A porous Br?nsted superacid as an efficient and durable solid catalyst

Article abstract of DOI:10.1039/c8ta06516k

The development of catalysts able to assist industrial chemical transformations is a topic of high importance. In view of the versatile catalytic capabilities of acid catalysts, extensive research efforts are being made to develop porous superacid materials with a high density of accessible active sites to replace molecular acid catalysts. Herein, we report the rational development of a porous Br?nsted superacid by combining important elements that target high strength acidity into one material, as demonstrated by grafting the sulfonic acid group onto a highly fluorinated porous framework, where the acid strength and stability are greatly enhanced by an electron-withdrawing environment provided by the polymer backbone, reminiscent of that seen in Nafion resin. In addition, the densely arranged acid groups that are confined in the three-dimensional nanospace facilitate the transfer of hydrons, thereby further increasing the acidity. By virtue of the pore structure and strong acidity, this system exhibits excellent performance for a wide range of reactions, far outperforming commercial acid resins under repeated batch and flow reaction conditions. Our findings demonstrate how this synthetic approach may instruct the future design of heterogeneous acid catalysts with advantageous reaction capabilities and stability.

Full text of DOI:10.1039/c8ta06516k

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10.1039/C8TA06516K.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C8TA06516K
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Porous Brønsted Superacid as an Efficient and Durable Solid
Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Qi Sun,*a¶ Kewei Hu,a¶ Kunyue Leng,b Xianfeng Yi,c Briana Aguila,d Yinyong Sun,c Anmin, Zheng,c
Xiangju Meng,a Shengqian Ma,d and Feng-Shou Xiao*a
DOI: 10.1039/x0xx00000x
The development of catalysts able to assist industrially chemical transformations is a topic of high importance. In view of
the versatile catalytic capabilities of acid catalysts, extensive research efforts are being pursued to develop porous
superacid materials with a high density of accessible active sites to replace molecular acid catalysts. Herein, we report the
rational development of a porous Brønsted superacid by combining important elements that target high strength acidity
into one material, as demonstrated by grafting the sulfonic acid group onto a highly fluorinated porous framework, where
the acid strength and stability are greatly enhanced by an electron-withdrawing environment provided by the polymer
backbone, reminiscent of that seen in Nafion® resin. In addition, the densely arranged acid groups that are confined in the
three-dimensional nanospace facilitate the transfer of hydrons, thereby further increasing the acidity. By virtue of the pore
structure and strong acidity, this system exhibits excellent performance for a wide range of reactions, far outperforming
commercial acid resins in repeated batch and flow reaction conditions. Our findings demonstrate how this synthetic
approach may instruct the future design of heterogeneous acid catalysts with advantageous reaction capabilities and
stability.
www.rsc.org/
their liquid structural analogues. Therefore, the development
of nanostructured strongly acidic catalysts, particularly those
with superacidity (acids stronger than 100% sulfuric acid), is
still in the portfolio of researchers and such materials will pave
the way toward alleviating the various existing burdens
imposed by current synthetic technologies pertaining to the
environment and the overall cost.18,19
To tackle the challenges of designing high performance
acid materials and especially those with superacidic character,
we were inspired by perfluororesinsulfonic acids, such as
Nafion®,20 well-known superacid catalysts, and numerous
developed elegant systems on the journey to achieve efficient
porous acid catalysts.21-28 From the established knowledge that
the strength of acidity is primarily determined by the nature of
the acid group (e.g. -SO3H > -COOH), the electronic properties
of the backbone and the density of acid groups in the materials,
whereby reducing the distance strengthens the acidity by
enhancing their cooperation, it is believable that combining all
these essential elements into one material may give rise to a
superior catalyst. We thus postulate that if the sulfonic acid
groups can be grafted onto a highly fluorinated porous
framework with preset position for modification, a catalyst
with a high density yet readily accessible acid sites that are
Introduction
The development of porous materials offers great
improvements in catalyst design for efficient large-scale
chemical processes.1-6 In particular, solid acid catalysts,7-12 as
represented by zeolites, have played an indispensable role in
the petroleum refinery industry for continuous throughput and
efficient separation.13,14 Yet, due to the lack of high
performance solid acid catalysts, a significant number of acid-
catalyzed reactions still mainly rely on molecular acids.15,16
Transferring the high activity of these catalysts into their solid
states remains challenging because the low grafting degree of
acid groups and the rigidity of solid supports prevents them
from cooperating in the transfer of protons, an essential
feature in the manifestation of superacidity.17 In addition, the
lack of suitable porous materials with unique properties to
compensate such an inevitable loss after immobilization, leads
to the acidity of the resultant solids being much weaker than
a.Key Lab of Applied Chemistry of Zhejiang Province and Department of Chemistry,
Zhejiang University, Hangzhou 310028, China. Email: sunqichs@zju.edu.cn;
fsxiao@zju.edu.cn.
