This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 7377–7379 7377

Cite this: Chem. Commun., 2011, 47, 7377–7379

Tuning the moisture stability of metal–organic frameworks

by incorporating hydrophobic functional groups at different

positions of ligandsw

Deyun Ma, Yingwei Li* and Zhong Li

Received 28th March 2011, Accepted 16th May 2011

DOI: 10.1039/c1cc11752a

The introduction of hydrophobic groups (e.g. methyl) at the most

adjacent sites of each and every coordinating nitrogen atom of the

bipyridine pillar linker in a carboxylate-based bridging MOF could

shield the metal ions from attack by water molecules, and thus

enhance the water resistance of the MOF structure significantly.

Metal–organic frameworks (MOFs) are a new class of porous

materials that have received widespread attention owing to their

potential applications in gas storage, separation, and catalysis.1

In particular, frameworks that incorporate carboxylate-based

bridging ligands were found to be most promising for such

applications.2 However, more recently a number of reports have

shown that many MOFs of this type lost their structures

and high surface areas quickly when exposed to air.3 The

decomposition of the MOF networks has been related to an

effect of water in the air. Water molecules could easily penetrate

the pores and disrupt the framework by hydrolyzing the

carboxylate groups coordinated to the zinc centers.4 This draw-

back greatly hinders the industrial applications of the MOFs

because water is very difficult to fully remove from industrial gas

resources.

So far, the issue of adsorption and stability of MOFs with

respect to moisture has been addressed in very limited research

work,3–6 and most of the studies focused on the hydrophobicity/

philicity of MOFs. It has been demonstrated that the

introduction of water repellent functional groups within the

frameworks can largely enhance the hydrophobic properties of

MOFs.6 The stability of the MOFs in humid air can also be

improved to some extent because less water could be adsorbed

within the more hydrophobic pores. However, it is extremely

difficult to fully exclude water from adsorbing on the frameworks

even with highly hydrophobic pores especially upon a long

period of exposure to moisture, and even a small quantity of

water adsorbed on the MOFs could disrupt the structures

significantly.6c Moreover, for a potential adsorption/separation

or catalysis application, the porosity and sorption capabilities of

the MOFs after incorporating a large amount of water repellent

functional groups should also be taken into account.6b,c

Our hypothesis is that the incorporation of hydrophobic

functional groups close to each and every coordinated metal

center within a MOF material could protect the relatively

weak bonds involving the metal ions from attack by water,

and thus improve the water resistance of the MOF efficiently.

In this study, three microporous Zn-MOFs, which were

constructed by 1,4-benzenedicarboxylate (BDC) and various

pillar linkers with methyl groups at different positions

(P1 = 4,40-bipyridine, P2 = 2,20-dimethyl-4,40-bipyridine,

and P3 = 3,30-dimethyl-4,40-bipyridine, respectively) were

investigated (Scheme S1, ESIw). To assess differences in the

moisture stability of the materials, each material was exposed

to humid air and then characterized using powder X-ray

diffraction (PXRD), thermal gravimetric analysis (TGA),

and N2 physical adsorption. The sorption capacities of toluene

on the MOFs have also been examined to evaluate the

porosity of the materials.

Zn2(BDC)2(P1) (MOF-508),7 Zn2(BDC)2(P

2) (SCUTC-18), and

Zn2(BDC)2(P3) (SCUTC-19) were synthesized under solvothermal

conditions. The structures were determined by single-crystal X-ray

diffraction at 298 K,z and guest molecules were determined by

TGA and IR (Fig. S1–S4, ESIw). As shown in Fig. 1, three

compounds have the similar a-Po-type 3D elongated primitive

cubic MOFs, in which each is comprised of dimeric zinc units

{Zn2(COO)4} bridged by BDC ligands in the a and b directions

and further pillared by bipyridine struts approximately in the

c direction (Scheme S1, ESIw). Crystallography also establishes

2-fold interpenetration with one-dimensional channels along the c

Fig. 1 Single network units for MOF-508 (a), SCUTC-18 (b), and

SCUTC-19 (c). The cyan polyhedra represent the zinc ions. Carbon,

black; oxygen, red; nitrogen, blue.

School of Chemistry and Chemical Engineering, South ChinaUniversity of Technology, Guangzhou 510640, China.E-mail: liyw@scut.edu.cnw Electronic supplementary information (ESI) available: Experimentaldetails, crystal data, TGA, FT-IR, XRD, N2 adsorption data,theoretical calculation, and the views of the crystal structures. CCDC806376 and 806377. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c1cc11752a

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7378 Chem. Commun., 2011, 47, 7377–7379 This journal is c The Royal Society of Chemistry 2011

axis (Fig. S5, ESIw). N2 adsorption measurements (HK model)

indicate that the pore sizes of MOF-508, SCUTC-18, and

SCUTC-19 are 6.4, 6.5, and 8.0 A, respectively (Fig. S6, ESIw).TGA studies indicate that the guest molecules can be readily

released in the temperature range of 25 to 200 1C to form the guest

free phases, which were stable up to 400 1C (Fig. S1, ESIw).The maintenance of the crystal structures after removing guest

molecules was confirmed by XRD (Fig. S7, ESIw).The kinetic trap effects of water on the samples were

monitored by TGA. The desolvated MOF materials were

saturated by water vapor at 298 K prior to TGA analysis.

