Plant-Mediated Synthesis of Zinc Oxide Supported Nickel-Palladium Alloy Catalyst for the Selective Hydrogenation of 1,3-Butadiene

Article abstract of DOI:10.1002/cctc.201601178

We report the green synthesis of ZnO-supported Ni-Pd alloy nanoparticles for the gas-phase selective hydrogenation of 1,3-butadiene. The supported catalysts were synthesized through a simple bioreduction route using Cinnamomum Camphora leaf extract. We used XRD, SEM, TEM, and energy-dispersive X-ray spectroscopy to characterize and verify the nature of the catalysts. The results showed that the size of the Ni-Pd alloy particles was (3.2±0.7), (3.4±0.3), and (3.8±0.6) nm for Ni₁Pd₁, Ni₁Pd₃, and Ni₃Pd₁ respectively. FTIR spectroscopy revealed that the presence of stretching vibrations from C?H, ?C=C?, O?H, and ?C?O?O groups that remained on the surface and act as stabilizers. The influence of some reaction variables, such as the type of S-Pd (S=Au, Ag, Ni) bimetallic catalyst, the type of metal oxide support, and the reaction temperature, on the hydrogenation activity and selectivity towards total butene (trans-2-butene, 1-butene, and cis-2-butene) is investigated. The bioreduced, supported catalysts showed excellent catalytic activity and selectivity to butene in the selective hydrogenation of 1,3-butadiene. The calculated total butene selectivity was above 80 % for all supported S₁-Pd₁ catalysts compared to 46.92 % for a monometallic Pd/ZnO catalyst. In addition, the Ni₁-Pd₁/ZnO catalyst presented the best butene yield of 88.90 %, which was 1.9 times that of the Pd/ZnO catalyst. Moreover, it remained stable over a 10 h durability experiment.

Full text of DOI:10.1002/cctc.201601178

Accepted Article
Title: Plant-mediated synthesis of ZnO supported Ni-Pd alloy catalyst
for the selective hydrogenation of 1, 3-butadiene
Authors: Tareque Odoom-Wubah, Xiaolian Jing, Daohua Sun, Jiale
Huang, Zhengqiang Gu, and Qingbiao Li
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To be cited as: ChemCatChem 10.1002/cctc.201601178
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10.1002/cctc.201601178
ChemCatChem
Plant-mediated synthesis of ZnO supported Ni-Pd alloy
catalyst for the selective hydrogenation of 1, 3-butadiene
Tareque Odoom-Wubah, a Xiaolian Jing, b Daohua Sun,a Jiale Huang,a Zhengqiang Gu,c
Qingbiao Lia, b
aDepartment of Chemical and biochemical engineering, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, PR China, bCollege of Chemistry& Life
Science, Quanzhou Normal University, Quanzhou 362000, PR China, c Department of
Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, PR China.
ABSTRACT
We report the green synthesis of ZnO supported Ni-Pd alloy nanoparticles for the gas-phase
selective hydrogenation of 1, 3- butadiene. The supported catalysts were synthesized through a
simple bio-reduction route using Cinnamomum Camphora leaf extract. X-ray diffraction,
scanning and transmission electron microscopy and energy dispersed X-ray were used to
characterize and verify the nature of the catalysts. The results showed that the Ni-Pd alloy
particles were 3.2 ± 0.7, 3.4 ± 0.3 and 3.8 ± 0.6 nm for Ni1Pd1, Ni1Pd3 and Ni3Pd1 respectively.
FTIR analysis revealed that stretching vibration bands such as C–H,–C=C–, O–H, –C–O– O
remained on the surface acting as stabilizer. The influence of some reaction variables, such as
type of S-Pd bimetallic catalyst, type of metal oxide support and reaction temperature, on the
hydrogenation activity and selectivity towards total butene (trans-2-butene, 1-butene and cis-2-
butene) is investigated. The bioreduction supported catalysts showed excellent catalytic activity
and selectivity to butene in the selective hydrogenation of 1,3 butadiene. The calculated total
butene selectivity was above 80% for all supported S1-Pd1 catalyst compared to 46.92 % for
monometallic Pd/ZnO catalyst. In addition, the Ni1-Pd1/ZnO catalyst presented the best butene
yield 88.90 % which was 1.9 times that of Pd/ZnO catalyst. Moreover, it maintained stability
over a 10 h durability experiment.
Keywords: Zinc Oxide supported catalyst; Selective hydrogenation; Biosynthesis; Ni-Pd alloy
Corresponding author. Tel.: (+86) 592-2183088; fax: (+86)592-2184822. E-mail address: cola@xmu.edu.cn.
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1. Introduction
Hydrogenation of 1, 3-butadiene is an important industrial process used for the purification of
butene streams to obtain butene (trans-2-butene, 1-butene and cis-2-butene), generated from
cracking naphtha or gas oil.1 Among which 1-butene is most desired for making polyethylene
with butane considered a by-product. The suppressing butane formation and the isomerization of
1-butene to attain the desired product is a challenging task.
Palladium catalyst supported on various inorganic materials has proven to be efficient in the
hydrogenation of dienes and unsaturated hydrocarbons. However, the process is limited by the
generation of large amounts of by-products due to double bond and geometric isomerization
reducing the overall yield and quality of the desired products. Due to the demand of high purity
products, the attainment of even a slight improvement in the selectivity of the desired products is
of significant importance. Over the past few decades tremendous research effort from both
industry and academia towards improving the selectivity of desired products has been carried out.
