Plant-mediated synthesis of AgPd/γ-Al2O3 catalysts for selective hydrogenation of 1,3-butadiene at low temperature

Article abstract of DOI:10.1039/c9nj01733j

In this dissertation, silver-palladium (AgPd) bimetallic nanoparticles were synthesized by a green biosynthesis method using Cacumen platycladi leaf extract, which provided both reductive and protective agents. Then the supported AgPd/γ-Al₂O₃ catalysts were obtained by the sol-immobilization method, and the as-biosynthesized AgPd/γ-Al₂O₃ catalysts with different particle sizes and compositions were used for 1,3-butadiene hydrogenation. Optimization of the catalyst preparation and selective hydrogenation parameters was performed. Using the catalyst, a 1,3-butadiene conversion of 98.2% and a butene selectivity of 88.1% were achieved. The durability experiment of AgPd/γ-Al₂O₃ catalysts was carried out for 50 h, and the activity decreased slightly and the selectivity almost remains the same during the 50 h, indicating their remarkable stability. The results of TEM and TG analysis showed that the size of the AgPd nanoparticles was nearly the same before and after the durability experiment, and the existence of residual biomolecules on the catalyst surface helped to prevent agglomeration and modification of the surface properties of the catalyst, which would promote desorption of the product, and then avoid intensifying the level of further hydrogenation.

Full text of DOI:10.1039/c9nj01733j

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DOI: 10.1039/C9NJ01733J
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Plant-mediated synthesis of AgPd/γ-Al2O3 catalyst for selective
hydrogenation of 1,3-butadiene at low temperature
Fenfen Lua, Daohua Sun*b and Xia Jiangb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In this dissertation, silver-palladium (AgPd) bimetallic nanoparticles were synthesized by a green biosynthesized method
using Cacumen Platycladi leaves extract, which provided both the reductive and protective agents. Then the supported
AgPd/γ-Al2O3 catalysts were obtained by sol-immobilization method, and the as-biosynthesized AgPd/γ-Al2O3 catalyst with
different particle sizes and composition were used for 1,3-butadiene hydrogenation. Optimization of the catalysts
preparation and selective hydrogenation parameters were tested. With the catalyst, 1,3-butadiene conversion of 98.2%,
and butene selectivity of 88.1% were achieved. Durable experiment of AgPd/γ-Al2O3 catalyst was carried out for 50 h, the
activity decreased slightly and the selectivity almost remains the same during the 50 h, which indicating the remarkable
stability. The results of TEM and TG analysis showed that the size of AgPd nanoparticles were nearly the same before and
after durable experiment, and the existence of residual biomolecules on the catalysts surface helped to prevent the
agglomeration and modify the surface properties of the catalyst, which would promote desorption of the product, then
avoid intensifying the level of further hydrogenation.
of catalyst. Generally, ways to improve the selective
hydrogenation include modifying the carrier, adding a second
metal and changing the particle size and valence of the active
1 Introduction
Selective hydrogenation is a very important reaction to remove
alkynes or alkadienes in the stream for polymer synthesis.
Palladium (Pd) catalysts have been demonstrated to be highly
efficient for hydrogenation reaction, which has received
considerable attention in recent decades, and become one of the
important research highlights in the field of catalysis.1-4
Selective hydrogenation of 1,3-butadiene is not only an
important industrial process to purify the alkene streams from
petroleum cracking in an excess of alkenes, but also an
metal.8,9 The effect of the addition of the second metal on
palladium is not yet clear, researchers generally believe that the
second metal modifies the surface structure and electronic
properties of Pd, improves the adsorption, diffusion and
permeation of hydrogen on catalysts, changes the hydrogen
species on the surface of the catalysts, and also reduces the
adsorption abilities of butene, which leading to increase the
selectivity of the catalyst to the single olefins.7,10 Bimetallic Pb-
Pd, Cr-Pd, Sn-Pd, Cu-Pd, Ag-Pd, Ni-Pd catalysts were proved
to have better catalytic performance than single metal Pd
catalysts.5,11 Goetz et al. found that PdPb/Al2O3 catalyst could
maintain higher butene selectivity even when the conversion of
1,3-butadiene was high then monometallic Pd/Al2O3, which
attributed to the decrease of the surface hydrogen coverage that
originated from electronic interactions between the bimetallic.12
Pattamakomsan et al. compared the hydrogenation performance
of Pd/Al2O3 and PdSn/Al2O3 catalysts for 1,3-butadiene
hydrogenation, and found that monometallic Pd catalysts could
produce deep hydrogenation and the products of butene were
easy to be isomerized. And the bimetallic catalysts could avoid
the formation of butane and the proportion of 1-butene is also
relatively high, by the reason that the change of the structure
and electronic properties between metals.13 CuPd bimetallic
catalysts supported on graphite were prepared by Cooper et al.
