Photocatalysis in dimethyl carbonate green solvent: Degradation and partial oxidation of phenanthrene on supported TiO2

Article abstract of DOI:10.1039/c4ra06222a

Dimethyl carbonate (DMC) is here proposed-for the first time-as a green organic solvent for photocatalytic synthesis. In this work, the photocatalytic partial oxidation of phenanthrene in dimethyl carbonate (DMC) by using anatase TiO₂as the photocatalyst is described as paradigmatic example of a green synthetic process starting from polycyclic aromatic hydrocarbons (PAHs). For comparison, the same reaction carried out also in ethanol, 1-propanol or 2-propanol is reported. The use of DMC as the solvent allowed us to achieve 19% and 23% selectivity towards 9-fluorenone and 6H-benzo[c]chromen-6-one, respectively. The proposed approach may represent both a new green synthetic process and an environmentally friendly route to degradation of PAHs. This journal is

Full text of DOI:10.1039/c4ra06222a

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Photocatalysis in dimethyl carbonate green
solvent: degradation and partial oxidation of
Cite this: RSC Adv., 2014, 4, 40859
phenanthrene on supported TiO2†
M. Bellardita,‡a V. Loddo,‡a A. Mele,‡bc W. Panzeri,‡c F. Parrino,‡a I. Pibiri‡d
a
*
and L. Palmisano‡
Dimethyl carbonate (DMC) is here proposed – for the first time – as a green organic solvent for
photocatalytic synthesis. In this work, the photocatalytic partial oxidation of phenanthrene in dimethyl
carbonate (DMC) by using anatase TiO2 as the photocatalyst is described as paradigmatic example of a
green synthetic process starting from polycyclic aromatic hydrocarbons (PAHs). For comparison, the
same reaction carried out also in ethanol, 1-propanol or 2-propanol is reported. The use of DMC as the
solvent allowed us to achieve 19% and 23% selectivity towards 9-fluorenone and 6H-benzo[c]chromen-
6-one, respectively. The proposed approach may represent both a new green synthetic process and an
environmentally friendly route to degradation of PAHs.
Received 25th June 2014
Accepted 18th August 2014
DOI: 10.1039/c4ra06222a
www.rsc.org/advances
Although photocatalytic reactions are usually carried out in
water, which is the green solvent par excellence,15–19 the low
Introduction
solubility of PAHs in water does not allow to use it. PAHs
oxidation is reported in solvents such as acetonitrile,5,6 water–
acetone mixtures,7,8,13 subcritical water,9 and anionic and non-
ionic surfactant solutions.10 Moreover, in some cases different
amounts of TiO2 were mixed with contaminated soil samples to
simulate the remediation of the soil surface.11,12 In recent years
the use of photocatalysis as synthetic route has been proven to
be a viable and sustainable alternative to classical chemical
methods as it offers, under suitable experimental conditions,
the possibility to stop the photoreaction before the total
mineralization of the substrate, obtaining oxygenated products
of industrial interest.20–24
Addressing the selectivity of photocatalytic reactions towards
specic products is a challenging issue. In fact, photocatalysis
involves radical chain reactions which produce oen various
products, although the selective formation of aromatic alde-
hydes from the corresponding alcohols has been reported.25 In
the last case tuning the surface properties of the catalyst
allowed desorption of the obtained oxygenated products,
avoiding their further oxidation and mineralization. A moderate
crystallinity of nanostructured TiO2 photocatalysts was found to
be a key factor to increase the selectivity of the photocatalysts
which, however, presented a low oxidant power.
Heterogeneous photocatalysis in the presence of TiO2 is an
increasingly attractive and competitive method for the oxida-
tion of many organic pollutants under mild conditions.1–4 This
technology has also been successfully used to degrade polycyclic
aromatic hydrocarbons (PAHs).5–12 PAHs are formed during the
incomplete combustion of coal, oil and gas, urban waste, or
other organic substances like tobacco or grilled meat.13 Phen-
anthrene is one of them and it is frequently found in the envi-
ronment14 as for instance in the soot. PAHs present a potential
genotoxicity and carcinogenicity, and cause damages to the
skin, body uids, and immune system in laboratory animals by
mere exposure or contact. The high stability and hydrophobicity
of PAHs leads to their accumulation in the environment and the
risks associated with their presence can be overcome by using
conventional removal techniques. However, these methods are
generally expensive, inefficient and they do not lead to degra-
dation but only to physical separation of the pollutants.
