Towards acid MOFs-catalytic performance of sulfonic acid functionalized architectures

Article abstract of DOI:10.1039/c3cy00272a

In this work, the inclusion of free sulfonic acid groups in highly stable MOFs is explored. The synthesized catalysts have been applied in a model esterification reaction. Two metal organic frameworks bearing sulfonic acid moieties are investigated: HSO₃-MIL-101(Cr) synthesized following different approaches and a new structure based on HSO₃-bdc and Zr. The acidic properties, catalytic performance, deactivation and stability of the different structures are critically evaluated. In the case of MIL-101(Cr), deactivation of the sulfonic groups via formation of butanol sulfonic esters has been observed. Due to the strong interaction between -SO₃ ^- and the Cr open metal site where usually fluorine (F^-) is located in the structure, the HSO₃-MIL-101(Cr) catalysts are not stable under acidic regeneration conditions. When using Zr as a metal node, a new and stable sulfonic acid containing porous structure was synthesized. This structure showed high activity and full re-usability in the esterification of n-butanol with acetic acid. In this case, deactivation of the catalyst due to sulfonic ester formation could be reversed by reactivation under acidic conditions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cy00272a

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Towards acid MOFs – catalytic performance of sulfonic
acid functionalized architectures†
Cite this: DOI: 10.1039/c3cy00272a
Jana Juan-Alcan˜iz,*a Robin Gielisse,a Ana B. Lago,b Enrique V. Ramos-Fernandez,a
Pablo Serra-Crespo,a Thomas Devic,b Nathalie Guillou,b Christian Serre,b
Freek Kapteijna and Jorge Gascon*a
In this work, the inclusion of free sulfonic acid groups in highly stable MOFs is explored. The synthesized
catalysts have been applied in a model esterification reaction. Two metal organic frameworks bearing
sulfonic acid moieties are investigated: HSO3-MIL-101(Cr) synthesized following different approaches and
a new structure based on HSO3-bdc and Zr. The acidic properties, catalytic performance, deactivation and
stability of the different structures are critically evaluated. In the case of MIL-101(Cr), deactivation of the
sulfonic groups via formation of butanol sulfonic esters has been observed. Due to the strong interaction
between –SO3 and the Cr open metal site where usually fluorine (Fꢀ) is located in the structure, the
ꢀ
HSO3-MIL-101(Cr) catalysts are not stable under acidic regeneration conditions. When using Zr as a
metal node, a new and stable sulfonic acid containing porous structure was synthesized. This structure
showed high activity and full re-usability in the esterification of n-butanol with acetic acid. In this case,
deactivation of the catalyst due to sulfonic ester formation could be reversed by reactivation under
acidic conditions.
Received 20th April 2013,
Accepted 3rd June 2013
DOI: 10.1039/c3cy00272a
www.rsc.org/catalysis
resulting in nearly infinite design possibilities.7 When it comes
to catalytic applications, as recently highlighted in several
Introduction
reviews,8,9 MOFs hold many promises: the inorganic connec-
tors may hold certain functional sites by themselves, as shown
for MOFs possessing coordinatively unsaturated sites (CUS),10
or may display photoactivity.11 Besides the intrinsic activity of
such CUS, they can also be utilized as anchoring points for
molecules to create catalytically active or sorption centers.12
Regarding the organic linkers, it is generally anticipated that
MOFs may bridge the gap between organic and inorganic
chemists,13,14 since almost any organic moiety of choice could
be immobilized. The formation of guest-accessible functional
organic sites (FOS) may be achieved following two different
strategies:15 direct incorporation of functionality during the
synthesis16 or post-functionalization.17
Different approaches have been followed for the introduc-
tion of strong Brønsted acidity in MOFs, these include mainly
post-synthetic and encapsulation routes, like controlled frame-
work destruction to increase the number and acidity of CUS,18
post-synthetic framework sulfation,19 post-functionalization of
amine groups with sultones20 and encapsulation of hetero-
polyacids.21 To date, Kitagawa and co-workers have reported the
direct synthesis of free sulfonic acid functionalized MOFs, namely
MIL-101(Cr) and doped UiO-66(Zr), via direct synthesis.22–24 Very
recently Biswas et al. presented a new direct synthesis procedure
During the last few decades, much effort has been put into the
development of acidic solid materials such as ion-exchange
resins based on sulfonic acid groups,1 sulfated oxides based on
zirconia, silica, or alumina2 and activated carbons with different
surface functional groups.3–5 However, the application window of
such solids is limited: some exhibit swelling in solvents (polymers)
or strong leaching issues (sulfated oxides and activated carbons),
while none of them possess a well-defined porosity. The develop-
ment of strongly acidic nano-structured catalysts is a long-standing
challenge for materials’ researchers and is only partially solved by
organic functionalized mesoporous silicas.6
In the field of synthetic nano-structured materials, metal–
organic frameworks, hereafter MOFs, are attracting a great deal
of attention. Next to a high surface area and pore volume, the
chemical environment in MOFs can be fine-tuned by selecting
the appropriate building blocks and/or by post-synthetic treatment,
a Catalysis Engineering, Chemical Engineering Department, Delft University of
Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands.
