Oxidation of dibenzothiophene by hydrogen peroxide or monopersulfate and metal-sulfophthalocyanine catalysts: An easy access to biphenylsultone or 2-(2′-hydroxybiphenyl)sulfonate under mild conditions

Article abstract of DOI:10.1039/b212152b

A catalytic system consisting of metal-sulfophthalocyanines (MPcS) and monopersulfate or hydrogen peroxide as oxidants was effective in the dibenzothiophene oxidative desulfurization with various yields and selectivities. Oxidations were conducted at room temperature in acetonitrile-water mixed solvent. The dibenzothiophene oxidation involved the step by step formation of dibenzothiophene dioxide and biphenylsultone (dibenzo-1,2-oxathiine 2,2-dioxide), followed by hydrolysis to 2(2′-hydroxybiphenyl)sulfonate and finally catalytic desulfurization to 2-hydroxybiphenyl (2-phenylphenol) and sulfuric acid; all the intermediate compounds were identified. Moreover, catalytic over-oxidation of 2-hydroxybiphenyl, with ring fission and formation of various oxidation products, among them carbon dioxide, oxalic and benzoic acid, was also observed. Among the various MPcS catalysts examined (M = Fe, Co and Ru), the ruthenium derivative exhibited the best performances with persulfate and iron derivative with hydrogen peroxide; in both cases the slow step of the process consisted in the oxidation of dibenzothiophene dioxide to biphenylsultone.

Full text of DOI:10.1039/b212152b

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Oxidation of dibenzothiophene by hydrogen peroxide or
monopersulfate and metal–sulfophthalocyanine catalysts: an easy
access to biphenylsultone or 2-(20-hydroxybiphenyl)sulfonate under
mild conditions
Nicola d’Alessandro,a Lucia Tonucci,a Monica Bonetti,a Milena Di Deo,a Mario Bressan*a and
Antonino Morvillob
a
`
Dipartimento di Scienze, Universita ‘‘G.d’Annunzio’’ di Chieti-Pescara, Viale Pindaro 42,
65127, Pescara, Italy. E-mail: bressan@sci.unich.it; Fax: +39 085 453 7545
Tel: +39 085 453 7548
`
Dipartimento di Chimica Inorganica, Universita di Padova, Via Marzolo 1, 35131,
Padova, Italy
b
Received (in Cambridge, UK) 6th December 2002, Accepted 4th April 2003
First published as an Advance Article on the web 14th May 2003
A catalytic system consisting of metal–sulfophthalocyanines (MPcS) and monopersulfate or hydrogen
peroxide as oxidants was effective in the dibenzothiophene oxidative desulfurization with various yields and
selectivities. Oxidations were conducted at room temperature in acetonitrile–water mixed solvent. The
dibenzothiophene oxidation involved the step by step formation of dibenzothiophene dioxide and
biphenylsultone (dibenzo-1,2-oxathiine 2,2-dioxide), followed by hydrolysis to 2(20-hydroxybiphenyl)sulfonate
and finally catalytic desulfurization to 2-hydroxybiphenyl (2-phenylphenol) and sulfuric acid; all the
intermediate compounds were identified. Moreover, catalytic over-oxidation of 2-hydroxybiphenyl, with
ring fission and formation of various oxidation products, among them carbon dioxide, oxalic and benzoic acid,
was also observed. Among the various MPcS catalysts examined (M ¼ Fe, Co and Ru), the ruthenium
derivative exhibited the best performances with persulfate and iron derivative with hydrogen peroxide; in both
cases the slow step of the process consisted in the oxidation of dibenzothiophene dioxide to biphenylsultone.
ous and/or sulfuric acid. The physiological role of enzyme
Introduction
systems is to obtain sulfur for growth; the sulfur product being
incorporated into cellular biomass via sulfur assimilation path-
ways. The oxidation pathway stops at HBP, which is released
into the medium; therefore, the desulfurization of fossil fuels
does not decrease the carbon content (‘‘fuel value’’). However,
several engineering concerns are not addressed fully and a
large aqueous phase is required. The other known technology
is based on the chemical oxidation (ODS) of PASHs (UniPure
Corp.). The UniPure process relies on the facile conversion of
thiophenic compounds, among them DBT, to the correspond-
ing sulfones, by using an oxidant carried in an aqueous phase
along with a dissolved catalyst.4 The oxidation is done at near
atmospheric pressure and mild temperatures, around 100 ꢀC,
and consumes a relatively insignificant amount of oxidant.
