Tailoring supported palladium sulfide catalysts through H 2-assisted sulfidation with H2S

Article abstract of DOI:10.1039/c3ta12277h

Supported palladium sulfide catalysts are of great interest in selective hydrogenation reactions. In this work, "Pd₄S", "Pd₃S", "Pd₁₆S₇" and "PdS" supported on activated carbon were selectively synthesized by tailoring the H₂-assisted sulfidation of Pd/C with H₂S at 150-750°C, and were characterized by means of XRD, XPS, TEM, HRTEM, EDS, BET and H₂-TPR techniques. The results indicated that the sulfidation atmosphere, the sulfidation temperature and the metal-support interaction all played important roles in determining the crystal structure and composition of the Pd_xS_y/C catalysts, which in turn gave different catalytic performances in the synthesis of N-(1,3-dimethylbutyl)-N′- phenyl-p-phenylenediamine (6PPD) by the reductive N-alkylation of aromatic amines. PdS/C showed the highest selectivity (>97%) and stability among all the Pd_xS_y/C catalysts.

Full text of DOI:10.1039/c3ta12277h

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Tailoring Supported Palladium Sulfides Catalysts through H2-Assisted
Sulfidation with H2S
Wei Xu, Jun Ni, Qunfeng Zhang, Feng Feng, Yizhi Xiang† and Xiaonian Li*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Supported palladium sulfides catalysts are of great interest in selective hydrogenation reactions. In this work,
“Pd4S”, “Pd3S”, “Pd16S7” and “PdS” supported on activated carbon were selectively synthesized by tailoring
the H2ꢀassisted sulfidation of Pd/C with H2S at 150ꢀ750 °C, and were characterized by means of XRD, XPS,
TEM, HRTEM, EDS, BET and H2ꢀTPR techniques. The results indicated that sulfidation atmosphere,
10 sulfidation temperature and the metalꢀsupport interaction all played important roles in determining the crystal
structure and composition of the PdxSy/C catalysts, which in turn gave different catalytic performances in the
synthesis of Nꢀ(1, 3ꢀdimethylbutyl)ꢀN'ꢀphenylꢀpꢀphenylenedianine (6PPD) by the reductive Nꢀalkylation of
aromatic amines. PdS/C showed the highest selectivity (>97%) and stability among all the PdxSy/C catalysts.
usually leads to supported palladium sulfides with a structure of
17ꢀ21
45 amorphous or mixed crystal phases.7,
The preparation of
1. Introduction
supported pallaidum sulfides with a single crystal structure seems
to be a great challenge.24 Up to now, only two papers have
reported the synthesis of supported palladium sulfides with single
crystal structure: 1) the formation of Pd16S7/Al2O3ꢀMoOx through
15
Transitional metal sulfides used for catalysts and in materials
have been found to be very important for a variety of applications
due to their special physicalꢀchemical properties.1ꢀ6 For example,
NiꢀMoꢀS, CoꢀMoꢀS and RuS2 have been used as catalysts for
°
50 the reaction of Pd/Al2O3ꢀMoOx with H2S/H2 at 400 C;22 2) the
2
hydrodesulfidation and hydrodenitrogenation reactions.1, PtS
formation of Pd4S/C by the sulfidation of Pd/C with an aqueous
20 and Rh2S3 have been used for the selective hydrogenation and
the sulfurꢀresistant hydrogenation of organic compounds.3, 4 Pd
sulfides have been proven to be more promising catalysts than
other transitional metal sulfides for thiophene hydrogenation and
the reductive Nꢀalkylation of aromatic amines due to their ease of
25 formation of active sites and have thereofore attracted increasing
attention recently. 5ꢀ7
According to the phase diagram of PdꢀS, there are five types of
PdꢀS crystalline phases, namely Pd4S, Pd3S, Pd16S7, PdS and
PdS2.8 Unsupported palladium sulfides can be generally
30 synthesized by introducing sulfurꢀcontaining compounds (CS2,
H2S, or Na2S) into an aqueous palladium salt solution, applying
the chemical vapor deposition (CVD) method, and by the
thermolysis of organometallic compounds.8ꢀ12 Their applications
have been found in fabricating photographic and lithographic
35 films, electrolytic deposition of Cu and hydrogen generation.6, 8, 9
However, the use of unsupported palladium sulfides used as
heterogeneous catalysts in the transformation of organic
compounds is scarcely reported because of their lower catalytic
activity and stability compared to supported palladium sulfides
40 catalysts.13, 14
°
Na2S solution in the presence of H2 at 200 C.23 However, the
factors that determine the final crystal structure of supported
palladium sulfides have not been investigated. There are also no
55 known correlations between the crystal structures of palladium
sulfides with their corresponding catalytic performances in
chemical reactions. Therefore, it seems necessary to develop
methods to synthesize a series of supported palladium sulfides
with single crystal structures, so that structureꢀproperty
60 relationships can be established.
