Three Series of Sulfo-Functionalized Mixed-Linker CAU-10 Analogues: Sorption Properties, Proton Conductivity, and Catalytic Activity

Article abstract of DOI:10.1002/chem.201501502

Ten mixed-linker metal-organic frameworks [Al(OH)(m-BDC-X)_1-y(m-BDC-SO₃H)_y] (H₂BDC=1,3-benzenedicarboxylic acid; X=H, NO₂, OH) exhibiting the CAU-10-type structure were synthesized. The compounds can be grouped into three series according to the combination of ligands employed. The three series of compounds were obtained by employing different ratios of m-H₂BDC-X and m-H₂BDC-SO₃Li. The resulting compounds, which are denoted CAU-10-H/Sx, -N/Sx and -O/Sx, show exceptionally high thermal stability for sulfonated materials of up to 350 C. Detailed characterization with special focus on polarity and acidity was performed, and the impact of the additional SO₃H groups is clearly demonstrated by changes in the sorption affinities/capacities towards several gases and water vapor. In addition, selected samples were evaluated for proton conductivity and as catalysts for the gas-phase dehydration of ethanol to ethylene. While only very low proton conductivities were observed, a pronounced increase in catalytic activity was achieved. Although reactions were performed at temperatures of 250 and 300 C for more than 40 h, no desulfonation and no loss of crystallinity were observed, and stable ethanol conversion resulted. This demonstrates the high stability of this material.

Full text of DOI:10.1002/chem.201501502

DOI: 10.1002/chem.201501502
Full Paper
&
Metal–Organic Frameworks
Three Series of Sulfo-Functionalized Mixed-Linker CAU-10
Analogues: Sorption Properties, Proton Conductivity, and
Catalytic Activity
Nele Reimer,[a] Bart Bueken,[b] Sebastian Leubner,[a] Christopher Seidler,[c] Michael Wark,[c]
Dirk De Vos,[b] and Norbert Stock*[a]
Abstract: Ten mixed-linker metal–organic frameworks
[Al(OH)(m-BDC-X)1Ày(m-BDC-SO3H)y] (H2BDC=1,3-benzenedi-
carboxylic acid; X=H, NO2, OH) exhibiting the CAU-10-type
structure were synthesized. The compounds can be grouped
into three series according to the combination of ligands
employed. The three series of compounds were obtained by
employing different ratios of m-H2BDC-X and m-H2BDC-
SO3Li. The resulting compounds, which are denoted CAU-10-
H/Sx, -N/Sx and -O/Sx, show exceptionally high thermal sta-
bility for sulfonated materials of up to 3508C. Detailed char-
acterization with special focus on polarity and acidity was
performed, and the impact of the additional SO3H groups is
clearly demonstrated by changes in the sorption affinities/
capacities towards several gases and water vapor. In addi-
tion, selected samples were evaluated for proton conductivi-
ty and as catalysts for the gas-phase dehydration of ethanol
to ethylene. While only very low proton conductivities were
observed, a pronounced increase in catalytic activity was
achieved. Although reactions were performed at tempera-
tures of 250 and 3008C for more than 40 h, no desulfonation
and no loss of crystallinity were observed, and stable etha-
nol conversion resulted. This demonstrates the high stability
of this material.
Introduction
functionalized linker molecules are incorporated in one frame-
work to selectively influence the characteristics of a com-
pound.[4]
Metal–organic frameworks (MOFs) have attracted much atten-
tion in the last few years due to their potential applications in
the fields of gas storage and separation, drug delivery, and cat-
alysis.[1] MOFs are built up from inorganic building units, for ex-
ample, metal ions or clusters, which are interconnected by or-
ganic linker molecules, forming, in general, three-dimensional
porous structures. This modular assembly allows for tuning of
properties by chemical functionalization of the organic part or
by replacement of the inorganic unit. This approach is called
isoreticular chemistry[2] and has previously been applied to sev-
eral well-known framework structures.[3] Recently, another con-
cept to design compounds with desired properties has been
established. In the so-called mixed-linker approach, differently
For example, Al-MIL-53, [Al(OH)(BDC-X)], is a flexible com-
pound based on (functionalized) 1,4-benzenedicarboxylic acid
[H2BDC(-X)] and chains of trans corner-sharing AlO6 octahe-
dra.[3a,5] Its flexibility and thermal stability strongly depend on
the nature of the functionalized linker molecule. By using a mix-
ture of H2BDC and H2BDC-NH2 it was possible to influence
these properties by means of the ratio of the incorporated
linker molecules.[6] In another recent study, mixed-linker MOFs
based on the CAU-10, [Al(OH)(m-BDC-X)], structure were syn-
thesized to fine-tune the sorption properties of this com-
pound.[7] CAU-10 is built up from helical chains of cis corner-
sharing AlO6 octahedra interconnected by isophthalate ions. A
series of isoreticular compounds has been synthesized with
several functionalized isophthalic acids. Depending on the
functional group of the linker molecule, the structure of each
derivative is slightly altered. This results in different uptake ca-
pacities, affinities, and accessibilities of the pores of each com-
pound towards different adsorbates.[3c]
[a] N. Reimer, S. Leubner, Prof. N. Stock
Institut für Anorganische Chemie
Christian-Albrechts-Universität zu Kiel
Max-Eyth-Strasse 2, 24118 Kiel (Germany)
E-mail: stock@ac.uni-kiel.de
[b] B. Bueken, Prof. D. De Vos
The incorporation of sulfonic acid groups in MOFs is of great
interest. Due to their strongly acidic protons, compounds bear-
ing these groups show potential for catalytic or proton con-
duction applications.[8] The sorption properties of such com-
pounds can also be influenced. Recently, it was reported that
the introduction of sulfo groups in the zirconium-based com-
pound UiO-66 led to a strong increase in the sorption selectivi-
Centre for Surface Chemistry and Catalysis
University of Leuven
Kasteelpark Arenberg 23, 3001 Leuven (Belgium)
[c] C. Seidler, Prof. M. Wark
Institut für Chemie, Carl von Ossietzky Universität Oldenburg
Carl-von-Ossietzky-Strasse 9–11, 26129 Oldenburg (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501502.
