Combined high degree of carboxylation and electronic conduction in graphene acid sets new limits for metal free catalysis in alcohol oxidation

Article abstract of DOI:10.1039/c9sc02954k

Graphene oxide, the most prominent carbocatalyst for several oxidation reactions, has severe limitations due to the overstoichiometric amounts required to achieve practical conversions. Graphene acid, a well-defined graphene derivative selectively and homogeneously covered by carboxylic groups but maintaining the high electronic conductivity of pristine graphene, sets new activity limits in the selective and general oxidation of a large gamut of alcohols, even working at 5 wt% loading for at least 10 reaction cycles without any influence from metal impurities. According to experimental data and first principles calculations, the selective and dense functionalization with carboxyl groups, combined with excellent electron transfer properties, accounts for the unprecedented catalytic activity of this graphene derivative. Moreover, the controlled structure of graphene acid allows shedding light upon the critical steps of the reaction and regulating precisely its selectivity toward different oxidation products.

Full text of DOI:10.1039/c9sc02954k

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ARTICLE
Combined high degree of carboxylation and electronic conduction
in graphene acid set new limits for metal free catalysis in alcohol
oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
b
Matías Blancoa*, Dario Mosconia, Michal Otyepkab,c, Miroslav Medveď , Aristides Bakandritsosb,
DOI: 10.1039/x0xx00000x
Stefano Agnolia* and Gaetano Granozzia.
Graphene oxide, the most prominent carbocatalyst for several oxidation reactions, has severe limitations due to the
overstoichiometric amounts required to achieve practical conversions. Graphene Acid, a well-defined graphene derivative
selectively and homogeneously covered by carboxylic groups but maintaining the high electronic conductivity of pristine
graphene, sets new activity limits in the selective and general oxidation of a large gamut of alcohols, even working at 5
wt% loading for at least 10 reaction cycles without any influence from metal impurities. According to experimental data
and first principle calculations, the selective and dense functionalization with carboxyl groups, combined with excellent
electron transfer properties, accounts for the unprecedented catalytic activity of this graphene derivative. Moreover, the
controlled structure of graphene acid allows shedding light upon the critical steps of the reaction and regulating precisely
its selectivity toward different oxidation products.
by the chemical oxidation of graphite,7 and its most accepted
structure consists of small patches of aromatic domains
separated by defective regions.8 Regarding the surface
Introduction
The term carbocatalysis stands for carbon-based catalysis,
embracing metal-free nanocarbon catalysis, where the carbon
material as a whole or some specific regions of it act as active
sites to speed-up chemical processes.1, 2 Research interest on
heterogeneous carbocatalysis is growing, since it is based on
extremely low-cost and abundant materials, without the need
of metal centers, therefore complying with the standards of
sustainability and environmental protection required by
modern society. The limited or abolished use of critical
materials, the implementation of “green” methods for their
synthesis, and easy recycle procedures are key aspects that
make carbocatalysts the most eligible materials for the
development of a sustainable circular economy.3-5
Despite the rapid progress in this field, there are still many
unmet challenges for the development of more efficient and
well-defined materials able to work at truly catalytic loadings,
and for the fundamental understanding of catalytic
mechanisms at the molecular scale. Graphene Oxide (GO) is
one of the most studied carbocatalysts,1, 6 commonly obtained
chemistry, hydroxyl and epoxy groups are present on the basal
plane, while carboxylic and carbonyl functions are located at
the edges and around defects.9 Therefore, this material, since
it is plenty of potential active sites that can be employed in
chemical transformations, has attracted widespread attention
in the catalysis community. Several studies have explored the
catalytic role of GO as a multifunctional catalytic platform, e.g.,
in the oxidation of alcohols10 and olefins,11 as oxygen activator
in the hydration of alkynes,12 as co-catalyst in the Michael-type
Friedel-Crafts alkylations13 and as solid acid in aldol
condensations.14 However, in most of these cases, the activity
of GO cannot be truly defined as catalytic, because many clues
suggest that GO acts indeed as a stoichiometric reagent, and
the effective “catalyst” loadings employed in such reports are
16
in the overstoichiometric range, typically 50–400 wt%.15,
Among the reactions catalyzed by GO, the selective oxidations
of alcohols are extremely appealing, being fundamental
processes in industrial processes that lead to key
intermediates for the production of polymers, pharmaceutical
synthons and fragrances.17 In the alcohol oxidation, the activity
can be triggered in the catalytic range by the addition of a
promoter, e.g. nitroxyl-based species combined with nitric
acid, usually in stoichiometric amounts.18 This synergistic
approach is common in the literature, but the combination
with inorganic solid acids19 or metal containing carbon
materials20 is usually required to obtain good performances,
making the process less attractive for applications. A general
HNO3-mediated mechanism was suggested21 based on the
a. Department of Chemical Sciences and INSTM Unit, University of Padova, Via F.
