Confinement of Br?nsted acidic ionic liquids into covalent organic frameworks as a catalyst for dehydrative formation of isosorbide from sorbitol

Article abstract of DOI:10.1039/c9gc02126d

The confinement of Br?nsted acidic 1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrosulfate ([PSMIm][HSO₄]) into the channel walls of two-dimensional (2D) COFs using a one-pot self-assembly strategy was achieved by incorporating an imine-linked TPB-DMTP-COF (TPB, triphenylbenzene; DMTP, dimethoxyterephthaldehyde) as the host. An appropriate loading of [PSMIm][HSO₄] is crucial for the BIL-COF hybrids to maintain proper geometry in the channel and sufficient acidic sites for the sorbitol substrate and sorbitan intermediate to enter and react. The best yield of isosorbide (97%) from sorbitol to date was obtained in the presence of BIL-COF-30 as the catalyst under optimized conditions. Besides, BIL-COF-30 can be recycled for at least five runs without activity loss.

Full text of DOI:10.1039/c9gc02126d

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DOI: 10.1039/C9GC02126D
Journal Name
ARTICLE
Confinement of Brønsted Acidic Ionic Liquids into Covalent Organic
Frameworks as Catalyst for Dehydrative Formation of Isosorbide
from Sorbitol
Received 00th January 20xx,
Accepted 00th January 20xx
Yi-Ran Du,a,b Bao-Hua Xu,a,b*Jia-Sheng Pan,a,c Yi-Wei Wu, Xiao-Ming Peng, Yao-Feng Wang, and
d
d
a
a,b*
DOI: 10.1039/x0xx00000x
Suo-Jiang Zhang
www.rsc.org/
The confinement of Brønsted acidic 1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrosulfate ([PSMIm][HSO ]) into the
4
channel walls of two dimensional (2D) COFs using a one-pot self-assembly strategy was addressed by incorporating imine-
linked TPB-DMTP-COF (TPB, triphenylbenzene; DMTP, dimethoxyterephthaldehyde) as the host. A proper loading of
[
4
PSMIm][HSO ] is crucial for the BIL-COF hybrids to maintain proper geometry in the channel and sufficient acidity sites for
sorbitol substrate and sorbitan intermediate to enter and react. The best yield of isosorbide (97%) from sorbitol to date was
obtained in the presence of BIL-COF-30 as the catalyst under optimized conditions. Besides, BIL-COF-30 can be recycled for
at least five runs without activity loss.
including acid resins,15-18 zeolites,19-22 supported metal oxides,
23
2
4-29
30-37
Introduction
sulphated/phosphate metal oxides,
metal phosphates
3
8
and mesoporous carbon-based acid have been employed. It
was found that higher reactivity could be achieved in the case
of catalyst bearing proper porosity, sufficient Brønsted acidity
and surface hydrophobicity. Noteworthily, the strong sulfonic
Cyclic ethers derived from catalytic dehydration of sugar
alcohols play privileged role as structural motifs of many bio-
1
-4
solvents, pharmaceuticals, surfactants and polymers.
Particularly, isosorbide, which is synthesized from sorbitol
3
9
acid resins such as Glu-Fe
3 4 3
O -SO H
(94%, isolated),
5
following an acid-catalyzed two-step dehydration process,
3
8
organosulfonic acid functionalized mesoporous carbon
represents the most promising replacement to bisphenol A in
the synthesis of polycarbonate with excellent optical properties.
Various homogeneous catalyst including mineral acids, organic
(
82.7%, HPLC) as well as hydrophobic mesoporous polymer-
4
0
41
3
based acid catalysts (P-SO H (87.9%, HPLC) and PDS (86.8%,
HPLC)) provided excellent yields of isosorbide from sorbitol so
far. Besides, a novel strategy reported by Xu and Lu using
ketone reagent as the solvent under the catalysis of H-beta
zeolite facilitated 93% GC yield.
