Active Sites on Nickel-Promoted Transition-Metal Sulfides That Catalyze Hydrogenation of Aromatic Compounds

Article abstract of DOI:10.1002/anie.201808428

Hydrogenation on Mo and W sulfides occurs at the edges of the sulfide slabs. The rate of hydrogen addition is directly proportional to the concentration of sulfhydryl (SH) groups at the slab edge and the metal atom attached to it. Sulfhydryl groups vicinal to edge-incorporated Ni hydrogenate with much higher rates than SH close to Mo and W. Each subset of SH groups, however, exhibits nearly identical intrinsic activity and selectivity, independent of the sulfide composition. The higher activity of Ni-WS₂ compared to Ni-MoS₂ stems from a higher concentration of SH groups on the former sulfide associated with a higher tendency of its surface vacancies to react with H₂.

Full text of DOI:10.1002/anie.201808428

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Accepted Article
Title: Active sites on Ni-promoted transition metal sulfides that catalyze
aromatics hydrogenation
Authors: Wanqiu Luo, Hui Shi, Eva Willnecker, Oliver Y. Gutiérrez, and
Johannes Lercher
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Angewandte Chemie International Edition
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COMMUNICATION
Active sites on Ni-promoted transition metal sulfides that catalyze
aromatics hydrogenation**
,†
Wanqiu Luo, Hui Shi*, Eva Willnecker, Oliver Y. Gutiérrez* and Johannes A. Lercher*
Abstract: Hydrogenation on transition metal sulfides occurs at
the edges of the sulfide slabs. The rate of hydrogen addition is
directly proportional to the concentration of sulfhydryl (SH) groups
at the slab edge and the metal atom attached to it. Sulfhydryl
groups vicinal to edge-incorporated Ni hydrogenate with much
higher rates than SH close to Mo and W. Each subset of SH
groups, however, exhibits nearly identical intrinsic activity and
selectivity, independent of the sulfide composition. The higher
metal sulfides with varying concentrations of Ni. This approach
allowed identifying two subsets of surface SH groups, with each
subset showing a characteristic activity and regioselectivity
toward the hydrogenation of phenanthrene (PHE). PHE is a C14
polycyclic aromatic hydrocarbon representative of those present
in heavy oil fractions, whose reaction pathway and mechanism
has been extensively studied.6,7
All catalysts used in this study were prepared by incipient wetness
impregnation followed by consecutive thermal treatment in
synthetic air and sulfiding the oxidic catalyst precursor with
activity of Ni-WS
concentration of SH groups on the former sulfide associated with
a higher tendency of its surface vacancies to react with H
2 2
compared to Ni-MoS stems from a higher
2
.
H
2
S/H
Supporting Information). The as-sulfided samples are denoted as
MS /γ-Al and Ni(x)MS /γ-Al with M being Mo, W or a mix of
Mo and W, and x being the measured concentration of Ni in wt. %.
Extensive characterization data for WS /Al , the corresponding
Ni-promoted series and the trimetallic sulfide are presented in the
Supporting Information, while those for (Ni)MoS /γ-Al samples
have been previously published.2e Importantly, the dimensions
and characteristic properties of these γ-Al -supported sulfides
2
mixtures in a continuous flow trickle bed reactor (details in
2
2
O
3
2
2 3
O
Co- and Ni-containing MoS
2
and WS
2
catalyze both
hydrogenation of aromatic rings and hydrogenolysis of carbon-
heteroatom linkages. The presence of Ni is used to achieve higher
hydrogenation rates compared to Co-containing counterparts,1,2
2
2 3
O
2
2 3
O
and Ni-containing WS
2
catalyzes hydrogenation with even higher
rates than Ni-MoS
2
.1,3 The hydrogenation of aromatic rings is
2
O
3
needed to adjust the quality of the hydroprocessed product and is
a conversion route parallel to the direct hydrogenolysis of C-S and
C-N bonds. Indeed, for the most refractory heterocompounds in
crude oils, aromatics hydrogenation prior to defunctionalization is
the only pathway with relevant rates. Thus, the hydrogenation of
aromatic rings is a key reaction toward the efficient conversion of
were largely similar. The most significant trend is a 10-40%
decrease in average slab diameter with increasing Ni
concentrations (0.3-11.1 wt.%) compared to the unpromoted
catalysts (Table S1). Overall, these samples span a wide range
-1
-1
of Ni (0.05-1.9 mmol g ), Mo (0.4-1.0 mmol g ) and W loadings
-1
(
0.4-1.1 mmol g ).