aDipartimento Energia, Ingegneria dell'Informazione e Modelli Matematici (DEIM),
`
Universita degli Studi di Palermo, Viale delle Scienze, Ed. 6, Palermo, Italy. E-mail:
leonardo.palmisano@unipa.it
bDepartment of Chemistry, Materials and Chemical Engineering “Giulio Natta”,
Politecnico di Milano, Piazza L. da Vinci 32, Milano, Italy
cCNR – Istituto di Chimica del Riconoscimento Molecolare, Via L. Mancinelli 7,
Milano, Italy
In this paper the use of dimethyl carbonate (DMC) is
proposed, for the rst time, as a green organic solvent in
heterogeneous photocatalysis using home prepared anatase
TiO2 as the photocatalyst. Phenanthrene photocatalytic oxida-
tion was carried out in a xed bed total recirculating batch
reactor containing Pyrex beads on which a home prepared TiO2
was supported. The partial photo-oxidation of phenanthrene is
dDipartimento di Scienze
e Tecnologie Biologiche, Chimiche e Farmaceutiche
`
(STEBICEF), Universita degli Studi di Palermo, Viale delle Scienze, Ed. 17, Palermo,
Italy
† Electronic supplementary information (ESI) available: Experimental details,
GC-MS and NMR analysis. See DOI: 10.1039/c4ra06222a
‡ All the authors contributed equally to this work.
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a paradigmatic example of photocatalytic synthesis using PAHs
as substrates. DMC is a non-toxic and biodegradable compound
and it can be synthesized without harmful reagents; its degra-
dation does not give rise to toxic compounds, and its reactivity
is tunable. DMC has been recently proposed as a green
solvent,26 but its use under photocatalytic conditions is
unprecedented. We demonstrate that the use of DMC instead of
water dramatically enhances the selectivity of anatase TiO2 for
the partial oxidation of phenanthrene, leading mainly to 9-u-
orenone and 6H-benzo[c]chromen-6-one. Conversely, the same
reaction was carried out also in short chain alcohols (ethanol, 1-
and 2-propanol) but it led to complex reaction mixtures.
Fluorene family compounds are widely applied in many
elds. They are used in applications of thermo and light
sensitizer, liquid crystal chemistry, luminescence chemistry,
polymers, spectrophotometric analysis, molecular chemistry
and organometallic-complexes. Fluorenones are used in the
formation of polyradicals for resins, in manufacturing antima-
laria drugs and other pharmaceuticals and as ligands in the
synthesis of metallocene catalysts. 9-Fluorenone, in particular,
has been investigated as an attractive element of organic solar
cells, and display devices.27 Some derivatives of uorenone have
been also detected in different orchidaceae plants.28 Most of the
Fig. 1 Fixed-bed photoreactor (a), cross-section micrograph (b) and
EDAX mapping (c) of a layer of TiO2 supported on a glass bead.
uorenones are prepared by Friedel–Cras acylation,29 remote 98%) was used as precursor for the catalyst without any further
metalation,30 and oxidation of uorenes31 or uorenols.32 Other purication. TiCl4 was slowly added to distilled water (volume
strategies for the synthesis of uorenones include radical ratios 1 : 10) at room temperature. The hydrolysis reaction was
cyclization,33 coupling reactions of arylpalladium,34 and intra- highly exothermic, produced high quantities of HCl fumes and
molecular dehydro Diels–Alder reactions.35 Chromenone deriv- a clear solution was obtained aer 10 h stirring. The Pyrex beads
atives are high value-added heterocyclic molecules which are were added to the solution and were kept in there for 30
very common in natural products as well as components of minutes. Aer that they were ltered in order to eliminate the
pharmacologically active compounds.36 Various synthetic supernatant liquid and placed in a Pyrex tube (maintained at a
methodologies have been devised for their synthesis.8,37
temperature of 423 K) having a porous frit at the bottom
through which a ow of nitrogen was fed for 1.5 h until the TiCl4
was completely hydrolysed to Ti(OH)4. A thermal treatment at
673 K for three hours in air was then performed and the
Experimental section
The photocatalytic oxidation of phenanthrene (Fluka >97%) was procedure was repeated in order to support two layers on the
carried out in an annular Pyrex home-made xed bed contin- beads. A thin layer of TiO2 as anatase phase was obtained, as
uous photoreactor (Fig. 1a) using DMC (Sigma-Aldrich 99%), conrmed by XRD analysis carried out with a Philips diffrac-
ethanol (Fluka $99.8%), 1-propanol (Sigma-Aldrich $99.5%) or tometer using Cu Ka radiation and a 2q scan rate of 1.2ꢁ minꢀ1
.
