Circular Upcycling of Waste Polystyrene into CO ₂ -Selective Thin-Film Composite Membranes for CO ₂ /CH ₄ Separation

The growing demand for sustainable energy recovery and greenhouse gas mitigation emphasizes the need for efficient, low-cost CO 2 /CH 4 separation technologies. In this study, waste-derived polystyrene (PS) was upcycled into gas separation membranes through structural modification with 2-methylimidazole and surface coating with PEBAX-1657. The membranes were systematically characterized to evaluate their molecular, structural, thermal, and morphological properties, demonstrating thermal stability up to 400 °C. Single-gas permeation experiments revealed that pristine PS exhibited moderate CO 2 permeance (209.53 GPU) and low CO 2 /CH 4 selectivity, indicating largely nonselective transport through the amorphous matrix. The incorporation of 2-methylimidazole significantly altered transport behavior, with M2 showing enhanced CO 2 permeance (669.56 GPU), CH 4 permeance (267.74 GPU), and improved CO 2 /CH 4 selectivity (2.50), indicating increased CO 2 affinity due to specific interactions within the modified polystyrene matrix. In contrast, the PEBAX-coated membrane (M4) achieved the highest CO 2 permeance (963.87 GPU), CH 4 permeance (196.70 GPU), and enhanced CO 2 /CH 4 selectivity (4.90), indicating that separation performance is predominantly governed by the selective PEBAX layer. Overall, this work establishes that waste PS effectively serves as a high-permeance membrane support, with surface engineering rather than bulk modification playing a dominant role in achieving improved gas separation performance, thereby supporting sustainable strategies for plastic waste valorization.