Light driven styrene epoxidation and hydrogen generation using H2O as an oxygen source in a photoelectrosynthesis cell

Article abstract of DOI:10.1039/c5gc01589h

A dye-sensitized photoelectrosynthesis cell (DSPEC) has been prepared for the oxidation of alkenes to epoxides and evolution of hydrogen using water as an oxygen source and sunlight. A Ru oxidation catalyst, 2,2⁺, is used in the homogeneous phase in the anodic compartment to oxidize a water-soluble alkene, 4-styrene sulfonic acid (4-HSS), to the corresponding epoxide that in acidic solution is hydrolyzed to the diol (4-(1,2-dihydroxyethyl)-benzenesulfonic acid). Concomitantly, protons are generated that diffuse through a proton exchange membrane to a Pt cathode where they are transformed into hydrogen. Illumination under 1.5 AMG (100 mW cm^-2) together with an external bias of 0.3 V vs. NHE after 24 h leads to the generation of 1.28 Coulombs together with the formation of 6.1 μmol of H₂ at the cathodic compartment that corresponds to a faradaic efficiency of 92%. In addition 0.7 mM of the 4-HSS substrate has been oxidized at the anodic compartment with a conversion yield of 7%. The rate of hydrogen evolution is limited by the oxidation of the organic substrate, and the TOF for both reactions is measured to be 0.8 ks^-1.

Full text of DOI:10.1039/c5gc01589h

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DOI: 10.1039/C5GC01589H
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ARTICLE
Light Driven Styrene Epoxidation and Hydrogen Generation Using
H O as an Oxygen Source in a Photoelectrosynthesis Cell
2
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
a
*,a,c
*,a,d
P. Farràs, C. Di Giovanni, J. N. Clifford, P. Garrido-Barrios, E. Palomares and A. Llobet
DOI: 10.1039/x0xx00000x
A dye-sensitized photoelectrosynthesis cell (DSPEC) has been prepared for the oxidation of alkenes to epoxides and
+
evolution of hydrogen using water as an oxygen source and sunlight. A Ru oxidation catalyst, 2,2 , is used in homogeneous
www.rsc.org/
phase in the anodic compartment to oxidize a water-soluble alkene, 4-styrene sulfonic acid (4-HSS), to the corresponding
epoxide that in acidic solution is hydrolyzed to the diol (4-(1,2-dihydroxyethyl)-benzenesulfonic acid). Concomitantly
protons are generated that diffuse through a proton exchange membrane to a Pt cathode where are transformed into
-
2
hydrogen. Illumination under 1.5 AMG (100 mW cm ) together with an external bias of 0.3 V vs. NHE after 24 h, leads to
the generation of 1.28 Coulombs together with the formation of 6.1 mol of H at the cathodic compartment that
2
corresponds to a faradaic efficiency of 92%. In addition 0.7 mM of the 4-HSS substrate has been oxidized at the anodic
compartment with a conversion yield of 7%. The rate of hydrogen evolution is limited by the oxidation of the organic
-
1
substrate, and the TOF for both reactions is measured to be 0.8 ks .
8
-1
equation below, which involves the input of 37.35 Kcal mol
INTRODUCTION
given its uphill thermodynamics that can be provided
9,10
11
Epoxides are highly useful intermediates for the production of thermically or photochemically with sunlight. This strategy
1
,2
a variety of important commercial products, and therefore also benefits from the formation of hydrogen as a byproduct
their synthesis is a subject of substantial academic and that is also a clean energy vector.12,13 Given the fact that the
industrial interest. Olefin epoxidation is one of the main paths worldwide production of ethylene oxide represents 20 million
that lead to the production of epoxides both at a laboratory tons per year, and assuming that the reaction is driven by
and at the industrial scale. While molecular oxygen can be sunlight and with a 100 % atom economy, it would represent
14
3
used for the epoxidation of ethylene, the majority of alkene around 0.9 million tons of hydrogen produced in that process,
epoxidation reactions are carried out using stoichiometric or 2500 tons of hydrogen per day. In terms of energy, this
amounts of alkylhydroperoxides or peracids, with the represents the production of an impressive 2700 MWh per day
formation of the corresponding alcohol or carboxylic acid just for this reaction. The same conceptual scheme can be
respectively as an undesired side product. In addition, from a applied for the oxidation of aldehydes to acids, the oxidation
safety perspective, peracids are relatively hazardous of organic sulfides to sulfoxides, etc.
chemicals. In this respect, there is a strong need for the To achieve the reaction shown in equation (1), a set of
development of new epoxidation methods that make use of reactions catalyzed by transition metal complexes is needed.
