Mechanisms of thiophene hydrodesulfurization on model molybdenum catalysts

Article abstract of DOI:10.1006/jcat.1998.2162

Hydrodesulfurization (HDS) activities and selectivities were measured for thiophene, tetrahydrothiophene (THT), and 1-butanethiol on silica-supported molybdenum catalysts at a pressure of 1 atm and temperatures ranging from 530 to 795 K. The model catalysts, which were previously characterized, feature isolated molybdenum atoms in the +2, +4, and +6 oxidation states and molybdenum dimers with each molybdenum atom in the +4 oxidation state. Silica-supported MoS₂ was used for reference. Activities for thiophene and THT HDS correlate with oxidation state. Mo(II) is most active among dispersed catalysts. 1-Butanethiol activities were much larger than thiophene or THT activities and were roughly equal on all dispersed catalysts. Apparent activation energies of 43.4 and 48.5 kJ/mol were determined for thiophene HDS on Mo(II) and MoS₂/SiO₂, respectively. Apparent activation energies of 132 and 174 kJ/mol were determined for THT HDS on Mo(II) and MoS₂/SiO₂, respectively. Dihydrothiophene, THT and 1-butanethiol were formed in thiophene HDS over Mo(II) and MoS₂/SiO₂. The major products of thiophene and THT HDS were 1-butene, 2-butene, and n-butene. Butadiene, i-butane, i-butene, methane, ethane, ethene, propane, and propene were formed in small amounts. Butadiene is thought to be the initial product of thiophene and THT desulfurization and undergoes subsequent hydrogenation and isomerization to yield the observed products. A common mechanism for HDS of thiophene and THT with 2,5-DHT as an intermediate is discussed.

Full text of DOI:10.1006/jcat.1998.2162

JOURNAL OF CATALYSIS 178, 226–233 (1998)
ARTICLE NO. CA982162
Mechanisms of Thiophene Hydrodesulfurization
on Model Molybdenum Catalysts
David L. Sullivan1 and John G. Ekerdt2
Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712
Received January 28, 1998; revised May 14, 1998; accepted May 18, 1998
reduced during sulfidation from the +6 oxidation state of
MoO3 to the +4 oxidation state of MoS2. Formation of
MoS2 has been shown by many techniques including X-
ray photoelectron spectroscopy (XPS), X-ray diffraction
(XRD), extended X-ray absorption fine structure spec-
troscopy, and Raman spectroscopy. MoS2 forms crystallites
with a layered structure under these conditions (6).
Hydrodesulfurization (HDS) activities and selectivities were
measured for thiophene, tetrahydrothiophene (THT), and 1-buta-
nethiol on silica-supported molybdenum catalysts at a pressure of
1 atm and temperatures ranging from 530 to 795 K. The model
catalysts, which were previously characterized, feature isolated
molybdenum atoms in the +2, +4, and +6 oxidation states and
molybdenum dimers with each molybdenum atom in the +4 oxi-
dation state. Silica-supported MoS2 was used for reference. Activ-
ities for thiophene and THT HDS correlate with oxidation state.
Mo(II) is most active among dispersed catalysts. 1-Butanethiol ac-
tivities were much larger than thiophene or THT activities and
were roughly equal on all dispersed catalysts. Apparent activation
energies of 43.4 and 48.5 kJ/mol were determined for thiophene
HDS on Mo(II) and MoS2/SiO2, respectively. Apparent activation
energies of 132 and 174 kJ/mol were determined for THT HDS
on Mo(II) and MoS2/SiO2, respectively. Dihydrothiophene, THT
and 1-butanethiol were formed in thiophene HDS over Mo(II) and
MoS2/SiO2. The major products of thiophene and THT HDS were
1-butene, 2-butene, and n-butene. Butadiene, i-butane, i-butene,
methane, ethane, ethene, propane, and propene were formed in
small amounts. Butadiene is thought to be the initial product of
thiophene and THT desulfurization and undergoes subsequent hy-
drogenation and isomerization to yield the observed products. A
common mechanism for HDS of thiophene and THT with 2,5-DHT
The active sites on MoS2/Al2O3 are thought to be molyb-
denum atoms on the perimeter of the MoS2 crystallites that
have sulfur vacancies and are in an oxidation state lower
than +4 (7, 8). We have used isolated Mo(II), isolated
Mo(IV), and isolated Mo(VI) to study oxidation state ef-
fects in thiophene HDS (9). The perimeter of MoS2 crystal-
¯
lites contains two distinct environments (8, 10). The (1010)
face contains sulfur atoms that are bound to only one
¯
molybdenum atom. The (1010) face contains sulfur atoms
that are bound to two molybdenum atoms. Active sites cre-
ated by removal of these sulfur atoms will present different
environments for thiophene adsorption. We have used iso-
lated Mo(IV), dimeric Mo(IV), and MoS2/SiO2 to study
structural effects in thiophene HDS (9). XPS and XRD
were used to show that the Mo(II) and Mo(IV) did not
change oxidation states or become sulfided during thio-
phene HDS. Thiophene HDS activity was shown to cor-
relate with the oxidation state, Mo(II) being the most ac-
tive. MoS2/SiO2 was much more active than Mo(II) but had
the same activation energy as Mo(II). The product distri-
butions of thiophene HDS on Mo(II), isolated Mo(IV),
dimeric Mo(IV), and MoS2/SiO2 were similar, indicating
that the catalyst oxidation state and structure do not sig-
nificantly affect the reaction mechanism. The activity of
isolated Mo(IV) for thiophene HDS shows that a site con-
taining multiple metal atoms is not needed for reaction and
the equivalent activities of isolated Mo(IV) and dimeric
Mo(IV) show that multiple metal atoms do not enhance
activity.
