Tetrahedral Nickel(II) Phosphosilicate Single-Site Selective Propane Dehydrogenation Catalyst

Article abstract of DOI:10.1002/cctc.201701815

Silica-supported Ni catalysts usually show poor stability, low selectivity, and short lifetime in high-temperature alkane dehydrogenation reactions owing to the reduction to Ni⁰ nanoparticles under the reaction conditions. The introduction of a phosphate ligand to silica-supported Ni^II provided single-site tetrahedral Ni^II phosphosilicate as a stable and selective propane dehydrogenation catalyst. The Ni^II?OSi bonds activate the C?H bonds of propane and make the Ni^II sites catalytically active, whereas the Ni?OP bonds prevent the reduction of Ni^II to Ni⁰ under the dehydrogenation conditions and help to achieve high stability and selectivity.

Full text of DOI:10.1002/cctc.201701815

Accepted Article
Title: Tetrahedral Nickel(II) Phosphosilicate Single Site Selective
Propane Dehydrogenation Catalyst
Authors: Guanghui Zhang, Ce Yang, and Jeffrey T Miller
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Tetrahedral Nickel(II) Phosphosilicate Single Site Selective
Propane Dehydrogenation Catalyst
Guanghui Zhang,[a] Ce Yang,[b] and Jeffrey T. Miller*[a]
Abstract: Silica-supported Ni catalysts usually show poor stability,
low selectivity, and short lifetime in high-temperature alkane
dehydrogenation reactions due to the reduction to Ni(0) nanoparticles
under reaction conditions. Introduction of the phosphate ligand to the
silica-supported Ni(II) provided single site tetrahedral Ni(II)
phosphosilicate as a stable and selective propane dehydrogenation
catalyst. The Ni(II)-OSi bonds activate C-H bonds of propane and
make the Ni(II) sites catalytically active, while the Ni-OP bonds
prevent the reduction of Ni(II) to Ni(0) under dehydrogenation
conditions and help achieve high stability and selectivity.
bonding.[10] We anticipate high stability and selectivity can be
achieved for alkane dehydrogenation by keeping Ni(II) catalysts
from reduction to Ni(0) under reaction conditions, which will
facilitate the alkene desorption and thus increase alkene
selectivity. We envisioned that introduction of a phosphate ligand
may help keeping Ni(II), and the choice was made based on
several reasons: 1) Ni-OP bonds should be more resistant to
cleavage than Ni-OSi bonds in the presence of H2 which should
help prevent the reduction; 2) H3PO4 is a stronger acid than silica
surface OH groups, and it should be able to react with the Ni-OH
or Ni-OSi bonds forming Ni-OP bonds; 3) although stronger than
silica surface –OH groups, H3PO4 is a weak acid, and the
presence of P-OH groups in the catalyst should not lead to
oligomerization or cracking of the olefin products; 4) nickel
phosphate is thermally very stable.
Nickel has a number of readily available oxidation states
commonly invoked in catalysis. In addition to the commonly seen
Ni(0)/Ni(II) catalytic cycles, easy accessibility to Ni(I) and Ni(III)
allows different modes of reactivity and radical mechanisms. Thus,
Ni catalysts have been extensively used in cross-coupling,[1]
oligomerization,[2] hydrogenation and reforming reactions.[3] But
compared with its group 10 counterparts, especially Pt, Ni does
not find a broad application in high-temperature (> 500 °C) light
alkane dehydrogenation reactions.[4] Under these conditions,
supported Ni catalysts are reduced to Ni(0) nanoparticles which
usually show poor stability, fast coke deposition, high methane
and low alkene selectivity.[5] To improve the stability and
selectivity, several Ni-based catalysts have been studied
including NiSn,[6] NiS,[7] and NiP for isobutane dehydrogenation
reaction.[8] NiAu bimetallic catalysts have been studied for more
challenging propane dehydrogenation reaction.[9] For each of
these, the active state is metallic Ni. By modifying the surface
composition and structure of the Ni catalysts, selectivity to
propylene was indeed improved, but fast deactivation was still
observed. To the best of our knowledge, high stability and
selectivity have not been achieved for propane dehydrogenation
reaction using Ni catalysts.
Scheme 1. Synthesis of the SiO2-supported Ni(II) catalysts.
Based on this hypotheses, we firstly synthesized the SiO2-
supported Ni(II), Ni/SiO2,using the strong electrostatic adsorption
which has been successfully used in the synthesis of Zn/SiO2 and
Co/SiO2 catalysts.[11] At pH = 11, the surface hydroxyl groups are
2+
deprotonated, and the Ni(NH3)6 cations are electrostatically
adsorbed on the surface which ensures the isolation of Ni(II) due
to the self-limiting nature of the reaction. Single site Ni(II)/SiO2
was obtained after thermal removal of the NH3 ligands at 300 °C.
