Ultrafine and Highly Dispersed Pd/SiO2 for Suzuki?Miyaura Cross-coupling Reactions

Article abstract of DOI:10.1007/s10562-020-03465-9

Abstract: Construction of heterogeneous Pd/SiO₂ catalyst via the pollution-free strategy marked strong electrostatic adsorption has been reported for the application to Suzuki–Miyaura cross-coupling reactions. The exposed negatively charged oxygen groups, which were converted from the hydroxyl groups on the surface of silica under the alkaline atmosphere, could effectively anchor palladium species to form ultrafine Pd nanoparticles (Pd NPs) with an average particle size of 1.3?nm and high dispersion (43%). Pd/SiO₂ catalyst was endowed with the excellent catalytic performance which was that the yield of the Suzuki–Miyaura reaction between bromobenzene and phenylboronic acid at 40?°C was > 99% for 30?min and the TOF was ~ 80,000?h^?1. The catalyst could be easily recovered and recycled by facile procedure without a significant decrease in catalytic activity, which was able to maintain the 90% yield after repeated for 8 times. In addition, a continuous flow reaction device was designed using the Pd/SiO₂ catalyst to effectively improve the production efficiency of biphenyl. Graphic Abstract: Pd/SiO₂ catalyst constructed with the strategy of strong electrostatic adsorption (SEA) possesses uniformly dispersed and highly exposed Pd sites which can be easily transformed into electron-deficient Pd^δ+ and strengthened stability for itself due to its strong interaction with the front surface of the carrier, and has been endowed the outstanding catalytic performance for Suzuki?Miyaura cross-coupling reaction. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03465-9

Catalysis Letters
https://doi.org/10.1007/s10562-020-03465-9
Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura
Cross‑coupling Reactions
Xizheng Fan1 · Jingyi Yang1 · Qingqing Pang1 · Zhongyi Liu1 · Panke Zhang1 · Jing‑He Yang1,2
Received: 20 September 2020 / Accepted: 12 November 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Construction of heterogeneous Pd/SiO2 catalyst via the pollution-free strategy marked strong electrostatic adsorption has
been reported for the application to Suzuki–Miyaura cross-coupling reactions. The exposed negatively charged oxygen
groups, which were converted from the hydroxyl groups on the surface of silica under the alkaline atmosphere, could efec-
tively anchor palladium species to form ultrafne Pd nanoparticles (Pd NPs) with an average particle size of 1.3 nm and
high dispersion (43%). Pd/SiO2 catalyst was endowed with the excellent catalytic performance which was that the yield of
the Suzuki–Miyaura reaction between bromobenzene and phenylboronic acid at 40 °C was>99% for 30 min and the TOF
was~80,000 h−1. The catalyst could be easily recovered and recycled by facile procedure without a signifcant decrease in
catalytic activity, which was able to maintain the 90% yield after repeated for 8 times. In addition, a continuous fow reaction
device was designed using the Pd/SiO2 catalyst to efectively improve the production efciency of biphenyl.
Graphic Abstract
Pd/SiO2 catalyst constructed with the strategy of strong electrostatic adsorption (SEA) possesses uniformly dispersed and
highly exposed Pd sites which can be easily transformed into electron-defcient Pdδ+ and strengthened stability for itself due
Xizheng Fan and Jingyi Yang have equally contributed to the
work.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03465-9) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

X. Fan et al.
to its strong interaction with the front surface of the carrier, and has been endowed the outstanding catalytic performance
for Suzuki−Miyaura cross-coupling reaction.
