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Equilibrium, kinetics and enthalpy of hydrogenadsorption in MOF-177

Dipendu Saha, Zuojun Wei1, Shuguang Deng*

Department of Chemical Engineering, New Mexico State University, P.O. Box 30001, MSC 3805, Las Cruces, NM 88003, USA

a r t i c l e i n f o

Article history:

Received 24 February 2008

Received in revised form

27 May 2008

Accepted 20 September 2008

Available online 12 November 2008

Keywords:

Metal–organic framework (MOF-177)

Synthesis

Characterization

Hydrogen adsorption

Equilibrium

Kinetics

Isosteric heat of adsorption

* Corresponding author. Tel.: þ1 575 646 434E-mail address: sdeng@nmsu.edu (S. Den

1 Permanent address: Zhejiang University,0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.09.053

a b s t r a c t

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for

hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen

adsorption equilibrium and kinetic data were measured in a volumetric unit at low pres-

sure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-

177 adsorbent was characterized with nitrogen adsorption for pore textural properties,

scanning electron microscopy for morphology and particle size, and X-ray powder

diffraction for phase structure. The MOF-177 synthesized in this work was found to have

a uniform pore size distribution with median pore size of 12.7 A, a higher specific surface

area (Langmuir: 5994 m2/g; BET: 3275 m2/g), and a higher hydrogen adsorption capacity

(11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported

values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low

and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated

from the hydrogen adsorption kinetics and equilibrium data measured in this work.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction one of the most challenging research endeavors these days.

Due to severe environmental impacts and ever-depleted

petroleum deposits, petroleum fuels-powered transportations

desperately need an alternative power source that is clean and

sustainable. Among many emerging new power sources for

transportations in the future, hydrogen fuel cell is probably the

best candidate. However, the main bottleneck of hydrogen fuel

cells for mobile application is developing efficient, economic

and safe on-board hydrogen storing systems. Without the new

hydrogen storage system, it is difficult for automobile manu-

facturers to switch to the hydrogen fuel cell engine because the

existing hydrogen storage systems including compression or

cryogenic storage are not economically viable and difficult to

implement. The quest for better hydrogen storage materials is

6; fax: þ1 575 646 7706.g).College of Material Scienational Association for H

Starting with the newly developed storing materials, hydrogen

can be ‘adsorbed’ either chemically or physically. In chemical

adsorption, hydrogen is reversibly bonded to a substance, like

complex metallic hydrides [1] or nitrides [2]. The chemical

storage systems have either low storage capacity or need high

temperature to release the adsorbed hydrogen [3]. In physical

adsorption, hydrogen is adsorbed inside the micropores of

porous materials including zeolites [4], carbon nanotubes [5],

ordered mesoporous carbon [6,7], and certain clathrates [8].

The basic advantage of physical adsorption is reversible and

fast kinetics of sorption compared to chemical adsorption. But

it also suffers from the problem of very low adsorption

enthalpy resulting in high storage capacity only at very low

temperatures.

ce and Chemical Engineering, Hangzhou, China.ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Nomenclatures

am Langmuir equation constant (wt.%)

b Langmuir equation constant (bar�1)

Dc Intracrystalline diffusivity (cm2/s)

E activation energy for diffusion (J/mol)

k Freundlich equation constant

Dm Observed mass change of adsorbent sample (g)

mA Adsorbed amount (g)

mA_absolute Absolute adsorbed amount (g)

mA_excess Excess adsorbed amount (g)

mS Mass of adsorbent sample (g)

mSC Mass of sample container (g)

mt Adsorbed amount per unit mass of adsorbent at

time t (wt.%)

m_max Maximum adsorbed amount per unit mass of

adsorbent at t¼N (wt.%)

n Freundlich equation constant

P Pressure (bar)

P0 Gas saturation pressure at temperature T (bar)

q Adsorbate concentration in the adsorbent

(wt.%)

q Average adsorbate concentration in the

adsorbent particle (wt.%)

q0

0 Initial adsorbate concentration in the

adsorbent particle (wt.%)

rc Radius of equivalent sphere of single crystal of

MOF-177 (cm)

t Time (s)

