Hydrogen storage using heterocyclic compounds: The hydrogenation of 2-methylthiophene

Article abstract of DOI:10.1016/j.cattod.2009.02.039

Alkyl substituted thiophenes are promising candidates for hydrogen carriers, as their dehydrogenation reactions are known to occur under mild conditions. Four types of catalysts, including supported noble metals, bimetallic noble metals, transition metal

Full text of DOI:10.1016/j.cattod.2009.02.039

Catalysis Today 149 (2010) 172–184
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Hydrogen storage using heterocyclic compounds: The hydrogenation
of 2-methylthiophene
a
a,
b
H.Y. Zhao , S.T. Oyama *, E.D. Naeemi
a
Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute and State University,
40 Randolph Hall, Blacksburg, VA 24061, United States
1
b
Asemblon Inc., 15340 NE 92nd Street, Suite B, Redmond, WA 98052-3521, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Alkyl substituted thiophenes are promising candidates for hydrogen carriers, as their dehydrogenation
reactions are known to occur under mild conditions. Four types of catalysts, including supported noble
metals, bimetallic noble metals, transition metal phosphides and transition metal sulfides, have been
investigated for 2-methylthiophene (2MT) hydrogenation and ring-opening. The major products were
Available online 5 April 2009
Keywords:
Hydrogen storage
Hydrogenation
tetrahydro-2-methylthiophene (TH2MT), pentenes and pentane, with very little C
selectivity towards the desired product TH2MT follows the order: noble metals > bimetallics >
phosphides > sulfides. The best hydrogenation catalyst was 2% Pt/Al which exhibited relatively high
reactivity and selectivity towards TH2MT at moderate temperatures. Temperature-programmed reaction
TPR) experimentsrevealed that pentanethiolbecame themajorproduct, especially with HDScatalysts like
CoMoS/Al and WP/SiO
5
-thiols observed. The
Alkyl substituted thiophenes
2 3
O
2
-Methylthiophene
Sulfides
(
Phosphides
Noble metals
Bimetallics
O
2 3
2
.
ß 2009 Elsevier B.V. All rights reserved.
1
. Introduction
Hydrogen has been suggested as an ecologically clean energy
High pressure storage needs high strength containers and has a
limited volume capacity. A conventional steel hydrogen cylinder
can hold only 1% by weight hydrogen and the boil-off of liquefied
hydrogen requires venting, reduces driving range, and produces
safety problems. Hydrogen liquefaction is also energy intensive at
an expense of 30% of the heating value of hydrogen [6].
Metal hydrides are difficult to apply because they are too
thermodynamically stable. This has two consequences. First, the
hydrides have to be heated to an inconveniently high temperature
to release hydrogen. Second, the heat of absorption is so high that a
large amount of heat must be removed during the refueling
process. In addition, the hydride systems contain strongly reducing
agents which can react vigorously with air, and thus must be leak-
free [7].
Adsorption of hydrogen onto high surface area materials, such
as carbon nanotubes, has been studied extensively but also has
barriers. These include the reproducibility of the synthesis of the
material and the hydrogen storage performance [8].
Compared with the above methods, chemical hydrogen storage
provides high gravimetric and volumetric hydrogen densities.
Additionally, it has the advantage that hydrogen storage and
transportation use conventional petrochemical substances [9].
Chemical hydrogen storage materials, also called organic
chemical hydrides, employ hydrogenation–dehydrogenation of
cyclic hydrocarbons or heteroaromatic compounds as a means to
store and transport hydrogen. Aromatic compounds such as
carrier because it does not produce air pollution or greenhouse
gases. It can be produced by conventional or emerging membrane
technology [1–3]. Hydrogen has almost three times the energy
content of gasoline based on weight, but only about a quarter
based on volume [4]. Lower-cost, lighter-weight and higher-
density hydrogen storage is one of the key requirements for
hydrogen energy use.
The US Department of Energy has set technology targets for
hydrogen storage for 2010 and 2015. It is desired by 2010 to
develop hydrogen storage systems achieving a gravimetric density
of 2 kWh/kg (6 wt.%), a volumetric density of 1.5 kWh/L, and a cost
of $4/kWh, and by 2015, corresponding quantities of 3 kWh/kg
(9 wt.%), 2.7 kWh/L, and $2/kWh [4]. Currently four methods are
being considered as promising candidates for hydrogen storage:
compression and storage in conventional cylinders or cryogenic
tanks, adsorption by metal hydrides, adsorption on high surface
area materials, and chemical hydrogen storage (including off-
board regeneration) [5].
*
Corresponding author. Tel.: +1 540 231 5309; fax: +1 540 231 5022.
E-mail address: oyama@vt.edu (S.T. Oyama).
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.02.039

