Efficient hydrogenation performance improvement of MoP and Ni2P catalysts by adjusting the electron distribution around Mo and Ni atoms

Article abstract of DOI:10.1039/c6ra09862b

MoP and Ni₂P catalysts were modified by elements with different electronegativities. Based on this method, two series of catalysts were designed and synthesized: Mo based phosphide catalysts (MoP, Mo₈WP₉, Mo₈CuP₉) and Ni based phosphide catalysts (Ni₂P, Ni₃₈W₂P₂₀). The consistency between synthetized and designed materials was mainly verified by XRD, SEM-EDS, TEM, EXAFS. Then, relative catalyst parameters were further characterized using BET, and CO chemisorption. The hydrogenation properties of verified synthesized catalysts are evaluated by hydrodeoxygenation of methyl palmitate. The activity of the catalysts followed the order: Mo₈WP₉ > MoP > Mo₈CuP₉, Ni₃₈W₂P₂₀ > Ni₂P. In particular, the W modified catalysts (Mo₈WP₉ and Ni₃₈W₂P₂₀) exhibited hydrogenation activities 2.83 and 3.68 times that of MoP and Ni₂P, respectively. Further, the product selectivities of different catalysts were analysed detailedly. The C16/C15 molar ratio was increased with the incorporation of W, which represented that the selectivity of the HDO pathway increased. While the C16/C15 molar ratio was decreased with the introduction of Cu, and this suggested that the selectivity of decarbonylation increased. These results indicate that the adjustment of the electron distribution around Mo and Ni atoms could successfully control the hydrogenation properties of catalysts.

Full text of DOI:10.1039/c6ra09862b

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Efficient hydrogenation performance improvement of MoP and
Ni2P catalysts by adjusting the electron distribution around Mo
and Ni atoms
Received 00th January 20xx,
Accepted 00th January 20xx
Mingyue Lu, a Lirong Zheng, c Rongguan Li, a Qingxin Guana and Wei Lia, b*
DOI: 10.1039/x0xx00000x
MoP and Ni2P catalysts were modified by elements with different electronegativities. Based on this method, two series of
catalysts were designed and synthesized: Mo based phosphide catalysts (MoP, Mo8WP9, Mo8CuP9) and Ni based phosphide
catalysts (Ni2P, Ni38W2P20). The consistency between synthetized and designed materials was mainly verified by XRD, SEM-
EDS, TEM, EXAFS. Then, relative catalyst parameters were further characterized using BET, and CO chemisorption. The
hydrogenation properties of verified synthetized catalysts are evaluated by hydrodeoxygenation of methyl palmitate.The
activity of the catalysts followed the order: Mo8WP9 > MoP > Mo8CuP9, Ni38W2P20 > Ni2P. In particular, the W modified
catalysts (Mo8WP9 and Ni38W2P20) exhibited hydrogenation activities 2.83 and 3.68 times that of MoP and Ni2P,
respectively. Further, the products selectivities of different catalysts were analysed detailedly. The C16/C15 molar ratio
was increased as the incorporation of W, which represented the selectivity of HDO parthway increased. While the C16/C15
molar ratio was decreased as the introduction of Cu, and this suggested that the selectivity of decarbonylation increased.
These results indicate that the adjustment of the electron distribution around Mo and Ni atoms could successfully control
the hydrogenation property of catalysts.
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modification. In surface modification, the modifier is attached
to the catalyst surface. The normal modified catalysts were
reported as Ni–P–S17, 18 and Ni–P–O19, which were recognized
1. Introduction
Numerous studies have been devoted to the studies of
the hydrogenation catalysts due to their wide applications in
many fields, such as synthesis of organic chemicals, crude oil
processing and production of bio-fuel1-6
as the real active phases in the process of HDS and HDO,
respectively. In crystal modification, the modifier is inserted
into the crystal of catalyst. Representative modified catalysts
were bi/trimetal phosphides20–24. These modified catalysts
exhibit higher hydrogenation activities than the original metal
phosphides.
. Among these
catalysts, transition metal phosphides, which exhibit
unexpected catalytic activity and excellent stability, have
attracted considerable attention as new catalysts for
hydrotreating process7-9. Particularly, metal phosphides have
definitely emerged as research focus because of their better
In our previous work19, we had inferred and provided an
electronegativity modification method, which could enhance
effectively the hydrogenation performance of the catalysts.