The addition of copper increased the selectivity of butene and
reduced the carbon deposition of the catalysts, thus reducing the
deactivation rate of the catalysts. They believed that the
electronic and geometric effects between Cu and Pd improved
interesting
model-reaction
for
unsaturated
alkene
hydrogenation and metal catalyst electronic structure research.
Currently, there are still some problems unresolved on 1,3-
butadiene hydrogenation, for example, there still remains
controversy on the active sites, butene further hydrogenation
and the catalytic stability.5,6 Selective hydrogenation of 1,3-
butadiene is a meaningful and challenging reaction, it is more
likely to produce saturated hydrocarbons than butene from a
thermodynamic point of view.7 Therefore, as for 1,3-butadiene
selective hydrogenation reaction system, improving the
selectivity to butene is the most important factor in the selection
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the active surface and surface hydrogen species of Pd
catalysts.14
The dried as-synthesized AgPd/γ-Al2O3 caVtaielwyAsrttsicle wOnelirnee
DOI: 10.1039/C9NJ01733J
analysed by XRD using an X-ray diffractometer (Ultima IV,
The method by using natural plants to synthesize Rigaku) with Cu Kα radiation (40 kV, 30 mA). TEM
nanomaterials is a new and eco-friendly technique.15-19 In characterization was performed on an electron microscope
recent years, our research group have successfully combine the (Tecnai F30, Netherlands) with an accelerating voltage of 300
bioreduction technology of metal nanoparticles with kV. The sizes of the AgPd nanoparticles were obtained based
preparation of supported metal catalysts, and realized that the on the statistical average from TEM images. XPS
preparation and loading of metal nanoparticles under mild characterization was obtained to determine the valence state on
conditions.20-22 In this work, the selectivity of butene in 1,3- a PHI-Quantum 2000 spectrometer (Physical Electronics,
butadiene hydrogenation reaction was enhanced by adding a America), the powder samples were characterized by X-ray
second active metal silver (Ag). And the bimetallic AgPd/γ- equipped with a hemispherical electron analyzer and a Al Kα
Al2O3 was obtained by the sol-immobilization method, with the (1486.6 eV) X-ray source. The binding energy was calibrated
mediating of plant extract of Cacumen platycladi leaves. The by C 1s as the reference energy (C 1s = 284.6 eV). The actual
structure of the AgPd/γ-Al2O3 catalyst was characterized by the metal loadings of the samples were determined by AAS (GFA-
analysis of X-ray photoelectron spectroscopy (XPS), X-ray 6880, Japan). TG studies were carried out on a SDT Q600
diffraction (XRD), transmission electron microscopy (TEM), (NETZSCH, Germany) thermo balance under flowing air
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o
CO chemisorption measurement techniques, thermogravimetry atmosphere at a heating rate of 10 C/min. The in-situ FTIR
(TG), atomic absorption spectroscopy (AAS) and Fourier experiments were performed using a home-built in-situ IR cell
transform infrared spectroscopy (FTIR). Besides, the influences with quartz lining and CaF2 windows in a FTIR spectrometer
of various catalyst parameters and catalytic reaction conditions (N6700, Nicolet) using an MCT/A detector. Prior to the
on the bimetallic catalytic performance were investigated.
adsorption experiment, the samples were dried and pretreated in
situ at 35 C in a vacuum for 1 h. The spectra prior to the
o
adsorption experiment were used as background.
