New Stable Catalytic Electrodes Functionalized with TEMPO for the Waste-Free Oxidation of Alcohol

Article abstract of DOI:10.1021/acs.oprd.8b00156

We report the first catalytically active and stable sol-gel electrode functionalized with the TEMPO moiety for the highly selective oxidation of alcohols with an electric current only. The method shows broad applicability to different substrates, opening

Full text of DOI:10.1021/acs.oprd.8b00156

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Communication
New Stable Catalytic Electrodes Functionalized with
TEMPO for the Waste-Free Oxidation of Alcohol
Babak Karimi, Mina Ghahremani, Rosaria Ciriminna, and Mario Pagliaro
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00156 • Publication Date (Web): 09 Aug 2018
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New Stable Catalytic Electrodes Functionalized with TEMPO for the Waste-
Free Oxidation of Alcohol
Babak Karimi,*† ‡ Mina Ghahremani, † Rosaria Ciriminna,§ Mario Pagliaro* §
† Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), PO-
‡
Box 45195-1159, Gava zang, Zanjan 45137-6731, Iran; Research Center for Basic
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Science & Modern Technologies (RBST), Institute for Advanced Studies in Basic Sciences
(IASBS). Zanjan 45137-66731, Iran; Istituto per lo Studio dei Materiali Nanostrutturati.
§
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CNR via U. La Malfa 153 90146 Palermo, Italy
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Abstract
We report the first catalytically active and stable solꢀgel electrode
functionalized with the TEMPO moiety for the highly selective oxidation
of alcohols with an electric current only. The method shows broad
applicability to different substrates, opening the route to the widespread
adoption of this eminently clean technology by the fine chemical and
pharmaceutical industries.
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Keywords: Alcohol oxidation, Electrocatalytic activity, EASA, TEMPO, Hydrophobicity
thin films.
Introduction
Due to the fundamental role of aldehydes, ketones and carboxylic acids in the chemical
industry as well as in several other industrial fields, the catalytic oxidation of alcohols to
carbonyl compounds or carboxylic acids using environmentally benign oxidants, catalysts
and reaction media continues to attract a great deal of research.1 Gone are the days in which
industry used highly toxic chromium (VI) reactants, having mostly replaced the latter and
other toxic stoichiometric oxidants with a catalytic amount of TEMPO (2,2,6,6ꢀtetramethylꢀ
1ꢀpiperidine Nꢀoxyl) or its nitroxyl radical derivatives thanks to their high selectivity and
pronounced versatility.2 In general, in TEMPOꢀlike mediated oxidations, diverse primary
oxidants are generally used to promote catalytic turnover, the most common of which
include bleach (NaOCl), hypervalent iodine reagents, and O2 in combination with NOx coꢀ
catalysts (Scheme 1A).3
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Scheme 1. Chemical (A) and electrochemical (B) alcohol oxidation mediated
by stable organic nitroxyl radicals.
Remarkable recent advances even include the mechanically induced oxidation of a broad
set of alcohols,4 via ball milling the substrates under air in the presence of Stahl’s catalyst.5
However, there are some limitations associated with these protocols such as the use of toxic
NOX sources and/or halogenꢀbased coꢀ oxidants and the employment of costly and barely
recoverable TEMPO in homogeneous form. Although, several elegant and successful
immobilization strategies have been developed, providing the possibility of reusing the
radicals to a great extent,6 the environmental concerns associated with the NOX materials
used in these metalꢀfree systems is still an important issue that remains to be addressed.7
A promising approach avoiding the use of toxic electronꢀcarriers or coꢀoxidant, was
first proposed by Semmelhack and coꢀworkers in 1983 who generated the catalytically
active cyclic nitrosonium ion by applying a relatively small electric potential (0.7 V vs.