b.School of Chemical Engineering and Technology, Harbin Institute of Technology,
Harbin Institute of Technology, Harbin 150001, China.
arranged
in
electron-withdrawing
three-dimensional
c. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences,
Wuhan 430071, China.
nanospaces will be achieved. Due to the strong
electronegativity of the fluorine atoms, it not only makes the
molecule less susceptible to chemical, electrophilic attack, but
also stabilizes the sulfonic acid anions and thereby facilitates
the leaving of the protons on them. This is anticipated to
d.Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue,
Tampa, Florida 33620, United States.
¶ These authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: Experimental methods and
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afford new acid materials with fast reaction kinetics, high acid thereby holding great promise to replace the existing
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DOI: 10.1039/C8TA06516K
strength, in conjugation with great stabilities, which surmount technologies.
the prime issues of low efficiency encountered by Nafion® as a
result of low surface area and low density of acid
Material and methods
Materials
Commercially available reagents were purchased in high purity and
used without purification. Solvents were purified according to
standard laboratory methods. THF and DMF were distilled over
LiAlH4 and CaH2, respectively.
Catalyst preparation
Synthesis of fluorinated porous polymer (FPOP). 2,2’,4,4’,6,6’-
hexafluoro-3,3'-divinyl-1,1'-biphenyl (1.0 g) was dissolved in
THF (10 mL), followed by addition of azobisisobutyronitrile
Scheme 1 Synthetic illustration of the fluorinated porous organic polymer (FPOP) and
its subsequent sulfonation with oleum.
(AIBN, 25 mg). The mixture was transferred into an autoclave
groups,29 as well as low acid strength of many other solid
at 100 C for 24 h. The title material was obtained after being
washed with CH2Cl2 and dried under vacuum.
catalysts.30 To fulfill these tasks, porous organic polymers
(POPs)31-33 give great promise as versatile candidates for acid
catalysis. Due to their amenable synthesis, high surface areas,
and exceptional chemical stability, they have emerged as one
of the prime contenders to traditional porous materials, such
as active carbon and silica gels, with a great potential to
address ongoing energy and environment challenges.34-40 In
this contribution, we demonstrate that the sulfonic acid
groups grafted on a highly fluorinated porous polymer
framework catalyze in a range of industrially important
Synthesis of sulfonated FPOP (FPOP-SO3H). Fuming sulfonic
acid (20%, 40 mL) was added dropwise to a 100 mL flask
containing 500 mg of FPOP. The mixture was stirred at room
temperature for 48 h, and then 50 °C for another 48 h, after
which the mixture was poured into ice water and washed with
an excess of water until the pH of the filtration tuned to
around 7.
transformations with a combination of activity and selectivity Characterization
that is premier among all benchmark heterogeneous acid
catalysts tested.
The gas adsorption isotherms were collected on the surface
area analyzer ASAP 2020. The N2 sorption isotherms were
measured at 77 K using a liquid N2 bath. Scanning electron
microscopy (SEM) images were collected using Hitachi SU 1510
or Hitachi SU 8000. TGA was carried out on a Q50
thermogravimetric analyzer under N2 atmosphere at a heating
rate of 10 °C min-1. IR spectra were recorded on a Nicolet
Impact 410 FTIR spectrometer. X-ray photoelectron
spectroscopy (XPS) spectra were performed on a Thermo
In developing versatile POP-based acid catalysts, we sought
to employ 2,2',4,4',6,6'-hexafluorobiphenyl as a building block
given that it is highly fluorinated yet molecularly designable.