2.4 Catalyst testing
2 Experimental Sections
2.1 Materials
The measurements of prepared catalysts were tested in the
hydrogenation of 1,3-butadiene reaction, which carried out at
Silver nitrate (Ag(NO)3) was purchased from Sinopharm
Chemical Reagent Co., Ltd. (China). Palladium nitrate
(Pd(NO3)2) was purchased from Aladdin Reagent Co., Ltd.
(China). Aluminium oxide (γ-Al2O3) was supplied by Cener
Tech Tianjin Chemical Research and Design Institute Co., Ltd.
The sundried Cacumen platycladi leaves in the previous study17
were used in this work.
2.2 Catalyst preparation
1.0 g of the milled Cacumen platycladi leaves powder was
dispersed into 100 mL deionized water with a 250 mL conical
o
35 C in a fixed-bed flow stainless reactor (i.d. Six mm) under
atmospheric pressure. The reaction system consists of a glass
tube covered Cr-Al thermocouple located in the center of the
catalyst bed to measure the reaction temperature. In a typical
experiment under gas-phase conditions, the sample (50 mg)
was placed into the reaction tube. A mixture of gases based on
1,3-butadiene (2.15%), H2 (4.30 %), and N2 (balance gas),
previously prepared in a gas cylinder, was injected into the
reactor, and the feed flow rate was adjusted to 13.5 mL/min
with mass flow meter (Sevenstar Electronics). The performance
of catalysts were monitored on line using a gas chromotograph
(GC) coupled with an Al2O3 column and an FID detector, and
the product stream was sampled every 20 min and analysed.
Conversions of 1,3-butadiene and selectivity of butene were
obtained from the GC areas corresponding to the reactant (1,3-
butadiene), main products (1-butene, cis-2-butene and trans-2-
butene) and by product (butane).
o
flask, and then kept in a water bath shaker at 30 C and 150 rpm
for 2 h. Then, the aqueous mixture was filtered to remove the
residual insoluble biomass and obtain the extract, and the
achieved extract was stored in the refrigerator for later use.
First,
a certain ratio of Pd(NO3)2(75.08 mM) and
AgNO3(58.87 mM) solutions were added into 100 mL conical
flask, then deionized water were added to keep the total volume
10 mL. The mixture was then placed in an oil-bath with
constant stirring rate. When the temperature reach the specified
value, 10 mL of the Cacumen platycladi leaves extract was
added. After 1 h constant stirring, AgPd colloid was achieved,
then the pH value of as-synthesized AgPd colloid was acidified
at 2 with nitric acid, and then certain amount of γ-Al2O3 support
(to keep the metal concentration 0.5 wt%) was added to the
AgPd colloid with vigorous stirring. After another 1 h, the
suspension was filtered using a cellulose filter membrane of
pore size 0.8 mm, and the retained solid was washed totally
with deionized water, and then dried at 50 °C in a vacuum oven
for 12 h, and the corresponding catalyst was labelled as
AgPd/γ-Al2O3.
3 Results and Discussion
3.1 Characterization of catalysts
3.1.1 Transmission electron microscopy (TEM) studies
The monometallic and bimetallic catalysts prepared by
immobilization of the as-prepared nanoparticles with γ-Al2O3
support were examined by TEM, which were performed to
examine the particle mean size and the size distribution of
catalysts. As shown in Fig. 1, it is obvious that the as-
synthesized nanoparticles were close to spherical particle. The
correlations between the particles mean size and the Pd molar
fraction is shown in Fig. 1F, clearly, the size of particles
became larger with increasing Pd molar ratio. Under the same
2.3 Catalyst characterization
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3.1.3 X-ray photoelectron spectroscopy (XPS) studies
View Article Online
DOI: 10.1039/C9NJ01733J
XPS elemental analysis was used to quantify the variation of
surface chemical composition and the chemical state of
supported catalysts.24-26 Fig. 3 and Table 1 show the X-ray
photoelectron spectroscopic patterns and binding energy of Ag
and Pd elements in monometallic and bimetallic catalysts with
different initial metal molar ratios. It is obviously that, Ag was
in metallic state in these catalysts, while Pd was in the presence
of metallic state and oxidation state.23,27 The coexistence of a
small amount of the oxidation state of Pd was beneficial for
1,3-butadiene hydrogenation.17 In the case of strong interaction
between metal nanoparticles and metal oxide support, their
Fermi energy levels are compromised to a new value. As a
result, immobilization led to lower the core-level binding
energy of Ag, Pd and AgPd bimetallic, which also become a
proof of the interaction between oxide support and supported
metal.