Synthesis
of
Nꢀ(1,3ꢀdimethylbutyl)ꢀN'ꢀphenylꢀpꢀ
phenylenedianine (6PPD) by reductive Nꢀalkylation of aromatic
amines is of importance to caoutchouc and synthetic rubber
industries. This reaction starts with a condensation reaction
65 between 4ꢀaminodiphenylamine (ADA) and 4ꢀmethylꢀ2ꢀ
pentanone (MIBK) to form a Ketimine compound (Shiff base),
followed by hydrogenation to 6PPD in the presence of noble
metal catalysts.25 But the concomitant undesired reductions, such
as the hydrogenolysis reaction of CꢀN and C=N bonds, and the
70 hydrogenations of carbonyl group and benzene ring are more
serious over supported noble metal catalysts due to their high
activity.26 It has been reported that the introduction of sulfur
could modify the electronic structure of noble metal catalysts,
which minimize the undesired reductions in the selective
75 hydrogenation. 4, 19, 20
Supported palladium sulfides are generally prepared through
the sulfidation of supported metallic palladium (or oxide)
precursors with H2S or Na2S.14ꢀ23 However, the sulfidation
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In this work, a series of activated carbon supported palladium
sulfides catalysts were prepared in which all catalysts had only
one crystal structure, being either of “Pd4S”, “Pd3S”, “Pd16S7” or
“PdS”. The catalysts were created through the H2ꢀassisted
sulfidation of Pd/C precursors with H2S at various temperatures.
The effects of the sulfidation temperature, the composition of
sulfidation atmosphere and the metalꢀsupport interaction on the
formation of PdꢀS crystalline phases were investigated. The
catalytic performances of these asꢀprepared PdxSy/C catalysts in
programmed reduction (H2ꢀTPR). The details of these
characterization methods can be found in the supplementary
material.
60 2.3 Catalytic performance test (Hydrogenation reaction)
5
The catalytic reaction was performed in a 75 mL stainless steel
autoclave reactor (Weihai Automaticallyꢀcontrolled Reaction
Kettle Co. Ltd.), in which 0.05g of catalyst, 3.68g of 4ꢀ
aminodiphenylamine (AR/99.7%, Shandong Shengao Chemical
65 Co. Ltd.) and 10 mL of 4ꢀmethylꢀ2ꢀpentanone (AR/99.5%,
Quzhou Juhua Reagents Co. Ltd.) were placed. The reactor was
then pressurized with H2 to the desired pressure (1ꢀ4 MPa) after
10 the synthesis of 6PPD that was used as a model reaction were
evaluated, and the structureꢀproperty relationships were also
investigated.
°
being heated up to 80ꢀ200 C. The beginning of the reaction was
indicated by the start of high speed stirring (1200 r/min). The
70 reaction products were analyzed by gas chromatography
(Shimadzu GCꢀ2014) equipped with an FID detector and a
capillary column HPꢀ5 (φ0.320 mm × 30 m).
2. Experimental
2.1 Catalyst preparation
15
Pd/C catalysts with a nominal Pd loading of 10.0 wt% were
prepared by the conventional impregnation method.27 Prior to
impregnation, a commercial activated carbon (China National
Medicines Corporation Ltd.) was pretreated with 2.5% HNO3 at
3. Results and discussion
3.1 Catalyst preparation and characterization
°
80 C for 6 h, followed by washing with distilled water until the
°
75
The evolution of the crystal structure of Pd/C in the sulfidation
with H2S/H2 in the temperature range of 30ꢀ950 °C is presented in
the supplementary material (Fig. S1ꢀ5). Here Fig. 1 shows the
20 pH was 7 and then outgassing in a vacuum at 120 C overnight.
