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Chemistry Letters Vol.32, No.5 (2003)
Hydrodesulfurization of 4,6-DMDBT in the High Boiling Fraction of Gas Oil
Ki-Hyouk Choi, Yozo Korai, and Isao MochidaÃ
Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580
(Received January 10, 2003; CL-030032)
A high boiling fraction of straight run gas oil, which con-
activity of the sulfur containing products may influence the deep
desulfurization.
tained 4,6-DMDBT by addition but free from dimethylbiphenyl
and dimethylcyclohexylbenzene of its HDS products, was de-
sulfurized over commercial catalysts to find principal products
of 4,6-DMDBT. Thus, HDS of 4,6-DMDBT in the high boiling
fraction of real feed was proved to proceed principally through
hydrogenative route. During the HDS of the fraction, 4,6-
DMDBT was found to be produced from trimethyldibenzothio-
phenes. This activity depended very much on the acidity of the
catalyst.
Fractionated feed oil (345 ꢀC+, 1.87 wt% S) used in this
study was prepared from straight run gas oil (SRGO,
1.54 wt% S). 4,6-DMDBT (Aldrich Chem.) was added to the
fractionated oil to obtain the total sulfur content of 1.93 wt%
S. The catalysts, which were supplied from commercial catalyst
vendors, were CoMoS supported on alumina (CM3, CMMS,
CMML) and silica-alumina (CMLX) with different specific sur-
face area and acidity as shown in Table 1. 10 g of feed oil, 2 g of
pre-sulfided catalyst and 60 kg/cm2 of H2 were charged into
100 mLof an autoclave-type reactor. Reaction temperature
and time were 340 ꢀC and 2 h, respectively. Reaction products
were analyzed by GC (HP 6890+) coupled with atomic emis-
sion detector (AED, HP G2350).7
Strict regulation on the sulfur content in the diesel fuel low-
er than 15 ppmS requires deep hydrodesulfurization of refrac-
tory sulfur species such as 4,6-dimethyldibenzothiophene (4,6-
DMDBT) and 4,6,x-trimethyldibenzothiophene (4,6,x-TMDBT)
which survive in the oil after conventional hydrodesulfurization
process.1{3 Their HDS has been reported to proceed principally
through the hydrogenation of one phenyl ring of dibenzothio-
phene in the model feed at around 300 ꢀC while higher reaction
temperature at 380 ꢀC enhanced the direct desulfurization.2
Such a scheme is a key information for the design of the better
catalyst and process for practical deep desulfurization below
350 ꢀC under the strong influences of inhibitors present in real
feed. It is easy in principle to distinguish the mechanism by
identifying products, dimethylbiphenyl (DMBP) and dimethyl
cyclohexylbenzene (DMCHB) from 4,6-DMDBT in the HDS
products. However, it is not practically easy since the gas oil
contains a variety of hydrocarbons in relatively high concentra-
tions compared to those of 4,6-DMDBT at a few hundreds of
ppm. Hence, no literature is found on the HDS mechanism of
4,6-DMDBT in real feed although there has been many reports
on the reaction network of 4,6-DMDBT in model feeds.2
The contribution of each pathways to desulfurization de-
pends on the reaction conditions and catalysts type.2 Recently
acidic catalyst showed higher activity on HDS of 4,6-
DMDBT[4,5]. Such a high activity is ascribed in the model
study partly to the migration of methyl groups at 4- or 6-posi-
tion to reduce their steric hindrance.4 Such results indicates
the significance of interconversion of methyl groups substituted
on the DBT ring. Hydrogenation is also enhanced by the acidity
of the catalyst.6
Table 1. Characteristics of Catalysts Used in This Study
Name
Support
CM3
CMLX
CMML CMMS
Alumina Silica-Alumina Alumina Alumina
Surface Area 217 m2/g
234 m2/g
3.5 nm
0.56
148 m2/g 202 m2/g
5.4 nm 3.2 nm
Average Pore
3.8 nm
Size
Aciditya
0.48
0.36
0.18
aAmount of NH3 Desorbed (mmol NH3/g catalyst)
A feed of 345 ꢀC+ fraction contained high boiling point
sulfur species whose retention times were longer than that of
4,6-DMDBT. Benzothiophenes (BT), dibenzothiophene
(DBT), mono methyl substituted DBT (C1-DBT) and 4,6-
DMDBT were absent in the 345 ꢀC+ fraction.
Figure 1 is the sulfur chromatogram of the reaction product
from 345 ꢀC+ fraction over CMLX catalyst. The product cer-
tainly contained 4,6-DMDBT as a product. Furthermore new
peaks besides 4,6-DMDBT, which were not observed in the
feed oil, were found in the product. Such peaks, which survived
2 h reaction, were regarded as refractory sulfur species having
alkyl substituents at 4- and 6-positions of dibenzothiophene.
The amount of 4,6-DMDBT detected in the products de-
pended on catalyst types. As shown in Figure 1, CMLX and
CM3 catalysts produced the largest amount of 4,6-DMDBT in
the product.
The addition of 4,6-DMDBT to 345 ꢀC+ fraction slowed
down the HDS of the fraction. The product from 345 ꢀC+ frac-
tion with added 4,6-DMDBT (600 ppmS) was analyzed as in
Figure 2. The sulfur chromatogram of the HDS product of
0.1 wt% 4,6-DMDBT in n-decane was also illustrated to indi-
cate the peak positions of HDS products. The product from
4,6-DMDBT in n-decane showed two peaks assigned to
DMCHB and another peak to DMBP. DMCHB appeared to car-
ry two isomers such as cis and trans-forms.8 DMCHB stayed al-
ways the dominant product regardless of the reaction time over
all catalysts. No methyl group migration with 4,6-DMDBT was
The aim of this study is to confirm the reaction network of
4,6-DMDBT in the practical desulfurization. On this objective,
cut fraction of gas oil free from 4,6-DMDBT above 345 ꢀC was
hydrodesulfurized over CoMoS catalysts by adding 4,6-
DMDBT to identify the biphenyls and cyclohexylbenzenes by
GC-AED in its products since the fraction was free from such
species. Characteristic features of the catalysts can be identified
by measuring the particular products. Reactivities of high boil-
ing sulfur species were also concerned since they may suffer the
methyl migration as well as desulfurization. Thus the HDS re-
Copyright Ó 2003 The Chemical Society of Japan
Choi, Ki-Hyouk
