REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
on nanoporous materials is an interesting option for
the separation of ethane/ethylene mixtures. The adsorption equilibria of ethane
(C2H6) and ethylene (C2H4) has been
studied over several adsorbents as activated carbons, zeolites, single-walled
carbon nanotubes. More recently, the adsorption characteristics of the C2H6/C2H4
pair on different metal organic frameworks (MOFs) have been reported [1, 2].
MIL-53 family is an interesting class of MOFs, consisting of a trivalent metal cation (Al, Cr, Fe, etc.) linked to the carboxylate group
of a terephthalate moiety. The adsorption equilibria data of C2H6
and C2H4 on MIL-53 MOFs present in the literature are
scarce, and in the present work we develop a self-consistent framework,
combining experimental and molecular simulations, to determine the adsorptive
characteristics of C2H6 and C2H4
onto a MIL-53(Al) MOF. The MIL-53(Al) is composed by corner-sharing AlO4(OH)2
octahedra interconnected with 1,4–benzenedicarboxylate
ligands, possessing one dimensional diamond-shaped nanopores
(Figure 1), whose large pore and narrow pore conformations exhibit free
dimensions of (0.85 x
0.85) nm2 and (0.26 x
1.36) nm2 [3 - 5].
adsorption equilibrium of the pure components over MIL-53(Al) was measured in a high-pressure
magnetic-suspension microbalance from Rubotherm GmbH
(Germany), spanning temperature and pressure ranges of 303 – 353 K and 0 – 8
bar for C2H6, and 323 – 373 K and 0 – 1.7 bar for C2H4.
Additionally, simulations were performed using the Grand Canonical Monte Carlo
(GCMC) algorithm with a parameterization of solid–fluid dispersive interactions
based on the TraPPE-UA force field . The GCMC results
were checked against the experimental data to validate the molecular
force-field employed and establish a thermodynamical
analysis by calculating the isosteric heat of
adsorption data (C2H6/C2H4) were determined
by simulation using the validated force-field; the Ideal Adsorbed Solution
Theory (IAST) was employed to evaluate the ideal behaviour of the adsorbed
II. Results and Discussion
absolute adsorption isotherms of C2H6 at 303, 323, and 353 K between 0 and 8 bar, and C2H4 at 323, 353, and 373 K between
0 and 1.7 bar are plotted in Figure 2. The results present
a slightly higher adsorption capacity of MIL-53(Al) towards C2H6
than to C2H4.
The experimental data and the GCMC calculations show good agreement, although the
latter slightly over predict C2H6 adsorption at 353 K and
C2H4 at 323 K above 1.5 bar. The
molecular simulations considered the modelled MIL-53(Al) in its large pore form,
MIL-53lp(Al), and the results demonstrate that this is the most stable
conformation in the whole (P, T) phase space studied; this is in
accordance with our previous results regarding CH4 adsorption .
isosteric heat of adsorption, Qst, was
obtained from molecular simulations considering the co-variance formulation of
Nicholas and Parsonage . Results show that the isosteric
heat, within the studied regions, is independent of temperature for both adsorbates and exhibits a linear dependence with adsorbate loading, increasing essentially with pore filling
as a consequence of molecular packing within the adsorbent channels. The energetical dissimilarity between ethylene and ethane is
only minorly reflected in the energetics of
adsorption, since their isosteric heat values are
similar, even in the very low pressure domain, which at zero-coverage results
= 22.5 kJ/mol (C2H6) and Qst =
20.5 kJ/mol (C2H4). In this
region, where entropic considerations can be neglected, the small DQst
= 2 kJ/mol difference can be rationalized as arising
from enhanced dispersive interactions between the benzene rings and ethane
was also calculated from the experimental data, employing the integrated form
of the Clausius-Clapeyron equation, and the obtained
data show a fairly good agreement with the GCMC results, although the experimental
values are slightly higher (ca. 5 kJ/mol).
validation of the employed force-field, GCMC computational simulations were performed
for C2H6/C2H4 binary mixtures. The
binary adsorption equilibria GCMC results were probed using the IAST and the Toth isotherm multicomponent extension. Figure 3 presents
the binary adsorption results obtained at 323 K and 1 bar. Both the IAST and
the extended multicomponent Toth model predict the
GCMC simulation data well, indicating that the mixture does have ideal
behaviour in the studied conditions.
Financial support from FCT/MEC (Portugal)
through projects EXCL/QEQ-PRS/0308/2012, PTDC/AAC-AMB/108849/2008,
is gratefully acknowledged.
References:  M. Jorge et al., Ind. Eng. Chem. Res., 53 (2014) 15475.  C. Gücüyener et al., J.
Am. Chem. Soc., 132 (2010) 17704.
 T. Loiseau et al., Chem. Eur. J.,
10 (2004) 1373.  Y. Liu, et al., J. Am. Chem. Soc., 130 (2008) 11813.  C. Serre, Adv.
Mater., 19 (2007) 2246.  M.G.
Martin, J.I. Siepmann, J. Phys. Chem. B,
102 (1998) 2569.  A. Lyubchyk et al., J.
Phys. Chem. C, 115 (2011) 20628.
 D. Nicholson, N.G. Parsonage, Computer
Simulation and the Statistical Mechanics of Adsorption, Academic Press, New
of the two solid conformations of the MIL-53(Al) framework. The large
pore (lp) form is
orthorhombic  and belongs to the Imma symmetry space group whilst the narrow pore (np)
analogue is monoclinic belonging to the C2/c
Figure 2. C2H6 and C2H4 absolute
adsorption isotherms on MIL-53(Al). Comparison of experimental (filled
symbols) and simulated by GCMC
(open symbols) absolute adsorption equilibrium isotherms of C2H6
over MIL-53(Al) at 303 (♦), 323 (●) and 353 K (▲) and C2H4
at 323 (●), 353 K (▲) and 373 K (■). Solid lines
are drawn as guide to the eye.
3. IAST and Toth model predictions and GCMC simulation adsorption
equilibrium data for C2H6/C2H4 binary mixture on MIL-53(Al) at 323 K and 1 bar. Symbols – GCMC simulation
results; Solid lines – IAST model predictions; Dashed lines – Toth model predictions