E-mail: j.gascon@tudelft.nl
b
´
Institut Lavoisier, UMR 8180 CNRS Universite de Versailles St Quentin en
Yvelines, 45 avenue des Etats-Unis, 78035 Versailles, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00272a
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for SO3H-UiO-66.25 Several works describe the benefits of Nitrogen adsorption at 77 K
sulfonic MOFs for applications like catalysis,26 proton conductivity
Nitrogen adsorption at 77 K was measured in a Quantachrome
Autosorb-6B unit gas adsorption analyzer.
or adsorption.23–25 In this work, we explore two synthetic routes
for the production of sulfonic structures like MIL-101(Cr) and
a Zr based MOF, along with a thorough assessment of their
acid catalytic activity, deactivation and influence of synthesis
conditions on framework stability and catalyst reusability.
Carbon dioxide adsorption at 273 K
Low-pressure adsorption was measured in a Micromeritics
TriStar II 3020 based on the volumetric technique. The samples
were pre-treated under vacuum for 16 h at different temperatures
depending on the sample.
Experimental section
All the chemicals and reference catalysts were obtained from
Sigma-Aldrich and used without further purification, unless
otherwise stated.
X-ray diffraction
X-ray diffraction was performed using a Bruker-AXS D5005 with
CoKa radiation. For HSO3-ZrMOF, high-resolution synchrotron
data were collected at ID31, ESRF, France. Subsequent pattern
indexing and Le Bail structure-independent fits were performed
using Topas software.
HSO3-MIL-101(Cr)HCl
HSO3-MIL-101(Cr)HCl has been synthesized according to the
procedure reported by Kitagawa and co-workers, a mixture of
monosodium 2-sulfoterephthalic acid (2 g, 7.5 mmol), CrO3
(0.75 g, 7.5 mmol), concentrated aqueous hydrochloric acid
(12 N, 0.546 g) was dissolved in water (30 g), and it was
hydrothermally treated at 453 K for 168 h.22
ICP-OES
Samples were digested in duplo in a mixture of 8 ml aqua regia
and 2 ml HF using microwave irradiation. After digestion, the
samples were diluted to 50 ml with MQ, and analyzed using an
ICP-OES Perkin Elmer Optima 5300dv.
HSO3-MIL-101(Cr)HF
HSO3-MIL-101(Cr)HF has been synthesized from a mixture of
chromium(III) nitrate nonahydrate (2.00 g, 5 mmol), mono-
sodium 2-sulfoterephthalic acid (Tokyo Chemical Company,
2.70 g, 10 mmol), deionized water (30 g) and hydrofluoric
acid (47–51 wt%, 0.3 g) by heating at 463 K for 24 h. The
as-synthesized solid was washed with deionized water and
dried in air at 393 K.
Diffuse reflectance IR spectroscopy
DRIFT spectra were recorded on a Nicolet model 8700 spectro-
meter, equipped with a high-temperature DRIFT cell (KBr
windows), DTGS-TEC detector and a 633 nm laser. The spectra
were registered from 4000 to 600 cmꢀ1 after accumulation
of 128 scans and a resolution of 4 cmꢀ1. A flow of helium at
20 ml minꢀ1 was maintained during the measurements. Before
collecting the spectra, the different samples were pre-treated in
the same helium flow at 393 K for 30 min. KBr was used to
perform background experiments.