acids, Lewis acidic molten salts and their mixtures were
4
2
investigated,6 wherein H
-13
2 4
SO showed the best activity and
was widely used in the synthesis of isosorbide in industry. To
address the serious drawbacks of environmental pollution
arising from base-acid neutralization post-treatment, significant
handling risk, corrosive nature of the catalyst and stability issues
of the products, considerable efforts have been made in the
development of more practical and efficient strategies. The
most attractive solution is to design a proper heterogeneous
catalyst attributed to convenient catalyst separation from
reaction system.14 To this end, many types of solid acid
NH2
Jiang's work
HAc (6 M, 0.1 mL)
o-DCB/n-BuOH/H2O
H2N
NH2
(5/5/1 by vol., 1.1 mL)
120 oC, 72 h
TPB-DMTP-COF
CHO
CHO
O
This work
O
[
PSMIm][HSO4]
(10/30 g/L)
a. Beijing Key Laboratory of Ionic Liquids Clean Processes, CAS Key Laboratory of
Green Processes and Engineering, State Key Laboratory of Multiphase Complex
Systems, Institute of Process Engineering, Chinese Academy of Sciences, 100190,
Beijing, China. Email: bhxu@ipe.ac.cn; sjzhang@ipe.ac.cn
School of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China
BIL-COF hybrids
b.
Scheme 1. Schematic illustration of the preparation process of BIL-COF
hybrids.
c.
State Key Laboratory of Chemical Engineering, East China University of Science and
Technology, Shanghai 200237, China
d.
Ionic liquids (ILs) as one of green candidates attract a broad
scientific attention because their significant role as efficient
Beijing Key Laboratory of Clean Fuel and Efficient Catalytic Reduction Technology
in Beijing, Beijing Institute of Petrochemical Technology, Beijing 102617, China
Y.-R. Du, Prof. Dr. B.-H. Xu, J.-S. Pan, Y.-W. Wu, Dr. X.-M. Peng, Dr. Y.-F. Wang, Prof.
†
4
3-45
catalysts in various reactions.
Our laboratory has been
Dr. S.-J. Zhang
Electronic Supplementary Information (ESI) available: [details of any supplementary focusing on constructing a continuous process of isosorbide
information available should be included here]. See DOI: 10.1039/x0xx00000x
production by using Brønsted acidic ionic liquids (BILs) as the
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
catalyst.46 However, it is still lack of practical value under the equiv.) in the presence of [PSMIm][HSO
] (X g/L; X = 10, 30 and
4
View Article Online
present homogeneous conditions because the simultaneous 50) under solvothermal conditions in DaOIm: 10ix.1t0u3r9e/Co9fGCn0-2b1u2t6yDl
generation of himins is significant and its amount will be further alcohol/o-dichlorobenzene/water (n-BuOH/o-DCB/H O, 5: 5: 1 by
2
increased during the reactive distillation operation. One way to vol., 1.1 mL). Here Brønsted acidic [PSMIm][HSO ] was supposed
4
circumvent these problems, particularly the catalyst separation
problem, is to confine BILs in nanoporous matrices with one or
more of their spatial dimensions subjected to a geometrical
restriction. Considering the structural requirement of the
carriers for a high catalytic activity, flexible regulation of the
pores and easy modification of functional reactive sites onto the
skeleton of covalent organic frameworks (COFs) render them
to take the catalytic role of acetic acid in the formation of COF
The parent TPB-DMTP-COF52 from the reaction
structure
catalysed by acetic acid was also prepared for convenient
comparison.
.
4
7,48
promising platforms
liquids (ILs).
for this nanoconfinement of ionic
To attain such an IL-COF hybrid system, besides the post
4
9
impregnation imposed by simple physisorption, the one-pot
synthesis recently reported by Fang’s group could be
5
0
considered as an effective synthetic methodology. Therein, IL
[BMIm]NTf ) functions not only as a green solvent but also a
(
2
guest molecule, thereby a series of three dimensional (3D) IL-
containing imine-linked COFs (3D-IL-imine-COFs) were
efficiently produced under mild conditions. Such a methodology
was also employed by Dong’s group in the synthesis of two
5
1
dimensional (2D) keto-enamine-linked COFs. Unfortunately,
-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate
[BSMIm][HSO ]) launched as the solvent was completely
1
(
4
removed during the post-treatment. Apparently, challenge still
remains in the search for a successful confinement of ILs into
COFs, which should suppress leakage of ILs during the reaction
and maintain sufficient space and proper geometry in the
channel for substrates/intermediates to enter. Here we address
this issue by incorporating imine-linked 2D-COF as the host to
reinforce the interaction with the guest BILs for the synthesis of
a new BIL-COF hybrids that combines outstanding stability with
satisfied porosity. Specifically, TPB-DMTP-COF (TPB, Fig.1 PXRD (a) and BET (b) studies of TPB-DMTP-COF and BIL-COF hybrids.
