Low temperature catalytic hydrogenation naphthalene to decalin over highly-loaded NiMo, NiW and NiMoW catalysts

Article abstract of DOI:10.1016/j.cattod.2016.03.024

Hydrogenation of naphthalene to decalin at low temperatures (140–240 °C) was studied over highly-loaded sulfided NiMo, NiW, and NiMoW catalysts with a supported NiMo/γ-Al₂O₃ catalyst as comparison. The NiMo, NiW, and NiMoW catalyst precursors were synthesized by hydrothermal reactions, and the corresponding highly-loaded catalysts were made by mixing the precursors with an alumina gel. The catalyst precursors, oxide and sulfided highly-loaded catalysts were characterized by XRD, N₂ adsorption-desorption, SEM, and TPR techniques. A highly crystalline phase of ammonium nickel molybdate was detected on the NiMo precursor, and the NiW precursor exhibited sharp XRD peaks attributed to a phase of hydrated tungsten oxide. Typical Ni₃S₂ and MoS₂/WS₂ nanoparticles were observed over the sulfided highly-loaded catalysts, and a main reduction peak was detected due to nickel sulfide as revealed by TPR. Catalytic results showed that more than 99.0% of naphthalene was hydrogenated over the sulfided highly-loaded catalysts at 200 °C or higher temperatures, with a very high selectivity to decalin (more than 99.9%) during the same temperature regions over the NiMo and NiMoW catalysts. As a comparison, the reference NiMo/γ-Al₂O₃ catalyst showed a moderate hydrogenation activity with a naphthalene conversion of 49.6% and a decalin selectivity of 40.1% at 300 °C. The ratios of trans- to cis-decalin on the highly-loaded catalysts and on the NiMo/γ-Al₂O₃ catalysts varied.

Full text of DOI:10.1016/j.cattod.2016.03.024

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Low temperature catalytic hydrogenation naphthalene to decalin over
highly-loaded NiMo, NiW and NiMoW catalysts
Huan Liu, Chenguang Liu∗, Changlong Yin, Bin Liu, Xuehui Li, Yanpeng Li, Yongming Chai,
Yunqi Liu
State Key Laboratory of Heavy Oil Processing and Key Laboratory of Catalysis of CNPC, China University of Petroleum, Qingdao 266580, PR China
a r t i c l e i n f o
a b s t r a c t
Hydrogenation of naphthalene to decalin at low temperatures (140–240 ◦C) was studied over highly-
loaded sulfided NiMo, NiW, and NiMoW catalysts with a supported NiMo/␥-Al2O3 catalyst as comparison.
The NiMo, NiW, and NiMoW catalyst precursors were synthesized by hydrothermal reactions, and the
corresponding highly-loaded catalysts were made by mixing the precursors with an alumina gel. The
catalyst precursors, oxide and sulfided highly-loaded catalysts were characterized by XRD, N2 adsorption-
desorption, SEM, and TPR techniques. A highly crystalline phase of ammonium nickel molybdate was
detected on the NiMo precursor, and the NiW precursor exhibited sharp XRD peaks attributed to a phase
of hydrated tungsten oxide. Typical Ni3S2 and MoS2/WS2 nanoparticles were observed over the sulfided
highly-loaded catalysts, and a main reduction peak was detected due to nickel sulfide as revealed by
TPR. Catalytic results showed that more than 99.0% of naphthalene was hydrogenated over the sulfided
highly-loaded catalysts at 200 ◦C or higher temperatures, with a very high selectivity to decalin (more
than 99.9%) during the same temperature regions over the NiMo and NiMoW catalysts. As a comparison,
the reference NiMo/␥-Al2O3 catalyst showed a moderate hydrogenation activity with a naphthalene
conversion of 49.6% and a decalin selectivity of 40.1% at 300 ◦C. The ratios of trans- to cis-decalin on the
highly-loaded catalysts and on the NiMo/␥-Al2O3 catalysts varied.
