Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Abstract

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as "boomerang" type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.

Endohedrally Functionalized Metal–Organic Cage-Cross-Linked Polymer Gels as Modular Heterogeneous Catalysts

“…Toward this end, the immobilization of homogenous catalysts onto amorphous cross-linked or self-assembled (co)­polymers is a common strategy that benefits from the compositional diversity, synthetic flexibility, and tunable physical properties of polymers (Figure a). ,− When such catalyst-functionalized polymer networks are in a highly swollen gel state, reagent mobility can approach that within typical solutions, facilitating reasonable reaction rates. − Moreover, the mechanical robustness of synthetic polymers can allow for repeated reuse of such materials and facile physical manipulation. Nevertheless, the amorphous nature of traditional polymers makes it difficult to control the local catalyst environment at molecular scales, giving rise to unpredictable outcomes and potential batch-to-batch variation.…”

Ligand-regulated metal–organic frameworks for synergistic photoredox and nickel catalysis

“…[10][11][12] Third, is the non-recyclability of catalysts, which raises substantial practical difficulties. 13,14 Metal-organic frameworks (MOFs) are a new class of crystalline-state materials with long-range ordered and welldefined structures, 15,16 which have been used as heterogeneous catalysts for various catalytic conversions. 17,18 The important advantage of metal-organic frameworks is the possibility of easily and systematically designing and regulating the functionality at a molecular level by applying reticular chemistry.…”

“…10–12 Third, is the non-recyclability of catalysts, which raises substantial practical difficulties. 13,14…”

Supramolecular Immobilization of Adamantyl and Carboxylate Modified N-Heterocyclic Carbene Ligand on Cucurbituril Substrates

“…The immobilization of homogeneous catalysts has been gaining traction in recent decades owing to their attractive features in recoverable and recyclable catalysts [ 1 , 2 ]. Key to this success is the sustained effort to obtain versatile ligands while ensuring their electronic properties are not negatively affected by modifications.…”

“…While traditional heterogeneous catalysts are widely used in the industries, the process is plagued with low surface area and diminished catalytic activities, thereby limiting their applications. In recent years, novel solutions emerged in the form of non-covalent Immobilizable NHC ligands, where homogeneous catalysts are tethered to solid supports via π–π stacking [ 10 ], electrostatic [ 11 ], physio-sorption [ 2 ] or host–guest interactions [ 7 ]. They can be designed to include chemical moieties with superior supramolecular binding affinity and modular designs.…”