Marzolo 1, 35131, Padova, Italy. Email: matias.blancofernandez@unipd.it;
stefano.agnoli@unipd.it
b. Regional Centre for Advanced Technologies and Materials, Faculty of Science,
Palacký University Olomouc, Šlechtitelů 27, 771 46 Olomouc, Czech Republic.
c.Department of Physical Chemistry, Faculty of Science, Palacký University
Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic
†
Electronic Supplementary Information (ESI) available: Characterizations,
oxidation of benzyl alcohol, computational details and monitoring of the catalysts.
See DOI: 10.1039/x0xx00000x
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formation of active N-oxides (NOx) facilitating the oxidation,22 and a 20 eV pass energy. The TEM images were acquired using
View Article Online
DOI: 10.1039/C9SC02954K
but no first principle calculations or definitive experimental a FEI Tecnai 12 microscope with an acceleration voltage of
confirmation have been provided.
100ꢀkV. The Raman spectra were collected using
a
Here, we propose Graphene Acid (GA) as a potent and well- ThermoFisher DXR Raman microscope using a laser with an
defined metal free carbocatalyst for the controlled and highly excitation wavelength of 532ꢀnm (5ꢀmW), focused on the
selective alcohol oxidation, and thanks to DFT calculations, we sample with
a 50ꢀ×ꢀobjective (Olympus). The UV−visible
delineate at the molecular scale the reaction steps involved in absorption spectroscopy data were acquired using a Cary 50
the catalytic cycle. Interestingly, we demonstrate the catalytic spectrometer (Varian), in the 200−800 nm range. In this case,
nature of the reaction and that the final oxidant is atmospheric powder samples were dispersed in 2-propanol, forming a
oxygen, which is the most eligible reactant for truly sustainable stable colloidal dispersion. Solid state Fourier Transformed
large scale alcohol oxidation processes.
Infrared (FT-IR, KBr disk technique) absorption spectra were
GA is a graphene (Gr) derivative with the basal plane densely recorded with a Nicolet Nexus FT-IR spectrometer. Inductive
covered by COOH groups (up to 9-13 at%, see figure S1).23 Coupled Plasma Mass Spectrometry (ICP-MS) metal analysis
Interestingly, in contrast to GO, GA is electronically was performed on a on an Agilent 7700x quadrupole ICP-MS
conductive,23-25 which is a key property in redox catalytic instrument. Samples were digested with mixture of acids using
reactions,26 but at the same time comprises oxygenated a microwave digestion unit Milestone MLS 1200 Mega. The
groups that are strong oxygen activators. The carbocatalytic Nuclear Magnetic Resonance (NMR) spectra were recorded on
1
activity of GA was tested in the benzyl alcohol (BA) oxidation a Bruker Avance 300 MHz (300.1 MHz for H, 298 K); chemical
as a model reaction. Both the overstoichiometric (up to GA shifts (δ) are reported in units of parts per million (ppm)
loading of 20 wt.%), and low loading (1-5 wt.%) regimes in relative to the residual solvent signals and coupling constants
presence of HNO3 were investigated to unravel the role played (J) are expressed in Hz.
by each component in the catalytic cycle. The influence of Overstoichiometric benzyl alcohol oxidation. The typical
metallic impurities was ruled out by recycling experiments catalytic test in the benzyl alcohol oxidation in the
coupled with ICP analysis. DFT calculations performed for a overstoichiometric regime was performed, if not otherwise
finite-size GA model corroborated the reaction mechanism stated, as follows: 1 mmol (0.108 g) of benzyl alcohol was
deduced by the experiments.
mixed, in a Teflon-capped vial, with the adequate amount of
GA powder to generate the corresponding 1-20 wt.% loading
of catalyst, and the mixture was heated at the target
temperature, typically 150 oC, for 24 h. Then, it was allowed to
cool to room temperature, the solid was filtered, CDCl3 was
added and immediately submitted to NMR analysis. If solvent
was employed, 1 M concentration on benzyl alcohol was used.
Benzaldehyde (CDCl3, 300.1 MHz, 298 K, δ ppm): 9.95 (s, 1H),
7.83 (d, J = 8.1 Hz, 2H), 7.57 (t, J = 8.2 Hz, 1H), 7.53 (d, J = 8.2
Hz, 2H).
