Hydrogenation of Aromatic Substrates over Dispersed Ni–Mo Sulfide Catalysts in System H2O/CO

Article abstract of DOI:10.1134/S0965544118070095

The activity of unsupported Ni–Mo sulfide catalysts formed during the in situ decomposition of oil-soluble precursors (molybdenum hexacarbonyl, nickel naphthenate) in the hydrogenation of aromatic and naphthene-aromatic compounds is studied. The catalysts are characterized by HRTEM and XPS. Catalytic experiments are conducted at temperatures of 340–380°C and an increased pressure of ?? in the presence of water providing the in situ generation of hydrogen via the water-gas shift reaction. The catalysts exhibit a high activity in the hydrogenation of model substrates (methyl- and dimethyl-substituted naphthalenes and anthracene).

Full text of DOI:10.1134/S0965544118070095

ISSN 0965-5441, Petroleum Chemistry, 2018, Vol. 58, No. 7, pp. 528–534. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © A.V. Vutolkina, D.F. Makhmutov, A.V. Zanina, A.L. Maximov, A.P. Glotov, N.A. Sinikova, E.A. Karakhanov, 2018, published in Nanogeterogennyi Kataliz,
2018, Vol. 3, No. 1, pp. 12–17.
Hydrogenation of Aromatic Substrates
over Dispersed Ni–Mo Sulfide Catalysts in System H2O/CO
A. V. Vutolkinaa, *, D. F. Makhmutova, A. V. Zaninaa, A. L. Maximovb, A. P. Glotova, c,
N. A. Sinikovaa, and E. A. Karakhanova
aFaculty of Chemistry, Moscow State University, Moscow, Russia
bTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
cGubkin Russian State University of Oil and Gas (National Research University), Moscow, Russia
*e-mail: annavutolkina@mail.ru
Received June 22, 2017
Abstract⎯The activity of unsupported Ni–Mo sulfide catalysts formed during the in situ decomposition of
oil-soluble precursors (molybdenum hexacarbonyl, nickel naphthenate) in the hydrogenation of aromatic
and naphthene-aromatic compounds is studied. The catalysts are characterized by HRTEM and XPS. Cata-
lytic experiments are conducted at temperatures of 340–380°С and an increased pressure of СО in the pres-
ence of water providing the in situ generation of hydrogen via the water-gas shift reaction. The catalysts
exhibit a high activity in the hydrogenation of model substrates (methyl- and dimethyl-substituted naphtha-
lenes and anthracene).
Keywords: hydrogenation, hydrodearomatization, nickel–molybdenum sulfide catalysts, dispersed catalysts,
water-gas shift reaction
DOI: 10.1134/S0965544118070095
Conventional catalysts applied in hydrocarbon nation of aromatic and naphthene-аromatic com-
feedstock hydroprocessing are mostly aluminum oxide pounds contained in distillate fractions.
supported sulfides of transition metals (molybdenum,
Molybdenum and tungsten compounds, including
tungsten) promoted by cobalt or nickel [1, 2]. A wide
Ni(Co)–Mo(W) dispersed sulfide catalysts, accelerate
use of these catalysts is associated with their excellent
the water-gas shift reaction [9–12]. This circumstance
resistance to poisoning by organosulfur compounds;
makes it possible to use the system H2O/CO as a
however, asphalt-resinous and organometallic com-
source of hydrogen for hydrogenation [13].
pounds contained in the petroleum feedstock lead to
the irreversible deactivation of these catalysts. This
problem may be solved owing to the use of dispersed
EXPERIMENTAL
unsupported catalysts distributed in the hydrocarbon
feedstock which are characterized by a high content of
Catalytic experiments were run in a steel autoclave
the active phase and, as a consequence, a high activity under vigorous stirring of the reaction mixture. The
in hydrogenation reactions. In the case of these cata- temperature of the reaction mixture was maintained
lysts, the effect of steric and diffusion limitations hin- constant using an electric resistance furnace equipped
dering access of a substrate to the active component of with a thermocouple and a temperature controller.
a catalyst is almost eliminated; therefore, a high resis- The reaction was mediated by the unsupported bime-
tance of the catalyst against the effect of catalytic poi- tallic Ni–Mo sulfide catalyst formed in situ during the
sons and coking is provided [3].
