Study of selective adsorption of aromatic compounds from solutions by the flexible MIL-53(Al) metal-organic framework

Article abstract of DOI:10.1007/s11172-015-0973-8

The dynamic method under conditions close to equilibrium was applied to study the liquid-phase adsorption in the Henry region for a series of aromatic compounds on the MIL-53(Al) metal-organic framework at different temperatures. The interpretation of the

Full text of DOI:10.1007/s11172-015-0973-8

Russian Chemical Bulletin, International Edition, Vol. 64, No. 5, pp. 1039—1048, May, 2015
1039
Study of selective adsorption of aromatic compounds from solutions
by the flexible MILꢀ53(Al) metalꢀorganic framework*
B. R. Saifutdinov,a V. I. Isaeva,b E. V. Alexandrov,c and L. M. Kustovb,d
aSamara State Technical University,
244 ul. Molodogvardeiskaya, 443100 Samara, Russian Federation.
Fax: +7 (846) 242 3580. Eꢀmail: sayf_br@mail.ru
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: sharf@ioc.ac.ru, lmk@ioc.ac.ru
cSamara Center for Theoretical Materials Science, Samara State University,
1 ul. Akademika Pavlova, 443011 Samara, Russian Federation.
Fax: +7 (846) 334 5417. Eꢀmail: aleksandrov_ev1@mail.ru
dChemistry Department, M. V. Lomonosov Moscow State University,
Building 1, 3 Leninskie Gory, 119992 Moscow, Russian Federation
The dynamic method under conditions close to equilibrium was applied to study the liquidꢀ
phase adsorption in the Henry region for a series of aromatic compounds on the MILꢀ53(Al)
metalꢀorganic framework at different temperatures. The interpretation of the obtained experiꢀ
mental adsorption data was based on the TOPOS analysis of the structure of the cavities in the
MILꢀ53(Al) framework using the Voronoi—Dirichlet polyhedra concept. It is shown that the
adsorption activity of the investigated material under the liquidꢀphase conditions is governed by
a possible expansion of the channels and cavities in the structure and by a breathing effect of the
structure caused by the temperature variation. The selectivity of adsorption shown by MILꢀ53(Al)
for a series of the studied compounds is due to the adsorbate—adsorbent —ꢀinteraction and
hydrogen bonding of adsorbate molecules with Brönsted acid sites of the metalꢀorganic frameꢀ
work. High adsorption selectivity of the MILꢀ53(Al) framework were found for compounds
differed in the number of aromatic rings in the molecule and the presence of the methyl
substituent, as well as for aromatic hydrocarbons and their sulfurꢀcontaining heterocyclic analogs.
Key words: metalꢀorganic frameworks, MILꢀ53(Al), adsorption, liquid chromatography,
temperature dependence, topology.
Many works describing hybrid nanomaterials,1 among
which metalꢀorganic frameworks (MOFs) occupy a speꢀ
cial position, have been published in recent years.2—7 Metꢀ
alꢀorganic frameworks represent a new type of hybrid orꢀ
ganic—inorganic supramolecular materials, whose ordered
network is built of electronꢀdonor organic linkers and metal
cations.6,7 The possibility of varying the linker nature favors
the preparation of numerous metalꢀorganic frameworks.8
These structures are characterized by unusually high
specific surface areas and pore volumes, the controlled
pore size and, as a consequence, the ability to adsorb molꢀ
ecules of various compounds.5—8 Owing to unique strucꢀ
tural and texture characteristics, MOFs are promising
functional materials for gas storage and separation,9,10
heterogeneous catalysis,11 systems for targeted drug delivꢀ
ery in living organisms,12 adsorption,13—15 chromatoꢀ
graphy,16—19 and other technological areas.
The promising use of metalꢀorganic frameworks for
addressing problems of adsorption separation and chroꢀ
matography is due to the possibility of purposeful changꢀ
ing selectivity of these materials by the variation of the
type of organic linker and, as a consequence, the size and
chemistry of pore surfaces.19,20 The efficiency of MOF
application in processes of selective adsorption and sepaꢀ
ration in the liquid phase is favored by the unimodal pore
size distribution and open hydrophobic channels in many
frameworks of the system.12,21 The works published to
date on this issue22—25 provide no information on the
mechanism of liquidꢀphase adsorption on metalꢀorganic
frameworks.
The purpose of this work is to reveal the physicochemꢀ
ical regularities of adsorption of a series of aromatic comꢀ
pounds from the liquid phase by the MILꢀ53(Al) metalꢀ
* On the occasion of the 100th anniversary of the birth of Acadeꢀ
mician N. K. Kochetkov (1915—2005).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1039—1048, May, 2015.
1066ꢀ5285/15/6405ꢀ1039 © 2015 Springer Science+Business Media, Inc.

1040
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
Saifutdinov et al.
a
I (abr. units)
16
organic framework. Many of the chosen adsorbates can
hardly be separated on the traditional adsorbents.26—28
Therefore, it seems interesting to study the selective propꢀ
erties of the MILꢀ53(Al) framework towards the comꢀ
pounds considered. The metalꢀorganic framework studied
as an adsorbent is built of octahedra AlO4(OH)2 linked by
terephthalate ions to form a regular system of elliptic chanꢀ
nels.29 Since the MILꢀ53(Al) framework has hydrophobic
pores formed by aromatic rings29 and is stable in an aqueꢀ
ous medium,21 this material can be used in liquidꢀphase
adsorption and HPLC.
