Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 23-29
doi:10.4236/jsemat.2011.12004 Published Online July 2011 (
Copyright © 2011 SciRes. JSEMAT
H2 and CH4 Sorption on Cu-BTC Metal Organic
Framewo rk s at Pressures up to 15 MPa and
Temperat ures between 273 and 318 K
Yves Gensterblum
Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Lochn er s tr Aachen, Germany.
Emai l:
Received March 17th, 2011; revised May 31st, 2011; accepted Jun e 9th, 2011.
Sorption isotherms of methane and hydrogen on Cu3(BTC)2 have been measured in the temperature range from 273 to
318 K and a t pr essu res up to 15 MPa. H2 excess sorption capacities of the Cu3(BTC)2 amounted to 3.9 mg/g at 14 MPa.
Promising maximum CH4 excess so rption capacities on the same sample were reached at approximately 5 MPa. They
amounted to 101, 100, 92 and 80 mg/g at 273, 278, 293 and 318 K, respectively. The sorbed phase density was essen-
tially the same for all temperatures and amounted to ~600 kg/m3. Structural changes of the Cu3(BTC)2 samples after
thermal activation and treatment with high pressure H2 and CH4 were tested. It was found that the initial micropore
structure has virtually disappeared as evidenced by a decrease of the Langmuir specific surface area by a factor ~3 and
CO2 micropore volume by a factor of ~4 for H2 and ~3 for CH4. This is in line with a n inc rease in the average pore di-
ameter from in itially 9.2 to 15.7 for H2 and 12.8 for CH4.
Keywords: Metal Organic Framework (MOF), Sorption, Methane, Hydrogen, Pore Structure
1. Introduction
The concept of reticular design in the synthesis of metal
organic frameworks (MOF) permits custom-tailoring of
regular pore structures on the nanometer scale [1]. This
approach opens new perspectives for the development of
gas and energy storage systems [2]. So far, the ga s sorp-
tion capacity of metal-organic frameworks (MOF) of
various chemical and structural compositions has mainly
been determined at low pressures (< 0.1 MPa) and tem-
peratures (77 to 87 K). Only a few studies [3,4] reported
H2 and CO 2 measurements at ambient temperatures (ideal
operating temperature).
Several authors reported the internal structure of dif-
ferent MOFs [5,6]; however, significant amounts of side
products were detected using the synthesis described by
Chui et al. (1999)[5]. One major problem concerning the
storage of gases on these materials at high pr essur es (> 1
MPa) and temperatures (especially at the activation tem-
perature of 458 K) is their structural instability and
hence, a decrease in specific surface area associated with
struc- tural rearrangements for Cu-(BTC) [6].
In this study the sorption capacities of CH4 and H2 on
Cu3(BTC)2 (Copper benzene-1,3,5-tricarbox ylate,C18H6
Cu3O12) have been investigated at temperatures between
273 and 318 K and pressures up to 15 MPa. Specific
surface areas (SSA) and pore size distributions (PSD)
have been determined on untreated samples before and
after high-pressure sorption experiments in order to de-
tect any structural changes of the sample due to the in-
teraction with the gases. A similar approach has been
used by [7], who reported changes in SSA and PSD on
MOF-5 during hydrogen sorption.
Two slightly different MOFs both consist of the same
organic ligands but with different basic metals (Al3+ and
Cr3+) [8]. At 77K the Al3+MOF shows higher sorption
capacities than the corresponding Cr3+ sa mp le. However,
both samples nearly show the same specific surface area
of ~1000 m2/g (Figure 1). This finding confirms the im-
portance of micropore volume and the electrostatical
potential of t he different metal s.
