Exploring the coordination confinement effect of divalent palladium/zero palladium doped polyaniline-networking: As an excellent-performance nanocomposite catalyst for C-C coupling reactions

Article abstract of DOI:10.1016/j.jcat.2020.02.021

A pre-formed catalyst Pd²⁺/PANI composite for C-C coupling reaction was synthesized by combining the self-stabilized dispersion polymerization method with the in-situ composite material. Experiments have confirmed that the relatively high reduced structure (75%) in the polyaniline carrier is more favorable for the coupling reaction. Raman spectroscopy, solid nuclear magnetic, and X-ray photoelectron spectroscopy were performed to characterize the structures. The pre-formed catalyst has uniform coordination of divalent palladium and nitrogen in different valence states of the carrier polyaniline, which shows a good synergistic effect in the catalytic Ullmann reaction, and greatly reduces the use of reducing agents such as hydrazine hydrate. Compared with other studies, we analyzed the catalytic reaction mechanism in detail through real-time online infrared and XPS characterization. The results show that the divalent palladium in the catalyst and the zero-valent palladium generated by the in-situ reaction synergistically promote the reaction, while the polyaniline support acts as a stabilizer and dispersant, which prevents the agglomeration of the metal particles and prolongs increased catalyst life. The prepared Pd²⁺/PANI composites will become the most attractive alternative to traditional organic materials due to their wide applicability, high catalytic activity, stable recycling and relatively low price. This work provides a new theoretical basis for the understanding of the essential driving force of PANI catalytic activity and the cognition of the micro mechanism of action.

Full text of DOI:10.1016/j.jcat.2020.02.021

Journal of Catalysis 384 (2020) 177–188
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Exploring the coordination confinement effect of divalent
palladium/zero palladium doped polyaniline-networking:
As an excellent-performance nanocomposite catalyst
for C-C coupling reactions
1
1
⇑
Gang Wang , Zhiqiang Wu , Yanping Liang, Wanyi Liu , Haijuan Zhan, Manrong Song, Yanyan Sun
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, National Demonstration Center for Experimental Chemistry Education, College
of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China
a r t i c l e i n f o
a b s t r a c t
A pre-formed catalyst Pd2+/PANI composite for C-C coupling reaction was synthesized by combining the
self-stabilized dispersion polymerization method with the in-situ composite material. Experiments have
confirmed that the relatively high reduced structure (75%) in the polyaniline carrier is more favorable for
the coupling reaction. Raman spectroscopy, solid nuclear magnetic, and X-ray photoelectron spec-
troscopy were performed to characterize the structures. The pre-formed catalyst has uniform coordina-
tion of divalent palladium and nitrogen in different valence states of the carrier polyaniline, which shows
a good synergistic effect in the catalytic Ullmann reaction, and greatly reduces the use of reducing agents
such as hydrazine hydrate. Compared with other studies, we analyzed the catalytic reaction mechanism
in detail through real-time online infrared and XPS characterization. The results show that the divalent
palladium in the catalyst and the zero-valent palladium generated by the in-situ reaction synergistically
promote the reaction, while the polyaniline support acts as a stabilizer and dispersant, which prevents
the agglomeration of the metal particles and prolongs increased catalyst life. The prepared Pd2+/PANI
composites will become the most attractive alternative to traditional organic materials due to their wide
applicability, high catalytic activity, stable recycling and relatively low price. This work provides a new
theoretical basis for the understanding of the essential driving force of PANI catalytic activity and the cog-
nition of the micro mechanism of action.
Article history:
Received 24 December 2019
Revised 21 February 2020
Accepted 24 February 2020
Keywords:
Coordination confinement effect
Palladium
c-c coupling reactions
Polyaniline
Dual active sites
Online infrared
Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction
The research shows that nitrogen atoms of different valence states
in polyaniline play a crucial role in the conductivity and catalytic
Palladium is one of the most effective catalysts for the forma-
tion of various carbon–carbon (C-C) bond reactions and is widely
used in the construction of natural products, pharmaceutical
inter-mediates, pesticides and advanced materials [1–5]. In recent
years, researchers have become more and more demanding for the
greening of reaction conditions, the repeated use of catalysts, and
the use of expensive organic ligands [5–7]. Among them, the
low-load precious metal palladium/polymer catalysts have become
the most promising strategy due to easy preparation, structural
diversity and good catalytic performance [8–11].
reaction of materials [15,16]. For example, recently Palatty group
have been confirmed polyaniline composites incorporating transi-
tion metals as a super capacitive material due to the formation of
metal-PANI hetero junctions [17]. Besides, The researchers used
different methods to synthesize Pd@PANI catalysts with different
morphologies and made some progress in the study of C-C cou-
pling reactions such as Heck and Suzuki reactions [18–21]. How-
ever, there are still many dis-advantages, such as the use of a
larger amount of catalyst and the use of toxic solvents. Researchers
pay more attention to the study of the synthesis of special mor-
phologies, and there are almost few systematic analyses of the
mechanism of action of metal palladium and polyaniline carriers.
In 2018, the Yu group first explored the effect of support of
polyaniline for palladium nanoparticles in the Ullmann reaction
[22]. They explain the relationship between the carrier and the
catalytic activity from the electronic effect of substituents on the
As we all know, polyaniline (PANI) has a variety of electronic
structures including oxidized, reduced, and eigenstates [12–14].
⇑
Corresponding author.
E-mail address: liuwy@nxu.edu.cn (W. Liu).
Gang Wang and Zhiqiang Wu contributed equally to this work.
1
https://doi.org/10.1016/j.jcat.2020.02.021
0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

