Iron oxide-loaded alginate-bentonite hydrogel beads as a green and sustainable catalyst for 4-nitrophenol reduction

The development of sustainable green catalysts is always a challenging task. Besides this, the catalyst should be recyclable and hence cost-effective. In the present work, alginate hydrogel beads were used as a support of iron oxide to form iron oxide-loaded alginate beads (Io-alg). Alginate is a biopolymer, cheap, and nontoxic. Iron is abundant in nature and a non-toxic element. To improve the mechanical stability of the beads, bentonite was used to form iron oxide-loaded alginate-bentonite composite beads (Io-alg-ben). Bentonite is an inorganic material, easily available. Both Io-alg and Io-alg-ben beads in hydrogel conditions were deployed as the supported catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in borohydride environment. It is interesting to observe that hydrogel beads showed excellent catalytic activity toward 4-NP reduction. The zemvalent iron (ZVI) produced in-situ during the 4-NP reduction under NaBH 4 medium was the active catalyst for 4-NP reduction. The reduction was examined using various catalyst doses and various initial concentrations of 4-NP, and in all cases the reaction followed zero-order kinetics. The catalyst could be reused up to 5 cycles without any loss of efficiency. The turnover number (TON) and turnover frequency (TOF) of the catalyst were found as 4.3 x 10(19) molecules/g and 3.09 x 10(17) molecules/g/s for Io-alg-ben hydrogel beads. The performance is comparable with other reported catalysts. The effect of reaction temperature was also investigated to find out the activation energy. Thus, a sustainable iron oxide-loaded alginate-bentonite supported catalyst has been developed for the efficient reduction of 4-NP to 4-AP. The novelty of the present work is that here the iron-oxide loaded alginate-bentonite hydrogel beads are stable, economical, nontoxic, and reusable for many cycles.

Automated summary

The development of sustainable green catalysts, using alginate hydrogel beads as support for iron oxide, has shown efficient reduction of 4-nitrophenol to 4-aminophenol. The catalysts exhibit stability, cost-effectiveness, non-toxicity, and reusability for multiple cycles.