Photocatalytic degradation of phenolic pollutants using N-methylquinolinium and 9-mesityl-10-methylacridinium salts

Article abstract of DOI:10.1016/j.cattod.2019.01.045

The photodegradation of a mixture of phenolic pollutants including: phenol (P), orto-phenylphenol (OPP), 2,4,6-trichlorophenol (TCP) and pentachlorophenol (PCP) was accomplished using two organic cationic photocatalysts, namely N-methylquinolinium (NMQ⁺) and 9-mesityl-10-methylacridinium (Mes-Acr-Me⁺) salts, due to their singular photophysical and redox properties. On one hand, NMQ⁺ exhibits more energetic excited states and accordingly more favorable redox potentials than Mes-Acr-Me⁺; on the other hand, NMQ⁺ absorption reaches only up to 380 nm, while Mes-Acr-Me⁺ extends in the visible up to 480 nm. Evaluation of the efficiency of both photocatalysts, revealed that the highest level of photodegradation was achieved when they were employed at 20% mol. Specifically, with NMQ⁺, removal of the pollutants was completed within 24 h of irradiation. Even more, irradiation time could be shortened from 24 to 8 h, since high levels of removal were already achieved (93%, 100%, 100% and 82% for P, OPP, TCP and PCP, respectively). Albeit, Mes-Acr-Me⁺ was not as effective, and best results were obtained using 20% mol upon 24 h of irradiation. Under these conditions, removal of PCP was 80%, while TCP was 40%, OPP 30% and P resulted in the most recalcitrant contaminant with only 10% of removal. Next, NMQ⁺ and Mes-Acr-Me⁺ were separately supported onto Zeolite Y, an inert inorganic support (Y-NMQ⁺ and Y-Mes-Acr-Me⁺), and elemental analyses revealed a loading of ca. 13% and 15% weight for NMQ⁺ and Mes-Acr-Me⁺, respectively. Upon heterogenization, in the case of Y-NMQ⁺, the extent of removal was lower than the one achieved in the homogeneous photodegradations. On the contrary, performance of Y-Mes-Acr-Me⁺ improved, because of its enhanced photostability; thus, upon 46 h irradiation, 98%, 80%, 40% and 26% for PCP, TCP, OPP and P, respectively, was achieved. Moreover, their efficiency was maintained upon second use. Steady-state and time-resolved fluorescence quenching revealed that every pollutant was able to quench the singlet excited state of both ¹(NMQ⁺)^* and ¹(Mes-Acr-Me⁺)^*, with kinetic rate constants in the order of the diffusion limit. Thus, Type I photooxidation happening through the singlet excited state of either photocatalyst was the main operating process in the photodegradation of the studied pollutants.

Full text of DOI:10.1016/j.cattod.2019.01.045

CatalysisTodayxxx(xxxx)xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Photocatalytic degradation of phenolic pollutants using N-
methylquinolinium and 9-mesityl-10-methylacridinium salts
R. Martinez-Haya, M.M. Luna, A. Hijarro, E. Martinez-Valero, M.A. Miranda⁎, M.L. Marin⁎
Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, E-46022, Valencia, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Electron transfer
Photocatalysis
Singlet excited state
Steady-state fluorescence
Time-resolved fluorescence
The photodegradation of a mixture of phenolic pollutants including: phenol (P), orto-phenylphenol (OPP), 2,4,6-
trichlorophenol (TCP) and pentachlorophenol (PCP) was accomplished using two organic cationic photo-
catalysts, namely N-methylquinolinium (NMQ+) and 9-mesityl-10-methylacridinium (Mes-Acr-Me+) salts, due
to their singular photophysical and redox properties. On one hand, NMQ+ exhibits more energetic excited states
and accordingly more favorable redox potentials than Mes-Acr-Me+; on the other hand, NMQ+ absorption
reaches only up to 380 nm, while Mes-Acr-Me+ extends in the visible up to 480 nm. Evaluation of the efficiency
of both photocatalysts, revealed that the highest level of photodegradation was achieved when they were em-
ployed at 20% mol. Specifically, with NMQ+, removal of the pollutants was completed within 24 h of irradia-
tion. Even more, irradiation time could be shortened from 24 to 8 h, since high levels of removal were already
achieved (93%, 100%, 100% and 82% for P, OPP, TCP and PCP, respectively). Albeit, Mes-Acr-Me+ was not as
effective, and best results were obtained using 20% mol upon 24 h of irradiation. Under these conditions, re-
moval of PCP was 80%, while TCP was 40%, OPP 30% and P resulted in the most recalcitrant contaminant with
only 10% of removal. Next, NMQ+ and Mes-Acr-Me+ were separately supported onto Zeolite Y, an inert in-
organic support (Y-NMQ+ and Y-Mes-Acr-Me+), and elemental analyses revealed a loading of ca. 13% and 15%
weight for NMQ+ and Mes-Acr-Me+, respectively. Upon heterogenization, in the case of Y-NMQ+, the extent of
removal was lower than the one achieved in the homogeneous photodegradations. On the contrary, performance
of Y-Mes-Acr-Me+ improved, because of its enhanced photostability; thus, upon 46 h irradiation, 98%, 80%,
40% and 26% for PCP, TCP, OPP and P, respectively, was achieved. Moreover, their efficiency was maintained
upon second use. Steady-state and time-resolved fluorescence quenching revealed that every pollutant was able
*
to quench the singlet excited state of both 1(NMQ+
)
and 1(Mes-Acr-Me+)*, with kinetic rate constants in the
order of the diffusion limit. Thus, Type I photooxidation happening through the singlet excited state of either
photocatalyst was the main operating process in the photodegradation of the studied pollutants.
