Polyphenylene-Based Solid Acid as an Efficient Catalyst for Activation and Hydration of Alkynes

Article abstract of DOI:10.1021/acs.chemmater.0c01763

Porous polymer catalysts possess the potential to combine the advantages of heterogeneous and homogeneous catalysis, namely, easy postreaction recycling and high dispersion of active sites. Here, we designed a -SO₃H functionalized polyphenylene (PPhen) framework with purely sp²-hybridized carbons, which exhibited high activity in the hydration of alkynes including challenging aliphatic substrates such as 1-octyne. The superiority of the structure lies in its covalent crosslink in the xy-plane with a π-πstacking interaction between the planes, enabling simultaneously high swellability and porosity (653 m²·g^-1). High acidic site density (2.12 mmol·g^-1) was achieved under a mild sulfonation condition. Similar turnover frequencies (0.015 ± 0.001 min^-1) were obtained regardless of acidic density and crosslink content, suggesting high accessibility for all active sites over PPhen. In addition, the substituted benzene groups can activate alkynes through a T-shape CH/πinteraction, as indicated by the 8 and 16 cm^-1 red shift of the alkyne C-H stretching peak for phenylacetylene and 1-octyne, respectively, in the infrared (IR) spectra. These advantages render PPhen-SO₃H a promising candidate as a solid catalyst replacing the highly toxic liquid phase acids such as the mercury salt.

Full text of DOI:10.1021/acs.chemmater.0c01763

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Article
Polyphenylene-based Solid Acid as an Efficient
Catalyst for Activation and Hydration of Alkynes
Yiyun Liu, Bolun Wang, Liqun Kang, Apostolos Stamatopoulos, Hao Gu, and Feng Ryan Wang
Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 01 May 2020
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Polyphenylene-based Solid Acid as an Efficient Catalyst for Activation
and Hydration of Alkynes
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Yiyun Liu, Bolun Wang, Liqun Kang, Apostolos Stamatopoulos, Hao Gu, Feng Ryan Wang*
Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK
ABSTRACT: Porous polymer catalysts possess the potential to combine the advantages of heterogeneous and homogeneous
catalysis, namely easy post-reaction recycling and high dispersion of active sites. Here we designed a -SO3H functionalized
polyphenylene (PPhen) framework with purely sp2-hybridized carbons, which exhibited high activity in the hydration of
alkynes including challenging aliphatic substrates such as 1-octyne. The superiority of the structure lies in its covalent
crosslink in the xy-plane with π-π stacking interaction between the planes, enabling simultaneously high swellability and
porosity (653 m2g-1). High acidic sites density (2.12 mmolg-1) was achieved at mild sulfonation condition. Similar turnover
frequencies (0.015 ± 0.001 min-1) were obtained regardless of acidic density and crosslink content, suggesting high
accessibility for all active sites over PPhen. In addition, the substituted benzene groups can activate alkynes through a T-
shape CH/π interaction, as indicated by the 8 cm-1 and 16 cm-1 red shift of the alkyne C-H stretching peak for phenylacetylene
and 1-octyne respectively in the IR spectra. These advantages render PPhen-SO3H a promising candidate as a solid catalyst
replacing the highly toxic liquid phase acids such as the mercury salt.
to replace the traditional highly-toxic mercury salt catalysts
INTRODUCTION
with recoverable and environment-friendly solid acid
29
catalysts.28, A number of attempts have been made in
Porous organic materials play an important role in gas
storage and separation,1, 2 drug release,3 catalysis, sensing4
and membrane separation5, due to their properties in
merging the benefits of porosity, processability, synthesis
diversity and organic nature.6 A number of novel polymeric
frameworks have been developed in recent years,6, 7 such as
developing polymer-based solid acids, however, their
catalytic activity are often limited by the contradiction lying
within the nature of polymeric framework.
