This article is part of the

Metal-organic frameworks web themed issue

Guest editors: Neil Champness, Christian Serre and Seth Cohen

All articles in this issue will be gathered together

online at www.rsc.org/metal-organic-frameworks

Dow

nloa

ded

by N

orth

wes

tern

Uni

vers

ity o

n 10

/04/

2013

17:

55:3

6.

Publ

ishe

d on

10

Sept

embe

r 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C2C

C35

711A

View Article Online / Journal Homepage / Table of Contents for this issue

10496 Chem. Commun., 2012, 48, 10496–10498 This journal is c The Royal Society of Chemistry 2012

Cite this: Chem. Commun., 2012, 48, 10496–10498

Understanding excess uptake maxima for hydrogen adsorption isotherms

in frameworks with rht topologywzDavid Fairen-Jimenez,ya Yamil J. Colon,ya Omar K. Farha,

bYoun-Sang Bae,

ac

Joseph T. Huppband Randall Q. Snurr*

a

Received 6th August 2012, Accepted 6th September 2012

DOI: 10.1039/c2cc35711a

For a series of metal–organic frameworks with rht topology, we study

computationally the effect of the linker length on the surface area,

pore size, and pore volume, relating them with the hydrogen adsorp-

tion properties. The results provide new insights about the excess

capacities and the pressures where the uptake maxima in the excess

isotherms occur. We found that, of the materials studied, NU-109/L7

has the optimal pore volume for excess gravimetric hydrogen uptake.

The development of new porous materials for adsorption

applications has turned increasingly to metal–organic frame-

works (MOFs) due to their structural diversity and the resulting

functionality. MOFs are obtained by the self-assembly of metal

clusters and organic linkers, resulting in tailored nanoporous

host materials. MOFs show great promise in many areas,

including industrial gas separation and storage. In particular,

hydrogen storage has received much attention.1 NewMOFs are

being synthesised at a very fast pace today, and for many

materials, hydrogen adsorption is one of the first properties

to be characterised. The high internal surface areas and large

pore volumes make MOFs promising candidates for hydrogen

storage, especially at cryogenic temperatures.2

Efforts to increase the surface area and pore volume of

MOFs have often focused on increasing the length of the organic

linkers. However, the possibility of catenation increases significantly

when using long linkers.3 Network catenation occurs when two or

more independent, identical networks are entangled, partially filling

each other’s pores, and cannot be separated without breaking

bonds. Catenation has been a major concern in the design of

low-density porous structures since it significantly reduces the

pore size and volume. In the last few years, several MOFs with

very large pore volumes and surface areas have been synthesized

using topologies that prevent the possibility of catenation.4–10

Among these materials, we focus here on a series of isoreti-

cular MOFs with (3,24)-paddlewheel-connected networks and

rht-topology, used independently by the groups of Eddaoudi,4

Schroder,5 Zaworotko,6 Zhou,7 and us.8 The rht topology can

be described as having either three or four cages, depending

on whether the curvature of the linkers is taken into account

(Fig. S3 and S4, ESIz).11 These structures share a common

cavity (yellow sphere in Fig. S3 and S4, ESIz) which is enclosed

by the copper paddle wheel corners and the R2 portion of

the linker (Scheme 1) and is, thus, almost independent of the

linker length. In this work, we study computationally the

influence of the linker length on the pore volume, pore size,

surface area, and hydrogen adsorption. We focus particular

attention on the hydrogen capacity and the pressures at which

the maxima in the excess adsorption isotherms occur. We then

compare our simulations with available experimental data and

propose new guidelines for the validation of future results.