4
heavy feedstocks.
Only four products were detected with all studied sulfide catalysts
at low to moderate conversions of PHE (1-25%), i.e., 9,10-
dihydrophenanthrene (DiHPHE), 1,2,3,4-tetrahydrophenanthrene
The Ni(Co)-Mo-S model, where Ni and Co cations substitute Mo
at the edges of sulfide crystals, is widely accepted for describing
the principal structure of the active mixed sulfide phase.2b,5 The
presence of the promoter atoms increases the concentration of S-
vacancies, i.e., active sites and/or precursors to active sites for
hydrodesulfurization (HDS). Studies have shown that similar
(
(
(
(
TetHPHE),
1,2,3,4,5,6,7,8-octahydrophenanthrene
symOHPHE), and 1,2,3,4,4a,9,10,10a-octahydrophenanthrene
asymOHPHE). The latter two are nearly absent on WS /γ-Al O
2 2 3
Figure S1) and MoS
2e,7
2
/γ-Al
2
O
3
.
The reaction network is
active site structures exist on (Ni)WS
However, it is unclear whether or not the high activity of Ni-WS
2
-based catalysts.3a–c,3e,3g
composed of two parallel routes, i.e., initial hydrogenation at the
middle ring (PHE → DiHPHE → asymOHPHE) and initial
hydrogenation at a lateral ring (PHE → TetHPHE → symOHPHE).
The dehydrogenation of DiHPHE to PHE and its hydrogenation to
TetHPHE were demonstrated for the same reaction catalyzed by
2
catalysts for hydrogenation is caused by differences in the
strength of the M-S bonding or by different active sites.
To connect the chemical structure and concentration of active
sites to their steady-state activity, we characterized the quality
2 2 3
/γ-Al O
.7 At low conversions, both DiHPHE reactions
(
Ni)MoS
2 3
and quantity of functional sites on γ-Al O -supported transition
2
e
can be neglected.
[
a]
W. Luo, Dr. H. Shi, Dr. E. Willnecker, Dr. O.Y. Gutiérrez, Prof. J. A.
Lercher
Department of Chemistry, Catalysis Research Center
Technische Universität München
Lichtenbergstr. 4 Garching D-85747 (Germany)
E-mail: hui.shi@mytum.de, oliver.gutierrez@pnnl.gov,
johannes.lercher@ch.tum.de
[†]
Present address: Institute for Integrated Catalysis
Pacific Northwest National Laboratory
902 Battelle Boulevard, Richland, WA 99352 (USA)
[
**]
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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COMMUNICATION
Table 1. Mass specific rates (extrapolated to zero contact time) of PHE
conversion via initial hydrogenation at the middle ring and at a lateral ring
that non-promoted WS
hydrogenation activities (normalized to metal content), Figure
(A) clearly shows that the effect of Ni promotion is significantly
more pronounced on WS -based catalysts. For example, a Ni-
promoted WS /γ-Al catalyst with the greatest promotion effect
Ni/(Ni+W) ~0.2) was 2.2 times as active as the most active Ni-
MoS /γ-Al catalyst (Ni/(Ni+Mo) ~0.3). Despite the difference in
2 2 3 2 2 3
/γ-Al O and MoS /γ-Al O have similar
(denoted in the Table as r
1 2 2 3
and r , respectively) on γ-Al O -supported sulfide
1
catalysts. Data for MoS -based catalysts are taken from ref.[2e].
