Effects of Pb dopant on structure and activity of Pd/K-OMS-2 catalysts for heterogeneous oxidative carbonylation of phenol

Article abstract of DOI:10.1016/j.catcom.2010.01.013

Palladium catalysts supported on Pb-cation-doped manganese oxide octahedral molecular sieves were prepared and used for heterogeneous oxidative carbonylation of phenol to diphenyl carbonate in the absence of homogeneous cocatalysts. The synthesized catalysts were characterized by ICP-AES, XRD and XPS techniques. The experimental results demonstrated that an enhanced activity was obtained when Pb²⁺ entered into the tunnels of cryptomelane by replacing K⁺ to form a new hollandite-type phase of Pb_2-xMn₈O₁₆. The promotion effect of Pb dopant on activity was ascribed to the increase of low-valence Mn³⁺ and OH group, which was favor of reoxidation of Pd⁰ to active Pd²⁺ species.

Full text of DOI:10.1016/j.catcom.2010.01.013

Catalysis Communications 11 (2010) 643–646
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Effects of Pb dopant on structure and activity of Pd/K-OMS-2 catalysts
for heterogeneous oxidative carbonylation of phenol
a,b
a
b
b
b
a,b,*
Xiaojun Yang , Jinyu Han , Zhiping Du , Hua Yuan , Fang Jin , Yuanxin Wu
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2009
Received in revised form 11 January 2010
Accepted 13 January 2010
Available online 18 January 2010
Palladium catalysts supported on Pb-cation-doped manganese oxide octahedral molecular sieves were
prepared and used for heterogeneous oxidative carbonylation of phenol to diphenyl carbonate in the
absence of homogeneous cocatalysts. The synthesized catalysts were characterized by ICP-AES, XRD
and XPS techniques. The experimental results demonstrated that an enhanced activity was obtained
2
+
+
when Pb entered into the tunnels of cryptomelane by replacing K to form a new hollandite-type phase
of Pb2ꢀxMn 16. The promotion effect of Pb dopant on activity was ascribed to the increase of low-
8
O
Keywords:
3+
0
2+
valence Mn and OH group, which was favor of reoxidation of Pd to active Pd species.
Diphenyl carbonate
Oxidative carbonylation
Cryptomelane
Ó 2010 Elsevier B.V. All rights reserved.
Hollandite-type manganese oxides
Pb dopant
1
. Introduction
in fact [10]. Although other single metal oxides, such as CuO [5],
CrO , MoO , OsO and RuO [2], were also chosen as redox cocata-
3
3
4
3
Diphenyl carbonate (DPC) is an important starting material for
producing polycarbonate. Several methods have been developed
to synthesize DPC [1]. Among them, oxidative carbonylation of
lysts, the activities were not satisfactory. The best yield was only
8.9% when using 5%Pd/C as catalyst and CuO as cocatalyst. Besides,
both the Pd catalyst and the Co cocatalyst were heterogenized and
proven to form an effective catalytic system using layered double
hydroxides as a support. Preliminary test showed a promising re-
sult as far as activity and Pd leaching is less than 5% [7]. Our re-
2
phenol with CO and O is a simple and environmentally benign
process that avoids the use of phosgene. In this catalytic reaction,
two or more metallic and/or organic homogeneous cocatalysts
should be added into the system in order to promote the reoxida-
search group placed emphasis on manganese oxides because only
0
0
2+
tion of reduced Pd because direct oxidation of Pd with gas oxygen
to regenerate active Pd2 species proved to be a slow process [2,3].
Although Pd complexes anchored on various supports have been
developed to facilitate the recovery of homogeneous palladium
catalysts [4–9], the problems in the separation and product refin-
ing still remain owing to the presence of homogeneous cocatalysts.
Therefore, some researchers attempted to use metal oxides instead
of homogeneous cocatalysts [2,4,5,10]. Insoluble metal oxides are
easily separated and the function of reoxidation of Pd0 is per-
formed simultaneously.
