Anion Exchange and Catalytic Functionalization of the Zirconium-Based Metal-Organic Framework DUT-67

Article abstract of DOI:10.1021/acs.cgd.8b00832

A postsynthetic treatment with diluted solutions of the inorganic HCl or H₂SO₄ acids was applied to functionalize the eight connected Zr-based metal-organic framework DUT-67 (DUT = Dresden University of Technology). During the treatment, it is possible to remove and exchange the pristine modulator (formate) by Cl^- or SO₄^2- anions. The position of the chlorine in the crystal structure of DUT-67 after HCl treatment could be determined by single-crystal X-ray diffraction analysis. Moreover, by means of the acidic treatment the polarity of the network as well as its Br?nsted acid strength are increased, which have a crucial impact on the catalytic performance. The improved catalytic activity of the acid-treated materials was demonstrated in the esterification of levulinic acid with ethanol.

Full text of DOI:10.1021/acs.cgd.8b00832

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Article
Anion exchange and catalytic functionalization of the
zirconium based metal-organic framework DUT-67
Franziska Drache, Francisco G Cirujano, Khoa D. Nguyen, Volodymyr
Bon, Irena Senkovska, Francesc X Llabrés i Xamena, and Stefan Kaskel
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00832 • Publication Date (Web): 26 Jul 2018
Downloaded from http://pubs.acs.org on July 26, 2018
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Anion exchange and catalytic functionalization of the zirconium
based metal-organic framework DUT-67
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a
b,c
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a
Franziska Drache, Francisco G. Cirujano, Khoa D. Nguyen, Volodymyr Bon, Irena
a*
b
a
Senkovska and Francesc X. Llabrés i Xamena, Stefan Kaskel
a
Department of Inorganic Chemistry, Dresden University of Technology, Bergstraße 66, 01062
Dresden, Germany
b
Instituto de Tecnología Química, Universitat Politècnica de València, Consejo Superior de
Investigaciones Científicas, Avda. de los Naranjos, s/n, 46022 Valencia, Spain
c
Present adress: Centre for Surface Chemistry and Catalysis, KU Leuven, 3001 Leuven,
Belgium
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Abstract
A postꢀsynthetic treatment with diluted solutions of the inorganic HCl or H SO acids was
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applied to functionalize the eight connected Zrꢀbased metalꢀorganic framework DUTꢀ67 (DUT ꢀ
Dresden University of Technology). During the treatment, it is possible to remove and exchange
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ꢀ
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the pristine modulator (formate) by Cl or SO4 anions. The position of the chlorine in the crystal
structure of DUTꢀ67 after HCl treatment could be determined by single crystal Xꢀray diffraction
analysis. Moreover, by means of the acidic treatment the polarity of the network as well as its
Brønsted acid strength are increased, which have a crucial impact on the catalytic performance.
The improved catalytic activity of the acid treated materials was demonstrated in the
esterification of levulinic acid with ethanol.
Introduction
Metalꢀorganic frameworks (MOFs) are discussed as materials of high potential for a broad range
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of applications such as gas storage, separation processes
or sensor applications,
mostly
because of their high porosity and modularity of the structure. The catalytic activity of MOFs
was recognised very early, and up to now MOFs have been applied as heterogeneous catalyst in
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32ꢀ37
a broad variety of catalytic reactions.
MOFs bearing open metal sites, such as HKUSTꢀ1,
3
8ꢀ40
40ꢀ42
MILꢀ100
or MILꢀ101
are ideal Lewisꢀacid catalysts. But also MOFs with saturated
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coordination environment, based on Zrꢀ or Hfꢀcluster (e.g. UiOꢀ66 ), turned out as active
catalyst because of high concentration of defects in this type of materials.
The majority of Zrꢀbased MOFs contain Zr ꢀ octahedra as inorganic building blocks, where the
6
triangular faces are alternately capped by µ ꢀO and µ ꢀOH groups. The edges of the octahedron
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are coordinated to twelve carboxylate linkers creating a coordinatively saturated cluster. The
catalytic activity of such materials relies on defects produced by missing linkers. Therefore, the
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concentration of active sites as well as the activity varies from batch to batch.
As usual, the catalytic activity of a MOF can be tuned either by linkerꢀ or by cluster
modification. The introduction of electronꢀwithdrawing groups into the linker positively
influences the Lewis acidity of the MOF. For instance, by the introduction of the NO group into
2
UiOꢀ66, a 56ꢀfold conversion rate increase of citronellal to isopulegol cyclization reaction could
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be achieved. Also additional Brønsted acid sites can be introduced into the framework by
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functionalization (e.g.ꢀSO H or similar groups), expanding the field of potential applications.
