Gated Channels and Selectivity Tuning of CO2over N2Sorption by Post-Synthetic Modification of a UiO-66-Type Metal–Organic Framework

Article abstract of DOI:10.1002/chem.201602318

The highly porous and stable metal–organic framework (MOF) UiO-66 was altered using post-synthetic modifications (PSMs). Prefunctionalization allowed the introduction of carbon double bonds into the framework through a four-step synthesis from 2-bromo-1,4-benzenedicarboxylic acid; the organic linker 2-allyl-1,4-benzenedicarboxylic acid was obtained. The corresponding functionalized MOF (UiO-66-allyl) served as a platform for further PSMs. From UiO-66-allyl, epoxy, dibromide, thioether, diamine, and amino alcohol functionalities were synthesized. The abilities of these compounds to adsorb CO₂and N₂were compared, which revealed the structure–selectivity correlations. All synthesized MOFs showed profound thermal stability together with an increased ability for selective CO₂uptake and molecular gate functionalities at low temperatures.

Full text of DOI:10.1002/chem.201602318

DOI: 10.1002/chem.201602318
Full Paper
&
Materials Science
Gated Channels and Selectivity Tuning of CO over N Sorption by
2
2
Post-Synthetic Modification of a UiO-66-Type Metal–Organic
Framework
[a]
[b]
[a]
[c]
Alexander Kronast, Sebastian Eckstein, Peter T. Altenbuchner, Konrad Hindelang,
[a]
[a]
Sergei I. Vagin, and Bernhard Rieger*
Abstract: The highly porous and stable metal–organic
framework (MOF) UiO-66 was altered using post-synthetic
modifications (PSMs). Prefunctionalization allowed the intro-
duction of carbon double bonds into the framework
through a four-step synthesis from 2-bromo-1,4-benzenedi-
carboxylic acid; the organic linker 2-allyl-1,4-benzenedicar-
boxylic acid was obtained. The corresponding functionalized
MOF (UiO-66-allyl) served as a platform for further PSMs.
From UiO-66-allyl, epoxy, dibromide, thioether, diamine, and
amino alcohol functionalities were synthesized. The abilities
of these compounds to adsorb CO and N were compared,
2
2
which revealed the structure–selectivity correlations. All syn-
thesized MOFs showed profound thermal stability together
with an increased ability for selective CO uptake and molec-
2
ular gate functionalities at low temperatures.
Introduction
combustion capture of CO is hampered by its low content (~
2
1
5%) in flue gases compared to the main component (N ) and
2
Metal–organic frameworks (MOFs) and their underlying con-
cept, which involves the combination of inorganic building
blocks with ubiquitous organic linking units, offer numerous
other components present in minor quantities (H O, O , CO,
2 2
[8]
NO , and SO ). Solid porous materials such as zeolites, func-
x
x
tionalized silica, activated carbon, and MOFs show distinct ad-
[
1]
possibilities for modifications. In general, MOFs provide
a high degree of crystallinity and high pore volumes accompa-
nied by extraordinary internal surface areas. These unique
properties make MOFs ideal candidates for gas storage and
vantages for post-combustion CO capture compared with the
2
very energy intensive scrubbing process that uses aqueous sol-
utions of alkanolamines, which results in substantial efficiency
[9]
losses in power plants equipped with this technology.
[
2]
[3]
[4]
[5]
separation, catalysis, drug delivery, and chemical sensing.
Although MOFs show lower chemical and thermal stabilities
than other inorganic porous materials, the ease of modification
of both the network architecture and functional groups make
them promising candidates as post-combustion adsorbents.
For MOFs to be an economically viable alternative in post-com-
The host–guest interactions can be tuned by the introduction
of functional groups into the framework; these groups are
mainly attached to the organic-linker units. Thus, the interac-
tions between the target molecule and the framework have
been systematically tuned previously and current studies
remain focused on the separation and storage of greenhouse
bustion applications, high capacities for CO adsorption, high
2
selectivity for CO over N , and robustness against the trace
2
2
[
2d,6]
gases.
amounts of water present in the flue gas are prerequisite prop-
[10]
The release of CO from anthropogenic sources has led to
erties that research has to tackle.
2
intensified concerns about the greenhouse effect and global
To match these requirements, convertible side chains, which
do not interfere with the network synthesis, have recently
been investigated. The use of these groups avoids the forma-
tion of additional crystal phases, decomposition, and other
[7]
warming and the implications for the global society. Post-
[a] A. Kronast, Dr. P. T. Altenbuchner, Dr. S. I. Vagin, Prof. Dr. B. Rieger
[11]
side reactions. Mainly pioneered by Wang et al. and Cohen
Wacker-Lehrstuhl fꢀr Makromolekulare Chemie
et al., post-synthetic modifications (PSMs) have become one of
the most promising and versatile routes to obtain the desired
features, for example, amines or other polarized groups, on
the framework surface. Recently, the concept of molecular
Technische Universitꢁt Mꢀnchen, Lichtenbergstrasse 4
8
5747 Garching bei Mꢀnchen (Germany)
[
b] S. Eckstein
Lehrstuhl fꢀr Technische Chemie II, Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching bei Mꢀnchen (Germany)
[12]
gates has been introduced. Molecular gates enable MOFs to
[
c] Dr. K. Hindelang
Consortium fꢀr elektrochemische Industrie
Wacker Chemie AG, Zielstattstrasse 20, 81379 Mꢀnchen (Germany)
discriminate between different gases during adsorption and,
therefore, increase the selectivity of the frameworks. Conse-
quently, frameworks offering this feature came into focus but
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602318.
