Separation of polar compounds using a flexible metal–organic framework

Abstract

A flexible metal-organic framework constructed from a flexible linker is shown to possess the capability of separating mixtures of polar compounds (propanol isomers) by exploiting the differences in the saturation capacities of the constituents. Transient breakthrough simulations show that these sorption-based separations are in favor of the component with higher saturation capacity.

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As featured in:
See R. K. Motkuri, P. K. Thallapally,
R. Krishna et al., Chem. Commun.,
2015, 51, 8421.
Showcasing collaborative research from the Laboratory of
Dr Radha Kishan Motkuri et al. at the Pacifi c Northwest National
Laboratory, Richland, USA and Prof. Rajamani Krishna at the
University of Amsterdam, The Netherlands
Separation of polar compounds using a fl exible metal–organic
framework
Inspired by the need of energy reduction in the area of separation
and purifi cation of organic liquid mixtures, we developed a
fl exible metal–organic framework that was shown to possess
the separation capability of polar compounds such as alcohols
and ketones, specifi cally propanol isomers. The experimental and
simulations studies revealed that sorption-based separations are
in favor for the component with higher saturation capacity.
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Cite this: Chem. Commun., 2015,
51,8421
Separation of polar compounds using a flexible
metal–organic framework†
Radha Kishan Motkuri,*
a
Praveen K. Thallapally,*
b
Harsha V. R. Annapureddy,
b
Liem X. Dang,
b
Rajamani Krishna,*
c
Satish K. Nune,
a
Carlos A. Fernandez,
a
Jian Liu
a
and B. Peter McGrail
a
A flexible metal–organic framework constructed from a flexible linker is
shown to possess the capability of separating mixtures of polar com-
pounds (propanol isomers) by exploiting the differences in the saturation
capacities of the constituents. Transient breakthrough simulations show
that these sorption-based separations are in favor of the component
with higher saturation capacity.
Separation and purification of organic liquid isomers are scientifi-
cally important industrial technologies and have received consider-
able attention worldwide.
1
Distillation is clearly the dominating
separation process, accounting for more applications than all the
other chemical-separation processes combined. In fact, distillation
columns consume more than 50% of the total energy used in the
chemical industry worldwide. Even more challenging is separation
of an azeotrope mixture that forms when certain compositions of
liquid isomers are present by weight. Specifically, the separation
of water, alcohols, and ketones is often made difficult because of
azeotrope formation. Separating these mixtures using fractional
distillation or using polymeric membranes is energy-intensive and
is highly complex.
2
Alternatively, these processes sometimes require
the addition of separating agents, called entrainers, that alter
the vapor/liquid equilibrium in a favorable manner to achieve the
desired separation, but the recovery of such entrainers later in the
process not only requires an additional distillation step but also
incurs an increased overall energy penalty. The largest opportunities
for energy reduction in this area are offered by replacing distillation
or membrane-based separations by low-cost adsorption-based
systems. The success of such replacement strategies is crucially
dependent on the development of suitable adsorbents, but there
is very limited information available on adsorption-based separa-
tion of azeotropes using porous media.
Recent developments in porous metal–organic frameworks
(MOFs) have gained much attention because of the outstanding
properties and ability to fine tune the pore apertures and high
stability towards the desired application.
3,4
Such remarkable
properties of MOFs make them an interesting class of materials
for adsorption,
5
and separation applications.
6
Specifically, studies
of the gas separation are extensively reported in the literature;
however, very limited information is available on the separation of
polar molecules, including azeotropic mixtures. For example,
Denayer et al. used highly stable zeolitic imidazole frameworks
(ZIF-8, ZIF-68) to separate butanol from aqueous mixtures in the
presence of organic contaminants like ethanol.
7
Jie Zhang et al.
reported the separation of alcohol and water mixtures using a
charge-polarized MOF that shows selectivity towards polar mole-
cules under an electric field gradient.
