Impact of structured catalysts in amine oxidation under mild conditions

Article abstract of DOI:10.1016/j.cattod.2016.05.001

A structured graphene/graphite catalyst grown on a commercial austenitic stainless steel sheet providing a micromonolith was obtained by submitting the nude stainless steel structure to a carbon-rich atmosphere (first 300 mL/min of a reductive H₂/N₂ (1:1) flow, then to 180 mL/min of a CH₄/H₂ (1:5)) at high temperature (900 °C) for 2 h. The preparation procedure resulted in a homogenous surface coated with a carbon-rich film as observed by EDX and SEM images. Further characterizations by Raman spectroscopy revealed characteristic Raman lines of graphene and crystalline graphite disposed in a hierarchical organization. The disposal of the obtained surface layers was also confirmed by grazing incidence X-ray diffraction. Besides this, XRD indicated the overlapping diffraction lines of graphite, cementite and M₇C₃ carbides. The graphene nature of the outermost layer was also confirmed by XPS. The catalytic behavior of the structured graphene/graphite catalyst was evaluated in the selective oxidation of heptylamine. At 200 °C it afforded a total conversion with a combined selectivity in heptanonitrile and N-heptylidene-heptylamine of 67% (10% heptanonitrile) that corresponds indeed to a very efficient system in the absence of any metal. Kinetic experiments with the scope to calculate the activation energies were also performed.

Full text of DOI:10.1016/j.cattod.2016.05.001

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Impact of structured catalysts in amine oxidation under mild
conditions
Jose Luis Santosa, Pablo Navarroa, Jose Antonio Odriozolaa, Miguel Angel Centenoa,∗
,
Octavian D. Pavelb, Bogdan Jurcab, Vasile I. Pârvulescub,∗
a Institute of Materials Science of Seville, CSIC-University of Seville, Avda. AmericoVespucio 49, 41092 Seville, Spain
b Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, 4-12 Regina Elisabeta Av., Bucharest,
Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
A structured graphene/graphite catalyst grown on a commercial austenitic stainless steel sheet providing
a micromonolith was obtained by submitting the nude stainless steel structure to a carbon-rich atmo-
sphere (first 300 mL/min of a reductive H2/N2 (1:1) flow, then to 180 mL/min of a CH4/H2 (1:5)) at high
temperature (900 ◦C) for 2 h. The preparation procedure resulted in a homogenous surface coated with a
carbon-rich film as observed by EDX and SEM images. Further characterizations by Raman spectroscopy
revealed characteristic Raman lines of graphene and crystalline graphite disposed in a hierarchical orga-
nization. The disposal of the obtained surface layers was also confirmed by grazing incidence X-ray
diffraction. Besides this, XRD indicated the overlapping diffraction lines of graphite, cementite and M7C3
carbides. The graphene nature of the outermost layer was also confirmed by XPS. The catalytic behavior
of the structured graphene/graphite catalyst was evaluated in the selective oxidation of heptylamine. At
200 ◦C it afforded a total conversion with a combined selectivity in heptanonitrile and N-heptylidene-
heptylamine of 67% (10% heptanonitrile) that corresponds indeed to a very efficient system in the absence
of any metal. Kinetic experiments with the scope to calculate the activation energies were also performed.
© 2016 Elsevier B.V. All rights reserved.
Received 17 December 2015
Received in revised form 29 April 2016
Accepted 2 May 2016
Available online xxx
Keywords:
Micromonolith
Carbon micromonolith
Graphene/graphite structured catalyst
Amines oxidation
1. Introduction
with NH3 [9], etc. However, these routes show some disadvantages,
like rather strong environmental issues: the harsh reaction con-
Naturally, nitriles are present in a diverse set of plant and ani-
mal sources. However, they are difficult to be extracted from these
sources and therefore the synthesis of nitriles has already a long his-
tory [1,2]. This is mainly connected to the large utilization of these
compounds in various fields, either as products or as intermediates
for the production of amines, amides, carboxylic acids, aldehydes,
esters, heterocycles, etc. For example, there are different types of
pharmaceuticals containing nitriles, especially those with antidia-
betic and anticancer effects. Also, nitrile rubber is highly resistant
to chemicals being used as a protective material.
ditions, the use of toxic agents, the generation of undesired toxic
by-products, etc.