Conversion to sulfones is completely accomplished with reac-
tor residence times of less than five minutes. However, separa-
tion of sulfones is somewhat complicated, since they dissolve at
least in part in the oil phase, which must therefore be sent
through an extraction step. The product diesel oil then con-
tains 5 ppm or less of sulfur.
The scientific challenge is to devise a one-step biomimetic
process operating at room temperature in a biphasic system
oil/water and using green co-oxidants, such as oxygen or
hydrogen peroxide, which may lead to formation of sulfuric
acid, without degrading the hydrocarbon part. The catalyst
and the oxidant should be water-soluble, to avoid pollution
of the organic phase and allow the regeneration of the catalyst,
as well as the products of DBT deep oxidation (sulfonic
and/or sulfuric acids). Unfortunately, whereas oxidation of
Dibenzothiophene (DBT) and its alkylated analogues are
among the most abundant polycyclic aromatic sulfur hydro-
carbons (PASH) in crude oils and are usually selected for
model studies of desulfurization, both reductive and oxida-
tive.1 In December 2000, the US EPA issued revised require-
ments for sulfur in on-road diesel fuel, with phase-in
beginning in mid-2006. This directive lowers the specification
from 500 ppm to 15 ppm sulfur.2 On January 30th, 2003, the
European Parliament passed a new resolution on diesel and
gasoline quality (P5_TA-PROV(2003)0029). In this regulation
the ‘‘sulfur-free’’ fuels are defined as having no more than 10
ppm sulfur content. Both ‘‘sulfur-free’’ gasoline and diesel
must be available throughout the EU from 2005, and become
mandatory in 2009.
Research has particularly focused on innovative routes for
desulfurization approaching cleaner technology and compli-
ance with increasingly stringent regulatory laws. In this context
the major issue concerning PASH removal is their recalcitrance
to hydrodesulfurization, which makes removal by these con-
ventional technologies unreliable. Two main technologies are
close to being introduced in the production stream for diesel,
down to the conventional hydrodesulfurization (HDS), both
exploiting the relatively facile oxidation of DBT. The first
is based on the biological oxidation (BDS) of PASHs (by
Enchira Biotech. Corp. and EniTecnologie).3 It exploits the
fact that some classes of bioengineered microorganisms (i.e.
Rhodococcus sp. IGTS8) are able to promote the aerobic trans-
formation of DBT into 2-hydroxybiphenyl (HBP) and sulfur-
DOI: 10.1039/b212152b
New J. Chem., 2003, 27, 989–993
989
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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Organic analyses were performed on a HP 6890 GLC instru-
ment equipped with FID, using a 30 m HP-5 capillary column
(0.32 mm id; 0.25 film thick) with the injection port kept at
250 ꢀC (carrier gas: He) on aliquots withdrawn with a micro-
syringe from the aqueous reaction mixtures either as such or
diluted 1 : 10 with acetone. The identity of each product was
confirmed by comparison of the fragmentation pattern in the
mass spectra obtained with a MD 800 Fisons mass spectro-
meter operating in the electron ionization mode at 70 eV. To
analyze the carboxylic acids as their isobutyl esters, the aqu-
eous reaction mixtures were first evaporated under vacuum
to small volumes, then refluxed for 3 h in the presence of a
Fig. 1 MPcS.
20 : 1 excess 2-methyl-1-propanol and some drops of HClconc .
Carbon dioxide evolved during the reactions was captured by
an aqueous solution of freshly calibrated Ba(OH)2 0.0125 M
and aliquots from the filtered solution were back-titrated with
HCl 0.1 M. Sulfate ions, produced during the reactions, were
analyzed by ionic chromatography with a DIONEX DX
100 instrument, equipped with IonPacAS4A-SC Analytical
Column (4 ꢂ 250 mm).
thioethers, among them DBT, to sulfones is a relatively facile
process, sulfones can be further transformed into sulfonic acids
only by strong oxidants, such as nitric acid, and oxidative
desulfurization is an even rarer event.5 PASHs are indeed
among the most refractory compounds at contaminated sites6
and the conventional activated sludge process does not
effectively degrade these toxic compounds;7 simple biological
systems, such as hemoglobin, horseradish peroxidase and
heme–bovine serum albumin, promote the oxidation of DBT
to dibenzothiophene monoxide (DBTO) only.8 A number of
effective combinations of catalysts and oxidants have been
used to achieve the oxidation of DBT, but most of them how-
ever only yield dibenzothiophene dioxide (DBTO2).9–11 Deep
oxidations of DBT are reported to occur upon sonication,
yielding desulfurized products, such as hydroxy- and dihy-
droxy-dibenzothiophenes,12 or upon exposure to sunlight, to
give acidic photoproducts, such as 2-sulfobenzoic acid.13
We present here a series of metal–sulfophthalocyanines
(Fig. 1) and other simple ruthenium complexes, among them
1
The reactions were also followed by H NMR on a Bruker
Avance 300 MHz spectrometer equipped with a BBO 5 mm
probe, by adding a small amount of D2O to the reaction mix-
tures; water suppression was carried out by a presaturation
sequence using a composite pulse (zgcppr Bruker sequence).