12
8
4
15
25
35
2/deg
Experimental
I (abr. units)
b
Synthesis of the metalꢀorganic framework MILꢀ53(Al).
A sample of MILꢀ53(Al) was synthesized under solvothermal
conditions at an autogenic pressure. A solution of Al(NO3)3•
•9H2O (1.30 g, 3.47 mmol) and 1,4ꢀbenzenedicarboxylic acid
(0.288 g, 1.73 mmol) in deionized water (5 mL) was transferred
to an autoclave with a Teflon bush and heated for 72 h at 220 C.
After the end of the reaction, a crystalline precipitate of
MILꢀ53(Al) was separated by centrifugation, washed with deionꢀ
ized water (3×10 mL), and calcined in air (220 C, 72 h).
Physicochemical properties of the synthesized sample of
MILꢀ53(Al). Prior to physicochemical studies, the MILꢀ53(Al)
sample was additionally activated for 3 h at 440 C with a heating
rate of 3 C min–1. The activated MILꢀ53(Al) sample was stored
in a drying box.
5
15
25
35
45 2/deg
Fig. 1. Comparison of the XRD powder pattern of the samples of
the MILꢀ53(Al) framework (a) with that published earlier29 (b).
The XRD powder pattern (Fig. 1) of the studied metalꢀorꢀ
ganic framework MILꢀ53(Al) was detected with an ARL X´TRA
diffractometer (Thermo Fisher Scientific, Switzerland) with the
CuꢀK radiation ( = 1.54 Å) with a scanning rate of 2 deg min–1
.
The SEM image (Fig. 2) of the MILꢀ53(Al) powder was obtained
on a JSMꢀ6390A scanning electron microscope (JEOL, USA).
A TG analysis of the MILꢀ53(Al) sample was carried out on
a Jupiter STA 449 F3 instrument (NETZSCH, Germany) under
the following conditions: heating from 20 to 800 C with a heatꢀ
ing rate of 10 C min–1 in an air flow with a flow rate of
100 mL min–1. The corresponding thermogravimetric curve is
shown in Fig. 3. The isotherm of lowꢀtemperature adsorption—
desorption of nitrogen (Fig. 4) for the MILꢀ53(Al) sample was
measured on an Autosorbꢀ1 instrument (Quantachrome Instruꢀ
ments, USA). The specific surface of the adsorbent was formally
calculated using the Brunauer—Emmett—Teller (BET) equation
(Ssp(BET)/m2 g–1) in the range of relative pressures from 0.05 to
0.3 and using the Langmuir equation (Ssp(Langmuir)/m2 g–1) in
a relative pressure range of 0.07—0.2.30 The size of mesopores in
the sample was estimated from the desorption branch of the
isotherm in the framework of the Barrett—Joyner—Halenda
(BJH) model30 (r(BJH)/Å), and the micropore size was deterꢀ
mined using the Horvath—Kawazoe (HK) model31 (r(HK)/Å).
The characteristics of the porous structure of the studied MILꢀ53(Al)
sample are as follows: Ssp(BET) = 1085 m2 g–1, Ssp(Langmuir) =
= 1569 m2 g–1, r(BJH) = 19 Å, and r(HK) = 3.36 Å.
10 m
Fig. 2. SEM image of the powdered sample of the MILꢀ53(Al)
metalꢀorganic framework.
The MILꢀ53(Al) sample with an average particle size of 7—8 m
(see Fig. 2) was used as an adsorbent without preliminary isolaꢀ
tion of a narrow fraction. Then the inlet to the column was
closed with the frit mounted by the nipple joint. The column was
connected to a pump of an LCꢀ20AD liquid chromatograph
(Shimadzu, Japan) and washed with an H2O—MeCN (9 : 1) solꢀ
vent for wetting and compacting of the adsorbent. Then the inlet
to the column was opened, an additional amount of the dry
powder of MILꢀ53(Al) was placed into a void formed due to the
shrinkage of the adsorbent, and the procedure on adsorbent wetꢀ
ting and compacting was repeated until no adsorbent shrinkage
Study of adsorption on the metalꢀorganic framework
MILꢀ53(Al). Prior to dynamic adsorption experiments, a dry
powdered sample of MILꢀ53(Al) was placed in a steel chroꢀ
matographic column (50×4.0 mm), the outlet from which was
closed with a porous membrane (frit) mounted by a nipple joint.

Liquidꢀphase adsorption on MILꢀ53(Al)
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
1041
m/mg
dyne dosage valve. Concentration of adsorptives was detected at
a wavelength of 254 nm.
0
–1
–2
–3
–4
100 200 300 400 500 600 700 T/С
Fig. 3. Thermogravimetric curve for the samples of the MILꢀ53(Al)
metalꢀorganic framework.
V/cm3 g–1
600
2
400
1
200
0.2
0.4
0.6
0.8
1.0 p/ps
Fig. 4. Isotherm of lowꢀtemperature adsorption (1)—desorption
of nitrogen (2) on the sample of the MILꢀ53(Al) metalꢀorganic
framework.
was observed after the next opening of the column. As a result,
the solvent volume in the column, the so called "dead" volume,
turned out to be VM = 350 L. The obtained column was characꢀ
terized by reproducible values of retention times for a series of
the studied substances. The pressure at the inlet of the prepared
column was 1.4 MPa at 313 K using H2O—MeCN (1 : 1) as
a solvent.