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
173
Scheme 1. Dehydrogenation of aromatic compounds.
benzene, toluene, and naphthalene can be easily hydrogenated by
using appropriate metal catalysts under relatively mild conditions,
e.g. about 100 8C and 2 MPa. However, the dehydrogenation of
cyclic hydrocarbons is highly endothermic and the reaction is
favored only at high temperatures (Scheme 1). Catalytic dehy-
drogenation under ‘‘liquid-film state’’ conditions has been
reported [10–13], where the reactant is supplied as a liquid so
that the surface of the catalyst is always wetted with a thin film.
Equilibrium limits were surpassed because of the evaporation of
the dehydrogenated reactants. Another method uses ‘‘wet-dry
multiphase conditions’’ to take advantage of multiple phases to get
over thermodynamic equilibrium limitations [14,15]. However,
these two processes still require relatively high temperatures for
vaporization of the volatile components of the process. An
important need is also an effective separation of hydrogen from
the mixtures to get a pure hydrogen product and to reuse the
hydrogen carrier materials.
thiophene, at the same time releasing hydrogen. For example, for
the case of 2-pentanethiol the reaction produces 2-methylthio-
phene (Scheme 4).
A method for catalytic hydrogenation of thiophenes needs to be
established to use thiophenes as hydrogen carriers. The further
cleavage of one of the C–S bonds, also called selective ring-opening
(SRO), is more desirable because an additional mole of hydrogen
can be carried than with the closed ring (Scheme 5).
In this work, 16 total catalysts are evaluated for the hydro-
genation and ring-opening of 2MT, including supported noble
metals, bimetallic noble metals, transition metal phosphides [22]
and transition metal sulfides. Temperature-programmed deso-
rption (TPD) of hydrogen provides insight into the relationship
between hydrogen activation and the hydrogenation activities of
the catalysts. Temperature-programmed reaction (TPR) is used to
elucidate the reaction mechanism over the supported catalysts.
Heteroatom aromatic rings for H
recently because the addition of electron-donating groups favors
release both thermodynamically and kinetically at moderate
2
storage were proposed
2. Experimental
H
2
2.1. Materials
temperatures. In the case of indoline, dehydrogenation is possible
at modest temperature (110 8C) [16]. Benzimidazolines, including
N,N -dimethyldihydrobenzimidazole, 1,3-dimethyl-2-phenylben-
Commercial noble metal catalysts including Pt/Al O , Pd/Al O ,
2
3
2 3
0
2 3
and Ru/Al O were provided by BASF Catalysts, Inc. Transition
zimidazoline, and 1,3-dimethylbenzimidazoline, were studied
metal sulfides MoS/Al O , NiMoS/Al O , CoMoS/Al O3 were
2
3
2
3
2
with different palladium catalysts, and were found to release H
even at room temperature [17] (Scheme 2).
2
obtained from Haldor Topsøe. Transition metal phosphides WP/
SiO , Ni P/SiO , MoP/SiO and bimetallics, PdPt/Al and PdRu/
Al were synthesized. The support, fumed silica EH-5, was
provided by Cabot Corp. The chemicals used in the synthesis of the
2
2
2
2
2 3
O
Hydrogen density is a very important factor in hydrogen
storage. A lower weight of the organic framework needs to be
developed while maintaining favorable thermochemical and
kinetic parameters. Smaller molecules such as 4-aminopiperidine
and piperidine-4-carboxamide are promising compounds for
reversible hydrogen storage [18]. The dehydrogenation and
hydrogenation of 4-aminopiperidine and piperidine-4-carboxa-
mide occur easily at low temperatures without by-products, such
as C–N cleavage and hydrogenolysis products. Density functional
theory calculations suggest that dehydrogenation is greatly
favored in five membered rings over six and by the incorporation
of N atoms into the rings, either as ring substituents or as ring
atoms, particularly in the 1 and 3 positions [19]. A number of
heteroaromatic ligands have been used for reversible hydrogena-
tion/dehydrogenation. For example, N-ethyl carbazole is hydro-
genated with Pd at 72 atm and 160 8C and dehydrogenated with Ru
at 50–197 8C [20] (Scheme 3).
2 3
O
catalysts were (NH
(Alfa Aesar, 99%), (NH
(NH HPO
99.9%), Pt(NH
xH O(Alfa Aesar, 99.9%) were the precursors for the bimetallic
4
)
6
W
12
O
39ꢀxH
2
O (Aldrich, 99%), Ni(NO
24ꢀ4H (Alfa Aesar, 99%),
ꢀxH (Alfa Aesar,
O (Alfa Aesar, 99.9%) and RuCl
ꢀx-
3
)
2
ꢀ6H
2
O
4
)
6
Mo
7
O
2
O
4
)
2
4
(Aldrich, 99%). Pd(NH
(NO
ꢀxH
3
)
4
Cl
2
2
O
3
)
4
3
)
2
2
3
2
catalysts. The chemicals used for the reactivity tests were 2-
methylthiophene (Alfa Aesar, 98%) and n-nonane (Alfa Aesar, 99%).
2
The gases employed were H (Airco, Grade 5, 99.99%), He (Airco,
Grade 5, 99.99%), CO (Linde Research Grade, 99.97%), 0.5% O /He
2
(Airco, UHP Grade, 99.99%), O
2
(Airco, UHP Grade, 99.99%), 10%
H S/H (Airco, UHP Grade, 99.99%) and N (Airco, Grade 5, 99.99%).
2
2
2
Chemical standards for GC and mass spectrometry were tetra-
hydro-2-methylthiophene (TH2MT) (Alfa Aesar, 98%), penta-
nethiol (Alfa Aesar, 99%), pentane (Alfa Aesar, 99%), 1-pentene
(Alfa Aesar, 99%), 2-pentene (Alfa Aesar, 99%).
The Asemblon hydrogen liquid storage system was proposed to
utilize substituted thiophenes as carriers for storage, transport,
and release of hydrogen [21]. This is based on the discovery that
finely dispersed supported metal catalysts readily carry out the
cyclization and dehydrogenation reaction of alkyl sulfides to
Scheme 3. Hydrogenation and dehydrogenation of N-ethyl carbazole.
0
Scheme 4. Dehydrogenation of pentanethiol.
Scheme 2. N,N -dimethyldihydrobenzimidazole dehydrogenation.

