Knitting an oxygenated network-coat on carbon nanotubes from biomass and their applications in catalysis

Article abstract of DOI:10.1039/c1jm10989h

In this work, we presented a new way for the functionalization of carbon nanotubes (CNTs) with the use of biomass as starting materials and introduced a novel concept of knitting process in the chemistry of CNTs for the first time. A mixture of aromatic compounds obtained from the hydrothermal treatment of biomass, rather than the traditional polymer monomers, was used as the nanoscale building blocks to knit an oxygenated network-coat on the CNTs layer-by-layer. It is an effective, mild, green and easily-controlled method for the functionalization of CNTs. The obtained f-CNTs were proved to be a promising catalyst support for metal catalysts, such as Ru/f-CNTs, showed high activity and selectivity for the hydrogenation of citral to unsaturated alcohol. More importantly, we opened a pioneering way for the conversion of low-cost, abundant and renewable biomass into a hydrophilic/chemical reactive network-coat on the inert surface of a wide range of sp² carbon materials, such as prevalent fullerene, carbon nanotubes, carbon nanohorns and hot graphene etc.

Full text of DOI:10.1039/c1jm10989h

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PAPER
Knitting an oxygenated network-coat on carbon nanotubes from biomass and
their applications in catalysis†
ab
Jun Ming,abc Ruixia Liu,ab Guanfeng Liang,abc Haiyang Cheng,ab Yancun Yuab and Fengyu Zhao
*
Received 7th March 2011, Accepted 4th May 2011
DOI: 10.1039/c1jm10989h
In this work, we presented a new way for the functionalization of carbon nanotubes (CNTs) with the
use of biomass as starting materials and introduced a novel concept of knitting process in the chemistry
of CNTs for the first time. A mixture of aromatic compounds obtained from the hydrothermal
treatment of biomass, rather than the traditional polymer monomers, was used as the nanoscale
building blocks to knit an oxygenated network-coat on the CNTs layer-by-layer. It is an effective, mild,
green and easily-controlled method for the functionalization of CNTs. The obtained f-CNTs were
proved to be a promising catalyst support for metal catalysts, such as Ru/f-CNTs, showed high activity
and selectivity for the hydrogenation of citral to unsaturated alcohol. More importantly, we opened
a pioneering way for the conversion of low-cost, abundant and renewable biomass into a hydrophilic/
chemical reactive network-coat on the inert surface of a wide range of sp2 carbon materials, such as
prevalent fullerene, carbon nanotubes, carbon nanohorns and hot graphene etc.
damaged at least in the process of oxidation. Although non-
Introduction
covalent functionalizations such as polymer wrapping and
The hydrophobic sidewalls of carbon nanotubes (CNTs) that
adsorption could do much for preserving the intact microstruc-
consist of sp2 hybridized carbon atoms often limit their solubility
tures, organic functionalities were always adopted and most
and dispersibility in solvents, which seriously restricts their
modification procedures were performed in harmful organic
manipulations and applications. Therefore, surface functionali-
solvents, which could burden the post-treatment and bring some
zation has been always demanded in most nanotechnologies,
unexpected biological toxicity to the final nanotube composites.6
such as in the fabrication of multifunctional CNTs based
Biomacromolecule-based functionalizations give researchers the
composites,1 template synthesis of tube materials,2 preparation
edification that environmental friendly functionalities, such as
of an effective catalyst support3 or a series of biocompatible
vehicles,4 and so forth.
simple saccharides, polysaccharides, proteins, enzymes, etc.,
could be adopted widely.7 However, directional adsorption or
However, to date, most functionalizations are started on the
wrapping of tangled biomacromolecules on the CNTs surface
harsh oxidation (usually using acid or other strong oxidant) of
just through p–p interactions, Van der Waals forces and char-
CNTs with the aim of forming functional groups (e.g., carboxylic
acteristic surface adsorption seems difficult to control and
groups) on the CNTs surfaces, and then followed by suitable
sometimes gives rise to an unstable form of the composite,
reactions with desired functionalities through the covalent
especially when they redispersed in solvent which has a strong
linkage of the formed groups.5 Inevitably, the sp2 nanotube
dissolving ability for the biomacromolecules. Therefore, an
structure and their electronic characteristics of CNTs were
effective, mild and green strategy that endows CNTs with
requisite functional groups without harsh treatment is pursued
vigorously, not only for the easy functionalization of CNTs but
aState Key Laboratory of Electroanalytical Chemistry, Changchun
also for preserving the intact micro-/macro-structure of the
Institute of Applied Chemistry, Chinese Academy of Sciences,
CNTs and the stability of composite.
