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Exceptional surface area from coordination copolymers derived from twolinear linkers of differing lengths†

Kyoungmoo Koh, Jacob D. Van Oosterhout, Saikat Roy, Antek G. Wong-Foy and Adam J. Matzger*

Received 25th January 2012, Accepted 1st May 2012

DOI: 10.1039/c2sc20407j

Although a multitude of microporous coordination polymers (MCPs) with ultrahigh surface area have

been reported in the last decade, none of these can come close to matching the cost/performance ratio of

conventional sorbents such as zeolites and carbons for most applications. There is a need to drastically

reduce the cost of MCPs and this goal cannot be achieved through complex linker synthesis strategies

so often used to boost MCP performance. Here two new MCPs: UMCM-8 (Zn4O(benzene-1,4-

dicarboxylate)1.5(naphthalene-2,6-dicarboxylate)1.5), and UMCM-9 (Zn4O(naphthalene-2,6-

dicarboxylate)1.5(biphenyl-4,40-dicarboxylate)1.5) are described and the concept of using mixtures of

readily available linear linkers that enforce different spacings between network nodes is introduced as

a means to reduce interpenetration. These new MCPs demonstrate Brunauer–Emmett–Teller (BET)

surface areas over 4000 m2 g�1 and high pore volumes over 1.80 cm3 g�1.

Introduction

The development of new crystalline microporous coordination

polymers (MCPs) has fascinated researchers in both academic

and industrial fields not only because of unique features such as

ultrahigh surface area1–3 and regular pore structure but also for

the potential commercial impact of this novel sorbent class for

various applications such as gas storage,4 separations,5,6 and

catalysis.7 The general synthetic approach to MCPs involves the

assembly of rigid molecular building blocks composed of metal

clusters/metal ions and organic ligands (linkers).8 Even though

various synthetic approaches have been explored for the

construction of MCPs, the rational design of new structures

remains a challenge for highly porous materials. Moreover, if the

constraint of using inexpensive and commercially available

feedstocks is introduced, a requisite step towards producing

commercially viable materials, there are no materials displaying

exceptional BET surface areas. In many cases, the synthesis of

new MCPs relies on serendipity, but there are a few cases in

which design of MCPs using building units with simple geometry

can be generally realized. For example, the combination of

octahedral metal clusters and linear organic linkers yields the

IRMOF-X (Isoreticular Metal Organic Framework-X) series9–25

wherein Zn4O clusters act as metal nodes and linear dicarbox-

ylates connect metal clusters to generate a cubic net. Theoreti-

cally, the elongation of an organic linker in the cubic net

Department of Chemistry, Macromolecular Science and Engineering,University of Michigan, Ann Arbor, Michigan 48109-1055, USA. E-mail:matzger@umich.edu; Fax: +1 734 615 6627; Tel: +1 734 615 6627

† Electronic supplementary information (ESI) available: Powder X-raydiffraction, 1H NMR spectrum, gas sorption isotherms, and disclaimer.See DOI: 10.1039/c2sc20407j

This journal is ª The Royal Society of Chemistry 2012

structure should lead to increasing pore volume and surface

area,26 but experimentally, this linker-elongation approach has

met with severe limitations with regard to improving surface area

because expanding free volume is accompanied by structural

interpenetration which reduces surface area by creating inac-

cessible regions where two frameworks are in contact.

Non-interpenetrated cubic MCPs have been produced by

solvothermal reaction under very dilute conditions27 or by

a surface-induced method.28 However, these approaches are not

suited to bulk production. An alternative means to reduce the

extent of interpenetration is random copolymerization using two

differentially substituted linear linkers of identical length;29

however bulky substituents decrease surface areas and cannot be

deployed with commercial feedstocks. Recently we have

demonstrated that breaking the symmetry of network nodes

suppresses structural interpenetration.30,31 By contrast, here we

demonstrate a novel copolymerization strategy using two linear

linkers with different lengths wherein the effects of suppressing

network interpenetration are realized without altering local node

symmetry.

Results and discussion

Combining two linear linkers of different lengths with octahedral

metal clusters yields two cubic cages and two tetragonal cages

(Fig. 1). Two cubic cages are generated with single bridging

components. Two tetragonal cages are generated: one cage is

built from eight long linkers and four short linkers whereas the

other cage arises from four long linkers and eight short linkers.

The assembly of these cages is determined by coordination

modes of the two linkers to a metal cluster and eight different

structures can be obtained (Fig. S1, ESI†).

Chem. Sci., 2012, 3, 2429–2432 | 2429

Fig. 1 Possible cage structures from the assembly of octahedral nodes

and two different linear linkers.

