A series of sulfonic acid functionalized mixed-linker DUT-4 analogues: Synthesis, gas sorption properties and catalytic performance

Article abstract of DOI:10.1039/c7dt02752d

In this work, we present the successful synthesis of a series of sulfonic acid functionalized mixed-linker metal-organic frameworks (MOFs) having the DUT-4 topology by using different ratios of 2,6-naphthalenedicarboxylic acid (H₂-NDC) and 4,8-

Full text of DOI:10.1039/c7dt02752d

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A series of sulfonic acid functionalized mixed-
linker DUT-4 analogues: synthesis, gas sorption
properties and catalytic performance†
Cite this: DOI: 10.1039/c7dt02752d
a
a
a,b
b
Guang-Bo Wang, Karen Leus,
Kevin Hendrickx, Jelle Wieme,
a
c
b
Hannes Depauw, Ying-Ya Liu, Veronique Van Speybroeck and
a
Pascal Van Der Voort
*
In this work, we present the successful synthesis of a series of sulfonic acid functionalized mixed-linker
metal–organic frameworks (MOFs) having the DUT-4 topology by using different ratios of 2,6-naphthalene-
2 2 3
dicarboxylic acid (H -NDC) and 4,8-disulfonaphthalene-2,6-dicarboxylic acid (H -NDC-2SO H) in
one-pot reactions. The obtained materials were fully characterized and their CO
2
adsorption properties at
low and high pressures were studied and compared with those of the pristine DUT-4 material. Generally,
adsorption capacities range from 3.28 and 1.36 mmol g− for DUT-4 to 1.54 and 0.78 mmol g−1
1
the CO
2
for DUT-4-SO
3
H (50) up to 1 bar at 273 K and 303 K, respectively. Computational calculations corrobo-
rated the structural changes of the material in function of the loading of sulfonic acid groups.
Furthermore, due to the strong Brønsted acid character, the resulting sulfonic acid based MOF material
was evaluated as a catalyst for the ring opening of styrene oxide with methanol as a nucleophile under
mild conditions, showing almost full conversion (99%) after 5 hours of reaction. A hot filtration experiment
demonstrated that the catalysis occurred heterogeneously and the catalyst could be recovered and
reused for multiple runs without significant loss in activity and crystallinity.
Received 28th July 2017,
Accepted 26th September 2017
DOI: 10.1039/c7dt02752d
rsc.li/dalton
before synthesis, has been examined extensively in recent
years since it preserves the structural integrity and is a very
Introduction
1
2–15
Metal organic frameworks (MOFs), also known as porous straightforward procedure.
Baiker and co-workers have
coordination polymers (PCPs), are hybrid materials widely used this mixed-linker approach to synthesize a series of
examined in various applications ranging from gas sorption amine functionalized MOF-5 materials with the general
1
–4
5–8
and separation
to heterogeneous catalysis.
They owe this formula [Zn O(BDC) (BDC-NH2)3−x] in which x can be varied
4 x
large interest to their unique structural tailorability, good crys- and the obtained materials were tested as catalysts in the reac-
9
tallinity and controlled surface area and porosity. Moreover, tion of propylene oxide and CO forming propylene carbon-
2
5
the concept of isoreticular synthesis allows one to introduce ate. Yaghi and co-workers successfully synthesized a series of
specific functionalities into the framework whilst retaining the multivariate (MTV) MOFs by incorporating eight different
1
0,11
topology of a given framework structure.
To date, tremen- functionalities into MOF-5 under solvothermal conditions, in
dous efforts and strategies have been reported to embed mul- which the amount of the introduced functional groups could
1
16
14
tiple functional groups into given MOFs. Amongst them, the be varied. In the work of Cohen et al. and Bordiga et al.,
mixed-linker approach, where different linkers are mixed the synthesis and characterization of amine functionalized
mixed-linker metal–organic frameworks having a UiO-66 topo-
logy was described. Another class of materials that has
received considerable attention in the field of isoreticular syn-
thesis is the MIL-53 type of material due to its interesting
a
Department of Inorganic and Physical Chemistry, Center for Ordered Materials,
Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281 (S3),
9
000 Ghent, Belgium. E-mail: pascal.vandervoort@ugent.be
structural flexibility upon external stimuli (e.g. guest molecule
adsorption/desorption).