c
as an intermediate is discussed.
ꢀ 1998 Academic Press
INTRODUCTION
Removal of sulfur from petroleum feedstocks is ac-
complished by hydrodesulfurization (HDS) in which the
petroleum is treated with hydrogen over a CoMoS/Al2O3
catalyst at temperatures from 573 to 723 K and pressures
from 35 to 100 atm. Several reviews of catalyst structure and
properties have been published (1–5). The industrial cata-
lyst consists of molybdenum oxide supported on ꢀ -alumina,
which is promoted with cobalt or nickel. The supported
metal oxides are presulfided before use. Molybdenum is
ꢁ1 and ꢁ5 binding of thiophene have been observed
in organometallic complexes (11) and on single crystal
metal surfaces (2). Binding of thiophene to an active site
is thought to weaken the C-S bonds, thereby promoting
reaction. In organometallic complexes, the C-S bonds of
1 Current address: SABIC Americas, 16200-A Park Row, Houston,
Texas 77084.
2 Corresponding author. E-mail: ekerdt@che.utexas.edu.
226
0021-9517/98 $25.00
c
Copyright ꢀ 1998 by Academic Press
All rights of reproduction in any form reserved.

THIOPHENE HDS MECHANISM
227
ꢁ5-thiophene ligands are weaker than those of ꢁ1-thiophene
ligands and ꢁ5-thiophene ligands are more reactive than
ꢁ1-thiophene ligands. ꢁ5-thiophene is a six-electron donor,
requiring a site with three vacancies. ꢁ1-thiophene is a two-
electron donor, requiring only a singly unsaturated site.
The activity of isolated Mo(IV) (9), which is only capa-
ble of accepting two electrons (1 coordination vacancy),
indicates that HDS can be initiated by ꢁ1-adsorption of
thiophene. Sulfhydryl (-SH) groups have been shown to
be weak Brønsted acids and are thought to be a source of
hydrogen for the hydrogenation and hydrogenolysis reac-
tions (12, 13). Metal hydryl (-MH) groups have been de-
tected by neutron scattering of unsupported RuS2 and may
also be involved in hydrogenation and hydrogenolysis (10,
14). The activity of isolated Mo(IV), which is not capable of
binding both thiophene and hydrogen, indicates that metal
hydryl groups are not required for HDS. For the dispersed
catalysts, which lack sulfhydryl groups, hydrogen for hy-
drogenation and hydrogenolysis probably comes from hy-
droxyl groups on the silica surface.
Thiophene is typically used as a model reactant because
it is representative of the aromatic sulfur-containing com-
pounds that are difficult to desulfurize. Mechanisms for
thiophene HDS have been proposed, based on results from
small scale reactors (15–21), single crystal metal surfaces
(22–25), organometallic complexes (11, 26), Chevrel phases
(27), and supported metal clusters (28). These mechanisms
are summarized in Scheme 1. Kolboe studied HDS of thio-
phene, tetrahydrothiophene (THT), and 1-butanethiolover
unsupported MoS2 and CoMoS/Al2O3 (17). He proposed
that thiophene and THT are not hydrogenated and do not
share a common intermediate and that HDS occurs by
transfer of the ꢂ-hydrogen atoms to the sulfur atom with
concurrent C-S bond cleavage. Thiophene would form di-
acetylene and THT would form butadiene, which would
be hydrogenated to butene and butane (17, 29 30). Lipsch
and Schuit proposed that the C-S bonds of thiophene un-
dergo hydrogenolysis to form butadiene, which is hydro-
genated to butene and butane (16). However, direct cleav-
age of the strong aromatic C-S bonds would be difficult (26,
30, 31). In the hydrogenation mechanism, hydrogenation
SCHEME 1
60% yield; the rest decomposes to surface carbon, sulfur,
and hydrogen (25). Formation of butadiene from 2,5-DHT
has also been observed on Re/Al2O3 and Mo/Al2O3 cata-
lysts and in organometallic complexes (21, 26). THT on
Mo(110) reacts through a 1-butanethiolate intermediate to
form 1-butene and butane (23). Only 25% of the THT pro-
duced hydrocarbons; the rest decomposed to surface car-
bon, sulfur, and hydrogen.