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Fig. 1 TEM images and size histograms (inlet) of biosynthesized
monometallic and bimetallic catalysts: (A) Ag/γ-Al2O3, (B)
Ag3Pd1/
γ-Al2O3, (C) Ag1Pd1/γ-Al2O3, (D) Ag1Pd3/γ-Al2O3, (E)
Pd/ -Al2O3, (F) particle mean size versus Pd molar fraction.
γ
Moreover, the positions of the AgPd bimetallic metal peaks
are close to those of the monometallic Ag and Pd peaks. The
metal peaks of AgPd bimetallic samples has revealed a tiny
negative binding energy shift of Ag 3d core level binding
energy in AgPd samples as compared to monometallic Ag
sample, as shown in Fig. 3, Which has been previously reported
in AgPd alloys both experimentally and theoretically.28-30 While
the binding energy of Pd 3d electrons was positively shifted for
AgPd samples as compared to that for monometallic Pd sample,
this positive shift may be caused by the partial electron transfer
from Ag to Pd atoms.31 The shifts of core level binding energy
for Ag and Pd elements consistent with the former reports.31,32
3.1.4 Fourier Transform Infrared Spectroscopy (FTIR) studies
To explicit the metal active sites, we focused on in-situ FTIR
study of CO adsorption on the supported catalysts. Fig. 4 shows
in situ FTIR spectra of CO absorbed on the Ag/γ-Al2O3, Pd/γ-
Al2O3 and AgPd/γ-Al2O3 catalysts prepared with different
Ag/Pd molar ratio. According to previous reports,13,33 the linear
CO species can be observed at around 2100 cm-1, and IR band
synthesizing condition, the bimetallic nanoparticle size
(10.5±1.4 nm, 8.5±0.8 nm, 7.6±0.8 nm) were all in between the
monometallic particle size of Ag (21.1±3.8 nm) and Pd
(6.9±0.6 nm), which suggesting that the molar ratio of
bimetallic has obvious influence on the particle size, and the
size of bimetallic nanoparticles decrease with increasing Pd
ratio.
3.1.2 X-ray diffraction (XRD) studies
Fig. 2 shows the powder XRD patterns of the AgPd/γ-Al2O3
with different initial Ag/Pd ratios (3:1, 1:1, 1:3), Ag/γ-Al2O3
and Pd/γ-Al2O3 catalysts. The characteristic peak of Ag catalyst
at 2θ position 38.1o pertains to (111) plane (ref. code: 04-0783),
which was covered up by the diffraction of γ-Al2O3 (19.3o,
37.5o, 39.3o, 45.7o, 60.5o, 66.6o, ref. code: 50-0741), and the
specific absorbing peak of Pd catalyst at 40.1o corresponds to
(111) plane (ref. code: 46-1043). Clearly, the metal diffraction
peaks of bimetallic catalysts all around 39.5o located between
38.1o(Ag(111))and 40.1o(Pd(111)) is assigned to the obvious
alloying structure with fcc crystal form,18,23 and the intensity
increased with the increasing Pd fraction. Since γ-Al2O3 has
many obvious characteristic absorption peaks, no other peaks
were found among the range of 2θ from 30 to 90o.
Fig. 3 XPS patterns of Ag and Pd elements in Ag/
Al2O3 and AgPd/ -Al2O3 catalysts: (A) Ag 3d regions of Ag/
Al2O3, (B) Pd 3d regions of Pd/
AgPd/ -Al2O3, (D) Pd 3d regions of AgPd/
γ-Al2O3, Pd/
γ-
-
Fig. 2 XRD patterns of Ag/
catalysts pretreated with different Ag/Pd molar ratio: (a)
Al2O3, (b) Ag/ -Al2O3, (c) Ag3Pd1/ -Al2O3, (d) Ag1Pd1/ -Al2O3, (e)
Ag1Pd3/ -Al2O3, (f) Pd/ -Al2O3.
