Acid-tolerant intermetallic cobalt-nickel silicides as noble metal-like catalysts for selective hydrogenation of phthalic anhydride to phthalide

Article abstract of DOI:10.1039/c8cy02258e

Chemoselective hydrogenation of phthalic anhydride is regarded as the most promising route for producing downstream high-performance phthalide. A series of intermetallic cobalt-nickel silicide catalysts embedded in a carbon matrix (Co_xNi_2-xSi@C) with acid-tolerance prepared by microwave-assisted chemical vapor deposition have been investigated in this reaction system. Activity measurements show a remarkable positive synergistic effect, forming a volcano-shaped plot over the nominal Co metal fraction for Co_xNi_2-xSi@C catalysts with a peak at x = 1.5, which corresponds to a 4.5-fold enhancement of the yield of phthalide over Co₂Si@C and a 2.0-fold enhancement over the Ni₂Si@C monometallic silicide catalysts, which can be correlated with the interaction of cobalt-nickel and metal-silicon bonds. In addition, the cobalt-rich bimetallic silicide exhibits an equal activity to the reference noble metal catalysts (Au, Pd, and Pt) and a significantly higher activity than transition metal catalysts. Moreover, the bimetallic silicide as an intermetallic compound catalyst has demonstrated robust acid corrosion resistance compared to the corresponding metal catalysts.

Full text of DOI:10.1039/c8cy02258e

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DOI: 10.1039/C8CY02258E
Journal Name
ARTICLE
Acid-tolerant Intermetallic Cobalt-nickel Silicides as Noble Metal-
like Catalysts for Selective Hydrogenation of Phthalic Anhydride
to Phthalide
Received 00th January 20xx,
Accepted 00th January 20xx
Liangliang Zhang,a, b,† Xiao Chen,a,† Yujing Chen,a Zhijian Peng,b and Changhai Liang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chemoselective hydrogenation of phthalic anhydride is regarded as the most promising route for producing of
downstream high-performance phthalide. A series of intermetallic cobalt-nickel silicides embedded in a carbon matrix
(CoxNi2-xSi@C) catalysts with acid-tolerance prepared by microwave-assisted chemical vapor deposition have been
investigated in this reaction system. Activity measurements show remarkable a positive synergistic effect, forming a
volcano-shaped plot over nominal Co metal fraction for CoxNi2-xSi@C catalysts with a peak at x = 1.5, which corresponds to
a 4.5-fold enhancement of yield of phthalide over Co2Si@C and a 2.0-fold enhancement over Ni2Si@C monometallic
silicide catalyst, which can be correlated to that the interaction of cobalt-nickel and metals-silicon bonds. In addition, the
cobalt-rich bimetallic silicide preforms an equal activity to the reference noble metal catalyst (Au, Pd, and Pt) and higher
activity than that of transition metal catalysts significantly. Moreover, the bimetallic silicide as an intermetallic compound
catalyst has been demonstrated robust acid corrosion resistance than the corresponding metal catalyst.
over noble-based homogeneous catalyst ([RuCl2(Ph3P)3]) at
o
Introduction
100 C and 1 MPa H2 pressure and the good yield to phthalide
[3]
was generated.
difficult to separation and contamination of products limited
the application of homogeneous catalysts. In recent years,
supported noble metal catalysts also aroused interesting in
this reaction.
However, the drawbacks of high prices,
Phthalide as an important industrial intermediate has been
widely applied in the manufacturers of pharmaceuticals, fine
chemicals, organic synthesis, etc. Compared with the
traditional methods including the chemical reduction over Zn-
acetic acid and the reaction of phthalimide and sodium
hydroxide, the chemoselective hydrogenation of phthalic
anhydride to phthalide is regarded as the most promising
[1a, 4]
Au/FeOx-TiO2 catalyst due to the interface
effect exhibited excellent activity with high selectivity (>92%)
to phthalide under mild reaction conditions. Unfortunately,
[4]
the high price and the inevitable loss of precious metal by
corrosion limit its extensive application. Therefore,
development of low-cost alternative for noble metal catalyst
with excellent yield of phthalide and robust acid corrosion
resistance has an enormous significance.
Generally, doping the second element into the parent
metal lattice to form alloy or intermetallic compound or using
the inert support to protect the active metal can improve the
acid resistance of catalysts significantly. [1b, 5, 6] For example, Liu
et al reported that doped Ni/activated carbon catalyst had the
best catalytic performance in the hydrogenation of phthalic
anhydride to phthalide and remarkably acid-resistant ability.
[1]
routine owing to its reduced pollutant production.
Researchers strive to exploit high efficient catalysts for
chemoselective hydrogenation of phthalic anhydride to obtain
high yield of phthalide. In addition, the acid corrosion
resistance of catalyst is also another key to evaluate the
catalyst in the hydrogenation of phthalic anhydride due to the
naturally acidic of substrate and the hydrolysis by-products (o-
toluic acid and hexahydro-o-toluic acid).
Traditional nickel-base catalysts have been widely
reported as commercial catalysts with high yield of phthalide.