KSO3-bdc
25 g (0.134 mol) of 2,5-dimethylbenzenesulfonic acid dehydrated
(Acros Organics, 99%) was dissolved in 100 ml of water and
KMnO4 (106 g, 0.67 mol) was slowly added. The reaction mixture
was heated under reflux for 48 h, and followed by filtration to
remove the residual MnO2. The solution was concentrated until
approx. 80%, and 12 N hydrochloric acid was added to the
filtrate (approx. 200 ml). Immediately, a precipitation of a
mixture of 2-sulfoterephthalic acid and the monopotassium salt
is formed. The white precipitate is washed with EtOH and dried
at 100 1C. 1H NMR (dmso-d6): d 8.34 (s, 1H), 7.99 (dd, 1H, J = 2 Hz,
X-ray photoelectron spectroscopy (XPS)
Measurements were made using VG-Microtech Multilab equip-
ment, MgKa (hn: 1253.6 eV) radiation and a pass energy of 50 eV.
The XPS system analysis pressure was kept at 5 ꢁ 10ꢀ10 mbar.
The binding energy (BE) and the kinetic energy (KE) scales were
adjusted by setting the C1s transition to 284.6 eV. BE and KE
values were determined with the peak-fit software of the spectro-
meter. The intensities were estimated by calculating the integral
of each peak, after subtraction of the S-shaped background, and
by fitting the experimental curve to a combination of Lorentzian
(30%) and Gaussian (70%) peak shapes. The surface atomic
ratios were estimated from the integrated intensities corrected
by the atomic sensitivity factors (D. A. Shirley, Phys. Rev. B.,
1972, 5, 4709).
1
J = 8.6 Hz), 7.69 (d, H, J = 8.5 Hz). MS-ESI: m/z (%) = 245 (100)
[HSO3-bdc]+. EDS analysis: (K : S/0.6 : 1). IR (cmꢀ1): 1710 vs,
1487 m, 1260 m, 1208 vs, 1120 m, 1386 m, 1200 w
HSO3-ZrMOF
Zirconium tetrachloride (1.17 g, 5 mmol) and KSO3-bdc (2.50 g,
8.5 mmol) were reacted in deionized water (25 g) at 423 K for
24 h. The resulting white solid was washed with hot deionized
water and dried in air under ambient conditions.
Acid–base titration
Scanning electron microscopy
Volumes of known solutions of two different bases (NaOH and
SEM was measured in a JEOL JSM 6500F setup coupled to an bicycloguanidine) were added to 100 mg MOF dispersed in
Energy Dispersive Spectrometer (EDS) for micro elemental 20 ml water, while analyzed using a pH-meter, at RT. The
analysis.
diffusion of the bases inside of the MOF was in agreement
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with their size, and in both cases long times were needed for
pH stabilization (1 ml needs at least one hour).
Esterification of acetic acid and n-butanol
The reaction was performed without solvent, using a molar
ratio mixture of acetic acid : n-butanol = 1 : 1. The reaction
mixture (25 ml) was introduced into a round bottom flask,
while being stirred under reflux the temperature was increased
to 343 K. A ratio of 3 g catalyst per mole of acetic acid was used.
After recovering the catalyst, it was filtered, and treated with an
acidic solution before use in consecutive runs. The acidic
treatment was performed by putting the MOF catalyst in con-
tact with a 0.2 M solution of HCl or pellets of an acidic ion
exchange resins 1 : 1 mass ratio (DOWEXs 50W X8 hydrogen
form) in water under stirring conditions for 30 minutes at room
temperature.