Adsorption of a series of aromatic compounds 1—18 was
studied under conditions close to equilibrium by HPLC on
a Prominence instrument (Shimadzu, Japan) at temperatures of
the column of 313, 323, and 333 K. The components of the
solution used to study adsorption were MeCN (HPLCꢀgradient
grade) (Panreac, Spain) and H2O obtained on a DMEꢀ1/B deꢀ
ionization system (BMT, Russia). The binary solvent H2O—MeCN
(1 : 1) was passed through a layer of the MILꢀ53(Al) adsorbent
(weight g = 0.14265 g) with a flow rate of 500 L min–1. All
studied aromatic compounds, viz., benzene (1), toluene (2), allylꢀ
benzene (3), thiophene (4), pyridine (5), naphthalene (6), benzoꢀ
thiophene (7), indole (8), 1ꢀmethylindole (9), quinoline (10),
phenanthrene (11), dibenzothiophene (12), 4,6ꢀdimethyldiꢀ
benzothiophene (13), phenol (14), oꢀcresol (15), pyrocatechol
(16), anisole (17), and guaiacol (18), are commercial reagents
(Sigma—Aldrich). Solutions of these substance in H2O—MeCN
(1 : 1) were introduced into the flow of the solvent with a Rheoꢀ
The conditions of adsorption experiment were selected in such
a way that equilibrium and equilibrium adsorption in the Henry
law region were ensured. The absence of changes in the reduced
retention time of the adsorbates with the variation of the flow
rate of the binary solvent from 50 to 500 L min–1 and the use of
solutions of the adsorptives with the concentration <3.3 mol L–1
confirm the assumption that the experiments were carried out
under the equilibrium conditions in the Henry law region.26,32
Based on the retention volumes of the adsorbates VR/L
determined taking into account the HPLC data, Henry´s conꢀ
stants of adsorption K1,c/L m–2 were calculated for the studied
aromatic compounds on the MILꢀ53(Al) samples using the
known formula33
K1,c = (VR – VM)Ssp–1g–1
,
where VR is the retention volume of the adsorbate (L), VM is the
dead volume of the column (L), Ssp is the Langmuir specific
surface (m2 g–1), and g is the adsorbent weight (g).
The obtained values of Henry´s constant of adsorption at
different temperatures are presented in Table 1.
Procedure of crystalꢀchemical analysis. The values of the
surface area SVDMP/Å2 and volume VVDMP/Å3 of the Voronoi—Diꢀ
richlet molecular polyhedron (VDMP) and the degree of sphericꢀ

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Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
Saifutdinov et al.
Table 1. Henry´s constants of adsorption of aromatic
compounds 1—10 and 14—18 from an H2O—MeCN
solution on the sample of the MILꢀ53(Al) metalꢀorꢀ
ganic framework at different temperatures of the column
was identified by the XRD method from the position of
the main peaks in the XRD pattern.37—39 A comparison of
the XRD pattern of the synthesized sample (see Fig. 1, a)
with the earlier published39 pattern (see Fig. 1, b) shows
that the sample of MILꢀ53(Al) corresponds to the variety
of the metalꢀorganic framework MILꢀ53(Al) in which one
water molecule is located at the center of each channel.38,39
This specimen of MILꢀ53(Al) is characterized by the comꢀ
position [Al(oꢀC6H4(CO2)2)(OH)]•(H2O), and is comꢀ
monly designated as MILꢀ53(Al) lt (soꢀcalled "low temꢀ
perature form").38,39
The SEM image (see Fig. 2) of the synthesized
MILꢀ53(Al) powder shows the presence of microcrystalꢀ
lites, with the size ranging from 1 to 10 m. Since particles
with an average size of 7—8 m predominate in the studꢀ
ied MILꢀ53(Al) sample, the latter can be used as an adꢀ
sorbent for HPLC.
The TG data (see Fig. 3) indicate that the studied
material has a high thermal stability and preserves its crystal
structure on heating to 550 C and that even the activated
sample contains water. In agreement with the attribution
based on the position of the main peaks in the XRD pattern
(see Fig. 1), the character of the thermogravimetric curve
of the sample of the metalꢀorganic framework (see Fig. 3)
confirms its assignment to the MILꢀ53(Al) lt form.38,39
Thus, the synthesized metalꢀorganic framework
MILꢀ53(Al) contains pores free of terephthalic acid molꢀ
ecules, which makes it possible to use it as an adsorbent in
selective adsorption from the liquid phase.