Our strategy to build the different structural models is similar

to the scheme we followed in a previous paper.8 We start with

the asymmetric unit ofNOTT-112, synthesized by Schroder and

co-workers.5 We maintain the same copper paddlewheel cluster,

the Fm3m group symmetry, and the rht network topology, but

we substitute the organic linker. Scheme 1 shows the hexa-

protonated precursor of the original NOTT-112 linker and the

Scheme 1 (a) Hexa-protonated precursor of the linker used to construct

the L1–L8 isostructural materials, and (b) the Lx linker fragments.

aDepartment of Chemical & Biological Engineering, NorthwesternUniversity, 2145 Sheridan Road, Evanston, Illinois 60208-3120,USA. E-mail: snurr@northwestern.edu; Tel: +1-847-467-2977

bDepartment of Chemistry and International Institute forNanotechnology, Northwestern University, USA

cDepartment of Chemical and Biomolecular Engineering, YonseiUniversity, 262 Seongsanno, Seodaemun-gu Seoul 120-749, S. Koreaw This article is part of the ChemComm ‘Metal–organic frameworks’web themed issue.z Electronic supplementary information (ESI) available: Simulationprocedures, structural information and adsorption data. See DOI:10.1039/c2cc35711ay These authors contributed equally.

ChemComm Dynamic Article Links

www.rsc.org/chemcomm COMMUNICATION

Dow

nloa

ded

by N

orth

wes

tern

Uni

vers

ity o

n 10

/04/

2013

17:

55:3

6.

Publ

ishe

d on

10

Sept

embe

r 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C2C

C35

711A

View Article Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 10496–10498 10497

new variants we used to build a series of isoreticular MOFs,

where some of these materials have been already synthesised: L1

(PCN-61),7 L2 (NU-111),11 L3 (NOTT-112),5 L4 (NOTT-119/

PCN-69),7,10 L5 (NOTT-116/PCN-68),7,9 L6 (PCN-610/

NU-100)7,12 and L7 (NU-109).13 After the asymmetric unit is

modified, the symmetry operations of the space group allow

the construction of the new structure. The unit cell was then

subject to geometry optimisation based on molecular mechanics,

modifying the size of the unit cell and the atomic coordinates

of the new structure.

The energy minimisations produced eight structures, named

here L1�L8, with cubic unit cell lengths in the range of 43 to

68 A. We performed grand canonical Monte Carlo (GCMC)

simulations on these structures to predict the H2 adsorption

isotherms at 77 K. It is important to note that simulations calculate

the absolute amount of gas adsorbed, NAbs, while experiments

measure the excess adsorption, NExc. These quantities are

related by:

NExc = NAbs � rVpore (1)

where r is the density of the bulk gas (i.e. hydrogen) at the

adsorption pressure and temperature, and Vpore is the pore

volume of the adsorbent. A popular way to estimate the bulk

density is using the Peng–Robinson (PR) equation of state

(EOS).14 Fig. S5 (ESIz) compares the density of hydrogen as a

function of pressure calculated with PR and the experimental

density obtained from the National Institute of Standards and

Technology (NIST).15 The densities match well, but with some

deviations at higher pressures. We focus here on results using

the NIST H2 density data to convert between excess and

absolute isotherms.

We first validated our results by comparing the simulated

isotherms at 77 K on L2 with experimental isotherms on the

analogous structure NU-111. Fig. 1 shows that the simulations

accurately predict the experimental absolute isotherm, calculated

using the H2 bulk density provided by NIST, across all pressures.

For the excess isotherms, the simulations also agree well with

experiment and correctly describe the location of the adsorption

maximum at ca. 40.6 bar when using the NIST data. There are

only small differences at high pressures (ca. > 40 bar) when

using PR due to a small overprediction of the true H2 bulk

density (Fig. S6, ESIz).

Fig. 2a and Fig. S7 (ESIz) present the gravimetric and

volumetric, excess and absolute adsorption isotherms on the

different materials. Tabulated data are available in the ESIz;Table S2 summarizes the main results. On a gravimetric basis,

structure L8 exhibits the highest absolute hydrogen uptake,

whereas L7 has the highest excess hydrogen uptake. It should

be noted that although L8 possesses the largest pore volume,

cavity size, and surface area, it does not exhibit the highest

uptake in terms of excess. This can be explained by eqn (1).

For L8, the large pore volume reduces the excess uptake.