Among these, alloying Pd with a second metal and the selecting of an appropriate support has
shown to improve the selectivity. Several metals including Ag, Au, Sn, Ni etc. acting as
modifiers improves selectivity and helps prevent “green oil formation” which is reported as the
causative agent for catalyst deactivation.2
In a previous work, El Kolli co-workers prepared Au-Pd on alumina and titania oxide using three
different methods and showed that their surface composition depended on the preparation
method. In addition, the alloyed catalyst had superior performances over their monometallic
counterparts for hydrogenation of butadiene in the presence of excess propene.3 It was also
observed that, co-deposition-precipitation (DP) method produced smaller uniform and
reproducible Au/Pd particles which also outperformed their counterparts produced using co-
impregnation in excess of solution (IES) in hydrogenation of 1,3- butadiene.2d In addition,
Pattamakomsan and co-workers reported a remarkable improvement in both activity and
selectivity in the gas-phase hydrogenation of 1, 3-butadiene over Pd–Sn catalyst supported on
mixed phase θ/α-(MA) and pure α-alumina (AA) catalyst.2b
Furthermore, Hou and co-workers showed from modeled (density functional theory (DFT)
calculations, temperature-programmed desorption (TPD) and experimental work that as-
produced Ni-Pd/ γ-Al2O3 outperformed their monometallic ones in both activity and selectivity to
butene.4 In addition, they observed that the Ni-Pd bimetallic bond formation on the γ-Al2O3was
very small (only 1.4) and this could be improved by utilizing other metal supports. In a related
work using different supports for Ni-Pd bimetallic catalyst, Hou and co-workers observed that
different supports had no significant influence on hydrogenation activity except Pd-Ni/CeO2
catalyst which showed a slightly lower activity. On the contrary, they showed marked difference
is the selectivity to total butene which was attributed to strong metal-support (SMSI) interaction,
oxygen defects, geometric and electronic properties.5
Moreover, Pt-Ni bimetallic catalyst supported on TiO2 and γ-Al2O3 showed different results for
hydrogenation of 1, 3-butadiene, with the former support performing lower than the later due to
structural stability of the catalyst caused by the support.6 In another study, Primet and co-workers
observed that the acidic properties of the support affected the selectivity to butene and butene
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distribution. Basic supports were more selective to butene and promoted 1-butene formation.
The above reports thus motivate research into the attainment of a suitable synthesis protocol and
the identification of other alternative metal oxide supports to the conventional utilized ones for
industrial application.
Various chemical and physical preparative means have been utilized to prepare bimetallic
nanomaterials for target applications with commercially obtained conventional oxide supports (γ-
Al2O3, TiO2, SiO2, CeO2, and ZrO2). These methods usually involve the reduction of metals
precursors using reducing agents and then the as-produced nanomaterials are stabilized with
organic, dendrimers or polymers. After which they are affixed unto the supports and the
stabilizers removed through calcinations. These methods have the advantages of morphological
control of the resulting nanomaterials and can produce large amounts of nanomaterials. However,
they require complex and tedious operations conditions, costly substrates, high temperatures and
surfactants or stabilizers (PVP, CTAB etc.) and non-biodegradable materials.
These drawbacks can be circumvented using biogenic means which is proven to be
environmentally benign, less costly and uses less sophisticated equipment.8 Our group in a
previous work studied the hydrogenation of 1, 3- butadiene using biosynthesized Ag-Pd/γ-Al2O3
catalyst.9 In this present study, we report the green synthesis of Ni-Pd alloy nanoparticles of sizes
3~6 nm affixed onto biosynthesized zinc oxide (ZnO) support as efficient catalyst for the
hydrogenation of 1, 3-butadiene using Cinnamomum camphora (C. camphora) leaf extract as
reducing and stabilizing agent. In addition, we studied the effects of different metal oxide
support, temperature and type of S-Pd bimetallic catalyst, where S= Au, Ag, Ni on catalytic
performance and selectivity to butene.
2. Experimental section
2.1. Materials and methods
Cinnamomum camphora leaves (C. Camphora), as in our previous work10 was obtained from
Xiamen Peony Fragrance Co Ltd, washed several times with DI water, dried and crushed into
fine powder. Zinc nitrate hexahydrate Zn (NO3)2.6H2O, Nickel (II) nitrate hexahydrate (Ni
(NO3)2.6H2O), Ascorbic Acid (AA) was purchased from Sangon Biotech (Shanghai) Co., Ltd.
Palladium nitrate Pd (NO3)2, and all other commercial supports (P25 TiO2, referred to as TiO2 in
this work and SiO2) used were obtained from Sinopharm Chemical Reagent Co. Ltd., and used as
received without purification. Deionized (DI) water was employed throughout the work.
2.2. Synthesis of the Ni-Pd Alloy Nanoparticles and the ZnO, CuO support
The water extraction method was used to obtain the C. camphora leaf extract. 1 g of the leaf
powder was dissolved in 100 mL DI water, placed in an oil bath rotating at 160 rpm with the aid
of a magnetic stirrer, at 60 °C for 6 hr. Then the resulting mixture was filtered through a 0.8 μm
membrane to obtain the plant extract. To synthesize the Ni-Pd alloy nanoparticles, 1 mL of 10
mM, Pd(NO3)2 and (Ni(NO3)2.6H2O) solution (molar ratio 1:1) was added to a 150 mL round
bottom flask and the volume adjusted to 40 mL with DI water. The flask was then placed in a
preheated oil bath at 90 °C, 1000 rpm for 10 min. After which, 10 mL of the C. camphora leaf
extract was added to the mixture under vigorous stirring for 20 min. Different Ni-Pd alloy
nanoparticles varying molar ratios were obtained by changing the ratio of the precursor solutions
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(3:1, 1:1 and 1:3). Monometallic Pd and Ni nanoparticles were prepared for comparison using
the same process by using only Pd (NO3)2 or (Ni (NO3)2.6H2O) respectively. The ZnO and CuO
supports used were prepared as in our previous works with little adjustments.10-11 In a typical
synthesis of ZnO and CuO, 30 ml of 0.2 mol/L Zn (NO3)2.6H2O and 0.1 mol/L of
Cu(NO3)2.3H2O solutions were added to 10 and 20 mL of C. camphora leaf extract respectively
in a 150 mL round bottom flask and kept at room temperature to age for 1 h. After, the respective
mixtures were dried in the oven at 80 °C for 10 h and then annealed at 450 °C 4 h. The obtained
metal oxides were milled using a lab mortar and pistil and then sieved with a 50 mm mesh and
stored for further use.