2
2
2
2 3
O
(
2
2 3
O
rates, the selectivities towards different products were very similar
over most sulfide catalysts with the notable exception of the
lowest Ni-loaded sample. The ratio of the two hydrogenation
routes decreased as the Ni/(Ni+M) ratio increased and
approached unity at higher ratios on both series of catalysts.
Additionally, the apparent activation energy measured on
-
1
-
Ni(2.2)WS
2
/γ-Al
2
O
3
was 127 kJ mol , identical to that (126 kJ mol
/γ-Al under the same conditions
1
)
obtained on Ni(2.7)MoS
2
2 3
O
Ni/(Ni+M)
molar ratio
r
1
r
2
Catalyst
(10-3 mol h
-1
cat-1)
(10-3 mol h cat-1)
-1
(Figure S4). Together, these observations point to variations in
the concentrations of active sites having identical intrinsic activity
for PHE hydrogenation on the two series of materials, reminiscent
of that suggested for thiophene HDS on carbon-supported Ni-W
and Ni-Mo catalysts.8
g
g
WS
2
0
0.40
0.67
1.48
1.78
1.66
1.61
1.39
1.21
0.20
0.30
1.53
0.52
0.66
0.78
0.55
0.06
0.16
1.47
1.78
1.66
1.61
1.55
1.35
0.04
0.05
1.57
0.06
0.47
0.76
0.53
Ni(0.3)WS
Ni(1.5)WS
Ni(2.2)WS
Ni(3.2)WS
Ni(4.5)WS
Ni(7.8)WS
2
2
2
2
2
2
0.05
0.14
0.25
0.33
0.41
0.55
0.67
0
Ni(11.1)WS
2
WS
2
-a
-b
WS
2
0
Figure 1. (A) Rates of phenanthrene (PHE) hydrogenation on unpromoted and
Ni(2.5)MoWS
MoS
2
0.34
0
Ni-promoted WS
ratio where M = Mo or W. Reaction conditions: 573 K, 60 bar total pressure, and
a H /PHE ratio (v/v) of 300. Note that the rates reported have been normalized
to the total metal content. (B) Percentage of Ni substitution at the slab edge of
Ni)WS /γ-Al (▲) and (Ni)MoS /γ-Al (■) with increasing Ni content. The
data of MoS -based catalysts are taken from Ref.[2e]. See details for the
2 2
- (▲) and MoS -based (■) catalysts as a function of Ni/(M+Ni)
2
2
Ni(1.3)MoS
Ni(2.7)MoS
Ni(8.8)MoS
2
2
0.19
0.33
0.62
(
2
O
2 3
2
2 3
O
2
calculation of Ni substitution degree in the SI.
2
Establishing structure-function relations requires reliable
quantification of the accessible functional sites (e.g., edge
vacancies and SH groups). In particular, NO adsorption has been
used to estimate the concentration of active metal sulfide
edges.2d,9 Since NO is adsorbed on both unpromoted and
promoted edges (those exposing S-vacancies), spectroscopic
discrimination of the adsorption sites is necessary.9a NO
Compared to WS
hydrogenation increased by a factor of 4.4 for the Phe → DiHPhe
asymOHPhe route (r ), and 32 for the Phe → TetHPhe →
symOHPhe route (r ) on the most active Ni(2.2)WS /γ-Al with
a Ni/(Ni+W) ratio of 0.25 (Table 1). Higher Ni contents led to
reduced promotional effects, which is attributed to the increasing
2 2 3
/γ-Al O , the mass specific rates of PHE
→
1
2
2
2 3
O
-
presence of catalytically inactive NiS
of NiS /γ-Al was at least two orders of magnitude lower than
the unpromoted MoS /γ-Al and WS /γ-Al under identical
x
(the mass specific activity
adsorbed on NiS /γ-Al O produced a main IR band at ~1830 cm
x
2
3
1
(Figure 2(A)), which was attributed to NO on Ni2+ in NiSx.9a,10a
x
2
O
3
2
2
O
3
2
2
O
3
2 2 3
For NO on WS /γ-Al O , the two main bands at 1695 and 1775
cm-1 are attributed to the symmetric and asymmetric stretching
conditions). The ratios of the initial rates along the two reaction
routes changed drastically with the addition of Ni, from
hydrogenation at the middle ring being the dominant route on
vibrations of dinitrosyl species on W4+ at the edges of WS (Figure
2
2(A)).9a In addition to the bands associated with WS and NiS , a
2
x
shoulder at 1875 cm-1 emerged in the spectra of the Ni-promoted
WS
on all Ni-promoted catalysts except Ni(0.3)WS
It is instructive to compare the impact of Ni promotion on the
hydrogenation rates on WS - and MoS -based catalysts. Noting
2
/γ-Al
2
O
3
to equivalent middle and lateral ring hydrogenation,
2
/γ-Al (Table 1).