The protocol is ‘modifying elements with relatively high
electronegativity will favour an increase in the hydrogenation
activity, provided crystal metallic properties are retained’.
Based on this method, the crystal-modified Ni–W alloy was
designed. However, this modification changed the original
hexagonal crystal of Ni2P. This was unfavourable for
determining whether the enhancement of catalyst activity was
caused by lattice change or by electronegativity change. In
addition, the metal phosphides represented by Ni2P were
reported to have superior activities to traditional metal
catalysts in some hydrotreating processes. It was important to
improve the activity of metal phosphides, while retaining their
original crystal structure and properties.
catalytic
activity
in
hydrodenitrogenation,
hydrodesulfurization, and hydrodeoxygenation (HDO)10-12, and
they have potential to be next-generation catalysts in
hydrogenation. Of all the phosphides, previous works have
shown that MoP and Ni2P are more active and stable than
others in HDO reactions13-16
.
The promising performance of MoP and Ni2P has
prompted many studies on their synthetic modification to
enhance the activity of the active phases. The promotion
methods can be divided into surface modification and crystal
In this paper, the electronegativity modification method
was further expanded and extended. We attempted to
maintain the original crystal structure of MoP, choosing the
higher electronegativity atom W to enter the crystal lattice to
1 | J. Name., 2012, 00, 1-3
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replace Mo atom, in accordance with the above 2.2 Material Characterization
electronegativity modification method. To prove further the
The catalysts were studied by a series of characterizations
as follows:
reliability of the theory, another common phosphide (Ni2P)
was also modified by W atom. In addition, we chose the lower
electronegativity atom Cu to modify the MoP catalyst to test
briefly this method from the opposite perspective. Preliminary,
we propose two assumptions about the Cu dopant in virtue of
the electronegativity of Cu being lower than those of Mo and
P: (1) Cu could partly enter the crystal lattice randomly,
applying the electronegativity modification to decrease the
catalyst activity, and (2) little Cu could enter the crystal lattice,
also causing the activity declined. In order to investigate purely
the effect of the incorporation of modified elements on the
catalysts, the catalysts were prepared in bulk form to eliminate
the influence of the support on the catalytic activity. In detail,
two series of catalysts were synthesized in this research: Mo-
based phosphide catalysts (MoP, Mo8WP9, Mo8CuP9) and Ni-
based phosphide catalysts (Ni2P, Ni38W2P20).
X-ray diffraction (XRD) was studied with a Bruker D8
FOCUS instrument using Cu-Kα radiation. The measurement
was made in the 2θ range of 10–80 ° at 40 kV and 40 mA.
The specific surface area of materials was analysed with
the BET method using nitrogen adsorption-desorption
isotherms measured at 77 K on a BELSORP Mini instrument.
CO uptake was obtained on Micromeritics Chemisorb
2750 instrument using 100 μL loop ring. CO chemisorption was
taken at 30 °C in the flowing He (100 mL/min). The samples
were pre-reduced in flowing H2 at corresponding temperature
for 1 h.
H2-TPD was carried out using the Micromeritics
Chemisorb 2750 instrument. 0.15 g of sample was reduced at
600 ºC for 1 h by a stream of gas containing 10% H2 and 90%
Ar (50 mL/min). After reduction, the sample was cooled down
to room temperature, and the sample was swept with a N2
flow (50 mL/min) for 1 h. The H2-TPD was conducted by
sample heating at a rate of 10 ºC/min from room temperature
to 850 ºC under a N2 flow (20 mL/min).
Nowadays, HDO reactions, as an effective way to upgrade
bio-oil, were commonly used to test the hydrogenation
performance of catalysts. Therefore, the activity of modified
catalysts was investigated in the methyl palmitate HDO
evaluation reaction. Methyl palmitate is a derivative of palm
oil and widely distributed in nature. It has a lower melting
point (30 ºC) compared with palmitic acid, and is more suitable
for use in laboratory studies25. The activity results showed that
the hydrotreating performance of the catalysts was controlled
by the electronegativity of the modifier. The electronegativity
modification method was consummated further.
The surface morphologies of the alloys were observed
using scanning electron microscopy (SEM, VEGA III SBH) linked
with energy-dispersive X–ray spectrometer (EDS). The samples
were plated with gold before measurement to avoid the
electric charge accumulation.
