Characterization and HDS activity of mesoporous MoS2 catalysts prepared by in situ activation of tetraalkylammonium thiomolybdates

Article abstract of DOI:10.1006/jcat.2002.3553

Unsupported molybdenum disulfide catalysts with unique morphological pore structure were synthesized from tetraalkyammonium thiometalates precursors by in situ activation during the hydrodesulfurization (HDS) of dibenzothiophene. The precursors used in this study are ammonium thiomolybdate (ATM), tetrapropylammonium thiomolybdate, tetrapentylammonium thiomolybdate, tetrahexylammonium thiomolybdate, tetraheptylammonium thiomolybdate (THepATM), and tetraoctylammonium thiomolybdate. The thermogravimetrical analysis carried out under nitrogen demonstrated that all precursors yield MoS₂ bulk structure at 623 K, after one or two decomposition steps. The morphology of these catalysts observed by scanning electron microscopy presents large hemispherical or ovoid cavities with a cheese-like porous arrangement, high surface area (from 255 up to 329 m²/g), high content of carbon (C/Mo = 2.7-4.0), and type IV adsorption-desorption isotherms of nitrogen. The nature of the alkyl group affects the surface area, the pore size distribution, and the HDS selectivity. The highest selectivity for direct C-S bond cleavage is observed for the molybdenum disulfide catalyst formed from THepATM. X-ray diffraction studies showed that the catalysts are poorly crystallized, with a very weak intensity of the (002) line of 2H-MoS₂ characteristic of exfoliated samples.

Full text of DOI:10.1006/jcat.2002.3553

Journal of Catalysis 208, 359–369 (2002)
doi:10.1006/jcat.2002.3553
Characterization and HDS Activity of Mesoporous
MoS Catalysts Prepared by in Situ Activation
2
of Tetraalkylammonium Thiomolybdates
∗
, ,1
∗
∗
∗
G. Alonso, † G. Berhault,‡ A. Aguilar, V. Collins, C. Ornelas, S. Fuentes,§ and R. R. Chianelli†
∗
Departamento de Cat a´ lisis, Centro de Investigaci o´ n en Materiales Avanzados, Chihuahua, C.P. 31109, Chih., M e´ xico; †University of Texas at El Paso,
Materials Research Technology Institute, 500 W. University Ave., El Paso, Texas 79968-0685; ‡Laboratoire de Catalyse en Chimie Organique—UMR
6
503 CNRS, Universit e´ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers cedex, France; and §Departamento de Cat a´ lisis, Centro
de Ciencias de la Materia Condensada (CCMC)—UNAM, Apartado postal 2681, 22830, Ensenada, B.C., M e´ xico
Received October 22, 2001; revised January 30, 2002; accepted January 30, 2002
widely used in the petroleum industry as hydrodesulfuriza-
Unsupported molybdenum disulfide catalysts with unique tion (HDS) catalysts (1, 2).
morphological pore structure were synthesized from tetraalky-
lammonium thiometalates precursors by in situ activation dur-
ing the hydrodesulfurization (HDS) of dibenzothiophene. The
precursors used in this study are ammonium thiomolybdate
Unsupported HDS catalysts have been prepared by dif-
ferent methods, including comaceration (3), homogeneous
sulfide precipitation (4), and thiosalt decomposition (5). In
these preparation methods, catalysts need to be activated
under a H2/H2S mixture before being active in catalytic re-
actions. The catalytic properties of MoS2 and WS2 obtained
by these methods are reported to depend strongly on the
reacting atmosphere, as well as on the heating conditions
(
ATM), tetrapropylammonium thiomolybdate, tetrapentylammo-
nium thiomolybdate, tetrahexylammonium thiomolybdate, tetra-
heptylammonium thiomolybdate (THepATM), and tetraoctylam-
monium thiomolybdate. The thermogravimetrical analysis carried
out under nitrogen demonstrated that all precursors yield MoS2
bulk structure at 623 K, after one or two decomposition steps.
(
3–5). Large variations in surface area have been observed
The morphology of these catalysts observed by scanning electron for MoS2 and WS2 catalysts, from a few to several tens of
microscopy presents large hemispherical or ovoid cavities with a square meters per gram, depending on the decomposition
cheese-like porous arrangement, high surface area (from 255 up to conditions (6, 7).
2
3
29 m /g), high content of carbon (C/Mo = 2.7–4.0), and type IV
Thiosalt decomposition is an interesting alternative
preparation since it provides a simple and reproducible
method for obtaining MoS2 catalysts with controlled sto-
ichiometry. Moreover, these thiosalts have already sulfur
bound to the metal atoms in a tetrahedral coordination,
and their decomposition has been reported to undergo a
topotactic reaction, whereby the c-axis of sulfide remains
the same as in the precursor (8). Some patents have re-
adsorption–desorption isotherms of nitrogen. The nature of the
alkyl group affects the surface area, the pore size distribution,
and the HDS selectivity. The highest selectivity for direct C–S
bond cleavage is observed for the molybdenum disulfide cata-
lyst formed from THepATM. X-ray diffraction studies showed
that the catalysts are poorly crystallized, with a very weak in-
tensity of the (002) line of 2H-MoS2 characteristic of exfoliated
samples.