2.3. Catalyst synthesis
To obtain the supported catalyst, bimetallic and monometallic Ni-Pd nanoparticles were affixed
onto the metal oxide supports through electrostatic attraction: typically 0.5 g of the support was
added to the as-prepared Ni-Pd colloidal suspension at room temperature and pH was adjusted
depending on the isoelectric points of the metal oxides.12 After stirring for 1 h, the mixture was
filtered and vacuum dried at 150 °C for 10 h to obtain the catalyst which were labeled NixPdy/M.
Where x, y and M are the molar ratios of Ni, Pd and support respectively. For comparison,
Au1Pd1 and Ag1Pd1/ZnO catalyst using the same procedures as above and three Ni1Pd1/TiO2
catalysts, synthesized using conventional impregnation or/and annealing at 450 °C, and chemical
reduction with AA.
2.4. Characterization of the Ni-Pd Alloy nanoparticles
Transmission electron microscopy (TEM), Energy dispersed X-ray (EDX) and selected area
(electron) diffraction (SAED) examinations were conducted on an electron microscope (Tecnai
F30, FEI; Netherlands) with an accelerating voltage of 300 kV. Diameter measurements of the
as-produced Ni-Pd NPs were done as in our previous report.13 XRD investigation of the dried
samples was done on an X-ray crystallography (Phillips; Netherlands) provided with Cu Kα
radiation (40 kV, 30 mA). FT-IR measurements were carried out on Nicolet 6700
spectrophotometer (Thermo Nicolet, U.S.A). Brunauer-Emmett-Teller (BET) analysis was
conducted using a Micromeritics Tristar 3020 system. H2-TPR profiles for the as-prepared
catalyst were measured using a Micromeritics Autochem II 2920 instrument. Atomic absorption
spectroscopy (AAS) analysis of Ni (II) and Pd (II) ions in the solutions before and Ni and Pd in
the obtained samples were conducted using (AAS Pgeneral, China).
Pulse CO and H2 chemisorption were conducted on ASAP 2020 (Micromeritics) as in a previous
report.14 Briefly, chemisorption uptakes of adsorbates (H2 and CO) on catalysts were measured
on a Micromeritics ASAP 2020 (M+C) analyzer using the static volumetric method. A measured
amount of catalyst (minimum of 500 mg) sample was reduced under a flow of high-purity
dihydrogen at 573 K for 2 h and then evacuated for 1 h to ensure the complete removal of
chemisorbed species. Under a precisely controlled temperature of 308 K, an initial isotherm was
measured in a pressure range of 100–600 Torr. A 30 min evacuation at 308 K was conducted to
remove the reversibly adsorbed adsorbate molecules on the catalyst surface prior to measuring a
repetitive isotherm under the same conditions as the initial one. The difference between the
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initial and repetitive isotherms reflects the uptake of the irreversibly chemisorbed adsorbates on
catalysts. The test was program such that the H2 chemisorption test preceded the CO
chemisorption test.
2.5. Catalyst activity test
The gas phase hydrogenation of 1, 3- butadiene using the Nix-Pdy/M catalysts were carried out
at 35 °C in a fixed bed reactor (diameter 1 cm) under atmospheric pressure using a reactant
concentration of 2.12% , 4.28% and (93.6%) of 1,3- butadiene, H2 and N2 respectively. 50 mg
of the catalyst was placed into the reactor and the reactor effluent was analyzed every 20 min for
4 h using a gas chromatograph (GC) having an Al2O3 column and a FID detector. The output
conversion of 1, 3- butadiene and the corresponding selectivity to total butene (cis-2-butene
trans-2-butene, and 1-butene) or its by-product butane were determined from their corresponding
GC peaks. The metal loadings of all catalyst were controlled at 1 wt. % and the results represent
an average of four tests. The overall yield was calculated as in our previous studies using the
formula below.
3. Results and discussions
3.1. Characterizations of Ni-Pd/ZnO catalysts
3.1.1. TEM, HRTEM and SAED Analyses
The structural properties of the as-produced bimetallic nanostructures were investigated using
low resolution TEM. TEM images of the bimetallic and their monometallic counterparts are
shown in Fig.1.The size of the Pd NPs is 2.8 ± 0.4 nm and the Ni NPs is 4.7 ± 0.6 (Fig.1A and E).
The sizes of the bimetallic alloy Ni-Pd NPs were 3.2 ± 0.8 nm, 3.4 ± 0.7 nm and 3.8 ± 0.3 nm
corresponding to Ni/Pd of initial molar ratios of 1:1, 1:3, 3:1 respectively (Fig.1B, C and D).
From Fig.3F (plot of average particle size of the as-produced nanostructures against the Pd molar
fractions), it is observed that, the average particle size decreases linearly with increasing
palladium amount.