Keywords Pd/SiO2 · Heterogeneous catalyst · Strong electrostatic adsorption · Suzuki–miyaura
size and dispersion of the supported Pd nanoparticles (Pd
NPs) are the primary factors afecting the activity, which
have been well documented by references [18, 19]. How-
ever, Pd clusters having high surface energy, are prone to
agglomeration with poor stability. It’s difcult to carry out
size-controlled synthesis for the particles on the surface
of the support. In general, the solution to such methods is
mainly to use surfactants, organic ligands and grafting on
the carrier during the preparation process, increasing costs
and difculties and the risk of polluting the environment
[20]. Therefore, an imperious demand has emerged for new
strategy that can manufacture highly active Pd catalyst which
can provide enjoyable results in the green solvent system by
using benign chemical additives, and is easily recycled and
cost-efective.
1 Introduction
The formation of carbon–carbon bonds is fundamentally
important reaction of synthetic organic chemistry, due to
the widespread bonds in lots of natural products, pharma-
ceuticals, bio-active molecules and advanced materials.
Among the array of methods developed over the years,
Suzuki–Miyaura cross-coupling reactions have been docu-
mented as the straightforward and efcient approach for the
preparation of carbon–carbon bonds, with mild conditions,
satisfactory selectivity, facile operation and extensive toler-
ance of functional groups [1–6]. Therefore, Suzuki–Miyaura
cross-coupling reactions have occupied more special posi-
tion based upon its versatility, compatibility and key contri-
butions to various disciplines including organic compounds
synthesis, materials science and drug synthesis, which is
publicly recognized for the efect to the rapid development
in the feld of chemical synthesis [7–9].
The Regalbuto etc. has developed the strong electro-
static adsorption (SEA) method (oxide surfaces become
protonated or deprotonated to adsorb such anions or cati-
ons) which brings the uplifting news for the preparation of
highly active heterogeneous nano-catalysts [21–24]. Com-
pared with the methods of traditional wet impregnation or
equal volume impregnation, the convenient and low-cost
SEA strategy can efectively solve the disadvantages of large
average size and wide size distribution without any explicit
surface functionalization and capping agents. So we pre-
pared palladium-loaded silica catalyst (Pd/SiO2) including
the superfne size (about 1.3 nm average value) and uniform
The variety and activity of metal catalysts are the main
factors for Suzuki–Miyaura cross-coupling reactions. Palla-
dium as a high-efciency catalyst can provide higher conver-
sion constant, which can be regulated to improve its catalytic
performance through the choice of ligands, surfactants or
carriers [10–15]. As the development of robust heterogene-
ous catalysts, multitudinous eforts have been focused on
nano-composite containing Pd to obtain the development
of more satisfactory performance [16, 17]. Obviously, the
1 3

Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura Cross-coupling Reactions
dispersion of Pd by SEA method. It is employed as catalyst
for Suzuki–Miyaura reaction, which can aford excellent
catalytic activity with highlighted advantages of structural,
to remarkably improve to 99.5% yield adding 0.05 mmol%
catalyst in the mild conditions. And it can be reused up 8
times with the robust catalytic performance, and further pro-
moted to practical application feld.
water were used as the reaction solvent, and ethyl acetate
(Aladdin, 99.99%) was as an extractant. The whole experi-
ment was done with deionized water. The same metal pre-
cursor and silica support were used for catalysts prepared by
the methods of strong electrostatic adsorption (SEA) and dry
impregnation (DI).
