Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Y. Liu et al.
Molecular Catalysis 505 (2021) 111487
development of various multi-functional porous materials to immobilize
and stabilize noble metal species [32–34]. Thereinto, hypercrosslinked
polymers (shorten as HCPs), a type of microporous organic polymer,
have received a staggering degree of attention in the field of catalysis
[35–41]. Thanking to the high surface area, tunable porous structure,
easy functionalization, low skeleton density, high hydrothermal stability
and mild operating conditions, HCPs have exhibited promising potential
as the supports of noble metal species to catalyze the reactions such as
alcohol oxidation, hydrogenation and Suzuki coupling [42–48]. In
general, HCPs are synthesized via Friedel-Crafts alkylation catalyzed by
Lewis acids, such as FeCl3, AlCl3 and SnCl4 [35–41]. The removal of
these metal species is usually carried out via Soxhlet extraction involving
large volumes of organic solvents [35–41]. Moreover, the potential re-
sidual metal species in HCPs would negatively influence the homoge-
neity and yield uncontrollable impurity. From this regard, establishment
of a facile metal-free synthesis route for functional HCPs is particularly
requirable and significant for the immobilization of noble metal
particles.
JEOL) at 200 kV. Morphologies were investigated using a field-emission
scanning electron microscope (FE-SEM, Hitachi S-4800). X-ray photo-
electron spectroscopy (XPS) was recorded on a PHI 5000 Versa Probe
instrument with Al Karadiation (1486.6 eV). The correction of binding
energy was based on C1s hydrocarbon peak of 284.80 eV. Thermogra-
vimetric analyses (TG) were carried out on an STA409 instrument under
-1
◦
the nitrogen atmosphere (heating rate: 5 C min ). Pd contents were
estimated on an OPTMA 20,000 V Inductive Coupled Plasma Mass
Spectroscopy (ICP). A Bruker AVANCE-III 400 NMR spectrometer (13C:
100 MHz, Bruker Corporation, Rheinstetten, Germany) was engaged for
the collection of Solid-state 13C NMR spectra.
2.3. Preparation of HCPs
The HCPs were solvothermally synthesized via hypercrosslinkage.
The follows are the details for the synthesis of HCP(PB). In general, PA
(0.83 g, 5 mmol), BP (1.18 g, 7.5 mmol) and s-trioxane (2.4 g, 26 mmol)
were in sequence added into a flask containing DCE (105 mL). After
sufficient stirring, chlorosulfonic acid (ClSO3H, 2.4 g, 20 mmol) was
dropped into the mixture at 273 K. Copolymerization was carried out at
313 K for 12 h and then at 313 K for 12 h. Subsequently, the liquid phase
was removed by filtration, while the isolated solid was washed with
methanol and water, and then dried at 353 K under vacuum for 12 h
(yield: ca. 80 %).
In the present investigation, we reported the straightforward syn-
thesis of dicarboxylic acid-functional HCPs through the copolymeriza-
tion of phthalic acid (PA) and biphenyl (BP) via Friedel-Crafts alkylation
catalyzed by Bronsted acid in the absence of any metal species. Con-
ventional metal Lewis acid-catalyzed Friedel-Crafts alkylation involves
an electrophilic substitution step, challenging the integration of mono-
mers with the electronic-withdrawing functional groups such as car-
By changing the monomers, other HCPs materials were prepared
through a similar process (Schemes S1-S3). HCP(B) was synthesized
through the self-polymerization of BP. HCP(BB) was synthesized
through the co-polymerization of BA and BP. HCP(DB) was synthesized
through the co-polymerization of DP and BP. Detailed descriptions are
presented in the Supporting Information.
–
boxylic acid groups (C OOH) [35–41]. Various supports have been
applied to immobilize Pd clusters, such as metal-organic frameworks
[49], covalent organic frameworks [50], and porous carbon materials
[51], however, HCPs supported Pd clusters are still to be explored.
