Sulfur-33 Isotope Tracing of the Hydrodesulfurization Process: Insights into the Reaction Mechanism, Catalyst Characterization and Improvement

Article abstract of DOI:10.1002/anie.201704027

The novel approach based on ³³S isotope tracing is proposed for the elucidation of hydrodesulfurization (HDS) mechanisms and characterization of molybdenum sulfide catalysts. The technique involves sulfidation of the catalyst with ³³S-isotope-labeled dihydrogen sulfide, followed by monitoring the fate of the ³³S isotope in the course of the hydrodesulfurization reaction by online mass spectrometry and characterization of the catalyst after the reaction by temperature-programmed oxidation with mass spectrometry (TPO-MS). The results point to different pathways of thiophene transformation over Co or Ni-promoted and unpromoted molybdenum sulfide catalysts, provide information on the role of promoter and give a key for the design of new efficient HDS catalysts.

Full text of DOI:10.1002/anie.201704027

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Accepted Article
Title: 33S label tracing for hydrodesulfurization process: insights
into the reaction mechanism, catalyst characterization and
improvement
Authors: Vitaly L. Sushkevich, Andrey G. Popov, and Irina I. Ivanova
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704027
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33S label tracing for hydrodesulfurization process: insights into
the reaction mechanism, catalyst characterization and
improvement
Vitaly L. Sushkevich,*,[ab] Andrey G. Popov[a] and Irina I. Ivanova[a]
Abstract: The novel approach based on 33S label tracing is
proposed for the elucidation of hydrodesulfurization (HDS)
mechanisms and characterization of molybdenum sulfide catalysts.
The technique involves sulfidation of the catalyst with dihydrogen
sulfide labeled with the 33S isotope, followed by monitoring the fate
of 33S label in the course of hydrodesulfurization reaction by on-line
mass spectrometry and characterization of the catalyst after the
reaction by TPO-MS. The results point to different pathways of
thiophene transformation over Co or Ni promoted and unpromoted
molybdenum sulfide catalysts, provide information on the role of
promoter and give a key for the design of new efficient HDS
catalysts.
bond cleavage.[9]
In the case of NiMo and CoMo catalysts the hydrodesulfurization
of thiophene proceeds via two possible mechanisms (Scheme 1,
pathways 1 and 2) that are generally accepted in the literature.
The first one corresponds to the hydrogenation route (HYD)
implying the stepwise addition of hydrogen to thiophene with the
subsequent S-C scission. The second one includes an
irreversible direct desulfurization (DDS) via a direct break of the
S-C bond leading to the formation of sulfur S* surface species
and butane, followed by the hydrogenation of sulfur species into
H2S. The impact of each pathway is governed by the reaction
conditions, hydrogen pressure and catalyst composition.[10]
HYD
+H2
+H2
1)
H2S +
Hydrodesulfurization (HDS) is one of the most important refinery
processes, with the annual capacity close to 4 billion tons.[1]
Nowadays, strict refinery regulations on the sulfur content in the
final feedstock require significant improvements in the
composition of catalyst and the process technology.[2] In addition,
the trends to use heavy oil for the fuel production demand new
approaches to refine high-temperature boiling and sour oil.[1]
These challenges motivate research in the field of hydro-
desulfurization, revealing several directions aimed at the
increase of the efficiency of the MoS2 based catalysts.[3-6] In the
general case, the efficiency can be increased by using different
supports,[4] variation of the dispersion of the MoS2 phase, using
different precursors for the catalyst preparation[5] and promoting
with various dopants.[6] For the industrial catalysts promoting
with cobalt or nickel is commonly used resulting in the 2-5 fold
increase of catalysts activity. However, the nature of this
promotion effect is still the subject of debate.
It is generally accepted that doping the MoS2 catalysts with Co
and Ni leads to the formation of (Co or Ni)-Mo-S surface species
with higher intrinsic activity with respect to the unpromoted Mo-S
samples.[7] These active species are located at the edges of the
slabs, giving coordinatively unsaturated sites (CUS).[8] The basal
planes are believed to be inactive. Another model of the active
sites, proposed in the recent years, postulates the existence of
the so-called “brim sites”. These sites are described as metal-
like regions located near the edges of the sulfide slabs and
suggested to be able to catalyze hydrogenation and the C−S
S
S
DDS
+H2
+H2
S
2)
+
H2S
S
*
Scheme 1. Main pathways of thiophene hydrodesulfurization over (Co,Ni)-
MoS2 catalysts. First route corresponds to hydrogenation (HYD) and second to
direct desulfurization (DDS) of thiophene molecules over the catalyst.