PXRD indicates that the frameworks of SCUTC-18 and

SCUTC-19 were mostly remained unchanged, while MOF-508

decomposed partially after water exposure for a short time

(ca. 10 h) (Fig. S7, ESIw). The TGA curve shows that the

partially decomposed MOF-508 still adsorbed a large number

of water molecules, and the structure fully collapsed with the

removal of water upon heating (Fig. S8, ESIw). SCUTC-19

exhibits enhanced hydrophobicity as compared with MOF-508.

However, the inflection point (4130 1C) is still higher than the

boiling point of water, indicating a strong adsorption of water

inside the MOF pores. Despite the same amount of water

repellent groups (i.e. methyl) in the unit cells of SCUTC-18

and SCUTC-19, SCUTC-18 displays a much different TG curve

from SCUTC-19. As shown in Fig. S8 (see ESIw), SCUTC-18

adsorbed up to 4 wt% water at 25 1C, and there is no additional

weight loss upon heating up to 300 1C. The inflection at below

85 1C is lower than the boiling point of water suggesting a weak

physical adsorption of water molecules inside MOF pores.

The desolvated MOF samples were exposed to air and exam-

ined using PXRD to provide insight on the structure integrity of

the materials under ambient conditions. PXRD shows that

MOF-508 was instable in air and its structure fully collapsed

after moisture exposure for one week (Fig. 2a). As expected,

the addition of methyl substituents clearly stabilizes the bulk

crystallinity of the structure in ambient air. After exposing to air

for 7 days, no new peaks appear for SCUTC-19, but all of the

peak intensities for SCUTC-19 decrease (Fig. 2c). In contrast,

PXRD patterns of SCUTC-18 remain essentially unchanged up

to 30 days, with no new peaks and apparent loss in peak intensity

at 2y = 8.41 (Fig. 2b). The structural stability of SCUTC-18 in

air was verified by TGA (Fig. S9, ESIw). The results indicate

that the incorporation of methyl groups enhances the stability

of MOF-508 under standard atmospheric conditions (Fig. 3a).

Furthermore, the moisture stability of the MOFs depends

strongly on the positions of the methyl substituents on the

4,40-bipyridine linkers. The hydrophobic groups at the ortho-

positions of the coordinating nitrogen atoms are located near the

Zn centers in SCUTC-18, which could shield the Zn ions from

attack by water molecules (Fig. 3b). In contrast, methyl groups at

the meta-positions are far from the metal centers in SCUTC-19,

which cannot provide efficient protection for the metal ions

obviously (Fig. 3c).

In order to further prove the importance of the protection of

the metal centers in constructing moisture-stable MOFs,

we prepared a previously reported MOF-5 type structure

(MTV-MOF-5-AF) from a mixture of BDC and methyl

substituted BDC ((CH3)2–BDC) linkers.2a MOF-5 is known

to be unstable in air.3a,c The incorporation of methyl groups

did not enhance the moisture stability of the structure

significantly. PXRD shows that the crystals have changed

apparently within 24 h (Fig. S13, ESIw). From the structural

analysis, it is apparent that the methyl groups are too far from

the Zn centers to obstruct water molecules from adsorbing on

the zinc ions (Fig. S14, ESIw). Recently, Wu et al. prepared a

new MOF-5 type structure (Banasorb-22) from 2-trifluoro-

methoxy terephthalate (2-CF3O–BDC), which exhibited

enhanced stability in humid air as compared with MOF-5.6c

However, Banasorb-22 showed a big drop in the surface area

(from 1113 m2 g�1 to 210 m2 g�1) after one week exposure to

moisture, which is indicative of the collapse of the structure. A

similar structural analysis suggests that the trifluoromethoxy

groups bend against the Zn centers, but cannot shield the

metal ions, like the methyl groups in MTV-MOF-5-AF, even

though they are longer than methyl (Fig. S14, ESIw).N2 sorption measurements were used to examine the

porosity of the MOF samples after exposure to air (Fig. S15,

ESIw). The BET surface areas of MOF-508 and SCUTC-19

after one week of exposure to air are 52 and 208 m2 g�1, which

show ca. 87% and 55% reduction, respectively, as compared

with those of the as-synthesized samples (398 m2 g�1 for

MOF-508, and 458 m2 g�1 for SCUTC-19, respectively).

However, no significant loss in the BET surface area was

observed (from 523 to 506 m2 g�1) for SCUTC-18 even after

30 days of exposure to humid air, indicating the maintenance

of the porous structures of SCUTC-18 in ambient air

(Fig. S15, ESIw). The N2 adsorption data further confirm the

stability of SCUTC-18 in air.