A good polymeric heterogeneous catalyst requires high
active sites density and good accessibility simultaneously,
which can be achieved in porous polymers by increasing the
porosity and swellability, respectively. While porosity of a
polymer generally increases as a function of crosslink
content, as seen in sp3 carbon linker groups,12 a three
dimensionally crosslinked framework reduces its ability to
swell and thus decreases the accessibility of the active sites.
polytriazine,8-10 hyper-cross-link polystyrene (HPS),11,
nanoporous
12
polydivinylbenzene
(PDVB),13
dihydroxybenzoic acid based polymer nanospheres14 and
porphyrin network polymers.15, 16 The polymeric structures
not only offer high surface area and well-controlled
porosity, but also provide an organic environment, the light-
weight advantage and a dynamic structure upon external
stimulation. Thus, they have offered an innovative way to
combine the advantages of homogeneous and
heterogeneous catalysis.17 On one hand, a solvent-like
reaction environment is established via organic ligands and
the geometric structure of the framework, providing well-
dispersed high-density active sites. On the other hand, the
heterogeneous nature of solid catalysts facilitates post-
reaction separation, rendering them environmental-
friendly alternatives for highly toxic homogeneous liquid
catalysts.
High swellability is usually obtained in polymers with lower
31
crosslink content,30,
within which pore formation is
difficult due to close stacking of the linear chains. This
further hinders the formation of high-density active sites in
the polymer matrix. Such a dilemma between high active
sites density and good molecular accessibility limits the
practical application of porous polymers. Here we
hypothesize that the solution to this dilemma lies in the
design of a polymer structure with covalent crosslinks at
two-dimensional level. This structure enables pore
formation with rigid chemical bonds, and pure molecular
interactions between crosslinking layers, which provides
good swellability, allowing reactant molecules to diffuse
into the polymer matrix which lowers the mass transfer
limitation.25
Various polymer-based solid acid catalysts have been
investigated in the recent years for hydrolysis,18
hydration19-21,dehydration22-24 and other acid-catalyzed
reactions25, 26in both gaseous and liquid phase, as the
replacement of liquid acids which leads to severe technical
and environmental problems in practical applications. Acid-
catalyzed alkynes hydration is a straightforward and atom-
economical method for the synthesis of ketones via
carbocation intermediates,27 where efforts have been paid
To achieve this target, we employed the polyphenylene
(PPhen)-based polymeric framework functionalized with -
SO3H acidic sites. The material has been investigated
previously as the ion-exchange resin, which shows that it
can facilitate water diffusion through the hydrophilic
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Scheme 1. The concept of PPhen structure, showing the covalent crosslinks in xy-plane (left), swelling along z axis
(middle) and activation of CC-H (right) via T-shape CH/π interaction. The scheme is a simplification of the real
PPhen-SO3H structure and does not indicate a laminar structure.
regions and possesses a rigid backbone preventing folding
of the structure.32, 33 All carbon atoms in the PPhen-based
polymer are sp2-hybridized, retaining covalent crosslinks in
the xy-plane (Scheme 1, left), while - stacking becomes
the main force at z axis (Scheme 1,middle). As a result,
PPhen achieves a high surface area of 653 m2g-1 and good
swellability. Meanwhile, it also provides an organic solvent-
like environment for reaction molecules via high density of
di-substituted (phenylene) and tetra-substituted benzene
groups,34 which activate alkynes through the edge-to-face
configuration (Scheme 1, right).35 The catalytic behavior of
sulfonated PPhen solid acid is evaluated with -the hydration
of phenylacetylene and 1-octyne, two model compounds to
study the structure-reactivity relationship and the
activations of aromatic and aliphatic alkynes, respectively.
Maximum 2.12 mmolg-1 acidic sites density was achieved at
mild sulfonation condition with sulfuric acid treatment
The mixture was degassed through three freeze-pump-
thaw cycles. K2CO3 (2.0 M, 15 mL) and Pd(PPh3)4 (0.3 g, 0.25
mmol) were then added with three subsequent freeze-
pump-thaw cycles. The mixture was then purged with Ar
and heated to 150 °C for 20 h under stirring. The product
precipitated in water, and was washed by water,
dichloromethane and methanol. Approximately 600 mg of
grey product was obtained in each batch. PPhen with
different crosslink contents were obtained by changing the
ratio
of
1,2,4,5-tetrabromobenzene
and
1,4-
dibromobenzene. Reactants used in the synthesis of
different polymers are summarized in Table S1.