Changchun, 130022, P. R. China. E-mail: zhaofy@ciac.jl.cn; Fax: +86-
431-85262410; Tel: +86-431-85262410
Furthermore, the functionalities (e.g., polymers) that linked
with branched carboxylic or hydroxyl groups could not fully coat
the CNTs surface in theory because they seemed more likely to
form a branched structure like that of a millipede or a brush.8
Moreover, CNTs-composites bundles were often formed in
traditional functionalization methods because the neighbouring
CNTs were easily assembled into bundles by the linear/tangled
functionalities, such as polymers and biomacromolecules.9 Thus,
designing a conformal coat compactly surrounded on the CNTs
bLaboratory of Green Chemistry and Process, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022,
P. R. China
cUniversity of the Chinese Academy of Sciences, Beijing, 100049, P. R.
China
† Electronic supplementary information (ESI) available: GC-MS
analysis of mixture solution. FTIR spectra of biomass/CNTs and
(decompositions of biomass)/CNTs. Comparison of the catalytic
activity of Ru catalysts in selective hydrogenation of citral. See DOI:
10.1039/c1jm10989h
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surface is necessary for preserving their macroscopic cylindrical
structure and individual dispersity, both of which are important
for the applications of CNTs in a wide field.10 Therefore, herein,
we used a mixture of small molecules of cyclic/heterocyclic
aromatic compounds as nanoscale building blocks for the first
time to knit an oxygenated network-coat conformally sur-
rounded on CNTs layer-by-layer. This method not only
successfully functionalizes the CNTs effectively without any
harsh treatment, but also perfectly preserves an intact micro-/
macro-structure and the individual dispersity of CNTs.
(5 mL) and catalysts (20 mg) were put into the reactor under
nitrogen atmosphere and the reaction was carried out with
continuous stirring at 80 ꢁC and a hydrogen pressure of 4 MPa.
After the reaction, the reaction mixtures were centrifuged and
analyzed by gas chromatography with an FID detector (GC-
Shimadza-14C, FID, Capillary column Rtx-Wax 30 m–0.53 mm–
0.25 mm) and gas chromatography/mass spectrometry (GC-MS,
Agilent 5890). The catalyst after reaction was washed with
deionized water and ethanol repeatedly, and then dried in
a vacuum oven at room temperature before the next recycling test.
Results and discussion
Experimental
Knitting an oxygenated network-coat on CNTs and the
mechanism analysis
Materials
The chemical reagent of glucose was purchased from Sinopharm
Chemical Reagent Beijing Co. Ltd, and cellulose was purchased
from Alfa Aesar. The multiwalled carbon nanotubes
(MWCNTs) with a length of 5ꢀ15 mm (purity > 95%, outer
diameter 40ꢀ60 nm) were purchased from Shenzhen Nanotech
Port Co., Ltd.