Ag/AgCl) to nitroxyl radicals dissolved in solution (Scheme 1B).8 The method remained
relatively unexplored until recently,9 when it was used for the electrocatalytic synthesis of
some carbonyls on preparative scale,10 as well as for the solarꢀdriven oxidation of 5ꢀ
hydroxymethylfurfural into 2,5ꢀfurandicarboxylic acid at the anode of
photoelectrochemical cell.11
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In this research context, then, the last issue to be addressed to make the oxidation
entirely clean and wasteꢀfree is that to immobilize TEMPO motif onto a solid electrode
surface, and use the modified electrode for the electrooxidation of alcohols.12 Among the
various electrode modifiers, periodic mesoporous organosilicas (PMOs) offer uniquely
attractive properties (i.e., ion exchange capacity, size and/or charge selectivity, hosting
capabilities, catalytic or redox activity, and selective recognition or permselective
properties) widely exploited most notably for electroanalytical applications.13 Indeed, a
most promising progress was reported in 2015 when a highly ordered TEMPO
functionalized mesoporous hybrid silica electrode (TGSE) film with perpendicular
mesochannels was found remarkably active and selective in the oxidation of primary
benzylic, allylic and aliphatic alcohols in aqueous bicarbonate solution at room temperature
without the need of any coꢀcatalyst.14
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Interestingly, the modified electrode could be conveniently employed also on
preparative scale and the catalytic process, with a TON of up to 3070 h−1, became the
fastest ever reported for TEMPOꢀmediated alcohol oxidations under chemical,
electrochemical, or aerobic conditions. However, in the alkaline solution buffered at pH 9
in which the reaction is performed, the TGSE electrode rapidly degrades due the known
chemical instability of purely inorganic silica comprising the MCMꢀ41 scaffold.
In addition to good selective activity and broad applicability to different substrates,
the electrode’s stability is perhaps the most important factor that will drive the adoption of
this eminently clean technology by the fine chemical and pharmaceutical industry.
Now, we report the development of such first catalytically active and stable
electrodes.
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Results and Discussion
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Previous studies on the activity and stability of amorphous ORMOSIL (organically
modified silica) and SiO2 electrodes functionalized with TEMPO employed at pH 9.3,
clearly showed that the latter electrode lost its activity completely after only two runs in the
electroꢀoxidation of benzyl alcohol, whereas the ORMOSIL electrode retained its modest
activity.15 Hence, a first attempt to stabilize the periodic mesoporous silicaꢀbased electrodes
was based on extending the electroꢀassisted selfꢀassembly (EASA)16 method to the
generation of oriented mesoporous thin films bearing both organic groups and TEMPO
groups covalently attached to the mesopore channels.
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Such organic groups were introduced into the film by direct coꢀcondensation of
alkyltriethoxysilane (AꢀTES), TEOS and precursor of organosilane bearing TEMPO group,
whose loading was varied to achieve the maximal possible functionalization level without
losing the oriented hexagonal mesostructure. Considering that solꢀgel coꢀcondensation in
electrochemical interfacial surfactant templating is only possible up to a certain level of
organoꢀfunctional group loading for hybrid ordered films prepared by EASA, the ordered
mesostructure and pore orientation was maintained for molar ratios of organosilaneꢀtoꢀ
TEOS up to 60% for methyl17 and 10% for propylamine groups.18
A preliminary study was thus necessary to optimize the different experimental
protocols. We focused initially on the preparation of mesoporous films onto GC electrode
surfaces according with organosilane (MTES + TPTES) TEOS molar ratio < 25% in the sol
precursor solution. The resulting electrode (HCSTE) accessibility was then examined by
voltammetric study of a dissolved redox probe. Figure 1 shows the voltammetric responses
3ꢀ
of Fe(CN)6 on bare GC electrode as well as on the modified electrode before and after
extraction of the templating surfactant. A significant current decrease is observed for the
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HCSTE electrode organically modified with both methyl and TEMPO moieties in
comparison to both previously reported TGSE electrode.
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Figure 1: Left: CVs of 1.0 mM Fe(CN)63− on (a) bare GC electrode, and of HTGSE modified electrode (b)
before and (c) after CTAB extraction; Supporting electrolyte 0.15 M phosphate buffer (pH=7),scan rate: 20 mV
s−1; Right: currents 1.0 mM Fe(CN)63− on HTGSE, TGSE, MCMꢀ41 and bare electrode (GC).
Now, the current is rather similar to that observed for the glassy carbon electrode coated
with MCMꢀ41 silica, which provides indirect evidence of a porous, accessible structure also
for the HCSTE film. The lower voltammetric currents for the HCSTE compared with the
TGSE is likely due to the absence of positively charged amine groups in the channels. Also,
we postulate that the presence of methyl groups inside the HCSTE channels is responsible
for the extraordinary decline in the diffusion of ionic Fe(CN)63ꢀ species.
To prove our assumption as well as to investigate the existence of TEMPO groups
in the channels of HCSTE modified electrode, the electrochemical response of TEMPO
groups using cyclic voltammetry was investigated (Figure 2). In the first two cycles (a and
b in Figure 2) only cathodic peaks were observed at 0.65 V vs. SCE, whereas in the
subsequent cycles (cycles c, d, etc.) both anodic and cathodic peaks were observed. This
observation is in contrast to the results of cyclic voltammetry studies of TGSE electrode,
showing both anodic and cathodic peaks from the first cycle on.
