A Mechanistic Study of Thiophene Hydrodesulfurization by the Parahydrogen-Induced Polarization Technique

Article abstract of DOI:10.1002/cctc.201500691

Thiophenic compounds are the least reactive organosulfur compounds in fossil fuels, and thiophene is widely used as a model substrate in studies of industrially important hydrodesulfurization (HDS) processes. It is generally presumed that the HDS process can proceed by two possible pathways, namely hydrogenation or direct desulfurization. In this work, the parahydrogen-induced polarization technique was successfully utilized in a mechanistic study of a hydrodesulfurization reaction by example of a heterogeneous hydrodesulfurization of thiophene over supported catalysts in the presence of parahydrogen. It was demonstrated that the HDS of thiophene on a MoS₂/γ-Al₂O₃ catalyst proceeds preferentially by the hydrogenation pathway to form tetrahydrothiophene, followed by desulfurization. In contrast, if a Pt/TiO₂ catalyst was used, direct desulfurization to 1,3-butadiene and the hydrogenation pathway both contributed to the overall reaction mechanism.

Full text of DOI:10.1002/cctc.201500691

DOI: 10.1002/cctc.201500691
Full Papers
A Mechanistic Study of Thiophene Hydrodesulfurization
by the Parahydrogen-Induced Polarization Technique
Oleg G. Salnikov,[a, b] Dudari B. Burueva,[a, b] Danila A. Barskiy,[a, b] Galina A. Bukhtiyarova,[c]
Kirill V. Kovtunov,*[a, b] and Igor V. Koptyug[a, b]
Thiophenic compounds are the least reactive organosulfur
compounds in fossil fuels, and thiophene is widely used as
a model substrate in studies of industrially important hydrode-
sulfurization (HDS) processes. It is generally presumed that the
HDS process can proceed by two possible pathways, namely
hydrogenation or direct desulfurization. In this work, the para-
hydrogen-induced polarization technique was successfully uti-
lized in a mechanistic study of a hydrodesulfurization reaction
by example of a heterogeneous hydrodesulfurization of thio-
phene over supported catalysts in the presence of parahydro-
gen. It was demonstrated that the HDS of thiophene on
a MoS2/g-Al2O3 catalyst proceeds preferentially by the hydroge-
nation pathway to form tetrahydrothiophene, followed by de-
sulfurization. In contrast, if a Pt/TiO2 catalyst was used, direct
desulfurization to 1,3-butadiene and the hydrogenation path-
way both contributed to the overall reaction mechanism.
Introduction
Sulfur-containing impurities in crude petroleum and its refined
products (gasoline, diesel oil) are undesirable because they are
able to poison catalysts used in fuel processing and to cause
corrosion of apparatus. In addition, sulfur oxides in exhaust
gases from sulfur-containing fuels are a major reason for envi-
ronmental pollution and acid rains. So, in view of stricter legis-
lation regarding the sulfur content of transportation fuels, hy-
drodesulfurization (HDS) is one of the most important catalytic
processes for the removal of sulfur-containing impurities from
petroleum fractions.[1–5] In a HDS process, CÀS bonds undergo
hydrogenolysis, which leads to the formation of hydrocarbons
and H2S. The industrial catalysts, consisting of Co(Ni)-promoted
MoS2 supported on g-Al2O3, have been developed and opti-
mized for several decades.[1,2,4,6] Direct and indirect insights
into the structure and activity of these catalysts were gained
by experimental[7–11] and theoretical[12,13] studies. The main
sulfur-containing compound in fluidized catalytic cracking gas-
oline is thiophene, which is also a most suitable model mole-
cule for studying the HDS reaction as a result of its relatively
simple structure.[14] However, despite the significant number of
thiophene HDS investigations, the mechanism of this reaction
is still debated.[15–17] For instance, both the hydrogenation
(HYD) pathway and the direct desulfurization (DDS) pathway,
Scheme 1. Possible pathways of thiophene hydrodesulfurization.
