Catalytic conversion of aliphatic alcohols on carbon nanomaterials: The roles of structure and surface functional groups

Article abstract of DOI:10.1134/S0036024417030293

Carbon nanomaterials with the structure of graphene and different compositions of the surface groups are used as catalysts for the conversion of С₂–С₄ aliphatic alcohols. The conversions of ethanol, propanol- 1, propanol-2, butanol-1, butanol-2, and tert-butanol on carbon nanotubes, nanoflakes, and nanoflakes doped with nitrogen are investigated. Oxidized and nonoxidized multiwalled carbon nanotubes, nanoflakes, and nanoflakes doped with nitrogen are synthesized. X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning and transmission electronic microscopies, Brunauer–Emmett–Teller method, derivatographic analyses, and the pulsed microcatalytic method are used to characterize comprehensively the prepared catalysts. It was established that all of the investigated carbon nanomaterials (with the exception of nondoped carbon nanoflakes) are bifunctional catalysts for the conversion of aliphatic alcohols, and promote dehydration reactions with the formation of olefins and dehydrogenation reactions with the formation of aldehydes or ketones. Nanoflakes doped with nitrogen are inert with respect to secondary alcohols and tert-butanol. The role of oxygen-containing and nitrogen-containing surface groups, and of the geometrical structure of the carbon matrix of graphene nanocarbon materials in the catalytic conversion of aliphatic alcohols, is revealed. Characteristics of the conversion of aliphatic alcohols that are associated with their structure are identified.

Full text of DOI:10.1134/S0036024417030293

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 3, pp. 448–454. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © E.A. Tveritinova, Yu.N. Zhitnev, S.A. Chernyak, E.A. Arkhipova, S.V. Savilov, V.V. Lunin, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91,
No. 3, pp. 429–435.
CHEMICAL KINETICS
AND CATALYSIS
Catalytic Conversion of Aliphatic Alcohols on Carbon Nanomaterials:
The Roles of Structure and Surface Functional Groups
E. A. Tveritinova*, Yu. N. Zhitnev, S. A. Chernyak, E. A. Arkhipova, S. V. Savilov, and V. V. Lunin
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
*e-mail: eatver@mail.ru
Received March 4, 2016
Abstract—Carbon nanomaterials with the structure of graphene and different compositions of the surface
groups are used as catalysts for the conversion of С2–С4 aliphatic alcohols. The conversions of ethanol, pro-
panol-1, propanol-2, butanol-1, butanol-2, and tert-butanol on carbon nanotubes, nanoflakes, and nano-
flakes doped with nitrogen are investigated. Oxidized and nonoxidized multiwalled carbon nanotubes, nano-
flakes, and nanoflakes doped with nitrogen are synthesized. X-ray diffraction analysis, X-ray photoelectron
spectroscopy, scanning and transmission electronic microscopies, Brunauer–Emmett–Teller method, deri-
vatographic analyses, and the pulsed microcatalytic method are used to characterize comprehensively the
prepared catalysts. It was established that all of the investigated carbon nanomaterials (with the exception of
nondoped carbon nanoflakes) are bifunctional catalysts for the conversion of aliphatic alcohols, and promote
dehydration reactions with the formation of olefins and dehydrogenation reactions with the formation of
aldehydes or ketones. Nanoflakes doped with nitrogen are inert with respect to secondary alcohols and
tert-butanol. The role of oxygen-containing and nitrogen-containing surface groups, and of the geometrical
structure of the carbon matrix of graphene nanocarbon materials in the catalytic conversion of aliphatic alco-
hols, is revealed. Characteristics of the conversion of aliphatic alcohols that are associated with their structure
are identified.
Keywords: carbon nanomaterials, catalysis, alcohols, dehydration, dehydrogenation
DOI: 10.1134/S0036024417030293
INTRODUCTION
structure of a carbon matrix itself on one catalytic pro-
cess or another are sparse and somewhat contradic-
tory. In [14], we investigated the conversion of С2–С3
alcohols on nanodiamond (ND) and its oxidized and
reduced forms, and showed that nanodiamond is a
catalyst for the conversion of aliphatic alcohols, and
the oxidative treatment of nanodiamond raises its cat-
alytic activity without changing its high-level selectiv-
ity toward the conversion of alcohols into dehydroge-
nation products. This allowed us to draw conclusions
regarding the formation of new catalytic sites on sur-
faces of ND in the form of bridging carbonyl groups.