MnO
2
/Mn pair with potentials of between the potentials of
+
2+
0
+
0
Pd /Pd and O
idized by O
2
+ H /H
2
O pairs can oxidize Pd and then be reox-
0
4+
2+
2
. The Pd formed was oxidized with Mn to form Pd
2
+
2+
2
and Mn , and then Mn was reoxidized by O . The dual role of
manganese oxides as cocatalyst and support for the metallic parts
of the catalytic system led to an easy separation process and the
DPC yield achieved 10.7% [11].
As mentioned above, the activities of catalysts with metal oxi-
des as cocatalysts are inferior to those using homogeneous cocata-
lysts; nevertheless the advantage of easy separation for metal
oxide as cocatalysts is very attractive. Thus, an enhanced activity
is essential in order to substitute for homogeneous cocatalysts,
and introduction of other metal dopant into metal oxides is a pow-
erful method. In this work, Pb was doped into potassium-contain-
ing manganese oxide octahedral molecular sieves (K-OMS-2), on
which the active palladium species were loaded by a precipitation
method. The supported catalysts were characterized to clarify the
modification effects of Pb on structure and activity of heteroge-
neous catalysts.
Takagi and his coworkers discovered that PbO effectively pro-
moted the activity in the Pd/CAN(C H ) Br system and 9.65%
4 9 4
DPC yield was obtained in combination of CuO as a second cocat-
alyst [4]. But PbO was likely to react with phenol to form an organ-
ic lead compound, which meant it was a homogeneous cocatalyst
*
Corresponding author. Address: School of Chemical Engineering and Technol-
ogy, Tianjin University, Tianjin 300072, PR China.
E-mail address: wyx@mail.wit.edu.cn (Y.X. Wu).
1
566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.01.013

6
44
X. Yang et al. / Catalysis Communications 11 (2010) 643–646
2
. Experimental
was cooled and the products were taken out. The yields of final
products were determined by a capillary gas chromatography with
a FID detector and a SE-54 capillary column.
2.1. Catalyst preparation
Hydrothermal, reflux, solvent free and high temperature calci-
3
. Results and discussion
nation methods were used to synthesize K-OMS-2. The details of
preparation were given in the literature [12]. To prepare lead-cat-
ion-doped K-OMS-2, 7000 g K-OMS-2 prepared by the reflux meth-
3.1. Activity studies
3 2
od was impregnated with 70.0 ml of Pb(NO ) aqueous solutions
Table 1 shows the catalytic activities of Pd/K-OMS-2 and Pd/Pb-
with different concentrations. And then the mixture was stirred
for 24 h, followed by evaporation. The lead modified supports (de-
signed as Pb-OMS-2) were obtained by calcination in a muffle fur-
nace at 400–900 °C for 6 h.
OMS-2 catalysts for oxidative carbonylation of phenol. As listed in
Table 1, the activities of Pd/K-OMS-2 catalysts prepared by differ-
ent methods were almost the same (entries 1–4). The yields of
DPC varied in a narrow range of 7.5%–9.2%.
It can be also observed from Table 1 that the DPC yields of Pd/
Pb-OMS-2 catalysts changed with Pb/Mn mass ratios and calcina-
tion temperatures. When the Pb/Mn mass ratio was 3/10 and the
calcination temperature of Pb-OMS-2 was 700 °C, the DPC yield
was up to 18.1%, indicating the best catalytic activity. The yield
of Pd/Pb-OMS-2(C) was almost twice as much as that of undoped
Pd/K-OMS-2 catalysts.
0
.042 g PdCl
the pH value was adjusted to 1.0 with concentrated hydrochloric
acid. K-OMS-2 or Pb-OMS-2 was dipped into the PdCl solution.