Synthesis of PPhen-SO3H catalysts. As prepared PPhen
polymers were treated with HCl/H2O2 (5:1, v:v) solution to
remove Pd residue from the cross-coupling reaction. The
mixture was stirred overnight under room temperature and
o
heated up to 60 C for two hours. The product was then
o
under 40 C. The conversion of phenylacetylene reached
washed with water and methanol. After drying in vacuum
oven overnight, 200 mg H2O2 treated polymer was added to
5 mL concentrated sulfuric acid (98 wt%) and stirred under
80±4% with 87±3% selectivity towards acetophenone at
100 oC, which is three times higher than that of the
commercial solid acid catalyst Amberlyst 15. A linear
increase in the hydration reactivity with increasing -SO3H
site density over PPhen-SO3H is achieved, resulting similar
turnover frequencies (TOF) regardless of acidic density and
crosslink content. This result suggests good accessibility of
all -SO3H sites for phenylacetylene. In addition, the
electrophilic phenylene and tetra-substituted benzene
groups in the PPhen framework weaken the CC bond,
showing an 8 cm-1 red shift in the alkyne C-H stretching for
phenylacetylene and 16 cm-1 for 1-octyne. As a result, the
PPhen-SO3H shows superior hydration activity even for
aliphatic 1-octyne, a reaction that is very challenging for
o
40 C for 16 hours. The product was poured into 200 mL
deionized water and filtered, followed by washing with H2O
and methanol until neutral pH to eliminate physically
absorbed sulfuric acid molecules within the pores. Obtained
PPhen-SO3H catalysts were dried in vacuum oven under 50
oC overnight.
Hydration reaction of alkynes. In a typical reaction, 20
mg of catalyst was added into glass tube reactor with 1 mL
of H2O and 110 µL (1 mmol) of phenylacetylene. 40 µL
(0.176 mmol) of dodecane was also added into the reactor
as the standard in the GC-MS measurement. The standard
o
reaction continued for 24 hours under 100 C. After the
36
solid acid catalyst.21, The PPhen-SO3H catalyst has the
reaction, 5 mL tetrahydrofuran (THF) was added and the
mixture was placed in the ultrasonic washer for 30 min. 50
μL was taken from the mixture and diluted with THF to be
measured with GC-MS for quantification. The conversion of
phenylacetylene, yield of acetophenone and selectivity of
catalysts mentioned were summarized in Table S4.
Hydration reactions with the same molar amount of 1-
octyne were carried out under the same conditions. In the
catalyst recycling experiments, the catalysts were
potential to replace Hg based catalysts in hydration
reactions.
MATERIALS AND METHODS
Synthesis of PPhen. To obtain a fully crosslinked PPhen-
100% polymer framework, 1,2,4,5-tetrabromobenzene
(1.531 g, 3.89 mmol) and benzene-1,4-diboronic acid (1.289
g, 7.78 mmol) were added into 120 mL dimethylformamide.
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separated by filtration, washed with ethanol, dried and NaHCO3 solution and stirred for 24 hours. The mixture was
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weighed.
filtered to get clear solution, which is then titrated with 0.01
mol∙L-1 HCl solution to get the amount of remaining NaHCO3.
The concentration of HCl standard solution is determined
by the titration with dry Na2CO3 standard. The
concentration of NaHCO3 is then titrated by HCl standard
solution. NaHCO3 is chosen because the scale of cross
Characterizations. Samples for High Angle Annular Dark
Field Scanning Transmission Electron Microscopy (HAADF-
STEM) were prepared by sprinkling a small amount of dry
sample powder on 400 mesh copper grids with lacey carbon
support film. High resolution aberration-corrected HAADF-
STEM images in Fig. 1, Fig. S2 and Fig. S4 were acquired on
probe-corrected (CEOS) JEM ARM 200CF (JEOL, Japan)
operated at 200 keV. We used a 40 μm probe-forming
aperture, resulting in 31.8 mrad probe convergence semi-
angle. The HAADF signal was gathered at 8.0 cm STEM
camera length, integrating the scattered electron intensity
between 71.5 and 257.8 mrad. Energy-Dispersive X-ray
Spectroscopy (EDS) data were obtained on the probe-
corrected JEM ARM 200CF (JEOL, Japan), which is equipped
with large solid-angle dual EDS detectors for X-ray
spectroscopy and elemental mapping. The EDS data
acquisition was carried out in STEM imaging mode, with a
probe current of 143 pA (probe size is 5C), with the
microscope operated at 200 keV acceleration voltage. Each
EDS spectrum image is 100 × 100 pixels in size, with 0.05
second exposure time per pixel. Gatan Microscopy Suite
Software was used for EDS spectrum imaging data
acquisition. Nitrogen adsorption–desorption isotherms
were recorded at 77 K using a Micromeritics 3Flex surface
characterization analyser. The samples were degassed in
vacuum at 200 oC overnight for removal of any
contaminates. Specific surface areas were determined
according to the BET model. Commercial Amberlyst 15 (20-
30 mesh) is used as the benchmark for the hydration
reactions, as it has been widely used as the solid acid
catalysts for the transformation of organic molecules.37-40
Acidic sites densities of as-synthesized PPhen-SO3H
catalysts and Amberlyst 15 are measured via chemical
titration. 50 mg catalyst is added to 50 mL 0.01 mol∙L-1
-
section for HCO3 ions (3.0 Å) is comparable to the reactants
phenylacetylene (4.3 Å) and 1-octyne (1.7 Å) (estimated
from bond length in Lange’s Handbook of Chemistry).
