The thermal and physicochemical properties of protic ionic liquids (PILs) are reported. It is highly evident that there has been an extensive range of alkylammonium, imidazolium, and heterocyclic cations paired with many organic and inorganic anions that have been employed to prepare PILs. There has been strong interest in modifying the properties of PILs through the addition of water or other molecular solvents. For many applications, the presence of some water in the PILs is not detrimental, and instead leads to enhanced solvent properties such as lower viscosity, higher conductivities, and lower melting points. It remains an issue of definition though of how to refer to these resulting protic solutions. There is also an ongoing difficulty surrounding how to describe the proton activity in the PILs, analogous to pH in aqueous systems. For a broad range of applications, it has been reported that the acidity/basicity of the PIL or PIL-solvent system is crucial for their beneficial properties. It is expected that the fundamental properties of PILs will continue to be explored, along with continued interest in many existing and new applications, such as in electrochemistry, organic and inorganic synthesis, and biological applications. In particular, there has been a significant interest in a broad- range of PILs for use as electrolytes and incorporation in polymer electrolytes for fuel cells, and other energy storage devices.

Protic Ionic Liquids: Evolving Structure–Property Relationships and Expanding Applications

In this manuscript, four common transition-metal derived metal–organic frameworks have been extensively investigated as heterogeneous catalyst supports for Knoevenagel condensation reactions. A simple post-synthetic modification strategy was employed for the rapid and facile introduction of a primary alkyl amino group. The resulting novel MOF–RNH2 catalysts showed greatly enhanced Knoevenagel condensation reactivities towards a variety of aldehyde electrophiles. IRMOF-3 proved to be an unsuitable heterogeneous catalyst support due to its fragile nature upon treatment with bases. The novel zirconium based UiO-66–NH–RNH2 and chromium based Cr-MIL-101–NH–RNH2 materials showed excellent catalytic reactivities, while being highly convenient to synthesize. The basic catalytic activity was further extended to the Henry reaction, and excellent catalytic reactivity was achieved. The size-selectivity was also studied to show that the Knoevenagel condensation occurred inside of the porous structure of the MOF catalyst. The recycling properties of zirconium, aluminum and chromium derived MOFs were evaluated and zirconium based UiO-66 and chromium based Cr-MIL-101 showed excellent catalytic efficiency after five reaction cycles.

A general post-synthetic modification approach of amino-tagged metal–organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction

A multifunctional carbazole-based conjugated microporous polymer MFCMP-1 is successfully prepared by oxidative coupling polymerization using a single monomer and structurally characterized. A new three-dimensional π-conjugated polymer framework can be combined with permanent microporous, highly luminescent properties and abundant nitrogen activated sites in the skeleton. It possesses a large BET surface area of over 840 m2 g−1 with a pore volume of 0.52 cm3 g−1, and displays a high carbon dioxide uptake capacity (up to 3.69 mmol g−1) at 273 K and 1 bar, with good selectivity towards CO2 over N2 and CH4. MFCMP-1 exhibits also strong fluorescent emission at 529 nm after excitation at 380 nm in THF solution and works as a luminescent chemosensor towards hazardous and explosive molecules, such as nitrobenzene, 2-nitrotoluene, and 2,4-dinitrotoluene. In addition, MFCMP-1 features a high concentration of Lewis base nitrogen sites on its internal surfaces; it thus acts as a highly efficient recyclable heterogeneous organocatalyst towards Knoevenagel reaction of malononitrile with aromatics, heterocyclic aldehydes, and cyclic ketones. Furthermore, we further highlight that the ease of synthesis and low cost, coupled with multifunctional properties, make MFCMP-1 an attractive functional material in practical applications.

Gas uptake, molecular sensing and organocatalytic performances of a multifunctional carbazole-based conjugated microporous polymer

Ionic liquids, as reaction media, and sonochemistry are two recently developing fields of chemistry that present some similarities. Firstly, they constitute separately unconventional approaches to reaction chemistry that, in many cases, generate improvements in yield, rate and selectivity compared to classical chemistry, or even change the mechanisms or products expected. In addition, both are often associated with green chemistry concepts as a result of their properties and their possible eco-friendly uses. A recent trend has been to combine these two technologies in a range of different applications and the results demonstrate very significant and occasionally surprising synergetic effects. Here we critically review the advantages and limitations of the ionic liquid/ultrasound combination in different applications in chemistry, to understand how, and in which respects, it could become an essential tool of sustainable chemistry in the future. Many practical and theoretical aspects associated with this combination of techniques are not understood or resolved and we discus where fundamental studies might further advance this field.

Ionic liquids and ultrasound in combination: synergies and challenges

Sonochemistry in non-conventional, green solvents or solvent-free reactions

An efficient and reusable protic-ionic-liquid solvent–catalyst system, HMTA–AcOH–H2O, has been developed and used in the Knoevenagel condensation reaction of aromatic aldehydes with ethyl 2-cyanoacetate. Under ultrasonic irradiation, the Knoevenagel condensation promoted by the protic-ionic-liquid solvent–catalyst system proceeds smoothly and cleanly. Moreover, the HMTA–AcOH–H2O solvent–catalyst system could be recycled for at least 6 times and no significant loss of activity was observed. This protocol has notable advantages, such as being eco-friendly, the ease of the work-up and reuse of the ionic liquid conveniently, which could help reduce disposal costs and contribute to the development of new catalysts for use in green and continuous chemical processes.

Sanhu Zhao

Papers

Rapid and efficient Knoevenagel condensation catalyzed by a novel protic ionic liquid under ultrasonic irradiation