triphenylbenzene;
reported by Jiang’s group was selected because of its excellent
performance in strong acid resistance, high porosity and large
specific surface area of COFs. We demonstrated this COF
successfully confines 1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-
ium hydrosulfate ([PSMIm][HSO
DMTP,
dimethoxyterephthaldehyde)
The power X-ray diffraction (PXRD) profile of all BIL-COF
hybrids except for BIL-COF-50 showed diffraction pattern that
was identical to that of TPB-DMTP-COF. It exhibited six
5
2
prominent diffraction peaks, with the most-intensive one at
o
2
2
.76 and the five other peaks at 4.82, 5.60, 7.42, 9.70 and
4
]) of a high content by one pot
o 52
5.2 ; these peaks were assigned to the (100), (110), (200),
in-situ synthesis to provide a series of mesoporous 2D BIL-
containing imine-linked COFs (2D-BIL-imine-COFs) (Scheme 1). (210), (220) and (001) facets, respectively (Fig. 1a). The
In comparison, a less loading of [PSMIm][HSO
] could be matching PXRD pattern with that of TPB-DMTP-COF indicated the
4
reached without losing the desired crystallinity and porosity in retention of the robust COF structure in the presence of
the case of post impregnation method. Most importantly, BIL- [PSMIm][HSO ] as substitutes for acetic acid. However, the
4
COF-30 exhibited impressively high reactivity towards the
dehydration of sorbitol yielding up to 97% isosorbide at 160 C
and can be recycled for at least 5 runs without activity loss.
intensity of the (100) peak was decreased with the addition of
4
PSMIm][HSO ]. And the larger the amount of [PSMIm][HSO ]
presented, the weaker the (100) peak was. This result suggested
the electron density inside the pores of TPB-DMTP-COF was
o
[
4
4
varied upon interaction with the guest [PSMIm][HSO ]. By
contrast, BIL-COF-50 displayed an amorphous framework
according to the PXRD measurements (Fig. 1a).
Results and discussion
Synthesis and characterization of BIL-COF hybrids
Nitrogen-sorption isotherms of TPB-DMTP-COF and BIL-
COF-X (X = 10, 30) hybrids measured at 77 K each exhibited a
typical type IV isotherm featured by a rapid uptake at a low
The synthesis of BIL-COF hybrids denoted as BIL-COF-X (X is the
concentration of BILs (g/L) that added to the reaction) was
conducted by condensation of TAPB (28.1 mg) and DMTP (1.2
0
relative pressure at P/P < 0.01, followed by a sharp step in 0.15
2
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<
(
P/P
Fig. 1b). As expected, the amorphous BIL-COF-50 displayed a sulphur atoms were uniformly doped inDOthI:
low uptake of N . The Brunauer-Emmett-Teller (BET) surface and [PSMIm][HSO ] was highly distributed throughout the
area was found dropped in the case of mesoporous BIL-COF-X channels (Fig. S2).