DHT, THT, and 1-butanethiolate are proposed interme-
diates in thiophene HDS (18–26, 29, 33–35). This paper ex-
amines the degree of thiophene hydrogenation during HDS
to gain a better understanding of the reaction mechanism.
The activities and selectivities of model molybdenum cata-
lysts and silica-supported MoS2 for thiophene, THT and
1-butanethiol HDS are presented. 2,5-DHT is shown to be
the key intermediate in thiophene and THT HDS.
==
of one or both C C bonds occurs before C-S bond scission
(11, 22, 32). Removal of the aromatic stability of thiophene
by hydrogenation would make C-S bond scission much eas-
ier. Hydrogenolysis of the C-S bonds of dihydrothiophene
METHODS
==
(DHT) may compete with hydrogenation of the C C bond
All samples were supported on Cab-O-Sil HS 5 silica
(Cabot, 325 m2/g) in a quartz U-tube that permitted sample
preparation in an inert atmosphere and flow-through reac-
tions. Mo(II), isolated Mo(IV), dimeric Mo(IV), Mo(VI),
and MoS2/SiO2 were prepared asbefore (9). For MoS2/SiO2,
XRD indicates that crystallites are formed with an aver-
(32). Similar product distributions have been observed in
the HDS of thiophene, DHT, and THT, suggesting that they
are desulfurized by the same mechanism (18, 29).
Thiophene, THT, and 2,5-DHT adsorbed on Mo(110)
react by independent mechanisms (23–25). Thiophene ad-
sorbed on Mo(110) decomposes to surface carbon, sulfur,
and hydrogen with no hydrocarbon products (23, 24). 2,5-
DHT adsorbed on Mo(110) eliminates butadiene in about
˚
˚
age size of 48 A–134 A, and approximately 10% of the
molybdenum atoms on the perimeter. Atomic absorption

228
SULLIVAN AND EKERDT
spectroscopy was used to determine the molybdenum con-
Hydrogenation and isomerization of 1-butene and buta-
tent of the catalysts. Weight loadings varied from 0.5% diene were studied to determine whether these reactions
to 5.0% molybdenum. The number of moles of molybde- might occur after desulfurization of thiophene, THT, and
num used to calculate turnover frequencies (TOFs) (mol 1-butanethiol. 1-Butene in helium (5400 ppm) and butadi-
reacted/mol Mo-sec) and turnover numbers (TONs) (to- ene in helium (3900 ppm) were mixed with hydrogen to give
tal mol reacted/mol Mo) for Mo(II), dimeric Mo(IV), and flow rates of about 20 ml/min and concentrations of about
Mo(VI) catalysts is the number of moles of molybde- 500 ppm, to approximate the flow rates and concentra-
num present on the catalyst. TOFs and TONs for isolated tions present during the HDS experiments. Hydrogenation
Mo(IV) were based on the number of moles of Mo(IV) and isomerization of 1-butene and butadiene with Mo(II),
formed by photoreduction. The activity of isolated Mo(IV) dimeric Mo(IV), and MoS2/SiO2 catalysts were studied at a
was corrected for the activity of the unreduced Mo(VI). pressure of 1 atm and a temperature of 530 K. Reaction of
TOFs and TONs for MoS2/SiO2 were based on the amount butadiene with simultaneous thiophene HDS was studied
of molybdenum on the perimeter.
to determine whether hydrogenation of butadiene was pos-
XRD and XPS were performed as before (9, 36). Ex situ sible in the presence of thiophene. Five ml/min butadiene
XRD was performed following HDS to determine whether in helium was mixed with 18 ml/min hydrogen and passed
MoS2 crystallites formed during HDS. The oxidation states through the thiophene saturator. Reaction of this mixture
of representative samples of Mo(II), dimeric Mo(IV), and with MoS2/SiO2 catalysts was studied at a pressure of 1 atm
Mo(VI) were determined before and after HDS by in situ and temperatures of 530 and 660 K.
