Conformation-controlled sorption properties and breathing of the aliphatic Al-MOF [Al(OH)(CDC)]

Article abstract of DOI:10.1021/ic500288w

The Al-MOF CAU-13 ([Al(OH)(trans-CDC)]; trans-H₂CDC = trans-1,4-cyclohexanedicarboxylic acid) is structurally related to the MIL-53 compounds that are well-known for their breathing behavior, i.e., the framework flexibility upon external stimuli such as the presence of adsorbate molecules. The adsorption properties of CAU-13 were investigated in detail. The sorption isotherms of N₂, H₂, CH₄, CO, CO₂, and water were recorded, and the adsorption enthalpies for the gases were determined by microcalorimetry. The structural changes upon adsorption of CO₂ were followed with in situ synchrotron powder X-ray diffraction (PXRD). The patterns were analyzed by parametric unit cell refinement, and the preferential arrangement of the CO₂ molecules was modeled by density functional theory calculations. The adsorption and separation of mixtures of o-, m-, and p-xylene from mesitylene showed a preferred adsorption of o-xylene. The structures of o/m/p-xylene-loaded CAU-13 were determined from PXRD data. The adsorption of xylene isomers induces a larger pore opening than that in the thermal activation of CAU-13. In the crystal structure of the activated sample CAU-13(empty pore), half of the linkers adopt the a,a confirmation and the other half the e,e conformation, and the presence of a,a-CDC^2- ions hampers the structural flexibility of CAU-13. However, after the adsorption of xylene, all linkers are present in the e,e conformation, allowing for a wider pore opening by this new type of breathing .

Full text of DOI:10.1021/ic500288w

Article
pubs.acs.org/IC
Conformation-Controlled Sorption Properties and Breathing of the
Aliphatic Al-MOF [Al(OH)(CDC)]
Felicitas Niekiel,† Jeroen Lannoeye,‡ Helge Reinsch,‡ Alexis S. Munn,§ Andreas Heerwig,∥ Ivo Zizak,⊥
Stefan Kaskel,∥ Richard I. Walton,§ Dirk de Vos,‡ Philip Llewellyn,# Alexandra Lieb,¶
Guillaume Maurin,*,▽ and Norbert Stock*,†
†Institut fur Anorganische Chemie, Christian-Albrechts-Universitat zu Kiel, Max-Eyth-Straße 2, 24118 Kiel, Germany
̈
̈
‡Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Arenbergpark 23, B-3001 Leuven, Belgium
§Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
∥Department of Inorganic Chemistry, Dresden University of Technology, Bergstraße 66, 01069 Dresden, Germany
⊥Institute of Nanometer Optics and Technology, Helmholtz Zentrum Berlin fur Materialien und Energie, Albert-Einstein-Straße 15,
̈
12489 Berlin, Germany
#Aix-Marseille Universite,
́ ́ ̂
CNRS, Laboratoire MADIREL (UMR7246), Centre de Saint Jerome, 13397 Marseille Cedex 2, France
¶Institut fur Chemie, Otto-von-Guericke-Universitat Magdeburg, Universitatsplatz 2, 39106 Magdeburg, Germany
̈
̈
̈
▽Institut Charles Gerhardt Montpellier, UMR CNRS 5253, UM2, ENSCM, Place E. Bataillon, 34095 Montpellier Cedex 05, France
S
* Supporting Information
ABSTRACT: The Al-MOF CAU-13 ([Al(OH)(trans-
CDC)]; trans-H2CDC = trans-1,4-cyclohexanedicarboxylic
acid) is structurally related to the MIL-53 compounds that
are well-known for their “breathing” behavior, i.e., the
framework flexibility upon external stimuli such as the
presence of adsorbate molecules. The adsorption properties
of CAU-13 were investigated in detail. The sorption isotherms
of N2, H2, CH4, CO, CO2, and water were recorded, and the
adsorption enthalpies for the gases were determined by
microcalorimetry. The structural changes upon adsorption of
CO2 were followed with in situ synchrotron powder X-ray
diffraction (PXRD). The patterns were analyzed by parametric unit cell refinement, and the preferential arrangement of the CO2
molecules was modeled by density functional theory calculations. The adsorption and separation of mixtures of o-, m-, and p-
xylene from mesitylene showed a preferred adsorption of o-xylene. The structures of o/m/p-xylene-loaded CAU-13 were
determined from PXRD data. The adsorption of xylene isomers induces a larger pore opening than that in the thermal activation
of CAU-13. In the crystal structure of the activated sample CAU-13(empty pore), half of the linkers adopt the a,a confirmation
and the other half the e,e conformation, and the presence of a,a-CDC2− ions hampers the structural flexibility of CAU-13.
However, after the adsorption of xylene, all linkers are present in the e,e conformation, allowing for a wider pore opening by this
new type of “breathing”.
as “breathing”, was discovered in 2002 for [Co(NCS)2(3-
PIA)2]·guest [PIA = N-(3-pyridyl)isonicotinamide],3
[VIII(OH)BDC]·guest,4 and [Cr(OH)BDC]·guest (BDC =
1,4-benzenedicarboxylate).5 The latter examples represent the
first MOFs with MIL-53-type structure, which have the general
composition [MIII(OH)BDC]·guest and which have now been
studied in various functionalized forms. They are built up of
zigzag chains of trans-corner-sharing MO6 octahedra, with M3+
being Al3+, Sc3+, Cr3+, Fe3+, Ga3+, and In3+.4−10 The one-
dimensional inorganic building units are interconnected via
BDC2− ions, forming a network with diamond-shaped channels.
INTRODUCTION
■
Owing to their potential applications in gas storage and
separation, catalysis, and drug delivery or as sensor materials,
metal−organic frameworks (MOFs) have become an important
class of inorganic−organic hybrid materials.1,2 These materials
are often (micro)porous and are built up from inorganic
building units such as metal ions or metal−oxygen clusters,
which are linked by polydentate organic molecules. The great
diversity of possible combinations of linkers with different
inorganic building units allows the formation of a large variety
of structures. Special interest lies in the investigation of MOFs
that undergo structural transformations upon temperature and
applied mechanical pressure changes or the inclusion of guest
molecules. This unusual behavior of crystalline solids, described
Received: February 5, 2014
Published: April 10, 2014
© 2014 American Chemical Society
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When MIL-53 is stored at ambient conditions, it adsorbs water,
leading to the so-called narrow-pore (np) form of the structure.
