Nanocell type Ru?quinone core-shell catalyst for selective oxidation of alcohols to carbonyl compounds

Article abstract of DOI:10.1016/j.apcata.2020.117693

Selective aerobic oxidation of alcohols to corresponding carbonyl compounds is one of the most important challenges in the modern chemical industry. The existing metal based heterogeneous catalysts provide low selectivity due to over-oxidation of aldehydes to acids and esters. We have found that coating of Ru nanoparticles by disodium anthraquinone-2,6-disulfonate (SQ) results in selective oxidation of aliphatic, unsaturated and aromatic alcohols to aldehydes. Analysis of core-shell Ru?SQ catalyst shows strong interaction between Ru and SQ leading to change of their electronic state and structure. In-situ study of alcohol oxidation using FTIR and electrochemistry indicates on hydrogen abstraction by shell quinone species with hydrogen transfer by quinone to Ru core for water generation. Thus, the catalyst behavior mimics nano-electrocell by separation of oxidation reaction over quinone and reduction of oxygen over Ru providing higher selectivity to aldehyde.

Full text of DOI:10.1016/j.apcata.2020.117693

Journal Pre-proof
Nanocell type Ru@quinone core-shell catalyst for selective oxidation of
alcohols to carbonyl compounds
´
J.P. Zhao, W.Y. Hernandez, W.J. Zhou, Y. Yang, E.I. Vovk, M. Wu, N.
Naghavi, M. Capron, V. Ordomsky
PII:
S0926-860X(20)30286-6
DOI:
https://doi.org/10.1016/j.apcata.2020.117693
APCATA 117693
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Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
6 February 2020
22 May 2020
3 June 2020
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Please cite this article as: Zhao JP, Hernandez WY, Zhou WJ, Yang Y, Vovk EI, Wu M,
Naghavi N, Capron M, Ordomsky V, Nanocell type Ru@quinone core-shell catalyst for
selective oxidation of alcohols to carbonyl compounds, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117693
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Nanocell type Ru@quinone core-shell catalyst for selective
oxidation of alcohols to carbonyl compounds
J.P. Zhao1,2, W.Y. Hernández2, W.J. Zhou2, Y. Yang3, E.I. Vovk3, M. Wu2, N. Naghavi2,
M. Capron1*, V. Ordomsky2*
1Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité
de Catalyse et Chimie du Solide, F-59000 Lille, France.
2Laboratoire, Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464
CNRS-Solvay, 201108 Shanghai, China
3School of Physical Science and Technology, Shanghai Tech. University, 201210
Shanghai, China.
Graphical abstract
Highlights




Ru nanoparticles coated by anthraquinone-2,6-disulfonate (SQ) have been prepared
This catalyst selectively oxidizes different types of alcohols to aldehydes
Strong interaction between Ru and SQ species changing electronic states was shown
Mechanism involves abstraction by quinone of H with transfer to Ru core to form
water
ABSTRACT:

Selective aerobic oxidation of alcohols to corresponding carbonyl compounds
is one of the most important challenges in the modern chemical industry. The existing
metal based heterogeneous catalysts provide low selectivity due to over-oxidation of
aldehydes to acids and esters. We have found that coating of Ru nanoparticles by
disodium anthraquinone-2,6-disulfonate (SQ) results in selective oxidation of aliphatic,
unsaturated and aromatic alcohols to aldehydes. Analysis of core-shell Ru@SQ catalyst
shows strong interaction between Ru and SQ leading to change of their electronic state
and structure. In-situ study of alcohol oxidation using FTIR and electrochemistry
indicates on hydrogen abstraction by shell quinone species with hydrogen transfer by
quinone to Ru core for water generation. Thus, the catalyst behavior mimics nano-
electrocell by separation of oxidation reaction over quinone and reduction of oxygen
over Ru providing higher selectivity to aldehyde.
Keywords: oxidation; alcohols; Ru; quinone; nanocell
Introduction
The selective oxidation of alcohols to versatile intermediates for the production of
chemicals such as aldehydes and ketones remains a challenging task in the industry. It
is of special interest the utilization of molecular oxygen as the green oxidant in the
presence of a heterogeneous catalyst, thus favoring the atom economy in contrast to the
use of toxic and expensive stoichiometric oxidants [1-5].
Different heterogeneous catalytic systems based on Pt [6-8], Pd [9, 10], Au [11,
12] have been reported for oxidation of alcohols. Among metallic catalysts, Ru has
shown to be quite efficient in oxidation reactions [13-15]. However, low selectivity to
aldehydes or ketones over heterogeneous catalysts is still a challenging problem due to
formation of corresponding acids and esters [16]. The oxidation mechanism for metallic
catalysts usually follows an oxidative dehydrogenation pathway, where both
abstraction of hydrogen from alcohols and oxygen reduction processes (regeneration of
the active site) simultaneously occur on the metal surface [17]. The aldehydes can be
further oxidized over the same active sites to acids with decrease of the selectivity and