In this work, we studied the catalytic conversion of
С2–С4 aliphatic alcohols on three types of nanocar-
bon materials (oxidized and nonoxidized carbon
nanotubes (CNTs, sp2-hybridization, bent structure),
Carbon materials, particularly activated carbons,
have long been used in catalysis, both as supports and
as catalysts. However, the nature of active sites of car-
bon catalysts has yet to be completely clarified. In con-
trast to activated carbons, the new generation of car-
bon nanomaterials (CNMs) has a definite regular
structure. Due to their properties (nanodimensions,
mesoporosity, and high specific surface), and to the
presence of surface functional groups whose composi-
tions are determined by their postsynthetic treatment
[1], carbon nanomaterials are used in very different
areas of physics, biology, and chemistry, especially in
catalysis [2]. These materials are mainly used as
active-phase carriers [3–7], since their functional
groups serve as adsorption sites for deposited metal
atoms, promoting their uniform distribution over the carbon nanoflakes (CNFs, sp2-hybridization, planar
surface of CNMs. However, CNMs themselves serve structure), and nanoflakes doped with nitrogen
as catalysts for a number of different processes, e.g., (CNF-Ns). The conversion of aliphatic alcohols
the oxidative dehydrogenation of ethylbenzene [8–11] (dehydration and dehydrogenation), a reaction used
and light alkanes [12, 13], and for the conversion of in industry and fine chemical synthesis, is also a test
aliphatic alcohols [14]. The authors ascribe the cata- reaction for the presence of acidic and basic sites on
lytic properties of carbon nanomaterials in the above the surfaces of catalysts that allows us to correlate the
processes to the effect different functional groups have catalytic properties of carbon nanomaterials with their
on their surfaces; however, data on the effect of the surface chemistries [15–19].
448

CATALYTIC CONVERSION OF ALIPHATIC ALCOHOLS ON CARBON NANOMATERIALS
449
Table 1. Elemental composition (at %) of nanocarbon cat-
pore structure and size were determined on an Auto-
sorb-1E/QMS analyzer (Quantachrome) using low-
temperature nitrogen adsorption data.
alysts by means of XPS
Catalyst
С
О
N
Cl
The elemental composition and composition of
surface groups of the studied catalysts were deter-
mined by means of X-ray photoelectron spectroscopy
(XPS) on a Kratos Axis Ultra DLD spectrometer
using a monochromatic AlKα radiation at transmis-
sion energies of 160 and 40 eV for separate lines. X-ray
diffraction analysis (XRD) of carbon nanomaterials
was performed on a DRON diffractometer with a
CоKα radiation source. The catalytic conversion of
alcohol was investigated using the pulsed microcata-
lytic method [21]. A quartz tube 0.5 cm in diameter
and 14.5 cm long was used as the pulse reactor. A
weighed catalyst sample (∼50 mg) was placed in the
isothermal zone of the reactor (bed height, ∼2.5 cm).
The temperature in the reaction zone was maintained
with an accuracy of 1 K. Alcohol vapors were
injected into the catalytic reactor with a syringe in
amounts of ∼5 μL. The volume of the injected sample
depended on the equilibrium vapor pressure of each
alcohol at room temperature. The carrier gas (nitro-
gen) flow rate was 50 mL/min. The mixture of the
products and unreacted alcohol was transferred from
the reactor to a column 1.3 m long and 0.5 cm in diam-
eter, filled with a Porapak-N sorbent. The column
temperature was 130°C. The products of the conver-
sion of alcohols were analyzed on a Chrom-5 chro-
matograph equipped with a flame-ionization detector.
ND
CNT
CNT-AO
CNT-NO
CNF-N
CNF
91.08
98.89
98.42
89.59
87.34
98.2
7.16
0.64
1.4
9.35
2.43
1.8
1.74
0.36
–
0.62
9.76
0.1
0.02
0.02
0.02
0.02
0.02
0.02
The aim of this work was to study the role played by
the geometrical structure of a carbon matrix (planar or
bent), and by the composition of the surface func-
tional groups of nanomaterials with the sp2-carbon
structure in the catalytic conversions of alcohols.