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Figure 2. CVs of HTGSE after CTAB extraction (a: cycle 1, b: cycle 2, c: cycle 3 and d: cycle 4); 0.15 M
phosphate buffer (pH=7) as supporting electrolyte, scan rate: 10 mV s−1.
Moreover, whereas the TGSE electrode shows a remarkable drop in the height of anodic
current already in the second cycle (b in Figure 3), the HCSTE electrode displays only a
slight decrease in the second cyclic run (Figure 2), which points to remarkable higher
stability of the latter electrode.
Figure 3. CVs of TGSE after CTAB extraction (a: cycle 1 and b: cycle 2) 0.15 M phosphate
buffer (pH=7) as supporting electrolyte, scan rate: 10 mV s−1.
The reduction in the anodic current, indeed, can be safely attributed to more pronounced
leaching (or quenching) of the TEMPO groups at the TGSE electrode’s surface due to the
low stability of its silica backbone in alkaline reaction medium. On the other hand, the
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presence of methyl groups inside the channels of HCSTE results in higher stability of this
electrode in the same basic aqueous medium, leading to a lower drop in electrode currents
in comparison to those observed with the hydrophilic TGSE electrode.
To investigate the electrocatalytic activity of the HCSTE electrode, we attempted
the electrochemical oxidation of benzyl alcohol (BA) dissolved in buffered aqueous
solution.
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Figure 4. CVs of bare GC electrode in the presence of BA (a), and of HCSTE in the presence
of 9.0 mM BA (b), and in the absence of BA (c); Supporting electrolyte 0.15 M phosphate buffer (pH = 7);
scan rate: 20 mV s−1.
The cyclic voltammograms of bare GC and HCSTE electrodes immersed in a 9.0 mM
solution of BA show a high anodic current peak at 0.7 V for the HCSTE electrode (Figure
4a), while the corresponding cathodic peak disappeared (Figure 4b). Furthermore, a low
anodic current peak (Figure 4c) at the same point appeared in the voltammogram of
HCSTE electrode in the absence of BA.
These observations clearly demonstrate the catalytic activity of HCSTE electrode
electroꢀoxidation as BA is oxidised inside the accessible ordered mesoporosity of the
organically modified HCSTE electrode wherein the TEMPO moieties are located.
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Figure 5. Multiꢀcyclic voltammograms of HCSTE in the presence of 9.0mM of BA (a: cycle 1, b: cycle 2, c:
cycle 3, d: cycle 4 and e: cycle 5) ; Supporting electrolyte 0.15 M phosphate buffer (pH = 7); scan rate: 20 mV
s−1.
Consecutive recycling of potential in the presence of BA at the surface of the same HCSTE
electrode shows (Figure 5) that the height of anodic current peaks decreases and shifts to
the higher voltages pointing to progressive inhibition of the electrocatalytic oxidative
process.
These preliminary results in the study of electroactive HCSTE electrode encouraged
us to investigate the potential of this electrode in the electrooxidation of BA and its stability
and durability in aqueous media on larger scale. We therefore coated larger graphite plate
electrodes (260 cm2 geometric area) with the active ORMOSIL layer via EASA
electrodeposition of organosilica (X% MeSi(OR)3 + 5% TEMPOꢀpropyltrialkoxysilane) in
the presence of CTAB. After surfactant extraction, the graphite electrodes functionalized
with mesoporous organosilica were named TMꢀX@MCMꢀ41 (T:TEMPO, M:methyl and
X:prechantage of methylꢀmodified silane in solꢀgel precursor solution).
A number of graphite electrodes with and without TEMPO moieties were deposited
using various solꢀgel precursor solutions, including TMꢀX@MCMꢀ41 containing 15%
methyl precursor and 5% TEMPO precursor (TMꢀ15@MCMꢀ41), and another
incorporating 15% methyl and no TEMPO moieties (Mꢀ15@MCMꢀ41).
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The nitrogen content of the scratched films of these electrodes was determined by
elemental (C, H, N) analysis. For the purpose of comparison with previously reported
electrode (TEMPO@MCMꢀ41),14 an electrode containing 5% TEMPO and bearing no
methyl functionality was also prepared according to previously reported method for TGSE
preparation, with slight modifications only.