generation of waste products. In general, these catalysts are very efficient in the
selective oxidation of activated alcohols (e.g. benzyl and allyl alcohols) but less active
and selective for oxidation of aliphatic primary alcohols [18-21].
Recently, it has been found that metal-free electron deficient organic compounds
like TEMPO (2,2,6,6-Tetramethylpiperidinyloxy) and DDQ (2,3-Dichloro-5,6-
Dicyanobenzoquinone) can be used for dehydrogenation of alcohols in stoichiometric
way, producing aldehydes and ketones together with the hydrogenated form of TEMPO
and DDQ [22-25]. However, the use of large amounts of organic oxidants and
difficulties related to their separation and regeneration strongly hinders the viability of
this oxidation approach. To solve this drawback, the in-situ regeneration of these type
of organic oxidants has been proposed, involving their combination with additional
electron transfer mediators (ETM) and the utilization of a different terminal oxidant
(e.g. oxygen). For example, Hu et al. explored an aerobic metal-free alcohol oxidation
with a catalytic system consisting of TEMPO/Br2/NaNO2, where the Br2/HBr redox
pair oxidizes TEMPOH with subsequent regeneration by NO2 [26]. In 1977, the DDQ-
catalyzed reaction has been reported for the transformation of allylic alcohols to α,β-
unsaturated ketones using 10 mol% DDQ in the presence of 30 mol% periodic acid
under biphasic conditions at room temperature [27]. Helquist et al. illustrated similar
reactivity using Mn(OAc)3 and DDQ for oxidation of allylic alcohols and benzylic
alcohols to the corresponding aldehydes and ketones at mild reaction conditions [28].
Despite this selective oxidation methodology favors the regeneration of the organic
oxidant, it still requires the presence of metal salts in the final liquid product. Earlier,
we have proposed application of anthraquinones as a catalyst for non-metallic oxidation
of alcohols, however, the reaction proceed at relatively high temperature with oxidation
of activated alcohols [29].
Hereby, we propose the preparation and application of hybrid heterogeneous
catalyst characterized by the presence of an organic shell in charge of the abstraction of
hydrogen during the selective oxidation of alcohols to carbonyl compounds and a
metallic core able to regenerate organic shell of the catalyst (Scheme 1). We have found
that deposition of sulfonated anthraquinone over Ru nanoparticles in core-shell catalyst
results in selective oxidation of alcohols to aldehydes. The alcohols are selectively
oxidized (oxidatively dehydrogenated) over the non-metallic anthraquinone, whereas
the production of water takes place on the metallic core, in presence of oxygen. This

system mimics the behavior of an electrochemical cell with oxidizing nano-anode in
the form of the shell and reducing nano-cathode in the form of the core.
Experimental Section
Reagents
The chemicals RuCl3·H2O (99.9 wt%, CAS: 14898-67-0), γ-Al2O3 (99 wt%, CAS:
1344-28-1), CTAB (99 wt%, CAS: 57-09-0), NaBH4 (96 wt%, CAS: 16940-66-2),
biphenyl (99.5 wt%, CAS: 92-52-4), benzyl alcohol (99 wt%, CAS: 100-51-6), 4-
chlorobenzyl alcohol (98 wt%, CAS: 873-76-7), 4-methylbenzyl alcohol(98 wt%, CAS:
589-18-4), cinnamyl alcohol (98 wt%, CAS: 104-54-1), 1,2-hexanediol (98 wt%, CAS:
6920-22-5), furfuryl alcohol (98 wt%, CAS: 98-00-0), HMF (95 wt%, CAS: 67-47-0),
1-octanol (>99 wt%, CAS: 111-87-5), 2-octanol (98 wt%, CAS: 6920-22-5),
cyclohexanol (99 wt%, CAS: 108-93-0), 1,4-benzoquinone (98 wt%, CAS: 106-51-4),
2-ethylanthraquinone (97 wt%, CAS: 84-51-5), 1,4-naphthoquinone (97 wt%, CAS:
130-15-4), anthraquinone-2-carboxylic acid (98 wt%, CAS: 117-78-2), disodium
anthraquinone -2,6-disulfonate (95 wt%, CAS: 212-719-9) are purchased from J&K
Scientific Ltd.
Synthesis of Catalysts
Reverse micellar microemulsions of CTAB/1-hexanol/water (as shown in Table
S1, SI) have been prepared [15, ]. Briefly, 10 g of microemulsion B containing 0.14 g
NaBH4 has been dropwise added to 10 g of microemulsion A containing 0.2 g RuCl3
during 1 h under vigorous stirring. Ru nanoparticles with uniform size distribution have
been formed inside the micelles. The catalyst Ru NPs has been separated by
centrifugation, washed with hot water for 5 times and dried at 60 °C in vacuum oven
for 12 h.
The catalysts Ru@SQ with varied shell thickness have been prepared by
deposition of SQ over Ru NPs. Briefly, different amounts of SQ have been
ultrasonically dissolved in 3 mL of water and subsequently mixed with 100 mg of Ru
NPs. The water has been removed from resulting slurry by rotary evaporator and
consequently dried at 60 °C in a vacuum oven overnight. A series of samples Ru@SQ-
X% (X = 10, 25, 50 and 80) have been prepared, where X is the theoretical weight
percentage of SQ. The sample with monolayer coating Ru@SQ-monolayer has been
prepared by deposition of 2 wt. % of SQ over Ru NPs.

