Influence of the substituent on selective photocatalytic oxidation of aromatic compounds in aqueous TiO2 suspensions

Article abstract of DOI:10.1039/b515853b

Experimental results are reported showing that the photocatalytic oxidation of aromatic compounds containing an electron-donor group (EDG) gives rise mainly to ortho- and para-monohydroxy derivatives while in the presence of an electron-withdrawing group (EWG) all the monohydroxy derivatives are obtained. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515853b

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Influence of the substituent on selective photocatalytic oxidation of
aromatic compounds in aqueous TiO2 suspensions{
Giovanni Palmisano,*a Maurizio Addamo,a Vincenzo Augugliaro,a Tullio Caronna,b Elisa Garc´ıa-Lo´pez,a
Vittorio Loddoa and Leonardo Palmisanoa
Received (in Cambridge, UK) 9th November 2005, Accepted 4th January 2006
First published as an Advance Article on the web 23rd January 2006
DOI: 10.1039/b515853b
Detector), equipped with a Luna 5 m Phenyl-Hexyl column
(250 mm long 6 2 mm i.d.); the flow rate was 0.2 ml min21 and
the identification was made by comparison with authentic
samples. TOC (total organic carbon) measurements were
carried out by using a TOC Shimadzu 5000 A analyser.
It is important to point out that two competing pathways were
present from the starting of irradiation: substrate degradation that
produced CO2 and formation of monohydroxylated species
released into the bulk of the solution. Particular care was devoted
to determine the concentrations in the liquid phase of each
hydroxylation product (ortho, meta and para).
Experimental results are reported showing that the photo-
catalytic oxidation of aromatic compounds containing an
electron-donor group (EDG) gives rise mainly to ortho- and
para-monohydroxy derivatives while in the presence of an
electron-withdrawing group (EWG) all the monohydroxy
derivatives are obtained.
Photocatalytic oxidation of aromatic compounds has been widely
studied,1–10 while photocatalytic syntheses represent an incoming
research field.11 In this work a systematic attempt was made to
understand the influence of the substituent group on the selectivity
of photo-oxidation of the aromatic molecules to hydroxylated
compounds, in the presence of polycrystalline TiO2.
The alternative oxidation route to complete mineralization was
reported in literature for toluene6 and it should be stressed that it
occurs at various extents from the beginning of irradiation when
the molecules of substrates are not very big, as in our case. The
evolution to CO2 occurs through various oxidation steps involving
unknown intermediates species strongly (photo)adsorbed onto
TiO2 surface without their release into the bulk of solution. An
experimental evidence of the occurrence of this parallel and
kinetically very fast process derives from the observation of TOC
results showing a decrease from the very beginning of irradiation
(see, for example, Fig. S1 in ESI{).
The results obtained with an aromatic compound containing an
EDG and an EWG are reported in Fig. 1(a) and (b), respectively.
Fig. 1(a) reports the concentration values of nitrobenzene and its
monohydroxylated products vs. irradiation time, while Fig. 1(b)
reports the results obtained with N-phenylacetamide.
Photocatalytic runs were performed in liquid–solid regime to
oxidise aromatic compounds containing an EDG (phenol,
phenylamine, and N-phenylacetamide) or an EWG (nitrobenzene,
cyanobenzene and 1-phenylethanone). The photocatalyst used was
commercial anatase TiO2 (Merck) with an amount of 0.4 g l21 and
the pH was the natural one (ca. 6.5). The experiments lasted 1.5 h
and were performed in a batch photoreactor of cylindrical shape,
containing 0.5 l of aqueous suspension, with pure O2 continuously
bubbling. A 125 W medium-pressure Hg lamp (Helios Italquartz,
Italy) was axially immersed within the photoreactor and it was
cooled by water circulating through a Pyrex thimble. The
temperature of the suspension was about 300 K. The radiation
energy impinging on the suspension had an average value of
10 mW cm22. The initial concentration of the substrates was ca.
500 mM. The extent of adsorption in the dark was measured by
monitoring the concentration of the substrates before adding the
catalyst and after 1 h of contact with it, when the physical
equilibrium was achieved. During the photocatalytic runs, samples
for analyses were withdrawn at fixed intervals of time and the
catalyst was immediately separated from the aqueous solution by
filtering through 45 mm Millex Millipore filters.
