Hydrodesulfurization of Alkyldibenzothiophenes over a NiMo/Al2O3 Catalyst: Kinetics and Mechanism

Article abstract of DOI:10.1006/jcat.1997.1732

The transformation mechanism of dibenzothiophene, 4-methyldibenzothiophene, 4,6-dimethyldibenzothiophene, and 2,8-dimethyldibenzothiophene has been studied in a batch reactor over an industrial NiMo/Al₂O3 hydrotreating catalyst at 573 K under 5 MPa of hydrogen pressure. A detailed mechanistic study including competitive catalytic experiments proved that the adsorption of the most refractory molecules at the catalyst surface was not the rate-determining step for their transformation. Our results imply that the hydrodesulfurization of these compounds occurs on one single type of sites by a flat adsorption, leading to a preliminary partial hydrogenation of one aromatic ring. Variations in reactivities of the dibenzothiophene derivatives were thus explained by different reaction rates for the C-S bond scission due to steric hindrance generated by the methyl substitution near the sulfur atom.

Full text of DOI:10.1006/jcat.1997.1732

JOURNAL OF CATALYSIS 170, 29–36 (1997)
ARTICLE NO. CA971732
Hydrodesulfurization of Alkyldibenzothiophenes over a NiMo/Al O
2
3
Catalyst: Kinetics and Mechanism
�
,
�
Val e´ rie Meille, † Emmanuelle Schulz,† Marc Lemaire,† and Michel Vrinat
�
Institut de Recherches sur la Catalyse, 2, Avenue A. Einstein, 69626 Villeurbanne Cedex, France; and †Laboratoire de Catalyse et Synth e` se Organique,
IRC, UCBL, CPE, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Received February 3, 1997; revised April 14, 1997; accepted April 14, 1997
one considers that the sulfur compounds have different ad-
The transformation mechanism of dibenzothiophene, 4-methyl- sorption constants according to their structure. The low re-
dibenzothiophene, 4,6-dimethyldibenzothiophene, and 2,8-di- activity of the alkyldibenzothiophenes could thus be due to
methyldibenzothiophene has been studied in a batch reactor over
an industrial NiMo/Al2O3 hydrotreating catalyst at 573 K under
their lower adsorption constant at the catalyst surface. A
second hypothesis suggests on the contrary that the differ-
5
MPa of hydrogen pressure. A detailed mechanistic study includ-
ent reactivities result from different surface reaction rates.
An unequivocal explanation for these different reactivities
is however difficult to be found by reading conscientiously
the corresponding literature. The researches run in this field
by the scientists have been conducted under different ex-
perimental conditions according to their speciality, lead-
ing competitive catalytic experiments proved that the adsorption
of the most refractory molecules at the catalyst surface was not the
rate-determining step for their transformation. Our results imply
that the hydrodesulfurization of these compounds occurs on one
single type of sites by a flat adsorption, leading to a preliminary
partial hydrogenation of one aromatic ring. Variations in reactiv-
ities of the dibenzothiophene derivatives were thus explained by ing however often to ambiguous and even contradictory
different reaction rates for the C–S bond scission due to steric hin- results.
drance generated by the methyl substitution near the sulfur atom.
�
c 1997 Academic Press
To illustrate the main results, we first report the work
of Houalla et al. published in 1980 (3), dealing with the
transformation of dibenzothiophene (DBT), 2,8-dimethyl-
dibenzothiophene (2,8-DMDBT), 3,7-dimethyldibenzo-
thiophene (3,7-DMDBT), and 4,6-dimethyldibenzothio-
phene (4,6-DMDBT). The authors noticed a decreasing
INTRODUCTION
Recent and future legislation in European countries con- reactivity in the order DBT, 2,8-DMDBT, 3,7-DMDBT, 4,6-
cerning air pollution by diesel exhaust gas prompts refiners DMDBT; this phenomenon was attributed to different ad-
to reduce considerably the sulfur content of gas oil, from sorption strengths of the studied molecules on the catalyst
0
.2 wt% in 1994 to 0.05 wt% in 1996 and probably lower surface. Ma et al. (4) observed the same inhibition by the
in the near future. For the sulfur compounds, the high con- neighboringmethylgroupson the sulfur atom and proposed
versions thus required can be achieved with the present hy- similarly a possible steric hindrance of the methyl groups
drodesulfurization processes only by using expensive and limiting the adsorption of the molecules on the catalyst.