Analysis of the structure of cavities in the metalꢀorganic
framework MILꢀ53(Al). To study the possibility of adꢀ
sorption of molecules of the studied compounds by the
metalꢀorganic framework MILꢀ53(Al), it is important to
determine the geometric arrangement of the system of
voids (cavities and channels) in the crystal structure. The
fact is that, the information on the porous structure deꢀ
rived from adsorption methods of measuring porosity in
the averaged form is insufficient analyzing the mechaꢀ
nism of adsorption on such supramolecular materiꢀ
als as MILꢀ53(Al). It is necessary to compare the geoꢀ
metric sizes of the cavities (minimum and maximum sizes
of cavities and channels) with dimensions of adsorbate
molecules. The comparison makes it possible to establish
a possibility of incorporation of these molecules in the
cavities and channels and, as a consequence, their adsorpꢀ
tion on the crystalline adsorbents, including the metalꢀ
organic framework MILꢀ53(Al). In addition, when anaꢀ
lyzing adsorption of molecules on MILꢀ53(Al), it is necꢀ
essary to take into account the soꢀcalled "breathing" efꢀ
fect,7,40 that is, possible variations of the pore diameter as
the response to the change in the size of adsorbed moleꢀ
cules and to the change in the temperature of the adsorpꢀ
tion system.2,7,40,41
Comꢀ
pound
K1,c/L m–2
313 K
323 K
333 K
1
2
3
4
5
6
7
8
13.2
17.4
36.7
11.8
12.4
76.3
65.1
14.6
93.9
15.0
11.9
16.4
11.3
17.2
11.2
13.9
18.6
40.6
12.0
12.6
79.0
65.8
15.3
90.4
15.2
11.7
16.4
11.3
16.9
10.9
14.6
19.0
40.9
12.3
12.7
74.9
60.8
14.3
79.0
13.9
11.9
16.2
11.4
17.0
10.9
9
10
14
15
16
17
18
Table 2. Geometric characteristics of molecules of the studied
aromatic compounds 1—6, 8, and 10—18
Compound
VVDMP/Å3
SVDMP/Å2
G3 (arb. units)
1
2
3
4
5
6
8
10
11
12
13
14
15
16
17
18
128.08
154.01
190.76
119.74
118.79
181.66
162.24
176.68
242.26
223.16
276.64
129.26
151.39
134.83
166.12
164.04
154.48
178.67
211.21
144.82
144.72
205.06
188.28
199.28
257.12
241.32
289.85
158.92
180.72
166.39
190.92
192.20
0.0952
0.0993
0.1039
0.0937
0.0936
0.1069
0.1034
0.1057
0.1155
0.1167
0.1180
0.0981
0.1003
0.1000
0.1051
0.1113
ity G3 (arb. units) of molecules of the studied aromatic compounds
(Table 2) were calculated using the TOPOS structural topologiꢀ
cal program packages34 for crystal structures from the Camꢀ
bridge Crystallographic Data Centre.35 An analysis of the structure
of cavities in MILꢀ53(Al) was based on the Voronoi—Dirichlet
concept of polyhedra36 using the TOPOS program package.
Results and Discussion
Physicochemical characteristics of the metalꢀorganic
framework MILꢀ53(Al). The crystalline phase of MILꢀ53(Al)
A powerful method for an analysis of the structure of
cavities in crystalline materials is the Voronoi—Dirichlet

Liquidꢀphase adsorption on MILꢀ53(Al)
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
1043
method of polyhedra,36 which was used for the analysis of
the system of cavities in MILꢀ53(Al). The construction of
the Voronoi—Dirichlet polyhedra for vertices of the atomic
Voronoi—Dirichlet polyhedra (centers of cavities) makes
it possible to establish specific features of arrangement of
the system of cavities (cavities and channels) in the crystal
structure. For example, the construction of the Voronoi—
Dirichlet polyhedra for cavities of the MILꢀ53(Al) frameꢀ
work expanded by terephthalic acid molecules using the
TOPOS program packages shows that the metalꢀorganic
framework with the code [SABVOH] in the database
(Table 3) contains parallel channels that can be approxiꢀ
mated by elliptic cylinders with sizes of 6.6 and 14.2 Å
(Fig. 5). These channels crossꢀlinked by narrower lateral
channels 2.6 Å in diameter (see Fig. 5).
The MILꢀ53(Al) framework can be simplified and clasꢀ
sified according to the known topological types. For this
purpose, we plotted the basic network of the framework
using the TOPOS structural topological program packꢀ
age.42 In this network, the sites correspond to aluminum
atoms and centers of gravity of terephthalate anions, and
the edges correspond to the bonds between them and the
bridging 2ꢀaqua ligands (Fig. 6). Then, to obtain the
basic network, we calculated the topological indices (coꢀ
ordination sequence, point symbol, and vertex symbol)
that make it possible to unambiguously identify its topoꢀ
logical type. The comparison of the calculated topological
indices with those written in the collection of topological
types of the TOPOS program package allowed us to estabꢀ
lish that the framework can be described by the topology
sehꢀ4,6ꢀImma.
It follows from the database of these representatives of
topological types of the TOPOS (collection TTO) that
topology sehꢀ4,6ꢀImma can be assigned to 106 crysꢀ
tal structures. Among them are five representatives of
MILꢀ53(Al) and many chemically modified frameworks
MILꢀ53(Al) and MILꢀ47(V). The latter metalꢀorganic
framework has the same topology as that of MILꢀ53(Al),
and the relevant information was deposited at the Camꢀ
bridge Crystallographic Data Centre.43 The distinction is
only the nature of the central metal atom. The crystalꢀ
lochemical information on seven crystal structures with
the MILꢀ53(Al) framework is presented in Table 3. When
molecules of terephthalic acid and water, 1,3,2ꢀbenzoꢀ
a
c
H
C
O
Al
b
c
a
a
b
c
b
d
b
b
a
a
c
c
Fig. 5. Fragment of the framework (a, b) and the system of channels of the framework (b—d) in the [Al(oꢀC6H4(CO2)2)(OH)]•
•(oꢀC6H4(COOH)2) [SABVOH] structure.
Note. Figures 5 and 6 are available in full color in the onꢀline version of the journal (http://www.springerlink.com).