2
O
3
WS catalysts. This band has been attributed either to partially
2
sulfided Ni vicinal to an oxidic W phase,10a or to dispersed Ni2+
ions in the sulfide.10b Taking into account the rapid oxidation-
2
2
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COMMUNICATION
reconstruction10c-10f of the Ni-promoted phase by NO (but not the
unpromoted WS , see Figure S7) and the IR bands of NO
2
line in (B) represents a trend. See Figure S9 for a full set of IR spectra and
Figure S12 for the changes of integrated areas of other bands as a function of
Ni/(Ni+W) ratio.
adsorbed on the oxide precursors (Figure S11) and sulfide
catalysts, we attribute the band at 1875 cm-1 to NO adsorbed on
Ni2+ cations embedded in the perimeter of the WS
slab. It
2
Sulfhydryl groups can be formed at or near edge vacancies by
dissociative adsorption of H and H S. While these SH groups are
2 2
Brønsted acidic, they are involved in hydrogen addition
reactions.11 In order to quantify the concentration of Brønsted
appears that these sites are transformed to oxidic W species
formed upon the interaction of NO. The integrated area of this
band increased first with Ni concentration, reaching a maximum
on Ni(2.2)WS
integrated areas of the bands at 1695 and 1775 cm (NO on WS
steadily decreased (Figure 2). The band at 1830 cm-1 (NO on
NiS ) also increased with the Ni concentration, suggesting that the
concentration of NiS increased (Figure S12). Because the
intensity at 1875 cm-1 dropped rapidly after reaching the
maximum (Figure 2(B)), NiS clusters are speculated to form at
2 2 3
/γ-Al O with a Ni/(Ni+W) ratio of 0.22, whereas the
(
BAS) and Lewis acid sites (LAS), we studied the IR spectra of
-1
2
)
adsorbed pyridine and 2,6-dimethylpyridine (DMP). Pyridine
adsorption on BAS of a sulfide surface generates rather unstable
pyridinium ions,12 often making their concentration undetectably
low and unreliable for quantification. In contrast, the higher base
strength of DMP suffices to probe the weak BAS, but produces
bands that do not correspond solely to adsorption on LAS (i.e.,
bands overlap those of physisorbed DMP and H-bonded species).
Thus, for quantification of BAS and LAS we combined information
from DMP and pyridine, respectively (Section S3.3).
x
x
x
2
+
edge sites and block the access to Ni . This implies that the
maximum absorbance observed here was already attenuated
from the real maximum, attained only if the site-blocking NiS
were entirely absent.