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In the synthesis of Zrꢀbased MOFs, modulators (monocarboxylic acids) are usually used to
increase the crystal size of the resulting MOFs and in some cases to reduce the connectivity of
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the metal cluster. The modulator supported synthetic approach also has been demonstrated to
52,57,58
control the amount of defects to some extent.
Nevertheless, Zr based MOFs based on
clusters with reduced connectivity should give a higher level of catalyst structure control and
tunability. One of the MOFs where the cluster connectivity is only eight instead of twelve is
DUTꢀ67 with the composition M O (OH) (tdc) (Fa) (M – Zr or Hf, tdc ꢀ 2,5ꢀ
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thiophenedicarboxylate, Fa ꢀ formate) (Fig. 1a). The charge balance is achieved by
coordination of monoꢀcarboxylate anions (modulator used in the synthesis), OH groups and the
linker molecules. Such cluster configuration is ideally suited for the postꢀsynthetic
functionalization approach since the anions can be easily exchanged by other anions in the
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synthesis or postꢀsynthetically.
With the aim to tune the catalytic properties of DUTꢀ67, herein we study the functionalization
and anion exchange in MOF by means of postꢀsynthetic treatment with diluted inorganic acids
(HCl or H SO ), since the high stability of Zrꢀbased MOFs against acids is widely reported in the
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2,64
ꢀ
literature.
As a result of such treatment, the pristine modulator anions (Fa ) in DUTꢀ67 are
ꢀ
2ꢀ
replaced by Cl or SO anions, respectively. The catalytic properties of the new catalysts in
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comparison to the pristine material were studied in a model reaction, the esterification of
levulinic acid, known to be catalyzed by Brønsted acids. The esterification of levulinic acid in
the presence of a suitable solid acid heterogeneous catalyst yields ethyl levulinate as the main
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reaction product, a compound recently reported as a potential biofuel.
Experimental part
General remarks
The metal precursors (ZrCl (98% purity) and HfCl (98 % purity)) and the linker (2,5ꢀ
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thiophenedicarboxylic acid (H tdc) (99% purity)) were purchased from Sigma Aldrich. The N,Nꢀ
2
dimethylformamide (DMF) (p.a. purity) and ethanol (99 % purity) were purchased from ABCR
GmbH and the Nꢀmethylꢀ2ꢀpyrrolidone (NMP) (99% purity) from ACROS Organics. All
chemicals were used without further purification.
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Nitrogen physisorption measurements were performed on BELSORPꢀmax apparatus
(MicrotracBEL, Japan) and on NOVA 4000e surface area & pore size analyser (Quantachrome
Co.) at 77 K up to 1 bar. Water vapour physisorption measurements were performed on
Hydrosorb 1000 apparatus (Quantachrome Co.) at 298 K up to 1 bar. Prior to the adsorption
measurements the samples were degassed in vacuum at 373 K for 16 hours.
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The thermogravimetric analysis (TG) and differential thermal measurements (DTA) were
performed in air on a STA 409 PC Luxx (Netzsch) thermal analyser with a heating rate of 2
°
C/min in a range from 25 °C to 1000 °C. C, H, N elemental analysis was performed using
CHNS 932 analyzer (LECO).
The powder Xꢀray diffraction data were collected on a STOE STADI P diffractometer with Cuꢀ
1
Kα1 radiation (λ = 1.5405 Å) at room temperature. HꢀNMR spectra were recorded on a Bruker
DRX 500 P at 500.13 MHz. The spectra were referenced against deuterated DMSO.
MOF synthesis
DUT-67(M)_HCOOH
In contrast to the synthetic procedure reported in ref. [66], the formic acid was used as
modulator instead of acetic acid. In a typical synthetic procedure ZrCl (1.38 g, 6 mmol) or
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HfCl (1.92 g, 6 mmol) was dissolved in 150 mL of DMF/Nꢀmethylꢀ2ꢀpyrrolidon (NMP)
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mixture (1:1) by ultrasonication for 10 minutes. Afterwards, 2,5ꢀthiophenedicarboxylic acid
(0.66 g, 4 mmol) (Scheme 1) was added and the mixture was sonicated again for 5 minutes. After
addition of formic acid (26.8 mL, 120 eq) the resulting
mixture was placed into an oven for 48 h at 80 °C. The
white precipitate was filtered off and washed several
times with DMF and ethanol. The samples were dried in
vacuum at 120 °C. Yield based on H tdc: 89 %.