drastic methods such as increasing the temperature and the
The second hypothesis is illustrated by the study of Kabe
pressure in the reactor or lowering the liquid hourly space et al. (5). They have also compared the reactivities of differ-
velocity. Development of new catalysts is therefore a chal- ent mono- and polyalkyldibenzothiophenes: 4-MDBT and
lenge for refiners and catalyst producers, but such a suc- 4,6-DMDBT were found to be the only ones to have a very
cess could be more easily achieved by a better knowledge low reactivity compared to that of DBT. This was explained
of the deep hydrodesulfurization process. Many researches by a steric hindrance generated by the methyl groups of the
have been performed in the past 20 years to find out which adsorbed molecule to proceed to the C–S bond scission.
compounds are refractory to the HDS process (1). These Unlike previously published results, they found more im-
molecules are now known to be alkyldibenzothiophenes, portant values for the adsorption equilibrium constants in
and especially compounds substituted in the 4 and/or 6 po- the cases of 4-MDBT and 4,6-DMDBT than in the case
sitions (2). Many authors have proposed hypotheses to ex- of DBT.
plain the lower reactivity of these compounds compared
The evident confusion and lack of information concern-
to the unsubstituted dibenzothiophene. The discussions are ing the understanding of the low reactivities of 4-methyl-
mainly based on two proposals: the most generally accepted and 4,6-dimethyldibenzothiophene are also probably
29
0021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

3
0
MEILLE ET AL.
related to a poor knowledge of the catalytic sites (nature,
number...) involved in their transformation.
Two modes of adsorption can be taken into account: a flat
adsorption by the �-electrons of the aromatic system or a
bonding via the sulfur atom of the molecule. It is generally
reported that the �-adsorption favors hydrogenation reac-
tions, whereas the sulfur adsorption is related to desulfur-
ization reactions. Several authors (6–8) claimed that these
two different kinds of adsorption modes are required for
the hydrodesulfurization of the studied molecules and con-
nected the low reactivity of 4,6-DMDBT to a difficult ad-
sorption by the sulfur atom on account of the steric hin-
drance.
However, the possibility of two different adsorption
modes for the transformation of DBT and its derivatives
has not yet been actually proven. Considering that sulfur
adsorption was improbable, Singhal (9) has proposed a
mechanism involving a preliminary hydrogenation of the
molecule thus occuring after a �-adsorption, leading to a di-
hydrodibenzothiophene, a common species to the both
routes of transformation usually observed. A study by
Zdrazil about the transformation of thiophene, benzothio-
phene, and dibenzothiophene supports this hypothesis of
such hydrogenated intermediates (10).
FIG. 1. Synthesis of 2,8-dimethyldibenzothiophene.
dride and methylated by methyl magnesium iodide in ether.
This last step is catalyzed by a nickel II complex (Kumada
type reaction).
For the alkyldibenzothiophenes hydrodesulfurization,
the catalyst used was an industrial NiMo/Al2O3 (Mo, 9 wt% ;
Ni, 2.4 wt% ). This catalyst was provided as extrudates (bulk
�
3
3
� 1
density, 0.71 g cm ; pore volume, 0.48 cm g ; surface area,
2
� 1
1
1
85 m g ). The extrudates were crushed, screened (80–
20 �m), and then sulfided ex situ by a H2–H2S (15% ) flow
at 673 K for 4 h before use.
Reactor and Analytical
3
Studies were performed in a 200-cm stirred slurry tank
reactor operating in batch mode. The autoclave is equipped
with a hollow-shaft six-bladed magnetically driven turbine
with four baffles on the wall to prevent vortex formation.
The samples are collected through a 1/16 in. diameter tube;
hydrogen is introduced through a pressure controller which
maintains a constant pressure during the course of the ex-
periment.