However, the weak corrosion resistance of nickel-base
catalysts limits its further application. [2] In the early 1980s, the
selective hydrogenation of phthalic anhydride was studied
[1b]
In addition, the active nickel protected by inert carbon
nitride can significantly avoid the corrosion of acid, which
presented stable for hydrogenation even under a strongly
acidic environment. [6]
Metal silicides, the family of refractory intermetallic
compounds between metal and silicon, as coating and thin-
film materials have been reported to present good acid
a. State Key Laboratory of Fine Chemicals, Laboratory of Advanced Materials and
Catalytic Engineering, School of Chemical Engineering, Dalian University of
Technology, Dalian 116024, P.R. China, E-mail: changhai@dlut.edu.cn
b. School of Engineering and Technology, China University of Geosciences, Beijing
100083, P.R. China
[7]
corrosion resistance in the acid solution (H2SO4 or HCl).
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [Powder XRD patterns, SEM
images, and TC-DTG curves of CoxNiy-MOFs-74]. See DOI: 10.1039/x0xx00000x
Recently, a number of hydrogenation reactions over metal
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silicides catalysts had been surveyed, which presented unique (99.999%), hydrogen (99.999%), and nitrogen (99.999%) were
View Article Online
catalytic properties different from its constituents. [8] Recently, purchased from Dalian Institute of Chemical Physics. DI water
DOI: 10.1039/C8CY02258E
Beller et al reported that supported nickel silicide catalyst could be was lab-synthesized. A domestic microwave oven (WBFY-201)
used for hydrogenations of structurally diverse unsaturated operated at 2.45 GHz with a power of 800 W was used for
compounds. The presence of silicon as a secondary component synthesis.
in the nickel metal lattice plays the key role in its properties and is
Synthesis of CozNi1-z@C samples
[8a]
of crucial importance for improved catalytic activity.
Transition
The CozNi1-z@C samples (z = 1, 0.75, 0.5, 0.25, 0) were
prepared by thermolysis of MOFs-74. Firstly, the MOFs-74 was
prepared according to reported method.
metals incorporated in the specific lattice site function as
catalytically active centers have great potential for modulating the
local structure and the electronic properties of active sites, and
enhancing the catalytic activity and stability. Hosono et al.
reported that a new copper-based intermetallic electride catalyst
(LaCu0.67Si1.33), which afforded selective hydrogenation of
[13]
In a typical
synthesis, H4DOT (0.15 g) and different ratios of cobalt nitrate
hexahydrate and nickel nitrate hexahydrate were treated in
ultrasonic and dissolved in 50 mL DMF, 3 mL EtOH, and 3 mL
H2O in a 100-mL beaker. This beaker was sealed tightly and
placed in an isothermal oven at 120 oC for 20 h for the
completion of the solvothermal reaction. The resulting MOF-
74 precipitates were washed with H2O and MeOH for three
times, and finally dried under vacuum at 80 oC for 24 h.
nitroarenes with above 40-times higher turnover frequencies than
[8b]
well-studied metal-loaded catalysts.
In our previous research,
transition metal silicides and noble metal silicides have been
synthesized and exhibited good performance in the
hydrogenation of naphthalene, cinnamaldehyde, and
phenylacetylene. [8c-8g] Especially, Ding et al. recently reported
the Ni-silicides nanoparticles as substitute for noble metals for
o
Secondly, 0.5 g of MOFs-74 was pyrolysis at 600 C for 4 h
under argon atmosphere. The temperature was raised from
room temperature to the target temperature at a heating rate
of 5 oC/min.
hydrogenation of nitrobenzene to p-aminophenol in sulfuric
[9]
acid.
The fundamental roles of metal silicides were
attributed to the unprecedented electronic, geometric, steric, Synthesis of Co-Ni bimetallic silicide catalysts
[10]
and ordering effects.
These results have fascinated us to
The CoxNi2-xSi@C catalysts were synthesized via microwave-
assisted chemical vapour deposition (MWCVD). An amount of
CozNi1-z@C alloy precursors were loaded in the quartz tube
and kept in argon atmosphere to exclude the air. Subsequently
organosilane (Cl2Si(CH3)2) was introduced to the reaction by
using argon (40 sccm) as the carrier gas that flows through a
bubbler containing organosilane and maintained at 20 oC.
After 30 min for the deposition of Cl2Si(CH3)2, the tube was
microwave processed for 5 min. The CoxNi2-xSi@C catalysts
were prepared after processed three-time cycles.
develop the promising silicides for the hydrogenation of
phthalic anhydride.
In our previous research, monometallic cobalt silicide has
been tested in the hydrogenation of phthalic anhydride, but
the original low catalytic activity and the low metal loading
and bigger crystallite size lead to the relatively low yield to
Taking into consideration the synergistic effect
of bimetallic silicides in the hydrodesulphurization,
[11]
phthalide.
[12]
we
might expect to observe the similar phenomenon in the
hydrogenation of phthalic anhydride. In this work, a series of
intermetallic cobalt-nickel silicides embedded in a carbon
Synthesis of noble metal catalysts
matrix (CoxNi2-xSi@C (x=2, 1.5, 1.0, 0.5, 0)) catalysts are The AC support were firstly washed to remove the impurities
prepared by microwave-assisted chemical vapor deposition in 0.5 mol/L aqueous nitric acid solution at room
a
(MWCVD). The structures and catalytic properties of CoxNi2- temperature for 24 h. The support powders were all sieved to
xSi@C catalysts for the hydrogenation of phthalic anhydride obtain particles smaller than 100 meshes before use and dried
have been explored. In addition, the high acid tolerance of in air at 140 oC overnight. The 1.5 wt.% noble (Pd and Pt)
cobalt-nickel silicides has been studied in-depth.
catalysts supported on AC were prepared by an incipient
impregnation method using the aqueous of the metal salt
PdCl2 and H2PtCl6·6H2O, respectively. After the impregnation,
o
Experimental
the catalysts were dried at 110 C in air for 12 h. Before the
test, the as-prepared noble metal catalyst was activated in
hydrogen at 300 oC for 2 h.