T Absolute temperature (K)

V Volume of gas (cm3)

VA Volume of adsorbed phase (cm3)

VS Volume of adsorbent sample (cm3)

VSC Volume of sample container (cm3)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 87480

Until recently, a new kind of porous material, known as

metal–organic framework (MOF) has shown quite high storage

capacity for hydrogen through physical adsorption. Among

the several MOF structures that have been investigated for

hydrogen storage, most of them are Zn-based, like MOF-5 (or

isoreticular MOF-1, IRMOF-1), IRMOF-20, IRMOF-6, etc. [9–14],

and a few other MOFs based on Cr, Cu or Mn [15–18]. Some

MOF adsorbents were reported to be very promising members

for hydrogen storage due to their extremely large specific

surface area and relatively high hydrogen uptake capacity.

MOF-5 shows a hydrogen uptake of 5 wt.% at 77 K and 90 bar

[12]. IRMOF-20 shows a H2 uptake of 6.7 wt.% at 77 K and 90 bar

[12]. MIL-101, which is a chromium (III)-based MOF, shows an

uptake of 6.1 wt.% at 77 K and 80 bar [15]. Another Mn-based

MOF, which is composed of Mn and 1,3,5-benzenetristrazolate

is reported to adsorb up to 6.9 wt.% of hydrogen at 77 K and

90 bar [18].

Yaghi et al. [19] reported a Zn-based MOF, MOF-177, which

has a specific surface area of 4500 m2/g and hydrogen

adsorption capacity of 7.5 wt.% at 77 K and 80 bar. It was

synthesized by attaching octahedral Zn4O clusters with 3-

benzene ring-structured benzene tribenzoate (BTB) ligand. In

a later publication, Yaghi’s group reported an improved MOF-

177 that has a Langmuir specific surface area 4526 m2/g and

hydrogen adsorption uptake of 1.25 wt.% at 77 K and ambient

pressure [11]. In a following paper [12], the same group

reported the highest hydrogen uptake of 7.5 wt.% at 77 K and

90 bar on MOF-177 with a Langmuir specific surface area

around 5500 m2/g. This MOF-177 has the largest specific area

of any metal–organic framework materials and the highest

hydrogen adsorption capacity amongst all the hydrogen

storing materials, either by physical adsorption or chemical

adsorptions. In 2007, Yaghi group made another progress on

MOF-177 by confirming the hydrogen excess adsorption of

7.5 wt.% and absolute adsorption of 11 wt.% [20]. They also

reported the hydrogen uptake of 0.75 wt.% at 87 K and

ambient pressure. Li and Yang synthesized the BTB ligand

with a different approach and obtained a MOF-177 with

a Langmuir specific surface area of 4300 m2/g, and reported

the highest hydrogen uptake of 1.5 wt.% at 77 K and ambient

pressure, 1.28 wt.% and 0.62 wt.% storage at 298 K and 100 bar

with and without the spillover effect, respectively [21]. They

also reported the heat of adsorption of 5.8–11.3 kJ/mol at

a hydrogen adsorption amount between 0.32 and 1.5 cm3/g.

In the present study, MOF-177 was synthesized following

the technique reported by Yaghi et al. [20] with some modifi-

cations. The objective of this work is to increase specific

surface area and hydrogen adsorption capacity by optimizing

the synthesis procedures. The pore textural properties of

MOF-177 were measured by nitrogen adsorption at 77 K. X-ray

powder diffraction (XRD) measurement was performed to

determine the phase structure and scanning electron

microscopy (SEM) was carried out to study the morphology

and particle size of MOF-177 synthesized in this work.

Hydrogen adsorption equilibrium and kinetic data were

measured in a volumetric apparatus at 77 K, 194.5 K and 297 K

and hydrogen pressure up to 1.05 bar. Hydrogen adsorption in

MOF-177 was also measured in a magnetic suspension

balance at 77 K and 297 K at hydrogen pressure up to 100 bar.