XPS. Mo(II) shows Mo 3d3/2 and 3d5/2 binding energies of
2,5-DHT was synthesized by reaction of 1,4-dichloro-2-
231.6 ꢁ 0.1 and 228.5 ꢁ 0.1 eV, respectively, and dimeric butene with sodium sulfide (38). cis-1,4-Dichloro-2-butene
Mo(IV) shows Mo 3d3/2 and 3d5/2 binding energies of (Aldrich, 95% ) in methanol (Aldrich, 99.9+% ) and sodium
235.0 ꢁ 0.2 and 231.8 ꢁ 0.1 eV, respectively (9). X-ray pho- sulfide (EM Science, 98% ) in methanol were added drop-
toelectron spectra were collected with a physical electronics wise to a stirred solution of methanol and methylsulfox-
system. Chamber pressure during measurement was on the ide (Aldrich, 99.9% ). The mixture was added to distilled
order of 10ꢂ9 Torr. Samples were transferred from the reac- water and extractions with pentane (EM Science, 98% )
tion U-tube to the XPS chamber under an inert atmosphere were performed. The combined extracts were washed with
by use of a dry box and an airtight sample transfer case. The distilled water and dried with anhydrous MgSO4 (EM Sci-
oxidation state of isolated Mo(IV) catalysts was not verified ence). Since the 2,5-DHT was only used to measure its gas
by XPS due to the presence of unreduced Mo(VI). Isolated chromatograph retention time, the 2,5-DHT was not sep-
Mo(IV) catalysts were prepared by photoreduction of iso- arated from the pentane. The successful synthesis of 2,5-
1
lated Mo(VI) samples in CO, which is known to produce DHT was verified by H NMR (ꢃ of 3.67 and 5.81 ppm)
isolated Mo(IV) (9, 36, 37).
(39). The NMR indicated a small amount of decomposition
Thiophene (Janssen, 99+% ), THT (Aldrich, 99% ), and of 2,5-DHT to 2,3-DHT (ꢃ of 2.74, 3.21, 5.59, and 6.14 ppm)
1-butanethiol (Janssen, 98+% ) were purified by several (40).
freeze/pump/thaw cycles before use. The major impurities
The effluent from the reaction U-tube was sampled
in the thiophene were methanethiol (0.1% ) and 1-buta- on-line with a Hewlett-Packard 5890 gas chromatograph
nethiol (0.04% ). The thiophene did not contain detectable equipped with a flame ionization detector. A 1-ml sample
amounts of THT or DHT. Thiophene was fed to the cata- from an automatic gas sampling valve was injected onto a
lysts by hydrogen flow at 4 to 8 ml/min through a saturator 50 m, 0.53 mm PLOT Al2O3/KCl column (Chrompak). The
containing liquid thiophene at 295 K and 1 atm, resulting detector was calibrated with a 1010 ppm CH4/1010 ppm
in 8.7% thiophene in hydrogen. Conversions for thiophene C3H8/He mixture (Matheson, certified). The relative sen-
HDS were between 1% and 7% . The major impurity in sitivity of the FID was corrected by using effective carbon
the THT was 1-butanethiol (0.21% ). The THT did not con- numbers (41). Effective carbon numbers of 3.0, 2.9, 3.0, and
tain detectable amounts of thiophene or DHT. THT was 3.5 were used for thiophene, DHT, THT, and 1-butanethiol,
fed to the catalysts by hydrogen flow at 8 to 18 ml/min respectively. A mixture of C1-C4 hydrocarbons in helium
through a saturator containing liquid THT at 295 K and (in-house) was used to calibrate retention times. Reten-
1 atm, resulting in 1.9% THT in hydrogen. Conversions for tion times for thiophene, DHTs, THT, and 1-butanethiol
THT HDS were between 1% and 6% . The only detectable were individually determined by injecting the genuine com-
impurity in the 1-butanethiol was thiophene (0.50% ). pounds.
1-Butanethiol was fed to the catalysts by hydrogen flow
at 8 to 20 ml/min through a saturator containing liquid
1-butanethiol at 1 atm and 273 K or 295 K, resulting in 1.5%
and 4.5% 1-butanethiol in hydrogen, respectively. Conver-
sions for 1-butanethiol HDS were between 4% and 24% .
RESULTS
Ex situ XRD and in situ XPS were used to determine
whether the catalysts changed oxidation state or were

THIOPHENE HDS MECHANISM
229
and 43.4 kJ/mol were determined for thiophene HDS on
MoS2/SiO2 and Mo(II), respectively. Apparent activation
energies of 174 and 132 kJ/mol were determined for THT
HDS on MoS2/SiO2 and Mo(II), respectively. THT has
higher activity than thiophene at temperatures above 515
and 580 K on MoS2/SiO2 and Mo(II), respectively. For all
catalysts, 1-butanethiol HDS is much faster than thiophene
and THT HDS.