The selective oxidation may provide an alternative to these syn-
thesis routes. The introduction of oxygen atoms in the substrate
molecule is a reaction moderately exothermic with water as the
sole by-product [10,11]. However, the exothermic effect should be
maintained under a severe control.
Several attempts to synthesize nitriles via selective oxidation
of primary amines (Reaction Eq. (1)) have been already reported.
Chen et al. used trichloroisocyanuric acid (TCCA) in the presence
of catalytic TEMPO [12], Reddy et al. [13] KI or I2 in combination
with TBHP, Shi et al. [14] NaClO and beta-cyclodextrin in presence
of water, Veisi [15] TCCA as an oxidant and reagent in aqueous
ammonia, Kim and Stahl [16] homogeneous (bpy) Cu/TEMPO for
selective oxidation, Schröder and Griffith [17] potassium ruthenate
(K2[RuO4]) in the presence of persulfate ion, and Bagherzade et al.
[18] pentylpyridiniumtribromide in aqueous NH4OAc. However, all
these procedures have the disadvantage of stoichiometric reactions
in which at least one of the reactants is consumed providing by-
products that are hard to be separated.
Nowadays, nitriles are usually produced by ammoxidation
(where
a hydrocarbon is partially oxidised in the presence
of ammonia) [3], hydrocyanation (from hydrogen cyanide and
alkenes) [4], dehydration of amides and aldoximes [5], condensa-
tion of aldehydes [6], alcohols [7], esters [8], of carboxylic acids
∗
Corresponding authors.
E-mail addresses: centeno@icmse.csic.es (M.A. Centeno),
vasile.parvulescu@chimie.unibuc.ro, v parvulescu@yahoo.com (V.I. Pârvulescu).
http://dx.doi.org/10.1016/j.cattod.2016.05.001
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.L. Santos, et al., Impact of structured catalysts in amine oxidation under mild conditions, Catal. Today
(2016), http://dx.doi.org/10.1016/j.cattod.2016.05.001

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Tang et al. [19] suggested the use of ruthenium trichloride in
tandem with dioxygen that offered a very high advantage due to
an enhanced atom economy. However, the catalyst is not easily
recyclable. To avoid this, it was proposed the replacement of homo-
geneous catalysts with heterogeneous ones. High catalytic activity
of Ru/Al2O3 catalyst [20] was thus reported in the oxidation of
both primary and secondary amines by molecular oxygen in liq-
uid phase at atmospheric conditions. Cristian et al. [21] proposed
the oxidation of both aliphatic and aromatic amines using a recy-
clable catalyst and dioxygen or air, as oxidants. The Ru2Cl4(az-tpy)2
complex is highly effective in this reaction. Furthermore, the asso-
ciation of this catalyst with supported ionic liquids (SILP) enhanced
the catalyst recyclability [22]. More recently, we demonstrated that
such reactions can be carried out in carbocatalysts, where graphene
oxide provided a very high activity and selectivity [23].
(1)
Except for the ammoxidation, all these reactions were carried
out under batch conditions. Therefore, transferring it under a con-
tinuous flow reactor, will offer more advantages for the practical
applications.
Fig. 1. Plug-flow reactor setup used to perform the catalytic tests on heptylamine
oxidation (atmospheric pressure).
Based on it, the aim of this study was the investigation of the
selective oxidation of primary amines under flow conditions using
a structured graphene/graphite catalyst. The catalyst is generated
by the in-situ growth of the graphene/graphite layer on a stainless
steel micromonolith substrate, acting the unit as a microreactor.
Accordingly, this study reports on the synthesis of a structured
graphene/graphite catalyst that has been incorporated in a flow
design, and on the results of the catalytic oxidation of heptylamine.
Kinetic evaluation of these results is also considered.