1
74
H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
Scheme 5. Hydrogenation and ring-opening of 2-methythiophene.
2
.2. Synthesis
for chemisorption. After pretreatment, the pressure was set to
3
2
MPa, a hydrogen flow rate of 150 cm NTP/min was started, and
3
WP/SiO
were prepared by TPR, following procedures reported previously
29,30]. Briefly, the synthesis of the catalysts involved two stages.
First, solutions of the corresponding metal phosphate precursors
were prepared by dissolving appropriate amounts of (NH
O, Ni(NO o, (NH Mo O, Pd(NH Cl
2
[23], Ni
2
P/SiO
2
[24–26], MoP/SiO
2
[27,28], PdP/SiO
2
the liquid reactant was fed at a rate of 2.4 cm /h. Reactivity testing
was carried out as a function of temperature, which was started at
the highest temperature 325 8C and was varied downwards and
upwards with the initial temperature repeated at the end.
Hydrotreating products were collected every few hours in sealed
septum vials and quantified by a gas chromatograph (Hewlett-
Packard, 5890A) equipped with a 0.32 mm i.d. ꢂ 50 m fused silica
capillary column and a flame ionization detector. The reactants and
products were identified by their retention time in comparison with
commercially available standards and confirmed by gas chromato-
graphy–mass spectrometry (GC–MS) (Hewlett-Packard, 5890-
5972A). EquilibriumcalculationswerecarriedoutusingtheCHETAH
program (ASTM International, West Conshohocken, PA).
[
4 6 12
) W
O
39ꢀxH
2
3
)
2
ꢀ6H
2
)
4 6
7
O24ꢀ4H
2
)
3 4
2 2
ꢀxH O
with ammonium phosphate in distilled water, and these solutions
were used to impregnate silica by the incipientwetness method. The
obtained samples were dried and calcined at 500 8C for 6 h, then
ground with a mortar and pestle, pelletized with a press (Carver,
Model C), and sieved to particles of 650–1180
2
at 2 8C min in flowing H
temperatures were 577 8C for WP/SiO
for MoP/SiO , 350 8C for PdP/SiO The samples were kept at the
reduction temperature for 0.5 h, followed by cooling to RT under He
flow [100 cm (NTP) min ], and then passivated at RT in a 0.5% O
Hefor4 h. The Ni, Mo, Wmolarloadingwereall1.6 mmol g (mmol
per g of support), corresponding to a weight loading of Ni P of 7.9%
mm diameter (16/
0 mesh). Second, the solid phosphates were reduced to phosphides
ꢁ1
3
ꢁ1 ꢁ1
g
2
[1000 cm (NTP) min
]. Reduction
2
, 568 8C for Ni
2
P/SiO , 494 8C
2
2
2
2.5. TPD and TPR
3
ꢁ1
2
/
2 3 2 2 2 3
The compounds Pt/Al O , PdPt/SiO , WP/SiO , and CoMoS/Al O
ꢁ
1
were the most active members of the four groups consisting of
noble metals, bimetallics and transition metal phosphides and
sulfides and were selected for further TPD and TPR tests. The TPD
experiments were performed in a U-shaped quartz reactor
connected to a mass spectrometer through a leak valve. The noble
metal catalysts were pretreated in a flow of hydrogen at 325 8C for
2 h, and then cooled in hydrogen to 225 8C. After a He flush at the
same temperature to remove the hydrogen background, the
heating temperature program was started 5 8C/min to 450 8C. The
mass 2 signal was monitored by MS. The sample was then cooled in
helium to 225 8C and 2MT was introduced to the system in a
helium carrier flow until saturation was reached. Then the system
was heated to 550 8C at 5 8C/min in hydrogen. A total of 12 masses
2
withaninitialNi/Pratioof1/2, MoP12.8%withinitialMo/P ratioof1,
WP 19.9% with initial W/P ratio 1.
The bimetallic catalysts (1/4) PdPt/SiO
2 2
, (4/1) PdPt/SiO [31]
and (4/1) PdRu/SiO of total loading 2 wt.% with Pd/metal weight
2
ratios (in parentheses) were synthesized by the incipient wetness
method from the corresponding precursors dissolved in distilled
water. After impregnation, the samples were dried in vacuum at
1
00 8C for 6 h, then calcined at 300 8C for 3 h [32].
2.3. Characterization
Irreversible CO uptake measurements were used to titrate the
were monitored: 2, 4, 34, 42, 43, 55, 70 (pentene, or C
fragment), 72 (pentane, or C 12 fragment), 87, 97 (2MT), 102
(TH2MT), 104 (pentanethiol). The TPD and TPR were done with
WP/SiO and CoMoS/Al in a similar way except that they were
pretreated at 450 8C in H and 400 8C in H /H S, respectively.
5 10
H
surface metal atoms and to provide an estimate of the active sites
on the catalysts for the noble metals and the transition metal
phosphides. Usually, 0.3 g of a passivated sample was loaded into a
5
H
2
2 3
O
quartz reactor. Noble metal catalysts were reduced in H
2
at 325 8C
2
2
2
for 2 h while passivated transition metal phosphides were reduced
at 450 8C for 2 h. After cooling in He, pulses of CO in a He carrier at
43 mmol s [65 cm (NTP) min ] were injected at RT through a
sampling valve, and the mass 28 (CO) signal was monitored with a
mass spectrometer. CO uptake was calculated by measuring the
decrease in the peak areas caused by adsorption in comparison
3. Results and discussion
3.1. CO chemisorption and O chemisorption
ꢁ ꢁ1
1
3
2
Uptakes of CO of the noble metal, bimetallic and transition
metal phosphide catalysts are given in Table 1. Earlier studies have
with the area of a calibrated volume (19.5
sulfides were characterized by the irreversible chemisorption of O
at dry-ice acetone temperature using the same techniques. Prior to
the measurement the samples were sulfided in a flow of 10% H S/
at 400 8C.
mmol). Transition metal
2
Table 1
Summary of catalyst characterization by CO chemisorption.
2
H
2
ꢁ1
Catalyst
CO uptake (
mmol g
) Dispersion (%) Particle size (nm)
0
5
0
.5% Pd/Al
% Pd/Al
.5% Pt/Al
2
O
3
10
120
8
21
25
31
42
47
77
55
84
8
4
4
2
.4. Reactivity studies
2
O
3
2
O
3
3
Hydrogenation activity was measured with a mixture containing
0 vol.% nonane and 90 vol.% 2MT at 2 MPa (300 psig) in a
2% Pt/Al
2
O
3
44
2
0
2
2
2
.5% Ru/Al
2
O
3
23
2
1
% PdRu(4/1)/SiO
2
140
83
1
continuous-flow trickle bed reactor. Briefly, the testing unit
consisted of three parallel reactors enclosed in heating ovens. The
dimensionsofthe reactorswere1.5 cmi.d. ꢂ 25.5 cmlong, and were
% PdPt(4/1)/SiO
% PdPt(1/4)/SiO
2
2
2
99
1
Ni
MoP/SiO
WP/SiO
PdP/SiO
2
P/SiO
2
95
11
5
200
42
18
4
generally loaded with 100
mmol active sites or 40 mmol for the
2
2
25
2
samples with low uptakes. To start a reaction, catalysts were placed
in the catalyticreactor and pretreated at the same conditions as used
2
74
56