In recent years, the interest of researchers has been
focused on highly dispersed Ni(Co)−Mo(W) sulfide
decomposition of oil-soluble precursors, namely,
molybdenum hexacarbonyl Mo(CO)6 and nickel(II)
naphthenate Ni(С10H7СOO)2. Catalytically active
unsupported catalysts formed in situ during the particles formed directly during the hydrogenation
decomposition of precursors [4–6]. This method reaction. To provide formation of the active phase of
enables the obtainment of stable dispersed particles the catalyst (molybdenum sulfide), 2.5 wt % elemental
with a high content of the active phase [3, 7, 8]. In this sulfur was added to the reaction mixture. The hydro-
study, the behavior of Ni–Mo sulfide unsupported genation of aromatic substrates was conducted at an
catalysts formed in situ during the decomposition of increased pressure of СО in n-hexadecane or toluene
oil-soluble precursors is investigated in the hydroge- solutions (Table 1).
528

HYDROGENATION OF AROMATIC SUBSTRATES
Table 1. Ratios and amounts of the initial reagents
529
ω(Н2О),
wt %
ω(Mo),
ω(substrate),
СО/Н2О
1.2
Substrate
Mo : substrate ω(S), wt % Mo : Ni
wt %
wt %
1-methylnaphthalene
2-methylnaphthalene
2,6-dimethylnaphthalene
Anthracene
1 : 115
20
5
0.18
2.5
3 : 1
20
1 : 58
The in situ synthesized sulfide nickel-molybdenum as anthracene, for which further conversion of par-
catalysts were studied by high-resolution transmission tially saturated compounds is hardly probable.
electron microscopy (HRTEM) and X-ray photoelec-
The hydrogenation of model substrates was con-
tron spectroscopy (XPS). The structure and morphol-
ducted at an increased pressure of CO in the presence
ogy of the in situ synthesized catalysts were investi-
of the Ni−Mo sulfide catalyst formed in situ during
gated on a LEO ABOMEGA analytical electron
the decomposition of oil-soluble precursors; the latter
microscope with the magnification factor from 80 to
are well dispersed in the hydrocarbon feedstock and
500000 and at image resolution of 0.2–0.34 nm. XPS
form nanosized particles showing a high catalytic
studies were conducted on a Kratos Axis Ultra DLD
activity in thermal decomposition conducted in the
electronic instrument equipped with a photoelectron
presence of a sulfiding agent [5, 6]. The catalyst activ-
analyzer with an OPX-150 retarding field. Photoemis-
ity depends on the dispersion, morphology, and struc-
sion was excited using X-ray radiation of the alumi-
ture of the active component [4, 6]. These data were
num anode (AlKα = 1486.6 eV) at a tube voltage of
obtained by TEM. TEM micrographs (Fig. 1) show a
12 kV and an emission current of 20 mA. Photoelec-
typical layered structure of the phase, which is com-
tron peaks were calibrated against the carbon C 1s line
at a binding energy of 285 eV.
posed of МоS2 nanoplates united to agglomerates. An
interplanar distance of 6.24 Å indicates the basal (002)
plane of МоS2 crystallite. Phase NiS (interplanar dis-
tance, 3.38 Å) is absent on the micrographs. For the
active Ni–Mo sulfide component, the average length
of particles is 6–7 nm and the length of particles with
a length above 13 nm does not exceed 5% (Fig. 2a).
The average number of active component layers in the
multilayer agglomerate for the catalyst synthesized in
the hydrocarbon feedstock is 4.0 (Fig. 2b). Thus, the
technique used for the synthesis of Ni–Mo sulfide
particles in situ in the hydrocarbon feedstock ensures
formation of small sulfide nanoparticles; as a result, a
high degree of catalyst dispersion is attained.