b
3,17–19
Center for Molecular Modeling, Ghent University, Technologiepark 903,
9
052 Zwijnaarde, Belgium
The incorporation of sulfonic acid functional groups into
MOFs has a significant impact on the gas sorption properties
and in potential catalytic and proton conduction appli-
c
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
1
16024 Dalian, People’s Republic of China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
0,21
c7dt02752d
cations.
Within this context, Zhou and co-workers reported
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on the introduction of sulfonic acid functional groups into a room temperature, sealed in a 23 mL Teflon-lined stainless
porous polymer network (PPN-6-SO H), which significantly steel vessel and heated to 150 °C for 24 hours. After cooling
3
2
2
increased the CO
2
adsorption capacity and CO
2
/N
2
selectivity.
down to room temperature, the obtained solid was collected by
Stock et al. synthesized a series of sulfo-functionalized mixed- membrane filtration and thoroughly washed with DMF, metha-
linker CAU-10 analogues, in which the polar groups signifi- nol and acetone. Thereafter, the material was dried under
cantly influence the sorption capacities towards N
2
, H
By using the mixed-linker approach, a series of sulfonic materials were denoted as DUT-4-SO
acid functionalized mixed-linker UiO-66 materials was success- ponds to the theoretical percentage of H -NDC-2SO H used
2
and vacuum at room temperature prior to analysis. The obtained
2
3
CO
2
.
3
H(x), in which x corres-
2
3
fully synthesized by the group of Kitagawa, after which an during the synthesis.
enhanced CO adsorption capacity and isosteric heat of CO
adsorption was noted. Moreover, the presence of sulfonic
2
2
2
4
Experimental and analytical techniques
X-ray powder diffraction (XRPD) patterns were collected on a
2
5
acid groups can be beneficial for various catalytic reactions.
The acid catalyzed ring opening of epoxides with various Thermo Scientific ARL X’Tra diffractometer, operated at 40 kV,
nucleophiles can provide an easy access to various intermedi- 30 mA using Cu-K radiation (λ = 1.5406 Å). Peak indexing and
α
ates, which are widely desired in the pharmaceutical indus- refinement of the unit cell parameters were performed with
2
6
33
try. Pioneering work on the use of MOFs in these reactions the TOPAS suite using the Pawley method. Peaks were fitted
was done by Baiker et al., who examined a copper based MOF with a TCHZ pseudo-Voigt profile and a simple axial model to
2
7
under ambient and solvent-free conditions. Later on, some correct for peak asymmetry. Fourier transform infrared spec-
commercially available MOFs were systematically studied by troscopy (FT-IR) in the region of 400–4000 cm− was performed
1
2
8,29
Garcia and co-workers.
functionalized MIL-101 material (MIL-101-SO
thesized exhibiting an exceptionally high efficiency and recycl- and a KBr beam splitter. Thermogravimetric analysis (TGA)
Recently, a ∼100% sulfonic acid with a Thermo Nicolet 6700 FT-IR spectrometer equipped with
3
H) was syn- a nitrogen-cooled Mercury Cadmium Telluride (MCT) detector
3
0
ability in the ring opening reaction of styrene oxide. In was done on a Netzsch STA-449 F3 Jupiter-simultaneous
another recent report of Farha and co-workers, seven Zr/Hf- TG-DSC analyzer in the temperature range of 20–800 °C in air
based UiO-type MOFs with different degrees of defects were at a heating rate of 2 °C min−1
.
1
H NMR spectra of all the
studied for their catalytic activity in the epoxide ring-opening obtained MOFs, digested in a mixture of D SO /DMSO-d , were
2
4
6
reaction, which proved that the catalytic activity improved with recorded on a Bruker 300 MHz Avance spectrometer. The reac-
3
1
increasing the numbers of defects in the MOFs.