Chem. Eur. J. 2015, 21, 12517 – 12524
12517
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ty for CO2 over CH4 compared to the unfunctionalized com-
pound.[9] The synthesis of SO3H-functionalized MOFs is not
always easily accomplished. Only a few examples are known in
which the acid group was directly introduced into the frame-
work by the use of a monosodium sulfoterephthalate organic
linker molecule.[8a,10] Another way to introduce sulfo groups is
through post-synthetic modification, which was demonstrated
by Chen et al. using Cr-MIL-101, Zr-UiO-66, and Al-MIL-53.[8c,11]
We focused our research on aluminum-based MOFs, because
these compounds are known for their high thermal and chemi-
cal stability.[3a,c,12] In addition, the starting materials are inex-
pensive and of low toxicity, which makes them interesting for
industrial usage.[13] The goal of the present study was to com-
bine the properties of Al-MOFs and sulfo-functionalized com-
pounds. Therefore, we chose CAU-10 as the parent structure
and employed isoreticular synthesis using 5-sulfo isophthalic
acid monolithium salt (m-H2BDC-SO3Li). Pure m-H2BDC-SO3Li as
the organic linker does not lead to the CAU-10 analogue;
hence, mixtures of linkers were used. By adopting the synthe-
sis conditions of the already known compounds CAU-10-H,
-NO2, and -OH, we were able to synthesize three series of
mixed-linker compounds with the CAU-10 structure with differ-
ent fractions of sulfonic acids incorporated into the framework:
[Al(OH)(m-BDC-H)1Ày(m-BDC-SO3H)y]·solvent (y=0.08, 0.15,
0.205, 0.24; CAU-10-H/S1 to CAU-10-H/S4); [Al(OH)(m-BDC-
NO2)1Ày(m-BDC-SO3H)y]·solvent (y=0.065, 0.10, 0.175, 0.215;
CAU-10-N/S1 to CAU-10-N/S4) and [Al(OH)(m-BDC-OH)1Ày(m-
BDC-SO3H)y]·solvent (y=0.08, 0.125; CAU-10-O/S1 and CAU-10-
O/S2); for further details see Experimental Section]. The ob-
tained compounds were characterized by IR and NMR spec-
troscopy, thermal and elemental analysis, powder XRD (PXRD),
and sorption measurements. Selected samples were investigat-
ed with regard to their proton conductivity and catalytic per-
formance in the dehydration of ethanol.
Figure 1. Structure of CAU-10. Al atoms are displayed in very light gray, O
atoms in gray, and C atoms in dark grey. The light gray atoms represent pos-
sible functional groups.
Results and Discussion
Figure 2. SEM image of CAU-10-H/S1.
PXRD investigations and crystal structure
The structure of the CAU-10 framework is built up from helical
chains of cis corner-sharing AlO6 polyhedra.[3c] The three-di-
mensionality of the framework results from the connection of
each helix to four adjacent helices with alternating rotational
orientation through the carboxylate groups of (functionalized)
isophthalate ions (Figure 1). Square-shaped channels with
a maximum diameter of about 7 Š are formed. The pores can
be modulated by the use of functionalized linker molecules[3c]
or in a mixed-linker approach by the use of differently func-
tionalized linker molecules.[7]
All mixed-linker compounds in this study were obtained as
microcrystalline powders. The SEM image of CAU-10-H/S1 is
shown in Figure 2 (SEM images of CAU-10-H/S2–4 are shown
in Figures S1–S3 of the Supporting Information). For example,
the PXRD patterns of the CAU-10-H/Sx samples are compared
to that of CAU-10-H in Figure 3. The mixed-linker compounds
are also highly crystalline. Compared to the PXRD pattern of
CAU-10-H, only changes in the relative intensities of the reflec-
Figure 3. PXRD measurements for the mixed-linker compounds CAU-10-H/Sx
compared to the theoretical and experimental patterns of CAU-10-H.
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12518
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
tions up to 208 (2q) are observed, which become more pro-
nounced with increasing amounts of SO3H groups. This can be
explained by the additional electron density within the pores
due to the incorporated sulfo groups.
The PXRD patterns of CAU-10-N/Sx and CAU-10-O/Sx are
compared with those of CAU-10-NO2 and CAU-10-OH in Fig-
ure S4 of the Supporting Information. The mixed-linker com-
pounds are obtained in high crystallinity, and the positions
and relative intensities of all reflections are in good accordance
with those of CAU-10-NO2 and CAU-10-OH. The high-resolution
PXRD patterns of all compounds were indexed by using TOPAS
Academics.[14] The resulting cell parameters were refined by
using the Pawley method,[14] and only slight changes com-
pared to those of the parent structures were observed
(Table 1). The Pawley fits are shown in Figures S5–S14 of the
Supporting Information.