[12a]
most lack the required stability.
Chem. Eur. J. 2016, 22, 1 – 9
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Chemically and thermally stable networks have recently at-
tracted attention in the field of MOF chemistry. The UiO-66
MOF synthesis. The UiO-66 analogue was prepared by the
combination of ZrCl with 2-allyl-BDC in DMF. The addition of
4
IV
MOF, a three-dimensional framework of Zr -carboxylate clus-
water to the reaction mixture was indispensable for the forma-
tion of the nanocrystalline powder, as a full hydrolysis of ZrCl4
is essential.
ters, Zr (m -O) (CO ) , and linking 1,4-benzenedicarboxylate
6
3
4
2 12
2À
(
BDC) units, is characterized by exceptional stability, high ad-
sorption volumes, and good functional-group tolerance during
The crystallinity of the sample was verified using powder X-
ray diffraction (PXRD) and the diffraction pattern obtained was
compared with literature data for the parent network. The pat-
tern clearly confirmed the integration of the previously un-
known 2-allyl-1,4-benzenedicarboxylate linker into the UiO-66
network (Scheme 2).
[
6d,13]
the MOF synthesis.
The surface area of unfunctionalized
2
À1
UiO-66 was determined to be greater than 1100 m g . Funda-
mental work on the influence of functional groups on the ad-
sorption properties of different gases in UiO-66 was performed
37
[13a,14]
by Cohen et al. and Walten et al.
According to their re-
sults, the interaction of guest molecules like nitrogen or CO2
rises with the polarity of the attached groups.
The reaction times for PSMs in MOFs fundamentally depend
on the diffusion limitations within the microporous solids. Fast
mass transport through the MOF cavities can be achieved by
small uniform MOF-particles and a high degree of activation.
Herein, we report the synthesis of the highly stable MOF
UiO-66-allyl and the enormous potential of PSMs for fine-
tuning of the network–guest interactions. The unique network
features allow the introduction of molecular gates at low tem-
peratures, accompanied by a remarkable selectivity for adsorp-
tion of CO in preference to N at elevated temperatures.
[13b]
The standard synthesis reported in the literature
yielded
MOF samples with low Brunauer–Emmett–Teller (BET) surface
areas. Behrens et al. developed a modulated MOF synthesis for
Zr-carboxylate networks using monocarboxylic acids to slow
down the crystallization rate and increase the accessible pore
2
2
[17]
volumes through improved crystallization.
Results and Discussion
After activation, acetic acid modulation increased the acces-
sible surface area in the unfunctionalized UiO-66 network from
We recently reported on the conversion of olefinic side chains
2
À1
[
15]
711 to 1189 m g (see Scheme S2 in the Supporting Informa-
in a Zn-based pillared MOF. As a model system, we modified
a bipyridine pillar ligand connecting square paddle-wheel
[17]
tion) through slowing down the crystallization rate.
The
[16]
maximum surface area was obtained by using 10 vol% of
acetic acid in the reaction solution; this concentration of acetic
acid was also used in the synthesis of the functionalized MOF
layers based on 9,10-triptycenedicarboxylic acid and Zn ions.
As this framework did not show high tolerances to basic and
acidic conditions, we decided to modify the exceptionally
stable UiO-66 system by attaching an olefinic side chain to
2
À1
UiO-66-allyl (736 m g ). The drop in the BET value for the de-
rivative can be attributed to the increased steric demand in
the pores by the attached side chains. Nevertheless, the allylic
side chains could be successfully introduced while preserving
both crystallinity and porosity (Figure 1).
benzenedicarboxylic acid H . The stability of the network is of
2
crucial importance for the fine tuning of CO2 adsorption
through PSM.
Linker synthesis
Post-synthetic modifications
Methyl ester protection of the commercially available 2-bromo-
0
1,4-benzenedicarboxylic acid, followed by a Pd -catalyzed Stille
The permanent porosity and stability of Zr-based MOFs make
the UiO-66-allyl network a perfect substrate for PSM. The inte-
grated C=C double bond can be converted through a variety
of PSMs (Scheme 2) to tune the adsorption properties for dif-
ferent gases like CO and N .
coupling reaction gave dimethyl 2-allylbenzene-1,4-dicarboxy-
late. Subsequent deprotection under basic conditions pro-
duced 2-allyl-1,4-benzenedicarboxylic acid (2-allyl-BDC) in good
yield (Scheme 1 and Scheme S1 in the Supporting Informa-
tion).