8
Similarly, Kitagawa et al.
synthesized a copper-based coordination polymer that selectively
adsorbs methanol and water from bioethanol.
9
Mostofthese
studies focused on purifying bioethanol, but there are few reports
that focus on the separation of mixtures of alcohols and other polar
molecules such as chloroform and acetone.
6g,10
Our experimental
adsorption studies coupled with transient breakthrough simulations
confirm the separation of propanol isomers and various azeotropes.
To our knowledge, this is the first report on the separation of
mixtures of propanol isomers and other binary mixtures containing
alcohols, chloroform, and acetone using flexible MOFs.
TetZB, the flexible porous framework used in this communica-
tion, was synthesized using a flexible tetrahedral organic linker,
tetrakis[4-(carboxyphenyl)-oxamethyl]methane 1(Scheme S1, ESI†),
and then was used effectively for the sorption and separation of
polar solvents. The synthesis method and associated sorption
properties of TetZB were reported by us previously.
11
For this study,
we chose adsorption experiments of polar solvents such as C1–C3
alcohols, water, acetone, chloroform, and benzene, respectively.
a
Energy and Environment Directorate, Pacific Northwest National Laboratory (PNNL),
Richland, WA 99352, USA. E-mail: radhakishan.motkuri@pnnl.gov
b
Fundamental and Computational Sciences Directorate, PNNL, Richland, WA 99352,
USA. E-mail: praveen.thallapally@pnnl.gov
c
Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam,
The Netherlands. E-mail: r.krishna@contact.uva.nl
†Electronic supplementary information (ESI) available: (a) Material synthesis,
characterization; (b) pure component isotherms and dual-Langmuir–Freundlich
models; (c) adsorption energy calculations; (d) GCMC simulation studies; (e) IAST
calculations; (f) transient breakthrough simulation methodology; and (g) video
animations for transient breakthroughs of several binary mixture separations. See
DOI: 10.1039/c5cc00113g
Received 6th January 2015,
Accepted 19th February 2015
DOI: 10.1039/c5cc00113g
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Experimentally measured vapor sorption capacities were obtained
using an Intelligent Gravimetric Analyzer (IGA) from Hiden Instru-
ments. The TetZB sample was activated at 473 K under dynamic
vacuum before sorption studies. To evaluate the separation efficiency
of TetZB, we initially considered 1-propanol/2-propanol isomers for
the sorption studies. After sample activation, the MOF sample was
exposed to 1-propanol vapors and the sorption behavior was plotted
against pressure. The adsorption curve shows a sudden increase in
uptake at a relative pressure (P/P
0
)of0.15reachingthefirstsatura-
tion capacity of 7.8 wt%. At a relative pressure (P/P
0
) of 0.27, TetZB
shows another step adsorption reaching the second saturation
capacity of 26 wt% or 4.55 mmol g
1
(Fig. 1). Such two-step
adsorption was observed in TetZB with other gases/vapors and was
shown to expand and contract the framework upon guest removal
and re-adsorption of the same or different guest molecule. The
flexibility arises from the twisting of benzoate moieties around the
central quaternary carbon atom through ethereal links of the tetra-
hedral building block, which result from diverse ligand geometries
such as tetrahedral, irregular, or near-flattened. Such building block
flexibility has been observed both by us and other researchers. The
desorption curve does not follow the adsorption, rather, it shows a
sudden decrease in the sorption capacity at a P/P
0
ratio of 0.03
(Fig. S1, ESI†). Similarly, when a freshly activated MOF sample was
exposed to 2-propanol vapors, the first uptake isotherm reached its
first plateau at P/P
0
= 0.3; which was followed by a step adsorption
with approximately 2.5 times higher capacity (25 wt%, 4.1 mmol g
1
at P/P
0
= 0.8), and then the saturation point was reached. Another
significant difference between these two sorption isotherms is the
rates at which they sorb onto the TetZB framework. Sorption profiles
indicate that both propanol isomers can enter the pores of TetZB,
but 1-propanol with its kinetic diameter of 4.7 Å has slightly higher
uptake when compared to 2-propanol with the same kinetic dia-
meter. This may be attributed to the flexibility of 1-propanol, which
is a linear chain that can enter the pore more easily than a branched
isomer. The density functional theory (DFT) estimated dipole
moment value of 2-propanol is slightly higher (1.56D) than that of
1-propanol (1.49D), which shows that the 2-propanol molecule is
more likely to be polarized by the TetZB pore structure, thus having a
sharper uptake at relatively low pressure compared to 1-propanol.