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
175
Table 2
2
Summary of catalyst characterization by O chemisorption.
ꢁ
1
Catalyst
MoS/Al
Composition
O
2
uptake (
mmol g
)
Dispersion (%)
Particle size (nm)
2
O
3
(TK-Mo)
Mo 2–4.7%
58
54
55–23
17–9
12–7
12–8
2–4
NiMoS/Al
NiMoS/Al
2
O
3
3
(TK-NiMo1)
, AlPO (TK-NiMo2)
(TK-CoMo)
Ni 0.8–2.4%; Mo 7–13%
Ni 1.6–3.9%; Mo 8–12%, Al
5–10
9–13
7–11
2
O
4
2
O
3
68–80%, AlPO
4
5–11%
68
CoMoS/Al
2
O
3
Co 2.4–4.7%; Mo12–16%, support: 70–80%
100
shown that uptakes of the SiO
29,33–35]. The CO chemisorption uptakes of the different samples
varied in a wide range from 8 to 200 mol/g. It was found that for
noble metals, higher loadings samples gave larger CO chemisorp-
tion uptakes but the average particle sizes (2–4 nm) did not vary
appreciably. Bimetallic noble metal alloys were highly dispersed
on the silica support surface and the particle sizes were small (1–
2
and Al
2
O
3
supports were negligible
lower dispersion, although the total number of active sites
increased with the metal loading, which is reasonable.
Assuming that each active site adsorbs one CO molecule or one
oxygen atom, the number of active sites in the catalysts followed
the order: sulfides > phosphides ꢃ bimetallics > noble metals.
[
m
3.2. Reactivity
2
nm). Among the phosphides the order of dispersion was WP/
SiO < Ni P/SiO < MoP/SiO < PdP/SiO . For transition metal sul-
fides dispersion was approximated based on the O chemisorption
uptakes (Table 2). It was found that higher metal loadings gave
2
2
2
2
2
A model feed containing 10 vol.% nonane and 90 vol.% 2MT was
used to test the HDS and hydrogenation (HYD) activities of the 16
different catalysts. All the catalysts showed expected responses to
2
2 3 2 2 2 3
Fig. 1. Catalytic activity as a function of temperature. (a) 2% Pt/Al O , (b) 2% PdPt (1/4)/SiO , (c) WP/SiO and (d) CoMo/Al O .