Experimental
General. All chemicals were purchased from Aldrich. Reagent
grade or better quality was employed in all experiments. GA
was synthetized following the procedure described
previously.23 All air-sensitive reactions were carried out under
N2 atmosphere employing Schlenk techniques with N2
degassed solvents. GO was synthetized according to
a
modified Hummers procedure.27 An amount of 3.0 g of
graphite was mixed with 75 ml of 9:1 mixture of H2SO4 and
H3PO4. Then an amount of 9.0 g of KMnO4 was slowly added
while cooling the mixture in an ice bath. The mixture was
stirred for 3 h at 0 oC and then overnight at room temperature.
After that, a volume of 150 ml of water was added heating at
80 oC. After stirring for 1 h, the obtained mixture was
Alcohol oxidation with HNO3. The standard catalytic test in
the alcohol oxidation with nitric acid was performed, if not
otherwise stated, as follows: 1 mmol of alcohol was mixed, in a
20 mL volume Teflon-capped vial (see Figure S4), with a certain
amount of graphenic material powder (typically GA, 5 mg), and
the mixture was suspended in 2 mL of solvent, normally 1,4-
dioxane. Then, 2 mmol of the oxidant, typically HNO3
(concentrated, >65%) were added, the vial was closed tightly,
o
sonicated for 1 h at 35 C. To quench the reaction, 3 mL of 30
o
and heated at the target temperature, usually 90 C, for the
%wt. of H2O2 was added, stirred for additional 2 h, sonicated
for 1 h and finally, diluted with additional water. The
supernatant was profusely washed with water, dialyzed 72 h
against DI water and 8 h against Milli-Q quality water, and
finally lyophilized.
Characterization techniques. The surface chemical
characterization of the catalyst has been carried out using X-
ray photoelectron spectroscopy (XPS) in a custom-made UHV
system working at a base pressure of 10−10 mbar, equipped
with an Omicron EA150 electron analyzer and an Omicron DAR
400 X-ray source with a dual Al−Mg anode. Core level
photoemission spectra (C 1s, N 1s and O 1s) were collected at
rt with a non-monochromated Mg Kα X-ray source (1253.6 eV)
and using an energy step of 0.1 eV, 0.5 s of integration time,
desired time. Then, it was allowed to cool to room
temperature, the solid was filtered, CDCl3 was added and
immediately submitted to NMR analysis.
The study of the catalytic cycle required the systematical
modification of the standard reaction parameters, such as
catalyst (GA, GO or any), solvent, temperature (from rt to
90 oC), oxidant (various acids, different amounts of HNO3,
different amounts of NaNO2 combined with different acids or
even NOBF4), reactor (pressured autoclave or open-to-air vial)
and atmosphere (air or inert). Given this set of modified
conditions, the changes in the performance of the reaction are
routinely compared with the standard catalytic run in the
Results and Discussion section.
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Recovery experiments were conducted after washing the distribution of the oxygen species, with smalVleierw AartricolemOanltinice
DOI: 10.1039/C9SC02954K
catalyst by 3 centrifugation cycles with water and 3 additional domains (53.4%) and a double amount of hydroxyl and epoxy
cycles with acetone, sonicating for 5 min between each cycle, groups compared to the GA (Table S1). Moreover, COOH
and vacuum-dried. After that, the solid was submitted to a groups in GO accounted only 7% of the total C 1s intensity.
new catalytic cycle without adding in any case new catalyst Metallic impurities are higher on this sample, in particular Mn
precursor. The procedure was repeated 10 times.
and Pt, as demonstrated by ICP-MS analysis (Table S5). The
Benzoic acid (CDCl3, 300.1 MHz, 298 K, δ ppm): 7.99 (d, J = 7.9 Raman spectra of both samples agreed with the prototypical
Hz, 2H), 7.45 (t, J = 8.1 Hz, 1H), 7.36 (d, J = 8.2 Hz, 2H).
pattern of an oxidized carbon material, having the most
Butyric acid (CDCl3, 300.1 MHz, 298 K, δ ppm): 2.34 (t, J = 7.3 prominent bands centered at 1350 cm-1 (D band) and 1580 cm-
1
Hz, 2H), 1.69 (sx, J = 8.1 Hz, 2H), 0.98 (t, J = 8.0 Hz, 3H).