2
2
Epoxidation of the double bond was chosen as the model
PSM, as it had been successfully applied in our previous
[15]
work.
The chemical stability of the network emerged as
MOF preparation
a challenge in the characterization of the PSM results. Besides
XRD and BET measurements, solution NMR was the method of
choice to quantify the degree of conversion of the allylic side
chains. Cohen et al. reported the acidic disintegration of UiO-
In Zr-carboxylates, the formation of undesirable crystal phases
is suppressed because of their structural homogeneity com-
pared with other metal–linker combinations that are used in
Scheme 1. Synthesis of 2-allyl-1,4-dicarboxylic-acid (L1).
&
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Scheme 2. Synthesis of and PSMs applied to UiO-66-allyl: bromination to yield UiO-66-dibromide, epoxidation with dimethyldioxirane to yield UiO-66-epoxide,
and a thiol–ene click reaction using EtSH to yield UiO-66-ethylsulfide. Tandem PSM with nucleophilic ring opening in UiO-66-epoxide to yield UiO-66-aminoal-
cohol, and nucleophilic substitution in UiO-66-dibromide to yield UiO-66-diamine.
[
18]
6
6 using hydrofluoric acid. The application of this method to
ing gas adsorption behavior. The basic workup of the dibro-
mide compound led to a substitution reaction resulting in the
formation of a glycol-containing product identical to the epoxi-
dation product.
the UiO-66-epoxide lead to side reactions hampering further
analysis. Cyclic esters and side products that were not further
investigated were found (Supporting Information, Scheme S3).
To avoid any further problems in product determination, we
developed a new disintegration method using deuterated
Polar functional groups, especially amine functionalities, are
well known to increase the interaction and selectivity with
[13a]
sodium hydroxide in D O. Under these conditions, the newly
guest molecules, such as CO2, in porous networks.
2
formed epoxide was quantitatively ring opened and converted
to the glycol (Figure 2). The nearly quantitative yields achieved
using the oxidizing reagent dimethyldioxirane, together with
the remaining crystallinity and the high nitrogen surface area
Nevertheless, the introduction of primary amines through PSM
has not been widely investigated, with the exception of the
post-synthetic deprotection of N-Boc-tagged networks. Boc-
protecting groups hamper the framework interpenetrations
that occur during MOF assembly, and subsequent removal of
these groups can both increase the pore volume and unmask
an unprotected amine functional group. Following this route,
Telfer and co-workers were able to unmask an amine function-
ality in IRMOF-12 as well as the secondary amine of proline in
2
À1
(
BET=617 m g ), emphasize the suitability of the MOF for
PSMs.
Both bromination with elemental bromine and a UV-initiated
thiol-ene click reaction using ethyl mercaptane proceeded in
nearly quantitative yields and good conversions of the double
bond, which were comparable to the results of our previous
[19]
IRMOF-Pro.
Although there are some examples of amine-
[
15a]
work.
Although crystallinity was retained in both cases ac-
tagged MOFs in the literature, there are, to our knowledge, no
cording to PXRD, the accessible BET surface area of the activat-
vicinal diamino- or amino alcohol-tagged frameworks known
2
À1
[13a,19b,20]
ed samples dropped to 5 and 52 m g , respectively. This
result will be discussed in detail below in the section describ-
until now.
Through the integration of these functionali-
ties, a variety of possible applications arise, especially as chelat-
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dry and inert atmosphere conditions. After drying the MOF
powder under vacuum at 1208C for 16 h in a steel autoclave,
the autoclave was pressurized with dry ammonia at room tem-
perature and heated to 808C overnight. After removing the
gas in vacuo, the samples were characterized using standard
methods (see the Supporting Information). The conversion of
the dibromo-functionalized UiO-66 was quantitative, forming
the diamine as proven by NMR spectroscopy.
Nevertheless, the PXRD of the functionalized network
showed a decrease in reflex intensity. This can be attributed to
a decrease in the crystal size, congestion of the pores by the
ammonium bromide byproduct (reflexes at 22.30 and 31.70
2
V, Figure 3), or an overall loss in crystallinity. BET measure-
Figure 1. PXRD and BET surfaces obtained by modulated MOF synthesis
using acetic acid as a modulator for UiO-66 and UiO-66-allyl.
Figure 3. PXRD analysis of the different UiO-66 derivatives before (UiO-66-
allyl) and after PSM (see the Supporting Information for enlarged versions of
the diffractograms).
ments were conducted and revealed only a low active surface
2
À1
for nitrogen adsorption (6 m g ). Although the combination
of these analytics indicated framework destruction, CO2 ad-
sorption measurements were inconsistent with these results
(
Figure 5) and showed a higher CO uptake than the unfunc-
2
tionalized UiO-allyl-network.
