Eco-friendly electrocatalytic oxidation of alcohols on a novel electro generated TEMPO-functionalized MCM-41 modified electrode

Article abstract of DOI:10.1039/c4gc01303d

The development of a highly efficient and waste-free system for the selective oxidation of alcohols without using expensive transition metal catalysts is an extremely important and highly challenging area in organic chemistry and particularly in the chemi

Full text of DOI:10.1039/c4gc01303d

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Journal Name
RSCPublishing
ARTICLE
Eco-friendly Electrocatalytic Oxidation of Alcohols
on a Novel Electro-Generated TEMPO- Functional-
ized MCM-41 Modified Electrode
Cite this: DOI: 10.1039/x0xx00000x
Babak Karimi,*a Mohammad Rafiee,*b Saber Alizadeh, and Hojatollah Valib
Received 00th January 2012,
Accepted 00th January 2012
The development of highly efficient and wasteꢀfree system for selective oxidation of alcohols
without using expensive transitionꢀmetal catalyst is an extremely important and ground
challenging area in organic chemistry and in particular for chemical industry. In this study, a
novel procedure for an environmentally friendly electrochemical oxidation of alcohols is
introduced, where thin layer of ordered mesoporous silica (MCMꢀ41) with well oriented
channels is constructed on the electrode surface and then functionalized with 2,2,6,6ꢀ
tetramethylpiperidineꢀ1ꢀoxyl (TEMPO) as an electroꢀactive organocatalyst. The electroꢀ
catalytic system operates efficiently in mild bicarbonate solution, at room temperature, without
the need to any coꢀcatalyst, which provides very fast, simple, selective and wasteꢀfree protocol
for the oxidation of a wide variety of alcohols. The proposed modified electrodes could be
easily employed in 20 mmol scale in one reaction run and shows very good stability and can be
successfully reꢀused in several runs. In term of electroꢀcatalytic activity, this system enables
unprecedented turnover frequencies (TOF) up to 3070 hꢀ1, which is much superior to the entire
ever reported nitroxyl radical under chemical, electrochemical, or aerobic oxidation
DOI: 10.1039/x0xx00000x
www.rsc.org/
conditions
.
and or halogen sources as coꢀcatalyst in order to ensure the fast
electron transfer from alcohol to molecular oxygen through a
catalysis relay mechanism.3 The essential role of these NOX and
or halogen sources can be simply understood by large oxidation
Introduction
The development of efficient catalytic systems for the selective
oxidation of alcohols is not only a major concern in basic
chemistry research, but it is also a grand challenge for the
chemical industry. In this context, heterogeneous compared to
homogeneous catalysis offers greater benefits for largeꢀscale
operations owing to an easier separation and recycling of the
catalysts as well as reduced metal contamination in the final
products. As a consequence, a significant effort has been
dedicated into the development of new, efficient and metalꢀ
based catalysts in recent years by emphasizing the use of
molecular oxygen as the most environmentally benign oxidant.1
In this way, the attentions have been mainly directed toward the
designing suitable support to improve the catalyst performance
while minimizing the catalyst deactivation arising from metal
leaching and/or morphological changes of the catalyst.2
Although significant advances have been achieved in the field
of metal/O2 heterogeneous catalyst systems, many of them are
still environmentally harmful due to the presence of toxic trace
of metals in the final products and/or catalyst deactivation. It is
therefore highly desirable to replace them with metalꢀfree
systems and greener alternatives. Along this line, several
seminal metalꢀfree aerobic oxidation protocols have been
reported, which centrally employ 2,2,6,6ꢀtetramethylpiperidineꢀ
1ꢀoxyl (TEMPO) as catalysts in combination with varied NOX
gap between molecular oxygen and TEMPO (Figure 1).
OH
H2O
NO2
Br2
R1
R2
N
O
O
N
O2
HBr
NO
R1
R2
OH
Figure 1. Proposed pathway for the aerobic oxidation of alcohols
mediated by TEMPO and nonꢀmetallic chemical coꢀcatalysts
However, the use of toxic NOX sources and/or halogenꢀbased coꢀ
oxidants combined with the high cost of the TEMPO in particular as
homogeneous form makes these protocols less attractive for
industrial applications from both sustainable and green chemistry
point of views. Although, the use of expensive homogeneous
TEMPO can be avoided to a great extent by employing varied
TEMPO supported catalyst systems,4 the environmental impact of
NOX materials used in these metalꢀfree protocols is still a key
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obstacle that needs to be addressed.5 Therefore, the development of wasteꢀfree electrochemical oxidation of alcohols. We predict that
alternative TEMPOꢀcatalyzed selective oxidation of alcohols by high accessibility and uniform nanoꢀsized thickness of this
avoiding the use of harmful mediators or coꢀoxidant while electrochemically derived ordered mesoporous film would not only
maintaining an adequate reaction kinetic profile is extremely enable more convenient diffusion of alcohols into the mesochannels
desirable. Considering the fact that TEMPO can be also oxidized where the TEMPO groups are located, but it may also provide a
electrochemically to its active Nꢀoxoammonium (OXO) intermediate means of faster and easier electron transfer from TEMPO to the
without significant kinetic barrier,6
a
wasteꢀfree selective electrode surface, thereby enhancing the overall oxidation of
electrochemically mediated TEMPOꢀcatalyzed oxidation of alcohols alcohols in comparison with the oxidations using conventional
would represent an appealing and highly promising alternative for modified electrode coated with amorphous organosilica
this type of transformation. Although, a few attempts have been functionalized TEMPO. If this hypothesis could be realized in
dedicated towards application of homogeneous TEMPO as an practice this approach would seem to open a new area in designing
electroꢀcatalyst in some oxidative transformations,7 however, the high performance modified electrodes for wasteꢀfree and
approaches in designing appropriate and highly effective modified environmentally benign oxidation of alcohols.