When water is removed, for most of the MIL-53 compounds, a
drastic increase of the unit cell volume occurs and the large-
pore (lp) form of the structure is observed.5 For Al-MIL-53, the
unit cell volume changes by 33% upon dehydration.11 The
overall expansion or shrinkage of the framework of MOFs is
often accompanied by a rotation of the organic and inorganic
building units around a part of the structure that plays the role
of a “knee-cap”. In MIL-53, this is the axis through the
carboxylate oxygen atoms.12 Layered structures that are
connected by flexible (aliphatic) pillar molecules can also
exhibit a breathing effect,13 which has, for example, been
realized by layered zirconium phosphates pillared by alkyl
diphosphonates.14 General descriptions of the concept of
breathing in inorganic−organic hybrid materials are given in
Figure 1. Ball-and-stick representations of the crystal structures and
structural formula of the incorporated linkers of (a) CAU-13·H2O
([Al(OH)CDC]·H2O) and (b) Al-MIL-53(np) ([Al(OH)BDC]·
H2O). AlO6 polyhedra are presented as gray octahedra, carbon
atoms as black spheres, and oxygen atoms (of adsorbed water
molecules) as red spheres. Hydrogen atoms are omitted. The
structures were drawn using the crystallographic data of ref 26 for
part a and ref 11 for part b. On the right, the linkers present in the
structures of CAU-13 and MIL-53 are shown: (a) e,e-CDC2− and a,a-
CDC2−; (b) BDC2−.
review papers by Kitagawa et al. and Fer
́
ey et al.12,13,15 The
breathing behavior of MIL-53 has been investigated by
combining several experimental approaches including in situ
powder X-ray diffraction (PXRD) during solvent exchange or
exposure to various gases and vapors, infrared and micro-
calorimetry measurements, and molecular simulations.16−18
Concerning practical applications, an understanding of the
19−22
adsorption of CO2
and the separation of C8 alkyl
aromatics (xylenes, ethylbenzene, etc.) is relevant.23−25
Recently, some of us have reported on the Al-MOF denoted
as CAU-13 ([Al(OH)CDC]·guest), which is isoreticular to
MIL-53 but contains the flexible aliphatic linker trans-1,4-
cyclohexanedicarboxylate (CDC2−).26 trans-1,4-Cyclohexanedi-
carboxylic acid is able to undergo conformational changes in
solution, leading to an equilibrium between the e,e and a,a
conformers. Both conformers are present in the structure of
CAU-13, and the same conformers are situated on opposing
sides of the pore walls. The structures of MIL-53 and CAU-13
are compared in Figure 1. MIL-53 exhibits “smooth” pore walls,
whereas the kinked a,a-CDC2− ions protrude into the channels
of CAU-13.
water upon treatment at the same conditions. Product purity and
preservation of crystallinity were confirmed by PXRD.
PXRD. PXRD experiments for product identification were carried
out on a Stoe Stadi P diffractometer in transmission geometry with Cu
Kα1 radiation, equipped with an image-plate detector system. The
high-resolution PXRD pattern of activated CAU-13 for structure
refinement was obtained at beamline P08 at PETRA III, DESY,
Hamburg, Germany. The sample was prepared as follows: The purified
and dried sample was filled into a borosilicate capillary (diameter: 0.7
mm), which was attached to a glass valve. The capillary was heated and
evacuated (200 °C, 2 × 10−3 kPa). After 2 h, the capillary was sealed.
The PXRD patterns of CAU-13 loaded with o-, m-, and p-xylene were
recorded on a PANalytical Empyrean diffractometer (Cu Kα /α
Here, we present a comprehensive study on the sorption
properties and framework flexibility of CAU-13 combining in
situ and ex situ PXRD, molecular modeling based on density
functional theory (DFT), and force-field-based calculations as
well as high-pressure adsorption experiments. The adsorption
of xylene isomers gave rise to a new manner of pore-size
variation upon adsorption of guest molecules, which is
accompanied by a conformational change of the cyclic aliphatic
linker.
1
2
radiation) using a PIXcel3D detector. Amounts of 30 mgworking
with TOPAS of the purified CAU-13 were placed in a DURAN Pyrex
tube and mixed with 3 mL of o-, m-, or p-xylene. The mixture was
heated at 100 °C for 5 h under stirring. After cooling to room
temperature, the mixture was centrifuged and the supernatant was
removed. The suspension was transferred into a glass capillary
(diameter: 0.5 mm). The capillary was then centrifuged to concentrate
the solid. Temperature-dependent PXRD measurements were
recorded under air using a PANalytical Empyrean diffractometer
equipped with an oven (TC-radiation, MRI) and a PIXcel3D detector.
The samples were loaded into Al2O3 sample holders, and the patterns
were recorded in reflection geometry with Cu Kα /α radiation every 10
EXPERIMENTAL SECTION
■
1
2
Synthesis. All reagents and solvents were purchased from
commercial sources and used without further purification. Aluminum
chloride hexahydrate (AlCl3·6H2O; puriss.) and N,N-dimethylforma-
°C, heating the sample from 30 to 200 °C, then cooling it to 30 °C,
and heating it again to 500 °C.
mide (DMF; 99.5%) were purchased from Grussing GmbH, Filsum,
̈
INVESTIGATION OF THE BREATHING BEHAVIOR
OF CAU-13 BY PXRD
■
Germany. trans-1,4-Cyclohexanedicarboxylic acid (trans-H2CDC;
99%) was purchased from TCI Deutschland GmbH, Eschborn,
Germany. The synthesis of CAU-13 was carried out as described in the
literature using microwave-assisted heating.26 Typically, 516 mg (3.00
mmol) of trans-H2CDC and 724 mg (3.00 mmol) of AlCl3·6H2O were
combined in a Biotage 10−20 mL glass reactor, and 16.0 mL of DMF
and 4.00 mL of water were added. The reactor was sealed with a
septum. The reaction mixture was heated in a Biotage Initiator
microwave oven for 45 min at 130 °C under stirring. Product recovery
was performed by centrifugation. The crude product was purified by
stepwise treatment with DMF and ethanol for 45 min at 130 °C using
the microwave oven. The compound was shown to also be stable in
Thermal Treatment. To quantify the changes in the unit
cell parameters upon thermal treatment, Pawley refinements
were carried out using each of the recorded PXRD patterns
between 30 and 350 °C. The method of parametric Rietveld
refinement, as implemented in the computer program Powder
3D Parametric by Rajiv et al.27 working with TOPAS-
Academic,28 was applied. Because no obvious reconstructive
transitions were observed, the number of phases was set to one.