ꢀc 2002 Elsevier Science (USA)
Key Words: MoS2; thiosalts; in situ activation; mesoporous; hy- ported the use of tetraalkylammonium thiometalates to
drodesulfurization.
generate carbon-containing MoS2 and WS2 catalysts with
high surface area and with improved activity (9–12). How-
ever, a detailed explanation about their structure and mor-
phology has not been proposed. Such catalysts prepared
from tetraalkylammonium thiomolybdates contain certain
amounts of carbon and are described with the general for-
mula MoS2−yCz, where 0.01 < y < 0.5 and 0.01 < z < 3.0
INTRODUCTION
Transition metal sulfides form a very important group
of materials exhibiting a number of interesting properties.
One particular chemical property of several of these sul-
fides is the ability, in the presence of hydrogen, to catalyze
sulfur removal from heterocyclic organic molecules such as
thiophene, benzothiophene, and dibenzothiophene (1). For
this reason, molybdenum disulfide-based solids have been
(
12). These catalysts exhibit improved activity for the HDS
of dibenzothiophene (DBT) (13).
The method of in situ decomposition of ammonium
thiometalates containing one or two metals (9–11) in a
hydrocarbon solution with sulfur organic compounds and
pressurized with hydrogen at 623 K yields active metal
sulfide. The role of carbon in the formulation of these HDS
catalysts is not completely understood, but recent studies
1
To whom correspondence should be addressed. Fax: +52(1)4931112.
E-mail: gnunez@utep.edu.
359
0
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.
ꢀ

3
60
ALONSO ET AL.
have shown that carbon, at least partly, is included in the with the reagents (5 vol% of DBT in decaline). The catalyst
arrangement of active sites, probably in the form of surface precursors were prepared in tablet form by uniaxial press-
carbides (14, 15). We have termed these catalysts sulfide- ingofthethiosaltsandwerecrushedinamortarbeforeplac-
supported carbides to reflect the catalytic stable state (16). ing them inside the reactor. The reactor was then pressur-
Previous studies on catalytic systems based on the in situ ized to 3.1 MPa with hydrogen and heated to 623 K at a rate
decomposition of tetraalkylammonium thiotungstates and of 10 K/min. After the working temperature was reached,
thiomolybdates have clearly evidenced the formation of sampling for chromatographic analysis was performed dur-
sulfur-deficient materials with excess sulfur replaced by car- ing the course of each run to determine conversion ver-
bon at the catalyst surface (14, 17). Moreover, molybdenum sus time dependence. The reaction run time averaged 5 h.
carbide catalysts are highly active for hydroprocessing re- The resulting products were analyzed using a Perkin–Elmer
actions (18–20). Indeed, Sajkowski and Oyama have shown Auto-system gas chromatograph with a 6-ft long, 1/8 in.
that Mo2C/Al2O3 was three times more active in HDS than packed column containing OV-3 (phenyl methyl dimethyl
MoS2/Al2O3 catalyst (20).
silicone, 10% phenyl) as a separating phase.
The aim of this work was to obtain MoS2 catalysts with
The main reaction products from the HDS of DBT are
a unique amorphous morphology from tetraalkylammo- biphenyl (BP) and phenylcyclohexane (PCH). Selectivity
nium precursors decomposed in situ during the HDS of for the main reaction products (BP, PCH) was determined
DBT. These as-formed MoS2-based catalysts were charac- for MoS2 catalysts prepared for each precursor and was cal-
terized using X-ray diffraction (XRD), energy-dispersive culated for a given product as the weight percentage of the
X-ray analysis (EDX), scanning electron microscopy, and product in the product mixture. The mean standard devia-
nitrogen adsorption at 77 K using the BET isotherm for sur- tion for catalytic measurements was about 2.5%.
face area measurement and the BJH method for pore size
Catalyst Characterization
distribution. Catalytic activity and selectivity in the HDS of
DBT was also reported.
Differential thermal analysis (DTA) and thermogravi-
metric analysis (TGA) of the thiosalt precursors were made
simultaneously on a TA Instrument SDT 2960 DTA–TGA
equipment under dry nitrogen flow from 293–1073 K at a
heating rate of 10 K/min.
EXPERIMENTAL
Precursor Preparation
The syntheses of the tetraalkylammonium salts
Characterization of catalysts was performed on samples
(
R4N)2MoS4 have been reported by McDonald et al. (21). obtained after the catalytic tests. The samples were sepa-
In the present work, an improved version of this method rated from the reaction mixture by filtration, then washed
was used. The precursors were synthesized at room tem- with isopropanol to remove residual hydrocarbons and
perature using an aqueous solution of ammonium tetra- dried at room temperature.
thiomolybdate (ATM) [(NH4)2MoS4] and quaternary am-
A JEOL JSM5800 LV scanning electron microscope was
monium halides [(R4N)Br, where R = propyl, pentyl, used to perform morphological and elemental analysis. Sev-
hexyl, heptyl, and octyl] with yields approximately 80%. eralfieldswereanalyzedatdifferentmagnificationsinorder
The precursor synthesis is reported elsewhere (22). The to aid in the recognition of the prevalent features.
tetraalkylammonium tetrathiomolybdate salts are referred
Specific surface area determination was made with a
as TProATM, TPenATM, THexATM, THepATM, and Quantachrome AUTOSORB-1 model by nitrogen adsorp-
TOctATM when the following respective alkyl groups are tion at 77 K using the BET isotherm. The pore distribu-
used: propyl, pentyl, hexyl, heptyl, and octyl.
tion was obtained from the desorption isotherm following
the BJH method. Samples were degassed under flowing
argon at 473 K for 2 h before nitrogen adsorption. The
mean standard deviation for surface area measurements
was about 2%.