electrodes based on supported TEMPO are very rare at present.8 In
this context, the work of Pagliaro et al. on the preparation of a thin
layer of amorphous organosilica functionalized TEMPO on the
surface of Indium Tin Oxide (ITO) coated glass electrodes for
Experimental Section
application in selective electrochemical oxidation (selelox) of
alcohols foreshadow future opportunities in this transformation in a
wasteꢀfree pathway.9 This organosilica modified electrode possesses
some desirable properties such as high durability and selectivity in
oxidation of alcohols, which are reflected by its suitable surface
hydrophobicꢀhydrophilic balance. Owing to both/either low
accessibility of the active sites arising from amorphous nature of the
organosilica layer and/or electrodes low surface area, it appears,
however, that prolonged reaction times up to 180 h were necessary
to ensure satisfactory conversion. In this case a significant numbers
of the catalytic groups might be supported within microporous
region of organosilica layer, leading to serious diffusion limitations.
Another problem regarding ITOꢀbased electrodes is the much higher
overꢀvoltage for the electron transfer of TEMPO at the surface of
this electrode. The use of ordered mesoporous silica (OMS) are
certainly preferred over amorphous silica due to their adjustable and
more accessible pores in the range of 2ꢀ50 nm that in turn confers
them a high specific surface area beside the other important unique
properties.10 However, when using OMS materials to modify the
electrode surface, the appropriate pore size as well as channel
orientation are challenging issues in order to facilitate the diffusion
of substrates into the system pores, where they favorably got reacted
at available active sites at the proximity of electrode surface,
followed by the departure of the reaction products from the channels
which leaves the reaction sites for the next available starting
molecules. To address the problem associated with the channel
orientation, Walcarius and his coꢀworkers have recently
demonstrated that an ordered mesoporous silica film consisting
hexagonally packed channels of MCMꢀ41 types perpendicular to the
electrode surface could be generated using a technique known as
ElectroꢀAssisted Self Assembly (EASA).11 This approach has thus
far been successfully employed for construction of varied high
performance modified electrodes in several electrochemical
studies.12 However, despite these unique characteristics, to the best
of our knowledge there is currently no report of preparing and
employing EASA-generated (modified) electrodes comprising
electro-catalytic moieties in electrochemical functional group
Materials:
Tetraethylorthosilicate
99%
(TEOS),
cetyltrimethylammonium bromide 98% (CTAB), potassium nitrate,
sodium bicarbonate, sodium acetate, sodium cyanoborohydride95%
(NaBH3CN), acetic acid, hydrochloric acid 37% and all of the
alcohols
were
purchased
from
Merck.
(3ꢀ
aminopropyl)triethoxysilane 99% (APTES) and 4ꢀOxoꢀ2,2,6,6ꢀ
tetramethylꢀ1ꢀpiperidinyloxy (TEMPON) were purchased from
SigmaꢀAldrich. All chemicals were used without further purification.
Typical Procedure for the preparation of 2,2,6,6-tetramethyl-4-
(3-(triethoxysilyl)propylamino)piperidin-1-ol (TPTES): The
material was synthesis by addition of 2.0 mmol APTES to a solution
of 2.0 mmol of TEMPON in dried ethanol (20 mL) in a reaction
flask kept closed under a nitrogen atmosphere after 3 h stirring 2.2
mmol of NaBH3CN was added to the reaction mixture and the
resulting mixture was stirred for 24 h, after which the excess of
sodium cyanoborohydride was quenched with 1.5 ml of 4.0 M
HCl.13 The ethanolic solution of this precursor was used without
further purification.