2.3. Plug-flow microreactor setup
The microreactor prepared as above was inserted in an “in house”
built installation (Fig. 1). It was placed inside a ceramic resistance
furnace where the temperature control was achieved with a Toho
TTM-204 controller (K-type thermocouple at the middle-level of
the ceramic jacket) and an Elko ELK-38 indicator (K-type thermo-
couple in contact with the microreactor body). Both instruments
(calibrated against a high temperature glass thermometer) were
computer interfaced through RS-232 to TTL level converters based
on Maxim MAX-232 IC allowing bidirectional Modbus communica-
tion. The reactants were fed to the reactor through two concentric
tubes: the inner one carried the solution of amine in dioxane and
the oxygen gas flow through the outer thicker tube. The flow con-
trol was performed for the liquid phase by the pump P1 (Knauer
K-501 HPLC pump, computer controlled by RS-232 serial interface)
and for the gas phase by an ensemble of a micrometer-knob nee-
dle valve (HokeMilli-Mite) and a capillary gas flowmeter. The gas
flow rate was checked before each experiment with a gas burette
connected to the microreactor outlet. A condenser was attached
by soldering directly on the distal microreactor tube. The liquid
output from the reactor was collected into an open glass vessel.
Samples were extracted every 10 min by fully depleting the con-
tent of the collector vessel with the peristaltic pump P2 (Heidolph
PD 5001, computer controlled through a parallel port relay inter-
face) and dispensed in individual glass tubes (covered with small
funnels to reduce evaporation) by the help of a fraction collector
(Teledyne ISCO Foxy Jr, computer connected by RS-232 serial link).
The operational parameters of the installation (temperature, liquid
flow rate, collection of samples) were controlled by a code devel-
oped in C-programming language running on GNU/Linux operating
system (32-bit CentOS 6).
2. Materials and methods
2.1. Materials
All chemicals were used as purchased, without preliminary
purification: n-heptylamine (Merck); 1,4-dioxane (Sigma-Aldrich)
as solvent for preparing heptylamine feed solutions and oxygen
(99.999%, Linde).
2.2. Preparation of the structured graphene/graphite catalyst
The catalyst was grown on a commercial austenitic stainless
steel sheet (AISI 304, 50 ␮m thick, Goodfellow). Typically these Fe-
based alloys contain 18 wt% Cr, 8 wt% Ni, percentages around 1 wt%
of Si and Mn as well as minor amounts of other transition met-
als, carbon, nitrogen and sulphur. Micromonoliths were built on
these sheets following a procedure described elsewhere [24,25].
The final micromonolith is a cylinder 3 cm height, 1.7 cm diameter
with 540 cm2 of total surface area and 320 cells/cm2. A native oxide
layer 1–3 nm thick grows naturally on air-stabilized stainless steel
sheets [26].
Carbon-coated micromonoliths were obtained by submitting
the nude stainless steel structure to a carbon-rich atmosphere at
high temperature. First the temperature was ramped to 900 ◦C at
15 ◦C/min under 300 mL/min of a reductive H2/N2 (1:1) flow, then,
the reductive atmosphere was switched to 180 mL/min of a CH4/H2
(1:5) mixture and the temperature kept for 2 h. Finally, a N2 flow
replaces the reactive flow and the system is allowed to cool down
to room temperature.
The structured catalyst was wrapped in Al foil and fitted at
the middle of the microreactor body. All the inner surface of the
microreactor (including the exposed metallic surface of the feeding
tube) was covered with the Al foil so that the reagent mixture does
not enter in contact with other heated metallic surface. The choice
of Al over Pyrex glass was made from preliminary blank tests: Al is
more effective than glass to avoid gas-phase free radicals [27] that
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Fig. 2. Selected SEM micrographs of the structured catalyst. a) Secondary, b) backscattering detector.
may originate from thermal reactions of the aliphatic amine. In this
way the auto oxidation was canceled.
X-ray diffraction (XRD) analyses were carried out on a X‘PertPro
PAnalytical diffractometer. Diffraction patterns were recorded
using Cu-K␣ radiation (␭ = 1.5404 Å) with an incident beam
parabolic mirror W/Si over a 10–90◦ 2␪-range and a PIXcel solid
state X-ray detector with 0.05◦ step size at a step time of 300 s.