The High-resolution TEM (HRTEM) and SAED patterns of the bimetallic NPs are presented in
Figure S1, Supporting Information. The HRTEM shows that the as-produced bimetallic NPs are
made up of clear crystallographic orientations in different directions which are confirmed by its
SAED results. In addition, the results shows that the Ni-Pd alloy NPs are presented by dominant
(111) facets. Moreover, an interplanar distance of 0.21 nm is observed for a single Ni-Pd NP
which lies between the d-spacing of Ni (0.204 nm) and Pd (0.225 nm) and corresponds to the
face centered cubic Ni (111) and Pd (111). The typical SAED pattern shows four Bragg
diffractions of (111), (200), (220) and (311).
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Figure 1. TEM images and diameter distribution histograms of the biogenic Ni-Pd NPs with molar ratios (B) 1:1, (C)
1:3, and (D) 3:1; and their monometallic NPs (A) Pd and (E) Ni; and (F) plot of average diameter distribution vs Pd
molar ratio
3.1.2. STEM and EDX Elemental Analyses
Scanning TEM (STEM) and energy dispersed X-ray Spectroscopy (EDX) were done to study
the elemental distribution of the Ni-Pd NPs. Figure 2 show the STEM images of the Ni-Pd NPs
with different initial molar ratios. The colored mapping Ni (brown Figure 2B, E and H) and Pd
(yellow Figure 2C, F and I) shows that, as-produced nanostructures contained Ni and Pd.
However, due to poor resolution the images were not very clear. Therefore, EDX analysis was
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conducted to further confirm their alloy nature. Typical EDX image of Ni3Pd1 NPs is showed in
Figure S2. Three peaks are observed for Pd, Ni and P probably from the plant extract.
The atomic percentage concentrations of the Ni-Pd solutions were estimated by AAS analysis of
the solutions before and Ni-Pd NPs after reaction, the results are shown in Figure S2 B and
tabulated in Table S1. It the case of Ni1-Pd1 and Ni1-Pd3, though, the calculated atomic
percentages deviated by about ± 5 %, the Ni: Pd ratios were close to the nominal ones. However,
in the case of Ni3-Pd1, the estimated ratio was about 1.6:1 which we attribute to the inability of
C.camphora to completely reduce the Ni(II) to Ni(0) under the current reaction time. It is known
that, Pd(II) can easily be reduced to Pd(0) compared to Ni(II) due to the differences in their
reduction potentials, the amount of leaf extract was not enough to totally reduce Ni in the case of
Ni3-Pd1.15 The observation agrees with our previous work using alfalfa leaf extract to synthesize
Ni NPs, it was observed that, by increasing the amount of leaf extract used, more Ni(II) were
reduced to Ni(0) and the particle sizes became larger.15b Furthermore, the calculated atomic
percentages from the AAS analysis were similar to the atomic percentage concentrations from
EDX analysis Table S1 without considering element P.
Figure 2. STEM images of the Ni-Pd bimetallic nanoparticles of molar ratio ratios (A) 1:1 (D) 1:3 and (G) 3:1and
their corresponding EDX elemental maps Ni (B, E and H) and Pd (C, F and I) concentrations
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3.1.3. XRD Analysis
XRD patterns of the bio-reduced bimetallic Ni-Pd NPs and their monometallic counterparts are
presented in Figure 3. The bimetallic Ni-Pd NP showed diffraction peaks at 42.1, 41.6 and 41.0 o
respectively corresponding to samples with initial molar ratio 3:1,1:1 and 1:3 respectively. These
peaks coincide with the (111) diffraction peak values between pure Ni (pdf2 card no: 01-089-
7128) and Pd (pdf2 card no: 01-087-0635) affirming the formation of an alloy structure.
Moreover, there is a gradual peak shift towards pure Ni with increasing Ni content.
Figure 3. XRD patterns of the monometallic (I) Pd NPs (V) Ni NPs and bimetallic Ni-Pd NPs with initial molar
ratio of (I) 1:3 (II) 1:1 and (IV) 3:1
3.1.4. XPS and DRUV-vis spectra Analyses
XPS analyses of the S-Pd NPs and the supported catalysts were conducted to check their
electronic state and composition. The XPS results for Ni-Pd NPs are shown in Figure 4; the
binding energy 2p3/2 at about 853.3 eV Figure 4A corresponds to the Ni (0) component while that
of 3d5/2 at about 335 eV Figure 4B corresponds to Pd (0). From the results, it is observed that as
Pd content increases, the binding energies shifts to higher energy levels (Figure 4B). For
example the Pd (0) 3d5/2 for metallic Pd were 335.50, 335.53 and 335.60 eV corresponding to Ni-
Pd NPs of molar ratios 3:1, 1:1 and 1:3 respectively. The Ni (0) 2p3/2 also shows similar shifts in
binding energies with increasing Ni content. In addition, a satellite peak is observed around
859.7 eV in all Ni-Pd samples probably due to the partial oxidation of Ni during the drying
process. The shits show possible changes in the electronic properties of the two metals. The
binding energies are summarized in Table S2 supporting information. The XPS data was used to
estimate the atomic percentage concentration of the Ni-Pd alloy NPs from the major peaks. The
results are shown in Table S3. The major peaks observed were Ni 2p, Pd 3d, C1s and O1s. The
results agreed with those from the AAS and EDX analysis.