Reagents and Equipment
2, 5-Dihydroxyterephthalic acid (H4DOT) (98.0%, Aladdin),
cobalt nitrate hexahydrate (99.0%, Kermel), nickel nitrate
hexahydrate (98.0%, Kermel), dimethyldichlorosilane (96.0%,
Aladdin), NaOH (96.0%, Kermel), N, N-dimethylformamide
(DMF) (99.7%, Kermel), methanol (MeOH) (99.5%, Kermel),
phthalic anhydride (99.7%, Sinopharm), phthalide (99%,
Aladdin), γ-butyrolactone (98%, Guangfu), cyclohexane(99.7%,
Kermel), ethanol (EtOH) (99.7%, Kermel), palladium chloride
(99%, Sinopharm) and chloroplatinic acid hexahydrate (99%,
BK) were purchased and used without further purification.
Activated carbon (AC) was also purchased commercially. Argon
Catalyst Characterization
The prepared samples were dissolved into aqua regia and the
chemical composition (Co, Ni, and Si) were determined by
inductively coupled plasma optical emission spectroscopy (ICP-
OES) and the contents of C, H, and N were measured by
elemental analysis with varioEL lll. Powder X-ray diffraction
(XRD) measurements were performed on a Rigaku D/Max-RB
diffractometer with a Cu Kα1 monochromatized radiation
source, operated at 40 kV and 100 mA. Raman spectra of the
samples were obtained at room temperature with a micro-
2 | J. Name., 2012, 00, 1-3
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Raman spectrometer (Renishaw England) with a laser of 532 Generally,
ARTICLE
a
highly crystalline metal and metal oxide
View Article Online
nm. BET surface areas and porous structure of the catalysts nanoparticles embedded in a carbon matDriOxI:c1a0n.1b03e9s/Cu8cCceY0ss2f2u5l8lEy
were measured using an autosorb iQ automated gas sorption synthesized by the controlled thermolysis of metal organic
[14]
analyzer from the nitrogen adsorption and desorption frameworks (MOFs).
isotherms at 77 K.
As shown in scheme 1, intermetallic
Thermogravimetric/derivative cobalt-nickel silicides are designed by microwave-assisted
thermogravimetric (TG/DTG) experiments were performed on chemical vapor deposition of CozNi1-z@C derived from the
a Mettler Toledo TGA/SDTA851 thermogravimetric analyzer. thermolysis of bimetallic MOFs-74. Initially, CozNi1-z-MOFs-74
The size and morphology of Co1.5Ni0.5Si@C sample were with different Co/Ni ratio was prepared by a solvothermal
studied by high-resolution field emission scanning electron method. [13] XRD patterns match well with the simulated peaks
microscopy (HRSEM, JEOL JSM-5600LV) equipped with an from single-crystal data in the 2θ range of 5-40o, confirming
energy-dispersive X-ray spectroscopy (EDX), and high- the pure phase of MOFs-74 with good crystallinity has been
resolution transmission electron microscopy (HRTEM, T2- successful synthesized according to the reported literatures
Tecnai F30). The X-ray photoelectron spectra (XPS) were (Figure S1). SEM images show that the as-synthesized CozNi1-z
-
performed on an ESCALAB250 (Thermo VG, USA) spectrometer MOFs-74 materials are hexagonal prisms, with length from 5
with Al Kα (1486.6 eV) radiation with a power of 150 W. All to 10 μm (Figure S2). In order to make sure the appropriate
core-level spectra were referenced as the C 1s neutral carbon pyrolysis temperature for CozNi1-z-MOFs-74, TG/DTG
peak at 284.6 eV and were deconvoluted into Gaussian experiment was performed in argon atmosphere and heated
o
o
component peaks.
at 5 C/min to 850 C. The peak in the TG-DTG curves (Figure
S3a-c) show the MOFs-74 precursors are completely
Procedure for hydrogenation of phthalic anhydride
o
decomposed at ca. 600 C. Therefore, the target temperature
The as-prepared CoxNi2-xSi@C nanocatalysts were treated by
5.0 mol/L NaOH solution at 50 oC for 5 h to etch the
presumable SiOx passivation layer. Before the test, the CoxNi2-
xSi@C nanocatalysts were activated in hydrogen at 400 oC for 2
h. Liquid-phase hydrogenation reaction was carried out in a
50-mL stainless autoclave containing 100 mg of catalyst, 1.0 g
o
o
was set at 600 C with a heating rate of 5 C/min from room
temperature. Figure S4 presents the XRD patterns of alloy
precursor CozNi1-z@C samples, which clearly demonstrates the
cubic structure of all samples. As compared to monometallic
Co (JCPD PDF 15-0806) and Ni (JCPD PDF 04-0850) patterns,
(5 wt.%) phthalic anhydride, 18.6
g of solvent (γ-
butyrolactone), 0.4 g (2 wt.%) cyclohexane as internal standard
substrate and 4.0 MPa of H2 at 180 C. The reaction system
121
220
021
(a)
o
C
Ni2Si@C
was stirred vigorously (700 rpm) to eliminate the diffusion
effect. The products were analysed by gas chromatography
using a FID with an OV-101 capillary column and identified by
GC-MS with an Agilent 7890B with 5977A MSD.
