Microwave Encounters Ionic Liquid: Synergistic Mechanism, Synthesis and Emerging Applications

Progress in microwave (MW) energy application technology has stimulated remarkable advances in manufacturing and high-quality applications of ionic liquids (ILs) that are generally used as novel media in chemical engineering. This Review focuses on an emerging technology via the combination of MW energy and the usage of ILs, termed microwave-assisted ionic liquid (MAIL) technology. In comparison to conventional routes that rely on heat transfer through media, the contactless and unique MW heating exploits the electromagnetic wave-ions interactions to deliver energy to IL molecules, accelerating the process of material synthesis, catalytic reactions, and so on. In addition to the inherent advantages of ILs, including outstanding solubility, and well-tuned thermophysical properties, MAIL technology has exhibited great potential in process intensification to meet the requirement of efficient, economic chemical production. Here we start with an introduction to principles of MW heating, highlighting fundamental mechanisms of MW induced process intensification based on ILs. Next, the synergies of MW energy and ILs employed in materials synthesis, as well as their merits, are documented. The emerging applications of MAIL technologies are summarized in the next sections, involving tumor therapy, organic catalysis, separations, and bioconversions. Finally, the current challenges and future opportunities of this emerging technology are discussed.

Intercalation of Ionic Liquids into LDH Structures for Microwave-Accelerated Polymerizations

Microwave-accelerated ring-opening polymerization (ROP) of cyclic esters catalyzed by ionic liquid (IL) anions, intercalated into layered double hydroxides (LDHs), has been recently described as a fast and environmentally friendly synthetic way to prepare biodegradable polyester/LDH nanocomposites. However, to observe this synergistic catalytic effect between microwaves and IL anions and to achieve a homogeneous structure of the final polymer nanocomposite, the IL anions must be efficiently intercalated inside the LDH structure. Herein, we investigate the effects of various metal compositions of M2+/Al3+ LDHs (M = Mg, Co, and Ca) and different LDH synthetic routes (one-step direct coprecipitation, two-step coprecipitation/anion exchange, and two-step urea/anion exchange) on the intercalation efficiency of trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate IL. The most effective IL anion intercalation was observed for Ca2+/Al3+ LDH prepared using the two-step method consisting of coprecipitation and subsequent anion exchange. After optimization, this synthetic pathway led to the production of LDHs with intercalated IL anions and a reduced amount of intercalated water (<0.6 wt %). The catalytic ability of thus optimized LDH particles was demonstrated on the microwave-assisted ROP of ε-caprolactone, showing rapid progress of polymerization. Within minutes, the polycaprolactones with an average molecular mass in the range of 20 000-50 000 g/mol containing fully delaminated and exfoliated LDH nanoparticles were obtained.

IL-Functionalized Mg3Al-LDH as New Efficient Adsorbent for Pd Recovery from Aqueous Solutions

Palladium is a noble metal of the platinum group metals (PGMs) with a high value and major industrial applications. Due to the scarce palladium resources, researchers' attention is currently focused on Pd ions recovery from secondary sources. Regarding the recovery process from aqueous solutions, many methods were studied, amongst which adsorption process gained a special attention due to its clear advantages. Moreover, the efficiency and the selectivity of an adsorbent material can be further improved by functionalization of various solid supports. In this context, the present work aims at the synthesis and characterization of Mg₃Al-LDH and its functionalization with ionic liquid (IL) (Methyltrialkylammonium chloride) to obtain adsorbent materials with high efficiency in Pd removal from aqueous solutions. The maximum adsorption capacity developed by Mg₃Al-LDH is 142.9 mg Pd., and depending on the functionalization method used (sonication and co-synthesis, respectively) the maximum adsorption capacity increases considerably, q_max-Mg₃Al IL-US = 227.3 mg/g and q_max-Mg₃Al IL-COS = 277.8 mg/g.

Effects of Immobilized Ionic Liquid on Properties of Biodegradable Polycaprolactone/LDH Nanocomposites Prepared by In Situ Polymerization and Melt-Blending Techniques

The high capacity of calcinated layered double hydroxides (LDH) to immobilize various active molecules together with their inherent gas/vapor impermeability make these nanoparticles highly promising to be applied as nanofillers for biodegradable polyester packaging. Herein, trihexyl(tetradecyl)phosphonium decanoate ionic liquid (IL) was immobilized on the surface of calcinated LDH. Thus, the synthesized nanoparticles were used for the preparation of polycaprolactone (PCL)/LDH nanocomposites. Two different methods of nanocomposite preparation were used and compared: microwave-assisted in situ ring opening polymerization (ROP) of ε-caprolactone (εCL) and melt-blending. The in situ ROP of εCL in the presence of LDH nanoparticles with the immobilized IL led to homogenous nanofiller dispersion in the PCL matrix promoting formation of large PCL crystallites, which resulted in the improved mechanical, thermal and gas/water vapor barrier properties of the final nanocomposite. The surface-bonded IL thus acted as nanofiller surfactant, compatibilizer, as well as thermal stabilizer of the PCL/LDH nanocomposites. Contrary to that, the melt-blending caused a partial degradation of the immobilized IL and led to the production of PCL nanocomposites with a heterogenous nanofiller dispersion having inferior mechanical and gas/water vapor barrier properties.