Figure 4. IR spectra of CAU-10-H/Sx compared to those of CAU-10-H and m-
H2BDC-SO3Li. The star marks the out-of-plane CH vibration of the 1,3,5-sub-
stituted linker molecules and the plus sign the same vibration for the 1,3-
substituted linker. The box marks the area of the SO3H vibrations.
Table 1. Results of the Pawley refinements.
Sample CAU-10-
Space group
a=b [Š]
c [Š]
Rwp [%]
H
I41
I41
I41
I41
I41
P41
P41
P41
P41
P41
P4
21.55(7)
10.38(3)
1.9
5.7
5.2
4.4
4.8
3.2
4.0
3.3
3.6
3.2
4.8
4.2
5.5
10-O/Sx are compared to those of the linker molecule and the
respective single-linker compound in Figures S15 and S16 of
the Supporting Information. Again, no unconsumed linker mol-
ecules are observed in the spectra, and the characteristic vibra-
tions for the sulfo groups are present.
H/S1
H/S2
H/S3
H/S4
NO2
N/S1
N/S2
N/S3
N/S4
OH
21.3846(6)
21.4262(8)
21.4786(6)
21.4896(5)
21.4707(3)
21.532(1)
21.539(1)
21.525(1)
21.514(2)
21.3072(5)
21.325(1)
21.285(2)
10.7424(5)
10.6918(6)
10.6635(5
10.6277(5)
10.3777(2)
10.409(1)
10.421(1)
10.416(1)
10.405(2)
38.6974(9)
38.793(2)
38.792(5)
Thermal Analysis
O/S1
O/S2
P4
P4
To investigate the thermal stability, thermogravimetric (TG)
measurements of all compounds were performed under air.
The results for the CAU-10-H/Sx samples are shown in Figure 5.
All TG curves exhibit a two-step weight loss. The first step up
to around 3008C can be attributed to the removal of incorpo-
rated H2O and DMF molecules. Around 4008C the frameworks
start to decompose, and X-ray amorphous Al2O3 is formed at
temperatures above 6008C. The TG measurements of the CAU-
10-N/Sx and -O/Sx samples are shown in Figures S17 and S18
Vibrational spectroscopy
IR spectra of all compounds were collected after washing the
samples with water. For example, the spectra of CAU-10-H/Sx,
m-H2BDC-SO3Li, and CAU-10-H are shown in Figure 4. The box
marks the characteristic vibrations of the SO3H groups and it is
clearly visible that, with increasing amount of SO3H groups,
the intensities of the characteristic bands increase.
There are no indications for uncoordinated linker molecules,
since no bands around 1700 cmÀ1 due to free carboxyl groups
are observed. The characteristic stretching modes of the car-
boxylate groups of the mixed-linker compounds are in accord-
ance with those of the single linker compound. The presence
of sulfo groups is verified by the vibrations between 1250 and
1000 cmÀ1 (black box). The asymmetric and symmetric vibra-
tions of the sulfo groups are observed at n˜O O =1223 and
= =
1160 cmÀ1, respectively, and at n˜SÀO =1044 cmÀ1S. The in-plane
skeletal vibration of the sulfonic acid substituted benzene ring
appears at n˜ =1080 cmÀ1 and the CÀS vibration at n˜ =
629 cmÀ1. The CÀH out-of-plane vibrations for the 1,3-substi-
tuted benzene rings, which are observed at n˜ =725 and
755 cmÀ1, are marked with stars. For the 1,3,5-substituted ben-
zene ring this vibration appears at n˜ =782 cmÀ1 and is marked
with a plus sign.[8b,15] The IR spectra of CAU-10-N/Sx and CAU-
Figure 5. TG measurements of the CAU-10-H/Sx samples.
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12519
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
of the Supporting Information. Again, two-step weight losses
were observed. The removal of incorporated solvent molecules
is completed at about 2508C for CAU-10-N/Sx and about
2308C for CAU-10-O/Sx. Above these temperatures the frame-
works slowly start to decompose into X-ray amorphous Al2O3.
Sorption properties
To investigate the influence of the incorporated sulfo groups
in the different functionalized CAU-10 analogues, sorption
measurements with different adsorptives were performed. N2
and H2 (up to 1 bar) measurements were carried out at 77 K
and CO2 (up to 1 bar), and H2O vapor measurements at 298 K.
For CAU-10-H/Sx it was only possible to record a N2 isotherm
for the compound with the lowest degree of doping due to
very long equilibration times (Supporting Information, Fig-
ure S19). The pore volume is decreased to 0.21 cm3gÀ1 and the
specific surface area to 372 m2gÀ1 compared to CAU-10-H
(Vmic =0.25 cm3gÀ1, aS =635 m2gÀ1). All CAU-10-N/Sx samples
show a type I isotherm characteristic of microporous materials
(Supporting Information, Figure S20). The specific surface area
decreases with increasing amount of sulfo groups incorporated
into the framework. The single-linker compound CAU-10-OH
does not adsorb any nitrogen. However, for the mixed linker
compounds a small uptake is observed (Supporting Informa-
tion, Figure S21). The incorporation of sulfo groups perhaps
leads to a more ordered orientation of the linker molecules
due to interactions of the functional groups, so that the pores
become accessible for nitrogen molecules.
Water-vapor sorption measurements for all mixed-linker
compounds are compared to those of the single-linker parents
in Figure 6. According to Canivet et al., three parameters
should be considered to characterize the samples concerning
their hydrophobicity/hydrophilicity.[16] The capacity c reflects
the accessible pore volume, indicator a=p/p0 gives the rela-
tive pressure at which 50% of the capacity is reached, and the
slope of the isotherm at low relative pressures is relevant to
describing the affinity.