JCPDS 50-0779 Ni2Si
Co0.5Ni1.5Si@C
Co1Ni1Si@C
Co1.5Ni0.5Si@C
Co2Si@C
Results and discussion
JCPDS 04-0847 Co2Si
20
30
40
50
60
70
2 Theta (deg.)
(b)
G-band
1572.8 cm-1
Ni2Si@C
D-band
1342.2 cm-1
Co0.5Ni1.5Si@C
Co1Ni1Si@C
Co1.5Ni1.5Si@C
Co2Si@C
800
1000
1200
1400
1600
1800
2000
Raman shift (cm-1)
Figure 1. (a) XRD patterns and (b) Raman spectra of CoxNi2-xSi@C samples
prepared by MWCVD.
Scheme 1. Schematic illustration of the procedure used to fabricate CoxNi2-xSi@C
samples.
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Table 1. Chemical and structure properties of the obtained metals and intermetallic Co-Ni silicides embedded a carbon matrix.
DOI: 10.1039/C8CY02258E
a
a
SBET
da
Vmeso
Elements composition （at. %）
Sample
Co/Ni
(m2/g)
(nm)
(cm3/g)
Cb
Hb
Nb
Coc
29.76
21.19
15.67
10.17
4.69
-
Nic
-
Sic
-
Co@C
Co2Si@C
197
116
127
132
138
168
299
2.45
3.19
3.47
3.69
3.75
3.15
2.40
0.24
0.19
0.22
0.24
0.26
0.26
0.36
44.19
32.65
33.80
34..76
35.70
35.82
46.17
24.10
18.66
18.63
18.21
18.70
20.30
25.72
1.95
1.41
1.63
1.83
2.00
2.14
2.74
-
-
26.09
25.18
24.96
24.09
23.05
-
-
Co1.5Ni0.5Si@C
Co1Ni1Si@C
Co0.5Ni1.5Si@C
Ni2Si@C
5.09
10.07
14.82
18.69
25.37
3.08
1.01
0.32
-
Ni@C
-
-
[a] BET surface area (SBET), average pore diameter (d) and total pore volume (Vtotal) as determined by N2 adsorption-desorption isotherms at -196 ℃ . [b]
Measured by elementary analysis. [c] As determined by the ICP-OES.
the diffraction peaks for the bimetallic Co-Ni nanoparticles are and crystallite growth of particles. With decreasing the Co/Ni
located in between their respective monometallic peaks, molar ratios, the specific surface area of the CoxNi2-xSi@C
[5]
which indicates the formation of Co-Ni alloys.
With samples is increased, which is in consistent with the result that
decreasing the Co/Ni ratio, the metal diffraction peak slightly the specific surface area of Ni@C is higher than that of Co@C.
shifts towards higher 2θ values, which can be attributed to the In addition, the pore diameter of all samples clearly indicates
decrease of interplanar spacing with Ni atoms replacing Co that all samples are mesoporous materials. A nitrogen uptake
atoms.
at P/P0 = 0.4~1.0 with a hysteresis loop (Figure S5). The pore
Due to the excellent microwave absorption property of a diameter of CoxNi2-xSi@C is main in mesoporous region with a
carbon matrix, CozNi1-z@C samples are good candidate for the few microspores in ca. 0~2 nm. The textural properties of the
synthesis of intermetallic cobalt-nickel silicides embedded in a Co@C, Ni@C, and CoxNi2-xSi@C samples are also shown in
carbon matrix by MWCVD using Cl2Si(CH3)2 as silicon source. Table 1. For all the CoxNi2-xSi@C samples, the actual metal
Figure 1a shows the XRD patterns of CoxNi2-xSi@C samples content is ca. 23 at. %, which is obviously higher than that of
[11]
with a range of Co/Ni ratio. The porous carbon matrix is previous CoSix/CNTs catalysts.
The actual proportion of
confirmed by the peak indexed to the (002) plane at 26o. As Co/Ni atomic ratios of CoxNi2-xSi@C samples is almost equal to
shown in Figure 1b, the two broad peaks at 1342.2 cm-1 and the theoretical values, which confirms that there is no losing or
1572.8 cm-1 are ascribed to the D and G bands of carbon in segregation of metals during MWCVD processing.
Raman spectroscopy, which further confirm the existence of
The high-angle annular dark-field scanning TEM (HAADF-STEM)
porous carbon matrix. The pattern of Co2Si@C sample image of representative Co1.5Ni0.5Si@C sample shows that the
primarily presents four typical diffraction peaks at 44.1o, 44.8o, rough surface consists of irregular particles. The elemental mapping
45.3o, and 46.0o, which can be indexed to the (021), (220), patterns further reveals that C, Co, Ni, and Si elements are
(301), and (121) crystal faces of Co2Si with orthorhombic
structure (JCPD PDF 04-0847), respectively. For the Ni2Si@C
C
Si
sample, three typical diffraction peaks at 43.5o, 44.3o, and
45.5o can be indexed to the (021), (220), and (121) crystal
faces of Ni2Si with orthorhombic structure (JCPD PDF 50-0779),
respectively. With decreasing the Co/Ni ratio, the diffraction
peak attributed to (301) crystal face disappears, while another
three diffraction peaks attributed to (021), (220), and (121)
crystal faces slightly shift towards lower 2θ values, which can
be attributed that the ionic radius of Ni2+ (0.69 Å) is bigger
than that of Co2+ (0.54 Å). It can be presumed that some Ni
atoms are substituted for Co in the CoxNi2-xSi@C samples.
