High gas storage capacities and stepwise adsorption in a UiO type metal-organic framework incorporating Lewis basic bipyridyl sites

Article abstract of DOI:10.1039/c3cc48275h

A UiO type MOF with Lewis basic bipyridyl sites was synthesized and structurally characterized. After being activated by Soxhlet-extraction, this MOF exhibits high storage capacities for H₂, CH₄ and CO₂, and shows unusual

Full text of DOI:10.1039/c3cc48275h

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High gas storage capacities and stepwise
adsorption in a UiO type metal–organic
framework incorporating Lewis basic
bipyridyl sites†
Cite this: Chem. Commun., 2014,
50, 2304
Received 29th October 2013,
Accepted 10th December 2013
Liangjun Li,ab Sifu Tang,a Chao Wang,ab Xiaoxia Lv,ab Min Jiang,a Huaizhi Wua and
DOI: 10.1039/c3cc48275h
Xuebo Zhao*a
www.rsc.org/chemcomm
A UiO type MOF with Lewis basic bipyridyl sites was synthesized and attractive due to their benefits of constructing isomorphic MOFs
structurally characterized. After being activated by Soxhlet-extraction, and incorporating functional Lewis basic sites into the pore surface
this MOF exhibits high storage capacities for H2, CH4 and CO2, and without declining their original pore spaces.8
shows unusual stepwise adsorption for liquid CO2 and solvents,
The UiO types of MOFs are an emerging class of MOFs that
indicating a sequential filling mechanism on different adsorption sites. attract broad interest.9 The highly porous structures and excellent
stability of these types of MOFs allow them to be promising
Developing an effective system for carbon dioxide capture from materials for the targets of CO2 capture and energy gas storage.
anthropogenic emissions and finding appropriate mediums for Up to now, extensive studies have been conducted based on UiO
energy gas (i.e., H2 and CH4) storage have been long term challenges, MOFs and their derivatives synthesized by functionalization/
and will be increasingly urgent in future.1 Physical sorption using modification on organic linkers.10 Nevertheless, the gas sorption
solid state absorbents provides efficient alternatives owing to fast studies on UiO type MOFs incorporating Lewis basic pyridyl sites
kinetics and high energy efficiency.2 Indeed, some pilot plants for have never been investigated so far. The outstanding structural
carbon dioxide capture and methane storage using solid state characteristics and excellent stability of UiO type MOFs motivate us
absorbents have been realized by some research groups.3
to incorporate the pyridyl moieties into the frameworks, with the
Established as a new class of crystalline porous materials, anticipation that it would further enhance the gas uptake capacities
metal–organic frameworks (MOFs) provide ideal platforms by anchoring the Lewis basic sites onto the pore surface but without
for such applications due to their intriguing structures, high sacrificing its original high porosity and exceptional robustness.
surface area and tuneable functional pore environments.4 In Herein, we synthesized a UiO MOF, Zr6(m3-O)4(OH)4(bpdc)12 (termed
order to achieve high gas storage capacities or high selectivity, as UiO(bpdc) in this communication), by using a N-heterocyclic
extensive efforts have been devoted to increase the affinity of carboxylate ligand: 2,2-bipyridine-5,50-dicarboxylate (bpdc), in which
frameworks with gas molecules, such as generating open metal the bipyridyl moieties were incorporated as free Lewis basic sites.
sites5 or tuning pore environments by immobilizing functional Upon being activated by Soxhlet-extraction, the adsorption proper-
groups on the pore surface.6 Immobilizing functional groups ties of N2, H2, D2, CO2 and CH4 were examined. Afterwards, the
appears to be a promising strategy to tune the adsorption properties, sorption behaviours towards CO2 at 195 K and organic solvents at
especially for enhancing CO2 binding strength by incorporating Lewis room temperature on UiO(bpdc) were investigated, and the adsorp-
basic sites. However, the conventional strategy of anchoring the tion mechanism was discussed.