C4Hx products of thiophene, THT, and 1-butanethiol
HDS on MoS2/SiO2, Mo(II), isolated Mo(IV), and dimeric
Mo(IV) at 530 K are shown in Figs. 2, 3, 4, and 5, re-
spectively. The temperature of 530 K was chosen for the
majority of experiments to try to minimize secondary hy-
drogenation and isomerization reactions. Product distribu-
tions at higher temperatures are similar to those at 530 K
TABLE 1
Turnover Frequencies for Thiophene, THT,
and 1-Butanethiol HDS at 530 K
Thiophene
(1/s ꢃ 106)
THT
1-Butanethiol
Catalyst
Mo(II)
Isolated Mo(IV)
Dimeric Mo(IV)a
Mo(VI)
(1/s ꢃ 106)
(1/s ꢃ 106)
56
29
27
14
4.1
5.6
0.22
460
460
480
360
280
3170
2.0
240
MoS2/SiO2
a Turnover frequency is per molybdenum atom. One dimer is counted
as two sites.
sulfided during HDS. Previous work has shown that Mo(II), but show small decreases in the amounts of butadiene
isolated Mo(IV), and dimeric Mo(IV) do not change oxi- and DHT and small increases in the amounts of butenes
dation state or become sulfided during thiophene HDS at and n-butane. 1-Butanethiol HDS produced only n-butane,
temperatures less than 800 K (9). Mo(VI) formed MoS2 1-butene, and 2-butene. The major products of thiophene
crystallites in thiophene HDS at temperatures above 650 K and THT HDS are 1-butene, n-butane, and 2-butene.
(9). XRD and XPS results (not shown) of catalysts used Butadiene, light products (methane, ethane, ethene, prop-
for THT and 1-butanethiol HDS are similar to those for ane, and propene) and cracking products (i-butene and
thiophene HDS. Electron binding energies and oxidation i-butane) were detected in thiophene and THT HDS. The
states of all catalysts measured before HDS agree with lit- C4Hx products of thiophene and THT HDS are similar.
erature values(42, 43) and have not changed followingTHT Table 2 shows the product distributions, including sulfur-
and 1-butanethiol HDS. Mo(II), isolated Mo(IV), dimeric containing compounds, for thiophene and THT HDS on
Mo(IV), and Mo(VI) do not change oxidation state or be- Mo(II) and MoS2/SiO2 at 530 K and low conversion (<2% ).
come sulfided during THT or 1-butanethiol HDS at tem- Thiophene HDS on Mo(II) and MoS2/SiO2 produces mod-
peratures less than 800 K.
erate amountsofTHT and light productsand smallamounts
Table 1 shows TOFs for thiophene, THT, and 1-buta- of 1-butanethiol, 2,5-DHT, and 2,3-DHT. 1-Butanethiol
nethiol HDS at 530 K. Figure 1 shows TOFs for thiophene was detected in the products of THT HDS on Mo(II)
and THT HDS on Mo(II) and MoS2/SiO2 at temperatures and MoS2/SiO2; however, it was present in amounts lower
from 530 to 795 K. Apparent activation energies of 48.5 than the impurity level of the THT feed. 2,5-DHT and
FIG. 1. HDS activities for thiophene on Mo(II) (ᮀ), THT on Mo(II) (1), thiophene on MoS2/SiO2 (᭿), and THT on MoS2/SiO2 (᭡).

230
SULLIVAN AND EKERDT
FIG. 2. C4Hx products of thiophene, THT, and 1-butanethiol HDS on
FIG. 4. C4Hx products of thiophene, THT, and 1-butanethiol HDS on
MoS2/SiO2 at 530 K.
isolated Mo(IV) at 530 K.
DHT and 2-butene-1-thiolate to form cis-2-butene; 2,3-
DHT and 3-butene-1-thiolate to form 1-butene and buta-
diene; or 1-butanethiolate to form 1-butene and n-butane
(Scheme 1). Formation of THT and 1-butanethiol in thio-
phene HDS on Mo(II) and MoS2/SiO2 (Table 2) agrees
with thiophene HDS on MoS2, MoS2/C, and MoS2/Al2O3
(19, 20, 26) and could suggest that thiophene HDS pro-
ceeds through THT and 1-butanethiolate intermediates.
However, the product distributions observed for thiophene
and THT HDS are not similar to those for 1-butanethiol
HDS. Butadiene, DHTs, and light products are formed in
thiophene and THT HDS, but not in 1-butanethiol HDS.
Furthermore, THT cannot be a major intermediate in thio-
phene HDS since the activation energy for THT HDS is
higher than that for thiophene HDS. Activation energies
for thiophene and THT HDS on Mo(II) and MoS2/SiO2 are
comparable to those with MoS2/Al2O3 and CoMoS/Al2O3
catalysts (20, 26, 29, 34, 35). If THT was a major interme-
diate in thiophene HDS, the activation energy for thio-
phene HDS would equal or exceed that for THT HDS.
The large amount of THT formed in thiophene HDS indi-
cates that complete hydrogenation of the thiophene ring is
2,3-DHT were not detected in THT HDS on MoS2/SiO2.
Thiophene was not detected in THT HDS on Mo(II) or
MoS2/SiO2.