High-resolution transmission electron microscopy
(HRTEM) images were obtained on a Tecnai F20 electron
microscope operated at an accelerating voltage of 200 kV
equipped with a field emission source.
2. Experimental
X-ray photoelectron spectroscopy (XPS) was performed
using a Kratos Axis Ultra DLD spectrometer employing a
monochromatic Al Kα X–ray source (hν = 1486.6 eV), hybrid
(magnetic−electrostaꢀc) opꢀcs, and a mulꢀchannel plate and
delay line detector. All XPS spectra were recorded using an
aperture slot of 300 × 700 μm, survey spectra were recorded
with a pass energy of 160 eV, and high resolution spectra with
a pass energy of 40 eV.
2.1 Material synthesis
MoP, as
a reference, was prepared in two steps:
precursor preparation, precursor reduction. In the preparation
of precursor, the Mo/P mole ratio = 1. (NH4)6Mo7O24·4H2O and
(NH4)2HPO4 were dissolved in an appropriate amount of water,
followed by stirring for 1 h. The mixture solution was marked
with solution A. Then, the solution A was dried adequately at
120 ºC for 12 h, calcined at 550 °C for 2 h. After that, the
catalyst precursor was obtained. In the precursor reduction,
the precursor was directly reduced at 700 °C for 1 h in flowing
H2. After cooled down to room temperature under H2
atmosphere, MoP catalysts were prepared.
Extended x-ray absorption fine structure (EXAFS) of the
catalysts were measured at the 1W1B beamline of Beijing
Synchrotron Radiation Facility (BSRF) using a Si (111) double-
crystal as monochromator. The storage ring was operated at
2.5GeV, with an average ring current of 200 mA. The Mo K-
edge spectra of Mo-based phosphides, the Ni K-edge spectra
of Ni-based phosphides, the W L3-edge spectra of W modified
catalysts, and the Cu K-edge spectra of Cu modified catalysts
were collected in transmission mode. During measurements,
Mo, Ni, W, and Cu foils were used to calibrate the energy,
respectively. The obtained spectra were back-subtracted and
converted to k space. Then the Fourier transforming was
performed on the k3-weighted EXAFS data using a Hanning
window function to get the radial structure functions (RSFs).
The XAFS data were analysed using the IFEFFIT26 software
package. Final fits of all the samples were made in Artemis.
W or Cu were introduced into the bulk MoP catalyst by
the mixture (NH4)6W7O24·6H2O or Cu(NO3)2·3H2O with the
solution A according to the corresponding ratio. The Mo/W/P
molar ratio = 8/1/9, and the Mo/Cu/P molar ratio = 8/1/9. The
above solution was dealt with following the same processes as
the preparation of MoP.
The syntheses of Ni2P and Ni38W2P20 were in the similar
procedures with Mo based phosphide catalysts. The Ni/P
molar ratio = 2, and the Ni/W/P molar ratio = 38/2/20.
Whereas, Ni(NO3)2·6H2O was used as the source of Ni. The
resulting catalysts were prepared from the precursors by
reducing at 600 °C for 2 h in flowing H2.
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2.3 Hydrogenation performance test
Methyl palmitate was chosen as the model compound to
investigate the catalysts hydrogenation performance. The HDO
reaction was carried out in a continuous flow fixed-bed reactor
equipped with a high-pressure pump.
Take the Mo based phosphide catalysts for example, the
reaction procedure was stated in detail here. The catalysts
were pelleted, crushed, and a particle size fraction of 20-40
mesh was used for the test. And the catalysts were diluted
with quartzs sand before loaded into the reaction tube. Before
reaction, the catalysts were reduced at the same temperature
as preparation for 1 h in flowing H2. After prereduction, the
system was adjusted to the desired reaction temperature (300
°C) and pressure (3.0 MPa). The feedstock containing methyl
palmitate in decalin (30 wt %) was continuously pumped into
the reactor at the rate of 0.1mL/min. The weight hourly space
velocity (WHSV) was equal to 6 h–1, and the H2/oil ratio was
kept at 1000 (V/V).
The hydrogenation reactions for Ni based phosphide
catalysts were conducted in the same procedure.
Nevertheless, the reaction temperature was desired as 370 °C.
The weight hourly space velocity (WHSV) was 3 h–1.
Liquid products were collected every hour in all assays.