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Table 1 displays that both the nitrogen content and the TEMPO loading in the
TEMPO@MCMꢀ41 and TMꢀ15@MCMꢀ41 electrodes are in good agreement with each
other.
Table 1. Elemental (CHN) analyses
Entry
Type modified electrode
Mꢀ15@MCMꢀ41
N(in 100 mg)
0.667
TEMPO(mmol)
1
ꢀꢀꢀꢀꢀꢀ
0.50
0.54
2
3
TEMPO@MCMꢀ41(piror report)
TMꢀ15@MCMꢀ41
1.970
2.225
Elemental (CHN) analyses of the nitrogen content in Blank (Mꢀ15@MCMꢀ41) and TEMPO functionalized thin
films (TEMPO@MCMꢀ41 and TMꢀ15@MCMꢀ41 electrode surfaces), and calculated TEMPO.
The electroꢀoxidation of 1 mmol BA was performed in 0.15 M bicarbonate solution using
TEMPO@MCMꢀ41 and TMꢀ15@MCMꢀ41 electrodes at 1 V vs. SCE controlledꢀpotential
shows lower activity for the organically modified TMꢀ15@MCMꢀ41 electrode in
comparison with the hydrophilic TEMPO@MCMꢀ41 electrode (Table 2).
Table 2. Screening of electroꢀoxidation of BA using TEMPO@MCMꢀ41 and TMꢀ15@MCMꢀ41
electrodes.
Entry
Electrode
T(h)
2
Conv.(%)a
1
2
3
TEMPO@MCMꢀ41
TEMPO@MCMꢀ41, second run
TMꢀ15@MCMꢀ41
100
74
4
4
64
aReaction conditions: Benzyl alcohol (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. The product yields were determined by gas
chromatography analysis (GC) after the extraction of all aqueous phase contents using an internal standard.
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Considering that the comparable loadings of TEMPO in both electrodes (TMꢀ15@MCMꢀ
41 and TEMPO@MCMꢀ41 supported on graphite plates), this outcome seems to point to
inhomogeneous formation of thin mesoporous film on the graphite electrodes under larger
scale preparation conditions. Indeed, when we attempted to prepare various type of TMꢀ
x@MCMꢀ41 using different proportion of MeSi(OMe)3, TEOS and TEMPO precursor
under the same EASA condition (Table 3, entries 2ꢀ4 vs. entry 1), we found that changing
in Me/TEMPO ratios as well as increasing the TEMPO loading had no significant effect in
improving the catalytic performance of the electrode.
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Such inhomogeneous formation of the mesoporous thin film on the graphite
electrodes may arise from the different rate of hydrolysis and coꢀcondensation of Si
precursors, resulting in the burying of TEMPO functionalities in non accessible (non
porous) parts of the organosilica layer. Indeed when in order to balance the rate of
hydrolysis/condensation of Si precursors, we managed to prepare the TMꢀx@MCMꢀ41
electrodes by employing TEMPOꢀTMS instead of TEMPOꢀTES (and using a new cellꢀ
divided technique having homeꢀdesigned Sꢀmembrane) we obtained slight improvements in
the electrocatalytic activity, though still far from satisfaction (entries 5ꢀ7 in Table 3).
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Table 3. Screening of TMꢀx @MCMꢀ41 electrodes in the electroꢀoxidation of BA.
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Entry
1
Content of Sol Solution for thin film on W.
TEMPO@MCMꢀ41
Conv. (%)
74
64
69
54
73
80
78
2
15% MeꢀTES + 5% TEMPOꢀTES + 80% TEOS
15% MeꢀTES +7.3%TEMPOꢀTES +78% TEOS
15% MeꢀTES + 10%TEMPOꢀTES +75% TEOS
15% MeꢀTES +7.3%TEMPOꢀTES +78% TEOS
15% MeꢀTES +7.3%TEMPOꢀTMS+78% TEOS
15% MeꢀTMS+7.3%TEMPOꢀTMS+78%TEOS
3
4
5[a]
6[a]
7[a]
Reaction conditions: Benzyl alcohol (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. The product yields were determined by gas
chromatography analysis (GC) after the extraction of all aqueous phase contents using an internal standard.
[a]
Deposited in the presence of Sꢀmembrane.
To overcome this problem, we decided to investigate the grafting method for
functionalization of thin film with the TEMPO moiety, fully retaining the oriented
hexagonal mesostructure. In this way, thin layer of periodic mesoporous organosilica
bearing alkyl groups at the surface of well oriented channels was constructed on the
electrode surface by direct coꢀcondensation using the EASA technique. The TEMPO
functionality was subsequently attached to the surface of the organically modified periodic
mesoporous film by direct grafting of TEMPOꢀTMS (Scheme 2).
