Study on the cis-trans isomerization of methyl oleate-rich biodiesel over sulfated tin(IV) oxide catalysts: the effect of iron and nickel modification

Nowadays, the world is undergoing a transition toward cleaner and more sustainable energy systems, with superior-quality biodiesel emerging as one of the promising alternatives. To realize this level, oxidation stability represents a major challenge in methyl oleate–rich biodiesel. In this work, cis - trans isomerization employing sulfated tin(IV) oxide-based catalysts was proposed to address this issue with minimal adverse impacts on cold flow properties. The effects of non-precious metal modification (undoped, iron, nickel, and iron−nickel), reaction temperature (200–250 °C), and reaction time (1–12 h) were investigated. The experimental results demonstrated that Fe − Ni/SO 4 /SnO 2 catalyst at reaction temperature of 200 °C and reaction time of 12 h provided the highest catalytic performance for the cis - trans isomerization, reaching a yield of 23.07%, a selectivity of 97.74%, and an isomerization turnover frequency (TOF) of 0.5 h − 1 . Co-impregnation of iron and nickel also effectively suppressed cracking reactions induced by the acidity of SO 4 /SnO 2 , although it slightly promoted hydrogenation. In this condition, a slight reduction was observed in the oxidation stability (ΔOS = − 0.1 h) due to the presence of olefinic cracking products. Meanwhile, the cloud and pour points were found to escalate (ΔCP = 1.5 °C; ΔPP = 1.0 °C), signifying a minor reduction in the cold-flow properties. According to these findings, sulfated tin(IV) oxide-based catalysts offer a promising prospect for further advancement to enhance catalyst performance and product oxidation stability.