1. Introduction
processes (AOPs) to deal with the organic matter present in the was-
tewaters [4]. Among AOPs, photocatalysis constitutes an emerging
Extensive agriculture is currently a motor of the economy for many
countries; however, it is accompanied with several environmental
problems such as massive use of pesticides, spoiled soils or use of huge
volumes of water. In the Mediterranean area, production of olives ac-
counts for approximately 63% of world production [1]. However, one
of the main drawbacks associated with this industry is the great amount
of wastewaters generated by olive oil production [2]. These waste-
waters contain high concentrations of phenolic compounds, lipids, su-
gars and polyphenols [3]; therefore, they require specific pretreatments
to prevent serious problems in the usual biological treatment processes.
Special attention has recently been paid to advanced oxidation
green process with a proven potential for water remediation [5]. More
specifically, organic photocatalysts offer the possibility of investigating
the reaction mechanisms involved in photooxidation upon studying the
reactivity of the photogenerated excited states by means of time-re-
solved techniques in the nano or micro-second scale [6].
With this background, the aim of the present work is to investigate
the photodegradation of a mixture of phenolic pollutants (Fig. 1) in-
cluding: phenol (P), orto-phenylphenol (OPP), 2,4,6-trichlorophenol
(TCP) and pentachlorophenol (PCP). They are generated in agriculture
due to the widespread use of antifungals or pesticides [7]. To achieve
the photodegradation of this group of pollutants, herein we have
⁎ Corresponding authors.
E-mail addresses: mmiranda@qim.upv.es (M.A. Miranda), marmarin@qim.upv.es (M.L. Marin).
https://doi.org/10.1016/j.cattod.2019.01.045
Received 14 July 2018; Received in revised form 5 January 2019; Accepted 15 January 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Martinez-Haya,R.,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.01.045

R. Martinez-Haya et al.
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Fig. 1. Chemical structures of the selected pollutants.
Fig. 2. Chemical structures, redox and photophysical properties of NMQ+ and Mes-Acr-Me+. aEquilibrium between the locally excited and the charge transfer singlet
species. bThe triplet species could be assigned to the locally excited and/or the charge transfer species [9–11].
focused our attention on two organic cationic photocatalysts, namely N-
methylquinolinium (NMQ+) and 9-mesityl-10-methylacridinium (Mes-
Acr-Me+) salts [8], due to their singular photophysical and redox
properties (Fig. 2) [9–11]. Among their potential advantages as pho-
tocatalysts, they do not contain metals and, being cationic, they should
allow for efficient photoinduced electron transfer, due to the lack of
Coulombic attraction between the resulting species. Moreover, al-
though both cations can generate singlet oxygen, previous results from
our group have demonstrated that the contribution of this reactive
oxygen species for the overall oxidation at typical pollutant con-
centrations is negligible compared to the photoinduced electron
transfer pathway at the typical pollutant concentration in water
[12,13]. Even more, although these photocatalysts have attracted some
attention in the recent years [9,12,14,15], to the best of our knowledge,
neither Mes-Acr-Me+ nor related acridinium salts have been employed
in pollutants treatment.
photocatalyst (Fig. 2). Nevertheless, NMQ+ absorption reaches only up
to 380 nm, while Mes-Acr-Me+ extends in the visible up to 480 nm
(Figure S1). Whether one or the other parameter is more important
would depend on the performance at the specific context and on the
energetic and cost balances. Furthermore, while NMQ+ must be syn-
thesized, Mes-Acr-Me+ is commercially available.
Therefore, photodegradation of the mixture of phenolic pollutants
has been evaluated in parallel with the two photocatalysts. In addition,
both NMQ+ and Mes-Acr-Me+, have been supported onto zeolite Y-100
to investigate the abatement of pollutants in heterogeneous phase.
Finally, a detailed time-resolved photophysical study has been under-
taken to elucidate the operating mechanism.
