Partial hydrogenation of polyunsaturated fatty acid methyl esters for biodiesel upgrading

Abstract

In this work, the improvement of biodiesel oxidative stability by partial hydrogenation of polyunsaturated fatty acid methyl esters (FAMEs) was studied, and can be divided into two parts based on the type of biodiesel feedstock. In the first part, partial hydrogenation of polyunsaturated FAMEs derived from palm oil was investigated. The Pd supported on carbon catalysts prepared from incipient wetness impregnation (IWI) technique, were used as a catalyst. The influences of Pd precursor, catalyst calcination condition, type of carbon support, and particle size of carbon support were studied for the partial hydrogenation of polyunsaturated FAMEs derived from palm oil. The results revealed that Pd particle size plays an important role on the hydrogenation activity of the catalyst. The Pd supported on activated carbon prepared from Pd (NO3)2.2H2O and calcined under N2 at 500°C, which possessed Pd particle size of ~17 nm, provided suitable performance for the partial hydrogenation of palm oil-derived polyunsaturated FAMEs and resulted in an improvement of the oxidative stability with small effect on the cold flow properties. The smaller particle size of activated carbon shows the higher hydrogenation activity. In addition, the effects of type of reactor (batch and continuous flow) and reaction conditions in continuous flow reactor (temperature, hydrogen partial pressure, and biodiesel feed flow rate) were investigated. It was found that a batch-type reactor provides higher selectivity towards monounsaturated C18:1 FAME than that of a continuous-flow reactor. However, at the conversion lower than 78%; selectivity of C18:1 obtained from both types of reactors were almost the same. The higher reaction temperature and hydrogen partial pressure, and lower biodiesel feed flow rate exhibited higher conversion of polyunsaturated C18:2 and C18:3 FAMEs. In the second part, biodiesel feedstock was changed to rapeseed oil, which contains higher polyunsaturated FAMEs composition when compared with palm oil. The Pd supported on mesoporous amorphous materials (SiO2, Al2O3, SiO2-Al2O3) prepared by IWI technique, were used as a catalyst. The effects of SiO2 pore size and support acidic properties were studied. The results showed that pore size of the SiO2 support had a significant effect on the activity and cis-trans selectivity of the catalyst. The Pd on ~45 nm pore size SiO2 exhibits the highest hydrogenation activity, whereas; cis-trans selectivity depends on contact probability between reactant and catalyst. Furthermore, the acidic properties of the support showed a significant effect on the sulfur (S) tolerance. The acidic supports (SiO2-Al2O3 and Al2O3) revealed a higher degree of S tolerance. In addition, the selectivity towards cis-isomers of the Pd catalysts could be increased by using acidic supports and the addition of S. Finally, the influence of metal type: Pt, Pd, and Ni, on catalytic activity and cis-trans selectivity was considered. It was found that Pd is the best catalyst in terms of partial hydrogenation, which results in a lower saturated FAMES composition when compared with the Pt and Ni catalysts at the same conversion. In addition, Pd shows the highest hydrogenation activity, which provides an advantage in terms of mild operating conditions.