2.2 S‑PdX/SiO2 Synthesis by SEA
2 Experimental Section
SEA catalyst was prepared by controlling the pH relative
to the surface point of zero charge (PZC) which decided
what kind of precursors to be used. The convenient and
reliable synthesis process for the Pd/SiO2 catalysts are
exhibited in Scheme 1 [22], and briefy described as fol-
lows. Tetraammine palladium chloride (1.26 g) as precur-
sor, was dissolved in 1000 mL of deionized water under
the ambient temperature for the whole operation. Based on
the theoretical loading of Pd species, the 9.70 mL above
solution and 290.30 mL deionized water were added into a
500 mL beaker. Then the pH of the precursor solutions was
adjusted to 11.5 using ammonium hydroxide. 1.0 g silica
was dispersed into the above liquid phase, then rapidly mag-
netic stirring for 1.0 h at room temperature to obtain the
homogeneous white gelatum. During this period, the pH of
above system was measured with a pH meter every 10 min,
and ensured that the pH was maintained at 11.5 by continu-
ously adding ammonium hydroxide. Then, the gelatum was
centrifuged and washed three times with deionized water
to remove redundant ammonium hydroxide. And the col-
lected precipitate was dried in a vacuum oven at 60 °C to
get the bulk named Pd2+0.8/SiO2. The Pd2+0.8/SiO2 ground
2.1 Chemicals
Aladdin Aerosil 300 (hydrophilic type, BET SA-300 m2/g,
Point zero charge = 4.2) was used as the support of cata-
lysts. Tetraamminepalladium (II) chloride monohydrate
(Pd(NH3)4Cl2, Pd ≥ 41%, Adamas-beta) was used as the
cationic precursor salt for the catalyst preparation. The sub-
strates (all are more than 99.00% pure) of Suzuki–Miyaura
cross-coupling reaction including aryl halides (TCI and
McLean), 2-bromine naphthalene (McLean), areneboronic
acids (TCI and McLean), etc., were directly used after pur-
chase without further purifcation. Base reagents contained
sodium carbonate (Na2CO3, Sinopharm), potassium car-
bonate (K2CO3, Sinopharm), cesium carbonate (Cs2CO3,
McLean), sodium bicarbonate (NaHCO3, Sinopharm),
sodium hydroxide (NaOH, Sinopharm), potassium hydrox-
ide (KOH, Sinopharm), ammonia (NH4OH, McLean),
sodium acetate (CH3COONa.3H2O), disodium hydrogen
phosphate (Na2HPO4·7H2O), urea (CON2H4, McLean); were
analytically pure. Ethanol (Aladdin, 99.90%) and deionized
Scheme 1 Illustration of Pd cat-
alyst assembled on the surface
of SiO2
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X. Fan et al.
into powder was calcined in muf furnace at 120 °C for 4 h.
Immediately, the sample was placed in a tube furnace with
the controllable temperature, reduced at 400 °C for 2 h in
the atmosphere of H2/N2 mixture gas (10 vol% in H2) and
collected, which was named the S-Pd0.8/SiO2 catalyst with
0.8 wt% Pd. The synthesis of S-PdX/SiO2 catalysts with dif-
ferent Pd quality content followed the same process (X is the
theoretical loading of Pd).
reduction of hydrogen) of metal oxides was confrmed on
the Quanrachrome Autosorb-IQ gas adsorption analyzer
attached to a Pfeifer Vacuum QME220 mass spectrometer.
The dispersion of metallic Pd was detected by CO pulse
chemisorption with 100 mg weighed sample (~ 100 mg),
which was calculated with the assumptions of Pd and CO
(stoichiometry of 1:1).
2.5 Evaluation of Catalysts for Suzuki–Miyaura
Coupling Reaction
2.3 D‑Pd0.8/SiO2 Synthesis by DI
Meanwhile, we also prepared the D-Pd/SiO2 with the uni-
form chemicals and weight load as the SEA sample (S-Pd0.8/
SiO2) by the procedure of dry impregnation [25, 26]. Pore
volume of the silica was determined to be 2.4 mL/g. So the
0.02 g of Pd(NH3)4Cl2 was dissolved in 2.4 mL of deion-
ized water for 1.0 g DI catalyst. The support and 2.4 mL
precursor solution were added into a centrifuge tube, and
the mixture slurry was shaken and churned to ensure com-
plete wetting the silica with the solution. And the remaining
formula was undiferentiated with the synthesis of S-Pd0.8/
SiO2 to manufacture the D-Pd0.8/SiO2.