(G band),34 owing a ID/IG ratio of 1.05 and 0.92 for GA and
Octanoic acid (CDCl3, 300.1 MHz, 298 K, δ ppm): 2.21 (t, J = 7.5 GO, respectively (see Figure S3). Both materials presented the
Hz, 2H), 1.54 (q, J = 7.9 Hz, 2H), 1.29 (m, 6H), 0.88 (t, J = 8.1 Hz, typical flake-like morphology of graphene derivatives in the
3H).
3-Butenoic acid (CDCl3, 300.1 MHz, 298 K, δ ppm): 6.07 (ddd, J S3). The lateral sizes ranged from hundreds of nm on GA to
= 16.0, 10.8, 6.3 Hz, 1H), 5.27 (m, 2H), 2.9 (d, J = 6.4 Hz, 2H). tens of µm in GO, in excellent agreement with previous
Adipic acid (CDCl3, 300.1 MHz, 298 K, δ ppm): 2.24 (t, J = 7.2 reports.23, 26
Hz, 4H), 1.50 (t, J = 7.4 Hz, 4H). Motivated by the overstoichiometric strategy of Bielawski et
al.,10 we tested the catalytic activity of the selectively −COOH
monolayer regime, as depicted by TEM analysis (see Figure
Computational details: All calculations were performed with covered and metal-free surface of GA without any promoter in
the Gaussian09 program28 employing the ωB97X-D/6-31+G(d) the BA oxidation, employing different GA loadings (from 1
level of theory.29, 30 GA was modelled using finite-size model
(ovalene) functionalized by carboxyl and hydroxyl groups
(Figure S7) according to the composition of the catalyst as
revealed by the experimental
C 1s photoemission line
deconvolution data reported in Table S1. To understand
different performance of the catalyst in various solvents, two
representative solvents (toluene and dimethyl sulfoxide
(DMSO)) were considered in theoretical models. The solvent
effects were included by using the universal continuum
solvation model based on solute electron density (SMD).31
Whereas the structures of molecular species (such as HNO3,
NO2, H2O, benzyl nitrite, benzaldehyde) were fully relaxed in
the geometry optimization, to mimic the semilocal rigidity of
graphene sheets, the GA model was obtained by constrained
geometry optimizations keeping the edge carbon atoms
frozen.
RESULTS AND DISCUSSION
GA was obtained by the controlled hydrolysis of
cyanographene, which was prepared by the substitution and
defluorination of fluorographite.23 GO, used as a benchmark,
was synthesized according to a modified Hummers protocol.32
The X-ray photoemission spectroscopy (XPS) investigation of
the C 1s spectrum of GA (Figure 1) showed the presence of
large aromatic domains (61.5% of C-C sp2 bonds, Table S1)33
together with a large fraction of COOH groups, as deduced by
the component centered at 289 eV (9.7% of the total C 1s
spectrum area, i.e. 11% functionalization degree). No traces of
metals were observed by XPS (Figure S2 and Table S2), as
expected from the synthesis protocol. Nevertheless, since our
goal is to employ the GA as carbocatalyst, ICP-MS analysis was
also performed to quantify the metallic content below the XPS
detection limit, revealing ppb levels of metals such as Cr, Mn,
Fe, Ni or Cu at the GA surface; however, our graphene
derivative does not contain Co, Pt or Pd (Table S5). On the
other hand, the GO sample presented the standard complex
wt.% to 20 wt.% with respect to BA).
Figure 1. XPS C 1s core level spectra with separation into chemically shifted
components, of GA (upper) and GO (down) with the corresponding structural
models.
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Under the standard catalytic conditions reported in Table 1, no compared to other metal- and carbo-catalysts reported in the
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conversion was detected with 1 wt.% GA loading, while the literature for this reaction (Figure S3DaOnI:d10.T10a3b9l/eC9SS3C)0.29T5h4eK
reaction progressively created the intended products till selectivity toward benzaldehyde was 70%, because the excess
reaching the 27% conversion when the amount of the added of HNO3 could lead to the competing formation of benzoic acid
catalyst was increased to the 20 wt.% loading. In all positive (Figure 2), which is progressively formed when the reaction
tests (i.e. from 5 wt.% to 20 wt.% loading), NMR analysis of the time is increased. Interestingly, in the corresponding catalytic
reaction crude confirmed 100% selectivity for benzaldehyde, test with benchmark GO, it was not possible to reach activity:
i.e., without detection of any trace of benzoic acid. The in this case, no NOx evolution was observed from the reaction
obtained conversion value in the GA-catalyzed test was very vessel (Figure S4), probably because of the inherent GO
similar (Table 1) to the results obtained with GO (25 % of insulating nature35 that blocks its activity as redox mediator,
conversion) and also to that reported by Bielawski et al which might prevent the decomposition of HNO3 at the GO
(27%).10 By a linear extrapolation of our “overstoichiometric” surface. Indeed, Larsen et al.36 proposed a mechanism for
catalytic data, we deduced a behaviour identical to that redox reactions involving organic molecules and carbon-based
reported in the literature,10 i.e. full conversion with high catalysts, where the carbon material acts as an electron
selectivity for benzaldehyde, at 200 wt.% loading of GA. reservoir that is able to store and successively inject electrons
However, we did not pursue further this route because we back to the reactants. This mechanism was later confirmed by
were interested to use only catalytic amounts of active theory and experiments.37 Obviously, this mechanism cannot
material. As a matter of fact, it has been decisively reported be applied with an insulating material acting as catalyst, thus
that, when working in overstoichiometric conditions, GO acts justifying the negligible activity showed by GO.