2-propanol (Fluka $99.9%) as the solvents with or without
A cross-section micrograph (b) and an EDAX mapping (c) of a
small (3% v/v) amounts of water and TiO2 anatase as the pho- layer of TiO2 supported on a glass bead are shown in Fig. 1. The
tocatalyst. It can be noticed that as far as DMC is concerned last pictures were obtained by using a Philips XL30 ESEM
higher amounts of water could not be used as a separation of scanning electron microscope (SEM), operating at 30 kV on
the aqueous phase occurred. The same quantity of water was specimens upon which a thin layer of gold was deposited. The
used also for the other solvents in order to compare the results. experimental runs were carried out in the following way. A ask
The use of a recirculating system with the immobilized catalyst containing 500 mL of 10 mM phenanthrene solution was used
avoided the need to separate the catalyst from the reaction to feed the photoreactor through a peristaltic pump. The solu-
mixture, and allowed to treat high reaction volumes of effluents tion was saturated with O2 during the photocatalytic runs and
with high concentration of substrate (magnitude order g Lꢀ1).
samples were withdrawn at xed intervals of time. Preliminary
A 1000 W medium pressure Hg lamp (Helios Italquartz, Italy) tests were carried out under the same experimental conditions
was axially positioned inside the reactor which was cooled by to ensure that the covered beads did not release appreciable
water circulating through a Pyrex thimble. The inner part of the amounts of catalyst during the runs.
annulus contained a xed bed of Pyrex beads (diameter 2 mm;
bed height 150 mm) covered by a thin layer of home prepared formed by means of a HPLC Beckman Coulter (System Gold 126
TiO2. Solvent Module and 168 Diode Array Detector), equipped with a
The quantitative determination of phenanthrene was per-
The photocatalytic bed was prepared according to a proce- Phenomenex Kinetex 5 mm C18 100A column (4.6 mm ꢂ 150
dure reported in literature.38 Titanium tetrachloride (Fluka mm) working at room temperature. The eluent consisted of a
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mixture of acetonitrile (VWR chemicals $99.9%) and 1 mM obtain 9-uorenone (0.123 g, 19% yield calculated with respect
triuoroacetic acid (Sigma-Aldrich $98%) aqueous solution to the converted phenanthrene) and 6H-benzo[c]chromen-6-one
(20 : 80 volumetric ratio) and the ow rate was 0.8 mL minꢀ1
.
(0.158 g, 23% yield calculated with respect to the converted
GC-MS analysis were performed in order to identify some phenanthrene), in addition to phenanthrene recovered (0.277 g)
intermediates in the reaction mixture. To this aim, a Thermo and trace amounts of unidentied by-products. Spectral data
Fisher ITQ900 instrument, equipped with a Thermo Scientic are reported within the paper and in the SI section. Some
TGWAXMS A 30 m ꢂ 0.25 mm ꢂ 0.25 mm column was used. A experiments were performed in order to clarify the last step of
temperatuꢀre1 ramp was applied as follows: 150 ꢁC for 4 minutes, the proposed reaction mechanism (see Scheme 1 below) in the
1.5ꢁC min up to 250 ꢁC and 40 minutes at 250 ꢁC. Electrospray following way: 0.05 g of 9-uorenone (Fluka >99%) were dis-
ionization mass spectrometry (ESI-MS) was carried out on a solved in 50 mL DMC and 28.6 mL of H2O2 (Sigma-Aldrich 30%
Bruker Esquire 3K+ instrument equipped with an ion trap w/w). This mixture was poured into a 100 mL round bottom
detector (ITD). Solutions were introduced in the ESI source by ask and reuxed 12 h by bubbling O2 at 50 ꢁC in the dark.
an infusion pump operating with a ow rate of 5 L minꢀ1. GC- Similar tests were carried out under the same experimental
MS analysis of the isolated products were performed on a Shi- conditions but (i) in the presence of 15 mg of the TiO2-covered
madzu GC-MS QP2010 Plus. 1H NMR spectra (250 and 400 MHz) Pyrex beads and (ii) by substituting DMC with the same amount
and 13C NMR spectra (62.5 and 100 MHz) were taken on Bruker of ethanol. In all cases the solvent was removed by freeze-drying
spectrometers. Flash chromatography was performed using aer the reaction and the residue was dissolved with ethyl-
silica gel (200–400 mesh) and mixtures of ethyl acetate (VWR acetate and analyzed by GC-MS.