Results and discussion
HSO3-MIL-101(Cr)
HSO3-MIL-101(Cr)HCl has been synthesized according to the
procedure reported by Kitagawa and co-workers, while a new
route has been developed for the synthesis of HSO3-
MIL-101(Cr)HF. Both catalysts exhibit the PXRD pattern expected
for MIL-101 structure (Fig. S1, ESI†). N2 adsorption isotherms
measured at 77 K (Fig. S2, ESI†) show the two characteristic steps
of MIL-101, related to the filling of the mesoporous cavities. At
very low relative pressures (P/P0 o 0.05) only the supertetrahedra
are filled. As pressure increases, the medium (P/P0 = 0.15) and
later the large cavities (P/P0 = 0.20) are filled. Samples synthe-
sized in the presence of HCl and at long synthesis times display a
lower surface area (circa 1200 m2 gꢀ1, calculated between 0.05
Fig. 1 FTIR spectra of the different HSO3-MIL-101(Cr) structures compared to
that of non-functionalized MIL-101(Cr). Top: full spectral range. Bottom: details of
the 1000–1750 cmꢀ1 region.
Infrared spectroscopy confirms the successful functionalization.
and 0.10 relative pressure) than samples synthesized in shorter Fig. 1 shows a comparison between the pristine MIL-101(Cr) and
times and in the presence of HF (circa 1800 m2 ꢀ1). In both both functionalized HSO3-MIL-101(Cr). Clear differences are already
cases, the surface area is smaller than for pure MIL-101(Cr)27 but revealed when observing the whole spectrum (4000–1000 cmꢀ1).
in good agreement with other functionalized MIL-101s.28 The n(OH) region (4000–3000 cmꢀ1) exhibits a broad band centered
g
SEM pictures (Fig. S3, ESI†) reveal a smaller particle size for at 3400 cmꢀ1 for the sulfonic materials, absent in the bare samples.
HSO3-MIL-101(Cr)HCl (E200 nm) than for HSO3-MIL-101(Cr)HF This vibration corresponds to water molecules retained in the pores
(E700 nm). Elemental analysis of the evacuated samples by strong hydrogen bonding with the sulfonic acid even after
demonstrates that ion exchange occurs during the synthesis, in situ treatment at 423 K for one hour. Specific sulfonic acid
resulting in an exchange of Na+ from the linker with protons, stretching can be observed in the fingerprint region (between
yielding a partial protonation of the sulfonic acid groups. 1800 and 1000 cmꢀ1).
This ion exchange is more effective for HSO3-MIL-101(Cr)HCl
New bands appear at 1190 and 1240 cmꢀ1, attributed to the
(1.28 wt% Na resulting in 0.3 Na+/SO3ꢀ) in contrast with HSO3- OQSQO symmetric and asymmetric stretching modes. The
MIL-101(Cr)HF (2.75 wt% Na resulting in 0.41 Na+/SO3ꢀ), probably peak at 1080 cmꢀ1 corresponds to the S–O stretching vibration,
due to the longer synthesis times, the stronger acidity of the with a slight shift of the band at 1020 to 1030 cmꢀ1, possibly
synthesis medium and the smaller particle size.
attributed to the influence of the SO3 substituted aromatic ring.29
The catalytic activity of the HSO3-MIL-101(Cr) catalysts has
Thermo-gravimetric analysis of both samples (Fig. S4, ESI†)
reveals a significantly higher amount of inorganic residue in been benchmarked against Amberlysts-15 by the liquid-phase
the case of the HSO3-MIL-101(Cr)HCl. While the remaining stoichiometric esterification of n-butanol and acetic acid at low
20 wt% in the case of HSO3-MIL-101(Cr)HF is in good agreement temperatures (343 K). Experiments were performed using the
with the expected molecular formula (Cr3OF(H2O)2[(O2C)- same ratio of 3 g molꢀ1 reactant for all catalysts. The evolution
C6H3(SO3NaxHy)-(CO2)]3), and the amount of water found in of the butyl acetate yield (the only reaction product next to
the sample (15 wt%), as indicated also by EDS with Cr/S of 1. water) with time for all studied catalysts is plotted in Fig. 2.
The residue for HSO3-MIL-101(Cr)HCl is higher than expected
The commercial catalyst (Amberlysts-15) displays a higher
and can be attributed to the presence of Cr2O3 in the material. activity on a weight basis, while a clear difference in activity
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catalyst) were observed, demonstrating the complete deactivation
of the HSO3-MIL-101(Cr) samples.