1044
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
a
Saifutdinov et al.
b
H
C
O
Al
c
c
c
d
a
b
a
b
Fig. 6. Fragments of the framework (a, b) and basis network (c, d) of the structure [Al(oꢀC6H4(CO2)2)(OH)] [Large].
dithiazolyl radical or 5ꢀmethylꢀ1,3,2ꢀbenzodithiazolyl radꢀ
ical are incorporated into the pores, the crystal structure
of MILꢀ53(Al) is deformed and takes symmetry Pnma,
Cc, and P21/n, which somewhat differs from the symmeꢀ
try of maximally compressed and expanded frameworks
(C2/c and Imma, respectively).
The sizes of the cavities in the MILꢀ53(Al) structure
respond to compression and expansion of the framework.
The compressed metalꢀorganic framework (structure [Narꢀ
row] in Table 3) contains broad channels with a dimenꢀ
sion of 16.2×4.4 Å (average diameter 8.4 Å) and narrow
channels 1.8 Å in diameter. The expanded structure of
Table 3. Results of crystallochemical analysis of the crystallographic data on the structure of the MILꢀ53(Al) metalꢀorganic coordinaꢀ
tion polymer
Code in
database
Compounda
Space
group
Pore sizes
Vsp
Ssp
Guest sizes
(average
Vb/Å3
(average pore /cm3 g–1 /m2 g–1
I
II
diameter)/Å
diameter)/Å
SABVOH0135 [Al(oꢀC6H4(CO2)2)(OH)]• Pnma
•(oꢀC6H4(COOH)2)
15×5.8 (9.3),
1.8
14.2×6.6 (9.7),
2.6
17.2×3.6 (7.9),
2.0
14.4×8.6 (11.1),
2.2
0.559
0.615
0.418
0.610
0.707
4262
4160
4569
4105
5139
4.0×7.6×11
(6.9)
4.0×7.6×11
(6.9)
1374.1
1383.1
1946.7
1462.4
1499.0
182.0
182.0
128.0
166.6
176.6
SABVOH25 [Al(oꢀC6H4(CO2)2)(OH)]• Pnma
•(oꢀC6H4(COOH)2)
SABWAU25 [Al(oꢀC6H4(CO2)2)(OH)]•
Cc
4.0
•(H2O)
HAFQOW39 [Al(oꢀC6H4(CO2)2)(OH)]• P21/n
4.0×7×9.6
(6.4)
4.0×7×10
(6.5)
•(bdta)
HAFQUC39 [Al(oꢀC6H4(CO2)2)(OH)]• P21/n
•(mbdta)
Pores:
10.8×10.8×4.0 (7.8),
windows:
4.2, 7×3.6
16.2×4.4 (8.4),
1.8
[Narrow]37
[Large]37
[Al(oꢀC6H4(CO2)2)(OH)]
[Al(oꢀC6H4(CO2)2)(OH)]
C2/c
0.388
0.531
4505
3940
—
—
1893.5
1423.8
—
—
Imma
13.4×6.6 (9.4),
1.8
a Designations: bdta is 1,3,2ꢀbenzodithiazolyl radical, and mbdta is 5ꢀmethylꢀ1,3,2ꢀbenzodithiazolyl radical.
b The volumes of the unit cell (I) and guest (II) are given.

Liquidꢀphase adsorption on MILꢀ53(Al)
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
1045
MILꢀ53(Al) (structure [Large] in Table 3) has broad chanꢀ
nels 13.4×6.6 Å in size (average diameter 9.4 Å) and
narrow channels 1.8 Å in diameter. According to pubꢀ
lished data,6,17,29 the MILꢀ53(Al) framework contains
channels and windows with a diameter of 8.5 Å. An analyꢀ
sis of the results of studying the lowꢀtemperature adsorpꢀ
tion—desorption of nitrogen using the Horvath—Kawazoe
model31 indicates that the studied sample of MILꢀ53(Al)
contains pores 6.72 Å in diameter, which is well consisꢀ
tent with the earlier published44 pore sizes for MILꢀ53(Cr).
Evidently, the diameter of broad channels in
MILꢀ53(Al) is fairly large, and terephthalic acid moleꢀ
cules with an average diameter of 6.9 Å and a volume of
182.0 Å3 can be incorporated into the channels. Let us
consider the results of crystallochemical analysis of caviꢀ
ties of the MILꢀ53(Al) framework incorporating (see
Table 3) water molecules (structure [SABWAU]), terephꢀ
thalic acid molecules (structure [SABVOH]), 1,3,2ꢀbenzoꢀ
dithiazolyl radical (structure [HAFQOW]), and 5ꢀmethꢀ
ylꢀ1,3,2ꢀbenzodithiazolyl radical (structure [HAFQUC]).
It turned out that the pore volume of the framework in this
series increases from 0.418 to 0.707 cm3 g–1 due to the
adsorption deformation of the MILꢀ53(Al) structure
caused by the penetration of these molecules into the cavꢀ
ities. The latter value is significantly 33.2% higher than
the pore volume (0.531 cm3 g–1) of the expanded frameꢀ
work (structure [Large] in Table 3). Note that the values
of specific pore volume and specific surface determined
for the MILꢀ53(Al) structures taking into account the crysꢀ
tallographic data35 and presented in Table 3 significantly
exceed the corresponding values measured on the basis of
adsorption of inert gases (see above). This is due to the
fact that the Voronoi—Dirichlet method of polyhedra
based on an analysis XRD data takes into account a higher
value of the free volume in the structure of the solid than
the volume calculated from the data of adsorption meaꢀ
surements of porosity using adsorbate molecules.