In all cases, with increasing amount of incorporated SO3H
groups the a parameter shifts to lower p/p0 values and the
slope at low relative pressures increases, which indicates in-
creasing affinity towards water vapor and therefore higher hy-
drophilicity. For CAU-10-H/Sx the capacity decrease is clearly
due to the additional space required by the SO3H groups. For
CAU-10-N/Sx only small changes in the sorption capacity are
observed, since the replacement of a NO2 group by an SO3H
group probably only slightly affects the pore volume. The
same can be assumed for the CAU-10-O/Sx samples. The
strong affinity towards water vapor suggests potential applica-
tion of these materials in the field of humidity sensing, and
therefore they are currently under investigation. For the CO2
measurements the same trend was observed. In all cases the
affinity increases with increasing amount of incorporated SO3H
groups (Supporting Information, Figures S22–S24). The capaci-
ties of CAU-10-H/Sx (Supporting Information, Figure S27) and
-N/Sx (Supporting Information, Figure S23) decrease but, inter-
estingly, that of CAU-10-O/Sx (Supporting Information, Fig-
Figure 6. Water-vapor sorption isotherms of the mixed-linker compounds
compared to those of the single-linker counterparts. Top: CAU-10-H/Sx.
Middle: CAU-10-N/Sx. Bottom: CAU-10-O/Sx.
ure S24) increases. Apparently, the presence of OH and SO3H
groups leads to strong interactions between the linker mole-
cules, which could lead to a polarizing effect of the pore sur-
face that strongly affects the affinity. This behavior has previ-
ously been observed for the mixed-linker compound CAU-10-
NO2/NH2.[7]
Since hydrogen is a small nonpolar molecule, the polarity of
the introduced sulfo groups does not affect the affinity to-
wards this gas, and only the reduced pore volume influences
the sorption behavior of the materials. Therefore, for all materi-
als a reduced sorption capacity for H2 is observed, but no
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12520
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Catalytic experiments
Table 2. Results of the sorption measurements.
Sample
N2 (77 K)
H2 (77 K) CO2 (298 K) H2O (298 K)
The combination of well-defined pore spaces, the presence of
strongly acidic SO3H groups and high thermal stabilities make
the mixed-linker CAU-10 MOFs potentially interesting catalysts.
Selected samples were evaluated as catalysts for the gas-phase
dehydration of ethanol to ethylene (Table 3).
CAU-10- as [m2 gÀ1]/Vmic [cm3 gÀ1
]
[wt%]
[wt%]
[cm3gÀ1
]
H
635/0.25
372/0.21
–
–
–
440/0.18
324/0.15
270/0.13
242/0.12
158/0.09
–
1.12
1.03
0.95
0.91
0.85
1.01
0.91
0.81
0.69
0.64
0.79
0.77
0.69
10.57
10.52
9.88
9.44
8.87
8.71
8.61
8.29
7.72
7.12
5.57
6.57
8.04
468
407
357
329
305
215[a]
318
351
332
382
390
379
377
H/S1
H/S2
H/S3
H/S4
NO2
N/S1
N/S2
N/S3
N/S4
OH
Table 3. Ethanol dehydration over mixed-linker CAU-10 samples.[a]
Sample CAU-10-
XEtOH [%]
SC2= [%]
SC2OC2 [%]
SC2 O [%]
=
H[b]
7
22
22
27
30
52
79
70
76
71
69
42
20
8
2
2
1
6
1
H/S2[b]
H/S4[b]
N/S4[b]
H[c]
27
52
15
56
91
O/S1
O/S2
67/0.04
74/0.05
[a] p/p0 =0.5
H/S4[c]
[a] XEtOH =ethanol conversion, SC2==ethylene selectivity, SC2OC2 =diethyl
ether selectivity, SC2 O =acetaldehyde selectivity. [b] 2508C; feed rate
=
change in their affinity (Supporting Information, Figures S25–
S27). Trend graphs comparing the affinities and capacities of
all compounds towards H2O vapor and CO2 gas as well as the
specific surface areas are shown in Figures S28–S30 of the Sup-
porting Information; Table 2 summarizes the results of the
sorption measurements of all compounds.
À1
6.9 mmolmmolg
h
À1; 300 mg catalyst. [c] 3008C.
MOF
The presence of SO3H groups in the mixed-linker CAU-10
materials clearly results in a strong increase in catalyst activity.
The parent CAU-10-H shows only a very modest ethanol con-
version of 7% at 2508C, mainly due to the presence of weakly
acidic m2-OH groups decorating the inorganic chains, a situation
similar to those in MIL-53(Al) and the aluminum fumarate
A520.[18] Under identical reaction conditions, however, CAU-10-
H/S2 and CAU-10-H/S4 achieved ethanol conversions of 27 and
52%, respectively, which indicate that in these materials the
more acidic protons of the SO3H groups dominate as the
Brønsted acidic active sites. Furthermore, a higher degree of
m-H2BDC-SO3H doping correlates with a higher activity. How-
ever, CAU-10-N/S4 only reaches a conversion of 15% (2508C),
in spite of its high SO3H content. It is likely that the relatively
bulky NO2 groups restrict diffusion through the one-dimen-
sional pore system and thus result in lower conversions. When
the reaction temperature is increased to 3008C, the reactivity
differences between CAU-10-H and CAU-10-H/S4 become
more pronounced, with ethanol conversions of 56 and 91%,
respectively.
