A Comprehensive Review and Experimental Study on Biodiesel Upgrade through Selective Partial Catalytic Hydrogenation

Introduction:
Biodiesel, primarily composed of fatty acid methyl esters (FAMEs), offers a renewable alternative to conventional diesel fuel. However, its widespread application is limited by its poor oxidative stability, largely due to the high content of unsaturated FAMEs, particularly polyunsaturated compounds. Partial hydrogenation can improve stability but often degrades cold flow properties by increasing the proportion of trans-monounsaturated and saturated FAMEs. Achieving selective formation of cis-monounsaturated FAMEs is essential for balancing oxidative stability and cold flow performance.

Methods:
This study reviews conventional and unconventional catalytic systems for selective partial hydrogenation of biodiesel FAMEs. Conventional methods involve hydrogen gas with metal-supported catalysts, while alternatives include catalytic transfer hydrogenation (CTH), simultaneous transesterification–hydrogenation, and biphasic aqueous/organic systems. Experimentally, biodiesel derived from waste cooking oil (WCO) was upgraded using a Ru-TPPTS biphasic catalytic system to enhance oxidative stability.

Results:
Conventional hydrogenation showed limited selectivity and often compromised cold flow properties. Unconventional methods, particularly CTH and biphasic systems, demonstrated improved selectivity and efficiency. The Ru-TPPTS biphasic system significantly modified the FAME profile of WCO biodiesel, reducing polyunsaturated compounds and increasing the proportion of saturated components. This led to an improvement in oxidative stability of over 100%.

Conclusions:
Selective partial hydrogenation is a promising route for enhancing biodiesel quality. Biphasic systems and CTH methods offer effective, efficient, and scalable solutions. Continued optimization of these approaches could support the broader adoption of biodiesel as a sustainable fuel.