To unravel the nature of the deactivation, the different
HSO3-MIL-101(Cr) catalysts were analyzed after several reaction
cycles: PXRD confirmed the integrity of the crystalline structure
(Fig. S5, ESI†) while SEM did not show any strong attrition of
the catalyst particles, both discounting framework collapse as a
reason for the deactivation (Fig. S6, ESI†).
On the other hand, BET measurements (Fig. S7, ESI†) show a
large decrease in surface area after every reaction cycle, even
after pre-treatment at 433 K under vacuum. This decrease in
accessible pore volume and the absence of particle disintegra-
tion point at chemisorbed reactants and products as possible
causes for deactivation.
Fig. 2 Esterification of n-butanol and acetic acid (1 : 1 molar fraction) using
In order to gain insight into the specific interactions
between both reactants and the HSO3-MIL-101(Cr) framework,
DRIFT spectroscopy has been performed during the adsorption
and thermal desorption of n-butanol and acetic acid. Fig. 3
shows the collected IR spectra at different temperatures.
Impregnation with acetic acid does not result in the appearance
of new vibrations other than those representative of the carboxylic
acid group at 2650, 2575 and 1725 cmꢀ1. All bands disappear after
heat treatment of the sample, demonstrating that the framework–
acetic acid interaction is rather mild. In contrast, when the HSO3-
MIL-101(Cr) catalysts are contacted with n-butanol, in addition to
the CH2 and CH3, symmetric and asymmetric stretching modes,
between 2960 and 2880 cmꢀ1, several new vibrations can be
observed after heat treatment, namely a doublet at 2650 and
2575 cmꢀ1 and a broadening of the 1725 cmꢀ1 and SQO bands
(1260 and 1180 cmꢀ1). Both the doublet and the broadening of
the 1725 cmꢀ1 bands can be ascribed to carbonyl vibrations
resulting from the partial hydrolysis of terephthalate–Cr bonds,
while the broadening of the SQO bands and the shoulder
formed at 1065 cmꢀ1 are attributed to a strong SO3–n-butanol
interaction.
3 grams of the catalyst (specified in the graph) per mole of reactant at 343 K.
between both HSO3-MIL-101(Cr) samples can be noticed. The
latter can be explained based on the amount of protonated
HSO3 groups in the HSO3-MIL-101(Cr)HCl catalyst. On the other
hand, the difference in activity found between commercial and
MOF based catalysts can be explained when the density of acid
sites is considered: the amount of H+ in Amberlysts-15 is
4.2 mmol gꢀ1, higher than 1.9 and 2.2 mmol gꢀ1 for HSO3-
MIL-101(Cr)HF and HSO3-MIL-101(Cr)HCl, respectively (Table 1).
TOF values of 0.36, 0.23 and 0.30 minꢀ1 were calculated based
on the amount of protonated sulfonic groups for Amberlysts-15,
HSO3-MIL-101(Cr)HCl and HSO3-MIL-101(Cr)HF, according to
the theoretical molecular formula. The TOF values for the
MOFs are systematically 15–30% lower, although equal values
could be expected, since all three catalysts are based on
benzene-sulfonic moieties and therefore their acidic strength
should not differ much. On the other hand, it has to be noted
that TOF values obtained for the MOF catalysts are even higher
than most heterogeneous catalysts reported in the literature,
including sulfonic functionalized mesoporous silicas.31,32 Recycling
tests were performed to assess the heterogeneous character of the
samples. These experiments suggest a strong deactivation of both
HSO3-MIL-101(Cr) samples, in contrast to a less pronounced
deactivation found for Amberlysts-15. A large decrease in activity
is observed for the MOF catalysts. Indeed, after 3 cycles, similar
conversion levels as those in the blank run (in the absence of
Table 1 Turn-over frequencies (TOF, minꢀ1) for the esterification of n-butanol
and acetic acid (1 : 1 molar ratio) using 3 gram of the catalyst per mole of reactant
at 343 K. TOF obtained for several catalytic runs
TOF/minꢀ1
Acidity
Catalyst
[mmol gꢀ1] 1st Run 2nd Run 3rd Run 4th Run
Amberlysts-15
4.2a
0.36
0.23
0.30
0.29
0.31
0.12
0.22
0.23
0.20
0.11
0.13
0.20
HSO3-MIL-101(Cr)HF 1.9b
HSO3-MIL-101(Cr)HCl 2.2b
HSO3-ZrMOF
2.6c
0.33
Fig. 3 DRIFTS (a) HSO3-MIL-101(Cr)HF (also observed for HCl samples) upon
contact with an esterification reactant at RT (b) and desorption under helium at
increasing temperature steps: (c) 30 min at 323 K, (d) 30 min at 373 K, (e) 30 min
at 423 K, (f) 120 min at 443 K. Left: n-butanol. Right: acetic acid.