12
To gain further insights into sorption behavior, we performed
grand canonical Monte Carlo (GCMC) simulations using the MuSic
program where the simulation box consisted of one unit cell of
MOF and the periodic boundary conditions were used in all three
dimensions.
13
Because the host framework considered has a rigid
structure, the breathing phenomenon was not observed, but the
overall solvent uptake matched the experimental results at 25 1C
(Fig. S6, ESI†). In agreement with experimental results, the simula-
tions of the 2-propanol sorption curve appear to be steeper at lower
loadings when compared to 1-propanol. To understand this
behavior, we computed the interaction energies between the
TetZB framework and propanol isomers as a function of loading.
The simulated results clearly showed more negative interaction
energies for 2-propanol when compared to 1-propanol at lower
loadings (Fig. S6, ESI†), but the overall uptake is slightly higher
for 1-propanol. These intriguing experimental and simulation
results suggest that vapor sorption experiments of 1-propanol
and 2-propanol using the flexible TetZB have potential for
separating propanol isomers, which motivated us to undertake
further IAST breakthrough simulations (Fig. 2).
We then focused our attention on lower chain alcohols such
as methanol and ethanol. The sorption isotherm of methanol
shows a sudden increase at a P/P
0
ratio of 0.18 and then reaches
Fig. 1 Adsorption isotherms of alcohol adsorbents and water in TetZB at
298 K.
Fig. 2 Adsorption and desorption isotherms of 1-/2-propanol in TetZB
(top) and the corresponding transient breakthrough simulation character-
istics of an adsorber packed with TetZB for separation of 50/50 mixtures of
1-propanol from 2-propanol (bottom).
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27 wt% at a P/P
0
ratio of 0.8. For ethanol after the first uptake
at low relative pressure, the isotherm reaches its first plateau
(7.8 wt% at a P/P
0
ratio of 0.12) followed by a step adsorption
with approximately four times higher capacity of ethanol
(25.7 wt%, 6 mmol g
1
at P/P
0
= 0.25). Because of the hydro-
phobic nature of the TetZB framework, water sorption studies
show a low uptake until the P/P
0
ratio reaches 0.7, and
the isotherm does not reach saturation even at a P/P
0
ratio of
0.95. The saturated loadings of alcohols decrease as the size
increases from methanol to propanol because adsorption near
saturation is mainly directed by the entropic (size) effect as
fewer propanol molecules can be adsorbed compared to methanol
(Fig. S2, ESI†). Furthermore, interestingly when TetZB is exposed to
acetone vapors, the first plateau is reached at a very low relative
pressure (P/P
0
= 0.05). Chloroform and benzene show a type-I
isotherm that exhibits significant uptakes at low vapor pressures.
The distinct behaviors of the solvent molecules with the host
framework definitely reveal potential for application in separation
technologies that should be studied.
To investigate the separation potential of TetZB, the experi-
mentally measured loadings of 1- and 2-propanols, methanol,
ethanol, acetone, benzene, chloroform, and water were fitted
with the dual-site Langmuir–Freundlich model, and the fits are
excellent over the entire range of pressures. The details of
simulation methodology and the breakthrough simulations
using IAST calculations are outlined in the ESI.†The transient
breakthrough simulations suggest that TetZB has the potential to
separate mixtures of alcohols by differentiating on the basis of chain
length and conformation as can be observed for 1-propanol/
2-propanol mixtures (Fig. S7–S9, ESI†). The separation of
1-propanol from 2-propanol is governed by molecular packing
effects that favor the adsorption of the linear alcohol when
operating under conditions corresponding to pore saturation.