Considering that the various explanations for the mech-
anism of DBT transformation are quite contradictory, the
aim of the present study is to reexamine the transformation
of the refractory alkyldibenzothiophenes in order to precise
their reaction scheme and to explain the lower reactivity of
In each run, the autoclave was charged with 0.1 to 0.5 g of
reactant dissolved in 80 ml dodecane, after having checked
that the reaction order toward the sulfur compound was
zero. Freshly sulfided catalyst (0.1 to 1 g) was added to the
solution. The reactor was flushed with nitrogen and heated
to reach the reaction temperature of 573 K; nitrogen was
then replaced by hydrogen and the stirring was started, in-
dicating the zero of the reaction time. Sample were period-
ically removed during the reaction course and analyzed by
gas chromatography. When the influence of H2S on the con-
vertion was studied, 0.35 g of dimethyldisulfide (DMDS)
was added to the solution containing in this case 1.3 mmol
4
,6-DMDBT.
For this purpose, we have synthesized the required
molecules and compared their reactivities in a batch re-
actor, on an industrial NiMo/Al2O3 hydrotreating catalyst,
in separate and competitive experiments.
EXPERIMENTAL PROCEDURE
Materials
of substrate. DMDS is rapidly hydrogenolyzed in H S in our
2
operating conditions providing a molar ratio H2S/DBT then
Dibenzothiophene (98% purity) was obtained from
Sigma–Aldrich–Fluka and used without further purifica-
tion. 4-Methyldibenzothiophene (93% purity) was syn-
thesized according to the method of Gerdil et al. after
optimization, by metallation and subsequent methylation
of DBT (11). 4,6-Dimethyldibenzothiophene (>99% pu-
rity) was prepared by a four-step method (25% global
yield) starting from a coupling reaction of ortho-thiocresol
and 2-bromo-3-nitrotoluene as previously described (12).
equal to 6, a value which is closed to the conditions existing
in a refinery process when an almost complete desulfuriza-
tion of a gas oil feed is achieved.
GC analyseswere performed usingan apolar column (HP
5
, 5% phenylmethylsilicone, 30 m � 0.53 mm) equipped
with a FID detector. The column temperature was pro-
grammed between 368 and 573 K. The different intermedi-
ates and products of the reaction were identified by GC/MS
analyses.
2
,8-Dimethyldibenzothiophene (>99% purity) was syn-
thesized in two steps with 30% global yield (Fig. 1).
The first step, described by Neumoyer (13), consists in a
bromination of DBT in refluxing acetic acid for 4 h. 2,8-
Control of the Kinetic Regime
A set of experiments was performed to check the absence
Dibromodibenzothiophene is separated from monobro- of intraparticle and interphase mass transfer limitations.
modibenzothiophene by recrystallization in acetic anhy- Different weights of catalyst were tested (0.1 to 1 g) and the

HYDRODESULFURIZATION OF ALKYLDIBENZOTHIOPHENES
31
results indicated that the kinetic regime was established in then the formation rate of C is
all cases. The effect of the stirring rate and the granulometry
vC = kA � [A] � [R],
where [A] can be expressed by
[2]
of the catalyst has been controlled (14). It appeared that
no diffusional problem could occur under our operating
conditions when the mass of the catalyst was limited to 1 g
and the stirring rate was fixed to at least 1500 revolutions
per minute.
k1A � [A] � [S]
[
A] =
.
[3]
k� 1A + kA � [R]
Kinetic Calculations
In this expression, [i] represents the concentration of com-
ponent i after a known reaction time, [S] is the concentra-
tion of active sites at the surface, and [R] is the concentra-
tion of the hydrogen adsorbed on the catalyst surface.
Similarly, at the stationary state of [B]
Some experiments were realized on single products and
others on equimolar mixtures of two compounds thus re-
acting in competition.
Hydrodesulfurization of a single compound. A typical
reaction course according to the time is pointed out in Fig. 2.
The initial rate of the DBT transformation can be obtained
by the slope to the DBT curve at the origin.
The yields of the different products can also be plotted
versus the conversion, in order to determine the reaction
scheme and the product selectivities, as will be illustrated
later in Figs. 4 and 5.
k1B � [B] � [S] = k� 1B � [B] + k � [B] � [R],
B
[4]
then the formation rate of D is
v = k � [B] � [R],
D
B
[5]
where [B] can be expressed by
k1B � [B] � [S]
Hydrodesulfurization in competitive experiments. In
competitive experiments, two compounds A and B are con-
sidered to compete for adsorption on the same sites. R is
the common reactant to the transformation of A and B.
[
B] =
.