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Scheme 2. Preliminary formation of the periodic mesoporous organosilica thin film (top), followed
by grafting of TEMPOꢀTMS on the accessible inner channel surface (bottom).
Several modified electrodes using either different proportion of methyl (Me, 0ꢀ 40%) or
phenyl (Ph, 0ꢀ20%) were constructed using again the EASA technique (Table 4). When
employing 30% PhSi(OMe)3, though, no significant deposition was observed (Table 4,
entry 7). We remind here that organic modification of periodic mesoporous organosilica
with organoꢀfunctional groups is only possible in a certain level of organoꢀfunctional group
loading (i.e., up to 60% for methyl groups,17 and 10% for amine18).
Above these threshold values, formation of the transient surfactant hemimicelle
assemblies on the electrode surface under potential control is disturbed and cannot properly
induce the growth of an ordered and oriented mesostructure. After extraction of the
surfactant by acidic ethanol solutions under moderate stirring for 20 min, the organically
modified MCMꢀ41 thin films were treated under reflux in toluene for 24 h with an ethanol
solution containing the previously synthesized TEMPOꢀfunctionalized organosilane for
direct grafting.14
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Table 4. Abbreviation of thin films on electrode surfaces.
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Entry
Composite of Solꢀsolution
Abbreviation
10ꢀMe
1
2
3
4
5
6
7
10% MeꢀTMS + 90% TEOS
20% MeꢀTMS + 80% TEOS
30% MeꢀTMS + 70% TEOS
40% MeꢀTMS + 60% TEOS
10% PhꢀTMS + 90% TEOS
20% PhꢀTMS + 80% TEOS
30% PhꢀTMS + 70% TEOS
20ꢀMe
30ꢀMe
40ꢀMe
10ꢀPh
10
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20ꢀPh
No deposition
Modified thin films with different proportion of Alkyl groups (Methyl and Phenyl) prepared via the EASA
technique on electrode surfaces and their abbreviation.
After 24 h the resulting functionalized and organically modified electrodes were rinsed in
ethanol for 45 min under moderate stirring. After drying at room temperature the electrodes
ready for use were denoted as xꢀA@TES (x: extent of alkyl group; A: methyl or phenyl; T:
TEMPO).
We used the resulting modified electrodes in the electroꢀoxidation of several
alcohols to investigate their catalytic activity, scope and durability. First, controlledꢀ
potential coulometry was performed with each modified electrodes in 0.15 M bicarbonate
solution containing 1 mmol of BA at 1 V vs. SCE (Table 5). Electrodes 4 and 6 with,
respectively, 30% methyl and 10% phenyl degree of organic modification exhibited the
highest activity in the electrochemical oxidation BA (Table 5, entries 4 and 6).
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Table 5. Screening of xꢀA@TES electrodes in the electroꢀoxidation of BA
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4
5
6
7
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9
Entry Electrode (W)
T(h)
Conv.(%) a
1
0ꢀR @TES
4
4
4
4
4
4
4
79
83
88
96
90
96
88
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2
3
4
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8
10ꢀMe@TES
20ꢀMe@TES
30ꢀMe@TES
40ꢀMe@TES
10ꢀPh@TES
20ꢀPh@TES
30ꢀPh@TES
No active layer deposition
aReaction condition : Benzyl alcohol (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. The product yields were determined by gas
chromatography analysis (GC) after the extraction of all aqueous phase contents using an internal standard.
We thus checked the stability and durability of the latter electrodes that, following the first
run, were washed with EtOAc and dried in an oven at 70 °C for 1 h, and immersed again in
a solution containing 0.15 M bicarbonate and 1 mmol of BA to repeat the electrolysis at 1
V vs. SCE (Run 2).
Both electrodes 4 and 6 with 30% methyl and 10% phenyl demonstrated much
higher stability and durability as compared to the electrode with no auxiliary organic
moieties (0ꢀR@TES), showing evidence that such groups at the outer surface of the
ORMOSIL channels significantly enhance the stability of the electrocatalytic thin film in
the alkaline aqueous media during the electroꢀoxidation (Table 6).
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Table 6. Screening of xꢀA@TES electrodes in the electroꢀoxidation of BA.
