Synergistic CPL/APS catalytic pyrolysis for fabricating high-yield and ultra-pure rayon-based carbon fiber mats

High-yield and ultra-pure rayon-based carbon fiber (RCF) mats were successfully fabricated via a synergistic catalytic pyrolysis strategy using ammonium persulfate (APS) and caprolactam (CPL), aiming to overcome the conventional trade-off between carbon yield and catalyst-derived impurities. The catalytic mechanism, pyrolysis behavior, and structural evolution and performance were systematically investigated. APS selectively oxidized C6 hydroxy groups of cellulose into carboxyl functionalities and generated an in situ acidic environment, which suppressed levoglucosan formation and reduced the apparent activation energy of pyrolysis. Under these conditions, CPL underwent acid-induced ring opening to form aminohexanoic acid intermediates that reacted with cellulose-derived species through Schiff-base condensation, leading to the formation of covalently cross-linked structures. This cooperative mechanism effectively inhibited depolymerization and enhanced carbon yield during thermal conversion. As a result, the R-C/A-2000 sample achieved a high carbon yield of 31.78%, and developed turbostratic graphite microcrystals with a d 002 spacing of 0.362 nm after graphitization at 2000 °C. Subsequent halogen purification reduced the total impurity concentration to 119.23 ppm, meeting the purity requirements for semiconductor thermal-field insulation materials. The results demonstrated that synergistic regulation of cellulose pyrolysis pathways via CPL/APS catalysis is an effective strategy to simultaneously improve carbon yield and product purity in RCF mat production. Graphical abstract