In this contribution, we report the synthesis and character- Interscience), coupled to
ization of a series of sulfonic acid functionalized mixed-linker (Thermo, Interscience). The first column consists of
tion products were identified with a TRACE GC × GC (Thermo,
a
TEMPUS TOF-MS detector
a
DUT-4 analogues by using various ratios of the linker 2,6- dimethyl polysiloxane package and has a length of 50 m, with
naphthalenedicarboxylic acid (H -NDC) and 4,8-disulfo- an internal diameter of 0.25 mm, whereas the second column
2
naphthalene-2,6-dicarboxylic acid (H
2 3
-NDC-2SO H) in one-pot has a length of 2 m with an internal diameter of 0.15 mm. The
reactions. The CO and N gas sorption properties of all the package of the latter is a 50% phenyl polysilphenylene-siloxane
2
2
obtained compounds were evaluated and compared with those and helium was used as a carrier gas. Inductively coupled
of the pristine DUT-4 material. Additionally, a selected plasma-optical emission spectroscopy (ICP-OES) measure-
material, named DUT-4-SO
3
H(30), was examined for its cata- ments were performed to determine the sulfur leaching using
lytic performance in the ring opening of styrene oxide under a Varian 715-ES. Scanning electron microscopy (SEM) images
mild reaction conditions.
of the catalyst before and after catalysis were recorded on a
JEOL JSM-7600F FEG-SEM. N adsorption isotherms were
obtained using Belsorp Mini apparatus measured at 77 K,
whereas CO adsorption measurements were carried out on a
2
Experimental section
Chemicals
2
Quantachrome iSorb-HP gas sorption analyzer. Prior to the gas
sorption measurements, all the materials were activated at a
heating rate of 2 °C min− up to 150 °C under vacuum for 3 h.
1
2 3
The organic ligand H -NDC-2SO H was synthesized according
3
2
to a previously reported procedure. All other chemicals were
purchased from Sigma Aldrich or TCI Europe and used
without further purification.
Catalytic setup
In a typical catalytic test, a 25 mL Schlenk flask was loaded
with 10 mL of methanol and 2 mmol of dodecane used as a
solvent and internal standard, respectively, 2 mmol styrene
3 x
Synthesis of Al(OH)(NDC)1−x(NDC-2SO H)
The detailed ratios and employed amounts of starting oxide and a loading (based on sulfonic acid groups) of 5 mol%
materials for the synthesis of the mixed-linker MOF series are of DUT-4-SO H(30) was employed. All the catalytic tests were
listed in Table S1, ESI.† Typically, 0.26 g (0.69 mmol) performed in air while stirring at 55 °C. The reaction was
Al(NO ·9H O and a specific ratio of H -NDC and H monitored periodically by GC until full conversion was
NDC-2SO
3
3
)
3
2
2
2
-
3
H were dissolved in 15 mL N,N-dimethylformamide obtained. Afterwards, the catalyst was recovered by means of
(DMF). The resulting mixture was stirred for 5 minutes at centrifugation and the filtrate was analyzed by ICP-OES to
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examine the sulfur leaching. The recovered catalyst was acti- the obtained materials, XRPD measurements were performed
vated at 150 °C under vacuum and reused in multiple runs.
(Fig. 1 and Fig. S1†). The mixed-linker MOFs are crystalline
and isostructural, having the same topology as the pristine
DUT-4 framework. It should be noted that a significant
decrease in the crystallinity is observed as the ratio of
Computational details
Two types of calculations were performed to obtain more
insight into the structural changes of the materials upon
loading with sulfonic acid groups and the corresponding cata-
lytic activity. First of all, periodic Density Functional Theory
2 3
H -NDC-2SO H is higher than 50% and a considerable amount
of amorphous phase was obtained at a substitution of 100% of
H -NDC-2SO H, showing that it is not possible to synthesize
2
3
the pure sulfonic acid functionalized DUT-4 material under
the examined reaction conditions. A simple Pawley refinement
of the experimental diffractogram allowed us to refine the unit
cell parameters, starting from the literature values for the pris-
tine DUT-4 (a: 18.825 Å; b: 6.787 Å; c: 16.901 Å) and using the
(
DFT) calculations were performed with the Vienna Ab Initio
3
4
Simulation Package (VASP) using the projector-augmented
wave (PAW) method on three materials with various loadings
of sulfonic acid groups (see Fig. 4). The ionic positions and
lattice cell parameters were relaxed at the experimental volume
3
5
4
5
3
6
same orthorhombic space group Pnna. After refinement,
of the material. The PBE functional is used together with
slightly smaller values of the unit cell dimensions were found
(a: 18.26 Å; b: 6.73 Å; c: 16.64 Å) yielding a unit cell volume of
D3 empirical dispersion corrections with Becke–Johnson
3
7,38
damping.
