Industrial preparation of phosphonium ionic liquids

While a great deal of attention has been given to imidazolium ionic liquids in recent years, very few investigations involving phosphonium ionic liquids have been reported in the journal literature. The same is not true in the patent literature, where, in addition to filings concerning phosphonium ionic liquids specifically, filings concerning imidazolium ionic liquids routinely claim the manufacture and/or use of phosphonium ionic liquids as well. Despite this activity, commercial applications, and hence commercial production, have not materialized for any ionic liquids to date. Here we present an account of our research into ionic liquids from the perspective of a future, large-scale producer of ionic liquids for industrial applications. Several phosphonium ionic liquids are discussed with respect to synthesis and physical characteristics, and broad comparisons are made to relevant imidazolium systems. Full synthetic and characterization data are reported for several representative compounds including trihexyl(tetradecyl)phosphonium chloride, trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, trihexyl(tetradecyl)phosphonium tetrafluoroborate, triisobutyl(methyl)phosphonium tosylate, and triisobutyl(methyl)phosphonium dimethylphosphate.