chemicals) and light petroleum (VWR chemicals, fraction
boiling in the range 40–60 ꢁC) in various ratios, and allowed to
Results and discussions
The time course for the photo-oxidation of phenanthrene in the
presence of ethanol and ethanol–H2O (3% v/v) mixture as the
solvents is illustrated in Fig. 2. The reaction rate was higher in
the presence of water, giving rise to an almost complete
degradation of phenanthrene aer about 50 h of irradiation.
Possibly, in this case the presence of water enhanced OH radi-
cals production and/or the recovery of surface active sites of the
photocatalyst which in turn contributed to phenanthrene
oxidation. In fact, the water molecules in the reacting mixture
may be oxidized by the photogenerated holes (h+) according to
the following equation:39
H2O + h+ / cOH + H+
(1)
In the presence of 1- or 2-propanol the phenanthrene oxidation
rate was lower than in the case of ethanol as only 60% conversion
was achieved aer 90 h irradiation. Nevertheless the co-presence of
Fig. 2 Phenanthrene concentration versus reaction time for repre-
Scheme 1 Proposed reaction pathways to 9-fluorenone and 6H- sentative runs carried out in the presence of ethanol (empty circles)
benzo[c]chromen-6-one.
and ethanol–H2O (3% v/v) mixture (full circles) as the solvents.
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water did not inuence the photoreaction rate in 1- or 2-propanol. phenanthrene. Among the many other products that were
The different behaviour of ethanol with respect to 1- and 2-propanol identied by GC-analysis, we report 9-hydroxyphenanthrene;
can be tentatively attributed to the chain length of propanol mole- 9,10-dihydroxyphenanthrene; 9,10-phenanthrenedione; 2,20-
cules which could compete more strongly with water for the diphenic acid; 1,2-benzenedicarboxylic acid monobutylester;
adsorption on the surface sites and/or to a different OH 1,2-benzenedicarboxylic acid bis(2-methylpropyl)ester; 1,2-ben-
radicals scavenger ability of 1- and 2-propanol with respect to zenedicarboxylic acid diisooctyl ester; didodecylphthalate;
ethanol. The oxidation by-products were detected and identied by bis(2-ethylhexyl)phthalate; 2-(1-oxopropyl)benzoic acid; farne-
means of both electrospray ionization mass spectrometry (ESI-MS) sol; phenanthrene-9-carboxaldehyde, but they were found only
and GC-MS analysis. When ethanol was used as the in very small concentrations or in traces and a detailed analyt-
solvent, several products were found aer ca. 80% conversion of ical quantitative investigation of all of these very minor prod-
phenanthrene: 2-methyl-9H-uorene; 9-methylene-uorene; ethoxy- ucts is out of the scope of this work. DMC did not appreciably
2,20-diphenic acid diethyl ester; ethyl-2,20-diphenic acid diethyl react with phenanthrene as no methylated products were
ester; phthalic acid ethyl ester; 2-phenylbenzoic acid ethyl detected by GC-MS analysis. Notably some reactivity of DMC has
ester; 2,20-diphenic acid diethyl ester; 2,20-diphenic acid ethyl ester; been reported for the catalytic transesterication of alcohols
9H-uorene; 3-hydroxyphenanthrene; 9,10-dihydroxyphenan- into methyl carbonates in the presence of TiO2 nanobers,40
threne; 9,10-phenanthrenedione; 2,20-diphenic acid; 9-uorenone differently from the photocatalytic process here reported.
and 6H-benzo[c]chromen-6-one. Most of the products found when
In our system 6H-benzo[c]chromen-6-one was rst identied
1- or 2-propanol were used as the solvents are the same of those by electrospray ionization mass spectrometry (ESI-MS) by direct
observed in the presence of ethanol except those resulting from the infusion of the reaction mixture in the mass spectrometer
insertion of the ethyl or ethanoate groups in the phenanthrene source as an intense peak at m/z 197, corresponding to
molecule which are replaced by compounds in which the propyl or [C13H9O2]+. Isolation and collision induced decomposition
the propylate groups are present. The presence of some of these (CID) of m/z 197 gave the ion at m/z 153 as the major fragment
compounds in comparable amounts, as evidenced by parallel GC- ions, consistent with neutral loss of 44 Da (see ESI, Fig. S1†).