γ-Al2O3, Pd/γ-Al2O3 and AgPd/γ-Al2O3
γ
γ
γ-
γ-Al2O3, (C) Ag 3d regions of
γ
γ
γ
γ
γ-Al2O3.
γ
γ
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Table 1 XPS peak positions of Ag and Pd elements in Ag/
Pd/ -Al2O3 and AgPd/ -Al2O3 catalysts.
Biding energy (eV)
γ-Al2O3,
View Article Online
DOI: 10.1039/C9NJ01733J
γ
γ
AgNO3/Pd(NO3)2
(mol/mol)
Ag
Pd
3d5/2
367.6
367.4
367.2
367.0
3d3/2
373.6
373.4
373.2
373.0
3d5/2
3d3/2
1:0
3:1
1:1
1:3
0:1
335.6
335.6
335.5
335.4
340.8
340.7
340.7
340.6
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Fig. 5 Catalytic performance of monometallic catalysts (Ag/
γ-
Al2O3, Pd/ -Al2O3) and bimetallic catalysts (AgPd/ -Al2O3) with
γ
γ
different initial Ag/Pd ratios.
3.2.2 Effect of Cacumen platycladi concentration
Adjusting the reactant concentration was an effective way to
regulate the reaction rate, and then accordingly control the size
of as-synthesized nanoparticles. Therefore, a series of catalysts
prepared with different concentration of Cacumen platycladi
were studied (Fig. 6). It is obvious that the conversion of 1,3-
butadiene firstly increases and then decreases with increasing
Cacumen platycladi concentration, clearly, 5.0 g/L was the best
concentration for the catalyst performance. It was observed
from TEM analysis (Fig. 7) result that nanoparticle size was
inversely proportional to the concentration of Cacumen
platycladi, since lower concentration (eg. 1.25 g/L and 2.5 g/L)
was insufficient to fully reduce the metal ions, when the
concentration gradually increase to 7.5 g/L and 10 g/L, there
were too much biomolecule residues on the catalysts, which
may cover the active sites and lead to the lower catalytic
activity. Thus, the concentration of 5.0 g/L achieved the best
catalytic performance.
Fig. 4 In situ FTIR spectra of CO absorbed on the Ag/
Pd/ -Al2O3 and AgPd/ -Al2O3 catalysts prepared with different
Ag/Pd molar ratio: (a) Ag/ -Al2O3, (b) Ag3Pd1/ -Al2O3, (c)
Ag1Pd1/ -Al2O3, (d) Ag1Pd3/ -Al2O3, (e) Pd/ -Al2O3.
γ-Al2O3,
γ
γ
γ
γ
γ
γ
γ
below 2000 cm-1 are ascribed to the bridge CO species
consisting of isolated bridged species, compressed bridged
species and three-metal bridged species. The result reveals that
the bridge bonded species dominate for catalysts with
increasing Pd content. The peaks intensity of the bimetallic
catalyst decreased with increasing Ag content, this is probably
the consequence of some Ag dilution affect associated with the
formation of AgPd alloy.
3.2.3 Effect of preparation temperature
The influence of preparation temperature on the performance of
1,3-butadiene hydrogenation was studied and shown in Fig. 8.
The conversion of 1,3-butadiene increased with increasing of
preparation temperature, when the temperature increased to
90oC, the conversion was 100%, while the butene selectivity
decreased slightly to 84.9%. Since the higher preparation
temperature could get higher reduction rate of metal precursor,
accordingly the yield of bimetallic nanoparticles and the active
site increased, which lead to the higher catalytic performance.
3.2 Catalyst activity studies
3.2.1 Effect of the Ag/Pd molar ratio
According to the early reports,34, 35 the relative proportion of the
bimetallic metals is important for the performance of the
catalysts, since the composition of bimetallic metals affects
their surface composition and exhibits different catalytic
properties. Thus, AgPd/γ-Al2O3 bimetallic catalysts with
different initial Ag/Pd ratios and monometallic catalysts (Ag/γ-
Al2O3 and Pd/γ-Al2O3) with 0.75% loading were tested,
respectively.