The chemical and structure properties of Co@C, Ni@C, and
CoxNi2-xSi@C samples were investigated by N2 adsorption-
desorption and ICP-OES, as shown in Table 1. The specific
surface area of Co@C and Ni@C is 197 m2/g and 299 m2/g,
respectively. After deposition of silicon, the specific surface
area of Co2Si@C and Ni2Si@C is 116 m2/g and 168 m2/g,
respectively, which is lower than that of the parent metal
particles embedded in a carbon matrix. The loss of surface
area upon MWCVD treatment is attributed to gradual sintering
Co
Ni
500 nm
Ni
Co
Si
70
60
50
40
30
20
10
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130
Posotion (nm)
Figure 2. HAADF-STEM image, the elemental mapping indicated by box in the
STEM image, cross-sectional compositional line profiles of Co1.5Ni0.5Si@C sample,
and the corresponding EDX spectrum indicated by cross in the STEM image.
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(0.1970 nm) and the (121) plane of the Ni2SiViepwhAartsicele Ownilitnhe
DOI: 10.1039/C8CY02258E
orthorhombic structure (JCPD PDF 50-0779) (0.1992 nm). It
indicates that the formation of bimetallic cobalt-nickel silicide
in the Co1.5Ni0.5Si@C sample (Figure 3b).
Before probing the catalytic properties of CoxNi2-xSi@C samples,
the surface composition of the CoxNi2-xSi@C catalysts were
investigated by XPS measurements. Figure 4 shows the XPS spectra
of Co 2p, Ni 2p, and Si 2p signal of CoxNi2-xSi@C samples. There are
three peaks at 778.0 eV, 779.5 eV, and 782.0 eV in the Co 2p3/2 of
Figure 3. (a) TEM and (b) HRTEM image of Co1.5Ni0.5Si@C. The corresponding size
distribution was the inset in (a).
Co@C sample, which can be attributed to the metallic Co, Co-O
[15]
bond, and Co satellite peak, respectively (Figure 4a).
After
MWCVD of silicon precursor, the peak due to metallic Co shifts
slightly to higher binding energy (778.2 eV), which can be attributed
to the interaction between Co and Si, leading to the electron
transfer from metallic Co to Si. [16] In addition, the peak due to Co-O
bonds disappears, which means both metallic Co and Co oxide are
transformed to cobalt silicide. With decreasing the Co/Ni ratios, the
peak corresponding to Co2Si slightly shifts towards higher binding
energy. In the Ni 2p3/2 region of Ni@C sample, there are also three
peaks at ca. 852.8 eV, 855.5 eV, and 858.4 eV (Figure 4b),
corresponding to metallic Ni, NiOx, and Ni satellite peak respectively.
uniformly distributed throughout the whole bimetallic
Co1.5Ni0.5Si@C silicide sample. Co and Ni atoms are well overlapped
in the entire part, fully indicating that intermetallic Co-Ni silicide
nanoparticles without phase segregation are homogeneous
embedded in a carbon matrix. The cross-sectional compositional
line profiles also indicate the Si, Ni, and Co elements are
homogeneous. The ratio of Co/Ni peak intensities is 2.94:1, which is
highly close to the ICP-OES result (3.08) and the theoretical value
(3:1). Meanwhile, the ratio of metal content (Co+Ni) to silicon
content is 2.2:1, which is also close to the theoretical ratio
according to phase composition (metal/silicon = 2:1), further
confirming the formation of intermetallic Co1.5Ni0.5Si@C silicide with
no segregation. The elemental composition also verified by the
corresponding EDX spectrum.
[17]
Opposite phenomenon existed in Co 2p3/2 region, the peak
[18]
shifts towards lower binding energy (852.2 eV) after doping Si.
With increasing Co/Ni molar ratios, the peak due to Ni-Si bonds
further shifts to lower binding energy. These offsets of binding
energy can be attributed to the Ni atoms replacing Co atoms in the
To investigate the dispersion and particle size of the
synthesized intermetallic Co-Ni silicide embedded a carbon
matrix, TEM observation was performed for Co1.5Ni0.5Si@C
sample. As revealed from Figure 3a, the intermetallic Co-Ni
silicide nanoparticles without any possible agglomeration are
uniformly dispersed on a carbon matrix. The corresponding
size distributions and HRTEM image shows the presence of
dispersed nanoparticles with diameter of ca. 8 nm. HRTEM
image reveals that the clear lattice fringes with an interplanar
spacing of 0.1990 nm, which is closing to the (121) plane of the
Co2Si phase with orthorhombic structure (JCPD PDF 04-0847)
[12a]
intermetallic cobalt-nickel silicides.
For the Si 2p spectra, there
are two peaks at ca. 98.9 eV and 101.6 eV in all CoxNi2-xSi@C
samples, corresponding to the metal-silicon bond and SiOx,
respectively (Figure 4c). [18b] The appearance of SiOx means that the
excess silicon deposited on the surface of CoxNi2-xSi@C is oxidized
[19]
due to expose in the air.
For exposing the active sites of
[12a]
intermetallic cobalt-nickel silicides.
For the intermetallic Co-Ni
silicides catalysts in hydrogenation reaction, the CoxNi2-xSi@C
samples have been treated by NaOH to etch the presumable SiOx
passivation layer before catalytic property testing.