[6]
k� 1B + kB � [R]
[
R] is supposed to be the same in both expressions; the
rate ratio is then expressed by
R
A
R
A
vD
vC
kB � k1B � [B] k� 1A + kA � [R]
d[B]
d[A]
k1A
kA
=
�
=
.
[7]
A + R ��→ C
kA � k1A � [A] k� 1B + kB � [R]
k� 1A
k1B
From integration of this equation, the reactivity ratio RB/A
between the considered molecules A and B can be obtained
15–17).
kB
B
B
B + R ��→ D,
k� 1B
(
�
!
where the components underlined (i.e., A) correspond to
the adsorbed species, k1i (respectively, k� 1i) represents the
rate constant of adsorption (respectively, desorption) of
component i and ki the rate constant of the surface reac-
tion of component i.
[B]
log [
B0]
kB � k1B k� 1A + kA[R]
=
= RB/A.
[8]
[A]
kA � k1A k� 1B + kB[R]
log [
A0]
The ratio R is, at constant temperature, independent of
At the stationary state of A
the mixture composition and extent of conversion. At rel-
atively low hydrogen pressure, the surface reaction is the
rate-determining step and R can be expressed as a function
of adsorption equilibrium constants and rate constants
k1A � [A] � [S] = k� 1A � [A] + kA � [A] � [R],
[1]
kB KB
RB/A =
[9]
kA KA
with
k1i
Ki =
[10]
k� 1i
representing the adsorption equilibrium constant of com-
pound i at the surface.
It has been checked (14) that under our experimental
conditions, a pressure of 5 MPa still corresponds to the
range of low hydrogen pressure and thus that the above
expression can be used. Therefore, a comparison of the R
FIG. 2. Transformation of a single molecule (DBT): conversion
versus time. Experimental conditions: 563 K, 50 MPa, NiMo catalyst. (+,
DBT; ᭹, BP; � , CHB; ᭝, HN.)

3
2
MEILLE ET AL.
values with the rate ratios kB/kA calculated for the trans- product. Hydrogenated products (HN) are only observed
formation of each product in separate experiments allows as traces. On the contrary, the products issued from the
the determination of the adsorption equilibrium constants transformation of 4-MDBT and 4,6-DMDBT are mainly
ratios KB/KA.
resulting from the hydrogenation pathway, leading to CHB
and HN. Moreover, concerning the transformation of 4,6-
DMDBT, the selectivity in BP remains very low, whatever
the conversion.
RESULTS
The same tendency is observed when H2S is present in
the reactor: at comparable conversion, 4,6-DMDBT leads
to the formation of less BP derivatives than DBT. It appears
also that the selectivities for the products resulting from the
hydrogenation route are increasing.
From the curves presented in Figs. 4 and 5, the selec-
tivities in the different compounds can be evaluated. The
values determined at 10% conversion are given in Tables 1
and 2.
The initial transformation rates of the different com-
pounds were calculated from the slope to the reactant trans-
formation curves similar to the one presented in Fig. 2. The
quantification of the rates according to each pathway was
performed using the initial transformation rates and the
selectivities calculated at 10% conversion, taking into ac-
count that, at such a level of conversion, the hydrogenation
ofbiphenylinto cyclohexylbenzene could be neglected (14).
The transformation rates of the studied molecules de-
creased in the order 2,8-DMDBT > DBT > 4-MDBT >
Transformation of the Single Molecules
The transformation study of the different compounds in-
dicates a reaction scheme in agreement with the proposals
of the literature (18, 19). As schematized in Fig. 3, DBT and
alkyl-DBT appear to be transformed according to two par-
allel routes. The first one, called the direct desulfurization
pathway (DDS) first gives biphenyl derivatives (BP). These
desulfurized molecules are then slowly hydrogenated into
cyclohexylbenzenes(CHB). The second pathway, called the
hydrogenation route (HYD) consists in a preliminary hy-
drogenation of one aromatic ring, giving tetrahydro- and
hexahydrodibenzothiophene or analogues (HN). These in-
termediates can then be desulfurized to CHB.