The modularity property of this compound promises its
feasibility to be constructed into a highly porous framework
and to be introduced with abundant active sites. Moreover,
the performance of the acid groups, in turn, could benefit from
the electron-withdrawing polymer backbone (Scheme 1). In
view of materials synthesis for practical applications, free-
radical induced polymerization of vinyl-functionalized
monomers are particularly attractive with these considerations:
in addition to the excellent monomer tolerance and cost-
effective synthesis, the adapted polymerization conditions
allow adjustment of the pore structure and introduce
hierarchical porosity for the enhancement of mass
transfer.41,42 We thus surmised that modifying sulfonic acid
moieties onto such a framework could serve as a very
proficient solid catalyst for a wide range of acid catalysis.
Indeed, in liquid phase processes, the resultant catalyst with
higher intrinsic acidity exhibits outstanding properties that are
many times better than that for the sulfonic acid groups
grafted on an analogous nonfluorinated porous polymer and
commercial acid resins, as well as approaching or exceeding
those observed for H2SO4 as a catalyst. Most notably, this
catalyst, with its hierarchical porosity, is highly active and
durable in fixed bed conditions, allowing for the elimination of
solvent and ease of separation of the reaction products,
ESCALAB 250 with Al K irradiation at θ = 90 for X-ray sources,
and the binding energies were calibrated using the C1s peak at
284.9 eV. CHNS elemental analyses were performed on a
Perkin-Elmer series II CHNS analyzer 2400. Diffuse reflectance
ultraviolet-visible (UV-vis) spectra were measured with
spectrometer of PE Lambda 20, and BaSO4 was an internal
1
standard sample. H NMR spectra were recorded on a Bruker
Avance-400 (400 MHz) spectrometer. Chemical shifts are
expressed in ppm downfield from TMS at
 = 0 ppm, and J
values are given in Hz. 13C (125 MHz) cross-polarization magic-
angle spinning (CP-MAS), and 19F (470 MHz) MAS solid-state
NMR experiments were recorded on a Bruker Avance-500
spectrometer. Before the 31P MAS NMR measurements,
samples were placed in a Pyrex cell equipped with a stopcock.
The sample cell was outgassed at 120 °C. A CH2Cl2 solution
containing a known amount of TMPO (100%, Alfa) was added
under inert atmosphere to prevent moisture absorption. After
being homogeneously mixed, CH2Cl2 solvent was removed by
evacuation at room temperature and then transferred into a
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NMR sample rotor. The (NH4)2HPO4 was chosen as reference exhibited a sharp uptake of N2 at low relative pressuVrieew(PAr/tPic0le<O0nl.in1e)
with 31P chemical shift at 1.0 ppm.
and a hysteresis loop at higher relative presDsuOrIe: 1(00..1403<9/PC/8PT0A<0065.9156K),
manifesting the hierarchically porous structure of FPOP44 and the
Brunauer-Emmett-Teller (BET) surface area was calculated to be
866 m2 g−1 (Fig. 1c).
Results and discussion
Synthesis of fluorinated porous framework (FPOP). In our attempts
to target a sulfonic acid grafted fluorinated porous framework, a
two-step synthesis was employed due to the incompatibility of the
highly active vinyl groups in the harsh conditions of the sulfonation
process. The fluorinated porous framework (FPOP) can be
quantitatively obtained by polymerization of the vinyl-
functionalized hexafluorobiphenyl under solvothermal conditions in
tetrahydrofuran (THF) at 100 °C for 24 h with the assistance of
azobisisobutyronitrile (AIBN), where the monomer was readily
synthesized from 1,3,5-trifluorobenzene, as detailed in the ESI†.