Product distributions for the reaction of 1-butene in hy-
drogen and butadiene in hydrogen with Mo(II), dimeric
Mo(IV), and MoS2/SiO2 catalysts are shown in Tables 3
and 4, respectively. Hydrogenation of butadiene is nearly
complete. Hydrogenation and isomerization of 1-butene
are rapid. The product distribution of thiophene HDS
with added butadiene for MoS2/SiO2 at 660 K is shown in
Table 5. Also shown in Table 5 are the product distribution
for thiophene HDS on MoS2/SiO2 at 660 K and the calcu-
lated product distribution for thiophene HDS with added
butadiene if no butadiene reacted. Similar results were ob-
tained for thiophene HDS with butadiene on MoS2/SiO2 at
530 K. The products of thiophene HDS with butadiene are
similar to those for thiophene HDS only.
DISCUSSION
HDS of thiophene and THT may occur through many
possible intermediates: 2,5-DHT to form butadiene; 2,5-
FIG. 3. C4Hx products of thiophene, THT, and 1-butanethiol HDS on
FIG. 5. C4Hx products of thiophene, THT, and 1-butanethiol HDS on
Mo(II) at 530 K.
dimeric Mo(IV) at 530 K.

THIOPHENE HDS MECHANISM
231
24, 27). The presence of trans-2-butene indicates that
isomerization is occurring since trans-2-butene cannot be a
direct product of thiophene, DHT, THT, or 1-butanethiol
HDS. We have shown that hydrogenation of butadiene
is rapid and gives product distributions similar to those
for thiophene and THT HDS. We have also shown that
butadiene hydrogenation is rapid in the presence of
thiophene. These results suggest that butadiene is the
initial product of thiophene and THT HDS and that it
undergoes subsequent hydrogenation and isomerization
reactions. Butadiene is probably formed by elimination
of sulfur from 2,5-DHT (11, 21, 25, 26). We suggest that
the major path for thiophene HDS is hydrogenation to
2,5-DHT and elimination of sulfur to produce butadiene.
The major path for THT HDS is dehydrogenation to
2,5-DHT and elimination of sulfur to produce butadiene.
The observed mixture of C4Hx products is formed by
subsequent hydrogenation and isomerization reactions.
We can infer that the activation energy for THT reaction to
1-butanethiolate is significantly higher than that for THT
dehydrogenation to DHT and butadiene elimination.
The cis-2-butene to trans-2-butene ratio of roughly 1 to
1 indicates that isomerization occurs by a carbenium ion
intermediate on a Brønsted acid site. Sulfhydryl groups
on MoS2 (12) and hydroxyl groups on SiO2 (44) are weak
Brønsted acids and may be sites for 1-butene isomerization
on MoS2/SiO2 and the dispersed catalysts, respectively. The
slight excess of cis-2-butene suggests that a small amount is
a primary desulfurization product. 2,5-DHT may undergo
hydrogenolysis to 2-butene-1-thiolate to yield cis-2-butene.
Similarly, a small amount of 2,3-DHT may undergo hy-
drogenolysis to 3-butene-1-thiolate to yield 1-butene and
butadiene. The difference in the amount of cis-2-butene
and trans-2-butene in thiophene and THT HDS is small,
so most of the 2-butene is produced by isomerization of
1-butene. Therefore, the butenethiolates are only minor in-
termediates in thiophene and THT HDS.
TABLE 2
Product Distributions (mol%) for Thiophene and THT HDS on
Mo(II) and MoS2/SiO2 at 530 K
Thiophene
reactant
Tetrahydrothiophene
reactant
Products
Mo(II)
MoS2/SiO2
Mo(II)
MoS2/SiO2
2,3-Dihydrothiophene
2,5-Dihydrothiophene
Tetrahydrothiophene
1-Butanethiol
Butadiene
0.04
0.29
0.22
0.28
23
0.23
0.40
—
0b
0a
0a
—
0b
25
1.2
0.13
0.77
61
0.21
0.31
78
0.13
81
Butene
Butane
59
8.1
6.6
8.0
6.4
15
16
C1-C3
5.6
2.0
a Not detected. Detection limit is approximately 0.02% .
b Present but at a concentration lower than the impurity level of the
feed.
energetically feasible. Formation of 1-butanethiol indicates
that hydrogenolysis of THT is possible, but the activation
energies and the small amount formed indicate that it is not
a major path for HDS. Evidently, the activation energy for
hydrogenolysis of THT is larger than that for dehydrogena-
tion of THT.
The similarity of product distributions for thiophene and
THT HDS suggests that they react by a common path.
Based on our results and the literature (11, 21, 25, 26),
2,5-DHT is probably the common intermediate in thio-
phene and THT HDS. Formation of DHTs in thiophene
and THT HDS on Mo(II) and MoS2/SiO2 is shown by us,
in agreement with the formation of DHTs in thiophene
HDS on MoS2/C and CoMoS/C (20). DHTs have not been
observed in thiophene or THT HDS on MoS2/Al2O3 or
CoMoS/Al2O3, but they must be formed in the intercon-
version of thiophene and THT (18, 26). The absence of
DHTs in thiophene and THT HDS on MoS2/Al2O3 and
CoMoS/Al2O3 is probably due to their high activities at the
temperatures typically used (20, 26). Our experiments are
at lower temperatures than others and allow us to observe
DHTs in thiophene and THT HDS.