As starting values, the cell parameters of CAU-13·H2O were
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Table 1. Details of the Rietveld Refinements of CAU-13(empty pore), CAU-13(mX), CAU-13(oX), and CAU-13(pX) Compared
to the Unit Cell Parameters of CAU-13·H2O
a
CAU-13(empty pore)
CAU-13(mX)
CAU-13(oX)
CAU-13(pX)
CAU-13·H2O
wavelength
0.825986 (synchrotron radiation)
Cu Kα /α
Cu Kα /α
Cu Kα /α
2
1
2
1
2
1
space group
a/Å
P1
P21
P21
P21
P1
̅
̅
6.6237(3)
9.7034(5)
9.4227(3)
108.362(2)
107.395(5)
93.685(3)
540.04(4)
2
6.6103(3)
17.8984(9)
11.0989(5)
90
6.6031(8)
18.060(2)
10.858(7)
90
13.225(2)
17.892(2)
11.0836(9)
90
6.6169(5)
9.4300(6)
9.4642(6)
107.577(3)
107.725(7)
93.209(5)
529.08(7)
2
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
91.246(4)
90
90.374(9)
90
89.80(2)
90
1312.8(1)
4
1294.8(2)
4
2622.6(5)
8
RBragg
Rwp
0.93
0.44
0.68
0.38
4.65
2.98
3.50
3.66
GOF
0.89
1.72
2.28
2.46
a
The cell parameters of CAU-13·H2O were taken from ref 26.
chosen and refined to the first recorded PXRD pattern. The
refined cell parameters are listed in Table S1 in the Supporting
Information (SI).
were then calculated following the same method that we fully
described in ref 21 to allow a direct comparison with the
microcalorimetry measurements.
CO2 Sorption. The in situ PXRD (synchrotron radiation, λ
= 1.5406 Å) study during CO2 adsorption at 195 K was
performed at beamline KMC-2 at BESSY, Berlin, Germany.
The experimental setup employing a BELSORP-max apparatus
and a homemade sample holder is described elsewhere.29
PXRD patterns were recorded at 16 points of the adsorption
branch and 6 points of the desorption branch of the isotherm.
The adsorption isotherm is presented in Figure S3 in the SI,
together with an overview of the CO2 pressures at which the
PXRD patterns were recorded. To quantify the changes of the
unit cell parameters, Pawley refinements were carried out using
each of the recorded PXRD patterns by the method of
parametric Rietveld refinement, as implemented in the
computer program Powder 3D Parametric.27 The number of
observed phases was set to one. As starting values, the cell
parameters of CAU-13(empty pore) were chosen because the
sample had been thermally activated. The refined cell
parameters are visualized in Figure S4 in the SI and listed in
Table S2 in the SI. To localize the pressure-dependent
positions of the CO2 molecules in the framework of CAU-13,
DFT calculations were carried out. For the calculations, a
structure model of CAU-13 present upon CO2 adsorption was
built, starting with the crystal structure of CAU-13(empty
pore) and fixing the unit cell parameters to the ones provided
in Table S2 in the SI at a CO2 pressure of 96.307 kPa using the
same force-field-based computationally assisted structure
determination as that described below [structure determination
of CAU-13(xylene)]. The structural models were further
geometry-optimized by DFT calculations in the absence and
presence of one and two CO2 molecules per unit cell. In a
manner similar to that in our previous investigation on Al-MIL-
53,19 several starting configurations were considered for CO2 in
order to probe the possible geometries of this guest in the
pores. These configurations include single interactions between
CO2 and either the OH groups of the inorganic structure parts
or the organic linker and double interactions in which the guest
molecule bridges two OH groups of opposing pore walls.
Geometry optimization was performed with the PW91 GGA
density functional30 and the double numerical basis set
containing polarization functions31 on the hydrogen atom, as
implemented in the Dmol3 code.29 The adsorption enthalpies
Xylene Vapor-Phase Adsorption. PXRD was used to
examine the flexible behavior of CAU-13 in response to xylene
vapor. A Bruker D8 diffractometer operating with Cu Kα /α
1
2
radiation was used for these experiments, with the hydrated
sample packed into a ceramic holder and data measured in θ−θ
geometry using a VANTEC solid-state detector, which
permitted a typical diffraction pattern to be accumulated in
around 3 min. The diffractometer was equipped with an Anton
Paar XRK 900 reactor chamber, which allowed accurate
temperature control and for gases to be passed over the
sample. For the adsorption experiments, nitrogen was bubbled
through the chosen xylene isomer in a Drechsel bottle to
introduce a vapor of xylene over the sample. The powdered
sample was packed into the holder to allow space for expansion,
and its height was checked after the experiment to ensure that
no movement had occurred. The desorption of each xylene
isomer was investigated by heating the xylene-loaded samples in
air in the range 60−150 °C. Scans were recorded every 1 °C.
STRUCTURE DETERMINATION
■
CAU-13(empty pore). The crystal structure of desolvated
CAU-13 denoted as CAU-13(empty pore) was solved and
refined from PXRD data obtained at beamline P08, PETRA III
at DESY, Hamburg, Germany (λ = 0.825986 Å). The sample
was measured in a borosilicate glass capillary (diameter: 0.7
mm) after thermal activation. The experimental setup of the
beamline is described in ref 32. After indexing the powder
pattern and refining the cell parameters using TOPAS-
Academic,28 the structure was solved by direct methods in the
space group P1 using the program EXPO 2009.33 With the
̅
extracted atomic positions set as the structural starting model, a
Rietveld refinement using TOPAS-Academic was carried out.
The profile was fitted using a Thompson−Cox−Hastings
pseudo-Voigt profile function and a simple axial model. Soft
distance restraints were applied to the C−C bonds of the
organic linker molecule (C2−C3, C2−C4, C3−C4, C5−C6,
C6−C7, C6−C8, and C7−C8). The Rietveld plot, an image of
the asymmetric unit, and a table showing selected bond lengths
are presented in the SI (Figures S6 and S7 and Table S3). The
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Figure 2. Final Rietveld plots of the structure refinements of CAU-13 loaded with xylene isomers (Cu Kα /α radiation). The observed powder
1
2
patterns are shown in black, the calculated powder patterns as an overlay in red, and the difference plots (observed − calculated) in blue. The tics
mark the allowed Bragg peak positions.
unit cell parameters of CAU-13(empty pore) are listed in Table
1.
This leads to the doubled value for the a axis, which indicates a
fully ordered arrangement of the guest molecules. For all
structures, the refinements converged to reasonable figure-of-
merit values. The details of the refinements are given in Table
1, and the final Rietveld plots are shown in Figure 2.
Figures of the asymmetric units as well as tables of selected
atomic distances are given in the SI (Tables S5−S7 and Figures
S9−S11).