Catalyst Characterization
The morphologies and particle size distribution of the catalysts and elemental
mapping analysis has been performed using JEOL 2100 high-resolution transmission
electron microscope (HRTEM) with an accelerating voltage of 200 kV.
X-ray diffraction (XRD) was used to characterize the lattice structure. XRD
patterns were recorded on a Rigaku D/max-2200/PC diffractometer using Cu Kα
radiation (λ = 1.5418 Å) operating at 20 kV.
X-ray photoelectron spectroscopy (XPS) spectra were acquired using a Thermo
Scientific ESCALAB 250Xi X-ray Photoelectron Spectrometer incorporating a 165
mm hemispherical electron energy analyzer. The incident radiation is Monochromatic
Al Kα X-rays (1486.6 eV) at 225 W (15 kV, 15 ma). The deconvolution was performed
on Thermo Scientific Avantage Software.
Thermogravimetric analysis (TGA) measurements were performed using a
SDT 2960 analyzer from TA instrument under air flow (50 mL/min) with a ramping
rate of temperature of 10 °C/min.
In-situ FTIR spectra were recorded using a Thermo Fisher Scientific Nicolet 6700
FTIR (16 scans at a resolution of 4 cm−1) equipped with MCT detector, high-vacuum
o
system and heating zone. The catalyst pellet has been pretreated at 150 C for 1 h in
vacuum at 10-5 Torr and cooled down to room temperature. The catalyst has been
treated by 20 Torr hydrogen or furfuryl alcohol vapors at 25 oC~100 oC for 20 min.
1
The H NMR spectra were measured using a Bruker ARX 400 or ARX 100
spectrometer at 400 MHz (1H) and 100 MHz (13C).
Catalysis
The catalytic tests were conducted in a 30 mL autoclave. For a typical test of 1-
octanol oxidation, 50 mg catalysts and 3 g of 1-octanol have been added in the reactor,
which was pressurized with 10 bar of O2 and heated to 100 ⁰ C for 1~48 h under
continuous stirring. The reference experiments with Ru NPs, pure SQ and the mixture
of Ru and SQ were carried out under the same reaction conditions. Other alcohols have
been oxidized in a similar way.
The catalytic oxidation of aldehyde has been performed in the same autoclave. The
catalysts containing 5 mg of Ru and 0.3 g of octanal were placed in the reactor with air

as oxidant at atmospheric pressure and solvent-free conditions. The mixture was stirred
at 60 °C for 20 min.
The products of the reaction were analyzed using Agilent 7820A gas
chromatography (GC) equipped with a HP-5 column and biphenyl as internal standard.
Cyclic voltammetry
Cyclic voltammetry (CV) was performed with a three-electrode setup controlled
by a potentiostat (Reference 3000, Gamry Instruments Inc.). The synthesized catalysts
containing Ru NPs or SQ were dispersed in ethanol to form a well-dispersed ink (10
mg/ml) using Nafion 117 solution (5 wt%) as a binder. 100 μL of the ink was deposited
onto a polished and pre-cleaned glassy-carbon electrode with a working area of 0.1256
cm2. For Ru@SQ-80%, the working electrode was prepared by coating of electrode
surface by 20 μL Ru NPs and subsequently 80 μL SQ ink. Ag/Ag+ was used as a
reference electrode in all measurements while Pt wire as a counter electrode. CVs were
carried out at 25 °C in 8 mL solution of 0.1 M LiClO4 in DMSO saturated with Ar at a
constant sweep rate of 100 mV/s.
Results and Discussion
Catalytic performances
Water-in-oil microemulsions built of water, hexanol and surfactant
cetyltrimethylammonium bromide (CTAB) with RuCl3 and sodium borohydride have
been used for the synthesis of metal nanoparticles. Ru nanoparticles form aggregates
and the size of Ru nanoparticles measured by TEM varies in the range 2-3 nm
(Figure 1).
Ru@quinone catalysts with core-shell structure have been prepared using rotary
evaporator for deposition of quinone over Ru nanoparticles by removal of solvent. The
prepared samples are denoted as Ru@quinone-X % (X=10, 25, 50, 80), where X is the
weight percentage of quinone in the samples. Figure 1 demonstrates TEM images of
Ru nanoparticles coated by different amount of disodium anthraquinone-2,6-
disulfonate (SQ) in Ru@SQ catalysts. There is a clear contrast between the heavier
atom of Ru in the core and the lighter SQ shell. Increase of the content of SQ phase
leads to increase of the thickness of the shell from 1 to 3 nm.
The catalytic performance of core-shell catalysts with different content of SQ has
been compared with the reference Ru nanoparticles and pure SQ in selective oxidation