It may be observed that the aromatic molecules were oxidised in
both cases and the concentrations of their oxidation products
increased with irradiation time. The whole molar yields of the three
hydroxylated species (OH-derivatives produced divided by the
reacted substrate) were ca. 20% and ca. 60% for nitrobenzene and
N-phenylacetamide, respectively.
Inside the investigated irradiation time, the monohydroxylated
isomers were the main reaction products, with yields in the 50–75%
range when the photocatalytic reaction was carried out by using an
aromatic compound containing an EDG. On the contrary, the yields
in monohydroxylated isomers were in the 20–30% range for the
aromatic compounds containing an EWG. It is worth noting that
oxidation of the –COCH3 and –NHCOCH3 substituents (although
in lesser extent) could also take place during the photocatalytic
process as unidentified peaks were observed in the chromatograms.
Table 1 summarizes the obtained results for all of the studied
compounds; it reports the amounts of substrates adsorbed in the
dark, the conversions and yields in monohydroxylated isomers
along with the HPLC-eluents appropriately chosen.
All the monohydroxylated species produced during the
reactions were analyzed with a HPLC Beckman Coulter
(System Gold 126 Solvent Module and 168 Diode Array
aDipartimento di Ingegneria Chimica dei Processi e dei Materiali,
Universita` degli Studi di Palermo, Viale delle Scienze, 90128, Palermo,
Italy. E-mail: giovanni_palmisano@yahoo.it;
palmisano@dicpm.unipa.it; Fax: +39 091 6567280; Tel: +39 091 6567246
bUniversita` di Bergamo, Facolta` di Ingegneria, Via Marconi 5, I – 24044,
Dalmine (Bergamo), Italy
{ Electronic supplementary information (ESI) available: TOC trend
during 4 (see Table 1) photo-oxidation; graphics with the concentrations
of compounds 1, 2, 5, 6 (see Table 1) and intermediates during their
photocatalytic oxidations. See DOI: 10.1039/b515853b
1012 | Chem. Commun., 2006, 1012–1014
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Fig. 1 (a) Photocatalytic oxidation of nitrobenzene: (r) nitrobenzene, ($) 2-nitrophenol, (m) 3-nitrophenol, (&) 4-nitrophenol. (b) Photocatalytic
oxidation of N-phenylacetamide: (r) N-phenylacetamide, ($) N-(2-hydroxyphenyl)acetamide, (m) N-(3-hydroxyphenyl)acetamide, (&) N-(4-
hydroxyphenyl)acetamide.
The data of Table 1 clearly indicate that the substituent nature
strongly influences: (i) the adsorption of the aromatic substrates
onto the surface of the catalyst, and (ii) the position of the
hydroxyl group entering the aromatic ring, giving rise to a
regioselectivity in the monohydroxylated reaction products.
The adsorption of aromatic compounds containing an EDG is
negligible in all cases. A notable adsorption, instead, was observed
with a strongly EWG. The behaviour of 1-phenylethanone, that
showed a negligible adsorption, can be explained by considering
that this substituent has only a slight electron withdrawing effect.
The above findings indicate that the presence of a certain
additional electron density, induced in the aromatic ring by the
presence of an EDG, is not beneficial for the molecule adsorption.
On the contrary, the presence of a strongly EWG induces a
delocalized electron density in the aromatic ring, lower with respect
to that of benzene, that promotes the adsorption of the molecule
onto the surface. This effect could support the reaction pathway to
complete mineralization onto TiO2 surface with consequent lower
yields in monohydroxylated species (see Table 1), observed for the
substrates containing an EWG.
revealed by FTIR spectroscopy,14,15 and consequently it is difficult
to establish which of them are the most important for the
measured adsorption in the dark.
The conversion values (calculated after 1.5 h of irradiation) were
not strongly influenced by the amount of adsorption in the dark
nor from the nature of the substituent. The highest and the lowest
conversions were those of phenol (y70%) and phenylamine
(y40%), respectively.
Another aspect that may be stressed concerns the selectivity of
the attack to the aromatic ring in positions ortho, meta and para: it
can be seen that it is strongly dependent from the type of
substituent (Table 1). It is worth noting that the yields and o:m:p
values reported in Table 1 were calculated as average percentages,
considering that during the first 45 min of irradiation time they did
not change significantly. The yield data of monohydroxylated
compounds obtained starting from aromatic substrates containing
an EDG indicate that the main products were ortho- and para-
isomers, while the meta-isomer was not present or it was present
only in very small amounts. Completely different is the behaviour
exhibited by the photocatalytic oxidation when an EWG is present
in the aromatic ring: no influence on the orientation of the
monohydroxy substituent was observed, given that ortho-, meta-
and para-isomers were all present in significant amounts (>20%).