Diffractrograms were taken varying the angle of incidence between
0.1 and 5◦. Additionally, the diffractrograms at 90◦ incidence of the
bare and carbon-coated micromonoliths were also taken.
Raman spectra were recorded in a dispersive Horiba Jobin-Yvon
LabRam HR800 Microscope, with a He-Ne green laser (532.14 nm)
working at 5 mW, and with a 600 g mm−1 grating. The microscope
used a 50x objective and a confocal pinhole of 1000 ␮m. In such con-
ditions, the calculated laser spot diameter results to be 0.721 ␮m.
The Raman spectrometer was calibrated using a silicon wafer.
XPS analyses were carried out on a photoelectron spectrometer
Leybold-Heraeus LHS-10/20 model, working with AlK␣ radiation
(1486.6 eV), a potential of 11 kV and a current of 30 mA. The analy-
sis chamber operates under ultra-high vacuum with a pressure of
about 5.10−9 Torr. The evacuation was performed for 1 h at room
temperature. Energy scale calibration was performed considering
the instrument work function.
Several experiments were performed as blank tests at high
temperature (200 ◦C) and low oxygen flow rate (2.5 mL/min or
102 ␮mol/min): a) with the catalyst load by passing dioxane
through the microreactor, which proved that the solvent is indeed
inert (no detectable products by GC/MS), and b) two runs with the
empty reactor for a molar ratio of O2/amine of 20/1, and passing
a solution of heptylamine (amine flow rate of 5.1 ␮mol/min) in
presence of i) a Pyrex glass tube fitted in the microreactor (amine
conversion 27.5%) and ii) Al foil covering the inner surface of the
microreactor (amine conversion 10.8%).
2.4. Analytical methods and characterization
The adherence of the catalytic layers to the metallic substrate
was evaluated by means of an ultrasonic method earlier described
[28]. Previously weighted micromonoliths are immersed in a bath
of deionized water and sonicated for 30 min at 30 ◦C. After the
ultrasonic test, micromonoliths are dried in a stove and weighted
again. The adherence is expressed as the percentage of coating that
remains on the micromonolith after the test.
SEM images were obtained on a scanning electron microscope
Hitachi S-4800 FEG-SEM, equipped with secondary and backscat-
tered electron detectors. Microanalysis was performed with an
energy dispersive spectrometer (OXFORD LINK) coupled to the
microscope. Micrographs are taken at 2k Vanda working distance
of 8 mm.
The identification of the reaction products in the liquid sam-
ples was performed using a GC/MS setup (Thermo Electron Trace
GC Ultra/Trace DSQ) equipped with a non-polar GC column (TG-5
SILMS) with He as carrier gas. The temperature profile of the elution
was set to a 10 min dwell at 60 ◦C, 10◦/min. ramp to 240 ◦C followed
by a dwell of 10 min. MS spectra were recorded for positive ioniza-
tion in full-scan m/z range 50–300. The catalytic reaction products
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2D
a
G
2D/G=2,51
D
'
*
2D
G
2D/G=2,44
b
c
2D1
2D2
2D/G=0,65
Fig. 3. EDX analysis of the structured catalyst.
500 1000 1500 2000 2500 3000
Raman shift (cm-1)
2600
2700
2800
Raman Shift (cm-1)
are identified as N-heptylideneheptylamine; heptylamide; hepta-
nenitrile and heptanal.
Fig. 4. Raman spectra of selected regions of the structured catalyst. The deconvo-
lution of the 2D line in two components (2D1 and 2D2) characteristic of bi-layer
graphene is visible in trace c.
The quantitation was performed with a GC/FID apparatus
(Thermo Electron Trace GC Ultra) on a non-polar column (DB5)
operated with N2 carrier gas and a temperature profile starting with
1 min dwelt at 160 ◦C followed by a 2◦/min ramp to 200 ◦C and a
final 1 min dwell.
cedure that results in a homogenous carbon-rich film coating the
steel surface.