Article history:
Received 14 October 2015
Received in revised form 26 February 2016
Accepted 3 March 2016
Available online xxx
Keywords:
Low temperature hydrogenation
Highly-loaded catalyst
Naphthalene
NiMo(W)
Decalin
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
which includes conventional hydrotreating catalysts (Mo or W sul-
fide promoted by Ni or Co and usually supported on ␥-Al2O3) in the
Clean environmental legislations on diesel not only put
impelling emphasis on the contents of heteroatoms-containing
compounds, like sulfur, nitrogen and oxygen [1], but also lay spe-
cial focus on the density (no more than 845 kg m−3) and cetane
number (higher than 51) [2–4]. The aromatics and their result-
ing emissions have potential hazardous and carcinogenic effects
[4]. The decease of density and increase of cetane number can
be simultaneously achieved by saturation of aromatics. For exam-
ple, only total hydrogenation of naphthalene (ca. 990 kg m−3) to
decalin (ca. 896 kg m−3) leads a sufficient decrease of density, an
increase of cetane number, and improved combustion character-
istics [2,3]. In addition, naphthalene hydrogenation and decalin
dehydrogenation is proposed as a potential catalysis pair for hydro-
gen storage, which could provide sufficient hydrogen content of
decalin (7.3 wt.%, 64.8 kg-H2/m3, more than Department of Energy
(DOE, USA) target values 6.5 wt.% and 62.0 kg-H2/m3) [5]. In refiner-
ies, a two-stage strategy has been adopted for deep hydrotreating,
first stage and noble metal catalysts in the second stage for deep
hydrogenation [1,3,6].
A lot of information about naphthalene hydrogenation and
formation of decalin have been reported in literature, which is
mainly limited to metallic Ni catalysts [6–8], and noble metal cat-
alysts, including Pt [5,6,9,10], Pd [6,11], PtPd [2,12], and Rh [13].
Meanwhile, new catalytic active phases, like molybdenum carbide
[14,15], cobalt silicide [4,16], and nickel phosphide [17], are also
testified for naphthalene hydrogenation. Noble metal-based cat-
alysts, including Pt, Pd, Rh, or bimetallic systems of these noble
metals, show distinctly high catalytic activities towards naphtha-
lene hydrogenation, due to the efficient ability for dissociating
hydrogen. However, high costs of these catalysts and low resis-
tances to poisons retard their industrial applications [2–4,11,14].
Molybdenum carbide and cobalt silicide catalysts show moderate
hydrogenation abilities compared to noble metal catalysts, which
could only convert naphthalene to tetralin [4,14]. The resistance
to sulfur-containing compounds of these novel active compounds
should also be improved and modified before large-scale applica-
tions. Besides, the preparation methods and also raw materials of
∗
Corresponding author.
E-mail address: cgliu@upc.edu.cn (C. Liu).
http://dx.doi.org/10.1016/j.cattod.2016.03.024
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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NiMo, NiW and NiMoW catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.024

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Fig. 1. XRD patterns of the NiMo, NiW, and NiMoW catalyst precursors.
high prices for synthesis of these novel compounds need further
modifications [4,14].
Transition metal sulfides (TMS) have been used for the removals
of heteroatoms from petroleum for a long period due to their unique
properties, like high cost-effectiveness, stability in high tempera-
ture and pressure, and long-time duration in reducing conditions,
and other unique properties [1,14,18,19]. However, TMS were tra-
ditionally regarded with moderate or low hydrotreating abilities
[14]. A breakthrough of TMS-based hydrodesulfurization (HDS)
catalysts called NEBULA occurred in 2002, which could produce
clean diesel less than 10 ␮g/g S in a unit originally designed for
350- or 500- ␮g/g S [18,20]. This catalyst showed high hydro-
denitrogenation (HDN) and hydrogenation activity [20]. In order
to search for a highly active HDS catalyst we synthesized ammo-
nium nickel molybdate as a potential precursor and highly-loaded
HDS catalysts were prepared by mixing this precursor and an alu-
mina gel for catalytic reactions [19,21,22]. The results showed that
the hydrotreating catalysts based on this precursor exhibited high
HDS, HDN and hydrotreating activities [19,21,22]. In this paper we
adopted this catalyst for naphthalene hydrogenation to decalin at
low temperatures. In order to discover more catalytic options, the
NiW and NiMoW precursors were also synthesized by hydrother-
mal reactions, and a reference NiMo/␥-Al2O3 catalyst was used for
comparison.
Fig. 2. N2 adsorption-desorption isotherms (Up) and pore size distributions (Down)
of the NiMo, NiW, and NiMoW catalyst precursors.
the as-synthesized precursors were first mixed with an alumina
gel (80/20 in weight) and then extruded into rods. According to
the chemical composition of as-synthesized material, the catalyst
precursors, and the corresponding highly-loaded catalysts were
named NiMo, NiW, and NiMoW.