CAU-13(xylene). The PXRD patterns of CAU-13 loaded
with an excess of liquid xylene of each isomer were recorded
with Cu Kα /α radiation on a PANalytical Empyrean
1
2
diffractometer. The patterns were indexed with the program
EXPO 2009,33 and the unit cell parameters were refined with
the Pawley method using TOPAS-Academic.28 A Split Pearson
VII profile function and a simple axial model were used for
profile fitting. The Pawley fits are presented in Figure S8 in the
SI, and the details of the refinements are listed in Table S4 in
the SI.
SORPTION EXPERIMENTS
■
Sorption Isotherms. Before the gas sorption experiments
were performed, the powdered samples were thermally
activated at 200 °C under vacuum for at least 2 h. However,
the xylene vapor-phase adsorption experiments were carried out
using the hydrated CAU-13·H2O. Sorption experiments up to 1
bar were performed with a BELSORP-max apparatus (BEL
Japan Inc.). The N2 and H2 sorption isotherms were recorded
at 77 K, while the CO2 and H2O sorption isotherms were
recorded at 298 K. The apparent specific surface area was
determined from the N2 adsorption isotherm, as described in
the literature.38 The micropore volume was obtained at p/p0 =
0.5. Calculations of the micropore volume were performed
using the computer program PLATON39 for the crystal
structure of CAU-13(empty pore), with hydrogen atoms
geometrically placed using Materials Studio 5.0.37
A structural model was constructed starting from the crystal
structure of the large-pore Al-MIL-53(lp) (space group
Imma).11 Using the program Powdercell,34 the original
orthorhombic crystal structure was converted subsequently
into hypothetical ones with lower symmetry, using subgroups
of Imma (sequence Imma−I212121−P212121−P21). The model
of lowest symmetry was used as a starting point for
constructing the structural model for CAU-13(oX) and CAU-
13(mX) using the same computationally assisted structure
determination strategy that some of us previously validated on
different MOFs.19,35 For that purpose, we considered the initial
atomic positions of the large-pore form of Al-MIL-53 converted
to the space group P21, the bonding scheme of the organic
linker was changed from aromatic to aliphatic single bonds, and
the unit cell parameters were changed to the respective indexed
ones for CAU-13(oX) and CAU-13(mX). The obtained models
were then geometry-optimized with fixed unit cell parameters
using the generic universal force field,36 as implemented in the
Forcite software.37 The indexing of the PXRD pattern of CAU-
13(pX) resulted in a doubling of the length of the a axis
compared to those of CAU-13(oX) and CAU-13(mX). The
structural model for CAU-13(pX) was therefore obtained by
doubling the a axis using again the supergroup−subgroup
relation employed in Powdercell, and the same geometry
optimization procedure as that described above was applied.
Two oX and mX molecules as well as four pX molecules per
unit cell were then introduced in the structure models. Their
positions were energy-minimized by force-field-based simu-
lations. Starting with the resulting structural models, Rietveld
refinements were carried out using TOPAS-Academic.28 The
organic linker and xylene molecules were defined as rigid
bodies, while the inorganic parts of the framework were freely
refined. Because the guest molecules inside the channels
arranged in close proximity to each other for the compounds
adsorbing o- and m-xylene, their occupancies were constrained
to reasonable values, indicating a disordered arrangement inside
the channels after the refinements had converged. In the case of
p-xylene, the guest molecules occupy well-defined positions.
Microcalorimetry. Adsorption enthalpies were measured
experimentally at 303 K and up to 40 bar using a Tian-Calvet
type microcalorimeter coupled with an in-house manometric
gas dosing system.40−42 This apparatus allows the simultaneous
measurement of the adsorption isotherm and the correspond-
ing differential enthalpies. Gas is introduced into the system
using a step-by-step method, and each dose is allowed to
stabilize in a reference volume before being brought into
contact with the adsorbent, located in the microcalorimeter.
The introduction of the adsorbate to the sample is
accompanied by an exothermic thermal signal, measured by
the thermopiles of the microcalorimeter. The peak in the
calorimetric signal is integrated over time to give the total
energy released during that adsorption step.
Xylene Liquid-Phase Adsorption. Liquid-phase batch
adsorption experiments were carried out according to a
literature procedure.23 A 1.8 mL vial with 25 mg of thermally
activated CAU-13 and an empty reference vial were filled with a
solution of one of the xylene isomers in mesitylene. The
contents of the vials were stirred for 2 h at 298 K. Gas
chromatography output data were used to determine the initial
and equilibrium xylene concentrations. Adsorption isotherms
were obtained by plotting the absorbed amount, in weight
percentage, versus the equilibrium concentration. For com-
petitive adsorption experiments, 0.2 mol L−1 binary mixtures of
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xylene isomers (0.1 mol L−1 each) and a 0.3 mol L−1 ternary
mixture in mesitylene were used. Selectivities αi,j were
calculated using eq 1,
heating to 200 °C. In accordance with the previously published
thermogravimetric data,26 this effect can be attributed to the
removal of water molecules from the channels. The reversibility
of the observed phenomenon is attributed to the uptake of
water from the air. Decomposition of the framework begins at
around 350 °C. The structural changes were quantified by
parametric refinement of the unit cell parameters. The plot of
the cell lengths versus temperature presented in Figure 3
(bottom) shows that mainly the b axis of the unit cell is
influenced by the thermal treatment.
qi cj
αi,j =
qj ci
(1)
with qi and qj being the amounts (mol g−1) of isomers i and j
adsorbed per gram of MOF adsorbent, and cj and ci being the
equilibrium concentrations (mol L−1) of isomers i and j present
in the liquid phase.
The length of the b axis increases upon the removal of
adsorbed water up to 200 °C and then decreases during
cooling. The reversibility of this effect could not be fully
reached within the time frame of the experiment. However, the
breathing effect is clearly observable. The finding that mainly
the b axis is affected shows that breathing occurs along the
shorter intersection of the lozenge-shaped pores of CAU-13,
thus causing a larger opening of the one-dimensional channels
(Figure 1a). Small changes in the c axis can be observed,
whereas the length of the a axis, which is parallel to the
inorganic building unit (i.e., chains of trans-corner-sharing AlO6
octahedra), remains almost unaltered. Similar observations can
be made regarding the unit cell angles, whose progress with the
temperature is presented in Figure S2 in the SI. The pore is
further enlarged upon heating to temperatures higher than 200
°C due to thermal motion of the atoms.