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Scheme 1: 2,5ꢀthiophenedicarboxylic
acid (H tdc).
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The composition of the sample was derived from combined data obtained from elemental
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analysis, TGA (Figure S1 ꢀ S6, Table S1, ESI) and H NMR after sample digestion (Figure S16 ꢀ
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8, Table S5, ESI).
Elemental analysis for Zr O (OH) (tdc) (HCOO) (DMF) calc.: Zr 34.33, C 24.11, O 30.11, H
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1.64, S 8.05, N 1.76; found: C 24.26 H 1.46 S 8.08 N 1.84.
Anion exchange: Synthesis von DUT67_HCl and DUT-67_H SO4
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50 mg of as made DUTꢀ67_HCOOH was suspended in 5 mL of HCl or H SO solution (2
2 4
wt.%) at room temperature for 4 hours. Afterwards the resulting products were washed several
times with ethanol and activated in vacuum at 100 °C for at least 12 hours.
Elemental analysis for Zr O (OH) (tdc) (Cl) (DMF)(EtOH) : calcd. %: C 22.34, H 1.87, S 7.69,
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N 0.84, Cl 8.51; found, %: C 22.40, H 2.34, S 8.23, N 0.72, Cl 8.89.
Hf O (OH) (tdc) (Cl) (DMF) (EtOH) : calcd. %: C 18.54, H 1.82, N 1.52, S 5.58, Cl 6.16;
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found %: C 19.11, H 1.26, N 1.55, S 6.34, Cl 6.15.
Elemental analysis for Zr O (OH) (tdc) (SO ) (DMF)(EtOH) : calcd. %: Zr C 20.51, H 1.83, S
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6
4
4 3
2
12.36, N 0.77; found %: C 20.38, H 2.03, S 12.35, N 0.63.
Elemental analysis for Hf O (OH) (tdc) (SO ) (DMF)(EtOH) : calcd. %: C 15.92, H 1.42, N
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6
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4 3
2
0
.60, S 9.60, found %: C 16.26, H 1.77, N 0.69, S 9.81.
Catalysis
Esterification reactions were performed as follows: 1 mmol of levulinic acid (LA), and 0.6 mL of
ethanol (15 mmol) were contacted with the MOF (0.018 mmol of either Zr or Hf based MOF) in
a batch reactor at 80 °C. The reaction was monitored by GC–MS (Varian 3900) equipped with a
BP20(WAX) column (15 m long, i.d. 0.32 mm) using dodecane as internal standard and
comparing retention times with those of commercial standards.
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Single crystal X-ray diffraction
The octahedral single crystal of DUTꢀ67(Zr)_HCl with dimensions of 20 x 20 x 20 µm was
placed into a boronꢀsilicon glass capillary (0.3 mm in diameter) with small amount of solvent.
The data collection was performed on the BL14.3 Beamline of the MXꢀfacility at Helmholtz
Zentrum Berlin für Materialien und Energie, equipped with 1ꢀcirlcle goniometer and MAR MXꢀ
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25 CCD detector (Rayonics, Illinois). The experiment was performed at room temperature (22
C) using monochromatic synchrotron radiation with λ = 0.89499 Å (E = 13.5 keV). 180 images
were collected using φꢀscan with oscillation steps ꢁφ = 0.5° and exposition time 10 s/frame. The
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dataset was processed in automatic regime using XDSAPP 2.0 software. The crystal structure
was solved by direct methods and refined with anisotropic ellipsoids for all nonꢀhydrogen atoms
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by fullꢀmatrix least squares on F using SHELXꢀ2016/4 program package. After refinement of
ꢀ
the framework, the position of the Cl anion was determined from the difference Fourier map.
ꢀ
The occupancy of the Cl anion was refined and then fixed in the final refinement cycle. In the
final refinement, SWAT instruction was used in order to model the diffused solvent molecules in
the pores. CCDCꢀ1838419 contains the supplementary crystallographic data for DUTꢀ
67(Zr)_HCl. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The main data are given in Table 1.
Table 1. Single crystal Xꢀray diffraction data for DUTꢀ67(Zr)_HCl.