The better packing efficiency of 1-propanol is reflected in its
higher saturation capacity compared to 2-propanol. It is impor-
tant to note that this separation is not dictated by differences in
binding energies that are higher for 2-propanol (Fig. S6b, ESI†).
For other mixtures of 1-alcohols, in the Henry regime, at
pressures below 1 kPa, selectivity favors alcohols with longer
chain lengths; however, at pressures above 10 kPa, selectivity
favors alcohols with shorter chain lengths. This is because of
the higher saturation capacity of the shorter chain alcohols.
The IAST calculations also imply that sharp separations of
alcohol mixtures are possible using TetZB provided the operating
pressures are greater than 10 kPa. This is confirmed in the
transient breakthrough simulations presented for 50/50 mixtures
of methanol/ethanol, ethanol/1-propanol, and ethanol/2-propanol
at a total pressure of 100 kPa (Fig. 3, Fig. S9–S12, ESI†). It is
interesting to compare the separations of TetZB with those
obtained with ZIF-8 and CHA zeolites. The shorter chain alcohol
is eluted later than the longer chain alcohol, which is in agreement
with the corresponding results for other microporous materials
such as SAPO-34, and ZIF-8 reported previously.
7b,14
Comparisons
of ethanol/1-propanol adsorption selectivity, and uptake capacity of
ethanol for equimolar ethanol/1-propanol mixtures in TetZB, ZIF-8,
and CHA zeolites are shown in Fig. S10 (ESI†). We note that TetZB
has both higher selectivity and higher uptake capacity, making it
more suitable for separation of mixtures of 1-alcohols (Fig. S11–S16,
ESI†). Fig. 3f shows the separations of water–ethanol mixtures of
azeotropic composition using TetZB. The separation is selective to
water that has the higher saturation capacity; similar water-selective
separations, achieved as a result of molecular packing effects, have
been reported for CuBTC.
15
The methodology adopted for the
breakthrough simulations is provided in the ESI.†Also available
in ESI†are seven video animations of the breakthroughs.
The experimental and modeling sorption analysis shows
that the hydrophobic –CH
2
and aryl groups of tectonic acid
and phenyl groups of the 4,40-bipyridine molecules are exposed
inside the pore, thereby creating a hydrophobic environment.
11
To illustrate such an environment, we painted all the hydrophobic
groups in green and the hydrophilic groups in red where it is clearly
evident that hydrophobic groups dominate the surface of the pore
(Scheme S1, Fig. S6, ESI†). The metal atoms and the carboxylate
groups that are more hydrophilic are buried deep inside and are
not easily accessible to the guest molecules. Polar alcohol, such as
methanol and ethanol, molecules consisting of hydrophobic and
Fig. 3 Transient breakthrough simulation characteristics of an adsorber
packed with TetZB for separation of ethanol from various solvent mixtures
of (a) methanol, (b) 1-propanol, (c) 2-propanol, (d) chloroform, (e) benzene,
(f) water at 298 K. The total pressure is 100 kPa.
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hydrophilic groups interact favorably with pore components,
leading to higher uptake rates with high adsorption energies.
To our knowledge, this is the first report of the separation of
propanol isomers, mixtures of 1-alcohols with acetone, and
chloroform ketones using MOFs.
