Synthesis of the zeolitic imidazolate framework ZIF-4 from the ionic liquid 1-butyl-3-methylimidazolium imidazolate.

A new synthesis route for the zeolitic imidazolate framework ZIF-4 using imidazolium imidazolate is reported. Additionally, the ionic liquid-derived material is compared to conventional ZIF-4 with respect to the powder X-ray diffraction pattern pattern, nitrogen uptake, particle size, and separation potential for olefin/paraffin gas mixtures. Higher synthesis yields were obtained, and the different particle size affected the performance in the separation of ethane and ethylene.

Continuous Separation of Light Olefin/Paraffin Mixtures on ZIF-4 by Pressure Swing Adsorption and Membrane Permeation.

In this study, two zeolitic imidazolate frameworks (ZIFs) called ZIF-4 and ZIF-zni (zni is the network topology) were characterized by sorption studies regarding their paraffin/olefin separation potential. In particular, equilibrated pure and mixed gas adsorption isotherms of ethane and ethene were recorded at 293 K up to 3 MPa. ZIF-4 exhibits selectivities for ethane in the range of 1.5-3, which is promising for continuous pressure swing adsorption (PSA). ZIF-4 shows high cycle stability with promising separation potential regarding ethane, which results in purification of the more industrial desired olefin. Furthermore, both ZIF materials were implemented in Matrimid to prepare a mixed matrix membrane (MMM) and were used in the continuous separation of a propane/propene mixture. The separation performance of the neat polymer is drastically increased after embedding porous ZIF-4 crystals in the Matrimid matrix, especially at higher feed pressures (3-5 barg). Due to the smaller kinetic diameter of the olefin, the permeability is higher compared to the paraffin.

Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks.

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths.

Experimental and Theoretical Analysis of the Influence of Different Linker Molecules in Imidazolate Frameworks Potsdam (IFP-n) on the Separation of Olefin-Paraffin Mixtures.

Four metal-organic frameworks with similar topology but different chemical environment inside the pore structure, namely, IFP-1, IFP-3, IFP-5, and IFP-7, have been investigated with respect to the separation potential for olefin-paraffin mixtures as well as the influence of the different linkers on adsorption properties using experiments and Monte Carlo simulations. All IFP structures show a higher adsorption of ethane compared to ethene with the exception of IFP-7 which shows no selectivity in breakthrough experiments. For propane/propane separation, all adsorbents show a higher adsorption for the olefin. The experimental results agree quite well with the simulated values except for the IFP-7, which is presumably due to the flexibility of the structure. Moreover, the experimental and simulated isotherms were confirmed with breakthrough experiments that render IFP-1, IFP-3, and IFP-5 as suitable for the purification of ethene from ethane.

Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4.

The microporous zeolitic imidazolate framework ZIF-4 has been synthesized, and its ethylene/ethane and propylene/propane separation potentials have been evaluated by single-component adsorption isotherms and breakthrough experiments of the respective binary mixtures. In all experiments, a higher selectivity for the paraffin is observed that is manifested by a steeper equilibrium isotherm as well as a later breakthrough in the fixed-bed adsorber experiments. Microporous adsorbents with paraffin selectivity are rare but highly interesting for cyclic adsorption processes such as pressure-swing adsorption (PSA).