Developing two-dimensional solid superacids with enhanced mass transport, extremely high acid strength and superior catalytic performance

Article abstract of DOI:10.1039/c9sc01988j

Solid acids have been widely used as heterogeneous catalysts in developing green and sustainable chemistry. However, it remains a challenge to improve the mass transport properties and acid strength of solid acids simultaneously. Herein, we report a class of two dimensional (2D) layered hybrid solid acids with outstanding mass transfer and extremely high acid strength by incorporating sulfonated polymers in-between montmorillonite layers. The 2D layered structure and broad distribution of pore sizes allow for highly efficient mass transport of substrate molecules into and out of the solid acids. The acid strength of these solid acids was found to be stronger than that of 100% H₂SO₄, H₃PW₁₂O₄₀ and any other reported solid acids to date, as determined by ¹H and ³¹P solid-state NMR. These 2D solid acids show extraordinary catalytic performance in biomass conversion to fuels, superior to that of H₃PW₁₂O₄₀, HCl and H₂SO₄. Theoretical calculations and control experiments reveal that H-bond based interactions between the polymer and montmorillonite facilitate the unusually high acid strengths found in these samples.

Full text of DOI:10.1039/c9sc01988j

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Developing two-dimensional solid superacids with
enhanced mass transport, extremely high acid
strength and superior catalytic performance†
Cite this: DOI: 10.1039/c9sc01988j
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Fujian Liu, *ab Xianfeng Yi, Wei Chen,
c
cd
cd
e
a
Zhiqiang Liu, Wei Chen, Chen-Ze Qi,
e
c
Yu-Fei Song
and Anmin Zheng
*
Solid acids have been widely used as heterogeneous catalysts in developing green and sustainable
chemistry. However, it remains a challenge to improve the mass transport properties and acid strength
of solid acids simultaneously. Herein, we report a class of two dimensional (2D) layered hybrid solid acids
with outstanding mass transfer and extremely high acid strength by incorporating sulfonated polymers
in-between montmorillonite layers. The 2D layered structure and broad distribution of pore sizes allow
for highly efficient mass transport of substrate molecules into and out of the solid acids. The acid
strength of these solid acids was found to be stronger than that of 100% H
2
SO
other reported solid acids to date, as determined by H and P solid-state NMR. These 2D solid acids
show extraordinary catalytic performance in biomass conversion to fuels, superior to that of H PW12
HCl and H SO . Theoretical calculations and control experiments reveal that H-bond based interactions
4 3
, H PW12O40 and any
1
31
Received 22nd April 2019
Accepted 20th May 2019
3
40
O ,
2
4
DOI: 10.1039/c9sc01988j
between the polymer and montmorillonite facilitate the unusually high acid strengths found in these
samples.
rsc.li/chemical-science
3,21–23
the substrate molecules during reactions.
However, the
Introduction
three dimensional, connected network of the pores may give
Solid acids are one of the most important catalysts used in rise to locally conned spaces and limit the diffusion of large
1
–4
24,25
industry. Compared to liquid acids, solid acids cause less molecules or macromolecules into and out of the system.
In
5–9
corrosion of the facility, can be easily separated from the addition to improving the accessibility of acidic sites and
10–14
reaction medium for recycling,
tunable for specic feedstock and products.
and their properties may be enhancing mass transport, it is also desirable to incorporate the
15–19
10,11,22,24
However, strongest possible acid centers into solid acids,
to lower
widely used solid acids such as natural clays, sulfated metal the activation energy in acid-catalyzed reactions. Integrating all
oxides, and ion-exchange resins have poor porosity that limits these designing factors into the molecular engineering of solid
the exposure of their acidic sites to the substrate mole- acids, however, remains challenging in the eld.
2,7,14,20
cules.
Solid acids such as zeolites and sulfonated meso-
The emergence of two dimensional (2D) materials provides
porous materials have abundant, nanometer-scaled pores, a new opportunity in designing new solid acids to overcome this
25–31
which considerably enhance the accessibility of acidic sites to challenge.
When organized as layered structures, 2D materials
such as graphene and its derivatives usually facilitate enhanced
a
mass transfer of guest molecules and can be chemically modied
Key Laboratory of Alternative Technologies for Fine Chemicals Process of Zhejiang
Province, College of Chemistry and Chemical Engineering, Shaoxing University, to incorporate specic acid groups. The enhanced mass transfer
Shaoxing, 312000, China
can be attributed to the improved wettability and unique nano-
b
25,26
National Engineering Research Centre of Chemical Fertilizer Catalyst (NERC-CFC),
sheet structures in these 2D materials.