Data provided in Figure 1 reflects the general correla-
tion between SSA (determined by low pressure N2-iso-
therms at 77 K) and H2 sorption capacity at 77 K. It is
known that this trend is ambiguous for isotherms re-
corded at higher temperatures [9]. One suggestion is that
H2 and CH4 Sorption on Cu-BTC Metal Organic Frameworks at Pressures up to 15 MPa and Temperatures
between 273 and 318 K
Copyright © 2011 SciRes. JSEMAT
[1] IRMOF-11
[1] IRMOF-6
[1] MOF-177
[1] IRMOF-1
[1] HKUST-1
[1] IRMOF-20
[3] Cu3BTC2
[1] MOF-74
[8] MOF-53 Cr
[8] MOF-53 Al
01000 2000 3000 4000 5000 6000
uptake / g g
BET Surface area / m² g
Figure 1. Comparison of high-pressure (~5 MPa) H2 sorp-
tion capacities at 77 K vs. N2-BET surface area for different
MOFs and carbon materials .
a better correlation can be obtained by separating the
hydrogen adsorbed in the micropores from that on the
surface of the mesopores. Furthermore, Nijkamp et al.
(2001) concluded that a higher storage capacity could be
achieved with adsorbents containing a large micropore
volume with suitable diameter [9].
In a st ud y on h ydr ogen stor ag e in c hemi call y ac tiva ted
carbons and carbon nanomaterials, Beneyto (2007) [10]
proposes that at 77 K the H2 adsorption capacity depends
on the SSA and the total micropore volume of the acti-
vated carbon. At 298 K it depends on both the micropore
volume and the micropore size distribution. To date, very
few high pressure/temperature isotherms and corre-
sponding micropore volumes have been determined on
In comparison to hydrogen, it was shown for methane
in different studies that Cu3(BTC)2 has a high CH4 stor-
age capacity at room temperature [11,12] (295 to 298 K,
Wang et al. 2002; Lin et al. 2006). Sorption values on
Cu-BTC for CH4 of up to 72 mg/g (at 295 K and 0.1
MPa) with nearly linear sorption isotherms up to 0.1
MPa [11]. Maximum CH4 sorption capacities of 31.4
mg/g at 0.9 MPa and 298 K on Zn–MOF and a hysteresis
between the sorption and desorption curves [12]. Senk-
ovska and Kaskel (2007) [13] reported high pressure CH4
adsorption on Cu3(BTC)2, Zn2(bdc)2dabco, Cr3F(H2O)2,
O(bdc)3. Among the three materials, Cu3(BTC)2 shows
the highest excess adsorption at 303 K (15.7 mg/g).
2. Experimental
2.1. High-Pressure Sorption Experiments
High pressure single-gas adsorption experiments have
been performed using a manometric experimental set-up
(Figure 2) consisting of a stainless-steel measuring cell
(MC) and a calibrated reference volume (RC) connected
with a set of actuator-driven valves and a high-precision
Figure 2 . S che mat i c diag r a m of s or pti o n a p par at us us e d fo r
high pressure/high temperature H2 and CH4 sorption ex-
periments on Cu-BTC MOFs. (RC = calibrated reference
volume ; MC = measuring cell).
pressure transducer (max. pressure 25 MPa, with a preci-
sion of 0.05% of the full scale value). The entire device
is placed in a temperature-controlled oven (variations in
temperature < 0.1 K). Calculation of the excess sorption
was performed based on equations of state by Setzman
and Wagner (1991) [14] and McCarty et al. (1981) [15]
for methane and hydrogen, respectively. Quality control
is ensured by participation in laboratory comparison
studie s (e. g. Gensterblum et a l. 2009 [16], 2010 [17] and
Goodman et al. 2004 [18]). For further details on the
experimental approach, please refer to e.g. Busch et al.
(2004) [19].
As pointed out by Férey (2003) [8] for the MIL-53 and
Schlichte et al. (2004) [6] for the MOFs - which were
also investigated in this study - the activation of the
metal-organic framework compounds is an important
issue affecting the qualit y and rep roducibilit y of sorption
capacity measurements. The activation procedure was
carried out in three stages: 12 h at 105˚C, 12 h at 155˚C,
and finally 18 h at 185˚C. This activation procedure led
to well-reproducible starting conditions for the isotherm
Error bars shown in the diagrams and error margins
listed in the Table 2 were determined based on the Gauss
error propagation method considering the individual er-
rors of the individual experimental parameters.