Introduction of sulfonic acid groups on the fluorinated porous
framework (FPOP-SO3H). With a porous and fluorinated framework
available, we proceeded to introduce sulfonic acid groups. By
treatment of FPOP with oleum at room temperature for 48 h and
then 50 °C for another 48 h, the resultant solid was denoted as
FPOP-SO3H. The successful sulfonation of FPOP was confirmed by X-
ray photoelectron spectroscopy (XPS), FT-IR, solid-state 13C NMR
studies, and elemental analysis. The appearance of the sulfur signal
at a binding energy of 169.1 eV in the XPS spectrum of FPOP-SO3H
suggests the existence of sulfonic acid groups (Fig. S3, ESI†).45 In
addition, the emergence of the characteristic bands of S=O at 1079
cm−1 and 1197 cm-1 compared with that of pristine FPOP45 verifies
the presence of sulfonic acid groups (Fig. S4, ESI†). The high
throughput of the sulfonation process was determined by the
significantly weakened peak assigned to the nonsubstituted
aromatic carbons at around 102.6 ppm and the concomitant
emergence of a peak at 129.4 ppm ascribed to the aromatic carbon
substituted by –SO3H group in the 13C MAS NMR spectrum of FPOP-
SO3H (Fig. S5, ESI†). The retention of F species of FPOP during the
sulfonation process was indicated by the high intensity of F signals
in the XPS and 19F MAS NMR spectra of FPOP-SO3H (Fig. S3 and S5,
ESI†). The slight shift of F signals in FPOP-SO3H compared with
those in FPOP can be a result of the introduction of electron-
withdrawing sulfonic acid groups, which lead to the benzene ring
becoming more negative and thus decreasing the electron density
of F species. The acid content of FPOP-SO3H was measured by ion
exchange with NaCl, followed by titration of the free H+ with NaOH,
showing that it contains 3.56 mmol g-1 of accessible H+ sites (Table
S1, ESI†). To validate the titration results, elemental analysis was
performed. Based on the S element content, the sulfonic acid group
amount in FPOP-SO3H was calculated to be 3.49 mmol g-1, which is
consistent with that from the titration experiments. SEM and TEM
images of FPOP-SO3H displayed similar morphologies as that of
FPOP, indicative of its structural integrity and retention of
hierarchical pore structure after sulfonation (Fig. S6, ESI†). N2
sorption results revealed that the BET surface area of FPOP-SO3H
was 340 m2 g-1 with a pore volume of 0.3 cm3 g-1, confirming its pore
structure (Fig. 1c). The decreased surface area can be reasonably
attributed to the increased mass after the functionality addition.
Based upon the calculations of the nonlocal density functional
theory method (NLDFT), the pore size is distributed in the range of
2.8-18 nm (Fig. S7, ESI†).
Structure characterization of fluorinated porous framework
(FPOP). The chemical composition and structure of FPOP was
determined by solid-state 13C and 19F NMR spectroscopy. 13C MAS
NMR spectrum of FPOP showed a characteristic signal centered at
31.2 ppm, which is assignable to the opened double bonds formed
after polymerization.43 The chemical shifts at 102.6, 116.0, and
160.2 ppm can be attributed to the nonsubstituted, substituted,
and adjacent aromatic carbons to the F atoms in the polymer
backbone, respectively, which are in accord with those of the
monomer (Fig. S1, ESI†). 19F MAS NMR spectrum of FPOP gave a
clear F signal, however, due to the chemical anisotropy of solid
samples, the well-separated 19F signals in the range of -110.6 to -
109.3 ppm of liquid NMR of the monomer appeared as a broad
signal at -111.6 ppm in solid NMR of the resultant polymer (Fig. S2,
ESI†). These results indicate that the hexafluorobiphenyl moieties
are maintained during the polymerization process. The morphology
of FPOP was examined by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM), showing that FPOP
displayed
a rough surface that is composed of randomly
agglomerated small particles with sizes ranging from several
nanometers to over tens of nanometers, giving rise to
interconnected meso- and macroporous objects (Fig. 1a and 1b). To
investigate the details of the pore characteristics of FPOP, N2
sorption isotherms at 77 K were measured. The adsorption curve
Taking a lesson from Nafion®, it is suggested that the
presence of electron-withdrawing F atoms in the structure
increases significantly both the thermal stability and acid
strength of the sulfonic acid groups.46 To determine the role of
fluorine species on the performance of sulfonic acid groups in
FPOP-SO3H, side-by-side comparisons were made with the
nonfluorinated analogue, POP-SO3H (BET : 561 m2 g-1, Fig. S8-
S10, ESI†). This material was synthesized by polymerization of
4,4'-divinylbiphenyl and then sulfonated as described above
Fig. 1 Porous structure and acid strength Characterization. (a) SEM and (b) TEM
of FPOP, (c) N2 sorption isotherms collected at 77 K, and (d) 31P MAS NMR
spectra of TMPO chemically adsorbed on FPOP-SO3H and POP-SO3H, respectively
.
