BiOBr photocatalyzed decarboxylation of glutamic acid: reaction rates, intermediates and mechanism

Article abstract of DOI:10.1039/c5ra09528j

The degradation of glutamic acid by BiOBr under both UV and visible irradiation was investigated and compared with degradation by TiO₂/UV. Analysis of the reaction rates and the distribution of intermediates was used to show that both BiOBr systems, unlike the TiO₂ system, catalyze direct substrate oxidation by valance band holes.

Full text of DOI:10.1039/c5ra09528j

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BiOBr photocatalyzed decarboxylation of glutamic
acid: reaction rates, intermediates and mechanism†
Cite this: RSC Adv., 2015, 5, 55727
Yanfen Fang,ab Hongwei Yang,ab Wei Zhou,ab Yue Li,c David M. Johnsonab
ab
*
and Yingping Huang
Received 21st May 2015
Accepted 19th June 2015
DOI: 10.1039/c5ra09528j
www.rsc.org/advances
The degradation of glutamic acid by BiOBr under both UV and visible on D-Glu and D-MeAsp bind with the metal atom and Arg96 of
irradiation was investigated and compared with degradation by TiO2/ protein phosphatase 1 (PP1) to inhibit protein phosphoryla-
UV. Analysis of the reaction rates and the distribution of intermediates tion.19,20 Thus, understanding the degradation mechanism of
was used to show that both BiOBr systems, unlike the TiO2 system, amino acids, particularly the decarboxylation process, is of
catalyze direct substrate oxidation by valance band holes.
practical signicance for water purication.
Glutamic acid (Glu) is one of the proteinogenic amino acids
and, with a second carboxyl group on the side chain, it is an
ideal substrate for comparing the degradation process of
carboxylic acids with that of amino acids. In this work, we used
BiOBr as the photocatalyst to degrade Glu under both UV and
Vis irradiation. The degradation process was examined with 1H
NMR and 18O isotope labeling and spin trapping ESR were used
to elucidate the reaction mechanism. These results were
compared with those from a TiO2 system to show the effect of
the valance band structure of BiOBr on the catalytic degradation
of amino acids.
D2O suspensions containing BiOBr and Glu were irradiated
with UV or visible light for a given time and then analyzed using
1H NMR analysis aer removing the photocatalyst. Compared
with parent substrate (Glu), the reacted solutions gave addi-
tional peaks at d of 1.93, 2.48, 3.26 and 8.35 with both UV and
visible light irradiated systems (Fig. 1). Using reference
Visible light photocatalysis has attracted continuing attention
because of potential applications in water purication.1,2
A
recently developed photocatalyst, BiOBr, is an efficient visible
light photocatalyst3–5 with visible light activity higher than that
of N-doped TiO2.6 As described previously, BiOBr has two
discreet valance bands produced from O-2p and Br-4p
orbitals.7,8 The two bands respond, respectively, to UV and
visible light excitation and holes of different oxidation potential
are generated, providing multiple mechanisms for photo-
catalytic degradation.7
Amino acids are biologically important organic compounds,
both as building blocks for proteins and as metabolic inter-
mediates. As biodecomposition products, amino acids are
distributed widely in natural waters.9,10 The concentration of
amino acids in surface water is generally in the range of
2.5–60 nM.11,12 Although amino acids are nontoxic, they can
form carcinogenic and mutagenic species during the water
purication process.13,14 For example, amino acids were con-
verted primarily to halomethanes and haloacetic acids by
chlorination.15–17 More importantly, with naturally occurring
toxins it is usually the carboxyl group of an amino acid that
binds to the affected enzyme.18–21 For example, with the well-
known cyanotoxin, microcystin-LR, the free carboxyl groups
aInnovation Center for Geo-Hazards and Eco-Environment in Three Gorges Area,
Yichang 443002, Hubei province, China. E-mail: huangyp@ctgu.edu.cn; Fax: +86
717 6395966; Tel: +86 717 6397488
bEngineering Research Center of Eco-environment in Three Gorges Reservoir Region,
Ministry of Education, China Three Gorges University, Yichang, 443002, China
cDepartment of Chemistry, Nankai University, Tianjin 300071, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
additional gures and table. See DOI: 10.1039/c5ra09528j
Fig. 1 1H NHR spectra of oxidative products of Glu in BiOBr/Vis and
BiOBr/UV systems, 1 g Lꢀ1 BiOBr, cG0lu ¼ 10 mmol Lꢀ1, 10 mL D2O.