Proton conductivity
Due to the presence of SO3H groups the proton conductivity
of selected compounds, that is, CAU-10-H/S1, CAU-10-N/S2,
and CAU-10-O/S2 were determined. Previous conductivity
measurements on Al-MIL-53-OSO3H (degree of sulfation of Al-
MIL-53: 50%) showed high proton conductivities at tempera-
tures up to 333 K, which were on the same order of magnitude
as observed for Nafion.[17] The samples in our study contain
smaller amounts of SO3H groups, but roughly the same frac-
tion. All investigated CAU-10 samples exhibited low proton
conductivity (Supporting Information, Figure S31). Only for
CAU-10-O/S2 was it possible to determine proton conductivi-
ties in the whole temperature range. The obtained values were
between 2.74”10À7 and 8.61”10À6 ScmÀ1. For CAU-10-N/S2,
conductivities ranging from 1.71”10À8 to 5.61”10À7 ScmÀ1
were found in the temperature range of 373–413 K. A proton
conductivity of 4.88”10À7 ScmÀ1 at 413 K was determined for
CAU-10-H/S1. Since the amount of SO3H groups is similar in all
three samples, the roles of the H, NO2, and OH groups deter-
mine the proton conductivity. In combination with hydroxyl
groups higher proton conductivities could be determined over
the whole temperature range. Observed values are one order
of magnitude higher, which could be due to the fact that both
functional groups can be protonated and deprotonated and
thus participate in the proton-conducting mechanism. In con-
trast, the amount of SO3H groups in CAU-10-H/S1 and CAU-10-
N/S2 does not lead to a sufficient density of functional groups
for a constant proton-conduction pathway. Only with increas-
ing temperature, and therefore increasing mobility of the func-
tional groups and the protons, can proton conductivity occur.
For this reaction to proceed selectively towards the olefin
product, elevated reaction temperatures and relatively strong
acidic sites are required.[19] Although at 2508C the ethylene se-
lectivity between CAU-10-H and the SO3H-bearing materials
does not differ strongly, this picture changes drastically at
3008C, at which CAU-10-H/S4 achieves an ethylene selectivity
of 79%. In stark contrast, CAU-10-H only achieves a selectivity
to ethylene of 52%. Notably, both at 2508C and 3008C, a signif-
icant amount of acetaldehyde is formed over CAU-10-H. This
dehydrogenation product is typically associated with the pres-
ence of basic sites. For instance, the carboxylate groups of iso-
phthalic acid could act as proton acceptor sites. For the mixed-
linker compounds, acetaldehyde is formed in far lower yield,
which indicates that mainly the Brønsted acidic SO3H sites
lining the pore interior participate in the dehydration reaction.
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12521
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Furthermore, only trace amounts of oligomerization products
were observed (butenes <0.2%). However, for CAU-10-N/S4 re-
action at 3008C, even for short reaction times, led to strong
browning of the material and appearance of compounds of
higher molecular weight in the gas chromatogram, which indi-
cate that this material promotes undesired condensation and
oligomerization side reactions.
Thus, the mixed-linker MOFs could serve as attractive alterna-
tives to these resins in more demanding applications such as
catalysis at elevated temperatures.
Conclusion
We have presented the synthesis of ten mixed-linker materials
exhibiting the CAU-10 structure. The known compounds CAU-
10-H, -NO2, and -OH were chosen and the mixed-linker ap-
proach was used to introduce SO3H groups in different frac-
tions up to a maximum amount of 24% for CAU-10-H/S4. The
SO3H groups do not affect the high thermal stability of the ma-
terials, but the polarity of these groups strongly influences the
sorption properties. Changes in sorption capacities and affini-
ties towards several gases (N2, H2, CO2) were observed, and
a strong increase in affinity towards water vapor reveals a po-
tential application in the field of humidity sensing, which is
currently under investigation. Due to the strong acidity of the
SO3H groups, selected samples were evaluated for their proton
conductivity and as catalysts for the gas-phase dehydration of
ethanol to ethylene. Whereas only low proton conductivity
was observed, the presence of SO3H groups clearly results in
a strong increase in catalytic activity. Although reactions were
performed at 3008C, stable conversion of ethanol, no loss in
crystallinity, and no desulfonation were observed. This high
thermal stability is in contrast to many SO3H-containing resins,
so these compounds could be an alternative candidate for cat-
alysis at elevated temperatures. In comparison to many other
MOFs, the CAU-10 compounds also exhibit exceptional stability
towards water. The materials neither showed any loss of crys-
tallinity during the water sorption experiments nor under the
severe conditions of the proton conductivity measurements
(up to 1408C under 100% R.H.) and the catalytic reactions (de-
hydration of ethanol at 3008C).
The stability of the mixed-linker CAU-10 samples under the
applied reaction conditions was confirmed by powder XRD
and the observation of stable conversion and selectivity pro-
files with extended reaction times, as is illustrated for CAU-10-
H/S4 in Figure 7 and for CAU-10-H in Figure S32 of the Sup-
porting Information. This was further confirmed by the lack of
desulfonation (Supporting Information, Figure S33) of the used
catalysts. The high thermal stability of the CAU-10-H/Sx materi-
als is in stark contrast to many organic SO3H-containing resins,
which typically suffer from desulfonation starting at 1508C.[20]
Experimental Section
Materials and methods
All chemicals are commercially available and were used without
further purification. For the synthesis, custom-made steel auto-
claves with Teflon inlets and a volume of 30 mL or glass reactors
with a volume of 100 mL for solvothermal reaction conditions
were used. The initial characterization by means of PXRD methods
was carried out on a STOE-Stadi-P Kombi diffractometer (CuKa1 radi-
ation) equipped with an xy stage and an image-plate detector. The
data for the indexing and Pawley refinements were collected on
a STOE-Stadi-P diffractometer (CuKa1 radiation) equipped with
a Mythen detector. The software used for Pawley refinements was
TOPAS Academics v4.1.[14] IR spectra were recorded on a Bruker
ALPHA-FT-IR A220/d-01 spectrometer equipped with an ATR unit.