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A similar phenomenon is observed when the Ni-Pd, Ag-Pd and Au-Pd NPs were immobilized on
to the various supports Figure 5. In our previous work, we observed a shift in the XPS spectra of
Ag after anchoring it onto CeO2 support,16 which we attributed to the changes in their Fermi
energy levels as a result of interactions between the Ag nanoparticles and the CeO2 support. This
is in agreement with other reports in literature.17 Habibi et.al observed a marked shift to a lower
binding energy from the XPS of metallic Ag (3d5/2 at 368.1 eV) compared to as-prepared Ag-
ZnO NPs (367.7 eV). And thus to a large extent, the supports surfaces were reduced by the
different S-Pd alloy NPs and they presented possible (SMIS). The binding energies of all six
catalysts are summarized in Table S4. From the table it is observed that, all the binding energies
of the metals shifted to higher energy levels. According to literature, the positive shift could be
as a result of change in particle size, chemical or charge effects.18 However, from the TEM
images of the S-Pd alloy supported catalyst (Figure S3) we can rule out effect of particle size
since no obvious change in particle size is observed. Therefore the shifts of binding energies to
higher values are attributed to charge and chemical effects and partially to the possible
interactions between the biomolecules from the leaf extract and the supported catalyst. In the
case of Au-Pd/ZnO catalyst compared to bimetallic Au-Pd, the 4f7/2 is not observed probably due
to its overlap with the Zn 3p binding energy level Figure 5 E.
In addition, the different oxide supports may present different surface oxygen vacancies when
the S-Pd alloy NPs are immobilized on the supports which could affect their possible catalytic
performance. A further study of their O1s XPS spectra could provide more information. The
asymmetric O1s XPS spectra of ZnO can be deconvoluted into three distinct peaks, oxygen
species in the lattice (OL) at 530.0 eV, oxygen vacancies OV at 531.28 eV and dissociated or
17, 19
chemisorbed oxygen OC at 532.36 eV, Figure 6.
The O1s of TiO2 also show three similar
peaks however, most of the peaks shifted to lower binding energies compared to the ZnO,
probably due to interactions with the residual organic groups. While that of SiO2 shows only (OV
and OL) and CuO (OL and OC). The intensity and number of oxygen defects were similar on the
TiO2 and ZnO support, suggesting they may have more surface oxygen defects compare to SiO2
and CuO.
3d5/2
A
2p3/2
Ni
B
Pd
3d3/2
Pure Pd
Ni-Pd 3:1
Ni-Pd 1:1
2p1/2
Ni-Pd 3:1
Pure Ni
Ni-Pd 1:1
Ni-Pd1:3
Ni-Pd 1:3
885
880
875
870
865
860
855
850
845
344
342
340
338
336
334
332
Binding Energy (e.V)
Binding Energy (eV)
Figure 4. XPS spectra of the Ni-Pd NPs
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DRUV-vis spectra analysis of the supported Ni-Pd catalysts is shown in Figure S4. The ZnO and
TiO2 show absorbance below 400 nm, while the supported catalyst shows slight red shifts to
higher absorbance to indicate the incorporation of the S-Pd NPs. Usually Ag displaces absorption
plasmon at about 420 nm while that of Au shows around 560 nm. 16, 20 However, the observation
is not marked since Pd in bimetallic nanoparticles usually suppresses the surface absorption
plasmon of Au, and Ag.9, 21
Figure 5. Comparison of the XPS spectra of the supported S-Pd catalysts and their corresponding bimetallic NPs
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Figure 6. The XPS spectra of the supports (A) ZnO (B) TiO2 (C) SiO2 and (D) CuO showing the O1s region
3.1.5. FTIR Analysis
FTIR absorption spectra of the different supported Ni1-Pd1 catalyst after reaction, and the plant
extract before reaction are shown in Figure S5. The spectrum (Figure S5 I) shows several clear
bands at 1059, 1109, 1245, 1384, 1735, 2852, 2921 and 3426 cm−1 within the region of 1000 to
4000 cm−1. The peaks around 1245 and 1059 cm−1 are ascribed to the C–O stretching vibration
from cyclic compounds and hydroxyl in carbohydrate respectively. Those centered at 1319 and
2921cm−1 are due to C–H bending and stretching vibration in alkanes. Moreover, the broad peak
at 3426 cm−1 can be attributed to the stretching vibration of (N–H group) in amides found in
proteins. The peak at 1109 cm−1 is attributed to the –C–O– groups of polyols like flavonoids,
polysaccharides and terpenoids in the plant biomass. In addition the absorption peak at 1634
cm−1 can be assigned to –C=C– stretch vibration. After the reaction, the bands at 1059 shits to
1053 in II and 1051 cm−1 in III while it disappeared completely in IV and V. In addition, the
bands at 1245, 1319, and 1731 cm−1 in Figure S5 I disappeared completely from Figure S5 II-V.
The FTIR analysis shows that, interactions occurred between all the catalyst and biomolecules
from the leaf extract. Therefore, from the FTIR analysis it is speculated that, the biomolecules
mostly streaching vibration bands from C–H,–C=C–, O–H, –C–O– O that remained on the
supported Ni-Pd catalysts after the bio-reaction acted as stabilizers.
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3.2. Catalytic Activity Evaluation
The possible hydrogenation routes of 1, 3-butadiene using S-Pd/M catalyst is shown in scheme 1
below. The process involves the hydrogenation and isomerization of 1, 3- butadiene over Pd
supported metal catalyst with butene (trans-2-butene, 1-butene and cis-2-butene) as the desired
reaction product and butane a by-product. Modifying the electronic and structural properties of
Pd by alloying with other metals tunes its selectivity toward butane. The average catalytic
performance of the as-produced mono and bimetallic Ni-Pd/ZnO catalysts in the selective
hydrogenation of 1, 3-butadiene at 35 °C after 4 h of reaction is summarized in Table S5, and
plotted in Figure 7. Ni/ZnO catalyst is almost inactive in 1, 3- hydrogenation achieving a very
low conversion of 2.54 ± 0.5 % compared to 100 % conversion for the Pd/ZnO catalyst.
However, it showed high selectivity to cis-2-butene, a low selectivity to 1-butane and no
selectivity towards trans-2-butene and butane. In addition, a marked improvement in both
selectivity towards butene and conversion is observed using the bimetallic Ni-Pd/ZnO catalysts.