General procedure for construction EASA-generated (modified)
electrodes with TEMPO function (TGSE):
a)
Initial sol preparation: various mixtures consist of 3.26
mmol CTAB, (10ꢀX) mmol TEOS, X mmol APS (the percent of
functional group containing silica precursor should be less than 10%
over TEOS), 15 ml ethanol and 15 ml aqueous solution of 0.1 M
NaNO3 and 3.0 mMHCl were used for sol preparation. CTAB and
APTS or TPTES should be added under stirring and vigorous
stirring respectively.12 The sol solutions were aged under stirring for
2.5 h at pH 3 prior to electroꢀdeposition.
b)
Thin film formation: The graphite or GC electrodes were
immersed in the above mentioned precursor solutions and electroꢀ
deposition was achieved by applying a suitable cathodic current
(−1.9 mA cm−2) for a defined period of time (typically 20 s). The
electrodes were rapidly removed from the solution and immediately
rinsed with distilled water. The electrodeposited surfactant template
film was then dried overnight in an oven at 130o C. Extraction of the
transformations. We envisioned that
a
properly designed
mesoporous film having electroꢀactive TEMPO functionality
distributed over the entire the surface of a given electrode might
result in an unprecedented and more efficient electrode system for
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surfactant template was carried out in acidic ethanol solutions under
moderate stirring for 10 min to give the modified electrodes with
aminopropyl functionalized MCMꢀ41 thin film (Figure 2A).12
Results and Discussion
To prove our hypothesis, we have chosen to employ EASA
technique for the construction of two kinds of modified
electrodes coated with TEMPOꢀfunctionalized MCMꢀ41 thin
film as outlined in Figure 2. The surface of either GC or
graphite plate electrodes was selected instead of ITO to make
the EASA modified electrodes, since it has been wellꢀ
documented that the former electrodes exhibit much lower
overvoltage for the electron transfer of TEMPO.7a,9a The
present EASA protocol consist of application of a cathodic
potential to an electrode immersed in a sol solution containing
appropriate silica precursor, tetraethylorthosilicate (TEOS), and
cetyltrimethylammonium bromide (CTAB) as structure
directing agent.11a
c)
Functionalization of mesoporous silica modified
electrode: The modified electrodes with aminopropyl functionalized
MCMꢀ41 thin film were immersed in 0.15 M acetate solution and
the cathodic potential of ꢀ.2.2 V (vs. Ag/AgCl for 2 min) were
applied for deprotonation of amine groups. The electrodes then
rinsed by distilled water and dried in an oven overnight. Thereafter
the electrodes were immersed in super dry CH3OH (150 mL)
containing 2.0 mmol TEMPON under nitrogen atmosphere. The
electrode immersion followed by the stepwise addition of 2.0 mmol
NaBH3CN (three times addition after 3, 27 and 51 h). The electrodes
were then removed after 72 h and rinsed with methanol and distilled
water to remove any surface adsorbed TEMPON. The resulting
modified electrode was denoted as TGSE.
General procedure for construction EASA-generated (modified)
electrodes with TEMPO function (CSTE): The preparation of
CSTE was achieved following the step a and b of the aboveꢀ
described protocol for TGSE except the fact that APS has been
replace with 2,2,6,6ꢀtetramethylꢀ4ꢀ(3ꢀ(triethoxysilyl)ꢀpropylamino)
piperidineꢀ1ꢀol (TPTES).
Instrumentation: The film characterizations were performed using
the following instruments: Elementar Analysen System GmbHꢀvario
EL Element Analyzer for elemental analysis (CHN), NETZSCH
STA 409 PC for Thermal Gravimetric Analysis (TGA), Philips
CM20 Microscope an acceleration voltage of 200 kV for
Transmission Electron Microscopy (TEM). A Metrohm pHꢀmeter
691 was utilized for measuring the solutions pH. Voltammetric
measurements were performed at room temperature using an
Autolab PGSTATꢀ101 monitored by the Electrochemical System
Software (Eco Chemie). Voltammetric experiments were carried out
in a classical threeꢀelectrode undivided cell configuration with a
platinum counter electrode and a GC working electrode (surface
area=3.14 mm2). Electrochemical preparative reactions were
performed in an undivided homeꢀmade threeꢀelectrode cell which the
working electrodeswere graphite plates, the auxiliary electrodes
consisted of a hollow tube and a centralized steel rod. Behpajooh
(Isfahan, Iran) coulometer with the maximum current of 300 mA
were used for the preparative electrolysis reactions. The working
electrodes potentials were measured versus a double junction
Figure 2. Proposed pathway of electrode modification with thin layer of
TEMPOꢀfunctionalized ordered mesoporous silica (TGSE electrode, top),
Ag/AgCl reference electrode in all of electrochemical experiments. and thin layer of TEMPOꢀfunctionalized amorphous silica (CSTE electrode,
bottom). The functionalized group are demonstrated bigger (exaggerated
size) than their actual sizes for clarification
All of the electrodes were manufactured by AZAR Electrode
Company, Urmia, Iran. The extent of the reaction during the
electrolysis experiments was monitored by internal standard method
using Varian CPꢀ3800 gas chromatograph equipped with a CPꢀSil 8
CB capillary column.