EXPERIMENTAL
Ethanol, propanol-1, propanol-2, butanol-1, buta-
nol-2, and tert-butanol of special purity grade
(Khimmed, Russia) were used as the substrates in the
studied catalytic reactions. Oxidized and nonoxidized
CNTs, CNF, and CNF-N synthesized in the present
work were used as the catalysts. CNMs were synthe-
sized in a reactor consisting of a quartz tube with an
inner diameter of 50 mm and a length of 1.5 m, placed
in a Carbolite TZF 12/100/90 tube furnace. For the
synthesis of CNTs, a Co0.03Mo0.01MgO catalyst was
placed in the center of the reactor, and the furnace was
heated to a temperature of 750°C for 1 h at a nitrogen
flow rate of 200 mL/min. After achieving the set tem-
perature, a nitrogen flow with a rate of 500 mL/min
passed through a Drechsel flask filled with hexane was
fed to the reactor for 5 h. Nondoped and doped CNFs
were synthesized by the pyrolitic decomposition of
benzene or acetonitrile, respectively, at a temperature
of 900°C over fine-crystalline magnesia catalyst [20].
The obtained CNMs were refluxed in a 15% HCl solu-
tion in order to remove the catalyst, then washed with
distilled water until neutral pH, and dried at 120°C to
a constant weight. The carbon nanotube surfaces were
functionalized via the oxidation of CNTs in air at
120°C for 5 h (CNT-AO), and by treating the CNTs
with concentrated nitric acid for 9 h (CNT-NO). The
obtained oxidized CNTs were washed with distilled
water and dried at 120°C.
RESULTS AND DISCUSSION
SEM and transmission electron microscope
(TEM) images of the carbon nanomaterial samples
are shown in Fig. 1.
The oxidized and nonoxidized CNTs (Figs. 1а–1d)
were more than 5 μm long and 18 to 25 nm in diame-
ter. Carbon nanoflakes, both pure and doped with
nitrogen, had identical morphologies and were
obtained in the form of rectangular three-to-six-lay-
ered graphene planes with rounded corners. They were
25 × 25 nm in size (Figs. 1e and 1f).
The elemental compositions of the studied carbon
nanomaterials, obtained from the XPS data, are given
in Table 1. The least amount of oxygen is observed in
nonoxidized CNTs, and some of the oxygen in the
CNTs was, according to the XPS data, bonded with
the metal that usually remains in nanotubes during
synthesis. The largest content of oxygen was observed
in UNT-NOs. The content of surface oxygen-con-
taining groups in CNFs and CNF-Ns was negligible,
but it was three to four times higher than in nonoxi-
dized CNTs.
The purity of carbon nanomaterials was checked by
means of synchronous thermal analysis on
STA449PCLUXX device (Netzsch) at an air flow rate
of 40 mL/min and a heating rate of 10 K/min. The
morphology of carbon nanomaterials was studied by
means of scanning electron microscopy (SEM) on a
JEOL JSM 6490 LV instrument (JEOL) equipped
a
The surface oxygen atoms of CNTs were constitu-
with an X-ray fluorescent detector. High-resolution ents of surface carbonyl, carboxylic, anhydride, and
electron microscopy studies were performed on a JEM ester groups, which play the roles of Lewis acid and
2100 F/Cs instrument (JEOL). The specific surface, Lewis base sites in catalysis.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 3 2017

450
TVERITINOVA et al.
The high-resolution O 1s- and N 1s-spectra are
rather complex, and their convolution does not always
give an unequivocal answer to the question of what the
composition of one group or another is. The use of
catalytic reactions, for which it is already known what
functional groups are responsible for either catalytic
process, allows us to specify the composition of func-
tional groups. The conversion of aliphatic alcohols
among such catalytic reactions is quite informative.
The catalytic properties of activated carbons in the
conversion of aliphatic alcohols have been studied by
many researchers [17, 18, 22–24]. To study these, acti-
vated carbons from different sources underwent addi-
tional oxidation treatment. It was found that activated
carbon catalyzes the conversion of aliphatic alcohols
with the formation of dehydration (olefins) and dehy-
drogenation (aldehydes and ketones) products.