a
b
c
Obtained from ref. 30. Obtained by molecular formula. Obtained by
the theoretical molecular formula, and confirmed accessibility of all protons
by acid–base titration with NaOH and 1,5,7-triazabicyclo[4.4.0.]dec-5-ene.
c
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Very recently, Fraile and co-workers3 demonstrated that in in the synthesis, a very small amount of fluor is found in the
the case of sulfonic functionalized microporous carbons the final solid, with a binding energy of 684.2 eV. Interestingly, in
formation of sulfonate esters accounts for the deactivation of the case of pristine MIL-101(Cr) (no sulfonic groups) and when
the catalysts in the presence of an alcohol. In our work, the only half of the linkers are sulfonated, a much larger amount of
infrared and adsorption results are in line with this formation fluor, with a binding energy of 689 eV, is observed.36,37 Thesꢀe
of sulfonate esters.4 In view of such a strong chemical inter- results demonstrate that in the fully sulfonic structure, SO3
action, we re-activated the HSO3-MIL-101(Cr) catalysts using plays the same role as Fꢀ in stabilizing the chromium trimers
either acidic ion exchange resins (DOWEXs 50W X8 hydrogen and that full proton exchange results in the collapse of the
form) or very diluted HCl–water mixtures. Infrared spectra of structure.
the reactivated samples show the disappearance of all n-butanol
It is indeed well known that fluorine is involved in the
related bands, demonstrating the successful exchange. However, terminal bond of the trimeric chromium species and partly
XRD and adsorptive characterization after several ion exchanges substitutes the terminal –OH molecules attached to chromium
show a clear loss of crystallinity along with a reduced N2 uptake, in MIL-100 and MIL-101. Although it is not fully clear yet, it
inferring the partial collapse of the framework. These results seems that the use of fluorine provides a strong interaction
demonstrate that although active during the first catalytic runs, with the chromium octahedral motif and enhances the for-
the application window of HSO3-MIL-101(Cr) is rather limited.
mation of MIL-101 during the hydrothermal reaction.38,39 In
Kitagawa et al. claimed that electroneutrality in the HSO3- our case, when a similar stabilization role is taken by the
MIL-101(Cr) framework is achieved by partial deprotonation of sulfonic groups, the final structure is more likely to collapse
SO3H groups, instead of halide or hydroxide inclusion in the Cr due to the weaker interaction with the sulfonic groups.40
ꢀ
trimers, resulting in one out of three SO3 moieties in anionic
form.22 If this is the case, it is easy to envisage that due to a
HSO3-Zr-MOF
strong interaction of these anionic moieties with n-butanol and Zr-carboxylate MOFs like UiO-66(Zr) and others have been
during the ion exchange with the acidic resin, charge neutrality shown to be among the most stable MOFs.41 Functionalization
may be compromised, resulting in a partial collapse of the of UiO-66(Zr) has been extensively studied, both by direct
framework. Two different approaches were followed to improve synthesis for groups like –NH2, –NO2 or –Br,42 and by post-
stability of the Cr3+ trimers and to achieve charge neutrality in synthetic treatment for more complex groups.43–45 Several
the framework without using the SO3ꢀ functional groups: in the theoretical studies already proposed the benefits of sulfonic
first approach, the as-synthesized catalysts were ion exchanged acid functionalization on the UiO-66(Zr)46 framework and very
with NH4F with the idea that Fꢀ would stabilize the Cr3+ trimers recently Kitagawa and co-workers and Biswas et al. described
+
and that NH4 would act as counter ion for the SO3ꢀ, but this the first synthetic route towards fully sulfonated and mixed-
approach resulted in the collapse of the framework. In a second linker UiO-66(Zr) and the effect of linker functionalization on
approach, we synthesized mixed linker MIL-101(Cr) using a adsorption properties.24,25 More precisely, whereas the mixed
50 : 50 mixture of monosodium 2-sulfoterephthalic acid and bdc/bdc-SO3H linker based compounds were shown to present a
terephthalic acid and the same synthesis conditions as in the rather high specific surface area, the fully substituted compound
case of HSO3-MIL-101(Cr)HF (Fig. S8, ESI†).