This finding is confirmed by the intermediates found during
photocatalytic oxidation of a large number of substituted aromatic
compounds, using different kinds of TiO2 under various experi-
mental conditions.1–10 It is furthermore reported8 that the presence
of both an ED and an EW group gives rise to a behaviour similar
to that observed when only an EDG is present.
Literature12–15 reports that aromatic molecules can adsorb
through the formation of Ti4+
p electron or OH(surface)
p
…
…
(surface)
electron type complexes,12,13 and the latter should be related to the
production of monohydroxylated species under irradiation before
their release in the solution bulk. For both types of adsorption, the
more or less significant basic nature of the substituents, related to
their withdrawing or donor capacity, could play a major role. All
types of Lewis and Brønsted–Lowry acid–base sites are generally
present at various extent on the surface of polycrystalline TiO2, as
Table 1 Adsorption in the dark (after 1 h), average yields in hydroxylated species, o:m:p average ratios (during the first 45 min of irradiation),
eluents used for HPLC-analyses of the substrates studied
Adsorption Conversiona,b Total yield (%)
HPLC-eluent
in OH-derivativesa o:m:p Ratioa A; B; C (%)c
Species
Group
Orientation in the darka (%)
Phenol (1)
Phenylamine (2)
–OH (EDG)
–NH2 (EDG)
ortho, para Negligible y70
ortho, para Negligible y40
y75
y50
y60
y20
y30
y30
54.5:0.5:45.0 65; 35; 0
49.7:0.0:50.3 90; 5; 5
20.0:3.0:77.0 60; 40; 0
29.0:34.0:37.0 60; 20; 20
45.0:30.0:25.0 70; 15; 15
38.5:21.0:40.5 65; 35; 0
N-Phenylacetamide (3) –NHCOCH3 (slightly EDG) ortho, para Negligible y50
Nitrobenzene (4)
Cyanobenzene (5)
–NO2 (EWG)
–CN (EWG)
—
—
—
y8%
y6%
y50
y60
1-Phenylethanone (6) –COCH3 (slightly EWG)
a
Negligible y55
b
c
Mol (%). Values calculated after 1.5 h of irradiation. A: 40 mM aqueous solution of KH2PO4; B: methanol, C: acetonitrile; vol. (%).
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Scheme 2 Main hydroxylated products obtained during the photocata-
lytic oxidation of aromatic compounds containing either an electron donor
or an electron withdrawing group.
In summary it was proved that the reaction of monohydroxyla-
tion of an aromatic ring occurs in all the three possible positions
when an EWG group is present, while only the formation of ortho-
and para-isomers virtually occurs when an EDG is present (see
Table 1 and Scheme 2).
Scheme 1 Resonance structures of radical intermediates produced
during the oxidation of a compound containing an EDG.
An interesting perspective is open not only to produce
monohydroxylated compounds with high conversions and yields
starting from aromatics with an EDG, but also to predict the
intermediate products through which the photo-oxidation of
aromatic compounds evolves.
It is well known that heterogeneous photocatalytic oxidations
canoccurthrougha mechanisminvolvingOHradicalsduring the
first steps.16 The reactivity of hydroxyl radical was a subject of
intensive studies17 and the correlation between the selectivity of
the attack and the nature of the substituent in homogenous
reactions was also recently studied with various experimen-
tal18–22 and theoretical23 methods. It is reported that the rate of
addition of the OH radical to the aromatic rings (k = 109–
1010 l mol21 s21) is very high and this could mean that the attack
should be rather unselective. It is, however, also reported that the
ratio between the different regioisomers strongly depends on
the oxidative power of the reaction medium. The oxidation of the
radical intermediates arising from the addition of a hydroxyl
radical to the aromatic ring of the substrates studied in this work
could occur in different ways: (i) by means of another hydroxyl
radical for hydrogen abstraction, (ii) by means of electron
transfer followed by proton elimination. Consequently the
further oxidation of the intermediates to the final products
depends on the nature of the substituent: an easier process would
take place in the case of an EDG, with larger difficulty when the
electron withdrawing ability of the group increases.