3. Results and discussion
The surface of the carbon-coated micromonolith was carefully
studied by Raman spectroscopy that allows discriminate between
graphite, graphene or metal carbide species as a result of the car-
burizing treatment. Spectra were taken along the x, y and z axis.
Characteristic Raman lines of graphene and crystalline graphite
were obtained in every case. However, some small but important
differences are observed. Besides this, Raman spectroscopy cannot
rule out the presence of iron or chromium carbides since vibrational
modes for these species are not Raman-active. In any case, the car-
bide vibrational modes should be expected in the 200–600 cm−1
region [30,31].
After carbon deposition, the bare microreactor structure
presents a 98.8 mg weight gain and a low surface area (SBET
< 5 m2/monolith). However, the amount of carbon deposited may
be higher since the native oxide layer that passivized the stainless
steel may be reduced, which would result in a negative weight gain
and the total amount of carbon deposited should be higher than
98.8 mg. The procedure of carbon coating is carried out over the raw
(non-oxidised) stainless steel monolith. The weight of such mono-
lith is 10.63 g. On the stainless steel surface, which is passivated,
exists a very thin oxygen layer (typically in the nanometer range).
This is the layer reduced upon treatment. However, the determi-
nation of such weight loss by gravimetry is uncertain and well
below the accuracy limit of the balance. The passive layer thickness
depends on the atmosphere and temperature of treatment, there-
fore the commercial treatment BA or 2B, for instance, modifies this
thickness. Assuming an upper limit of 10 nm for a passive layer
formed by a compact Cr2O3 layer, well above the usually reported
values [26,29], a mass of ca. 5·10−6g cm−2 may be reduced. As the
monolith has typically ca. 500 cm2, the maximum weight reduction
should be of around 1,5 mg which is less than 0,1% weight loss of the
monolith. Adhesion tests resulted in weight losses below 0.2 wt%
in every case indicating and excellent adherence of the coated film
to the metallic substrate.
Fig. 2 shows SEM images of the obtained surface both in sec-
ondary (a) and backscattered (b) modes. Secondary electron images
show a homogeneous surface composed of micron-sized rounded
crystals. This supposes a morphological change with respect to
the stainless steel passive layer typically composed of octahedral
and plate-like manganese-containing spinels, and chromium-iron
oxides corundum-type crystals, respectively [24]. Backscattered
images of the same region points out the presence of a carbon-
rich thin film covering heavier chromium-rich crystals as evidenced
by the small and disperse bright spots on the sample surface. EDX
analysis of the coated specimen, Fig. 3, shows the presence of sig-
nals corresponding to the main alloy elements Cr, Mn, Ni and Fe
together with a very intense signal at low energies associated to
the presence of carbon. This supports a monolith preparation pro-
Fig. 4 compiles a selection of characteristic Raman spectra of
the carbon-coated monoliths. Two intense carbon features, the G
band at 1587 cm−1 and the 2D band at 2700 cm−1, dominate the
spectra. The G band is due to the bond stretching of all pairs of
sp2 carbon atoms in both rings and chains, while the 2D band is
the overtone of the D band at 1356 cm−1, being this band due to
the breathing modes of sp2 carbon atoms in rings [32]. Besides
this, peaks at 2451 and 3249 cm−1 due to G* and 2Dꢀ modes are
also observed [33]. It is well known that the D-to-G bands intensity
ratio, as well as the shape and intensity of the 2D feature are signa-
tures of the carbon nature, either graphite or graphene as well as
of the number of graphene layers. The absence of the D line and a
very high intensity of the 2D line characterize graphene. Typically,
the intensity of the 2D feature is four times more intense than the
intensity of the G one [34]. Moreover, the 2D line of graphene can
be fitted with a single Lorentzian curve. The 2D line needs to be
decomposed at least in two components, 2D1 and 2D2, on increas-
ing the number of graphene layers. On the limit, more than 5 layers,
the 2D2 intensity dominates the spectrum being indistinguishable
from that observed for bulk graphite. The shape of the 2D line in
the spectrum closer to graphite (Fig. 4, trace c) has been reported
as characteristic of bi-layer graphene [34]. Therefore, these results
points to the formation of 1 or 2 layers of grapheme on the metal-
lic substrate. However, in all spectra, the presence of the D line is
always observed indicating a certain disorder or defects in the car-
bonaceous layer. The presence of the D line may be explained by
the amorphization trajectory in the sequence graphene-graphite-
nanocrystalline graphite [35]. As the diameter of the laser spot is
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Fig. 5. Selected diffractrograms of the structured catalyst obtained as a function the incidence angle. a. 90◦ b. 5◦ c. 2.5◦ d. 0.1◦ e. bare substrate, 90◦.