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For example, when three short and three long linkers coordi-

nate to an octahedral cluster, two possible coordination modes

with meridional and facial fashion can be considered (Fig. 2).

The framework of the facial coordination mode is built with the

assembly of two cubic cages in a corner sharing fashion and the

assembly of two tetragonal cages in a face sharing fashion. On

the other hand, in the meridional mode the framework is built

with two tetragonal cage layers alternating.

When benzene-1,4-dicarboxylic acid (H2BDC) is used as the

short ditopic linker and naphthalene-2,6-dicarboxylic acid

(H2NDC) as the long ditopic linker, the distance between

carboxylate carbons of H2NDC is 1.3 times longer than for

H2BDC. As background, it is noted that in the presence of zinc

nitrate and diethylformamide (DEF), pure H2BDC reacts to

generate Zn4O(BDC)3 (MOF-5) with a simple cubic net. The

BET surface area of MOF-5 is approximately 3530 m2 g�1.32

H2NDC reacts with zinc nitrate to yield a material which has

been formulated as Zn4O(NDC)3 (IRMOF-8).9 The structure of

this material is somewhat ambiguous and it has been suggested

that IRMOF-8 in fact has an interpenetrated structure.33 The

experimental BET surface area (�1466 m2 g�1) of IRMOF-834 is

much lower than the accessible surface area derived from the

(non-interpenetrated) crystal structure (4390 m2 g�1)26 consistent

with at least partial interpenetration not detected by crystallog-

raphy. In this study, H2BDC and H2NDC were reacted with zinc

nitrate in the presence of DEF at 85 �C. After reaction, the

resultant crystalline solids were washed with dimethylformamide

(DMF) and stored in CH2Cl2. The composition of linkers in the

Fig. 2 Three short linkers and three long linkers coordinated octahedral

nodes with facial (a) and meridional (b) fashion and the resultant struc-

tures from the coordination modes.

2430 | Chem. Sci., 2012, 3, 2429–2432

product was ascertained by decomposing the fully dried

compound in 1 M NaOH in D2O solution. To determine the

effect of linker mixing on surface area, the BET approximation

was applied based on N2 sorption experiments. The samples,

dispersed in CH2Cl2, were charged into tubes for measurement

and were evacuated at room temperature to remove all guest

molecules. At a BDC content of 45 mol%, the BET surface areas

of the product increase to over 4000 m2 g�1. At a molar content of

NDC >50%, the BET surface area decreases towards that of

IRMOF-8 (Fig. 3). From the perspective of controlling the

degree of structural interpenetration using two topologically

similar organic linkers, the results in this study are similar to our

previous findings for the copolymerization of a sterically bulky

and non-bulky linker.29 However, it should be noted that the

driving force for suppressing the interpenetration in this study is

fundamentally different and is the result of a change in spacing

between metal clusters.

PXRD analysis of the crystalline product at 45 mol% BDC

revealed that the structure of the product was different from that

derived from the pure linkers (Fig. S2, ESI†). Single crystal X-ray

diffraction data were unsatisfactory due to low resolution of

multiple crystals studied at a variety of temperatures and with

different solvent exchange conditions; therefore, PXRDwas used

to determine product structures. Pawley refinement using

a simulated PXRD pattern from the model structure (Fig. 2) and

the experimental PXRD data revealed the product has a struc-

ture assembled with MOF-5 cages and IRMOF-8 cages in

a corner-sharing fashion (Fig. 4a). The framework of the mate-

rial consists of Zn4O metal clusters linked together by three BDC

and three NDC linkers arranged in an octahedral geometry in

a facial mode. The structure includes two cubic and two tetrag-

onal microporous cages (Fig. 5). The product constructed with

BDC and NDC linkers is denoted as UMCM-8 (University of

Michigan Crystalline Material-8). The BET surface area of

UMCM-8 is 4030 m2 g�1, which matches well with the theoretical

surface area (4005 m2 g�1) calculated from the model structure

using the accessible surface area method established by Snurr

and co-workers (Fig. S7–8, ESI†).35

Using H2NDC as a short linker and biphenyl 1,4-dicarboxylic

acid (H2BPDC) as a long linker in the presence of zinc nitrate, an

extended (isoreticular) analogue of UMCM-8 was produced

Fig. 3 BET surface areas of products prepared from various mole

fractions of H2BDC and H2NDC in the presence of zinc nitrate.

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 (a) Pawley refinement results from simulated and experimental

powder X-ray diffraction patterns of UMCM-8. (b) Pawley refinement

results from simulated and experimental powder X-ray diffraction

patterns of UMCM-9.