However, it is usually
School of Chemical Engineering, Fuzhou University, Fuzhou, 350002, China. E-mail:
difficult to gra abundant acidic sites onto the surface of gra-

liu@fzu.edu.cn
2
phene and its derived materials, due to the inert nature of the sp
carbon.
c
Wuhan Center for Magnetic Resonance, Key Laboratory of Magnetic Resonance in
Biological Systems, State Key Laboratory of Magnetic Resonance and Atomic and
25,26
In addition, the regeneration of graphene-based
Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy materials is challenging as they are not readily separated from the
2
5,26
of Sciences, Wuhan 430071, China. E-mail: zhenganm@wipm.ac.cn
reaction mixture.
Although some early studies, e.g., based on
d
University of Chinese Academy of Sciences, Beijing 100049, China
sulfated graphene, showed promising results, none of these
studies have yet succeeded in controlling wettability, improving
mass transportation and enhancing acidity simultaneously.
In general, the acid strength of solid acids could be
enhanced by introducing strong electron-withdrawing moieties
e
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, 100029 Beijing, China
25–29
†
Electronic supplementary information (ESI) available: detailed descriptions of
the apparatus and synthetic methods, 29 gures, and tables. See DOI:
0.1039/c9sc01988j.
5
1
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3 3 3 3 4
Scheme 1 Synthetic route for 2D hybrid solid superacids MMT@PAN-[C N][SO CF ] and MMT@PPY-[C N][SO H].
2,23
(
e.g., uorine containing groups) on their surface. In partic- acidity, and excellent mass transporting properties. The
ular, graing uorine-containing groups onto solid acids is procedure for the synthesis of the 2D hybrid solid acid is
6,22
+
a good choice, due to the improved stability.
A variety of illustrated in Scheme 1. We rst use ion exchange to replace K
polymeric acids have also been incorporated into 3D silica with protonated aniline in the interlayer of MMT. The inter-
materials, which exhibit enhanced acidity due to the conne- calated aniline was then self-polymerized in-between the MMT
10,24,32–36
ment effect.
connement effect in layered 2D materials and molecular
engineering of the local electronic environment may provide which results in the formation of
Alternatively, taking advantage of the layers to give a MMT@PAN composite, and then the released
+
À
H
combines with O anchored onto the network of MMT,
hydroxyl group.
a
new routes to 2D solid acids with a desired acid strength and MMT@PAN was further sulfonated with 1,3-propanesultone
surface wettability. Herein, we design a class of hybrid solid and ion exchanged with triuoromethanesulfonic acid or
superacids based on a 2D montmorillonite (MMT) scaffold by sulfuric acid to give 2D hybrid solid acids MMT@PAN-[C N]
3
polymerizing nitrogen-containing monomers such as aniline [SO
3
CF
3
]
3 4
and MMT@PAN-[C N][SO H]. Besides aniline,
and pyrrole in-between the MMT layers (Scheme 1). The poly- protonated pyrrole can also be used as the monomers and
mers are sulfonated subsequently to facilitate extremely high polymerized in-between MMT to obtain MMT@PPY, and
À
acidity, because of the unique hydrogen bonding between the [SO
4
H] can be incorporated as the anion in this system
MMT and sulfonic groups in polymer species. Their acid (Scheme 1). Altogether, three different samples, MMT@PAN-
strength was stronger than that of 100% H SO , phospho- [C N][SO CF ], MMT@PAN-[C N][SO H] and MMT@PPY-[C N]
2
4
3
3
3
3
4
3
tungstic acid and any other reported solid acids to date. The [SO H] were prepared. We control the extent of sulfonation of
4
interstices in-between the MMT layers are 2D in nature and vary the polymers in MMT to tune the surface wettability and
in the domain sizes. Aer aniline and pyrrole molecules are porosity of the samples. The morphology and structures of the
intercalated into the interstices and polymerized, the resultant hybrid solid acids are shown in Fig. S1.† All the samples
polymers can be further functionalized to include superacidic exhibit periodic 2D layered structures.