0,721 ␮m the recorded Raman spectrum may be a convolution of
the spectra of different carbon structures. Taken this into account
a sequential process for the formation of the carbon film may be
envisaged. The graphene initial layer evolved with time in thick
graphite layers covering the metallic substrate. This sequence is
also supported by the presence of intense lines, characteristic of
the metal oxides from the stainless steel passive layer (manganese-
containing spinels, and chromium and iron oxides [35]) (Fig. 4, trace
a).
The hierarchical organization of the obtained surface layers is
confirmed by grazing incidence X-ray diffraction (Fig. 5). The char-
acteristic diffraction lines of austenite accounting for the initial
metallic surface dominate the XRD pattern. Besides these, a broad
signal peaking at 2␪ ≈ 24◦ reflects the amorphous nature of the
stainless steel passive layer.
Grazing incidence XRD experiments allow determine the hier-
archical nature of the carburized layer. The XRD diffractogram at
the lowest incidence angle utilized (Fig. 5, curve d) shows diffrac-
tion lines at 16.7, 26.6, 43 and 44.6◦ ascribed to the (110) plane
of graphene, and to the (002), (100) and (101) planes of crys-
talline graphite, respectively [36,37]. Besides this, other incipient
diffraction lines are observed that may be associated to the exis-
tence of metallic carbides. On increasing the incidence angle to
2.5◦, the diffraction line associated to graphene decreases in inten-
sity while the rest of reflections clearly increase their intensities
indicating the surface nature of the graphene phase. At this inci-
dence, angle diffraction lines characteristics of Fe3C (cementite)
and Cr7C3 phases are clearly observed. The graphite as well as the
cementite phases are unequivocally identified by the presence of
the diffraction lines at 2␪ ≈ 26.6 and 37.8◦ associated to the (002)
and (210) planes of graphite and cementite, respectively. M7C3
carbides, mainly containing chromium, are identified through the
reflections at 2␪ ≈ 48.5◦, 50.7◦, 70.3◦, 80.2◦, 82◦ and 83.7◦ corre-
sponding to the (212), (161), (143), (282), (092) and (480) planes,
respectively [38]. Besides this, diffraction lines at ca. 40.6, 43 and
45◦ may be the result of overlapping diffraction lines of graphite,
cementite and M7C3 carbides.
Fig. 6. C1s XPS spectra of the structured catalyst.
those of graphite and metal carbides. It is interesting to note that
the relative proportion of graphite with respect to metal carbides is
maximized at 5◦ [39]. In any case, the austenite diffraction lines are
completely absent for the incidence angles in the range 0.1–5◦.The
diffraction lines of the austenite phase are clearly detected when
the XRD pattern is recorded at 90◦ incidence angle. At this angle
the diffraction lines of graphite and metal carbides are also visi-
ble. Here, the relative intensity of the metal carbides reflections
increased, with respect to those of graphite.
These data corroborates the hierarchical nature of the layers
resulting after depositing carbon on the micromonolith. A graphene
layer accounts for the outermost phase grown on the carbon-coated
micromonolith. Underneath the graphite layer, a chromium-rich
carbide phase that is mainly the result of carburizing the mixed
oxide phase forming the initial stainless steel passive layer is
detected as well.
The graphene nature of the outermost layer is also confirmed by
XPS. The C1s level of the micromonolith surface is shown in Fig. 6.