RESULTS AND DISCUSSION
■
MIL-53-type compounds exhibit a large breathing effect upon
adsorption and desorption of the guest molecules. For example,
the pore diameter of Al-MIL-53·H2O increases drastically upon
dehydration, whereas for Fe-MIL-53, an even narrower pore is
observed.43 On the other hand, different guest molecules also
lead to various pore sizes.16 To check whether CAU-13 shows a
breathing effect, at first the structural changes were studied
upon thermal dehydration. Furthermore, the influence of
various gases and vapors on the structure of CAU-13 was
studied.
Thermal Behavior. The thermal behavior of CAU-13·H2O
was investigated with temperature-dependent PXRD measure-
ments under air. To determine the framework flexibility, the
sample was at first heated from 30 to 200 °C. The sample was
then cooled to 30 °C and subsequently heated to 500 °C.
PXRD patterns obtained at selected temperatures are shown in
Figure 3 (top). The complete pattern series is presented in
Figure S1 in the SI.
On the basis of the temperature-dependent PXRD study, the
crystal structure of CAU-13(empty pore) was determined using
synchrotron PXRD data. CAU-13·H2O was transferred into a
borosilicate glass capillary and thermally activated under
vacuum. The structure was solved with direct methods and
refined using the Rietveld method. CAU-13(empty pore)
The temperature-dependent PXRD patterns show a
reversible shift of some of the reflections’ positions upon
crystallizes in the space group P1. Thus, no change of the space
̅
group could be observed for CAU-13 upon the removal/
addition of water molecules. The main difference of the
structures is the elongation of the b axis in CAU-13(empty
pore) due to the loss of hydrogen-bonding interactions
connecting two inorganic building units via adsorbed water
molecules. The desorption of water causes an increase of the
unit cell volume of about 2% (Table 1). This breathing effect is
very small compared to the 33% increase of the unit cell volume
of Al-MIL-53 upon thermal activation.11 The “kinked” a,a
conformation of half of the CDC2− ligands in CAU-13 seems to
prevent further pore opening.
Sorption Properties. Breathing may be triggered by guest
molecules and observed as steps in the respective sorption
isotherms because of the presence of different structures (and
pore sizes) during the adsorption process. We investigated the
sorption properties of CAU-13 towards various gases and
vapors at low (p ≤ 1 bar) and high pressures (p ≤ 40 bar).
From the adsorption isotherm of N2 at 77 K, the specific
surface area (380 m2 g−1) and the micropore volume (0.15 cm3
g−1) were determined. These experimental data are in good
agreement with the calculated pore volume of 0.17 cm3 g−1
using PLATON39 and the simulated accessible surface area
(450 m2 g−1) estimated using a Monte Carlo algorithm,
following the same strategy as previously reported by Duren et
̈
al.44 The H2O sorption isotherm of CAU-13 at 298 K is
presented in Figure 4, while the isotherms for N2, CO2, and H2
obtained up to a pressure of 1 bar are shown in Figure S13 in
the SI.
The H2O sorption isotherm exhibits an S shape. Because of
the hydrophobic character of the pore walls, adsorption does
Figure 3. (Top) PXRD patterns of CAU-13 at selected temperatures
(Cu Kα /α radiation). The dashed lines at 12.5 and 22.5° (2θ) are
introduced to guide the eye. (Bottom) Evolution of the unit cell
lengths over temperature: circles, a axis; squares, b axis; diamonds, c
axis.
1
2
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not show a stepwise adsorption of the investigated adsorbates,
which implies that there are no guest-induced sudden structural
changes. The observed capacities for the adsorbates are
relatively small. At 303 K and 23 bar, CAU-13 adsorbs 2.78
mmol g−1 CO2 (0.60 mol mol−1), similar to the amount
adsorbed by the np form of Al-MIL-53, which is only stable up
to a CO2 pressure of 6 bar. However, the lp form of Al-MIL-53
is formed above this pressure and adsorbs almost 4 times the
amount of CO2 (∼10 mmol g−1, 2.1 mol mol−1).46 The
relatively flat enthalpy profiles observed for all four samples
during pore filling suggests a relatively homogeneous energetic
surface. A comparison of the adsorption enthalpies obtained for
the adsorption of CO2 shows that the adsorption enthalpy of
CAU-13 (−39 kJ mol−1) is similar to that observed in the first
step in the experiments with the np form of Al-MIL-53 (−35 kJ
mol−1)46 or with STA-12 (−32 kJ mol−1), which features open
metal centers.47 In contrast, the adsorption enthalpy for
methane is remarkably higher for CAU-13 (−32 kJ mol−1)
compared to Al-MIL-53 (−17 kJ mol−1).46 Furthermore, the
methane uptake is high with respect to that observed for
nitrogen and carbon monoxide. This can be explained by the
smaller pore diameter of CAU-13 and thus a higher degree of
confinement, which leads to an increase of the host−guest
interactions. However, these drastic differences in the methane
adsorption enthalpies and uptakes of methane by CAU-13 may
also be related to a contribution of the aliphatic pore
environment.
In Situ PXRD and CO2 Adsorption. The adsorption of
water and CO2 in MIL-53 leads to similar structural changes.48
Because CAU-13 has already shown a small breathing effect
upon the uptake of water, a similar effect was expected for the
adsorption of CO2. Because the adsorption isotherms measured
at 298 and 303 K gave no direct evidence for a breathing effect,
the CO2 sorption of CAU-13 was additionally followed in situ
by synchrotron PXRD at 195 K. At this lower temperature, a
larger amount of CO2 can be adsorbed. The experiments were
performed at beamline KMC-2 at BESSY, Berlin, Germany.
The corresponding adsorption isotherm is presented in Figure
S3 in the SI (left) together with an overview of the relative
pressures at which PXRD patterns were recorded. At the
conditions used for this study (195 K, p ≤ 1 bar), CAU-13
adsorbs 24 wt % (1.2 mol mol−1) CO2. During the experiment,
PXRD patterns were recorded. Significant parts of the patterns
are shown in Figure 6 (top). The full patterns are presented in
Figure S3 (right) in the SI. A noticeable shift of the reflections
Figure 4. H2O sorption isotherm of CAU-13 recorded at 298 K.
not begin at low partial pressures but is shifted to p/p0 = 0.15.
This effect has also been observed for functionalized MIL-53
derivatives Al-MIL-53-Cl, -Br, and -CH3.45 The steep increase
at p/p0 = 0.2 corresponds to the adsorption of one water
molecule per formula sum (Figure 4) and is in accordance with
the water molecule that was localized in the structure of CAU-
13·H2O by Rietveld refinement.26 At higher relative pressures, a
maximum of 1.8 mol mol−1 water are adsorbed. The maximum
amounts of CO2 and H2 adsorbed at 298 K and 1 bar are 75
and 6.8 mg g−1, respectively. These values correspond to less
than one molecule per formula sum (0.36 and 0.73 mol mol−1).