178
G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
aniline group in the molecular perspective. It is also proposed that
the introduction of an electron-donating group on polyaniline is
more advantageous for improving the catalytic activity. It’s worth
noting that the role of nitrogen atoms in different valence states
in the PANI molecular chain. That is, the microscopic structure–ac-
tivity relationship of this nitrogen valence state with the palladium
valence state is not clear. In fact, the PANI-supported palladium cat-
alysts all begin with Pd (II), but the influence mechanism of the
coordination mechanism of divalent palladium/zero palladium
and PANI on the performance of the catalyst is not fully understood.
It makes the Pd/PANI material greatly hindered in practical applica-
tions. This is a subject that urgently needs to be studied and solved.
Here, we report divalent palladium/zero palladium doped
Polyaniline-Metal-networking nanocomposite as high perfor-
mance catalyst for C-C coupling reaction with a low-cost, efficient,
and easy-to-process. By in-situ reaction, using small molecular
acids and trace amounts of PdCl2 as a directing agent and dopant,
a well dispersed palladium/polyaniline preformed catalyst (Pre-
PdCI2/PANI) was obtained using self-stabilizing dispersion poly-
merization (SSDP). The experimental results show that the catalyst
can be applied to various C-C coupling reactions such as Heck,
Suzuki and Ullmann than other’s more greening and more effective
(Scheme 1). In particular, the weak organic base used in the Ull-
mann reaction, such as ethyldiisopropylamine, has a particularly,
synergistic effect with the catalyst pre-pared in the present study,
and can completely replace the auxiliary catalyst such as hydrazine
hydrate and the catalyst can be reused more than 8 times, and can
achieve a gram-level preparation reaction. More importantly, we
found that the higher the proportion of the reduced state of
polyaniline in the catalyst, the more favorable the catalytic reac-
tion to construct the C-C bond. Further, the coordination mode of
metal palladium, of an intrinsic structural unit in polyaniline and
the mechanism of electronic inter-action between them was
demonstrated by Raman and solid nuclear magnetic methods. In
addition, we tracked the Ullmann reaction for the first time
through an infrared real-time online detection system, and studied
the reaction process more care-fully and visually. It was found that
the divalent palladium in the catalyst has a synergistic effect with
the zero-valent palladium formed by the reaction, which together
promotes the progress of the reaction.
in the next round of reaction. Organic phase was collected, add sat-
urated salt water (2 mL), extracted with ethyl acetate (15 mL), then
the organic phase was dried with anhydrous Na2SO4. The product
was separated and purified by silica gel column chromatography
(eluent: petroleum ether).
3. Results and discussion
It should be noted that, low-temperature self-stabilizing disper-
sion polymerization (SSDP) method has been reported for the
preparation of highly conductive polyaniline molecular chains
[23–25], the microemulsion state is formed at a low temperature
using water and an incompatible organic solvent, and in-situ poly-
merization occurs at the aqueous/organic phase interface with ice
as a core. However, there are few reports that were using palla-
dium chloride as a Lewis acid directly doped with polyaniline by
the SSDP method. We have reported the PdCl2/PANI nano-
catalysts which preparation of in situ supported by using camphor
sulfonic acid as a directing agent [26]. This catalyst exhibits excel-
lent properties in reducing the nitro compound in the aqueous
phase. Based on our previous research, the PdCl2/PANI catalyst
(Pre-PdCI2/PANI) synthesized by in-situ SSDP reaction, using small
molecular acids such as hydrochloric acid and trace amounts of
PdCl2 as a directing agent and dopant. The prepared Pre-PdCI2/
PANI catalyst has obvious d-
p conjugation effect. Furthermore,
the prefabricated catalyst was partially reduced to zero-valent pal-
ladium by the in-situ reaction in the subsequent coupling reaction,
lead to the original type of divalent palladium and the new zero
valent palladium species were uniformly loaded on the PANI net-
work. And then, the new catalyst (Pd2+-Pd0/PANI) was obtained.
It is a catalyst with double active sites that can be recycled many
times (Scheme 2).
To start with, the Ullmann coupling reactions using aryl iodide,
and PdCl2/PANI Pre-catalyst was chosen as the model reaction.
Results showed PdCl2/PANI Pre-catalyst by in-situ easily forming
a new catalyst species with double active sites (Pd2+-Pd0/PANI)
and the biaryl product, leading to successful C-C bond formation.
Therefore, the solvent effect was tested initially, and it was surpris-
ing that the yield in DMF and DMSO was superior to that in other
previous findings (Table S1 and S2).
After obtaining the optimal reaction conditions, we investigated
the universality of the catalytic Ullmann reaction of PdCl2/PANI
catalyst. It was found that the substituent group on the benzene
ring has a certain influence on the reaction. A higher yield can be
obtained when the substituent group is an electron withdrawing
group such as a halogen or a cyano group (Table 1, Entries 2–4,
10). When the substituent group is an electron-donating group
such as methyl or methoxy, only a moderate yield can be obtained
(Table 1, Entries 7–8). At the same time, we found that the yield of
the para substituent is higher than the meta and ortho position.
This may be because when the substituent is an electron-
withdrawing group, the electron-withdrawing effect of the para-
substitution on the C-I bond is the strongest, so that the catalyst
is easily inserted into the C-I bond to cause a coupling reaction.
Next, we investigated the iodide aromatic hydrocarbons with dif-
ferent functional groups and found that the catalytic system has
good tolerance to ketone carbonyl, aliphatic and hydroxyl groups.
In addition, for the highly hindered substituent group, the corre-
sponding coupling product can also be obtained. For example, 2-
iodine can be reacted to produce 40% of the product. We are
pleased that when the substituent group is a heterocyclic iodothio-
phene, an excellent yield of 95% is obtained. Except for iodoben-
zene, PdCl2/PANI catalysts can also be applied to the coupling
bromobenzene, the yield is between 45% and 70%. The catalyst
has a high TON value and TOF value in the Ullmann reaction, and
2. Experimental
2.1. Preparation of the acid-PdCl2/PANI catalysts
Aniline (1.864 g, 20 mmol), acid (20 mmol), deionized water
(40 mL) and chloroform (50 mL) were added to the jacket reactor.
Mechanical stirring was opened and cooling was lowered until the
solution was white paste. PdCl2 (89.29 mg, 0.5 mmol), (NH4)2S208
(4.564 g, 20 mmol) and acid (20 mmol) were added to the reactor.
Continuous stirring until the solution was completely blue-black,
and static inversion was carried out at low temperature. Generated
PdCl2/PANI was isolated by centrifugalization, washed by deion-
ized water for 4 times and ethanol for 4 times, take 1 mol/L sodium
hydroxide (50 mL) for acid removal treatment, then wash with
water and ethanol to neutral, vacuum dried at 60 °C for 4 h, collec-
tion for use (Acid-PdCl2/PANI).
2.2. Typical procedure for the Ullmann coupling reaction
To a 15 mL reaction tube, ArI (1 mmol), (i-Pr)2NEt (2 mmol),
DMF (0.5 mL) and 5 mg of PdCl2/PANI were added. The reaction
mixture was stirred at 140 °C under air for 2.5 h. After the reaction,
the catalyst was isolated by centrifugalization, washed by deion-
ized water and ethanol, vacuum dried at 60 °C and then reused