9
safer oxidants and minimize waste products. The employment For the thermal case, Ru pincer type of complexes have been
of hydrogen peroxide is an attractive option both on used for the oxidation of aldehydes to carboxylic acids and
4-7
environmental and economic grounds giving just water as a formation of hydrogen with a single catalyst in homogeneous
byproduct. Another option is to use water as the source of phase.
oxygen for the epoxidation reaction with the formation of A different strategy consist on taking advantage of the intrinsic
hydrogen as a valuable side product as indicated in the redox nature of the chemical reactions occurring and separate
the single reaction shown in equation (1) into two half
reactions. This would allow building an electrochemical cell
where the oxidation of the alkene will take place at the anode
and the proton reduction will occur at the cathode, as
depicted for styrene in equations (2) and (3) respectively.
a.
Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16,
E-43007 Tarragona, Spain.
Molecular Photonics Laboratory, School of Chemistry, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK.
ICREA, Avinguda Lluís Companys 28, Barcelona E-08030, Spain.
Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del
Vallès, E-08193 Barcelona, Spain.
b.
c.
d.
†
Electronic Supplementary Information (ESI) available: Experimental section
containing the preparation of photoanodes and modified membranes, cell
construction and additional spectroscopic, electrochemical, and catalytic data. See
DOI: 10.1039/x0xx00000x
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+
Furthermore, if the reaction is to be driven photochemically, a Figure 1. Drawn structure for 2,2 complex and catalytic
View Article Online
DOI: 10.1039/C5GC01589H
light harvesting molecule is needed since sunlight interacts scheme proposed for the epoxidation of styrene by the active
with H O and alkenes mainly at a vibrational level. Thus a species derived from 2,2 . In the scheme the 2,2’:6’,2”-
series of additional reactions involving light harvesting terpyridine (trpy) and pyrazolate-3,5-dicarboxylate (pdc )
molecules capable of interacting with suitable catalysts will ligands are not shown.
need to be designed in order to break and form the desired
+
2
3-
bonds at a reasonable rate.
The oxidation of substrates induced by visible light using water
as an oxygen source has been previously described in the
2+
literature in homogeneous phase using [Ru(bpy)3] (bpy is
,2’-bypyridine) as a light harvesting molecule coupled to a
sacrificial electron acceptor (SEA), such as sodium persulfate or
2
2+ 15-19
Co(III) complexes like [Co(NH ) Cl] .
Other examples
3
5
include the use of catalyst-photosensitizer dyad systems that
2
0-25
can oxidize alcohols, sulfides and alkenes.
On the other
hand the reduction of protons has also been achieved with a
26,27
completes the octahedral geometry around the metal center.
variety of sacrificial electron donors (SED).
In both cases
+
Complex 2,2 has been shown to be a powerful epoxidation
the sacrificial agents react fast and in an irreversible manner so
that they favor only the desired reactions. The development of
photoelectrosynthesis cells (DSPEC) to carry out the reaction in
equation (1) implies necessarily the absence of sacrificial
agents and thus introduces further difficulties in its overall
-
catalyst for a variety of alkenes including styrene (TOF > 2.9 s
)
i
1
38
in organic solvents and using PhIO as sacrificial oxidant.
The catalytic mechanism proposed is shown in Figure 1. In
+
order to introduce the 2,2 catalyst into a DSPEC cell it is
-
28-37
indispensable that it also works in water and is activated by 1e
implementation.
Today there is an urgent need to build
3
+
outer sphere electron transfer oxidants such as [Ru(bpy) ] .