1
76
H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
temperature with higher conversions at higher temperatures and
reasonable stability over the time course of the reactions. The
Fig. 2d shows the selectivity towards the desired product TH2MT.
The order was noble metals > bimetallics > phosphides > sulfides.
The order of selectivity towards TH2MT was opposite the order of
the number of active sites of the catalysts, which indicates that
major products were TH2MT, pentenes and pentane, but C
could barely be observed. For convenience, all of the HDS products
will be referred to as C HDS products. Fig. 1 gives the conversion
and selectivity of the four most active catalysts 2% Pt/Al , 2%
out of the four groups of
5
-thiols
5
2
active sites of the catalysts titrated by CO or O chemisorption
2
O
3
grossly favor HDS more than the hydrogenation product TH2MT.
The most active catalysts were the sulfides with up to 80%
conversion but with low selectivity to TH2MT of around 10%
(Fig. 2a and d). This is expected, as these commercial sulfides are
optimized for HDS. The noble metals (Pt and Pd) had conversion of
10–20% at lower temperatures and selectivity of 80–90%. At higher
temperatures the noble metals had conversion of 40–50% but
selectivity of 50–60%. The transition metal phosphides had very
low conversions around 5% at low temperature with selectivity of
70–90%. At higher temperature transition metal phosphides had a
conversion of 10–30% with selectivity of 30–80%. For the
bimetallics, the conversions were about 10–15% at lower
temperature with selectivity of 90%, while at a high temperature
the conversion increased to 40–50% while selectivity decreased to
60–80%.
2 2 2 3
PdPt (1/4)/SiO , WP/SiO , CoMoS/Al O
catalysts.
Fig. 2 summarizes reactivity results for all catalysts, presented
as the total turnover frequency (TOF) for the 2MT reaction, the TOF
of formation of TH2MT, the conversion of 2MT, and the selectivity
to TH2MT. Fig. 2a shows that activities of the catalysts expressed as
turnover frequency for the 2MT reaction follow the trend: noble
metals > sulfides > bimetallics > phosphides. Fig. 2c shows that in
terms of total conversion, the order is different, sulfides > pho-
phosphides ꢄ noble metals > bimetallics. Fig. 2c also shows that
TH2MT is the major hydrogenation product, followed by the HDS
products, pentane and pentene. Very little pentanethiol was ever
observed with all the catalysts, indicating that once this ring-
opening reaction occurred, the thiols underwent desulfurization.
Fig. 2. Reactions of catalysts in the hydrogenations of 2-methylthiophene. (a) Turnover frequency of 2MT reaction, (b) rate of Formation of TH2MT, (c) conversion of 2MT and
d) selectivity to TH2MT.
(