Adsorption isotherm models were used to correlate the

adsorption equilibrium data, and a pore diffusion model was

used to extract hydrogen diffusion time constants from the

kinetic data measured in this work. Isosteric heat of adsorp-

tion was estimated from two hydrogen adsorption isotherms

measured at low pressure.

2. Experimental method

2.1. Synthesis of MOF-177

Synthesis of MOF-177 was performed following the reported

procedures [20] with a few modifications. All chemicals used

in this work are of commercially available highest purity and

purchased from Fisher Scientific. The synthesis portion can be

divided into two steps: synthesis of BTB ligand and formation

of MOF-177. BTB ligand was synthesized in our lab following

the procedures reported by Yaghi et al. [20]. To produce MOF-

177, 0.32 g of zinc nitrate hexahidrate and 0.07 g BTB ligand

were dissolved in 20 ml of dimethyl formamide. The mixture

was degassed thrice using freeze–pump–thaw method and

then stored in a 20-ml reaction vial that was fully filled with

mixture and capped tightly. The vial was then put in an oven

maintain at 67 �C for 7 days. At the end of this step, clear and

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 8 7481

transparent MOF-177 formed and became visible in the wall as

well as on base of the vial. The vial was then removed from the

oven, decanted and washed with dimethyl formamide to

remove the unreacted zinc nitrate. After this step, the vial was

filled with chloroform, capped tightly and put in an oven at

70 �C for another three days, and it was replenished with fresh

chloroform every day. Because the MOF-177 crystals are

vulnerable to moisture and air [22], they were stored and

transported in a Schlenk flask under vacuum or in a vial filled

with chloroform.

2.2. MOF-177 characterizations

The MOF-177 samples were characterized for their pore

textural properties with a Micromeritics� ASAP 2020 adsorp-

tion apparatus at liquid nitrogen temperature. The pore

textural properties including specific Langmuir and BET

surface area, pore volume and pore size distribution were

obtained by analyzing the nitrogen adsorption and desorption

isotherms with Micromeritics� ASAP 2020 built-in software.

Before starting the nitrogen adsorption measurements, the

MOF-177 sample was degassed in situ at about 353 K for 10 h to

remove majority of the guest molecules (i.e. chloroform) from

the sample. To examine the sample crystallinity, the MOF-177

samples were analyzed with a Hitachi TM-1000 bench-top

scanning electron microscopy device without prior gold

deposition. The MOF-177 samples in as-synthesized condi-

tions were also investigated for their phase structure by

a Rigaku� Miniflex-II X-ray diffractometer with CuKa emis-

sion, 30 kV/15 mA current and kb-filter.

2.3. Hydrogen adsorption study

2.3.1. Hydrogen adsorption at low pressureHydrogen adsorption equilibrium and kinetics in MOF-177

samples at low pressures were measured volumetrically in

the Micromeritics� ASAP 2020 adsorption apparatus at three

temperatures, liquid nitrogen temperature (77 K), dry ice

temperature (194.5 K) and room temperature (297 K) and

hydrogen pressure up to 1.05 bar. About 0.5 g of MOF-177 was

used in this experiment. The adsorbent was degassed under

a vacuum and at 100 �C for 12 h before the hydrogen adsorp-

tion measurement. Ultra-high purity hydrogen (99.99%) was

introduced into a separate gas port of the adsorption unit for

the hydrogen adsorption measurements. The changes of

hydrogen pressure with time were recorded and converted

into the transient adsorption amount as a function of time.

The transient adsorption uptakes generated the adsorption

kinetics, and the final adsorption amount at the terminal

pressure determined the adsorption equilibrium amount at

a given hydrogen pressure. For MOF-177 sample, adsorption

isotherms were measured at 77 K, 194.5 K and 298 K with

hydrogen pressure ranging from 0 to about 1.05 bar, and

fractional adsorption uptake curves were collected at same

temperature and three different hydrogen pressures.

