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detector (FID), PIONA capillary column (25 m 6 0.25 mm) and
autosampler.
Table 2 Desulfurization efficiency of ODS followed by extraction and by
catalytic decomposition
The effect of the reaction temperature on the oxidative
conversion of model sulfur compounds is shown in Table 1,
which indicates that the conversion rate increases with an increase
in temperature. It is also evident from the data that the trend in
oxidation reactivity of the model compounds is as follows:
thiophene , DBT , 4-MDBT. During the oxidation process,
performic acid is formed by the reaction of H2O2 and HCOOH.
The performic acid oxidizes the sulfur compounds into sulfoxides,
and then sulfones. The products formed in the current process
were only the sulfones respective to the model sulfur compounds,
which have been confirmed by GC-MS analysis in the laboratory.
As air cannot directly oxidize sulfur compounds, it can participate
in the oxidation only when it forms a hydroperoxide from the
hydrocarbon molecule present in the medium. In the case of the
model oil, n-heptane is resistant to autoxidation under the current
temperature, which is lower than 100 uC, however, it acts as a co-
oxidant with performic acid.
The results shown in Table 2 indicate that the desulfurization
yield of the extractive ODS process was 87.92%, whereas the yield
of the process involving the oxidation of model oil followed by
catalytic decomposition with Ni2B was 88.67%. It is evident from
these results that the efficiency of both the processes was almost
the same. However, in the former process the spent solution
contains sulfones, whereas in the latter process the sulfones are
decomposed into the parent hydrocarbons with elimination of the
sulfur atom. The reaction involves the formation of solid Ni2B and
gaseous H2, from the reaction of NiCl2 and NaBH4 in the presence
of methanol. The highly reactive hydrogen is adsorbed onto the
surface of Ni2B, which attacks the aC and results in the C–S bond
cleavage (Scheme 1).11
Furthermore, it was found that the desulfurization yield of the
model oil was very low for the direct reduction with Ni2B without
prior oxidation, as compared to the oxidation followed by
reduction route. Upon the direct reduction of the model oil, the
rate of decomposition of the sulfur compounds was very poor, but
in the oxidation followed by reduction route, it increased
significantly (Scheme 1).
Using the direct reduction route, the % reductive decomposi-
tion of thiophene, DBT and 4-MDBT was found to be 57, 53 and
68%, respectively (Fig. 1). However, in the process involving prior
oxidation followed by reduction, the decomposition of thiophene,
DBT and 4-MDBT was 86, 88, and 96%, respectively. These results
show that the ease of the C–S bond cleavage increases when S is
oxidized to the respective sulfones and sulfoxides. In other words,
Sulfur content
(mg g21 a
%
Treated sample
)
Desulfurization
ODS followed by
154.1 ¡ 1.4
144.5 ¡ 0.62
87.9
88.7
extraction with MeOH
ODS followed by reduction
with Ni-boride
Sulfur content in model oil 1275 (mg g21).
a
due to the S–O linkage in the oxidized sulfur compounds, the
neighbouring C–S bond weakens, and can be easily cleaved by
reduction as compared to a normal C–S bond. This phenomenon
was explained by Gilman,12 who proposed that during the course
of the decomposition of DBT sulfones, the dianion intermediate
formed is stabilized, which is not the case for DBT. Therefore, the
yield of biphenyl by decomposition of DBT sulfone is more than
that of DBT.
The reduction with Ni2B requires a protic medium, which is
provided in excess when carried out in an oxidation medium, i.e.
in the presence of aqueous formic acid solution. Additionally,
sulfones have high reactivity towards reduction, thus a high
desulfurization yield is attained in the process. In the case of the
reduction without oxidation, the same medium in not available
and the quantity of methanol available is very small (merely used
for the dissolution of NiCl2), which is insufficient for the
reduction, and leads to
a low desulfurization efficiency.
Therefore, the current process involving oxidation followed by
reduction does not need additional protic solvents.
Next, we carried out desulfurization of commercial oil samples,
i.e. untreated naphtha, light gas oil (LGO) and heavy gas oil (HGO),
through oxidation followed by extraction, as well as oxidation
followed by reduction. The oxidation of commercial oil (20 ml) was
carried out in a 100 ml three neck flask, to which hydrogen
peroxide (2 ml) and formic acid (2 ml) were added, and was
magnetically stirred (750 rpm speed) with continuous air bubbling
(150 ml min21) for 60 min at a temperature of 60 uC. In the case of
HGO, 5 ml benzene was also added to reduce the viscosity of the
feed and facilitate the mixing of the oxidant and oil. A batch of
oxidized oil was extracted with an acetonitrile–water mixture
(80 : 20) followed by another extraction with a methanol–water
mixture (80 : 20). To another batch of oxidized oil, Ni chloride (3.5
Table 1 Oxidative conversion of model sulfur compounds using anair assisted
performic acid oxidation system
% Conversion at different temperatures
Sulfur compounds
40 uC
60 uC
80 uC
Thiophene
DBT
4-MDBT
47
73
87
78
96
98
94
98
99
Scheme 1 Reaction scheme of deoxydesulfurization using Ni2B.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.
Shakirullah, Mohammad