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(see details in the ESI†). Titration experiments were carried out acid-catalyzed conversions, carbocationic mechanisms are generally
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to value the accessible H+ content in POP-SO3H, giving as high accepted. The strong acidity facilitatesDOtI:h1e0.10g3e9n/Cer8aTtAio0n6516oKf
as 3.84 mmol g-1, indicative of a similar acid content in POP- carbocations which may also result in the formation of coke.
SO3H and FPOP-SO3H. To compare the thermal stability of However, improved diffusion by introduction of pore structures in
FPOP-SO3H and POP-SO3H, thermogravimetric analysis (TGA) the solid acid catalysts has proven to be an effective way to inhibit
under N2 atmosphere was performed and both samples such a side reaction and thereby the overall efficiency.[12]
showed a multi-step decomposition pattern (Fig. S11 and S12,
ESI†). The first step mass loss before 150 °C is attributed to
Catalytic evaluation. To demonstrate the usefulness and
effectiveness of FPOP-SO3H, the esterification of palmitic acid with
removal of adsorbed H2O. A second mass loss is likely
ethanol was used as a test platform to compare its catalytic
associated with desulfonation and the remaining is likely due
performance against that of POP-SO3H, two benchmark solid acid
to decomposition of the polymer matrix. It is interesting to
catalysts Amberlyst-15 and Nafion® NR50, as well as a mineral acid,
find that FPOP-SO3H exhibits higher decomposition
H2SO4. This particular reaction was chosen because esterification of
higher fatty acids has spurred a great deal of interest, due to the
potential utilization of the product as a biofuel. Also, the reaction is
temperatures for both the sulfonic groups (290 °C vs 230 °C)
and the polymer framework (430 °C vs 350°C) in relation to
those of POP-SO3H, promising for FPOP-SO3H to have long-
especially important in the synthesis of biodiesel from low-cost
term stable performance under high temperature conditions.
feedstocks containing a large amount of free fatty acids, as such
The increased thermal resistance after introduction of F
compounds must undergo acid-catalyzed esterification before
species could be a result of the hydrogen bonding interactions
transesterification of the larger triglycerides being carried out using
between H and F species that stabilize the overall structure.[47]
a base.52 Fig. 2a shows the time course of ethyl palmitate formation
Acidity strength test. To evaluate the acid strength, the Hammett over various catalytic systems with a catalyst loading level of 2.0
method was employed by using a range of closely related UV-vis mol% accessible H+ sites relative to palmitic acid (Table S2, ESI†).
probes. Specifically, the Hammett acidity (H0) was tested by The initial turnover frequencies (TOF) from this figure are 9.4 h−1 for
immersing a solid in a specific indicator solution, if the color of the FPOP-SO3H, 3.3 h−1 for POP-SO3H, 1.5 h−1 for Amberlyst-15, and 2.3
solid changes to that of the acid form of the indicator or on the h−1 for Nafion® NR50, indicating that it is at least four and six times
change in position of the indicator absorption band in the visible higher than Nafion® NR50 and Amberlyst-15 catalysts, respectively,
spectrum, the H0 value of that solid is the same or lower than the in the initial
pKa of the conjugate acid of the indicator. After being soaked in a
benzene solution of 4-nitrofluorobenzene, FPOP-SO3H displayed a
new UV-vis peak at around 460 nm, which is attributed to the acid
form of 4-nitrofluorobenzene, indicating H0 ≤ −12.4 and placing it in
the superacid region (H0 ≤ −12.0), while FPOP-SO3H exhibited
moderate acidity (−4.4 ≥ H0 ≥ −8.1), as evidenced by the fact that it
showed a colour change in 2,4-dinitroaniline benzene solution but
not in anthraquinone solution.48 To provide more qualitative
information, probe molecule studies were investigated. Given the
sensitivity and isotropicity of 31P chemical shift of the probe
Fig. 2 Catalytic kinetics and water resistance tests. (a) Ethyl palmitate yield verse time
plots over various catalysts. Reaction conditions: palmitic acid (2.0 mmol), ethanol (2.0
mL), catalyst (0.04 mmol accessible H+, 2.0 mol%), and 60 C. (b) Water sensitivity of
trimethylphosphine oxide (TMPO) according to its interaction
strength with a Brønsted acid site, it has proven to be an
informative tool for identifying the acidity of multiple acid sites.49,50
31P MAS NMR spectrum of TMPO after interaction with FPOP-SO3H
sample shows a singlet peak at 87.1 ppm, appearing to possess an
acid strength higher than the threshold of superacidity (86.0 ppm).