2.2. Characterisation of Pore Structure and
Surface Area
A Quantachrome Autosorb 1 instrument was used to
characterise the pore structure and surface properties of
the samples. The latter has been performed using the
equation proposed by Brunauer, Emmet and Teller
(1938, BET) [20]. The pore size distribution for 1.2 up to
40 nm pore diameters were derived by N2-isotherms
measured at 77 K. Pore size distributions (PSD) were
analysed [21,22] and the nonlinear density functional
theory (NLDFT, Thommes et al. 2006) [ 23]. The submi-
cro- and micropores, in the range of 0.3 to 1.5 nm, were
charac- terised by CO2 isotherms measured at 273 K.
These iso- therms were also evaluated according to the
Dubinin and NLDFT methods. For the NLDFT evalua-
tion, slit and/or cylindrical pores were assumed which
H2 and CH4 Sorption on Cu-BTC Metal Organic Frameworks at Pressures up to 15 MPa and Temperatures
between 273 and 318 K
Copyright © 2011 SciRes. JSEMAT
represents a com- promise between availability of
mathematical PSD-al- gorithms with spherical pore
structure and real pore structures.
3. Results and Discussion
3.1. Sorption Capacities
3.1.1. Hydrogen
Figure 3 shows the H2 excess sorption isotherms for
Cu3(BTC)2 (HKUST-1), measured at 318 K, in compare-
son with similar literature data reported for Cu3(BTC)2 at
298 K [7]. The higher sorption capacities reported by
Panella et al. (2005) [7] can be explained by lower ex-
perimental temperatures used. The rather linear trend
(Freundlich or Langmuir-Freundlich type) of the sorption
isotherms is comparable to other isotherm data (see Fig-
ure 1), but it is not a cla ssical Langmuir type as would be
expected for porous materials.
Structural changes (pore structure, specific surface
area) of the Cu3(BTC)2 after H2 sorption have been de-
termined; the results are provided in Table 1. It is obvi-
ous that specific surface areas (SSA) decrease by a factor
of 3-4, N2 pore volumes by a factor of 2-3, and CO2 mi-
cropore volumes by a factor of 4-5, while an increase in
the Dubinin-Astakhov (DA) average pore diameter from
9.2 to 15.7 Å (1010 m) can be observed.
Compared to the N2 micropore volumes for Cu3(BTC)2
determined in this study 0.63 cm3g1 (Table 1), several
authors obtained values between 0.41 and 0.76 cm3g1 on
virgin samples [13,24,25]. Other studies report BET sur-
face areas for this material of 1781 m2/g (single-point
BET) [26], 1154 m2/g [7] and 1239 m2/g [11] which is
similar to the BET surface area of 1246 m2/g ascertained
in this study (all measured wit h N2 at 77 K).
3.1.2. Met hane
Methane sorption isotherms on Cu3(BTC)2 have been
measured at temperatures between 273 and 318 K and
pressures up to 15 MPa and are provided in Figure 4.
Maximum excess sorption values obtained in these
measurements are listed in Table 2. They show a gene-
rally decreasing trend with temperature from 101 to 79
mg CH4/g MOF. After passing a maximum value, the
excess sorption isotherms in Figure 4 decrease slightly
until the final pressure value is reached. This decline
above ~6 MPa can be attributed to a volumetric effect
(non-negligibl e volume of the sorbed phase) that needs to
be considered when calculating absolute sorption capaci-
ties. Among others, Humayun and Tomasko (2000) [27]
proposed a method to determine the sorbed phase density,
which has been applied in this study. This procedure re-
sulted in nearly identical values of 597 to 601 kg/m3 for
036912 15
excess s or ption / mg g
Pressure / MPa
this study (318K)
Panella et al.(2006) (298K)
Figure 3. H2 excess sorption capacity measured at 318K on
Cu3(BTC)2. For comparison the H2 excess sorption data at
298 K on Cu3(BTC)2 reported by Panella et al.7 has been
Table 1. Structural cha nges of Cu3(BTC)2 after high-pressure H2 s orpt ion e x peri ment at 318 K. DR = Dubinin R ad ushkewic;
NLDFT = non-local density functional theory.