DUT-67(Zr)_HCl
Empirical formula
Formula weight
C H Cl O S Zr
24 8 0.86 32 4 6
1514.23
ꢀ
Crystal system, space group
Cubic, Fm3m
Unit cell dimensions, Å
a = 38.650(4)
3
Volume, Å
57736(20)
24
Z
3
Calculated density, g/cm
1.045
ꢀ1
µ, mm
1.452
0.972, 0.972
1.149 – 32.48
T , T
min
max
θ range, deg
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ꢀ
47 ≤ h ≤ 39
Limiting indices
ꢀ48 ≤ k ≤ 35
50 ≤ l ≤ 44
ꢀ
Reflections collected / unique
R(int)
54700 / 3520
0.0995
3520 / 93
Data / parameters
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GooF on F [I>2σ(I)]
1.019
2
GooF on F (all data)
1.019
R1 [I>2σ(I)]
wR2 [I>2σ(I)]
R1 (all data)
wR2 (all data)
0.0872
0.3450
0.1154
0.4062
ꢀ
3
Largest diff. peak / hole, eÅ
2.451 / ꢀ3.041
Results and discussion
Recently, solvent assisted linker incorporation (SALI) has been introduced as promising strategy
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to functionalize [Zr (µ ꢀOH) (OH) ] node of NUꢀ1000, an eight connected framework where
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the terminal ꢀOH groups were replaced by carboxylates containing functional group.
To
ꢀ
2ꢀ
obtain the DUTꢀ67 materials containing Cl or SO4 counterꢀions, also a postꢀsynthetic exchange
route was chosen. This route involves a treatment of the parent DUTꢀ67 MOF, containing
formate as monodentate ligand (modulator), with hydrochloric or sulfuric acid, respectively. As a
result, highly crystalline materials could be obtained where the modulator molecules, as well as a
ꢀ
2ꢀ
part of the DMF molecules, are replaced by Cl or SO4 and the solvent of choice. In such a way
the comparability of the samples could be guaranteed and the influence of synthetic conditions
could be minimized. The samples obtained in the postꢀsynthetic treatment are referred to as
DUTꢀ67_HCl (sample treated with HCl) and DUTꢀ67_H SO (sample treated with H SO ).
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4
In general the degree of anion exchange depends on the crystal size, time of treatment, and
concentration of the acidic solution. Since the crystal size of DUTꢀ67, which was used for the
modulator exchange, was small (~ 1.2 µm, Fig. S19), low concentrated acid solution (2 wt.%)
and short time (four hours only) were sufficient to remove the majority of the modulator
1
molecules. A kinetic study, where the modulator/linker ratio was monitored via HꢀNMR, shows
that more than half of the formates are removed by hydrochloric acid after 10 minutes (Section
S2 ESI, Fig. S7).
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66
Figure 1. a) Crystal structure of DUTꢀ67_HCOOH. The colored spheres represent three
different pores: small octahedral micropore in yellow, small cuboctahedral pore – in violet, big
cuboctahedral pore – in red. b) Local cluster environment of DUTꢀ67 (Zr)_HCl. Zr atoms are
shown in green, carbon in grey, oxygen in red, sulfur in yellow, chloride in orange. c – d)
Positions of chlorine anions in the small octahedral pore (shown in yellow in Fig. 1a)
In case of treatment of the parent material with 5 wt.% H SO solution for 24 hours, the
2
4
crystallites were completely dissolved. If the material is treated with 2 wt.% H SO solution, the
2
4
MOF retains the crystallinity, but shows already after four hours of treatment missing linker
defects, according to TG analysis (Figures S3, S4).
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To get more insights into anion exchange process, time resolved PXRD measurements (Figure
2a, Figure S8a, ESI) were performed during the HCl treatment. The measurements show that
after a short time of 10 ꢀ 30 min the crystallinity decreases and the reflections are broadened.
However, after two hours of treatment the reflections become wellꢀdefined and a highly
crystalline material is formed after four hours. Furthermore, a sample of DUTꢀ67 with poor
quality and very broad reflections show clearly defined and sharp reflections after the acidic
treatment with HCl (Figure 2b, Figure S8b, ESI), which indicates a partial recrystallizationꢀ and
healing process. Thus, extended reaction time (up to 24 h) of HCl treatment leads to a healing of
the crystals resulting in a material with high crystallinity and porosity (Figure S10, Table S2,
ESI). The acidic treatment influences not only the crystallinity and porosity, but also water
vapour adsorption behavior (Figure 3, Figure S12, ESI). The adsorption isotherm of pristine
DUTꢀ67 contains three steps, representing the filling of the three different pore types: the small
octahedral pore (yellow sphere in Figure 1a), the cuboctahedral pore with open metal sites (red
1
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1
1
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0
sphere in Figure 1a) and the cuboctahedral pore without open metal sites (purple sphere in Figure
63
1
a). The water adsorption isotherms of DUTꢀ67_HCl and DUTꢀ67_H SO have much steeper
2
4
slope in the low p/p region, indicating a more hydrophilic character of the treated samples in
0
comparison to the pristine material (Figure 3). This higher hydrophilicity of the acid treated
samples with respect to the parent material is very relevant for the resulting catalytic properties,
as we will discuss later.