An energy cutoff of 500 eV is used together with
3
2
064 Å . The weighted profile R-factor (Rwp) was 15.15 after
a 2 × 6 × 2 k-point mesh similar to the work of Vanpoucke
2
39
refinement and a χ of 1.78 was obtained. Additionally, the
successful incorporation of the functionalized linker mole-
cules into the DUT-4 structure was confirmed by FT-IR
et al. A Gaussian smearing scheme with σ equal to 0.05 eV is
−
6
employed. An electronic convergence criterion of 10 eV is
−
4
used together with an ionic convergence of 10 eV. The tex-
tural properties such as pore diameter and accessible surface
area are calculated with Zeo++ with a probe with diameter of
−
1
measurements (Fig. S2†). The bands at 1205 cm
and
1
1
145 cm− are attributed to the OvSvO stretching vibrations
−
1
4
0
whereas the vibration at 1038 cm can be assigned to the S–O
3
.64 Å, resembling the kinetic diameter of N
2
.
4
6,47
stretching vibration.
The absence of the band at
To obtain more insight into the size of the reactant mole-
1
∼
1680 cm− confirms that there is no residual free carboxylate
cules (styrene oxide and 2-methoxy-2-phenylethanol) for cataly-
sis and their fitting into the various materials, molecular cal-
culations were done within the Gaussian09 program using a
linker in the structure. The actual composition of the mixed
1
linkers in the resulting MOFs was determined via H NMR
4
1,42
spectroscopy by digesting the obtained mixed-linker MOFs
B3LYP hybrid functional
and a Pople 6-311+G(d,p) basis
1
4
3
into a mixture of D
2
SO
4
–DMSO-d
6
. The obtained H NMR
set. Furthermore, solvent effects were included using a polar-
izable continuum model (with ε = 32.613)
4
4
spectra are shown in Fig. 2, while ratios calculated via integrat-
ing their respective proton peaks are listed in Table S1.† By
increasing the ratio of the H -NDC-2SO H linker (from 0 to
to mimic the
methanol solvation of the reactants during a catalytic run.
2
3
5
0%) used in the initial reaction mixture, a progressive
1
increase of the
H NMR signals associated with the
Results and discussion
Synthesis and characterization of the DUT-4-SO H(x) materials
3
H -NDC-2SO H linker is observed. Notably, in all cases, the
quantity of the H -NDC-2SO H linkers observed in the resulting
2 3
2
3
The synthesis of the sulfonic acid functionalized mixed-linker
DUT-4 analogues was performed in a one-pot reaction by
varying the initial ratio of H -NDC and H -NDC-2SO H linkers
2
2
3
(
Scheme 1), applying similar reaction conditions as reported
4
5
for the synthesis of the pristine DUT-4 material. Table S1†
shows that at a reaction temperature of 150 °C for 24 hours, up
to 50% of the H -NDC-2SO H linker can be introduced into
2 3
the DUT-4 framework. To confirm the framework topology of
Scheme 1 Schematic representation of the synthesis of the mixed-
linker metal–organic frameworks possessing the DUT-4 topology,
Fig. 1 XRPD patterns of the sulfonic acid functionalized mixed-linker
DUT-4-SO H(x) materials, x = 0, 10, 20, 30, 40, 50, and 100.
which is denoted as DUT-4-SO
3
H(x).