The reactant and products were analysed using
a gas
chromatograph (Agilent 7890A), equipped with an HP-5
capillary column and a flame ionization detector (FID). The
products compositions were identified by GC-MS (Shimadzu
QP2010 SE). The hydrogenation performance of catalysts was
determined by the methyl palmitate conversion and turnover
frequency (TOF). The conversion, selectivity and TOF value
were calculated following Eq. (1) Eq. (2) and Eq. (3).
Figure 1. XRD patterns of different catalysts.
MoP phase. No peak shifting was observed in the curve of
Mo8WP9 compared with MoP, because of the atom size of W
(atomic radius, 137 pm), which is similar to Mo (atomic radius,
137 pm). Although the atom size of Cu (atomic radius, 128 pm)
is smaller than that of Mo, no peak shifting was found in the
plot of Mo8CuP9. This could probably be explained as the
retention the original MoP lattice structure, It could be
ascribed that part of Cu atom might enter MoP lattices by a
random replacement of Mo without causing the original lattice
structure collapsed. For the Ni-based phosphide catalysts
(Figure 1b), a series of Ni–W–P materials with different Ni/W
ratios was prepared (Figure S1, ESI†). When the Ni/W ratio =
19/1, there was no Ni5P4 detected in the pattern, and the
tungsten could be well dispersed. The diffraction peak
positions of Ni38W2P20 slightly shift to a lower angle compared
with Ni2P. This shows that the lattice parameters of Ni38W2P20
are larger than those of Ni2P calculated using the Bragg
Conversion ꢀ ꢁ1 ꢂ ꢃꢃ ꢅ ꢆ 100%
(1)
(2)
(3)
ꢄ
ꢃꢇ
Product selectivity =
∑
ꢃꢇ
ꢋꢉ
TOF = ꢈ
ꢉꢄ
ꢊ
ꢌꢍꢎꢏꢐꢑꢒꢓꢔ
Where n and n0 denote the moles of methyl palmitate in
the product and feed, respectively, and ni is the moles of
product i. FA0 is the molar rate of reactant pumped into the
reactor (ꢀ
mol s−1), W is the catalyst weight (g), COuptakes is the
uptake of chemisorbed CO (
ꢀ
mol g−1), and XA is the reactant
conversion (%).
3. Results and discussion
3.1 Catalyst Characterization
3.1.1 X-ray Diffraction
equation (2dsinθ = nλ), as has been reported for the NiMoP
system27. This lattice expansion was formed by the
substitution of smaller Ni atoms (atomic radius, 125 pm) by
bigger W atoms (atomic radius, 137 pm).
Figure 1 shows the XRD patterns of both series of
catalysts. As seen from Figure 1a, all the Mo-based phosphide
samples exhibited peaks due to the phases of MoP. This
suggested the new phase preferred to remain more like the
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Figure 2. SEM (a, b, c, d, e) and HRTEM (f, g, h, I, j) micrographs of MoP (a, f); Mo8WP9 (b, g), Mo8CuP9 (c, h), Ni2P (d, i) and Ni38W2P20 (e, j). Insets of figure (f); (g), (h), (i) and (j)
show the corresponding FFT diffraction pattern.
3.1.2 SEM and TEM
MoP. The samples give rise to two peaks at distances of 2.02 Å
and 2.85 Å roughly, which correspond to Mo–P and Mo–Mo
bond distances28. This indicates that Mo in Mo8WP9
maintained the
The morphologies and microstructures of catalysts were
observed by SEM and HRTEM images (Figure 2). Mo based
phosphide catalysts (Figures 2a–c) showed similar
appearances, and there were no significant differences in
particle sizes among MoP, Mo8WP9 and Mo8CuP9 samples.
Comparing SEM images of Ni2P (Figure 2d) with Ni38W2P20
(Figure 2e), it can be seen that the particle sizes of Ni38W2P20
are much smaller than those of Ni2P. The results correspond to
the XRD patterns (Figure 1b), in which Ni38W2P20 showed
weaker and broader peaks than Ni2P. This was attributed
essentially to the substitution of the larger W atom for smaller
Ni atom, leading to lattice expansion. This expansion, which
could be seen from XRD spectra (Figure 1b), partially broke the
original crystal, and destroyed the long-range ordered
structure to make the crystal become smaller. Moreover, in
the SEM–EDS results (Table S1, ESI†), Mo8WP9 contained 19.86
wt% W and Ni38W2P20 contained 13.93 wt% W, while there
were no W diffraction peaks detected in the XRD patterns. The
results demonstrated that the W atom might enter MoP and
Ni2P lattices. In Figures 2f–j, the crystal lattice fringe was
clearly observed in the TEM images. In the fast Fourier
transform diffraction patterns of MoP, Mo8WP9, Mo8CuP9, Ni2P
and Ni38W2P20 (inserts of Figure 2f–j, respectively), one set of
distinguishing points was regularly distributed. This possibly
elucidates that W- and Cu-modified catalysts maintained their
original crystal forms.