sites and to possess good wettability. We found that both small
2
Fig. S2† shows N isotherms and the pore size distribution of
organic molecules and large macromolecules can have efficient the synthesized samples. MMT shows a type II isotherm with 2D
37
mass transport into and out of the 2D solid superacids. The layered characteristics, showing an upsurge at a relative
synergistic effect of good wettability, record-high acid strength, pressure (P/P ) between 0.5 and 1.0. Aer intercalation of
0
and efficient mass transportation thus gives rise to an extraor- polymers into MMT (e.g., MMT@PAN and MMT@PPY) and
dinary catalytic performance in the degradation of cellulose, subsequent sulfonation, the resultant 2D solid acids exhibit
a major component of biomass and, a class of macromolecules similar isotherms with 2D layered characteristics, but reduced
that are usually very difficult to deal with by using solid acids. volume of adsorption resulting from the incorporation of
polymers into the interstices. In comparison, MMT has a BET
2
À1
surface area of 101 m g , while MMT@PAN, MMT@PPY, and
the obtained 2D solid acids all show reduced BET surface areas
ranging from 29 to 97 m g (Table S1†). Besides, the hierar-
chical structures in these samples possess a wide size distri-
bution ranging from 2.3 to 129.2 nm (Fig. S2†).
Results and discussion
2
À1
Our proposal is to incorporate sulfonated polymers in-between
MMT layers and nd the conditions under which the resulting
materials may simultaneously possess good wettability, super
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The successful graing of sulfonic group onto the 2D hybrid of structural defects and locally distorted structures with
solid acids was conrmed by XPS and FT-IR spectroscopy. different domain sizes in the samples. The mentioned hierar-
Except for MMT, additional signals of S , C , N , O and F1s chical structures may provide the opportunity for guest mole-
2
p
1s
1s
1s
could also be observed in these samples (Fig. S3†). The signals cules of different sizes to diffuse into and out of the solid acids
of S2p at around 168–170 eV were assigned to the sulfonate and afford diverse local environments for acid catalyzed reac-
À
À
group, [SO
tted and deconvoluted into several peaks centered at around and different domain sizes (Fig. S7†) were observed by scanning
84.5, 286.6, 289.7 and 292.3 eV, corresponding to the C–C, C– electron microscopy (SEM). The HR-TEM images and STEM
4 3 3
H] and [SO CF ] anions. The signals of C1s were tions. Similar hierarchically layered structures with abundant

2
22
N, C–S and C–F bonds, respectively. The signals of N1s were images of the synthesized 2D solid acids are shown in Fig. 1, in
tted and deconvoluted into two peaks, assigned to aniline or which highly ordered, periodic layer-by-layer structures coexist

pyrrole N, and quaternized N with 1,3-propanesultone. FT-IR with locally distorted structures with different domain sizes in
spectra also conrm the successful incorporation of sulfonic these samples. The hybrid layered structures were also
groups into PAN and PPY (Fig. S4†). The additional broad peaks conrmed by elemental linear scanning (Fig. S8†). The
À1
at around 1040, 1086 and 1186 cm were assigned to C–S and elemental maps of MMT@PAN-[C
3
N][SO
4
H] in Fig. S8† indicate
22
S]O bonds, in good agreement with XPS results. The sulphur the relatively homogeneous distribution of Si, Al, C, N, O and S
contents of the 2D solid acids were measured to be from 1.24 to in the crystalline domains of the samples, consistent with XPS
À1
+
1
.36 mmol g , and their H ion capacities range from 1.32–1.83 and FT-IR spectra.
À1
mmol g (Table S1†), lower than that of Amberlyst-15, but
We rstly compared the diffusion properties of the 2D hybrid
higher than that of Naon NR50. The above results indicate that solid acids with those of 3D mesoporous solid acids of SBA-15-
the hybrid solid acids have well-dened structures with SO H-0.2, commercial Amberlyst-15 and superacid of Naon
3
adjustable concentrations of acid groups.
NR50 in the esterication of acetic acid with cyclohexanol. As
H-0.2,
The structures of the synthesized 2D hybrid solid acids are shown in Fig. 2, the catalytic efficiencies of SBA-15-SO
3
characterized by X-ray diffraction (XRD) and high-resolution Amberlyst-15 and Naon NR50 in the reaction depend on the
transmission electron microscopy (HR-TEM). A well-dened stirring rates, indicating that the process is limited by diffusion
layered structure could be detected before and aer the intro- of molecules into the inner pores of solid acids. In contrast, the
duction of sulfonated polymers into MMT. Fig. S5† shows the catalytic efficiencies of the 2D solid acids are almost indepen-
XRD patterns of MMT, MMT@PAN, MMT@PPY and the three dent of the stirring rate, and can reach very high levels even
2
D solid acids samples: MMT@PPY-[C N][SO H], MMT@PAN- without stirring. This phenomenon used to only exist in strong
3
4
[C
3
N][SO
4
H] and MMT@PAN-[C
3
N][SO
3
CF
3
2 4
]. Compared with liquid acids such as H SO (Fig. 2). The result indicates the
pristine MMT, the a peaks associated with (001) diffraction outstanding mass transporting properties of 2D hybrid solid
became wider for MMT@PPY-[C N][SO H], MMT@PAN-[C N] acids. In addition, the 2D hybrid solid acids have much higher
SO H] and MMT@PAN-[C N][SO CF ], and the peak positions TOF values and better activities than H SO , Naon NR50 and
shied from 5.77 to 6.67 . This change indicates that the Amberlyst-15 under the same reaction conditions (Table S2†).
