From expired metformin drug to nanoporous N-doped-g-C3N4: Durable sunlight-responsive photocatalyst for oxidation of furfural to maleic acid

Abstract

Following the waste management approach and increasing contamination by the pharmaceutical industry, expired metformin drug, a nitrogen-rich organic scaffold, was employed as a nitrogen dopant for the synthesis of valuable N-doped g-C3N4 under very mild conditions. To this end, various amounts of expired metformin drugs were combined with urea, providing N-doped g-C3N4 with a high N/C ratio, confirmed by X-ray photoelectron spectroscopy (XPS). It was found that the presence of the waste doping agent not only resulted in nitrogen-rich graphitic carbon nitride but also elegantly improved its physical and optical properties such as surface area and the rate of recombination of photogenerated charge carriers, as well as band gap. The photocatalytic potential of prepared samples was evaluated in the oxidation of furfural under sunlight irradiation. In this line, prepared material containing 1 wt% expired metformin drug, which benefits from high surface area and efficient separation of photo-generated electron-hole pairs, was a more efficient photocatalyst to deliver the maleic acid product with high selectivity and yield (98% yield) using hydrogen peroxide as an oxidizing agent. In addition, the photocatalytic system exhibited high stability and could be reused seven times. The key role of hydroxyl and superoxide radicals in the progression of the reaction was established based on control experiments along with proposing the mechanism pathway. Indeed, this study demonstrates a deeper understanding of waste management concepts in chemical applications, particularly in the synthesis of privileged catalysts using green and sustainable approaches.