To date, many results have demonstrated that a mixture solu-
tion of aromatic heterocyclic/cyclic compounds, such as 5-
hydroxymethyl-2-furaldehyde, furfural, 5-methylfurfural, and
1,2,4-benezenetriol, etc.11 could be formed during the hydro-
thermal carbonization (HTC) of biomass, like carbohydrates,
crude plant materials and cellulose (see the Supporting Infor-
mation, Fig. S1†). These intermediates with a high density of
reactive groups (C–OH, C]O, COOH, etc.) have a strong
tendency to intra-/inter-molecular dehydration, aromatization
and subsequently act as a surfactant for self-assembling to form
a specific structure if the concentration of intermediates ach-
ieved a critical value.12a The self-assembly polymers could
undergo further intermolecular dehydration to form a series of
carbonaceous materials with different morphologies.11,12 While
in the presence of CNTs, these intermediates could preferen-
tially adsorb on the CNTs surface through p–p interactions
because of the p–p conjugated surface of CNTs (Fig. 1a–c),13
and then followed by dehydration and aromatization to knit
Functionalization of MWCNTs with the use of carbohydrates
(e.g., glucose)
0.3 g pristine MWCNTs (p-CNTs) were well dispersed in 30 L
deionized water under stirring and ultra-sonic treatment for 30
min respectively, and then 3.75 g glucose was dissolved in the
solution under vigorous stirring. After 2 h, the mixed solution
was transferred toꢁa 50 mL Teflon-sealed autoclave and then
maintained at 180 C for 4ꢀ12 h. After the reaction, the mixed
solution was filtered and washed completely with deionized water
ꢁ
and ethanol, and dried at 50 C overnight, giving rise to func-
tional carbon nanotubes (f-CNTs).
Functionalization of MWCNTs with the use of crude plant
materials (e.g., grass) and cellulose
2.0 g grass or cellulose were treatmed hydrothermally with 35 mL
H2O in a 50 mL Teflon-sealed vessel at 220 ꢁC for 5 h, and then
filtration of the mixture after cooling down, giving rise to a clear
yellow solution of aromatic compounds. Subsequently, 75 mg
pristine MWCNTs were dispersed in such solution (30 mL) and
were processed by the same procedures as with the glucose used.
Preparation of catalyst
The catalysts of Ru/f-CNTs and Ru/p-CNTs were prepared by
a wet impregnation method. The supports (0.27 g) were added to
the aqueous solution of RuCl3$1.8H2O (33.2 mg/100 mL H2O)
and the resulting slurry was stirred vigorously for 24 h at room
temperature. The solid materials were separated by filtration and
then washed thoroughly with deionized water and ethanol. Then,
the obtained solid materials were dried in a vacuum oven at 80 ꢁC
overnight. The as-prepared samples were reduced in a H2
(99.999%) stream at 150 ꢁC for 3 h before being used for reaction.
Fig. 1 (a,b) Hydrothermal treatment of a series of biomass gives rise to
a mixture solution of heterocyclic/cyclic aromatic compounds (a small
typical part of chemicals was listed on bottle); (c–f) the schematic
behaviours and knitting mechanism of the aromatic compounds on
surface of the CNTs; (g) the schematic structure and compositions of the
knitted layer.
Selective hydrogenation of citral
The catalysis reaction was performed in a 50 mL stainless steel
autoclave reactor with a magnetic stirrer. Citral (2 mmol), solvent
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a conformal layer surrounded on the CNTs surface compactly
(Fig. 1d).11d–f These intermediates acting as nanoscale building
blocks could be continuously knitted layer-by-layer on the
CNTs because of the presence of carbon nanotubes, which act
as nuclei. The formed layers could be knitted compactly
because the dehydration could occur between adjacent layers;
in addition, the p–p interactions, Van der Waals forces and H-
bond were also responsible for the stacking of them (Fig. 1e,f).
The mixed cyclic/heterocyclic compounds that experienced
a series of intra-/inter-dehydration, aromatization and subse-
quent cross-linking polymerization/carbonization are more
preferential to form a network structure like that of graphene
(Fig. 1g),11d–f rather than a linear one that is polymerized from
traditional polymeric monomers.6 Therefore, a novel concept of
knitting process is suitable for interpreting the complex
behaviors of the aromatic compounds coating on the CNTs
surface and showing their perfect combination with CNTs
sidewalls.