3
DSPECs and understand and master all the kinetic and
thermodynamic parameters involved in such a cell so that
efficient devices can be obtained.
For this purpose we have carried out the photochemically
3
+
induced generation of [Ru(bpy)3]
in water using
2+
III
[
Co(NH ) Cl] (Co ) as a sacrificial electron acceptor in the
3
5
With all this in mind, we have designed a DSPEC using
presence of the water soluble sodium 4-styrene sulfonate as
substrate. Since we have carried out all the reactions at pH =
FTO/TiO -P-bpy-Ru (where FTO is fluorine doped tin oxide, P-
2
II
2+
bpy-Ru is [Ru (P-bpy)(bpy) ] , P-bpy is 2,2'-bipyridine-4,4'-
2
1
.0, the latter is protonated forming 4-styrene sulfonic acid (4-
bis(phosphonic acid), bpy is 2,2’-bypyridine) as a photoanode
and a Pt mesh as a cathode. The oxidation catalyst will be
present in homogeneous phase since this configuration gives a
high degree of flexibility with regard to the selection and use
of potential catalysts and reactions. The latter has to comply
with two indispensable requirements: a) the catalyst has to act
efficiently with regard to substrate oxidation and b) the active
site of the catalyst needs to be regenerated very fast by the
photoanode. Finally, a specific proton exchange membrane
3
9
HSS).
+
A deoxygenated solution containing 2,2 0.01 mM/4-HSS 10
mM/[Ru(bpy)3] 0.1 mM/Co 10 mM in 4 mL of a
2+
III
D O:CF SO D mixture (pD 1.0) was irradiated with visible light
2
3
3
for 30 min and generated 1.1 mM 4-(1,2-dihydroxyethyl)-
benzenesulfonic acid. The latter is a known phenomenon due
to the ring-opening hydrolysis of the epoxide under acidic
conditions, and was already reported by Fukuzumi et al. for
40
the chemical oxidation of epoxides using a Ru catalyst. The
conversion of the initial substrate reached 30% that represents
based on
a
combination of Nafion® and poly(3,4-
ethylenedioxythiophene) (PEDOT) has been designed so that
cationic species in the anodic compartment do not block the
proton channel.
Here on we report a two compartment DSPEC cell for H2
generation and epoxidation of alkenes using visible light and
water and its complete kinetic and thermodynamic
characterization.
-1
a TOF of 0.17 s (see Figure S1 for details). This results shows
3+
that [Ru(bpy)3] is capable of activating the catalyst to its
+
active state, 4,4 (see Figure 1 and equation (4)), and that the
latter is capable of oxidizing the styrene substrate to styrene
RESULTS AND DISCUSSION
+
2
.1 Alkene epoxidation catalyzed by 2,2 in homogeneous
phase.
+
We have recently reported a dinuclear Ru-aqua 2,2 complex,
whose structure is shown in Figure 1 and that contains a
dinucleating trianionic ligand backbone (pyrazolate-3,5-
3-
dicarboxylate, pdc ) that bridges the two Ru metal centers and
a tridentate meridional ligand (2,2’:6’,2”-terpyridine, trpy) that
2
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DOI: 10.1039/C5GC01589H
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Figure 2. Left, schematic drawing of a photoanode. Right, transient absorption spectroscopy (TAS) kinetics of FTO/TiO -P-bpy-Ru,
2
TiO -P-tBu/ photoanodes in a 0.1 M LiClO /HClO aqueous solution (pH 1.0) in the presence of 42.5 mM 4-HSS. Black, in the
2
4
4
+
+
absence of catalyst; red, with the addition of 30
under 10 mW cm , using laser excitation pulses at 450 nm and recorded at 650 nm.