A NiMo/␥-Al2O3 (BET surface area 230 m2 g−1) catalyst devel-
oped by our group was chosen as a reference catalyst. This catalyst
was prepared by sequent impregnation of ammonium heptamolyb-
date ((NH4)6Mo7O24·4H2O) and nickel nitrate (Ni(NO3)2·6H2O) to
obtain MoO3 and NiO loadings of 20.0 and 5.0%, respectively. After
each step of impregnation, this NiMo/␥-Al2O3 catalyst was first
dried at 100 ◦C for 4 h and then calcined at 500 ◦C for 4 h in an air
atmosphere.
2. Experimental
2.1. Catalyst preparation
The NiMo, NiW, and NiMoW catalyst precursors were synthe-
sized by the hydrothermal reaction method as described before
[21]. The reagents used were purchased from Sinopharm Chemi-
cal Reagent Company (PR China) without any further purification
(Grade AR). In a typical run, nickel nitrate (Ni(NO3)2·6H2O) was first
mixed with ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) or
(or and) ammonium metatungstate ((NH4)6H2W12O40·4H2O) to
obtain Ni:Mo (or Ni:W) in a molar ratio of 1:1 or Ni:Mo:W in a
molar ratio of 2:1:1, then urea and polyethylene glycol additive
were added and the mixture was stirred with a proper amount of
deionized water (250 mL). The liquid mixtures were transferred to
an autoclave of 500 mL. After reacting at 150 ◦C for 5 h, the catalyst
precursors were obtained. As for traditional TMS-based supported
catalysts, the total loadings of active components were usually no
more than 30 wt.%. In order to prepare the highly-loaded catalysts
2.2. Catalyst characterization
Characterizations of the NiMo, NiW, and NiMoW samples,
including the catalyst precursors, oxide catalysts, and sulfided cat-
alysts, have been described in detail elsewhere [19,21,22]. In short,
X-ray powder diffraction (XRD) characterizations were carried out
on an X’Pert Pro MPD equipment with Cu K␣ at a scan rate of
2◦ min−1. Nitrogen adsorption-desorption experiments were taken
on a ChemBET 3000 workstation (Quantachrome) using nitrogen as
absorbent. The scanning electron microscope (SEM) analysis was
carried out on an FEI Quanta200 equipment. The temperature-
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Table 1
3
programmed reduction characterizations (TPR) were performed
on a Micromeritics RS232 workstation equipped with a thermal
conductivity detector (TCD).
BET results of the NiMo, NiW, and NiMoW catalyst precursors.
Precursor BET Surface
Area (m2 g−1
Pore Volume (cm3 g−1
)
Average Pore Size (nm)
)
2.3. Naphthalene hydrogenation reactions over the highly-loaded
NiMo, NiW, and NiMoW catalysts
NiMo
NiW
NiMoW
89
142
171
0.26
0.24
0.25
5.7
3.5
3.2
Naphthalene hydrogenation reactions were carried out in a high
pressure fixed-bed down-flow microreactor, and 5 mL of the oxide
highly-loaded NiMo, NiW, or NiMoW catalysts (20–40 mesh) were
loaded in the center. Both ends of the reactor were filled with quartz
sand (20–40 mesh). Before catalytic reaction, the catalysts were
first in-situ sulfided at 330 ◦C for 12 h with a liquid stream contain-
ing 3.0 wt.% CS2 in petroleum ether (90–120 ◦C) and a H2 pressure of
3.0 MPa. Then 10.0 wt.% naphthalene-petroleum ether (90–120 ◦C)
solution was pumped into the reactor at a rate of 15.0 mL/h. The
amount of hydrogen into the reactor was controlled mass flow
meter and kept at a rate of 4500 mL/h. Due to the distinctly
high activities of the highly-loaded catalysts, the hydrogenation
reactions were carried out in moderate conditions at a hydrogen
pressure of 3.0 MPa, a H2/oil of 300 (v/v), temperatures of 140, 160,
180, 200, 220, and 240 ◦C, and a liquid hourly space velocity (LHSV)
of 3.0 h−1. As for the reference NiMo/␥-Al2O3 catalyst, the tem-
peratures were raised up to 220, 240, 260, 280, and 300 ◦C, while
the other reaction conditions were kept the same. The liquid prod-
uct obtained from the high-pressure separator was analyzed by a
Varian 3800 gas chromatograph (flame ionization detected, FID)
coupled with a quadrupole mass spectrometer (Finnigan SSQ710)
(GC–MS).