As aqueous monoethanolamine is the current commercially
used CO -scrubbing reagent, this functional group promises
2
a high interaction potential with CO as a guest molecule. We
2
therefore carried out the nucleophilic ring-opening of UiO-66-
epoxide with the goal of preparing an amino alcohol-function-
alized UiO-66 network. The nucleophilic attack of ammonia
leads to the corresponding amino alcohol without the forma-
tion of any side products. The reaction product showed re-
maining crystallinity and a conversion of the epoxide of 45%,
determined by NMR spectroscopy, which could not be acceler-
ated by varying the reaction conditions. However, BET surface-
area measurements revealed an accessible surface of only
1
Figure 2. H NMR spectra of the tandem functionalized linkers after disinte-
2
gration using NaOD in D O: a) UiO-66-epoxide (for comparison), b) UiO-66-
aminoalcohol and unreacted UiO-66-epoxide, c) UiO-66-diamine.
ing ligands for catalytic active metal ions and CO -sorption
2
substrates.
2
À1
As there are different literature examples for both the nucle-
ophilic ring-opening of epoxides and the substitution of hal-
ides using ammonia, we developed tandem PSMs for the con-
version of our epoxy- and dibromide-tagged networks. On ac-
count of the fragility of the network under strongly basic con-
ditions, it was necessary to avoid any traces of water by both
activating the network before modification and working under
3 m g , which will be discussed below.
The introduction of diamino and amino alcohol functionali-
ties offers not only a high potential for host–guest interactions
but also the potential to introduce catalytically active metal
[11]
ions by inclusion of donor atoms during PSM.
The wide range of PSMs that were available for the UiO-66
system is demonstrated by the multitude of different reaction
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conditions applied, and the exceptional chemical stability of
the system was proven by the crystallinity remaining after
treatment with ammonia.
been no studies looking at the influence of allyl, epoxide, di-
bromide, ethylsulfide, diamine, or ethanolamine groups in UiO-
66 MOFs on the capacity for N and CO sorption.
2
2
The modified MOF systems were analyzed by the common
methods used for PSM experiments, including PXRD, thermog-
The N2 adsorption isotherms recorded at 358C show
a strong dependence on the attached side chains in the frame-
work cavities (Figure 5). While diamine and ethanolamine dis-
1
ravimetric analysis, solution H NMR spectroscopy, and nitro-
gen adsorption measurements using the BET method (see the
Supporting Information). Because UiO-66-allyl was prepared as
a microcrystalline powder, no single-crystal analysis was possi-
ble. To clarify the structure of this material, the crystal phase
was compared to that of the original UiO-66 using PXRD.
The disintegration using NaOD offered direct access to linker
solutions in deuterated water following removal of the Zr-con-
taining salts using centrifugation. This method also revealed
that the allylic side chain was preserved during the MOF syn-
1
thesis, as shown in the Supporting Information. The H NMR
spectra of the corresponding PSM products can be attributed
to either the direct reaction products or tandem products ob-
tained as a result of the basic workup. The conversion of the
C=C double bond can also be evaluated using this method.
Gas adsorption
Nitrogen adsorption isotherms revealed the influence of the
newly introduced functional groups on the accessible surface
area in the network. In comparison with the parent compound
2
À1
(
723 m g ), all PSM products showed a remarkable decrease
in BET values. The smallest decrease was determined for UiO-
2
À1
6
6-epoxide, which had a pore surface area of 498 m g . UiO-
6-thioether exposes accessible pores, with the surface area re-
6
2
À1
duced to 52 m g . UiO-66-dibromide, UiO-66-diamine, and
UiO-66-ethanolamine show only low or no accessible pore sur-
face areas at 77 K in nitrogen adsorption experiments
Figure 5. N
2 2
and CO high-pressure adsorption isotherms recorded for UiO-
(Figure 4).
66-allyl and PSM derivatives at 308 K.
Different groups have previously reported that CO adsorp-
2
tion depends on the interaction of the integrated side chains
in UiO-66 with the corresponding gas. Polar groups were
play uptakes are comparable to the parent framework, dibro-
mide, thioether, and epoxide show remarkably increased up-
takes (up to 200% higher). As the selectivity of the substrate
plays an essential role in any possible gas separation applica-
found to be more attractive for CO sorption than nonpolar
2
[
9,13a,14,21]
functionalizations.
However, until now, there have
tions, the CO adsorption isotherms of the PSM products and
2
UiO-66-allyl were also recorded, and are shown in Figure 5. The
isotherms clearly illustrate the influence of polar groups on the
adsorption properties. Although it shows the highest BET sur-
face value, UiO-66-allyl shows the lowest CO loading of all the
2
substrates investigated. The amino alcohol-tagged MOF (UiO-
66-aminoalcohol) exhibited the highest adsorption of CO2
combined with one of the lowest values for N , followed by
2
UiO-66-epoxide and UiO-66-thioether. The total CO loading of
2
UiO-66-aminoalcohol was about 51 wt% at 358C and 20 bars.