3
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This observation is consistent with previously published
reports on functionalized MOF-5 and sulfonic acid functiona-
1
,24
lized mixed-linker UiO-66 type of material.
TGA was used to
determine the thermal stability of the resulting mixed-linker
MOF materials and the thermogravimetric curves are depicted
in Fig. 3. From this figure, the thermal stability of the
functionalized materials gradually decreases with increasing
amount of H -NDC-2SO H. The pristine DUT-4 material has a
2
3
decomposition temperature of 450 °C, while the DUT-4-SO
50) starts to decompose at 300 °C. A similar trend in stability
has also been observed for a series of mixed-linker MOFs
3
H
(
1
7
48
having the MIL-53 and IRMOF-3/MOF-5 topology.
In order to gain more insight into the structural changes
induced by the functional groups, structural relaxations on
3
different materials were performed. We studied the pristine
material of DUT-4 and two materials with different loadings of
1
Fig. 2 H NMR spectra of the digested mixed-linker DUT-4 analogues.
sulfonic acids namely, DUT-4-SO H(25) and DUT-4-SO H(100),
3
3
corresponding to a substitution pattern of 25 and 100%. For
the 25% functionalized material, the SO H groups were placed
MOFs is somewhat less than the amount that was added along a chain parallel to the b-axis. Fig. 4 shows that a torsion
initially to the reaction mixture, demonstrating that the non- of the functionalized linkers occurs, and hence the SO
3
3
H
functionalized organic linker is preferentially incorporated. group points inside the pore, which is beneficial for its accessi-
bility during catalysis. However, this also induces a contraction
of the pore along the c-axis, which becomes even more pro-
nounced when looking at the 100% loaded material. Here, the
SO H groups can interact via hydrogen bonds, blocking the
3
entire pore system of the material. This strong interaction also
gives a hint why the materials with a high SO H loading form
3
less crystalline or even amorphous materials. We think that
the strongly interacting sulfonic groups could interfere during
the synthesis and hinder the crystallization process. Moreover,
interaction between the groups within the framework could
lead to increased strain and lower the crystallinity of the
material in that way. Table S3† depicts the calculated internal
diameters of the three materials, showing a gradual decrease
of this parameter.
Gas sorption properties
The porosity of the resulting mixed-linker MOFs was deter-
mined by means of N
2
adsorption. As shown in Fig. 5, all the
Fig. 3 TGA curves of the mixed-linker DUT-4 analogues measured resulting materials exhibit a typical type I isotherm, which is
under an air flow at a heating rate of 2 °C min−
1
.
characteristic of microporous materials. As expected, the
3 3
Fig. 4 Representation of the 3 materials considered in the computational study. DUT-4 (left), DUT-4-SO H(25) (middle) and DUT-4-SO H(100)
(
right).
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at 273 and 303 K, respectively. It should be noted that the
incorporation of sulfonic acid groups does not increase the
CO
2
adsorption capacity of the studied materials, which
mainly follows the trend of the available pore volume of the
compounds. This behavior has been previously observed and
2
3
reported by Stock et al. Additionally, to provide a better
understanding of the CO adsorption properties, the isosteric
heats (Q ) of CO adsorption were calculated from the CO2
2
st
2
adsorption isotherms measured at different temperatures
4
9
using the Clausius–Clapeyron equation (see ESI†).
As
expected, the trend of the Qst for the series of MOFs is the
2
same as the CO adsorption capacity discussed above, decreas-
ing from 20.4 kJ mol− for DUT-4 to 14.3 kJ mol for DUT-4-
1
−1
SO H(50) (Fig. S3†). High-pressure CO adsorption isotherms
3
2
were also recorded at 273 and 303 K and are shown in Fig. S4.†
At both temperatures, the obtained isotherms show a type I
Fig. 5
N
2
adsorption (solid symbols) and desorption (open symbols)
H(x) materials.
isotherms of the resulting mixed-linker DUT-4-SO
3
profile with final CO adsorption capacities ranging from 13.7
2
and 10.7 mmol g− for DUT-4 to 6.3 and 5.4 mmol g for
1
−1
3
DUT-4-SO H(50) at 273 and 303 K, respectively, and a pressure
of up to 20 bar. Notably, the saturation capacity significantly
decreases as the loading of sulfonic acid groups reaches 50%,
which is mainly due to the severely reduced pore volume and
surface area of the materials.