as illustrated in Scheme 1, have been proposed for thiophene
HDS.[18] In the DDS route, the CÀS bonds of thiophene are di-
rectly broken by hydrogenolysis, with the formation of unsatu-
rated C4 hydrocarbons,[19] whereas in the HYD route, hydroge-
nation of one or both C=C bonds occurs before the CÀS bond
scission.[17]
The thiophene hydrodesulfurization process involves hydro-
gen addition, so it is very promising to study the mechanism
of this reaction by the parahydrogen-induced polarization
(PHIP) technique.[20,21] PHIP is based on the pairwise addition of
a parahydrogen (p-H2) molecule to an unsaturated bond of
1
a substrate molecule followed by H NMR detection of the re-
[a] O. G. Salnikov, D. B. Burueva, Dr. D. A. Barskiy, Dr. K. V. Kovtunov,
Prof. I. V. Koptyug
action products. Pairwise addition means that the reaction
product molecule incorporates two atoms from one and the
same H2 molecule. If the reaction meets the requirement of
pairwise hydrogen addition, the NMR signals of the reaction in-
termediates and product molecules become significantly en-
hanced.[22] This enhancement of the NMR signals, along with
the characteristic antiphase shape of the polarized NMR pat-
terns, makes the PHIP technique a unique tool for mechanistic
studies of catalytic reactions and for the indirect identification
of reaction pathways.[23] The applications of PHIP for mechanis-
International Tomography Center SB RAS
Insitutskaya Street, 3A, 630090 Novosibirsk (Russia)
E-mail: kovtunov@tomo.nsc.ru
[b] O. G. Salnikov, D. B. Burueva, Dr. D. A. Barskiy, Dr. K. V. Kovtunov,
Prof. I. V. Koptyug
Novosibirsk State University
Pirogova Street, 2, 630090 Novosibirsk (Russia)
[c] Dr. G. A. Bukhtiyarova
Boreskov Institute of Catalysis SB RAS
Lavrentieva Avenue, 5, 630090 Novosibirsk (Russia)
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tic investigations of homogeneous hydrogenations catalyzed
by transition-metal complexes are well established.[24] In con-
trast, for heterogeneous hydrogenations, the observation of
PHIP effects was considered impossible for a long time be-
cause the accepted Horiuti–Polanyi mechanism for hydrogena-
tion over supported metal catalysts involves dissociative hydro-
gen adsorption.[25] The latter should lead to the loss of spin
correlation between the parahydrogen-derived H atoms and,
therefore, to an absence of any PHIP effects. However, it was
found that, in reality, some fraction of H2 (about 1–3%) can be
added to unsaturated substrates in a pairwise manner, which
leads to PHIP effects in the NMR spectra if p-H2 is used in het-
erogeneous hydrogenations over supported metals. Recently,
the range of catalytic systems suitable for heterogeneous PHIP
studies was broadened with the demonstration that ligand-
capped metal nanoparticles,[26] bulk metals and metal
oxides,[27] metal-containing zeolites,[28] and supported ionic-
liquid-phase catalysts[29,30] can also produce PHIP effects. More-
over, the PHIP technique was utilized for mechanistic studies
of heterogeneous hydrogenation of hydrocarbons such as pro-
pene, propyne, 1,3-butadiene, and 1-butyne over supported
metal catalysts.[31] Recently, the mechanism of heterogeneous
hydrogenation of a,b-unsaturated carbonyl compounds was
addressed by the PHIP technique.[32] The important features of
these reaction mechanisms were revealed with the help of
1H NMR signal enhancement and the characteristic shape of
NMR lines.
Figure 1. a) The reaction scheme of propene hydrogenation; b) 1H NMR
spectra acquired during propene hydrogenation with p-H2 at various tem-
peratures over a commercially available MoS2 catalyst. All spectra were ac-
quired while the gas was flowing (17 mLsÀ1) with eight signal accumulations
and are presented on the same vertical scale.
vation of PHIP effects during heterogeneous hydrogenation of
propene over the MoS2 catalyst gave us an opportunity to use
the PHIP technique for the investigation of reactions catalyzed
by metal sulfide catalysts.
Next, the supported MoS2/g-Al2O3 catalyst was utilized for
thiophene hydrodesulfurization with parahydrogen. Several re-
action products were observed including 1-butene, 2-butene,
and butane (Figure 2). However, all attempts to observe PHIP
effects (polarization) in this reaction by varying the reaction
temperature (200–6008C) and the gas flow rate (0.1–14 mLsÀ1)
were unsuccessful. There are two possible explanations for this
observation. First, nuclear spin relaxation processes that, with-
out doubt, become operative after the pairwise hydrogen ad-
dition, can completely destroy the polarization of the initially
polarized products, which would result only in thermally polar-
Herein, the PHIP technique was applied for mechanistic
studies of thiophene hydrodesulfurization. The unique capabili-
ties of this method allowed us to establish that thiophene HDS
over MoS2/g-Al2O3 proceeds mainly by the HYD route, whereas
the contribution of the alternative DDS route was found to be
more pronounced on Pt/TiO2.