The conversion of aliphatic alcohols on carbon
nanomaterials with the sp2-carbon structure, specifi-
cally CNT, CNF, and CNF-N, were studied in this
work. These materials have mesoporous graphene
structures with highly developed surfaces (see
Table 2).
The conversion of С2–С4 alcohols on carbon
nanotubes with minimum oxygen content (0.64%)
was investigated in [25]. Nevertheless, nonoxidized
CNTs yield high degrees of conversion, particularly in
the case of secondary alcohols (propanol-2 and buta-
nol-2) and tert-butanol.
The temperature dependences of the degree of
conversion of alcohols on air-oxidized CNTs are
shown in Fig. 2a. Comparison to the similar depen-
dences given in [25] for the degree of conversion of
alcohols on nonoxidized CNTs shows that raising the
oxygen content on the CNT surfaces from 0.64 to
1.4% increases the degree of conversion. With primary
alcohols, it grows substantially, by 35% on average.
The temperature curves of the degree of conversion of
secondary alcohols and tert-butanol, the catalytic
activity of which on nonoxidized CNTs is already
quite high, are shifted to the lower temperature region.
The 100% conversion of tert-butanol is moved from
200°C for nonoxidized CNT to 120°C for CNF-AO
with the formation of a single product, isobutene. The
temperature dependence of the conversion of primary
С2–С4 alcohols only is shown in Fig. 2b, since nearly
100% conversion is observed for secondary alcohols
and tert-butanol over the investigated range of tem-
peratures. The products of the conversion of С2–С4
alcohols and the selectivity of oxidized and nonoxi-
dized CNTs toward the formation of dehydration and
dehydrogenation products are given Table 3.
5 nm
2 nm
2 nm
0.2 µm
0.2 µm
0.2 µm
(а)
(b)
(d)
(f)
(c)
(e)
Fig. 1. Electron microphotographs of (а) CNT (SEM), (b)
CNT (TEM), (c) CNT-NO (SEM), (d) CNT-NO
(TEM), (e) CNF-N (SEM), and (f) CNF-N (TEM) car-
bon nanomaterials.
Analysis of the spectra of C 1s- and O 1s-electrons
of CNF-Ns showed that the presence of oxygen was
basically due to the C–O groups. Nonoxidized CNTs
contain nitrogen in the form of pyridine surface
groups and quaternary nitrogen, but the content of
these groups was negligible (0.36%). The nitrogen
content in the oxidized CNTs was higher (0.62%) and
mainly represented by quaternary nitrogen (0.29%)
and nitrogen bonded with oxygen in the form of NO3
groups (0.21%). Most of the nitrogen in CNF-Ns fell
on quaternary nitrogen included in the CNF structure
(3.25%), and on the nitrogen of the surface pyridine
groups (2.32%). The content of nitrogen bonded with
oxygen was 2.08%.
Table 2. Surface characteristics of sp2-carbon nanomaterials
according to BET (V is pore volume, and R is pore radius)
Ssp, m2/g
sp2-CNM
V, cm3/g
R, Å
The main products of the conversion of primary
alcohols on nonoxidized CNTs are aldehydes, the
selective formation of which in a homologous series of
primary alcohols is reduced upon an increase in the
number of carbon atoms in an alcohol molecule. The
oxidation of CNTs in air does not change the selectiv-
CNT-AO
CNT-NO
CNF
197
249
326
456
0.58
0.58
2.36
4.88
18.4
4.95
18.6
15.8
CNF-N
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CATALYTIC CONVERSION OF ALIPHATIC ALCOHOLS ON CARBON NANOMATERIALS
451
butanol-2, and tert-butanol, 100% conversion and
100% selectivity toward the formation of dehydration
products were preserved over the investigated range of
temperatures. The temperature dependences of the
conversion of primary alcohols on CNT-NO are given
in Fig. 2b. With ethanol, the selectivity toward the for-
mation of dehydration products grew from 3% on
nonoxidized and air-oxidized CNTs to 88% on CNT-
NO. The selectivity toward the formation of dehydra-
tion products from propanol-1 and butanol-1 on
CNT-NO was close to 100%, but considerably lower
on nonoxidized and air-oxidized CNTs: 26% for pro-
panol-1 and 28% for butanol-1 on nonoxidized
CNTs, and 35% for propanol-1 and 52% for butanol-
1 on air-oxidized CNTs.