Langmuir surface area and pore volume of the obtained
materials gradually decreases with increasing the amount of
sulfonic acid functionalized linker. This could be attributed to
the presence of the extra functional groups as well as a contrac-
tion of the pores, reducing the total available pore volume. The
Catalytic performance of DUT-4-SO H(30)
3
2
−1
Langmuir and BET surface areas and pore volume of 1950 m g , The presence of strong Brønsted acid sites within the obtained
2
−1
3
−1
1
650 m
g
and 0.63 cm g , respectively, are obtained DUT-4-SO H(x) materials allows us to examine them as a cata-
3
for DUT-4, while the Langmuir and BET surface areas of lyst in the ring opening of styrene oxide under mild conditions
2
−1
2
−1
3
−1
8
20 m g and 546 m g and pore volume of 0.23 cm g
3
(Fig. 7a). Based on the structural study, the DUT-4-SO H(30)
are obtained for DUT-4-SO H(50). The detailed Langmuir and was chosen as a catalyst due to its good crystallinity, high
3
BET surface areas and pore volumes of all the mixed-linker surface area and pore volume as well as the desirable content
MOFs are summarized in Table S1.†
of sulfonic acid groups. Furthermore, we showed via structural
Furthermore, to investigate the influence of the sulfonic optimizations on the pristine and functionalized materials
acid functional groups on the gas sorption properties, low that upon increasing the amount of functional groups in the
pressure CO
samples were recorded up to 1 bar at 273 and 303 K (see (Table S3†). If we compare this diameter with the size of the
Fig. 6). From this figure one can see that the CO adsorption reactant molecules (styrene oxide), they are in the same order
2
adsorption isotherms for all the activated unit cell, the internal diameter of the material decreases
2
capacity of all the samples increases linearly with the pressure, of magnitude (Fig. S5†). The pristine DUT-4 has a larger
without reaching a saturation in the studied pressure region. internal diameter than the reactant size, however, the 25%
−
1
The CO
2
adsorption capacity ranges from 3.28 and 1.36 mmol g
loaded material of the computational study already has a
−
1
for DUT-4 to 1.54 and 0.78 mmol g for DUT-4-SO
3
H(50) slightly smaller pore opening than the long axis of the reac-
Fig. 6 CO
2
adsorption isotherms (up to 1 bar) of the mixed linker DUT-4-SO
3
H(x) materials measured at 273 K (left) and 303 K (right), respectively.
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Fig. 7 (a) Styrene oxide ring-opening reaction with methanol, catalyzed by MOFs; (b) time–conversion plot for the ring opening reaction of styrene
oxide using DUT-4 and DUT-4-SO H(30) as a catalyst and the performed hot filtration test; and (c) reusability of the catalyst in 3 successive runs for
3
the ring opening of styrene oxide with methanol.