Figure 3. (a) Comparison of Fourier transforms of the Mo K-edge EXAFS spectra for MoP
and Mo8WP9 catalysts. (b) Comparison of Fourier transforms of the Mo K-edge and W
L3-edge EXAFS spectra for Mo8WP9 catalysts. (c) Comparison of Fourier transforms of
the Ni K-edge EXAFS spectra for Ni based phosphide catalysts. (d) Comparison of
Fourier transforms of the Ni K-edge and W L3-edge EXAFS spectra for Ni38W2P20
catalysts. (e) Comparison of Fourier transforms of the Mo K-edge EXAFS spectra for
3.1.3 EXAFS
The main objective of EXAFS characterization was to
investigate the structure of the catalysts. Figure 3a shows the
magnitude of the
k
3-weighted Fourier transforms (FTs) of the MoP and Mo8CuP9 catalysts. (f) Comparison of Fourier transforms of the Cu K-edge
and W L3-edge EXAFS spectra for Mo8CuP9 catalysts.
EXAFS spectra of the Mo K-edge on the Mo-based phosphide
catalysts. From this figure, it can be seen that the FT of the
EXAFS spectrum of the Mo8WP9 is very similar to that from
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MoP catalyst retained its original structure. The Cu K-edge FT of
the EXAFS spectrum shows a broad peak at around 2.1 Å,
approximating to the first peak for the Mo K-edge of Mo8CuP9
derived from Mo–P. Therefore, it is speculated that this peak
corresponds to the Cu–P bond. However, in the Cu K-edge
spectra, a small broad peak was found between the position of
the Cu–Cu bond (2.24 Å) on the Cu K-edge spectrum of Cu foil
and the position of the Mo–Mo (2.85 Å) bond on the Mo K-
edge spectrum. This indicates that there were probably a few
Cu-Mo bonds in Mo8CuP9 crystallite. It is assumed that Cu may
be partly enter the MoP crystallite. We also need further
experiments to research the local structure of Cu in Mo8CuP9
catalyst.
The Ni K-edge FTs of EXAFS collected in Ni-based
phosphide catalysts are given in Figure 3c. Both samples give
two dominant peaks located at 1.77 and 2.27 Å characteristic
for Ni–P and Ni–Ni, respectively30, 31. Analogously, Ni38W2P20
maintained the original structure and co-ordination of Ni2P
after doping with W. Figure 3d shows the FTs of the Ni K-edge
and W L3-edge EXAFS spectra for Ni38W2P20 catalysts. The W L3-
edge EXAFS spectrum shows a broad radial distribution
between about 1.15 Å and 2.80 Å, which comprises both W–P
and W–M (M = W or Ni) contributions. The W L3-edge EXAFS
spectra for Ni38W2P20 catalyst was fitted with Ni2P as a model.
From the fitting parameters (Table 1), the bond lengths of W–P
(2.46 Å) and W–M (M = W or Ni) (2.72 Å) were longer than
those of Ni–P (2.24 Å) and Ni–Ni (2.62 Å). These could be
attributed to the atom size of W (atomic radius, 137 pm) being
longer than that of Ni (atomic radius, 125 pm). These results
demonstrated that W entered the Ni2P crystal lattice to
replace Ni.