Further considering the nonpolar interaction between the
reactants and organic substrate of polymer, the acidic sites
included in the titration should be accessible to the
reactants.
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Thermogravimetric Analysis (TGA) was measured on
PerkinElmer Pyris 1 TGA. 2-10 mg sample is heated from 30
o
o
oC to 700 C with a heating rate of 10 C∙min-11. Attenuated
Total Reflection Infrared (ATR-IR) spectroscopy is
measured with Bruker ALPHA FTIR spectrometer. The
spectra were taken within 4000-400 cm-1 range. The
spectra of polymers absorbed with phenylacetylene were
taken with 10 mg polymers absorbed with 5 µL
phenylacetylene.
RESULTS AND DISCUSSION
The PPhen-SO3H Catalysts: Structure and Chemical
Properties. PPhen is synthesized through the palladium-
catalyzed
cross-coupling
reaction
of
1,2,4,5-
tetrabromobenzene and benzene-1,4-diboronic acid as
reported previously.34 To study the role of crosslink content
in catalysis, 1,2,4,5-tetrabromobenzene is selectively
replaced with 1,4-dibromobenzene. This results in a
controlled ratio between phenylene and tetra-substituted
benzene groups in the PPhen framework and thus
determines the crosslink content, which is denoted as
PPhen-SO3H-X% where X% is the normalized crosslink
Figure 1. (a) The schematic diagram shows the synthesis of PPhen and loading of SO3H. (b) HAADF-STEM image of PPhen-SO3H. (c-
e), EDS map of carbon (c), sulfur (d) and carbon + sulfur (e). Region difference between STEM and EDS map is due to sample drifting.
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Figure 2. (a) N2 physisorption of fresh PPhen-100% (black), PPhen-100% treated with H2O2 (red) and PPhen-SO3H-100% (orange).
(b) Comparison of N2 physisorption for H2O2 treated PPhen-100%, PPhen-82%, PPhen-65%, PPhen-36% and PPhen-0%. (c)
Thermogravimetric Analysis (TGA) of corresponding PPhen polymers in a). (d) TGA and the first derivatives of weight loss for PPhen-
SO3H-100% and PPhen-SO3H-0%.
content (Table S1, Supporting Information). To avoid the
influence of Lewis acidity of Pd in the hydration reactions,
as-prepared PPhen is treated with H2O2-HCl to remove the
Pd residue (Fig. 1a). -SO3H sites are introduced by treating
PPhen with concentrated sulfuric acid (95-97%) (Fig. 1a),
the duration of which decides the acidic site density over
the fully crosslinked polymer (PPhen-SO3H-100%). The
crosslinked polymer has irregular shapes in the form of
curved aggregates (Fig. S1, Supporting Information). High
angle annular dark field-scanning transmission electron
microscopy (HAADF-STEM) images show the candyfloss
shape of the polymer particles in the range of 100 to 400 nm
(Fig. 1b and Fig. S2, Supporting Information). -SO3H site
disperse uniformly across the PPhen framework, as
confirmed by energy-dispersive x-ray spectroscopy (EDS)
element mapping of C and S (Fig. 1c-e and Fig. S3,
Supporting Information). The linear polymer (PPhen-SO3H-
0%) has very similar morphology and -SO3H distributions
(Fig. S4, Supporting Information). PPhen-SO3H-100%
shows amorphous structure, as indicated in the X-ray
diffraction (XRD) measurement (Fig. S5, Supporting
Information). The absence of a Pd signal in the EDS
spectrum and the XRD pattern confirms the complete
removal of Pd residues.