0
< 0.25, which was characteristic of mesoporous material the elemental mapping image of BIL-COF-30 demoVnieswtrAartticeledOtnhlianet
e
10
c
.1
a
0
r
3
b
9
o
/C
n
9
s
G
k
C
e
0
l
2
e
1
t
2
on,
6
D
2
4
(
X = 10 and 30) hybrids as compared to that of TPB-DMTP-COF
4
The concentration of [PSMIm][HSO ] in the synthesized BIL-
2
(2231 m /g) and the value decreased in an increasing order of COF hybrids was quantified with elemental analysis again taking
2
amounts of [PSMIm][HSO
1
of BIL-COF-10 (2.5 nm, 1.34 cm /g
cm /g
4
] loaded (1550 m /g (BIL-COF-10)  sulphur as the indicator atom. As a result, only traces sulphur
2
266 m /g (BIL-COF-30)). The pore size and volume evaluation mass fraction was detected for TPB-DMTP-COF, whereas BIL-
3
)
and BIL-COF-30 (2.2 nm, 1.03 COF hybrids featured values of 2.13% (BIL-COF-10), 6.10% (BIL-
3
)
indicated the channel of TPB-DMTP-COF (2.7 nm, 1.75 COF-30) and 7.89% (BIL-COF-50) (Table S1). The calculated
] therein (Fig. S1). concentration of [PSMIm][HSO therein was 0.33 mmol/g, 0.95
3
cm /g) was partially occupied by [PSMIm][HSO
4
4
]
By contrast, the amorphous BIL-COF-50 showed a BET surface mmol/g and 1.23 mmol/g, respectively. Thermogravimetric
2
3
area and pore volume of only 39 m /g and 0.03 cm /g, analysis (TGA) of TPB-DMTP-COF and BIL-COF hybrids was carried
respectively.
out under nitrogen atmosphere. TPB-DMTP-COF showed a single
decomposition temperature at 440 C, typical structural indication of
o
COF
(Fig. S3S6). For BIL-COF hybrids, especially BIL-COF-10 and
BIL-COF-30, the degradation of [PSMIm][HSO
C, followed by the skeleton collapse at 440 C.
4
] occurred below 280
o
o
Fig. 3 FT-IR spectra of TPB-DMTP-COF and BIL-COF-hybrids.
The structural integrity of all BIL-COF hybrids was examined by
FT-IR studies. For all hybrids, the C=O stretching vibration (1680
−1
−1
cm ) of TAPB and the NH stretching vibration (3345 and 3434 cm )
of DMTP are disappeared, coincidently with the appearance of a new
peak at around 1614 cm−1 assigned to the C=N bond, suggesting the
formation of imine linkers by polycondensation reaction
worthy to note that the apparent red shift of C=N absorption from
614 cm−1 to 1609 cm−1 for BIL-COF hybrids may be attributed to the
hydrogen bonding (H-bond) interaction between the TPB-DMTP-
(Fig. S7). It’s
Fig. 2 The morphology and microstructure of BIL-COF (a) SEM images of
TPB-DMTP-COF and BIL-COF. (b) HR-TEM images of BIL-COF-30.
1
The morphology and microstructure of BIL-COF hybrids as
well as TPB-DMTP-COF were studied by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM).
There was no remarkable difference in morphology that was
observed between the mesoporous BIL-COF-X (X = 10, 30) and
TPB-DMTP-COF as presented by the SEM image, suggesting
4
COF backbone and the confined [PSMIm][HSO ] through C=NH
unit.53 In particular case of BIL-COF-50, the potentially formed C=N
−1
peak (1609 cm ) is merged with C=C stretching bands at around
−1
1
594 cm , thus forming a broad shoulder represented to the
amorphous structure.54 We also found each spectra of BIL-COF
hybrids contained a band at the range of 2300 cm−1 to 3700 cm−1,
[
4
PSMIm][HSO ] was successfully confined into the COF channel
4
corresponding to the presence of [PSMIm][HSO ]. The intensity was
rather than coated on the wall. The persistence of crystallinity
and porosity in these two samples was thus further evidenced.
By sharp contrast, the amorphous BIL-COF-50 exhibited random
morphology. In the high-resolution transmission electron
microscopy (HR-TEM) images of BIL-COF-30, we can clearly see
the lattice fringes and the spacing of these COF lattice fringes
was 0.35 nm (Fig. 2b), which was attributed to the plane of (100)
reflection as seen in the corresponding PXRD profile (Fig. 1a). It
further proved an ordered alignment of BIL-COF-30. Besides,
relatively increased with the loading amount of [PSMIm][HSO
expected.