After the pretreatment, the impurities of the activated carbon can
be removed, which increased structural parameters, such as SBET
,
°
°
XRD patterns of Pd/C sulfided by H2S/H2 at 150 C (or 550 C),
Vmicro, Vtotal, and D. It was also beneficial to create an appropriate
density of the total acidity on the activated carbon for the
25 preparation of Pd/C catalysts.27 Three ml of H2PdCl4 (Hangzhou
Kaida Metal Catalyst & Compounds Co. Ltd.) aqueous solution
(0.05 g/mL) was then added dropwise into 100 ml of aqueous
suspension containing 1.5 g of pretreated activated carbon, and
then the pH of the solution was adjusted to 10 by the addition of
30 NaOH aqueous solution. This was followed by washing with
distilled water until the pH of filtrate was 7 and no chlorine could
be observed in filtrate by AgNO3 titration method. The sample
was subsequently reduced with hydrazine hydrate at room
°
°
250 C and 750 C, which gave rise to the formation of Pd4S/C,
80 Pd3S/C and Pd16S7/C, respectively. When fresh Pd/C catalyst was
°
sulfided at 150 C, only a poorly crystallized “Pd4S” phase was
observed (Fig. 1b). This “Pd4S” phase diminished at the
°
sulfidation temperature of 250 C and “Pd3S” became the only
existing phase (Fig. 1d). When the temperature was increased to
°
85 550 C (Fig. 1c), the “Pd4S” phase reappeared at the expense of
the “Pd3S” phase and a single well crystallized structure was
°
formed. At temperatures above 600 C, a “Pd16S7” phase started
°
to develop and reached its maximum at 750 C, at which neither
°
“Pd4S” nor “Pd3S” were present (Fig. 1e). Further increasing the
90 sulfidation temperature led to a mixture of “Pd16S7”, “Pd4S” and
“Pd3S” phases. Hence, an appropriate sulfidation temperature can
generate the desired single phase of “Pd4S”, “Pd3S” or “Pd16S7”
supported on activated carbon.
temperature and was dried at 110 C for 7 h. For comparison
35 purposes, a 10.0 wt% Pd/Al2O3 sample was also prepared. This
was accomplished by impregnating 1.5g of 20-80 mesh γꢀAl2O3
(Shandong Alumina Factory) with 3 mL of H2PdCl4 aqueous
solution (0.05 gꢁmL−1) overnight, followed by drying at 120 C
°
°
The sulfidation of Pd/C with H2S/Ar in the absence of H2 shows
95 significantly different results as those with H2S/H2. Single phases
of “Pd4S”, “Pd3S” and “Pd16S7” could not be obtained within the
for 3 h and calcination in air at 500 C for 4 h. This sample was
40 then reduced in H2 at a flow rate of 30 mLꢁmin−1 at 400 C for 5
°
h. Unsupported palladium particles were prepared by the direct
reduction of 200ꢀ400 mesh palladium chloride (PdCl2) particles
in H2 at a flow rate of 30 mLꢁmin−1 at 80 °C for 2 h.
°
sulfidation temperature range of 150ꢀ750 C (Fig. S7). On the
°
contrary, a single “PdS” phase was observed at 300 C (Fig. 1f).
This suggests that the formation of palladium sulfides supported
100 on activated carbon is highly denpendent on the composition of
the sulfidation atmosphere and the temperature.
The sulfidation of catalysts was carried out by placing 1.5g of
45 sample into an isothermal region of a silica tube (D=20 mm) and
°
heating it to the desired temperature (30ꢀ950 C) at a rate of
°
10 Cꢁmin−1 in an N2 stream (30 mLꢁmin−1). Subsequently, the
The presence of single PdxSy phases supported on activated
carbon supported catalysts is further evidenced by HRTEM
micrographs (Fig. 2) of the lattice structures of palladium
105 sulfides. EDS measurements indicate that the S/Pd atomic ratios
are slightly higher than their nominal stoichiometry value (Table
1). It is due to the accumulation of trace amounts of sulfur on the
top of the sample surface evidenced by XPS characterization
(Table 1).10 Another possible explanation is that a trace amount
sulfidation was initiated by switching the N2 stream to a mixed
stream of 10%ꢀH2S/H2(Ar) at the same flow rate and lasted for
50 3.5 h. Finally, the sample was cooled down to room temperature
in an N2 stream at a flow rate of 30 mLꢁmin−1.