The N2 adsorption-desorption isotherm of fresh fully
crosslinked PPhen (PPhen-100%) (Fig. 2a, black) shows a
type I shape of isotherm, corresponding with microporous
materials. The increase in adsorbed volume between 0.2 to
0.8 P/P0 indicates a large external surface, contributing to
roughly 10% of the BET-equivalent specific surface area.34
Though BET surface area calculations are not
representative for microporous materials, here they are
used to make a relative comparison between different
PPhen systems. The steep increase from P/P0 = 0.9 stems
from the interparticulate void. The removal of the Pd
residues by H2O2 not only reduces the weight of PPhen, but
also creates new pores and voids which boost the specific
surface area from 529 m2g-1 to 653 m2g-1 (Fig. 2a and Table
S2, Supporting Information). Noticeably, the adsorption and
desorption branches do not close completely. This
phenomenon is consistent with other polymer isotherms in
41-44
the literature,34,
and has been attributed to the
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S2, Supporting Information). Compared with commercial
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catalyst Amberlyst 15, which lost all -SO3H sites below 350
C,12, 39the superior thermal stability of PPhen-SO3H-100%
enables reactions catalyzed at higher temperatures. The
decomposition of -SO3H in the PPhen-SO3H-0% starts at 280
C, which is 80 C higher than that of the PPhen-SO3H-100%
(Fig. 2d). The results agree with the literature that the
stability of -SO3H increases with the decrease in crosslink
content.12 There is no obvious water evaporation for PPhen-
SO3H-0%, possibly due to its low acidic site density (0.48
mmolg-1) and porosity (76 m2g-1).
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The structural changes from fresh PPhen to PPhen-SO3H
are characterized by Attenuated Total Reflection Infrared
(ATR-IR) spectroscopy for fully crosslink polymer (Fig. 3)
and linear polymer (Fig. S8, Supporting Information). The
absorption bands at around 3030 cm-1 in all spectra
represent the aromatic C-H stretch. The successful loading
of -SO3H sites is confirmed by the new peaks at 1032 cm-1
45,
and 1100 cm-1, corresponding to the S=O stretching.26,
46The emergence of O-H stretching (3400 cm-1) and O-H
bending (610 cm-1, 1650 cm-1) bands for PPhen-SO3H-100%
shows the absorbance of H2O molecules via hydrogen
bonds.45, 46This IR feature of H2O is consistent with the H2O
loss in the TGA measurement, which shows H2O loss of
around 3.3 per -SO3H ligand. All of the vibrational modes are
also visible in the diffuse reflectance infrared fourier
transform spectroscopy (DRIFT) (Fig. S9). The solid-state
13C NMR spectrum of PPhen shows two signals at 140.1 and
129.2 ppm in the characteristic region of sp2 carbon
atoms,47 which correspond to the connecting (140.1 ppm)
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Figure 3. (a)ATR-IR spectra of fresh PPhen-100%, H2O2 treated
PPhen-100%, and PPhen-SO3H-100%. (b) Solid-state13C NMR
spectra of PPhen-100% and PPhen-SO3H-100%. The inset
shows the shift of NMR peaks after the sulfonation.
roughness within the mesopores.26 Sulfonation reduces the
specific surface area to 461 m2g-1 with closed adsorption
and desorption branches and small hysteresis (Fig. 2a,
orange). Decrease of crosslink content reduces specific
surface areas to 435, 407, 224 and 36 m2g-1 for PPhen-82%,
PPhen-65%, PPhen-36% and PPhen-0%, respectively (Fig.
2b and Table S1, Supporting information). In contrast to
PPhen-100%, the surface area of PPhen-0% increases from
36 to 76 m2g-1 after sulfonation (Fig. S6, Table S2
Supporting Information). This is possibly due to the
increased repulsive force between –SO3H in linear chains,
creating new pore systems.
The PPhen polymer framework shows outstanding
thermal stability (Fig. 2c, red) with an initial decomposing
temperature of 400 C. PPhen-SO3H-100% undergoes rapid
weight loss (11%) below 100 C due to the evaporation of
hydrogen-bonded H2O at -SO3H sites. At 200 oC, -SO3H starts
to decompose and reaches the highest rate of weight loss at
around 400 oC, as indicated by the peak in the first
derivatives of TGA (Fig. 2d, Fig. S7, Supporting Information).