Specifically, the conversion of 1, 3- butadiene was 98.8 ± 0.2 % for Ni1-Pd1/ZnO catalyst and
100% for both Ni1-Pd3/ZnO and Ni3-Pd1/ZnO catalyst. In terms of selectivity towards total
butene (trans-2-butene, 1-butene and cis-2-butene), the Ni1-Pd1/ZnO catalyst recorded the
highest selectivity of about 90.0 ± 0.4 % and 40.7 ± 0.8 % for single 1-butene with an overall
calculated yield of 88.9 ± 2%. And thus, alloying with Ni in the case of Ni1-Pd1/ZnO could
prevent full hydrogenation while increasing selectivity to butene. Table S6 presents six catalytic
performance assessment of the Ni1-Pd1/ZnO catalyst, the last four were considered in calculating
the average performance.
The addition of Pd greatly enhanced the conversion efficiency of 1,3- butadiene and the
selectivity to butene compared to the monomettalic Ni/ZnO catalyst proving Pd is the main
active constituent for the reaction. The observed marked improvement of the bimetallic
Ni1Pd1/ZnO catalyst over their monometallic counterparts is attributed to the catalyst
composition, modification of the electronic and structural properties of Pd and the enhance
bimetallic-support interaction (Ni-Pd bimetallic bonding) as support by the XPS analysis above
(Figure 5). As mentioned above, the enhancement in both catalytic performance and selectivity
to butene in the hydrogenation of 1, 3- butadiene on bimetallic catalyst over monometallic Pd has
been proven from both experimental and DFT calculations.2b, 4, 22 The present works thus
confirms this trend using biosynthesized Ni-Pd bimetallic catalyst on a less conventional
biosynthesized ZnO as support.
For comparison, Ni1-Pd1/TiO2 catalyst prepared by impregnation, impregnation and subsequent
calcinations at 450 °C and chemical reduction using 1 mL of 1 mol/L, ascorbic acid as reductant
are shown in Table S5. Compared to the plant-mediated synthesis in our current work, the Ni-
Pd/TiO2 synthesized using impregnation were very selective towards total butene with a
selectivity of about 99 ± 0.1 and 71.6 ± 3 % for single 1-butene, nevertheless, it showed low
conversion of about 10 %. After annealing at 450 °C, no obvious change in activity was
observed. However, the distribution of selectivity towards total butene changed, for example
selectivity to single 1-butene reduced from 71.6 ± 3 to 45.3 ± 4 % while that of single cis-2-
butene increased from 16.1 ± 0.3 to 33.37 ± 5 %. When it was reduced with ascorbic acid, a
conversion of 100 % was observed, with a drop in the total selectivity to butene to about
44.8 ± 1 and 22.8 ± 4 %for single 1-butene respectively. The observation highlights the
importance of preparation conditions of catalyst in the selective hydrogenation of 1,3 butadiene.
Moreover, the Ni1-Pd1/ZnO showed good reusability performance (Table S 7), with about
73 ± 5% conversion and 95 ± 2 and 52 ± 4% selectivity towards total butene and single 1-butene
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respectively after a week. A gradual increase in the selectivity to total butene from 90.0 ± 0.4 to
95 ± 2 was accompanied with a drop in the conversion of 1,3 butadiene from 98.8 ± 0.2 to about
73 ± 5%.
Scheme.1 Possible reaction routes for 1, 3- butadiene
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Figure 7. A plot of the catalytic performance of the Ni-Pd/ZnO catalyst and their monometallic counterparts in
1, 3-hydrogenation
Table 1. Physical properties of the different supported Ni/Pd catalyst
2
Sample
ZnO
SBET (m g-1 )a
Pore Volumeb (cm3g-1)
0.16
22.0
TiO2
SiO2
CuO
Ni1Pd1
Ni1Pd1/ZnO
Ni1Pd1/ TiO2
Ni1Pd1/SiO2
Ni1Pd1/CuO
52.0
1.0
18.0
4.0
25.0
50.0
2.00
15.0
0.73
0.04
0.07
0.02
0.14
0.46
0.03
0.05
a. The BET surface area is determined by the BET method using the adsorption data in the relative pressure (P/P0)
b. BJH Adsorption cumulative volume of pores
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Table 2. Summary of the metal dispersion, average particle sizes and CO and H2 uptake values of the supported
Ni/Pd catalyst
Catalyst
Average
Particle size (ANPs)
Metal Dispersion CO-Uptake
from CO (µmol/g)
H2-Uptake
(µmol/g)
CO uptake/ANPs
chemisorptions
(%)
Ni1Pd1/ZnO 3.6
Ni1Pd1/TiO2 2.6
Ni1Pd1/SiO2 3.2
Ni1Pd1/CuO 2.9
Au1Pd1/ZnO 4.1
Ag1Pd1/ZnO 4.5
7.2
20.4
3.4
54.0
3.9
3.6
8.4
7.0
3.6
32.4
7.1
9.9
6.5
2.5
2.0
14.1
6.5
5.7
2.0
2.8
1.1
11.2
1.7
2.2
3.2. 1. Effect of metal oxide support on the Catalytic Activity
TiO2, SiO2 and CuO were selected to compare the effects of different supports on the catalytic
activity. The biosynthesized Ni-Pd bimetallic catalysts supported on different supports were
evaluated in the selective hydrogenation 1, 3- butadiene and the results are shown in Table S8
and plotted in Figure 8. The average 1, 3- butadiene conversion over a 4 h period at 35 °C in
order of maximum activity was as follows TiO2 (100 %) , ZnO (98.8 ± 0.2 %), CuO ( 63.3 ± 2 %)
and SiO2 (44.0 ± 3 %). In terms of average calculated yield, the Ni1-Pd1 bimetallic catalyst
supported on ZnO presented the best yield about 88.9 ± 2 % and TiO2 recorded a yield of 80.5 ±
3%, followed by CuO (53.9 ± 1%) and SiO2 (41.3 ± 2 %) respectively. Thus the hydrogenation
activity was close in the case of ZnO and TiO2 support but very wide for SiO2 and CuO.