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
177
those obtained at standard conditions (20 MPa, 300 psig) shows
that the selectivity for hydrogenation of 2-MT was slightly
increased at lower temperatures (225 and 275 8C). However, at
higher temperature (325 8C) the pressure change almost did not
make a difference for the selectivity towards hydrogenation. The
pressure did not show an obvious impact on the TOF either.
Therefore, the activity studies for all the other catalysts were
carried out at the lower pressure of 300 psig.
Thiophene reactions have been studied extensively in the field
of hydroprocessing, because they are models for desulfurization of
sulfur-containing heterocycles in petroleum feedstocks. However,
the goal in hydroprocessing is to remove the sulfur atom (for a
recent review see [36]) and the hydrogenation and ring-opening
reactions have been considered just intermediary steps in the
overall process. Studies on hydrogenation of thiophenes in the gas
phase with Mo and W sulfides promoted with Ni and Co indicate
that hydrogenolysis is facile and the hydrogenated products are
only formed in small amounts [37], as found here. At 240 8C and
2
MPa, the activities of metal sulfides for the formation of
tetrahydrothiophene are in the order: Pd » Mo > Rh ꢅ Ru
Re > W > Co > Ni [38]. The higher activity of the palladium
Fig. 3. Comparison of theoretical equilibrium constants for HDS and HYD.
>
catalyst compared to the other sulfide catalysts is likely due to the
ease of PdS reduction, which favors the formation of metallic sites
for hydrogenation.
Table 3
Comparison of active site determinations.
2% PdPt(1/4)/SiO
2
2
2% Pt/Al O
3
2
CoMo/Al O
3
WP/SiO
2
The methyl group at the a-carbon of 2MT increases the electron
density on the aromatic ring and facilitates the activation of the
ring. As a result, the rate of 2MT hydrogenation was reported to be
almost an order of magnitude higher than that of thiophene
hydrogenation [38]. The transformation of 2MT is believed to occur
by a consecutive-parallel network on PdS because the selectivity to
the hydrogenation product TH2MT and the hydrogenolysis
product pentane are constant until the conversion reached 50%
H
2
TPD desorption
mol/g)
CO chemisorption
mol/g)
chemisorption
mol/g)
5
3
30
12
(
m
a
44
99
–
42
(
m
a
a
a
O
2
–
–
100
–
(
m
a
Not measured.
[39] (Scheme 6).
Desulfurized products are favored at high temperatures and
Supports with strong Br o¨ nsted acid surface sites like SiO
2
,
TH2MT is favored at low temperatures for all of the catalysts,
which is consistent with theoretical equilibrium results: HDS was
less favored at low temperatures (Fig. 3). As reaction temperature
increased, the reactivity of 2MT increased while selectivity
towards TH2MT decreased.
aluminosilicates, and zeolite HNaY showed much larger specific
2
catalytic activity for the hydrogenation of 2MT than Al , TiO ,
2
O
3
and carbon [40]. A commercial CoMo sulfide catalyst produced
TH2MT and HDS products (pentenes and pentane) as the primary
products [41]. The product 1-pentanethiol from the SRO of TH2MT
was present in very small amounts.
The results of the effect of pressure on the activity of 0.5% and
2
% Pt/Al
2
O
3
are summarized in Table 4. Comparison of the results
Hydrogen dissociatively chemisorbs on the surface of sulfide
catalysts and forms hydrogen atoms and ions [42]. 2MT is
2 3
on 0.5% and 2% Pt/Al O of higher pressure (3.7 MPa, 550 psig) to
Table 4
2 3
Effect of pressure on the activity of 0.5% and 2% Pt/Al O .
Pressure (MPa, psig)
Catalyst
498 K
548 K
598 K
ꢁ
1
ꢁ1
ꢁ1
TOF (s )
TOF (s
)
Selectivity (%)
TOF (s
)
Selectivity (%)
Selectivity (%)
2
.0, 300
.7, 550
0.5% Pt/Al
2
O
3
3
0.01
92
94
0.04
0.02
83
84
0.06
0.06
53
60
2
2% Pt/Al O
3
0.009
3
0.5% Pt/Al
% Pt/Al
2
O
0.01
96
95
0.03
0.02
84
87
0.05
0.04
50
59
2
2
O
3
0.006
Table 5
Product distribution for TPR of 2MT.
TPR of 2MT
2% PdPt(1/4)/SiO
(K)
2
2% Pt/Al
2
O
3
CoMoS/Al
(K)
2
O
3
WP/SiO
2
T
p
m
mol/g
T
p
(K)
mmol/g
T
p
mmol/g
T
p
(K)
mmol/g
1
-pentene (
Pentane ( mol/g)
MT ( mol/g)
TH2MT ( mol/g)
Pentanethiol ( mol/g)
mol/g)
m
mol/g)
590
0.06
0
670
0.3
0
560
550
550
640
640
0.3
0.1
0.04
0.04
5
560
0.1
0
a
a
a
m
–
–
–
a
2
m
–
a
0
580
0.02
0
550
640
640
0.3
0.03
4.2
4.6
a
m
–
0
–
m
m
630
0.16
0.2
720
0.7
1
Total product (
Conversion (%)
5.6
100
98
99
99
a
Not observed.