.0% NaOH solution as a precipitant was dropped slowly till the
2
was dissolved in 50.0 ml aqueous solution when
2
5
pH value was kept at 9.5. After stirred for 2 h, the solid was sepa-
rated by filtration, then dried at 80 °C overnight and calcined at
3
00 °C for 3 h. The powdery catalysts prepared were labeled as
Pd/K-OMS-2 or Pd/Pb-OMS-2, respectively.
2.2. Characterization
3.2. XRD characterization
The phase identification and crystalline structure analysis were
Fig. 1 illustrates the results of XRD analysis for Pb-OMS-2 with
determined by X-ray diffraction (XRD) on a PANalytical X’Pert PRO
X-ray diffractometer with high-intensity Cu radiation
k = 1.54060 Å). The chemical shift and valence of element on sur-
face were studied by X-ray photoelectron spectra (XPS) in a Per-
kin–Elmer PHI 1600 ESCA system with Mg K X-ray radiation
1253.6 eV, 250 W). The bulk chemical composition of the samples
different Pb/Mn mass ratio at the calcination temperature of
00 °C. The reference pattern of K-OMS-2 synthesized by a reflux
method was given in Fig. 1a, corresponding to cryptomelane [12].
K
a
4
(
K-OMS-2 has a one-dimensional tunnel structure formed by 2 ꢁ 2
a
6
edge shared MnO octahedral chains and a general composition
(
8
KMn O16. After Pb was doped into K-OMS-2 with the Pb/Mn mass
was determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) on a Perkin Elmer ELAN DRC-e instrument.
ratio at 1/10, diffraction pattern of Pb-OMS-2(A1) sample (Fig. 1b)
was almost identical to that of K-OMS-2. When the Pb/Mn mass ra-
tio increased to 3/10, however, the intensity of the peak at
Samples were dissolved in the mixture of HCl and HNO
3
, and then
diluted with ultrapure water.
h = 37.5o was suppressed and the intensity at 2h = 28.7 was en-
o
2
hanced. Fig. 1c showed that the strongest peak appeared at
o
o
2.3. Activity test
2h = 28.7 , not at 37.5 . It indicated that a new hollandite-type
phase of Pb2ꢀxMn 16 generated through ion exchange in tunnels.
In the light of the results of activity studies shown in Table 1 (entry
6), the reason for increasing catalytic activity might be ascribed to
8
O
The oxidative carbonylation reaction was performed in a 250 ml
stainless steel autoclave equipped with a magnetic stirrer. A typical
reaction condition was as follows: phenol 47 g (0.5 mol), 4A molec-
ular sieves (MS) 2 g, tetrabutylammonium bromide (TBAB) 1 g
8
the formation of Pb2ꢀxMn O16 after the doping of Pb. When the Pb/
Mn mass ratio was over 3/10, the crystalline phase of
o
(
3 mmol) and Pd (in the catalyst) 0.09 mmol were introduced into
the autoclave. Then the autoclave was sealed and heated to 65 °C.
Subsequently, the gas mixture of CO and O (CO/O = 12/1 M ratio)
was charged. It must be noted that the conditions employed in this
procedure fell within the explosion range of CO in O . After the
Pb
appearance of Pb
CO and H O in air with Pb , which did not enter into tunnels.
Combined with the activity of Pd catalyst supported on Pb-OMS-
3 3
(CO )
2
(OH)
2
, as shown in Fig. 1d, was found at 2h = 35.6 . The
3
(CO (OH) was possibly due to the reaction of
3
)
2
2
2
+
2
2
2
2
2
2(A3) (entry 7), it was found that the formation of Pb
3 3 2 2
(CO ) (OH)
reaction lasted for 4 h at the pressure of 4.8 MPa, the autoclave
was detrimental to the catalytic activity.
Table 1
Catalytic activities of palladium catalysts supported on K-OMS-2 and Pb-OMS-2.