The comparatively low loading of UiO-66-diamine may be
caused by congestion of the pore system resulting from the
large amino groups, as well as the NH Br byproduct of the
4
tandem PSM. As expected, the dibromo-functionalized MOF,
without polar groups, displayed the lowest CO loading of the
2
PSM derivatives. Nevertheless, the CO loading was still higher
Figure 4. N
2
adsorption isotherms recorded for UiO-66-allyl and PSM deriva-
2
tives at 77 K.
than that observed for the parent system. Compared to litera-
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ture data, UiO-66-aminoalcohol hosts up to ten percent more
UiO-66-allyl. The CO adsorption of UiO-66-aminoalcohol was
2
CO2 than the amino tagged UiO-66-NH2 at 258C and
nearly four times higher than that of the mother compound,
and was exceptionally high when compared with other com-
pounds reported in the literature.
[
13a]
20 bars.
To the best of our knowledge, this is the highest CO ad-
2
sorption loading of a functionalized UiO-66 MOF in the litera-
ture, and points out the importance of adjustable host–guest
interactions in MOFs that are available through PSMs. Chemi-
The PSMs developed herein illustrate the importance of re-
active side groups, introduced in pre-synthetic modifications,
which do not hinder the MOF crystallization process, but may
be converted in high yields to functionalities that are not ac-
cessible through pre-synthetic functionalization. Moreover, our
work highlights the possibilities of tailoring host–guest interac-
tions through PSMs and, therefore, enabling applications in
gas sorption and separation.
sorption of CO was excluded for all substrates using ATR-IR
2
measurements (see the Supporting Information, Scheme S7).
Thus, UiO-66-aminoalcohol represents a promising candidate
for possible CO –N gas separation.
2
2
The astonishing differences in the adsorption properties of
the frameworks at different temperatures point to a gated-
pore effect in the MOFs similar to that found by Fischer
[
12a]
Experimental Section
et al.
This group described a honeycomb-like zinc-dicarbox-
ylate-bipyridine network with flexible ether side chains. The
effect was attributed to the low thermal energy of the ether
substituents and subsequent congestion of the pore windows,
General
All reactions containing air- and/or moisture-sensitive compounds
were performed under dry argon using standard Schlenk or glove-
box techniques. All chemicals were purchased from Aldrich, ABCR
or Acros. Solvents were obtained from an MBraun MB-SPS solvent
purification system.
which prevented N diffusion at low temperatures. Neverthe-
2
less, these examples did not employ covalent PSM for the in-
troduction of the molecular gates.
The tandem PSMs performed by epoxidation and nucleo-
philic ring-opening impressively demonstrate the versatility
and viability of PSM to precisely adjust adsorption properties
UV irradiation for the photoinduced thiol–ene click reaction was
executed with a MAX-302 ASAHI SECTRA. NMR spectra were re-
corded on a Bruker Avance 500 UltraShield (500 MHz) and a Bruker
Avance 300 (300 MHz). They were recorded in ppm and the sol-
vents residual proton signal and carbon signal were used as the in-
ternal standard. X-ray powder diffraction was measured on a STOE
STADI P system with a DECTRIS MYTHEN 1 K detector. TGA was car-
in MOFs. UiO-66-allyl showed only a low capacity for CO ad-
2
sorption, at about 13 wt% at 358C and 20 bars. The oxidized
UiO-66-epoxide exhibited an increased CO2 adsorption of
2
À1
4
8 wt% with a preserved BET surface of 498 m g . The final
ried out on a Texas Instruments TGA-Q500 with a heating rate of
product, UiO-66-aminoalcohol, displayed the highest CO load-
2
À1
1
0 Kmin . Brunauer-Emmett-Teller (BET) adsorption measurements
ing of 51 wt% in combination with a closed-gate effect for ni-
were obtained by using a Quantachrome Nova 4000e sorption ap-
paratus and a PMI automatic Sorptometer. The samples were acti-
vated in vacuum at 1358C for 2 h before measurement. Apparent
surface area was calculated by applying the BET theory. CO and N
trogen diffusion at 77 K and the lowest N loading at 358C.
2
2
2
Conclusion
isotherms were obtained at 358C in a range of 0.5–20 bar using
a Rubotherm magnetic suspension balance. Approximately 300 mg
of the sorbent was placed in a steel crucible and dried at 1358C
for 8 h under vacuum prior to adsorption. To correct for buoyancy,
reference adsorption isotherms of nonadsorbing glass spheres
We have successfully used PSMs to introduce molecular gates
into the MOF UiO-66-allyl. The obtained functionalities enable
a powerful tuning for the adsorption of CO over N .