M
3,3 ; blue, with the addition of 220
M
3,3 . TAS kinetics were recorded
-
2
oxide that is finally hydrolyzed to the corresponding diol has a broad absorption band. In black is shown the kinetics in
equations 4-6). The reaction mechanism for this catalyst was the absence of catalyst whereas the red trace contains 30 M
(
+
+
previously studied in our group.38 If no 4,4+ had been 3,3 and the blue one 220
produced, only the oxidation by [Ru(bpy)3] would have been oxidation state 3,3 since, as we have shown previously, the
observed. In that case, the reaction without catalyst is not lower oxidation states are precursor to the real catalytic
M 3,3 . We add the catalyst in
3+
38
+
+
selective and produced only the aldehyde derivative (see species that are in oxidation states 3,4 and 4,4 as indicated at
Figure S1).
the right hand side of Figure 1.
In Figure 3 is depicted a scheme that summarizes the most
significant reactions that can occur at the photoanode
2
.2 The performance of the photoanode
As a photoanode we use FTO coated with TiO containing both compartment and that will be analyzed in detail here on. In the
2
II,2+
P-bpy-Ru and tert-butylphosphonic acid (P-tBu), (see Figure absence of catalyst the system decays with a half lifetime of
S3 for UV-vis spectroscopic characterization). The latter about 4.1 ms which is attributed to the recombination reaction
minimizes interactions of the catalyst with the TiO surface and shown in eq. 7 (red arrow 6 in Figure 3) once the electron is
2
thus avoids undesirable back electron transfer (ET) reactions injected into the TiO conduction band. The latter is known to
2
4
3
improving the yield of the productive reactions. The same happen within the picosecond time scale.
strategy has been used in dye sensitized solar cells (DSC) with In the presence of low concentration of catalyst 3,3 the bpy-
+
4
1,42
III,3+
phosphinate amphiphiles.
We label this photoanode as Ru
signal decays at a similar rate. However as the catalyst
M the decay of the
signal becomes faster by almost an order of
FTO/TiO -P-bpy-Ru, TiO -P-tBu/ and a schematic drawing of concentration is increased from 30 to 220
its structure is offered in Figure 2. Transient absorption bpy-Ru

2
2
III,3+
spectroscopy (TAS) was carried out in 0.1 M LiClO /HClO4 magnitude with a t1/2 ≈ 0.02 ms. A dependence of dye
4
aqueous solution at pH = 1.0 with this photoanode in the
presence of the catalyst and substrate in order to check the
capacity of this anode to activate the catalyst with light and to
kinetically characterize the reactions involved.
Figure 2 shows the results obtained upon illuminating under
-
2
1
0 mW cm , using laser excitation pulses at 450 nm and
3+
recording at 650 nm where the anchored [Ru(bpy)3] moiety
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regeneration on red/ox couple concentration is also not Pt mesh as cathode. The anodic compartment contains a 0.1 M
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44,45
+
DOI: 10.1039/C5GC01589H
uncommon in DSCs.
The notable increase in signal intensity solution of LiClO and 2,2 in a 0.1 M perchloric acid solution
4
Figure 3. Scheme showing the most significant reactions that Figure 4. Up, linear sweep voltammetry or I-V curve of the
can occur at the photoanode compartment. Red arrows DSPEC upon 1.5 AMG irradiation with a Xe lamp ( > 400 nm,
indicate unproductive reactions whereas blue arrows are 100 mW cm ) (red line) or without light (black line) at a scan

-
2
-
1
desired reactions.
rate of 10 mV s . The anodic half-cell contains FTO/TiO -P-
2
-8
-2
bpy-Ru, TiO -P-tBu/ ( = 8.8 x 10 mol cm ) as the working
2
+
electrode, 0.01 mM 2,2 as catalyst and 4-HSS 20 mM as
substrate. The cathodic half-cell consists of a Pt mesh as
auxiliary electrode. In both cases the supporting electrolyte
solution is made of a 0.1 M LiClO /HClO aqueous solution (pH
4
4
1
.0) up to a total volume of 9 mL in each compartment. Down,
hydrogen evolution and current intensity measured at 0.3 V vs.