Ni-Si
(b)
(a)
(c)
98.6
Co0.5Ni1.5Si@C
Co1.5Ni0.5Si@C
Ni2Si@C
Co1.0Ni1.0Si@C
Co1.5Ni0.5Si@C
Co2Si@C
Co1Ni1Si@C
Co0.5Ni1.5Si@C
Co0.5Ni1.5Si@C
Co1Ni1Si@C
Ni-Si
852.2
Co-Si
778.2
Ni2Si@C
Ni
Co1.5Ni0.5Si@C
Co0
Ni0
Co-Si
Ni2+
Co2+
SiOx
sate
98.9
Co
sate
Co@C
Co2Si@C
Ni@C
788 786 784 782 780 778 776 774
106
104
102
100
98
96
94
864
861
858
855
852
849
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Figure 4. XPS spectra of (a) Co 2p of Co@C and CoxNi2-xSi@C samples, (b) Ni 2p of Ni@C and CoxNi2-xSi@C samples, (c) Si 2p of CoxNi2-xSi@C.
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DOI: 10.1039/C8CY02258E
100
(a)
Co1.5Ni0.5Si@C
80
60
40
20
Ni2Si@C
Co2Si@C
0.00
0.25
0.50
0.75
1.00
Scheme 2. The reaction pathways for phthalic anhydride hydrogenation.
Nickel Metal Fraction(Ni/(Ni+Co))
100
90
(b)
Co2Si@C
Pt/AC
Pd/AC
Au/FeOx-TiO2
Au/TiO2
Co@C
The as-prepared intermetallic Co-Ni silicides embedded a
carbon matrix catalysts are applied in the chemoselective
hydrogenation of phthalic anhydride. The general
Co1.5Ni0.5Si@C
Ni@C
Co0.5Ni1.5Si@C
Co1Ni1Si@C
hydrogenation pathways of phthalic anhydride are shown in
80
[1]
Scheme 2.
Phthalic anhydride is converted to phthalide
through C-O cleavage initially, while varying amounts of
hydrogenation byproducts (identified by GC-MS as shown in
Figure S6), such as methyl benzoic acid, aromatic ring-
hydrogenated derivatives of the components of the reaction
mixture (adjacent cyclohexyl methyl formate, hydrogenated
phthalide). As shown in Table 2, it is clear that the conversion
70
Co1.5Ni0.5@C
Co2Si@C+Ni2Si@C(3:1)
Ni2Si@C
60
0
5
10
15
20
25
30
Activity (mmolPhthalic Anhydride/(gcatal. h))
Figure 5. (a) Yield of phthalide versus nominal Ni metal fraction for CoxNi2-xSi@C
catalysts and (b) selectivity to phthalide versus hydrogenation activity of phthalic
of phthalic anhydride over Co2Si@C catalyst (ca. 21.1%) is
o
much high than that of Co@C catalyst (ca. 10.6%) at 180 C, 4 anhydride over technical catalysts (reaction conditions: 100 mg catalyst, 180 oC，
4 MPa H2, 1.0 g phthalic anhydride in 18.6 g γ-butyrolactone (solvent), stirring
speed = 700 rpm).
MPa, and 4 h. The result indicates the formation of cobalt
silicide significant improves the catalytic activity comparing
with the parent metal catalyst. However, this rule might not
apply for the nickel silicide. The activity of Ni2Si@C catalyst is
(Ni/(Ni+Co)) in Figure 5a. Starting from the CoSi2 catalyst, the
lower than that of Ni@C catalyst. For the completely
yield of phthalide increases significantly and is achieved the
conversion of phthalic anhydride, it need a long reaction time
highest over the Co1.5Ni0.5Si@C, subsequently the yield
for Ni2Si@C catalyst, which is consistent with previous work.
gradually decreases with increasing of Ni content. This result
[11]
With decreasing Co/Ni molar ratios, the conversion of
indicates that new sites are created, probably by the
phthalic anhydride over CoxNi2-xSi@C increases significantly,
[12a]
synergistic effect between Ni2Si and Co2Si.
In addition, the
while the selectivity to phthalide decreases due to the over-
hydrogenation. The yield of phthalide over the CoxNi2-xSi@C
catalysts is plotted as a function of the Ni metal fraction
key point is that the selectivity to phthalide and the activity
Table 2. The catalysis performances of as-prepared catalysts for the hydrogenation of phthalic anhydride.a
Selectivity（%）
Catalyst
Time (h)
Conv (%)
Yield of phthalide (%)
Phthalide
-
Others
-
0.9
Blank
Co2Si@C
Co1.5Ni0.5Si@C
Co1Ni1Si@C
Co0.5Ni1.5Si@C
Ni2Si@C
Co@C
Co1.5Ni0.5@C
Ni@C
4
4
4
4
4
4
4
4
2
4
3
4
9
2
0.1
-
21.1
97.7
99.2
99.7
99.9
10.6
96.7
91.6
99.6
98.0
13.5
97.6
100
99.1
91.8
64.8
66.3
45.5
95.7
66.9
82.7
65.6
97.4
99.2
95.2
95.2
20.9
89.6
64.3
66.0
45.5
10.1
64.7
75.6
65.3
95.5
13.4
92.9
95.2
8.2
35.2
33.7
54.5
4.3
33.1
17.3
34.4
2.6
75%Co2Si@C+25%Ni2Si@C
Pd/AC
Pt/AC
0.8
4.8
4.8
b
Au/FeOx-TiO2
NiFe/C c
[a] Reaction conditions: 100 mg catalyst, 180 oC, 4 MPa H2, 1.0 g phthalic anhydride in 18.6 g γ-butyrolactone (solvent), stirring speed = 700 rpm. The results from [b]
Ref. 1a. and [c] Ref. 1b.