A co-axial capillary tube containing 30 mM water (D2O) solu-
tion of 3-(trimethylsilyl)propionic acid-2,2,3,3-d4 sodium salt
(TSP) was used as reference. The identity of each product
and their quantitation were confirmed by comparison of the
position and intensity of suitable signals after adding measured
amounts of the pure compounds to the reaction mixtures.
Average reproducibility of quantitative measurements was
within 5%.
UV–visible spectra were measured out on a HP 8452A
Diode Array, with a 200–820 nm spectral window.
a
Keggin-type heteropolyoxotungstate, previously used
successfully for the deep oxidation of chlorophenols,14 chloro-
olefins15 and of cyclohexane to adipic acid,16 that catalyzed
also the oxidation of DBT and DBTO2 into biphenylsultone
(DBTO3) in near quantitative yields using both monoper-
sulfate and hydrogen peroxide as the oxidizing sources, in very
mild conditions and in an aqueous–acetonitrile homogeneous
phase; DBTO3 easily undergoes hydrolysis into 2-(20-hydroxy-
biphenyl)sulfonate (HBPS), which eventually desulfurizes to
HBP and sulfuric acid. To the best of our knowledge, there
is no previous literature report on oxidation of sulfones to
sultones by chemical means.
ESI-MS (electrospray ionization MS) measurements were
performed on a LCQ Advantage Thermofinnigan apparatus.
Injections of the samples were done directly into the ESI detec-
tor at the flow rate of 6 ml minꢃ1
.
The ionization voltage was set at +4500 V; capillary tem-
perature was set at 180 C; capillary voltage and tube lens offset
were set at ꢃ10 V and ꢃ50 V respectively. Instrument control
and data acquisition were performed with a Dell Optiplex
GX400 computer using XCALIBUR 1.2 Thermofinnigan
software. The Mass Spectrometer was calibrated with reserpine
obtained from Thermofinnigan and the ratio signal : noise was
set 10 : 1.
Experimental
Results and discussion
Materials: RuPcS was prepared by template synthesis starting
from RuCl3ꢁ3H2O, 4-sulfophthalic acid trisodium salt and
urea.14 Cobalt–sulfophthalocyanine (CoPcS),17 cis-[RuCl2{(CH3)2-
SO}4] (RuDMS),18 K5[Ru(H2O)PW11O39] (RuPW)19 and
biphenylsultone (DBTO3)20 were prepared by published proce-
dures. FePcS, the organic substrates and chromatographic and
spectroscopic standards were grade reagents from Aldrich.
A procedure for the catalytic oxidations is as follows. A
water–acetonitrile (4 : 6) solution (8.5 ml) containing the sub-
strates, 5 mM, and the metal catalysts, 1 mM, was stirred
magnetically in a vial together with commercial Oxone1 (cor-
responding to 0.05 M concentration of active oxygen as
KHSO5) or with hydrogen peroxide (0.05–0.5 M), as deter-
mined by iodometric titration. The reactions were carried out
at 20 ꢀC and were not affected by the presence of air. The pro-
gress of reaction was monitored by NMR and/or GC-MS. The
products were identified by comparing their spectral data with
those reported in the literature or of authentic compounds pur-
posely prepared; conversion, yields and rates were reproduci-
ble to within 10–15%.
Our investigations on the oxidation of DBT and its oxidation
products, i.e. DBTO2 , DBTO3 , HBPS and HBP were carried
out at ambient temperature and an appropriate catalyst was
always necessary in order to effectively promote the reactions.
Conversions and product distributions for the examined cata-
lysts, i.e. MPcS (M ¼ Fe, Co, Ru), RuDMS and RuPW, did
vary on a large scale and are shown in Tables 1 and 2, respec-
tively. Initial work on the optimization of reaction conditions
led to a standard protocol that was used throughout this study.