Cu3(BTC)2 method N2 surface area
(m2/g) CO2 micropore surface
area (m2/g) N2 pore volume
(cm3/g) CO2 micropore
volume (cm3/g) DA pore
diam et er (Å)
befor e H2 sor ptio n test DR
NLDFT 1731
1994 2708
2340 0.63
0.64 0.94
0.89 9.2
after H2 sorption test DR
576 752
641 0.23
0.33 0.26
0.20 15.7
Table 2. CH4 Sorptio n p arameters of Cu3(BTC)2 at 273 to 318 K considering helium densities.
T (K) max. excess CH4 sorption capacity (mg/g) absolute CH4 sorption capa city (mg/g) CH4 sor be d phase de ns i ty (kg / m 3)
273 101 ± 7 11 3 ± 3 599 ± 10
278 100 ± 7 11 2 ± 3 601 ± 8
293 92 ± 2 103 ± 2 59 7 ± 5
318 79 ± 2 90 ± 3 n.d.
H2 and CH4 Sorption on Cu-BTC Metal Organic Frameworks at Pressures up to 15 MPa and Temperatures
between 273 and 318 K
Copyright © 2011 SciRes. JSEMAT
036912 15
Pressure / MPa
Excess sorption / mg g
Langmuir fit 273K
Langmuir fit 278K
Langmuir fit 293K
Langmuir fit 318K
Figure 4. CH4 sorption isotherms measured on Cu3(BTC)2
at 273, 278, 293, and 318 K.
the CH4 sorbed phase density at three of the four experi-
mental temperatures (Figure 5, Table 2). For the iso-
therm measured at 318 K, the pr essure i s not high enough
for a graphical d etermination. This suggests that t he den-
sity of the sorbed CH4 phase is independent of the ex-
perimental temperature. Maximum absolute sorption
capacities, taking into consideration the volume of the
sorbed phase, have been determined to be as high as 113,
112, 103 and 90 mg/g at 273, 278, 293 and 318 K, re-
spectively (Table 2), by applying the following relatio n-
abs gas
denotes the excess sorption capacity,
the gas phase density, “
” is the Langmuir parameter
(1/MPa) and sorbed
the sorbed phase density as deter-
mined graphically in Figure 5.
The Langmuir curves shown in Figure 4 were calcu-
lated with explicit consideration of the sorbed phase den-
sity and therefore, reproduce the decline of the isotherms
at high pressures.
In comparison to the study by Senkovska and Kaskel
(2007) [13], the CH4 sorption values in this study are
lower by a factor of ~1.4. One explanation might be the
reference to the sample mass (or sample density). Sen-
kovska and Kaskel (2007) [13] used a crystallographic
density of 0.88 g/cm3. In this study, sorption values are
related to initial weight, while the sample density after
activation is 1.77 ± 0.03 g/cm³ as determined by helium
pycnometry. This lead to a slightly lower sorption
amounts for this study in comparison to Senkovska and
Kaskel (2007) [13].
0100 200300 400500 600700
excess sorpt ion / mg g
Gasphas e dens ity / kg m
Figure 5. CH4 excess sorption capacities vs. CH4 gas phase
density at different temperatures (Cu3(BTC)2.
3.2. Isosteric Heats of Adsorption
Based on the sorption isotherms measured between 273
and 318 K, isosteric heats of adsorption for CH4 have
been calculated using the Clausius-Clapeyron relation
(Equation (2)). In this first approximation, the depen-
dence of the adsorption enthalpy on surface coverage
was neglected.
12212 1
21121 2
ln ln
ads R TTpR TTa
hTTpTT a
 
∆= =
 
 
where T1 and T2 are 273 and 318 K, respectively, p1 and
p2 are the CH4 pressures for each isotherm corresponding
to an equal fractional coverage Θ and R denotes the uni-
versal gas constant. The parameters a1 and a2 are con-
stant s de rived fr om t he li near ise d La ngmu ir c urve for the
two temperatures. For further details, please refer to Pa-
nella et al. (2006) [3].