Unlike thiophene and THT, 1-butanethiol does not show
a strong correlation between activity and the oxidation
state. 1-Butanethiol is much more reactive than THT. The
only products of 1-butanethiol HDS are 1-butene, cis-2-
butene, trans-2-butene, and n-butane. Thiophene, THT,
DHTs, butadiene, light products, and cracking products are
It has been shown that 1-butene is present in thiophene
and THT HDS in amounts in excess of equilibrium (9, 18,
TABLE 3
Product Distributions (mol%) for Hydrogenation of 1-Butene at 530 K
Products (mol% )
Catalyst
Mo(II)
Dimeric Mo(IV)
MoS2/SiO2
C1-C3
i-Butane
N-Butane
1-Butene
c-2-Butene
t-2-Butene
i-Butene
0.87
0.02
0.55
0
0
0.21
58
11
28
16
42
39
12
30
20
10
17
12
0
0
0

232
SULLIVAN AND EKERDT
path for HDS is the elimination of sulfur from 2,5-DHT to
TABLE 4
produce butadiene. 1-Butene, 2-butene, and n-butane are
formed by hydrogenation and isomerization. Some reac-
tion of thiophene and THT through thiolate intermediates
does occur, but these pathways are minor.
Product Distributions (mol%) for Hydrogenation
of Butadiene at 530 K
Products (mol% )
Catalyst
C1-C3
Butane
Butene
Butadiene
ACKNOWLEDGMENTS
Mo(II)
Dimeric Mo(IV)
MoS2/SiO2
0.93
1.4
0.62
44
51
18
53
48
72
2.4
0.02
9.7
This work was supported by grants from the U.S. Department of
Energy, Office of Basic Energy Sciences (Grant DE-FG03-95ER14570)
and the Exxon Foundation. We gratefully acknowledge the assistance of
Sandra Whaley and Dr. Deborah Hess of the Department of Chemistry
and Biochemistry for assistance with the XPS.
not formed in 1-butanethiol HDS. These results agree with
those on CoMoS/Al2O3 (23) and suggest that 1-butanethiol
HDS proceeds by a different mechanism than thiophene
and THT HDS. 1-Butene and n-butane are probably the
initial products with 2-butene formed by isomerization of
1-butene (23).
REFERENCES
1. Grange, P., Catal. Rev. Sci. Eng. 21, 135 (1980).
2. Prins, R., de Beer, V. H. J., and Somorjai, G. A., Catal. Rev. Sci. Eng.
31, 1 (1989).
Isolated Mo(II), which is tetrahedrally coordinated with
two Mo-O bonds and two vacancies, is an 8-electron cen-
ter. Isolated Mo(IV), which has four Mo-O bonds and one
vacancy, is a 10-electron center. Thiophene HDS activities
per coordination vacancy are similar for the Mo(II) and
Mo(IV) catalysts (Table 1), suggesting that oxidation state
may not be the dominant factor in determining the activ-
ity of a site. Rather, the number of coordination vacancies
may control the activity of a site. This would imply that each
atom of the Mo(IV) dimers is an active site and that the two
vacancies of Mo(II) act as two active sites. Unfortunately
lower oxidation state and increased coordination vacancy
are coupled and cannot be studied independently. The THT
HDS activities per vacancy for the Mo(II) and Mo(IV) cata-
lysts are close but not similar, which could suggest that the
identical thiophene HDS activities per vacancy are merely
coincidental. We believe that the higher activity of Mo(II),
compared to Mo(IV), is due to a stronger interaction of
thiophene with more reduced molybdenum atoms, thereby
increasing the rate of hydrogenation of thiophene. It ap-
pears that the oxidation state does not affect the reaction
mechanism, merely the rate of reaction.
3. Ratnasamy, P., and Sivasanker, S., Catal. Rev. Sci. Eng. 22, 401
(1980).
4. Topsøe, H., and Clausen, B. S., Catal. Rev. Sci. Eng. 26, 395 (1984).
5. Chianelli, R. R., Catal. Rev. Sci. Eng. 26, 361 (1984).
6. Topsøe, N.-Y., and Topsøe, H., J. Catal. 139, 631 (1993).
7. Daage, M., and Chianelli, R. R., J. Catal. 149, 414 (1994).
8. Drew, M. G. B., Mitchell, P. C. H., and Kasztelan, S., J. Chem. Soc.
Farad. Trans. 86, 697 (1990).