NHE applied bias for the same DSPEC under exactly the same
conditions.
at slower timescales (0.1-1 ms) is attributed to the formation
+
of a new species which we ascribe to the 4,4 state of the
catalyst according to Equations (8) and (9) (blue arrows 3 and 4
+
in Figure 3). The decay of the 4,4 state is due to the oxidation
of the 4-HSS substrate concomitant with the generation of the
+
catalyst, 3,3 species (blue arrow 5 in Figure 3). Due to the low
signal to noise ratio of the kinetic traces we were not able to
record the transient spectra of the long-lived species,
+
however, we note that the 4,4 state of the catalyst (generated
in solution by bulk electrolysis at 1.12 V) also has an
absorption band at 650 nm (see Figure S2).
The fast kinetics observed in the catalyst oxidation all the way
+
to 4,4 together with the generation of oxidized substrate (vide
infra), indicates that the undesired back electron transfer from
the (-)TiO and the catalyst (red arrows 7 and 8 in Figure 3)
2
takes place at longer than ms timescale. On the other hand
this fast forward kinetics enables the use of this set up for the
construction of a DSPEC, as described in the next section.
whereas the cathodic compartment contains the same
solution in the absence of the catalyst. LiClO is added to
4
enhance current densities since it decreases recombination
2
.3 The photoelectrosynthesis cell
III
®
events by stabilizing the Ti trap sites (see Figures S5 and S6
As a proton exchange membrane, Nafion has been widely
used in fuel cells.
species present in solution would adsorb at the surface of the
Nafion film blocking the membrane and sequestering the
catalyst. To avoid this, we have deposited a film of poly(3,4-
ethylenedioxythiophene) (PEDOT) on the surface of Nafion
and generated a new membrane with practically the same
conductivity and proton transfer kinetics as Nafion and where
48,49
11,46
for photocurrent enhancement using LiClO
).
4
However, in our case, catalytic cationic
For this type of cells based on TiO as semiconductor, a small
2
50
®
external electrical bias is generally needed. In the present
case we apply a 0.30 V vs. NHE bias (all redox potentials in this
work are reported vs. NHE), which should be an enough bias as
judged by the LSV voltammetry shown in Figure 4 (up) in the
presence and absence of sun simulated light irradiation. Once
all parameters had been optimized, the DSPEC is run for 24 h
until no more hydrogen is produced under 1.5 AMG (Xe lamp,
4
7
®
®
the catalysts is not adsorbed (see SI and Figure S3 for details).
With this membrane we have built a two compartment DSPEC
using FTO/TiO -P-bpy-Ru, TiO -P-tBu/ as a photoanode and a
-
2

> 400 nm, 100 mW cm ) with the mentioned 0.30 V applied
2
2
4
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external bias. Both photocurrent intensities and hydrogen epoxidation catalysis with simulated sunlight and small
View Article Online
DOI: 10.1039/C5GC01589H
evolution are monitored and are depicted in Figure 4 (down).
After 24 h, 1.28 C (13.2
through the external circuit together with the formation of 6.1 thermodynamics and kinetics of the main reactions involved in
mol of H at the cathodic compartment that corresponds to a the cell. We also underline the unproductive reactions as well
potential applied bias. In addition the byproduct is hydrogen

mols of electrons) have passed that can be used as a solar fuel. Here we establish the

2
faradaic efficiency of 92%. In addition 0.5 mM of the 4-HSS as the decomposition pathways that need to be improved to
substrate have been oxidized to 4-(1,2-dihydroxyethyl)- come up with a technologically useful device. It is also
benzenesulfonic acid (see Figure S7) at the anodic important to highlight that the present model DSPEC
compartment. Overall, a 7% conversion yield was found from configuration possesses a high degree of chemical versatility
the disappearance of the starting material, corresponding to since the catalyst is used in homogeneous phase and thus it
7
0% selectivity towards diol formation, which is in accordance can be adapted to other substrates provided the right catalyst
+
with the selectivity of catalyst 2,2 in chemical oxidation is used.
reactions. This represents 70 TON with a TOF of 0.8 ks for
the oxidation of the organic substrate and 68 TON with a TOF
of 0.8 ks for the production of hydrogen. In the absence of
the organic substrate or Ru-catalyst or Ru-photosensitizer, the
currents obtained were negligible in all cases (see Figure S6).