The isosteric heat of adsorption was calculated based
on a Langmuir fit of the experimental data. The decrease
in CH4 excess sorption at pressures above 10 MPa is re-
produced by introducing a term that accounts for the in-
crease in sorbed phase volume with increasing loading
(Equation (1)). For this purpose, the density of the sorbed
phase was determined graphically (see Figure 5). The
procedure resulted in a very good fit (Figure 4) of the
experimentally determined excess sorption values. The
Langmuir parameters a1 and a2 obtained with this met ho d
for isotherms at two different temperatures were used in
Equation (2) for the calculation of the isosteric heat of
Isosteric heats of adsorption for CH4 show an average
value of 11.2 ± 0.7 kJ/mol. In this case, a constant den-
sity of the sorbate phase was assumed. Using the pa-
rameters for the best least square Langmuir fit, an aver-
age value of 12.2 ± 0.7 kJ/mol was obtained. Evaluation
of the isosteric heat of adsorption in the low pressure
H2 and CH4 Sorption on Cu-BTC Metal Organic Frameworks at Pressures up to 15 MPa and Temperatures
between 273 and 318 K
Copyright © 2011 SciRes. JSEMAT
range (0 - 4 MPa) w
the linearised Langmuir fit (pro-
viding the same values of absolute adsorption as the
graphical evaluation) results in a significantly higher
value for the heat of adsorption (17.9 ± 1.3 kJ/mol).
3.3. Impact of CH4 Sorption on Pore-Size
The pore-size distributions of the initial Cu3(BTC)2 and
the samples after successive CH4 and H2 sorption mea-
surements were determined by N2 adsorption at 77 K. In
order to describe adsorption on a wider range of mic-
roporous materials the Dubinin-Astakhov (DA) equation
was used (Dubinin and Astakhov, 1971) [21]. It is a ge-
neralized form of the Dubinin-Radushkevich (DR; Du-
binin, 1975 [22]) equation and was found to reasonably
fit adsorption data for heterogeneous micropores. The
resul ts of the evaluatio ns according to the DA method are
displayed in Figure 6; the pertaining fitting parameters
are listed in Table 3. The heterogeneity value (n) is
higher for homogeneous distributions and is typically
between 1 and 3. For instance, zeolite has an n-value
higher t han 3, d e monstrating that these materials are very
homogeneous. In this study, a value of n = 1 was dete r-
mined for Cu3(BTC)2. The characteristic energy E after
Dubinin (1975) was determined to be 20.97 kJ/mol for
the initial (unspoiled) sample and ranged between 8 and
18 kJ/mol after high-pressure CH4 and H2 sorption tests.
The interaction constant is given as k = 2.96 kJ·nm3/mol
for nitrogen.
From Figure 6 it is evident that CH4 affects the stabi-
lity of the MOF to a lesser extent than H2. Although a
significant decrease in total pore volume is shown after
two successive CH4 sorption experiments, accompanied
by a shift to larger mean pore radii (4.7 Å for the original
sample; 6 . 4 Å afte r si x CH4 so rptio n isother ms; Table 3),
a reduction comparable to the H2 treatment is only ob-
served after 6 CH4 sorption isotherms. As observed for
the H2 treatment, the pore volume in the 5 to 15 Å range
is most heavily affected. In this study it is likely that we
had chosen a slightly too high activation temperature.
However the reason for the different influence of CH4
and H 2 on the sample structure is not clear will be invest-
tigated in further detail in a fol low-up st udy.
Results of the evaluation of the micropore structure of
the Cu3(BTC)2 before and after high-pressure CH4 sorp-
tion i so therms b ase d o n the D R met hod a nd the no nl i near
density functional theory (NLDFT) are summarised in
Table 4 . The se results a lso supp ort a signi fica nt decrease
in micropore volume and subsequent reduction in spe-
cific surface area of the sample after high-pressure CH4
sorptio n tests.