2.3.2. Hydrogen adsorption at high pressureHigh pressure adsorption was performed gravimetrically in

a Rubotherm magnetic suspension balance at liquid nitrogen

temperature (77 K) and room temperature (297 K) and hydrogen

pressure up to 100 bar. This balance is equipped with an

automatic flow gas dosing and pressure control system, it can

be used to measure both adsorption equilibrium and kinetics

automatically at temperatures ranging from 77 K to 723 K, and

pressure ranging from a vacuum to 500 bar. A booster pump

was used to compress the supply gas (H2) from a gas cylinder

from 30–50 bar to a maximum of 500 bar. The pressurized gas

from the booster pump was then introduced to the inlet of the

flow gas dosing system for flow rate and pressure control. The

temperature, gas pressure and sample weight are recorded by

a computer data acquisition software. In this work, 0.28 g of

MOF-177 was loaded for the hydrogen adsorption equilibrium

and kinetic measurements. A routine procedure for

measuring gas adsorption on an adsorbent using the Rubo-

therm magnetic suspension balance includes:

1) Performing a blank run with an empty sample container

and a high purity helium gas (99.999%) to determine the

mass (mSC) and volume (VSC) of the sample container.

2) Loading an adsorbent sample and activating the sample

under vacuum for at least 12 h. If the following adsorption

run is to be performed in the low temperature chamber (0–

150 bar, 77 K) at liquid nitrogen temperature, the sample

needs to be activated under a vacuum only because the low

temperature chamber is not designed to be heated. If the

following adsorption run is to be performed in the high

pressure chamber (0–500 bar), the adsorbent sample is

activated under a vacuum and at temperature ranging from

ambient to 450 �C.

3) Doing a buoyancy run of the sample in the container with

a high purity helium at ambient temperature to determine

the sample mass (mS) and sample volume (VS).

4) Performing the adsorption of hydrogen or other gases on

the adsorbent under controlled conditions.

A special data processing procedure is needed to obtain the

adsorption amount (mA) from the observed sample mass

change (Dm) under specific condition using the following

equation:

mA¼ ðDm�mS �mSCÞ þ ðVS þ VSC þ VAÞrðT;PÞ (1)

where mA, Dm, mS, mSC, VS, VSC, VA, r(T,p) are mass of the

adsorbed phase, balance reading of sample mass change,

mass of the sample, mass of the sample container, volume of

the sample, volume of the sample container, volume of the

adsorbed phase and density of the analysis gas (hydrogen) at

particular temperature and pressure, respectively.

The mass and the volume of the sample container were

calculated from the slope and intercept of the weight versus

density plot which was found by exposing the empty sample

container to the helium gas in different pressure levels at

room temperature in the blank run. Similarly, mass and

volume of the sample were found from the slope and intercept

of a similar curve where the sample container with a sample

was exposed to helium gas at room temperature in the

buoyancy run. Peng–Robinson equation-of-state was used to

calculate the density of helium or hydrogen gas at different

conditions in this work.

If the adsorbed phase volume is assumed to be negligible as

compared with the volume of sample and sample container,

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 87482

the calculated adsorbed amount mA_excess is defined as the

excess adsorption amount.

mA excess ¼ ðDm�mS �mSCÞ þ ðVS þ VSCÞrðT;PÞ (2)

The excess adsorption amount is close to the absolute

adsorption amount when the adsorption amount is small or

the gas density is small as compared with the density of the

adsorbed phase. The excess adsorption deviates from the

absolute adsorption significantly as the gas pressure

increases. In order to calculate the absolute adsorption

amount, the following buoyancy correction was performed by

assuming the adsorbed phase has a similar density as the

liquid phase of the adsorbate molecules at saturation

temperature and ambient pressure.

mA absolute ¼ mA excess=ð1� rðT;PÞ=rLÞ (3)

Both excess and absolute hydrogen adsorption isotherms on

MOF-177 were reported in this work.

Fig. 2 – Pore size distribution of MOF-177 obtained from

nitrogen adsorption/desorption isotherms at 77 K.

Fig. 1 – Nitrogen adsorption and desorption isotherms in

MOF-177 at 77 K.

3. Adsorption theories

3.1. Adsorption equilibrium

Langmuir and Freundlich isotherm models can be used to

correlate the adsorption isotherms of hydrogen in MO-177.