of 1-octanol. Besides aldehyde as the main product of the reaction, there is an octanoic
acid as the product of deeper oxidation of aldehyde and octyl octanoate as the product
of further esterification (Scheme 2). 1-Octanol in the presence of acid also undergoes
etherification reaction to dioctyl ether.
The catalytic activity of 1-octanol oxidation is the highest over Ru NPs with
decrease of the activity after deposition of SQ (Figure 2). Increase of SQ loading leads
to decrease of the catalytic activity. Almost no activity has been observed for pure SQ.
Thus, presence of Ru NPs in the core of the catalyst affects the catalytic activity in
alcohol oxidation. Comparison of the catalytic activity of core-shell Ru@SQ-80% with
mechanical mixture Ru+SQ-80% indicates on SQ coating as the reason of low activity
(Figure 2). It is interesting to note that activity normalized by Ru is comparable in a
series of core-shell Ru@SQ catalysts (Figure 2). It means that the thickness of SQ
coating does not affect the catalytic activity and the rate limiting step could be in Ru-
SQ interface zone. Non-coated Ru NPs provide significantly higher activity indicating
on different mechanism of oxidation with and without SQ coating.
The parent Ru NPs demonstrate (Figure S1, SI) strong decrease of the selectivity
to octanal with increase of the conversion of 1-octanol with formation of acid and ester
as side products. Figure 3 shows comparison of the selectivities over Ru NPs with core-
shell Ru@SQ catalysts at the similar conversions about 60 %. At this conversion, the
selectivity to octanal is only about 20 % over Ru NPs. Pure SQ provided even lower
selectivity due to intensive over-oxidation to acid and ester. It is interesting to note that
the selectivity to octanal was gradually increasing with increase of the content of SQ in
the core-shell catalysts from 30 % for Ru@SQ-10% to 74 % for Ru@SQ-80%. Thus,
the selectivity increases up to 3 times in comparison with pure Ru NPs and SQ. The
catalysts with low loading of SQ still could contain accessible Ru surface, which
explains lower selectivity. High selectivity at high SQ loading could be explained by
full coverage of Ru surface by SQ.
Usually the aldehyde formation during 1-octanol oxidation is accompanied by
subsequent non-catalytic and catalytic oxidation, which results in the formation of acid
and esters. In our previous study of alcohols oxidation over Ru NPs we have
demonstrated protective role of RuOx species from adsorption and deeper oxidation of
aldehydes to acids [15]. Similarly, the presence of SQ coating markedly reduces the
exposure of active Ru NPs, which is likely to suppress the deep catalytic oxidation
reaction. In order to verify this concept, we have conducted oxidation of 1-octanol

using mechanical mixture of Ru NPs with 80 % of SQ (Ru+SQ-80%). It is interesting
to note that the selectivity to aldehyde was significantly lower than in the case of core-
shell system and comparable with Ru NPs.
SQ is almost insoluble in 1-octanol, however, it could be soluble during the
catalytic reaction in the presence of water. The catalytic stability has been studied by
separation of the catalyst Ru@SQ-80% after the first cycle with subsequent test using
fresh 1-octanol for different reaction time to provide comparable conversions. Figure
S2, SI demonstrates decrease of the selectivity to octanal from 75 to 37 % for the second
cycle at the selectivity 23 % for pure Ru NPs. It means partial dissolution of SQ during
the catalytic reaction, which makes catalytic performance similar to Ru NPs. It is
interesting to note that the catalytic performance of Ru@SQ-80% after two cycles could
be recovered by recoating of SQ shell (Figure S2, SI). It confirms solubility issue
during the catalytic reaction for this composite material, however, provides simple way
to recover the catalytic performance.
Characterization of Catalysts
Powder X-ray diffraction (XRD) has been used to study the crystallinity of the
catalysts. Figure S3, SI demonstrates broad peaks at 38.4 °, 42.2 °, 43.8 °, 58.2 ° and
69.4 °, which correspond to (100), (002), (101), (102) and (110) planes, respectively,
of the hexagonal close-packed (hcp) small size Ru metal nanoparticles. SQ does not
change the patterns of Ru, however, increase of SQ content leads to appearance of new
signals at 16.2°, 19.5° and 26.2° corresponding to the indices of (210), (201) and (301),
respectively, of orthorhombic phase of crystalline SQ [30, 31].
The composition and thermal stability of the Ru NPs and Ru@SQ catalysts has
been examined by TGA (Figure S4, SI). The curves show weight losses in two steps.
o
At low temperature (25~300 C) the weight loss is the result of removal of water, 1-
hexanol and CTAB used for the catalyst preparation. Subsequently, the weight losses
of about 45 wt%, 23 wt%, 14 wt% and 9 wt% for Ru@SQ-X% samples (X=80, 50, 25,
10), respectively, have been observed between 400 °C and 600 °C, which could be
ascribed to the combustion of SQ layer. The lower weight loss in comparison with the
theoretical content of SQ should be ascribed to the generation of sodium sulfate as a
product of oxidation.
XPS analysis of Ru nanoparticles covered by SQ (monolayer and 80%) in
comparison with Ru nanoparticles and pure SQ has been performed to investigate the