tant, thus, the epoxide can only diffuse into the material in 5 h of reaction. To verify whether the reaction is completely
one way, which limits the rate of the reaction. For the 100% heterogeneous, a hot filtration test was done in which the
SO H loaded material, the pores contract even further and the mixture was filtered off after 1 h of reaction (Fig. 7b). After the
3
reactants are now completely inhibited to enter the material or removal of the catalyst, no further conversion of styrene oxide
in other words the SO H groups which are located on the outer was observed in the filtrate, demonstrating that the catalysis is
3
surface of the material would be only accessible. The loading heterogeneous. ICP analysis of the filtrate showed a negligible
of 25–30% is therefore an optimal trade-off between the pres- amount of sulfur leaching (∼0.4%) after the first run. In
ence of sufficient functional groups and still enough opening Table 1, a comparison is made with some other literature
of the pores to allow for diffusion of the reactant molecules reported MOF materials that have been used for the ring
used in this study. For comparison, we also examined the cata- opening of styrene oxide. The active acid sites of the examined
lytic activity of the DUT-4 material. Fig. 7b shows that the MOFs are either Lewis or Brønsted acids and most of them can
DUT-4 material gave a negligible conversion after 8 h of reac- achieve full conversion within a reasonable time except for the
3
+
tion. This is due to the fact that the Al ions in DUT-4 are HP-CuBTC-0.25 for which only a conversion of 35.6% was
fully saturated by NDC linkers and therefore no free metal obtained after 4 h of reaction. Although it is difficult to make a
sites are available as Lewis acids to promote the reaction. For fair comparison as different reaction conditions were used, the
3 3
the DUT-4-SO H(30), almost full conversion was observed after MIL-101-SO H seems to be the best catalyst and has a relatively
Table 1 Comparison of the catalytic performance of DUT-4-SO
3
H(30) with other reported MOF catalysts used for the ring opening of styrene
oxide
TON/TOF (h−
1
)
Ref.
Entry
Catalyst
T (min)
T (°C)
Conv. (%)
1
2
3
4
5
6
Cu-MOF
Fe(BTC)
HKUST-1
HKUST-1/ATP
MIL-101-SO
3 h
60
2.5 h
20
30
20
RT
40
40
50
RT
40
99
>99
90
—/5.3
—/10
—
—/3.32–4.44
—/99
—
27
29
50
51
30
52
98.9
>99
99 (dropped to
3
H
MIL-101(HPW)
rd
62 in the 3 run)
7
8
9
1
1
HP-CuBTC-0.25
Zr-UiO-66
Zr-MOF-808
4 h
40
55
55
50
55
35.6
—
—
—
—
53
31
31
28
a
24 h
24 h
24 h
6 h
40
100
96
a
b
0
1
UiO-66-X (X = H, NH
2 2
, NO , Br, Cl)
c
d
3
DUT-4-SO H(30)
>99
20 /8
This work
a
d
b
c
Isopropanol as the nucleophile. The conversion is based on the catalyst of UiO-66-Br. The TON value was calculated after 5 h of catalysis.
The TOF value was calculated after 0.5 h of catalysis.
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rd
Fig. 8 (a) XRPD patterns of the catalyst before and after each run; (b) N
2
sorption isotherms of the catalyst before and after the 3 run; (c) FT-IR
rd
−1
spectra of the fresh catalyst (black) and the catalyst after the 3 run (red). Inset: Details on the region of 900–1350 cm . (d) SEM images of the
rd
fresh catalyst (a) and the catalyst after the 3 run (b).
fast reaction rate and a high TOF of 99 h− . This is probably
due to the fact that the compound has mesopores and higher
1
Conclusion
pore volumes in comparison with other reported MOF In conclusion, a series of sulfonic acid functionalized mixed-
materials, which will allow a better diffusion and mass linker metal–organic frameworks (MOFs) possessing the
transfer.
DUT-4 topology have been successfully synthesized using well-
-NDC and H -NDC-2SO H. The resulting
Additionally, the stability and reusability of the catalyst was defined ratios of H
2
2
3
examined. After obtaining full conversion, the catalyst was materials were thoroughly characterized by means of XRPD,
1
recovered by membrane filtration, washed with acetone (10 mL TGA, FT-IR spectroscopy, SEM, H NMR spectroscopy and
×
3) and activated at 150 °C under vacuum before it was used
2
N sorption. The changes in the pore and textural properties of
in additional runs. As shown in Fig. 7c, only a slight decrease the materials were studied in detail via computational calcu-
in the catalytic activity is observed after 3 runs. This somewhat lations and provided a structural explanation for the observed
lower catalytic activity in the additional runs is probably due to experimental trends. The effect of the functional groups on
a partial loss of the catalyst during the catalyst recovery and the gas sorption properties was examined by CO
2
adsorption
also because of a partial clogging of the pores after the first measurements. Generally, the CO adsorption capacities
2
1
−
2
run, which was confirmed by N adsorption analysis. A slight range from 3.28 and 1.36 mmol g for DUT-4 to 1.54 and
decrease in the Langmuir surface area was noted in the reused 0.78 mmol g− for DUT-4-SO3H (50) up to 1 bar at 273 K and
1
catalyst in comparison with the fresh DUT-4-SO H(30) catalyst 303 K, respectively. Furthermore, due to the strong acidic char-
3
3
(see Fig. 8a). Moreover, in the additional runs, no leaching of acter of the sulfonic acid groups, the DUT-4-SO H(30) material
sulfur was observed demonstrating once more the hetero- was evaluated as a catalyst for the ring opening of styrene
geneous character of the catalysis. The stability of the catalyst oxide under mild conditions reaching full conversion (99%)
after multiple runs was confirmed by XRPD measurements, after 5 h of reaction. Moreover, the DUT-4-SO H (30) catalyst
3
which is shown in Fig. 8b. The crystallinity of the recovered could be recovered and reused for three additional cycles
catalyst after each catalytic test is well preserved, which without significant loss in activity and stability.