An overview of the adsorbed amounts in mol mol−1, mg g−1,
and wt % is listed in the SI (Table S8). Except from the water
sorption experiment, no stepwise adsorption of the guest
molecules could be observed in the low-pressure region. In
order to investigate whether such adsorption steps occur at
higher adsorbate concentrations, high-pressure isotherms at
303 K were recorded using N2, CH4, CO2, and CO.
Simultaneously, the adsorption enthalpies were determined
by microcalorimetry. The adsorption isotherms and a plot of
the adsorption enthalpies against the adsorbed amount are
reported in Figure 5. The exact adsorbed amounts and
adsorption enthalpies are presented in Table S8 in the SI.
The high-pressure adsorption isotherms show that CAU-13
exhibits porosity towards all investigated gases. All isotherms
show a steeper increase at lower pressures than at higher
pressures, which is most distinct regarding the adsorption of
CO2 and CH4. Unlike the MIL-53 compounds, CAU-13 does
corresponding to the crystallographic b axis (i.e., 010 and 011)
̅
can be observed, whereas the position of the 001 reflection is
not remarkably influenced.
To gain deeper insight into the structural changes that
underlie the observations, the unit cell parameters of CAU-13
were determined at various CO2 loadings using parametric cell
parameter refinement. In Figure 6 (bottom), evolution of the
length of the b axis over increasing partial pressure of CO2 is
presented together with the sorption isotherm.
Upon adsorption of about 0.5 CO2 molecules per formula
sum (CAU-13·0.5CO2), i.e., one molecule per unit cell, the
length of the b axis reaches a minimum of 9.316(2) Å. This is
even shorter than the length of the b axis of CAU-13·H2O with
9.4300(6) Å. At higher pressures, the b axis is elongated while
more CO2 is adsorbed, indicating a change in the interactions
between CO2 and the framework of CAU-13. For adsorption of
the first CO2 molecule per unit cell, the reduction of the pore
opening suggests stronger host−guest interactions, whereas
Figure 5. Adsorption isotherms (top) and enthalpies (bottom) for the
adsorption of N2 (gray pentagons), CH4 (red diamonds), CO2 (blue
circles), and CO (green squares). Repetitions of one experiment are
marked by symbols in different shades of the same color.
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cell, respectively. The calculated arrangements of the CO2
molecules inside the channels are shown in Figure 7.
The DFT calculations show that the presence of one CO2
molecule per unit cell leads to a double interaction between the
quadrupolar CO2 and the OH groups of the framework via its
CO2 oxygen atoms. The end-on interaction of one CO2
molecule with one OH group is found to be energetically less
favorable. The characteristic OCO −HOH distances of 2.78 and
2
2.82 Å between the CO2 molecules and the framework OH
groups are significantly longer than those we previously
simulated for Al-MIL-53 at the same CO2 concentration
(2.07 and 2.10 Å).18,19 This observation can be explained by
the larger-pore aperture of CAU-13 (8.45 Å; see Figure S5 in
the SI) compared to Al-MIL-53 in its narrow-pore form (6.78
Å). This results from the proximity of two CDC2− ions
belonging to two neighboring chains, which prevents the pores
from further closing and leads to a less confined environment
for the guest molecules. These geometric considerations are
consistent with a calculated adsorption enthalpy of −34 kJ
mol−1, which remains lower than the one previously simulated
for CO2 in Al-MIL-53 (−41 kJ mol−1).21 Nevertheless, the
averaged experimentally observed adsorption enthalpy of Al-
MIL-53 in the first adsorption step is −35 kJ mol−1,46 thus
similar to both the experimental and calculated adsorption
enthalpies of CAU-13.
When in the case of CAU-13·CO2 two CO2 molecules per
unit cell are introduced (Figure 7, right), the binding scheme
switches from the double interaction to an arrangement in
which only one CO2 oxygen atom is interacting with an OH
group of the inorganic building unit. This is similar to previous
Figure 6. (Top) Synchrotron PXRD pattern series obtained during
the adsorption (red patterns) and desorption (blue patterns) of CO2.
The PXRD patterns were measured using radiation with λ = 1.5406 Å.
From left to right, the 010, 001, and 011 reflections are shown. The
̅
findings for Al-MIL-53.20,21 The calculated OCO −HOH
dashed lines were added to guide the eye. (Bottom) Evolution of the
length of the crystallographic b axis with increasing pressure of CO2
(squares). In addition, the corresponding CO2 adsorption isotherm
measured at 195 K (circles) is shown. Filled symbols mark the
adsorption and open symbols the desorption.
2
distances of 2.34 and 2.33 Å are shorter than that in the case
of CAU-13·0.5CO2, leading to a slightly higher adsorption
enthalpy (−36 kJ mol−1). The interactions between the CO2
molecules lined up along the direction of the channel are
relatively weak, represented by a characteristic CCO −CCO
2
2
distance of 3.76 Å.
upon the adsorption of more CO2 molecules, the guest−guest
interactions seem to dominate. The possible binding schemes
were modeled by DFT calculations on the basis of the cell
parameters observed at the partial pressures corresponding to
the compositions of CAU-13·0.5CO2 and CAU-13·CO2, i.e.,
CAU-13 with one and two adsorbed CO2 molecules per unit
Indeed, the differences in sorption behavior between Al-MIL-
53 and CAU-13 are mainly based on the space the respective
ligands occupy in the structure. This spatial arrangement
influences the interactions between the CO2 molecules and the
framework. The interactions of CO2 with CAU-13 and MIL-53
Figure 7. DFT-optimized arrangements of CO2 in CAU-13: left, CAU-13·0.5CO2; right, CAU-13·CO2. The aliphatic cyclohexane rings are omitted
for clarity. The characteristic interatomic distances are provided in angstroms. Color code: aluminum, blue; oxygen, red; carbon, black; hydrogen,
light gray.
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mainly occur between the adsorbate and the OH groups that
are part of the inorganic building units.
selectivities (CAU-13:1.3, Al-MIL-53:1.2). This fact can be
attributed to the shape-selective interactions of o-xylene in Al-
MIL-53,23 which are not possible in CAU-13.
