Closed-Loop Valorization of Spent Mo(VI)-Loaded Alginate-Based Hydrogel as Reusable Adsorbent and Efficient Catalyst for Sustainable Benzimidazole Synthesis

Abstract Efficient recovery of molybdenum(VI) and the sustainable utilization of metal-loaded spent adsorbents are important for reducing secondary pollution and promoting circular chemistry. A sodium alginate/poly(dimethylacrylamide) semi-interpenetrating polymer network hydrogel (SSP) was developed for Mo(VI) adsorption and catalytic valorization. SSP showed a Mo(VI) adsorption capacity of 108.2 mg g–1 at pH 3 and 298 K, reached equilibrium within 120 min, and retained 86.8 mg g–1 after five cycles. Mechanical testing, 10-fold scale-up preparation, and simulated high-chloride wastewater adsorption supported its practical potential. The spent Mo-loaded SSP was directly used as a supported Mo catalyst for the oxidative synthesis of 2-substituted benzimidazoles without regeneration or post-modification. Using ethanol and H2O2, 2-phenylbenzimidazole was obtained in 90.2% yield, and substituted products were obtained in 47.2-93.5% yields. Apparent TON and TOF values were 4.44–8.80 and 1.48–5.86 h–1, respectively, with Mo leaching of approximately 0.027%. DPBF trapping and TEMP-EPR indicated that singlet oxygen was the dominant reactive species. This work establishes a Mo(VI)-specific adsorption-to-catalysis closed-loop system integrating metal removal, immobilization, and value-added organic synthesis.