loses its crystallinity upon activation. Exploring the reactivity
Elemental analysis (XPS) on the resulting solids demon- towards a MOF formation of zirconium- and potassium 2-sulfo-
strates that both linkers are present and that Fꢀ incorporation terephthalic in water rather than DMF, a new crystalline porous
in the framework took place during synthesis (Fig. S9, ESI†). solid could be isolated, denoted HSO3-ZrMOF (see Experimental
However, the resulting solids did not display any activity in the section).
esteꢀrification of n-butanol and acetic acid, inferring that most
The XRD powder pattern of the new structure could be
%
SO3 moieties are used to stabilize the framework Cr trimers indexed (Fig. S10, ESI†) within a I cubic unit-cell (Im3m, a =
and confirming the rather limited availability of protons in 41.5331(2) Å), first indicating that the product is a single
HSO3-MIL-101(Cr) for catalysis.
crystalline phase. Although this cell presents similarities with
%
XPS analysis of the S 2p shows one peak centered at 168 eV, the one of UiO-66 (cubic, SG: Fm3m, a = 20.7004(2) Å) with a
indicating the presence of sulfonic groups.33 These peaks doubling of the cell parameter, preliminary structural analysis
decrease significantly for the mix-linker system, suggesting suggests that it does not represent a UiO-66 type topology built
the rather low inclusion of sulfonic linkers compared to the up from 12-connected Zr6 clusters solely, but is rather based on
100% functionalized linker sample. For both structures, con- 8-connected Zr6 clusters, similar to the ones observed in some
tributions from two different S 2p peaks are observed after very recently reported Zr-MOFs.47–49 This is furthermore in
precise deconvolution: 169 eV and 167.8 eV. Although these accordance with the Zr/ligand ratio extracted from EDS analysis
binding energies are typically attributed to the S 2p3/2 and S 2p1/2 (Zr/S B 1.5, repeated at least 10 times with equal results), which
transitions of sulfonic groups (R–SO3H),34 several publications suggests a Zr6(bdc-SO3H)4 formula rather than a Zr6(bdc-
partially attribute this transition to sulfone (R–SO3–R0) groups,35 SO3H)6 one. Using state of the art structure solution methods
suggesting a possible interaction of sulfonic groups with another (charge flipping), some Zr atoms could be allocated. This
part of the framework. This hypothesis is emphasized when unambiguously confirms the presence of Zr6 clusters similar
looking at the F 1s spectra. When full sulfonic linker is present to those found in many Zr-based MOFs (Fig. S11, ESI†); one can
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nevertheless not preclude that some clusters are still missing in
our model. Moreover, the ligands could not be localized yet.
Nevertheless, the examination of the intercluster distances
gives information about their connection. The shortest inter-
cluster Zr–Zr distances (11.4 Å) are indeed almost identical to
those found in UiO-66, indicating that at least one part of the
cluster is connected in a similar way to that in the UiO-66 solid,
i.e. through Zr–terephthalate–Zr bridges. This proves that at
least one part of the sulfonic groups is not connected to Zr ions
and is thus available for catalysis.
Nitrogen adsorption at 77 K proved not to be the best
method to characterize porosity of the new Zr MOF: N2 uptakes
depended on the pre-treatment prior to the measurement.
For instance, after pre-treatment at room temperature under
vacuum, samples presented a decent adsorption capacity
Fig. 5 Esterification of n-butanol and acetic acid (1 : 1 molar ratio) using 3 grams
of the catalyst per mole of reactant at 343 K. Before every use the catalyst has
been pre-treated with 0.2 M HCl solution for 1 h at room temperature and
thoroughly washed with H2O until neutral pH.