Conversion, %
6
100
(a)
80
5
3
4
60
40
20
0
2
1
Nondoped CNFs and CNTs (sp2-hybridization)
have the same carbon structure but different morphol-
ogies. The graphene layers in nondoped CNFs are
planar, but are rolled into tubes in CNTs. The specific
surface area of nondoped CNFs is much larger than
those of any of the investigated types of CNTs.
According to the XPS data, nondoped CNFs are 1.8%
oxygen. This value exceeds the oxygen content in non-
oxidized and air-oxidized CNTs. The surface charac-
teristics of carbon nanomaterials with the sp2-carbon
structure, determined by the Brunauer–Emmett–
Teller (BET) method, are given in Table 2. The spe-
cific surface area of nondoped CNFs considerably
exceeds this value for nonoxidized and oxidized car-
bon nanotubes; nevertheless, nondoped CNFs dis-
played no catalytic activity in the conversion of the
investigated С2–С4 alcohols. The large specific sur-
face area and high porosity of carbon materials are still
insufficient for displaying catalytic activity. The pres-
ence of oxygen-containing groups is also not enough
for catalysis. It is important which kind of functional
groups include oxygen. Based on the composition of
products from the conversion of alcohols, and on an
analysis of high-resolution XPS spectra, these are car-
boxylic, carbonyl, ester, and anhydride groups in the
case of CNTs. The absence of catalytic activity of non-
doped CNFs indicates these kinds of groups are not
found on their surfaces. Oxygen-containing groups in
CNFs are most likely C–OH groups, which are not
active in acid–base catalysis. CNF-Ns have developed
surfaces (456 m2/g) and are 2.43% oxygen; this greatly
exceeds the oxygen content in nonoxidized and air-
oxidized CNTs. However, CNF-Ns are inert in the
conversion of secondary alcohols and tert-butanol.
They display catalytic activity only in the conversion
of primary alcohols. The temperature dependences of
the degrees of conversion of ethanol, propanol-1, and
butanol-1 on CNF-N are shown in Fig. 3.
60 80 100 120 140 160 180 200 220 240 260 280 300 320
100
80
60
40
20
0
(b)
3
2
1
140 160 180 200 220 240 260 280 300 320
Temperature, °C
Fig. 2. Temperature dependences of the degrees of conver-
sion of (1) ethanol, (2) propanol-1, (3) propanol-2, (4)
butanol-1, (5) butanol-2, and (6) tert-butanol on oxidized
CNTs; (a) the conversion of all С –С alcohols on CNT-
2
4
AO and (b) the conversion of primary С –С alcohols on
2
4
CNT-NO.
ity toward the formation of acetic aldehyde in the case
of ethanol; it somewhat lowers the selectivity of the
formation of propionaldehyde from propanol-1; and it
substantially reduces the selectivity of the formation of
butyraldehyde in the case of butanol-1 (from 71 to
48%). To a lesser extent, the oxidation of CNT in air
affects the selectivity of the formation of conversion
products from secondary alcohols. A high selectivity
toward the dehydration products (70% for propanol-2
and 88% for butanol-2) is observed with nonoxidized
CNTs and CNT-AO.
As can be seen from Fig. 3, the conversion of pri-
Oxidizing CNTs with nitric acid for 9 h greatly mary alcohols on CNF-Ns proceeds only at relatively
changed the selectivity of the formation of dehydra- high temperatures, and the degree of its completion is
tion products from primary and secondary alcohols not as high as when using CNTs. The character of con-
toward products of dehydration. With propanol-2, version differs from that on CNTs. While ethanol dis-
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TVERITINOVA et al.
Table 3. Selectivity (S) and the composition of the products
of conversion of С2–С4 aliphatic alcohols on oxidized and
nonoxidized CNTs (α is the maximum degree of conver-
sion)
plays high selectivity toward the formation of acetalde-
hyde on nonoxidized and air-oxidized CNTs, the
products of the conversion of ethanol on CNF-Ns are
diethyl ether and, to a lesser extent, ethylene (Fig. 4).