Catalytic Activity and Selectivity
The HDS of DBT has been extensively studied as a model
reaction of hydrodesulfurization of petroleum feedstock
XRD studies were performed using a Phillips X Pert
MPD diffractometer, equipped with a curved graphite mo-
nochromator, using CuKα radiation and wavelength λ (λ =
(
2). Laboratory-scale studies have been performed in pres-
surized flow (23) and batch reactors (9–13, 17, 20). By using
batchreactors, usefulinformationsuchasrateconstantsand
selectivity can be obtained by following the composition of
the reaction mixture as a function of time. The HDS of DBT
was then carried out in a Parr model 4522 high-pressure
batch reactor. The appropriate amount of precursor to yield
˚
1
.54056 A) operating at 43 kV and 30 mA.
RESULTS
Thermal Analysis
1
5
g of MoS2 catalyst (1.6 g for ATM, 3.73 g for TProATM,
.11gforTPenATM, 5.82gforTHexATM, 6.52gforTHep-
The data obtained from the DTA–TGA studies of dif-
ATM, and 7.7 g for TOctATM) was placed in the reactor ferent precursors are summarized in Tables 1–5. DTA

IN SITU ACTIVATED MESOPOROUS MoS2 CATALYSTS
361
1
00
4
100
90
80
70
60
50
40
30
20
10
0
2
TProATM
TPenATM
a)
b)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
3
1
2
0
1
-1
-2
-3
0
H O
2
-1
3
00
400
500
600
700
800
300
400
500
600
700
800
Temperature (K)
Temperature (K)
1
00
2
100
90
80
70
60
50
40
30
20
10
0
2
1
0
c)
THexATM
d)
THepATM
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1
0
H O
2
-1
-1
3
00
400
500
600
700
800
3
00
400
500
600
700
800
Temperature (K)
Temperature (K)
100
4
e)
TOctATM
90
80
70
60
50
40
30
20
10
0
2
0
H O
2
-2
300
400
500
600
700
800
Temperature (K)
FIG. 1. DTA–TGA curves of the decomposition of the different tetraalkylammonium precursors.
curves corresponding to the decomposition of the catalyst some amount of water may be expected to contaminate the
precursors [from TProATM (Fig. 1a) to TOctATM thiosalts. Such an early run weight loss has been ascribed
(
Fig. 1e)] are reported in Fig. 1. Small weight losses (2% to the dehydration of the starting material (24, 25).
for TProATM, 6% for TOctATM, and less than 1% for Two steps of weight loss are distinguishable from the ther-
THepATM) were observed at low temperatures, around mal decomposition of the TProATM and TPenATM pre-
50 K, with endothermic peaks that may be attributed cursorswithacomplicateddecompositionpatterninvolving
3
to the elimination of impurities. Indeed, since these ma- intramolecular rearrangements and the interaction with
terials were crystallized from a saturated aqueous solution, neighboring units.

3
62
ALONSO ET AL.
TABLE 1
TABLE 2
DTA–TGA Results for the TProATM Precursor Decomposed
DTA–TGA Results for the TPenATM Precursor Decomposed
under Nitrogen Atmosphere
under Nitrogen Atmosphere
Determinant
TProATM
Determinant
TPenATM
T−1 (K)
360
2.0
440
493
T1 (K)
T2 (K)
320
403
21.6
21.3
ꢀ
w−1, wt% (exp), assuming water
T1 (K)
T2 (K)
ꢀw1, wt% (exp)
ꢀw1, wt% (theor), assuming loss
ꢀw1, wt% (exp)
38.0
[CH3(CH2)4]2S
ꢀw1, wt% (theor), assuming loss
38.8
T3 (K)
623
[
CH3(CH2)2]3N + CH3(CH2)2N=S
ꢀw2, wt% (exp)
ꢀw2, wt% (theor), assuming loss
60.4
59.5
T3 (K)
623
ꢀ
w2, wt% (exp)
w2, wt% (theor), assuming loss
34.0
34.2
[CH3(CH2)4]6N2S
ꢀ
ꢁꢀw, wt% (exp)
Residual, wt% (exp)
Residual, wt% (theor), assuming residual as MoS2
82.0
18.4
19.2
[
CH3(CH2)2]2S + CH3(CH2)4CH3
ꢁ
ꢀw, wt% (exp)
72.8
27.2
27.0
Residual, wt% (exp)
Residual, wt% (theor), assuming
residual as MoS2
Note. ꢀw1 and ꢀw2 are experimental and theoretical weight losses of
sample during the first step of thermal decomposition at the T1–T2 tempe-
rature range and during the second step at the T2–T3 temperature range,
respectively.
Note. ꢀw−1 is the experimental weight loss of water observed at temper-
ature T−1, ꢀw1, and ꢀw2 are experimental and theoretical weight losses
of sample during the first step of thermal decomposition at the T1–T2 tem-
perature range and during the second step at the T2–T3 temperature range, DTA analysis in Fig. 1b shows four endothermic peaks dur-
respectively.
ing the decomposition reactions. The final product of the
thermal decomposition corresponds to nearly stoichiomet-
ric MoS2.