The metal dispersion, average particle sizes and CO uptake of the supported Ni-Pd catalyst are
presented in Table 2 above. The CO uptake values are similar in the case of ZnO and TiO2 but
very high in the case of CuO and low in the case of SiO2. The Ni1Pd1/CuO catalyst recorded the
highest metal dispersion (54%), followed by Ni1Pd1/TiO2 (20 %), Ni1Pd1/ZnO (7.2 %) and
Ni1Pd1/SiO2 (3.4 %). Hydrogenation activity of 1, 3 butadiene is reported to strongly depend on
Pd metal dispersion while 1-butene selectivity is insensitive to dispersion.23 Moreover,
hydrogenation activity is reported to decrease when dispersion is higher than 20% and is similar
at dispersions 20 % and lower regardless of metal oxide support type. Hou et al also observed
that Pd-Ni/CeO2 with a higher metal dispersion resulted in a lower TOF compared to Pd-Ni
bimetallic supported on (γ-Al2O3, TiO2, SiO2 and ZrO2) with lower dispersion.5 The higher metal
dispersion 54 % recorded for the Ni1Pd1/CuO catalyst causes the significant reduction in
hydrogenation activity. The observation can be ascribed to less exposed active sites on the Ni-Pd
bimetallic surface23 and the strong binding of 1,3 butadiene on the Ni-Pd bimetallic catalyst due
to electronic effect.24 In the case of the Ni1Pd1/SiO2 catalyst the low CO uptake is not surprising
since a small surface area of (2 m2/g) could affect dispersion. Moreover, the O1s XPS analysis
revealed that, the Ni-Pd supported on TiO2 and ZnO possessed more oxygen defects compared to
SiO2 and CuO this could be a possible attribute to the high hydrogenation activity in the case of
the former supports. However, further studies are needed to support this claim and since
interpretation is yet to be reported in literature we take notice of the results and plan to carry out
further studies.
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Comparing the selectivity to total butene the biosynthesized Ni1-Pd1 bimetallic catalyst
supported on SiO2 recorded the highest selectivity 93.6 ± 2 %, followed by ZnO with a
selectivity of about 90 ± 0.4 %, then CuO 85.17 ± 1 % with TiO2 recording the lowest 80.5 ± 3%.
It is worth nothing that the selectivity to total butene was above 80 % for all bimetallic supported
catalyst compared to 46.9 ± 2 % for monometallic Pd/ZnO catalyst. And there was a slight
reduction in the selectivity at 100 % conversion of 1, 3- butadiene. Furthermore, in the case of
selectivity to single 1-butene the trend was as follows SiO2, TiO2, CuO and ZnO. As mentioned
above selectivity to butene is independent on metal dispersion. Therefore factors such as metal
dispersion and particle size can be ruled out since the Ni-Pd bimetallic catalysts are of similar
size 2.6 ~ 3.6 nm.
The apparent difference in the selectivity towards total butene and to a large extent single 1-
butene on the dissimilar supports is attributed to their composition, the nature of the supports and
their interaction with the biosynthesized bimetallic catalyst. Strong metal-support interaction was
inferred from the ability of the oxide to suppress the chemisorption of H2 onto the metal. SMSI
has been shown to occur when small metal particles are dispersed on metal oxide supports.25
Kang et.al reported that the selectivity to butene isomers in the hydrogenation of 1, 3 butadiene
increases when Cu was added to Pd/CeO2 to form Cu-Pd/CeO2 catalyst.26 The bimetallic Cu-
Pd/CeO2 could suppress the extent of SMIS effect by reducing the amount of chemisorb H2.
Previous reports showed that, all the supports (SiO2, TiO2, CuO and ZnO) used in the present
study present SMIS effect when reduced at higher temperature.27 However, alloying as well as
preparation conditions could reduce the required reduction temperature for SMIS to occur on
catalyst.26, 28 H2-TPR profiles of the catalyst Figure S6, shows several peaks between 80 to
850 °C which we attribute to different Pd, Ni, Pd-Ni, Ni-Pd and leaf extract biomolecules
interactions with the support. Therefore, possible SMIS effect on the catalyst affected the
selectivity to total butene and 1-butene.
In addition, different supports may affect the geometric and electronic properties of the
supported Ni-Pd catalyst as observed from the different shifts in binding energies (Figure 5 XPS
results).
Figure 8. Catalytic performance of the Ni-Pd bimetallic catalyst on different supports in 1, 3- butadiene
hydrogenation
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Moreover, the O1s region (Figure 6) of the supports show different levels of oxygen defects
existed on the different supports. The SiO2 oxide support with the least oxygen defects showed
the best selectivity while TiO2 with the highest level of oxygen effect presented the worst
selectivity to total butene. This suggests a correlation between 1,3 hydrogenation activities,
butene selectivity and the oxygen defects of the supports.
3.2.2. Effect of type of supported S-Pd bimetallic on catalytic activity
The nature of bimetallic catalyst in terms of electronic state, synergistic effect, and alloy
formation could affect both activity and selectivity in hydrogenation reactions. Two different
biosynthesized Pd bimetallic catalyst (Ag1Pd1 and Au1Pd1) supported on ZnO (sizes between 3.6
~ 4.5 nm) were investigated in the hydrogenation of 1, 3- butadiene and compared to the
Ni1Pd1/ZnO catalyst (Figure 9A). The results are also tabulated in Table S 9. Compared to
monometallic Pd, Au, Ni and Ag are all inactive for 1, 3- hydrogenation.