In conclusion, we reported that hydrophobic TetZB, a flexible
metal organic framework generated from a flexible tetrahedral
building block, shows remarkable affinity and separation capability
of alcohols and ketones, specifically separation of propanol iso-
mers. If the operating conditions are chosen such that pore
saturation is achieved, separation using TetZB strongly favors the
component with the higher saturation capacity. For mixtures of
alcohols, the separation is selective for the alcohols with the shorter
chain length. For separation of water–alcohol mixtures, the separa-
tion favors water. Of particular interest is the separation of azeo-
tropic water–ethanol mixtures; see Fig. 3(f).
This work was performed at the Pacific Northwest National
Laboratory (PNNL) and was supported by the U.S. Department
of Energy (DOE). L.X.D. acknowledges funding from the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences, and
Biosciences. PNNL is operated by Battelle for the U.S. Depart-
ment of Energy under Contract DE-AC05-76RL01830.
Notes and references
1 Industrial Technologies Program; US-DOE EERE, 2005, 14.
2 F. P. Mccandless and W. B. Downs, J. Membr. Sci., 1987, 30, 111–116.
3(a) O. K. Farha, C. E. Wilmer, I. Eryazici, B. G. Hauser, P. A. Parilla,
K. O’Neill, A. A. Sarjeant, S. T. Nguyen, R. Q. Snurr and J. T. Hupp,
J. Am. Chem. Soc., 2012, 134, 9860–9863; (b) M. Eddaoudi, J. Kim,
N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe and O. M. Yaghi, Science,
2002, 295, 469–472; (c) H. X. Deng, S. Grunder, K. E. Cordova,
C. Valente, H. Furukawa, M. Hmadeh, F. Gandara, A. C. Whalley,
Z.Liu,S.Asahina,H.Kazumori,M.O’Keeffe,O.Terasaki,J.F.Stoddart
andO.M.Yaghi,Science, 2012, 336, 1018–1023; (d) G. Ferey, Chem. Soc.
Rev., 2008, 37,191–214;(e)N.StockandS.Biswas,Chem. Rev., 2012, 112,
933–969; ( f) D. Banerjee, A. J. Cains, J. Liu, R. K. Motkuri, S. K. Nune,
C. A. Fernandez, R. Krishna, D. M. Strachan and P. K. Thallapally, Acc.
Chem. Res., 2015, 48, 211–219.
4(a) J. R. Li, J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869–932;
(b) H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi,
Science, 2013, 341, 1230444; (c) T. Li, M. T. Kozlowski, E. A. Doud,
M. N. Blakely and N. L. Rosi, J. Am. Chem. Soc., 2013, 135, 11688–11691;
(d) R. Haldar, S. K. Reddy, V. M. Suresh, S. Mohapatra,
S. Balasubramanian and T. K. Maji, Chem. – Eur. J., 2014, 20,
4347–4356; (e) R. K. Motkuri, J. Liu, C. A. Fernandez, S. K. Nune,
P. Thallapally and B. P. Mcgrail, Industrial Catalysis and Separations,
Apple Academic Press, 2014, pp. 61–103.