G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
179
Scheme 1. Different synthetic methods of C-C coupling reactions.
Scheme 2. Configuration of the PdCl2/PANI Catalyst.
the TOF value is ten times larger than that of the same type of cat-
alysts in the literature.
In order to investigate the application of PdCl2/PANI catalyst in
other C-C coupling reactions, we applied the catalyst to the Suzuki
coupling reaction, the polyaromatic coupling reaction and the Heck
reaction. The Suzuki coupling reaction can achieve cross-coupling
of two aromatic compounds with different functional groups,
which makes up for the deficiency of Ullmann reaction; The pro-
duct of polyaromatic coupling reaction, polybiphenyl is the main
material for preparing liquid crystal; The Heck reaction is a key
step in the synthesis of many natural products and is widely used
in organic synthesis.
After simple optimization, the PdCl2/PANI catalyst was success-
fully applied to the above C-C coupling reaction, and the results are
shown in Table S3, Table S4 and Table S5. It can be seen from the
table that the PdCl2/PANI catalyst has excellent reactivity, both
electron-withdrawing and coelectron groups can obtain good
yields. At the same time, the PdCl2/PANI catalyst exhibits good
Next, we evaluated the cross-coupling reaction of the aryl
iodide using a PdCl2/PANI catalyst. As shown in Table 2, different
combinations of the two aromatic hydrocarbons showed different
selectivity. When iodobenzene reacts with an electron-donating
group (Entries 1 and 2), the product tends to Homo-coupling;
when reacted with an electron withdrawing group, the product
tends to the cross-coupling reaction. At the same time, we found
that the total yield of iodobenzene reacted with iodothiophene
was 81%, and 45% of the them was unsymmetrical product. The
results of reaction of different electron withdrawing groups (F, Cl,
Br,) with the electron donating group 4-OMe-Ph were shown at
entries 6–8. It was found that as the electron-withdrawing ability
of the substituent group was weakened, the selectivity of the
cross-coupling reaction was lowered (F: 45%, Cl: 44%, Br: 39%).