In this regard, electroꢀgenerated hydroxyl ions under cathodic
conditions are known to function as the catalyst for polyꢀ
condensation of the precursors and formation of thin mesoporous
silica (MCMꢀ41) film at the electrode surface. The aminopropyl
functionalized silica film (APS) with the maximum possible (3ꢀ
aminopropyl)triethoxysilane (APTS) to TEOS ratio (10%
APTS/TEOS in sol solution) was first deposited following the
reported EASA procedure on both GC and graphite electrodes.11a,12a
The accessibility of film deposition was examined by voltammetric
Typical procedure of electro-catalytic oxidation of alcohols: The
oxidation of alcohols (1 to 20 mmol) was carried out in 80 to 120
mL of aqueous carbonate buffered (NaHCO3 0.10 M and Na2CO3
0.01 M) using the Modified TGSE or CSTE modified graphite
electrodes. In order to attain better solubility for cinnamyl alcohol
and 4ꢀnitrobenzyl alcohol mixture of acetonitrile/water solution
(20:80 v/v) was employed. The electrolysis reactions were carried
under stirring at 1.0 V vs. Ag/AgCl as the optimum potential
3ꢀ
3+
study of Fe(CN)6 and Ru(NH3)6 as electroꢀactive charged probe
and composition of the scratched film after surfactant extraction,
have been approved by thermogravimetric (TGA) and elemental
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3ꢀ
(CHN) analyses which were in good agreement with the previously Figure 4 presents the voltammetric responses of Fe(CN)6 on
reported results.12a
bare, MCMꢀ41 modified electrode, CSTE, and TGSE. As it is
evident from the Figure 4, the TGSE showed considerable
currents, comparable to the bare electrode and higher than
previously reported MCMꢀ41 modified electrodes as well as
CSTE electrode after CTAB extraction.11a This is a strong
evidence for the presence of porous structure of the film and
electrode accessibility through TGSE. Moreover, higher
voltammetric currents for the TGSE compared with either
MCMꢀ41 or surfactant extracted CSTE samples, is an evidence
that despite the partial filling of mesopores by pendant TEMPO
groups, the presence of positively charged amine groups and
The resulted APS was then allowed to react with 4ꢀoxoꢀ2,2,6,6ꢀ
tetramethylpiperidineꢀ1ꢀoxyh (TEMPON) in the presence of
NaBH3CN to afford the corresponding TEMPOꢀgrafted silica
modified electrode, which is described as TGSE (Figure 2a).
TGA analysis (See Supplementary Figure 1) of the scratched
thin film of TGSE exhibited 7% increase in the weight loss
o
between 300ꢀ500 C in comparison with that observed for the
initial APS. Although, it is very difficult to quantify the precise
amount of incorporated TEMPO moieties alone by TGA, due to
the overlapping matrix weight loss, the amount of TEMPO
estimated by TGA did show good consistency with the results
obtained by elemental analysis (See Supplementary Table 1).
High resolution transmission electron microscopy (HRTEM),
Electron Tomography (ET) as well as 3Dꢀreconstruction
analysis were conducted to get further insight into the structural
characteristics of TGSE thin film (Figure 3)
electrostatic interaction enhance the electrode accessibility and
3ꢀ 12c
voltammetric currents of negatively charged Fe(CN)6
.
Figure 4. CVs of 1.0 mM Fe(CN)63ꢀ on bare GC electrode (a), CSTE
before (b) and after CTAB extraction (c), TGSE (d) and MCMꢀ41
modified electrode (e); Supporting electrolyte 0.15 M KNO3, scan rate:
100 mVs−1
The voltammetric analysis of the CSTE was approximately the
same but considerably less than TGSE (Figure 4b), which is
most likely due to the disordered structure of the film (See also
Supplementary Figure 4). It should also be noted that both
CSTE and APS modified electrodes did not show any
considerable currents before the extraction of CTAB, indicating
the appropriate coverage of the electrode surface by the
deposited films.11a,12a
Figure 3. TEM images (A,C and E)and reconstructed images (B, D) of
TGSE
As can be clearly seen, the fragment of the separated film from
the surface of electrode shows a highly porous wellꢀdefined
2Dꢀhexagonal mesostructures perpendicular to the electrode
surface. As the 3D reconstruction derived from tomography
data obtained from thin film reveals (Figure 3D), the channels
are perfectly aligned perpendicular to the surface of electrode.
For the purpose of comparison, EASA technique was also
employed in hydrolysis combined with coꢀcondensation of
2,2,6,6ꢀtetramethylꢀ4ꢀ(3ꢀ(triethoxysilyl)ꢀpropylamino)
piperidineꢀ1ꢀol (TPTES, 8%), TEMPO containing silica
precursor denoted as TPTES, and TEOS (92%) as silica sources
to furnish the respected TEMPOꢀfunctionalized which is
referred to as CSTE (Figure 2b). The total incorporated
TEMPO function was again estimated to be 12 mmolgꢀ1 by
TGA and CHN analysis (See Supplementary Table 1 and
Supplementary Figure 2). However, our attempts in obtaining
highly ordered mesoporous thin film by changing the
TPTES/TEOS molar ratios in the initial sol solution were
unsuccessful as evidenced by TEM analysis (See
Supplementary Figure 3).
Figure 5. CVs of TGSE in a blank bicarbonate solution (a) 1.0 mM of
dissolved TEMPO (b) and TEMPON (c) on GC electrode in bicarbonate
solution; scan rates 100 mV s−1
The electrode accessibility of both TEMPOꢀfunctionalized
EASA electrodes were then examined and compared by
voltammetric study of the dissolved electroꢀactive probes.
