M.J.B. Souza et al. / Catalysis Communications 69 (2015) 217–222
219
Table 1
Textural properties of the catalysts.
Samples
Suport
ao (nm) SSA (m2 g−1
)
Wt (nm) Dp (nm) Vp (cm3 g−1
)
8.42
1325
989
0.87
0.77
0.50
0.56
1.32
0.52
1.20
3.70
3.71
3.71
3.50
2.72
3.76
2.30
0.19
0.16
0.17
0.18
0.17
0.14
0.12
PtMo/MCM-48b 8.11
Pt/MCM-48b
Mo/MCM-48b
7.29
7.15
1053
1075
546
PtMo/MCM-48a 8.29
Pt/MCM-48a
Mo/MCM-48a
7.43
7.28
735
534
Where: a = after sulfidation and b = before sulfidation, ao = mesoporous parameter,
SSA = specific surface area; Wt = silica wall tickness; Dp = pore diameter and Vp
=
pore volume.
The nitrogen adsorption–desorption isotherms for the M/MCM-48
(where M = Pt, Mo or PtMo) before and after sulfidation are shown
in Fig. 3. The M/MCM-48 catalysts display a type IV isotherms in the
IUPAC classification and a sharp inflection on the range of P/P0
=
0.15−0.3, which takes places due the capillary condensation of nitro-
gen in the mesoporous [23]. The textural properties data of the catalysts
are shown in Table 1. It is possible to observe that for all M/MCM-48
catalysts there was a decrease in the values of total area and pore
volume in relation of the support. This decrease is more significant for
sulfide samples, and in general can be related to significant blockage
of the MCM-48 pores by platinum and–or molybdenum species and
also due to partial loss of structural ordering of the mesoporous
channels.
Table 2 shows the XPS analysis results of the PtMo/MCM-48 cata-
lysts (in the oxide form, in the sulfided form and after HDS reaction),
as the binding energies (eV) and the surface atomic concentration of
core electrons.
In Fig. 4., Pt 4f7/2 core level spectra of Pt/MCM-48 and PtMo/MCM-48
catalysts as unsulfided, sulfided and after reaction show two contribu-
tions: a main contribution located about 71.5 eV, which is attributed
to metallic platinum and a second contribution of lesser extend cen-
tered about 73.0 eV assigned to a small amount of Pt(II) in the form of
PtO [24,25]. After the sulfidation step, both contributions remain unal-
tered for Pt/MCM-48 and PtMo/MCM-48 catalysts. Comparing the
data presented in Table 2 with the data obtained by Dembowski and
collaborators [26] it was not observed the formation of PtS or PtS2
(Pt 4f7/2 = 72.55 and 74.16 eV, respectively) in the sulfided catalysts
(S) and too in the after reaction catalysts (SPR). This fact suggests that
platinum base catalysts present a high sulfur resistance which could
favor the stability of the active phase during the catalytic test [8,26,
27]. This can be correlated as a result of the differential interaction of
platinum species formed with the support which has been attributed
to an electronic deficiency of the metal, resulting of interaction with
the support. Analysis of S 2p3/2 core level spectrum of PtMo/MCM-48
shows an unique contribution located at 162.3 eV which has been
attributed to sulfide species.
In the case of containing molybdenum oxide precursor samples
(Mo/MCM-48 and PtMo/MCM-48), Fig. 5 shows Mo 3d5/2 core level
spectra XPS data of both catalytic precursor show an unique contribu-
tion located between 231.6 and 232.0 eV assigned to Mo6+ species in
the form of MoO3 [15].
When the samples are sulfided, both catalysts present two contribu-
tion: the main contribution, located about 228 eV, has been attributed to
Mo4+ species in the form of MoS2 and a minor contribution between
231.2 and 231.9 eV is ascribed to Mo6+ species in the form of unsulfided
species and/or oxysulfide species. In the same way, S 2p3/2 core level
spectra present the typical contribution ascribed to sulfide species,
about 162 eV, corroborating the deep sulfidation of molybdenum
species for the formation of MoS2 species [15].
Fig. 2. XRD of the catalysts at high angle: a) before sulfidation and b) after sulfidation.
set of catalytic tests were performed at 350 °C during 8 h on stream
(Table 4). With the increasing temperature occurred an increase in the
DBT conversion and the yield also BP N CHB. These results possibly indi-
cate that at high temperatures may have been promoted BP via direct
hydrogenation of DBT, while production of CHB was observed in lesser
amounts via the THDBT intermediary. BP product was obtained in larger
quantities in all cases, reaching yields of 68.0, 17.8 and 0.8% for catalysts
PtMo/MCM-48, Pt/MCM-48 and Mo/MCM-48, respectively after 8 h on
stream. The HDS of DBT over sulfided CoMo/MCM-41 and CoMo/Al2O3
at 350 °C was previously reported and showed conversions of 58.5
and 30.7%, respectively [28].
The main reaction product was biphenyl (BP) in all catalysts, coming
from the direct hydrogenolysis or direct desulfurization (DDS) pathway.
Ciclohexylbenzene (CHB) also appears as by-product, although is
formed in a lesser extend. This product is obtained from a hydrogena-
tion of the rings prior the desulfurization (HYD) pathway through to
tetrahydrodibenzothiophene (THDBT) intermediate. However, THDBT
as intermediate possesses a very short life time in the catalytic cycle
and thus it is expected does not appear in significant amounts in the
products [29].
Table 3 shows the data of conversion of DBT over Pt/MCM-48,
Mo/MCM-48 and PtMo/MCM-48 catalysts. In order to evaluate the
stability of the catalysts as function of the time on stream [19], a