The weight loss in this region (16%) is well-matched with
the 1.81 mmolg-1 acidic site density (corresponding to
14.5wt% of -SO3H) measured by chemical titration (Table
Figure 4. (a) Conversion of phenylacetylene and yield of
acetophenone as the function of time for PPhen-SO3H-100%
and Amberlyst 15. Reaction condition: H2O
1
mL,
phenylacetylene 110 L, dodecane as standard 40 L, catalyst
20 mg, temperature 100 C. (b) Selectivity of acetophenone as
the function of conversion for PPhen-SO3H-100% at 100 C. (c)
Phenylacetylene conversion, acetophenone yield and
selectivity as a function of surface area. (d) TOF as a function of
acidic site density for all PPhen-based polymers (blue, black
and red) and Amberlyst 15 (orange).
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and nonconnecting (129.2 ppm) carbons in the substituted 67%, respectively. The positive relationship between
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benzene groups (Fig. 3b). Although not well distinguished
due to large band width, both peaks have shifted towards
the middle after the sulfonation by 0.4 ppm to 139.7 and
129.6 ppm respectively, suggesting the emergence of an
NMR signal for aromatic carbon coordinated with –SO3H
ligands.
activity and crosslink content is different to conventional
polymeric solid acid systems, in which catalysts with less
crosslink content usually have higher activity due to the
flexibility of the polymer frameworks which allows the
31,
diffusion of reactants and products.12,
48Studies on
polystyrene-divinylbenzene based solid acids showed that
the diffusion coefficient through the polymer framework
decreased strongly above 8% divinylbenzene content.31 In
comparison, PPhen-SO3H-100% is stacked via -
interactions, which can swell at z axis, offering high
accessibility of phenylacetylene, even at high crosslink
content (Scheme 1). It achieves 1.81 mmolg-1 of acid
density with sulfuric acid at 40 C, which is much milder
than conventional sulfonation conditions, including the use
of oleum or SO3 at elevated temperatures.12, 48, 49
Catalytic
Performance
of
PPhen-SO3H
in
Phenylacetylene Hydration. We first studied the
structure-reactivity relationship through the hydration of
aromatic alkyne phenylacetylene. The conversion of
phenylacetylene and yield of acetophenone are 22% and
16% for PPhen-SO3H-100% at 100 C over 2 h, which is 20
times higher than that of macroreticular resin Amberlyst 15
(20-30 mesh) (Fig. 4a and Table S3). Both conversion and
yield gradually increase, reaching 80% and 67% at 24 h,
respectively. Overall, over 80% of selectivity is achieved
between 40% and 80% conversion for PPhen-SO3H-100%,
showing superior hydration performance (Fig. 4b). 95%
conversion and selectivity are reached when raising the
temperature to 120 C. The catalytic activities of the
commercial gel-type resins are also evaluated under the
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To justify the increasing catalytic activity with higher
crosslink content, PPhen-SO3H catalysts with a range of
acidic sites density are investigated. They include the
aforementioned PPhen-SO3H catalysts with varied crosslink
contents (Fig. 4d, blue circle), PPhen-SO3H-100% with
different sulfonation durations (Fig. 4d, black square), and
PPhen-SO3H-0% with higher sulfonation temperature (Fig.
4d, red star). Amberlyst 15 is also included as a comparison
(Fig. 4d, orange triangle). Determined by titration, the acidic
site density is 0.48 mmolg-1 for PPhen-SO3H-0%, which
increases to 1.43 mmolg-1, 1.73 mmolg-1, 1.72 mmolg-1 and
1.81 mmolg-1 for PPhen-SO3H-36%, PPhen-SO3H-65%,
PPhen-SO3H-82% and PPhen-SO3H-100% (Table S2,
Supporting Information). This shows that higher porosity
enables higher-SO3H sites density, as -SO3H groups are
more easily grafted onto the polymer frameworks.50 With
extended sulfonation duration, a highest density of 2.12
same
reaction
conditions.