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As mentioned above, TEMPO has a wellꢀdefined voltammetric
response and thus cyclic voltammetry (CV) was also employed
for the investigation of these TEMPO functionalized films.
Figure 5 (curve a) shows the CV of TGSE in a slightly basic
blank solution containing only supporting electrolyte (0.15 M
sodium bicarbonate). Notably, the constructed TGSE showed
one anodic and corresponding cathodic peaks with the midꢀ
peak potential (Emid 0.62 V vs. Ag/AgCl, ꢁEp = 0.14 V) being
more positive than TEMPO and less positive than TEMPON
moiety (Figure 5, curves b and c respectively). From the
considerable faradaic currents in Figure 5, it is apparent that the
acceptable loading of electroactive groups has been achieved
and the anchored groups have sufficient ability for the electron
transfer. Moreover, the supported form of TEMPO exhibited a
significantly better reversibility in cyclic voltammogram as
compared with that obtained from soluble TEMPON, which is
attributed to a great extent to the superior stability of electroꢀ
generated oxoammonium salt of supported TEMPO and most
likely to the conversion of carbonyl to amine groups during the
reductive amination.4a In addition, voltammetric studies at
various pH values demonstrated that the initial currents of the
electrodes at mild acidic solutions were to some extent less than
the value measured in basic solutions. Moreover, in contrast to
either TEMPO or TEMPON the midꢀpoint potential of
anchored TEMPO was dependent on the solution pH, for the
first voltammetric cycle only. This may be due to the reduction
loading of TEMPO moiety in our prepared CSTE thin film is
significantly higher than that measured in TGSE sample as it is
evidenced from the elemental analysis (See Supplementary
Table 1). In order to make a better comparison and to find the
best electrochemical performances and accessibility of CSTE,
we attempt to conduct experiments in various sol solution
compositions. To do this, the composition of sol solution was
changed with respect to TPTES/TEOS ratio to achieve the
maximum possible voltammetric currents. The results showed
that the optimum ratio of TPTES/TEOS was 7.5/92.5. At the
lower ratios; the voltammetric currents of electrodes decreased
proportional to TPTES concentrations, whereas at the higher
concentration it gradually decreased until 20% of TPTES where
voltammetric current completely declined for the constructed
films (See Supplementary Figure 5). This observation clearly
highlights the notion that the use of higher than 8%
concentration of TPTES could indeed strongly block the system
pores thus preventing the electroꢀactive species to diffuse into
the pores and causing a significant loss of voltammetric current.
A more appealing observation, however, was that in the
presence of 1 mM of benzyl alcohol (BA), TGSE displayed a
considerably enhanced voltammetric response under mild basic
conditions (Figure 6D). The amounts of anodic peak currents,
which were proportional to the loading of TEMPO groups is
considered as criteria for the leaching stability of the electrode.
However, it was found that the decrease in current associated
with anodic peak for CSTE was about 13% after 5 h exposed to
the buffer solution. The successive potential scans and the peak
currents decreased to 80% of the initial values after being
exposed for additional 24 hours. In contrast, the current of
TGSE anodic peak was decreased to 11% of its initial value
only, even being exposed to the mild basic buffer solutions for
3 days. This observation clearly shows that TGSE display much
superior leaching stability under the described experimental
conditions. The loadings of TEMPO groups incorporated in
thin silica film on the electrode surface were estimated by
determining the charges under the voltammetric oxidation peak
recorded at low scan rate, 10 mV s−1.13 The charge under the
TEMPO peak shows the appropriate loading of 1.5×10ꢀ8 mol
cmꢀ2 (geometric area of electrode) for CSTE. The total amount
of active TEMPO functionalities in TGSE was also estimated to
be 1.1×10ꢀ8 mol cmꢀ1 by charge analysis under the voltammetric
oxidation peak, a value that shows a good agreement with those
estimated by CHN analysis. The voltammetric studies at
various scan rates demonstrated also the presence of diffusion
like behavior for the anchored electroꢀactive groups at the
electrode surfaces. This behavior may be attributed to the "layer
by layer" diffusion of electrons through the anchored electroꢀ
active groups inside the channels of silica support.14 The
superior leaching stability and excellent electrochemical
performance of TGSE in addition to wellꢀknown remarkable
catalytic properties of TEMPO makes us ask if the modified
electrode having TGSE thin film can function as an effective
electroꢀcatalytic system for wasteꢀfree oxidation of alcohols.
Figure 7 demonstrates the voltammetric response of TGSE in
the absence and presence of benzyl alcohol (BA) under mild
basic conditions (aqueous bicarbonate solution). What is
appealing in this experiment is that the anodic current of
of
N
ꢀoxyl radicals under reductive amination of TEMPON and
electroꢀinactive nature of the reduced
N
ꢀhydoxy form.4a
Remarkably, the electrodes showed approximately the same
currents in both acidic and basic solution after applying one
voltammetric cycle in basic solutions. On the other hand, we
found that despite the disordered structure of CSTE, a sample
of GC electrode with thin film of CSTE constructed using
appropriate concentration of TPTES (less than 8%) in initial
sol, exhibited relatively higher voltammetric currents as
compared with TGSE ) in bicarbonate solution (Figure 6C).
