Biodiesel modified by catalytic transfer hydrogenation improving combustion performance

Abstract: Biodiesel has received extensive attention as a kind of renewable and clean fuel. However, because of its intrinsic unsaturated composition, it is prone to auto-oxidation and corruption in long-term storage. Fortunately, the partial hydrogenation of biodiesel, in which the high unsaturated fatty acid esters are selectively converted to the low unsaturated or saturated fatty acid esters, has been an effective measure to improve the oxidation stability as well as the cetane number. In this study, the partially hydrogenated soybean methyl ester (PHSME) was produced from soybean methyl ester (SME) via the catalytic transfer hydrogenation. The catalytic transfer hydrogenation of SME was implemented using isopropanol as the hydrogen donor, water as the reaction medium and Raney-Ni as the catalyst. The ratio of solvent water, isopropanol and SEM was 100:32:7, and the catalyst loading accounted for 13% of SME approximately. The hydrogenation reaction was progressing under the water bath of (85±1) ℃ with a magnetic stirring speed of 600 r/min. After about 100 min, the degree of hydrogenation for biodiesel was found to reach the maximum, and the final product PHSME was collected by suitable separation. By the gas chromatography-mass spectrometry (GC-MS) analysis, methyl palmitate (C16:0), methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2), methyl linolenate (C18:3), methyl eicosanoate (C20:0) and methyl docosanoate (C22:0) were detected out in sequence for SME sample, however, C18:3 did not exist in the PHSME. The total amount of unsaturated components C18:3, C18:2 and C18:1 in the SME was 70.9%. After moderate hydrogenation, the high unsaturated components C18:3 and C18:2 containing 3 and 2 double bonds were converted into C18:1 and C18:0 preferentially, and the conversion rate could reach 37.8%. In view of the number of unsaturated double bonds in carbon chain, the unsaturation degree of SME was reduced by 46.2%. Compared with SME, although the kinematic viscosity of PHSME increased slightly, its oxidation stability was improved significantly, and the cetane number of PHSME rose to a desirable level as well. In air atmosphere, the oxidation and combustion characteristics of SME and PHSME were comprehensively explored in a thermal analyzer. Due to the molecular structure change and increased kinematic viscosity, the start of weight loss for PHSME was a little late, whose TG (thermogravimetry) profile shifted to the high temperature region with respect to that for SME, however eventually the finish of weight loss advanced by 7.2 ℃, which affirmatively indicated that PHSME, owning a greater average oxidation rate than SME, was more prone to be oxidized and burned. Meanwhile, in DSC (differential scanning calorimeter) profiles, due to the desirable cetane number, the exothermic onset temperature of PHSME was 10.7 ℃ earlier than that of SME. In summary, the fuel properties including the oxidation stability, iodine value and cetane number of SME are beneficially upgraded by moderate hydrogenation. The better quality of partially hydrogenated biodiesel makes it more popular in the fuel blend market.