Influence of electron acceptors on the kinetics of metoprolol photocatalytic degradation in TiO2 suspension. A combined experimental and theoretical study

Article abstract of DOI:10.1039/c5ra10523d

Metoprolol (MET) belongs to a group of frequently used β₁-blockers, which often occur in waste waters. The objective of this work was to employ liquid chromatography (LC) and total organic carbon methods to study the photocatalytic degradation of MET in UV irradiated aqueous suspensions of TiO₂ (Wackherr's "Oxyde de titane standard" and Degussa P25), in the presence of different electron acceptors such as molecular oxygen, hydrogen peroxide, potassium bromate, and ammonium persulfate. The degradation rates were found to be strongly influenced by the kind of electron acceptor and the type of catalyst. The optimal amount of hydrogen peroxide and potassium bromate was investigated as well. MET photocatalytic degradation was the fastest in the presence of O₂ and potassium bromate with TiO₂ Degussa P25, while mineralization was most efficient in the presence of molecular oxygen alone. In all investigated cases, degradation followed a pseudo-first order kinetics. Reaction intermediates of MET degradation in the presence of different electron acceptors with both catalysts were studied in detail and a number of them were indentified using LC-ESI-MS/MS. The interactions with MET of reactive radical species relevant to this study (O₂?^-, ?OH, BrO₂?, and SO₄?^-) were theoretically investigated by means of density functional theory (DFT) computations.

Full text of DOI:10.1039/c5ra10523d

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Influence of electron acceptors on the kinetics of
metoprolol photocatalytic degradation in TiO2
suspension. A combined experimental and theoretical
study§
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2014,
Accepted 00th January 2014
S. J. Armaković,a S. Armaković,b N. L. Finčur,a F. Šibul,a D. Vione,c J. P. Šetrajčićb and
B. F. Abramovića
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metoprolol (MET) belongs to a group of frequently used β1ꢀblockers, which often occur in waste waters.
The objective of this work was to employ liquid chromatography (LC) and total organic carbon methods
to study the photocatalytic degradation of MET in UV irradiated aqueous suspensions of TiO2
(Wackherr’s “Oxyde de titane standard” and Degussa P25), in the presence of different electron
acceptors such as molecular oxygen, hydrogen peroxide, potassium bromate, and ammonium persulfate.
The degradation rates were found to be strongly influenced by the kind of electron acceptor and the type
of catalyst. The optimal amount of hydrogen peroxide and potassium bromate was investigated as well.
MET photocatalytic degradation was fastest in the presence of O2 and potassium bromate with TiO2
Degussa P25, while mineralization was most efficient in the presence of molecular oxygen alone. In all
investigated cases, degradation followed a pseudoꢀfirst order kinetics. Reaction intermediates of MET
degradation in the presence of different electron acceptors with both catalysts were studied in detail and a
number of them were indentified using LC–ESI–MS/MS. The interactions with MET of reactive radical
•
species relevant to this study ( O•2− , OH, BrO•2 , and SO•4− ) were theoretically investigated by means of
density functional theory (DFT) computations.
The h+ can either directly oxidize pollutants, or oxidize water
1.
Introduction
(preferably the OH− groups adsorbed on the solid surface) to
•
produce OH. In contrast, e– reduces surfaceꢀadsorbed O2. The
oxidative and reductive reaction steps taking place with the
irradiated photocatalyst (TiO2) are expressed as follows:5–8
Studies dating more than 30 years ago dealt with photocatalytic
reactions, which irrupted into the scientific literature when they
were proposed as a suitable tool to promote separation of
molecular H2 and molecular O2 from water using solar
irradiation.1,2 Heterogeneous photocatalysis using TiO2
powders has become a subject of increasing interest during the
past twenty years, mainly in the field of environmental
protection and wastewater decontamination.3 Photocatalysis in
the presence of semiconductors is triggered by the interaction
of electrons and holes, generated in a photochemically activated
solid, with the surrounding medium. Activation is the
consequence of light absorption: the irradiation of the
photocatalytic material with sufficient energy leads to the
formation of holes (h+) in the valence band and electrons (e−) in
the conduction band.
TiO2 + hν → e– + h+
(1)
h+ + organic compound → CO2 + H2O + inorganic ions (2)
H2O + h+ → •OH + H+
O2 + e– → O•2−
(3)
(4)
A practical problem in using TiO2 as a photocatalyst is the
energy waste due to the e−−h+ recombination, which results in
lower degradation efficiency. The prevention of recombination
is thus very important, and it can be achieved by adding proper
electron acceptors. In the simplest systems, in aerated solution,
molecular O2 acts as electron acceptor to prevent e−−h+
a
recombination.9 Additional oxidants, such as H2O2, S2O82− , and
Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences,
University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia
Eꢀmail: biljana.abramovic@dh.uns.ac.rs; Tel.: +381 214852753; Fax: +381 21454065.
Department of Physics, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 4,
BrO3−
, can act as electron acceptors to enhance the
b
21000 Novi Sad, Serbia
photodegradation efficiency. These electron acceptors can have
several effects including: (I) avoidance of e−−h+ recombination
because of scavenging of conductionꢀband electrons; (II)
c Dipartimento di Chimica, Università di Torino, Via Pietro Giuria 5, 10125 Torino, Italy
§ Electronic supplementary information (ESI) available: Influence of electron acceptors on the kinetics
of metoprolol photocatalytic degradation in TiO2 suspension. A combined experimental and theoretical
study.
•
increase of the concentration of OH and (III) production of
The two species can either recombine or participate in reductive
and oxidative reactions that lead to the decomposition of
contaminants.4
other oxidizing species that can enhance the oxidation rate of
the substrate and of its intermediate compounds.10
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DOI: 10.1039/C5RA10523D
1
1 30
Poland (51–155 ng L− ),29 UK (7–11 ng L− ) , Sweden (60–70
By employing computer simulations within the framework of
the density functional theory (DFT), it is possible to gain
insight into the changes of the investigated structures as a
consequence of the presence of other molecules in the
system.11,12 The information thus obtained is very important to
further understand the degradation mechanisms of the
ng L− ),30 and Germany (exceeding 1000 ng L− ).31
Previous work has shown that the rate of MET photocatalytic
1
1
degradation tended to a plateau at about 0.5−1.0 mM initial
concentration of the substrate, and that photodegradation was
most efficient at a photocatalyst loading of 1.0 mg mL− . The
1 13
aim of this work was to compare the kinetics of
photodegradation of MET, sensitized by TiO2 Wackherr or
Degussa P25 in aqueous suspension under the same
experimental conditions, in the presence of different electron
investigated compounds.13,14
Fukui functions and Fukui indices are often used as local
quantumꢀmolecular descriptors. Fukui functions describe the
changes in the molecular electron density as a consequence of
the addition or removal of charge, while Fukui indices represent
scalar values for each atom. Fukui functions are visualized as
isoꢀsurfaces, and larger Fukui values indicate higher
reactivity.15,16 It should be emphasized that values of Fukui
functions are sensitive to changes in basis sets and population
analysis. Therefore, one shouldn’t use these values as absolute
but rather as comparative parameters.
The molecular electrostatic potential (MEP) is related to the
charge distribution and it is a very useful descriptor to
determine potential sites prone to electrophilic attack and
nucleophilic reactions.14,17,18 In the field of pollutant
degradation, MEP enables the localization of parts of a
molecule that are prone to various types of attacks, also giving
information on how molecules interact with other molecules or
radicals. The MEP (hereafter indicated as V(r)), when
neglecting polarization and nuclear rearrangement effects due
acceptors (O2, H2O2, S2O82− , and BrO3− ). To monitor MET
removal and mineralization, liquid chromatography (LC) and
total organic carbon (TOC) analysis were used, respectively.
An attempt has also been made to identify the intermediates
formed during the photooxidation of MET in the presence of
different electron acceptors with both catalysts, and a number
of them were indentified using LC–ESI–MS/MS. Employing
DFT computations, the interactions of radical species ( O•2−
•OH, BrO•2 , and SO•4− ) with MET and their possible effects on
,
its degradation were investigated from the aspect of structural
considerations, charge distribution, NBO analysis, Fukui
functions and Fukui indices.