phase of ammonium nickel molybdate ((NH4)HNi2(OH)2(MoO4)2,
JCPDS card No. 50-1414). The sharp peaks detected at 2␪ of 14.9,
28.8, 30.1, and 50.2◦ in the NiW catalyst precursor were attributed
to the hydrated tungsten oxide (WO3·0.5H2O, JCPDS card No. 44-
0363). The ternary NiMoW catalyst precursor exhibited a quite
lower intensity of XRD peaks with comparison to the NiMo and
NiW precursors. Some XRD peaks due to the hydrated tungsten
oxide with 2␪ at 14.9, 28.8, 30.1, and 50.2◦ were also confirmed on
the NiMoW precursor, while XRD peaks attributing to the phase
of ammonium nickel molybdate were not detected. Compared to
the NiMo and NiW precursors, the less crystallinity of the NiMoW
precursor might be attributed to the suppression effect of Mo (or
W) addition to the NiW (or NiMo) systems.
The N2 adsorption-desorption isotherms and pore size distri-
butions of the NiMo, NiW, and NiMoW catalyst precursors are
depicted in Fig. 2. The hysteresis loops of isotherms (Type IV) were
confirmed over these catalyst precursors, indicating the detections
of meso-pores on these three precursors [22,24]. Hysteresis loop of
these catalyst precursors was typical for a H4 Type, which implied
that these materials had narrow slit-like pores [24]. Pore size dis-
tributions results revealed that a narrow pore size distribution was
detected over the NiMo, NiW, and NiMoW precursors. The textural
properties of the oxide precursors, including BET surface area, pore
volume, and pore size, were reported in Table 1. Compared to the
binary NiMo and NiW precursors, the ternary NiMoW precursor
had the largest BET surface area (171 m2 g−1).
3. Results and discussion
3.1. Catalyst precursor and oxide highly-loaded catalyst
characterization
The NiMo, NiW and NiMoW catalyst precursors were charac-
terized by XRD, and the results are present in Fig. 1. The three
precursors exhibited various XRD patterns. As reported before
[21,23], several well-resolved XRD peaks of the NiMo catalyst pre-
cursor, centered at 17.4, 23.5, 26.5, and 29.6◦, were ascribed to the
The SEM micrographs of the NiMo, NiW, and NiMoW catalyst
precursors (Fig. 3) were recorded to provide information on the
morphologies of these catalyst precursors. It could be easily found
that the small NiMo, NiW, or NiMoW nanoparticles were formed
during the hydrothermal reactions. The NiMo precursor showed
Fig. 3. SEM photographs of the NiMo, NiW, and NiMoW catalyst precursors.
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Fig. 5. XRD patterns of the sulfided NiMo, NiW, and NiMoW catalysts and the ref-
erence NiMo/␥-Al2O3 catalyst.
Table 2
Fig. 4. TPR patterns of the oxide NiMo, NiW, and NiMoW catalysts.
BET results of the sulfided highly-loaded NiMo, NiW, and NiMoW catalysts.
Catalyst BET Surface
Area (m2 g−1
Pore Volume (cm3 g−1
)
Average Pore Size (nm)
small nanoparticles about 30 nm attached to larger ones, whose
sizes were several hundred nanometers, and the NiW precursor
exhibited more uniform nanoparticles and the sizes were about
50 nm. The nanoparticles of about 20 nm of the NiMoW precursor
agglomerated to form some larger clusters.
)
NiMo
NiW
103
103
112
0.16
0.19
0.18
3.3
3.6
3.4
NiMoW
H2-TPR experiments were employed to characterize the
reducibility of the oxide highly-loaded NiMo, NiW, and NiMoW cat-
alystsm and the results are depicted in Fig. 4. The TPR patterns of
the oxide NiMo and NiMoW catalysts have been reported recently
[25]. A broad hydrogen consumption peak was in the temperature
range from 375 to 610 ◦C, and this TPR peak was centered at 495 ◦C,
which was attributed to the reduction of molybdenum-containing
species [22,25,26]. Due to limitations of our TPR instruments, sig-
nals at high temperature (more than 800 ◦C) were not recorded, but
as reported by other groups [27,28], the full reduction of molybde-
num or tungsten-containing species might occur at a temperature
higher than 1000 ◦C. As for the oxide NiW catalyst, the TPR peak
was distinctly different from that of the oxide NiMo catalyst. A
wide but not centered hydrogen consumption peak from 415 to
762 ◦C was detected. Two small TPR peaks were confirmed at 536
and 686 ◦C, which were ascribed to the reduction of WO3 crystal-
lites to WO3 − x (0 < x < 1) [29] and NiWO4 [27], respectively. The
oxide ternary NiMoW catalyst showed a broad TPR peak from 334
to 743 ◦C. Similar to the oxide highly-loaded NiW catalyst, a main
hydrogen consumption peak was detected at 539 ◦C, which was also
attributed to the reduction of WO3 crystallites to WO3 − x (0 < x < 1)
[29].
