A Hybrid Pre-Assessment Assists in System Optimization to Convert Face Masks into Carbon Nanotubes and Hydrogen

Abstract

With extensive attention being paid to the potential environmental hazards of discarded face masks, catalytic pyrolysis technologies have been proposed to realize the valorization of wastes. However, recent catalyst selection and system design have focused solely on conversion efficiency, ignoring economic cost and potential life-cycle environmental damage. Here, we propose an economic–environmental hybrid pre-assessment method to help identify catalysts and reactors with less environmental impact and high economic returns among various routes to convert discarded face masks into carbon nanotubes (CNTs) and hydrogen. In catalyst selection, it was found that a widely known Fe–Ni catalyst exhibits higher catalytic activity than a cheaper Fe catalyst, potentially increasing the economic viability of the catalytic pyrolysis system by 38%–55%. The use of this catalyst also results in a carbon reduction of 4.12–10.20 kilogram CO₂ equivalent for 1 kilogram of discarded face masks, compared with the cheaper Fe catalyst. When the price of CNTs exceeds 1.49 × 10⁴ USD·t^–1, microwave-assisted pyrolysis is the optimal choice due to its superior environmental performance (in terms of its life-cycle greenhouse gas reduction potential, eutrophication potential, and human toxicity) and economic benefits. In contrast, conventional heating pyrolysis may be a more economical option due to its good stability over 43 reaction regeneration cycles, as compared with a microwave-assisted pyrolysis catalyst with a higher conversion efficiency. This study connects foundational science with ecological economics to guide emerging technologies in their research stage toward technical efficiency, economic benefits, and environmental sustainability.