Analogous porous metal–organic frameworks:synthesis, stability and application inadsorptionSung Hwa Jhung,* Nazmul Abedin Khan and Zubair Hasan

DOI: 10.1039/c2ce25760b

So far, a huge number of metal–organic frameworks (MOFs) have been synthesized andstudied very widely for various applications like gas adsorption/storage, separation,catalysis, drug delivery, luminescence, magnetism, etc. Some of the MOFs areisomorphous, isostructural or isoreticular in topologies having nearly similar (analogous)framework structures. On the other hand, some of the MOFs also have very similarstructures with different functional groups via direct synthesis or post-modification. Inthis highlight, MOFs having very similar structures will be classified into three categories:(1) analogous MOFs with different metallic components; (2) analogous MOFs withdifferent linkers; (3) analogous MOFs with different functional groups. Moreover, variousMOFs with very similar structures composed of different metallic, organic or functionalgroups will be compared especially with regard to their synthesis kinetics, chemical/thermal stability and their applications in the adsorption of hydrogen, acetylene,propylene, carbon dioxide and sulfur-containing compounds, and so on. The synthesisrate and chemical stability of analogous MOFs depend on the lability and inertness,respectively, of metal ions. On the other hand, thermal stability may be explained withthe bond strength of metal–oxygen in common oxides. The thermal or chemical stabilityof analogous MOFs having extra functional groups depends on the functional groupstagged on the linkers; however, no comprehensive explanation is available. Adsorptiondepends strongly on the property of the metallic or organic moiety of analogous MOFs,and important parameters (size, binding strength, ionic character, density, redox ability,softness and acidity of the metal ions; length, polarity, and hydrophobicity of the linkers)for adsorption can be suggested. Based on the analysis of the reported results, it can beconcluded that the metallic, organic or functional groups of analogous MOFs havedominant roles in the synthesis, stability and adsorption even though contradictoryresults were also reported. Understanding the effects of metallic, organic or functionalmoieties of very similar MOFs on the synthesis, stability and adsorption will lead to a newway to develop MOF materials that have various commercial applications.

1. Introduction

The number of materials exhibiting per-

manent nanoporosity has rapidly

expanded in recent years, due in large

part to the introduction of porous materi-

als including metal–organic frameworks

(MOFs) or coordination polymers.1–3 The

porous MOFs have attracted considerable

attention due to an easily tunable crystal-

line hybrid network with a high and

regular porosity. Moreover, MOFs have

lots of potential applications including

adsorption/storage of carbon dioxide,4,5

hydrogen storage,6 adsorption of vapours,7

separation of chemicals,8 drug delivery/

biomedicine,9 polymerization,10 magnet-

ism,11 catalysis,12 luminescence,13 and so

on. The MOFs have recently been studied

in depth over a wide range of fields

including synthesis, characterization and

applications, as evidenced by several com-

prehensive review articles in the field.1–13

So far, a huge number of publications

related to MOFs have been reported.

However, the main topics of the research

on MOFs have usually been synthesis,

characterization, modification and appli-

cation, and so on. Most of the research

was on a special structure or component

of metal ions or organic linkers. In some

cases, MOFs with very similar structures

Department of Chemistry and Green-NanoMaterials Research Center, KyungpookNational University, Daegu, 702-701, Korea.E-mail: sung@knu.ac.kr; Fax: 82-53-950-6330

CrystEngComm Dynamic Article Links

Cite this: DOI: 10.1039/c2ce25760b

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(with different metallic species or linker

materials etc.) were reported simulta-

neously by a research group or indepen-

dently by several groups. Moreover, some

MOFs were modified to impart function-

ality. In this highlight, analogous MOFs

having very similar structures (such as

isostructural,14 isomorphous14 and isore-

ticular15 MOFs) will be explained includ-

ing their classifications and properties.

Especially, the effect of the central metal

ions, organic linkers or functional moi-

eties on the synthesis kinetics, stability

and adsorption of hydrogen, acetylene,

propylene, carbon dioxide and hazardous

materials will be reviewed. This highlight

will lead to an idea or way to develop new

MOF materials for viable applications.

2. Classifications of analogousMOFs

Analogous MOFs may be classified in a

few ways depending on functionality

(like acidic, basic, neutral, cationic,

anionic etc.), composition (transition

metals, lanthanides, etc) or stability, etc.

In this highlight, analogous MOFs are

classified in three categories: (1) analo-

gous MOFs having different cations or

cation clusters; (2) analogous MOFs with

different organic linkers; (3) analogous

MOFs with different functional groups.

Fig. 1 shows the schematic illustration of

the classified analogous MOFs. More

detailed explanation with typical exam-

ples will be shown in the following

sections. Even though there are many

structures that can be classified or

grouped, this highlight will mainly focus

on porous, stable and widely-studied

MOFs.

2.1. Analogous MOFs with differentmetal ions

Several metal ions or clusters may lead

to MOF materials with analogous

(isomorphous or isostructural) topology

such as CPO-27 (or MOF-74), MIL-53,

MIL-96, MIL-100, MIL-68, etc. Several

highly studied structures can be

described as follows:

CPO-27 or MOF-74 (Me-DHTP):

Trigonal MOF named CPO-27, com-

posed of Me (metal) ions and 2,5-

dihydroxyterephthalate (DHTP), has a

1-dimensional channel (y1.1 nm) shown

in Fig. 2. The structure is very interesting

because of the presence of either a CUS

(coordinatively unsaturated site) or OMS

(open metal site) and the possibility to

accommodate many metallic components

(Co,16 Fe,17–19 Ni,20 Mg,21,22 Mn,23 Zn24)

in the oxidation state of +2. The CPO-27

has been used in various applications

including adsorption, which will be

described in the next section.

MIL-100 (Me-BTC): MIL-100s are

another typical analogous MOF having

a cubic structure and the materials are

composed of various Me(III) ions (Al,25

Cr,26 Fe,27 V28) and BTC (1,3,5-benzene-

tri-carboxylate). The MIL-100s are

highly porous and have not only micro-

pores but also mesopores. These struc-

tures also have CUS; therefore, can be

used in modifications and in various

applications (see below).