However, they show 100 % selectivity to total butene as to 46.9 ± 2% for Pd. Addition of Pd
increased their respective conversions drastically to 98.8 ± 0.2, 96.2 ± 0.8, and 95.9 ± 0.6 % for
Ni1Pd1, Au1Pd1 and Ag1Pd1 respectively. And thus the bimetallic catalysts irrespective of
composition had almost similar conversions slightly lower than the biosynthesized monometallic
Pd/ZnO catalyst. In addition, they exhibited higher selectivity over monometallic Pd. The ZnO
supported Ni1Pd1 and Au1Pd1 catalyst showed the highest selectivity towards total butene about
90 %, (3.54 %) slightly higher than the Ag1Pd1/ZnO catalyst (86.5 %). Nonetheless, the Au1Pd1
/ZnO showed the highest selectivity to single 1-butene, followed by Ni1Pd1 and Ag1Pd1/ZnO.
The observed higher selectivity to butene species for the bimetallic catalysts over monometallic
Pd is ascribed to synergistic effect of the alloy in hydrogenation. Moreover, the adding of the
second metal (Ni, Au and Ag) could increase selectivity to butene by favoring isomerization and
subduing complete hydrogenation.29 In addition, the observed difference in selectivity to total
butene of the Ag1Pd1/ZnO catalyst compared to Ni1Pd1, and Au1Pd1/ZnO catalysts is as a result
composition and to a large extent Ag1-Pd1 bonding with ZnO.
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Figure 9. Catalytic performance of different S-Pd bimetallic catalyst on ZnO supports in 1, 3- butadiene
hydrogenation
3.2.3. Effect of reaction temperature on catalytic activity
The catalytic performance of the Ni-Pd/ZnO catalyst at different reaction temperature (25 to
50 °C) is plotted in Figure 10 and the results over a 4 h is tabulated in Table S 10. Butane
selectivity increased linearly with increasing temperature while selectivity to total butene
decreased. The selectivity to 1-butene is observed to linearly decrease with increasing
temperature while Trans-2-butene and cis-2-butene present an initial increase in selectivity from
25 to 35 °C and then decreases rapidly after 35 °C Figure 10 A.
Figure 10 B compares 1,3 butadiene conversion and yield against temperature. It is observed that,
1,3 butadiene conversion increases markedly from 59.6 % at 25 °C to 90.8 % at 30 °C then to
98.8 % at 35 °C then stabilizes to 100 % with increasing temperature. The calculated yield
Figure 10 B shows an initial linear increase from 57.0 % at 25 °C to a maximum of 88.9 % at
35 °C, and then it decreases rapidly to 29.5 % at 50 °C. As reaction temperature increase from 25
to 50 °C, conversion of 1,3 butadiene and butane selectivity increased. This implied higher
temperature promoted hydrogenation of 1,3 butadiene to butane and inhibited isomerization to
obtain butene species.
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Figure 10. Effect of reaction temperature on catalytic Activity (A) selectivity to butane and butene (B) conversion
and yield of 1, 3butadiene
3.2.4. Catalyst stability
The durability of the catalyst was tested at 35 °C within a time on stream of 10 h. Figure 11 plots
the selectivity to butene and conversion of 1,3- butadiene versus reaction time. As can be seen no
apparent loss in both the selectivity to total butane and conversion of butadiene is observed over
the 10 h period. Thus the biosynthesized Ni1-Pd1/ZnO catalyst possessed desired stability and
excellent catalytic activity.
Figure11.Time on stream plot of conversion of 1, 3- butadiene and butene selectivity over the biosynthesized
Ni1-Pd1/ZnO catalyst
4. Conclusion
In conclusion, vapor phase selective hydrogenation of 1,3-butadiene was studied on different
biosynthesized S-Pd/M catalysts using Cinnamomum Camphora leaf extract as reducing and
stabilizing agent. The process is simple, inexpensive and environmentally-friendly. Comparative
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studies using different S-Pd bimetallic catalysts, and metal oxide supports were carried out. The
bimetallic Ni1-Pd1/ZnO catalyst exhibited excellent selective hydrogenation of 1,3-butadiene
with an overall butene yield of 88.93 compared to 46.92 % for Pd/ZnO which we attributed to
the bimetallic synergistic effect. The effect of different support was more significant on
hydrogenation activity and selectivity to total butene than S-Pd bimetallic composition. The
chemical properties of the support, oxygen defects and strong metal-support (SMSI) interaction
are assigned to be possible reasons for the marked difference is both hydrogenation performance
and selectivity to butene. TiO2 with the highest oxygen defect presented the maximum
1,3-butadiene conversions (100 %) while the highest butene selectivity was observed on the SiO2
with the least number of oxygen defect. Finally, it was observed that, 1,3-butadiene conversion
increased as reaction temperature increased and inhibited butene selectivity. The total 1-butene
selectivity reduced from a maximum of 55.47 % at 25 °C to about 14.75 % at 50 °C.
Acknowledgments
This work was supported by the NSFC project (No.21536010) and the Natural Science
Foundation of Fujian Province of China (No.2015J01059).
The authors thank Jianwei Zheng PhD, Prof. Haiqiang Lin and Prof. Youzhu Yuan of the State
Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for
Green Chemical Productions of Alcohols-Ethers-Esters, and iChEM, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen for help in conducting the TPR test, H2 and
CO chemisorption test.
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