Surface properties of amphiphilic carbon nanotubes and study of their applicability as basic catalysts

Article abstract of DOI:10.1039/c6ra08032d

Amphiphilic hybrid carbon nanotubes (CNTs) containing a hydrophobic undoped section connected to a hydrophilic N-doped segment were synthesized. These amphiphilic hybrid CNTs were characterized by elemental analysis, nitrogen physisorption (BET), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), CO₂ adsorption coupled with microcalorimetry and X-ray photoelectron spectroscopy (XPS). The point of zero charge (PZC) for the different materials was also determined. The presence of basic sites on the surface of N-doped nanotubes is evidenced both from the CO₂ adsorption measurements and the PZC determinations. Their catalytic activity in the aldol condensation of furfural derivatives and acetone was evaluated. The results of the catalytic tests show significant specific activities (amount of reactant converted per gram of solid) when compared with strong basic solutions of NaOH. A clear correlation between the characterization data and the catalytic behavior was found. Furthermore, the selectivity values towards the two main reaction products, C₈ and C₁₃ aldol condensation adducts, are close to that of the NaOH solution. All together, these findings suggest these materials might be useful basic catalysts.

Full text of DOI:10.1039/c6ra08032d

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Surface properties of amphiphilic carbon nanotubes and study of
their applicability as basic catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,*
a
b
c
C. Ramirez-Barria , A. Guerrero-Ruiz , E. Castillejos-López , I. Rodríguez-Ramos , J. Durand , J.
c
c,*
Volkman , P. Serp
DOI: 10.1039/x0xx00000x
Amphiphilic hybrid carbon nanotubes (CNTs) containing a hydrophobic undoped section connected to a hydrophilic N-
doped segment were synthesized. These amphiphilic hybrid CNTs were characterized by elemental analysis, nitrogen
www.rsc.org/
2
physisorption (BET), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), CO adsorption coupled
with microcalorimetry and X-ray photoelectron spectroscopy (XPS). The point of zero charge (PZC) for the different
materials was also determined. The presence of basic sites on the surface of N-doped nanotubes is evidenced both from
2
the CO adsorption measurements and the PZC determinations. Their catalytic activity in the aldol condensation of furfural
derivatives and acetone was evaluated. The results of the catalytic tests show significant specific activities (amount of
reactant converted per gram of solid) when compared with strong basic solutions of NaOH. A clear correlation between
the characterization data and the catalytic behavior was found. Furthermore, the selectivity values towards the two main
8
reaction products, C and C13 aldol condensation adducts, are close to that of the NaOH solution. All together, these
findings suggest these materials might be useful basic catalysts.
2
by Van Dommele et al. Additionally other catalytic
applications, such as transesterification, dehydrogenation and
7–9
Introduction
aldol-condensation have also been reported. Base catalyzed
Carbon materials have been widely employed as solid catalysts, aldol reactions of oxygenated platform molecules obtained
either as active phases or supports. It is well known that the from biomass are amongst the most important recent
9
surface properties of carbon materials can be tuned by applications of basic catalysts. The challenge of developing
1
covalently bonded adatoms. The presence of nitrogen atoms bulk chemicals from biomass or other renewable resources is a
1
0–12
in the basal planes of graphite or graphene layers can change field of growing interest.
many of their chemical properties. In particular, carbon Furfural and its derivatives, such as 5-hydroxymethylfurfural
materials with nitrogen doped graphitic structures displayed (HMF), are obtained by dehydrating pentoses and hexoses
2
13
surfaces with basic properties. Among carbon materials, isolated from lignocellulosic biomass. Furfural can react with
carbon nanotubes (CNTs) have become one of the most active organic carbonyl compounds such as acetone to form C cross–
8
research topic in nanoscience and nanotechnology due to their aldol condensation adducts (FAc, Scheme 1). These products
3
unique electronic, chemical, and mechanical properties. N- can further condensate with a second molecule of furfural to
doped CNTs have recently been reported as metal-free form C13 aldol adducts (F Ac, Scheme 1). These adducts are
2
catalysts in the aerobic oxidation of cyclohexane to adipic acid, important oxygenated precursors for the synthesis of C –C15
8
the selective oxidation of H S, and the oxidative liquid alkanes obtained after a series of coupled hydrogenation
2
4
–6
14
dehydrogenation of propane (ODH).
and hydrodeoxygenation reactions.