The evolution of the reactant transformation is followed
by regular samplings which are analyzed by gas chromatog-
raphy. Product concentrations are determined and the se-
lectivities are calculated. The curves representing the yield
of products versus the conversion of the studied molecule,
without or with H2Spresent in the reactor, are given, respec-
tively, in Figs. 4 and 5. The rates being drastically lowered
by the presence of H2S the conversion was in the latter case
limited to 30% .
4
,6-DMDBT whatever the operating conditions (Tables 1
and 2).
Without H2S (Table 1), the 4,6-DMDBT was found to be
about six times less reactive than the DBT and the 4-MDBT
about three times less reactive. These results confirm that
hydrodesulfurization of substituted DBT is inhibited by the
presence of methyl groups at the carbon � to the sulfur
atom. However, under the same conditions, the transfor-
mation of 2,8-DMDBT is more than two times faster than
that of DBT, thus providing additional proof that the loss
of reactivity in the case of 4,6-DMDBT was not due to the
number of substituents but to their position. It must be
pointed out that the calculation of rates according to each
pathway clearly shows that the methyl-inhibition in the 4
and 6 positions only affects the DDS pathway, the hydro-
genation rates being indeed similar for all the compounds.
When H2S is present in the reactor (H2S/DBT = 6), the
globaltransformation rate isgreatlylowered for allthe com-
pounds (Table 2), the order of reactivities remaining how-
ever similar. For example, the transformation rate of DBT
Considering first the case of DBT and 2,8-DMDBT trans-
formations without H2S, the major product observed is
biphenyl (or dimethylbiphenyl), CHB appearing signifi-
cantly only at conversions higher than 30% as a secondary
�
8
� 1 � 1
under these conditions reaches only 7.5 � 10 mol s
g
�
8
� 1 � 1
instead of 180 � 10 mol s
g
without H2S present in the
reactor. This decrease in reactivity is similar concerning the
�
8
� 8
transformation of 2,8-DMDBT (22 � 10 and 400 � 10
�
1
� 1
mol s g ) but is less important for the compounds methy-
lated at positions close to the sulfur atom. By comparing the
rates according to each pathway under both operating con-
ditions, it is obvious that the desulfurization route is more
FIG. 3. Transformation scheme of dibenzothiophene.

HYDRODESULFURIZATION OF ALKYLDIBENZOTHIOPHENES
33
FIG. 4. Diagrams of yield versus conversion obtained for the transformation of DBT, 4-MDBT, 4,6-DMDBT, and 2,8-DMDBT without H2S in the
�
reactor. (᭹, BP; , CHB; ᭝, HN.)
inhibited by the presence of H2S than the hydrogenation tion (A and B). According to the competitive theory (see
one.
above), curves of ln(A/A0) versus ln(B/B0) are thus avail-
Competitive studies were realized in order to understand able. The straight lines obtained demonstrate the validity
if the decreasing reactivities of 4-MDBT and 4,6-DMDBT of the competitive experimental approach and allow the
were due to a problem of adsorption on a catalytic site or corresponding reactivity ratio R to be calculated (Fig. 6).
to differences in the surface reaction rates.
The rate constants ratio kA/kB is easily available through
ri values (initial transformation rate of compound i) since
the considered reactants are present in equimolar propor-
Competitive Experiments
An equimolar mixture of two dibenzothiophene deriva- tions; this rates ratio, compared to the reactivity ratio ob-
tives (noted A and B for simplification) in dodecane was in- tained in competition, allowed the calculation of the ad-
troduced into the reactor and converted under the standard sorption equilibrium constants ratio KA/KB between the
conditions without H2S (NiMo catalyst, 50 MPa, 573 K). two considered molecules (see Table 3).
Samples were periodically removed at increasing conver-
In all cases, the rate ratio kA/kB calculated from separate
sion to allow the calculation of each reactant concentra- experiments is similar to the reactivity ratio R calculated
TABLE 1
Transformation of the Compounds without H2S in the Reactor
Initial transformation
rate ( � 10 mol/(s � g))
S(BP)a
(% )
S(CHB)a
(% )
S(HN)a
(% )
Desulfurization rate
( � 10 mol/(s � g))
Hydrogenation rate
( � 10 mol/(s � g))
8
8
8
DBT
180
57
30
87
43
10
93
6
27
30
3
7
30
60
4
157
24
3
23
33
27
28
4
4
2
-MDBT
,6-DMDBT
,8-DMDBT
400
372
a
Selectivities of each product calculated at 10% conversion.