Atomic absorption spectroscopic (AAS) analysis suggests that the
Ni is well dispersed on the SiO2 surface with a density of ~ 0.6 Ni
nm-2. Ni-P/SiO2 was synthesized by incipent wetness
impregnation (IWI) of excess H3PO4 (2 eq. to Ni) solution to
Ni/SiO2 followed by a 5°C/min ramp heating to 600 °C in air
(Scheme 1). AAS analysis of the Ni-P/SiO2 confirmed the Ni
density on the surface remained ~ 0.6 Ni nm-2.
Ni(0) forms many stable alkene complexes with a synergistic
alkene→Ni donation from the occupied alkene π orbital to the
empty d orbitals of Ni, and a Ni→alkene back-donation from the
occupied Ni d orbital to the empty alkene π* orbital. In many cases,
the strong olefin bonding comes mainly from the Ni→alkene back-
donation, while the relative contribution of the alkene→Ni
donation is much smaller. On the contrary, Ni(II) alkene
complexes are much less common due to the weak Ni(II)-alkene
As expected, Ni remained +2 oxidation state after thermal
removal of the NH3 ligands at 300 °C confirmed by the X-ray
absorption near edge structure (XANES) edge energy of 8342.1
eV and the pre-edge energy of 8333.1 eV. Ni2+ also remained in
the Ni-P/SiO2 with the edge energy of 8342.8 eV and the pre-
edge energy of 8333.0 eV (Figure 1 and S1). The 0.7 eV shift of
the edge to higher energy is consistent with the introduction of the
more electron withdrawing phosphate ligand. The energy and
calculated area of the pre-edge for Ni reference compounds of Oh
and Td coordination geometry as well as the catalysts are
summarized in Table 1 (Table S1 and Figure S2). Square planar
[a]
[b]
Dr. G. Zhang, Prof. Dr. J. T. Miller
Davidson School of Chemical Engineering
Purdue University, 480 Stadium Mall Drive, West Lafayette, IN
47907, United States
E-mail: mill1194@purdue.edu
Dr. C. Yang
Chemical Sciences and Engineering Division
Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL
60439, United States
Supporting information for this article is given via a link at the end of
the document.
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is very common for d8 Ni(II), which typically shows a very small
pre-edge area (< 2 X 10-2 eV) and a well-resolved strong 1s-4pz
transition.[12] However, it is not observed in the spectra of Ni/SiO2
or Ni-P/SiO2. The pre-edge area of NiO is 3.8 X 10-2 eV consistent
with its octahedral geometry, while the larger pre-edge areas of
Ni/SiO2 and Ni-P/SiO2 of4.2 and 5.6X 10-2 eV are consistent with
the tetrahedral geometry in which d-p mixing can occur to help
increase the probability of the dipole-forbidden 1s-3d transition.
Also consistent with the tetrahedral geometry, the pre-edge
energies of Ni/SiO2 and Ni-P/SiO2 are very close to the
tetrahedral reference compound Ni(PPh3)2Cl2 and are about 0.5
eV below the octahedral NiO due to the lower energy level of half-
filled t2 orbitals in the tetrahedral than the eg orbitals in the
octahedral geometry.
consistent with the formation of well-dispersed Ni(II) on the silica
surface. An even smaller second-shell peak at ~2.6 Ȧ was
observed in the Ni-P/SiO2 spectrum, and the wavelet analysis
suggests that the contribution centers at k ~ 4 Ȧ-1. The quick
decay of the EXAFS signal at higher k suggests that the Si/P shell
has higher structural disorder. Different from the Ni3(PO4)2
spectrum, no higher shell above 3 Ȧ was observed in the Ni-
P/SiO2 spectrum which is also consistent with the high Ni
dispersion.
4
3
2
4
3
2
0.40
0.30
0.20
0.10
0.0
0.40
0.30
0.20
0.10
0.0
Ni-P/SiO2
Ni/SiO2
Si
Si/P
O
O
1
4
1
4
2
4
6
8
10
12
2
4
6
8
10
12
k (Å-1
)
k (Å-1
)
0.60
0.48
0.36
0.24
0.12
0.0
1.0
NiO
Ni3(PO4)2
0.80
0.60
0.40
0.20
0.0
3
2
1
3
2
1
Ni
P
Figure 1. XAS spectra. a) Pre-edge region of the XANES spectra; b) k2-
weighted Fourier transform (FT) magnitudes of the EXAFS spectra. FT range
3.0 – 11.0 Ȧ-1.
O
O
2
4
6
8
10
12
2
4
6
8
10
12
k (Å-1
)
k (Å-1
)
Figure 2. Wavelet analysis of the XAS data with Morlet mother wavelet.
Table 2. Summary of the EXAFS fitting results.
Table 1. Summary of the XANES data of the catalysts and references.