An initial investigation of the Knoevenagel condensation
reaction with basic N-doped CNTs, as catalyst, was reported
Scheme
1 Representation of the reaction products obtained during the aldol
condensation of acetone and furfural
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Conventional base such as NaOH have been reported as Three CNT samples were produced with (i) ethylene for 30
catalyst for the aldol condensation of furfural derivatives and minutes (E30); (ii) acetonitrile/Ar/H for 30 minutes (A30); and
for 20 minutes followed by ethylene for
drawbacks of these systems is the large amount of 10 minutes (A20E10). The produced CNTs were suspended in
homogeneous catalyst required. Alternatively, the an aqueous solution of H SO (50 vol%) under reflux for 3 h to
2
1
5
acetone in biphasic systems. However, one of the major (iii) acetonitrile/Ar/H
2
2
4
development of heterogeneous catalysts based on oxides of dissolve the alumina and partially eliminate of exposed metal
alkaline earth metals has been reported for aldol condensation particles.
1
6
reactions.
Hydrotalcites
and
zeolites
(MCM-41)
functionalized with amines were also described as basic solid Characterization of carbon materials
7,18
catalysts for aldol condensation reactions.
1
The CNTs were characterized using transmission electronic
microscopy (JEOL JEM-2100F microscope at 200 kV), chemical
analysis (C, H and N) using a Perkin-Elmer elemental analyser,
and Thermal Analysis (recorded in TG/DTA mode in a TA
Instruments SDT Q600) under air. The textural characterization
In particular, a variety of mixed oxides have been evaluated as
solid base catalysts for the aldol-condensation of furfural and
1
9,20
HMF with acetone.
Thus, in the experiments reported by
2
1
Faba and coworkers using basic mixed oxides (Mg–Zr, Mg–Al
and Ca–Zr), a condensation yield greater than 60% for C13,
under mild conditions, was obtained. Resasco et al. studied
hybrid amphiphilic carbon nanotubes (CNTs) doped with basic
(porosity distribution and BET surface areas, SBET) of the
materials was based on N adsorption isotherms, which were
2
obtained using a Micromeritics ASAP 2020 instrument. The
samples were also analysed by X-ray photoelectron
spectroscopy (XPS) using an Omicron ESCA Probe
2
2
2 3 2 2 5 x 2
oxides (MgO, MgO/Al O , TiO , ZnO, V O and Ce Zr1-XO ).
These nanohybrids, when stabilized as emulsions between
organic and aqueous phases, display an increased catalytic
activity by augmenting the surface area between the two
phases. This system has also been shown to catalyze aldol
condensation-hydrogenation cascade reactions occurring in
the aqueous and organic phases respectively. Recently it has
been reported that sequential addition of carbon and nitrogen
precursors during the synthesis of CNTs results in the
spectrophotometer, which
monochromatized Mg K source (1253.6 eV).
The amount and strength of the basic sites were determined
by CO adsorption using a volumetric equipment described in
detail elsewhere. . Successive doses of CO
operated
with
a
non
α
2
27
2
were introduced
into the system to titrate the surface of the carbon
nanotubes.Prior to the measurements, aliquots (200 mg) of
the three synthetised CNTs were pretreated for 2 hours under
vacuum at 573 K and outgassed overnight. The adsorption
temperature was maintained at 300 K.
2
3–25
formation of heterojunctions of CNTs and N-CNTs.
Several
studies have been dedicated to establish correlations between
2
the activity and basic properties of carbon materials.
,26
However, a clear understanding of the surface properties
requires well-established methodology to determine and
quantify the exposed surface active species.