2.2 Catalyst Characterization
The samples were characterized using Xꢀray powder
diffraction (XRD), transmission electron microscopy (TEM), 110 of sulfur penetrated into the activated carbon layers during
55 energyꢀdispersive Xꢀray spectroscopy (EDS), N2 physisorption,
Xꢀray photoelectron spectroscopy (XPS) and H2 temperatureꢀ
sulfidation, as suggested by XRD results (Fig. 1). The XRD
patterns show that the peak of (0 0 2) facet of the activated
2
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carbon of sulfided samples shifted to a lower angle compared to
that of fresh sample at 2θ of 26º.28
5
Fig. 1 XRD patterns of samples: (a) fresh Pd/C, (b) Pd4S/C (formed at
150 °C in H2S/H2), (c) Pd4S/C (sulfided at 550 °C in H2S/H2), (d) Pd3S/C
(sulfided at 250 °C in H2S/H2), (e) Pd16S7/C (sulfided at 750 °C in
H2S/H2), (f) PdS/C (sulfided at 300 °C in H2S/ Ar). JCPDS PDF: 73ꢀ1387,
Pd4S; PDF: 75ꢀ2228, Pd16S7; PDF: 73ꢀ1831, Pd3S; PDF: 74ꢀ1060, PdS.
10
The mean particle size of the various palladium sulfides
calculated by applying the Scherrer formula was about 30ꢀ50 nm
(Table 1). This is consistent with the TEM analysis (Fig. 2),
which shows the palladium sulfides particles having a size of 5ꢀ
15 70 nm distributed on activated carbon surface. These results
indicate an increase in the average particle size after sulfidation,
since the lattice constant of palladium sulfides is much larger than
that of pure Pd.29 Meanwhile, the adsorption of H2S decreased the
metal–support interaction, making the migration and growth of
20 palladium sulfides particles easier at high sulfidation
temperature.30 Additionally, except for Pd4S/C obtained at
°
150 C, no significant difference in the SBET values and the pore
diameter were measured (Table 1), indicating that activated
carbon was stable during the sufidation process.
25 Table 1 Summary of characterization results of activated carbon
supported palladium sulfides
S/Pd atomic ratio
Particle
SBET Pore diameter
Catalyst
Pd/C
size/nma /m2.gꢀ1
/nm
3.19
3.79
3.17
3.16
3.17
3.26
EDS
0
XPS
0
1.02
0.59
0.49
0.56
1.12
29.7
32.1
35.5
44.7
48.6
33.6
1563
1268
1598
1551
1407
1473
Pd4S/C(150 °C) 0.25~0.37
35
Pd3S/C
0.33~0.42
Pd4S/C(550 °C) 0.25~0.33
Fig. 2 HRTEM images of the lattice structure and TEM images of the
Pd16S7/C
PdS/C
0.44~0.51
1~1.14
°
particle size distribution of (a) Pd4S/C (sulfided at 150 C in H2S/H2), (b)
°
°
Pd3S/C (sulfided at 250 C in H2S/H2), (c) Pd4S/C (sulfided at 550 C in
a
:
Determined by XRD.
°
H2S/H2), (d) Pd16S7/C (sulfided at 750 C in H2S/H2), (e) PdS/C (sulfided
40 at 300 °C in H2S/ Ar).
In summary, a series of activated carbon supported palladium
sulfides catalysts with single crystal structures can be prepared by
30 tuning the sulfidation conditions. To better undertand the
formation mechanism of these palladium sulfides, we have
investigated the factors that affect the final composition of PdxSy,
such as sulfidation atmosphere, temperature and metalꢀsupport
interaction.
3.2 The effect of sulfidation atmosphere and temperature
The “Pd4S”, “Pd3S” and “Pd16S7” could be sequentially formed
with the increase in sulfidation temperatures using H2 as the
balance gas. According to the phase diagram, it was difficult to
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obtain single “Pd4S” and “Pd3S” phases through the reaction Pd
metal and sulfur powder under vacuum.8 For example, single
“Pd4S” phase could not be obtained at temperatures below 767 °C
under vacuum, because a mixed phase of “Pd3S” and “Pd4S”
dissociation rate of H2S was influenced by the sulfidation
temperature.16 In addition, a high sulfidation temperature may
improve the degree of lattice perfection, resulting in the
formation of wellꢀcrystallized phase. For example, “Pd4S” could
°
°
5
could be formed when the atomic percent of sulfur is above 45 be formed at 150 C and 550 C in the presence of H2S/H2.