average layer distance of MMT decreases from 1.54 to 1.33 nm To understand the molecular basis of the mass transport
3
4
3
[
4
3
ꢀ
3
3
2
4
aer introducing the sulfonated polymers. In general, the properties, we used theoretical calculations to compare the
intercalation of polymer additives into the interlayer of MMT mass transport properties of 2D MMT with those of 3D con-
usually results in the extension of its interlayer distance. The nected zeolites such as MFI, MOR and VFI. The mass transport
nearly 14.4% shrinkage of the layer distance should come from properties of different solid acids were studied by molecular
the strong interactions between sulfonated polymers and the dynamics (MD) (Fig. S9 and S10†). We found that the diffusion
MMT support. Interestingly, aer calcination of MMT@PAN- of small molecules such as methanol or acetic acid in MMT was
3 3 3
[C N][SO CF ] with oxygen to remove the sulfonated polymer, at least 1 order faster than that in zeolites with 3D micropores
the residual MMT support shows a similar pattern to pristine (Table S3†). Presumably, the unique 2D layered structures,
MMT. This phenomenon further conrms the successful together with a controllable and good surface wettability
intercalation of the sulfonated polymer into the interlayer of (Fig. S11†), promote the mass transportation of molecules into
MMT and the strong interactions between the sulfonated and out of the MMT support.
polymer and MMT layers. The successful intercalation of the
In addition to mass transport, the acid strength plays a crit-
sulfonated polymer into the interlayer of MMT can be further ical role in the catalytic performance of solid acids. We found
conrmed by fabricating an inverted structure of MMT, in that the acid strength of the 2D solid acids was effectively
which sulfonated polymer wrapped MMT (designated as MMT- enhanced via the molecular interactions between the
PAN-[C N][SO CF ]) was prepared for comparison. Both sulfonated polymer and MMT in close proximity. The acidity of
3
3
3
1
MMT@PAN-[C N][SO CF ] and MMT-PAN-[C N][SO CF ] were the synthesized 2D solid acids was investigated by using H and
carbonized and etched with HF for the removal of the MMT
scaffold. The resultant MMT@carbon shows a much enhanced determination of the acidity of solid acid catalysts.
porosity in comparison with MMT-carbon (Fig. S6†). The particular, P trialkylphosphine (TMP) NMR has been used to
3
3
3
3
3
3
3
1
P MAS NMR, a reliable and state-of-the-art technique for
6
,9,24,38,39
In
3
1
incorporation of the polymers into MMT increases the hetero- distinguish Brønsted and/or Lewis acid types. When TMP
3
1
geneity of the system and leads to the decreased crystallinity of molecules are bound to Lewis acid sites, they give rise to
P
MMT (broadening of the a peak), which results in the formation resonance ranging from ca. À20 to À60 ppm. In contrast,
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Fig. 1 HR-TEM images of (A and B) MMT@PPY-[C
3
N][SO
3
3 3 4 3 3 3
CF ], (C and D) MMT@PAN-[C N][SO H], and (E) MMT@PAN-[C N][SO CF ], and STEM
3 4 3 3 3 3 3 3
images of (F) MMT@PAN-[C N][SO H], (G) MMT@PPY-[C N][SO CF ] and (H and I) MMT@PAN-[C N][SO CF ].
+
TMPH ions, which are protonated by the Brønsted acidic
3
1
proton, normally lead to P resonances at ca. À2 to À5
6
,9,24,38,39
31
ppm.
We found that P NMR only resides at À2.9 ppm
3 3 3
for TMP adsorbed on MMT@PAN-[C N][SO CF ], suggesting
6
,9,40
a Brønsted acid (Fig. S12a†).
In general, both Lewis and
Brønsted acid sites are expected to exist in MMT due to the
3
+
41,42
presence of Al ions in low coordination.
This assessment
3
1
was conrmed by P TMP NMR; the Lewis acid site in MMT
shows a broad and weak signal (Fig. S12b†). However, no signal
of the Lewis acid site was detected in MMT@PAN-[C
SO CF ], likely due to the loss of most Lewis acid sites during
the preparation process involving the treatment with a strong
3
N]
[
3
3
41,42
acid (HSO CF ).