Fig. 2b further illustrates this concept. While larger pore

volume increases the absolute uptake, there is an optimum

pore volume for the excess uptake, which corresponds to

3.61 cm3 g�1 (NU-109/L7). For hydrogen storage applications,

the volumetric capacity is also of great importance, especially for

transportation applications, where the volume of the reservoir is

limited. In terms of volumetric capacity, it is remarkable that the

material ranking inverts, with L1 (34.98 mg cm�3) showing the

highest uptake and L8 (17.92 mg cm�3), the lowest (Fig. S7, ESIz).The relation between the pressure where the maximum

gravimetric excess is found and the different textural proper-

ties of the materials is illustrated in Fig. 3 and Fig. S8 (ESIz).Excess maxima are located in the range 33.1–51.5 bar, corres-

ponding to L1 (shortest linker) and L8 (longest linker),

respectively. There is a clear relation between these maxima

and the corresponding geometrical surface area (Fig. 3), unit

cell size and pore volume (Fig. S8, ESIz). The pressures of theexcess maxima are also correlated with the sizes of the larger

cavities of the rht topology, but not with the common cavity

Fig. 1 Absolute (diamonds) and excess (triangles) H2 adsorption

isotherms at 77 K. Simulations (red) were performed in L2,

and experiments (black) were performed on its analogue, NU-111.11

Bulk hydrogen densities required in eqn (1) were calculated using

NIST data.

Fig. 2 (a) Calculated excess adsorption isotherms for H2 at 77 K on the

materials L1–L8, and (b) calculated absolute (black circles) and excess (red

circles) adsorption capacity maxima of H2 isotherms on L1–L8 obtained at

77 K. The H2 bulk density was obtained from the NIST data.

Dow

nloa

ded

by N

orth

wes

tern

Uni

vers

ity o

n 10

/04/

2013

17:

55:3

6.

Publ

ishe

d on

10

Sept

embe

r 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C2C

C35

711A

View Article Online

10498 Chem. Commun., 2012, 48, 10496–10498 This journal is c The Royal Society of Chemistry 2012

that is defined by the copper paddle-wheels. It seems obvious

that, in the absence of catenation, an increase in the linker length

implies an enlargement of the pore size, the pore volume, the

surface area and the absolute H2 capacity. The size of the linker

and therefore the pore volume affect, in turn, not only the

number of molecules that can adsorb in the material, but also

the relation between absolute and excess amounts (eqn (1)).

Finally, we compared our simulations on L5 with available

experimental data from the Schroder, NOTT-116,9 and Zhou,

PCN-68,7 groups. As shown in Fig. S9 (ESIz), the experimental

hydrogen adsorption isotherms for the nominally composition-

ally identical NOTT-116 and PCN-68 are generally similar, but

the pressures where the maxima in the excess isotherms occur are

ca. 30 and 46 bar, respectively. Moreover, after reaching the

maximum in the excess isotherm, the amount adsorbed decreases

much faster in NOTT-116 than in PCN-68. From Fig. 3, we

can observe that some experimental data in rht MOFs follow

the expected trend. Deviations from this trend might be related

to experimental challenges in the adsorption measurements

(e.g. instrumental calibration issues related to assessment of empty

reactor, manifold and dead volumes, thermal transpiration, etc.),

or they might reflect difficulties in sample activation.16

In this work we have shown, using computational methods,

the effect of linker length on the textural properties of rht-type

MOFs such as surface area, pore size, and pore volume and

their relation to hydrogen adsorption properties. We showed

that although absolute uptake capacity increases linearly with pore

volume, excess uptake capacity maximizes at a pore volume of

ca. 3.61 cm3 g�1 (L7/NU-109). Further increases in pore volume

and pore size increase the pressure at which the maximum occurs,

but not the excess uptake capacity. We predicted the pressure at

which the excess maximum occurs as a function of the textural

properties of the structures. We believe that these relationships

will serve as guides for the synthesis of novel materials with

the rht topology and for understanding their hydrogen

uptake. We further suggest that gross ‘‘real world’’ deviations

from computationally observed trends could be indicative of

experimental issues, and thus may prove to be an instructive,

corroborative, or (alternatively) usefully critical, diagnostic of

experimental findings for yet-to-be-developed high-hydrogen-

capacity metal–organic framework materials.