(400 cm3
g
ꢀ1), whereas when pre-treated at 353 K under
vacuum, samples did not show any N2 uptake. In contrast,
adsorption of carbon dioxide at 273 K demonstrated sample
micro-porosity, independent of the pre-treatment used (2 mmol
CO2 per g at 100 kPa, see Fig. 4).
experiments were performed using two different bases: sodium
hydroxide and 1,5,7-triazabicyclo[4.4.0.]dec-5-ene; differing very
much in size. Titration results indicated a similar consumption
of both bases, and were in agreement with the amount of sulfonic
groups in the theoretical molecular formula (B3 mmol H+ per g).
These results demonstrate that even though pores might not be
accessible to N2 adsorption after sample activation, in the liquid
phase much bulkier molecules like triazabicyclo[4.4.0.]dec-5-ene
can enter the pores and titrate all sulfonic groups.
The acidic performance of the new Zr MOF has been
assessed in the esterification of acetic acid and n-butanol.
Fig. 5 shows a very high initial catalytic activity, well in
comparison with the benchmark catalyst Amberlysts-15.
Reusability of the catalyst was tested. Similar to that for MIL-
101(Cr) samples, formation of sulfonic esters resulted in a
strong decrease in catalytic activity. In addition, both the Na+
and K+ salts of the sulfonic linker were tested in blank experi-
ments, and did not show any catalytic activity. In order to
regenerate the spent catalyst, acid treatments were applied after
each reaction cycle with a 0.2 M HCl solution at room tempera-
ture for one hour followed by thorough cleaning with water
until pH 7 was reached. In the case of the Zr-MOF, re-usability
and full recovery of the catalytic activity upon regeneration is
possible (Fig. 5), with crystallinity and porosity being intact
after 4 catalytic-regeneration cycles, demonstrating the much
higher chemical stability of the new MOF. Possible dissolution
of the framework could be discarded by 13C, 1H NMR and
infrared analysis of the reaction solution: the presence of a
sulfonic linker in solution could not be detected using any of
the above-mentioned techniques.
Compared to the pristine UiO-66(Zr), the accessible surface
area is lower: 496 m2 gꢀ1 for UiO-66(Zr) vs. 339 m2 gꢀ1 for
Zr6(bdc-SO3H)4, as calculated from CO2 adsorption data using
the Dubinin Radushkevich model. This adsorption capacity
is however higher than the values shown by fully sulfonated
UiO-66(Zr) presented by Kitagawa et al.24 In addition, since the
synthesis takes place in the presence of the highly acidic
precursor ZrCl4, almost full K+ exchange takes place during
synthesis (B0.36 wt% residual K compared to the 2.5 wt% Na
in the MIL-101 samples). In fact, after one single use of the
catalyst in reaction all the K+ is exchanged (o0.01 wt% K).
DRIFT spectroscopy demonstrates the presence of sulfonic
groups, with the same new vibrations related to Caromatic–SO3
bonds (Fig. S12, ESI†) as observed for the MIL-101 sulfonic
samples.
In order to demonstrate that all sulfonic groups present in
the new Zr structure are accessible for catalysis, titration
Conclusions
In this work the inclusion of free sulfonic groups is explored
in, a priori, highly stable MOFs. When applied in a model
esterification reaction, deactivation of the sulfonic groups via
formation of butanol sulfonic esters has been observed. In the
Fig. 4 Carbon dioxide adsorption isotherms (273 K) of the HSO3-Zr-MOF
pre-treated under different conditions compared to UIO-66(Zr) pretreated at
427 K for 16 hours.
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20 D. Britt, C. Lee, F. J. Uribe-Romo, H. Furukawa and
case ꢀof MIL-101(Cr), due to the strong interaction between
–SO3 and the Cr open metal site where usually fluorine (Fꢀ)
is located in the structure, the catalysts are not stable under
acidic regeneration conditions. When using Zr as a metal node,
a new and stable sulfonic acid containing porous structure was
synthesized. This structure showed high activity and full
reusability in the esterification reaction of n-butanol with acetic
acid. In this case, deactivation of the catalyst due to the
formation of sulfonic esters could be tackled by reactivation
under acidic conditions.
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