3.1.3 XPS
Figure 4 shows the XPS spectra in the Mo 3d and Ni 2p3/2
regions for the phosphide catalysts. For MoP, the Mo 3d5/2
peaks at 227.8, 229.5 and 232.0 eV are attributed to the Moδ+
Figure 4. XPS spectra of Mo 3d and Ni 2p in phosphide catalysts.
original structure and co-ordination of MoP after doping with
W. Nevertheless, the peak intensity of further shells (Mo–Mo)
in Mo8WP9 is lower, demonstrating that the disorder of the
sample increased with W into the crystal lattice. Figure 3b
compares the FTs for the k3-weighted Mo K-edge and W L3-
edge EXAFS spectra obtained from Mo8WP9 catalyst. The W L3-
edge EXAFS radial structure plot of Mo8WP9 gives one peak at
around 2.12
Å
due to W–P29. Moreover, another two
distinctive peaks are located at similar positions (2.91 Å, 4.23
Å) with Mo K-edge spectra of the Mo8WP9 sample. This
indicates that the W in the Mo8WP9 samples took a structure
with lattice parameters close to MoP. Similar trends are
evident from the values presented in the fitting parameters (Table
S2, ESI
which is considerably longer than the W
crystal (2.86
29, demonstrating that W hardly existed in the form
†
). The W
–
M (M = W or Mo) fitted bond distance is 3.21
Å,
–W bond distance in WP
Å
)
Figure 5. H2-TPD profiles of the phosphide catalysts.
of WP. Above all, it could be concluded that W occupied the site
of Mo in the Mo8WP9 catalyst.
Considering the Mo8CuP9 (Figure 3e and 3f), from the spectra
of K-edge FT of EXAFS, it could be observed that the Cu-modified
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species in MoP, Mo4+ and Mo6+, respectively14. The peak
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due to Moδ+ in Mo8WP9 was located at about 228.2 eV, which
was higher than that in MoP. Analogously, In the Ni 2p3/2
spectra of Ni2P, two peaks were visible at 852.9 and 855.9 eV,
3.2 Hydrogenation activity
which were assigned to Niδ+ species in Ni2P and Ni2+ ions, 3.2.1. Hydrogenation Performance
respectively14. The peak attributed to Niδ+ in Ni38W2P20 was at
To evaluate the hydrogenation performance of these
catalysts, they were tested on the HDO reaction of the model
compound, methyl palmitate. As the activity of the Ni-based
about 852.8 eV, higher than that in Ni2P. These results
indicated that the positive charge of Moδ+/ Niδ+ in W modified
phosphides was more than that in original catalysts. These
could be ascribed to the electron transfer from Mo/Ni to W,
which was consistent with that the electronegativity (2.16) of
Mo/Ni is lower than that (2.36) of W.
phosphides was lower than Mo-based phosphides, the
hydrogenation performance of the Mo-based phosphides were
performed at 300 °C, while the activity test of the Ni-based
phosphides catalysts was employed at an elevated
temperature (370 °C). The variations of the conversion and
products selectivity with the reaction time were observed to
Conversely, the Mo 3d spectra of Mo8CuP9 exhibited one
peak at about 227.7 eV corresponding to Moδ+ species, which
was slightly lower than that of MoP. This meant that the
positive charge of Moδ+ in Mo8CuP9 was less than that of MoP,
partly due to the addition of Cu leading to an increase in the
electron density around the Mo.
tend to stable (Figure S2-S6, ESI
catalysts are
†). The catalytic activities of
presented as conversions at in Table 1. It is clearly observed
that both of the W-modified catalysts (Mo8WP9 and Ni38W2P20)
showed higher activities, whereas Cu-modified catalysts
exhibited lower activities than those of the reference catalysts,
as anticipated. In practical terms, the conversions of methyl
palmitate on Mo8WP9 were approximately 2.83 times those on
MoP, while the conversion on Mo8CuP9 was more than two
times lower than those on MoP. In terms of Ni-based
phosphide catalysts, the conversion of Ni38W2P20 increased
from 6.92% on Ni2P to 25.48%.