20%.8 Furthermore, according to the values of ꢂG for the
formation of different phases of palladium sulfides, the formation
rate follows the order: “PdS”> “Pd16S7”> “Pd3S”> “Pd4S”. And it
has been reported that “Pd16S7” is an intermediate of “PdS” in the
Diffraction peaks of “Pd4S” formed at 550 C showed higher
°
°
intensity than those of “Pd4S” formed at 150 C, indicating that
“Pd4S” formed at 150 °C was poorly crystallized and its
lattice may have imperfections, such as dislocations and
10 direct sulfidation of Pd with sulfur powder.31 But our studies 50 vacancies.31 Note that the broadening appearance of diffraction
showed that when using H2 as the balance gas, the “PdS” phase
could not be formed by the further sulfidation of “Pd16S7” with
H2S even after prolonged sulfidation time (8 h) or at higher
profile of Pd4S/C(150°C) could be attributed to the fact that this
sample contains polycrystalline agglomerates carried out during
the sulfidation process. 33
°
temperature (800ꢀ950 C) (Fig. S5ꢀ6). In contrast, when using Ar
Therefore, the sulfidation atmosphere and temperature played
15 as the balance gas, the final composition of PdxSy were 55 a crucial role in the shift of phase equilibrium, which tailored the
significantly different from the results obtained using H2 as
balance gas, and the single “PdS” phase could be formed at a
sulfidation temperature of 300 C. Therefore, the formation of
formation of single “Pd4S”, “Pd3S”, “Pd16S7” or “PdS” phase. But
it should be noted that the formation of “PdS2” that was supposed
to form according to the phase diagram, was not observed in our
study, indicating that “PdS2” may only be formed through the
60 reaction of Pd metal and sulfur powder.8, 34
°
single phase is highly dependent on the sulfidation atmosphere.
20
Fig. 3 XPS analysis of Pd/C treated with H2S/Ar at 30 °C for 3.5 h
Fig. 4 H2ꢀTPR analysis of samples: (a) Carbon (sulfided at 30 °C in
H2S/H2); (b) PdꢀPdSx/C (sulfided at 30 °C in H2S/H2); (c) Carbon
65 (sulfided at 150 °C in H2S/H2); (d) Pd4S/C (sulfided at 150 °C in H2S/H2);
(e) Pd3S/C (sulfided at 250 °C in H2S/H2); (f) PdS/C(sulfided at 300 °C in
H2S/ Ar); (g) Pd4S/C (sulfided at 550 °C in H2S/H2); (h) Pd16S7/C
(sulfided at 750 °C in H2S/H2).
XPS spectra (Fig. 3) shows that there were three forms of
sulfur species with peaks, including 162.1 eV (S2ꢀ), 164.1 eV (Sn)
°
25 and 168.0 eV (S6+) for the sulfided Pd/C formed at 30 C in a
flow of H2S/Ar.32 These findings indicate that H2 and
chemisorbed sulfur species could easily be produced via the
dissociation of HS and H2S at low temperatures. Therefore,
sulfidation under both H2S/H2 and H2S/Ar can be considered as
30 H2ꢀassisted sulfidation. Hensen reported that the presence of
different Pd sulfide phases is mainly dependent on the H2S/H2
ratio.16 They observed Pd metal was formed at a H2S/H2 ratio
below 0.008, Pd4S was formed at H2S/H2 ratios between 0.008
and 0.025, and PdS at higher ratios.16 Therefore, the formation of
35 the “PdS” phase using Ar as the balance gas can be attributed to
the higher H2S/H2 ratio in the sulfidation atmosphere than that
required for the formation of “PdS1ꢀx” (0<x<1) that is normally
formed using H2 as the balances gas.