4
] as
4
The confinement of [PSMIm][HSO ] in the BIL-COF hybrids
and host-guest interaction mode were further confirmed by the
X-ray photoelectron spectroscopy (XPS) spectra, wherein the
blank TPB-DMTP-COF, mesoporous BIL-COF-30 and amorphous
BIL-COF-50 were studied and compared. In consistence with the
elemental analysis, the sulphur signal in the survey XPS spectra
of both BIL-COF-30 and BIL-COF-50 was well identified and the
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same signal was not detected in that of TPB-DMT-COF (Fig. S8).
Journal Name
(
Fig. S9). It could be observed that upon confinemeVnietwoAfrtiBclIeLO, ntlhinee
Besides, the corresponding N1s XPS spectra of three samples absorption edge of the hybrids shifted
D
t
O
owI: 1
a
0
r
.
d
10
s
39la/Crg
9
e
G
r
C0
v
2
isible
1
2
6
D
were apparently different from each other (Fig. 4). TPB-DMTP- light domain (> 550 nm) in comparison to that of the TPB-DMTP-
COF features one typical symmetric band of the imine signal at COF spectra. This was also supported by an observable change
3
98.8 eV. In contrast, BIL-COF-30 showed a broad and in the colour of TPB-DMTP-COF, that was bright yellow to
asymmetric band at the relatively high level, which was reddish brown on incorporation of [PSMIm][HSO . The shift in
4
]
deconvoluted into three peaks of intensity ratio ca. 2.0: 1.0: 4.0 the absorption spectra confirmed the improved visible light
with different binding energies, containing values assigned to absorption of BIL-COF hybrids and can be attributed to the host-
the imidazolium nitrogen at 401.4 eV, ammonium-like nitrogen guest H-bond interaction and a higher degree of conjugation
at 400.3 eV and imine nitrogen at 398.8 eV. The middle peak resulting from delocalization along as well as across the plane in
was imposed by a C=NH unit, wherein the binding energy of the extended frameworks.56
imine group is positively shifted by ca. 1.5 eV when it was
subjected to [PSMIm][HSO
, confirming the electron was further and BIL-COF-30 could be transformed back to yellow as
delocalized by H-bond interaction with the guest demonstrated by TPB-DMTP-COF upon exposure to strong base
].55 The identical nitrogen species was recognized for 1,4-diazabicyclo [2.2.2] octane (DABCO) (Fig. 6), suggesting the
PSMIm][HSO
BIL-COF-50, but the ratio was changed to ca. 2.0: 1.0: 2.0, which is inherent H-bond in the hybrids being broken and a good
In experiments, the red colour of mesoporous BIL-COF-10
4
]
[
4
attributed to a high loading amount of [PSMIm][HSO
4
].
endurance of TPB-DMTP-COF as a skeleton. Characterization of
the resulted product, such as FT-IR, XPS and UV/Vis DRS,
displayed a typical feature of TPB-DMTP-COF (Fig. S10S13). By
sharp contrast, the colour of amorphous BIL-COF-50 remained
intact under identical conditions (Fig. 6).
Fig. 6 The reaction of BIL-COF-X (X = 30, 50) with basic DABCO.
Fig. 4 N1s XPS spectra of TPB-DMTP-COF, BIL-COF-30 and BIL-COF-50.
Black: experimental line; red: fitted line; pink: background; green:
imidazolium N at 401.4 eV; blue: ammonium-like N at 400.3 eV; brown:
imine N at 398.8 eV.
Information on the acidity of BIL-COF hybrids could help
predict the intermolecular interactions of nucleophiles or
electron-rich substrates with the labile protons in the confined
[
4
PSMIm][HSO ]. In this study, the acidity of BIL-COF hybrids was
determined by 31P MAS NMR spectroscopy, using slightly
excessive trimethylphosphine oxide (TMPO) as the weakly basic
5
7
o
probe molecule. Only one symmetric signal appeared at 25 C
in each case besides that for TMPO itself, suggesting
[
4
PSMIm][HSO ] was homogeneously dispersed inside the channel of
TPB-DMTP-COF (Fig. S14). Following the equation elucidated for
the relationship between the proton affinity (PA) and 31P
chemical shift,57 the PA values of BIL-COF-X (X = 10, 30 and 50)
was 357, 343 and 339 kcal/mol with 31P chemical shift featured
at 43.74, 48.98 and 50.81 ppm, respectively.