The unique features of f-CNTs
HRTEM images of the f-CNTs (Fig. 2a–f) demonstrate that
a conformal coat was compactly knitted on the surface of the
CNTs when a wide variety of low-cost biomass, including the
carbohydrate of glucose, grass and cellulose, was used as green
precursors. The thickness of the layers was controlled precisely
over a wide range within an extra-small change of Angstrom
ꢀ
meter (Am) (2.03–24.83 nm) through varying the reaction time or
the concentration of the precursors. Clearly, the conformal coat
compactly surrounded on the CNTs surface was incomparably
continuous, uniform and smooth, perfect like graphitic layers,
therefore allowing the f-CNTs to preserve a good individual
dispersity and a perfect macroscopic cylindrical structure, as
confirmed by FESEM (Fig. 3, a1). Furthermore, the outer layer
of the f-CNTs has a large amount of chemically reactive groups
originated from the knitted aromatic compounds, as demon-
strated by FTIR and XPS. For example, the absorption bands
around 1000–1460 cmꢂ1 (Fig. 3a, yellow area) belonged to the C–
O stretching vibrations and/or O–H bending vibrations con-
firming the presence of the hydroxyl (C–OH, 1079 cmꢂ1), ester, or
ether groups (C–O–C, 1258 cmꢂ1). Moreover, the absorption
band near 1563 cmꢂ1 is attributed to the C]C stretching
vibrations of condensed aromatic skeleton, which proved that
the layer indeed resulted from the intra-/inter-dehydration,
aromatization and subsequent cross-linking polymerization/
carbonization of the heterocyclic/cyclic aromatic compounds.
Meanwhile, the bands around 1717 cmꢂ1, attributed to the C]O
vibrations corresponding to carbonyl, quinone, ester or carboxy
groups (Fig. 3a, tangerine area), also show that a series of
reactions occurred between aromatic compounds on the CNTs
surface. Additionally, the outer layer seems also to possess
aliphatic structures, which were confirmed by the band at 2855
cmꢂ1 and 2925 cmꢂ1 (Fig. 3a, gray area).11,14,15 Except the proof in
FTIR spectra, the peak intensity and area of the C1s spectrum of
the f-CNTs, which is much lower than that of p-CNTs, further
demonstrate that the chemical compositions of layers are
carbonaceous materials and rich in oxygenated groups.14 As
shown in Fig. 3, b1, the C1s peak of f-CNTs contains five signals
which are attributed respectively to the aromatic carbon groups
(C]C/C–C) (284.6/285.0 eV), hydroxyl groups (C–OH/C–O–C)
(285.7 eV), carbonyl groups (C]O) (287.3 eV) and carboxylic
groups, esters or lactones (O–C]O) (289.2 eV).11,14 Simulta-
neously, the higher peak intensity and area of the O1s spectrum
of f-CNTs than that of p-CNTs also prove that the layers contain
a large amount of oxygen atoms (Fig. 3, b2). Moreover, the kind
of oxygenated groups could be confirmed by the O1s spectrum,
in which two signals were identified at 531.7 and 533.0 eV. The
first signal corresponds to C]O groups, whereas the peak at
533.0 eV is mainly attributed to C–OH/C–O–C groups.11 The
above results strongly prove that there was an abundance of
oxygenated groups on the outer layer of hte f-CNTs (Fig. 1g),
and hence the hydrophobic/inert CNTs were successfully coated
with a hydrophilic/chemically reactive coat. Therefore, the
solubility and dispersity of the f-CNTs could achieve a high
concentration, to 2.0 mg mLꢂ1 in deionized water and 2.5 mg
mLꢂ1 in ethanol, which were drastically improved when
compared to the pristine CNTs (<0.2 mg mLꢂ1) (Fig. 3, a2, inset).
Moreover, to prove that the coating layer is a new carbonaceous
Fig. 2 HRTEM images of f-CNTs functionalized at 180 ꢁC (a–d) for 4 h,
6 h, 8 h, 11 h with the use of glucose; (e,f) with the use of grass and cellulose,
respectively. Inset is the thickness and percentage loading of the knitted
layer. The percentage loading ¼ (mf-CNTs ꢂ mp-CNTs)/mp-CNTs; mf-CNTs
¼
the weight of f-CNTs, mp-CNTs
¼
the weight of CNTs before
functionalization.