MS, HPLC and TLC analysis, indicates that the photo-reaction is Identical CID pattern was found on a standard sample of 6H-
not selective and the alcohols used as the solvents work also as benzo[c]chromen-6-one synthesized on purpose, thus providing
reagents.
Fig. 3 shows the degradation of phenanthrene in DMC with full characterization, the two main products (9-uorenone and
and without water as the co-solvent. 6H-benzo[c]chromen-6-one) were isolated by preparative
a ngerprint conrmation of the molecular structure. For the
Phenanthrene conversion occurred only in the presence of column chromatography with silica ash and mixtures of
water, as the reaction rate was negligible in pure DMC. The petroleum and ethyl acetate in various ratios, identied by GC-
photo-oxidation rate was slower than when ethanol was used as MS (Fig. 4) and extensive NMR analyses (see ESI, Fig. S2–S8†)
the solvent. Nevertheless, although phenanthrene reacted faster and compared with standards. Herein the spectroscopic data of
in the alcohols than in DMC, in the latter case the reaction gave the two main isolated products are reported.
two main products only, thus showing a greatly enhanced
9-Fluorenone, yellow solid, 1H NMR (250 MHz, CDCl3): d 7.67
selectivity compared to the alcohols. For instance, a run lasting (d, J ¼ 7.3 Hz, 2H), 7.45–7.55 (m, 4H), 7.26–7.33 (m, 2H). 13C NMR
250 h (ca. 60% conversion of phenanthrene) produced 9-uo- (62.5 MHz, CDCl3): d 193.8, 144.4, 134.6, 134.2, 129.1, 124.3,
renone and 6H-benzo[c]chromen-6-one with 19% and 23% w/w 120.3. GC-MS (m/z, relative intensity): 180 (M+, 100), 152 (50), 126
yield, respectively, calculated with respect to the converted
Fig. 3 Phenanthrene concentration versus reaction time for repre-
sentative runs carried out in the presence of DMC (empty squares) and Fig. 4 GC-MS analyses of the isolated 9-fluorenone (a) and 6H-benzo
DMC–H2O (3% v/v) mixture (full squares) as the solvents.
[c]chromen-6-one (b).
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(10), 76 (24), 63 (13). 6H-Benzo[c]chromen-6-one, white solid, 1H species, likely formed by reaction between DMC and photo-
NMR (400 MHz, CDCl3): d 8.43 (dd, J ¼ 8.0 and 1.3 Hz, 1H), 8.15 generated H2O2, the second one in the presence of TiO2 as the
(br d, J ¼ 8.1 Hz, 1H), 8.09 (dd, J ¼ 7.9 and 1.5 Hz, 1H), 7.86 (ddd, catalyst and H2O2.
J ¼ 7.5, 7.5 and 1.5 Hz, 1H), 7.62 (ddd, J ¼ 7.6, 7.6 and 1.4 Hz, 1H),
DMC can play an active role as precursor for the in situ
7.52 (ddd, J ¼ 7.7, 7.7 and 1.5 Hz, 1H), 7.40 (dd, J ¼ 7.9 and 1.2 formation of organic peroxo-species which in turn induce
Hz, 1H), 7.37 (ddd, J ¼ 7.1, 7.1 and 1.4 Hz, 1H). 13C NMR (100 chromenone formation. Formation of monoperoxy carbonic
MHz, CDCl3): d 161.2, 151.3, 134.8, 134.8 (q), 130.6, 130.4, 128.9, acid monomethyl ester from DMC oxidation through H2O2 was
124.5, 122.8, 121.7, 121.3, 118.1, 117.8. GC-MS (m/z, relative reported in literature44 only in the presence of an enzymatic
intensity): 196 (M+, 100), 168 (60), 139 (65), 113 (13), 70 (13).
The reaction mechanism hypothesized involves the initial
catalyst.