2.
Materials and methods
Chemicals and solutions
2.1.
to the presence of a unit test charge at the distance
r
, is given as
All chemicals were of reagent grade and were used without
further purification. The drug (±)ꢀMetoprolol(+)ꢀtartrate salt
(SigmaꢀAldrich) was used as received (≥99% purity); 85%
H3PO4 was purchased from Lachema, Neratovice; acetonitrile
(ACN) was a product of J.T. Baker. Other chemicals were as
follows: 30% H2O2 from SigmaꢀAldrich; KBrO3 and
(NH4)2S2O8 from Merck. All solutions were made using doubly
distilled water. The used catalysts were TiO2 Degussa P25
(75% anatase and 25% rutile, surface area 50 ± 1.0 m2 g−1,
crystallite size about 20 nm, nonꢀporous) and TiO2 Wackherr’s
"Oxyde de titane standard" (100% anatase, surface area 8.5±1.0
m2 g−1, crystallite size 300 nm, hereafter “TiO2 Wackherr”),
produced by the sulfate process.32
follows:
r
r
Z A
r
ρ
(
r'
)
dr
r
V
(r
)
=
−
∑
r
r
∫
(5)
RA − r
r'−r
where the summation runs over all nuclei A with charge ZA and
coordinate RA, while r') is the electron density of the
molecule. ) represents the potential exerted at the coordinate
by the nuclei and the electrons. The sign of ) at any point
ρ(
V(r
r
V(r
depends on whether the effects of the nuclei or the electrons are
dominant.19,20
Natural bond order (NBO) can be used for efficient
investigation of intraꢀ and interꢀmolecular bonding and
interactions. It is a convenient basis for the investigation of
charge transfer or conjugative interactions in molecular
systems.21–23 NBO analysis is carried out by examining all
possible interactions between 'filled' (donor) NBOs and 'empty'
(acceptor) NBOs, estimating their energetic importance by 2ndꢀ
order perturbation theory. In this way one obtains the energies
of delocalization of electrons from filled NBOs into empty
NBOs, e.g. stabilization energies gained by donation from the
2.2.
Photodegradation procedures
Photocatalytic degradation was carried out in a cell made of
Pyrex glass (total volume of ca. 40 mL, liquid layer thickness
35 mm), with a plain window on which the light beam was
focused. The cell was equipped with a magnetic stirring bar and
a water circulating jacket. A 125 W highꢀpressure mercury
lamp (Philips, HPLꢀN, emission bands in the UV region at 304,
314, 335 and 365 nm, with maximum emission at 365 nm)
together with an appropriate concave mirror was used as the
radiation source. The output of the mercury lamp was
donor NBO to the acceptor NBO. For each donor NBO (
acceptor NBO ( ), the stabilization energy associated with i→j
i) and
j
delocalization can be estimated on the basis of the secondꢀorder
perturbation theory:24–26
9
1
1
calculated to be ca. 8.8
ferrioxalate actinometry).
×
10− Einstein mL− min− (potassium
2
F
(
i
,
j
)
Experiments were performed using 20 mL of 0.05 mM MET
E
(
2
)
= ꢀE = q
ij
(6)
i
1
containing 1.0 mg mL− of TiO2 (Wackherr or Degussa P25),
ε ε
i j
except for the study of direct photolysis. The aqueous
suspension of TiO2 was sonicated (50 Hz) in the dark for 15
min before irradiation, in order to uniformly disperse the
photocatalyst particles and to attain adsorption equilibrium.
Before irradiation, the suspension thus obtained was
thermostated at 25±0.5 ºC in a stream of O2 (3.0 mL min−1),
except for a control run in the absence of electron acceptors
when N2 was bubbled (3.0 mL min−1) to remove dissolved
where qi is the donor orbital occupancy, εi
,
εj are diagonal
elements (orbital energies) and F(i,j is the offꢀdiagonal NBO
)
Fock matrix element.
Metoprolol (MET) is a selective β1ꢀblocker of the cardiac
adrenergic receptors.27 Due to the frequent use, MET is present
in sewage waters, in rivers from Netherlands (25–100 ng L− ),28
1
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DOI: 10.1039/C5RA10523D
oxygen. During irradiation, the mixture was stirred at a constant Gaussian 03. For all systems, calculations were performed
rate under continuous gas flow. All experiments were employing the B3LYP exchange and correlation functional
performed at the natural pH which changed during the with 6ꢀ31 G+(d) basis set.35 Two stages took place for all
photodegradation, from pH 7 to pH 4 in the case of TiO2 configurations. Firstly, equilibrium geometry of the
Wackherr and from pH 7 to pH 5 in the case of Degussa P25. In investigated systems was located using default convergence
the investigation of the influence of electron acceptors, apart criteria and, secondly, the harmonic vibrational spectrum was
from constant O2 bubbling, solutions of H2O2, KBrO3 or checked to assure that the true minimum of potential energy,
(NH4)2S2O8 (at typical 3.0 mM initial concentration) were characterized by positive frequencies, was located.
added to the MET solution.
In this work we investigated five systems: MET, MET/ O•2−
,
MET/ O•2− /•OH, MET/ O•2− BrO•2 , and MET/ O•2− SO•4− . To
/ /
2.3.
Analytical procedures
make the simulations more realistic, solvent effects of water
were taken into account using the default Polarizable
Continuum Model (PCM). Initially, in all cases O•2− was placed
above the hydroxyl group located at the tail of MET, while
•OH, BrO•2 and SO•4− were placed above the aromatic ring.
The photodegradation of MET was monitored by liquid
chromatography–photodiode array detection (LC–PDA). To do
so, aliquots of 0.30 mL were taken from the reaction mixture at
the beginning of the experiment and at regular time intervals.
Aliquot sampling caused a maximum volume variation of ca.
10% in the reaction mixture. The suspensions were filtered
through Millipore (MillexꢀGV, 0.22 ꢁm) membrane filters to
eliminate the photocatalyst. Lack of adsorption of MET on the
filters was preliminarily checked. Afterwards, a 10ꢀµL sample
was injected and analyzed using a Shimadzu UFLC–PDA,
equipped with an Eclipse XDBꢀC18 column (150 mm × 4.6
mm i.d., particle size 5 µm, 25 °C). The UV/vis PDA detector
was set at 225 nm (wavelength of MET maximum absorption),
as well as at 210, 260, 270 and 280 nm for the monitoring of
the intermediates. The mobile phase (flow rate 0.8 mL min−1)
was a mixture of ACN and water (the latter acidified with 0.1%
H3PO4), with the following gradient: 15% ACN at 0 min, which
was increased to 30% ACN in 5 min, after which 30% ACN
was constant for 5 min; post time was 3 min. The retention time
for MET was 6.0 ± 0.1 min. Reproducibility of repeated runs
Interesting sites containing a significant amount of charges
were located through MEP surfaces, which were obtained using
Molekel after geometry optimization and frequency check.36
3.
Results and discussion
Effect of electron acceptors
3.1.
Apart from O2, which is the most frequently applied electron
acceptor, in this work we also investigated the influence of
H2O2, BrO3− , and S2O82− . These compounds operate as e−
scavengers and should be able to prevent e−−h+ recombination,
thereby enhancing the formation of •OH and other reactive
species.5,6 However, compounds such as H2O2 are also able to
scavenge the photogenerated transients.37
was around 3−10%.