This was as follows: 0.001 M catalyst and 0.005–0.01 M sub-
strate in the acetonitrile–H2O solution ([H2O] ¼ 40 vol%) in
the presence of 0.05 M oxone or 0.05–0.5 M H2O2 ; 20 ꢀC tem-
perature, and 24 h reaction time. At [H2O] around 40 vol% in
the acetonitrile–H2O mixture, the reaction system was homo-
geneous; at higher contents of acetonitrile the catalyst (and
also the oxone oxidant) precipitated, whereas at lower contents
it was the substrate (or the products) that separated from the
solution. The chosen concentration range of the substrates
(5–10 mM, corresponding to a sulfur content of 160–320
990
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Table 1 Catalytic oxidation of dibenzothiophene and dibenzothio-
Table 2 Catalytic oxidation of dibenzothiophene and dibenzothio-
phene dioxide by monopersulfatea
phene dioxide by hydrogen peroxidea
Dibenzothiophene
Dibenzothiophene
Composition of the reaction
Composition of the reaction
mixture/mol%
mixtures/mol%
Time/
Catalyst
None
h
DBT
DBTO2
DBTO3
HBP
Otherb
Catalyst
Time/h
DBT
DBTO2
DBTO3
Otherb
1
47
2
53
98
—
—
—
—
—
—
—
—
—
—
—
—
RuPcSd
FePcSd
24
3
90
75
15
2
10
13
24
63
62
35
—
—
—
1
—
12
61
34
36
63
6
24
3
—
—
100
100
24
3
CoPcS;
RuDMS;
RuPW
FePcS
6
24eh
—
—
2
2
24
3
—
3
100
95
92
88
70
48
—
1
—
—
—
4
—
1
Dibenzothiophene dioxide
FePcS
RuPcS
24
1
—
—
—
—
2
6
Composition of the reaction
mixture/mol%
1
7
6
26c
2
—
5
23
50
2
Catalyst
FePcS
Time/h
DBTO2
DBTO3
Otherb
Dibenzothiophene dioxide
3
6
24eh
76
70
43
2
2
1
22
28
56
Composition of the reaction
mixtures/mol%
Time/
a
Otherb
Catalyst, 1 mM; DBT or DBTO2 , 10 mM; H2O2 , 1 M; in H2O–CH3CN 4 : 6
(vol) 20 ꢀC. b Mostly HBPS, together with minor amounts of CO2 and products
from the oxidative fission of the aromatic moieties. c No reaction in the absence
Catalyst
CoPcS
FePcS
h
DBTO2
DBTO3
HBP
3
91
87
—
—
3
8
1
10
11
15
32
40
50
—
—
d
of catalyst or in the presence of CoPcS 1 mM (24 h). In the same conditions,
24
3
3
e
except that DBT, 5 mM, and H2O2 , 0.1 M. Reaction still running. CO2
f
86
81
—
—
3
g
evolved: 3% (mol per mol of initial DBT). No reaction in the absence of cat-
24
3
4
alyst or in the presence of CoPcS, RuPcS, RuPW, RuDMS or Fe(III) acetate, 1
h
mM or of FePcS, 0.1 mM (24 h). Sulfate ions produced: 27% (mol per mol of
RuPcS
65
—
—
1
6
55
40
5
initial DBTO2).
24
3
9
RuPW
100
98
—
2
—
—
24
a
Most of the metal complexes investigated were able to
further oxidize DBTO2 (Table 1). The reactions proceeded
slowly, the most active catalyst being RuPcS, for which an
initial oxidation rate of ca. 0.002 mM minꢃ1 was estimated,
i.e. almost three orders of magnitude smaller than the rate of
the RuPcS-catalyzed transformation of DBT to DBTO2 (ca.
1 mM minꢃ1). Although conversions at 24 h never exceeded
60%, the result was nevertheless interesting since under the
same reaction conditions, ‘‘naked’’ ruthenium ions, arising
upon the fast oxidation of the RuDMS catalyst, were found
completely inactive for the further oxidation of DBTO2 .