70
In order to further understand the effect of sulfidation
atmosphere and temperature on the formation of palladium
sulfides, the PdꢀS bond strength of PdxSy/C was analyzed by H2ꢀ
TPR as shown in Fig. 4. Since there is no desorption of H2S over
the activated carbon sulfided at 30 °C (Fig. 4a), the peak at 327 °C
°
75 observed over the PdꢀPdSx/C sulfided at 30 C can be assigned to
chemisorbed S species on the Pd surface (Fig. 4b).35 A broad
°
peak located at 434 C can be seen over the activated carbon
°
sulfided at 150 C (Fig. 4c), which was ascribed to the reduction
of sulfur in the layers of the activated carbon support, as
80 evidenced by XRD (Fig. 1).28 According to the hydrogen
consumption during H2ꢀTPR experiments (Fig. S8 and Table S1),
the peak at 379 °C is referred to the reduction of “Pd4S” formed at
°
Meanwhile, mixed Pd–S phases were obtained at 550 C and
40 750 °C using Ar as the balance gas, indicating that the
4
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°
°
150 C (Fig. 4d). The reduction peak of “Pd3S” formed at 250 C
“Pd3S” should have the strongest metalꢀsupport interaction
should be composed of multiple peaks with a maximum at 338 60 among all the catalysts studied, followed by the interaction of
°
°C and a shoulder peaks at 380 C (Fig. 4e). This result indicates
that “Pd3S” was first reduced into “metal Pd” with a minor
amount of “Pd4S”, and then “Pd4S” was reduced to “metal Pd”
with the increasing of temperature. In addition, “PdS” showed
convoluted reduction peaks offsetting at 289 °C up to 427 °C with
catalysts with “Pd4S” stronger than those with “Pd16S7”. This
5
°
°
a maximum at 393 C and two shoulder peaks at 343 C and 384
°C, respectively (Fig. 4f). They correspond to the transition from
10 a part of “PdS” to “PdꢀPd4SꢀPdS” and then “PdꢀPdS” at reduction
temperature of 343 °C and 384 °C, respectively, which was
followed by the direct reduction of the remaining “PdS” to “metal
°
°
Pd” at 393 C. It is also found the peak at 379 C referred to the
°
reduction of “Pd4S” formed at 150 C (Fig. 4d) disappears on the
°
°
15 “Pd4S” formed at 550 C (Fig. 4g), and only the peak at 327 C
could be detected. Furthermore, since almost no reduction is
observed on Pd16S7/C (Fig. 4h), this catalyst is assumed to be the
most stable among all the studied catalysts in H2. Therefore, a
high sulfidation temperature is generally associated with a high
20 stability of PdxSy/C in H2. And it is thus concluded that the PdꢀS
bond strength (stability in H2) of palladium sulfides follows the
order: “Pd16S7” (sulfided at 750 °C) > “Pd4S” (sulfided at 550 °C)
> “PdS” (sulfided at 300 C) > “Pd4S” (sulfided at 150 C) >
“Pd3S” (sulfided at 250 °C) > “PdꢀPdSx” (sulfided at 30 °C). Since
25 all the studied palladium sulfides are stable below 250 °C in H2, it
is reasonable to suggest that no decomposition of sulfides would
occur during the catalytic hydrogenation.
°
Fig. 5 XRD patterns of samples treated with H2S/H2 at 300 C for 3.5 h.
°
°
(a) Pd16S7 (b) Pd3SꢀPd4S/C (c) Pd4S/Al2O3
65 trend indicates that the interaction between Pd metals and
activated carbon is stronger than Al2O3. This is because the
pretreatment of activated carbon with HNO3 produced a great
amount of acidic oxide groups assembling densely on the
activated carbon, resulting in a strong interaction between the
70 oxygenꢀcontaining groups and the Pd metal precursor.27 On the
contrary, the unsupported Pd particles that gave rise to “Pd16S7”
after sulfidation have the least metalꢀsupport interaction (actually
no metalꢀsupport interaction) among these three catalysts.
3.3 The effect of metal-support interaction
We have demonstrated that the “Pd16S7” phase could not be
30 formed over Pd/C until the sulfidation temperature was above
°
600 C, whilst an unsupported single “Pd16S7” phase (Fig. 5a)
could be obtained through the reaction of unsupported Pd
particles with H2S/H2 at 300 °C. We propose that the formation of
PdxSy/C was influenced by the presence of support. Koki reported
35 that Pd/TiO2 and Pd/Al2O3 sulfided at room temperature displayed
a significant difference in sulfur tolerance and PdꢀS bond strength.
This was mainly due to the different electron densities of the Pd
metals influenced by the interaction between the Pd metals and
the support.36 In general, a weak PdꢀS bond indicates a low
40 electron density of Pd metals, which is induced by a strong metalꢀ
support interaction, and vice versa. This is due to the fact that a
strong metalꢀsupport interaction induces the electron transfer
from Pd metals to support, the electron density of Pd is thus
reduced and the PdꢀS bond is weakened.36 If the metalꢀsupport
45 interaction is weak, Pd metals have a high electron density and
tend to bond strongly with sulfur. Therefore, the higher
sulfidation temperatures required for the formation of Pd16S7/C
compared to that of unsupported Pd16S7 particles can be attributed
to the metalꢀsupport interaction.