1
78
H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
Scheme 6. Hydrodesulfurization reaction mechanism for 2-methylthiophene.
Scheme 7. Hydrogenation mechanism for 2-methylthiophene.
protonated at the
a-carbon and produces a thiophenium cation or
a
p
-complex with coordinatively unsaturated surface cations to
2
Fig. 5. Relation between turnover frequency and H TPD desorption.
form the thiophenium cation. In the presence of a hydride ion
donor, for example, a metal hydride, thiophene hydrogenation
occurs by hydride transfer (Scheme 7) [38]. A similar 2MT
hydrodesulfurization mechanism was proposed on metal sulfide
catalysts [43]. The rate-determining step before the rupture of the
C–S bond is the transfer of the first hydrogen to the adsorbed
thiophenic compound.
The proton sites are very important for 2MT activation.
Hydrides are thought to be a source of hydrogen for the
hydrogenation and hydrogenolysis reactions. The TH2MT ring
can be protonated and then made to undergo rupture through an
elimination process to produce thiols.
ring-opening of methylcyclopentane to the corresponding C
6
paraffins [44]. Ring-opening of naphthenes was seen to occur
exclusively on the Br o¨ nsted acid sites of zeolites [45]. The number
and strength distribution of Br o¨ nsted acid sites [46,47], the
crystallite size [48], and the zeolite topology were all important
parameters for the desired ring-opening products. When an
effective acid functionality is combined with a high-activity
hydrogenolysis metal, the bifunctional catalyst system will be
greatly selective to the ring-opening products [49]. The commer-
cial process for selective ring-opening involves such bifunctional
catalysts. The acidic sites catalyze dehydrogenation, cracking,
isomerization and dealkylation, while the metal sites carry out
adsorption and desorption. Noble metals supported on acidic
oxides are highly active catalysts for selective ring-opening [50].
All these transformations though occur on aromatics not contain-
ing heteroatoms, when heteroatoms like sulfur are present these
molecules are removed by hydrogenolysis.
Hydrogenation of aromatic and consecutive ring-opening can
be carried out on certain noble metals. Pt, Pd, Ir, Ru and Rh
2 3
supported on Al O have been found active and selective for the
3
3
.3. TPD and TPR
.3.1. TPD of H
Based on their high reactivity and selectivity, 2% PdPt (1/4)/
SiO , 2% Pt/Al , CoMo/Al , and WP/SiO were chosen for
further study by TPD of H and TPR of 2MT. The TPD experiments
were started 498 K instead of room temperature so as to
depopulate the low energy binding state of H [51] and to only
2
2
2
O
3
2
O
3
2
2
2
Fig. 4. TPD of H
2
profile.
Fig. 6. Product Distribution for TPR of 2MT.