3
3
The acid strength of MMT@PAN-[C N][SO CF ] was deter-
3
3
3
3
1
mined by using P trimethylphosphine oxide (TMPO) NMR,
where a larger P chemical shi of adsorbed TMPO indicates
3
1
stronger acidity. As shown in Fig. 3, the MMT support displayed
3
1
31
two distinct P resonances at d P of 46 and 55 ppm, which are
Fig. 2 The yields of cyclohexyl acetate in the esterification of acetic
acid with cyclohexanol catalyzed by various solid acids at different assigned to the physisorbed TMPO and TMPO adsorbed on
stirring rates (0.11 mol of cyclohexanol, 0.305 mol of acetic acid, 0.15 g weaker acidic sites, respectively. MMT possesses sites with
ꢀ
+
2 4
of solid acids, 100 C for 5 h; H SO : the same number of H sites as
weaker acidity comparable to those of HY zeolites (55–65
3 3 3
MMT@PAN-[C N][SO CF ]).
6,41
ppm).
Meanwhile, the sulfonated polymer of PAN-[C N]
3
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1
Fig. 3 (A) Room temperature P MAS NMR spectra of trimethylphosphine oxide (TMPO) adsorbed on (a) MMT support, (b) PAN-[C
3 3 3
N][SO CF ]
6
and (c) MMT@PAN-[C N][SO CF ]; (B) the comparison of the acid strength of MMT@PAN-[C N][SO CF ] with that of various reported acids.
3
3
3
3
3
3
3
1
[
SO CF ] showed enhanced acidity, giving P spectra with show the kinetic curves and calculated activation energy in the
3 3
TMPO adsorption at 84 ppm. The intercalation of the transesterication of tripalmitin with methanol over various
sulfonated polymer into the MMT layer results in a signicant catalysts, where methyl palmitate was the only product. The 2D
3
1
increase of the acid strength with P chemical shis found at solid acids showed far better catalytic activities than Amberlyst-
6 and 93 ppm (Fig. 3A and S13†), corresponding to two types of 15, SBA-15-SO H-0.2, Naon NR50, porous SO /ZrO and
superacid sites formed in MMT@PAN-[C N][SO CF ], which 40, as well as H SO . The acid strength of the solid
should be attributed to the sulfonated polymer intercalated in acids was found to be correlated with the activation energy of
both locally distorted structures and the interlayer of MMT. A the reaction. For instance, MMT@PAN-[C N][SO CF ] shows an
chemical shi above 86 ppm in P TMPO NMR was commonly activation energy of 34.32 kJ mol , much lower than those of
8
3
4
2
3
3
3
H
3
PW12
O
2
4
3
3
3
3
1
À1
6
À1
À1
À1
À1
used to identify superacids. As far as we know, the measured H SO (45.84 kJ mol ), H PW O (50.05 kJ mol ), Naon
2
4
3
12 40
3
1
d P value of 93 ppm is by far the largest chemical shi reported NR50 (59.74 kJ mol ), Amberlyst-15 (88.36 kJ mol ) and SBA-
6
À1
3
for TMPO adsorbed on solid acids (Fig. 3B). Therefore, the acid 15-SO H-0.2 (95.55 kJ mol ) under experimental conditions.
strength of the 2D solid acid is stronger than any other reported The acid strength (evaluated from P chemical shis) and
3
1
solid acids to date, which is even higher than that of 100% activation energy in this reaction show a strong correlation
3
1
H
2
SO
at nearly 89.4 and 92.1 ppm.
strength of MMT@PAN-[C N][SO CF ] has been further strong acid strength and outstanding mass transport proper-
4 3
and H PW12O40, both of which have a P chemical shi (Fig. S17†), suggesting that the acid strength is regulated by the
6,43,44
The unusually high acid activation energy of solid acid catalysts. Combining the ultra-
3
3
3
1
1
conrmed by the Hammett indicator method. The UV spectra ties, much higher TOF values and initial reaction rates were
of the solutions with the indicator (4-nitrouorobenzene) were found in MMT@PAN-[C N][SO CF ] than H SO , Amberlyst-15
collected, showing a much stronger adsorption peak for and Naon NR50 (Table S2 and S4†).