Figure 6. CVs of CSTE in the absence (a) and in the presence of 1.0
mM of BA (b) and CVs of TGSE in the absence (c) and in the presence
of 1.0 mM of BA (d) in bicarbonate solution, scan rate 100 mV sꢀ1
These superior voltametric responses of CSTEꢀbased electrodes
can be simply understood by considering the fact that the total
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electrode was increased 2.5 times in the presence of dissolved appears however, that conversion of carbonyl to amine group
BA. Notably, BA didn’t show any oxidation current on bare GC during reductive amination improves the electrochemical
and both MCMꢀ41 and APS modified electrodes. Figure 7 also reversibility of the TPTES and enhances its catalytic activity.
shows a clear trend of increasing the anodic currents by The ultimate goal of this study was to enable the
increasing the BA concentration. This finding corroborates the electrochemical conversion of alcohols to their corresponding
indirect or mediated (by anchored TEMPO) oxidation of BA on carbonyl compounds in preparative scale through a wasteꢀfree
TGSE. It is worth of mentioning that the extent of this behavior protocol. In order to attain this objective, an individual
was indeed inferior in the case of employing CSTEꢀbased homemade electrochemical cell comprising 8 wellꢀseparated
modified electrode achieved by increasing the BA graphite plates as working electrode in combination with a
concentration. By emphasizing the fact that the loading of hollow tube and a centralized rod of steel as an auxiliary
TEMPO moieties in TGSE was found to be around 72% of that electrode were designed and assembled. An array of TEMPO
estimated for CSTE, it is apparent that the modified electrode modified graphite plates, which were constructed exactly in a
comprising TGSE thin film demonstrates higher catalytic similar way as our previously described TGSE, has been
activity per TEMPO unit as compared with our CSTEꢀbased deposited using the essentially the same EASA protocol with
electrode (See Supplementary Figure 7).
uniform distances to both steel anodes (See Supplementary
Figure 7). The geometric area of the working electrodes was
124 cm2 considering the dimensions of electrodes and the
height with which the electrodes have been immersed in
electrolysis solution.
Figure 7. CVs of TGSE in the absence,(a) and in the presence of
9.0mM of BA(e) and CV of BA at bare GC electrode (b); CVs of TGSE
in the presence of 1.0 (c), 4.0 (d), 9.0 (e) and 16.0 (f) mM BA, in
bicarbonate solution, scan rate 100 mV sꢀ1
Figure 8. Images of employed electrochemical cell with 260 cm2
geometric area of working electrode; inner (a) and outer (b) parts
Taking into account the consistent higher electroꢀcatalytic
behavior of TGSE in the oxidation of BA, it would be
reasonable to speculate that the increase in accessibility of
ordered mesoporous silica thin film, where TEMPO groups are
located in close proximity to the electrode surface might be
indeed one of the major reasons for this favorable behavior in
By employing this new setup, complete conversion and
selectivity (a 100% BA conversion with >99% selectivity
toward benzaldehyde) were obtained by using 1.0 mmol of BA
within 10 hours under optimal conditions (slightly 0.1 M
bicarbonate solution or buffer pH=9). Remarkably, this method
was also equally applicable for the quantitative oxidation of 7.0
mmol of BA in 10 hours. These results indicate that the desired
electrodes have no restriction with respect to kinetic and mass
transfer throughout the ordered mesoporous thin film. In other
words, the limiting factor may occur by mass transfer in
solution and the cell design. Therefore, to shed light to this
issue we decided to improve the electrochemical cell device by
employing additional graphite plates in a hollow tube with
greater radius and adding more steel plates between the
graphite electrodes (Figure 8). Using this new cell design we
were able to increase the electrode functional surface up to 260
cm2 by maintaining the alternative array of working and
auxiliary electrodes. Gratifyingly, by employing this new
electrode system, practically quantitative conversions of BA
with excellent selectivity toward benzaldehyde were achieved
for all the solutions containing 2.0, 7.0 and even 12.0 mmol of
BA within only 100 minutes. By employing this new unit, the
comparison with CTSEꢀbased electrode. In
a separate
experiment, we also found that the voltammetric currents of
electrodes increase slightly by increasing the potential scan
rates while their normalized currents decreased drastically at
high scan rates. Normalized CVs (I/v1/2) were obtained by
dividing the current by the square root of the scan rate. These
voltammetric data very well match with the characteristics of
electrocatalytic (EC’) mechanism.14 The amounts of catalytic
currents as a criterion for the catalytic activity of the desired
electrodes have been studied at different pHs and various
alcohol substituents. The best catalytic activities were observed
for primary alcohols under mild basic (carbonate solution)
condition that are consistent with the results obtained for
homogeneous study of the TEMPO.7a It was also exciting that
TEMPON itself doesn’t have a reversible electrochemical
behavior and catalytic activity toward the oxidation of BA
under the same conditions (See Supplementary Figure 7). It
6 | J. Name., 2012, 00, 1-3
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oxidation of 20.0 mmol BA was also achieved within 4 hours, to employ this system for oxidation of a wide variety of
whereby the limiting factor was the maximum current of the alcohols, which the results are summarized in the Table 2. As
employed coulometer (0.3 A was the maximum tolerant of the can be seen, TGSE equally functioned in highly efficient
utilized coulometer). These conversions were obtained under electrochemical oxidation of substituted benzyl alcohols with
potentiostatic conditions and the potential of working excellent selectivity to yield the corresponding aldehyde as the
electrodes was 1.0 V versus Ag/AgCl. Based on these results, sole products (Entries 1ꢀ4, Table 2).