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
179
2
Fig. 7. TPR of 2MT on 2% PdPt/SiO profiles.
examine the effective active sites in the range of temperatures used
for the reaction of 2MT. Hydrogen desorption peak temperatures
metals than on the transition metal compounds, as expected.
Hydrogen was also observed to desorb over a broad range of
temperature on these four catalysts. The broad desorption spectra
are possibly due to the site heterogeneity in the samples [53].
Table 3 compares the numbers of active sites measured by using
for 2% PdPt (1/4)/SiO
30, 540, 550, 560 K, respectively (Fig. 4), which are comparable
with results in the literature [52]. The order desorption of H
indicates that hydrogen is held slightly less strongly on the noble
2 2 3 2 3 2
, 2% Pt/Al O , CoMo/Al O , and WP/SiO were
5
2
2
different methods. The numbers measured by TPD of H were much

1
80
H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
2 3
Fig. 8. TPR of 2MT on 2%Pt/Al O profiles.
smaller than those titrated by CO or O
because the lower energy binding states of hydrogen were excluded
by the higher TPD starting temperature as mentioned above. H TPD
desorption amounts for these four catalysts followed a different
order from the CO and O chemisorption results.
It was found that the H TPD desorption amount was inversely
related to the rate of TH2MT formation (Fig. 5). Since the H
2
chemisorption This is mainly
desorption amount reflects the coverage of hydrogen at reaction
conditions, this may indicate that hydrogen was consumed to a
greater degree on the more active catalysts.
2
2
3.3.2. TPR of 2MT and H
Temperature programmed reduction experiments of 2MT were
carried out with 2% PdPt (1/4)/SiO , 2% Pt/Al , CoMo/Al , and
2
2
2
2
2
O
3
2 3
O

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
181
2 3
Fig. 9. TPR of 2MT on CoMo/Al O profiles.
WP/SiO
2
(Figs. 7–10) and the desorption peaks were integrated to
on the catalysts followed the order: CoMo/Al
Pt/Al > 2%PdPt (1/4)/SiO
for the amount of desorbed H
indicates that the sites that are active for binding hydrogen are also
active for 2MT hydrogenation. The major product in the TPR of 2MT
2
O
3
> WP/SiO
2
> 2%
quantify the corresponding products (Table 5). The unreacted 2MT
desorbed at a low temperature of around 550 K in a narrow peak
but over 90% of the adsorbed 2MT reacted in the broad
temperature range 498–823 K. The total amounts of 2MT adsorbed
2
O
3
2
. This order matches the order found
in the TPD experiment, which
2

1
82
H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
2
Fig. 10. TPR of 2MT on WP/SiO profiles.
was the ring-opened product, pentanethiol, with some pentene
also formed. The selectivities to TH2MT were no more than 6% and
very little or no pentane was formed (Fig. 6). This is a reasonable
finding. Generally the desulfurization of thiophenes involves two
pathways [54]. One is the direct desulfurization pathway which in
the case of 2MT gives pentenes or pentane. The other is the
N
Scheme 8. S 2 mechanism for 2MT ring-opening.

H.Y. Zhao et al. / Catalysis Today 149 (2010) 172–184
183
Scheme 10. Metal hydride ring-opening of 2MT.
Scheme 9. E2 mechanism for 2MT ring-opening.
Scheme 11.
hydrogenation pathway which gives TH2MT followed by desul-
furization of TH2MT. It has been reported that the reactivities of
thiophenes and tetrahydrothiophenes are similar while those of
thiols are about 15–40 times more reactive [55]. Thus, the
formation of pentene or pentane and TH2MT is expected, not
pentanethiol.
tion metal sulfides. The major products were TH2MT, pentenes and
pentane, and C -thiols could barely be observed. The selectivity
towards the desired product TH2MT follows the order: noble
metals > bimetallics > phosphides > sulfides. The order of selec-
tivity towards TH2MT was opposite the order of the number of
active sites of the catalysts, which indicates that the active sites of
5
The products distribution of TPR of 2MT is shown in Fig. 6.
Pentanethiol formed through the ring-opening of TH2MT was the
major product. There are three pathways to ring-open TH2MT
similar to the mechanisms of aromatic ring-opening, such as
naphthalene [49]. Protonation of TH2MT followed by attack by a
nucleophile will open the ring to form 2-pentanethiol because the
the catalysts titrated by CO or O
than the hydrogenation product TH2MT.
TPD of hydrogen indicated that the H
inversely related to the rate of TH2MT formation. TPR showed that
pentanethiol was the major product on the 2MT preadsorbed
2
chemisorption favor HDS more
2
desorption amount was
2 3
surface, especially on excellent HDS catalysts like CoMoS/Al O
S
N
2 process favors the less hindered carbon (Scheme 8). On the
other hand, attack by a base will tend to open the ring on the more
substituted side by elimination (Scheme 9). A third potential
2
and WP/SiO .
b
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