3
3
3
2
4
MMT@PAN-[C
result conrms that the acidity of MMT@PAN-[C
4
SO . Furthermore, which eight methyl ester products with varying lengths of the
3
N][SO
3
CF
3
] than for H
2
SO
4
(Fig. S14†). The
The 2D solid acids also showed impressive activities for the
3
N][SO CF
3
3
]
conversion of low quality plant oil to biodiesel (Table S5†), in
(
H
0
< À12.4) is stronger than that of H
2
potentiometric titration was used to characterize the acidity of carbon chain and unsaturated levels were detected. The yields
MMT@PAN-[C N][SO CF ]; the very high initial electric poten- of biodiesel reach 94.2–99.1% in the reactions catalyzed by the
3
3
3
tial (617.8 mV) of MMT@PAN-[C N][SO CF ] further conrms 2D solid acids, far better than those of H PW O (69.8–74.1%),
3
3
3
3
12 40
45
31
its superacidity (Fig. S15†), in good agreement with P solid PAN-[C
3
N][SO
4
4 2
H] (86.5–89.2%), porous SO /ZrO (29.3–40.9%),
state NMR. Likely, the superacidity of the 2D hybrid solid acids SBA-15-SO
3
H-0.2 (33.5–58.2%), Amberlyst-15 (29.8–51.7%) and
is a result of the interactions between the sulfonated polymer H-USY (9.3–20.6%). Once again, the activities of the synthesized
+
and the MMT scaffold, which promotes the release of H in the 2D solid acids are better than those of strong liquid acids such
sulfonate group.
2 4
as H SO (95.9–98.1%). Unlike liquid acids, these 2D solid acids
The enhanced mass transfer and superacidity facilitate the could be easily recovered by centrifugation. Their activities
excellent catalytic efficiency of 2D solid acids, and their catalytic largely remained the same even aer ve times of recycling. The
performance was evaluated in important acid-catalyzed reac- excellent reusability of the 2D solid acids should be due to their
tions of biomass conversion to fuels, which include towards superior mass transfer and enhanced hydrothermal stabilities;
transesterication to biodiesel and depolymerization of crys- the sulfonic groups in the 2D solid acids only start to decom-
ꢀ
talline cellulose to sugars and ne chemicals. Fig. 4 and S16† pose at a temperature above 250 C under ambient conditions
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a 3 3 3
Fig. 4 (A) The yields of methyl palmitate catalyzed by various acid catalysts, and (B) activation energies (E ) of (a) MMT@PAN-[C N][SO CF ], (b)
H SO , (c) H PW12O40, (d) Nafion NR50, (e) Amberlyst-15 and (f) SBA-15-SO H-0.2 in the transesterification of tripalmitin with methanol.
2 4 3 3
Table 1 Yields of dehydration products in the depolymerization of 2D distorted structures, which can be veried by the adsorption
Avicel catalyzed by various solid acids
of Avicel. The mentioned hierarchically 2D nanostructures were
c
large enough for the Avicel macromolecules to diffuse into and
out of the sample (Fig. S21†), and thereby facilitate fast
deconstruction of Avicel in the reaction (Fig. S22†).
Glucose
Cellobiose Cellobiose TRS
a
a
b
Catalysts
yield (%) yield (%) yield (%) (%)
Amberlyst-15
27.5
37.2
22.7
66.5
55.5
79.5
77.6
76.9
19.1
18.5
13.8
13.7
11.6
9.1
3.9
4.6
4.5
9.3
7.9
8.5
7.7
6.3
56.2
To understand the unusual acid strength of the synthesized
D solid superacids, H solid state NMR was used to investigate
H
3
PW12
O
40
68.7
47.2
94.8
83.4
99.5
98.2
96.3
1
2
SBA-15-SO
HCl
3
H-0.2
the behavior of –SO
3
H in MMT@PAN-[C
3 3 3
N][SO CF ]. As shown
1
in Fig. 5, the H peak at 4.4 ppm was assigned to the acidic
3
PAN-[C N][SO
3
CF
3
]
46
1
MMT@PAN-[C
MMT@PAN-[C
MMT@PPY-[C
3
N][SO
N][SO
3
CF
H]
H]
3
]
proton in –SO
to hydrogen atoms in the –NH
ppm were attributed to the –CH – group bound to –SO H. In
3
H. The H peaks at 6.7–8.0 ppm were assigned
3
4
8.9
9.4
– group, and the peaks at 0.8–2.5
2
3
N][SO
4
2
3
a
ꢀ
Measured by the HPLC method; the reaction was performed at 100 C contrast to PAN-[C N][SO CF ], the intensities of peaks at ca.
3
3
3
b
c
for 5 h. Yields of HMF were measured by GC-MAS. Yields of TRS were
measured by the DNS method.