Notes and references
1 M. A. Gonza´lez, S. G. Howell and S. K. Sikdar, J. Catal., 1999, 183,
159.
2 A. M. Peiro´, J. A. Ayllo´n, J. Peral and X. Dome´nech, Appl. Catal. B:
Environ., 2001, 30, 359.
3 N. N. Rao, A. K. Dubey, S. Mohanty, P. Khare, R. Jain and S. N. Kaul,
J. Hazard. Mater., 2003, 101, 301.
4 P. Piccinini, C. Minero, M. Vincenti and E. Pelizzetti, Catal. Today,
1997, 39, 187.
5 V. Augugliaro, A. Bianco Prevot, V. Loddo, G. Marc`ı, L. Palmisano,
E. Pramauro and M. Schiavello, Res. Chem. Intermed., 2000, 26, 413.
6 G. Marc`ı, M. Addamo, V. Augugliaro, S. Coluccia, E. Garc´ıa-Lo´pez,
V. Loddo, G. Martra, L. Palmisano and M. Schiavello, J. Photochem.
Photobiol. A: Chem., 2003, 160, 105.
7 G. Mailhot, L. Hykrdova´, J. Jirkovsky´, K. Lemr, G. Grabner and
M. Bolte, Appl. Catal. B: Environ., 2004, 50, 25.
8 A. Di Paola, V. Augugliaro, L. Palmisano, G. Pantaleo and E. Savinov,
J. Photochem. Photobiol. A: Chem., 2003, 155, 207.
9 D. S. Bhatkhande, V. G. Pangarkar and A. A. C. M. Beenackers, Water
Res., 2003, 37, 1223.
Furthermore, by taking also into account the position of the
attack, the different intermediates will show different abilities to
be oxidized. In fact the behaviour of aromatic compounds
containing an EDG can be explained by the stabilization of the
radical intermediate, whose resonance structures are shown in
Scheme 1. In particular the highest contribution to the
stabilization is given by the formula with the unpaired electron
on the carbon bonded to the EDG. This resonance structure can
exist only when the hydroxyl radical enters ortho- and para-
positions. On the other hand, a predominance of the meta-
isomer would have been expected in the presence of an EWG for
which the above reported resonance structure is particularly
unstable. Nevertheless this behaviour was not observed and all
the three isomers were found: this could be explained
hypothesising that the further oxidation process of the radical
intermediate is more difficult in this case, especially considering
that in the whole mechanism the presence of TiO2 plays an
important role.
10 A. H. C. Chan, C. K. Chan, J. P. Barford and J. F. Porter, Water Res.,
2003, 37, 1125.
11 T. Caronna, C. Gambarotti, L. Palmisano, C. Punta and F. Recupero,
Chem. Commun., 2003, 18, 2350.
12 V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra,
L. Palmisano and M. Schiavello, Appl. Catal. B: Environ., 1999, 20, 15.
13 M. Nagao and Y. Suda, Langmuir, 1989, 5, 42.
14 M. Primet, P. Pichat and M. Mathieu, J. Phys. Chem., 1971, 75, 1221.
15 M. Primet, P. Pichat and M. Mathieu, J. Phys. Chem., 1971, 75, 1216.
16 L. Linsebigler, G. Lu and J. T. Yates, Jr, Chem. Rev., 1995, 95, 735, and
references therein.
17 C. Walling, Acc. Chem. Res., 1975, 8, 125.
18 C. Walling and R. A. Johnson, J. Am. Chem. Soc., 1975, 97, 363.
19 S. Ito, A. Mitarai, K. Hikino, M. Dirama and K. Sasaki, J. Org. Chem.,
1992, 57, 6937.
20 M. P. DeMatteo, J. S. Poole, X. Shi, R. Sachdeva, P. G. Hatcher,
C. M. Hadad and M. S. Platz, J. Am. Chem. Soc., 2005, 127, 7094.
21 D. Vione, C. Minero, V. Maurino, M. E. Parlotti, T. Picanotto and
E. Pellizzetti, Appl. Catal. B: Environ., 2005, 58, 79.
22 G. Albarran and R. H. Schuler, J. Phys. Chem. A, 2005, 109, 93.
23 A. B. J. Parusel, R. Schamschule and G. Kolher, Croat. Chem. Acta,
2000, 73, 359, and references therein.
1014 | Chem. Commun., 2006, 1012–1014
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