electronic state of the ruthenium species (Figure 4). According to the XPS analysis
(Table S2, SI), the ratios of S/Ru in both Ru@SQ-monolayer and Ru@SQ-80%
samples are 0.1:1.0 and 2.3:1.0, respectively, which correlates well with the difference
in SQ loading. Ru 3p core level spectra show the presence of ruthenium in two oxidation
states: metallic Ru0 (Ru 3p3/2 BE 462.0-463.3 eV) and lower contribution of oxidized
Ru species with lower electronic density (Ru 3p3/2 BE 464.6-465.5 eV). This
assignment was made according to earlier studies of various Ru compounds including
metallic Ru, RuO2 and Ru(OH)3 demonstrating different Ru 3p3/2 BEs [32].
After SQ deposition over the surface of Ru NPs, the metallic Ru 3p doublet shifts
to lower binding energy values. Additionally, the oxidized Ru peak appears
demonstrating partial oxidation of Ru. Increase of SQ deposition leads to decrease of
the intensity of Ru peaks due to the attenuation of Ru signal by SQ shell. It should be
noted that with the increase of SQ coating the metal Ru 3p doublet shifts to lower
binding energy values from 462.7 eV to 462 eV. This shift can be associated with the
effect of electron transfer from SQ to Ru leading to increase of the electronic density
over Ru.
Pure SQ sample demonstrates only one O 1s peak at 531.8 eV although it is
expected to have two types of oxygen in SO32- and C=O. Most probably it is due to the
strong interaction and electron transfer between oxygen in SO32- [30] and oxygen in
C=O [31] groups in SQ. Ru NPs demonstrate a similar O 1s peak with 531.8 eV BE,
which is associated with RuO2 or surface Ru(OH)3 [33]. SQ coating over Ru NPs leads
to appearance of two additional peaks: the broad peak at 534.1 eV, which can be
assigned to oxygen in sulfo groups [34], and well defined peak at 530.2 eV (529.8 eV
after 80% SQ loading). The 530.2 eV peak we associate with the bridge Ru-O-SQ
species, which appearance indicates strong interaction between SQ and Ru (Scheme 3).
This peak is attenuated as well as Ru 3p doublet with the increase of SQ loading.
Structure-catalytic performances relationship
In order to understand the role of Ru in the core-shell catalyst for oxidation of
alcohols, we have varied the metal core. We have used Pd and Pt nanoparticles prepared
in the same way for subsequent deposition of SQ to check the effect of metal of the core
on the catalytic performance during oxidation of 1-octanol. Figure S5, SI demonstrates
selectivity-conversion curves for Pd@SQ and Pt@SQ catalysts with different content

