Critical assessment of copper-alginate hydrogel beads as recyclable and heterogeneous catalysts for aqueous azide‒alkyne cycloaddition.

Introduction: We report a reproducible and sustainable catalytic system based on copper ion-crosslinked alginate hydrogels for the synthesis of 1,2,3-triazoles via aqueous 1,3-dipolar cycloaddition at room temperature. The catalyst, derived from a biodegradable matrix, is prepared through a simple, energy-efficient method and operates under mild, eco-friendly conditions. Methods: Copper(II)-alginate hydrogels were prepared by ionic crosslinking. Sodium ascorbate was identified as a key reducing agent to generate catalytically active Cu(I) species. Different catalyst morphologies (hydrogel, aerogel, xerogel) were evaluated. Structural and morphological characterization was performed using FT-IR, SEM, EDX, TGA, and AAS. Results: The Cu(II)-alginate hydrogel (Cu(II)-AHG) exhibited the highest performance, delivering >95% yields within 3 h using only 2 mol% catalyst. The system tolerated a broad range of terminal alkynes and operated under predominantly heterogeneous conditions. The catalyst was recyclable up to four times with minimal loss in activity. Discussion: The catalyst's efficiency is attributed to its mesoporous structure and chemical integrity. These findings support the potential of alginate-based materials as sustainable supports for transition metal catalysis and contribute to the advancement of green and circular synthetic methodologies.