3.4 Catalytic performances
75
The reaction scheme for the synthesis of Nꢀ(1, 3ꢀ
dimethylbutyl)ꢀN'ꢀphenylꢀpꢀphenylenedianine (6PPD) is shown
in Fig. 6. As shown in Table 2, ADA was completely converted
in only 0.4 h, but a lower selectivity (21.7%) was obtained over
Pd/C catalyst under the reaction conditions of 200 °C and 4 MPa.
80 The main byꢀproducts were Nꢀ(1, 3ꢀdimethylbutyl)ꢀN'ꢀ
cyclohexaneꢀpꢀphenylenedianine, 4ꢀmethylpentanꢀ2ꢀol and 2ꢀ
methylpentane, which were derived from the hydrogenation of
6PPD, MIBK and the hydrogenolysis of ketimine,
20, 37
respectively.19,
In contrast, a higher selectivity (≥94.6%)
85 could be obtained over activated carbon supported palladium
sulfides catalysts under the same reaction condition but with a
longer reaction time (5ꢀ6 h). Furthermore, Fig.7 showed that the
binding energies of Pd 3d5/2 of PdxSy/C with different phase
structures (Table S2), featured at lower (336ꢀ336.8 eV) and
90 higher binding energy (337.2ꢀ337.9 eV), are attributed to Pd
atoms bonded with Sn&S6+ and S2ꢀ, respectively.28, 32 It is worth
50
In order to gain an understanding of the effect of the metalꢀ
support interaction, Pd/Al2O3, Pd/C and unsupported Pd particles
sulfided with H2S/H2 at 300 C were compared. It is found that
°
Pd4S/Al2O3, Pd4SꢀPd3S/C and unsupported Pd16S7 particles were
formed after the sulfidation (Fig. 5), which suggests that the
55 metalꢀsupport interaction indeed affected the formation of
palladium sulfides during the sulfidation process. According to
the PdꢀS bond strengths of palladium sulfides as demonstrated in
Section 3.2: “Pd16S7” > “Pd4S” > “Pd3S”, the catalysts with
Fig. 6 Reaction scheme for the synthesis of Nꢀ(1, 3ꢀdimethylbutyl)ꢀN'ꢀ
phenylꢀpꢀ phenylenedianine (6PPD)
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to note that the yield of 6PPD over PdxSy/C with different phase
structures was in the order: PdS/C ＞ Pd4S/C(150 °C) ＞
°
Pd4S/C(550 C) ＞Pd3S/C ＞ Pd16S7/C, which was consistent
with the order of the binding energy of Pd 3d5/2 (Fig.7 and Table
S2).
5
It is well known that the adsorption behaviour of substrates,
which is highly dependent on the electronic structure of the
surface of catalysts, plays an important role in the yield of
products.37 Sideꢀon via σꢀdonation and πꢀbackdonation (i.e.σ/πꢀ
10 adsorbed imine) are the most possible coordination states for the
adsorption of the C=N bond and the C=O bond of ketimine and
MIBK, respectively. Then, the activation energies of C=N bond
formation and C=O bond cleavage are lowered due to the
presence of the favorable coordination states.38, 39 And the most
15 stable adsorption model for the phenyl group in the vicinity of the
C=N bond is a flatꢀlying state by coordinating the phenyl group
to the Pd surface, which also promotes the activation of phenyl
ring.38 The high yield to 6PPD obtained over catalysts can be thus
attributed to the relatively low backdonation from electron
20 deficient palladium atoms to the adsorbates, thereby reducing the
activation of phenyl ring, C=O bond and C=N bond.
