Display omitted •Pure α-Al2O3 with high thermal stability and OSC synthesized via combustion method.•4–10% of Power and 2–6% of BTE reduced by using B5 and B20 compared to B0.•Inclusion α-Al2O3 enhanced the power (up to 5% for B5Al) and BTE (up to 3.5% for B20Al).•The HC and CO emissions reduced using blend fuels and dropped 50% and 10% by adding α-Al2O3.•Although biodiesel increased the NOx emissions, it was reduced over 23% with the addition of α-Al2O3. Alumina has been recently studied as a fuel additive in diesel–biodiesel blends to enhance combustion characteristics in the engine chamber and reduce emissions. However, concerns have been raised about the potential formation of high NOx emissions. Alumina exists in different phases, each with distinct properties. In this research, the pure alpha phase of alumina was synthesized using the solution combustion method and mixed into diesel–biodiesel blends (B0, B5, and B20). Performance parameters (power, BSFC, BTE, and EGT) and emissions (CO2, CO, HC, and NOx) of a single-cylinder diesel engine were then examined. The prepared alumina exhibited alpha phase with high thermal stability over 500 °C and proper oxygen storage capacity (1741.8 μmol g−1) which provide it as a suitable catalyst to mix with fuel. The addition of α-Al2O3 to the fuel slightly increased the density, calorific value, and cetane number, while decreasing the viscosity and flash point. These changes in fuel properties influenced the engine performance, resulting in a 4.9 % (B5Al fuel) increase in power and a 3.5 % (B20Al fuel) increase in BTE while B5 and B20 fuels showed less power and BTE compared to diesel fuel at all engine loads. Furthermore, a 3.2 % and 16.2 % reduction in BSFC and EGT was respectively observed using B20Al due to the decrease in viscosity and flash point of fuels by adding α-Al2O3. The inclusion of α-Al2O3 also impacted emissions, resulting in reductions of 16–50 % in CO, 3–12 % in HC, and 1–23 % in NOx emissions. The results demonstrate the potential of α-Al2O3 as a catalyst for fuel combustion, owing to its high OSC and its ability to regulate engine temperature and promote complete combustion reactions.