all of the potentials higher than 0.62 V vs. Ag/AgCl (midꢀpoint
potential of the supported TEMPO) should be appropriate for
Table 2. Electrochemical oxidation of alcohols using TGSE [a]
the oxidation of TEMPO and promotion of the reaction.
However, in order to find the most efficient electrochemical
pathway, the applied potential has been optimized for the
electrolysis reactions, which the results were summarized in
Table 1. From the data presented in Table 1, it is apparent that
the higher overꢀvoltages lead to higher conversions at a fix
time. But the nonꢀselective overꢀoxidation of alcohols was
considerable at the positive potentials higher than 1 V. While
the control experiments at bare graphite electrodes applying 1.0
V potential resulted in only 4 % conversion of BA at 100
minutes; the same experiments at 1.2 V gave 30% conversion
under otherwise identical reaction conditions.
Entry
Substrate
Product
Time/min
Product
yield
1
2
3
100
100
100
100
100
100
OH
O
OH
O
Cl
O
Cl
OH
O
O
4
5
6
7
8
200
50
100
48§
100§
95
OH
O
Table 1. Electrochemical oxidation of BA at various applied potential of
working electrode, vs. Ag/AgCl, using TGSE
O2N
O2N
Conversion
on TGSE
Conversion
on Graphite
O
OH
OH
Applied
potential
Substrate
Time (min)
100
O
100
500
1000
0.7
0.85
1.0
28
70
--
--
OH
OH
OH
OH
N
N
100
100
100
OH
O
O
O
85
OH
O
100
100
4
OH
O
1.2
26
9
300
500
91
48
10
OH
O
Applying the less positive potentials, however, is more
favorable from the economical point of views, in particular for
the preparative scale reactions. Therefore, the potential range of
0.9 to 1.0 V was selected as the optimum applied potential for
the rest of studies. The other important parameter that should be
considered in electrochemical reactions is the current yield.
Current yield of a reaction is usually measured by the number
of coulomb or electrons consumed for the desired reaction by
the total passage of coulomb. The current yield of 95% was
calculated for conversion of 7.0 mmol of BA whereas it was
57% for conversion of 1.0 mmol BA. One of the reasons of
obtaining these excellent current yields, using an undivided
cell, may be the immobilization of the TEMPO as the only
electroꢀactive component of reaction and prevention of back
reactions. Another explanation for this excellent favorable
current yield at higher BA concentrations could be the fact that
the side reactions and background currents are insignificant at
higher ratio of BA to TEMPO. Having optimized the electroꢀ
catalytic system based on TGSE electrode, we next proceeded
100
20
OH
OH
O
O
+
+
12
100
[a]
Reaction conditions: alcohols (1.0 mmol), water (100 mL), NaHCO3 (10
mmol, 0.84 g), Na2CO3 (1 mmol, 0.106 g), E=1.0 V vs. Ag/AgCl unless
otherwise stated.
[b] The product yields were determined by GC analysis after the extraction of all
aqueous phase contents.
[c]
Reaction conditions: alcohols (1.0 mmol) water (80 mL), CH3CN (20 mL),
NaHCO3 (10 mmol, 0.84 g), Na2CO3 (1 mmol, 0.106 g), E=1.0 V vs. Ag/AgCl.
The only exception in the case of primary benzylic alcohols
was the oxidation of 4ꢀnitrobenzyl alcohol, which furnished 4ꢀ
nitrobenzaldehyde in moderate yields of 48%, possibly due to
some electrochemical side reactions at nitro moieties (Entry 5,
Table 2). The present wasteꢀfree electrochemical oxidation
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was also suitable for the highly selective oxidation of
cinnamyl alcohol to cinnamaldehyde as a model for allyic
alcohols within relatively short reaction times (Entry 6, Table
2). This system was also successfully extended to the
oxidation of heterocyclic as well as primary aliphatic alcohols
with excellent selectivity and good activity (Entries 7ꢀ9, Table
2).
For the purpose of comparison, electrochemical oxidation of
BA was also performed using CSTE and complete conversion
has been achieved in 2 hours. However, the electrocalytic
activity of CSTE was declined faster than that observed for
TGSEꢀbased electrode. Ease of recovery and recycling that is of
great importance and often vital for the economic viability of
the described catalytic processes. Recovery of the desired
electroꢀcatalytic system has been examined by consecutive
addition of BA to the cell.