The DTA–TGA results for THexATM and THepATM
are reported in Tables 3 and 4, respectively. Contrary to
TProATM and TPenATM precursors, the thermal decom-
position of THexATM occurs in a single step from 423 to
According to Table 1, the transitions observed for
TProATM are consistent with the following series of re-
actions:
[
(CH3CH2CH2)4N]2MoS4
[CH3(CH2)2]3N + CH3(CH2)2NS
[CH3(CH2)2]4MoS3,
CH3CH2CH2)4MoS3
(CH3CH2CH2)2S + CH3(CH2)4CH3 + MoS2. [2]
623 K. It is suggested that during the transition step, two
→
molecules of trihexylamine, and one molecule of dihexyl-
[1] disulfide, are directly removed, as shown in the following
decomposition reaction:
+
(
→
{
[(CH3(CH2)5]4N}2MoS4
→
2[(CH3(CH2)5]3N + [CH3(CH2)5]2S2 + MoS2. [5]
The first step (440–493 K) is proposed to involve elimina-
tion of tripropylamine and propylamine sulfide. The second
step, at 493–623 K, suggests the elimination of dipropylsul-
fide and hexane. As shown in Fig. 1a, both steps of decom-
position are characterized by an endothermic peak, at 450
and 590 K, respectively.
As shown in Table 4, for THepATM, the transition steps
TABLE 3
DTA–TGA Results for the THexATM Precursor Decomposed
For TPenATM, the transitions are consistent with reac-
tions [3] and [4] (cf. Table 2):
under Nitrogen Atmosphere
Determinant
THexATM
{
[CH3(CH2)4]4N}2MoS4
[CH3(CH2)4]2S + {[CH3(CH2)4]6N2}MoS3, [3]
[CH3(CH2)4]6N2}MoS3
2[CH3(CH2)4]3N + S + MoS2.
T1 (K)
T2 (K)
423
623
79.1
82.8
→
ꢀw, wt% (exp)
{
ꢀw, wt% (theor), assuming loss
→
[4]
2[CH (CH ) ] N + [CH (CH ) ] S
3
2
5
3
3
2 5 2 2
Residual, wt% (exp)
Residual, wt% (theor), assuming
residual as MoS2
20.9
17.2
Reaction [3] occurs in the 320- to 403-K temperature range
and it is proposed to involve elimination of dipentylsulfide.
Reaction [4] suggests the elimination of two molecules of
Note. ꢀw is the experimental and theoretical weight losses of sample
tripentylamine plus sulfur from 403 to 623 K. The TGA– during the thermal decomposition at the T1–T2 temperature range.

IN SITU ACTIVATED MESOPOROUS MoS2 CATALYSTS
363
TABLE 4
molecule of dioctyldisulfide:
DTA–TGA Results for the THepATM Precursor Decomposed
under Nitrogen Atmosphere
{[CH (CH ) ] N} MoS
3
2 7 4
2
4
→
2[(CH3(CH2)7]3N + [CH3(CH2)7]2S2 + MoS2. [7]
Determinant
THepATM
T1 (K)
T2 (K)
423
573
83.0
83.2
Figure 1e shows that the TOctATM DTA curve presents
some undefined endothermic peaks during the decomposi-
tion reaction occurring from 400 to 630 K. As observed be-
ꢀw, wt% (exp)
ꢀw, wt% (theor), assuming loss
2
[CH3(CH2)6]3N + [CH3(CH2)6]2S2
2−
fore with the THexATM and THepATM salts, the S ions
Residual, wt% (exp)
Residual, wt% (theor), assuming
residual as MoS2
17.0
16.8
2
4
−
of the MS unit would react with the octyl groups to give
disulfide compounds.
Note. ꢀw is the experimental and theoretical weight losses of sample
during the thermal decomposition at the T1–T2 temperature range.
Elemental Analysis
The S/Mo and C/Mo atomic ratios were determined us-
ing EDX analysis and are reported in Table 6. A crystalline
MoS2 flake was used for calibration of Mo and S signals.
The S/Mo ratio for all catalysts stays constant to a stoichio-
metric value of 2.0. EDX analysis reveals high C/Mo ratios
are consistent with the reaction
{
[CH3(CH2)6]4N}2MoS4
→
2[CH3(CH2)6]3N + [CH3(CH2)6]2S2 + MoS2. [6] (2.7–4.5) for the catalysts activated through the in situ de-
composition of the tetraalkylammonium salts during the
This reaction occurs from 423 to 573 K in a single step. HDS of DBT whereas a low C/Mo ratio is found for the
Two molecules of triheptylamine and one molecule of di- ATM-derived catalyst (0.5). Even if the amount of carbon
heptyldisulfide are directly removed. In both cases, the varies strongly with the type of precursor, no clear relation-
2
−
hexyl or heptyl groups would react with the S ions of ship could be observed between the size of the alkyl group
2
4
−
the MoS unit to give disulfide compounds. Figures 1c and in the thiosalt precursor and the final amount of carbon.
1
d report the TGA–DTA analysis of THexATM and THep- However, it appears that C/Mo ratios slightly increase with
ATM precursors, respectively. In both cases the reactions of the number of C atoms initially present in the precursor,
decomposition are endothermic. However, the THexATM with the noticeable exception of TPenATM. Indeed, the
DTA curve occurring from 400 to 600 K appears more com- MoS catalyst formed from TProATM has a lower C/Mo
2
plicated, while for THepATM, a quite important wide peak ratio than the catalyst derived from TOctATM. They have
is observed at 500 K.
a stoichiometry, respectively, of MoS2.0C3.5 and MoS2.0C4.5.
Finally, according to Table 5, the thermal decomposition The TPenATM salt gives rise to a final stoichiometry of
of TOctATM also occurs in a single step from 423 to 623 K MoS2.0C2.7. The high amount of residual carbon may be
with the formation of two molecules of octylamine and one present in the structure as well as on the surface of the cata-
lysts. Moreover, no relationship could be clearly observed
when comparing carbon content and surface area.