MIL-53/MIL-47(V) (Me-BDC): Orth-

orhombic MIL-53s are composed of

Me(III) (Al,29 Cr,30 Fe31,32) and BDC

(1,4-benzene-di-carboxylate). The dehy-

drated/desolvated MIL-53s(Al, Cr) have

a 1-dimensional channel structure (Fig. 3,

top). Even though the structure of MIL-

53(Fe) is not the same as MIL-53s

Nazmul Abedin Khan receivedhis BS and MS in Chemistryfrom the University of Dhaka,Bangladesh. In 2012, he receivedhis PhD in physical chemistryfrom Kyungpook National Uni-versity, South Korea under thesupervision of Prof. Sung HwaJhung. He is currently a postdoc-toral researcher at the samegroup. His major research in-volves the facile synthesis, func-tionalization or modification ofporous materials like metal–organic frameworks, zeolites, me-tal phosphates etc. for selectiveadsorption of hazardous organiccompounds and for various het-erogeneous catalytic purposes.

Zubair Hasan studied Chemistryat the University of Dhaka,Bangladesh and received his BSand MS (in 2009) degrees. In2010 he was awarded with aBK21 scholarship and then joinedthe group of Prof. Sung HwaJhung as a PhD student atKyungpook National University,South Korea. His current re-search interest is concerned withthe synthesis and functionaliza-tion of porous materials likemetal–organic frameworks foradsorption and catalysis, biodieselproduction through heterogeneouscatalysis.

Nazmul Abedin Khan Zubair Hasan

Sung Hwa Jhung received his BS from Seoul NationalUniversity and MS and PhD (in 1990) degrees inChemistry from Korea Advanced Institute of Scienceand Technology. Then he worked for ETRI, SamsungGeneral Chemicals Co. and Korea Research Institute ofChemical Technology. Since September 2007, he hasbeen a Professor in the Department of Chemistry ofKyungpook National University (KNU). He has beenthe director of Green-Nano Materials Research Centerof KNU since March 2011. He is interested in catalysis,green chemistry, adsorptive removal, adsorption/separa-tion with nanoporous materials. Since 2004, he haspublished about 120 peer-reviewed journal papers.

Sung Hwa Jhung

CrystEngComm This journal is � The Royal Society of Chemistry 2012

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(Al, Cr), the materials are quite similar to

one another in their composition

(MeIII(OH){O2C–C6H4–CO2}) and con-

nections. Orthorhombic V-BDC called

MIL-47(V)33 is very similar to MIL-

53s(Al, Cr) because of their similar com-

positions (VIV(O){O2C–C6H4–CO2}and

MeIII(OH){O2C–C6H4–CO2}) and the fact

that they have the same (orthorhombic)

crystal structures. The main difference is

the bridging groups of m2(OH) and m2(O)

for MIL-53s(Al, Cr, Fe) and MIL-47,

respectively (Fig. 3, bottom). Because of

the presence of the –OH group in the MIL-

53s, the materials show a very interesting

breathing phenomenon34 and adsorption

properties which will be explained in the

next sections. Similar MOFs having the

MIL-53 structure, composed of Ga35 or

In36 were also reported.

Other typical analogous MOFs, com-

posed of different metallic species, are (1)

Hexagonal MIL-96 (Me-BTC) where

Me(III) is Al,37 Cr,38 Ga39 or In40; (2)

Cubic Me3(BTC)2 like Cu3(BTC)2

(HKUST-1),41 Cr3(BTC)242 and Mo3

(BTC)2 (TUDMOF-1);43 (3) Tetragonal

Ln-BTC [Ln(BTC)(H2O)?4.3H2O] (Ln:

lanthanides) like Ce-BTC,44 Tb-BTC (or

MOF-76)24 and Y-BTC;45 (4) Orth-

orhombic MIL-68 (Me-BDC)(solvent)

where Me(III) is Al,46 Fe,47 Ga,48 In48

or V.49

Some of the analogous MOFs can also

be obtained with an ion-exchange pro-

cess. For example, a single-crystal to

single-crystal (scsc) interchange of Pd-

MOF and Cd-MOF was complete and

reversible.50 A MOF, which cannot be

obtained by a direct synthesis, may be

obtained with an ion exchange.51

Moreover, it has been suggested that

MOFs can be utilized to remove metal

ions with the ion-exchange of MOFs.52,53

Therefore, a few analogous MOFs can

be obtained with simple ion-exchange

procedures.

2.2. Analogous MOFs with differentlinkers

Analogous MOFs can also be produced

from different organic linkers and the same

metallic components. Isoreticular MOFs

may represent this kind of analogous

MOFs since the isoreticular (IR) MOFs

have been defined by Eddaoudi et al.15 in

2002 as being MOFs having the same

network topology. They presented the

synthesis, structural determination and

methane adsorption of several IRMOFs

like IRMOF-1, 2, 3 and 8 etc. In this

highlight, analogous MOFs with different

linkers will represent very similar MOFs

composed of different linkers (mainly with

different size). Analogous MOFs called

MIL-88-AyD were synthesized with var-

ious dicarboxylates like fumarate, BDC,

NDC (naphthalene-dicarboxylate) and

BPDC (biphenyl-dicarboxylate),54 respec-

tively. Stable Zr-MOFs such as UiO-66,

-67 and -68 were synthesized by using

three dicarboxylates like BDC, BPDC and

TPDC (terphenyl-dicarboxylate), respec-

tively.55 Kitagawa et al.2 used several

linker materials as pillars in the CPL-n

series MOFs. Chun et al.56 synthesized

analogous MOFs with different pillars and

linker materials and evaluated their per-

formances in hydrogen adsorption.

2.3. Analogous MOFs with differentfunctional groups (or tagged MOFs)

MOFs with functionality have been

mainly prepared by two ways of direct

synthesis and post-modification. In

direct synthesis, functionalized linker

materials have been applied in the

synthesis, very similar to the synthesis

using simple linkers without functional-

ity. Devic et al.57 prepared eight MIL-

53(Fe)–X (X denotes the functional

group used such as –Cl, –NH2, –OH

etc.) MOFs using the linkers in Fig. 4.

