Ethyl esters obtained from pequi and macaúba oils by transesterification with homogeneous acid catalysis

•Biodiesel production from macaúba and pequi oils in different reaction times.•Ethanol is the most viable alcohol in transesterification because it is produced by renewable sources.•Acid homogeneous catalysis is the best route for oils with high% FFAs.•Use of techniques and analysis to identify and quantify the number of ethyl esters in a sample of biodiesel.•Conversions of 95–98% for different reaction times. In this work, it was investigated the presence of ethyl esters in transesterified pequi (PO) and macaúba (MO) oils. To obtain the biodiesel, an acid-catalyzed homogeneous reaction was used. Yields of 95% for MO were obtained using an oil:ethanol ratio of 1:30 and sulfuric acid (H2SO4), reaction times of 4, 8, 12, and 24 h were used. The biodiesel samples obtained from the PO are denoted BP4, BP8, BP12, and BP24, and those from MO are denoted BM4, BM8, BM12, and BM24 according to the reaction time. The kinematic viscosity of the biodiesels was satisfactory and within the specifications stipulated by ANP. The density analysis showed that the biodiesels presented values that are within the limits stipulated by ANP and the European Union. With 1H NMR measurements, the presence of ethyl esters and the conversion rates were verified. The results by oxidative stability showed that the biodiesel obtained for 24 h are close to the values required by the ANP. The analysis of the ester content showed that the biodiesels obtained with the longest reaction time are in accordance with ANP resolution specifications. Raman spectroscopy revealed the stretching band of the C-O-C group in the ethyl ester molecule at 870 cm−1 for all samples. Using thermogravimetry, the maximum degradation temperatures were 243 and 205 °C for BP and BM, respectively. The characteristic absorption bands of the transesterified oils were identified by FT-IR at 743 cm−1 (CO group of the ethyl esters) and 800–1700 cm−1 (CO of esters).