Sample
Pre-edge
Energy
(eV)
Pre-edge Area
(10-2 eV)
Edge
Energy
(eV)
Geometry
Sample[a]
Scattering
pairs
Coordination
number
Bond
distance (Ȧ)
σ2 (Ȧ2)
Ni/SiO2
8333.1
8333.0
8333.0
8333.5
8333.4
4.2
5.6
9.4
3.8
2.7
8342.1
8342.8
8340.7
8341.0
8342.8
Td
Td
Td
Oh
Oh
Ni-O
3.5 ± 0.5
4.4 ± 0.9
4.2 ± 0.4
2.3 ± 1.0
2.03 ± 0.02
3.24 ± 0.02
1.99 ± 0.04
3.18 ± 0.05
0.005 ± 0.002
0.003 ± 0.002
0.007 ± 0.002
0.010 ± 0.006
Ni-P/SiO2
Ni(PPh3)2Cl2
NiO
Ni/SiO2
Ni-Si
Ni-O
Ni-P/SiO2
Ni-Si/P
Ni3(PO4)2
[a] EXAFS data were taken at room temperature after thermal treatment at
600 °C in He.
As shown in the extend X-ray absorption fine structure
(EXAFS) spectra in Figure 1b, the smaller FT magnitudes at ~ 1.5
Ȧ (phase uncorrected distance for Ni-O bonds) of Ni/SiO2 and Ni-
P/SiO2 are consistent with the four-coordination Td geometry
compared with the six-coordination NiO and Ni3(PO4)2. For the
NiO spectrum, the second-shell peak at ~ 2.6 Ȧ corresponds to
the Ni-Ni scattering; while for Ni/SiO2, the peak at ~ 2.7 Ȧ
corresponds to the Si scattering from the SiO2 support suggested
by the wavelet analysis which provides a correlation of R- and k-
space data (Figure 2).[13] For NiO, the Ni scattering features center
at R ~ 2.6 Ȧ and k ~ 7.0 Ȧ-1, while for Ni/SiO2 the Si scattering
paths produce features center at R ~ 2.7 Ȧ and k ~ 5.0 Ȧ-1. No
significant Ni-Ni scattering contribution was observed which is
Bond distances and coordination numbers were obtained by
fitting of the EXAFS data (Figure S3), and the results are
summarized in Table 2 and S2. For Ni/SiO2, 3.5 ± 0.5 oxygen
atoms were determined at 2.03 Ȧ in the first shell, and 4.4 ± 0.9
Si atoms at 3.24 Ȧ were determined in the second shell. The best
fit for Ni-P/SiO2 suggests a slightly shorter Ni-O average bond
distance at 1.99 Ȧ which is consistent with the expected formation
of stronger Ni-OP bonds. The fit of the second shell included both
Si and P atoms, and an average distance of 3.18 Ȧ was obtained.
Note that the mean square disorder of 0.010 Ȧ2 is much higher
than that in the Ni/SiO2 consistent with the higher structural
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disorder suggested by the wavelet analysis. Surface P-OH groups
(3670 cm-1) were detected in the diffuse reflectance infrared
Fourier transform (DRIFT) spectra, along with free silanol (Si-OH)
groups (3745 cm-1) which is consistent with the proposed reaction
between Ni-OSi with H3PO4 generating Si-OH.[14] The difference
IR spectrum between Ni-P/SiO2 and Ni/SiO2 is consistent with the
presence of phosphate on the catalyst surface (Figure S4 and
S5).[15]
90 % at 10 % C3H8 conversion, while only 60 % C3H6 selectivity
was observed for Ni/SiO2. At 5.5 % conversion without co-feeding
H2, the turn-over frequency (TOF) for C3H6 production of Ni-
P/SiO2 was calculated as 2.9 h-1 which is similar to other SiO2-
supported 3d and 4p single site catalysts under similar reaction
conditions.[11, 16] In addition, Ni-P/SiO2 is also active for propylene
hydrogenation reaction, the microscopic reverse reaction of
propane dehydrogenation, at 200 °C with a TOF of 0.35 h-1.
Although
a
calcium-nickel phosphate was used for
dehydrogenation of n-butene and ethylbenzene, it was not active
for butane or propane dehydrogenation,[17] which suggests the
catalyst with Ni-OP bonds but no Ni-OSi bonds can only activate
allyl and benzyl C-H bonds. For comparison, we synthesized a
tetrahedral Ni(II) catalyst with four Ni-OP bonds, Ni/P-SiO2, which
was achieved by impregnation of excess H3PO4 on SiO2 prior
adsorbing Ni(II) ions (Figure S3, Table S1 and S2). Ni/P-SiO2 is
not active for propane dehydrogenation under the same reaction
conditions. These results suggest that Ni-OSi bonds are crucial
for activating alkane C-H bonds for dehydrogenation and H-H
bonds for hydrogenation at lower temperature (200 °C).