The reaction products were analyzed on a Kristal-
lyuks 4000 M gas-liquid chromatograph equipped
with a flame ionization detector and a PetrocolTM
(Supelco) capillary column (0.25 mm × 30 m) with
programmed heating. Helium was used as a carrier
gas.
RESULTS AND DISCUSSION
The specific features of hydrogenation processes
involving aromatic hydrocarbons are associated with
difference in the rates of hydrogenation of individual
compounds and thermodynamic limitations arising
during the process [14–16]. The limiting stage of
hydrogenation is interaction between an adsorbed
organic molecule and hydrogen. The introduction of
alkyl substituents into the aromatic ring affects the rate
of reaction. This fact may be explained both by the
presence of hindrances at the stage of substrate sorp-
tion and by the formation of more stable π complexes
with metals. As a result, the rate of desorption of mol-
ecules of organic compounds from the catalyst surface
declines, the number of accessible active sites
decreases, and, eventually, the reaction decelerates
[17]. For the hydrogenation of middle-distillate frac-
tions, it is necessary that the catalyst be active in the
transformation of such sterically hindered substrates;
therefore, the chosen model compounds were substi-
tuted bicyclic аromatic hydrocarbons: 1- and 2-meth-
According to the XPS data, the spectrum of the
sample exhibits signals corresponding to sulfur,
molybdenum, nickel, oxygen, and carbon compounds
(Table 2).
The spectrum of Mo3d electrons (Fig. 3a) shows
two signals corresponding to molybdenum disulfide
MoSх and Mo6+ (oxide phase MoOx and/or interme-
diate state in the form of molybdenum oxysulfide
MoOxSy) [15, 16] with 94% of molybdenum being
present in the form of molybdenum disulfide. This
finding provides evidence that the degree of sulfiding
of the in situ formed catalyst is high. Nickel on the cat-
alyst surface may occur in the form of nickel sulfides
NiSx (Ni2S3, Ni9S8, NiS) and oxide phase NiOx and as
a component of complex sulfide NiMoS. In the spec-
trum of Ni2p electrons (Fig. 3b), signals correspond-
ing to NiSx, NiMoS, and NiOx are seen [18, 19]. More
ylnaphthalenes and 2,6-dimethylnaphthalene as well than 60% of nickel is present in the sulfide environ-
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530
VUTOLKINA et al.
(а)
(b)
20 nm
10 nm
Fig. 1. TEM micrographs of Ni–Mo sulfide catalysts synthesized in situ in the hydrocarbon feedstock.
(а)
(b)
40
35
30
25
20
15
10
5
25
20
15
10
5
0
0
3
4
5
6
Number of MoS2 plates, units
Particle length, nm
Fig. 2. Size characteristics of sulfide particles synthesized in the hydrocarbon feedstock: (а) length of particles and (b) the number
of layers in multilayer agglomerates.
ment. Given this, 50% of nickel is included in the the generation of excess hydrogen required for imple-
composition of the NiMoS phase. This observation mentation of hydrogenation reactions.
suggests a high degree of promotion of molybdenum
For all of the substrates, the degree of conversion
disulfide crystallites by nickel.
regularly grows with increasing reaction temperature
(Fig. 4). For example, in the event of 2-methylnaph-
thalene hydrogenation at a temperature of 360°С, the
substrate conversion is 21%; as the temperature is
increased to 380°С, the degree of conversion attains
65%. In the case of 1-methyltetralin more severe con-
ditions are required for hydrogenation. This is proba-
bly related to steric hindrances arising during the
adsorption of substrate on the catalyst surface: as the
temperature is increased from 360 to 380°С, substrate
conversion increases by 13%. For 2,6-dimethylnaph-
thalene, this parameter is 20%. In the case of anthra-
cene, the effect of temperature is less pronounced—
conversion grows by 8%.
Sulfur on the catalyst surface may be present in the
form of sulfur S2− (MoS2, NiS, NiMoS phase) and
phase 2− (MoOxS). The presence of three doublets in
S2
the S2p spectrum of the test sample indicates that sul-
fur occurs in three states (Fig. 3a) and a peak at 169 eV
indicates that the content of sulfur in the sulfate state
is small (no more than 10%).