Figure 2. a) PXRD patterns of DUTꢀ67(Zr), which was treated with a 2 wt.% solution of HCl for
different time. b) PXRD patterns of as synthesized DUTꢀ67(Zr) sample of pure crystallinity
(dark grey) and after a treatment with a 2 wt.% solution of HCl for 24 hours (light grey).
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In addition, the stability of the treated MOFs against water removal is increased. While total pore
volume of pristine DUTꢀ67(Zr) decreases for 37% after direct water removal, DUTꢀ67(Zr)_HCl
shows a porosity loss of 20% and DUTꢀ67(Zr)_H SO of 7% only (Table 2, Figure S11, ESI).
2
4
The same trend is observed for the Hf based DUTꢀ67. Therefore we conclude that exchange of
ꢀ
2ꢀ
0
1
2
3
4
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0
1
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4
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8
9
0
modulator molecules by Cl or SO4 does increase the hydrolytic stability/robustness of
investigated MOFs. However, the effect on thermal stability is more complex (Figure S9, ESI).
DUTꢀ67_HCOOH preserves its crystallinity up to 275 °C. DUTꢀ67_HCl decomposes at 200 °C,
ꢀ
which is the temperature region where Cl starts to be released from the metal cluster, according
to TG/MS analysis (Figure S2, S5, ESI). DUTꢀ67_H SO seems to be stable at even 300 °C.
2
4
To get a deeper look into the chemical nature of the DUTꢀ67 cluster, DUTꢀ67_HCOOH and
DUTꢀ67_HCl were analyzed by the diffuse reflectance infrared Fourier transform spectroscopy
ꢀ1
ꢀ1
(
DRIFTS). Two sharp peaks at 3672 cm and 731 cm appear in the spectrum of DUTꢀ67_HCl,
which can by assigned to the free ꢀOH stretching and ꢀOH bending vibrations, respectively,
while no signals relating to such terminal ꢀOH groups were observed in DUTꢀ67_HCOOH
ꢀ1
(
Figure S15, ESI). In addition, the peak at 1561 cm present in DUTꢀ67_HCOOH, which can be
attributed to the carboxylic groups of formate, disappears in spectrum of DUTꢀ67_HCl. It
points on complete removal of modulator molecules as well as DMF molecules from the cluster
to form open zirconium cluster coordinated to terminal hydroxyl groups.
Figure 3. Water adsorption isotherms of DUTꢀ67(Zr)_HCOOH (black circles) and DUTꢀ
6
7(Zr)_HCl (red diamonds) and DUTꢀ67(Zr)_H SO (green triangles) at 298 K.
2 4
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ꢀ1
The band at 1670 cm can be attributed to the not coordinated carboxylic groups of the tdc
linkers. It is present in all samples pointing of some amount of free acidic groups available on
the surface of MOF or on defect sites.
A single crystal of DUTꢀ67(Zr)_HCl was subjected to the single crystal Xꢀray diffraction
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0
ꢀ
analysis with the purpose to localize the electron rich Cl anion in the crystal structure. Indeed,
ꢀ
after refinement of the framework structure, Cl atoms could be localized with the partial
occupancy of 10.7%. The chlorine atoms are located on a mirror plane m, which corresponds to
the 96k Wyckoff position in the unit cell. All these positions are located in the smallest and
hydrophilic octahedral pore of DUTꢀ67(Zr). The chloride anions are localized next to the metal
node of the MOF and separated from the µ ꢀoxygen atoms O6 by a distance of 3.353 Å. (Figure
3
1
b, Figure S20, ESI). Such interatomic distance is characteristic for the weak OH…Cl
ꢀ
interaction, which is probably the main reason for the defined positions of the Cl in the crystal
structure. As a consequence, the Zr cluster is less shielded by anions in comparison to pristine
DUTꢀ67_HCOOH and are more accessible for substrate molecules. In contrast, in Zrꢀbased
6
2
2ꢀ
4
MOFs containing sulphate anions, according to Jiang et al. , the SO are located at the corners
and/or the edges of the octahedral Zr cluster (Figure S20).