Figure 3. C3H8 catalytic dehydrogenation results at 600 °C. a) stability; b)
selectivity (see SI for experimental details).
To further probe the possible structural changes of the
catalysts under dehydrogenation conditions, XAS was carried out
for Ni/SiO2 and Ni-P/SiO2. No changes were observed in either
the XANES or the EXAFS spectra of Ni-P/SiO2 after 1 hour on
stream, as shown in Figure 4; however, the formation of Ni(0)
nanoparticles was observed in the XANES and EXAFS spectra of
Ni/SiO2, as expected. The introduction of phosphate ligand to the
SiO2-supported Ni catalyst successfully prevented the reduction
of Ni(II) during C3H8 dehydrogenation at 600 °C, and thus
achieved high stability and C3H6 selectivity. To further investigate
its stability against reduction, Ni-P/SiO2 was treated with H2 at
700 °C for 30 minutes. A shift of the edge to lower energy, as well
as a decrease of both Ni-O and Ni-Si/P scattering peaks was
observed (Figure 4c and 4d) suggesting a partial reduction of the
Ni(II) which is consistent with previous reports.[18] A good linear
combination fit (R factor = 2 X 10-4) was obtained using the
XANES spectra of Ni2P and Ni-P/SiO2 which suggests 34% of the
Ni(II) was reduced to Ni2P, while the other 66% remained Ni(II).
Figure 4. XAS spectra. a) XANES spectra after dehydrogenation; b) EXAFS
spectra after dehydrogenation; c) XANES spectra after 700 °C reduction with
H2 and linear combination fit; d) EXAFS spectra after 700 °C reduction with H2.
STOP
The catalytic performance of Ni/SiO2 and Ni-P/SiO2 was
investigated at 600 °C for C3H8 dehydrogenation (Figure 3, S6
and S7). High stability was observed with an initial conversion of
~ 18 %, and the conversion dropped to ~ 16 % after 3 hours on
stream. Further stability test was carried out with the same C3H8
space velocity but co-feeding H2 where the reduction of Ni(II) is
more likely to occur. Ni-P/SiO2 still showed high stability under the
strong reducing conditions, and the C3H8 conversion dropped
from 14 % to 12 % after 3 hours on stream. On the contrary,
Ni/SiO2 deactivates very fast and lost the activity in 30 minutes
under the same reaction conditions. High C3H6 selectivity was
also observed for Ni-P/SiO2, and the C3H6 selectivity reaches
Scheme 2. Reaction of the SiO2-supported Ni(II) catalysts with H2.
The reaction of SiO2-supported Ni(II) with H2 likely occurs
following the mechanism shown in Scheme 2[11a, 16a-d]. Heterolytic
cleavage of the H-H and Ni-O bonds generates O-H and Ni-H
bonds. Further Ni-O bond cleavage may occur in the presence of
H2 or following an O-H reductive elimination generating Ni(0)
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around 200 °C, as suggested by the propylene hydrogenation
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bonds cleave under reaction conditions, but the Ni-OP bonds are
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In summary, we have designed and synthesized a silica-
supported Ni(II) phosphosilicate catalyst employing a two-step
process involving strong electrostatic adsorption of Ni(II) followed
by impregnation of H3PO4 solution and thermolysis. AAS, XAS
and IR analysis allowed for assignment of the highly dispersed Td
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Ni(II)-OSi and Ni(II)-OP bonds, while the former activate alkane
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This work was supported by the Davidson School of Chemical
Engineering, Purdue University. GZ and JTM were supported in
part by the National Science Foundation under Cooperative
Agreement No. EEC-1647722. Any opinions, fingds and
conclusions or recommendations expressed in this material are
those of the authors and do not necessarily reflect the views of
the National Science Foundation. Use of the Advanced Photon
Source was supported by the U.S. Department of Energy, Office
of Basic Energy Sciences [grant number DE-AC02-06CH11357].
MRCAT operations, beamline 10-BM, are supported by the
Department of Energy and the MRCAT member institutions.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: nickel phosphate • alkane dehydrogenation •
heterogeneous catalysis • single-site catalyst • EXAFS
spectroscopy
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Phosphate dispels nickel’s
G. Zhang, C. Yang, J. T. Miller*
catalytic demons: Single site
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tetrahedral Ni(II) phosphosilicate is
highly stable and selective in propane
dehydrogenation. The Ni(II)-OSi
bonds activate the alkane C-H bonds
and make the Ni(II) sites catalytically
active, while the Ni-OP bonds prevent
the reduction of Ni(II) to Ni(0) under
dehydrogenation conditions and help
achieve high stability and selectivity.
Tetrahedral Nickel(II) Phosphosilicate
Single Site Selective Propane
Dehydrogenation Catalyst
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