Green and recyclable mesoporous silica supported WO3–ZrO2 solid acid catalyst for biodiesel production by transesterification of Ankol seed oil with methanol

Abstract Biodiesel, an important sustainable fuel used in the transportation sector, demands a stable, recyclable and green catalyst for its economical and environmentally benign production. A novel green heterogeneous acid catalyst was developed by extracting sodium silicate from bamboo leaf ash (BLA), using which SBA-16 (BLA) was synthesized and then impregnated with 10 wt% each of WO3 and ZrO2, characterized and evaluated for the transesterification of Ankol seed oil with methanol to biodiesel. XRD, SEM, TEM and pore size characterization indicated that impregnated WO3 and ZrO2 were present outside the mesopores of SBA-16 (BLA) as monoclinic phases, thus 3D cubic cage-like Im3m mesopores of SBA-16 were unaltered. NH3-TPD indicated the presence of acid sites of two distinct strengths, attributed to the Lewis and Brønsted acidity of WO3–ZrO2 impregnated into SBA-16 (BLA) and hence gave the highest biodiesel yield of 98 %. In contrast 10 wt% of WO3 and 10 wt% of ZrO2 separately impregnated into SBA-16 gave 65  and 57 % of biodiesel yield respectively, possibly due to the presence of Lewis acidity alone in them. Among the WO3(10 %)–ZrO2(10 %) impregnated mesoporous supports viz. SBA-16 (BLA), SBA-16 (synthesized using tetraethyl orthosilicate), SBA-15, MCM-41, MCM-48, KIT-6, FDU-5, and TUD-1, the highest biodiesel yield of 98 % was given by SBA-16 (BLA), attributed to its spherical morphology and strong interaction with WO3–ZrO2 as inferred from SEM and XPS characterizations respectively. From the effect of process parameters on the WO3(10 %)–ZrO2(10 %)/SBA-16 (BLA) catalyst, maximum biodiesel yield was obtained at the temperature of 65 °C, catalyst amount of 200 mg, methanol:oil weight ratio of 10:1 and reaction time of 3 h. Under these reaction conditions, it retained the same biodiesel yield for six recycles after regeneration every time, confirmed its catalytic stability and recyclability.