The TG analyses were carried out with a NETZSCH STA 409 CD ana-
lyzer. The samples were heated in Al2O3 crucibles at a rate of
48CminÀ1 under a flow of air (25 mLminÀ1). The contents of
carbon, hydrogen, nitrogen, and sulfur were determined by ele-
mental chemical analysis on a EuroVector EuroEA Elemental Ana-
lyzer. Atomic absorption spectroscopy (AAS) for analysis of lithium
was carried out using a PerkinElmer AAnalyst 300 spectrometer.
Gas sorption experiments were performed with a BEL JAPAN INC.
Figure 7. Top: Ethanol conversion (black) and selectivities to ethylene (gray)
and diethyl ether (light gray) over CAU-10-H/S4 (feed rate:
À1
6.9 mmolg
h
À1. Bottom: PXRD pattern of CAU-10-H/S4 before (black) and
MOF
after reaction (light gray).
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12522
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 4. Linker ratios used in the synthesis and for the final compounds with the resulting molecular formulas for all compounds.
1
1
Sample
Ratio used in synthesis
Ratio calcd by H NMR
Molecular formula calcd by H NMR and TG analysis
CAU-10-
H/S1
H/S2
H/S3
H/S4
m-H2BDC-H
0.825
0.75
0.625
0.5
m-H2BDC-SO3Li
0.125
0.25
0.375
m-BDC-H
0.92
0.85
0.795
0.76
m-BDC-SO3H
0.08
0.15
0.205
[Al(OH)(m-BDC-H)0.92(m-BDC-SO3H)0.08]·3.5H2O·0.1DMF
[Al(OH)(m-BDC-H)0.85(m-BDC-SO3H)0.15]·2.5H2O·0.1DMF
[Al(OH)(m-BDC-H)0.795(m-BDC-SO3H)0.205]·2H2O·0.2DMF
[Al(OH)(m-BDC-H)0.76(m-BDC-SO3H)0.24]· 1.5H2O·0.2DMF
0.5
0.24
CAU-10-
N/S1
N/S2
N/S3
N/S4
m-H2BDC-NO2
0.825
0.75
0.625
0.5
m-H2BDC-SO3Li
0.125
0.25
0.375
0.5
m-BDC-NO2
0.935
0.90
0.825
0.785
m-BDC-SO3H
0.065
0.10
0.175
0.215
[Al(OH)(m-BDC-NO2)0.935(m-BDC-SO3H)0.065]·2.23H2O
[Al(OH)(m-BDC-NO2)0.9(m-BDC-SO3H)0.1]·3.08H2O
[Al(OH)(m-BDC-NO2)0.825(m-BDC-SO3H)0.175]·2.97H2O
[Al(OH)(m-BDC-NO2)0.785(m-BDC-SO3H)0.215]·2.93H2O
CAU-10-
O/S1
O/S2
m-H2BDC-OH
0.825
0.75
m-H2BDC-SO3Li
0.125
0.25
m-BDC-OH
0.92
0.875
m-BDC-SO3H
0.08
0.125
[Al(OH)(m-BDC-OH)0.92(m-BDC-SO3H)0.08]·4.7H2O
[Al(OH)(m-BDC-NO2)0.875(m-BDC-SO3H)0.125]·4.68H2O
Belsorpmax instrument. Measurements with N2 and H2 were per-
formed at À1968C, and measurements with CO2 and H2O vapor at
258C. Prior to each measurement all samples were activated at
2008C overnight at 10À2 kPa. Solution-state 1H NMR spectroscopy
was performed on a Bruker DRX500/Bruker Advance 200 spectrom-
eter. Prior to the measurements 2–3 mg of each sample was dis-
solved in a mixture of NaOD (5%) and D2O (95%). SEM images of
gold-sputtered samples were collected on a Philips XL30 FEG mi-
croscope. Catalytic investigations were performed on an in-house-
constructed continuous flow reactor (Supporting Information, Fig-
ure S34). For each MOF, the reactor was loaded with a sample con-
taining adsorbed atmospheric water, which was removed prior to
reaction by outgassing the sample overnight at 1508C under an N2
flow (5 mLminÀ1). In each case, the resulting amount of dry materi-
al was 300 mg. An N2 flow (10 mLminÀ1) was saturated with etha-
nol by passing it through a bubbler containing pure ethanol (Fisch-
er), which was kept thermostatically at 408C. This N2 flow was al-
lowed to equilibrate with ethanol for 4 h prior to reaction. The re-
sulting gasÀ1mixture was passed over the catalyst bed (feed rate:
perature. The syntheses of the CAU-10-N/Sx and the CAU-10-O/Sx
samples were carried out in 100 mL glass reactors equipped with
a screw cap. In a typical procedure for CAU-10-N/S1, a mixture of
a 2m aqueous solution of AlCl3·6H2O (3400 mL, 6.8 mmol), m-
H2BDC-NO2 (1260 mg, 5.97 mmol), m-H2BDC-SO3Li (214.9 mg,
0.853 mmol), 4.0 mL of DMF, and 12.6 mL of H2O was placed in
one of the vessels. In a typical procedure for CAU-10-O/S1, a mix-
ture of a 2m aqueous solution of AlCl3·6H2O (2800 mL, 5.6 mmol),
m-H2BDC-OH (874.9 mg, 4.80 mmol), m-H2BDC-SO3Li (173.3 mg,
0.686 mmol), 4.0 mL of DMF, and 13.2 mL of H2O were used. In
both cases the reaction was performed in an oven at 1208C for
12 h with 2 h heating and cooling ramps. The resulting precipitate
was filtered off, intensively washed with water, and dried in air at
room temperature. IR spectroscopy showed no residues of uncoor-
dinated linker molecules. To determine the fraction of the incorpo-
rated linker molecules and thus to assign the molecular formula,
1
solution H NMR measurements were performed (Supporting Infor-
mation, Figure S35–37) on digested samples. On the basis of the
integral ratios of the protons of the different linker molecules, the
actual ratios were calculated (Table 4). The results of the elemental
analysis agree well with the compositions of all compounds deter-
mined from thermogravimetric (TG) and 1H NMR measurements
(Supporting Information, Table S2). The acidic character of the sam-
ples was verified by a simple indicator test. On addition of H2O/
methyl red a color change from yellow to pink was observed, indi-
cating an acidic pH value (Supporting Information, Figure S38). The
exchange of the Li+ ions against H+ ions at the SO3H groups was
verified by AAS.