of SQ in comparison with Pd and Pt nanoparticles. In comparison with Ru catalysts, the
deposition of SQ over Pt and Pd leads either to low effect in selectivity or significant
decrease of the selectivity to aldehyde with increase of the contribution of acid and ester.
Pt and Pd are well-known catalysts for hydrogenation/dehydrogenation reactions
in comparison with Ru, which is used for hydrogenolysis reactions. Pt and Pd
nanoparticles do not demonstrate strong interaction with SQ as demonstrated by XPS
analysis of the samples before and after SQ impregnation (Figure S6, SI). The reason
could be in strong oxophiliticy of Ru leading to easy adsorption of oxygen on the
surface with formation of Ru-O species. In order to confirm it, we have examined O 1s
XPS spectra. SQ covered Pt and Pd nanoparticles in O 1s spectra do not show peak
around 530 eV, which has been assigned to Me-O species (Figure S7, SI). Pd 3d and
Pt 4f spectra demonstrate the metallic state of the elements with no changes after SQ
loading. Thus, one can conclude that there is no interaction between Pt or Pd and SQ in
contrast to Ru nanoparticles.
Thus, strong interaction of Ru with SQ could be the key factor of high selectivity
during oxidation of alcohols to carbonyl compounds. The mechanism of the reaction
has been further studied using FTIR and electrochemical methods.
Mechanism
According to our earlier studies, oxidation of alcohols over Ru nanoparticles
proceed according to Mars-van Krevelen mechanism by interaction of surface RuOx
species over Ru nanoparticles with alcohol leading to the aldehyde formation and
subsequent re-oxidation of Ru nanoparticles [15]. In order to verify if SQ could be
involved in this mechanism of oxidation of alcohols, we have performed the 1-octanol
oxidation reaction in inert atmosphere. The only product of the reaction is octanal.
Figure 5 demonstrates the conversion of 1-octanol depending on the content of SQ in
the catalyst. The yield of octanal was gradually decreasing from 9 to 1 % with increase
of the content of SQ up to 80 %. It indicates on different mechanism of alcohol
oxidation in this case and absence of direct contact of 1-octanol with Ru core.
In addition, considering alcohols oxidation to acids as consecutive reaction with
intermediate aldehydes formation, the higher selectivity to octanal could be explained
by suppression of further aldehyde oxidation over core-shell catalysts. In order to
support this assumption, we have performed model reaction of octanal oxidation at mild
reaction conditions to avoid autoxidation (Figure 6). Blank experiments without Ru

o
species show almost no conversion of octanal at 60 C and air instead of oxygen. Ru
NPs provide conversion of octanal 26 % to acid. Increase of the amount of coated SQ
leads to suppression of octanal oxidation and correlates with the results of 1-octanol
accessibility to Ru core. It proves that SQ shell protects Ru core from interaction with
aldehydes and suppresses over-oxidation, which could be the reason of high selectivity.
The key question in this case is how Ru@SQ catalyst provides oxidative
dehydrogenation of alcohols. In-situ FTIR has been used to study the state of SQ during
interaction with alcohol. FTIR spectrum of parent SQ (Figure 7) shows the peak at
1277 cm-1 attributed to the sulfonate group [30]. The peak at 1514 cm-1 can be assigned
to the stretching vibration of aromatic conjugate C=C band [35]. The strong
characteristic peaks at 1695 cm-1 and 1624 cm-1 can be assigned to the stretching C=O
vibrations in non-chelated and a chelated quinone carbonyl group, respectively [36].
The non-chelated quinone group disappears after core-shell structure fabrication, which
could be explained by strong interaction between C=O and Ru NPs in accordance with
XPS results (Figure 4). It is interesting to note that the spectrum of mechanical mixture
of SQ with Ru is similar to pure SQ and confirms specific interaction of SQ with Ru
only in core-shell material (Figure S8, SI).
Presence of hydrogen in IR cell leads to decrease of the intensity of the C=O group
at 1624 cm-1, which could be recovered after the heating treatment at 100 °C in vacuum.
It indicates on easy reduction of C=O to C-OH and subsequent decomposition in the
absence of hydrogen.
Pure SQ and mechanical mixture of SQ with Ru does not show any interaction
with hydrogen (Figure S8, SI). Hydrogenation of SQ could be initiated by hydrogen
dissociation over Ru [37] with subsequent hydrogen transfer to SQ through Ru-O-SQ
linkers. In order to study hydrogen transfer from alcohol molecule, we have introduced
furfuryl alcohol vapors in the cell with subsequent heating from 25 to 100 °C (Figure
7). The carbonyl group characterized by the peak located at 1624 cm-1 started to
decrease in intensity at 80 °C in comparison with no effect for pure SQ or mechanical
mixture Ru+SQ-80% (Figure S8, SI). Thus, carbonyl group of SQ in core-shell catalyst
can be involved in hydrogen transfer from alcohol with generation of aldehyde and
hydrogenated carbonyl group in the shell. Subsequent transfer of hydrogen through
carbonyl group of SQ to Ru and decomposition with regeneration of C=O in SQ could
be proposed as mechanism of oxidation of alcohols without direct contact of metal with
alcohol (Scheme 1).