3
4
MEILLE ET AL.
FIG. 5. Diagrams of yield versus conversion obtained for the transformation of DBT, 4-MDBT, 4,6-DMDBT, and 2,8-DMDBT with H2S present
�
in the reactor. (᭹, BP; , CHB; ᭝, HN.)
from competitive experiments. Therefore, the adsorption presence of methyl groups near the sulfur atom, in accor-
equilibrium constantsare almost the same for allthe studied dance with all the published studies (3).
molecules, taking into account the limit of the precision
during catalytic activity determination (� 10% ).
Moreover, the methyl substitution and the inhibiting ef-
fect of H2S do not affect similarly the two parallel routes of
transformation (DDS and HYD), the DDS one being prin-
cipally inhibited (Tables 1 and 2). The inhibition by H2S
is particularly noticeable if we observe the DBT transfor-
DISCUSSION
It is difficult to compare our results (rate values) with mation which is essentially governed by this pathway. The
those of the literature. The operating conditions (temper- global transformation of this compound is drastically low-
ature, pressure, reactor...) are indeed very different from ered (24-fold) if H2S is present in the reactor, though only
one group to another. Comparing only the classification a 7-fold decrease is observed in the case of 4,6-DMDBT.
for the reactivity of alkyldibenzothiophenes, it is clear that
Both methyl substitution and H2S concentration signifi-
the transformationof dibenzothiophene is inhibited by the cantly affect the direct desulfurization route. To afford deep
TABLE 2
Transformation of the Compounds with H2S Present in the Reactor
Initial transformation
rate ( � 10 mol/(s � g))
S(BP)a
(% )
S(CHB)a
(% )
S(HN)a
(% )
Desulfurization rate
( � 10 mol/(s � g))
Hydrogenation rate
( � 10 mol/(s � g))
8
8
8
DBT
7.5
5.5
4.4
22
70
25
8
20
30
23
9
10
45
69
27
5
2
4
4
8
4
4
2
-MDBT
,6-DMDBT
,8-DMDBT
1.5
0.4
14
64
a
Selectivities of each product calculated at 10% conversion.

HYDRODESULFURIZATION OF ALKYLDIBENZOTHIOPHENES
35
FIG. 6. ln(A/A0) versus ln(B/B0) obtained in competitive experi-
ments. (᭞) A, DBT; B, 2,8-DMDBT. (� ) A, 4-MDBT; B, 4,6-DMDBT.
FIG. 7. Reaction scheme of DBT transformation involving a partially
hydrogenated intermediate.
(
� ) A, DBT; B, 4-MDBT. (+) A, DBT; B, 4,6-DMDBT.
and 372 � 10� mol s
8
� 1 � 1
g for 2,8-DMDBT without H2S)
HDS, the hydrogenation route then appears to be of great
importance for the conversion of alkyldibenzothiophenes
and has to be developed (20, 21).
clearly demonstrates that only the substitution near the sul-
fur atom inhibits the desulfurization route. To explain this
rising activity, an analogy can be proposed with the hydro-
genation of aromatics. Toluene is known to be more easily
hydrogenated than benzene on sulfide catalysts (22) and we
have also shown that on a NiMo catalyst, 3-methylbiphenyl
was faster hydrogenated than biphenyl (14). In the case
of 2,8-DMDBT, the transformation can then be exalted by
an easier hydrogenation of an aromatic ring. The mech-
anistic scheme described by Singhal et al. (9) is in accor-
dance with our observations. They proposed a common
intermediate to the two routes of transformation, such as
a dihydrodibenzothiophene. However, this very unstable
species has never been detected in solution, it can prob-
ably only exist as an adsorbed intermediate. Markel and
Angelici proposed a similar intermediate (dihydrothio-
phene) to explain the transformation of thiophene (23).
The dihydrodibenzothiophene could then either be further
hydrogenated into tetrahydro- and hexahydrodibenzothio-
phene or desulfurized by elimination to provide biphenyl.
This last pathway has been proposed by Kasztelan to il-
lustrate the HDS and HDN mechanisms (24). A possible
structure for this adsorbed dihydrodibenzothiophene and
its transformation according to the two pathways are pre-
sented in Fig. 7.
