Porous Carbon Fiber Functionalization and Thermochemical Degradation of Polystyrene-based Plastics for Resource Recovery

Abstract

This dissertation explores the chemical transformation of polyacrylonitrile (PAN)- and polystyrene (PS)-based polymers into sustainable functional materials and chemical products. The research is motivated by two global challenges: the increasing demand for rare earth elements (REEs) for high technology applications and the environmental issues caused by plastic waste accumulation. First, porous carbon fiber (PCF) derived from PAN-based block copolymer was synthesized and functionalized for the extraction of REEs. Second, the thermochemical degradation of PS and PS-based engineering plastic into benzene and alkylbenzene were explored. In the first project, a greener REE leaching method followed by solid-phase extraction using functionalized PCF was developed. To dissolve REE in NdFeB magnets, citrate-assisted leaching was developed and achieved over 90% leaching efficiency, reducing the use of mineral acid. A block copolymer-based PCF with high surface area and well-controlled porous structure was synthesized and functionalized with diglycolamide (DGA) ligands. Two different functionalization strategies, using silane-modified and azide-modified DGA, were employed. Although functionalization of PCF with both strategies was successful, limited improvement in extraction capacity was achieved. The poor performance was attributed to low ligand loading, partial pore blocking, and strong citrate-REE complex interaction. This study provided PCF functionalization strategies but showed challenges of REE extraction with functionalized PCF as an adsorbent. The second project focused on the degradation of PS into benzene and its subsequent upcycling into alkylbenzene. PS was thermochemically degraded using AlCl3 as a catalyst to produce benzene. The effect of Lewis acid type, solvent, and AlCl3 concentration was investigated. The optimized reaction conditions for benzene production were identified as 100 mol% AlCl3 in cyclohexane at 100 °C for 5 h. The recovered benzene was further upcycled to alkylbenzene by Friedel-Crafts alkylation with 1-dodecene or 1-chlorodedecane. Both one-step and two-step alkylation reactions for alkylbenzene production were explored, but a low alkylbenzene yield was achieved. GC-MS analysis revealed the formation of alkylbenzene isomers and alkane side products, indicating low selectivity and competing side reactions as the limitation of alkylbenzene production. The third project extended the thermochemical degradation strategy to an engineering plastic, acrylonitrile-butadiene-styrene (ABS), containing a PS block. Compared with PS, ABS is a multiphase polymer, containing two other polymer blocks, PS and polybutadiene (PBD), which make the degradation pathway more complicated. Under similar conditions of PS degradation using AlCl3, the benzene yield was lower in ABS degradation. Size exclusive chromatography (SEC) analysis revealed the incomplete degradation with crosslinked structures. The effect of solvent on benzene yield was investigated, showing that the benzene yield was affected by both aromaticity of the solvent and the solvent-AlCl3 interactions. Deuterated benzene as an aromatic solvent enhanced benzene yield due to charge-transfer interaction with AlCl3. Degradation studies on PAN and PBD homopolymers showed that both PAN and PBD underwent partial backbone degradation, forming short-chain fragments. Nitrile groups in PAN remained intact, and PBD underwent crosslinking. These results showed that non-PS blocks altered the degradation pathways and hindered the benzene production. Overall, this dissertation advances the understanding of polymer-derived porous carbon materials and thermochemical plastic degradation for sustainable material recovery. While challenges remain in the improvement of REE extraction capacity and achieving high alkylbenzene and benzene yield, the results highlighted the potential of PCF-based adsorbents for REE extraction and thermochemical degradation pathways for plastic recycling. These studies not only provide fundamental insights into structure-property relationships but also suggest future directions toward greener and more efficient strategies for addressing critical metal recovery and plastic waste management.