The presence of other by-products in trace amounts evi-
hydroxylation of phenanthrene by radicals produced through denced by GC-MS analysis, can be explained by taking into
TiO2 photocatalysis. In fact, the charge carriers photogenerated account further reaction of some of the intermediates illus-
upon TiO2 excitation can give rise both to direct oxidation of the trated in the Scheme 1. For instance 2,2-diphenic acid could be
substrate and to indirect oxidation by highly oxidizing species produced by OH radicals attack to the diradical, whereas 3-
produced through water oxidation or oxygen reduction. It is hydroxyphenanthrene could be likely produced by a different
worth noting that the rst attack to phenanthrene occurs at regiodirected hydroxylation in position C3 of phenanthrene.
position 9 and 10 according to the Dewar molecular-orbital The ethyl esters produced when ethanol–H2O (3% v/v) was used
theory.41 The hydroxylated intermediates in turn undergo as the solvent, are likely due to the attack of EtO radicals to the
different pathways, strongly depending on the different solvent various intermediates. Finally, it cannot be excluded a parallel
present. In order to justify the formation of the two main prod- pathway affording CO2 and water aer various oxidant attacks
ucts isolated when the photocatalytic reaction was performed in to adsorbed intermediates species. In summary, the unprece-
DMC–H2O (3% v/v), the reaction mechanism shown in Scheme 1 dented use of DMC as reaction medium for TiO2 photocatalyzed
was hypothesized. The suggested path involves the initial double partial oxidation of phenanthrene showed the possibility to
hydroxylation at position 9 and 10 of phenanthrene, affording achieve both good conversion yield and selectivity.
9,10-dihydroxyphenanthrene which was detected by means of
GC-MS analysis when both DMC–H2O (3% v/v) and ethanol–H2O
(3% v/v) were used as the solvents. This intermediate undergoes
Conclusions
oxidation giving the corresponding diketone which was detected
in the aforementioned analysis as well. The diketone gives rise,
through homolytic rupture of the C9–C10 bond, to a not isolable
double radical intermediate, which in turn, aer decarbonylation
The photocatalytic partial oxidation of phenanthrene in
dimethyl carbonate was carried out in a xed bed total recir-
culating batch reactor containing Pyrex beads covered with
TiO2 anatase. This system avoids the separation of the catalyst
and ring closure, affords 9-uorenone.
from the reaction mixture, and allows to treat high reaction
This latter compound is converted by a Baeyer–Villiger
volumes of effluents with high concentration of substrate. For
reaction to 6H-benzo[c]chromen-6-one, involving hydroperoxide
comparison, the same reaction was also performed in ethanol,
species which are likely present in the reaction medium
1-propanol or 2-propanol. The use of dimethyl carbonate as
(possibly deriving from water which interacts under irradiation
the solvent allowed to achieve 19% and 23% of selectivity
towards 9-uorenone and 6H-benzo[c]chromen-6-one, respec-
tively. On the contrary aliphatic alcohols took part in the
in the presence of O2 on the TiO2 surface). The last step of the
proposed mechanism consists in a dark reaction between per-
oxo-species and 9-uorenone. Some qualitative tests were per-
reaction producing a large variety of products. These ndings
formed in order to understand the inuence of TiO2 in the
open the route to a sustainable process targeted to both the
degradation of polycyclic aromatic hydrocarbons as disposal
strategy and to the synthesis of potentially interesting
products.
Baeyer–Villiger step (details are reported in the Experimental
section). A mixture of DMC, H2O2 and 9-uorenone was stirred
by bubbling O2 overnight at 50 ꢁC in the presence or in the
absence of the TiO2-covered Pyrex beads. Surprisingly 6H-benzo
[c]chromen-6-one was detected by GC-MS analyses, both in the
presence and in the absence of TiO2. This nding indicates that Acknowledgements
the Baeyer–Villiger oxidation occurs in the presence of DMC
NMR experimental data were provided by Centro Grandi
and H2O2 also without catalyst.42
`
Apparecchiature – UniNetLab – Universita di Palermo funded by
On the other hand, 6H-benzo[c]chromen-6-one was found
only in the presence of TiO2 when ethanol was used as the
solvent, whereas it was absent without catalyst. This last result
indicates that Baeyer–Villiger oxidation could be catalyzed with
low yields in the presence of TiO2, analogously to what reported
in the literature.43
Therefore the production of 6H-benzo[c]chromen-6-one in
our reacting system can occur by the Baeyer–Villiger reaction
through two pathways: the rst one through organic peroxo-
P.O.R. Sicilia 2000–2006, Misura 3.15 Quota Regionale. M.B.,
V.L., F.P. and L.P. wish to thank MIUR (Rome) for nancial
support (Project: PON02_00153_2849085).
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40864 | RSC Adv., 2014, 4, 40859–40864
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