Concerning TOC analysis, 10 mL aliquots of the reaction Fig. 1 and Table 1 show the time trend and degradation kinetics
mixture were taken at regular time intervals, diluted to 25 mL of MET, upon irradiation in the presence of TiO2 (Wackherr,
1a, and Degussa P25, 1b) and several electron acceptors. In all
and analyzed after filtration on an Elementar Liqui TOC II
analyzer, according to Standard US 120 EPA Method 9060A. investigated cases, the degradation process followed a pseudoꢀ
For the LC–ESI–MS/MS evaluation of intermediates after 10 first order kinetics. Note that the role of O2 as electron acceptor
min irradiations, 100ꢀµL samples were analyzed on an Agilent was investigated by comparing the system in air (blue triangles)
with the one in which oxygen was bubbled to create an O2
Technologies 1200 series LC with Agilent Technologies 6410A
series electrospray ionization tripleꢀquadrupole MS/MS, using atmosphere (purple stars; in such a system, the concentration of
Agilent Technologies Zorbax XDBꢀC18 column (50 × 4.6 mm
i.d., particle size 1.8 µm, 40°C). The mobile phase (flow rate
0.5 mL min–1) consisted of 0.05% aqueous formic acid and
MeOH (gradient, 0 min 20% MeOH, 10 min 60% MeOH, 12
min 100% MeOH, postꢀtime 3 min). Analytes were ionized
using the electrospray ion source, a capillary voltage of 4.0 kV
and with nitrogen as the drying gas (temperature 350°C, flow
10 L min–1) and nebulizer gas (45 psi). Highꢀpurity nitrogen
was used as the collision gas. Full scan mode (m/z range 50–
800, scan time 100 ms, fragmentor voltage 100 V) in positive
ion mode was used to select precursor ions for the starting
compound and each degradation intermediate, as well as to
examine isotopic peak distribution. Then, the product ion scan
MS2 mode (fragmentor voltage 135 V, scan time 200 ms,
collision energy 10–40 V in increments of 10 V) was used to
elucidate the structure of each degradation intermediate.
both gasꢀphase and dissolved O2 is expected to be
∼5 times
higher than in the case of air equilibrium). For further
comparison, the behaviour of the deoxygenated system (N2
bubbling, pink triangles) was also studied.
As expected, all the studied electron acceptors increased the
efficiency of MET photocatalytic removal. It was determined
that the influence of electron acceptors on the efficiency of
degradation followed the order: O2/H2O2 > O2
≈
O2/ S2O82−
≈
O2/ BrO3− using TiO2 Wackherr, and O2/ BrO3− > O2/H2O2
>
O2/ S2O82− > O2 using Degussa P25 (Fig. 1, Table 1). The added
oxidants (H2O2, BrO3− S2O82−
did not cause MET
transformation in the dark.
,
)
Moreover, MET degradation using TiO2 under N2 atmosphere
was very slow, and only marginally faster compared to the
direct photolysis (MET irradiation in air without TiO2). As
reported in Fig. 1 and Table 1, a considerable enhancement of
MET degradation was observed, with irradiated TiO2, in air
compared to N2 atmosphere: the increase of the pseudoꢀfirst
order rate constant k' was around 7 times with TiO2 Wackherr
and around 12 times with Degussa P25.
2.4.
Computational details
All DFT calculations were carried out using the Gaussian 03
software package,33 except for Fukui functions and Fukui
indices that were calculated at the same level of theory using
Jaguar, version 8.4,34 as implemented in the Schrödinger
Materials Suite, release 2014ꢀ2. For the purpose of NBO
analysis, it was used the NBO 3.0 program as implemented in
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structure (presence of the rutile phase in Degussa P25). These
issues could lead to differences in the interaction of O2 and
other e− scavengers with the photocatalyst surface. In the case
of TiO2 Wackherr, it appears that O2 bubbling already brought
the reaction to its optimum and that any further enhancement
was difficult. The situation was very different for Degussa P25.
As far as the role of O2 as electron acceptor is concerned,
photoelectrons can be captured by O2 to produce O•2− , H2O2
(reactions 4, 7, and 8) and then eventually hydroxyl radicals
(reactions 9ꢀ11):8,10,38
O•2− + H+ → HO•2
(7)
2HO•2 → H2O2 + O2
(8)
H2O2 + hν → 2•OH
(9)
H2O2 + e– → •OH + OH–
(10)
H2O2 + O•2− → •OH + OH– + O2
(11)
When comparing the systems O2 and O2/H2O2 (oxygen
bubbling in both cases, reported as purple stars and red circles,
respectively, in Fig. 1), one can notice a small acceleration (by
a factor of 1.2) caused by H2O2 addition in the case of TiO2
Wackherr, and a more marked H2O2 effect (2.2 times) with
Degussa P25. Despite the more important role of H2O2, MET
degradation with Degussa P25 and O2/H2O2 was still a bit
slower compared to TiO2 Wackherr with otherwise identical
conditions. The fact that a photocatalyst with lower surface area
(TiO2 Wackherr) could induce faster MET degradation
compared with Degussa P25 is most likely accounted for by
lower radiation scattering, which allows a better use of the
incoming photons.13
The increase of MET degradation rate upon addition of H2O2 to
Fig.
1
Kinetics of the photolytic/photocatalytic removal of MET using TiO2 Degussa P25 (Table 1) was likely due to increased generation
•
of OH. Indeed, H2O2 can enhance •OH production through the
Wackherr (a) and Degussa P25 (b) under UV irradiation, in the absence/presence
of different electron acceptors.
following pathways: (I) direct photolysis of H2O2 under UV
irradiation (reaction 9); (II) reaction between H2O2 and e−:
H2O2 is a more effective electron acceptor than oxygen because
its reaction with e− yields •OH and OH− (reaction 10), while the
corresponding process with O2 produces the weaker oxidant
Table 1. Values of the firstꢀorder decay constant, k’, for the photolytic/
photocatalytic removal of MET (initial concentration 0.05 mM) using TiO2
(where applicable) and UV radiation, as a function of the absence/presence of
different electron acceptors
O•2− (reaction 4); (III) reaction with O•2− , also yielding OH
•
k’ (min–1)*
(reaction 11).8,10,38
Direct photolysis
0.0029
TiO2 Wackherr
In the presence of Degussa P25, the O2/ BrO3− system was the
most efficient for the degradation of MET. In contrast, the
addition of BrO3− did not have an important effect on MET
Degussa P25
0.007
N2
Without bubbled O2
O2
0.008
0.054
0.184
0.224
0.086
0.092
0.199
degradation with TiO2 Wackherr. When operational, the
enhancement of photodegradation efficiency would likely be a
consequence of the reaction between BrO3− and conductionꢀ
band electrons. A first effect is the inhibition of e−−h+
recombination, which prolongs the life time of the
photogenerated holes. One should additionally consider the
formation of the reactive species BrO•2 (reaction 12).38,39 The
O2/H2O2
BrO−
0.172
0.180
0.299
0.133
O2/
3
S O2−
O2/
2
8
* k’ determined after 10 min of irradiation.
In all cases, correlation coefficient was higher than 0.95
possible role of BrO•2 in MET degradation was investigated
Moreover, while MET degradation with Degussa P25 was little
influenced by a further increase of the oxygen concentration
(compare the runs with and without bubbled O2), a further
significant increase of the MET rate constant (over three times)
was observed with TiO2 Wackherr. Interestingly, in the case of
TiO2 Wackherr the degradation of MET did not undergo a
further important enhancement upon addition of other e−
scavengers. In contrast, with Degussa P25 the additional effect
of the scavengers was substantial. The differences between the
two photocatalysts may be connected with the different surface
area (much larger for Degussa P25) and different crystal
with DFT methods (vide infra).
BrO3− + 2 H+ + 2 e−
→
BrO•2 + H2O
(12)
The addition of persulfate to the O2ꢀbubbling system (to obtain
O2/ S2O82− ) had a limited enhancement effect in the case of
Degussa P25 (1.4 times) and practically no effect with TiO2
Wackherr. The reaction (13) between S2O82− and e− yields the
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strong oxidant SO•4− , but SO24− is also formed and it can act as
a hole scavenger (reaction (14)).10,38 The tradeꢀoff between h+
and SO•4− could result into a limited enhancement effect, or
even into no effect.10
The results reported, and in particular the much faster
transformation of MET with O2 compared to N2 bubbling,
highlight the importance of electron acceptors in the
degradation process. The addition of further electron acceptors
may enhance degradation, but the reaction pathways are
probably modified as well. Fig. 2 reports chromatograms
obtained using TiO2 Wackherr and Degussa P25 in the presence
of the studied electron acceptors, showing that the occurrence
of certain intermediates (represented by unassigned peaks)
significantly depended on the applied electron acceptor and
catalyst. Moreover, according to literature data,10,38,40,41 it is
suggested that the application of an optimal concentration of
electron acceptors is of great importance to achieve the
maximum removal of organic compounds from the system. For
this reason, the effects of the acceptor concentrations were
studied in this case as well.