9. Sullivan, D. L., and Ekerdt, J. G., J. Catal. 172, 64 (1997).
10. Kasztelan, S., Langmuir 6, 590 (1990).
11. Angelici, R. J., Bull. Soc. Chim. Belg. 104, 265 (1995).
12. Topsøe, N.-Y., and Topsøe, H., J. Catal. 119, 252 (1989).
13. Topsøe, N.-Y., and Topsøe, H., J. Catal. 139, 641 (1993).
14. Jobic, H., Clugnet, G., Lacroix, M., Yuan, S., Mirodatos, C., and
Breysse, M., J. Am. Chem. Soc. 115, 3654 (1993).
15. Mitchell, P. C. H., Catalysis (London) 4, 175 (1981).
16. Lipsch, J. M. J. G., and Schuit, G. C. A., J. Catal. 15, 179 (1969).
17. Kolboe, S., Can. J. Chem. 47, 352 (1969).
18. Desikan, P., and Amberg, C. H., Can. J. Chem. 42, 843 (1964).
19. Kieran, P., and Kemball, C., J. Catal. 4, 394 (1965).
20. Hensen, E. J. M., Vissenberg, M. J., de Beer, V. H. J., van Veen, J. A.
R., and van Santen, R. A., J. Catal. 163, 429 (1996).
21. Sauer, N. N., and Angelici, Inorg. Chem. 26, 2160 (1987).
22. Weigand, B. C., and Friend, C. M., Chem. Rev. 92, 491 (1992).
23. Roberts, J. T., and Friend, C. M., J. Am. Chem. Soc. 108, 7204
(1986).
24. Zaera, F., Kollin, E. B., and Gland, J. L. Surf. Sci. 184, 75 (1987).
25. Liu, A. C., and Friend, C. M., J. Am. Chem. Soc. 113, 820 (1991).
26. Markel, E. J., Schrader, G. L., Sauer, N. N., and Angelici, R. J., J. Catal.
116, 11 (1989).
In summary, the activation energies and product distribu-
tions for thiophene and THT HDS suggest that the major
TABLE 5
27. Ekman, M. E., Anderegg, J. W., and Schrader, G. L., J. Catal. 117, 246
(1989).
28. Neurock, M., and van Santen, R. A., J. Am. Chem. Soc. 116, 4427
(1994).
Product Distributions (mol%) for Thiophene and Butadiene
Reaction on MoS2/SiO2 at 660 K
Products (mol% )
29. Hargreaves, A. E., and Ross, J. R. H., J. Catal. 56, 363 (1979).
30. Kraus, J., and Zdrazil, M., React. Kinet. Catal. Lett. 6, 475 (1977).
31. Zdrazil, M., Collect. Czech. Commun. 40, 3491 (1975).
32. Schulz, H., Schon, M., and Rahman, N., in “Catalytic Hydrogenation”
(L. Cerveny, Ed.), p. 201. Elsevier, Amsterdam, 1986.
33. “Sulphide Catalysts, Their Properties and Applications” (O. Weisser,
and S. Landa, Eds.), p. 181. Pergamon, New York, 1963.
34. Carvill, B. T., and Thompson, L. T., Appl. Catal. 75, 249 (1991).
35. Owens, R. J., and Amberg, C. H., Can. J. Chem. 40, 947 (1962).
C1-C3
Butane
Butene
Butadiene
Thiophene HDS
3.3
2.3
14.2
14.3
81.9
80.9
0.6
1.5
Thiophene HDS with
butadiene reaction
Thiophene HDS with
butadiene (as diluent)
2.1
9.2
54.2
35.8

THIOPHENE HDS MECHANISM
233
36. Sullivan, D. L., Ph.D. dissertation, The University of Texas at Austin, 41. Sternberg, J. C., Gallaway, W. S., and Jones, D. T. L., in “Gas Chro-
1998.
matography” (N. Brenner, J. E. Callen, and M. D. Weiss, Eds.), p. 231.
Academic Press, New York, 1962.
37. Williams, C. C., and Ekerdt, J. G., J. Phys. Chem. 97, 6843 (1993).
38. Everhardus, R. H., Grafing, R., and Brandsma, L., J. Royal Neth. 42. Aigler, J. M., Brito, J. L., Leach, P. A., Houalla, M., Proctor, A., Cooper,
Chem. Soc. 95, 153 (1976). N. J., Hall, W. K., and Hercules, D. M., J. Phys. Chem. 97, 5699 (1993).
39. Korver, P. K., van der Haak, P. J., Steinberg, H., and de Boer, Th. J., 43. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy,
Recl. Trav. Chim. 84, 129 (1965). Perkin-Elmer Corp., Eden Prairie, MN, 1992.
40. Sauer, N. N., Angelici, R. J., Huang, Y. C. J., and Trahanovsky, W. S., 44. Ramani, N. C., Sullivan, D. L., and Ekerdt, J. G., J. Catal. 173, 105
J. Org. Chem. 51, 113 (1986). (1998).