Catalytic experiments on the hydrogenation of
aromatic substrates in the presence of unsupported
Ni–Mo sulfide catalysts formed in situ during the
decomposition of oil-soluble precursors were run in
the temperature range of 360–380°С and an increased
pressure of СО. The content of water in the reaction
mixture was 20 wt % at H2O/CO = 1.2. In accordance
In the hydrogenation of 1- and 2-methylnaphtha-
lenes, a low conversion is observed for the first two
with our experiments, this ratio is optimal and ensures hours (Table 3). This finding may be attributed to the
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HYDROGENATION OF AROMATIC SUBSTRATES
Table 2. XPS data for Mo3d, Ni2p, and S2p levels
531
Element
Element content, at % Binding energy, eV
Fraction, at %
State
229.2
6.5
0.4
1.3
MoS2
MoOx
NiSx
Mo3d5/2
6.91
232.6
853.3
Ni2p3/2
S2p3/2
4.22
854.3
856.7
162.0
1.3
NiMoS
NiOx
1.6
S2–
14.4
S22−
(SO4)2–
163.3
169.3
1.4
1.8
17.59
initial stage of formation of the active phase of the sul- group is observed. It should be emphasized that the
fide catalyst: sulfide particles, which are characterized
by a lower dispersion and, as a consequence, by a
smaller number of edge and corner sites, a lower pro-
portion of the active phase, and a smaller degree of
promotion, are initially formed [5, 6]. The rate of
hydrogenation grows in the course of time. For exam-
ple, for the hydrogenation of 2-methylnaphthalene,
an increase in the time of reaction from 4 to 6 h causes
a sharp rise in conversion from 30 to 60%. This is pre-
sumably associated with the formation of finely dis-
persed particles of the catalyst and a rise in the con-
centration of active sites on its surface. At small reac-
tion times, a low conversion may also be explained by
the shortage of hydrogen in the system because of a
low ratio H2O/CO in the initial reaction mixture.
rate of hydrogenation of alkyl naphthalene derivatives
to tetralins is much higher than the rate of hydrogena-
tion of tetralins to decalins; as a result, tetralins accu-
mulate in the reaction mixture [16]. Selectivity for
5-tetralin and 6-methyltetralin throughout the studied
temperature range is higher than that for 1-methylte-
tralin and 2-methytetralin. This fact may be explained
by steric hindrances appearing during adsorption of
the latter compounds on the catalyst surface. For
example, in the case of 1-methylnaphthalene, the
ratio of tetralins is 30 : 70; for 2-methylnaphthalene,
40 : 60. However, in the case of 2-methylnaphthalene
hydrogenation, a decrease in temperature to 360°С
leads to a reduction in selectivity of the process and
entails an increase in the fraction of 2-methyltetralin
in the reaction system (Fig. 5). Under these condi-
tions, no decalins are formed even if the reaction time
is increased to 8 h. This fact may be related to both
The main products in the hydrogenation of meth-
ylnaphthalenes (the chromatography data) are
methyltetralins. Note that, for 1-methylnaphthalene
and 2-methylnaphthalene, no migration of the alkyl thermodynamic limitations arising at high tempera-
(а)
(b)
1 S2s
1 NiSx
2 NiMoS
3 Ni2+
2 MoS
34000
32000
30000
28000
26000
50000
40000
30000
20000
10000
3 Mo6+
3
2
1
3
1
2
236
232
228
224
850
240
860
Binding energy, eV
Binding energy, eV
Fig. 3. XPS spectra of the test sample: (a) Mo3d and S2s and (b) Ni2p.
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532
VUTOLKINA et al.
1-methyltetralin
2-methyltetralin
Conversion, %
1-methylnaphthalene
2,6-dimethylnaphthalene
2-methylnaphthalene
Anthracene
5-methyltetralin
6-methyltetralin
90
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
4 h 6 h 8 h
4 h 6 h 8 h
2 h
2 h
350
360
370
380
340
Products
Products
Temperature, °С
of 1-methylnaphthalene
hydrogenation
of 2-methylnaphthalene
hydrogenation
Fig. 4. Temperature dependence of substrate conversion
(the time of reaction is 8 h).