The Langmuir isotherm is written as:

q ¼ ambP1þ bP

(4)

where q is the adsorbed H2 amount on MOF-177 (wt.%), P is the

H2 pressure (bar), am (wt.%) and b (bar�1) are the Langmuir

isotherm equation parameters. They can be determined from

the slope and intercept of the linear Langmuir plot of (1/a)

versus (1/P).

Freundlich isotherm is given by:

q ¼ kP1=n (5)

where k and n are the Freundlich isotherm equation param-

eters that can be determined from the experimental H2

adsorption isotherms.

3.2. Adsorption kinetics

It should be pointed out that hydrogen adsorption kinetics is

as important as the hydrogen adsorption equilibrium

although meeting the DOE specifications for hydrogen storage

capacity is more challenging [23,24]. So far there is no

hydrogen adsorption kinetic data on MOF-177 has ever been

reported. In order to process the hydrogen kinetic data to

extract the intracrystalline diffusivity, a diffusion model was

described according to Ruthven [25]. By neglecting the heat

transfer between particle and surrounding fluid, the diffusion

equation in a spherical coordinate is written as [25]:

vqvt¼ 1

r2

v

vr

�r2Dc

vqvr

�(6)

where r is the radius of the equivalent sphere, Dc is the

intracrystalline diffusivity and q(r,t) is adsorbed amount time t

and radial position r. For constant diffusivity for a particular

pressure, Eq. (6) can be converted to

vqvt¼ Dc

�v2qvr2þ 2

rvqvr

�(7)

The solution of the Eq. (4) for the equivalent radius rc is given

by

q� q00

q0 � q00¼ mt

m max¼ 1� 6

p2

XNn¼1

1n2

exp

��p2Dct

r2c

�(8)

where, q is the average adsorbate concentration in the

particle, given by

q ¼ 3

r3c

Z rc

0

qr2 dr (9)

For fractional uptake mt=m max greater than 70%, Eq. (9) can

be reduced to

Table 1 – Comparison of pore textural properties and hydrogen adsorption on MOF-177 with literature data

Properties Literature value This work

Langmuir specific surface area (m2/g) 5640 [20] 5994

4300 [21]

BET specific surface area (m2/g) 4630 [20] 3275

3100 [21]

Pore diameter (A) 10.6 A [20] 12.7

Cumulative pore volume (cm3/g) 1.69 [20] 2.65

1.58 [21]

H2 uptake at 77 K, 1 bar (wt.%) 1.25 [11] 1.32

1.50 [21]

H2 uptake at 77 K, high pressure (wt.%) 7.5 wt.% (excess), 11 wt.%

(absolute) at (80 bar) [20]

11.0 wt.% (excess), 19.67 wt.%

(absolute) at 120 bar

H2 uptake at 298 K, high pressure (wt.%) 0.62 wt.% (absolute) at 100 bar [21] 0.35 wt.% (absolute) at 40 bar

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 8 7483

1� mt

m max¼ 6

p2exp

��p2Dct

r2c

�(10)

3.3. Activation energy for diffusion and heat ofadsorption

The activation energy for hydrogen diffusion can be estimated

from the Eyring equation by assuming hydrogen diffusion

inside MOF-177 to be an activation process.

Dc ¼ D0cexp

�� E

RT

�(11)

where D0c is an equation constant, E is the activation energy for

diffusion.

Isosteric heat (enthalpy) of adsorption is an important

index characterizing the interactions between the adsorbate

molecules and the adsorbent material. It is believed that the

heat of adsorption of hydrogen on MOFs and carbonaceous

adsorbents is so small that very low temperature is required to

achieve a significant hydrogen adsorption amount in the MOF

materials. So it is necessary to quantify the heat of adsorption

Fig. 3 – Scanning electron microscopy image of MOF-177

sample.

experimentally and to compare it with heat of adsorption for

other adsorption systems. The definition of isosteric heat of

adsorption can be obtained from the van’t Hoff’s equation [25]:

DHRT2¼ �

�vln p

vT

�q

(12)

where DH is the isosteric heat of adsorption (kJ/mol), T (K) is

temperature, p is adsorbate pressure (bar), q refers to

a constant adsorption amount, and R is universal gas

constant. Integrating Eq. (12) gives:

ln p ¼ �DHRTþ C (13)

where C is constant of integration.

