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.