Stijn Van der Perre

Adsorptive Characterization of the ZIF-68 Metal-Organic Framework: A Complex Structure with Amphiphilic Properties

In this experimental study, the adsorption behavior of the ZIF-68 heterolinked zeolitic imidazolate framework has been explored. Vapor phase adsorption isotherms of linear C1-C6 alcohols, C6 alkane isomers, aromatics (benzene, toluene, xylene isomers, 1,3,5-trimethylbenzene, and 1,3,5-triisopropylbenzene), and polar adsorbates (water, acetonitrile, and acetone) are reported and discussed. The complex pore structure of ZIF-68, with two one-dimensional channels, each with a different polarity, displays an overall hydrophobic character. Its two-pore system results in S-shaped isotherms for small polar adsorbates (small alcohols, acetone, and acetonitrile), while longer alcohols and nonpolar molecules, such as aromatics and C6 alkane isomers, lead to type I adsorption isotherms. Bulky molecules, with a kinetic diameter significantly larger than the pore windows, are adsorbed in large amounts, which gave reason to think that this ZIF-68 material has a certain degree of framework flexibility to enlarge the free aperture of the channels. Besides, diffusion coefficients from vapor phase uptake and infrared experiments point to a different adsorption mechanism for polar and nonpolar adsorbates. Liquid phase adsorption experiments demonstrated the separation of alcohol mixtures (ethanol/1-butanol) at low concentration from water, with a clear preference for 1-butanol.

Adsorption and separation of CO2 on KFI zeolites: effect of cation type and Si/Al ratio on equilibrium and kinetic properties.

Selective separation of CO2 is becoming one of the key technologies in the (petro-) chemical industry. This study focuses on the adsorption and separation of CO2 from CH4 on a new low-silica (LS) type of the eight-membered ring KFI zeolite. A series of alkali (Li, Na, K) and alkaline-earth (Mg, Ca, Sr) exchanged samples of the new LS KFI were synthesized and characterized. LS Li-KFI showed the largest pore volume, whereas LS Na-KFI and LS K-KFI were inaccessible for Argon at 87 K. Adsorption of CO2 at 303 K demonstrated the dominant quadrupolar interaction on alkali-exchanged LS KFI samples. LS Li-KFI showed the largest capacities upon high pressure isotherm measurements of CO2 (4.8 mmol/g), CH4 (2.6 mmol/g), and N2 (2.2 mmol/g) up to 40 bar at 303 K. The performance of the new LS KFI was compared to a KFI sample (ZK-5) with a higher Si/Al ratio. Isotherm measurements and dynamic breakthrough experiments demonstrated that ZK-5 samples show larger working capacities for CO2/CH4 separations at low pressure. Li-ZK-5 and Na-ZK-5 show the highest capacities and high selectivities (similar to benchmark 13X).

Prediction of Molecular Separation of Polar–Apolar Mixtures on Heterogeneous Metal–Organic Frameworks: HKUST-1

Due to the combination of metal ions and organic linkers and the presence of different types of cages and channels, metal-organic frameworks often possess a large structural and chemical heterogeneity, complicating their adsorption behavior, especially for polar-apolar adsorbate mixtures. By allocating isotherms to individual subunits in the structure, the ideal adsorbed solution theory (IAST) can be adjusted to cope with this heterogeneity. The binary adsorption of methanol and n-hexane on HKUST-1 is analyzed using this segregated IAST (SIAST) approach and offers a significant improvement over the standard IAST model predictions. It identifies the various HKUST-1 cages to have a pronounced polar or apolar adsorptive behavior.

Separation properties of the MIL-125(Ti) Metal-Organic Framework in high-performance liquid chromatography revealing cis/trans selectivity

Monodisperse MIL-125(Ti) Metal-Organic Framework crystals were synthesized and studied as stationary phase in high performance liquid chromatography (HPLC). Different pure compounds and model mixtures (including stereoisomer mixtures) were injected from which chromatographic parameters, including selectivities and resolution factors, were determined to evaluate the adsorption properties and separation performance of MIL-125(Ti) in liquid phase. The MIL-125(Ti) framework displayed a trans selectivity for cis/trans difunctionalized cyclohexane molecules with high selectivity and resolution for 1,3-dimethylcyclohexane and 4-ethylcyclohexanol. The slurry-packed column was further characterized via van Deemter analysis. Fitting of the van Deemter equation through the experimental points allowed to define the contributions of the different processes to plate height with a significant proportion of the A-term, reflecting the importance of a good crystal packing. Although high in comparison to commercial HPLC stationary phases, a very good plate height of around 50μm was found.

Intensified Biobutanol Recovery by using Zeolites with Complementary Selectivity.

A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.