as a reagent, undergoing important changes in its C:O ratio, Therefore, the key of the superior catalytic activity of GA is not
rather than being a true catalyst that remains essentially simply related to the presence of carboxyls groups, which are
unaltered at the end of the chemical conversion.16 Here, we known to be oxygen activators,38, 39 but to the fact that these
are interested only on the study of the true catalytic activity of catalytically active centers are electronically connected to a
GA, and not on the possible stoichiometric reactions that may reduced carbon scaffold that can be used as an electron
be happening when it is used in an extremely large excess.
reservoir/sink. GO bears 23 % less carboxyl groups than GA
No conversion was detected when the reaction was performed and located only at the nanosheet edges on very defective
in a compatible solvent such as dimethylformamide (DMF) areas, whereas on GA they are directly placed on the basal
(Table 1).
plane, maximizing the possibility of an easy and fast electron
transfer.
Metallic impurities do not seem to have any effect on the
catalytic activity for both samples. Indeed, the GO control
sample carries higher metal amounts than GA as
demonstrated by ICP analysis (Table S5), but its activity is
almost negligible in this oxidation, confirming the outstanding
carbocatalytic nature of GA. Finally, the reagents used in the
catalytic reaction (HNO3 and 1,3-dioxane), despite the
significant content of Cr, and Fe, (Table S5), also displayed
undetectable conversion (Figure 2) suggesting that these
impurities cannot be considered as the source of the catalytic
activity.
Table 1. Overstoichiometric benzyl alcohol oxidation
a
OH
GX
O
Solvent or neat
T, 24 h
Catalyst
Loadingb
Solvent
Tc
rt
Yieldd
GA
GA
GA
GA
GA
GA
GO
GOe
10
10
1
DMF
DMF
Neat
Neat
Neat
Neat
Neat
Neat
-
-
-
6
15
27
25
27
150
150
150
150
150
150
150
5
10
20
20
20
a) 1 mmol of BA and the amount of catalyst to reach the desired loading, b) wt.%
vs BA, c) oC, d) %, determined by 1H-NMR spectroscopy, e) Reported by Bielawski
et al.10
However, this scenario completely changed when nitric acid
was added to the reaction mixture. When 2 equivalents of
HNO3 as promoter were introduced in the reaction medium,
GA (5 wt.% loading) oxidized quantitatively BA in 75 minutes
under air atmosphere (Figure 2), and the formation of brown
NOx fumes demonstrated an outstanding performance
Figure 2. Time evolution of the Benzyl Alcohol oxidation catalysed by graphene
materials.
