Tuning the adsorption behaviors of water, methanol, and ethanol in a porous material by varying the flexibility of substituted groups

Article abstract of DOI:10.1039/c6dt00658b

Exploring the adsorption and separation of water, methanol, and ethanol is important concerning the use of a sustainable energy source from biofuel. In this paper, the effects of the flexibility of substituted groups have been studied based on three iso-reticular metal-organic frameworks (MOFs), in which the pore surface is decorated with propargyl (-CH₂-C≡CH), allyl (-CH₂-CH=CH₂), and propyl (-CH₂-CH₂-CH₃) groups respectively. These substituted groups stretch into the channel, acting as gates, and the gate-opening for guests is controlled by the flexibility as well as host-guest interactions. Our study results indicate that (i) the adsorption capacity of water, methanol and ethanol enhances accordingly with the increase of the flexibility of substituted groups; (ii) the adsorptive discrimination of water, methanol, and ethanol on this porous sorbent could be tuned by varying the substituted groups.

Full text of DOI:10.1039/c6dt00658b

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Tuning the adsorption behaviors of water,
methanol, and ethanol in a porous material by
Cite this: DOI: 10.1039/c6dt00658b
varying the flexibility of substituted groups†
Received 18th February 2016,
Accepted 1st April 2016
Yunfei Sha,‡a Shizhe Bai,‡b Jiaying Lou,a Da Wu,*a Baizhan Liua and Yun Ling*b
DOI: 10.1039/c6dt00658b
www.rsc.org/dalton
Exploring the adsorption and separation of water, methanol, and opening adsorption caused by the intrinsic framework
ethanol is important concerning the use of a sustainable energy flexibility,11–14 in which the host structure can show a dynamic
source from biofuel. In this paper, the effects of the flexibility of response to the guests, showing an abrupt increase of uptake
substituted groups have been studied based on three iso-reticular amount when the structure changes from a closed pore to an
metal–organic frameworks (MOFs), in which the pore surface is open one at a specific pressure. This guest-dependent un-
decorated with propargyl (–CH2–CuCH), allyl (–CH2–CHvCH2), conventional sorption behavior15 makes MOFs more promising
and propyl (–CH2–CH2–CH3) groups respectively. These substituted for selective adsorption of guests whose chemical properties
groups stretch into the channel, acting as gates, and the gate- are similar to each other. Along this way, adsorption behaviors
opening for guests is controlled by the flexibility as well as host– of water, methanol and ethanol have been studied recently, in
guest interactions. Our study results indicate that (i) the adsorption which gate-opening adsorption and highly selective separation
capacity of water, methanol and ethanol enhances accordingly with have been reported.16–29 Despite these achievements, rational
the increase of the flexibility of substituted groups; (ii) the adsorp- tuning of their adsorption behaviors, which is important for
tive discrimination of water, methanol, and ethanol on this porous achieving high adsorption capacity and selectivity to meet
sorbent could be tuned by varying the substituted groups.
specific separation needs, is still a great challenge. In addition,
this reported discriminable adsorption on MOFs could be cate-
gorized by host framework breathing,16–18 subnetwork
displacement,19–21 and framework swelling22–25 according to
the classification of different flexibility modes of MOFs.11,30,31
The effect of flexible substituted groups on the scaffold of
rigid host frameworks, which is one of the flexibility modes of
MOFs, has rarely been studied.32–36
In this communication, we aimed to explore the effects of
the flexibility of substituted groups in a MOF structure on the
adsorption and separation of water, methanol, and ethanol
(Scheme 1). Based on 5-hydroxy-1,3-dicarboxylate benzoic acid,
its derivatives with the same number of carbon atoms but pre-
senting different flexibilities are designed (ESI†), which are
5-(prop-2-yn-1-yloxy)isophthalic acid (H2L1), 5-(allyloxy)iso-
phthalic acid (H2L2), and 5-propoxy-isophthalic acid (H2L3).
Due to the different types of terminal C–C bonds, these carbon
chains display different flexibilities in the order of
H2L1 < H2L2 < H2L3 defined by the number of rotatable σ bonds.
The hydrothermal reaction of the H2L1 ligand with copper
nitrate in the solution of ethanol and water at 90 °C for three
days gives crystal products of [Cu(L1)H2O] (Cu-L1, ESI†).
Single-crystal X-ray diffraction analysis indicates that Cu-L1
Separation of water, methanol, and ethanol is the key process
concerning the use of a sustainable energy source from
biofuel.1,2 Owing to the large energy demands in distillation,
separation by adsorption is regarded as an energetically
efficient, environmentally friendly, and economically competi-
tive approach.3–5 However, selective sorption by traditional
adsorbents is difficult because the chemical properties of
water, methanol and ethanol are similar to each other. In this
context, the development of novel porous materials with
significantly improved adsorption and separation capability is
an extremely important aspect.
As a new member of sorbent materials, metal–organic
frameworks (MOFs, also known as porous coordination poly-
mers, PCPs) have received considerable interest.6–10 Their
unique feature, compared with other sorbents, is the gate-
aTechnical Center, Shanghai Tobacco Group Co., Ltd., Shanghai 200082, China.