To obtain the point of zero charge (PZC), the electrophoretic
mobility (μ) vs. pH of the samples was measured in a Zeta
Meter 3.0+ at 298 K. The amount of sample used in each point
was approximately 100 mg suspended in 100 ml of water. As χ.
a > 1 (χ is the reciprocal of Debye length and a is the particle
Herein, the synthesis and characterization of hybrid N-doped
CNTs were carried out. Chemical and thermochemical
methods, as well as electron microscopy and spectroscopic
studies techniques were used to accurately identify the CNTs
active surface sites. The catalytic features of these hybrid
nanotubes have been investigated for aldol-condensation of
furfural and acetone, obtaining relevant catalytic results in
terms of activity and selectivity.
r 0
radius), the Smoluchowski equation μ=ε ε ξ/η was applied to
obtain the zeta potential (ξ). The pH was adjusted adding
either HCl or NaOH to the suspensions, which were stabilized
for 15 h before μ measurements.
Finally, in order to detect the amphiphilic properties of these
materials CNTs samples (30 mg) have been suspended in 10
mL of a biphasic system (1:1, v/v) composed by THF: water
(NaCl saturated).
Experimental
Synthesis of carbon nanotubes
Catalytic test
CNTs were synthesized by a catalytic CVD process in a fluidized
bed reactor using ethylene as carbon precursor and
The aldol condensation of furfural and acetone was performed
in a 100 mL stainless steel Parr reactor. The reaction mixture
was stirred and heated with a silicone oil bath. In a typical run,
2
5
acetonitrile as carbon/nitrogen source. An iron catalyst
supported on Al
2
O
3
was pre-reduced by hydrogen during 30 the reaction mixture contained 0.1 g of catalyst
min at 650 C and placed in contact with ethylene/H flow (375 furfural, 0.6 mmol of acetone and 22 mL of decalin. The
mL min ) at the same temperature, to produce hydrophobic reactor temperature was increased to 453 K (reaching a final
, 1.2 mmol of
◦
2
–1
–
pressure of 0.25 MPa) and the reaction was allowed to
proceed for 15 hours. Thereafter the reaction mixture was
cooled down to room temperature and 25 mL of ethyl acetate
were added in order to increase the solubility of the products.
The samples were filtered and diluted in a stock solution of
hexadecane (internal standard) in THF. The analysis was
performed with a Clarus 500 gas chromatograph, equipped
carbon nanotubes or with acetonitrile/Ar/H
1
2
flow (375 mL min
)
to grow N-CNTs. In order to produce hybrid CNTs, after a
first reaction stage, the gas inlet was switched to continue the
nanotube growth and produce heterojunctions between
undoped and doped nanotubes.
2
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with a split/splitless injector, a capillary column (Elite 5 fused Concerning the material preparation it should be noted that
silica 30 m, 0.32 mmi.d.) and a flame ionization detector (FID).
Quantitative responses were determined using standard
calibration mixtures. 1,5-bis(2-furanyl)-1,4-pentadien-3-one
the introduction of nitrogen in the CNT structure decreases the
yield of the reaction and increases the tube diameter (Table 1),
30
presumably due to different growth mechanisms.
(
F
2
Ac) standard for GC-FID analysis was synthesized as
described in the supplementary information and its structure
In addition, the specific surface area (SBET) (Table 1) of E30 is
slightly greater than the ones measured for A30 and A20E10.
This is coherent with the larger external diameters (measured
by TEM) observed for the N-doped samples. Also the narrower
1
confirmed by H and C NMR spectroscopy.
13
Finally, for comparative purposes catalytic tests using 5 mg
and 100 mg of powdered NaOH as catalyst were carried out
under the same conditions described (vide supra). Blank
experiments with the reactor without any catalyst added, were
also performed; the very low conversion values obtained in
this experiment was subtracted from conversions achieved in
the presence of catalysts.
CNTs of E30 samples probably favor the formation of
31
2
bundles. The N adsorption–desorption isotherms of the
samples E30, A20E10 and A30 correspond to a type IV nitrogen
adsorption isotherms according to the IUPAC classification (Fig.
S2 of the supplementary information), which is typical of
32
mesoporous structures.
Table 1. Yield, outer diameter and BET surface area of the produced CNTs.