A typical probe of Suzuki–Miyaura cross-coupling reac-
tion, used to evaluate the catalytic activity of the catalysts,
was that bromobenzene (1.0 mmol) and phenylboronic acid
(1.5 mmol) as the substrates were dissolved in the mixed
liquor of ethanol (5 mL) and water (5 mL) containing base
(1.5 mmol) and Pd/SiO2 catalyst (0.5 mmol‰). The reaction
was taken place in a fask under a mild environment. After
removing the excess phenylboric acid by sodium hydrox-
ide, the products were obtained through extraction procedure
using ethyl acetate. Finally, the fnal yield of target product
was determined by the gas chromatograph. Bromobenzene
could be replaced by aryl halides with a variety of substitu-
ents and bremnaphthalene, and phenylboronic acid was also
replaced by areneboronic acids containing the para substitu-
ent groups.
2.4 Characterizations
The morphology and structure of the material were obtained
by the following methods. The crystal structures of the cata-
lysts were determined by X-ray powder difraction (XRD) on
a D8ADVANCE in the range of 10~80°. The morphology of
the samples was observed using the high resolution transmis-
sion electron microscopy (HR-TEM) on a FEI TalosF200S
operating at an acceleration voltage of 200 kV, and high
angle annular dark feld-scanning transmission elec-tron
microscopy (HAADF-STEM) on a spherical aberration cor-
rection electron microscope (Titan Cubed Themis G2 300).
X-ray photoelectron spectroscopy (XPS) measurement was
performed with a Thermo Scientifc-ESCALAB 250XI mul-
tifunctional imaging electron spectrometer to research the
valence states of metal. Brunauer–Emmett–Teller (BET)
data of catalysts degassed at 120 °C for 6 h, were obtained
by nitrogen sorption experiments conducted at 77 K using
the Quantachrome Autosorb Gas Sorption analyzer. The UV
spectra were scanned with Shimadzu UV-2450 UV–Visible
spectrophotometer at wavelength of 240 ~ 800 nm with a
speed of 0.5 nm/s. The metal loadings was determined by
EDX spot scanning on the */Zeiss/Auriga FIB SEM. Infra-
red spectroscopy experiments were performed on a Bruker
Vertex 70 V vacuum Fourier infrared spectrometer with
the test conditions: hydrogen reduction at 400 °C for half
an hour, nitrogen purge for half an hour to cool down, CO
adsorption at room temperature until saturated adsorption,
nitrogen purge at room temperature until the free gas phase
CO was desorbed clean. H2-TPR (temperature programmed
2.6 Recyclability and Stability of Catalysts
The stability of the catalyst was identifed by the representa-
tive reaction of bromobenzene and phenylboronic acid as
the substrates. The recycle of catalyst was achieved through
the following procedures: (i) Product was separated from
the reaction system by extracting with 0.02 mol/L sodium
hydroxide and ethyl acetate. (ii) The catalyst was gathered by
centrifugation, and washed three times by sodium hydroxide
solution and ethanol to remove the absorbed substrate. (iii)
The collected solid was dispersed in a little amount of etha-
nol and reused for the next run.
In addition, a continuous fow reaction was performed,
which was simply described as follows. The reactor was
placed in a controlled temperature water bath, where a
well-proportioned solvent was added to disperse a certain
amount of catalyst. The substrates and base (1.5 equiv. of
bromobenzene) were dissolved in the prepared solvent and
stirred continuously. The mixture was transported to the bot-
tom of the reactor by a constant fux pump with adjustable
fow rate and pressure. Under constant pressure, the mixture
after reacting was squeezed from the top of the reaction solu-
tion and collected in a conical fask. Finally, the product was
obtained by extraction, separation and purifcation. In order
to ensure the product does not precipitate from the reaction
1 3

Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura Cross-coupling Reactions
liquid and does not afect the reaction, we use the solvent of
XRD pattern (Fig. 1a), corresponding to the primary dif-
water and ethanol (volume ratio=1/4).