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To gain a deeper insight into the reaction mechanism and Given these first experimental evidences, weViecwarArrtiieclde Onoliunet
DOI: 10.1039/C9SC02954K
understand the excellent behavior of GA as carbocatalyst, we density functional theory (DFT) calculations to unravel the
systematically changed several reaction parameters (Figure 3). reaction mechanism. We assumed that the high temperature
We found that a minimum GA loading of 5 wt.% was necessary needed to yield the products (Figure 3b) was likely related to
to start the conversion, while increasing the loading to 10 wt.% the decomposition of the HNO3 on the GA surface, which
or 20 wt.% did not significantly affect the obtained mixture of might be the initiating step of the catalytic cycle. This was
products (Figure S4). On the other hand, at least 2 equivalents firstly addressed by investigating the possible binding modes
of HNO3 employing conventional vessel under air atmosphere of HNO3 on GA (Figure S6). As expected, the most favorable
were required to achieve a good oxidation performance, while binding mode corresponded to the formation of two hydrogen
when the acid content was reduced (0.25 – 1.5 eq), the activity bonds (2HB) between the HNO3 and GA (Figure S6a). In the gas
was largely quenched (Figure 3a). Adding an excess of HNO3 (3 phase and an almost non-polar solvent such as toluene, a
or 4 eq) however, led to a slight increase in the benzoic acid binding mode involving one hydrogen bond (1HB) was also
production as the oxidizing strength of the medium is found to be favorable (Figure S6b). Interestingly, in the case of
progressively increased. Notably, the nature of the acid played DMSO, a “parallel” (P) binding mode was identified, in which
a very important role: 2 equivalents of HCl, H2SO4 or acetic the orientation of the HNO3 molecule was highly convenient
acid only yielded traces of products. Thus, the decomposition for the formation of NO2 and H2O (Figure S6c). For all three
of the nitric acid in NOx derivatives seems to be a key step to binding modes, a decrease of the partial charge on N, (which is
initiate the BA oxidation. In addition, if the reaction was a predisposition for reduction), compared to isolated HNO3 (qN
performed at a temperature lower than 90oC (from rt to 70 oC) = +0.12e; in DMSO) was observed. The energy differences
no conversion was observed (Figure 3b). Moreover, the between the identified local minima indicated that, under the
reaction was also solvent dependent, because full conversion experimental conditions (T = ~90 °C), the HNO3 molecule was
was achieved only with 1,4-dioxane and dimethylsulfoxide not fixed in one particular position and could also adopt
(DMSO). On the contrary, in toluene, DMF and water, no orientations convenient for the reaction, such as, e.g., the P
conversion could be achieved even after overnight reactions binding mode with the most pronounced decrease of qN
(Figure S5).
(-0.22e). In the next step, the reaction energies for the GA
assisted transformation of HNO3 to NO2 were calculated (Table
2). In general, all reactions resulted thermodynamically
favorable, nevertheless, the most negative reaction energies
were obtained for the DMSO solvent, in line with the
experimental observations. Moreover, the HNO3 reduction
was accompanied by a partial decarboxylation of GA and
production of NO2 that hydrolysed to give nitric and nitrous
acid.21 To confirm this theoretical prediction, we performed a
spectroscopic analysis of the GA catalyst after reaction. The
XPS, Raman and FTIR measurements on the sample after BA
oxidation suggested a small decrease of carboxyl groups
accompanied by a parasitic oxidation of the material (Figure
S8-S9, Table S2). Let us also note that due to the complexity of
the simultaneous electron transfer, hydrogen transfer and
decomposition process, the kinetics of the reduction was not
addressed computationally.
Table 2. Reaction energies (kcal/mol) of the transformation of HNO3 to NO2 assisted by
GA for the binding modes displayed in Figure S6 calculated at the ωB97X-D/6-31+G(d)
level of theory. Parentheses: Energies corresponding to the formation of GA-COO•
after releasing •NO2 and H2O.
Gas phase
Toluene
DMSO
Reaction
2HB mode
GA-COOH…HNO3 
GA• + •NO2 + H2O + CO2
1HB mode
-16.1
-18.4
-19.8
(41.5)
(28.4)
(19.5)
GA-COOH…HNO3 
GA• + •NO2 + H2O + CO2
P mode
-19.5
-20.6
(38.0)
(26.2)
-
Figure 3. Oxidation conversion (%) of Benzyl Alcohol under standard conditions
with 5wt.% GA with nitric acid promoter changing a) the nature and amount of
the acid and b) the temperature.
GA-COOH…HNO3 
GA• + •NO2 + H2O + CO2
-28.1
-
-
(11.2)
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In addition, the selectivity to the aldehyde ViwewaAsrtichleiOgnhliener
DOI: 10.1039/C9SC02954K
Based on the experimental and theoretical evidences compared to the standard run (2 eq. of HNO3) due to the
discussed above, we can propose the following catalytic cycle milder generation of the co-catalyst, allowing a better control
19
(Figure 4): HNO3 decomposes on the material and forms NO2
on the selectivity. Moreover, if the reaction intermediates
(step 1). This brown gas dissolved in the reaction medium must corresponding to the step 3 are carefully analyzed during the
hydrolyze to HNO2 as consequence of the aqueous BA oxidation, the -CH2-(ONO) group belonging to the benzyl
a
environment in which the reaction takes place (step 2).21 The nitrite is detected as a small singlet at δ ≈ 6.0 ppm in the NMR
in situ formed HNO2 can react with benzyl alcohol to give spectra (Figure S11), confirming the set of experimental data
benzyl nitrite (step 3), which eventually yields the final discussed above.