Mean diameters
–1
Results and discussion
–
1
2
Sample
Yield (gCgcat )
S
BET (m g )
Hybrid CNTs synthesis and characterization
(nm)
9.7
E30
A20E10
A30
5.1
2.2
1.1
296
230
227
TEM images of the E30 samples showed the presence of very
regular multi-walled CNTs with an average external diameter
of ca. 9.7 nm (Fig. 1a). Samples A30 presented a “bamboo-like”
structure of N-doped CNTs, with larger diameters (ca. 12.9 nm,
Fig. 1b). Such structure has been reported in the literature
15.1
12.9
2
8,29 As far as the chemical composition of these samples is
concerned, E30 contains 93.5% of carbon with no nitrogen
being detected. Samples A20E10 contain up to 2.0% of N,
which further increase to 4.1% for A30 (Table 2).
when nitrogen is incorporated into the carbon walls.
Hybrid samples A20E10 present sections of two different types
in the same material: a regular section suggesting the presence
of an undoped CNT, and a bamboo-like structure, suggesting
The thermal reactivity under air of the prepared samples was
the presence of an N-doped CNT (Fig. 1c). Heterojunctions of investigated by TG analyses (Fig. S3). DTG analysis for E30
CNT and N-CNTs, an interesting morphological feature, can showed a well-defined peak near 895 K that could be
also be observed by TEM. CNTs outer diameter size attributed to the oxidation of a well-organized carbon
structure. In comparison, sample A30 showed an oxidation
temperature of 796 K, which suggests the presence of a more
defective and reactive carbon structure. Hybrid CNTs showed
two peaks, suggesting the presence of two sections. For the
sample A20E10 the main peak at 780 K is related to the N-
doped part, and the second contribution at 860 K is related to
the carbon sections (Fig. S4). From the measured weight loss,
distribution is also provided in Fig. S1.
the remaining metal content can be estimated for A30 (
∼
∼3.2%),
A20E10 ( 8.6%), and E30 ( 10.5%). As iron species are not
∼
detected by XPS, the iron is likely to be encapsulated carbide
iron structures. The surface charge of carbons is governed by
the nature of the surface groups and the pH. PZC, was used to
1
estimate the CNTs surface chemistry (Fig. 2). The PZC values
(Table 2) increase as the N content increases from 0 to 4.1%.
These results seem to indicate that N-CNTs surfaces present a
greater basic character than non-doped CNTs. Since nitrogen
adatoms have an additional electron in comparison with
carbon atoms, p electron delocalization will occur easily in N-
CNTs. This excess of electrons changes the hydrophobicity of
the carbon surface. Therefore, electronic density of the N-
doped carbon materials increases, which would lead to higher
PZC values. As electron-donor properties are related to basicity,
the stronger the electron donating is, the greater the
2
7
Figure 1. TEM images of CNTs (a) E30; (b) A30; and (c) A20E10
basicity.
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uptakes were not sufficient to measure adsorption heats with
enough precision to distinguish between chemisorption and
physisorption surface sites.
Figure 2. PZC for (a) E30; (b) A30; and (c) A20E10.
Table 2. N content (from microanalysis), isotherm slope, and PZC for the
produced CNTs.
2
Figure 3. Isotherms of CO adsorption on CNTs samples at 330 K (a) A30; (b)
Slope CO
2
A2E10; and (c) E30
Sample
%N EA
PZC
isotherms
0.38
E30
A20E10
A30
0
6.1
8.2
9.1
Dispersion test in different biphasic systems such as THF:
water (NaCl saturated) showed that after a brief sonication
and 6 hours of decantation the best stabilized material at the
liquid interphase is A20E10. While E30 tends to be located in
the organic phase and A30 remains in the water phase. In the
supplementary information an illustrative picture of these
differences is provided (Fig. S6).
2.0
4.1
1.45
1.23
The chemical composition was also obtained from the
characteristic XPS peaks corresponding to C, N and O. These
results are shown in Table 3. Nitrogen peak deconvolution (Fig.
S4) indicated the presence of four elementary peaks: pyridinic
nitrogen (399.5–398.5 eV), pyrrolic nitrogen (400.8–399.8 eV),
Aldol condensation of furfural and acetone
9
404.9–405.6 eV). Of all these species, the pyridinic nitrogen is The performance of these hybrid CNTs as basic catalyst was
quaternary nitrogen (403.0–401.0 eV), and NOx groups
(
believed to have the stronger basic character. Accordingly, it is tested for the aldol condensation reaction of furfural with
generally assumed that the basicity of carbon catalysts is acetone. Overall reaction activities and the selectivities toward
2
,33
linked to the amount of pyridinic groups.