S2O82− + e− → SO24−
+
SO•4−
(13)
(14)
SO24− + h+ → SO•4−
Another issue is that sulfate, formed in reaction (13), can be
adsorbed on the TiO2 surface and decrease the photocatalytic
activity of the oxide.10 Since S2O82− turned out to be quite
ineffective as electron acceptor in the removal of MET, it was
not subjected to further investigation. The very low interaction
between SO•4− and MET, compared to other systems, was
confirmed through theoretical analysis as well (vide infra).
Fig. 2 Chromatograms obtained after 10 min of MET exposure to UV irradiation in the presence of TiO2 Wackherr (a) and Degussa P25 (b), using different electron
acceptors (1: O2 2: H2O2; 3: S2O28− ; 4: BrO3− ).
det = 225 nm
λ
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mM BrO3− concentration range. While e− scavenging and a
potential involvement of BrO•2 (formed in reaction (12)) could
possibly enhance degradation, at elevated BrO3− levels the
3.2.
peroxide
Effect of the initial concentration of hydrogen
The influence of the concentration of H2O2 on the efficiency of
MET removal was investigated in the range of 1.0 to 5.0 mM
(Fig. 3). For both Degussa P25 and TiO2 Wackherr, the optimal
concentration of H2O2 was 3 mM. Under such conditions, after
10 min irradiation of the O2/H2O2 system, the degradation of
MET reached 89% with TiO2 Wackherr and 86% with Degussa
P25 (Fig. 3).
The presence of H2O2 at concentration values above 3 mM
resulted in lower MET degradation compared to the optimal
conditions. The most likely reason is that H2O2 acts as an OH
system reactivity could be limited by the formation of bromide
(reaction (17)). The latter could both adsorb on the
photocatalyst surface and be involved in h+ and •OH
scavenging.10,38,41,43,44
BrO3− + 6H+ +6 e− → [ BrO2− , HOBr] → Br− + 3H2O
(17)
•
scavenger,
generating
the
much
less
reactive
hydroperoxyl/superoxide radicals ( HO•2
/
O•2− , reaction 15).
Moreover, HO•2 can further react with •OH to form oxygen and
water, which are not directly involved into MET degradation
(reaction 16).10,40
Fig.
4
Effect of the initial concentration of BrO3− on MET removal using
O2/UV/TiO2 for the first 10 min of irradiation.
In the studied systems the electron acceptors are expected to
inhibit the e−–h+ recombination processes, but they could have
additional effects (e.g. production of reactive species by
•
photolysis, such as OH from H2O2, see reaction (9)). These
effects can be highlighted in the absence of TiO2. For this
reason, MET was irradiated alone and in the presence of
Fig. 3 Effect of the initial concentration of H2O2 on MET removal using
O2/UV/TiO2 for the first 10 min irradiation.
O2/H2O2 and O2/ BrO3− , without TiO2 (Fig. 5). The degradation
H2O2 + •OH → HO•2 + H2O (or O•2− + H+ + H2O) (15)
of MET was considerably slower compared to photocatalytic
conditions, but in the presence of H2O2 around 50% of MET
was removed from the system after 60 minutes irradiation. In
this case, acceleration of degradation compared to MET direct
HO•2 + •OH → O2 + H2O
(16)
Additionally, H2O2 at elevated concentration could react with
TiO2 to form peroxo compounds, which are detrimental to the
photocatalytic action.42
•
photolysis would probably be accounted for by OH, generated
by H2O2 irradiation. Indeed, the hydroxyl radical reacts rapidly
and nonꢀselectively with most organic compounds, either by Hꢀ
abstraction or by addition to C=C unsaturated bonds.45
Although slower compared to MET photocatalytic degradation,
the use of H2O2 under irradiation could be attractive due to its
simplicity.
Some enhancement of MET degradation was also observed in
the presence of irradiated bromate, in particular at longer
irradiation times (> 30 min), in agreement with literature
reports.46
3.3.
Effect of the initial concentration of potassium
bromate
Fig. 4 reports the MET degradation efficiency in the presence
of different initial concentrations of BrO3− . In the case of TiO2
Wackherr, as already discussed, the bromate effect was very
limited. In contrast, Degussa P25 showed a degradation
increase that quickly tended to a plateau. In the latter case the
most effective MET degradation was observed for 3 mM
bromate, but very little difference could be detected in the 3ꢀ5
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obtained in the present work indicate that the adsorption of
MET on the catalysts doesn’t differ more than 10%, which
leads to conclusion that the adsorption of the investigated
compound doesn’t have significant influence on the efficiency
of photocatalytic degradation. As far as mineralization is
concerned, soon after complete removal of MET from the
system, 88% of organic compounds (measured as organic
carbon) were still present with TiO2 Wackherr in the absence of
H2O2, and even 94% (that is, only 6% mineralization) with 3
mM H2O2 (Fig. 7a). The degree of mineralization after MET
disappearance was significantly higher using Degussa P25,
which gave 33% residual organic carbon without H2O2 and
72% with H2O2 (Fig. 7b). Photonic efficiencies48 calculated
from data obtained for 60 min of mineralization of MET in the
presence of TiO2 Wackherr and Degussa P25 without H2O2
were 0.068%, and 0.223%, respectively. Also, for the systems
TiO2 Wackherr and Degussa P25 in the presence of H2O2, the
photonic efficiency was 0.012% and 0.066%, respectively. The
better performance of Degussa P25 toward mineralization,
compared to TiO2 Wackherr, could be accounted for by its
higher surface area. Indeed, a photocatalyst with low surface
area, such as TiO2 Wackherr, could undergo surface poisoning
by the degradation intermediates.13 As an alternative or in
addition, the mineralization of MET could be connected with
reactions involving h+. The latter are favoured in the presence
of Degussa P25 compared with TiO2 Wackherr, which induces
•OH reactions to a higher extent.13 This issue would be
consistent with the previously discussed finding (Fig. 2) that
different chromatographic peaks, corresponding to different
intermediates, could be detected with the two photocatalysts.
Fig. 5 Kinetics of the direct and indirect photolysis of MET under UV irradiation in
the presence of O2, O2/ BrO3− and O2/H2O2
3.4.
Evaluation of the degree of mineralization
3.4.1
Systems with hydrogen peroxide
H2O2 was poorly retained by the analytical column, but its
considerable absorption at 225 nm allowed a rough estimation
of its concentration to be derived from the chromatograms. By
so doing, a considerable decrease of H2O2 concentration under
photocatalytic conditions could be highlighted with both TiO2
Wackherr and Degussa P25. Note that such an approach could
overestimate H2O2 in the degraded systems, because of the
possible interference by other poorly retained compounds.
Therefore, the extent of H2O2 degradation could be
underestimated. The chromatograms reported in Fig. 6 also
show that the variety and the amount of intermediates depended
on the type of photocatalyst. We made similar conclusions also
in our previous works, where degradation mechanism13 and
toxicity of investigated systems47 were studied in details.
Interestingly, most intermediates formed with both
photocatalysts had lower retention times compared to MET:
they mainly consisted of ringꢀhydroxylated compounds and of
derivatives arising from oxidative cleavage of the shorter lateral
chain of MET (that containing the methoxy group).13 The
mixture of intermediates with Degussa P25 was considerably
more toxic than that obtained with TiO2 Wackherr.47
Fig. 6. Chromatograms obtained after 10 min of MET irradiation in the presence
of TiO2 Wackherr (a) and Degussa P25 (b): O2/H2O2 system in both cases (O2
bubbling, 3 mM H2O2), λdet = 225 nm
Fig. 7. Kinetics of photocatalytic degradation of MET under UV irradiation and O2
bubbling, in the presence of TiO2 Wackherr (a) and Degussa P25 (b): (1) MET
trend, no H2O2; (2) MET trend, 3 mM H2O2; (3) TOC trend, no H2O2; (4) TOC trend,
3 mM H2O2.