5 H. T. Kwon and H. K. Jeong, J. Am. Chem. Soc., 2013, 135, 10763–10768.
6(a) R. K. Motkuri, H. V. R. Annapureddy, M. Vijaykumar,
H. T. Schaef, P. F. Martin, B. P. McGrail, L. X. Dang, R. Krishna
and P. K. Thallapally, Nat. Commun., 2014, 5, 4368; (b) Z. Y. Gu and
X. P. Yan, Angew. Chem., Int. Ed., 2010, 49, 1477–1480; (c) Z. R. Herm,
B. M. Wiers, J. A. Mason, J. M. van Baten, M. R. Hudson, P. Zajdel,
C. M. Brown, N. Masciocchi, R. Krishna and J. R. Long, Science, 2013,
340, 960–964; (d) M. Latroche, S. Surble, C. Serre, C. Mellot-
Draznieks, P. L. Llewellyn, J. H. Lee, J. S. Chang, S. H. Jhung and
G. Ferey, Angew. Chem., Int. Ed., 2006, 45, 8227–8231; (e) Z. Y. Gu,
C. X. Yang, N. Chang and X. P. Yan, Acc. Chem. Res., 2012, 45,
734–745; ( f) L. C. Lin, J. Kim, X. Q. Kong, E. Scott, T. M. McDonald,
J. R. Long, J. A. Reimer and B. Smit, Angew. Chem., Int. Ed., 2013, 52,
4410–4413; (g) Z. J. Lin, R. Q. Zou, J. Liang, W. Xia, D. G. Xia,
Y. X. Wang, J. H. Lin, T. L. Hu, Q. Chen, X. D. Wang, Y. S. Zhao and
A. K. Burrell, J. Mater. Chem., 2012, 22, 7813–7818; (h) P. Pachfule,
Y. F. Chen, J. W. Jiang and R. Banerjee, Chem. – Eur. J., 2012, 18,
688–694; (i) N. Chang, Z. Y. Gu and X. P. Yan, J. Am. Chem. Soc., 2010,
132, 13645–13647; ( j) R. Krishna, Phys. Chem. Chem. Phys., 2015, 17,
39–59.
7(a) S. Van der Perre, T. Van Assche, B. Bozbiyik, J. Lannoeye, D. E. De
Vos, G. V. Baron and J. F. M. Denayer, Langmuir, 2014, 30,
8416–8424; (b) J. C. Saint Remi, T. Remy, V. Van Hunskerken,
S. van de Perre, T. Duerinck, M. Maes, D. De Vos, E. Gobechiya,
C. E. A. Kirschhock, G. V. Baron and J. F. M. Denayer, ChemSusChem,
2011, 4, 1074–1077.
8 J. K. Sun, M. Ji, C. Chen, W. G. Wang, P. Wang, R. P. Chen and
J. Zhang, Chem. Commun., 2013, 49, 1624–1626.
9 A. Shigematsu, T. Yamada and H. Kitagawa, J. Am. Chem. Soc., 2012,
134, 13145–13147.
10 (a) X. F. Zheng, L. Zhou, Y. M. Huang, C. G. Wang, J. G. Duan,
L. L. Wen, Z. F. Tian and D. F. Li, J. Mater. Chem. A, 2014, 2,
12413–12422; (b) K. Zhang, L. L. Zhang and J. W. Jiang, J. Phys. Chem.
C,2013,117, 25628–25635; (c) T. Borjigin, F. X. Sun, J. L. Zhang,
K. Cai, H. Ren and G. S. Zhu, Chem. Commun.,2012,48, 7613–7615.
11 (a)P.K.Thallapally,J.Tian,M.R.Kishan,C.A.Fernandez,
S.J.Dalgarno,P.B.McGrail,J.E.WarrenandJ.L.Atwood,J. Am. Chem.
Soc., 2008, 130, 16842–16843; (b)R.K.Motkuri,P.K.Thallapally,
S.K.Nune,C.A.Fernandez,B.P.McGrailandJ.L.Atwood,Chem.
Commun., 2011, 47, 7077–7079.
12 M. Sadakiyo, T. Yamada and H. Kitagawa, J. Am. Chem. Soc., 2011,
133, 11050–11053.
13 A. Gupta, S. Chempath, M. J. Sanborn, L. A. Clark and R. Q. Snurr,
Mol. Simul., 2003, 29, 29–46.
14 (a)R.KrishnaandJ.M.vanBaten,Sep. Purif. Technol., 2011, 76, 325–330;
(b)T.Remy,J.C.SaintRemi,R.Singh,P.A.Webley,G.V.Baronand
J. F. M. Denayer, J. Phys. Chem. C, 2011, 115, 8117–8125; (c)R.Krishna,
Microporous Mesoporous Mater., 2014, 185, 30–50.
15 J. J. Gutierrez-Sevillano, S. Calero and R. Krishna, J. Phys. Chem. C,
2015, 119, 3658–3666.
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