<![CDATA[Waste cooking oil (WCO)-derived biodiesel represents a promising renewable fuel because it simultaneously supports waste valorization, reduces dependence on petroleum-derived diesel, and lowers feedstock costs. However, its relatively high viscosity, oxygenated ester structure, and lower energy density compared with conventional diesel can limit its fuel performance. Most existing upgrading strategies address these limitations through catalytic cracking or deoxygenation at temperatures above 250–350 °C, frequently involving pressurized hydrogen and extensive conversion of fatty acid methyl esters (FAME) into hydrocarbon-rich fuels, leaving limited understanding of whether biodiesel properties can instead be improved through low-severity treatment while preserving its FAME-rich character. This study investigated H₂-free mild catalytic upgrading of WCO-derived biodiesel using untreated bentonite and HCl-activated bentonite at catalyst loadings of 0.20, 0.40, 0.60, and 0.80 wt% in a rotary reactor operated at 150 °C for 1 h, followed by density, kinematic viscosity, calorific value, flash point, FTIR, and GC–MS analyses. The treatment caused only negligible variation in density, from 0.842 g/cm³ for the initial biodiesel to 0.843–0.844 g/cm³ after upgrading, indicating that the bulk molecular characteristics of the fuel were largely retained, while the kinematic viscosity decreased substantially from 4.94 cSt to minimum values of 3.54 and 3.55 cSt at 0.20 and 0.40 wt%, respectively. The calorific value increased from 41.46 MJ/kg to a maximum of 42.39 MJ/kg at 0.80 wt%, corresponding to an improvement of approximately 2.24%, while the 0.60 and 0.80 wt% samples exhibited flash points of 57.5 and 56.5 °C, respectively. GC–MS analysis revealed a compositional redistribution from 65.48% FAME and 16.13% aliphatic hydrocarbons in the initial biodiesel to approximately 53.46% FAME and 28.48% aliphatic hydrocarbons at 0.80 wt%; meanwhile, the 0.40 wt% sample showed the formation of approximately 3.80% C12 hydrocarbons, consistent with its lower viscosity and greater volatility. FTIR spectra retained the characteristic ester C=O and long-chain aliphatic bands, supporting limited molecular transformation rather than extensive destruction of the FAME framework. These findings demonstrate that low-temperature, low-catalyst-loading bentonite treatment can selectively redistribute biodiesel components and tune fuel properties without deep conversion, providing a low-severity upgrading pathway that bridges conventional biodiesel processing and high-temperature hydrocarbon-fuel production.]]>
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