Thus, the results of analysis of the structure of the
cavities in MILꢀ53(Al) using the TOPOS program packꢀ
age and preliminary calculations of linear sizes of the adꢀ
sorbate molecules studied in the present work, whose geꢀ
ometry was optimized by quantum chemical methods,
show that the studied aromatic compounds are capable of
penetrating into pores of the MILꢀ53(Al) metalꢀorganic
framework.
Dependence of Henry´s constant on the structure of
adsorptives. Now we can combine the results of crystalꢀ
lochemical analysis of the cavity structures in the metalꢀ
organic framework with the experimental data on adsorpꢀ
tion of aromatic compounds from an H2O—MeCN soluꢀ
tion on MILꢀ53(Al). As can be seen from Table 1, moleꢀ
cules of adsorptives containing two condensed aromatic
rings (adsorbates 6—10) are adsorbed on MILꢀ53(Al) subꢀ
stantially more strongly than analogs with one aromatic
ring (adsorptives 1—5 and 14—18). Aromatic compounds
with three rings in a molecule (adsorptives 11—13) are not
desorbed within an appropriate time of HPLC experiment,
which indicates an irreversible pattern of adsorption. Thus,
it follows from the data in Table 1 that the pores of the
adsorbent MILꢀ53(Al) should be accessible for all studied
adsorptives, regardless of the number of aromatic rings in
their molecules.
Indeed, the performed preliminary calculations of linꢀ
ear sizes of molecules of the adsorptives studied indicate
that the cavities of MILꢀ53(Al) can incorporate moleꢀ
cules of both monoꢀ and dicyclic aromatic compounds
with linear sizes of 6×4 and 7×5 Å, respectively, and
tricyclic adsorbates, whose dimensions (9×6 Å) are closꢀ
est to the pore diameter of the metalꢀorganic framework.
As a result, these adsorbates can form the maximum numꢀ
ber of contacts with the pore "walls," favoring a strong
interaction of the pore walls with adsorbed molecules. As
shown by the HPLC data, tricyclic adsorbates are adꢀ
sorbed more strongly on the metalꢀorganic framework than
other studied aromatic compounds.
Equal accessibility of cavities in the MILꢀ53(Al) strucꢀ
ture for molecules of monoꢀ, diꢀ, and tricyclic aromatic
compounds affects the pattern of the observed dependence
of Henry´s constant of adsorption K1,c of the studied subꢀ
stances on the number of aromatic rings in the adsorptive
(see Table 1). The value of K1,c increases regularly with an
increase in the number of aromatic rings in the molecule,
but the increase is noticeably faster than the increase in
the volume and surface area of the Voronoi—Dirichlet
molecular polyhedron for the studied compounds (see
Tables 1 and 2). The both latter parameters determine the
ability of adsorbate molecules to participate in the disperꢀ
sion interaction with hydrophobic cavities of the adsorꢀ
bent. For example, on going from benzene (adsorbate 1)
to naphthalene (adsorbate 6), VVDMP and SVDMP increase by
a factor of 1.3—1.4, respectively, from 128.08 to 181.66 Å3
and from 154.48 to 205.06 Å2, whereas K1,c increases by
a factor of 24: from 3.2 to 76.3 L m–2. Note that the
further same slight increase in VVDMP and SVDMP on going
from naphthalene (adsorbate 6) to phenanthrene (adsorꢀ
bate 11) results in irreversible adsorption of the latter on
MILꢀ53(Al). Thus, a substantial increase in K1,c with an
increase in the number of condensed rings in the adsorpꢀ
tive molecule is due to two factors. First, weak dispersion
adsorbate—adsorbent interactions are enhanced due to an
increase in the size of the molecule. Second, as additional
aromatic rings appear, the —ꢀinteractions between the
aromatic systems of the adsorbate molecules and terephꢀ
thalate ions forming "walls" of the hydrophobic cavities in
the MILꢀ53(Al) structure become stronger.45 An addiꢀ
tional argument in favor of the substantial role of —ꢀ
interactions adsorbate—adsorbent in adsorption by the
metalꢀorganic framework is a multiple increase in K1,c
observed on going from benzene (adsorbate 1) to allylbenzꢀ
ene (adsorbate 3). Transition from benzene to allylꢀ

1046
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
Saifutdinov et al.
benzene, like that from benzene to naphthalene and
phenanthrene, produces an increase in K1,c by an order of
magnitude (see Table 1) and an insignificant increase in
the surface area and volume of the Voronoi—Dirichlet
molecular polyhedron (see Table 2). This fact proves that
the —ꢀinteractions are the main factor leading to a stronger
adsorption of allylbenzene compared to that of benzene. It
cannot be excluded that —ꢀinteractions adsorbate—adꢀ
sorbent are not the only reason for a stronger adsorption of
diꢀ and tricyclic aromatic compounds compared to that of
monocyclic analogs. A compatibility between the shape of
the adsorbate molecule and that of the cavity of the
MILꢀ53(Al) metalꢀorganic framework also plays a subꢀ
stantial role.
The methyl radical exerts a strong effect on the adꢀ
sorption behavior of the studied substances on MILꢀ53(Al).
