Supramolecular Hydrogels for Reagent and Catalyst Delivery

Hydrogels have found increasing use in widespread modern-day applications, including in the field of catalysis. Low-molecular weight gelators can be mixed and spatially shaped with polymer gelators to form a hybrid hydrogel bead. Beads of DBS-CONHNH2/sodium alginate had an average diameter of 3 mm, and the beads made up of DBS-CONHNH2/agarose had a larger distribution in diameter between 2-5 mm.
Hybrid hydrogel beads have been used for the support and growth of silver and palladium nanoparticles, which were used for the catalysis of nitro-aromatic reduction and Suzuki-Miyaura cross-couplings respectively. Both reactions were able to operate at room temperature in aqueous conditions. The hydrogel support was used for a silver nanoparticle catalysed 4-nitrophenol reduction, which achieved 43% yield, and a palladium nanoparticle catalysed 4-iodophenyl cross-coupling with phenyl boronic acid, which achieved 52% yield. The silver nanoparticle loaded alginate-based beads could be recycled to reduce 4-nitrophenol for a second time, although mechanical degradation of the beads prevented further recycles.
The two nanoparticle species were further added onto the same hydrogel support, giving a multi-nanoparticle loaded bead. The beads loaded with both silver and palladium nanoparticles were used for the Suzuki-Miyaura cross-coupling and 4-nitrophenol reduction, achieving a 52% and 43% yield respectively.
Palladium nanoparticle uptake was measured by UV-Vis and studied using Transmission Electron Microscopy (TEM), while silver nanoparticle uptake was measured using titration with sodium chloride and potassium chromate. Both nanoparticle species were also closely studied by TEM, where the silver nanoparticles were found to be 20-50 nm in diameter, while the palladium nanoparticles were between 1-10 nm.
The hybrid hydrogel bead is a versatile, simple-use catalyst tool for catalysis, that additionally provides a green approach to the field by improving recyclability for multiple reactions and negating the need for potentially dangerous and expensive ligands.