fraction of (111), (200) and (220). It can be further con-
frmed that the irregular face-core cube for Pd is larger
with average size of 9.8 nm (Fig. S1) [27]. Amazingly, to
3 Results and Discussion
the catalyst prepared by SEA with 0.8 wt% Pd (S-Pd0.8/
SiO2), obvious peaks do not appear in the result of XRD,
because of containing the smaller Pd NPs which can be
observed directly from Fig. 1b, d, and have an average
of 1.3 nm from the distribution histogram in Fig. 1c [28,
29]. The uniform dispersion of Pd species is also dem-
onstrated by the elemental distribution test results of the
3.1 Structural Investigations
The catalyst prepared by controlling dry impregnation
(DI) with 0.8 wt% Pd (D-Pd0.8/SiO2) contains three rep-
resentative peaks of Pd at 40.1°, 46.6° and 68.1° in the
Fig. 1 Morphology and structure analysis of the Pd/SiO2 samples: a XRD patterns of SiO2, S-Pd0.8/SiO2 and D-Pd0.8/SiO2; b HADDF-STEM, c
particle size distribution histogram, d AC-STEM and (e~g) EDX mapping images of S-Pd0.8/SiO2
1 3

X. Fan et al.
catalyst through the mapping diagram (Fig. 1e ~ g). Fur-
thermore, the EDX result in Fig. S2 testifes that 0.78 wt%
Pd content of S-Pd0.8/SiO2 is consistent with the theoreti-
cal load. The as-prepared catalysts by SEA with diferent
Pd content (S-PdX/SiO2) have same phenomenon and pos-
sess uniformly dispersed and weeny particles, which were
observed from results of XRD patterns (Fig. S3A) and
HADDF-STEM (Fig. S3B). Besides, the dispersion of Pd
NPs on the surface of Pd0.8/SiO2 calculated by CO pulse
Pd and O species in D-Pd0.8/SiO2 does not give signifcant
change.
TPR profle demonstrates that the D-Pd0.8/SiO2 exhibits a
low reduction temperature of~150 °C, but the SEA sample
requires a higher temperature of~348 °C for Pd reduction,
which also verifes that the Pd atoms have the much stronger
interaction with the silica support (Fig. 2c), implying the Pd
NPs of S-Pd0.8/SiO2 have better stability in the reacting sys-
tem with stronger support interaction in common with XPS.
Infrared (IR) spectroscopy with CO as a probe mol-
ecule (Fig. 2d) shows that the S-Pd0.8/SiO2 has the obvious
absorption peaks, which are labeled as bridge CO peak at
1948 cm−1 and line CO peak at 2087 cm−1 [33, 34]. Com-
pared with the D-Pd0.8/SiO2, the peak value is signifcantly
stronger to demonstrate that S-Pd0.8/SiO2 has more adsorp-
tion sites to suggest that the surface of silica is Pd-rich.
In contrast, the weakly infrared peak for D-Pd0.8/SiO2 at
1981 cm−1 corresponding the Pd specie with contiguous
dual sites for CO absorption, which is closely related to the
larger Pd NPs resulting in the fewer number of Pd atoms
exposed on the surface in per unit mass [35, 36].
chemisorption is 43%, which is fourfold that of D-Pd0.8
/
SiO2 (9.7%), due to the ultrafne NPs and excellent disper-
sion to provide more active sites for the adsorption of CO
[30]. The above results obviously prove that Pd catalysts
prepared by SEA have the great structural advantage over
the catalyst of DI.