By contrast, notable decrease in acid strength was observed for
POP-SO3H catalyst, as revealed by the TMPO resonance upfields to
81.7 ppm, providing a consistent interpretation of the acidity
evaluated by Hammett analysis. These results were further
validated by their performance in catalytic α-pinene isomerization
(Scheme S1, ESI†), where FPOP-SO3H showed appreciable
conversion of α-pinene with low camphene yield and high
secondary reactions of limonene, while POP-SO3H afforded inferior
conversion and relatively high selectivity towards camphene and
limonene (Fig. S13, ESI†).51 It is worth mentioning that FPOP with a
low degree of sulfonation (2.04 mmol g-1) gave an adsorbed TMPO
31P NMR signal at 87.1 ppm. Together, these results indicate that
both F species and the amount of sulfonic acid groups introduced
play critical roles in the acidity of the resultant materials. In the
palmitic acid esterification catalyzed by H2SO4 and FPOP-SO3H. Reaction conditions:
palmitic acid (10.0 mmol), ethanol (10.0 mL), catalyst (0.04 mmol accessible H+), 60 C,
and in the presence of various amounts of water.
1 h. In addition, the reaction using FPOP-SO3H goes to a full
conversion after 13 h, while at that time, POP-SO3H, Amberlyst-15,
and Nafion® NR50 only gave the product yields of 88.7%, 47.0%, and
65.1%, respectively. The catalytic tests section in the ESI† has the
TOF formula and a detailed procedure on calculating yield from GC.
Considering the same amount of active sites employed, the much
higher activity observed for FPOP-SO3H relative to other solid
catalysts tested should be attributed to the combined contributions
from its high acidity and accessibility of active sites. In comparison
with H2SO4, FPOP-SO3H exhibited an inferior initial activity, giving a
TOF value by a factor of 1.3 lower than that of H2SO4 in the
beginning 1 h (9.4 h-1 vs 12.5 h-1). However, their discrepancy in
kinetics became smaller along with the reaction. Specifically, FPOP-
SO3H reached around 90% and 95% product yields within 6 h and 10
h, respectively, and it also took the same time for H2SO4 to
accomplish such conversions. We assumed that this phenomenon
should result from their different level of tolerance towards water.
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As an esterification byproduct, water was found to significantly Table 1. Catalytic data in the Friedel-Crafts acylation of benzoyl chloride and
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inhibit H2SO4 involved catalysis.52 To prove this hypothesis, their anisole over various catalysts.a
DOI: 10.1039/C8TA06516K
resistance against water poisoning was evaluated. The effect of
water on FPOP-SO3H- and H2SO4-catalyzed esterification was
studied using initial reaction kinetics (<15% conversion) with
varying amounts of initially added water. This permits a better
determination of the effect of water on the catalyst, because the
Entry
Catalyst
Conv. (%)
Sel. (%)
rate is little affected by the reverse reaction when only a small
amount of the ester products are present. In Fig. 2b, the water
sensitivities of FPOP-SO3H and H2SO4 are compared. The catalytic
activity of H2SO4 was significantly inhibited with increasing water
concentration in the reaction mixture. After 1 h of reacting, the
system with 10 wt% initial water content had a conversion around
80% lower than that with no initial water added (13.6% vs 3.0%).
By contrast, FPOP-SO3H exhibited less deactivation by water than
H2SO4, as demonstrated by less drop in activity (10.5% vs 5.3%).
After proving the water resistance, we assessed the heterogeneity
of FPOP-SO3H under reaction conditions by evaluating the possible
leaching of acid species. To this end, the filtrate obtained by
catalyst recovery after 3 h was continued to react. Negligible ester
formation was observed, providing evidence against leaching of a
soluble active catalyst (Fig. S14, ESI†). Furthermore, the retention of
reactivity upon recycling reaffirm that the material is stable under
the catalytic conditions. The above results underscore the
advantages of FPOP-SO3H as an environmental friendly solid acid
catalyst (Fig. S15, ESI†). To verify the structural stability of the
catalyst during the reaction, we carried out the N2 sorption tests. It
was revealed that negligible changes were observed in terms of
both surface area and pore volume (Fig. S16, ESI†).
