Catalytic Upgrading of Biodiesel by Partial Hydrogenation of Its FAME Composition: A Systematic Review

Biodiesel is one of the most popular biofuels as it is a promising substitute for conventional diesel fuel, but for now, it cannot be used as a stand-alone fuel due to its low oxidative stability. For this reason, there have been efforts to upgrade biodiesel in the last few years through the partial hydrogenation of its fatty acid methyl esters (FAMEs). This procedure succeeds in resolving the problem of low oxidative stability, but in some cases, a new issue arises as non-selective hydrogenation deteriorates the fuel's cold flow properties, which are crucial for vehicle's operation without damaging the vehicle's engine. More specifically, the problem of deteriorated cold flow properties is caused by the formation of trans-monounsaturated and fully saturated FAMEs during the hydrogenation reaction. Hydrogenated biodiesels are preferred to contain more cis-monounsaturated FAMEs because these are considered to have the best combination of high oxidative stability and good cold flow properties. As a result, various systems and methods have been tested to achieve selective partial hydrogenation of biodiesel FAMEs. In this review article, the catalytic systems and processes that stood out in various research studies are presented, and the factors that lead to the best possible outcome are investigated and discussed.