Adsorptive Separations on Metal-Organic Frameworks

Abstract

Metal-organic frameworks are a class of microporous materials with exceptionally high pore volumes. Their composition of metal ions and organic linkers creates a chemically diverse environment allowing various potential interactions with adsorbing molecules. Much research has been performed on this class of materials concerning catalysis and gas phase adsorption. The storage of hydrogen gas in metal-organic frameworks is one of the key research topics concerning MOFs. Recently, liquid phase adsorption and separation on MOFs gained interest, although not many data are available yet. This thesis will focus on various separation and purification problems. The separation of a steam cracker s C5-cut will be discussed first using a combination of MOFs and zeolites. MIL-96, an Al-trimesate, is able to separate di-olefins from the other fractions due to sterical and possibly entropical reasons, while [Cu3(BTC)2] can separate olefins from paraffins based on interactions of the olefinic bond with metal sites present in the host material. Using a high silica chabasite zeolite, the separation of linear and branched C5-isomers can be attained due to shape selectivity. A second major separation studied is the separation of ethylbenzene vs styrene and of vinyltoluenes from the corresponding ethyltoluenes. MIL-47, MIL-53 and [Cu3(BTC)2] have been studied and all three materials have similar uptake capacitites and separation factors, but the adsorption in all three materials is based on a different mechanism. The adsorption is entropy driven on MIL-47, while it is enthalpy driven on the analogous structure of MIL-53. On [Cu3(BTC)2], the interaction with the free metal sites as well as the number of p-electrons in the adsorbate seem to be playing an important role. Apart from separations, purification is another important application of liquid phase adsorption. The removal of phenolic compounds from wastewater is an example of this and it is shown that a water stable MOF like MIL-53(Cr) can perform this purification. Due to its organic composition, MIL-53(Cr) is capable of interacting with phenolic compounds and as water is coadsorbed, the regeneration of this material is more straightforward than with classical activated carbon. Finally, the removal of heterocyclic aromatic contaminants from fuel feeds is discussed for a variety of MOFs. It turns out that the Lewis acidity of the free metal sites of a MOF is of crucial importance to adsorb either both sulfur and nitrogen containing contaminants or only nitrogen containing contaminants, underlining the enormous potential that MOFs have for adsorptive applications.status: publishe