Molecular Imprinting of Metal-Organic frameworks (MOFs) for Selective Separations
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2022-04
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Abstract
This thesis explores the idea of using chromium(III) ions and an orthogonal polyaromatic phosphonate linker to produce a stable metal-organic framework (MOF) for the separation of xylene isomers. This began by creating a charge-assisted hydrogen-bonded metal-organic framework (HMOF) that uses weaker interactions to hold the framework together. The orthogonalized phosphonate creates pores due to its bulky nature that prevents efficient ligand packing. In tandem, this allows for the inclusion of guest molecules within the HMOF structure. Once the guest-incorporated structure is formed, heating can remove water molecules from the HMOF to form coordination bonds between the chromium and the phosphonate ligand. This dehydration locks the template in place as it converts to a coordinated metal-organic framework (MOF). The template for this technique is of great importance since you can imprint the pore to a specific molecule. For this study, the xylene isomers ortho, meta, and para, were looked at for separation. The separation of xylenes is of great importance since the different isomers are used in a variety of different applications, such as para-xylene for terephthalic acid (TPA) to manufacture polyethylene terephthalate (PET). The problem with separating xylenes is how expensive the separation is. Since the xylene isomers have very similar boiling points and sizes, either cryogenic distillation or recrystallization has to be performed to acquire a pure product. On this note, three different chromium phosphonate HMOFs were developed. The first structure incorporated a para-xylene template (HCALF 50αP) while the second structure incorporated ortho-xylene as the template (HCALF 50α)). The final structure used meta-xylene as a template (HCALF 50M). These HMOFs were dehydrated into the corresponding MOF structures and used to test selective xylene separation. Initial studies using a static separation and nuclear magnetic resonance (NMR) spectroscopy were conducted followed by a flow-through separation using high-pressure liquid chromatography (HPLC) and a MOF-packed column. The findings were intriguing showing selectivity towards para-xylene for some of the materials. Most notable is that HCALF 50βM showed anti-selectivity towards para-xylene allowing it to flow freely through and making it easier to collect.
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Evans, D. (2022). Molecular imprinting of metal-organic frameworks (MOFs) for selective separations (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.