characteristic XRD peaks of the MoS2 and WS2 nanoparticles were
overlapped, thus it seemed difficult to distinguish one from the
other by XRD characterization. As for these highly-loaded sulfided
catalysts, some very sharp XRD peaks at around 21, 31, 44, 50, and
55◦ were detected, which were attributed to the phase of Ni3S2
nanoparticles [21,22,32]. The highly-stacked MoS2/WS2 and Ni3S2
nanoparticles on the highly-loaded NiMo, NiW, and NiMoW cata-
lysts were mainly attributed to the agglomerations of TMS under
high temperatures. Compared with the results reported before [31],
the sulfided NiMo and NiW catalysts exhibited more intense XRD
peaks of the Ni3S2 phase than the catalysts after HDS reactions. The
sulfided NiMo/␥-Al2O3 catalyst exhibited an amorphous XRD pat-
tern, and no typical peaks due to Ni or Mo-containing sulfides were
detected.
The N2 adsorption-desorption characterizations of the sulfided
NiMo, NiW, and NiMoW catalysts (Fig. 6) showed that the typical IV
isotherms were maintained [22,24]. Besides, the hysteresis loop of
these sulfided highly-loaded catalysts was due to a H4 Type, indi-
cating the narrow slit-like pores [22]. The pore size distributions of
the sulfided highly-loaded catalysts also showed a narrow pore size
distribution, which was similar to those of the catalyst precursors.
Table 2 exhibits that the BET surface areas of the sulfided catalysts
were from 103 to 112 m2 g−1, while the average pore diameters
were 3.3, 3.6, and 3.4 nm for the NiMo, NiMoW, and NiW catalysts,
respectively, which could provide efficient pathways for reactants
and products.
3.2. Sulfided highly-loaded catalyst characterization
The sulfided highly-loaded NiMo, NiW, NiMoW and the refer-
ence NiMo/␥-Al2O3 catalyst after hydrogenation reactions were
characterized by XRD, and the results are exhibited in Fig. 5. The
characteristic sharp peak at about 14.4◦ was detected for the sul-
fided NiMo catalyst, which was due to the typical (0 0 2) peak of
MoS2 nanoparticles (JCPDS card No. 89-5112) [21,30,31]. The XRD
peaks at about 34, 41 and 58◦ were also attributed to the MoS2
nanoparticles. An XRD peak around 14◦ was also detected in the
sulfided NiW and NiMoW catalyst, which was ascribed to the phase
of WS2 nanoparticles [30,31]. Two other broad XRD peaks at about
33 and 58◦ were also due to the WS2 nanoparticles. Therefore, the
The sulfided NiMo, NiW, and NiMoW catalysts were further
characterized by TPR, and the results are depicted in Fig. 7. In the
temperature region 50–650 ◦C, the sulfided catalysts exhibited two
major hydrogen consumption peaks, one was centered at about
250 ◦C, and the other was located around 300 ◦C. The first TPR peak
(around 250 ◦C) might be attributed to the reduction of nickel sul-
fide located at Ni-Mo-S (or Ni-W-S) active phase [22,33], while the
following TPR peak (around 300 ◦C) is ascribed to the reduction of
the small MoS2 (or WS2) nanoparticles [34].
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Scheme 1. Reaction routes of naphthalene hydrogenation.
3.3. Catalytic results
It has been widely reported that the naphthalene hydrogenation
reaction is a typical consecutive reaction as shown in Scheme 1
[2,8,11]. Naphthalene is first hydrogenated to tetralin (1,2,3,4-
tetrahydronaphthalene), then tetralin is further hydrogenated to
decalin [2]. Octa-hydro-naphthalene is reported to be the most
reactive intermediate product in the reaction tetralin → decalin
[2,6]. The decalin has two isomers, which are trans- and cis-decalin,
and the trans-decalin is more stable in point of thermodynam-
ics [6,35]. The hydrogenation rate of naphthalene to tetralin was
an order of magnitude faster than that of tetralin to decalin as
reported by Pang et al. [14]. Furthermore, Beltramone et al. studied
naphthalene hydrogenation over a supported NiMo/Al2O3 catalyst
and found that the kinetic constant k of naphthalene to tetralin
was almost four times as high as that of tetralin to decalin [36].