180
G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
Table 1
Ullmann coupling reactions of aryl iodide and aryl bromide over PdCl2/PANI catalystsa.
b
e
f
Entry
Ar
Time (h)
Yield (%)
TON
TOF (hÀ1
)
1
2
3
4
5
6
7
8
Ph
4-FC6H4
4-ClC6H4
4-BrC6H4
3-ClC6H4
2
85
80
85
85
85
30
67
76
70
82
55
63
40
61
35
95
1124
1058
1124
727
1124
396
886
873
793
1084
727
562 (50)
423
450
290
187
66
354
349
132
433
121
138
211
112
77
2.5
2.5
2.5
6
2-ClC6H4
6
4-MeC6H4
4-MeOC6H4
4-NO2C6H4
4-CNC6H4
4-MeCOC6H4
4-MeOOCC6H4
1-C10H7
2.5
2.5
6
2.5
6
6
2.5
6
9c
10
11c
12c
13
14c
15
16
833
529
674
463
4-OHC6H4
2-Cl-4-FC6H4
6
6
1256
209
17d
18d
19d
Ph
12
12
12
50
70
45
661
926
595
55
77
49
4-ClC6H4
4-MeOC6H4
Reaction Conditions: a1 mmol of ArI, 2 mmol of (i-Pr)2NEt, PdCl2/PANI (0.075 mol% Pd), 0.5 mL of DMF, 140 °C. bIsolated yields. cRecrystallization yield. d1 mmol ArBr, 1 mL
dioxane. eTurnover numbers (TONs) were calculated based on the ICP results and molar percentages of the catalysts. fTOF is the turnover frequency, represents the amount of
reactant that is converted per unit time by the unit active site.
Table 2
a.
Cross-coupling reactions of aryl iodide over PdCl2/PANI catalysts
b
Entry
Ar1
Ar2
Ratio (3:4:5)
Yield (%)
TON
1
2
3
4
Ph
Ph
Ph
Ph
4-OMe-Ph
4-Me-Ph
17:36:47
15:40:45
21:48:31
22:45:33
65
70
72
81
866
933
960
1080
4-NO2-Ph
S
5
6
7
8
4-OMe-Ph
4-OMe-Ph
4-OMe-Ph
4-OMe-Ph
4-Me-Ph
4-F-Ph
4-Cl-Ph
4-Br-Ph
20:34:46
28:45:27
27:44:29
28:39:33
61
78
74
74
813
1040
986
986
Reaction Conditions: a1 mmol of Ar1I, 1 mmol of Ar2I, 2 mmol of (i-Pr)2NEt, PdCl2/PANI (0.075 mol% Pd), 1 mL of DMF, 140 °C. bIsolated yields.
functional group tolerance in different reactions. And the use of
precise metal Pd is greatly reduced (Suzuki: 0.15 mol%, Heck:
0.07 mol%, Ullmann: 0.07 mol%) [27]. The solvent in the Suzuki
coupling reaction is ethanol, which is greener and cleaner than
toluene [28,29]. This is sufficient to demonstrate by using micro
acid molecules as the directing agent, the in situ synthesis PdCl2/
PANI material by interfacial polymerization at low temperature,
Because of the regulation of hydrochloric acid, the content of
reduced state in polyaniline is increased, and polyaniline acts as
the ligand and the cocatalyst to enhance the reactivity of metal pal-
ladium. So, the PdCl2/PANI material is a highly efficient, green, reu-
sable catalyst for the C-C coupling reaction.
experiment, we also considered the situation that green oxidant
(H2O2) [30] replaced (NH4)2S2O8. However, the prepared materials
are easy to dissolve in the reaction solvent, and exhibit low activity
in the coupling reaction, so we gave up further research and explo-
ration experiments. During the research, we found that using dif-
ferent acids in the initial preparation of the catalyst can control
the morphology of the catalyst, which will greatly affect the reac-
tivity of the catalyst, this is consistent with similar literature
reports [31,32]. Therefore, we use hydrochloric acid, camphor sul-
fonic acid, acetic acid, citric acid, pyromellitic acid as the initial
doping acid, and hope to regulate the morphology of polyaniline
during the growth process. About 8 h, the in-situ loading of Pd
and the polymerization of aniline were completed at a low temper-
ature. In order to study the difference in the intrinsic structure of
polyaniline, we used NaOH to deacidify the PdCl2/PANI material,
eliminating the influence of doping acid. The micro acid molecules
only act as a directing agent and a template. The loading of
palladium was measured by ICP-MS, and it was found that the
loading amount of metal palladium was between 1.0% and 1.6%
(Table 3).
3.1. Comparative analysis of the primary structure of catalysts
To shed light on that how the pre-PdCl2/PANI catalyst promotes
the C-C coupling reaction. It is of great importance and indispens-
ability to figure out its intrinsic structural characteristics and the
electronic effect between the complexes with double active sites
as active species. It should be noted that at the beginning of our

