TiO2/UV Photocatalytic Degradation of Michler’s Ketone
J. Chin. Chem. Soc., Vol. 56, No. 4, 2009 737
acid with fit values of 92%, 83%, 97% and 97%, respec-
tively, found by searching the mass spectra library. The for-
mer intermediates (compounds A¢-B¢) are the results of the
cleavage of the MK and N-de-methylated MK species,
leading to N-methylaminobenzene and aminobenzene. The
findings confirm other authors’ work. These results are co-
herent with some given by Muneer.36 The latter intermedi-
ates consist of compounds C¢-D¢ formed by cleavage of the
aromatic derivatives, leading to aliphatic products.
Concentration of compounds A¢ and B¢ can be deter-
mined from SPE and GC/FID experiment. The mean recov-
ery values of N-methylaminobenzene (compound A¢) and
aminobenzene (compound B¢) were 75.5% and 51.2% with
R.S.D. values of 9.8% and 15.8% (n = 3) respectively,
which made estimated quantitation feasible. The concen-
tration of N-methylaminobenzene and aminobenzene in
MK/TiO2 solution after 8 h of irradiation, determined by
SPE-GC/FID method, were 10.1 and 33.8 mgL-1, respec-
tively.
de-methylated intermediates, MMBP and DBP. The de-
methylation process as described above continues until
formation of the completely de-methylated species, BP.
In addition to the N-de-methylation degradation route,
an alternative pathway was also identified. A plausible
mechanism for the formation of degradation products A¢-
B¢ involving electron transfer reactions and reaction with
hydroxyl radicals formed in the photocatalytic system is
proposed in Scheme I (route B). Upon the transfer of an
electron the MK and its N-de-methylated species can form
the radical anion, which can undergo the addition of a
hydroxyl radical and form the anionic species, which upon
cleavage can lead to the formation of N-methylaminoben-
zene and aminobenzene. This is in agreement with mecha-
nisms proposed by Muneer et al. for gentian violet.36
It is known from previous photocatalytic studies that,
after the formation of various aromatic derivatives, cleav-
age of the benzene or other organic rings takes place, and
different aliphatic products are subsequently formed before
complete mineralization.25 Even though further oxidation
leads to the ring-opening and the formation of aliphatic ox-
idation products (compounds C¢-D¢), these species will not
be discussed here.
Initial photooxidation pathway
It is well accepted that oxidation in photocatalytic
systems (TiO2, O2, hn) is mediated to a large extent by the
hydroxyl radical (•OH) formed when either adsorbed water
or hydroxide is oxidized at a valence band hole.41,42 Based
on the LC/MS and GC/MS analyses of the various photo-
catalytic intermediates, the degradation pathways for MK
are proposed in Scheme I. It involves two different path-
ways (routes A and B, respectively), corresponding to the
two possible sites for the attack of the •OH radicals on the
MK molecule.
CONCLUSION
Our studies suggest that TiO2 photocatalysis should
be an effective technique for the destruction of the MK in
aqueous solutions. The percentage decreases in MK con-
centration, resulting from the photocatalytic reactions con-
ducted for 20 h, was 98.1%. Nine intermediates have been
identified and characterized using LC/MS and GC/MS
techniques, giving insight into the early steps of the degra-
dation process. The photocatalytic degradation of MK pro-
ceeds through competitive reactions such as N-de-methyla-
tion and destruction of the bis-aminobenzophenone struc-
ture. The former path generates a carbon-centered radical
upon H-atom abstraction, which further reacts with O2 gen-
erating peroxy and alkoxy radical intermediates to result in
a series of N-de-methylated products. The hypsochromic
effects resulting from N-de-methylated intermediates of
MK occurred concomitantly during irradiation. In the latter
path, the electron in the conduction band of TiO2 can be
picked up by the adsorbed MK, leading to the formation of
radical anion, which can undergo addition of a hydroxyl
radical forming the anionic species, which upon cleavage
can lead to the formation of N-methylaminobenzene and
The N-de-methylation of the MK occurs mostly by
the attack of the •OH species on the N,N-dimethyl groups of
the MK, as shown in Scheme I (route A). Hydroxyl radicals
yield carbon-centered radicals upon the H-atom abstraction
from the methyl group, or they react with the lone-pair
electron on the N atom to generate cationic radicals, which
subsequently convert into carbon-centered radicals.19 The
carbon-centered radicals react rapidly with O2 to produce
peroxy radicals that subsequently transform into alkoxy
radicals through the bimolecular Russell mechanism.43 The
fragmentation of the alkoxy radical produces a de-methyl-
ated product. The mono-de-methylated species, DMBP,
can also be attacked by •OH species and be implicated in
other similar events (H-atom abstraction, oxygen attack,
and the bimolecular Russell mechanism) to yield the bi-