To get an insight into the acid strengths of the materials obtained after acidic treatment, the
72,73
Hammett indicator method
was applied (Table S3, ESI). The Hammett acidity (H ) allows a
0
qualitative statement about the ability of a material to convert a neutral base to its conjugated
acid (Brønsted acidity). If an adsorbed indicator changes the colour to its acidic form, it means
that the acid strength of the material is equal or lower than the pK value of the indicator. For the
a
UiOꢀ66 and UiOꢀ67 materials, as well as the (Zr and Hf based) pristine DUTꢀ67, the H acid
0
strength ranges between +6.8 > H > +3.3. In contrast, the DUTꢀ67 materials, which were treated
0
with sulphuric and hydrochloric acid, the acid strength is increased to +1.5 > H > ꢀ3 (Table S4,
0
ESI).
Table 2. Textural properties of pristineꢀ and acid treated DUTꢀ67.
3
-
Surface area
tPV /
tPVH2O
/
Porosity
loss/ %
tPV / cm g
N2
N2
2
-1
3
-1
3
-1
1
/
m g
cm g
cm g
experimental
after water experimental
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experimental desorption
DUTꢀ67(Zr)
DUTꢀ67(Zr)_HCl
DUTꢀ
1110
1013
1153
0.49
0.46
0.48
0.31
0.37
0.45
0.40
0.42
0.37
37
20
7
0
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0
6
7(Zr)_H SO
2 4
DUTꢀ67(Hf)
763
791
798
0.33
0.33
0.33
0.29
0.31
0.29
0.30
0.28
0.28
12
6
DUTꢀ67(Hf)_HCl
DUTꢀ
12
6
7(Hf)_H SO
2 4
tPVN2
tPV
ꢀ pore volume derived from nitrogen adsorption isotherm at 77 K;
ꢀ pore volume
H2O
derived from water vapor adsorption isotherm at 298 K.
In order to evaluate the catalytic properties of Zr and Hf containing DUTꢀ67 compounds and to
investigate the effect of the postꢀsynthetic anion exchange treatments, we decided to use the
esterification of levulinic acid with ethanol as a test reaction. We find this test reaction
particularly interesting, since it has been recently shown that it can be catalyzed by UiO
7
4
materials, proving that catalytic sites are accessible to reactant molecules. Given the high
connectivity of the Zr clusters in UiOꢀ66 and UiOꢀ67 and the closed coordination sphere of Zr
6
ions, the ideal structures lack the available coordination vacancies, so that open metal sites can
only be created at missing linker defects. In contrast, a clear advantage of DUTꢀ67 compounds
over UiOꢀ66 is that the lower connectivity of their clusters provides (once the formate
4
+
modulators are removed) suitable coordinatively unsaturated Zr metal centres that can act as
the active sites for the esterification reaction, thus avoiding the need to create lattice defects to
display catalytic properties. In addition, coordination of H O molecules to coordinatively
2
unsaturated ZrꢀMOFs also increases the concentration of Brønsted sites provided by hydroxyl
groups coordinated to the Zr atoms, as it has been recently demonstrated by a combined
75
experimental and first principles calculations study.
In this sense, an increase of the
hydrophilic character of the MOF (as a consequence of the treatment with strong mineral acids)
is then expected to provide an increase of the Brønstedꢀinduced acidity of the compound. The
treatment of DUTꢀ67_COOH with HCl or H SO leads to different local coordination
2
4
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environment of the cluster (Fig. S20), which have also to be reflected in different catalytic
activity, as we show herein.
For catalytic experiments, batches of pristine DUTꢀ67_HCOOH and acidꢀtreated DUTꢀ67_HCl
and DUTꢀ67_H SO with similar porosity were chosen. The total pore volumes for DUTꢀ67(Zr)
2
4
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0
3
ꢀ1
3
ꢀ1
and its acid treated analogues range between 0.46 cm g and 0.48 cm g and for DUTꢀ67(Hf)
3
ꢀ1
3
ꢀ1
between 0.32 cm g and 0.33 cm g (Table 2).