Stock et al. synthesized various MIL-

53(Al)s having functional groups like

–Cl, –NO2, – (OH)2 and so on.58,59

MIL-53s(Al, Fe), tagged with various

functional groups, were prepared from

functionalized linkers and showed the

importance of the pKa values of sub-

stituted benzoic acids (see below) in

determining the proton conductivity of

the MOFs.60 Substituted MIL-53(Fe)s

were synthesized and showed a striking

difference in the adsorption capacity of

hydrocarbons (see below).61 Some of

Fig. 3 (top) The framework structure and

(bottom) inorganic subunit of analogous Me-

BDCs such as MIL-53s(Al, Cr, Fe) and MIL-

47(V). Metal, oxygen, and carbon atoms are

shown in green, red, and black, respectively.

The one difference between the MIL-53s and

MIL-47 structures is highlighted with a dotted

blue circle. The blue circle corresponds to OH

and O in the MIL-53s and MIL-47, respec-

tively. Reproduced with permission from

ref. 98. Copyright 2009 American Chemical

Society.

Fig. 1 Schematic presentations of analogous

MOFs: (a) Analogous MOFs with different

cations or cationic clusters; (b) Analogous

MOFs with different linkers; (c) Analogous

MOFs with tagged functional groups. Black

sphere and gray bar mean metallic part and

linker, respectively, of a basic MOF which is

shown in the center of the scheme. Blue sphere

and dotted line represent new metallic part

and new linker, respectively, of analogous

MOFs. Green sphere with thin gray bar

represents tagged functional group.

Fig. 2 Crystal structure of desolvated CPO-

27(Me) as viewed along the [001] direction.

Orange, gray, and red spheres represent Me,

C, and O atoms, respectively; H atoms have

been omitted for clarity. Reproduced with

permission from ref. 17. Copyright 2011

American Chemical Society.

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analogous or isoreticular MOFs such as

IRMOF-2–IRMOF-715 may also be clas-

sified into this group. Post-synthetic-

modification (PSM) of MOFs was also

applied to produce functional MOFs, and

this method was reviewed very recently by

Cohen.62 Covalent PSM has been usually

carried out on amine or aldehyde-tagged

MOFs. Dative PSM could be carried out

on organic linkers or secondary building

units usually after dehydration or deso-

lvation to lead to CUS.63 MOFs having

functional groups via PSM can also be

classified as a part of this group.

3. Effect of metal ions, linkers orfunctional groups on propertiesof analogous MOFs

Starting here, the effects of different metal

ions, different linkers or tagged materials

(mainly simple groups like –NH2, –NO2,

–COOH etc.) on the synthesis (especially

synthesis rate), stability and adsorption

characteristics of analogous MOFs will be

described. This highlight will mainly focus

on the porous, stable and widely-studied

MOFs even though there may be other

relevant research results.

3.1. Synthesis rate of analogous MOFs

Even though there are many studies on

new or facile syntheses of MOFs,64,65 so

far, there have been only a few studies to

understand the relative synthesis kinetics

of analogous MOFs. Haque et al.66

quantitatively analyzed the synthesis

rates of two analogous MOF structures

like MIL-53/47s (Al, Cr, V-BDCs) and

CPO-27s (Co, Ni, Zn-DHTPs). They

synthesized the MOFs from the very

same reactant mixtures and compositions

(only excluding the type of metal ions) at

the very same temperatures to under-

stand the effect of cation species on the

synthesis kinetics. The synthesis rates for

MIL-53/47s were r MIL-47(V) . r MIL-53(Al)

. r MIL-53(Cr) for both the nucleation and

crystal growth stages at the same tem-

perature. The main reason for the high

synthesis rates of a MOF was low

activation energy.66 Similarly, the rate

(both nucleation and crystal growth) for

the syntheses of CPO-27s decreased in

the order of r CPO-27(Zn) . r CPO-27(Co) .

r CPO-27(Ni).67 They also confirmed that

the relative rates of MOFs synthesis were

in the order of ultrasound . microwave

. electric heating,67 suggesting the

potential applications of ultrasound and

microwave in the rapid syntheses of

MOFs. Khan et al.44 studied the synth-

esis of Ln (Ce, Tb, Y)-BTCs (Ln:

lanthanides) in a very similar process

under ultrasound at room temperature.

The relative rates were r Ce–BTC . r Tb–BTC

. r Y–BTC for both the nucleation and

crystal-growth stages.44

Irrespective of the structures (MIL-53/

47, CPO-27 and Ln-BTC), linkers (BDC

for MIL-53/47, DHTP for CPO-27 and

BTC for Ln-BTC) and solvents (water

for MIL-53/47 and DMF/water for CPO-

27 and Ln-BTC) of the synthesis, the

synthesis rates44,66,67 correlate nicely with

the lability68,69 of cations (summarized in

Table 1), suggesting the importance of

lability of the metal ions in the synthesis

rate of a MOF material. The importance

of the lability of metal ions may suggest

that the deprotonation rate of dicar-

boxylic or tricarboxylic acids (to dicar-

boxylate or tricarboxylates, respectively),

even in acidic conditions, is relatively fast

compared with the complexation rate

of carboxylates on metal ions to form

MOFs. Fig. 566 shows the schematic

presentation of the relative synthesis

rates of analogous MOFs (Me-BDCs),

the importance of the lability of cations

and the easy or rapid deprotonation of

dicarboxylic acids.

Ahnfeldt and Stock70 studied the

synthesis of MOFs called CAU-1-NH2

and CAU-1-(OH)2 under microwave and

conventional electric heating to under-

stand the effect of linkers and heating

methods on the synthesis kinetics.