As can be observed in Figure 4 (down), as the photocatalytic
reaction proceeds the current intensity decreases up to
practically zero after 24 h. The main deactivation pathway is
de-attachment of the Ru-phosphonate dye from the surface of
38
-1
-1
Acknowledgements
We thank MINECO (CTQ-2013-49075-R, SEV-2013-0319,
CTQ2014-52974-REDC), the EU COST actions CM1202 and
CM1205, and "LaCaixa Foundation". PG-B thanks "LaCaixa
Foundation" for a PhD grant. CDG is grateful for an FPI grant
from MINECO. EP and JC would like to thank ICIQ and ICREA
for their financial support.
the TiO due to the hydration of the phosphonate bipyridyl
2
ligands (see Figure S8). It can thus be inferred that the current
intensity decrease over time shown in Figure 4 (down) References
basically correlates with Ru leaching. This low stability of the
1
G. Grigoropoulou, J. H. Clark and J. A. Elings, Green Chem.,
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K. Bauer, D. Garbe and H. Surburg, In Common Fragrance
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G. Sienel, R. Rieth and K. T. Rowbottom, In Ullmann’s
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dye on the TiO -surface is a phenomenon that has been
2
1
5
5
reported earlier and that needs to be dramatically minimized
in order to build more stable devices. Promising solutions
2
5
2
involve the use of atomic layer deposition 5 (ALD),
3
3
electropolymerization of alkene functionalized dyes or dip-
coating of polymers, all of which have proven to improve the
stability of photosensitizers in DSPEC.
54
1
999.
4
5
6
K. Sato, M. Aoki, M. Ogawa, T. Hashimoto and R. Noyori, J.
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This model DSPEC represents the first example of a molecular
device that performs the epoxidation of an alkene and
production of hydrogen simultaneously using the most
abundant and green reagents available: sunlight and water. It
is also the first example where the catalysts operates in
homogeneous phase and thus gives a large flexibility to the
system since the catalyst can be easily changed. The
configuration used here in turn also allows foreseeing a large
7
8
9
scope, of not only different type of catalytic oxidative reactions 10 J. H. Choi, L. E. Heim, M. Ahrens and M. H. Prechtl, Dalton
but also of substrates to be converted.
Trans., 2014, 43, 17248-17254.
1 P .Farràs, C. Di Giovanni, J. N. Clifford, E. Palomares and A.
Llobet, Coord. Chem. Rev., 2015, 304, 202-208.
2 D. Kalita, B. Radaram, B. Brooks, P. P. Kannam and X. Zhao,
ChemCatChem, 2011, 3, 571– 573.
1
1
The literature related to using water as oxygen source and at
the same time generate dihydrogen, is extremely scarce and
9,36,55
limited solely to the oxidation of alcohols.
An interesting
example is described for the oxidation of benzylalcohol to 13 L. Zang, In Energy Efficiency and Renewable Energy Through
Nanotechnology; Springer: Berlin, 2011.
14 "Ethylene Oxide". WP Report. SRI Consulting. January 2009.
Retrieved 2009-09-29.
benzaldehyde and hydrogen genation but with much lower
operation times and much lesser faradaic efficiencies.
3
6
1
1
1
5 S. Fukuzumi, T. Kishi, H. Kotani, Y. M. Lee and W. Nam, Nat.
Chem., 2011, , 38-41.
6 F. Li, M. Yu, Y. Jiang, F. Huang, Y. Li, B. Zhang and L. Sun,
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7 S. Ohzu, T. Ishizuka, Y. Hirai, S. Fukuzumi and T. Kojima,
Chem. Eur. J., 2013, 19, 1563-1567.
3
Conclusions
The use of water as an oxygen source in combination with
sunlight for the oxidation of organic substrates is one of the
hot topics the green oxidation chemistry is facing today. In the 18 X. Zhou, F. Li, X. Li, H. Li, Y. Wang and L. Sun, Dalton Trans.,
present paper we present for the first time a model DSPEC
2014, 44, 475-479.
that transform alkenes into the corresponding diols via
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1
2
2
2
2
2
2
2
2
9 P. Farràs and A. C. Benniston, Tetrahedron Lett., 2014, 55
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