Herein, by virtue of a Bronsted acid-catalyzed route, efficient copoly-
–
merization of PA and BP was reached to enable abundant dual C OOH
groups on HCP with large surface area and pore volume. Ultrafine and
2.4. Immobilization of Pd clusters on HCPs
highly dispersive Pd◦ clusters were anchored on the constructed dual
The HCPs supported Pd clusters, shorten as Pd◦@HCPs, were syn-
thesized according the following procedure. In a typical preparation of
Pd◦5@HCP(PB) (5 denotes the original Pd(OAc)2/support weight per-
centage ratio), HCP(PB) (0.5 g) was mixed with 20 mL acetone con-
taining Pd(OAc)2 (0.025 g, 0.1125 mmol). Under nitrogen atmosphere,
the mixture was stirred at room temperature. After that, the liquid phase
was removed and the remained solid Pd2+5@HCP(PB) was collected,
washed with abundant acetone, and then freeze-dried in a freeze-dryer
for 36 h. Finally, Pd◦5@HCP(PB) was obtained by reducing
Pd2+5@HCP(PB) at 503 K for 4 h under 20 % H2/N2 with a flow rate of
50 mL minꢀ 1. Pd loaded on HCP(B) was prepared similarly and the
obtained product was denoted as Pd◦5@HCP(B). Pd loaded on the other
HCPs including HCP(BB), HCP(DB) and HCP(B) were prepared under a
similar process and the products were denoted as Pd◦5@HCP(BB),
Pd◦5@HCP(DB) and Pd◦5@HCP(B), respectively.
–
C
OOH groups functional HCP and demonstrated excellent activity in
the oxidative homocoupling of benzene under O2, creating an attractive
case for Pd◦ clusters-catalyzed benzene coupling. Several control HCPs
with different surface groups were prepared and engaged in the Pd
loading and catalysis evaluation, revealing that modulating the surface
groups of HCPs is crucial for the fabrication of efficient Pd clusters for
the benzene coupling.
2. Materials and methods
2.1. Materials
Phthalic acid (PA), biphenyl (BP), disodium phthalate (DP), benzoic
acid (BA), and palladium acetate (Pd(OAc)2) were purchased from
Aldrich, while s-trioxane was purchased from Acros. Other chemicals
and reagents were commercially available.
2.5. Aerobic homocoupling of benzene into biphenyl
2.2. Characterization
Pd-catalyzed homocoupling of benzene with O2 was performed in a
25 mL stainless steel reactor. Typically, catalyst (0.03 mol% Pd to
benzene), benzene (15 mmol), trifluoromethanesulfonic acid (CF3SO3H,
0.1 g), acetic acid (3 mL), and water (2 mL) were successively added into
the reactor. The reactor was purged and pressured with 8 bar O2. The
coupling was carried out at 418 K for 2 h. After the reaction, the solid
was removed by filtration. The filtrate was analyzed by an Agilent
7890B gas chromatography (GC). The column was HP-5 capillary col-
Fourier transform infrared spectroscopy (FT-IR) analysis in the
wavenumber range of 4000–800 cmꢀ 1 were performed on an Agilent
Cary 660 FTIR Spectrometer. X-ray diffraction (XRD) patterns were
tested on a Smart Lab diffractometer (Rigaku) equipped with a 9 kW
rotating anode Cu K
α
radiation (λ =1.5406 Å) from 2θ = 5◦ to 80◦ (scan
rate: 0.2◦ s-1, beam voltage: 45 kV; beam current:200 mA). Elemental
analyses (EA) were performed on a Vario EL cube Elemental Analyser.
–
The amounts of carboxylic acids (C OOH) groups were measured via
umn (30 m*0.25 mm*0.25 μm). The detector was the flame ionization
the inverse acid-base titration method by using the indicator of methyl
orange. Nitrogen adsorption experiments were performed on BELSORP-
MAX analyzer. Before analysis, the samples were pre-activated at
moderate temperature (below 373 K to avoid the decomposition of the
organic matrix) under vacuum. Transmission electron microscopy
(TEM) images were recorded on an electron microscope (JEM-2100,
detector (FID). An internal standard method was applied for the quan-
titative analysis by using 1,4-dioxane as an internal standard.
Hot filtration experiment was applied to identify the heterogeneous
nature of Pd05@HCP(PB). During a Pd05@HCP(PB) catalyzed benzene
coupling with O2 (8 bar), the reaction was prematurely stopped when
the reaction proceeded 418 K for 1.0 h. Through filtration, the catalyst
2