Microwave-synthesis, compared with

conventional electrical heating, led to

shorter induction periods and shorter

crystallization times, in accordance with

Fig. 4 Modified terephthalate linkers BDC-X used in the synthesis of MIL-53(Fe)–X.57

Table 1 Effect of lability68,69 of cations on the synthesis rates of analogous MOFs such as MIL-53/47,66 CPO-2767 and Ln-BTC44

Type of MOFs Cation Lability of cation (s21)a Relative nucleation rateb Relative crystal growth rateb

MIL-53/4766 V(III) 103.0 74 51Al(III) 101.3 31 18Cr(III) 1026.2 1 1

CPO-2767 Zn(II) 107.2 9.0 15Co(II) 106.2 2.2 3.7Ni(II) 104.3 1 1

Ln-BTC44 Ce(III) 107.9 9.1 5.6Tb(III) 107.4 1.1 1.6Y(III) 107.1 1 1

a Water exchange rate constants of aqua ions.68,69 b Relative rates of analogous MOFs syntheses in nucleation and crystal growth stages. Therates of the most inert ions were set as standards of each analogous MOF.

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previous results. Under conventional

electrical synthesis, linkers had only a

minor or no influence on the rates of

product formation. Interestingly, the

microwave-synthesis of CAU-1-NH2

showed a shorter induction period and

a higher rate of crystal growth compared

with the CAU-1-(OH)2; however, no

explanation for the different kinetics

was given. Based on this result, it may

be concluded that further work is needed

to understand the effect of functional

groups (such as acid, base and neutral

ones), attached to linkers, on the kinetics

of MOFs synthesis.

3.2. Stability of analogous MOFs

Similar to synthesis kinetics, there are

only a few studies to understand the

thermal and chemical stabilities of ana-

logous MOFs even though several results

have been reported for the stability of

each MOF material.

Kang et al.71 compared the relative

chemical and thermal stability of MIL-

53/47 to understand the effect of central

metal ions on the stabilities of the Me-

BDCs. Chemical stability to acids, bases

and water decreased in the order of MIL-

53(Cr) . MIL-53(Al) . MIL-47(V),

suggesting that the stability increased

with increasing inertness68,69 of the cen-

tral metal ions. Thermal stability, how-

ever, decreased in the order of MIL-

53(Al) . MIL-53(Cr) . MIL-47(V), and

this tendency could be explained by

neither the inertness of the metal ions

nor the average bond strength between

the metal and oxygen. On the other

hand, the tendency might be explained

by the strength of the metal–oxygen

bond in common oxides like Al2O3,

Cr2O3, and V2O5 since the strengths of

the metal–O bonds in Al2O3, Cr2O3, and

V2O5 are 514, 447, and 383 kJ mol21,

respectively.72 The importance of the

strength of the metal–O bond in common

oxides to explain the thermal stability

was also suggested by Low et al.72 Fig. 6

shows the summarized relative stabilities

of Me-BDCs to show a dominant effect

of metal ions on stability.71

The effects of functional groups tagged

to MOFs on the thermal or chemical

stability of analogous MOFs have been

reported recently. Kandiah et al.73 eval-

uated the stability of tagged UiO-66s

under a variety of a wide range of

conditions. Thermal stability, determined

with TGA and temperature-dependent

XRD, decreased in the order of UiO-66

y UiO-66-Br & UiO-66-NO2 ¢ UiO-

66-NH2. However, the chemical stability

to a base (pH = 14) was UiO-66 y UiO-

66-NO2 . UiO-66-Br ¢ UiO-66-NH2

even though all the materials were stable

to an acidic solution (pH = 1). Moreover,

there has been no clear explanation

regarding which properties of the func-

tional group are responsible for the

thermal and chemical stabilities of the

tagged MOFs. The detrimental effects of

the amino group on the thermal stability

of tagged MOFs such as MIL-53(Al)-

NH274,75 and MOF-5-NH2

76,77 were

Fig. 5 Synthesis steps of Me-BDCs to show

the relative synthesis rates of the Me-BDCs

such as MIL-53s(Al, Cr) and MIL-47(V). The

deprotonation step is faster than complexation

to lead to MOFs.66

Fig. 6 Schematic presentation of relative chemical and thermal stabilities of Me-BDCs such as MIL-53s(Al, Cr) and MIL-47(V).71

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observed a few times mainly by using

TGA results. On the other hand, by

using more branched ligands, the stabi-

lity of the MOFs could be improved.78

However, there has been little explana-

tion as to why the stability of the tagged

MOFs can be changed with the func-

tional groups. Therefore, more detailed

works will be needed to understand the

relative stabilities of analogous MOFs

because the stability of MOFs is very

important for commercial applications.

3.3. Adsorption with analogous MOFs

3.3.1. Adsorption with analogousMOFs composed of different metal ions.So far, several studies on the adsorption

of gases have been carried out using

analogous MOFs like CPO-27s. Zhou

et al.23 studied the hydrogen adsorption

over CPO-27s having CUS. They showed

the strong effect of metal ions on the

binding strength and adsorption capa-

city. The binding strength of hydrogen to

the MOFs was in the order of CPO-

27(Ni) . CPO-27(Co) . CPO-27(Mg) .

CPO-27(Mn) . CPO-27(Zn), and these

tendencies might be explained with the

ionic radius of the cations (The 2QST

values for hydrogen on the MOFs were

12.9, 10.7, 10.1 8.8 and 8.5 kJ mol21; and

the ionic radii of the metal ions are 0.63,

0.67, 0.66, 0.75 and 0.68 A, respectively).

The high binding strength of H2 with

Ni(II), compared with Mg(II), was also

observed,79 and a calculation also sug-

gested the high binding strength of

hydrogen with Ni(II).80 Recently,

FitzGerald et al.81 also demonstrated

the importance of the identity of metal

ions (Co, Mg, Mn, Ni, Zn) of CPO-27s in

isosteric enthalpy of adsorption. Even

though there was no detailed explana-

tion, MIL-53(Al) showed higher hydro-

gen adsorption than analogous MIL-

53(Cr).82 The effect of metal ions of

MOFs on hydrogen adsorption was also

observed with PCN-9 as the PCN-9(Co)