4. Results and discussion

4.1. Physical properties of MOF-177

4.1.1. Pore textural propertiesPore textural properties for MOF-177 were calculated from the

nitrogen adsorption–desorption isotherms plotted in Fig. 1.

The maximum nitrogen uptake in our MOF-177 sample is

1700 mg/g at 1.05 bar, which is significantly higher than the

Fig. 4 – Powder X-ray diffraction pattern of MOF-177

sample.

Fig. 5 – Low pressure hydrogen adsorption isotherms in

MOF-177 at 77 K, 194.5 K and 298 K.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 87484

previously published result of 1200 mg/g at similar conditions

[19–21]. The corresponding pore size distribution for the MOF-

177 is shown in Fig. 2. Table 1 summarizes the pore textural

properties for the MOF-177 including Langmuir specific

surface area, BET specific surface area, maximum pore

volume and median pore diameter calculated by H–K method

[26]. It also compares the pore textural properties and

hydrogen adsorption on MOF-177 sample synthesized in this

work with the literature data. All of these values were calcu-

lated using the ASAP 2020 built-in software. Langmuir surface

area of our MOF-177 sample is 5994 m2/g, which is higher than

the published Langmuir specific surface areas for MOF-177

[11,19–21]. The BET specific surface area of 3275 m2/g for our

MOF-177 sample is consistent with the recently reported

result [21]. The cumulative pore volume calculated by the H–K

method is 2.65 cm3/g, which is significantly higher than

1.58 cm3/g for a MOF-177 sample reported by Li and Yang [21].

The MOF-177 synthesized in this work has a quite uniform

pore size with a median pore diameter of 12.7 A, which is

consistent with the median pore diameter of 10.6 A reported

by Yaghi’s group [20].

4.1.2. SEM imagesThe scanning electron microscopy image of MOF-177 after

removing all guest molecules is shown in Fig. 3. The MOF-177

Table 2 – Isotherm model parameters for hydrogen adsorption

Isothermmodel

Parameters T¼ 77 K(low pressure)

T¼ 194.5 K(low pressure

Langmuir am (wt.%) 1.80 14.8

b (bar�1) 1.35 0.07

Freundlich k 1.34 0.93

n 1.14 0.99

shown in the SEM image has visible crystal structures with an

average particle size about 200 mm, which is consistent with

the previous results on MOF-177 [19–21].

4.1.3. XRD patternThe X-ray powder diffraction pattern for the MOF-177 sample

after extended degassing under vacuum is shown in Fig. 4.

The main peaks at 4.85�, 5.84�, 7.75�, 11.15� and 11.54� are well

identified and consistent with the results obtained by Yaghi’s

group [27]. It must be pointed out that the XRD pattern

obtained on, as synthesized MOF-177 in the chloroform is

quite different and shows no crystal peaks at all. This is

basically because the guest molecules occupying the internal

pores of MOF-177 shift the atomic orientation in crystal plane

[14,22,28].

4.2. Adsorption equilibrium

The hydrogen adsorption isotherms at 77 K, 194.5 K and 298 K

with hydrogen pressure up to 1.05 bar were plotted in Fig. 5. As

shown in Table 1, the hydrogen adsorption uptake on MOF-

177 synthesized in our lab is 1.36 wt.% at 77 K and 1 bar, which

is higher than the uptake of 1.25 wt.% reported by Yaghi’s

group [11], but lower than a recently published result of

1.5 wt.% [21] at similar conditions. Hydrogen uptake

of 0.92 wt.% at 194.5 K is higher than reported uptake of

0.75 wt.% at 87 K [20]. Uptake at room temperature (297 K) is

about 0.012 wt.%, which was not reported by previous

researchers.

Both Langmuir and Freundlich equations were used to

correlate the hydrogen adsorption isotherms in MOF-177. The

adsorption isotherm equation parameters for both Langmuir

and Freundlich equations obtained from linear regression of

the hydrogen adsorption isotherms shown in Fig. 5 were listed

in Table 2. It appears from the correlation results that

Freundlich equation fits the low pressure hydrogen adsorp-

tion isotherms better than the Langmuir equation.