2
2
The functionalization of the linking 1,4-benzenedicarboxylate
ligand, as well as the optimization of the MOF synthesis using
acetic acid as a modulator, was intensively investigated to opti-
mize both crystallinity and pore accessibility. The known bro-
mination (UiO-66-dibromide), epoxidation (UiO-66-epoxide),
and thiol-ene click reaction (UiO-66-ethylsulfide) PSMs were ap-
plied UiO-66-allyl and resulted in the formation of the desired
products in good to quantitative yields. Furthermore, we have,
for the first time, introduced a vicinal 1,2-diamino functionality
into UiO-66 using a tandem PSM by the reaction of the dibro-
mide with dry ammonia at elevated temperatures (UiO-66-dia-
mine). In another tandem PSM, we successfully ring-opened
the epoxide with ammonia to give the corresponding vicinal
amino alcohol functionalized compound (UiO-66-aminoalco-
hol). These tandem PSM steps illustrate the exceptional stabili-
ty of the UiO-66 system.
(
300 mg, particle size: 425–600 mm) were determined and subtract-
ed from the isotherms. Elemental analyses were measured at the
Laboratory for Microanalytics at the Institute of Inorganic Chemis-
try at the Technische Universitꢁt Mꢂnchen. ESI-MS analytical meas-
urements were performed with acetonitrile, THF, isopropanol, or
toluene solutions on a Varian 500-MS spectrometer. IR spectra
were recorded with a liquid nitrogen cooled Bruker Vertex 70 FTIR-
spectrometer with a attenuated total reflectance unit Platinum ATR
of Bruker using 32 scans in a region of 4500 to 600 cm
À1
.
Linker synthesis
2-Bromoterephthalic acid dimethylester: To a stirred suspension
of 2-bromoterephthalic acid (6.2 g, 25.3 mmol, 1.0 equiv) in dry
methanol (100 mL), thionylchloride (7.3 mL, 101 mmol, 4.0 equiv)
was added dropwise at room temperature. The reaction mixture
was heated to reflux for 16 h and excess of thionylchloride and the
solvent were evaporated in vacuo. To the residue, 50 mL of a satu-
rated sodiumbicarbonate solution was added and extracted with
ethyl acetate (50 mL) three times. The organic extracts were com-
bined, dried over sodium sulfate, and evaporated in vacuo to give
6.26 g 2-bromoterepthalic acid (22.9 mmol; 92%) as a colorless
All the products resulting from PSM displayed increased CO2
adsorptions, and, with the exception of UiO-66-epoxide, dis-
played gated pores with a major decreased in the uptake of ni-
trogen gas at 77 K in comparison with the mother compound
&
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1
solid. H NMR (300 MHz; CDCl ): d=3.94 (s, 3H, ÀCH ), 3.95 (s, 3H,
Postsynthetic modifications
3
3
3
ÀCH ), 7.80 (m, 1H, H ), 8.01 (dd, 1H, J=8.1 Hz, 1.7 Hz, H ),
3
arom
arom
3
13
UiO-66-epoxide: Activated UiO-66-allyl (300 mg, 0.95 mmol) was
treated with a solution of dimethyl-dioxirane (DMDO) (0.05m,
8
5
1
.31 ppm (d, 1H, J=1.7 Hz, Harom); C NMR (75.5 MHz; CDCl ): d=
3
2.81, 52.91, 121.57, 128.22, 131.14, 133.83, 135.32, 136.25, 165.12,
66.26 ppm.
4
0 mL, 2.0 mmol) in acetone for 72 h at 48C. At the end of the re-
action time, the solution was removed using centrifugation and
the MOF powder was washed three times with 10 mL of fresh ace-
2
-Allylterephthalic acid dimethylester: 2-Bromoterephthalic acid
dimethylester (12.0 g, 43.9 mmol, 1.0 equiv) and 1.0 g tetrakis-(tri-
phenylphosphine)palladium(0) [Pd(PPh ) ] (0.88 mmol; 0.02 equiv)
were dissolved in 200 mL of degassed toluene under an argon at-
mosphere. 15.0 mL allyltri(n-butyl)tin were added and the reaction
solution was heated to reflux for 5 days. The reaction mixture was
quenched using 100 mL of a 4% cesium fluoride solution and ex-
tracted with ethyl acetate. After drying over sodium sulfate, the
raw product was purified using flash chromatography (silica gel,
hexane/ethyl acetate 95:5) to give 9.07 g 2-allylterephthalic acid di-
tone. Afterwards, the product was dried in vacuo. Conversion
3
4
1
(
H NMR): 95%.
Proton NMR shows signals for the corresponding glycol through
nucleophilic ring-opening of the epoxide.