G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
181
Table 3
As shown in Fig. 3, blue is the benzene structure in polyaniline,
and green is the quinoid structure. The content of reduced and oxi-
dized states in polyaniline materials directly affects the morphol-
ogy and color of polyaniline materials. We can find that the PANI
materials prepared with different acids have different levels of oxi-
dation structure and reduction structure. Among the materials pre-
pared by using hydrochloric acid, the ratio of the reduced state to
the oxidized state is approximately 3:1. The content of reduced and
oxidized states in polyaniline materials directly affects the mor-
phology and color of polyaniline materials. It is well known that
materials of different structures will have different electron trans-
port properties. Our research proves the use of different acids as a
directing agent can regulate the content of reduced and oxidized
states in polyaniline, thereby affecting the morphology of the
Acid-PdCl2/PANI catalysts.
Preparation of PdCl2/PANI catalysts by different acids.a
b
Entry
Acid (1 mol/L)
Acid- PdCl2/PANI
Pd (wt.%)
1
2
3
4
5
6
HCl
PdCl2/PANI
1.6
1.5
1.2
1.0
1.3
Camphorsulfonic acid
Acetic acid
citric acid
Pyromellitic acid
HCl
CSA-PdCl2/PANI
AcA-PdCl2/PANI
CA-PdCl2/PANI
PA-PdCl2/PANI
Pd@PANI-H
9–10 [19,22]
a
PdCl2 (0.5 mmol), Ar-NH2 (20 mmol), acid (40 mmol), Chloroform (50 mL).
Weight content of palladium in the PdCl2/PANI catalysts was measured by ICP-MS
b
analysis.
The SEM and HRTEM image of PdCl2/PANI catalysts that pre-
pared by using different acid as schematically depicted in Fig. 1.
From the SEM image (a1, b1, c1, d1, e1), we see that using the dif-
ferent acids can directly affect the morphology and structure of the
polyaniline material. Due to the metal palladium is loaded in situ
during the polymerization process, the different growth modes of
aniline during the polymerization process can directly change the
coordination mode of metal palladium and nitrogen in PdCl2/PANI
catalysts. Different coordination structures can affect the reactivity
of the PdCl2/PANI catalysts. When using hydrochloric acid as a
directing agent, polyaniline grows into a porous thorn-like nano-
sphere (Fig. 1(a1)), the diameter is about 100–200 nm. It is found
to be a mesoporous material by BET test (pore diameter: 9.7 nm,
surface areas: 58.9 m2/g). It is seen from the HRTEM (Fig. 1(a2)
and (a3)) that the palladium particles are uniformly distributed
in the material, and the 111-plane spacing of the palladium is
about 0.238 nm. At the same, when the camphorsulfonic acid,
acetic acid, citric acid, and pyromellitic acid were used as the
directing agent, the polyaniline composites respectively exhibited
nano-mesh, flake, filament, and sponge-like structures [33–35].
Firstly, it can be found from Fig. 2 that the PdCl2/PANI materials
synthesized by different acid-directed are still retained the original
characteristics of polyaniline, the amorphous diffraction peak of
polyaniline appeared between 15.5° to 23.8°, but the crystallinity
of the polyaniline materials of different acid-directed synthetic
was significantly different. PANI synthesized from small inorganic
molecule hydrochloric acid as a guiding agent has a high degree of
crystallinity. This means that the PANI molecular chain grows
more orderly, and the dispersion of palladium is more uniform.
However, PANI prepared by using organic acids with larger molec-
ular weights has poorer crystallinity, which indicates that different
acids may cause different formation mechanisms of PANI molecu-
lar chains, further forming different morphological features. 40.0°
is the diffraction peak of the supported metal palladium. It can
be seen from the figure that the acetic acid, citric acid, and
pyromellitic acid directed synthetic materials exhibit sharper
peaks of palladium diffraction. The diffracted peaks of palladium
in the materials synthesized by hydrochloric acid and camphorsul-
fonic acid are relatively gentle. We already know that the content
of palladium in five different PdCl2/PANI materials is basically
equivalent from ICP-MS test. Combined with HRTEM analysis, we
believe that this is closely related to the degree of dispersion of
metallic palladium on the polyaniline carrier. It can be considered
that the metal palladium is uniformly dispersed on the PdCl2/PANI
material pre-pared by the directing of hydrochloric acid and cam-
phorsulfonic acid. Therefore, the materials have low crystallinity
and the diffraction peak in XRD is relatively gentle.
The above scientific view is further verified by the following
experiments. The catalytic activity tested results for the different
acid-directed synthetic PdCl2/PANI catalysts are shown in Fig. 4.
It is found that different PdCl2/PANI materials have significant dif-
ferences in the performance of catalytic C-C coupling reaction:
PdCl2/PANI
> PA-PdCl2/PANI > CA-PdCl2/PANI > CSA-PdCl2/
PANI > AcA-PdCl2/PANI. It is a particularly valuable finding, the
higher the content of the reduced structure in Acid-PdCl2/PANI
materials, and the higher the activity of the catalysts. It can be
found that the reduced structure of the polyaniline is more benefi-
cial to improve the catalytic activity of the Acid- PdCl2/PANI mate-
rials. The possible reason is that the C-N structure in the reduced
state of polyaniline is easier to combine with the active center
metal palladium than the C = N structure in the oxidation state.
In addition, we found that the use PdCl2 to catalyze the iodoben-
zene coupling reaction can get the yield at 68% from Fig. 4. Only
59% yield can be obtained by using AcA-PdCl2/PANI catalyst, but
the 95% yield can be achieved by using the HCl-PdCl2/PANI cata-
lyst. This indicates that the polyaniline carrier in different states
has a great influence on the activity of the catalyst. Because the
polyaniline carrier synthesized by acetic acid has a lower content
of reduced state and the relatively higher content of oxidation
state, which will inhibit the activity of metal palladium to some
extent. The PdCl2/PANI catalyst synthesized by using hydrochloric
acid as the directing agent has a higher content of reduced state,
the catalytic activity is higher, and can greatly improve the yield
of the C-C coupling reactions.
Then, we investigated the recyclability of the catalyst in the Ull-
mann reaction and the yield after the amplification of the reaction
using iodobenzene as the raw material. It was found that the cata-
lyst after repeated use for 8 times still has high reactivity. This may
be because the catalyst we prepared uses an in-situ loading
method in which the metal palladium is coordinated with the
nitrogen atom on the aniline, so that the metal palladium is firmly
supported on the polyaniline carrier. At the same time, polyaniline
has excellent electronic conduction and capacitance properties,
which can synergistically promote the reactivity of metal palla-
dium in the reaction. From the thermogravimetric analysis of the
catalyst (Fig. 5b), it can be found that the polyaniline material is
relatively stable below 300 °C [38]. Therefore, when catalyzing
the Ullmann reaction, the temperature reaches 140 °C, the bulk
structure of polyaniline does not change due to heat. At the same
time, we found that the spatial arrangement of polyaniline may
change due to the coordination of palladium with the support, so
the pyrolysis temperature is reduced from 400 °C to 300 °C. How-
ever, as the process progresses, the pyrolysis temperature returns
to 400 °C. Fig. 5(c–f) is the SEM and HRTEM of the catalyst after
repeated use 8 times. It can be seen that the original porous thorn
structure of the catalyst changed during the repeated use from
Fig. 5(c), but the metal palladium is still well dispersed on the
polyaniline carrier, and the dark particles in the HRTEM should
We used a Gaussian function to fit the infrared absorption spec-
tra of five different acid-directed synthetic PdCl2/PANI materials in
the range of 1300–1700 cmÀ1. 1584 cmÀ1 is a C = C, C = N stretch-
ing vibration peak of a quinoid ring structure,1502 cmÀ1 is a C = C,
C-N stretching vibration peak of a benzene ring structure [36,37].

