maleic anhydride results in the appearance of anhydride peaks on
FTIR (Fig. S4, ESIw) and decreased %C on elemental analysis
(Table S1, ESIw). The modification of HTC carbon via the use
of dienophiles and its subsequent functionalization reactions
could be further extended as an attractive approach to prepare
novel carbon-based materials.
Entry
C%
H%
N%
S%
A-@180
B-@180
C-@180
A-@550
B-@550
C-@550
57.2
56.9
57.4
86.7
86.0
83.8
3.4
3.5
3.9
2.9
2.8
2.9
6.0
5.5
5.5
2.6
2.6
2.7
9.1
8.9
8.2
2.7
2.4
2.3
We would like to thank Dr Camillo Falco, Linghui Yu,
Dr Magdalena-Maria Titirici for providing HTC@180 and @550
precursors as well as for their help with taking the SEM
images. We would also like to thank Dr Marek Grzelczak
and Dr Stephanie A. Wohlgemuth for the numerous exciting and
insightful discussions. We also thank Dr Eugenia Maxinova for
her help with the fluorescent microscopy.
Furthermore, SEM images (Fig. S5, ESIw) showed no apparent
morphological differences between the unfunctionalized-,
DCDTO-functionalized-, and the hydrolyzed-HTC samples.
To prove the accessibility of surface bound thiols, polyethylene
glycol (PEG480) acrylate was used as the substrate for the
conjugate addition. The elemental analysis (Table 2, C-180 and
-550) and FTIR (Fig. S6, ESIw) of the PEGylated carbons
compared to thiol-functionalized HTC carbons (B-180 and -550)
indicated the success of the conjugate addition. Obvious changes in
the C% between HTC@180 samples were not observed due to the
similarity in C% between PEG acrylate and HTC@180 samples.
However, the decrease in S% is observed from C-180 supports the
successful conjugate addition of PEG acrylate onto B-180. To
further support the conjugate addition, anthracene acrylate was
prepared and utilized as a conjugate addition substrate. Although
the changes observed from the elemental analysis results were not as
obvious as with PEG acrylate, the change seen in the percent
composition of the HTC@180 sample was as anticipated (Table S2,
ESIw). However, no apparent change was observed from the
HTC@550 samples likely due to the inherent low degree of
available thiol groups for the conjugate addition.
Notes and references
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To visualize the functionalization, fluorescent images of
anthracene acrylate functionalized HTC@180 and @550 were
taken. The samples were excited at a wavelength of 361 nm and
were detected at the emission wavelength of 402 nm. The images
of HTC@180 samples (Fig. 2c) clearly show the fluorescence
emission, further supporting the presence of anthracene acrylate
covalently attached to the carbon surface. On the other hand,
no fluorescence was observed from the HTC@550 samples
(Fig. S7, ESIw). The lack of fluorescence was attributed to a
low degree of functionalization due to the limited amount of
thiol groups in its precursor.
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In conclusion, we have shown the facile functionalization of
HTC materials using reactive dienophiles (TCNE, MI). DCDTO
was utilized as a dienophile as well as a precursor to obtain
surface-thiol accessorized HTC carbons. The presence of the
thiol groups on the surfaces of HTC@180 and @550 was
demonstrated by successful conjugate addition with Michael
acceptors. To date, we have confirmation of other dienophiles,
such as maleic anhydride and tricyano-functionalized molecules,
being able to modify the HTC carbon surface in the preparation
of functional materials. Initial results show that the addition of
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c
10986 Chem. Commun., 2012, 48, 10984–10986
This journal is The Royal Society of Chemistry 2012