products through a GA mediated reaction involving the The step 4 of the catalytic cycle was investigated by DFT
adsorption of the benzyl nitrite on the GA surface and its calculations. As for the HNO3 reduction, firstly we identified
subsequent oxidation (steps 4 and possibly 5).22 As a result, a the binding modes of the benzyl nitrite on GA (Figure 5). In
NO molecule is released (step 6),19,20 which is then re-oxidized solvents, the mode with a hydrogen bond between the oxygen
to NO2 by molecular oxygen (step 7), closing the catalytic bound to the benzyl moiety and the hydrogen atom from the
cycle.18 It is noteworthy mentioning that this reaction, GA’s COOH was the most favorable. Obviously, this structure
especially steps 1 and 2, require stoichiometric amounts of was convenient for a subsequent release of NO. Although the
nitric acid to yield appreciable conversion of the oxidation cleavage of the N–O bond required c.a. 40 kcal mol-1 according
products, in accordance with our theoretical and experimental to our calculations, it could be compensated by the
evidences, and also with previous works (see table S3 for more energetically favorable stabilization of the benzyloxy radical by
examples). Nevertheless, the proposed cycle in Figure 4 should its reaction with O2 or NO2 (Table S4 and Figure S7) to form
be sustained even with catalytic quantities of HNO3 or NOx benzaldehyde. Moreover, the relatively high energy of the N–
since they do not seem to be consumed after closing the mass
O bond dissociation can be overcome at moderate
balances. However, key catalytic species such as NO2, due to temperature, which in fact represents a limiting factor for BA
their gaseous nature, are prone to be quickly released from oxidation.
the reaction environment when operating under standard We also explored the experimental conditions that allowed to
conditions. Therefore, to confirm the true catalytic nature of oxidize the aldehyde to the corresponding carboxyl acid (step
the reaction, we ran a test employing 10 mol% of HNO3 in an 5). When the BA oxidation was performed with stoichiometric
autoclave to secure that all the gases remained in the reaction amounts of HNO3 in an open vessel, where gases can be easily
vessel. Operating in such a way, excellent performances were released and oxygen constantly provided, full conversion was
obtained (95% conversion with 98% selectivity) (see Figure reached and the reaction was also fully selective toward the
S10). Therefore, the NOx derivatives or even the nitric acid acid. (Figure 5d) Thus, the release of gases accelerated the
would truly act as a co-catalyst in the benzyl alcohol oxidation oxidations reaching the level when two oxidations (1 for the
and not simply as a promoter. On the other hand, the HNO2 alcohol and 1 for the aldehyde) occurred successively.
-
can be mildly generated if NO2 ions are externally added.
Thus, reactions with NaNO2 were performed under standard
conditions and air atmosphere. We observed that when 1
equivalent of NaNO2 was mixed with 1 equivalent of HNO3 (or
any other strong acid to provide HNO2 under standard
conditions, Figure S10), high conversion with excellent
selectivity to the aldehyde was achieved. Therefore, these
tests additionally confirmed that the steps 1 and 2 of the
reaction cycle (Figure 4) are actually taking place as initiating
processes of this catalytic oxidation.
Figure 5. a-c) Binding modes of benzyl nitrite on GA. The binding energies (in
kcal/mol) were calculated at the ωB97X-D/6-31+G(d) level of theory; the
black/blue/red values correspond to the gas phase/toluene/DMSO, respectively.
Figure 4. Catalytic cycle involving the GA.
6 | J. Name., 2012, 00, 1-3
d) cycle terminating steps analysis.
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The terminating step of the cycle (step 6) was explored (Figure UV-Vis spectrum after reaction), and submitted to a new
View Article Online
DOI: 10.1039/C9SC02954K
5d) performing new catalytic tests changing a variety of catalytic cycle. This procedure can be repeated at least 10
parameters. On the one hand, the presence of NO in the times (Figure S12) without detecting an increase in the
catalytic cycle after the BA oxidation (step 6) was confirmed by oxidation of the material. The metal impurities were also
running a catalytic test employing the [NO:BF4] complex as analyzed by ICP-MS during the cycling procedure. The initial
oxidant (standard vessel, air atmosphere, stoichiometric in crude catalyst (GA) displayed significantly higher content of
oxidant). In this particular case (Figure 5d), a complete metal contaminants than the catalyst after recycling (GA*,
conversion with full selectivity to the aldehyde was obtained Table S5) as a consequence of the additional washings and
as well. This simple test ratifies the cyclic nature of the successive treatment with fresh HNO3. Nevertheless, the
reaction due to the possibility of producing the intended recycled catalyst showed identical activity as the “fresh” one
products starting even from the last released substance (i.e even after ten recycling experiments, because there were no
NO), according to the proposed cycle (see Figure 4). On the differences detected in conversion within the experimental
other hand, O2 represents
a key intermediate of this error (Figure S12). These facts rule out again a potential effect
transformation since this gas is proposed to regenerate the of metal impurities on the catalytic performance of GA.