1
2
1
2
3
4
5
FPOP-SO3H
POP-SO3H
Amberlyst-15
Nafion® NR50
H2SO4
95.3
55.3
44.8
61.3
39.3
>99.5
82.4
78.2
79.6
62.4
n.d.b
17.6
21.8
20.4
37.6
a
Reaction conditions: benzoyl chloride (2.0 mmol), anisole (1.0 g), 100
catalyst (0.04 mmol accessible H+). b Not detectable.
C, 10 h,
Similarly, in the Friedel-Crafts alkylations between benzyl
alcohol and toluene, FPOP-SO3H also exhibited the most
satisfactory results compared with other catalysts tested by
taking into account both activity and selectivity. In this
reaction, along with the formation of the desired benzylation
product (monobenzylated aromatic), it usually accompanies
with the dehydrative coupling of benzyl alcohol to yield
dibenzyl ether.56 As shown in Table S2 (ESI†), FPOP-SO3H
exhibited a full benzyl alcohol conversion after 6 h with a
satisfyingly preferred product yield rivaling to that of Nafion®
NR50 (76.8% vs 83.4%), but far surpassed it in terms of kinetics.
In contrast, the benzylation product/ether selectivity of
Amberlyst-15 and Nafion® NR50 were in the range of 1.6-1.9
and H2SO4 only gave a ratio of around 1:1. These results
suggest that FPOP-SO3H constitutes a competitive alternative
to replace some traditional homogeneous catalysts.
The fast reaction rate combined with the stability and
recyclability (Table S3 and S4, ESI†) of FPOP-SO3H prompted us
to evaluate its performance under continuous fixed-bed
reaction conditions to reduce the footprint of chemical plants
and increase their productivity, modularity, and flexibility. To
highlight the superiority of FPOP-SO3H, the performance of
other heterogeneous catalytic systems were compared, taking
the benzylation reaction of benzene with benzyl alcohol as an
example. Fig. 3a and 3b show the profile of reaction
conversion and selectivity as a function of time over FPOP-
SO3H, POP-SO3H, Amberlyst-15, and Nafion® NR50. FPOP-SO3H
experienced a remarkable increase in catalytic activity in the
initial stage, approaching to a full conversion after 150 min
(this delay is likely because when the instrument is initially
started it requires time to stabilize, Fig. S17 ESI†). Impressively,
the activity and selectivity toward the target product
diphenylmethane remained essentially constant after that; this
is in striking contrast to POP-SO3H, which gave rise to a steady
decrease in conversion from 97% to 60% and consistent
selectivity at around 90% during the reaction time. Under
identical conditions, the two nonporous resin catalysts,
To further demonstrate the versatility of FPOP-SO3H towards
other acid catalyzed reactions we have studied its
performance on Friedel-Crafts alkylation and acylation
reactions which are one of the top research priorities in
industry and academia.53-55 The traditional catalysts used in
acylation chemistry suffer from several drawbacks from the
use of stoichiometric amounts of catalysts (AlCl3, BF3) to
extensive problems with the disposal of the associated waste
stream, motivating more direct and atom-economical
procedures employing a catalytic amount of activator. Using
acylation of anisole with benzoyl chloride as a model reaction,
once again, FPOP-SO3H is the most active material, giving rise
to a 95.3% benzoyl chloride conversion within 10 h. Even more
interesting was the finding that the desired (4-
methoxyphenyl)(phenyl)methanone (1) was the only product
observed (Table 1, entry 1). By contrast, other catalysts,
Amberlyst-15, Nafion® NR50, POP-SO3H, and H2SO4 were found
to be much less effective, affording the conversion of benzoyl
chloride at 44.8%, 61.3%, 55.3%, and 39.3%, respectively,
under identical conditions. In addition, being different from
FPOP-SO3H, both of the para and ortho acylated products,
whose ratios were in the range of 1.66 to 4.68, were detected
for the other catalytic systems studied (Table 1, entries 2-5).
The observed unique catalytic performance of FPOP-SO3H
should be as a result of the combination of strong acidity and
unique pore structure.