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Fig. 3 (a) FTIR spectra of p-CNTs (1) and f-CNTs with different thickness of layers (2–7: 2.03, 3.02, 8.94, 9.84, 14.66, 23.83 nm, respectively) or
functionalized with different precursors (2,3,6,7: glucose; 4: grass; 5: cellulose). Inset is the FESEM images of f-CNTs (6), and the picture of dispersion
ability of p-CNTs and f-CNTs in solvent. (b1, b2) Comparing the XPS (C1S, O1s) of p-CNTs and f-CNTs (6).
material but not the biomass or the decompositions of biomass,
we further checked the FTIR spectra for the mixture of biomass
and CNTs, as well as the decompositions of biomass and CNTs.
Clearly, the spectra of them (see the Supporting Information,
Fig. S2†) are totally different from those of the CNTs@layers
(Fig. 3a), which strongly confirm the results that the knitted
network-coat on CNTs was really a new-formed carbonaceous
material, rather than being the original biomass or the decom-
positions of the biomass.
developed for the conversion of low-cost, abundant and renew-
able biomass into a hydrophilic/chemical reactive network-coat
on the inert surface of a wide variety of sp2 carbon materials,
such as fullerene, carbon nanohorns/nanotubes and graphene
because they consist of the same sp2 hybridized carbon atoms and
have the same p–p conjugated surface. It is well known that the
functionalization of these typical sp2 carbon materials is neces-
sary for constructing carbon-based materials and expanding
their applications.17 Therefore, with our presented method, these
inert carbon materials could be effectively endowed with the
requisite functional groups for further preparation of new
carbon-based materials while still preserving their intact micro-/
macro-structures perfectly.
Compared to the previous methods of the functionalization of
CNTs,5–7,16 our present strategy has several distinctive advan-
tages: (i) it is a green process because the used functionality
precursors and solvent are environmentally-friendly; (ii) the
functionalization conditions are mild and can be tuned over
a wide range, making the functionalizations more specific and
easy; (iii) the formed aromatic compounds adsorbed on the
CNTs through p–p interactions could undergo further intra-/
inter-molecular dehydration, aromatization, and subsequent
cross-linking polymerization/carbonization, thereby allowing
them to knit a network like a bag-coat, which strongly bonded on
the CNTs surface conformally as a graphitic layer. The strong
interactions including p–p interactions, Van der Waals forces,
H-bonds, as well as the unique physical structure of the bag-like
coat could greatly improve the stability of the CNTs@layers
composite, even in the solvent during the process of further
preparing CNT-based materials; (iv) the whole knitting process
was a self-catalyzed polymerization/carbonization without any
initiators or catalysts, and the degree of the process could be
finely controlled via varying the reaction conditions; and (v) the
knitted layers could coat the CNTs conformally, continuously
and compactly as a graphitic-like layer, therefore preserving the
cylindrical structure and individual dispersity of the CNTs very
well. Based on the knowledge of the source and the knitting
mechanism of the intermediates, an efficient and simple way was
The application of f-CNTs in catalysis
The obtained f-CNTs were an effective support for the deposition
and dispersion of metal catalyst because the high density of
oxygenated groups could act as anchoring centres and specific
nucleation sites for the deposition of precursors. As illustrated in
Fig. 4a, the Ru nanoparticles on the f-CNTs surface were highly
dispersed with a narrow size distribution of ꢀ0.9 nm, which is
smaller and more uniform than those on the p-CNTs (ꢀ1.8 nm)
(Fig. 4b). Moreover, the coated layer with the riched oxygenated
groups on the surface of f-CNTs make the resulted Ru/f-CNTs to
be a hydrophilic catalyst and have a well dispersion in water,
a clean medium in the chemical reactions. While for the p-CNTs,
the formed Ru/p-CNTs could not disperse in water due to their
rather inert surface (Fig. 4b, inset). The good dispersity, ultra-
small size and incomparable uniformity of Ru nanoparticles of
Ru/f-CNTs, together with their good dispersion in water, make
them a promising candidate for catalytic reactions. Herein, the
catalytic performance of Ru/f-CNTs was tested and compared
with the Ru/p-CNTs and a commercial Ru/C catalyst in a selective
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Fig. 4 TEM images of 1.5 wt% Ru/f-CNTs (a) and 1.2 wt% Ru/p-CNTs
(b), the size distribution of Ru nanoparticles and the dispersion ability of
catalyst in water (inset).