C8 (FAc) and C13 (F Ac) products are given in Table 4.
2
Materials containing nitrogen groups show significant activity
as basic catalysts. Satisfyingly, we obtained a very low specific
activity for E30 CNTs, in contrast to N-CNTs. Therefore, the
activities of N-CNTs allow us to confirm that basic nitrogen
functional groups are involved in this catalytic process. When
comparing A30 and A20E10 catalysts, in spite of the higher
nitrogen content in the former (Table 2), the specific catalytic
activities are very similar. This observation can be rationalized
by considering the number of basic nitrogen surface groups
Table 3. Nitrogen content and contribution species detected by deconvolution of
spectra
E30
A20E10
91.0
4.9
A30
88.1
6.3
%
C XPS
95.1
%
O XPS
N XPS
C/N
4.9
%
0.0
4.1
5.7
-
-
-
-
-
22.2
29.7
30.6
11.1
28.6
15,5
29.9
32.2
17.2
20.7
%
Pyridinic-N
Pyrrolic-N
(pyridinic species). The pyridinic nitrogen content, as revealed
%
by the XPS study of the N1s peaks, is similar for A20E10 and
A30 samples (information shown in Fig. S4 and also reported in
Table 3).
%
Quaternary-N
N-Ox groups
%
For comparison purposes two additional experiments were
carried out using NaOH as catalyst (5mg and 100 mg) (i.e.
homogenous catalytic tests). Interestingly, activities achieved
with N-doped samples were higher than those obtained
utilizing NaOH as homogeneous catalyst assuming that basicity
on the surface of NCNTs is fully related to content of nitrogen.
Concerning selectivity, in all cases FAc was identified as the
Volumetric isotherms corresponding to CO2 adsorption on
CNTs samples are depicted in Figure 4. The isotherm slopes
increase from non-doped CNTs to N-doped CNTs (Table 2).
Since CO possesses a Lewis acidic character, it is expected to
2
2
7
be adsorbed on basic sites of the N-CNTs surfaces. The
variations in the isotherm slope are thus directly proportional
to the amount of surface adsorption sites on the samples. In
short, the amount of basis surface sites in our samples follows
the order: A20E10>A30>>E30. Unfortunately, the low CO2
2
main reaction product. However, selectivity toward F Ac
4
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increases for N-doped catalysts when compared to undoped
E30 sample.
Notes and references
The present results demonstrate that hybrid CNTs are an
attractive material that can be used as metal-free basic
catalyst for organic synthetic reactions. Furthermore, their
applicability in biphasic media enables heterogeneous catalysis
for reaction processes that at present apply homogeneous
catalysts, i.e. in continuous operations.
1
2
3
4
5
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Serp, P.; Machado, B. F. Nanostructured Carbon Materials
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Yu, H.; Peng, F.; Tan, J.; Hu, X.; Wang, H.; Yang, J.; Zheng, W.
Specific
activity
C
Activity
Sel
FAc
(%)
77
Sel
Angew. Chemie - Int. Ed. 2011, 50 (17), 3978–3982.
Catalysts
Balance (mol/mol cat)
F
2
Ac
(μmol/g)
(%)
Chizari, K.; Deneuve, A.; Ersen, O.; Florea, I.; Liu, Y.;
Edouard, D.; Janowska, I.; Begin, D.; Pham-Huu, C.
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A20E10
1.4
95.9
98.3
98.4
98.0
97.6
-
23
33
37
31
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1
1
25.6
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and the catalytic performances demonstrated the potential of
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doped CNTs. Besides, the correlation between the
experimental adsorption data and the catalytic results is
noteworthy and can be used as a criterion to assess the
basicity of the surface.
9
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Acknowledgements
Carolina Ramirez-Barria gratefully acknowledges financial
support by the Erasmus program (UNED-Toulouse University)
and a grant for Master study from the Sociedad Española de
Catálisis (SECAT). Also the financial support from the Spanish
Ministerio de Economía y Competitividad under projects
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