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(Fig. 5). This result was confirmed by 1H NMR analysis and

Pawley refinement of the PXRD pattern (Fig. 4b). Dubbed

UMCM-9, this compound has the formula: (Zn4O(NDC)1.5-

(BPDC)1.5). Based on its structure, the accessible surface area of

UMCM-9 is calculated to be 4900 m2 g�1. However, the experi-

mental BET surface area of UMCM-9 was determined to be only

1330 m2 g�1 after immersing the product in CH2Cl2 and evacu-

ation. The altered PXRD pattern after solvent removal and

accompanying decrease in surface area results from structure

collapse. To attempt a more mild activation procedure, a super-

critical CO2 activation was applied to UMCM-9 to remove the

solvent. In contrast to the conventional batch supercritical CO2

activation protocol,36 a flow system was employed to reduce the

sample purge time required before converting to the supercritical

Fig. 5 Structure comparison of MOF-5, IRM

This journal is ª The Royal Society of Chemistry 2012

state. After activation, the BET surface area of UMCM-9 was

4970 m2 g�1 (Fig. S9–10, ESI†). No structural changes after

supercritical CO2 activation of UMCM-9 were found by PXRD

analysis.

Conclusions

The results reported here amplify earlier conclusions that

combinatorial approaches involving coordination copolymeri-

zation are likely the best route to newmaterials discovery; amajor

finding is that the considerable library of ditopic linkers may

suffice to produce high performance sorbents and this opens the

possibility to use a plethora of commercially available starting

materials. The potential for cost reduction of the sort MCPs need

for commercial viability is therefore coming within reach.

Experimental

Synthesis of UMCM-8

H2BDC (13.1 mg, 0.0789 mmol) and H2NDC (17.2 mg,

0.0796 mmol) were dissolved in 6 mL of DEF and the solution

was clarified by filtration. Zn(NO3)2$6H2O (0.142 g, 0.477 mmol)

was added to the solution in a 20 mL vial which was then capped.

The mixture was sonicated for 15 min and heated to 85 �C. After

3 days, crystals of a single phase were obtained. After cooling to

room temperature over the course of 30 min, the product was

isolated by decanting the mother liquor and washing with DMF

(3 � 6 mL). The resulting solid was then immersed in 6 mL of

CH2Cl2 for 2 days, during which time the CH2Cl2 was replaced

three times. The solvent was removed under vacuum at room

temperature, yielding the porous material. The yield of the

reaction, determined from the weight of the solvent-free material

(26.4 mg), is 39.6% based on H2BDC. Anal. calcd for

C30H15O13Zn4: C, 42.63; H, 1.78. Found: C, 42.65; H, 1.74.

Production of UMCM-8 on a larger scale was conducted as

above with H2BDC (0.460 g, 2.77 mmol), H2NDC (0.598 g,

2.77 mmol), and Zn(NO3)2$6H2O (4.25 g, 14.28 mmol). The yield

of the reaction, determined from the weight of the solvent-free

material (1.13 g), is 48.0% based on H2BDC.

Synthesis of UMCM-9

H2NDC (28.7 mg, 0.132 mmol) and H2BPDC (35.6 mg,

0.147 mmol) were dissolved in a mixture of 6.7 mL of DEF

and 13.3 mL of N-methyl pyrrolidone. Zn(NO3)2$6H2O

(0.238 g, 0.800 mmol) was added to the solution. The mixture

OF-8, -9, and -10 and UMCM-8 and -9.

Chem. Sci., 2012, 3, 2429–2432 | 2431

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was sonicated for 15 min and heated to 85 �C. After 4 days,

crystals of a single phase were obtained. After cooling to room

temperature the product was isolated by decanting the mother

liquor and washing with DMF (3 � 20 mL). The resulting solid

was then immersed in 20 mL CH2Cl2 for 2 days, during which

time the CH2Cl2 was replaced three times. The sample was stored

in CH2Cl2. The yield of the reaction, determined from the weight

of the solvent-free material (57.9 mg), is 41.8% based on H2NDC.

Anal. calcd for C39H21O13Zn4: C, 48.82; H, 2.19. Found: C,

47.77; H, 2.05.

Supercritical CO2 flow activation of UMCM-9

Activation was performed with a Jasco PU-1580-CO2 delivery

pump equipped with a back pressure regulator (Jasco BP-1580-

81). The CH2Cl2-soaked sample (0.1 g) was placed in a metal

column and CH2Cl2 was exchanged with liquid CO2 at 100 bar;

the liquid CO2-charged column was heated at 35 �C for 30 min.

CO2 was vented over 30 min via the back pressure regulator to

obtain the activated sample.

Acknowledgements

This material is based upon work supported by the U.S.

Department of Energy (DE-SC0004888).

Notes and references

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