Table 3 and Figure 4 summarize the results obtained for the esterification of levulinic acid with
ethanol using either pristine or acid treated DUTꢀ67 materials as catalysts. For comparison, the
7
4
results obtained with a UiOꢀ66(Zr) sample containing around 12% of missing linker defects
were also included. As it can be clearly observed, the catalytic activity of both asꢀprepared Zr
and Hf based DUTꢀ67_HCOOH MOFs is much lower than that of the reference UiOꢀ66
compound (compare entries 2, 3 and 6 in Table 3). In terms of turnover frequencies (TOF), the
ꢀ1
calculated values for Zr and Hf containing DUTꢀ67 are 6 and 2 h , respectively, as compared to
ꢀ1
2
2 h obtained for UiOꢀ66(Zr). The lower activity of the asꢀprepared DUTꢀ67 materials can be
attributed to the blocking of the active sites by the formate and DMF molecules, which are still
coordinated to the metal sites, thus hampering the adsorption and activation of the reaction
substrates. On the other hand, UiOꢀ66 contains a relatively high concentration of missing linkers
(ca. 12%), and hence also a significant amount of open metal sites, which provides the necessary
active centers to carry out the reaction efficiently.
The situation is completely different for the acid treated DUTꢀ67 samples. As we have
commented above, acid treatment of the MOFs produces the displacement of both formate and
DMF molecules formerly coordinated to the metal ions in the asꢀprepared material, by the
corresponding anions. It translates into a marked increase of their catalytic activity (as compared
with the corresponding asꢀprepared compounds), largely surpassing that of the reference UiOꢀ66
compound (see entries 4, 5, 7 and 8 in Table 3). The corresponding TOFs of the acidꢀtreated
DUTꢀ67 compounds are between three and five times higher than the reference UiOꢀ66. In
general, Hf materials are found to be less active than the Zr analogues, while the compounds
treated with HCl are only slightly more active than the corresponding H SO treated materials.
2
4
The explanation for such minor deviation in the catalytic activity can be attributed to the pivotal
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2ꢀ
ꢀ
role of Brønsted acid sites, since the shielding effect of SO4 anions is higher with respect to Cl
also in conjunction with estimated Hammett acidity.
Table 3. Esterification of levulinic acid with ethanol over pristine and acid treated DUTꢀ67
compounds. Results obtained in a blank experiment and over UiOꢀ66(Zr) (ca. 12% missing
0
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5
6
7
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0
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a
linker defects) are also included for comparison.
ꢀ1 c
Catalyst
Blank
Time
2 h
EL yield
mol%)
TOF (h )
b
(
1
2
3
4
5
6
7
8
1.5
9.7
54
99
14
47
44
84
34
78
6.2
21.6
33
77
28
72
ꢀ
2
0 h
2 h
0 h
DUTꢀ67(Zr)_ HCOOH 2 h
UiOꢀ66(Zr)
22
6
2
20 h
DUTꢀ67(Zr)_HCl
15 min
2 h
15 min
110
85
2
DUTꢀ67(Zr)_H SO
2
4
2
h
DUTꢀ67(Hf)_ HCOOH 2 h
20 h
DUTꢀ67(Hf)_HCl
15 min
2 h
15 min
100
72
DUTꢀ67(Hf)_H SO
2
4
2
h
a
Reaction conditions: Levulinic acid (116 mg, 1 mmol), ethanol (0.9 mL, 15 mmol), catalyst
b
(
1.8 mmol.% metal with respect to levulinic acid), 80ºC. Determined by GC. Ethyl levulinate
c
ꢀ1
was the only product detected. Turnover frequencies (TOF, in h ), calculated as moles of
levulinic acid converted per mole of metal and per unit time at short reaction time (at conversion
levels below 20 mol%).
Note also that the larger availability of open metal sites in the acidꢀtreated DUTꢀ67 materials,
with respect to the pristine material, translates into a stronger coordination of water molecules
(
as evidenced from the water adsorption isotherms shown in Figs. 3 and S12), which in turn will
increase the Brønsted induced acidity of the acid treated materials, and thus, the observed
catalytic activity. All materials were found to remain highly crystalline after the reaction, as
confirmed by XRD of the used solids, while leaching of Zr (or Hf) from the solid to the reaction
medium was ruled out according to ICPꢀOES analysis of the liquid filtrate. Accordingly, hot
filtration experiments at partial conversion showed in all cases that the reactions do not proceed
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any further when the solid catalysts are removed from the reaction medium, thus evidencing the
heterogeneity of the reaction.
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1
2
3
4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Yield of ethyl levulinate obtained over Zr (part a) and Hf (part b) based DUTꢀ67
compounds: pristine compounds (ꢀ■ꢀ); HClꢀtreated samples (ꢀ◊ꢀ); and H SO ꢀtreated samples (ꢀ
2
4
○ꢀ). Results obtained over a UiOꢀ66(Zr) compound with ca. 12% missing linkers (ꢀ*ꢀ) and in a
blank experiment without catalyst (+) are also included for comparison.