E-mail: wud@sh.tobacco.com.cn
bShanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Fudan University, Shanghai, 200433, China.
E-mail: yunling@fudan.edu.cn
†Electronic supplementary information (ESI) available: Synthesis, measure-
ˉ
crystallizes in a trigonal system, P3m1 space group (Fig. S1 and
ments, CIF data, figures for PXRD, TGA, etc. CCDC 1452219. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c6dt00658b
‡These authors contributed equally to this paper.
Table S1†). It is built of L1 ligands and paddle-wheel units, the
same as that in HKUST-1.37 Three paddle-wheel units are con-
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of the neck is around 2.3 Å, which is small enough to block other
guests outside if coordinated water molecules remain (Fig. 1d).
By the replacement of H2L1 with H2L2 and H2L3 in the
reaction process respectively, crystal products of Cu-L2 and Cu-
L3 have been obtained accordingly. Although isolation of
detectable crystals failed, powder X-ray diffraction (PXRD)
measurements revealed that Cu-L2 and Cu-L3 have the same
diffraction peaks at 2θ = 5.5, 9.5, 11.0°, etc., as that of Cu-L1,
indicating that they are iso-structures with similar cell para-
meters (Fig. 2a). FT-IR spectra (Fig. 2b) show similar vibration
bands in the range of 1750 to 400 cm−1 as well as character-
istic vibration bands of C–H stretching for different terminal
substituted groups. For Cu-L3, the multi-bands in the range of
2900–2700 cm−1 are the typical vibration bands of alkane C–H,
while for Cu-L2 and Cu-L1, it shows the typical vibration band
of alkene C–H at about 3050 cm−1, and alkyne C–H at
3300 cm−1 respectively, which confirms the presence of
different substituted carbon chains. Thermogravimetric ana-
lyses under N2 flow (Fig. S2†) and variable-temperature PXRD
(Fig. S3†) were also performed to assess the framework
thermal stability. These results suggest an enhanced frame-
work thermal stability with the increase of the flexibility of
substituted groups from Cu-L1 to Cu-L3.39 The enhanced
thermal stability might be related to the increase of the flexi-
bility of substituted groups, which exhaust more thermal
energy by molecular vibration.
Scheme 1 A view to present the strategy for studying the effects of the
flexibility of substituted groups on the adsorption and separation.
nected to each other via three L1 ligands, forming an approxi-
mately calixarene-like motif (Fig. 1a), in which three paddle-
wheel units are at the narrow end, and the wide end is delim-
ited by three propargyl groups. It should be mentioned that
the propargyl group on the phenyl ring is in a disordered state
by symmetry over two positions because of an equal prob-
ability of its location. This motif is then extended into a 2D
layered structure by alternating the motif in up–down–up
orientations through the connection of the paddle-wheel units
and L1 ligands (Fig. 1b). This host framework is similar to the
previously reported MOF structure.38 There are two different
void spaces in Cu-L1. One is the hydrophobic hexagonal
channel with alternating large cages and small necks formed
by six SBUs with six ligands surrounded by six substituted
groups, three of which point down and the other three point
up. The size of a small neck (formed by three substituted
groups) is around 3.3 Å (Fig. 1c). The other one is the triangu-
lar hydrophilic channel formed by the packing of the motifs
with three SBUs at the narrow end from adjacent layers. The size
Owing to the relatively low thermal stability of Cu-L1, as well
as excluding the uncertain effects of the small channel (Fig. 1d)
on adsorption, all compounds are activated at 100 °C (ESI†).
Single-component vapor-phase adsorption of water, metha-
nol and ethanol at 298 K was then respectively carried out on
the three iso-structures (ESI†). Their absorption isotherms
show that both the uptake amount and sorption isotherm of
water, methanol, and ethanol change greatly when substituted
groups are changed (Fig. 3a–c). For methanol, a characteristic
gate-opening adsorption isotherm is observed on Cu-L1, which
Fig. 1 (a) The calixarene-like motif in Cu-L1; (b) the 3D packing structure of Cu-L1 viewed along the c axis, showing a hexagonal pattern (the blue
surface: calculated Connolly surface with the radius of 1.4 Å for the probe atom. The window size for the hexagonal pattern is ∼3.3 Å formed by
three substituted groups, and the window size for the triangular channel is ∼2.3 Å formed by three paddle-wheel units with coordinated water mole-
cules); (c) the cage structure formed by six substituted groups, three of which point up and three of which point down (the radius of the inserted
yellow ball: 5 Å); (d) the channel structure in Cu-L1 (the radius of the inserted green cylinder: 2.5 Å).
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P/P0 = 0.90, respectively. For ethanol, the uptake amount
increases linearly and the capacity is only 0.085 mmol g−1 at P/P0
= 0.90 on Cu-L1, indicating negative adsorption. However, it
reaches 1.9 and 2.5 mmol g−1 on Cu-L2 and Cu-L3, respectively.