oxidant agents in the last step of the cycle (step 7). Obviously,
an inert atmosphere inhibits the reaction. As a matter of fact,
in this case no conversion was observed because the absence
Conclusions
of O2 prevented the propagation step to regenerate the NO2.
Finally, we also challenged GA in the catalytic oxidation of
aliphatic alcohols (Table 3), which are less reactive than BA.
Remarkably, after 2 h of reaction, the alcohols were fully
transformed into the nitrites, as demonstrated by the
detection of the sharp NMR peak at δ ≈ 6.0 ppm of the
withdrawn aliquots. This suggests that the oxidation of the
organic nitrite is likely the rate-determining step. Eventually,
GA converted all alcohols listed in Table 3 with >99 %
selectivity to the corresponding acid in 16 h. Even the less
reactive higher alcohols, such as 1-octanol, did not
compromise GA activity. Furthermore, the reactive groups of
the catalytic substrates were not affected during the oxidation:
for example, 3-butenoic acid was produced with 98%
selectivity starting from 3-butenol (i.e. less than 2% of the α,β-
aldehyde was detected). Diols, like 1,6-hexanediol, were also
oxidized to the corresponding adipic acid in 16 h.
By a judicious choice of the experimental conditions, GA can
catalyze with unprecedented efficiency the conversion of BA
either to the aldehyde with catalytic amounts of HNO3 as co-
catalyst or to the carboxyl acid, in open air conditions and
stoichiometric amount of HNO3. After thorough comparisons
with other carbocatalysts (even with metal nanoparticles) as
summarized in Table S3, GA unequivocally delivers an
outstanding catalytic activity. On top of that, GA can be
recovered after the reaction and reused in new catalytic runs
with uncompromised activity for at least 10 cycles.
Remarkably, GA demonstrated to be a versatile catalyst,
performing flawlessly on the oxidation of a variety of alcohols.
Accumulated experimental evidence and DFT calculations on
the reaction mechanism unraveled key aspects of the reaction:
i) the redox decomposition of HNO3 on the GA surface; ii) the
generation of key NOx intermediates and organic nitrites; iii)
the evolution of the intermediates on GA surface; iv) the
release of NO, which is oxidized by O2 to NO2 for re-entering in
the reaction cycle as propagation step, and iv) equilibrium
shifting by gas-release to direct the selectivity. Based on this
understanding, the critical parameters were identified and
optimized, turning the reaction fully selective to each of the
two possible products of the oxidation: aldehyde or acid (when
BA is employed), or carboxylic acids, in the case of aliphatic
alcohols.
Table 3. Scope of the catalysta
Substrate
Product
Timeb
2
Yieldc
2
O
OH
OH
16
2
>99
3
OH
O
16
2
>99
4
OH
Conflicts of Interest
OH
O
OH
16
2
98
-
There are no conflicts to declare
O
OH
O
OH
HO
16
>99
Acknowledgements
OH
This work was partially supported by the following projects:
Italian MIUR (PRIN, SMARTNESS, 2015K7FZLH), and MAECI
Italy-China bilateral project (GINSENG, PGR00953). M. B.
gratefully acknowledges the Clarin CoFund Program (ACA17-
a) 5 mg of GA, 1 mmol of alcohol, 2 mmol of HNO3 in 1,4-dioxane (0.5 M) at 90 oC,
b) h, c) %, determined by 1H-NMR to the corresponding product.
GA can successfully be recycled as catalyst of the BA oxidation. 29, 600196), funded by Gobierno del Principado de Asturias
After reaction, sample was recovered and washed profusely and Marie Curie Actions. MO acknowledges ERC Consolidator
with acetone to remove any contaminant (see Figure S9 for grant 683024 from the European Union’s Horizon 2020 and
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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25.
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D. Matochova, M. Medved’, A. BakandritsoVsi,ewT.ASrtticelekOlyn,linRe.
the Operational Programme Research, Development and
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Education, Youth and Sports of the Czech Republic. Mgr. Radka
Pechancová and Dr. David Milde for the digestion of solid
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