Fig. 5. Distribution of products in the hydrogenation of
1- and 2-methylnaphthalenes.
tures and an insufficient partial pressure of hydrogen
in the system as well as catalyst dispersion [6, 14, 16].
Note that, at a temperature of 360°С, an increase in
the time of reaction does not cause the formation of
decalin, whereas at 380°С its fraction in the reaction
products is as high as 20%.
The features of anthracene hydrogenation are sim-
ilar to those observed for substituted naphthalenes. At
a temperature of 380°С, an almost quantitative con-
version is attained over 4 h (Fig. 7).
A decrease in the substrate/catalyst ratio causes an
increase in the conversion of substrates. For example,
in the event of 2,6-dimethylnaphthalene hydrogena-
tion, the conversion of dimethyl-substituted naphtha-
lene achieves 70% and the substrate content in the
reaction mixture decreases by a factor of 2 (Fig. 6).
As in the case of methyl-substituted naphthalenes,
The main reaction product is octahydroanthra-
the rate of hydrogenation for 2,6-dimethylnaphtha- cene; selectivity for this product in the temperature
lene grows with increasing temperature and reaction range of 340–360°С is as high as 98%. An increase in
time. Note that at low temperatures the time factor
temperature to 380°С leads to a gain in the proportion
becomes more crucial. For example, at 360°С, an of perhydro- and tetrahydroanthracenes in the reac-
increase in the time of reaction from 4 to 6 h entails a
marked rise in substrate conversion from 12 to 61%,
tion products; the content of the latter in the reaction
mixture grows in the course of time. Dihydroanthra-
whereas at a temperature of 380°С the increase in the cene is formed in trace amounts throughout the stud-
degree of conversion is no greater than 18%.
ied temperature range.
The main product of reaction is 2,6-dimethylte-
Thus, the dispersed Ni–Mo sulfide catalysts
tralin; selectivity for this product attains 80–100%. formed in situ during the decomposition of oil-soluble
Table 3. Hydrogenation of methylnaphthalenes in the system H2O/CO (p(CO) = 3 MPa)
1-Methylnaphthalene
selectivity, %
1-methyltetralin 5-methyltetralin
2-Methylnaphthalene
selectivity, %
2-methyltetralin 6-methyltetralin
T, °C t, h
conversion, %
conversion, %
380
360
2
4
6
8
6
8
0
5
0
0
10
29
61
85
5
45
47
41
38
50
57
55
53
59
62
50
43
30
36
35
0
70
64
65
0
11
23
0
10
30
70
21
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HYDROGENATION OF AROMATIC SUBSTRATES
533
2,6-dimethyltetralin
2,6-dimethyldecalin
Conversion, %
70
60
50
40
30
20
10
0
4 h 6 h 8 h
4 h 6 h 8 h
3 h
2 h 3 h
360°С
380°С
Fig. 6. Distribution of 2,6-dimethylnaphthalene hydrogenation products.
Anthracene
Dihydroanthracene
Tetrahydroanthracene
Octahydroanthracene Perhydroanthracene
100
90
80
70
60
50
40
30
20
10
0
2 h 4 h 6 h
340°С
2 h 4 h 6 h
360°С
2 h 4 h 6 h
1 h
1 h
1 h
380°С
Fig. 7. Distribution of anthracene hydrogenation products.
precursors are active in the water-gas shift reaction, as
evidenced by the occurrence of hydrogenation/hydro-
dearomatization of alkyl-substituted aromatic and
naphthene-aromatic substrates in the system H2O/CO
in the absence of hydrogen in situ. It is shown that, in
the hydrogenation of methyl-substituted naphthalenes
run in the temperature range of 360–380°С and at
increased pressure of CO in the presence of water, the
main products are the respective tetralins. For anthra-
cene, hydrogenation occurs with a high selectivity for
octahydroanthracene.
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Translated by T. Soboleva
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2018