Results and Discussion
As mentioned above, the typical catalyst for thiophene hydro-
desulfurization is MoS2 supported on g-Al2O3.[1,2,4,6] To date,
there are no reports about the utilization of sulfide catalysts in
heterogeneous catalytic processes involving parahydrogen.
Therefore, it is not known if the spin correlation between the
two hydrogen atoms can be preserved upon hydrogenation
on this type of catalyst. With this taken into consideration,
commercially available MoS2 (Sigma–Aldrich) catalyst was used
for the hydrogenation of propene gas, which is a standard
substrate for heterogeneous PHIP studies.[33–35] Propene hydro-
genation was performed at various temperatures (from T=200
to 4008C with a 508C increment) with parahydrogen. PHIP ef-
1
fects were successfully observed for the propane H NMR sig-
nals, which implied that H2 can be added to the propene C=C
bond in a pairwise manner with preservation of the parahydro-
gen spin order over the MoS2 catalyst (Figure 1).
Notably, this is the first report of PHIP effects in a hydrogena-
tion over a metal sulfide catalyst. The intensities of the hyper-
polarized NMR signals were found to increase with an increase
in the reaction temperature from 200 to 3508C; this was ac-
companied by an increase in propane yield. Successful obser-
Figure 2. a) The reaction scheme of thiophene hydrodesulfurization;
b) 1H NMR spectrum acquired during thiophene hydrodesulfurization with p-
H2 at T=6008C over the MoS2/g-Al2O3 catalyst. The spectrum was acquired
while the gas was flowing (1.4 mLsÀ1) with 16 signal accumulations.
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ized signals that are detected by NMR spectroscopy. Second,
whereas the general feasibility to observe PHIP over metal sul-
fide catalysts was successfully shown above (Figure 1), the hy-
drogenation of thiophene can occur through a reaction path-
way that does not involve pairwise hydrogen addition.
thiophene, which leads to 2,5-dihydrothiophene. The next step
can be hydrodesulfurization of 2,5-dihydrothiophene to form
2-butene, which can be subsequently further hydrogenated to
butane or can isomerize to 1-butene. Alternatively, butane can
be formed as a result of 2,5-dihydrothiophene hydrogenation
to tetrahydrothiophene and subsequent hydrodesulfurization.
For any of these mechanisms, the parahydrogen-derived pro-
tons are chemically equivalent in the respective product mole-
cules (2,5-dihydrothiophene, 2-butene, butane). Therefore, the
PHIP effects for these compounds should be largely sup-
pressed because of the symmetry of these molecules and
chemical equivalence of the p-H2 nascent protons.[36,37]
As mentioned in the Introduction, there are two possible
pathways of thiophene hydrodesulfurization (Scheme 1). The
hydrogenation (HYD) pathway involves thiophene hydrogena-
tion to dihydrothiophenes and tetrahydrothiophene with sub-
sequent hydrodesulfurization to butenes and butane, respec-
tively.[17,18] The second pathway, the direct desulfurization
(DDS), begins with thiophene hydrodesulfurization to 1,3-buta-
diene, which is further hydrogenated to butenes and butane.
To establish which pathway is involved with our catalytic
system, 1,3-butadiene was hydrogenated with p-H2 over the
MoS2/g-Al2O3 catalyst. The acquired 1H NMR spectra clearly
show characteristic PHIP patterns for the various reaction prod-
ucts (1-butene, 2-butene, and butane; Figure 3).
It is known that supported metal catalysts also catalyze thio-
phene hydrodesulfurization.[18] Previously, it was shown that
they can provide PHIP effects in the hydrogenation of such hy-
drocarbons as propene, propyne, 1,3-butadiene, and 1-
butyne.[31] Therefore, we examined several supported metal
catalysts in thiophene hydrodesulfurization. The Rh/TiO2 cata-
lyst was found to be completely inactive in the reaction even
at temperatures as high as 5008C. The Pd/TiO2 catalyst pro-
duced some amounts of 2-butene, but no PHIP effects were
observed. However, the Pt/TiO2 catalyst was found to produce
polarized lines for all hydrodesulfurization products, namely 1-
butene, 2-butene, and especially butane (Figure 4).