Differences of activities might be well explained by consider-
ing resonance energy. The resonance energy per ring decreases
with the increasing ring numbers of aromatics, thus leading to
higher activities [11]. Besides, adsorption constant is also to highly
be emphasized. The adsorption constant K (atm−1) decreased in
the following order: naphthalene (70) > tetralin (50) » trans-decalin
(10) ∼ cis-decalin (8) as reported by Beltramone et al. [36]. This is
in principle in accordance with the heat of adsorption of tetralin,
cis-decalin, and trans-decalin reported by Santana et al. who stud-
ied tetralin hydrogenation over a Pt supported on alumina catalyst,
which was 83.7, 66.7, and 65.3 kJ mol−1, respectively [37].
Fig. 6. N2 adsorption-desorption isotherms (Up) and pore size distributions (Down)
of the sulfided NiMo, NiW, and NiMoW catalysts.
Figs. 8 and 9 exhibit that the compounds, including naphtha-
lene, tetralin, and decalin (cis- and trans- isomers), were detected
and confirmed by the combination of GC–MS. No cracking or ring-
opening products were observed over the highly-loaded NiMo,
NiW, and NiMoW catalysts, possibly due to the moderate reaction
conditions (3.0 MPa, LHSV 3.0 h−1 and 140–240 ◦C). The interme-
diate octa-hydro-naphthalene was also not detected [2,6]. The
conversion of naphthalene increased significantly with the increas-
ing reaction temperatures over the highly-loaded NiMo, NiW, and
NiMoW catalysts. With the reaction temperature rising from 140
to 200 ◦C, the conversion of naphthalene increased from 86.0 to
99.9% on the NiMo catalyst, and during the same temperature
region, the conversions of naphthalene were from 79.6 to 99.1%
and from 74.8 to 99.8% on the NiW and NiMoW catalysts, respec-
tively. Therefore, compared with the other two sulfided catalysts,
the highly-loaded NiMo catalyst exhibited higher activities towards
naphthalene hydrogenation and decalin formation. However, the
reference NiMo/␥-Al2O3 catalyst showed a low hydrogenation
activity towards naphthalene, with a naphthalene conversion of
49.5% at 300 ◦C. This implied the high hydrogenation abilities of
the highly-loaded catalysts.
Tetralin and decalin were the products in the naphthalene
hydrogenation reactions. Fig. 8 shows that increasing reaction
temperatures led to reverse trends of these two products due to
property of consecutive reactions. The content of tetralin was 21.1%
at 140 ◦C over the NiMo catalyst, and then it dropped to 0.1 and
less than 0.05% at 200 and 240 ◦C, respectively. On the other hand,
Fig. 7. TPR patterns of the sulfided NiMo, NiW, and NiMoW catalysts.
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Fig. 8. Catalytic results of the highly-loaded NiMo, NiW, NiMoW and reference NiMo/␥-Al2O3 catalysts.
Scheme 2. Possible reaction illustration.
decalin was the abundant product, occupying 64.9% at 140 ◦C, and
increased to more than 99.8% at 200 ◦C or higher temperatures.
The NiW and NiMoW catalysts also exhibited a similar trend as
to the NiMo catalyst. However, the NiW catalyst exhibited lower
hydrogenation activities than the NiMo or NiMoW catalysts. The
content of tetralin was 42.0% at 140 ◦C over the NiW catalyst, and
increasing the temperature to 240 ◦C, the naphthalene could be
fully converted while the content of tetralin was more than 2.0%. As
for the NiMo/␥-Al2O3 catalyst, the contents of tetralin and decalin
increased from 2.0 to 9.4% and from 11.6 to 40.1%, respectively,
when the temperatures increased from 220 and 300 ◦C.
The ratios of trans-decalin to cis-decalin over the highly-loaded
NiMo, NiW, and NiMoW catalysts are depicted in Fig. 9. Lai and Song
reported that trans-decalin was superior to its cis-isomer due to the
stability at high temperature when serving as a jet fuel component,
and that trans-decalin had a desirable ability of inhibiting the solid
deposit formation from jet fuels at high temperatures [35]. With the
increasing reaction temperatures (from 140 to 240 ◦C), the ratios of
trans- to cis-decalin increased from about 0.6–1.4 over the highly-
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7
Table 3
Content of trans-decalin and cis-decalin over the NiMo/␥-Al2O3 catalyst and calculated compositions at equilibrium.