TABLE 5
DTA–TGA Results for the TOctATM Precursor Decomposed
under Nitrogen Atmosphere
TABLE 6
Determinant
TOctATM
Specific Surface Areas, Total Pore Volume, and Elemental Analy-
sis of Mo, S, and C Atomic Ratios for in Situ Prepared Molybdenum
Sulfide Catalysts
T−1 (K)
w−1, wt% (exp), assuming water
383
6.0
423
623
86.5
86.2
ꢀ
T1 (K)
T2 (K)
Elemental
analysis
ꢀ
w1, wt% (exp)
w1, wt% (theor), assuming loss
MoS2 catalysts
from
Specific surface
area (m /g)
Total pore
volume (cm /g)
ꢀ
2
3
S/Mo
C/Mo
2
[CH3(CH2)7]3N + [CH3(CH2)7]2S2
Residual, wt% (exp)
Residual, wt% (theor), assuming
residual as MoS2
13.5
13.8
ATM
60
61
329
255
267
278
0.092
0.175
0.247
0.429
0.472
0.546
1.7
2.0
2.0
2.0
2.0
2.0
0.5
3.5
2.7
3.7
4.0
4.5
TProATM
TPenATM
THexATM
THepATM
TOctATM
Note. ꢀw−1 is the experimental weight loss of water observed at temper-
ature T−1. ꢀw1 is the experimental and theoretical weight loss of sample
during the thermal decomposition at the T1–T2 temperature range.

3
64
ALONSO ET AL.
0.07
Surface Area and Pore Size Distribution
TOctATM
THepATM
TPHexATM
TPeATM
ATM
The size of the alkyl group in the precursor compoundhas
an important effect on the surface area and the total pore
volume of the MoS2-based catalysts (cf. Table 6). The total
pore volume exhibits a direct correlation with the length
of the organic chain of the precursor since the total pore
volume increases from 0.092 cm /g for ATM to 0.546 cm /g
for TOctATM. As observed in Fig. 2, with the exception of
ATM and TProATM, all the adsorption–desorption curves
correspond to type IV isotherms with a desorption step
characteristic of mesoporous materials above the relative
pressure of 0.3 for TPenATM and 0.4 for THex-, THep-,
and TOctATM.
0
0
0
0
.06
.05
.04
.03
3
3
0.02
0.01
0.00
For ATM- and TProATM-made solids, a poorly deve-
loped porous system is observed with a surface area of only
2
0
20
40
60
80
100
6
0 m /g. Type I isotherms can be considered for both sam-
Pore Diameter [Å]
ples. Nevertheless, their different isotherm profiles reveal
that the higher pore volume of TProATM is due to the pres-
ence of very large mesopores while no difference between
these solids can be found in the microporous region.
The nitrogen adsorption isotherm for TPenATM shows
FIG. 3. BJH pore size distribution of MoS2 catalysts formed by in situ
decomposition of the different thiometalate precursors.
bution for the development of the pore volume is related
to mesopores and that the proportion of micropores versus
mesopores probably decreases with increasing alkyl size
length. Finally, the hysteresis loops shown by all these cata-
lysts corresponds to cylindrical pores open at both ends.
Figure 3 shows the desorption BJH pore size distribution
observed for the MoS2 catalysts formed from the different
thiometalate precursors. Results about TProATM have not
been reported since only a very broad pore size distribution
is obtained. These catalysts derived from THex-, THep-,
and TOctATM precursors present narrow pore size distri-
2
the highest surface area (329 m /g) but a moderate pore
3
volume (0.247 cm /g). This indicates that the surface area
is mainly due to microporosity since this solid presents the
highest nitrogen adsorption values at low relative pressures
(
P/P0) corresponding to the filling of both large and small
micropores.
Compared to TPenATM, for catalyst based on THex-,
THep-, and TOctATM precursors, the specific surface area
decreases. However, the BET values hardly change (255–
2
2
78 m /g) whereas the total pore volume increases when
increasing the size of the alkyl group from hexyl to octyl.
This result suggests that for these solids the main contri-
˚
butions, ranging from 30 to 50 A, while the mean pore size
value for TPenATM shifts downward, to 29 A. Moreover,
˚
its profile suggests a possible bimodal porous distribution
3
3
2
2
1
1
50
00
50
00
50
00
with another maximum in the microporous region.
(
(
(
(
TOctATM)
THepATM)
THexATM)
TPenATM)
Scanning Electron Microscopy
Images of the final MoS catalysts prepared from (a)
2
(TProATM)
ATM)
TProATM, (b) TPenATM, (c) THexATM, (d) THepATM,
(
and (e) TOctATM are reported Fig. 4. These solids appear
very porous, with bubblelike cavities probably resulting
fromtheeliminationofgasproducts. Cavitysizeseemstoin-
crease from TProATM to TPenATM and THexATM while
for larger-size alkyl groups, the cavities present slightly
smaller diameters. For the THex-, THep-, and TOctATM-
made solids, walls between cavities appear thicker than for
catalysts made from TPenATM.
5
0
0
0
X-Ray Diffraction
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Figure 5 shows the XRD patterns of the final MoS2 cata-
lysts prepared from tetraalkylammonium thiometalate pre-
cursors. All these patterns are in agreement with those
reported for the poorly crystalline MoS2 structure (24).
Relative Pressure (p/p0)
FIG. 2. Adsorption–desorption isotherms for MoS2 catalysts formed
by in situ decomposition of the different thiometalate precursors.