The introduction of the Me radical into an adsorbate molꢀ
ecule results in a more noticeable increase in K1,c than
changes in SVDMP and VVDMP (see Tables 1 and 2). On
going from benzene (adsorbate 1) to toluene (adsorbate 2),
from phenol (adsorbate 14) to oꢀcresol (adsorbate 15) and
anisole (adsorbate 17), and from pyrocatechol (adsorbate
16) to guaiacol (adsorbate 18), the value of K1,c increases
several times (see Table 1). For example, the values of K1,c
for toluene is about 5.4 times higher than that of benzene.
Probably, such a multiple enhancement of adsorption is
due to the appearance of the Me radical in the adsorbate
molecule are electronꢀdonor properties of Me, which inꢀ
crease the electron density in the aromatic fragment of the
molecules and thus enhance the —ꢀinteractions adsorꢀ
bate—adsorbent. In addition, the appearance of the Me
radical in the adsorbate molecule evidently favors an inꢀ
crease in the number of intermolecular contacts of the
adsorbate with the hydrophobic cavity of the MILꢀ53(Al)
framework, due to which the interaction of the adsorbate
molecule with the adsorbent cavity is enhanced.
Intermolecular interactions of adsorbate molecules in
solution can explain weak adsorption from an H2O—MeCN
solution on MILꢀ53(Al) observed for compounds, the
molecules of which contain sites able to a stronger specific
solvation by the components of an H2O—MeCN medium
(adsorbates 5, 8, 10, and 14) compared to their closest
analogs (adsorbates 1, 9, 6, and 17, respectively). These
interactions decrease the strength of adsorption in the hyꢀ
drophobic cavities of the metalꢀorganic framework.46 For
example, pyridine (adsorbate 5) can form a hydrogen bond
with the components of solution due to the unshared elecꢀ
tron pair of the nitrogen atoms and, this is the reason of
a weaker adsorption of pyridine compared to benzene (adꢀ
sorbate 1). It should be mentioned that thiophene (adꢀ
sorbate 7) is also somewhat weakly adsorbed from an
H2O—MeCN solution compared to benzene (adsorbate 1)
and naphthalene (adsorbate 6), respectively. In this case,
a weaker adsorption of the heterocyclic analogs compared
to the corresponding aromatic hydrocarbons is due to the
nonꢀspecific solvation of molecules of the heterocyclic
adsorbates by the components of an H2O—MeCN soluꢀ
tion because of orientational interactions. The data of preꢀ
liminary quantum chemical calculations indicate that molꢀ
ecules 4 and 7 have dipole moments. In the course of
adsorption on MILꢀ53(Al), aromatic hydrocarbons and
their sulfurꢀcontaining heterocyclic analogs are separated
with a high selectivity, which was not attained for adsorpꢀ
tion on the traditional adsorbents26—28 (see Table 1).
Thus, in the course of adsorption of the studied series
of aromatic compounds, the pores of the MILꢀ53(Al)
metalꢀorganic framework are equally accessible for moleꢀ
cules with different sizes; i.e., no exclusion effects are
observed in this case. For the adsorption of the studied
series of compounds from the polar liquid phase on the
MILꢀ53(Al) metalꢀorganic framework, the determining
factor is the ability of the adsorbate molecules to particiꢀ
pate in the —ꢀinteractions with aromatic fragments of
hydrophobic cavities of the adsorbent. In addition, the
presence of substituents increasing the number of interꢀ
molecular contacts with the "walls" of channels and pores
or, on the contrary, weakening adsorption due to interꢀ
molecular interactions with the components of solution
plays an important role.
Temperature effect on Henry´s constant of adsorption
of aromatic compounds on the MILꢀ53(Al) metalꢀorganic
framework. Under conditions of the liquidꢀphase adsorption
and HPLC, the temperature increase results, as a rule, in a
regular decrease in adsorption interactions on the adꢀ
sorbent surface.30,32,47 A minor number of endotherꢀ
mic processes is known in which adsorption is enhanced
with temperature. Note that for adsorption by the
MILꢀ53(Al) metalꢀorganic framework the temperature
change exerts different effects on Henry´s constant of adsorpꢀ
tion K1,c of the studied aromatic compounds (see Table 1).
This is due to the fact that the effect of temperature on the
strength of adsorption interactions depends on the nature
of the adsorbate. An unusual temperature dependence of
retention on the MILꢀ53(Al) adsorbent under the HPLC
conditions has been found previously,48 whereas other reꢀ
searchers49 established that retention on the considered
metalꢀorganic framework is weakened with temperature.
For the adsorbates with one aromatic ring in the moleꢀ
cule (adsorbates 1—5), K1,c increases with temperature.
This enhancement of adsorption with temperature is reꢀ
lated, most likely, to the expansion of pores6 in the
MILꢀ53(Al) metalꢀorganic framework due to the "breathꢀ
ing" effect of the structure.7,40 The compounds, molecules
of which contain two condensed aromatic rings (adsorꢀ
bates 6—8 and 10), are adsorbed somewhat more strongly
with the temperature increase from 313 to 323 K, whereas
their adsorption is weakened with the further temperature
increase to 333 K. A similar behavior of the diaromatic
compounds is probably due to the fact that the temperaꢀ
ture increase favors trend to desorption.