3.1.3 H2-TPD
H2-TPD was performed to investigate the effects of
modifier (W and Cu) on the interaction between metal and
hydrogenation. The H2-TPD profiles recorded on different
samples are shown in Figure 5. Usually, the hydrogen species
desorbed below 400 °C are related with those adsorbed on the
metal sites, while those desorbed above 400 °C are attributed
to the spilt-over hydrogen species27. It was worth noting that
the desorption peak below 400 °C of the W modified sample 3.2.2. Catalyst TOF
slightly shifted to lower temperature, and the amount of
In order to get rid of the influence of different exposed active
desorbed H2 of the W modified sample decreased compared
with the unmodified catalyst (MoP). This shift is attributed to
the decreased binding energy between the adsorbed hydrogen
and active sites, which due to the W modifier effects, leading
to the decrease of electron density around metal sites. On the
contrary, in the case of Cu modified phosphide, the area of the
sites on different catalysts, we chose TOF as a significant the
activity of per active site in unit time. The Brusnaeur–Emmett–
Teller (BET) surface area, CO uptake and Turn over frequency
(TOF) value of different catalysts are given in Table 1.The
distinctions reflected among these BET values of catalysts
were in accordance with the SEM images. The number of
active sites was determined by the CO uptake value. Mo8WP9
and Ni38W2P20 chemisorbed more CO than MoP and Ni2P,
respectively, indicating that the amount of active sites
increased through W modification. Crucially, TOF displays a
same variation tendency as the conversion. The TOF value of
Mo8WP9 was 1.9 times that of MoP, and Ni38W2P20 maintained
a TOF value 1.2 times that of Ni2P, while Mo8CuP9 maintained
a TOF value less than half that of MoP. Indeed, the active site
of Mo8WP9 was intrinsically more active than that of MoP for
the conversion of methyl palmitate, the same for Ni38W2P20
and Ni2P. These phenomena can possibly be explained by the
higher electronegative W entering the original crystal and
giving rise to electron transfer from Mo (or Ni) to W, leading to
the electron density around Mo (or Ni) decreased and causing
the interaction between Mo (or Ni) and
peak below 400 °C was much smaller than that of MoP though
the peak shifted toward a low temperature. This could suggest
that the interaction between the adsorbed hydrogen and
active sites enhanced with the Cu modification, resulting in the
increase of electron density around metal sites.
Table 1. The physical properties and catalytic activities of different samples.
Samples
BET Area
[m2/g]
CO uptake
Conversion
(%)
TOF
[s−1
[ꢀmol/g]
]
MoP
Mo8WP9
Mo8CuP9
Ni2P
8.70
9.67
3.29
0.42
1.92
8.05
11.65
2.24
0.04
0.11
26.71a
75.67a
3.50a
0.21
0.40
0.09
6.92b
11.86
14.29
Ni38W2P20
25.48b
areaction conditions: 3 MPa, 300 °C, WHSV = 6 h−1, H2/oil = 1000
breaction conditions: 3 MPa, 370 °C, WHSV = 3 h−1, H2/oil = 1000
6 | J. Name., 2012, 00, 1-3
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the reaction conditions of Ni-based phosphides were: 3 MPa, 370 °C, WHSV = 3 h−1
H2/oil = 1000
,
atomic hydrogen weakened. It could be investigated in the H2-
TPR results. The interaction between active sites and atomic
hydrogen has been proved to be the lynchpin to determine the
activation of atomic hydrogen, which revealed the
improvement of hydrogenation properties19. Conversely, the
activity of active sites decreased after the MoP catalyst
modified by Cu. On the one side, this is might due to the
electron transfer from Cu to Mo, caused by the interaction
between Mo and Cu entered the crystal. On the other, it is
supposed that the introduction of lower electronegativity Cu
likely favoured more electrons around Cu to transfer to P,
which could be shown by the existence of the Cu–P bond
obtained from the EXAFS results. Instead, the electrons
transferred from Mo to P might be reduced. The two
approaches of electron transfer make the electron density
around Mo increased, which resulted in the enhancement of
the interaction between Mo and hydrogen. This is not
conducive to the activation of hydrogen. In summary, these
results testified that the addition of elements W and Cu with
different electronegativities
successfully controlled the hydrogenation performance of
catalysts.
3.2.3. Product selectivity
Figure 6 compares the products distribution of different
catalysts resulted at steady state. Analysis of the products
showed that in the presence of all these catalysts, the major
products were pentadecane (C15) and hexadecane (C16),
which were generated from the decarbonylation (DCO) and
HDO pathways, respectively32, 33. The detected oxygenated
intermediates were hexadecanol, hexadecanoic acid (trace)
and palmityl palmitate (trace). Hexadecanol was an important
intermediate in the process of HDO reaction. The selectivity of
different products showed remarkable dependency on
composition of catalysts. Figure 7 shows the C16/C15 molar
ratios on different catalysts, which represents the selectivity
between the HDO and the DCO pathways. For Mo based
phosphides (MoP and Mo8WP9), the C16/C15 ratio was larger
than 1. While the C16/C15 ratio obtained from the Ni based
phosphides (Ni2P and Ni38W2P20) was less than 1. It could turn
out that MoP and Ni2P preferentially catalysed HDO and DCO
reaction, respectively14. This could also be partly due to the
higher reaction temperature employed in the case of Ni-based
phosphides than that of Mo-based phosphides catalysts.