The active sites of HDS catalysts are usually characterized by
STM and TEM microscopy.[11] These techniques give important
information on the structure of the active sites and the location of
a promoter over fresh and used catalysts. However, to get a
deeper insight into the mechanism of the HDS process, the
information on the catalyst and promoters and their interaction
with substrate under real reaction conditions is highly desirable.
This information can be best obtained by the in situ isotope
tracing technique, which allows following the fate of different
atoms of the catalyst and substrate directly during the catalytic
reaction. Thus, in the case of the HDS reaction, tracing of S-
isotopes could be particularly useful since sulfur atoms are
contained both in the catalyst and the reactants and are involved
in their interaction.
Kabe et al. and Dobrovolsky et al. have used the 35S radioactive
isotope tracing for the investigation of hydrodesulfurization of
dibenzothiophene over (Co,Ni)-MoS2.[12] Their experiments
confirmed the lability of sulfur atoms, and pointed to sulfur
exchange between the catalyst and the reaction products.
However, the application of the 35S radioactive isotope tracing
experiments was limited to several examples[12] due to the low
resolution of this technique, large characteristic time for the
measurements and difficulties in determination of the
contributions from the different compounds into the total
radioactivity. In addition, these experiments require special
expensive equipment to protect the environment from strong
[a]
[b]
Dr. V.L. Sushkevich, Dr. A.G. Popov and Prof. I.I. Ivanova,
Chemistry Department of Lomonosov Moscow State University
119991, Leninskye Gory, 1, bld, 3, Moscow, Russia
Dr. V. L. Sushkevich, Paul Scherrer Institut, 5232 Villigen,
Switzerland
Email: vitaly.sushkevich@psi.ch
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201704027
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irradiation.
Herein we propose to use the 33S isotope, which is commercially
available and not radioactive and can be easily monitored by on-
line mass spectrometry. The application of mass spectrometry
appears to be beneficial with respect to radioactivity
measurements since it allows accessing the sulfur distribution in
the reactants and products, which is extremely important for
analyzing the mechanism of the sulfur exchange between the
catalyst and the substrate. Furthermore, for the quantitative
determination of the contents of labile and rigid sulfur in different
catalysts, we propose to examine the samples obtained after
tracing experiments by the TPO-MS technique.
By using this novel approach, this study is aimed to clarify the
mechanism of sulfur exchange over the unpromoted and the
cobalt and nickel promoted MoS2/Al2O3 catalysts that are the
most commonly used catalysts for hydrodesulfurization. The
results point to the existence of a new undiscovered pathway of
thiophene transformation over MoS2/Al2O3 catalysts implying the
reversible S exchange in the aromatic ring. Furthermore, they
provide information on the role of Co and Ni promoters and give
hints for characterization and improvement of molybdenum
sulfide catalysts.
Table 2. Hydrodesulfurization of thiophene over MoS2 based catalysts.
Catalyst
Mo
CoMo
NiMo
Yield of H2S*, %
8.9
33.7
0.19
0
29.1
0.24
0
Alkane to alkene ratio*
Yield of [33S]thiophene**, %
Labile sulfur content***, wt%
Rigid sulphur content***, wt%
0.20
62.9
4.39
2.51
5.13
2.97
4.32
2.58
* calculated from GC data
** calculated from MS-data
*** calculated from TPO-MS data
For the 33S label tracing experiments oxide precursors were
sulfidized in H2[33S] yielding the catalysts with the 33S-labeled
active phase. After sulfidation all physically adsorbed H2[33S]
was purged with hydrogen at 663K for 1h and then the flow was
switched to the [32S]C4H4/H2 mixture. The H2[32S] and H2[33S]
zero time corrected MS responses observed after the switch are
shown in Fig. 1. Before the introduction of thiophene, the
evolution of H2[33S] was not detected. After the switch, the
formation of both labeled and unlabeled hydrogen sulfide was
observed hence confirming the lability of the surface sulfur
atoms of the catalysts. The response of the unlabeled H2S for
CoMo and NiMo catalysts was ca. 3 times higher than in the
Table 1. Catalysts characteristics
Catalyst
Metal content in oxide
precursor, wt%
Surface
Sulfur
content,
wt% [a]
area, m2/g
Mo
Co, Ni
Mo
11.0
12.6
10.3
0
165
190
205
6.90
8.10
6.90
CoMo
NiMo
2.6
3.0
case of MoS2 catalyst, which confirms
a
higher
hydrodesulfurization activity of promoted catalysts (Table 2). The
yield of labeled H2[33S] passed through the maxima after ca. 1
min on stream indicating a fast exchange process. It is important
to mention that the MS response of H2[33S] for Mo catalyst did
not decrease to zero, stabilizing at a considerable level. This
behavior could point to several different mechanisms of sulfur
exchange over MoS2 catalysts.