The advantage of SQ in comparison with the strong electrophilic organic
scavengers like TEMPO or DDQ is its intermediate reduction potential leading to lower
stability of hydrogenated form, which results in efficient hydrogen transfer and
regeneration of SQ over Ru surface.
Electrochemical experiments have been used to further evaluate the state of SQ in
the core-shell catalyst. Cyclic voltammograms (CVs) of the bare glassy carbon (GC)
electrode, Ru NPs/GC, SQ/GC, and Ru@SQ-80%/GC electrodes in Ar-saturated
DMSO solution with 0.1 M LiClO4 as electrolyte were studied in the presence/absence
of 0.1 M furfuryl alcohol (FA). The cyclic voltammetry data for SQ are characterized
by two anodic and cathodic waves between -1.6 V and -1 V vs Ag/Ag+ corresponding
to the two consecutive one-electron reversible transfer process of the oxydo-reduction
of anthraquinones to hydroquinone (Figure 8, Table 1). However, when 0.1 M of
furfuryl alcohol (FA) is added a 3rd anodic wave occurs at -0.14 V for SQ-FA and at -
0.21 V for Ru@SQ-80%-FA corresponding to the oxidation of FA.
At the same time, blank Ru NPs/GC and bare GC electrodes have no CVs
responses (Figure S9, SI). For the Ru@SQ-80%-FA, the lowest oxydo-reduction peak
around -1.62 V shifts to higher voltage -1.25 V, which shows that only one electron
transfer is involved in the oxidation of FA. This leads to increase of the current density
of FA oxidation at -0.21 V. Moreover, the resulting current density for FA oxidation
over Ru@SQ-80% is much higher in comparison with pure SQ. The negative shift and
higher current density of this peak for Ru@SQ-80% combined with the shift of the peak
of SQ to higher voltages indicates increase of the catalytic activity of Ru@SQ-80% in
comparison to pure SQ for the oxidation of FA. The improved electrocatalytic activity
in oxidation of FA could be due to the presence of Ru in the core, which accelerates the
electron transfer via improved conductivity and change of the electronic state of SQ in
the core-shell catalyst observed by FTIR and XPS.
Substrate Scope
The selective oxidation of a wide range of alcohols has been studied over Ru@SQ-
80% catalyst in the presence of O2. For benzyl alcohol, methyl-substituted and
chlorinated benzylic alcohol conversions of 92 %, 95 % and 95 % have been achieved
within 6 h, respectively (Table 2). The selectivity to the target products was higher than
95 %. Similarly, unsaturated cinnamyl alcohol was converted to cinnamaldehyde with
95 % selectivity. In the case of oxidation of furfuryl alcohol and 2,5-

hydroxymethylfurfural (HMF) the selectivity about 90 % was achieved to furfural and
2,5-diformylfuran (DFF), respectively, at high conversion. Non-activated aliphatic
alcohol (i.e., 1-octanol) and cyclic alcohol (i.e., cyclohexanol) over Ru@SQ-80% have
been converted to carbonyl compounds with a moderate activity and excellent
selectivity. Secondary 2-octanol, can be oxidized with a conversion of >90 % and
selectivity of 95 % to corresponding ketone, which is comparable to the selectivity
achieved using TEMPO [38].
The excellent catalytic activity towards various oxygenate substrates demonstrates
that the uniform organic shell around the metal nanoparticles suppressed over-oxidation
of aldehydes toward acids and esters. Nanocell Ru@SQ exhibits much better catalytic
behavior than conventional Ru catalysts [39]. Isolation of metal nanoparticles from
direct contact with alcohols decreases activity, however, results in significant increase
of the selectivity toward carbonyl compounds. The catalytic system can be further
optimized by variation of the shell type for more efficient alcohols dehydrogenation
and hydrogen transfer to combine high activity and selectivity of the catalyst.
Conclusion
In conclusion, we have demonstrated that it is possible to build nanocell with core-
shell structure containing Ru nanoparticles in the core and electro-deficient
anthraquinones in the shell for selective oxidation of alcohols to carbonyl compounds.
XPS analysis shows strong interaction between quinone species and Ru nanoparticles
leading to transfer of the electronic density to Ru and easy activation of alcohols over
the shell. In-situ mechanism study confirms transfer of hydrogen to carbonyl groups of
quinone species from alcohol with subsequent transfer to the core. This concept can be
used for other applications, where contact of the target product of the reaction with
metal results in decrease of the selectivity.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgments

J.P. Zhao thanks Solvay and CIFRE (2016-1082) for providing him a stipend for
PhD study
CRediT authorship contribution statement
V. Ordomsky, W.Y. Hernández and M. Capron conceived the idea for this work. All
authors contributed to the design of the experimental set-up and experimental
procedures. J.P. Zhao and W.J. Zhou prepared the catalysts and performed catalytic
tests. Y. Yang and E.I. Vovk performed XPS analysis. M. Wu and N. Naghavi
performed electrochemical measurements. J.P. Zhao and V. Ordomsky wrote the
draught, and all the authors worked on improving the manuscript
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Figure 1. TEM images of (a) Ru NPs; (b) SQ; (c~f) Ru@SQ-X% catalysts, X=10, 25,
50, 80.