The
best
1,3-butadiene
hydrogenation
performance was obtained with the initial Ag/Pd ratio of 1:3
(Fig. 5), the conversion of 1,3-butadiene was up to 100% and
the butene selectivity was 68.1%. However, the catalysts with
the initial Ag/Pd ratio of 3:1 and 1:1 showed low activity, and
Ag/γ-Al2O3 was hardly active for this reaction. As for catalyst
Pd/γ-Al2O3, the activity was up to 100%, but the butene
selectivity was only 32.3%. Clearly, the addition of Ag dilutes
Pd active sites, which supress the further hydrogenation to
some degree, and leading to better catalytic activity.
Fig. 6 1,3-butadiene hydrogenation over AgPd/
with different concentration of biomass.
γ-Al2O3 catalysts
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calcination temperatures. It can be seen from theVdieiwagArrtaicmle Otnhlinaet
DOI: 10.1039/C9NJ01733J
the intensity of the specific absorbing peak of (111) crystal
plane (39.6o) in the catalysts decreased slightly when the
calcination temperature was increasing. When the calcination
o
temperature increased to 300 C, the intensity of the diffraction
peak decreased a lot. Combine the characterization of TEM and
XRD, we supposed that the structure of bimetallic nanoparticles
has some changes after calcination, but it cannot be clearly
displayed because of the lower loadings.
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Fig. 7 TEM images and size histograms (inlet) of biosynthesized
AgPd nanoparticles under different Cacumen platycladi
concentration: (A) 2.5 g/L, (B) 5.0g/L, (C) 7.5g/L, (D) 10g/L.
Fig. 9 1,3-butadiene hydrogenation over AgPd/
at different calcination temperature.
γ-Al2O3 catalysts
Fig. 8 1,3-butadiene hydrogenation over AgPd/
with different preparing temperature.
γ-Al2O3 catalysts
Therefore, the 1,3-butadiene conversion and butene selectivity
both take into account, higher preparation temperature is
benefit for 1,3-butadiene hydrogenation.
3.2.4 Effect of calcination temperature
The influence of calcination temperature on the catalytic
performance was studied and showed in Fig. 9. It can be seen
from the diagram that when the calcination temperature was
under 250 oC, the catalytic activity remained almost unchanged,
and the butene selectivity gradually decreased with the
Fig. 10 TEM images and size histograms (inlet) of AgPd/
catalysts with different calcination temperature: (A) Ag1Pd3/
γ-Al2O3
γ-
Al2O3, (B) 100-Ag1Pd3/
γ-Al2O3, (C) 200-Ag1Pd3/
γ
-Al2O3, (D) 225-
increasing calcination temperature; when the calcination Ag1Pd3/
γ-Al2O3, (E) 250-Ag1Pd3/γ-Al2O3, (F) 275-Ag1Pd3/γ-Al2O3,
temperature continues to increase to 275 oC, the catalytic
(G) 300-Ag1Pd3/
γ-Al2O3.
activity decreased sharply and remained only about 2%. When
the calcination temperature further increased to 300 oC, the
catalyst hardly shows activity.
Herein, to figure out the possible reasons for the influence of
calcination temperature, TEM and XRD analysis has been
studied. Fig. 10 shows the TEM diagram of bimetallic catalysts
with different calcination temperatures and the corresponding
particle size distribution diagram. It is obviously that when
o
calcination temperature was less than or equal to 250 C, the
particle size of nanoparticles was basically unchanged, and the
dispersion of nanoparticles shows good. When the calcination
temperature continues to rise to 275 oC and 300 oC, the
diameter of the nanoparticles increased slightly, and the
dispersion of nanoparticles was relatively poor. Thus,
bimetallic particle agglomeration in higher calcination
Fig. 11 XRD patterns of AgPd/
calcination temperature: (a) Ag1Pd3/
Al2O3, (c) 200-Ag1Pd3/ -Al2O3, (d) 225-Ag1Pd3/
γ
-Al2O3 catalysts with different
γ-Al2O3, (b) 100-Ag1Pd3/γ-
γ
γ-Al2O3, (e) 250-
temperature resulted in the deactivation of catalysts; Fig. 11 Ag1Pd3/
was the XRD diagram of bimetallic catalysts with different
γ-Al2O3, (f) 275-Ag1Pd3/γ-Al2O3, (g) 300-Ag1Pd3/γ-Al2O3.