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DOI: 10.1039/C8TA06516K
Fig. 3 Fixed-bed reaction evaluation. (a and b) Conversion of benzyl alcohol and selectivity of diphenylmethane and (c) plots of diphenylmethane yields versus time over various
catalysts (50 mg of FPOP-SO3H, POP-SO3H, Amberlyst-15, or 300 mg of Nafion® NR50) in the benzylation reaction of benzene with benzyl alcohol. Reaction conditions were detailed
in the catalytic tests in the ESI.
In summary, a hierarchically porous superacid material with
Amberlyst-15 and Nafion® NR50, exhibited very low conversion high surface area and abundant accessible active sites was
and selectivity and even worse, the performance is not stable, synthesized by grafting sulfonic acid groups on a highly
as demonstrated by the fact that the conversion of the fluorinated porous polymer framework. The resultant material
reaction underwent a sharp decrease and both of the catalysts has a much higher surface area and accessible H+ content than
gave less than 5% conversion after 180 min. These results the benchmark superacid Nafion® NR50, acting as a remarkable
thereby underscore the importance of the introduction of solid acid catalyst with superior long-term stability in a range
hierarchical porosity on the maintenance of the catalyst’s high of industrially important and molecular catalyst-dominated
performance over extended reaction times by improving reactions. The improved catalytic performances are attributed
accessibility of reactants to catalytic sites and providing faster to combined contribution of the faster transport paths of
transport paths for removal of products and byproducts. In reagents to active sites resulting from the porous structure,
addition to this, the presence of F species also plays an and increased acid strength and stability of sulfonic acid group
essential role to maintain the long-term stability of active sites. grafted on a highly fluorinated framework. We believe these
To further assess the productivity of the catalysts, the yield features will profoundly advance the application and
according to the conversion and selectivity was calculated. fabrication of solid superacid catalysts. In light of the excellent
FPOP-SO3H exhibited a yield that increased with reaction time catalytic performance, porous fluorosulfonic acids, as
and reached a stable value of about 97% after 150 min. In presented here, might be promising alternatives to replace
contrast, a steady decrease in yield over the reaction time was hazardous and corrosive homogeneous acid catalysts, thus
observed for all of the other catalysts tested. Specifically, in making a major contribution to green chemistry.
terms of the amount of product after the reaction time period
of 120 min, FPOP-SO3H far exceeded all of the other catalysts,
producing 1.83, 91.3, and 131.7 times as much product as that
by POP-SO3H, Amberlyst-15, and Nafion® NR50, respectively at
Conflicts of interest
There are no conflicts to declare
300 min. It is worth mentioning that due to the hieratical
porosity of FPOP-SO3H, diffusion is not a rate determining step
given the fact that we increased the reactant benzoyl alcohol
Acknowledgements
injection rate from 0.06 mL min-1 to 0.07 mL min-1 and
We acknowledge National Natural Science Foundation of China
performed the catalytic evaluations under otherwise identical
(21720102001, 21333009, and 21422306).
conditions. A benzoyl alcohol conversion of 93% was achieved
within 60 min (Fig. S17 ESI†). The synergistic effects of high
acidity and hierarchical porosity afford FPOP-SO3H with the
highest values known among porous-material-based solid acid
catalysts, placing it within striking distance of the all-time
benzylation record and thereby presenting a new benchmark
for acid materials.57-62
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Journal of Materials Chemistry A
Page 8 of 8
View Article Online
DOI: 10.1039/C8TA06516K
Porous Brønsted Superacid as an Efficient and Durable Solid
Catalyst
Qi Sun,*a¶ Kewei Hu,a¶ Kunyue Leng,b Xianfeng Yi,c Briana Aguila,d Yinyong Sun,b
Anmin, Zheng,c Xiangju Meng,a Shengqian Ma,d and Feng-Shou Xiao*a
a Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University,
Hangzhou 310028, China
b School of Chemical Engineering and Technology, Harbin Institute of Technology,
Harbin Institute of Technology, Harbin 150001, China
c State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan
430071, China
d Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue,
Tampa, Florida 33620, United States
¶ These authors contributed equally to this work
*E-mail: sunqichs@zju.edu.cn; fsxiao@zju.edu.cn
A porous superacid material with high surface area and abundant accessible active
sites is rationally designed, showing outstanding catalytic activities and durability in
industrially chemical transformations.