For reusability experiments, the solid catalysts were recovered by filtration, washed with ethanol
and dried at room temperature overnight. In all cases, it was found that strong deactivation of the
acidꢀtreated DUTꢀ67 samples occurs after the first catalytic cycle (see Section 6 of ESI).
However, starting from the third catalytic run, the progressive deactivation of the catalyst is
much less pronounced, as shown in Figure 5 for the DUTꢀ67(Zr)_HCl compound (the behavior
of all the other materials was similar). When the spent catalyst recovered after 6 consecutive
reuses was washed with H O instead of ethanol, it partially recovered its activity, which
2
evidences the relevant contribution to the observed catalytic activity of the Brønstedꢀinduced
4
+
acidity upon water coordination on the Zr open metal sites. Meanwhile washing with an
ethanolic solution of 2 wt.% HCl produced a complete recovery of the catalytic activity to the
same level of the fresh catalyst.
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9
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Figure 5. Yield of ethyl levulinate obtained (after 3 h, 8 h and 20 h of reaction) in consecutive
recycling experiments of DUTꢀ67(Zr)_HCl. Between two consecutive runs, the solid catalyst
was washed with ethanol (runs 1 to 6), or with either H O or a 2wt% ethanolic solution of HCl,
2
and dried at room temperature overnight.
Therefore, this (reversible) deactivation of the catalyst can be attributed to the regeneration of
Brønsted sites by acidic treatment, as well as desorption and washing of eventually adsorbed
products on the catalyst surface. Meanwhile, according to the XRD analysis, the crystallinity of
the solid was maintained throughout the whole reusability tests, which evidences the high
stability of the MOFs under the selected reaction conditions.
Summary and conclusions
Due to the labile character of ligands in Zrꢀbased MOFs, promoting ligand exchange, a treatment
of DUTꢀ67 with an ethanol solution, containing small amount of HCl shows a healing effect.
This recrystallization process results in improved crystallinity and porosity. An acidic treatment
with HCl or H SO changes furthermore the coordination environment of the metal cluster,
2
4
insofar as modulator molecules are removed from the framework and corresponding anions of
the inorganic acids are incorporated for charge balance. Thereby, relevant properties of the MOF
are tuned: the hydrophilicity as well as the acid strength is increased. These modifications have a
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huge impact on the catalytic activity. While pristine DUTꢀ67 has relatively small TOFs in the
ꢀ1
ꢀ1
esterification of levulinic acid with ethanol (6 h for Zr based – and 2 h for Hf based DUTꢀ67),
DUTꢀ67_HCl shows a ~18ꢀfold increase of TOFs (Zr based) and even a 50ꢀfold increase for the
Hf based MOF. The sulphate bearing DUTꢀ67 is slightly less active in comparison to DUTꢀ
1
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7_HCl, likely due to steric factors. Recycling tests showed some decrease in the catalytic
activity, but the activity could be completely regained by a subsequent washing procedure,
involving a diluted HCl solution. The full recovery illustrates the importance of absence of
molecules, like modulators or other metal coordinating molecules, which block catalytic active
centers.
Acknowledgments
This project has received funding from the European Union’s Horizon 2020 research and
innovation programme under the Marie SklodowskaꢀCurie grant agreement no. 641887 (project
acronym: DEFNET). The Spanish Ministry of Economy and Competitiveness (program “Severo
Ochoa”), the Spanish Ministry of Science and Innovation (project MAT2014ꢀ52085ꢀC2ꢀ1ꢀP),
and the German Research Foundation (project KA 1698/19ꢀ1) are also acknowledged for
financial support.
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75) Caratelli, C.; Hajek, J.; Cirujano, F. G.; Waroquier, M.; Llabrés i Xamena, F. X.; Van
Speybroeck, V., Nature of active sites on UiOꢀ66 and beneficial influence of water in the
catalysis of Fischer esterification, J. Catal., 2017, 352, 401.
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Crystal Growth & Design
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Anion exchange and catalytic functionalization of the zirconium based metal-organic
framework DUT-67
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Franziska Drache, Francisco G. Cirujano, Khoa D. Nguyen, Volodymyr Bon, Irena Senkovska
and Francesc X. Llabrés i Xamena, Stefan Kaskel
By postꢀsynthetic treatment of Zr and Hf based DUTꢀ67 MOFs with inorganic acids, the polarity
as well as Brønsted acid strength are increased, giving materials highly active in the
esterification of levulinic acid with ethanol.
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