These results indicate that SCUTC-18 could have a

potential application for gas-adsorption from air, where

moisture always presents as a common interference. We

selected toluene for the study. As is well known, toluene isFig. 2 Representative PXRD patterns comparing peak intensity

changes for (a) MOF-508, (b) SCUTC-18, and (c) SCUTC-19 over time.

Fig. 3 Views of the MOF structures along the same directions.

(a) MOF-508, (b) SCUTC-18, (c) SCUTC-19. Zinc, cyan; carbon,

black; hydrogen, white; oxygen, red; nitrogen, blue.

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 7377–7379 7379

one of the typical indoor volatile organic compounds (VOCs)

that threaten human health, and it is desirable to effectively

remove toluene vapor from indoor air by adsorption.8

The sorption behavior of toluene on SCUTC-18 features a

reversible type-I isotherm (Fig. 4). In the low P/P0 region, the

adsorption reached saturation indicating a strong guest–host

interaction. In contrast, the adsorption of toluene on MOF-508

or SCUTC-19 shows weak interaction with the framework as

supported by the observation that the amount of adsorption

increases gradually along with the increasing relative pressure

of toluene (Fig. 4). The adsorbed amount of toluene for

SCUTC-18 is 170 mg g�1 at P/P0 = 0.90, which is 6–11 times

of that for MOF-508 or SCUTC-19. The differences in

adsorption capacity can be related to the molecular sieving

effects of the MOFs.1d The minimum rectangle of toluene

is ca. 4.0 � 6.6 A2,9 which could enter into the windows of

SCUTC-18 with a size of 8.0 A, while could not easily

enter into the channels of MOF-508 or SCUTC-19 with a

size of 6.4 or 6.5 A respectively. According to the Dubinin–

Radushkevich (DR) equation, the isosteric heat of adsorption,

qst,F = 1/e, for SCUTC-18 is calculated to be 62.8 kJ mol�1

(Fig. S16, ESIw), which can be comparable to those for some

MOFs with excellent toluene sorption capability.10 The kinetic

trap effects of toluene were monitored by TGA. As shown in

Fig. S8 (see ESIw), toluene was not completely removed from

SCUTC-18 until the temperature exceeded 200 1C, which is

about 100 1C higher than its boiling point, further confirming

a strong interaction with the host framework. The combination

of high stability in air and hydrophobicity as well as high

adsorption capacity for toluene implies that SCUTC-18 has

potential capabilities for air-cleaning applications.

In conclusion, we have demonstrated the significance of the

positions of substituent groups on organic linkers in constructing

moisture stable MOF materials. The introduction of water

repellent groups (e.g.methyl) at the most adjacent sites of each

and every coordinating atom of the organic ligand in a MOF

could efficiently shield the metal centers from attack by water

molecules, and thus enhance the water resistance of the MOF

structure significantly. Moreover, the incorporation of short

functional groups at such positions doesn’t affect the porosity

and adsorption capability of the MOF material remarkably.

This valuable finding provides us a facile but highly efficient

method for the preparation of highly water-resistant MOF

materials for specialized and industrial applications. Further

work, e.g. quantum mechanical model calculations,4 will be

required in order to fully understand the interactions of water

molecules with the MOFs.

This work was supported by NSF of China (20803024,

20936001, and 21073065), Doctoral Fund of Ministry of

Education of China (200805611045), Guangdong Natural

Science Foundation (10351064101000000), the Fundamental

Research Funds for the Central Universities (2009ZZ0023,

2011ZG0009), and the program for New Century Excellent

Talents in Universities (NCET-08-0203).

Notes and references

z Crystallographic data for SCUTC-18: C34H36Zn2N4O11, colorlessblock-shaped, Mr = 806.4, crystal size 0.30 � 0.25 � 0.21 mm,tetragonal, space group P43212, a = 10.9288(4) A, c = 56.2967(2) A,V= 6724.0(4) A3, Z= 4, m(Mo Ka) = 1.484 mm�1, r= 1.410 g cm�3,T = 293 K, reflection numbers collected = 12 976, unique reflections(Rint) = 5626 (0.0437), R1 [F

2 4 2sF2] = 0.0518 and wR2 (all data) =0.1726, GOF = 1.073, Flack = 0.34(3). The crystal data for SCUTC-19:C31H31Zn2N3O11, colorless block-shaped, Mr = 751.0, crystal size0.30 � 0.23 � 0.18 mm, triclinic, space group P�1, a = 10.925(2) A,b = 10.946(2) A, c = 13.972(3) A, a = 95.19(3)1, b = 99.23(3)1, g =99.89(3)1, V = 1612.7(6) A3, Z = 2, m(Mo Ka) = 1.532 mm�1, r =1.325 g cm�3, T = 293 K, reflection numbers collected = 12 733,unique reflections (Rint) = 5736 (0.1001), R1 [F2 4 2sF2] =0.0759 and wR2 (all data) = 0.1807, GOF = 0.941. CCDC 806376(SCUTC-18) and CCDC 806377 (SCUTC-19).

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Fig. 4 Adsorption and desorption isotherms of toluene vapor on the

MOF samples at 298 K.

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