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Fig. 1. SEM and HRTEM image of different Acid-PdCl2/PANI catalysts: (a1-a3) PdCl2/PANI catalyst; (b1-b3) CSA-PdCl2/PANI; (c1-c3) AcA-PdCl2/PANI; (d1-d3) CA-PdCl2/PANI;
(e1-e3) PA-PdCl2/PANI.
Fig. 2. XRD image of different Acid-PdCl2/PANI catalysts.
be zero-valent palladium (Figure S8). When the reaction is
increased to 40 mmol, the excellent yield of 85% can still be
obtained (Scheme 3). In addition, we studied the dissolution of
the catalyst and the loss of metal through the ICP-MS test, the
results are shown in Table S7. We found that the PdCl2/PANI cata-
lyst is relatively stable, and the average metal loss rate in the mate-
rial is less than 5%, which may be one of the reasons for the good
recycling performance of the catalyst. From the experimental

G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
183
Fig. 3. Infrared absorption spectra of different Acid-PdCl2/PANI catalysts at 1300–1700 cmÀ1.
Fig. 4. The Catalytic activity of C-C coupling reaction by using different PdCl2/PANI catalysts.

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G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
Fig. 5. Reuse of PdCl2/PANI catalyst.
polyaniline material acts not only as a carrier but also as a ligand
for the metal. The Pd and polyaniline are bonded together by a
coordinate bond. In addition, the metal palladium in the fresh
material mainly exists in a divalent form (Fig. 6(a2)). It can be seen
from Fig. 6(a3) that the PdCl2/PANI catalyst after repeated use 8
times, the peak of the = N-bond (398.3 eV) substantially disap-
pears, which may be that the oxidation state of the polyaniline is
converted to reduced state during the reaction. And it is also
proved that the complex of reduced nitrogen and palladium is
more stable, so palladium mainly forms the active center with
the reduced polyaniline.
The structure activity relationship can be further confirmed by
FT-IR test (Figure S5). After the reaction, the amino peak at
3386 cmÀ1 is enhanced, and the peaks of 1495 cmÀ1 and
1584 cmÀ1 are shifted toward the long wavelength (1495 cmÀ1
to 1502 cmÀ1, 1584 cmÀ1 to 1595 cmÀ1). From the XPS, it can be
found that the peak of the N-Pd bond at 399.8 eV still exists, which
further confirms our concept. In the PdCl2/PANI catalyst, the metal
palladium is bound by a relatively strong N-Pd bond, so the higher
content of the reduced state in polyaniline, the more favorable to
form N-Pd bonds. This is one of the reasons why the catalyst can
be used stably 8 times instead of deactivated. It can also be found
from the XPS of Pd 3d of the catalyst after repeated use that 76% of
the original divalent metal palladium was reduced to zero valences
during the reaction, but 24% of the divalent palladium is still pre-
sent in the catalyst after repeated using 8 times. Furthermore, we
used the enough hydrazine hydrate to reduce the PdCl2/PANI cata-
lyst. Then, put the zero-valent catalyst into the reaction. Finally,
only a yield of 78% was obtained, and this is obviously lower than
the yield of the catalyst greater than 85%. This gives us reason to
Scheme 3. The gram-scale preparation reaction of biphenyl.
results, it can be inferred that the PANI polymer carrier prepared
by the SSDP method has a high molecular weight and polymeriza-
tion degree [39], and therefore it can be used in a strongly polar
DMF solution at 140 °C for many times without being dissolved.
This surprising discovery has made PdCl2/PANI catalysts promising
for wider application in practical organic synthesis production.
3.2. Comparative analysis of micro-coordination structure of catalyst
The Pre-PdCl2/PANI catalyst exhibits excellent catalytic activity
in various C-C coupling reactions. In addition to the macroscopic
primary structure of the catalyst, we are more interested in the
micro-coordination structure of the trace palladium element, the
mechanism of the interaction between the two active sites of diva-
lent palladium and zero-valent Palladium produced by in situ cou-
pling reaction. Fig. 6(a1) is an analytical diagram of N 1 s in the
fresh material. The peak of N 1 s can be resolved into four main
peaks: 399.9 eV, 399.3 eV, 398.7 eV, 398.1 eV; line width is:
1.13 eV, 0.92 eV, 0.83 eV, 0.88 eV. The peaks of 398.2 eV and
399.3 eV correspond to the C = N-C and C-NH-C structures, respec-
tively. The peaks of 398.7 eV and 399.9 eV are formed by coordina-
tion of N and Pd [10,40,41]. This further indicates that the