[a] After sulfidation in a flow of H2/H2S
The catalysts were prepared by the insipient wetness
impregnation of the commercial alumina with the solutions of
corresponding metal salts followed by the calcination at 773K
(for details see Supporting Information). The catalysts
characteristics are summarized in Table 1. Metal loadings were
selected basing on the compositions reported in the literature.[12]
Before the catalytic tests the oxide precursors were preliminary
sulfidized in a flow of H2S/H2 mixture at 673K for 1 h. The H2S
enriched with the 33S isotope was synthesized in situ by reacting
the 33S sulfur powder with paraffin (see Supporting Information).
Catalytic properties were studied in the thiophene
hydrodesulfurization reaction under atmospheric pressure in a
continuous flow fixed bed reactor at 633K using the hydrogen to
thiophene molar ratio of 13.
The results pointed to 2-3 fold higher activity of Co and Ni
promoted catalysts in the HDS reaction of thiophene with
respect to the unpromoted MoS2 catalyst (Table 2). The main
products observed were H2S, linear butenes and n-butane. The
selectivity and the alkane to alkene molar ratio were similar for
all studied catalysts. In the case of the NiMo sample a slightly
higher content of n-butane was observed due to the
considerable hydrogenation properties of nickel.[13]
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Figure 1. Variation of intensities of the MS molecular ions of H2[32S] (solid
lines) and H2[33S] (dashed lines) with time on stream during the
hydrodesulfurization of unlabelled thiophene over the catalysts sulfidized with
H2[33S]. Inset: zoomed fragment from 45 to 75 s region.
(Table 2). The values calculated for promoted catalysts are very
close to those of unpromoted pointing to the similar content of
labile S in all catalysts. Thus, although the amount of H2[33S]
originating from the unpromoted catalyst during the HDS
reaction was found to be much lower as compared to the
promoted catalyst, the amount of labile and rigid sulfur was
found to be the same in both cases (Table 1). This observation
implies different pathways of the sulfur exchange over the
promoted and unpromoted catalysts and points to the possibility
of a reversible sulfur exchange in thiophene over the latter.
The sulfur exchange in aromatic compounds was not reported
previously for MoS2 catalyst. Such process was found to
proceed over Al2O3 at the temperatures higher than 700K by the
Juriev reaction.[15] Under the conditions used in the present
study the impact of this process is negligible. It is generally
assumed that once adsorbed on the active sites, the S-
containing aromatics undergo irreversible hydrogenation or/and
the C-S cleavage forming the hydrogenation products and
hydrogen sulfide.[7, 11d] However, our results suggest that MoS2
catalysts can reversibly exchange S atoms in thiophene.
The position of the H2[33S] response maximum is different for
promoted and unpromoted catalysts. The detailed examination
of this region (see inset) shows that for Mo catalysts the sulfur
exchange process is much faster with respect to CoMo and
NiMo samples despite the higher activity of the latter. The
difference in behavior could be associated with several reasons:
i) different amount of labile sulfur and ii) different mechanisms of
the S exchange over the MoS2 and (Co,Ni)-MoS2 catalysts.
The information on the amount of labile sulfur can be obtained
by examination of the catalysts used in the HDS reaction. To
estimate the amount of 32S and 33S retained by the catalysts
after the reaction, the catalysts were oxidized in situ in a
temperature programmed regime and the formation of [32S]O2
and [33S]O2 was monitored by on-line mass spectrometry. The
results are presented in Fig. 2.
To get more details on this pathway, the MS responses of
thiophene isotopomers were studied during the HDS over the
Mo, CoMo and NoMo catalysts preliminary sulfidized with H2[33S].
The results are shown in Fig. 3.
Figure 2. TPO-MS profiles observed over the catalysts sulfidized with H2[33S]
and subjected to HDS reaction of unlabelled thiophene for 1h. Solid lines
correspond to the evolution of
evolution of [33S]O2.
[
32S]O2, dashed lines correspond to the
All catalysts demonstrated similar TPO curves both for the
[32S]O2 and [33S]O2 evolution. This observation suggests that the
promoters do not influence the oxidation properties of the
catalysts significantly. In the case of CoMo and NiMo small
peaks in the range of 600-750K were also observed due to the
individual phases of metal sulfides.[14]
Figure 3. Variation of intensities of the MS molecular ions of the [32S] and [33S]
thiophene with time on stream during the hydrodesulfurization of unlabelled
thiophene over the catalysts sulfidized with H2[33S]. Solid lines correspond to
m/z = 84 ([32S] thiophene), dashed lines - to m/z = 85 ([33S] thiophene).