Liquidꢀphase adsorption on MILꢀ53(Al)
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 5, May, 2015
1047
In these cases, an increase in Henry´s constant with
temperature in the course of adsorption of some substances
on the MILꢀ53(Al) metalꢀorganic framework possibly inꢀ
dicates that the adsorption is endothermic. This is assoꢀ
ciated with the fact that energy consumed by desorption of
the solvent exceeds the energy released upon adsorption of
hydrophobic molecules. In this case, the driving force of
adsorption is evidently an increase in the entropy of the
adsorption system observed when solvent molecules are
desorbed.
A particular position is occupied by 1ꢀmethylindole
(adsorbate 9), which is characterized by a decrease in K1,c
with the temperature increase, and phenols (adsorbates
14—18), the adsorption of which is almost temperatureꢀ
independent. It can be assumed that some contribution to
the adsorption of phenols on MILꢀ53(Al) is made by the
interaction of hydroxyl groups of their molecules with the
Brönsted acid sites AlO4(OH)2 occupying the nodal posiꢀ
tions of the network of the metalꢀorganic framework. It is
known that the energy of hydrogen bond changes but
slightly with temperature and, therefore, adsorption of
phenols on the MILꢀ53(Al) sample is hardly sensitive to
a temperature change. The participation of the Brönsted
acid sites of the MILꢀ53(Al) framework in the interaction
with the hydroxyl groups of adsorbate molecules is also
indicated by the fact that guaiacol (adsorbate 18) is adꢀ
sorbed on this sample more strongly than anisole (adsorꢀ
bate 17), although a guaiacol molecule contains the hyꢀ
droxyl group capable of intermolecular interacting with
the polar liquid phase.
Thus, the unusual pattern of the temperature depenꢀ
dence of the strength of adsorption interactions of moleꢀ
cules of the studied compounds with the MILꢀ53(Al) samꢀ
ple characterizes the expansion of the initial system of
pores (channels) in the framework existing under the
liquidꢀphase conditions and the appearance of new adꢀ
sorption sites. In our opinion, this expansion can be due to
"swelling" of the framework network50 in the liquid phase,
which is not observed for the MILꢀ53(Al) structure in the
initial activated state.29 Adsorption of components of an
H2O—MeCN solution leads probably to the expansion of
accessible pores (channels) with the temperature change,
which favors the adsorption of such bulky molecules
as molecules of diꢀ and tricyclic aromatic adsorbates.
This assumption is confirmed by the data,21 according to
which the pore size in the MILꢀ53(Al) structure increases
after contact with an aqueous medium at different pH and
temperatures. It should be expected that wetting of the
metalꢀorganic framework sample on contact with an
H2O—MeCN medium can result in a more signifiꢀ
cant expansion of pores than that observed upon wetting
in pure water, since the hydrophobic cavities of the
phenylenecarboxylate frameworks51 should more efficientꢀ
ly be solvated by MeCN molecules containing the hydroꢀ
phobic fragment.
The adsorption activity of the metalꢀorganic frameꢀ
work MILꢀ53(Al) is determined by its elasticity, i.e., the
capability of the structure of "breathing" which the temꢀ
perature change. The accessibility of the system of pores
(channels) of this metalꢀorganic framework depends on
the temperature of the medium and on the size and form
of adsorbates. The adsorption selectivity of MILꢀ53(Al)
towards the studied series of organic compounds is due to
—ꢀinteractions of the aromatic systems of adsorbate
molecules with terephthalate ions and the possibility of
interaction of the adsorbate molecules with the Brönsted
acid sites of the metalꢀorganic framework. The metalꢀ
organic framework MILꢀ53(Al) shows a high adsorption
selectivity towards compounds differing in the number of
aromatic rings in the molecule, and in the presence of the
Me substituent. The selectivity is also shown towards aroꢀ
matic hydrocarbons and their sulfurꢀcontaining heteroꢀ
cyclic analogs. No such selectivity is observed for adsorpꢀ
tion on the convertional carbon and polymer materials.
Irreversible adsorption on the MILꢀ53(Al) adsorbent of
tricyclic aromatic compounds makes it possible to use this
adsorbent, for example, for the decontamination of oil
fractions and petroleum products from heavy hardly reꢀ
movable components.
The MILꢀ53(Al) sample was synthesized at the N. D.
Zelinsky Institute of Organic Chemistry, Russian Academy
of Sciences, the physicochemical and adsorption properties
of the metalꢀorganic framework were studied at the Samara
State Technical University, and the porous structure of
MILꢀ53(Al) was simulated in terms of the Voronoi—Diꢀ
rechlet concept of polyhedra at the Samara State University.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 14ꢀ03ꢀ31979ꢀ
mol_a and 14ꢀ03ꢀ97077ꢀr_povolzh´e_a) and the Ministry
of Education and Science of the Russian Federation in
terms of the basic part of the state task (Project No. 1288).
In addition, B. R. Saifutdinov thanks the Council on
Grants at the President of the Russian Federation for
the research grant of the President of the Russian Federaꢀ
tion to young scientists and postꢀgraduates (Project
No. SPꢀ134.2015.1). V. I. Isaeva and L. M. Kustov are
grateful to the Russian Science Foundation for the finanꢀ
cial support (Project No. 14ꢀ50ꢀ00126). E. V. Alexandrov
also thanks the Government of the Russian Federation for
financial support in terms of the megagrant (Project
No. 14.V25.31.0005).
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