showed the strongest adsorption among

the PCN-9s(Co, Fe, Mn).83

Analogous CPO-27s were compared

for acetylene adsorption by two research

groups. Xiang et al.84 showed the adsorp-

tion capacity was CPO-27(Co) . CPO-

27(Mn) . CPO-27(Mg) . CPO-27(Zn),

due to a strong affinity of acetylene to

Co(II). Chavan et al.85 showed the

strongest adsorption of acetylene over

CPO-27(Ni), compared with CPO-

27(Co) or CPO-27(Fe), due to the small

size of Ni(II). Bae et al.86 showed a very

high selective adsorption of propylene,

compared with propane, over CPO-

27s(Co, Mg, Mn). The unusual selectivity

of propylene over CPO-27s could be

explained partly by the proper match

between the pore size of CPO-27s and the

size of the adsorbates. Compared with

CPO-27(Mg) and CPO-27(Mn), CPO-

27(Co) had a remarkable selectivity

(propylene/propane) of 46 due to the

strong adsorption of propylene over

CPO-27(Co), similar to the acetylene

adsorption.84 Very recently, Bloch

et al.87 showed that CPO-27(Fe), because

of its softer metal character and higher

surface area, compared with analogous

CPO-27(Mg), could be used in the

separation of ethylene/ethane and propy-

lene/propane mixtures. Moreover, they

suggested that a mixture of methane,

ethane, ethylene, and acetylene could be

separated by using just three-packed beds

of CPO-27(Fe) (Fig. 7). Yoon et al.

showed selective adsorption of propy-

lene, compared with propane, over the

reduced MIL-100(Fe) because of a

strong interaction between Fe(II) and

the unsaturated molecules having either

a double or triple bond.88 This selective

adsorption over MIL-100(Fe) probably

may not be observed over the analogous

MIL-100(Al) due to the lack of redox

property, suggesting the importance of

central metal ions in the selective adsorp-

tion.

The adsorption of carbon dioxide was

also studied with CPO-27s.21 The

adsorption capacity was CPO-27(Mg) .

CPO-27(Co) y CPO-27(Ni) . CPO-

27(Zn), and the best performance over

CPO-27(Mg) could be explained with the

increased ionic character of the Mg–O

bond (The electronegativities of O, Mg,

Co, Ni and Zn are 3.5, 1.2, 1.9, 1.9 and

1.6, respectively) and the low density of

Mg.21 A similar beneficial effect of the

low density of Mg was reported in the

adsorption of carbon dioxide and

methane over CPO-27s.89 Britt et al.90

also studied CO2 adsorption over CPO-

27s, and found a high efficiency over

CPO-27(Mg) due to a favourable inter-

action between CO2 and the Mg-ion of

the MOF.

Carbon dioxide and methane were

adsorbed over MIL-53s(Al, Cr) and

MIL-47(V).91 Even though methane

showed type-I adsorption isotherms over

the three MOFs, carbon dioxide had a

type-I isotherm over only MIL-47(V).

Instead, MIL-53s(Al, Cr) showed a step-

wise adsorption for polar CO2 due to a

breathing property originated from m2-

OH34 of MIL-53s(Al, Cr). By the grand

canonical Monte Carlo calculation, the

result of CO2 adsorption over the MIL-

53s(Al, Cr) and MIL-47(V) was inter-

preted.92 The step-wise adsorption of

CO2 over MIL-53(Al) was due to an

interaction between CO2 and m2-OH. On

the contrary, in the case of MIL-47(V),

there was no preferential adsorption site

for CO2, and only a weak interaction

between CO2 and m2-O was possible.92

By applying elastic layer-structured

MOFs, Kondo et al.93 also found that

the adsorption of permanent gases

depended on the types of metal ions of

MOFs.

n-Hexane and n-nonane were adsorbed

over MIL-53(Cr) and MIL-47(V), and

MIL-53(Cr) showed sub-steps in adsorp-

tion due to the breathing property34 of

the MOF; however, MIL-47(V) had a

type-I adsorption isotherm.94 Light

hydrocarbons (C1–C4) also showed simi-

lar results to have steps in the adsorption

Fig. 7 Schematic representation of the separation of a mixture of methane, ethane, ethylene, and acetylene using just three packed beds of CPO-

27(Fe) in a vacuum swing adsorption or temperature swing adsorption process.87

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over the MIL-53(Cr), which was different

from that over MIL-47(V).95 MIL-53(Fe)

and MIL-53s(Al, Cr) showed different

characteristics in the adsorption of short

linear alkanes.96 MIL-53(Fe) showed

multi-steps in adsorption isotherms

because of the existence of four discrete

pore-openings and an initially closed

structure.96

Various toxic gases like ammonia,

cyanogen chloride, and sulfur dioxide

were adsorbed over the analogous CPO-

27s(Co, Mg, Ni, Zn). It was reported that

the metal centers had a strong influence

on the adsorption capacity.97 However,

there was no explanation for this and the

surface areas of the MOFs were very

different from one another. Hamon

et al.98 did a comparative study of H2S

adsorption over MIL-53s(Al, Cr, Fe) and

MIL-47(V). Even though the adsorption

capacities at high pressure were similar

with one another, the adsorption char-

acteristics at low pressure were not

similar with one another. MIL-47(V)

had a type-I isotherm; however, the

MIL-53s showed steps in isotherms due

to the breathing property.34 H2S inter-

acts strongly with the m2-OH of inorganic

chain of MIL-53s because of the polarity

of H2S. At low loading the pore was

closed; and reopened with increasing

pressure; and finally the entire pore was

filled with H2S. Interestingly, MIL-

53(Fe) was not stable against H2S

because of the formation of an iron–

sulfur compound and terephthalic acid.98

Decorated or doped cations in MOFs

also have an influence on the adsorption

capacity and enthalpy of hydrogen,99,100

showing the importance of cations in

adsorption.

Liquid–phase adsorption over analo-

gous MOFs were also done to remove

sulfur-containing compounds or nitro-

gen-containing compounds from fuels.

As shown in Fig. 8, Khan et al.101

showed that benzothiophene adsorption

capacity over MIL-47(V) was much

higher than that over MIL-53s(Al, Cr),

and they explained the good performance

of MIL-47(V) with the high acidity of the

phase.101 Moreover, different to MIL-

53s(Al, Cr), MIL-47(V) was very efficient

in the selective reduction of Cu(II) to

Cu(I), which was very effective for

adsorbing benzothiophene via p-com-

plexation.102,103

Maes et al. studied the adsorptive

removal of nitrogen-containing com-

pounds using analogous MOFs, MIL-

100s(Al, Cr, Fe) and CPO-27s(Ni,

Co).104 There was little effect of metal

ions on the adsorptive denitrogenation.