IN SITU ACTIVATED MESOPOROUS MoS2 CATALYSTS
365
FIG. 4. Scanning electron micrographs of the MoS2 catalysts prepared from (a) TProATM, (b) TPenATM, (c) THexATM, (d) HepATM, and
(e) TOctATM.
However, for catalysts obtained from ATM and TProATM cordingly, the c-direction layer stacking in the ATM and
◦
precursors, aquitestrong(002)diffractionpeakat2ꢂ = 14
TProATM-derived catalysts is then higher than for the cata-
can be observed, while for the series from TPenATM- to lysts made from TPenATM, THexATM, THepATM, and
TOctATM-made catalysts, the (002) peak is very weak. Ac- TOctATM precursors.

3
66
ALONSO ET AL.
lectivity results, with a HYD/DDS ratio of 1.5–1.7. Inter-
1
000
(
100)
estingly, from THexATM to TOctATM, a strong change
in selectivity is observed, with lower HYD/DDS values
(0.3–0.6). The MoS2 catalyst prepared from THepATM
presents the highest selectivity for the direct desulfuriza-
tion pathway. Hydrogenation of dibenzothiophene to form
tetrahydrodibenzothiophene (THDBT) strongly decreases
between the series of ATM-, TProATM-, and TPenATM-
made catalysts and catalytic systems based on the decom-
position of THex-, THep-, and TOctATM precursors.
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
(
002)
(110)
(
103)
TOctATM
THepATM
THexATM
TPenATM
TProATM
ATM
DISCUSSION
0
10 20 30 40 50 60 70 80 90
Tetraalkylammonium thiometalate precursors were used
2
θ
in situ to prepare MoS2-based catalysts during the HDS re-
FIG. 5. XRD patterns of MoS2 catalysts formed by in situ decompo- action. The in situ preparation of active sulfide catalysts
sition of the different thiometalate precursors.
2
leads to very high surface area solids (from 60 to 329 m /g)
with narrow pore size distribution. After the HDS reaction,
all these catalysts present large amounts of excess carbon.
However, the activity variations are not really marked com-
pared to results reported for ex situ methods of preparation
Catalytic Activity and Selectivity
The HDS of DBT yields two main products: biphenyl
(
(
BP) through the so-called direct desulfurization pathway
DDS), and phenylcyclohexane (PCH) through the hydro-
(
27). Ex situ decomposition of precursors (before HDS test)
lead to catalytic systems with a very low surface area, in
contrast to in situ decomposition, and to a final carbon con-
tent directly proportional to the size of the precursor’s alkyl
group. Variations of activity would then be expected to de-
pend on the carbon content of the ex situ decomposed cata-
lyst. Anyway, for in situ experiments, no clear relationship
could be observed between surface area, alkyl size length,
and catalytic activity. A slight increase in surface area could
be observed when varying the organic chain from propyl to
hexyl groups. These results show that even if more active
sites could be expected from an increase in surface area,
activity change could not be really detected. This situation
is common for anisotropic layered materials like MoS2 and
this represents a major ongoing characterization challenge
in this field.
genative pathway (HYD). Since these two pathways are
parallel (26), the ratio between HYD and DDS can be
approximated in terms of the experimental selectivity by
means of the equation
HYD/DDS = (PCH)/(BP).
Table 7 summarizes the selectivity for BP and PCH through
HYD/DDS ratios measured for different MoS2 catalysts
formed by the in situ decomposition of tetraalkylammo-
nium thiometalate precursors. Contrary to ex situ experi-
ments, for which thiosalt precursors are decomposed before
being used for HDS reactions, in situ experiments do not
reveal significant changes in activity between the ATM pre-
cursorandothercarbon-containingATMprecursors. ATM,
TProATM, and TPenATM precursors lead to similar se-
One may propose that the excess carbon formed dur-
ing the HDS reaction could play a negative role in the
activity by reducing the number of accessible sites. How-
ever, the stoichiometry of the catalyst prepared from ATM
TABLE 7
Initial Rate Constants, Selectivity (HYD/DDS Ratio), and
(
MoS1.7C0.5) is similar to the value expected for stabilized
[
THDBT]/[PCH] Ratio of MoS2 Catalysts Prepared by in Situ De-
active phases during the course of an HDS run, as reported
recently by Berhault et al. (15). Indeed, it was observed
that starting initially from a MoS2+X stoichiometry, the ac-
composition of Tetraalkylammonium Precursors during the HDS
Reaction of DBT
MoS2 catalysts
from
k (specific)
HYD/DDS [THDBT]/[PCH] tive phase of a sulfide catalyst progressively accommodates
(×10− , mol · g · s−1
7
−1
)
ratio
ratio
structural carbon by replacing surface sulfur atoms at the
edges of MoS2+X layers. This replacement occurs in typical
hydrotreating conditions, leading to a final stoichiometry of
MoS C , with y + z = 2 + x. Sulfur replacement by struc-
ATM
5.9
5.0
5.7
5.2
4.4
5.4
1.5
1.5
1.7
0.5
0.3
0.6
1.0
1.2
1.7
0.4
0.2
0.3
TProATM
TPenATM
THexATM
THepATM
TOctATM
Y
Z
tural carbon should be viewed as a process limited to the
edgesofthelayers. Bulksulfuratomsinsidetheslabsarenot
easilyreplacedbycarbon. Infact, usingAugerspectroscopy,
previous studies (14, 17) about the in situ decomposition
Note. T = 623 K, P = 3.1 MPa.