functional groups has some drawbacks. Along with the modification
UiO(bpdc) is prepared from the solvothermal reaction of the
of the pore environments, the space occupation or blockage of bpdc ligand with ZrCl4 via a modulated method (see experimental
functional groups always decreases the pore volume as well as specific details in ESI,† S2), and a pure phase of octahedral shaped crystals
surface areas significantly, which contrariwise lower the gas uptake were obtained. Single crystal analysis reveals that UiO(bpdc) crystal-
capacities.7 In this respect, the N-heterocyclic ligands are more lizes in cubic space group Fm3 which has a lower symmetry in
%
%
contrast to the commonly encountered Fm3m space group in UiO
a Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Shandong, 266101, China. E-mail: zhaoxb@qibebt.ac.cn;
Fax: +86 0532-80662728
b University of Chinese Academy of Sciences, Beijing, 100049, China
† Electronic supplementary information (ESI) available: Details of experiments,
IR spectra, TGA and calculation details. CCDC 968930. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c3cc48275h
series MOFs,11 because the inclusion of hetero N atoms in bpdc
linkers lowers the symmetry of the crystal system. As expected, a
six-nuclear 12-connected SBU: Zr6O4(OH)4(CO2)12, is formed by
the assembly of six zirconium atoms with eight m3-O or m3-OH
bridged oxygen atoms and twelve carboxylate groups (as shown
in Fig. 1a). Due to the oxo-philic nature of the early transition
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Fig. 1 (a) Ball and stick representation of the bpdc ligand and SBU:
[Zr6(m3-O)3(OH)3(COO)12] (gray: C; red: O; blue: N; purple: Zr); (b) the
tetrahedral cage; (c) the octahedral cage; (d) packing of two types of cages.
elements, the Zr(IV) ions are preferably coordinated to the oxygen
donor atoms of the carboxylate groups in bpdc, while the
2,20-bipyridine moieties are left free, which could act as the Lewis
basic sites. Then, each vertex of the 12-connected SBUs is bridged
by the bpdc linkers through the carboxylate groups, forming a
three-dimensional network with the fcu topology. The formula of
UiO(bpdc) can be determined to be Zr6(m3-O)4(OH)4(bpdc)12 on
the basis of single crystal X-ray analysis and elemental analysis.
Fig. 2 Adsorption isotherms for UiO(bpdc). (a) N2 (77 K); (b) H2 (77 K)
(excess) and D2 (77 K) (inset); (c) CO2 (273, 283, 293, 303 and 313 K)
(excess); (d) CH4 (273, 283, 293, 303 and 313 K) (excess).
Two types of cages, the octahedral cages with a diameter of 1.6 nm among the top ranks of the hydrogen uptakes on MOF materials,
and the tetrahedral cages with a diameter of 1.2 nm, are formed comparable to the uptakes on NOTT-10212 and Be12(OH)12(BTB)4.13
by the assembly of the SBUs and linkers (as illustrated in Fig. 1b This uptake is also 26.6% higher than that on UiO-679a (4.5 wt% at
and c). The theoretical void ratio of the framework is 68.5%, with 77 K and 20 bar). D2 isotherm for UiO(bpdc) upto 1 bar at 77 K (see
a crystallographic density of 0.765 g cmꢀ3
.
inset in Fig. 2b) shows a similar shape to H2 with D2/H2 ratios
The as synthesized sample prepared in this work is of high (mol ratio) of about 1.05 times that are the reasonable uptake ratios
purity, as confirmed by good match between the simulated and of D2 to H2, confirming the reliability of the above H2 adsorption
experimental PXRD patterns (as shown in ESI,† S4). TGA analysis measurements.14 Moreover, the uptake ratio of D2 to H2 shows an
reveals that UiO(bpdc) is stable up to 512 1C under an N2 increasing trend with the decreasing pressure and reaches a
atmosphere (see TGA curves in ESI,† S5), which is comparable maximum ratio of 1.19 at 20 mbar (as shown in Fig. S5 in ESI,†
with UiO-66 and its analogues,7b indicating its excellent thermal S8). Former studies revealed that the quantum effects of D2 and H2
stability. The unvaried PXRD patterns of the samples exposed to adsorption in confined nano-porous structures led to the difference
moisture and treated by soaking in common solvents also show in their adsorption properties.14 As for UiO(bpdc), the fine structure
good chemical stability for this MOF (see PXRD patterns in ESI,† of nano-corners around SBUs might attribute to the different
S4). To fully activate the as synthesized sample, we employed adsorption isotherms of D2 and H2 at low pressures. At 293 K
Soxhlet-extraction to extract the high boiling point solvents and and 20 bar, it absorbs 79.7 wt% of CO2 (as shown in Fig. 2b),
other compounds involved in the framework, using ethanol as corresponding to 310 v(STP)/v (using a crystallographic density of
the extraction agent. Then, the guest molecules were removed by 0.765 g cmꢀ3). This value is the among the top uptakes of CO2
heating the extracted sample under ultrahigh vacuum before on MOF materials, comparable to the uptakes on MOF-17715 and
adsorption measurements.