XYLENE ADSORPTION
The lower capacity of CAU-13 is attributed to the smaller
accessible pore volume due to the bulkier nature of the CDC2−
ion in comparison to the BDC2− ion of Al-MIL-53. Not only
does the aliphatic six ring of carbon atoms in the chair
conformation need more space than the rigid planar aromatic
ring of the BDC2− ions but also the number of hydrogen atoms
has doubled. Because of the sp3 configuration of the carbon
atoms, the hydrogen atoms are not in the same plane as the
carbon atoms but point in different directions. To find out if
any structural changes occur when xylene is adsorbed, the
vapor-phase adsorption of the xylene isomers was monitored by
in situ PXRD. Figure 9 shows the contour maps of PXRD data
collected during the adsorption as well as the desorption of
xylene by CAU-13·H2O. The adsorption experiments were
performed using a steady flow of xylene-saturated nitrogen, and
the PXRD patterns were recorded with a time resolution of ∼3
min scan−1. Desorption was achieved by stepwise heating.
Upon adsorption of the three different xylene isomers,
structural changes of CAU-13 are observed. The beginning of
the phase change is first seen for m-xylene (15 scans,
approximately 45 min) followed by o-xylene (20 scans,
approximately 60 min) and p-xylene (25 scans, approximately
75 min). During the adsorption of o- and m-xylene, the
coexistence of two phases continues over a range of about 15−
20 scans, while the phase change in the adsorption of p-xylene
is faster (within less than 10 scans). The desorption
experiments show that p-xylene is retained to the highest
temperature followed by o- and m-xylene. Thus, only one step
can be seen in each of the experiments with CAU-13, while Al-
MIL-53 undergoes two steps in the adsorption of xylenes.24
The observed phases during the adsorption are CAU-13·H2O
in the beginning of the experiment and one phase for each of
the xylene-loaded samples. During the desorption experiments,
the resulting phase is CAU-13(empty pore) instead of CAU-13·
H2O because desorption was achieved via thermal treatment.
Interestingly, upon the adsorption of xylene, the reflections
shift to lower angles, indicating an increase of the unit cell to
values larger than those observed for CAU-13(empty pore)
(Figure 10). To gain deeper insight into the structural changes
triggered by the xylene adsorption of CAU-13, powdered
samples were treated with an excess of the pure liquid xylene
isomers and PXRD patterns were recorded. A comparison of
the PXRD patterns of CAU-13·H2O, CAU-13(empty pore),
and CAU-13 loaded with xylene, collected during the in situ
experiments and obtained with an excess of xylene, is shown in
Figure 10.
■
So far, the steric properties of the aliphatic CDC2− ions
dominated the sorption behavior of CAU-13. To study possible
interactions with the aliphatic pore environment, xylene
adsorption experiments were performed. In competitive
adsorption experiments, the MIL-53 compounds showed a
preference for o-xylene over the other isomers.23,25 Crystal
structure refinements of xylene-loaded Fe-MIL-53 indicated
π−π interactions not only between the adsorbate molecules but
also between the adsorbate and the aromatic pore walls.25
Because the pore walls of CAU-13 are aliphatic, such an
interaction is improbable and should result in different sorption
behavior.
We studied the liquid- and vapor-phase single-component
xylene adsorption on CAU-13 and also the competitive
adsorption from the liquid phase. In Figure 8, the liquid-
Figure 8. Liquid-phase batch adsorption isotherms of CAU-13 for o-
xylene (squares), p-xylene (circles), and m-xylene (triangles) from
mesitylene at 298 K.
phase batch adsorption isotherms of CAU-13 are presented.
The experiments were carried out at 298 K using mesitylene as
the eluent. For o-xylene, a slightly higher uptake (17 wt %, 0.34
mol mol−1) than those for p-xylene (15 wt %, 0.29 mol mol−1)
and m-xylene (14 wt %, 0.28 mol mol−1) can be observed.
In competitive adsorption experiments from a mixture of o-
and p-xylene in hexane, the o-xylene uptake of Al-MIL-53 is
higher (45 wt %, 0.88 mol mol−1).23 The selectivities given in
Table 2 show only a slight preference of CAU-13 for o-xylene
over the other isomers. While the separation factors of o/m-
xylene and o/p-xylene are lower than those observed for Al-
MIL-53, the separation of m/p-xylene results in similar
Table 2. Comparison of the Selectivities Calculated for the
Adsorption from Equimolar Binary and Ternary Mixtures of
Xylene Isomers by CAU-13 and Al-MIL-53
The PXRD patterns exhibit a shift of the reflections to lower
angles in the order CAU-13·H2O < CAU-13(empty pore) <
CAU-13(xylene, vapor phase) < CAU-13(excess xylene).
Indexing of the powder patterns obtained for CAU-13 loaded
with the xylene isomers and Pawley refinements gave rise to cell
volumes of 1350 Å3 for the m-xylene-loaded sample and 1317
Å3 for the o-xylene-loaded sample. The unit cell of p-xylene-
containing CAU-13 is doubled in the direction of the one-
dimensional pores (a axis), giving a cell volume of 2654 Å3. The
results of the refinement and final Pawley plots are presented in
Table S4 and Figure S8 in the SI. With a cell content of Z = 4
[8 for CAU-13(pX)], this corresponds to a relative expansion
by ∼25% compared to CAU-13(empty pore) (V = 540 Å3, Z =
2; Table 1). The volume changes are accompanied by an
α(o/p)
α(o/m)
α(p/m)
CAU-
Al-MIL-
53
CAU-
Al-MIL-
53
CAU-
Al-MIL-
53
a
b
a
b
a
b
mixture
13
13
13
binary, o/p
1.5
2.7
binary,
o/m
1.9
3.5
binary,
p/m
1.3
1.3
1.2
ternary
1.7
2.1
a
0.2 mol L−1 binary or 0.3 mol L−1 ternary mixtures in mesitylene.
b
Batch experiments at 298 K. 0.028 mol mol L−1 binary mixtures of
xylene isomers in hexane. Batch experiments at 298 K. Experimental
data from ref 23.
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Figure 9. Contour maps of the in situ PXRD investigation of the adsorption (bottom) and desorption (top) of xylene isomers (Cu Kα /α radiation).
1
2
In the adsorption experiment, one scan number corresponds to approximately 3 min.
Figure 10. Comparison of the theoretical PXRD patterns of CAU-13·H2O (black) and CAU-13(empty pore) (gray) with the PXRD patterns of
xylene-loaded samples: cyan, final pattern of vapor-phase adsorption experiments; blue, pattern of CAU-13 loaded with an excess of liquid xylene.