The
conversion
of
phenylacetylene are 5.2%, 15% and 20% for three chosen
resins (Dowex 50WX2, Dowex G26, Dowex Marathon)
respectively (Table S4, Supporting Information), which
again shows the potential of PPhen-SO3H as a promising
solid acid.
PPhen-SO3H
catalysts
show
an
increase
in
phenylacetylene conversion from 23% to 80% when the
surface area increases from 36 to 653 m2∙g-1, corresponding
to a raise in crosslink content from 0% to 100% (Fig. 4c).
Similarly, the yield of acetophenone increases from 13% to
Figure 5. (a) ATR-IR spectra for PPhen-SO3H-100%, PPhen-SO3H-0%, Amberlyst 15 catalysts showing their interaction with
phenylacetylene. (b) Zoom-in of (a) with wavenumber between 3400 and 3175 cm-1.
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Chemistry of Materials
benzene and hydrocarbons, which showed the preference
mmolg-1 is achieved (Table S2, Supporting Information),
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of C-H bonds for pointing towards the benzene ring and that
the benzene-acetylene complex bore the strongest
interaction among all complexes studied.35 The high density
of phenylene and tetra-substituted benzene groups in
PPhen-SO3H catalysts maximizes such absorption and
activation of phenylacetylene.
corresponding to one -SO3H site per 4.5 substituted
benzene groups. Under the same reaction condition, similar
TOF at 0.015 ± 0.001 min-1 is achieved for all PPhen-SO3H
catalysts at different acidic density and crosslink contents
(Fig. 4d), suggesting that PPhen-SO3H offers high
accessibility to both H2O and phenylacetylene molecules
during the reaction. This is attributed to the high
swellability of PPhen-SO3H even at 100% crosslink content.
In comparison, the TOF for Amberlyst 15 is only 0.004 min-
1 (Fig. 4d).
In addition, PPhen-SO3H can be well dispersed in water,
in which the hydrophilicity of the –SO3H group and
hydrophobicity of the PPhen framework (Fig. S11 and
Movie S1, S2 Supporting Information) enable the
simultaneous adsorption of H2O and alkynes. This creates a
“solvent-like” environment, which enables alkynes to
accumulate around the surface of PPhen and facilitates the
subsequent reaction with hydrogen bonded water at –SO3H
sites. The excess amount of water will further dilute the
local concentration of product ketone as it is more soluble
than alkynes and drive the reaction forward.
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The -SO3H ligands also exhibit stability on PPhen-SO3H
series solid acids, in which PPhen-SO3H-100% maintains
phenylacetylene conversion in the range of 60-75% for
three cycles, with over 85% selectivity towards
acetophenone (Fig. S10). The generally lower selectivity for
all tested catalysts is due to the carbon balance problem
associated with the reaction system.
Alkyne Activation over PPhen. PPhen-SO3H catalysts
also benefit from a high density of phenylene and tetra-
substituted benzene groups, for which the interaction with
alkynes is studied via infrared spectra (Fig. 5) by absorbing
a small amount of reactants within the PPhen-SO3H
catalysts (Fig. 5a). The peaks for CC bond (2100-2260 cm-
1) are difficult to identify due to weak intensity, while alkyne
C-H stretching is identified at around 3289 cm-1 for pure
phenylacetylene. It is red-shifted to 3281 cm-1 for
phenylacetylene over both PPhen-SO3H-100% and PPhen-
SO3H-0%, respectively. In comparison, the alkyne C-H
stretching of phenylacetylene over Amberlyst 15 remains at
3289 cm-1. This effect can be understood as the interaction
between the substituted benzene groups in the PPhen and
the aromatic alkyne in Amberlyst 15 with an edge-to-face
(T-shape) configuration (Scheme 1, right), in which
phenylacetylene is vertically adsorbed on the alkynyl
group(-CC-H) pointing to the substituted benzene group in
order to minimize the repulsive force with adjacent
framework benzene groups. As a result, the CC bond is
weakened and shifts the alkyne C-H peak towards a lower
frequency region. The existence of this configuration is
rationalized by research on the CH/π interaction between
Hydration of Aliphatic Alkynes. The CH/ interaction is
not limited to aromatic alkynes, as it is proposed by studies
on direct interaction between aliphatic substituents and the
counterpart aromatic groups.51-56 Therefore, we propose
that the PPhen-SO3H can also activate aliphatic alkynes. The
hydration of 1-octyne was studied with PPhen-SO3H-100%
catalysts with highest acidic sites density (2.12mmol∙g-1)
and Amberlyst 15 (2.4 mmol∙g-1). The PPhen-SO3H-100%
obtains a conversion of 51% and yield of 30% as expected
while Amberlyst 15 shows almost no activity towards the
hydration of the aliphatic alkyne (Fig. 6a), again showing
the exceptional activation of alkynes over a PPhen
framework. The reduced activity compared with
phenylacetylene substrate can be understood as the lack of
the conjugation between phenyl and CC groups, which
activate the CC in phenylacetylene. The interaction
between PPhen framework and 1-octyne is also studied
with ATR-IR (Fig. 6b, Fig. S12, Supporting Information),
which shows a red shift of C-H stretching peak from 3309
cm-1 for pure 1-octyne and that adsorbed in Amberlyst 15 to
3293 cm-1.