This work was funded by the Department of Energy’s Office

of Energy Efficiency and Renewable Energy, Fuel Cell Technol-

ogies Program under Grant DE-FC36-08GO18137 and as a

cooperation project No. KK-1201-F0 (Synthesis of Porous

Hybrids and Their Applications) and supported by the Korea

Research Institute of Chemical Technology (KRICT). YJC

thanks the National Science Foundation Graduate Research

Fellowship Program. Computational work was supported

through resources provided by Information Technology at

Northwestern University as part of its shared cluster program,

Quest (p20320), and by the National Energy Research Scientific

Computing Center.

Notes and references

1 M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev.,2011, 112, 782–835.

2 H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi,A. O. Yazaydin, R. Q. Snurr, M. O’Keeffe, J. Kim andO. M. Yaghi, Science, 2010, 329, 424–428.

3 N. L. Rosi, M. Eddaoudi, J. Kim, M. O’Keeffe and O. M. Yaghi,Angew. Chem., Int. Ed., 2002, 41, 284–287.

4 F. Nouar, J. F. Eubank, T. Bousquet, L. Wojtas, M. J. Zaworotkoand M. Eddaoudi, J. Am. Chem. Soc., 2008, 130, 1833–1835.

5 Y. Yan, X. Lin, S. Yang, A. J. Blake, A. Dailly, N. R. Champness,P. Hubberstey and M. Schroder, Chem. Commun., 2009, 1025–1027.

6 B. Zheng, J. Bai, J. Duan, L. Wojtas and M. J. Zaworotko, J. Am.Chem. Soc., 2010, 133, 748–751.

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

8 O. K. Farha, A. O. Yazaydın, I. Eryazici, C. D. Malliakas,B. G. Hauser, M. G. Kanatzidis, S. T. Nguyen, R. Q. Snurr andJ. T. Hupp, Nat. Chem., 2010, 2, 944–948.

9 Y. Yan, I. Telepeni, S. Yang, X. Lin, W. Kockelmann, A. Dailly,A. J. Blake, W. Lewis, G. S. Walker, D. R. Allan, S. A. Barnett,N. R. Champness and M. Schroder, J. Am. Chem. Soc., 2010, 132,4092–4094.

10 Y. Yan, S. Yang, A. J. Blake, W. Lewis, E. Poirier, S. A. Barnett,N. R. Champness and M. Schroder, Chem. Commun., 2011, 47,9995–9997.

11 O. K. Farha, C. E. Wilmer, I. Eryazici, B. G. Hauser, P. A. Parilla,K. O’Neill, A. A. Sarjeant, S. T. Nguyen, R. Q. Snurr andJ. T. Hupp, J. Am. Chem. Soc., 2012, 134, 9860–9863.

12 O. K. Farha, A. O. Yazaydin, I. Eryazici, C. D. Malliakas,B. G. Hauser, M. G. Kanatzidis, S. T. Nguyen, R. Q. Snurr andJ. T. Hupp, Nat. Chem., 2010, 2, 944–948.

13 O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer,A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen, A. O. Yazaydin andJ. T. Hupp, J. Am. Chem. Soc., DOI: 10.1021/ja3055639.

14 R. C. Reid, J. M. Prausnitz and B. E. Poling, The Properties ofGases and Liquids, McGraw-Hill, New York, 4th edn, 1987.

15 E. W. Lemmon, M. O.McLinden and D. G. Friend, ‘‘ThermophysicalProperties of Fluid Systems’’, in NIST Chemistry WebBook, NISTStandard Reference Database Number 69, ed. P. J. Linstrom andW. G. Mallard, National Institute of Standards and Technology,Gaithersburg, MD, p. 20899, http://webbook.nist.gov, (July 10, 2012).

16 C. Zlotea, P. Moretto and T. Steriotis, Int. J. Hydrogen Energy,2009, 34, 3044–3057.

Fig. 3 Pressures corresponding to the maxima in the gravimetric excess

adsorption isotherms of L1–L8 at 77 K as a function of the geometrical

surface areas. Experimental data from PCN-68, red triangle, NOTT-116,

purple diamond, and NU-111, green square, are also included. The bulk

H2 density was obtained from the NIST data.

Dow

nloa

ded

by N

orth

wes

tern

Uni

vers

ity o

n 10

/04/

2013

17:

55:3

6.

Publ

ishe

d on

10

Sept

embe

r 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C2C

C35

711A

View Article Online