Evaluation of catalytic performance of BIL-COF hybrids
Fig. 5 UV-DRS spectra of TPB-DMTP-COF, BIL-COF-10, BIL-COF-30 and BIL-
COF-50
.
The catalytic performance of these characterized BIL-COF
hybrids was subsequently evaluated in terms of dehydrative
transformation of sorbitol to isosorbide (Table 1 and Table S2).
We found additional solvent is required in order to make the
catalyst powder (2030 wt%) completely impregnated by the
reaction solution under the experimental scale (sorbitol: 50 mg).
Thus, the solvent type was optimized for the model reaction
The UV/Vis diffuse reflectance spectrum revealed TPB-
DMTP-COF absorbs light in the blue and purple part of the
visible region and shows an absorption profile with an edge at

490–530 nm (Fig. 5), thereby suggesting an optical band gap
of around 2.01 eV as determined by the Kubelka-Munk function
4
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that conducted in the presence of BIL-COF-30 with 30 wt% COF-30 (20 wt%) (entry 2). Subsequently, the perVfieowrAmrtaicnlecOenlionef
loading amount at 160 °C for 24 h (Table S2), wherein a mixture TPB-DMTP-COF (entry 3) and the otherDOtwI: o10.B10IL3-9C/CO9FGCh0y2b1r2i6dDs
of 1,4-sorbitan and isosorbide was obtained as the major (entries 4 and 5) was also evaluated for comparison. We found
product, together with a part of other undefined side-products TPB-DMTP-COF had almost no catalytic activity, which was
upon dehydration, such as 1,5-sorbitan and 3,6-sorbitan. comparable to that of catalyst-free case (entry 6). Besides,
Neither protic solvent, such as MeOH, nor aprotic polar solvent, neither BIL-COF-10 nor BIL-COF-50 was better than BIL-COF-30
such as DMSO and 1,4-dioxane, could provide satisfied under identical conditions. The poor performance of BIL-COF-
reactivity, which mainly generated 1,4-sorbitan with 4669% 10 was supposed because of its insufficient acidic sites in the
yield. Xu’s strategy by using ketone as the solvent was also pore, whereas that of BIL-COF-50 was most likely imposed by
4
2
examined in this study. However, the optimized 4-methyl-2- the amorphous structure that render the reaction mainly
pentanone therein did not perform the same efficiently. Only occurring outside the surface. The dehydrative transformation
6
3% sorbitol was converted, generating isosorbide with 5% of sorbitol to isosorbide was also conducted in the presence of
yield under the above-mentioned conditions. By contrast, the BIL-COF-soaked hybrids, which were synthesized by direct
reactivity was significantly increased using dimethyl carbonate impregnation of [PSMIm][HSO ] into TPB-DMTP-COF and
DMC) as the solvent, wherein isosorbide was obtained in a denoted as BIL-COF-S. For comparison, BIL-COF-S-30 (Fig.
yield of 76%. We proposed that the formation of methoxyl S15S16) was prepared with the same concentration of
4
(

58,59
anion (CH
3
O ) from reaction of Brnsted acid with DMC,
4
[PSMIm][HSO ] (30 g/L) as that used in the one-pot in-situ
followed by sequential proton abstraction of hydroxyl groups in synthesis of BIL-COF-30. PXRD analysis suggests the crystallinity
sorbitol and sorbitan, accounts for such a high reactivity. To our of TPB-DMTP-COF retains in BIL-COF-S-30, but only 4.52%
delight, the excellent result was obtained in nonpolar solvents, sulphur mass fraction was detected by elemental analysis (Table
such as toluene, ethylbenzene, m-xylene and mesitylene, with S1). In fact, such a loading amount was found approximate to
a full conversion of sorbitol and an up-to 97% yield of isosorbide, the maximum value upon absorption as indicated by
respectively. We proposed that nonpolar solvents did not impregnation of TPB-DMTP-COF with [PSMIm][HSO ] of a
4
participate in the interaction between BILs within the pores of concentration range between 1 and 50 g/L. On the other hand,
BIL-COF-30 and substrate as compared to those polar ones, so TPB-DMTP-COF was found collapsed during the impregnation
4
that the active acidic site could directly protonize hydroxyl process when the concentration of [PSMIm][HSO ] was
groups of substrates for an efficient dehydration and cyclization. increased up to 90 g/L. A significant mass loss (ca. 70 wt%) was
detected for BIL-COF-S-90 following the standard preparation
Table 1 Catalytic dehydrative formation of isosorbide from sorbitola
procedure and the PXRD study indicated it was an amorphous
material consisting of 3.52% sulphur mass fraction (Fig. S17).