6.9 mmolgMOF h
À1) at different reaction temperatures (250, 275, and
3008C). By using a gas-sample loop, samples were collected at
30 min intervals (5 s sampling time) and analyzed with an on-line
gas chromatograph (GC Shimadzu 2010 plus chromatograph with
FID detector, GsBP-1 column; 100 m, 250 mm inner diameter). Data
handling was performed with the GCsolution Analysis software (v
2.3). The proton conductivity was determined by electrochemical
impedance spectroscopy[21] in a two-electrode cell setup with
a Zahner Zennium workstation. Measurements were carried out
between 1 Hz and 1 MHz (100 mV) at temperatures ranging from
333–413 K at 100% relative humidity (R.H.).[22] Further details are
presented in the Supporting Information.
Acknowledgements
We acknowledge Dr. Helge Reinsch (CAU Kiel) for fruitful dis-
cussions. This work has been supported by the DFG (SPP 1362,
WA 1116/17-2 and STO 643/5-2). B.B. and D.D.V. gratefully ac-
knowledge the Research Foundation Flanders (FWO) for finan-
cial support (Aspirant grant).
Synthesis
The exact amounts of starting materials for the synthesis of all
compounds are listed in Table S1 of the Supporting Information.
According to the synthesis conditions of CAU-10-H,[3c] the synthe-
ses of the CAU-10-H/Sx samples were carried out in custom-made
steel autoclaves with Teflon inlets and a volume of 30 mL. In a typi-
cal procedure for CAU-10-H/S1, a mixture of a 1m aqueous solu-
tion of Al2(SO4)3·18H2O (920 mL, 1.38 mmol), m-H2BDC (200.6 mg,
1.21 mmol), m-H2BDC-SO3Li (43.5 mg, 0.172 mmol), 1.15 mL of
DMF, and 3.68 mL of H2O was placed in one of these vessels and
the reaction was performed in an oven at 1358C for 12 h with 1 h
heating and cooling ramp. The resulting precipitate was filtered
off, intensively washed with water, and dried in air at room tem-
Keywords: aluminum · conducting materials · heterogeneous
catalysis · metal–organic frameworks · microporous materials
[1] a) Themed issue on Metal-Organic Frameworks, Chem. Soc. Rev. 2009,
38, 1201–1508; b) Themed issue on Metal-Organic Frameworks, Chem.
Rev. 2012, 112, 673–1268; c) Themed issue on Metal-Organic Frame-
works, Chem. Soc. Rev. 2014, 43, 5415–6172.
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12523
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[2] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim,
Nature 2003, 423, 705–714.
[10] a) Y. Jin, J. Shi, F. Zhang, Y. Zhong, W. Zhu, J. Mol. Catal. A 2014, 383–
384, 167–171; b) M. Lin Foo, S. Horike, T. Fukushima, Y. Hijikata, Y.
Kubota, M. Takata, S. Kitagawa, Dalton Trans. 2012, 41, 13791–13794.
[11] a) B. Li, Y. Zhang, D. Ma, L. Li, G. Li, G. Li, Z. Shi, S. Feng, Chem. Commun.
2012, 48, 6151–6153; b) W. J. Phang, H. Jo, W. R. Lee, J. H. Song, K. Yoo,
B. Kim, C. S. Hong, Angew. Chem. 2015, DOI: 10.1002/ange.201411703.
[12] a) T. Ahnfeldt, D. Gunzelmann, J. Wack, J. Senker, N. Stock, CrystEng-
Comm 2012, 14, 4126–4136; b) N. Reimer, B. Gil, B. Marszalek, N. Stock,
CrystEngComm 2012, 14, 4119–4125.
[13] M. Gaab, N. Trukhan, S. Maurer, R. Gummaraju, U. Müller, Microporous
Mesoporous Mater. 2012, 157, 131–136.
[14] A. A. Coelho, Coelho Software, Brisbane, Australia, 2007.
[15] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables
and Charts, 3rd ed., Wiley, Hoboken, 2004.
[16] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Chem. Soc. Rev.
2014, 43, 5594–5617.