Temperature (◦C)
Trans-decalin (%)
Cis-decalin (%)
Equlibrium compostionsa
Trans-decalin (%)
Cis-decalin (%)
140
160
180
200
220
240
260
280
300
–
–
–
–
–
–
–
–
0.0
0.0
0.0
0.9
1.3
97.09
96.55
95.96
95.35
94.71
94.04
93.35
92.65
91.94
2.91
3.45
4.04
4.65
5.29
5.96
6.65
7.35
8.06
100.0
100.0
100.0
99.1
98.7
a
Calculated according to Ref. [33].
loaded NiMo and NiMoW catalysts. The contents of trans- and cis-
decalin over the reference NiMo/␥-Al2O3 catalyst are reported in
Table 3. Compared with the data in Fig. 9, it could be found that the
decalin over the NiMo/␥-Al2O3 catalyst was almost all the trans-
isomer, and that the content of cis-decalin was no more than 1.5%
in the studied temperature regions (220–300 ◦C).
According to Lai and Song [35], a general expression for deter-
mining equilibrium compositions of trans-decalin and cis-decalin
was proposed as follows:
15370
T
R ln K =
+ 14.83 ln T − 0.0365T + 1.885
× 10−5T2 − 4.49 × 10−9T3 − 85.2
(1)
where R is the gas constant (8.314 J (mol K)−1), K is equilibrium con-
stant, and T is the temperature in K. We used this expression for
calculating the equilibrium compositions of trans- and cis-decalin
at various temperatures (220–300 ◦C for the NiMo/␥-Al2O3 cata-
lyst and 140–240 ◦C for the highly-loaded NiMo, NiW, and NiMoW
catalysts). The results of equilibrium compositions at various tem-
peratures are also reported in Table 3.
Fig. 9. Ratio of trans- to cis-decalin over the highly-loaded NiMo, NiW, and NiMoW
catalysts.
Table 3 shows that due to thermodynamic equilibriums, increas-
ing reaction temperatures lead to the decreasing content of
trans-decalin at equilibrium [6,35]. However, combined the data in
Fig. 9 and Table 3, it could be clearly found that the contents of trans-
and cis-decalin are not at equilibrium. As for the highly-loaded
NiMo, NiW, and NiMoW catalysts, the contents of trans-decalin
were far lower than those at equilibrium, while for the NiMo/␥-
Al2O3 catalyst, contents of trans-decalin were a little higher than
those at equilibrium. A similar trend of trans-/cis-decalin ratios was
detected by Dokjampa et al. studying the hydrogenation of tetralin
to decalin over supported Pt and Ni catalysts [6]. They found that
the intrinsic trans/cis-decalin ratios depended much on the hydro-
genation ability of catalysts, and more active catalysts led to the
closer trans/cis-decalin ratio to unity (at low conversions) [6]. This
means that not only the thermodynamic equilibrium but also the
reaction kinetics plays a vital role in the trans/cis-decalin ratio.
As aforementioned, octa-hydro-naphthalene is widely regarded
as the intermediate in the reaction tetralin → decalin [2,6], there-
fore, the formations of trans- or cis-decalin are closely related to this
intermediate. Based on the conformation between the intermedi-
ate octa-hydro-naphthalene and the catalyst surface, the possible
formation of trans- or cis-decalin could be inferred considering the
syn nature of hydrogen addition, which means that when the H
atoms are added to a double bond, both H atoms are always on the
same side [6]. Dokjampa et al. proposed that the formation mech-
anisms of trans- or cis-decalin depended on the hydrogen atom in
position 10 facing towards or away from the catalyst surface [6].
When this hydrogen atom faces towards the catalyst surface, cis-
decalin is formed and trans-decalin is produced in the case of this
hydrogen atom facing away from the catalyst surface. Furthermore,
they pointed out that the Pt and Ni catalysts with intrinsic high
hydrogenation ability could produce trans- and cis-decalin close to
unity because once the intermediate octa-hydro-naphthalene hits
the catalyst surface, it could be quickly converted. As for the Pd
catalyst with less active hydrogenation ability, this intermediate
might have time to roll over on the catalyst surface and thus lead
to more hydrogen atom in position 10 facing away from the catalyst
surface. This would produce more trans-decalin [6].