1
H NMR (500 MHz, Deuterium Oxide): d=2.56–2.82 (m, 2H), 3.15–
3
3
3.35 (m, 2H), 3.60 (td, J=9.4, 7.8, 4.5, 1H), 7.16 (d, J=7.8, 1H),
7.41–7.51 (m, 2H); elemental analysis calcd (%) for
[Zr O (OH) (C H O ) ] [Zr O (OH) (C H O ) ] : C 39.72, H 2.63, O
6
4
4
11
8
5 6 0.95
6
4
4
11
8
4 6 0.05
methylester (38.7 mmol, 88%) as a colorless oil. TLC: R =0,16
30.22, Zr 27.43; found: C 41.02, H 2.89.
f
1
(
3
hexane/ethyl acetate=95:5) [UV]; H NMR (300 MHz; CDCl ): d=
3
UiO-66-dibromide: Activated UiO-66-allyl (300 mg, 0.95 mmol) was
placed in a Schlenk flask, suspended in 5 mL of fresh trichlorome-
thane and cooled to 08C. 0.2 mL bromine (4.12 mmol) was added
dropwise to the MOF and the reaction mixture was stirred at room
temperature for 16 h. The solvent was removed using centrifuga-
tion and decantation. The MOF powder was washed several times
using trichloromethane and dichloromethane and dried under
3
.77 (d, 1H, J=6.4 Hz, ÀCH À), 3.90 (s, 3H, ÀOCH ), 3.93 (s, 3H, À
2
3
3
OCH ‘), 5.01–5.05 (m, 2H, -CH=CH ), 5.99 (ddt, 1H, J=16.7, 10.1,
6
3
2
13
.4 Hz, ÀCH=CH ), 7.90–7.94 ppm (m, 3H, Harom);
C NMR:
2
(
75.5 MHz; CDCl ): d=38.33, 52.39, 52.51, 116.35, 127.32, 130.64,
3
1
32.05, 133.05, 133.88, 136.77, 141.69, 166.44, 167.57 ppm; elemen-
tal analysis calcd (%) for C H O : C 66.66, H 6.02, O 27.32; found:
13
14
4
C 66.57, H 6.09; IR (ATR): u=3080–2840 (w, multiple weak maxima,
ArÀH, CÀH), 1718 (vs, C=O), 1638 (w, C=C str), 1571 (w), 1492 (w),
1
vacuo. Conversion ( H NMR): 100%.
Proton NMR shows signals for the corresponding glycol through
nucleophilic substitution of bromide by hydroxide.
1
434 (m), 1405 (w), 1257 (s, CÀO), 1194 (m), 1111 (s), 1072 (m), 992
(
m, CH=CH twist), 963 (w), 915 (s, CH=CH wag), 883 (w), 857 (w),
2
2
À1
1
8
15 (m), 750 (s), 717 (m), 654 cm (w).
H NMR (500 MHz, Deuterium Oxide): d=2.79–2.53 (m, 2H), 3.34–
3
3
.14 (m, 3H), 3.63–3.52 (m, 1H), 7.15 (d, J=7.9, 1H), 7.48–7.40 (m,
2
-Allylterephhalic acid (L1): 2-Allylterephthalic acid dimethyl ester
3
H); elemental analysis calcd (%) [Zr O (OH) (C H Br O ) ]: C 27.69,
(
6.28 g, 26.8 mmol, 1.0 equiv) were dissolved in 100 mL of a 1:1
6
4
4
11
8
2
4 6
H 1.83, Br 33.49, O 17.88, Zr 19.12; found: C 26.25, H 2.15.
mixture of THF and methanol. After the addition of sodium hydrox-
ide solution (65 mL, 1m, 65.0 mmol, 2.4 equiv) the mixture was
stirred for 24 h at room temperature. The organic solvents were re-
moved in vacuo and the residue was dissolved in 50 mL of water.
The aqueous solution was neutralized using concentrated hydro-
chloric acid and the formed precipitate was isolated by filtration.
The white powder was repeatedly washed with water and dried in
vacuo to obtain 5.30 g 2-allylterephthalic acid (25.7 mmol, 96%) as
UiO-66-ethylsulfide: Activated UiO-66-allyl (250 mg, 0.79 mmol)
was placed in a quartz Schlenk tube to ensure UV light transparen-
cy. Degassed ethylmercaptane (3.5 mL, 47 mmol) was added at
room temperature and the mixture was allowed to stay in the dark
for 2 h impregnating the pores with the reactant. Subsequently,
the reaction mixture was irradiated with UV light for 8 h under
moderate stirring. After the reaction, the MOF powder was washed
three times using THF (5 mL) and dried in vacuo. Conversion
1
3
a colorless solid. H NMR (300 MHz; [D ]DMSO): d=3.74 (d, 1H, J=
6
1
1
6
7
.6 Hz, ÀCH À), 5.03 (m, 2H, ÀCH=CH ), 5.95 (m, 1H, -CH=CH ),
( H NMR): 80%. H NMR (500 MHz, Deuterium Oxide): d=0.81–0.93
2
2
2
13
3
.82–7.86 (m, 3H, H ), 13.26 ppm (s, 2H, ÀCOOH); C NMR
(m, 3H), 1.48–1.68 (m, 2H), 2.19–2.33 (m, 4H), 2.56 (t, J=7.6, 2H),
arom
3
(
75.5 MHz; [D ]DMSO): d=37.39, 116.19, 127.10, 130.27, 131.35,
7.07 (dd, J=12.8, 7.8, 1H), 7.36–7.44 ppm (m, 2H); elemental anal-
6
1
34.70, 137.18, 140.58, 166.67, 168.31 ppm; elemental analysis
ysis calcd for [Zr O (OH) (C H O S) ] [Zr O (OH) (C H O ) ] : C
6
4
4
13 14
4
6 0.80
6
4
4
11
8
4 6 0.20
calcd (%) for C H O : C 64.07, H 4.89, O 31.04; found: C 63.93, H
41.22, H 3.70, O 23.24; S 6.99, Zr 24.85; found: C 39.13, H 3.68, S
5.65.