To understand which phenomenon could explain these
inhibitions, and principally the effect of the methyl near the
sulfur atom, a competitive study was performed. The results
indicate that the adsorption equilibrium constants of all
the dibenzothiophene derivatives are similar and that these
compounds are identically adsorbed on the catalyst surface,
probably via the �-electrons of the aromatic rings. Thus, the
difference of reactivity observed between the alkyldiben-
zothiophenes cannot be explained by a different adsorption
strength at the catalyst surface.
Taking into account these results, an adsorption by the
sulfur atom isunrealistic. The constantsofadsorption would
be otherwise greatly different for molecules sterically hin-
dered by methyl groups around the sulfur atom. Following
this conclusion, we can then assume a flat adsorption of the
DBT derivatives by the �-electrons of the aromatic system
as proposed by Singhal et al. (9). This supposes a subse-
quent destabilization of the molecule, leading probably to
a preliminary hydrogenation of at least one double bond.
If the adsorption strength is not responsible for the differ-
ence of reactivity observed between the various alkyldiben-
thiophenes, an explanation must be searched for in the
mechanism of evolution of the adsorbed species.
Therefore, the higher reactivity of 2,8-DMDBT is ex-
plained by an easier and faster formation of the partially
hydrogenated intermediate. According to Houalla et al. (3),
The promotion of the desulfurization route in the case
of the 2,8-disubstitution (157 � 10� 8 mol s� 1
� 1
g
for DBT
2
,8-DMDBT has almost the same reactivity as DBT. They
TABLE 3
use however for its transformation a CoMo catalyst that
is known to be less hydrogenating than a NiMo one. We
assume that in this case the poor hydrogenating properties
of CoMo are not sufficient to exalt the transformation of
Competitive Experiments (573 K, 50 MPa, No H2S)
Rates ratio
kA/kB
Two compounds A–B (single products)
Reactivity ratio
RA/B (competitive
experiments)
2
,8-DMDBT compared to unsubstituted DBT.
For all the dibenzothiophene derivatives, the modi-
KA/KB
DBT–4-MDBT
-MDBT–4,6-DMDBT
DBT–4,6-DMDBT
DBT–2,8-DMDBT
3.2
1.9
6
4
2.5
8
1.3
1.3
1.3
1.2
fications in the global transformation rates with H S partial
pressure could be explained by the competitive adsorption
of the organic sulfur molecule and H2S on the Mo coordina-
tively unsaturated sites. However, the DDS route was found
2
˙
˙
4
0.45
0.53

3
6
MEILLE ET AL.
to be more inhibited than the HYD one. This can be corre- hydrogenating properties of the catalyst involved in the
lated to the mechanism proposed in Fig. 7, which requires HDS of refractory molecules are of great importance. We
the presence of a basic species. In such a mechanism, H2 assume, that a more hydrogenating catalyst could allow an
and H2S are considered to be dissociatively adsorbed on increasing global transformation rate through an increase
the catalytic surface, over the coordinatively unsaturated of hydrogenation reactions. Experiments in this way are in
molybdenum sites and over the sulfur ions according to
progress.
M + H2 + �S ��→ MH� + �SH�
�
2�
ACKNOWLEDGMENTS
�
2�
�
�
M + H2S + �S ��→ MSH + �SH
This work was carried out in the framework of the contract “Hy-
drod e´ sulfuration des gazoles.” It received support from CNRS-Ecotech,
Elf, IFP, Total and Procatalyse.
In agreement with Kasztelan (24), we propose that the hy-
drogenation steps leading to partially hydrogenated DBT
are a H or H addition, whereas the desulfurization is
the result of a basic attack by S species. Therefore, the
inhibiting effect of H2S on the DDS route can be related to
the lowering concentration of S .
Moreover, for the transformation of 4-MDBT and 4,6-
DMDBT, the inhibition of the only desulfurization pathway
can be explained by a steric hindrance generated by the
methyl groups for the basic attack.
�
+
2�
REFERENCES
1
2
3
4
5
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2�
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DBT or 2,8-DMDBT lead indeed preferentially to biphenyl
derivatives whereas 4-MDBT and even more 4,6-DMDBT
are transformed according to a hydrogenation pathway.
We have proven that the drastic decrease in the global
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1
2
2
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2
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