Byproduct Formation Mechanisms and Effect of Mass Transfer in Plant Sterol Hydrogenation

Plant sterols, a mixture of several des-4-methyl sterols, were hydrogenated over a Pd/C catalyst by varying catalyst amount and stirring rate. Reactions were carried out under kinetic regime and under the influence of external mass transfer limitations. All reaction were done in the absence of internal diffusion limitations. Under external mass transfer limitations of hydrogen more byproducts due to hydrogenolysis and double bond migration were formed. Under mass transfer limited reactions higher catalyst amounts led to a more extensive byproduct formation. Because of double bond migration two very similar byproducts from sitosterol were formed having trans- and cis-fused rings. Interestingly hydrogenolysis also resulted in two similar stereoisomers of sitostane, in the same way as sitostanone, indicating that the hydrogenolysis occurred during the double bond migration when the double bond is in the Delta 3 position. An intermediate product of stigmasterol was observed, in which the ring structure is hydrogenated but the alkyl chain double bond is intact, proving that the ring double bond is hydrogenated prior to alkyl chain double bond hydrogenation. The results show that under mass transfer limitation an optimal amount of catalyst should be used to minimize unwanted byproduct formation.