Zinc carboxylic salts used as catalyst in the biodiesel synthesis by esterification and transesterification: Study of the stability in the reaction medium

Abstract

Biodiesel is produced by the catalytic transesterification of renewable sources such as vegetable oils andanimal fats. It is an attractive alternative to diesel fuel because of its environmental benefits.In this work, the transesterification of soybean oil with methanol using zinc carboxylic salts as Lewisacid catalysts was studied. The esterification of fatty acids was also researched. In both reactions, thestability of the catalyst in the reaction medium was examined.Zinc carboxylic salts of different chain lengths, Zn(CnH2n+1COO)2 with n = 1, 11, 15, and 17, andZn(C17H33COO)2 (zinc(II) acetate, laurate, palmitate, stearate and oleate, respectively) were preparedand characterized by X-ray diffraction, FTIR spectrometry and thermogravimetric analysis.The Zn salts were tested in the transesterification of soybean oil at 100 .C for 2 h. They presented oilconversions between 88 and 94% with fatty acid methyl ester (FAME) yields between 71 and 74%, andthey were stable in three consecutive tests. Zn acetate was not stable. The salts transformed into Znglycerolate at 140 .C in the reaction medium (zinc laurate, palmitate, and stearate only partially).It was observed that, in the presence of fatty acids, the carboxylates of the salts were graduallyexchanged for the acid to be esterified. With oleic acid, this process was completed at 60 .C for all theZn(CnH2n+1COO)2 salts.Zinc laurate, palmitate and stearate are crystalline solids, but soluble in the reaction medium at 100 .C,and they recrystallize rapidly at room temperature, presenting certain advantages as regards decreasingmass transfer resistance during reaction, and easy separation and recovery from reaction medium.