The complex signal clearly indicates the existence of more than
one carbon form in the surface layer as evidenced by the shoulder
at ∼286.5 eV in the high binding energy side of the graphitic carbon
signal peaking at 284.6 eV. Several approaches have been proposed
in the literature for deconvoluting these complex carbon signals.
Different functional C O groups and a ␲ → ␲* transition together
In general the XRD pattern of the diffractogram taken at 2.5◦ is
similar to that taken at 0.1◦, but showing a decrease in the intensity
of the reflections associated with graphene, and an increase in those
of crystalline graphite and metal carbides. The same trend is contin-
ued with the increment of the incidence angle up to 5◦; the higher
the angle, the lower the intensity of the graphene lines, and higher
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Fig. 7. Conversion of heptylamine vs. temperature for high flow rates of reagents.
Fig. 8. Arrhenius plots and apparent activation energies for high flow rates of O2
and heptylamine.
with components associated to sp2 and sp3 carbons have been pro-
posed [40–42]. In these models, the high binding energy shoulder at
286,5 eV is associated to the presence of graphene oxide. Assuming
just the presence of graphene oxide and graphite the deconvolution
of the XPS signal results in ca. 23% of graphene. This result is con-
sistent with the XRD and Raman data that indicates the presence
of 1 or 2 layer graphene phase on the monolith surface.
times the flow rate of oxygen (25 mL/min or 1020 ␮mol/min) and
ten or twenty times that of heptylamine (51 and 102 ␮mol/min)
while keeping a mole ratio O2/heptylamine of 20/1 and 10/1,
respectively.
When setting the operating parameters of the reactor, the
concentration of the reported reaction products presented a contin-
uous change for the samples collected at 10, 20 and 30 min. It was
then stabilized to a plateau-like as it has been determined for the
samples collected at 40, 50 and 60 min, respectively. Such a behav-
ior accounts for an initial transient regime of the reactor that, for
the investigated flow rates and temperatures, reached a steady-
state operation after approximately 30 min. The reported values of
the selectivity and conversion (further used to estimate the appar-
ent activation energy) correspond to samples extracted when the
reactor operated under steady-state conditions.
3.1. Evaluation of the catalytic performances
Very recently it was reported the liquid phase oxidation of
amines to the corresponding imines and nitriles on graphenes as
catalysts [23]. Although showed very high activity, selectivity and
stability, their separation from the reaction mixture and work-
ing under liquid phase presented some inconveniences. Therefore,
with the disadvantage of a partial loose in the activity the transfer
to a monolith reactor shows a step forward.
The calculus of the activation energy was made from the slope
dependency of ln(conversion) vs. 1/T taking
The catalytic experiments targeting high conversions were per-
formed at 200 ◦C for a large oxygen excess (20/1) and low flow rates
(O2: 2.5 mL/min or 102 ␮mol/min; heptylamine: 5.1 ␮mol/min).
Under these conditions the conversion of the amine was total and
the selectivities in heptanonitrile and N-heptylidene-heptylamine
reached 10, and 57%, respectively, which represented indeed excel-
lent achievements in the absence of any metal. The catalyst did not
deactivated during a 24 h continuous run.
Radical homogeneous oxidation of the hydrocarbons was early
reported by Sobalik et al. [43] and occurs in glass, quartz and
metallic reactors, although is very rarely reported. These authors
demonstrated that aluminum may largely attenuate the formation
of the radicals. Grubbs and George also confirmed this behavior
using glass reactors [27]. However, as shown in the present study
the radical homogeneous oxidation is not completely avoided by
aluminum or stainless steel. Data we report in this study gave the
overall conversion of heptylamine.
Literature does not report a reactor material completely inert to
the production of radicals during the gas-phase oxidation of hydro-
carbons and derivatives at temperature. In the present study, at
200 ◦C, the empty glass reactor afforded a conversion of 27.5%, the
aluminum reactor of 10.8% while the monolith reactor inside alu-
minum afforded a total conversion. However, at 165 ◦C the empty
aluminum reactor afforded a conversion of 0.8% and the mono-
lith reactor inside aluminum afforded a conversion of 50.3%. Under
these conditions we decided to perform the kinetics at lower tem-
peratures, where the contribution of the homogeneous reaction can
be neglected.