Therefore, the ability of the systems to achieve MET
mineralization is particularly important in the case of the
Degussa P25 photocatalyst. Further experimental results
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MET/ O•2−
SO•4−
.
,
MET/ O•2− /•OH,
The optimized
With both TiO2 types, mineralization further increased up to the
longest irradiation time (4 h). However, while H2O2 enhanced
MET degradation (at least with Degussa P25), it slowed down
mineralization with both photocatalysts. If the above hypothesis
following
MET/ O•2−
systems:
BrO•2 and MET/ O•2−
/
,
/
molecular geometries of the investigated systems, together with
the specific dihedral angles, are given in Fig. 10. The presence
of O•2− (Fig. 10b) modifies dihedral angles, which reduces
MET stability. Moreover, the interaction between O•2− and
MET is modified by other electron acceptors (Fig. 10cꢀd).
•
concerning h+ vs. OH is correct, the addition of H2O2 would
shift the system reactivity towards the hydroxyl radical and the
inhibition of mineralization would be automatically explained.
3.4.2
Systems with potassium bromate
In a similar way as H2O2, bromate was poorly retained by the
analytical column but it could be detected due to significant
radiation absorption at 225 nm. An approximate assessment of
BrO3− concentration could thus be obtained from the
chromatograms, similarly to H2O2 and with the same
limitations. Contrary to H2O2, for which clear evidence of a
concentration decrease under photocatalytic conditions was
obtained, no evidence was available from the chromatograms of
a
change in bromate concentration. Fig. 8 shows the
chromatograms obtained after 10 minutes of MET degradation
in the system O2/ BrO3− . In addition to the constant area of the
peak to which bromate contributes, chromatograms obtained in
the presence of TiO2 Wackherr and Degussa P25 show
significant differences in the amount and presence of peaks
related to the degradation intermediates. This finding is in
general agreement with earlier suggestions that different
photocatalysts induce not only different transformation kinetics,
but also different degradation mechanisms.13
TOC measurements (Fig. 9) show that bromate slightly
increased mineralization with TiO2 Wackherr and slightly
decreased it with Degussa P25. After 240 minutes of irradiation
without BrO3−
, the percentage of the residual organic
compounds was reduced to 17% for TiO2 Wackherr and to 4%
for Degussa P25. In the presence of BrO3− , the corresponding
values were 5% for TiO2 Wackherr and 13% for Degussa P25.
Photonic efficiencies of mineralization of MET after 60 min in
the presence of TiO2 Wackherr and Degussa P25 with KBrO3
were 0.096%, and 0.194%, respectively.
Fig. 9. Kinetics of photocatalytic degradation of MET under UV irradiation with O2
bubbling, in the presence of TiO2 Wackherr (a) and Degussa P25 (b): (1) MET
trend, no BrO3− ; (2) MET trend, 3 mM BrO3− ; (3) TOC trend, no BrO3− ; (4) TOC
trend, 3 mM BrO3−
.
The interaction distance between O•2− and MET was the
•
shortest (1.612 Å) when OH was also present, while it was the
longest (3.904 Å) in the presence of SO•4− . Concerning other
radicals, the interaction distance with MET was ca. 3.4 and 6.4
Å for BrO•2 and SO•4− , respectively. The most interesting
situation for the interaction of radicals with MET was found in
•
Fig. 8. Chromatograms obtained after 10 min of UV irradiation of MET in the
presence of TiO2 Wackherr (a) and Degussa P25 (b), in the O2/ BrO3− system (3
mM bromate, O2 bubbling); λdet = 225 nm
the case of OH, where a new bond was formed. In various
experimental studies carried out with molecules similar to
MET, it was concluded that •OH binds to the aromatic ring.13,49
However, it was still unclear which carbon atom formed a bond
•
3.5.
Influence of radicals on MET – DFT insight
with OH. According to our study, a bond is formed with the
carbon atom number 4 (Fig. 10).
Structural properties indicate that the highest interaction
between MET and radicals takes place in the MET/ O•2− /•OH
3.5.1. Structural and reactivity properties ꢀ Fukui functions
and indices
To better understand the interaction between MET and the
radicals produced by reaction between e− and electron system. These data are in overall agreement with the
experimental results reported before, which emphasized the
acceptors, we conducted a DFT computational analysis of the
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important role of O2 vs. deoxygenated systems and the effect of molecule is subjected to nucleophilic attack, or when it acts as
electron acceptors in the enhancement of •OH (and h+)
electrophile.
occurrence, through inhibition of e−–h+ recombination. This
−
The fNN HOMO indices are related to the
f
Fukui function,
fact is also confirmed by the NBO analysis provided in section
3.5.3 (vide infra).
while the fNN LUMO ones are related to
f
+ . In other words, a
high positive value of fNN HOMO indicates that the relevant
atom can donate electrons, thereby acting as a nucleophile,
while an elevated fNN LUMO indicates that the atom can
receive electrons, thus acting as an electrophile. Having in mind
the fact that a new bond was formed between MET and OH,
special attention was paid to the C4 atom of the MET aromatic
In order to understand the reactive properties of MET, we will
refer to Fukui functions and Fukui indices. Fukui functions are
presented in Fig. 11, while Fukui indices (fNN HOMO and fNN
LUMO) are given in Table 2. Fukui indices are commonly used
since they describe the electron density when the molecule is
subjected to a reaction that modifies the electron density
−
•
ring, namely the atom involved in bond formation. According
itself.50–52 Concerning the Fukui functions (
f
and
f
+ ), the
+
to positive surface of Fukui
f
function, the MET molecule
red colour corresponds to the negative values while the blue
has an electrophilic nature on both tail parts. On the other hand,
−
colour corresponds to the positive ones. Negative values of
f
−
the negative value of
f
located at the aromatic ring, and
correspond to regions that lose electron density when the
molecule is subjected to electrophilic attack, or when the
automatically at the C4 atom, suggests that this atom could act
as a nucleophile as well. Significant values of both Fukui
functions are located at the oxygen atom O7 (Fig. 11), while its
Fukui indices emphasize nucleophilic nature (Table 2).
+
molecule itself acts as a nucleophile. Positive values of
correspond to areas that gain electron density when the an
f
Fig. 10 Optimized geometries of the investigated structures with specific angles (degrees) and distances (Å) for: a) MET; b) MET/ O•2− ; c) MET/ O•2− /•OH; d)
MET/ O•2− BrO•2 ; and e) MET/ O•2−
SO•4−
/
/
.
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One should be careful when interpreting the results of Fukui
functions and indices, since they are not reliable to absolutely
identify the most reactive electrophilic or nucleophilic sites
within a particular molecule. In such a case, they can serve only
as qualitative indicators of reactivity. However, according to
both Fukui functions and indices, the C4 atom of the MET
aromatic ring is a potential reaction site, which is confirmed in
the case of the MET/ O•2− /•OH system.
3.5.2.
Molecular Electrostatic Potential (MEP) surfaces
Representative MEP surfaces of the investigated structures are
given in Fig. 12. The blue colour corresponds to the areas with
the highest electrostatic potential, the red colour corresponds to
the lowest electrostatic potential, while the green colour
indicates intermediate values. MEP surfaces clearly indicate
that significant changes of charge distribution would occur in
MET in the presence of radicals. In particular, the introduction
of radical species would change significantly the maximal and
minimal MEP values, and the associated change would be the
highest for SO•4− . In the MEP surface of MET alone (Fig. 12)
there are two specific sites containing a significant negative
charge, located in the near vicinities of oxygen and nitrogen
atoms. As expected, these sites would be the centres of
reactivity with radicals. In the equilibrium state, O•2− was
located above the hydroxyl group and the closest adjacent H
atom. The hydroxyl radical was bound to the aromatic ring, and
the H atom of •OH acquired a significantly positive MEP value.
