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Understanding Biodegradability of Phosphonium and Ammonium-Based Ionic Liquids from Electronic Structure Calculations

Understanding Biodegradability of Phosphonium and Ammonium-Based Ionic Liquids from Electronic Structure Calculations

Grant Spencer

Chemical Engineering

Ionic Liquids (ILs) are compounds that are ionic in nature and have melting points below 100 oC. Due to negligible vapor pressures of most of the ILs they are attractive solvents for a variety of processes in petroleum recovery, refining, and other chemical processes. Furthermore, their negligible vapor pressure means application of ILs in industry has the potential to nearly eliminate air emissions. While ILs reduce emissions by sustaining in the liquid phase, there is still a possibility to pollute water sources and/or soils. Hence a rational design of novel ILs must include biodegradability considerations from the beginning. In this poster presentation, the focus is on using electronic structure calculations to correlate the frontier orbital energies and the potential for biodegradation for two families of ionic liquids: tetra-alkylphosphonium and tetra-alkylammonium (quaternary ammonium). One of the four alkyl groups these ILs was systematically varied to assess the effect of alkyl chain length on the frontier orbital (HOMO- Highest occupied molecular orbital and LUMO – lowest unoccupied molecular orbital) energies. Additionally, isomers for a given chain length were also examined to determine the influence of branching on these energies. Further, the placement of the HOMO and LUMO on the molecules was probed in order to identify possible sites for biodegradation. Our results indicated that as the alkyl chains were extended, the HOMO and LUMO energies for both classes of the ILs increase. It was also noted that the HOMO begins to stray from the center of compound (either phosphorus or nitrogen), particularly towards the far end of the longest alkyl chain. Furthermore, the LUMO is located around the center of the compounds where the positive charge is concentrated on the primary heteroatom of the compound irrespective of the chain length. Similar results were obtained for isomeric ILs as well. The straying of HOMO along the alkyl chain provides an early indication that the alkyl chains are likely to be the reactive sites and that the reactivity and hence biodegradability of the ILs is enhanced with longer alkyl chains – a result that has been experimentally reported in the literature for another class of ionic liquids.