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3.2.5 Effect of metal loading
View Article Online
DOI: 10.1039/C9NJ01733J
The dosage of AgPd bimetallic catalyst is an important factor
for its catalytic performance. The AAS analysis was carried by
measuring the total amount of metals in the catalyst filtrate, and
then the actual loading of bimetallic catalysts was calculated.
Table 2 showed that the actual loading and theoretical loading
of bimetallic catalysts were similar when the catalyst dosage
increased from 0.2% to 1.0%.
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The effect of metal loading on the performance of selective
hydrogenation of 1,3-butadiene was shown in Fig. 12. Clearly,
the best performance of catalysts were achieved at loading of
0.4%, namely, 1,3-butadiene conversion of 98.2%, and butene
selectivity of 88.1% were obtained. Furthermore, it can be seen
from the diagram that the hydrogenation selectivity decreases
gradually with the increase of metal loading, while the
conversion of 1,3-butadiene reached 100% at loading of 0.5%,
and remains unchanged at 100% thereafter. Comparing the
XRD spectra of AgPd bimetallic catalysts with different
loadings (Fig. 13), the diffraction peaks of catalysts were not
obvious when the loadings was less than 0.5%, indicating that
the bimetallic nanoparticles are uniformly distributed on the
surface of the support. With increasing of metal loading, the
characteristic peak intensity of (111) crystal face (39.6o) of
AgPd bimetallic nanoparticles increased gradually. At lower
metal loading, the catalyst usually showed lower activity since
the insufficient number of metal active sites; while the catalyst
with higher metal loading has more active sites, it was easy to
make the catalyst further hydrogenation, which leads to the
decrease of selectivity of butene.
Fig. 13 XRD patterns of AgPd/
AgPd loadings: (a)
0.4%Ag1Pd3/ -Al2O3, (d) 0.5%Ag1Pd3/
Al2O3, (f) 1.0%Ag1Pd3/ -Al2O3.
γ
-Al2O3 catalysts with different
γ
-Al2O3, (b) 0.2%Ag1Pd3/
γ-Al2O3, (c)
γ
γ-Al2O3, (e) 0.6%Ag1Pd3/γ-
γ
3.6 Durability of the catalysts
Whether the catalyst can run stably for a long time is one of the
most important factors to be considered in industrial production.
In many cases, the biggest obstacle to popularize the high-
efficiency catalyst studied experimentally to actual production
is the stability problem, clearly, the stability testing and
research is very necessary. In this section, the stability of
AgPd/γ-Al2O3 catalyst prepared by plant reduction method with
0.4% metal loading was investigated. As shown in Fig. 14, the
conversion of 1,3-butadiene stayed close to 100% for the first
12 h, then decrease slightly to 96% for another 24 h, and slowly
decreased to 90% at 50 h. But the butene selectivity remains
basically unchanged for 50 h.
From the TEM diagram of the catalyst before and after the
stability testing (Fig. 15), the particle size of nanoparticles on
Fig.12 1,3-butadiene hydrogenation over AgPd/
catalysts at different metal loading.
γ-Al2O3
Fig. 14 1,3-butadiene hydrogenation over AgPd/
catalyst as a function of time-on-stream.
γ-Al2O3
Table 2 Determination of actual AgPd loading on AgPd/γ-Al2O3
catalysts.
Samples
Actual AgPd loading (%)
0.2% AgPd/γ-Al2O3
0.4% AgPd/γ-Al2O3
0.5% AgPd/γ-Al2O3
0.6% AgPd/γ-Al2O3
1.0% AgPd/γ-Al2O3
0.20
0.40
0.50
0.59
0.99
Fig. 15 TEM images and size histograms (inlet) of AgPd/
catalyst: (A) before testing, (B) after testing.