The temperature of the 32S species oxidation was ca. 50K lower
with respect to 33S indicating that labile sulfur undergoes easier
oxidation. It could be suggested that labile sulfur is located at the
edges of the slabs, which facilitates the exchange and oxidation.
Integration of the [32S]O2 and [33S]O2 TPO curves gives the
content of labile and rigid sulfur for all considered catalysts
The results show that in the case of the CoMo and NiMo
catalysts the molecular ion at 84 m/z corresponding to unlabeled
thiophene is mostly observed. Slight ramp for 85 m/z after the
introduction of thiophene could be associated with the 13C
natural abundance impact arising from unlabeled thiophene.
This observation suggests that sulfur from the active sites of the
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Ni or Co promoted catalysts does not undergo the reversible
exchange with thiophene. Once adsorbed on the active site,
thiophene molecule decomposes into C4 hydrocarbons and
sulfur species as shown in Scheme 1. The latter can exchange
with the surface Mo[33S]2 species giving H2[33S] observed during
the HDS reaction (Fig. 1).
A rather different result is observed over the Mo catalyst without
a promoter. The relative intensity of 85 m/z peak increases
immediately after the introduction of unlabeled thiophene and
then passes through the maximum. Simultaneously, the
unlabeled thiophene concentration reaches the lowest value and
then gradually increases. This behavior clearly evidences the
sulfur exchange between the catalyst surface and thiophene
molecules. Comparing the intensities for 84 and 85 m/z ions
right after the introduction of thiophene, the conversion of [32S]
into [33S]-thiophene could be estimated. The value obtained is
close to 63%, which is significantly higher (Table 2) than the
conversion of thiophene into hydrogen disulfide (8.9%).
catalytic properties. Most importantly, the current results
emphasize the crucial role of the metal promoter, which is able
to change the chemical environment of the MoS2 active sites
and thus influence the mechanism of the HDS reaction. This
opens an exciting possibility of using the metals with different
electronic structures to not only tune the catalytic activity but
even to change the entire reaction mechanism. This specifically
renders the MoS2-based catalyst a natural choice for rational
design of novel catalytic materials with engineered properties.
Another important issue arises from the application of the TPO-
MS technique for the analysis of the catalysts labeled with 33S
isotope and subjected to the HDS reaction. This approach
allows for quantitative determination of the amount of labile and
rigid sulfur atoms in the catalyst, which could be related to the
dispersion of an active phase. Assuming that the rigid sulfur
atoms are located on the top of the slabs, the stacking of MoS2
species can be estimated and related to the accessibility of
active sites and the catalyst activity in the HDS reaction. This
novel tool can be easily used to control the quality of the HDS
catalyst at industrial scale and evaluate the feasibility of novel
MoS2-based catalysts.
Summarizing, our data suggest that Mo catalyst demonstrates
rather high activity in the sulfur exchange in thiophene, providing
the reversible C-S cleavage reaction (RCS, Scheme 2).
However, after the cleavage only a small part of the newly
formed S* and C4* surface species undergo further
hydrogenation into H2S and C4 hydrocarbons, accounting for low
hydrodesulfurization activity of this catalyst.
We believe that the proposed approach opens new perspectives
for the improvement of existing hydrodesulfurization catalysts
and the development of novel efficient heterogeneous catalysts
for this process.
The mechanism of reversible C-S cleavage might include the
acid-mediated steps similar to observed for Paal-Knorr synthesis
of thiophene. However, basing on the present results the
conclusions about the detailed mechanism of the reversible C-S
cleavage can not be delivered and further study in this field is
required.
Acknowledgements
The authors gratefully acknowledge the Russian Science
Foundation for the financial support (Grant №14-23-00094).
Keywords: hydrodesulfurization • isotope tracing •
heterogeneous catalysis • mass spectrometry • 33
S
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hydrodesulfurization over unpromoted MoS2 catalysts
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Entry for the Table of Contents
COMMUNICATION
The novel approach based on 33
label tracing is proposed for the
S
Vitaly L. Sushkevich*, Andrey G. Popov
and Irina I. Ivanova
elucidation of hydrodesulfurization
(HDS) mechanisms and
Page No. – Page No.
characterization of molybdenum
sulfide catalysts. The results point to
different pathways of thiophene
transformation over Co or Ni promoted
and unpromoted molybdenum sulfide
catalysts, provide information on the
role of promoter and give a key for the
design of new efficient HDS catalysts.
33S label tracing for
hydrodesulfurization process:
insights into the reaction mechanism,
catalyst characterization and
improvement
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