Considering our studies of the highly-loaded NiMo, NiW, and
NiMoW catalysts and also the reference NiMo/␥-Al2O3 catalyst,
the hydrogenation abilities of these TMS catalysts cannot be com-
pared with those of the noble ones, like Pt or Pd [14]. However, the
highly-loaded catalysts showed high conversions towards naph-
thalene hydrogenation (more than 99.0%) and high selectivities
towards decalin (more than 98.0 and 99.5% as for the NiMoW
and NiMo catalysts, respectively) at 200 ◦C or higher temperatures.
Tospøe et al. proposed that the NiMoS active phase was the main
specie for catalytic reactions in TMS-based catalysts [38,39]. Based
on the stacking numbers of MoS2 nanoparticles and the strength
between the active components and support, the NiMoS active
phase could be divided into two types (NiMoS Type I and NiMoS
Tpye II), and the NiMoS Type II active phase with higher stack-
ing of MoS2 nanoparticles and weaker interactions between active
components and support was more active in catalytic reactions
[40,41]. The XRD results of sulfided catalysts indicated that more
agglomerated MoS2/WS2 and Ni3S2 nanoparticles were detected
on the highly-loaded NiMo, NiW, and NiMoW catalysts than on the
reference NiMo/␥-Al2O3 catalyst, which implied that more NiMoS
Tpye II active phases might be formed on the highly-loaded cat-
Please cite this article in press as: H. Liu, et al., Low temperature catalytic hydrogenation naphthalene to decalin over highly-loaded
NiMo, NiW and NiMoW catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.024

G Model
CATTOD-10118; No. of Pages9
ARTICLE IN PRESS
8
H. Liu et al. / Catalysis Today xxx (2016) xxx–xxx
alysts. Besides, the formation of Ni3S2 phase might be attributed
to the decomposition of NiMoS active phase [32,42]. Therefore,
we inferred that the higher hydrogenation abilities of the highly-
loaded catalysts were attributed to more NiMoS Type II active
phases.
alysts and the NiMo/␥-Al2O3 catalyst, which might be due to
morphological effects of the agglomerated MoS2/WS2 and Ni3S2
nanoparticles. Among the highly-loaded catalysts, the NiMo cata-
lyst with high activity towards naphthalene conversion and high
selectivity to decalin shows potential industrial application.
Pang et al. studied naphthalene hydrogenation to tetralin and
concluded that the selectivity of tetralin was most likely a func-
tion of adsorption nature and hydrogenation ability of the catalyst
[14]. Since the highly-loaded NiMo, NiW, and NiMoW catalysts
and the NiMo/␥-Al2O3 catalyst are all TMS catalysts, we thus sup-
pose the adsorption/desorption nature between TMS catalysts and
naphthalene (or its hydrogenated intermediates) is almost the
same. Therefore, we infer that the morphological differences of the
agglomerated active components (MoS2/WS2 and Ni3S2 nanopar-
ticles) might be main reason. A possible reaction illustration is
exhibited in Scheme 2. The highly-loaded NiMo, NiW, and NiMoW
catalysts exhibited much larger MoS2/WS2 and Ni3S2 nanopar-
ticles than the supported NiMo/␥-Al2O3 catalyst as revealed by
XRD. Therefore, in the consecutive reaction from naphthalene to
decalin we supposed that the hydrogenated intermediate tetralin
did not need to desorb from the catalyst surface in the highly-
loaded catalysts, since there might exist enough space for the total
hydrogenation reactions, so the formations of trans- and cis-decalin
were in unity to some extent over the highly-loaded NiMo, NiW,
and NiMoW catalysts. As for the supported NiMo/␥-Al2O3 catalyst,
the hydrogenated product tetralin might desorb from the catalyst
surface and then absorb again to undergo further hydrogenation
reactions. Therefore, the NiMo/␥-Al2O3 catalyst showed a higher
selectivity towards tetralin than the highly-loaded catalysts, and
a preferred conformation between the possible intermediate octa-
hydro-naphthalene from tetralin and the catalytic surface might
be chosen, which means that the hydrogen atom in position 10 is
facing away from the catalytic surface. In this case, trans-decalin is
preferred.
Acknowledgements
This work was financially supported by the National Nature Sci-
ence Foundation of China (Grant No. U1162203 and No. 21106185),
and the Fundamental Research Funds for the Central Universities
(14CX06032A, 15CX02023A, and 15CX06051A). Financial support
from PetroChina Corporation Limited was also greatly appreciated.
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The NiMo, NiW, and NiMoW catalyst precursors were syn-
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