6
.7–8.0 ppm were enhanced (Fig. 5b), and a new lower-eld peak
at 16.7 ppm appeared in MMT@PAN-[C N][SO CF ]. We believe
3
3
3
that this is due to the connement effect of 2D layered MMT,
which allows for the formation of strong H-bonds between
intercalated PAN-[C
the acid strength. It is noteworthy that –SO
C N][SO CF ] cannot fully interact with the MMT scaffold
because of the intrinsic steric hindrance effect, which results in
the presence of a lower chemical shi of TMPO at around 86
3
N][SO
3
CF
3
] and MMT, further enhancing
(
Fig. S18 and 19†), while the polymer networks start to
ꢀ
3
H in MMT@PAN-
decompose at a temperature above 450 C due to their large
molecular weights (Mw of PAN-[C N][SO CF ] and PPY-[C N]
[
3
3
3
3
3
3
3
À1
[SO
4
H] was measured to be ꢁ22k and ꢁ23k g mol , respec-
tively, Fig. S20†) and strong interactions with the MMT scaffold,
which can prevent the decomposition of the sulfonated polymer
ꢀ
in a low temperature range (<250 C).
To further demonstrate the capability of the 2D solid acids,
we investigate their catalytic efficiency in the depolymerization
of crystalline cellulose (Avicel), which is usually very difficult to
deal with using traditional solid acids. When the reaction is
catalyzed by MMT@PAN-[C N][SO CF ], the total reducing
3 3 3
sugar was up to 99.5%, with the yields of glucose, cellobiose and
HMF measured to be 79.5, 9.1 and 8.5%, respectively. High
catalytic efficiencies could also be found in MMT@PAN-[C
3
N]
[
SO H] and MMT@PPY-[C N][SO H]. The yields of glucose,
4
3
4
cellobiose and HMF reach ꢁ77%, ꢁ9%, and ꢁ7% within 5 h of
reaction, and the total reducing sugar was almost ꢁ98%. The
catalytic activities of the 2D solid acids were much higher than
those of Amberlyst-15, H 40 and SBA-15-SO
3
PW12
O
3
H-0.2, as well
1
Fig. 5 H solid-state NMR spectra of (a) PAN-[C
3
N][SO
3 3
CF ], (b)
as HCl and PAN-[C N][SO CF ] (Table 1). The superior catalytic
3
3 3
MMT@PAN-[C
MMT@PAN-[C
denote spinning sidebands.
3
N][SO
N][SO
3
CF
CF
3
]
and (c) trifluoroacetic acid treated
activities of the 2D solid acids should be attributed to their
extremely high acid strength and the presence of hierarchically
31
3
3
3
]. The asterisks in the P NMR spectra
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Fig. 6 The optimized structural parameters of sulfonated aniline and pyrrole in MMT (a and c) and sulfonated aniline and pyrrole monomers (b
and d). The calculated O–H interatomic distances (in A˚ ) are indicated.
ppm (as depicted in Fig. 3A). To further conrm the intermo- mechanism to understand the drastically enhanced acid
3
1
lecular hydrogen bonding interaction, MMT@PAN-[C
3
N] strength of the 2D solid superacids and is consistent with
P
1
[
SO CF ] was treated with triuoroacetic acid, an acid and H solid-state NMR, Hammett indicators and potentio-
3
3
commonly used to interrupt H-bond interactions in mole- metric titration methods.
4
7
cules. We found, aer treating MMT@PAN-[C N][SO CF ] with
Besides the MMT scaffold, graphite oxide can also serve as
triuoroacetic acid, that the intensities of peaks at ca. 6.7–8.0 the layered nanostructure for incorporation of PAN-[C N]
ppm decreased, and the lower-eld peak at 16.7 ppm dis- [SO CF ] to give the 2D solid acid GO@PAN-[C N][SO CF ]. The
appeared. This result conrms the existence of H-bonds product was conrmed from XRD patterns, Raman spectra,
between intercalated PAN-[C N][SO CF ] and MMT. electron microscopy and XPS spectra (Fig. S24–S27†). However,
The molecular nature of H-bond interactions between no H-bond interaction was observed between PAN-[C N]
sulfonated polymers and MMT was further investigated by [SO CF ] and GO in the synthesized GO@PAN-[C N][SO CF ]
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
using DFT calculations. The length of the O–H bond in –SO H of (Fig. S28†), which may be attributed to the lower oxygen
3
sulfonated aniline and pyrrole ions is determined to be 0.981 A˚ . contents and electron-rich network of GO in comparison with
Aer intercalation of sulfonated aniline and pyrrole ions into the MMT scaffold. As a result, GO@PAN-[C
3
N][SO
3
CF
3
] shows
3
CF ]
the interlayer of MMT, the length of the O–H bond in –SO H is a much weaker acid strength than MMT@PAN-[C
3
3
N][SO
3
˚
˚
calculated to be 1.060 A and 1.022 A respectively, which are (Fig. S29†). The control over the molecular interactions between
longer (as shown in Fig. 6). On the basis of the optimized acidic sites and the layered scaffolds plays a crucial role in the
structure, –SO
3
H in sulfonated aniline and pyrrole monomers enhancement of the acid strength of 2D solid acid catalysts.