IN SITU ACTIVATED MESOPOROUS MoS2 CATALYSTS
367
of tetraalkylammonium thiometalate salts have confirmed predict that selectivity will change with particle size along
that in the catalytic system studied here, sulfur atoms are the basal plane. Large molecules like DBT will make in-
replaced by carbon, leading to stabilized sulfur-deficient appropriate the assumptions that each layer is chemically
carbon-containing MeSY CZ catalysts. Then, one may sug- independent since, in such cases, steric hindrances during
gestthatinthepresentcatalyticsystems, acertainamountof adsorption with neighboring molybdenum atoms in the
structural carbon is included in the stabilized active phase. next layer have to be expected. Finally, even if the particle
Nevertheless, after the replacement of all the surface sulfur size along the basal plane cannot be measured using X-ray
atoms available, no more carbon replacement should occur diffraction, the crystalline order along the basal plane could
since the MoS2 bulk structure would stay sulfidic by nature. be estimated using the widening of the (110) peak, as shown
In this case, the carbon excess would be formed by blocking Fig. 5. It could then be observed that the crystalline order
active sites. Consequently, the beneficial effect of structural along this plane is hardly changed for all precursor-made
carbon would be counterbalanced by the formation of car- catalysts. This would mean that the fraction of corner sites
bon in excess, resulting in the absence of improved HDS to edge sites is relatively constant and that according to the
activity.
corner–edge model, selectivity would not be modified when
More interestingly, while there is no change in selectiv- increasing the alkyl chain length of the precursor, which it
ity for catalysts prepared from TProATM and TPenATM does not. Anyway, according to the rim–edge model, steric
6
compared to ATM, strong change is observed for catalysts constraints will hinder the initial η -adsorption of the aro-
prepared by in situ decomposition of THex-, THep-, and matic rings, prerequisite for their hydrogenation. A low
TOctATM precursors. These catalysts present characteris- HYD/DDS ratio would then be expected for MoS2 cata-
tic type IV isotherms while TProATM-made catalysts do lysts derived from ATM and TProATM precursors, while in
not present a type IV isotherm and TPenATM presents the same spirit, the less stacked morphology of MoS2 cata-
only a small hysteresis. This could be related to the different lysts formed from THex-, THep-, and TOctATM would
decomposition modes between TProATM and TPenATM, lead to more hydrogenating catalytic properties. In fact, a
with two steps of decomposition, and THex-, THep-, and reverse effect is observed to what could be predicted based
TOctATM, with only a single step. Moreover, according to on the rim–edge model. This result leads to the considera-
XRD results, THex-, THep-, and TOctATM-made catalysts tion that a simple morphological effect modifying only the
demonstrate large morphological modifications compared relative proportion of rim sites versus edge sites is not suf-
to ATM and TProATM-made catalysts. Indeed, their lay- ficient to explain the strong change in selectivity observed
ered structure is largely less stacked than for catalysts de- in this study. Consequently, the nature itself of the sites is
rived from ATM and TProATM precursors. The rim–edge probably altered during the in situ process of formation of
model was previously developed by Daage and Chianelli MoS2 catalysts derived from THex-, THep-, and TOctATM
(
28) in order to explain selectivity changes for the HDS of precursors. This result might be explained through the for-
dibenzothiophene using geometrical considerations. Since mation of a “sulfocarbide” phase, as already reported by
the MoS2 particles are composed of stacked layers, differ- different authors (13–15, 32–34). Indeed, it was proposed
ent types of active sites could be found. While extremities that in typical hydrotreating conditions carbide and sulfide
of the stacked layers possess active sites (called rim sites) catalysts would tend to a common active surface presenting
for both hydrogenation and HDS reactions, other active both S and C moieties (32) and with a more favored selec-
sites located on “internal” stacked layers (called edge sites) tivity along the direct desulfurization pathway, leading to
are only active for HDS reactions. It should be underlined biphenyl. This mixed carbosulfide phase was also observed
that the terminology used here could differ from previous in recent studies by Schwartz et al. (32) and by Hsu et al.
studies. Indeed, Massoth et al. (29) and Topsøe et al. (30) (35). However, in this case, the formation of a sulfocar-
have envisaged that corner sites would be responsible for bide phase would be expected for any in situ catalyst (de-
HDS reactions while hydrogenation occurs on edge sites rived from both ATM and tetraalkyl–ATM precursors) pre-
(
equivalent in that case to both edge and rim sites, accor- pared during the course of the HDS of DBT. Therefore, this
ding to the rim–edge model). However, these results were would not explain the marked difference in selectivity ob-
obtained for the gas-phase HDS of thiophene whereas the served between the series of catalysts made from ATM,
rim–edge model, as in our study, was based on liquid-phase TProATM, and TPenATM and those made from THex-
DBT HDS results. A recent study by Hensen et al. (31) has ATM, THepATM, and TOctATM. TGA–DTA results have
demonstrated that considerations based on the rim–edge shownthatthedecompositionmodeofthetwoseriesofpre-
model should not be used to explain selectivity changes in cursors differs strongly. This may explain the mesoporosity
the HDS of thiophene. Moreover, “corner–edge” models of the second series of catalysts in contrast to catalysts made
were based on the assumption that each layer is chemi- from ATM and TProATM, for which a similar porous struc-
cally independent of each other since interlayer forces are ture was not observed, the TPenATM-made catalyst being
only due to van der Waals interactions. These models then an intermediate case. As observed in Fig. 3 and Table 7, the

3
68
ALONSO ET AL.