x
DMP adsorbed and protonated on SH groups give rise to bands
Figure 1(B) shows the percentage of Ni substitution at the WS
2
-1 13
at 1650 and 1627 cm . Deconvoluted spectra of DMP adsorbed
on WS /γ-Al and Ni(x)WS /γ-Al are shown in Figures S13
and S14. Exposing the DMP-saturated catalyst to H S and H (1
edge (e.g., Ni-S-W), calculated from the integrated area of 1875
cm-1 band and the estimated molar extinction coefficient (~1.0 cm
2
2
O
3
2
2 3
O
2
2
-1
μmol ) of NO adsorbed at the Ni-substituted W-edge sites (see
Section S4, Supporting Information, for details of calculation and
assumptions made). Also presented is the percentage of Ni
mbar or 1 bar at equilibrium, respectively) increased the intensity
of the bands of protonated DMP and concomitantly decreased the
-1
intensity of the bands at 1615 and 1580 cm for DMP on LAS. For
both adsorbates, the increase in the SH group concentration
substitution as a function of the bulk Ni/(Ni+Mo) ratio for MoS
based samples as determined previously.2e Interestingly, the
degree of Ni substitution at both MoS and WS slab edges was
2
-
(
ΔCSH) was most pronounced for Ni(2.2)WS
also showed the highest concentration of accessible Ni-
substituted edge sites. On Ni(11.1)WS /γ-Al , the increase in
2 2 3
/γ-Al O . This sample
2
2
quite similar for any given Ni/(Ni+M) ratio. Consequently, the
similar slab sizes and dispersions as well as the similar Ni
substitution degrees for WS - and MoS -based catalysts would
2 2
imply similar concentrations of such Ni-associated edge sites for
the two sets of catalysts.
2
2 3
O
the concentration of SH was the smallest (ΔCSH = 11 and 7 µmol
g-1 for the two adsorbates), in parallel with the lowest
concentration of accessible edge-embedded Ni (Figure 2(B)).
2 2
In comparison to WS -based catalysts, the (Ni)MoS series
showed much smaller increase in SH concentration after
adsorption/dissociation of H
applied here, but the trend with respect to Ni loading was similar.
2
For example, after co-adsorption of H S, Ni(2.2)WS /γ-Al O
2
S or H
2
at the same conditions as
2e
2
2
3
-1
showed a SH concentration of 170 mol gcat , which was higher
than that of Ni(2.7)MoS /γ-Al
(119 mol gcat-1).14 Similarly, after
co-adsorption of Ni(2.2)WS /γ-Al showed SH
concentration of 162 mol gcat , also higher than that of
Ni(2.7)MoS /γ-Al (107 mol
cat-1).14 The much higher
concentration of sulfhydryl groups with the Ni-WS (than for Ni-
MoS ) is hypothesized to reflect a higher concentration of CUS
which are LAS by definition) responsible for the formation of SH
groups upon H and H S dissociative adsorption. Indeed, a linear
2
2 3
O
H
2
,
2
2
O
3
a
-1
2
2
O
3
g
2
2
(
2
2
correlation is observed between the concentration of SH groups
and that of strong LAS (Figure 3(B)). The higher the concentration
of strong LAS, the higher the SH concentration at the edges of Ni-
2 2
WS than Ni-MoS . This shows a relation between (a fraction of)
accessible cations and the SH at the edge of the sulfide slab,
pointing to a well-defined site, rather than to two independent
variables, i.e., the LAS and the SH concentration. An illustration
of the proposed site structure is shown in Figure 3(C).