These results suggested there should be a potential loading of
[
4
PSMIm][HSO ] into the porous channel of TPB-DMTP-COF by
impregnation, which was less than that one-pot synthesis
reaches (Table S1). BIL-COF-S-30 does not offer a comparative
reactivity towards the dehydrative formation of isosorbide as
that of BIL-COF-30 (entry 7). Finally, the catalytic performance
Conv.
Yield (%)b
isosorbide
97, 91d
Entry
Catalyst
b
(%)
sorbitan
0
others
3
1
2
3
4
5
6
7
8
BIL-COF-30
BIL-COF-30c
TPB-DMTP-COF
BIL-COF-10
BIL-COF-50

100
100
47
5
0
90
0
5
47
3
4
of equal molar [PSMIm][HSO ] as that in entry 1 (ca. 5 mol%)
under free state was evaluated, which generated a dark black
solution with 77% isosorbide (entry 8). In fact, isosorbide was
obtained with only a yield of 47% by internal standard method
using mannitol as the standard molecule, suggesting a
significant amount of himins being produced. We also
investigated the catalytic dehydrative transformation of
sorbitol on a scale of 1 gram, which offers isosorbide with 90%
isolated yield under optimized conditions.
100
100
35
73
0
24
77
0
23
35
3
0
BIL-COF-S-30
100
100
17
0
78
]e
77, 47d
[PSMIm][HSO
4
23
a
Reaction conditions: sorbitol 50 mg, catalyst 15 mg, toluene 2 mL, 160 °C, 24 h;
Determined by HPLC with a Phenomenex Rezex RCM-monosaccharide column
8 %; Ca , 300 × 7.80 mm) at 80 °C, degassed demi water as eluent; catalyst (10
mg); Yield determined by internal standard method using mannitol as internal
standard; catalyst (4.3 mg, 5 mol%).
Recyclability of BIL-COF-30 was thereafter examined (Fig.
b
7
). The catalyst powder was readily separated from the reaction
2
+
c
(
d
suspension through centrifugation and filtration. The recycled
solid was thoroughly washed with ethanol and dried under high
vacuum at 80 °C for 6 h before the reaction. The conversion and
selectivity of each run were the average values of at least three
parallel samples to reduce the experimental error. The results
showed that BIL-COF-30 has excellent catalytic activity in the
conversion of sorbitol to isosorbide. In five consecutive runs,
both the conversion of sorbitol and yield of isosorbide was
relatively stable at above 90%, respectively. In addition, PXRD
and FT-IR studies (Fig. S18S19) were conducted on BIL-COF-30
e
The desired quantitative synthesis of isosorbide from
sorbitol was nearly approached with BIL-COF-30 (30 wt%) under
a decreased temperature at 160 C (Table 1, entry 1). A yield of
7% isosorbide was obtained therein, which was the highest
among those with other catalysts reported to date. The catalytic
efficiency was found slightly reduced by a lower loading of BIL-
o
9
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J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
after recycling, suggesting both of [PSMIm][HSO
remain.