This result clearly indicates that the absence of PHIP effects
in thiophene hydrodesulfurization over the MoS2/g-Al2O3 cata-
lyst is a result of the peculiarities of the reaction mechanism
and not, for instance, because of the relaxation of hyperpolari-
zation during the transfer of reaction products from the reac-
tor to the NMR instrument.[38] Thus, in thiophene hydrodesulfu-
rization on Pt/TiO2, both possible reaction pathways can be re-
alized, which is in accordance with results of Wang and Iglesia,
Figure 3. a) The reaction scheme of 1,3-butadiene hydrogenation; b) and
c) 1H NMR spectra acquired during 1,3-butadiene hydrogenation with p-H2
at T=5008C over the MoS2/g-Al2O3 catalyst (b) while the gas was flowing
(4.0 mLsÀ1) and (c) after termination of the gas flow. Both spectra were ac-
quired with 16 signal accumulations and are presented on the same vertical
scale.
These results confirm that PHIP effects should be observed
for the butenes and butane if thiophene hydrodesulfurization
proceeds via the 1,3-butadiene intermediate (DDS pathway).
Therefore, we may conclude that 1,3-butadiene is not formed
during thiophene hydrodesulfurization over the MoS2/g-Al2O3
catalyst. Based on these observations, it can be supposed that
the alternative (HYD) pathway is preferentially realized. This is
in agreement with the results obtained by Hensen et al. for
thiophene hydrodesulfurization over several transition-metal
sulfides.[17] The absence of PHIP effects in thiophene hydrode-
sulfurization over MoS2/g-Al2O3 can then be explained as fol-
lows. The first step in the HYD pathway is 1,4-addition of H2 to
Figure 4. a) The reaction scheme of thiophene hydrodesulfurization; b) and
c) 1H NMR spectra acquired during thiophene hydrodesulfurization with p-H2
at T=5008C over the Pt/TiO2 catalyst (c) while the gas was flowing
(14 mLsÀ1) and (b) after termination of the gas flow. Both spectra were ac-
quired with 64 signal accumulations and are presented on the same vertical
scale.
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who found that thiophene hydrodesulfurization over Pt/SiO2
proceeds by both pathways with comparable turnover rates.[18]
Conclusion
In this work thiophene hydrodesulfurization was studied by
means of the parahydrogen-induced polarization (PHIP) tech-
nique. In the case of thiophene hydrodesulfurization over the
MoS2/g-Al2O3 catalyst, no PHIP effects were observed despite
the formation of 1-butene, 2-butene, and butane as reaction
products. However, if this catalyst was utilized for 1,3-buta-
diene hydrogenation, PHIP effects were observed for all reac-
tion products. These results unambiguously show that 1,3-bu-
tadiene is not formed during thiophene hydrodesulfurization
over MoS2/g-Al2O3. Therefore, the main reaction pathway in
this case should be hydrogenation to dihydro- and tetrahydro-
thiophenes and their subsequent hydrodesulfurization. On the
other hand, in thiophene hydrodesulfurization over the Pt/TiO2
catalyst, PHIP effects were observed for all reaction products,
that is, 1-butene, 2-butene, and butane. This fact suggests that
the hydrodesulfurization reaction on Pt/TiO2 proceeds by both
possible pathways (hydrogenation and direct desulfurization)
with comparable rates. The obtained results demonstrate the
unique capabilities of the PHIP method for mechanistic studies
of heterogeneous catalytic hydrodesulfurization reactions.
Figure 5. TEM image of the sulfided Mo/g-Al2O3 catalyst.
like fringes visualized by TEM correspond to the layered structure
of MoS2 nanoparticles with different stack heights and lengths
evenly distributed on the surface of Al2O3. The average slab length
of the sulfide nanoparticles is about 4.5 nm; the mean number of
layers is 1.9.
Commercially available hydrogen, thiophene (Sigma–Aldrich,
ꢀ99%), 1,3-butadiene, and propene were used in the catalytic ex-
periments as received. For PHIP experiments, H2 gas was enriched
with parahydrogen up to 50% by passing it through a spiral tube
containing FeO(OH) powder (Sigma–Aldrich) maintained at liquid
nitrogen temperature.
For 1,3-butadiene hydrogenation, a substrate/p-H2 (1:4) mixture
was supplied to a 1/4“ outside diameter (OD) straight stainless
steel tube heated with an electric tube furnace. The MoS2/g-Al2O3
catalyst (30 mg) was packed between two pieces of fiberglass
tissue in the middle of this steel reactor. The resulting gas mixture
was supplied through a 1/16” OD polytetrafluoroethylene capillary
to a 10 mm NMR tube located in the NMR spectrometer. For pro-
pene hydrogenation, the procedure was generally the same except
that a 1:3 substrate/p-H2 mixture and commercially available MoS2
catalyst (100 mg; Sigma–Aldrich, 99%) were used. In this case, the
catalyst was activated in 18% H2/He flow at T=3008C for 4 h and
then evacuated at T=5008C for 4 h.