X-ray photoelectron spectroscopy (XPS), has been
employed to analyze Pd 3d and O 1 s of S-Pd0.8/SiO2
(Fig. 2a, b, S4). The peak of Pd0 has been signifcantly
deviated from the standard metallic Pd (335.2 eV) to higher
binding energy (336.2 eV), which can be proofed by the
result of Pd0 3d for S-Pd0.8/SiO2 [31]. Meanwhile, the bind-
ing energy of O 1 s has the shift to lower binding energy,
which is from 530.6 eV to 530.3 eV, because that O tends
to get electrons from Pd species [32]. Hence, it implies that
there is close interaction between the Pd atoms with the O
atoms in the surface of silica. Unsurprisingly, the peak of
Comparing the UV–Vis spectrogram of Pd2+ in S-Pd2+/
SiO2 and D-Pd2+/SiO2 in Fig. 3a, the inconsistent absorp-
tion peak is at 351 nm and 399 nm (the normal Pd ions
absorption peak at 410 nm) for S-Pd2+/SiO2 and D-Pd2+/
SiO2, respectively [37]. This indicates that Pd atoms in
Fig. 2 XPS spectra of a Pd 3d,
b O 1 s, c TPR and d CO in-situ
IR of Pd0.8/SiO2 preparation by
SEA and DI, respectively
1 3

Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura Cross-coupling Reactions
Fig. 3 UV–Vis spectroscopic
images of a SiO2; Pd2+/SiO2
and Pd/SiO2 prepared by SEA
and DI, b S-PdX/SiO2(X=0.1,
0.2, 0.4, 0.8, 1.3) with diferent
content of Pd, c N2 adsorption–
desorption isotherms and d Pore
size distribution calculated by
the BJH method with Physical
adsorption apparatus for SiO2,
S-Pd0.8/SiO2, D-Pd0.8/SiO2
S-Pd2+/SiO2 have a stronger interaction with superfcial
3.2 Catalytic Activities of Pd/SiO2 in Suzuki–
O and cause the absorption peak to have the conspicu-
ous blue shift. The absorption intensity of S-Pd0.8/SiO2
in infrared and visible regions is signifcantly lower than
that of D-Pd0.8/SiO2, but opposite in ultraviolet region,
which can be afrmed by showing plasmon band at around
300 nm for the ultrafne Pd particles [38]. All S-PdX/SiO2
samples with diferent Pd content also have such perfor-
mance (Fig. 3b), and as the content of Pd increased, the
color of appearance for samples gradually deepen.
The information of the pore structure from N2 adsorp-
tion–desorption isotherms illustrates that the amorphous
silica is the typical mesoporous material (Fig. 3c and d)
[39, 40]. The pore structure parameters in Table S1 show
that the surface area of S-Pd0.8/SiO2 and D-Pd0.8/SiO2
have sufered from a decline compared with SiO2, and the
reduction are more remarkable for D-Pd0.8/SiO2, because
of the alkali etching and the adsorption of Pd NPs.
The characterization results fully confirm that the
ultrafne Pd NPs prepared by SEA beneft from the strong
interaction between the electron-rich O element on the
carrier and the electron-defcient Pd complex under the
alkaline atmosphere, which can reduce the surface forces
between the Pd NPs. However, due to the poor binding
force between the carrier and Pd species and the high sur-
face energy, the Pd NPs of DI catalyst can continuously
grow larger with the adjacent Pd particles by dry impreg-
nation method.
Miyaura Reactions
To examine the activity of the as-prepared catalysts for
Suzuki–Miyaura, the model reaction of benzeneboric
acid and bromobenzene was selected for the optimization
of the reaction conditions. As shown in Fig. 4, Fig. S5
and Table S2, S3, large variations for the reactivity were
observed depending on the catalyst, time, temperature, base
and solvent, used in the reactions [41–43]. As summarized,
it was found that the following conditions as the standard
reaction conditions: bromobenzene (1.0 eq.), phenylboronic
(1.3 eq.), S-Pd0.8/SiO2 (0.05 mmol %), KOH (1.5 eq.), H2O/
EtOH (volume ratio is equal to 1:1) as the solvent. Biphenyl
was synthesized with up to>99% yield, at 40 °C for 30 min.