F0X
−r ∝
¯
mL
where:
mL = mass of catalyst, g
F0 = molar flow rate, mol/min.
X = fractional conversion
−r¯ = average molar rate of amine consumption, per unit mass of
catalyst, mol/(min·g)
Since F0 and mL are constant for each experiment, one may
assume −r¯ ∝ X.
Figs. 7 and 8 show the variation of the conversion and of
ln(conversion) as a function of the temperature. At temperatures
smaller than 140 ◦C, the conversions showed high values but
under these conditions no nitrile has been formed. After 140 ◦C,
the nitriles start to be formed and the conversion raised again.
Accordingly, the isolated points in Fig. 8 do not fit the Arrhe-
nius dependence. Checking experiments at different flow rates
suggested that the reactions were out of the diffusional control.
However, the values of the apparent activation energies were,
indeed, rather small. This might merely attributed to the activation
of the substrates onto the graphene/graphite. Molecules containing
–NH2 moieties are susceptible to interact with graphene surfaces,
as suggested by theoretical calculations [44]. Such interactions
occur via weak interactions.
4. Conclusion
With the aim to determine the activations energies, a series of
experiments were performed at lower temperatures raising ten
A structured graphene/graphite catalyst was prepared via an
original strategy by its growing on a commercial austenitic stainless
Please cite this article in press as: J.L. Santos, et al., Impact of structured catalysts in amine oxidation under mild conditions, Catal. Today
(2016), http://dx.doi.org/10.1016/j.cattod.2016.05.001

G Model
CATTOD-10188; No. of Pages7
ARTICLE IN PRESS
J.L. Santos et al. / Catalysis Today xxx (2016) xxx–xxx
7
steel sheet. The carbon-coated micromonoliths were obtained by
exposing the stainless steel structure to a carbon-rich atmosphere
(CH4/H2 (1:5)) at high temperature (900 ◦C). The preparation proce-
dure resulted in a homogenous surface coated with a carbon-rich
film as observed by EDX and SEM images. Further characteriza-
tions by Raman spectroscopy revealed characteristic Raman lines
of graphene and crystalline graphite disposed in a hierarchical
organization. The disposal of the obtained surface layers was also
confirmed by grazing incidence X-ray diffraction. Besides this, XRD
indicated the overlapping diffraction lines of graphite, cementite
and M7C3 carbides. The graphene nature of the outermost layer
was also confirmed by XPS.
The oxidation of heptylamine onto the structured
graphene/graphite catalyst occurred selectively with the for-
mation of heptanonitrile and N-heptylidene-heptylamine. The
hierarchical organization of the graphene and graphite layers have
as main advantage the organization of these entities in a robust
reactor in which the their separation from the reaction mixture
is not any more necessarily. In this way the oxidation of amines
is transferred into a continuous process. At 200 ◦C it afforded a
total conversion with selectivity in N-heptylidene-heptylamine of
57% and 10% in heptanonitrile that corresponds, indeed, to a very
efficient system in the absence of any metal. Kinetic experiments
allowed to calculate the activation energies that correlate with
the activation of the substrates onto the graphene/graphite. No
deactivation of the catalyst has been detected for a range of time
of 24 h.
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The authors gratefully acknowledge the financial support from
the 275/2011PN-II-ID-PCE-2011-3-0060 project and from Span-
ish Ministerio de Economía y Competitividad (MINECO) project
ENE2013-47880-C3-2-R, co-financed by FEDER funds from the
European Union. J.L. Santos also acknowledges the Spanish Minis-
terio de Economía y Competitividad for his predoctoral fellowship
(BES-2014-068244).
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Please cite this article in press as: J.L. Santos, et al., Impact of structured catalysts in amine oxidation under mild conditions, Catal. Today
(2016), http://dx.doi.org/10.1016/j.cattod.2016.05.001