γ-Al2O3
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the surface of the catalyst remained almost unchanged,
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View Article Online
DOI: 10.1039/C9NJ01733J
indicating that the active center of the catalyst does not change
during the durability test. Also, from XPS (Fig. 16) and XRD
(Fig. 17) analysis of the AgPd/γ-Al2O3 catalyst before and after
testing, the results showed that the metallic state basically
considered maintaining invariable, thus this may responsible for
the good catalytic stability. From the TG diagram of Fig. 18, it
is obviously the quality of the catalyst decreased with
increasing temperature. A weightlessness peak appeared around
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o
100 C for all samples, which was caused by the presence of a
small amount of free water desorption. The apparent mass loss
peaks near 300 oC indicated that the residual biomass molecules
are mainly decomposed by combustion at this temperature.
Meanwhile, TG analysis showed that the residual biomass of
Cacumen platycladi leaves on AgPd/γ-Al2O3 catalyst was
approximately 3.6%. We assume that the 3.6% residual
biomass molecules have a positive impact on promoting the
catalytic reaction and maintaining the stability of the catalyst.
On the one hand, the residual plant biomass molecules on the
catalyst surface protect nanoparticles from agglomeration and
growth during the catalytic reaction. Besides, the residual plant
biomass molecules play a role in modifying the catalyst, which
is more conducive to the timely desorption of butene from the
catalyst surface, thereby reducing the probability of deep
hydrogenation of butene. Furthermore, taking the metal loading
and turnover frequency into consideration, Table 3 lists the
rough comparison of catalytic performance in selective
hydrogenation of 1,3-butadiene. Among the catalysts, the
turnover frequency of AgPd/γ-Al2O3 is relatively high, while
the metal loading and testing temperature are low. Hence, the
88.1% butenes selectivity and 98.2% 1,3-butadiene conversion
were obtained at relatively low temperature and metal loading.
Fig. 17 XRD patterns of AgPd/
(b) after testing.
γ
-Al2O3 catalyst (a) before testing,
Fig.18 TG profiles of support γ-Al2O3 (a), AgPd/γ-Al2O3 catalyst (b).
4 Conclusions
In this paper, AgPd/γ-Al2O3 catalysts were prepared by sol-gel
method using Cacumen platycladi leaves extract. Then the
catalysts were characterized by XRD, UV-Vis, FTIR, TEM and
TG analysis. The performance of the as-synthesized catalysts
was evaluated by selective hydrogenation of 1,3-butadiene. The
influence of preparation conditions (including the molar ratio of
silver to palladium, the concentration of Cacumen platycladi
leaves extract, preparation temperature, the loading of catalysts)
and durability experiment were investigated. The result showed
that the AgPd/γ-Al2O3 bimetallic catalyst exhibited good
stability for the hydrogenation of 1,3-butadiene. The activity of
the catalyst decreased a bit and the butene selectivity remained
basically unchanged for 50 h. The changeless particle size,
invariable metallic state and the residual biomass molecules
may be responsible for the good performance of AgPd/γ-Al2O3
catalysts.
Fig. 16 XPS patterns of Ag and Pd elements in of AgPd/γ-Al2O3
catalyst: (A) Ag 3d regions of AgPd/γ-Al2O3, (B) Pd 3d regions of
AgPd/γ-Al2O3.
Table
3 A rough comparison of catalytic performance in
hydrogenation of 1,3-butadiene.
loading
(wt.%)
0.3
Reaction
Con. TOFa
Acknowledgements
Catalyst
Ref.
temp.(oC)
(%)
100
100
100
90
(s-1)
The authors are thankful for the financial support of the Natural
Science Foundation of Fujian Province (2018J05021), Science
and Technology Program of LongYan (2017LY89), and
Science and Technology Project of Education Department of
Fujian Province (JAT170563).
Pd/Al2O3
Pd/γ-Al2O3
25
35
35
70
0.101
0.092
0.102
0.149
1
17
36
0.5
PdCu/Mn2O3
PdNi/γ-Al2O3
1.0
0.91
35
This
work
AgPd/γ-Al2O3
a Calculation of TOF (s-1) after 1h of reaction.
0.4
35
100
0.153
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29 I. A. Abrikosov, W. Olovsson and B. Johansson, Phys. Rev.
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DOI: 10.1039/C9NJ01733J
Graphic abstract:
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Plant-mediated synthesis of bimetallic AgPd/γ-Al2O3 catalyst for selective hydrogenation of
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low-temperature 1,3-butadiene was reported.
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