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begins with oxidation of the amino-carboxyl structure of Glu.
These results indicate that the amino-carboxyl structure is
susceptible to oxidation in BiOBr photocatalytic systems. Due to
the reactivity of this structure, amino acids are more readily
degraded by BiOBr than are free carboxylic acids.
The BiOBr/Vis and BiOBr/UV systems display different
degradation kinetics, but the visible light irradiated system also
shows lower selectivity for SA and a markedly higher selectivity
for MA (Table 1). This phenomenon is attributed to the discrete
valance band structure of BiOBr. The holes generated by UV
(hO-2p+) and visible light (hBr-4p+) excitation have different
oxidation potentials, leading to different secondary reactions
and the observed differences in rate. The results obtained in
these systems were also compared with those of the classic
TiO2/UV system to show the unique properties of BiOBr pho-
tocatalysis. It was observed that the TiO2/UV photocatalyzed
oxidation of Glu gave remarkably low intermediate concentra-
tions. The total selectivity of SA and MA in the TiO2 photo-
catalyzed system is only 2.8%, which is much lower than that of
BiOBr/Vis and BiOBr/UV systems (37.4% and 21.8%, respec-
tively, Table 1). TiO2 has a valance band (Evb ¼ 2.7 V) more
oxidizing than either of the two valance bands of BiOBr and its
hole oxidizes H2O to cOH. It was reported that cOH plays a
signicant role in TiO2 photocatalyzed degradation of amino
acids.23–25 Considering the valance band potentials and differ-
ences observed between the BiOBr and TiO2 systems, we assume
that the valance band hole of both BiOBr systems initiates the
degradation of Glu by direct oxidation rather than by cOH
mediated reactions.
Fig. 2 Concentration change of substrate and oxidation products
during photocatalytic oxidation of Glu in (a) BiOBr/Vis system and (b)
BiOBr/UV system.
Since the photocatalytic degradation of Glu starts from the
amino-carboxyl end and leads initially to SA, the decarboxylated
and deaminated product, we anticipated that the source of
oxygen atoms in the carboxyl group formed in this process
could give useful information about the mechanism of the
reaction. These experiments were carried out in 18O-enriched
water (H218O) and atmospheric 16O2. Samples from the three
systems were collected at times that resulted in similar
substrate conversion (20–30%), and analyzed by derivative
GC-MS (Fig. S2–S4 (ESI†)). As shown in Table 2, O atoms from
both H2O and O2 were incorporated into SA under BiOBr pho-
tocatalysis condition. The SA formed in BiOBr/UV and BiOBr/
Vis systems have similar isotope abundances of carboxyl O
atoms (16O% ¼ 13–14), which illustrates that these two systems
react with similar mechanisms. In contrast, the SA formed in
compounds, these peaks were assigned to acetic acid (AA),
succinic acid (SA), malonic acid (MA) and formic acid (FA). The
change in the 1H NMR spectrum with reaction time also
provides the kinetics of substrate consumption and interme-
diate formation in the BiOBr/UV and BiOBr/Vis systems (Fig. 2).