[17] M. G. Goesten, J. Juan-AlcaÇiz, E. V. Ramos-Fernandez, K. B. Sai Sankar
Gupta, E. Stavitski, H. van Bekkum, J. Gascon, F. Kapteijn, J. Catal. 2011,
281, 177–187.
[18] a) E. Alvarez, N. Guillou, C. Martineau, B. Bueken, B. Van de Voorde, C. Le
Guillouzer, P. Fabry, F. Nouar, F. Taulelle, D. de Vos, J.-S. Chang, K. H. Cho,
N. Ramsahye, T. Devic, M. Daturi, G. Maurin, C. Serre, Angew. Chem.
2015, 127, 3735–3739; b) U. Ravon, G. Chaplais, C. Chizallet, B. Seyyedi,
F. Bonino, S. Bordiga, N. Bats, D. Farrusseng, ChemCatChem 2010, 2,
1235–1238.
[3] a) S. Biswas, T. Ahnfeldt, N. Stock, Inorg. Chem. 2011, 50, 9518–9526;
b) M. Lammert, S. Bernt, F. Vermoortele, D. E. De Vos, N. Stock, Inorg.
Chem. 2013, 52, 8521–8528; c) H. Reinsch, M. A. van der Veen, B. Gil, B.
Marszalek, T. Verbiest, D. de Vos, N. Stock, Chem. Mater. 2013, 25, 17–
26; d) H. Deng, S. Grunder, K. E. Cordova, C. Valente, H. Furukawa, M.
Hmadeh, F. Gµndara, A. C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M.
O’Keeffe, O. Terasaki, J. F. Stoddart, O. M. Yaghi, Science 2012, 336,
1018–1023; e) P. Horcajada, H. Chevreau, D. Heurtaux, F. Benyettou, F.
Salles, T. Devic, A. Garcia-Marquez, C. Yu, H. Lavrard, C. L. Dutson, E.
Magnier, G. Maurin, E. Elkaim, C. Serre, Chem. Commun. 2014, 50, 6872–
6874; f) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe,
O. M. Yaghi, Science 2002, 295, 469–472; g) S. J. Garibay, S. M. Cohen,
Chem. Commun. 2010, 46, 7700–7702.
[4] a) A. D. Burrows, CrystEngComm 2011, 13, 3623–3642; b) W. Lu, Z. Wei,
Z.-Y. Gu, T.-F. Liu, J. Park, J. Park, J. Tian, M. Zhang, Q. Zhang, T. Gentle Iii,
M. Bosch, H.-C. Zhou, Chem. Soc. Rev. 2014, 43, 5561–5593; c) X. Kong,
H. Deng, F. Yan, J. Kim, J. A. Swisher, B. Smit, O. M. Yaghi, J. A. Reimer,
Science 2013, 341, 882–885.
[5] T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Ba-
taille, G. Ferey, Chem. Eur. J. 2004, 10, 1373–1382.
[6] a) S. Marx, W. Kleist, J. Huang, M. Maciejewski, A. Baiker, Dalton Trans.
2010, 39, 3795–3798; b) T. Lescouet, E. Kockrick, G. Bergeret, M. Pera-
Titus, D. Farrusseng, Dalton Trans. 2011, 40, 11359–11361; c) T. Les-
couet, E. Kockrick, G. Bergeret, M. Pera-Titus, S. Aguado, D. Farrusseng,
J. Mater. Chem. 2012, 22, 10287–10293.
[19] a) M. Zhang, Y. Yu, Ind. Eng. Chem. Res. 2013, 52, 9505–9514; b) A.
Corma, J. Perez-Pariente, Clay Miner. 1987, 22, 423–433.
[7] H. Reinsch, S. Waitschat, N. Stock, Dalton Trans. 2013, 42, 4840–4847.
[8] a) G. Akiyama, R. Matsuda, H. Sato, M. Takata, S. Kitagawa, Adv. Mater.
2011, 23, 3294–3297; b) Y. Zang, J. Shi, F. Zhang, Y. Zhong, W. Zhu,
Catal. Sci. Technol. 2013, 3, 2044–2049; c) J. Chen, K. Li, L. Chen, R. Liu,
X. Huang, D. Ye, Green Chem. 2014, 16, 2490–2499; d) P. Ramaswamy,
R. Matsuda, W. Kosaka, G. Akiyama, H. J. Jeon, S. Kitagawa, Chem.
Commun. 2014, 50, 1144–1146; e) Z. Li, G. He, Y. Zhao, Y. Cao, H. Wu, Y.
Li, Z. Jiang, J. Power Sources 2014, 262, 372–379.
[20] P. F. Siril, H. E. Cross, D. R. Brown, J. Mol. Catal. A 2008, 279, 63–68.
[21] a) E. Barsoukov, J. R. MacDonald, Impedance Spectroscopy, 2nd ed.,
Wiley, Hoboken, 2005; b) J. Grehn, J. Krause, Metzler Physik, 3rd ed.,
Schroedel Verlag, Hannover, 1998.
[22] G. Alberti, M. Casciola, L. Massinelli, B. Bauer, J. Membr. Sci. 2001, 185,
73–81.
Received: April 17, 2015
Published online on July 17, 2015
[9] S. Biswas, J. Zhang, Z. Li, Y.-Y. Liu, M. Grzywa, L. Sun, D. Volkmer, P. van
der Voort, Dalton Trans. 2013, 42, 4730–4737.
Chem. Eur. J. 2015, 21, 12517 – 12524
www.chemeurj.org
12524
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim