42
H. Wang, R. Prins / Journal of Catalysis 264 (2009) 31–43
of DMDBT and its intermediates were about 1.4 to two-fold lower
(Tables 1 and 2). Furthermore, the rate constants of dehydrogena-
tion of HHDBT and HHDMDBT decreased with decreasing pressure.
Thus, hydrogen influences the rate of (de)hydrogenation not only
through its partial pressure, but also through its effect on the struc-
ture of the catalyst surface. The number of catalytically inactive Ni
atom on the catalyst surface, in square-planar sulfur coordination
with an additional sulfur atom on top, increases with decreasing
H2/H2S ratio [32]. Nevertheless, a shift of the (quasi) equilibrium
of THDMDBT and HHDMDBT, toward THDMDBT, was observed
when the H2 pressure was decreased from 5.0 to 3.0 MPa. The ratio
of THDMDBT/HHDMDBT at 3.0 MPa was around 2.4 to 2.8, as cal-
culated from the rate constants and reaction profiles of the HDS
of DMDBT and the two intermediates. This ratio is higher than
the ratio (2.0) at 5.0 MPa, most likely because of thermodynamics.
place by hydrogenolysis of the aryl C–S bond, followed by cleavage
of the cycloalkyl C–S bond of the resulting thiol by elimination to
(DM)CHEB and by hydrogenolysis to (DM)CHB. The selectivity of
CHEB over Ni–MoS2/c-Al2O3 was higher than that over MoS2/c-
Al2O3 at 5.0 MPa and 35 kPa H2S, possibly because elimination is
favored over Ni–MoS2/c-Al2O3.
5. Conclusions
The mechanism of the desulfurization of DBT and its hydroge-
nated intermediates (THDBT and HHDBT) over Ni–MoS2/ -Al2O3
is similar to that over MoS2/ -Al2O3, and to that of DMDBT and
its intermediates (THDMDBT and HHDMDBT) over Ni–MoS2/
c
c
c-
Al2O3. DBT, THDBT, DMDBT, and THDMDBT undergo desulfuriza-
tion by hydrogenolysis of both C–S bonds. HHDBT and HHDMDBT
undergo desulfurization by cleavage of the aryl C–S bond by
hydrogenolysis, and then cleavage of the cycloalkyl C–S bond by
elimination as well as by hydrogenolysis.
4.4. Mechanism of hydrodesulfurization
Although BP and DMBP behaved as primary products in the HDS
of DBT and DMDBT (Figs. 1–4), respectively, it is unlikely that the
two C–S bonds of DBT and DMDBT break simultaneously. The
DDS of DBT and DMDBT will, therefore, occur in two hydrogenoly-
sis steps, with C–S bond breaking to 2-phenyl-thiophenol for DBT
and to 2-(2-methylphenyl)-6-methylthiophenol for DMDBT, fol-
lowed by C–S bond breaking to BP and DMBP [44]. As explained be-
fore [18,19], it is very unlikely that arylthiols such as thiophenol,
DBT, and DMDBT undergo desulfurization by hydrogenation to a
dihydro intermediate followed by elimination of H2S, as proposed
earlier [9,10].
TH(DM)DBT underwent desulfurization to CHB and CHEB (Figs.
5 and SM1). The yield of CHB increased continuously with weight
time, and the yield of CHEB passed through a maximum, indicating
that CHEB was an intermediate and was hydrogenated to the final
desulfurized product CHB. The selectivity of CHEB at low weight
time was much higher in the presence of MPi, because MPi inhib-
ited the hydrogenation of CHEB to CHB. DMCHB was a primary
product in the HDS of THDMDBT (Figs. 6 and SM2), whereas
DMCHEB had low selectivity (and behaved as a primary product)
in the presence of MPi (Fig. SM2D). The hydrogenation of DMCHEB
was probably much faster than that of CHEB because of the
electron donation by the methyl groups. The reaction of THDBT
to CHEB and THDMDBT to DMCHEB may proceed by hydrogenoly-
sis of the two C–S bonds (an aryl C–S bond and a vinyl C–S bond)
[19]. Thereafter, fast hydrogenation of CHEB to CHB and of
DMCHEB to DMCHB occurs.
The rate constants of all the steps of the kinetic networks of the
HDS of DBT and DMDBT were measured. Ni promoted the desulfur-
ization of DBT, THDBT, and HHDBT (DBT > THDBT > HHDBT), but
hardly promoted their (de)hydrogenation. The methyl groups at
the 4 and 6 positions of DMDBT, THDMDBT, and HHDMDBT sup-
pressed their desulfurization by steric hindrance in the order
DMDBT > THDMDBT > HHDMDBT and promoted hydrogenation.
These different degrees of steric hindrance are due to the hydroge-
nation of a phenyl ring, which makes the THDBT and HHDBT rings
flexible, especially the cyclohexyl ring of HHDBT.
H2S strongly inhibited the rates of the desulfurization of the six
molecules in the order (DM)DBT > TH(DM)DBT > HH(DM)DBT, but
affected their (de)hydrogenation only slightly. DMDBT and its
intermediates were less affected because of electron donation of
methyl groups. MPi inhibited the (de)hydrogenation of the six mol-
ecules in the order (DM)DBT > TH(DM)DBT > HH(DM)DBT, which
can be explained by the special structure of the Ni–MoS2 catalyst.
MPi promoted the desulfurization of DBT and HHDBT, whereas it
inhibited that of THDBT and DMDBT and its intermediates. The
promotion effect can be explained by the transformation of hydro-
genation sites to desulfurization sites by the perpendicular adsorp-
tion of MPi.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
CHB and CHEB behaved as primary products in the HDS of
HHDBT. The yield-time curves of CHEB and CHB (Figs. 7 and
SM3) indicate that CHEB was an intermediate and reacted to
CHB. In the HDS of HHDMDBT, DMCHEB was observed only with
low selectivity and behaved as a primary product in the presence
of MPi (Fig. SM4D), similar to the HDS of THDMDBT. Several reac-
tion pathways are possible in the desulfurization of TH(DM)DBT
and HH(DM)DBT, depending on which C–S bond breaks first, and
whether elimination or hydrogenolysis of the aliphatic C–S bond
occurs, all of which can lead to the simultaneous formation of
(DM)CHEB and (DM)CHB. A Density Functional Theory (DFT) calcu-
lation of the desulfurization of dihydrobenzothiophene over a
Mo3S9 cluster showed that the activation energy for hydrogenoly-
sis of the aryl C–S bond in dihydrobenzothiophene is lower than
that for breaking the alkyl C–S bond [45]. Cristol et al. calculated
a negligible difference in the energy of 2-ethylbenzenethiolate
and 2-phenylethanethiolate, the products of alkyl and aryl C–S
bond breaking of dihydrobenzothiophene [46]. Dihydro-benzo-
thiophene is similar to HHDBT, with one part of the molecule being
aromatic and the other part being aliphatic. Therefore, the desul-
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