The high pressure adsorption isotherm on MOF-177 was

determined gravimetrically by the Rubotherm magnetic

suspension balance. Both excess and absolute adsorption

isotherms of hydrogen on MOF-177 at 77 K and 297 K with

pressure up to 100 bar were plotted in Figs. 6 and 7, respec-

tively. It is very encouraging to find out the highest excess

adsorption amount of hydrogen on MOF-177 synthesized in

our lab is 11.0 wt.% at 77 K and 100 bar; and the corresponding

absolute adsorption amount is 19.67% at the same conditions.

Both of them are the highest hydrogen adsorption capacities

on MOF-177 or any other adsorbents ever reported at these

in MOF-177 at different temperatures

)T¼ 297 K

(low pressure)T¼ 77 K

(high pressure)T¼ 297 K

(high pressure)

– 17.0 0.7

– 0.06 0.02

0.05 1.71 0.03

0.56 1.95 1.39

Fig. 6 – High pressure hydrogen adsorption isotherms in

MOF-177 at 77 K.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 8 7485

conditions [12,20,21]. From the trend of the absolute adsorp-

tion isotherm the MOF-177 adsorbent was not saturated, more

hydrogen could be adsorbed at higher hydrogen pressure.

Hydrogen uptakes at 297 K and 46 bar are 0.35 wt.% and

0.37 wt.% for excess and absolute adsorption, respectively.

The significantly higher hydrogen adsorption capacity

obtained in our MOF-177 is probably related to the extremely

high specific surface area and large pore volume. It is believed

from molecular simulation results that hydrogen adsorption

in MOF materials are decided by heat of adsorption at low

pressure, specific surface area at medium pressure and pore

volume at high pressure [29]. Further experimental studies are

being carried out to elucidate the hydrogen adsorption

mechanism in MOF-177 and other porous media.

Both Langmuir and Freundlich isotherm models were used

to correlate the absolute adsorption isotherm and Freundlich

equation fits the data better than the Langmuir as shown in

Fig. 7 – High pressure hydrogen adsorption isotherms in

MOF-177 at 297 K.

Figs. 6 and 7. The isotherm equation constants for all

isotherms shown in Figs. 6 and 7 are given in Table 2. It should

be pointed out that the adsorption isotherm describes only the

absolute adsorption, not the excess adsorption that is signif-

icantly lower than the absolute adsorption amount due to

buoyancy correction for the adsorbed hydrogen molecules at

high pressure.

4.3. Adsorption kinetics

The hydrogen adsorption kinetic data were collected at the

same time when the low pressure hydrogen adsorption

isotherms shown in Fig. 4 were measured in the Micro-

meritics� ASAP 2020 adsorption unit. The fractional adsorp-

tion uptake curves at various hydrogen pressures and three

temperatures (77 K, 194.5 K, 297 K) were plotted in Fig. 8(A), (B)

and (C). It was assumed that the primary MOF-177 crystals are

perfect spheres in order to apply the diffusion model given in

Eq. (10) to correlate the hydrogen adsorption kinetic data. This

equation allows us to calculate hydrogen diffusion time

constants (Dc=r2c, s�1) inside MOF-177 from the slope of a plot

of lnð1� ðmt=mmaxÞÞ versus t at different pressures. Only data

points with fractional adsorption uptake more than 70% were

used in the correlation. Due to the fluctuation of the data

points in the adsorption kinetics at 194.5 K, 297 K shown in

Fig. 8(B) and (C), it was difficult to differentiate the kinetics

curves at different pressures, so an average was taken for the

kinetic curves at 194.5 K and 297 K. The diffusion time

constants (Dc=r2c, s�1) at different pressures for 77 K and their

average values for 194.5 K and 297 K were summarized in

Table 3 and plotted in Fig. 9. It can be seen from Fig. 9 that

diffusion time constants increase with hydrogen pressure at

77 K, the average diffusion time constant also increases with

temperature. The temperature effect is expected due to higher

kinetic energy at higher temperature, while the pressure

effect is probably caused by the surface diffusion where the

increased hydrogen adsorption amount facilitates hydrogen

transport in the pores of MOF-177 at a higher hydrogen

pressure.