demonstrates the good stability of the selected catalyst under
the applied reaction conditions. Aside from the XRPD
measurements, FT-IR spectra (Fig. 8c) and SEM images
(
Fig. 8d) of the fresh catalyst and after the third run were col-
Conflicts of interest
lected revealing no significant changes in the morphology of
the catalyst.
There are no conflicts to declare.
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Dalton Trans.

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Dalton Transactions
1
4 S. M. Chavan, G. C. Shearer, S. Svelle, U. Olsbye, F. Bonino,
J. Ethiraj, K. P. Lillerud and S. Bordiga, Inorg. Chem., 2014,
53, 9509–9515.
Acknowledgements
The authors are grateful for financial support received from
the Research Board of Ghent University (BOF) and BELSPO in 15 M. Du, C.-P. Li, C.-S. Liu and S.-M. Fang, Coord. Chem. Rev.,
the frame of IAP/7/05. G.-B. Wang gratefully acknowledges the 2013, 257, 1282–1305.
Chinese Scholarship Council (CSC) for financial support. K. L. 16 M. Kim, J. F. Cahill, K. A. Prather and S. M. Cohen, Chem.
acknowledges financial support from the Ghent Commun., 2011, 47, 7629–7631.
University. J. W. acknowledges the Research Foundation 17 S. Marx, W. Kleist, J. Huang, M. Maciejewski and A. Baiker,
Flanders (FWO-Vlaanderen) for financial support. H. D. Dalton Trans., 2010, 39, 3795–3798.
acknowledges the Research Foundation Flanders 18 T. Lescouet, E. Kockrick, G. Bergeret, M. Pera-Titus and
FWO-Vlaanderen) Grant (No. G.0048.13N.). Y.-Y. L. is grateful D. Farrusseng, Dalton Trans., 2011, 40, 11359–11361.
for financial support from the National Natural Science 19 M. Pera-Titus, T. Lescouet, S. Aguado and D. Farrusseng,
Foundation of China (No. 21403025) and the Scientific J. Phys. Chem. C, 2012, 116, 9507–9516.
Research Foundation for Returned Scholars, Ministry of 20 G. Akiyama, R. Matsuda, H. Sato, M. Takata and
Education of China. The computational resources and services S. Kitagawa, Adv. Mater., 2011, 23, 3294–3297.
used in this work were provided by the VSC (Flemish 21 W. J. Phang, H. Jo, W. R. Lee, J. H. Song, K. Yoo, B. Kim
(
Supercomputer Center), funded by the Research Foundation
and C. S. Hong, Angew. Chem., Int. Ed., 2015, 54, 5142–
Flanders (FWO) and the Flemish Government – department
5146.
EWI. The authors thank Jeffrey Paulo Perez and Sander Clerick 22 W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna and
for ICP-OES and SEM measurements respectively.
H.-C. Zhou, J. Am. Chem. Soc., 2011, 133, 18126–18129.
3 N. Reimer, B. Bueken, S. Leubner, C. Seidler, M. Wark,
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