formation of mesopores is clearly accompanied by a strong disulfides are well-known to be highly efficient presulfi-
change in selectivity for the HDS of DBT. Indeed, the com- ding agents leading to HDS-enhanced active phases (36). It
parison of catalysts made from TPenATM and THexATM should be underlined that a similar DDS-favored selectivity
shows that change in pore size corresponds to a strong mod- change effect was recently observed using CoMo catalysts
ification in the selectivity, and the HYD/DDS ratio moves supported on mesoporous Si–MCM-41 (37).
from 1.7 to 0.5. The pore size optimum does not change for
If mesoporosity has mainly been previously observed in
THex-, THep-, or TOctATM-made catalysts and the selec- oxide-typesolidssuchasalumina, silica, andpillaredclays, it
tivity is similar between these three solids. A relationship has also been recently observed in sulfide materials (38, 39).
between mesoporous structure and DDS-selective catalytic The cause for a type IV isotherm in some solids has been
systems is evidenced.
extensively studied by Everett, who gave possible explana-
It is now well admitted that the prerequisite for hydro- tions to the existence of large and wide hysteresis loops
genation is a π-complexation through the aromatic ring. (40). One case deserving great attention for amorphous
This flatwise mode of adsorption covers a large part of materials is the study by Mayagoitia et al. (41, 42), which
the catalyst surface. Only rim sites present the suitable ge- correlates type IV hysteresis loops with the presence of an
ometry for adsorbing reactants through the aromatic ring extensively interconnected porous network of mesoporous
whereas the C–S bond-breaking mechanism first needs a σ cavities and connecting channels. In MoS2 catalysts derived
bond between the metal center and the sulfur atom. These from tetraalkylammonium thiometalates, mesoporous cav-
different modes of adsorption led Daage and Chianelli (28) ities are formed because of the accumulation of gases dur-
to consider that hydrogenation of DBT is a steric demand- ing the precursor decomposition while channels are cre-
ing reaction. This steric requirement does not seem to be ated during the escape of such gases to the exterior. Such
achieved in the mesoporous cavities of our catalytic sys- a porous network in amorphous materials may be consi-
tems, leading to a confinement effect restricting adsorption dered equivalent to the porous structure of molecular
of reactants on rim sites and change in selectivity. The steric sieves, which very likely induce confinement effects. Due to
hindrance for hydrogenation could be directly determined such properties, the high-surface-area MoS2 materials re-
when considering the ratio between tetrahydrodibenzoth- ported in this work are also called “amorphous mesoporous
iophene (THDBT), the primary product formed by hydro- sulfides.”
genation of one of the aromatic rings along the HYD path-
way, andbiphenyl(BP), theprimaryproductformedbyC–S
bond hydrogenolysis along the DDS pathway. For instance,
CONCLUSIONS
this ratio was used for the rim–edge model to show a direct
MoS2-based catalysts were prepared by in situ decompo-
linear relationship between stacking of the layers and varia-
sition of different tetraalkylammonium thiometalate pre-
tion in the hydrogenation/hydrogenolysis functions for the
cursors (with alkyl = propyl, pentyl, hexyl, heptyl, and
HDS of DBT. In the present study, in which stacking is not
octyl). The solids obtained using this procedure present in-
the main parameter involved in the change in selectivity,
teresting morphological properties with high surface area,
formation of mesopores for THex-, THep-, and TOctATM-
characteristic type IV isotherms, narrow pore size distribu-
made catalysts is accompanied by a strong decrease in the
tion, and a very dispersed active phase with low-stacked
[THDBT]/[BP] ratio, from 1.70 for the TPenATM-made
layers. Moreover, for catalysts derived from tetrahexyl-,
tetraheptyl-, and tetraoctylammonium precursors, high se-
lectivity along the direct desulfurization pathway was
observed while activity was preserved, as compared to
ATM-derived MoS2 catalysts. Results suggest a possible
confiningeffectmodifyingtheHDSselectivityforthemeso-
porous catalysts formed when decomposing thiometalate
precursors with long alkyl groups. This new type of sulfide
materials could be named “amorphous mesoporous sul-
fide” catalysts.
catalyst to 0.35 for the THexATM-made one (cf. Table 7).
The steric demanding adsorption preceding hydrogenation
of DBT seems hindered by confinement inside mesopores
and is then responsible for the less hydrogenating proper-
ties of these solids. However, in this case, apart from the hy-
pothesis of excess carbon blocking active sites, the apparent
absence of activity variation appears surprising since, first,
hydrogenation sites would be less accessible for DBT and,
second, the number of edge sites would be smaller since
the stacking is decreasing when mesoporosity is formed. A
total decrease in activity would then be expected, in con-
trast to experimental results. To solve this contradiction, an
increase in the intrinsic activity of the edge sites has to be
envisaged. This would be the result of a better “HDS benefi-
cial” interaction of the active phase with the as-formed alkyl
disulfides during the in situ decomposition of the THex-
ACKNOWLEDGMENTS
The authors appreciate the valuable technical assistance of J. J.
Calderon, L. De la Torre, M. Perez De la Rosa, H. Esparza, and A. Reyes.
This work was financially supported by Conacyt project J31397-U, FIES-
9
8-27-III, the Robert A. Welch Foundation, and the USA–Mexico Mate-
ATM, THepATM, and TOctATM precursors. In fact, alkyl rials Corridor Program (GSA and DOE)

IN SITU ACTIVATED MESOPOROUS MoS2 CATALYSTS
369
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