Fig. 7 XPS spectra of PdxSy/C with different phase structures: (a) Pd4S/C
(sulfided at 150 °C in H2S/H2) (b) Pd3S/C (sulfided at 250 °C in H2S/H2)
(c) Pd4S/C (sulfided at 550 °C in H2S/H2) (d) Pd16S7/C (sulfided at 750 °C
55 in H2S/H2) (e) PdS/C (sulfided at 300 °C in H2S/ Ar)
Table 2 Catalytic performances of activated carbon supported palladium
sulfides and Pd/C catalysts in the synthesis of 6PPDa
S/%
Catalyst
T/h
C/%
Ketimine
4.1
ꢀꢀꢀ
ꢀꢀꢀ
0.2
0.5
6PPD
78.9
21.7
96.6
95.3
96.1
97.5
96.5
96.9
97.1
97.5
Pd/Cb
Pd/C
3
0.4
5
6
6
99.5
100
100
98.9
98.3
97.6
91
4. Conclusions
Pd4S/C(150 °C)
Pd4S/C(150 °C) Reused 2
Pd3S/C
A H2ꢀassisted sulfidation method for the preparation of
supported palladium sulfides with single crystal structures was
developed. “Pd4S”, “Pd3S” and “Pd16S7” supported on activated
Pd4S/C(550 °C)
Pd16S7/C
6
6
0.6
1.7
°
°
60 carbons were prepared in H2 atmosphere at 150 C (or 550 C),
°
°
PdS/C
PdS/C Reused 2
PdS/C Reused 3
6
6
6
100
100
99.7
ꢀꢀꢀ
0.1
0.2
250 C and 750 C, respectively. “PdS” supported on activated
°
carbon was formed in Ar atmosphere at 300 C. The results
indicated that sulfidation atmosphere and temperature are the
determining factors in the selective formation of palladium
65 sulfides phase supported on activated carbon. Moreover, in the
comparison of the sulfidation results of Pd/C, Pd/Al2O3 and
unsupported Pd particles, the PdꢀS bond strength of palladium
sulfides phase was found dependent on the interaction between
metal and support. The catalytic performances of PdxSy/C
70 catalysts are highly dependent upon their crystalline phase and
composition, and PdS/C showed a higher yield and stability than
other PdxSy/C catalysts in the synthesis of 6PPD.
a: Reaction conditions: 4ꢀaminodiphenylamine (ADA) 3.68 g, 4ꢀmethylꢀ2ꢀ
25 pentanone (MIBK) 10 mL, catalyst 0.05 g, 200 °C, 4 MPa.
b: Reaction b was carried out under the reaction conditions of 80 °C and 1
MPa.
°
In addition, since PdS/C and Pd4S/C (150 C) catalysts showed
30 better catalytic properties, their reusability in this reaction was
investigated under the same reaction conditions. ADA could not
be completely converted even after longer reaction time (6 h)
under the same reaction conditions and the yield of 6PPD was
°
decreased to 94.2% over Pd4S/C (150 C) catalyst. The phase
°
35 structure of the used Pd4S/C (150 C) catalyst did not change as
Acknowledgements:
compared to that of fresh one (Fig. S9), but XPS results (Fig. S10
and Table S3) shows that its lower (higher) binding energy of Pd
3d5/2 and surface S/Pd atomic ratio decreased to 335.9 eV (336.9
eV) and 0.28, respectively. It indicates that the high activity of
40 fresh Pd4S/C (150 °C) may be attributed to the high sulfur content
on the catalyst surface compared to that of other PdxSy/C
catalysts, because the surface sulfur atoms could increase the
dissociation rate of H2 on Pd atoms during the hydrogenation.10
On the other hand, PdS/C could be reused at least for three times
45 without any significant loss of catalytic activity and yield (≥97%).
Moreover, there was no change in the phase structure (Fig. S9)
and the binding energy of Pd 3d5/2 between fresh and used PdS/C
(Fig. S11 and Table S3). The better catalytic properties of PdS/C
This work was financially supported by National Natural
75 Science Foundation of China (NSFCꢀ20976164) and National
Basic Research Program of China (973 Program) (No.
2011CB710800). The authors would also like to express their
gratitude to Prof. Xiaoliang XU and Stephen Louis Romano for
their help improve the English of this paper.
80 Notes and references
State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology, Zhejiang University of Technology, Hangzhou 310014, PR
China. Fax/Tel: 86 0571 88320409. E-mail: xnli@zjut.edu.cn
†
Current address: Chemical Physics of Materials (Catalysis-Tribology),
°
catalyst than that of Pd4S/C (150 C) can thus be attributed to its
50 more stable surface electronic structure.
85 Université Libre de Bruxelles, Campus Plaine, CP 243, B-1050 Bruxelles,
Belgium
6
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DOI: 10.1039/C3TA12277H
Tailoring Supported Palladium Sulfides Catalysts Through H2-Assisted
Sulfidation with H2S
Wei Xu, Jun Ni, Qunfeng Zhang, Feng Feng, Yizhi Xiang and Xiaonian Li
*
State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,
Industrial Catalysis Institute, Zhejiang University of Technology, Hangzhou, P. O.
Box 310014, P.R. China.
Table of contents
Pd4S/C, Pd3S/C, Pd16S7/C and PdS/C catalysts were selectively prepared and the
structure-property relationships were established.