As shown in the Fig. 4A and Fig. 4B, the catalytic perfor-
mance of the S-Pd0.8/SiO2 was signifcantly better compar-
ing with D-Pd0.8/SiO2. Specifc result was that D-Pd0.8
/
SiO2 has 22.9% yield in 40 °C for 30 min to be ascertained
poorer performance. In addition, a more intuitive result in
Fig. 4c was expressed by TOF values calculated at a conver-
sion rate of≤50%. The conversion frequency of the S-Pd0.8/
SiO2 (80,388 h−1) was around 87 time for the D-Pd0.8/SiO2
(917 h−1) and 34 time for commercial Pd/C (2288 h−1) under
the same conditions. As shown in Table S4, the signifcantly
high TOF (even reaching 105 h−1 at 60 °C) was given using
the SEA catalysts with diferent Pd loads under diferent
temperatures. Furthermore, the expansion of substrates for
1 3

X. Fan et al.
Fig. 4 Investigation of the
factors infuencing the Suzuki–
Miyaura coupling reactions,
including a temperature and
b time, c comparison for TOF
values of S-Pd0.8/SiO2, D-Pd0.8
SiO2 and commercial 10 wt%
Pd/C, d cyclic properties of
S-Pd0.8/SiO2 and D-Pd0.8/SiO2
/
Suzuki–Miyaura coupling reactions had been carried out at
the best experimental conditions. A series of aryl halides and
boronic acids were tested. As summarized in Table 1 and
Table S5, the S-Pd0.8/SiO2 catalyst exhibited highly activ-
ity for the diferent substrates with varieties of functional
groups, giving the corresponding products with higher yields
in mild conditions. High dispersion and ultrafne size of Pd
NPs aforded more reactive sites to increase the interacting
chance for substrates with the nano-metal and made contri-
butions to improving catalyst activity. When the substrates
and catalyst were simultaneously expanded by a factor of 10,
the yield was still 99% (Table 1, Entry 19).
and production efciency. And the process of reaction could
be controlled by the feed fow and the thermostat. The pro-
duction results were shown in Entry 20, 21 of Table 1, which
still gave the satisfactory yield more than 90% according to
30 mL/h fow rate at 40~60 °C.
3.3 Recyclability of Catalyst
To determine the cycling performance and stability of cata-
lysts, which were the highly important indicators for catalyst
quality, the cyclic experiment was carried out. As shown in
Fig. 4d, the S-Pd0.8/SiO2 could be circulated more than 8
times, which still maintained the yield of more than 90%. On
the contrary, the D-Pd0.8/SiO2 did not exhibit catalytic activ-
ity again after nearly 4 cycles. The amorphous structure of
silica is damaged to some extent after the catalytic reaction
for the carrier silica because of its slightly worse alkali toler-
ance and there is a certain degree of agglomeration of nano-
palladium to increase the size of nano-particles (Fig. 5a),
which is the main reason for the reduction of activity. Mean-
while, the catalyst still containing a large number of fne
Pd NPs with particle size less than 3 nm, does not increase
the difraction property of palladium to X-ray (Fig. 5b). In
addition, the hot fltration experiment for S-Pd0.8/SiO2 was
performed to check Pd-leaching behavior. The result testifed
that the fltrate from the S-Pd0.8/SiO2 catalyst did not given
the detectable conversion of biphenyl products.
To illustrate the superiority of the as-prepared Pd/SiO2 by
the SEA strategy, a comparison was made between S-Pd0.8/
SiO2 with the recently reported Pd nano-catalysts. As shown
in Table S6, the S-Pd0.8/SiO2 catalyst synthesized preferably
gave the excellent yield with the short reaction time, secure
solvent, as well as low temperature. This fully demonstrates
that S-Pd/SiO2 catalyst is one of the best classes of hetero-
geneous catalysts, afording the highest yield with the lower
catalyst loading of 0.05 mmol% under the mild environment.