2. Experimental
In spite of the fact that NMQ+ and Mes-Acr-Me+ share several
advantages, they have certain properties that make them different en-
ough to deserve a deeper evaluation. For instance, NMQ+ exhibits more
energetic excited states than Mes-Acr-Me+; this combined with the
more favorable redox potential makes NMQ+ more attractive as Type I
2.1. Chemicals
Phenol,
orto-phenylphenol,
2,4,6-trichlorophenol,
penta-
chlorophenol, p-xylene and 9-mesityl-10-methylacridinium perchlorate
were purchased from Sigma Aldrich. Zeolite Y 100 was obtained from
Zeolyst International. Water used in electrochemical measurements and
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photodegradation experiments was Milli-Q grade; acetonitrile (ACN)
was of HPLC quality from Scharlau and dimethyl sulfoxide (DMSO) was
from Across. N-Methylquinolinium tetrafluoroborate salt was synthe-
sized from quinoline (Sigma Aldrich) in two steps as previously de-
scribed [16,17]. Briefly, quinoline (0.42 mol) and methyl iodide
(0.64 mol, 1.5 eq) were heated at 65 °C, under reflux for 24 h; after-
wards, the resulting solid crude (N-methylquinolinium iodide) was
washed with ether (see spectroscopic details in Figure S2). The solid N-
methylquinolinium iodide was treated with BF3·Et2O (1.1 mol, 2.6 eq)
at 50 °C under stirring for 2.5 h, under nitrogen atmosphere. An addi-
tional amount of BF3·Et2O (1.1 mol) was added and the mixture was
stirred for further 2.5 h. The resulting solid was washed with ether and
recrystallized from ethanol to yield N-methylquinolinium tetra-
fluoroborate (see spectroscopic details in Figure S3).
Photocatalysts adsorption onto zeolite Y 100 was performed ac-
cording to a previously described procedure [18]. Briefly, for the het-
erogeneization of NMQ+ (Y-NMQ+), Y-zeolite 100 (6.2 g) was sus-
pended on an aqueous solution (25 mL) containing NMQ+
tetrafluoroborate (1.0 g). The mixture was stirred at 40 °C for 24 h in
the dark. Then, the solid was filtered, washed with water (30 mL) and
dried at 100 °C for 72 h. Heterogeneization of Mes-Acr-Me+ (Y-Mes-
Acr-Me+) was performed as follows [19]: Y-zeolite 100 (5 g) was sus-
pended on a ACN:H2O (1:1) solution (20 mL) containing Mes-Acr-Me+
perchlorate (0.82 g). The mixture was stirred at 40 °C for 24 h in the
dark. Then, the solid was filtered, washed with water (30 mL) and dried
at 100 °C for 72 h. Loading of NMQ+ and Mes-Acr-Me+ on Y-NMQ+
and Y-Mes-Acr-Me+ were 13% wt and 15% wt, respectively, according
to the elemental analysis.
For the heterogeneous photoreactions, aqueous mixtures of 15 mL
containing the four pollutants (5 × 10−5 M each) and the hetero-
geneous photocatalyst (93 mg L−1
,
26% mol, for Y-NMQ+ or
160 mg L−1, 30% mol in the case of Y-Mes-Acr-Me+), were stirred for
60 min in dark and then irradiated under air, using the above described
Luzchem photoreactor. Aliquots, at different irradiation times, were
centrifuged twice at 6000 rpm for 15 min to remove the photocatalyst,
and the supernatant submitted to the HPLC analysis.
For the study of the recyclability of the heterogeneous photo-
catalysts, they were recovered as follows: An aqueous mixture of 15 mL
containing the four pollutants and Y-NMQ+ or Y-Mes-Acr-Me+ was
subjected to irradiation as described above. After 46 h, an aliquot was
taken to check that the progress of the reaction was as expected. Then,
the reaction mixture was centrifuged (8000 rpm for 5 min), the super-
natant was removed, and the heterogeneous photocatalyst was washed
with 25 mL of clean distilled water for 1 h upon stirring. Then, it was
centrifuged again (8000 rpm for 5 min), the supernatant was removed,
and the photocatalyst was dried in the oven for 24 h at 100 °C, prior to
second use.
The HPLC used for monitoring the progress of the photodegrada-
tions was an Agilent 1100 Series model with quaternary pump
G1311 A, photodiode detector VWD G1314 A, standard liquid auto-
sampler G1313 A and degasser G1322 A. A Mediterranea Sea 18 column
(25 cm × 0.46 cm, 5 μm particle size) was employed. The mobile phase
was fixed at 1.5 mL min−1 with an isocratic mixture of water pH 3
(30%) and acetonitrile (70%). To monitor removal of the pollutants,
100 μL of a ACN:H2O (4:1) solution of p-xylene (3.2 × 10−4 M) were
added as internal standard to every sample (500 μL), prior to injection.
Then, aliquots of 90 μL from these samples were injected, and detection
wavelength was fixed at 215 nm.
2.2. Cyclic voltammetry
Cyclic voltammetric experiments were carried out using a cylind-
rical three-electrode quartz cell on a VersaSTAT 3 (Princeton Applied
Research) electrochemical workstation with a glassy carbon (GCE)
working electrode, a Pt wire counter electrode and a AgCl/Ag (sat KCl)
as the reference electrode, in a one compartment electrochemical cell.
The GCE and the Pt electrodes were polished using diamond spray
(particle size 0.05 μm) before each experiment.