CONCLUSION
In conclusion, SO3H-groups functionalized PPhen are
designed with adjustable crosslink contents and acidic site
densities. The advancement of such catalysts features high
porosity, which allow environmental-friendly synthesis
using a mild sulfonation process, as well as excellent
swellability which ensures accessibility of all acidic sites in
the pores. The phenylene and tetra-substituted benzene
groups can activate reactants through CH/π interactions
with a T-shape configuration and provide “an organic
solvent-like environment”. Being able to activate aliphatic
CC under mild conditions, PPhen-SO3H serves as a
promising candidate for alkyne chemistry, in replacement
of highly toxic liquid acids in the synthesis of valuable
chemicals.
Figure 6. (a) Conversion of 1-octyne and yield of 2-octanone.
Reaction condition: 0.2 mmol acidic sites, 100 mg PPhen-SO3H-
100% or 83 mg Amberlyst, 0.5 mmol 1-octyne, 1 mL H2O, 24h,
120 oC. (b) ATR-IR spectra between wavenumber 3400 and
3175 cm-1 for PPhen-SO3H-100% and Amberlyst 15 catalysts
showing their interaction with 1-octyne.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
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Page 8 of 10
Calculation of TOF, acidic sites density and crosslink
of Methane to Methanol. Angew. Chem. Int. Ed. 2009, 48, (37),
6909-6912.
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content, characterizations of PPhen-SO3H-0% and
PPhen-SO3H-100%, catalytic performance of PPhen-
SO3H and commercial acidic resins (PDF)
Movies demonstrating hydrophobicity of PPhen and
PPhen-SO3H (MP4)
(11) Sidorov, S. N.; Volkov, I. V.; Davankov, V. A.; Tsyurupa, M.
P.; Valetsky, P. M.; Bronstein, L. M.; Karlinsey, R.; Zwanziger, J.
W.; Matveeva, V. G.; Sulman, E. M.; Lakina, N. V.; Wilder, E. A.;
Spontak, R. J., Platinum-Containing Hyper-Cross-Linked
Polystyrene as a Modifier-Free Selective Catalyst for l-Sorbose
Oxidation. J. Am. Chem. Soc. 2001, 123, (43), 10502-10510.
(12) Richter, F. H.; Pupovac, K.; Palkovits, R.; Schüth, F., Set of
Acidic Resin Catalysts To Correlate Structure and Reactivity in
Fructose Conversion to 5-Hydroxymethylfurfural. ACS Catal.
2012, 3, (2), 123-127.
(13) Zhang, Y.; Wei, S.; Liu, F.; Du, Y.; Liu, S.; Ji, Y.; Yokoi, T.;
Tatsumi, T.; Xiao, F.-S., Superhydrophobic nanoporous
polymers as efficient adsorbents for organic compounds. Nano
Today 2009, 4, (2), 135-142.
AUTHOR INFORMATION
Corresponding Author
9
* E-mail: ryan.wang@ucl.ac.uk (F.R.W.)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
The project is funded by EPSRC (EP/P02467X/1 and
EP/S018204/2), Royal Society (RG160661, IES\R3\170097,
IES\R1\191035), the Newton Fellowship (NF170761). We
acknowledge Diamond Light Source beamtime (EM22572 and
EM21641). This research has been performed with the use of
facilities at the Research Complex at Harwell including DRIFTS
equipment. The authors would like to thank the Research
Complex for access and support to these facilities and
equipment.
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