Activity,mol octanol/g catalyst · h
Activity,mol octanol/mol Ru · h
18
16
14
12
10
8
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
6
4
2
0
Figure 2. The catalytic activity of 1-octanol oxidation over different catalysts. The
activity was calculated at low conversion (~15%) by the converted moles of 1-octanol
(red) per gram of the whole catalyst per hour; (blue) per mol of Ru per hour. Reaction
o
conditions: 3.0 g 1-octanol, 50 mg catalyst, solvent-free, p(O2)=10 bar, T=100 C,
1~36h.

Octanal
Octanoic acid
Octyl octanoate
Ether
Selectivity, mol.%
Conversion of 1-octanol, mol.%
100
80
60
40
20
0
75
60
45
30
15
0
Figure 3. The selectivity during 1-octanol oxidation over different catalysts. Reaction
o
conditions: 3.0 g 1-octanol, 50 mg catalyst, solvent-free, p(O2)=10 bar, T=100 C,
1~36 h.

O 1s
Na KLL
Ru 3p
(b)
(a)
531.8
462.0
464.6
529.8
Ru@SQ-80%
Ru@SQ-80%
534.1
530.2
462.7
Ru@SQ-monolayer
465.5
Ru@SQ-monolayer
Ru NPs
531.8
463.3
SQ
Ru NPs
490
480
470
460
450
538 536 534 532 530 528
Binding energy, eV
Binding energy, eV
Figure 4. XPS spectra of Ru NPs, Ru@SQ-monolayer and Ru@SQ-80%. (a) Ru 3p, (b)
O 1s core level spectra.

10
8
6
4
2
0
Figure 5. Oxidation of alcohol by Ru NPs, pure SQ and Ru@SQ-X% catalysts in inert
atmosphere. Reaction conditions: amount of catalyst equivalent to 50 mg of Ru, 0.1 g
1-octanol, 2.0 g toluene as solvent, T=100 oC, 3 h.

25
20
15
10
5
0
Figure 6. Oxidation of 1-octanal with different catalysts. Reaction conditions: amount
of catalyst equivalent to 5 mg of Ru, 0.3 g octanal, solvent-free, air atmospheric
pressure, T=60 oC, 20 min.

1326 cm-1
1277 cm-1
1624 cm-1
1695 cm-1
1514 cm-1
1374 cm-1
SQ
Ru@SQ
Ru@SQ-Vacuum-100oC
Ru@SQ-H2-25oC
Ru@SQ-Vacuum-100oC
Ru@SQ-FA-25oC
Ru@SQ-FA-40oC
Ru@SQ-FA-60oC
Ru@SQ-FA-80oC
Ru@SQ-FA-100oC
1800
1650
1500
1350
1200
Wavenumber(cm-1)
Figure 7. In-situ IR spectra of Ru@SQ-80% by H2 and furfuryl alcohol (FA) treatment
at different temperatures

0.03
0.02
0.01
0.00
-0.01
-0.02
Ru@SQ-80%
Ru@SQ-80%-FA
SQ
SQ-FA
Epa2
Epa3
Epa1
Epc1
Epc2
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Ewe/V vs. Ag/Ag+
Figure 8. Cyclic voltammograms of the studied catalysts on a glassy carbon electrode
in 0.1 M LiClO4 in the DMSO solution with/without 0.1 M furfuryl alcohol (FA) inside
in the range of -2.0 to 0.8 V at 100 mV/s scan rate.

Scheme 1. Proposed mechanism of alcohol oxidation by nanocells.

Scheme 2. The reaction of alcohols oxidation.

Scheme 3. Scheme of interaction between Ru and SQ.

Table 1. The values of cathodic Epc and anodic Epa peak potentials in DMSO solution.
Anodic potential (V vs. Ag/Ag+) Cathodic potential (V vs Ag/Ag+)
Epa1
Epa2
Epa3
Epc1
Epc2
SQ
SQ-FA
Ru@SQ-80%
Ru@SQ-80%-FA
-1.54
-1.51
-1.56
-1.12
-1.13
-1.08
-1.11
-1.62
-1.61
-1.71
-1.25
-1.19
-1.18
-1.22
-1.05
-0.14
-0.21

Table 2. Substrate scope oxidation by Ru@SQ-80%
Conv
Alcohols
/g
Target
T
/oC
Time
/h
Sele.
/%
Alcohols
.
Molecule
/%
1.0
1.0
1.0
100
100
100
6
6
6
92
95
95
95
95
95
a
a
a
1.0
0.5
0.4
100 18
90
80
69
95
92
89
a
90
3
b
110 16
b
c
c
3.0
1.0
1.0
100 36
115 60
115 60
57
74
>90
>90
>90
>90
a
a Reaction conditions: catalyst 50.0 mg, toluene 2.0 g as solvent.
b Reaction conditions: catalyst 50.0 mg, toluene 1.0 g + ethanol 1.0 g as solvent.
c Reaction conditions: catalyst 50.0 mg, solvent-free.