Figure 2. (A) Background-corrected infrared spectra of NO adsorbed on (a) γ-
Al
2
O
3
,
(b) NiS
/γ-Al
cm (i.e., NO adsorbed on oxidized Ni-promoted edge sites; the deconvoluted
x
/γ-Al
2
O
3
,
(c) WS
2
/γ-Al
2
O
3
,
(d) Ni(2.2)WS
2
/γ-Al
2
O
3
,
(e)
Ni(11.1)WS
2
2 3
O . (B) Integrated areas of the deconvoluted bands at 1875
-
1
-
1
band is marked in red in (A)) and at 1695 cm (i.e., NO adsorbed on non-
promoted sites) as a function of the molar ratio of Ni/(Ni+W). The continuous
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COMMUNICATION
In summary, the unequivocal structure-activity relation
established for aromatics hydrogenation pinpoints the direct role
of sulfhydryl groups as the sites that lead to H-addition. The
results show that the SH groups vicinal to edge-incorporated Ni
are much more reactive than SH associated with non-promoted
sites. This leads to higher hydrogen addition rates as well as a
much greater extent of enhancement on hydrogenation of one of
the lateral rings in PHE. These reactive species exhibit the same
intrinsic activity and selectivity in hydrogenation, regardless of the
2 2 2
slab composition (MoS , WS or mixed MoWS ). Our results on
PHE hydrogenation show that the higher activity of Ni-WS
2
primarily results from its higher concentration of SH groups, which
is in turn related to an inherently higher concentration of strong
LAS on Ni-WS than on Ni-MoS , but not to the intrinsic rate
2 2
constant for the rate-determining H-addition.
This study not only provides an answer to the question as to why
Ni-promoted WS
catalyst in general than the MoS
2
manifests itself as a better hydrogenation
-based counterpart, but should
Ni
2
also enable a more rigorous comparison of intrinsic activities
among different transition metal sulfides in their supported or even
unsupported forms.
Figure 3. (A) Correlation between corrected reaction rates and SH
concentrations (see Table S2 for tabulated values), with non-promoted MoS
2
or
Acknowledgements
WS labelled by ▼, and ■ for Ni-promoted phases. (B) Correlation between
2
This work is financially supported by the Chevron Energy
Technology Company. The authors would like to thank Drs.
Alexander Kuperman, Axel Brait, and Jinyi Han for fruitful
discussions, Dr. Marianne Hanzlik for TEM measurements and
Xaver Hecht for technical support. We also thank Professor Gary
L. Haller (Yale University) for his careful reading of the manuscript
and helpful suggestions.
total SH concentration (associated with both Mo/W and Ni sites at the slab edge)
and concentration of pyridine adsorbed on strong LAS (10-7 mbar and 623 K).
Blue: MoS
▲: SH concentration measured after adsorption of H
measured after adsorption of H ). Lines represent trends. The data measured
for MoS -based catalysts are taken from Refs. [2e,13]. (C) Illustration of the
proposed site structure after H or H S dissociation for Mo(W)S and Ni-
Mo(W)S slabs (note the size reduction after Ni incorporation based on our
2 2
-based catalysts; Orange: Ni-MoWS ; green: WS
2
-based catalysts
(
2
S, ●: SH concentration
2
2
2
2
2
2
experimental data). Grey: H; yellow: S; blue: Mo or W; red: Ni.
Keywords: • Hydrotreating • Transition metal sulfides • Active
Site • Hydrogenation • Ni promotion
The observed difference in SH concentration parallels the higher
hydrogenation activity of Ni-WS
of the Ni-promoted MS phase was corrected by subtracting the
contribution of non-promoted MS sites. Similarly, the
concentration of SH groups on the promoted phases was
corrected by subtracting those associated with unpromoted MS
linear correlation between the reaction rates and SH
concentrations was observed (Figure 3(A)), including SH groups
on mono-, bi- and trimetallic sulfides. Non-promoted WS and
MoS exhibit a similar rate dependence on the SH concentration
or the strong LAS concentration), yet significantly lower than that
2 2
than Ni-MoS . The reaction rate
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Wanqiu Luo, Hui Shi*, Eva Willnecker,
Oliver Y. Gutiérrez*, and Johannes A.
Lercher*
Distinct active sites on MoS
2 2
and WS
consisting of accessible Mo or W cations
and SH groups at the slab edge have been
identified. Addition of Ni on both sulfides
Page No. – Page No.
x
generates a Ni-(SH) site that is significantly
more active than the sites associated with
Mo or W and has a distinct regioselectivity
for H-addition. For all sites the ratio between
the metal cation and the SH groups is
constant.
Ni
Active sites on Ni-promoted transition
metal sulfides that catalyze aromatics
hydrogenation
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