Support
Method of preparation
[Pb/Mn]b
Pdc%
DPC Yieldd (%)
1
2
3
4
5
6
7
8
9
0
K-OMS-2
Solvent free
Reflux
High temperature
Hydrothermal
Calcine at 400 °C, Pb/Mn = 1/10
Calcine at 400 °C, Pb/Mn = 3/10
Calcine at 400 °C, Pb/Mn = 5/10
Calcine at 550 °C, Pb/Mn = 3/10
Calcine at 700 °C, Pb/Mn = 3/10
Calcine at 850 °C, Pb/Mn = 3/10
4.92
4.88
4.92
4.79
4.99
4.87
5.00
4.95
4.95
4.89
9.2
8.7
8.0
7.5
10. 0
13.2
10.3
17.0
18.1
7.0
a
Pb-OMS-2(A1)
Pb-OMS-2(A2)
Pb-OMS-2(A3)
Pb-OMS-2(B)
Pb-OMS-2(C)
Pb-OMS-2(D)
0.106
0.321
0.546
0.332
0.311
0.320
1
a
b
c
Theoretical value of mass ratio of Pb to Mn.
Mass ratio of Pb to Mn detected by ICP-AES.
Actual loading of active Pd species in heterogeneous catalysts detected by ICP-AES.
DPC yield based on the amount of charged phenol.
d

X. Yang et al. / Catalysis Communications 11 (2010) 643–646
645
XRD was also undertaken to study the influence of calcination
temperature on crystal structure for the same Pb content (shown
in Fig. 2). At 550 °C, two groups of peaks were observed with the
location of the most intensive peaks at 2h of 28.7o and 32.7 ,
respectively, indicating that the presence of crystalline phase of a
with the results of catalytic activities shown in Table 1 (entries 9
and 10), it was concluded that effective support was not bixbyite,
but Pb2ꢀxMn O .
8 16
o
large amount of Pb2ꢀxMn
The peaks after 60 degrees were suppressed with the increase of
temperature because Pb2ꢀxMn 16 and bixbyite generated at a
8
O16 and a small amount of bixbyite.
8
O
high temperature. It was well known that K-OMS-2 was progres-
sively converted into bixbyite with the increase of temperature
[
13]. Therefore, the growth of bixbyite crystals was observed based
o
on the increasing intensity of the peak at 2h = 32.7 when the cal-
cined temperature increased to 700 °C. The majority of phase re-
mained Pb2ꢀxMn O16 and the best yield of DPC was obtained.
8
However, at 850 °C sharp diffraction peaks were detected, suggest-
ing a fully developed crystalline structure of bixbyite. Combined
Fig. 3. XPS spectra of Pd3d (a) K-OMS-2, (b) Pb-OMS-2(A1) and (c) Pb-OMS-2(A2).
Fig. 1. XRD patterns of doped catalysts with different mass ratio of Pb to Mn. (w)
KMn
Pb (CO
(A2) and (d) Pb-OMS-2(A3).
8
O
16
,
JCPDS file 29-1020; (5) Pb2ꢀxMn
8 16
O , JCPDS file 42-1349; (d)
(OH) , JCPDS file 13-0131. (a) K-OMS-2 (b) Pb-OMS-2(A1), (c) Pb-OMS-
3
3
)
2
2
2
Fig. 2. XRD patterns of doped catalysts at different calcination temperature. (w)
KMn 16, JCPDS file 29-1020; ( ) Pb2ꢀxMn 16, JCPDS file 42-1349; (Ç) bixbyite,
JCPDS file 10-0069. (a) K-OMS-2, (b) Pb-OMS-2(B), (c) Pb-OMS-2(C) and (d) Pb-
OMS-2(D).
8
O
r
8
O
Fig. 4. XPS spectra of Pb4f, Mn2p, O1s (a) K-OMS-2, (b) Pb-OMS-2(A1) and (c) Pb-
OMS-2(A2).

