Ethene/ethane separation by the MOF membrane ZIF-8: Molecular correlation of permeation, adsorption, diffusion

doi:10.1016/j.memsci.2010.12.001

Journal of Membrane Science

Volume 369, Issues 1–2, 1 March 2011, Pages 284-289

https://doi.org/10.1016/j.memsci.2010.12.001 Get rights and content

Abstract

The newly developed MOF membrane ZIF-8 separates an equimolar ethene/ethane mixture at room temperature for 1 and 6 bar feed pressure, respectively, with a selectivity of 2.8 and 2.4. Independent sorption uptake studies of an ethene/ethane mixture on a big ZIF-8 single crystal by IR microscopy detection show in combination with grand canonical Monte Carlo simulations that this moderate ethene selectivity of the ZIF-8 membrane can be explained by the interplay of a preferential ethane adsorption selectivity competing with a preferential ethene diffusion selectivity. This means, that ethane adsorbs stronger than ethene, but ethene diffuses faster and overcompensates the adsorption preference of ethane, resulting in a membrane permeation selectivity for ethene.

Graphical abstract

Research highlights

▶ ZIF-8 membrane can separate ethene/ethane mixtures with selectivities between 2 and 3. ▶ From mixture adsorption and diffusion data, the membrane selectivity can be predicted. ▶ Both IR microscopy and grand canonical Monte Carlo simulation can give the mixture data.

Introduction

The paraffin/olefin separation by cryogenic distillation is one of the most energy and cost intensive processes. Separation by adsorption is an energy-efficient alternative. Two concepts can be applied: (i) the preferential uptake of the olefin under equilibrium condition e.g. by Cu modified adsorbents and (ii) the kinetic based separation by different diffusion rates, which result in the extreme case to steric size exclusion.

As a new type of nanoporous materials, metal–organic frameworks (MOFs) have been examined inter alia in their olefin/paraffin separation performance by adsorption [1]. By chlorine or bromine functionalization of organic linker molecules, the pore openings of zeolitic imidazolate frameworks (ZIFs) could be fine-tuned. The modification resulted in different diffusion rates – propene is slightly smaller and hence is diffusing faster than propane – and allowed kinetic separation of the mixture [2]. In contrary, copper containing MOFs like Cu₃(BTC)₂ showed favored adsorption of i-butene above i-butane, allowing the separation of the binary mixture in packed bed adsorber [3]. For liquid C₅ paraffin/olefin mixtures, Cu₃(BTC)₂ shows a clear olefin selectivity as well [4].

Very recently, first MOF membranes with selectivities higher than the Knudsen selectivity have been developed [5], [6], [7], [8], [9], [10], [11], [12]. In our recent works, we focused on the development of thermally stable and steam resistant ZIF membranes like ZIF-7 [13], [14], [15], ZIF-8 [16] and ZIF-22 [17]. Despite pioneering works in the field of computer modeling to predict separation behavior of MOF adsorbents [18] and membranes [19], the experimental separation factors often differ considerably from the predictions [20]. These deviations might be correlated to the framework flexibility of the MOFs.

The in situ study of sorption uptake/desorption of guest molecules on large MOF or zeolite crystals detected by IR microscopy (IRM) supported by theoretical studies by grand canonical Monte Carlo (GCMC) simulations appears to be a powerful tool for determining loading dependent transport diffusion coefficients under mixed gas conditions [20], [21]. Using the well-known relationship “permeability = mobility × solubility” as a rough estimation, the membrane selectivity can be expressed as the product of a diffusion and adsorption selectivity [22]. Here, we study ethene/ethane separation on a supported ZIF-8 membrane and give a molecular interpretation of the adsorption and diffusion contributions from GCMC supported IRM (GCMC-IRM) studies on large ZIF-8 single crystals.

Section snippets

Experimental

As recently reported, continuous ZIF-8 layers can be grown as membrane on-top of porous titania supports [16]. However, the discoid titania supports of 1 mm thickness turned out to be too brittle for application at pressures difference across the membrane >3 bar. Hence, in this work a special composite support (Fraunhofer IKTS, Germany) consisting of a smooth titania layer on top of a mechanically strong alumina support was used for membrane preparation.

A typical synthesis solution contains 0.538

Results and discussion

For the correlation of the permeation selectivity with the GCMC-IRM diffusion and adsorption data, it has to be ensured that only the ZIF-8 layer on-top of the macroporous support controls the permeation. Hence, a relative thick ZIF-8 layer of about 25 μm has been prepared on top of the asymmetric titania support (Fig. 1). However, it should be noted that using secondary growth crystallization, ZIF-8 membrane layers with a few μm in thickness can be realized [13].

Fig. 2 shows the pressure

Conclusions

Membrane selectivities can be predicted with sufficient accuracy as the product of adsorption and diffusion selectivity. Both can be obtained from sorption uptake experiments on large single crystals by IR microscopy. We demonstrated this procedure for the ethene separation from an equimolar ethene/ethane mixture for different feed pressures at room temperature. At 6 bar feed pressure, the product of the ethene/ethane adsorption selectivity (0.5) and diffusion selectivity (2.7) gives an

Acknowledgements

This work is part of the DFG Priority Program SPP 1362 “Porous Metal–Organic Frameworks”, organized by S. Kaskel. The financial support is gratefully acknowledged.

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Ethene/ethane separation by the MOF membrane ZIF-8: Molecular correlation of permeation, adsorption, diffusion

Abstract

Graphical abstract

Research highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Chem. Eng. J.

Sep. Purif. Technol.

Micropor. Mesopor. Mater.

Chem. Eng. Sci.

Micropor. Mesopor. Mater.

Selective gas adsorption and separation in metal–organic frameworks

Chem. Soc. Rev.

Zeolitic imidazolate frameworks for kinetic separation of propane and propene

J. Am. Chem. Soc.

Adsorptive separation of isobutene and isobutane on Cu3(BTC)2

Langmuir

Separation of C5-hydrocarbons on microporous materials: complementary performance of MOFs and zeolites

J. Am. Chem. Soc.

“Twin copper source” growth of metal–organic framework membrane: Cu3(BTC)2 with high permeability and selectivity for recycling H2

J. Am. Chem. Soc.

Microporous metal organic framework membrane on porous support using the seeded growth method

Chem. Mater.

Highly permeable zeolite imidazolate framework-8 membranes for CO2/CH4 separation

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Highly permeable zeolite imidazolate framework-8 membranes for CO₂/CH₄ separation