G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
185
Fig. 6. (a1) and (a2) the XPS spectra of fresh PdCl2/PANI catalysts; (a3) and (a4) the XPS spectra of 8-recycled PdCl2/PANI catalysts; (b) Raman spectroscopy of catalysts; (c) 13
C
CP-MAS-NMR spectra of catalysts; (d) the possible coordination model for catalysts.
believe that zero-valent palladium and divalent palladium act
together as a catalyst during the catalytic reaction. A part of the
divalent palladium and the polyaniline are stably combined by
the N-Pd bond, and the zero-valent palladium is reversibly gener-
ated during the reaction. Therefore, the catalysts we prepared
showed excellent catalytic activity and stability in the Ullmann
reaction and others.
In order to further expand the catalytic properties of the mate-
rial, we also studied the Raman spectral of the catalyst at a wave-
length of 785 nm, and the results are shown in Fig. 6b [11,42]. The
Ring deflection or C-H wag at 424 nm; Quinoid deflection or C-N-C
torsion at 530 nm; Quinoid ring bend and C-H (out-of-plane) are at
758 nm and 789 nm; Benzenoid ring and quinoid ring deflection at
852 nm; The C-H Bending benzenoid (quinoid) and C-N stretching
are at 1165 nm and 1222 nm; C-N+ stretching at 1340 nm; C = N
quinoid ring stretching at 1472 nm; C = C stretching and C-C
stretching at 1562 nm and 1596 nm. It can be clearly seen that
when the palladium is loaded with the polyaniline carrier, the peak
at 1340 nm is weakened, which is attributed to the coordination of
the metal palladium with the nitrogen of the amine structure to
form an N-Pd bond. The variation in the spectrum from 400 nm
to 1000 nm indicates that the palladium and PANI molecular
chains enhance the electron cloud density of the polyaniline mole-
cules by complex coordination, and at the same time prevent the
agglomeration of the active palladium catalytic sites and inacti-
vate. When the catalyst was repeatedly used for 8 times, a part
of the palladium was reduced to zero valence, and the coordination
of the zero-valent palladium was weakened compared with the
divalent palladium, and a part of the C-N+ structure was released
again, so that the peak at 1340 nm was enhanced.
And finally, we can find from 13C CP-MAS-NMR spectra of cata-
lysts (Fig. 6(c)) that when palladium is complexing-doping onto
polyaniline, the carbon on the benzene structure is coordinated
with palladium, so the peak center moves toward the low wave-
number (124.6 ppm to 122.9 ppm) [43–46]. Simultaneously, the
carbon attached to the imine nitrogen in the quinoid structure of
polyaniline is weakened at the peak of 147.9 ppm due to the weak
coordination of palladium. In the material after repeated use for 8
times, since part of the palladium was reduced to zero valence, the
C-Pd coordination was weakened, and the peak originally shifted
to 122.9 ppm recovered to 124.6 ppm. In addition, during the reac-
tion, the polyaniline carrier itself also changed, the quinoid struc-
ture was reduced to a benzene structure, so the carbon peak of
the benzene structure at 138.0 ppm was enhanced. From the 13C
NMR test results and compared with the polyaniline materials
reported in the literature, [47–50] it can be speculated that the

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G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
PdCl2/PANI catalyst prepared by the low-temperature SSDP
method has the high molecular weight (Mw greater than 15000)
and polymerization degree.
Based on the above analysis, we speculate that the possible
coordination model of polyaniline and palladium is shown in
Fig. 6d [51–53]. Combined with previous studies, we believe that
molecular chain. The partial divalent palladium is reduced to the
zero-valent element palladium in subsequent catalytic reactions.
The zero-valent palladium appears in the form of nanometers
and exhibits an effect of promoting the reaction. The electron effect
mechanism of catalysis is confirmed in the following experiments
for the first time.
the original structural form of PdCl2/PANI is
a
networked
Fig. 7(a) shows the online infrared spectrum of the reaction at
different time periods. We found that the characteristic peak of
the iodobenzene reactant gradually began to decrease with time,
and the characteristic peak of the product began to increase grad-
ually. We monitored the trends of the reactants and products with
typical characteristic peaks of 688.9 cmÀ1 and 703.8 cmÀ1, respec-
tively. It can be seen from Fig. 7(b) that the product was not formed
immediately after the feedstock was added and heated to 140 °C.
There is approximately 30 min of response time, during this pro-
cess, the peak intensity of the reactants is weakened. We believe
that the reactants are firstly adsorbed on the catalyst and adsorp-
tion activation is mainly through the complex dynamics and ther-
modynamics of electron conduction. The divalent palladium may
be reduced to zero in this process, and part of the palladium has
been inserted into the middle of the C-I bond. Therefore, after this
period the product began to increase linearly, and after 100 min,
the reaction basically ended. The changes in the infrared peaks at
the 652–775 nm and 897–1322 nm bands over time are shown
in Fig. 7c and d. From the three-dimensional spectrum, we can
clearly observe the changes in the individual peaks.
supramolecular organometallic complex of PANI. Compared with
other non-macromolecular imine ligands, the polyaniline as a
macromolecular ligand has coordination between layers and
between molecular chains, and forming a coordination form simi-
lar to a clamp compound in a localized range. During the catalytic
reaction, part of the divalent palladium is reduced to zero valence,
and divalent palladium and zero-valent palladium act together to
catalyze. At the same time, the quinoid structure in the polyaniline
structure is also reduced to the benzene structure. It is precise
because polyaniline has the controllable properties of this elec-
tronic structure, which inhibits the agglomeration inactivation of
zero-valent palladium to some extent. The polyaniline carrier acts
as a dispersant and stabilizer for the metal palladium, and func-
tions as a nitrogen-containing ligand in the catalytic reaction.
So far, empirical research on the mechanism of palladium cat-
alytic reaction process is still lacking. Here, we firstly report infra-
red online monitoring of iodobenzene coupling reaction as a model
for response. The result shows that in the catalytic Ullmann cou-
pling reaction, PdCl2/PANI catalyst forms a micro-emulsion with
a polar and highly soluble solvent DMF (the solvent used in the
reaction), which has the characteristics of semi-homogeneous
catalysis. As mentioned above, the low-loaded divalent palladium
is uniformly coordinated with the two nitrogen species in the
reduced and oxidized states (the ratio of about 3:1) in the PANI
Little has been revealed about the mechanism of action of sol-
vents or cocatalysts in such reactions, making it difficult to deeply
understand the mechanism of such reactions. To this end, we have
further studied the effects of solvents and bases on the reaction.
The action of the base and the solvent during the reaction can be
Fig. 7. Infrared online monitoring of iodobenzene coupling reaction.

