Nano-engineering in Ionic Liquid: From Fundamentals to Advanced Catalysis

Abstract

Harnessing nanostructured materials of metals has led to the development of various synthetic methodologies. Although various strategies have been developed, challenges still remain obtaining desired shape, size, and morphology of the particles. Presently, several synthetic approaches for the preparation of metal nanoparticles are available, yet there is still room for improvement, in particular toward the realization of nano-systems with desired properties. The combination of solvated metal atom dispersion (SMAD)1 and digestive ripening (DR)2 approaches, in particular, stands out from the rest in terms of scalability, purity, sustainability, and additionally, diversity of nanostructured materials. Capping ligands with a functional group, typically amine, alcohol, thiol, carboxylic acid, etc., at one end and a long alkyl chain are used for the surface coverage of nanoparticles, which prevents them from getting agglomerated. These capping agents, also referred to as digestive ripening agents, further play a significant role in the realization of nearly monodisperse nanoparticles from a polydisperse colloid through the process of digestive ripening. However, use of such ripening agents demands the use of high temperatures and prolonged reaction times. To mitigate this issue, environmentally benign capping agents, i.e., ionic liquids (ILs), could serve as an alternative. Ionic liquids offer subtle benefits compared to conventional capping agents, like the ease of synthesis, tunability of composition, and better “electro-steric” stability due to charged counterparts. In the present work, using a combination of SMAD and DR, the ability of ILs as ripening agents to transform a polydisperse colloid of metal nanoparticles into a nearly monodisperse colloid within a short period of time (~60 min) and at ambient temperatures has been demonstrated for the first time. By applying this strategy, the preparation of nearly monodisperse Au, Ag, and Cu nanoparticles was carried.3 Success of the ripening process in the monometallic NPs leverages the freedom to explore bimetallic NPs in similar synthetic protocols. Often bimetallic NPs offer better catalytic abilities compared to their monometallic counterparts due to the “synergistic effects” between the two metals.4 Considering this factor as an opportunity, the preparation of spherical Au–Pd alloys was carried out in IL with varying compositions of Au and Pd by combining SMAD and DR processes. These Au–Pd alloys were then screened for the catalytic 4-nitrophenol reduction to 4-aminophenol, which elucidated that the IL-capped bimetallic catalysts possess better activity. The IL-capped Au–Pd catalysts were then tested for an industrially relevant reaction, phenylacetylene reduction, and the catalyst with equimolar composition of Au and Pd revealed maximum conversion and selectivity towards styrene production.5 Further studies were carried out to explore the anisotropic growth of nanoparticles by modifying their surface. In this regard, Au–AgBr Janus heterostructures were developed by tuning the Au:Ag ratio in a bromide containing IL.6 The effects of different variables will be discussed in the talk alongside the mechanistic details of heterostructure formation. In addition, a new IL supported on SiO2 to generate a supported ionic liquid phase (SILP) has been designed and developed; this system showed high thermal stability up to 500 oC. Copper oxide (CuxO) nanoparticles prepared by SMAD were then incorporated into the SILP, and the CuxO@SILP catalyst was used for CO2 capture and valorization to yield drug molecule skeletons.7 The robustness and benefits of the catalyst will be discussed in the talk.