The concentration of acetaldehyde does not exceed
0.5%. As for propanol-1 and butanol-1, the selectivity
toward the formation of dehydrogenation products in
their conversion on CNF-Ns is in contrast higher than
in their conversion on CNTs: the selectivity of the for-
mation of propionaldehyde from propanol-1 is 89% at
a 49% degree of conversion, and the selectivity of the
formation of butyraldehyde from butanol-1 reaches 63
at a 66% degree of conversion. This difference in
selectivity for ethanol is perhaps due to the character
of its adsorption on CNF-N surfaces, which is deter-
mined in turn by the differences in the polarizability of
alcohols growing along with the number of carbon
atoms in the alcohol hydrocarbon chain. The proba-
bility of the dissociation of the adsorbed alcohol mol-
ecules increases upon an increase in polarizability.
CNF-Ns are 9.76% nitrogen; based on the high-reso-
lution XPS data, it is present on their surfaces in the
form of quaternary nitrogen, the nitrogen of pyridine
bases, and nitrogen bonded with oxygen. The conver-
sion of linear С1–С4 alcohols on nonporous carbon
fibers, and on CNFs synthesized from polyacryloni-
trile and modified by means of electrochemical and
oxidizing treatment, were investigated in [19] by
means of programmed thermal desorption. In contrast
to our results, the authors of that work did not observe
the formation of dehydration products. The adsorp-
tion behaviors of linear alcohols on CNF-Ns and on
nonporous carbon fibers are apparently different. The
catalytic function was attributed to the formed oxy-
gen-containing surface groups, while the effect of
nitrogen-containing groups was not considered.
Alcohol
Т, °C
α, %
, %
, %
SH O
2
S
H2
CNT
Ethanol
200–300
220–300
140–300
200–300
120–240
35.8
53.0
93.0
64.5
97.0
74.0
29.9
71.4
11.6
–
3.0
Propanol-1
Propanol-2
Butanol-1
Butanol-2
tert-Butanol
26.0
70.1
28.6
88.4
100
50–220 100
CNT-AO
100
Ethanol
180–300
160–300
130–300
120–280
130–250
58.8
73.6
99.5
100
100
97.0
65.3
25.6
48.2
93.0
3.0
34.7
74.4
51.8
90.7
Propanol-1
Propanol-2
Butanol-1
Butanol-2
tert-Butanol
50–200 100
CNT-NO
100
Ethanol
210–300
160–300
50–100
160–300
50–150
50–160
85.0
86.0
100
71.0
100
100
12.0
7.2
88.0
92.8
100
96.6
100
100
Propanol-1
Propanol-2
Butanol-1
Butanol-2
tert-Butanol
3.4
groups; i.e., oxidation in air increases the number of
both Lewis acid and Lewis base sites. Oxidizing CNTs
with nitric acid for 9 h results in the oxidation of not
only the available carbonyl, ester, and other groups
that serve as Lewis base sites, but also in the formation
of new Lewis acid sites. This is confirmed by the 100%
conversion and 100% selectivity toward the formation
of dehydration products from secondary alcohols and
tert-butanol, and by the substantial increase in selec-
tivity towards the formation of dehydration products
from primary alcohols.