As demonstrated in Table 3, excellent conversion of
approximately 95% was achieved in 5 successive reaction runs.
Knowing the loading of the TEMPO and colummetric
measurements during the electrolysis or total amount of
products could simply provide the possibility to calculate both
Turnover Number (TON) and Turnover Frequency (TOF).
Considering the 1.1×10ꢀ8 mol per cm2 geometric area of the
TGSE and the whole geometric area of 260 cm2 which have
been utilized in electrolysis reaction the total amount of
anchored catalyst was calculated as 2.8×10ꢀ3 mmol.
designed electrochemical system and the support have no
considerable restriction for its activity.
Conclusions
In summary, based on the electrocatalytic oxidation results presented
in Table 2, TGSEꢀbased electrode could be considered as an efficient
electrode system in the electrochemical oxidation of alcohols. In this
context, it was found through several compelling electrochemical
studies that the voltammetric responses of the CSTE was
considerably less than TGSE with ordered mesoporous structure
(Figure 4b), which could be most likely due to the disordered
(amorphous) structure of the film in CSTE that resulted in less
accessibility of the anchored electroꢀactive groups. Since the loading
of TEMPO moieties in TGSE was found to be around 72% of the
estimated value for CSTE, then the effect of TEMPO loading on the
enhanced electroꢀcatalytic activities could be excluded. The higher
accessibility of TEMPO units in the modified electrode comprising
TGSE thin film combined with its highly organized nanoꢀ
architecture should be the origin of its prominent performance. This
is further supported by the TEM, HRTEM, electron tomography
(ET) as well as 3Dꢀreconstraction data of the isolated film of TGSE,
which reveals
a highly porous wellꢀdefined 2Dꢀhexagonal
mesostructures perpendicular to the electrode surface. In general the
advantages of this new electrode system can be summarized as
following:
(1) As indicated by cyclic voltammetric studies, the current of TGSE
anodic peak was decreased only by 11% of the initial value even
after 3 days remaining in mild basic buffer solutions. This
observation clearly shows that TGSE display much superior
leaching stability and relatively high durability under the
described experimental conditions.
(2) The proposed TGSEꢀmodified electrodes could be easily
employed in electrochemical oxidation of benzyl alcohol in 20
mmol scales in one reaction run using our homeꢀmade
electrochemical cell unit.
(3) In term of electroꢀcatalytic activity, this system affords
unprecedented turnover frequencies (TOF) up to 3070 hꢀ1, which
is much superior to the entire reported nitroxyl radical under
chemical or aerobic oxidation conditions. In addition, this TOF is
also far superior than TEMPO@DE reported earlier by Pagliaro
and coꢀworkers.9a
Table 3. Subsequent runs for oxidation
of BA on TGSE
Run
Conversion
Time (min)
1
2
3
4
5
100
100
95
100
125
158
200
240
95
95
Conversion of 12.0 mmol of BA has been achieved in 2 hours
that present an effective and impressive TON of 5130 and the
unique TOF of 3070 hꢀ1. The TOF of 1570 that were obtained
using CSTE, was considerably less than the value obtained by
TGSE. This decrease in TOF can be certainly attributed to the
disordered structure of the film and diffusion problem.
Obtaining the TOF for all of the entries embodied in the Table
2S (See Supplementary Table 2) presents a significant trend;
the desired electrocatalytic system has higher efficiency toward
the oxidation of primary alcohols and among them; the better
reactivity was observed for benzylic and allylic substrates
rather than the aliphatic ones. The obtained trend is
approximately identical to those found in a homogeneous
kinetic study.7a The obtained trend and superior activity provide
solid evidences that the supported TEMPO can be considered
as the key and only catalyst appeared inherently and the
(4) The use of water as the reaction solvent in most cases and the
absence of any expensive and/or toxic transition metal coꢀ
catalyst make this protocol as an attractive environmentally
benign alternative for this important transformation.
(5) To the best of our knowledge the present protocol could be
considered as the first example of employing EASAꢀgenerated
(modified) electrodes having electroꢀcatalytic moieties such as
TEMPO in electrochemical oxidation of alcohols in a preparative
pathway.
Considering all of the aboveꢀmentioned advantages, we suggest
that the present discovery would open a new challenging area in
the design of new types of modified electrodes for application
in varied functional group transformations.
Acknowledgements
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DOI: 10.1039/C4GC01303D
5
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The authors greatly acknowledge Iran National Science
Foundation (INSF), IASBS research council, and Natural
Sciences and Engineering Research of Canada for the financial
support of this work. We have also appreciate Dr. Mojtaba
Khorasani for graphical presentations in the Figure 2 and
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a
Department of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), P.O. Box 45137ꢀ66731, Gava Zang, Zanjan, Iran;
Fax: (+98)ꢀ241ꢀ4214949; eꢀmail: karimi@iasbs.ac.ir; rafiee@iasbs.ac.ir.
b
Department of Anatomy and Cell Biology and Facility for Electron
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This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9