The situations changed when BrO•2 and SO•4− were present in
the system. The location of the radical O•2− was similar to
previous cases, but the MEP values around O•2− were much
higher than before and also the values around oxygen atoms
increased significantly. These different charge distributions
indicate interesting chemical interactions between MET and the
studied radicals, which will be further discussed in the
following section.
Fig. 11 Fukui functions of MET: a) Fukui f + function and b) Fukui f − function.
Table 2. Atomic Fukui indices of MET
Atom
fNN HOMO
fNN LUMO
C1
C2
C3
C4
C5
C6
O7
C8
C9
O10
C11
N12
C13
C14
C15
C16
C17
O18
C19
0.0344
0.2415
0.0714
0.1006
0.1795
0.1184
0.1739
0.0012
0.0000
0.0007
0.0013
0.0061
0.0004
0.0002
0.0000
0.0071
0.0219
0.0025
0.0018
0.2021
0.0115
0.2885
0.1777
0.0070
0.2969
0.0006
0.0014
0.0048
ꢀ0.0026
0.0011
ꢀ0.0008
0.0002
0.0001
0.0001
0.0016
ꢀ0.0035
0.0004
0.0004
3.5.3. Natural Bond Order (NBO) analysis
The nature of molecular interactions can be investigated
employing the NBO analysis, specifically based on the
observation of the electron delocalization energies between
NBO orbitals.11,12,53–56 The significance of the chemical or
physical interactions between MET and radical species can be
assessed by using the total sum of the delocalization energies,
ΣE(2), obtained by summing the electron delocalization
energies of all the significant interactions between the NBO
orbitals of interacting molecules.57,58 The ΣE(2) values are
reported in Table 3. A larger ΣE(2) means that the interactions,
resulting from electron delocalization from donor to acceptor
NBO between MET and radicals, are more important. As a
consequence, ΣE(2) values are a measure of the importance of
chemical interaction between the investigated species. On the
other side, lower ΣE(2) might suggest electrostatic interaction
between the studied species.
Concerning Fukui indices, the value of fNN HOMO for the atom
C4 again emphasizes its nucleophilic nature, while the highest
value of this index is recorded for the C2 atom. The highest
value of fNN LUMO is obtained for the atom C6. Overall, Fukui
indices have significantly high values for atoms belonging to
the aromatic ring.
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Fig. 12 Representative MEP surfaces of the investigated systems: a) MET; b) MET/ O•2−
; / ; / .
c) MET/ O•2− /•OH; d) MET/ O•2− BrO•2 and e) MET/ O•2−
SO•4−
from O•2− to MET NBOs in the case of the MET/ O•2− system,
as suggested by the quite elevated ΣE(2)
34 kcal mol–1. This
is the highest value of ΣE(2) among all the cases investigated in
this work, and it clearly indicates a chemical interaction
According to NBO results, the highest and the most important
chemical interactions took place in the MET/ O•2− /•OH system.
This is consistent with above findings, which suggested that a
new bond was formed in this system between the C4 atom of
MET and •OH. The interactions resulting from electron
delocalization from O•2− to MET NBOs are significant, since
≈
between O•2− and MET. This means that dissolved oxygen, in
addition to inhibiting e−–h+ recombination and enhancing, as a
ΣE(2)
≈
28 kcal mol–1. In contrast, chemical interactions
•
consequence, the occurrence and subsequent reactivity of OH
and h+, could further contribute to MET degradation through
resulting from electron delocalization from MET to O•2− NBOs
are insignificant. There is a significant electron delocalization the formation of O•2−
.
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Table 3. ΣE(2) of interacting units between MET and radicals
electron scavenging by persulfate would be offset by drawbacks
connected with SO42
formation (adsorption on the
−
Donor
unit
(NBOs
of)
Acceptor unit
(NBOs of)
ΣE(2)
System
[kcal mol–1]
photocatalyst surface, which reduces the photocatalytic
activity), while SO•4− would not be able to favour degradation
by significantly reacting with MET.
MET
3.14
O•2−
MET/ O•2−
O•2−
MET
34.31
0.23
3.6.
LC–ESI–MS/MS
identification
of
degradation
MET
O•2−
intermediates
•OH
MET
MET
MET
•OH
O•2−
Bonded
Bonded
28.21
By using the LC–ESI–MS/MS technique, intermediates of
MET degradation in the presence of different electron acceptors
(O2, O2/H2O2 and O2/KBrO3) with both catalysts were
investigated (see ESI,§ Figs. S1ꢀS22 and Table S1).
Intermediates formed in the absence of electron acceptors
(systems with N2) were not investigated because the rate of
MET degradation was very low. Therefore, the concentration of
intermediates was low as well and they couldn’t be identified.
Because the collisionꢀinduced dissociation patterns of MET
degradation were defined previously,59 it was possible to
identify the detected peaks by using the product ion MS2
spectra. The retention times of all identified degradation
MET/ O•2− /•OH
•OH
<0.05
<0.05
<0.05
<0.05
0.64
O•2−
•OH
O•2−
O•2−
MET
MET
BrO•2
MET
O•2−
MET/ O•2− / BrO•2
MET
6.08
BrO•2
O•2−
intermediates P1–P10 (1.18–3.98 min) were shorter than that of
MET (4.43 min), due to the cleavage of the molecule and the
formation of polar moieties.
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
BrO•2
O•2−
BrO•2
MET
O•2−
During the degradation of MET with TiO2 Wackherr/O2, a total
of seven peaks (labeled P1–P7) corresponding to degradation
intermediates were detected (ESI,§ Table S1 and Fig. 13).
Intermediate P1 represents a compound with Mmi = 133,
indicating the presence of a nitrogen atom in its structure.
Based on its fragmentation pattern (ESI,§ Table S1) and
literature data, it was concluded that this compound is 3ꢀ
MET
SO•4−
MET
O•2−
MET/ O•2− / SO•4−
MET
SO•4−
O•2−
SO•4−
O•2−
SO•4−
(propanꢀ2ꢀylamino)propaneꢀ1,2ꢀdiol,
already
identified.13
Intermediates P2 and P3 both correspond to compounds with
Mmi = 253, but with different MS2 spectra. Based on MS2
spectra and literature data13,60 it could be stated that P2 is
In the case of MET/ O•2− BrO•2 , the ΣE(2) of electron
/
delocalization from BrO•2 to MET NBOs is ca. 6 kcal mol–1,
which suggests weak chemical interaction when compared with
previous cases. The chemical interaction between O•2− and
MET in this system turned out to be insignificant, as indicated
hydroxy
propoxy]benzaldehyde, that was previously identified as MET
degradation product. Fragment 212 (C10H14NO4) present in P2
derivative
4ꢀ[2ꢀhydroxyꢀ3ꢀ(propanꢀ2ꢀylamino)
,
is formed by loss of water and isopropyl moiety, which was
further dehydrated to the m/z 177 (i.e., loss of water and
methylamine group). Radjenović et al. also identified this
compound and they stated that the characteristic fragment ion
m/z 133 wаs detected in the spectrum of the parent compound
and degradation products,60 which is also in agreement with our
results. The herewith provided MS2 spectra of P2 (Table S1)
are in very good agreement with those previously reported.13,60
It was not possible to determine the positions of the hydroxyls,
1
by a ΣE(2) value from O•2− to MET of only 0.64 kcal mol− .