3
4
5
W
C
OH
O
6
7
pH 9 aq. bicarbonate buffer
1 V vs Ag/AgCl
8
9
Conv.(%)
Entry
Electrode (W)
Run 1
79
Run 2
62
Run 3
63
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1
2
3
4
5
6
7
0ꢀR @TES
10ꢀMe@TES
20ꢀMe@TES
30ꢀMe@TES
40ꢀMe@TES
10ꢀPh@TES
20ꢀPh@TES
83
76
63
88
76
65
96
93
87
90
80
79
96
97
88
88
72
63
Reaction conditions in all Runs: Benzyl alcohol (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. After 4 h the product yields were
determined by gas chromatography analysis (GC) after the extraction of all aqueous phase contents using an
internal standard.
The contact angle value of each electrode with a drop of water confirm (130°±3° for 30ꢀ
Me@TES and 130°±6° for the 10ꢀPh@TES) their good hydrophobic character in
comparison with 0ꢀR@TES (MCMꢀ41) having a 28°±4° contact angle (Figure 6). A contact
angle >90°, indeed, is characteristic of an hydrophobic materials.19
Figure 6. Contact angle measurement for 30ꢀMe@TES, 10ꢀPh@TES and MCMꢀ41 thin films.
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The structure and morphology of both 30ꢀMe@TES and 10ꢀPh@TES is revealed by TEM
analysis showing (Figure 7) the uniform thicknes of both films and the hexagonal pattern of
their channels oriented perpendicularly on the electrode surface.
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Figure 7. Top view (A, C) and side view (B, D) TEM images of 30ꢀMe@TES and 10ꢀPh@TES
thin films on electrode surfaces,respectively, (Scale bar: 100 nm).
To investigate the scope of these new electrocatalysts, we tested them in the consecutive
electrochemical oxidation of several different benzyl alcohols under optimal conditions (1
mmol alcohol in 0.15 M hydrogencarbonate solution at room temperature electrolyzed at
E=1 V vs. SCE). The electrocatalytic electrode efficiently mediated the wasteꢀfree
electrochemical oxidation of substituted benzyl alcohols such chloro substituted benzyl
alcohols (Table 7).
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Table 7. Scope of the electroꢀoxidation of the benzyl alcohol derivatives using xꢀA@TES
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5
electrodes.
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OH
O
pH 9 aq. bicarbonate buffer
1 V vs Ag/AgCl
30ꢀMe@TES
10ꢀPh@TES
Entry Electrode (W)
T(h)
4
Conv.(%)
T(h)
4
Conv.(%)
1
H
96
84
99
97
96
97
96
66
97
96
96
97
2
3
4
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6
4ꢀmethoxy
4ꢀchloro
2ꢀchloro a
3ꢀchloro b
2,4ꢀdichloro c
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6
3
3
4
4
8
8
8
8
a Electrode washed with EtOAc and dried at 70 °C for 1 h , reused to oxidise 2ꢀchlorobenzylalcohol (Run 2),
bElectrode washed with EtOAc and dried at 70 °C for 1 h, reused to oxidise 3ꢀchlorobenzylalcohol (Run 3),
c
Electrode washed with EtOAc and dried at 70 °C for 1 h, reused to oxidise 4ꢀchlorobenzylalcohol (Run 4).
Again, after each run the electrodes were washed with EtOAc and dried for 1 h at 70 °C,
prior to reuse the electrodes immersed in solution containing 0.15 M bicarbonate and 1
mmol of another chloro benzyl alcohol derivative carrying out the electrolysis at 1 V vs.
SCE. Entries 3ꢀ6 in Table 6 show that activity of the electrocatalyst was fully retained even
after four consecutive runs.
Figure 8. Comparison of the electrochemical oxidation of 0.1 M benzyl alcohol by 30ꢀMe@TES (blue)
and 10ꢀPh@TES (red), at 1 V/vs. Ag/AgCl, supporting electrolyte is 0.15 bicarbonate buffer at rt.
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The kinetics of electroꢀoxidation of BA using both 30ꢀMe@TES and 10ꢀPh@TES
electrodes under described optimal electroꢀoxidation conditions show (Figure 8) higher
activity for the 10ꢀPh@TES electrode which may not be surprising given the affinity
between the phenyl group in the supported thin film and the diffusing BA molecules.