Experimental Section
The MoS2/g-Al2O3 catalyst was prepared by impregnation of cylin-
drical Al2O3 granules (1.5 mm in diameter, BET surface area of
208 m2gÀ1, pore volume of 0.68 cm3gÀ1, average pore diameter of
13.1 nm; JSC “Promkataliz”, Ryazan, Russia) with an aqueous solu-
tion containing the required amounts of MoO3 (gradeꢀ99.0%;
Vekton, Saint-Petersburg, Russia) and citric acid (gradeꢀ99.8%;
Vekton, Saint-Petersburg, Russia). The concentration of molybde-
num in the solution was 2.8 molLÀ1; the citric acid/Mo molar ratio
was adjusted to 0.7. The granules were dried at T=110 8C for 4 h
and then crushed and sieved to provide 0.14–0.25 mm particles.
According to the inductively coupled plasma atomic emission
spectroscopy (ICP-AES) data, the Mo/Al2O3 material contained
12.1 wt% of Mo as determined after sample calcination. However,
to produce the final catalysts with a highly dispersed MoS2 phase
on the alumina surface, the calcination step of the Mo/Al2O3 sam-
ples was avoided.[39] The catalyst was activated by the standard
presulfidation procedure in the gas phase at T=4008C with a mix-
ture of gases containing H2S (5 vol%) and H2 (95 vol%).
For thiophene hydrogenation, the substrate/p-H2 mixture (ꢁ1:7.6,
according to vapor pressure data) was obtained by bubbling para-
hydrogen through a two-necked flask containing liquid thiophene.
The resulting gas mixture was supplied to the same stainless steel
reactor containing either the MoS2/g-Al2O3 (150 mg) or Pt/TiO2
(100 mg) catalyst. (In preliminary experiments with supported
metal catalysts, Rh/TiO2, Pd/TiO2, or Pt/TiO2 (30 mg) were used.)
The gas mixture was then supplied to the NMR tube located in the
NMR spectrometer and maintained at T=1008C to prevent con-
densation of the reactant and products. The MoS2/g-Al2O3 catalyst
was pretreated in the reaction mixture overnight at room tempera-
ture before the reaction.
The elemental analysis of the catalysts was performed with ICP-
AES on an Optima 4300 DV instrument (Perkin–Elmer, France);
before analysis, the sample was calcined at T=5008C for 4 h. The
sample in the sulfided state was studied with high-resolution trans-
mission electron microscopy (HRTEM) with a JEM-2010 transmis-
sion electron microscope (JEOL, Japan) with an accelerating volt-
age of 200 kV and resolution of 0.14 nm. The catalyst samples were
applied to copper gauze by dispersing an alcoholic suspension of
the sample with an ultrasonic disperser. To obtain statistical infor-
mation, the structural parameters of about 500 particles were mea-
sured.
All hydrogenation experiments were performed at atmospheric
pressure, with the reaction volume located outside the NMR
magnet (an ALTADENA[40] experiment). The gas flow rate was mea-
sured with an Aalborg rotameter. 1H NMR spectra were acquired
on a 300 MHz Bruker AV 300 NMR spectrometer with a p/2 rf pulse
in the case of 1,3-butadiene and thiophene hydrogenation and
a p/6 rf pulse in the case of propene hydrogenation.
A TEM micrograph depicting the typical surface fragments of the
sulfided Mo/g-Al2O3 catalyst is shown in Figure 5. The black thread-
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Acknowledgements
O.G.S. and D.A.B. thank RSCF (grant 14-13-00445) for support of
the design and construction of the experimental setup for per-
forming catalytic hydrogenation processes of industrial impor-
tance with parahydrogen. K.V.K. and D.B.B. thank RFBR
(grants 14-03-00374-a, 14-03-31239-mol-a, and 14-03-93183
MCX_a) for support of the thiophene hydrogenation studies. I.V.K.
thanks RSCF (grant 14-35-00020) for supporting the NMR experi-
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Received: June 16, 2015
Revised: July 8, 2015
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Published online on September 14, 2015
ChemCatChem 2015, 7, 3508 – 3512
www.chemcatchem.org
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