Spent–fresh FCC catalyst blending for temperature-tuned valorization of HDPE into fuel-range hydrocarbons and light olefins

The rapid accumulation of polyolefin waste and the simultaneous generation of spent refinery catalysts pose serious environmental and resource management challenges, demanding integrated valorization strategies. Herein, a blended spent–fresh fluid (sFCC–fFCC) catalytic cracking system is proposed as a dual-waste solution for temperature-controlled catalytic pyrolysis of high-density polyethylene (HDPE). Physicochemical characterization (XRD, ED-XRF, NH 3 -TPD) showed that fresh FCC possessed high total acidity (1.177 mmol g −1 ), while spent FCC exhibited lower acidity (0.294 mmol g −1 ) but contained metal-containing deposits that may influence dehydrogenation and hydrogen-transfer reactions. Calcination at 900 °C reduced the total acidity and high-temperature acid-site population, thereby suppressing excessive secondary cracking and improving liquid selectivity. Non-catalytic pyrolysis required ≥540 °C to achieve maximum oil yield (71.33 wt%), whereas the best-performing 50S50F catalyst blend reduced the optimal oil production temperature to 435 °C, delivering 67.94 wt% oil with negligible char (<1 wt%). Excess catalyst loading (15 wt%) promoted over-cracking and increased gas yield to 62.6 wt%, while 10% provided optimal selectivity. GC-DHA analysis revealed catalytic pyro-oil enriched in olefins (32–38%) and aromatics (8–14%). The pyro-oils exhibited high heating values (41–42 MJ kg −1 ) and low viscosity (0.49–0.86 mPa s), indicating effective in situ upgrading. Gas analysis showed enhanced light-olefin production, with propylene selectivity reaching 26.07%. The 50S50F blend exhibits a favorable balance between acidity and metal functionality, facilitating the selective conversion of HDPE into fuel-range hydrocarbons and light olefins.