Microwave-assisted upgrading of fluid catalytic cracking-derived vacuum gas oil with Fe-modified biochar microspheres

• Fe-biochars achieved >77 % viscosity reduction of FCC-VGO via microwave heating. • Amphiphilic surface enhanced microwave absorption and intermediate stability. • FeBM12 showed high colloidal stability and balanced wettability. • Catalyst retained performance over multiple reuse cycles. Fe-modified biochar microspheres (FeBM) were developed as sustainable catalysts for the microwave-assisted upgrading of Fluid Catalytic Cracking-derived Vacuum Gas Oil (FCC-VGO), both in the presence and absence of decaline as hydrogen donor source. The biochars were synthesized by hydrothermal carbonization of starch at 200 °C for 6 and 12 h, followed by incipient wetness impregnation with 1.8 wt% Fe(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O. Structural and surface analyses (FTIR, XPS, Raman, SEM) confirmed the formation of carbonaceous microspheres with homogeneously dispersed Fe species, surface oxidation (Fe-O, SO 4 2- ), and stability under microwave irradiation. These features enhanced interfacial polarization and amphiphilic balance, facilitating the cleavage of S-C and aromatic C O bonds. Raman spectra confirmed the preservation of carbon domains after Fe incorporation. Under mild conditions (80 °C, 6 min, 800 W), FeBM12 exhibited the best catalytic performance, achieving a 77.03% viscosity reduction (from 444 to 102 mPa·s), increasing saturates to 65.6 wt%, and reducing asphaltenes to 2.2 wt%. Zeta potential measurements (-30.7 mV for FeBM12) and contact angle analysis (60.4°) demonstrated enhanced colloidal stability and surface polarity. These parameters favored catalyst dispersion and oil-catalyst interactions, which are critical for microwave responsiveness and catalytic efficiency. In comparison to conventional systems requiring high temperatures, long reaction times, or pressurized hydrogen, FeBM12 offers a low-cost, low-temperature, and energy-efficient platform for upgrading FCC-derived VGO.