MIL-101(Cr)16 under the similar conditions, and is also larger than
The N2 isotherm measured at 77 K of UiO(bpdc) shows the experimental or theoretical CO2 uptake capacities on UiO-67
a reversible typical type-I isotherm with a pore volume of and UiO-68.9b Considering the almost identical configurations as
1.057 cm3 gꢀ1
.
The Brunauer–Emmett–Teller (BET) and well as theoretical pore volumes of UiO-67, the significant enhance-
Langmuir surface areas are 2646 m2 gꢀ1 and 2965 m2 gꢀ1
,
ment of CO2 uptakes on UiO(bpdc) could be attributed to the
respectively (see the calculation process in ESI,† S7). These combinational benefits of high surface area and Lewis basic bipyridyl
values are significantly higher than that of UiO-67 (BET surface sites. The CO2 uptakes at 0.15 and 1 bar are 1.5 and 8.0 wt%,
area: 1877 m2 gꢀ1 9a 1575 m2 gꢀ1 (ref. 9b)). It is noteworthy that respectively. These values are higher than those on MIL-10116b
,
the experimental pore volume of UiO(bpdc) is identical to the and MOF-17715b under similar conditions, but are lower than those
theoretical value of UiO-67 (1.05 cm3 gꢀ1 9b
which has almost the on MOFs with narrower pore size, high dense open metal sites or
)
same ligand length and SBUs, demonstrating that UiO(bpdc) is fully –NH2 functional groups,17 indicating the different roles of surface
activated and the Soxhlet-extraction is of great help for completely chemistry and surface area in gas adsorption. The methane
extracting the guest compounds involved in the framework.
isotherms for UiO(bpdc) measured at room temperature show
The outstanding characteristics of structures and pore nearly linear trends up to 20 bar, indicating the promise of high
environments of UiO(bpdc) enable it to possess high uptake working capacity under practical conditions (as shown in Fig. 2d).
capacities for H2, CO2 and CH4. As shown in Fig. 2b, it adsorbs At 293 K and 20 bar, the adsorbed amount of CH4 is 12.2 wt%
5.7 wt% of H2 at 77 K and 20 bar, which has not reached (7.63 mmol gꢀ1), which is far from saturation. The methane
saturation yet. It should be pointed out that this uptake is isotherms fit well with the Langmuir equation at a higher pressure
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that the sequential filling mechanism is much related to the
dipole moments of the sorbates.
In conclusion, a UiO type MOF with incorporated Lewis basic
bipyridyl sites was synthesized by the solvothermal reaction of bpdc
with Zr salts. The Soxhlet-extraction activation allows UiO(bpdc) to
be fully evacuated. The highly porous structure combined with Lewis
basic sites incorporated on the pore surface enables UiO(bpdc) to
possess high uptake capacity for H2, CO2 and CH4. The exceptional
stability and excellent adsorption capacities make this MOF a very
promising material for CO2 capture and energy gas storage. Further-
more, the stepwise isotherms observed at low pressure for adsorp-
tion of CO2 at 195 K and adsorption of organic solvents at room
temperatures reveal a sequential filling mechanism of adsorbates on
different adsorption sites.
Fig. 3 (a) CO2 isotherm at 195 K; (b) ethanol isotherm at 293 K; (c)
This work was supported by grants from the Natural Science
Foundation of China (Grant No. 21073216 and 21173246) and
the ‘‘Hundred-talent Project’’ (KJCX2-YW-W34) of the Chinese
benzene isotherm at 293 K; (d) cyclohexane isotherm at 293 K.
range (see details in ESI,† S10). The predicated methane storage Academy of Sciences for the financial support.
capacity from Langmuir simulation is 10.52 mmol gꢀ1 at 293 K and
35 bar, corresponding to 180 v(STP)/v. This value reached the DOE
target of 180 v(STP)/v for methane storage at ambient temperature
and 35 bar, making UiO(bpdc) one of the few MOFs that meet the
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