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over the Co1.5Ni0.5Si@C catalyst are all higher than the
mechanically mixed catalyst (Co2Si@C+Ni2Si@C (3:1)), which
clearly indicates the bimetallic synergistic effect in the
Co1.5Ni0.5Si@C catalyst. Some Ni atoms are substituted for Co
atoms in the CoxNi2-xSi@C samples, which changes the
geometric structure to some extent, may providing more
hydrogenated active sites. In addition, because of the different
electronegativity between Ni (1.91) and Co (1.88), small
addition of Ni into Co2Si causes the modification of structure
and electronic properties, which obviously enhances the
capacity of active sites to adsorb phthalic anhydride,
promoting the hydrogenation activity. This phenomenon is
consistent with the reported catalytic performance of 15Ni-
5Fe/C catalyst. [1b]
In order to further evaluate the catalytic properties of
CoxNi2-xSi@C, activated carbon supported noble metal
catalysts have been synthesized and studied in the
hydrogenation of phthalic anhydride. Pd/AC presents good
catalytic properties for the hydrogenation of phthalic
anhydride to phthalide and achieve ca. 98.0% conversion of
phthalic anhydride and ca. 97.4% selectivity to phthalide at
180 oC, 4 MPa for 3 h. Although Pt/AC catalyst has high
selectivity to phthalide (99.2%), the conversion of phthalic
anhydride is only 13.5% for 4 h, which is far below the activity
of Pd/AC catalyst. In addition, the reported Au/FeOx-TiO2
Figure 6. (a) Conversion of phthalic anhydride, (b) selectivity to phthalide, and
(c) yield to phthalide vs. reaction time for CoxNi2-xSi@C samples and (d)
products distribution of the hydrogenation of phthalic anhydride over
Co1.5Ni0.5Si@C sample (reaction conditions: 100 mg catalyst, 180 oC，4 MPa H2,
1.0 g phthalic anhydride in 18.6 g γ-butyrolactone (solvent), stirring speed =
700 rpm).
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
[1a]
catalyst can achieve high yield of phthalide over 9 h.
Compared with the noble metal catalysts with as-prepared
intermetallic Co-Ni silicide catalyst, it is not hard to find out the
activity of Co1.5Ni0.5Si@C catalyst is matchable that of the
Pd/AC catalyst, while much higher than that of Pt-based and
Au-based catalysts. Figure 5b shows the selectivity to
phthalide versus hydrogenation activity of phthalic anhydride
over technical catalysts. The activity of Co1.5Ni0.5Si@C catalyst
presents 15 mmolphthalic anhydride/ (gcatal. h), which is slightly
lower than that of Pd/AC catalyst (21 mmolphthalic anhydride/ (gcatal.
h)). However, the selectivity to phthalide over both
Co1.5Ni0.5Si@C catalyst and noble metal catalysts is higher than
90%. It means that Co1.5Ni0.5Si@C catalyst presents noble
mental-like catalytic property in the chemoselective
hydrogenation of phthalic anhydride to phthalide.
For understanding the hydrogenation processing of
phthalic anhydride over intermetallic Co-Ni silicides catalysts,
the conversion and selectivity versus related reaction
parameters (reaction time, reaction temperature, H2 Pressure)
have been investigated. As shown in Figure 6a, Co2Si@C
catalyst presents only 21.2% conversion of phthalic anhydride
at 4 h reaction time, which is far lower than that of other
intermetallic Co-Ni silicides catalysts, indicating the doping of
nickel obviously promotes the catalytic activity. Figure 6b
shows the selectivity to phthalide over CoxNi2-xSi@C catalysts.
The Co2Si@C catalyst processes top to 99% selectivity of
phthalide. With increasing of nickel content in CoxNi2-xSi@C
catalysts, the selectivity of phthalide gradually decreased. It
can be found that Co1.5Ni0.5Si@C catalyst presents the highest
yield to phthalide in all CoxNi2-xSi@C catalysts (Figure 6c). The
products distribution of the hydrogenation of phthalic
others
phthalide
others
phthalide
160
170
180
190
200
2
3
4
5
6
Temperature (oC)
H2 Pressure (MPa)
Figure 7. Conversion of phthalic anhydride and selectivity to phthalide over
Co1.5Ni0.5Si@C versus (a) reaction temperature and (b) reaction pressure at 2 h
(Reaction conditions: 100 mg catalyst, 1.0 g phthalic anhydride in 18.6 g γ-
butyrolactone (solvent), stirring speed = 700 rpm).
anhydride over Co1.5Ni0.5Si@C sample is shown in Figure 6d. It
is clear that the product of phthalide is dominant, while bits of
over-hydrogenated methyl benzoic acid is the main byproduct.
The yield to phthalide over optimal Co1.5Ni0.5Si@C catalyst
increases with increasing of time and reaches to the maximum
at 4 h.