Pollutant stock solutions (1 mM) were prepared in mixtures
H2O:DMSO (24:1) for P, OPP and TCP, and in ACN for PCP. The cyclic
voltammetries were carried out at room temperature, under a constant
flux of N2 using solutions of 30 μM of each pollutant in 0.1 M aqueous
phosphate buffer pH 7 for P, OPP and TCP or in 0.1 M tetrabutyl am-
monium perchlorate in ACN for PCP. The speed for the measurements
was fixed at 0.05 V·s−1. The values of the redox potentials were cal-
culated as the average between the maximum and minimum of the
cyclic potential scan curves when the redox reactions were reversible,
or from the maximum obtained in the voltammograms for the irrever-
sible processes (Figure S4). The obtained data from the AgCl/Ag (sat
KCl) were converted into redox potential values vs standard calomel
electrode (SCE) as follows: E (vs SCE, in V) = E (vs AgCl/Ag, in V)
-0.045 (Table S1).
2.4. Photophysical instrumentation
A Shimadzu UV-2101PC spectrophotometer was employed to obtain
the UV/Vis absorption spectra of the photocatalysts (Figure S1) and the
pollutants (Figure S5). Steady-state and time-resolved fluorescence ex-
periments were performed with a Photon Technology International
(PTI) LPS-220B spectrofluorometer and with a EasyLife V spectro-
fluorometer from OBB, respectively. In the case of time-resolved
fluorescence, the excitation source was equipped with a pulsed LED
(λexc = 310 nm and 407 nm for NMQ+ and Mes-Acr-Me+, respec-
tively); residual excitation signal was filtered in emission by using a cut-
off filter (50% transmission at 320 nm and 435 nm for NMQ+ and Mes-
Acr-Me+, respectively). Monoexponential decay functions that use a
deconvolution procedure to separate them from the lamp pulse profile
provided the fitted kinetic traces except in the case of NMQ+ with OPP
in which the decay function was fitted to a biexponential relationship. A
pulsed Nd: YAG SL404G-10 Spectron Laser Systems at the excitation
wavelength of 355 nm was employed to carry out the laser flash pho-
tolysis (LFP) experiments. The energy of the single pulses (∼10 ns
duration) was lower than 15 mJ pulse−1. The laser flash photolysis
system consists of the pulsed laser, a pulsed Lo255 Oriel Xenon lamp, a
77,200 Oriel monochromator, an Oriel photomultiplier tube (PMT)
housing, a 70,705 PMT power supply and a TDS-640 A Tektronix os-
cilloscope.
2.3. Photocatalytic degradations
Homogeneous photochemical reactions were carried out in test
tubes with magnetic stirring using a Luzchem photoreactor (model LZC-
4 V) equipped with lamps emitting at 350 nm (9 bulbs, FL8BL-B model
from Hitachi) or 420 nm (7 bulbs, LZC420 model from Luzchem
Research Inc.), for NMQ+ or Mes-Acr-Me+, respectively. Aqueous so-
lutions (9 mL) containing a mixture of the four pollutants (P, OPP, TCP
and PCP, 5 × 10−5 M each, 2 × 10−4 M in total) were irradiated under
air, in the presence of different photocatalyst ratios (NMQ+ or Mes-Acr-
Me+ at 5, 10 or 20% mol, referred to the total pollutant concentration).
The removal of the pollutants at different irradiation times was mon-
itored by HPLC.
Photophysical measurements were run in solution (ACN:H2O, 4:1,
for the UV spectra experiments and in ACN and for the fluorescence and
LFP experiments), at room temperature, under nitrogen, using quartz
cells of 1 cm optical path length. For the fluorescence quenching ex-
periments, solutions of each photocatalyst with absorbance lower than
0.15 at λexc = 317 nm and 310 nm for NMQ+ (steady-state and time-
resolved, respectively), or 407 nm for Mes-Acr-Me+, were treated with
increasing concentrations of pollutant, up to 5.8 mM, 5.5 mM, 6.1 mM
and 4.6 mM for P, OPP, TCP and PCP, respectively. For the LFP ex-
periments, solutions of Mes-Acr-Me+ with absorbance lower than 0.3 at
λexc = 350 nm, were treated in the absence and in the presence of
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Fig. 3. Plot of the relative concentration of P
(■), OPP (●), TCP (▲) and PCP (♦) at initial
global concentration of C0 = 2 × 10−4 M vs
350 nm irradiation time in the presence of 20%
mol NMQ+ (A) or 420 nm irradiation in the
presence of 20% mol Mes-Acr-Me+ (B) under
aerated atmosphere in aqueous media.
pollutants, up to 1 mM.
stability of both photocatalysts was also monitored by UV absorption
spectra (Figure S10). After 24 h of irradiation, the absorption spectrum
of NMQ+ still contained the main bands, although their intensity de-
creased about 15%, whereas for the case of Mes-Acr-Me+ a significant
degradation was observed (about 60%), in agreement with previous
reports [20,21].
Diffuse reflectance of heterogeneous Y-NMQ+ and Y-Mes-Acr-Me+
were recorded using a Cary 5000 from Agilent Technologies (Figure
S6).
3. Results
Therefore, the higher efficiency of NMQ+ to produce the photo-
degradation of the mixture of pollutants is not only due to its higher
redox potential compared to Mes-Acr-Me+, but also to its higher pho-
tostability.