4
] and porosity Certificate (20180200052), and K. C. Wong EducatioVniewFoAruticnledaOtniloinne
(GJTD-2018-04) is gratefully acknowledged. DOI: 10.1039/C9GC02126D
Experimental section
Preparation of BIL-COF hybrids (one pot in-situ synthesis)
BIL-COF hybrids were synthesized according to the previous
literature with a slight modification. An n-BuOH /(o-DCB)/H
0.5 mL/0.5 mL/0.1 mL) mixture of TAPB (0.080 mmol, 28.1 mg)
and DMTP (0.120 mmol, 23.3 mg) in the presence of
PSMIm][HSO ] (10 g/L; 30 g/L; 50 g/L) in a Pyrex tube (15 mL)
2
O
(
[
4
was degassed through three freeze–pump–thaw cycles. The
tube was sealed by torch and then heated at 120 °C for 3 days.
After the reaction, the precipitate was collected through
centrifugation and washed sequentially with anhydrous THF (3
×
5 mL) and methanol (5 × 10 mL) until the pH of filtrate is
neutral. The obtained residue was then Soxhlet extracted in THF
for 1 day and dried under vacuum at 80 °C for 12 h to produce
Fig. 7 Conversion of sorbitol and yield of isosorbide from the
dehydration of sorbitol with BIL-COF-30 as the catalyst (sorbitol, 50 mg;
BIL-COF-30, 15 mg; toluene, 2 mL; 160 °C; 24 h) in consecutive recycling orange or dark brown powders.
cycles
.
Preparation of BIL-COF-S hybrids (impregnation method
TPB-DMTP-COF (40 mg) was dried at 100 °C for 12 h, which was
then dispersed with methanol of [PSMIm][HSO
)
Conclusions
a
4
]
In this study, mesoporous BIL-COF hybrids were successfully (1/2/4/6/30/50/90 g/L, 5 mL) in a round flask (25 mL). After
prepared via one-pot in-situ incorporation of Brønsted acidic stirring at room temperature for 12 h, the suspension was
[
(
PSMIm][HSO
TPB-DMTP-COF), wherein [PSMIm][HSO
4
] into the channel wall of imine-linked 2D-COF filtrated and washed with methanol until the pH of filtrate was
] functions both as the neutral. The obtained residue was dried under vacuum at 80 °C
4
catalyst of catalytic condensation for the formation of COF for 12 h to provide red to dark brown powders, denoted as BIL-
skeleton and the guest molecule capsuled in TPB-DMTP-COF COF-S-6/30/90.
host. A high loading of BIL could be reached without losing the
desired crystallinity and porosity in this case as compared to the
post impregnation method imposed by simple physisorption.
Catalyst performance evaluation and procedure for recycling the
catalyst
Here, the interaction of BIL between the parent 2D-COF was A mixture of sorbitol (50 mg, 0.27 mmol), BIL-COF hybrids (15
reinforced by incorporating imine instead of keto-enamine as mg) and toluene (2 mL) was added in a stainless-steel autoclave
the linker,51 which make BIL remaining in the channel upon (30 mL), which was sealed and heated up to 160 °C with a
vigorous washing. On the other hand, after the base treated, magnetic bar at 400 rpm. After reaction for 24 h, BIL-COF
mesoporous BIL-COF hybrids retain their crystallinity, porosity, hybrids was separated from the mixture upon centrifugation.
and stability, which make them attractive for solid acid catalysts The residue was washed with ethanol for several times and
applications. As an example, mesoporous BIL-COF-30 exhibited dried under high vacuum at 80 °C for 6 h, which was employed
impressively high reactivity towards the dehydration of sorbitol for the sequential catalytic run. Noteworthily, the recycled
o
yielding up to 97% isosorbide at 160 C and can be recycled for catalyst was accumulated by repeating the same reaction of last
at least 5 runs without activity loss. In BIL-COF-30, run because of the partial loss of catalyst in the recovery
[
PSMIm][HSO
the imine sites on the parent TPB-DMTP-COF backbone. The H- amount of 15 mg catalyst.
bond between [PSMIm][HSO ] and TPB-DMTP-COF was
4
] was found highly distributed, corresponding to process. Make sure that each run was performed with an
4
supposed significantly attenuated in the presence of polyol
substrates during the dehydration reaction and the limited
space in the channel renders the rate limiting dehydration of
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