Figure 7 shows the effect of reaction temperature and
hydrogen pressure on the conversion of phthalic anhydride
and selectivity to phthalide over Co1.5Ni0.5Si@C catalyst. The
conversion increases gradually and the selectivity to phthalide
decreases slightly with increasing the temperature from 160 oC
to 200 oC. When the reaction temperature is 200 oC, the
conversion of phthalic anhydride is ca. 90% and the selectivity
to phthalide is ca. 90% at 2 h (Figure 7a). High reaction
temperature promotes the hydrogenation of phthalic
anhydride, while leads to the over-hydrogenation, decreasing
the selectivity to phthalide. Similar with the effect of reaction
temperature, high H2 pressure also enhances the conversion of
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Journal Name
a much better
100
80
60
40
20
0
100
80
60
40
20
0
contrast, Co1.5Ni0.5Si@C catalyst exhibits
(a)
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stability with 90.9% conversion of phDthOaI:li1c0.1a0n3h9/yCd8rCidYe022a5n8dE
89.8% selectivity to phthalide after reused six times. The
reacted liquid presents non-discoloring. These results indicate
that the cobalt nickel bimetallic silicide catalyst sustains the
robust stability in the hydrogenation of phthalic anhydride,
owing to the modification of geometric structure and
electronic properties of bimetals by silicon. In addition, the
carbon matrix may be modified by silicon during MWCVD and
presents hydrophobization property, which may prevent the
produced water from contacting the bimetal silicide surface to
some extent, enhancing the acid-resistance of catalyst. [1b]
others
phthalide
1
2
3
4
5
6
Number of runs
100
80
60
40
20
0
100
80
60
40
20
0
(b)
others
phthalide
Conclusions
In summary, highly dispersed intermetallic Co-Ni silicides
embedded in porous carbon matrix catalysts (CoxNi2-xSi@C)
with high activity and stability for chemoselective
hydrogenation of phthalic anhydride to phthalide have been
successfully synthesized by MWCVD method. The cobalt-rich
Co1.5Ni0.5Si@C catalyst exhibits an excellent conversion of
97.7% and 91.8% selectivity to phthalide, which is much better
than those of Ni2Si and Co2Si catalyst, and presents noble
metal-like behaviors. The superior catalytic activity can be
attributed to the synergistic effect between the two metal
silicides. The small addition of Ni modifies the geometric and
electronic structure of Co2Si and further creates new sites,
which enhance the adsorption of phthalic anhydride,
promoting the hydrogenation activity. In addition,
intermetallic Co-Ni silicide sustains the robust stability
comparing the corresponding metal catalysts in this acid
system, which is owed to the formation of intermetallic metal
silicides. Moreover, the silicon modified carbon matrix with
hydrophobization property may prevent the produced water
from contacting the bimetal silicide surface and enhance the
acid-resistance of catalyst. These results arouse to design
intermetallic compounds as low-cost alternative for noble
metal catalysts in the rush catalytic reaction system.
1
2
3
4
5
6
Number of runs
100
80
60
40
20
0
100
(c)
80
60
40
20
0
others
phthalide
1
2
3
4
5
6
Number of runs
Figure 8. Stability results and the images of reacted liquid after six cycle
reaction of (a) Co@C, (b) Ni@C, and (c) Co1.5Ni0.5Si@C catalysts for
hydrogenation of phthalic anhydride. Reaction conditions: 100 mg catalyst, 180
oC, 4.0 MPa H2, 1.0 g phthalic anhydride in 18.6 g γ-butyrolactone (solvent),
stirring speed = 700 rpm.
phthalic anhydride but inhibits the selectivity to phthalide. As
show in Figure 7b, the conversion increases from 30% to 80%
with increasing hydrogen pressure from 2 to 6 MPa, while the
selectivity to phthalide decreases from 100% to 95%. The
chemoselective hydrogenation of phthalic anhydride to
phthalide over Co1.5Ni0.5Si@C catalyst is found to be optimum
at 180 oC and 4 MPa hydrogen pressure.
As mentioned in the introduction, the hydrogenation of
phthalic anhydride is an acidic reaction system, the acid
corrosion resistance of catalyst is a key factor to evaluate. The
recycling stabilities of Co1.5Ni0.5Si@C catalyst as well as metallic
Co@C and Ni@C catalysts in phthalic anhydride hydrogenation
Conflicts of interest
There are no conflicts to declare.
o
were contrastively checked at 180 C and 4.0 MPa H2 for 4 h.
Acknowledgements
After the reaction, all catalysts were easily separated from the
reaction solution by a magnet due to the good magnetic
properties of these catalysts. The remaining solid was washed
with ethanol and then reused under identical reaction
conditions. As shown in Figure 8a, the Co@C catalyst shows
only 10.8% conversion of phthalic anhydride initially, while the
conversion decreases to 7.3% after six runs. The color of the
reacted liquid obviously changes to pink, which means metallic
Co is corroded and dissolved in the reaction system. Similarly,
the conversion of phthalic anhydride over Ni@C catalyst
decreases immediately from 91.6% to 78.3% after six runs and
the color of reacted liquid changes to green (Figure 8b). In
We gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (21373038
and 21403026), and the Fundamental Research Funds for the
Central Universities (DUT18GJ206).
Notes and references
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DOI: 10.1039/C8CY02258E
Acid-tolerant Intermetallic Cobalt-nickel Silicides as Noble Metal-like
Catalysts for Selective Hydrogenation of Phthalic Anhydride to Phthalide
Liangliang Zhang,a, b,† Xiao Chen,a,† Yujing Chen,a Zhijian Peng,b and Changhai Liang*a
Intermetallic Co-Ni silicides embedded in a carbon matrix catalysts have been successfully
synthesized by a green microwave-assisted chemical vapor deposition. For the selective
hydrogenation of phthalic anhydride, Co1.5Ni0.5Si@C catalyst with the unique synergistic
effect exhibits the highest yield to phthalide (90.0%), which is match to noble metal catalysts.
Due to the modification of geometric structure and electronic properties by silicon, it exhibits
excellent acid corrosion resistance.