While important chemical reaction between MET and BrO•2
seems to be excluded, it should be reminded that bromate was
able to significantly enhance MET photocatalytic degradation
in the case of Degussa P25. In this case, the effect of bromate
should probably be ascribed to its mere ability to scavenge e−,
which inhibits the e−–h+ recombination and enhances the
•
occurrence and subsequent reactivity of OH and h+. Moreover,
the enhancement effect with Degussa P25 combined with no
bromate effect on TiO2 Wackherr suggests that an important due to the small number of fragments formed in MS2
role in MET degradation could be played by different
properties of the photocatalyst, lifetime of h+ and concentration
of radical species.
experiments. However, based on the theoretical results (Table 3
and Figs. 10 and 12), we can assume that •OH might be bonded
to the C4 atom of the aromatic ring. Fragment 116 (C6H14ON),
common to many MET degradation products, and observed in
In the case of MET/ O•2− SO•4− , the interactions between MET
/
the MS2 spectrum of P3, corresponds to
oxopropanꢀ1ꢀaminium and indicates the intact
Nꢀ(1ꢀmethylethyl)ꢀ2ꢀ
and the corresponding radicals would just be of an electrostatic
nature, as suggested by the very low values of the ΣE(2)
energies, all below the default threshold of 0.05 kcal mol–1.
This result excludes chemical reactivity between MET and
SO•4− , and it helps explaining the very limited effect of
S2O82− on the photocatalytic degradation of MET. Indeed,
O
ꢀbound moiety
i.e. hydroxylation of either benzene ring or C2ꢀchain. Ion 177
fragments by consecutive loss of water (m/z = 18) and ethene
(
m/z = 28), indicating the presence of an hydroxyethyl moiety.
The presence of ion 159 fragments in MS2 spectra of MET and
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possibly occurred at αꢀ and βꢀposition of 2ꢀmethoxyethyl
moiety, i.e. that peaks P4a and P4b possibly represent isomers
P3 indicates preserved aromatic ring of MET. Detailed
fragmentation of MET is described in the literature, where m/z
159 corresponds to the loss of 109 (18+42+17+32) mass units:
water, propene, ammonia are lost from the right side and
methanol from the left side of the chain.61 Thus, P3 represents
B
and C. The presence of abundant fragment 116
(corresponding to intact ꢀ(1ꢀmethylethyl)ꢀ2ꢀoxopropanꢀ1ꢀ
N
aminium, formed by cleavage of phenoxy bond) seems to
support the assumption. However, at the moment, it was not
possible to determine the exact oxygenation site for these two
compounds.
either
1ꢀ[4ꢀ(1ꢀhydroxyethyl)phenoxy]ꢀ3ꢀ(propanꢀ2ꢀ
ylamino)propanꢀ2ꢀol or its 2ꢀhydroxyethyl isomer. This
compound was previously identified62,63 as the 1ꢀhydroxyethyl
isomer.
In the presence of H2O2 three new intermediates were observed
(
P8
, P9 and P10), while in the presence of KBrO3 only P9 and
P10 were detected. Intermediates P7 and P8 both have Mmi
=
The three peaks, labelled P4, P5, and P6, corresponding to
299, which is by 32 mass units greater than for MET. This
implies that they are dihydroxy MET derivatives.13,49
Significant differences in fragmentation patterns of the two
compounds allow the determination of the positions of the
hydroxyl groups.
compounds with Mmi = 281, were detected. Upon further
evaluation of EICs for the observed fragment ions, it was
determined that peak P4 contains two closelyꢀeluting
compounds labelled P4a and P4b. Due to close elution, it was
impossible to obtain pure MS2 spectra of these compounds.
Therefore, EIC traces for each ion had to be checked, to
confirm the presence/absence of specific fragments in each
chromatographic peak. To ease the comparison of spectra and
interpretation, composite MS2 spectra were prepared by
summing MS2 spectra obtained at different collision energies.
Molecular mass, higher by 14 units than that of MET, implies
the introduction of one oxygen and the abstraction of two
hydrogen atoms (i.e. either introduction of an oxo group, or
introduction of hydroxyl and oxidation of the existing
hydroxyl). Early loss of isopropylamine and water (ꢂm/z = 77,
yielding fragment 205) and/or propene (ꢂm/z = 42, yielding
Intermediate P7 was identified as 1ꢀ{4ꢀ[2ꢀhydroxyꢀ3ꢀ(propanꢀ
2ꢀylamino)propoxy]phenyl}ꢀ2ꢀmethoxyethaneꢀ1,2ꢀdiol. Initial
loss of 62 mass units can be attributed to cleavage of the CꢀC
bond between CHOH and CH(OH)OCH3 units, leading to loss
of methoxymethanol and formation of formyl group on benzene
ring. Subsequent loss of propene (ꢂm/z 42) leads to formation
of ion 196. This ion further fragments by cleavage of the PhꢀO
bond, to yield two complementary ions at m/z 74 and 105.
Spectra of P8 are dominated by a series of fragments
characteristic for MET – 159, 133, 116, 74, 72 and 56. Since
these ions correspond to preserved C2ꢀC6ꢀOꢀC3ꢀN moiety
(without any additional substitution, compared to MET), we
assume that the two hydroxyls are located at peripheral groups
– methoxy and/or isopropyl. The value of m/z 282 corresponds
to the parent compound after loss of water. However, it was not
possible to determine where one or both hydroxyls are bound,
due to the lack of any other diagnostic fragments. Intermediate
P9 has Mmi = 273, namely six mass units higher than MET.
This is a consequence of C3H6 loss and the attachment of three
hydroxyl groups. However, at lower m/z range, a series of
fragments was observed – 116 (C6H14NO), 98 (C6H12N), 74
fragment 240) indicate the intact iPrNH–moiety. There are five
possible isomers of oxoꢀMET (excluding the one with
oxygenated isopropyl), hereby designated A–E. Isomer A: 2ꢀ
{4ꢀ[2ꢀhydroxyꢀ3ꢀ(propanꢀ2ꢀylamino)propoxy]phenyl}ethyl
formate, isomer B: methyl {4ꢀ[2ꢀhydroxyꢀ3ꢀ(propanꢀ2ꢀ
ylamino)propoxy]phenyl}acetate, isomer C: 1ꢀ{4ꢀ[2ꢀhydroxyꢀ
3ꢀ(propanꢀ2ꢀylamino)propoxy]phenyl}ꢀ2ꢀmethoxyethanone,
isomer D: 4ꢀ(2ꢀmethoxyethyl)phenylꢀ2ꢀhydroxyꢀ3ꢀ(propanꢀ2ꢀ
ylamino)propanoate, and isomer E: 2ꢀhydroxyꢀ3ꢀ[4ꢀ(2ꢀ
methoxyethyl)phenoxy]ꢀNꢀ(propanꢀ2ꢀyl)propanamide. Based
(C3H8NO), 56 (C3H6N) – corresponding to
Nꢀ(1ꢀmethylethyl)ꢀ
on differences in observed spectra, we tentatively assigned
structures to detected peaks. The loss of H2O (ꢂm/z = 18,
yielding fragment 264), was observable only in peak P6, and
was followed by subsequent loss of propylamine (ꢂm/z = 59,
yielding fragment 205). In all the other peaks the latter loss is
immediate, and thus only ion 205 is observable, leading to the
conclusion that ion 264 is stabilized in P6. Thus, we propose
that P6 represents either isomer D or E, that would – after water
loss – form an α,βꢀunsaturated carbonyl compound, stabilized
through electron delocalization. Peak P5 was characterized by
the absence of two otherwise common ions for MET and
derivatives: 133 (corresponding to ion produced by loss of H2O,
2ꢀoxopropanꢀ1ꢀaminium ion and its fragment, indicating a
preserved 2ꢀhydroxyꢀ3ꢀ[(1ꢀmethylethyl)amino]propoxy group.
