Chemical Engineering Seminar

Chelating agents are widely utilized as functional materials in various industrial, environmental and biomedical processes including (i) selective extractants in hydrometallurgy, (ii) polymeric ligands in water treatment and (iii) contrast agent carriers in magnetic resonance imaging. Metal ion complexation is a ligand exchange reaction that depends on several parameters including (i) metal ion size and acidity, (ii) ligand basicity, chemistry and molecular architecture and (iii) solution physical chemical properties. Four milestones in metal ion coordination chemistry were the discoveries of the Hard and Soft Acids and Bases (HSAB) principle, the chelate effect, the macrocyclic effect and the cryptate effect. The invention of dendrimers may be viewed as another significant milestone in ligand architecture and coordination chemistry. Dendrimers are highly branched 3-D macromolecules with controlled composition and architecture consisting of three components: a core, interior branch cells and terminal branch cells. These soft nanostructures (1-20 nm in size) can serve as supramolecular hosts for cations, anions, organic solutes and bioactive molecules. Poly(amidoamine) (PAMAM) dendrimers were the first class of dendrimers to be commercialized. They possess functional nitrogen and amide groups arranged in regular "branched upon branched" patterns, which are displayed in geometrically progressive numbers as a function of generation level. During the last two decades, a broad range of experimental and computational tools have been employed to probe metal ion binding to PAMAM dendrimers including ultrafiltration coupled with atomic absorption spectroscopy, potentiometric titrations, UV-Visible spectroscopy, EPR spectroscopy, XAFS spectroscopy and DFT calculations. These studies showed that higher generation (≥ G4) PAMAM dendrimers behave as macroligands with large binding capacity and selectivity for metal ions in aqueous solutions including Cu(II), Pt(II), Pd(II), Ag(I), Au(I) and U(VI).

In this seminar, I will discuss the supramolecular chemistry of metal ion binding to dendritic macromolecules using PAMAM dendrimers and Cu(II) as model systems. In the first part of my presentation, I will argue that the exceptional Cu(II) binding capacity and selectivity of high generation PAMAM dendrimers could be attributed to a dendritic effect in metal ion coordination. This dendritic effect might be a characteristic signature of higher generation PAMAM dendrimers as soft colloids with covalently bonded polymeric chains. This endows these macromolecules with the ability to serve as high capacity and selective macroligands that can form both coordination and inclusion complexes with metal ions such Cu(II). In the second part of my seminar, I will highlight recent work that my group has initiated to prepare a new generation of mixed matrix membranes and films with in situ synthesized dendrimer-like particles (DLPs) that can serve as supramolecular hosts and containers for metal ions [e.g. Cu(II)] and zero valent metallic clusters [e.g. Cu(0)] using low generation (G0-G1) PAMAM dendrimers as precursors. More specifically, I will discuss how these membranes/films could be utilized to address important problems in Sustainable Chemistry, Engineering and Materials (SusChEM) including (i) the extraction of dissolved Cu(II) ions from liquid waste streams (e.g. chemical mechanical planarization wastewater from semiconductor fabs) and (ii) the preparation of novel Cu-based electrode materials for CO₂ reduction.