shows distances from surface oxygen atoms on the MMT scaf-
˚
˚
fold of 2.504 A and 2.593 A, which are assigned to hydrogen-
Conclusions
48
bonding interactions with relatively high strength. Detailed
electronic structure information was used to understand the In summary, a unique type of 2D hybrid solid superacid was
nature of the elongated O–H bond as shown in Fig. S23;† the developed by incorporating sulfonated polymers in-between
deformation charge density between the sulfonated polymer MMT layers, with extensive H-bond interactions between MMT
and MMT mainly appears in the (MMT) O/H (sulfonated and the –SO
aniline and pyrrole ions) H-bond, and electron transport occurs weakens the O–H bond in –SO
3
H moiety in polymers. This molecular interaction
+
3
H, promotes the release of H ,
more extensively between sulfonated aniline and MMT than and thereaer allows for the preparation of solid acids with one
between sulfonated pyrrole and MMT, which may explain the of the record high acid strengths. In addition, the 2D layered
stronger acidity of MMT@PAN-[C N][SO CF ] found in the structures in the solid acids possess a broad distribution of pore
3
3
3
experiments. While this preliminary study ignores the hetero- sizes and tunable wettability, which promote the mass trans-
geneity of the system, the DFT calculation suggests a plausible portation of molecules into and out of the system and provide
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good accessibility of the acidic sites. As a result, the 2D hybrid 17 M. Dusselier, P. V. Wouwe, A. Dewaele, P. A. Jacobs and
solid acids show excellent performance for biomass conversion B. F. Sels, Science, 2015, 349, 78–80.
to fuels. This work may provide a new concept for designing 18 R. Rinaldi, R. Palkovits and F. Sch u¨ th, Angew. Chem., Int. Ed.,
solid acid catalysts with both highly efficient mass transport 2008, 47, 8047–8050.
and extremely strong acidity, which may nd useful industrial 19 M. Toda, A. Takagaki, M. Okamura, J. N. Kondo, S. Hayashi,
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Conflicts of interest
1 F. J. Liu, W. P. Kong, C. Z. Qi, L. F. Zhu and F.-S. Xiao, ACS
Catal., 2012, 2, 565–572.
2 F. J. Liu, L. Wang, Q. Sun, L. F. Zhu, X. J. Meng and F.-S. Xiao,
J. Am. Chem. Soc., 2012, 134, 16948–16950.
There are no conicts to declare.
Acknowledgements
23 M. A. Harmer, W. E. Farneth and Q. Sun, J. Am. Chem. Soc.,
996, 118, 7708–7715.
1
We acknowledge the nancial support from the National
Natural Science Foundation of China (grant no. 21573150,
2
2
2
2
2
2
4 X. M. Zhang, Y. P. Zhao, S. T. Xu, Y. Yang, J. Liu, Y. X. Wei
and Q. H. Yang, Nat. Commun., 2014, 5, 3170.
5 F. J. Liu, J. Sun, L. F. Zhu, X. J. Meng, C. Z. Qi and F.-S. Xiao, J.
Mater. Chem., 2012, 22, 5495–5502.
6 J. Ji, G. Zhang, H. Chen, S. Wang, G. Zhang, F. Zhang and
X. Fan, Chem. Sci., 2011, 2, 484–487.
21203122, 21522310, 21473244 and 91645112), the Natural
Science Foundation of Zhejiang Province (LY15B030002), the
Natural Science Foundation of Hubei Province (2018CFA009),
the Key Research Program of Frontier Sciences, CAS (No.
QYZDB-SSW-SLH026) and the Program for Qishan Scholar of
Fuzhou University (GXRC-18043). We thank Prof. Yao Lin at the
University of Connecticut for his helpful suggestions and
discussions.
7 F. Lin, J. Zhang, D. Liu and Y. Chin, Angew. Chem., Int. Ed.,
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