Ethane and Ethylene Adsorption on MIL-53(Al): Experiments and Molecular Simulations

Ethane and Ethylene Adsorption on MIL-53(Al): Experiments and Molecular Simulations

Rui P. P. L. Ribeiro, Bárbara C. R. Camacho, Andriy Lyubchyk, Isabel A. A. C. Esteves, Fernando J. A. L. Cruz, José P. B. Mota

LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.

pmota@fct.unl.pt

I. Introduction

Adsorption 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].

The 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].

The 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 [6]. 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.

Binary 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 mixture.

II. Results and Discussion

Single component Equilibria

The 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 [7].

Adsorption Energetics

The isosteric heat of adsorption, Q_st, was obtained from molecular simulations considering the co-variance formulation of Nicholas and Parsonage [8]. 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 in Q_st = 22.5 kJ/mol (C₂H₆) and Q_st = 20.5 kJ/mol (C₂H₄). In this region, where entropic considerations can be neglected, the small DQ_st = 2 kJ/mol difference can be rationalized as arising from enhanced dispersive interactions between the benzene rings and ethane molecules.           

The Q_st 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).

Binary Adsorption

Upon 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.

Acknowledgements: Financial support from FCT/MEC (Portugal) through projects EXCL/QEQ-PRS/0308/2012, PTDC/AAC-AMB/108849/2008, PTDC/CTM/104782/2008, PEst-C/EQB/LA0006/2013 and UID/QUI/50006/2013 is gratefully acknowledged.

References: [1] M. Jorge et al., Ind. Eng. Chem. Res., 53 (2014) 15475. [2] C. Gücüyener et al., J. Am. Chem. Soc., 132 (2010) 17704. [3] T. Loiseau et al., Chem. Eur. J., 10 (2004) 1373. [4] Y. Liu, et al., J. Am. Chem. Soc., 130 (2008) 11813. [5] C. Serre, Adv. Mater., 19 (2007) 2246. [6] M.G. Martin, J.I. Siepmann, J. Phys. Chem. B, 102 (1998) 2569. [7] A. Lyubchyk et al., J. Phys. Chem. C, 115 (2011) 20628. [8] D. Nicholson, N.G. Parsonage, Computer Simulation and the Statistical Mechanics of Adsorption, Academic Press, New York, 1982.

Figure 1. Structure of the two solid conformations of the MIL-53(Al) framework. The large pore (lp) form is orthorhombic [4] and belongs to the Imma symmetry space group whilst the narrow pore (np) analogue is monoclinic belonging to the C2/c category [5].

 

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.

Figure 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