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Conclusion
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In 1999, Schacham, Mandler and Avnir reported a surprising discovery that inaugurated the
field of solꢀgel electrochemistry:20 it is enough to apply a suitable negative potential to an
electrode immersed in a hydrolyzed Si alkoxide solution, to locally generate the hydroxide
ions necessary to catalyze the solꢀgel polycondensation of the precursors and rapid
deposition on the electrode’s surface of an highly porous and accessible uniform silica (or
organosilica) layer. In 2007, Walcarius extended the method to prepare periodic
mesoporous silica by carrying out the process in the presence of a templating surfactant.16
Now, formation of a transient surfactant hemimicelle induces the growth of hexagonally
packed oneꢀdimensional channels of about 2nm in diameter, perpendicularly to the
electrode surface. The latter method will lead to the introduction of the most active
TEMPOꢀbased alcohol oxidation catalyst comprised of highly ordered, hexagonally packed
mesopore channels with perfect orthogonal orientation with respect to the film plane.14 The
electrode (TGSE), alas, loses its spectacular activity after one run only.
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Now we have discovered a straightforward nanochemistry route to stabilize the high
selective activity of periodic mesoporous solꢀgel electrodes functionalized with the TEMPO
moiety, via deposition of an organically modified thin film on the surface of an electrode
via the EASA method, followed by functionalization of the channels of the resulting
organosilica via direct grafting with a TEMPOꢀbearing silicon alkoxide solution.
Alkylation degrees consisting of 10% phenyl modification and 30% methylation are
particularly stable and active. The new thin films do not suffer from mass transfer and
diffusion limitations of amorphous ORMOSIL electrodes,15 ensuring quick substrate
diffusion and electroꢀoxidation. Yet, owing to better surface hydrophilicꢀlipophilic balance,
now the electrodes retain both accessibility to the active sites and exquisite stability,
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opening the route to the entirely wasteꢀfree conversion of alcohols into carbonyls or
carboxylic acid on industrial scale in the fine chemical and pharmaceutical industries.21
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Experimental section
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Materials: Tetraethylorthosilicate 99% (TEOS), cetyltrimethylammonium bromide
98% (CTAB), sodium nitrate, sodium bicarbonate, sodium acetate, sodium
cyanoborohydride 95%, acetic acid, hydrochloric acid 37% and all alcohol substrates were
purchased from Merck. 4ꢀOxoꢀ2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy(TEMPON), (3ꢀ
aminopropyl)triethoxysilane 99% (APTES), (3ꢀaminopropyl)trimethoxysilane 99%
(APTMS), triethoxymethylsilane 99% (MTES), trimethoxymethylsilane 99% (MTMS) and
triethoxyphenylsilane 99% (PTES) and trimthoxyphenylsilane 99% (PTMS) were
purchased from SigmaꢀAldrich. Analytical grade potassium hexacyanoferrate (III)
(K3Fe(CN)6, Fluka) was used as redox probes for the electrochemical monitoring of the
film mass transport properties. All purchased chemicals were used without further
purification.
Typical precursor synthetic procedure. For the preparation of the precursor
2,2,6,6ꢀtetramethylꢀ4ꢀ(3ꢀ(trialkoxysilyl)propylamino)piperidinꢀ1ꢀol(TPTAS),
the
Si
alkoxide precursor bearing the nitroxyl radical were synthesis by addition of 2.0 mmol
APTAS (APTMS or APTES) to a solution of 2.0 mmol of TEMPON in dried EtOH (20
mL) in a tightly closed reaction flask under a N2 atmosphere. After 3 h stirring, NaBH3CN
(2.2 mmol) was added to the reaction mixture and the resulting mixture left under stirring
for 24 h, after which the excess cyanoborohydride was quenched with aqeous HCl (4.0 M,
1.5 mL). The ethanolꢀbased solution of the precursor thereby obtained was used without
further purification.
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General procedure for construction of electro-assisted self-assembly-generated
and organically modified electrodes functionalized with TEMPO.
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a) Initial sol preparation. Various mixtures consisting of 3.26 mmol CTAB, (10ꢀ
8
9
(X+Y))mmol
TEOS,
Xmmol
Organoalkoxysilanes
(Methylalkoxysilane
or
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Phenylalkoxysilane), Y mmol TPTAS were combined under vigorous stirring at molar
ratios up to 30 mol%, further adding ethanol (15 mL) and 15 mL of an aqueous solution 0.1
M in NaNO3 and 3.0 mM in HCl. The sol mixtures were allowed to hydrolyze for 2.5 h at
pH 3 under stirring at room temperature.
b) Thin film formation. Graphite or glassy carbon electrodes were immersed in the
above mentioned sol precursor mixtures. Electrodeposition of the solꢀgel hybrid silica film
was achieved by applying a suitable cathodic current (−1.9 mA cm−2) for a defined period
of time (typically 20s). The electrodes were rapidly removed from the solution and
immediately rinsed with distilled water. The electrodeposited surfactantꢀ templated film
was then dried overnight in an oven at 130 °C. Extraction of the surfactant template was
carried out in acidic ethanol solutions under moderate stirring for 10 min affording the
organically modified periodically mesoporous electrode functionalized with both organic
and TEMPO moieties (HCSTE).