For water, the uptake amount increases nearly linearly with the
increase of pressure on Cu-L1 and Cu-L2, showing a low capacity
of 0.5 and 1.0 mmol g−1 at P/P0 = 0.95. While on Cu-L3, it shows
a type-I like sorption isotherm with an uptake amount of
3.7 mmol g−1 at P/P0 = 0.95.
According to the structural analysis, there are three substi-
tuted groups that point up and other three point down,
forming small necks for the cage. Therefore, these substituted
groups will greatly affect the adsorption behavior. In Cu-L1,
the relatively rigid propargyl groups form a small neck of d =
3.3 Å (Fig. 2b). It can prevent methanol (dkinetic = 3.626 Å) to be
adsorbed into a channel. Therefore, it shows a low uptake
amount in the low pressure region. While, at high pressure,
methanol adsorption occurs, giving a gate-opening sorption
isotherm. This behavior might be ascribed to the vibration of
the propargyl groups through the rotation of the C–O σ bond
owing to the hydrogen bonding interaction between the
methanol guest and host framework. Thus, it makes the
pore size large enough to permit methanol in. On the other
hand, the opening pore size is still too small to permit ethanol
(dkinetic = 4.53 Å) diffusion in, even at the saturated vapor
pressure, indicating a size-exclusive effect on ethanol. When the
substituted group changes from propargyl groups to allyl groups
in Cu-L2 and propyl groups in Cu-L3, the sorption isotherm of
methanol is greatly changed from gate adsorption to type-I
mode. Furthermore, the observation of ethanol sorption on
Cu-L2 and Cu-L3 indicates that the opening size is larger than
that of propargyl groups in Cu-L1 when the substituted groups
are allyl and propyl groups. Since they have the same framework,
the changes could be ascribed to the increase of the flexibility of
the substituted groups that allow a larger opening size.
Fig. 2 The PXRD patterns (a) and the FT-IR spectra (b) of as-made Cu-
L1, Cu-L2, and Cu-L3.
Hydrogen bonding interactions, for example C–H⋯O,
O–H⋯O, play a significant role in the adsorption of water
vapour on porous materials. In general, oxygen-containing
groups are able to act as primary adsorption centres due to the
formation of hydrogen bonds with adsorbing water mole-
cules.40 When water molecules attach to the centre, it will
form a secondary adsorption site on which additional water
molecules can be further adsorbed.17,41 The adsorption will be
enhanced if the location of the site enables clusters of water
molecules to form “bridges” between the sites. Such a bridge
can itself act as a nucleation centre for more water mole-
cules.42,43 In this framework, the coordinated water molecules
as well as the oxygen atoms on the phenyl group would serve
as the primary centres for water vapor adsorption. The neigh-
Fig. 3 Sorption isotherms for methanol (a), ethanol (b), and water (c)
on Cu-L1, Cu-L2, and Cu-L3 respectively; (d) the sorption isotherms of
methanol, ethanol, and water on Cu-(L1)0.25(L3)0.75
.
shows a small uptake amount at low pressure. At P/P0 = 0.54, it bouring C–H sites play the role of enhancing the adsorption
reaches 0.47 mmol g−1, then increases abruptly to 1.82 mmol g−1 via C–H⋯O bonding. The propyl group has more C–H sites,
at P/P0 = 0.77, suggesting a gate-opening response to methanol.
and are more adaptable to the surroundings because of its
Finally, it reaches 1.91 mmol g−1 at P/P0 = 0.90. In contrast, on flexibility, which, in our opinion, can contribute to more
Cu-L2 and Cu-L3, it shows a type-I sorption isotherm with a uptake amount, thus giving the order Cu-L1 < Cu-L2 < Cu-L3.
steep increase in the range of P/P0 = 0–0.2, and then remains
The above studies have demonstrated that the sorption
steady, and finally reaches a capacity of 3.1 and 4.0 mmol g−1 at amount of each species, from water to ethanol, can be effec-
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tively tuned by the substituted groups. As a kind of potential
sorbent, each one (Cu-L1, Cu-L2, and Cu-L3) displays different
abilities of adsorptive discrimination of these three adsorbates
(Fig. S4†), suggesting that different separation needs would be
satisfied by tuning the ratio of the substituted groups in a
porous material. To examine this point, the substituted groups
inside the channel were further tuned by incorporating H2L1
and H2L3 into the same crystal via the mixed-linker synthetic
strategy (ESI†).39,44,45 The sample Cu-(L1)0.25(L3)0.75 was iso-
lated, and the ratio was verified by 1H NMR (Fig. S5†). We
failed to isolate a series of samples with an adjustable ratio of
L1 and L3 in the structure by changing the feed ratio. The
sorption isotherm of methanol presents a type-I model,
similar to that of Cu-L3, with an uptake amount of 3.6 mmol
g−1 at P/P0 = 0.90 (Fig. 3d and S6†). In contrast, the uptake
amount of water in the low pressure range is very low. At P/P0 =
0.5, gate-opening adsorption for water is observed, which
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We gratefully acknowledge the financial support from the
NSFC (no. 21471035, 21201039) and the Shanghai Leading
Academic Discipline Project (B108).
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