3.1. Homogeneous photochemical degradations
First, homogeneous photodegradation of aerobic aqueous mixtures
of the four pollutants (P, OPP, TCP and PCP) in the presence of different
molar ratios of each photocatalyst (5, 10 and 20% mol) was evaluated
for 24 h. Irradiation sources were centered at 350 nm or 420 nm, when
using NMQ+ or Mes-Acr-Me+, respectively (Figs. 3, S7 and S8). In both
cases, the highest level of photodegradation was achieved when pho-
tocatalyst was employed at 20% mol, being NMQ+ more efficient than
Mes-Acr-Me+, at every molar ratio. Specifically, with NMQ+, removal
of the pollutants was completed within 24 h of irradiation. Even more,
after 8 h of irradiation, high levels of removal were already achieved
(93%, 100%, 100% and 82% for P, OPP, TCP and PCP, respectively),
which suggests that irradiation time could be shortened from 24 to 8 h.
Yet, Mes-Acr-Me+ was not as effective, and best results were obtained
using 20% mol upon 24 h of irradiation. Under these conditions, re-
moval of PCP was 88%, while TCP was 45%, OPP 35% and P resulted in
the most recalcitrant contaminant with only 16% of removal. From the
irradiation mixtures, no clear-cut peaks were found in the HPLC ana-
lysis in any case, pointing out that under these conditions the pollutants
disappear giving probably rise to a complex mixture of minor photo-
products, with insufficient individual concentrations to be detected.
Control experiments performed in the absence of light showed that
the pollutants remain stable after equivalent reaction periods (results
not shown). Those carried out in order to check direct photolysis re-
vealed that at 350 nm irradiation, direct absorption of light is partially
responsible for the observed photodegradation, since it produced re-
moval of PCP (up to 50%), P (40%), OPP (20%) and TCP (20%) (Figure
S9). Conversely, at 420 nm direct photolysis was marginal, in agree-
ment with the UV spectra of the pollutants (Figure S5). Moreover, the
3.2. Heterogeneous photochemical degradations
A typical strategy to improve the photostability and easy recovery of
the photocatalysts that at the same time increases their potential for
future applications involves heterogeneization, most commonly based
on the adsorption of the organic compound onto a solid inert material,
such as a zeolite [22–25]. For this purpose, NMQ+ and Mes-Acr-Me+
were separately supported onto an inert inorganic support (zeolite Y
100), elemental analysis of the new materials (Y-NMQ+ and Y-Mes-Acr-
Me+) revealed a loading of ca. 13% and 15% weight for NMQ+ and
Mes-Acr-Me+, respectively. Moreover, diffuse reflectance demonstrated
the incorporation of the photocatalysts onto the zeolites as evidenced
by the presence of the typical chromophores absorption (Figure S6).
Next, to evaluate the performance of Y-NMQ+ and Y-Mes-Acr-Me+
,
mixtures of the pollutants (5 × 10−5 M each) in aerobic aqueous
media, were irradiated using 350 nm or 420 nm centered light in the
presence of Y-NMQ+ (26% mol) or Y-Mes-Acr-Me+ (30% mol), re-
spectively (Fig. 4). In the case of Y-NMQ+, the extent of removal was
lower than the one achieved in the homogeneous photodegradations for
all the pollutants, even with longer irradiation times, except in the case
of PCP, whose photodegradation was fully achieved after 30 h. Lower
abatement yields have been reported upon heterogeneization of organic
photocatalysts, and have been attributed to the more difficult diffusion
of the pollutants through the channels of the supports [18]. On the
contrary, for the case of Y-Mes-Acr-Me+, heterogeneization resulted in
better performance, thus within 46 h irradiation, abatement levels
Fig. 4. Plot of the relative concentration of P
(■), OPP (●), TCP (▲) and PCP (♦) at initial
global concentration of C0 = 2 × 10−4 M vs
irradiation time in aerated aqueous media,
upon irradiation at 350 nm in the presence of
Y-NMQ+ (26% mol) (A) and at 420 nm in the
presence of Y-Mes-Acr-Me+ (30% mol) (B).
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Fig. 5. Steady-state (left column) and time-resolved (right column) 1(NMQ+
)
* fluorescence quenching upon addition of increasing concentrations of P (A), OPP (B),
TCP (C) and PCP (D) in ACN under N2; λexc = 310 nm. Insets: Stern Volmer plots obtained from the corresponding steady-state (left) and time-resolved (right)
experiments.
achieved were 98%, 80%, 40% and 26% for PCP, TCP, OPP and P,
respectively, demonstrating the potential of heterogeneous Y-Mes-Acr-
Me+ as a visible light photocatalyst. Control experiments to check the
adsorption of the pollutants onto the zeolites Y-NMQ+ and Y-Mes-Acr-
Me+ indicated that adsorption was lower than 5%, therefore the ob-
served photodegradation was safely attributed to the photocatalyzed
degradation. Thus, the better performance of Y-Mes-Acr-Me+ upon
incorporation into the zeolite could be attributed to the prevention of
nucleophilic attack resulting in lower percentage of photobleaching
[10,19].
respectively, of abatement accomplished in the second use (see figure
S11).