Typical Procedure for the Grafting of 2,2,6,6-tetramethyl-4-(3-(trialkoxysilyl)propylamino)
piperidin-1-ol (TPTAS) on (Modified) Hydrophobic Electrodes. The modified electrodes
obtained by the initial sol containing Si source of (10ꢀX) mmol TEOS and X mmol
Organoalkoxysilanes, were immersed in the ethanolic solution of TPTAS precursor and
o
toluene (1:10) then refluxed for 24 h at 110 C. The resulting functionalized electrodes
were rinsed under mild stirring in ethanol for 45 min. After drying at room temperature
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modified electrodes bearing both alkyl and TEMPO functionalities were obtained and
denoted as xꢀA@TES (x: extent of alkyl group; A: methyl or phenyl; T: TEMPO)).
Typical procedure of electro-catalytic oxidation of alcohols. The oxidation of
alcohols (1 to 20 mmol) was typically carried out under magnetic stirring in a 80 to 120 mL
buffered solution of aqueous bicarbonate (NaHCO3 0.10 M and Na2CO3 0.01 M) using the
Modified xꢀA@TES or HCSTE graphite electrodes at 1.0 V vs. Ag/AgCl as the optimum
potential.
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ASSOCIATED CONTENT
Supporting Information
Detailed experimental methods, Large size of TEM images, Calculation of contact angle
measurements
Author Information
ORCID
Mina Ghahremani, 0000ꢀ0002ꢀ2003ꢀ0560
Babak Karimi, 0000ꢀ0002ꢀ0453ꢀ0047
Rosaria Ciriminna, 0000ꢀ0001ꢀ6596ꢀ1572
Mario Pagliaro, 0000ꢀ0002ꢀ5096ꢀ329X
Corresponding Authors
Prof. B. Karimi
Department of Chemistry and Research Center for Basic Science & Modern Technologies
(RBST), Institute for Advanced Studies in Basic Sciences (IASBS)
POꢀBox 45195ꢀ1159, Gava zang, Zanꢀjan 45137ꢀ6731, Iran.
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Fax: (+98ꢀ241ꢀ4214949)
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Eꢀmail: Karimi@iasbs.ac.ir
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Dr. M. Pagliaro
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Istituto per lo Studio dei Materiali Nanostrutturati, CNR
via U. La Malfa 153 90146 Palermo, Italy.
Eꢀmail: mario.pagliaro@cnr.it
Acknowledgments
This article is dedicated to the memory of Professor István E Markó (1956ꢀ2016), for all he
has done to advance catalysis for clean chemical productions at the School of Chemistry of
Université Catholique de Louvain. The authors also acknowledge Dr. Maniya Maleki from
IASBS for her generous help in measuring the contact angle of electrodes and also
interpretation of the contact angle results.
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List of Abbreviations
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AꢀTES, Alkyl (methyl or phenyl) triethoxysilane
BA, benzyl alcohol
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9
CTAB, cetyltrimethylammonium bromide
CV, cyclic voltammogram
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GC, glassy carbon
EASA, Electroꢀassisted selfꢀassembly
MTES, Methyltriethoxysilane
MeꢀTMS, Methyltrimethoxysilane
ORMOSIL, Organically modified silicate
PMO, Periodic mesoporous organosilicas
TEMPO, 2,2,6,6ꢀtetramethylꢀ1ꢀpiperidine Nꢀoxyl
TGSE = TEMPO functionalized mesoporous hybrid silica electrode
TEOS, Tetraethylorthosilicate
TPTES, 2,2,6,6ꢀtetramethylꢀ4ꢀ(3ꢀ(triethoxysilyl)propylamino)piperidinꢀ1ꢀol
TEMPOꢀTMS, 2,2,6,6ꢀtetramethylꢀ4ꢀ(3ꢀ(trimethoxysilyl)propylamino)piperidinꢀ1ꢀol;
TEMPOꢀTES, 2,2,6,6ꢀtetramethyl ꢀ4ꢀ(3ꢀ(triethoxysilyl)propylamino)piperidinꢀ1ꢀol ;
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