Fig. 5 Plots of selectivity vs. conversion for the selective hydrogenation
of citral over 1.5 wt% Ru/f-CNTs (a) and 1.2 wt% Ru/p-CNTs (b).
Ru/p-CNTs catalyst (Fig. 5b). The higher selectivity to G&N
obtained on the Ru/f-CNTs catalyst should be ascribed to the
adsorption mode of the reactant molecule. The outer surface of
the f-CNTs has a large amount of hydrophilic oxygenated groups,
including C–OH, COOH, C]O and C–O–C, etc. The uniformity
and continuity of the knitted oxygenated coat not only makes the
catalyst well dispersed in solution, but also facilitates the direc-
tional adsorption of the polar part of the citral molecule (C]O
bond) on the surface of the catalyst through H-bonding and then
preferential hydrogenation of the C]O bond to produce an
unsaturated alcohol (Fig. 6).21 While the part of C]C bond keeps
away from the hydrophilic surface of the catalyst, due to the C]C
bonds existing in the hydrophobic alkyl chain and near the
branched methyl groups which have a certain degree of steric
hindrance for the surface adsorption, therefore it successfully
hydrogenation of citral, an unsaturated aldehyde, which is an
important reaction in the flavour, fragrance and pharmaceutical
industries.18 The hydrogenation of citral usually produces citro-
nellal (CAL), citronellol (COL), menthol, geraniol and nerol
(G&N), and other products due to it involving two C]C and one
C]O unsaturated bonds (Scheme 1).19,20 It is therefore important
to control the product selectivity as well as the overall conversion
in this reaction, and thus it was selected as a model reaction to
evaluate the catalytic performance of Ru/f-CNTs.
The unsaturated alcohols of G&N were produced as the main
products over the Ru/f-CNTs catalyst, and it presented a high
level of around 90% at the conversion of ꢀ55%, and reduced
slightly to 75% at complete consumption of citral because of its
further hydrogenation to COL (Fig. 5a). By contrast, the selec-
tivity of G&N was lower (ꢀ55%), and decreased quickly over the
Fig. 6 The probable directional adsorption and hydrogenation of trans-
& cis-citral on 1.5 wt% Ru/f-CNTs catalyst in water.
Scheme 1 Reaction pathway of hydrogenation of citral.
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comparison to the commercial 5 wt% Ru/C catalyst, the 1.5 wt%
Ru/f-CNTs still show a higher selectivity to G&N with a high TOF
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of 400 hꢂ1, which is about 4 times higher than that (TOF, 83 hꢂ1
)
obtained on the 5 wt% Ru/C catalyst under the same reaction
conditions. Obviously, using the support of CNTs or f-CNTs
could largely enhance the activity and selectivity of the hydroge-
nation of citral, but even they themselves did not show any cata-
lytic activity in the reaction. It is noteworthy that the catalyst of
1.5 wt% Ru/f-CNTs showed a good activity and relative stability
in the recycling test (see the Supporting Information, Table S1†).
Therefore, the f-CNTs with such an oxygenated coat were an
effective support for the metal catalyst and presented outstanding
catalytic activity and selectivity in a typical selective hydrogena-
tion of unsaturated aldehydes.
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Conclusions
In summary, a network-coat of carbonaceous materials with
a high density of oxygenated groups was successfully knitted on
CNTs layer-by-layer from the hydrothermal treatment of
biomass, which was absolutely different to the linear polymers
polymerized from the monomers. The thickness, compositions,
and surface properties of the conformal coat could be easily
controlled via varying the reaction conditions. The functional-
ized CNTs could still keep the micro-/macro-structure of the
pristine CNTs and were proved to be a promising catalyst
support for the hydrogenation of citral to unsaturated alcohol.
With this strategy, carbon materials like fullerene, graphene and
the carbon horns could also be homogeneously endowed with the
requisite effective functional groups both more easily and
precisely whilst still preserving their intact micro-/macro-struc-
tures, and thus constructing new carbon-based materials and
exploring their applications.
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This work was financial supported by the One Hundred Talent
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