During photocatalytic oxidation, Glu forms SA initially and
further reaction of the primary intermediate gives MA, AA and
FA. However, no signal for aspartic acid, the decarboxylation
product of Glu, was recorded, a clear indication that degrada-
tion begins with the amino group rather than the carboxyl. The
different decarboxylation of Glu in BiOBr/Vis and BiOBr/UV
systems was proposed in Scheme 1 (additional details are
shown in Fig. S1 (ESI†)). Similarly, during the BiOBr photo-
catalyzed oxidation of microcystin-LR,22 degradation also
Table 1 The formation rate and selectivity of intermediates produced
in the photocatalytic degradation of Glu
r
b (mmol Lꢀ1 hꢀ1
)
Sel.c (%)
SA
f
System
r
a (mmol Lꢀ1 hꢀ1
)
SA
MA
MA
d
BiOBr/Vis
BiOBr/UV
TiO2/UV
0.199
1.071
13.33
0.019
0.152
0.197
0.055
0.081
0.173
9.6
14.2
1.5
27.8
7.6
1.3
a
b
c
Scheme 1 Difference in the decarboxylation of Glu in BiOBr/Vis (gray
arrows) and BiOBr/UV (black arrows) systems. Solid arrows shows a
major reaction route, dashed arrows represent a minor reaction route.
Decomposition rate of Glu. Formation rate of intermediate. Ratio
of consumption rate of substrate to accumulation rate of intermediate.
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+
the photogenerated hole (hO-2p or hBr-4p+) oxidizes Glu to a
cation radical, which then reacts with either O2 or H2O to
produce the carboxyl group on the product.
Table 2 Average isotope abundances of oxygen atoms in the carboxyl
group of SA in H218O isotope labeling experimentsa
Abundanceb
(%)
To further conrm that direct oxidation of Glu accounts for
the larger pool of intermediates and higher proportion of O2-
derived oxygen in SA observed in the BiOBr systems, spin-
trapping ESR spectroscopy was used to detect the formation
of cOH. The results were again compared with those of TiO2 and
are shown in Fig. 3. In contrast to the TiO2/UV system, the
signals from trapped cOH recorded in the BiOBr systems was
either weaker or nonexistent. Because neither of the valance
band holes of BiOBr can oxidize H2O, the small amount of cOH
is attributed to the reduction of O2 by conduction band elec-
trons (O2 / cOOH / H2O2 / cOH) and cOOH was detected
(Fig. S8 (ESI†)).
Time
(min)
Substrate conv.
(%)
SA yield
(%)
System
16O2
H218O
BiOBr/Vis
BiOBr/UV
TiO2/UV
480
90
20
29.9
23.8
20.8
11.9
23.8
20.8
14.2
13.1
6.4
85.8
86.9
93.6
a
1 g Lꢀ1 photocatalyst, cG0 lu ¼ 10 mmol Lꢀ1, 2 mL H218O. b Average value
of the two O atoms of the formed carboxyl group, corrected with the
oxygen isotope abundance of solvent H218O and the natural isotope
abundance of aerial O2.
the TiO2/UV system gave an 16O abundance (16O% ¼ 6.4) less
than half that of the BiOBr systems. TiO2 photocatalysis clearly
incorporates more H2O derived oxygen to the product than the
BiOBr systems. We also performed 18O2 isotope labeling
experiments and similar results were obtained (Table S1 and
Fig. S5–S7 (ESI†)). Since the valance band hole of TiO2 can
oxidize H2O to cOH and incorporate O atoms from H2O to the
product, the higher proportion of H2O derived oxygen in the
TiO2/UV system is reasonable. These results also corroborate
the direct oxidation mechanism proposed for BiOBr systems.
We propose that, in both the BiOBr/UV and BiOBr/Vis systems,
Conclusions
In summary, we studied the BiOBr catalyzed degradation of Glu
under UV and visible light irradiation. Results indicate that, in
both BiOBr/UV and BiOBr/Vis systems, the degradation process
is initiated by direct substrate oxidation by the valance band
hole. This, in turn, leads to the same primary product with the
same source of oxygen in the carboxyl group formed on SA.
However, the difference in the hole oxidation potentials of
BiOBr/UV and BiOBr/Vis leads to different degradation rates,
different secondary degradation processes and different distri-
butions of degradation intermediates.
Acknowledgements
This work was supported by the NSFC (21207079, 21377067 and
21307062).
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