6
46
X. Yang et al. / Catalysis Communications 11 (2010) 643–646
oxygen species were presented in Fig. 4 O1s and the proportion
of different oxygen species derived from fitted O1s spectrum was
displayed in Table 2, which referring to the XPS results obtained
by Peluso [17]. From Table 2, it can be seen that two Pd/Pb-OMS-
ꢀ
2
samples possessed more molecular H
2
O and OH compared with
Pd/K-OMS-2. The literature reported that hollandite-type Pb-OMS-
should contain more molecular H O because it had more unoccu-
pied tunnel sites than cryptomelane-structure K-OMS-2 [15]. In
2
2
Fig. 5. Schematic diagram of transformation process of the crystal structure.
-
2-
3+
view of charge equilibrium, OH species replaced O with Mn
4
+
ꢀ
replacing Mn . As a result, the increases of OH species was ob-
served. On the one hand, H O was an inhibitor of the oxidative car-
2
Table 2
bonylation of phenol; on the other hand, OH groups may be active
Proportion of different oxygen species derived from fitted O1s spectrum.
*
oxygen species and hot oxygen-containing radicals like HOO gen-
Sample
O1s(eV)
FWHMa
%O
+
ꢀ
erated according to Eq. MnO
2
+ H + e ? MnOOH, which was
capable of oxidizing Pd [2]. Thus, a large proportion of OH groups
and a small proportion of inhibitor H O were possible to interpret
a
529.8
531.6
1.5
2.3
2.7
1.3
2.6
2.2
1.5
2.7
1.9
62.0 O2
33.3OH
ꢀ
ꢀ
0
K-OMS-2
2
533.5
2
4.7 H O
2ꢀ
the highest activity of Pd/Pb-OMS-2(A2) catalyst among three
samples.
b
529.2
531.2
49.2 O
ꢀ
Pb-OMS-2(A1)
33.4 OH
533.0
2
17.4 H O
c
529.3
531.5
46.7 O2
ꢀ
4
. Conclusion
ꢀ
Pb-OMS-2(A2)
42.7 OH
2
10.6 H O
533.3
The effects of Pb dopant on structure and activity of the palla-
a
Full width at half maximum.
dium catalyst have been investigated for heterogeneous oxidative
carbonylation of phenol to DPC without homogeneous cocatalyst.
The DPC yield increased to 18.1% after the doping of Pb. This sug-
3
.3. XPS characterization
8
gested that the new support of Pb2ꢀxMn O16 was responsible for
2
+
+
the enhanced catalytic performance. Pb replaced K in tunnels
XPS spectra showed further information on the structure of
through ion exchange, resulting in lattice strain and the changes
doped Pd/Pb-OMS-2 catalysts. Pd/K-OMS-2, Pd/Pb-OMS-2(A1)
and Pd/Pb-OMS-2(A2) were selected as target samples considering
interference of impure phase. Fig. 3 showed that no change in
binding energy of Pd species was observed. The Pd3d binding ener-
gies at 337.5 eV and 343.0 eV were almost the same before and
after the doping of Pb.
3+
of Mn valence and oxygen species. The increase of Mn and the
formation of more OH groups benefited the redox cycle between
active Pd species and Pb doped supports.
Acknowledgements
As summarized in Fig. 4, Pb4f7/2 and Pb4f5/2 of two doped cata-
lysts were centered at 137.8 eV and 142.8 eV, respectively. They
We are grateful for the financial support from Natural Science
Foundation of China (Grant No. 20906073) and Hubei Provincial
Natural Science Foundation of China (Grant No. 2008CDA009).
2
+
were attributed to Pb in oxides and the possible Pb configuration
in doped catalysts was Mn AOAPb or MnAOAPb [14]. On the basis
of Pb4f XPS spectrum and XRD analysis, we inferred that Pb en-
2
2
+
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+
to Pb in the entire process. The transformation process of Pb-
OMS-2 was schematically shown in Fig. 5.
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