Because it has been reported that the increase of the
temperature in HDO reactions would favor the DCO
parthway34.
As to the MoP catalyst, the selectivity of C15 was 21% and
the selectivity of C16 was 27%, while the selectivity of the
intermediate hexadecanol was 53%. For comparison, the
selectivity of C16 was markedly increased to 67% over Mo8WP9
after the incorporation of W into MoP, while the selectivity of
hexadecanol showed an apparent decline. Simultaneously, the
C16/C15 molar ratio increased significantly. This indicated that
the addition of W promoted the formation of C16 proceeded
via the dehydration and hydrogenation of hexadecanol.
Analogously, in case of Ni based catalysts, the selectivity of
C16 increased and the selectivity of C15 decreased as the
dopant of W, resulting in the enhancement of C16/C15 molar
ratio. In summary, the HDO reaction on Mo (or Ni) sites was
promoted and the DCO pathway was inhibited to a certain
degree. It can partially be explained by the electronic
interaction between Mo (or Ni) and W (i.e., electron transfer
from Mo or Ni to W), which lead to the decrease of electron
density around metal sites (Mo and Ni). The Mo and Ni sites
with higher electrophilicity were more easily combined with
electronegative oxygen in methyl palmitate or oxygen-
containing intermediates (hexadecanol) and were favourable
to the activation of C = O and C–O group, predominately giving
Figure 6. The methyl palmitate conversions and products selectivity of two different
series catalysts at different reaction conditions. The reaction conditions of Mo-based
phosphides were: 3 MPa, 300 °C, WHSV = 6 h−1, H2/oil = 1000; and the reaction
conditions of Ni-based phosphides were: 3 MPa, 370 °C, WHSV = 3 h−1, H2/oil = 1000
rise to the HDO product (C16)14, 35
.
Figure 7. The C16/C15 molar ratios of two different series catalysts. The reaction
conditions of Mo-based phosphides were: 3 MPa, 300 °C, WHSV = 6 h−1, H2/oil = 1000;
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In regard to Cu with lower electronegativity modified
RSC Advances
6
7
8
C. Tu, M. Li, H. Li, Y. Chu, F. Liu, H. Nie and D. Li, RSC Adv.,
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F. Han, Q. Guan, and W. Li, RSC Adv., 2015, 5, 107533-
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MoP catalyst, the selectivity of C15 increased and the
selectivity of hexadecanol decreased (i.e., C16/C15 molar ratio
declined). The introduction of Cu to MoP catalysts facilitated
the decarbonylation and inhibited the HDO pathway. This may
partially because of the interaction between Mo and Cu
entered the crystal, the electron transferred from lower
electronegative Cu to higher higher electronegative Mo.
Meanwhile, the interaction between P and Cu not entered the
crystal made more electrons transfer from Cu to P, and lead to
the electrons transferred from Mo to P reduced. In both cases,
the electrophlicity of Mo was decreased to give rise to the
promotion the DCO pathway on Mo site36.
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gave the results that the TOF value of W-modified catalyst
(Mo8WP9) was higher than that of the original catalyst (MoP).
Meanwhile, the C16/C15 ratio on W-modified catalyst
(Mo8WP9) was increased, which represented the enhancement
of the selectivity of HDO route. A similar TOF and C16/C15
ratio changing trends were observed between Ni38W2P20
catalyst and Ni2P catalyst. However, both the TOF value and
C16/C15 ratio of the Cu-modified catalyst (Mo8CuP9) were
lower than those of MoP. It turned out that the
hydroprocessing performance of the catalyst could be
controlled by the addition of modifiers with higher or lower
electronegativities. This finding may give valuable information
for designing more effective hydrogenation catalysts.
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Acknowledgements
This work was financially supported by the NSFC
(21376123, U1403293), MOE (IRT-13R30 and 113016A), and
the Research Fund for 111 Project (B12015).
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8 | J. Name., 2012, 00, 1-3
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The catalyst (MoP) was modified by higher electronegativity element
W
and lower
electronegativity element Cu, realizing the control of hydrogenation performance of the catalysts.