They also checked the adsorptive desul-

furization, and they could explain per-

formance of adsorption with Pearson’s

HSAB (hard/soft acid/base) concept.

Therefore, thiophenes could be adsorbed

on soft MIL-100(Fe(II)), produced from

MIL-100(Fe(III)) by careful reduction,

even though the virgin MIL-100(Fe(III))

did not adsorb the thiophenes noticeably.

This may also show the effect of central

ions on adsorption because MIL-100(Al)

is not reducible under similar conditions.

Horcajada et al. used MIL-53s(Cr, Fe) in

ibuprofen storage/delivery, and they

found little effect of the central metal

ions on adsorption/delivery.105

Therefore, it can be concluded that

central metal ions or ion clusters have a

dominant role in adsorption even though

there is little effect in some cases.

Important factors to improve adsorption

capacity and selectivity might be size,

binding strength, ionic character, density,

redox ability, softness/hardness, and

acidity/basicity of metal ions.

3.3.2. Adsorption with analogousMOFs composed of different linkers.Lee et al.106 could kinetically separate

propylene from propane by controlling

the diffusion rates in plate-shaped

MOFs. The separation factor could be

increased by tuning the pore apertures

(with changing organic linkers) and

aspect ratios of the MOF crystals,

showing the importance of organic lin-

kers in the separation. Very recently, the

selective uptake of CO2, compared with

CH4, was achieved even in the absence of

CUS by optimizing linkers in the synth-

esis of MOFs.107

Chun et al.56 synthesized various

analogous MOFs by changing the linkers

and pillars, and found that wavy chan-

nels and small openings were beneficial

for hydrogen physisorption. Yuan

et al.108 suggested that a high surface

area could be obtained by using linkers

with lengths of 11.2–13.8 A in the

synthesis of PCN-series MOFs. More-

over, they showed that the surface area

of MOFs could be improved by using

more branched ligands.108 It was also

reported that longer ligands led to

MOFs with low density, high poro-

sity and a high hydrogen adsorption

capacity.109,110

Even though there are not many

results to understand the effect of linkers

on adsorption or porosity, it is clear that

an adequate selection of organic linkers

is very important regarding the capacity

and selectivity of various adsorption.

3.3.3. Adsorption with analogousMOFs with tagged linkers. Some inter-

esting results have been reported to show

the effect of functional groups tagged

onto analogous MOFs on adsorption.

For example, Ramsahye et al.61 have

demonstrated that modified MIL-

53(Fe)s (having functional groups of

–NH2, –CH3, –Cl, Br) showed very

striking differences, compared with vir-

gin MIL-53(Fe), in the adsorption of

normal alkanes (Fig. 9) by facilitating

pore filling.

MIL-53(Al)-NH2, obtained by using

amino-terephthalic acid in synthesis, was

effective in the separation of CO2 from

CH4 because of the strong interaction

(electron donor–acceptor complex for-

mation) between CO2 and the amino

group of the MOFs.111 Moreover, it was

reported that the polar functional group

was effective in CO2 adsorption because

of the CO2 quadrupole (and dipole

functional group). Eddaoudi et al.15 sug-

gested that a hydrophobic substituent

like a C2H4 unit in IRMOF-6 was

beneficial for increasing the methane

adsorption capacity. Devic et al.57

showed that MIL-53(Fe)-(CF3)2 could

adsorb nitrogen even though virgin

Fig. 8 Adsorption isotherms for benzothio-

phene adsorption over the Me-BDCs like

MIL-53s(Al, Cr) and MIL-47(V) at 25 uC.

Reproduced with permission from ref. 101.

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MIL-53(Fe) and all other modified MIL-

53(Fe)s (with –Cl, –Br, –CF3, –NH2,

–OH, –COOH) could not readily adsorb

nitrogen, suggesting the importance of

the functional group in adsorption.

There were also a few attempts to

increase hydrogen adsorption by fluor-

ination of the MOFs; however, not only

positive but also negative results were

also observed.112,113

3.3.4. Other properties with analo-gous MOFs. Shigematsu et al. studied

the proton conductivity of MIL-53s(Al,

Fe) tagged with various functional

groups.60 They found that proton con-

ductivity was dependent on the func-

tional groups tagged on ligands, and

proton conductivity increased with low-

ering the pKa value of meta-substituted

benzoic acids (Fig. 10).

The photocatalytic activity of the

analogous MOFs, MIL-53s(Al, Cr, Fe),

was evaluated to compare the perfor-

mance to degrade methylene blue by UV

irradiation.114 There was little effect of

the metal ions on the photocatalytic

activity even though the band gap was

in the order of Eg(MIL-53(Al)) .

Eg(MIL-53(Cr)) . Eg(MIL-53(Fe)).

Analogous Y- and Ln-BTC, MIL-78

(monoclinic),115 could be synthesized

from various metal ions (Y and Ln: Pr,

Nd, Sm, Eu, Gd, Tb, Dy, Er). The unit

cell volume increased linearly with the

ionic radii of the metal ions. The MIL-78

could also be synthesized with mixed

cations to lead to MOFs like Y12xEux-

BTC. Lanthanide doped Y-BTCs had

very efficient luminescent properties to

show red, green and blue emission for the

Eu,3+ Tb3+ and Dy3+ doped Y-BTCs,

respectively, showing the importance of

metal ions in the optical properties of

analogous MOFs.

4. Summary and conclusions

In this highlight, analogous MOFs hav-

ing different central metal ions, linkers

and tagged functional groups have been

classified in three categories. Moreover,

the effects of the central metal ions,

linkers and tagged functional groups of

analogous MOFs on synthesis kinetics,

chemical/thermal stability and adsorptive

performance have been reviewed.

From this highlight, it is clear that

central metal ions, linkers and tagged

functional groups have a noticeable

influence on synthesis, stability and

adsorption. Firstly, synthesis kinetics

for analogous MOFs can be explained

with the lability of metal ions. Even in

acidic conditions, the deprotonation of

dicarboxylic acids or tricarboxylic acids

to dicarboxylates or tricarboxylates,

respectively, is faster than complexation

for the formation of MOF structures.