3.3. Involvement of singlet excited species
Steady-state and time resolved fluorescence measurements were
carried out in order to determine the participation of the singlet excited
state of the photocatalysts in the removal of the contaminants, see
Figs. 5 and 6. From these experiments it is clear that every pollutant is
able to quench the singlet excited state of both 1(NMQ+ * and 1(Mes-
)
Acr-Me+)*. Special attention is required to the case of the quenching of
1(NMQ+)* by OPP (Fig. 5B left). The new band observed upon addition
Accordingly, successful results were obtained for recycled Y-Mes-
Acr-Me+: 77%, 29%, 39% and 23% for PCP, TCP, OPP and P,
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*
Fig. 6. Steady-state (left column) and time-resolved (right column) 1(Mes-Acr-Me+
)
fluorescence quenching upon addition of increasing concentrations of P (A),
OPP (B), TCP (C) and PCP (D) in ACN under N2; λexc = 310 nm. Insets: Stern Volmer plots obtained from the corresponding steady-state (left) and time-resolved
(right) experiments.
Table 1
of OPP corresponds to the emission of OPP, obtained from the partial
direct excitation of OPP at 317 nm. Therefore, the time-resolved mea-
surements required a two-exponential fitting for the decay (see Fig. 5B
right). Table 1 reveals an efficient kinetic quenching constants between
the pollutants and the singlet excited states of both photocatalysts, with
Rate constant values for the reaction between the pollutants and NMQ+ and
Mes-Acr-Me+ obtained from steady-state and time-resolved experiments.
kq (M−1s−1
)
*
*
*
*
1(NMQ+
)
1(NMQ+
time-
)
1(Mes-Acr-Me+
steady-state
)
3(Mes-Acr-Me+
time-resolved
)
values close to the diffusion limit (in the order of 1010
M
−1s−1) [26].
steady-state
resolved
3.4. Involvement of triplet excited species
P
2.7 × 1010
1.6 × 1010
4.8 × 1010
1.1 × 1010
8.7 × 109
1.5 × 1010
1.5 × 1010
4.1 × 109
5.2 × 108
7.7 × 109
7.5 × 109
2.4 × 109
1.4 × 109
OPP 2.7 × 1010
TCP 2.1 × 1010
Next step was the evaluation of the involvement of the triplet ex-
cited species of the photocatalysts in the photodegradations. NMQ+ is a
special photocatalyst as previously described: it exhibits a long lifetime
PCP
8.6 × 1010
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Fig. 7. Transient absorption spectra obtained upon laser flash photolysis excitation in deaerated CH3CN:H2O (4:1) (Abs = 0.3 at 355 nm) in the absence (A) or in the
presence of 1 mM of the pollutants: P (B), OPP (C), TCP (D) and PCP (E).
Table 2
Estimated free energy changes for the photoinduced electron transfer from the
pollutants to the singlet excited state of each photocatalyst, calculated ac-
cording to Eq. (1).
ΔGeot NMQ+ (eV)
ΔGeot Mes-Acr-Me+ (eV)
Pollutant
P
−2.04
−1.55
−1.57
−1.03
−1.52
−1.03
−1.05
−0.51
OPP
TCP
PCP
Scheme 1. Photoredox catalytic cycle of NMQ+ and Mes-Acr-Me+ (re-
participation of the triplet in the photodegradation. First, the transient
spectrum of Mes-Acr-Me+ was obtained (Fig. 7A), where the corre-
sponding band located at 450–500 nm can be observed (attributed to
the local or charge transfer triplet species) [9–11]. Next, the spectrum
of the photocatalyst in the presence of each one of the pollutants was
obtained (Fig. 7B-E), revealing the formation of the reduced radical
species of the photocatalyst [9,11], and, therefore, providing evidence
of the redox processes. However, kinetic analysis of the participation of
the triplet in the photodegradations was difficult due to the overlap of
the triplet and radical signals (Fig. 7).
presented as PC+) involving photooxidation of the pollutants (Q).
of its singlet excited state and a high fluorescence quantum yield [6,17].
This fact together with the high values of the kinetic constants obtained
from the fluorescence experiments, points out an efficient quenching
from the singlet, which accordingly, prevents formation of the triplet at
the pollutants concentration employed for the photophysical experi-
ments. On the contrary, for the case of Mes-Acr-Me+, which exhibits
lower kinetic constant values for the singlet excited state quenching,
laser flash photolysis experiments were performed to investigate the
7

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Scheme 2. Potential mechanistic pathway to explain the photooxidation of the pollutants.
3.5. Thermodynamic feasibility of the redox processes
Acknowledgements
As mentioned above, although NMQ+ and Mes-Acr-Me+ are able to
generate singlet oxygen [11,12], photoinduced electron transfer or the
generated superoxide anion must be responsible for the observed pho-
todegradations at the employed pollutant concentration (Scheme 1)
[27,28]. Nevertheless, a thermodynamic analysis of the potential par-
ticipation of the singlet excited state of the photocatalysts can be per-
formed using the photoinduced Gibbs free energy Eq. (1) [29], the
experimentally measured redox potentials values (Figure S4, and table
S1) and the energy of the excited states (Fig. 2).
Financial support from Spanish Government (Grant SEV-2016-
0683) is gratefully acknowledged. Financial support from VLC/Campus.