Figure 11. Ball-and-stick representations of the crystal structures of xylene-loaded CAU-13. (a) View along the channels of CAU-13(mX). Note the
e,e conformation of all of the linker molecules. (b−d) Representative images of the arrangement of the xylene molecules in (b) CAU-13(mX), (c)
CAU-13(oX), and (d) CAU-13(pX). In parts b−d, the carbon atoms of the cyclohexane rings are omitted for clarity.
increase of symmetry from the triclinic space group P1 to the
molecule of the two conformers [the Ccarboxy−Ccarboxy distance
of the e,e conformer is ∼5.6 Å and that of the a,a conformer is
∼4.6 Å, taken from the crystallographic data of CAU-13(empty
pore)]. Rietveld refinements applying rigid bodies for both the
linker and guest molecules proved the validity of the
assumption. Representative images of the crystal structures of
̅
monoclinic space group P21. On the basis of these findings,
structural models have been set up in the space group P21 using
Materials Studio 5.0.37 To allow for the large unit cell
dimensions, all of the linker molecules had to be introduced
using the e,e conformation, which represents the longer
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the three xylene-loaded CAU-13 samples are given in Figures
11 and S12 in the SI.
of the xylene isomers from the gas phase was monitored with in
situ PXRD, which shows that upon desorption p-xylene is
retained inside the pores to the highest temperature. Structure
determinations of CAU-13 loaded with the xylene isomers
reveal a remarkable breathing effect. The CDC2− ions that show
the a,a and e,e conformations (ratio 1:1) in all other
investigated forms of CAU-13 change their conformation
upon loading with xylene to adopt the e,e conformation only.
This results in a symmetry increase and allows for a wider pore
opening and a cell volume increase by about 25%. This new
type of breathing behavior has not been observed for any other
MOF so far.
As in CAU-13(empty pore), each chain of trans-corner-
sharing AlO6 octahedra is connected to four other chains by
two types of crystallographically independent CDC2− ions; i.e.,
the topology of the structure is maintained. In the xylene-
loaded CAU-13, both CDC2− ions are in the e,e conformation;
however, in CAU-13(empty pore), half of the linker molecules
are in the a,a- and e,e conformations, respectively. The pore
aperture of CAU-13 is characterized by the Al−Al distance of
two opposing inorganic building units and measures 9.70 Å for
CAU-13(empty pore). The increase to around 11 Å
determined for the xylene-loaded CAU-13 is only made
possible by a conformational change of the flexible aliphatic
linker molecules from the a,a conformation to the e,e
conformation. In contrast, the breathing of MIL-53 is just
accompanied by a rotation around the O−O axis of the O−M−
M−O and O−C−O planes.15 Breathing involving conforma-
tional changes of the flexible aliphatic linkers has so far only
been reported for layers of zirconium phosphates pillared by
aliphatic connectors such as 1,2-decanediphosphonates.14
However, these layered compounds are of low crystallinity,
and the pillaring connectors are statistically distributed
throughout the layers, mixed with short monophosphonate
molecules acting as space holders. Hence, the breathing of
CAU-13 upon the adsorption of xylene represents a new kind
of flexibility in MOFs.
The xylene molecules are located in the center of the pores in
two different types of stacking. In CAU-13(oX) and CAU-
13(mX), the positions of the xylene molecules are disordered,
whereas in CAU-13(pX), an ordering of p-xylene can be
observed. Therefore, the a axis of the unit cell of CAU-13(pX)
is doubled compared to the structures of CAU-13(oX) and
CAU-13(mX). The observed arrangements of the xylene
molecules compare well with the behavior of the compound
during the in situ PXRD experiments, where p-xylene is
desorbed at the highest temperature followed by the other two
isomers. The well-ordered array indicates relatively strong
intermolecular adsorbate−adsorbate interactions, in good
agreement with retention of the adsorbed molecules during
the desorption experiment. The fact that only p-xylene is well
ordered in CAU-13 might be due to its linear shape and its
kinetic diameter, which is the smallest of the three isomers.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format, details of the
parametric refinement of the unit cell parameters for the
temperature-dependent experiments as well as the in situ CO2
sorption experiments (tabulated and graphical representations
of the unit cell parameters, full PXRD patterns, and CO2
sorption isotherms), images of the asymmetric units as well as
bond length tables of CAU-13(empty pore), CAU-13(mX),
CAU-13(oX), and CAU-13(pX), N2, CO2, and H2 sorption
isotherms recorded up to 1 bar, and a tabulated overview of all
adsorbate uptakes of CAU-13 in the different experiments. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data for CAU-13(empty pore),
CAU-13(mX), CAU-13(oX), and CAU-13(pX) have also been
deposited with the Cambridge Crystallographic Data Centre
(CCDC 980340, 980341, 980342, and 980343). The data can
be obtained free of charge via the Internet at ww.ccdc.cam.a-
c.uk/conts/retrieving.html or from the CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax, +44 1223 36033; e-mail,
deposit@ccdc.ac.uk).
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: guillaume.maurin@univ-montp2.fr.
*E-mail: stock@ac.uni-kiel.de. Tel.: +49-431-880-1675. Fax:
+49-431-880-1775.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
CONCLUSION
■
The sorption properties and breathing behavior of the Al-MOF
CAU-13 have been studied extensively and have been
compared to those of Al-MIL-53. In summary, the adsorption
capacities of CAU-13 are lower than those of Al-MIL-53, which
adopts the large-pore form under similar experimental
conditions. The micropore volume of CAU-13 is relatively
small because of the bulkier aliphatic CDC2− ions in CAU-13
compared to the BDC2− ions in Al-MIL-53. A small breathing
effect under thermal treatment in air (i.e., the desorption of
water) and upon the adsorption of CO2 was revealed. DFT
calculations evidenced the most preferred arrangements of CO2
within the pores of CAU-13 for different loadings, resulting in
simulated adsorption enthalpies only slightly lower than those
previously reported for Al-MIL-53, consistent with the
microcalorimetry data. The adsorption of o-, m-, and p-xylene
as single components and mixtures was investigated using
solutions of the adsorbate in mesitylene. From mixtures in the
liquid phase, o-xylene is adsorbed preferentially. The adsorption
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research leading to these results has received funding from
the European Community’s Seventh Framework Programme
(FP7MACADEMIA/2007-2013) under Grant 228862. D.d.V.,
J.L., and H.R. are grateful for support by IWT Belgium
(MOFshape) and FWO (Vlaanderen). G.M. acknowledges the
Institut Universitaire de France for financial support. A.H. and
S.K. acknowledge the Federal Ministry of Education and
Research for financial support (Project BMBF 05K10OD3).
F.N. and N.S. are thankful for the support of the DFG (SPP
1362). We acknowledge the Helmholtz Association for
beamtime on KMC-2 at BESSY and P08 at DESY.
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