A series of oxidation products were detected, including
DBTO3 , HBPS and HBP, leading us to study in detail the
reactions of each one of the above substrates. DBTO3 was
not always measurable by GC, since in aqueous media it
Catalyst, 1 mM; DBT or DBTO2 , 5 mM; KHSO5 , 0.1 M; in H2O : CH3CN 4 :
b
6 (vol) 20 ꢀC. Mostly HBPS, together with minor amounts of CO2 and pro-
ducts from the oxidative fission of the aromatic moieties (oxalic acid, benzoic
c
d
acid). CO2 evolved: 11% (mol per mol of initial DBT). No reaction in the
absence of catalyst or in the presence of RuDMS, 1 mM (24 h).
ppm) is the normal concentration range of the effluent coming
from conventional HDS treatment of fuels. Increasing the oxi-
dant concentration led to significant changes in rates only in
the case of the H2O2 oxidations. The increasing of the tempera-
ture (up to 60 ꢀC) had little effect on the reaction rates and only
led to an enhanced dismutation of the H2O2 oxidant. Lowering
the catalyst concentration (to 0.1 mM) resulted in an early
deactivation of the catalysts.
Biphasic oxidation of DBT (and DBTO2) in water–dichlor-
omethane (1 : 1) was completely unsuccessful, apart for the
RuPcS catalyst, which exhibited a moderate activity for the
oxidation of DBTO2 by monopersulfate, with a 6% conversion
after 24 h. Addition of conventional quaternary ammonium
salt phase-transfer agents, such as trimethylcetyl ammonium
hydrosulfate, did not improve yields and conversion rates;
on the contrary, the catalytic systems led the alkyl chain of
the phase transfer agents to a fast and complete oxidative
degradation, thus making the experiments useless.
Oxidation with persulfate
The DBT persulfate-oxidation by metal catalysis involved
initial fast formation of DBTO2 , probably via DBTO, however
it was never detected in the reaction mixtures with only negli-
gible over-consumption of monopersulfate, due to the metal-
promoted dismutation of the oxidant to dioxygen and sulfate.
In the absence of added catalyst, DBT was found to undergo
oxidation to DBTO2 with monopersulfate, although slow, with
100% conversion after ca. 8 h reaction at room temperature
(see Fig. 2).
Fig. 2 Time course for the oxidation DBT, 5 mM, by KHSO5 , 0.05
M, in water–acetonitrile 40 : 60; 20 ꢀC. Uncatalyzed reaction (---):
DBT (filled triangles) DBTO2 (filled circles). Reaction catalyzed by
RuPcS, 1 mM (—): DBT (triangles) DBTO2 (circles) HBP (crosses).
New J. Chem., 2003, 27, 989–993
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The stability of DBTO3 was also tested in acidic media
(‘‘pH’’ ca. 1, H2SO4 in aqueous acetonitrile), since under the
reaction conditions oxone behaved as a strong Brønsted acid
(‘‘pH’’ ca. 2): fast hydrolysis to HBPS was again observed,
although not complete, with an estimated equilibrium constant
(UV spectra) of ca. 1.
Aqueous acetonitrile solutions of DBTO3 and also those
deriving from the complete alkaline hydrolysis of DBTO3 to
HBPS (after neutralization with H2SO4) were also tested for
the catalytic oxidation by the MPcS complexes (M ¼ Fe,
Ru) at the same concentrations used in all the experiments:
conversions up to 60% were observed after short times (1 h
or less), with no product however detected by GC, apart small
amounts of HBP, clearly indicating desulfurization. Further-
more, we noticed that HBP was quite unstable in the oxidizing
media, by undergoing 100% oxidative degradation within very
short times ( < 1 h). Among the oxidation products, we were
able to identify only CO2 (11% mol per mol of initial sub-
strate), oxalic (8%) and benzoic acid (4%), the latter two quan-
tified as their isobutyl esters. The nature of the identified
products clearly pointed to an effective oxidative fission of
the aromatic moieties, in agreement with the previously
reported general behavior of the persulfate/MPcS systems
towards phenolic substrates (M ¼ Fe,22 Ru14), which typically
underwent extensive oxidative fragmentation.
Fig. 3 UV spectra of DBTO3in acetonitrile–water 6 : 4. As such, 0.5
mM at ‘‘pH’’ 6.4 (—) after treatment at 80 ꢀC for 4 h in the presence of
KOH 2.5 M; spectra were run at 2.5 mM concentration and at ‘‘pH 8’’
(---); 20 ꢀC.