The characteristics of the conversion of aliphatic
alcohols on CNTs and CNF-Ns are most likely asso-
ciated with the structural features of their carbon
matrices. As was mentioned above, CNTs consist of
graphene planes, modified by rolling into tubes in
which carbon is in the sp2-hybridization state. The
rolling changes their electronic properties in compari-
son to graphite and alters the π-electron cloud [9]. In
addition, the rolling of graphene planes creates defects
in the form pentagonal and heptagonal rings, and
We may assume that the absence of catalytic activ-
vacancies in the stable hexagonal carbon structure. ity in the conversion of alcohol on CNFs is associated
The probability of defects forming in multiwalled with both the type of oxygen-containing groups and
CNTs is higher, since the rolling of each tube contrib- the lack of those defect sites that are available on the
utes to the formation of defect sites. As catalysts, surfaces of CNTs. We also cannot exclude the possi-
CNTs thus contain two types of active sites: oxygen- bility that the absence of catalytic activity is due to the
containing surface groups, which are not so numerous character of adsorption of alcohol on CNFs. Despite
in nonoxidized CNTs, and defects of the carbon struc- the high specific surface areas of CNFs, alcohol mol-
ture itself. A less significant change in selectivity ecules can be adsorbed but not chemisorbed on them
toward the formation of dehydration and dehydroge- with the formation of a catalytic complex. Doping
nation products during the conversion of alcohols on CNFs with nitrogen adjusts their catalytic activity with
air-oxidized CNTs indicates that the ratio of Lewis respect to the conversion of alcohols. The catalytic
acid and Lewis base sites basically remains unchanged activity in the conversion of primary alcohols and its
upon an increase in the number of oxygen-containing absence in the conversion of secondary alcohols must
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CATALYTIC CONVERSION OF ALIPHATIC ALCOHOLS ON CARBON NANOMATERIALS
Conversion, % Selectivity, %
453
100
80
60
40
20
0
2
50
40
30
20
10
0
dе-H2O
dе-H2
3
1
240
260
280
300
1
2
3
T, °C
Fig. 4. Selectivity of CNF-N toward the formation of
dehydration and dehydrogenation products in the conver-
sion of primary alcohols at 300°C: (1) ethanol, (2) propa-
nol-1, and (3) butanol-1.
Fig. 3. Temperature dependences of the degrees of conver-
sion of (1) ethanol, (2) propanol-1, and (3) butanol-1 on
CNF-N.
and Lewis base sites, the surface groups on CNFs dis-
play no such properties. Hence, both the carbon sur-
faces themselves and their oxygen-containing groups
perform catalytic functions in CNTs. The oxidation of
CNTs leads to the appearance of new catalytic sites, as
is revealed in increased activity and a change in selec-
tivity toward the formation of dehydration products.
CNF-Ns synthesized by the pyrolysis of acetonitrile
contain nitrogen in their elemental composition, the
presence of which affects their catalytic function. Only
primary alcohols are converted on CNF-Ns, making
this a characteristic trait of the conversion of С2–С4
alcohols on such catalysts. Propanol-2, butanol-2,
and tert-butanol are apparently not converted on
CNF-Ns, due to the nonlinear structure of these alco-
hols.
be due to the presence of nitrogen groups in CNF-Ns,
which include different forms of nitrogen contained
both in the carbon matrix structure (quaternary nitro-
gen and the nitrogen of pyridine bases) and in the form
of surface NO2 and NO3 groups bonded with carbon
atoms. Nitrogen atoms provide additional electron
density affecting the catalytic activity. However, the
catalytic activity of CNF-Ns is apparent only in the
conversion of primary alcohols with the formation of
dehydrogenation products from propanol-1 and buta-
nol-1. The bond dissociation energies of the О–Н
bonds in aliphatic alcohols are close; they vary from
104.7 kcal/mol for ethanol to 106.3 kcal/mol for tert-
butanol [26]. The absence of catalytic activity of
CNF-Ns in the conversion of propanol-2, butanol-2,
and tert-butanol is most likely associated with the
adsorption ability of nonlinear alcohols (secondary
and tertiary) on CNF-Ns, which is determined in turn
by the molecular structure of these alcohols; i.e., steric
factors come into play.
ACKNOWLEDGMENTS
The authors would like to thank K.I. Maslakov and
E.A. Nesterova for recording the XPS spectra of nano-
materials, and A.V. Egorov for preparing our electron
microphotographs. This work was performed on
equipment purchased using the financial resources of
the Development Program of Moscow State Univer-
sity.
CONCLUSIONS
Comparative analysis of the conversion of С2–С4
aliphatic alcohols on carbon nanomaterials with the
sp2-carbon configuration shows that CNTs are effec-
tive catalysts for the conversion of alcohols, while
CNFs do not display catalytic activity. This leads us to
conclude that the morphological features of the CNT
carbon structure that are associated with its curvature
resulting in the formation of defects which serve as
catalytic active sites play a key role in catalysis. The
structure of oxygen-containing surface groups in
CNTs and CNFs are different. While CNTs contain
surface functional groups able to serve as Lewis acid
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