Further fragmentation of ion 116 into 98 by the loss of water
(ꢂm/z =18) supports the presumption that the three hydroxyls
are bonded to the methoxyethylbenzene moiety. The absence of
common ions 159, 133 and 121 supports the oxidative cleavage
of benzene ring, with the loss of a C2 unit. While this reaction
was previously described,63 the hereby detected compound P9
exhibit MS2 spectra different from the published ones,
indicating differences in the structure and suggesting that there
are three OH groups bound to a benzene ring. This leads to the
conclusion that the compound is a trihydroxy derivative of 1ꢀ
aminoꢀ3ꢀ[4ꢀ(2ꢀmethoxyethyl)phenoxy]propanꢀ2ꢀol.
iPrNH2 and phenylꢀbound chain) and 116 (corresponding to
loss of ꢀsubstituted phenoxy moiety). Fragment 177, produced
p
Intermediate P10, eluting at 1.57 min, corresponds to a
compound with Mmi = 315. The short retention time suggests
that it is very polar, which would be accounted for by the three
hydroxyl groups present in its structure, since its molecular
mass is 48 mass units higher than that of MET.13 The fragments
116, 98, 74, and 56 in its MS2 spectra indicate that the
propanylaminopropane moiety stays intact, while the three
hydroxyls are bonded to the methoxyethylbenzene part of the
molecule. This compound has been previously identified as a
trihydroxy derivative of 1ꢀ[4ꢀ(2ꢀmethoxyethyl)phenoxy]ꢀ3ꢀ
(propanꢀ2ꢀylamino)propanꢀ2ꢀol.13,49
by loss of 28 mass units (CO2/C2H4) from ion 205. We
concluded that CO loss is to be expected if ion 205 has 1ꢀ
formylꢀ2ꢀ[4ꢀ(2ꢀmethoxyethyl)phenoxy]ethenylium
structure,
i.e. if P5 could represent the isomer E (in that case, P6 would
be the isomer D). The spectra of P4a and P4b differ from
general pattern observed for P5 and P6 by much more
pronounced fragments corresponding to loss of propene (ꢂm/z
= 42) or water and propylamine (ꢂm/z = 77), and absence of
otherwise common fragment 98 (either due to preferred loss of
Nꢀcontaining part as a neutral molecule, or due to low general
abundance). Another common fragment, m/z 121,
corresponding to protonated pꢀvinylphenol, was also absent in
both peaks, which lead us to the conclusion that the oxidation
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Fig. 13 Degradation intermediates of MET with electron acceptors, in the presence of TiO2 Wackherr and Degussa P25. P1, P3
–
P7 were identified in all cases; P2* was
identified, in all cases, only with TiO2 Wackherr; P8** was identified only with TiO2 Wackherr/O2/H2O2; P9*** was identified in all cases except TiO2 Wackherr/O2;
P10**** was identified in all cases except TiO2 Wackherr/O2 and Degussa P25/O2/KBrO3
The occurrence of different intermediates under different photocatalysts.13 Indeed, P2 and P8 were not detected in the
conditions could be due to their different stabilities, and/or to presence of Degussa P25/O2, while P8, P9 and P10 were not
differences in the degradation mechanism of MET. One issue found with TiO2 Wackherr/O2. By comparing the O2/H2O2
could be the interaction of O•2− with MET (Table 3). Compared
systems with Degussa P25, P2 and P8 were again not
identified, while in the case of TiO2 Wackherr all intermediates
to the case of O2 alone, the addition of H2O2 lowered such
were identified. In the presence of O2/KBrO3 with Degussa P25
interaction by 1.2 times and that of KBrO3 by 54 times.
However, NBO analysis already suggested that in the case of
the intermediate P10 was absent, while in the presence of TiO2
Wackherr P10 was present, but P8 was absent.
MET degradation with Degussa P25, besides the reactive
In the present work, a lower number of intermediates was
radicals, the properties of the catalyst and the lifetime of h+
detected in comparison with previous work.13 That is probably
would play an important role. This issue was confirmed by the
a consequence of the significantly lower concentration of MET
analysis of the mechanism. Namely, in the presence of Degussa
P25/O2 the intermediates P1, P3–P7, P9, and P10 were
(by 60 times) used in the present work, which prevented the
formation of dimeric species. Moreover, the concentrations
(peak areas) of some intermediates in this work were very low
and thus their identification was not possible.
identified. Moreover, they were also identified in the presence
of H2O2. This issue suggests that H2O2 in this case just
increased the degradation rate of the parent compound (Fig. 1),
which resulted in the faster formation of the intermediates
•
4. Conclusions
because of an increased occurrence of OH. The intermediate
P10 was not detected in the presence of KBrO3, which might be
This study shows that TiO2 is a very effective photocatalyst for
MET degradation, in the presence of electron acceptors such as
O2, H2O2, and BrO3− . The effect of the investigated electron
due to the lower •OH concentration in the system.
Similarly to our previous work, here it was confirmed that
different sets of MET intermediates are formed with different
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ARTICLE
DOI: 10.1039/C5RA10523D
3. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann,
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acceptors depends on their initial concentration and on the
nature of the photocatalyst. Moreover, while enhancing the
transformation of MET (at least with Degussa P25), H2O2
decreased the rate of MET mineralization with both TiO2 types.
This contrasting effect might be accounted for by differences in
•
reaction pathways (h+ vs. OH) induced by the photocatalysts
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degradation intermediates.
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Journal of Molecular Modeling, 2012, 18, 4491ꢀ4501.
•
DFT calculations and NBO analysis suggested that OH and
possibly O•2− could undergo chemical reaction with MET. It is,
therefore, suggested that dissolved oxygen would not only
enhance •OH/h+ reactivity by scavenging e− and, therefore,
inhibiting e−–h+ recombination; oxygen could additionally
favour MET degradation through the formation of O•2− . In
15. R. G. Parr and W. Yang, Journal of the American Chemical Society
1984, 106, 4049ꢀ4050.
,
contrast, reaction between MET and BrO•2 or SO•4− would be
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limited effect of S2O82− on MET degradation: e− scavenging by
S2O82− would be largely offset by the formation of sulfate in
16. K. Chandrakumar and S. Pal, International Journal of Molecular
Sciences, 2002, , 324ꢀ337.
3
17. S. H. Rosline Sebastian Sr, M. I. Attia, M. S. Almutairi, A. A. Elꢀ
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Chemical Physics Letters, 2013, 578, 156ꢀ161.
19. E. Kose, A. Atac, M. Karabacak, C. Karaca, M. Eskici and A. Karanfil,
Spectrochimica Acta Part A: Molecular and Biomolecular
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Chemical Physics, 1998, 109, 6854ꢀ6860.
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addition to SO•4− . While sulfate decreases the photocatalytic
activity by adsorbing on the surface of TiO2, SO•4− would not be
able to take part in MET transformation.
Interestingly, both DFT calculations and the approach based on
Fukui functions and Fukui indices consistently suggested that
•OH would react with the MET aromatic ring, and particularly
with its C4 atom.
Experimental study by LC–ESI–MS/MS indicated bonding of
OH groups to different parts of MET, while results of
theoretical analysis obtained in this investigation indicated
bonding of OH group to the aromatic ring. According to the
theoretical results, bonding of OH group to the C4 atom of
benzene ring of P2 was suggested. It was shown experimentally
that the binding of OH groups does not occur on the
propanylaminopropane group chain (P8, P9, and P10) and that
ring opening doesn’t occur as well, which is also in agreement
with theoretical analysis. Besides, intermediates that are
peculiar for the systems containing H2O2 and KBrO3 were
identified as well. Namely, P8, P9, and P10 were detected with
TiO2 Wackherr/O2/H2O2 and P9 and P10 with TiO2
Wackherr/O2/KBrO3. While the same intermediates were
identified with Degussa P25/O2 and Degussa P25/O2/H2O2, the
intermediate P10 was not identified with Degussa
P25/O2/KBrO3.
29. B. KasprzykꢀHordern, R. M. Dinsdale and A. J. Guwy, Water
Research, 2009, 43, 363ꢀ380.
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Hazardous Materials, 2005, 122, 195ꢀ204.
31. S. Wiegel, A. Aulinger, R. Brockmeyer, H. Harms, J. Löffler, H.
Reincke, R. Schmidt, B. Stachel, W. von Tümpling and A. Wanke,
Chemosphere, 2004, 57, 107ꢀ126.
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Pelizzetti, Applied Catalysis B: Environmental, 2005, 58, 79ꢀ88.
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Cheeseman, J. Montgomery Jr, T. Vreven, K. Kudin and J. Burant,
Gaussian.
Acknowledgements
The authors greatly appreciate the financial support from the
Ministry of Education, Science and Technological
Development of the Republic of Serbia (Project No. 172042).
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