11
10
4
4
.79; IR (ATR): u=3200–2500 (s, broad signal with multiple
maxima, OÀH, ArÀH, CÀH), 1681 (s, C=O), 1634 (w, C=C str), 1568
UiO-66-aminoalcohol:
Activated
UiO-66-epoxide
(300 mg,
(
w), 1495 (w), 1414 (m), 1284 (s, CÀO), 1206 (w), 1193 (w), 1134 (w),
0
.90 mmol) was placed in a 25 mL steal autoclave and pressurized
1
8
105 (w), 1070 (w), 993 (m, CH=CH twist), 914 (s, CH=CH wag),
60 (w), 784 (m), 748 (m), 706 (w), 678 (w), 654 cm (w).
2
2
with ammonia (8 bar). The autoclave was heated to 808C for 72 h
and the ammonia was released followed by sequential washing
with methanol (3ꢃ5 mL) and drying in vacuo. Conversion
À1
1
1
(
(
H NMR): 45%. H NMR (500 MHz, Deuterium Oxide): d=2.21–2.42
MOF-synthesis
3
m, 2H), 2.62–2.76 (m, 2H), 3.49–3.53 (m, 1H), 7.14 (d, J=3.3, 1H),
7
.44–7.47 ppm (m, 2H); elemental analysis calcd (%) for
UiO-66-allyl: A solution of 1.50 g 2-allyl benzene-1,4-dicarboxylylic
acid (7.27 mmol, 1.00 equiv) in 160 mL DMF was added in one por-
[
[
Zr O (OH) (C H NO ) ] [Zr O (OH) (C H O ) ]
Zr O (OH) C H O4)6]0.05: C 38.87, H 2.95, N 1.77, O 29.57, Zr 26.84;
6
4
4
11 11
5 6 0.43
6
4
4
11
8
5 6 0.52
tion to a solution of 1.61 g ZrCl (6.91 mmol, 0.95 equiv) in a mix-
6
4
4( 11
8
4
found: C 36.12, H 3.13, N 1.97.
ture of 164 mL DMF, 36 mL acetic acid, and 393 mL water. The mix-
ture was placed in a glass autoclave and heated under static condi-
tions to 1308C for 20 h. After cooling down the microcrystalline
powder was isolated using centrifugation and repeatedly washed
with DMF, methanol, and DCM. Every washing step included the
treatment in an ultrasonic bath for 5 min prior to drying in vacuo
at 1208C. Elemental analysis: calcd (%) for [Zr O (OH) (C H O ) ]: C
UiO-66-diamine: Activated UiO-66-dibromide (300 mg, 0.63 mmol)
was placed in a 25 mL steal autoclave and pressurized with ammo-
nia (8 bar). The autoclave was heated to 808C for 16 h and the am-
monia was released followed by sequential washing with methanol
1
(3ꢃ5 mL) and drying in vacuo. Conversion ( H NMR): 100%.
1
H NMR (500 MHz, Deuterium Oxide): d=2.15–2.39 (m, 2H), 2.54–
6
4
4
11
8
4 6
3
4
1.62, H 2.75, O 26.88, Zr 27.74; found: C 40.92, H 2.28.
2.76 (m, 2H), 3.43–3.55 (m, 1H), 7.12 (d, J=7.9, 1H), 7.38–7.43 (m,
Chem. Eur. J. 2016, 22, 1 – 9
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2
H); elemental analysis calcd (%) for [Zr O (OH) (C H N O ) (HBr) ]:
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Keywords: carbon dioxide sorption · gate effects · metal–
organic frameworks · post-synthetic modification · UiO-66
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Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 9
www.chemeurj.org
8
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ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
The highly porous and stable metal–
organic framework (MOF) UiO-66 was al-
tered using post-synthetic modifications
&
Materials Science
A. Kronast, S. Eckstein, P. T. Altenbuchner,
K. Hindelang, S. I. Vagin, B. Rieger*
(PSMs). The abilities of the compounds
to adsorb CO and N were compared
2
2
&& – &&
(see scheme), which revealed the struc-
ture–selectivity correlations. All synthe-
sized MOFs showed profound thermal
stability together with an increased abil-
Gated Channels and Selectivity Tuning
of CO over N Sorption by Post-
Synthetic Modification of a UiO-66-
Type Metal–Organic Framework
2
2
ity for selective CO uptake and molecu-
2
lar gate functionalities at low tempera-
tures.
Chem. Eur. J. 2016, 22, 1 – 9
www.chemeurj.org
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