G. Wang et al. / Journal of Catalysis 384 (2020) 177–188
187
Fig. 8. A possible mechanism for Ullmann coupling reaction over PdCl2/PANI Catalyst.
con-firmed by XPS test analysis of the catalyst after the reaction.
Figure S6 shows the X-ray photoelectron spectroscopy of Pd3d in
the initially prepared catalyst. We can see that the metal palladium
exists in the form of divalent in the PdCl2/PANI catalyst before the
reaction. However, in the catalyst after repeated use for six times,
Pd mainly exists in a zero-valent valence state. In the well-
recognized Ullmann reaction mechanism, the first step is to reduce
the divalent palladium to zero by using a reducing agent, and then
only the zero-valent palladium can be oxidized and inserted into
the middle of the carbon-halogen bond to allow the reaction to
proceed.
However, it’s worth noting that PdCl2/PANI pre-catalyst does
not require the addition of additional reducing agent during the
reaction. Therefore, we noticed that diisopropylethylamine itself
also has a certain reducing power [54], the zero-valent palladium
produced by the reduction during the reaction is the real catalyti-
cally active substance. From Table S2 entries 5–8, it can be found
that when inorganic base is used, the reaction yield is extremely
low. This is because the inorganic base cannot effectively reduce
the divalent palladium to zero, so the reaction cannot continue.
This view can be confirmed by XPS (Figure S6). Using sodium car-
bonate as the base and DMF as the solvent, after the reaction, the
catalyst was subjected to XPS test. We found that only divalent pal-
ladium was present in the material. Similarly, in the strong polar
protic solvent, diisopropylethylamine cannot play a reducing role,
leading to a decrease in reaction yield. Compared with other
polyaniline-supported palladium catalysts, the PdCl2/PANI catalyst
is prepared by in-situ low-temperature polymerization, due to the
regulation of hydrochloric acid and PdCl2, the polyaniline material
has a higher content of reduced N state, and the C-N bond is more
favorable for coordination with metal palladium, that the activity
of the metal palladium is enhanced. Therefore, the reaction can
be continued only by relying on the weak reducing ability of diiso-
propylethylamine without using other auxiliaries such as hydra-
zine. Combining the literature [7,55,56] and the result of
conditions in Table S1, we also propose that the possible reaction
mechanism of the PdCl2/PANI catalysts in Ullmann reaction is
shown in Fig. 8.
4. Conclusions
In summary, we in situ prepared the Pr-PdCl2/PANI catalyst by
the SSDP method using small molecular acids such as hydrochloric
acid and trace amounts of PdCl2 as a directing agent and dopant.
Palladium and aniline form a unique nanometer array through

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sites in subsequent reactions. Subsequent research demonstrates
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The catalyst exhibits excellent catalytic performance in various C-C
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can be reused more than 8 times, and can achieve the preparation
of a gram-level reaction.
Besides, the coordination mechanism of metal palladium and
polyaniline carrier was analyzed in detail by means of Raman spec-
troscopy and solid nuclear magnetic characterization. The palla-
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coordinated with nitrogen in polyaniline. During the catalytic reac-
tion, part of palladium was reduced to zero valence, and the qui-
none structure in the polyaniline carrier was also reduced to
benzene structure. It is precisely because of this unique structural
change of the polyaniline carrier that the agglomeration of the
metal palladium is prevented to some extent, and the divalent pal-
ladium in the material together with the zero-valent palladium
formed by the reaction promotes the progress of the reaction. In
addition, the reaction mechanism of Ullmann was explored
through the infrared real-time online detection system, which
illustrated the changes in the reaction process more intuitively
and visually, this also opens new prospects for a better under-
standing of reaction mechanisms. The above research is expected
to provide new design and synthesis strategies for polymer-
metal composites in other field of catalysis. It is worth mentioning
that research on designing and preparing M@PANI catalysts with
better performance by changing different oxidants, preparation
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Gang Wang and Zhiqiang Wu contributed equally to this work.
Notes
The authors declare no competing financial interest.
Acknowledgment
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This work was financially supported by the Major Innovation
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
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