Figure 7 compares the high-pressure CH4 exce ss sorp-
010 20 30 40 50 60
Differential pore volume / cm³ g
Pore diameter / A
initial sa mp le
after 2 CH4 s orption cycles
after 6 CH4 s orption cycles
after 2 H2 s orption cycles
Figure 6. Dubinin Astakov differential pore volume distri-
butions of Cu3(B TC)2 before and after high-pressure CH4
and H2 sorption isotherms. Evaluation based on low-pres-
sure N2-sorption at 77 K.
0246810 12 14 16
excess sorpt ion / mg g
Pressure / MPa
initial sa mp le
after treatment
m = 39.7 mg g
Figure 7. Comparison between initial and CH4 treated
Cu3(BTC)2 at 293 K.
tion isotherms measured on the initial Cu-BTC MOF
with a decomposed sample of the same material at 293
K. The treated sample shows significantly lower sor ption
values (decrease of up to 40 mg/g) compared to the ini-
tial samples, while the shape of the isotherms is similar
between the two isotherms.
4. Conclusions
At pressures up to 7 MPa, Cu-BTC MOF is one of the
most promising MOF materials for metha ne storage with
a large uptake capacity and an excess sorption of 9.2
wt% at 293 K. The repeated thermal activation and high-
pressure sorption tests with CH4 on HKUST-1 at tem-
peratures between 273 and 318 K resulted in a reduction
of specific micropore volumes and surface area. The
processes behind the reduction of the specific micropore
volumes and surface area during each CH4 sorption test
(Table 4) are ambiguous and further research on this
topic is needed.
Recent studies indicate ther mal deter ioratio n effect s and
H2 and CH4 Sorption on Cu-BTC Metal Organic Frameworks at Pressures up to 15 MPa and Temperatures
between 273 and 318 K
Copyright © 2011 SciRes. JSEMAT
Table 3. Fitting parameters of the DA equation for micropore characterisation of Cu3(BTC)2 before and after CH4 and H2
high-pressure sorpt ion i sotherms.
Cu3(BTC)2 N2 char acteristi c energ y E [kJ/mol] DA h etero geneous va l ue n average pore radius [Å]
initial sample 20.97 1 4.7
after 2 Ch4 isotherms 18.66 1 4.9
after H2 isotherms 11.17 1 5.8
after 6 CH4 isotherms 8.66 1 6.4
Table 4. Results of pore structure analysis of Cu3(BTC)2 before and after high-pressure so rption isotherms with CH4.
method N2 surface ar ea
(m2/g) N2 pore volume
(cm3/g) CO2 surface area
(m2/g) CO2 micropore
volume (cm3/g) N2 DA por e
dia meter (Å)
initial sample DR
NLDFT 1731
1994 0.62
0.64 2708
2340 0.94
0.89 9.2
after 2 CH4 isotherms DR
NLDFT 1174
1186 0.42
0.55 1226
1069 0.43
0.35 12.4
after 6 CH4 isotherms DR
591 0.26
0.48 997
887 0.35
0.28 12.8
by interaction with atmospheric air could cause the ob-
served degradation of the sample.
Table 4 shows a stepwise deterioration of the sample
after successive CH4 sorption isotherms and the thermal
activation process on the identical sample. Further
changes caused by exposure to the atmosphere (e.g. oxi-
dation of the sample) can be neglected, since samples did
not come in contact with atmospheric air from the begin-
ning of the thermal activation process (only H2, CH4 and
He). However, it is unclear as of yet why micropores
volumes decrease drastically and larger pores remain
Finally, this study has shown that the CH4 sorbed
phase density is, within reasonable errors, independent of
5. Acknowledgments
I am greatly indebted to Stefan Kaskel for providing
2007 the samples and Joachim Borchardt for technical
support. The fruitful discussion and remarks of Bernd
Krooß, Andreas Busch, Susan Giffin and Dirk Prinz are
greatly appreciated.
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