The chemical stability of analogous

Fig. 9 Comparison of properties of the modified MIL-53(Fe)-X in the adsorption of n-hexane. MIL-53(Fe)-4H means the non-modified MIL-

53(Fe). Reproduced with permission from ref. 61. Copyright 2011 American Chemical Society.

Fig. 10 Arrhenius plots of the proton con-

ductivities of MIL-53(Al) (blue, square),

MIL-53(Al)-NH2 (pink, inverse triangle),

MIL-53(Al)-OH (green, triangle), and MIL-

53(Fe)-(COOH)2 (red, circle) under 95% rela-

tive humidity conditions. Least squares fits are

shown as dotted lines. Reproduced with

permission from ref. 60. Copyright 2011

American Chemical Society.

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MOFs with different metal ions can be

explained with the inertness of central

metal ions. The thermal stability of

analogous MOFs with different metal

ions can be explained by neither inert-

ness nor the average bond strength

between metal and oxygen. Instead,

the strength of the metal–oxygen bond

in common oxides may be used to

explain the thermal stability of analo-

gous MOFs. Analogous MOFs with

different linkers or tagged functional

groups has shown different thermal or

chemical stabilities (excluding the poor

thermal stability of amino-MOFs).

However, so far, a clear explanation

is unavailable, suggesting the necessity

of further studies for commercial

applications.

Analogous MOFs have been applied in

various ways for both gas/vapor-phase

and liquid-phase adsorption. Metal ions,

organic linkers or tagged functional

groups have shown a remarkable impact

on adsorption. The effects of metal ions

may be explained with size, binding

strength, ionic character, density, redox

ability, softness/hardness, and acidity/

basicity of metal ions. The effect of

linkers or functional groups may be

explained with length, polarity, hydro-

phobicity, etc. However, no comprehen-

sive explanation for the effect of metal

ions, organic linkers or tagged functional

groups on various adsorption is avail-

able. Therefore, it is strongly recom-

mended to study adsorption in more

detail to understand the effects of metal

ions, organic linkers or tagged functional

groups on adsorption.

Acknowledgements

The authors would like to express their

sincere thanks to Prof. G. Ferey for his

valuable comments for this highlight.

This research was supported by Basic

Science Research Program through the

National Research Foundation of Korea

(NRF) funded by the Ministry of

Education, Science and Technology

(grant number 2012004528).

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63 Y. K. Hwang, D.-Y. Hong, J.-S. Chang,S. H. Jhung, Y.-K. Seo, J. Kim, A.Vimont, M. Daturi, C. Serre and G.Ferey, Angew. Chem., Int. Ed., 2008, 47,4144–4148.

64 N. Stock and S. Biswas, Chem. Rev.,2012, 112, 933–969.

65 J. Klinowski, F. A. A. Paz, P. Silva and J.Rocha, Dalton Trans., 2011, 40, 321–330.

66 E. Haque, J. H. Jeong and S. H. Jhung,CrystEngComm, 2010, 12, 2749–2754.

67 E. Haque and S. H. Jhung, Chem. Eng. J.,2011, 173, 866–872.

68 J. E. Huheey‘‘Inorganic Chemistry’’, 3rded., Harper & Row, Cambridge, 1983, pp547-550. The kinetic constant (s21) ofwater exchange rates of aqua Cr3+, Al3+,V3+, Ni2+, Co2+ and Zn2+ ions are about1026.2, 101.3, 103.0,104.3, 106.2 and 107.2,respectively.

69 F. A. Cotton and G. Wilkinson‘‘AdvancedInorganic Chemistry’’, 4th ed., John Wiley &Sons, Inc., New York, 1980, pp. 1188-1189.

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70 T. Ahnfeldt and N. Stock, CrystEngComm,2012, 14, 505–511.

71 I. J. Kang, N. A. Khan, E. Haque andS. H. Jhung, Chem.–Eur. J., 2011, 17,6437–6442.

72 J. J. Low, A. I. Benin, P. Jakubczak, J. F.Abrahamian, S. A. Faheem and R. R.Willis, J. Am. Chem. Soc., 2009, 131,15834–15842.

73 M. Kandiah, M. H. Nilsen, S. Usseglio,S. Jakobsen, U. Olsbye, M. Tilset, C.Larabi, E. A. Quadrelli, F. Bonino andK. P. Lillerud, Chem. Mater., 2010, 22,6632–6640.

74 S. J. Garibay and S. M. Cohen, Chem.Commun., 2010, 46, 7700–7702.

75 S. Marx, W. Kleist, J. Huang, M.Maciejewski and A. Baiker, DaltonTrans., 2010, 39, 3795–3798.

76 W. Kleist, F. Juth, M. Maciejewski andA. Baiker, Eur. J. Inorg. Chem., 2009,3552–3561.

77 W. Kleist, M. Maciejewski and A. Baiker,Thermochim. Acta, 2010, 499, 71–78.

78 D. Yuan, D. Zhao, D. Sun and H.-C.Zhou, Angew. Chem., Int. Ed., 2010, 49,5357–5361.

79 P. D. C. Dietzel, P. A. Georgiev, J.Eckert, R. Blom, T. Strassle and T.Unruh, Chem. Commun., 2010, 46,4962–4964.

80 M. Kosa, M. Krack, A. K. Cheethamand M. Parrinello, J. Phys. Chem. C,2008, 112, 16171–16173.

81 S. A. FitzGerald, B. Burkholder, M.Friedman, J. B. Hopkins, C. J. Pierce,J. M. Schloss, B. Thompson and J. L. C.Rowsell, J. Am. Chem. Soc., 2011, 133,20310–20318.