R. Martinez-Haya thanks financial support for a predoctoral contract
from Apadrina la Ciencia Association and Ford España/Ford Motor
Company Fund.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.cattod.2019.01.045.
ΔGeot = − [Eroed (PC+/PC·) − Eroed (Q·+/Q)] − EP*C
+
(1)
References
where PC+ represents NMQ+ and Mes-Acr-Me+ and Q is any pollutant.
Table 2 reveals that photooxidative degradations are exergonic for
both photocatalysts acting from their singlet excited states. To close the
photocatalytic cycle, formation of superoxide anion from the reduction
of O2 (Eºred = -0.33 V vs SCE) [30] would be exergonic in both cases
(-0.52 eV and -0.16 eV for NMQ+ and Mes-Acr-Me+, respectively)
[27,28].
[1] G. Banias, C. Achillas, C. Vlachokostas, N. Moussiopoulos, M. Stefanou,
Environmental impacts in the life cycle of olive oil: a literature review, J. Sci. Food
Agric. 97 (2017) 1686–1697.
[2] M. Beccari, F. Bonemazzi, M. Majone, C. Riccardi, Interaction between acidogenesis
and methanogenesis in the anaerobic treatment of olive oil mill effluents, Water
Res. 30 (1996) 183–189.
[3] N. Adhoum, L. Monser, Decolourization and removal of phenolic compounds from
olive mill wastewater by electrocoagulation, Chem. Eng. Process. Process. Intensif.
43 (2004) 1281–1287.
[4] C. Amor, M.S. Lucas, J. García, J.R. Dominguez, J.B. De Heredia, J.A. Peres,
Combined treatment of olive mill wastewater by Fenton’s reagent and anaerobic
biological process, J. Environ. Sci. Health 50 (Part A) (2015) 161–168.
[5] S. Malato, P. Fernandez-Ibanez, M.I. Maldonado, J. Blanco, W. Gernjak,
Decontamination and disinfection of water by solar photocatalysis: Recent overview
and trends, Catal. Today 147 (2009) 1–59.
The obtained values reveal that, as expected, singlet excited states of
both photocatalysts could, in principle, participate in the photoinduced
electron transfer. Nevertheless, it is mandatory to perform kinetic stu-
dies to evaluate the viability of the electron transfer in the time scale of
those excited states.
[6] M.L. Marin, L. Santos-Juanes, A. Arques, A.M. Amat, M.A. Miranda, Organic pho-
tocatalysts for the oxidation of pollutants and model compounds, Chem. Rev. 112
(2012) 1710–1750.
3.6. Mechanistic proposal
[7] M.A. Miranda, F. Galindo, A.M. Amat, A. Arques, Pyrylium salt-photosensitised
degradation of phenolic contaminants present in olive oil wastewaters with solar
light: Part II. Benzoic acid derivatives, Appl. Catal. B 30 (2001) 437–444.
[8] S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N.V. Tkachenko, H. Lemmetyinen,
Electron-transfer state of 9-mesityl-10-methylacridinium ion with a much longer
lifetime and higher energy than that of the natural photosynthetic reaction center,
J. Am. Chem. Soc. 126 (2004) 1600–1601.
[9] N.A. Romero, D.A. Nicewicz, Mechanistic Insight into the Photoredox Catalysis of
Anti-Markovnikov Alkene Hydrofunctionalization Reactions, J. Am. Chem. Soc. 136
(2014) 17024–17035.
[10] S. Fukuzumi, K. Ohkubo, T. Suenobu, Long-lived charge separation and applications
in artificial photosynthesis, Acc. Chem. Res. 47 (2014) 1455–1464.
[11] A.C. Benniston, A. Harriman, P. Li, J.P. Rostron, H.J. van Ramesdonk,
M.M. Groeneveld, H. Zhang, J.W. Verhoeven, Charge shift and triplet state for-
mation in the 9-mesityl-10-methylacridinium cation, J. Am. Chem. Soc. 127 (2005)
16054–16064.
[12] R. Martinez-Haya, C. Sabater, M.-Á. Castillo, M.A. Miranda, M.L. Marin, A me-
chanistic study on the potential of quinolinium salts as photocatalysts for the
abatement of chlorinated pollutants, J. Hazard. Mater. 351 (2018) 277–284.
[13] R. Martinez-Haya, M.A. Miranda, M.L. Marin, Type I vs Type II photodegradation of
pollutants, Catal. Today 313 (2018) 161–166.
[14] H. Kotani, K. Ohkubo, S. Fukuzumi, Photocatalytic oxygenation of anthracenes and
olefins with dioxygen via selective radical coupling using 9-mesityl-10-methyla-
cridinium ion as an effective electron-transfer photocatalyst, J. Am. Chem. Soc. 126
(2004) 15999–16006.
[15] H. Kotani, K. Ohkubo, S. Fukuzumi, Formation of hydrogen peroxide from coal tar
as hydrogen sources using 9-mesityl-10-methylacridinium ion as an effective pho-
tocatalyst, Appl. Catal. B 77 (2008) 317–324.
[16] P.F. Donovan, D.A. Conley, Some organic tetrafluoroborates, J. Chem. Eng. Data 11
(1966) 614-&.