Importantly, industrial production of chemicals is of great
signifcance to value maximization. According to the con-
stant pressure principle of the reaction vessel, we designed a
facile continuous production device (as shown in Fig. S6) to
ensure that the reactant feed was in an equilibrium state with
reaction mixture discharge, improving the reaction volume
1 3

Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura Cross-coupling Reactions
a
Table 1 Suzuki–Miyaura cross-coupling reactions of various Ar-X and Ar-B(OH)2 catalyzed by S-Pd/SiO2
Entry
R1
X
R2
t/min
T/°C
Yieldb (%)
1
2
3
4
5
6
7
8
H
Br
Br
Br
Br
Br
I
Br
Br
Br
Br
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
H
H
H
H
H
H
4-F
4-CH3
4-COCH3
H
H
4-F
4-CH3
4-COCH3
4-F
4-CH3
4-COCH3
H
30
30
30
60
30
30
30
30
30
30
60
60
30
30
30
30
30
30
30
/
40
40
40
60
40
40
40
40
40
40
60
60
40
40
40
40
40
40
40
40
60
>99.5
>99.5
97
99
99
99
>99.5
99
>99.5
98
97
95
99
99
96
98
99
95
99
4-COCH3
4-CN
4-CHO
4-OCH3
4-OCH3
4-CH3
H
9
H
H
10
11
12
13
14
15
16
17
18
19c
20d
21d
4-OH
4-OH
4-COCH3
4-COCH3
4-COCH3
4-CH3
4-CH3
4-CH3
H
H
H
H
H
90
97
/
aReaction conditions: Ar-X (1.0 mmol), Ar-B(OH)2 (1.3 mmol), KOH (1.5 mmol), S-Pd0.8/SiO2 (0.05 mmol% Pd), H2O/EtOH (5 mL+5 mL)
bYields were determined by GC
cThe substrates and catalyst were expanded by a factor of 10
dThe reaction result of the fow are obtained under 30 mL/h fow rate with 0.5 mmol% catalyst. The ratio of H2O/EtOH is 1/4
Fig. 5 a HAADF-STEM image
of the S-Pd0.8/SiO2 catalyst after
8th cycle; b XRD patterns of
the S-Pd0.8/SiO2 catalyst before
and after 8th cycle
reactions with respect to aryl halides and areneboronic
acids under the mild conditions. The versatile SEA
strategy has determined that the S-Pd/SiO2 catalyst as
a high-performance, inexpensive and easily recyclable
nano-materials, shows the substantial improvement in
this kind reactions, due to high dispersion and ultrafne
4 Conclusions
In summary, the practical and efcient approach for the
synthesis of various biphenyl compounds has been devel-
oped based on S-Pd/SiO2-catalyzed Suzuki–Miyaura
1 3

X. Fan et al.
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possibilities for constructing the high-efciency hetero-
geneous Pd-based catalyst without any ligands and cap-
ping reagents, which has great potential and superiority for
industrial production because of the remarkable activity
and cycling stability.
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Acknowledgments This research was supported by the National Nat-
ural Science Foundation of China (Grant nos. 21403053, 21502175
and U1404503), Dalian Institute of Chemical Physics (N-19-11), Key
Laboratory of Henan Province Drug Quality Control and Evaluation.
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Compliance with Ethical Standards
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Conflict of interest The authors declare that they have no confct of
interest.
Appendix A. Supplementary material
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contains TEM images, XRD pattern, EDX, ftting results of
XPS spectra and XPS, tables and fgures of experimental
datum, literature data for comparison, and schematic dia-
gram of fow device.
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1 3

Ultrafne and Highly Dispersed Pd/SiO2 for Suzuki−Miyaura Cross-coupling Reactions
Afliations
Xizheng Fan1 · Jingyi Yang1 · Qingqing Pang1 · Zhongyi Liu1 · Panke Zhang1 · Jing‑He Yang1,2
1
* Panke Zhang
College of Chemistry, and Institute of Green
Catalysis, Zhengzhou University, Zhengzhou 450001,
People’s Republic of China
pkzhang@zzu.edu.cn
* Jing-He Yang
2
jhyang@zzu.edu.cn
School of Chemical Engineering, Zhengzhou University,
Zhengzhou 450001, People’s Republic of China
1 3