4.4. Activation energy for diffusion and heat ofadsorption

If the MOF-177 crystal diameter is assumed to be 200 mm

according to the SEM image of MOF-177 shown in Fig. 3, the

intracrystalline diffusivity can be calculated from the average

diffusion time constant listed in Table 3. The average intra-

crystalline diffusivities are 9.6� 10�5 cm2/s, 2.1� 10�4 cm2/s,

and 3.0� 10�4 cm2/s at 77 K, 194.5 K, 297 K, respectively. From

the diffusivity data at three different temperatures, we esti-

mated the activation energy for hydrogen diffusion inside

MOF-177 according to Eq. (11). It was found that the activation

energy for hydrogen diffusion in MOF-177 is 0.94 kJ/mol, which

is much smaller as compared with the activation energy for

small hydrocarbon molecules diffusion in zeolite [25].

Hydrogen adsorption isotherms at 77 K, 194.5 K and 297 K

shown in Fig. 5 were used to calculate the heat of adsorption.

The hydrogen adsorption isotherms were first converted to

hydrogen adsorption isosteres (P versus T at a given adsorp-

tion amount), the heat of adsorption was then calculated

Fig. 8 – Hydrogen adsorption kinetics in MOF-177 at 77 K (A), 194.5 K (B) and 297 K (C).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 87486

from the slopes of the isosteres according to Eq. (12). The

effect of adsorbed hydrogen amount on isosteric heat is

demonstrated in Fig. 10. The heat of adsorption of hydrogen

in MOF-177 synthesized in this work is less than 4 kJ/mol,

which is lower than the reported heat of adsorption of 4–7 kJ/

mol for various MOF materials [3,15,30] and significantly

lower than the recent published result of 5.8–11.3 kJ/mol for

MOF-177 [21]. The small heat of adsorption is responsible for

Table 3 – Diffusion time constants for hydrogenadsorption in MOF-177 at different temperatures

T¼ 77 K T¼ 194.5 K T¼ 297 K

P(bar)

Dc=r2c

(s�1)P

(bar)Dc=r2

c

(s�1)P

(bar)Dc=r2

c

(s�1)

0.38 0.0891

0.65 0.0961

1.00 0.1039

Average 0.0964 0.2140 0.3023

the drastic fall of hydrogen adsorption uptake at ambient

temperature; therefore it is important to increase the heat of

adsorption in order to improve hydrogen storage capacity at

ambient temperature and low to intermediate pressures.

Fig. 9 – Effect of pressure of hydrogen diffusion time

constants in MOF-177.

Fig. 10 – Effect of adsorption amount on isosteric heat of

hydrogen adsorption in MOF-177.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 7 4 7 9 – 7 4 8 8 7487

5. Conclusion

MOF-177 was successfully synthesized with an improved

procedure as a potential adsorbent for hydrogen storage. The

MOF-177 prepared in this work has the largest Langmuir

5994 m2/g) and BET specific surface area (3275 m2/g), and more

importantly, the highest hydrogen adsorption capacity

(11.0 wt.% excess, and 19.67 wt.% absolute adsorption) among

all physical adsorbents at 77 K and 100 bar. Freundlich

adsorption equation fits well both low pressure and high

pressure hydrogen isotherms. Adsorption kinetic data were

reported and diffusion constant and diffusivity for hydrogen

adsorption in MOF-177 adsorbent were estimated. Activation

energy for diffusion and isosteric heat of adsorption for

hydrogen adsorption in MOF-177 were also estimated from

the kinetic and equilibrium data obtained in this work. The

hydrogen diffusion activation energy is 0.94 kJ/mol and the

heat of adsorption is less than 4 kJ/mol.

Acknowledgements

The authors greatly appreciate the support from US Army

Research Office through grant W911NF-06-1-0200.

Appendix ASupplemental material

Supplementary information for this manuscript can be

downloaded at doi: 10.1016/j.ijhydene.2008.09.053.

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