82 G. Ferey, M. Latroche, C. Serre, F.Millange, T. Loiseau and A. Percheron-Gue gan, Chem. Commun. , 2003,2976–2977.

83 S. Ma, D. Yuan, J.-S. Chang and H.-C.Zhou, Inorg. Chem., 2009, 48, 5398–5402.

84 S. Xiang, W. Zhou, Z. Zhang, M. A.Green, Y. Liu and B. Chen, Angew.Chem., Int. Ed., 2010, 49, 4615–4618.

85 S. M. Chavan, G. C. Shearer, E. Blochand S. Bordiga, ChemPhysChem, 2012,13, 445–448.

86 Y.-S. Bae, C. Y. Lee, K. C. Kim, O. K.Farha, P. Nickias, J. T. Hupp, S. T.Nguyen and R. Q. Snurr, Angew. Chem.,Int. Ed., 2012, 51, 1857–1860.

87 E. D. Bloch, W. L. Queen, R. Krishna,J. M. Zadrozny, C. M. Brown and J. R.Long, Science, 2012, 335, 1606–1610.

88 J. W. Yoon, Y.-K. Seo, Y. K. Hwang,J.-S. Chang, H. Leclerc, S. Wuttke, P.Bazin, A. Vimont, M. Daturi, E. Bloch,P. L. Llewellyn, C. Serre, P. Horcajada,J.-M. Greneche, A. E. Rodrigues and G.Frey, Angew. Chem. Int. Ed., 2010, 49,5949–5952.

89 P. D. C. Dietzel, V. Besikiotis and R.Blom, J. Mater. Chem., 2009, 19,7362–7370.

90 D. Britt, H. Furukawa, B. Wang, T. G.Glover and O. M. Yaghi, Proc. Natl.Acad. Sci. U. S. A. , 2009, 106 ,20637–20640.

91 S. Bourrelly, P. L. Llewellyn, C. Serre, F.Millange, T. Loiseau and G. Ferey,J. Am. Chem. Soc. , 2005, 127 ,13519–13521.

92 N. A. Ramsahye, G. Maurin, S.Bourrelly, P. L. Llewellyn, T. Devic, C.Serre, T. Loiseau and G. Ferey,Adsorption, 2007, 13, 461–467.

93 A. Kondo, A. Chinen, H. Kajiro, T.Nakagawa, K. Kato, M. Takata, Y.Hattori, F. Okino, T. Ohba, K. Kanekoand H. Kanoh, Chem.–Eur. J., 2009, 15,7549–7553.

94 T. K. Trung, I. Deroche, A. Rivera, Q.Yang, P. Yot, N. Ramsahye, S. D. Vinot,T. Devic, P. Horcajada, C. Serre, G.Maurin and P. Trens, MicroporousMesoporous Mater., 2011, 140, 114–119.

95 N. Rosenbach Jr, A. Ghoufi, I. Deroche,P. L. Llewellyn, T. Devic, S. Bourrelly,C. Serre, G. Ferey and G. Maurin, Phys.Chem. Chem. Phys., 2010, 12, 6428–6428.

96 P. L. Llewellyn, P. Horcajada, G.Maurin, T. Devic, N. Rosenbach, S.Bourrelly, C. Serre, D. Vincent, S.Loera-Serna, Y. Filinchuk and G.Feerey, J. Am. Chem. Soc., 2009, 131,13002–13008.

97 T. G. Glover, G. W. Peterson, B. J.Schindler, D. Britt and O. Yaghi, Chem.Eng. Sci., 2011, 66, 163–170.

98 L. Hamon, C. Serre, T. Devic, T.Loiseau, F. Millange, G. Ferey andG. D. Weireld, J. Am. Chem. Soc.,2009, 131, 8775–8777.

99 K. L. Mulfort and J. T. Hupp, Inorg.Chem., 2008, 47, 7936–7938.

100 T. A. Maark and S. Pal, Int. J. HydrogenEnergy, 2010, 35, 12846–12857.

101 N. A. Khan, J. W. Jun, J. H. Jeong andS. H. Jhung, Chem. Commun., 2011, 47,1306–1308.

102 N. A. Khan and S. H. Jhung, Angew.Chem., Int. Ed., 2012, 51, 1198–1201.

103 N. A. Khan and S. H. Jhung, 2012, to besubmitted.

104 M. Maes, M. Trekels, M. Boulhout, S.Schouteden, F. Vermoortele, L. Alaerts,D. Heurtaux, Y.-K. Seo, Y. K. Hwang,J.-S. Chang, I. Beurroies, R. Denoyel, K.Temst, A. Vantomme, P. Horcajada, C.Serre and D. E. De Vos, Angew. Chem.,Int. Ed., 2011, 50, 4210–4214.

105 P. Horcajada, C. Serre, G. Maurin, N.A.Ramsahye, F. Balas, M. Vallet-Regı, M.Sebban, F. Taulelle and G. Ferey, J. Am.Chem. Soc., 2008, 130, 6774–6780.

106 C. Y. Lee, Y.-S. Bae, N. C. Jeong, O. K.Farha, A. A. Sarjeant, C. L. Stern, P.Nickias, R. Q. Snurr, J. T. Hupp andS. T. Nguyen, J. Am. Chem. Soc., 2011,133, 5228–5223.

107 S. D. Burd, S. Ma, J. A. Perman, B. J.Sikora, R. Q. Snurr, P. K. Thallapally, J.Tian, L. Wojtas and M. J. Zaworotko,J. Am. Chem. Soc., 2012, 134, 3663–3666.

108 D. Yuan, D. Zhao, D. Sun and H.-C.Zhou, Angew. Chem., Int. Ed., 2010, 49,5357–5361.

109 S. Chavan, J. G. Vitillo, D. Gianolio, O.Zavorotynska, B. Civalleri, S. Jakobsen,M. H. Nilsen, L. Valenzano, C. Lamberti,

CrystEngComm This journal is � The Royal Society of Chemistry 2012

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110 I. Senkovska, F. Hoffmann, M. Froba, J.Getzschmann, W. Bohlmann and S.Kaskel , Microporous MesoporousMater., 2009, 122, 93–98.

111 S. Couck, J. F. M. Denayer, G. V.Baron, T. Remy, J. Gascon and F.

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112 P. Pachfule, Y. Chen, J. Jiang and R.Banerjee, Chem.–Eur. J., 2012, 18,688–694.

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This journal is � The Royal Society of Chemistry 2012 CrystEngComm

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