[17] U.C. Yoon, S.L. Quillen, P.S. Mariano, R. Swanson, J.L. Stavinoha, E. Bay,
Exploratory and mechanistic aspects of the electron-transfer photochemistry of
olefin-N-heteroaromatic cation systems, J. Am. Chem. Soc. 105 (1983) 1204–1218.
Scheme 2 shows the postulated mechanism to explain photooxida-
tion of the phenolic pollutants. Quenching of the singlet excited states
resulted to be dynamic acting as a proof of the participation of these
excited states in the photoinduced electron transfer. Thus, Type I
photooxidation happening through the singlet excited state of either
photocatalyst is the main operating process in the photodegradation of
the studied pollutants, although participation of the triplet cannot be
precluded.
4. Conclusions
Homogeneous photodegradation of phenolic compounds can be ef-
ficiently achieved with both photocatalysts, NMQ+ and Mes-Acr.-Me+
;
however, the rate of the process depends on the nature of the photo-
catalyst. Thus, the highly oxidizing NMQ+ leads to higher degradation
levels, which in part is due to direct photolysis. In the case of Mes-Acr-
Me+, its lower redox potential and photostability results in a reduced
photoactivity and therefore in a slower degradation; however, its cap-
ability to absorb visible light may be advantageous for environmental
applications. After heterogenization, the stability of Mes-Acr-Me+ in-
creases and the removal of the pollutants becomes more efficient.
Steady-state and time resolved fluorescence quenching reveal that both
photocatalysts are capable of oxidizing the pollutants from their singlet
excited states, with kinetic rate constants in the order of the diffusion
limit.
8

R. Martinez-Haya et al.
CatalysisTodayxxx(xxxx)xxx–xxx
[18] A. Arques, A.M. Amat, L. Santos-Juanes, R.F. Vercher, M.L. Marin, M.A. Miranda,
Abatement of methidathion and carbaryl from aqueous solutions using organic
photocatalysts, Catal. Today 144 (2009) 106–111.
[19] S. Fukuzumi, K. Doi, A. Itoh, T. Suenobu, K. Ohkubo, Y. Yamada, K.D. Karlin,
Formation of a long-lived electron-transfer state in mesoporous silica-alumina
composites enhances photocatalytic oxygenation reactivity, Proc. Natl. Acad. Sci.
109 (2012) 15572–15577.
from charge-transfer ion pairs by zeolite supercages, J. Am. Chem. Soc. 113 (1991)
1419–1421.
[25] A.M. Amat, A. Arques, S.H. Bossmann, A.M. Braun, S. Gob, M.A. Miranda, A "Camel
through the eye of a needle": direct introduction of the TPP+ ion inside Y-zeolites
by formal ion exchange in aqueous medium, Angew. Chem., Int. Ed. 42 (2003)
1653–1655.
[26] S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, 2nd ed., Marcel
[20] T. Hering, T. Slanina, A. Hancock, U. Wille, B. König, Visible light photooxidation of
nitrate: the dawn of a nocturnal radical, Chem. Commun. 51 (2015) 6568–6571.
[21] A.C. Benniston, K.J. Elliott, R.W. Harrington, W. Clegg, On the photochemical
stability of the 9-Mesityl-10-methylacridinium cation, European J. Org. Chem. 2009
(2009) 253–258.
Dekker, New York, 2009.
[27] H. Huang, A.H. Reshak, S. Auluck, S. Jin, N. Tian, Y. Guo, Y. Zhang, Visible-light-
responsive sillén-structured mixed-cationic CdBiO2Br nanosheets: layer structure
design promoting charge separation and oxygen activation reactions, J. Phys.
Chem. C 122 (2018) 2661–2672.
[22] V. Ramamurthy, J.V. Caspar, D.R. Corbin, Generation, entrapment, and spectro-
scopic characterization of radical cations of. alpha.,.omega.-diphenyl polyenes
within the channels of pentasil zeolites, J. Am. Chem. Soc. 113 (1991) 594–600.
[23] F. Gessner, J.C. Scaiano, Intrazeolite photochemistry VII: Laser photolysis of stil-
bene and some aromatic hydrocarbons in the cavities of NaX zeolite studied by
time-resolved diffuse reflectance, J. Photochem. Photobiol. A: Chem. 67 (1992)
91–100.
[28] H. Huang, S. Tu, C. Zeng, T. Zhang, A.H. Reshak, Y. Zhang, Macroscopic polar-
ization enhancement promoting photo- and piezoelectric-induced charge separation
and molecular oxygen activation, Angew. Chem. Int. Ed. 56 (2017) 11860–11864.
[29] D. Rehm, A. Weller, Kinetics of fluorescence quenching by electron and H-atom
transfer, Isr. J. Chem. 8 (1970) 259–271.
[30] Y.A. Ilan, D. Meisel, G. Czapski, The redox potential of the O2–O−2 system in
aqueous media, Isr. J. Chem. 12 (1974) 891–895.
[24] S. Sankararaman, K.B. Yoon, T. Yabe, J.K. Kochi, Control of back electron transfer
9