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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HyLITE (Hydrophobic Ionic Liquid Technologies)

Teaser

Summary

Over the past twenty years hydrophobic ionic liquids with little, or no vapour pressure have been developed as alternatives to volatile organic compounds solvents VOCs for use in chemical and materials separation processes from water. Applications include removal of organic compounds and metal ions for purification and pollution remediation, separation of radioactive nuclei for waste management, biotransformation and liquid chromatography for analytical and preparative synthesis. However, while many successes while many successful applications of hydrophobic ionic liquids have been demonstrated at a laboratory scale, with the exception of analytical chromatography, commercial applications have not followed. The main reason for this is because most hydrophobic ionic liquids contain complex (and expensive) perfluorinated anions that may have significant negative environmental impacts, for example generation of bio-persistant non-degradable products or, when decomposition takes place during use, formation of hydrofluoric acid.

The aim of this project was to investigate how new hydrophobic ionic liquids, for application in ionic liquid-water separations processes, in which the liquid/liquid separations could be produced that incorporate favourable performance characteristics, but remove the need for using fluorinated components. The research program examined synthetic strategies to prepare a library of ionic liquids, and then characterised their co-miscibility with water - the key selection criteria for a hydrophobic material - using this to iteratively improve properties.

New ionic liquids based on a family of phenyl-5-tetrazolate anions have been developed which exhibit the relatively rare behaviour in ionic liquids of having a lower critical solubility temperature (LCST) with water. That is, they are completely miscible with water at low temperatures and, on heating, separate into two phases (see attached image from the first publication on these materials from 2017 in the journal ChemPhysChem., DOI: 10.1002/cphc.201700942). The critical temperature and miscibility ranges can be controlled by modification of the tetrazolate anions enabling temperature responsive separations to be achieved.

Work performed

Using the concept of biostere substitution taken from pharmacy, ionic liquids containing anions derived from functionalised 5-tetrazoles were investigated ans analogues of ionic liquids with carboxylate anions.

Ionic liquids with small carboxylate anions, such as acetate, are water miscible, but lead to interesting characteristics including the ability to dissolve cellulose. Ionic liquids with longer chain alkyl-carboxylate anions have been shown to be capable of dissolving sulfur, and the series of tetrabutylphosphonium benzoate, salicylate and phthalate ionic liquids, with aromatic carboxylate anions show a progression from completely water miscible (benzoate) through to partial miscibility with LCST behaviour.

The key work performed during the project covered five major activities:

1. Synthesis of the tetrazole anion precursors using [2+3] cycloaddition reactions of azides and nitriles.

2. Synthesis of ionic liquid combining tetrazolate anions with a matrix of representative cations (1-alkyl-3-methylimidazolium, N,N-dialkylpyrrolidinium, tetraalkylammonium and tetraalkylphosphonium) and thermophysical characterisation, including rapid identification of chemically unstable materials allowing research pathways to be defined.

3. Behaviour of each ionic liquid with water, including LLE and SLE phase behaviour was determined and critical point phase diagrams were constructed. For the key prototype ionic liquid target, tetrabutylphosphonium 5-phenyltetrazolate, the full LLE/SLE phase behaviour was determined using optical cloud-point and melting point observation, differential scanning calorimetry and single crystal X-ray diffraction identifying LCST phase behaviour and SLE with novel eutectic and peritectic points that have not previously been reported in ionic liquids. This work was published in ChemPhysChem DOI: 10.1002/cphc.201700942.

4. The effects of phenyl-substituent groups on the tetrazolate anions has been assessed, examining ionic liquid/water compositions using ionic liquids prepared from commercially available and synthesised functionalised phenyltetrazoles to derive structure-property correlations that identify how the critical temperatures for miscibility and composition limits can be controlled through selective functionalisation. This will allow, for example polymerisable ionic liquid hydrogel membranes to be produced with defined and predictable gellation and separation characteristics.

5. Phase behaviour of these ionic liquids in contact with electrolyte solutions was evaluated. Lower critical temperatures were observed from brines, compared to contacting with pure water demonstrating potential for the removal and recovery of water from aqueous electrolyte solutions using a temperature-swing mechanism.

Final results

New ionic liquid anions were demonstrated, drawing on bioisostere concepts common in pharmacy to identify target motifs and, for the key model system, an unprecedentedly rich LLE and SLE phase behaviour with water was observed. Beyond current state or the art, scheduled neutron scattering studies at the ISIS Neutron and Muon Source will probe directly the association of water molecules with the ionic liquid giving unprecedented atomic insight into behaviour that has only been described previously in thermodynamic terms.

The key results, to-date, from this project demonstrate that there are many opportunities still available to use chemical knowledge to rationally design new working fluid systems. Ionic liquids with the phenyltetrazolate motif will lead to new research applied to water treatment and security (temperature-swing forward osmosis desalination and waste water treatment), CO2-gas capture and recovery (applied to biogas separations for anaerobic digestion energy generation) and for the design of new thermal fluid working-pairs for heat adsorption units. Moreover, integrating the antifungal and antimicrobial characteristics of both tetraalkylphosphonium cations and tetrazole-derived anions could present new opportunities to develop antifouling coatings and additives in future work.