underwent more or less complete hydrolysis, depending upon
pH and temperature. Hydrolysis was favored by alkaline
media and indeed we proved that KOH water–acetonitrile
solutions (0.25 M) led to 100% conversion of DBTO3 to the
corresponding sulfonate HBPS in few hours (at 80 ꢀC). HBPS
cannot be detected via GC, but dramatic changes were
observed in the 1H and 13C NMR, UV and ESI spectra of
the reaction mixtures. In particular, UV spectra exhibited the
progressive disappearance of the distinctive absorptions of
DBTO3 at 265 and 290 nm, which were replaced by a new
spectral pattern with a maximum at 275 nm, attributed to
HBPS and clearly reminiscent of the reported UV spectra of
2-hydroxybiphenylsulfinate.21 (Fig. 3)
Oxidation with hydrogen peroxide
In contrast to persulfate, DBT was not affected (24 h) by H2O2
in the absence of catalysts. All the MPcS-catalyzed reactions
with H2O2 suffered from heavy over-consumption of the oxi-
dant, due to the dismutation of H2O2 to water and oxygen,
and only with FePcS were acceptable results obtained. Test
experiments showed indeed that the MPcS catalysts (1 mM
in acetonitrile–water) behaved very differently as dismutating
agents of H2O2 (1 M): whereas RuPcS promoted almost com-
plete dismutation in less than 1 h, FePcS and CoPcS acted
much slower, by at least one order of magnitude (specific runs,
conducted in the presence of tetrafluoroboric acid at ‘‘pH’’ ca.
2, where dismutation was expected to slow down, did not show
any significant improvement).
Indeed, ESI-MS measurements (negative pattern) of the
final reaction mixtures showed two intense peaks, at m/z
249, corresponding to the monopotassium cluster of HBPS,
and at m/z 169 (ꢃ80), corresponding to the loss of SO3 ,
together with various other potassium cluster ions (Fig. 4).
The various substrates examined, i.e. DBT, DBTO2 ,
DBTO3 and HBP, were oxidized by the FePcS/H2O2 system
at very different rates (Fig. 5): the DTBO2 ! DBTO3 oxidation
Fig. 4 ESI-MS spectrum of HBPS.
992
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Fig. 6 Proposed reaction pathway for the oxidation of DBT.
HBPS, insoluble in organic media. Effective biphasic oxida-
tion, which could allow easier product separation, was not
achieved in the present work: novel phase transfer agents,
more resistant to the oxidizing media could make viable this
crucial step.
Fig. 5 Time course of the conversion of various substrates (10 mM)
by oxidation with hydrogen peroxide (1 M) in the presence of FePcS
(1 mM) catalyst; water–acetonitrile 40 : 60; 20 ꢀC. DBT (circles)
DBTO2 (squares) DBTO3 (triangles) HBP (crosses).
Acknowledgements
We thank Mr A. Ravazzolo, Istituto CNR Scienze Molecolari,
Sezione di Padova, for technical assistance.
(t1/2 , ca. 20 h) was the rate limiting step of the entire process,
with the oxidation of both DBT and HBP being much faster
(t1/2 , less than 1 h) and that of DBTO3 somewhere in between
(t1/2 , ca. three times faster than the latter). The mechanism of
the H2O2 oxidations was closely related to that of the reactions
conducted with oxone. For the H2O2-oxidations, also sulfate
ions, produced by the probable desulfurization of the sub-
strate, could be measured, which accounted for 27% (mol
per mol of substrate) it was however uncertain whether all
the sulfate came from the desulfurization of the substrate, since
blank runs conducted on FePcS/H2O2 system in the absence of
added substrates showed significant production of sulfate
upon direct desulfurization of the PcS ring.
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Conclusion
A survey of the recent literature showed that liquid-phase oxi-
dations of DBT by chemical means only led to DBTO2 ,
whereas enzymatic catalysts were able to effectively desulfurize
the substrate in the presence of molecular oxygen. Although
the present catalytic system should be tested in the presence
of a commercial diesel sample to understand the action of
the hydrocarbon molecules on the catalytic activity, the study
demonstrated that metal catalyzed oxidation of DBT with
common peroxidic oxidants was feasible, with formation of
HBPS and also desulfurization products, such as HBP, in good
yields and under mild conditions. Oxidation of DBT occurred
in aqueous acetonitrile, either with H2O2 and catalyzed by
FePcS or with oxone and catalyzed by RuPcS, at substrate
loading down to 160 ppm of sulfur content (w/w). In the pre-
sence of oxone RuPcS was more effective than FePcS, whereas
with H2O2 the opposite was found; this poorer performance
was however not due to an intrinsic, poorer reactivity of the
RuPcS/H2O2 system, but to the fact that the complex pro-
moted a very effective dismutation of the oxidant, thus making
less profitable RuPcS, if compared to FePcS. The oxidation
pathway for the present reactions consisted in the conventional
multistep process (Fig. 6), controlled, as expected, by the slow
oxygenation of DBTO2 to DBTO3 .
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